Sedimentological Characteristics and Shale Gas Potential of Carboniferous Mudstones in Ireland: The Clare and Northwest Carboniferous Basins

Taylor, Kevin; Khattab, Sami; Nolan, Kath; Redfern, Jonathan; Williams, Brian; Warshauer, Steven; Hill, Jim; Armstrong, James

The Carboniferous basins in Ireland contain thick mudstone intervals that offer potential shale gas targets. Dinantian organic rich mudstones in the Northwest Carboniferous Basin (the Bundoran, Benbulben, Carruan and Dergvone Shales) and the Namurian Clare Shale of the Clare Basin have up to 8% TOC (present day) and are thermally mature.

The sedimentology and mineralogy of these mudstone successions have been documented using available outcrops, to characterise the resource potential and provide analogues to the subsurface shale reservoirs. Thin section microfacies analysis shows that the mudstones have significant variability. Advection currents were the dominant sedimentological process responsible for sediment deposition, as evidenced by graded beds and ripple features. Bioturbation is abundant, indicating dominantly well-oxygenated depositional bottom waters and suggest high productivity was the major driver for organic-enrichment, not anoxia. Diagenetic cements include dolomite and quartz which significantly increase the fraccability of the rock. Integration of these observations with thermal maturity considerations can significantly decrease the risk within shale basins.

Thermal maturity measurements and basin modelling indicate that the Dinantian shales of the Northwest Basin are likely to have generated significant gas, even though measured present-day TOC values at outcrop are low (<2% in most places). The key risks are present burial depths, which can be less than 300m, TOC distribution and understanding if the outcrops are representative of possible subsurface composition, and mineralogical composition (many shales are ductile). In the Clare Basin, Namurian shales are thicker, more deeply buried and have consistently higher TOC (3 to 8%). However, the effect of Late-Carboniferous upliftand possible gas leakage in the basin is critical to the shale gas potential. Modelling suggests that significant gas would have been generated, but thermal maturities recorded in the past from the Clare Shales suggest they are over-mature. The Dinantian shales offer better prospectivity, but still have a risk of gas leakage following inversion.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013