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Lesli J. Wood1
(1) The University of Texas at Austin, Austin, TX

Abstract: Falling-stage tidal-bar complexes and incised valleys of the Sego Sandstone, Upper Cretaceous, Utah: Implications for reservoir development in heterolithic deltaic systems

The Upper Cretaceous Sego Sandstone of Utah and Colorado is composed of three genetic sequences bounded by major flooding events, which enable regional correlation westward along outcrop exposures into more proximal facies and northwestward into subsurface data located adjacent to the outcrop. Four key surfaces—the regressive surface of marine erosion (RSME), lowstand surface (LS), marine flooding surface (MFS), and maximum MFS (CS)—form the stratigraphic framework for reservoir architectural analysis, and their importance is supported by regional analysis of rock property trends in both outcrop and subsurface data. Oldest to youngest, Sequence One is composed of falling-stage and lowstand tidal-bar complexes deposited across the RSME and overlain by an LS characterized by estuarine distributary channelization. Sequence Two, thinner than Sequence One, contains thick falling-stage tidal-bar deposits deeply incised by sand-rich lowstand valleys. Sequence Three, deposited on the backstepping limb of the third-order lower Sego cycle, is thinner than the previous two sequences, consisting of thin tidal-bar complexes shallowly incised by valleys containing fine-grained heterolithic fills. Transgressive and highstand oyster beds are regionally extensive and pervasively cemented. Tidal bars contain internal inclined shales that influence fluid flow. Maximum lowstand valleys, deeply incised and filled by homogeneous, high net sands form primary reservoir targets and act as flow connections linking older tidal-bar complexes. Valleys developed on the rising limb of the Sego third-order cycle are more shallowly incised, and fills are more fine-grained and heterolithic. These valleys dissect falling stage tidal-bar complexes and flow-partition interfluves.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana