Comprehensive assessment of source rocks in the Bohai Sea area, eastern China

ABSTRACT The Bohai Sea area, offshore of the Bohai Bay Basin, is one of the most petroliferous regions in China, with proven original oil in place of approximately 2.4 × 109 m3 (150.94 × 108 bbl) and proven original gas in place of over 5 × 1012 m3 (1.76 × 1013 ft3). Cumulative oil production is over 50 million tons (3.5 × 108 bbl). In this study, using the limited data on source rock thickness, core samples, and Rock-Eval pyrolysis along with sedimentary facies analysis, source rock characteristics of different depositional settings were identified, and the thickness, richness, organic matter type, and thermal evolution of four sets of source rocks in the Bohai Sea area— the second member of Dongying Formation (E3d2), the third member of Dongying Formation (E3d3), the first and second members of Shahejie Formation (E2s1-2), and the third member of Shahejie Formation (E2s3)—were predicted and evaluated. Subsequently, the intensity and history of hydrocarbon expulsion for different sags was systematically compared and analyzed. The greatest thickness of the four sets of source rocks in the Bohai Sea area is 400–800 m (1300–2600 ft). The average richness of the organic matter of these source rocks is 1.74%–2.87%. The E2s3 set has the highest organic matter abundance; E2s1-2 has the lowest. The organic matter of these source rocks is mainly type I and type II, but their evolutions differ. The vitrinite reflectance of E3d2 is 0.5%–1.0%, that of E3d3 is 0.7%–1.25%, that of E2s1-2 is 0.75%–1.75%, and that of E2s3 is 0.75%–2.0%. The cumulative hydrocarbon expulsion of the four sets of rocks is 4.14 × 1010 t (2.90 × 1011 bbl). The E2s1-2 set has the greatest expulsion amount: 1.75 × 1010 t (1.22 × 1011 bbl). The peak stages of hydrocarbon expulsion of the four sets of source rocks were during Neogene Minghuazhen Formation (12.2–2.0 Ma) and Neogene Guantao Formation (16.6–12.0 Ma). The Bozhong sag expelled the most hydrocarbons, followed by the Liaozhong, Qikou, and Huanghekou sags.