Abstract

Located on the southeastern flank of Rock Springs uplift, Wyoming, the Brady Unit produces oil, condensate, and gas from seven reservoir intervals in two separate closures. These closures are in the hanging wall of a basement reverse fault and have total reserves on the order of 80 MBO* and 150 Gcf* gas. Discovered in 1973, the Brady Unit is characterized by several enigmas that remained unsolved until very recently. The Pennsylvanian Weber Sandstone is a retrograde condensate reservoir in the southern, structurally higher closure, but is a gas reservoir in the north. Significant compositional differences are evident among the gases produced at these two fields. In addition, the Jurassic Entrada formation is gas productive at the lower, northern closure, but porous and wet at Brady South. A three-dimensional seismic survey shot over the Brady Unit in 1993 has helped resolve these apparent discrepancies by allowing accurate correlation between fluid contacts and structure, and by supporting detailed reconstruction of migration and structural history. Resulting information indicates this history was highly complex, involving the creation and destruction of multiple closures, with migration occurring at two separate times. Such complexity has not been diagnosed previously in this area. Other foreland basement uplifts and basins in the greater Rocky Mountain region likely will have similarly complex histories and should be reviewed from this perspective for possible future prospects.

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