--> --> Depositional Facies of the Deep Lacustrine, Oligocene Syn-rift Succession of the ENRECA-3 well, Bach Long Vi Island in the Song Hong-Beibuwan Basins

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Depositional Facies of the Deep Lacustrine, Oligocene Syn-rift Succession of the ENRECA-3 well, Bach Long Vi Island in the Song Hong-Beibuwan Basins

Abstract

Significant petroleum shows at the intersection between the Song Hong and Beibuwan basins demonstrate an active petroleum system. However, well information on the syn-rift succession is very limited. Thus this paper focuses on the depositional facies of the syn-rift succession with particular emphasis on the formation of source rocks, reservoir rocks and carrier beds. The ENRECA-3 core well was drilled on the Neogene Bach Long Vi inversion structure forming a small island in the Gulf of Tonkin. The island exposes Oligocene syn-rift sedimentary rocks comprising oil-prone, organic-rich lacustrine mudstones interbedded with sandstones, and the island provides a unique geological window in to the deeply buried source rock facies in the Song Hong and BeibBuwan basins. The well cored approx. 500 m of the syn-rift succession consisting of very dark grey, medium and light grey mudstones, and dark grey to light grey sandstones. The sandstones are quartz-rich, very fine to coarse grained arkoses that were deposited by sediment gravity flows including low- and high-density turbidites, hybrites and debris flows. Visual porosity is mostly good. The mudstones show an average TOC content of 2.91 wt.% and the average HI is 575 mg HC/g TOC. Bioturbation is almost absent except for very small burrows related to sandstone laminae. The mudstones represent deposition from pelagic mud, fluid mud with minor input from hyperpycnal flows. Common soft sediment deformation structures (ball and pillows, slump folds, sandstone injectites, mudstone flames and deformed rip-up clasts) and the sediment gravity flows indicate rapid sedimentation in a soft-bottomed basin. The sedimentary facies, the large amount of fresh-water algae, the absence of marine signals and signs of subaerial exposure indicate that the deposition occurred in a deep, elongated, narrow and subsiding freshwater graben with a large organic production. Oxygen deficiency at the lake-bottom, nearly absence of a bentic fauna and a rapid deposition facilitated preservation of organic-rich mudstones. The grain size and sorting of the sandstones suggests that the sand was supplied and sorted by river processes prior to the re-sedimentation by density currents, and the absence of pebbles suggest that the faulted rift-margins and shoulders did not supply material to the basin at the time of deposition. The study is part the ENRECA Capacity Building Project, which is a long-term research cooperation between VPI and GEUS.