reservoir characterization and facies prediction within the late cretaceous doe creek member valhalla field west central alberta canada oil resources at valhalla field of west central alberta canada are stratigraphically trapped within the upper cretaceous doe creek member of the kaskapau formation the reservoir is subdivided into four thin 1 10 m 3 33 ft cyclic alternations of offshore mudrock and shoreface sandstone that are designated the i 1 i i 1 and i 2 units the thickest and most widespread i sandstone is the primary reservoir optimum reservoir quality corresponds to coarser grain shoreface sandstone however reservoir quality may be diminished by postdepositional calcite cement commonly observed near the top of shoreface sandstones open hole well logs are used to predict depositional facies and calcite cement occurrence in wells that lack core control decreasing shale volume vsh and increasing deep resistivity values correspond to progressively shallower water deposits zones of calcite cemented shoreface sandstone greater than 0 5 m 1 6 ft thick are interpreted when the neutron porosity exceeds the density porosity by more than 7 facies distributions predicted for the i sandstone closely match trends of the sandstone gross pore volume and daily total fluid production and suggest that open hole well logs may be used to anticipate reservoir quality and continuity regional and local observations support previous interpretations that attribute the doe creek to forebulge erosion and southwestward sediment transport toward a foredeep where shoreface sandstones accumulated within a coastal embayment to the western interior seaway regionally the doe creek interval thins northeast of valhalla and is truncated beneath the k1 unconformity and shoreface sandstone bodies are encased within offshore mudrocks and detached from their contemporaneous shoreline locally at valhalla the doe creek reservoir progrades toward the southwest and is extensively and commonly uniformly burrowed by a relatively diverse assemblage of trace makers

discontinuity surfaces clinoforms and facies architecture in a wave dominated shoreface shelf parasequence abstract detailed outcrop analysis of a wave dominated shoreface shelf parasequence exposed in the book cliffs utah reveals minor stratigraphic discontinuities which define clinoforms two types of discontinuity are recognized each with a distinctive lithologic character and geometry nondepositional discontinuities are marked by an abrupt decrease in the thickness and amalgamation of storm generated event beds and are interpreted to record hiatuses in sedimentation these discontinuities define clinoforms with a concave upward geometry that dip gently 0 02 0 58 over distances of 800 6000 m down depositional dip erosional discontinuities are marked by an abrupt increase in event bed amalgamation grain size and sand content and are interpreted as enhanced storm wave scours these discontinuities define more steeply dipping 0 22 0 95 concave upward clinoforms that extend over 100 1600 m down depositional dip and 500 1500 m along depositional strike the distribution and amalgamation of minor stratigraphic discontinuities defines linear zones of distinctive facies architecture that are oriented parallel to the paleoshoreline trend using the simple assumption that the shoreface shelf equilibrium profile remained approximately constant for each type of discontinuity throughout shoreface migration the bruun rule intra parasequence facies architecture can be speculatively interpreted in terms of shoreline trajectory which reflects the balance between sediment supply and accommodation the resulting interpretations support the notion that shoreline trajectory exerts a strong control on intra parasequence facies architecture and preservation of the shoreface shelf profile

patterns of early post depositional and burial cementation in distal shallow marine sandstones upper cretaceous kenilworth member book cliffs utah usa there is a limited understanding of the spatial linkage between cementation processes in shoreface coastal plain successions and those operating in distal offshore successions to address this an integrated sedimentological sequence stratigraphic and diagenetic study was undertaken on a distal shallow marine succession the upper cretaceous kenilworth member book cliffs utah usa laterally extensive carbonate cement is the most volumetrically significant diagenetic feature it constitutes up to 55 vol of the distal lower shoreface sandstone and sandy siltstone units beneath major flooding surfaces during rising relative sea level major flooding events resulted in low sedimentation which extended residence time of sediments in early diagenetic zones and promoted enhanced cementation beneath these boundaries carbonate for early calcite cements was probably derived from sulphate reduction marine water and oxidation of organic matter stable isotopic composition of later ankerite 18o 11 9 to 6 0 vdpb 13c 4 5 to 0 3 vdpb suggests precipitation during progressive burial c 1 7 to 3 4 km 72 to 122 c from evolved marine fluids the carbonate and other mineralizing solutes were derived increasingly with burial from decarboxylation of organic matter and clay mineral transformations in adjacent siltstones and shales these interbedded overlying and underlying facies provided an important source of mineralizing solutes that influenced the extent of pervasive cementation during burial the combination of stratigraphic and lithofacies control resulted in laterally extensive cements that differ to shoreface successions although are similar in form to those described for many offshore mudstone dominated successions limited early concretionary ferroan dolomite beneath the expression of the sequence boundary and some flooding surfaces however suggests cementation by meteoric fluids at times of relative sea level lowstand similar processes have been demonstrated in more proximal successions in the book cliffs

origin of nickel in water solution of the chalk aquifer in the north of france and influence of geochemical factors in the north of france high registers of nickel are sometimes recorded within the chalk aquifer in a confined context the presence of pyrite in the covering clays or in the marcasite nodules encrusted in the clay may constitute a natural source of trace metals with an objective of sanitary control the limits of chemical contents regulating the quality of water destined for human consumption have been lowered by the european framework directive in the field of water policy 2000 60 ec as a result nickel limits have been reduced from 50 to 20 g l the analyses carried out on three water catchment fields in our area of study were centred on variable parameters eh o2 d ph conductivity t major elements so4 no3 and metals fe ni mn co the acquired data enabled us to identify from one hand the conditions which are presented within the site special thanks to the evolution of nitrate and iron contents and on the other hand the natural origin geological of nickel for two of the three sites studied based essentially on the evaluation of the nickel cobalt ratio thus on the first site the evolution of nickel content and nitrate content showed the influence of the phenomenon of denitrification on the re mobilisation of the nickel whereas on the second site a high variation of total iron content and oxygen dissolved in solution highlighted a particular phenomenon of oxidation of the pyrite through molecular oxygen finally the correlation with the sulphates clearly showed behaviour of the nickel once released that was entirely dependent on the phenomenon of adsorption on the iron and manganese hydroxides

influence of host rock texture on the morphology of carbonate concretions in a meteoric diagenetic environment abstract the integrated stratigraphic petrographic geochemical study of quaternary marine terraces that have undergone well constrained sea level and water table oscillation s over geologically short time periods can provide important clues on diagenetic processes at the marine continental interface the punta di i cani marine terrace quaternary saint florent corsica unconformably overlies the well cemented low permeability arenites of the miocene monte s angelo formation and is erosively overlain by unlithified high permeability holocene continental sediments the terrace is composed of a retrogradational fining upward succession from fluvial pebbly sandstone to beach and shoreface deposits the entire thickness of the terrace hosts concretions made of low mg calcite concretion geometry is linked to the sedimentological facies of the host sediment i massive fluvial pebbly sandstone at the base of the terrace contains knobby subhorizontal concretions somewhat resembling turbidite flute casts ii very low angle cross bedded well sorted beach deposits contain pipy subhorizontal concretions iii trough crossbedded upper shoreface deposits in the upper part of the terrace contain a subtle diagenetic lineation on the laminae locally obliterated by a patchy isotropic cementation overall the degree of cementation decreases from the bottom to the top of the terrace cement chemistry and morphology as well as stable carbon and oxygen isotopic signatures indicate that the concretions formed by precipitation of meteoric phreatic water flowing toward the coast elongate concretions are invariably parallel to the topographic gradient consistent with the notion that the growth of concretions was controlled by advection smectitic vadose cement is locally present its crosscutting relationships with calcite cement point to periodic oscillations of the water table the results of this study i show that elongate concretions can be used as interstitial paleoflow indicators and ii suggest that concretion geometry in an advection dominated diagenetic environment can depend on the small scale permeability distribution as suggested by the fact that each type of concretion is invariably associated with a specific sedimentary facies

basin scale diagenetic alteration of shoreface sandstones in the upper cretaceous spring canyon and aberdeen members blackhawk formation book cliffs utah abstract leached zones whitecaps and dolomite cement bodies are volumetrically and areally significant features in shoreface sandstones of the upper cretaceous spring canyon and aberdeen members blackhawk formation book cliffs utah quantitative spatial and stratigraphic data of their distribution together with petrography and geochemistry allow development of a model for their formation within a sequence stratigraphic framework leached zones up to 5 m thick and 15 km in lateral extent are present beneath coals capping parasequences concretionary dolomite cement is present in shoreface sandstones downdip of parasequence capping coals and associated whitecaps these concretions are up to 2 m thick and 6 m in length and are present for up to 7 km downdip of these coals petrographic and field data indicate an early diagenetic timing for whitecap and dolomite cement formation 13c data suggest carbonate derivation from detrital dolomite whereas 18o data suggest that cement precipitated from porewater with a significant meteoric component although later diagenetic re equilibration and recyrstallization cannot be rejected we propose that organic acid generation during early diagenesis of coals led to the leaching of detrital dolomite from underlying sandstones and the formation of whitecaps meteoric water flowed basinward in response to base level lowering and sequence boundary development in the upper part of the aberdeen member remobilizing dolomite into shoreface sandstones dolomite precipitation resulted from the mixing of these meteoric fluids with downdip marine pore fluids and the presence of detrital dolomite in the shoreface sandstones that acted as nucleation sites for dolomite precipitation the geometry and extent of dolomite cement in the spring canyon and aberdeen members is in marked contrast to the much greater spatial extent of such cement in low accommodation space settings such as the stratigraphically younger desert member blackhawk formation the relatively simple aggradational to progradational evolution of the spring canyon and aberdeen member parasequences with only minor sequence boundary development in its upper part resulted in spatially limited meteoric fluid input into shoreface sandstones this resulted in dolomite cementation of shoreface sandstones up to a maximum of 7 km downdip of leached zones in the desert member extensive meteoric fluid migration as a result of major sequence boundary development resulted in dolomite cementation of shoreface sandstones for over 25 km downdip of leached zones this highlights that the degree and extent of early fluid diagenesis can be significantly controlled by the nature of sequence development having clear implications for developing predictive models for porosity permeability evolution in sandstone reservoirs

role of microbial processes in linking sandstone diagenesis with organic rich clays abstract numerous chemical reactions within clay sequences have been proposed to produce dissolved material for diagenesis of deeply buried sands however reactions responsible for solubilizing inorganic and organic constituents in clays at intermediate depths and their importance to sandstone diagenesis have not been evaluated results from this study show that the processes of microbial organic acid production via fermentation in clays and microbial organic acid consumption via sulfate reduction in sands effectively link organic rich clays to sandstone diagenesis in the black creek formation of south carolina diagenetic processes have resulted in the formation of 10 volume percent calcite cement 0 1 volume percent authigenic pyrite and 1 5 volume percent secondary porosity in black creek sands however the distribution of these diagenetic processes is not uniform resulting in net destruction of porosity in some parts of the sand and net porosity enhancement in other parts mass balance derived rates of microbial organic acid production 10 5 mmole carbon 1 1 yr 1 and microbial co2 production 4 10 5 mmole l 1 yr 1 show that microbial processes can account for all organic carbon in the calcite cements at least 11 of carbonate carbon based on isotope balance calculations all observed authigenic pyrite and all observed secondary porosity these findings show that microbial processes can serve to link organic rich clays with sandsto e diagenesis at intermediate depths

presence of sulfate does not inhibit low temperature dolomite precipitation abstract the hypothesis that sulfate inhibits dolomite formation evolved from geochemical studies of porewaters from deep sea sedimentary sequences and has been tested with hydrothermal experiments we examined the sulfate inhibition factor using aerobic culture experiments with virgibacillus marismortui and halomonas meridiana two moderately halophilic aerobic bacteria which metabolize independent of sulfate concentration the culture experiments were conducted at 25 and 35 c using variable so42 concentrations 0 14 28 and 56 mm and demonstrate that halophilic aerobic bacteria mediate direct precipitation of dolomite with or without so42 in the culture media which simulate dolomite occurrences commonly found under the earth s surface conditions hence we report that the presence of sulfate does not inhibit dolomite precipitation further we hypothesize that if sedimentary dolomite is a direct precipitate as in our low temperature culture experiments the kinetic factors involved are likely to be quite different from those governing a dolomite replacement reaction such as in hydrothermal experiments consequently the occurrence and presumably growth of dolomite in so42 rich aerobic cultures may shed new light on the long standing dolomite problem

petrography geochemistry and origin of burial diagenetic facies siluro devonian helderberg group carbonate rocks central appalachians petrographic and geochemical studies of pore filling cements and replacement products in the helderberg group upper silurian lower devonian central appalachians document the diagenetic history of these rocks shallow ramp skeletal limestones and buildups were partially lithified by marine cements however most pore filling cement formed under shallow 300 m to deep burial 300 4 000 m conditions regional cathodoluminescent zonation patterns in early calcite cements indicate meteoric ground waters were involved in shallow burial cementation early zoned calcite cements consist of updip nonluminescent cements with thin luminescent laminae in limestone or sandstone that pass downdip into subzoned dull cements interlayered bright and dull laminae and then into nonzoned dull cement in basinward limestone the regional distribution stable isotopic compositions and timing of early zoned cements relative to other diagenetic events suggest that all of the early cements formed synchronously from meteoric ground waters that became progressively more reducing as they flowed downdip at least 150 km helderberg sandstone tongues probably were preferential conduits for meteoric gro nd waters because early calcite cements formed only on scattered skeletal grains in sandstones late void filling dull cement formed from deep burial pore fluids at burial depths of 300 to 4 000 m deep burial pore fluids were dilute to saline na ca cl brines from primary fluid inclusion data with stable isotopic compositions that probably were similar to isotopic compositions of formation waters from modern oil fields void filling dull calcite cements occluded nearly all remaining porosity before late paleozoic fracturing and deformation latest diagenetic phases include dolomite fluorite silica and fracture filling dull calcite this study shows that integrated petrographic and geochemical approaches to diagenetic studies are useful for relating various diagenetic phases and fabrics to burial history and paleoaquifer distribution

basin scale dolomite cementation of shoreface sandstones in response to sea level fall this study documents a link between sea level fall and basin scale dolomite cementation of shoreface sandstones in the upper cretaceous desert member book cliffs utah by integrating sedimentology sequence stratigraphy and sedimentary geochemistry within the desert member shoreface sandstones a number of high frequency sequence boundaries formed as a result of relative sea level falls that led to localized incised valley formation subaerial exposure and meteoric leaching of upper shoreface sandstones shoreface sandstones up to 10 km downdip from these subaerial exposure surfaces contain ferroan dolomite concretion bodies as much as 8 m thick basinward of this smaller as much as 1 m in size less abundant dolomite concretions can be traced at the same horizons some 1 8 o data suggest that cement precipitated from porewater with a significant meteoric component although reequilibration and recyrstallization cannot be eliminated some 1 3 c data suggest carbonate derivation from detrital dolomite leached from beneath organic rich coastal plain strata deposited during high frequency sequence boundary development we propose that during high frequency sequence boundary formation meteoric fluids migrated basinward into shoreface sandstones as a result of the relative fall in sea level the precipitation of dolomite cement in the shoreface sandstones was promoted by the updip dissolution and remobilization of detrital dolomite by the meteroic fluids mixing with marine pore fluids and the presence of detrital dolomite in the shoreface sandstones which acted as nucleation sites for dolomite precipitation this study illustrates the impact of stratigraphic development upon basin scale early diagenetic cementation of siliciclastic succcesions

early diagenesis and its relationship to depositional environment and relative sea level fluctuations upper cretaceous marshybank formation alberta and british columbia early diagenesis of the upper cretaceous late coniacian to early santonian marshybank formation was controlled by depositional environment composition of depositional water fe and organic content of the sediment sedimentation rate proximity to the shoreline and influx of meteoric water related to relative sea level fall five depositional environments each characterized by a distinct early diagenetic mineral assemblage have been recognized offshore shelf sediments that were deposited in a dysaerobic environment are characterized by abundant framboidal pyrite and rare septarian concretions composed of early calcite and siderite intense sulphate reduction promoted by the dysaerobic depositional water was the primary influence on early diagenesis offshore shelf sediments deposited under aerobic conditions are characterized by abundant concretions composed of two generations of siderite s1 and s2 in this environment methanogenesis rather than sulphate reduction was more important early diagenesis of the inner shelf sands was generally limited however in sands deposited proximal to the shoreline mixing of marine and meteoric waters promoted crystallization of fe rich chlorite and siderite the shoreface was characterized by dissolution of detrital minerals in the upper portion and precipitation of kaolinite or illite smectite in the lower portion in the coastal plain environment brackish water and early reducing conditions resulted in formation of abundant euhedral pyrite ankerite rather than siderite is the typical early diagenetic carbonate the 18o values of the earliest cements i e early calcite siderite s1 inner shelf siderite indicate crystallization from a low 18o marine derived porewater assuming crystallization at 25 c a 18o value of about 7 smow can be estimated for the seaway during marshybank formation time similar calculations for the overlying dowling member puskwaskau formation suggest that the 18o value of the seaway increased to about 4 smow consistent with its transgressive nature very low 18o values are exhibited by siderite s2 these results indicate crystallization during intermediate diagenesis 60 c from meteoric water 15 smow that entered the marshybank formation during sea level lowstand

investigation of collar properties on data acquisition scheme for acoustic logging while drilling abstractwe have used the wavenumber integration velocity time semblance and dispersion methods to investigate the influence of collar properties including velocities density and attenuation on acoustic logging while drilling wavefields we have found that when the velocities of the collar wave and the p wave of the formation are similar they interfere however the interference disappears when the velocity difference increases having a collar with large velocities especial large shear velocity and density makes the direct p velocity determination possible in a fast formation even without isolators for a slow formation the interference of the collar flexural wave with the formation flexural and leaky p waves is slight for a dipole tool when collar velocities are large for this case the s velocity can be determined by the flexural formation wave at low frequency approximately 2 khz based on these observations we propose that the measurement of the p and s velocities can be easier if the col

patterns of groundwater salinity changes in a deep continental oceanic transect off the southeastern atlantic coast of the u s a abstract manheim f t and paull c k 1981 patterns of groundwater salinity changes in a deep continental oceanic transect off the southeastern atlantic coast of the u s a in w back and r letolle guest editors symposium on geochemistry of ground water 26th international geological congress j hydrol 54 95ndash 105 investigations of formation fluid salinities in a transect from western georgia to the edge of the blake plateau off the coast of georgia show surprisingly similar hydro chemical features offshore and onshore a fresh brackish wedge of groundwater 25g kg total dissolved solids lies beneath the shelf to a depth of 900 m on land brackish waters extend to a maximum depth of 1 2 km below sea level in lowndes county georgia in deeper horizons hypersaline brines 100g kg occur in lower cretaceous strata these strata have a pronounced evaporitic anhydritic character in the offshore segment strong salinity gradients in interstitial waters signify buried evaporite deposits at drill sites beneath the blake plateau

meteoric burial diagenesis of pennsylvanian arkosic sandstones southwestern anadarko basin texas pennsylvanian arkosic sandstones at mobeetie field were deposited in fan deltas that prograded onto a shallow shelf in the southern anadarko basin the detrital minerals of the sandstones reflect the composition of the precambrian granites and granodiorites that were exposed in the nearby amarillo uplift distal margins of some fan delta lobes were reworked by marine processes and carbonate fossil fragments and oolites were mixed with terrigenous clastics the diagenetic history and hence reservoir quality of the distal marine reworked sandstones differ from those of the more proximal nonreworked sandstones the earliest diagenetic events formation of chlorite ooids and precipitation of fibrous submarine mg calcite cement took place in the depositional environment of the marine reworked sandstones on the basis of isotopic data the remainder of the diagenesis evidently occurred in fluids of meteoric origin shallow early meteoric diagenesis induced precipitation of iron poor calcite spar in reworked sandstones nonreworked sandstones lack calcite spar but contain early pore lining chlorite cement dissolution of aragonite by fresh meteoric ground water created moldic porosity within reworked sandstones as burial increased porosity in both the reworked and nonreworked fan delta sandstones was reduced by precipitation of authigenic quartz feldspar kaolinite fe calcite and ankerite oxygen isotope data suggest that these cements also precipitated from fluids of meteoric origin that became increasingly hot but remained relatively constant in isotopic composition the last diagenetic events were influenced by the overlying permian evaporites sodium rich fluids caused partial albitization of detrital plagioclase and sulfate derived from the evaporites precipitated as anhydrite and celestite

sequence stratigraphic significance of sharp based lowstand shoreface deposits kenilworth member book cliffs utah the kenilworth member of the blackhawk formation upper cretaceous campanian is nearly continuously exposed from price to hatch mesa in the book cliffs utah five main depositional environments are recognized coastal plain foreshore and upper shoreface lower shoreface inner shelf and incised valley fill deposits the shelf to shoreface deposits combine to form nine coarsening upward wave to storm dominated parasequences that are 7 32 m thick and that split basinward into smaller coarsening upward successions or bedsets that are 2 18 m thick a maximum of four bedsets occurs within an individual parasequence bedset bounding surfaces are occasionally marked by thoroughly bioturbated iron rich carbonate cemented sandstones within the parasequences in the subsu face similar bedset bounding surfaces may act as permeability barriers or baffles leading to reservoir compartmentalization parasequences 1 6 form an aggradational to progradational parasequence set of the highstand systems tract parasequences 7 and 8 occur as sharp based lowstand shoreface deposits at price river and battleship butte forming the attached lowstand systems tract and parasequence 9 forms the transgressive systems tract the base of each lowstand shoreface is marked by an organic rich lag deposit that consists of granules coarse grained sandstone wood fragments coal intraclasts and shell debris the sharp base basal lag deposit and basinward shift in facies delineate a sequence boundary at the base of each lowstand shoreface deposit the presence of two sharp based lowstand shorefaces suggests that the kenilworth lowstand was punctuated by a stillstand the sharp based lowstand shoref ce sandstones form the bulk of the attached lowstand systems tract and represent one end member in a continuum of lowstand deposits the other end member is the detached lowstand systems tract revising existing sequence stratigraphic models to include both types of lowstand systems tracts is a first step toward improving the accuracy of these models which are often used as prospecting tools in lowstand deltaic environments

disseminated jigsaw piece dolomite in upper jurassic shelf sandstones central north sea an example of cement growth during bioturbation unusual textural and chemical characteristics of disseminated dolomite in upper jurassic shelf sediments of the north sea have provided the basis for a proposed new interpretation of early diagenetic dolomite authigenesis in highly bioturbated marine sandstones the dolomite is present throughout the franklin sandstone formation of the franklin and elgin fields as discrete non ferroan generally unzoned subhedral to highly anhedral jigsaw piece crystals these are of a similar size to the detrital silicate grains and typically account for 5 of the rock volume the dolomite crystals are never seen to form polycrystalline aggregates or concretions or ever to envelop the adjacent silicate grains they are uniformly dispersed throughout the sandstones irrespective of detrital grain size or clay content dolomite authigenesis predated all the other significant diagenetic events visible in thin section the dolomite is overgrown by late diagenetic ankerite and bulk samples display stable isotope compositions that lie on a mixing trend between these components extrapolation of this trend suggests that the dolomite has near marine delta18o values and low positive delta13c values the unusual textural and chemical characteristics of this dolomite can all be reconciled if it formed in the near surface zone of active bioturbation sea water provided a plentiful reservoir of mg and a pore fluid of regionally consistent delta18o labile bioclastic debris e g aragonite mg calcite supplied isotopically positive carbon to the pore fluids during shallow burial dissolution such dissolution took place in response to the ambient calcite sea conditions but may have been catalysed by organic matter oxidation reactions bioturbation not only ensured that the dissolving carbonate was dispersed throughout the sandstones but also prohibited coalescence of the dolomite crystals and consequent cementation of the grain framework continued exchange of mg2 and ca2 with the sea water reservoir maintained a sufficient mg ca ratio for dolomite rather than calcite to form irregular crystal shapes resulted from dissolution of both the dolomite and the enclosed fine calcitic shell debris before ankerite precipitation during deep burial diagenesis

geochemistry of early carbonate cements in the cardium formation central alberta abstract authigenic minerals and diagenetic textures in the marine conglomerates and sandstones of the cretaceous cardium formation in the northeast pembina area preserve a complex sequence of diagenetic events textural relationships observed in thin section and under the scanning electron microscope were used to determine the relative timing of diagenetic events in these marine sediments paragenetic sequences are similar for the conglomerate sandstone and siderite nodules in the enclosing shales extensive cementation by siderite and calcite occurred early in the diagenetic history before any significant compaction the geometry and distribution of lithofacies in the cardium may have influenced the diagenesis and internal stratigraphy of the conglomerate overlying shale matrix conglomerate may have trapped upward migrating fluids increased in buoyancy by dissolved co2 produced by decay of organic matter in the enclosing shales these fluids could have infiltrated the upper portion of open matrix conglomerate causing cementation petrographic evidence shows alternating precipitation of siderite and pyrite implying fluctuating activities of dissolved carbonate and hydrogen sulfide probably caused by bacterially mediated processes during early diagenesis early sequences of siderite and pyrite precipitation can be related to zones of iron reduction sulfate reduction and possibly decarboxylation during diagenesis carbon and oxygen isotopic data indicate a systematic change in the isotopic compositions of calcite and siderite from 25 to 3013c and 018o for cements early in the paragenetic sequence to 0 to 513c and 15 18o for cements which are interpreted to occur later in the paragenetic sequence this observation is consiste t with the influence of meteoric water on later stages of cement deposition in the cardium a unit considered to be marine and deposited well offshore on a shallow marine shelf the intrusion of meteoric water far offshore may be related to changes in sea level many variables must be accounted for but it is feasible using the estimated regional dip of the cardium at the time of deposition 0 005 for a 1 m drop in sea level to push the freshwater seawater interface seaward on the order of 100 km

oxygen and carbon isotopic composition of marine carbonate concretions an overview abstract compilation of stable isotope data 18o and 13c for marine calcite siderite and dolomite concretions from a large number of studies reveals consistent and unanticipated isotopic trends although many of the dolomites have 18o values that are compatible with precipitation from unaltered seawater at low temperatures most of the calcites and siderites are far too depleted in 12o for such an origin this is surprising as most of the concretions appear to have formed very early in the diagenetic history of the sediments often within several meters of the sediment water interface the most likely cause of the anomalously depleted oxygen values is a decrease in pore water 18o values during early diagenesis a result perhaps of a significant influx of meteoric water into the marine shelf environment or water mineral interaction e g precipitation of 18o enriched minerals the carbon isotopic field for dolomite is quite different from that for calcite and siderite highly positive 13c values are fairly common only for dolomites this indicates that while dolomite concretions often form in sediments where methanogenesis is rapid precipitation of calcite and siderite concretions is restricted to sediments where either n methanogenesis takes place or where methanogenesis is not extensive enough to generate highly positive pore fluid 13c values this variation in carbon isotopic composition among the different minerals suggests that the rates of organic carbon oxidation and sedimentation may control the mineralogy of carbonate concretions high organic carbon oxidation rates favor dolomite and lower oxidation rates favor calcite or siderite

fluid flow due to the advance of basin scale silica reaction zones the conversion of biogenic silica opal a to opal ct cristobalite and tridymite in biosiliceous sediment causes increased rates of water expulsion because of the reduction in sediment porosity and dehydration of the amorphous opal a phase this release of water occurs over large tracts of sedimentary basins during sediment burial within discrete diagenetic reaction zones analysis of two dimensional and three dimensional seismic data sets from basins in the northern hemisphere provides geophysical evidence for a variety of fluid conduits and roughly circular erosional depressions at the contemporaneous seabed we interpret these features as indicative of water expulsion and focused fluid flow emanating from opal a to opal ct reaction zones at burial depths within the range 200 800 m the rate at which water is expelled depends upon the degree of porosity reduction and the weight fraction of bound water at the reaction zone as well as the rate of advance of the reaction zone where the reaction is actively taking place within homogeneous biosiliceous sediment the rate of water expulsion is independent of the reaction rate this is because water is released across the entire reaction zone therefore slow reaction rates are compensated for by expulsion of water across wider reaction zones we calculate the rate and volume of water expulsion for the faeroe shetland basin where the sediment immediately below the reaction zone contains on average 30 opal ct by weight the estimated volumetric rate of water expulsion per unit surface area at the present day is 6 m3 my 1 per square meter which is greater than the vertical flux of water at the same depth from compaction of the deeper basin fill the average volumetric rate of water expulsion is 120 km3 my 1 across the whole basin biogenic silica is particularly rich in neogene successions in high latitude and equatorial regions and where silica reaction zones are identified they should be factored into sediment compaction and fluid flow histories

distribution of diagenetic alterations in fluvial deltaic and shallow marine sandstones within a sequence stratigraphic framework evidence from the mullaghmore formation carboniferous nw ireland abstract the distribution of diagenetic alterations in the fluvial deltaic and shallow marine arkosic to subarkosic sandstones average q72f26l02 of the mullaghmore formation carboniferous nw ireland can be predicted within a sequence stratigraphic framework eogenetic calcite 18opdb 13 3 to 6 5 13cpdb 3 0 to 3 4 and 87sr 86sr 0 706721 to 0 709227 and ferron dolomite feco3 8 12 mol 18opdb 14 2 to 7 8 13cpdb 1 4 to 1 0 and 87sr 86sr 0 709051 to 0 709167 occur in bioclast rich transgressive lag deposits at parasequence boundaries and transgressive surfaces and in wave influenced deltaic highstand systems tract hst deposits mesogenetic illite chlorite baroque dolomite feco3 16 mol 18opdb 14 2 to 12 7 13cpdb 3 8 to 1 0 quartz and calcite 18opdb 15 7 to 12 5 13cpdb 5 8 to 3 7 and 87sr 86sr 0 709016 to 0 709122 were formed mainly in the bioclast poor deposits which were not pervasively cemented by carbonates during near surface eodiagenesis these deposits include fluvial incised valley sandstones of lowstand systems tract lst and fluvial dominated deltaic sandstones of transgressive systems tract tst and hst illite is the dominant diagenetic clay mineral in the fluvial incised valley sandstones of lst possibly because of simultaneous albitization of k feldspars conversely chlorite dominates in the fluvial dominated deltaic sandstones of tst and hst because of the presence of suitable precursor clays the integration of diagenesis into sequence stratigraphic framework of clastic sequences should improve the ability to predict the spatial and temporal distribution of diagenetic alterations and related reservoir quality modifications of sandstone deposits

spatial distribution of carbon and oxygen isotopes in laterally extensive carbonate cemented layers implications for mode of growth and subsurface identification abstract carbonate concretions and laterally extensive cemented layers are common features in the prairie canyon member of the mancos shale at its type locality in western colorado stable isotope and petrographic data were obtained from these cemented zones in an attempt to determine 1 how the cemented layers form 2 whether or not they can be chemically correlated and 3 if it is possible to differentiate them from concretions in the subsurface the cement in both the layers and concretions consists of ferroan dolomite and lesser amounts of stoichiometric dolomite and ankerite samples for stable isotope analysis were drilled from 10 grids with a typical grid spacing of 10 cm10 cm gridded samples were obtained from two continuous cemented layers at several locations along the layers 2 3 m to 1 5 km apart on the same layer and from isolated and stratabound concretions taken together all the cements regardless of spatial position show a remarkably linear correlation r2 0 98 between 13c and 18o systematic center to edge decreases in both 13c and 18o occur in each of the cemented zones and are persistent over lateral intervals of at least 1 5 km the cores of the cemented zones have positive 13c values up to 10o oo pdb which result from incorporation of 13c enriched carbon during methanogenesis the edges of the cemented zones have negative 13c values down to 9o oo pdb which suggests incorporation of 13c depleted carbon from thermocatalytic decarboxylation reactions oxygen isotope paleothermometry indicates that precipitation may have started at moderate temperatures 25 c and ended at temperatures as high as 92 c the spatial distribution of the isotopic data in the cemented layers can be explained as resulting from the coalescence of stratabound concretions followed by growth above and below the coalesced concretions such growth may have occurred in a complex manner in which early cements precipitated throughout the thickness of the layer but were more abundant in the core zone given the lateral persistence of the pattern of isotopic variation in the cemented layers geochemical correlation of the layers may be possible in the subsurface in addition because laterally persistent cemented layers in the prairie canyon member exhibit patterns of isotopic variation different from concretions layers have a more asymmetrical vertical distribution it should be possible to distinguish between the two styles of cementation from subsurface core data alone

stratigraphic implications of meteoric sphaerosiderite 18o values in paleosols of the cretaceous albian boulder creek formation ne british columbia foothills canada abstract the md 80 08 coal exploration core from the albian boulder creek formation of northeastern british columbia contains 90 m of alluvial strata that underwent extensive pedogenic modification gleyed colors pedogenic slickensides clay coatings mottling sphaerosiderite and sedimentary pyrite characterize the paleosols the isotopic compositions of 30 sphaerosiderite bearing horizons commonly show trends of invariant 18o and variable 13c values equal area plots of 18o vs 13c were used to construct meteoric sphaerosiderite lines msls the msls indicate that meteoric groundwater compositions ranged from approximately 11o oo to 16o oo smow the sphaerosiderite 18o values indicate formation in shallow groundwaters predominantly recharged by local precipitation the sphaerosiderite isotopic compositions are a proxy record of mid cretaceous precipitation and are useful for quantifying changes in the hydrologic cycle during humid greenhouse periods micromorphology of a 1 15 meter interval 69 75 68 6 m with a 1 31o oo increase in the average 18o values records a polygenetic history of paleosol development the pedocomplex has a four stage developmental history characterized by 1 an initial well drained phase with wet dry cycles and relatively low baselevel 2 erosion baselevel rise and coarser grained sedimentation 3 saturated reducing soil conditions high baselevel with some marine influence on pedogenesis and 4 meteoric water dominated hydromorphic soil conditions high baselevel and sphaerosiderite precipitation the 18o excursion recorded through this interval reflects minor mixing of marine and meteoric groundwaters the modal abundance of pyrite and increased mg ca mg and mg fe cation ratios in the sphaerosiderites further substantiate marine influence on pedogenesis the alluvial section of the boulder creek formation is aggradational and several changes in baselevel recorded in the paleosols are interpreted to have been driven by minor parasequence scale changes in relative sea level

calcite cemented concretions in cretaceous sandstone wyoming and utah u s a abstract spheroidal cannon ball calcite cemented concretions some of gigantic size were studied from three cretaceous shelf sandstone units ferron frontier and second frontier the concretions have diameters between 2 cm and 6 m those larger than 40 cm in diameter in the ferron and frontier have septarian structure rare concretions in the second frontier with cone in cone also have septaria the pattern of carbon and oxygen isotopic data and intergranular volume igv data across concretions indicate that concretion growth was complex not all concretions grew progressively from their centers toward their edges in many concretions displacive fascicular optic high mg calcite foc precipitated simultaneously throughout the concretions pervasive growth pattern although the earliest cementation was concentrated in and near their centers remaining pores were subsequently filled by poikilotopic calcite of lower mg content displacive foc cement in the bodies of concretions and in cone in cone structure and the large igv values indicate that these carbonate phases formed at burial depths no greater than tens of meters igv values indicate that spar cement continued to depths of at least 50 m and possibly deeper the association of organic matter and microbial structures or microbial byproducts with displacive foc and cone in cone calcite raises the possibility that biogenic processes were responsible for the apparently rapid growth of the displacive calcite the oxygen isotope composition of pore waters was fairly uniform in the range of 18o of 4 to 6 smow during 80 of concretion growth but became even more depleted in 18o during the last stage of growth and during septarian fracture filling ranging into values as depleted as 14 smow 13c in calcite is negative 4 to 18 v pdb but variable reflecting a range of microbial processes under both oxidizing and reducing conditions earlier calcites fall mostly in the range of 10 to 16 v pdb relatively later calcite in veins and on concretion margins is more enriched in 13c fe and mn concentrations in calcites have a rough positive correlation with higher 13c values the concentration of mg derived from seawater and altered volcanic rock fragments covaries positively with the degree of meteoric influence lower mg contents correlate positively with relatively depleted 18o values 87sr 86sr values are typical of sr derived from coeval seawater or marine skeletal debris growth of concretions 4 to 6 m in diameter apparently in 5 my requires some spatial localization of growth sites by a self organization process and probably a large supply of carbonate shells in the original sediment elongation of larger concretions in the plane of bedding suggests that advective rather than diffusive supply of cement components dominated late in concretion growth history concretions formed close to the tops of parasequence boundaries

isolated highstand shelf sandstone body of turbiditic origin lower kenilworth member cretaceous western interior book cliffs utah usa abstract an isolated highstand shelf sandstone body of turbiditic origin has been identified in the lower kenilworth member hatch mesa area book cliffs utah relative to most other marine mudstone encased isolated sandstone bodies the hatch mesa succession has an abundance of bouma like turbidite beds a greater proportion of mudstone and siltstone is thinner 6 20 m and is weakly coarsening upward these attributes in combination with locally abundant hummocky cross stratified sandstones a paucity of wave ripples in the background fair weather deposits carbonaceous matter in all facies a lack of clinoforms and a low diversity abundance trace fossil suite suggest deposition in a prodelta environment between fair weather and storm wave base a three component shoreface to shelf model consisting of delta front subaqueous channel and prodelta turbidite deposits is proposed to explain the depositional environment and setting of the hatch mesa succession regional correlation has demonstrated that the hatch mesa succession is time equivalent with the second parasequence in the kenilworth kps2 which is characterized by inner shelf turbidite bodies at a number of other localities paleogeographic reconstructions reveal that the hatch mesa succession was deposited at least 16 km basinward of the kps2 lower shoreface in approximately 20 m of water the kps2 hatch mesa sandstone body package is part of the early highstand systems tract indicating that isolated or stray sandstone bodies are not solely restricted to the falling stage or lowstand systems tracts

use of isotope fractionation of sulfate sulfur and sulfate oxygen to assess bacterial desulfurication in a sandy aquifer abstract the unconfined aquifer of the fuhrberger feld consists of 20 30m sands and gravelly sands with unevenly distributed reduced sulfur compounds and small lignitic pebbles in vertical direction two hydrochemically different zones with specific solute transformations are present an upper portion with bacterial denitrification with reduced sulfur compounds as electron donors and a deeper portion with hydrochemical indications of reduction of the aqueous sulfate in the study area ten multilevel groundwater sampling wells were installed sampling and interpretation was based on a two dimensional vertical plane groundwater flownet and on the knowledge of the landuse upgradient of the multilevel wells the concentration and isotopic composition of the sulfate at the entrance into the zone of sulfate reduction is controlled by sulfate of groundwater recharge as well as sulfate formed during denitrification the recharge sulfate shows clear landuse specific differences in concentration and isotopic composition under arable land an average concentration of 60 mg so 4 l 1 with values of 34 s close to 3 cdt diablo meteorite standard canon and of 18 o about 8 standard mean ocean water smow was measured whereas under coniferous forest an average of 100 mg so 4 l 1 with 34 s 2 and 18 o 4 5 was found in the denitrification zone no significant sulfur isotope fractionation occurs during oxidation of reduced sulfur the oxygenisotopic composition of the newly formed sulfate denitrification sulfate depends on the 18 o content of nitrate acting as oxygen donor and on the 18 o content of groundwater in which the oxidation occurs under certain assumptions a 18 o 0 54 was calculated for the oxygen of the newly formed sulfate consequently with increasing sulfate concentrations i e with the admixture of denitrification sulfate a decrease of the 18 o values from about 8 to 2 was observed in the zone of sulfate reduction both the 34 s and 18 o values of the residual sulfate increase with decreasing sulfate concentrations thus proving the existence of bacterial reduction of the aqueous sulfate this sulfate reduction follows a rayleigh process with calculated enrichment factors of 34 9 7 for 34 s and 18 6 for 18 o respectively a plot of 34 s vs 18 o gives a good linear correlation with a slope close to 1 1 4 data on concentration and isotopic composition of sulfate and on the hydraulically derived groundwater age are used for kinetic considerations the results of a multilevel well for depths between 10 and 17m are given as an example the bacterial reduction can be considered as first order reaction with a half life between 75 and 100 years

a sedimentological and sequence stratigraphic reinterpretation of the upper cretaceous prairie canyon member mancos b and associated strata book cliffs area utah usa the mancos shale book cliffs eastern utah usa represents the open marine mudstones of the cretaceous western interior seaway and encloses the prairie canyon member which is located over 50 km seaward of interpreted contemporaneous highstand shoreline deposits in the blackhawk formation examination of the member reveals that it does not wholly represent offshore deposition as previously interpreted but instead contains three nearshore facies associations 1 tidally influenced fluvial channel fills 2 fluvial dominated delta fronts and 3 weakly storm influenced shorefaces tidally influenced fluvial channel fills are commonly stacked into multistory bodies that can be traced for tens of kilometers at discrete stratigraphic levels defining incised valley fill networks four such incised valley networks are identified at outcrop fluvial dominated deltas and weakly storm influenced shorefaces are eroded into by and lie at the down dip terminations of incised valley fills and are interpreted as forced regressive and lowstand shoreface deposits one incised valley fill appears to be onlapped by additional fluvial dominated deltas which represent pulses of shoreface progradation during an overall transgression forced regressive lowstand and transgressive shorefaces in the prairie canyon member differ significantly from highstand shorefaces in the blackhawk formation the former are sand poor and weakly wave storm influenced whereas the latter are sand rich and wave dominated this change in shoreface style reflects increased mud supply and an enhanced embayment paleogeography during periods of relative lowering of sea level

systematic sequence scale controls on carbonate cementation in a siliciclastic sedimentary basin examples from upper cretaceous shallow marine deposits of utah and coloradao usa whilst the relationship between stratigraphic development and carbonate cementation within siliciclastic succession has been documented through a number case studies these studies have been generally restricted to observations upon individual sequences and or limited sub surface data in this paper long term 5 million years large scale 200 km stratigraphic controls on carbonate cementation patterns are documented from the upper cretaceous panther tongue member blackhawk formation and castlegate sandstone exposed in the book cliffs in utah and colorado usa together these comprise eight progradational wedges of sandstones which interfinger with the mancos shale deposited within the western interior seaway foreland basin petrographic analyses of ferroan dolomite cement bodies within these sandstone wedges show that the ferroan dolomite cements are all early relative to burial diagenesis within the host sandstones stable isotope analyses indicates that a significant meteoric component was present in precipitating fluids and this is consistent with the observation that cements are always present down dip of sequence boundaries and or leached whitecaps beneath coals in addition the lateral distribution of cement bodies increases consistently up succession from less than 5 km in extent in the older sequences to 30 km in extent in the youngest sequences these changes in distribution are in response to the increased progradation and increased and more aerially extensive sequence boundary development in younger sequences the implications of these data are that whilst localized spatial patterns of diagenesis and in particular carbonate cementation are predictable and controlled by the nature and presence of individual stratal surfaces systematic diagenetic alteration patterns are also present at the sedimentary basin scale and controlled by the nature of larger scale stratigraphic development and basin evolution this evolution may be driven by eustatic shifts or through tectonic or climatic driven base level shifts these observations allow an improved insight into the basin scale processes that control the macroscopic diagenetic properties of sedimentary successions and sub surface hydrocarbon reservoirs

shoreface tongue geometry constrains history of relative sea level fall examples from late cretaceous strata in the book cliffs area utah progradational shoreface tongues preserve a near complete depositional record of relative sea level highstands falls and lowstands two distinct styles of progradational shoreface tongue are examined in an extensive outcrop and subsurface dataset from late cretaceous strata of the book cliffs area utah representing i highstand through attached lowstand progradation and ii highstand through detached lowstand progradation using this dataset key geometrical attributes of the shoreface tongues and their internal facies architecture are identified and quantified that enable the reconstruction of relative sea level fall history for example attached wave dominated lowstand shoreface deposits record a slow 0 2 0 3 mm yr 1 low magnitude 14 m relative sea level fall punctuated by minor rises detached weakly wave influenced lowstand shoreface deposits record a more rapid 0 4 0 5 mm yr 1 high magnitude 45 m relative sea level fall synchronous with a marked change in sediment delivery and depositional process regime at the shoreline

physical and chemical growth conditions of ordovician organogenic deep water dolomite concretions implications for the δ18o of early palaeozoic sea water the estimated depth of formation of authigenic dolomite concretions in the middle ordovician cloridorme formation quebec ranges from 350 mbsf 25 17 porosity in the lower ordovician levis formation outward decreasing 13cvpdb values 10 2 0 8 suggest precipitation in the methane generation zone with an increasing contribution of light carbonate derived by advection from thermocatalytic reactions at depth anomalously low 18ovpdb values centre to margin variations of 0 4 to 7 5 give reasonable temperatures for the concretion centres only if the 18o of ordovician sea water was negative 6 and the bottom water was warm 15 c the 3 5 lower values for the concretion margins compared with the centres can be explained if in addition volcanic ash alteration organic matter decomposition and or advection of 18o depleted water lowered the 18o of the pore water further by 2 0 4 0 during the first 25 200 m of burial reasonable growth temperatures for the margins of 17 20 c are compatible with a lowering of the isotopic ratios by 1 to 1 3 as a temperature effect the systematic concentric isotope zonation of the concretions suggests that the well ordered near stoichiometric dolomite is a primary feature and not the result of recrystallization diagenetic dolomite beds of the cloridorme formation appear to have formed by coalescence of concretions as shown by randomly sampled traverses that indicate formation at different subsurface depths growth of the cloridorme dolomites was probably limited by calcium availability at least 50 of which was derived from connate water and the remainder by diffusion from sea water dolomite precipitation was favoured over calcite by very high sedimentation rates the abundance of marine organic matter in the host sediment and a correspondingly thin sulphate reduction zone deep seated concretion growth in the levis formation required either internal sources for the participating ions carbonate dissolution event or porewater advection along faults

hydrochemical evolution of the east midlands triassic sandstone aquifer england a detailed hydrogeochemical interpretation has been made of the red bed sherwood sandstone triassic aquifer of the east midlands of england on which a radiocarbon model has been used to establish a chronology of groundwaters along the flow line groundwater evolution is dominated by carbonate and sulphate mineral solution and precipitation and by redox reactions cation exchange reactions are negligible and the hydrogeochemistry has evolved in the absence of residual saline water three distinct zones are recognised 1 i recent groundwater mainly 100 y oxidising dominated by congruent dissolution of dolomite and by inputs from atmospheric and anthropogenic sources 2 ii groundwater age 103 104 y predominantly reducing dominated by incongruent dissolution of dolomite 3 iii groundwater age 104 y reducing conditions approach to isotopic and chemical equilibrium in the carbonate system dominance of gypsum dissolution the minor and trace element variation can be interpreted in relation to the principal geochemical reactions and recharge history na cl f and b are controlled by atmospheric inputs sr2 and mn2 trends can be explained by carbonate and sulphate mineral dissolution ba2 is derived from carbonates but controlled by barite equilibrium fe so42 no3 and u are strongly influenced by redox reactions the concentrations of transition metals and other elements remain very low only li and k are considered to represent additions by reaction of silicate minerals the extent of direct anthropogenic influence is probably limited to enhanced so4 no32 and possibly cu zn and pb in zone i

sequence stratigraphy and marine to nonmarine facies architecture of foreland basin strata book cliffs utah u s a the desert member of the blackhawk formation and the castlegate sandstone in the book cliffs of utah and colorado were analyzed to develop a model of sequence stratigraphy and facies architecture in foreland basins stratal architecture of these rocks is controlled by two regionally extensive surfaces of incision and subaerial exposure interpreted as major sequence boundaries and at least six surfaces of incision and subaerial exposure with more limited lateral extent interpreted as minor sequence boundaries each sequence boundary is a regionally correlatable single surface major sequence boundaries can be traced from marine into proximal alluvial plain environments sequence boundaries have different physical expressions depending upon where in the basin they are observed strata beneath sequence boundaries are unrelated to strata above and time lines cannot be carried across these boundaries flooding surfaces called parasequence or parasequence set boundaries also exert an important control on facies architecture based on parasequence stacking patterns position of sequence boundaries and facies associations the desert member and castlegate sandstone have been divided into sequences with durations of 200 000 to 300 000 yr there are an order of magnitude more sequences in this interval than are predicted by the global sea level chart of haq et al 1988 the major sequences have been divided into the following systems tracts or sequence sets 1 grassy highstand sequence set below the desert sequence boundary named for the grassy member of the blackhawk formation 2 desert lowstand transgressive and highstand systems tracts between the desert and castlegate sequence boundaries 3 castlegate lowstand sequence set resting on the castlegate sequence boundary and 4 castlegate transgressive systems tract parasequence stacking patterns suggest that the grassy highstand sequence set and the desert highstand systems tract were deposited during a relative rise in baselevel the grassy highstand sequence set is punctuated by minor relative falls in baselevel both of these stratal units contain thick

evidence of across shelf transport of fine grained sediments turbidite filled shelf channels in the campanian aberdeen member book cliffs utah usa the shore normal transport of fine grained sediments by shelf turbidity currents has been the focus of intense debate over the last 20 years many have argued that turbidity currents are unlikely to be a major depositional agent on the shelf however sedimentological architectural stratigraphic and palaeogeographic data from the campanian aberdeen member book cliffs eastern utah suggests otherwise and clearly demonstrates that storm generated and river flood generated underflows can transport a significant volume of fine grained sediments across the shelf these across shelf flowing turbidity currents cut large subaqueous channel complexes up to 7 m deep tens of kilometres basinward of their time equivalent shoreface the shelf channels were filled with organic rich siltstones mudstones and very fine to fine grained bouma like sandstone beds including wave modified turbidites hyperpycnites and classical turbidites deposition was above storm wave base palaeocurrent data reveal an overwhelmingly dominant across shelf east south east offshore directed transport trend tectonic activity and or concomitant palaeogeographic reorganization of the basin may favour the generation of these turbidite rich shelf deposits by altering the relative balance of wave versus fluvial energy increased erosion and sediment supply rates because of tectonic uplift of the hinterland may have increased the probability of fluvial dominance along the coastline and hence the possibility of submarine channelization in front of the river mouths additionally the coastline may have become more sheltered from direct wave energy thus allowing the fluvial processes to dominate seasonal increases in rainfall and storm activity may also favour the generation of across shelf underflows on wave dominated shorelines isolated shelf channels and lobes are most likely to be found down dip of fluvial feeder systems in relatively high sediment supply settings these features are also most likely to occur in systems tracts that straddle a sequence boundary especially those which are tectonically generated as these would enhance the potential for altering basin morphology and hence the balance of fluvial and wave energy isolated shelf channels are recognized in older and younger strata in the book cliffs region implying that wave supported gravity flows were a recurrent phenomena in the campanian of utah it is probable that isolated shelf bodies are preserved in other stratigraphic intervals in the cretaceous western interior of north america and other basins worldwide and are currently being overlooked or misidentified shoreface to shelf facies models should be revised to incorporate turbidite rich shelf deposits in some shelf settings

microbial acetogenesis as a source of organic acids in ancient atlantic coastal plain sediments field and laboratory evidence shows that deeply buried 90 888 m fine grained sediments of the atlantic coastal plain contain viable acetogenic microorganisms and that these microorganisms actively produce organic acids concentrations of formate acetate and propionate in pore waters extracted from fine grained sediments ranged from 50 m to 5 m m and were much higher than in adjacent pore waters associated with sandy sediments m laboratory studies showed that asceptically cored fine grained sediments incubated under a h 2 atmosphere produced formate and acetate and that h 14 co 3 was converted to 14 c acetate and 14 c formate over time an enrichment culture of these acetogenic microorganisms was recovered from one long term incubation that showed the presence of several morphologically distinct gram positive rod shaped bacteria these microorganisms were capable of growth under autotrophic h 2 co 2 heterotrophic syringate and mixotrophic h 2 co 2 syringate conditions these results suggest that microbial acetogenesis rather than abiotic processes is the most important organic acid producing mechanism during low temperature 30 c diagenesis of atlantic coastal plain sediments

mudrock hosted carbonate concretions a review of growth mechanisms and their influence on chemical and isotopic composition existing interpretations of cement textures and isotopic compositions may significantly under estimate the depth and duration of concretionary growth minus cement porosities can commonly under estimate depths of concretionary growth for some or all of the following reasons i cements might not passively replace host sediment porosity ii non cement carbonate phases such as replaced bioclastic carbonate can be significant iii sediment compaction models over estimate rates of porosity loss at shallow carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from 30 to 15 and thus overlaps 13 c depleted values normally considered characteristic of sulphate reduction many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis indicating a deeper and more prolonged cementation history the necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters the equilibrium model or be generated in situ by organic matter decay the local equilibrium model or created where external fluids are introduced the fluid mixing model

sequence stratigraphic distribution of diagenetic alterations in coal bearing paralic sandstones evidence from the rio bonito formation early permian southern brazil linking siliciclastic diagenesis to sequence stratigraphy allows a better understanding of the parameters controlling the spatial and temporal distribution of diagenetic alterations and hence of reservoir quality a study of the coal bearing alluvial deltaic estuarine and shallow marine sandstones of the rio bonito formation early permian parana basin southern brazil reveals that the distribution of diagenetic alterations and of related reservoir quality evolution can be constrained within a sequence stratigraphic framework calcite dolomite siderite kaolinite and pyrite cementation is consistently linked to sequence and parasequence boundaries transgressive and maximum flooding surfaces and is systematically distributed within lowstand transgressive and highstand systems tracts diagenesis of coal layers at parasequence boundaries has promoted the formation of stratabound calcite detectable in resistivity wire line logs concretionary pyrite and kaolinite and of silicate grain dissolution in sandstones located above and below these boundaries particularly in the transgressive systems tract meteoric water diagenesis caused grain dissolution and the formation of kaolinite in sandstones below sequence boundaries and in lowstand systems tract sandstones carbonate bioclasts and low sedimentation rates in lag deposits at parasequence boundaries transgressive and maximum flooding surfaces favoured the formation of grain rimming siderite the results of this study are relevant to the exploration of coal bed methane and other coal bearing reservoirs where it is crucial to unravel and predict the distribution and quality of reservoirs and compartments

coastal mixing zone dolomite forward modeling and massive dolomitization of platform margin carbonates abstract dolomite of mixed water origin occurs in three late pleistocene raised reef terraces in southeastern barbados west indies dolomite concentrations range from trace quantities to locally complete with a general increase in dolomite percentage with increasing terrace age most of the dolomite is replacive both mimetic and fabric selective with limpid pore lining dolomite cements common in the oldest terrace stratigraphic relationships suggest that mixing zone dolomitization occurred locally during three temporally separate sea level events the rapidity of quaternary glacio eustasy constrains dolomitization to have occurred within about 5 000 yr in each terrace rapid recurring dolomitization resulted from development of similar hydrologic and hydrochemical conditions during each ea level event barbados data are used as conceptual input for a computer based forward model which simulates the development of thick platform margin sections of mixing zone dolomite through the interaction of sea level fluctuations sedimentation rapid recurring dolomitization and subsidence sensitivity tests of the model indicate that the most important parameters for producing thick sections of platform margin dolomite are 1 dolomitization rate 2 sea level still stand duration and 3 rate of sea level change dolomite thus formed may provide nuclei for later burial dolomitization or may provide an intrabasinal magnesium source for stepwise stabilization of platform interior dolomites

calcite cement distribution and its effect on fluid flow in a deltaic sandstone frontier formation wyoming precipitation of extensive calcite cement during burial diagenesis can strongly modify the depositional permeability of a sandstone reservoir and affect fluid flow during production to predict subsurface flow through cemented reservoirs permeability distributions used in fluid flow models must reflect this diagenetic overprint calcite cements in sandstones commonly occur as irregularly distributed concretions which makes it difficult to predict diagenetic permeability modifications in the subsurface from typically spaced wells outcrops can provide a continuous image of heterogeneity produced by concretionary calcite cements the size and distribution of calcite concretions were mapped in outcrops of the frewens sandstone frontier formation in central wyoming large tabular calcite concretions in this deltaic sandstone generally follow basinward inclined bedding median thickness of the concretions is 0 6 m length is 4 2 m and width is 5 3 m the highest cement fraction is in the high permeability facies at the top of the sandstone body concretion centers are approximately poisson distributed within the sandstone the upward increasing cement fraction is caused by upward increasing concretion size lateral variation in the fraction of the sandstone cemented by calcite has a normal distribution with a mean of 12 s 5 spatial distribution of calcite cement in the frewens sandstone was modeled using indicator geostatistics variograms were inferred from outcrop maps of cement indicator semivariograms of cement have a range of 30 m horizontally and 2 5 m vertically dimensions that correspond approximately to the size of the largest concretions stochastic images of cement were created using indicator simulation with vertically varying cement proportion flow models indicate that concretions make flow paths more tortuous and retard flow in the coarser facies near the top of the sandstone the fastest path through the sandstone is in the lightly cemented high net to gross center of the sandstone body because the cement mainly occurs within the highest permeability facies in the sandstone body a model based on depositional facies alone would overestimate upscaled permeability of the frewens sandstone

modelling of thermal convection in sedimentary basins and its relevance to diagenetic reactions abstract mathematical calculations of thermal convection have been carried out using a porous three player model to simulate pore water flow in a sedimentary basin with layers of different permeabilities the calculated flow lines demonstrate that even very thin layers eg around salt domes and igneous or hydrothermal intrusions analyses of formation waters from sedimentary basins like the north sea often show evidence of a crude stratification of the pore water with respect to salinity and isotopic composition this is evidence suggesting that large scale convection or other types of mixing does not take place and positive salinity depth gradient may help to physically stabilize the formation water the fact that many of the shallow reservoir rocks from the north sea have formation water with low salinity and negative 18 o values suggest that this is modified meteoric water and that migration of petroleum from deeper parts of the basin occurred as a separate phase along restricted migration pathways and was not associated with a high flux of pore water in the absence of thermal convection the total pore water flux through sandstones will be rather small except locally where we may have focused compactional flow through small cross sections diagenetic reactions will therefore normally be relatively isochemical during deeper burial