The Emerging Potential of Re-Os Isotope Geochemistry for Source Rocks and Maturation-Migration Histories

Abstract

Just a few years ago, the petroleum industry relied almost solely on lithostratigraphy, biostratigraphy, and U-Pb dating of volcanic tuffs and flows for correlations across basins in the sedimentary record. Re-Os technology was first applied to synsedimentary pyrite to obtain depositional ages for sedimentary rocks. Subsequently, organic-rich shales with their anomalous metal contents became the target for direct dating of source rock deposition. If organic material extracted from a source rock provides depositional ages, so too should the creation of new hydrocarbon be datable. Precise dating of organic-rich shales using Re-Os has immediate applications for hydrocarbon exploration by placing time pins at intervals of earth crises marked by anoxia and euxinia. These are the events that create rich source rock. In addition, unlike other geochronometers, Re-Os isochron ages carry key information on environmental conditions during source rock deposition through trace metal concentrations, stable isotope compositions, and the Os initial ratio. Trace metal abundances reflect the extent of anoxia and sedimentation rates, both of which impact concentration and preservation of organic material. Stable isotope compositions reflect the type of organic material. The Os initial ratio is a powerful proxy for marine and lacustrine conditions during shale deposition – for example: What was eroding and how fast? What are the depositional rates for accumulation of organic-rich material? What kinds of fluids were present and mixing in a basin and for how long? The nuances derived from this kind of information are far-reaching. For shales at maturation, with some or all of the generated hydrocarbons still residing in the shale (unconventional hydrocarbon), the timing of maturation can be teased from the Re-Os data. For conventional migrated hydrocarbon, good scientific interpretation requires the creativity to analyze players along migration paths before the reservoir is reached. We will present an accessible and understandable overview of Re-Os geochemistry and geochronology in the context of case studies. The field is new and the potential is enormous for petroleum exploration.