Kerogen variation in a Devonian half graben system

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Description/Abstract

The Middle Devonian Rousay Flagstone Formation of Orkney is a 200m thick lacustrine
succession that contains abundant preserved organic matter. It was deposited into a series of half
graben formed by the collapse of over-thickened Caledonian crust. The 14 lake cycles that
comprise the Rousay Flagstone Formation (RFF) were correlated across Orkney during 6 months
of fieldwork. Two of the lake cycles were sampled in detail for geochemical analysis. This work
has enabled an understanding of the sedimentary and tectonic processes that controlled the
distribution of facies and the quality of source rocks within the Orcadian Basin during this time.
The processes and environments present in Orkney during this time may be compared to the
Horton Group in Nova Scotia (Hamblin & Rust 1989).
The main finding of this research has been the extent to which the East Scapa Fault (ESF)
caused variation in sedimentation. By slowly extending throughout the RFF, the half graben
bounding fault caused certain areas of Orkney to experience continued relative uplift.
Two aspects of the structure of the half graben were of greatest influence. Firstly the uplifted
footwall of the half graben provided an environment away from the influence of inflowing
sediment and oxygen rich turbidity currents. The quiescent environment in this area allowed the
greatest amount of laminite facies accumulation. It was found that high TOC (total organic
carbon), H/C (hydrogen/carbon ratio) and spore numbers were associated with these areas of
enhanced laminite deposition.
The second area was a transfer zone located at the northern splay of the ESF. The zone acted as
a linkage zone between the ESF and a half graben to the north. Because of its location between
adjacent half graben depocentres, sedimentation was affected by the relative uplift of the area in
a manner similar to the uplifted footwall area to the west ofthe ESF.
The main agents that were detrimental to the formation of source rocks were turbidity currents.
These currents originated from the three main areas of alluvial fan input in the basin. Turbidity
currents carrying sediment and oxygenated water from these fans would bypass the shallower
and more uplifted areas and preferentially deposit in the more distal and downthrown areas. The
area immediately to the east of the ESF was the main location to have experienced reduction in
kerogen quality (TOC, H/C and spore numbers) because of turbidite deposition.
The preserved organic matter is predominantly composed of amorphous organic matter, making
the main kerogen type Type I. About 40% of each Rousay Flagstone Formation lake cycle
contains measurable organic matter, on average about 0.8%. The amount of organic matter is
controlled by facies type, with laminite facies having the highest average TOC (1.55%) and grey
silts having least (0.3%).
Exinite reflectivity and spore colour variation analyses from across Orkney indicate that the
thermal maturity of the sediments is within the thermal range of hydrocarbon generation.
Additionally the uniform spread of maturity values across Orkney indicates that fault movement
was never great enough to cause differential thermal maturity regimes to form across Orkney.