In Northeastern Scotland, the Orcadian Basin hosted large lacustrine systems which developed during Devonian times (c. 400 Ma). The pre-Devonian metamorphic basement unconformity is only exposed in a small number of places around the basin margin and therefore the characterization of the nature of this important surface has received relatively little attention. We have utilized vintage onshore seismic to gain a better understanding of the pre-Devonian basement physiography. Onshore exposures of the top Moine, base Devonian unconformity surface is exposed have been visited to ground truth our subsurface interpretations.
The studied deposits have been deeply buried then exhumed so that they are exposed widely onshore. Post Caledonian tectonism has faulted and folded the Devonian succession making it challenging to reconstruct the stratigraphy and the basin architecture from geological data only. The Devonian sediments were deposited in a continental environment and fluvial and alluvial deposits are interbedded with lacustrine units. These lacustrine facies contain fishbeds which are organic rich mudstones with moderate source potential. Variations in burial history have resulted in variations in the source rock maturity of the fishbeds.
A large dataset of 2D seismic reflection profiles are available from across the study area. The dataset is of mid 80's vintage and covers the Caithness and the Ross and Cromarty areas (NE Scotland). The survey consists of 33 profiles making up 416,92 km of data on an area of 140 X 40 km. The vintage data have limited horizontal and vertical resolutions. To enhance the images, we have used a dip-steering median filter. To depth convert the horizons, stacking velocity analysis has been performed. In parallel, newly acquired field data are used to correlate with the seismic profiles. Correlations made to deep borehole data (Tain-1 well) have allowed marker beds to be attributed to specific seismic reflections. Finally, gravimetric data are used to calculate the depth to basement.
The main focus of this work is to fully interpret the seismic architecture of the Devonian basins by integrating seismic and potential fields based depth to basement mapping techniques. Within the basin fill specific stratigraphic horizons and structural domains have been mapped. Seismic velocity analysis has contributed to the production of the basin maps. Understanding the stratigraphic architecture of the basin will bridge the gap between onshore field data and offshore North Sea wells and seismic. We expect to provide new insights into the North Sea’s geological history, as well as a better understanding of the offshore Devonian hydrocarbon play.