Residual oil zones (ROZS) are widespread in the Permian Basin and the result of multiple episodes of diagenetic modifications to reservoirs by the introduction of massive volumes of meteoric recharge into the basin (Figure 1).
In the Late Eocene-Early Miocene (40-16 Ma) western North America experienced uplift as the Southern Rocky Mountain Epeirogen (SRME) formed (Eaton, 2008), which uplifted the western half of the Permian Basin to form a massive meteoric recharge area that extended to the present-day Rio Grande River (Figure 2). Uplift was created by emplacement of igneous intrusive and extrusive bodies to form the Trans Pecos Magmatic Province (TPMP). Additional intrusive bodies emplaced and uplifted the entire length of western North America.
Meteoric water heated as it passed intrusive bodies and created hot, high pressure, high volume meteoric recharge into the subsurface through porosity fairways in ramp/shelf margins (Figure 2). Meteoric recharge created a large hydraulic head of energy, capable of sweeping mobile oil out of structural closures and reducing oil saturation to a residual oil zone (ROZ) by mother-natures-waterflood.
Later, meteoric recharge was drastically reduced following development of the Rio Grande Rift in the Middle-Late Miocene (16-5 Ma), which down faulted and destroyed the large recharge area (Figure 2). This resulted in cool, low pressure, low volume meteoric recharge into the subsurface from isolated, small mountain ranges. As energy dissipated, previously swept reservoirs within structural closures that contain a ROZ were able to back fill and resaturate partially to completely with mobile oil. Some ROZs did not resaturate with mobile oil and remained a ROZ or greenfield. One ROZ resaturated with gas instead of oil.
As mobile oil partially back filled and resaturated a reservoir with mobile oil to form a brownfield, the underlying ROZ was slightly back filled and resaturated with mobile oil. This explains why ROZ plays exist in the Permian Basin. A ROZ can be subdivided into an upper ROZ that is potentially productive and a lower ROZ that remains a ROZ.
Two examples of ROZs on the Central Basin Platform (CBP) are: 1) Eunice Monument South Unit (northwest corner Central Basin Platform, CBP) that was completely swept of mobile oil to form a ROZ and later back filled and partially resaturated with mobile oil and left a ROZ at the base of the reservoir (brownfield); and 2) McElroy field (southeast margin CBP) that was also completely swept of mobile oil to form a ROZ and later completely back filled and resaturated the entire oil column with mobile oil and left no ROZ behind (Figure 1).