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

Recent exploration by Battery Mineral Resources Limited has located numerous cobalt-arsenic-(silver, gold, nickel, copper) deposits in eastern Ontario. The deposits are located within a 250 kilometre zone located north of Sudbury and trending east across the Quebec border. Deposit types include sulphide-carbonate vein systems, skarn, massive sulphide and sulphide breccia. This talk provides an update on the geology and exploration of several high-grade cobalt deposits within this belt.

Ground Magnetotelluric (MT) data acquired today are typically broadband, covering 0.001 to >1000 Hz with inter-site spacing typically at 500 to 1000 m. Airborne Z-axis tipper data (ZTEM) are sampled at higher spatial density but usually band-limited to frequencies >30Hz. We analyze a pair of overlapping 3D surveys to examine lateral and vertical spatial sensitivity.

The MT data include a 2D line and a 3D survey. The line data also has magnetic tipper data that allows for a direct comparison with ZTEM; in the overlapping frequency range the agreement between the two magnetic data sets is good, with ZTEM showing higher lateral smoothness.

CGG’s RLM-3D non-linear conjugate gradient MT-CSEM inversion engine was extended to accurately model the ZTEM data, using measured sensor altimetry data and detailed 3D topography. Both single domain and joint inversions of the ZTEM and MT data were carried out. A suite of inversions were run to test the influence of starting resistivity and regularization parameters on output models, equally for MT, ZTEM, and joint MT+ZTEM inversions to allow for direct comparison.

ZTEM single domain inversion results depend strongly on the starting resistivity value, confirming that the method maps relative variations rather than absolute resistivity values, as expected for magnetics-only measurements. Shallower lateral structure shows qualitative agreement with the MT, but at depth resistivity from ZTEM inversion is driven by model regularization only. Joint inversion improved the relatively shallow section, calibrating the ZTEM resistivities and adding continuity between the MT sites. Below around 1000m depth, the 3D resistivity model is controlled by the MT data alone. Our overall conclusion is that today’s 3D broadband MT only benefits from joint MT-ZTEM acquisition and inversion workflows in the case of sparse MT station spacing.

Regional airborne electromagnetic (AEM) surveying in the southern Thomson Orogen of southwest Queensland and northwest New South Wales highlights the efficacy of AEM for mapping near-surface Paleozoic basement rocks, the basement-cover interface, and the hydrostratigraphy of cover rocks of the Eromanga and Lake Eyre basins. Interpretations of AEM data are applied to model the basement-cover interface and answer the important question “how thick is cover” in this underexplored region, and encourage the mineral exploration industry to explore more widely. This paper shows some of the interpretation highlights from regional AEM surveys conducted in 2014 and 2016. The interpretations, when considered with regional borehole basement intersections, reduce exploration risk by demonstrating that large portions of the basement are within reach of exploration drilling.

Efficient dewatering operations depend on reliable predictions of expected inflow and likely water level behaviour. These predictions stem from the conceptual understanding of the hydrogeology of the area, which itself is derived from studies into the aquifer extents, hydraulic parameterisation and connectivity with adjacent aquifer units. Due to their isolated nature, structural controls on these inputs to the conceptual understanding are amongst the most challenging to determine.

An investigation was undertaken on structurally controlled flow into an open pit iron ore mine in the Pilbara region of Western Australia. Bore yields, groundwater salinity and water level behaviour of in-pit bores unmistakably indicated flows in excess of normal aquifer throughflow. Airborne magnetic data allowed regional identification of a potential lineament which was confirmed with high density grade control drilling results. These indicated the presence of a trough-like mineralised feature likely to enhance connection to a regional aquifer system down dip of the iron ore body, or with a fractured rock aquifer beneath the ore body. A detailed ground magnetic survey was conducted along strike from the interpreted lineament, drastically improving on the aeromagnetic data, further validating the conceptualisation of the structure and providing greater accuracy with which to target future hydrogeological works around the current pit. Subsequent quantitative magnetic modelling constrained with close spaced drilling information and downhole magnetic susceptibility logging, increased the understanding of the characteristic basement magnetic response, and encourages the use of the magnetic method for local hydrogeological studies.

A crustal-scale, fully restored section across the PNG orogenic belt reveals the Oligocene to Recent compressional deformation of the margin. The northern end of the section comprises the Landslip Metamorphics, an accreted continental terrane, separated from the main part of the Fold Belt by the Jurassic April Ultramafics and Om Metamorphics, interleaved with Eocene volcanics which together constitute an accretionary prism. Existing maps show that the suture is overlain by distal Miocene sediments indicating Oligocene docking and probable compression prior to Early Miocene subsidence. The latter is consistent with Early Miocene extension in PNG and the emplacement of metamorphic core complexes in the Sepik area, but is also related to dynamic topography causing subsidence of the whole northern margin of Australia. Neogene compression commenced around 12 Ma with ~70km shortening in the Om terrane and ~38km shortening in the Fold Belt. Existing thermochronology data indicate shortening of ~12mm/year from 12-4 Ma, but only 2.5mm/year from 4-0 Ma, consistent with a change in structural style in the Fold Belt from thrust to more ductile, fold-dominated deformation. The model also requires substantial thickening of the continental crust beneath the Muller Ranges, here represented by ‘basement’ underthrusting. Gravity modelling indicates the presence of sedimentary graben up to 10 km deep beneath the Fold Belt, which were strongly inverted, such as beneath the Lavani Valley. A key issue is when this inversion occurred, in the Oligocene or Pliocene, as this has a significant influence on the timing of hydrocarbon generation and migration.

A 75 m-thick section of Early Carboniferous Tournaisian Ungani Dolomite reservoir was continuously cored in Ungani Far West 1, an appraisal well 3 km west of the Ungani field located on the southern flank of the Fitzroy Tough in the Canning Basin. The objective of this coring program was to better understand the pore systems, mineralogy, and diagenetic history of the reservoir to allow calibration and extrapolation of the petrophysical evaluation of the reservoir at the Ungani field. Petrography, stable isotopic, XRF, hyperspectral logging, grain density and CT scan studies were conducted on the core. The core consists of a 12m overlying sealing shale and 63 m of vuggy, fractured and dolomitised reservoir. The reservoir is commonly bioclastic-rich but pervasive dolomitisation hinders recognition of earlier depositional features. The upper carbonate facies is interpreted as shallow to moderate depth marine ramp-type deposits while the lower carbonate facies is suggestive of shallow platform top settings with ‘reefal’ constructing organisms. Bio-mouldic, vuggy, fracture, cavern and inter-crystalline porosity resulting from multistage brecciation, fracturing, dolomitisation and dissolution events are all present and critical to reservoir development. Based on hyperspectral logging and thin section petrography, the reservoir is predominantly dolomite with late phase cements comprising of quartz, calcite, gypsum, anhydrite, chalcedony, dickite and pyrite. Variable grain densities that correspond with porosity have been noted throughout the core but no obvious relationship between the mineralogy and porosity can be established. Shallow to moderate burial and marine or evaporative reflux fluids are likely responsible for the pervasive dolomitisation. Subsequent leaching of calcite is also key to reservoir development.

Hydraulic fracturing in many Australian Basins, particularly the Cooper Basin has been successful in higher permeability, structured conventional plays. However, adaptation of North American strategies to Australia’s complex, and highly stressed unconventional areas has resulted in less than adequate performance to progress further investment into widespread development these resources. This presentation will explore the obvious differences between Australian and North American stress settings, and the problems manifested by those differences in hydraulic fracture containment and behaviour. Further, as more unconventional targets are attempted, complementary strategies need to be considered based on fundamental geomechanical principles, relative to these basinal environments.

This presentation will explore several problems, emerging potential solutions, and areas of ongoing research with the purpose of aiding Australia to tap into a vast supply of potential unconventional resources, which are currently under-appraised.

The nature of the geological mechanisms allowing mineralising fluids to flow from depth and form localized mineral deposits is uncertain and a matter of debate. Traditional assumptions of fluids travelling through highly permeable faults raise interesting questions about the existence of highly permeable faults at depths below the brittle-ductile transition, for example. A recent model of multi-physics oscillation can explain the behaviour of impermeable shear zones in such environments, under specific conditions where temperature sensitive endothermal reactions trigger in-situ release of fluids that lubricate the fault and lead to their reactivation. The response of such systems can be of various nature, including slow creep, one-off reactivation events, or episodic reactivation events during which the permeability increases by several orders of magnitudes and allows fluids from depth to flow upwards. In this contribution, we review briefly the previous studies on this chemo-mechanical oscillator and place the findings in the context of mineral exploration. We extend the parameter sensitivity analysis to the main two dimensionless parameters controlling the chemical reactions, the Arrhenius and Damkohler numbers, to understand how they affect the location of episodic slip instabilities in the global parameter space. We show that lower values of the Damkohler number reduce the range of Gruntfest values in which the oscillator operates and we propose some fitting relationships between the main parameters in various interesting areas of the parameter space.

During 2017 Pioneer intersected Australia’s first significant intersection of pollucite in RC drilling, resulting in the definition of the Sinclair Zone caesium deposit, within 1 of 7 identified pegmatite suites with LCT mineral affinities that occur along the Eastern margin of the Pioneer Dome, Western Australia.

At the start of 2017, the Pioneer Dome was a clean slate in respect of LCT pegmatite knowledge. Using first principals, soil sampling and mapping, successive LCT pegmatites were identified and are now drill-ready.

The Company has been at the forefront of using modern techniques such as pXRF, SWIR and RAMAN to assist in its exploration advances.

The southern McArthur Basin in Australia’s Northern Territory is host to some Tier-1 sediment-hosted base metal mineral deposits including the McArthur River Zn-Pb-Ag mine. Airborne electromagnetic (AEM) data sets have been employed as a key exploration technology in the search for these mineral systems. A geological interpretation of results arising from the use of different inversion techniques, including a 1, 2.5 and 3D methods, was undertaken on a helicopter EM data set acquired over a structurally complex sediment package in the Batten Fault Zone north of the McArthur River Mine. The exploration targets were conductive, mineralised units (HYC pyritic shale member) associated with the Barney Creek Formation. Results from this study suggested that although the model fits were good, the derived conductivity models for the 2.5D and 3D inversions appeared to be smooth representations of geological reality, particularly when compared with data from drilling and surface geological mapping. Superficially, the 1D smooth model layered Earth inversions appear to map geological variability and structural complexity in greater detail even though the structures are more 3D in nature. IP effects are observed in the data and influence the modelled structure, but can be accounted for and complement the non IP 1D inversion results. The outcome of this study also indicates that when employing higher order inversion methods in the interpretation of AEM data sets, there may be significant benefit in asking a contractor/consultant for 1D inversion results as well. In the resulting interpretations if conductors appear in one but not the other, it is worth asking the question why?

Potential sites for up to sixteen stratigraphic boreholes were selected to intersect the basement geology throughout the southern Thomson Orogen as part of the Southern Thomson Project. At each of these sites high resolution estimates of cover thickness were derived by applying refraction seismic, Audio-MagnetoTelluric, Targeted Magnetic Inversion and passive seismic geophysical techniques. Estimating cover thickness in this way reduced the technical risk associated with drilling and allows for the various geophysical techniques to be compared at each site.

A comparison of the estimates derived from the applied geophysical techniques with the actual cover thicknesses determined from borehole geological and geophysical logs, together with an analysis of the uncertainties for each method, has highlighted the effectiveness of each geophysical technique. These new data and interpretations contribute to an Explorers’ Toolkit of techniques to help reduce the technical risk to the mineral exploration industry in searching for new mineral deposits in covered terrains in general, and in particular in the underexplored terrain of the southern Thomson Orogen.

The various geophysical estimates highlight that the basement-cover interface throughout the southern Thomson Orogen can be recognised by its seismic velocity, electrical conductivity and magnetic susceptibility contrasts. However, the cover thickness estimates determined by the geophysical techniques shown here provide non-coincident estimates in some cases and, as such, it is important to take into account their unique limitations and uncertainties. When comparing results from the GSQ Eulo 1 and GSQ Eulo 2 boreholes it is clear that the refraction technique produced the most accurate estimates due to a strong contrast between the velocity structure of the Eromanga Basin sediments and the basement geology throughout the study area.

Two airborne electromagnetic (AEM) surveys were undertaken in the Musgrave Province in South Australia in 2016 with the objective to increase knowledge about cover characteristics, thereby helping reduce exploration risks and to gain an understanding of the groundwater resource potential of the area. The Province is highly prospective for magmatic Ni-Cu-PGE and IOCG deposits, where a transported regolith imposes a significant challenge to exploration. Effective exploration through this region requires an understanding of that cover, its character and its spatial variability. This cover is also a source of groundwater that supports community and environment but our understanding of this resource is compromised by the limited information we have about it.

Two different systems, TEMPEST and SkyTEM, were used for the survey, each covering around 8000 line km with a line spacing of 2 km. The line spacing was deliberately chosen to provide a spatially coherent picture of the subsurface conductivity structure, particularly the buried palaeovalleys known to be present in the region. The two datasets were processed and inverted and the results assessed against known information from drill holes. Both systems map the palaeovalley systems in the area well and provide information about the location and geometry of these. Furthermore the results indicate that it is possible to map variability within the cover using AEM, as well as structural controls on the orientation of the palaeovalleys. Airborne electromagnetic surveys used in logistically challenging areas can therefore be a useful mapping tool for areas with varied but unknown cover sequence thickness and thereby reducing exploration risks, as well as increasing the information content about groundwater resources.

Rapid convergence between the Indo-Australian, Southeast Asian, and Pacific plates in the Cenozoic has resulted in a complex tectonic evolution of Australia’s northern margin. A lack of available geologic data leads to large uncertainties, such as the timing of the Sepik collision with the New Guinea margin, currently constrained to sometime between 50 and 30 Ma. Previous work suggested a link between the Sepik collision and a voluminous fast seismic anomaly presently in the mantle beneath Lake Eyre. Following from previous work, this study uses coupled plate reconstruction and numerical geodynamic models to test 50 Ma and 30 Ma collision timings of the Sepik terrane, along with an upper extent back-arc basin, to further refine our understanding of the origin and trajectory of the slab beneath Lake Eyre and address uncertainties in the plate reconstructions. The results of mantle flow models indicate that the Eocene collision timing (~50 Ma) is more likely than an Oligocene collision (~30 Ma). In addition, dynamic topography results support previous suggestions that dynamic subsidence relating to the down-going Sepik slab has influenced the evolution of the Eyre Basin, with up to ~100 m of dynamic subsidence since ~20 Ma. However, further work is required to address numerical issues relating to rapid thermal diffusion of slab material, and to investigate reasonable trench retreat velocities for intermediate (~3000 km) subduction zone lengths. This work highlights the role of numerical experiments in understanding transient plate-mantle processes and their effect on basin evolution.

Well, core and seismic data from the Lennard Shelf and Fitzroy Trough were integrated to produce a predictive sequence stratigraphic framework of the Laurel Formation consisting of Lower, Middle and Upper depositional sequences.

Significant encouragement for potentially commercial flow rates was achieved by the hydraulic stimulation of the Middle Laurel in the Eastern Gas Province (Valhalla North 1 and Asgard 1 wells) in late 2015 and in the Western Gas Province (Yulleroo) in 2010. The Laurel tight gas play extends over a 20,000km² area developed within a 2000m thick succession of marine clastics and carbonates. Condensate rich wet gas associated with overpressure is encountered regionally within low porosity and permeability sands at depths below 2000m. This was correlated across the basin from Yulleroo to the Meda Embayment and Northern Gas Province where a number of prospective plays were identified.

Upper Laurel shallow marine sandstones commonly possess good oil and gas shows and have potential for tight gas where overpressured. A prominent Middle Laurel lowstand prograding shelf slope wedge supported by the presence of conglomerates in updip wells suggest better clastic sediment supply and the potential for the development of conventional lowstand topset sandstone reservoirs in the Northern Province.

The Middle Laurel interval in the Northern Gas Province likely consists of a similar and potentially better tight gas reservoir than the interval stimulated in the Eastern Gas Province. Further drilling is required to confirm this and also to prove the viability of the tight gas play in the Northern Gas Province.

The diagnostic fracture injection test, commonly known as a DFIT, is frequently used in conventional and unconventional reservoirs (e.g., tight gas, shale gas, tight coals) to calibrate the hydraulic fracture treatment. In a normal stress regime, a single test can calibrate the in-situ stress profile and provide parameters such as reservoir pressure and transmissibility. However, in strike-slip regimes a single test cannot adequately derive strain values to develop an accurate stress profile as compared to multiple, precise, well-designed multi-DFIT program. Thus, if more consideration were given to the design process and stepwise implementation, a more robust stress profile and definitive reservoir characterization can result from implementation of DFITs in low permeability, unconventional gas reservoirs.

This presentation will define the workflow of a multi-DFIT program and the governing equations for stress profiling to allow practitioners to incorporate DFIT data with other available data to derive accurate geomechanical parameters. Further, a well-defined program can provide insight for hydraulic fracturing modelling and key information regarding natural fracturing and transmissibility for reservoir modelling. For unconventional, non-normal, tectonically-stressed reservoirs this includes defining minimum and maximum horizontal strains as well as intermediate stress values acting on known natural fracturing azimuths. Overall, a comprehensive set of recommendations and references are made for the practical application of DFITs to illustrate the overall benefit for the well design processes.

We have developed an algorithm and released open-source code for 1D inversion of airborne electromagnetic data incorporating spatial and conductivity log constraints. The deterministic gradient based inversion algorithm uses an all-at-once approach, in which whole datasets or flight lines are inverted simultaneously. This allows spatial constraints to be imposed while also ensuring the inversion model closely matches any downhole conductivity logs that are near to the flight lines. The intent of the algorithm is to improve consistency along and across flight lines by taking advantage of the assumed coherency of the geology.

Instead of roughness constraints, ‘sameness’ constraints are used. To implement these the regularization penalizes differences between the conductivity of 1D model/layer pairs and the weighted average conductivity of every other neighbouring 1D model within a user selected radius of their position. The neighbour averages are computed with inverse distance to a power weighting. The comparisons can be made over equivalent elevations. Downhole conductivity log constraints are imposed in a similar fashion, by penalizing the differences between conductivity logs, averaged over selected intervals, with their respective neighbouring 1D models. Overall the regularization encourages the final 1D conductivity models to be as similar as possible to their neighbours and to conductivity logs.

It is demonstrated with an example that the method enhances geological interpretation by improving the model’s continuity along and between flight lines, and its match to conductivity logs.

The Thirlmere Lakes include five natural wetlands within a Blue Mountains World Heritage listed national park, where there are concerns over an apparent possibility of a long term decline in water levels. Lake levels correlate with rainfall variability and are historically known to have dried several times during prolonged droughts. However, the effects of long term hydrological changes on the Lakes are unclear, as are uncertainties associated with extraction of water for local uses and the possible effects of nearby longwall coal mining.

This study is part of a large multi-disciplinary research program, of which this part focuses on groundwater conditions insedimentary strata, and the possibilities of interactions with sediments below the Lakes. Surface geophysical techniques and mapping of geological structures will be combined with deep drilling, wireline logging, geological and hydrogeological investigations. Characterisation of sedimentary strata include permeability, bulk density, moisture content, porewater stable water isotopes and XRD mineral identification. New deep drillholes are planned to obtain information on hydraulic properties of formations. A staged geophysical survey program is designed to complement the geological investigation. Resistivity imaging, time-domain electromagnetics and ground penetrating radar (GPR) will be employed to define sedimentary structure within the unconsolidated alluvium (sand, clay and peat layers) and depth estimates to the underlying rock. A combination of these geophysical methods and contextural geological information, will be used to identify structural and sedimentological anomalies and their hydrogeological properties such as permeability and connection to deeper strata. The results of this work provide new data on groundwater conditions in structured rock that underlie the sediments of Thirlmere Lakes.

As exploration moves into areas of increasing geological complexity and limited legacy data, reservoir evaluation is often based on the interpretation of one seismic image. Building a suitable velocity model followed by pre-stack depth migration plays an important role in the creation of this image on which economic evaluations are often based. In many cases drilling commitments are planned long in advance. Geologists have a good idea about the geometry and size of a potential reservoir but require accurate interpretation and positioning in the depth domain.

Typically, the amount of uncertainty associated with an image is poorly quantified. During a depth migration velocity model building project, such as shown here, we deliver a single final velocity model and its associated seismic products. The only quantitative measure of the reliability of the data would be through comparison with available auxiliary data or from analysis of volumetric residual move-out.

This may provide an indication of how well the model converges to a solution which satisfies the observations on the data. The high non-linearity inherent within the tomography used to generate the model yield multiple solution realizations. These honour the constraining data and yield the same convergence criterion. In isolation such data provides little useful evidence of the reliability of any one individual model.

We aim to rectify this by employing a workflow which assesses the uncertainty in our tomography process. This initially establishes both the resolution, and the degree to which the tomography fails to recover an implied perturbation. Using these criteria we generate a volume of models which equally conform to the observed data and derive confidence attributes assigned to the target model governing the image.

Here we present data from Taranaki Basin, Offshore New Zealand, and show how a model uncertainty workflow could de-risk exploration and development decisions.

Geothermal energy potential in China is high, and although they currently lead the way in direct heat production, geothermal power generation is still low. Hot spring analysis and surface heat flux data indicate significant potential resources for the major industrial province of Guangdong, South China. This pilot study investigates the Heyuan Fault, Guangdong, as a potential site for a geothermal power plant. Here we line out (i) preferred locations of possible hot spots on fault intersections, (ii) the possible sources of the heat anomalies, (iii) potential pathways for hot fluid circulation in the upper crust, (iv) available hot spring data and (v) the future work plan to investigate the geothermal hot spots.

We find that hot springs occur along the NE trending Heyuan Fault, clustering where NNW striking faults crosscut the Heyuan. The increased heat flow can be explained partly by radioactive decay of a large granite pluton beneath the fault, however, additional heat sources may need to be considered to explain the heat flow maxima of above 85 mWm^-2. We postulate that advective (topographically driven) and convective (deep fluids ponding at the brittle-ductile transition) processes may be operating to generate these heat anomalies.

Expansive quartz reef systems exposed on the Heyuan Fault, are proposed here, to represent uplifted sections of these deep fluid circulation patterns. A detailed systematic analysis of reef structures will reveal (i) the fluid provenance, (ii) precipitation conditions and (iii) deformation mechanisms, which will ultimately help us understand how fault intersection relations control fluid flow; which is of key significance if it can be utilised for targeting geothermal energy.