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

During November 2016, in the South Sumatra Basin, CGG acquired two airborne Full Spectrum Gravity surveys, over 11,300km². Full Spectrum Gravity combines the high resolution Falcon Airborne Gravity Gradiometry (AGG), and the sGrav airborne gravity systems. The sGrav is a proprietary strap-down gravimeter that recovers long wavelength gravity information that full tensor gradiometers cannot provide, and is a product a recent CGG R&D project.

The South Sumatra basin is a mature area in terms of hydrocarbon exploration, with many known and producing oil and gas fields. One of the key reservoir units in this basin are the uplifted areas of Mesozoic and Eocene fractured and weathered basement consisting of granites and quartzites. These uplifted basement blocks provide a positive density contrast.

The Full Spectrum Gravity data has provided detailed structural information, while preserving the regional context in terms of gravity signal. It shows an impressive correlation between the gravity highs and the known oil and gas fields, and has provided new leads in this mature area.

This paper demonstrates how Full Spectrum Gravity data can drastically improve the information and interpretation achievable from 2D seismic, and even provide clarity where imaging on 3D seismic is poor. We show how the data allows better decisions to be made throughout the exploration process; from seismic positioning, through integrated interpretation projects, and ultimately drilling decisions, providing tangible cost savings to any exploration project.

Research area is located in Bayah District, Lebak Regency, Banten Province. The research location is located approximately 80 kilometers southwest of Jakarta. This area has a complex geological structure, as well as found many intrusive and metamorphic rocks. In this research area, geothermal manifestations were found in the form of four hot springs (APPC-1, APPC-2, APPC-3, and APC) as well as two cold springs (ADC-1 and ADC-2). This study aims to identify the relationship of geological structure control with the occurence of manifestations in the research area, as well as to determine the Bayah non-volcanic geothermal prospects. The method used is Fault and Fracture Density analysis for structural analysis of research area and magnetic map analysis for interpretation of geothermal prospect prospect. Structural analysis methods performed in the form of lineament delineation, determination of lineament density and major trends, and application of structural sequence model. The results of structural analysis will be correlated with the occurences of geothermal manifestations with the aim of identifying the most influential structural patterns as the pathway for geothermal fluid to reach surface in the study area. Magnetic data is also used to determine the possibility of Bayah non-volcanic geothermal prospects. The developing structure in the research area has NE-SW and NW-SE directions. The FFD analysis shows that high-density lineament is located in the southeast of research area where 3 hot springs manifestation APPC-1, APPC-2, APPC-3 are present. This manifestation appears in the lineament with NE-SW direction. Magnetic data also obtained negative magnetic anomalies in the southeast of the study area. It can be concluded that the lineaments with NE-SW direction influence the fluid outflow the most, and Bayah non-volcanic geothermal prospect areas are located around APPC manifestations.

Regional scale flooding of New Guinea has occurred episodically since the Jurassic. The most recent flooding event during the Miocene occurred despite falling long-term eustatic sea levels. Recent work has suggested dynamic topography, the long-wavelength low-amplitude topographic response to mantle flow, as a factor in the emergence and flooding of this region, and therefore influencing the depositional history of New Guinea basins. The link between deep Earth and surface processes has not yet been explored for this region. We use forward numerical models coupling plate kinematics, mantle convection, paleogeography and eustasty to investigate the time-dependent topographic response of the New Guinea margin. Dynamic topography estimates derived from mantle convection models are then coupled with surface process modelling code Badlands to study the landscape evolution of New Guinea and the adjacent Australian continent. Reproducing the inundation history of New Guinea, our models show that continental scale dynamic topography plays a significant role in the development of drainage systems, and erosion-deposition regimes. Our work demonstrates the necessity in linking geological processes that operate across wide spatial and temporal scales to better understand how the interplay between deep Earth and surface processes control the source to sink evolution of basins.

Compared to the success of Mesozoic play in Westralian Superbasin (WASB), the lack of hydrocarbon discovery in West Timor within Timor Trough and North West Shelf Australia is still an enigma. The West Timor is still frontier petroleum province with problems in uncertainties of working petroleum system play as well as the hydrocarbon prospectivity.

This paper tries to approach this issue by integrating of fieldwork data with published well data and offshore seismic data from recent publication to re-evaluating potential hydrocarbon prospectivity in this area. Using the dataset, this study identifies structural framework across West Timor Island and offshore area, as well as potential petroleum system plays including source rock, reservoir presence and trap configuration.

The results of this study identify two potential petroleum province region including Timor deformation front and Australian passive continental margin. Within these areas, three main plays based on structural configuration were identified which are fold related fault, sub-thrust, and tilted fault block. Reservoir targets for these main plays are Jurassic sequences including sandstone of the Early-Middle Jurassic of Plover equivalent and Late Triassic Malita equivalent with seal rock including Early-Middle Jurassic shale of Wai Lui Formation and Early Cretaceous shale interval. These plays are expected to be charged from source rock interval of Triassic Formation.

The novelty of hydrocarbon prospectivity in this study will guide exploration screening of petroleum system analysis in West Timor area where current play analysis has not been tested yet.

The Houtman Sub-basin lies adjacent to the Wallaby-Zenith Transform Margin, an under-explored region of Australia’s continental margin located at the transition between the non-volcanic margin of the northern Perth Basin and volcanic province of the Wallaby Plateau. New seismic data acquired in the northern Houtman Sub-basin enables better understanding of the structural architecture and rifting development along a rifted-transform margin and provides the framework for a detailed integrated margin-scale basin evaluation. Profile modelling of potential field data, combined with 2D seismic, reveals complex along-strike and dip variability in the crustal thinning of the Houtman Sub-basin, with extreme thinning (<5 km thick) beneath the main Permian depocentre. Outboard of this hyperextended zone, along the basin margin, is a zone of volcanic SDRs. Five different structural domains have been mapped across the margin, reflecting abrupt change in crustal thinning and volcanic emplacement. These domains trend roughly NW-SE to NNW-SSW, parallel to major basement terrane boundaries. Magnetic modelling suggests that the nature of the basement underlying the proximal domain and the hyperextended domain in the central Houtman Sub-basin are different and that a major Proterozoic basement terrane boundary lies beneath the necking domain.

The margin was structured during polyphase Permian and Late Jurassic rifting events which led to hyperextension prior to continental magmatic break-up and formation of oceanic crust during the Early Cretaceous. Our results suggest that the distribution of Early Permian rifts localised strain during Jurassic-Early Cretaceous rifting and strongly controlled the location and style of rifted margin during Valanginian continental break-up.

The northern Houtman Sub-basin is an under-explored region of Australia’s western continental margin. It is located at the transition between the non-volcanic margin of the northern Perth Basin and the volcanic province of the Wallaby Plateau and lies adjacent to the Wallaby-Zenith Transform Margin. In 2014, Geoscience Australia acquired new 2D seismic data (GA-349, 3455 km) across the northern Houtman Sub-basin to assess its hydrocarbon prospectivity. Previous studies of the Houtman Sub-basin indicated that en-echelon basin bounding N-NW trending faults are associated with the Permian half graben complex, however, it was not known if this structural style continued into the northern area of the Houtman Sub-basin. This study integrated interpretation of the recently acquired survey, with regional interpretation of the Houtman Sub-basin. This was further supported by well data and geophysical modelling and a regional 2D structural and stratigraphic interpretation developed. Structural mapping was done for the basement, Early Triassic (Woodada Formation) and Early Jurassic (Eneabba Formation).

The basement structure of the northern Houtman Sub-basin is controlled by a series of large en-echelon NW-SE trending SW dipping faults, some of which have a throw of more than 10 km. These basement-involved faults control a series of Permian half graben separated by transfer zones and fault ramps. This basement architecture is similar to the inboard part of the southern Houtman Sub-basin, however the structures are larger. The Early Triassic and Early Jurassic faults trend NW-SE similar to the basement-involved faults, however major faults within the Jurassic succession lie about 50 km to the west of the Permian faults.

Interpretation of the northern Houtman Sub-basin reveals a structurally complex basin containing a wide range of structural and stratigraphic traps at several stratigraphic levels. Potential plays have been identified in the upper Permian, Triassic and Jurassic successions. They include large stratigraphic plays in the Upper Permian/Lower Triassic, rollover anticlines within the Lower Triassic and Jurassic, and fault propagation folds and fault block plays in the Jurassic.

The northern Houtman Sub-basin is an under-explored region of Australia’s western continental margin. It is located at the transition between the non-volcanic margin of the northern Perth Basin and the volcanic province of the Wallaby Plateau, and lies adjacent to the Wallaby-Zenith Transform Margin. In 2014-15, Geoscience Australia acquired new 2D seismic data (GA-349) across the northern Houtman Sub-basin to assess its hydrocarbon prospectivity.

This study integrated interpretation of the recently acquired GA-349 survey, with Geoscience Australia’s existing regional interpretation of the Houtman and Abrolhos sub-basins, to develop a 2D structural and stratigraphic interpretation for the study area. As there are no wells in the northern Houtman sub-basin, the age and lithologies of the mapped sequences were derived from regional mapping, stratal relationships and seismic facies. The new data clearly images a large depocentre, including a much thicker Paleozoic section (up to 13 km) than previously recognised. Extending the length of the inboard part of northern sub-basin are a series of large half-graben (7-10 km thick), interpreted to have formed as a result of Permian rifting. Overlying these half-graben, and separated by an unconformity, is a thick succession (up to 6 km) interpreted to represent a subsequent late Permian to Early Jurassic phase of the thermal subsidence. A second phase of rifting started in the Early Jurassic and culminated in Early Cretaceous breakup. The sedimentary succession deposited during this phase of rifting is highly faulted and heavily intruded in the outboard part of the basin, adjacent to the Wallaby Saddle, where intrusive and extrusive complexes are clearly imaged on the seismic. In contrast to the southern part of the Houtman Sub-basin, which experienced rapid passive margin subsidence and regional tilting after the Valanginian breakup, the northern sub-basin remained mostly exposed sub-aerially until the Aptian while the Wallaby Zenith Fracture Zone continued to develop.

Understanding the geological evolution and resource prospectivity of a region relies heavily on the integration of different geological and geophysical datasets. Geochronology is one key dataset, as it underpins meaningful geological correlations across large regions, and also contributes to reconstruction of past tectonic settings. Using geochronology in combination with other datasets requires the geochronology data to be available in a unified dataset with a consistent format. Northern Australia is a vast and relatively underexplored area that offers enormous potential for the discovery of mineral and energy resources. The area has a long and variably complex tectonic history, which is yet to be fully understood. Numerous geochronology studies have been completed at various scales throughout northern Australia over several decades; however, these data are scattered amongst numerous sources, limiting the ease with which they can be used collectively. The objective of this work is (1) to combine Uranium-Lead (U-Pb) data across north-northeastern Australia into one consistent dataset, and (2) to visualise the temporal and spatial distribution of the U-Pb age data through thematic maps as a tool for better understanding the geological evolution and resource potential of northern Australia.

In this contribution, over 2000 U-Pb ages from the Northern Territory, Queensland, eastern Western Australia and northern South Australia have been compiled into a single, consistent dataset. Data were sourced from Geoscience Australia, State and Territory geological surveys and from academic literature. The compilation presented here includes age data from igneous, metamorphic and sedimentary rocks. Thematic maps of magmatic crystallisation ages, high-grade metamorphic ages and sedimentary maximum depositional ages have been generated using the dataset. These maps enable spatial and temporal trends in the rock record to be visualised up to semi-continental scale and form a component of the ‘Isotopic Atlas’ of northern Australia currently being compiled by Geoscience Australia.

This article provides instruction on how to access large repositories of geophysical data held by the New South Wales Government. This data was acquired by government and private companies. Various products have been produced from this data and made available to the public; most are free. Each product was produced for different purposes and is delivered through different systems, which are not broadly utilised by the geophysical community. The following products are available to the public.

(1) Regional project data comprises airborne magnetic, radioelement, digital elevation, gravity, hyperspectral, electromagnetic (EM) and ground gravity data acquired using government funding. This data is available on portable hard drive and some is available through the Geoscience Australia GADDS portal. This data is ideal for producing potential field models, or custom imagery.

(2) Statewide grids, images and maps are merged regional airborne magnetic, radioelement, digital elevation and ground gravity data. These are available respectively through portable hard drive, MinView and DIGS. This data is suited to large-scale geological interpretations, or producing custom clipped imagery.

(3) The 250K geophysical imagery suite includes zip files of prepared standard imagery clipped from our statewide grids that enhance subtle features not readily visible in the statewide images. The 250K suite can be downloaded through DIGS.

(4) The open-file company data repository contains approximately 1000 surveys (of various geophysical techniques) submitted by title holders. The survey location can be viewed in MinView and requested by email. This data usually has higher resolution than NSW Government regional project data. It is excellent for producing models, small-scale geological interpretation and desktop studies.

Airborne time domain electromagnetic (ATEM) surveys have reached the stage where full waveform streamed data are recorded and delivered in addition to traditional survey products. One result of this advance in technology is that the line between the acquisition and processing phases has become more flexible and many parameters that used to be hardwired in acquisition can now be adapted during the processing phase. In order to make use of this opportunity the interpreter needs a clear description and understanding of the system specific corrections required to isolate geological responses as well as the effects of filters and other digital signal enhancement options that are available.

Validating procedures on a synthetic data set is one way of ensuring that all geological responses falling within similar parameter ranges would be accurately presented after processing. In this study the effects of three time-series and four spatial filters were analyzed. Streamed full waveform data were simulated by adding measured high altitude data to synthetic models. The various filters were applied and the deviations from the true models compared with that of the unfiltered data. The results were evaluated based on whether the filtered results showed more or less deviation than the unfiltered data from the original noise-free models.

Crustal silicate rocks at sub-solidus temperatures normally have high electrical resistivities. However, although upper crust is typically > 10³ Q.m, it is not unusual for lower crust to be < 10² Q.m, and in places < 10⁰ Q.m. That lower crust (below 10-15 km) can be as electrically conducting as seawater is remarkable, and indicates a substantial and highly-connected mineral, melt or aqueous phase. To date, temporal and spatial mechanisms to give rise to the low resistivity are speculative and poorly constrained by observation and laboratory measurement.

We present new maps of the Australian crust resistivity inferred from the regional EM responses. The project addresses the question as to whether the low resistivity is primary in the formation of the crust, or overprint due to melt and fluid migration from a deeper thermal source. A secondary question is how regions of low resistivity from an interconnected phase can be preserved through time-scales of billions of years. Observations are drawn from: the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP); 2D MT transects; and legacy MT and geomagnetic depth sounding (GDS) data.

Our research demonstrates a strong spatial correlation of crustal resistivity with tectonic domains in Australia. Lowest resistivities are often imaged just below the rheological boundary between upper and lower crust at -10-15 km. Below, low resistivity appears as a broad zone, tens or hundreds of kilometres wide, and tens of kilometres thick; above the boundary, regions of low-resistivity appear as narrower pathways. Such maps are correlated with long-wavelength Bouguer gravity data, suggesting a common origin that changes both density and resistivity.

Regional-scale continental magnetotelluric (MT) programs such as AusLAMP are naturally bounded by the continental shelf and electrically-conducting seawater. Within a few hundred kilometres of the coastline, long-period MT data may be strongly influenced by induction in the seawater, a phenomenon known as the coast-effect. Thus, 3D inversion of gridded long-period MT data for continental lithosphere models requires good constraints on the resistivity of the seawater, oceanic crust and upper mantle, and into the asthenosphere.

In this paper, we discuss the concept of a horizontal adjustment distance. This is the horizontal distance away from a major contrast in electrical conductance at which the anomalous electric fields are attenuated by a factor of 1/e from a 1D response, and is somewhat analogous to the more widely-known skin-depth concept. For seafloor MT, this adjustment distance can be thousands of kilometres. Inland, the effect depends on the conductance of the sedimentary cover, and the depth-integrated resistivity of the upper crust, and can vary from a few kilometres to hundreds of kilometres. We discuss the implications in terms of 3D smooth inversion that inherently minimises gradients in subsurface resistivity and suggest that the coast effect may be significantly underestimated in some continental models.

A typical product from an airborne electromagnetic (AEM) survey is a conductivity depth image (CDI) along each of the flight lines. These CDIs overcome the problem of non-uniqueness by choosing one model that fits the data, typically a smoothest model. However, in the case of using AEM for describing the stratigraphy of the regolith, an understanding of the landscape evolution processes that formed the regolith gives us knowledge about what the stratigraphic units are that make up the regolith, and also something about their likely geometry. In addition, knowledge of their mineralogy tells us something about their likely ranges of conductivity, and understanding of the processes that formed them tells us about their geostatistical properties. For example, materials which are well mixed, such as channel clays, will typically be homogeneous over large distances, whereas material that has formed by in-situ weathering could be much more heterogeneous. It therefore makes sense to try to invert the AEM data for stratigraphic boundaries and conductivity variations within stratigraphic units rather than smooth models. This immediately gives estimates, with uncertainty bounds, for the depths to various interfaces, which are of more direct interest to a geologist than the conductivity values in the CDI.

The work presented here shows how the use of geological contextual information can produce improved inverted models of the regolith vertical and lateral stratigraphy from AEM data. A better understanding of regolith architecture then allows for optimisation of drilling sites for geochemical sampling, focussing on stratigraphic units where the geochemical footprint of an orebody is likely to have been concentrated.

The magnetotelluric (MT) method is becoming more widely used in the geoscience community as it becomes increasingly recognised as a useful exploration tool. However, while the analysis and inversion tools available to the MT community have increased over recent years, the software available to work with these tools is still somewhat limited and often costly in comparison to some of the more mature techniques like gravity, magnetics and seismic.

The MTpy python library is open source software that aims to assist MT practitioners in carrying out the processing and analysis steps that need to be carried out with MT data and in working with the various inversion codes that are available. However, MTpy still contains coding issues, bugs and gaps in functionality, which have limited its use to date. We are currently developing MTpy to rectify these problems and expand the functionality, and thus facilitate the use of MT as an exploration technique. Key improvements include adding new functions and modules, refactoring the code to give better quality and consistency, fixing bugs and adding new Graphic User Interfaces.

The effect of the Permian onset of the Hunter-Bowen Orogeny on sedimentation patterns in the eastern Galilee Basin of Queensland was investigated through a comparison of age populations of detrital zircon grains in sandstones in the Cisuralian (early Permian) Joe Joe Group and the Lopingian (late Permian) Betts Creek beds and a potential shift in sediment provenance has been recognised. One key well (OEC Glue Pot Creek 1) was selected from the eastern Galilee Basin for detrital zircon analysis. The well intersected both Cisuralian and Lopingian strata. Nine sandstone samples were collected (three from the Cisuralian and six from Lopingian) and zircon grains were extracted. U-Pb isotopic data was gathered using the Laser Ablation - Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) technique.

A total of 286 concordant ages were obtained from the nine samples. The Cisuralian samples have a varied age population range, with multiple peaks between 300 and 1200 Ma, suggesting numerous sources and orogenic recycling. In contrast, the Lopingian samples have a dominant peak of 250 to 300 Ma zircons, with a singular minor peak of 1500 Ma zircons, suggesting a transition in provenance between the Cisuralian and Lopingian.

The recording of raw or streamed EM survey data, as done by CGG during MEGATEM surveys, allows for the reprocessing of the acquired EM data, including square-wave processing. During the latter, the recorded EM response to the actual half-sine waveform is replaced by the EM response to a square-wave, derived via deconvolution/convolution in the frequency domain. This makes the on-and early-time information more accessible for data modelling, including 1D inversions and conductivity-depth transformations. Square-wave EM data can also be corrected for survey height, transmitter-receiver offset and transmitter attitude. That correction allows for the interpretation of early-time EM response grids, which generally offer better spatial resolution than derived conductivity-depth slices.

The advantages of square-wave processing are demonstrated on a MEGATEM data set acquired in 2013 in South America. With survey terrain clearance ranging from 100 - 1600 m, due to the rugged topography, early-time grids of elevation-corrected square-wave data outlined the shallow conductivity structure, whereas early-time grids of the original half-sine data mostly reflected the variable system elevation. Further, derived conductivity-depth sections of the square-wave data show more lateral continuity than the sections derived from the original half-sine data. These results show that the early-time information of square-wave is more accessible than in the original data, facilitating interpretation of shallow conductivity structures.

Mercury and soil carbon dioxide are two of many elements that can be used to determine geothermal source potential. The simple concept is that these elements are commonly present in geothermal fluids and usually reside on rocks or soil along its migration path while going onto the surface through porosities. The research is located at Mt.Telomoyo, Central Java Province, located approximately 400 kilometers east-southeast of Jakarta. Around the mountain, four hot springs and four cold springs were found, as well as 144 soil samples. This study aims to determine the geothermal source by using soil geochemical analysis which uses mercury and carbon dioxide as analysed elements. Methods of mercury and carbon dioxide analysis were also aided by Fault-and-Fracture analysis in the study area. The anomaly map of both elements and the fracture density shows anomalies in the same location thas is at the north side of Mt. Telomoyo. This anomaly zone is indicated as a geothermal prospect area in the study area.

Magnetotelluric (MT) inversions are inherently non-unique. Due to the large computational requirement of 3D MT inversion, there is always a trade-off between exploring model space and the amount of time invested in the inversion process. A standard approach is a two-stage inversion, where coarse features are resolved first, and the outputs from the coarse model are used as the starting model for a finely discretized inversion. Inversion may be followed by a combination of sensitivity analysis and forward modelling to test the robustness of features. This approach quickly leads to a low RMS model but has a limited capacity to explore the potential range of acceptable models.

In this paper, we propose a revised two-stage process which enables fast comparison of inversion results from a series of inversion parameters. Three inversions were run using a starting half space with only the input data varied: one inversion using un-rotated full tensor and tipper data; one inversion using only tipper data; and the third inversion using rotated tensor and tipper data. In addition to these half-space models, an inversion was run using starting models based on geological constraints. It used three domain boundaries with roughness penalties turned off between units. The modified two stage inversion process suggested in this paper offers a balance between arriving at final inversion within a reasonable time frame while still allowing the user to explore a larger range of possible models than the original methodology.

Previous studies have used downhole resistivity logs either as a direct constraint during magnetotelluric inversion or as a qualitative validation of inversion accuracy. This study instead uses synthetic 1D magnetotelluric modelling based on downhole resistivity to better understand results of 1D, 2D and 3D inversion.

One-dimensional models with representative geology for the area were generated directly from downhole resistivity data. These models were used to generate synthetic data, which was then inverted using a range of methods. Synthetic modelling made a significant contribution to selecting an appropriate inversion technique and the quality of the interpretation. Joint use of SimPEG MT1D and Occam 2D inversions proved the most effective combination to understand the geology of the project area.