ASEG Extended Abstracts - 1st Australasian Exploration Geoscience Conference – Exploration Innovation Integration, 2018

The Darlot-Centenary gold deposit is one of the larger known mineralised systems in the southern end of the West Australian Yandal Greenstone Belt, with an estimated 2.7 Moz having been extracted from the Darlot Centenary Mine since 1988. The area is well explored near surface but given the proven endowment there is potential for significant additional mineralisation at depth. With current proven reserves dwindling, Gold Fields recognised the need to identify a technology to fast-track target generation in order to more rapidly evaluate the nearby rock volume.

In August 2016 Gold Fields began investigating the potential for 3D reflection seismic to accelerate evaluation of the rock volume accessible via existing workings. In November 2016 a seismic crew was on ground acquiring approximately 150km3 of 3D seismic data (25km2 surface area x 6km depth). The survey coverage was designed to image the local steeply dipping geology and structures. Processing of the seismic dataset was completed in Q1 2017 and Gold Fields has completed preliminary interpretation of the 3D cube.

The seismic data has provided a rich 3D picture of the Darlot structural framework to depth, which could not be obtained by any other geophysical method. It has highlighted a number of features with similar characteristics to known mineralisation and has provided a better defined structural framework that has greatly assisted the fundamental geological understanding and further aided ranking of these targets in terms of prospectivity.

Saline aquifers in the Murray River or SE Australia are traversed by freshwater rivers, with adjoining riparian and floodplain regions containing freshwater lenses. Bore data and more recent Airborne electromagnetic (AEM) surveys have determined that these lenses are spatially extensive, but have widely varying geometries. The maintenance of these lens systems is important as they support ecologically significant riparian vegetation communities such as Red Gum and Black Box. A more complete understanding of their hydrogeology is required to ascertain how they develop and degrade. Limited ground investigations including ¹⁴C geochemistry have determined that the lens systems contain recent water, indicating that they are dynamic systems with their development defined by the relative rates of recharge from the river and mixing with groundwater. Changes in groundwater gradients and depth, floodplain extent, and topography are believed to control their initial location. The same controls also govern their stability.

The potential of AEM systems for defining the geometry of these lens systems in 3D is considered along with an assessment of their value for monitoring variations associated with these ecosystems. The advent of “calibrated” AEM systems and robust inversion tools have given added impetus to their use for environmental monitoring. Spatio-temporal variations are observed in the near surface (top 20m) from a multi-temporal assessment of Clark’s Floodplain, adjacent to the Bookpurnong irrigation area, with co-incident AEM surveys acquired between 2008 and 2015. Spatial changes in ground conductivity, attributed to changing groundwater quality have been observed. The freshwater lens systems appear to have contracted significantly over the last decade. This is attributed, in part, to land use patterns and the development of an irrigation-related groundwater mound on the highlands adjacent to the floodplain, and an increased hydraulic gradient towards the river. The results indicate the geometry of the hyporheic zone may have also changed along the river.

In Australia’s semi-arid and arid interior, groundwater resources provide water supply security for agriculture and community consumptive use and are critical for underpinning economic development. . The Southern Stuart Corridor Project in central Australia, is an inter-disciplinary study which aims to better characterise regional groundwater systems and identify the location, quantity and quality of new groundwater resources. The main aims of the project are(1) to de-risk investment in development of a potential agricultural precinct in the Western Davenport Basin, and expansion of horticulture in Ti-Tree Basin, (2) to identify future water supplies for Alice Springs and Tennant Creek, and (3) for regional water supplies for mineral resource development.

The project is funded by Geoscience Australia (GA) as part of the Exploring for the Future (EFTF) Programme. The project integrates airborne electromagnetic (AEM), ground geophysics (ground magnetic resonance (GMR) and borehole geophysics (Induction, gamma and nuclear Magnetic Resonance (NMR)) with drilling and pump testing; hydrochemistry and geochronology; and geomorphic, geological, hydrogeological and structural mapping and modelling. Advancements in temporal remote sensing technologies for surface hydrology, vegetation and landscape mapping are also used to facilitate the identification of recharge and discharge zones and groundwater-dependent vegetation.

This paper reports on initial AEM inversion results for the Alice Springs, Ti-Tree Basin, Western Davenport and Tennant Creek areas and the use of a machine learning approach for rapid geological and hydrogeological interpretation of the AEM data. These machine learning approaches have the potential to significantly reduce interpretation time and facilitate the rapid delivery of project results.

Gassmann fluid substitution is widely used in geophysical practice. In the last few years, the topic of fluid/solid substitution has emerged, where the substances filling the pore space can be solids, fluids, or visco-elastic materials, such as heavy oils. Solid substitution cannot be accomplished with the Gassmann theory because the finite rigidity of the pore fill (either solid or viscoelastic) prevents pressure communication throughout the pore space, which is a key assumption of the Gassmann theory. In this paper we explore applicability of solid substitution techniques by using a sandstone saturated with a solid substance, octodecane. This substance is a hydrocarbon with a melting point of 28°C, making it convenient to use in the lab in both solid and fluid form. Our approach is to measure a dry sandstone sample under different confining pressure, then saturate it with liquid Octodecane at 35°C and measure again. After that, we cool it to 20-25°C and carry out the measurement once more. The dry properties can be used to obtain parameters necessary for fluid and solid substitution. The results show that moduli of the dry sandstone exhibit significant pressure dependency, which is reduced for the solid filled rock. Also the prediction of the Gassmann theory and Ciz and Shapiro theory underestimate the velocities. This suggests that stiffening occurs due to substantial reduction of compliance of grain contacts by the solid infill. This effect is accounted for by the solid squirt theory. The results give direct evidence of the solid squirt effect and can be used to verify and calibrate theoretical solutions for rocks saturated with solid or viscoelastic substances.

The Precipice Sandstone in the Surat Basin is being appraised for CO2 geological storage owing to its high porosity and permeability and expected high injectivity. Generally it is quartz rich with variable kaolinite, however detailed characterisation of core shows that it contains minor to trace amounts of potentially reactive minerals including carbonates, plagioclase, chlorite, and muscovite, increasing towards the overlying Evergreen Formation top seal. The Evergreen Formation is more variable, with interbedded low porosity and permeability mudstones, fine-grained sandstones, and calcite cemented zones. Injected CO2 dissolves into formation water forming carbonic acid. The geochemical reactivity of drill core samples affects the predicted pH and the dissolution or precipitation of minerals which could permanently trap CO2 as carbonates such as siderite, or dynamically change porosity and permeability altering CO2 migration.

Comparative kinetic geochemical modelling of the CO2 reactivity of four representative mineralogies from drill core samples from deeper parts of the central basin indicates that the Evergreen Formation is potentially more reactive to CO2 than the Precipice Sandstone, especially for calcite or siderite containing zones. In the Precipice Sandstone small amounts of albite and siderite dissolved with traces of siderite and kaolinite precipitated. Dissolution of calcite and siderite in the Evergreen Formation favourably buffered acidity, with predicted precipitated minerals including siderite, kaolinite, ankerite, and smectites. The geochemical models indicate overall changes to porosity are however minor.

Recent data from capture technologies has reported that CO2 from coal combustion may retain NOx impurities in the form of NO. Simulations with the addition of 30-100 ppm NO in the CO2 stream indicated the generated pH in the quartz rich Precipice Sandstone is similar to that on injection of pure CO2 after 30 years. The precipitation of Fe-rich smectite clays was additionally predicted.

The analysis of new 3D seismic and the acquisition of unique core data enables an in-depth interpretation of outboard Eocene mounds in the central Ceduna Sub-basin and a more accurate model of their origin.

Seismic attributes and spectral decomposition clearly image the 3D morphology and internal architecture of the mounds and enable building episodes to be defined. Mounds have length of 535km, width of 1-4km and heights of 50-110m and developed at water depth 300-600m. Gravity core of the top of one mound indicates that it consists, at least partly, of corals typical of a deep-water reef. This initial interpretation is based on comparison with modern reef-forming biota, the presence of suspected brachiopods and lack of gastropods.

The underlying faults control the initial localisation of the mounds. These faults were active in the Cretaceous and reactivated in the Tertiary and intersect sequences modelled as oil- and gas-mature.

One main control for deep-water coral is the need for suitable hard substrates for initial attachment. The distribution of the mounds supports a development mechanism that relies on carbonate hardgrounds produced by chemosynthetic communities metabolizing nutrients from natural hydrocarbon cold seeps along reactivated faults and segments intersections. A hydrothermal feedstock for these communities is possible but less likely due to the distance to the nearest volcanic bodies. Once the substrates are in place the mounds growth is not directly dependant on ongoing cold seeps and could be as well related to the specifics of hydrodynamics in the areas.

Results from helicopter VTEM time-domain electromagnetics that include aeromagnetics and gamma ray spectrometries and later ZTEM natural field helicopter electromagnetics are compared over the Dolly Varden Mine region that hosts both potential VMS Pb-Zn base metal and low sulphidation epithermal silver mineralization, beyond the known vein-type Ag deposits and showings.

There are few well-defined discrete targets within the VTEM data set. The magnetic data have defined a network of older fault structures trending NNE, ENE, WNW, and NW. These structures are interpreted to be related to extensional basin formation. Prominent in the radiometrics is a potassium anomaly over the Red Point area, consistent with a quartz-K-feldspar-chlorite-pyrite zone, interpreted as a VMS feeder. ZTEM resistivity and magnetic geophysical anomalies suggest the presence of broad, generally flat lying resistive and magnetic units at depth. At Red Point and along the Tiger-Evindsen Corridor, ZTEM displays moderate to high resistivity and low magnetics, which suggest the presence of strong potassic-silicic alteration, related to low sulphidation epithermal systems.

The airborne geophysical results over the Dolly Varden mine region provide valuable insights on the detectability of similar Ag rich Eskay Creek type HS VMS and Brucejack style LS epithermal deposits. The principal VMS deposits seem immune to clear or discrete identification as EM conductors using VTEM, likely due to their Pb-Zn rich/Cu poor mineralogy; whereas, unlike VTEM, the ZTEM seems to clearly define high resistivity regions surrounding the known deposits that would seem to be consistent with their K-Si-altered low sulphidation epithermal origin.

Vertical seismic profiling (VSP) is commonly used in the oil and gas industry for better subsurface imaging and characterisation, as well as for providing depth calibration for surface seismic. The use of VSP in mineral exploration and mine planning is not very common mostly due to the small diameter and stability of the boreholes, as well as the relatively high cost of such surveys. These issues can be mitigated by using cheap and potentially disposable borehole sensors, such as fibre-optic cables utilised in distributed acoustic sensing (DAS).

The questions we want to answer in this work are how the quality of DAS data compares to other types of borehole measurements and what are the operational benefits and constraints for the use of this technology in mineral exploration settings. To this end, we have tested performance of DAS measurements in one of the boreholes of the Mineral Systems Drilling Program in South Australia and compared them to hydrophone measurements. The DAS measurements provide data quality that is much better than a hydrophone string, in particular it has consistent amplitudes at different depths, shows less cable and tube waves, and the reflections are much clearer. The acquisition of DAS data is quicker than any other borehole measurements that require multiple pulls of the receivers. The reduction of the acquisition time increases with the depth of the borehole. This case study demonstrates that DAS measurements show big potential for mineral exploration and exploitation.

Data from AEM surveys carried out in Norway, to support ground investigations for infrastructure projects, were used in this study. In large infrastructure projects, knowledge of sediment thickness is vital, along with information about sediment type as possible occurrence of highly sensitive clay. The acquisition systems, calibration and data processing are continuously improved to increase the sensitivity of the AEM systems.

In an area with conductive shales over resistive bedrock, the recently introduced system response method was tested. It is applied in the inversion of SkyTEM data and makes it possible to utilize the very earliest time gates, providing information about the shallower layers. The models showed to give more pronounced structures in the near-surface, reflecting true structures observed in resistivity borehole measurements. The same outcome was observed when conducting synthetic modelling.

In another setting AEM measurements were carried out along a planned road project to provide information about the extent of very conductive, possible alum shale. A volume estimate of excavated masses was sought, as alum shale poses an environmental and health risk due to the decomposition to sulfuric acid by weathering and high uranium content giving radon gas. Preliminary AEM models had a tendency to overestimate the thickness of the very resistive overburden. Experimenting with and optimizing the inversion settings resulted in models better fitting a priori information from the survey area. Limited low moment data were available due to a noisy environment. This affected the reliability of the models, illustrated by modelling and resulting real data models.

The East Kimberley Region of north-western Australia has been identified as a priority for potential agricultural development. Within this region, the Ord Bonaparte Plain is remote, with limited access in an area of great cultural and environmental sensitivity. Initially, spatio-temporal mapping using remote sensing (and potential field) data, combined with data on the deeper basin geology was used to plan an airborne electromagnetics (AEM) survey. The relatively resistive nature of the basin sediments has enabled the AEM to map the hydrostratigraphy to depths of 300-500m, except in the coastal zone affected by seawater intrusion. Two overlying aquifers, separated by a faulted, ‘leaky’ aquitard, have been identified.

The AEM and remote sensing data were subsequently used to plan a ground magnetic resonance (GMR) survey. The latter has enabled a water table map to be constructed in an area with almost no drilling, while also enabling key aquifer properties to be determined. The target aquifer has a high free water content and high transmissivity. The GMR results have been validated by drilling, borehole Nuclear Magnetic Resonance (NMR), and induction logging.

Integration of AEM, GMR and temporal (Landsat) remote sensing data has enabled rapid mapping and characterisation of the groundwater system in a data-poor, culturally and environmentally sensitive area. These data have also revealed complex faulting within and bounding the aquifer system, delineated the sea-water intrusion interface, and mapped groundwater dependent ecosystems. These data have been used to target drilling and pump testing that will inform groundwater modelling, water allocations and development decisions.

This work studies seismic response of injection of super-critical CO2 in Perth Basin, WA for the purpose of CO2 sequestration. We aim to propose the most suitable way of monitoring and verification of such storage. To this end, we generated synthetic seismic datasets based on static geological models reflecting various hypothesis about the subsurface properties and fluid flow simulations for different injection scenarios. We investigated in detail two cases:

1. Reference case - the injected CO2 remains confined in the injection interval, which we aim to characterise quantitatively;
2. Relatively small leakage (~10 kt) into the shaley overburden through a major fault, which we merely aim to detect.

Existing theories of fluid substitution predict small seismic contrasts caused by the injection. Effectively, we cannot rely on the time-lapse changes of the reflection strength, which makes conventional surface-based time-lapse seismic inefficient. However, the fluid flow simulations predict that the buoyancy-driven plumes have significant thickness to allow for the robust detection of the time shifts, which makes seismic pull-down effect more efficient for the time-lapse seismic monitoring of the injection. Furthermore, this feasibility study proves high efficiency of a surface-to-borehole monitoring systems. In particular, results of the full-waveform inversion of the synthetic borehole seismic datasets shows that such a system will allow for the quantitative characterisation of the injected plume.

Despite a recent renewal in exploration, the Ceduna Sub-basin is underexplored with high uncertainty regarding lithofacies distribution. This results in limited understanding of reservoir and top seal coupling, trends for fluid migration and structural trapping for the Late Cretaceous marine and deltaic interval. In order to address these uncertainties a stratigraphic forward model was developed to recreates the development and preservation of stratigraphic successions. Over an area of interest in the centre of the sub-basin, an early investigation tool consists in a rapid screening of the stratigraphic forward model to forecast vertical and lateral stratigraphic trends which outline structural trapping opportunities.

The model shows an overall north-west to south-east net-to-gross distribution trend and stacked reservoir-seal successions predicted in both the marine and deltaic intervals. Structural trapping potential was assessed with Shale Gouge Ratio calculation and triangle diagrams. The marine-dominated Tiger and lower Hammerhead Supersequences are predicted with the higher potential for structural traps with probability of juxtaposition and membrane fault seals. A predicted sand-rich Early Campanian nearshore marine sequence is likely to act as a migration fairways.

The success of the DHEM method in detecting the Gossan Valley mineralisation, south of Gossan Hill mine, in 2008/2009 led to the systematic application of the DHEM method across the Golden Grove lease from 2011 to 2014. The method proved successful in identifying several new zones, including the Grassi resource. During these surveys it was noted that the EM method failed to elicit either in-hole or off-hole responses in a number of holes with economic intersections of lead, zinc and precious metal ore. It became clear that not all economic ore zones contained sufficient conductive sulphide to ensure detection using DHEM. This triggered an assessment of available methods to determine if other down-hole technologies could be used to complement the DHEM method. A program of core petrophysic measurements and petro-physical borehole logging led to the realisation that because the host rocks were very resistive there existed sufficient contrast for high frequency EM imaging to be viable. This led to a trial of the Radio Imaging Method at the Xantho resource of the Gossan Hill Mine in December 2016. The results of the trial suggested direct detection of the massive sphalerite ore is possible. Further work is being undertaken to better understand the optimum survey methodology in the Golden Grove Mine environment with a view to providing specific recommendations that if approved will see the use of the method expanded on the mine leases, both at Gossan Hill and Scuddles mines, as well as on the surrounding mine leases

Using geophysical data for recognising targets for testing and accurately mapping the geology are equally dependent on petrophysics, which constitutes a link between the geologist’s largely mineralogical ‘view’ of the Earth and the geophysicist’s physics-based ‘view’.

The availability of portable petrophysical instruments and spectral scanners allow co-located multiple geochemical, mineralogical and physical property measurements and allow larger volumes of petrophysical property data to be collected, and in a better geological context, than has been possible in the past.

Many rock physical properties are heterogeneous and a large number of data is required. Accurate interpretation of the data requires analysis of the data as populations and in the context of all of lithology, alteration, stratigraphy and spatial location. This requires close integration of the petrophysical data with the geochemical and mineralogical data.

A recommended interpretation workflow will be demonstrated using two examples: ultramafic rocks from greenstone terrains and carbonate successions hosting base-metal mineralisation.

Exmouth, a regional centre located 1260km north of Perth, Western Australia relies entirely on groundwater for its water supply. Its borefield extracts groundwater from an unconfined limestone aquifer within the Cape Range Group. Groundwater flows easterly from Cape Range into Exmouth Gulf where it discharges above a saline wedge at the base of the aquifer. The current borefield extraction has insufficient capacity to meet increased water demand from population growth and tourists during holiday periods.

In 2016-2017, Water Corporation decided to investigate options for improving borefield production from the existing infrastructure. This study comprised an airborne electromagnetic (AEM) survey, a desktop review, 3D hydrogeological modelling and pumping tests.

The AEM survey and new hydrogeological modelling have established a clear relationship between the extent of the saltwater interface and the location of karstic features. The AEM survey effectively mapped the saline water distribution. It identified existing bores in thicker lower salinity areas and away from the saline wedge. Twenty-four hour pumping tests of these bores were undertaken, producing flow rates much higher than the established normal production rates. Hydrogeological modelling indicated that these bores could accommodate substantial additional sustainable production. The AEM survey and study also identified bores in areas of higher conductivity and salinity where extraction rates should not be increased, or should be reduced.

This study reports the results of a comparative evaluation of 1D, 2.5D and 3D AEM inversions for resolving hydrostratigraphy and structural elements in two contrasting settings: unconsolidated Quaternary floodplain sediments affected by Neogene deformation; and a tectonically inverted Palaeozoic sedimentary basin.

Previous studies have demonstrated the importance of airborne electromagnetic (AEM) data optimization to ensure that key elements of the hydrogeological system, including geological faults, are appropriately represented in inversion models. In the inverted sedimentary basin study, 1D inversions of AEM data indicated greater structural complexity than previously known. Initially, a suite of equivalent 1D inversion models produced very similar inversion model results. However, 2.5D inversions produced a disparity in solutions in key locations. To resolve these differences, 3D AEM inversion methods have been trialled. In the second study (floodplain setting), 3D inversions have helped resolve the geometry of hydrostratigraphic units and tectonic elements (folds and faults). In both study areas, independent validation of inversion results has involved an inter-disciplinary approach incorporating a range of borehole and ground geophysics techniques (e.g. passive seismic and Ground Magnetic Resonance (GMR)), tectonic mapping and analysis, hydrochemistry and drilling.

In summary, comparative evaluation of 1D, 2.5D, and 3D AEM inversions in two contrasting settings demonstrates the importance of optimizing inversion procedures, taking into consideration all available geological, hydrogeological and tectonic data. The benefits of using 2.5D and/or 3D inversion procedures are particularly evident in areas of structural complexity. Confidence in 3D inversions is maximised when all elements of the system response are modelled appropriately.

Aside from historical issues of mechanical durability and efficiency, the design of marine vibrators (MVs) for towed streamer operations are confronted by several practical challenges to their different possible applications: 1. Alternatives to conventional air gun arrays for flexible and creative acquisition geometries, 2. Low power alternatives to air gun arrays for environmentally sensitive applications, and 3. High power ultra-low frequency sources specific to Full Waveform Inversion (FWI) optimization.

One relevant consideration to MV operations is the volume of water that must be displaced per cycle to achieve a desired Sound Pressure Level (SPL); increasing exponentially as the frequency of interest decreases, and becoming significant at frequencies less than about 5 Hz. This is particularly relevant for FWI optimization as the frequencies of interest are in the range of 1-6 Hz. Another consideration is that ultra-low frequency output theoretically benefits from deeper towing enhanced by the well-known free-surface ghost effect, but in practice, deeper towing is confronted by an air spring effect that increases the force required per cycle to generate a desired SPL, and is due to the surrounding hydrostatic pressure at depth.

Environmental motivations to develop low power vibrator concepts are driven by regulatory restrictions upon received SPL and Sound Exposure Level (SEL), and we demonstrate how low power MVs can be configured to yield low SPL and SEL metrics without compromising geophysical performance-in contrast to the use of air gun arrays with few elements. MVs enable independence from large compressors and may be more easily deployed in spatially distributed geometries than air gun arrays. We present examples of flextensional MV development verified by numerical modelling, tow tank testing, and field verification; collectively supporting the principles discussed herein.

In natural reservoir rocks, fabric heterogeneity can further induce heterogeneous geometrical distribution of immiscible multi-phase fluid mixture, since fluid migration may be affected by lithological variation (mainly permeability) in geological time scales, causing patchy saturation of fluids. Both structure heterogeneity and patchy-saturation can lead to strong seismic wave dispersion and attenuation. In this work, a double double-porosity model is presented to describe the overlapping effect of the two heterogeneities on wave dispersion and attenuation. The wave propagation equations are derived from the Hamilton’s principle, and the numerical results for a tight sandstone are compared with corresponding low-frequency experimental data, which shows good agreements. This new model allows for a comprehensive description of wave propagation process in highly complex reservoirs.

Reservoir and geomechanical analysis of Kyalla Formation core and wireline logs acquired at Beetaloo W-1 indicate the presence of two potential Source Rock Reservoir (SRR) intervals; the middle Kyalla SRR and the lower Kyalla SRR. The key properties of the SRRs include TOC 1-3%wt, PHUT 5-10%BV, SWT 30-60%PV, free gas porosity 2-5%BV, and 20-40scf/ton adsorbed gas content. A Poissons Ratio of 0.15-0.2 and Young’s Modulus of 30-35GPa suggest favourable geomechanical properties for effective hydraulic fracture stimulation. The lower Kyalla SRR has the greatest potential as it displays consistently positive geomechanical properties over the entire SRR interval as well as consistently high free gas porosity (>2-3%BV). Mudgas and core analysis indicate that the reservoir hydrocarbon phase is likely to be a wet gas. Potential retrograde condensation may yield significant condensate and LPG fractions from the produced gas stream.

The positive reservoir and geomechanical properties within the Kyalla Formation SRR intervals at Beetaloo W-1 suggest further appraisal work is warranted to determine the deliverability of the SRRs and to determine what LPG and Condensate yields can be recovered at the well head.