The Stratigraphic Organization of Basin-Floor Fans from the Tanqua Sub-Basin, South Africa and Ainsa Basin, Spain

Darren Box, Benjamin Sheets, Anthony Sprague, Kirt Campion, Chris Edwards, David C. Hoyal, Shauna K. Oppert, and Gregory D. Robertson
URC, ExxonMobil, Houston, TX

The ability to model increasingly fine-scale stratigraphic heterogeneity in reservoir models of basin-floor fans requires a more detailed understanding of these systems than currently available.

To characterize stratigraphic variability in deep-water fans, 2 systems were studied: ‘Fan 3’ of the Permian, Skoorsteenberg Formation in the Tanqua Karoo was studied in the Los Kop region where it is subdivided into 9 intervals (storey sets). Each storey set displays dramatic lateral facies changes and is bound by avulsion surfaces. The 9 storey sets are organized into 3 larger scale packages (complexes) progressively offset to the west which are bound by significant semi-regional, avulsion surfaces. The 3 complexes that form the sand-rich portion of fan 3 stack into a complex set which together with the overlying abandonment shales constitutes a single sequence.

The Eocene Ainsa 2 Formation of NE Spain offers an exposure across the proximal region of a coarse-grained basin-floor fan. Despite the difference in setting and grain size the Ainsa 2 deposits display, similar stratigraphic characteristics to the Tanqua ‘Fan 3’. The exposure is subdivided into 6 intervals (storey sets) which display rapid lateral facies changes. These intervals stack initially compensationally and then in a laterally offset manner. The 6 intervals are organized into 3 larger scale packages (complexes) bound by significant avulsion surfaces.

The organization of basin-floor fan systems is predictable. Each level of stratigraphic organization displays a similar progression of architectural elements and lithofacies, from up-dip scour dominated coarser grained facies, through depositional mounding to finer grained sheet like bodies which downlap. This predictable organization of the stratigraphic record of basin-floor fans suggests that a common process is responsible for their deposition.

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