Study On The Characteristics And Distribution Of Sedimentary Facies Of Xujiahe Formation In The Northeast Of Sichuan Basin

The Xujiahe Formation of Upper Triassic is one of the main distribution,exploration and development layers for tight sandstone gas in the Sichuan Basin,with high natural content and wide distribution range.In recent years,the oil and gas exploration of the Xujiahe Formation has achieved steady growth in production and reserves in Yuanba,Bazhong and Tongnanba areas in the northeastern Sichuan Basin.But there are few important breakthroughs in the southern study area due to its low research degree.Based on logging identification,lithology analysis,seismic interpretation,field outcrop investigation,and drilling core observation,this thesis systematically studies the isochronous stratigraphic framework,sediment source direction,sedimentary face types,and sedimentary facies belt distribution of the Xujiahe Formation in the northeastern Sichuan Basin,and analyzes the relationship between sedimentary facies and the dominant enrichment areas or sections of hydrocarbon source rocks.The results show that:（1）The Xujiahe Formation in the northeastern Sichuan region has generally accepted sedimentation since the second member of the Xujiahe Formation,while the first member of the Xujiahe Formation is not developed.Taking the regional unconformity surface,large-scale scouring surface and lithologic lithofacies discontinuity surface as the interface,it can be divided into two ultra long-term base level cycle sequences（SLSC₁,SLSC₂）and five long-term base level cycle sequences（LSC₁～LSC₅）.The five lithological segments from the Xu2 to Xu6 sections correspond to five long-term base level cyclic sequences.The distribution characteristics of gradually thinning from northwest to southeast on the stratigraphic plane reflect that during the sedimentary period of the Xujiahe Formation,the northeast of Sichuan was an ancient landform with high southeast and low northwest.（2）The source area of the Xujiahe Formation in the northeastern Sichuan region is not the Daba Mountains,as previously believed,but the southern Qinling Mountains in the northeast direction and the ancient uplift in the southeast direction.The main source of sediment is the Nandong phase,and the sediments south of the Jinzhu 2 Well,Shallow 20 Well,Shuangshi 1 Well,Longgang 166 Well,and Longgang 16 Well line all come from this source;The northeast source is the secondary source,and only a small portion of the sediment north of the Jinzhu 1 Well,Dongsheng 1 Well,Maoba 2 Well,and Jiepai 1Well line comes from this source;The intersection area between two sources is located between the two lines.（3）It is clear that the Xujiahe Formation in northeastern Sichuan is a shallow braided river delta plain subfacies,which can be further divided into distributary channel microfacies and distributary channel microfacies.The microfacies of distributary channels mainly develop core beach sedimentation and channel filling sedimentation,while the microfacies between distributary channels mainly develop swamp sedimentation,with occasional crevasse fan sedimentation.The distribution of sedimentary facies belts is controlled by the direction of the source,and overall has the characteristic of being spindle shaped or strip shaped along the northwest southeast direction.（4）Type III kerogen is the main type of organic matter in the Source rock of the Xujiahe Formation.The thermal evolution of organic matter has reached the mature high mature stage.The TOC mass percentage of the Source rock is mainly between 1%and 3%,belonging to the medium good Source rock.The thickness of the Source rock is generally>10m,with good gas generation potential.The dominant enrichment area and section of Source rock are obviously controlled by sedimentary facies.The Source rock of distributary channel marsh deposits is thick and has high TOC content,while the distributary channel microfacies and crevasse splay deposits basically do not develop Source rock.