Constraining Source Rock Quality Through Integrating Sedimentology, Geochemistry And Palynology: A Case Study from the UK Pennsylvanian

Davies, S. J.¹, T.S. Brewer¹, R.H. Davies¹, M. Norry¹, K. Hawkins¹ (1) University of Leicester, Leicester, United Kingdom

In the UK Pennsylvanian basins, high-frequency glacio-eustatic sea-level changes influenced depositional systems. Potential source rocks were deposited during maxiumum flooding events and subsequent highstands. In offshore settings continuous deposition of fine-grained sediments occurred through complete eustatic cycles (rise-fall-rise). An approach using sedimentology, geochemistry and palynology recognises these eustatic events and findings have implications for understanding source rock quality.

High TOC values (3-6%) and elevated abundances of elements, including Vanadium, Arsenic, Lead, and Molybdenum, reflect periods of water-column and sediment anoxia during maximum flooding events. Systematic changes in elemental abundances track the return to oxygenated conditions and consequently poorer source rocks. U, commonly used to identify maximum flooding surfaces from gamma-ray logs, does not consistently recognise the most organic-rich sediments. In this offshore setting the sequence boundary is a correlative conformity reflected by only a very minor grainsize increase. However the key surface is represented by the highest values of Silica and Zirconium, and the occurrence of older Mississippian spores. Therefore shelfal incision and a basinward shoreline shift during sea-level fall are reflected by the presence silt-sized detrital quartz and zircon grains and reworked spores.

Pseudo-gamma ray logs, constructed from the whole-rock geochemical data, illustrate the limitations and problems in using U, K and Th to identify potential source rocks. This research demonstrates that variability in mudstone successions reflects changing bottom-water oxygen content and siliciclastic sediment supply variations, and highlights the need to better understand the mineralogical controls on the distribution of U, K and Th in mudstones.