Constraining the depositional age of Neoproterozoic stratigraphy in western North America has implications for correlating global glaciation and tectonic events. The depositional ages of the Neoproterozoic Mutual and Browns Hole formations of northern Utah are controlled by two conflicting datapoints. However, new U-Pb geochronological data from 95 detrital apatite grains refines the maximum depositional age of the volcanic member of the Browns Hole Formation to 613 ± 12 Ma (2σ). This places new restrictions on the time available for the deposition of underlying units. Due to debate regarding the age models for underlying stratigraphy, two scenarios for sediment accumulation rates are explored. These results highlight a need for further exploring regional unconformities. Evidence for several post-depositional fluid-rock interaction events are observed in the Mutual and Browns Hole formations. Cross-cutting relationships identified via petrography, scanning electron microscopy, and electron microprobe analysis show at least seven fluid mediated events: (1) early grain-rimming hematite cement, (2) quartz overgrowth and cement development, (3) feldspar dissolution, (4) phosphate dissolution, (5) partial quartz dissolution, (6) authigenic mineral precipitation in cluding clays, sericite, monazite, and apatite cement, and (7) later hematite cementation. Constraining the timing of these events is challenging due to a limited of datable material. Using basic geochemical modeling and consideration of expected mineral formation conditions, a paragenetic sequence is placed into context of the known geologic history.

No description available.

Thesis (Ph. D.)--Oregon State University, 2009. / Printout. Includes bibliographical references (leaves 161-169). Also available on the World Wide Web.

Master of Science / Department of Geology / Matthew Totten / The Viola Limestone is a prominent petroleum reservoir in the Mid-Continent Region, particularly in Oklahoma and Kansas. Coral Coast Petroleum established production from the Viola Ls. in 2011 in their Stephens Ranch lease in Clark County, south-central Kansas. Development of this lease has been hindered by the unpredictable production rates encountered in each of the subsequent eleven development wells. Infield drilling locations to date were chosen by favorable structural position as determined by 3D seismic. The best reservoir conditions, however, do not necessarily coincide with structural position. It was the purpose of this study to determine whether the ideal porosity and permeability are controlled by depositional environment, diagenetic alterations, or a combination of these factors. Several approaches to solve this question were implemented and utilized, including well log analysis, petrographic inspection of well cuttings and thin sections, and the application of the Scanning Electron Microscope (SEM). An exploration model of the Viola Ls. in this field was developed, where the Viola A and B zones were dolomitized during during marine transgressions by mixing of sea water with other Mg-rich fluids. Reservoir conditions are found where these facies were preserved as paleotopographic highs during a subsequent sea level low-stand. These preserved dolomitized facies correspond to the seismic facies identified by seismic attributes in a 3D seismic study by Vohs (2016).

Parmi la diversité des types microstructuraux que l’on peut trouver dans les coquilles des Mollusques, les architectures entrecroisées (lamellaire-croisé, lamellaire-croisé complexe, folié-croisé) sont de loin les plus abondantes. Mais elles restent, de par leurs organisations tridimensionnelles complexes, peu documentées : la majorité des études de biominéralisation, et les différents modèles qui en découlent, sont en effet essentiellement basés sur des architectures plus simples (telles que les couches nacrées ou prismatiques). Or il convient, si l'on souhaite aboutir à une meilleure compréhension des mécanismes fondamentaux qui dirigent la minéralisation de ces biocarbonates, de vérifier dans quelle mesure les modèles développés à partir de coquilles ”simples” sont aussi applicables à des organisations microstructurales plus complexes. Cette étude se focalise donc sur les couches entrecroisées des coquilles de quelques Mollusques, dans le but de mettre en évidence les différents niveaux du contrôle biologique que l'organisme exerce sur leur formation et leur croissance. À cette fin, des techniques de caractérisation in-situ des assemblages organiques sont privilégiées, en relation étroite avec l’analyse des organisations microstructurales et des mécanismes de biocristallisation à fine échelle. Quelques aspects de la diagénèse de ces microstructures seront aussi abordés, à travers l’étude de coquilles fossiles de Patella sp (~100 ka) et Velates perversus (~50 Ma). / Among the variety of microstructural types that can be found within Molluscs shells, intercrossed structures (crossed-lamellar, complex crossed-lamellar and cross-foliated) are by far the most commonly found. They are, however, still poorly documented - mainly due to their complex 3D organization. The majority of biomineralization studies, and the resulting models, are indeed essentially based on simple architectures (such as nacreous or prismatic layers). In order to achieve a better understanding of the fundamental mechanisms that drive the mineralization of such biocarbonates, it is therefore mandatory to check to which extent models developed from « simple » shells stay consistent when applied to more complex microstructural organizations. The present study focuses on intercrossed layers of several Mollusk shells, in order to highlight the various levels of biological control exerted by the organism on their formation and growth. In-situ techniques are used to characterize biochemical compositions, in close correlation with microstructural patterns, as well as fine-scale biocrystallization processes. Some peculiar features of the diagenesis of these microstructures are illustrated, through the study of fossil shells from Patella sp (~100 ky) and Velates perversus (~50 My).

L'apport de contraintes chronologiques absolues sur les différents évènements et processus qui ont modelé la Terre constitue un des défis majeurs dans de nombreux domaines des sciences de la terre et de l'environnement. C'est en particulier le cas pour les bassins sédimentaires qui jouent un rôle économique majeur en étant sources d'importantes ressources naturelles, en particulier en hydrocarbures. L'objectif de cette thèse est de développer la datation absolue des minéraux secondaires carbonatés par la méthode U-Pb. Ces minéraux, quasi ubiquistes dans les réservoirs, témoignent des processus diagénétiques, tectoniques et des circulations fluides qui ont affecté ces systèmes. Au cours de cette thèse, plusieurs développements méthodologiques ont été mis en œuvre avec pour objectifs de repousser les limites de la méthode U-Pb. Des techniques d’analyses innovantes, comme le couplage ablation laser/SF-ICP-MS ou la sonde ionique, ont été testées et ont permis d’accroître de plusieurs ordres de grandeur la résolution spatiale des analyses U-Pb. Ces méthodes, couplées à la méthode plus classique de dilution isotopique ont été appliquées avec succès dans différents contextes d’exploration pétrolière permettant notamment d’apporter des contraintes temporelles sur des éléments clés du système pétrolier, comme le timing de la création/préservation du réservoir ou encore de la migration des hydrocarbures. Les différents résultats obtenus ont permis de dresser une synthèse des contextes les plus favorables à l’application de la méthode U-Pb, et de donner ainsi une vision d’ensemble du potentiel de la datation U-Pb sur carbonates secondaires appliquée aux réservoirs pétroliers. / Absolute chronological constraints on the different events and processes that have shaped the Earth constitute a major challenge in numerous realms in Earth and Environmental sciences. In particular this is the case for sedimentary basins that play a major economic role as being source of important hydrocarbon resources. The objective of this thesis is to develop absolute uranium-lead dating methodology on secondary carbonate minerals. This ubiquitous mineral phase in petroleum reservoirs testifies their complex geodynamic and diagenetic histories. In this study, several developments were implemented in order to circumvent the limits of U-Pb methodology. Innovative analysis techniques such as laser ablation coupled with SF-ICP-MS or ion probe were tested to increase the spatial resolution of the U-Pb analysis by several orders of magnitude. These methodologies coupled to the more traditional isotope dilution was successfully applied in different oil exploration context allowing to bring absolute constraints on key diagenetic events such as creation/preservation of reservoir properties or hydrocarbon migration. The results obtained during this study allowed to draw a synthetic model of the most favorable contexts for U-Pb method and gives an overview of the U-Pb dating potential to secondary carbonates applied to petroleum reservoirs.

Arenitos do Cretáceo Superior da Bacia de Santos correspondem aos principais reservatórios clásticos de hidrocarbonetos da bacia, localizada na margem leste brasileira. Alguns desses arenitos exibem porosidades anormalmente altas, considerando suas atuais profundidades maiores que 4000 m. A preservação da porosidade nesses arenitos, assim como em diversos outros reservatórios clásticos profundos vem sendo atribuída à inibição da cimentação por crescimentos de quartzo e da dissolução por pressão exercida por franjas e cutículas de clorita autigênica. Por esse motivo, o estudo da distribuição espacial e temporal das cloritas, assim como de seus hábitos, relações paragenéticas e condições genéticas, é de grande importância. Os arenitos cretácicos de Santos são arcósios e arcósios líticos, ricos em fragmentos de rochas vulcânicas (FRV). Clorita é o constituinte diagenético mais abundante, ocorrendo principalmente como franjas e cutículas cobrindo os grãos (pore-lining), rosetas preenchendo parcialmente os poros e agregados microcristalinos substituindo grãos. A precipitação de clorita foi favorecida pela presença de cutículas eogenéticas de argila esmectítica, que foram parcialmente preservadas ao longo de contatos intergranulares apertados. Além disso, a composição primária dos arenitos exerceu um controle fundamental na autigênese das cloritas. O enriquecimento de clorita nos arenitos arcósios líticos, ricos em FRV, sugere que esses grãos são uma significativa fonte de íons de Fe e Mg para a precipitação de clorita nesses arenitos. Além dos FRV, minerais pesados instáveis, biotita e intraclastos lamosos atuaram como fonte e/ou substrato para a autigênese da clorita. Adicionalmente à composição primária, a história térmica e de soterramento, assim como o padrão de fluxo de fluidos, certamente tiveram um papel importante na distribuição da diagênese da clorita e na evolução da qualidade dos reservatórios. Prováveis fontes externas de íons devem ter envolvido as reações de transformações de argilominerais esmectíticos em lutitos associados, e a espessa seção de evaporitos Aptianos. Os diferentes hábitos das cloritas exerceram impacto distinto na qualidade dos reservatórios. Cloritas pore-lining inibiram a cimentação por quartzo, isolando as superfícies dos grãos de quartzo e reduzindo a nucleação de crescimentos secundários, contribuindo assim para a preservação da porosidade. No entanto, franjas e cutículas finas e descontínuas não foram capazes de inibir efetivamente a cimentação por quartzo, enquanto que as muito espessas reduziram severamente a permeabilidade dos reservatórios. Rosetas de clorita, por sua vez, não inibiram a cimentação de quartzo e ainda reduziram a porosidade intergranular. Estudos específicos são necessários para um melhor entendimento dos diferentes processos diagenéticos nos arenitos da Bacia de Santos, assim como de sua distribuição temporal, estratigráfica e espacial, de modo a basear o desenvolvimento de modelos que possam efetivamente contribuir para a redução dos riscos durante a exploração por esses reservatórios. / Upper Cretaceous sandstones from the Santos Basin, eastern Brazilian Margin, correspond to the main clastic hydrocarbon reservoirs of the basin. Some of these sandstones show abnormally high porosities, considering their present depths larger than 4000 m. The preservation of porosity in these sandstones, as in other deep clastic reservoirs, has been ascribed to the inhibition of quartz overgrowth cementation and pressure dissolution by rims and coatings of authigenic chlorite. Therefore, the study of the space and time distribution of the chlorites, as well as of their habits, paragenetic relations and genetic conditions, is of great importance. Santos cretaceous sandstones are arkoses and lithic arkoses, rich in volcanic rock fragments (VRF). Chlorite is the most abundant diagenetic constituent, occurring mostly as rims and coatings covering the grains (pore-lining), as rosettes partially filling the pores, and as grain-replacive microcrystalline aggregates. Chlorite precipitation was favored by the presence of eogenetic coatings of smectitic clays, which were partially preserved along tight intergranular contacts. Besides these, the primary composition of the sandstones exerted a key control in chlorite authigenesis. Chlorite enrichment in the lithic arkoses, rich in VRF, suggests that these grains are significant source of Fe and Mg ions for chlorite precipitation in these sandstones. Besides VRF, unstable heavy minerals, biotite and mud intraclasts acted as source or substrate for chlorite authigenesis. In addition to the primary composition, thermal and burial histories, as also the fluid flow patterns, certainly played an important role in chlorite diagenesis and in the evolution of reservoir quality. Probable external ionic sources must have involved the transformation of smectitic clays in associated mudrocks, and the thick section of Aptian evaporites. The diverse chlorite habits exerted distinct impact on the quality of the reservoirs. Pore-lining chlorites inhibited quartz cementation isolating quartz grains surfaces, reducing the nucleation of overgrowths, thus contributing to porosity preservation. However, thin and discontinuous rims and coatings were not able to effectively inhibit quartz cementation, while those very thick severely decreased the permeability of the reservoirs. In turn, chlorite rosettes have not inhibited quartz cementation, but have reduced intergranular porosity nonetheless. Specific studies are required for a better understanding of the diverse diagenetic processes wthin Santos Basin sandstones, as also of their time, stratigraphic and space distribution, in order to support the development of models that may effectively contribute to the reduction of risks during the exploration for these reservoirs.

Carbonate rocks consider as significant reservoirs for hydrocarbon. More than 60% of the world's hydrocarbon is placed in carbonate reservoirs. Carbonate rocks are heterogeneous and contain complex pore system. This complexity causes the hydrocarbon recovery from these reservoirs difficult; having less than 35% of hydrocarbon is being recovered. The heterogeneity and the variation in pore system are a result of various depositional settings and successive diagenetic overprints. Diagenetic overprints account for most of the pore system complexity in subsurface. This project undertakes one of the important diagenetic processes, calcite cementation, which though to have major impact on reservoir quality. The project aims to better understand the controls on calcite cementation in five Lower Cretaceous Reservoirs, in particular the role of calcite cementation with relation to oil charge in reservoir quality. Other diagenetic processes were also asses including dolomitization, dissolution, micritization and chemical compaction. The five reservoirs (A, B, C, F & G) are from Field A which is located in Abu Dhabi, UAE. The reservoirs comprise of interbedded porous "Reservoir" and low porosity-permeability "Dense" limestones deposited in broad range of settings, ranging from restricted to open marine platform. Reservoir intervals are part of HSTs, deposited during higher sea level time. The dense intervals were deposited during TST and thought to be cemented early resulting in early compartmentalization in all reservoirs. The mMg/Ca and in-situ (SIMS) δ18OVPDB were measured through complete calcite cement stratigraphy obtained from equant, syntaxial and blocky calcite in all reservoirs. Both mMg/Ca and δ18OVPDB of oldest calcite cement zone are matching with published mMg/Ca and δ18OVPDB of Lower Cretaceous, suggesting precipitation from Lower Cretaceous seawater. The mMg/Ca and δ18OVPDB also vary from reservoir to another reflecting change in Cretaceous seawater. These data also coincide with trace element observations particularly Mn and Sr. All these parameters show fluctuations in Cretaceous seawater during 135-123Ma caused by global changes in climate and oceanic crust production rates. Lower reservoirs including F (133Ma) and C (130Ma) were more probably affected by the Hauterivian global cooling which resulted in higher δ18OVPDB in the early precipitated cements. Precipitation in upper Reservoir B (126Ma) was most likely affected by the abrupt warm episode just before the OAE1. Reservoir A (123Ma) precipitation may be affected by the Early Aptian cooling episode and the OAE1. Reservoir G is the only one not recording δ18OVPDB similar of Cretaceous seawater. Cementation in Reservoir G was affected by depleted δ18OVPDB fluids from early stage, perhaps hot, basinal waters. More interestingly, the mMg/Ca, δ18OVPDB Mn and Sr means of younger calcite cement zones which thought to be evolved during burial show similar trend to the oldest cement zones with various offsets. This suggests that calcite cement in each reservoir evolved in a relatively close system inferring by this that the reservoirs are compartmentalized. The compartmentalization is probably due to the sysedimentary or early cemented hardgrounds in the Dense Zones. The Dense Zones acted as seals for the reservoirs from early stage causing the later precipitated calcite cement which is diagenetically affected to behave in predictable and similar way. Moreover, calcite precipitation temperatures inferred from mMg/Ca and δ18OVPDB show progressive increase towards younger cement zones indicating fluid evolution with burial in also relatively close system. In-situ δ18OVPDB and oil inclusions suggest earlier oil charge in the shallower reservoirs compared with deeper reservoirs and coeval water leg. Consequently, cementation in the shallower reservoirs continued with lower rate and hence preserved some primary and secondary pores. Conversely, in the water leg cementation continue to occlude most of the pore spaces. Furthermore, early oil emplacement in the shallower reservoirs increased the cementation temperature of calcite in the oil leg to reach maximum precipitation temperatures of ~144˚C. Whereas, in the water leg and deeper reservoirs, cementation continued to a temperature of ~110˚C. Overall, reservoir quality in Thamama Group was affected by various diagenetic processes. Some have resulted in reservoir quality enhancement such as dolimitization which involves formation of microporosity as a result of replacive rhombic dolomite, dissolution particularly the late one which believed to be due organic acid, and micritization with yield microporosity particularly in Reservoir B. Open fractures might have also enhanced reservoir quality to some extent. Diagenetic events that have deteriorated reservoir quality include calcite and saddle dolomite cementation as well as stylolitization. Greater calcite cementation can be found in water leg compared with oil leg because oil thought to decrease cementation rate.

In the next 20 years, the global demand for oil is forecast to grow by 0.7% every year, and the demand for natural gas will increase by 1.6% annually. But as we continue to produce oil and gas, the resources of our current oilfields are depleting. To meet the rising global energy demand, it is essential that we can keep discovering more petroleum resources in the future. The primary aim of this PhD project is to deepen our understanding of hydrocarbon reservoirs and enhance our ability to explore. The first project looked at the geological risks in hydrocarbon exploration. It reviewed and statistically analysed the data of 382 unsuccessful boreholes in the UK offshore area. The results suggest that the most significant risk for an exploration well is encountering a thin or absent target reservoir. This risk happened to 27 ± 4% of the past unsuccessful wells. The following most common risks are low-porosity reservoirs (22 ± 4% of all cases) and the lack of a closed trap (23 ± 4%). The probability of a target reservoir having a leaky caprock is 5 ± 2%. The study has calculated the probability of occurrence of all the geological risks in exploration, and this risk data can be applied to predict the potential geological risks in future exploration. One challenge in developing saline aquifers as CO2 storage reservoirs is the lack of subsurface data, unless a well has been drilled. Drawing on the experience of hydrocarbon exploration, a potential CO2 storage site identified on seismic profiles will be subject to many uncertainties, such as thin or low-porosity reservoirs, leaky seals, which are analogue to the geological risks of an undrilled hydrocarbon prospect. Since the workflow of locating CO2 storage reservoirs is similar to the exploration for hydrocarbon reservoirs, the risk data of hydrocarbon exploration wells can be applied to infer the geological risks of the exploration wells for CO2 storage reservoirs. Based on this assumption, the study of Chapter 3 estimated that the probability of a borehole encountering a reservoir suitable for CO2 storage is c. 41-57% (90% confidence interval). For reservoirs with stratigraphic traps within the UKCS, the probability of success is slightly lower, at 39 ± 10% (90% confidence). Chapter 4 studies the porosity and diagenetic process of the Middle Jurassic Pentland Formation in the North Sea. The analysis data come from 21 wells that drilled and cored the Pentland Formation. Petrographic data suggest the content of detrital illite is the most important factor affecting the porosity of the Pentland Sandstone - the porosities of the sandstones with more than 15% of illite (determined by point-count) are invariably low (< 10%). Quartz cement grows at an average rate of 2.3 %/km below the depth of 2km, and it is the main porosity occluding phase in the deep Pentland Sandstone. Petrographic data shows the clean, fine-grained sandstones contain the highest amount of quartz cement. Only 1-2 % of K-feldspar seems to have dissolved in the deep Pentland Sandstone (> 2 km), and petrographic data suggest that K-feldspar dissolution does not have any substantial influence on the sandstone porosity. There is no geochemical evidence for mass transfer between the sandstones and shales of the Pentland Formation. Chapter 5 investigates the high porosity of the Pentland Sandstone in the Kessog Field, Central North Sea. The upper part of the Kessog reservoir displays an anomalously high porosity (c. 25 %, helium porosity) that is 10 % higher than the porosity of other Pentland sandstones at the same depth (c. 15 %, 4.1 - 4.4 km). Petrographic data show these high porosities are predominantly primary porosity. The effects of sedimentary facies, grain coats, secondary porosity and overpressure on the formation of the high porosity are considered to be negligible in this case. Early hydrocarbon emplacement is the only explanation for the high porosity. In addition to less quartz cement, the high-porosity sandstones also contain more K-feldspar and less kaolin than the medium-porosity sandstones of the same field. This indicates that early hydrocarbon emplacement has also inhibited the replacement of K-feldspar. The last chapter studies the potential mass transfer of silica, aluminium, potassium, iron, magenesium and calcium at sandstone-shale contacts. The study samples include 18 groups of sandstones and shales that were collected from five oilfields in the North Sea. The interval space between the samples of each group varies from centimetres to meters. The research aim is to find evidence of mass transfer by studying the samples' variation of mineralogy and chemistry as a function of the distance to the nearest sandstone-shale contact. The sandstones are mostly turbidite sandstones, and the shales are Kimmeridge Clay shales. Petrographic, mineralogical and chemical data do not provide firm evidence for mass transfer within any group of the samples. The result indicates that the scale of mobility of silica, aluminium, potassium, iron, magenesium and calcium in the subsurface may be below the scale of detection of the study method, i.e. < 5 cm.

Numerous detailed geochemical studies of Ediacaran (~635 - 541 Ma) marine successions provide snapshots into the palaeoenvironmental redox conditions which accompanied examples of the earliest metazoans in the fossil record. Spatial heterogeneity with respect to palaeomarine redox is evident from reconstructions of geographically-widespread Ediacaran environments. This project provides new data of local-scale redox within a paleogeographic and sequence stratigraphic framework in order to explore the mechanisms which controlled water column redox variations and the potential impact on early macro-benthic ecosystems. Lower than present atmospheric and oceanic oxygen concentrations enabled some shallow marine settings to remain poised at iron reduction until well into the Cambrian and likely influenced regional-scale ecosystem structure and stability. Many basins had a shallow and highly dynamic chemocline above anoxic (ferruginous or euxinic) or low oxygen (manganous) waters. Regional differences in palaeoredox were likely controlled primarily by local detrital nutrient provision and organic matter remineralisation and the redox state of the global deep ocean was most likely similarly heterogeneous (but this remains uncertain). It is suggested that cratonic positioning and migration throughout the Ediacaran Period, in combination with gradually increasing dissolved oxygen loading, may have provided a long-term control on redox evolution through regulating circulation mechanisms in the Mirovian Ocean. Some unrestricted lower slope environments from mid-high latitudes benefited from sustained oxygenation via downwelling, whilst cratonic isolation or transit towards more equatorial positions stifled pervasive ventilation either through ineffective surface ocean mixing, Ekman-induced upwelling, elevated surface ocean productivity, or a combination of these processes. Co-preservation of largely-enigmatic fossil forms within sedimentary rocks of the late Ediacaran Nama Group of southern Namibia have allowed the four-dimensional reconstruction of local redox dynamics and associated biotic establishment. This has been made possible through collation of previously published fossil occurrence and geochemical information alongside new palaeoredox and palaeoproductivity estimates based on iron speciation, major element and carbonate-bound iodine data. This is further supplemented by the first detailed assessment of the paragenetic sequence and diagenetic relationships of carbonates which precipitated within the earliest metazoan reef framework. Skeletal invertebrate taxa in the Zaris Sub-Basin of the lower Nama Group (~550-547 Ma), grew above wave base where micritic carbonate sediment often shows evidence for early dolomitisation. Mid-ramp Cloudina reefs composed of open, highly porous structures formed multiple, successive assemblages. Thin layers of dolomitised sediment and dolomite cement terminate each assemblage. Reef cements show a paragenetic sequence from synsedimentary, early marine cement through to final burial, each of which were precipitated under dynamic redox conditions. These cements likely record a general shallow to deeper water transect, from oxic shallow waters to low oxygen manganous waters and finally to oxic, shallow burial conditions. Transient incursions of upwelled, anoxic, ferruginous and dolomitising waters may have occurred during short-term, transgressive cycles, although the timing for this is poorly constrained. Such incursions may have terminated Ediacaran benthic communities that grew close to the chemocline. Viewed in its entirety, the palaeoredox record of the Nama Group reveals evidence for a pronounced shift in the depth of the ferruginous redoxcline from shallow to deeper levels in the water column through time, which was accompanied by a reduced frequency of anoxic incursions onto the shallow shelf. This transition approximately coincided with the first appearance and subsequent diversification of novel sediment bioturbators in the Lower Urusis Formation (~547-542 Ma). It is proposed that the observed coevolution of palaeoredox and ichnofossil diversity may directly relate to the impact of bioturbation on phosphorus retention. In this way, the diversification of burrowing forms effectively oxygenated the sediment column, prevented efficient P recycling to the water column and limited the detrimental impact of productivity-induced anoxia in the local environment. However, this hypothesis remains to be tested and would benefit from a focused study of palaeoproductivity employing targeted analyses of total organic carbon and sedimentary phosphorus speciation. It is further proposed that the persistent spatial separation of anoxic deep waters from habitable ecospace, implied by the fossil distribution of phylogenetically-enigmatic soft-bodied forms, qualitatively supports the inference that at least intermittently oxic conditions (at or above EH typical of ferrous iron oxidation) were a metabolic requirement of these organisms. Finally, four new sections of the late Ediacaran, deposited approximately time-equivalent to aforementioned sediments of the Nama Group, are described and preliminary geochemical data reported. These include two shallow marine carbonate-dominated sections of the southeast Siberian Craton which correspond to the Yudoma Formation and two sections of the Dengying and lower Zhujiaqing (and correlative) Formations deposited on the Yangtze Block, South China. Integrated proxy methods are able to distinguish palaeoredox heterogeneity between and within early animal ecosystems and test the influence of anoxia on ecosystem structure. The first and last appearances of Treptichnus pedum and Cloudina respectively, which globally bracket the boundary between the Ediacaran and Cambrian Periods, show no identifiable range overlap in any sections analysed in this study. This suggests that the first appearance of the organism responsible for characteristic T. pedum may have lived approximately contemporaneous in oxic habitable refuges of all regions in this study, regardless of the dominance of reducing conditions that persisted in coeval deeper environments in many areas.