Age and Composition of Source Rocks: New Steps toward Tracking Hydrocarbon Origin
Authors: Hannah J. L. , Stein H. J. , Xu G. , Galimberti R. , Nali M. 
Published: March 2015

A variety of chemical fingerprints link migrated hydrocarbons to their source rocks, defining the ends of migration pathways. Rhenium (Re) and osmium (Os), redox sensitive elements concentrated in organic material, add unique information - time. Decay of 187Re to 187Os provides a radiometric clock measuring time since chemical closure of the organic material. Here we show that Re-Os geochemistry of source rocks defines the age of deposition and tracks environmental changes through time. This geochronometer also reduces ambiguity with a fingerprint for migrated hydrocarbons: evolving 187Os/188Os in migrated hydrocarbons, dependent on the 187Re/188Os ratio and age of both source rock and hydrocarbons, constrains models for the timing of migration.

Black shales from the lower Streppenosa Formation, deposited in a deep euxinic intraplatform basin, yield a Re-Os age of 200.3 Ma and initial 187Os/188Os of 0.87. This Hettangian age aligns perfectly with the known biostratigraphic age, is nominally younger than the 201.3 Ma Triassic-Jurassic boundary[1], and postdates major magmatic pulses of the Central Atlantic Magmatic Province (CAMP, 201.6 and 200.9 Ma)[2].

Primitive CAMP magmatism produced sharp decreases in both 87Sr/86Sr and 187Os/188Os ratios of seawater[3], as both track relative inputs from continental versus chondritic sources. Osmium, however, has a much shorter seawater residence time than Sr; our data reveal that 187Os/188Os returns to high ratios within 1 m.y. of cessation of CAMP magmatism. High 187Os/188Os at 200.3 Ma documents reduced contribution of Os from CAMP, and may also reflect enhanced continental runoff from uplift along newly rifted margins. Osmium isotope variations in seawater, archived in organic-rich shales, provide a sensitive record of tectonic and environmental changes during source rock deposition.

Introduction

Hydrocarbon source rocks are sediments rich in organic material, notably black shales. These same rocks are a treasure trove of information about depositional setting, paleogeography, paleoclimatology and paleooceanography. Biomarkers and unique chemical/isotopic compositions can tie migrated hydrocarbons to their source rocks. Fundamentally, chemical and biological signatures are collected from the water column and trapped in chemically reducing organic-rich sediment and, ultimately, in black shale. Among the elements sequestered from the water column in organic-rich muds are rhenium (Re) and osmium (Os).

The radiometric Re-Os pair, parent 187Re and daughter 187Os, expands the wealth of information held in source rocks. The steady radioactive decay of 187Re to 187Os marks time, and the inexorable increase of 187Os/188Os over time yields an isotopic tracer, similar to the more familiar 87Sr/86Sr. The half-life of 187Re is long, 41.6 billion years, but 187Re/188Os ratios are typically high, from 100 to &gt;5000 in most shales. Accordingly, the geochronometer is useful for rocks of almost any age. The high 187Re/188Os in most crustal rocks, in contrast to 187Re/188Os &lt; 1 in the earth's mantle, imparts another value to the system: the 187Os/188Os ratio in crustal rocks is generally an order of magnitude higher than primitive mantle rocks, and increases very quickly with time. Thus, it is a very sensitive chemical tracer.