Characteristics of sedimentary microfacies of the third and second members of Shahejie Formation in nearshore subaqueous fan in Chengbei step-fault zone,Dagang offshore area

Sand body in the third and second member of the Shahejie Formation in Chengbei step-faut zone of Dagang off-shore area is the nearshore subaqueous fan controlled by braided stream. The petrographic facies including matrix-supported gravelly sandstone,grain-supported gravelly sandstone,normal grading bedding pudding stone,gravelly sandstone,inverse grading pudding stone,gravelly sandstone,argillaceous-suppoted sandstone and siltstone are mainly characterized by coarse graind sedimention;depositional bedding such as level bedding,massive bedding,parallel bedding,cross-bedding,wave bedding,convolut bedding and deformed bedd- ing are very developed;sand ball,load cast,sandstone vein and scoured base are also developed very well;electrical resistivity log shows positive rhythm with bell-tepy and box-like shapes changing respectively from high amplitude to medium amplitude which are closely related to different lithology and different depositional facies. On the basis of lithology association,sediment character and well logging,the nearshore subaqueaus fan in this area can be subdivided into two subfacies and five micro-facies:fan-middling including braided stream,spacebetween braided stream and fan-middle leading edge;fan-end include fan-end mud and fan-end sheet stone; braided stream is main reservoir.