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Effects of Hydrodynamic Flow on Carbonate Stratigraphic Traps, Mission Canyon Formation, Billings Nose Fields, North Dakota

Robert R. Berg, Alan R. Mitsdarffer

Downdip hydrodynamic flow has produced a plume of relatively fresh water that is invading the Mission Canyon aquifer below oil accumulations in thin dolomites. Water resistivities range from 0.09 ohm-m at the south margin of the fields to 0.01 ohm-m at the north, and correspond to NaCl salinities of 20,000 to 200,000 ppm, respectively. Salinities were established from well logs by plotting true resistivities as a function of porosity, and salinity distributions agree with measured salinities of produced waters. Reservoir pressure was determined by extrapolation of drill-stem-test pressure buildups; then hydraulic heads were calculated. The change in salinities causes a head gradient of 50 ft/mi (10 m/km) across the oil fields, a gradient that would result in an oil-water ilt much greater than observed. The present oil-water tilt of 25 ft/mi (5 m/km) corresponds to a lower head gradient of 10 ft/mi (2 m/km) under constant density conditions. Therefore, we concluded that oil accumulated under high-salinity conditions and that the freshwater plume has only recently reached the field area. The existing head gradient is capable of flushing the oil accumulation and suggests that greater structural, rather than stratigraphic closure would be required to trap oil in updip locations. Furthermore, saturation will be difficult to interpret from well logs in similar zones of salinity change because of uncertainties in formation-water resistivity.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.