Wettability and its major determinants of shale reservoirs in the Shahejie Formation, Dongying Sag, Bohai Bay Basin

Wettability, a factor influencing the occurrence and seepage patterns of shale oil in reservoirs, is an important indicator of shale reservoir evaluation. Using contact angle measurements and spontaneous imbibition experiments, we quantitatively characterize the wettability of shale reservoirs in the Shahejie Formation, Dongying Sag, Bohai Bay Basin, while delving into the primary determinants of wettability, and selecting the optimal reservoirs for assessment. The findings suggest that the shale reservoirs in the Shahejie Formation exhibit moderate pore connectivity and fractional wettability, generally proving to be water-wet to weakly water-wet. The reservoir wettability is jointly determined by organic matter characteristics, mineral components, pore size, and shale oil composition. The impacts of organic matter and mineral composition on reservoir wettability depend on the interfacial tension of solid particles. Specifically, a higher abundance of organic matter and a higher calcium content in minerals are associated with reduced water wettability and stronger oil wettability of the reservoirs. The oil-bearing capacity and shale oil components alter the surface tension of liquids, further influencing the reservoir wettability. A higher oil-bearing capacity of reservoirs and the presence of more polar components in crude oil suggest stronger oil wettability. Pore structure influences the solidliquid interfacial tension through capillary pressure, further affecting the reservoir wettability. A larger pore sizecorresponds to weaker water wettability and stronger oil wettability of the reservoirs. The wettability assessment results demonstrate that lamellar organic-rich calcareous shales exhibit the strongest lipophilicity. Shale oil tends to be enriched and accumulate in these shales under weakly water-wet conditions, creating conducive conditions for shale oil production. Therefore, these shales serve as favorable targets for shale oil exploration and production.