Study On Lithofacies Palaeogeography Of Feixianguan Age Of The Lower Triassic In The Middle And Upper Yangtze Region

The Middle and Upper Yangtze basin is a large-scale oil-gas bearing basin, were the strata form the Upper Sinian to Middle Triassic are well developed.Through decades of expolration, it has been found such as Dukou River, Tieshan Po, Luojia Zai, Pu Guang, Mao Ba, Tongnan Ba, Long Gang, Yuan Ba and other gas fields as well as numerous oil seeps and the ancient reservoirs, which are all supposed that the upper and middle Yangtze basin has the prospects of the good oil and gas explorationand development.The study of lithofacies palaeogeography is the basis for petroleum exploration. According to the detailed and systematic studies, it is hopeful to ascertain the distribu-tion and evolution of the lithofacies palaeogeography, clarify the relation between litho-facies palaeogeography and the development of the resource rocks, reservoirs and cap rocks.In recent decades, researches has been worked such as in the sedimentary facies evolution, petrology and structure characteristics, types of source rocks, reservoirs and so on in the Middle and Upper Yangtze Region. But, in a long period, the study is still mainly based on qualitative and less quantitative research for the study of lithofacies palaeogeography. Therefore, it is necessary to make a comprehensive and thorough study of the quantitative lithofacies palaeogeography in the Middle and Upper Yangtze Region, which also can provide adequate basic datas for oil and gas exploration.The research object of the dissertation is the Feixianguan stage of the Lower Triassic in the Middle and Upper Yangtze Region, by the means of measure, observation, drilling cross section analysis, combined with a variety of analytical tools, it has deeply studied the characteristics,origins of the rocksand to clarified their sedimentary environments; Based on the "single factor analysis and multifactor comprehensive mapping method" promoted by Professor Feng, the quantitative lithofacies palaeogeography study will be taken in the Feixianguan stage in this area, and on the basis, the hydrocarbon generation conditions, reservoir conditions and seal conditions of the Feixianguan stage in this area have also been analyzed.The Lower Triassic in the Middle and Upper Yangtze Region can be divided into Jialingjiang stage and Feixianguan stage from the bottom to the top. Feixianguan stage includes Feixianguan formation, Yelang formation, Daye formation and so on; the stage can be divided into four members from the bottom to the top, namely as member 1, member 2, member 3,and member4 of Feixianguan formation.The dominant rock type of the Feixianguan stage of the Lower Triassic in the study area is carbonate rocks, with minor clastic, volcaniclastic rocks and evaporites. Among carbonate rocks limestone are more developed, based on rock composition, structure, construction, limestone can be divided into grainstone (including oolitic limestone, intraclasts limestone, bioclastic limestone), plaster limestone, argillaceous limestone, worm-like limestone, nodular limestone, dolomitic can be divided into grain quality dolostone;fine-medium dolomite, dolomicrite-crystal powder dolomite; clastic can be divided into sandstone, siltstone, mudstone, shale; volcaniclastic rocks are mainly tuff; evaporite rocks are mainly gypsum.The sedimentary environment of Feixianguan Age of the Lower Triassic in the Middle and Upper Yangtze Region can be divided into the alluvial plain, clastic platform, Shallow-water carbonate platform, slope and deep water basin; Shallow-water carbonate platform can be further divided into beach,quasi-beach, young beach, cloud floor, Cream containing salt lake and other secondary environment unit.The article selects six single factors:thickness, deepwater sedimentary rock content, shallow-water carbonate rock content, carbonate content of the particles, penecontemporaneous dolomite content, the content of gypsum and rock salt,and then we select four units to draw the single factor maps. By univariate analysis,regional maps of geological data and other studies, we conducted the maps of quantitative lithofacies paleogeography of Feixianguan Age One to Four. The study area is a regressive process, the unit of the ancient geography of age One from west to east develops flood plains, clastic platform, shallow-water carbonate platform, ramp carbonate rock salt basin, carbonate platform develops a beach, young Beach, quasi beach, dolomitic flat and other secondary environment unit; the spreading of age Two from west to east is similar to age One, carbonate platform develops a beach, young Beach, quasi beach, Cloud mudflat and other secondary environment unit; the spreading of age Three from west to east is similar to age One and Two, carbonate platform develops a beach, young Beach, quasi beach, Cream containing salt lake and other secondary environment unit; the unit of the ancient geography of age Four from west to east develops flood plains, clastic platform, shallow-water carbonate platform, basin and slope are gradually disappearing, carbonate platform develops a beach, young Beach, quasi beach, Cream containing salt lake and other secondary environment unit.On the basis of study of lithofacies palaeogeography, the source rocks, reservoir rocks and relationship with lithofacies paleogeography of Feixianguan Age of the Lower Triassic are discussed. Feixianguan stage in the study area develops favorable source rocks and reservoir rocks, favorable source facies belt is mainly inf deep water basin, slope and restricted platform; reservoir rocks are widely distributed, with great thickness, favorable reservoir facies is mainly in the Midwest ofthe research area, and develops along the grain beach facies; In the sichuan basin, the gypsum rocks of Jialingjiang and Leikoupo formation is direct caprock of Feixianguan formation, the thousands of meters thick mudstone and silty mudstone of Triassic to the Jurassic can be a good regional cap rocks.