CHARACTERISTICS AND CONTROL FACTORS OF PALAEOKARST RESERVOIR OF THE MAJIAGOU FORMATION OF LOWER ORDOVICIAN IN NORTHERN JINGBIAN GAS FIELD OF ORDOS BASIN

The Characteristics of palaeokarst reservoir of O1m541 of lower Ordovician in northern Jingbian gas field of Ordos Basin are investigated by the analysis of core,cast thin section,SEM,CLM,porosity and permeability test.It is showed that intergranular pore,dissolved pore,dissolved fracture and moldic pore are the main primary pore types with the porosity between 15.91% and 0.22%,average 3.73%,with the permeability ranging from 79.13×10-3 μm2 to 0.001 3×10-3 μm2,average 1.24×10-3 μm2.The pore structures are divided into 4 types: dissolved pore-intergranular pore-dissolved fracture type,intergranular pore type,micropore-intergranular pore-dissolved fracture type,and compact micro-pore type.The development of reservoir is influenced by microfacies,diagenetic facies and karst palaeogeomorphology,with the sedimentary facies of the basis of its development.Diagenetic facies controls final reservoir space and palaeogeomorphology controls the degree of reservoir development and conservation.The composite parts of the three favorable areas are the key target for oil and gas exploration.