Petroleum source and accumulation of WZ12 oils in the Weixi'nan sag, south China sea, China

Source rocks are found in the Eocene Liushagang Formation (El(1: bottom), El(2) and El(3:top) members), in the Weixi'nan sag, Beibuwan Basin. The source rocks are mainly mudrocks, except El(2), which consists of organic-rich shales at the bottom and mudrocks in the middle-upper part. Total organic carbon (TOC) and Rock-Eval analyses of 146 samples indicate that El(2) rocks have the best hydrocarbon generating potential, especially for organic-rich shales that contain up to 10% TOC with HI > 800 mg/g. The vitrinite reflectance analysis suggests that El source rocks are marginally mature to mature. Geochemical analyses were conducted on 18 El rock extracts and 18 oil samples from the WZ12 field. El(2) and El(3) source rock have similar organic facies and are distinct from El(1). El(2) and El(3) have higher C-27 beta alpha (20R + 20S) diasteranes/C-27 alpha beta beta (20R + 20S) steranes (C-27 DiaS/C27S), Gammacerane/Hopane (G/H) and (C-28 + C-29) tricyclic terpane/((C-28 + C-29) tricyclic terpane + Ts) (ETR), and low C-23 tricyclic terpane/hopane (C-23/H) and C-24 tetracyclic terpanes/C-26 tricyclic terpane (C-24/C-26). These biomarker distributions indicate that El(2) and El(3) rocks were deposited under more alkaline-saline environment and had less import of terrestrial organic matter relative to El(1). El(2) can be distinguished from El(3) as having higher 4-methylsteranes/C-29 aaa steranes (4MSI) and G/H. WZ12 oils all exhibit similar compositions and display biomarker characteristics that correlate with the El(2) and El(3) rock extracts. 1D thermal and burial history model shows that El(2) and El(3) began to generate a great deal of hydrocarbon at similar to 11.5 Ma and 23 Ma, respectively. At this stage, traps formed and faults actively extended downwards into the El(2) and likely into El(3). Oils expelled from El(2) and El(3) migrated upward into shallow intervals along faults then laterally along the structural slopes and accumulated in areas with good fault sealing primarily in the Miocene age Nx and Nj strata. Meanwhile, under the effect of overpressure in the middle of organic-rich shales, oils from El(2) may have migrated downward into reservoirs in the upper part of El(3). Lithologic traps in El(1) reservoir may be charged with relatively immature oil derived from El(1).