Geological controls and mechanism of shale gas and shale oil accumulations in Liaohe western depression, China

Recently, the accumulation of shale gas and shale oil in the rifted lacustrine basin of China has garnered increasing interest. In this paper, the shale located in the Shahejie Formation of the Liaohe western depression of the Bohai Bay Basin was selected as the focus of a comprehensive evaluation of the geological controls of shale gas and shale oil accumulation. (1) In a rifted lacustrine basin, the subsidence rate of the stratigraphy is rapid, which results in a massive sedimentation of organic-rich shale. The shale that developed in the deep and semi-deep lacustrine facies is characterized by a high concentration of organic matter, with an average total organic carbon (TOC) content over 2.0% being measured. The TOC consists primarily of type I-II1 kerogen, which contains abundant sapropelic materials and has relatively low thermal maturity, ranging from 0.4 to 0.9%R-0, due to the shallow burial depth and the young deposition epoch. (2) Various pore-fractures in the core samples and erosion occurring in the calcareous and dolomitic shales were observed, which provide storage space for oil-gas accumulations. Furthermore, the mechanisms of the accumulation of shale gas and shale oil in the study area were established according to analyses of the geochemistry characteristics and the sedimentary environment, as well as a thermal pressure simulation experiment. Overall, the accumulation model of shale oil-gas is characterized by "upper oil and lower gas". In addition, oil-gas resources in siltstone, dolomite and limestone, which are adjacent to or interbedded with organic-rich shale, are also important targets in shale reservoirs.