Lodgepole reef potential seen in Montana Williston basin

Dec. 16, 1996
Larry Brogdon H.A. Hedberg Trust Fort Worth Stanton M. Ball, David S. Ball Ball Exploration Inc. Fort Worth The Williston basin Mississippian Lodgepole oil play has suffered a string of dry holes lately eroding the confidence of explorationists to find these prolific reefs, particularly in North Dakota. Detailed mapping of the Lodgepole trend suggests more Lodgepole reefs will be found in the Montana part of the trend than in North Dakota. Companies seeking impact plays should certainly give
Larry Brogdon
H.A. Hedberg Trust
Fort Worth

Stanton M. Ball, David S. Ball
Ball Exploration Inc.
Fort Worth

The Williston basin Mississippian Lodgepole oil play has suffered a string of dry holes lately eroding the confidence of explorationists to find these prolific reefs, particularly in North Dakota.

Detailed mapping of the Lodgepole trend suggests more Lodgepole reefs will be found in the Montana part of the trend than in North Dakota. Companies seeking impact plays should certainly give this area strong consideration.

Lodgepole fairway

A regional isopach map on the lower Lodgepole (Fig. 1 [27342 bytes]) delineates a fairway in which the Lodgepole reefs occur. To date, all Lodgepole reefs equivalent in age to those found near Dickinson, N.D., have been found within the 200 ft or greater contour of the lower Lodgepole section (Fig. 2 [40812 bytes]).

This fairway extends from eastern Stark County, N.D., west and northwestward into Dawson and McCone counties, Mont. The 200 ft or greater contour encompasses all or parts of 71 townships and may extend farther west but could not be defined due to lack of well control.

Key wells

Four wells encountered Lodgepole reef facies by serendipity while exploring for deeper objectives, including the Conoco 74 Dickinson State well in Stark County, N.D., which was completed for 2,045 b/d of oil and 1.16 MMcfd of gas. This well set off the Lodgepole play, and seven additional reef complexes have been found nearby, accounting for 50 million bbl of new reserves. The other three random penetrations of reefs were found in Dawson and McCone counties, Mont.

The Towner Petroleum 1 Dillon well (Fig. 3 [59053 bytes]) was drilled in 1982 for a Red River (Ordovician) objective in 29-17n-51e, Dawson County. This well encountered 283 ft of reef facies exhibiting clean gamma ray, tan to buff colored samples, oil shows in the samples, and log characteristics similar to productive wells in Stark County. Thin section studies indicate vug development and similar allochem assemblages to productive reefs in Stark County. A 3D survey has been completed to image this mound, and drilling is expected by the first quarter of 1997.

The E.L.K. 12-7 NPRR (Fig. 4 [51689 bytes]) in 7-20n-52e, Dawson County, and drilled in 1964 to the Red River, encountered an anomalously thick 288 ft of lower Lodgepole section with the bottom 160 ft exhibiting clean gamma ray. Unfortunately samples were not collected through the Lodgepole; however, log analysis indicates porous zones are present. This well is very similar to the State A-83 well in Dickinson field, Stark County, and is interpreted to be in a reef flank position.

In 1965 Gulf Oil drilled the 1 Federal (Fig. 5 [51022 bytes]) for a Red River objective in 29-23n-47e, McCone County, Mont. The lower Lodgepole in this well is unusually thick (318 ft). The lower 273 ft exhibit clean gamma ray. Samples through this section are tan to buff in color and are strikingly different from the dark gray color of the normal nonreef lower Lodgepole section. Cuttings show numerous calcite, dolomite, and pyrite crystal terminations indicative of vugular porosity with vugs being larger than the size of the cuttings. Bryozoans, crinoids, and ostracods are the main faunal constituents, and pale yellow oil fluorescence has been observed. As is the case with the two Dawson County wells, no tests of the Lodgepole were conducted in this well.

Reef clusters

Lower Mississippian reefs (including the Lodgepole reefs) throughout the world tend to be found in clusters. The importance of these key wells found in Montana is that they identify reef clusters and not just one individual reef.

Many more wells have been drilled through the lower Lodgepole section in North Dakota than in Montana, yet three reefs were encountered randomly in Montana and only one in North Dakota. This suggests that reefs are more numerous in Montana.

Source

Many geoscientists working this area believe the Lodgepole is self-sourcing. The False Bakken underlying the lower Lodgepole has been identified in the Dickinson area as a contributing source.

Lodgepole reefs are encased in organic rich carbonates. As early as the 1940s, field geologists recognized the petroliferous character of the lower Lodgepole section.

Lower Lodgepole samples throughout the Montana outcrop belt were dissolved in acid and almost universally yielded heavy films of oil. In eastern Montana, production has been found both above and below the Lodgepole section. Although source rocks are not a problem, identifying the reefs is critical.

Better Montana seismic

In North Dakota a big problem with identifying Lodgepole reefs is overlying salts corrupting the seismic data. Finding reefs encased by carbonates is a challenge in itself without the added velocity problems caused by overlying salt.

Fortunately in Dawson and McCone counties, Mont., Permian and Triassic sections containing erratic salt deposits in the Dickinson area of North Dakota are not present. The quality of seismic data is superior in the Montana part of the Lodgepole trend. Relatively gentle surface topography is suitable for Vibroseis acquisition, thereby lowering costs of 3D surveys as compared with dynamite data.

Activity

Several 3D surveys have been conducted or are planned in Montana.

The only recent notable Lodgepole test in Montana was the Vastar 1-2 Black Dog, in 2-23n-51e, Richland County. This well was reportedly drilled on an obvious large Lodgepole anomaly derived from a 240 sq mile 3D survey. This survey was outside of the lower Lodgepole fairway, and the well found no reef facies and was completed as a dry hole.

Lenticular deposits interpreted as debris flows in the lower Lodgepole section may account for the distinct seismic anomalies in this area. Several large lease blocks have been acquired in the fairway; however, many opportunities remain for those with interest in the area.

Conclusion

Delineation of a lower Lodgepole fairway extending into Montana with identification of reef facies in key wells (reef clusters), good source rocks, high quality seismic data, and impact reserve potential makes Montana good hunting ground for significant new discoveries.

The Authors

Larry D. Brogdon is a petroleum geologist and is working with the H.A. Hedberg Trust and Fortson Oil Co. on a contract basis. He was formerly president of Alamosa Oil & Gas Corp. and has been a working geologist for 22 years. He has a BA in geology from the University of Texas at Austin.

Stanton M. Ball has been a petroleum explorationist since 1964, when he joined Pan American Petroleum Corp. (now Amoco). Before that he was a Kansas Geological Survey geologist. He left Amoco in 1989 and with his son, David, started Ball Exploration Inc. in 1990. Ball Exploration specializes in generating large exploration plays and prospects and also consults. Ball has a PhD in geology (with honors) from the University of Kansas.

David S. Ball is a petroleum geologist with Ball Exploration Inc. He previously worked from 1984-90 with Union Pacific Resources Co. He has BS and MS degrees in geology from the University of Kansas.

Copyright 1996 Oil & Gas Journal. All Rights Reserved.