Evaporative Fractionation As the Important Formation Mechanism of Light Oil Reservoirs in the Dongying Depression, NE China

Light oil, a high-quality energy resource abundant in deep basins, is prevalent in the northern zone of the Dongying Depression. To elucidate the formation mechanism of light oil reservoirs, this study investigates the molecular and stable isotope composition, biomarkers, light hydrocarbons, and diamondoid compositions of petroleum. The results reveal that the gas primarily consists of oil-cracking gas from a late filling event, mixed with oil-associated gas generated during the source rock’s “oil window” maturity phase. Methane exhibits enriched light carbon isotopes, indicating noticeable migration fractionation effects. The crude oil in the same deep strata exhibits high maturity, originating from both terrestrial and aquatic organic matter in the source materials. Molar proportions of n-alkanes and light hydrocarbon indices (Tol/nC7, nC7/MCH) indicate significant evaporative fractionation in the petroleum reservoirs. This fractionation process modified early-formed oil reservoirs due to the late filling of highly mature gas reservoirs. The evaporative fractionation at different stages has varying effects on the diamondoid ratio (1- + 2-MA)/(3- + 4-MD). It is considered a pivotal mechanism in the formation of deep condensate reservoirs and volatile oil reservoirs.