Petroleum migration and accumulation in the Liuchu area of Raoyang Sag, Bohai Bay Basin, China

Abstract This study focused on vertical migration of petroleum in the Liuchu area, China. The physical properties of petroleum and changes in the geochemical characteristics of the study area were also investigated. Biomarkers such as, nitrogen compounds, n-alkanes, triterpenoids, steroids, hopanes, and cholestanes were analyzed in this regard. Petro-physical characteristics, such as, density, viscosity, concentrations of wax, resin, and sulfur of oil were studied. The rNa+/rCl− ratio of Ed2 formation water ranged from 0.25 to 5.7, oil density of Ed2 formation ranged from 0.85 to 0.95 g/cm3, and oil resin concentration 10–50%, oil wax content in Ed2 formation ranged from 2 to 29% were lower than that of Es1. The rNa+/rCl− ratio and mineralization of formation water data were examined to demonstrate the migration pathways. Geochemical and crude oil physical indexes were utilized to investigate the relationship between petroleum migration and seismic data. Cholestane αββ, ααα cholestane, diacholestane, steroid, and low ring terpenes in the formation were utilized to trace petroleum migration. Recent studies on petroleum maturity of have focused on comparison of source rocks and have reported that the maturity effects of source rock and reservoir oil are weak while those effects are high in the Liuchu aera. Analysis of triterpenoid and steroid hydrocarbon biomarkers elucidate the underground petroleum migration pathways to the reservoirs. Geochemical analyses of hopane and cholestanes were used. The ααα C29 (20S)/(20S + 20R) and C29 (ββ)/αα + ββ) ratios ranged from 0.2 to 0.35 Further, C31 (22S)/(22S + 22R), C32(22S)/(22S + 22R), and C29 (20S)/(20S + 20R) ratios ranged from 0.3 to 0.8, and 0.131 and 0.81, respectively. Petroleum migrated along migration channels. This suggested that vertical migration occurred after formation of the source rock fault and lateral migration occurred along the sandbody. Reservoir sandstone content, overlying mudstone content, and thickness of caprock are likely limited lateral petroleum migration. Petroleum migration and accumulation primarily occurred by vertical migration and lateral migration was weak. The results of this study enabled optimization of uncertain petroleum migration pathways and improved the petroleum migration and accumulation model.