Petroleum potential of wilderness lands in Wyoming-Utah-Idaho Thrust Belt

ACKNOWLEDGMENTS

The many, detailed reports on the surface geology and mineral potential of wilderness areas published by colleagues in the U.S. Geological Survey were a valuable source of information. I would like particularly to thank Steven S. Oriel for his helpful discussions on the overall surface geology of the thrust belt. Jerome W. Boettcher of Exxon Co. USA provided me with up-to-date information on the status of new-field wildcat gas discoveries in the area west of La Barge, Wyoming, that aided greatly in the evaluation of tracts in that area.

INTRODUCTION

A review has been made of the oil and gas potential of various categories of Wilderness Lands in the Wyoming-Utah-IdahoThrust Belt province (fig. 1). The types of Wilderness Lands are shown on maps of Wyoming, Utah and Idaho prepared by the U.S. Bureau of Land Management (1981a,b,c). Tract numbers evaluated in the present study are shown on these maps. Six of the wilderness tracts have a high oil and gas potential, one has a medium potential, and the remainder have either zero, low, or unknown oil and gas potential.

The Wyoming-Utah-Idaho thrust belt straddles parts of western Wyoming, north-central Utah, and eastern Idaho and covers an area of 15,000 square miles (fig. 1). The northern boundary is placed at the south edge of the Snake River volcanic plain west of Jackson, Wyoming; the southern boundary is at the intersection of the Uinta Mountains just east of Salt Lake City; the eastern boundary is at the surface trace of the Darby-Prospect thrust fault, which separates the structurally deformed thrust belt from the undeformed Green River basin on the east; the western boundary is the surface trace of the Willard-Paris thrust on the east side of the Bear River Range (fig. 2).

GEOLOGIC FRAMEWORK

During a span of geologic time (Paleozoic-early Mesozoic) ranging from about 570 to 140 million years (m.y.) before the present, more than 60,000 feet of sand, silt, mud, and limy material was de-

II

Jurassic(?)

and Triassic (?) Triassic

Oil 'lbickness Formation Geologic Age or or Group Range Gas Green R1ver Fm. Tertiary o-8,000' Wasatch-EvanstonFms. Adaville Fm. 6,000'- Billiard Fa. Late 16,000' RSe Frontier Fm. ~ Aspen Shale RS ~ ~ Bear River Fm. RS* Q) Early 10,000' u Gannett Group -,. Stump Fm. R 500'- Preuss Ss. ~Saltl R Jurassic Twin Creek Ls. RS * 1,QOO' IGypsumSpringMbr. 3,500' * 1,200'- R Jurassic(?) 500'- and Nugget Ss. 2,000 Triassic (?) R Ankareh Fm. Triassic 2,000'- RS ~ lbaynes Fm. 7,000' Early Triassic Woodside Fm.

Precambrian e Wasatch-EvanstonFms.

:2"0 Lodgepole Ls. RS ~ 7,000'

Darby lbree Forks Formation or Group

Adaville Billiard Fa. Frontier Aspen Shale Bear River

Stump Preuss Ss. Twin Creek IGypsumSpringMbr.

Nugget Ss.

Ankareh Fm. lbaynes Woodside Dinwoocly Phosphoria Fm.

Weber Ss.

FM4 Jefferson Fm. Bighorn Dolomite Gallatin Gros. Ventre Flathead Uinta Mtn. Group Oil or Gas

o-8,000'

500'- 1,QOO' 1,200'- 3,500'

)20,000' However, the identical sulfur content and chromatographic character of condensate from the Paleozoic and Jurassic-Triassic reservoirs suggest that both were generated from the same Cretaceous source rocks (Warner, 1982), or possibly at the same time from rocks of different ages.

The key factors that led to the presence of oil and gas fields in the thrust belt appear to be (1) the presence of an extensive area of organic-rich Cretaceous source rocks at peak maturation in the footwall of, and in contact with, the Absaroka thrust, (2) generation of oil and gas from these rocks after being overridden and buried by the Absaroka plate, and consequent expulsion of the oil and gas, (3) migration of the expelled hydrocarbons laterally and upward into Jurassic, Triassic, and Paleozoic reservoir rocks in available hangingwall traps, and (4) sealing over the traps by impermeable shale, anhydrite, or halite (salt) caprocks (Royse, 1979).

As discussed earlier, a wedge of Paleozoic and Mesozoic sedimentary rocks was compressed from west to east into a zone about one-half of its original width, resulting in the thrust folds of the present Wyoming-Utah-Idaho thrust belt. However, in this province only the sedimentary rock section is involved in folding and thrusting and is structurally detached from basement crystalline rocks (Precambrian granite) by a regional "decollement." This decollement condition has also been referred to as "thin-skinned" structure by Rodgers (1963) in describing folds and faults in a thrust belt involving only the upper strata lying on a decollement, beneath which the structure differs. In other words, the basement is passive and only the overlying sedimentary rock sequence is actively involved in the shortening of the total section in the province (fig. 3). Nearly all of the present oil and gas fields in the thrust belt have been trapped in asymmetric and overturned anticlinal folds in the hanging wall of the Absaroka thrust plate. Most of the folds have numerous, additional imbricate thrust faults included within their overall configuration. Vertical structural relief or amplitude of the traps ranges from 500 to more than 4,500 feet, and areal size ranges from about 3 to more than 50 square miles. The greater the vertical relief and area of the fold, the larger the amount of oil or gas it can trap. Many of the new discoveries in the thrust belt are major fields and some are estimated to be giant fields in size. A major field is defined informally as one that is estimat~d to ultimately produce 50 MMBO or more, or 300 BCF or more of combustible gas (Johnston, 1980, p. 1303). A giant field is defined as one that is expected to produce more than 100 MMBO, or 1 TCF of combustible gas (Halbouty, 1980, p. 1).

Prior to 1975, lack of success in finding oil or gas traps in most of the prominent surface anticlines present in the thrust belt eventually led to the conclusion that perhaps the structures that held oil and gas did not necessarily lie directly beneath the surface structures. The problem then became a matter of finding the right tool, or method, that would enable explorationists to look thousands of feet below the surface and identify anticlines similar to those on the surface that might trap oil or gas. The problem was solved by applying an improved method of subsurface mapping employing advanced seismic-reflection tools that were the result of a breakthrough in seismic data processing and mapping, coupled with modern, sophisticated computer technology.

OIL AND GAS FIELDS

Table 1 summarizes basic data relating to new fields and indicated new-field wildcat discoveries in the Wyoming-Utah-Idaho thrust belt since 1975. One of the largest of these fields is the Whitney Canyon-Carter Creek gas field in Uinta and Lincoln Counties, Wyoming, located 13 miles north of the town of Evanston (fig. 2, table 1). The field was discovered in late 1977 on a major northsouth-trending, slightly overturned anticline on the hanging wall of the Absaroka thrust plate. The overall anticline is actually made up of three individual structures with total structural closure (vertical relief) exceeding 4,500 feet (fig. 5). The anticline is 16 miles long and 4 miles wide occupying an area of 60 square miles (one and one-half townships).

An unusual feature of this field is that production of sour gas, averaging 12 percent hydrogen sulfide (H2S), and condensate comes from six separate reservoirs, including the Permian Phosphoria Formation, Pennsylvanian Weber Sandstone, Mississippian Mission Canyon and Lodgepole Limestones, Devonian Darby Formation and Ordovician Bighorn Dolomite (figs. 4 and 6). A seventh reservoir, the Triassic Thaynes Formation, which is productive of sweet gas and con-

N8

densate, is the exception in this dominantly sour gas field. Depth to production ranges from 9,200 feet to 14,000 feet in these formations. Warping (folding) of the upper, main Absaroka thrust sheet has evidently been the main reason for the great amount of structural closure on the overall anticline.

To gain some idea of the productive capability of this field, one well, the Amoco-Champlin No. 457- A (fig. 6), was flow-tested in the Paleozoic reservoirs for a total recovery rate of nearly 75 million cubic feet (MMCF) of gas and 1,294 barrels of condensate (B.C.) per day; even more significant is the fact that a 30-foot-thick section of the Mission Canyon Limestone reservoir, alone, flowed 32 MMCF of gas per day.

A fairly well defined, arcuate northeast-southwest pattern or trend exists of fields discovered in the southern part of the thrust belt (fig. 2). In addition, within this trend, a separation occurs between predominantly oil and sweet gas fields in Jurassic and Triassic reservoirs east of the Tunp thrust and dominantly sour gas and condensate fields in Paleozoic reservoirs west of the Tunp thrust (fig. 2, table 1). However, all these fields are common to the Absaroka thrust trend. Only

N9 one gas field, Hogback Ridge, is located on the Crawford-Meade thrust trend and only one recently discovered (sweet) gas field in a Mississippian reservoir (Horsetrap field), and three indicated sour gas discoveries (not listed) are located on the Darby-Hogsback thrust trend (fig. 2, table 1). Although temporarily abandoned at present, the Mill Creek field (table 1, no. 9) is also located on the Darby-Hogsback trend. This two-well field seems to be the exception to all of the fairly well defined patterns discussed above in that the discovery well flowed 154 barrels of sweet, 46 degree A.P.I. gravity green oil from a Paleozoic sandstone reservoir (Devonian Three Forks Formation). Interruptions in the established pattern of fields, trends, hydrocarbon type, and age of reservoir as shown by the anomalous Mill Creek field may indicate the possibilty of a different oil and gas field trend on the Darby-Hogsback thrust to the north and northwest along the leading edge of this thrust plate, as well as the Absaroka thrust trend.

QUALITATIVE RATINGS OF WILDERNESS LANDS

Tracts were rated on the basis of available geologic information, both published and unpub-

West East

High potential.-Geologic environment highly

Medium potential.-Geologic Contains known reservoir rocks and hydrocarbon-source beds. Includes some areas of sparse subsurface control or areas where known or expected field size will be small.

Low potential.-Geoiogic environment interpreted to have low potential for oil and gas. Includes areas of poor or unknown hydrocarbon-source bed richness and reservoir quality. Generally includes areas of sparse or no well control and (or) expected thin section of sedimentary rocks.

Zero potential.-Mostly comprises areas with exposed Precambrian rocks or very thin sedimentary cover with no potential for occurrence of sealed structural or stratigraphic traps.

Unknown potential.-Generally includes areas of no well control where Tertiary volcanic intrusions and volcaniclastic rocks are present on the surface. This cover, plus lack of subsurface well and geophysical control, makes prediction of hydrocarbon potential nearly impossible. Includes some areas where Precambrian igneous and metamorphic rocks are thrust over Phanerozoic sedimentary rocks of unknown potential. Lack of control does not mean that no oil and gas potential exists, but only that the potential can not reasonably be determined with present data.

Six wilderness tracts within the thrust belt are rated to have a high oil and gas potential on the basis of possessing geologic characteristics favorable for the occurrence of petroleum. These six tracts, as identified by the Bureau of Land Management (1981a--c), are discussed in the following section for four cluster groupings, three clusters in Wyoming and one cluster in Idaho.

1. Tracts 040--221 and 040--223 (U.S. Bureau of

Land Mangement, 1981c), in cluster 15 (C. W. Spencer, Wyoming, chapter M), western Lincoln County, Wyoming.-Two wildcat tests were drilled in the northern part of tract 040--221 by Gulf Oil Co. and Sohio Petroleum (1978, 1982) on the Crawford thrust. Gulf drilled to a total depth of 15,992 feet and reported minor gas shows in Mesozoic rocks. Sohio set production casing in its well and tested perforations in the Permian Phosphoria Formation (fig. 4) at a total depth of 17,131 feet. Good gas shows were indicated in this well, but the amounts were not sufficient to justify commercial production.

2. Tract 040--110 (U.S. Bureau of Land Mange-

ment 1981c), in cluster 15 (C. W. Spencer, Wyom-

N12 ing, chapter M), western Sublette County, Wyoming.-Exxon drilled a new-field wildcat discovery (Graphite Hollow) within the east half of this tract in 1982, on the Darby-Hogsback thrust. The discovery flowed 5 MMCF of low BTU gas (25 percent methane) from the Madison Group (fig. 4) below 16,000 feet. In addition, Exxon drilled two new-field wildcat discoveries, Lake Ridge and Fogarty Creek, directly north of the northern boundary of the tract. The Lake Ridge discovery well flowed 6 MMCF of low Btu gas from the Madison at 16,318 feet, and Fogarty Creek flowed 20 MMCF of similar type gas from the Madison at 16,291 feet, including a small percentage of helium.

3. Tract 4--102 (U.S. Bureau of Land Mange-

ment, 1981c), in cluster 17 (C. W. Spencer, Wyoming, chapter M), southern Teton County, Wyoming.-Only the southwestern portion of this tract, which lies between the Darby and Prospect thrusts where they form separate thrust sheets in the northern part of the thrust belt, is rated as having a high potential. Directly east of the tract Rainbow Resources Inc. completed a new-field wildcat gas discovery in the Frontier Formation (fig. 4) in 1977. The well is shut-in at present pending future field development.

4. Tract 4--613 (U.S. Bureau of Land Manage-

ment, 1981c), in cluster 16 (C. W. Spencer, Wyoming, chapter M), and tract W4-613 (U.S. Bureau of Land Management, 1981a) in Idaho cluster 7 (C. A. Sandberg, Idaho, chapter F).- These tracts straddle the Wyoming-Idaho State line in the vicinity of Alpine, Wyoming, but are grouped together for this discussion on the basis of. geologic similarity. The Darby thrust trends northwest in the eastern part of the tracts and the Absaroka thrust parallels the Darby trend in the central part of the tracts. A wildcat well, the Allday No. 1 Government, · was drilled in the westernmost part of tract W4-613 in 1966 to a depth of 5, 760 feet immediately northeast of the Palisades Creek picnic area in the Targhee N ationa} Forest, Bonneville County, Idaho. Live oil shows were encountered in porous and fractured Ordovician limestones from 1,252 to 1,256 feet, 1,348 to 1,354 feet, and 1,368 to 1,375 feet in the well; live oil was bleeding from fractures in the lower zone. Surface mapping indicates that the well was located on the hanging wall (fig.3) of the Thompson imbricate thrust above the buried Absaroka thrust plate (fig. 2). Live oil shows at these shallow depths are a positive indication that oil and gas are being generated and are migrating within the rock system on the Absaroka thrust plate in this general area. The same formations, reservoir and source rocks (fig. 4), and trapping structures are present here, as well as a favorable thermal-maturation history, in a framework similar to that in the productive southern area of the thrust belt (figs. 2 and 3).

Tracts 34-3 Islands and 34-4 Islands (U.S. Bureau of Land Management, 1981a) in cluster 6 (C. A. Sandberg, Idaho, chapter F), eastern Bonneville County, Idaho, are rated to have a medium oil and gas potential. The surface area of this cluster consists of volcanic cover (not prospective for oil and gas); however several old, shallow wildcat wells drilled nearby indicate that sedimentary rocks are present in the subsurface. These tracts, therefore, are rated as possessing medium oil and gas potential. The small part of a tract, referred to as cluster 18, in Utah at the western end of the Uinta Mountains, is rated as having low petroleum potential. (C. M. Molenaar and C. A. Sandberg, Utah, chapter K). The remaining tracts in the Wyoming-Utah-Idaho thrust belt.are rated to have a zero or unknown petroleum potential.

SUMMARY

The Wyoming-Utah-Idaho thrust belt possesses the following critical requirements that make it a major oil and gas producing province:

1. The presence of four dominant regional thrust

trends that act as controlling factors in localizing oil and gas fields.

2. Proved occurrence of similar trap types on the

hanging wall (overthrust block) of three of the regional thrust trends.

3. Organic-rich, thermally mature source rocks

ranging in age from Cretaceous through Devonian.

4. Thick, porous and permeable clastic and car-

bonate reservoir rocks productive of oil and gas.

5. The discovery of 26 new oil and gas fields and indicated new production in additional recent wildcat wells, during the past 8 years.

N13 There are approximately 500,000 acres of qesignated and proposed Wilderness Lands within the Wyoming-Utah-Idaho thrust belt. Eighty-five percent of these lands are assessed as having high to medium petroleum potential (high potential, 78.5 percent, and medium potential, 6.5 percent). An additional 1.2 percent are judged to have low petroleum potential, with the remaining 13.8 percent as having zero petroleum potential. ·

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