A LOOK AT BACON FLAT, GRANT CANYON OIL FIELDS OF RAILROAD VALLEY, NEVADA

Eric H. Johnson
Erec-Balcron Oil Division
Billings, Mont.

Bacon Flat and Grant Canyon oil fields are Milestones in Nevada oil history. The Bacon Flat discovery well produced the first oil from Devonian dolomites, and Grant Canyon field demonstrated the remarkable reservoir potential of those rocks.

Grant Canyon field has produced over 19 million bbl of oil so far from an area of about 300 acres, including 9.5 million bbl from a single well, the 3 Grant Canyon.

The prolific wells at Grant Canyon, and the puzzling geology have intrigued explorationists and promoters. Many a Nevada prospect has Been touted as "another Grant Canyon." But what processes formed Grant Canyon, and can others be found?

Last August, Equitable Resources Energy Co.'s Balcron Oil Division spudded a well at Bacon Flat, a mile west of Grant Canyon. A one well field, Bacon Flat had been abandoned in 1988. But just 900 ft north of the field opener, Balcron's well tested oil at a rate of 5,400 b/d.

It turns out that Bacon Flat and Grant Canyon fields have a common geological history and, in fact, share the same faulted horst. However, they formed by an unusual combination of events that may be unique to those fields.

HISTORY

Located along the east side of Railroad Valley, 65 miles southwest of the town of Ely, Bacon Flat field was discovered in July 1981 by Northwest Exploration Corp. The 1 Bacon Flat discovery well (originally named the 8 Railroad Valley) was drilled in C SW 17-7n-57e. From the first day it was pumped, the well was plagued by high water production. During the first year it averaged only 135 b/d of oil plus 830 b/d of water.

In September 1983 Northwest Exploration found Grant Canyon field just a mile southeast of the Bacon Flat discovery with a wildcat drilled in SW NW 21. Surprisingly, the nominal oil production at Bacon Flat was dwarfed by the 1 Grant Canyon well, completed flowing 1,816 b/d of oil and no water.

But even that amazing rate was eclipsed the following year by the 3 and 4 Grant Canyon wells, each capable of 3,000 to 4,000 b/d of oil. Including the No. 7 and 9 wells completed in 1991 and 1992, a total of five copious producers were drilled in the field.

Daily production from Grant Canyon field was constrained by trucking capacity to around 8,000 b/d of oil. The field showed no decline or pressure depletion, so its ultimate yield could not be predicted. Estimated reserves were regularly exceeded and revised. Finally, after more than 19 million bbl of oil, the field may have nearly watered out. The only well still producing is the No. 9 at around 1,500 b/d of oil.

In October 1988, the one well Bacon Flat field was shut in after producing 311,428 bbl of oil and 3,558,681 bbl of water. Four dry offsets had been drilled around the discovery well on 160 acre spacing. None of the offsetting wells hit the Bacon Flat reservoir.

In January 1992 Balcron Oil drilled the 24-17 Bacon Flat Federal 800 ft southeast of the abandoned discovery well. The well came in 73 ft low but found the Devonian reservoir and good oil shows. Tying this new well to seismic data helped to identify the eroded top of the reservoir, obscured by west dipping seismic reflections.

In August 1992, Balcron drilled 900 ft north of the original producer to test a seismically defined knoll. The Balcron 23-17 Bacon Flat Federal well, in NE SW 17 found the Devonian reservoir 140 ft high to the 1 Bacon Flat at a depth of 5,154 ft (-420 ft subsea). The well flowed clean, 26 API gravity oil at a rate of 5,400 b/d. Following completion, production was restricted to 1,000 to 2,000 b/d due to trucking capacity and refinery access.

INTERPRETATIONS

Read and Zogg 1 published an interpretation of Grant Canyon and Bacon Flat showing fault blocks rotated on a detachment surface. Subsequent erosion exposed the Devonian reservoirs in the east-tilted blocks.

Veal, Foster and Vincelette, and Bortz 2 3 published three different cross sections that all showed Grant Canyon and Bacon Flat to be individual horsts, separated by a graben that preserved Pennsylvanian age strata between them. Veal incorporated a detachment to place Devonian rocks over Mississippian shales, while the others showed various degrees of tilting or erosion of the horsts to expose the Devonian reservoir rocks. Montgomery 4 invoked Mesozoic thrusting, followed by extensive erosion, to position the Devonian reservoir rocks prior to horst and graben development.

The diversity of previous interpretations underscores the complexity of the area. This publication is one more attempt to explain the geology at Grant Canyon and Bacon Flat.

GEOLOGICAL SUMMARY

Grant Canyon and Bacon Flat have a complicated geological history.

To begin with, a mass of Devonian dolomites slid westward over younger Paleozoic shales and carbonates. They apparently detached, in the form of a landslide, from an adjacent highland to the east. Their movement likely was triggered by arching and faulting during Paleogene time, possibly associated with gradual uplift of an underlying Cretaceous intrusive. 5 6

Following the emplacement of the Devonian dolomites, normal faults broke the area into horst and graben blocks. Erosion scoured the dolomites capping the blocks, and cut through the dolomites between Bacon Flat and Grant Canyon, isolating the fields and exposing the underlying Pennsylvanian Ely limestone.

As Railroad Valley developed, the horst and graben blocks were subsequently sealed and buried by Neogene age "valley fill" deposits of volcanics, conglomerates, sands, and clays. Within the last 5 million years, oil was generated from Mississippian Chainman shales after they were buried deep enough in the valley to reach thermal maturity. The oil migrated into the overlying reservoirs in the horst blocks.

Grant Canyon and Bacon Flat fields produce from remnants of detached dolomites that cap a faulted horst. The reservoirs have been fractured and brecciated by tectonics and karsting. 1 Geothermal processes indicated by a geothermal gradient at least triple the regional average, may have aided the accumulation of oil in the reservoirs and augmented the reservoir seals as described by Hulen and others. 3

STRUCTURE MAP

On a structure map of Grant Canyon and Bacon Flat fields, dashes outline the limits of the detached Devonian reservoir rocks (Fig. 1). Generally, those limits coincide with the major bounding faults, except for an erosional valley cut between the fields. Contours reflect the Devonian reservoir topography.

The horst containing the fields is a rectangular block, covering about 800 acres, divided into two segments by a fault. The Bacon Flat segment has been dropped down 1,000 ft on the northwest flank of the Grant Canyon segment.

Due to regional dip and erosion, the crest of the Bacon Flat reservoir, at a depth of about 3,000 ft (-250 ft elevation), is 1,500 ft lower than the apex of Grant Canyon field, which, at an elevation of about + 1,250 ft, is covered by 3,500 ft of valley fill.

The reservoir at Grant Canyon caps a horst segment about 500 acres in size. The original oil accumulation occupied around 300 acres, assuming an oil/water contact at + 250 ft. The Bacon Flat reservoir covers about 160 acres of a 300 acre fault block. Assuming an oil/water contact at -660 ft, the oil field originally covered around 100 acres.

JEBCO C SEISMIC LINE

Part of seismic line Jebco C is reproduced here (Fig. 2) and its location is shown on the structure map. The line contains excellent quality data for the Grant Canyon area. It was acquired in 1989 by Jebco Seismic Inc. of Houston using a Geo-Systems crew with a 1,028 channel sign-bit recording system and a Vibroseis source.

The large number of channels permitted the simultaneous recording of a "swath" layout of four parallel lines spaced 80 ft apart. Both outside lines were vibrated. Effectively, eight conventional 60-fold 2-D seismic lines were recorded along the same traverse. In the version shown, the eight lines were summed together to enhance signal strength.

Reflections from the valley fill sediments exhibit amplitude variations and discontinuities, typical of mixed lacustrine and fluvial deposits of shale, sand, and conglomerate. Major changes in rock density (lithology) produce strong seismic reflections, such as the one associated with the basalt flow above Bacon Flat field.

Unfortunately, the unconformity that forms the top of the Devonian oil reservoir is not readily distinguishable on the seismic data. One reason is that the rugose erosional topography of the dolomites disperses seismic energy. Another reason is that much of the early valley fill is composed of carbonate conglomerate eroded from outcrops. Those deposits minimize the density contrast at the valley fill contact with the dolomites. Sometimes, the unconformity can only be located by tying the seismic data with sonic data from wells.

At Bacon Flat field, the base of the detached Devonian reservoir is obvious because the underlying Paleozoics dip uncomformably. Also, the Pennsylvanian Ely limestone contact with the Upper Mississippian Chainman shale produces a strong reflection. Other reflections within the Paleozoics parallel that west-dipping interface.

At Grant Canyon field, strong reflections define the contacts between the Devonian reservoir and the underlying Chainman shale, and between the Chainman shale and the Lower Mississippian Joana Limestone. A major detachment surface between the horst complex and the underlying Cretaceous intrusive was located by tying well data and projecting the surface, coincident with seismic reflection truncations.

GEOLOGIC CROSS SECTION

Geologic cross section A-A' (Fig. 3) incorporates data from eight wells. The traverse of A-A' is shown on the structure map. Some of the minor faults have been omitted.

Formation thicknesses were estimated to be around 700 ft for the Ely 600 ft for the Chainman, and 600 ft for the Joana, based on surrounding well data and seismic data. The detached Devonian reservoir rocks could be up to 1,300 ft thick at Grant Canyon and 1,100 ft thick at Bacon Flat, based on the seismic data.

On the cross section, well tops are honored by northwest dips of 7-10 in the Paleozoics. The seismic line exhibits northwest dips of around 10 in the Paleozoics. This is consistent with west dips of 5-15 within the Ely formation, in the 5 Bacon Flat well in NW SE 17 just north of the seismic line.

A normal fault, with over 1,000 ft of throw, bounds the northwest side of Bacon Flat field and separates the field from the BTA Oil Producers I Railroad well drilled in NE SE 18. No shows were reported in the BTA well, which encountered a remnant of detached Devonian rocks 1,200 ft low to the 1 Bacon Flat well.

Another fault, with around 1,000 ft of throw, separates the Bacon Flat and Grant Canyon fault blocks. Adjacent to that fault, an erosional valley cut away the Bacon Flat Devonian dolomites and exposed the underlying Ely limestone. That gorge was then filled by fanglomerates and volcanics, forming a lateral separation and seal between the Bacon Flat reservoir and the updip Paleozoics.

Other faults bound the northeast and southeast sides of the Grant Canyon horst, forming grabens with the detachment fault at the surface of the intrusive. It appears that the grabens developed due to movement along the detachment fault. The grabens have been filled by sediments that sealed the flanks of the Grant Canyon reservoir. Slumping in the valley fill above the grabens reflects late slippage on the detachment fault.

DISCUSSION

From the geometry exhibited by the structure map, seismic line, and cross section, some inferences can be made.

The detachment at the base of the Devonian reservoir rocks appears to have cut "up-section" to the west, through the Mississippian Chairman and the Pennsylvanian Ely. While this upcutting geometry could be explained by a westward moving back thrust or a major gravity slide sheet, the simplest explanation is that it is an erosional surface that exposed west dipping beds.

The rock mass that slid over this surface could have been larger than its present size of about one mile north-south by over two miles east-west. But, since detached Devonian rocks are not found in the grabens to the north and south of Grant Canyon and Bacon Flat, a larger detached mass must have been nearly completely removed by erosion. Why then could 1,100-1,300 ft of those rocks be preserved at Bacon Flat and Grant Canyon?

The shape of the detached mass may provide the answer to its origin. If its present shape reflects its original shape, the rock mass would be more akin to a localized gravity slide (landslide) than a thrust fault or large gravity slide sheet, since it is much longer than it is wide.

By whatever mechanism the Devonian rocks moved, they were emplaced as a continuous mass before the horst and graben fault development. The rocks have been subsequently displaced by the faults and exhibit extensive erosion adjacent to the fault scarps. Remarkably, their present location is almost exclusively coincidental with the Bacon Flat and Grant Canyon faulted horst.

Little or no reactivation of the bounding faults is evident in the overlying valley fill. Therefore, the horst and graben topography already had developed before, or contemporaneously with, the deposition of the earliest valley fill sediments. The rounded shapes of the Devonian remnants capping the horst suggest a long period of subaerial exposure before burial. So also may the solution collapse brecciation due to karsting, reported by Read and Zogg. 1

CONCLUSIONS

An extraordinary sequence of events produced the unique geology of Grant Canyon and Bacon Flat field;. It is unlikely that the same sequence has been duplicated elsewhere.

Certainly, other horsts of Paleozoic rocks are buried in the Tertiary valleys of Nevada. But what are the chances that another horst formed beneath a landslide of Devonian reservoir rocks, on the flank of an intrusive, and coincidental with recent geothermal activity? Still, the fact that a 300 acre reservoir could yield 20 million bbl of oil is alluring, and the search for other "Grant Canyons" will continue.

More oil fields will be found in Nevada, such as the recent Cenex Sans Spring discovery 3 miles west of Bacon Flat. The discovery well potentiated for about 1,400 b/d while the first offset about 1,100 ft to the west failed to find the oil column, suggesting that the size of the discovery may be limited.

Whether any future discoveries will ultimately be as prolific as Grant Canyon field still remains to be seen.

ACKNOWLEDGMENTS

Jebco C is part of a nonproprietary seismic study of Railroad Valley oil fields. I thank Grant Lichtman of Jebco Seismic Inc. for permission to publish the data. I thank Bob Schalla, Don French, and Bob Grabb for many enlightening debates on Nevada geology, and EREC-Balcron management for consent to publish.

REFERENCES

Read, D.L., Zogg, W.D., Description and origin of the Devonian dolomite oil reservoir, Grant Canyon field, Nye County, Nev., in S.M. Goolsby and M.W. Longman, eds., Occurrence and petrophysical prties of carbonate reservoirs in the Rocky Mountain region, Rocky Mountain Association of Geologists, 1988, pp. 299-240.
Veal, H.K., Duey, H.D., Bortz, L.C., and Foster, N.H., Grant Canyon and Bacon Flat oil fields Railroad Valley, Nye County, Nev., OGJ, Mar. 28,1988, pp. 67-70.
Hulen, J.B., Bortz, L.C., and Bereskin, S.R., Geothermal processes in the evolution of the Grant Canyon and Bacon Flat oil reservoirs, Railroad Valley, Nye County, Nev., in D.M.H. Flanigan, eds. Hansen, and T.E. Flanigan, eds., Nevada Petroleum society 1991 field trip guidebook, Nevada Petroleum Society, pp. 4--54.
Montgomery, S.L., Nevada, the next great awakening?--part 2: field summaries and exploration considerations: Petroleum Frontiers, Vol. 5, No. 2, 1992, 64 p.
Fryxell, J.E., Structural development of the west-central Grant Range, Nye County, Nev., The University of North Carolina at Chapel Hill, PhD thesis, 1991.
Lund, K., Beard, L.S., and Perry Jr., W.J., Structures of the northern Grant Range and Railroad Valley, Nye County, Nevada: implications for oil occurrences, in D.M.H. Flanigan, M. Hansen, and T.E. Flanigan, eds., Nevada Petroleum Society 1991 field trip guidebook, Nevada Petroleum Society, 1991, pp. 1-6.