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Abstract

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AAPG Bulletin, V. 85, No. 6 (June 2001), P. 971-988.

Copyright ©2001. The American Association of Petroleum Geologists. All rights reserved.

Process controls on the development of stratigraphic trap potential on the margins of confined turbidite systems and aids to reservoir evaluation

William McCaffrey,1 Benjamin Kneller2

1University of Leeds, School of Earth Sciences, Leeds LS2 9JT, United Kingdom; email: [email protected]
2Institute for Crustal Studies, University of California, Santa Barbara, California, 93106; email: [email protected]

AUTHORS

William (Bill) McCaffrey is a senior research fellow at the University of Leeds. His principal research area is deep marine clastics, with specific interests in the mechanics of sediment transport and deposition from turbidity currents; development of predictive models for geometries, thicknesses, internal characteristics, and textural character of turbidite sandstones; submarine channel development; and mechanisms of submarine debris flow generation. He received a B.A. degree in geology from the University of Oxford in 1987 and a Ph.D. from the University of Leeds in 1991.

Ben Kneller, formerly a lecturer at University of Leeds, United Kingdom, is currently a researcher at University of California, Santa Barbara. He gained his B.Sc. degree from Sheffield University, United Kingdom, in 1981, and a Ph.D. from Aberdeen University in 1988. From 1992 to 1998 he headed an industry consortium research project investigating the dynamics of turbidity currents and their interactions with sea floor topography. His current research interests include the structure of gravity currents and the architecture and evolution of deep marine clastic systems.

ACKNOWLEDGMENTS

This work was carried out under the auspices of the Phase 2 Turbidites Project at Leeds University, supported by Amerada Hess, Amoco, Arco, British Gas, BHP, BP, Chevron, Conoco, Elf, Enterprise, Fina, Mobil, Shell, and Texaco. We thank Jeff May and Chuck Stelting for reviews that were detailed, constructive, and perceptive. We thank Lawrence Amy for discussion, sharing field data, and allowing us to use the photographs in Figure 9; Chris Clayton for discussion and assistance in the field; Hugh Sinclair for discussion; and Juan Pablo Milana for discussion in the field and help in drafting Figure 8.

ABSTRACT

Stratigraphic trapping at pinch-out margins is a key feature of many turbidite-hosted hydrocarbon reservoirs. In systems confined by lateral or oblique frontal slopes, outcrop studies show that there is a continuum between two geometries of pinch-out configuration. In type A, turbidites thin onto the confining surface--although the final sandstone pinch-out is commonly abrupt--and individual beds tend not to erode into earlier deposits. In type B, turbidite sandstones commonly thicken toward the confining slope, and beds may incise into earlier deposits. These two types may occur in combination, to give a wide spectrum of pinch-out characteristics. Our analysis suggests the principal control in determining pinch-out character is flow magnitude, with smaller flows producing type A and larger flows producing type B.

In areas of poor seismic control it can be difficult to assess either pinch-out character or the proximity of wells to confining slopes. Because estimates of paleoflow magnitude can be made from core or high-quality log image data, however, it is possible to make reasonable estimates of pinch-out character even from wells such as exploration wells, which may be placed conservatively, away from the field margins. Furthermore, systematic paleoflow variations and thickness trends are commonly seen in individual turbidite sandstones as they approach confining slopes. For example, dispersal directions indicate flow deflection parallel with the strike of confining topography; beds thin toward type A onlaps and thicken toward type B onlaps. These relationships can be exploited via analysis of vertical successions to constrain well position with respect to the slope. Similarly, the presence, location, and frequency of locally derived debrites can provide information on the presence and proximity of confining slopes.

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