Gas exploration potential of tight carbonate reservoirs: A case study of Ordovician Majiagou Formation in the eastern Yi-Shan slope, Ordos Basin, NW China

On the basis of comprehensive analysis of drilling, gas testing, laboratory analysis and testing data, the characteristics and genesis of tight carbonate reservoirs in Ma5(1+2) Member of Ordovician Majiagou Formation, eastern Yi-Shan slope, Ordos Basin were examined, and the potential of natural gas exploration and development were analyzed. The tight carbonate reservoir is defined as the reservoir with a porosity of less than 2% and permeability of less than 0.1x10(-3) mu m(2). The Ma5(1+2) reservoirs are dominantly gypsum mud dolomite, muddy dolomite and Karst-breccia dolomite and has strong heterogeneity, pore types being mainly composed of fracture-dissolution pores and fracture-intercrystalline pores, and thin reservoir layers are distributed in a large area. The unconformity structure adjustment at the top of the Ordovician caused pore creation and pore filling effects, and the joint effect of dissolution pore increase and pore reduction by filling is the major reason for extensive reservoir densification. The thin tight dolomite reservoirs and the overlying adjacent coal source rock in the Upper Paleozoic formed extensive tight carbonate gas with shallow depth ( 1 900-2 500 m) and formed a three-dimensional gas containing pattern combined with the Upper Paleozoic tight sandstone gas. The eastern Yi-shan slope in the Ordos Basin has great exploration and development potential.