Genetic and Sealing Mechanisms of Calcareous Sandstones in the Paleogene Zhuhai–Enping Formations, Panyu A Sag, Pearl River Mouth Basin

Calcareous sandstones, acting as sealing layers, play a crucial role in hydrocarbon accumulation of formations with high sand content (sand content > 80%). However, the genetic mechanisms, sealing mechanisms, and effectiveness of calcareous sandstones remain unclear. This study takes the Zhuhai–Enping formations in the Panyu A Sag as an example. By comprehensively analyzing data from well logs, cores, cast thin sections, elemental geochemical analysis and carbon–oxygen isotopes, the genetic mechanisms, development patterns, and controlling effects on hydrocarbon accumulation of calcareous cement layers are investigated. The main findings are as follows: (1) The calcareous sandstone cements are mainly composed of dolomite, ankerite, and anhydrite. With increasing burial depth, dolomite transitions from micritic dolomite to silt-sized and fine-crystalline dolomite, and finally to coarse-crystalline dolomite. (2) The local transgression provided ions such as Ca2+ and Mg2+, forming the material basis for early dolomite formation. As burial depth increased, the diagenetic environment shifted from acidic to alkaline, leading to the dolomitization of early-formed calcite and the formation of ankerite. (3) The high source-reservoir displacement pressure difference effectively seals hydrocarbon accumulation. Vertically interbedded tight calcareous sandstones and thin marine transgressive mud-stones collectively control efficient hydrocarbon preservation and enrichment. This research addresses the current limits in the study of “self-sealing sandstone layers,” and provides new geological insights and predictive models for hydrocarbon exploration in sand-rich settings.