Organic facies variations in the Triassic shallow marine and deep marine shales of central Spitsbergen, Svalbard

Shallow marine shales of the Lower Triassic Sticky Keep Member and deep marine shales of the Middle Triassic Botneheia Member of the Barentsoya Formation, central Spitsbergen have been subjected to detailed organic petrological and organic geochemical study. An assessment, based on organic facies characteristics, has been carried out on these sediments, in order to distinguish between those deposited in shallow vs deep marine depositional settings. The methods employed include evaluation of organic carbon content, Rock-Eval pyrolysis, extractable organic matter, biomarker distributions and petrographic data. The maturity assessment of the samples analysed is mainly based on vitrinite reflectance and T-max data. Organic facies criteria such as TOC, HI, PP (Petroleum Potential), hydrocarbon yield, kerogen assemblage and ratios of pristane/n-C-17, phytane/n-C-18, T-m/T-s, C-24 tetracyclic/C-26 tricyclic, C-34/C-29 hopane, C-35/C-34 hopane, hopanes/steranes, diasteranes/steranes, C-21/C-22 alpha beta beta sterane and C-21 beta alpha alpha/C-21 alpha beta beta sterane were found to be useful in differentiating between the two depositional environments. The biomarker ratios in particular, are thought to reflect more of the depositional condition, such as the extent of anoxicity/oxicity, and less on source input, probably as a result of enhanced microbial degradation on the organic matter deposited. (C) 1999 Elsevier Science Ltd. All rights reserved.