--> Temperature of Dolomitization Derived From Clumped Isotopes: A Word of Caution

AAPG ACE 2018

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Temperature of Dolomitization Derived From Clumped Isotopes: A Word of Caution

Abstract

Since the work of Ghosh et al., (2006), proposing clumped isotope thermometry for reconstruction of precipitation temperature in recent carbonates, several researchers have applied the technique not only on recent sediments; the technique has also been applied to deeply buried carbonates or carbonate successions affected by fault-related diagenetic alteration, to constrain precipitation temperatures of successive diagenetic stages.

Dolostone samples from a Permian hydrocarbon reservoir have been selected for this study to test the utility of clumped isotope thermometry on a simple case study of reflux dolomitization. Petrographic features (conventional and cathodoluminescence) support the preservation of original homogeneous pristine dolomite crystals. The early diagenetic origin of dolomite, not affected by recrystallization, is supported by the following observations: (i) euhedral dolomite crystals with homogeneous luminescence patterns, and (ii) δ13C, and more specifically δ18O values. However, clumped isotope analysis revealed a range in temperatures between 40° and 80oC within the sampled interval (25 m); with higher temperatures where formation water saturation is significant. While temperatures around 40oC are postulated as a most-likely value for original temperatures of precipitation, higher temperatures (up to around 80oC) are interpreted as an effect of diagenetic alteration in presence of formation water; the only plausible evidence for such diagenetic alteration being the slightly larger crystal size observed in samples recording the higher temperatures .

The potential applications of the clumped isotope technique range from the interpretation of depositional/diagenetic environments, to constraining burial histories and basin models. However, this study shows that even when diagenetic alteration is not detected, great care needs to be taken to use clumped isotopes to interpret temperatures of precipitation and consequently predicting the dimension of the geobodies of interest (e.g. dolostone geobodies formed by reflux, vs. burial and/or inflow of hydrothermal fluids along faults), or to constraining the thermal history during burial and using the data for thermal evolution and basin modelling.

NAM is appreciated for access to samples and reservoir data.