Ciaran J. O'Byrne1
(1) BP Amoco Oil Corporation, Houston, TX
Abstract: Re-entrant fans and bypass aprons: depositional response to dynamic slope topography
Proprietary 3D coherency technology has produced remarkable map view realizations of Miocene slope to base of slope drainage across 3000 sq. miles of the Mississippi Fan Fold Belt, Gulf of Mexico. Inhouse seismic facies and rock properties databases provide constraints on predictive models of reservoir distribution within associated deepwater fan systems.
Re-entrant fans form at the mouth of structurally controlled re-entrants within base of slope fold belts or coalescing salt canopies. Structural relief plays a critical role in determining the sediment flux delivered to the fan. Narrow, high relief, point sourced windows segregrate coarse sediment on the stoss side of the barrier from an accelerated, fine grained plume feeding the lee side fan. Broad, low relief gateways to the base of slope are more efficient at dispersing coarser material across the fan effecting sheet and channel architecture and depositional relief.
Bypass aprons are sourced by multiple feeder valleys which more effectively bypass slope rugosity to the abyssal plain. As such they tend to have a coarser sediment distribution and form compensatory sheet and channel systems. Bypass slope roughness is a function of spatial frequency of salt diapirs and salt cored fold structures together with depositional topography created by local mass failures and base of slope fan lobe switching. The dynamic link between syn-fold elevated plateau areas, dip and axial drainage capture and flanking slope bypass with the MFFB is discussed. Simple graded slope profiles are not applicable here and emphasize the need for accurate paleotopographic reconstruction in predictive reservoir models.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana