Residual oil zones (ROZ) form due to various geologic conditions and are located below the oil/water contact (OWC) of main pay zones (MPZ). Since ROZs usually contain immobile oil, they have not typically been considered commercially attractive for development by conventional primary recovery methods used in the initial phases of oil production. However, during the last decade some operators of these viable fields that also contain ROZs have extended carbon dioxide enhanced oil recovery (CO₂-EOR) to below the MPZ to commercially recover oil from the associated ROZ. Increased interest in ROZs is also due to the application of anthropogenic CO₂ for oil recovery, leading to the subsurface sequestration of CO₂, which can be part of the current net-zero carbon oil and climate change objectives.

Several detailed studies of selected formations in the Permian Basin of the United States have shown that ROZs can be as common as traditional conventional oil reservoir traps, suggesting significant resources for potential additional hydrocarbon recovery and subsurface CO₂ sequestration via CO₂-EOR. However, applications of CO₂-EOR to ROZs have been limited despite the estimation of significant oil resources considered recoverable through CO₂-EOR, and the benefit of concurrent geologic CO₂ storage that would help offset carbon emissions from the produced oil. The combination of insufficient economic incentives and technical reasons related to data scarcity, such as lack of penetration of wells and well logs, for locating ROZs has limited development of their resource potential when compared to known fields.

This paper presents a probabilistic methodology for identifying and evaluating ROZ resources for CO₂-EOR and CO₂ sequestration potential with the use of public and proprietary data sources. The methodology was developed during a pilot study that focused on the ROZ in the San Andres Formation of a nine-county area in the Permian Basin in West Texas. The pilot study estimated a mean oil in place of 25 × 10⁹ barrels (bbl) of oil and a mean potential incremental oil recovery and CO₂ utilization of 2.6 × 10⁹ bbl and 28.2 Tcf (1.46 × 10⁹ tons), respectively, with 1 hydrocarbon pore volume (HCPV) of injection using the water alternating gas (WAG) method. The results of this pilot study are consistent with reported volumes in the literature for a similar area in the Permian Basin. The pilot study demonstrated that this methodology could be used to identify and assess the recoverable oil and coincident CO₂ storage volumes of ROZs in other formations and regions.