--> Abstract: Timing is Right for Thrust-Belt Plays in Western Montana and Northwestern Wyoming, by E. H. Johnston, K. M. Reddy, and T. D. Anderson; #90937 (1998).

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Abstract: Timing is Right for Thrust-Belt Plays in Western Montana and Northwestern Wyoming

JOHNSON, ERIC H., Johnson Geophysical; KEVIN M. REDDY, High Prairie Geotechnical; TOM D. ANDERSON, BHB Oil and Gas

Introduction

Many promising structures remain to be tested in the thrust terrains of the Rocky Mountains, Some of the structures are so compelling that it is surprising they have never been drilled, until one examines their history. Cycles in the oil business, corporate mergers, buyouts, and changes in strategy, have often and arbitrarily curtailed exploration activity in prospective oil and gas plays. Now, a rekindled interest in Rocky Mountain thrust-belt plays is anticipated due to an upturn in the oil industry combined with a good knowledge base for the plays.

When oil was discovered at Pineview field in 1975, the timing was right for a rapid succession of new field discoveries in the thrust belt of northeastern Utah and southwestern Wyoming. The groundwork had been laid by knowledge gained from hundreds of dry boles and, at the same time, developing seismic technology had become better able to image the structures. Also, fortuitously, the Pineview discovery coincided with an upturn in the oil business. That combination spurred years of exploration for thrust structures in the Rocky Mountains. By the time exploration activity was stifled by an abrupt downturn in the industry about 1986, a significant knowledge base had evolved for a number of other thrust-belt plays along the Western Cordillera.

Now, the timing is right for a resurgence of thrust-belt exploration in western Montana and northwestern Wyoming. Much of the groundwork already exists. Hundreds of miles of 2-D seismic data have been acquired and some key exploratory wells have been drilled. An upturn m the oil industry implies that promising structures will be tested, and analogs to producing fields suggest that large oil and gas discoveries will be made.

Western Montana

The Disturbed Belt of western Montana contains numerous large structures comparable to producing fields in the Canadian thrust-belt to the north. Although environmental factors have prevented a direct extension of the Canadian play into northwestern Montana, the same thrust-belt trend offers accessible areas 150 miles south of the prolific Canadian fairway that contain similar structures, source rocks, and reservoir rocks. Nothing precludes the chance of significant oil and gas discoveries in western Montana except a paucity of wells. In a 1,500 mi2 area, covering T14-to-20N and R3-to-8W, only 18 wells have been drilled. Only 8 of those wells penetrated the Mississippian Madison Formation, a primary reservoir in the area that is present at depths between 6,000 and 14,000 ft. Several wells encountered good oil shows in Cretaceous and Mississippian strata.

Mostly acquired between the mid-1970s and the mid-1980s, hundreds of miles of 2-D seismic data grid the area and delineate a series of east-verging thrust-sheets that created numerous large, untested structures. One prospective structure is a doubly-plunging anticline, about 16-mi long by 4-mi wide, that exhibits several thousand feet of closure. The structure is defined by good-quality seismic data and is flanked by four dry holes, drilled on misinterpretations of the seismic data. Those dry holes contribute knowledge for a new phase of exploration; they provide geologic ties to the seismic data and help corroborate the structure. Regional hydrocarbon maturation trends indicate flint this virgin structure could contain oil; its size suggests that potential reserves easily could exceed 100 million barrels.

Northwestern Wyoming

Other oil and gas opportunities are to be found in northwestern Wyoming. There, Cenozoic volcanic deposits that originated from the Yellowstone caldera, mask structures where regional trends project beneath the volcanic cover. Adjacent fields in the Big Horn Basin should be representative of the size of some of the hidden structures.

In the vicinity of Lava Mountain near Dubois, Wyoming, structures buried by Tertiary volcaniclastics were indicated from post-WWII gravity surveys, but could not be imaged by early, single-fold seismic data. During the 1970s, CDP seismic lines began to delineate the large structures in the area formed by west-verging thrust-sheets, but the seismic interpretation needed to be tested by drilling. During 1977, a well drilled by KN Energy and Amerada Hess (Sec. 24, T43N-R110W) encountered lower Paleozoic rocks in a breached anticline beneath horizontal Tertiary strata. A subsequent well drilled by MCOR during 1981 (Sec. 15, T43N-R110W) penetrated Cretaceous rocks beneath the unconformable Tertiary strata, in a lower thrust-plate likely to contain intact structures.

However, even as prospective structures were being delineated during the late 1980s and early 1990s by high-resolution seismic lines, the simultaneous slump in the oil industry caused interest to wane. The play changed hands several limes as corporations redirected their activities. Eventually, a wildcat well was drilled during 1996 that confirmed the presence of hydrocarbons in a closure, but also resulted in a reinterpretation of the structure. Currently, an offset well is underway, even as another change in ownership underscores the dynamic nature of the oil business.

Conclusions

On the road to off and gas discovery, the classic progression of seismic-drill-reinterpret-drill again, was interrupted in western Montana and northwestern Wyoming by the last downturn in the oil and gas industry. As another upturn in the industry gains momentum and exploration activity accelerates, oil and gas opportunities in those promising areas are just waiting for explorationists to put the plays back together.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah