Formation of an anomalously high‐porosity zone of a very fine‐grained deep clastic reservoir in the Qiongdongnan Basin, South China Sea

Abstract

Development of anomalously high porosities in very fine‐grained deep clastic reservoirs is still largely unknown. The reservoirs in the S13 block of the lower Meishan Formation in the Qiongdongnan Basin, South China Sea, developed a zone of anomalously high porosity up to 28.5%, and thus offers an ideal opportunity to quantitatively study the formation mechanism of the abnormally high porosity but fine‐grained deep clastic reservoir from the diagenetic perspective. Our study demonstrates that the first phase charging of oil and organic acid promoted the early‐stage dissolution. This process, however, restrained the early‐stage compaction, cementation, and transformation of clay minerals in sandstone reservoirs. CO2‐rich thermal fluid at the fourth phase enhanced the dissolution of late‐stage feldspar and early‐stage cements and authigenic clay minerals. The massive Ca2+, Mg2+, Fe2+, and other ions generated by the dissolution facilitated the formation of late‐stage siderite and ankerite, which filled the pore throats and lowered the reservoir permeability. Furthermore, the rapid deposition produced high primary porosities, and the early‐stage overpressure also inhibited the compaction, cementation, and clay transformation in the S13 block. All these processes promoted the preservation of primary pores, and the S13 block developed reservoirs with anomalously high porosity and low permeability. By contrast, the C24 block was only charged by oil and hydrocarbon‐rich gas at the second and third phases, and experienced strong compaction, weak dissolution, and strong cementation. Consequently, fewer primary and secondary pores, but higher contents of cements and authigenic clays were generated in the C24 block. The reservoirs in the C24 block exhibit medium porosity but low permeability.