Forming Mechanism And Distribution Of Ordovician Reservoirs In Fuxian Area,Ordos Basin

Fuxian area is located in the southeast of the Ordos Basin.The Caledonian movement has caused the uplifting of the strata to be denuded,resulting in a large-scale paleokarst.The main producing interval of Ordovician karst reservoirs is Ma 5 Member of Majiagou Formation.Due to tight reservoir quality and severe heterogeneous of the reservoir,the degree of accumulation of natural gas varies greatly in different regions,and reservoir research is weak.The formation mechanism and distribution law of effective reservoirs are not clear and directly restrict the progress of exploration.In view of this,it is imperative to conduct further research on karst reservoirs in the study area.Based on the core and microscopic observation of thin sections,this paper fully utilized drilling,well log,and combined the results of production on the karst paleogeomorphology and karst reservoir characteristics of the Ordovician palaeokarst in the Fuxian area of the Ordos Basin.Through strata division and correlation,the authors analyzed the sedimentary environment of Ma 5 Member,summarized the types of sedimentary microfacies,sedimentary characteristics,and the assembles of rock and minerals;applied “impression method” and “residual thick method” in combination with ancient geological maps.The paleotopography of karst was characterized;the characteristics of different karst facies were used to divide the karst facies,and the distribution rules of palaeokarst were determined;on this basis,the physical properties of karst reservoirs,types of reservoir space,and diagenesis were detailed.The discussion summarized the main controlling factors for the development of karst reservoirs.The following results are obtained: 1)the sedimentary subfacies in the study area include the restricted platform and the evaporite platform subfacies.The sedimentary microfacies are mainly supratube sboha,intertidal cloud flat,intertidal muddy flat,gypsum salt flat and subtidal ash flat.2)through the restoration of the ancient landforms in the study area,it is clear that the paleoglandomorphology in the study area is mainly divided into karst Highlands,and the ancient landforms are characterized by the west,the East and the lower.3)5 karst facies are mainly developed in the study area,including cracked breccia facies,mosaic breccia facies,granular breccia facies,complex breccia facies and cavern gravel facies.Different karst facies have different breccia structure,breccia component and breccia filling type,which leads to differences in electrical characteristics and physical properties.4)the main controlling factors for the development of karst reservoirs are sedimentary facies,palaeognomomorphic factors,karst facies and cave facies factors.The karst remnant developed by the crack breccia facies and the mosaic breccia is a favorable area for the development of karst reservoirs in the study area.The results of the study further reveal the characteristics of the middle to lower composite karst reservoirs and the control factors of their development,which can be used for reference for the exploration of the next step of natural gas in this area.