The Origin and Resource Potential of Residual Oil Zones

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Abstract Tectonics and active aquifers beneath oil-bearing reservoirs can be powerful forces in trapping large quantities of residual oil. In regions of the world where basin uplift (or subsidence) has created basinal tilt or where the oil columns are underlain by active aquifers, significant quantities of oil may have been swept from the original oil accumulation. In these situations, the once horizontal producing oil-water contact (OWC) transitions to a tilted interface across the field, creating a new and higher OWC. Below the new and now tilted OWC, there now exists a thick reservoir interval of immobile oil, called the residual oil zone (ROZ). In the past, the ROZ was only water productive and therefore avoided. Today, with the advent of CO2 EOR and several demonstration projects, this oil zone (often quite comparable in residual oil saturation with the waterflood swept interval in the main pay zone) has been shown to be a technically viable target for additional oil recovery. In the Permian Basin, OWC tilts have been mapped in many of the oil fields, particularly in the Permian age San Andres and Grayburg reservoirs. Examination of well logs clearly shows the presence of significant ROZs (often with 100's of feet in thickness) below the tilted producing OWC. Throughout the middle portion of the interval, the oil saturation is near residual oil saturation. The oil in place, due to ROZ thickness, is often on par with the original oil in place in the MPZ, representing a large, significant undeveloped oil resource. This paper discusses the origin and resource potential of ROZs. The paper examines how: 1) regional or local basin tilt; 2) breached and reformed seals; and/or 3) altered hydrodynamic flow fields can form ROZs. Subsequently, the paper examines, using geologic and reservoir modeling, how key oil reservoir and aquifer properties influence the shape, size and resource potential of ROZs.