124 D64. Advances in Stable Isotope Geochemistry–Applications to Earth Science Systems

Monday, 5 November 2018
1:30 PM - 5:30 PM
Room 135

EVALUATION OF MOLYBDENUM ISOTOPES AS A PETROLEUM TRACER: THE PHOSPHORIA PETROLEUM SYSTEM, BIGHORN BASIN, U.S.A

Paper number: 124-10
Program: Technical Sessions
Time: 3:45 PM – 4:00 PM

Authors

Abstract

The molybdenum (Mo) isotope composition of organic-rich mudrocks is now extensively used as a tracer of ocean redox conditions but has received relatively little attention as a petroleum tracer. Using a revised analytical methodology for crude oil digestion, we obtained precise Mo isotope data for crude oil NIST RM 8505 (average δ98Mo = 0.8 ± 0.1 ‰ relative to NIST SRM 3134 = 0.25‰; n = 4; 2SD). Subsequently, we measured the Mo isotope compositions of 15 crude oil samples as well as samples of organic-rich source rocks (Meade Peak Member and Retort Member of the Permian Phosphoria Formation) from the Phosphoria petroleum system. The effect of reservoir processes on the Mo isotope compositions of crude oils and the potential of Mo isotopes to serve as an oil-source rock correlation tool were evaluated. The crude oils have Mo concentrations of <0.01-0.39 μg/g that are significantly lower than concentrations of 17-443 μg/g in the most Mo- and organic carbon-rich mudrocks (2.4-16.6 wt% total organic carbon; TOC). Crude oils affected by biodegradation and water washing (0.9-1.5‰) have δ98Mo values that overlap with minimally altered crude oils (1.0-1.3‰). Five of six crude oils affected by thermochemical sulfate reduction have higher δ34S (> –2‰ relative to VCDT) and lower δ98Mo (0.5-0.9‰) compared to minimally altered crude oils, suggesting introduction of external fluids containing isotopically light Mo and isotopically heavy S into the oil reservoir. An inverse correlation is observed between δ98Mo and δ34S of crude oils minimally affected by thermochemical sulfate reduction (δ34S < –2‰) and potentially represents a maturity trend for oil generation. Crude oils that are minimally altered by reservoir processes (1.0-1.3‰) have lower δ98Mo than the Mo- and TOC-rich source rocks (1.3-2.0‰). This observation suggests Mo isotope fractionation during oil generation and/or oil migration. Hence, Mo isotopes may not be useful as an oil-source rock correlation tool.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-320149

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