www.acsprf.org

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 growth 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 coexisting fluids, pressure, or time. A single ¹³C-¹⁸O clumping measurement provides independent estimates of the growth temperature, ¹³C and ¹⁸O of a carbonate mineral, enabling the ¹⁸O of the waters from which the mineral grew to be calculated (Eiler, 2007).  In order to evaluate the utility of this geothermometer in structural diagenesis studies, we examined diagenetic calcite cements associated with fault systems in a well-studied carbonate reservoir, the Paradox Basin, southeast Utah.

We sampled calcite vein cements near the Moab Fault, a major Laramide normal fault system in the Paradox Basin and an important natural laboratory for structural diagenesis studies (e.g., Eichhubl et al., 2009; Chan et al., 2000; Garden, 2001; Davatzes et al., 2005). Previously published vitrinite reflectance, Rock-Eval Pyrolysis, fluid inclusion, and stable isotope data indicate that the diagenetic calcites formed during fault-parallel hydrocarbon migration from deeply circulating meteoric waters (Eichhubl et al., 2009; Chan et al., 2000; Garden, 2001; Davatzes et al., 2005). Our preliminary clumped isotope analyses, which focused on large homogenized samples of calcite cements collected in the Moab Fault deformation zone, are consistent with this general picture and reveal cement growth temperatures of 43-97°C and a narrow range of calculated ¹⁸O of water values (-11.7±1.8 ä SMOW). However, our recent results of petrographic (including cathodoluminescence (CL) microscopy) and clumped isotope analysis of samples that underwent post-depositional alteration on the Colorado Plateau (Huntington et al., 2011) highlight the importance of isolating distinct generations of cement for clumped isotope thermometry. Thus we collected Moab Fault samples for CL microscopy and microsampling.

Using CL microscopy we isolated distinct generations of fault cement, and in several cases analyzed "temporal transects" microsampled across a single calcite crystal. Three calcite crystals from different veins were sampled with increasing distance from the vein wall, and clumped isotope thermometry of these subsamples indicates that calcite growth temperature varied less than 4°C across each crystal. The samples discussed below are limited to a single generation of the most texturally simple cements.

Vein calcite samples were collected along transects perpendicular to the strike of major fault segments (Figure 1) to investigate the history of fluid migration along the fault. The d¹³C values of the samples have a mostly inorganic signature but are slightly depleted with respect to formation limestones, suggesting some of the carbon came from hydrocarbons at depth (e.g., from the Pennsylvanian Ismay and Desert Creek sections). The ¹⁸O values of the calcite samples vary widely from 0 to 20 m distance from the fault (range -19 to -8ä) and become uniform in ¹⁸O at distances >20 m from the fault. ¹⁸O values of the water from which the cements grew (calculated from the independently measured ¹⁸O of calcite and temperature from clumped isotope thermometry) are meteoric to slightly enriched, suggesting a dominantly meteoric source for fluid migrating along the fault.

It is thought that meteoric waters were driven into the basin along the topographic gradient created by uplift of the La Sal Mountains southeast of the Moab Fault. Therefore we expected to find that calcite cements near the fault precipitated from warm fluids derived from depth, and that fluid temperatures equilibrated with the host rock with increasing distance from the fault. However, new temperature data from clumped isotope thermometry reveal the opposite trend (Figure 3). Low-temperature calcite sampled within 5 meters of the fault may represent late cements precipitated from meteoric water near the surface. The temperature of calcite precipitation increases with distance from the fault, to a high temperature of ~70¼C, which is within 10¼C of the maximum burial temperature of the host rock.

We suggest two possible explanations for these observations. (1) Fluids migrated along the Moab Fault and precipitated calcite during ascension and CO₂ degassing when the Moab Fault was active in the mid-Tertiary. To precipitate calcite <30°C, cool fluids must have migrated extremely quickly through the basin and along the fault to prevent thermal equilibration with hot country rock. Slow migration of the fluid away from the fault would have allowed thermal equilibration leading to hotter cements farther from the fault. However, it is more likely that (2) deformation and calcite precipitation were long lived. Hot fluids migrated along the Fault and precipitated calcite during fluid ascension and CO₂ degassing in the mid-Tertiary. Precipitates far from the fault thermally equilibrated with the host rock (70-80¼C; Chan et al. 2000, Nuccio and Condon, 1997). Later stage calcite precipitated after the fault had been exhumed, thus explaining the cooler temperatures near the fault. This model implies late-stage, shallow fault activity.

Our ongoing work involves direct comparisons of fluid inclusion homogenization temperatures and clumped isotope results in the same sample (collaboration with Peter Eichhubl, UT Austin). We are also improving the precision of clumped isotope thermometry by analyzing synthetic calcite grown at known temperatures between 50-200°C. We completed calcite precipitations at <100°C at UW and at 100, 150, 175, 200, 225°C at UT Austin with Jim Gardner.

In summary, encouraging results from the Paradox Basin and Colorado Plateau (Huntington et al., 2011) 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.