Study On Sedimentary Facies Of The Yanchang Formation Of The Upper Triassic Series In Central West Region Of Erdos Basin

The main research target of this paper is Yanchang formation in Trassic systemof central west Ordos basin.In this paper the author uitilized the ourcrop research,drillcore data and logging data to analyze the sequence stratigraphy, sedimentology androck studies in study area. In Midwest of Ordos Basin the yanchang formation hadbeen divided to5Quasi secondary sequence and18third-order sequence.The authorsummarized the the identification of typical sedimentary facies and detailedlyanalyzed the various sedimentary microfacies.The study obtained that the Midwest ofbasin have several major sedimentary facies,wihich include braided river, braidedriver delta,meandering river delta, lacustrine and underwater gravity flow facies.Everysedimentary facies and the individual characteristics of the microfacies weredescribed detailly.The main sand type of Yanchang formation in Midwest region isunderwater distributary channel sand and underwater gravity flow deposits sand.Thesand distributes as strip in east-west and north-south direction.The sand bodiesdistributed continuously and looked as lobate.Through the comparison of thehorizontal profile and longitudinal profile in the Ordos Basin and in combination withregional sedimentary evolution process.The author recovered the sedimentaryevolution process in Yanchang period.In Yanchang sedimentary period,the whole lakebasin had a complete water into and out of the process.Chang10period was theinitial subsidence stage of lake basin.Starting from the Chang9period the basin hadentered the rapid subsidence phrase.At that period the sedimentary center is at theeastern basin.At the chang9period in the western regions of basin the mainsedimentary facies is shore and shallow lake.After the Chang8and Chang7periodthe basin subsided continuously.At the beginning of Chang7period the lake basindevelopment reached peak.The sedimentary center from east region migrated to thesouthwestern region.The Chang7period is an important turning point amongthe sedimentary evolution period.From the Chang7late period to Chang4+5period,the lake basin entered the gradually shrinking stage. From Chang3period to Chang1period the lake basin narrowed quickly,over the Chang1period the lake basingot into the comprehensive swampiness.After analyzed the control action to thesedimentary evolution and oil and gas reservoirs. The distribution of hydrocarbonsource rock, reservoir and hydrocarbon accumulation process is controlled bysedimentary evolution.The topography of basin is gently.The delta front sand scale isvery large in lake basin.The sand contacted adequately with the effectivehydrocarbon source rocks.There are two main source-reservoir-cap assemblages,oneis Chang7-Chang8assemblage another is Chang7-Chang6assemblage.At theChang7and Chang6period,in the half deep lake–deep lake facies of Chang7andChang6section,there are large thickness range of the underwater gravity flowdeposits.And that became the lithologic reservoir with huge potential.