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 elastic 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 prior/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.It is invariably the case that there are insufficient geological, geophysical and petrophysical data to create an unique reservoir description or reservoir engineering model. Consequently, geological models of a variety of kinds and scales, whose effectiveness is dependent upon the validity of the assumptions incorporated into them -assumptions which are often themselves models -must be relied upon. Central amongst these assumptions concerning deep water depositional systems are models for transport and deposition of sediment by turbidity currents; these influence the way we think about (and make predictions of) a whole range of sedimentary phenomena, from the character of individual beds to the geometry of turbidite systems and their components. They also affect predictions concerning lateral changes in reservoir-significant rock properties such as grain size, and the way correlation of lithofacies and individual beds is approached. Clearly, the reliability of larger-scale models comes into ques-

Sediment Gravity Flows: Study Based on Experimental Simulations

“…As consequence, a large billow (shear vortex) takes place behind the head (high mixing zone). The body presents the peak of velocity and after this point the flow starts to decelerate gradually (waning flows - Kneller, 1995). Concomitantly, the concentration of sediment within the flow follows the velocity behaviour.…”

“…The current accelerates (waxing flow - Kneller, 1995) due to the buoyancy flux with clearly defined head at the front. The thickness of the head is greater than the body, indicating the flow undergoes a large resistance of the ambient fluid and also from gravitational forces acting over the body.…”

Turbidite event history—Methods and implications for Holocene paleoseismicity of the Cascadia subduction zone

“…These events are interpreted as an earthquake, followed by a hyperpycnite that resulted from the breaching of a landslide dam caused by the original earthquake (St-Onge and others, 2004). Dam breaching is a variant of the more common hyperpycnal scenario involving waxing and waning depletive flow (Kneller, 1995), but likely would result in a similar flow hydrograph ( fig. 49; St-Onge and others, 2004).…”

Massive sand beds attributed to deposition by dense water cascades in the Bourcart canyon head, Gulf of Lions (northwestern Mediterranean Sea)

“…From bottom to top of the sand bed, the coarsening upward (including the shell lag) would be formed by a waxing (accelerating with time) flow (Kneller, 1995) (KIGC19/KGMO14 cores on Fig. 5 or 6).…”