Origin and Alteration of Deep Heavy Oil in the Dongying Depression, Bohai Bay Basin

This study conducts a comprehensive geochemical analysis of natural gas, crude oil, and mudstone to investigate the origin and alteration of recently obtained deep heavy oil from the Dongying Depression, Bohai Bay Basin. High contents of β-carotene, gammacerane (gammacerane index = 4.41), and dibenzothiophene (71.22%) and the low value of pristane/phytane (0.32) suggest that the deep heavy oil is mainly generated from in situ source rocks that formed in saline and reducing environments. According to the molecular maturity indicators, the deep heavy oil is at a low maturity level (%Ro ≈ 0.58). The occurrence of complete series n-alkanes and the absence of 25-norhopanes, normal C7 ratio K1 (1.0), high ααα(20R)-C29 sterane (8394.22 μg/g) and low (3- + 4-) methyldiamantane (41.81 μg/g) concentrations, along with the high values of toluene/nC7 (2.73) and diamantine/adamantane (16.6) in the deep heavy oil, suggest that the reservoir did not undergo noticeable biodegradation, TSR, or thermal cracking, but suffered from phase fractionation, respectively. The formation of the deep heavy oil is associated with its low maturity, which is further thickened by phase fractionation due to the recharge of excessive gas. The moderate reservoir temperature (90–150 °C) after charging performs an essential function in the preservation of the deep heavy oil by inhibiting both biodegradation and thermal cracking.