J. Casey Moore1,
(1) UC Santa Cruz, Santa Cruz, CA
(2) Univ. Texas, Austin, TX
(3) Amoco E&P Tech Group, Tulsa, OK
(4) Univ. Hawaii, Honolulu, HI
Abstract: Compartmentalization of a thrust decollement by high-angle faulting: Evidence from seismic amplitude and coherence images, and Logging While Drilling
The decollement zone of the Northern Barbados accretionary thrust system shows anomalies in porewater salinity and hydrocarbons indicating migration of deeply sourced fluids. Extractions of seismic amplitude of the fault surface from 3D seismic data show compartments of strong negative and positive polarity. Logging While Drilling measurements through the compartments indicate that the negative polarity compartments are approximately 15 m thick, low density, fluid-enriched intervals in the fault zone that model as tuned negative polarity seismic reflections. The positive polarity compartments show a positive impedance contrast in the decollement zone that models as positive polarity seismic reflection. The fluid enriched compartments apparently represent regions of arrested consolidation due to rapid burial by underthrusting and simultaneous fluid flow from depth. The largest fluid-enriched compartment is overlain by a seismic volume of high amplitudes which may be caused by fluids migrating from the decollement to the overlying thrust stack.
Seismic coherence imaging of the decollement surface indicates that the compartments of strong negative polarity are bounded by low coherency lineaments. Vertical seismic amplitude sections through these lineaments indicate that they are high angle fault zones cutting the thrust stack, the decollement zone, and the underthrusting plate. These fault zones offset the isopach of the thrust stack in a right lateral sense about 800 m, occur in muddy lithologies, and seal or partially seal the fluid-enriched compartments. The right lateral faults are predictable as shears due to the stress field associated with the plate convergence.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana