Given an honest explanation of the unique problems involved in producing the gas cap of this combination reservoir, the working interest owners, the gas purchasers, and the royalty owners all rose to the challenge, voluntarily agreeing to restrict sales until a unit could be formed. The fruits of their cooperation amount to an estimated 3 million barrels of additional oil.
Introduction and Geology
The Northwest Avard Hunton limestone reservoir lies approximately 10 miles west of the town of Alva, Woods County, Okla. Fig. 1 is a structure map of the top of the Hunton in the field. First Hunton production in the general area was established in March, production in the general area was established in March, 1967, in Section 23, T27N-Rl5W. Later development has proved this well to be geologically separated proved this well to be geologically separated from the unit area. A gas well in Section 16, completed in May, 1967, was the first producer from the eventual unit. The first oil production was in July, 1967, from Section 21. A total of nine oil wells and three gas wells was completed in the unit area. The average depth to the Hunton is approximately 6,200 ft.
The Hunton reservoir producing in the Northwest Avard field is correlative with the Silurian Chimney Hill section in other areas where the entire Hunton group is present. This area of the Anadarko basin contains the extreme northern edge of the Hunton formation, which is truncated against the unconformity immediately overlying it.
The accumulation of oil and gas in this field is associated with a stratigraphic trap characterized by truncation of the total Hunton zone in all directions except to the south where a water table is present. General dip of the Hunton is approximately 80 ft/mile to the south. (See Fig. 1.)
The eastern and western limits of the reservoir are fairly well defined by dry holes or thin Hunton intervals. These limits were possibly formed by erosional channels, or drainage streams, during the time of the unconformity. The reservoir's northern limit appears by the unconformity to be defined by either a cross-drainage stream or complete Hunton erosion. Pressure surveys show a definite separation between the Hunton outlier in Sections 7 and 8 and the main reservoir.
Electric logs of Unit Well 5–3 indicated the water-oil contact at a subsea elevation of −4,694 ft. The size of the aquifer is indicated by water recovery on DST's of correlative Hunton zones in dry holes from 3 to 10 miles from the field. A gas-oil contact at a subsea elevation of −4,633 ft was established by well completion data and by early performance history of some of the oil wells. Unit Well 4–1 originally produced with a high GOR until extremely high produced with a high GOR until extremely high withdrawal rates from the gas cap caused upward movement of the gas-oil contact. Subsequently, the GOR in this well fell from approximately 2,200 to 525 cu ft/bbl. A similar but less pronounced drop in GOR was exhibited by Well 5–1.
Fig. 2 is an isopachous map of the gross Hunton zone. Most of the better porosity results from dolomitization of the original limestone. Intervals of porosity in excess of 20 percent occur randomly in porosity in excess of 20 percent occur randomly in the section, along with intervals as low as 2 percent. Core analysis indicates good permeability, even in the lower porosity ranges.