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AAPG Bulletin

Abstract


Volume: 56 (1972)

Issue: 7. (July)

First Page: 1216

Last Page: 1222

Title: Oil and Gas Developments in Northern Rockies in 1971

Author(s): C. A. Balster (2), L. B. Henderson (3), G. C. Crews (4), H. T. Ashmore (5)

Abstract:

Exploratory drilling during 1971 amounted to only 65% of the number of tests drilled during 1970. The number of discoveries showed a corresponding decline. Only 780 exploratory tests were drilled during 1971, and of these 46 found oil and 27 found gas. South Dakota showed the greatest decline with only 35 (42% of the 1970 total) wells drilled. Wyoming had only 345 (59% of the 1970 total), Montana only 303 (71% of the 1970 total), and North Dakota was nearest the 1970 level with 97 wells (90% of the 1970 total). Significant discoveries are shown on Figure 1.

Stratigraphic traps still represent the major target for explorationists. Demand for gas reserves has caused a decided increase in exploration for gas fields.

Development drilling decreased to only 69% of the 1970 level, but success rates remained essentially the same. The reduction of total new discoveries for 1971 will result in probable further reduction of development drilling for 1972.

An overall decrease in drilling activity in the region characterized 1971. Success rates in North Dakota and South Dakota showed a marked increase. Montana showed a significant drop in successful efforts.

In preview, 1972 appears to present a somewhat brighter picture for exploratory drilling. Assuming that success rates do not change significantly, discoveries for the coming year should exceed 1971.

WYOMING

All of Wyoming's basins had some exploratory drilling during 1971, but most of the effort (and success) was in the eastern Powder River basin (Table 2). Total drilling decreased for the second consecutive year, with total completions at 893; 36.9% less than last year's 1,416 completions.

Of 272 new-field wildcats, 28 were successful, down 30.0% from the 40 successful new-field wildcats in 1970. New-field wildcat success improved slightly, however, to 10.3% in 1971 compared with 8.5% in 1970. Most exploratory drilling was aimed at stratigraphic or combination traps, with objectives about the same as in the past several years. An exception was the deep Jurassic discovery at Hat Creek (23) (FOOTNOTE 6) in the Powder River basin. The Jurassic has been relatively untested in the deeper parts of the basin. Total exploratory drilling was down to 345 completions compared with 586 in 1970, a drop of 41.1%. Exploratory successes totaled 43 in 1971 down from 73 in 1970, also a drop of 41.1%.

Development drilling decreased to 548 completions from 830 in 1970, a drop of 33.7%. Significant local production increases resulted from development of several newer oil fields (Table 3). However, preliminary figures indicate a decrease in Wyoming's oil production during 1971, primarily because of declines at Hilight, Recluse, and Kitty fields.

Powder River Basin

Of Wyoming's 28 successful new-field wildcats completed in 1971, 22 are in the Powder River basin. As in recent years, Cretaceous Muddy Sandstone exploration and success were dominant, with 11 new fields. The 11 remaining new-field wildcat discoveries in the basin were: 3 in the Cretaceous Teapot Sandstone, 2 in the Cretaceous Sussex Sandstone, 2 in the Cretaceous Dakota Sandstone, 1 in the Jurassic Canyon Springs Sandstone, and 3 in the Pennsylvanian Minnelusa Formation.

Several Muddy Sandstone discoveries were followed by significant development drilling: 8 successful offsets were completed at the Chan Muddy pool (10); 11 were completed in the Mill-Gillette area (6). Both of these new Muddy fields are still developing. One of the most significant developments of the year in terms of immediate results was the House Creek extension (11) which resulted in 18 development wells and a dramatic increase in production (Table 3). Also, the House Creek development improved the stature of the Sussex Sandstone as a stratigraphic exploration target. The Hat Creek discovery (23) may open new Jurassic potential in the southern Powder River basin. The discovery is on the down side of a thrust-faulted area which separates the Lance Creek-Lightning Creek area from the deeper part of the Powder River basin. An offset well is being drilled, but no details have been released.

FOOTNOTE 6. Numbers in parentheses indicate Map No. from Table 6.

End_Page 1216------------------------------

The discovery well produced an average of about 170 BOPD in December 1971.

Continuing development at Kaye field has resulted in steadily increasing production from the Teapot Sandstone (Table 3). Two of the Teapot discoveries listed as new-field wildcats in Table 6 (17 and 22) proved to be Kaye field extensions.

Waterflooding was begun in the north part of Hilight field (Grady Unit) and 3 other waterflood units in the field are in the planning stage. A waterflood program also was begun at Recluse field.

Big Horn Basin

Most of the 30 exploratory wells drilled in the Big Horn basin were tests of the Permo-Pennsylvanian Phosphoria and Tensleep Formations. Exploration was dispersed around the south, east, and west flanks of the basin. One new-field discovery resulted, an oil producer from the Phosphoria Formation (1) which was abandoned after a short time.

Development drilling continued at several of the older fields, including Garland, Little Buffalo Basin, and Cottonwood Creek.

Considerable leasing activity in the central part of the basin followed approval of the Colorado Interstate Gas Company (CIG) pipeline, which is now under construction. The gas pipeline will tie into the CIG system on the south.

Wind River Basin

Exploratory drilling resulted in 2 new-field wildcat discoveries and 3 successful outposts from established production. The new fields were both completed in the Tertiary Fort Union Formation (18, 19). The successful outposts were: Fort Union gas at Poison Creek field, Phosphoria oil at Pilot Butte field, and Phosphoria gas at Beaver Creek field. The Phosphoria extensions had surprisingly high IP rates: 8,400 MCFGD with 930 b/d of condensate.

Development drilling continued in some of the older fields, particularly Winkleman Dome field, which had 12 successful development completions in the Phosphoria and Tensleep Formations.

Green River Basin

Exploratory drilling resulted in 4 successful completions: 3 new fields and 1 new pool. The Masterson field discovery (26) has had 1 successful offset and 4 dry offsets. The discovery well has been producing about 5,000 MCFGD in the first few weeks of production, from the upper Dakota (Muddy equivalent) Sandstone. The Red Desert gas-field discovery (27) is completed in the Cretaceous Almond Sandstone from a gross interval of 191 ft. Robin field (28) also is a gas completion in the Almond, structurally downdip from Almond oil production on Table Rock anticline.

All of these discoveries are on the east side of the Rock Springs uplift. On the east side of the basin, Sugar Creek field had a new-pool discovery with a dual gas completion in the Muddy and Frontier.

The west side of the basin had no discoveries during the year, but a wildcat south of Church Buttes field was an indicated Dakota gas discovery at year end.

Development drilling during the year continued to improve Jurassic Nugget Sandstone production at Dry Piney field (Table 3).

Hanna Basin

One exploratory completion was recorded, an unsuccessful wildcat that tested the Cretaceous Frontier Formation on the west side of the basin. There were no development completions.

Laramie Basin

Exploratory tests were drilled to Cretaceous Shannon and Dakota Sandstones, Jurassic sandstones, and Pennsylvanian Casper (Tensleep) Formation objectives at 7 wildcats scattered around the basin. There were no successful exploratory completions, but Big Hollow field had 2 successful development completions in the Muddy Sandstone.

Denver Basin

Exploratory drilling for Muddy J sandstone stratigraphic traps resulted in 9 unsuccessful completions. An indicated Muddy J sandstone discovery in Colorado, just across the state line southeast of Borie field, provided some encouraging results for the area.

Conclusion

Despite the decreased activity there were several significant developments in Wyoming during 1971. The new fields are, for the most part, still incompletely developed, so their long-term importance is not known. Many new fields appear noneconomic after partial development, then a prolific outpost or new pool improves the field or area's "image." This appears to be a characteristic of stratigraphic exploration in

End_Page 1217------------------------------

complex intertonguing marine and nonmarine environments.

MONTANA

Drilling decreased for the third straight year in Montana. Only 427 wells (Table 4) were completed. This total is 176 less than the previous year, and 437 wells (51%) less than the record year of 1968.

Exploratory Drilling

The emphasis in exploration was on gas. The most active play was the search for shallow gas in the Upper Cretaceous Eagle Sandstone on the east flank of the Sweetgrass arch and the west margin of the Williston basin. This play had spread by the end of the year across an area of 250 townships in north-central Montana. Ten (1-10) new field wildcats were successful at depths of 600 to 1,650 ft.

A second shallow gas play gained momentum along the flanks of Bowdoin dome on the west side of the Williston basin. One new-field wildcat (13) and 2 outpost extensions (11, 12) were successful. The objectives were the Upper Cretaceous Bowdoin and Phillips Sandstones at depths of 1,000-1,500 ft.

Oil exploration was at a low level throughout the state. The two most important discoveries were in the Williston basin on the downthrown south side of the Weldon fault. Sawtooth Oil (14) made a successful completion in carbonates of the Mississippian Charles Formation. True Oil completed the state's best oil discovery (15) from the Mississippian Kibbey sandstone. This is only the fourth occurrence of Kibbey production in Montana.

A significant exploratory failure was drilled in the Crazy Mountain basin of western Montana. The Scat Drilling No. 1 State-Willsal, Sec. 30, T3N, R7E, Gallatin County, was plugged at a depth of 7,154 ft in the Cretaceous(?). Good oil and gas shows were tested from an upper sandstone zone and gas from a lower sandstone zone. These zones were not precisely identified, but are probably of Late Cretaceous age. The well is in a very sparsely drilled area that is 70 mi from any production.

Development Drilling

The development well total (Table 4) was the lowest of the past 20 years. There were two important programs in progress as the year closed.

Fifteen Eagle gas wells were completed in the west part of the Tiger Ridge field, along the east flank of the Sweetgrass arch. Tiger Ridge field has expanded significantly each year since its discovery in November 1966.

Seven Pennsylvanian Tyler Sandstone oil wells were completed in the Jim Coulee field of central Montana. Jim Coulee had 15 completed, or near-completed, wells at the end of 1971. Proved recoverable oil was placed at 4 million bbl. These figures established the field as the best central Montana discovery since 1960.

NORTH DAKOTA

Exploratory drilling decreased 10% from the 1970 total, but the success rate increased from 4.6 to 8.2%. This is a result of a decrease in random-type drilling for the shallower sandstones and a more selective drilling of prospects defined by geologic subsurface studies and by detailed geophysics.

SOUTH DAKOTA

Although there was a 25% decrease in exploration drilling compared with 1970, there was 1 discovery of significance. The Depco No. 42-27 Federal is the first well to find commercial production from the Red River outside the limits of Buffalo field. This well produced over 67,000 bbl of oil in the first 6 months after discovery.

End_Page 1218------------------------------

Table 1. Completion Summary, Northern Rockies, 1971

Table 2. Completion Summary, Wyoming, 1971

Table 3. Some Developing Western Oil Fields With Production Increases During 1971

Table 4. Completion Summary, Montana, 1971

Table 5. Completion Summary, North and South Dakota, 1971

End_Page 1219------------------------------

Table 6. Important Discoveries in Wyoming, Montana, and South Dakota, 1971

End_Page 1220------------------------------

Table 6. Continued.

End_Page 1221------------------------------

Fig. 1. NORTHERN ROCKIES 1971

End_of_Article - Last_Page 1222------------

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