Gas-in-place content and occurrence state of deep shale gas in the Luzhou area, Sichuan Basin, China

Abstract

The exploration and development of deep shale gas has made significant breakthroughs, but due to the characteristics of deep shale gas, there is no recognized and effective method of evaluating gas-bearing. To effectively determine the gas-in-place (GIP) content and occurrence state of deep shale gas, a total of 909 shales were collected for the study utilizing pressure-holding and conventional coring technologies from Wufeng-Longmaxi Formations in the Luzhou area, Sichuan Basin, China. The GIP content of the above shales was evaluated using various methods, including the USBM, polynomial fitting, ACF, MCF, process analysis and carbon isotope fractionation (CIF) methods. The accuracy of the above methods was verified through comparison with the gas content measured by pressure-holding coring. The results demonstrate that (1) the degassing content obtained from conventional coring ranges 0.06–2.13 cm3/g, averaging 0.64 cm3/g; the measured gas content obtained from pressure-holding coring ranges 2.19–11.91 cm3/g, averaging 7.16 cm3/g. The gas content obtained is significantly higher because pressure-holding coring samples have excellent sealing and minimal lost time. (2) The GIP content is underestimated by the USBM method, while the polynomial fitting, ACF, and MCF methods overestimate it. The process analysis and CIF methods can evaluate the GIP content of deep shales more accurately. Based on the comparative results of the adsorbed gas ratio calculated using the process analysis and the CIF methods (33.49% and 24.53%, respectively), it is recommended that the CIF method be given priority when evaluating deep shale gas-bearing. (3) The GIP content of deep shales in the Luzhou area calculated by the CIF method ranges 2.52–11.99 cm3/g, averaging 7.19 cm3/g; the adsorbed gas ratio accounts for 3.41%–53.32%, averaging 24.53%; and the lost gas ratio accounts for 54.01%–97.45%, averaging 86.38%. Compared with medium and shallow shale gas reservoirs in other areas, deep shale gas reservoirs in the Luzhou area have gas-bearing characteristics of high GIP content, low adsorbed gas ratio and high lost gas ratio, which restricts the applicability of traditional evaluation methods.