ASEG Extended Abstracts - 2nd Australasian Exploration Geoscience Conference: Data to Discovery, 2019

We present initial results from a physically self-consistent 3D geomechanical model for the Fitzroy Trough in the Canning Basin, Western Australia. Our research aims to improve the understanding of in-situ stresses in the trough, in particular the overall stress regime and spatial changes of the stress magnitudes due to basin geometry. We investigate the in-situ stresses first by utilising the classic “one-dimensional” approach to geomechanics and second by building a 3D geomechanical-numerical model. The boundary conditions of the 3D model are calibrated against stress data from wells within the trough. We show that the resulting overall stress regime is most likely strike-slip, although stress data from a small subset of wells would suggest a reverse faulting regime when considered in isolation. Further, basement depth and distance to basin boundary influence the stress magnitudes and lead to significant lateral variability of the stress magnitudes.

This paper reviews three different collaborative approaches that have been applied to acquire precompetitive geophysical data in the Northern Territory. The collaborative structures involve the resource exploration industry, Northern Territory Geological Survey (NTGS) and Geoscience Australia (GA), classified as Industry-led, NTGS-led and GA-led dependent on which group takes the lead role in survey design and project management.

Each collaborative structure has different benefits, trading off between project targeting, impact and scale. All three collaborative approaches have successfully contributed to increasing the coverage, resolution, quality and variety of pre-competitive geophysical data within the NT and will continue to be refined and applied into the future.

Timor’s Lolotoi Metamorphic Complex outcrops regionally on Timor and is critical to the interpretation of hydrocarbon prospectivity, which is obscured by complex geology. Reprocessing of 1994 data by conventional DMO workflow in 2017 improved imaging of relatively undeformed coastal Neogene basins, and hinted at coherent energy in the deeper section. Seeking further improvement, Timor Resources engaged Geomage whose Common-Offset Multi-Focussing (CO-MF) processing is a successful technical solution for low-fold data and in structurally complex areas. The multi-focussing workflow is described, with additional improvements gained by a novel method to remove linear noise.

The vastly improved imaging provided by CO-MF processing has been interpreted by integration with surface geology mapping and wells. Beneath a bland zone interpreted as allochthonous Lolotoi lies reflectivity interpreted as faulted blocks within a large sub-thrust anticline and drilling target.

It is well known that inversions of gravity data are non-unique and this means that if one model can be found that fits data then there is also a set of alternative models that fit the data equally well. This non-uniqueness also extends to the choice of model parameterisation, a voxelised model is likely to explain the data as well as a model with layers with constant density and variable thickness.

A practical consequence of the non-uniqueness of gravity inversions is that a measured gravity response above a sedimentary basin can be explained by (1) sediments with constant densities above a highly heterogeneous basement or by (2) a basement with a constant density combined with variations of densities within the sedimentary strata. Understanding the sedimentary cover’s gravity response and its variability is therefore central to localise the cause for anomalies in the observed gravity response particularly in the context of whether or not they are caused by density anomalies in the basement or the sediment cover.

Here, we use stratigraphic modelling to assess the variability of sedimentary strata’s gravity response. We employ process-based numerical simulations to generate density distributions and focus on geologically plausible models. Results from a study of a 2D section in the Northern Carnarvon basin (offshore of north-western Australia) show that even with a wide range of input parameters for the stratigraphic models, the gravity response has a limited variability when compared with the gravity response of a model where the subsurface density distribution and its uncertainty is parameterised using voxels or layers. Further to this, our numerical experiments provide insight into the global sensitivity of the gravity response to stratigraphic model parameters. We find that the gravity response is most sensitive to parameters related to creation and filling of the accommodation space.

The magnetotelluric method is the most sensitive geophysical tool in detecting the hydration state of the mantle. Therefore, improved interpretations of electrical conductivity distribution within the Earth is a key measure that has to be taken in order to have a better grasp on lithopsheric-scale geodynamic concepts and the nature of mineralising agents. Progress towards this goal requires detailed comparisons between MT models and xenolith data in order to understand the controls on mantle electrical conductivity.

In this study, new magnetotelluric models from southern Africa were utilised to constrain the composition and hydrogen content by comparing forward models based on the experimental studies made on mantle minerals at high P-T conditions. Many relations between the experimental parameters and information from xenolith data were tested to improve the capabilities of magnetotellurics as an exploration tool in the lithospheric mantle of cratons.

We present a depth to basement case study which incorporates knowledge of magnetotelluric inversion uncertainty into the final interpretation.

Inversion with Occam2D and rj-McMC was conducted for over 1600 magnetotelluric sites from the Isa Extension MT survey in western Queensland. The combination of 2D deterministic inversion and 1D probabilistic inversion offered surety that the 2D data were being modelled adequately, and provided a means of estimating inversion uncertainty. Synthetic modelling was also used to establish the resolvability of the basement interface based on the resistive character of the project area; derived from downhole resistivity data.

The final depth to basement surface was interpreted jointly from the 1D and 2D inversion results. Two sources of uncertainty were independently estimated for the interpretation. (1) Uncertainty due to inversion nonuniqueness was estimated by assessing the scatter in 1D inversion results. (2) Maximum and minimum possible basement depths for the final interpretation were estimated from the probability distributions from 1D inversion combined with knowledge of basement resolvability from synthetic modelling.

Our study provides a new depth to basement interpretation with robustly determined error margins. The final interpretation is paired with maximum and minimum bounds to reflect the poorly constrained nature of the basement interface. Comparison to drilling results suggests the error estimates are appropriate. The study demonstrates the limitations of using magnetotelluric data to model depth to basement in the absence of independent constraining information.

Micro X-ray fluorescence (μXRF) is a rapid and nondestructive technique used to acquire qualitative and quantitative data at high spatial resolution (i.e. μm scale). To date this technology has yet to be utilised to its full potential in the exploration, mining and metallurgical industries. Micro-XRF instrumentation can acquire a wealth of geochemical, mineralogical and petrological information for a versatile range of sample types and sizes, due to the large sample chamber and small X-ray beam size. The non-destructive nature of the instrument combined with the minimal sample preparation required is a major advantage over other geochemical techniques and makes μXRF ideally suited for rapid and inexpensive studies aimed at investigating micromorphology, chemical variability and mineral alteration in rock samples.

In this paper, we present the findings of an ongoing case study on Lithium-Caesium-Tantalum (LCT) pegmatites, primarily focused on the Sinclair Caesium Deposit. The analyses were conducted on split diamond drill-core and highlight the quality of data that can be acquired by μXRF and the ability to accurately identify the mineralogy and quantify the chemical compositions of geological samples. The resulting element distribution and mineral maps enabled mineralogical discrimination, the identification of previously unknown mineralogical features and the detection of chemical differences at a micro-scale. Therefore, μXRF is a versatile tool that can be utilised in the exploration and mining industry for the rapid acquisition of high-quality qualitative and quantitative geochemical data on a variety of geological and environmental samples.

The Jaguar VHMS camp represents a cluster of small subsea- floor replacement style VHMS deposits sandwiched between two closely spaced, craton-scale, shear zones. Despite preserving evidence for growth faults and feeder structures, the country rocks are overprinted by two regional folding events with associated cleavage and reworking by transcurrent shearing. Understandably sulphide mineralisation is substantially modified by structure. This involved local folding and boudinage of banded tectonised ores, and remobilisation into shear zones enriched in high-tenor sphalerite within both the footwall and hanging wall rocks. Studies of ancient VHMS systems need to acknowledge that structural modification can be significant and have major implications for exploration targeting.

As part of Western Australia’s State and Coastal Planning Policies, local councils are formulating Coastal Hazard Risk Management and Adaptation Plans (CHRMAP) with the aim of rating areas of concern with a risk level and to assist in future planning of land use and infrastructure.

To assist with geological modelling of future coastal erosion a suite of geophysical methods was utilised to model current sand thickness and depth and condition of underlying limestone bedrock. Seismic Refraction, Multichannel Analysis of Surface Waves (MASW) and Ground Penetrating Radar (GPR) were utilises and all proved to provide reliable data in suitable conditions. However, all methods showed limitations which were noted to improve productivity and planning of future coastal geophysical investigations. These limitations included dealing with loss of useable data due to interaction with saline, saturated sands, inaccessibility due to topography or local scrub, and data resolution.

A combination of Seismic Refraction and MASW proved to be the most robust and reliable combination of geophysical methods with GPR utilised only when well above mean sea level and where high resolution modelling of karst and pinnacles was required. When combined with Cone Penetrometer Tests (CPT), selected methods showed to provide significant value to CHRMAP.

This study presents the results from several case studies on the application of passive seismic Horizontal to Vertical Spectral Ratio (HVSR) surveying methods for Heavy Mineral Sand (HMS) deposit subsurface layer detection for exploration and mining. The results from these case studies demonstrate the usefulness of this rapid and low cost survey method to complement HMS deposit mapping and its ability to provide additional stratigraphic information in gaps between drillholes.

HMS deposits typically occur in geological settings that are ideal for the application of the passive seismic HVSR method, because HMS deposits are typically shallow and may demonstrate acoustic impedance contrasts relative to surrounding sedimentary deposits or underlying acoustic bedrock. Trial HVSR survey results vary between different styles of heavy mineral sand deposits, from providing a direct estimate of the depth to the top of known HMS mineralisation based on a positive HVSR response from more dense and higher velocity HMS lenses, to detecting parallel silt and clay horizons, sometimes producing an inverted HVSR response, to be used as a bounding marker horizons for HMS deposits, and in many cases detecting the acoustic hard rock basement forming the base to the unconsolidated, young sedimentary deposits and basin fill containing HMS layers.

In each case study, the use of a lightweight, self-contained and simple to use seismometer has allowed HMS explorers to carry out surveys quickly and cost effectively, in some remote areas with difficult access, mostly using company field staff following a short training session. The techniques and approaches to process and model HVSR data for shallow stratigraphic mapping during these trial surveys have contributed to advancing the passive seismic HVSR surveying method to become more commonly used for large production surveys.

Standard practices in scaling time-domain induced polarization (IP) chargeability estimates are frequently inconsistent or poorly supported. Antiquated M331 units (milliseconds) and scaling standards in particular, represent: a) ill-informed assumptions, b) conflicting scaling standards and c) a lack of adequate flexibility in time-gate start-time choices for mitigating inductive (EM) coupling.

Improvements are recommended that would ensure practitioners and the exploration geophysics industry in general, employ sound and consistent chargeability scaling or calibration practices.

This ongoing research evaluates a fractured carbonate reservoir in the Beekeeper Formation, Perth Basin, focused mainly on the Woodada Field. Previous reports identified a fractured carbonate system as the main hydrocarbon reservoir in the Woodada Field, yet there is no published detailed documentation of reservoir development. The aim of this research is to evaluate the depositional and diagenetic characteristics of the Beekeeper Formation, its fracture system development and their combined impacts on carbonate reservoir quality.

This study employs multi-method geological analyses. Subsurface core description, standard microscopy, and acetate peel analyses have been conducted, while scanning electron microscopy, cathodoluminescence petrography, stable isotope geochemistry and fluid inclusion analyses are planned for the coming months.

Preliminary results show that the carbonate of the Beekeeper Formation consists of packstone, rudstone, packstone-rudstone, floatstone-packstone and packstone-grainstone. The development of the Beekeeper Formation was affected by tectonic activity. The level of influence of tectonic processes, versus diagenesis and primary sedimentary facies on the development of fracture systems, pore system generation and reservoir quality is still being studied in detail. It is anticipated that this ongoing study will increase our understanding of the Woodada Gas Field and fractured carbonate plays in general.

There has been extensive mining for hematite mineralisation in Mauritania since the 1950s, focused on the Kediat Ijil and Mhaoudat regions in northern Mauritania. These discoveries were largely made during the colonial period, with only limited additional discoveries in more recent years.

In an effort to allow the discovery of additional hematite mineralisation in the district, the Société Nationale Industrielle et Minière (SNIM) has obtained regional airborne VTEM and magnetic data and has tested a variety of ground-based geophysical methods on and near existing mineralisation.

A re-interpretation of these datasets alongside structural mapping on the ground has allowed the development of a series of conceptual models for targeting high-grade hematite mineralisation, and the development of suitable exploration strategies to locate the next generation of prospective hematite iron ore mines to develop into the future.

Scanning drill cores using combined data from X-Ray Transmission and X-Ray Fluorescence, automatically measuring the weight, and performing 3D tomography of the core at the same time, gives a high-resolution 3D visualisation of the structures, textures and mineral distribution as well as elemental and density distribution along the drill hole.

Data such as planar and linear structures can be annotated in the 3D volume and later exported into regional modelling or other software packages together with elemental concentrations and density.

Particle sizes and distribution can be exported to produce stereo nets.

An example from the European Union Horizon 2020 funded X-Mine project (2017), where a work flow is illustrated.

Limitations on the vertical resolution in seismic data and fluid-flow models challenges the accurate interpretation of time-lapse (4D) seismic signals to identify fluid saturation changes within hydrocarbon reservoirs. Conventional streamer seismic data typically have a vertical resolution of 10-20 m at reservoir levels due to the lack of both lower and higher frequencies in the seismic spectrum, while fluid-flow simulations commonly run with significantly finer-scale vertical model grid spacings on the order of meters. Recently developed seismic acquisition methods (i.e., broadband seismic) have improved vertical seismic resolution relative to conventional acquisition by enhancing both the low- and high- frequency components. We investigate whether this improved seismic vertical resolution will result in potential benefits for fluid-flow and reservoir monitoring. We apply a synthetic 3D/4D seismic forward modelling procedure to the fine-scale fluid-flow model UNISIM-I-R, developed from observations of the Namorado field, Campos Basin, Brazil. Our procedure involves 4D seismic forward modelling, seismic amplitude map extraction, and quantitative 3D and 4D seismic amplitude analysis. Our results indicate that typical broadband seismic data can identify reservoir sands approximately 6 m thick in comparison to the 10-20 m commonly recovered in conventional seismic bandwidth. The improved seismic vertical resolution is closer to the fluid-flow model vertical grid spacing, which reduces seismic interpretation uncertainties associated with upscaling/downscaling procedures and ultimately improves the reliability of reservoir model predictions.

The Carajás Mineral Province in Brazil hosts the highest concentration of major IOCG deposits in the world. Airborne and ground electromagnetic (EM) methods are frequently applied when exploring for these deposits using relatively high operating frequencies (25Hz - 30Hz). This frequency range limits the survey to effectively detect shallow, weakly to moderately conductive sources.

An airborne EM survey (VTEM) using a 30Hz base frequency has been flown by Falconbridge Ltd over the Pedra Branca deposit in 2005, identifying two discrete anomalies in the area. The high frequency nature of the VTEM system did not provide good depth resolution of the anomalies and follow-up ground EM surveys were conducted.

An orientation fixed loop EM (FLTEM) survey was carried out at Pedra Branca to compare the effectiveness of using lower frequency (2Hz) ground EM against existing VTEM and previous high-frequency ground EM surveys. The application of EM methods using frequencies of 2Hz and below enables the detection of late time conductors by recording much later in time compared to high frequency systems. These typically detect stronger, deeper conductors which are commonly related to higher grade and/or thicker massive sulfide mineralisation.

The results of this new survey significantly increased the resolution of the conductor. Modelling of the FLTEM data shows excellent correlation with the high-grade mineralisation surface, down to a depth of approximately 550m.

An FLTEM survey was also completed over the Azevedo prospect and provided a significant improvement in resolution compared to the historical, poorly defined VTEM anomaly. The FLTEM data provided an 80m lateral adjustment to the initial anomaly, allowed detailed modelling, and reduced drill testing uncertainty.

Ground and downhole EM surveys at lower frequencies are now being used extensively to effectively explore for massive sulfides with a greatly increased depth of investigation compared to historical airborne and ground EM surveys.

The future of mineral resources in Australia relies on the discovery of deposits under sedimentary cover. Traditional surface geochemistry techniques are of limited use in this context, and alternative exploration tools such as the detection of soil gases are gaining increasing interest. Previous studies have highlighted the potential of soil gases, such as sulphur gases and soil gas hydrocarbons, for locating buried mineralisation. Here, we performed laboratory weathering experiments of sulphides under sterile and non-sterile conditions to gain insights into the origin of these gases.

The experiments revealed that hydrocarbon gases could not be detected, suggesting they commonly originate from microbial ecosystems in the cover and/or in the soil. In addition, equilibrium thermodynamic predictions indicate a larger range of sulphur gases than detected, which suggests the experimental system did not reach thermal equilibrium. Our results also reveal that CS2 is the most abundant gas produced, and could be of particular interest as a pathfinder for mineral exploration through cover.

The logging and biostratigraphic characterization of the Foura Sandstone type section, Timor-Leste, show it is Carnian–early Norian (Samaropollenites speciosus Zone). Sedimentary structures indicate a turbidite origin and petrographic analysis reveals a high proportion of volcanic lithic grains.

Palynological material presents a variable preservation suggesting a complex pre-burial history involving longdistance transport. The presence of prasinophytes suggests anoxic and euxinic depositional settings.

Full Spectrum Falcon combines a Falcon AGG system and an sGrav AG system to deliver a low noise gravity dataset across all the exploration wavelengths. The noise of the vertical gravity data from an AGG system will increase as its wavelength increases, while the noise from an AG system decreases as wavelength increases. Where these two error responses meet is the wavelength to use in a pair of matched high-pas and low-pass filters to use before merging the two gravity datasets. The two systems have different noise measurements: Difference Noise for the Falcon AGG data and Test Line Repeatability for the sGrav AG data. These noise estimation methods do not allow for a common noise measurement across both systems. Using a common noise estimation is necessary to determine an appropriate cross-over wavelength. We propose a modified Odd-Even Difference Method as that common measurement of the vertical gravity noise. This approach is demonstrated using results from a recent survey flown in Victoria Australia.