Shale gas potential and CO₂ storage prospectivity in the Carboniferous of southwest Ireland and southwest Portugal

Abstract

With increasing energy demand and declining oil production, shale gas and other unconventional resources are becoming progressively more important. Conventional hydrocarbons account for approximately 20% of the world's estimated fossil fuel reserves, whereas unconventional hydrocarbons account for ca. 80%. Carboniferous rocks in the Portuguese ‘South West Portuguese Zone' and the onshore Clare Basin of southwest Ireland have recently become the focus for shale gas exploration with extremely optimistic statements having been made concerning their hydrocarbon potential. The primary targets are thick, mid-Carboniferous black shale units, the Quebradas Formation in the former basin and the Clare Shale Formation in the latter.

New results are presented from TOC, Vitrinite Reflectance (VR), XRD, Rock-Eval pyrolysis and adsorption (CO₂ and methane) investigations. The data generated suggest that the Carboniferous rocks in both basins range from marginal to post-mature in terms of dry gas generating potential. Slightly lower, and more variable VR values were obtained from the Clare Shale and Quebradas formations than from the overlying stratigraphic units in both basins. The principal cause for the poor quality VR data from the back shales is believed to be the small particle size and poor preservation of the vitrinite in these rocks, together with difficulties in differentiating vitrinite from other macerals. The possibility that the maturity of the overlying rocks has been overestimated due to the incorporation of reworked vitrinite of higher maturity is firmly discounted on the basis of VR data from thin coals within the overlying clastic sequences in both basins. VR results from these coals (Romax and Roran) are fully consistent with Roran determinations from associated mudrocks in the same sections.

Adsorption experiments for the Clare Shale Formation from the Clare Basin confirmed that both CO₂ and methane adsorption increase with TOC content. CO₂ was adsorbed preferentially to methane at a ratio of ca 1.8 : 1. (Go to www.searchanddiscovery.com to see figures.)

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015