The Altamont field is a major, naturally fractured, overpressured oil reservoir situated on the gently northward-dipping flank of the asymmetric Uinta basin. Low porosity Tertiary clastic and carbonate rocks form a stratigraphic trap beneath thick Green River Formation carbonate mudstones. The reservoir occurs at depths between 2,450 and 5,200 m, and thickness of the producing interval commonly exceeds 700 m. Permeability is derived mainly from vertical fractures in sandstones and carbonate rocks.

Joints in surface rocks within the basin occur predominantly as orthogonal sets whose orientations correlate with major tectonic features bordering the basin. Fractures in oriented core of reservoir rock exhibit a single dominant north-northwest trend. Rock mechanics tests on samples from the core indicate anisotropy coincident with the trend of microcracks.

Timing of subsurface fracture development relative to basin subsidence and uplift is interpreted from fluid-inclusion thermometry conducted on quartz and carbonate crystals which line open fractures. Results indicate that fractures opened when strata were near their maximum burial depth. The fracture system became more extensively developed as uplift and erosional unloading continued.

Various geologic processes interact during the burial, diagenetic, tectonic, and unloading history of rocks in a sedimentary basin. Their combined effect determines the state-of-stress in stratigraphic units. At Altamont the effect of overpressuring has been critical in fracture genesis. Thorough evaluation of the relative influence of individual geologic processes on the stress history of rock units in a basin can be usefully applied in exploration for fracture reservoirs.

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