CHARACTERIZATION OF THE KEROGEN FRACTION OF ARNSBERGIAN (LATE MISSISSIPPIAN, SERPUKHOVIAN) MUDSTONES IN THE UK PENNINE BASIN

The composition and amount of organic matter in shales and mudstones determines the quality and influences the quantity of hydrocarbons contained in shale reservoirs. The kerogen fraction in Serpukhovian-aged Carboniferous mudstones in the south-eastern part of the Pennine Basin (northern England) includes up to 95% amorphous organic matter (AOM), heterogeneous material of unknown origin lacking a distinct morphology. A better understanding of the amount and origin of AOM is required to allow predictions to be made regarding the hydrocarbon potential of these rocks and is therefore relevant to both industry and government in understanding this currently unexploited source of indigenous energy.

Here we present palynological and geochemical results of the Edale Shales (Mississippian, Serpukhovian) from two boreholes: the Carsington Dam Reconstruction Borehole C3, located in the Widmerpool Gulf (Derbyshire) and the Karenight 1 Borehole drilled near the southern edge of the Edale Gulf (Derbyshire). Both depocentres were part of the Pennine Basin which in Mississippian times consisted of a mosaic of small, inter-linked basins formed due to crustal extension north of the Hercynic orogenic zone. Namurian deposits in these basins show a remarkable cyclicity thought to represent responses to eustatically controlled fluctuations of intra-basinal sea-level. The investigated intervals from both boreholes are representative of such a lithological cycle, for which we present new kerogen and Rock-Eval data.

During low stands, the kerogen fraction is characterized by higher levels of phytoclasts and generally lower % Total Organic Carbon (TOC). The high stands are characterized by high levels of granular AOM, higher TOC values, and a higher oil saturation index. This observation suggests that higher granular AOM counts are linked to higher %TOC and may represent intervals of elevated hydrocarbon generative potential. Further palynological investigations are required to clearly delineate the different AOM phases and to understand how these different phases affect hydrocarbon prospectivity. This could then be applied to understanding broader trends in Carboniferous Shales in the U.K. and can be integrated into models identify zones of richer hydrocarbon potential in the highly fragmented Pennine Basin.