ABSTRACT

Hydrocarbons in Mission Canyon reservoirs in the Elkhorn Ranch field area are trapped by hydrodynamic flow of waters downdip and to the northeast. The Elkhorn Ranch field is located on a north-plunging anticline with only very minor crestal closures. Mission Canyon production is localized on the north and northeast side of the structure. Subtidal/ strandline facies of the Mission Canyon, as defined by anhydrite and porosity isopachs, trend south-southwest across the plunge of the structure at a very low angle, so low as to preclude trapping by stratigraphic pinch-out.

The lower Mohall, upper Mohall, and Sherwood porosity intervals of the Mission Canyon are all productive within the field. Each has a unique distribution of oil production, but the apparent oil-water contact for each zone is tilted the same direction and amount. Plots (3 cycle, semi-log) of porosity versus core-derived permeability data for the upper and lower Mohall porosities show a linear relation above a threshold of about 12 percent porosity, below which no permeability (<1 md) exists. The Sherwood has an empirically-determined effective cut-off of 8 percent. The configuration of structure and porosity pinch-outs (and permeability barriers defined from the core data) for these productive intervals does not provide a hydrocarbon trap. Tilt of the oil accumulation appears to be in the order of about 25 feet per mile to the northeast. Tilts of oil-water contacts determined for the Elkhorn Ranch field are consistent with tilts for the Billings Nose field (Berg and Mitsdarffer, 1986).

A regional map of formation water salinity shows a region of fresher waters south and southwest of the field. A regional potentiometric map constructed using Horner-plot extrapolated shut-in pressures, and corrected for salinity variations, shows a tilt of about 60 feet/mile to the northeast of the Elkhorn Ranch field. The calculated tilt of the oil column is 150 ft/mi, while the observed tilt is one-sixth of that gradient. This discrepancy may result from the field not having reached equilibrium with the invading waters, a conclusion first stated by Berg and Mitsdarffer (1986) for Billings Nose.

Exploration and production geologists may be able to expand their prospects by incorporating this hydrodynamic model into their Mission Canyon play concepts.