| Title | INVESTIGATING CONTROLS ON GEOTHERMAL UPFLOW BENEATH THE ROTORUA LAKES |
|---|---|
| Authors | S. Pearson-Grant and G. Leonard |
| Year | 2021 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | TOUGH2, geothermal reservoir modelling, Rotorua lakes, Okataina Volcanic Centre, convective fluid flow, structural controls |
| Abstract | Seven of the 11 largest lakes in the Rotorua region have geothermal manifestations. All seven are associated with collapse caldera features, while the other four which do not have any identified geothermal manifestations are situated some distance from any caldera structures. Six of the geothermally influenced lakes are close to the boundaries of the Okataina Volcanic Centre (OVC), which is a composite caldera collapse feature partly filled with lava domes. Lake Rotorua, the seventh, sits to the west and fills the centre of a single volcanic caldera. In this project, we use heat and fluid flow models to explore the role the OVC might play in determining the locations of upflow beneath the Rotorua lakes. We created highly simplified TOUGH2 models to identify individual processes that could affect upflow locations. The model area encompasses the six lakes that lie close to the OVC boundary and the four cold lakes to the west and south. The base of the model is a uniform hot plate with 700 mW/m2 heat input. The top of the model follows water table elevation, which is a slightly muted reflection of topography. The model initially had uniform rock properties, with permeability variations added later to replicate an OVC boundary that acts as a barrier or conduit to flow. Model results show that topography alone can drive convection so that geothermal upflows occur beneath Lakes Tarawera, Rotomahana and Okataina, along the western edge of the OVC boundary. Upflows were too far to the north for Lakes Rotoiti, Rotoehu and Rotoma, but if a barrier to horizontal flow was included at the inferred location of the OVC boundary, model upflows occurred in approximately the right places. It therefore appears that geothermal manifestations beneath the Rotorua Lakes are influenced by a combination of topographically-driven convection and structural controls related to the OVC boundary. |