Quantifying CO2 Storage and Enhanced Oil Recovery for the Thick Cypress Sandstone: Outcrops in Southern Illinois Investigated for Further Sedimentological Study
Nathan Webb, assistant petroleum geologist at the Illinois State Geological Survey (ISGS); Kalin Howell, master’s student in the Department of Geology at the University of Illinois at Urbana-Champaign; and John Grube, retired ISGS geologist and consultant for the project, conducted field investigations of Cypress Sandstone outcrops in southern Illinois to find suitable locations for further detailed sedimentological studies, specifically opportune areas for acquiring new core and geophysical logs. This fieldwork is part of an ISGS project sponsored by the U.S. Department of Energy’s National Energy Technology Laboratory titled “A Nonconventional CO2 EOR Target in the Illinois Basin: Oil Reservoirs of the Thick Cypress Sandstone” and is the team’s second investigation of Cypress outcrops. Combining data from core samples of the Cypress Sandstone from the interior of the Illinois Basin, held at the ISGS Samples Library, with data from new samples acquired and observations made as a result of the present fieldwork provides evidence that allows geologists to properly characterize the geologic architecture of thick Cypress Sandstones, which are oil reservoirs and potential carbon dioxide (CO2) storage targets in the Illinois Basin. An accurate conceptual model of the geology of the sandstone beds is necessary to create precise three-dimensional geostatistical models, which ultimately will be used as input for reservoir simulations that quantify the potential for CO2 storage in and enhanced oil recovery from thick Cypress oil reservoirs.
According to Webb, principal investigator of the project, not every outcrop of the Cypress Sandstone in Illinois has a clean exposure, and the challenge is to find two to three outcrops where further investigation, specifically detailed characterization of sedimentary features, can be conducted. One of the best outcrops will be selected as the site for acquiring a new core and geophysical logs through the Cypress Sandstone interval, which will provide valuable insight into linking outcrop observations to sedimentary features found in existing Cypress core in the Illinois Basin. Scouting trips such as this one also allow project members to anticipate the logistics of coring, including working with landowners to allow drilling equipment on their land.
Webb, Howell, and Grube examined three outcrops during their one-day fieldwork, including one along the west bank of the Ohio River and another that is part of a geologic feature called the New Columbia Bluff. As with many outcrops, weathering and vegetation present challenges because they obscure features in the sandstone, such as cross- or ripple-bedding, which might reveal the depositional environment of the rock, and thus the properties that control the internal architecture and flow of fluids within oil reservoirs.
Although weathered, the outcrops do allow geologists to get a sense of the scale of the sandstone beds, which range from massive to thin within the Cypress Sandstone, and this can help guide the process of making subsurface correlations on geophysical logs. Webb noted that, when examining Cypress outcrops in Illinois, geologists “rarely get the kind of context needed for understanding the stratigraphic succession within the Cypress Sandstone interval” but that observations made during fieldwork can “simply [provide] a statement of X to X size beds in the Cypress.” Webb further noted that identifying the scale of the beds in this way, and understanding how they change across the length and height of an outcrop, provides geologists with a better three-dimensional picture of the features observed in core samples.
More information about this project can be found on its website located at http://www.isgs.illinois.edu/research/oil-gas/doe.