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ABSTRACT: Downhole Measurements and the Generation of Modal Mineralogy Logs in ODP Hole 798B.

James F. Bristow, Peter deMenocal

Ocean Drilling Program Hole 798B situated on the Oki Ridge, Japan Sea, provides an excellent opportunity for mineralogical evaluation of the sequence using downhole measurements. A full suite of high-quality physical and chemical logs was obtained at this hole including bulk density, sonic velocity, neutron porosity, resistivity, natural gamma, Formation Microscanner, and geochemical tools (giving elemental yields of K, Th, U, Al, Si, Ca, Fe, Cl, H, S, Gd, Ti, and Mg+Na). The tool-derived chemistry, representing continuous in situ measurements, will be compared and calibrated with a suite of analyzed core samples. These core samples are taken from intervals that have high percentage recovery and contain numerous ash layers which can be used to accurately equate core and l gging depths using microresistivity images from the recently introduced Formation Microscanner. This study should provide further information on the accuracy of downhole geochemical measurements and hopefully evaluate any improvement in the quality of the data by the recent introduction of the boron sleeve to the tool. The core analysis should allow the isolation of the chemical and possibly physical log responses of the individual sediment components. The sediment component log responses will be used in an inversion of the logging data to generate modal mineralogy logs for the entire logged sequence of the hole.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990