Hydrocarbon Generation Kinetics of a Heterogeneous Source Rock System: Example from the Lacsutrine Eocene-Oligocene Shahejie Formation, Bohai Bay Basin, China

Source rock heterogeneity is a common and important feature that requires attention in shale oil resource evaluation. The Eocene-Oligocene Shahejie Formation in the Jiyang Superbasin, Bohai Bay Basin, has been studied to investigate vertical variations in source rock characteristics and their thermal decomposition behaviors in relation to lacustrine shale oil resource potential. Fifty-four core samples from Luo 69 Well were analyzed using Rock-Eval 6 equipment, and the resulting FID (flame ionization detector) hydrocarbon pyrograms were examined to study their generation kinetics. Differences in bulk geochemical characteristics, petroleum generation capacities, and petrophysical properties allow for subdivision of the samples into three "homogeneous" groups: (a) a stratigraphically higher facies, corresponding to samples with HI value around 600 mg HC/g TOC and high S1/TOC (due to either exogenous hydrocarbon input or thermally less stable kerogen, or both); (b) stratigraphically a facies in the middle of the section, representing thermally more stable facies group with similar generation potential, but much lower S1/TOC; and (c) a stratigraphically lower facies, corresponding to source rock with low TOC (<2%) and HI (<350 mg HC/g TOC) and high S1/TOC, and thermally less stable organic matter. Such a grouping is consistent with a statistical classification by Principal Component Analysis (PCA) based on the Rock-Eval data set. Bulk generation kinetic parameters from samples in the higher and lower stratigraphic groups seem to be affected by the high SI-associated heavy hydrocarbon residues in default S2 temperature regime. Although the unaffected samples in the middle of the section show "homogeneous" bulk geochemical characteristics in general, the temperature of onset (and offset) of major hydrocarbon generation can vary up to 15 degrees C among the samples, indicating a kinetically heterogeneous source rock. For a geothermal gradient of 30 degrees C/km, this translates to a difference of 500 m in burial depth for the onset of hydrocarbon generation for the same source rock unit in the extreme case.