An ancient stratigraphic trap for petroleum exists in continental deposits at Rangely oil field where the eolian Weber Sandstone (Pennsylvanian-Permian) inter-tongues with the fluvial Maroon Formation. The stratigraphic trap developed as a result of the progradation of eolian dunes toward the ancient Uncompahgre uplift. Layers of fine silt and conglomeratic material that formed along the margins of the dune field created a permeable barrier, owing to diagenetic cementation and their intrinsic textural properties. The conditions which created the stratigraphic trap at Rangely may have developed in other areas along the margins of ancient Pennsylvanian uplifts in Colorado, Wyoming, and Utah.

Analysis of core indicates that porosity and permeability within the oil-producing sandstone are affected by diagenetic processes. Burrowed and contorted intervals are more intensely cemented and have reduced porosity and permeability values relative to undisturbed intervals.

Evidence for eolian origin of the Weber Sandstone

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near Dinosaur National Monument includes: (1) large-scale tabular-planar cross-stratified units with many low-relief, asymmetric ripples oriented up and down slip-face deposits; (2) raindrop imprints on slip-face deposits; (3) contorted stratification, lamination style, and burrowing in the Weber match those of modern eolian deposits; (4) well-sorted quartz sandstones interbedded with poorly sorted, micaceous siltstones and conglomerates at Rangely oil field; interpreted to be interbedded eolian dune and fluvial sediments respectively; (5) consistent southward transport directions in the Weber Sandstone considered more compatible with a wind-driven depositional system than a marine depositional system; (6) the general lack of appreciable clay or chert in the study in contrast to the o currence of these minerals in marine rocks of the Weber farther west; (7) thin lenticular carbonate rocks (commonly only 0.30 to 0.60 m thick) restricted to extensive diastems indicating deposition in nonmarine ponds associated with interdune areas; and (8) striking differences between the lenticular, brecciated, nonfossiliferous dolomites in the Weber Sandstone here and the thick, fossiliferous cherty limestones of equivalent age in marine deposits within the Weber and the older, underlying Morgan Formation elsewhere.

Shortly after deposition, the eolian deposits of the Weber Sandstone became saturated with water and were then subjected to penecontemporaneous deformation, which produced complex folding and breaking of laminations.

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