Main factors controlling the wettability of gas shales: A case study of over-mature marine shale in the Longmaxi Formation

Wettability plays a significant role in the exploration and exploitation of shale gas. However, it is challenging to evaluate the wettability of gas shale due to the complexity and heterogeneity of the mineral components, physical properties, and pore structures. In this study, we conduct rock composition analysis, field gas desorption, and air-liquid contact angle measurements for 15 core samples, and analyze the petrophysical properties of 8 core samples from the YY3 well drilled in the Longmaxi Formation, Hunan Province, Southern China. A strong parabolic correlation was found between water contact angles and total organic carbon (TOC) content. We divided the samples into two analysis groups using the symmetry axis with TOC approximate to 1.5%. Scanning Electron Microscope (SEM) analysis demonstrated that migratory organic matter is sparsely distributed in samples with TOC < 1.5%. However, in the samples with TOC > 1.5%, the migratory organic matter appeared to be like a uniformly distributed network. This suggests that migratory organic matter increases the water contact angle and is a significant factor explaining strong oil-wetting with high TOC. A negative correlation was found between the content of illite and the water contact angle (R-2 = 0.9070), while a positive correlation exists between the illite/smectite mixed layer and the water contact angle (R-2 = 0.8048). This relationship is likely due to the large inner surface area of the structure layer in the illite/ smectite mixed layer, allowing for chemical adsorption of organic matter. Additionally, a positive correlation between TOC and total gas content was identified. Pore throat analysis and SEM imaging demonstrated that organic pores constitute a significant portion of porosity. Nevertheless, the negative correlation between total gas content and the water contact angle indicated that gas occupies the pore space which previously stored degraded asphalt. Eventually, identifying the main factors that influence the water contact angle of gas shales provides a new mode diagram to analyze the wettability of over-mature marine gas shales.