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The AAPG/Datapages Combined Publications Database

Williston Basin Symposium

Abstract

MTGS-AAPG

Seventh International Williston Basin Symposium, July 23, 1995 (SP12)

Pages 69 - 88

Relationship of Salt Patterns to Hydrocarbon Accumulations, North Dakota Williston Basin

Julie A. LeFever, North Dakota Geological Survey, University Station, Grand Forks, North Dakota 58202
Richard D. LeFever, Department of Geology and Geological Engineering, University of North Dakota, Grand Forks, North Dakota 58202

ABSTRACT

Salts have had a profound effect on hydrocarbon accumulation within the Williston Basin. There are a number of successful fields where hydrocarbons have been trapped due to dissolution and related collapse of underlying salts. There are numerous examples where potential oil reservoirs have been destroyed by salt occlusion of the available porosity and permeability.

Salts are widespread throughout the Williston Basin. Traceable salts range in age from Silurian through Jurassic. For this study, thicknesses of 24 major and minor salts were obtained from about 4300 wireline logs throughout North Dakota. From these data, isopach maps of all of the salts were constructed. Areas of salt dissolution were identified by linking thinning on the salt isopach maps with thicker compensating sections in the overlying strata.

The lowermost salts in the stratigraphic section are located within the Inter lake (Silurian) and the Ashern, Prairie, Duperow and Souris River (Devonian) Formations. The Interlake salts reach a maximum thickness of 7 ft (2 m) and are scattered through southern Williams County. Five salts occur in the Ashern Formation to the east of the Nesson anticline in Mountrail County; each of the salts reaches a maximum thickness of about 20 ft (6 m). In North Dakota, the Duperow and Souris River salts are restricted to northern Divide County, although they are extensive to the north of the international border.

The most significant salt in the basin occurs within the Prairie Formation. Trapping of hydrocarbons and significant production related to dissolution of the Prairie Salt has been documented in several cases in Montana, Saskatchewan, and North Dakota. Dissolution is, in part, multi-stage. The limit of the Prairie Salt in central and western North Dakota has long been considered to be the depositional limit. Close examination of Devonian and Mississippian strata surrounding the Prairie edge shows the limit is largely, if not entirely, the result of dissolution, and that the edge of the salt gradually migrated basinward from the Devonian until the early stages of Charles salt deposition.

Distribution of the upper Charles salts (F-A) becomes more limited upsection, and reflects the withdrawal or restriction of the Charles sea. This sequence of 7 salts locally shows dissolution related to tectonic movement along structural elements, including the Nesson anticline and a probable extension of the Hummingbird trough from Saskatchewan into Montana and North Dakota.

The uppermost salts, the Pine, "G" and Dunham Salts, also reflect basement movement that may affect hydrocarbon trapping and production. Many of the salts appear to have been affected by a Late Jurassic-Early Cretaceous event. Some dissolution of the Pine Salt has also occurred locally in Late Cenozoic time.

The best-known examples of hydrocarbon trapping and significant production related to salt dissolution involve the Prairie Salt. However, examination of other salts within the North Dakota part of the basin suggests that such trapping can be documented for several other salts; in many cases, such dissolution appears to have occurred as a multi-stage process.

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