The Clare Shale Formation, West Ireland:The Sedimentological Variability of a Potential Shale Gas Reservoir

Abstract

The Clare Shale Formation, west Ireland, forms the lowest stratigraphical unit of the Upper Carboniferous Shannon Basin. This basin has received much attention in the past, mostly due to its spectacular cliff exposures of sedimentary deposits that have been used as analogues for deep-water reservoirs in the petroleum exploration industry.The Clare Shale Formation, with its high total organic carbon (TOC) content, significant thicknesses (from 180m at outcrop to over 280m in the subsurface), and reasonable depth of burial (up to 700m at least) instantly appears to be an attractive target for shale gas exploration. However, available thermal maturity data indicates that the formation may be over-mature for hydrocarbons, even gas. Its high thermal maturity values indicate that the formation was even higher originally in organic carbon. The excellent exposures provided in the basin makes this a perfect formation for studying the sedimentology of a source rock formation and how variability in the sedimentological processes affects the distribution of shale gas reservoir characteristics.Little is known about the sedimentology of the mudstones of the Clare Shale Formation. Through this research some of the first details have emerged regarding the sedimentology of the formation. Field work and subsequent laboratory analytical techniques revealed a variety of mudstone microfacies exist and this in turn leads to the interpretation that deposition of the Clare Shale Formation involved more than just simple suspension fallout as has been previously suggested.Analysis of the microfacies and their corresponding total organic carbon (TOC) contents and mineralogies revealed that at times the environmental conditions near the seafloor were more oxygenated than previously presumed and sedimentary processes such as density flows and turbidity currents provided silt from the shelf to the deeper parts of the basin.

Date of Award	1 Aug 2013
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jonathan Redfern (Supervisor) & Kevin Taylor (Supervisor)