BACH HO FIELD, A FRACTURED GRANITIC BASEMENT RESERVOIR, CUU LONG BASIN, OFFSHORE SE VIETNAM: A BURIED-HILL PLAY

A combination of seismic, wireline, FMI and core data shows that Bach Ho field in the Cuu Long Basin, offshore SE Vietnam, is an unusual buried hill reservoir. There is little or no production from associated siliciclastic grus or granite wash, and the fractured reservoir matrix is largely made up of unaltered acid igneous lithologies (mostly granites and granodiorites). A major NE-SW late Oligocene reverse fault system cross-cuts the field, with about 2000 m of lateral displacement in the highly productive Central Block. The associated fracture meshwork greatly enhances reservoir quality. Transpressional wrench faulting in the late Oligocene in this part of the field emplaced a block of brittle granitic rock on top of organic-rich Eocene - Oligocene mudstones, and facilitated the early migration of hydrocarbons into the fracture network. Structure, not erosion, set up the 1000 m column of liquids in the fractured granodiorites which form the reservoir at Bach Ho. Faulted intervals with associated damage zones create an enhanced secondary porosity system in the granodiorite; effective porosities range from 3-5% and occasionally up to 20%. Some associated fractures are partially blocked by authigenic calcite and kaolinite. Features that degrade reservoir quality at Bach Ho include: (i) a thin, low-permeability clay-plugged rind created by surface-related (meteoric) Eocene - Oligocene weathering - this rind variably overprints the uppermost 10-40 m of exposed basement throughout the Cuu Long Basin; and (ii) widespread hydrothermal cements which largely predate late Oligocene wrench faulting; cementation mostly took place during post-magmatic cooling and precipitated zeolites, carbonates and silica in fractures which cut across both the igneous and the country rocks. Porosity-occluding hydrothermal and authigenic precipitates developed in pre-existing fractures in the Bach Ho granodiorite. These pre- late Oligocene mineral-filled fractures acted as zones of structural weakness during and after subsequent late Oligocene structural deformation. Together with new fractures formed during thrusting, the older fractures may have reopened during thrust emplacement, and subsequent gravitational settling of, the Central Block.