Effect of liquid hydrocarbons on gas adsorption in alacustrine shale: A case study of the Yanchang Formation, Ordos Basin

The effect of liquid hydrocarbons on the gas adsorption capacity of lacustrine shales from the Yanchang Formation in the Ordos Basin has been studied using a series of integrated experimental methods including X-ray diffraction of bulk and clay minerals, Soxhlet extraction, focused ion beam scanning electron microscopy(FIB-SEM), low-temperature N₂ adsorption and high-pressure CH₄ adsorption methods.The results show that the lacustrine shale from the Yanchang Formation is in the middle diagenetic stage A.The thermal maturity is relatively low, and contributes to the absence of organic matter pores.However, it is sufficiently high enough forthe production of liquid hydrocarbons which can block up pores with a diameter of 4 nm.Methane can dissolve in these liquid hydrocarbon, whereas nitrogen cannot.As a result, the specific area and adsorbed capacity acquired in low-temperature N₂ adsorption analysis show great differences between samples before and after extraction.However, the difference does not exist in the CH₄ adsorption analysis.This means that a certain proportion of CH₄ exists in these samples in a dissolved form.It is necessary to pay attention to free hydrocarbons during studies of gas adsorption capacity of shales with low thermal maturity usinghigh pressure CH₄ adsorption methods.