URANIUM SPECTRAL GAMMA-RAY RESPONSE AS A PROXY FOR ORGANIC RICHNESS IN BLACK SHALES: APPLICABILITY AND LIMITATIONS

Autor: S. Kolonic, Sebastian Lüning
Rok vydání: 2003
Předmět:
Zdroj: Journal of Petroleum Geology. 26:153-174
ISSN: 1747-5457
0141-6421
DOI: 10.1111/j.1747-5457.2003.tb00023.x
Popis: In many organic-rich, low-carbonate hemipelagic shales, there is a stable and close correlation between the uranium and TOC contents. In this paper, we present a number of case studies using our own data and that from previous publications to investigate black shales with (1) good, (2) fair-to-good and (3) poor U/TOC correlations. U/TOC ratios in the different black shale units are compared to each other, and possible reasons for the observed variations are discussed. In general, the U/TOC ratio in a black shale is controlled by a number of factors which include for example the primary uranium content of the water body, the carbonate content and the sedimentation rate. The development of a stable U/TOC ratio may be inhibited by the presence of phosphate, by a high carbonate or sand content, by dissolution (“burn-down”) of uranium during intermittent oxic periods, and by large-scale diagenetic remobilisation of uranium. In suitable black shale systems, vertical variations in organic richness can be approximated by measuring the uranium content using spectral gamma-ray measurements. This may be especially important in outcrop studies because gamma-ray logging is a straightforward field technique. Before the uranium content can be used as a proxy for TOC content in a black shale system, however, a thorough calibration of uranium and TOC is necessary, in order to determine the stratigraphic and regional limits of the derived U/TOC ratios and to establish the presence of a stable U/TOC correlation.
Databáze: OpenAIRE
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