PORE CHARACTERISTICS OF THE RESERVOIR O_1m~5_4~1 (LOWER ORDOVICIAN MAJIAGOU FORMATION) IN NORTHERN JINGBIAN GAS FIELD,THE ORDOS BASIN

The reservoir O1m541 in the Lower Ordovician Majiagou Formation is one of the major gas pay zone in the northern part of Jingbian gas field,the Ordos basin.The determination of its main types of pores and pore combination and analysis of the factors influencing the reservoirs are critical to reservoir prediction.Based on core observation,cast thin-section and SEM analysis,the pore spaces can be divided into two types,including pore and fracture,and 9 subtypes,including intercrystal pore,intercrystal solution pore,solution pore and cavity,mould pore,dissolution fracture,interbreccia pore or fracture,structural fracture,suture and shrinkage joint.In combination with mercury penetration analysis,5 types of pore combinations are recognized,including dissolution pore/cavity-fracture,pore-fracture,pore,micropore-microfracture,and tight micropore.The pore development in the study area is mainly controlled by sedimentary microfacies and diagenesis.Sedimentary microfacies are the basis of pore formation,with supratidal dolomitic flat as the best facies belt for pore development followed by supratidal gypseous-dolomitic flat and calciferous-dolomitic flat.Diagenesis determines the reservoir capability,with dolomitization,dissolution,degypsumization and tectonic stress as the main constructive diagenesis,while dedolomitization,filling process,cementation and metasomatism as the main destructive diagenesis.