Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Reservoir Modeling of Highly Heterolithic Tidal Shoreline Deposits in the Sego Sandstone, Utah; Translating Detailed Outcrop Knowledge to Subsurface Understanding of Flow in Stratigraphically Complex Systems
(1) Bureau of Economic Geo, Univ of Texas at Austin, Austin, TX.
Tidally-influenced deposits form reservoirs worldwide, but their heterolithic nature makes recovery numbers low and emphasizes the need for improved flow modeling to predict well placement and maximize recovery. Two sequences of the Lower Sego Sandstone of eastern Utah provide an excellent setting for modeling facies distributions in tidally influenced deposits. We used 39 measured sections along >2 km of exposed outcrop in the San Arroyo Canyon section of the Lower Sego and 36 wells in the adjacent subsurface for modeling. These strata were originally deposited along the shoreline and in estuaries of the Upper Cretaceous Interior Seaway. Exposed at the outcrop are five units of the Lower Sego, including a lower sequence comprising a highstand systems tract (ST) topped by a falling-stage ST. The latter has minor channelization and a limited transgressive ST and an upper sequence of a highstand ST topped by a falling-stage ST overlain by a significant lowstand surface of erosion and transgressive estuarine fills forming the fifth and final unit. Facies were interpreted in each well log using a template derived from outcrop-gamma and facies logs. Following this process, we generated facies-log curves to provide input into facies simulation.
Multiple simulations were generated using six lithofacies. Facies relationships and variograms were used to distribute facies laterally while honoring well and outcrop data. The two highstand units where characterized in simulations by along-shore banks of muds and muddy sands with small, isolated, shoreline perpendicular distal bars. This highstand geomorphology contrasts with falling-stage geomorphology, which shows high-energy derived heterolithic and homolithic cross-stratified sands that built large (>15,000 ft in length and 2,200 ft in width) migrating, shoreline perpendicular tidal bars. The uppermost unit is the lowstand incised valley that downcuts into the upper falling-stage ST and well down into the underlying highstand ST. It is marked in the model by multiple tidal bars, stacked and laterally translated within the lowstand/transgressive fill. Bars in the lowstand/transgressive estuary fill of unit five, smaller than those in the falling-stage ST, are capped by bioturbated sands.