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

Unconventional gas shale resources recently became a standard in the American oil and gas industry and are studied throughout the world. However, gas shales are still poorly characterised with respect to other reservoir rock types. The different parameters of organic matter (OM) in organic shales needs to be better constrained in all theircomplexity and variability. This study focuses on the influence of OM maturity on the dielectric and elastic response of shale using a hydrous pyrolysis process to increase their maturity. We then incorporated the OM in a shale-like mineral matrix that was artificially compacted under control. Elastic properties and their anisotropy were measured during compaction while dielectric measurements were acquired post-compaction. The results showed that OM affects the onset of the liquid/plastic transition of compacting sediment. The immature OM lessens the development of P-wave anisotropy. On the other end, mature OM strongly decreases the imaginary dielectric permittivity at low frequencies while it increases electrical conductivity.

Cassiterite and rutile strongly resist physical and chemical weathering and can be used as indicator minerals for mineralisation. These minerals are residually concentrated in a near-surface silcrete duricrust capping a deeply weathered profile developed over the Scuddles Cu-Zn-Pb VMS deposit, Golden Grove, Western Australia. These minerals were extracted by panning of rock chips from Reverse Circulation drill holes and studied by optical, scanning electron microscopy and mapped and analysed by electron probe. The formation of silcrete involved destruction of all labile minerals, removal of clays and leaching of alkali, alkaline earth and transition (e.g. Fe, Mn, V, Ni, Co, Cu, Zn) elements. This is followed by residual concentration of high field strength elements in resistate minerals and silica cementation. The main driving force in the formation of silcrete is the oxidation of massive sulphides generating strongly acidic and highly saline fluids. Under these conditions, silica was probably transported as either silicic acid from destabilising aluminosilicates or as an aqueous sol under neutral to low pH conditions. In addition, Au and Ag were remobilised from the massive sulphide mineralisation as a halide complex.

Cassiterite, rutile and quartz in silcrete contain inclusions of pyrite, chalcopyrite, sphalerite, galena and argentite identical to those in the underlying VMS. Thus, they are considered as indicator minerals for VMS exploration. Bioturbation and organic acids produced by decay of plants from the upper lateritic profile remobilised clays downward as colloid, recycled Au from Ag halides and attacked resistant heavy minerals. Oxidation of sulphide inclusions in cassiterite, rutile and quartz intensely corroded these minerals and liberated trace elements (i.e., Sn, Ti, Pb, Sb, Bi, Hg, Se, Ge, W, Mo and Te) to form cements of complex chemical compositions. These cements and Au-Ag halides form a multi-element anomaly in silcrete delineating the underlying VMS deposits at Scuddles.

Brine mining is an important source of elements such as potassium, iodine, lithium, and bromine that occur in solution in groundwater, typically in shallow brines occurring beneath saline or dry lakes or in deep brines in sedimentary basins. Where feasible, brine mining is an attractive alternative to conventional mining due to lower surface and environmental impact and lower OPEX than conventional mining operations.

As with any resource, evaluating brine deposits requires developing an understanding of how much resource is present and how it can be most economically produced. How much resource is present is a function of the bulk aquifer volume, the specific yield, and the brine composition, while the primary subsurface control on economic production is hydraulic conductivity, which dictates the rate at which the brine can be produced to surface. Specific yield and hydraulic conductivity are analogous to the free fluid volume and permeability quantities that are of interest in oil and gas resource assessment.

Borehole magnetic resonance has been applied in the oil and gas industry for the evaluation of bound and free fluid volumes and permeability for over twenty years. These same methodologies are equally applicable in the evaluation of brine deposits, however the hypersaline brines that are targets for commercial development cause highly conductive borehole environments that can be extremely challenging for magnetic resonance measurements. Nevertheless, use of borehole magnetic resonance measurements to help evaluate a sulphate of potash brine deposit currently under assessment shows that such measurements can be employed successfully in these environments.

Sub-Audio Magnetics (SAM) is a rapid sampling survey technique capable of simultaneous acquisition of data related to the magnetic and electrical properties of the earth. SAM surveys have historically been used for the acquisition of high resolution magnetometric resistivity (MMR) and total magnetic intensity (TMI) data. Recent developments in SAM receiver instrumentation and signal processing have now made the extraction of electromagnetic (EM) data possible due to the exceptionally high data quality now being achieved. SAM data are acquired from a moving platform which makes the technique amenable to applications ranging from ultra-detailed, man-carried or vehicle-towed ground surveys to helicopter-borne acquisition.

This paper describes ground-based and helicopter-borne SAM Fixed Loop EM (FLEM) trials conducted over the Forrestania EM Test Range in Western Australia and compares the results with conventionally-acquired (stationary) SAMSON surveys. The trials have demonstrated that SAM FLEM surveys are able to detect high conductance ore bodies at significant depth from a moving survey platform. In either ground or airborne acquisition mode, the SAM technique is shown to be a significant advance toward reducing the cost of deep exploration for high conductance orebodies.

The principle aim of the research was to overcome the challenges faced by modern geophysical data analysts, particularly those working with large multivariate datasets using Self Organising Maps (SOM). SOM is an unsupervised learning technique for multivariate data, which works by taking multiple geophysical datasets for an area of interest, and integrating them to illustrate trends. Once developed, our method drastically lowered the time required for an analyst to examine and identify trends and relations across a broad range of geophysical, geochemical and other data layers. It also revealed hidden relations and distinct populations within correlated layers.

Our study shows that SOM continues to be a powerful tool in accelerating the interpretation process. This includes the separation of features into distinct geological units, even without any preliminary map inputs to the SOM process. It also highlights SOM’s ability to highlight variation in cover, which has been identified as a key aspect moving forward in Australia’s mining future, when considering the vast expanses of Australia covered in sub cropping rock. In the future as data continue to grow and overlap, SOM will play an important role in highlighting these relations in soil cover and outcrop geology.

Geophysical web applications are highly underrated. There are many potential benefits for transitioning to a cloud based web applications including compatibility improvement, low on-going operating costs and better access to existing third-party code and tools for facilitating rapid development. For users, time spent on installation, general IT maintenance such license management and upgrading is minimized.

While web applications for business style applications are deeply established, scientific applications on the web are only just emerging. This research builds upon our previous feasibility study (Pethick and Harris 2015), which was the development of a 1D MT web application. This is to be redevelop into a commercial grade cloud hosted geophysical inversion web app that can cooperatively invert seismic and magnetotelluric (MT) data. Our software, ECloud, is designed to be user friendly where geoscientists and drillers can upload MT field data directly from their laptop, tablet or mobile to obtain subsurface geo-electrical distributions quickly with minimal input. This application is designed to be scalable, suiting cloud environments and is currently hosted on an Amazon EC2 instance. The preconceived notion that web applications are slow will be challenged. The purposefully designed MT algorithm and software structure will hopefully result in lower computation times while minimizing restrictions based on hardware requirements (i.e., primary memory).

Exploration fault seal analysis of prospects is often focused on generating a probability of success. This risking considers sealing hydrocarbons against faults over geological periods of time, rather than production time. Typically the risking is based on cross-fault juxtaposition and/or sealing shale development on the faults, on a single “best” technical model, commonly referred to as a deterministic model. Considerable work has been done by a number of workers to calibrate the sealing shale development, for example, the Shale Gouge Ratio (SGR) algorithm, to predict free water contacts. These calibrations involve back-calculating the seal potential as SGR and determining a resulting across fault pressure difference (AFPD), to trap an observed free water level. Importantly, this back-fitting of SGR and AFPD has been conducted on single “best” technical models. In general, application of SGR methods on sealing across faults in prospects increases predicted column heights and enhances pre-drill chance of success. Prospects with large columns are typically generated and then discounted through geologic risk factors. If wells do not find the predicted columns, this is often “explained” by lack of charge or trap breach. It is proposed that the fault and stratigraphic uncertainties are significant and need to be included in the modelling of fault seal risk and inferred column heights. A process of model validation will be presented in which observed free water levels are compared with the results of single “best” technical versus probabilistic models for both juxtaposition and SGR. Case studies from the Gippsland, Taranaki, Otway & Southern North Sea Basins show that probabilistic models can accurately predict free water levels (sub 10m accuracy) and identify leaking faults. Probabilistic models better predict free water levels and are thus better define prospect fault seal risk than models such as SGR based on back-calculating from single ‘best” technical models. Incorporating uncertainties in a stochastic analysis typically yields smaller but much lower risk traps, rather than high risk traps based on overly optimistic calculations. Applying these models and methods to fault seal analysis will allow explorers to better define risks and rewards on prospects.

The Pelarang Anticline is part of the NNE-SSW oriented Samarinda Anticlinorium, a detached thrust-and-fold belt in the Tertiary Kutai Basin. Results from an airborne gravity gradiometer survey over the Pelarang Anticline are presented herein. The Pelarang Anticline is interpreted as a detachment fold ~30km long with steeply dipping (70°-80°) flanks. However, seismic imaging on existing 2D data is poor.

In October 2016 Cue Energy acquired airborne gravity gradiometer survey data over the anticline. The survey revealed a large (~10mGal) gravity signal range, and that the anticline is associated with a strong, positive gravity anomaly. Subsequent application of potential field enhancement filters clearly delineated the crest and the flanks of the feature.

2D modelling of selected profiles across the anticline suggests that it can be modelled as a 1,500m-2,000m wide, by ~2,000m high shale body that is close to breaching the surface in places. This is in alignment with an interpretation that the feature is cored by high-pressure shales, resulting in un-prospective areas.

However, 3D modelling has revealed significant along-strike variations in the depths to the crest of the anticline, suggesting the presence of several anomalous structural lows. Further investigation suggests these features are pull-apart mini-grabens, formed in response to localized shear movements. At least two commercial hydrocarbon accumulations, Sambutan and Mutiara, appear to be genetically related to the newly recognized structural anomalies.

This survey has led to the recognition of a new exploration play in the region, and provided a tool to pursue it.

Recent exploration studies on the stratigraphic framework, depositional environment and tectonic setting of the Mesoproterozoic Roper Group in the McArthur Basin, have led to an improved understanding of the highly prospective organic-rich shales of the Velkerri and Kyalla Formations. From a review of open file drill core and well logs, several major regressive-transgressive (R-T) cycles have been identified within the Roper Group. The R-T cycles show sedimentary features typical of deposition within a clastic dominated marine deltaic setting and show marked lateral variation in facies and thicknesses. The lateral thickness variations are particularly noticeable across regional 2D seismic lines. Seismic interpretation and well correlation confirm the extensive and relatively undeformed nature of the Velkerri and Kyalla Formations in the subsurface, they appear thickest in the southeast of the Greater Beetaloo Basin.

The oxidation of Fe-containing sulfide mineralization can lead to the development of natural galvanic cells that are characterized by a reduced acid chimney above the mineralization itself and pH-redox controlled chemical haloes surrounding the chimney. Such chimneys may extend through overlying transported regolith cover of varying thicknesses and composition, altering regolith mineralogy (changes to clays, loss of carbonates), element distribution patterns and geophysical characteristics such as electrical conductivity (EC). This paper compares EM and geochemical profiles in transported regolith over porphyry style Au-Mo-Cu mineralization at Mandamah (NSW) and MVT style Pb-Zn mineralization at Pitchi (Iran).

At Mandamah, mineralisation is buried under 30 m of residual regolith and a further 50 m of re-weathered alluvium. Mineralised zones that intersect the base of weathering are characterized by low EM and EC in the upper 6 m of the transported regolith profile. This can be related to changes in clay mineralogy and the destruction of carbonates caused by an acid chimney that formed during a “prograde” phase of regolith development. These patterns are also variably reflected in soil pH and the distribution of selectively-extracted Ca, Mg, S, Ba and REE in the upper part of the transported regolith profile, but generally not in the elements of economic interest (Au, Cu or Mo). At Pitch carbonate-hosted Pb-Zn mineralisation is covered by 80 m of barren dolomitic sedimentary rocks and a thin layer of alluvium and colluvium. There is an increase in the EM and EC as well as Na and S in the transported regolith above mineralisation and adjacent low EC values, but again no detectable patterns in the main ore-related metals. The combined geochemical and (EM) geophysical approach provides a new approach to exploration in areas with economically prospective geology but where mineralization is buried by various forms of cover including transported regolith.

Recent development and re-design of the Helitem helicopter time domain system have resulted in the release of the Helitem3oc and Helitem35c systems, both of which employ MultiPulse^™. We describe the Helitem30C system, which was designed for efficient operation and is applicable to the majority of TEM surveys. The Helitem35C was designed to increase sensitivity to both resistive and conductive targets (increased geologic bandwidth) while achieving maximum power (and depth). A redesigned receiver suspension system significantly reduces coil motion at low frequencies, allowing for 12.5/15 Hz operation with no increase in coil motion noise, important for exploring under cover. We show case histories using both systems to illustrate their performance.

Random Forests, a supervised machine learning algorithm, provides a robust, data driven means of predicting lithology from geophysical, geochemical and remote sensing data. As an essential part of input selection, datasets are ranked in order of importance to the classification outcome. Those ranked most important provide, on average, the most decisive split between lithological classes. These rankings provide explorers with an additional line of reasoning to complement conventional, geophysical and geochemical interpretation workflows. The approach shows potential to aid in identifying important criteria for distinguishing geological map units during early stage exploration. This can assist in directing subsequent expenditure towards the acquisition and further development of datasets which will be the most productive for mapping.

In this case study, we use Random Forests to classify the lithology of a project in the Central African Copper-Belt, Zambia. The project area boasts extensive magnetic, radiometric, electromagnetic and multi-element geochemical coverage but only sparse geological observations. Under various training data paradigms, Random Forests produced a series of varying but closely related lithological maps. In this study, training data were restricted to outcrop, simulating the data available at the early stages of the project. Variable ranking highlighted those datasets which were of greatest importance to the result. Both geophysical and geochemical datasets were well represented in the highest ranking variables, reinforcing the importance of access to both data types. Further analysis showed that in many cases, the importance of high ranking datasets had a plausible geological explanation, often consistent with conventional interpretation. In other cases the method provides new insights, identifying datasets which may not have been considered from the outset of a new project.

Hydraulic behaviour of an aquifer is defined in terms of the volumes of water present, both producible and not (specific yield and specific retention), and the productivity of the water (hydraulic conductivity). These parameters are typically evaluated using pumping tests, which provide zonal average properties, or more rarely on core samples, which provide discrete point measurements. Both methods can be costly and time-consuming, potentially limiting the amount of characterisation that can be conducted on a given project, and a significant measurement scale difference exists between the two.

Borehole magnetic resonance has been applied in the oil and gas industry for the evaluation of bound and free fluid volumes, analogous to specific retention and specific yield, and permeability, analogous to hydraulic conductivity, for over twenty years. These quantities are evaluated continuously, allowing for cost-effective characterisation, and at a measurement scale that is intermediate between that of core and pumping tests, providing a convenient framework for the integration of all measurements.

The role of borehole magnetic resonance measurements in hydrogeological characterisation is illustrated as part of a larger hydrogeological study of a coal measures unit and associated overburden. Borehole magnetic resonance has been used for aquifer and aquitard identification, and to provide continuous estimates of hydraulic properties. These results have been compared and reconciled with pumping test and core data, considering the scale differences between measurements. Finally, an integrated hydrogeological description of the target rock units has been developed.

Between 2015 and 2017, Searcher Seismic acquired 32,478 km of long offset PSDM 2D seismic data and reprocessed an additional 12,972 km of previously acquired 2D data in the Gulf of Papua, Papua New Guinea (PNG). The new data has resulted in a significant improvement in subsurface imaging and areal coverage, providing the foundation for a new integrated analysis of the region. In addition, a regional drop core geochemistry and heat flow survey provides important clues regarding the existence of working petroleum systems in the Gulf of Papua. The evaluation of these new datasets has improved the current understanding of the stratigraphy, plate tectonics, local structure petroleum prospectivity of the Gulf of Papua.

New seismic allowed identification of several depositional packages that are often bounded by regional unconformities related to the tectonic development of the area. Seismic and shipborne gravity/magnetics analyses allowed a confident identification of the following events/packages:

1. Moho event allowing estimation of the crustal thickness and differentiation between oceanic and crust and calibration of the heat flow measurements;
2. Paleozoic severely folded succession analogous to eastern Australia accretionary terrains;
3. Permian analogous to the Bowen Basin in Queensland, Australia;
4. Triassic to Jurassic succession supported by the existence of the Jurassic seep identified by the Davaria geochemical survey;
5. Presence of previously unidentified block faulted highs with Miocene reefs and carbonate platform build-ups;
6. Pliocene and younger basin sandstone floor fans; and
7. Extension of the compressional front into deep water Gulf of Papua.

These observations have been integrated into an updated plate tectonic model that predicts widespread deposition of the Permian and Triassic to Tertiary source rocks estimated to be often within the hydrocarbon generative window.

The underthrust fold belt of the Andes contains some very large gas fields in which the Lower Palaeozoic Sandstone reservoirs are buried to depths of over 4-6km. The foreland depositional environment received high amounts of metamorphic and igneous rock fragments and deep burial was accompanied by substantial tectonic folding and fracturing. The combined effects of Temperature, Pressure and time on these labile sediments have reduced typical porosities <7% and matrix permeabilities <1mD so that these reservoirs are ultra-tight and effectively unconventional. Nevertheless, the fields contain very large recoverable amounts of gas with minor liquids per well. They provide an intriguing case study that contrasts with the typical concept of a conventional petroleum sandstone prospect and pose the question how many more large gas fields fit this model?

These complex structural fields required modelling by 3D-GEO using fractured and folded simulation models. The micro-porosity and micro-permeability has been investigated by Curtin University using nano-scale special core analysis in an attempt to identify where the hydrocarbons reside in these rocks, how they migrate out on production, and how best to estimate and optimize ultimate recovery?

High technology characterization included TIMA-SEM to map mineralogy and texture at the nano-scale; X-ray Micro-CT analysis of 3D microstructure; NMR, ultra-low Helium porosity and permeability, Hg injection for capillary pressure; Elastic and Electrical Properties to tie the seismic and log data for the modelling. Tri-axial tests helped understand the structural and tectonic history and its relation to the burial history of the reservoirs.

Conventional pre-stack depth migration applied to the broadband Kraken 3D Marine Seismic Survey was unable to fully resolve short-wavelength velocity anomalies below the sea floor causing obvious imaging problems and limiting depth conversion and amplitude interpretation. Improved imaging was achieved by initiating tomography using a velocity model built by combining geomechanics with rock physics appropriate for shallow carbonates and mudrocks.

3D gravity modelling using high-resolution bathymetry and compaction trends constitutes a new approach for iteratively building a 3D geomechanical model. Effective stress is derived by applying Terzaghi’s principle within an integration (along depth) involving the model bulk and fluid densities and the vertical component of gravity (all of which may vary spatially, using more refined models).

Carbonate and mudrock rock physics models, believed to be appropriate for Neogene sediments along much of the NW Shelf of Australia were derived from abundant core and wireline data acquired during the recent IODP Expedition 356. These models provide the necessary link between effective stress and P-wave velocity with Backus averaging handling the “seismic scale” mixing of different lithologies expected in the Kraken 3D area.

Kirchhoff prestack depth migration was revisited from archived preprocessed gathers using the geomechanical model combined with Common reflection angle gathers (CRAM) to initiate tomography. CRAM gathers were very effective in the presence of multiples and complex near surface topography to achieve a better update in tomography. Heavy smoothing of velocities was imposed where sequence stratigraphic interpretation suggests only distal mudstone facies. Tomography in shallower layers was then revisited to restore geologically plausible depth structures and deliver a clear improvement in imaging relative to previous processing efforts.

The ability to easily analyse for Au in the field at meaningfully-low levels for mineral exploration has not been achievable until now. Portable X-Ray Fluorescence (pXRF) has been a game changer for explorers when it was first introduced about a decade ago. However, whereas pXRF can analyse for metals such as Cu, Zn, As and Fe at concentration levels relevant to mineral exploration, it can only effectively analyse for Au above 10 ppm and only then when interfering elements are in low concentrations. Here, new patent-pending detectORE technology is presented that can detect Au using a pXRF at five orders of magnitude lower. The method has been tested against certified reference materials and whilst not a “total” Au method like fire assay or neutron activation analysis, it provides the mineral explorer with crucial analytical results in the field; geochemical exploration relies on comparative rather than total Au results. Having data available during the execution of a soil sampling or drilling program allows the geologist to make critical decisions on where to collect next “on-the-fly”. For example, changing the drilling plan to define the extent of the mineralisation in a first pass campaign. Currently, mineral explorers may have to wait weeks or months before the results of the Au analysis are returned from the analytical laboratory, and by this time drilling is over, possibly for another year. Medium to small exploration companies can ill afford to wait that long where reporting of positive results is important to their success.

The recording of raw or streamed data, as done by CGG during MEGATEM and HELITEM surveys, allows for the extraction of passive EM responses, inadvertently recorded during AEM surveys. These include powerline responses in data sets acquired in the vicinity of strong powerlines, VLF responses in data sets recorded with sufficiently high sampling frequencies and potentially AFMAG responses in the frequency range 25-600 Hz.

The recording of the three-component AEM data allows for the vector processing of these passive EM responses, including the derivation and modelling of the tipper data. Conductivity information can be derived from the tipper data with an apparent conductivity transformation and, more rigorously, with 2D and 3D inversions that take into account the terrain’s topography.

The extraction of passive EM responses is demonstrated on a number of data sets. A powerline apparent-conductivity grid derived from a MEGATEM survey near Timmins, Canada indicates conductivity structures not evident in the corresponding active-source EM data. VLF responses derived from South American MEGATEM and North American HELITEM data show a strong correlation to topography. The former were successfully modelled with 2D and 3D inversions, and the derived shallow conductivity structures confirm and complement the information extracted from the active-source EM data.

The terrain correction is a critical component of the complete Bouguer anomaly in Tasmania. New terrain correction values for the entire Tasmanian onshore gravity database have been recalculated using automatic methods. An improved Statewide digital elevation model was produced for this purpose.

The magnitude of the terrain correction adjustment ranges as high as 23 mGal, with the average approximately 2 mGal. The size of this change is such that significant alterations to extant gravity models are indicated.