Origin of seismic reflections in a carbonate gas field, Lower Miocene, offshore Myanmar
Abstract The geological origin of seismic reflections, within a carbonate gas reservoir, has been determined on the basis of an integrative analysis of cores, well-logs, pre-production and 4D time-lapse-seismic data. The gas reservoir is subdivided into meter to decameter-thick diagenetic units which are characterized by distinct acoustic properties. Low impedance units are dominant and consist of highly microporous and vuggy foraminiferal/coralline algal floatstones intervals whose porosity has been significantly enhanced in burial diagenetic environments. High to moderate acoustic impedance units (thickness: 1–20 m) are related to various depositional and diagenetic fabrics: 1) early-lithified coral floatstones, 2) foraminiferal rudstone whose intergranular space is occluded by shallow burial sparry calcite and 3) carbonate breccia with neomorphosed corals occurring below an intra-formational subaerial exposure surface. Within the gas zone, seismic reflection horizons may form at the boundary between two diagenetic units with contrasted acoustic impedance or may result from the interference between reflection at base and top of such intervals. A stratigraphy-cross-cutting reflection horizon has been shown to form at a major and laterally continuous shift in porosity which is related to a differential diagenetic evolution between the gas and the water zone, during or after hydrocarbon emplacement. Finally, the analysis of the geological origin of seismic reflections has been proved to help predicting and mapping the major heterogeneities with significant impact on flow during production.