Abstract: Abnormal Electric Resistivity and Fluid Pressure in Bakken Formation, Williston Basin, and its Relation to Petroleum Generation, Migration, and Accumulation

Fred F. Meissner

Geochemical data show that organic shales in the Bakken Formation (lower Mississippian) are excellent petroleum source rocks where they have been buried sufficiently to have achieved thermal maturity.

Anomalous variations in electric resistivity in the Bakken shales are believed to be caused by the indigenous generation of hydrocarbons. Changes from low and normal shales resistivities (2 to 15 uhm-meters) to nearly infinite resistivity occur abruptly at a subsurface temperature of approximately 165°F (74°C). This temperature appears compatible with that for the critical temperature of oil-generation, on the basis of recently developed thermodynamic time-temperature relations and indicates that generation is currently active. The dramatic change from low to high resistivity suggests that conductive pore water in immature source-rock shales has been replaced by nonconductive hydrocarbons as oil is generated and the source rock becomes mature. The phenomenon further suggests that the hydrocarbons may be expelled as a continuous fluid phase saturating an oil-wet solid matrix. Variations in geothermal gradient associated with the resistivity-derived maturity criterion show that depth to oil generation ranges from approximately 8,500 ft (2,590 m) on the east side of the Williston basin to 7,000 ft (2,134 m) on the western side.

Drill-stem tests within the mature area of the Bakken Formation show that formation fluids are substantially overpressured and that hydrocarbons are the only producible fluid species present. Collapse of the rock matrix, as overburden-supporting solid organic material is converted partly to nonexpelled fluid, is believed to be the dominant process responsible for high fluid pressures. The amount and distribution of fluid overpressure in the Bakken may be determined from sonic-log transit times. DST-measured and log-determined fluid-pressure gradients range from 0.47 to 0.80 psi per foot, and all values greater than 0.47 (hydrostatic) are confined to the area of source-rock maturity and active hydrocarbon generation. Although the Bakken is sandwiched between seemingly impermeable rocks in the overlying Lodgepole and underlying Three Forks Formations, the fact that maximum fluid overpressure is less than lithostatic (1.0) suggests that some fluid expulsion is taking place from the system.

The area of Bakken oil generation contains several oil fields which produce from fracture-type reservoirs developed within or adjacent to the source-rock unit. Fracturing is believed to be controlled by the combined presence of high pore-fluid pressure and high differential rock stress, as predicted by the Hubbert-Rubey effective-stress concept and the Mohr-Coulomb-Griffith failure theory. Differential stress at Antelope field is caused by drape-fold bending over an uplifted basement fault block.

Fracturing related to oil generation and associated fluid overpressure may be an important mechanism allowing migration of oil from Bakken source rocks vertically upward through overlying dense rocks in the Lodgepole to reservoirs in the Mission Canyon or downward through dense rocks in the Three Forks to reservoirs in the Nisku.

AAPG Search and Discovery Article #90971©1976 AAPG-SEPM Rocky Mountain Sections 25th Annual Meeting, Billings, Montana