Geological Characteristics and Hydrocarbon Accumulation Models of the Tight-Sand Gas Reservoirs in Upper Triassic Xujiahe Formation in Western Sichuan Depression of China

Abstract

Summary URTeC 1619368 Western Sichuan Depression, located in western of Sichuan basin in China, is an area rich in natural gas. Typical deep tight sandstone gas reservoirs are developed in upper Triassic Xujiahe Formation, and their total resources are evaluated of approximately 27.5TCF. The main reservoirs of T3x2 and T3x4 Members have undergone intensive densification because of the severe diagenesis such as pressolution, compaction and cementation so that the Xujiahe reservoirs mainly exhibit the property of low porosity (average porosity less than 5%) and low permeability (average permeability less than 1mD) generally. Moreover, according to statistical data, the average gas-saturation is about 30% and the gas reservoirs in Xujiahe Formation generally present abnormal overpressure (with the pressure coefficient over 1.5). Exploration and development also confirm that the Xujiahe Formation is characterized by tremendously complicated distribution of water and gas. The main hydrocarbon expulsion time of the major source rock beds range from the middle-early stage of the Late Jurassic to the Early Cretaceous. The main densification period of T3x2 and T3x4 reservoirs is at the middle-early stage of the Late Jurassic. In addition, the analysis of fluid inclusions suggests that the main hydrocarbon accumulation in T3x2 and T3x4 Members occurred at the late stage of the Late Jurassic—the Early Cretaceous. Based on the matching relationship of the above three key factors, five hydrocarbon accumulation models are built. Xiaoquan-xinchang model— Previous structural-lithological reservoir was rebuild and superposed into deep basin gas reservoir owing to reservoir densification and readjusted by fracturing in the late stage. Hexingchang model — Previous lithological reservoir was rebuild and superposed into deep basin gas reservoir and then destroyed partly by faults. Dayi-yazihe model— Previous structural reservoir was rebuild and superposed into deep basin gas reservoir and then destroyed partly by faults in the late stage. Luodai model— Former deep basin gas reservoir was destroyed partly by faults later. Majing model— Former deep basin gas reservoir then became condensable further.