Identifying Causes of Disturbances in the Re-Os Geochronometer in Black Shales: A Case from the Jurassic Agardhfjellet Formation, Svalbard.

Abstract

Black shales of the Middle to Upper Jurassic Agardhfjellet Formation in Svalbard, Norway are of interest for two reasons: 1) they act as a seal formation for a series of CO₂ sequestration wells, and 2) they correlate with Upper Jurassic source rocks which have generated enormous hydrocarbon reserves throughout the North, Norwegian, and Barents Seas. Chronostratigraphy of the southern correlative Jurassic source rocks has been characterized by previous workers through a variety of methods including biostratigraphy and rhenium-osmium (Re-Os) geochronology. The reducing environments under which black shales are formed are ideal for the sequestration of Re and Os from the surrounding sea water, which starts the Re-Os isotopic clock. The purpose of this study is to extend the regional timeline northward by applying Re-Os geochronometry to drill core from the Agardhfjellet Formation near Longyearbyen, Svalbard. Sections of core with organic-rich shales were taken from two wells seven kilometers apart (DH2, DH5) which were drilled for the Longyearbyen CO₂ Lab pilot project. The core selections represent the upper and lowermost black shales of the Agardhfjellet Formation and intermediate segments thought to be near geologic stage boundaries defined by biostratigraphy. The purpose of these selections is to apply Re-Os geochronology to 1) determine the nominal age range of the Agardhfjellet Formation near Longyearbyen, 2) correlate these ages with related units in the region, and 3) date the stage boundaries currently defined by biostratigraphy. Four initial ages from DH5 are nominally similar to expected ages; however, the uncertainties (5-16%) are unusually large for well-stratified, organic-rich drill core. This implies that the Re-Os geochronometer in this region of the Agardhfjellet Formation has been disturbed either at the time of deposition (e.g. through changing redox conditions or discontinuous sedimentation), or post-depositionally, (e.g. through fluid migration or tectonic disruption). In order to address these possibilities, a variety of information from CT scans, optical microscopy, Rock-Eval, trace element, and stable isotope analyses will help identify the possible sources of uncertainties leading to the imprecise ages. Using alternative data to determine perturbations in the Re-Os system may lead to improved sampling or analytical methods in the future. 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