Geological controls and estimation algorithms of lacustrine shale gas adsorption capacity: A case study of the Triassic strata in the southeastern Ordos Basin, China

• Methane sorption capacity of shale exhibit good positive correlation with TOC content.
• Methane sorption capacity of shale has no correlation with clay content but illite.
• Temperature and pressure control methane sorption capacity of shale.
• Methane sorption capacities under geological conditions were estimated.
• Methane sorption capacities with varied TOC as function of depth were obtained.

High-pressure methane adsorption experiments on a series of Triassic lacustrine shale moisture-equilibrated samples from the southeastern Ordos Basin, China, were conducted at pressure up to 20 MPa, two of which were measured at 30 °C, 40 °C, 50 °C, 60 °C, and 70 °C, and seven were performed under reservoir temperature (from 48 °C to 62 °C) to investigate the effect of organic matter content, maturity, mineralogical compositions and reservoir conditions (temperature and pressure) on the methane sorption capacity. The total organic carbon contents (TOCs) range from 0.91 wt.% to 6.11 wt.%. The thermal maturities, as inferred from Rock-Eval Analysis, range from low mature to high mature. The minerals of the shale samples are dominated by clays (36–57 wt.%) and quartz (19–44 wt.%). For the entire shale samples the dominant clay minerals are mixed-layer illite/smectite with some illite and no smectite exists, corresponding to a stage of late diagenesis. The studied samples have N2 BET surface areas ranging between 1.47 and 9.21 m2/g and pore volumes of 0.013–0.034 cm3/g.The methane sorption capacities of moisture-equilibrated shale samples show a positive correlation with TOC contents and BET surface areas. No relationship was observed between the clay contents and methane sorption capacities, indicating that clay minerals do not significantly contribute to methane sorption capacity in these organic shales. The Langmuir pressure (PL) increases exponentially with temperature and the Langmuir volume (VL) decreases linearly with temperature. A computational scheme has been developed to calculate the methane sorption capacity of shales as a function of TOC content, temperature and pressure based on Langmuir sorption isotherm function. Using this algorithm methane sorption capacity of organic shales as function of depth can be obtained. Due to the predominating effect of pressure the methane sorption capacity increases with depth initially, through a maximum and then decreases due to the influence of increasing temperature at a greater depth. The maximum gas sorption capacity typically occurs at a depth range between 400 and 900 m. With TOC content increasing, the maximum methane sorption capacities of organic shales and the corresponding depths increase.