Characterization of thickness anomalies in the Bakken and Three Forks formations, north central North Dakota, USA

Abstract

Isopach maps of the Three Forks and Bakken formations show thickness anomalies at or near their depositional and/or erosional limits in north central North Dakota. Thickness anomalies align near to and outside of the Prairie salt zero edge, which is shown to be a dissolutional edge by several lines of evidence. Prairie salt dissolution, collapse of overlying beds, and compensating infill is the primary accommodation mechanism for the formation of thickness anomalies in the study area. Isopach maps of the Devonian Dawson Bay through Mississippian Mission Canyon formations show scattered thickness anomalies resulting from episodic and sporadic Prairie salt dissolution through geologic time. Cross-sections show compensating infill in the Bakken and Three Forks formations within both single-stage and multi-stage collapse structures. Enhanced groundwater flow through faults, fractures, and Winnipegosis reef mounds is considered to have influenced Prairie salt dissolution. Residual structure maps of Devonian horizons are indicative of paleotopography, and show depositional thickening within a paleotopographic low resulting from Prairie salt-dissolution collapse. Additionally, depositional thinning occurs above the Burleigh and Churchill-Superior boundary paleohighs. Three basement faults are proposed using Devonian through Mississippian isopachs and cross-sections. An aeromagnetic anomaly map of North Dakota, which is interpreted for basement terranes and faults, substantiates the existence and locations of these basement faults. The Churchill-Superior boundary is highly coincident with the Prairie salt zero edge, suggesting that associated faulting and fracturing likely influenced salt dissolution. The aeromagnetic anomaly map shows excellent correlation to isopachs in the study area, suggesting that basement faults influenced thickness variations in the overlying strata. This is primarily interpreted as the result of enhanced Prairie salt dissolution over basement faults, but may also be the result of deposition over upthrown and downthrown basement fault blocks. Increased surface lineament density occurs over an area of greatest salt-dissolution collapse, suggesting that the collapse of overlying strata forms faults and fractures that are expressed at the surface. A collapse breccia observed in the California Blanche Thompson 1 (SWSE S31-T160N-R81W) core grades into an extensive vertical fracture network that persists for hundreds of feet upsection. Vertical fractures observed in core of the Nisku Formation, directly underlying Bakken and Three Forks thickness anomalies, may also be evidence for dissolution-collapse fracturing. This study proposes that strata overlying areas of salt dissolution and underlying areas of compensating infill will have collapsed and fractured. This conclusion may be used as an exploration model for fractured sweet spots in the Bakken and Three Forks formations. The sequence stratigraphy of the Bakken Formation was interpreted from several core descriptions within the study area. A sequence boundary and regressive surface of erosion at the upper Three Forks surface is responsible for the erosional extent of the Three Forks Formation in the study area. A sequence boundary and regressive surface of erosion in the lower Middle Bakken Member is responsible for the erosional extent of the Lower Bakken Shale in the study area. The extents of the Middle Bakken and Upper Bakken Shale members are interpreted to be depositional in the study area. The Lower Bakken Shale thickness anomalies are not the result of an incised-valley fill. Additionally, the Sanish sandstone facies of the Pronghorn Member is not a fluvial deposit, but is a transgressive sand sourced from erosion of the upper Three Forks Formation during the subsequent sea level rise.