Project Location Map and generalised geological setting for the Torrens’ GELs. [click on the image to enlarge]

Torrens Energy understands that for an area to be prospective for Hot Fracture Rock geothermal resource exploration, three key factors are needed:

• an active heat source (high heat flow);
• insulating sedimentary cover over the heat source; and
• proximity to infrastructure and market.

Torrens Energy has secured the rights to explore in areas that are known for high heat flow, which have sediment cover with known insulating properties, and are located on the National Power Grid.

Torrens has been granted 15 GELs, over three project areas encompassing 7100km² of highly prospective HFR ground within the SAHFA.

The geological setting is comprised of the insulating rocks of the Torrens Hinge Zone and Adelaide Geosyncline. These areas are underlain by “basement” rocks of the Eastern Gawler Craton and Barossa Complex.

Temperature Modelling to 5,000 metres

Temperature modelling undertaken by Torrens Energy based on data gathered to date has yielded results comparable or better than most other Hot Fracture Rock projects around the world.

Torrens Energy commissioned an independent consultant to model the temperature down to a depth of 5,000 metres at eight locations within Torrens Energy’s tenements. The consultant was able to predict the temperature profiles based on expected heat flow and thermal conductivity variation.

The thickness and rock types of the geological layers expected to lie in the top 5,000 metres of crust at eight locations were deduced by Torrens Energy from information contained in previous reports on the area. Torrens Energy was also able to obtain samples of many of the relevant geological formations from the PIRSA core library at Glenside, South Australia. A laboratory was commissioned to measure the thermal conductivity of those samples and Torrens was thus able to construct a thermal conductivity profile for the top 5,000 metres at each of the eight locations.

Thermal conductivity is sensitive to changes in temperature, and this fact was incorporated into the modelling process. By applying a constant heat flow through the sequence of rock, estimating average surface temperature from Bureau of Meteorology records, and calculating the expected thermal gradient in each layer, the consultant determined that a heat flow value of about 90 mW/m² is sufficient to achieve a temperature of 200°C at shallower than a depth of 5,000 metres over most of Torrens Energy’s South Australian GELs.

The results of the shallow water-bore heat-flow estimates, give Torrens Energy confidence that areas exist within Torrens Energy’s tenements where heat flow is greater than 90 mW/m². Torrens Energy is therefore confident that it will locate attractive drilling targets for HFR development.

Temperature Modelling completed for each of the GELs shows temperatures of 200˚C or more at depths of less than 5,000m are possible. The modelling indicates that finding high temperature areas depends on the location of high heat flow zones in the GELs. The lines represent modelled temperatures at selected locations within each project area, assuming a heat flow of 90mW/m². [click on the image to enlarge]

Adelaide Project

The Adelaide Project consists of four GELs totalling 1,700 km². The Project targets predicted thick sequences of insulating sediments of the Torrens Hinge Zone. Existing data on the subsurface geology is limited. The project area lies on the coast and the southern part of it is in the northern suburbs of Adelaide itself. The Northern Adelaide Plains, the majority of the Project area, are largely undeveloped, flat and barren; yet lie in very close proximity to the city. There are existing fossil fuel power stations adjacent to the Adelaide Project, including the Torrens Island power station.

Uranium occurrences in emergent Barossa Complex of the Adelaide Fold Belt east of the plains make this area highly prospective for high heat flow. The diagram below is a schematic of the Adelaide Plains subsurface geology. It shows that there is likely to be 3,500 to 5,000m of sedimentary cover in the key target areas, with potential for heat producing basement rocks, the active heat source. Torrens has undertaken heat flow estimates using data from existing shallow drillholes in the area. Heat flow estimates from locations immediately south of the Project area lie in a narrow range between 80 and 100 mW/m². Temperature modelling based on existing data and that collected by Torrens Energy has found that a temperature of approximately 200˚C may exist at a depth of 5,000 metres close to the northern suburbs of Adelaide.

In addition, by securing tenure on the Northern Adelaide Plains, Torrens Energy has achieved a strategic position on the coast next to a major city. The coastal location gives Torrens Energy potential access to seawater. Many of the world’s power plants are located on the coast because seawater is used as a coolant in the power generation process. To date Torrens is the only Torrens Energy in South Australia to have licences in a coastal locality, and has secured the entire Northern Adelaide Plains.

The work program will determine the 3D geological structure as well as the detailed heat flow distribution in the Project area. Targeted geological formations are covered by thin (200–400 m thick) sediments of the Saint Vincent Basin, and as such cannot be directly mapped. Existing seismic data will be reprocessed, and new seismic data collected if necessary to determine the geological profile to 5000 m depth. Existing drillholes will be assessed for temperature data, and a programme of new core drilling will be undertaken to determine the heat flow regime.

Adelaide Plains schematic subsurface geology. Several kilometres of insulating sediments are expected above the heat producing basement rocks comprising the Eastern Gawler Craton, providing the active heat source.

Barossa-Clare Project

The Barossa-Clare Project’s four GELs totalling 2,000 km² target the insulating sediments of the Adelaide Geosyncline and potentially overlie the heat producing Barossa Complex. Geosyncline sediments include a majority of potentially insulating Saddleworth Formation, Mintaro Shale and Tapley Hill Formation siltstones and shales. Torrens Energy has undertaken thermal conductivity measurements on core samples from units found in the area and the results indicate that the sediments have good insulating properties.

The Project is also located on or adjacent to infrastructure and services, including the National Power Grid. The geological nature of the basement is not well understood, and is expected to lie well below the depth of any located deep drilling target. Published heat flow values north and south of this area are all high, and heat flow estimates undertaken by Torrens Energy in the immediate vicinity of the Project average 89 mW/m², within a small range of the target level of 90+ mW/m².

The work program will involve similar activities as those to be undertaken in the Adelaide Project, but with a key difference. The determination of the 3D geological geometry will involve detailed field mapping rather than seismic data collection because the relevant geology is not covered by younger sediments, but outcrops in the area.

The temperature modelling undertaken so far by Torrens Energy has found that one part of this Project area has the highest modelled temperature in Torrens Energy’s GELs – around 240°C at a depth of 5,000 metres.

Torrens Project

The Torrens project has 7 GELs of 3,500km². The Project is located on a strip of land on the eastern edge of Lake Torrens, targeting the thermally anomalous Eastern Gawler Craton (EGC) within the Torrens Hinge Zone. The EGC in this area is part of the Olympic Domain; an area characterized by the occurrence of uranium rich radiogenic iron oxide (RIO) deposits such as the enormous Olympic Dam orebody. Such deposits have extremely high heat generating capacity.

Published heat flow data from the Olympic Domain west of Lake Torrens is variable but around the known RIO deposits heat flow is very high. Published heat flow data from east of Lake Torrens in locations north and south of the Torrens Project are all above the stated target of 90+ mW/m². There is potentially 3,000 to 5,000 metres of insulating sediments in this area, including the Wonoka, Bunyeroo, Brachina and Tapley Hill Formations. The thermal conductivities of these units have been measured from existing core samples and their good insulating properties have been confirmed.