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Geophysical information gathered near the western edge of the Williston
basin by ARCO Exploration Co. indicates the presence of a structural anomaly resembling a meteorite impact feature. Many anomalous features believed to be subsurface astroblemes are documented in the literature. Controversy exists concerning these astroblemes, not only as to their existence but also to their potential as hydrocarbon reservoirs. There have been many hypotheses generated concerning the origins of such structural anomalies. Upon examination of the seismic data, surrounding well control, and literature, the most reasonable interpretation remains that of an astrobleme.
ARCO's Chimney Prospect, located in Garfield County, Montana, has seismic features similar to those seen at Red Wing Creek field, a well-documented probable astrobleme in North Dakota. These similarities formed the basis for the interpretation of Chimney Prospect as an astrobleme feature. Both impacts occurred in the Jurassic, and seismic evidence indicates that neither feature resulted from basement tectonic movement. Seismic data also indicate that Chimney Prospect has a central uplift with approximately 250 ft (76 m) of structural closure. It is surrounded by an identifiable rim syncline and a much less developed outer rim. Chimney Prospect encompasses approximately 2,000 acres (800 ha.). The time of impact has been determined to be Jurassic, with deformation found in pre-impact se iments as old as the Mississippian Kibbey Formation. The deposition of post-impact sediments has been affected by the rebound of the central uplift. The ARCO Coastal/BNRR 1-9 Skeleton Creek, located 3 mi (5 km) southeast of the prospect, has been used as a control well representing normal nonimpact sedimentation in the region of the anomaly.
Chimney Prospect has been tested by the ARCO-1-1 BNRR/Coastal well. No significant hydrocarbons were encountered. Geologic evidence indicates that a small meteorite landed in a shallow Jurassic sea impacting soft, plastic sediments which dispelled much of the impact force. The underlying sediments at the time of impact were elastic enough to brecciate. Subsequent to impact, the open fractures were either healed or filled with calcite, thereby destroying the porosity and permeability in the potential reservoir prior to oil migration. By contrast, the fracturing at Red Wing Creek field was more extensive because the meteorite body was larger; and it impacted more brittle, lithified carbonates.
Using Donofrio's classification system, Chimney Prospect must be considered to be a possible rather than a probable meteorite impact crater. Borehole samples did not confirm the presence of any shock metamorphic features.
The occurrence of astroblemes in the subsurface is rare. When detected, five main criteria must be met to enhance the possibilities of an economic reservoir: (1) a meteorite body of sufficient size and velocity to produce a brecciated reservoir, (2) preservation of open fractures and pore space through time, (3) effective seals for trapping hydrocarbons, (4) oil migration after impact and deposition of the seal, and (5) open-minded management aggressive enough to drill such features. One obviously does not actively explore for these features; however, once stumbled upon, they should be considered as an unusual opportunity to explore and test for hydrocarbons.
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