Borehole gravity surveys in the Gebo, Garland, and Big Polecat oil fields of Wyoming uniquely assess the density and porosity of the reservoir and associated rocks. The borehole gravity method is unique because (1) unlike other well-logging techniques, gravity measurements depend directly on rock bulk density; (2) the large radius (and volume) of investigation ensures that the measurements are unaffected by borehole fluids and by rugosity, casing, cement, or any area close to the borehole that may be modified by flushing or invasion by drilling fluids; and (3) the high precision of the measurements makes this method sensitive to very small variations in formation density (usually < 0.005 to 0.04 g/cc, depending on length of the borehole interval).

Interval density and porosity profiles determined from the Bighorn basin surveys were compared with gamma-gamma density logs, neutron porosity logs, and density and porosity measurements of core samples. Discrepancies between the density and porosity methods arise because borehole gravity, owing to its large radius of investigation, measures an average porosity that includes the irregularly distributed component (e.g., vugular porosity of reservoir rocks in the Madison Limestone at Garland) which is less effectively evaluated by conventional shallow-penetration logs or core samples. Other discrepancies are usually due to the inherent limitations of one or several of the methods and are mostly dependent on the composition and coherence of the rocks.

Variations in the contribution of fracture porosity to total porosity in the Tensleep Sandstone reservoir at Gebo are masked by much larger fluctuations in intergranular porosity caused by differences in the cementation and abundance of dolomite. Whatever the cause, the magnitude of fracture porosity is probably below the threshold of detection with borehole gravity. High-porosity (> 15%) and/or gas-filled sandstone units, principally in the Frontier Formation, were easily detected behind casing in the three oil fields. An abrupt and possibly widespread downward increase in porosity in the upper part of the Frontier Formation may (1) reflect lithologic and mineralogic variations owing to changes in the depositional environment, (2) be related to a previously proposed unconformity, nd (3) have exploration significance.

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