Abstract The Ceduna subbasin forms part of the under-explored but highly prospective frontier Bight Basin located on the southern margin of South Australia. Structural mapping of the Ceduna subbasin reveals two separate delta lobes/systems deposited in the late Albian-Santonian and late Santonian-Maastrichtian. Each system is comprised of an updip delta top linked via a shale detachment to a down-dip delta toe or deep-water fold-thrust belt. These delta lobes are separated by a transgressive sequence of Turonian-Santonian age that deposited a thick marine mud, up to 2000 m in places. Like the Niger Delta, this marine mud forms the detachment for the overlying Santonian-Maastrichtian delta—deep-water fold-thrust belt system and is also a proposed source rock. An Albian marine mud forms the detachment for the older Cenomanian system and is also thought to exhibit source rock potential. We examine the differences in structural style between the western and eastern parts of the basin, the west being dominated by the Cenomanian lobe and the east by the Santonian-Maastrichtian lobe. Recent work on the sedimentary provenance of the basin suggests two different mechanisms were responsible for deposition of the delta lobes in the west and east, as well as significant changes in regional tectonics dominating the basin fill history. This has resulted in the deposition of two delta—deep-water fold-thrust belt systems rather than a continuous system as is commonly observed elsewhere in Cenozoic analogues such as the Niger Delta. Evidence from the drilling of Gnarlyknots-1A on the delta-top suggests excellent reservoir quality in the Santonian-Maastrichtian system; the potential for seal and source development increases farther offshore toward the deep-water fold-thrust belts. The abundant availability of deep-water contractional targets, combined with modeled increase in source and sealing potential farther offshore, results in a highly prospective system in both the Cenomanian and Santonian-Maastrichtian deep-water fold-thrust belts.

Abstract The purpose of this Seals Atlas is to present the microstructural, petrophysical, and geomechanical properties of selected examples of cap rocks and fault seals for use as analogs in the prediction of seal capacity or containment potential. Similar atlases exist; however, this is the first such atlas to focus specifically on the characteristics of cap rocks. The atlas is primarily based on extensive mercury injection capillary pressure (MICP) analyses, but also includes thin section, XRD, grainsize distribution, SEM/EDS, and 'V shale' data. The samples included in this atlas are a result of APCRC and CO2CRC (Cooperative Research Centres) research programs focusing on top and intraformational seals and some fault seals (cataclasites) throughout Australia and New Zealand. The hydrocarbon/carbon dioxide seal examples are grouped by basin localities and further distinguished by formation, well, then depth. Where multiple examples are available, a range of lithologies and MICP data are included in the sample selection. This atlas also can be used in an evaluation of integrated seal potential for prospect risking and reservoir management.

The Tookoonooka impact structure is a subsurface structure of the Eromanga Basin in Australia. Impact ejecta have recently been discovered in the stratigraphy proximal to the structure. The ejecta includes accretionary and armored impacto-clasts. They are observed at multiple locations in drill core across central Australia, spanning 375,000 km 2 within possible impact tsunami deposits. Typical characteristics of the accretionary impactoclasts include a distinctive brownish-gray color, flattened shapes, concentric zonation, and a variety of morphologies with and without obvious nuclei. Some complex accretionary impactoclasts include melt components. Apparent diameters of these impactoclasts in drill core are commonly less than 2 cm, but may be up to 9 cm. They occur in a variety of depositional contexts, including clast-supported breccia-conglomerate layers and “floating” within massive and planar-bedded sandstones. Microscopic and geochemical investigations reveal that they are pervasively altered. Many resemble the types of accretionary lapilli recognized from hydroclastic volcanic environments, which implies the presence of significant water at the time of impact. Tookoonooka is interpreted to have been a marine (likely paralic to shallow) impact event. It is proposed that hydroclastic types of accretionary impactoclasts at impact sites may be an indicator of wet or marine targets. Complex forms of accretionary impactoclasts may also lead to new understanding of impact vapor plume processes. The impactoclasts studied at Tookoonooka are consistent with an impact origin of the candidate ejecta. The consistent first occurrence of the impactoclasts at the base of the Wyandra Sandstone Member stratigraphically constrains the Tookoonooka impact age to 125 ± 1 Ma in the Lower Cretaceous.

Abstract In petroleum exploration, an important task is to estimate both the reserves likely to be discovered in a prospect and the chance of success of finding them. Published advice on estimating reward and risk can be very misleading when applied to a prospect with a risk of membrane seal failure. The literature mostly recommends assessing the reward on the basis of the mean reserve volume of the mapped closure and the risk on the chance of finding a minimal amount of petroleum. This method breaks down for prospects where the perceived chance of success of the top seal being effective varies with prospect’s estimated column height. If the chance of retaining a column that will give a mean reserves volume is much less than the chance of retaining a minimal column, the value of the prospect will be grossly overestimated. This chapter presents several alternative methods. The preferred method of obtaining an expected monetary value (EMV) is to construct a curve of chance of top seal against column height and calculate a graph of the net present value and EMV for numerous levels. If the EMV is positive at any level, the prospect can be considered for drilling.

Coals of Cretaceous age are preserved within the fill of several Australian sedimentary basins. Presently, Cretaceous coal is mined in only one area, although numerous coalfields have been active over the past century. Cretaceous organic facies in the subsurface of the Gippsland Basin, offshore southeast Australia, are thought to have sourced the major oil and gas accumulations of that area. Cretaceous coal-bearing basins in Australia fall into four groups: interior basins, notably the Eromanga Basin where the greatest occurrence of coal is in the Cenomanian Winton Formation; east coast basins, including the Laura Basin (Late Jurassic to Early Cretaceous Battle Camp Formation), Styx Basin (Albiah to Cenomanian Styx Coal Measures), Stanwell Outlier (Albian Stanwell Coal Measures), and Maryborough Basin (Albian Burrum Coal Measures); south coast basins, notably the Great Australian Bight, Otway Basin (Otway and Sherbrook Groups), Bass Basin, (Otway and Eastern View Groups), and Gippsland Basin (Strzelecki and Latrobe Groups), where coal is known from throughout the Cretaceous system; west coast basins, notably the Perth Basin, which contains minor, Early Cretaceous coal. The major control on the formation and distribution of Cretaceous coal in Australia was the development of widespread, rapidly-subsiding lowland environments during passive margin breakup between Australia and Antarctica, and between Australia and Lord Howe Rise. The widespread stratigraphic distribution of coal resources suggests that fluctuating climate and evolving vegetational communities did not fundamentally affect coal development. The role of eustatic sea-level changes is difficult to assess at present. Within individual basins, structural regime and the distribution of depositional systems also played an active role in controlling coal distribution.