Colorado Rockies Yield an Intriguing Oil-Ring Accumulation
Authors: Adams Norman W. 
Published: December 2007

Abstract
The McCallum Unit anticline, located near Walden in Jackson County, Colo., possesses highly unusual fluid characteristics because the produced gas is 92 per cent carbon dioxide. The retrograde condensate cap of the Dakota-Lakota reservoir on the structure was discovered in 1926, but the oil ring along the anticline flank was not encountered until 1960. Production is now primarily from the 30 degrees API oil ring which contains the some nearly pure carbon dioxide gas in solution.Although the oil-ring discovery was not made until the condensate reservoir was well below the dew point, the oil accumulation is not a result of retrograde condensation. The reservoir condensate is extremely lean and analyses have indicated a maximum retrograde liquid content of less than 1 per cent of the total reservoir volume. Therefore, this unusual oil ring has been in association with the condensate cap throughout the life of the reservoir.The retrograde condensate cap also exhibits a rare performance history. Initial gas-oil ratios averaged between 50 and 70 Mcf/bbl. Dry gas cycling took place above the dew point for 10 years and was followed by partial pressure depletion of the condensate reservoir. Gas-cap withdrawals ceased during 1962 to conserve reservoir pressure and prevent further upstructure migration of the oil ring.Dakota-Lakota reservoir production prior to 1963 totaled nearly 2.9 million bbl of both oil-ring crude and gas-cap condensate and over 300,000 MMcf of carbon dioxide.
Introduction
Continental Oil Co.'s McCallum Unit is located about 150 miles northwest of Denver and 4 miles northeast of Walden, Colo., at the base of the east range of the Rockies. The field is unique in that the produced gas is 92 per cent carbon dioxide, 4 per cent nitrogen, and 4 per cent condensable hydrocarbons. The anticline structure lies in the northeast portion of the North Park area of Jackson County, where the ground elevation averages over 8,000 ft, and all facilities are designed strictly for cold weather operations. Production from the anticline was limited to high-gravity condensate until 1960, when a 30 degrees API oil ring was discovered along one structural flank at Lakota depth. Other unit reservoirs which commercially produce are the Morrison and the Muddy. Both produce gas containing the same high CO2 concentration.With reference to Fig. 1, additional production in the area includes Lion Oil Co.'s Battleship field, 1 mile north of McCallum; Cabeen's Canadian River field 4 miles east; and Gulf's South McCallum field adjacent to the southeast boundary of the unit. Production from the Battleship field is a dead 33 degrees API crude with only negligible gas volumes from Dakota-Lakota depth. Canadian River produces mainly 21 degrees API crude, with nearly pure methane gas, from the Dakota-Lakota interval. Gulf's South McCallum field is a retrograde condensate, CO2-rich reservoir of Dakota-Lakota origin similar to the McCallum Unit gas cap. Production is currently 53 degrees API condensate with no known oil ring present.Within the McCallum Unit, the practice has been to strip the condensate from the reservoir gas and flare the remaining carbon dioxide to the atmosphere. Volumes flared have exceeded 80 MMcf/D of CO2 from the Dakota-Lakota reservoir. Actual commercial use of the gas has been limited.
JPT
P. 927^