Fault enhancement using probabilistic neural networks and Laplacian of a Gaussian filter: a case study in the Great South Basin, New Zealand.

Fault identification is critical in defining the structural framework for both exploration and reservoir characterization studies. Interpreters routinely use edge-sensitive attributes such as coherence to accelerate the manual picking process, where the actual choice of a particular edge-sensitive attribute varies with the seismic data quality and with the reflectivity response of the faulted geologic formations. CMY color blending provides an effective way to combine the information content of two or three edge-sensitive attributes when more than one attribute is sensitive to faults. We evaluate whether combining the information content of more than three attributes using probabilistic neural networks (PNN) provides any additional uplift. We employ a training data consisting of manually picked faults on a coarse grid of 3D seismic lines, and then we employ an exhaustive search PNN to identify the optimal set of attributes to create a fault probability volume for a 3D survey acquired over the Great South Basin, New Zealand. We construct a suite of candidate attributes using our understanding of the attribute response to faults seen in the data and examples extracted from the published literature to use the list as the analyzed attributes. Using a subset of picked faults as training data, we evaluate which suite of attributes and hyperparameters exhibit the highest validation on the remaining training data. When used together, we find that volume aberrancy magnitude, GLCM homogeneity, GLCM entropy, Sobel filter similarity, and envelope best predict the faults for this dataset. The PNN supervised classification creates a seismic image volume that exhibits fault probabilities providing a simple combination of multiple seismic attributes. We also find that applying a directional Laplacian of a Gaussian and skeletonization filters to the PNN fault volumes provides a superior result to simple CMY blending techniques.

Sedimentology and depositional environments of Murihiku Supergroup sediments exposed in the Southland Syncline, New Zealand: Implications for reservoir potential in the Great South Basin

<p>A considerable amount is known about the biostratigraphy and organic geochemistry of the Murihiku Supergroup sediments exposed in coastal outcrops of the Southland Syncline, New Zealand. Much less work has been undertaken on the sedimentology of these strata, or understanding their depositional environments and depositional trends through time. What these implications are for reservoir prospectivity in the adjacent Great South Basin, has also had little study focused on it. This thesis addresses these issues by undertaking outcrop-based sedimentological and facies interpretations of these rocks, thin-section based petrographic composition and provenance analysis, augmented by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), as well as porosity and permeability measurements from outcrop core plugs. Petroleum industry seismic data has additionally enabled seismic facies mapping of Murihiku rocks in the offshore Great South Basin. Outcrop observations point to a progressive change in depositional setting, from shelf / upper slope settings during the Late Triassic, to base of slope turbidite deposition in the Early Jurassic. This transgression is followed by regression into fluvial settings in the youngest outcropping Murihiku rocks in the study of Middle Jurassic age. Petrographically the sandstones are feldspathic and lithic arenites and feldspathic and lithic wackes. Provenance suggests derivation from an evolving, intermediate arc that was becoming more siliceous through Late Triassic and Middle Jurassic time. Diagenesis is characterised by early calcite and chlorite precipitation which have almost completely destroyed any primary porosity. Any secondary micro porosity has subsequently been infilled through dissolution of framework grains and zeolitization. SEM and core plug porosity and permeability measurements corroborate the diagenetic changes observed petrographically, with only fluvial facies of Middle Jurassic (Upper Temaikan) age showing any measureable porosity or permeability. As a result, reservoir potential for the Late Triassic to Middle Jurassic, Murihiku Supergroup rocks analysed in this study is low. Younger Murihiku sandstones which are postulated to occur offshore in the Great South Basin are likely to be less influenced by burial diagenesis. As shown from North Island occurrences, these younger successions hold some potential.The reservoir potential for these youngest portions of the Murihiku succession therefore remains positive, both in the Great South Basin, as well as other frontier areas of Zealandia, and continue to provide an exploration target for the petroleum industry.</p>

Sedimentology and depositional environments of Murihiku Supergroup sediments exposed in the Southland Syncline, New Zealand: Implications for reservoir potential in the Great South Basin

<p>A considerable amount is known about the biostratigraphy and organic geochemistry of the Murihiku Supergroup sediments exposed in coastal outcrops of the Southland Syncline, New Zealand. Much less work has been undertaken on the sedimentology of these strata, or understanding their depositional environments and depositional trends through time. What these implications are for reservoir prospectivity in the adjacent Great South Basin, has also had little study focused on it. This thesis addresses these issues by undertaking outcrop-based sedimentological and facies interpretations of these rocks, thin-section based petrographic composition and provenance analysis, augmented by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), as well as porosity and permeability measurements from outcrop core plugs. Petroleum industry seismic data has additionally enabled seismic facies mapping of Murihiku rocks in the offshore Great South Basin. Outcrop observations point to a progressive change in depositional setting, from shelf / upper slope settings during the Late Triassic, to base of slope turbidite deposition in the Early Jurassic. This transgression is followed by regression into fluvial settings in the youngest outcropping Murihiku rocks in the study of Middle Jurassic age. Petrographically the sandstones are feldspathic and lithic arenites and feldspathic and lithic wackes. Provenance suggests derivation from an evolving, intermediate arc that was becoming more siliceous through Late Triassic and Middle Jurassic time. Diagenesis is characterised by early calcite and chlorite precipitation which have almost completely destroyed any primary porosity. Any secondary micro porosity has subsequently been infilled through dissolution of framework grains and zeolitization. SEM and core plug porosity and permeability measurements corroborate the diagenetic changes observed petrographically, with only fluvial facies of Middle Jurassic (Upper Temaikan) age showing any measureable porosity or permeability. As a result, reservoir potential for the Late Triassic to Middle Jurassic, Murihiku Supergroup rocks analysed in this study is low. Younger Murihiku sandstones which are postulated to occur offshore in the Great South Basin are likely to be less influenced by burial diagenesis. As shown from North Island occurrences, these younger successions hold some potential.The reservoir potential for these youngest portions of the Murihiku succession therefore remains positive, both in the Great South Basin, as well as other frontier areas of Zealandia, and continue to provide an exploration target for the petroleum industry.</p>

Trace Metal Variations through the Tartan and Waipawa Formations: Implications for the Environment of Deposition

<p>An inorganic geochemical study of the Late Paleocene organic matter-rich Waipawa and Tartan formations was undertaken in order to investigate the depositional environment. The formation varies in thickness between 2 and 50 metres and is distributed across many of New Zealand’s Cenozoic basins, where it forms an important potential hydrocarbon source rock. This study measured major and trace elements which can be loosely grouped into redox sensitive, biologically influenced, terrestrially sourced, and rare earth elements (REE). The study focused on three sections through the Waipawa and Tartan formations: Angora Quarry in the East Coast Basin, and the Great South Basin hydrocarbon exploration wells Kawau-1A and Pakaha-1. At Angora Quarry, x-ray fluorescence (XRF) was used to measure the major constituents Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, SO₃, K₂O, CaO, TiO₂, MnO and Fe₂O₃. inductively-coupled plasma mass spectrometry (ICP-MS) was used to measure Li, Ca, Ti, V, Cr, Co, Ni, Cu, Zn, Ga, Ge, As, Rb, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Ba, REE, Hf, Tl, Pb, Th and U. For Pakaha-1 and Kawau-1A side wall core samples, ICP-MS was used to measure Ti, V, Cr, Mn, Co, Ni, Cu, As, Se, Rb, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Hf, Ta, W, Tl, Pb, Bi, Th and U. Insufficient sample was available for XRF on these samples. No major changes in oxygen concentration during deposition were recorded by redox-sensitive elements from Angora Quarry and Pakaha-1 sediments; however samples from Kawau-1A and from a section 1 km upstream from Angora Quarry were deposited under somewhat oxygen-depleted conditions. As the anoxic and suboxic indicators show significantly lower variations than under present day anoxic environments, and in Angora Quarry CaO and SO₃ are significantly depleted with higher aluminosilicates a rapid deposition is required to explain the preservation of the organic matter. In the Great South Basin wells, the clay content correlates directly with increased gamma ray levels measured by well logs. Increased influx of terrestrial clays has been linked to marine transgressions in many New Zealand sediments and is been taken to mean the same for the Waipawa and Tartan formations. The oxygen depletion indicates that water depths during deposition exceeded 50 metres. The depositional model proposed here, therefore, is that of a major marine transgression that flooded and eroded near-shore swamps, re-depositing the terrestrial organic matter offshore. The increased nutrients released by this would have stimulated bioproductivity and locally, where conditions were suitable, depleted the oxygen content of the water column. This study also suggests ternary diagrams are valuable for calculating the enrichment of elements affected by two processes, such as Sr, which is related to both detrital Al and related to biological Ca. Ga, Ba and Al content are also related on a ternary diagram indicating the similar terrestrial and biological relationships for Ba and Ga. W was found to behave in a similar way to Bi. Enrichment factors proved less useful than absolute enrichment for Kawau-1A, where detrital input varied greatly and was found to be significantly different in composition to average shale as defined by Wedephol (1971).</p>

Trace Metal Variations through the Tartan and Waipawa Formations: Implications for the Environment of Deposition

<p>An inorganic geochemical study of the Late Paleocene organic matter-rich Waipawa and Tartan formations was undertaken in order to investigate the depositional environment. The formation varies in thickness between 2 and 50 metres and is distributed across many of New Zealand’s Cenozoic basins, where it forms an important potential hydrocarbon source rock. This study measured major and trace elements which can be loosely grouped into redox sensitive, biologically influenced, terrestrially sourced, and rare earth elements (REE). The study focused on three sections through the Waipawa and Tartan formations: Angora Quarry in the East Coast Basin, and the Great South Basin hydrocarbon exploration wells Kawau-1A and Pakaha-1. At Angora Quarry, x-ray fluorescence (XRF) was used to measure the major constituents Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, SO₃, K₂O, CaO, TiO₂, MnO and Fe₂O₃. inductively-coupled plasma mass spectrometry (ICP-MS) was used to measure Li, Ca, Ti, V, Cr, Co, Ni, Cu, Zn, Ga, Ge, As, Rb, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Ba, REE, Hf, Tl, Pb, Th and U. For Pakaha-1 and Kawau-1A side wall core samples, ICP-MS was used to measure Ti, V, Cr, Mn, Co, Ni, Cu, As, Se, Rb, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Hf, Ta, W, Tl, Pb, Bi, Th and U. Insufficient sample was available for XRF on these samples. No major changes in oxygen concentration during deposition were recorded by redox-sensitive elements from Angora Quarry and Pakaha-1 sediments; however samples from Kawau-1A and from a section 1 km upstream from Angora Quarry were deposited under somewhat oxygen-depleted conditions. As the anoxic and suboxic indicators show significantly lower variations than under present day anoxic environments, and in Angora Quarry CaO and SO₃ are significantly depleted with higher aluminosilicates a rapid deposition is required to explain the preservation of the organic matter. In the Great South Basin wells, the clay content correlates directly with increased gamma ray levels measured by well logs. Increased influx of terrestrial clays has been linked to marine transgressions in many New Zealand sediments and is been taken to mean the same for the Waipawa and Tartan formations. The oxygen depletion indicates that water depths during deposition exceeded 50 metres. The depositional model proposed here, therefore, is that of a major marine transgression that flooded and eroded near-shore swamps, re-depositing the terrestrial organic matter offshore. The increased nutrients released by this would have stimulated bioproductivity and locally, where conditions were suitable, depleted the oxygen content of the water column. This study also suggests ternary diagrams are valuable for calculating the enrichment of elements affected by two processes, such as Sr, which is related to both detrital Al and related to biological Ca. Ga, Ba and Al content are also related on a ternary diagram indicating the similar terrestrial and biological relationships for Ba and Ga. W was found to behave in a similar way to Bi. Enrichment factors proved less useful than absolute enrichment for Kawau-1A, where detrital input varied greatly and was found to be significantly different in composition to average shale as defined by Wedephol (1971).</p>

Stable Isotope Geochemistry of Paleocene to Early Eocene Strata around Southern New Zealand

<p>The Late Teurian (Paleocene) Tartan Formation is an organic-rich mudstone that has been identified in five of the eight exploration wells drilled in the Great South Basin, and three of four exploration wells drilled in the Canterbury Basin. In this study, the geochemistry of two wells from the Great South Basin (Pukaki-1 and Rakiura-1) and four wells from the Canterbury Basin in southern New Zealand (Resolution-1, Clipper-1, Galleon-1, and Endeavour-1) have been investigated using elemental analyser isotope ratio mass spectrometric (EA-IRMS) analyses on selected sidewall core and cuttings samples. This study builds on previous geochemical work by the author from five other wells from the Great South Basin (Takapu-1A, Toroa-1, Pakaha-1, Kawau-1A, and Hoiho-1C). All wells except Rakiura-1, Takapu-1A, and Resolution-1 showed geochemical characteristics that allowed recognition of the Tartan Formation. The formation is characterised by enrichments in TOC (typically above 3%) and 13C (generally delta13C ratios are between -21 and -17 per 1000), indicating a significant marine contribution. C/N ratios recorded within the Tartan Formation are all above 20, which suggest that the organic matter contains a significant contribution from terrestrial and/or altered marine material. Geochemical evidence of samples within the Tartan Formation suggests that it contains a mixture of marine bacterial/plant/algal and C3 terrestrial plant source components. This is consistent with the findings of Killops et al. (2000), who reported from biomarker studies that the organic matter of some Great South Basin samples contained organic matter derived from a marine source with varying degrees of terrestrial contribution. The Tartan Formation is distinct from enclosing formations which are characterised by low organic contents (generally below 2%), isotopically light delta13C values (typically around -26 per 1000), which is indicative of terrestrial C3 plant matter, and a wide range of C/N ratios (ranging from 4 to 64). The latter suggests that there were varying degrees of preservation of the deposited organic matter within these formations. Organic matter within enclosing formations appears to be derived from a combination of C3 land plants and marine material. The high TOC content of Tartan Formation sediments compared to the underlying formation suggests that it represents a profound change in depositional conditions. Conditions for the preservation and accumulation of organic matter were more favorable prior to deposition of the Tartan Formation than following it. The enrichment of 13C and the high TOC contents within the Tartan Formation are similar to those for the mid to Late Teurian Waipawa Formation that has been identified throughout many of New Zealand's major sedimentary basins; however, TOC and delta13C values for the Tartan Formation exceed those previously reported for the Waipawa Formation. Geochemical changes characteristic of the Tartan Formation are recognised below the lithological base of the formation in some wells, contemporaneous with the onset of the Paleocene Carbon Isotope Maximum (PCIM), and represent different lithostratigraphic expressions of that event. Termination of the environmental effects associated with the PCIM around New Zealand appears to have been diachronous and differences between the exact ages and stratigraphic positions of the Tartan and Waipawa formations are attributed to local environmental variations during deposition. TOC and delta13C enrichments associated with the Tartan Formation are not ubiquitous, and the formation has variable thickness throughout the Great South and Canterbury basins. It is concluded that the Tartan and Waipawa formations are correlatives.</p>

Stable Isotope Geochemistry of Paleocene to Early Eocene Strata around Southern New Zealand

<p>The Late Teurian (Paleocene) Tartan Formation is an organic-rich mudstone that has been identified in five of the eight exploration wells drilled in the Great South Basin, and three of four exploration wells drilled in the Canterbury Basin. In this study, the geochemistry of two wells from the Great South Basin (Pukaki-1 and Rakiura-1) and four wells from the Canterbury Basin in southern New Zealand (Resolution-1, Clipper-1, Galleon-1, and Endeavour-1) have been investigated using elemental analyser isotope ratio mass spectrometric (EA-IRMS) analyses on selected sidewall core and cuttings samples. This study builds on previous geochemical work by the author from five other wells from the Great South Basin (Takapu-1A, Toroa-1, Pakaha-1, Kawau-1A, and Hoiho-1C). All wells except Rakiura-1, Takapu-1A, and Resolution-1 showed geochemical characteristics that allowed recognition of the Tartan Formation. The formation is characterised by enrichments in TOC (typically above 3%) and 13C (generally delta13C ratios are between -21 and -17 per 1000), indicating a significant marine contribution. C/N ratios recorded within the Tartan Formation are all above 20, which suggest that the organic matter contains a significant contribution from terrestrial and/or altered marine material. Geochemical evidence of samples within the Tartan Formation suggests that it contains a mixture of marine bacterial/plant/algal and C3 terrestrial plant source components. This is consistent with the findings of Killops et al. (2000), who reported from biomarker studies that the organic matter of some Great South Basin samples contained organic matter derived from a marine source with varying degrees of terrestrial contribution. The Tartan Formation is distinct from enclosing formations which are characterised by low organic contents (generally below 2%), isotopically light delta13C values (typically around -26 per 1000), which is indicative of terrestrial C3 plant matter, and a wide range of C/N ratios (ranging from 4 to 64). The latter suggests that there were varying degrees of preservation of the deposited organic matter within these formations. Organic matter within enclosing formations appears to be derived from a combination of C3 land plants and marine material. The high TOC content of Tartan Formation sediments compared to the underlying formation suggests that it represents a profound change in depositional conditions. Conditions for the preservation and accumulation of organic matter were more favorable prior to deposition of the Tartan Formation than following it. The enrichment of 13C and the high TOC contents within the Tartan Formation are similar to those for the mid to Late Teurian Waipawa Formation that has been identified throughout many of New Zealand's major sedimentary basins; however, TOC and delta13C values for the Tartan Formation exceed those previously reported for the Waipawa Formation. Geochemical changes characteristic of the Tartan Formation are recognised below the lithological base of the formation in some wells, contemporaneous with the onset of the Paleocene Carbon Isotope Maximum (PCIM), and represent different lithostratigraphic expressions of that event. Termination of the environmental effects associated with the PCIM around New Zealand appears to have been diachronous and differences between the exact ages and stratigraphic positions of the Tartan and Waipawa formations are attributed to local environmental variations during deposition. TOC and delta13C enrichments associated with the Tartan Formation are not ubiquitous, and the formation has variable thickness throughout the Great South and Canterbury basins. It is concluded that the Tartan and Waipawa formations are correlatives.</p>

Water column stability as an important factor controlling nitrite-dependent anaerobic methane oxidation in stratified lake basins

Anaerobic oxidation of methane (AOM) with nitrate/nitrite as the terminal electron acceptor may play an important role in mitigating methane emissions from lacustrine environments to the atmosphere. We investigated AOM in the water column of two connected but hydrodynamically contrasting basins of a south-alpine lake in Switzerland (Lake Lugano). The North Basin is permanently stratified with year-round anoxic conditions below 120 m water depth, while the South Basin undergoes seasonal stratification with the development of bottom water anoxia during summer. We show that below the redoxcline of the North Basin a substantial fraction of methane was oxidized coupled to nitrite reduction by Candidatus Methylomirabilis. Incubation experiments with 14CH4 and concentrated biomass from showed at least 43-52%-enhanced AOM rates with added nitrate/nitrite as electron acceptor. Multiannual time series data on the population dynamics of Candidatus Methylomirabilis in the North Basin following an exceptional mixing event in 2005/2006 revealed their requirement for lasting stable low redox-conditions to establish. In the South Basin, on the other hand, we did not find molecular evidence for nitrite-dependent methane oxidizing bacteria. Our data suggest that here the dynamic mixing regime with fluctuating redox conditions is not conducive to the development of a stable population of relatively slow-growing Candidatus Methylomirabilis, despite a hydrochemical framework that seems more favorable for nitrite-dependent AOM than in the North Basin. We predict that the importance of N-dependent AOM in freshwater lakes will likely increase in future because of longer thermal stratification periods and reduced mixing caused by global warming.

Seismic Characteristics of Paleo-Pockmarks in the Great South Basin, New Zealand

Globally, a wide range of pockmarks have been identified onshore and offshore. These features can be used as indicators of fluid expulsion through unconsolidated sediments within sedimentary basin-fills. The Great South Basin, New Zealand, is one such basin where paleo-pockmarks are observed at around 1,500 m below the seabed. This study aims to describe the characteristics of paleo-pockmarks in the Great South Basin. Numerous paleo-pockmarks are identified and imaged using three-dimensional seismic reflection data and hosted by fine-grained sediments of the Middle Eocene Laing Formation. The paleo-pockmarks are aligned in a southwest to northeast direction to form a fan-shaped distribution with a high density of around 67 paleo-pockmarks per square kilometre in the centre of the study area. The paleo-pockmarks in this area have a similar shape, varying from sub-rounded to a rounded planform shape, but vary in size, ranging from 138 to 481 m in diameter, and 15–45 ms (TWT) depth. The origin of the fluids that contributed to the paleo-pockmark formation is suggested, based on seismic observations, to be biogenic methane. The basin floor fan deposits beneath the interval hosting the paleo-pockmark might have enhanced fluid migration through permeable layers in this basin-fill. This model can help to explain pockmark formation in deep water sedimentary systems, and may inform future studies of fluid migration and expulsion in sediment sinks.