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All Fill—No Spill: Slope-Fan Sand Bodies in Growth-Faulted Subbasins: Oligocene Frio Formation, South Texas Gulf Coast

Ursula Hammes, Hongliu Zeng, Robert Loucks, and Frank Brown, Jr.
Bureau of Economic Geology, Jackson School of Geosciences
The University of Texas at Austin
10100 Burnet Rd., Austin, Texas 78758

Growth-faulted subbasins in the Oligocene Frio Formation are major exploration targets along the South Texas Gulf Coast. Historically, exploration has targeted on-shelf highstand and transgressive systems tracts and lowstand prograding-wedge systems tracts with great success. Companies have recently become interested in exploring for slope-fan sandstone reservoirs in lowstand growth-faulted subbasins. However, the distribution, thickness, and pathways of these gravity-transported slope-fan sandstones are not well understood and are more complex than highstand transgressive systems tracts or lowstand prograding-wedge systems tracts. This paper presents the occurrence of slope-fan complexes in Oligocene growth-faulted subbasins and their characteristics.

Prior to subbasin formation, incised rivers transported sandy bedloads via terminal deltas onto slopes at different locations, resulting in deposition of deepwater systems basinward of incised shelf edges. Initially, basin-floor-fan sandstones were deposited. Next, as the system prograded, slope-fan systems with amalgamated channels and levees formed along the slope and terminated as lobe-shaped fan deposits. Deposition of these early slope-fan sediments overloaded a ductile substrate (basinal shale or salt), leading to mobilization and fold development. Resultant sediment ridges created localized subbasins and prevented younger slope-fan and prograding-wedge sediments from spilling farther out onto the more distal basin floor. Consequently, after the sediment ridge formed, most of the gravity-flow sedimentation was restricted to these subbasins. Correlation of these slope-fan and prograding-wedge sandstones outside the subbasin is not possible because of the mechanics of their origin. Additionally, because slope fans are point sourced, they are not correlatable laterally for long distances along slope. This lack of lateral correlation of slope sandstones is important to note when exploring in growth-faulted subbasin settings underpinned by mobile substrate.


AAPG Search and Discover Article #90069©2007 GCAGS 57th Annual Convention, Corpus Christi, Texas