Cretaceous ‘USM’ Reservoir, F-O Gasfield, Offshore South Africa: Sedimentological Factors Affecting Economic Viability

Roger Higgs
Geoclastica Ltd, Marlborough, United Kingdom

The F-O Field (Bredasdorp Basin) is a seismically defined, low-relief antiform. The last two of the four drilled wells found surprisingly poor porosity (<12%) in clean sands of the ‘Upper Shallow Marine’ (USM) reservoir, highlighting the need for a reliable predictive sedimentological model. Subsequent wireline correlations, core facies analysis and diagenetic studies have revealed or confirmed the following interpretations. (1) The depositional environment was a shallow marine tidal sand sheet (dunefield), for which suitable analogs are the modern North Sea and the Peninsula Formation of South Africa (Paleozoic, e.g. Table Mountain). Net-sand mapping for each of eight reservoir zones defined from log-shape correlations reveals lateral shifts in the sand-sheet axis. (2) The base of the USM is a sequence boundary, probably representing the 126 Ma eustatic superlow (Valanginian), consistent with imprecise USM microfossil dating. (3) The top of the USM is a diachronous flooding surface overlain by shales. (4) Not far (<100 m) above the USM is a low-relief angular unconformity (evident on dipmeter), formed during F-O antiform growth, reflecting Hauterivian early transpressive movement on the Agulhas-Falklands Fracture Zone, after USM “quiescent rift” deposition. The USM is partly to entirely eroded at the unconformity when traced laterally. (5) Porosity is secondary and generally poor due to deep burial (currently 3.5 km). Enhanced porosity (12-16%) can occur in the cleanest facies (central dunefield sand), but only near (<40 m below) the unconformity, indicating that it is telodiagenetically related. Overlaying of two isopach maps (net sand, and “sub-unconformity overburden”) will help to guide future well placement. Reservoir modeling and frac-suitability studies are underway.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery