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Abstract: All Fill—No Spill: Slope-Fan Sand Bodies in
Growth-Faulted Sub-basins: Oligocene Frio
Formation, South Texas Gulf Coast
Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX, [email protected]
Growth-faulted sub-basins in the Oligocene Frio Formation are major exploration targets along the South Texas Gulf Coast (Fig. 1). Historically, exploration has targeted on-shelf highstand and transgressive systems tracts and lowstand progradingwedge systems tracts with great success. Companies have recently become interested in exploring for slope-fan sandstone reservoirs in lowstand growth-faulted sub-basins. 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 (Hammes et al., 2005, 2007a).
Slope fans are prolific reservoirs in the deep waters of the Gulf of Mexico and other types of continental margin settings (e.g., Mitchum et al., 1993; Straccia and Prather, 2000). The typical slope and basin-floor-fan models in Pliocene and Pleistocene deepwater Gulf of Mexico basins are interpreted to exhibit a fill-and-spill sequence within one 3rd/4th-order minibasin (e.g., Pirmez et al., 2000; Hooper et al., 2002).
In contrast, Frio slope fans in growth-faulted sub-basins fill the present accommodation space but rarely spill into the next sub-basin within a 3rd-order sequence because of an evolving sediment ridge. interpreted The growth-faulted Frio Formation sub-basins resulted from early slope-fan sediments overloading a ductile substrate (basinal shale or salt) above a detachment surface (Brown et al., 2004; Hammes et al., 2005, 2007a). This led to mobilization and fold development of a sediment ridge during one 3rdorder lowstand of sea level (Fig. 2). Slope-fan systems with amalgamated channels and levees formed along the slope and terminated as lobe-shaped fan deposits. This produced downslope sediment ridges which ponded slope-fan sediments and kept them from spilling farther downslope onto the deeper basin floor (Fig. 3). Consequently, after a sediment ridge formed, all gravity-flow sedimentation was contained within its attendant sub-basin.
Overall, slope fans have limited lateral continuity because of avulsion of lobes in the slope-fan system (Brown et al., 2004). When correlating more proximal sub-basin slopefan bodies to more distal slope-fan bodies, time stratigraphic rather than lithostratigraphic correlations
Figure 1. South Texas regional tectonic map and study area. Displayed are the growth faults that parallel the coastline. Note the absence of salt domes in the study area (indicated by the box). This area is dominated by mobile shale. The study is based on data from South Texas Bay areas. (Modified from Ewing, 1991)
must be performed (Brown et.al., 2004, their Fig.9). Correlating “first sands” likely leads to an erroneous interpretation. As the prograding-wedge system prograded over these slope fans later during the lowstand, sediment-ridge and growth-fault movement ceased. Transgressive and highstand systems tracts completed the sub-basin depositional sequence. A new sequence will then begin with the next sea-level lowstand.
Production from slope fans in the south Texas Gulf Coast has been uncommon except in a few wells. Cumulative production ranges between 132 MMCF and 3.3 BCF and 5-130 thousand barrels of condensate. Porosities are typically between 10 and 25%, permeabilities range from
Figure 2. Conceptual model of slope-fan deposition in growth-faulted su-basins (from Hammes et al., 2007b). Lowstand systems tract commences with basin-floor-fan deposition onto fine-grained basinal sediments. Slope failure along incipient growth fault creates a depression that serves as ponding basin for subsequent lowstand deposits. Slope fans are being deposites into the sub-basin mobilizing the unconsolidated shaley basinal sediments. Growth fault initiates movement and slope fans develop growth and rollover into fault. Depositional systems prograde over the slope-fans, establishing a prograding wedge. Sediment ridge and growth-fault movement cease. Transgressive and highstand systems tracts complete sub-basin deposition until a new sea-level lowstand occurs.
Figure 3. Frio growth-faulted sub-basin model showing slope-fans being trapped behind sediment ridge and slope channels aligning parallel to slope (from Hammes et al., 2007a). Slope-fans pond behind the sediment ridge that rose owing to loading of coarser-grained sediment onto fine-grained, muddy sediments. Note that no spill occurs beyond the sediment ridge into a subsequent sub-basin.
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