Kinetic pyrolysis simulation of hydrocarbon generation in shale system: A case study on Pearl River Mouth Basin, China

Pyrolysis experiments were performed in a closed system of sealed gold tube using both whole rock and its kerogen separated from the low maturity carboniferous shale of the Lower Oligocene Enping Formation in the Pearl River Mouth Basin, China. The pyrolysis temperatures range from 373.5 to 525.0 ℃ and from 343.0 to 489.2 ℃ with heating rate of 20 ℃/h and 2 ℃/h, respectively. The pyrolysis pressure keeps constant at 24.1 MPa. The gaseous hydrocarbons released in pyrolysis have been analyzed for their yields and chemical compositions. The simulation data were used to calculate kinetic parameters(activation energy distribution and pre-exponential factor) in the generation of gaseous hydrocarbons by software Kinetics 05. The results showed that the yields of CH4 from whole rock and its kerogen continually increase with increasing heating temperatures. Whereas, the yields of C2H6, C3H8 and C4H10 firstly increased at the early stage of kerogen cracking but decreased thereafter. The whole rock shows significantly lower yields of gaseous hydrocarbons and lower conversion rate of C2–C4 than its kerogen at the same temperature. But whole rock showed conversion rate of CH4 similar to its kerogen. At the same temperatures, the yields and conversion rate of gaseous hydrocarbons at the heating rate of 2 ℃/h are higher than those at the heating rate of 20 ℃/h. Whole rock and its kerogen show similar distributions of activation energies in the generation of CH4, C2H6, C3H8 and C4H10, ranging from 52×4.1840 kJ/mol to 64×4.1840 kJ/mol. Hydrocarbon generation kinetics of shale and its kerogen show that gaseous hydrocarbon is formed by kerogen cracking at two temperature intervals of 135.0–165.0 ℃ and 165.0–190.0 ℃, respectively. Similarly, gaseous hydrocarbon is formed by cracking of liquid hydrocarbon at two temperature intervals of 190.0–225.0 ℃and 225.0–260.0 ℃. The continuous increase of the yield of methane is accompanied by the decrease of heavy hydrocarbons at the late stage of pyrolysis. It could be related to the decomposition of high carbon number hydrocarbons that are generated at the early stage of hydrocarbon generation. The kinetic parameters of gaseous hydrocarbon generation had been extrapolated to the geological conditions of the Pearl River Mouth Basin. It suggests that the Pearl River Mouth Basin started to generate gaseous hydrocarbon by kerogen cracking at ca. 19.6 Ma, and it is currently at the stage of kerogen cracking.