Reservoir Characteristics And Their Effects On Gas Accumulation In The Mid-lower Ordovician Assemblages From East-Central Ordos Basin

Based on the theory of sedimentology and combined with the previous research results,this paper makes a comprehensive study on the characteristics of the mid-lower assembled reservoirs in the Ordovician Majiagou group from east-central Ordos basin,by using the data of logging interpretation,gas production tests,core observation,thin section identification,core physical property analysis and cathodoluminescence.This paper analyses the effects of reservoir on the formation and distribution of natural gas from the aspects of reservoir sedimentary facies,lithology,physical property and source-reservoir configuration.Affected by denudation,the middle assemblage strata in the western part of the study area were denuded,and the denudation line gradually moved eastward from bottom to top.Stratigraphy of the Ma4 member in the lower assemblage gradually thickened from the east to the central paleo-uplift,and gradually thinned from the bottom to the top.In the western area,the dolomitization is strong,and it develops dolomite.The limestone,gypsum and salt rock developed in the eastern part.The sedimentary facies centered on the eastern depression and showed a ring ribbon distribution.The mid-lower assemblages of high-quality reservoirs are mainly developed in grain-beach and dolomite flat.The main rock types are crystal powder dolomite,silty-fine dolomite and arenite dolomite.The type of reservoir storge space is mainly intercrystalline pores,intercrystalline dissolved pores and dissolved hole.The plane porosity of intercrystalline pores is largest with an average of 1.56%.Controlled by a variety of diagenesis,the reservoir physical property are generally poor.Among them,the reservoir physical property of Ma5₉ is the best,followed by Ma5₅ and Ma5₇,and Ma4 is the worst.The effects of the mid-lower assemblages reservoirs on the formation and distribution of gas reservoirs in the study area is mainly manifested in reservoir sedimentary facies,lithology,physical property and source-reservoir configuration.The source-reservoir configuration controls the rich and poor of natural gas,the distribution of natural gas is controlled by the sealing conditions of gypsum and salt rocks and the shielding conditions of limestone.Reservoir sedimentary facies,dolomite thickness and physical properties control the enrichment of gas.With the increase of the distance between the mid-lower assemblages strata and the Upper Paleozoic coaly source rocks strata,the gas production situation gradually becomes worse.When the source-reservoir configuration,sealing conditions of gypsum and salt rocks and lateral shielding conditions of limestone are similar,gas reservoirs are mainly distributed in grain-beach and dolomite flat,with larger dolomite thickness and better physical properties.Gas can be accumulated in poorer reservoirs when the sealing conditions of gypsum,salt rocks and limestone are favorable.It is noteworthy that gas does not accumulate in the places where the reservoir sedimentary facies,lithology,physical properties and source-reservoir configuration are all the optimal.Gas is most likely to accumulate in the areas formed by the optimal allocation and mutual compensation of the four factors.