Geochemical characteristics of the extremely high thermal maturity transitional shale gas in the Southern North China Basin (SNCB) and its differences with marine shale gas

Abstract

A great number of studies have been carried out on the geochemical characteristics of marine shale gas from the lower Silurian Longmaxi Formation in the Sichuan Basin. However, a systematic study on geochemical and isotopic characteristics of transitional shale gas (gas from the alternating marine-terrigenous shale facies) has not been well addressed. In the present study, transitional shale gas from the lower Permian Shanxi (P1s) and Taiyuan (P1t) Formation in the Southern North China Basin (SNCB) were collected and analyzed for their geochemical and isotopic compositions. The hydrocarbon gases are dominated by methane, with small amounts of ethane, without propane and butane, which is consistent with the extremely high thermal maturity of the gas shales (RO% values between 3.2 and 4.2%). Compared with marine shale gas, the relatively high non-hydrocarbon components may be associated with the humic source rocks. The δ13C1 values range from −31.6‰ to −21.0‰ and the δ13C2 values range from −35.9‰ to −26.1‰, the δ2HCH4 values range from −221‰ to −138‰. These results indicate that the gases are of thermogenic origin, methane with anomalous heavy carbon isotopic and light hydrogen isotopic values was interpreted as redox reactions of gases with transition metals and water at maximum burial resulting in Rayleigh-type fractionation. The difficulties of identifying marine and transitional shale gas may be due to the complete or partial carbon isotopic reversal caused by secondary alteration and the extremely high thermal maturity of the gas shales. CO2 concentration and δ13C(CO2) values suggest that CO2 in the transitional shale gas was mainly thermogenic origin and formed together with hydrocarbon generation. Furthermore, significant differences can be observed from the CO2 in different types of shale gas, CO2 in the transitional shale gas is mainly distributed in areas with CO2 > 5% and δ13C(CO2) < −8‰, while CO2 in the marine shale gas is mainly distributed in areas with CO2 < 5% and δ13C(CO2) > −8‰. The δ15N(N2) values and high illite and illite-smectite mixed clay (I/S) content suggest that nitrogen in transitional shale gas was possibly generated during thermo-ammoniation of organic matter and/or the breakdown of NH4-rich clay minerals. In addition, based on the geochemical characteristics of marine and transitional shale gas, a geochemical pattern of shale gas is presented in this paper.