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ABSTRACT: Petrophysical and Petrographic Analysis of the 14,660 ft to 16,310 ft Interval in tho Nellie #1 Well, Pecos County, Texas


The North American Royalties, Nellie #1 well is located in Pecos County, Texas at the southern end of the Central Basin Platform. The Nellie #1 well penetrated 1.4 km of Phanerozoic sedimentary rocks and 4.5 km of Precambrian (1100 Ma) layered mafic rocks of the Nellie intrusion. The composition of the Nellie intrusion is predominately norite with some peridotite and pyroxenite.

The Nellie intrusion also has numerous thin (0.6-3 meters) zones of high radioactivity (where gamma ray > 300 API units). Density, neutron, sonic, and photoelectric absorption logs were used to construct lithology cross plots in order to determine the proportions of the three minerals. The low gamma ray (< 30 API units) values, which are indicative of mafic and ultramafic rocks, consistently plotted in the mineral triangles defined by plagioclase (An[53]), orthopyroxene (Fs[23]), clinopyroxene or plagioclase (An[53]), hornblende or orthopyroxene (Fs[23]). In contrast the high gamma ray zones, which are indicative of felsic compositions, plotted in the mineral triangles defined by quartz - orthoclase plagioclase (An[10]), thus suggesting that these high radioactivity zones are granitic in composition. This granitic composition is verified by petrographic analysis which indicates that these thin zones of high radioactivity may be granitic dikes, granitic sills, or wall rock xenoliths.

AAPG Search and Discovery Article #90947©1997 AAPG Southwest Section Meeting, San Angelo, Texas