Petroleum generation kinetics for Permian lacustrine source rocks in the Junggar Basin, NW China

The Junggar Basin is a major oil producing province in China. Most of the oil originated from Permian lacustrine oil-prone source rocks in the basin. Kinetic parameters for oil and gas generation were obtained from confined pyrolysis experiments using gold capsules at heating rates of 20 °C/h and 2 °C/h on kerogen samples from two Permian lacustrine oil-prone source rocks (J23S3 and SS1S3) having hydrogen indices of 663 and 698 mg HC/g TOC, respectively. The kinetic properties for oil and gas generation vary substantially between these two source rocks. Assuming a burial heating rate of 5 °C/My, the temperature for 50% transformation ratio to oil is 132 °C for J23S3 and 168 °C for SS1S3. The maximum oil yields are about 604 and 770 mg/g TOC, while the maximum yields of total gaseous hydrocarbons are about 306 and 348 mg/g TOC for J23S3 and SS1S3, respectively. Gaseous hydrocarbons from the confined pyrolysis experiments include primary cracking components from kerogen and secondary cracking components from the generated oil. The secondary cracking gaseous hydrocarbons comprise up to 84.9% and 95.1% of the total, respectively, for the two source rocks in the highest temperature experiments. A practical method estimates the amount of gaseous hydrocarbons generated from these source rocks under geological conditions based on: (1) kinetic parameters for gas generation obtained from kerogen confined pyrolysis experiments in the present study, (2) the maximum yield of gaseous hydrocarbons from oil confined pyrolysis experiments from previous studies, and (3) the maximum amount of expelled oil. The maximum amounts of expelled oil range from 509-588 and 724-750 mg/g TOC with oil expulsion efficiencies in the ranges 84.3-97.4% and 94.0-97.4% for J23S3 and SS1S3, respectively, assuming that the amounts of retained oil range from 30-160 and 50-110 mg/g TOC for the two source rocks after oil expulsion based on the amounts of extracted bitumen. Consequently, the maximum yields of total gaseous hydrocarbons could be in the ranges 53.1-87.0 and 25.5-36.8 mg/g TOC, respectively, for J23S3 and SS1S3 in nature. Using the kinetic parameters for these two source rocks determined from pyrolysis experiments, oil and gas generation and expulsion were modeled for source rocks within the Lower Permian Fengcheng (P1f) and Middle Permian Lower Wuerhe formations (P2w) in the central area of the Mahu Depression, the source kitchen for most oilfields in the basin.