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Mineralogy and Geochemistry |
Stratigraphy and Sedimentology Research Group, British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK
1 Lemigas, P.O. Box 89/Jkt., Cipulir-Kebayoran Lama, Jakarta 10002, Indonesia
The results of laboratory mineralogical analyses have been compared with downhole log responses to determine the possibility of detecting individual minerals using the gamma-ray, natural gamma-ray spectroscopy, formation density and litho-density logs. Cored sequences from the Murchison, Thistle, Kittiwake, Gannet and Tartan Fields, North Sea, are used as case examples. K-feldspars, micas, detrital clays (illite, and possibly kaolinite) and phosphatic cements are shown to produce a measurable response on the gamma-ray and natural gamma-ray logs. Authigenic kaolinite did not impart radioactivity to the sandstones in the examples studied. The photoelectric absorption index (Pe) was found to be particularly sensitive to Fe-bearing minerals and baryte. A number of problems constrain full characterization of the mineralogy of a sandstone: specific detection of quartz and plagioclase feldspars is not possible by nuclear logs, distinction between minerals is problematical when a log responds to several minerals simultaneously, and calibration for minerals with a variable composition such as ferroan dolomite and mixed-layer illite-smectites is difficult.
