G. Alan Petzet
Exploration Editor
Impact craters, also called astroblemes, are the most common landform in the solar system, and several on earth have produced oil and gas.
Significant cross-disciplinary study is being aimed at understanding the origin, structure, and economic potential of surface and buried craters worldwide. The Oklahoma Geological Survey and the U.S. Department of Energy organized a workshop to discuss the Ames structure, a feature in Major County, Okla., along the sprawling, giant Sooner Trend/Ringwood oil producing complex.
This rundown is designed to provide basic information gleaned from speakers and abstracts presented at the workshop. Attendees from a dozen states and six countries included oil and gas operators, geologists, planetary, university, government researchers, organic geochemists, and service and supply company representatives. OGS is to publish proceedings later.
METEORS VS. VOLCANOES
The Ames feature itself has been known to some oil people as a Silurian Hunton graben for decades, but oil and gas production related to its supposed origin as a postulated meteorite impact structure started in 1990-91.
One speaker said 145 meteorite impact structures have been located on earth, and more than 10% are producing or have produced economic resources, whether oil and gas, uranium, copper, diamonds, gold, lead, and zinc. These resources range from minor to world class ore bodies.
Craters are being discovered at the rate of three to five per year. Most are buried or obscure, and dozens if not hundreds more are likely to be found, another speaker said.
Many questions remain about these generally circular structures, and the more than 225 people attending the Norman, Okla., workshop in late March made no secret that they differ widely on a lot of the answers.
One big point of contention is whether any given structure is the site of a meteorite impact, volcanic activity, or a combination. A majority of those attending the workshop seemed to firmly accept the impact theories, but the nonimpacters were vociferous.
The Ames crater contains so-called shock metamorphic features such as planar deformation features in quartz and glass inclusions, vestiges of the extreme temperatures and pressures present at the time of the impact. Impacters maintain that volcanic processes do not involve temperatures or pressures high enough to affect rocks in this manner.
OKLAHOMA CRATER
The Cambro-Ordovician Arbuckle oil and gas discoveries at Ames have proved exciting in numerous aspects.
Ultimate recovery is estimated in excess of 50 million bbl of oil plus 20 bcf of gas, which would make the feature Oklahoma's largest oil discovery in many years.
The feature, 8-10 miles across and buried to 8,500-9,500 ft, is said to be the most studied, the most oil productive, and one of the most drilled astroblemes on earth.
Ames has been the subject of, among other studies, 2D and 3D seismic surveys, gravity and magnetics, satellite and radar image analysis, mineralogical and source rock geochemistry, borehole imaging logs, petrographic and shock metamorphism studies, reservoir characterization, and argon age dating.
Oil company geologists said the Ames crater produces from 17 reservoirs including 11 in lithified granodiorite and dolomite breccias, five in the Arbuckle dolomite of the crater rim, and one substantial accumulation in recrystalized dolomite ejecta on the crater's west rim.
An Ordovician Oil Creek black shale that exists only in the crater in this general area is believed to have been the source of oil to the underlying Arbuckle.
One well, the 1 Gregory in 20-21n-9w, is possibly the largest oil well established from a single pay in Oklahoma, one speaker said, with conservatively estimated ultimate recovery of as much as 10 million bbl. Production has involved no water and little decline.
QUESTIONS ABOUT AMES
One oil company speaker divided up the economic drilling results like this: excellent wells five, good 13, fair 10, poor 22, dry 42, and unknown four.
He concluded that spending has totaled $41.7 million, including 3D seismic surveys and four horizontal wells and excluding acreage costs. Combined net revenue, based on production of 3 million bbl of net $15/bbl oil and 7 bcf of net $1.35/bbl gas, topped $54 million through mid-1994.
However, two wells with little or no decline curve as yet will have produced most of the oil. They were drilled before wide recognition of the impact theory and 3D surveys and produce from non-Arbuckle formations.
An overall return of three or four to one is likely, but profitability or lack of it has varied widely for the independent oil and gas operators that drilled there. One speaker said many post 1991 wells that turned up dry were drilled largely because operators of acreage held by production were under great pressure to drill due to the Arbuckle discoveries nearby.
MORE QUESTIONS
The two best wells at Ames produce from non-Arbuckle crater lithologies known as exotics, which are believed to be basement rock talus deposits on the flank of the central uplift.
Arbuckle carbonates alone have not been very commercial, casting doubt on the economic viability of other Arbuckle structures on the Anadarko basin northern shelf.
Explorers are aware of the existence of 16-18 look-alike structures to Ames within 75 miles northwest of the Nemaha ridge.
Ames is still tectonically active. A 2.8 magnitude earthquake centered along the crater's southeastern rim occurred in April 1994.
PRODUCING ASTROBLEMES
Here is a list of impact craters (containing shock metamorphism) associated with oil and gas production:
1. Ames structure (oil and gas), Major County, Okla.
2. Avak structure (Barrow gas fields), Alaska.
3. Calvin-28 oil and gas field, Cass County, Mich.
4. Marquez structure (gas), Leon County, Tex.
5. Newporte structure (Des Lacs oil field), Renville County, N.D.
6. Sierra Madera (Elsinore gas field), Pecos County, Tex.
7. Lyles Ranch gas field, Zavala County, Tex.
8. Red Wing Creek oil field, McKenzie County, N.D.
9. Steen River oil and gas field, northwestern Alberta.
10. Chicxulub (oil), Yucatan peninsula, Mexico.
Other postulated impact craters outlined at the workshop:
- Viewfield oil field, southwestern Saskatchewan, shock metamorphic features not yet detected.
- Panther Mountain, Ulster County, N.Y. (gas potential).
- Big Basin craters, Clark County, Kan.
- Boltysh depression, Ukraine, oil shale potential.
- Haswell hole, Bent and Kiowa counties, Colo.
- Manson crater, northwestern Iowa.
- Merna crater, Custer County, Neb.
- Ries crater, Bavaria, southern Germany.
- Wells Creek and nearby features, northwestern middle Tennessee.
THE FUTURE
The locations of impact features on earth has seemed random to many workers, but theories are emerging that indicate that if all craters could be identified that locational patterns might emerge.
Scientists familiar with the 300 km diameter Chicxulub impact basin said they strongly believe the impact there did not initiate volcanic activity, but they agreed this might occur in areas where the earth's crust is relatively thin.
Researchers are to drill three more wells to study the Chicxulub structure this spring using Mexican funds. The deepest penetration at Chicxulub is 3,500 m, and the nearest production is about 100,000 b/d of oil from Mexico's Bay of Campeche fields, which some believe to be producing from impact ejecta rocks.
One speaker said it is possible that the residual heat of an impact would be sufficient to mature a pre-existing source rock; oil entrapment could be afforded by the structure created by the impact. The hydrocarbon seeps on the east side of the Siljan structure in Sweden probably originated in this manner, he said.
More significant, however, is that astroblemes can create their own basins where source rocks can accumulate, he added.
Origin, however, doesn't concern geologists as much as economic significance. Of 20 craters with shock metamorphism that are found in petroleum basins, 10 are commercial oil and gas fields. That should be a significant incentive to drill impact structures.
One geologist estimated the potential reserves of undiscovered impact craters in North America at upwards of 5 billion bbl.
Another geologist with a major oil company reminded those who intend to pursue oil and gas in postulated impact craters to map in circular trends.
Copyright 1995 Oil & Gas Journal. All Rights Reserved.