Sedimentary Facies And Reservoir Characteristics Of Lower Ordovician Outcrop Carbonate Rocks In The Western Margin Of Ordos Basin

Ordovician in Ordos Basin is one of the key areas of marine carbonate oil and gas exploration in China.In recent years,new gas fields have been discovered continuously in the western margin of Ordos Basin,such as Jingbian gas field,where the lower Paleozoic carbonate rocks are the main gas-producing horizons,indicating that the lower Paleozoic Ordovician carbonate reservoirs in Ordos Basin have good gas exploration potential.Therefore,the analysis of sedimentary facies characteristics and reservoir development of Lower Ordovician carbonate rocks in the western margin of Ordos Basin is of guiding significance for gas exploration and research in the western margin of the basin.On the basis of fully understanding the previous research results and regional geological background data,through field survey of the Ordovician in the western margin of the basin,the Ordovician profiles of Gangou-Sandaogou in Pingliang,Goutanggou in Qinglongshan and Shizuizi in Xiamaguan were selected to carry out field surveys,and the related data of stratum thickness,stratum lithology,sedimentary characteristics and paleontology were obtained.On the basis of making full use of these data,the stratigraphic division criteria are defined,the stratigraphic division scheme is formulated,and the study intervals are divided into Yeli Formation,Liangjiashan Formation and Majiagou Formation from bottom to top.Then the vertical correlation of different sedimentary units is made,and the stratigraphic correlation framework is established.On the basis of stratigraphic division,through identification of sedimentary facies markers such as petrological characteristics,sedimentary structure and paleontology,three sedimentary facies and seven sedimentary microfacies are identified in different strata,and the sedimentary facies model of Ordovician epicontinental sea carbonate platform on the western margin of Ordos Basin is established.Based on the observation and analysis of casting slices and scanning electron microscopy of different sedimentary facies zones,the petrological characteristics and pore types of reservoirs in different facies zones are determined.Combined with the analysis of physical characteristics of reservoirs in different sedimentary facies zones,the reservoirs in different facies zones are comprehensively evaluated.It is concluded that the lower Ordovician carbonate rocks in the western margin of Ordos Basin are more favorable reservoir facies in platform margin reef facies.