Significance of sedimentary organic matter input for shale gas generation potential of Mississippian Mudstones, Widmerpool Gulf, UK

• Organic matter assemblages from Namurian mudstones of Derbyshire are described.
• Two amorphous organic matter (AOM) types are distinguished.
• Total organic carbon was controlled by bioproductivity.
• Ratio marine/terrestrial organic matter was controlled by sediment delivery process.
• Mudstone facies rich in granular amorphous organic matter (AOMGr) have high gas generation potential.

Carboniferous mudstones in central and northern England are shale gas prospects but the controls on the amount and composition of organic matter are not well understood, even though these parameters define the volumes of gas generated in fine-grained sediments. Organic matter in samples from basinal late Mississippian (Arnsbergian) mudstones in the Widmerpool Gulf was characterised by using semi-quantitative (n = 58) and quantitative palynofacies (n = 16) analyses, sporomorph counts and bulk rock geochemistry (total organic carbon, δ13C of bulk organic matter, Rock-Eval Pyrolysis).The results of this study suggest that most organic matter at this location was delivered to the sediment–water interface as aggregates of a granular translucent type of amorphous organic matter (AOMGr, mean = 66.7% ± 19.3%) via hemipelagic suspension settling. AOMGr represents fragments of algal material with subordinate inclusions of small plant fragments and pyrite framboids held together by microbial colonies. AOMBr (brown granular amorphous organic matter) is the second most abundant group (mean = 15.6% ± 8.5%) comprising similar microbial colonies that grew on suspended land plant-derived fragments in the water column. Palynofacies components representing clearly terrestrial organic matter are much less abundant and include gelified organic matter (G, mean = 9.6% ± 12.6%), black phytoclasts (PhBl mean = 2.7% ± 4.7%), brown phytoclasts (PhBr, mean = 3.3% ± 3.6%) and sporomorphs (mean = 1.4% ± 1.3%). Sediment delivery processes influence the balance between terrestrial organic matter and AOMGr. During low sea-level times, turbidity currents and debris flows delivered terrestrial organic matter (representing 12% to 40% of the palynofacies). Kerogen composition varies between Type II and III. In contrast, thin-bedded carbonate-bearing mudstones deposited during rising and high sea-level contain up to 95% AOMGr and these high abundances correspond to higher total organic carbon. Carbonate and AOMGr were generated by high bioproductivity in the water column. Type II (oil- and gas-prone) kerogens are dominant in these mudstones and therefore these intervals represent the best potential targets for thermogenic shale gas.