| For the characteristics of the Triassic Xujiahe formation in the middle of the western Sichuan depression, which has super dense, super depth, high pressure and the history of trap formation and the complexity of oil and gas accumulation, this paper starts from the basic information, on the basis of the basic forming conditions of the study area, through the detailed study and contrast of fluid geochemical characteristics, we known the causes and sources of natural gas, and analyzed migration mechanism of longitudinal changes, the sources of the formation water and the mechanism of water-rock interaction. Based on the recovery of formation eroded thickness, through many methods, such as the history of the main hydrocarbon source rock generation and expulsion, the package body with homogenization temperature, and K/Ar dating, etc, we identified the forming age of major gas accumulation. Finally, through the anatomy of typical gas reservoir, we analyzed the dynamic process of natural gas accumulation of Xujiahe in the study area, and summed up the gas accumulation model. The main conclusions and innovative achievements are as follows:(1)The characteristics of hydrocarbon source rock of Xujiahe formation was large thickness, wide distribution, high organic matter abundance, high maturity, and single type(the main type wasâ…¢kerogen).(2)The characteristics of gas of member 2 compared with other members of Xujiahe formation shown that the methane carbon isotope of member 2 was higher than that of other members, and ethane carbon isotope of member 2 was lower than that of other members. This shown that gas of member 2 udergoed higher organic maturation evolution and came from better mother substance, which hint the blending of the ear1y paleo-oil pools pyrolysis gas.(3)The longitudinal regular change of gas mainly embodyed the gas maturity and fractionation effect in migration process. The longitudinal migration mechanism of the primary pool and the secondary pool had great difference. The main resource of the upper Jurassic gas was the underneath Xujiahe formation gas, and faults were the first migration pathway. The migration manner of the middle Jurassic gas was complex, one part came from Xujiahe formation gas through high speed migration pathway, the other came from the underneath Xujiahe formation gas in hydrotrope. The mainly gas migration manner of member 4 of Xujiahe formation was diffusion, which reflected that the reservoir bed was fine and close when the gas accumulated. The mainly gas migration manner of member 4 of Xujiahe formation was transfusion, in which faults played an important role.(4)Gas source comparison indicated that the Jurassic gas of HeXingChang mainly came from the member 5 of Xujiahe formation hydrocarbon source rocks, and the middle Jurassic source rock was its powerful supplement. Jurassic gas of LuoDai mainly came from the middle and the lower Jurassic system, the member 4 of XuJiahe formation had certain contribution. The Jurassic gas of JinMa mainly came from the middle and the lower Jurassic. The Jurassic gas of XinChang mainly came from the member 5 of XuJiahe formation hydrocarbon source rocks, the middle and the lower Jurassic source rock were its powerful supplement. Jurassic gas of LuoDai mainly came from the middle and the lower Jurassic, the member 4 of XuJiahe formation had certain contribution. Jurassic air hydrocarbon source rocks of Ducks river area mainly came from the member 4 of XuJiahe formation. The Jurassic gas of ZhongJiang mainly came from the member 5 of XuJiahe formation hydrocarbon source rocks. The Jurassic gas of the member 4 of XuJiahe formation of FengGu area mainly came from its own hydrocarbon source rocks. The gas of the member 4 of XuJiahe formation of XinChang area was mainly provided by gas source of the member 4 formation source rocks, and had nature of self-generation and self-storage. The gas of the member 2 of XuJiahe formation of GaoMiaoZi area was mainly provided by gas source of both XiaoTangZi-Saddle group and the member 2 of XuJiahe formation. The gas of the member 2 of XuJiahe formation of XinChang mainly came from the member 2 of XuJiahe formation and XiaoTangZi-Saddle group hydrocarbon source rocks. The gas of the member 2 of XuJiahe formation of XinChang and ZhongJiang mainly came from the member 2 of XuJiahe formation and XiaoTangZi-Saddle group hydrocarbon source rocks. However, for the nature of self-generation and self-storage of the gas of the member 4 of XuJiahe formation, we think that the main reason was the gas samples mainly came form the upper mild-segment of T3x4, and the mother rock of the member 3 of XuJiaHe formation should make great contribution to the lower mild-segment of member 4 of Xujiahe formation.(5)The main formation water of Xujiahe formation was atmosphere fresh water, but at the later stage there was marine facies formation water intruding, and argillutite and the coal bed compaction water affected water properties tremendously. The geochemical characteristics similarity of formation water of member 2 and member 4 of Xujiahe formation was caused not only by cross-formation flow, but also mainly by the common resource such as marine facies formation water and argillutite compaction released water. The geochemical characteristics divergence of formation water of member 2 and member 4 of Xujiahe formation was mainly caused by different water-rock interaction, for the member 2 had more open ended water-rock interaction than that of member 4.(6)The average eroded thickness of Himalayan movement formation was around the 1400m, in the plane, there had the following changeable characteristics: the eroded thickness gradually became smaller from north to south in front of Longmen Mountain, Dayi region had relatively minimal the eroded thickness. From in front of the Longmen Mountain to the east, the eroded thickness gradually reduced. The western Sichuan depression had the characteristics of the eroded thickness decreased from north to south. From west to east, the performance shown larger after smaller, the erosion of Duck River region was stronger, from east to the Horse Well region, the eroded thickness was smaller, and then east to the zhongjiang region, the eroded thickness became larger.(7)The homogenization temperature of inclusion enclave of great quantity sample, especially the fissure sample, overtook the palaeogeothermal far away, the main reason of which was hydrothermal fluid through the faults and fissures.(8)The gas of member 2 of Xujiahe formation had the characteristic of persistent infusion, which had 3 important communicating stages, as follows: member 5 of Xujiahe formation-early Jurassic sedimentation, middle Jurassic-early Crataceous sedimentation and Himalayan movement.Middle Jurassic-early Crataceous sedimentation was the foremost stage of gas communication, the other were less important relatively.The gas of member 4 of Xujiahe formation had the same characteristic of persistent infusion, and the corresponding communicating stages are the middle sedimentation of member 5 of Xujiahe formation-early Jurassic sedimentation, late Jurassic-early Crataceous sedimentation and Himalayan movement. Late Jurassic-early Crataceous sedimentation was the foremost stage of gas communication, and Himalayan movement mainly reformed the early primary gas pool.(9)The master control of natural gas concentration of member 4 of Xujiahe formation was: 1) Structure nowadays controlled the gas and water distribution of local structure, which was the key factor of oil and gas concentration, and ancient and modern structure eminence was the most favorable place for gas concentration. 2) Faults were the key factor of late period regulatory of gas pool. Fissure, especially the high angle fissure was not only the key factor of gas pool high production, but also the main invasion channel of formation water. 3) The reservoir heterogeneity was the fundamental reason of gas horizon heterogeneity. 4) The beneficial deposition microfacies were the base of superior quality reservoir development.(10)The master control of natural gas concentration of member 4 of Xujiahe formation was: 1) Ancient and modern structure was the base of gas and water distribution; 2) Cracks and favorable sedimentary facies superimposed the controlled superior quality sandstone reservoir was the key factor of high production and stable yields of the lower mild-segment of member 4 of Xujiahe formation; 3) The preservation conditions, especially the preservation conditions of reservoiring critical period, was the most key factor of gas and water distribution of the upper mild-segment of member 4 of Xujiahe formation.
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