Deposition, stratigraphic sequences and spatial distribution of coal gas reservoirs—A case study of the Gubei block in Jiyang

Abstract：High-frequency sequence stratigraphic division of the Permian-Carboniferous in the Jiyang area has been made. Seven depositional sequences have been distinguished. Sequence I, II and III are of binary structure, that is, they are composed of transgressive systems tracts and highstand systems tracts. Sequences Ⅳ to Ⅶ are of tertiary structure, that is, they are composed of lowstand systems tracts, transgressive systems tracts and highstand systems tracts. The main coal gas reservoirs are developed in the lowstand systems tracts and highstand systems tracts in sequences IV to VII. The main frame sandstones are: the deltaic sand body of the deltaic plain dominated by distributary channel deposits; filling deposits of the river channel in the river system which are dominated by point-bar deposits; distributary channel deposits and sandy deposits of lakeshores and shallow lakes of the lake system dominated by lake delta and lakeshore sandy deposits; channel filling deposits and sandy deposits along the shore in the composite river-lake sedimentary system; and also flood fan deposits. The coal gas accumulations occurring in Permian sandstone (compact sandstone) are the most important natural gas accumulations. As the diagenesis of the Permian-Carboniferous sandstone was strong, scarce primary intragranular pores have been preserved and most intragranular pores formed by late-stage corrosion. Such secondary intragranular pores formed by late-stage corrosion of matrix or cement filling between the intragranular pores.