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

A comprehensive understanding of the sources of uncertainty is essential in stochastic inversion workflows of magnetotelluric data. Input uncertainty related to the electromagnetic noise and measurement biases can be reliably estimated statistically during processing. Uncertainties related to limitations and oversimplifying assumptions made on the physics and geometry by the forward solver employed are usually lumped into error floors in magnetotelluric inversion workflows.

Here we propose a workflow for using 1D trans-dimensional Markov chain Monte Carlo samplers for estimating subsurface conductivity and its associated uncertainty. Our methodology replaces error floors with site-specific likelihood functions which are calculated using a machine learning algorithm trained on a set of synthetic 3D conductivity training images. The learning method quantitatively compensates for the bias caused by the 1D earth assumption. This is achieved by exploiting known dimensional properties of the magnetotelluric phase tensor.

We apply this workflow to synthetic data to quantify the improvement in reliability compared to classical 1D probabilistic inversion.

A new workflow of bore data re-interpretation that increases the value of old data with only a minimal cost has been developed. The workflow estimates hydraulic and petrophysical parameters from historical bore data, providing clay volume, total porosity, effective porosity or free fluid, permeability, hydraulic conductivity and salinity profiling.

Hydraulic parameters were calculated for bores installed in the Leederville-Parmelia aquifer in the northern Perth Basin during 2018 and 2019 by using natural gamma ray, resistivity and borehole magnetic resonance logs. The bores were sampled and the laboratory analysis of water salinity was used to verify the accuracy of the computations.

Using a regression analysis to correlate the “old with the new”, estimates for these parameters were then calculated for older bores that only had natural gamma logs, resistivity logs and geological logging details available. The calculated salinity values from the historical bores were validated using existing chemistry sampling. This workflow facilitates the calculation of hydraulic parameters across a regional area and potentially reduces the timing and resourcing of investigation programs by increasing the information available from historical bore data.

Syn- and post-collision between Indian and Eurasian plates, several peripheral foreland basins were formed that accommodate the detritus from the adjoining regions. The geochronological study is a precise tool for understanding the provenance in this regards several studies have been carried out in the central and western Himalayan foreland basin, while very little in the eastern Himalaya. The present study carried out in the Siang river valley, eastern Himalaya by means of detrital zircon U-Pb geochronology. The Late Paleocene to Early Eocene Lower Yinkiong Formation and Early to Mid-Eocene Upper Yinkiong Formation, deposited in the distal foredeep and foredeep deopzone of the foreland basin respectively. The detrital ages of the Lower Yinkiong Formation are dominantly older than late Paleozoic, resembling the cratonic and early Himalayan Thrust Belt (HTB) affinity. However, there is the presence of Cenozoic age grains in Upper Yinkiong Formation indicate the HFB source and possible the granitic body within the Asian plate. This shifting of source regions signifies the India-Asia collision in the Eastern Himalaya began before or immediately after the Early Eocene (~56-50 Ma).

We discuss the implementation of multi-resolution framework to 3-D Direct Current (DC) problem. Commonly used staggered (SG) grid fixes the horizontal grid resolution for all depths. Thus, employing the fine horizontal resolution may lead to an over-discretised forward problem, subsequently affecting the performance of the inversion. We implemented a novel multi-resolution (MR) grid approach to the 3-D DC modeling and inversion problem, which allows adjustment of the horizontal resolution with depth. By using finer resolution for the near-surface regions, MR grid can ensure the modeling accuracy and describe the shallow features in the inversion model as well. The ability to use relatively coarser horizontal resolution for the deeper regions reduces the computation costs compare to the SG grid modeling. As a result, modeling and inversion can be accelerated several times by solving a smaller problem. Our grid resembles non-conformal rectangular grid, which commonly used in finite-elements modelling.

The Cunaloo Limestone Member of the Locker Shale (Kockatea Shale facies) from the Carnarvon Basin contains a distinctive conodont zone also seen in the informally named ‘Limestone Marker’ in the lower Kockatea Shale of the northern Perth Basin. The boundary between the Induan (Dienerian) and the Olenekian (Smithian) is selected at the base of the incoming of the conodont Novispathodus waageni eowaageni in this core and provides an important biostratigraphic correlation point between the two basins.

The Cunaloo Limestone Member contains Novispathodus dieneri-Neospathodus waageni- Scythogondolella milleri conodont zone species and this correlates with upper Bed 32 at the type section of the Permian-Triassic transition at the Meishan Permian-Triassic stratotype section D in China, dated by zircon U/Pb as about 251.5 Ma. This suggests an absolute age correlation of the lower part of the Kraeuselisporites saeptatus palynological zone in the southern Carnarvon Basin. This conodont zone is Smithian in age.

A thin, apparently discontinuous, previously un-named limestone appears above the Cunaloo Limestone Member and is within the K. saeptatus zone. This carbonate unit becomes more prominent towards the north and contains conodonts, mostly fragmentary, of Smithian age. It is here named the Lawley Limestone Member.

The Chiosella timorensis conodont zone occurs within Core 1 in the Candace Member of Cunaloo-1 and lies within the basal Tigrisporites playfordii palynozone. The First Appearance Datum (FAD) of the conodont C. timorensis has been proposed as an index for the worldwide recognition of the Olenekian-Anisian Boundary (OAB), although the species occurs first with upper Spathian Haugi Zone ammonoids. Nevertheless, it is a good approximation of the OAB, and therefore places the earliest occurrence of the T. playfordii palynozone in Western Australia at around the end of the Spathian-earliest Anisian (Aegean), about 247.2 Ma.

A younger limestone, the Sholl Limestone Member of the Locker Shale facies, is recognised only from the Carnarvon Basin. This carbonate lies within the T. playfordii zone, but conodonts recovered from the unit are not age diagnostic. The Sholl Limestone is missing in several wells below sandstones of younger Triassic age, and in one case may be faulted out (e.g. Hampton-1).

Recognition of these Early Triassic limestones allows a better stratigraphic understanding of those regions from the marine realm in the northern Perth and Carnarvon basins.

The coalfield of the foreland basins is distributed in the different region of the central Himalaya (Nepal Himalaya). For the first time, we applied Total Reflection Fourier transform infra-red spectroscopy (ATR–FTIR), X-ray diffraction, high-resolution transmission electron microscopy (HR–TEM), and scanning electron microscopy (SEM) to study their chemical and nanostructural characteristics of the Nepalese coals. The coal samples from two coalfields, Jhadewa and Tosh coalfield (i.e., Palpa and Dang district) of central Himalaya have been studied. A total of 6 coal samples, 3 for each coalfield were collected directly from the underground coal mine and outcrop coal seams in the western and central Nepal Himalaya. FTIR spectra illustrate the presence of stretching vibrations of aliphatic –CH, –CH2 and –CH3 absorptions, –OH bonds, C=C and –CH of aromatic structures and C=O stretching vibrations of carbonyl groups. X-ray diffraction signifies the presence of crystalline carbon having turbo-stratic structure along with some highly disordered amorphous carbon whereas HR–TEM study shows the formation of carbon nanostructures of 1.73–8.03 nm size and nominal areas in the range of 11–14 nm2. Single-wall nano-tubes (SWNTs) of 12.67–36.23 nm diameters were also formed. The nominal size of the SWNTs is in the range of 16–20 nm. Micro-pores, micro-fissures, and micro-cracks were discovered in the SEM study

Low-frequency measurements of the elastic properties rocks based on the forced-oscillation method is an emerging approach for calibration of well log and seismic measurements. The dispersive properties observed in the low-frequency experiments are usually dependent on the boundary conditions of a tested rock sample, whether the sample surface is properly sealed, or surrounded by rigid pore-fluid containers connected with the pore space of the rock, so-called “dead volumes”. By treating the dead volume as a specific porous material, we analysed the impact of the dead volumes on the strains in the sample via numerical simulation of the forced oscillation tests, our simulation is based on the models incorporating the properties and physical characteristics of the laboratory samples. The numerical results are in good agreement with the analytical solutions.

Knowledge of seismic anisotropy is highly important for Otway site as it can improve the quality of seismic imaging and thus provide better 4D signal/noise ratio. The presence of anisotropy has already been reported at the Otway site. Comparison of geophone and DAS VSP data and its application for anisotropy analysis is presented in this article based on 3D VSP surveys acquired during Stage 2C of the Otway Project.

Definition of the basement architecture within the Ceduna Sub-basin is poorly understood due to the depth of the basin and limited deep crustal geophysical datasets. This study uses seismic interpretation of BightSPAN™ deep crustal seismic lines combined with 2D forward models of the gravity to define the basement architecture in the Ceduna Sub-basin. Gravity forward models identify two depocentres with maximum depths of 25 km overlying thinned continental crust. Syn-rift sedimentary packages are several km thick and underlie up to 10 km of postkinematic Cretaceous deltaic sequences. Regional Moho models contain only sparse data points offshore and isostatic residual gravity data suggests substantial local variation in the depth to Moho. This local variation in the Moho surface is interpreted as boudinage of a weak lower crust along crustal shear zones. A gradual increase in basement density towards the continent-ocean transition suggests a diffuse rather than distinct boundary between continental and oceanic crust.

The results of this work are used to constrain the tectonic evolution, particularly in terms of the subsidence history and uplift and erosional pulses recorded on the seismic data in the Ceduna Delta (Hill et al., this volume) and then used as input for finite element numerical models of rifting (Farrington et al., this volume).

Bringing a new well on-line is an expensive proposition and the need to ensure optimal performance is critical. Saturation and contamination of the drilling fluid with fines and subsequent invasion of the formation has historically resulted in hundreds of hours per well of invisible lost time during the clean-up phase of the well. New technologies, workflows and methods that can reduce costs/turnaround on projects were conducted on a large multi-well project in Perth, Western Australia, to maximise well efficiencies.

Data from multiple sources were used to optimise and validate well clean-up operations with the aim to maximising production. This case study uses integration of well testing transient pressure data, Borehole Magnetic Resonance (BMR) derived transmissivity and flow logging to quantify success of clean-up/development of water wells, to reduce cost and optimise productivity.

This case study demonstrates the successful implementation of an integrated approach to well clean-up using several scales of permeability data from core to wireline BMR to well test. This case study demonstrates that, in this particular setting, the use of the presented methodology was cost effective, yielded positive confirmation of asset delivery, and has led to a 90% reduction in clean-up associated time.

The Lachlan Orogen’s mineral wealth is a direct result of tectonic processes that took place in the early Palaeozoic, but the exact nature and timing of events is widely contested. Here, we apply new methods of deforming tectonic reconstruction modelling to the area. The resulting reconstructions enable us to consistently compare alternative, previously-proposed models and test them against new and old data. This approach highlights model self-inconsistencies and incompatibilities with available data. We adopted an approach where the most valid components of individual tectonic reconstructions were combined to produce a new reconstruction model constrained by the most recent data. The new model invokes two concurrent subduction zones from the Early Cambrian to the Late Ordovician. It includes a consistent continent-dipping subduction at the Eastern Gondwanan margin, and an outboard subduction complex, which experiences multiple reversals. These are responsible for an unnamed Cambrian Arc and its obduction in Tasmania, which is part of the microcontinent VanDieland before accretion to Gondwana in the late Cambrian. The Macquarie Arc later develops in the Ordovician over the Cambrian Arc. A single continent-dipping system then resumes following the Benambran Orogeny, when oroclinal folding occurs across south-eastern Australia followed by east-west shortening of the Tabberabberan Orogeny.

The magnetotelluric method has been successfully used as a complementary tool in the exploration of foothill plays but its intrinsic non-uniqueness and its reduced structural fidelity, with respect to a seismic section, make its interpretation more complex.

Since the solution of a non-unique inverse problem is heavily dependent on its starting point, a traditional method to tackle the non-uniqueness relies on the careful definition of the starting model, used as a tool for injecting the available a-priori information into the inverse problem.

We present an alternative approach to the exploitation of the a-priori knowledge that, instead of embedding it into the starting model, relies on the continuous assimilation of the structural/geological information into the resistivity volume. The method we propose for the inversion of the MT soundings tries to maximize, simultaneously, the data-fit and the structural fidelity. We discuss the advantages deriving from the application of this technique by mean of a synthetic example representative of a foothills play.

The North West Shelf, situated in the north-west of Western Australia, is a world-renowned offshore hydrocarbon province with investments of more than A$200 billion to date. However, the quality of the legacy seismic often suffers from severe residual multiple, limited bandwidth and poor S/N ratio within the reservoir level. This is mainly due to the existence of a complex shallow overburden with a strong water bottom, tertiary carbonates and channel systems.

With the recent advancement in seismic imaging technologies, it is now feasible to mitigate these challenges through high-end reprocessing. In this paper we will demonstrate how a tailored processing flow was applied on a development project on the North West Shelf. The key technologies that we will review here are (1) demultiple workflow with 3D curvelet domain subtraction, (2) hybrid Tomo-FWI velocity model building, and (3) Least-Squares Q Pre-SDM (LS Q-PSDM). The application of these high-end technologies significantly improved the quality of the final imaging, which helps with reducing uncertainties and minimizing risk in the field development.

Magnetic field anomalies measured by the Gawler Craton Aeromagnetic Survey (GCAS) have revealed anomalies of amplitude > 18,000 nT over Paragon Bore. The flying height is 60 metres above ground and depth to basement is 150 metres below ground, so the causative basement sources clearly have magnetizations of extreme intensity.

We apply an iterative processing of the GCAS TMI data to a vector-consistent TMI. This also supplies vector component grids which we downward continue to the ground surface and then transform to declination and inclination maps. We invert the measured TMI using a model of multiple ellipsoids to enable inclusion of substantial self-demagnetization effects. Vector components forward computed from the inversion model at ground level are also transformed to declination and inclination maps which closely match those derived from the filter transform. Deviations of declination and inclination about the regional values are -15° to +21° and -14° to +5° respectively.

High magnetic susceptibility values reported from borehole intersections (up to 1.6 SI in 2 boreholes) are mostly associated with banded iron formation (BIF) and metasomatic magnetite-rich rocks. These values are about 1/3rd of the equivalent inversion model intersection susceptibilities. We suggest that this apparent discrepancy is due to self-demagnetization effects in the susceptibility measurements and the presence of substantial (possibly viscous) remanent magnetization.

Following advances in magnetometer instrumentation and inversion software we re-explore the feasibility of a field methodology utilising relationships between magnetization and dynamic (time-variation) magnetic field anomalies to recover structural information from magnetic field data over thin sheets. We show that we can estimate the dip of a sheet from a reliably determined dynamic magnetic anomaly induced by the range of the magnetic field through its diurnal variation. The strength and inclination of remanent magnetization in the plane perpendicular to the sheet can be subsequently estimated from the static magnetic anomaly using that pre-determined dip angle. The principal challenge in working with dynamic magnetic anomalies is their low amplitude and the requirement for multiple measurements at each station. These relationships between magnetizations and fields apply equally to field gradients. SQUID magnetometers and gradiometers can provide a feasible solution to the high resolution measurements required for this application

We present a multidisciplinary study investigating depositional and diagenetic impacts on pore system variability in carbonate systems from the Canning Basin. With some of Australia’s only known commercially viable carbonate reservoirs the Canning Basin is a region of renewed industry interest and considerable national importance having both recent discoveries (Ungani Far West-1, 2011) and recent historical production from the Blina field (discovered 1981). Detailed core, microscopy, geochemical and petrophysical studies combined with some seismic work are beginning to reveal significant variations in pore system development from the shallow-water carbonate systems on the northern versus southern margins of the Fitzroy Trough within the broader Canning Basin. A strong association is emerging between reservoir quality, dolomitisation and/or earlier cavity system development on the basis of preliminary results.

We have quantified the use of finite electric dipole lengths from the point measurement assumptions typical in 3D MT inversion modeling. Electric fields are measured across dipoles of typically 50 m to 200 m at MT soundings. Modeling algorithms, however, normally use point electric field values at the surface of single cells to calculate MT transfer functions. This is perfectly reasonable for the majority of cases, but there are situations with strong shallow variability of resistivity, where measurements may not be simulated well by point electric fields, and detailed information might not be used optimally. We explore the consequences of this omission by quantifying the difference between point solutions and electric field integrations across dipoles in 3D forward calculations for selected cases. The topic ties closely with galvanic distortion and inversion for related parameters, lateral magnetic field variations, and the benefit of providing shallower constraints for the imaging of deeper targets. As a side product, the analysis led us to focus on the fields output from the 3D modeling, and we illustrate electric current systems through the cases analyzed. We observe that in the presence of strong topography and outcropping inhomogeneities, finite dipole solutions can differ considerably from point solutions, while over a variable regolith case the effect appears more contained

The Grapple and Bumblebee mineralisation at the Lake Mackay Project have been discovered using a combination of routine fine fraction soil sampling, drilling and focused ground electromagnetic methods. Soil sampling initially provided the target areas with subsequent EM surveys delineating basement conductors. Bumblebee returned sub economic intersections when drill testing, while the third drill hole at Grapple returned a significant Cu-Au intersection.

The methodology has been expanded to use airborne electromagnetic methods to rapidly screen the large tenement holding, assist in the understanding of the soil geochemistry results and plan ground EM surveys going forward. The airborne EM method also allows us to test areas deeper under cover than soil sampling.

The increasing depth of investigation for sediment hosted copper mineralisation in the Central African Copperbelt requires the use of adequate geophysical methods such as natural source audio magnetotellurics (NSAMT) as opposed to conventional electrical geophysics techniques (resistivity/IP and time domain electromagnetics) which have limited depth of investigation. A NSAMT survey was carried out at the Solwezi East project near the Kansanshi copper mine in north-western Zambia in order to map a possible undercover domal structure which is thought to control mineralisation at the mine. A total of 50 line kilometres of NSAMT successfully identified a confined domal structure 1.2 km long and 700m wide buried at about 700m below surface. The mapped dome awaits a drill test to confirm the presence of economic mineralisation. NSAMT is proven to be successful for mapping buried domes at considerable depth.