In this study we investigated the effect of tidal forcing on iron cycling in intertidal saltmarsh sediments (ISS). Historically, sulfate has been considered the major terminal electron acceptor involved in organic carbon remineralization in ISS. Although sulfate is a more efficient electron acceptor for organic matter degradation in anoxic ISS, irons rapid recycling at the surface of ISS may allow it also to be an important electron acceptor for the remineralization of organic matter. Bioturbation, macrophyte-mediated irrigation, and semidiurnal tidal forcing in this environment may increase the abundance of O2 in the top few cm of the sediment, rapidly oxidizing iron and inhibiting sulfate reduction. To determine if the cycling of iron may be faster than previously thought in these sediments, we combined sediment core chemical profiles of reduced and oxidized insoluble iron with in-situ electrochemical profiles of O2, Fe2+, soluble organic-Fe3+ complexes, FeS(aq), and hydrogen sulfide in the top few centimeters of unvegetated creek bank sediments over several tidal cycles. We also installed monitoring wells in the tidal creek bank to quantify tidal forcing and to investigate tidal direction in the sediments. We built a transient, reactive transport model to simulate measured geochemical profiles and test our understanding of diagenetic processes. Additional tests were run on the model to investigate the importance of bioirrigation compared to tidally-induced porewater advection. Results indicate that tidal action is a more dominant transport process. It affects the cycling of iron in ISS by flushing reduced species out of the sediment during flood tide, and allowing oxygen and oxidized species deeper into the sediment during ebb tide. As a result, amorphous iron oxides are replenished at the sediment surface, and microbial iron reduction may be the main respiratory process in the first tens of centimeters of creek bank saltmarsh sediments subjected to intense tidal forcing.

The purpose of this study was to describe depositional and diagenetic characteristics of the (Jurassic) Smackover formation and subsequently identify and rank the quality of flow units within Grayson field, Columbia County, Arkansas. The field has production from the Smackover, a reservoir which consists mainly of highly altered peloidal grainstones. This was a four part study including a lithological analysis of ten cores, a petrographical study of 97 thin sections, a petrophysical study of reservoir properties from core analyses and borehole logs, and predictive mapping of quality ranked flow units across the field. Examination of the cores and thin sections revealed H1a as the main pore type in Grayson field, which was a hybrid of both depositional and diagenetic processes with dominantly interparticle pores. The lowest ranked reservoir quality corresponded to intraparticle and intercrystalline pore types, which occurred mainly in the wackestone/mudstone and packstone/wackestone facies. The highest ranked reservoir quality corresponded to the H1a pore type which occurred mainly in the grainestone/packstone facies 1 and 2. The reservoir quality maps identified the spatial distribution of the facies within the field, which could be used to locate zones for possible in-fill drilling. These results should aid in the economical development of Grayson field and other similar fields.

<p> Four measured sections of the Whirlpool Sandstone were prepared from outcrops in the Niagara Gorge and Hamilton areas. Sedimentary structures and constituents present in the lower two-thirds of the unit are consistent with the sandy braided fluvial depositional model. proposed by Salas [1983]. The upper one-third of the unit has been deposited in a near shore, shallow marine environment. All samples have been classified as Quartzarenites, or Sublitharenites after Folk [1974] and the source of the Whirlpool lies to the southeast in primarily pre-existing sediments, with some input from low grade metamorphic and hydrothermally veined terrains. </p> <p> Cathodoluminescent microscopy has proven to be a safe, relatively inexpensive, easy to use method, that offers a great deal of new information. The technique's only drawback is the gradual destruction of thin sections by the electron beam. The CL study demonstrated that pressure solution was not the source of the massive, pore occluding, mesodiagenetic quartz cement. Since very low diagenetic temperatures have been calculated for the Whirlpool in the study area [360C]; the local generation of silica would be impossible. Instead, it has been suggested that silica was carried in by saturated pore fluids that had migrated up-dip from source areas deep within the depositional basin to the southeast. Similarly, pyrite was precipitated as H2S bearing fluids migrated through the unit. These reducing fluids also produced the reduced zone at the top of the Queenston Formation. The H2S was produced during the maturation of hydrocarbons. Calcite cement is more abundant in the upper marine units of the Whirlpool. This suggests that the source of the calcite was local detrital carbonate in the upper marine units. Quartz cementation ceased when the porosity had been reduced sufficiently to inhibit the passage of the migrating pore fluids. Thus, the calcite cement precipitated from static pore fluids. The local detrital carbonate was dissolved by the acidic fluids that carried in the silica. This Ca+2 rich fluid was prevented from mixing with the bulk porewater and calcite precipitation occurred due to an increase in C02 by the decay of organic detritus in the upper marine units. The major proportion of secondary porosity was formed during mesodiagenesis by the dissolution of calcite. The pore fluids became undersaturated with respect to calcite when local intershale water was released into the porewater. The formation of dolomite cement was in response to a decrease in the amount of available iron relative to magnesium due to the precipitation of ferroan calcite. The zonation of the dolomite reflects rapid changes in porewater composition. Four morphologies of illite have been identified: two represent direct precipitation from alkaline, K+ rich solution; one may be detrital in origin, or it could represent illite that has been mechanically infiltrated down into the sand after deposition; and the fourth is a mixed layer assemblage that has been formed by the replacement of earlier clays by illite. The oil and gas found in the Whirlpool Sandstone in the Lake Erie area have probably migrated up-dip from source areas deep within the depositional basin to the southeast. </p> / Thesis / Bachelor of Science (BSc)

Diagenetic and structural aspects of tight gas sandstones must be addressed concurrently in order to fully understand low-permeability sandstones and to better predict their reservoir quality attributes that arise from a combination of pore-scale and fracture distribution characteristics. This dissertation focuses on aspects of rock evolution that are germane to concurrent structural and diagenetic evolution, such as loading and thermal history, rock mechanical property evolution, and fracture timing. I tested the hypothesis that the cement precipitation step, governed by thermal exposure and grain surface attributes, governs how sandstone attributes evolve using observations from the Late Cretaceous Williams Fork sandstones from the Piceance Basin, Colorado. My research shows that essential information for predicting and understanding fracture patterns in sandstone can be obtained by unraveling cement precipitation (diagenetic) history. Fractures depend on the mechanical properties existing during fracture growth. I show that key rock mechanical properties (subcritical crack index, Young's modulus and Poisson's ratio), petrophysical behavior, and reservoir quality depend in a systematic way on time-temperature history and the intrinsic grain surface attributes of these sandstones. I classified the Williams Fork lithofacies petrographically and correlated those with log responses to create a model that can be used to predict reservoir quality and diagenesis directly from well logs. I determined rock mechanical characteristics by measuring the subcritical crack index (SCI), a mechanical property that influences fracture distribution characteristics, and by examining log-derived bulk mechanical properties. To quantify the influence of quartz cementation on the SCI and to determine the range of SCI values for sandstone of given framework composition at different diagenetic stages, I measured SCI on Williams Fork core samples and their outcrop equivalents. Diagenetic modeling is applied to determine the sandstone characteristics during fracturing. / text

Many low matrix-porosity hydrocarbon reservoirs are productive because permeability is controlled by natural fractures. The understanding of basic fracture properties is critical in reducing geological risk and therefore reducing well costs and increasing well recovery. Unfortunately, neither geophysics nor borehole methods are, so far, accurate in the acquisition of key fracture attributes, such as density, porosity, spacing and conductivity. This study proposes a new protocol to predict key fracture characteristics of subsurface carbonate rocks and describes how using a relatively low-cost but rock-based method it is possible to obtain accurate geological information from rock samples to predict fracture attributes in nearby but unsampled areas. This methodology is based on the integration of observations of diagenetic fabrics and fracture analyses of carbonate rocks, using outcrops from the Lower Cretaceous Cupido Formation in the Monterrey Salient of the Sierra Madre Oriental, northeastern Mexico. Field observations and petrographic studies of crosscutting relations and fracture-fill mineralogy and texture distinguish six principal coupled fracturing-cementation events. Two fracture events named F1 and F2 are characterized by synkinematic calcite cement that predates D2 regional dolomitization. A third fracture event (F3) is characterized by synkinematic dolomite fill, contemporaneous with D2 dolomitization of host strata. The fourth event (F4) is characterized by synkinematic D3 baroque dolomite; this event postdates D2. The fifth fracture event (F5) is characterized by C3 synkinematic calcite, and postdates D3 dolomite. Finally, flexural slip faulting (F6) is characterized by C3t calcite, and postdates D3 dolomite. Carbon and oxygen stable isotopes were used to validate the paragenetic sequences proposed for the Cupido Formation rocks. The dolomite isotopic signatures are consistent with increasing precipitation temperatures for the various fracture cements, as is expected if fractures grew during progressive burial conditions. Three main groups of calcite cement can be differentiated isotopically. Late calcite cement may have precipitated from cool waters under shallow burial conditions, possibly during exhumation of the SMO. The development of the Structural Diagenetic Petrographic Study protocol, and its integration with geological, geophysical and engineering data, can be applied to oil fields in fractured carbonates such as those located in Mexico, to validate its applicability. / text

The Atlantis II Deep is a location of modern submarine hydrothermal activity along the slowly spreading Red Sea rift axis. Venting of high-temperature hydrothermal fluids, similar to those associated with black smokers, takes place in a brine pool and has led to the accumulation of 90 Mt (dry, salt free) of stratiform, metalliferous sediment. The conditions of mineralization are unique in the modern oceans, but have been widely suggested as a possible analog of some important ancient stratiform base metal ore deposits. This study shows that many of the proposed genetic models for these ancient deposits may be highly simplified and do not take into account rapid diagenetic transformations, widespread non-equilibrium processes, and many other aspects of metal deposition. Sediment cores of the Atlantis II muds were last studied more than 30 years ago. High-resolution sampling and careful re-examination of the mineralogy and geochemistry of the sediments, using modern analytical techniques has significantly improved the understanding of the different processes responsible for the formation of the finely layered metallifeous sediments. The geochemistry of the individual layers is controlled by highly variable detrital, hydrogenic and hydrothermal input. Primary depositional pathways from the brine pool are the main control on basin-wide metal distribution (e.g., increasing Cu/Zn away from the vents) including variable enrichment in trace metals via scavenging from the brine pool and from the enriched pore waters. Cu and Zn have been deposited not only as sulfides but also with poorly crystalline Si-Fe-(oxy)hydroxides. A significant proportion of the original non-sulfide Cu and Zn are diagenetically transformed into sulfides, but also carbonates and clays, in large part reflecting sulfide deficient pore waters. Negative δ34S values, previously unrecognized in the sulfide- and metal-rich units, indicate a source of bacteriogenic sulfide. Syn-diagenetic processes also appear to have been responsible for the sharp laminations in the sediments, as well as distinctive zoning of carbonate and clay minerals around the vent source. The early diagenetic transformations observed in the Atlantis II Deep may not be preserved in the ancient rock record but nevertheless have important implications for metal deposition in some of the world’s largest and richest base metal ore deposits.

<p> The transition from the Whirlpool Sandstone to the Manitoulin Dolomite represents a marine transgression. Within the Manitoulin Formation, the proportion of siliclastics to carbonates shows a marked upward decrease. The Manitoulin Formation consists of four main facies. The lower two facies indicate an inner shelf environment, while the upper two represent a middle to outer shelf environment. </p> <p> These sediments have been almost totally dolomitized. Cathodoluminescent microscopy was employed to determine the diagenetic history of quartz, calcite, and dolomite cements. The petrographic characteristics of the dolomite using CL and normal light indicate a late stage (epigenetic) dolomitization. </p> <p> Dolomitization is thought to have been a late diagenetic process brought about by Mg-rich fluids expelled during compaction of adjacent shales and supplemented by brines circulating through fracture systems. </p> / Thesis / Bachelor of Arts (BA)

As coquinas de bivalves do Grupo Lagoa Feia (Eoaptiano) ocorrem na área sudoeste da Bacia de Campos e constituem reservatórios de hidrocarbonetos. Foram descritos 1030 metros de testemunhos e 368 lâminas delgadas para definir os controles estratigráficos dos eventos e produtos diagenéticos observados e propor um método de predição da paragênese diagenética. Inicialmente foram definidas 34 fácies sedimentares posteriormente agrupadas em 7 associações de fácies nas quais foram reconhecidos os conjuntos de processos geneticamente relacionados, que levaram à interpretação do paleosistema deposicional. Sete produtos eodiagenéticos foram observados na seção informalmente denominada Coquina Inferior e desses, quatro alteram bastante as características permoporosas primárias das rochas, sendo eles: cimentação por calcita blocosa em mosaico, cimentação por sílica microcristalina (quartzo), cimentação por calcedônia e dissolução. Interpreta-se um forte controle do(s) tipo(s) de produto(s) diagenético(s) com a variação do nível base do lago, conforme as seguintes premissas: o cimento de sílica microcristalina ocorre preferencialmente quando o nível base está alto e as rochas sob condições lacustre freáticas, pois, devido ao grande escoamento superficial, o lago neste momento encontra-se enriquecido em Si (proveniente do intemperismo e lixiviação das seções de rochas anteriores), porém ainda diluído (com um volume de água muito amplo), favorecendo a precipitação do quartzo e não da calcedônia. A partir do momento em que o nível do lago cai relativamente, a concentração de Si aumenta e a calcedônia passa a ser o cimento principal. Neste momento, as rochas estão sob condições de ambiente de zona de mistura, pois o nível do lago estaria mais baixo, porém ainda não totalmente em condições de domínio meteórico. A seguir, com nível do lago mais baixo ainda, as rochas passam para o ambiente meteórico e ocorre a precipitação de calcita blocosa (provavelmente necessitando de um tempo de residência baixo, pois o mineral ocorre em todo o intervalo da Coquina Inferior) no meteórico freático, e dissolução no meteórico vadoso. Quando o nível do lago volta a subir, as condições favorecem novamente a precipitação de calcedônia, e, em um nível posterior, mais alto ainda, de sílica microcristalina, iniciando assim um novo ciclo de alterações diagenéticas. A correlação entre a paleobatimetria deposicional da associação de fácies sedimentar e o nível base requerido para determinado(s) tipo(s) de produto(s) diagenético(s) encontrado(s) levou à criação de uma curva de \"velocidade relativa de variação do nível base\", que pôde também ser construída sinteticamente a partir das fácies sedimentares e dos perfis de raios gama e sônico. Tendo em mãos essa curva sintética, foi possível fazer o \"caminho inverso\" e criar um modelo preditivo (ou seja, sem descrição da diagênese em lâminas delgadas) da paragênese diagenética a partir da associação de fácies sedimentar e da \"velocidade relativa de variação do nível base\" em escala de poço (1D) e também em um grid geocelular (3D). A seção Coquina Inferior dos reservatórios da Bacia de Campos apresenta elevado grau de heterogeneidade vertical, gerada pela ciclicidade estratigráfica e pela intensa modificação eodiagenética. O entendimento dos fatores controladores da diagênese levou à criação de um método preditivo da paragênese diagenética em escala de poço (1D) e também em um grid geocelular (3D) que poderá ser usado para a caracterização e gerenciamento de outros campos petrolíferos em carbonatos. / The bivalve coquina from Lagoa Feia Group (Eoaptian), occurs in southwestern part of Campos Basin and consists petroleum reservoirs. 1030 meters of core and 368 thin sections were described to define the stratigraphic controls of the diagenetic events and then create a diagenetic paragenesis predictive method. Firstly, 34 sedimentary facies were defined and then grouped in seven facies associations. These associations have the same genetically depositional process relationship and they were used to define the depositional paleosystem. Seven eodiagenetic products were observed in the informally called Lower Coquina section and four from these products cause an important primary rock permeability and porosity alteration: block calcite cementation, microcrystaline silica (quartz) cementation, chalcedony cementation and dissolution. There is a strong control of the diagentic(s) product(s) type(s) and the base level variation. Microcrystaline silica occurs when the base level is high and the conditions are under lacustrine phreatic environment. Under these conditions, the lake has a Si enrichment due to run off great rates but still diluted, because the huge volume of water. This situation favors the precipitation of quartz instead of chalcedony. When the lake level falls, Si concentration rises, and chalcedony becomes the main cement phase. At this time, the conditions are mostly under mixing zone. When the lake falls again, meteoric environment becomes predominant. At the meteoric phreatic environment blocky calcite occurs as cement and at the meteoric vadose zone environment dissolution is the main process. When the lake level raises again, the conditions favor the precipitation of chalcedony and, in the next level, (higher base level than previous) the conditions are good to microcrystaline silica precipitation. At this moment, a new diagenetic cycle is started. The correlation between depositional paleobathymetry of sedimentary facies association and the lake level required for different types of diagenetic paragenesis led to the creation of a curve called \"relative velocity of base level variation\". This curve may also be calculated synthetically from sedimentary facies, gamma ray and velocity logs. With this synthetic curve it was possible to do a \"reverse path\" procedure and generated a diagenetic paragenesis predictive method. The coquina reservoir has a high level of heterogeneity due to stratigraphic cyclicality and due to eodiagenetic transformations. The understanding of diagenetic controls led to a diagenetic predictive method creation on 1D (well) and 3D (geocell grid). This knowledge can be used for the characterization and management of others carbonates associated oil fields.

As coquinas de bivalves do Grupo Lagoa Feia (Eoaptiano) ocorrem na área sudoeste da Bacia de Campos e constituem reservatórios de hidrocarbonetos. Foram descritos 1030 metros de testemunhos e 368 lâminas delgadas para definir os controles estratigráficos dos eventos e produtos diagenéticos observados e propor um método de predição da paragênese diagenética. Inicialmente foram definidas 34 fácies sedimentares posteriormente agrupadas em 7 associações de fácies nas quais foram reconhecidos os conjuntos de processos geneticamente relacionados, que levaram à interpretação do paleosistema deposicional. Sete produtos eodiagenéticos foram observados na seção informalmente denominada Coquina Inferior e desses, quatro alteram bastante as características permoporosas primárias das rochas, sendo eles: cimentação por calcita blocosa em mosaico, cimentação por sílica microcristalina (quartzo), cimentação por calcedônia e dissolução. Interpreta-se um forte controle do(s) tipo(s) de produto(s) diagenético(s) com a variação do nível base do lago, conforme as seguintes premissas: o cimento de sílica microcristalina ocorre preferencialmente quando o nível base está alto e as rochas sob condições lacustre freáticas, pois, devido ao grande escoamento superficial, o lago neste momento encontra-se enriquecido em Si (proveniente do intemperismo e lixiviação das seções de rochas anteriores), porém ainda diluído (com um volume de água muito amplo), favorecendo a precipitação do quartzo e não da calcedônia. A partir do momento em que o nível do lago cai relativamente, a concentração de Si aumenta e a calcedônia passa a ser o cimento principal. Neste momento, as rochas estão sob condições de ambiente de zona de mistura, pois o nível do lago estaria mais baixo, porém ainda não totalmente em condições de domínio meteórico. A seguir, com nível do lago mais baixo ainda, as rochas passam para o ambiente meteórico e ocorre a precipitação de calcita blocosa (provavelmente necessitando de um tempo de residência baixo, pois o mineral ocorre em todo o intervalo da Coquina Inferior) no meteórico freático, e dissolução no meteórico vadoso. Quando o nível do lago volta a subir, as condições favorecem novamente a precipitação de calcedônia, e, em um nível posterior, mais alto ainda, de sílica microcristalina, iniciando assim um novo ciclo de alterações diagenéticas. A correlação entre a paleobatimetria deposicional da associação de fácies sedimentar e o nível base requerido para determinado(s) tipo(s) de produto(s) diagenético(s) encontrado(s) levou à criação de uma curva de \"velocidade relativa de variação do nível base\", que pôde também ser construída sinteticamente a partir das fácies sedimentares e dos perfis de raios gama e sônico. Tendo em mãos essa curva sintética, foi possível fazer o \"caminho inverso\" e criar um modelo preditivo (ou seja, sem descrição da diagênese em lâminas delgadas) da paragênese diagenética a partir da associação de fácies sedimentar e da \"velocidade relativa de variação do nível base\" em escala de poço (1D) e também em um grid geocelular (3D). A seção Coquina Inferior dos reservatórios da Bacia de Campos apresenta elevado grau de heterogeneidade vertical, gerada pela ciclicidade estratigráfica e pela intensa modificação eodiagenética. O entendimento dos fatores controladores da diagênese levou à criação de um método preditivo da paragênese diagenética em escala de poço (1D) e também em um grid geocelular (3D) que poderá ser usado para a caracterização e gerenciamento de outros campos petrolíferos em carbonatos. / The bivalve coquina from Lagoa Feia Group (Eoaptian), occurs in southwestern part of Campos Basin and consists petroleum reservoirs. 1030 meters of core and 368 thin sections were described to define the stratigraphic controls of the diagenetic events and then create a diagenetic paragenesis predictive method. Firstly, 34 sedimentary facies were defined and then grouped in seven facies associations. These associations have the same genetically depositional process relationship and they were used to define the depositional paleosystem. Seven eodiagenetic products were observed in the informally called Lower Coquina section and four from these products cause an important primary rock permeability and porosity alteration: block calcite cementation, microcrystaline silica (quartz) cementation, chalcedony cementation and dissolution. There is a strong control of the diagentic(s) product(s) type(s) and the base level variation. Microcrystaline silica occurs when the base level is high and the conditions are under lacustrine phreatic environment. Under these conditions, the lake has a Si enrichment due to run off great rates but still diluted, because the huge volume of water. This situation favors the precipitation of quartz instead of chalcedony. When the lake level falls, Si concentration rises, and chalcedony becomes the main cement phase. At this time, the conditions are mostly under mixing zone. When the lake falls again, meteoric environment becomes predominant. At the meteoric phreatic environment blocky calcite occurs as cement and at the meteoric vadose zone environment dissolution is the main process. When the lake level raises again, the conditions favor the precipitation of chalcedony and, in the next level, (higher base level than previous) the conditions are good to microcrystaline silica precipitation. At this moment, a new diagenetic cycle is started. The correlation between depositional paleobathymetry of sedimentary facies association and the lake level required for different types of diagenetic paragenesis led to the creation of a curve called \"relative velocity of base level variation\". This curve may also be calculated synthetically from sedimentary facies, gamma ray and velocity logs. With this synthetic curve it was possible to do a \"reverse path\" procedure and generated a diagenetic paragenesis predictive method. The coquina reservoir has a high level of heterogeneity due to stratigraphic cyclicality and due to eodiagenetic transformations. The understanding of diagenetic controls led to a diagenetic predictive method creation on 1D (well) and 3D (geocell grid). This knowledge can be used for the characterization and management of others carbonates associated oil fields.

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