Tidal Influence in Shelf-Edge Deltas of the Karoo Basin, South Africa; Implications for Reservoir Location

Shelf-edge deltas are thought to be the primary conduit for sediment delivery to Deepwater and basin floor fan construction. Though uncommon, facies distributions in shelf edge deltas characterized by strong tidal influence present reservoir locations which differ from their more commonly wave-and-fluvial influenced and dominated counterparts.

The Tanqua depocenter of the Karoo Basin in South Africa provides exceptional 3D outcroppings of the Permian Kookfontein Formation stacked deltas. Three clinothems were studied through 13 detailed stratigraphic columns correlated by “walking out” progradational tops along Miederberg, Grasberg, Valberg, and Bitterberg. The main focus of this work is on the tidally-influenced Clinothem 3, and implications for down-dip and lateral reservoir location.

Strong presence of facies displaying bidirectional ripple lamination, mud-drapes, and flaser bedding suggests Clinothem 3 is tidally influenced. Thick, 3 to 12m successions of heterolithic, lower delta-front turbidites are overlain by 2 to 6m of tidally reworked turbidites. These are overlain by elongated, shore-perpendicular mouth bars displaying bidirectional cross strata and bidirectional rippled tops. Lack of significant mud drapes on cross strata or accretion surfaces in these bars is thought to result from high energy associated with shelf-edge environments.

High sediment supply and effective, tidally-driven sediment redistribution resulted in rapid subaqueous progradation of the deltaic package; suggested by high volumes of associated mouth bars and collapse features and an overall thinner package than Clinothems 1 and 2. Thick 0.2 to 1.5m sand rich surge type turbidites located on the upper to lower slope down-dip of paleo-flow direction are interpreted to have been initiated by up-dip mouth-bar collapse. Along-strike, tidal influence strengthens with thickening successions of interbedded, tidally reworked heterolithics. Mud content increases lateral to fluvial input due to multi-scale topographic lows between tidal bars and between deltaic lobes, which amplify tidal slack-water periods. Study of Clinothem 3 thus suggests that favorable sand-rich turbidite systems are located directly down-dip of tidally influenced shelf-edge deltas while mud and silt prone heterolithics successions dominated areas lateral to the fluvial input focal-point.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California