20 results on '"Pe‐Piper, Georgia"'
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2. Variability in mineralization of the petrified forests in the UNESCO Global Geopark of Lesvos, Greece.
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Pe-Piper, Georgia, Piper, David J.W., Anastasakis, George, Zouros, Nickolas, and Lyras, George
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HYDROTHERMAL alteration , *MINERALIZATION , *FOSSIL trees , *CARBONATE minerals , *GEOCHRONOMETRY , *SULFIDE minerals - Abstract
Miocene petrified tree fossils in tuffs from Lesvos Island show a variety of types of mineralization. Trees from the older part of the volcanic succession, including the well-known Petrified Forest near Sigri, are brilliant yellows, reds and browns as a result of Fe–Mn hydrothermal alteration post-dating silicification. Trees from the younger part of the volcanic succession on the southeastern flank of the volcanic chain include some in which the original carbon has not been replaced, others that are completely silicified or completely replaced by calcite, and yet others with cell walls silicified and the lumina filled with a variety of carbonate and sulfide minerals. Widespread Fe–Mn hydrothermal veins are found only in the older part of the volcanic succession and in underlying basement rocks and date from the inversion of the Sigri basin in the mid-Burdigalian and the intrusion of the Mesotopos dykes and laccoliths. The variable petrifaction of fossil wood is thus essentially a consequence of the variability of diagenetic and hydrothermal processes. This study demonstrates the importance of tectonic history in localising volcanic hydrothermal circulation and hence diversity in style of petrifaction. • Silicified trees in tuffs in western Lesvos show late Fe–Mn mineralization. • Mineralization at depth after basin inversion and laccolith intrusion. • In tuffs in eastern Lesvos, incomplete tree petrifaction by silica and/or carbonates. • Mineralization at shallow depths, no basin inversion, some basement leaching. • Petrified trees provide record of diagenetic and hydrothermal history of host tuffs. [ABSTRACT FROM AUTHOR]
- Published
- 2024
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3. Significance of the chemistry and morphology of diagenetic siderite in clastic rocks of the Mesozoic Scotian Basin.
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Pe‐Piper, Georgia, Piper, David J. W., and Hendry, Jim
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PARAGENESIS , *CLASTIC rocks , *SIDERITE , *ELECTRON probe microanalysis , *CHEMISTRY , *SEDIMENTARY basins - Abstract
Siderite (FeCO3) is a widespread minor diagenetic mineral in clastic sedimentary basins. Although eodiagenetic authigenesis of siderite is well‐known, siderite formed during burial diagenesis shows habits and chemical compositions that are poorly understood. This study tests the hypothesis that diagenetic siderite cements in sandstones in the Scotian Basin, offshore eastern Canada, show systematic variability in chemistry and habit that is a response to recrystallization and changing composition of basinal fluids. Mineral textures were determined from backscattered electron images, and chemistry mostly from electron microprobe analyses. Five chemical types of siderite are identified using k‐means cluster analysis, based on the amount of substitution of Ca, Mg and Mn for Fe. Eodiagenetic microcrystalline coated grains, concretions and intraclasts in sandstones are principally Fe‐rich siderite and locally have recrystallised to blocky equant crystals. Mesodiagenetic Mg‐rich siderite partly replaced these equant crystals and also framework mica and K‐feldspar grains, showing textural evidence for coupled dissolution–reprecipitation. Slender Mg‐rich siderite rhombs (lozenges, bladed or wheat‐seed siderite) have precipitated before and after the formation of quartz overgrowths in geochemical microenvironments. Magnesium substitution reflects Mg‐rich formation waters resulting from smectite to illite conversion. Equivalent Ca‐rich siderite occurs where sandstones overlie a Jurassic carbonate bank. Late Mn‐rich siderite has complex textures resembling those of Mississippi‐Valley type ores, with spheroidal rims, a honeycomb‐like mesh and concentric infill around secondary pores. It also occurs in veins or replacing intraclasts, post‐dating late ferroan‐calcite cements in sandstones that show strong dissolution by hot basinal brines. The Ca, Mg and Mn content of diagenetic siderite, coupled with textural evidence for recrystallization, can thus be used to track changes in ambient formation fluids. Siderite habits and chemistry described from the Scotian Basin are found in many clastic basins, suggesting that the observed recrystallization textures and variation in chemical type are of broad application. [ABSTRACT FROM AUTHOR]
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- 2020
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4. Diagenetic F-rich ferroan calcite and zircon in the offshore Scotian Basin, eastern Canada: Significance for understanding thermal evolution of the basin.
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Pe-Piper, Georgia, Sangster, Christopher, and Zhang, Yuanyuan
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DIAGENESIS , *ZIRCON , *FLUORINE - Abstract
In the Scotian Basin, offshore eastern Canada, an unusual combination of high heat flow in the Cretaceous and the abundance of halite has resulted in unusual diagenetic minerals such as sphalerite. The Newburn H-23 well is the most distal well in the basin with good core samples and has two previously unknown diagenetic mineral occurrences: fluorine-rich ferroan calcite and diagenetic zircon. This study uses SEM backscattered electron images and EDS analyses, EMP WDS mineral analyses and Raman spectroscopy to determine mineral chemistry and textures to investigate the diagenetic and thermal significance of these minerals. Late diagenetic Fe-calcite contains 1 -- 2.5 wt% fluorine, mostly from adsorption, but rarely as small fluorite crystals. Fluoride is also adsorbed on the surfaces of some framework minerals and chlorite. Fluoride was transported in highly saline formation brines derived from the Argo salt Formation. Zircon grains, 20 -- 40 mm in size, have crystal outlines that are straight adjacent to pores, partially lobate filling porosity, and cross cutting other grains: these may be diagenetic. Some zoned detrital zircon grains show 1 -- 3 mm wide diagenetic outgrowths. Neoformation of diagenetic zircon requires temperatures of >250 °C. Transport of zirconium is favored by ligands in low-pH solution, principally fluoride and phosphate anions, with zirconium mobilized during the alteration of metamict detrital zircon under low-grade metamorphic conditions. The presence of diagenetic sphalerite and the documented mid-Cretaceous thermal event in the Scotian Basin indicate conditions that could have been suitable for the formation of diagenetic zircon in this well. Suitable geological conditions for such diagenetic formation of zircon will be found in a subsiding rift basin with early evaporites that are affected by a subsequent phase of volcanism due to new rifting or subduction. [ABSTRACT FROM AUTHOR]
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- 2017
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5. Controls on sandstone diagenesis, Scotian Basin, eastern Canada: basinal fluids and salt tectonics.
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PE-PIPER, GEORGIA
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SALT tectonics , *DIAGENESIS , *APATITE , *SANDSTONE , *PARAGENESIS , *HUMIC acid , *RESERVOIR rocks , *FLUID inclusions - Abstract
Sedimentation and diagenesis in the Scotian Basin were strongly influenced by salt tectonics. Thick Upper Jurassic-Lower Cretaceous deltaic sandstones are important reservoir rocks. Published fluid inclusion data provide a record of changing salinity and temperature of migrating formation water, with the most saline fluids at times of active salt deformation. Recognised high temperature events correspond to high regional mantle heat flow in the middle Cretaceous and the Paleogene release of over-pressured fluids known from apatite fission track studies. Geographic variations in timing of salt deformation resulted in variable diagenetic conditions along the strike of the basin. In reservoir sandstones, albitization of detrital K-feldspar and plagioclase resulted from the high Na+ content of formation waters that migrated through permeable pathways at the time of silica and carbonate cementation, with strong albitization coinciding with transport-controlled dissolution of K-feldspar. Such saline fluids recorded in fluid inclusions created widespread dissolution of silica and carbonate grains and cements and this secondary porosity was host to a variety of late diagenetic minerals including minerals transported by halogen-rich fluids such as sphalerite and titania (TiO2) minerals. Among the titania polymorphs, anatase is mostly early diagenetic whereas brookite is usually late diagenetic and is abundant in reservoir sandstones above the free-water level. Detrital Ti-bearing minerals are unusually abundant in the Scotian Basin and provide a source of Ti for chelation by organic acids. Early diagenesis of titania minerals is related to humic acids in meteoric water at marine lowstands. Late diagenetic mobility of Ti and precipitation of brookite is enhanced by Ti complexing with organic acids in saline, hydrocarbon-rich fluids. [ABSTRACT FROM AUTHOR]
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- 2023
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6. How sandstone porosity and permeability vary with diagenetic minerals in the Scotian Basin, offshore eastern Canada: Implications for reservoir quality.
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Zhang, Yuanyuan, Pe-Piper, Georgia, and Piper, David J.W.
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SANDSTONE , *POROSITY , *PERMEABILITY , *DIAGENESIS , *STRATIGRAPHIC geology , *MINERALS - Abstract
The Scotian Basin is an under-explored offshore basin that has produced gas and minor oil from Jurassic–Cretaceous deltaic sandstones. Reservoir quality is an important exploration risk in the basin. This study evaluates whether there are systematic stratigraphic or geographic variations in diagenetic mineral assemblage control of reservoir porosity and permeability. Three data sets are used: a compilation of all available core-plug porosity and permeability measurements (3271 in total), a subset of 577 corresponding to thin sections with lithofacies assignments, and a subset of 35 thin sections principally from reservoir sandstones, in which modal abundance of diagenetic minerals has been determined by image analysis of scanning electron microscope backscattered electron images. Abundances above a 4% threshold of diagenetic kaolinite, calcite and ankerite correlate inversely with permeability, but chlorite and possibly siderite correlate positively. Diagenetic assemblages show no systematic variation with stratigraphy or geography, both of which may involve variable detrital supply. Permeability shows the well-known variation with grain size and depth of burial, but after correcting for these effects it is greatest in estuarine channel and river-mouth turbidite sandstones. This is because these facies preferentially develop chlorite rims on detrital quartz grains and are also the sites of enhanced flow of late basinal fluids resulting in secondary porosity. Local effects such as facies distribution, architecture of channel sandstones, fluid pathways related to complex salt tectonics, and the effects of overpressure are more important than predictable basin-wide effects in controlling sandstone reservoir quality. [ABSTRACT FROM AUTHOR]
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- 2015
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7. Temperature and basinal fluid controls on feldspar diagenesis, Lower Cretaceous sandstones, Scotian Basin, Canada.
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Pe-Piper, Georgia and Piper, David J.W.
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FLUID control , *FELDSPAR , *DIAGENESIS , *SALT tectonics , *SANDSTONE , *SOIL salinity , *PARAGENESIS - Abstract
The passive-margin Scotian Basin accumulated thick Upper Jurassic–Lower Cretaceous deltaic sandstones and shales, which were influenced by high heat flow in the mid-Cretaceous and deformed by salt tectonics. This study takes advantage of the complex thermal history and record of saline fluid flow in the basin to determine the relative importance of burial depth, temperature, salinity of basinal brines, overpressure, and sandstone permeability in controlling the dissolution of detrital feldspar and growth of authigenic K-feldspar and albite. Feldspar grains and volcanic lithic clasts from two transects, with differing thermal and salt tectonic histories, were studied by scanning electron microscope (SEM) backscattered electron images, energy-dispersive spectroscopy, SEM cathodoluminescence (CL) imaging, and hot-cathode CL microscopy. Published fluid inclusion measurements are integrated with burial history curves and timing of deformation on salt detachment surfaces to provides a new synthesis of the post-Jurassic thermal and saline fluid flow history of the basin. Surface-controlled dissolution of K-feldspar and growth of authigenic K-feldspar rims occur from 1.5 to 3.1 km. More destructive transport-controlled dissolution, down to 3.9 km, is more effective in thick-bedded permeable sandstones that acted as fluid pathways. Albitization of detrital K-feldspar and sodic plagioclase, and of feldspars in volcanic lithic clasts was facilitated by high Na+ content of formation waters migrating preferentially through such pathways at the time of silica and carbonate cementation, with strong albitization coinciding with transport-controlled dissolution of K-feldspar. Authigenic albite partially fills pores created by K-feldspar dissolution. Overpressured conditions enhance albite authigenesis, as a result of slower fluid advection. Episodic release of hot, saline, overpressured fluids in the zone of surface-controlled dissolution of K-feldspar favoured albitization but had no detectable effect on K-feldspar dissolution. Basin hydrology plays a definitive role in feldspar diagenesis, producing patterns that cannot be interpreted as a simple consequence of temperature and activity of Na+. • Fluid inclusions related to diagenetic paragenesis, salt mobility and thermal events. • More K-feldspar dissolution in permeable sandstone pathways for basin fluids. • Albitization favoured by Na + rich fluids, permeable pathways and overpressure. • Release of hot overpressured fluids enhanced albitization but not K-feldspar dissolution. • Definitive role of basin hydrology in feldspar diagenesis. [ABSTRACT FROM AUTHOR]
- Published
- 2022
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8. Sediment geochemistry as a provenance indicator: Unravelling the cryptic signatures of polycyclic sources, climate change, tectonism and volcanism.
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Zhang, Yuanyuan, Pe‐Piper, Georgia, Piper, David J.W., and Sheldon, Nathan
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GEOCHEMISTRY , *SEDIMENTS , *PROVENANCE (Geology) , *PLATE tectonics , *VOLCANISM & climate , *JURASSIC Period , *POLYCYCLIC aromatic compounds - Abstract
Interpretation of bulk-sediment geochemistry is one of several approaches for determining sediment provenance. This study investigates the value added by bulk-sediment geochemical analysis in interpreting provenance in a passive margin clastic basin, the Upper Jurassic-Lower Cretaceous deltaic sediments of the Scotian Basin. Provenance studies in this basin are challenging because source tectonic terranes are parallel to the basin margin and polycyclic sediment sources are abundant. More than 400 samples of mudstone and sandstone representing the geographical and stratigraphic range of interest were analysed for 57 elements. Diagenetic processes added calcium to many samples and removed potassium in rocks buried below 3 km, thus impacting principal component analysis and published weathering indices. However, multiple geochemical approaches to assessing the degree of weathering showed climatically controlled changes in weathering in the Tithonian and Barremian, and changes in supply from major tectonic events, such as the top-Aptian uplift in the Labrador rift. Covariance of elements in heavy minerals demonstrates the varying magnitude of polycyclic supply and stratigraphic changes in sources. Geochemical analyses revealed a previously unsuspected Tithonian alkali volcanic sediment source, characterized by high niobium and tantalum. The lack of highly contrasting sources means that geochemistry alone is inadequate to determine sediment provenance. Published discrimination diagrams are of limited value. Statistical analysis of geochemical data is strongly influenced by diagenetic processes, episodic volcanic inputs and polycyclic concentration of resistant heavy minerals in sandstones. Single indicator elements for particular sources are generally lacking. Nevertheless, careful consideration of geochemical variability on a case by case basis, integrated with detrital mineral studies, provides new insights into palaeoclimate, sediment provenance and, hence, regional tectonics. Although there is no simple template for such analysis, this study demonstrates an approach that can be used for other basins. [ABSTRACT FROM AUTHOR]
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- 2014
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9. Lateral variation in sandstone lithofacies from conventional core, Scotian Basin: implications for reservoir quality and connectivity1,2.
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Gould, Kathleen M., Piper, David J.W., Pe-Piper, Georgia, Dehler, Sonya, Deptuck, Mark, and Karim, Atika
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SANDSTONE ,LITHOFACIES ,RESERVOIRS ,DIAGENESIS ,TURBIDITES ,LANDSLIDES - Abstract
Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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- 2012
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10. Controls on regional variability in marine pore-water diagenesis below the seafloor in Upper Jurassic–Lower Cretaceous prodeltaic sandstone and shales, Scotian Basin, Eastern Canada
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Okwese, Ann C., Pe-Piper, Georgia, and Piper, David J.W.
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DIAGENESIS , *RESERVOIRS , *JURASSIC stratigraphic geology , *CRETACEOUS stratigraphic geology , *SEDIMENTOLOGY , *SANDSTONE , *GEOCHEMISTRY , *MINERALOGY , *GEOLOGICAL basins - Abstract
Abstract: Diagenesis in the uppermost Jurassic to Lower Cretaceous deltaic sandstones and shales of the Scotian Basin is an important control on reservoir quality. Ferruginous zone (sub-oxic) marine pore-water diagenesis controls the initial formation of Fe2+-silicates that are the precursors of grain-rimming chlorite that preserves porosity. This study assesses the regional controls on the type of marine pore-water diagenesis by studying the sedimentology, mineralogy, and geochemistry of the retrogradational units and underlying progradational units in parasequences from conventional cores in two wells in different parts of the basin. Coated grains preserve a record of whether marine pore-water diagenesis below the seafloor was dominantly in the ferruginous or sulphidic geochemical zone. Four types of coated grain were distinguished, each with a different mineral paragenesis. Mineralogical and chemical evidence of ferruginous zone diagenesis includes the presence of diagenetic chlorite and siderite, and the correlation of P with Fe or Ti. Pyrite and Fe-calcite are found where the sulphidic zone is more significant than the ferruginous zone. Ferruginous zone diagenesis was common in low-sedimentation rate retrogradational sediments with low organic carbon, and in delta-front turbidites and river-mouth sandstones. Estuarine, tidal flat and prodeltaic facies that are directly supplied by riverine sediments have a lower Fe:Ti ratio than do fully marine shoreface and open shelf facies as a result of input of detrital ilmenite and its alteration products. The relative contribution of colloidal iron (hydr)-oxides appears greater in distal low-sedimentation rate environments. Where large changes in sedimentation rate occurred at ravinement surfaces, the underlying progradational rocks have evidence of ferruginous zone diagenesis, whatever their facies. Rapid upward migration of the pore-water profile resulting from the change in sedimentation rate reduced the time available for mineral products to form in the deeper pore-water zones. This study has shown that the availability of Fe and organic carbon varying in a complex manner in marine deltaic sediments, but that the resulting diagenesis by marine pore-water can be predicted from facies and paleogeographic setting. [Copyright &y& Elsevier]
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- 2012
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11. Thermal and hydrocarbon-charge history and the relationship between diagenesis and reservoir connectivity: Venture field, offshore Nova Scotia, eastern Canada.
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Karim, Atika, Pe-Piper, Georgia, Piper, David J. W., Hanley, Jacob J., and Bentley, Samuel
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DIAGENESIS , *FLUID inclusions , *HYDROCARBONS , *GAS reservoirs , *JURASSIC paleopedology , *SCANNING electron microscopy , *TEMPERATURE - Abstract
Fluid inclusions in diagenetic cements in Upper Jurassic - Lower Cretaceous sandstones offshore Nova Scotia provide constraints on the fluid migration history in gas reservoirs of the Scotian basin. Diagenetic minerals from six wells in the Venture field were analysed by optical petrography, scanning electron microscopy (SEM), and electron microprobe. A total of 122 primary and secondary fluid inclusions were analysed from different cements. Primary aqueous inclusions in quartz overgrowths have homogenization temperatures (Th) of 111.8 ± 7.1 °C (1σ) and in later carbonate cements 126.5 ± 2.1 °C; inclusions in both cements are highly saline (16-26.1 wt.% NaCl equivalent). Secondary aqueous and hydrocarbon-bearing inclusion trails crosscutting silica cement and detrital quartz have Th of 121.6 ± 13.6 °C and low salinities (8.7 ± 6.0 wt.%). Secondary carbonic inclusions have CO2 melting temperatures (-56.6 ± 0.1 °C) and Th (-9.3 ± 0.8 °C) indicating a high-density carbonic phase. Late carbonate cements in the same sandstone units vary in chemical composition in different wells, and connected reservoirs show similar late carbonate assemblages, suggesting that the late carbonate cementation may be partly controlled by the reservoir fill and spill sequence. Silica and late carbonate cementation involved highly saline fluid flow, likely at about ∼135 Ma. Hydrocarbon migration postdated silica cementation and was associated with secondary fracturing, suggesting that it corresponded to the onset of overpressure. [ABSTRACT FROM AUTHOR]
- Published
- 2011
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12. Controls on diagenesis of Lower Cretaceous reservoir sandstones in the western Sable Subbasin, offshore Nova Scotia
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Karim, Atika, Pe-Piper, Georgia, and Piper, David J.W.
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DIAGENESIS , *CRETACEOUS stratigraphic geology , *SANDSTONE , *GAS reservoirs , *LITHOFACIES , *SEA level , *SIDERITE - Abstract
Abstract: Lower Cretaceous deltaic sandstones of the Scotian Basin, offshore eastern Canada, are important gas reservoirs. The influence of several factors on diagenesis has been investigated: depositional lithofacies, sea level changes, chemistry of basinal sediments and basinal fluid flux during burial. The distribution and chemistry of diagenetic minerals was determined from nine wells located along a dip section of the Sable Subbasin. Mineral type and paragenesis were characterized using a combination of optical petrography, back-scattered electron images, and electron microprobe analyses. Siderite is unusually abundant in marine sediments of the Scotian Basin and has thus been studied in detail. Siderite occurs in several generations. Early and late siderites are similar in chemical composition, suggesting buffering by pre-existing siderite, but rare low-Mg siderite is related to a greater contribution of meteoric water. Siderite has locally dissolved to create microporosity and has suppressed quartz overgrowths. Siderite is most common in those muddy prodeltaic lithofacies where there is the highest availability of detrital ilmenite. Reactive Fe released by breakdown of this ilmenite is responsible for the unusual presence of early siderite in marine sediments, with the Ca and Mg content of the siderite indicating fully marine waters. Lithofacies have a strong influence on early diagenetic mineral assemblages. Lithofacies deposited in the transgressive system tract have abundant early Fe-calcite and siderite. Early kaolinite occurs principally in proximal (fluvial and river mouth) lithofacies, where meteoric water was most likely available during the deposition. Contrary to other studies, we find little impact of sequence stratigraphy on diagenetic minerals except in the transgressive system tract. Mesogenetic minerals are related to flux of formation water and maturing hydrocarbon products, resulting first in pyrite and siderite and later in ankerite and ferroan calcite. The principal controls are interpreted to be high Fe2+ and low Ca2+ of formation waters and vertical movement of the waters along faults. Overall, the bulk chemical composition of terrigenous sediments and the depositional lithofacies are the most important factors controlling diagenetic minerals in the Lower Cretaceous of the Scotian Basin. [Copyright &y& Elsevier]
- Published
- 2010
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13. Relationship of diagenetic chlorite rims to depositional facies in Lower Cretaceous reservoir sandstones of the Scotian Basin.
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GOULD, KATHLEEN, PE-PIPER, GEORGIA, and PIPER, DAVID J. W.
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SANDSTONE , *CHLORITE minerals , *ELECTRON probe microanalysis , *GEOCHEMISTRY , *MUDSTONE , *SEDIMENTOLOGY - Abstract
The relationship between diagenetic chlorite rims and depositional facies in deltaic strata of the Lower Cretaceous Missisauga Formation was investigated using a combination of electron microprobe, bulk geochemistry and X-ray diffraction data. The succession studied comprises several stacked parasequences. The delta progradational facies association includes: (i) fluvial or distributary channel sandstones (some with tidal influence); (ii) thick-bedded delta-front graded beds of sandstone interpreted as resulting from fluvial hyperpycnal flow during floods and storms; and (iii) more distal muddier delta-front and prodeltaic facies. The transgressive facies association includes lag conglomerate, siderite-cemented muddy sandstone and mudstone, and bioclastic sandy limestone. Chlorite rims are absent in the fluvial facies and best developed in thick sandstones lacking mudstone baffles. Good quality chlorite rims are well correlated with Ti in bulk geochemistry. Ti is a proxy for Fe availability, principally from the breakdown of abundant detrital ilmenite (FeTiO3). Under conditions of sea floor diagenesis, the abrupt decrease in sedimentation rate at transgressive surfaces caused progressive shallowing of the sulphate-depletion level and of the overlying Eh-controlled diagenetic zones, resulting in conditions suitable for diagenetic formation of berthierine to migrate upwards through the packet of reservoir sandstones. This early diagenetic berthierine suppressed silica cementation and later recrystallized to chlorite. Thick euhedral outer chlorite rims were precipitated from formation water in sandstone lacking muddy baffles on this chlorite substrate and inhibited late carbonate cementation. This study thus shows that the preservation of porosity by chlorite rims is a two-stage process. Rapidly deposited delta-front turbidite facies create early diagenetic conditions that eventually lead to the formation of chlorite rims, but the best quality chlorite rims are restricted to sandstones with high permeability during burial diagenesis. [ABSTRACT FROM AUTHOR]
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- 2010
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14. Early diagenesis of inner-shelf phosphorite and iron-silicate minerals, Lower Cretaceous of the Orpheus graben, southeastern Canada: Implications for the origin of chlorite rims.
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Pe-Piper, Georgia and Weir-Murphy, Shawna
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DIAGENESIS ,PHOSPHATE rock ,SILICATES ,CHAMOSITE ,GEOLOGICAL basins - Abstract
Wells in the Orpheus graben encountered the most proximal part of the deltaic Lower Cretaceous rocks of the Scotian Basin. More distal sandstones are important gas reservoir rocks, with good reservoir quality where Fe-rich chlorite (chamosite) rims on framework grains have inhibited quartz cementation. Cutting samples from the Orpheus graben show the presence of Fe-rich sheet silicates (berthierine or chamosite) and early diagenetic phosphorite. These minerals were analyzed by electron microprobe, and their textures were mapped with backscattered electron images. Studies in the North Sea have shown a relationship between high phosphorus and the presence of good chlorite rims in reservoir rocks. The mineralization of pore-water phosphorus, instead of its return to seawater, is favored by Fe-rich sediments and sorption on iron oxides during shallow sea-floor diagenesis. The Fe, Ti, and P contents are uncommonly high in Scotian Basin shales compared with global average shale compositions. The uncommon occurrence of inner-shelf phosphorite in this study is interpreted to be a consequence of the same high Fe content of the sediment that also favors the formation of Fe-rich sheet silicates. In rapidly deposited deltaic sandstones of the offshore reservoirs, the dominance of type 3 kerogen led to sulfate depletion occurring at depths of tens of meters and a corresponding great thickness for overlying Eh (oxidation potential)-controlled diagenetic zones. The thick Fe-reduction zone allowed the formation of early diagenetic berthierine, which on burial formed the chamosite rims that resulted in the improved reservoir quality in sandstones. The distribution of phosphorus minerals may be an indicator of conditions suitable for berthierine formation. [ABSTRACT FROM AUTHOR]
- Published
- 2008
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15. Diagenetic history and provenance of Devonian terrestrial sandstones at the margin of Gondwana: Padeha Formation, Eastern Alborz, Iran.
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Poursoltani, Mehdi Reza and Pe-Piper, Georgia
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HEAVY minerals , *SANDSTONE , *MAFIC rocks , *GEOCHEMISTRY , *CONTINENTAL margins , *QUARTZ ,GONDWANA (Continent) - Abstract
• The northern margin of Gondwana in Iran rifted in the Devonian, forming a passive continental margin to the Paleotethys Ocean. • The Padeha sandstones were sourced by the late Neoproterozoic granitic gneisses of Iran. • Further mesodiagenesis was terminated by the Eocimmerian collision. The northern margin of Gondwana in Iran rifted in the Devonian, forming a passive continental margin to the Paleotethys Ocean, the details of which are obscured by later collision. A record of the early phase of opening of Paleotethys is preserved in the 600-m thick Padeha Formation, a Middle Devonian terrestrial rift basin succession. The sandstones are quartz arenite, sub-arkose and arkose and contain minimal detrital matrix (~1%). Bulk-rock geochemistry, according to major elements, suggests a source to the sandstones that was dominated by I-type granite, but also, based on trace elements, included small amounts of intermediate or mafic rocks. Low Zr and HREE contents suggest little or no sandstone in the source area and thus low abundance of fine-grained heavy minerals (<1%). The sandstones were sourced by the widespread granitic gneisses of central and northern Iran that formed at the northern margin of Gondwana in the late Neoproterozoic–Ediacarian. Diagenesis of the sandstones is dominated by compaction and fracturing of framework quartz without widespread stylolites in the sandstones, and cementation by silica, carbonates and Fe-oxides. Fractures within quartz grains are filled mostly by silica, but locally by carbonate and authigenic K-feldspar. Albitization of plagioclase, K-feldspar overgrowths and partial dissolution of detrital K-feldspar post-date silica cements and suggest temperatures between 70 and 120 °C. The particular diagenetic assemblage of altered K-feldspar and limited quartz stylolites was a consequence of a high geothermal gradient of ~70 °C/km at a burial depth of <1.5 km. Further mesodiagenesis was terminated by the Eocimmerian collision. [ABSTRACT FROM AUTHOR]
- Published
- 2020
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16. Diagenesis, burial history, and hydrocarbon potential of Cambrian sandstone in the northern continental margin of Gondwana: A case study of the Lalun Formation of central Iran.
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Poursoltani, Mehdi Reza, Gibling, Martin R., and Pe-Piper, Georgia
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DIAGENESIS , *HYDROCARBONS , *SANDSTONE , *POROSITY , *PERMEABILITY - Abstract
Graphical abstract Highlights • Primary porosity was largely occluded by early quartz overgrowths and clay cement. • More carbonate cement present in marine sandstones due to facies-related diagenesis. • Rapid subsidence of NE Gondwana buried fluvial sandstones to >4 km by the Devonian. • Rapid burial and lack of eustatic lowstands limited early meteoric dissolution. • Few permeability pathways were available to generate secondary porosity. Abstract Undeformed Cambrian–Ordovician sandstones on the northern margin of Gondwana are in places important hydrocarbon reservoirs. The factors that favoured diagenetic preservation of porosity in such sandstones are reviewed and compared to the Lower Cambrian Lalun Formation of central Iran, in order to assess what aspects of geological history have controlled the cementation and porosity of the sandstones. The Lalun Formation is up to 640 m thick and comprises braided-fluvial sandstones with minor shales that are overlain by marine shales, sandstones and carbonates. In four sections along a ∼650 km traverse, the sandstones are predominantly quartzarenites, locally arkosic and lithic, and contain minimal detrital matrix. Provenance analysis and paleoflow suggest derivation from the Arabian Shield. Diagenesis was dominated by quartz overgrowths and abundant intra-grain microfractures healed by silica cement, with modest amounts of poikilotopic and mosaic calcite, authigenic clay, and iron oxide as early grain rims and late-stage fracture fills. Minor phases include dolomite, ankerite, siderite, magnetite and barite. Marine strata contain more carbonate cement, with silica cement prominent in topmost nearshore sandstones. Burial-history analysis suggests relatively rapid burial in a cratonic setting to more than 4 km by the end of the Silurian and a maximum temperature of ∼150 °C during burial diagenesis, sufficient for hydrocarbon generation by the end of the Paleozoic. Dissolution of K-feldspar, ferromagnesian grains and, locally, calcite cement generated secondary porosity, but porosity and permeability values are low, averaging 1.5% and <0.1 mD, respectively. Rapid subsidence of thick Cambrian fluvial successions at the northeastern Gondwanan margin may have limited early dissolution and promoted occlusion of primary porosity by quartz overgrowths, limiting permeability pathways for fluids that might have generated secondary porosity later in the basinal history. In contrast with Cambrian and Ordovician sandstone reservoirs across North Africa and the Middle East, the existence of Paleozoic source rocks may be problematic in central Iran. [ABSTRACT FROM AUTHOR]
- Published
- 2019
- Full Text
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17. Cement timing and distribution in Lower Cretaceous sandstones: Glenelg, Thebaud and Chebucto fields, offshore Scotian Basin.
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KARIM, ATIKA, PE-PIPER, GEORGIA, and PIPER, DAVID J. W.
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CEMENT , *DIAGENESIS , *SANDSTONE , *LITHOFACIES , *GEOLOGICAL basins , *CLAY minerals , *MUDSTONE - Abstract
Diagenetic cements have been studied in conventional core of Lower Cretaceous sandstone from the Glenelg, Chebucto and Thebaud fields in the Sable Subbasin, offshore Nova Scotia. The spatial and temporal distribution of diagenetic cements and para-sequences has been constrained in relationship to lithofacies, depth and the well position in both the distal (Glenelg and Chebucto) and proximal (Thebaud) parts of the basin. Clay minerals in the sandstones include early grain-coating clays, kaolin, illite and chlorite. Grain-coating illitic clays occur in Glenelg N-49 forming coated grains cemented, initially, by Fe-rich calcite (CI), then low Fe-calcite (CII). Kaolinite occurs as booklets and vermicular stacking textures. It fills large inter-granular pores in the Chebucto well and in some samples from Thebaud I-93 and Thebaud #3. Kaolinitized mica exhibits expanded texture that inflates into adjacent intergranular pores. Illite occurs also as fibrous crystals, which in the Chebucto K-90 well are included by ankerite. Fe-rich chlorite (chamosite) rims are found only in the Thebaud samples examined and demonstrably have developed from earlier Fe-rich clay. Early pore-filling chlorite occurs in contact with detrital quartz and lithoclast grains and is often associated with illite. Both this chlorite and chlorite rims are formed around quartz grains lacking quartz overgrowths. Quartz cement (overgrowths) is well developed principally in medium and coarse sandstones. It postdates kaolinite cement and predates most of the other cements. Carbonate cements (calcite, Fe-calcite, Mg-calcite, ankerite and siderite) are the major cementing minerals filling the large intergranular pores in Glenelg, Chebucto and Thebeaud wells. In Glenelg H-59, two siderite cements were defined; the earliest one is formed by large and corroded crystals and it is low in Mg. The late microcrystalline siderite (< 10 µm) is Mg-rich (8 to 9%). It forms the tiny crystals that fringe detrital grains and fill intercrystalline micropores between quartz and Fe-calcite cement. In Thebaud I-93 siderite nodules contain less Mg than the siderite cement (1%, 8.5% respectively). The neo-formation of framboidal pyrite in carbonate cement indicates a burial under both reducing and alkaline conditions. In samples from the Glenelg field, perthite is partially replaced by Fe-calcite, with only K-feldspar patches and albite left. Rare traces of francolite (samples with 1 to 6 wt.% P2O5) are found in the Glenelg wells associated with illite and calcite cements. These observations on diagenetic minerals are related to the position of host sediments within parasequences. Coated grains are restricted to transgressive system tracts. Abundant early kaolinite and siderite are found principally in sandstones immediately beneath transgressive system tracts, particularly in cross-bedded coarse channel sandstones.Early calcite cement, predating quartz overgrowths, is found principally in bioturbated sandstones and mudstones with bioclasts, typical of the HST. [ABSTRACT FROM AUTHOR]
- Published
- 2008
18. Chlorite diagenesis in reservoir sandstones of the Lower Missisauga Formation, offshore Nova Scotia.
- Author
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GOULD, KATHLEEN, PE-PIPER, GEORGIA, and PIPER, DAVID J. W.
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CHLORITE minerals , *DIAGENESIS , *SANDSTONE , *RESERVOIRS , *GEOLOGICAL basins , *MINERALS , *GEOLOGICAL formations - Abstract
Diagenetic chlorite rims on quartz grains preserve porosity by preventing the formation of secondary, pore-filling quartz overgrowths in wells from the Venture and Thebaud fields. Elsewhere, in the Norwegian Sea and the US Gulf Coast, such chlorite rims have been interpreted as an early burial diagenetic feature related to the input of iron from rivers or volcanic activity, or to later diagenesis by basinal fluids. The purpose of this study is to evaluate which hypothesis is applicable to the Scotian Basin. A set of 45 sandstone samples from conventional cores were analyzed for mineralogy in thin section, mineral composition by electron microprobe, whole-rock chemistry, and X-ray diffraction. From analytical data, it can be argued that a precursor iron-rich clay has diagenetically altered to form chlorite rims during early burial diagenesis, before widespread precipitation of pore-filling kaolinite and quartz overgrowths. The depositional environment, including the degree of bioturbation, may influence formation of early Fe-rich clay coatings. The quality of the final chlorite rim depends on the sea floor diagenetic environment, apparent from the correlation between the quality of chlorite rims and phosphorus. The conditions that favour precipitation of phosphate must also result in Fe-rich clay coatings and may also make some coatings a better precursor than others for the conversion to chlorite during burial diagenesis. The presence of other Fe-rich minerals may also indicate an abundant supply of iron in the early diagenetic environment. [ABSTRACT FROM AUTHOR]
- Published
- 2008
19. Lateral variation in sandstone lithofacies from conventional core, Scotian Basin: implications for reservoir quality and connectivity1,2.
- Author
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Gould, Kathleen M., Piper, David J.W., Pe-Piper, Georgia, Dehler, Sonya, Deptuck, Mark, and Karim, Atika
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SANDSTONE , *LITHOFACIES , *RESERVOIRS , *DIAGENESIS , *TURBIDITES , *LANDSLIDES - Abstract
Facies analysis in outcrops on land is strongly dependent on the lateral variability of lithofacies. Interpretation of conventional core in wells relies principally on the vertical succession of lithofacies. To better understand depositional environments and reservoir sandstone connectivity, the lithofacies of reservoir sandstones sampled in conventional core were correlated laterally through two sets of closely spaced wells in the Scotian Basin: in the Barremian-Albian succession around the Panuke-Cohasset field and in the Late Jurassic succession west of the Venture field. Regional correlation by gamma logs is confirmed by lithologically similar transgressive units including shelly mudstones or coals. A standard scheme of lithofacies and recognition of three types of parasequences were used for comparisons between wells. Some transgressive surfaces are of limited extent and may represent delta distributary switching and subsidence rather than regional changes in sea level. Major sandstone packets extend at least 10-30 km laterally, but commonly show lateral changes in lithofacies, and some are bounded by the margins of incised valleys. Such packages show poor correlation of lithofacies with porosity and permeability, probably because of the variable effects of diagenesis. Lateral transitions from tidal estuary sandstones in Panuke B-90 to thick-bedded river-mouth turbidites in Lawrence D-14, over a distance of 15 km, demonstrates the scale of delta lobes and confirms that sharp-based sandstone beds are turbidites related to river floods, not storm deposits. Similar lateral transitions in the Venture field are on a similar scale and pass distally into prodeltaic muddy landslide deposits. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
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20. The roles of pedogenesis and diagenesis in clay mineral assemblages: Lower Cretaceous fluvial mudrocks, Nova Scotia, Canada
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Piper, David J.W., Hundert, Thian, Pe-Piper, Georgia, and Okwese, Ann C.
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CLAY minerals , *MUDSTONE , *DIAGENESIS , *SOIL science , *CRETACEOUS stratigraphic geology - Abstract
Abstract: Clay mineral assemblages in alluvial mudrocks are important for paleoclimatic interpretation and for understanding burial diagenetic cementation in sandstones, but it is commonly difficult to unravel the relative importance of source weathering, pedogenesis and diagenesis in their origin. The clay mineral assemblages in fluvial overbank mudrocks from the Lower Cretaceous Chaswood Formation in central Nova Scotia, investigated by X-ray diffraction analysis of the <2 µm fraction of 45 samples, include kaolinite, illite, vermiculite, and mixed layer kaolinite/expandable clay and mica/vermiculite. The assemblages vary with depositional facies. Wetland organic-rich mudrocks have large amounts of amorphous material and kaolinite is the dominant clay mineral. In the eastern part of the basin, where overbank mudrocks were episodically uplifted by syn-sedimentary strike-slip faulting, cumulate ultisol and alfisol paleosols are common. In the ultisols, hematite is enriched and kaolinite increases at the expense of illite in the B horizon. Alfisols contain more illite and vermiculite and the B horizon is enriched in goethite. In the western part of the basin, where thin sandstones with abundant diagenetic kaolinite cement are interbedded with the mudrocks, the distinctive clay mineral assemblage of mica/vermiculite mixed layer, vermiculite with 15.5 Å peak, and kaolinite/expandable mixed layer clay with a 17.7 Å peak is interpreted to result from bacterially-mediated oxidation of organic matter below the paleo-water table during early burial diagenesis. Deeper burial diagenesis may lead to slightly higher kaolinite crystallinity. Volcanic ash appears to alter to kaolinite/expandable mixed layer clay with a 7.9 Å peak. Comparison with the continuously subsiding and rapidly accumulated Wessex Formation of southern England, formed at a similar paleolatitude, shows the strong role of pedogenic processes and early diagenesis by meteoric water in development of clay mineral assemblages in the locally tectonically uplifted Chaswood Formation. [Copyright &y& Elsevier]
- Published
- 2009
- Full Text
- View/download PDF
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