Mineralogy and Geochemistry of Some Lacustrine Devonian and Carboniferous Source Rocks, Scotland, United Kingdom

Martin H. Link, Susan Palmer

Eighteen lacustrine samples from the Middle Devonian Old Red Sandstone and Carboniferous oil shales of Scotland were analyzed for mineralogy and organic geochemistry to determine source rock potential and possible seismic character of these rocks in the Moray Firth and Shetland Shelf area. Fourteen outcrop samples were from fish beds of the Middle Devonian Old Red Sandstone in the Shetland and Orkney Islands and northeastern Scotland. X-ray diffraction analyses indicate they contain 22% quartz, 12% feldspar, 10% clay (mainly illite and chlorite), 35% calcite, 20% dolomite, and lesser amounts of pyrite and siderite. These lacustrine carbonates may be seismically recognized in the subsurface by their high velocity potential. Total organic carbon ranges from 0.4 to 2.6% by w ight, the kerogen is primarily amorphous, and average vitrinite reflectance values are 0.52 to 0.70%. Nonwaxy n-alkane distributions and biomarker features indicate low maturity, relatively carbonate-rich lacustrine rocks, and poor to very good oil source rock potential.

The Carboniferous outcrop samples are from the Midland Valley oil shales near Edinburgh, Scotland, and are distinctively more clastic and pyrite-rich than the Devonian samples. Four x-ray diffraction analyses indicate 14% quartz, 7% feldspar, 58% clay (mainly kaolinite with some illite and chlorite), 6% calcite, 11% pyrite, 5% siderite, and trace amounts of dolomite. High total organic carbon (3.6 to 21.8%), abundant oil-prone amorphous kerogen with some land-derived exinite and vitrinite, and waxy n-alkane distributions characterize these samples which are similar to Eocene oil shales of the United States. Biomarker distributions reflect the aquatic and continental origin of the kerogen. These samples rate as excellent low maturity (% R_o ^approx 0.6) oil source rocks.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.