PALYNOLOGICAL AND GEOCHEMICAL ANALYSIS OF CARBONIFEROUS BOREHOLE AND OUTCROP SAMPLES FROM THE ISLE OF MAN

Abstract

Palynological and geochemical analyses were performed on Namurian (mid‐Carboniferous) samples from the Ballavaarkish and Shellag Point boreholes from the north of the Isle of Man, and on Dinantian/Namurian outcrop samples from Black Marble Quarry in the south of the island.The boreholes yielded a rich and diverse (over 60 species) Namurian B‐C (Kinderscoutian‐Yeadonian) palynomorph assemblage equivalent to the KV/FR Zones of Clayton et al. (1977). Reworked Brigantian‐Namurian A palynomorphs of the NC/TK Zones were recorded in the Shellag Point borehole. Offshore seismic data indicates a period of major fault activity in the early Namurian which may support this concept of early Namurian reworking. Amorphous organic matter indicating anoxic/dysoxic depositional conditions together with prasinophyte algae and goniatites suggesting a marine environment were recorded in the Shellag Point borehole.Vitrinite reflectance and spore colouration measurements coupled with Tmax data from Rock‐Eval pyrolysis indicate that the Namurian shales from the Shellag Point borehole are currently in the oil window. Rock‐Eval data and kerogen analyses indicate that these shales represent moderate to good gas‐prone source rocks which have yet to attain full maturity. The Namurian samples from Ballavaarkish are mainly in the oil window or late mature zone and have poor gas potential. Extract chromatography indicates a relatively mature product and the possible onset of hydrocarbon biodegradation, particularly in the Ballavaarkish borehole samples.Outcrop samples from near the top of the succession at Black Marble Quarry underwent thermal alteration as a result of dyke intrusion and subsequent volcanism. Although previously believed to be Brigantian (end‐Dinantian) in age based on goniatites, corals and conodonts, the samples have produced a sparse palynological assemblage indicating an apparent age no older than Namurian. The samples are late‐ to post‐mature in terms of hydrocarbon generation and have no remaining source potential.The results indicate that where Namurian sediments are preserved beneath the base‐Permian unconformity, for example in the Solway Basin, hydrocarbons are likely to have been generated.