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

This case study illustrates the use of an exploration method that leverages modern exploration techniques for deep-seated VMS and Ni-Cu deposits in a 978 km² survey area in the highly prospective Areachap Belt, South Africa.

The exploration method comprises the use of a high power helicopter TEM (“HTEM”) system, a large fixed loop TEM (“FLTEM”) ground system and drilling that are employed in a staged process to keep down risks and costs and more importantly to optimize the chances of discovering viable economic mineral resources.

Based on the HTEM survey 19 high priority VMS targets were detected for ground follow up with FLTEM. The FLTEM data confirmed all of these targets, and 2D plate modelling refined the parameter for optimal drilling of the targets. The first result of the drilling campaign led to the discovery of a significant Ni-Cu deposit.

The prospect of this case study is that the employed method may be useful for certain types of mineral exploration. Provided that the electrical conductivity contrast between the mineralisation and the host rock is sufficient then the use of HTEM and FLTEM is a viable choice. The staged exploration process ensures that potential resources are detected and that risks and costs at each step are kept down.

Rapid, field-portable analytical instruments provide potentially powerful tools to support exploration for so-called battery and other critical minerals, but the effectiveness of particular techniques for identifying specific mineral species or elements is not well understood.

We have undertaken case studies of graphite, Li- and Ni-mineralized rocks in drill core and hand specimens, comparing and contrasting results from infrared reflectance (IR) spectroscopy, X-ray diffractometry (XRD) and X-ray fluorescence spectroscopy (XRF). The results allow us to recommend analytical workflows that integrate IR, XRD and XRF techniques for the most appropriate detection of a suite of battery and other critical minerals.

Understanding the kinematic processes around the formation of sedimentary basins is central to quantifying their resource potential. Building accurate kinematic models is a subjective and time-consuming process which requires incorporation of both vertical and lateral motions within sedimentary basins. Majority of plate reconstruction studies mostly focus on lateral plate motions hence, methods of subsidence analysis have seen slow evolution. So far subsidence analysis has been performed in one dimension only using borehole data and the results are interpolated or extrapolated over an area to give subsidence map of a region. This approach is prone to errors as it may not incorporate the effects of differential compaction within basins with complicated geology. We propose the introduction of an algorithm that performs decompaction and backstripping on 2D seismic cross-sections to improve the accuracy of subsidence analysis in the sedimentary basins. The decompaction component of the algorithm is based on the bisection method of a Python programming language. We applied the algorithm on an 87km wide seismic cross-section from the Exmouth Sub-basin of the Northern Carnarvon Basin in the Australian North West Shelf. The algorithm highlighted effects of differential compaction and quantified the spatial and temporal variability of subsidence within the area. One unique feature in the results is the variation of the basement morphology with time. This may be caused by faulting of the basement rocks from tectonic events and may also be due to the differential deflection of the basement from sediment loading. The tectonic subsidence periods coincide with the major tectonic events within the region as proposed by literature

Information in the third dimension is intrinsic to potential-field data. Potential-field interpretations and modelling can provide crucial regional and sub-surface geological constraints in areas considered to be structurally ‘well’ understood. These approaches should not be restricted to regions with little to no outcrop.

The geophysical interpretation of the Leichhardt River Fault Trough provides new insight into the tectonic evolution of the region that was not apparent from the geological relationships alone. Evidence for a major inversion event over a larger region of the Western Fold Belt pre-dating ca. 1710 Ma. This requires a re-assessment of how we interpret the evolution of the eastern parts of the North Australian Craton.

In the Mt Painter Inlier, geophysical inversion modelling indicates that significant additional Ordovician aged felsic intrusions occur at depth. The intrusion of this additional material in the Palaeozoic either could be the product of; or contributed to, an increased local geotherm and heat flow in the region during the Palaeozoic. There is a potential link between these intrusives and Palaeozoic hydrothermal mineralisation in the region (e.g. Mt Gee, Yudanamutana).

Carlow Castle is a Cu-Co-Au deposit situated within the western Pilbara Craton of Western Australia. Whilst Carlow Castle is the oldest discovered copper deposit in the Pilbara region, having been initially discovered in 1882, no detailed study of the ore mineralisation has ever been undertaken. After being long abandoned, a recent drilling campaign through 2018 uncovered an economically significant and geologically complex system of Cu-Co-Au mineralisation with a current resource estimate for Carlow Castle of 7.7Mt @ 1.06g/t Au, 0.51% Cu, and 0.08% Co, making it one of Australia’s most significant Cu-Co-Au deposits. This mineralisation was analysed using a variety of geochemical and mineralogical techniques in order to provide the first constraint on its genesis. This analysis suggests that Carlow Castle is a hydrothermal Cu-Co-Au deposit, with mineralisation hosted in sulphide-rich quartz veins throughout a pervasively chloritised shear zone in an Archaean mafic volcano-sedimentary sequence. Within these ore veins, the sulphide mineralogy is dominated by pyrite (FeS2), chalcopyrite (CuFeS2), chalcocite (Cu2S), and cobaltite (CoAsS). Here we present the findings of the first detailed study on the nature of the Cu-Co-Au mineralisation at Carlow Castle and propose an orogenic model for the genesis of this unique deposit. It is proposed that the orogenic event that gave rise to Carlow Castle is related to the initial assembly of the Pilbara Craton during the Archean.

The Canning Basin in Western Australia is classified as a frontier basin. Of the 345 wells drilled onshore, only eight have intercepted small commercial hydrocarbon deposits. The offshore Canning Basin is one of the world’s most underexplored Paleozoic basins with six wells in a 75,000 km² area with no commercial oil deposits discovered. To overcome the limited stratigraphic data from offshore wells, the well-constrained onshore stratigraphy has been mapped offshore through the development of a seamless onshore/offshore stratigraphic model.

The model is an amalgamation of over 5000 km of 2D seismic data, 25 petroleum wells and potential field data. Large-scale structures that cross the coastline were mapped and allowed for the transfer of stratigraphy from onshore to offshore. Key Paleozoic stratigraphic units were continuously mapped across the onshore/offshore transition and were identified to thin towards the Roebuck Basin as Mesozoic and Cenozoic sediments increase in thickness. Thick Ordovician and Devonian units that host commercial hydrocarbon deposits onshore could not be confidently identified in the offshore Oobagooma Sub-Basin due to the attenuation of the seismic signal by inferred overlying volcanic units. Ordovician sedimentary units, however, can be mapped offshore across the adjacent Broome Platform but are comparatively thin (~2 km). Geochemical data acquired from mafic volcanic units indicate that they share a similar source and are more extensive than previously reported.

This model has produced a more accurate stratigraphic model of the offshore Canning Basin. A multifaceted approach of mapping stratigraphy and structures across the onshore/offshore transition and gathering new geochemical data has provided new insights into the offshore Canning Basin that can inform future petroleum exploration activities.

Rapid field identification of mineralogy is critical for exploration and ore extraction. For LCT pegmatites, a significant challenge is presented as many dominant minerals are visually similar and many of these minerals cannot be identified by commonly used methods such as SWIR or pXRF analysis. Therefore, being able to readily identify full mineralogy, especially of key lithium bearing minerals such as eucryptite, petalite and spodumene, is invaluable.

Field portable Raman spectroscopy was evaluated, and an application developed to identify and delineate the dominant mineralogy of the Pioneer Sinclair deposit. Raman is a form of vibrational spectroscopy and can readily identify key pegmatite minerals such as quartz, feldspars, micas, Li-silicates, beryls, tourmalines etc.

Over 8,000 Raman spectra were collected from RC, diamond drill holes and grade control samples. The main mineral components were identified by spectral matching to a custom LCT pegmatite reference library and were further delineated by the extraction of key spectral features.

The mineralogical results from these samples were then applied for the identification of pegmatite zones and the development of mineralogical maps of the deposit. Raman analysis of grade control samples identified the core pollucite zone in agreement with pXRF analysis and is being used to augment geological mapping and mineral zonation definition ahead of further exploration. Field portable Raman spectroscopy was found to be a powerful tool for rapid mineral identification and will have useful applications for the exploration and mining of LCT pegmatites and other commodities.

Woodside Exploration routinely conducts large regional study projects to inform business decisions regarding new acreage, data acquisition and drilling. Whilst varied, these studies generically comprise phases of data collection and conditioning, analysis, collaboration and actions leading to assurance and finally completion. These studies are usually managed in a linear, or Waterfall, approach and time taken to complete these phases ranges from weeks to months depending upon complexity. An ‘Agile’ approach, originally developed for rapid software development, has been modified to fit geoscience studies insofar as we define a ‘Rapid Study Sprint’ (RSS) that combines the analysis and collaboration phases of the project into a facilitated workshop, at the end of which preliminary products must be completed. The RSS is analogous to a hackathon and leverages intense collaboration to drive fit-for-purpose outcomes. Woodside Exploration Australia held its first RSS in 2017 to produce a play-based exploration evaluation of the Jurassic (J50) Macedon play interval in the Exmouth Sub-basin, offshore Western Australia. In one day of intense collaboration, the RSS generated maps of equivalent quality to traditional regional studies. This paper compares the actual time and cost of this RSS with a notional project plan for a conventional regional study with the same objectives and deliverables. Comparison demonstrates that the RSS and conventional regional project has a similar ultimate duration. However, the distribution of effort and reward varies considerably; with a 40% cost saving attributed to the reduction in worked hours and delivery of usable products two weeks earlier. Savings are attributed to the core principles of Agile driving collaborative behaviours and simplifying business processes to focus effort on the highest priority areas to address the key project objectives.

The Braveheart-1/ST1 well (2009) was drilled on a pronounced Class III AVO anomaly identified on the Braveheart 2D and Braveheart infill 2D seismic data in the Browse Basin, Australia. The well was dry with no indication of hydrocarbons. During a post-drilling analysis of the well, the seismic synthetics generated displayed no Class III AVO anomaly at the well location. The failure of seismic to predict a correct AVO response at the well raised major questions around the legitimacy of AVO and Quantitative Interpretation (QI) approach for prospect generation and maturation in the Browse Basin.

In 2011, Searcher Seismic acquired the long offset (8km) Vampire 2D seismic survey in the Browse Basin that tied 60 wells (including Braveheart-1/ST1). The assessment of the Vampire PSDM (2018) data at Braveheart-1/ST1 reveals that the Class III AVO anomaly is absent. Looking back at the well result it is likely that the generated Class III AVO in the vintage 2D data was caused by insufficient acquisition parameters and an inappropriate processing workflow.

This paper demonstrates how the acquisition of long streamer seismic, with appropriate processing could help to avoid misidentification of false AVO anomalies and also help identify true AVO anomalies, which have been masked by unsuitable acquisition and processing workflows. This paper could help support a more successful drilling program.

The Vampire long streamer 2D PSDM seismic data with preserved amplitude, provides great opportunities for the geoscientist to reassess the exploration potential in the Browse Basin. Using this dataset with 60 well ties also offers profound opportunities to run a 3-term AVO Inversion for reservoir characterisation and field development planning of the main fields in the Browse Basin.

Monitoring of passive seismic sources generated by mining activities and hydraulic fracturing has an important role in hazard analysis and in development of unconventional reservoirs. Surface arrays are vastly used in such monitoring scenarios with the advantage of wider spatial monitoring aperture, thus monitoring larger volumes over downhole arrays. However, signal-to-noise ratio of surface array records is naturally low. That makes application of coherency-based techniques an appropriate option for surface monitoring.

Polarity variations corresponding to the source mechanism across the moveout curves/surfaces is a complicating task in the use of coherency-based monitoring methods to locate passive seismic events. To overcome this issue, we suggest a straight-forward approach that applies semblance, as a coherency analysis tool, on separate clusters of stations followed by averaging the results from all the clusters. To evaluate the performance of the suggested approach, we applied it on a semi-synthetic passive seismic data example generated from a reverse-oblique source and compared the result with the outcome from application of the classic coherency-based technique. It shows the ability of the suggested method to overcome polarity variations task, without conducting any polarity correction step.

Fibre optic sensing is an emerging technology enabling reliable long term monitoring and surveillance of the subsurface. In this contribution, we detail the first Australian installation of a fibre optic sensing system as part of a stratigraphic well decommissioning or plug and abandon (PandA) process including preliminary results of the cementing operations. Additionally, we propose options for long term enduring research at the resulting facility.

The transient electromagnetic method (TEM) survey was carried out within one of the private house area to allocate the fresh water reservoirs in Greece.

There are two challenges that make the problem complex. The first is noisy settings for geophysical survey: populated area with power lines, pipes, houses etc. The second challenge is uncertainties with water-bearing reservoirs.

According to geological settings and water-bearing reservoirs location, geophysical survey had to maintain penetration depth up to 250-300 m. For this task DC (direct current) methods are not applicable whereas induction electromagnetic sounding like Transient electromagnetic method (TEM) in the near field zone can show superior results. Therefore TEM survey was carried out.

To ensure the high quality of TEM data the special algorithms of electromagnetic noise attenuation were applied.

From TEM results it was found, that at a depth of about 180–280 m in the southeastern part of the study area, one can expect the presence of fresh water, and the resistance values are 80 Ω·m.

Palynostratigraphy is one of the primary methods employed to date and correlate strata on the Northwest Shelf. In this setting dinocysts provide the highest resolution for the Mesozoic, particularly for the Middle–Upper Jurassic interval. This period is characterized by a global-scale dinoflagellate radiation, which is reflected, in the Bathonian–Kimmeridigian strata of the Northwest Shelf by the highly diverse W. indotata–D. swanense assemblages

Transmitted light and scanning electron microscopy were used to document the palynological assemblages of 235 palynological strew slides prepared from conventional and sidewall cores from wells in the Bonaparte and Northern Carnarvon. Previously undescribed species of dinocyst were identified, and their geographic and stratigraphic ranges were documented. The data from all wells was then integrated to i) characterize the morphological variations of species across different basins, ii) identify common biostratigraphic events and iii) produce an updated regional biostratigraphic scheme.

Eleven new species of dinocyst and one species of acritarch with stratigraphic values have been formally described as a result of this work. An additional fourteen species, and two dinocyst genera were identified, of which four have been identified as potential biostratigraphic markers.

Most of the new species present a consistent stratigraphic distribution in both the Bonaparte and the Northern Carnarvon basins. These new markers increase the resolution of the regional biostratigraphic scheme.

The events observed in the Bonaparte Basin, increase the accuracy of well to well correlations and further characterise the evolution of the local area during the Bathonian–Kimmeridgian interval. Further investigation is likely to yield additional, significant biostratigraphic events.

The Paleozoic extensional Petrel Sub-basin within the Bonaparte Basin, Australia is relatively underexplored. Historical two-dimensional seismic data and 59 exploration wells provide a substantial residual subsurface uncertainty. Woodside’s WA-522-P exploration permit comprises 7806 km² on the western flanks of the Petrel Sub-basin in shallow-water (<150 m) that hosts proven oil (Turtle and Barnett fields). Furthermore, the Torrens-1 well located within this permit encountered Paleozoic oil shows (KUFPEC, 1994).

Polarcus, on behalf of Woodside, acquired the 2846 km² Zenaide three-dimensional seismic survey between February and April 2018. The aim of the survey was to better resolve the Paleozoic exploration potential within the north-eastern area of WA-522-P. Final pre-stack depth migrated data received in December 2018 followed preliminary (fast-track) seismic data received in July. This modern seismic data contributes to the polarisation of key subsurface uncertainties.

We report the application of an Agile approach to exploration delivering regional interpretation, gross depositional environment mapping, charge modelling and biostratigraphic analysis in support of play-based exploration attribution of risk and uncertainty prior to receipt of seismic data. Integration of play and prospect analyses will support faster portfolio maturation.

The greater Phoenix area in the Bedout Sub-basin has experienced recent exploration success on Australia’s North West Shelf (NWS). Oil and gas discoveries in the Triassic reservoirs of the Keraudren Formation and Locker Shale have revived interest in mapping the distribution and lateral facies variation of the Triassic succession from the Bedout Sub-basin into the adjacent underexplored Beagle and Rowley sub-basins. This multi-disciplinary study integrating structural architecture, sequence stratigraphy, paleogeography and geochemistry has mapped the spatial and temporal distributions of Triassic source rocks on the central NWS. The Lower-Middle Triassic paleogeography is dominated by a deltaic system building from the Bedout Sub-basin into the Beagle Sub-basin. The oil sourced and reservoired within the Lower‒Middle Triassic sequences at Phoenix South 1 is unique to the Bedout Sub-basin, compared to other oils along the NWS. Its mixed landplant and algal biomarker signature is most likely sourced locally by fluvial-deltaic mudstones within the TR10‒ TR14 or TR15 sequences and represents a new petroleum system on the NWS. A Middle Triassic marine incursion is recorded in the Bedout Sub-basin with the development of a carbonate platform while in the Rowley Sub-basin, volcanics have been penetrated at the top of the thick Lower‒Middle Triassic sediment package. The Late Triassic paleogeographic map suggests a carbonate environment in the Rowley Sub-basin distinct to the clastic-dominated fluvial-deltaic environment in the Beagle Sub-basin. This information combined with results of well-based geochemical analyses highlights the potential for hydrocarbon generation within the Upper Triassic in these sub-basins.

Relief shading has long been recognised as being an effective way of presenting data. However, the interaction of relief shading and colour is perhaps not widely understood, and many software packages do not combine shading and colour in a physically correct manner. When relief shading and colour are correctly exploited an enhancement of the perception of 3D is obtained and, at the same time, a dynamic range compression of the data can be achieved while still retaining a sense of the metric values of the data. However, on the other hand, an inappropriate use of colour has the potential to disrupt the 3D perception induced by relief shading.

To obtain the maximum benefit from combining colour with relief shading a low contrast colour map should be used and the shading pattern should be applied to the colour image multiplicatively, not via a transparency blending as is often implemented in GIS packages.

The timing and distribution of gold mineralization in Proterozoic orogens is influenced by crustal architecture which is often established long before gold mineralization occurs. Gold occurrences in such settings are commonly associated with crustal-scale faults formed at cratonic margins. Once established, these faults provide critical pathways for hydrothermal and mineralizing fluids which during repeated fault reactivations can result in remobilization or introduction of new auriferous fluids resulting in overprinting gold events. Recently published geochronological data for the northern part of the Proterozoic Capricorn Orogen in Western Australia show it has experienced at least three episodes of gold mineralization occurring at c. 2400, 1770 and 1680 Ma. Many of the gold deposits are associated with intracratonic reworking during the 1820–1770 Ma Capricorn Orogeny and 1680–1620 Ma Mangaroon Orogeny. Intracratonic settings are not normally considered prospective for gold mineralization due to a lack of input of juvenile material. However, it appears that repeated hydrothermal fluid flow during intracratonic events, has the potential to upgrade gold mineralization or increase gold endowment throughout the orogen, either through gold remobilization or through introduction of new gold, increasing the potential for economic gold deposits.

The Acropolis prospect is a vein-style magnetite (±apatite ±hematite) system located ~20 km southwest from the giant Olympic Dam iron-oxide copper gold (IOCG) deposit, South Australia. A whole rock dataset comprising 4,864 core samples from fourteen drillholes was analysed using multivariate statistics to understand and identify geochemical signatures of mineralization at Acropolis. Statistical analyses included principal component analysis, hierarchical and k-means clustering. The results of statistical analyses are overlaid and interpreted relative to a 3D implicit geological model of the prospect, and encompass a projection of mineralization signature as PC1.

The mineralization footprint of Acropolis is multi-element and typified by a distinct ‘magnetite’ signature of Fe-V-Ni-Co. Such a signature is developed in the western part of the prospect and represents samples containing >60 wt% Fe. In contrast, the ‘hematite’ signature displays an association of REE, W, Sn, Sb, U, Th, Ca, and P and is present throughout the prospect. Furthermore, kriged values of Cu (> 200 ppm) demonstrate an offset from Ferich veins, thus supporting a genetic model in which (earlier) Cu-Au-deficient veins in which magnetite is the dominant Fe-oxide are subsequently overprinted by Cu-bearing hematite-dominant mineralization.

Results obtained provide insights into the evolution from magnetite- to hematite-dominant IOCG systems and may offer a proxy for exploration and discovery of economically significant IOCG deposits at shallower levels in the eastern Gawler Craton.

Criticism of the exploration and mining industry for failing to find enough new deposits and open new mines is misplaced. Significant growth in the size of gold Resources on many mines of the Yilgarn Craton has occurred well after the deposit was discovered and after mining had commenced. Brownfields exploration is commonly (but not always) socially, environmentally and politically acceptable. It is easier to add to an operation than it is to permit and fund a new operation. The record of successful brownfield exploration in Western Australia is outstanding with 100% to 1000% additions to endowment recorded at many gold operations in Western Australia in recent years. This success more than covers the perceived, but not proven, short fall from greenfield exploration.