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ABSTRACT: Characterizing tidally influenced reservoirs through integrated sequence stratigraphic study of subsurface and outcrop: Sego Sandstone, Book Cliffs, Utah, U.S.A.

Wood, Lesli J.1 and Brian Willis2
(1) The University of Texas at Austin, Austin, TX 
(2) State University of New York at Oswego, Oswego, NY

The Sego Sandstone (Campanian) is exposed extensively in eastern Utah, immediately adjacent to several hundred wells that provide conventional logs and core collected over the outcropping interval. It consists of four fourth-order genetic sequences deposited by a series of prograding tidal deltas and dissected by a network of valleys. Regional correlation of these data provides a three-dimensional framework of key surfaces within which to examine the distribution of architectural and depositional elements and their petrophysical properties. Gamma scans and velocity data from outcrop and core provide a proxy for inferring petrophysical character and facies from subsurface log suites. We integrated these data and came to the following conclusions:

1. Five depositional elements are mappable in subsurface wells through the Sego: deep marine, wave-dominated shelf, incised valleys, tidal bars, and transgressive complexes. Each has a distinct petrophysical character that can influence production.

2. Transgressive systems tracts (ST's) are dominated by low-permeability, bioturbated sandstone. Falling-stage ST's are dominated by sandy, homogeneous, crossbedded bars. Highstand ST's contain marine muds and hummocky, cross-stratified sandstones. Lowstand ST's are dominated proximally by fluvial facies and tidal bars and distally by heterolithic, bioturbated tidal bars .

3. Bioturbated transgressive complex sands (facies 5) can have a dramatic impact on reservoir quality and can be the most difficult to differentiate in subsurface logs. Velocity contrast between facies 5 and other facies may provide a tool for mapping facies 5 in subsurface sonic and density logs.

4. The character of lowstand incised valleys and channels varies temporally and spatially in a continuum as a function of changing accommodation.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia