Calling impact cratering the “most violent geological process we know,” a leading expert in the field says the process has the broadest “implications for Earth history.”
Not to mention what it means for the oil and gas industry and the implications for finding hydrocarbons.
AAPG member David T. King Jr., professor of geology at Auburn University, has been studying such craters since the late 1990s – and he says the future regarding hydrocarbon exploration could be very bright.
“Impact structures have the potential to be, and in some instances are, important reservoirs for oil and gas,” he said, “and are probably much more common than we expect them to be, so they may be encountered in exploration activities and thus should be explored for their potential.”
To further amplify, King cites a 1998 article in the Oil and Gas Journal by A. D’Onofrio that concluded there were 11 impact structures in North America that had proved reserves totaling over 30,000 Mbbl of oil and over 15,000 bcf of natural gas.
King’s own latest research (“Impact Structures and Craters of the Gulf Coastal”) was presented in a paper and poster at last year’s annual meeting of the Gulf Coast Association of Geological Societies. His co-author was Lucille W. Petruny.
The debate over impact structures is not if they exist or if there are more yet to be found, or even if there’s a potential for petroleum exploration and development – but whether they can explain, as some theorize, the Gulf of Mexico itself.
First, though, some definition:
For an impact structure or crater to be confirmed, King said, there must be evidence in four categories.
- The structure or crater must be (or have been) circular to oval shaped and have concentric faults and overturned beds on their rims.
- They must show geophysical signature consistent with impact (e.g., gravity profile, seismic cross section, magnetic contour, etc.).
- There must be hypervelocity-shocked materials, such as microscopic shocked quartz or megascopic shatter cones.
- There should be physical traces of the impactor, such as meteoritic fragments or geochemical traces of the meteorite in the bedrock.
Structures lacking such high levels of proof generally are “suspected” sites until the proof comes out (or not).
“For petroleum or other resource exploration,” King said, “there are features associated with impacts that are very important. Production of deep fractured zones (impact-induced porosity and permeability) can occur in regions below the impact structure’s rim, central peak or ring faults. Larger impacts infuse thermal energy in the crust that persists for some time.
“This can cause maturation of hydrocarbons,” he added, “and/or induce fluid flow that accentuates economic mineralization.”
Evidence in the Gulf
One of the places where drilling in an impact structure has occurred in the Gulf Coast is at the Marquez Dome in Texas, where, according to King, the success rate as of 1998 was 60 percent. King points to the successful drilling at Sierra Madera and Bee Bluff (Lyles Ranch).
As for where the structures and craters are in the Gulf region, Texas has the most confirmed (Marquez, Odessa and Sierra Madera) and suspected impact features (Bee Bluff, Hico and one unnamed).
Among the other Gulf coastal states, the list is as follows:
- Louisiana (Brushy Creek; suspected).
- Mississippi (Kilmichael; suspected).
- Alabama (Wetumka; confirmed).
- Georgia (Woodbury; suspected).
Collectively – including the drilling at Marquez – King said the three structures in Texas have primary reserves estimated to be about 280 bcf of natural gas.
What surprised King were the number of wells already drilled at Texas impact structure sites (89) and the number across the nation (1,161).
Personally, for King, during the mid- 1990s he shifted his research from facies analysis and sequence stratigraphy to impact geology.
“I started working on the Cretaceous-Tertiary boundary in Belize in 1996 and Wetumpka impact structure in Alabama in 1997,” he said.
Over the past 10 years he has visited impact structures and outcrops of impact ejecta in many parts of the world.
As to those larger claims – particularly those that the entire GOM, in fact, was formed from a cosmic impact from above rather than traditional mechanisms from below, and that the impact crater theory of the Gulf would explain many of the “perplexing problems in the Gulf” – King says, “I am reminded of what Carl Sagan often said: ‘Extraordinary claims require extraordinary proof.’”
King believes theories about the origins of GOM are an “interesting idea,” but believes there are other valid ideas on the origin of the Gulf as well.
He challenges those who make these spectacular claims to send in their geophysical data, have the samples examined petrographically for shocked minerals and perform age-dating work.
“Let’s get this question answered.”
King says he knows no specific plans to exploit further impact structures or craters for exploration beyond what has been done already with the 11 structures.
He cites 180-kilometer diameter Chicxulub impact structure in Mexico as the most drilled and most productive structure, but says that’s just the tip of the geologic iceberg.
“Smaller craters have potential reservoir rocks as well,” he said. “There are probably dozens, if not hundreds, of impact structures and craters out there that we do not know about, or are known but are proprietary.”
Clearly King believes these structures are an untapped potential, but one not yet seized upon by the industry.
“I am not sure that impact structures are on the radar screen for most exploration groups,” he said.
And here King issues something of a challenge.
“Over the Mesozoic-Cenozoic history of the Gulf region, there must have been dozens of significant impacts in the Gulf region. Where are all these craters? Do petroleum companies have data and samples that can help us answer this question?
“There should be many, many more impact structures out there in the Gulf area than we know today,” he said. “I think many are not identified because geologists do not consider looking for them.”