Reservoir Characteristics Of Middle Triassic Leikoupo-4 Formation In The Central Part Of Western Sichuan Basin

As the last layer of marine carbonate evolutionary phase in the superimposed Sichuan Basin, Middle Triassic Leikoupo Formation has realicastic signifince and exploration value in prospecting deep marine carbonate. As the Indosinian Moment in Middle Triassic, Leikoupo Fm complete reserve in Western Sichuan Basin only. This thesis used 3 drillings(XNS1, XS1 and PZ1) deployed in Western Sichuan Basin by Sinopec as research object. Combined former researches on Leikoupo-4 Fm in Western and Northwestern Sichuan Basin, and based on core observation, section identify, SEM, CL, C-O-Sr isotope analysis, this thesis described sedimentary microfacies, dissolution phases, dolomitization mechanism, reservoir space and pore evolution history and discussed main control factors of forming of high quality reservoirs in Leikoupo-4 Fm in Western Sichuan Basin.12 microfacies(MF) first be distinguished by microscopic characteristics through thin sections in Leikoupo Fm. In these types thrombolite(MF1), stromatolite(MF2), thrombolitic stromatolite(MF3), oncoid wackestone(MF6) and peloid grainstone(MF11), are related to microbial activities. Luzhou-Kaijiang paleouplift was nearly formed in late Leikoupo stage. Surrounded by the paleouplift, Western Sichuan Basin was in extreme environment of restricted-platform, which was extended NE-SW along edge of the basin.Dissolution and dolomitization consisted the main constructive diagenesis of Middle Triassic Leikoupo Fm. Compaction, pressure solution, cementation and filling composed the main deconstructive diagenesis of Leikoupo Fm. Section identify and chemical analysis shows that there are contemporaneous- penecontemporaneous dissolution which affected by fresh water and burial stage dissolution affected by acid fluid. Dolomitization are happened mainly in Penecontemporaneous and burial phase. And diagenetic fluid may be seawater sealed in the pore or formation water similar to seawater.Confirmed that microbialte carbonate is the new type of reservoir in Leikoupo-4. Through the observation of casting slices of Leikoupo FM, the reservoir mainly distribute in 43 submember. Main lithology of reservoir rocks is microbial dolomite and residual crystalline dolomite. The main types of reservoir space are microbial framework pore, intercrystalline pore, intergranular pore and fracture. Physical parameters analysis showed reservoir properties are affected by lithology and rock structure. The reservoir properties of dolostone are better than limestone and gypsum stone. Among dolostone, microbial dolomite and residual sand-sized introclast crystalline dolomite have the best reservoir properties. Among microbial dolomite, thrombolitic stromatolite have the best reservoir properties, and when it comes to stromatolite, thrombolite, oncoid wackestone the reservoir properties are lower.Based on the match of diagenesis history and pore variation, pore evolutionary phase are recognized, they are Penecontemporaneous dolomitization(increase porosity)?undersea cementation(decrease porosity)?contemporaneous disolution(increase porosity) ? compaction/recrystallization/filling(decrease porosity) ? dolomite cementation(decrease porosity) ?burial dolomitization(increase porosity) ?burial dissolution(increase porosity)?calcite cementation(decrease porosity).Leikoupo-4 reservoir is controlled by tectonic framework, sedimentary facies and diagenesis. In Western Sichuan Basin, which have hot, shallow water and high salinity environment, have fine microbial and grain basement. Dissolution and dolomitization played a key role in reservoirs' modification. Furthermore, as a new type of reservoir stone of Leikoupo Fm in Western Sichuan Basin, microbial dolomite has fine reservoir properties and it has broad prospects for oil and gas exploration.