Sedimentological and reservoir characteristics of Carboniferous–Permian shales in the northern Songliao Basin and surrounding areas, Northeastern China

The Carboniferous–Permian shale gas represents a new target for hydrocarbon exploration in the Songliao Basin. The reservoir quality of these shales remains poorly understood. In this paper, we describe the field relationships of the Carboniferous–Permian succession and characterize the shales in terms of sedimentology, petrology, organic geochemistry, and petrophysics use drilling, logging, and shale gas experimental data. The Carboniferous–Permian succession is mainly composed of clastic rocks, carbonate rocks, and shales, and has been slightly metamorphosed. The shales contain types II 1 and II 2 kerogen and have high vitrinite reflectance values and moderate total organic carbon content, with regional means ranging from 3.34% to 4.68% and 0.59% to 1.29% within the basin, respectively. Porosity values are mostly below 3% (mean 1.05%), but the maximum reaches up to 10.18%. Specific surface areas range from 0.15 to 15.13 m 2 /g (mean 3.22 m 2 /g). Quartz content varies between 7% and 60% (mean 31.6%). When feldspar and pyrite are also classified as brittle minerals, the mean brittle mineral content reaches 41.94%. Young’s modulus of shales ranges from 27.82 to 66.73 GPa (mean 49.09 GPa), and Poisson’s ratio ranges from 0.18 to 0.328 (mean 0.265). Gas logging confirmed that the shales are rich in gaseous hydrocarbon, with a maximum total hydrocarbon anomaly of 48.2%. Methane adsorption capacity varies from 0.94 to 6.02 m 3 /t (mean 3.17 m 3 /t). The northern slope and southern margin of the Songliao Basin where preservation conditions are favorable, such as in Mingshui–Suihua, Fuyu, Ar Horqin, and Jiutai counties, are the most promising areas for shale gas exploration, owing to weak deformation, well-developed fracture reservoirs, effective cap rocks, and relatively low thermal maturity.