Abstract: A Complex Shallow-Marine Gas Reservoir: A South African Case Study

James R. T. Turner, William Mc Aloon

The gas-field was discovered by Soekor in 1989 and is located in the Bredasdorp basin, off the south coast of South Africa, 110 km from the nearest landfall, at a water depth of 160 m. Domal structural trapping is formed by segmented roll-over against a major bounding fault. The reservoir itself is highly faulted. Three boreholes have intersected the reservoir at depths of between 3616 and 3714 m. Gas sand thicknesses vary between 24 m and 70 m and consist of stacked, blocky and upward-coarsening shallow-marine sandstone cyclic units, representing an overall transgressive phase of barrier-bar progradation. The reservoir is highly overpressured.

The appraisal challenge has been to reconcile the fact that, although pressure data indicates that the three boreholes are in hydraulic communication, a gas-water contact has yet to be intersected, and gas-down-to depths differ by as much as 54 m. There is also a dramatic regional variation in reservoir quality across the field. Permeability, in particular, varies for a given flow unit from an average of 1 MD to 250 MD at different boreholes. The reservoir has been characterised using a genetic approach in order to investigate the spatial relationship of reservoir parameters. In calculating in-place volumes and constructing appropriate development scenarios, emphasis has been placed on geological/depositional modelling, as well as the use of mapping seismic attributes using impedance ata derived from an extensive 3D seismic survey.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France