Evaluation of Radiogenic Noble Gas Isotope Composition of Quartz in the Marcellus Shale
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Date
2015-05
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The Ohio State University
Abstract
Unconventional energy resources have become a major source of domestic oil and natural gas in the United States. Throughout the last decade, hydrocarbon production has increased by more than 30%. Nonetheless, the success rates of individual wells remain highly variable. This necessitates work toward improving exploration techniques capable of understanding the style, extent, and thermal conditions of fluid migration in low permeability shales. Noble gas geochemistry provides a promising geochemical approach toward characterizing fundamental reservoir properties (porosity and permeability), crustal fluid migration, and thermal evolution of unconventional basins, specifically in the Marcellus Shale. Before this can occur, fundamentals of the behavior of radiogenic noble gases in mineral grains must be determined and validated. This study investigates the fundamentals of 4He and 21Ne* release from quartz grains in the Marcellus across 3 distinctive burial settings and in proximity to deformational features, and explores the use of radiogenic isotope measurements for geothermochronological purposes in quartz grains.
Diffusional release profiles of 4He and 21Ne* are constructed by using a step-heating experiment where quartz grains from the Marcellus are incrementally heated at temperatures of 50oC, 75oC, 100oC, 150oC, 200oC, and 500oC. The diffusional release rates of 4He and 21Ne* are determined empirically by reporting accumulated concentrations of 4He and 21Ne* at each temperature step via Arrhenius plots. Our preliminary results suggest a diffusional rate of 228.3 x 10-6 cc STP/g/yr at 50oC, 3786.8 x 10-6 cc STP/g/yr at 75oC, and 24135.5 x 10-6 cc STP/g/yr at 100oC for He with a ~19% standard error between two analyses. Ne exhibits a lower rate of diffusion at given temperature intervals and our results suggest a diffusional rate of 8.91 x 10-12 cc STP/g/yr at 50oC, 9.59 x 10-12 cc STP/g/yr at 75oC, and 7.85 x 10-10 cc STP/g/yr at 100oC for Ne with ~15% standard error between two analyses. Determination of these diffusional rates provides a necessary first step in validating the (U-Th)/He system in quartz as a potential thermochronological tool.
Measurements of 4He and 21Ne* concentrations show different levels of fractionation for each burial setting from which the samples were collected. Our preliminary results suggest that samples collected from Marcellus core (~1.2km depth) retained the greatest amount of helium and exhibit 4He/21Ne* ratios that are essentially at production value (2.2 x 107). Conversely, samples collected from sub-aerially exposed Marcellus outcrop show ~90% helium loss and 4He/21Ne* values much lower than production values.
Measurements of 4He and 21Ne* concentrations from Marcellus samples collected with proximity to a low angle thrust fault show a significant drop in 4He/21Ne* values in samples closest to the fault. This indicates that helium is being preferentially lost relative to neon from rock that is in close proximity of the fault.
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Keywords
Geochemistry, Noble Gases, Marcellus Shale, Energy