Hydrodynamic Trapping in Mission Canyon Formation (Mississippian) Reservoirs: Elkhorn Ranch Field, North Dakota

DEMIS, WILLIAM D., Marathon Oil Company, Houston, TX

Hydrocarbons in Mission Canyon dolomite reservoirs in the Elkhorn Ranch field area are trapped by downdip flow of formation water to the northeast.

Elkhorn Ranch field is located on a north-plunging anticline with only 10 ft (3 m) of crestal closure. The Mission Canyon is a regressive, shallowing upward sequence of subtidal dolomitized mudstones and wackestones grading upward into sebkha-salina evaporites. Mission Canyon oil production is localized on the north and northeast side of the structure.

Maps of porosity pinch-outs and permeability barriers defined from core data, superimposed upon the Mission Canyon structure, show that most of the oil cannot be trapped by stratigraphic facies change. South-west-trending, updip porosity pinch-outs cross the north-plunging structural axis at an angle so low that hydrocarbons would leak out to the southwest under hydrostatic conditions. Downdip hydrodynamic flow to the northeast provides the critical trapping component.

Regional maps of apparent formation water resistivity and water salinity show a region of fresher water south and southwest of the field. A regional potentiometric map constructed using Horner-plot extrapolated shut-in pressure data indicates a head gradient of about 20 ft/mi (4 m/km) to the northeast at Elkhorn Ranch field. This gradient corresponds to a calculated water-oil tilt of about 50 ft/mi (20 m/km). Observed tilt of the oil accumulation is actually about 25 ft/mi (5 m/km) to the northeast. This discrepancy might be the result of the field having not yet reached equilibrium with the invading water.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)