New Evidence of Fluid Inclusions for the Chemical Reaction Pathways for the Generation of Methane in Highly Mature Gas-Pools from C2–C4 Hydrocarbons

Focused on the significant scientific question regarding the chemical reaction pathway of methane in high maturity gas-pools generation from C2–C4 hydrocarbons, the fluid inclusions trapped within hydrothermal quartz which developed in Dengying Formation were analyzed. The paleo-oil and gas pools of the Dengying Formation exhibit high maturity, with the crude oil having transformed into methane-dominated gas pools completely. The selected fluid inclusion assemblage (FIA) consisted of one pyrobitumen inclusion and less than 20 methane inclusions. This suggests that the FIA was formed through the cracking of precursor oil-H2O immiscible inclusions. Importantly, the FIA track was confined to the interior of the host mineral quartz, indicating that the thermal cracking of oil occurred within a closed system inside the quartz. The composition of the methane inclusions primarily consisted of CH4, with trace amounts of CO2, H2S, and aromatic hydrocarbons. The absence of pyrobitumen in methane inclusions refutes the notion that methane in highly mature gas pools is directly generated by the fracture of carbon–carbon bonds through C2–C4 hydrocarbons. The presence of aromatic hydrocarbons in the methane inclusions suggests that methane in highly mature gas pools may be generated through the demethylation of aromatic ring systems.