Sedimentary Facies Characteristics Of Upper Triassic Xujiahe Formation In Wubaochang Area,Northeast Sichuan Basin

For a long time,the oil and gas exploration in the northeastern Sichuan region has mainly focused on marine strata,and the exploration degree of the continental Xujiahe Formation is low,with few coring data,only 2 wells were coring.Many wells are used as pass-through wells for drilling marine strata,and the lack of lithologic logging data of Xujiahe Formation brings some difficulties to the research of this thesis.Guided by theories of sedimentary petrology,lithofacies paleogeography,reservoir geology,etc.,this thesis adopts a variety of methods for comprehensive analysis,and makes full use of the existing drilling cores,well logging,lithology,seismic and related data,systematically studied the stratigraphic characteristics and plane distribution characteristics of Xujiahe Formation,discussed its provenance direction,and clarified the vertical and horizontal distribution characteristics of sedimentary microfacies.The main achievements and understandings were as follows:（1）The Late Triassic Xujiahe Formation was gradually overlain from southwest to northeast from early to late,so the Xu1 member is missing in the study area.The top of the Xujiahe Formation is in unconformable contact with the overlying Zhenzhuchong Section and the bottom with the underlying Leikoupo Formation,and the bottom is divided from the Xu2 member by the Leikoupo Formation carbonates with high resistance and low natural gamma;A layer of pure quartz sandstone,only a few meters thick,is developed at the bottom of the Zhenzhuchong member,which serves as a boundary with the Xu6 member.Within the Xujiahe Formation,the Xu2 member is mainly developed as a thick layer of overlapping fluvial sand;the bottom and top of the Xu3 member are extensively developed as fluvial marsh-phase carbonaceous shale,and the central fluvial sand is relatively developed;the bottom of the Xu4 member is developed as a thick layer of sandstone with obvious scouring surface.The middle and upper parts of Xu5 member are widely developed with marsh-phase carbonaceous mudstone sandwiched by coal seams,and the bottom of Xu5member is developed with a set of medium to thick-bedded medium-fine sandstone.（2）The plane variation trend of stratum thickness,sand body thickness,grain size,sand-to-ground ratio,carbonaceous mudstone and coal seam thickness revealed by drilling shows that the Xujiahe sedimentary period was a paleogeomorphic background with high southeast and low northwest and southwest.Field outcrops around the area,a large number of quartz spin resonances,⁴⁰Ar-³⁹Ar dating,apatite and zircon fission tracks and other data suggest that During the Xujiahe depositional period in the study area,the provenance mainly came from the southeast,not the northeast.（3）According to the vertical rhythm of the color,lithology and electrical properties of sand and mudstone,it is considered that the sedimentary facies of the Xujiahe Formation in the study area are mainly braided river delta plain subfacies,and the microfacies are mainly delta plain braided channels and channels.Between rivers（and swamps）,occasionally leading estuarine dams develop.（4）Planar distribution of sedimentary facies:The braided river channels of the delta plain in the Xu2 member are mainly developed in the direction of well Luojia 4-well Qilibei 102.In Xu3 member,the channel is mainly developed in the southeast of the study area,and the distribution range of the channel is significantly smaller than that of the Xu2 member,and the thickness of the sediment is thinner.The sedimentary thickness of braided river channels in the delta plain of Xu4 member is larger,second only to that of Xu2 member.In Xu5 member,the distribution range of swamp facies between distributary channels is wide,and the distribution range of distributary channels is small.The river channel was relatively developed only in the early stage of Xu5,and the river channel developed from well Luojia 3 to well Qibei 101.