Abstract
Globally there is an abundance of geothermal resources that are suitable for low enthalpy geothermal energy generation, such as ground source heating however, very few of these have been utilised. Specific to São Miguel, The Azores, there are plenty of geothermal manifestations in the form of hot springs and fumaroles, and one current geothermal power plant (Ribeira Grande) within the central volcanic complex of Fogo. Neighbouring this is the basaltic Nordeste volcanic system. Few detailed studies have been published on the facies and facies architecture of Nordeste, with this study aiming to providing an analysis of the exposed ‘a‘ā lavas, specifically the cavities within. With a wide range of sizes and origins, the vesicles, segregation vesicles, clinker cavities, drainage cavities and fractures or jointing associated with cooling or vesiculation each provide the sequences with a degree of porosity, influencing the overall porosity and permeability of the entire sequence.The vesicles and fracture networks found within the cores and clinkers of Nordeste lavas, coupled with the auto brecciated clinker flow margins significantly increase the porosity and permeability of the sequences. Micro CT analysis of ‘a‘ā clinker samples demonstrates porosity values ranging between 13.73% - 18.74%, whilst porosity values of ‘a‘ā core samples range between 4.36% - 9.41%. The unaltered sequences are known to form some of the most effective lava reservoirs, with any jointing or fracturing associated with emplacement of the lava flows forming the critical fluid pathways. However, the secondary alteration, burial and fracturing of the sequences often results in the sealing of vesicle and fluid networks due to mineral precipitation, although the alteration can also lead to mineral dissolution. Additionally, any secondary tectonic fracturing can form critical areas of high permeability and fluid pathways.
Since the porosity and permeability of the subsurface sequences at Nordeste is unknown, the likelihood of the complex providing a geothermal resource is uncertain. Whilst it is unlikely that the Nordeste sequences could feasibly sustain a geothermal reservoir, the porosity analysis of samples suggests that over 103 million kilolitres of water could be held within the Nordeste sequences, and the theoretical permeability provided by the porous areas, coupled with fractures, jointing and dykes would theoretically sustain a low enthalpy heating system.
| Date of Award | 2024 |
|---|---|
| Original language | English |
| Supervisor | Ian Skilling (Supervisor) & Duncan Pirrie (Supervisor) |