diagenesis of the hild brent sandstones northern north sea isotopic evidence for the prevailing influence of deep basinal water abstract the deeply buried 3 8 4 3 km brent reservoir sandstones of the hild field northern north sea have undergone intense diagenetic alteration fluid inclusion o c h sr isotope and k ar age data combined with petrography and burial history constrain the conditions and timing of the main diagenetic transformations as well as the nature and origin of diagenetic fluids non ferroan calcite cement formed at shallow depth the reconstructed diagenesis of the hild brent reservoirs displays notable differences with diagenetic models developed in many fields of the northern north sea in particular diagenesis in hild is characterized by the prevailing influence of deep basinal water a feature related to the immediate proximity of the viking graben this study further illustrates that diagenetic processes related to compaction derived fluids commonly reported in sediments located within rift basins should also be expected to occur in neighboring sub basins along rift margins

an integrated thermal and isotopic study of the diagenesis of the brent group alwyn south u k north sea abstract the petrography fluid inclusion thermometry and isotope geochemistry of diagenetic cements are used to reconstruct the pore fluid history of the middle jurassic brent group reservoir sandstones in the alwyn south area of the u k north sea the study focuses on a relatively limited area of three adjacent reservoir compartments at successively higher structural levels the cement assemblage of kaolinite quartz and illite has resulted in severe deterioration of otherwise good reservoir quality early precipitation of vermiform and late blocky kaolinite was succeeded by a period of relatively intense illite precipitation temperature estimates for kaolinite precipitation of 80 c and 18o of 15 3 suggest co existing fluids of 18o 3 quartz cementation overlapped both kaolinite and illite formation fluid inclusion data indicate that quartz cementation took place at temperatures of 109 7 c pore fluid salinities were 4 wt nacl with a h2o o isotopic composition of 1 o 0 5 smow the fluids which precipitated coexisting illite were compositionally homogeneous with equilibrium 18o water compositions of 0 5 smow illite sd values range from 33 to 50 smow these fluid inclusion and isotopic data suggest that both quartz and illite were precipitated from pore waters with a uniform marine signature this is consistent with the predominantly marine to paralic depositional context of the brent group in alywn south illite precipitation was followed by hydrocarbon emplacement between the middle eocene and lower oligocene

moored observations of western boundary current variability and thermohaline circulation at 26 5 n in the subtropical north atlantic abstract a 5 8 year time series of moored current meter observations is used with hydrographic section data cme model results and gridded wind fields over the north atlantic to describe the mean structure and variability of circulation and volume transports east of abaco bahamas at 26 5 n a mean antilles current with 5 sv of northward transport is confined against the bahamas boundary in the upper 800 m and combines with approximately 19 sv of florida current transport to balance the sverdrup interior circulation and does not contribute to interhemispheric exchange the mean transport of the deep western boundary current dwbc off the bahamas is approximately 40 sv of which 13 sv compensates the upper branch of the thermohaline circulation requiring a 27 sv deep recirculation robust annual and semiannual cycles of meridional are found in both moored observations and model results with remarkable agreement in amplitude 13 sv and phase maximum northward transports occur in winter and summer

a note on the calculation of errors in conventional kar dating abstract an evaluation is made of the various formulae available in the literature for calculation of errors in k ar dating guidelines are laid down for the use of the most suitable formulae

what is the thermohaline circulation the term thermohaline circulation is widely used in the context of past and future climate change in his perspective wunsch argues that there are many different and inconsistent definitions for this term he defines ocean circulation in terms of mass fluxes which are largely driven by surface forces such as wind and tides and argues that thermohaline circulation should only refer to the separate transport of heat and salinity in the ocean

comment on glacial interglacial circulation changes inferred from 231pa 230th sedimentary record in the north atlantic region by j m gherardi et al

circulation and deep water export at the western exit of the subpolar north atlantic the current system east of the grand banks was intensely observed by world ocean circulation experiment woce array acm 6 during 1993 95 with eight moorings reaching about 500 km out from the shelf edge and covering the water column from about 400 m depth to the bottom more recently a reduced array by the institut fur meerskunde ifm at kiel germany of four moorings was deployed during 1999 2001 focusing on the deep water flow near the western continental slope both sets of moored time series each about 22 months long are combined here for a mean current boundary section and both arrays are analyzed for the variability of currents and transports a mean hydrographic section is derived from seven ship surveys and is used for geostrophic upper layer extrapolation and isopycnal subdivision of the mean transports into deep water classes the offshore part of the combined section is dominated by the deep reaching north atlantic current nac with currents still at 10 cm s 1 near the bottom and a total northward transport of about 140 sv sv 106 m3 s 1 with the details depending on the method of surface extrapolation used the mean flow along the western boundary was southward with the section mean north atlantic deep water outflow determined to be 12 sv below the 27 74 kg m 3 isopycnal however east of the deep western boundary current dwbc the deep nac carries a transport of 51 sv northward below 27 74 kg m 3 resulting in a large net northward flow in the western part of the basin from watermass signatures it is concluded that the deep nac is not a direct recirculation of dwbc water masses transport time series for the dwbc variability are derived for both arrays the variance is concentrated in the period range from 2 weeks to 2 months but there are also variations at interannual and longer periods with much of the dwbc variability being related to fluctuations and meandering of the nac a significant annual cycle is not recognizable in the combined current and transport time series of both arrays the moored array results are compared with other evidence on deep outflow and recirculation including recent models of different types and complexity

potassium metasomatism of precambrian paleosols abstract independent of substratum composition ancient regoliths formed by subaerial weathering are characterized by accumulation of k and other alkaline elements potassium enrichment due to the formation of clay minerals is a trend typically observed in regoliths developed by modern hypergene processes however the k content in precambrian paleosols is much higher than in modern weathering profiles k metasomatism affecting buried hypergen targets is a possible mechanism for k accumulation a novel reaction transport dissolution precipitation model designed as part of this work proved able to reproduce many essential features of post burial alterations of a subaerial weathering profile formed on basaltic rocks including potassium enrichment dehydration compaction and pore filling that impacted the buried regolith the phenomena of preservation of the primary basaltic structure the apparent selective alteration of soil levels and immobility of al ti and p could be explained by the model

similar rates of modern and last glacial ocean thermohaline circulation inferred from radiochemical data today the ocean thermohaline circulation transports half of the 231pa produced by radioactive decay in the atlantic ocean water column to the southern ocean this export respectively imparts low and high 231pa 230th ratios to the surface sediments of these oceans ocean sediments from the last glacial maximum bear a similar isotopic fingerprint implying that advection of north atlantic deep intermediate water into the circumpolar deep water of the southern ocean occurred at a similar or slightly higher rate during the last glacial period

deglacial diatom productivity and surface ocean properties over the bermuda rise northeast sargasso sea 1 diatom assemblages document surface hydrographic changes over the bermuda rise between 19 2 and 14 5 ka subtropical diatom species and chaetoceros resting spores dominate the flora as in north atlantic productive regions today from 16 9 to 14 6 ka brackish and fresh water diatoms are common and their contribution is generally coupled with total diatom abundance this same interval also contains rare grains of ice rafted debris coupling between those proxies suggests that successive discharge of icebergs might have stimulated productivity during heinrich event 1 h1 iceberg migration to the subtropics likely created an isolated environment involving turbulent mixing upwelled water and nutrient rich meltwater supporting diatom productivity in an otherwise oligotrophic setting in addition the occurrence of mode water eddies likely brought silica rich waters of southern ocean origin to the euphotic zone the persistence of lower salinity surface water beyond the last ice rafting suggests continued injection of fresh water by cold core rings and advection around the subtropical gyre these results indicate that opal productivity may have biased estimates of meridional overturning based on 231pa 230th ratios in bermuda rise sediments during h1

geochemistry and origin of sour gas accumulations in the northeastern sichuan basin sw china significant natural gas reserves have recently been discovered in the lower triassic oolitic reservoirs from northeastern sichuan basin sw china in the wake of the december 2003 sour gas well blow out this study presents an overview on the petroleum geology and geochemistry of the sour gas accumulations in the study area two types of natural gas accumulations were identified in the lower triassic oolitic reservoirs both containing highly mature thermogenic gases with their hydrocarbon source rocks in upper permian strata natural gases from the area south of the ancient kaijiang liangping seaway are generally sweet gases formed as the result of thermal maturation whereas those discovered from north of the seaway are products of both thermal maturation and thermochemical sulfate reduction of early accumulated oils in the feixianguan formation reservoirs the proposed origins of the gases are supported by their chemical and stable carbon isotope compositions as well as the presence or absence of pyrobitumens in the reservoir the distribution of gas accumulations is controlled predominantly by the combination of lithologic and structural factors the regional variation in the concentrations of h2s in the gases appears related to the presence and thickness of anhydrite bearing evaporitic rocks interbedded or intercalated with the oolitic reservoirs

k ar dating of time of gas emplacement in rotliegendes sandstone netherlands the time of emplacement of gas into each of two reservoirs in the rotliegendes sandstone one in the groningen field and one in the broad fourteens basin was estimated from potassium argon k ar ages of diagenetic illitic clays and careful petrographic observations of the rocks in which the clays occurred in each reservoir the intensity of diagenetic alteration increased downward within a relatively thin 15 to 175 m or 50 to 580 ft interval apparently reflecting a longer period of alteration for deeper samples this is expected in cases where a gradually accumulating gas column expels pore water from progressively deeper parts of the reservoir rock illite is a common diagenetic phase in the rotliegendes because detrital contamination in these rocks has been infer ed to be minimal k ar ages of very fine grained illite should indicate the time the illite forming process ceases in each case reported here the age of the deeper sample is younger than that of the shallower we interpret the age of the finest illite fraction in the gas zone of each reservoir to indicate the time at which illite formation ceased as the result of gas emplacement and pore fluid displacement in the rock our results indicate that gas emplacement in the groningen field commenced prior to 150 ma and that the growth of diagenetic illite continued in the present day water zone from that time until at least 120 ma

identification of groundwater flow paths in complex aquifer systems a methodology was developed and applied to the tindouf southwestern algeria and the annaba bouteldja aquifers northeastern algeria in order to understand better the hydrogeology of the complex aquifers despite the scarcity of the available data graphical representation of deuterium versus oxygen 18 and principal components analysis pca are statistical techniques used to combine various disciplinary data in order to identify chemical and isotopic groups which are in turn used to define groundwater flow paths the results of this study agree with the generally accepted hydrogeological conceptual model of the aquifers in addition we obtained new results using the pca method 1 a description of the complex flow system by grouping various qualitative and quantitative parameters 2 the definition and characterization of the main groundwater flow paths from their sources to the discharge zones these flow paths are defined by their water categories which are represented by salinity and origin of groundwater this approach is useful for analysing aquifers despite the lack of important database and may also be helpful for studying other complex groundwater basins copyright 2006 john wiley sons ltd

petroleum evolution within the tarim basin northwestern china insights from organic geochemistry fluid inclusions and rhenium osmium geochronology of the halahatang oil field the newly discovered halahatang oil field in the northern tarim basin has a potential resource of more than 70 billion bbl of oil oil organic geochemical data from the halahatang oil field indicate that the oils are of moderate maturity biodegraded and represent one oil family derived from the same paleozoic marine source modeling of coeval aqueous and hydrocarbon bearing inclusion data provide fluid trapping temperatures and pressures of 100 c to 110 c and approximately 39 to 59 mpa 5656 8557 psi respectively the fluid inclusion data coupled with the previous basin model studies suggest a single prolonged oil migration event during the permian the re os isotope data of the oil yields an early permian re os age of 285 48 ma the age agrees with the timing of maturation of the paleozoic source via burial history modeling but is slightly older 5 55 m y than the oil migration and accumulation timing implied by the basin modeling coupled with fluid inclusion analysis and the published reservoir illite k ar dates thus the oil re os date suggests that oil generation in the halahatang depression of the tarim basin occurred during the early permian rather than the silurian as previously proposed with subsequent oil migration and accumulation occurring during the middle late permian as recorded by basin modeling coupled with fluid inclusion analysis and illite k ar dating in addition to promoting petroleum exploration in the tarim basin this study which combines crude oil re os isotope dating and traditional analytical methods organic geochemistry and fluid inclusion analysis to constrain petroleum evolution is applicable to hydrocarbon systems worldwide

glacial interglacial circulation changes inferred from 231pa 230th sedimentary record in the north atlantic region 1 studies from the subtropical western and eastern atlantic ocean using the 231pa 230th ratio as a kinematic proxy for deep water circulation provided compelling evidence for a strong link between climate and the rate of meridional overturning circulation moc over the last deglaciation in this study we present a compilation of existing and new sedimentary 231pa 230th records from north atlantic cores between 1710 and 4550 m water depth comparing sedimentary 231pa 230th from different depths provides new insights into the evolution of the geometry and rate of deep water formation in the north atlantic during the last 20 000 years the 231pa 230th ratio measured in upper holocene sediments indicates slow water renewal above 2500 m and rapid flushing below consistent with our understanding of modern circulation in contrast during the last glacial maximum lgm glacial north atlantic intermediate water gnaiw drove a rapid overturning circulation to a depth of at least 3000 m depth below 4000 m water renewal was much slower than today at the onset of heinrich event 1 transport by the overturning circulation declined at all depths gnaiw shoaled above 3000 m and significantly weakened but did not totally shut down during the bolling allerod ba that followed water renewal rates further decreased above 2000 m but increased below our results suggest for the first time that ocean circulation during that period was quite distinct from the modern circulation mode with a comparatively higher renewal rate above 3000 m and a lower renewal rate below in a pattern similar to the lgm but less accentuated moc during the younger dryas appears very similar to ba down to 2000 m and slightly slower below

palaeohydrodynamics of fluids in the brent group oseberg field norwegian north sea from chemical and isotopic compositions of formation waters generally the history of past sub surface fluid movements is difficult to reconstruct however the composition of oil field waters characterizes the origins and mixing processes that allow such a reconstruction we have investigated present day formation waters from brent group sedimentary rocks of the oseberg field in order to assess both their geochemical variations and their origin s water samples sampled at the separator produced from immediately above the oil water contact and from the aquifer water saturated zone below the oil water contact were taken from 11 wells across the field in addition 3 trace water samples were extracted from oil produced from higher up in the oil column the water samples were analysed for their chemical components and isotopic compositions conservative tracers such as cl br deltad and delta o 18 were used to evaluate the origin of the waters all formation waters can be characterised as na cl brines the separator samples are of aquifer origin indicating that aquifer water drawn up by the pressure reduction near the well is produced from the lower few tens of metres of the oil zone by defining plausible endmembers the waters can be described as mixtures of seawater 60 90 meteoric water 10 30 evaporated seawater primary brines 3 5 and possibly waters which have dissolved evaporites secondary brines alternatively using multidimensional scaling the waters can be described as mixtures of only 3 endmembers without presupposing their compositions in fact they are seawater very dilute brine and a secondary brine confirming the power of this approach meteoric water was introduced into the reservoir during the end brent and early cretaceous periods of emergence and erosion and partially replaced the marine pore fluids lateral chemical variations across the oseberg field are extremely small the waters from closer to the erosion surfaces show slightly stronger meteoric water isotopic signatures the primary and secondary brines are believed to come from permian and triassic evaporitic rocks in the deeply buried viking graben to the west and to have been modified by water rock interactions along their migration path these primary basinal brines have not been detected in the oil zone waters suggesting that the brines entered the reservoir after the main phase of oil migration there are indications that these external fluids were introduced into the reservoir along faults present day aquifer waters are mixtures of waters from different origins and hardly vary at a field scale they are different in composition to the water trapped in the present oil zone one of the oil zone samples is a very dilute brine it is thought to represent a simple mixture of seawater and meteoric water due to oil emplacement this geochemical signature was preserved in the waters trapped within the oil zone another oil zone water shows a very similar chemical signature to the aquifer waters but the chlorine isotopic signature is similar to that of the dilute oil zone water this water is interpreted to represent a palaeo aquifer water that is it was within the aquifer zone in the past but was trapped by subsequent emplacement of more oil these vertical differences can be explained by two features i emergence of the brent group sedimentary rocks in the early cretaceous allowed ingress of meteoric water ii subsequent rapid burial of viking graben rocks caused migration of petroleum and aqueous fluids into the adjacent less deeply buried oseberg field c 2001 elsevier science ltd all rights reserved

mapping diagenetic fluid flow within a reservoir kar dating in the alwyn area uk north sea abstract the alwyn area demonstrates that variation in the timing of diagenesis on the field scale depends on the structure and the proximity to the sources of diagenetic fluids variation in reservoir quality within the structure is the result of facies control on fluid migration illite in alwyn south formed in the middle palaeocene to middle oligocene between 60 and 38 ma at this time the brent group was at depths of between 2400 and 3000 m and reservoir temperatures ranged from 110 to 120 c extensive k ar dating shows that illitization pre oil emplacement proceeded from the deepest reservoir panels along and up structure fluid migration may have occurred along the deep seated alwyn south ninian fault system where the reservoir is lithologically homogeneous illite precipitation proceeded systematically base upwards at rates of 5 5 m ma 1 the duration of illite formation at any one level was between 2 and 8 ma in contrast in heterolithic reservoirs fluid migration occurred initially along high permeability fairways and consequently illite ages do not vary systematically with depth

the principal components statistical method as a complementary approach to geochemical methods in water quality factor identification application to the coastal plain aquifer of israel abstract in general schoeller and piper diagrams as well as other classical techniques facilitate the description of the various geochemical groups of water and help to explain quality changes in an aquifer we propose the use mainly of principal components analysis for identification of relevant groups of water and the factors that bring about a change in their quality the major advantage of this method is its suitability for simultaneous analysis of a great number of variables and observations it is here being applied for the investigation of the dan metropolitan region of israel s coastal plain the variables involved include major ions as well as the physical factors of depth of the well intake filter below sea level distance from the sea and aquifer recharge the primary purpose of this work is to demonstrate the use and reliability of principal components analysis pca as a method complementary to classical approaches for hydrogeochemical research subsidiary purposes involve determination of the major water groups characterizing the coastal plain aquifer identification of some of the principal variables that influence changes in water quality in the aquifer and description of some of the geochemical phenomena contributing to change in aquifer water quality the following results were achieved for the test area 1 1 two major groups of water were identified a low salinity calcium bicarbonate water occurring in the phreatic portion of the coastal plain aquifer and a more saline sodium chloride water characterizing the neighboring cenomanian aquifer and the confined portions of the coastal plain aquifer 2 2 the major water input sources such as rain injection water waste water irrigation etc may vary in their influence upon resulting water quality this water quality is affected by such physical factors as distance from the sea depth of well intake filters below sea level proximity to streams and the lithology of the saturated and unsaturated zones 3 3 the evolutionary alteration in content of specific ions with time can be clearly visualized on the same graph by means of pca the universal applicability of such a statistical approach to other hydrological settings is noted

textures and geochemistry of authigenic albite from miocene sandstones louisiana gulf coast feldspars in louisiana miocene sandstones at depths of 2 400 to 6 000 m 80 to 150 degrees c show progresssive reaction with increasing burial plagioclase undergoes both dissolution and replacement by albite whereas k feldspar undergoes only dissolution reactions do not reach completion by 6 000 m 150 degrees c where a small amount of oligoclase and k feldspar remain within an assemblage of nearly pure authigenic albite dissolution of plagioclase gradually becomes volumetrically important near 3 600 m 100 degrees c and for the most part ends by 5 000 m 130 degrees c albite replacement of plagioclase also occurs primarily between 3 600 and 5 000 m 100 to 130 degrees c but continues to the limit of sample control at 6 000 m 150 degrees c k feldspar dissolution is negligible until 4 800 to 5 600 m 125 to 140 degrees c below 5 600 m k feldspar is nearly absent in the rocks discontinuous zones of blocky to tabular extinction characterize grains of partially albitized plagioclase backscattered electron images of these grains coupled with electron microprobe analyses indicate that these zones represent domains of unreacted calcic plagioclase an sub 10 30 interspersed with domains of nearly pure authigenic albite an sub 0 2 sem images show that some albite domains form from precipitation of small albite overgrowths on nucleation sites within secondary intragranular megascopic pores in thin section the overgrowths are optically clear other highly vacuolized albite domains may develop from direct replacement of calcic domains by dissolution reprecipitation in submicroscopic pores without the formation of megascopic pores here pore fluid may have been trapped between large numbers of subcrystals that grew together with imperfect boundaries operating on both megascopic and submicroscopic scales albite replacement of plagioclase grains can be completed

tracing biogeochemical processes in stream water and groundwater using non linear statistics summary stream water and groundwater solute concentration are subject to a multitude of biogeochemical processes that act at different scales and are often characterized by non linear relationships and feedback loops different multivariate statistical methods were applied to investigate the interplay of different processes the data set from the lehstenbach catchment in south germany comprised 2641 stream water and groundwater samples from 38 different sites in the catchment where 13 different solutes had been determined according to the correlation dimension analysis the number of dominant processes was four the first four components determined via principal component analysis comprised 88 of the total variance whereas the non linear isometric feature mapping explained 92 with the first four components these components were ascribed to prevailing biogeochemical processes and were used to investigate spatial and temporal patterns redox processes and contamination by road salt explained 35 of the variance each another 13 were ascribed to near surface runoff in the acidified topsoil and 9 to the impact of contaminated filter gravel in some of the groundwater wells the redox component exhibited clear seasonal patterns at most stream water and groundwater sampling sites with the most reduced conditions in late summer immediately before the onset of re wetting there was clear evidence that redox processes especially denitrification play an important role even in the oxic aquifer during discharge peaks stream water exhibited higher values of the near surface runoff component however the associated lower values of the redox component pointed to near surface runoff in the riparian wetlands as the predominating runoff generation process rather than to a contribution of upslope soil water a series of major rain storms in fall 1998 altered groundwater and stream water solute concentration for months stream water and groundwater became more oxic and more acidified and showed a higher impact of road salt contamination at some sites overall the spatial and temporal patterns of the predominating components gave a consistent picture and helped considerably to better understand the interplay between biogeochemical and hydrological processes the chosen approach is a promising tool for investigating apparently complex hydrological and biogeochemical systems

a renewed look at calcite cement in marine deltaic sandstones the brent reservoir heather field northern north sea uk wireline and seismic acoustic impedance imaging show that the marine part of the clastic brent group reservoir in the heather field northern north sea contains much calcite cement in the flank parts of the structure the non marine ness formation and crest parts of the structure contain negligible calcite cement this localised calcite cement has led to relatively poor reservoir performance since first oil in 1978 although a new suite of wells has boosted production with plans to keep the field active until 2030 understanding the origin and distribution of calcite cement would help the development of more realistic reservoir models and boost production rates through optimum well location we have thus used a suite of techniques including standard point counting sem eds mineralogy bse microscopy fluid inclusion thermometry and stable isotope analysis to develop new and improved models of calcite distribution calcite seems to have attributes of both early and late diagenetic cement 30 to 40 intergranular volume in calcite cemented beds seems to support pre compactional growth but high temperature fluid inclusions and the presence of primary oil inclusions suggests late growth much calcite may have developed early but it seems to have recrystallized and possibly undergone redistribution at close to maximum burial or had a late growth event calcite cement probably originated as marine derived micrite bioclasts or early marine cement but adopted the isotopic characteristics of high temperature growth as it recrystallized quartz grains have corroded outlines in calcite cemented areas with one sample with 79 calcite cement displaying signs of nearly total replacement of quartz grains by calcite the flank localisation of calcite remains to be explained although it could be due to primary depositional factors early diagenetic loss of calcite from crestal regions or late diagenetic loss of calcite from crestal regions controversially the growth of calcite seems to be associated with quartz dissolution although the geochemical and petrophysical cause of this remains obscure diagenetic loss of quartz from sandstones cannot easily be explained by conventional modelling approaches and yet seems to be an important phenomenon in heather sandstones

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

diagenesis and reservoir quality prediction the modern concept of diagenesis was introduced by walther one hundred years ago to encompass the postdepositional processes that affect sediments from deposition to low grade metamorphism since walther 1893 diagenetic studies have been concerned with an extraordinary variety of chemical and physical interactions each of which has the potential to significantly affect the final composition and texture of a sedimentary rock the issues to which those studies have been applied are equally varied and include 1 improved techniques to predict the producibility of hydrocarbon reservoirs 2 improved techniques to predict subsurface fluid pressures 3 better constraints on the timing of fluid flow events 4 reconstruction of global climate change and 5 evaluation of groundwater contamination commercial applications of diagenetic studies have traditionally focused on the producibility of hydrocarbon reservoirs as such a review of this topic serves as an effective illustration of how basic research has been commercialized and a useful focus for the following dicussion however it is worth noting that decreased industrial research funding coupled with increased government funding for environmental research have recently resulted in a gradual evolution of diagenetic publications away from the producibility of hydrocarbon reservoirs toward the producibility of uncontaminated groundwater and other environmental issues despite the shift in emphasis many of the basic research issues remain unchanged whether one is focused on deep hydrocarbon reservoirs or shallow aquifers

timing and conditions of permian rotliegende sandstone diagenesis southern north sea k ar and oxygen isotopic data illitic clay ranging from pure illite to highly illitic illite smectite i s is the most abundant diagenetic phase in the eolian and sabkha facies of the rotliegende formation of the southern north sea and northeastern netherlands most k ar ages of diagenetic illite and i s in the eolian sandstones are between 100 and 175 ma illite formation is related to two major phases of tectonic activity the jurassic kimmerian orogenic movements and late cretaceous early tertiary inversion movements the burial depth of a sample at the time of diagenetic illite formation can be derived from the k ar age and the appropriate burial history curve illite formed in different localities at different times and at different depths generally diagenetic illite is more abundant in samples more deeply buried at the time of illite formation the correlation between illite formation and tectonic disturbance and the correlation between illite abundance and burial depth may be useful for predicting reservoir properties specifically we would expect reservoir permeability of the rotliegende in the study area to be least impaired by illite growth in those samples less deeply buried from the late jurassic through the late cretaceous calculated dgr18o values of most illite forming fluids lie between 0 and 4 pmil smow indicative of meteoric or marine waters modified by interaction with rock however 18o depleted fluids as low as 5 pmil indicative of a major meteoric component are inferred to have been involved in the formation of illite in the groningen gas field at a time when nearby sections of the rotliegende were eroded and exposed at the surface