The Emerging Potential of Re-Os Isotope Geochemistry for Source Rocks and Maturation-Migration Histories
Authors: Stein H. J. , Hannah J. L. 
Published: March 2015

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 syn-sedimentary 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.

Introduction

The rhenium (Re) - osmium (Os) isotope system is applicable for geochronology and tracer studies of organic-rich shales because both Re and Os are fixed by reduction in organic matter at or below the sediment-water interface in anoxic environments. Parent isotope 187Re decays to daughter 187Os with a long half-life of 46.1 Gy (?187Re = 1.666×10-11 yr-1[1]); thus the Re-Os isotopic system can provide highly precise depositional ages for organic-rich shales from Archean to Cenozoic. In addition, shales acquire the 187Os/188Os ratio of contemporaneous seawater or lake water in lacustrine settings. Given the short residence time of Os in seawater (10 to 60 Ky[2]), short-term fluctuations in 187Os/188Os in the water column are captured in the sedimentary record. The 187Os/188Os ratio of the source rock at the time of deposition thus serves as a fingerprint of the paleoenvironment during deposition of organic matter, providing a starting point for tracking produced hydrocarbons. Caution is required in restricted environments, however, as the short residence time also captures local compositional variations in Os input where mixing with open oceans may be limited or episodic[3].