Research On Petroleum Accumulation Geochronology And The Characteristics Of Fluid Evolution Of Upper Triassic In The Middle Part Of Western Sichuan Depression

The west sichuan depression belongs to a typical Superimposed foreland basin. It has experienced complicated tectonic movement since Upper Triassic Xujiahe Formation. Numerous oil and gas pools are widely distributed in this field. Because of specific complicated geological situation and experienced complicated geological changes, bringing up clastic reservoirs the Formation Time,the characteristics of Fluid Evolution and distribution are very complicated,and also determines serious heterogeneity of reservoir distribution in the field.The paper is starting from regional tectonic evolution, geochemical characteristics and distribution of main source rocks in clastic rock formations in west sichuan depression.Reararch on formation and petrological assemblage, characteristics of hydrocarbon source rock, fluid characteristics and gas-sources tracing ,and so on, the clastic rock gas-bearing system in west sichuan depression is divided into three reservoir-forming system ,namely the Xiaotangzi and Ma'antang formation—the Xujiahe formation second and third member reservoir-forming subsystem; the Xujiahe formation third member ,fourth and fifth member reservoir-forming subsystem; the Xujiahe formation fifth member—Jurassic-Cretaceous reservoir-forming subsystem.On the base of the above,we research on system of the source rock of the Xujiahe formation fourth member and second member,and also study on the reservoir-forming subsystem with the Xujiahe formation fourth member.Further analysis about gas-sources of main gas reservoir, especially according to the lower degree of element fractionation of natural gas, study on gas source tracing by the means of source rock and light hydrocarbon fingerprints -isotopic analysis.Using various method and technique combine together, research on evolution features and matching relationship of source rock, reservoir, cap rock, trap in the two reservoir-forming system of deep the Xujiahe formation. Which combined with fossil records of petroleum emplacement and isotopic dating established the main reservoir forming geochronology of complicated reservoiring process of the Xujiahe formation second and fourth member.The Study on System of the characteristics of fluid evolution in clastic rock formations, obtained the characteristics of fluid potential energy and composition change which reflect gas accumulation, relationship between fluid composition evolution and servoir evolution. the results showed that the Xujiahe formation in the middle part of western sichuan depression exist three periods ancient hydrodynamic field. Based on the research of the relationship between pore-fluid evolution characteristics and reservoir quality in the member second and fourth of the Xujiahe formation and some jurassic gas reservoir, we established evolution model of the main reservoir in jurassic and Xujiahe formation in western Sichuan depression. Through the research, we found the quality reservoir development in Shaximiao formation may be related to structure uplift in Yanshan period. The quality reservoir development in the member second of the Xujiahe formation is mainly influenced by rock components and early fracture system. The quality reservoir development in the member fourth of the Xujiahe formation, and the uncomformity between the member third and fourth of the Xujiahe formation are all closely related to the exposure of the end of sedimentation in the member fourth of the Xujiahe formation.Quantitative study on free gas in the Xujiahe formation shows that gas migration phase of gas bearing domain of clastic rock is the chief free phase in western Sichuan depression.Combing with gas generation intensity, paleo tectonic, distributing graph of gas potential and so on, it can make sure of hydrocarbon migration pathway in key period of the Xujiahe formation. It's beneficial to guide exploration targets of gas bearing domain of compact and hypercompact clastic rock in the Xujiahe formation. These research fruits are important in gas exploration and development in western Sichuan depression.