Geologists partner with stakeholders, collect data to test new method of identifying oil reserves

A partnership months in the making has paid off. Geologists from the Prairie Research Institute collected data, called pulsed-neutron logs, on the thick Cypress Sandstone in collaboration with the Podolsky Oil Company and Schlumberger. They will use the logs to validate the accuracy of new methods they have been developing to quickly and cheaply identify residual oil zones, or ROZs, in the Cypress.

Residual oil zones have not been investigated in Illinois because of their low saturation unlike other areas such as the Permian Basin in West Texas. A process called carbon dioxide (CO2) enhanced oil recovery and storage—where CO2 is used to produce oil and stored underground—could be used to economically produce oil from ROZs in Illinois, just like Texas. Before that can happen, oil field operators in Illinois need a method to identify ROZs using existing data rather than costly logging—which is why geologists are developing the new method.

“Collecting this data is an important step before we acquire new core in the summer, which will allow us to physically verify that our methods can predict the presence of oil in a part of the formation where it hadn’t been detected before,” said Nathan Webb, principal investigator of the project at the Illinois State Geological Survey, a division of the Prairie Research Institute at the University of Illinois at Urbana-Champaign.

Engineers preparing to lower the tool for pulsed-neutron loggin

Schlumberger engineers preparing to lower the tool for pulsed-neutron logging into the well. Photo credit: Nathan Webb, ISGS.

Geologists are collaborating with CountryMark, another oil company active in Illinois, to drill a new well and collect the core. They hypothesize this new well will drill through an ROZ, which means the core will hopefully be stained brown instead of the normally white sandstone—a physical sign of oil. But the amount of oil in an ROZ may be so low that staining isn’t apparent, which is when logs can help geologists and oil field operators correctly identify the presence of oil.

The partnership helps the Podolsky Oil Company to understand how much more oil could be produced from the Cypress and where the oil may be located. According to Michael Podolsky, owner of Podolsky Oil Company, the pulsed-neutron logging helped identify that a formation in one of their wells did not contain an oil reservoir, even though initial data suggested that might be the case.

“When we ran these Schlumberger logs we were able to say that that’s not where the [oil] saturated sand came from,” said Podolsky. “So down the road we won’t waste money trying a zone that doesn’t have anything in it. In a small-scale sense there was a direct immediate benefit to us not to count on something or worry about looking at it in future wells.”

“We also have a better understanding of the kind of ultimate reserves we have within our productive Cypress reservoir in this area,” he added.

Roustabouts removing tubing from the well

Because the well is an active oil producer, roustabouts needed to remove tubing that remains in the well during production. Photo credit: Nathan Webb, ISGS.

During logging, an engineer lowers a tool with sensors into a well. The tool is then slowly raised while it “pulses” neutrons a few feet into the surrounding rock. The neutrons create gamma rays when they contact the rock and fluids it contains. Different materials and fluids emit varying intensities of gamma rays, which the tool measures. Geologists and engineers interpret measurements from the tool to discern how the proportion of oil and water within the rock varies with depth. Four wells in Noble Field, Illinois, were logged for the project.

This data collection and analysis is the first step to a regional assessment of CO2 enhanced oil recovery and storage potential in ROZs in the Cypress, according to Webb. Both the field- and regional-scale work of the project help stakeholders gauge the economic feasibility of that potential.

Ideal water saturation profile

Water saturation profiles like this one are what geologists hope to find when assessing an oil reservoir. Water saturation, which is very low in a conventional oil reservoir, increases and stabilizes in a residual oil zone, before increasing to 100% in the brine-saturated base. Image credit: Nathan Grigsby, ISGS.

“On a larger scale, I think it’s extremely important because Colborn [Noble] oil field where this was done was a multi-million barrel field and there’s a tremendous amount of oil still in place. When Nathan is through with his work, if we can find that there are even more barrels left behind after the water has swept through it, then it certainly makes the likelihood of a future CO2 tertiary flood much more economic, much more likely,” Podolsky said.

One of the keys to carbon storage is to find ways to monetize the CO2 being captured, according to Podolsky. If projects can accelerate the process of monetizing captured CO2 by identifying more oil reserves and making the process more economically feasible, then this may help initiate other projects.

“That’s one of the risks in a lot of these old fields. Once they’re plugged out, it may never be feasible to re-establish them both on a lease hold basis because there has been so many splintered mineral interests and the capital costs of redrilling,” Podolsky said. “It’s important that, if these projects are going to go forward, that they go forward while those things are intact because the projects become much less likely if you have to redo everything from the beginning.”

The researchers would like to thank Michael Podolsky and Stephanie Storckman (geologist) at the Podolsky Oil Company without whom this research would not have been possible. Specifically, Stephanie acted as the main contact with the Illinois State Geological Survey, screened the wells, chose the logging intervals, and coordinated field logistics.

This material is based upon work supported by the Department of Energy under Award Number DE-FE0024431. More information about the project can be found at

Contact: Nathan Webb

By Dan Klen