A new interpretation of a very old area may turn traditional views and expectations for a Midwest U.S. basin upside down.
New interpretations of potential field, seismic and other data in the Illinois Basin, according to an Oklahoma geologist, indicates unrecognized natural gas reservoirs may exist there.
AAPG member T. Joshua Stark, an independent consultant in Tulsa, along with a team of geoscientists, believe they have discovered new structural styles and unpenetrated rocks based on the review of a wide spectrum of geological data.
"The idea now is to utilize this new data to drill deeper for Cambrian and Precambrian-aged targets," he said of the historic basin that has been active for more than a century.
"Looking at the scale of the thrusts and intra-basin faults, there could be recoverable gas reserves in the 100 bcf range."
"On the Eastern Shelf of the Illinois Basin within the newly discovered English Basin, there exist two unexplored rock groups," he said. "The basal strata is the Mesoproterozoic Centralia Group, which is truncated by a major angular unconformity. Upon this erosional surface, the overlying Marengo formation is locally deposited within the English Basin."
The English Basin contains a central depocenter filled with up to 3,000 meters of Marengo sediments.
"While the Centralia and Lower Marengo appear to be Mesoprotozoic in age, the upper part of Marengo is interpreted to be Neoproterozoic-aged," he said.
"What became evident was that there were great changes in the understanding of structure and stratigraphy in the eastern shelf of the Illinois Basin."
Stark, who won the Eastern Section Leverson Award in 1998, and co-authors Steve Rowley, Carl Steffenson, James Drahovzal, Louis Schultz, Glenn Bear (who coined the name "Marengo" in his Ph.D. dissertation) and Steve Bergman, presented their paper on these new concepts in September at AAPG's Eastern Section meeting in Indianapolis.
He also recently talked with officials at the Indiana Department of Natural Resources to inform them of his findings.
Seismic Data Clues
Stark said that since 1990 there has been a renaissance in the eastern Mid-continent, linked to the recognition of the East Continent Rift Complex, which may contain up to 10 kilometers of unpenetrated strata. It extends from Oklahoma to Ohio and has formed, among other things, the Cincinnati Arch.
"The Cincinnati Arch is not underlain by a basement ridge," he said. "Instead, a major rift system has become buoyant beneath it, resulting in the deformation of the overlying Paleozoic rocks, creating the arch."
Simultaneous to this discovery, Stark recognized another major depocenter in the eastern shelf area of the Illinois Basin, based upon interpretation of total intensity aeromagnetic and bouguer anomaly gravity maps
Stark published his initial findings in 1990. Several years later, Vastar acquired a proprietary regional seismic survey over the English Basin area. Ultimately, Stark and his team convinced Vastar and Williams Gas Pipeline Texas Gas to allow the team to analyze the regional seismic database and publish the findings.
"It demonstrated conclusively the existence of the depocenter," he said.
He and other researchers named it the English Basin for the small town of English located near the basin's center.
"One exciting discovery is that the underlying Centralia rocks are extensively thrusted," he said. "These thrusts are eastward verging with west dipping fault planes. They cover the two southern tiers of counties in Indiana, and have been named the Hoosier Thrust Belt."
In the deeper subsurface there are potential traps in the thrust anticlines of the Centralia.
"Some of the seismic suggest there may be gas in place," he said. "In addition, the Cambrian-age strata of the English Basin demonstrates syn-tectonic growth faulting, linked with Cambrian-aged rifting during the formation of the Iopetan Ocean.
'Very Defined Thrusts'
Stark maintains that the English Basin's deep central depocenter is margined by large faults that are positioned favorably with respect to regional gas migratory pathways. This may provide a mechanism for significant accumulations of gas.
The Illinois Basin, he noted, is one of the oldest working basins in the country besides Pennsylvania and Ohio.
Stark said that his team's research demonstrates the existence of new structural mechanisms.
"Within the Cambrian-aged rocks there is a tremendous amount of deformation," he said, "primarily due to thrust ramp folds and fracturing generated during Permian-aged compression. There's also a great deal of undrilled section, not only in the expanded Cambrian rocks but in the clastic Marengo rocks and in the Centralia thrusted rocks."
While most of the Permian-aged thrusting is evident at the edge of the basin, there is good evidence to suggest that ramping has also occurred to the west in the basin's deeper part, linked with significant production.
"They're very defined thrusts," said Stark, who has some large-scale prospects associated with the features.
Going Deep
Another significant discovery, Stark said, is that the eastern margin of the Illinois Basin is bounded by two major uplifts.
The southern block, the Louisville Uplift, is interpreted to have been emplaced as a foreland-style thrust block during the Neoproterozoic, about 600 million years ago. Vertical movement of this 1.5 billion-year-old block was about eight kilometers.
Since the earliest days of area oil exploration, the Mount Carmel Fault has been considered to be the hinge of the Illinois Basin, bounding the basin as a normal fault with a maximum down to the west displacement of 250 feet.
"We've learned that the Mount Carmel Fault is not the major fault of the system," he said. "In fact, it's antithetic to the master fault of the system, which dips to the east into the East Continent Rift Basin.
"The fault system appears to be defined by early wrench deformation, and occurs on the eastern margin of another major basement uplift block called the Mount Carmel Uplift."
Between the master fault and the Mount Carmel Fault is a central down-dropped graben -- a late-stage remobilization of the fault system, he said.
On the fault system's down-side, numerous gas storage domes are developed in Devonian-aged strata upon the Leesville Anticline, a major fold system that extends for almost 50 miles. Since the late 1800s, it was believed that the folds were derived from deformation associated with the Mount Carmel Fault.
"It turns out that the fault and anticline are essentially unrelated," Stark said. The fault may have formed during Neoproterozoic block uplift, while the folds came into existence during later Permian compression.
"There probably exists a wider belt of anticlinal structures that are susceptible to entrapment of hydrocarbons.
"We're also seeing additional folds to the west of that final stage fold system," he added.
The Devonian closures have been linked with the propagation of forced folds, generated by the thrust deformation of the underlying Cambrian-aged sediments.
"This realization gives us a major reason to drill considerably deeper," he said. "Everyone on our team is a great champion of drilling deeper in the Illinois Basin.
"It's viewed as being a mature basin," he continued. "However, over 90 percent of wells only penetrate into Mississippian rocks, so very few wells evaluate Devonian and older strata."
That means, he said, the basin is manifestly unexplored at depth.
Stark noted that Indiana currently cannot meet its demand for gas -- and that these new findings may provide an excellent opportunity for the state to increase the supply of natural gas in the Eastern Shelf area.
Stark asks: In the United States' mature basins, are we running out of reserves, or are we simply running out of ideas?
"At least in the Illinois Basin," he said, "the application of modern technology to a historically mature area is generating large-scale prospects and hopeful expectations.