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Mineralogy and Lithology of Utah Oil Shale and Their Influence on Logging Response

Harry M. Dahl, Christie A. Callender, Kerry D. Savage

Detailed mineralogic and lithologic investigations of the Green River Formation oil shale in Uintah County, Utah, indicate unusual properties which strongly influence certain wireline logging responses. Those logging parameters considered include density, gamma, relative dielectric constant, resistivity, and magnetic susceptibility. Two 160-ft cores, comprising the Mahogany Zone and an adjacent section approximately 40 ft above and below this unit, were cut, slabbed, and analyzed.

Lithologically, the cores consist predominantly of laminated oil shale and marlstone with minor sandstone and altered tuff. Mineralogically, the oil shale contains dolomite, calcite, quartz, Na-feldspar, K-feldspar, smectite, illite, iron sulfides, and scattered siderite, aragonite, and analcime. Petrographic studies (thin section, SEM/EDS and XRD) indicate that iron sulfides occur both dispersed in the oil shale and concentrated along certain laminae.

These iron sulfides, identified as pyrrhotite, pyrite, and marcasite, form blades, pods, cubes, and aggregates. In particular, pyrrhotite is highly conductive and magnetic and, when concentrated along thin, continuous laminae, markedly affects the electrical properties of the oil shale. Results of the study determined that the logging response reflects the bulk composition of the rock and is strongly influenced by the pyrrhotite-rich laminae and the amount of moisture in the shale.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.