Beyond the turbidite paradigm: physical models for deposition of turbidites and their implications for reservoir prediction

Abstract

The simple physical models which are popularly used to describe deposition from turbidity currents are based on the notion of unidirectional waning flow, resulting in the familiar Bouma sequence (Ta-e) or its high-density counterpart, the Lowe sequence (S1-3). Most geologists working on turbidite successions know only too well how wide is the range of facies which do not fit into the standard facies models. The application of simple equations of motion shows that deposition can occur beneath flows that are steady or even waxing. The various combinations of different spatial and temporal accelerations produce markedly different vertical and lateral variations in the resulting turbidite. On these grounds alone, we should expect not one but at least five basic types of sequence in turbidite beds, for which likely candidates can be found in many deep water clastic systems. This has important implications for deposit geometry, gradients in reservoir properties, and geological interpretation and correlation of well cores. Since many basins are confined, a priori reasoning tells us to expect interactions between turbidity currents and topography, producing anomalous intrabed vertical sequences, multiple current directions and locally enhanced deposition. Flume experiments with radially spreading scaled sediment gravity flows demonstrate how the geometry of sandstone deposits may be controlled by topography. Interactions between the flow and topographic features are dependent upon the shape and orientation of the obstacle, and upon its size relative to the height of the flow. Some of the results are counter-intuitive, and may have significant impact on location of reservoir sands around basin-floor and marginal topography.