Study On Sedimentary Microfacies And Reservoir Characteristics Of The Third Member Of Majiagou Formation In Central Ordos Basin

Ordos Basin is rich in oil and gas resources.A large number of oil and gas have been found in Carboniferous Permian,Triassic and Jurassic.Large gas fields with proven reserves of more than 200 billion m³ have also been found in the weathering crust at the top of Ordovician in the middle of the basin.Majiagou Formation is an important exploration target layer,and its carbonate reservoir characteristics play an important role in natural gas exploration.In this paper,in the third member of Majiagou Formation,the characteristics of strata and small layers,types of sedimentary microfacies,reservoir genesis and main controlling factors are studied.Based on the drilling data,outcrop and core observation,microscopic slice identification,etc.Combined with the carbon,oxygen and strontium isotope analysis data of samples,the field profile,typical single well and profile between wells are analyzed.The lithology of the target layer is mainly gypsum dolomite,with some salt rocks and a small amount of pure dolomite,argillaceous dolomite,dolomitic limestone and gypsum salt thin layer.According to the lithology,lithology combination and sedimentary structure,the third member of Majiagou Formation is divided into three small layers of Masan 1,Masan 2and Masan 3.Sedimentary microfacies mainly include mud-dolomite flat,gypsum-dolomite flat,gypsum-containing dolomite flat,gypsum-salt rock lagoon and granular beach facies.Anhydrite nodule dolomite is developed in gypsum-dolomite flat and gypsum-containing dolomite flat,which is a favorable sedimentary microfacies for reservoir development.The reservoir type developed in the third member of Majiagou Formation in the study area is gypsum solution pore dolomite reservoir.Its average porosity is 1.73%,and the average permeability is 0.23×10^-3μm.The formation of reservoir is controlled by quasi syngenetic dolomitization,atmospheric fresh water dissolution and mineral filling.The reservoir of the third member of Majiagou Formation in the study area is distributed in a northeast direction along the Shenmu-Yulin-Jingbian area.The thickness of a single layer is generally 0～3m,and the cumulative thickness can be up to 14.7m.