Petroleum phase evolution at high temperature: A combined study of oil cracking experiment and deep oil in Dongying Depression, eastern China

Numerous light oil reservoirs have been discovered in the lower part of Mbr 4 of Paleogene Shahejie Fm in the Dongying Depression, Bohai Bay Basin, eastern China. The petroleum phases and reservoir-forming models are crucial to understanding the hydrocarbon accumulation mechanisms in the deep lacustrine layers. The deep formation temperature and pressure, the PVT phase diagrams, as well as the burial-thermal history were simulated and restored based on the results of oil cracking experiments and the analysis of measured data of geological sections in the Dongying Depression. The process and laws of hydrocarbon accumulation were summarized. The cricondentherm envelopes depict a series of variations when the molar percentages of CO2, CH4, C2-5, and C6+ in the fluid composition change. In the process of oil cracking, the cricondenbar increases with the increase in the molar percentage of CH4. With the increase in molar percentage of C2-5 and C6+, the cricondenbar decreases continuously. According to simulations of fluid composition and phase state, the oil reservoirs in this area are primarily oil-rim condensate gas reservoirs and volatile oil reservoirs. Based on burial thermal history recovered under the constraint of measured vitrinite reflectance (VRo), the homogenization temperature of fluid inclusions suggests two main periods of petroleum charging: in the late deposition of the Mbr 3 of the Paleogene Shahejie Fm and Guantao Fm in the Eocene. In the second charged stage, the oil cracking gas from deep places in the depression was injected into the primary oil and gas reservoirs, causing the changes and differentiation of fluid phases in the reservoirs. The gas invasion and evaporative fractionation caused by late natural gas filling are key factors for the formation of light oil reservoirs.