Tectono-thermal evolution and gas source potential for natural gas hydrates in the Qilian Mountain permafrost, China

Abstract

The gas source of natural gas hydrates in the Qilian Mountain permafrost was determined based on analyses of the source rocks using model simulations and observed data. The thermal history, maturity history, and hydrocarbon generation/expulsion history of the basin were modelled in an artificial well using the BasinMod 1D software. The modelled geothermal gradient presents a gradual increase from the Permian to the mid-Jurassic (29–51 °C/km) and then a decrease to present day (51–31 °C/km). The mid-Jurassic geothermal fields were shown to play an essential role in the evolution of source rock maturity, with the peak of maturity occurring in the late period of the mid–Jurassic. The Upper Triassic Galedesi Formation is the most important source rock in the South Qilian Basin and experienced two periods of oil generation during the Late Triassic and mid-Jurassic and one period of gas generation during the mid–Jurassic, but only one period of oil and gas expulsion during the mid-Jurassic. The natural gas hydrates in the Qilian Mountain permafrost include relatively high concentrations of methane, ethane and high carbon-number alkanes (e.g., propane, butane); a small amount of heavy hydrocarbons that are heavier than C6; and some CO2 and N2. These compounds are consistent with the characteristics of pyrolysis and wet gas. The gas source of natural gas hydrates in this area was found to come from the deep source rocks of the Upper Triassic Galedesi Formation based on measurements and modelling. This study also reveals that the South Qilian Basin not only contains natural gas hydrates, but may also be a potential oil and gas basin, and hence, further studies may be worthwhile to identify oil and gas resources in the South Qilian Basin.