Dissolution pores in shale and their influence on reservoir quality in Damintun Depression, Bohai Bay Basin, East China: Insights from SEM images, N2 adsorption and fluid-rock interaction experiments

Shale pore characterization is crucial for understanding the mechanism of shale hydrocarbon accumulation and its effective exploitation. X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), pore extraction, nitrogen (N-2) adsorption, and fluid-rock interaction experiments were used to study the shale pore structure of the fourth member of the Paleogene Shahejie Formation in the Damintun Depression, East China. The Damintun shale included three types of pores: organic matter pores, mineral matrix pores, and microfractures. The dissolution pores were mainly developed between and inside the matrix minerals, such as calcite, dolomite, and feldspar. The average shale visible dissolution porosity determined using SEM was 1.12%. N(2 )adsorption analysis revealed that the less than 20 nm pores were the main reservoir space in the Damintun shale. Fluid-rock interaction experiments indicated that the degrees of dissolution of the calcite and dolomite were similar and stronger than those of the K-feldspar and plagioclase. The shale with a high content of carbonate minerals (calcite and dolomite) exhibited more intense dissolution. The degree of dissolution pores development was positively correlated with total organic carbon (TOC) and carbonate mineral (calcite and dolomite) contents. The average contribution of the dissolution porosity to the total porosity of the shale was 18.20%. However, the contribution of the dissolution pores increased with increasing pore size only when the average pore diameter was larger than 12 nm. This may be due to the fact that small pores (diameter < 12 nm) are more easily to be filled by re-precipitated minerals. The total porosity and the contribution of the dissolution pores were positively correlated with the dissolution porosity, revealing that dissolution pores are an important factor affecting the quality of shale reservoirs.