November 2007 Houston Geological Society Bulletin 17 G rowth-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 prograding- wedge systems tracts with great success. Companies have recently become interested in exploring for slope-fan sandstone reser- voirs in lowstand growth-faulted sub-basins. However, the distri- bution, thickness and pathways of these gravity-transported slope-fan sandstones are not well understood and are more com- plex 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 3rd- order 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 slope- fan bodies to more distal slope-fan bodies, time stratigraphic rather than lithostratigraphic correlations by Ursula Hammes, Hongliu Zeng, Robert Loucks and Frank Brown, Jr. Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX, Ursula.Hammes@beg.utexas.edu HGS Northsiders Luncheon Meeting Tuesday, November 13, 2007 Crowne Plaza Hotel (former Sofitel Hotel) • 425 Sam Houston Pkwy. North Social 11:15 a.m., Luncheon 11:30 a.m. Cost: $31 Preregistered members; $35 non-members & walk-ups The HGS prefers that you make your reservations on-line through the HGS website at www.hgs.org. If you have no Internet access, you can e-mail reservations@hgs.org, or call the office at 713-463-9476 (include your name, e-mail address, meeting you are attending, phone number and membership ID#). Northsiders Luncheon Meeting All Fill—No Spill: Slope-Fan Sand Bodies in Growth-Faulted Sub-basins: Oligocene Frio Formation, South Texas Gulf Coast Northsiders Luncheon Meeting continued on page 19 Correlating “first sands” likely leads to an erroneous interpretation. 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)