Overpressure characteristics and classification of the tight gas reservoirs in upper Triassic Xujiahe formation in the middle section of west Sichuan Depression,China
The upper Triassic Xujiahe formation is the main gas production interval in west Sichuan Depression,has two main characteristics of ultra-tight and overpressure.Most of the gas reservoirs are found in the positive tectonic units in the middle section of west Sichuan Depression.Pressure tests result and drilling and logging data have shown that the overpressure character differs in different intervals of Xujiahe formation;the pressure coefficient is higher in the cap rock than in the reservoirs,and higher in the upper Xujiahe formation than in the lower.Different gas reservoirs have different vertical overpressure characteristics.The Xujiahe gas reservoirs in the middle section of west Sichuan Depression are classified into 4 pressure associations according to typical pressure development characteristics,i.e.Xinchang type,Hexingchang type,Yazihe type and Dayi type,which respectively represent different levels of pressure and hydrocarbon preservation conditions,and have different implications in gas exploration.