Kinetic study of marine and lacustrine shale grains using Rock-Eval pyrolysis: Implications to hydrocarbon generation, retention and expulsion

The kinetic parameters of marine and lacustrine shale grains (4 mm) were retrieved by using Rock-Eval pyrolysis in comparison with finely ground powder (<0.178 mm) and kerogen of same samples. Results of grains show remarkable differences from powder and kerogen. Grains of Pingliang marine shale exhibit a relatively broader distribution of activation energies than powder and kerogen while grains of Yanchang lacustrine shale show higher dominant activation energies than powder and kerogen. At laboratory heating rates (5-25 °C/min), the corresponding temperatures to the maximum hydrocarbon generating rate of grains are 3-8 °C higher than powder and kerogen for marine shale and 6-8 °C higher for lacustrine shale, respectively. Extrapolated to geological heating rate (3 °C/my), the corresponding maturity and geological temperature to the maximum hydrocarbon expulsion rate of grains lags powder 0.02 Ro% and 3 °C, as well as 0.05 Ro% and 6 °C for marine shale and lacustrine shale, respectively. After the peak of hydrocarbon generation (Ro = 1%), the retention percentage for grain and powder of marine shale reach 7.33% and 0.09% while those for lacustrine shale reach 16.50% and 10.85%, respectively. These results suggest grains enjoy higher expulsion threshold and higher retention ability. The results suggest that Yanchang lacustrine shale exhibits stronger retention ability and weaker expulsion ability than Pingliang marine shale. The results presented in this study show that grain-based pyrolysis provides a novel method for evaluating the residual oil and gases for shale, which can study the hydrocarbon generation, expulsion and retention comprehensively.