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

Gas hydrates consider as one of the prospective natural resource that can supply energy. The seismic method has successfully been applied to locate gas hydrates. Their presence is revealed by bottom simulating reflectors (BSR) which represent the seismic signature of the base of gas-hydrate saturated sediments, with a layer partially saturated with free gas below the BSR. It is observed that the seismic response of the BSR is characterized by low frequency, which it is called the low frequency shadow (LFS). The possible causes of LFS are the presence of free gas or normal moveout (NMO) stretching. In order to have a deep understanding of the low frequency shadow causes, 1D and 2D synthetic seismograms and spectrograms are computed. The 1D and 2D simulations of seismic signal are based on rock physics and numerical modelling, considered the effect of the seismic attenuation and NMO stretch. The results of numerical seismograms and spectrograms show that attenuation affects the lower interface with minimum amplitudes for lower values. The quantification of the maximum frequency is obtained as the shift of the centroid spectrum. Moreover, the non-stretch NMO corrections improve the resolution to detect the BSR layer, and a stacked trace can be achieved without loss of frequencies. As a result, using an appropriate rock physics method is required to obtain valuable knowledge about the effect of the different parameters on the wave properties.

Hydrocarbon exploration in the Exmouth Sub-basin, North West Shelf, Australia, has resulted in the discovery of a variety of oil and gas accumulations mainly in Upper Jurassic and Lower Cretaceous intervals. However, the distribution of the different petroleum system elements, including Jurassic and Triassic intervals, is poorly determined, but required for improved understanding of the complex charge history, as indicated by the variety of hydrocarbon types encountered in the basin.

As illustrated herein, various stages of processing and imaging have provided a more accurately imaged 3D seismic volume covering the entirety of the Exmouth Sub-basin. An increased signal-to-noise ratio for all depths now allows, 1) detailed picking of events within the entire Mesozoic (Cretaceous through to Triassic) section, enabling interpreting and correlating key events across the area, and 2) accurate investigation of the complexity of different aged fault networks and their relationships across the full Exmouth Sub-basin for the first time.

This large-scale multimeasurement survey provides a detailed insight into the deeper basin architecture of the Exmouth Sub-basin. The seamless volume imaged to depth allows accurate mapping which is critical to unravel the complex evolutionary history in a basin with proven and remaining hydrocarbon potential.,

A late Jurassic pulse of igneous activity occurred within the Exmouth Sub-basin, with several submarine volcanic extrusive centres evident, along with contemporaneous intrusive feeder systems that fed them. The extrusive volcanics have been mapped on 3D seismic datasets and include cone-shaped vents up to 8 km in diameter and >250 m high with flanking lava flows and volcanoclastic facies. Feeder dykes and more stratiform sill systems have also been mapped, and in some cases the former can be directly related to individual volcanic extrusive complexes. Several wells have penetrated lateral equivalents to seismically defined volcanic intervals. These wells intersected thin sequences of distal volcanic facies – invariably interpreted as “crystal tuffs” which is consistent with them being distal fall deposits from a submarine basaltic/intermediate volcanic centre. Timing of the volcanic activity has been relatively well constrained by the ages of onlapping marine sedimentary sequences. A depositional model in which mafic-intermediate volcanic extrusives were erupted into a submarine setting within intermediate water depths (~100s m) is proposed, and modern-day analogues discussed.

The tools of modern exploration developed in the post war and cold war periods are no longer delivering mineral discoveries at the required rate for future economic production. The near surface search space is becoming mature. A new exploration toolkit backed by holistic systems thinking will be required to evolve Modern Exploration 1.0 into Discovery 2.0. Significant discoveries under moderate to deep cover will require new knowledge, new industry and government collaboration, new business and socio-economic models to deliver a step change in discovery rates.

To develop Discovery 2.0 we will need to deal with both the good methods and failing thinking of the past. To be truly great at discovery in 21^st century geoscientists will have to step up from being simply good explorers of the modern exploration era.

Discovery thinking will need to embrace systems thinking, risk taking, and cope with ambiguity and uncertainty. Systematic area reduction may not be possible and exploration will rely on recognition of patterns more than anomalies. Lucky teams can be grown and forged by the right exploration leadership. Discoveries will come from good team based business decision making and risk management as much as brilliant geoscience and new technology. Better thinking and more thinking time will be critical factors.

We integrate seismic tomographic models and filtered gravity anomaly maps to investigate the compositional variations in the SCLM of Australia. We find zones of relative enrichment and depletion coinciding with the spatial extent of major thermal events. Anomalous regions of enrichment may represent subduction metasomatism of the lithospheric mantle, or refertilisation due to asthenospheric upwelling. Regions of apparent depletion signify high-degree melt extraction from a previously more enriched lithosphere.

The majority of discharged mine waste from the Mt Lyell copper mine was deposited as deltaic sediments at the mouth of the King River, Macquarie Harbour, Tasmania. These waste materials are composed of sulphide bearing tailings and are a significant environmental hazard due to their potential to generate highly acidic waters and mobilise heavy metals. This project aims to understand the internal structure of tailings deposited in the King River Delta (KRD) using coincident near surface geophysical methods, such as EM38, DC Resistivity, seismic refraction and shear-wave velocity analysis. This information is required to generate robust estimates of the volume of mine waste in the KRD, which will be used in a larger project that is assessing the environmental and economic feasibility of future tailings processing.

Inverted geophysical models are able to resolve four near surface layers within the KRD to a depth of ~20 m: (1) dry unconsolidated tailings to 2 m depth, (2) saturated unconsolidated tailings to 4-5 m depth; (3) salt water within unconsolidated sediments at depths >4-5 m; and (4) the transition to bedrock and/or consolidated sediments from >12 m depth. Unfortunately, the significant attenuation of seismic energy in the near surface and a decrease in seismic velocity with depth coupled with very low apparent resistivities of salt water prevent confident identification of the interface between mine tailings and the inferred underlying Quaternary fluvial sediments.

Geodynamic models, geological-geophysical interpretations, and isotope analysis illustrate that there are links between the nickel and gold mineral systems in the Halls Creek Orogen, Western Australia. Whole-rock Nd and Ar-Ar analysis of rocks throughout the region, when compared with the geodynamic models suggest that nickel and gold mineralization in the Halls Creek Orogen can be related to basin development and subsequent basin closure during the convergence of North Australian Craton and Kimberley Craton, respectively. Whole-rock Nd analysis confirmed the input of juvenile melts in the centre of the orogen before the 1835-1805 Ma Halls Creek Orogeny, supporting the upwelling of decompression mantle melts during the basin development. These analyses also revealed the spatial links between nickel and gold mineralization and lithological units with positive εNd values. The results of geodynamic models, geophysical interpretation, and isotopic analysis are used to understand the critical processes in the gold and nickel mineralization, which are presented by predictor maps. The GIS-based knowledge-driven fuzzy method used to integrate the predictor maps and create the prospectivity maps. Herein we show that mafic-ultramafic units prospective for nickel mineralization formed by upwelling of decompression mantle melt during crustal thinning and extension during basin development, and typically consist of the most juvenile magmas in the region. Whereas, gold deposits formed during the compressional regime and basin closure, and are located along a major shear zone separating two terranes. Another critical element that appears to be related to gold prospectivity is the presence of lithologies with a juvenile signature. In contrast to nickel analyses which are closely related to mafic-ultramafic units, the source component seems less influential when attempting to target orogenic gold deposits.

Head wave amplitudes are dominated by an extremely rapid decrease with increasing source-to-receiver separation. Nevertheless, the standard corrections for geometric spreading are usually quite adequate and are not critical with multi-fold reversed refraction data.

The head wave coefficients (HWC), which are the refraction analogue of the reflection coefficients, are essentially equivalent at coincident sources and receivers. It supports a 2D revision of the standard 1D head wave expression. HWCs can be readily computed with a variety of methods using both the measured head wave amplitudes and full waveform refraction images.

The head wave coefficient (HWC), the refraction analogue of the reflection coefficient, is a complex function of the densities and the P- and S-wave velocities in both the weathered and sub-weathered regions. In general, the HWC increases with increasing P- and S-wave velocities in the weathered layer, but it decreases with increasing P- and S-wave seismic velocities in the sub-weathered layer.

Unscaled S-wave velocities in the weathered and sub-weathered regions can be computed with the HWCs for each interface and the detailed P-wave seismic velocities in each layer, using various approximations for the HWC. In general, there is excellent agreement between the measured and computed HWCs. However, some form of traveltime-based estimate of the S-wave velocities is required to calibrate the amplitude-based estimates.

The signal-to-noise ratios (SNR) of the first arrivals are a function of the source-to-receiver distance and the head wave coefficient (HWC), the refraction analogue of the reflection coefficient. In turn, the precision of the first break traveltimes is a function of the SNR.

Stacking with the refraction convolution section (RCS) can significantly reduce the effects of geometrical spreading. However, the effects of the HWC still remain.

The time models of the base of the weathering computed with the common reciprocal method are comparable to those computed with the RCS. However, only the RCS is able to detect small thrusts within the Fresnel zone.

Reflected guided borehole radar waves can be observed when a borehole radars (BHR) is either suspended by a conductive communication cable or run in a borehole filled with saline water. They are often referred as unwanted contaminations to the conventional BHR surveying and should be avoided or suppressed. However, as a type of reflected waves, they contain geological information about the surroundings of the borehole and can be used to recognise geological boundaries such as lithological interfaces and fractures intersecting the borehole. The guided BHR reflections have different phase characteristics for stratigraphic boundaries and fractures. The reflections from both sides of a fracture have the same phase, whereas the reflections from both sides of a stratigraphic interface have opposite phases. Such characteristics enable us to differentiate fractures from bedding boundaries down the borehole using the borehole guided waves. This is demonstrated by both synthetic and real field data.

In Australia, in particular, the consensus is that most, if not all, of the world-class non-ferrous mineral deposits that are either at or near the surface have been found. This means that we need to look deeper in the new greenfield search space characterised by thicker post-mineralisation cover. But in doing so the technical and financial risk increase considerably. One way of reducing the risk to be able to make an assessment of the “economic value” of any deep orebody much earlier in the discovery process. But what will it take to be able to achieve this? The answer lies in improving existing geophysical technology and applying it in novel ways, developing more effective 3D multi-modal inversions tightly calibrated to all available geological facts, and the application of machine learning approaches that permit robust assessment of uncertainty.

The purpose of this paper is to examine the remaining oil potential of the Eromanga Basin, specifically in the Cooper region.

The key factor is the timing of dominant oil charge. If the oil moved after the late compression and anticlinal structures are critical for commercial success, then the potential is low since almost all these features have been identified and drilled. If the oil moved before the late structuring then except for a few mappable paleostructures, the oil must have been trapped stratigraphically. The late structuring role would then tilt or remobilize the oil locally. If this is the case, then the large unexplored areas of the basin with little or no late structure are just as prospective as the explored structural areas. Additionally, even the discovered oil fields may not be fully appraised as the stratigraphic variability in reservoir has not been fully accounted for.

The examination method chosen was to map and postmortem a semi-regional 9000 sq km area with 6 anticlines with numerous oil fields, near successes and dry holes. The area also has extensive 3D seismic.

It was found that a strong case could be made that all the oil discoveries were stratigraphically trapped and the late structure only rearranged some of the already trapped oil.

This conclusion not only upgrades the future exploration potential of the Basin but points the way to a fundamentally different way of mapping and high grading prospects.

Seafloor hydrothermal chimneys from back-arc basins are important hosts for metals, e.g. Cu, Zn, Pb, Ag and Au, and bear potential for deep-sea mining. A solid understanding of the distribution of metals requires appreciation of detailed mineralogy and chimney growth histories. This study reports the mineralogy and microstructures of chalcopyrite-lined conduit wall of a multi-conduit hydrothermal chimney from the PACMANUS hydrothermal field (eastern Manus basin, Papua New Guinea). New observations revealed that the conduits are dominated by thick chalcopyrite walls with bi-directional growth (towards and away from the conduit) which are bounded by a thin layer dominated by fine-grained sphalerite. Clustered pyrite grows outwards from the sphalerite substrate. The mineralogy records the early growth stage of chimneys during the initial mixing between hydrothermal fluids and seawater. Late-stage sphalerite and barite then overgrew the conduits at the waning stage. Four types of native gold are observed within the conduit walls, three of which are associated with the sphalerite-rich layer and have not been reported before. Native gold is interpreted to have precipitated by various mechanisms. This study bears important potential for searching for native gold in fossil hydrothermal chimneys.

In this study, we performed rock physics analysis and modelling in the Browse Basin, in which we analysed the relationships among elastic properties of end-member (EM) sandstone (SST) and EM-shale, and then, modelled the properties of non-EM-SST and non-EM-shale to simulate seismic amplitude responses at boundaries of realistic litho-facies.

We found that the elastic properties of SST of the basin has similar trends to those in other basins; therefore, we adopted existing rock physics relationships with minor adjustments. On the other hand, it was found that the careful consideration of mineralogy and overpressure is required in the EM-shale trend analysis. The observed data was well defined by a semi-empirical rock physics model including the effect of the volume of clay (Vcl) variation and by an “Equivalent depth method” which accounts for overpressure.

To express the elastic behaviour in mixed sand-clay systems, we adopted a “Triangular diagram model” and the established trends of defined EM facies. Simulated properties from this approach agree well with actual data from the Browse Basin.

The most northeastern Yamarna Terrane consists of deeply weathered Archean basement and is covered by weathered Permian glacial sediments and recent aeolian sand dunes. This transported cover setting and regolith-landscape influences the effectiveness of metal migration mechanisms in and through cover, and hence the selection of the best sample media for exploration. The development of authigenic ferruginous pisoliths near-surface in the aeolian sands and Permian sediments identifies proximal Au and As anomalies over Au mineralisation in the Yamarna Terrane. An arsenic anomaly was also delineated using Eucalyptus foliage over the mineralisation. Furthermore, sampling across the unconformity between Permian sediments and saprolite or bedrock also identified a Au anomaly in areas where pisoliths are transported.

Recent advances in development of automated tools and machine learning algorithms based on artificial intelligence (AI) have revolutionized our interpretation approach of big data by making it faster, more objective and more reliable than tedious manual processes.

In this paper, we show results derived from machine learning applications to the recently acquired high-resolution airborne geophysical data of Burkina Faso. The results are represented as country-wide prospectivity maps for various mineral resources including gold, uranium, base metals and strategic metals.

The new mapping products indicate that Burkina Faso has a diversified and significant mineral potential.

The application of a growing body style of inversion within the program VPmg is presented. The body begins by a seed and grows until there are no eligible neighbours or the data misfit criterion is fit. In this case, the assumptions of source location and physical property are specifically given by drillhole intersects with the body. This style of inversion is useful for reinverting data after drilling in order to produce a more realistic model. The program VPmg allows for density and magnetic susceptibly to be recovered through physical property domains. The algorithm is shown to be promising through two examples: one with density contrast and one with magnetic susceptibility. In summary, the application is another style of inversion that is added to the current hands-on approach to inversion that is performed through VPmg.

We describe how the world-class Proterozoic Century zinc deposit in northwest Queensland has been sculptured by an Ordovician meteorite impact, the Lawn Hill Impact Structure. The deposit is located at the SW edge of the crater, is dismembered by crater-related faults and is overlain by breccias (suevite) related to fall-back and resurge processes into the crater. Using drilling and newly acquired IP data, we interpret a five-fold thickening of slumped Cambrian carbonate breccias in the crater, to a depth of 600 m. The Century deposit is known to have been larger than has been so far discovered. A restoration of the post-ore faults and Zn:Pb metal ratios of the mined Northern and Southern ore blocks, indicates an original enlarged form of the world-class deposit and that parts of the orebody may have spalled into the Cambrian-filled crater. This points to significant discovery potential in an otherwise intensely explored, near-mine setting.