Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Recognition of Dryland Fluvial and Terminal Splay Systems in the Pinda Formation, Offshore Angola: Implications for Predicting Subsurface Reservoir Connectivity
(1) Chevron Energy Technology Company, Chevron, Perth, WA, Australia.
(2) Chevron Energy Technology Company, Chevron, San Ramon, CA.
(3) Chevron Energy Technology Company, Chevron, Houston, TX.
(4) Southern Africa Strategic Business Unit (SASBU), Chevron, Houston, TX.
The Albian Pinda Formation is a complex mixed-lithology carbonate and siliciclastic system comprised of highly variable marine, tidal, and fluvial to nonmarine depositional environments. Over three decades of Pinda exploration and production have generated a wealth of data, including ~35,000 ft of core, hundreds of well penetrations, and broad 3-D seismic coverage. Recent core acquisition has led to the recognition of dryland “ephemeral” river systems and associated terminal splays (e.g., sub-aerial deposition at terminus of confined flow) as an important Pinda reservoir, one of only a handful of examples recognized in the subsurface. This study demonstrates how the incorporation of these systems into a previously well-defined depositional model (developed from detailed core analysis, process sedimentology, well data, and use of subsurface and modern analogs) greatly improves our predictive capability of reservoir quality, geometry, and lateral continuity. Terminal splay “complexes” consist of amalgamated proximal lobes: 10-30 ft thick, upward-fining, laterally extensive sheet sands deposited under flash discharge conditions. These deposits are sharp-based and consist of planar laminated to current rippled fine sand with abundant basal mudclast lags. Cross-bedded fine sands indicate bedform migration within thin distributary channels, whereas thin 1-3 ft thick distal lobes display current ripples and occasional sub-aqueous wave modification. The integration of observations from core with ancient and modern analogs (e.g., Lake Eyre, central Australia) provides context for the sedimentary processes at play, and constrains the depositional geometry and lateral continuity of these systems. A case study demonstrates how the evolution of the Pinda depositional model influenced our ability to predict the connectivity of fluvial/splay reservoirs. Early stratigraphic models for this field were challenged due to sparse well coverage (one well in 25 km2). The integration of both modern process-based analogs and subsurface analogs from nearby assets predicted the presence of laterally continuous (~5-10 km) splay sandbodies. Recent development drilling confirmed these predictions while an additional core validated the interpretation of terminal splay reservoirs. These results were cycled into a new iteration of earth models (refined geometry and dimension estimates) and were incorporated into a regional evaluation of future exploration and development opportunities.