Early-Middle Jurassic Stratigraphy And Palynological Assemblages Of The Taibei Depression In The Turpan-Hami Basin

Lies in the eastern Xinjiang,the Turpan-Hami Basin is one of the three most important large oil-enriched basins in Xinjiang district.Jurassic strata are of fluvial,lacustrine,and swamp facies,and they are the best developed and thickest deposits most widely distributed among other strata within the Turpan-Hami Basin.In addition,it is an important target for oil and gas exploration.Over the years,detailed investigation of Jurassic deposits greatly enhanced geologists' understanding of the basin.However,controversies still remain on stratigraphic correlation and subdivision of the Jurassic in the.basin,especially in the Taibei Depression,a prospect district for exploration of oil and gas.The poor understanding on regional geology limits our evaluation and selection for new target strata and exploiting fields. These limitations together make it difficult to reshape the petroleum geology and hence to obtain a better understanding of the principle of hydrocarbon accumulation in the Turpan-Hami Basin.Thus,it is of great importance to have a comprehensive study of the Jurassic strata in the this area and to constrain an ideal stratigraphic framework and stratigraphic correlation.In this paper,we had a systematic study on palynology assemblages from the Taibei Depression on the basis of a combination of both previous research data on this area and modern stratigraphic and palynological principle,as well as lithological,paleontological,and geochronological constraints.Our study focuses on the Well Le15and the Well Sadong 2,and these two wells are considered as key exploratory wells in the Taibei Depression.A systematic study of the Lower Jurassic in the Taibei Depression leads to a new stratigraphic correlation of the Lower and Middle Jurassic(the Lower Jurassic Badaowan and Sangonghe formations; the Middle Xishanyao,Sangjianfang,and Qiketai formations) and identification of five sporopollen assemblages in the Taibei Depression.i.e.Deltoidospora-Osmundacidite-Pinuspollenite, Leiotrites-Pinuspollenites-Quadraeculina,Baculatisporites-Pinuspollenites-Abietineaepollenites, Cyathidites-Pinuspollenites-Abietineaepollenites,and Cyathidites-Baculatisporites -Pinuspollenites sporopollen assemblages,respectively.The affinities between sporopollen assemblages indicate that Early-Middle Jurassic vegetation in the Taibei Depression consisting mainly of herbaceous filicinae,arbor,bush, cycad,coniferales,ginkgo,and these vegetations together suggest a paleoclimatic transition from warm and humid subtropical of the Early and early Middle Jurassic to drought subtropical of the middle-late Middle Jurassic.A detailed stratigraphic correlation of the Earlyand Middle Jurassic strata between the Taibei Depression and neighboring area is also documented in this paper.