AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Stratal Patterns in Salt-Withdrawal Minibasins: Simple Modeling and Implications for Reservoir Prediction
(1) Clastics Research Team, Shell International E&P Inc., Houston, TX.
(2) Shell Nigeria E&P, Lagos, Nigeria.
Although it long has been known that salt-withdrawal minibasin fills in the Gulf of Mexico record the interplay between subsidence and sediment input, so far no effort has been made to undertake a rigorous analysis of how these parameters influence stratigraphic geometries and stratal termination patterns. We use a simple three-dimensional model to show how subsidence rate and episodic sediment input result in variable degrees of convergence and onlap. Our model includes (1) coarser-grained turbidite deposits restricted to the basin fill and (2) mud-dominated slope sediments that extend as drapes onto the basin flank. The basin-fill turbidite deposits are assumed to form horizontal depositional surfaces, an assumption supported by the small depositional gradients observed in most present-day basins and the sudden pinchouts of sands at minibasin margins in both outcrops and high-resolution seismic sections. Subsidence rates are assumed to be constant or changing at a slower rate than sediment input.
Model results illustrate that periods when high sediment input overcomes subsidence result in limited convergence and well-defined onlaps at basin margins. Smaller rates of sediment input result in stronger convergence and an increased clustering of onlap points, with all stratal boundaries converging toward the same termination point when sediment input matches exactly the rate at which new accommodation is created through subsidence. Convergent baselapping and convergent thinning seismic facies probably reflect such variations in sediment input relative to subsidence, and are unlikely to be the result of flow-scale phenomena like high run-ups of muddy turbidity currents on basin flanks. When sediment input is variable but overall large enough to keep up with subsidence, periods of full sediment retention will alternate with periods of bypass, creating asymmetric cycles consisting of (1) ‘ponded’, (2) bypass, and (3) condensed section deposits. This kind of cyclicity has been observed in the Auger and Mars Basins in the Gulf of Mexico.
Previous work has shown that the main seismic facies types present in minibasins correspond to different net-to-gross distributions. Our results suggest that seismic facies typical of higher N/G values form during times of large basin-wide sedimentation rates. Assuming that some age information is available, this insight could be used to improve sand prediction in salt-withdrawal minibasins.