Re-Os Systematics in Petroleum Systems: What Are We Measuring

Abstract

The rhenium-osmium (Re-Os) isotopic system has been applied to organic matter in sedimentary and petroleum systems for over a decade. Geochronology of organic carbon-rich sedimentary rocks (OCRs) and tracking the Os isotopic composition of seawater are presumed by many to be routine, though pitfalls abound. More challenging is application of Re-Os systematics to migrated hydrocarbons to determine the timing of maturation and migration. One of the great unknowns? Exactly what are we measuring? Challenges arise from a host of, at best, partially answered questions. What chemical and biological process control the concentrations of Re and Os and the Re/Os ratio in OCRs? Which minerals or organic molecules hold Re and Os, and in what form? What syn-depositional or early diagenetic processes disrupt Re-Os systematics in OCRs? Given that oxidative weathering will disturb the system, can we design methods to use surface samples or is drill core imperative? How are Re, Os, and Os isotopes fractionated during hydrocarbon maturation and fluid interactions during migration? What is the significance of apparent isochrons derived from migrated hydrocarbons? Most literature on Re-Os applications to petroleum systems is based on measurement of natural systems. Most published Re-Os ages for OCRs fit known biostratigraphy or other age constraints; some do not, however, or suffer from high uncertainties. Similarly, published Re-Os data on migrated hydrocarbons are based on multiple crude oil samples with uncertain relations, and with one exception1, are not used to determine specific ages. Limitations arise primarily because we lack experimental data on the behavior of Re and Os in these systems. We are building on preliminary experimental work 2, 3 to determine the extent of fractionation of Re, Os, and their isotopes among organic compounds and between organics and waters under a range of conditions. Using well-defined fractions of hydrocarbons, we are also determining where Re and Os reside within organic molecules using FTIR and NMR. These parameters are essential to interpretation of data from natural samples, and ultimately employing Re-Os data for tracking and timing hydrocarbon maturation and migration. 1 Lillis & Selby, 2013, Geochim. Cosmochim. Acta 118: 312-330; 2 Mahdoaui et al., 2013, Chem Geol. 358: 90-100; 3 Mahdoaui et al., 2015, Geochim. Cosmochim. Acta 158: 1-21. This work was supported by the Norwegian petroleum industry under the CHRONOS project.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016