Formation mechanism and sealing capacity evaluation of calcareous cements in the Paleogene fault zone,southwestern Huizhou Sag, Pearl River Mouth Basin

The Paleogene strata in the southwestern Huizhou Sag of the Pearl River Mouth Basin (PRMB) are characterized by a high sand content and the predominance of fault traps, where hydrocarbon accumulation is closely related to the lateral sealing capacity of faults. Drilling data reveal that in parts more adjacent to fault zones, reservoirs feature more developed calcareous cements within sandstones. However, the impact of cementation on the lateral sealing capacity of faults in the southwestern Huizhou Sag remains poorly understood. By integrating multiple techniques, including carbon and oxygen isotope analysis, cathodoluminescence, homogenization temperature measurements of fluid inclusions, and clumped isotope thermometry, we reveal the genetic mechanisms of calcareous cements within fault zones and clarify their role in modifying the seepage properties of the fault zones. Accordingly, a comprehensive evaluation index (F_lcs) is developed to evaluate the lateral sealing capacity of trap-bounding faults. The research findings indicate that the formation of calcareous cements resulted from a combination of basement-derived hydrothermal fluids and the decarboxylation of sedimentary organic matter, as suggested by their carbon and oxygen isotope compositions. Specifically, these cements were formed in a weakly reducing to reducing environment, and their formation period (postdating the deposition of the Yuehai Formation) is roughly consistent with the hydrocarbon charging periods. Governed by the vertical differences in the thermal maturity of sedimentary organic matter, cementation within fault zones exhibits a distinct zonation. In zones with intense cementation, calcareous cements effectively block the fluid seepage pathways within fault zones, whose permeability decreases significantly with increasing cement content. Index F_lcs is established by comprehensively considering the dual impacts of compaction and cementation on the seepage mechanisms of faults while combining fault-reservoir displacement pressure differences and cement zones determined by vitrinite reflectance (R_o). Validation confirms high consistency between Flcs-based sealing evaluations and actual hydrocarbon-water distributions. Notably, F_lcs values exceeding 0.4 suggest effective hydrocarbon sealing capacity.