Abstract Introduction Geologic Setting Stratigraphic Setting Porosity Determination Data Set Porosity vs. Depth Depth vs. Age Effect of Pore- Water Composition Limestone & Dolomite Porosity Summary References

The upper parts of wells 1 and 2, and wells 3 through 5 (Fig. 1; Table 1), were logged by the authors with the U.S. Geological Survey LaCoste and Romberg³ borehole gravity meter. Porosities were computed from the borehole-gravity data using standard methods (McCulloh, 1966; Robbins, 1981), with grain densities determined from lithologic logs prepared by the well-site geologists. Details of these surveys and porosity computations are given by Schmoker et al (1979). The borehole-gravity data of this study are limited to the upper 2,740 ft (835 m) of section.

Borehole-gravity data are not significantly influenced by the wellbore rugosity or formation alteration that often occurs when drilling high-porosity formations. Also, the borehole gravity meter has a large radius of investigation and can sample heterogeneously distributed pore volumes typical of high-porosity carbonate rocks. For these reasons, it was thought that porosities of the shallow, very high-porosity carbonate strata of the study area could be more representatively determined from borehole-gravity measurements than from conventional wire-line logs.

Considering the fundamental accuracy of borehole-gravity measurements (Schmoker, 1978) and possible errors in determining grain density in limestone-dolomite sequences, individual porosity values calculated from the boreholegravity data are estimated to have an experimental error of about ± 3 porosity units.

The porosity and lithology of strata in the lower parts of wells 1 and 2 and in wells 6 through 15 (Fig. 1; Table 1) were determined from suites of conventional acoustic, neutron, and density logs. The conventional-log data range downward in depth from 2,260 ft (689 m); there is little depth overlap with the borehole-gravity measurements. Although data are limited, the transition from borehole-gravity to conventional-log porosity determinations appears smooth when expected differences between limestone and dolomite porosity are taken into account. (These differences are discussed in a following section.)

Porosity was computed from log-determined values of formation density and grain density, with formation density obtained from the density log and grain density obtained from lithology estimates derived from the Schlumberger M-N plot (Burke et al, 1969). The M-N plot utilizes acoustic, neutron, and density logs to help identify the components of ternary mineral mixtures. Pores were assumed to be water saturated.

Because the intent was to study limestones and dolomites unaffected by non-carbonate minerals, all intervals that were indicated to contain shale or anhydrite by the M-N plot were rejected. The limestone-dolomite percentages of the carbonate intervals comprising the final data set are estimated to have an experimental error of about ±20%. Considering the resulting uncertainty in grain density, and the basic accuracy of the density log, individual interval-porosity values calculated from the acoustic, neutron, and density-log data are estimated to have an experimental error of about ±3 porosity units.