| Western Sichuan Depression is a foreland basin which develops on the Triassic basement fold cover and NE-SW direction since the Late Triassic. The middle section of western Sichuan depression is the main part of foreland basin, including Xiaoquan-Xinchang-Fenggu welt of east-west direction. The upper Triassic can be divided into T3t+m, T3x (T3x2, T3x3, T3x4, T3x5) formations.There is source-reservoir-cap rock in deep T3x in mid-west Sichuan depression: abundant gas source and higher hydrocarbon generation intensity provide a favorable material base; oil and gas transporting system provide favorable conditions for migration; timely ancient uplift, the development of ancient slopes and traps and thick large area distribution of the reservoir for the accumulation of gas; Widespread regional cap provide favorable capping conditions.Reservoir rock in this depression, thick, widely distributed and source storage symbiotic, is main source rock. Natural gas prospective reserves currently are only about 7.7% of total resource, which is mainly in shallow and middle of the Jurassic, although predecessors carried out a large number of explorations in structure, reservoir, accumulation, and other aspects. Apart from super-dense, high pressure of the source rock, usually greater than 3,000 meters depth, and multi-stage forming caused by tectonic movement, petroleum exploration not clear enough are the deeper reasons for low natural gas proved reserves. The reasons for indefinite clue are relatively weakness of the study in fluid causes, distribution, oil and gas accumulation period and its dynamic accumulation process.This paper, guided by sediment logy, petroleum geochemistry theory, resorted to geochemical analysis of drilling data, and combined with the tectonic evolution and sedimentary characteristics in mid-west Sichuan depression, completed geochemical analysis of T3x in mid-western Sichuan depression and gas source tracking work:(1) Collect the related information of organic carbon, organic sulfur, chloroform bitumen "A", chloroform bitumen ethnic composition, kerosene maceral, saturated Hydrocarbons, saturated hydrocarbons, carbon isotopes, pyrolysis, virginities reflectance, C, H, O elements, sand classification, reservoir properties and cover of T3x in mid-western Sichuan depression, definite the geochemical characteristics of basic source rocks in study area T3x and study the organic matter abundance, organic matter type, thermal evolution extent of hydrocarbon source rocks in research areas, and determined the effective source rock system.Systematic division of clastic rock in gas field in the mid-western Sichuan depression: T3t+m -T3x2-T3x3, T3x3-T3x4-T3x5, T3x5-Jurassic, Cretaceous, three main reservoir systems. Based on this division, two reservoirs-T3x4 and T3x2-have been systematically studied and pointed out that T3x develop internal subsystems.(2) Deeply researched geochemistry of T3x source rock, laid a foundation for analysis of the characteristics of the reservoir system. Organic carbon of source rock of T3x has the characteristic of the high organic carbon content, high maturity, high mature stage of most of the, over mature of little source rocks, typeâ…¢of the main type of organic matter,â…¡type with a small amount of organic matter.(3) Collected gas composition, isotopes and hydrocarbon and other data, analyzed the basic characteristics of natural gas in the study area and fined contrast basic characteristics of natural gas (including the contrast in and outside the region). Divide genetic type of natural gas according to carbon isotope characteristics, hydrogen isotopes, light hydrocarbon features and noble gas characteristics. The result showed that the natural gas in the mid-western Sichuan depression was of relatively homogeneous and was mainly high evolution levels coal-type gas; A systematic comparison between primary gas pool and secondary gas pool was carried out according to Satiating a large number geochemistry data of natural gas, and the results showed that the natural gas in T3x in study area belonged to the original reservoir gas pools.(4) Analyze the gas source of main gas reservoir deeply, especially for the natural gas of low degree of differentiation characteristics. Carry out fingerprint analysis of light hydrocarbon, isotope analysis and other means within and source rocks to track the original rock. Results show that the natural gas in T3x2 is mainly of self-generating and self-preserving shall not exclusive of the contribution of subjacent T3t+m original rock; the natural gas in sub-section T3x4 is also mainly of self-generating and self-preserving, while under sub-section of natural gas mainly from T3x3.(5) Established of a reservoir system and gas accumulation in the typical pattern and forecasted exploration area of T3x based on analysis of summary of reservoir in T3x in the typical regions. Pointed out T3x2 has exploration potential, Type I mainly in Dayi-Jinma-Yazihe-Mianzhu,Xiaoquan-Xinchang-Hexingchang-Gaomiaozi-Fenggu structural belt, and north-south tectonic zone Luodai. Type II areas mainly in Majing and Zhongjiang, Which of T3x4 mainly in Jinma-Yazihe-Mianzhu area, Xiaoquan-Xinchang-Hexingchang-Gaomiaozi, including the western Xiaoquan-Xinchang a better structure relatively.
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