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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 (nonther-mogenic) 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 reflector (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 ofsediment 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 depres-surizes 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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