Assessing the Spatial and Temporal Variability of Lithofacies and TOC in a UK Carboniferous Mudstone Succession
Mississippian to Pennsylvanian strata exposed in the UK are broadly age-equivalent to the Barnett Shale in the lower Mississippian of central Texas. This study examines temporal and spatial changes in the character and distribution of lithofacies in a stratigraphically well-constrained Bashkirian mudstone succession at locations across the UK onshore Carboniferous outcrop belt.
Textures and mineralogy identified by optical and electron optical methods of unusually thin thin sections, combined with geochemical and total organic carbon (TOC) data, highlight a range of lithofacies. Key lithofacies, common to all locations, include homogeneous clay-rich mudstones, pelleted silt-bearing/silt-rich mudstones and thinly-bedded mudstones. Thinly-bedded mudstones can be carbonate-rich (distal setting) or carbonate-poor (proximal setting), with beds commonly <3mm. Individual lithofacies packages vary in thickness from tens of millimetres to a few metres in thickness, and represent changes in the predominant delivery mechanisms for a given period, from suspension settling to advective transport by turbidity and debris flows.
The distribution of the different lithofacies varies between locations. Closer to the sediment supply, mudstones contain a higher proportion of silt-sized grains, exhibit a greater proportion of homogeneous and bedded lithofacies and have average SiO2 of 52%. In contrast, pelleted mudstones dominate the more distal location and are characterised by lower SiO2 (ca. 45%). Organic matter is primarily derived from terrestrial plant debris and microscopic spores with a component of marine algal matter. Higher TOC abundances (typically >2%; up to 6.5%) are associated with the pelleted lithofacies in all locations and the thinly bedded lithofacies also contain significant (>2%) TOC. TOC abundances (for the bedded lithofacies) range from 1.5-2.5% proximally to 2-8.5% in a more distal location.
The proximity to the sediment source has an important control on the abundance of siliciclastic material but similar sediment delivery processes occur across the basin. The pelleted mudstones are seen in both proximal and distal settings and are associated with higher TOC; typically >2%. This lithofacies forms thicker packages in distal settings, creating the potential for a thicker source/reservoir rock. In addition, the association with carbonate-rich mudstones in the distal setting may improve the porosity and permeability of the source/reservoir rock.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California