Synopsis

Potential hydrocarbon source rocks are evaluated from the Devonian Orcadian Basin (plus a small suite from the Hornelen Basin, Norway) for organic matter richness, type and maturation level. Techniques used include total organic carbon determination, pyrolysis, visual kerogen typing, spore colour and vitrinite reflectance. The sequences of SE Shetland, the Walls Sandstone, Fair Isle and the Hornelen Basin only contain small amounts of organic matter and were probably thermally mature before any significant cover was established and any minor hydrocarbons generated were soon lost. The main development of the Orcadian Basin does contain significant Middle Devonian and minor Lower Devonian hydrocarbon source rocks with oil and gas prone materials which are still in the oil window for the majority of the succession. Speculations are made concerning the possibility of hydrocarbons accumulating from this part of the sequence and prospects include reservoirs in the Upper Old Red Sandstone, Eday Beds, John O'Groats Sandstone, and the Lower Old Red Sandstone although deep ‘weathering’ effects are probably detrimental to preservation. Aeolian beds associated with extensive organic rich units may also be significant. Possible sites for hydrocarbon accumulation are the offshore half graben structures beneath the Moray Firth and in the onshore Devonian sequence. The timing of hydrocarbon generation is thought to be early.