Key core samples acquired for identifying carbon dioxide enhanced oil recovery and storage opportunities in Illinois

Illinois State Geological Survey (ISGS) researchers acquired roughly 162 feet of core samples from the Cypress Sandstone, a geologic formation in the Illinois Basin, for sedimentological and petrophysical study. These core samples will assist researchers in their efforts to assess whether carbon dioxide (CO2) can be stored in the Cypress Sandstone to reduce the effects of climate change while simultaneously using CO2 to recover oil from the formation—a process called nonconventional CO2 enhanced oil recovery (EOR) and storage.

Leo Giannetta with an intact core sample of the Cypress Sandstone of roughly 5 feet in length. ISGS driller Jack Aud is in the background on the drill rig. Photograph courtesy of Leo Giannetta, ISGS.

The site where drilling operations took place. The ISGS drilling crew, seen in the photo, conducted the drilling operations.

“These core samples provide a key piece of information about the thick Cypress Sandstone,” said principal investigator Nathan Webb, “They are the only example of a core through the entire formation. You can’t go to any outcrop for this kind of information.” The ISGS project, which is sponsored by the Carbon Storage Program under the U.S. Department of Energy, focuses on a specific part of the Cypress that contains thick sandstones with oil-bearing reservoirs. Of the 162 feet of core acquired from the Cypress, 100 feet was from this thick sandstone part of the formation. In total, 260 feet of core was acquired during the drilling operations.

Carbon dioxide EOR and storage is particularly helpful for the kinds of oil-bearing reservoirs being investigated, called residual oil zones, because those oil reservoirs can overlie brine formations that “steal” the CO2 conventionally used to enhance oil recovery. Typically, the loss of large volumes of CO2, low oil production, and high salt water production from the underlying brine formation cause the reservoirs to be uneconomic. However, because the aim of CO2 EOR and storage is to produce oil while simultaneously injecting large volumes of CO2 underground, the loss of CO2 is actually beneficial and enough CO2 is usually injected to contact oil and increase recovery (read more about it here). To ensure the technical and economic feasibility of an oil-bearing reservoir for CO2 EOR and storage, researchers must characterize its geology. Core samples play a critical role in this.

Core samples are collected through a coring process whereby cylindrical pieces of rock are cut from geologic formations by drilling into the Earth’s subsurface. Joe Devera (ISGS) scouted the drilling location, acquired permission from the landowner to drill, and coordinated drilling activities with the ISGS drilling team. The drilling location (near Anna, IL) was chosen based on geologic mapping done by the ISGS and the experience of ISGS geologists. Previous fieldwork conducted by ISGS geologists for the project “cemented in our minds what the mapping showed,” according to Webb, namely that the core taken from the Cypress in the area of the drilling location would be most representative of the oil-bearing reservoirs of interest.

The drilling location was two to three miles north of the Cypress Creek outcrops (see the topographic map below), from which the Cypress Sandstone gets its name. Geologists call the location that gives a geologic formation its name a “type area.” As is the case with the Cypress, a type area is often where a formation outcrops on the Earth’s surface and where the formation was first identified and described.