Constructive burial diagenesis and significance of dolomite reservoir from the Ma5₄¹ member in Gaoqiao area, Jingbian gas field, Ordos basin

Influenced by paleokarst, stable porous dolomite reservoir layer was formed in Gaoqiao area, Jingbian gas field, Ordos Ba-sin. In this paper, the authors conducted systematical research by means of well core, cathodoluminescence image and geochemistry of burial diagenesis of the Ma5₄¹ member in Gaoqiao area. The results indicate that the constructive diagenesis in a burial environment consists of burial dolomitization, burial dissolution and fracture effect. The carbon and oxygen isotope values of crystal grain dolomite are obviously negative compared with those of the Ordovician sea water. So it is obvious that the strata experienced burial dolomitization. The burial dolomitization which reduced the volume of the rock and increased the porosity was conducive to the formation of intergranular pore. The burial dissolution caused a series of storage spaces with holes and seams, and resulted in meaningful places for oil and gas reservoir. In addition, the development of fissures increased the connectivity of the pores and effectively increased the porosity and permeability of the gas reservoir. These three kinds of constructive diageneses were superimposed upon each other and the cracks linked the existing pore. This pore-hole-seam-crack system seems to have been a decisive control function for the formation of oil and gas storage reservoir.