A Study On Sedimentary And Reservoir Characteristicd Of Carboniferous Gas Reservoirs In Wanshunchang Block,Eastern Sichuan

The Wanshunchang gas field belongs to the high and steep structural belt in eastern Sichuan,and is a secondary structure in the northeast of Dachi dry well structural belt.It is a NEE-SWW-trending narrow and elongated anticlinal complex oil and gas field.Nine major faults mainly develop in the block,which basically controls the form and scale of structural traps.Based on the full assimilation of previous research results,this paper,guided by the theory of carbonate reservoir sedimentology and classical sequence stratigraphy,carried out the study on the sedimentary and reservoir characteristics of the Carboniferous gas reservoir in Wanshunchang block by means of the whole well section core observation and description,rock section identification and analysis,logging data,rock,mineral and physical property analysis and testing data.Huanglong Formation is only developed in the Carboniferous system in the study area,which is generally divided into the first section of the Huanglong Formation,the second section of the Huanglong Formation and the third section of the Huanglong Formation.All the third section of the Huanglong Formation in the area has suffered denudation,and only the second section of the Huanglong Formation and the first section of the Huanglong Formation remain.The thickness distribution of the Huanglong Formation in the study area is stable,and the residual thickness ranges from18m to 40m,with an average of 28.7m.According to the characteristics of lithofacies association and the variation of sedimentary microfacies,the second section of the Huang Formation is subdivided into three sub-sections.Combined with the regional sedimentary background,the sedimentary facies and microfacies types of the Carboniferous system in the study area are defined by using the identification marks of sedimentary facies equivalent to lithofacies,paleontological characteristics and logging.The results show that the Huanglong Formation in the study area is mainly a limited set of Carbonate tidal flat depositional system within a confined bay,which is similar to the classic Laporte tidal mode.According to the difference of sedimentary characteristics,it can be further divided into supratidal flat,intertidal flat and subtidal flat subfacies.The first section of Huanglong Formation（C₂hl¹）mainly developed supratidal flat subfacies,while the second section of Huanglong Formation（C₂hl²）developed intertidal flat subfacies and subtidal flat subfacies.Based on the analysis of single well sedimentary facies and Correlation of sedimentary facies between wells,the stratigraphic thickness distribution map and grain rock thickness distribution map of sequence framework were compiled.Combined with the regional sedimentary background,the plane distribution characteristics and distribution law of sedimentary facies in different periods were analyzed,and the sedimentary facies model was established.The Carboniferous gas reservoir in Wanshunchang area belongs to fractured-porous reservoir.The rock types mainly include granular dolomite,powder-fine crystalline dolomite and dolomitic karst breccia,and the pore types are mainly intergranular（dissolved）pores,intergranular（dissolved）pores and intra granular（dissolved）pores.The fractures of Carboniferous gas reservoirs in the high point of Wanshunchang structure are mainly network fractures,followed by high Angle fracture and low Angle fractures.The development of Carboniferous gas reservoirs in Wanshunchang area is controlled by high energy facies zone,carbonate diagenesis and tectonics.The high energy shoal microfacies in subtidal flat and intertidal flat algal sand flat microfacies in sedimentary facies,dolomitization and dissolution in diagenesis,and a large number of unfilled/half-filled fractures in tectonic movement（especially the Himalayan movement in Quaternary period）are the main factors for the formation of favorable reservoir,among which the high energy facies zone is the foundation for the development of favorable reservoir.In Wanshunchang area,the Carboniferous well area of C30-C10-C18 has the largest effective porosity,and the distribution pattern of effective reservoir thickness is similar to the average porosity distribution.The first sub-section of the second section of the Huanglong formation（C₂hl₁²）has the largest effective reservoir thickness,which is a favorable area for reservoir development.