Investigating Full-Range Pore Size Distribution, Movable Fluids Occurrence, and Pore-Fracture Systems in Volcanic Rocks: Insights From Eight Drilling Core Samples

Summary The objective of this study is to achieve a comprehensive characterization of the full-range pore size distribution (PSD) and to elucidate the occurrence of mobile fluids within the pore-fracture systems of volcanic reservoirs. Eight representative volcanic rock samples were selected from the Shenhu Formation within the Huizhou Sag of the Pearl River Mouth Basin (PRMB). A multitechnical analysis was conducted, encompassing low-temperature nitrogen adsorption (LTNA), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM). By introducing an innovative approach from fractal theory, we successfully translated the NMR T2 spectrum from the relaxation time domain to the spatial domain of PSD using LTNA, thereby accurately depicting the full range of pore sizes in volcanic rocks. The PSD was found to be consistent with the reservoir storage space pore sizes observed through cast thin-sections and SEM imagery. Our findings indicate that the reservoir storage spaces in volcanic rocks comprise four types: intercrystalline pores of clay minerals (IPCM), dissolution pores (DP), microfractures (MIF), and macrofractures (MAF). Mobile fluids in volcanic rocks are predominantly encountered within the 101–105 nm range, with their occurrence characteristics being closely linked to rock type and regulated by various reservoir storage space types. Tuff and vesicular amygdaloidal lava reservoirs, characterized by abundant IPCM and DP, exhibit significant gas storage capacity but limited producibility, making them potential reservoirs. Block lava reservoirs, depending on the development of MIF and MAF, may serve as potential reservoirs if fractures are well-developed or as potential seals if fractures are poorly developed. Cryptoexplosive breccia reservoirs exhibit a more stochastic behavior, with variable gas storage and producibility depending on the degree of fracturing and dissolution. MIF significantly influences the degree of dissolution, while MAF provides crucial pore volume (PV) for mobile fluids and conduits for fluid migration at the micrometers to millimeters range. By analyzing the inflection points between fractal trend lines and statistically assessing the sizes of different reservoir storage spaces, we have established thresholds between small, medium, and large pore-fracture systems at 86 and 596 nm. The small pore-fracture system is primarily constituted by IPCM and DP, the medium system by DP and MIF, and the large system by MIF and MAF. This research offers novel insights into the full-range PSD, composition of reservoir storage spaces, and characteristics of fluid occurrence within the pore-fracture systems of volcanic rock reservoirs.