Abstract: Integrating Genetic Stratigraphic Principles in the Interpretation of Deep Water Sediments

Johnson, Stephen D. - Statoil

Introduction: Testing of sequence stratigraphic models against well exposed outcrop datasets provides an evaluation of their general applicability. A study of Permian deep water clastic systems in the Tanqua Karoo foreland sub-basin in South Africa provided valuable insight into the stratigraphic control on depositional style and facies distributions.

Basin summary: Depositional style is progradational. The main components include: 4 basin floor fans separated by shales/silts, a stable slope and intra slope basin, an intra slope conduit, and younger pro-delta and tidally influenced delta front mouth bars and shorefaces (Fig. 1).

Sedimentology and genetic stratigraphy: Sands are well sorted, fine to medium grained with bed thickness of 1cm to 50cm (max. 1m). Processes include high and low density turbidite flows, minor slurries and debris flows. Net:gross for each fan is 50 to 85 %. Depositional events were frequent and cyclical with a mature, long lived, possibly river generated supply. A genetic stratigraphy based on the recognition of 3 types of starvation zones was developed.

Type 1 horizons occur in the fine intervals between fans and comprise diagenetic concretions that represent depositional hiatus caused by allocyclic processes. Type 2 horizons comprise shale, silt and thin dilute turbidites in zones up to 3 m thick that can be traced over 20 km+. They are interpreted to represent sediment starvation related to allocyclic processes. Type 3 horizons are similar to type 2 horizons but are thinner (<1m) and occur over <5 km and represent local sediment starvation due to autocyclicity.

Mapping Type 2 zones defined an internal genetic stratigraphy and the upper surfaces of ?lobate geometry? units within each fan. Successive lobate units step basinward (progradation) with pinch outs defined by small isolated channels with a dendritic outcrop pattern. The internal architecture of a lobate unit is affected by autocyclicity which causes variable vertical stacking patterns at different geographic positions within a single genetic unit.

Subsurface implications of genetic stratigraphy based correlations: Type 2 horizons define potentially significant permeability barriers/seals whereas type 3 horizons cause baffles. Significant sand input occurs between depositional hiatus (type 2 horizons). Risking of vertical and lateral connectivity in the reservoir is improved because genetic stratigraphy defines high resolution packages within which autocyclic variation can be assessed. In this study progradational stacking of lobate units influenced sand distribution but the internal architecture of each lobate unit reflects autocyclicity. e.g., main sand fairways are controlled by channels. Use of genetic stratigraphy can significantly improve geological models developed for the subsurface.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil