Density and porosity of oil reservoirs and overlying formations from borehole gravity measurements, Gebo Oil Field, Hot Springs County, Wyoming

High-precision measurements of relative gravity were made at 75 locations over a temporarily shut-in 5,083foot cased producing oil well in the Gebo oil field in the southern Big Horn Basin, Wyoming. Gravity and depth differences between adjacent stations were used to calculate the fluid-saturated bulk density of the horizontal layers of rock bracketed by each pair of measurements that extend tens to hundreds of feet outward from the well. Interval bulk density was converted to interval total porosity by assuming values for the densities of pore fluids and mineral grains. The Gebo well penetrates sandstone, shale, carbonate, and evaporite units that range in age from Pennsylvanian to Late Cretaceous. Oil production comes from carbonate (Park City Formation) and sandstone (Tensleep Sandstone) reservoirs. Bulk density ranges from 2.33 g/crn^ in a Cretaceous sandstone unit to 2.80 g/cm3 in a Permian anhydritic dolomite and averages 2.55 g/cm^ for the entire section. Porosity reaches 15-21 percent in sandstone units in the Frontier Formation, Cleverly Formation, Crow Mountain Sandstone, and Tensleep Sandstone. Several dolomite units in the Park City Formation have porosities as high as 14 to 18 percent. Judging from maximum observed porosities of sandstone and shale units, the Cretaceous section appears to be overcompacted an observation that supports independent evidence that thousands of feet of overburden has been removed from the section at Gebo by late Cenozoic erosion. Fluctuations of density and porosity are related to lithology and known variations in the abundance of bentonite, carbonate minerals, and anhydrite. For example, Cretaceous formations with abundant bentonite are less dense and more porous than Cretaceous formations without bentonite. A conspicuous and abrupt change in density and porosity near the top of the Frontier Formation appears to be due to lithologic and mineralogic variations related to a change in the environment of deposition of the rocks. Large fluctuations of intergranular porosity in the Tensleep Sandstone due to variations in cementation and abundance of dolomite completely mask the very small, possibly negligible contribution of fracture porosity. Qualitative comparison of the gravimetric density and porosity profiles of this study with conventional density and porosity well logs primarily illustrates the difference in the lateral radius of investigation between these conventional types of logs and the borehole gravity method.