School of Earth Sciences - Theses

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    Measurement and modelling of heat flow in the Gippsland Basin, Victoria
    Geothermal energy has the potential to provide significant quantities of highly available power with little environmental impact. Enhanced geothermal system (EGS) technologies have the potential to become major contributors to global energy production, with a much wider geographic scope than the more established conventional geothermal systems. Certain favourable geological conditions have been recognised as providing improved prospectivity for EGS resources, primarily elevated heat flow rates and thermally insulating cover sequences. Most attention has been given to the location of regions having anomalously high heat flow, typically due to increased heat production in basement rocks such as granites rich in radiogenic isotopes. Little attention has been paid to extremes in thermal insulation, which offer an alternative but equally effective mechanism for increasing geothermal gradients and thus reducing the depth to a geothermal resource. Large and thick deposits of highly insulating coal present a unique thermal environment, where the commonly assumed one-dimensional relationship between surface heat flow and temperature at depth described by Fourier's Law is not maintained due to heat refraction -- a three-dimensional process. Understanding the effects of heat refraction due to thermal insulation and its effect on surface heat flow is a crucial element of exploration strategies in coal-bearing sedimentary basins. The onshore Gippsland Basin, and in particular the Latrobe Valley, is an ideal setting to study the effects of buried confined insulators on surface heat flow and thermal structure. This thesis combines the results of observational insights from empirical field data collection with mathematically driven insights of theoretical models, and simulation-driven insights of numerical finite-element modelling. Additionally, it explores the relatively modern paradigm of data-driven statistical science to generate predictions of rock properties from related intrinsic variables. Measured surface heat flow is moderately variable, with the ten most reliable calculations from borehole data having an interquartile range of 61--78 mW/m², with a mean and standard deviation of 72±14 mW/m², slightly higher than previous estimates. Groundwater advection identified in previous studies appears to affect the thermal structure of only the Cainozoic stratigraphy. Losses of up to 37 mW/m² in the vertical heat flow in the sandy Balook Formation of borehole Rosedale-301 represents a local maximum of heat transfer associated with groundwater advection. Only minor thermal effects are observed in the uppermost Mesozoic section, indicating a return to a dominantly conductive thermal regime there. The self-organising map technique was applied to the prediction of lithostratigraphy and thermal conductivity from well-log data. It successfully identified 91.3% of lithostratigraphy samples from a supervised mapping of well-log data. Mapping of thermal conductivity with corresponding well-log data produced more variable results compared with a petrophysical log interpretation technique over a large cohort of boreholes. However, the SOM analysis returned predicted values with a better correlation with measured values at sampled depths, required less pre-processing of log data, and was able to perform with non-standard log data and legacy tools. With further refinements of the technique, potential improvements may be made with its prediction performance of thermal conductivity and other rock properties. Heat flow theory applied to an idealised simulation of the Latrobe Valley coal seams showed that temperature increases of 35°C beneath the coal are possible over a reference model having no such insulation. Finite-element forward models of cross-sections and 3D volumes through the onshore Gippsland Basin identified highly variable surface heat flow, having up to ±30 mW/m² variance from the basal flux input. Complex patterns resulting from heat refraction were produced, with two common features indicative of confined insulators: 1. the greatest increase in subsurface temperature is correlated with the greatest decrease in surface heat flow, however, 2. surface heat flow tends to be slightly increased above the margins of buried insulators. The main implication from these results is the identification of an end-member insulation-dominated geothermal resource style, requiring new strategies for exploration and resource targeting.