ASEG 09 © 2004 BRGM Ray Seikel (Intrepid Geophysics), Kurt Stüwe (Graz University), Helen Gibson (Intrepid Geophysics), Betina Bendall (Petratherm), Louise McAllister (Petratherm), Peter Reid (Petratherm), Anthony Budd (GeoScience Australia) Forward Prediction of Spatial Temperature Variation From 3D Geology Models

ASEG 09 © 2004 BRGM 1) Building 3D geology models 2) Prediction of spatial temperature variation from 3D geology model. Develop a method for rapid computation directly from a 3D geology model 3) Case Study - Compare predictions with measured Collaboration

ASEG 09 © 2004 BRGM Conduction Production Radiogenically Mechanically Chemically Advectionby Fluids by Erosion by Deformation by Magma Heat Transfer Processes + S chem + S mech Summary of heat flow theory

ASEG 09 © 2004 BRGM  Heat production via radioactive sources is important  In contrast no highly active tectonism, metamorphism or volcanism is occuring in the upper crust today, which might otherwise contribute to mechanical or chemical heat production. So we do not take these into account Assumptions for Australia

ASEG 09 © 2004 BRGM  It is sufficient to consider only the case of thermal steady state for the Australian crust  We must take into account the variation of conductivity with rock types Assumptions for Australia (Cont)

ASEG 09 © 2004 BRGM Conduction * Production Radiogenically * Mechanically Chemically Advectionby Fluids * by Erosion by Deformation by Magma Heat Transfer Processes + S chem + S mech XX Simplified Equations

ASEG 09 © 2004 BRGM Software Implementation Fourier’s first and second laws Steady state Variable thermal conductivity (k) & heat production rate (S) The final equation allows us to solve in 3D using finite difference approximation

ASEG 09 © 2004 BRGM Surface Topography Surface: Mean surface temperature Sides: Neumann-type Base: Constant Heat Flow or Constant Temperature Support for surface topography fixed internal temperatures Isotherms with Increasing depth Boundary Conditions

ASEG 09 © 2004 BRGM Solve in Voxet space <Assign: Thermal Conductivities Heat Production Rates <Assign: Boundary Conditions

ASEG 09 © 2004 BRGM Case 1 - Constant qbase and layered geology Case 2 – Constant temperature at base and layered geology Case 3 – Uniform thermal conductivity and heat production rate through out Case 4 – Step heat production rate Case 5 – Same as Case 2 expect one voxel is held at fixed temperature Case 6 – Topo test Case 7 – Uniform advection through out Case 8 – Advection through a 3x3 vertical column Unit testing: 8 cases

ASEG 09 © 2004 BRGM To validate FD approximations against analytical solutions and expected T distributions Different initial settings and boundary conditions all passed  Unit testing: 8 cases

ASEG 09 © 2004 BRGM Example: Unit Test 3 Results Uniform conductivity Uniform radiogenic heat production Temperature Heat Production Depth Conductivity Depth

ASEG 09 © 2004 BRGM Heat Production Conductivity Temperature Depth Uniform conductivity No heat production Setting drill hole temperature data as fixed (Unrealistic scenario but ok for testing!) Example: Unit Test 5 Results

ASEG 09 © 2004 BRGM Example: Test Localised advection Assuming Uniform conductivity and constant basal heat flow Fluid flow upwards through 150x150m vertical column Properties adjusted to give visible results

ASEG 09 © 2004 BRGM 1) Compare with measured values 2) Consider potential field data, re-fine the model, repeat This case study assists in software testing Brief overview of Paralana Case Study

ASEG 09 © 2004 BRGM Petratherm Ltd’s Paralana Project Tenements ~20 km east of Mt Painter Inlier Northern Flinders Ranges South Australia Paralana Project Mt Painter Inlier Adelaide

ASEG 09 © 2004 BRGM Generalised W-E section: Poontana Graben mW/m 2 Paralana-1B

ASEG 09 © 2004 BRGM Paralana Case Study Geology model constraints 9 interpreted seismic sections (Petratherm) simple, linear depth conversion (in GeoModeller) Paralana-1B well (Petratherm) ~50 shallow drill holes (SARIG dataset, PIRSA) SEEBASE economic basement depth (PIRSA / SRK) 1:700,000 Basement map Arrowie Basin (PIRSA / SRK)

ASEG 09 © 2004 BRGM Tops and faults from seismic

ASEG 09 © 2004 BRGM Paralana-1B SeeBase: Top Curnamona Paralana Fault(s) Shallow drill holes Seismic

ASEG 09 © 2004 BRGM Solid geology model

ASEG 09 © 2004 BRGM Forward temperature modelling Conduction  Heat Production  (U, Th, K) Advection x (but soon possible) Possible small heat contribution from fluids fluxing via Paralana Fault and (?) deeper fracture networks/pathways

ASEG 09 © 2004 BRGM Steps <Assign Thermal Conductivities Heat Production Rates <Set Boundary Conditions <Discretise the model <Input Voxet <Forward Model 3D Temperature <Output Voxet(s)

ASEG 09 © 2004 BRGM model inputs run24 Thermal Conductivity Watts m –1 K –1 Heat Production Rate Watts / m 3 Rec - Mesozoic1.5~1 x10 -6 Carboniferous2.0~1 x10 -6 Lake Frome Gp5.3~1 x10 -6 Lwr Arrowie3.2~1 x10 -6 Brachina Sh2.0~1 x10 -6 Lwr Adelaidean2.4~1 x10 -6 Moolawatana3.2~22 x10 -6 Mt Painter MesoP3.2~22 x10 -6 U-depleted base3.2~2 x10 -6 BOUNDARY CONDITIONS Top: 19°CBottom: Wm -2 Constant heat flow

ASEG 09 © 2004 BRGM Model discretisation: run 24 Input model extents Number of cells Discretisation cell size X 55 km m Y 30 km40750 m Z 10 km40250 m Total voxels: 64,000 Run-time: 14 mins

ASEG 09 © 2004 BRGM Set iterations controls (run24) Maximum residual in Degrees C: (the maximum change allowed in temperature in any cell) Maximum Iterations: 15,000 When either condition is met, iterations cease, as thermal equilibrium is said to be reached

ASEG 09 © 2004 BRGM Model outputs Voxet: x, y, z, lithologies (initial earth model) Voxet of results: temperature, vertical heat flow, vertical temperature gradient, total horizontal temperature gradient jpegs (for every 2D section in the geology model) grid files (ditto) record of run (inversions.xml; COMPUTE_LOG.txt)

ASEG 09 © 2004 BRGM ‘Verify‘: W-E section Temperature Modelled geology Wells (projected to section) Result ~103°C at bottom hole / Paralana–1B (compared measured 109 °C) 286 degC 19 degC Paralana-1B Section line: 55 km long

ASEG 09 © 2004 BRGM 140 mWm mWm -2 Horizontal section at -500m Vertical Heat Flow Result ~108 mWm -2 (compared measured 129 mWm -2 within Paralana–1B) Paralana-1B 55 km x 30 km

ASEG 09 © 2004 BRGM Concluding points Initial geology model is reasonable Together with estimated thermal properties: Measured T data in Paralana-1B can be matched Surface Heat Flow data can be (~) matched Software implementation performs to specifications Geology model still needs refining (assisted by forward modelled gravity and magnetics in GeoModeller) Geology dominates the T distribution - Hence true 3D modelling is crucial !

ASEG 09 © 2004 BRGM grateful acknowledgement !

ASEG 09 © 2004 BRGM Disclaimer Data have been manipulated to show software features and may not reflect actual conditions at Paralana