Heterogeneities Controlled by Interbedding of Eolian and Fluvial Deposits and Its Impact on Reservoir Quality, Weber and Maroon Formations, Rangely Field, Colorado, U.S.A.

Diego J. Van Berkel and Kitty L. Milliken
University of Texas at Austin, Austin, TX

Contrasting sedimentological textures and stratigraphic styles within interbedded eolian and fluvial deposits give rise to vertical and lateral heterogeneities that control the reservoir quality at Rangely Field, Colorado. Composition and compaction parameters were determined for the two facies types in fifty thin sections taken across 2000' of core. The average sandstone architecture for eolian facies is F₇₇M₁C₁₃P₉; framework grain composition is Q₇₉F₁₂R₉; Ø_total=12% (Ø_prim/Ø_prim+Ø_sec ratio=0.6); average IGV=15.5% and an Index of Compaction averaging 0.69 (though is quite variable). Both eolian and fluvial facies produce characteristic responses on radioactive and resistivity logs that enable a reliable distinction of each facies in any vertical succession. Average GR and porosity values for eolian facies range from 25 to 65 API units and 8 to 15% respectively, whereas fluvial ones average about 5% porosity and quite higher GR values with exceptional readings in the order of 150 API units. The latter is associated with Fe-micas and metamorphic rock fragment content. The eolian/fluvial+eolian ratio in the upper 500' of the Weber-Maroon Formations reveals a predominance of eolian facies in the north area of Rangely (ratio=0.67) which progressively decreases to the south (ratio=0.48).

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005