Slurry Flows VS Turbidity Corrents: A Key Distinction in Evaluating the Reservoir Potential of Thin-Bedded Deepwater Sequences, Modern and Ancient
D. R. Lowe
Stanford University, Stanford, CA, [email protected]
Although turbidity currents and debris flows form end members in the spectrum of sediment gravity flow types, a more significant distinction from the perspective of reservoir development is the difference between turbidity currents and slurry flows. Both flow types are turbulent, but slurry flows possess enough cohesion due to the presence of entrained mud that it influences sedimentation and the structuring, texture, and reservoir potential of the resulting deposits. Whereas thick-bedded sands remain one of the principal targets of deepwater exploration, thin-bedded sequences are coming increasingly into play, and the distinction between these flow types becomes correspondingly significant. Thin-bedded deepwater sequences develop in a variety of settings at the transition between high energy sites of transport and sedimentation and lower energy areas of mud deposition. The main thin-bedded sequences with reservoir potential include levee deposits, a variety of splays, and off-axis sections within major channels and canyons. Levee sequences seldom include slurry-flow beds because most levees form through deposition from the dilute, turbulent tops of turbidity currents. Splay deposits, however, form through deposition from the entire flow and in many areas flows depositing thick-bedded sands in more "proximal" parts of the splays evolve downslope into slurry flows. Hence, exploration toward the margins of fields producing from thick-bedded channel or proximal splay sands may encounter thin-bedded splay sequences dominated by either mud-rich slurry flows with little reservoir potential or relatively clean, sandy turbidites that may be excellent reservoirs. Fortunately, there are ways to predict downslope potential from upslope deposits. Examples of modern deepwater systems characterized by slurry-flow deposits include medium- to thin-bedded sands in the Santa Monica Basin, southern California. Ancient examples are widespread, including the classic Cretaceous slurry-flow sequences in the Britannia Field, North Sea, and portions of the Pennsylvanian Jackfork Group, Ouachita Mountains, Arkansas and Oklahoma.
AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009