Sedimentary and diagenetic facies of tidal flat in epeiric sea and its related descriptive method-a case of Ma51-4 submember in Jingbian gas field of Ordos basin, China

The Ma51-4 submembers of Jingbian gas field in Ordos basin of China belongs to epeiric sea evaporative carbonate tidal flat facies. Supratidal zone of gypsum nodule dolostone and upper part of intertidal zone of gypsum crystal dolostone are most favorable reservoir. The carbonate rocks of Ma51-4 had undergone complex diagenesis. Penecontem-poraneous dolomitization and gypsification provided the material foundation of the reservoir. Epidiagenesis selective dissolution of gypseous dolostone constructs the origin shape of pore structure. The dissolution and filling of various minerals in the burial period determine whether the early dissolution pores can be preserved, and the final state of the reservoir. Burial dissolution and filling of various minerals determined the preservation of early dissolved pores and the final condition of the reservoir. The concept of “minus cement porosity” directly reflects that sedimentary facies have great influence on the growth of reservoir in Ma51-4. Combined with sedimentary microfacies and diagenetic facies, a concise numerical symbol is used to express semiquantitatively sed-imentary microfacies and diagenesis facies. This descriptive method has great benefit in reservoir evaluation and prediction.