Abnormal Pressure in the Antelope Field, North Dakota
Authors: Finch William C. 
Published: December 2007

This southeast spur of the Nesson Anticline field produces oil from two reservoirs that demonstrate anomalous pressure relationships. This abnormal pressure is induced by tectonic forces during structural growth and may be expected in other parts of the Williston basin wherever similar structural and stratigraphic conditions exist.

Introduction

The purpose of this paper is to present evidence of abnormally high pressure in a single reservoir that lies laminated between normally pressured reservoirs of an oil field in North Dakota (Fig. 1) and to offer a possible reason for it. possible reason for it. However, before proceeding, it is necessary to define what is implied here by the words "normal" and "abnormal".

Personal experience, gained from several years of regional hydrodynamic and oilfield studies in the Rocky Mountain region, embracing the area from southern Saskatchewan to northern New Mexico, reveals that, with some marked exceptions, the pressure gradient in the sedimentary column of that region normally approximates 0.433 psi/ft of depth. Exceptions occur in western Montana, the Denver basin, the Powder River basin, and the San Juan basin, mostly in Cretaceous rocks. These exceptions indicate that in some regional reservoirs pressures are subhydrostatic for a column of fresh water. In the Willisten basin in North Dakota, where saline waters up to 356,000 ppm occur, the gradient runs to about 0.512 lb/ft. In view of this experience, the occurrence of a pressure of 7,670 psi at a depth of 10,540 ft is considered to be abnormally high. This pressure yields a gradient of 0.725 psig and provides good reason for study and speculation.

An unlimited reservoir with high pressure can be a plus for production and a minus for drilling. Is this particular pressure regional or local? What causes it? particular pressure regional or local? What causes it? Can it be anticipated? These are some of the questions that call for answers.

History of Development

Fig. 2 is a structural contour map of a limestone reservoir above the Sanish zone. It coincides very closely with a map of the Sanish pool published by G. H. Murray and is used here because it is considered to be more detailed.

Oil was discovered at Antelope on Dec. 6, 1953, when Pan American Petroleum Corp. completed Woodrow Starr No. 1 from a perforated interval (10,525 to 14556 ft) in a silty dolomite of Devonian age. Initial production was 1,420 BOPD flowing, 43.8 degrees API gravity oil. Initial BHP at 8,400 ft was 7,670 psig. Subsequently, 43 Sanish wells were completed, and 8,541,102 bbl of oil had been produced as of Jan. 1, 1968.

Amerada Petroleum Corp. discovered oil in the Upper Mississippian Nesson zone on May 12, 1956, when the Lacey Norby No. 1 was completed, flowing at 356 BOPD through perforations from a limestone reservoir at 9,057 to 9,088 ft. This well made 39.8 degrees API gravity oil. Initial BHP at 6,750 ft was 4,207 psig. To date, 53 Mississippian wells have produced 9,177,980 bbl of oil. produced 9,177,980 bbl of oil. Just below the Sanish zone, oil was discovered by Amerada in the Devonian Nisku formation on Feb. 16, 1960.

JPT

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