Borehole elemental analysis: A comparison of core and log data

Neutron induced gamma ray spectroscopy, including prompt capture and delayed activation measurements, provides borehole elemental analysis which, together with natural gamma ray detection, characterizes rock matrix composition. The study involved extensive core and log data in two wells. One was drilled with a barite weighted, oil based mud through shallow marine sand. The other was drilled with freshwater based mud through a channel sand-mudstone sequence overlying limestone. The results illustrate the importance of a suitable core sampling strategy and the problems associated with matching core to log data. A method for correcting the total measured aluminium concentration for that due to the borehole mud has been tested successfully on aluminium concentrations measured in the cleaned core samples. The most accurate determination of the elemental concentrations includes assumptions about the minerals present. Elemental analyses of core material provide important data on limitations in these assumptions. Pyrite must be treated differently than most other minerals, since it does not contain any unmeasured elements. A comparison is presented of statistical uncertainties calculated from the measurements with estimates of the overall accuracy and precision of the elemental concentrations obtained by comparing the log results with those obtained from the laboratory core analysis. A comprehensive core elemental analysis can also provide a useful insight into the way other logs, such as photoelectric factor, or formation thermal neutron macroscopic absorption cross-sections, are influenced by minor and trace elements. This is particularly useful, since a sufficiently complete set of elemental concentrations can now be determined to allow these macroscopic parameters to be calculated. Differences between the calculated values and the log measurements provide additional data for a more detailed understanding of the rock properties and their influence on logging tool performance. (author). 15 refs, 7 figs, 4 tabs