The Origin and Resource Potential of Residual Oil Zones

摘要

Tectonics and active aquifers beneath oil-bearing reservoirscan be powerful forces in trapping large quantities of residualoil. In regions of the world where basin uplift (or subsidence)has created basinal tilt or where the oil columns are underlainby active aquifers, significant quantities of oil may have beenswept 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 newand higher OWC. Below the new and now tilted OWC, therenow exists a thick reservoir interval of immobile oil, called theresidual oil zone (ROZ).In the past, the ROZ was only water productive and thereforeavoided. Today, with the advent of COCO2 EOR and severaldemonstration projects, this oil zone (often quite comparablein residual oil saturation with the waterflood swept interval inthe main pay zone) has been shown to be a technically viabletarget for additional oil recovery.In the Permian Basin, OWC tilts have been mapped in manyof the oil fields, particularly in the Permian age San Andresand Grayburg reservoirs. Examination of well logs clearlyshows the presence of significant ROZs (often with 100's offeet in thickness) below the tilted producing OWC.Throughout the middle portion of the interval, the oilsaturation is near residual oil saturation. The oil in place, dueto ROZ thickness, is often on par with the original oil in placein the MPZ, representing a large, significant undeveloped oilresource.This paper discusses the origin and resource potential ofROZs. The paper examines how: 1) regional or local basintilt; 2) breached and reformed seals; and/or 3) alteredhydrodynamic flow fields can form ROZs. Subsequently, thepaper examines, using geologic and reservoir modeling, howkey oil reservoir and aquifer properties influence the shape,size and resource potential of ROZs.