Analysis of the Differences in Feldspar Dissolution in Different Zones of Sandstone Reservoirs: A Case Study from the Zhuhai Formation, Wenchang A Sag, Pearl River Mouth Basin, China

The Wenchang A Depression in the Pearl River Mouth Basin is a confirmed area with abundant oil and gas resources. The exploration breakthroughs of some wells have revealed the exploration prospects of the deep clastic rock reservoirs in the Wenchang A Depression. Previous studies have focused more on the reservoir research results of individual blocks, but there has been a lack of comparative research among different blocks, and the differences in the dissolution effects and formation reasons of the reservoirs in each block have not been clearly identified. Through the comparative study of the Zhuhai Formation reservoirs in Zones 9 and 10 of Wenchang A Sag, this paper clarifies the differences in the dissolution effects of the reservoirs between the two blocks. Based on rock thin section identification, scanning electron microscopy, trace element analysis, and fluid inclusions homogenization temperature testing, combined with seismic data and formation water data, the study of the differences in the dissolution effects of the Zhuhai Formation reservoirs in Zones 9 and 10 was carried out. The key conclusions are as follows: (1) Feldspar grain dissolution is the dominant dissolution process in the study area. In Zone 10, intense feldspar dissolution is observed in the thick-bedded sandstones in the middle part of the braided river delta front, whereas weak dissolution effects are noted in the fan delta and braided river delta plain reservoirs within the same zone. In Zone 9, vertical variations in dissolution intensity are insignificant; however, dissolution is enhanced in the fault transition zone horizontally, accompanied by weak authigenic kaolinite precipitation. Near the No. 6 Fault Zone, the dissolution effect is attenuated, while authigenic kaolinite extensively fills intergranular pores. (2) Elements associated with feldspar, such as Al, are detected in authigenic quartz overgrowths in Zone 10. Combined with fluid inclusion thermometry of quartz overgrowths, the results demonstrate that dissolution in Zone 10 occurred relatively late, primarily driven by organic acid derived from the thermal evolution of organic matter. (3) Elements including K, Ca, Fe, and Al are identified in quartz overgrowths in Zone 9, with the Al content significantly higher than that in Zone 10. During the deposition of the Zhuhai Formation, the Zhu III South Fault and the Fault No. 6 entered their peak activity. The dextral rotation of the stress field triggered a series of strike-slip faults, transitioning the strata from a closed system to a semi-open system. Faulting, characterized by stronger intensity in the southern segment and weaker in the northern segment, facilitated the migration of deep CO₂ fluids into the reservoirs. Integrated with numerical simulation experiments, the results demonstrate that the secondary faults associated with oil-source faults connect organic acids derived from the thermal evolution of source rocks and CO₂ fluids. This combined fluid action induces extensive dissolution of feldspar grains and enables effective migration of kaolinite within the semi-open system.This study provides a critical foundation for understanding diagenetic fluid evolution and reservoir development mechanisms in the study area, with implications for future hydrocarbon exploration in similar geological settings.