Permo-Carboniferous coal measures in the Qinshui basin: Lithofacies paleogeography and its control on coal accumulation

Abstract

The Qinshui basin in southeastern Shanxi Province is an important base for coalbed methane exploration and production in China. The methane reservoirs in this basin are the Carboniferous and Permian coals. Their thickness is strongly controlled by the depositional environments and the paleogeography. In this paper, sedimentological research was undertaken on the outcrop and borehole sections of the Taiyuan and Shanxi formations in the Qinshui basin and the basin-wide lithofacies paleogeography maps for these two formations have been reconstructed. The Taiyuan Formation is composed of limestones, aluminous mudstones, siltstones, silty mudstones, sandstones, and mineable coal seams, with a total thickness varying from 44.9 m to 193.48 m. The coal seams have a thickness ranging between 0.10 and 16.89 m, averaging 7.19 m. During the deposition of the Taiyuan Formation, the northern part of the basin was dominated by a lower deltaic depositional system, the central and southern parts were dominated by a lagoon environment, and the southeastern corner was occupied by a carbonate platform setting. Coal is relatively thick in the northern part and the southeastern corner. The Shanxi Formation consists of sandstones, siltstones, mudstones, and coals, with the limestones being locally developed. The thickness of the Shanxi Formation is from 18.6 m to 213.25 m, with the thickness of coal seams from 0.10 to 10 m and averaging 4.2 m. During the deposition of the Shanxi Formation, the northern part of the Qinshui basin was mainly dominated by a lower deltaic plain distributary channel environment, the central and southern parts were mainly an interdistributary bay environment, and the southeastern part was occupied by a delta front mouth bar environment. The thick coals are distributed in the central and southern parts where an interdistributary bay dominates. It is evident that the thick coal zones of the Taiyuan Formation are consistent with the sandstone-rich belts, mainly located in the areas of the northern lower deltaic plain and southeastern barrier bar environments, whereas the thick coal zones of the Shanxi Formation coincide with the mudstone-rich belts, located in the areas of the central and southern interdistributary bay environments.