Porosity Evolution Differences of the Lingshui Formation Reservoir between Baodao and Changchang Sag, Qiongdongnan Basin

Sandstone reservoirs of Lingshui formation in Baodao-Changchang depression are currently the key targets for oil and gas exploration in Qiongdongnan Basin. In this paper, the differences of diagenesis evolution in sandstone reservoirs of Lingshui formation between the northern slope of Baodao sag and Changchang sag are studied by means of casting thin sections, laser particle size, element geochemistry, X-ray diffraction, scanning electron microscopy, fluid inclusions and electron probe analysis. The results show that the sandstone reservoirs on the northern slope of Baodao sag are delta deposits under the provenance system of Hainan Island which have strong hydrodynamic characteristics of "coarse grain size and high stable component", while the sandstone reservoirs in Changchang sag have weak hydrodynamic characteristics of "fine grain size and heavy argillaceous", which are submarine fan deposits under the provenance system of Shenhu uplift and Xisha uplift. Mechanical compaction results in linear or convex contact of sandstone particles in Lingshui formation in the northern slope of Baodao sag. The high heat flow leads to the chemical compaction stage of sandstone in Lingshui formation of Changchang depression under the condition of shallow burial below sea floor (about an depth of 1 400 m below sea floor) and point-line contact of particles, accompanied by hydrothermal minerals such as barite and dawsonite. There are two stages of hydrocarbon filling (14.5~10.0 Ma, 2~0 Ma) in the northern slope of Baodao sag, which were separated by one stage of CO₂ filling (7~3 Ma). In Changchang sag, only early hydrocarbon filling (14.5~6.0 Ma) and subsequent CO₂ filling (5~0 Ma) were developed. The evolution of sandstone reservoirs in Lingshui Formation on the northern slope of Baodao Sag was generally characterized by "slowly deep-buried below sea floor and slow temperature rise". Natural gas filling promoted dissolution and inhibits cementation, while Changchang Sag was characterized by " shallow-buried below sea floor and rapid temperature rise" and continuous CO₂ filling promoted cementation of carbonate or clay or hydrothermal minerals.