Southwest Section AAPG Annual Convention

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A “cookbook” Approach to Exploring for, and Evaluating Residual Oil Zones in the San Andres Formation of the Permian Basin


Residual Oil Zones (ROZ's), defined as a more general term for those intervals below the oil/water contact, have recently emerged as a viable commercial target for oil production. A number of operators have been successful in extending CO2 EOR production into these intervals beneath major San Andres fields (Seminole, Wasson, and others) in the Permian Basin. And, armed with the knowledge of how these ROZs were formed, other operators have extended the exploitation to the areas between the existing fields (known as Greenfields). Examples of successful Greenfield projects are the Tall Cotton (Gaines), George Allen (Gaines), and Platang (Yoakum) projects and highlight the different approaches operators have taken to producing Greenfield ROZ oil. Tall Cotton is a classic vertical 5 spot patterned CO2 flood; George Allen is also a CO2 project with ROZ flooding both beneath and peripheral to an existing field; and Platang is composed of a series of horizontally drilled “DUROZ” wells. The geology and reservoir attributes of each of these will be discussed.

Exploring for ROZ's in the formerly uneconomic areas (“Goat Pasture”) is similar to, yet different from, conventional exploration for new field or new pool targets. As part of a DOE/RPSEA supported study of the Greenfield potential in the Permian Basin, a step-by-step guide was created to assist companies in an evaluation to determine: 1) whether a Residual Oil Zone (ROZ) might exist beneath their producing field; 2) where a ROZ might exist outside of the limits of a field, and 3) determine what are the properties of the fluids and rocks within the ROZ that might be exploited with EOR methods.

Technology and innovation has, once again, redefined the world of prospect opportunities. A comparison of the “classic” interpretation of “Goat Pasture” with the new understanding of Greenfield ROZ occurrences, and the science behind these new explanations is presented.