Modeling of tectono-thermal evolution of Permo-Carboniferous source rocks in the southern Qinshui Basin, China: Consequences for hydrocarbon generation

Abstract

As a main coalbed methane-producing area in China, Qinshui Basin has undergone typical tectonic thermal evolution as a result of the destruction of the North China Craton since the Late Paleozoic. This work describes the construction of a comprehensive tectono-thermal model for the southern Qinshui Basin (SQB) with which to reveal the maturation and hydrocarbon generation of the coal-bearing source rocks. The organic geochemical results show that in the present day, the source rocks comprise Type III kerogen containing more than 2.0 wt % TOC and that the Ro ranges from 2.05% to 3.24%, suggesting appropriate thermal maturity and excellent gas generation potential. Modeling results indicate that from the Late Carboniferous to the Late Triassic, the coal-bearing strata entered the first rapid subsidence stage and reached a burial depth of about 5000 m in the Late Triassic, with a geothermal gradient of 42 °C/km and maximum temperature of 118 °C. Consequently, the maturity of source rocks increased to the first peak and entered the late oil stage. Due to the destruction of the North China Craton, lithospheric thinning, and associated magmatic activity during the Late Jurassic to Early Cretaceous, the study area experienced an abnormal thermal event, with an average geothermal gradient of 58 °C/km and a maximum heat flow of 124 mW/m2, and the source rocks reached the dry gas stage rapidly, so that secondary large-scale hydrocarbon generation occurred in the Early Cretaceous. Overall, the coal-bearing source rocks of the SQB underwent deep burial thermal metamorphism in the Late Triassic and intense magmatic thermal metamorphism in the Early Cretaceous, and the latter was completely controlled by magmatic events caused by the Craton destruction.