Unscrambling the source kitchen and hydrocarbon prospect information of the ultra-deepwater area in southern Pearl River Mouth basin, South China Sea: Insights from the first ultra-deepwater exploration well W21

Abstract

Recent breakthroughs in Pearl River Mouth basin (PRMB) regarding offshore hydrocarbon exploration have shifted the focus from deepwater area to ultra-deepwater area (>1500 m water depth), and the southern sub-sag of Baiyun sag, Liwan sag, and the Shunhe Uplift between them constitute the front area for PRMB ultra-deepwater hydrocarbon exploration. This study investigated the source-rock strata (i.e., Eocene Wenchang Formation and Enping Formation) and reservoir fluids (mainly CO2 with small amounts of oil and alkane gas) in the first exploratory well (namely W21, drilled at Shunhe Uplift), aiming to unscramble the source kitchen and hydrocarbon prospect information in this ultra-deepwater area. Biomarker and transgressive history analysis of source-rock strata jointly determined the Wenchang Formation and Enping Formation to be marine sediments in the Liwan sag, while marine and lacustrine sediments in the Baiyun southern sub-sag, respectively. The oil biomarkers and isotopes of alkane gas evidenced prospect for mature oil and oil-associated gas in this ultra-deepwater area, and the correlated source analysis determined the source rocks in the two sags have an advantage of aquatic organic input, indicating a good hydrocarbon-generating potential. Additionally, the regional distribution of terrestrial source-specific biomarkers (i.e., oleanane and bicadinanes) in PRMB deep to ultra-deep water areas traced the terrestrial source area and revealed a significant less input of terrigenous organic matter at the two ultra-deepwater sags. The isotopic analysis of reservoir non-hydrocarbon gases determined the CO2 to be mantle-derived magmatic origin. The present reservoirs are a result of displacement of early charged oils by posterior migrated CO2. In conclusion, this PRMB ultra-deepwater area has a considerable prospect for mature oil; however, the frequent volcanism since the Pliocene poses great risks to hydrocarbon exploration.