Evolutionary process of the wettability of low-permeability sandstone reservoirs under the control of diagenesis and its mechanism: A case study of the Dongying Sag, Bohai Bay Basin

Wettability plays a critical role in dictating the fluid percolation within low-permeability sandstone reservoirs. However, the wettability evolutionary mechanisms remain ambiguous given the frequent changes in the fluid properties and mineral types and compositions in these reservoirs throughout diagenesis. This opacity severely hampers the research on the mechanisms of hydrocarbon accumulation in the reservoirs. This study centers on the upper submember of the 4th member of the Paleogene Shahejie Formation in the Dongying Sag, Bohai Bay Basin. Based on the analysis of geological data, we undertake a systematical analysis of diagenesis’s controlling effects on the wettability of lowpermeability sandstone reservoirs through observation of casting thin sections and the X-ray diffraction analysis of mineral compositions, Amott wettability tests using nuclear magnetic resonance (NMR) equipment, and experiments of contact angles in a solid-oil-water system, both under high temperature and pressure. The results highlight distinct major pore types across various diagenetic states attributable to complex diagenesis. Specifically, the early diagenetic stages A and B evidenced the major pore types of residual pores by compaction and both intragranular pores from feldspar dissolution and intergranular pores from carbonate dissolution, respectively, while middle diagenetic stages A1, A2, and B witnessed the presence of dissolution pores at quartz edge, both intragranular pores from feldspar dissolution and intergranular pores from carbonate dissolution, and fractures, respectively as major pore types. As diagenesis advanced, residual pores by compaction, dissolution pores at quartz edge, and intragranular pores from feldspar dissolution grew increasingly hydrophilic. Furthermore, intergranular pores from calcite dissolution trended toward lipophilicity, while intergranular pores from dolomite dissolution evolved from water wetting to intermediate wetting. The overall wettability of the low-permeability sandstone reservoirs is governed by the major pore types and their surface wettability throughout the diagenetic timeline. The wettability of low-permeability sandstone reservoirs was dominated by water wettability across all the diagenetic stages, showcasing strong hydrophilicity, weak hydrophilicity, hydrophilicity, intermediate wetting, and hydrophilicity sequentially. Finally, a wettability evolution model of sandstone reservoirs under the action of diagenesis is established, which will guide the prediction of sweet spots in low-permeability sandstone reservoirs.