diagenesis and hydrocarbon accumulation brent sandstone jurassic bergen high area north sea diagenesis of brent group sandstones in the bergen high area was studied and related to the history of hydrocarbon maturation and pore fluid environment calcite and kaolinite are low temperature diagenetic precipitates that formed as meteoric pore water entered the brent at the end of brent deposition and later during late cimmerian erosion a major period of feldspar dissolution associated with quartz and illite cementation occurred at huldra field precipitation of diagenetic illite in brent sandstones was closely linked with hydrocarbon maturation in the adjacent viking graben and accumulation of generated fluids in traps at structural highs at huldra field illite k ar ages suggest onset of illite diagenesis in the latest cretaceous initial reservoiring of hydroca bons during the latest paleocene to early eocene 58 ma and filling of the reservoir by early oligocene 38 ma pore water character during illite diagenesis interpreted from oxygen isotope and quartz overgrowth fluid inclusion data was warmer than predicted by burial history analysis and one and a half to two times seawater salinity the incursion of saline isotopically evolved fluids at huldra suggests replacement of paleometeoric water by basin compaction fluid illite k ar ages from veslefrikk field indicate a minimum age of hydrocarbon accumulation at about 30 ma modern pore water chemistry in the brent at veslefrikk is unlike the suggested water chemistry at huldra and indicates that migrating compaction water did not completely displace older fresh connate water this sugge ts that hydrocarbons migrated into veslefrikk as a discrete phase without great volumes of associated compaction water

ankerite cementation in deeply buried jurassic sandstone reservoirs of the central north sea abstract upper jurassic fulmar formation shelf sandstones of the high temperature high pressure franklin and elgin fields south central graben north sea contain abundant disseminated and concretionary ankerite in contrast most jurassic north sea reservoirs contain only minor amounts of dispersed ankerite disseminated ankerite cement in the franklin and elgin fields has a fairly uniform isotopic composition 18o 10 to 12 5o oo pdb 13c 3 to 5o oo pdb ankerite concretions have 18o values similar to disseminated cements but a wider range of 13c values 1 to 5 5o oo pdb they also have highly variable intergranular volumes which together with the 13c data are interpreted as a combination of pore filling cementation and in situ replacement of comminuted bioclastic debris by ankerite fluid inclusion 18o and paragenetic evidence suggests that ankerite formed during deep burial c 3 5 to 4 5 km 140 170 c after the onset of overpressuring but before hydrocarbon emplacement in the reservoirs the regionally consistent 18o data suggest that ankerite formed via a temperature influenced mechanism and the relatively uniform 13c cement value indicates that organic matter and marine bioclastic carbonate contributed to the dissolved carbon reservoir in constant proportion this can be explained by calcite dissolution in response to ph decrease during thermal breakdown of organic acids such acids were derived from adjacent mudrocks undergoing hydrocarbon maturation and clay mineral transformations and are likely to have been transported in pore fluids with mg2 and fe2 the presence of these cations in solution upon thermal decarboxylation is inferred to have stabilized ankerite at the expense of calcite a relative paucity of ankerite in other fulmar formation reservoirs may reflect different sedimentological compositions less bioclastic debris and or lower burial temperatures less advanced decarboxylation

mesoscale features create hotspots of carbon uptake in the antarctic circumpolar current the influence of eddy structures on the seasonal depletion of dissolved inorganic carbon dic and carbon dioxide co2 disequilibrium was investigated during a trans atlantic crossing of the antarctic circumpolar current acc in austral summer 2012 the georgia basin downstream of the island of south georgia 54 55 s 36 38 w is a highly dynamic region due to the mesoscale activity associated with the flow of the subantarctic front saf and polar front pf satellite sea surface height and chlorophyll a anomalies revealed a cyclonic cold core that dominated the northern georgia basin that was formed from a large meander of the pf warmer waters influenced by the saf formed a smaller anticyclonic structure to the east of the basin both the cold core and warm core eddy structures were hotspots of carbon uptake relative to the rest of the acc section during austral summer this was most amplified in the cold core where greatest co2 undersaturation 78 atm and substantial surface ocean dic deficit 5 1 mol m 2 occurred in the presence of high wind speeds the cold core eddy acted as a strong sink for atmospheric co2 of 25 5 mmol m 2 day 1 waters of the warm core displayed characteristics of the polar frontal zone pfz with warmer upper ocean waters and enhanced co2 undersaturation 59 atm and depletion of dic 4 9mol m 2 a proposed mechanism for the enhanced carbon uptake across both eddy structures is based on the ekman eddy pumping theory i the cold core is seeded with productive high chlorophyll a waters from the antarctic zone and sustained biological productivity through upwelled nutrient supply that counteracts dic inputs from deep waters ii horizontal entrainment of low dic surface waters biological uptake from the pfz downwell within the warm core and cause relative dic depletion in the upper water column the observations suggest that the formation and northward propagation of cold core eddies in the region of the pf could project low dic waters towards the site of antarctic intermediate water formation and enhance co2 drawdown into the deep ocean

kinetic constraints on illitization reactions and the effects of organic diagenesis in sandstone shale sequences abstract based on water rock interaction modelling and kinetic considerations the present study is aimed at testing the impact of organic matter maturation on two kinds of diagenetic reactions leading to the alteration of the petrophysical properties of sandstones 1 clay mineral conversion to illite with k feldspar being a local potential source of k and 2 dissolution precipitation processes produced in sandstones by the action of water soluble organic species derived from adjacent shales assuming firstly that the chemical reactions take place in closed systems the nature and timing of diagenetic reactions in marine sandstone shale formations were modelled for a 50 120 c temperature range in order to improve understanding of the factors that control the illitization reaction with k feldspar coexisting with aluminous clay illite is modelled here as a muscovite type mineral we tested the effects of an energy barrier on illite growth by allowing or preventing the muscovite illite precipitation reaction to occur while using several illitization reaction rates we also compared the stable mineral parageneses predicted for organic material free systems with those predicted in the case of organic diagenesis release of co 2 ch 4 acetic and oxalic acids similarities and discrepancies between numerical results and natural mineral assemblages suggest that the illitization reaction depends on the nature of the reacting clays kaolinite conversion to end member illite involves high energy conditions 2 kcal mol 1 which are not met when the pore water equilibrates with the mineral matrix from undersaturated conditions in a closed system to overcome this barrier the fluid should be oversaturated with respect to k feldspar an external source of k or a ph increase in an open system is necessary for this reaction no particular effect of organic diagenesis on this reaction was found in the present study on the other hand smectite to illite conversion involves a lower energy barrier and can operate in closed systems where k feldspars are the source of k the maturation of organic matter may speed up the smectite to illite conversion rate by increasing the gibbs free energy of illite growth interactions between sandstone and shale were examined by modelling the transfer of aqueous species of organic origin from shale to sandstone fluid expulsion from shale had little or no effect on diagenesis in adjacent sandstones during each flushing cycle mainly because organic protons and ligands were neutralized in the source rock however the diffusion of shale derived cations through sandstone as organometallic complexes mg ca fe al appears to be an efficient process during carbonate cementation in sandstones where oxidizing conditions enhance the decomposition of such complexes

using rare earth elements to monitor sediment sources from a miniature model of a small watershed in the three gorges area of china abstract understanding soil erosion processes at different landscape positions is important in order to predict and control watershed soil losses rare earth elements rees can be used to trace eroded soil sources but their efficacy may be soil dependent we constructed a miniature watershed model of a small watershed located in the three gorges area of china and used oxides of eight rees to trace the erosion of a purple soil the miniature watershed was divided into eight regions containing a different landform type as a potential sediment source a different ree was applied in each region redistributions of the rees under three successive simulated rainfall events with intensities of 1 0 1 5 or 2 0 mm min 1 were examined the percentage contribution from each region to the total soil loss from the watershed fluctuated relative to landform type during the three rainstorms contributions from the lower main gully decreased before stabilizing while those from the upper main gully increased before decreasing and those from other sources all increased before stabilizing overall the contribution of the gully system comprising main and branch gullies was greater than that of the slopes contributions from the gully system tended to decrease with increases in rainfall intensity and rainstorm duration while those from the slopes increased a comparison of the calculated and actual soil loss masses indicated that the accuracy of the ree tracing method was less for the coarse textured purple soil than those previously found for fine textured soils the increased errors likely due to the assumption used in the calculation that there is no particle size selectivity during erosion needs to be addressed this pilot study provided a technical reference for the use of rees in monitoring sediment sources from a natural watershed and a theoretical basis for soil conservation in the three gorges area

simulation of potassium feldspar dissolution and illitization in the statfjord formation north sea prior work has suggested that potassium mobility is an important control on the formation of late stage illite in deep reservoirs of the north sea we developed a one dimensional finite difference reactive transport model to simulate the concurrent dissolution of k feldspar and illitization of kaolinite in statfjord formation reservoirs the kinetics of k feldspar dissolution is set to be faster than diffusion consistent with the observed behavior and the proposition that the reaction was transport limited k feldspar dissolution produced dissolved potassium and silica that diffused and then precipitated to form illite and quartz areas that had higher initial reactant k feldspar produced higher solute concentrations and formed chemical gradients having areas that had lower initial feldspar content in typical north sea cases this means mass transfer would be from sandstone to shale the diffusive transport moved significant amounts of potassium and silica over distances of several meters and created observable diagenetic alteration simulations of an oil filled reservoir showed that oil emplacement did not stop illitization but did limit potassium mobility and maximized illite formation within the sandstone preservation of preillitization permeability was favored in water filled reservoirs that had slower illite formation where diffusion could export potassium from the sandstone before precipitation similar patterns of transport and precipitation were predicted for silica from k feldspar dissolution the results of the model showed good agreement with field data from other north sea reservoirs

long term development of holocene and pleistocene gullies in the protva river basin central russia abstract the specific geomorphic structure a combination of flat or gentle watersheds with short steep valley sides and the high resistance of surface material in the case study area in the centre of the russian plain are responsible for the specific pattern of holocene erosion no or minor sheet erosion the occasional appearance of new gullies on the valley sides four out of 19 studied and a concentration of erosion activity in old gullies that had existed since pre holocene times 15 out of 19 studied the erosion activity is studied using radiocarbon dating of gully sediments summed probability density distribution spdd of 65 radiocarbon dates is applied to detect changes of erosion rates over the last five millennia three millennium scale phases of erosion activity are distinguished phase 1 1200 years bp to the present intensive erosion phase 2 2800 to 1200 years bp weak erosion and phase 3 4800 to 2800 years bp intensive erosion short episodes or single events of relatively strong erosion have been found at around 4700 3800 3000 2200 1800 and 900 years bp erosion during phase 3 coincided with the holocene lowest population density in the whole region and the start of phase 1 coincided with a population gap at the case study area which suggests other than anthropogenic causes for changing erosion regimes these may be climatic factors because changes between millennium scale phases of erosion activity coincide with pronounced climatic changes the subboreal subatlantic transition and the start of the medieval warm period however a direct correlation between erosion activity and climatic parameters warmer cooler wetter drier has not been found presumably because the available palaeoclimatic reconstructions do not contain enough information on changing frequencies and magnitudes of hydrological extremes according to population dynamics charcoal frequency in erosion derived sediments and pollen data a human impact on erosion is suggested to have occurred from the 11th century ad and more confidently from the 14th 16th centuries this contributed to erosion acceleration that began some two centuries earlier apparently for climatic reasons

physical controls on biogeochemical zonation in the southern ocean the primary control on the n s zonation of the southern ocean is the wind induced transport of the antarctic circumpolar current acc the acc divides the southern ocean into three major zones the subantarctic zone saz north of the acc the acc transport zone and the zone south of the acc saccz the zone of acc transport is most often subdivided into two zones the polar frontal zone pfz and the antarctic zone aaz but it may be appropriate to define more subzones or indeed only one at some longitudes to maintain geostrophic balance isopycnals must slope upwards to the south across the acc thus raising nutrient rich deep water closer to the surface as one goes polewards in addition silicate concentrations increase polewards along isopycnals because of diapycnic mixing with silicate rich bottom water surface silicate concentrations therefore decrease northwards from high levels in the saccz to low levels in the saz within the saz and pfz and even in the northern part of the aaz silicate levels may drop to limiting levels for siliceous phytoplankton production during summer nitrate concentrations also decrease northwards but only become limiting in the subtropical zone north of the saz the second circumpolar control is the changing balance of stratification with temperature dominating near surface stratification in the saz and salinity dominating further south because of fresh water input to the surface from melting ice this results in circumpolar features such as the subsurface 2 c temperature minimum and the subduction of the salinity minimum of antarctic intermediate water which are often but not always associated with frontal jets and large transports the transport of the acc is dynamically constrained into narrow bands the number and latitudinal location of which are controlled by the bathymetry and so vary with longitude thus it is not the fronts that are circumpolar but the total acc transport and scalar properties of the salinity and temperature fields evidence of summer silicate and nitrate uptake in all zones saz pfz and aaz shows that there is productivity despite their high nutrient low chlorophyll status blooms covering large areas say 400 km across in the pfz and aaz are found in the vicinity of submarine plateaux which suggest benthic iron fertilizatio

chemostratigraphy of upper jurassic reservoir sandstones danish central graben north sea abstract a chemostratigraphic study of upper jurassic sandstones in the northern danish central graben has been undertaken within the framework of a well defined stratigraphic sedimentological model based particularly on cored well sections two reservoir sandstone units are recognised the transgressive marginal marine to shoreface sandstone of the gert member and the regressive to transgressive shoreface sandstone of the ravn member both members belong to the heno formation which is equivalent to the fulmar formation uk and the ula formation norway multivariate analysis of geochemical data from 264 core samples from 8 wells reveals the distinction between the two reservoir sandstones gert and ravn members and the two offshore claystones farsund and lola formations specific elements have proven to be important for this separation and these elements demonstrate differences even in 2 dimensional cross plots the farsund formation is characterised by higher v u and p 2 o 5 and lower mgo and k 2 o when compared with the lola formation the gert member typically has higher maximum amounts of cr and tio 2 than the ravn member the high cr and tio 2 content probably from chrome spinel and ti minerals might be related to a source of exposed carboniferous sediments in the gert ridge area the ravn member is characterised by higher na 2 o p 2 o 5 and th contents than the gert member which may reflect a higher content of plagioclase na 2 o and a changed heavy mineral assemblage the mid north sea high is a likely source for the heavy mineral suite that characterises the ravn member the rita 1 well situated closest to the mid north sea high seems to have been more influenced by this source than the other wells in the hejre area a volcanic source supplying k feldspar may be responsible for the relatively high k 2 o al 2 o 3 observed in both the gert and ravn members hence in addition to differentiation between the two reservoir sands and between the two offshore claystones this study also illustrates the use of geochemical data for evaluation of source characteristics and dominance of different sediment source areas

evidence for reduced quartz cementation rates in oil filled sandstones when quartz rich sands are buried and heated pore space is gradually filled by precipitation of quartz cement from aqueous formation fluids here we examine whether the presence of oil in the pore space can retard or halt this loss of porosity by slowing or stopping quartz cementation the effect of oil fill on quartz cementation is examined by using the distribution of quartz cement in the brae formation deep water sandstone reservoir of the miller oil field north sea petrographic data demonstrate that sandstones from the oil zone have much less quartz cement and more porosity than sandstones from the water zone sandstones in both oil and water zones are compositionally and texturally identical and have been affected by a similar burial history kinetic modeling of the cementation process suggests that progressive oil charging has slowed quartz cement growth rates by at least two orders of magnitude halting it completely in the most extreme cases our data demonstrate that early oil charging in the crestal part of an anticline can preserve porosity in deeply buried sandstones this knowledge is especially relevant to porosity prediction for petroleum exploration in deeply buried sandstones

recent gully erosion in the el cautivo badlands tabernas se spain abstract detailed geomorphic observations in el cautivo badland area in the semiarid se of spain show a recent renewal of linear channel erosion following a period of aggradation prior to this renewal the landscape was characterized by smooth colluvial pediments on the north east facing aspects and relatively steep crusted microhillslopes on south west facing aspects first order valleys within the badland landscape were smooth floored and filled with silty deposits connected with pediment colluvial deposits the conditions under which valley bottom deposition was replaced by channel incision were analyzed by a study of flow hydraulics magnitude frequency of eroding events vs resistance of valley floor to water flow and the pollen sequence in the recently cut deposits changes in the vegetation carpeting the valley bottom seem to be responsible for the deposition erosion alternation rather than changes in the magnitude frequency of major runoff events given the high sensitivity of flow hydraulics to changes in roughness present rainfall rates would not on their own be able to erode a well vegetated valley bottom yet the pollen sequence in the valley fills demonstrates a progressive deterioration in climatic conditions with the eventual disappearance of groups that require a constant level of soil moisture alnus corylus cyperaceae an increase in yearly rainfall or a change in temporal patterns would increase infiltration leading to permanent denser vegetation on footslopes and valley floors helped by the natural water harvesting system this vegetation cover would reduce the flow velocity and induce the deposition of the colluvium and valley fills whereas erosion on headslopes would have been active enough to supply the sediments that were deposited downslope or on the valley bottoms the reactivation of linear erosion would seem therefore to be a consequence of the decay of this permanent vegetation induced by an increase in the duration or severity of drought periods which meant that the water storage capacity of the valley fillings was insufficient to support it

relationship between diagenesis and reservoir quality in sandstones of the garn formation haltenbanken mid norwegian continental shelf a quantitative model is proposed for the dependence of porosity and permeability on diagenesis in subarkosic arenites of the middle jurassic garn formation porosity variation is controlled by the combined effects of compaction and quartz cementation in general more porosity has been lost by compaction than by quartz cementation but compactional porosity loss appears to have occurred relatively early in the burial history before burial below 2 km below the sea floor thus progressive reduction of the total porosity with increasing burial below 2 km results mainly from quartz cementation permeability is a function of the abundance of intergranular macroporosity which is interpreted to be a measure of effective porosity at depths less than 3 5 km below the sea fl or intergranular macroporosity generally comprises more than 50 percent of the total porosity but at greater depth as total porosity decreases below about 16 percent the ratio of effective to ineffective porosity drops below one this depth corresponds to the onset of extensive illitization and grain dissolution which results in major reorganization of the pore system generation of secondary porosity by feldspar dissolution appears not to have been beneficial to reservoir quality in the garn formation but it is regarded as an intrinsic part of the overall process of porosity destruction with increasing diagenesis

climatic controls on holocene lake level changes in europe sensitivity experiments with a simple water balance model were used to constrain the possible climatic causes of distinct holocene patterns of lake level variation in different regions of europe lakes in s sweden were low at 9 ka high around 6 5 ka low again around 4 ka and are high now lakes in estonia show similar but weaker trends lakes in s france were highest around 9 ka lowest around 4 ka intermediate now lakes in greece were also high around 9 ka but continued rising until 7 5 ka then fell gradually from 5 ka with a brief high phase around 3 ka and are low now the model was forced with insolation anomalies deduced from orbital variations temperature anomalies inferred from the pollen record and cloudiness anomalies derived from changes in the position of the subtropical anticyclone inferred from reconstructed changes in the equator to pole temperature gradient in order to evaluate the effects of resultant evaporation changes on catchment water balance the resulting simulated changes in runoff precipitation minus actual evapotranspiration were slight and frequently opposite to the observed trends larger changes in precipitation are plausible and are required to explain the data the required precipitation increase in n europe from 9 ka low to 6 ka high is suggested by gcm experiments to have been a consequence of interacting insolation and residual ice sheet effects on the atmospheric circulation over the north atlantic the explanation of other observed changes including the drying trend during the holocene in s europe has not been provided by gcm experiments to date explanations may lie in changes in mesoscale circulation sea surface temperature patterns and the coupling between these phenomena that may not follow orbital changes in any simple way

physical and biogeochemical controls on the variability in surface ph and calcium carbonate saturation states in the atlantic sectors of the arctic and southern oceans polar oceans are particularly vulnerable to ocean acidification due to their low temperatures and reduced buffering capacity and are expected to experience extensive low ph conditions and reduced carbonate mineral saturations states in the near future however the impact of anthropogenic co2 on ph and will vary regionally between and across the arctic and southern oceans here we investigate the carbonate chemistry in the atlantic sector of two polar oceans the nordic seas and barents sea in the arctic ocean and the scotia and weddell seas in the southern ocean to determine the physical and biogeochemical processes that control surface ph and high resolution observations showed large gradients in surface ph 0 10 to 0 30 and aragonite saturation state ar 0 2 to 1 0 over small spatial scales and these were particularly strong in sea ice covered areas up to 0 45 in ph and 2 0 in ar in the arctic sea ice melt facilitated bloom initiation in light limited and iron replete dfe 0 2 nm regions such as the fram strait resulting in high ph 8 45 and ar 3 0 along the sea ice edge in contrast accumulation of dissolved inorganic carbon derived from organic carbon mineralisation under the ice resulted in low ph 8 05 and ar 1 1 in areas where thick ice persisted in the southern ocean sea ice retreat resulted in bloom formation only where terrestrial inputs supplied sufficient iron dfe 0 2 nm such as in the vicinity of the south sandwich islands where enhanced ph 8 3 and ar 2 3 were primarily due to biological production in contrast in the adjacent weddell sea weak biological uptake of co2 due to low iron concentrations dfe 0 2 nm resulted in low ph 8 1 and ar 1 6 the large spatial variability in both polar oceans highlights the need for spatially resolved surface data of carbonate chemistry variables but also nutrients including iron in order to accurately elucidate the large gradients experienced by marine organisms and to understand their response to increased co2 in the future

chlorite meniscus cement implications for diagenetic mineral growth after oil emplacement the possible influence on diagenesis of oil migration into reservoir sandstones is important for understanding their development although many studies have addressed the question of continued growth of diagenetic minerals after oil emplacement there is little clear evidence of the timing of mineral growth relative to oil emplacement nor of the possible mechanisms of continued mineral growth this study presents an example of continued cementation by pore lining chlorite during and after the migration of oil into a palaeocene turbidite sandstone reservoir near the faroe islands chlorite developed in two different textures one texture occurs on framework grain surfaces adjacent to open pore space in contact with oil the other texture occurs close to framework grain contacts where contact with oil appears to have been prevented by residual pore water menisci the meniscus crystals are larger than those from open pore implying that diagenetic clay mineral growth has continued after oil emplacement

an oxygen isotope study of illite and calcite in three appalachian paleozoic vertic paleosols abstract the oxygen isotope compositions of pedogenic calcite and illite in three paleozoic vertic red bed paleosols record a complex history of pedogenic and diagenetic processes the paleosols are preserved in the upper mississippian chesterian mauch chunk pennsylvania hinton west virginia and pennington tennessee formations in the appalachian basin and were buried to 7 8 km 230 260 c 3 4 km 100 145 c and 1 5 3 km 70 100 c respectively the submicron size clay fractions of the paleosols are presently composed of r3 ordered illite smectite i s inferred from k ar dates and soil morphology to have been authigenically produced by the reaction of smectite to illite oxygen isotope fractionations calculated pore fluid compositions and k ar dates were used to constrain the conditions and timing of i s and pedogenic calcite re crystallization in each of the paleosols the data were interpreted in light of three possible end member environments for formation and recrystallization 1 the pedogenic environment 2 the burial diagenetic environment and 3 a burial environment dominated by hot tectonically driven fluid the 18o values of pedogenic calcite and authigenic i s in vertic red bed paleosols generally reflect recrystallization and exchange during burial diagenesis and are thus not suitable for interpretation of paleoenvironment or paleoclimate the involvement of orogenic fluids during burial cannot be ruled out but it is not required by any of the paleosols in the coastal margin pennington formation paleosol i s formation may have been initiated in the pedogenic environment in response to cyclical wetting and drying of the soil clay matrix continued smectite transformation and coarsening and ordering of i s during a long period of shallow burial led to its isotopic reequilibration with pore fluids at slightly elevated temperature but oxygen isotope compositions of pedogenic calcite in the paleosol are preserved

origin of quartz cements in some sandstones from the jurassic of the inner moray firth uk the extent of quartz cementation in shallow marine sandstones of the brora arenaceous formation oxfordian is closely related to the occurrence and abundance of rhaxella perforata sponge spicules three cement morphologies are identified chalcedonic quartz microquartz and mesoquartz chalcedonic quartz forms matrix supported cements which preserve moulds of rhaxella spicules chalcedonic quartz crystals have inequant development of crystal faces on average 0 1 m in diameter and are the first formed cement and reveal homogeneous dark grey tones on the sem cl bei microquartz forms 5 10 m diameter crystals which commonly grow on chalcedonic quartz substrates and show various grey tones under sem cl bei mesoquartz crystals grow in optical continuity with their host grains have 20 m a axial diameter crystals and exhibit distinctly zoned luminescence although no opaline silica is preserved the quartz cement is interpreted to have formed from an opaline precursor detrital quartz has an average 18o composition of 12 2 and mesoquartz syntaxial overgrowth has an average 18o composition of 20 0 estimates of the 18o compositions of microquartz and chalcedonic quartz are complicated by the problem of isolating the two textural types mixtures of the two give consistently higher 18o compositions than mesoquartz the higher estimate being 39 2 from oxygen isotope data the formation of quartz microquartz and chalcedonic quartz is interpreted to have taken place between 35 and 71 c in marine derived pore waters organic and inorganic maturation data constrain the upper temperature limit to less than 60 c

diagenetic controls on reservoir quality in middle to upper jurassic sandstones in the south viking graben north sea the deeply buried synrift play of the south viking graben is characterized by highly variable reservoir quality an integrated approach incorporating petrophysics petrography and one dimensional basin modeling methods was applied to investigate these variations analysis shows that average porosities below 4000 m 13 123 ft vertical depth below sea floor range from approximately 5 to as much as about 25 in comparable quartz arenitic sandstones from porosity depth trends three porosity categories can be recognized normal low and high porosity sandstones normal porosity sandstones fall along the regional average porosity depth trend low porosity sandstones have been subject to extensive quartz cementation as a consequence of a higher degree of thermal maturity and plot below the regional porosity depth trend high porosity sandstones plot above the regional porosity depth trend here quartz cementation has been inhibited by grain coating microquartz and thus porosity has been preserved hydrocarbon emplacement has previously been thought to have inhibited quartz cementation in the study area but this study concludes that the reservoirs are mainly water wet allowing for continued quartz cementation despite the presence of hydrocarbon pore fluids predicting the distribution of microquartz coated sandstones and the degree of thermal maturity is therefore fundamental for successful exploration in the deeply buried parts of the synrift play this study presents a regional and stratigraphic framework for such predictions that may be incorporated into play models in the area

the role of fluid pressure and diagenetic cements for porosity preservation in triassic fluvial reservoirs of the central graben north sea anomalously high porosities and permeabilities are commonly found in the fluvial channel sandstone facies of the triassic skagerrak formation in the central north sea at burial depths greater than 3200 m 10 499 ft from which hydrocarbons are currently being produced the aim of our study was to improve understanding of sandstone diagenesis in the skagerrak formation to help predict whether the facies with high porosity may be found at even greater depths the skagerrak sandstones comprise fine to medium grained arkosic to lithic arkosic arenites we have used scanning electron microscopy petrographic analysis pressure history modeling and core analysis to assess the timing of growth and origin of mineral cements with generation and the impact of high fluid pressure on reservoir quality our interpretation is that the anomalously high porosities in the skagerrak sandstones were maintained by a history of overpressure generation and maintenance from the late triassic onward in combination with early microquartz cementation and subsequent precipitation of robust chlorite grain coats increasing salinity of pore fluids during burial diagenesis led to pore filling halite cements in sustained phreatic conditions the halite pore filling cements removed most of the remaining porosity and limited the precipitation of other diagenetic phases fluid flow associated with the migration of hydrocarbons during the neogene is inferred to have dissolved the halite locally dissolution of halite cements in the channel sands has given rise to megapores and porosities of as much as 35 at current production depths

silica diagenesis i solubility controls abstract the diagenetic sequence of silica is examined in terms of several thermodynamic and kinetic concepts the relationship between solubility and surface area or particle size is sufficient to explain simple opal a opal ct quartz transformations suggesting that the role of temperature and other physical parameters may not be as critical as previously thought complexation and adsorption in an impure system when added to the model allow for possibilities of opal a transformation directly to quartz of clay and zeolite formation from an opaline silica source and of variations in silica transformation rates due to presence of impurities in the system

thirty years of studies on badlands from physical to vegetational approaches a succinct review the scientific subjects addressed in the research into badlands since the publication of bryan and yair s book bryan r b and yair a editors 1982 badland geomorphology and piping geo books norwich 408 pp are briefly analysed with a steadily increasing number of papers per year attention to some aspects such as runoff generation piping and the role of lithology and soils varied whereas the percentage of studies on erosion rates increased steadily over time vegetation was not a subject of research in early papers as most of the papers dealing with vegetation were rather descriptive and published in local or limited diffusion media which contrasts with the present situation when about 40 of papers published in international journals about badlands address some aspect of vegetation however the growth in the attention paid to vegetation and erosion rates was not linked to any increase in papers dealing with badland reclamation badlands are now seen as a group of systems with some common features but diverse dynamics depending mainly on climatic and lithological drivers more attention should be paid in the future to investigating the evolution of these landforms over time mostly through modelling studies whereas emerging technologies foster detailed analyses that were not possible formerly although some badland areas determine on and off site environmental issues others are spots of landscape diversity that merit protection rather than reclamation

the age of illite cement growth village fields area southern north sea evidence from k ar ages and 18o 16o ratios in this paper we describe k ar radiometric ages and oxygen isotope ratio measurements of illite cements from eolian sandstone samples of the permian lower leman sandstone formation rotliegendes group the samples come from a large about 100 100 km part of the southern north sea basin village fields area and from a range of burial depths 2 8 3 4 km subsea bed mean illite ages in the gas fields of ravenspurn north 164 ma 20 1 2 sgr n 11 hyde 161 ma 12 3 n 7 cleeton 155ma 20 2 n 3 and hoton 152 ma 14 0 n 14 fields and in two wells located in block 47 5a 162 ma 16 1 n 4 are all in the middle to late jurassic apart from two early cretaceous ages considered to represent unreliable analyses the m an for all ages is 158 ma 18 6 2 sgr n 39 we largely avoided contamination with old detrital k bearing minerals by careful sample selection and monitoring to a first approximation the assumptions of k ar dating appear to be fulfilled so the measured ages are likely to date illite cement precipitation oxygen isotope ratios measured on the dated illite samples show small but significant variations among fields and a weak correlation with depth during the middle jurassic when illite grew both k ar dating and oxygen isotope ratios are compatible with a regional middle to late jurassic phase of illite growth during which the mineral precipitated over a large area and over a range of temperatures governed by the depth of the formation at that time the age of illite cementa ion coincides with rifting which ultimately may have caused illite growth by driving pore water flow

location and dynamics of the antarctic polar front from satellite sea surface temperature data the location of the antarctic polar front pf was mapped over a 7 year period 1987 1993 within images of satellite derived sea surface temperature the mean path of the pf is strongly steered by the topographic features of the southern ocean the topography places vorticity constraints on the dynamics of the pf that strongly affect spatial and temporal variability over the deep ocean basins the surface expression of the pf is weakened and the pf meanders over a wide latitudinal range near large topographic features width and temperature change across the front increase and large scale meandering is inhibited elevated mesoscale variability is seen within and downstream of these areas and may be the result of baroclinic instabilities initiated where the pf encounters large topographic features the strong correlations between topography and pf dynamics can be understood in the context of the planetary potential vorticity ppv or f h field mean ppv at the pf varies by more than a factor of 2 along its circumpolar path however at the mesoscale the pf remains within a relatively narrow range of ppv values around the local mean away from large topographic features the pf returns to a preferred ppv value of 25 10 9 m 1 s 1 despite large latitudinal shifts the mean paths of the surface and subsurface expressions of the pf are closely coupled over much of the southern ocean

theory and observations of ekman flux in the chlorophyll distribution downstream of south georgia 1 a large phytoplankton bloom occurs downstream of south georgia an island on the northern edge of the scotia sea atlantic sector of the southern ocean this is due to natural iron fertilisation being advected downstream in the antarctic circumpolar current mapping of seawifs chl a in dynamic height longitude space reveals the trajectory of surface water under the dual influences of geostrophic flow and ekman flux a theoretical estimate of the trajectory shows good agreement with observations the trajectory is dependent on the wind stress mixed layer depth ratio but independent of the latitudinal separation of dynamic height contours the chl a distribution suggests that iron reaches 20 w below the mixed layer away from the effects of ekman flux but beyond the iron is confined to the surface layer chl a reduces monotonically east of south georgia with the signal detectable to 15 e over 3500 km downstream indicating the basin scale biogeochemical influence of the iron supply

surface exchange between the weddell and scotia seas within drake passage the southern flank of the antarctic circumpolar current acc hosts the ventilation of deep water the injection of antarctic shelf waters and interactions between westward and eastward boundary currents this exchange is explored through the trajectories of forty surface drifters released in january 2012 in the northwestern weddell sea the drifters detail lagrangian transport pathways between the eastern antarctic peninsula and sites of elevated chlorophyll in the scotia sea acc frontal currents in particular the southern acc front act as dynamical transport barriers to the drifters and influence surface chlorophyll distributions indicating that acc fronts partition weddell source waters in the scotia sea interannual fluctuations in surface chlorophyll in the south scotia sea and the northern weddell sea covary this suggests that scotia sea ecosystem dynamics are linked to water properties injected from the tip of the antarctic peninsula and respond to weddell gyre circulation changes

discontinuous ephemeral streams abstract many ephemeral streams in western north america flowed over smooth valley floors before transformation from shallow discontinuous channels into deep arroyos these inherently unstable streams of semiarid regions are sensitive to short term climatic changes and to human impacts because hillslopes supply abundant sediment to infrequent large streamflow events discontinuous ephemeral streams appear to be constantly changing as they alternate between two primary modes of operation either aggradation or degradation may become dominant attainment of equilibrium conditions is brief disequilibrium is promoted by channel entrenchment that causes the fall of local base level and by deposition of channel fans that causes the rise of local base level these opposing base level processes in adjacent reaches are maintained by self enhancing feedback mechanisms the threshold between erosion and deposition is crossed when aggradational or degradational reaches shift upstream or downstream extension of entrenched reaches into channel fans tends to create continuous arroyos upvalley migration of fan apexes tends to create depositional valley floors with few stream channels less than 100 years is required for arroyo cutting but more than 500 years is required for complete aggradation of entrenched stream channels and valley floors discontinuous ephemeral streams have a repetitive sequence of streamflow characteristics that is as distinctive as sequences of meander bends or braided gravel bars in perennial rivers the sequence changes from degradation to aggradation headcuts concentrate sheetflow a single trunk channel conveys flow to the apex of a channel fan braided distributary channels end in an area of diverging sheetflow and converging sheetflow drains to headcuts the sequence is repeated at intervals ranging from 15 m for small streams to more than 10 km for large streams lithologic controls on the response of discontinuous ephemeral streams include 1 amount and size of sediment yielded from hillslopes 2 infiltration capacity of valley floor alluvium that influences both the unit stream power available for upstream headcut migration and the attenuation of flashy streamflow events by riparian vegetation in sheetflow reaches and 3 cohesiveness of alluvium which affects headcut and streambank morphology and rates of arroyo extension downcutting and widening initiation of arroyo cutting may be too complex to be attributed to a single cause such as change in mean annual precipitation or grazing by livestock but is most likely associated with a decrease in density of protective plant cover on hillsides and along valley floors relatively larger unit stream power makes downstream reaches more susceptible to initial entrenchment during floods than headwaters reaches and favors persistent arroyos entrenchment continues until an equilibrium longitudinal profile is briefly attained then channel widening occurs streambanks are undercut and aggradation begins

factors controlling permeability variation in sandstones of the garn formation in trestakk field norwegian continental shelf abstract a comparison of sandstone cores from trestakk field was carried out to better understand the difference in permeability between a well near the oil filled crest average 26 md and a second well located mainly within the water zone average 4 md plug samples distributed throughout similar 80 90 m cored intervals in each well were analyzed by x ray diffraction bulk chemical analysis point counting of thin sections and sem image analysis of mineralogy and porosity no differences were found between the two wells in either primary sand quality or the degree of diagenetic alteration including illitization however permeability is strongly correlated with both pore size distribution and abundance of intergranular macroporosity which in turn appear to be controlled by the volume occ pied by illitic clay despite the similar degree of illitization in the two wells the illite may be more widely distributed in the intergranular pore system in the lower permeability well thus reducing intergranular macroporosity and permeability this difference is suggested to result from growth of illite in completely water filled porosity in the flank position as opposed to illite growth only in residual water saturated porosity on the oil filled structural crest

fronts and habitat zones in the scotia sea abstract the fronts in the antarctic circumpolar current acc comprise the southern acc front saccf the polar front pf and sub antarctic front saf which together with the southern boundary sb separate zones with distinct water mass properties in the southern ocean they are important for describing habitat ranges of oceanic species over the range of trophic levels and are also key habitat zones in their own right for a variety of species frontal positions were studied along a transect running s to n across the scotia sea during three hydrographic cruises in spring 2006 summer 2008 and autumn 2009 subsurface data from these cruises and argo data are here combined with aviso satellite absolute dynamic topography data to locate these fronts facilitating study of their ecological significance within the scotia sea northern and southern dynamic height limits are found for each front as opposed to the more usual one dimensional line thus the sb was found between 132 to 116 dyn cm the saccf between 115 to 99 dyn cm and the pf between 71 to 45 dyn cm argo data allow the sensitivity of the dynamic heights found to be tested against seasonal interannual and longitudinal effects and each is found to be small although the saf lay beyond the northern extent of the transect we sampled along argo profiles allowed us to determine its limits as lying between 5 and 29 dyn cm the dynamic height limits found are thus suitable for assessing the location in relation to fronts of any sample or tracked animal within this area the chlorophyll distribution across the scotia sea is assessed relative to frontal positions and known iron sources the physical conditions sea surface temperature sea ice and mixed layer depth along the sampling transect are also presented allowing the oceanographic conditions of the stations occupied across the scotia sea to be assessed

asphaltene rich residues in sandstone reservoirs of haltenbanken province mid norwegian continental shelf abstract thin zones of asphaltene rich residue are present near the crests of two adjacent oil and gas filled structures in one structure the smorbukk sor field multiple thin 5 50 cm dark coloured zones are observed within the lower 20 m of the 88 m thick garn formation these dark coloured zones tend to occur immediately above stylolitized mica clay laminations and become gradually lighter upwards apparently reflecting gradual upward decrease in residue abundance in the other structure the smorbukk field the lowermost 1 m of the formation is dark coloured and the sandstone gradually becomes lighter in colour upwards for a distance of about 3 m similar but more diffuse residues are also present throughout the oil zone in two other fields nearby organic geochemical analyses show that the residue rich dark coloured zones differ from the sorrounding lighter coloured sandstone mainly in having a higher content of asphaltenes whereas the total content of extractable organic matter is often similar thus the darker rocks might be regarded more as zones of conspicuous oil staining due to asphaltene deposition rather than as small tar mats the residues typically appear to have accumulated under the influence of gravity at points where the downward flow was hindered by a lithological discontinuity present day zonation upward decreasing gas to oil ratio in the oil column of the smorbukk sor reservoir is consistent with a model of asphaltene precipitation in the lower part of the reservoir in response to upward leakage of gas from an underlying gas condensate reservoir in the ile formation this is also consistent with the available organic geochemical data in thin section the asphaltene rich material is seen to preferentially coat feldspar and clay but not quartz surfaces thus illustrating why sandstone wettability should be expected to depend on both mineralogy and the asphaltene content of the petroleum involved

authigenic albite formation due to water rock interactions case study magnus oilfield uk northern north sea abstract it is the aim of this contribution to test whether organic inorganic interactions could induce the formation of authigenic albite this concept and related results are being compared with modelling scenarios which are purely based on inorganic geochemical reactions in order to unravel the pathway of authigenic albite formation this paper presents results of a multidisciplinary study from imaging geochemistry mineralogy and hydrogeochemical modelling the jurassic reservoir sandstones of the magnus oilfield uk north sea were chosen as a test site albite occurs with 4 18 wt in the magnus sandstones and its contents vary with depth however albite contents increase with increasing k feldspar contents and decreasing grain size it occurs in three forms 1 as lamellae in perthite 2 as overgrowth on in corroded feldspar and 3 as cloudy replacing albite patches in k feldspar the albite overgrowth has the highest chemical purity 100 albite whilst albite lamellae and replacing albite patches are slightly less pure containing 1 4 anorthite albite appears non altered and has a euhedral morphology and dull cathodoluminescence it commonly co occurs with corroded k feldspar grains the precipitation of diagenetic albite in the magnus sandstones is attributed to deep burial 80 ma ago and may have continued until today at temperatures between 90 120 c the results of hydrogeochemical modelling offer two possible pathways for the authigenic albite formation 1 dissolution of unstable minerals such as kaolinite and chalcedony coupled to reduction of ferric iron minerals by products generated during oil generation migration and degradation 2 dissolution of non end member feldspar such as k feldspar with 10 albite coupled to illite formation can account for trace amounts of albite due to an elevated na k activity ratio in the pore water

rapid assessment of gully sidewall erosion rates in data poor catchments a case study in australia knowledge of erosion rates and sediment yields from gullies is critical for the effective prioritisation of management efforts aimed at reducing the impact of gully derived sediments on water quality this paper describes a rapid assessment method for estimating sidewall gully erosion rates for already established gully networks it combines a conceptual empirical model of the cycle of gully initiation and stabilisation with an easy to apply technique for assessing gully extent and dimensions the method is illustrated using a case study of the avon richardson catchment of north central victoria australia it is estimated that a total of 4 6 106tonnes of sediment have been generated in that catchment from gully erosion since the latest major phase of gullying in the mid 1800s this is equivalent to 8 9tha 1y 1 if sediment generation was evenly distributed spatially and temporally across the 3300km2 catchment over 160years prior to 2010 current rates of gully erosion are estimated to be 4700ty 1 equivalent to 1 4tha 1y 1 the results are comparable with observed rates of sidewall erosion in the avon richardson catchment further testing is required to build confidence in its applicability to assess sediment yields and for assisting regional authorities to plan remediation efforts

the effect of grain coating microquartz on preservation of reservoir porosity clay coatings have been widely accepted by many workers as an explanation for preserving high porosity in deeply buried sandstones but few workers have realized that similar effects can be produced by microcrystalline quartz coatings this phenomenon can be expected only under special circumstances but in such cases it can have profound consequences for exploration in the central graben area of the southern north sea unusually high porosity 20 27 and permeability 100 1000 md are found in certain zones in upper jurassic sandstones at depths of 3 4 4 4 km the porosity in these zones is 5 15 higher than expected based on average porosity depth trends from brent and haltenbanken sandstones we propose that the high porosity is due to continuous grain coatings of euhedral microcrystalline quartz crystals that are 0 1 2 m thick the distribution of microcrystalline quartz coatings is controlled by the presence of siliceous sponge spicules rhaxella which implies a sedimentological control on the reservoir quality we present a thermodynamic model showing how continuous microcrystalline quartz coatings inhibit development of no mal macrocrystalline quartz overgrowths sourced mainly from stylolites high porosities in parts of various upper jurassic oil fields ula and gyda have previously been explained by inhibition of quartz cementation by early hydrocarbon charge we suggest that the microcrystalline quartz coatings provide a more plausible explanation

widespread intense turbulent mixing in the southern ocean observations of internal wave velocity fluctuations show that enhanced turbulent mixing over rough topography in the southern ocean is remarkably intense and widespread mixing rates exceeding background values by a factor of 10 to 1000 are common above complex bathymetry over a distance of 2000 to 3000 kilometers at depths greater than 500 to 1000 meters this suggests that turbulent mixing in the southern ocean may contribute crucially to driving the upward transport of water closing the ocean s meridional overturning circulation and thus needs to be represented in numerical simulations of the global ocean circulation and the spreading of biogeochemical tracers

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

a fluid inclusion study of quartz cemented sandstones from offshore mid norway possible evidence for continued quartz cementation during oil emplacement abstract jurassic sandstones from the oil zone in a well from the haltenbanken area offshore mid norway contain both aqueous and hydrocarbon fluid inclusions within quartz overgrowths homogenization temperatures for the inclusions range from 92 c up to the present formation temperature of approximately 118 c since the inclusions mostly are located at the boundaries between quartz clasts and quartz overgrowths the homogenization temperatures indicate that quartz cementation started at temperatures slightly below 92 c poikilotopic ferroan calcite cement usually predates formation of quartz overgrowths and most calcite cement must be precipitated at temperatures below 90 c although in one calcite cemented zone ferroan calcite encloses quartz overgrowths with inclusions trapped at temperatures of up to 105 c none of the examined hydrocarbon inclusions homogenized below 102 c and this suggests that oil started entering the reservoir at temperatures of around 100 c the burial history of the examined sandstones indicates that temperatures of 100 c were not reached before the late pliocene and that filling of the reservoir has taken place during the last few million years homogenization temperatures approximately equal to present formation temperature suggest that quartz cementation may still be in progress and that emplacement of oil did not necessarily stop precipitation of quartz cement possible continued quartz cementation after oil filled the reservoir could be explained by dissolution of quartz at stylolites and grain contacts as transport of dissolved quartz from these sources might possibly take place by diffusion through irreducible water

substantial primary production in the land remote region of the central and northern scotia sea the scotia sea area has high productivity relative to the southern ocean as a whole but this displays strong latitudinal and longitudinal gradients elucidating the extent of these from a single cruise is problematic given the high variability of bloom timing and location in this region therefore this study used data from transects across the central scotia sea in spring summer and autumn of 2006 2008 and 2009 combined with satellite data to obtain a larger scale appreciation of the latitudinal contrasts in phytoplankton standing stock and primary production across the region concentrations of nitrate phosphate and particularly silicic acid increased towards the south of the transect with the latter showing a step change at the southern antarctic circumpolar current front saccf changes in seasonal nutrient concentrations indicated increasing phytoplankton uptake north of 57 s that peaked at 53 s in the georgia basin based on seasonal depletions of nitrate relative to phosphate the highest relative nitrate uptake occurred northwest of south georgia on the periphery of the georgia basin indicating efficient nitrate use here due to iron replete conditions an integrative approach to examine these gradients was with the use of 10 year satellite climatology data these showed that the lowest mean chlorophyll a chl a values were in the central northern scotia sea but these were still substantial values 67 of values within the georgia basin bloom cruise data on chl a and on microplankton biomass from cell counts support this finding of substantial biomass in the central scotia sea since these averaged half of values in the iron fertilised bloom of the georgia basin downstream of south georgia given that our transect was nearly 1000 km long and in parts was land remote with low iron concentrations the relatively high production in the central and northern scotia sea is surprising iron levels may be maintained here by efficient recycling and irregular injections possibly for example from dust deposition or shelf derived inputs from the south the moderate chl a concentration across the mid scotia sea and southwest of south georgia reflect periodic non ice associated blooms that occur in some years and not others these may provide a connection between the large populations of krill inhabiting the northern and southern fringes of the scotia sea

anomalously high porosity and permeability in deeply buried sandstone reservoirs origin and predictability porosity and permeability generally decrease with increasing depth thermal exposure and effective pressure however a significant number of deep 4 km approximately 13 000 ft sandstone reservoirs worldwide are characterized by anomalously high porosity and permeability anomalous porosity and permeability can be defined as being statistically higher than the porosity and permeability values occurring in typical sandstone reservoirs of a given lithology composition and texture age and burial temperature history in sandstones containing anomalously high porosities such porosities exceed the maximum porosity of the typical sandstone subpopulation major causes of anomalous porosity and permeability were identified decades ago however quantification of the effect of processes responsible for anomalous porosity and permeability and the assessment of the predictability of anomalous porosity and permeability occurrence in subsurface sandstones have rarely been addressed in published literature the focus of this article is on quantification and predictability of three major causes of anomalously high porosity 1 grain coats and grain rims 2 early emplacement of hydrocarbons and 3 shallow development of fluid overpressure grain coats and grain rims retard quartz cementation and concomitant porosity and permeability reduction by inhibiting precipitation of quartz overgrowths on detrital quartz grains currently prediction of anomalous porosity associated with grain coats and grain rims is dependent on the availability of empirical data sets in the absence of adequate empirical data sedimentologic and diagenetic models can be helpful in assessing risk due to reservoir quality such models provide a means to evaluate the effect of geologic constraints on coating occurrence and coating completeness required to preserve economically viable porosity and permeability begin page 302 in a given play or prospect these constraints include thermal history and sandstone grain size and composition the overall effect of hydrocarbon emplacement on reservoir quality is controversial it appears that at least some cements quartz and illite may continue to precipitate following emplacement of hydrocarbons into the reservoir our work indicates that integration of basin modeling with reservoir quality modeling can be used to quantify prior to drilling the potential impact of hydrocarbon emplacement on porosity and permeability the best case scenario for significant reservoir quality preservation due to fluid overpressure development is in rapidly deposited tertiary or quaternary sandstones our models suggest that significant porosity can be preserved in sandstones that have experienced continuous high fluid overpressures from shallow burial depths the models also indicate that the potential for porosity preservation is greatest in ductile grain rich sandstones because compaction tends to be the dominant control on reservoir quality in such rocks the case for significant porosity preservation associated with fluid overpressures in pre tertiary basins however is more problematic because of the complexities in the history of fluid overpressure and the greater significance of quartz cementation as a potential mechanism of porosity loss

predictive permeability variations in a hermod sand reservoir stine segments siri field danish north sea abstract the hermod sand of the stine segments siri field danish north sea displays large permeability variations 1 600 md these permeability variations represent horizontal layering in the reservoir largely formed by diagenetic processes they have a significant impact on reservoir performance because horizontal producers in fairly thin reservoirs 30 m are protected against bottom water influx and can produce water free for long periods of time this paper presents the results of a detailed multidisciplinary study where the observed variation in permeability can be explained by two main diagenetic phases silica as opal cement and microquartz and berthierine a serpentine mineral that is closely related to chlorite opal microquartz and berthierine cements dominate in separate parts of the reservoir bodies opal microquartz cemented sands have high permeability typically 500 600 md berthierine cement has a different growth pattern in the oil and water zones resulting in intermediate permeability in the oil zone typically 50 100 md and very low permeability in the water zone typically 1 10 md

how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones observations and experiments observations and hydrothermal experiments were used to derive new information about how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones samples of deeply buried porous sandstones with different types of clay coats were split in two coats removed from one of each pair of splits and grain surfaces inspected with scanning electron microscopy quartz grains in a fluvial deltaic sandstone buried to 115c had no visible authigenic quartz on grain surfaces cleaned of diagenetic chlorite coats though well developed overgrowths occurred on nearby naturally uncoated grains however in similar sandstones buried to 164c quartz grain surfaces exposed by chlorite coat removal were covered with small 5 m mainly anhedral syntaxial quartz overgrowths similar overgrowths were observed under various detrital and diagenetic clay coats in porous eolian sandstones buried to temperatures up to 215c we conclude that clay coats may retard quartz nucleation at moderate temperatures but at high temperatures many coats permit quartz nucleation and preserve porosity by limiting cement growth to investigate cement growth limitation mechanisms samples with coats removed were subjected to quartz cementing conditions in a hydrothermal reactor during experiments the naturally occurring small overgrowths on clay cleaned grains coalesced and grew suggesting that clay particles in coats inhibit cement growth by forming barriers to early overgrowth coalescence although the fraction of grain surface coverage is the primary control on cement inhibition by coats cement growth interference textures vary with coat type providing a mechanism by which coat composition may be a secondary control on inhibitory effectiveness in deeply buried sandstones quartz cement can fill significant microporosity within diagenetic chlorite coats potentially affecting mechanical and petrophysical rock properties

triassic jurassic weathering and clay mineral dispersal in basement areas and sedimentary basins of southern sweden abstract the basement rocks of southern sweden were deeply weathered in late triassic through cretaceous times and possibly even earlier sub mesozoic saprolites formed presumably due to long term warmth and humidity in conjunction with decomposition of abundant organic matter and subsequent infiltration of acidic water the upper parts of the weathering profiles were subjected to intense leaching and kaolinitisation whereas the less evolved deep weathering front is enriched in smectite thus any erosion prior to full regolith development or extraordinary deep erosion released less developed smectite rich weathering material to the late triassic jurassic receiving basins mesozoic sedimentary successions in southern sweden show high detrital smectite and or kaolinite contents moderate illite contents and low chlorite contents compatible with expected mid latitude warm equable greenhouse conditions norian and older triassic haematite cemented continental arkoses and smectite dominated clays smectite kaolinite were clearly formed in arid settings from the rhaetian and throughout the jurassic humid onshore conditions sustained abundant plant and peat accumulation early diagenetic meteoric flushing and intense chemical weathering kaolinite is therefore a dominating detrital clay mineral in these sedimentary successions kaolinite illite chlorite and smectite i s minor quantitative variations in clay mineralogy within the humid climate rhaetian jurassic sedimentary successions are difficult to interpret in deltaic coal bearing successions the detrital clay mineral composition is conspicuously constant kaolinite illite chlorite possibly due to post depositional clay mineral transformation conversely in shallow marine deposits some quantitative variation is notable kaolinite illite variable smectite chlorite this may reflect that the original detrital composition was better preserved in shallow marine settings smectitic clay minerals associated with gypsum and caliche nodules ambiguously indicate increased aridity towards the end of the jurassic

provenance of gravity flow sandstones from the upper jurassic lower cretaceous farsund formation danish central graben north sea abstract through the upper jurassic and lower cretaceous gravity flow sandstones were deposited in the danish central graben these gravity flow sandstones were deposited within mudstones of the farsund formation which is the main source rock in the area and they are thus potential oil and gas reservoirs the petrography and geochemistry of the sandstones show good intra well relations and several local geographical relations which may indicate that they were derived from multiple locally sourced flows and not merely by one basin axial flow system the gravity flow sandstones deposited in the eastern part of the basin are less mature and richer in especially carbonates and albite compared to previously deposited jurassic sandstones and simultaneously deposited sandstones in the western part of the danish area this may indicate a division of the basin in an eastern and a western provenance province this study addresses the provenance of the gravity flow sandstones by petrographical and geochemical analyses in order to derive possible shared detrital sources as a means of understanding the transport system s of the gravity flows

the physical environmental conditions for biogeochemical differences along the antarctic circumpolar current in the atlantic sector during late austral summer 2012 the physical and biological carbon pumps in the different hydrographic and biogeochemical regimes of the atlantic sector of the southern ocean are controlled by a series of coupled physical chemical and biological processes and a project named eddy pump was designed to study them the eddy pump field campaign was carried out during rv polarstern cruise ant xxviii 3 between january and march 2012 particular emphasis was laid on the differences which occur along the axis of the antarctic circumpolar current acc with its associated mesoscale eddy field the study sites were selected in order to represent 1 the central acc with its regular separation in different frontal jets investigated by a meridional transect along 10 e 2 a large scale bloom west of the mid atlantic ridge which lasted several months with conspicuous chlorophyll poor waters to its immediate east studied by a three dimensional mesoscale survey centred at 12 40 w and 3 the georgia basin north of the island of south georgia which regularly features an extended and dense phytoplankton bloom was investigated by a mesoscale survey centred at 38 12 w while eddy pump represents an interdisciplinary project by design we here focus on describing the variable physical environment within which the different biogeochemical regimes developed for describing the physical environment we use measurements of temperature salinity and density of mixed layer turbulence parameters of dynamic heights and horizontal current vectors and of flow trajectories obtained from surface drifters and submerged floats this serves as background information for the analyses of biological and chemical processes and of biogeochemical fluxes addressed by other papers in this issue the section along 10 e between 44 s and 53 s showed a classical acc structure with well known hydrographic fronts the subantarctic front saf at 46 5 s the antarctic polar front apf split in two at 49 25 s and 50 5 s and the southern polar front spf at 52 5 s each front was associated with strong eastward flows the west mid atlantic ridge survey showed a weak and poorly resolved meander structure between the apf and the spf during the first eight days of the survey the oceanographic conditions at the central station at 12 40 w remained reasonably constant however after that conditions became more variable in the thermocline with conspicuous temperature inversions and interleavings and also a decrease in temperature in the surface layer at the very end of the period of observation the conditions in the thermocline returned to being similar to those observed during the early part of the period with however the mixed layer temperature raised the period of enhanced thermohaline variability was accompanied by increased currents the georgia basin survey showed a very strong zonal jet at its northern edge which connects to a large cyclonic meander that itself joins an anticyclonic eddy in the southeastern quadrant the water mass contrasts in this survey were stronger than in the west mid atlantic ridge survey but similar to those met along 10 e with the exception that the warm and saline surface water typical of the northern side of the saf was not covered by the georgia basin survey mixed layers found during eddy pump were typically deep but varied between the three survey areas the mean depths and standard variations of the mixed layer along the 10 e were 77 2 24 7 m at the west mid atlantic ridge 66 7 17 7 m and in the georgia basin 36 8 10 7 m

the influence of variations in illite morphology on the permeability of middle jurassic brent group sandstones cormorant field uk north sea abstract sem examination of core samples from brent group sandstones in the cormorant field reveals that critical point drying preserves delicate illite morphologies that are damaged during air drying this damage may lead to an increase in permeability as defined by the drying effect air permeability air dried air permeability critical point dried the cormorant reservoir sandstones contain four illite morphologies early diagenetic pore lining illite formed before hydrocarbon emplacement and occurs ubiquitously pore bridging illite blooming illite and illite that replaces kaolinite all formed later and occur preferentially at or below the oil water contact damage to these late illites causes an average drying effect of 3 5 and explains why measured permeabilities predict higher water injection rates than occur in the field comparison with data from other reservoirs reveals that the drying effect is influenced by variations in illite particle dimensions as well as illite morphology tem shadowing techniques reveal that illite particle sizes describe a trend from small particles in reservoirs with large drying effects to large particles in reservoirs with small drying effects these size variations probably reflect differing illite growth rates permeability in illite bearing sandstones thus reflects not only the amount of illite present but also the size of the individual particles and their morphology

diagenetic alterations related to marine transgression and regression in fluvial and shallow marine sandstones of the triassic buntsandstein and keuper sequence the paris basin france abstract the distribution of diagenetic alterations in triassic fluvio deltaic quartzarenitic to sublitharenitic lowstand systems tract lst sandstones of the gres a voltzia formation anastomosing fluvial quartzarenitic transgressive systems tract tst sandstones of the gres a roseaux formation and shallow marine quartzarenitic to sublitharenitic tst sandstones of the gres coquiller formation the paris basin france can be linked to transgression and regression events and thus to the sequence stratigraphic context near surface eogenetic alterations which display a fairly systematic link to the depositional facies and sequence stratigraphic framework include i cementation by meteoric water calcite 18 o 8 9 and 13 c 9 1 in the fluvio deltaic lst sandstones ii cementation by mixed marine meteoric calcite 18 o 5 3 to 2 6 and 13 c 3 9 to 1 3 and dolomite 18 o 4 6 to 2 6 and 13 c 2 9 to 2 3 in the foreshore tst sandstones and below parasequence boundaries pb and transgressive surface ts and in the shoreface tst sandstones below maximum flooding surfaces mfs being facilitated by the presence of carbonate bioclasts iii dissolution of detrital silicates and precipitation of k feldspar overgrowths and kaolinite particularly in the fluvio deltaic lst sandstones owing to effective meteoric water circulation and vi formation of autochthonous glauconite which is increases in abundance towards the top of the fluvio deltaic lst sandstones and along ts and in the shoreface tst sandstones by alteration of micas owing to the flux of seawaters into the sandstones during transgression whereas parautochthonous glauconite is restricted to the ts sandstones owing to marine reworking mesogenetic alterations which include cementation by quartz overgrowths and illite display fairly systematic link to fluvio deltaic lst sandstones this study has revealed that linking of diagenesis to transgression and regression events enables a better understanding of the parameters that control the spatial and temporal distribution of diagenetic alterations in sandstones and of their impact on reservoir quality evolution

assessing the response of watersheds to catastrophic logging and possible secular global temperature change perturbations using sediment chemical chronologies abstract geochemical profiles of sediment cores from two oligotrophic lakes elk and mullett in northern lower michigan were studied to examine the response and recovery of watersheds to large scale logging that occurred between 1850 and 1920 specific questions addressed were can the impact of extensive clear cutting of forests be recognized in the sediment chemical chronologies can states of system stability be identified prior to the logging and are there indications that the systems are recovering and possibly returning to a stable state to answer these questions elements were put into four groups as proxies for watershed runoff or export e g al mg pollution e g pb cu redox e g fe as and productivity e g ca p it was observed that vertical patterns of all proxies were influenced by logging and the early increases in concentration of pollution proxies were due to increased watershed export not pollution system stability might be recognized by relatively symmetrical vertical patterns among all of the proxies or secular changes of slowly increasing or decreasing vertical concentration trajectories some pre logging trajectories were punctuated by episodes of slightly elevated concentrations that appear to be related to comparatively warmer periods during the little ice age iron and mn enrichments caused by increased watershed runoff might be misinterpreted as paleo redox horizons results are interpreted to indicate that 1 reference conditions may be better defined as the temporal trends among proxy groups and not individual concentrations 2 simply assuming pre 1800 conditions as a reference may not be appropriate 3 inter proxy group comparisons are needed to help for interpretations of intra proxy group patterns 4 the possible regime shift identified here might be expected for other ecosystems because of the intensity of human disturbances and secular changes and 5 without consideration of a possible regime shift recovery from logging is estimated to be on the order of 75 130 a but shorter if regime shifts are considered

clay cutans and the origin of illite rim cement an example from the siliciclastic rotliegend sandstone in the dutch southern permian basin

an isochemical model for formation of authigenic kaolinite k feldspar and illite in sediments abstract although thermodynamic calculations indicate a narrow range of conditions for coprecipitation of kaolinite and k feldspar coexistence of authigenic kaolinite and k feldspar is common in jurassic sandstones of the north sea this paper shows that the coexistence of authigenic k feldspar and kaolinite may indicate that this narrow range of conditions did prevail and that the system behaved as a closed system these conditions are presence of muscovite in the sediment pore waters supersaturated with respect to quartz and temperatures lower than 50 c at higher temperatures the authigenic kaolinite and authigenic k feldspar may react and form authigenic illite muscovite and quartz cement the direction of the isochemical reaction is thermodynamically determined by the degree of supersaturation with respect to quartz in the formation water published data on the degree of supersaturation with respect to quartz in formation waters suggest that the illitization of kaolinite may take place at temperatures as low as 50 c much lower than previously believed

the influence of a sudden climatic change on marine deposition in the kimmeridgian of northwest europe a sudden change from humid style to semi arid style deposition markedly affected the accumulation of the upper kimmeridge clay in southern england many of the changes appear to be related to a change in sedimentation rate at this time thus softground faunas are replaced by firmground faunas diagenetic dolostones formed in the methanogenic zone are replaced by sulphate reduction zone carbonate nodules and depositional gradients recorded by lateral biofacies changes becomes steeper the evidence available is in accord with a decline in offshore sedimentation rates during this interval other changes such as a decline in kaolinite abundance were more directly controlled by the drying out of the hinterland similar changes elsewhere in the marine geological record could be used as climatic indicators the climatic change is part of a wider northern hemisphere dry event which affect a broad area in the late jurassic the kimmeridge clay of southern england was one of the last depositional environments to be influenced by the climatic change at this latitude

authigenic quartz microfabrics in cretaceous turbidites evidence for silica transformation processes in sandstones abstract lower cretaceous sandstone turbidites in the scapa field north sea contain a complex variety of authigenic quartz microfabrics early diagenetic microfabrics predate burial calcite cements and compaction they comprise abundant grain rimming microquartz intergranular aggregates of microquartz with authigenic illite smectite and localized isopachous cryptocrystalline quartz and chalcedonite fringes these precipitates contain evidence of precursor opal ct in the form of dissolved recrystallized or pseudomorphed lepispheres and rim cements late diagenetic quartz microfabrics are postcompactional and either replace or overgrow burial calcite cements they display a paragenetic trend from microporous fibrous silica quartzine or chalcedonite to equant syntaxial and mosaic meso uartz cements none of these contain evidence for opal ct precursors and they are likely to have precipitated directly from silica rich pore fluids cathodoluminescence examination of calcite cemented sandstone intervals reveals that siliceous sponge spicules were a major depositional component and were the most likely silica source for opal ct and subsequent quartz cementation dissolution of biogenic silica and formation of opal ct began shortly after deposition with replacement of opal ct by quartz during burial of the scapa member to 1 2 km the early diagenetic silica transformations are identical to those recorded from cenozoic porcelanites and cherts thermodynamic models developed for the diagenesis of biogenic cherts can therefore be applied to the scapa sandstones and qualitatively explain the development of contrasting quartz microfabrics on a bed to bed or smaller scale in contrast direct precipitation of chalce onic quartz during late diagenesis required a resurgence of high supersaturations this is difficult to account for without some import of silica to the sandstones and or cooling of pore fluids precipitation most probably occurred from allochthonous deep basinal brines associated with early stages of hydrocarbon migration

illite distribution and morphology explaining basinal variations in reservoir properties of upper jurassic sandstones danish north sea abstract growth of fibrous illite may destroy an otherwise promising sandstone reservoir even sandstones characterized by microquartz coatings therefore detailed petrographical and diagenetic investigations were performed in order to understand the factors that control illite precipitation and other factors affecting the reservoir properties in the upper jurassic heno formation the heno formation is encountered in large parts of the danish central graben where its lower part the gert member was mainly deposited in back barrier environments and its upper part the ravn member was deposited in a shoreface environment in total 215 core samples from 15 wells were investigated petrographically by optical and scanning electron microscopy this was supplemented by plug porosity and permeability measurements and x ray diffraction of bulk and clay fraction the investigations show among other things that microquartz coatings may help to preserve porosity and permeability whereas the permeability is reduced when microquartz occurs combined with fibrous illite illite precipitation is largely governed by the amounts of k feldspar present during burial consequently the initial abundance of k feldspar in the sediment source area and its alteration during and immediately after deposition in particular during subaerial exposure at sequence boundaries has a major influence on the final reservoir properties of the sandstones the risk of illite precipitation can largely be predicted from the available sediment source material depositional environment and burial depth

mesozooplankton in the southern ocean spatial and temporal patterns from discovery investigations mesozooplankton samples taken during the discovery investigations in the southern ocean in the 1930s were analysed from a series of 5 transects along 80w the samples provide a unique level of depth discrete resolution across large spatial scales over most of the productive austral season stratified net hauls were taken between 0 and 1000 m within the period december 1933 to november 1934 within the epipelagic 0 100 m median zooplankton abundance 278 ind m 3 was 22 times greater than at 1000 m a 3 4 fold variability of abundance in the epipelagic contrasted with depths 250 m where variability was 1 fold depth was the strongest factor separating samples anosim r 0 66 p 0 1 with a clear biological distinction between epipelagic and upper and lower mesopelagic horizons results from multidimensional scaling indicated that when plankton abundance was integrated over all depth horizons 3 different groups could be identified these communities were consistent with the spatial extents of antarctic polar frontal zone and sub antarctic water mass regimes such groupings became less distinct when considering only deeper horizons 500 1000 m and excluding seasonal migrants seasonal signals across all data became less distinct with depth rarefaction analysis indicated that diversity increased with depth although depth alone was the most important influence on sample diversity r 2 0 60

sequence stratigraphy of upper jurassic reservoir sandstones in the northern part of the danish central trough north sea abstract the jurassic succession of the northern part of the danish central trough is subdivided into six depositional sequences based on a multi disciplinary study including sedimentology biostratigraphy seismic interpretation structural interpretation log correlation geochemistry and palynofacies analysis three of these sequences include significant sandstone deposits that represent potential reservoirs the lowermost sequence 1 contains upper kimmeridgian backbarrier and shoreface sandstones of the basal sandstone unit and the heno formation deposited within a transgressive and a highstand systems tract respectively sequence 2 includes upper kimmeridgian middle shoreface sandstones of the heno formation deposited within a transgressive systems tract within sequence 6 middle upper volgian distal turbidite sandstones have been recognized these are interpreted as having been deposited within a lowstand and transgressive systems tract and may constitute a reservoir elsewhere in the basin the sequence stratigraphic boundaries of this study do not match those proposed by the exxon group in this rift setting sequence development was probably controlled predominately by tectonism eustasy was of subordinate importance

evaporite evidence of a mid holocene c 4550 4400 cal yr bp aridity crisis in southwestern europe and palaeoenvironmental consequences sedimentological evidence for an abrupt dry spell in south eastern spain during the middle holocene from c 4906 to 4384 cal yr bp is presented this phase was determined primarily from halite beds deposited between muddy slimes in a lagoon system of puerto de mazarron murcia province with a peak phase from c 4550 to 4400 cal yr bp a multi core multi proxy study of 20 geotechnical drills was made in the lagoon basin to identify the main sedimentary episodes and depositional environments the results suggest that this halite bed more than 80 cm thick was conditioned by climate change and was accompanied by a generalized drying out of the basin halite precipitation was linked with palaeoecological changes including forest and mesophyte depletions and increasing cover and diversity of xerophytic plant species archaeological evidence indicates a demise of the population at this period probably due to resource exhaustion an overall picture of the biostratigraphy and palaeoclimates of the region is given in a broader geographical context

development of early diagenetic silica and quartz morphologies examples from the siri canyon danish north sea abstract the siri canyon has proved to be a perfect area for investigating various morphologies of diagenetic silica in sandstones the development in silica morphologies can be observed from very shallow 1700 m to increased burial depth 3000 m and increased proximity to the central graben distance from 0 to 65 km hydrocarbons and pore fluids now found in the siri fairway have at least partly originated from the central graben the siri canyon is a submarine canyon system eroded into the uppermost chalk deposits and filled with palaeogene hemipelagic and turbiditic marls and mudstones interbedded with sandstone units deposited from sandy mass flows and sandy turbidites which originated on the stavanger platform several hydrocarbon exploration and production wells have been drilled in the siri canyon seven of which are included in this study nini 3 nini 1 na 2p sofie 1 siri 4 celilie 1a and augusta 1 the reservoir sandstones in these wells all contain authigenic silica of various morphologies identified with a combination of traditional optical microscopy and scanning electron microscopy the silica morphologies in some places are classic and well documented in the literature whereas others at least to our knowledge have never previously been described some of the silica morphologies presented here show gradual transition from one to another and others are stand alone forms without clear relationships to other forms the silica morphologies can be expressed in the following way 1 opal rims characteristic of the initial phase of the silica diagenesis in most sandstone units in the siri canyon thick opal rims characterise the sandstone parts adjacent to the mudstone units in the stine segment of the siri field 2 microquartz quartz crystals with a size of 1 5 m seen as coatings on the opal rims both ordered and random 3 cavity overgrowth found as quartz outgrowths in circular and angular cavities formed by dissolution of early authigenic phases angular cavities in the microquartz coatings origin from dissolution of clinoptilolite possibly with a source in volcanic ash whereas dissolution of lepispheres probably resulted in circular cavities 4 quartz ridges syntaxial quartz overgrowths a few micro millimetre in width in parallel lines defined in this paper 5 quartz mountains syntaxial irregular 5 10 m large quartz crystals defined in this paper developed in the water filled sandstone intervals where fluids from the central graben were introduced possibly together with the hydrocarbon and where the continued growth was not retarded by the presence of hydrocarbons in the pore fluids 6 macroquartz syntaxial quartz overgrowths 20 m seen as continued growths from quartz mountains this takes place in the deeper reservoir sandstones promoted by fluids originating from the central graben and increased burial depth 7 quartz with sutured grain contacts occurring only in the deepest well 3000 m

thermogenetic degradation of early zeolite cement an important process for generating anomalously high porosity and permeability in deeply buried sandstone reservoirs abstract diagenetic explanations to anomalously high porosities and permeabilities in deeply buried sandstones have been presented for many reservoirs for example grain coatings of microquartz or chlorite can preserve porosity by preventing succeeding quartz overgrowths and dissolution of grains or cement can generate secondary porosity anomalously high porosity and permeability up to 25 and 500 md respectively characterize the deeply buried 5 km upper jurassic shoreface sandstones of the gert member heno formation in the hejre field in the danish central graben optical cathodoluminescence and scanning electron microscopy were used to characterize the petrography and diagenetic changes that resulted in this high pressure and high temperature hpht reservoir this was supplemented by chemical analyses icp oes and icp ms mineralogical composition xrd and oxygen isotope composition of quartz and calcite cement sims the petrographic investigation shows that none of the known explanations for anomalously high porosity and permeability can be applied hence a new hypothesis for deeply buried sandstones in contact with lavas and volcaniclastic lithologies is proposed the new hypothesis includes reducing mechanical compaction due to early patchy zeolite cement which was sourced from adjacent lavas and volcaniclastic rocks locally stylolites developed where zeolite cement was less effective hence the present overpressure formed at a later stage increased temperatures during burial resulted in thermal instability of the zeolite cement which was dissolved and authigenic k feldspar and quartz precipitated instead this resulted in a reduction of the cement volume of up to 70 if the zeolite was k heulandite and a corresponding relative porosity increase thus a reduction in cement volume was generated by replacing zeolite of an open crystal structure with mineral phases k feldspar and quartz of denser crystal structures the thermogenetic breakdown of zeolite was forced by increased temperatures during burial this process may explain previously enigmatic highly porous and permeable deeply buried sandstone reservoirs

modelling the occurrence of gullies in rangelands of southwest spain gully erosion has been recognized as an important soil degradation process in rangelands of southwest spain in this paper a relatively new data mining technique called multivariate adaptive regression splines mars was applied to construct a model capable of predicting the location of gullies a large database was used to support the model composed of a target variable presence or absence of gullies and 36 independent variables related to topography lithology soils rainfall land use and vegetation cover the performance of the model was evaluated using the receiver operating characteristic roc curve for five external datasets the model had high predictive power with values for the area under the roc curve of the external validation datasets varying from 0 75 to 0 98 1 0 being perfect prediction the most important variables explaining the spatial distribution of gullies were lithology and soil type finally the model was compiled and implemented into a geographical information system to obtain maps of susceptible areas for gully erosion these maps show that approximately 7 of the study area presents favourable conditions for the development of gullies the results demonstrate that mars constitutes a valuable model in geomorphic research and could also be a useful tool for assessing the impacts of changing climate and land use on gully erosion copyright 2009 john wiley sons ltd

circumpolar structure and distribution of the antarctic circumpolar current fronts 1 mean circumpolar paths 1 high resolution hydrographic sections and maps of the gradient of sea surface height ssh reveal that the antarctic circumpolar current acc consists of multiple jets or frontal filaments here we use a 15 year time series of ssh observations to determine the circumpolar structure and distribution of the acc fronts the jets are consistently aligned with particular streamlines along the entire circumpolar path confirming and extending the results of an earlier study restricted to the region south of australia the intensity of the fronts as measured by the cross front gradient of ssh varies along the fronts and the individual branches merge and diverge often in response to interactions with bathymetry maps of absolute velocity at 1000 m depth derived from argo trajectories confirm the existence of multiple current cores throughout the southern ocean high resolution hydrographic sections and profiles of temperature and salinity from argo floats are used to show that the front locations derived from fitting ssh contours to maps of ssh gradient are consistent with locations inferred from the traditional criteria based on water mass properties suitably modified to account for multiple frontal branches three regions are examined in detail the crozet plateau the kerguelen plateau and the scotia sea these examples show how recognition of the multiple jets of the acc can help resolve discrepancies between previous studies of acc fronts

diagenetic history of eocene wilcox sandstones south central texas abstract sandstones and shales of the lower eocene wilcox group extend from outcrop to depths greater than 5 km where temperatures exceed 200 c eight authigenic minerals have been identified quartz kaolinite calcite ankerite and albitized plagioclase are volumetrically significant whereas chlorite illite albite overgrowths and dolomite are present only in trace amounts quartz kaolinite and ankerite cements are present over the entire depth range sampled 1528 to 4446 m calcite cement occurs only in sandstones shallower than 3300 m albitized plagioclase is the predominant feldspar in samples deeper than 2600 m whereas detrital potassium feldspar has been completely dissolved out of sandstones deeper than about 3200 m no systematic variations in the chemical composition of calcite with depth no zoning within calcite or ankerite and no compositional differences between pore filling and grain replacing calcite or ankerite were found calcite cements have 87 sr 86 sr ratios generally less than 0 708 whereas ankerite cements are significantly enriched in 87sr the oxygen isotopic composition of carbonate cement ranges between 9 and 14 pdb for calcite and between 8 and 12 for ankerite with no systematic variation with depth calculated 18o values for authigenic quartz and kaolinite range from 22 to 31 smow and from 14 to 17 respectively volumetrically important amounts of authigenic quartz and kaolinite precipitated during early burial before calcite cementation secondary porosity generation or smectite to illite transformation in encasing shales the isotopic composition of strontium in calcite cement shows most of it to be of eocene or older marine carbonate origin ankerite cementation followed the generation of secondary porosity and occurred after the products of organic maturation and shale diagenesis plagioclase albitization or potassium feldspar dissolution significantly affected the sandstone pore water system

mean sea surface height of the antarctic circumpolar current from geosat data method and application the mean sea surface height across the antarctic circumpolar current has been reconstructed from height variability measured by the geosat altimeter without assuming prior knowledge of the geoid for this study an automated technique has been developed to estimate mean sea surface height for each satellite ground track using a meandering gaussian jet model and errors have been estimated using monte carlo simulation the results are objectively mapped to produce a picture of the mean subantarctic and polar fronts which together comprise the major components of the antarctic circumpolar current the meandering jet model explains between 40 and 70 of the height variance along the jet axes the results show that the fronts are substantially steered by topography and that the jets have an average gaussian width of about 44 km in the meridional direction and meander about 75 km to either side of their mean locations the average height difference across the subantarctic front saf is 0 7 m and across the polar front pf 0 6 m the mean widths of the fronts are correlated with the size of the baroclinic rossby radius

geochemical constraints from formation water analyses from the north sea and the gulf coast basins on quartz feldspar and illite precipitation in reservoir rocks

transport and variability of the antarctic circumpolar current in drake passage the baroclinic transport of the antarctic circumpolar current acc above 3000 m through drake passage is 107 3 10 4 sv and has been steady between 1975 and 2000 for six hydrographic sections 1993 2000 along the world ocean circulation experiment woce line sr1b the baroclinic transport relative to the deepest common level is 136 7 7 8 sv the acc transport is carried in two jets the subantarctic front 53 10 sv and the polar front pf 57 5 5 7 sv southward of the acc the southern antarctic circumpolar current transports 9 3 2 4 sv we observe the pf at two latitudes separated by 90 km this bimodal distribution is related to changes in the circulation and properties of antarctic bottom water three realizations of the instantaneous velocity field were obtained with lowered adcps from these observations we obtain near bottom reference velocities for transport calculations net transport due to these reference velocities ranges from 28 to 43 sv consistent with previous estimates of variability the transport in density layers shows systematic variations due to seasonal heating in near surface layers volume transport weighted mean temperatures vary by 0 40 c from spring to summer a seasonal variation in heat flux of about 0 22 pw finally we review a series of papers from the international southern ocean studies program the average yearlong absolute transport is 134 sv and the standard deviation of the average is 11 2 sv the error of the average transport is 15 to 27 sv we emphasize that baroclinic variability is an important contribution to net variability in the acc

chemistry of oil field brines in relation to diagenesis of reservoirs 2 reconstruction of palaeo water composition for modelling illite diagenesis in the greater alwyn area north sea abstract input data for water composition are required in order to numerically simulate diagenetic episodes in rock dominated closed system modelling the composition of water initially present in the pores has at most only a minor influence on the stability of the system and is controlled by the mineral composition in open system modelling the composition of infiltrating water can play an important role brent water salinities of the greater alwyn area exhibit a regional trend from the north to the south the k cl ratio of these waters varies along two trends it is higher in the north than in the south alkalinity and published pco2 values are used to calculate ph under subsurface conditions very few silica measurements are available aluminium analyses of pore water are not available some reservoirs of the greater alwyn area experienced a major reduction in permeability due to illite and quartz precipitation coeval with kaolinite and k feldspar dissolution saturation of present day brent waters with respect to these minerals is examined the method proposed in part 1 is applied to reconstruct a range of possible compositions for ancient waters in brent reservoirs between several extreme conditions one of these extremes is provided by buffering water with a set of minerals chosen from petrographic considerations the other extremes correspond to reasonable states of over saturation with respect to particular minerals e g quartz and concern the values of dissolved si and al which are the least well characterised concentrations from the routine analyses of formation waters

the impact of diagenesis on the heterogeneity of sandstone reservoirs a review of the role of depositional facies and sequence stratigraphy diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability linking the types and distribution of diagenetic processes to the depositional facies and sequence stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diagenetic alterations controlling quality and heterogeneity the heterogeneity patterns of sandstone reservoirs which determine the volumes flow rates and recovery of hydrocarbons are controlled by geometry and internal structures of sand bodies grain size sorting degree of bioturbation provenance and by the types volumes and distribution of diagenetic alterations variations in the pathways of diagenetic evolution are linked to 1 depositional facies hence pore water chemistry depositional porosity and permeability types and amounts of intrabasinal grains and extent of bioturbation 2 detrital sand composition 3 rate of deposition controlling residence time of sediments at specific near surface geochemical conditions and 4 burial thermal history of the basin the amounts and types of intrabasinal grains are also controlled by changes in the relative sea level and therefore can be predicted in the context of sequence stratigraphy particularly in paralic and shallow marine environments changes in the relative sea level exert significant control on the types and extent of near surface shallow burial diagenetic alterations which in turn influence the pathways of burial diagenetic and reservoir quality evolution of clastic reservoirs carbonate cementation is more extensive in transgressive systems tract tst sandstones particularly below parasequence boundaries transgressive surface and maximum flooding surface because of the abundance of carbonate bioclasts and organic matter bioturbation and prolonged residence time of the sediments at and immediately below the sea floor caused by low sedimentation rates which also enhance the formation of glaucony eogenetic grain coating berthierine odinite and smectite formed mostly in tst and early highstand systems tract deltaic and estuarine sandstones are transformed into ferrous chlorite during mesodiagenesis helping preserve reservoir quality through the inhibition of quartz cementation the infiltration of grain coating smectitic clays is more extensive in braided than in meandering fluvial sandstones forming flow barriers in braided amalgamated reservoirs and may either help preserve porosity during burial because of quartz overgrowth inhibition or reduce it by enhancing intergranular pressure dissolution diagenetic modifications along sequence boundaries are characterized by considerable dissolution and kaolinization of feldspars micas and mud intraclasts under wet and warm climates whereas a semiarid climate may lead to the formation of calcrete dolocrete cemented layers turbidite sandstones are typically cemented by carbonate along the contacts with interbedded mudrocks or carbonate mudstones and marls as well as along layers of concentration of carbonate bioclasts and intraclasts commonly hybrid carbonate turbidite arenites are pervasively cemented proximal massive turbidites normally show only scattered spherical or ovoid carbonate concretions improved geologic models based on the connections among diagenesis depositional facies and sequence stratigraphic surfaces and intervals may not only contribute to optimized production through the design of appropriate simulation models for improved or enhanced oil recovery strategies as well as for co2 geologic sequestration but also support more effective hydrocarbon exploration through reservoir quality prediction

erosion and stabilisation sequences in relation to base level changes in the el cautivo badlands se spain abstract the complex multiple age badlands at el cautivo are cut in upper miocene marls of the uplifted and dissected tabernas basin almeria in semi arid se spain previous work identified six episodes of erosion and subsequent stabilisation with ages ranging from the late pleistocene to the present day this paper uses newly acquired digital elevation models orthorectified aerial photographs and field survey data to illustrate the development sequence of a series of gullies which drain into the rambla de tabernas changing drainage directions and phases of incision and stabilisation are related to the filling and subsequent dissection of the tabernas lower lake sediments differential material strengths changing process mixes on hillslopes and in more recent times land use changes we propose that the erosional phases differed considerably in both time span and depth of incision a long period of stability during the existence of the lake led to extensive pediment development in the area subsequent incision into the lake sediments by the rambla de tabernas produced a limited amount of localised pediment incision in the tributary catchment subsequent aggradation occurred in the lower reaches of gullies when incision by the rambla de tabernas reached the underlying bedrock following incision through the lip of the lake the rambla de tabernas cut rapidly into the bedrock leading to an altered drainage direction in the tributary catchments and the major phase of badland development visible today stabilisation of the pediments and some of the north facing slopes occurred subsequently increased sediment loads caused by climatic shift a change in land use or both gave rise to a later phase of valley fill in the gullies the modern channels have reduced this fill to isolated terrace benches and there are also localised remnants of a lower more recent fill whilst supporting the general interpretation of the site s development by alexander et al alexander r w harvey a m calvo a james p a cerda a 1994 natural stabilisation mechanisms on badlands slopes tabernas almeria spain in millington a c pye k eds environmental change in drylands biogeographical and geomorphological perspectives wiley chichester pp 85 111 this re analysis highlights the importance of changing drainage direction to the pattern of landform development and attempts to assess the magnitude and overall rate of incision

evaluating southern ocean carbon eddy pump from biogeochemical argo floats the vertical transport of surface water and carbon into ocean s interior known as subduction is one of the main mechanisms through which the ocean influences earth s climate new instrumental approaches have shown the occurrence of localized and intermittent subduction episodes associated with small scale ocean circulation features these studies also revealed the importance of such events for the export of organic matter the so called eddy pump however the transient and localized nature of episodic subduction hindered its large scale evaluation to date in this work we present an approach to detect subduction events at the scale of the southern ocean using measurements collected by biogeochemical autonomous floats bgcargo we show how subduction events can be automatically identified as anomalies of spiciness and apparent oxygen utilization aou below the mixed layer using this methodology over more than 4 000 profiles we detected 40 subduction events unevenly distributed across the sothern ocean events were more likely found in hot spots of eddy kinetic energy eke downstream major bathymetric features moreover the bio optical measurements provided by bgcargo allowed measuring the amount of particulate organic carbon poc being subducted and assessing the contribution of these events to the total downward carbon flux at 100 m ep100 we estimated that the eddy pump represents less than 19 to the ep100 in the southern ocean although we observed particularly strong events able to locally duplicate the ep100 this approach provides a novel perspective on where episodic subduction occurs that will be naturally improved as bgcargo observations continue to increase

late jurassic climate and its impact on carbon cycling abstract the climate of the late jurassic has been characterized by high atmospheric co2 levels and by a monsoonal rainfall pattern in this study we traced the evolution of the global carbon cycle through the late jurassic with the help of carbonate carbon isotope stratigraphy the oxfordian tithonian 13c curve is marked by one major positive carbon isotope excursion with an amplitude 13 c 1 0 middle to late oxfordian and a second minor positive excursion with an amplitude 13 c late kimmeridgian the early kimmeridgian and early tithonian 13c values fluctuate around 13 c 2 0 3 and contrast with the less positive 13c values of early oxfordian and late tithonian age 13 c 1 0 5 a comparison of the late jurassic carbonate carbon isotope curve with the occurrence of organic rich sediments suggests that not only fluctuations in organic carbon burial but also in carbonate carbon burial had an impact on the c isotope record the oxfordian c isotope excursion appears to correspond to a time of overall increased organic carbon burial triggered by increased nutrient transfer from continents to oceans during a time of rising global sea level however episodes of enhanced organic carbon burial during the kimmeridgian and early tithonian are not reflected by prominent spikes in the c isotope record favourable conditions for carbonate platform growth at a time of high global sealevel may have resulted in the stabilisation of the c org c carb burial ratio and hence maintained the 13c record at steady but relatively positive values the middle and late tithonian c isotope values drop below 13 c 1 5 a similar shift to less positive c isotope values was recognized in other c isotope records from the tethys and atlantic oceans and reflects a decrease in the c org c carb burial ratio possibly related to a reorganisation of the global climate system globally widespread marine black shale mature quartzose sandstone assemblages suggest that the relative efficiency of the late jurassic carbon pumps was controlled by weathering erosion and runoff causing widespread marine eutrophication eutrophication favoured the organic carbon pump but it diminished the carbonate platform growth potential the monsoonal rainfall distribution pattern may explain why late jurassic carbonate platforms experienced less severe growth crises than early cretaceous carbonate platforms

diagenetic clinoptilolite and opal ct from the middle eocene wittering formation isle of wight u k abstract thinly interbedded estuarine sandstones and mudstones in the middle eocene wittering formation bracklesham group isle of wight contain authigenic clinoptilolite opal ct and possibly authigenic smectite over a thickness of 8 5 m the clinoptilolite and opal ct cements have resulted in these beds being the only ones tightly cemented in the wittering formation the rest being soft sands and muds the unit contains abundant siliceous sponge spicules many of which are preserved as voids and were the likely source of the opal ct the presence of an organic rich rooted paleosol histosol immediately overlying the unit is believed to have contributed significantly to the early diagenetic history of the estuarine sediments in particular the formation of zeolite although the ph and dissolved silica of unmodified seawater are too low for zeolite formation high concentrations of dissolved silica are believed to have resulted from dissolution of siliceous sponge spicules opal a and formation of silica organic acid complexes organic acid dissolution of feldspars absent from the zeolite bearing interval is the most probable source of alumina and cations for zeolite formation ree data show that volcanic ash was never present and cannot therefore have been the source of the zeolite this investigation therefore demonstrates a novel route by which the pore fluid chemistry required for zeolite formation can be achieved

rapid southern ocean front transitions in an eddy resolving ocean gcm the formation of persistent multiple fronts is an established feature of the antarctic circumpolar current acc front strength and location are closely linked to eddy properties and therefore have important implications for the eddy driven closure of the southern ocean meridional overturning circulation acc front structure is analyzed here by calculating regional probability density functions pdfs of potential vorticity diagnosed in an eddy resolving ocean general circulation model rapid spatial transitions in the number of fronts and in the density classes over which they occur are found front transitions are associated with the major topographic obstacles kerguelen island campbell plateau and drake passage multiple fronts are preferentially found downstream of these features these findings highlight the significant departure from zonal symmetry of the acc front structure and emphasize the importance of local dynamics on large scale southern ocean properties

diagenetic albitization of detrital k feldspar in jurassic lower cretaceous and tertiary clastic reservoir rocks from offshore norway i textures and origin abstract the origin of albitized grains in sandstones has been the topic of considerable discussion in recent years some workers believe that the albitization of feldspar can occur in situ during burial diagenesis but others argue that albitization is a feature inherited from source clastic grains our petrological and chemical studies of subsurface samples of the jurassic lower cretaceous and tertiary reservoir sandstones from offshore norway show that detrital grains of potassium feldspar have been albitized during burial diagenesis criteria suggesting that the albitization has occurred during burial diagenesis include 1 euhedral habit of albite crystals with sharp edges and comers and markedly smooth crystal faces 2 generally untwinned albitized grains mostly riddled with minute br wnish inclusions 3 lack of cathodoluminescence in albite 4 homogeneous and pure albite composition 99 ab 5 absence of albitized grains in carbonate cemented zones and 6 increase in the percentage and degree partial to complete of albitized k feldspar with depth under sem albitized grains display two types of fabrics type i albitized grains are composed of numerous tiny albite crystals 1 30 m growing along cleavage planes in parent grains and they have abundant dissolution porosity while type ii albitized grains show blocky albite crystals 5 80 m forming pseudomorphs of detrital k feldspar and they lack any dissolution porosity type i albitized grains are found commonly at shallower depths between 2 2 and 3 km and temperatures varying from 65 to 90 c whereas type ii albitized grains dominate at greater depths 3 5 km and at temperatures 90 c albitization proceeds by a dissolution precipitation mechanism at shallower depth dissolution of the parent k feldspar is more rapid than the precipitation of albite and rates of albite precipitation increase with depth favoring pseudomorphic replacement of k feldspars other factors that influence albitization processes are ana ak permeability reduction and surface dissolution kinetics of the parent grain

holocene fluctuations of lake levels in the french jura and sub alpine ranges and their implications for past general circulation patterns temperature and precipitation variations reflected in lake level fluctuations in the french jura and pre alps during the last nine millennia are qualitatively assessed by reference to 1 climatic oscillations reconstructed in the alps and 2 modern processes behind alpine tree limit variations the summer season and mainly the late summer appears to have a deterministic impact on the holocene environment correlations between palaeoclimatic proxy data recorded in the north atlantic area from high and middle latitudes and climatic dynamics illustrated by the little ice age and the recent warming suggest 1 the existence of climatic oscillations due to varying solar activity superimposed on larger plurimillennial climatic trends and 2 that these minor climatic phases correspond with two main types of general circulation pattern alternating over western europe during the last six millennia the climatic deteriorations correspond with cooling at high latitudes inducing a stronger thermal grad

antarctic circumpolar current frontal system in the south atlantic monitoring using merged argo and animal borne sensor data 1 we describe large scale features of the antarctic circumpolar current acc in the atlantic part of the southern ocean by merging argo data and data obtained by novel animal borne ctd sensors twenty one of these ctd satellite relay data loggers ctd srdls were attached to southern elephant seals mirounga leonina on south georgia the merged data yield unified gridded hydrogaphic fields with high temporal and spatial resolution enabling the determination of features absent in each of the data sets separately the structure and variability of the frontal field revealed by this data set were compared with those in daily quarter degree optimally interpolated sea surface temperature fields and fields of weekly gridded sea level anomaly in general the frontal positions derived using our data set are in agreement with previous work especially where the pathways are constrained by topography e g at the north scotia ridge and the south scotia ridge however with the improved temporal and spacial resolution provided by the ctd srdls we were able to observe some novel features all frontal positions are more variable than previously indicated across the scotia sea and west of the mid atlantic ridge on seasonal time scales the merged data set shows the temporal variability of the southern acc front saccf north of south georgia and in its position east of the island where the saccf lies further north than has been suggested in previous work in addition the subantarctic front crosses the mid atlantic ridge about 400 km further north when compared to previous work