Abstract:

Gas composition and carbon isotopes of hydrocarbon gases thermally simulated from cores (mudstone, oil shale and coal) were compared with gases from gas hydrate at gas hydrate-bearing intervals in the drilling of DK-2 and DK-3 in Qilian Mountain permafrost. The purpose is to explore possible links between these gas source rocks and gas sources of gas hydrates. The results indicate that thermally simulated gases are mainly composed of CO₂ with a small amount of hydrocarbon gases on the low temperature conditions (＜300 ℃); the yields of hydrocarbon gases from the mudstone, oil shale and coal gradually increase, displaying differences between adsorbing gas with different rock. With increasing of thermal simulation temperatures, yields of hydrocarbon gases from the mudstone, oil shale and coal obviously increase and reach maximum values at 500 ℃; conversely, yields of CO₂ vary a little. With increasing of thermal simulation temperatures, carbon isotope values of hydrocarbon gas firstly become lighter and then heavier and are characteristics of normal carbon isotopic series of δ¹³C₁ ＜δ¹³C₂＜δ¹³C₃. The consequence suggests that hydrocarbon gases from mudstone in the range of 350 ℃ to 400 ℃ or from oil shale in the range of 380 ℃ to 400 ℃ have similar features of gas composition and carbon isotope with gas hydrate, indicating that the gas source of gas hydrate has geochemically genetic relation with deep mudstone or oil shale. In contrast, the gas composition of hydrocarbon gases from coal and gas hydrate are similar, but their carbon isotope values are incomparable, suggesting that the relationship between coal and gas source of gas hydrate is not closely associated.

Key words: Qilian Mountain permafrost, gas hydrate, thermal simulation, gas component, carbon isotope