Geochemical characteristics of the Paleozoic natural gas in the Yichuan-Huanglong area, southeastern margin of the Ordos Basin: Based on late gas generation mechanisms

Abstract

As a new expansion area for natural gas exploration, the Yichuan-Huanglong area improves the complete evolutionary sequence of the Paleozoic natural gas in the eastern part of the Ordos Basin, but the geochemical characteristics and the late gas generation processes involved have not been discussed. By collecting and analyzing gas samples from existing wells, the following insights were obtained. The alkane components had a dichotic characteristic, the wetness of the O1m5–C2b natural gas (average: 3.85%) was relatively higher than that of the Permian natural gas (average: 0.88%). The carbon isotopic compositions exhibited partial reversal (δ13C-CH4 >δ13C-C2H6), and the Permian natural gas from the coal-measure strata had abnormally low δ13C-C2H6 values (<−28‰). The O1m5–C2b natural gas had characteristics typical of oil-associated gas, whereas the Permian natural gas had the mixed characteristics of coal-derived gas and oil-associated gas. The mixing and diffusion effects illuminated the isotopic anomalies of the alkane gas. According to the source rocks and the thermal maturity, mechanisms such as the depletion of 13C in the residual ethane, water-reforming, and the Fischer-Tropsch-like mechanism, and the demethylation of aromatic hydrocarbons also provided important information about the late gas generation of the Paleozoic natural gas in the Yichuan-Huanglong area.