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

A detailed understanding of the frequency response of geophones is critical for the design and quality control of broadband seismic surveys. We illustrate a pragmatic methodology for accurate identification of geophone spurious resonances. We include tap-test examples from conventional 10 Hz geophones, and from 5 Hz and 10 Hz high-output nodal sensors. High-frequency noise sources, including high-voltage harmonics can interfere with the accurate identification of the spurious events. This problem can be overcome by subtraction of matched noise spectra

This study aims to highlight sources of uncertainty in the determination of vertical stress estimates using density log data and examines the compounding impacts of various workflows for calculating the vertical stress. To achieve this, we use petroleum data sets from the Moomba Gas field in the Cooper Basin, South Australia, Australia. The datasets encompass density, gamma and sonic ‘check-shot’ logs that enable stepwise analysis of overburden lithologies. The approach employed in this study helps to determine how the accuracy of vertical stress estimates can be influenced by (1), The calibration of sonic to density transforms; (2) Check-shot survey data and preparation (3) The application of contrasting methods for sonic-density transforms (e.g. the Gardner and Nafe-Drake methods) and (4) The use of filters to remove data in zones with poor borehole conditions (e.g. coals) and interpolation between voids in data. We find that the largest source of uncertainty in vertical stress determinations is the calibration of sonic to density transforms. These transforms are used to estimate the density of rock mass above which the density logging tools start recording. with the other factors contributing to uncertainty in decreasing magnitudes. Our analyses suggest that at 3 km (i.e. reservoir) depths, workflow and data processing decisions can introduce uncertainty in vertical stress magnitude determination equivalent to 3.5 MPa.

CO2 injection into clastic brine-saturated reservoirs leads to a detectable reduction of the elastic moduli of the reservoir rocks. However, quantitative interpretation of the time-lapse seismic anomalies obtained for CO2 storage projects is challenging, because the injected gas can form thin plumes with low saturated narrow streaks. That is why, the time-lapse interpretation is often limited to qualitative detection of CO2 leakages. This paper is concerned with two questions: what CO2 plume parameters can be estimated from realistic bandlimited seismic data and how noise in the data affects the quality of the estimates. To this end we perform stochastic rock physics simulations of the injection reservoirs. The reservoir realisations differ in thickness, net-to-gross, contrast between the permeable and impermeable sediments and vertical distribution of the CO2. The rock physics analysis suggests that maximum and integral value of the relative acoustic impedance changes are most sensitive to the parameters of the plume. The remainder of the analysis of the noise focuses on the survey repeatability and errors in the wavelet estimation. We show that both of the noise types strongly affect the accuracy of the timelapse inversion. The proposed workflow provided rigorous means to estimate limitations of the time-lapse seismic inversion for CO2 storage projects. It may be easily adapted to real projects and guide the monitoring system design or optimisation of data analysis workflows.

The development of Northern Australia has been identified as a national priority, with water availability being fundamental to economic development. Surface water options are limited hence identification of new groundwater resources and water banking options is essential. Over the past four years, Geoscience Australia, in concert with State and Territory partners, has been involved in focussed groundwater investigations in 10 geographic areas as well as broader regional investigations (Figure 1). New data acquisition has included airborne electromagnetics (AEM); drilling (sonic, rotary mud, air core and diamond); slug and bore testing; ground geophysics (surface nuclear magnetic resonance, microgravity, passive seismic, seismic reflection and ground penetrating radar); borehole geophysics (induction, spectral gamma, nuclear magnetic resonance); hydrochemical and hydrodynamic analysis; age dating of water and landscape materials; and mapping (geomorphic, morphotectonic, regolith, geological and hydrological). These investigations inform hydrogeological assessments and water allocation planning.

Overall, this multi-physics, inter-disciplinary approach has been critical in enabling the rapid identification and assessment of significant new potential fresh groundwater resources within tectonically inverted Palaeozoic sedimentary basins in the Fitzroy Basin (WA), Bonaparte Basin (WA-NT), Wiso Basin (NT), and Southern Georgina Basin (NT), and helped define the extents of a new groundwater resource for the town of Alice Springs (NT). Potential brackish and saline groundwater resources have also been identified in Cenozoic paleovalleys and Tertiary and Paleozoic Basins.

The work is aimed at estimating the porosity of shale from the Jhuran formation (Kachchh Basin) through non-destructive technique. Shale is a sedimentary rock formed by compaction of fine grained silt and clay sized mineral particles. In recent times, free gas is present in clay-mud shale complexes also within laminae that are enriched in bioturbation. However, a significant amount of natural gas is present in organic pores located within insoluble organic matter which is called kerogen. And also, there is still a lack of knowledge on the effect of bioturbation on porosity and pore size distribution in unconventional reservoir, especially in Indian context. The shales of Jhuran formation are characterized by low permeability on account of bioturbation. The purpose of the paper is to demonstrate use of non-destructive technique for defining effect of bioturbation on pore volume of shale sample. The Scanning Electron Microscope (SEM) images were examined for a porosity and pore size distribution analysis using an open source image processing software (ImageJ). The methodology developed allows estimation of porosity and pore size distribution from the image processing technique which validated with routine core analysis (Helium porosimeter). SEM image analysis is essential as shale properties can be directly observed and key details regarding unconventional reservoir conditions better understood.

This abstract reports on the insights gained from numerical simulations which have been used to place constraints on the nature of critical geological processes responsible for sediment-hosted base metal mineralisation in the southern McArthur Basin, Australia. The study was undertaken as part of a multidisciplinary study which also included the interpretation of new and existing geophysical datasets; detailed sedimentological and stratigraphic analysis; petrography; geochemistry; and the basin-scale numerical modelling of fluid flow described here. The integration of information from this multidisciplinary study has been used to refine and constrain these coupled 3D deformation – heat transport – fluid flow simulations to determine 1) the relative effects of deformation and heat transport (convection) in driving fluid flow, 2) how extensional versus contractional deformation effects fluid flow direction within faults and 3) conditions conducive for syngenetic vs diagenetic mineralisation. Numerical models suggest that the convective flow (and potential mineralisation) would have continued during extensional deformation, unless the deformation occurred at an extremely high strain rate, while contractional deformation would have enhanced the upward convective flow. These results indicate that it is not necessary to invoke inversion to explain upward flow of mineralising fluids in this system, although an inversion event may have enhanced mineralisation in areas of convective upwelling. Geochemistry and petrography support a diagenetic origin for the mineralisation, implying low permeability at the top of the Emu Fault which is consistent with stratigraphic and sedimentological analysis of the Barney Creek Formation. Numerical modelling confirms that such a permeability scenario would result in upwelling fluids being diverted out of the Emu Fault into the Barney Creek Formation.

The Darlot gold mine is an Archean orogenic deposit located in the world-class gold and nickel terrain of the Yandal granite-greenstone belt within the Yilgarn Craton in Western Australia.

In 2016-2017, HiSeis designed, acquired and processed a high-resolution 3D seismic survey centred on the Darlot-Centenary mineralised system with the objective of improving lithological and structural interpretation of the project area and to support targeting of mineralisation.

The capability of modern 3D seismic surveys to image formational contacts and structures in hardrock environments has had a game-changing impact on the effectiveness of brownfields exploration programs because the geometry of mineralised systems can now be directly imaged over large volumes of ground. More specifically, partially-preserved relative seismic amplitudes can allow the direct imaging of alteration associated with gold mineralisation in fertile structures, allowing discrimination between these and barren structures and dramatically improving drilling success rates.

We present the results of a structural interpretation of the seismic data which focused on identifying drilling targets prioritised with seismic amplitude variation.

Exploration geophysics and reservoir production strategies rely on a robust knowledge of reservoir petrophysical properties and their uncertainties. We have developed a new approach that not only quantifies uncertainties of petrophysical properties in a static scenario but also assesses their dynamic changes under tectonic, chemical and production loads. We address this challenge with a new Multi-scale and Multiphysics data assimilation approach integrating experiments, numerical simulations and rock physics theory. This paper focusses on data integration from molecular to laboratory scale to provide a solid physics foundation for the multiscaling approaches presented in companion papers for the larger scales.

Banded iron formations (BIFs) and granular iron formations (GIFs) are the two primary types of sediment hosted iron ore deposits and provide the primary source for iron, globally. Although both deposit types are comprised of bedded chemical sediments and chert layers, these deposits show notable differences in mineralogy and texture which can affect grade and processing behaviour.

Hyperspectral imaging technology is able to map key iron ore minerals as well as the textural relationships between these minerals. This paper presents a case study comparing the mineralogy and texture of two distinct iron ore regions across the globe from one another: the Biwabik Iron Formation in Minnesota, U.S.A. (GIF) and the Pilbara Region in Western Australia (BIF). Using hyperspectral imaging data, the mineralogy and texture of these two distinct regions are compared to one another and to traditional iron ore characterisation from core logging. The results show that hyperspectral imaging technology is a useful tool in mapping the mineralogical, textural, and grade characteristics of both BIFs and GIFs. The application of this technology in the iron ore industry would provide rapid, accurate, and cost-effective ore characterisation to meet the ever-increasing production demand.

The application of Nuclear Magnetic Resonance (NMR) to investigate the distribution of moisture in materials has not normally been applied to iron ores due to the magnetic properties of iron. However recent test work on blast hole chip samples of iron ore using the very low field Corona system has shown that it is possible to obtain useful information on moisture content. Key to obtaining useful results has been operating the Corona with “burst-mode”, replicating the style of data acquisition used in wireline NMR. The greatly increased signal-to-noise-ratio allowed for more robust inversions and more realistic representation of the moisture distribution in the samples being tested in a manageable time frame. This has allowed us to investigate the relationship between magnetic susceptibility and detected water content using the Corona.

The flood basalts and gold-bearing basal sediments of the 2775-2629 Ma Fortescue Group unconformably overlie the Mesoarchean West Pilbara Superterrane to the south of Karratha, Western Australia. Fresh exposures of the sedimentary units are lacking and their geochemical, mineralogical composition and sequence stratigraphy under cover are largely unknown. This research outlines the results of integrated downhole geochemistry and hyperspectral mineralogy of two new diamond drill holes, 4 km apart, which intersect the Fortescue group stratigraphy into the Mesoarchean Pilbara basement. This has been combined with trace element geochemistry, and high-resolution XRF mapping of representative samples provide micro-structural insights and in-situ geochemistry with textural context.

The lithostratigraphy of the holes was defined using automated geochemical logging and tessellation methods, which provide objective classification of geologic units based on geochemistry and mineralogy. Individual basalt flows within the Kylena Formation can be correlated across holes based on chlorite content and geochemistry, and a chromium-rich geochemical marker horizon has been identified at the top of the siliciclastic Hardey Formation. The granitic basement rocks show evidence of significant fluid flow are interpreted to be a southern extension of the Maitland River Supersuite, consistent with regional geophysical anomalies that show a continuous gravity low.

This dataset provides an unprecedented view into the Fortescue Group stratigraphy and its geochemistry. Correlation of this dataset with previous drilling provides insights into the regional tectonic and depositional history of the basin. The depth to basement increases from 650 m to >2200 m over ~12km N-S and the thickness of the clastic sedimentary basal Hardey Formation increases from ~150 m to 1050 m respectively. This indicates significant variation in the geometry of the basement during the deposition of the basal Hardey Formation, which was likely influenced by synsedimentary rifting. This has implications on the distribution of potential gold-bearing conglomerate facies unconformably overlying the basement, and in the Hardey Formation.

The Abra sedimentary replacement Pb-Ag-Cu-Au deposit is located in the Paleoproterozoic Edmund Basin, 900km NNE of Perth. Mineralisation at Abra has no surface expression and the deposit was discovered in 1981 by drill testing coincident magnetic and gravity anomaly highs. The deposit is hosted within siliciclastic and carbonate deposits of the Edmund Group and consists of a stratabound apron of lower grade Pb-Ag-Ba mineralisation in a laminated iron oxide and barite altered siltstone unit that overlies a funnel shaped feeder zone of chlorite altered, brecciated and veined carbonaceous siltstone containing high-grade Pb-Ag in the core, transitioning to Pb-Cu and Cu-Au at depth. As at December 2018, the Abra deposit remains unmined and has an estimated resource of 37.4Mt at 7.5% Pb and 18g/t Ag.

Mutton and McInerney (1987) and McInerney et al. (1994) described the geophysical expression of Abra, and recent geophysical survey results are presented here. Abra is characterised by discrete anomaly responses in magnetic, gravity and TDEM survey data. A +450nT magnetic anomaly is observed in ground and airborne magnetic data, which is caused by magnetite within the lower part of the stratabound zone. Dense galena, barite and iron oxide mineralisation in the stratabound zone, and galena in the feeder zone, is surrounded by low-density sedimentary host rock, resulting in a +1mGal gravity anomaly. TDEM surveys have resolved massive sulphide mineralisation as EM conductors, and petrophysical testing on core samples show this is mostly caused by galena. Inverted AMT-MT data sections resolved the deposit halo as a conductive anomaly. ZTEM data failed to resolve a distinct anomaly response. DDIP surveying failed to resolve a chargeable anomaly coincident to known high-grade mineralisation, despite significant disseminated sulphide mineralisation occurring within the deposit. An IP chargeability anomaly observed on the southern side of the deposit is thought to be associated with an alteration zone and low-grade disseminated sulphide mineralisation in a fault zone.

A 2D seismic reflection survey line resolved the deposit envelope as strong seismic reflectors surrounded by a seismically bland zone, and this is related to the significant density contrast between the high-density stratabound mineralisation in contact with low-density sedimentary host rocks, as the mineralisation and host rock have similar seismic velocities. Passive seismic HVSR surveying resolved the top of Abra as a subtle HVSR response below a flat impedance contrast horizon interpreted as weathered siltstone over diagenetic cemented siltstone.

Volcanic layers within sedimentary basins cause significant problems for petroleum exploration because the attenuation of the seismic signal masks the underlying geology.

A test study was conducted for the South Australia Government to map the thickness of volcanics and sub-volcanic geology over a large area in the Gawler Range Volcanics (GRV) province. The area is covered by good quality magnetic data. The thickness of volcanics and basement configuration was unknown as there has only been a limited amount of drilling.

The Automatic Curve Matching (ACM) method was applied to located magnetic data and detected magnetic sources within different rock units, providing their depth, location, geometry and magnetic susceptibility. The magnetic susceptibilities detected by ACM allowed the differentiation of the volcanics and the underlying basement. The base of volcanics and the depth to the top of basement was mapped along 75km North-South profiles, that were spaced 1km apart over a distance of 220km. The volcanic and basement magnetic susceptibilities and the magnetic source distribution pattern, were used as key determinants to interpret the depth to the two interfaces. After the results for each interface were gridded, the surfaces were used to produce a 3D voxel model of the volcanics. A magnetic field was generated from the model and subtracted from the RTP data to produce the field generated by the sub-volcanic geology.

Conventional seismic data are naturally mainly sensitive to the very smooth velocity variations that alter transmission traveltimes (low-model wavenumbers) and very abrupt discontinuities that cause reflections (high-model wavenumbers). Full-waveform inversion (FWI) of seismic data inherits this lack of middle model wavenumber illumination, which results into ringy artifacts in the gradients. Multiple methods have been suggested to overcome this issue. Here we tackle the problem of missing wavenumbers with a deep-learning approach. Namely, we filter out the wavenumbers that are expected to be missing from the acquisition design and then train a deep convolutional neural network to provide the missing wavenumbers trace-by-trace. We test several network configurations and several training sets derived from the Marmousi II model. The neural network shows limited capabilities in generalizing from the input data sets. We also report a tradeoff between the generalization abilities and accuracy on the training data set.

The In-Situ Laboratory Project (In-situ Lab) entails a configuration of wells at approximately 400 m depth for monitoring the controlled release of CO2 in a fault zone at the South West Hub CCS Flagship project in Western Australia. The project aims to evaluate the ability to monitor and detect unwanted leakage of CO2 from a storage complex. The In-Situ Lab consists of three instrumented wells up to 400 m deep: 1) Harvey-2 – primarily for CO2 injection, 2) a fiberglass geophysical monitoring well with behind-casing instrumentation, and 3) a shallow groundwater well for fluid sampling.

A controlled- release test involving the injection of 38 tonnes of CO2 between 336-342 m depth was conducted successfully in February 2019. Monitoring during the CO2 controlled-release test included: a) continuous downhole pressure and temperature recording in the injection well, b) recording of pressure and temperature at the wellhead and at various points in the injection system, c) regular distributed temperature measurements, d) multiple vertical seismic profiling surveys using the behind-casing distributed acoustic sensor fiber-optic cable and geophones, e) electric resistivity imaging, f) groundwater sampling, g) comprehensive soil flux and atmospheric monitoring surveys, h) collection of gas samples from the surface injection facilities, i) recording of passive seismic data close to the injection well and in the wider area around the well lease, j) downhole video camera surveys, and k) pulsed neutron and induction logging.

The In-Situ Lab has the potential to form an enduring research facility at the South West Hub to enable further research of the characterisation of CO2 migration in fault zones and the shallow groundwater environment.

Electromagnetic (EM) systems are often described with varying technical specifications and standards, making it difficult to directly compare and assess their application to practical field examples. Exemplar case studies provided by contractors, whilst highlighting system capabilities, do not necessarily help to refine the suitability of the system across different geological targets and environments. Test ranges provide an opportunity for direct and consistent comparison of multiple systems for objective assessment.

The Forrestania and Nepean EM test ranges in Western Australia consist of readily accessible land, openly available for testing by airborne, ground and downhole EM systems. Multiple conductors at varying depths beneath 10-20 Siemens (S) conductive overburden provide challenging, real-world conductive targets. Surveying using different EM systems allows for a direct comparison of system detection and resolution capabilities in a conductive regolith environment. The conductors have been well defined by drilling and provide a large range of metrics available for measurement, varying from 60-400 m in depth, 5,000-10,000 S in conductance, and with variable lateral profiles and depth extents.

Multiple airborne, ground and downhole EM systems have utilised these test ranges, and several have made their data freely available for review. These include ground methods such as moving loop EM, fixed loop EM, SAMSON, downhole EM, and helicopter systems including HeliSAM FLEM, SkyTEM, VTEM, HELITEM, HeliGEOTEM, XTEM, HoistEM and AeroTEM. The SPECTREM, and Xcite airborne systems plan to fly the test range in the near future. Comparison of the airborne results, show that most of the post-2007 systems have been adequate to good at detecting the shallow IR2 conductor at Forrestania under conductive regolith. Only the hybrid grounded loop HeliSAM system has successfully detected the deep IR4 conductor at Forrestania.

The Darlot gold mine is an Archean orogenic deposit located in the world-class gold and nickel terrain of the Yandal granite-greenstone belt, part of the Yilgarn Craton in Western Australia.

A 3D seismic survey centred on the Darlot-Centenary mineralised system was acquired in 2016-2017 with the objective of improving lithological and structural interpretation, and to generally extend understanding of the Darlot 3D mineralised system to support targeting.

The capability of modern 3D seismic surveys to image formational contacts and structures in hardrock environments can have a game-changing impact on the effectiveness of brownfields exploration programs because the geometry of mineralised systems can be directly imaged over large volumes of ground. Furthermore, because the formational and structural geometry revealed by 3D seismic also provides the primary control on the physical property variations that magnetic, gravity, and electrical or EM methods respond to, seismic interpretation provides ideal constraints and guidance on the interpretation of other geophysical data. This is particularly valuable when an objective of geophysical interpretation is imaging of second-order controls on physical property variation, such as the effects of alteration.

We present the preliminary results of an an integrated geological, petrophysical, seismic and non-seismic geophysical program to effectively support brownfields targeting in hardrock environments.

The Mineral Systems Atlas and Guide are recent online systems developed and curated by the Geological Survey of Western Australia (GSWA). The objective of the Atlas and Guide is to more effectively deliver relevant precompetitive geoscience information on mineral systems to explorers and researchers.

Recently discovered nugget gold (Au) in boulder conglomerate between the Mesoarchean West Pilbara Superterrane basement and the overlying volcano-sedimentary stratigraphy of the Neoarchean Fortescue Group in Western Australia have drawn comparisons with the famous Witwatersrand conglomerate Au deposits. Links have also been made between the Pilbara conglomerate gold and nugget Au occurrences in Quaternary colluvial deposits throughout the region. However, little is known about the origin of these as they are hitherto critically unstudied. Therefore, any genetic link is uncertain. Understanding the source and deposition of these nugget Au deposits is critical to aid further exploration in the region.

Here we present a detailed study on the texture, composition and sedimentology of the nugget Au and their host rocks. The Archean conglomerate Au is comprised of a central nugget that is overgrown by a barren chloritic halo, which is further enveloped by a wider halo of Au-bearing chlorite. The central nuggets show no evidence for sedimentary transport, and have faceted surface textures consistent with chlorite imprinting. We argue these represent a modified placer deposit, with surface evidence for sedimentation removed by partial dissolution post-deposition. The Quaternary colluvial nugget Au is hypogene in origin, with minor flattening and limited silver (Ag) leaching on their surface indicative of limited colluvial-fluvial transport from source. Furthermore, preserved facets on their surface are similar to those in the Archean conglomerate deposits.

We propose the source of the Quaternary colluvial nugget Au was a modified placer deposit within a proximal Archean colluvial-conglomerate that has been eroded in situ.