Assessment the gas potential of coal-bearing mudstones from upper paleozoic in ordos basin via gold-tube pyrolysis

Abstract

Previous natural gas explorations in Ordos Basin were conducted in areas with thick coal deposited. Recently, natural gas flow were discovered in southern basin where very thin coal seams were developed, implying the contribution from other source rocks, e.g. the coal measure mudstones. In order to thoroughly assess the quality and gas potential of coal measure mudstones, 58 mudstones from borehole were analyzed by Rock-Eval method and gold-tube pyrolysis was carried out on immature carbonaceous and dark mudstones with detailed geochemical characteristics of hydrocarbon products detected.The results indicated the mudstones were dominated by type III kerogen and assisted by type II kerogen and were fair to good source rocks. Pyrolysis data demonstrated the peak yield of C1-5 for carbonaceous mudstone was very close to that of coal, whereas the maximum C1-5 yield for dark mudstone accounted for 62.5% of that of carbonaceous mudstone, indicating the good gas potential of carbonaceous mudstone. The generated gaseous hydrocarbons were predominately derived from primary kerogen cracking.The computed activation energy of C1-5 for carbonaceous mudstone showed a more discrete distribution compared to dark mudstone. The modeled methane conversion for dark mudstone was higher than that of carbonaceous mudstone since entering main gas generation stage, which was consistent with the concentrated and lower activation energies for dark mudstone. Furthermore, the modeled gas in southern basin was much higher than that in central basin, due to the more severe thermal alteration that coal measures in southern basin had experienced.On the basis of the integrated analysis of methane generation and adsorption, the areas with thick carbonaceous mudstone deposited had fair exploration prospects and further gas explorations should pay more attention on.