The USGS estimates the source rocks of the Permian-aged Phosphoria Formation have generated more than 225 billion barrels of oil (Kirischbaum, 2008). The Phosphoria Total Petroleum System (TPS) is stratigraphically defined by the Phosphoria Formation and surrounding sub-Cretaceous reservoirs. Migration from this system is extensive and encompasses the western portion of Wyoming and the edges of surrounding states (Maugham, et. al., 1994). Although this system contains some of the most prolific source rocks and migration pathways in the western US it is yet to be developed into a major resource play. The Phosphoria "Ervay" carbonate reservoir is complex and much of it has low porosity (<8%) and permeability (<1 md). Nevertheless much of the Phosphoria is oil-saturated and is a promising target for horizontal development of migrated oil.
Unconventional plays can be divided into three types based on relative position to hydrocarbon source—
In Situ, produces directly from the source, e.g., Eagleford Shale, Maverick Basin,
Migrated, produces from reservoirs containing oil that is migrated some distance horizontally and/or vertically, e.g., Mississippian carbonates, Anadarko Basin shelf, or
Hybrid, produces for reservoirs juxtaposed to the source, e.g., the Bakken Shale, Williston Basin (Figure 1).
Exploring for in-situ and hybrid plays requires a thorough understanding of maturity and maturation based on geochemical analysis, burial depth, thermal history, oil gravity, GOR, organo-porosity, water saturation, pore pressure gradient and recovery factor. It is still important to understand all of these characteristics for migrated resource plays but to explore for this resource type it also is important to know migration pathways, structural history, natural fracturing, and reservoir paragenetic sequences. In addition, a thorough understanding fluid history and dynamics is need to evaluate migrated resource reservoirs.