Petrographic and Geochemical Analysis of the Carboniferous (Namurian) Holywell Shale of northeast Wales

Abstract

The potential of the Holywell Shale as a source-rock reservoir was explored using a multidisciplinary approach which focused on understanding the depositional setting of the Holywell Shale within the UK Namurian (mid-Carboniferous) Pennine Basin. The deposition of the Holywell Shale was examined to understand the supply and preservation of organic matter and detrital input into the basin both temporally and spatially. Both outcrop and borehole material was sampled and analysed using organic geochemical (e.g. total organic carbon, carbon isotopes, and RockEval™), inorganic geochemical (e.g. X-ray fluorescence) techniques in combination with detailed petrographic analysis.
Organic matter content of the Holywell Shale was highly variable (0.1 wt % to 10.3 wt %), with an average 2.0 wt % and predominantly Type III kerogens present. Carbon isotopes revealed a change in supply of organic matter to the basin from the strongly marine influenced Lower Holywell Shale (-31.1 ‰ to -25.6 ‰, average -28.8 ‰; containing some Type II kerogen) to the terrestrial dominated Upper Holywell Shale (-28.0 ‰ to -22.4 ‰, average -24.5 ‰; containing some Type IV kerogens). Trace element and RockEval™ pararmeters indicate that the Holywell Shale was deposited under predominantly oxic conditions with some periods of hypoxia. This resulted in relatively poor preservation of organic matter (low HI values < 301 mg/g and high OI values < 121 mg/g).
There is no clear relationship between organic matter source, quantity and quality within the Holywell Shale. The highest organic matter quantity with greatest preservation occurs within fossiliferous, clay-rich lithofacies associated with marine highstands. Although, the small-scale variability and heterogeneity of both organofacies and lithofacies mean that the reservoir quality of the Holywell Shale is inherently difficult to predict.