| Abstract |
A study was carried out in 2004 and 2005 to develop a database and series of index maps for the geothermal resources of Ireland. The objective of the project was to produce a GIS-linked geothermal database, an up-to-date map series, and a report with recommendations for the next steps necessary in expanding the use of Ireland’s geothermal resources for both shallow resources and the somewhat unknown deeper resources. Some of the difficulties involved in finding a best approach to such an exercise are discussed and some of the short comings of such a study are presented. However the value of carrying out such an exercise is highlighted as a first tool in geothermal resource assessment. The study started with a review of the available data sources in the relevant government departments of the Geological Survey of Ireland within the geothermal archives, The Exploration and Mining Division and the Petroleum Affairs Division. Previous studies were reviewed and date incorporated into the study – in particular the Irish Geothermal project 1981-1983. Mineral exploration companies were contacted to gather any available information from recent or current exploration programmes which were not yet publically available. A short monitoring programme was also carried out on any boreholes open and available. The study included geological, structural and hydrodynamic analysis of the areas with potential and attempted to use these data to provide a more reliable estimate of the resources and potential. Ireland has had a very active mineral exploration and mining sector for over 50 years and as such there has been plenty of shallow (50-300m) drilling in parts of the country though somewhat confined to particular rock types, generally the Carboniferous. However there is a sparsity of drilling and therefore geological detail at depth (>800m) due to limited onshore oil and gas exploration. As a result the data available are patchy and somewhat biased to particular lithologies. The result of this is that extensive extrapolation of data was necessary in order to produce a dataset that could be used for mapping geothermal potential. Extrapolation of temperature and geothermal gradient was applied to data from shallower boreholes of 300-800m (with a small number of deeper boreholes to a maximum depth of 2300m. Aside from not having the temperature data the lack of lithology information at the location of an extrapolated data point increases the uncertainty in judging the appropriateness of the extrapolated temperature or geothermal gradient value. The project resulted in the provision of a dataset which could then be used for modelling temperature and geothermal gradient to extrapolate in areas and at depths for which there were no data. A number of attempts to model the data using various modelling techniques were made with data before a best approach was found. These data were then modelled for temperature and geothermal gradient and a series of maps were produced for surface groundwater temperatures, 100m, 500m, 1,000m, 2,500m, 5,000m. Warm spring data and surface/shallow groundwater temperatures show two main anomalous zones with temperatures between 15ºC and 21ºC and some indication of a significant Variscan aged E-W structural trend. Outside of these areas average shallow groundwater temperatures vary regionally from 12ºC in the south to 9ºC in the north of the country. Modelled temperatures at depths of 500m to 5000m show a consistent NE-SW break across the centre of Ireland with higher temperatures in the north central and north of the country and maximum temperatures of 90ºC in the north and 45ºC in the south of the country at a depth of 2,500m. |