Evaporative fractionation and biodegradation impacts on a complex petroleum system: QHD29-2 oil field, Bohai Sea area

By studying the tectonic background of QHD29-2 oil field and the geochemical characteristics of oil and gas and its source rocks, the dissimilarity of crude oil physical properties was discussed from the perspective of evaporative fractionation and biodegradation. Oil source comparison confirmed that the oils in Paleogene and Neogene had the same source. However, there were significant differences in light hydrocarbon composition, family composition and the chromatographic distribution of saturated hydrocarbons, indicating that they had experienced a strong evaporative fractionation. Combined with the analyses of source rock thermal evolution, accumulation stage and fault activity history, it was believed that the intrusion of mantle-derived CO₂ gas had caused evaporative fractionation in the area. The evaporative fractionation and biodegradation jointly controlled crude oil physical properties in the reservoir. Evaporative fractionation and biodegradation were the main factors controlling the differences of crude oil physical properties in reservoirs, and the differences were mainly controlled by evaporative fractionation, whereas, the oil in shallow reservoirs was controlled by both evaporation and biodegradation. Extra-heavy, light, condensate and heavy oils distributed in sequence from deep to shallow reservoirs. The two mechanisms had an impact on the same reservoir simultaneously. The early reservoirs in deep formations were affected by evaporative fractionation. After the shallow reservoirs of the post-accumulation were biodegraded, they were continuously charged in the late stage of the pre-existing reservoir.