| Abstract: | A number of oil- and gasfields have recently been discovered on the continental shelf of Southern Vietnam, the largest of which is White Tiger. The main pay-zone in this field is in the basement, which is composed of granites and granodiorites, lithologies which are found in the basement elsewhere on the shelf. The granites have undergone severe alteration as a result of tectonic, hydrothermal and surface weathering processes. Reservoirs are related to cavernous fracture porosity in deep basement zones, and to “porous cavernous” fracturing at more shallow levels. Total reservoir thickness exceeds 1 km, and the oil-water contact has not yet been located. Flow-rates reach 2,000 cu. m/d. Drilling data indicates that the granitoids are overlain by argillaceous-terrigenous rocks of Oligocene and younger ages which have a thickness of 2.5–4.4 km (up to 8 km in places, according to seismic data). Source rocks are mostly early Oligocene argillites, from which oil probably migrated into upthrown basement blocks. |
