Cara Danis

Gunnedah Basin 3D architecture and upper crustal temperatures

The Gunnedah Basin in New South Wales has long been an important coal and gas resource, but limited information exists on the temperature structure or crustal architecture at depth to enable development of its geothermal potential. Here we combine gravity modelling, seismic-reflection surveys and borehole drilling results to develop a 3D depth to basement structural map and geological model of the basin. The 3D structure of the Gunnedah Basin is characteristic of a typical intracontinental rift basin. Gravity modelling of the Lachlan Fold Belt basement, using borehole and seismic-reflection controls, shows a 2–3.5 km-deep approximately north–south-oriented channel between the basement highs of the Rocky Glen Ridge in the west and Boggabri Ridge in the east. Extensional basal volcanics during the Late Carboniferous–Early Permian fill this channel. Borehole data and gravity modelling show up to 1 km of Permian to Jurassic sedimentary rocks overlying the rift volcanics. Preliminary thermal modelling, incorporating the geological model and limited deep borehole temperatures, indicates temperatures at the top of basement are in the range 105–165°C.

Retrograde metamorphism of the Wongwibinda Complex, New England Fold Belt and the implications of 2.5D subsurface geophysical structure for the metamorphic history

Garnet-bearing schists and migmatites sampled from the high-T, low-P Wongwibinda Complex in the New England Fold Belt, northern New South Wales, contain S1 and S2 assemblages that are inferred to have formed within error of each other at T = 700 and 650°C, respectively, and P = 400 and 380 MPa, respectively. Garnet grains commonly display a zoning profile that includes a flat unzoned interior with narrow (<350 μm) rims of variable composition. We interpret the unzoned cores as resulting from elemental homogenisation at peak D1 metamorphic conditions and the narrow rims (with increased Mn) as resorbed grain edges that formed during retrograde conditions (D2 and thereafter). The retrograde overprint is nearly pervasive across the complex and is most notable nearer to shear zones and intrusive rocks that cut S1, including the Hillgrove Plutonic Suite. A gravity traverse across the complex determined the Wongwibinda Fault is best modelled with a dip of 65° towards the west but did not identify any substantial concealed mafic plutons, suggesting that the heat source for the shallow crustal thermal perturbation is not imaged beneath the complex today.

Use of groundwater temperature data in geothermal exploration : the example of Sydney Basin, Australia

Multidimensional simulations in geothermal exploration require vast quantities of measurements, including temperature, to produce realistic estimates. In Australia, the database of temperature measurements is small, limited by sparse distribution and tainted by non-equilibrium conditions. Groundwater temperature data from the groundwater-monitoring/water-supply bore network provide a creative cost effective way to bridge the information gap. Down-hole temperature profiles are valuable when thermal equilibrium conditions are present. Equilibrium conditions are common in groundwater bores as they are installed to be long term. Effective use of groundwater temperature data for geothermal exploration requires an understanding of (1) the thermal conditions being measured, (2) the factors that affect the measurement, and (3) how the measurements can be used. Highly constrained models, rather than extrapolation maps, are the cost effective, risk-reducing solution for geothermal exploration in Australia. The Sydney Basin provides a case study of how an undervalued, ‘cold’ coal-bearing sedimentary basin became ‘hot’ through high-resolution modelling using groundwater temperature measurements. Groundwater temperature data are the new information source capable of filling the gaps left by the limited deep temperature measurements. Hydrogeological data play a critical role in geothermal exploration, as models representing a highly complex world approach reality.RésuméLes simulations multidimensionnelles dans l’exploration géothermique requièrent de grandes quantités de mesures dont la température pour fournir des estimations réalistes. En Australie, la base de données des mesures de température est peu importante à cause de la distribution éparse et de plus marquée par les conditions de non équilibre. Les données de température des eaux souterraines provenant du réseau de points de suivi des eaux souterraines et des forages d’alimentation en eau permettent de façon rentable de combler le manque d’information. Des profils de température en forage sont précieux lorsque les conditions d’équilibre sont présentes. Les conditions d’équilibre sont courantes dans les forages d’eaux souterraines lorsque elles sont mis en place sur le long terme. L’utilisation efficace des données de température des eaux souterraines pour l’exploration géothermique nécessite une compréhension de (1) des conditions thermales mesurées, (2) des facteurs qui affectent la mesure et (3) comment les mesures peuvent être utilisées. Des modèles fortement contraints, plutôt que des cartes d’extrapolation, sont des solutions rentables permettant de réduire les risques pour l’exploration géothermique en Australie. Le bassin de Sydney constitue un cas d’étude illustrant comment un bassin sédimentaire avec des strates carbonifères désigné comme « froid » devient un bassin « chaud » à partir de la modélisation à haute résolution des mesures de températures des eaux souterraines. Les données de température des eaux souterraines sont les nouvelles sources de données capables de combler les lacunes dues au nombre limité de mesures de température en profondeur. Les données hydrogéologiques jouent un rôle critique dans l’exploration géothermique, les modèles représentant une réalité hautement complexe.ResumenLas simulaciones multidimensionales en la exploración geotermal requieren vastas cantidades de mediciones, incluyendo temperatura, para producir estimaciones realistas. En Australia, la base de datos de mediciones de temperaturas es pequeña, limitada por una distribución dispersa y afectada por condiciones de no equilibrio. Los datos de temperatura del agua subterránea de la red de pozos de monitoreo de agua subterránea y de abastecimiento de agua proveen una manera creativa poco costosa para salvar la carencia de información. Los perfiles de temperatura en profundidad en los pozos son valiosos cuando las condiciones de equilibrio termal están presentes. Las condiciones de equilibrio son comunes en los pozos de agua subterránea cuando son instalados para ser utilizados a largo plazo. El uso efectivo de los datos de temperatura de agua subterránea para la exploración geotermal requiere una comprensión de (1) las condiciones termales a ser medidas, (2) los factores que afectan las mediciones, y (3) como pueden ser usadas las mediciones. Los modelos altamente restrictivos, más que la de los mapas de extrapolación de, son una solución de costo razonable y de riesgo reducido para la exploración geotermal en Australia. La Sydney Basin provee un caso de estudio de como una subvaluada cuenca sedimentaria carbonífera “fría” se convierte en “caliente” a través de un modelado de alta resolución usando mediciones de la temperatura del agua subterránea. Los datos de temperatura del agua subterránea son la nueva fuente de información capaz de completar las carencias dejadas por las limitadas mediciones en profundidad de la temperatura. Los datos hidrogeológicos juegan un papel crítico en la exploración geotermal, como modelos que representan un enfoque de la realidad universal altamente compleja.ResumoSimulações multidimensionais na exploração geotérmica requerem grandes quantidades de medições, incluindo a temperatura, para produzir estimativas realistas. Na Austrália, as bases de dados de valores de temperatura são escassas, limitadas por uma distribuição esparsa e inquinadas por condições de não equilíbrio. Os dados de temperatura da água subterrânea obtidos a partir das redes de captação de água de abastecimento ou de redes de monitorização de águas subterrâneas proporcionam um modo criativo de custo reduzido para colmatar esta lacuna de informação. Perfis de temperatura dentro de furos são também valiosos quando estão presentes condições de equilíbrio térmico. Como os furos de água subterrânea são instalados a longo prazo, são comuns condições de equilíbrio dentro dos mesmos. O uso efetivo da temperatura da água subterrânea para exploração geotérmica requere o entendimento (1) das condições térmicas em que são medidas, (2) dos fatores que afetam as medições, e (3) de como as medições podem ser usadas. Modelos altamente restritos, mais do que mapas extrapolados, são as soluções mais económicas e que apresentam menos riscos para a exploração geotérmica na Austrália. A Bacia de Sidney proporciona um estudo de caso sobre o modo como uma bacia sedimentar carbonífera “fria” subavaliada se tornou “quente”, através de modelação de alta resolução usando medições de temperatura de água subterrânea. Os dados de temperatura de água subterrânea são a nova fonte de informação capaz de preencher as lacunas deixadas pelas medições limitadas de dados de temperatura em profundidade. Os dados hidrogeológicos desempenham um papel fundamental na exploração geotérmica, uma vez que os modelos representam uma abordagem altamente complexa à realidade global.

Deep 3D structure of the Sydney Basin using gravity modelling

A detailed deep 3D geological model is an important basis for many types of exploration and resource modelling. Renewed interest in the structure of the Sydney Basin, driven primarily by sequestration studies, geothermal studies and coal seam gas exploration, has highlighted the need for a model of deep basin geology, structure and thermal state. Here, we combine gravity modelling, seismic reflection surveys, borehole drilling results and other relevant information to develop a deep 3D geological model of the Sydney Basin. The structure of the Sydney Basin is characteristic of a typical intracontinental rift basin, with a deep north–south orientated channel in the Lachlan Fold Belt basement, filled with up to 4 km of rift volcanics, and overlain with Permo-Triassic sediments up to 4 km thick. The deep regional architecture presented in this study will form the framework for more detailed geological, hydrological and geothermal models.

Geothermal state of the Sydney Basin: assessment of constraints and techniques

The thermal structure of sedimentary basins is largely dependent on complex three-dimensional effects encompassing architecture, geology and groundwater, making it difficult to describe in a one-dimensional model. New equilibrated down-hole temperature measurements in the Sydney Basin, in conjunction with regional scale thermal modelling using the geodynamics simulation software Underworld, can provide an accurate assessment of the thermal structure of the basin. When compared with extrapolation maps, these results highlight important limitations of utilising extrapolation maps as an unaccompanied geothermal exploration tool. The extrapolated temperature method creates a ‘temperature-at-depth’ map, which propagates and exaggerates near surface variations, and is limited by coverage and number of boreholes that have temperature measurements recorded. Numerical simulations of basin heat flow, using basic material properties, combined with a deep three-dimensional geological model and calibrated by measured equilibrated temperature data are not limited by the borehole coverage but rather the chosen resolution of the model. The Underworld thermal model provides a realistic estimation of temperature at depth within the Sydney Basin, a clearer understanding of thermal structure and allows a more comprehensive assessment of potential geothermal targets.

Building 3D geological knowledge through regional scale gravity modelling for the Bowen Basin

Regional scale gravity modelling is an effective and fast way to gain geological understanding of large scale structures like the Bowen Basin. Detailed deep 3D geological knowledge has become an important component of many types of exploration and resource modelling. Current interest in the Bowen Basin for geothermal exploration highlights the need for a complete basin scale model which is compatible with thermal modelling software. The structure of the Bowen Basin is characteristic of a typical asymmetrical extensional rift basin, with up to 5 km of sediment overlying the basement. By combining gravity modelling, calibrated by boreholes and seismic reflection profiles, we produce geologically reasonable 3D surfaces and structures to create a model of the Bowen Basin. This model is the final part in the completion of the 3D Sydney–Gunnedah–Bowen Basin system geological model and provides both an important framework from which detailed thermal models can be derived and a platform from which to expand with new information.

Sydney–Gunnedah–Bowen Basin deep 3D structure

Studies of the Sydney–Gunnedah–Bowen Basin (SGBB), one of the largest extensional rift sedimentary basins on the east coast of Australia, lack an understanding of the 3D upper crustal structure. Understanding of the subsurface structure is essential for many areas of resource exploration, development and management, as well as scientific research. Geological models provide a way to visualise and investigate the subsurface structure. The integrated regional scale gravity modelling approach, which uses boreholes and seismic data constraints, provides an understanding of the upper crustal structure and allows the development of a 3D geological model which can be used as the architectural framework for many different applications. This work presents a 3D geological model of the SGBB developed for application in high resolution thermal models. It is the culmination of geological surfaces derived from the interpolation of previous regional scale 2D gravity models and numerous borehole records. The model outlines the basement structure of the SGBB and provides information on depth to basement, depth to basal volcanics and thickness of overlying sediments. Through understanding the uncertainties, limitations, confidence and reliability of this model, the 3D geological model can provide the ideal framework for future research.