Significance of the Toolebuc gamma ray anomaly in the search for and evaluation of oil shale in the Eromanga Basin

B. L. Dickson, A. R. Ramsden

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Chemical and radiometric analyses have been made on core samples from 15 drill holes intersecting the Toolebuc Formation at Julia Creek. The holes are located in the vicinity of the St Elmo Structure and include intersections of a potentially economic oil shale deposit west of this structure as well as non-economic mixed coquinite/shale intersections to the east. The gamma ray activity of the formation is due largely to uranium in the shales. Wire-line gamma ray logs, available for eight of these holes, reflect the distribution of organic carbon. Despite this, there is poor correlation between uranium and organic carbon in the oil shales as a result of remobilization of uranium from organic matter into phosphate during diagenesis. In consequence wire-line gamma ray logs have no potential for evaluating oil yield. Density, on the other hand, is a good indicator of oil potential. On a hole by hole basis, the correlation between uranium and organic carbon in the formation ranges from 0.22 west of the St Elmo Structure (the potential oil shale resource) to 0.95 east of the structure. This reflects a systematic change in sedimentology from oil shale that is thick and well separated from coquinite (low correlation) to laterally equivalent but finely interbedded coquinite and shale (high correlations). Such trends could be used in exploratory drilling to indicate favourable areas elsewhere in the Toolebuc Formation.

Original languageEnglish
Pages (from-to)429-441
Number of pages13
JournalAustralian Journal of Earth Sciences
Issue number4
Publication statusPublished - 1986



  • Eromanga Basin
  • Oil shale
  • Oil yield
  • Queensland
  • Toolebuc Formation
  • Uranium
  • Wire-line logging

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