Quantitative Characterization, Classification, and Influencing Factors of the Full Range of Pores in Weathering Crust Volcanic Reservoirs: Case Study in Bohai Bay Basin, China

Weathering crust volcanic oil and gas reservoirs were revealed in the Bohai Bay Basin, and the physical properties changed considerably in different volcanic reservoirs. To understand better the pore structure of weathering crust volcanic reservoirs, it is important to characterize quantitatively the full-range pore size distribution (PSD). In this study, six Lower Cretaceous volcanic samples from the Bohai Bay Basin were measured by scanning electron microscopy, low-temperature nitrogen adsorption (LTNA), mercury intrusion porosimetry (MIP), and nuclear magnetic resonance (NMR) to uncover the full-range PSD of volcanic rocks. The results showed the following. Firstly, four types of pores were observed in the volcanic samples, and in ascending order of the pore size, these pores were devitrification pores, inter-crystalline pores, dissolution pores, and amygdales. Secondly, ink-bottle and slit-type nano-sized pores measured by LTNA generally accounted for 25% of the volcanic porosity, and in tight volcanic rocks with porosity lower than 5%, nano-sized pores accounted for more than 95%. Thirdly, LTNA was combined with NMR corrected by MIP to characterize the full-range PSD, and the pores were divided into three ranges according to boundaries of 0.05 and 1 µm, respectively. Fourthly, the clay-related inter-crystalline pores dominant in dacitic tuff and basalt reservoirs and the dissolution pores dominant in dacite and trachyte reservoir were characterized by “larger pores connected by narrow throats” and “honeycomb-like type,” respectively. Lastly, mafic minerals were prone to alteration under the weathering crust geological conditions, and the iron and magnesium ions promoted the spontaneous generation of clay minerals in the pores, thereby worsening the physical properties of the reservoir. Therefore, we propose that alkali-rich acidic volcanic lava is a favorable reservoir for oil and gas exploration.