Characteristics and Classification Evaluation of Dolomite Reservoir of Lower Ordovician Ma5 Member in Northwestern Jingbian Gasfield

Pore type of Lower Ordovician Majiagou Formation in northwestern Jingbian Gasfield,Ordos Basin,is complicated and consisting of a wide range since the Majiagou Formation has been weathered,denuded and leached for approximately 130 Ma in the process of its geological evolution.Ma5₁₊₂ and Ma5¹₄ submembers of Majiagou Formation are considered as the major natural gas reservoirs in this district,hence research on its reservoir characteristics and evaluation is significant to not only the theoretical study,but also the practical production.A deep study is carried out on dolomite reservoir by drilling cores,casting thin section,scanning electron microscope and mercury injection test.The results show that pore and fracture are the major reservoir space,and it could be divided into intercrystalline pore,intercrystalline solution pore,solution pore,gypsum mouldic pore,structural fracture,pressolution fracture and dissolution fracture.Pore structure includes four types,namely,macropore and medium-sized throat,mesopore and medium-fine throat,small pore and fine-micro throat,micropore and microthroat.On the basis of such complicated reservoir characteristics,parameters including porosity,permeability,pore structure parameter,displacement pressure,median radius and maximum mercury instrusion saturation are comprehensively considered and selected,according to which the reservoir is classified into four categories with clustering analysis,so that its storage-permeation ability is reflected in a more objective degree.