Sedimentary Facies Of The Changxing-Feixianguan Formation In The Northeastern Sichuan Basin

Focus on sedimentary facies of the upper Permian Changxing formation-lower Triassic Feixianguan formation of northeastern Sichuan area,through the combination with outcrop,drill cores and thin sections observations,drilling and logging,and seismic data,guided by the theories of geology.This study demonstrates the development of reef and shoal facies because of sedimentary facies.The characteristics and distribution of sedimentary facies and the favorable distribution area of reef and shoal in the study will be determined.At last,provides a geological basis for reservoir distribution prediction and later exploration and development.The Permian strata in the upper Paleozoic erathem in the northeastern Sichuan Basin are well preserved,with the lack of Devonian and Carboniferous stratas.Besides,the Triassic and Jurassic stratas in the Mesozoic erathem are well preserved.Changxin formation,in the study area,covered the Feixianguan formation and unders the Wujiaping formation(Longtan formation)for the integration of contact.The Feixianguan formation is continuously deposited with the overlying Jialingjiang formation.There are obvious differences between the two sets of strata and the characteristics of the lithology,and the boundary is clear and easy to identify.A large number of compressional structures were developed in the study area.On the north side of Micang mountain in area is from east to west extension.The high-steep fold belt in eastern Sichuan was extended from the north to the west of the arc.A large area of exposed platform residual layer developed at the late Permian Changxing period because of the significant decline of sea-level,northeastern Sichuan area mainly shows a smaller scale of two sea-level cycles,whicn belongs to a shallow water carbonate platform that north and northeast sides are surrounded by Guangyuan-Wangcang trough and Kaijiang-Liangping trough,and the west edge of platform locates at the line of Tieshanpo-Puguang-Qilibei,and extends to the southwest,developes a lenticular,bead-like distribution of Puguang,QiliBei and other reefs;The depositional environment gradually changes into an open platform environment from the platform margin;In the period of Feixianguan,the whole upper Yangtze region appears a slow and sustained extensive transgressive process,from west to east,the trend of water deepening gradually clearly;From the platform margin to deep water,the environment quickly evolves into Kaijiang-Liangping trough and Exi-Chengkou trough,there ia a connected trend of the two trough in the south of Luojiazhai;The slope between trough and platform margin is narrow,and distributes around the platform margin.From the background of the deposition in the northeastern Sichuan Basin,platform margin reef and shoal of Changxin formation and Feixianguan formation developed around the edge of the shelf,especially the development of thick area should be the best regional exploration.The distribution and development of reservoirs in this area are controlled by the distribution of sedimentary facies.The reservoir is mainly composed of grain-shoal and reef.The favorable zones of these reservoirs are mainly developed in the platform margin facies,which are mainly developed in the open platform,restricted platform facies of the platform shoal(reef),the inner point shoal(reef)subfacies.It is worth noting that,In the slope facies zone,the gravity flow deposits are composed of oolitic dolomite,can also be used as a reservoir to study.