Quantitative characteristics of nanoscale pores in mafic-intermediate volcanic rock reservoirs in the Changling fault depression, Songliao Basin

Abstract

Tight volcanic reservoirs can have porosities of 2–8%, and the pore space primarily consists of nanoscale pores. Thin section observations, scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and low-temperature nitrogen adsorption (LTNA) were used to qualitatively and quantitatively characterize the nanoscale pore size distribution (PSD). The MIP results show that the nanoscale pore diameters in the mafic-intermediate volcanic rocks mainly range from 4 to 100 nm. Moreover, different lithologies have different PSD characteristics. The LTNA results indicate that the nanoscale pore morphologies are mainly the narrow slit type and irregular type. Our results suggest that nanoscale pores are favorable for gas accumulation. Methane adsorption theory indicates that the lower pore diameter limit for methane adsorption is 18.5 nm and the lower filling porosity limit in the volcanic rock reservoir is 2.80%. Based on the PSD and gas testing results, a model of nanoscale pores’ PSD and gas-bearing properties is established.