Megan Lech

Geochemical mapping ‘down under’: selected results from pilot projects and strategy outline for the National Geochemical Survey of Australia

A series of regional pilot geochemical survey projects have been carried out in Australia over recent years. These pilot projects targeted a number of landscape and climate settings that are challenging for geochemical mapping, and applied increasingly lower sampling densities, from c. 1 site/300 km2 to c. 1 site/ 2100 km2. Selected results are presented from pilot geochemical surveys in the Riverina, Gawler and Thomson regions, as well as results from modelling aimed at testing the robustness of ultra-low sampling densities in Australia. The salient conclusions are that: (1) geochemical surveys sampling surface and near-surface transported regolith are capable of identifying major lithologies and mineralization below the cover; and (2) a sampling strategy targeting outlet sediments of large catchments allows the application of ultra-low sampling densities in Australia. These findings overcome the main hurdles met by earlier attempts at designing a nation-wide geochemical survey. The strategy for the resulting National Geochemical Survey of Australia (NGSA) project, recently approved for funding under the Australian Governments Onshore Energy Security Initiative, is outlined. By 2011, the NGSA will produce a publicly available multi-element, internally consistent and state-of-the-art regolith geochemical database and atlas to support decision-makers.

Baseline geochemical survey of the Riverina region of New South Wales and Victoria, Australia: concentrations and distributions of As, Ba, Br, Cd, Co, Cr, F, Ga, Mo, Sb, U and V compared to national and international guidelines

This study discusses element excesses and deficiencies in the regolith of the Riverina region, which may impact on plant, animal and human health. Top (0–10 cm depth) and bottom (c. 60–90 cm depth) overbank sediment samples were collected near the outlets of 142 catchments and the composition of the <180 μm fractions determined. Total concentrations of As (0.8–159.8 mg/kg), Ba (189– 1263 mg/kg), Br (<1–89.5 mg/kg), Cd (<0.1–2.33 mg/kg), Cr (29–200 mg/kg), F (150–610 mg/kg), Ga (6.3–26.1 mg/kg), Sb (0.37–10.8 mg/kg), U (1.26– 8.49 mg/kg) and V (31–145 mg/kg) are locally elevated above national and international guidelines and these elements therefore have potential for health concerns. Concentrations in at least half of the samples are above the guidelines for Ba, Cr, F, Ga and V. Based on total concentrations, most elements are unlikely to cause harm due to their low plant uptake and/or strong adsorption onto soil particles; however, Cd, V and U in particular require further investigation. Total concentrations of Co (2.96–34.2 mg/kg) and Mo (0.5–1.9 mg/kg) are potentially deficient in parts of the region and can be remedied with the application of appropriate fertilizers.

The 2017 offshore acreage release areas: petroleum geological overview

In 2017, 21 new offshore petroleum exploration areas have been released. The majority of the areas are located along the North West Shelf spanning the Westralian Superbasin from the Bonaparte Basin in the north-east to the Northern Carnarvon Basin in the south-west. New areas have been released in offshore south-eastern Australia with new opportunities provided in the Otway, Bass and Gippsland basins. Two large areas in the northern Perth Basin, an offshore frontier, complete the 2017 Acreage Release. All Release Areas are supported by industry nominations and one new cash bid area has been offered in the Dampier Sub-basin. Geoscience Australia continues to support industry activities by acquiring, interpreting and integrating pre-competitive datasets that are made freely available as part of the agency’s regional petroleum geological studies. A new regional 2D seismic survey was acquired in the Houtman Sub-basin of the Perth Basin, forming the basis of the latest prospectivity study carried out by Geoscience Australia. The results of the study are presented in the technical program of the 2017 APPEA conference. A wealth of seismic and well data, submitted under the Offshore Petroleum and Greenhouse Gas Storage Act 2006 (OPGSSA) are made available through the National Offshore Petroleum Information Management System (NOPIMS). Additional datasets are accessible through Geoscience Australia’s data repository.

Paleogeographic Evolution of Early Campanian to Maastrichtian Supersequences in the Caswell Sub-Basin—Implications for CO2 Storage and Hydrocarbon Entrapment

The Caswell Sub-basin is a northeast-trending Paleozoic to Cenozoic depocentre in the Browse Basin, on the Northwest Shelf, offshore Western Australia. As part of a recent study to investigate the CO2 storage potential of the sub-basin, sequences within the latest Cretaceous section have been high-graded as potential CO2 storage targets based in part on their sequence stratigraphic and paleogeographic characteristics. Sequence stratigraphic analyses were used to improve the understanding of sequence architecture, facies and palaeogeography during early Campanian-Maastrichtian, a time of relative sea level fall in the basin. This phase is characterised by two supersequences (K60a and K60b), with maximum regression reached by the end of the Maastrichtian Sequence (K60b). During the early Campanian (K60a), a major sea level fall resulted in deep incision and channeling at the base of the supersequence. Wide fluvial systems draining to the north from the Kimberley Craton were captured by east-west channels that fed large submarine fan complexes across the Caswell Sub-basin. This was followed by highstand fluvio-deltaic systems that prograded to the north over the structurally controlled ramp-like inner shelf. Supersequence K60b (near base to late Maastrichtian) was initiated by eustatic sea level fall punctuated by deep incision into K60a. Large EW-trending incised valleys and wide fluvial belts provided sediment pathways from to the basin during lowstand conditions. Outboard, transgressive pro-delta shales of the younger supersequence (K60b) provide a seal for the submarine fans. As for the K60a supersequence and in comparison with sediment transport direction for the lowstand fans, the fluvio-deltaic highstand successions are characterised by rapid northward progradation along the shelf margin. The submarine fan complexes of these two supersequences formed isolated sand-rich bodies sealed by thick transgressive and downlapping highstand shales. Containment risk for the evaluation of these fans as potential CO2 storage targets considered updip continuity of sand bodies, incision by channels, faulting and top/base seal. The studies identified a variety of CO2 storage sites and examined the area for potential conflict with active hydrocarbon exploration.

The role of seal integrity in the Vlaming Sub-basin (Perth Basin) for preservation of hydrocarbon accumulations

The offshore Vlaming Sub-basin, located in the southern part of the Perth Basin, is a Mesozoic depocentre estimated to contain over 12 km of sediments. It has several potential source rock intervals, good reservoir and seal pairs and an active petroleum system. The reasons for a lack of exploration success in this basin have been re-assessed by analysing fault reactivation and signs of hydrocarbon seepage. A recently completed study integrated structural mapping with analysis of fluid inclusion results. New data and interpretations show that a number of synrift faults with signs of reactivation in seismic data also have Fluid Inclusion Stratigraphy (FIS) anomalies above the regional seal. Many previously identified plays rely on the post-rift South Perth Shale for a seal. Our analysis suggests that many faults were reactivated after the deposition of the South Perth Shale, with some showing signs of present-day reactivation. Reactivated faults provided migration pathways for generated hydrocarbons; therefore, no accumulations were formed at these locations. The study provides insight into the location of leaky structures and areas with potentially valid plays in the Vlaming Sub-basin.

Geomorphology and seismic stratigraphy of the early Cretaceous delta in the Vlaming Sub-basin and implications for seal quality

The early Cretaceous South Perth Shale has been previously identified as the regional seal in the offshore Vlaming Sub-basin. The South Perth Shale is a deltaic succession, which infilled a large palaeotopographic low in the Early Cretaceous through a series of transgressive and regressive events. A study undertaken at Geoscience Australia has shown that the seal quality varies greatly throughout the basin and in places has very poor sealing properties. A re-evaluation of the regional seal based on seismic mapping determined the extent of the pro-delta shale facies within the South Perth Shale succession, which provides effective sealing capacity. New sequence stratigraphic interpretation, seismic facies mapping, new and revised biostratigraphic data and well log analysis were used to produce palaeogeographic reconstructions which document the distribution of depositional facies within the South Perth Shale and reveal the evolution of the early Cretaceous deltas. Our study documents spatial variations in the seal quality and re-defines the extent and thickness of the regional seal in the offshore central Vlaming Sub-basin. It provides an explanation for the lack of exploration success at some structural closures and defines constraints on the possible location of valid plays.