Process-Based Interpretation of Basin Floor Fan Stratigraphy, Tanqua Sub-Basin, South Africa

Benjamin Sheets¹, Darren Box², Chris Edwards², David C. Hoyal², Anthony Sprague², Kirt Campion², Shauna K. Oppert², and Roger Bloch^2
1ExxonMobil Production Company, Houston, TX
²ExxonMobil Upstream Research, Houston, TX

Accurate prediction of 3D architecture and facies distribution is of utmost importance to the construction of reservoir models. On one hand, despite increasingly high-quality seismic data, interpretation of 3D stratigraphy is limited to seismic resolution. On the other hand, outcrop studies provide immense detail, but primarily 2D information. Therefore, we focus on the relationship between stratigraphy and sedimentary processes, with the goal of better 3D extrapolation of 2D outcrop exposures. Here we interpret a medial ‘Fan 3’ exposure of the Permian Skoorsteenberg Formation on Los Kop and Kleine Gemsbok Fontein farms in the Tanqua Karoo in the context of results from analogous physical experiments. The study locality is interpreted as medial, due to the coincidence of relatively conformable and lobate deposition with relatively confined, erosionally-based deposits at the storey set scale.

The experimental analogue is a deep water distributive system, self-formed by a saline density current flowing over and modifying a sloped bed of 0.18 - 0.25 mm melamine sediment (SG = 1.5). Salt was used as a substitute for fine-grained sediment in order to better visualize transport processes. The experimental distributive dynamics are characterized by a cyclical process of channel extension, lobe deposition, channel backfilling and avulsion at multiple scales.

The Fan 3 deposits in the study area show remarkably similar stacking patterns to those formed in the experimental runs. In particular, both show a vertical progression of compensationally stacked and increasingly erosional storey sets as time progresses. Interpretation of the Skoorsteenberg deposits in the process context provided by the experiments allows extrapolation of outcrop exposures into three dimensions, and better estimation of the regional extent and character of major stratigraphic packages and surfaces.

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