Characteristics and evolution of sub-member Ma₅⁴ of Majiagou Formation in southern Jingbian gas field, Ordos Basin

As one of the critical replacement production layers following the Sub-member Ma₅⁵ of the Ordovician Majiagou Formation in the Jingbian gas field, the sub-member Ma₅⁴ is limited in the reasonable and accurate prediction for the carbonate reservoir development and natural gas distribution because of the weak analysis on the sedimentary microfacies. Taking the southern Jingbian gas field as an example, the sedimentary microfacies type were identified and the planar distribution law was described, and the evolution law of sedimentary microfacies of different layers from the sub-member MaMa₅⁴ was made clear in this paper through core observation, thin section identification and logging interpretation for sedimentary facies. The results show that the sedimentary subfacies of the sub-member MaMa₅⁴ is dominated by restricted platform in the central and western study area, including lime-bearing dolomite flat, calciferous dolomite flat, mud-bearing dolomite flat, argillaceous dolomite flat, dolomitic mud flat, and mud-lime bearing dolomite flat. The evaporation platform is mainly distributed in the eastern study area, and dominated by gypsiferous dolomite flat and gypsiferous lake. The Ma₅^4-3 layer is dominated by restricted platform, with the largest area of mud-bearing dolomite flat and mud dolomite flat development. During the sedimentary period of Ma₅^4-2 layer, the evaporation platform area in the eastern study area increased. The content of terrigenous mud increases again when in the Ma₅^4-1b layer sedimentary period resulting in the extensive development of mud dolomite flat in the central study area, while decreases during the Ma₅^4-1a layer sedimentary period resulting in the extensive distribution of calciferous dolomite flat development with local development of evaporation platform. The natural gas distribution is controlled obviously by sedimentary microfacies. The calciferous dolomite flat and mud-bearing dolomite flat are the favorable sedimentary microfacies for natural gas distribution of sub-member Ma₅⁴ in the study area. The research results is helpful for the prediction of favorable carbonate reservoir distribution areas and the deployment of natural gas exploration and development scheme in Jingbian gas field.