Abstract

The canyons of the San Juan River provide impressive outcrops exposing the Upper Pennsylvanian strata of the Paradox and Honaker Trail Formations, and the Lower Permian Group. These outcrops reveal the Upper Paleozoic geology of the Paradox Basin; the same units are productive in the subsurface. They are the analog to the prolific Greater Aneth Oil Field.

Four vertical sections along the canyon of the San Juan River were measured, sampled, and described. The data collected was used to build detailed stratigraphic columns. Eight depositional environments were identified and they include: fluvial, eolian, beach, lagoon, tidal flat, high energy shoal, proximal open shelf, distal open shelf.

The Upper Pennsylvanian Paradox and the Lower Honaker Trail Formations (Desmoinesian through early Missourian) are dominated by transitional – marginal marine and open marine carbonates with intermittent tidal flat sandstones and siltstones. The Upper Ismay Member is dominated by marine sediments that become shallower from west to east. The Lower Honaker Trail Formation is deeper marine on the west and shallow to transitional environments eastward. As the cycles transitioned into the Middle Missourian – Virgilian, siliciclastic content increased. The Upper Honaker Trail Formation and the Lower Permian all appear to be predominantly non-marine sands. These upper cycles are composed of flood plain, fluvial, and eolian sandstones.

The various lithofacies observed in outcrop sections were bundled into depositional cycles that represent intervals of sea level rise and fall and exhibit a deepening to shallowing succession of facies. These cycles represent a mixed system of carbonates and sandstones that were deposited on the shelf of the Paradox Basin (Grammer and others, 1996). The best reservoir units appear to be shallow marine grain-dominated carbonates and fluvial and eolian sands.

The Pennsylvanian strata on the Eastern Shelf of the Permian Basin are very similar to those located on the shelf of the Paradox Basin. The results from this study can be directly applied to the deposits on the Eastern Shelf and in North Central Texas.