Restoration Of The Primary Ordos Basin In Yan'an Period Of Jurassic

The Mesozoic Ordos basin is a residual interior cratonic basin that superimposed on the Palaeozoic large North China craton and experienced multistaged deformation by different styles. The discussion about the primary features of this basin in Mesozoic has been paid more attention. Various conclusions have been proposed.The Yan'an period in middle Jurassic is an important stage in the development and evolution of Ordos basin.It's the main period for the hydrocarbon and coal seams forming and transforming,but the study about Yan'an formation is very little.On the base of predecessor's work,taking integrated,dynamic and comprehensive analysis and the research theories about reformed basin as guidance,starting from the nowadays' remanent sedimentary entities and structural framework of the basin and its adjacent areas,by the systemic stratigraphic partition and compare,sedimentary facies analysis,provenance analysis,identify of marginal facies,seismic profile interpretation,sediment speed compare,and fission-track dating, the cognition about the sedimentary features of primary basin is acquired as a result.Effected by the intense and diversified denudation in later stages,the stratum distribution and thickness pattern of Yan'an formation in Ordos Basin and its synchronal stratum in adjacent areas altered largely.In the residual basin,Yan'an formation distributed extensively,the primary thickness is from 160m to 300m,appearing west thicker than east and north thicker than south generally.The synchronal stratum that can be compared sparsely distributed in the adjacent areas of Ordos basin,including Shanxi,Henan Province,inner Mongolia,Ningxia autonomous region.It is early middle Jurassic according to sporomorph,ancient plant and bivalve fossils in Yan'an formation in Ordos basin.Based on the characteristic of lithology,color,marker bed, depositional cycle and coal seam,the Yan'an formation in the residual basin is divided into four segments or ten reservoirs and existing Y10-Y4 reservoir except for eastern outcrops nowadays. The synchronal stratum that distributed in the adjacent areas of Ordos basin can be compared with the Yan'an formation on the characteristic of lithology,segmentation and paleontology.According to the analysis of great deal of the well cores,outcrops observation,well logging data interpretation,and combined with other methods,such as lithology identify and grain size analysis,some sedimentary system types as alluvial fan,meandering river,braided river,delta and shallow lake facies are identified in Yan'an formation in residual Ordos basin.There is a evolution process of filling to regional sedimentation and uplift slowly from Fuxian to Yan'an period in Ordos Basin.So it can be divided into alluvial fan-river sedimentary system from Fuxian to Y10 period,delta-lake sedimentary system from Y9 to Y6 period and anastomosing river-lake sedimentary system in Y4+5 period.On the basis of analysis of sedimentary facies of synchronal stratum in adjacent areas,Yima formation shows characteristic of alluvial fan,river,and lake facies in Yima.Yangshuzhuang formation shows shallow lake facies in Jiyuan.Datong formation shows from a set of coal-bearing viver-delta facies to none coal-bearing lake facies in Datong to Ningwu of Shanxi Province. Yan'an formation in Rujigou can be compared with synchronal stratum in western of Ordos Basin on the characteristic of lithology,coal-bearing and segmentation.Wudanggou formation shows characteristic of a complete set of alluvial fan-river-shallow lake-deep lake facies in Shiguai.There are many provenances of sediments and it is main provenance in west or northwest of Ordos Basin in Yan'an period by studying sedimentary facies,paleocurrent direction,content of light and heavy mineral,quartz cathode luminescence and content of rare earth elements.It shows a unified catchment center with eastern outcrops because paleocurrent direction in Datong formation of Datong basin is eastward to Yan'an.Paleocurrent direction in Yan'an formation of Rujigou mine is consistency with synchronal stratum in western of Ordos Basin.Paleocurrent direction of synchronal stratum in Yima and Shiguai basin is inner of Ordos Basin nowadays.It shows thickness of present stratum is higher in ancient river area by drawing depth isoline graph and lower surface structure graph in Yan'an and Fuxian formation of Ordos Basin.It shows valley terrace and distribution location of ancient river of lower surfaces in Yan'an and Fuxian formation visually by using 3D modeling basin of Surfer 8 software.It is high east and low west of monocline,upwarping appreciably in northeastern and south margin of Ordos Basin.Based on multidisciplinary analysis and research,the boundary of the original basin in Yan'an period is approximately concluded:the eastern borderline is in west of Taihangshan, including Datong,Ningwu,Yima and Jiyuan basin;the northwestern borderline is fault zone in west margin of Helanshan;the southwestern borderline is fault zone in west margin of Liupanshan; the northern borderline lie in Wulangeer-Hantaichuan of Yimeng liftup;the southern borderline lie in Longxian-Qianyang-Linyou-Binxian-Xunyi-Yijun of Weihe basin.