Study on the Sedimentary Facies of Conglomeratic Sandstone Member in CIII Oil Group of Northwest of Tazhong Area

In the northwest region of Tazhong area,a number of oil fields have been developed in the Carboniferous marine clastic reservoirs,including Tazhong 10 and Tazhong 40 oil fields,with the conglomeratic sandstone member in CⅢ oil group(Carboniferous Ⅲ oil group) as one of their major oil-producing layers.In order to study the sedimentary facies of conglomeratic sandstone member,an integrated analysis including cores,well logging,seismic data and laboratory data has been performed.According to the analytical result,the conglomeratic sandstone member is composed of multistage superimposed sand bodies in positive rhythm,formed by fine and pebbly fine sandstone.Its rock type is dominated by lithic quartzite and litharenite with moderate maturity.Sedimentary structures such as scour surfaces,lag deposits,parallel bedding and cross bedding can be found.The sandstone grain size probability curve is mainly in the two-segment pattern,featuring large content of suspension population.Both grey and brown mudstones are developed,and the trace elements in mudstones reflect the palaeosalinity of fresh water and salt water.Both logging curves combination pattern and 3D seismic attributes display the characteristics of channel deposit.Referring to the regional geologic background,it is considered that the conglomeratic sandstone member of CⅢ oil group in the study area was braided river delta deposits,and it developed microfacies such as braided channel and overbank deposits in the delta plain subfacies,as well as underwater distributary channel,interdistributary bay,river mouth bar,sheet sand deposits in the delta front subfacies.The frequent lateral migration of braided channels resulted in the multistage superimposition of sand bodies in the vertical direction and wide distribution on the plane,making them favorable reservoirs.Therefore,looking for lithologic-stratigraphic reservoirs in the channel sand bodies should be the main target for progressive exploration and development.