Development mechanism of dissolution-type volcanic reservoirs in the Shenhu formation, Huizhou Sag, Pearl River Mouth basin: insights from geochemical and stable isotope analyses

The quality of unconventional volcanic reservoirs depends on dissolution spaces formed by diagenesis, yet their genesis and evolution are poorly understood. Here, we recover the formation and evolution processes of the dissolution pores using thin sections, reservoir porosity and permeability, major element analysis, electron probe microanalysis, in situ micro-scale carbon and oxygen stable isotopes, well logging, and seismic data. In the Palaeogene Shenhu Formation volcanic reservoir of the Huizhou Sag, Pearl River Mouth Basin (PRMB), dissolution pores dominate the reservoir space. During the epigenetic diagenetic stage, volcanic rocks underwent a prolonged weathering and dissolution (60.8–47.8 Ma), which increased the porosity. Primary pores were partially filled and damaged by first-phase quartz. During the burial diagenetic stage, secondary minerals, including chlorite, laumontite, second-phase quartz and calcite, filled the residual primary and weathering-dissolution pores. The diagenetic sequence of these minerals, combined with mineral thermometers and thermal history, indicates that chlorite formed at approximately 13–17 Ma and the subsequent laumontite were the primary causes of reservoir degradation. However, the extensive dissolution pores in chlorite aggregates and the volcanic matrix suggest post-filling dissolution by fluids, which formed the effective reservoir pores. The δ13C (−4.296‰ to −11.987‰) and δ18O (−10.182‰ to −17.934‰) values of calcite associated with dissolution pores indicate that the dissolution fluid was organic acid derived from the Wenchang Formation’s hydrocarbon generation process. Temperature-driven transformation of secondary minerals may also provide some effective porosity, but organic acid-driven dissolution is likely the dominant mechanism for reservoir formation. A favourable relationship with source rocks is the necessary condition for forming promising reservoirs, and weathering unconformities and faults likely served as migration channels for organic acids. This study, by revealing the complexity of the formation process of dissolution-type volcanic reservoirs, provides important theoretical foundations and practical guidance for oil and gas exploration and development in other regions.