Evaluation of pore structures in volcanic reservoirs: a case study of the Lower Cretaceous Yingcheng Formation in the Southern Songliao Basin, NE China

Different pore types and variable pore structures can determine reservoir potentials of volcanic rocks. Therefore, it is of great importance to understand the pore structures of volcanic reservoirs. With the Lower Cretaceous Yingcheng volcanic rocks, this study attempts to investigate and evaluate the micro pore structures of a volcanic reservoir through: (1) measuring porosity and permeability of core samples; (2) identifying pore types within thin sections; (3) characterizing pore structures with pressure-controlled mercury injection. The Lower Cretaceous Yingcheng volcanic rocks exhibits low porosities (on average 7.39%) and very low permeability (on average 0.283 × 10⁻⁶ m²). Three major pore types were identified from the Lower Cretaceous Yingcheng volcanic reservoir, including primary pores, secondary pores and fractures. Additionally, the pore structures of the Lower Cretaceous Yingcheng volcanic reservoir can be classified into three types. Type I pore structure primarily occurs in the volcanic breccia, crystal tuff and tuffites. The corresponding pore system is dominated by primary intergranular pores, intragranular dissolved pores and devitrified micropores with porosity and permeability higher than 5.4% and 0.05 × 10⁻⁶ µm², respectively. Corresponding mercury intrusion curves are characterized by low displacement pressure, high maximum mercury saturation and mercury withdrawal efficiency. Type II pore structure is commonly observed in the tuff and tuffites, while primary intergranular pores and secondary micropores dominate pore system with porosity of 2.5–5.4% and permeability of 0.012–0.05 × 10⁻⁶ m². The mercury intrusion curves are similar to the type I. Type III pore structure is also pervasively developed in the crystal tuff and tuffites. Intragranular dissolved micropores are most important ones with porosity and permeability less than 2.5% and 0.01 × 10⁻⁶ m², respectively. Different from type I and II pore structure, the mercury intrusion curves of the type III have high-displacement pressure, low maximum mercury saturation and mercury withdrawal efficiency, indicating poor reservoir quality.