Development of a Carbonate Platform with Potential for Large Discoveries - An Example from Vietnam

M. Mayall, A. Bent, B. Dale, J. Evans, C. Reddick, D. Roberts, G. Seljeskog, and M. Vincent

In offshore central and southern Vietnam a number of different carbonate accumulations can be recognised. Platform carbonates form basin-wide units of carbonate characterised by strong, continuous parallel seismic reflectors. Facies are dominated by bioclastic wackestones with poor-moderate reservoir quality. On the more isolated highs, large buildups developed. These are typically 5-10km across and 300 m thick. They unconformably overlie the platform carbonate facies which are extensively karstified. In places these are pinnacles, typically 2-5km across, 300+ meters thick with chaotic or mounded internal seismic facies.

The large carbonate buildups are characterised by steep sided slopes with talus cones, reef-margin rims usually developed around only part of the buildup, and a prominent back-stepping geometry. Buildup interior facies form the main potential reservoirs They are dominated by fine to coarse grained coralgal packstones. Fine grained carbonates are associated with deeper water events and multiple karst surfaces can also be identified. Reservoir quality is excellent, largely controlled by extensive dissolution and dolomitisation believed to be related to the exposure events. Gas has been found in a number of reservoirs. Heterogeneities can be recognised which could potentially effect production. These include the extensive finer grained facies, cementation or open fissures associated with the karst surfaces, a more cemented reef rim, shallowing upwards facies cycles and faults.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California