Triassic mudstones (Gråklint Beds) containing hydrocarbons with Early Barremian and Santonian ages, Jameson Land, East Greenland

Knowledge of source rock and timing of hydrocarbon migration is highly relevant to our understanding of hydrocarbon resources. Biomarkers are an essential tool for linking migrated hydrocarbon to potential source rocks, but this approach is compromised if the source rock hydrocarbon is modified. Here we show evidence from Re-Os isotope geochemistry for open system exchange of organic compounds in a potential source rock.

This study initially set out to determine the disputed depositional age for the Middle-Upper Triassic Gråklint Beds in northern Jameson Land, at Buch Bjerg, adjacent to Carlsberg Fjord. The Gråklint mudstones have been cited as source rocks. They are interpreted as deep lacustrine mudrocks, with organic, silty and carbonate-rich laminations reflecting seasonal additions to quiet anoxic bottom waters in a potentially stratified water body. Bioturbation is absent.

Our Re-Os work shows that the organic component in the analyzed Gråklint mudstones no longer reflects original Triassic organic material. Based on Re-Os analyses of the hydrogenous component of samples from three stratigraphic levels spanning much of the 10 meter section of Gråklint Beds at Buch Bjerg, we conclude:

(1) Hydrocarbon was introduced into Triassic rocks at about 130 Ma with an ¹⁸⁷Os/¹⁸⁸Os ratio that is lower than any known Boreal Permian and Triassic marine shales in the Arctic region (Xu et al. 2009, EPSL 288: 581-587; Georgiev et al. 2011, EPSL, in press). The mechanism of exchange, equilibration, replacement, and/or mixing of original hydrocarbon with introduced hydrocarbon is not well understood.

(2) Jurassic shales are the only known source rock in the region with ¹⁸⁷Os/¹⁸⁸Os ratios (eg. Troms III, Norwegian shelf) that are low enough, even after post-deposition in-growth of ¹⁸⁷Os, to serve as a hydrocarbon source at 130 Ma; yet this scenario conflicts with current structural interpretations.

(3) External hydrocarbon was again introduced into the Triassic mudstones in one of the samples (SSA0036) at about 82 Ma. Re-Os systematically relates both the 130 Ma and 82 Ma hydrocarbon events to the same source (Os in-growth soundly matches time lapsed).

(4) 130 Ma and 82 Ma ages for hydrocarbon migration coincide with changes in plate motions recorded by paleomagnetic data in the North Atlantic Ocean region.