鄂尔多斯盆地奥陶系碳酸盐岩-膏盐岩层海相含气系统近源勘探进展与思考OA

Against the bottleneck issues in the Ordovician subsalt marine gas-bearing system of the Ordos Basin,including doubtful quantity of gas generated by low-abundance source rocks,and unclear gas accumulation and preservation patterns,this study investigates the reservoir-forming conditions and near-source exploration practices of the gas-bearing system.First,the argillaceous dolomite and argillaceous gypsum dolomite of the third member of the Ordovician Majiagou Formation(Ma-3 Member)are the main subsalt marine source rocks,and the Dingbian sub-depression and its periphery are the most favorable gas-generating centers,hosting source rocks of 10-80 m thick cumulatively,dominated by Type Ⅰ kerogen with total organic carbon(TOC)content of 0.58％-1.39％and vitrinite reflectance of 1.62％-2.16％.Second,reservoirs are controlled by paleogeomorphology and penecontemporaneous dissolution,with anhydrite nodule dissolution mold pores,intergranular pores,and intercrystalline pores.Regional and direct caprocks of gypsum-salt rocks are widely developed.The dense NNE-trending strike-slip faults in the east and sparse X-type strike-slip faults in the central area effectively connect source rocks and reservoirs.Third,the south-north fault-uplift and east-west nose-uplift structural setting,combined with the gypsum-bearing dolomitic flat-salt sag facies transition zone,control natural gas accumulation and preservation.Based on these findings,a new accumulation model characterized by near-source gas supply,facies transition sealing,and structural convergence is established for the Ma-3 Member,and favorable exploration zones with multi-type trap groups in low-relief structures are identified.The carbonate-gypsum-salt rock strata in the Ordos Basin exhibit distinct characteristics of low-abundance source rocks coupled with strong gypsum-salt rock sealing.Near-source exploration offers a new pathway for the exploration in the Ordovician subsalt marine gas-bearing system.