Nitrogen-rich gas shale logging evaluation and differential gas-bearing characterization of lower Cambrian formation in northern Guizhou, south China

Abstract

Abstract There are significant differences in the gas content between the two sets of lower Paleozoic shale in south China. The nitrogen content of the lower Silurian shale gas that has been commercially developed is very low, whereas that of the lower Cambrian shale (LCS) is relatively high. Shale has complex gas-bearing mechanistic characteristics. The genesis of nitrogen is complex and sources are diverse. A gas containing high nitrogen and low hydrocarbons increases the risk and difficulty of developing a shale gas resource. Based on the key geological parameters of the LCS in the northern Guizhou area (NGA), this study investigates the characteristics of nitrogen in the highly mature shale and discusses the logging evaluation method for nitrogen-bearing shale gas reservoirs in this complex tectonic area. Changes in the gas-bearing characteristics of the LCS in the study area are complex. The shale gas composition was found to vary widely due to differential enrichment; the methane content was between 1.2% and 82.4%, and the nitrogen content was between 0.03% and 97.4%. The nitrogen-rich shale formations exhibited a high compensation density (2.46–2.58 g/cm3), high compensation neutron (10.2–17.9%), and low acoustic time difference (192–230 μs/m). There were two different types of resistivity responses in nitrogen-bearing shale gas reservoirs, which had distribution characteristics of 21–56 Ω m and 100–1000 Ω m. With these response characteristics, the logging evaluation of nitrogen-bearing shale gas reservoirs was interpreted by selecting differential gas-sensitive parameters, and different gas-bearing types were divided into dry, nitrogen, mixed, and methane gas layers. Furthermore, the identification and computational evaluation of the gas composition (methane/nitrogen) and content were carried out quantitatively. Differential gas-bearing property was characterized and that is beneficial to the exploration practice in the study area.