The Arkoma Basin has long been a popular drilling focus for the industry. As the saying goes, everything “old” is new again – and the Arkoma is no exception.
Lately it’s been the focus of some renewed attention, including the evaluation of 3-D seismic inversion data as an aid in natural gas exploration.
The basin occurs in southeast Oklahoma and extends east from there into western Arkansas. Production is principally from upper Paleozoic reservoirs, including the lower Atokan Spiro, Panola and mid-Atokan Red Oak sandstones, according to AAPG member Ibrahim Cemen, professor and chair of the geological sciences department at the University of Alabama in Tuscaloosa, Ala.
He noted the Arkoma is a foreland basin of the Ouachita fold-thrust belt. It contains middle Cambrian to Late Mississippian miogeoclinal rocks about 5,000 feet thick, overlain by about a 20,000-foot thickness of Pennsylvanian flysch and molasse deposits.
The basin harbors over four Tcf of undiscovered natural gas, mostly within the shallow marine to deltaic Pennsylvanian sandstones of the Atoka formation, according to Cemen. The lower Atokan Spiro-Wapanucka reservoir has been a major exploration target.
The Alabama professor has been studying the basin since 1993, when he was a professor at Oklahoma State University (OSU). He was particularly intrigued with the Spiro sandstone, which he said is an especially good reservoir when it contains chamosite facies, which preserve porosity.
He embarked on a major project in the area, which was funded by the Oklahoma Center for Advancement of Science and Technology (OCAST).
“At OSU, we constructed balanced cross sections and placed the chamosite facies on the cross sections based on our study from cores in the Arkoma Basin with my colleague, the late Zuhair Al-Shaieb,” Cemen said. “We restored the cross sections and found out there is probably chamosite facies to the south of the big structures everyone was looking at.
“When we submitted our final report to OCAST in 1995, a lot of oil companies got interested,” he noted.
“During a field trip to the basin in 2004, I was talking about the importance of chamosite facies and pointing out the possibility of it being present beyond the structures already extensively drilled,” Cemen said.
“A couple of BP geologists who were on the trip said BP had tested this hypothesis and discovered the South Hartshorn gas field,” Cemen said. “They had more data, of course.
“That is exactly why OCAST supported our project,” he added, “to help the oil and gas industry in Oklahoma.”
Porosity and Acoustic Impedance
Cemen noted that in 2006, his structure group entered into a project interpreting 3-D seismic data provided by Devon Energy. Both 2-D and 3-D seismic, well log and core data were used during the project to construct the structural sections to illustrate the Arkoma Basin’s complex structure.
Structure is a big factor in Spiro productivity, but understanding and identifying changes in rock properties over an area of interest is on par with knowledge of structure.
Major Pennsylvanian sand units of the Arkoma Basin.
“In the particular area of the structurally complex Arkoma where this study was done there’s a number of low angle thrusts, and you get repeated Wapanucka-Spiro formations,” said AAPG member Rod Gertson, senior geophysical adviser at Devon.
“Since the ’50s, at least, there’s been a lot of production in the Arkoma Basin from there,” he said.
Cemen pointed out that they used an acoustic impedance inversion volume calibrated to well control to map porosity changes in the Spiro sandstone. Every well with a sonic log within the survey area was analyzed to understand velocity changes in the Spiro along strike and dip of the principal thrusting direction.
“Looking at the seismic inversion to understand which physical characteristics of the rocks and fluids might be predictable, we found an acceptable correlation between porosity and acoustic impedance,” Cemen said.
The Good Predictor
One of the results of the interpretation of the seismic inversion data is that in areas where the Spiro sandstone experienced facies changes, the acoustic impedance value is a good predictor of porosity.
“We’re finding things with the data that maybe we didn’t know before,” Gertson said. “From a scientific point of view, it’s interesting to see if you can determine some things about areas of higher porosity within particular producing formations using the 3-D seismic.
“There appears to be somewhat of a correlation between acoustic impedance and the porosities of the Spiro,” Gertson said. “The Spiro sandstone is hard to distinguish from the Wapanucka because both are very high velocity, high density units.
“When there is some porosity in the Spiro as we saw from well logs, we hoped there was come correlation with inversion also, and that’s what we appeared to see,” he noted. “We haven’t been out to test it in other ways, as our plays we’re drilling right now are the shales.”
Gertson and Cemen both emphasized that the 3-D seismic data used at OSU have been used by several students for theses focusing on structure and stratigraphy.
Cemen noted that he is continuing his Arkoma Basin project at Alabama, where he has one master’s student working on the project.
Gertson serves on the students’ thesis committee.