Stratigraphic Development of the Upper Fan From the Brazos-Trinity Slope System (Basin IV), Western Gulf of Mexico

Abstract

An ultra-high resolution seismic volume collected over Basin IV of the Brazos-Trinity series of mini-basins, Gulf of Mexico, reveals the internal stratigraphy of its upper submarine fan down to the scale of small distributary channels and lobe elements (as small as 10-100 meters in areal extent and a few meters thick). Gravity cores collected from 16 locations across the upper portion of the fan were also used to understand the transition in facies from proximal amalgamated channel sands to distal distributary lobes. Using skeletonization techniques we were able to identify lobes with embedded distributary channels and lobe elements contained within lobe complexes that were interpreted manually. The result of this work is a new depositional model with a complete hierarchy tied to facies, architecture and inferred sedimentary processes. The upper fan stratigraphy is composed of five lobe complexes accumulated during the last glacial maximum that overlie series of mass-transported complexes originating from the collapse of the steep flanks of this classical halokinetic minibasin. The early fan deposition is represented by two overlapping lobe complexes with limited lateral offset. Multi-horizon slicing reveals that channels were poorly-developed at this time, and occurred as numerous, disorganized, short-length segments on relatively steep slopes inherited from the original basin bathymetry. Later fan development is characterized by a transition from overlapping to laterally offset lobe complexes developing on much lower slopes and reaching to the distal confines of the basin. These complexes show increasingly evolved channel patterns with longer, deeper, and more sinuous single-thread channel features feeding well developed lobe bodies through a network of ever smaller distributary channels. Gradient analysis augmented by studies of analogous tank experiments demonstrate an inverse relationship between channel length and slope such that longer channels are expected to develop on lower slopes. In addition, the observed upward evolution of depositional style is inferred to represent a self-organized transition from supercritical to subcritical hydraulic regimes controlled by depositional gradients that generally decrease with basin filling during the progradation of a perched, deep-water fan. Accurately establishing paleobathymetry is therefore a critical process in the prediction of facies distributions and sand body connectivity in lower resolution datasets.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017