Chris H. Peterson, Robert C. Nims
Unocal Corp.
Los Angeles
The Unocal 1-B30 Federal Canyon Creek well was drilled in the Montana thrust belt about 27 miles northwest of Helena to test Mississippian carbonates in a seismic structure in the footwall of the Eldorado thrust (Fig. 1).
The well penetrated more than 12,000 ft of thrusted Precambrian belt metasedimentary rocks before encountering a faulted and fractured footwall section of Cretaceous through Mississippian shales, sandstones, and carbonates.
The 17,818 ft test, the deepest well ever drilled in Montana, was plugged and abandoned in 1989 after recovering fresh water during production tests in the Madison group.
The original structural interpretation of the Canyon Creek prospect, based solely on 24- and 48-fold seismic data, was that of a relatively simple thrust faulted, doubly plunging anticline in the footwall of the Eldorado thrust.
The 1-B30 well demonstrated that a more complex duplex structure was present under the Eldorado thrust and that the Madison was deeper than anticipated with a true vertical depth of 16,688 ft.
Vitrinite reflectance (VR) data showed the Madison to be in the dry gas window in the 1-B30 well, and offsets in VR to depth plots marked major fault locations in the well.
Dead and live oil shows were present in extensively fractured Madison carbonates in the 1-B30 well, indicating possible hydrocarbon migration through the area. However, production tests of fractured sections in the Madison recovered only relatively fresh water, which probably indicates deep meteoric groundwater flow along faults and fractures.
REGIONAL STRUCTURE
The Unocal 1-B30 Federal is in the Montana thrust belt province, in which a very complex series of low angle thrust sheets moved easterly during late Cretaceous through Paleocene time.
The Eldorado thrust, one of the major thrust faults, outcrops approximately 10 miles east of the well forming a major northwest-southeast trending fault contact between overlying Precambrian and underlying Mesozoic rocks (Fig. 2).
Paleozoic rocks in the footwall outcrop in the Big Belt uplift about 20 miles southeast of the well.
STRATIGRAPHY
The Mississippian Madison group carbonates were the potential reservoir objective of the Unocal 1-B30 since they produce at several fields on the Sweetgrass arch, at Blackleaf Canyon and Two Medicine fields in the Disturbed Belt, and at several fields in the Alberta foothills in Canada.
The well reached total depth in the Mississippian Mission Canyon formation of the Madison group (Fig. 3). Limestone was the primary lithology, but numerous dolomitic intervals were encountered.
The limestones appeared to have little porosity except for fractures. Dead and live oil staining was fairly common on these fracture surfaces in drill cuttings.
The overlying Mississippian Sun River dolomite and the Mississippian Big Snowy group, as well as the Pennsylvanian and Permian section, were missing in the well due to an erosional unconformity at the base of the Jurassic Ellis group.
A relatively normal section of Jurassic and Cretaceous shales, siltstones, and sandstones was penetrated above the Mission Canyon except for an interval of multiple small-displacement thrusts in the Cretaceous Blackleaf formation. These thrusts were recognizable on logs as multiple repeated to overturned sections.
Marine shales in the Cretaceous Blackleaf formation are the probable source rocks for any hydrocarbons in the Mississippian in this area. To locally serve as a source rock, these Cretaceous shales would probably have to be faulted juxtaposed to the potential reservoir rocks.
An igneous sill approximately 200 ft thick was penetrated above the Blackleaf formation. It was composed of hydrothermally altered diabasic or basaltic rocks and appears to have been intruded along a thrust fault that placed the older Kootenai formation over the younger Blackleaf formation.
Kootenai formation shales extended up to the Eldorado thrust, which has placed Precambrian Belt metasediments over these Cretaceous rocks.
GEOPHYSICS
Due to a complete lack of well control in this rank wildcat area, subsurface mapping was done with seismic data.
Of the 10 lines used to directly map the structure, eight were shot with heliportable surface charges (Poulter method) because of the mountainous topography. The other fines were vibroseis lines shot along highways.
The Canyon Creek proprietary lines were shot in 1982 and 1983 using the Poulter method with 2 lb. charges hung on twenty four 3 ft stakes connected by primacord.
Station spacing was either 110 ft or 165 ft, and CDP coverage was 24 or 48 fold. Testing of some parameters such as shot and receiver arrays was done in the field at the start of operations.
Reflection events on these lines are discontinuous due to the complex folding and faulting. Reflections were identified by synthetic seismograms made in wells penetrating Paleozoic rocks in the undisturbed area to the north, then extended into the prospect area.
Thrust segments were character-correlated to the identified events. The reflection event marking the Eldorado thrust plane was identified by extrapolation from surface outcrop.
For depth conversion, velocity estimates of Belt rocks ranged from 13,000 ft/sec to 17,500 ft/sec. A study of first break velocities on proprietary lines across the prospect showed a mean velocity of about 13,000 ft/sec. The 17,500 ft/sec velocity was derived from data in the Belt basin west of Glacier National Park.
A median estimate of 15,000 ft/sec was selected for depth maps using data from the ARCO 1 Silver Creek well, which is located about 20 miles southeast of the Unocal 1-B30 (Fig. 2). This well drilled 2,400 ft of Belt rock with an average velocity of 15,000 ft/sec.
Canyon Creek Line 6 (Figs. 4, 5) is a dip line oriented southwest to northeast across the prospect. The low angle Eldorado thrust, located at a time of just over 1 sec, separates the belt with its chaotic seismic signature from the more coherent Cretaceous reflectors below. The package of reflectors representing west-dipping undeformed Paleozoics is located at about 2.5 sec.
In the center of the line at about 1.5 sec is the Canyon Creek anticline formed by somewhat discontinuous reflectors that were interpreted to be the Paleozoic section based on higher amplitudes and character correlations with regional reflectors.
Seismic mapping showed the Canyon Creek structure to be a doubly plunging anticline about 12 miles long and 6 miles wide trending roughly N. 15 W. It was originally interpreted as an imbricate stack of Paleozoic rocks riding on a large thrust fault informally designated the Canyon Creek thrust.
Mapping on the Eldorado thrust reflector indicated a gentle anticline located over the Canyon Creek structure. This feature probably formed as the Eldorado thrust was folded by later movement along the underlying Canyon Creek thrust.
RESULTS OF THE WELL
The Unocal 1-B30 Federal was a geologic success proving the presence of Mississippian carbonates under Precambrian belt rocks thrusted along the Eldorado thrust (Fig. 6).
For the most part the well penetrated the expected section of thrusted belt metasedimentary rocks and Cretaceous shales overlying Mississippian Madison carbonate.
The average velocity of the Belt section was found to be about 16,750 ft/sec, which was considerably higher than the 15,000 ft/sec velocity that was originally used. Thus the Eldorado thrust was encountered deeper than expected.
The reflector initially interpreted to be the first Madison turned out to be an igneous sill. However, the Madison was eventually encountered at a true vertical depth of 16,688 ft.
Structurally the prospect was found to be similar but more complex than expected. Sample lithology, well logs, and VR data indicated the presence of several thrust faults in the section below the Eldorado thrust (Fig. 8). These data have led us to our current interpretation of the structure as a duplex.
Sharp offsets in the VR values at various depths clearly showed the Eldorado thrust fault and a lower, lesser thrust fault (Fig. 7). However, carbonaceous material in the Precambrian Belt rock that was analyzed for VR was probably not vitrinite and is not quantitatively comparable with the true vitrinite data.
The base of the oil window was identified at a depth of about 15,500 ft in the Cretaceous Blackleaf formation. The base of the wet gas window occurred at about 16,500 ft in the Jurassic Ellis group.
The targeted Madison group was found to be in the dry gas window at total depth. Although extensively fractured, the Madison carbonates had very little matrix porosity.
Live and dead oil staining was common along the numerous fractures indicating possible oil migration through the area. However production tests of three of the most highly fractured intervals recovered only 141 to 288 bbl of fresh water with Rw values ranging from 2.55 to 3.75.
Such fresh water probably indicates deep meteoric ground water flow along faults and fractures possibly recharging from the Big Belt uplift about 20 miles southeast of the well.
The few wells drilled so far in the Montana thrust belt have not begun to test the hydrocarbon potential of this vast frontier area, but the Unocal 1-B30 Federal has contributed one more piece to the puzzle.
ACKNOWLEDGMENTS
The authors thank the management of Unocal Corp. for allowing publication of this paper, and Unocal's Science & Technology Division for interpretations of the geochemical data. Also, special thanks goes to Unocal geologist Steve Sperry for the original geological interpretation of the prospect.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
