Abstract

The Desert Creek substage of the Desmoinesian (Pennsylvanian) Paradox Formation is the most important oil-producing interval in the Paradox Basin. It produces mainly from porous carbonates containing phylloid algae. Reservoir rocks rich in phylloid algae in the older, larger fields such as Aneth, and the satellite fields around Aneth, are widely recognized as carbonate buildups. Lower Desert Creek reservoirs discovered more recently closer to the center of the Paradox Basin, however, such as Bug, Cutthroat, Island Butte, and Spargo, produce from a previously unreported type of phylloid-algal deposit with depositional facies and reservoir geometries quite different from those of the carbonate buildup reservoirs.

These lower Desert Creek discoveries are located more than 20 miles basinward (northeast) of the Aneth area. The reservoir rocks were deposited in somewhat deeper, quieter, and more saline waters than those of the buildup reservoirs nearer to the basin margin. In the lower Desert Creek reservoirs, Kansasphyllum rather than Ivanovia is the dominant form of phylloid algae.

Sedimentologic and stratigraphic relationships indicate that the lower Desert Creek reservoirs and associated anhydrite beds were deposited in sea-floor lows rather than on paleobathymetric highs, Isopach maps of intervals within and associated with the lower Desert Creek show that the sea-floor lows resulted from dissolution of halite in the subjacent Akah substage. Thickness of the phylloid-algal reservoirs that formed in these paleo-lows was controlled by the amount and timing of halite dissolution. Up to three fifth(?)-order parasequences in the thickest reservoir intervals each record deposition resulting from episodic halite dissolution over periods of a few thousand years.

Diagenetic events shaping the lower Desert Creek reservoirs include: 1) marine cementation at the base of the phylloid algal deposits; 2) complete dolomitization during or shortly after deposition (which contributed to preservation of primary porosity); 3) partial plugging of pores by early anhydrite cement; 4) burial and formation of compaction-induced fractures; 5) late-stage dissolution which enhanced the fracture porosity; and 6) late-stage cementation by anhydrite and halite that has locally destroyed reservoir quality.

Most of the lower Desert Creek reservoirs trend in a northwest-southeast direction. They are from 10 to 15 miles long, 1 to 3 miles wide, and 20 to 60 feet thick. Porosity and permeability average 12% and 400 md, respectively. Wells productive from the lower Desert Creek typically produce 200 to 1000 MBO and 0.2 to 3.0 BCF of gas on primary recovery from depths of about 6000 feet.