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1 James Armstrong & Associates, 30 Linden Walk, Prestatyn, Clwyd, LL19 9EB, UK
2 Geolab (UK) Ltd, Unit 14, Enterprise Court, Nelson Industrial Estate, Cramlington, Northumberland, NE23 2LZ, UK
Outcrops of the Namurian Holywell Shale have been sampled and geochemically analysed to determine source potential, maturity and ability to generate hydrocarbons. The extensive outcrops at Pen-y-Maes, Holywell, have been sampled on a 1–2m interval basis, where exposure allows, and these samples have been utilized in this study.
Initial results show that the Holywell Shale is quite heterogeneous in terms of source potential, varying from poor (<2.0 mg/g S2 pyrolysate yield) to very good/rich (>10.0 mg/g S2 pyrolysate yield). By contrast, maturity and, more importantly, organic matter type appear to be consistent over the sampled intervals. There is an apparent link between source richness and the occurrence of carbonate-rich source facies; the richest source potential is exhibited by calcareous-rich claystones. The authors believe that there is a repetitive depositional cycle akin to that observed by Bloxham & Thomas (1969). However, a more detailed analysis of the section on a smaller scale (50 cm interval) is required to confirm this.
The environment of deposition is interpreted as being dominantly anoxic. There is little or no evidence of any benthonic fauna in the macrofossil assemblages but there are numerous occurrences of goniatites, although these are poorly preserved and are frequently pyritized. There are also sporadic assemblages of plant detritus which have probably been swept from onshore settings and have accumulated in deeperwater environments. Organic matter is considered to be a mix of indigenous marine debris and transported terrestrially derived material.
In addition to the Holywell Shale samples, two other samples have also been analysed: a relatively
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