Study On Carboniferous-Permian Sedimentary System In The Eastern Gansu Province,Ordos Basin

The ordos basin is a large intracontinental depression basin located in the west of north China craton.It is rich in oil,gas,coal and other energy resources.It is an important energy base in China.The eastern Gansu Province is located in the southwestern part of the basin,due to the low degree of exploration of the Carboniferous-Permian gas reservoirs in the area,there are some problems,such as unclear distribution of sedimentary facies and unclear understanding of sand body scale,It has greatly affected the exploration and development of Carboniferous-Permian natural gas in this area.Therefore,the study of Carboniferous-Permian sedimentary system in the eastern Gansu Province of Ordos Basin has been carried out,Grasping the spatial distribution law of sedimentary facies and sand bodies in this area and looking for favorable reservoir facies belts can provide an important reference for the exploration of natural gas in Permian in the eastern Gansu Province.On the basis of consulting relevant geological literature,this paper focuses on drilling,logging,field profile observation and analysis testing,mainly conducts the following research:The isochronous stratigraphic framework of the study area was established by stratigraphic division and comparison of the Carboniferous-Permian strata.The source direction of the Carboniferous-Permian system in the study area is determined by heavy mineral analysis,thin slice identification and particle size analysis.Based on the logging and field data,the sedimentary facies marks were identified,the field profile and single well phase analysis were carried out,and the types and characteristics of sedimentary facies in the study area were grasped.By drawing sand/ground ratio map,sandstone thickness contour map and the sedimentary facies plane distribution map,the characteristics and laws of the sand body and sedimentary facies belt in the study area are grasped.Through the above-mentioned research,the following main results and understandings are obtained:?Based on the study of the division and correlation of Carboniferous-Permian in the eastern Gansu Province,it is found that the distribution range of Benxi formation and Taiyuan formation in this area is relatively limited,in the Qingyang-Lingtai area of the study area,there is no deposition of Benxi formation,which is speculated to be ancient land.With the expansion of the transgression range,the Taiyuan formation is gradually overlaping to Qingyang ancient land.After the filling of the Benxi Formation-Taiyuan Formation,the stratigraphic distribution of the Shanxi Formation and the Shihezi Formation is stable.Because of the later denudation of Shiqianfeng formation,the thickness of strata in different areas varies greatly;?The source of Shanxi formation,Shihe formation is analyzed emphatically.It is found that the source of the eastern Gansu Province is mainly south source and that of north source is auxiliary,it is considered that the detrital mainly come from Qilian-Qinling uplift in the southwest and Yinshan ancient land in the north;?The sand thickness in the study area is affected by the distribution of sedimentary facies and source,and the sand bodies of Benxi formation and Taiyuan formation are thinner and lack of Continuous property.The sand body thickness of Shanxi formation is gradually thickened and the sand body of Shanxi formation is distributed in a strip along the direction of source,and the whole sand thickness of Shihezi formation and Shiqianfeng formation is continuously increased compared with that of previous periods;?The bottom-up sedimentary environment of Carboniferous-Permian experienced the sedimentary evolution process of continental sea-coastal lake basin-inland lacustrine basin.The Benxi and Taiyuan periods were affected by marine transgression,the development barrier coastal facies,and the Shanxi sedimentary environment from marine facies to continental facies,the sedimentary environment of Shanxi period was changed from marine facies to continental facies,and the sedimentary environment of Shanxi period was changed from marine facies to continental facies.The lacustrine-delta facies developed,the Shihezi period completely entered the continental environment,the lacustrine-braided river delta facies developed,and the lacustrine-fan delta facies developed in the Shiqianfeng period.