HYDROCARBON MIGRATION IN THE SOLWAY BASIN

Carboniferous rocks in the north Solway onshore outcrop show evidence of hydrocarbon migration. Oil residues occur as intergranular bitumen in sandstones, thoriferous bitumen nodules in sandstones and uranifero us bitumens in fractures associated with the North Solway Fault at the northern margin of the Solway Basin. Oil was emplaced in sandstones a fter the dissolution of dolomite, calcite and clay cements. The radioe lement-rich bitumens represent deposition following interactions betwe en migrating hydrocarbons and metalliferous fluids. The close proximit y of the Caledonian Criffel granodiorite, unroofed by Carboniferous ti mes, suggests that the pluton was the source of uranium and thorium. H ydrocarbon source rocks are identified further east in the more comple te succession exposed in the Esk Valley region, along with further oil residues. Time-temperature modelling suggests that oil was generated from Lower Carboniferous source rocks during the Permo-Triassic. Chemi cal age dating of uraninite in bitumen yields early Jurassic ages, whi ch may reflect the time of hydrocarbon migration along the North Solwa y Fault system, and may be related to a mid-Jurassic episode of region al uplift. Analyses of thorite in bitumen nodules give a wider spread of data, probably due to the migration of daughter lead.