G. Backe

Detrital zircon data reveal the origin of Australia's largest delta system

The Late Cretaceous Ceduna Delta is the largest deltaic system on the Australian continent, yet its source is unknown. Apatite fission-track data reveal widespread Late Cretaceous exhumation across the southern Australian margin. New detrital zircon analysis of 786 grains from the Gnarlyknots-1 well, which penetrated the offshore delta top, show that the upper part of the delta (Santonian–Maastrichtian) was sourced largely from recycled Permian to Early Cretaceous cover and underlying basement eroded from the margin, proximal to the basin. This challenges the widely accepted model involving distal provenance of >2000 km from the eastern margin of Australia. Supplementary material The 2D seismic reflection data, results for detrital zircon LA-ICP-MS and zircon fission-track analyses, including the LA-ICP-MS method, and a list of sample intervals and ages are available at www.geolsoc.org.uk/SUP18582.

Tectonic evolution of the northern Bonaparte Basin: impact on continental shelf architecture and sediment distribution during the Pleistocene

The Bonaparte Basin (NW Australia) forms a rare, recent example where Neogene deformation shaped a very wide platform (630 km wide) in which a mixed carbonate-silliciclastic sedimentary sequence developed. This study combines structural and stratigraphic analysis and provides new insights as to the role of tectonics in controlling platform shape and sediment distribution in wide shallow water settings. Detailed analysis of the structure and stratigraphy of the northern part of the Bonaparte Basin allowed identification of the main regimes and phases of deformation and their control on sedimentation during the Neogene. The results reveal that the distribution of Neogene sediments across the northern Bonaparte Basin is mainly controlled by flexure-induced deformation mechanisms associated locally with extensional faults and low-strain, left-lateral strike-slip. These processes ultimately shaped the geometry and sedimentary architecture of the wide continental shelf. They led to the development of two different types of tectonically induced shelf depocentres that controlled the gross distribution of Quaternary sediments. In particular, deformation processes enhanced the formation of the carbonate-dominated, ∼200 m-deep Malita intra-shelf basin. The Bonaparte Basin is a prime natural laboratory to describe the links between tectonics and sedimentation along a very large, mixed carbonate/clastic platform and could be used as a modern analogue to similar settings in the past Earths history.

Remote sensing of subsurface fractures in the Otway Basin, South Australia

Naturally occurring fractures were remotely detected in a 3-D seismic volume from the Penola Trough in South Australias Otway Basin and validated through an integrated approach. Identified in image logs are 508 fractures and 523 stress indicators, showing maximum horizontal stress orientation in the Penola Trough is 127°N. Two fracture types were identified: (1) 268 electrically conductive (potentially open to fluid flow) fractures with mean NW-SE strikes and (2) 239 electrically resistive (closed to fluid flow) fractures with mean E-W strikes. Core from Jacaranda Ridge-1 shows that open fractures are rarer than what image logs indicate, due to the presence of fracture-filling siderite, an electrically conductive cement which may cause fractures to appear hydraulically conductive in image logs. The majority of fractures detected is favorably oriented for reactivation under in situ stresses, although it is demonstrated that fracture fills primarily control which fractures are open. Seismic attributes calculated from the 3-D Balnaves/Haselgrove survey are mapped to the Pretty Hill Formation to enhance observations of structural fabrics, showing linear discontinuities likely representing faults and fractures. Discontinuity orientations are consistent with natural fracture orientations identified in image logs, striking E-W and NW-SE, limited to zones around larger faults. However, it is unlikely that a large proportion of these fractures are open given observations of core and image logs, limiting possible fracture connectivity and therefore significant secondary permeability in the Penola Trough. The integrated methodology presented herein provides an effective workflow for remote detection of subsurface fractures and determining if electrically conductive fractures are also hydraulically conductive.

Cenozoic post-breakup compressional deformation and exhumation of the southern Australian margin

We present results from a margin-wide analysis of the history of post-breakup Cenozoic compressional deformation and related exhumation along the passive southern margin of Australia based on a regional synthesis of seismic, stratigraphic and thermochronological data. The Cenozoic sedimentary record of the southern margin contains regional unconformities of intra- Lutetian and late Miocene-Pliocene age, which coincide with reconfigurations of the boundaries of the Indo-Australian Plate. Seismic data show that post-breakup compressional deformation and sedimentary basin inversion, characterised by reactivation of syn-rift faults and folding of post-rift sediments, is pervasive from the Gulf St Vincent to Gippsland basins, and occurred almost continually since the early-to-mid Eocene. Inversion structures are absent from the Bight Basin which we interpret to be the result of both the unsuitable orientation of faults for reactivation with respect to post-breakup stress fields, and the colder, stronger lithosphere that underlies that part of the margin. Compressional deformation along the southeastern margin has mainly been accommodated by reactivation of syn-rift faults resulting in folds with varying ages and amplitudes within the post-rift Cenozoic succession. Many hydrocarbon fields in the Otway and Gippsland basins are located within these folds, the largest of which are often associated with substantial localised exhumation. Our results emphasise the importance of constraining the timing of Cenozoic compression and exhumation in defining hydrocarbon prospectivity of the southern margin.

Geomechanical modelling of fault reactivation in the Ceduna Sub-basin, Bight Basin, Australia

Abstract The Ceduna Sub-basin is located within the Bight Basin on the Australian southern margin. Recent structural analysis using newly acquired two-dimensional (2D) and three-dimensional (3D) seismic data demonstrates two Late Cretaceous delta–deepwater fold–thrust belts (DDWFTBs), which are overlain by Cenozoic sediments. The present-day normal fault stress regime identified in the Bight Basin indicates that the maximum horizontal stress (SHmax) is margin parallel; Andersonain faulting theory therefore suggests the delta-top extensional faults are oriented favourably for reactivation. A breached hydrocarbon trap encountered in the Jerboa-1 well demonstrates this fault reactivation. Faults interpreted from 3D seismic data were modelled using the Poly3D© geomechanical code to determine the risk of reactivation. Results indicate delta-top extensional faults that dip 40–70° are at moderate–high risk of reactivation, while variations in the orientation of the fault planes results in an increased risk of reactivation. Two pulses of inversion are identified in the Ceduna Sub-basin and correlate with the onset of rifting and fault reactivation in the Santonian. We propose a ridge-push mechanism for this stress which selectively reactivates extensional faults on the delta-top, forming inversion anticlines that are prospective for hydrocarbon exploration.

A balanced 2D structural model of the Hammerhead Delta–Deepwater Fold-Thrust Belt, Bight Basin, Australia

The Hammerhead Delta–Deepwater Fold-Thrust Belt is located in the Ceduna Sub-Basin of the Bight Basin, offshore southern Australia. It is synonymous with the Hammerhead Supersequence and consists of three, Campanian to Maastrichtian, deltaic sediment packages. The Hammerhead Delta–Deepwater Fold-Thrust Belt is a short-lived gravity-gliding system that exhibits a distinctive spoon-shape in cross-section. The system detaches on a master horizon at the top of the Tiger Formation. Finite Element Method based two-dimensional restorations show that the Hammerhead Delta–Deepwater Fold-Thrust Belt is a near-balanced system with near equal amounts of up-dip extension and down-dip compression. Overall, there is only 2.4% additional extension in the Hammerhead Delta–Deepwater Fold-Thrust Belt. This near-balanced system is unusual in comparison with other passive margin Delta–Deepwater fold-thrust belts, which generally demonstrate large amounts of extension compared with shortening, due to the regional-scale progradational nature of the systems. The results suggest that sediment input to the Hammerhead Delta–Deepwater Fold-Thrust Belt was not sufficient to result in the regional-scale progradation of fault activity and that the sediment supply shutdown before the system could develop in an extensive passive margin Delta–Deepwater fold-thrust belt, hence demonstrating that it is sediment supply that drives ongoing gravitational deformation in Delta–Deepwater fold-thrust belts and not slope gradient.

Stress deflections around salt diapirs in the Gulf of Mexico

Abstract Delta–deepwater fold–thrust belts are linked systems of extension and compression. Margin-parallel maximum horizontal stresses (extension) on the delta top are generated by gravitational collapse of accumulating sediment, and drive downdip margin-normal maximum horizontal stresses (compression) in the deepwater fold–thrust belt (or delta toe). This maximum horizontal stress rotation has been observed in a number of delta systems. Maximum horizontal stress orientations, determined from 32 petroleum wells in the Gulf of Mexico, are broadly margin-parallel on the delta top with a mean orientation of 060 and a standard deviation of 49°. However, several orientations show up to 60° deflection from the regional margin-parallel orientation. Three-dimensional (3D) seismic data from the Gulf of Mexico delta top demonstrate the presence of salt diapirs piercing the overlying deltaic sediments. These salt diapirs are adjacent to wells (within 500 m) that demonstrate deflected stress orientations. The maximum horizontal stresses are deflected to become parallel to the interface between the salt and sediment. Two cases are presented that account for the alignment of maximum horizontal stresses parallel to this interface: (1) the contrast between geomechanical properties of the deltaic sediments and adjacent salt diapirs; and (2) gravitational collapse of deltaic sediments down the flanks of salt diapirs.

Petroleum Geoscience of the West Africa Margin

The West Africa margin, formed by the progressive separation of the South American and African continents, has enjoyed a rich and varied exploration history and become a significant hydrocarbon-producing region. The amalgamation of hydrocarbon exploration approaches and imaginative ideas, leveraged with modern technologies, is yielding significant scientific and economic successes within the region. The main objective of this Special Publication is to provide an overview of the advancement in understanding of the crustal structure, tectonic evolution and Mesozoic to Cenozoic stratigraphy of the West Africa margin both onshore and offshore, with a particular focus on the petroleum geology.

The petroleum geology of the West Africa margin: an introduction

Abstract The continental margin of West Africa formed as result of the south-to-north rifting of Gondwana and the progressive separation of the South American and African continents. This margin has enjoyed a rich and varied exploration history and, in the 70 years or so since the first significant exploration began in the onshore area, the margin has emerged as a significant hydrocarbon-producing region. The amalgamation of hydrocarbon exploration approaches and imaginative ideas, leveraged with modern technologies, is yielding significant scientific and economic successes. The main objective of this Special Publication is to provide an overview of the advances in our understanding of the crustal structure, tectonic evolution and Mesozoic to Cenozoic stratigraphy of the West Africa margin both onshore and offshore, with a particular focus on the petroleum geology. The papers contained in this Special Publication represent a selection from the 37 abstracts presented at the original conference in March 2014, which covered the entirety of the margin from South Africa to Morocco as well as stratigraphy from the crystalline basement to the most recent strata. The original abstracts from the conference are available through the Geological Society of London website at: http://www.geolsoc.org.uk/pgresources

3D seismic analysis of complex faulting patterns above the Snapper Field, Gippsland Basin: implications for CO2 storage

Mechanical damage (e.g. faults and fractures) related to tectonic forces and/or variations in formation pore pressures may enable the leakage of fluids through otherwise effective seal rocks. Characterisation of faults and fractures within seals is therefore essential for the assessment of long-term trap integrity in potential CO2 storage sites. 3D seismic reflection data are used to describe a previously unrecognised network of extensive, small Miocene-age faults with displacement of generally <30 m and lengths that vary between ∼300 and 2500 m above the Snapper Field, in the Gippsland Basin. The Snapper Field is a nearly depleted oil and gas field that presents an attractive site for potential CO2 storage due its structural closure and because it has effectively retained significant natural hydrocarbon (including CO2) columns over geological time-scales. Volume-based seismic attributes reveal that this fault system is located within the Oligocene Lakes Entrance Formation of the Seaspray Group, which acts as the regional seal to the Latrobe Group reservoirs in the Gippsland Basin. Detailed analysis of fault lengths and linkages suggests that the Miocene faults are non-tectonic, polygonal faults, although the displacement analysis of fault segments reveals strong correlations with the both the structure of the underlying Top Latrobe surface and normal faults that segment the Latrobe Group reservoirs, suggesting that the development of this fault system has been influenced by underlying structures. The geological evidence for long-term retention of hydrocarbons within the Snapper Field suggests that this fault system has not compromised the integrity of the Lakes Entrance Formation seal, although elevated pore pressures during CO2 injection could potentially lead to reactivation of these structures.