Indicative significance of biomarker parameters on crude oil cracking rate from the Dongpu Depression, Bohai Bay basin of North China

Molecular markers have a strong ability to maintain their own structure. The side chain fracture, isomerization and aromatization show regular response characteristics for the organic matter evolution process, which can measure oil maturity and identify oil cracking degree. In this research, 43 crude oil samples were collected for geochemical investigation in the Dongpu Depression from Bohai Bay Basin of North China. By the gas chromatography (GC) experiments of saturated hydrocarbon and aromatic hydrocarbon, a variety of molecular marker parameters were selected for quantitative evaluation of crude oil maturity and cracking rate, including pristane isomerization index (PIR), C29 sterane parameters, monoaromatic sterane, rearranged sterane (20 S/(20 S + 20 R)), hopane (Ts/(Tm + Ts)), methyl phenanthrene ratio (MPR), Methyl phenanthrene index (MPI), alkyl naphthalene, and alkyl dibenzothiophenes (DBTs). Thus, the crude oil cracking rate in the different regions of Dongpu Depression were calculated quantitatively. The results show that the increase of temperature, the pressure (<40Mpa) and thermochemical sulfate reduction (TSR) action promotes crude oil cracking, whereas the pressure (>40 MPa) generally inhibits the crude oil cracking. In addition, well Wei 20 does not experience prominent thermal history temperature and burial history pressure, but its oil cracking rate is higher than the average cracking rate curve value, suggesting other factors also have an important impact on crude oil cracking, which needs to be further discussed.