The composition and its impact on the methane sorption of lacustrine shales from the Upper Triassic Yanchang Formation, Ordos Basin, China

The organic geochemistry, mineralogy and methane sorption of lacustrine shales of the Upper Triassic Yanchang Formation, collected from the south-eastern Ordos Basin, were investigated to characterize them and clarify the effects of shale composition on their sorbed gas capacity. These Yanchang shales have recently been selected as a target area for shale gas exploration in typical terrestrial strata in China. The two main sections of these shales containing type II organic matters, Chang 7 and Chang 9, have relatively high total organic carbon content (TOC) of 2–10%. The two shales also have similar mineralogies, mainly comprising quartz, clay minerals and feldspars. Both the Chang 7 and Chang 9 shales are generally in the oil window; Chang 9 is slightly more mature than Chang 7. Higher methane sorption capacity was observed for Chang 9 than for Chang 7 shales, determined on a dried basis at 50 °C. Methane sorption measurements were further performed on three samples from which the residual bitumen had been extracted, and their corresponding kerogen fractions, to gain insight into the effects of shale composition on methane sorption. This was significantly higher in solvent-extracted samples than in raw samples, indicating that residual bitumen largely restricts methane sorption on such shales. A positive correlation between the amount of clay minerals and methane sorption capacity of bulk rocks was evident, suggesting that clay mineral content is relevant to methane sorption. This result was also supported by the much higher methane sorption capacity of solvent-extracted shales compared to the extracted kerogen from those shales, when measured sorption data was normalized to TOC values. The effects of both residual bitumen and clay mineral on the methane sorption of bulk rocks have complicated the evaluation of methane sorption on organic matter in these mature shales.