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Study On The Characteristics And Formation Mechanism Of Middle-Triassic Natural Gas Pools In The West-Central Sichuan Basin

Posted on:2010-09-30Degree:DoctorType:Dissertation
Country:ChinaCandidate:H WangFull Text:PDF
GTID:1100360278960752Subject:Mineral prospecting and exploration
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So far, there were only two medium-scale gas fields found in the Middle Triassic Series (Leikoupo formation) of Sichuan Basin. The Middle Triassic Leikoupo formation is composed up of dolostone, limestone, shale and anhydrite, which could be divided into five members (Lei-1, Lei-2, Lei-3, Lei-4, and Lei-5). Lei-5 is also called Tianjingshan fromation. The lithology of Leikoupo formation varies vertically and horizontally.The restricted-platform and open-platform facies are predominant facies of Leikoupo formation in the west-Central Sichuan Basin, and the evaporative platform facies could be found as well. The main subfacies are lagoon and tidal flat. The western Sichuan basin has platform marginal bank microfacies, while the central Sichuan basin has platform inner bank microfacies. In the Leikoupo formation, compacting shrink & pressolution, cementation, recrystallization, the forming of authigenic mineral (pyrite, celestine, and anhydrite) and filling of pore or fracture are five destructive diagenesis for the pore formation. The constructive diagenesis for the pore formation is dolomitization, dissolution (anhydrite dissolution, karst) and fracturing.The Leikoupo reservoir in the Central Sichuan Basin is medium porosity and low permeability. The main reservoir space is dissolved pore, poor connectivity between throat and pore, and fracture is not developed. The non-match of porosity and permeability is the vital geological factor of low gas production rate. On the contrary, the Leikoupo reservoir in the western Sichuan basin has high porosity, and fracture is well developed with good reservoir connectivity. It is a type of fracture-pore reservoir, and the reservoir space is intergranular & intragranular dissolved pore, while fracture is the main percolation channel.There are four types of good reservoir in the Middle Triassic Series in the western-central Sichuan basin: alga arenitic dolomite reservoir of bank facies, karst reservoir, reef & bank reservoir and hydrothermal dolomite reservoir. The exploration target is used to looking for alga arenitic dolomite reservoir of bank facies in the Middle Triassic Series of Sichuan Basin. The paleokarst improved reservoir conditions effectively, thus the paleokarst reservoir are becoming the recent exploration target in Leikoupo formation. The reef & bank reservoir is possibly developed in Western Sichuan basin. Reef & bank lithology combination is discovered in the well of Longshen 1 and Zhong 46. Furthermore, the hydrothermal dolomite reservoir may be developed in the Middle-Central Sichuan resulting from the discovering of hydrothermal minerals in the well of Longshen 1, Zhong 46, Qinglin 1, Lai 1, Mo 16, Mo 22, Nvshen 2, Sui 47 and Ying 21 etc.The Moxi Middle Triassic gas field is a mix-source gas which contains main coal-genetic gas and a few petroliferous gas. The coal-genetic gas is from Upper Permian Longtan formation coal measure source rock, while the petroliferous gas is from Permain sapropel source rock. The Zhongba Middle-Triassic gas field is also a mix-source gas, but contains main petroliferous gas and a few coal-genetic gases. The petroliferous gas is possibly from Permian sapropel source rock. The coal-genetic gas is from Upper Triassic Xujiahe formation coal measure source rock .Both Moxi and Zhongba gas reservoirs have abundant hydrocarbon source, good reservoir rocks, early formed trap and good preservation condition. Moxi T2l11 gas reservoir has undergone three phases:â‘ middle-late Triassic source rock generating hydrocarbon continuously and releasing hydrocarbon episodically,â‘¡the structure trap forming in the end of late Triassic Epoch, and gas trapped in the Yinzhi-Yanshan period,â‘¢trap finally formed in Himalaya period and the natural gas reservoir formed explosively. Moxi Middle Triassic gas reservoir forming model is: the Permian source hydrocarbon of Longnv structure migrated vertically to the T2l11 reservoir and then migrated to Moxi structure laterally through fractures caused by abnormal high formation pressure.Zhongba T2l3 gas reservoir has undergone four phases:â‘ epidiagenetic stage in the end of Middle Triassic Epoch to the near surface pore build-up stage,â‘¡ancient shielding forming stage in the Middle-Late Triassic Epoch,â‘¢the deep buried, big scale, complex hydrocarbon forming stage during the Yanshan period,â‘£hydrocarbon reservoir ultimately formed during the Himalaya period. Zhongba Middle Triassic gas reservoir forming model is as follows. The Permian oil cracking gas in the hanging wall of Zhangming fault migrated upwards to T2l3 reservoir. When the Upper Triassic coal-genetic gas in the footwall of Zhangming fault migrated upwards and passed through Zhangming fault, the gas moved downwards to Lei-3 reservoir and formed the gas field.Main factors controlled hydrocarbon accumulation of middle Triassic in the western-central Sichuan Basin. Rich hydrocarbon source, good conservation and conduit system are the base and precondition of hydrocarbon accumulation. Paleo-uplift and paleostructure had obviously controlled the middle Triassic hydrocarbon distribution. Depositional facies, Diagenesis and structural fracturing controlled the forming and distribution of high-quality reservoir.
Keywords/Search Tags:Natural Gas Reservoir Characteristics, Reservoir Mechanism, Leikoupo Formation, Middle-Triassic Series, Sichuan Basin
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