Research On Lower Paleozoic Reservoir Evaluation In Shenmu Gasfield,Ordos Basin

Reserviors of Majiagou formation in Shenmu Gasfield are dense carbonate gas reservoirs with deeply buried,strong heterogeneity,wide extent of gas-bearing area.The single wells'output of the reservoirs are quite different.This paper takes Ma5_1-4sub-member of Majiagou Formation of Lower Ordovician in Shenmu Gasfield as the research object.Reservoir formation,sedimentary microfacies,karst palaeogeomorphology,basic characteristics,main controlling factors of reservoirs,synthesized estimate thereservoir and natural gas accumulation are deeply studied by using geology,drilling,well logging,gas,core and scanning electron microscope data.The main results and new ideas of this study are as follows:Firstly,stratigraphic division and correlation of Ma5_1-4sub-member of Majiagou Formation are made and the target layers are divided into five sub segments,which are subdivided into 12small layers.The whole stratigraphic framework of the study area is analyzed to be more comprehensive and accurate.The formation condition of Karst in study area is defined:The karst erosion in the northwest of the study area is strong,a larger range relative old strata are well exposed,such as Ma5₁⁴-Ma5₃²layers.The karst erosion in the east and south of the study area is weak,a larger range relative young strata are well exposed,such as layers overlying of Ma5₁³.Secondly,Sedimentary subfacies and microfacies are determined by analyzing the sedimentary characteristics of the target segment.It can be divided into 3 subfacies of supralittoral zone,inter tidal zone and sub tidal zone,and 8 microfacies.The characteristics of sedimentary microfacies and the distribution of micro facies in the plane of different stages are studied.The type of the most favorable sedimentary microfacies is hard gypsum nodules and the favorable facies zone are Ma5₁^1-4,Ma5₂^1-2,Ma5₄¹layers.Thirdly,the residual condition of karst formation in the study area is defined by counting formations.The division of the karst landform units is completed at the base of the boundary of the residual strata,the thickness of the remaining strata and the thickness of the Carboniferous strata.The main body of the study area is distributed in the hill and the groove.Larger hill mainly developed in the southern part of study area while small developed in the northwest part of study area.The distribution of the ancient groove extend from South East to North West.Fourthly,the reservoir rocks are mainly anhydrite nodules of micrite dolomite and reservoir space is mainly anhydrite nodule moldic pore,dolomite intercrystal pore.The reservoir is characterized by low porosity and extra low porosity and low permeability.The porosity is mainly distributed in 1%?7%while the permeability is mainly distributed in 0.005?1m D.Through the study of reservoir characteristics of rock type,space type et.Northwest reservoir physical properties are better than that of the eastern and Southeast in general.Fifthly,the reservoir is divided into 4 types,namely,the gas layer,the gas-bearing layer,the gas-water layer and aquifer.The Ma5₁^1-4,Ma5₂^1-2layers develop gas layer,gas-bearing layer which are distributed in the area of karst hill.The distribution of gas water in Ma5₄¹layer is obviously,the gas and gas bearing layers are mainly distributed in the west while the distribution of the gas-water layer and aquifer are in the East.