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Abstract

Warren, John K., Alwyn Cheung, and Ian Cartwright, 2010, Organic geochemical, isotopic, and seismic indicators of fluid flow in pressurized growth anticlines and mud volcanoes in modern deep-water slope and rise sediments of offshore Brunei Darussalam: Implications for hydrocarbon exploration in other mud- and salt-diapir provinces, in L. Wood, ed., Shale tectonics: AAPG Memoir 93, p. 163196.

DOI:10.1306/13231314M933424

Copyright copy2010 by The American Association of Petroleum Geologists.

Organic Geochemical, Isotopic, and Seismic Indicators of Fluid Flow in Pressurized Growth Anticlines and Mud Volcanoes in Modern Deep-water Slope and Rise Sediments of Offshore Brunei Darussalam: Implications for Hydrocarbon Exploration in Other Mud- and Salt-diapir Provinces

John K. Warren,1 Alwyn Cheung,2 Ian Cartwright3

1International Graduate Program in Petroleum Geoscience, Department of Geology, Chulalongkorn University, Bangkok, Thailand; Fomer address: Shell Chair in Carbonate Studies, Sultan Qaboos University, Muscat, Oman
2Petronas Malaysia, Kuala Lumpur, Malaysia
3School of Geosciences, Monash University, Melbourne, Australia

ACKNOWLEDGMENTS

The authors thank the Brunei Petroleum unit, Total Brunei, BHP Billiton, and Shell International Deepwater for their support of this project. The senior author also thanks his former fellow faculty at Universiti Brunei Darussalam, Chris Morley, Martin Gee, Joe Lambiase, and Angus Ferguson for the lively discussions and helpful criticisms that helped shape the ideas in this article.

ABSTRACT

Modern slope and rise sediments of offshore Brunei contain thermogenic gases and bitumens atop or adjacent to actively growing mud-cored compressional ridges. This is particularly so where pressurized fluid and mud periodically breakout onto the sea floor as mud volcanoes or chimneys along ridge axes. Where modern sediment is actively accumulating in areas away from the ridges, the sediment contains organic signatures and gases that are biogenic (nonthermogenic) and related to the bacterial breakdown in and below the sulfate reduction zone.

Seismic signature implies that ridge-top volcanoes are regions of pressure buildup, and incipient fluid breakout is indicated by a combination of convex-upward seismic reflectors and zones of discontinuity in the bottom-simulating Previous HitreflectorTop (BSR, a seismic indicator of the base of stability of methane hydrates). Areas where the BSR weakens or disappears are likely zones where rising warm fluids have melted part or all of the hydrate layer. Following fluid pressure release at the sea floor and a subsequent period of inactivity, sediment layers within and adjacent to a mud volcano collapse, sediment reflectors bend downward into the neck of the volcano, and a BSR re-establishes in the now-inactive neck.

Mud volcanoes in the continental slope and rise of Brunei occur along a series of gravity-driven compressional ridges. Seismic geometries are similar to the salt-cored compressional ridges of the slope and rise setting of the circum-Atlantic salt basins. In the case of the shale-cored ridges and mud chimneys of Brunei, however, the disrupted ridge and chimney cores are zones of sediment disruption created by the presence of pressurized gas and fluid. This disrupted ridge sediment has a different set of rheotropic properties compared to that of a salt-cored ridge. Where the disrupted zone has reached the Brunei sea floor, the mud-filled structure depressurizes and sediment flow ceases. Little or no evidence exists for widespread lateral flow of disrupted sediment across the Brunei sea floor, although evidence of debris-flow initiation exists in zones of instability created by ridge collapse. In contrast, where halokinetic salt breaks out onto the land surface or the sea floor, the halite mass continues to flow and spread across the deep-sea floor and ultimately constructs large allochthonous tongues and namakiers.

Active mud and salt diapirs create highs in the landscape and zones of adjacent withdrawal; this means that most reservoir styles and traps are similar in intervals above the source kinetic layer, whether it is composed of mud or salt. Once salt reaches the surface, it can flow over the landscape, but pressurized mud cannot, so no known argillokinetic equivalents to suballochthon traps are observed, nor argillokinetic equivalents to cap-rock traps.

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