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Quantifying the temperature conditions under which diagenesis and fluid flow modulate hydrocarbon maturation, porosity, and permeability is important for predicting hydrocarbon formation and preservation in tectonically active basins.  Clumped isotope thermometry determines the precipitation temperature of carbonate minerals based on the abundance of ¹³C-¹⁸O bonds in the carbonate crystal lattice, potentially enabling the thermal conditions of fracturing, fluid flow, and growth of diagenetic minerals in hydrocarbon reservoirs to be determined independent of the composition of co-existing fluids, pressure, or time. A single ¹³C-¹⁸O clumping measurement provides independent estimates of the growth temperature and ¹⁸O of a carbonate mineral, enabling the ¹⁸O of the diagenetic or surface waters from which the mineral grew to be calculated (Eiler, 2007).  In order to evaluate the utility of this geothermometer in structural diagenesis studies, this project examines the growth conditions of diagenetic calcite cements associated with fault systems in a well-studied carbonate reservoir, the Paradox Basin, Utah.

The Paradox Basin, an intraforeland flexural basin in SE Utah, hosts 3 km of Pennsylvanian-Jurassic sediments that underwent burial, diagenesis and exhumation in the last 120 Myr.  In Spring 2010, we sampled calcite veins developed along sections of the Lisbon Valley and Moab Faults, two major Laramide normal fault systems in the Basin.  Previously published vitrinite reflectance, Rock-Eval Pyrolysis, fluid inclusion, and stable isotope data indicate that the diagenetic calcites formed during fault-parallel hydrocarbon migration at 30–125°C, from deeply circulating meteoric waters with little variation in O-isotopic composition (Eichhubl et al., 2009; Chan et al., 2000; Garden, 2001; Davatzes et al., 2005).  Preliminary clumped isotope results and our ongoing work on outcrop and core samples serve to test this model.

Clumped isotope analyses of 26 samples of calcite veins from the footwall of the Moab fault performed at Caltech in September 2010 indicate a wide range of ¹⁸O values for the calcites (-24.4 to -6.7 per mil PDB).  Since ¹⁸O of calcite is a function of both temperature and ¹⁸O of water, in the absence of independent temperature data one would not know whether the calcite isotopic values record a >50°C range of growth temperatures, >15 per mil range in fluid composition, or some combination of temperature and fluid variation.  The clumped isotope data reveal temperatures of 43-97°C and a narrow range of calculated ¹⁸O of water values (-11.7±1.8 per mil SMOW). These temperatures agree with predictions from previous basin models and other proxies, but are lower than fluid inclusion homogenization temperatures of 84-125°C. The ¹⁸O of carbonate and ¹³C-¹⁸O clumping are strongly correlated (R²=0.83), and the slope of the relation is consistent with precipitation of the calcites at equilibrium from waters with a limited range of isotopic compositions (Figure 1).  Importantly, this constraint on the ¹⁸O of diagenetic waters enables conventional ¹⁸O thermometry (e.g., Urey, 1947) to be applied. The ability to micro-sample for conventional ¹⁸O thermometry complements our investigation, since clumped isotope measurements require much larger samples that can homogenize multiple diagenetic phases.

Future work on this aspect of the project will involve careful documentation of cross-cutting relationships in outcrop and thin section, additional clumped isotope measurements, CL microscopy, and micro-sampling of the cements for ¹⁸O thermometry in order to constrain the causes of the large temperature variation and investigate the difference between fluid inclusion homogenization temperatures and clumped isotope results.  We will complement our sampling from the Moab and Lisbon Valley faults with additional sampling from outcrop (spring 2011) and from the USGS Core Research Center in Denver (November 2010).

Following the recommendations of the reviewers of the original PRF proposal, in parallel with our work in the Paradox Basin, additional petrographic (including CL) observations of diagenetic calcites in Eocene limestones from the Colorado Plateau were conducted to complete the pilot study that motivated this PRF project. This work provided a test of the methodology to be used in the Paradox Basin case study, in particular highlighting the need to interpret clumped isotope results in the context of careful textural analysis and CL because samples can integrate multiple diagenetic phases. Results are detailed in a manuscript by Huntington, Budd, Wernicke, and Eiler to be submitted to the Journal of Sedimentary Research in October 2010, and the major conclusions of this work are as follows: Conventional ¹⁸O and petrographic/CL analysis would suggest all diagenetic calcites in the samples grew from meteoric waters at Earth surface temperatures (Figure 2). While clumped isotope thermometry measurements indicate that the earliest cements formed at 14-19°C, temperatures of 49-122°C for the remaining phases (cements and calcified shells) require that the samples were influenced by diagenetic alteration in different diagenetic fluids during a post-depositional thermal perturbation.  Combined with isotopic modeling, clumped isotope data for calcified gastropod shells constrain water-rock ratios ( 0.01 wt %) in the diagenetic microenvironment (aragonite-calcite transformation across thin films; Figure 3).

Motivated by these findings, we analyzed thin sections of the Paradox basin samples petrographically, and initiated CL-SEM work summer 2010 at the University of Puget Sound.   The CL and petrographic analysis will be completed at CU Boulder in collaboration with Prof. David Budd, an unfunded collaborator in November 2010. 

In addition to natural case studies, we proposed to improve the precision of clumped isotope thermometry by analyzing synthetic calcite grown at known temperatures between 50-200°C. Since submission of the proposal, Guo et al. (2009) published theoretical calibrations for several carbonate minerals including calcite based on transition state theory and statistical thermodynamics, and Dennis and Schrag (2010) corroborated the prediction for calcite up to 60°C. Our apparatus for calcite precipitations <100°C is operational, and precipitation experiments >100°C are planned at UT Austin (collaboration with Jim Gardner).     

In summary, encouraging results from the Paradox Basin and Colorado Plateau demonstrate the utility of clumped isotopes for determining fluid migration temperatures and isotopic compositions in the crust during deformation and thermal perturbation. Upon completion, this work will serve as a model for combining clumped isotope thermometry with isotopes, other temperature proxies, petrography, and structural geology of basins to constrain the thermal and chemical history of fluids in tectonically active reservoirs.