Sedimentary Environment And Reservoir Characteristics Of The Ma 5 Member In Fuxian Area,Ordos Basin

Fuxian area is located in the southeastern part of Ordos Basin,and tectonically on the northeast slope of the central paleo-uplift.Affected by Caledonian movement,the basin was uplifted in the Late Ordovician,and experienced weathering and erosion for 140 million years.The study area is located in the palaeokarst upland,which is characterized by long duration,high intensity of karstification,and low crustal-opening strata.At the present time,exploration and development of Ordovician in the basin are mainly aimed at the karst slope,and the exploration degree of the karst highland is low.Many exploration wells in Fuxian area have shown good potential in Ma 5 Member.However,due to the great fluctuation of paleotopography and the presence of lithologies such as limestone,dolostone,paste dolostone and gypsum dolostone,the controlling factors of reservoir development are unclear and the distribution of reservoir is uncertain in this area.Therefore,it is necessary to systematically study the weathering crust reservoir of Ma 5 member in Karst highland.Based on the previous research results,using the core data,logging data and related analytical testing techniques,the sedimentary characteristics,paleo-karst characteristics and reservoir characteristics of the Ma 5 Member were systematically studied and the following research results were obtained:(1)The Ma 5 Member is a subtidal-intertidal deposition.The subtidal zone is located at the bottom of the parasequence,which mainly composed of micrite sandy clastic limestone,bioclastic micrite limestone,micrite limestone and dolostone.The intertidal zone is located at the upper part of the parasequence,which lithology is mainly clayey dolostone,paste dolostone and gypsum dolostone;(2)The brecciation of the Ma 5 Member is serious.It can be divided into four kind of karst facies and three kind of cave facies.The intensity of karst gradually weakens from west to east.The second-level karst paleogeomorphic unit can divide into four third-level karst paleogeomorphic unit;(3)The diagenesis of the Ma 5 Member is complex.Dolomitization and gypsum metasomatism are the material basis of reservoir formation,dissolution is the key to reservoir development,calcite replacement of dolomite and pore space filling are important reasons for reservoir quality deterioration;(4)The development of reservoirs in Ma 5 Member is mainly controlled by four factors: Lithology,karst and cave facies,paleotopography and diagenesis.Lithology(controlled by sedimentary environment)and paleotopography control the development and distribution of karst facies and cave facies,and further control the distribution of karst breccia.The differences of diagenesis processes between different sedimentary environments,karst paleogeomorphic units and karst facies-cavities,ultimately determines the development and distribution of reservoirs;(5)The Ma 5 Member has poor physical properties,and the reservoir space is mainly consist of not completely filled vugs,gypsum moldic pores,fractures,intercrystalline pores and intergranular dissolution pore.High-quality reservoirs mainly distribute in the crackle-mosaic breccia facies at the top of Mawu-6 in the north.