Most of the work in promoting technologies for development of basin-centered, low-permeability reservoirs focuses on identifying production sweet-spots, commonly through appraisal of the natural fracturing.  However, not all areas are fractured.  Basin-centered settings do possess isolated pockets of enhanced reservoir quality.  Furthermore, contrary to earlier thinking on basin-centered systems, recently drilling results indicates that simply finding the sweet spots in reservoir quality is not sufficient to ensure success - it is also necessary to assess whether the reservoir will produce gas or water.  Therefore, the Office of Natural Gas Technologies is committed to testing all promising avenues that may help in allowing these resources to become reserves. 

The recently-completed study in the Wind River basin of Wyoming is testing the validity of an exploration rationale for basin-centered gas that does not rely on fracture detection. Instead, the concept involves the careful mapping of the 3-d structure of the boundary between normal and anomalous pressures combined with prediction of areas of enhanced secondary porosity and permeability related to diagenetic or other events.  The goal is to determine where reservoir-quality rock may be, and most importantly, whether these "sweet spots" are likely to contain gas or water.

The Wind River basin work has shown that a regional pressure boundary can be detected by inversions in the gradient of sonic velocity with depth in both well logs and seismic data. Two 3-d seismic surveys from the area were analyzed, and from them, areas of anomalously-slow velocity were detected. These anomalies suggest where significant volumes of gas may be present.  Conversely, areas lacking in the anomaly are those that are likely to contain significant volumes of water.

Ongoing drilling in the area has supported the validity of this approach. During the course of the study, six Muddy Formation wells were completed in the study area. Three wells located within a mapped anomaly initially produced from 3 to 4 million cubic feet (MMcf) of gas per day. One well on the edge of the anomaly produced 1 MMcf per day. Two wells outside the anomaly produced less than 1 MMcf per day.

A second, ongoing project is further investigating the phenomenon of water in basin-centered accumulations.  Although many investigators had settled on a concept for basin-centered accumulations that suggested water should be rare, recent drilling experience has countered this belief.  It must now be determined how prevalent water is in these formations, where it is coming from, and what relation water occurrence may have to natural fracturing and other issues.

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