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Making Sense of Turbidite Reservoirs: A Multi-basin Perspective on What Drives Architecture and Rock Properties

David C. Jennette, Bureau of Economic Geology, Austin, TX

Our knowledge of turbidite reservoirs has advanced rapidly over the past 15 years, owing largely to the petroleum industry's acquisition of ever larger and higher frequency 3-D marine seismic surveys. These surveys provide a stunning portfolio of high-resolution snapshots of ancient and modern submarine landscapes and illustrate the intricate details of their accompanying turbidite systems. As 3-D volumes are stitched together and time-stratigraphy across large parts of continental slopes established, subsurface workers can reconstruct characteristics of the ancient shelf, shelf edge, slope, and basin floor environments and deduce the suite of processes and controls that led to the development of the spectrum of turbidite reservoirs. Understanding these basic controls improves our ability to generate models that better predict the broad range of attributes required to ensure commerciality in costly offshore operating environments and/or settings where resolution of the objective is compromised by salt or deep burial.

Subsurface systems like those encountered in the Paleogene of the North Sea and the Tertiary of the Gulf of Mexico, offshore west Africa, and offshore Egypt, together with the outcropping systems of the Permian of west Texas and the Carboniferous Claire Group of western Ireland, provide comparative data sets from which to evaluate the principal mechanisms that establish turbidite reservoirs. We observe contrasting styles of architecture (channel-dominated to sheet-dominated), pattern (straight vs. highly sinuous, dendritic vs. lobate), sand percent, bed thickness, and grain size and sorting. These characteristics can be tied to (1) the sediment delivery system that, in conveyer belt fashion, controls the composition and volume of sediment available to the shelf edge, (2) triggering mechanisms at the shelf edge that control the volume, feed rate, and concentration of the flows, and (3) sea floor gradients that influence the acceleration, steadiness, or deceleration of flows. Within any one system, much of the reservoir architecture we observe can be tied back to the sand:mud ratio of flows and to sea floor gradient. A fourth factor influencing final architectural character, particularly on the upper slope, is modification by slumping or headwardly migrating erosional nickpoints.

Amid all of the volume-based characterization tools, classical seismic sequence stratigraphy remains one of the stratigrapher's most important characterization tools. Whether applied to outcrops or subsurface data sets, it provides a method to establish reservoir properties of genetically related deposits. Increasing seismic resolution allows subsurface stratigraphers to identify a highly detailed array of reservoir building blocks and systems tracts that lead to better prediction of individual fluid flow units. Sequence stratigraphy also helps identify changes in reservoir characteristics that occurred over a longer unit of geologic time. Such evolutionary changes record systematic variations in the sediment delivery system, shelf edge character that may be driven by a eustasy, climate, or accommodation space.

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