Mineralogy and Gas Content of Upper Paleozoic Shanxi and Benxi Shale Formations in the Ordos Basin

To directly measure the gas content in the Benxi and Shanxi subformations of the Ordos Basin in NW China, a series of canister desorption tests were carried out on 33 over-mature Lower Permian to Upper Carboniferous fresh shale cores (>3000 m) at both the reservoir temperature (75–80 °C) and an elevated temperature of 95 °C. Organic chemistry and X-ray diffraction analyses of 33 replicate samples were used to establish relationships between the gas content and rock composition. Geochemical measurements show that the total organic carbon (TOC) contents range from 0.49 to 13.7 wt %. The organic matter is mainly type III arising from lagoon and delta depositional settings. The dominant minerals are clay (25–97 wt %, average 59 wt %) and quartz (1–62 wt %, average 33 wt %). A new ternary diagram is proposed based on the origin and brittleness of the minerals. Multiple linear regressions of emitted gas volumes with respect to the full mineralogy and TOC show a strong positive correlation with TOC and a weak one with clay composition. This is consistent with independent high-pressure methane adsorption experiments in the literature. Elevating the temperature resulted in an incremental gas production of 12% for the Lower Permian Shanxi facies versus 62% from the Upper Carboniferous Benxi shale (with a weighted average of 43%). This may be indicative of more significant gas adsorption (related to the pore size distribution and specific surface areas) in the Benxi lagoon environment, which has more functional components (TOC and clay) and micropore volume than the Shanxi delta deposits, which are more quartz-rich.