Abstract

The Glorieta-Clearfork dolomite in the Monahans field, Ward County, Texas represents an upward shoaling carbonate platform sequence. The main facies are skeletal mudstone, peloid packstone/grainstone, peloid wackestone/mudstone, and stromatolitic boundstone. The major mineral constituents in these four rock types are dolomite and anhydrite. Petrographic analysis reveals mainly intercrystalline/intergranular pore geometries with minor vuggy/moldic pore types.

The cementation exponent (m) was determined for 60 samples and cross-plotted against porosity (ø). Even though the m vs. ø plot shows scatter, two transforms could be established. Fifty one samples (85% of data) cluster along a transform of lower m values and 9 samples (15% of data) cluster along a transform of higher m values. These two transforms represent different pore geometries. The rocks in the lower transform mainly exhibit connected intercrystalline/intergranular porosity; the rocks in the upper transform exhibit a greater degree of porosity isolation. The porosity isolation is caused by the dissolution of grains, mosaic and sucrosic dolomitization, and anhydrite cementation.

The lower transform was used to determine variable m values because it represents the dominant reservoir type. It would have been better to use each of the two transforms with their concomitant reservoir type, but log signature alone could not be used to distinguish the rock types represented by the two transforms.

Once the values of m were calculated for each depth, the saturation exponent “n” was determined, “n” was calculated using a dielectric fog (Sxo) and an averaging technique (Asquith and Saha, 1991; Maute et al., 1992) to minimize the error in water saturation. The values of n for the Glorieta, upper Clearfork, lower Clearfork are 3.64, 2.98, and 3.58, respectively. These values are indicative of water-wet reservoirs.