Kinetic analysis of hydrocarbon generation components in shales from the Bonan sub-Sag and its significance for hydrocarbon exploration

The Paleogene shales in the East China contain Type I organic matter comprised mainly of planktonic algae. A kinetic analysis of the hydrocarbon generation of this source rock was made using a pyrolysis-gas chromatograph. The hydrocarbon generation potential, composition and oil saturation of shales in the Bonan sub-Sag were studied in the context of the thermal history of the Shahejie Formation. The hydrocarbon conversion rates of light and heavy hydrocarbons in source rocks in the middle section of the third member of Shahejie Formation (Es³) are 31.1% and 43.8%, respectively, indicating the oil generation period. The oil saturation index might reach 273.24 mg/g, showing some shale oil potential. However, the oil is too heavy for exploitation. The lower section of Es³ (and the upper section of Es⁴) has a high oil saturation up to 571.17 mg/g and a high elastic force with coexisting oil and gas, and hence is an excellent target for shale oil exploitation. The middle and lower sections of Es⁴ arrived at the peak of methane generation. Any residual oil would have been cracked. Shale gas should be the main exploration focus unless an overpressure system developed.