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Use of Clumped Isotopes to Reduce Uncertainties in Reconstructing the Thermal History of Carbonate Reservoirs

John, Cedric M.; Jourdan, Anne-Lise; Kluge, Tobias; Davis, Simon; Vandeginste, Veerle

The ability to reconstruct the thermal history of a basin is essential in modeling reservoir and source rock quality. This presentation will focus on a very promising new technique for carbonate minerals called the "carbonate clumped isotopes paleothermometer". The field of clumped isotopes is concerned with the state of ordering of natural substances and aims at constraining the abundance of 18O-13C bonds (i.e. a ‘clump') within the lattice of carbonate minerals. Measuring the difference between the abundance of 18O-13C bonds in a sample relative to a stochastic distribution of isotopologues offers a single-phase paleothermometer applicable to all carbonate phases.

Clumped isotopes provides an independent measurement of the temperature of precipitation of a carbonate phase that coupled with the traditional δ18O measured simultaneously on the same carbonate allows the reconstruction of the isotopic composition of the diagenetic fluid. This greatly reduces ambiguities when interpreting paragenesis, fluid flow history, and thermal history of basins.

The presentation will synthesize the results of several projects within QCCSRC that aim at understanding and applying clumped isotopes to diagenetic systems in reservoir analogue outcrops. In particular, we will show results from a study of low-temperature diagenetic phases in a salt dome in Oman (Jebel Madar). Jebel Madar is an ideal case study as large calcite crystals in fractures can be sampled for both clumped isotopes and fluid inclusions. Results reveal the history of mixing between fluids of distinct composition and temperature. Based on the clumped isotope data, inference on the degree of mixing and depth at which the fault was cemented can be made, thus guiding the use of Jebel Madar as a subsurface analogue.

Applications of clumped isotopes at high-temperatures are currently more challenging because of the lack of empirical calibrations above 50°C. The second part of the presentation will focus on a series of pressure-controlled lab precipitations for calibrations at high temperature, as well as experiments with different fluid composition representative of subsurface brines. The long-term aim of our research efforts is thus calibration at subsurface conditions to establish clumped isotopes as a reliable proxy in the reservoir geologist toolkit.

We would like to acknowledge the financial support of QCCSRC (funded jointly by Qatar Petroleum, Shell and the Qatar Science & Technology Park).


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013