Generation Potential and Kinetics of Gaseous Hydrocarbons from Upper Paleozoic Carboniferous Benxi Formation Mudstone in Ordos Basin, China.

Industrial quantities of tight gas from the Carboniferous Benxi Formation have been found in the southern Ordos Basin. The source and contributions from mudstone and coal in the Benxi Formation to the tight gas are still unclear, and the hydrocarbon generation potential and kinetics of the Benxi Formation mudstone have rarely been reported, which has halted resource evaluation of tight gas. Confined pyrolysis experiments were performed to determine the yields and kinetic parameters for gaseous hydrocarbon formation for a representative sample of a Benxi Formation mudstone from the Ordos Basin, with a hydrogen index (HI) of 137 mg/g TOC and of 434 °C. The maximum yield of C-C hydrocarbons is 143 mg/g TOC. For samples with similar HI, Upper Paleozoic mudstone samples might have a higher total gas generation potential than Upper Paleozoic coal samples because of the more reducing environment during mudstone deposition, which is beneficial for the preservation of sedimentary lipids. Even though the HI of Upper Paleozoic coals is apparently higher than that of Upper Paleozoic mudstones, they have a similar late gas generation potential. Basin modeling shows that the amount of natural gas generated from Benxi Formation mudstone increases southward. Coaly source rocks from the Ordos Basin with a lower oil generation potential have more negative δC values when the vitrinite reflectance is lower than 1.4-1.7%. This phenomenon might be related to the more negative δC of wet gas or the small hydrocarbon molecules incorporated into the kerogen compared with the components generating primary cracking gases. This study deepens the understanding of gaseous hydrocarbons generated from mudstone and coal in transitional depositional environments and provides the key parameters for tight gas resources in this area.