hydrocarbons. High-intensity and large-scale migration and charging occurred in the study area, and large-scale accumulation occurred in Mahu sag and its periphery. The second stage of hydrocarbon charging is in the early–middle Cretaceous, which is blue-white fluorescent high maturity crude oil and is also widely charged and accumulated. These results are consistent with previous studies [20,21].

**Figure 9.** Homogeneous temperature histogram of brine inclusion.

**Figure 10.** Burial history of well DT1.

#### *5.2. Organic Acid Dissolution and Its Control on Accumulation*

Hydrocarbon source rocks can produce a large amount of organic acids before generating a large amount of oil. The organic acids formed have a great influence on the diagenesis of reservoirs, which has been recognized by many scholars [22–25]. The kerogen in the source rock will remove oxygen-containing functional groups (such as carboxyl and phenol) and form a large number of organic acids (such as oxalic acid and acetic acid) under the thermal action of 80~120 ◦C. These organic acids are easy to form complexes with Al3+, which increases the activity of Al3+ and promotes the dissolution of aluminosilicate and calcite. When the temperature rises to 120~160 ◦C, the carboxylic acid anion will undergo thermal decarboxylation and crack into hydrocarbons and CO2, which will increase the concentration of CO2 in the strata water and reduce the concentration of organic

acids [22,25]. At the same time, the concentration of acid in strata water will increase due to the dissolution of CO2 in water to form carbonic acid. As a result, alkaline minerals will continue to be dissolved.

As shown in Figure 11, yellow fluorescent mature hydrocarbon inclusions and blue fluorescent highly mature hydrocarbon inclusions can be observed simultaneously. It is speculated that it may be due to the formation of new dissolution pores by organic acids dissolving alkaline minerals during the migration of mature oil in the first stage, which provides space for the capture of highly mature hydrocarbon in the second stage, resulting in the occurrence of yellow fluorescent inclusions and blue fluorescent inclusions in the same part of the reservoir. It provides strong evidence for organic acid dissolution minerals to improve reservoir physical properties.

**Figure 11.** Fluorescence photos of hydrocarbon inclusions coexisting with different maturity. (**a**) Well M19, 3534.1 m, distributed along the microcracks of quartz grain, showing yellow or bluegreen fluorescence; (**b**) well M217, 4010.02 m, distributed along micro-fractures of analcime cement, showing blue-green or yellow-green fluorescence; (**c**) well M217, 4010.02 m, distributed in belts along micro-fractures of quartz grain, showing blue-green fluorescence; (**d**) well YB4, 3871 m, distributed in belts along the micro-fractures of quartz particles, showing yellow-green or bluish-green fluorescence; (**e**) well YB4, 3914 m, distributed in belts along micro-fractures of quartz particles, showing yellow or yellow-green fluorescence; (**f**) well D15, 4249 m, distributed in belts along micro-fractures of quartz particles, showing yellow, blue or yellow-green fluorescence.

A large number of dissolution pores are formed in the reservoirs of the Baikouquan formation and the Lower Wuerhe formation in the study area, which greatly improves the reservoir physical properties. There are three main favorable conditions for this phenomenon: (1) The study area is located in the upward direction of the hydrocarbon generation center, which is conducive to the migration of organic acids and makes it easier to react with alkaline minerals. (2) Development of faults and unconformable surfaces: faults connecting source rocks are developed in the study area, providing a vertical channel for the migration of organic acids. In addition, the unconformity surface between the Baikouquan formation and the Lower Wuerhe formation and stratigraphic pinch-out line are developed in the study area [26], which provides an important channel for the lateral migration of organic acids. (3) The laumontite cementation particularly developed in the study area. Previous studies on the distribution of zeolite in this area suggest that the

study area is close to the ancient lake center, and the high salinity and alkalinity in the early diagenesis are the two major factors for the development of laumontite, resulting in the formation of laumontite development zone in the study area [27]. The more laumontite cement, the greater the probability of acidic fluid contacting laumontite through fractures, unconformity surfaces and residual intergranular pores, as well as being easier to form dissolution pores.

The above-mentioned studies indicate that the dissolution during diagenesis in the study area is closely related to the hydrocarbon charging process. It can be seen from Figure 12 that after the organic acid reaches the unconformity surface with hydrocarbon along the faults connecting to source rock, it will migrate to the high part of the structure, so the sandstone reservoir in the high part will first develop secondary pores. The concentration of organic acids in the high part is relatively higher and dissolution is more intense. As shown in Figure 12, there are faults connecting source rocks between wells MZ1 and MZ2. Organic acids migrate vertically with hydrocarbon through faults to the unconformity surface, and then migrate laterally along the unconformity surface (or sandstone) to the high part of the structure. Therefore, a large number of secondary pores dissolved by organic acids are developed in the reservoir of well MZ2, which is located in the high structural position. There is no fault connecting source rocks in the downdip direction of well MZ1, so there is almost no organic acid passing through. As a result, the reservoir is basically not dissolved and the physical properties is poor. In addition, the sedimentary subfacies of the Lower Wuerhe formation in well XY3 is in the fan delta plain, and the reservoir is very tight. So even if there is organic acid fluid passing through, dissolution is not easy because there is no chance for organic acids to contact with alkaline cements.

**Figure 12.** Relationship between diagenesis and hydrocarbon charging.

In summary, it suggests that the sandstone of fan delta front subfacies rich in alkaline cements is the basis for the formation of secondary pores, the dissolution of organic acids is the main factor controlling the development of secondary pores, and the dominant migration channel is a necessary condition for the formation of a large number of secondary pores.

#### **6. Conclusions**

(1) The sandy conglomerates reservoirs of the Lower Wuerhe formation–Baikouquan formation in the study area are characterized by low porosity and permeability, and both of them have low textural maturity and compositional maturity and poor size classification. However, the reservoir physical properties of the Lower Wuerhe formation are more affected by laumontite cementation and volcaniclastic dissolution.

**Author Contributions:** Conceptualization, W.J.; methodology, H.L. and W.J.; validation, P.S., H.L. and B.B.; formal analysis, P.S.; investigation, H.L.; resources, P.S. and N.W.; data curation, P.S. and W.G.; writing—original draft preparation, W.J.; writing—review and editing, W.J.; visualization, X.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Major projects of PetroChina science and technology [2021DJ0206].

**Data Availability Statement:** Data is unavailable due to confidentiality.

**Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

#### **References**

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## *Article* **Petroleum Geology and Exploration of Deep-Seated Volcanic Condensate Gas Reservoir around the Penyijingxi Sag in the Junggar Basin**

**Baoli Bian 1, Ablimit Iming 1, Tianze Gao 2, Hailei Liu 1, Wenlong Jiang 1, Xueyong Wang 1 and Xiujian Ding 2,\***

**Abstract:** Many types of volcanic rock oil and gas reservoirs have been found in China, showing great petroleum exploration potential. Volcanic reservoir also is one of the key fields of exploration in the Junggar Basin and mainly concentrated in the middle and shallow layers, while the deep volcanic rock and natural gas fields have not been broken through. Based on comprehensive analysis of core observation, single well analysis, reservoir description, source rocks evaluation, combined with seismic data and time-frequency electromagnetic technology, multiple volcanic rock exploration targets were identified, and industrial oil and gas flow was obtained in the well SX 16 of the Penyijingxi Sag, western Junggar Basin. It is believed that the deep Permian source rocks have relatively higher natural gas generation potential and volcanic breccia usually have large reservoir space. And the mudstone of the Upper Wuerhe Formation played as the role of caprock. The success of exploration well SX16 has achieved a major breakthrough in natural gas exploration in the Penyijingxi Sag, which has essential guiding significance for the exploration of deep volcanic rocks and large-scale gas exploration in the Junggar Basin.

**Keywords:** condensate gas; volcanic rocks; Penyijingxi Sag; Junggar basin; Carboniferous

#### **1. Introduction**

Both deep-seated petroleum reservoirs and unconventional petroleum are hotspot for exploration and development [1–3]. And deep-seated petroleum exploration is an important strategic replacement field for oil and gas exploration in China [4,5]. With the deepening of theoretical understanding of high-quality reservoir formation mechanism, deep oil and gas hydrocarbon generation and reservoir formation, deep oil and gas exploration has gradually attracted extensive attention. Volcanic rock is an essential field of oil and gas exploration in China [6,7]. Many types of volcanic rock oil and gas reservoirs have been found in the Songliao, Bohai Bay, Hailar, Erlian, and Sichuan basins of China [8–10], showing great petroleum exploration potential.

In China, as early as the 1970s to 1980s, it was recognized that volcanic rocks can be used as reservoir rocks and volcanic reservoirs such as Huangshatuo, Oulituozi, and Rehetai were successively discovered. The Junggar Basin is one of the large petroliferous basins in northwestern China [11,12]. More than sixty years of petroleum exploration show that the Junggar Basin is rich in oil and poor in natural gas [13]. Volcanic reservoir is one of the key fields of oil and gas exploration in Junggar Basin [14,15]. In the early stage of exploration, many oil reservoirs were found in the northwest margin of the basin [16]. Volcanic oil and gas reservoirs are mainly concentrated in the middle and shallow layers, the deep seated volcanic rock and natural gas fields have not been broken through. In order to find large-scale oil and natural gas reservoirs, based core observation, single well analysis, reservoir description and hydrocarbon source rock evaluation, combined with

**Citation:** Bian, B.; Iming, A.; Gao, T.; Liu, H.; Jiang, W.; Wang, X.; Ding, X. Petroleum Geology and Exploration of Deep-Seated Volcanic Condensate Gas Reservoir around the Penyijingxi Sag in the Junggar Basin. *Processes* **2022**, *10*, 2430. https://doi.org/ 10.3390/pr10112430

Academic Editors: Ping Gao, Yidong Cai, Yingfang Zhou and Quan Gan

Received: 16 October 2022 Accepted: 15 November 2022 Published: 17 November 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

seismic data and time-frequency electromagnetic technology, several volcanic rock targets were identified for the deep volcanic rocks in the Penyijingxi Sag of the Junggar Basin. The exploration well was successfully deployed, and high-yield industrial oil and gas flow was obtained, revealing the great potential for oil and gas exploration in deep volcanic rocks, which has essential guiding significance for the exploration of deep volcanic rocks and large-scale gas exploration in the Junggar Basin.

In summary, many types of volcanic rock oil and gas reservoirs have been found in China, showing great petroleum exploration potential. Volcanic reservoir also is one of the key fields of exploration in the Junggar Basin and mainly concentrated in the middle and shallow layers, while the deep volcanic rock and natural gas fields have not been broken through. The study has essential guiding significance for the exploration of deep volcanic rocks and large-scale gas exploration in the Junggar Basin.

#### **2. Exploration History of Volcanic Rocks in the Junggar Basin**

Junggar basin is located in the west of China, with Junggar Boundary Mountain in the northwest, Altai Mountain in the northeast and North Tianshan Mountain in the south [17,18]. It is a slightly triangular closed basin with an area of 130,000 km2 (Figure 1a,b). It is one of the four major oil-gas basins with oil and gas resources of more than 10 billion tons in China [11,12]. It has been rich in oil and poor in gas for a long time [13,19].

**Figure 1.** Distribution of structural units, oil fields, gas fields, and sample wells distributions in the northwestern Junggar Basin (After reference [20]). (**a**) geotectonic setting of the Junggar Basin; (**b**) tectonic unit of the Junggar Basin; (**c**) study area tectonic unit and petroleum, exploration well distribution. Reprinted with permission from Ref. [20].

Before 1990, the volcanic exploration in Junggar Basin mostly belonged to the early exploration stage, and the exploration discoveries were mainly oil reservoirs, which were all located in the northwest margin region of the Junggar Basin [20]. From 1990 to 2000, with the continuous improvement of geological knowledge and exploration technology, targeted exploration was carried out in Shixi area and Chepaizi area of Junggar basin [21]. The exploration discoveries were mainly oil and some natural gas. Since 2000, volcanic exploration in Junggar basin has stepped into an accelerated stage, and breakthroughs have been made successively in the Liuzhong Area of the northwest margin [22]. In 2007 and 2008, large-scale breakthroughs in natural gas have been made in the northwest margin, and significant progress and breakthroughs have been made. The early volcanic exploration in Junggar Basin mainly focused on the middle and shallow layers such as Shixi uplift. Due to the recognition accuracy and engineering technology, the deep layer has not been explored yet.

With the increase of exploration degree, deep volcanic rocks have gradually become the focus of exploration and experienced a tortuous exploration process [23]. In 2006, the Moshen-1 well was deployed and the design well depth was 7380 m. After more than one year of drilling, nearly 400 m tuff was drilled in the Carboniferous system. The gas logging showed good results. Gas invasion occurred during the drilling process of the Carboniferous system, but oil and gas flow could not be obtained during the oil test. Although the exploration of Moshen-1 well did not obtain industrial oil and gas flow, the good gas logging shows increased confidence of deep volcanic rock exploration. In 2015, Datan 1 well was implemented, with a drilling depth of 6226 m. The Carboniferous-Jurassic system has obtained extremely active oil and gas display, with a longitudinal span of more than 2500 m, and no industrial oil flow has been obtained from igneous rocks. In view of the active oil and gas display of well Datan 1, well Yantan 1 was deployed in 2016. Frequent interbedding of tuff andesite and basalt was encountered in Carboniferous, unfortunately there was no oil and gas display in igneous rocks.

In 2016, the exploration focus was adjusted from Dabasong Uplift to the surround of the Penyijingxi Sag, and a comprehensive study on the deep volcanic rocks in the Penyijingxi Sag was carried out. The study shows that the Permian hydrocarbon source rocks have a high degree of deep organic matter thermal evolution and large-scale gas generation potential. And well SX 16 exploration well was deployed for deep volcanic rocks in 2019. Formation testing was conducted in the 4800 m deep andesitic breccia well section of Carboniferous system, and high-yield oil and gas flow was obtained. Since then, a major breakthrough has been made in the exploration of deep volcanic rocks and deep natural gas in the basin.

#### **3. Data and Method**

Based on comprehensive analysis of core observation, microscopic slice observation and physical property, combined with logging and seismic data, volcanic reservoirs were distinguished and predicted. Source rocks distribution was defined by seismic data and petroleum filling intensity was defined by the data of grain with oil inclusions. With seismic data and time-frequency electromagnetic technology, multiple volcanic rock exploration targets were identified.

#### **4. Result of Reservoir and Source Rock Characteristics**

#### *4.1. Types and Characteristics of Volcanic Reservoir Space*

The volcanic reservoir in the Penyijingxi Sag is mainly developed in andesitic and basaltic volcanic rocks, mainly including andesitic cryptoexplosive breccia, andesitic volcanic breccia, andesitic tuff, stomatal andesite, massive andesite, stomatal basalt, and massive basalt. The reservoir space and association law of andesitic volcanic rocks in Shixi area are systematically summarized by using core and cast thin section analysis methods (Figure 2).

Andesitic cryptoexplosive breccia is mainly developed near the volcanic channel. It is formed by blasting the early volcanic rocks in situ during the later magma rising process. The magama filled between the self-broken breccias has poor crystallinity and is easy to dissolve, forming a large number of intergranular solution pores. It is characterized by large dissolution pores, and its shape is limited by the boundary of the self-broken breccias (Figure 2a). Andesitic volcanic breccia and andesitic tuff are formed by volcanic eruption. The volcanic debris is composed of crystal debris and rock debris, and the matrix is volcanic dust. Because volcanic dust is composed of volcanic glass with poor stability, it is prone to devitrification to form intergranular micropores. It is easy to dissolve under the action of fluid, forming a large number of dissolution pores in the matrix (Figure 2b,c).

Stomatal andesite and stomatal basalt are developed in the upper part of lava flow unit, some primary pores are filled by amygdaloid, and secondary dissolution occurs in the later stage, forming a large number of dissolution pores in amygdaloid (Figure 2d,e). In addition, the curved condensation shrinkage cracks between the amygdaloid body and the pore wall are also common in the stomatal andesite and the stomatal basalt, which are formed by the condensation shrinkage of the amygdaloid body (Figure 2f). Massive andesite and massive basalt are developed in the middle and lower part of the lava flow unit, with similar reservoir space characteristics. They are characterized by massive structure, less porosity and lack of primary pores. Under the action of weathering fluid, phenocrysts and matrix can undergo dissolution to a certain extent, forming intracrystalline solution pores (Figure 2g) and matrix dissolved pores with sieve-like distribution (Figure 2h). In addition, structural fractures of massive basalt and andesite are relatively developed (Figure 2i). Structural fractures are formed by regional tectonic stress and deformation. The fractures are generally flat or in micro saw tooth shape (tracking tensional joints), and the filling content is generally not high (Figure 2i). In summary, there are many types of pores in the volcanic reservoir, showing good reservoirs developed in deep-seated volcanic.

**Figure 2.** Types and characteristics of volcanic reservoir space around the Penyijingxi Sag. (**a**) Interparticle dissolution pore of andesitic cryptoexplosive breccia. (**b**) Interparticle dissolution pore of andesitic volcanic breccia. (**c**) Matrix solution pore of andesitic tuff. (**d**) Pore inner stomatal basalt amygdaloid. (**e**) Pore inner andesite amygdala. (**f**) Basalt amygdaloid contraction joint. (**g**) Intracrystalline solution pore of andesite. (**h**) Andesite matrix solution pore. (**i**) Structural fracture.

#### *4.2. Physical Property Characteristics of Volcanic Reservoir*

The reservoir physical property statistics of volcanic reservoirs with different lithology in the study area are shown in Figure 3. The matrix porosity of andesitic volcanic breccia formed by eruption is between 10 and 20%, and the maximum permeability can reach 100 × 10−3 μm2, which is the best reservoir. Stomatal andesite, stomatal basalt, and andesitic cryptoexplosive breccia have similar physical properties. Their matrix porosity is between 10 and 20% and permeability is 0.1 × 10−3~1 × 10−3 μm2, which is also a relatively favorable volcanic reservoir. The matrix porosity of massive andesite is between 3 and 10% and the seepage rate is 0.01 × 10−3 μm2~1 × 10−3 μm2, the reservoir physical property is poor. The porosity of massive basalt matrix is between 1 and 5% and the permeability is 0.01 × 10−3 μm2~0.1 × 10−3 μm2, which basically cannot form an effective reservoir. Compared with clastic rocks and dolomitic rocks, the Carboniferous deep volcanic rocks are less affected by compaction, and the widely developed unconformity at the top of the Carboniferous system will significantly improve the physical properties of the reservoir. It is comprehensively considered that the deep volcanic rocks develop large-scale reservoirs.

**Figure 3.** Crossplot of porosity and permeability of volcanic rocks in Shixi area of the Junggar Basin.

#### *4.3. Distribution and Evolution of Petroleum Source Rocks*

At present, the Permian source rocks were widely drilled in the Mahu Sag and no well has been drilled in the Penyijingxi Sag. The Permian source rocks in the Mahu Sag have high organic matter abundance and are mainly medium-good hydrocarbon source rocks, with organic matter type II, and are developed from low to high maturity [24]. Through the comparison between Mahu Sag and Penyijingxi Sag, it is considered that the depositional environment of Permian is similar as a whole. It is concluded that two sets of source rocks of the Fengcheng Formation and lower Wuerhe Formation are developed in the Penyijingxi Sag, which is similar to the Mahu Sag.

According to the seismic data, the formation thickness of the Fengcheng Formation and Lower Wuerhe Formation is identified. The mudstone of the Fengcheng Formation and Lower Wuerhe Formation is widely distributed, with a thickness of more than 200 m.

In the Mahu Sag, the burial depth of source rocks of the Fengcheng Formation in the Mahu Sag is mostly less than 6500 m, and decade's years petroleum exploration had confirmed that the source rocks are in the stage of oil generation, and has not entered in the stage of condensate gas generation stage.

From the Mahu Sag to the Penyijingxi Sag, the burial depth of source rocks gradually increases, and the buried depth of Fengcheng Formation source rocks in the Penyijingxi Sag is more than 9000 m. Combined with the burial depth of source rocks and the thermal evolution simulation of organic matter, it is considered that the evolution degree of source rocks in the Lower Wuerhe Formation is dominated by high maturity, mainly in the condensate generation oil and gas and wet gas stages (Figure 4a), and the evolution degree of Fengcheng Formation source rocks is mainly high mature and over mature, mainly in the stage of generating wet gas and dry gas (Figure 4b).

**Figure 4.** Plane distribution of organic matter maturity (Ro) of Permian main source rocks. (**a**) Ro of the Lower Wuerhe Formation; (**b**) Ro of the Fengcheng Formation.

#### **5. Discussion on Petroleum Accumulation and Natural Gas Exploration**

#### *5.1. Distribution Model of Volcanic Rocks Reservoir*

The Penyijingxi Sag is surrounded by Dabasong Uplift, Xiayan Uplift, Shixi Uplift, Mobei Uplift, and Mosuowan Uplift. According to the lithological characteristics of Upper Carboniferous revealed by drilling, Xiayan Uplift and Dabasong Uplift in the west of Penyijingxi Sag are mainly basic basalt, and tuff, intermediate acid andesite and dacite with a certain thickness are developed. Affected by the ancient landform, some low-lying areas also developed sedimentary volcaniclastic rocks and sedimentary rocks.

The identification of volcanic lithofacies is the basis for establishing volcanic institutions and making high-quality reservoir predictions [25]. Based on core observation, thin section identification, logging data and seismic, the lithology and lithofacies of volcanic reservoir in the Shixi Uplift was classified and identified [25]. The volcanic rocks in Shixi Uplift, Mobei Uplift and Mosuowan Uplift in the east of Penyijingxi Sag are mainly medium acid andesite, dacite and rhyolite, and volcanic breccia and tuff are developed. Under the influence of volcanic mechanism, volcanic lava and pyroclastic rocks are interbedded, as shown in Figure 5. The volcanoes in Shixi Uplift and Mobei Uplift are continental eruptions and do not develop sedimentary pyroclastic rocks and sedimentary rocks. However, thick sedimentary rocks are developed in the upper part of Mosuowan Uplift, and there are sedimentary pyroclastic rocks and sedimentary rock intercalations in the lower volcanic rocks. On the plane, the lithology of the Upper Carboniferous volcanic rocks around the Penyijingxi Sag has obvious cyclicity.

#### *5.2. Oil and Gas Filling Intensity*

The industrial oil and gas flow is obtained from the formation testing of well SX 16. And the gas test has the characteristics of low gas/oil ratio, high oil production and high gas production.

The formation testing results of well SX 16 showed that condensates are the primary petroleum type. Through the observation of reservoir thin sections, it is found that the hydrocarbon phases are diverse, with residual black oil (Figure 6a), black brown heavy oil inclusions (Figure 6b), light yellow fluorescent condensate oil and gas (Figure 6c) and black brown gas hydrocarbon inclusions (Figure 6d). Grain with oil inclusions (GOI) is an essential parameter to evaluate the oil and gas filling intensity. It is generally considered that the GOI of oil layer is mostly greater than 5%, the GOI of the oil-water intermediate layer is 1~5%, the GOI of SX16 well is 8%, and the grain with gas inclusion is 4%, revealing a relative stronger oil and gas charging intensity.

**Figure 5.** Regional distribution of volcanic rocks in the Penyijingxi Sag and its surroundings.

**Figure 6.** Microscopic characteristics and inclusions characteristics of volcanic reservoir in well SX16. (**a**), well SX 16, 4806.9 m, black oil, plane polarized light; (**b**), well SX 16, 4806.9 m, heavy oil inclusion, plane polarized light; (**c**), well SX 16, 4806.9 m, light yellow, condensate, fluorescent light; (**d**), well SX 16, 4806.9 m, gas hydrocarbon inclusion, plane polarized light.

#### *5.3. Natural Gas Preservation and Caprock Condition*

Most deep-seated oil and gas reservoirs in Junggar basin have experienced multi-stage tectonic movements, and most of hydrocarbon are light oil and natural gas, thus caprock and petroleum reservoir preservation are often one of the key controlling factors. For instance, the oil and gas show of well Datan-1 ranges from the Carboniferous to the Jurassic, with a depth range of more than 2500 m. However, there is and no industrial oil flow has been obtained in the Carboniferous volcanic rocks of well DT 1.

The regionally distributed mudstone of the Upper Wuerhe Formation, Fengcheng Formation are developed on the Carboniferous volcanic rocks, played as the regional cap rocks of volcanic oil and gas reservoirs. The Carboniferous igneous rock of the well SX16 is overlaid with mudstone of the Upper Wuerhe Formation (P3w) with a cumulative thickness of more than 100 m, which has good preservation conditions and is conducive to the preservation of oil and gas reservoirs (Figure 7).

**Figure 7.** Formation testing results and comprehensive histogram of upper caprocks of well SX 16 around the Penyijingxi Sag.

#### *5.4. Key Techniques for Deep Volcanic Gas Reservoir Exploration*

Compared with the medium and shallow layers, the deep oil and gas exploration technology is much more difficult. The improvement of exploration technology has advantageously guaranteed the breakthrough of oil and gas exploration.

Due to the limitations of previous geophysical exploration acquisition techniques and the shielding effect of middle and shallow Jurassic coal rock strata, the existing 3D seismic data have relatively low deep resolution and signal-to-noise ratio (SNR), which cannot meet the requirements for deep volcanic oil and gas reservoir exploration. The progress of 3D seismic acquisition and processing technology for deep volcanic rocks is the key to exploration.

The new seismic imaging technology named "wide frequency, wide azimuth and highdensity seismic exploration" have a significant effect on improving the imaging quality [26]. Recently, the "wide frequency, wide azimuth and high-density seismic exploration" new technology had been used to discover exploration target (Figure 8).

**Figure 8.** Comparison diagram of seismic profile through well SX16 with conventional (left) and new (right) technology.

Time frequency electromagnetic method (TFEM) is an artificial source electromagnetic detection method based on the principle of electromagnetic induction to observe and study the response field in time domain and frequency domain [27]. This method can effectively eliminate the influence of complex surface conditions and large horizontal variation of underground geological structure, and obtain resistivity and polarizability parameters. It can cooperate with seismic to identify reservoir target lithology, implement geological structure, predict oil, and gas potential, etc. In recent years, this technology has been widely applied in many regions at China and abroad, and has become an indispensable and important means in oil and gas exploration.

Through statistical analysis of the physical property differences between the lithology of different strata and the specific lithology of Carboniferous in the Junggar basin, the resistivity anomaly information template of different types of Carboniferous volcanic rocks is established. Then the abnormal information of time-frequency electromagnetic resistivity and polarizability in SX16 well area is inversed and interpreted (Figure 9). The Carboniferous volcanic rocks were characterized in detail, which effectively improved the drilling success rate of deep and complex volcanic reservoirs and reduced the drilling risk. Through the TFEM exploration application, the distribution of volcanic rocks is determined, which provides a favorable support for the exploration of deep volcanic gas reservoirs.

**Figure 9.** TFEM (time frequency electromagnetic method) profile through well SX16.

#### *5.5. Deep Volcanic Gas Reservoir Exploration Inspiration*

The source rocks in the Penyijingxi Sag are characterized by large thickness and wide area, but the source rocks have not been drilled in the study area, and the gas generation capacity of the source rocks is questionable. The great breakthrough which has been made in the exploration of natural gas in deep volcanic rocks confirms that the Permian source rock in the Penyijingxi Sag has the large-scale natural gas generation potential.

The Carboniferous distributed in the Penyijingxi Sag develops large-scale volcanic rocks, and the buried depth gradually increases from north to south. The statistical analysis of physical properties shows that with the increase of depth, the volcanic rocks are supported by the rock framework and volcanic breccia, and the physical properties change relatively little. The exploration breakthrough of well SX16 reveals that the volcanic reservoir has good physical properties, large rock mass thickness, and high gas production, and may become an important exploration area for large-scale natural gas exploration.

#### **6. Conclusions**

Based on comprehensive analysis of core observation, single well analysis, reservoir description, and source rocks evaluation, it could be concluded that the source rocks have relatively higher natural gas generation potential and supply a relative stronger oil and gas charging intensity. The deep volcanic rocks may develop large-scale weathered crust reservoirs and the Upper Wuerhe Formation played as the role of caprock and controlling natural gas reservoirs preservation.

Based on comprehensive analysis of petroleum geology elements and the new seismic imaging technology named "wide frequency, wide azimuth and high-density seismic exploration", a major breakthrough has been made in the exploration of deep volcanic rocks and deep natural gas in the basin. This is the first breakthrough of deep seated gas reservoirs exploration in volcanic reservoir. The success has achieved a major breakthrough in natural gas exploration in the Penyijingxi Sag, which has essential guiding significance for the exploration of deep volcanic rocks and large-scale gas exploration in the Junggar Basin.

**Author Contributions:** Conceptualization, B.B. and A.I.; methodology, T.G.; validation, H.L., W.J. and X.W.; formal analysis, X.W.; investigation, B.B.; resources, B.B.; data curation, B.B.; writing—original draft preparation, B.B.; writing—review and editing, X.D.; visualization, H.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Major projects of PetroChina science and technology [2021DJ0206].

**Acknowledgments:** We thank Xinjiang Oil Field Company, PetroChina, for providing access to the cored rock samples.

**Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

#### **References**

## *Article* **The Whole-Aperture Pore Structure Characteristics and Their Controlling Factors of the Dawuba Formation Shale in Western Guizhou**

**Kun Yuan 1,2, Wenhui Huang 1, Xianglin Chen 2, Qian Cao 3, Xinxin Fang 4,\*, Tuo Lin 2,\*, Chunshuang Jin 2, Shizhen Li 2, Chao Wang 2 and Ting Wang 2**

**Abstract:** Since shale gas mainly occurs in shale pores, research on pore structure characteristics is the key to understanding the shale gas accumulation mechanism. The pore structure of the Lower Carboniferous Dawuba Formation shale in the Qianxi area, represented by the well QSD-1, which obtains a daily shale gas flow of 10,000 m3 and represents an important breakthrough in the investigation of marine shale gas in the Upper Paleozoic region of southern China, is characterized by high-pressure mercury compression experiments and low-temperature gas adsorption (N2 and CO2) experiments with whole pore size. The main controlling factors affecting the pore development of the shale are discussed. (i) The micropores and mesopores are more developed in the shale, and the macropores are the second most developed in the shales of the Dawuba Formation in the Qianxi area, among which the mesopores and macropores contribute most to the pore volume and the micropores and mesopores contribute most to the pore-specific surface area. (ii) The microfractures and interlayer pores of clay minerals are developed in the shales of the Dawuba Formation, which are the main storage spaces for hydrocarbon gases. (iii) The main factors affecting the adsorption capacity of the shales of the Dawuba Formation in the Qianxi area are the organic carbon content and clay mineral content of the shales, both of which have an obvious positive correlation with the variation of pore structure.

**Keywords:** taphrogenic trough; Qianshuidi 1 well; carboniferous strata; pore structure; total pore volume

#### **1. Introduction**

Since the "shale gas revolution", China has carried out a lot of exploration work on shale gas resources [1–9]. However, there are few breakthroughs in Carboniferous strata in South China with great oil and gas potential [10–14], and there are few studies on the pore development and its main controlling factors of Carboniferous shale [13,14]. In 2021, well QSD-1 was deployed in the Liupanshui area of Qianxi, which was tested in the Carboniferous Dawuba Formation and obtained 10,000 m3/D shale gas flow, achieving a major breakthrough in the investigation of shale gas in the Lower Carboniferous area of South China. Drilling revealed that the shale thickness of the Dawuba Formation was nearly a kilometer, and 53 sets of gas reservoirs were comprehensively interpreted, with the highest total hydrocarbon value of 63.4%. As an example of typical shale gas drilling in the area, this paper takes the core samples from the Dawuba Formation in Well QSD-1 as the research object, characterizes the whole pore size of the shale pore structure, and

**Citation:** Yuan, K.; Huang, W.; Chen, X.; Cao, Q.; Fang, X.; Lin, T.; Jin, C.; Li, S.; Wang, C.; Wang, T. The Whole-Aperture Pore Structure Characteristics and Their Controlling Factors of the Dawuba Formation Shale in Western Guizhou. *Processes* **2022**, *10*, 622. https://doi.org/ 10.3390/pr10040622

Academic Editor: Carlos Sierra Fernández

Received: 9 February 2022 Accepted: 18 March 2022 Published: 22 March 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

discusses its influencing factors, in order to provide reference for shale gas investigation in the Qianxi area.

Shale gas occurs mainly as free gas in pores and natural fractures, adsorbed gas in organic matter and clay minerals, dissolved gas in residual oil, and in water in shale reservoirs [15,16]. The size, volume, shape, connectivity and permeability of the pores significantly influence shale gas enrichment and exploration [17]. The pore structure and fracture systems have been characterized by researchers based on various models [18–20]. For example, the low temperature CO2 and N2 adsorption are widely used to characterize the pore distributions of micropores (<2 nm) and mesopore size (2–50 nm), respectively. The high-pressure mercury injection method is used to characterize the pore distributions of macropores (>50 nm).

The shale characteristics of the Lower Paleozoic Silurian strata represented by the Sichuan Basin have been extensively studied [21]. In contrast, due to the large difference in mineral composition and strong heterogeneity of shale in the Upper Paleozoic marine rocks in southern China, the pore structure characteristics and their main controlling factors are quite different from those in the Lower Paleozoic strata [22].

In order to understand the pore structure characteristics of shale in the Dawuba Formation in Qianxi area and to understand the enrichment mechanism of shale gas in the Lower Carboniferous, qualitative and quantitative studies of the shale in the Dawuba Formation were carried out by FE-SEM, low temperature gas adsorption (N2 and CO2) and high-pressure mercury injection. The pore structure of the shale in the Dawuba Formation was characterized from different scales. The pore structure parameters such as specific surface area, pore size distribution and pore volume of the shale were obtained, and the main controlling factors were analyzed.

#### **2. Experimental Scheme and Sample Selection**

#### *2.1. Sample Selection*

The shale samples selected in this paper were taken from the cores of different layers of the Dawuba Formation in Well QSD-1, which was located in the Qiannan depression, southwestern China (Figure 1). The study area is located in the west part of the Qiannan depression, and confined by the Ziyun and Guiyang fault. Under the tectonic processes of multiple stresses, the Carboniferous, Permian and Triassic strata have undergone strong structural deformation and denudation of different extents. In the Indosinian-Yanshan and Himalayan stages, strong folds and thrust nappes resulted in massive shortening deformation.

According to the lithology and electrical characteristics, and combined with the outcrop situation of the surrounding strata, the shale samples can be divided into four sections from bottom to top [23,24]. (1). The first member of the Dawuba Formation is the thickest and most uniform lithology member of this formation, which is mainly composed of thick black mudstone. (2). The shale in the second member of the Dawuba Formation is thin interbedded sandwiched in marl, and the shale lamellation is well developed. (3). The third member of the Dawuba Formation is grayish-black calcareous shale, mudstone, and dark gray argillaceous limestone mixed with grayish-black carbonaceous mudstone, and the interbedded structure of calcareous mudstone and micritic limestone is developed. (4). The fourth member of the Dawuba Formation is characterized by dark gray and grayish-black marl with carbonaceous shale and calcareous mudstone (Figure 2).

In order to ensure the distinguishability of the samples, nine samples from different members of the Dawuba Formation were selected for the high-pressure mercury injection experiment and the low-temperature gas adsorption experiment. At the same time, the organic carbon content, maturity, mineral composition and rock density of the samples were tested (Table 1) in order to fully reflect the pore structure characteristics of Dawuba Formation shale.

**Figure 1.** Study area and well location of Qianshuidi 1.

**Figure 2.** Stratigraphic characteristics and division scheme of the Dawuba Formation in Well QSD-1.

#### *2.2. Mineral Composition Analysis*

A commonly used and statistically valid method to evaluate minerals in shale reservoirs is X-ray diffraction. The diffract grams were recorded in detail from 4◦ to 70◦ at a scanning speed of 1◦ 2θ/min by a Rigaku automated powder diffractometer (D/MAX-RA) equipped with a Cu X-ray source (40 Kv, 35 mA). The samples from the Dawuba shale were analyzed for whole-bulk and clay fraction mineralogy by quantitative X-ray diffraction following two independent processes. First, the bulk mineral composition of the powder sample was determined at this stage to only include the total clay content. The whole-rock samples were analyzed over an angular range of 4◦ to 70◦ 2θ at a scanning speed of 1◦ 2θ/min. Second, the individual clay mineral content of the clay fractions separated from the rock powder sample was determined. The clay minerals were analyzed over an angular range of 3◦ to 65◦ 2θ at a scanning speed of 1.5◦ 2θ/min. The quantification of the minerals was based on calculations of the integrated area using JADE 5 software.

#### *2.3. SEM Observation*

A ZEISS Sigma 300 (Oberkochen, Germany) SEM and Bruker Quantax 200G (Berlin, Germany) spectrometer were used to visualize the shale minerals and micropores of the Dawuba Formation. The organic matter, porosity, fracture distribution, size and pore connectivity were quantitatively described [25]. The porosity of the dry samples was determined from the grain density obtained from helium pycnometry (skeletal density) and the bulk volume of the plugs was calculated from mercury immersion (bulk density). The total porosity was calculated from the difference between the bulk and skeletal densities. A Gatan model 685.C (Pleasanton, CA, USA) was used for argon ion polishing in the SEM observations. Ar ion polishing is a common processing procedure carried out before observing under SEM. The samples were milled through a two-step process: (1) source operated at 2.5 kV for 2 h, followed by (2) source operated at 1.0 kV for 1 h. Each ion milling step used a 40% focus and 5◦ tilt angle, and the samples were continuously rotated 360◦ to ensure the entire surface was milled and common induced features such as curtaining were minimized.

#### *2.4. High-Pressure Mercury Injection Experiment*

A high-pressure mercury injection experiment is mainly used to analyze the macropore development in shale. The AutoPore 9500 mercury injection instrument from Micrometrics Instrument Co., Ltd. (Shanghai, China) is used for testing and analysis. The maximum external pressure is 400 MPa (60,000 psia). The experimental steps are based on the national standard. Before the test, the cylindrical samples (diameter 2.5 cm, length 3 cm) were first dried (dried at 150◦ or higher temperature for an hour), and mercury was injected into the sample tube under vacuum conditions. The test analysis was carried out at low-pressure and high-pressure stages, respectively. During the low-pressure test, the pressure was increased incrementally. When the maximum external pressure was reached, the pressure was reduced to atmospheric pressure, and the sample tube was moved into the highpressure station for analysis. At the same time, mercury was filled into the dilatometer under vacuum conditions. The continuous pressure gradually increased the mercury to fill the pores, so as to obtain the relationship curve between the press-in amount and the pressure [26,27]. The distribution of the pore structure characteristic parameters of the different samples could then be obtained by analysis. To minimize the potential effects of the formation of artificially induced microfractures in the cylindrical sample processing, samples without any microfractures on the surface were used. However, it is hard to be certain that no artificially induced microfractures occurred in these samples.

#### *2.5. Low-Temperature N2 and CO2 Adsorption Experiments*

N2 and CO2 were selected as the adsorbed gas, and the isothermal adsorption curve was obtained by measuring the adsorption capacity of the sample under different pressures. The distribution of the pore structure characteristic parameters such as the pore size, specific

surface area and pore volume of the sample was calculated by BET and BJH models [27]. Among them, the liquid nitrogen adsorption–desorption test was used to characterize the mesoporous distribution characteristics of the sample. The instrument was an ASAP 2460 automatic specific surface and pore size analyzer. The relative pressure range of the isothermal adsorption–desorption experiment of the instrument is 0.00–0.995. The pore size range that can be tested is 0.35 nm–400 nm, and the minimum specific surface area that can be tested is 0.0005 m2/g.

The samples were ground to powder with the particle size of 60–80 mesh. The experimental temperature is 77.3 k when the experimental adsorbate is liquid nitrogen, and 273.15 k when the experimental adsorbate is carbon dioxide. Before the experimental test, the appropriate amount of sample was weighed and put into a specific long tube (high-temperature and low-temperature resistance), and the vacuum degassing treatment was carried out at 150 ◦C for 4–6 h in order to remove the gas adsorbed on the surface of the material. After the degassing was complete, the sample was cooled to room temperature and fixed on the instrument to backfill helium and nitrogen (carbon dioxide) in turn. Then, the corresponding gas adsorption amount was determined by gradually changing the pressure at the test temperature, and the adsorption and desorption tests were carried out to obtain the adsorption–desorption isotherm curves of different samples.

#### **3. Results**

#### *3.1. Mineral Composition Characteristics*

The overall performance of the Dawuba Formation in Well QSD-1 was dominated by carbonate minerals and clay minerals, followed by quartz minerals, and by fewer iron and magnesium minerals. The carbonate rock content was 4–93%, average 53%; the clay mineral content was 5–82%, average 33%; the quartz mineral content was 4–27%, average 11%; some samples contained iron–magnesium minerals and plagioclase: the pyrite content was 1–3%, average 2%, and the plagioclase content was 1–3%, average 1%.

Vertically, the fourth and third members of the Dawuba Formation are dominated by carbonate minerals (58–93%, average 70%), clay minerals (6–15%, average 10%) and felsic minerals (6–11%, average 7%). Carbonate minerals (32–61%, average 49%) and clay minerals (37–48%, average 40%) are the main minerals in the second member of the Dawuba Formation, followed by felsic minerals (average 8%) and iron–magnesium minerals (average 3%). The first member of the Dawuba Formation is dominated by clay minerals (54–82%, average 69%), followed by felsic minerals (16–33%, average 27%), and carbonate minerals (4–13%, average 6%) (Figure 3).

**Figure 3.** Mineral component content of the Dawuba Formation in Well QSD-1.

#### *3.2. Pore Types and Morphological Characteristics*

The SEM observation results show that several pore types are mainly observed in the Dawuba Formation of Well QSD-1: primary intergranular pore, organic matter pore, dissolved pore and intergranular pore of carbonate minerals, diagenetic fracture and interlayer pore fracture of clay minerals (Figure 4). It was found that the internal/intergranular pores of inorganic minerals such as clay minerals, quartz and feldspar, and the internal and marginal pores of organic matter differ greatly in size and morphology. The SEM observation shows that the distribution of different organic matter is quite different, and most of them are distributed in the shale matrix independently in the form of lumps and ribbons. Some organic matter grows together with the surrounding matrix minerals, filling in inorganic mineral particles (crystals) such as siliceous minerals, clay minerals or pyrite, and their boundaries are mostly mixed with the boundary between the mineral particles/crystals. There are a certain number of pores in the internal and edge of the organic matter, with large morphological differences. Ellipsoid, needle-like or irregular shapes are distributed, and the pore scale is relatively small. The content of the clay minerals in the samples is relatively high, and the pores in the crystal (grain) are most developed in the illite and illite–montmorillonite mixed layer. The intergranular pores of the clay minerals mostly exist in the natural faults between the laminated structures, particles and the internal plates of the particles. The bending lamellar pore aggregates are developed in the mineral crystals such as the illite and illite–montmorillonite mixed layer. Due to compaction, some clay minerals show a flake shape, and there are a large number of flat or linear pores between these flake clay minerals [28]. Compared with organic matter pores, the microfractures and clay mineral interlayer pore fractures in the Dawuba Formation shale are more developed, providing a good reservoir space for hydrocarbon gas occurrence [29].

#### *3.3. Experimental Curve of Mercury Injection*

It can be seen that the shape of the mercury injection–withdrawal curve of the samples in each member of the Dawuba Formation is quite different. The "hysteresis loop" formed by the inconsistency of the mercury injection curve and the mercury withdrawal curve is also the performance of the distribution of parameters such as the shape and connectivity of the reservoir space in the samples [30], reflecting the different characteristics of the strata in each member of the Dawuba Formation. The mercury withdrawal efficiency reflects the capillary effect recovery of the non-wetting phase, which indicates that the throat volume accounts for the percentage of the total volume of pores and throats in the core [31]. The larger the mercury withdrawal efficiency, the more uniform the size distribution of the pores and throats in the core. The maximum mercury saturation reflects the connectivity of the pore-fracture system and the degree of pore development.

The mercury injection test results show that there are macropores in the first, second and third members of the Dawuba Formation, the macropores in the second and third members are more developed than those in the first member and the pore connectivity is better in the first member. The pores in the first member are relatively developed and the pore size distribution is more uniform (Figure 5a).

The first member of the Dawuba Formation: when the pressure increases to about 10 MPa, the mercury curve shows a slow increasing trend, the pore sorting is relatively good and the pore size distribution is relatively uniform. The hysteresis loop of the mercury injection and withdrawal is wide, the volume difference of mercury injection and withdrawal is small and the efficiency of the mercury injection and withdrawal is relatively low, indicating that the corresponding throat is thin and the connectivity is poor in the mercury injection test range.

**Figure 4.** Microscopic characteristics of shale pores in the Dawuba Formation of Well QSD-1. (**a**) Black shale, member 4 of the Dawuba Formation, 1490 m, banded organic matter developed, micropores developed inside; (**b**) black shale, member 3 of the Dawuba Formation, 1531 m, organic matter is distributed in mineral grains, pores are developed in its interior and edge and some pores are connected to each other; (**c**) black shale, member 3 of the Dawuba Formation, 1660 m, clay mineral intergranular pore development; (**d**) black shale, member 2 of the Dawuba Formation, 1930 m, mixed distribution of organic matter and clay minerals; (**e**) black shale, member 1 of the Dawuba Formation, 1940 m, clay mineral interlayer fractures developed in a narrow strip; (**f**) black shale, member 1 of the Dawuba Formation, 1940 m, clay mineral interlayer fractures and micropores developed.

**Figure 5.** Pore distribution characteristics at different scales of the Dawuba Formation shale in Well QSD-1. (**a**) The mercury injection–withdrawal curve of each member of the Dawuba Formation shale; (**b**) nitrogen adsorption–desorption curve of shale in each member of the Dawuba Formation; (**c**) Based on the distribution of specific surface area of shale in each member of Dawuba Formation by N2 adsorption; (**d**) shale pore volume distribution based on N2 adsorption; (**e**) shale pore volume change rate distribution based on N2 adsorption; (**f**) specific surface area distribution of shale in each member of the Dawuba Formation based on N2 adsorption.

The second member of the Dawuba Formation: with the increase of pressure, the mercury injection curve showed a slow ladder increase trend; the sorting coefficient is about 4.45 and the displacement pressure is small. The corresponding maximum pore throat radius is 15.65 μm, and the maximum mercury saturation of the sample is 75%. The hysteresis loop of the mercury injection and withdrawal is wide and the volume difference between the mercury injection and withdrawal is large. The mercury withdrawal efficiency is about 25%, indicating that the macropores and mesopores of the sample are relatively developed in the pressure test range, and the pore connectivity is good.

The third member of Dawuba Formation: with the increase of pressure, the mercury curve shows a certain increasing trend. When the pressure increases to about 30 MPa, the mercury curve shows a ladder increasing trend. The sorting coefficient is about 4.21, the displacement pressure is 0.172 MPa and the maximum pore throat radius is 4.268 μm. It can be seen that the macropores of the third member samples are less developed than those of the second member samples. The maximum mercury saturation of the sample is about 70%, and the hysteresis loop of the mercury injection and withdrawal is wide. The volume difference of the mercury injection and withdrawal is large, and the mercury withdrawal efficiency is about 27%, indicating that the pore connectivity is good within the pressure test range.

The fourth member of Dawuba Formation: with the increase of pressure, the mercury injection curve exhibits little change. When the pressure increases to 400 MPa, the mercury saturation is less than 10%, indicating that the pores in the sample are relatively undeveloped.

#### *3.4. Experimental Curve of N2 and CO2 Adsorption at Low Temperature*

Under the condition of low temperature (77 K), the N2 adsorption–desorption curve can reflect the pore morphology. According to the four types of pores summarized by IUPAC [31], the main types of pores in the third and fourth members of the Dawuba Formation are close to the H3 type, that is, the open slit pores on four sides, while the main types of pores in the first and second members of the Dawuba Formation are close to the mixture of H1 and H2 types, indicating that the shales in the first and second members of the Dawuba Formation are mainly composed of regular tubular pores with openings at both ends and ink bottle-shaped pores with wide, thin necks (Figure 5b).

Based on the N2 adsorption curve, the BJH adsorption model was used to calculate the pore volume and specific surface area of the shale samples in different layers (Figure 5c,d). The calculation results show that the pore volume and specific surface area of the second member of the Dawuba Formation are the largest, which are 14.3060 m2/g and 0.0236 cm3/g, respectively, followed by the first, third and fourth members. It can be seen from the analysis of the change rates of the pore volume and specific surface area with pore size (Figure 5e,f) that with the increase of pore size, the change rate of the pore volume gradually decreases, indicating that the pore volume mainly changes significantly in the pores within the range of 2 nm to 5 nm. The specific surface area also shows a similar rule with the pore volume, indicating that the pore volume and specific surface area of the mesopore are mainly provided by the pores with pore size (2 nm–5 nm), and the contribution of a large pore size (5 nm–100 nm) to the pore volume and specific surface area is low.

#### **4. Discussion**

#### *4.1. Distribution Characteristics of Total Pore Size of Shale*

Based on the experiments of high-pressure mercury injection and low-temperature gas adsorption (N2 and CO2), the pore distribution characteristics of shale from the nano scale to the micron scale were obtained [32]. Different experimental data have different characterization ranges and accuracy for pores. Micropores are mainly characterized by CO2 adsorption data, mesopores are characterized by N2 adsorption data and macropores are characterized by high-pressure mercury injection data [33]. Thus, the full-aperture distribution characteristics of shale in each member of the Dawuba Formation are obtained.

It can be seen from the comparison that the pore volume of the shale pores in the Dawuba Formation in the study area is relatively developed in the mesopores and macropores, followed by the micropores. Among them, the shale pore characteristics of each member of the Dawuba Formation are similar, and the shale pore volume is dominated by medium pores (pore volume ratio > 50%), followed by macropores and micropores. The change of pore volume has three relatively stable peaks, which are 0.4 nm~0.6 nm, 5.6 nm~9.6 nm and 32.8 nm~500 nm (Figure 6, Table 2). The mesopores and macropores contribute about 89.78% of the pore volume, and the micropores contribute about 10.22% of the pore volume (Table 2).

The distribution of the dV/dD pore volume and pore size in the full pore size range of the shale samples in each member of the Dawuba Formation shows that the pores of the samples are mainly distributed in the micropore members and some mesopore members, among which the number of mesopores with a pore size of about 10 nm in the first and third members is more than that in the second and fourth members (Figure 7).

**Figure 6.** Whole pore size distribution of shale pore volume of the Dawuba Formation in the Qianxi area. (**a**) Member 1 of the Dawuba Formation; (**b**) member 2 of the Dawuba Formation; (**c**) member 3 of the Dawuba Formation; (**d**) member 4 of the Dawuba Formation.

**Table 2.** Statistics of shale pore volume of the Dawuba Formation in the Qianxi area.

**Figure 7.** Variation rate curve of total pore volume with pore diameter of Dawuba Formation shale in the Qianxi area. (**a**) Member 1 of the Dawuba Formation; (**b**) member 2 of the Dawuba Formation; (**c**) member 3 of the Dawuba Formation; (**d**) member 4 of the Dawuba Formation.

From the analysis of the specific surface area provided by the shale pores, the specific surface area of the shale pores in the Dawuba Formation in the study area is mainly provided by micropores and mesopores. Pores contribute 93% of the specific surface area of the shale in the Dawuba Formation, and pores less than 0.6 nm in each member are the main contributors to the specific surface area. With the increase of pore diameter, the specific surface area shows a downward trend; the specific surface area provided by the mesopores is generally less than 0.5 m2/g, and the specific surface area provided by the macropores is one order of magnitude smaller than that of the mesopores (Figure 8, Table 3).

**Figure 8.** Distribution of specific surface area and full aperture of Dawuba Formation shale in the Qianxi area. (**a**) Member 1 of the Dawuba Formation; (**b**) member 2 of the Dawuba Formation; (**c**) member 3 of the Dawuba Formation; (**d**) member 4 of the Dawuba Formation.

#### *4.2. Discussion on Shale Pore Control*

The development of nanopores in shale may be affected by many factors [34]. Based on the analysis of the relationship between pore-specific surface area, pore volume and total organic carbon (TOC), it is considered that TOC has a good positive correlation with the development of pores in shale. The specific surface area of shale micropores has the best correlation with TOC. As TOC increases from 0.53% to 1.22%, the specific surface area of the micropores increases from 1.03 m2/g to 6.07 m2/g, and the correlation coefficient is 0.88 (Figure 9a). The correlation between mesopore-specific surface area and TOC is 0.61, and the correlation between the macropore-specific surface area and TOC is poor. The relationship between pore volume and the TOC of shale is similar to that of the specific surface area. The pore volume of the micropores and mesopores has good correlation with TOC, followed by macropores (Figure 9b). It indicates that the organic matter contributes more to the micropores, which was also observed from all of the densely packed organic matter pores in Figure 4.

**Figure 9.** Relationship between pore-specific surface area, pore volume and TOC of Dawuba Formation shale in the Qianxi area. (**a**) Relationship between pore-specific surface area and TOC; (**b**) Relationship between pore volume and TOC.

Through the comparison of the shale mineral content and pore characteristics, it is found that there is a certain correlation between shale pores and mineral content in different pore size ranges [35]. The specific surface area and pore volume show a positive correlation with quartz and clay minerals, but a negative correlation with carbonate minerals. In view of the highest correlation coefficient, the correlation between clay mineral content and pore-specific surface area and pore volume is the best regardless of pore size (micropore, mesopores or macropores), followed by carbonate minerals and quartz mineral content, indicating little effect on pore development and specific surface area (Figure 10). For pores of different size ranges, the clay mineral content is positively correlated with the specific surface area of micropores and mesopores, and the correlation coefficient is more than 0.8 (Figure 10e). The clay mineral content has a good correlation with the micropore and mesopore volume of the sample (Figure 10f). Quartz minerals contribute greatly to the pore volume of macropores (Figure 10b). In shale reservoirs, quartz acts more likely as a rock framework, with clay minerals as interstitial materials. Hence, clay minerals have a stronger impact on pores of small sizes. In addition, due to the strong chemical cementation of carbonate minerals, the development of nano–micropores will be inhibited to varying degrees. In addition, the pores between carbonate particles are easily cemented and filled by clay minerals and organic matter, so the content of carbonate minerals is negatively correlated with the pore volume and specific surface area (Figure 10c,d).

**Figure 10.** Relationship between pore-specific surface area, pore volume and quartz, carbonate and clay minerals of shale in the Qianxi area. (**a**) Relationship between pore-specific surface area and Quartz mineral content; (**b**) Relationship between pore volume and Quartz mineral content; (**c**) Relationship between pore-specific surface area and Carbonate mineral content; (**d**) Relationship between pore volume and Carbonate mineral content; (**e**) Relationship between pore-specific surface area and Clay mineral content; (**f**) Relationship between pore volume and Clay mineral content.

The analysis shows that the organic carbon content and clay mineral content of the Carboniferous Dawuba Formation in the Qianxi area are the main controlling factors for the specific surface areas of micropores and mesopores, and carbonate minerals and quartz minerals are positively correlated with the macropore volume of the samples. The analysis shows that the TOC and mineral components, to some extent, mainly affect the adsorption capacity of shale by affecting its specific surface area and pore volume and other pore structures. The porosity of organic matter makes it form a huge internal surface area. Due to the development of a layered structure, clay minerals also have large specific surface area and micropore volume. The specific surface area of organic matter pores and pores between clay minerals plays a major role in the specific surface area of the sample, which can provide more adsorption points for methane adsorption and is the main factor affecting the adsorption capacity of the sample. In the process of diagenetic evolution, brittle minerals such as quartz and carbonate play a supporting and protecting role in intergranular organic matter, intergranular pores and interlayer fractures, and play a decisive role in the enrichment of free gas in shale.

The intersection analysis of the clay mineral content of the samples with the total hydrocarbon values at the locations shows that the high total hydrocarbon values rise at the sites with a high clay content (Figure 11). It indicates that the shale gas is fully enriched and effectively preserved at the high clay sites. In addition, the comparison of total hydrocarbon with the specific surface area and pore volume shows that the gas is mainly adsorbed on the micropores and mesopores with a larger specific surface area, and is mainly enriched in the pore volume provided by the micropores. Combining the SEM and FMI results, it is concluded that clay minerals—especially those with microfractures and interlayer joints—are the main storage space for shale gas in the Dawuba Formation.

**Figure 11.** *Cont*.

**Figure 11.** Cross plot of total hydrocarbon, clay minerals, specific surface area and pore volume. (**a**) Relationship between Clay mineral content and Total Hydrocarbon; (**b**) Relationship between pore-specific surface area and Total Hydrocarbon; (**c**) Relationship between pore volume and Total Hydrocarbon.

#### **5. Conclusions**

**Author Contributions:** Conceptualization, K.Y. and W.H.; methodology, Q.C.; software, X.F.; validation, S.L., C.J. and T.W.; formal analysis, T.L. and C.W.; investigation, T.L. and K.Y.; resources, K.Y.; data curation, T.L. and X.C.; writing—original draft preparation, K.Y.; writing—review and editing, K.Y.; visualization, X.F.; supervision, W.H.; project administration, K.Y.; funding acquisition, K.Y. and S.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the China Geological Survey Projects "Geological survey of shale gas in Guizhong-Nanpanjiang area"(grant no.DD20190088).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The data has been included in the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

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**1. Introduction**

Broccoli, Brussels sprouts, cauliflower and kohlrabi are members of the Brassicaceae family, which is widely cultivated throughout the world. Some consumers reject *Brassica* vegetables due to their characteristic flavor, which is genetically related [1]. The consumption of Brassicaceae is high; however, it is mostly in the cooked form because of significant changes in flavor. Thermal treatment makes vegetables easily digestible and more acceptable for consumers. Nevertheless, some of them, e.g., broccoli, cauliflower or kohlrabi, are sometimes eaten raw, usually as an ingredient in salads. Thermal treatment affects the concentration of many beneficial compounds in vegetables, such as vitamins or phenolic compounds [2], while at the same time significantly changing the flavor profile of these vegetables. Many *Brassica* vegetables are sold as frozen florets or cubes, often found as ingredients in vegetable mixes, thus ensuring their supply year-round in a convenient form. Raw vegetables are rich in aromatic compounds, which usually are produced as a result of enzymatic reactions. Their storage in the frozen form can significantly alter biochemical reactions occurring in vegetables, thus

influencing flavor also after cooking. However, there are practically no studies on the comparison of volatile compound profiles of frozen, fresh and cooked *Brassica* vegetables. In the case of the analyzed plants, the specific interest focused on bioactive molecules is particularly high and they have been extensively studied recently, mainly due to the presence of isothiocyanates. These are the products of enzymatic hydrolysis of glucosinolates and their beneficial influence on human health has been widely examined [3–8].

A wide array of analytical techniques have been used to analyze volatile compounds in plants, including fruit and vegetables. *Brassica* volatiles have been collected using simultaneous distillation-extraction (SDE) [9,10], sorbent (Tenax) trapping [11], solvent extraction [12], as well as solid phase microextraction (SPME) [13,14]. The latter is the most widely applied solvent-free isolation method for plant materials and food flavors, thanks to its sensitivity, preconcentration abilities, ruggedness, potential automation and ease of use [15,16].

The aim of the present study was to evaluate differences in the profiles of volatile compounds in fresh and cooked *Brassica* vegetables. Thanks to the results obtained for 15 different cultivars of kohlrabi, broccoli, cauliflower and Brussels sprouts, generalized conclusions may be drawn in terms of changes in volatile compounds in the main representatives of *Brassica* vegetables. The volatile compounds were analyzed using solid phase microextraction and comprehensive two-dimensional gas chromatography with time of flight mass spectrometry (SPME-GC×GC-ToFMS).

#### **2. Results**

#### *2.1. Identification of Volatiles*

To compare the profiles of volatiles for the investigated vegetables, the compounds were isolated using headspace analysis, where the volatiles were adsorbed using SPME fiber and then separated by comprehensive two-dimensional chromatography-mass spectrometry (HS-SPME-GC×GC-ToFMS). This approach allows for the specific separation and (tentative in this case) identification of volatile compounds, thus, it is widely used in volatilome fingerprinting and profiling [17,18]. As the peak capacity of GC×GC systems can reach 10,000, the number of identified compounds is usually very high. GC×GC-ToFMS provides plenty of data, for this reason extracting useful information is often a very challenging task. For our work, only the most abundant compounds with a relatively high signal to noise ratio (S/N > 250), high similarity and retention indexes comparable to literature data were taken into consideration and the comparison was based on the peak area values and the peak area percentage. This serves the assumed comparative purposes well; however, it has to be remembered that SPME is a partition coefficient based extraction method and the area percentage may be different from the data provided by exhaustive extraction methods.

Figure 1 shows the differentiation between various *Brassica* vegetables using principal component analysis (PCA) based on the profiles of volatile compounds. Each point represents a mean of three replicates analyzed for a given cultivar. One can observe obvious differences in the profiles of particular vegetables, both raw and cooked. However, there are also significant differences between particular cultivars of some investigated vegetables, which is especially noteworthy for raw broccoli and Brussels sprouts. Examination of the PCA graphs shows a similarity between cauliflower and broccoli volatiles in raw vegetables, despite the between-cultivar differences for broccoli, while kohlrabi and Brussels sprouts differ substantially in terms of their volatile compound profiles. When the data are compared for cooked vegetables, the similarities between cauliflower and broccoli can still be observed (close distance between clusters), whereas the other vegetables form distant clusters. Interestingly, after cooking, the differences between cultivars are much less pronounced. This would indicate the more uniform profile of the main volatiles. A detailed list of main compounds for all the examined cultivars is provided in Tables S1–S4 in the supplementary files. The compounds corresponding to the number in the PCA loading plots are presented in Table S5 in the supplementary files. As can be observed in the tables, the compounds can be classified into several main groups: Aldehydes (with alkanals, 2-alkenals

and 2,4-alcadienals), alcohols (mainly unsaturated), isothiocyanates (mainly aliphatic, although also aromatic), other sulfur compounds (mainly sulfides and thiophenes) and nitriles. To a certain extent, the classification of volatile compounds into these classes is also associated with the sensory properties and acceptance of these compounds. Some of the identified aldehydes and alcohols may contribute to the "green", "sweet", "fatty", "soup" and other notes in the vegetable aroma. Isothiocyanates are products of the enzymatic hydrolysis of glucosinolates, which are widely investigated because of their beneficial effect on human health. However, they are also partially responsible for the specific pungent notes. Nitriles, which are an abundant group in all analyzed samples, are also found as degradation products of glucosinolates, instead of isothiocyatates [1]. Other sulfur components include mainly products of the cysteine pathway, which are sulfides. All the detected sulfides, i.e., disulfides, trisulfides, tetrasulfides, pentasulfides, and others have been collected under the general name of "sulfides", simply because of their labile nature and mutual transposition. To facilitate an easy visual comparison of raw and cooked cultivars, Figure 2 shows the main groups of the detected compounds in the four examined vegetables.

**Figure 1.** Principal component analysis (PCA) graphs of volatiles extracted from the raw (**a**) and cooked (**b**) *Brassica* vegetables investigated. Numbers on loadings plot represent compounds characteristic for analyzed vegetables. Compound numbering is uniform with data provided in the Supplementary Table S5.

**Figure 2.** Percentage contents of isothiocyanates, alcohols, aldehydes, other sulfur compounds, nitriles, others (miscellaneous) in all analyzed samples. Broccoli (**a**): Covina, Malibu, "2970"; Brussels sprouts (**b**): Ajax, Marte, Maximus, Neptuno, Profitus; Cauliflower (**c**): Charlotte, Liria, Oviedo; Kohlrabi (**d**): Kolibri, Konan, Konmar, Kordial.

#### 2.1.1. Concentrations of Isothiocyanates and Nitriles in Raw and Cooked *Brassica* Vegetables

Very small amounts of isothiocyanates were detected in all of the raw broccoli cultivars (<2% in all samples). Additionally, the nitrile levels were relatively low (<5%), which confirms very low concentrations of myrosinase hydrolysis products in the gas phase. The diversity of isothiocyanates was also very limited, barely four different isothiocyanates were identified using GC×GC. Both 2-methylbutyl isothiocyanate and isobutyl isothiocyanate were found in the cultivars Malibu and "2970", while cyclopentyl isothiocyanate and n-hexyl isothiocyanate were present in all the analyzed broccoli samples. The composition of isothiocyanates was different in cooked vegetables, as propane, 1-isothiocyanato-, butane, 1-isothiocyanato-, 1-butene, 4-isothiocyanato- and benzene, (2-isothiocyanatoethyl)- were detected in all the analyzed cooked vegetables, instead of most isothiocyanates present in raw vegetables. The most interesting result was found in the cooked cultivar "2970", where all the described isothiocyanates were present, along with a significant amount of allyl isothiocyanate (which was not present in the raw broccoli). This unexpected finding was connected with the absence of sulforaphane, one of the most explored isothiocyanates present in broccoli. Because of the health benefits of sulforaphane [6], interest in this component concentration is particularly high. Solvent assistant flavor evaporation (SAFE) extraction was performed for one broccoli cultivar to check if the sulforaphane was present in the sample. In the SAFE extract, analyzed by GC×GC-ToFMS,

sulforaphane and also sulforaphane nitrile were present as glucoraphanin degradation products, although in very small concentrations. The absence of sulforaphane in the SPME extract may either be due to the labile nature of this component [19], or more probably, due to its small K value (partition coefficient sample-headspace-fiber). SAFE is an exhaustive extraction type technique, therefore other/more components can be identified.

The concentration and diversity of isothiocyanates was much greater in the cooked broccoli. The amounts were almost two times higher than in the raw vegetables. It was reported that myrosinase in the broccoli matrix is inactivated after 20 min at 60 ◦C [20]. No reports have been found in available literature on the stability of nitrile specific proteins. Generally, the total amount of glucosinolate degradation products decreased, especially the concentration of nitriles. Cooking caused a change in the amount of isothiocyanates, which is beneficial from the consumer's point of view.

In cauliflower, the major isothiocyanate was allyl isothiocyanate, representing 2–4% of the total volatile fraction, resulting from the high concentration of sinigrin, its precursor, in cauliflowers [12]. Allyl isothiocyanates are some of most frequently studied isothiocyanates, with many publications confirming their anticancer activity in both cultured cancer cells and animal models. Moreover, the bioavailability of this compound is very high, as almost 90% of oral intake is absorbed [21]. Other isothiocyanates were also present, although in much lower concentrations, usually less than 1%. In the Lira and Charlotte cultivars, somewhat higher amounts of isothiocyanates were recorded in the cooked vegetables compared with the raw vegetables, while in the Oviedo cultivar the situation is opposite. An abundant group in all raw cauliflower cultivars was composed of nitriles, which constituted almost 10% in all analyzed samples. It proves again, that also in raw cauliflower after tissue disruption, nitriles are formed as the main products of glucosinolate hydrolysis. It suggests that after tissue disruption, the hydrolysis of glucosinolates yields nitriles instead of bioactive isothiocyanates. The concentration of 2-methylbutyl isothiocyanate was higher in the cooked vegetables in all analyzed varieties, whereas the allyl isothiocyanate contents were higher in the raw vegetables (except for the Liria cultivar). The total amount of isothiocyanates was higher in the cooked Lira and Charlotte cultivars than in the raw vegetables (Table S2). An opposite situation was observed in the Oviedo cultivar. The nitrile concentration was significantly smaller in all analyzed cooked cauliflower samples.

The third group of analyzed plants comprised four varieties of kohlrabi: Kolibri, Kordial, Konan and Konmar. The percentage concentration of isothiocyanates in the raw vegetables was unexpectedly varied, as it was 4.16% for Konan, 18.96% in Kolibri, 6.15% Konmar and 3.71% in Kordial. The content slightly exceeded that in cauliflower. This fact was also correlated with high nitrile concentrations (more than 10% in all cases). In cooked kohlrabies, this amount increased in the Konmar and Kordial cultivars. Benzene, (2-isothiocyanatoethyl)-, which is present in uncooked vegetables at significant levels, almost disappeared in the cooked kohlrabies. The increase in isothiocyanate concentrations (Table S3) in the Konmar and Kordial cultivars was mostly caused by the increase of the n-pentyl isothiocyanate peak area in those two cultivars. The peak area (i.e., amounts) of nitriles decreased significantly in all analyzed cooked kohlrabi varieties, compared to the raw varieties.

The uncooked Brussels sprout cultivars contained mostly nitriles in their volatile fractions. Isothiocyanates constituted the second most abundant group. In the cooked plants, the isothiocyanates accounted for more than 50% in the Maximus, Marte, Ajax and Neptuno cultivars. However, the percentage contents of isothiocyanates increased, while their summary peak area changed significantly only in the Maximus cultivar. In the case of nitriles, the summary peak area decreased almost two-fold when compared with the raw varieties. Only in the Profitus cultivar did the levels of nitriles remain practically unchanged.

The important issue described in this part of the study is related with the high concentration of nitriles in some of the analyzed samples. This fact suggests that after tissue disruption in raw, defrosted vegetables, the hydrolysis of glucosinolates occurs in favor of the formation of nitriles instead of bioactive isothiocyanates. This seems to be important from a biological point of view. This is because of the health-promoting nature of isothiocyanates, which are bioactive molecules that

are known for having many positive effects on human health. These isothiocyanates are found as a minority in the volatile fraction with respect to nitriles. Based on the actual in vitro data, nitriles have less beneficial health potential [19,22,23], or even harmful effects on consumers [24]. In cauliflower, the percentage of nitriles was more than 10% and the percentage of isothiocyanates was about 3% in all samples, with a similar situation observed in broccoli. In the kohlrabi cultivars, the level of isothiocyanates was slightly higher; however, the nitrile concentration was still two times higher, and in the Konan cultivar it was even four times higher than that of the isothiocyanates. As presented, the enzymatic degradation of glucosinolates leads to the formation of isothiocyanates, and in some cases, to nitriles, as the main product. The presence of modifying proteins such as the epithiospecifier protein or the nitrile-specifier proteins results in the enzymatic degradation of glucosinolates being altered in favor of nitriles [8,25]. It is important to highlight here that all vegetables were frozen before analysis; the impact of freezing on the composition of volatiles is presented in the next part of the paper. The changes in isothiocyanates and nitriles induced by cooking are presented for selected cultivars in Figure 3. It illustrates the general decrease in nitriles caused by cooking and also the different behavior of isothiocyanates in these cultivars. Initially, research on glucosinolate degradation products was mainly focused on their toxic, antinutritive and goitrogenic properties. More recently, attention has shifted to investigations concerning their beneficial effects against various diseases. Most studies are focused on different *Brassica* vegetables and the detection of glucosinolates, which are biologically non-active molecules. Still, a higher glucosinolate content does not always guarantee an increment of desirable isothiocyanates after tissue mastication. The formation of beneficial isothiocyanates depends on a variety of factors, such as the activity of myrosinase and nitrile specific proteins or domestic processing. It is worth mentioning that even if glucosinolates are not degraded, their consumption is beneficial, since they can be hydrolyzed by a healthy intestinal microbiome [26]. Studies focusing on the health-promoting effects of isothiocyanates are numerous; however, they are mostly focused on sulforaphane [27], allyl isothiocyanate [21] and benzyl isothiocyanate [28], while as presented the qualitative diversity of isothiocynates is considerable.

**Figure 3.** Proportions between isothiocyanates and nitriles in selected raw and cooked *Brassica* vegetables. The results are the mean of three replicates and the error bars show standard deviation. Amounts were expressed in total (sum of) peak areas.

#### 2.1.2. Other Sulfur Volatiles

Derivatives from the *S*-alk(en)yl-l-cysteine pathway compounds occurred very frequently in the volatile fraction in all analyzed vegetables. In all raw kohlrabi cultivars, sulfides formed the most abundant group of volatile components. They accounted for more than 50% of all the volatiles present in the analyzed kohlrabies (Figure 2). In the Konmar cultivar, it was almost 80%. Surprisingly, a small total percentage of sulfides was noted in raw Brussels sprouts cultivars, where "other sulfides" represented less than 15% in all analyzed samples. Numerously represented sulfides (di, tri-, tetra-) are shown in the supplementary tables. The average percentage content of sulfides was approximately 10% in all analyzed *Brassicaceae*. Taking into account the extremely low odor threshold of these compounds, which are detectable at levels as low as one part per trillion by the human nose [29], their crucial role in the *Brassicaceae* characteristic flavor formation seems to be obvious. The smell of all sulfides detected here is described as extremely unpleasant [29]. The comparison of sulfide concentrations in raw and cooked vegetables was inconclusive, because in broccoli, the amount of sulfides was higher in the cooked form than in the raw form, while in kohlrabi, cauliflower and Brussels sprouts, the concentration was usually higher in the cooked vegetable. Additionally, there were some exceptions for some cultivars, where the proportions were reversed.

#### 2.1.3. Aldehydes and Alcohols

All the analyzed cauliflower and broccoli cultivars contained high percentage levels of aldehydes. In broccoli, the dominating aldehydes were hexenal, 2-hexenal (more than 10% in all samples), 2,4-heptadienal and benzaldehyde. According to [29], those aldehydes contribute the "green" type aromas to the final broccoli fragrance. In the cauliflower cultivars, the content of aldehydes was even higher than in broccoli, with 2,4-heptadienal, 2-hexenal, hexanal, benzaldehyde, propanal and nonanal being dominant (found in the highest percentage levels). The presented profile of aldehydes in cauliflower was quite similar to that in broccoli. A marked decline in the diversity and percentage contents of aldehydes was observed in all kohlrabi varieties. In all of the analyzed cultivars, the level of aldehydes was below 5%. The magnitudes of the aldehyde peaks on the chromatograms were also relatively small. In comparison with cauliflower and broccoli, which contain respectively 22 and 16 different aldehydes, seven aldehydes were identified in kohlrabi after GC×GC analysis, seemingly to be a very small number. The most abundant aldehyde in all varieties was 2-butenal, with a flower-type odor [29]. In Brussels sprouts, the aldehyde percentage content was low; however, their peak areas and diversity were relatively high. Fifteen different aldehydes were identified by GC×GC, with the ratios of all aldehydes being similar and with benzaldehyde being the most abundant. Twelve different alcohols were also present in the Brussels sprouts chromatograms and their percentage contents were significantly higher than those of the aldehydes (upper 20%). Only seven different aldehydes were identified in the kohlrabi samples, at less than 5% in all samples. Despite the small amounts of aldehydes, which are mostly important for "green", "sweet" and "nice" aromas, the intensity of "raw kohlrabi" and "green" odors detected by the panelists was relatively high. Alcohols were present in trace amounts, so they were disregarded in the percentage calculations. In cooked broccoli and cauliflower, the number of aldehydes was noticeably lower in the cooked vegetables, compared to raw varieties. In contrast, the number of aldehydes in the kohlrabi was significantly greater in the cooked vegetables. The percentage contents of different aldehydes in cooked kohlrabi varied between cultivars. In Brussels sprouts, the levels of the lipoxygenase (LOX) pathway products decreased in all cooked cultivars.

Figure 4 shows the peak areas of the main groups of volatile compounds investigated in the analyzed vegetables, both raw and cooked. Total peak areas were used to indicate the differences between particular vegetables, so it can be seen that aldehydes are dominant in broccoli and cauliflower, whereas in kohlrabi, sulfides constitute the most abundant fraction. Additionally, the dominance of nitriles and isothiocyanates in raw Brussels sprouts is easily noted. The error bars indicate differences caused by different cultivars taken for comparison, therefore they are in some instances relatively high. Similarly, the main fractions of the examined cooked vegetables can be compared, indicating an overall decrease in the amounts of volatiles caused by cooking, with some exceptions discussed above.

**Figure 4.** Mean values in different cultivars of analyzed raw (**A**) and cooked (**B**) vegetables. For broccoli, it is the mean value from three cultivars: Covina, Malibu and "2970", for cauliflower, from three cultivars: Charlotte, Oviedo, Liria, for kohlrabi, from four cultivars: Kolibri, Konan, Konmar, Kordial and for Brussels sprouts, from five cultivars: Ajax, Marte, Maximus, Neptuno, Profitus. The results are the mean of three replicates for each cultivar (i.e., nine analyses for broccoli and cauliflower, 15 for Brussels sprouts and 12 for kohlrabi), error bars show standard deviation. Amounts were expressed in total (sum of) peak areas.

### 2.1.4. Freezing-Thawing Effect on the Volatile Fraction

Prior to cooking, all samples were stored at −20 ◦C, which is why the impact of freezing needed to be evaluated. The comparison of the volatile composition in raw, fresh and frozen-thawed vegetables was performed based on Brussels sprouts. Figure 5 shows a comparison of nitrile amounts in raw (fresh) and raw frozen-thawed vegetables for all 5 Brussels sprout cultivars. Moreover, the percentage contents of aldehydes was also higher than those of alcohols in the frozen vegetables, whereas the opposite situation was observed in fresh tissue. It is worth highlighting here that in the fresh product, the concentration of isothiocyanates was more than 50% of the total value, whereas in the frozen vegetables the amount was approximately 30%. The reason for the change in the isothiocyanate levels and an increase of nitrile contents is unknown at the moment. It may be connected with cell degradation during the freezing process, leading to changes in the activity/stability of enzymes responsible for the hydrolysis of glucosinolates.

**Figure 5.** Mean values from all Brussels sprout cultivars (Ajax, Marte, Maximus, Neptuno and Profitus) for total peak areas of the main groups in raw non-frozen and frozen-thawed vegetables. The results are the mean of three replicates for each cultivar (i.e., 15 analyses), error bars show standard deviation. Amounts were expressed in total (sum of) peak areas.

#### **3. Materials and Methods**

#### *3.1. Brassica Cultivars*

Three cultivars of fresh broccoli (Covina, "2970", Malibu), three cultivars of cauliflower (Charlotte, Oviedo, Liria), four of kohlrabi (Konmar, Kolibri, Konan, Kordial) and five cultivars of Brussels sprouts (Ajax, Maximus, Profitus, Naptuno, Marte) were used for analysis. They were harvested during the same 2016 autumn season: Broccoli, cauliflower and kohlrabi were harvested in September and the Brussels sprout cultivars were harvested in December. The vegetables were delivered to the laboratory within 24 h after harvest and stored at −20 ◦C until analysis (approximately 4 weeks).

#### *3.2. Cooking Process*

All vegetables were cooked in the same way. After washing and chopping, about 200 g of plant material was placed in 1 L of boiling pure water with 7 g of salt. Cooking times differed among vegetables. It was 5 min for the broccoli, 7 min for the cauliflower and 10 min for the kohlrabi and Brussels sprouts.