Quantitative calculated shale gas contents with different lithofacies: A case study of Fuling gas shale, Sichuan Basin, China

Abstract

To accurately evaluate pore structure and gas content characteristics of shale reservoir, a series of tests, including XRD, TOC, porosity, FE-SEM, N2 and CO2 adsorption, and high-pressure methane sorption, were conducted on the Wufeng-Longmaxi shale in the Fuling area. The principal objective of this work is to reveal the development mechanisms of shale pores, and quantitatively calculated both free gas and absorbed gas contents in different lithofacies. A total of three main lithofacies, i.e., clay-rich siliceous shale lithofacies (S-3), argillaceous/siliceous mixed shale lithofacies (M-2) and silica-rich argillaceous shale lithofacies (CM-1) were identified in the study area. The highest values of TOC, helium porosity, pore volume, and specific surface area are recognized in the S-3 shales, with a mean of 3.94%, 5.12%, 22.13 × 10−3 cm3/g, and 35.23 m2/g, respectively. The pore volume and specific surface area of S-3 shales are primarily contributed by OM pores, with an average proportion of 66.99% and 66.38%. However, the pore volume and specific surface area of clay-related pores, a significant component of CM-1 shales, occupied an average of 64.30% and 52.72%, respectively. Owing to the change of lithofacies from S-3 to M-2 and CM-1 shales, the pore volume is reduced by 28.74% and 22.37%, while the specific surface area is reduced by 30.28% and 32.89%, respectively. The total shale gas contents of S-3 shales are calculated to be 5.54–7.72 m3/t, with an average of 6.64 m3/t, and that of M-2 shales and CM-1 shales are ranging from 4.28 m3/t to 5.03 m3/t and 2.43 m3/t to 4.33 m3/t, with a mean of 4.61 m3/t and 3.51 m3/t, respectively. However, due to the difference of pore types, the CM-1 shales are characterized by the highest proportion of free gas (average 59.08%), whereas that of M-2 shales and S-3 shales are recognized with 54.96% and 50.93%, respectively.