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

Abstract

Abstract Source rocks are found in the Eocene Liushagang Formation (El1: bottom, El2 and El3:top members), in the Weixi'nan sag, Beibuwan Basin. The source rocks are mainly mudrocks, except El2, 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 El2 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. El2 and El3 source rock have similar organic facies and are distinct from El1. El2 and El3 have higher C27 βα (20R + 20S) diasteranes/C27 αββ (20R + 20S) steranes (C27DiaS/C27S), Gammacerane/Hopane (G/H) and (C28 + C29) tricyclic terpane/((C28 + C29) tricyclic terpane + Ts) (ETR), and low C23 tricyclic terpane/hopane (C23/H) and C24 tetracyclic terpanes/C26 tricyclic terpane (C24/C26). These biomarker distributions indicate that El2 and El3 rocks were deposited under more alkaline-saline environment and had less import of terrestrial organic matter relative to El1. El2 can be distinguished from El3 as having higher 4-methylsteranes/C29 ααα steranes (4MSI) and G/H. WZ12 oils all exhibit similar compositions and display biomarker characteristics that correlate with the El2 and El3 rock extracts. 1D thermal and burial history model shows that El2 and El3 began to generate a great deal of hydrocarbon at ∼11.5 Ma and 23 Ma, respectively. At this stage, traps formed and faults actively extended downwards into the El2 and likely into El3. Oils expelled from El2 and El3 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 El2 may have migrated downward into reservoirs in the upper part of El3. Lithologic traps in El1 reservoir may be charged with relatively immature oil derived from El1.