Gas adsorption capacity calculation limitation due to methane adsorption in low thermal maturity shale: A case study from the Yanchang Formation, Ordos Basin

A series of integrated and repeatable methods were used to demonstrate the methane adsorption limitations associated with calculating low thermal maturity shale gas adsorption capacities. These methods include X-ray diffraction of bulk and clay minerals, Soxhlet extraction, Ar-ion polishing, focused ion beam scanning electron microscopy (FIB-SEM), low pressure N 2 adsorption and high pressure CH 4 adsorption. The laboratory data suggest that the Yanchang Formation shale primarily consists of clay minerals (≈47.4%) and detrital minerals (≈51.1%). An illite–smectite mixed layer is the principal clay mineral constituent, while quartz is the major detrital mineral component. The thermal maturity is relatively low with Ro ranging from 0.84%Ro to 1.1%Ro, suggesting the absence of organic matter pores and the presence of soxhlet-extractable bitumen and oil, which clog pores with diameters around 4 nm and 50 nm. The specific surface areas and quantities of adsorbed N 2 significantly higher for samples extracted using a mixed solution (CH 2 Cl 2 and CH 3 OH) compared to non-extracted samples. However, the methane adsorption volumes, including volume that were measured and calculated using Langmuir fitting parameters, minimally varied during high pressure CH 4 adsorption experiments. CH 4 dissolution in bitumen and oil causes the difference exhibited by these two gas adsorption analyses. Non-extracted samples produce inaccurate results using methane adsorption procedures and do not effectively represent practical shale reservoirs. Extracted samples are limited by CH 4 adsorption when calculating gas adsorption capacities for low thermal maturity shales.