Geochemical and Preliminary Reservoir Characteristics of the Carboniferous–Permian Coal-Bearing Strata in the Junger Area, Northeastern Ordos Basin, China: Source Implications for Unconventional Gas

Unconventional natural gas development, including coalbed methane, shale gas, and tight sand gas, has received extensive attention recently in coal-bearing basins. This study provides a case study of the hydrocarbon generation potential (coal and shale) and reservoir characterization (coal, shale, and sandstone) of samples from Carboniferous–Permian coal-bearing strata in the Junger area, northeastern Ordos Basin, China. Results show that the coal is dominated by vitrinite (9.3–84.1%) and inertinite (14.0–82.5%) with an average total vitrinite and inertinite content higher than 90% and maximum vitrinite reflectance (R o,max) values from 0.44% to 0.81%. The shales contain mainly type II kerogen with high total organic carbon content (0.36–31.58%) and relatively low R o,max (0.45–0.72%). The hydrocarbon generation potential of all the shale samples ranges from 3.25 to 82.82 mg/g rock, and the hydrogen index values range from 100.31 to 768.06 mg HC/g. Three types of fluid inclusions were detected in the microfractures in/through quartz grains, particle pores, dissolution holes of calcite cement, and microfractures in calcite veins, including liquid, gas–liquid, and gas hydrocarbon inclusions, with yellow-green and blue fluorescence. The homogenization temperatures range from 60 to 140 °C, and combined with the geothermal evolution of the research area and the salinity of the fluid inclusions, the continuous charging history of unconventional gas can be divided into three stages, two hydrocarbon charging stages (80–110 °C, corresponding to Late Triassic–Middle Jurassic, and 110–140 °C, corresponding to Middle Jurassic–Middle Cretaceous) and one adjustment stage (60–80 °C, Late Cretaceous). The coal samples show the highest porosity (10.4–18.0%, avg. 14.3%) and permeability (2.17–2.93 mD, avg. 2.6 mD), followed by sandstones with porosities ranging from 10.1–13.0% (avg. 11.6%) and permeability varying between 0.51 and 1.16 mD (avg. 0.84 mD). Shale samples have the lowest porosity (1.1–4.7%, avg. 2.48%) and permeability values (0.003–0.37 mD, avg. 0.1 mD). The extensively developed and thickly deposited source rocks and multiple lithological cycles of coal-bearing strata set a good foundation for the development of unconventional gases, and the western Junger area should be newly considered as a target area for its appropriate burial depth, great hydrocarbon potential, and favorable preservation conditions.