Roderick W. Tillman¹

(1) Consulting Geologist, Tulsa, OK

ABSTRACT: Muddy Sandstone, Tidal Paleovalley-Fill Sandstone Reservoir, Sun Ranch Field, Wind River Basin, Wyoming

Sun Ranch Field produces from the Lower Cretaceous age Muddy (Grieve) Sandstone in the Wind River Basin of Wyoming. Detailed geological analysis, using seven cored wells, indicates that the reservoirs in the field were deposited primarily by tidal back-filing of an incised paleovalley.

The discovery well, the Sun Ranch No.1, initially produced 1048 STB/D. Within a year most of the 10 wells that produce in the field were completed and a number of significant production problems were observed indicating the presence of reservoir heterogeneity and compartmentalization. Compartmentalization of reservoirs occurs on varying scales. A dozen laterally baffled or flow-separated sandstones are identifiable within the field. Failure of large amounts of gas injected in updip wells to affect any downdip wells, production of oil above gas in some wells, and highly variable production among adjacent wells result from compartmentalization.

The presence of clay deposited during slack tides and shales deposited in sheet like and topography filling deposits cause much of the compartmentalization. These very thin relatively widespread horizontal beds are easily recognizable in core but not on most logs. The tidal bar sandstones contain detrital clay as thin beds, drapes, clasts and "grains." Log porosities may calculate significantly lower where clay occurs as thin drapes or clasts in otherwise high quality reservoir sandstones. Tidal accretion bars are the dominant fill of the the earlier paleovalley sand filling event. Subfacies of the accretion bars which are recognizable include: (1) slurried, (2) slumped, (3) incipient slumped, (4) undeformed. The final sand-filling event, which yields a large proportion of the field's production, probably resulted from redeposition of shoreline sands which were swept into the mouth of the estuary.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado