Sedimentary Facies Of Taiyuan-Shanxi Formation In Cangxian Uplift Block Of North China Oilfield And Its Impact On Sandstone Reservoir

The Cangxian uplift in the study area is located in the center of the Bohai Bay Basin,with the Jizhong Sag on the west and the Huanghua Sag on the east.As shallow oil and gas reservoirs were continuously exploited,reserves began to decline,so scholars turned their attention to the deeper Paleozoic strata.The Upper Paleozoic oil and gas reservoirs were successively discovered in the Bohai Bay Basin in the 1990s,indicating that the Paleozoic The stratigraphic stratum also has great potential for oil and gas exploration,but previous studies on the Paleozoic stratum are relatively weak.In this paper,the lithology data,paleontology,logging curves and other data of the Upper Paleozoic strata were used to analyze sedimentary facies,reservoir analysis and diagenesis on the physical properties of the Dagang exploration area.During the Ordovician to Early Carboniferous period in the North China Basin,due to the influence of the Caledonian movement,the uplift of the strata occurred weathering and erosion,resulting in the loss of the Upper Ordovician,Silurian,Devonian,and Lower Carboniferous formations in the study area,making the Lower Ordovician The limestone is in unconformity contact with the overlying strata.During the Late Carboniferous period,under the influence of the Hercynian movement,the strata began to subsidence to be deposited.From the Late Carboniferous to Early Permian,the Benxi Formation,Taiyuan Formation and Shanxi Formation were deposited.The Taiyuan Formation mainly deposits limestone,grey sandstone,mudstone and coal seam.The Shanxi Formation mainly deposits mudstone,carbonaceous mudstone,sandstone and coal line.During the Taiyuan Formation,seawater was injected into the study area from the southeast.The Taiyuan Formation mainly developed barrier island facies,lagoon facies,tidal flat facies,and swamp-like sedimentary facies.During the Shanxi Formation,seawater withdrew from the southern part of the study area.The Shanxi Formation mainly developed delta plain facies and delta front facies,but the front delta did not develop.The microscopic observation of the thin section shows that the sandstones of Shanxi Formation are mainly detrital sandstone and feldspathic debris sandstone.The sandstone has extremely poor physical properties,and the pores are almost completely filled with hetero-base and cement.The analysis is mainly affected by the sedimentary environment and diagenesis.The sandstones of Shanxi Formation are mainly delta-distributary channel sand bodies,with low maturity and poor porosity.Sandstone diagenesis in Shanxi Formation mainly includes compaction and cementation,and the impact of compaction on the physical properties of the reservoir occupies a dominant position.In the early stage of diagenesis,compaction caused rearrangement of debris particles and plastic particles were squeezed and deformed.The rigid particles break up,causing the original pores of the sandstone to be filled,the porosity becomes smaller,and the permeability decreases.In the late period of diagenesis,cementation became the main factor affecting the physical properties of the reservoir.The cement consolidated the loose sediment into rock in the early stage of sedimentation,and the cement filled the remaining pores in the late stage of sedimentation,and the porosity beca-smaller again.Therefore,the physical properties of the Shanxi Formation are extremely poor.