Application of Quantitative Back-Scattered Electron Image Analysis in Isotope Interpretation of Siderite Cement: Tirrawarra Sandstone, Cooper Basin, Australia

  1. Sadoon Morad
  1. M. R. Rezaee and
  2. J. P. Schulz-Rojahn

Published Online: 17 APR 2009

DOI: 10.1002/9781444304893.ch20

Carbonate Cementation in Sandstones: Distribution Patterns and Geochemical Evolution

Carbonate Cementation in Sandstones: Distribution Patterns and Geochemical Evolution

How to Cite

Rezaee, M. R. and Schulz-Rojahn, J. P. (1998) Application of Quantitative Back-Scattered Electron Image Analysis in Isotope Interpretation of Siderite Cement: Tirrawarra Sandstone, Cooper Basin, Australia, in Carbonate Cementation in Sandstones: Distribution Patterns and Geochemical Evolution (ed S. Morad), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304893.ch20

Author Information

  1. Australian Petroleum Cooperative Research Centre (APCRC), National Centre for Petroleum Geology and Geophysics (NCPGG), Thebarton Campus, University of Adelaide, SA 5005, Australia

  1. Shell Development (Australia) Pty Ltd, 1 Spring Street, Melbourne, Victoria 3000, Australia

Publication History

  1. Published Online: 17 APR 2009
  2. Published Print: 29 MAY 1998

ISBN Information

Print ISBN: 9780632047772

Online ISBN: 9781444304893

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Keywords:

  • quantitative back-scattered electron image analysis of siderite cement;
  • palaeoenvironmental interpretations;
  • general diagenetic characteristics;
  • siderite cement characteristics;
  • bulk-rock isotope data with electron microbe and image analysis results

Summary

A new method is presented to improve the interpretation of bulk-rock oxygen and carbon isotope data in isotopically heterogeneous samples. In the fluvio-deltaic Tirrawarra Sandstone of the Fly Lake and Moorari Fields, Cooper basin, Australia, volumetric estimation of individual cement generations of siderite is accomplished using image analysis techniques in conjunction with electron microprobe data. Results show that bulk-rock isotope values are controlled by the relative proportions of three main siderite cement generations. The variation in δ18O can be expressed by the equation δ18O(bulk) = (VS1 × δ18OS1) + (VS2 × δ18OS2) + (VS3 × δ18OS3). The variation in δ13C can be expressed by the same type of equation, but the correlation coefficient is lower (0.64) than the one for δ18O (0.82). The δ18O and δ13C end-member values for each cement generation are characterized by a narrow range, allowing a precise definition of the conditions under which individual generations formed. Integration of petrographic and fluid inclusion results has led to the identification of the following siderite cementation events: (i) an early, homogeneous Fe-rich siderite with a δ13C signature of +1.45‰, indicative of low-temperature methanogenic processes (≤30°C); (ii) an Mg-rich inhomogeneous siderite characterized by a complex zoning, with a δ13C signature of −8.5‰ produced mainly by the decarboxylation of organic matter at temperatures between 64 and 76°C; (iii) an Mg-rich, relatively homogeneous pore-filling siderite with a δ13C character of −11‰ that was produced during kerogen maturation, at more elevated temperatures (98−110°C). Both the first and second generation of siderite cement were followed by a period of cement dissolution. The technique presented here has particular applications in cases where pure or nearly pure samples of end-member siderite cement generations are not available for isotope analysis, provided the various cement generations have different chemical compositions.