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Organic geochemistry, petrology and palynofacies of Middle Devonian lacustrine flagstones in the Orcadian Basin, Scotland: depositional environment, thermal history and petroleum generation potential

Published online by Cambridge University Press:  09 November 2016

ASSAD GHAZWANI ,
RALF LITTKE ,
VICTORIA SACHSE ,
REINHARD FINK ,
NICOLAJ MAHLSTEDT  and
CHRISTOPH HARTKOPF-FRÖDER
ASSAD GHAZWANI
Affiliation:
Institute of Geology and Geochemistry of Petroleum and Coal, Energy and Mineral Resources Group (EMR), RWTH Aachen University, D-52062 Aachen, Germany
RALF LITTKE*
Affiliation:
Institute of Geology and Geochemistry of Petroleum and Coal, Energy and Mineral Resources Group (EMR), RWTH Aachen University, D-52062 Aachen, Germany
VICTORIA SACHSE
Affiliation:
Institute of Geology and Geochemistry of Petroleum and Coal, Energy and Mineral Resources Group (EMR), RWTH Aachen University, D-52062 Aachen, Germany
REINHARD FINK
Affiliation:
Institute of Geology and Geochemistry of Petroleum and Coal, Energy and Mineral Resources Group (EMR), RWTH Aachen University, D-52062 Aachen, Germany
NICOLAJ MAHLSTEDT
Affiliation:
German Research Centre for Geosciences (GFZ), Telegrafenberg, D-14473 Potsdam, Germany
CHRISTOPH HARTKOPF-FRÖDER
Affiliation:
Geological Survey North Rhine-Westphalia, De-Greiff-Str. 195, D-47803 Krefeld, Germany
*
†Author for correspondence: [email protected]
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Abstract

During Middle Devonian time a thick succession of organic-rich, mainly lacustrine flagstones developed within the Orcadian Basin. These petroleum source rocks crop out in northern Scotland. Nineteen samples were studied using organic petrology, palynology and organic geochemistry in order to characterize kerogen type, depositional environment, thermal maturity and petroleum generation potential. Corg, carbonate and sulphur content as well as hydrogen index (HI) values are quite variable (e.g. HI from 79 to 744 mg HC/g Corg). Based on biomarker data, organic material mainly originates from aquatic organic matter deposited under lacustrine conditions with oxygen-depleted, but not permanently anoxic, bottom waters. Petrography reveals small quantities of vitrinite particles, indicating minor input of terrestrial material. This is supported by biomarker data and the palynofacies, which is characterized by a high amount of oil-prone amorphous organic matter (AOM) and generally few miospores. Maturity of the succession studied in Caithness and Orkney is between immature and oil mature. One-dimensional basin modelling shows that a significant remaining hydrocarbon generation potential exists within the Middle Devonian succession. In contrast to the low hydrocarbon generation in the onshore area, offshore oil generation was significant, especially after deposition of thick Upper Jurassic – Upper Cretaceous sediments. At the end of Cretaceous time, hydrocarbon generation ceased due to uplift. The contribution to known oil fields from the Devonian flagstones is a realistic scenario, including a contribution to the Beatrice oil field in the south of the modelled area.

Keywords

Orcadian BasinDevoniannorthern Scotlandpetroleum source-rock analysispetroleum generationbasin modelling
Type
Original Articles
Information
Geological Magazine , Volume 155 , Issue 4 , May 2018 , pp. 773 - 796
Copyright
Copyright © Cambridge University Press 2016 

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