Petroleum Accumulation Conditions Of Triassic Leikoupo Formation,in The Depression Of Western Sichuan Basin

Some gas fields have been discovered in Leikoupo formation of Middle Triassic in the depression of western Sichuan Basin, like Zhongba, Daxingchang, Yanjinggou and Pingluoba gas-bearing structure. Recently, the exploration of gas reservoirs of karst weathering crust in the top of Leikoupo formation made a breakthrough because of the good effect of XS1, CK1 and PZ1 exploratory test well. The lithology of Leikoupo formation has mainly been proved to be dolomite rock, gypsum rock and shale supplementing. Longitudinally, leikoupo formation has been divided into five sections and the fifth section is Tianjingshan group.The main sedimentary system of western Sichuan depression has confirmed to be carbonate platform which include restricted platform, open platform, evaporation platform facies and lagoon, tidal subfacies. Leikoupo formation is the final stage of evolution of marine carbonate rocks in western Sichuan depression, then would become into continental clastic deposition stage. There could be there kinds of constructive diagenesis of Leikoupo formation, such as dissolution, dolomitization and tectonic movement; meanwhile there would be five kinks of diagenesis, such as pressure solution compaction, recrystallization, cementation, authigenic mineral development and filling the pore.The properties of Leikoupo formation reservoirs are middle porosity and low permeability, small throat and poor connectivity, besides the main space of reservoir are secondary pores. The crack- porosity gas reservoirs are well developed in Zhongba area of northern depression where secondary pores.are developed, reservoir space type is varied, and microfractures made by tectonic activities could improve connectivity. Unconformity between T3 x and T2 l was made by tectonic uplift and paleo-karstification in central depression where weathering crust karst reservoirs are developed, the main reservoir space is dissolution pore. Weathering crust karst reservoir could be used as reservoirs and also served as migration channel communicating the upper and under source rocks. And thickness of unconformity have a major impact on the effectiveness of weathering crust karst reservoir. The weathering crust karst reservoirs and sand beach facies algal dolomite reservoirs are the favorable reservoirs which are controlled by sedimentary facies and diagenesis in western sichuan depression.The main type of gas of Leikoupo formation is mature-over mature gas of mixed source. The gas of Zhongba is predominantly sapropelic gas which came from the underlying shale of Permian, pyrolysis oil even much deeper, and the overlying coal-bearing strata of Xujiahe formation. The gas of middle depression is humic gas which came from its own source rocks or the overlying coal-bearing strata of Xujiahe formation.The gas accumulation process of Leikoupo formation of Zhongba: the first stage, the period of epidiagenesis pore construction in the late of Middle Triassic; the second stage, the period of ancient anticline formation in the middle and late of Late Triassic; the third stage, the period of paleouplift inherited development-gas accumulation in Yanshan movement; the fourth stage, the period of final gas reservoir formation.The weathering crust karst gas accumulation process of Leikoupo formation of middle depression: the first stage, the period of epidiagenesis pore construction in the late of Middle Triassic; the second stage, the period of paleouplift structure which was favorable to hydrocarbon accumulation formation; the third stage, the period of paleouplift inherited development-a large scale of gas accumulation in Yanshan movement; the fourth stage, the period of final gas reservoir formation.The main control factors of gas accumulation of Leikoupo formation of Middle Triassic in western sichuan area:(1) the background of palaeohigh affected the early migration and accumulation of gas and has a significant pointing role to gas accumulation;(2) favorable sedimentary facies and constructive diagenesis control the development of quality reservoir;(3) favorable migrating system is a necessary condition of hydrocarbon accumulation.(4) the abundant hydrocarbon source is the precondition of gas accumulation.(5) the cap of trap and preservation conditions are the keys to the effectiveness of large- medium gas reservoir.