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ABSTRACT: Fluid and Hydrocarbon Migration at Active Margins: A Conceptual Synthesis of Results from the Barbados Ridge and Pacific Northwest

J. C. Moore, D. Orange, S. Stiles, D. Geddes

At active convergent margins, offscraped sediments are imbricated at the deformation front and mature with attendant loss of fluids. Underthrust sediments slide beneath the accretionary prism at the velocity of the oceanic plate and continuously release fluids and/or hydrocarbons during descent. The continuous renewal of sediment by underthrusting beneath the accretionary prism suggests large cumulative fluid and hydrocarbon flux beneath any area of the prism.

The asymmetrical loading of underthrust sediments by the tapered accretionary prism causes updip migration of fluids and hydrocarbons through stratigraphic conduits or the decollement zone at the base of the accretionary prism. Probable thermogenic hydrocarbons indicate long-distance lateral migration along these conduits from beneath the Barbados prism. Pore-water and temperature anomalies (Barbados) and biological indicators of fluid expulsion (Oregon) indicate that faults effectively capture fluid flow from the underthrust sequence (Barbados) and permeable sedimentary layers within the accretionary prisms (Oregon). Permeability measurements and hydrogeological modeling indicate that fault-capture of fluids requires permeabilities two to five orders of magnitude higher in the fault ones than in the source sediments.

Complex deformation and diagenesis in accretionary prisms rapidly destroys the continuity of permeable stratigraphic conduits; faults become the principal migration conduits. The basinward growth of accretionary

prisms require that they deform internally, commonly by out-of-sequence thrusts; such thrusts may be migration conduits connecting underthrust or underplated source sediments to shallow reservoirs. Both in Barbados prism and the Pacific Northwest, out-of-sequence thrusts leak hydrocarbons and could feed reservoirs in overlying slightly deformed shelf and basinal sequences.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990