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

Hammes, Ursula¹, Hongliu Zeng¹, Robert Loucks¹, Frank Brown¹ (1) Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

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 the lowstand prograding-wedge systems tract with great success. Recently companies have 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 sands are not well understood. This paper presents characteristics of slope-fan complexes and their lateral extent in Oligocene growth-faulted subbasins.

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 sands 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 prevented younger slope-fan and prograding-wedge sediments from spilling farther onto the basin floor and created localized subbasins. Consequently, after the sediment ridge formed, all gravity-flow sedimentation was restricted to these subbasins. Correlation of these slope-fan and prograding-wedge sandstones outside the subbasin is not possible on the basis of the mechanics of their origin. Additionally, because slope fans are point sourced, they are not correlatable laterally along slope. This lack of lateral correlation of slope sandstones is important to understand when exploring in growth-faulted subbasin settings underpinned by mobile substrate.