Primary biological controls on UK lower namurian shale gas prospectivity: A step towards understanding a major potential UK unconventional gas resource

Deep-water mudstones from ancient epicontinental settings are significant repositories for organic matter, but the primary controls underpinning the amount and type of organic matter preserved in these basins have not been examined. Comprehensive analysis of late Mississippian mudstones from the Widmerpool Gulf, UK, was undertaken using thin section analysis, bulk rock geochemistry (total organic carbon (TOC), δ[superscript 13]C[subscript org], Rock-Eval), palynofacies typing, sporomorph counts, δ[superscript 13]C of isolated organic matter fractions and gas-chromatography of extracted n-alkanes and kerogen pyrolysates. Late Mississippian mudstones, deposited across central and northern England, are known oil and gas source rocks but also represent important onshore exploration targets for shale gas. Using these data, the processes that delivered and buried organic matter during glacio-eustatic sea-level cycles are interpreted. During sea-level lowstands, thin-bedded silt-bearing clay-rich mudstones with up to 4.1% TOC (average=2.3±0.8%; mean δ[superscript 13]C[subscript org]=–28.2±1.0‰) are intercalated with graded silt-bearing mudstones and sand-bearing silt-rich mudstones (average TOC=1.7±0.6%; mean δ[superscript 13]C[subscript org]=–26.2±0.7‰). The clay-rich mudstones contain significant proportions of a granular translucent type of amorphous organic matter (AOMGr), comprising algal material and bacteria, and were deposited via dilute turbidity flows and hemipelagic settling of flocculated organo-mineralic material. The intervening coarser facies contain significant land plant-material deposited via intermittent sediment density currents. Kerogen composition varies between type II and III. During sea-level highstands, thin-bedded carbonate-bearing mudstones are the dominant facies, with up to 6.6% TOC (average=4.6±1.3%; mean δ[superscript 13]C[subscript org]=–28.5±0.9‰). Hemipelagic suspension settling was the main depositional process. These mudstones are rich in AOM[subscript Gr] which was abundant due to high bioproductivity in the water column; kerogen is consistently of type II. Mudstones accumulated during sea-level highstands represent the best potential targets for thermogenic shale gas. This study explains the systematic biological and sedimentary mechanisms for the delivery and preservation of organic matter in such prospective intervals.