PEXMOD Technical Report: Starting Point report of Component C1, C2 & C3. Summarizing and integrating existing knowledge from completed analyses of ENRECA-3 samples - the starting point and working hypotheses

The Vietnamese Bach Long Vi Graben is located in the Gulf of Tonkin near the intersection between the Song Hong and Beibuwan basins and linking the two basins. Owing to very pronounced inversion tectonic movements, part of the deeply buried rift succession is exposed on the remote Bach Long Vi Island that delineates the crest of the intra-basinal Neogene inversion structure. The island and the sea-bottom around it expose oil-prone mudstones and offer a unique geological window into the otherwise deeply buried Paleogene syn-rift succession containing matured source rock facies in places in the two basins. In order to study in detail the depositional evolution of the syn-rift succession formed during the rift climax the ENRECA project decided to drill a stratigraphic research core well, EN-RECA-3, on the island in 2013. The well obtained 500 m of high-quality continuous cores consisting of lacustrine mudstones interbedded with various types of density flow facies that range from mm- and cm-scale, mud-rich density flow deposits to m-scale classic hybrid beds showing the typical bed motifs known from marine hybrid event beds. The mudstones form excellent to good oil source rocks with a cumulative thickness of minimum 233 m with an average TOC content of 2.91 wt.% and an average HI of 575 mg HC/g TOC. The inter-bedded sandstones constituting potential migration paths and reservoirs are quartz-rich, very fine to coarse-grained sandstone with dominantly angular–subrounded grains, mostly moderate to well-sorted arkoses with 10–22% gas porosity and Klinkenberg permeabilities of 0.1–38 mDarcy excluding the tight debrites. Bioturbation is almost absent except for tiny shallow burrows related to the gravity flows. Only freshwater palynomorphs are found and the Sulphur content is low. Soft sediment deformation structures including ball and pillow structures, mud-flames, sand injectites and deformed rip-up mud clasts are very common. Detailed biomarker analyses of mudstone laminae and mudstone clasts in density flow beds reveal generally no significant terrestrial organic input.

The analyses thus indicate that deposition occurred in a deep, elongated, narrow sub-siding freshwater graben system where mud accumulated on a soft, oxygen-poor lake bot-tom. Mud-rich flows and sand-dominated density flows possibly originated from graben margin aprons that owing to tectonically-induced slope instability delivered sediment flows that rapidly decelerated due to gradient change, high sediment concentration and the fluid mud prone lake floor. In combination with evidence from the studied seismic and well data it is suggested that source rock pods were mainly generated and preserved in areas with prolific algae production, limited fluvial sediment input and increased subsidence. In such settings, even very thick, highly oil-prone lacustrine successions may form as shown by the cored succession.