Discovery and Forming Pattern of the Upper Triassic Xujiahe Formation Oilcracking Gas Reservoirs in Western Sichuan Depression, China

Key words: 
• reservoir asphalt /
• paleooil pools /
• forming pattern /
• Upper Triassic /
• West Sichuan Depression

Abstract: There are a number of sets of source rocks ,a number of hydrocarbon source area,multistage hydrocarbon generation and multistage hydrocarbon accumulation in the middle part of the West Sichuan Depression and it also experienced a number of tectonic stages adjustment and havoc. On it's vertical, multiple sets of sourcereservoircap assemblages gave birth to multilayer gasbearing series. According to the results of gas carbon isotopic analyses, part of upper Triassicseries Xujiahe Formation's gas ethane carbon isotopic have obvious negative skewness characteristics in this area.Relative photos' discrimination show that the gas has the feature of oil cracking gas. Meanwhile, a large amount of reservoir asphalt fillings have been discovered in upper Triassicseries Xujiahe formation in this area. Through the analysis of core observation, micrograph, elements, pyrolysis and so on,we can see reservoir asphalt fillings have many characteristics such as multitude types, several shapes of distribution, a more clear boundary and so on. Reservoir asphalt fillings also have a characteristic that these reservoir asphalt fillings have a higher reflecting especially in the second member of Xujiahe Formation's reservoir bitumen. This characteristic shows residue's characteristic which is similar to the coal tar pitch's basic characteristic after oil reservoir cracking. The discovery of oil cracking gas and the coal tar pitch confirms the existence of paleooil pools of Xujiahe Formation in middle part of the West Sichuan Depression. On this basis, through the generalized analysis of genetic oil and cracking condition and so on, we carried out the preliminary discussion on the forming pattern of paleooil pools cracking gas of the Xujiahe Formation.We can divide this process into three consecutive phases: Late Indosinian paleooil pools's formation, Middle Yanshan paleooil pools's cracking and Late YanshanHimalayan period's gas reservoir adjustment. It is of certain significance to deepening recognition on the formation mechanism and distribution of gas of the middle part of the West Sichuan Depression.