permeability prediction from geologic models permeability is a critical parameter for the petroleum geologist by simulating the processes of compaction and cementation in a model porous medium we have gained a new understanding of how permeability is controlled in reservoir sandstones this understanding can be used predictively for simple sandstones the geometry of the model pore system is completely defined so permeability can be calculated directly using a network model for flow the calculation is based on first principles and is physically rigorous in contrast to many previous efforts to predict permeability there are no adjustable parameters in the calculations and no additional measurements or correlations e g capillary pressure data or pore system data from thin sections are required the model derived porosity permeability trend for a compacted or quartz cemented sandstone or a sandstone having a combination of these processes closely matches measurements on fontainebleau sandstone samples whose permeabilities span nearly five orders of magnitude the model also correctly predicts mercury injection measurements of pore throat size distribution for the fontainebleau sandstone pore scale geometric features of the model are found to be spatially correlated and this departure from randomness significantly affects macroscopic properties such as permeability the agreement between predictions and measurements suggests that spatial correlation is inherent in granular porous media and consequently uncorrelated or arbitrarily correlated models of transport in such media are unlikely to be physically representative we also discuss extending the model to predict properties of more complicated rocks

sediment transport resuspension and accumulation rates in the northeastern skagerrak results of watercolumn light transmission measurements in the skagerrak and of 210pb excess and 137cs determinations in boxcore sediments of the eastern skagerrak are combined to study regional sediment transport resuspension and sedimentation patterns and rates in the upper water layer a 20 m thick surface nepheloid layer formed by concentration of particles at the thermocline and pycnocline is best developed in the eastern skagerrak and less well in the southwestern skagerrak off the danish coast bottom nepheloid layers are extensively developed in the southeastern skagerrak and indicate additional sediment input via resuspension in shallow water areas and near bottom sediment transport in suspension following the prevailing current regime locally the transmission in the bnl is reduced by more than 50 compared to clear water transmission values between 85 and 90 210pb and 137cs derived sedimentation rates are highest in the eastern and northeastern skagerrak and least in the central deep basin there is generally a good agreement between 137cs and 210pb data indicating that sediment accumulation on the short 30 year and longer 100 year time scale is of comparable magnitude

patterns rates and possible causes of saltmarsh erosion in the greater thames area uk the estuary dominated coast of the greater thames in england has experienced rapid lateral erosion and internal dissection of saltmarshes this paper provides an overview of saltmarsh development in this area and re examines the role of environmental and human forcing factors it draws on documentary evidence including historical maps survey data and time series data of forcing factors lateral marsh retreat began in the 19th century in the medway and blackwater estuaries followed by other estuaries in the greater thames region at the beginning of the 20th century the outer estuaries and the wider parts of the inner estuaries especially have experienced erosion erosion has been modest in wave sheltered areas e g the colne and the inner crouch in the 1960s and more widely the 1970s a phase of rapid erosion took place with erosion rates of up to ca 16 ha year 1 per site notably along the open coast of dengie and foulness and in the blackwater and thames estuaries at all sites vertical sediment accretion was well able to keep up with sea level rise over the past century evidence indicates that there may have been several causes for the erosion of saltmarshes these are notably land claim and embankment construction increasing the tidal range and current velocities and a continuous rise of especially high and extreme water levels the latest episode of rapid erosion in the 1970s is largely attributed to changes in the wind wave climate for example erosion at wave exposed sites coincided with a peak in high magnitude waves combined with a high incidence of southeasterly waves the study shows that many factors including natural forcing factors and human activities have to be taken into account when explaining saltmarsh development

dispersal concentration and deposition of suspended matter in the north sea suspended matter comes into the north sea from the atlantic ocean from the channel rivers seafloor erosion coastal erosion the atmosphere and from primary production the contribution of the last source is temporarily large when phytoplankton growth is abundant but is soon decomposed mineralized or consumed so that over the year the net effect is small accumulation and deposition of suspended matter occurs predominantly in only a few areas the waddensea the german bight the skagerrak norwegian channel the remainder of the north sea floor consists of pleistocene and early holocene relict deposits and of reworked sands and gravels the mechanisms resulting in accumulation and deposition are discussed there are strong indications that the amount of suspended matter flowing out from the north sea into the norwegian sea along the norwegian coast is only little more or the same as the amount coming in from the north atlantic although its composition may be different

evidence for episodic cementation and diagenetic recording of seismic pumping events north coles levee california u s a abstract bands of calcite and dolomite cements alternating with zones of nearly carbonate free sand occur in the stevens sandston aat north coles levee san joaquin valley california temperatures calculated from o isotopes suggest that the calcite cement bands were emplaced episodically as a result of repeated injections of hot water from deeper in the section burial analysis suggests that these cements precipitated from 7 ma to the present over the temperature range of 45 to 95 c carbon isotope data suggest that the c in the cements is a mixture derived from two sources detrital shell material 13c pdb and co2 liberated from maturing kerogen 13c 24 plots of 13c vs time and depth of crystallization show that the cementation sequence was 1 dolomite cements possibly concretionary precipitated at depths a log of 13c vs calculated depth of crystallization correlates with the stratigraphic column at north coles levee if the correlation is valid the light 13 in each cement sample can be tied to its source a model based on this interpretation suggests that the early light c was derived from maturing kerogen in the kreyenhagen formation eocene as it passed through the oil window between 4 and 5 ma the subsequent passage of younger sediments with less organic material produced correspondingly smaller amounts of light co2 which was reflected in the relatively heavier c isotopes in the later cements it is suggested that the epidsodic injections of hot water carried dissolved gases and minerals principally calcite upward from rocks as deep as 2 3 km below the stevens sandstone and reprecipitated the calcite in more permeable zones in the rock degassing of co2 from rising pore waters likely triggered the precipitation and accounts for the relatively large volumes of cement the sibson model for seismic pumping of pore fluids is considered a likely explanation for the observed cementation

ultrasonic characterization of marine sediments a preliminary study abstract ultrasonic waves 1 mhz and 2 25 mhz were used to identify fluid mud and partially consolidated fine grain sediment beds the preliminary laboratory results indicate that both fine grain sediments and coarse grain sediments display distinguishable properties in terms of acoustic wave speed and attenuation rate this approach can clearly detect the water sediment interface with high resolution it can also distinguish the consolidation status for a fine grain sediment bed although the ultimate goal of measuring the vertical bulk density profile of a sediment bed still depends on more studies the preliminary results are encouraging and indicate a new feasible method for field applications

petrography and reservoir physics ii relating thin section porosity to capillary pressure the association between pore types and throat size porosity in reservoir rocks is configured into a few types of pores whose size and shape are controlled by depositional fabric and postdepositional processes the size shape and abundance of each pore type can be objectively determined from thin section using image analysis and pattern recognition procedures each pore type tends to be associated with a limited range of throat sizes the association between pore type and throat size can be determined using regression procedures linking pore type data obtained from thin section with capillary pressure data to do so a set of samples is required wherein the association between pore type and throat size is fixed but where pore type proportions vary between samples this condition is met by a sample suite representing res rvoir facies from a single core or in many cases from a single field the relationship between pore type and throat size is an effective means to relate reservoirs in terms of the efficiency of the porous microstructure to multiphase flow parameters derived from the relationship can be used to construct accurate physical models that subdivide physical response in terms of the contributions of each pore type

transport of clay minerals and anthropogenic compounds into the german bight and the provenance of fine grained sediments se of helgoland a large mud body 500 km 2 and up to 21 m thick se of helgoland has been investigated using geochemical mineralogical and sedimentological methods the analysis of 27 sediment cores from this area revealed that overlying older fine grained sediments there is a 9cake9 of muddy polluted sediments up to 3 m thick which has accumulated over the last 100 years it is assumed that human activities like dredging and dumping are responsible for this rapid rate of deposition generally it has been estimated that all of this mud has been supplied by rivers entering this part of the north sea but analyses of sediments from the ems weser and the elbe rivers show that under normal conditions the river sediments do not enter the open sea but are trapped in the estuaries it may therefore be concluded that much of the underlying unpolluted sediments of the mud area were derived from elsewhere or were laid down during an earlier period one possibility is that the lower sediments were deposited when the german bight was a sheltered area protected by a barrier of morainic sediments in the north between helgoland and schleswig holstein this barrier must have been breached more than 2000 years ago

recent developments in dating ancient crustal fluid flow several isotopic techniques have recently been developed or substantially improved allowing the accurate determination of the timing of fluid related processes these include k ar rb sr and sm nd dating of diagenetic minerals in clastic rocks u pb dating of carbonates sm nd dating of fluorite and uraninite re os and rb sr dating of sulfides 40ar 39ar and rb sr dating of fluid inclusions and u pb dating of ilmenite magnetite sphene and rutile while many of these techniques require further evaluation some have already resulted in new constraints on a variety of crustal fluid flow models

putting transport into water rock interaction models new quantitative models for water rock interaction are needed that explicitly couple fluid flow and chemical reactions one such model is based on the finite difference method and is applicable to both one and two dimensional reactive flow in applying the model we find that ignoring the coupling between hydrodynamic dispersion and chemical reactions leads to incorrect predictions of the rates of reactions along fluid flow paths and in cases of which reactions will occur

sediment transport pathways in the skagerrak and kattegat as indicated by sediment chernobyl radioactivity and heavy metal concentrations abstract the concentration of the heavy metals hg cu pb and zn as well as chernobyl cs 137 radioactivity was determined in surface sediments from the skagerrak and kattegat sediment samples from adjacent swedish fjords were analysed as well the sampling was carried out between spring 1988 and autumn 1990 highest concentrations of heavy metals and largest cs 137 activities occur in fine grained organic rich 62 2 c org sediment from swedish inshore waters and locally also in the kattegat regional trends of relative enrichment of the various heavy metals and chernobyl cs 137 with respect to organic carbon was used to determine transport patterns of material enriched with respective elements it can be proven that deposition of fine grained material transported from the southern north sea by the jutland current is the main mechanism determining the regional accumulation of lead and chernobyl derived cs 137 this mechanism principally controlls the areal distribution of mercury enriched material as well but it is regionally of less importance as far as the accumulation of cu and zn concerns the chernobyl cs 137 data show that a large proportion of the fine grained material transported by the jutland current originally is river borne suspension load from central western europe the areal enrichment pattern of copper and on a smaller scale those of mercury and zinc too points to westerly directed suspension load transport across the kattegat material enriched with these elements apparently is being discharged from swedish industrial and urban centers in particular the goteborg area

subcritical creep compaction of quartz sand at diagenetic conditions effects of water and grain size 1 compactional creep was measured for aggregates of quartz sand that were subjected to an effective pressure of 34 5 mpa and temperature of 150 c for times up to 6 months the effects of grain size and water on creep were determined by loading quartz aggregates of 255 60 130 18 and 35 12 m grain size under nominally dry water vapor static liquid water and flowing water conditions all aggregates displayed transient decelerating creep volume strain rates as low as 2 10 10 s 1 were achieved volume strain rate increases systematically with decrease in grain size and increase in exposure to water at comparable strain or time despite the fact that the effective pressure applied was far less than the critical pressures for short term cataclastic compaction of the quartz aggregates grain scale microstructures indicate that the underlying mechanism of creep is crack growth creep rates are explained by subcritical crack growth as governed by water silicate reactions at crack tips controlled by access of water at dry and vapor conditions and solute chemistry under static and flowing liquid water conditions extrapolation of the experimental results to natural compaction of quartz sand during burial and diagenesis over times of tens of millions of years indicates that porosity loss through subcritical cracking and grain rearrangement of medium grained porous wet quartz sands can reach 10

lead in sediments and suspended particulate matter of the german bight natural versus anthropogenic origin sediments 568 and suspended particulate matter spm 302 samples of the southern german bight and the adjacent tidal flat areas were analysed for selected major elements al fe k trace metals mn pb and 206pb 207pb ratios using xrf icp oes icp ms for selected samples a leaching procedure with 1 m hcl was used to estimate the pb fraction associated with labile phases e g mn fe oxihydroxide coatings in contrast to the resistant mineral matrix enrichment factors versus average shale efs reveal elevated pb contents for all investigated sediments and spm in the following order holocene tidal flat sediments htf human unaffected 5 mg l 1 offhore spm 5 mg l 1 besides pollution rtf contain elevated amounts of natural pb rich materials k feldspars and heavy minerals due to a man made high energy environment dike building in comparison to htf 206pb 207pb ratios of rtf 1 192 0 019 are similar to the local geogenic background determined from htf 1 207 0 008 in contrast pb isotope ratios of nearshore spm 1 172 0 007 and offshore spm 1 166 0 012 show a distinct shift towards the anthropogenic atmospheric signal of 1 11 1 14 this difference between rtf and spm supports the assumption of low deposition rates of fine material in the intertidal systems as the 206pb 207pb ratios of spm do not reach the pure anthropogenic signal the adsorbed pb fraction was examined leaching however the leachates also contained large amounts of geogenic pb spm 40 recent sediments 60 the authors assume that the uptake of natural pb occurs in nearshore waters presumably in the turbid intertidal systems possible sources for dissolved pb are mobilisation during weathering geogenic signal and dissolution of oxihydroxide coatings with subsequent release from porewaters and unspecific riverine input comparatively small parts of spm leave the coastal water mass and reach the open north sea this process therefore leads to a decontamination of the tidal flat sediments due to more pronounced atmospheric input the offshore spm becomes enriched in anthropogenic pb as indicated by decreasing 206pb 207pb ratios with increasing distance from the coast

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

indications of long term episodic suspension transport of sand across the norfolk banks north sea abstract predicted 50 year maximum storm surge currents and the currents resulting from their combination with strong tidal currents are veered and amplified as they pass over the norfolk banks they therefore drive sand off the crest of well bank in a wide arc of directions between north east and south southwest this arc corresponds to that seen in the cross bedding dip directions known from the lee slope of well bank and to the northeasterly growth of the well bank the strength of these currents is such that some sand may even pass from bank to bank in the series this is facilitated by the proximity of the upper part of the sand banks to the sea surface such that their crests act as stepping stones where sand suspensions originate during storm surge episodes it is postulated that this new sand transport path oblique to the path of the peak tidal currents probably commences at the coast of east anglia and provides a route for transporting sand for up to about 100 km seaward

geothermometry from brine analyses lessons from the gulf coast u s a abstract geothermometers based on mineral water equilibria are of limited use at temperatures characteristic of hydrocarbon production from sedimentary basins metastable detrital phases such as alkali feldspars and plagioclase persist to temperatures characteristic of the deepest wells sasmpled approaching 200 c and equilibria involving stable phases such as albite illite and quartz are not commonly achieved empirical relations such as that between mg and li are more successful in predicting subsurface formation water temperatures than are equilibrium relations involving quartz carbonates or feldspars the concentrations of most components in saline formation waters seem to be governed by kinetic rather than by equilibrium considerations

numerical models of fluid flow in carbonate platforms implications for dolomitization abstract the role of fluid flow in dolomitization is evaluated through hydrologic simulations designed to capture and test numerically the essential elements of some common dolomitization models a two dimensional finite difference code bethke 1985 was used to simulate single phase flow of variable salinity fluids driven by compaction convection and topography in carbonate platforms and adjacent basinal sediments a simple hydrostratigraphic model consisting of a platform reef complex surrounded by basin filling shale was constructed to examine the magnitude and direction of compaction flow during the initial phase of subsidence most pore fluids in the shale migrate to the surface at velocities less than 0 1 mm yr and basin derived fluids are focused into the reef from only a limited distance of 10 km relatively porous and permeable basinal limestones and sandstones may act as conduits for updip fluid flow into the platform reef complex magnesium mass balance constraints coupled with hydrologic simulations suggest that burial compaction is not an efficient mechanism for regional dolomitization two layer hydrostratigraphic models involving variable salinity fluids show that evaporation of seawater results in downward and seaward fluid migration the magnitude of flow is a function of fluid density even fluids at salinities marginally elevated above normal seawater values are capable of penetration several hundreds of meters into the underlying sediments albeit at rates of less than 1 cm yr paleohydrologic simulations coupled with thermodynamic arguments support that reflux of surface derived waters evaporated past gypsum saturation is a viable dolomitization mechanism calculations utilizing rates of mg mass flux show that dolomitization of large carbonate platforms 10 km3 by fluids below halite saturation requires millions of years of reflux driven flow p thermal convection of seawater through carbonate platforms has been simulated by varying the basal heat flow the thermal pump produces a convection cell in which lateral and vertical flow achieve rates of 75 0 and 13 0 mm yr respectively the potential for dolomitization is high because a large reservoir of mg seawater is tapped elevated temperatures drive fluids further into the stability field for dolomite and kinetic inhibition of dolomite nucleation and growth is decreased simulations of high frequency glacio eustatic sea level oscillations suggest that a dynamic hydrologic regime may be established a sea level fall of 2 m lasting for 10 000 yr can drive lateral flow at rates up to 0 15 m yr dolomitization is possible because marine waters migrate through the carbonate platform in response to gradients of hydraulic head created by eustatic sea level fluctuations

illite smectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the texas gulf coast paleocene eocene abstract mudrock samples from a well penetrating eocene and paleocene strata 1235 4455 m of the texas gulf coast have been studied to assess geochemical redistribution in mudrocks during burial diagenesis and the mechanism by which detrital smectitic illite smectite i s is transformed into diagenetic illitic i s idealized smectite and illite end member compositions for interstratified i s are estimated to be k0 00x 10 56mg0 39fe0 57al1 13si3 90o10 oh 2 and k0 53x 10 18mg0 17fe0 16al2 28 si3 40o10 oh 2 respectively there is no obvious difference in the amount of k addition to i s in random and ordered interstratified i s aluminum substitution into both tetrahedral and octahedral sites suggests that the smectite to illite reaction is a complete dissolution precipitation reaction rather than a solid state k 1 al 3 for si 4 substitution reaction depth relat d mineral trends and mass balance calculations suggest that illitic i s may form from kaolinite and possibly illite or mica as well as from smectitic i s although most whole rock element abundances are invariant over the sampled depth interval k2o content increases from ca 2 0 weight percent at 1500 m of burial to ca 3 8 weight percent at 4000 m some of this increase is probably caused by import of k into the shale via fluids derived from interbedded sandstones implying that gulf coast mudrocks behave as open chemical systems during burial diagenesis if this is the case a minimum of 103 pore volumes of fluid must have passed through the most k enriched shales to have introduced the added potassium provided that the k content of present day formation water is representative of that of ancient formation water

differential mobility of elements in burial diagenesis of siliciclastic rocks abstract mass balance calculations involving smectite rich siliciclastic mudstones sampled from boreholes show that significant k2o addition and cao loss correlate strongly with the progress of illitization of smectite and with the dissolution of calcite the calculations are based on the immobility of al2o3 and require a chemically homogeneous sediment at deposition such that the shallower samples represent protolith compositions of deeper samples constancy of ratios of tio2 al2o3 are used to 1 establish that both tio2 and al2o3 have similarly low differential mobilities and 2 identify stratigraphic sections available in the literature that are sufficiently homogeneous to allow the calcu ations our calculations show that most major oxide components are immobile at the 5 limit of resolution of the calculations exceptions are k2o which is almost universally added to mudstones and cao which is commonly removed from mudstones during diagenesis the addition of up to 50 k2o correlates with percent illite in mixed layer illite smectite indicating that illitization of smectite involves fixation of k2o from diagenetic fluids removal of 40 cao from some sections correlates with decreasing calcite mode and is attributed to dissolution of calcite together these reactions lead to a 10 15 loss of mineral volume a volume loss that must be added to porosity loss when modeling compaction illitization and carbonate dissolution may be linked because k fixation during illitization decreases the ph of the local fluid a reverse weathering reaction which enhances carbonate dissolution addition of up to 20 na2o in some rocks may be caused by albitization of plagioclase but na2o addition is not pervasive the apparent lack of mass transfer associated with dissolution of kaolinite and feldspars or precipitation of chlorite implies that these reactions are balanced by other reactions such that their oxide components in mudstones are conserved limited data from sandstone mudstone pairs are consistent with transfer of k2o from sandstones to mudstone via rising diagenetic fluids except for cao and the alkalis diagenesis of siliciclastic mudstones appears isochemical 5 relative for most major oxide components the much larger changes recorded in some reservoir sandstones are clearly exceptions and may not reflect changes occurring at the scale of kilometers

evidence for vertical movement of fluids gulf coast sedimentary basin sediments between 10 and 15 km thick have prograded approximately 400 km into the small oceanic basin which formed along the southern margin of north america subsequent to early mesozoic rifting redistribution of mass during burial diagenesis and the chemistry of present day formation waters in jurassic through pleistocene formations document local kilometer scale and in cases basin scale vertical fluid transport within the sedimentary wedge fluid flow is in part driven by porosity reduction which takes place by both physical and chemical processes during burial vertical and horizontal transport also occurs as the result of thermally and salinity generated buoyancy effects input of fluids into the sedimentary wedge from the underlying crust is predicted but not documented

preservation of primary porosity in deeply buried sandstones a new play concept from the cretaceous tuscaloosa sandstone of louisiana abstract

sem imaging of bacteria and nannobacteria in carbonate sediments and rocks abstract bacteria and nannobacteria 0 1 m dwarfed forms are abundant in both non etched and gently etched travertines deposited by hot springs in viterbo italy they are also plainly visible in bahama ooids and hardgrounds and in great salt lake aragonite ooids and cements in addition nannobacteria are found in paleozoic and mesozoic rocks because of their extraordinary abundance in some specimens they probably play a prominent role in catalyzing the precipitation of carbonate materials

late holocene coastal hydrographic and climate changes in the eastern north sea we present a high resolution palaeoenvironmental reconstruction covering the late holocene from the skagerrak and other sites in the north sea area the data which are based on the analyses of marine sediment cores reveal a marked environmental shift that took place between ad 700 and ad 1100 with the most pronounced changes occurring at ad 900 both surface and bottom waters in the skagerrak were subject to major circulation and productivity changes at this time due to an enhanced advection of atlantic waters to the north sea marking the beginning of the mediaeval warm period mwp the observed increase in bottom current strength is especially remarkable as there is hardly any comparable signal in the older part of the record going back to 1000 bc at the transition to the little ice age lia the bottom current strength remains at a high level now probably forced by atmospheric circulation thus despite opposite temperature forcing these two consecutive climate scenarios are apparently able to generate distinctly stronger bottom currents in the skagerrak than observed in the preceding 2000 years and demonstrate the significance of climatic forcing in shaping the marine environment indeed both the mwp and the lia are reported as strong climatic signals in northwest europe being the warmest except the late twentieth century and coldest periods respectively during at least the last 2000 years

quantification of quartz cements using combined sem cl and image analysis quartz cement content is often difficult to quantify using conventional petrography developments in equipment and technique reported here show how excellent high contrast images of quartz cements can be produced using combined scanning electron microscopy and cathodoluminescence sem cl in most cases quartz cement can be differentiated readily from detrital quartz image analysis allows the percentage of authigenic quartz to be quantified advantages of this method are 1 it can be readily automated 2 backscatter electron bse and cl images can be compared directly and 3 images can be improved through acquiring and integrating multiple images comparison of results from conventional optical petrography with those from sem cl suggests that the percentages of quartz cement determined by conventional methods are overestimated where quartz cement is present in high proportions and are underestimated where quartz cement is less abundant by adopting a statistically rigorous approach to image collection the abundance of quartz cement can be determined by sem cl with a known accuracy

carboxylic acid anions in formation waters san joaquin basin and louisiana gulf coast u s a implications for clastic diagenesis reply to discussion by p d lundegard and l s land abstract macgowan and s urdam 1990a suggested some modifications to the model of lundegard and land 1989 to make it more geologically and geochemically reasonable the predictive power of such a geochemical model is wholly dependent on the species modeled and the constants used any model that excludes important species or important thermodynamic data or one that couples certain reactions in an unrealistic way may produce results which are not geologically or geochemically reasonable w k harrison pers commun 1988 y k kharaka pers commun 1991 we have long recognized that under early burial diagenetic conditions aluminosilicate hydrolysis generally controls formation water ph surdam and eugster 1976 mariner and surdam 1970 taylor and surdam 1981 and that during intermediate burial either aqueous co2 or caa species in the absence of aqueous s species or other weak conjugate acid base pairs will dominate formation water alkalinity and control ph surdam et al 1989c we reassert that the model of lundegard and land 1989 does not take into account the relative importance of pco2 and of concentrations of both ca2 and caa and their relative organic metal complexes to carbonate mineral stability in sandstones in the zone of intermediate burial clastic diagenesis cf the models of surdam et al 1984 and surdam and crossey 1985 the usefulness of such models is predicted on the completeness of the model and the use of the best most accurate thermodynamic data also geologically realistic concentrations of critical species are required for reasonable modeling to be done although their model is vigorously defended in the discussion of lundegard and land 1989 lundegard and land 1993 we continue to disagree that their analysis of their model conditions are either geologically or geochemically satisfying we agree with the fundamental approach and philosophy of lundegard and land 1989 lundegard and land 1993 it is of the utmost importance to determine from experimental geochemical petrographic and geological data what the controls on ph and alkalinity in formation waters are as well as the exact thermodynamic speciation of aqueous moieties and the stability of detrital and authigenic minerals lundegard and land 1993 raise an additional point about caa reaction with carbonate minerals in shales although fisher and lewan 1989 lewan 1989 and macgowan and surdam 1990b have demonstrated that caa generated in shale likely migrate in the oil phase along incipient shale microfractures to the sandstone reservoir and thus are likely to not react much with the shale finally we agree with lundegard and land that these areas require much additional experimental and field analysis and petrographic study

physical and chemical controls on the composition of waters in sedimentary basins abstract the composition of subsurface waters is determined not only by diagenetic reactions but also by physical processes of fluid convection and hydrodynamic dispersion the south west louisiana gulf coast provides an instructive field example of the net effects of diagenetic reaction and solute transport on pore water compositions in a regional siliciclastic sequence most formation waters here have compositions totally unlike the compositions of the connate meteoric and marine fluids that were buried with their host sediments at the time of deposition dissolved chloride has been generated by the subsurface dissolution of salt domes and has been pervasively transported by fluid flow throughout most of the upper 3 km of the sedimentary section the simultaneous systematic variation in dissolved na k mg ca and alkalinity with chloride in these waters supports the hypothesis that metastable thermodynamic buffering by silicate carbonate mineral assemblages is a first order control on fluid compositions the chemical potential of chloride or alternatively total anionic charge appears to be a master variable which ranks in importance with such other variables as pressure and temperature in driving diagenesis in this region this variable is controlled largely by physical processes of advection and dispersion in the upper 3 km of the section and by dehydration reactions in deeper mudstone dominated sediments where the composition of the fluid is largely rock buffered the ultimate origin of the fluid and its pathway of chemical evolution may be obscured at least in terms of major solute composition non buffered components such as cl and br or isotopic compositions are more likely to retain information on original end member fluid compositions and reaction pathways

salinity variations in north sea formation waters implications for large scale fluid movements abstract sodium chloride is the major salt dissolved in formation waters even at a considerable distance away from any evaporites chloride anions are not consumed to any significant extent by diagenetic reactions outside the salt deposits chloride ions are therefore preserved in the pore waters and the distribution of pore water salinities in sedimentary basins may help to constrain pore water flow and the transport of dissolved species of silica and carbonate minerals in the north sea basin and haltenbanken mid norway salinity gradients can be inferred from formation water samples measurements of pore water salinity from wireline logs are less accurate than formation water samples but have the advantage that a much larger database can be obtained data from 150 wells in the north sea and haltenbanken show that the salinities measured from well logs range from 20000 to 300000 ppm in the southern part of the norwegian north sea and haltenbanken there is a clear trend towards higher salinities with greater burial depth there is a pronounced salinity increase in the 500 m of section closest to the evaporites in the northern north sea where underlying evaporites are not known there is only a slight increase in the measured salinities with depth there are however no measurements of high salinity pore water 100000 ppm at depths shallower than 2200 m this pattern of salinity distribution in the pore water precludes large scale convection of the pore water during diagenesis it also indicates limited compaction driven flow log derived salinity data from the north sea suggest that the pore waters in upper jurassic and tertiary reservoirs are generally more saline than those of middle and lower jurassic reservoirs at the same depth the constraints on pore water flow that can be deduced from salinity variations puts severe limitations on the mechanisms of transport of dissolved solids in sedimentary basins and thus also on aspects of diagenetic models

evidence for resetting of fluid inclusion temperatures from quartz cements in oilfields abstract quartz is a major pore occluding cement in north sea oilfield reservoir sandstones fluid inclusions yield homogenization temperatures t h which have been thought to represent growth temperatures for the host quartz however early diagenetic inclusions from the detrital grain overgrowth boundary yield excessively high palaeotemperatures which do not match the growth temperatures calculated from the depth of cementation the temperatures of these inclusions increase with present day depth of burial and approach present day reservoir temperatures this suggests that the inclusions have begun to reset towards present day pressure temperature conditions the t h of the inclusions hence represent burial temperatures rather than growth temperatures for the quartz experimental studies indicate that the amount of resetting undergone by an inclusion varies depending on its size shape and fluid composition a relationship between inclusion t h and these variables has been observed in inclusions from deeply buried quartz cements in the north sea

rare earth element fractionation by selective sediment dispersal in surface sediments the skagerrak abstract neutron activation analysis was carried out to study the abundance distribution and fractionation of ree rare earth elements in surface sediment samples of 28 box cores collected along transects across the skagerrak results show that the clays have 3 5 times higher contents of k sc cr fe co rb k cs ta and th lree la and ce and mree sm eu and tb compared to sands and silts the hree yb and lu are only up to two times enriched in the clays and silts compared with the values measured in sands shale normalized ree patterns vary from flat for the sands towards enrichment 1 3 1 5 times of lree relative to hree in the clays this is consistent with published ree data for oceanic and riverine sediments and evidences fractionation hf and zr are up to thee times enriched in sands and silts compared to clays samples with highest contents of hf show hree enrichment in shale normalized ree patterns simple model calculations show that on the average zircon grains in skagerrak silts and sands contain 800 1200 ppm yb assuming that this value is also valid for the zircon grains in the shales this indicates that lower hree contents observed in atlantic ocean shelf and oceanic sediments compared to shales cannot be related to a greater abundance of zircon in shales

bioerosion and carbonate mud production on high latitude shelves low latitude carbonate muds often are composed either of entire units of skeletons e g algal muds or of precipitates whereas high latitude carbonate muds are bioerosional or result from maceration bioerosion at high latitudes is most intense in the photic zone particularly down to 25 m depth shelly substrata may be crushed bitten drilled bored or scraped clionid sponges endolithic algae acmaeid gastropods and regular echinoids are the most significant agents clionids produce distinctive facetted carbonate silt chips when boring which have been described from both high and low latitudes faecal pellets break down to yield mud sized carbonate particles that are more irregular than those produced by maceration exhumed infaunal bivalves are often preferred to epifaunal organisms as substrata bioerosion occurs very rapidly shells may be totally infested with boring algae in three months a moth eaten appearance therefore does not denote a relict grain reliable rates of fine sediment production are not yet available the mud fraction of northwest european shelf sediment generally contains 10 20 caco3 though an inshore and offshore belt with higher values may be identified some holocene supratidal mud flats exceed 50 caco3 much of the shelf represents a modern day equivalent of the calcareous shale facies common in the geological record instances of synsedimentary cementation are not uncommon particularly in association with heavily burrowed muds

holocene sedimentation in the skagerrak interpreted from chirp sonar and core data high resolution chirp sonar profiling in the northeastern skagerrak shows acoustically stratified sediments draping a rough surfaced substratum a 32 metre long sediment core retrieved from the survey area encompasses the entire holocene and latest pleistocene the uppermost seismo acoustic units in the chirp profiles represent holocene marine sediments the lowermost unit is interpreted as ice proximal glacial marine sediments rapidly deposited during the last deglaciation the end of ice proximal sedimentation is marked by a strong reflector interpreted to have been formed during latest pleistocene time as a consequence of rapid ice retreat and drastically lowered sedimentation rate the subsequent distal glacial marine sediments were deposited with initially high sedimentation rates caused by an isostatic rebound associated sea level fall based on correlation between the core and the chirp sonar profiles using measured sediment physical properties and ams 14 c dating we propose a revised position for the pleistocene holocene boundary in the seismo acoustic stratigraphy of the investigated area copyright 2005 john wiley sons ltd

material flux and porosity changes during sediment diagenesis understanding the changes in porosity that occur during burial of sediments is of importance both in modelling fluid flow in sedimentary basins and for prediction of petroleum reservoir quality when deposited up to 50 of the volume of sands is intergranular pore space porosity generally decreases with depth because of compaction and precipitation of diagenetic minerals but some sandstones retain an anomalously high porosity whether this is due to dissolution and removal of material or to its local redistribution is not clear we originally intended to quantify losses of major chemical components during burial of a complete sedimentary unit we have investigated ten reservoir sandstones of permian to tertiary age from oilfields worldwide these show consistent results and here we present detailed data from four of them we found that the silica content of these sandstones actually increased by 220 to 350 kg m 3 following compaction no statistically significant changes were observed for aluminium potassium or sodium the flux of silica cannot be modelled satisfactorily using currently accepted values of permeability silica solubility and flow rates posing a challenge for existing models of fluid flow and ore emplacement

physical and acoustic properties of shelf sediments the south sea of korea abstract laboratory determinations of compressional wave velocity porosity density and sediment texture have been conducted on samples collected from the inner shelf holocene mud sediment in the south sea and southern part of the east sea of korea thirty five piston core samples were used for the measurement mean grain size decreases seaward reflecting the distance from source velocity decreases with increasing water depth porosity and clay content increase gradually with deepening bathymetry on the inner shelf the study area can be divided into three different areas namhae pusan and ulsan by sediment source texture and physical properties velocity increases from west to east namhae pusan ulsan porosity has a pattern that is inverse to that of velocity sediment texture mean grain size and clay content is a very important factor to determine physical and acoustic properties of sediment the geoacoustic properties of the three study areas generally coincide well with that of hamilton s 1970 north pacific continental terrace sediment ulsan area is a little different from the hamilton s model which has a relatively higher velocity at the same textural range the difference is believed to be related to the lowest clay content in ulsan area

40ar39ar dating of very fine grained samples an encapsulated vial procedure to overcome the problem of 39ar recoil loss 40ar39ar dating of very fine grained materials is compromised by the loss of 39ar by recoil and also sometimes other ar isotopes during irradiation an encapsulated sample 40ar39ar procedure is described which overcomes this problem and which produces a k ar equivalent date measurements indicate that the procedure yields reliable and reproducible results it offers the potential for dating small amounts of material of many types for example very fine grained authigenic sheet silicates from rock cores which may suffer ar loss during irradiation

benthic organic matter supply and metabolism at depositional and non depositional areas in the north sea in 1994 four stations in the southern north sea the german bight and the skagerrak were visited with the aim to get insight into spatial and temporal variation in the supply of fresh organic matter to the benthos and its subsequent metabolic reaction stations were chosen on the basis of differences in sediment type depth and depositional regime sediments were sampled sliced and analysed for phytopigments fatty acids and nucleic acids on deck sediment core incubations provided the sediment oxygen demand sod fatty acids as well as chlorophyll a indicated a seasonal variation of phytodetritus sedimentation with highest values in may reflecting the spring bloom sedimentation the two stations in the depositional areas german bight and skagerrak had the highest input of algal detritus into the sediment these stations also contained higher concentrations of bacterial and long chain fatty acids probably related to the relatively higher proportion of fine particles in the sediment and to the input of terrestrial organic matter and or refractory marine organic matter respectively the metabolic indices viz the sod and the rna concentrations reflected the benthic metabolic reaction to the labile organic matter supply the three stations in the southern and south eastern north sea had a sod congruent with the phytodetrital inventories with highest sod measured at the german bight surprisingly the skagerrak sediment did not have a high oxygen demand although it contained the highest inventory of labile organic matter and high rna concentrations the fluxes of carbon to the sediment at station sk calculated from the chlorophyll a inventory were about four times higher than the sediment oxygen demand we hypothesise two mechanisms for this discrepancy one is the decreased mineralisation rate of labile organic matter due to sorption to clayey sediment particles the other is advection of labile organic matter from the shallower coastal waters of northern denmark to and subsequent removal from the deeper skagerrak sediments by periodically occurring high near bottom water currents

compaction of lithic sands experimental results and applications compaction experiments showed that the degree of physical compaction of a lithic sand is related to the amount and type of lithic material present these experiments produced a product that is texturally indistinguishable from natural lithic sandstones generally sedimentary lithic fragments are more ductile than metamorphic lithic fragments volcanic lithic fragments when altered to phyllosilicate minerals by weathering or diagenesis are extremely ductile mineralogy and intragranular microporosity influence ductility ratios based on weight percent of soft ductile minerals clay and micas to hard brittle minerals quartz and feldspar within the lithic fragments provides a good indication of ductility a ratio greater than 1 indicates the lithic material will be h ghly ductile microporous lithic fragments are more ductile than dense lithic fragments physical compaction models were developed based on experimental data from well sorted lithic sandstones for which the percentage of preserved porosities were functions of the effective stress the type of lithic material and the quantity of lithic material these models are derived from laboratory sandstones with at least 25 lithic grains in the following categories moderately ductile metamorphic lithic fragments highly ductile shale lithic fragments and extremely ductile altered volcanic lithic fragments application of the compaction models of the experimental compaction techniques to modern subsurface sand samples or outcrop equivalents can provide valuable estimates of the preserved porosities i e reservoir potential for lithic sandstones that may be exploratory objecti es in frontier basins examples using the models and experimental compaction techniques for exploration are given experimental data showed that precompaction cement and overpressure may be beneficial to porosity preservation an early formed partial cement can retard compaction and preserve porosity by stabilizing the sand pack a later cement does not have this beneficial effect overpressure when developed early retards compaction by reducing the effective stress of course a late developing overpressure is not effective at preserving porosity because compaction is an irreversible process

experimental simulation of clay precipitation within reservoir sandstones 1 techniques and examples experiments have been performed in which cores of clay free sand and clay forming mineral phases placed beneath the cores have been treated hydrothermally with various solutions using experimental conditions in the range 150 500 degrees c and 0 75 1 0 kbar 75 100 mpa illite illite smectite kaolinite 7 a chlorite and trioctahedral smectite have been precipitated in the sand pore space with morphologies similar to those of authigenic clay minerals in sandstone petroleum reservoirs this study demonstrates that the mechanism rate of clay precipitation and controls on clay morphology and mineral stability can all be determined by experimental studies these results have direct application to the interpretation of clay mineral paragenesis in reservoir sandstones the study demonstrates 1 authigenic clays in sandstones are direct precipitates from solution and do not require a preexisting clay coating 2 grain surface effects such as abrasion and detrital clay coatings do promote nucleation of clays and 3 clay morphology is controlled by precipitation mechanism specifically the formation of metastable clays which are subsequently modified the synthesis of authigenic type illite and illite smectite is by direct precipitation from solution following dissolution of mineral phases such as detrital illite or feldspar both platy and fibrous illite morphotypes are synthesized platy illites are more readily formed the compositions of synthetic illite and illite smectite clays are controlled by temperature and a sub k super a sub h super fluid composition ratio platy illites show subtle changes in morphology with compositional variation illite smectite and illite with low k content forms a crenulated boxwork texture while more k rich illites approaching muscovite composition form larger discrete plates no correlation between illite composition and fibrous formation is recognized kaolinite and chlorites were also precipitated in experiments vermiform kaolinite appears to be a direct precipitate kaolinite morphology however is controlled by precipitation mechanism isolated discrete kaolinite plates are associated with early formation of a mixed layered smectitic phase chlorite synthesis proceeds via the formation of metastable 7 a chlorite and expandable phases which have previously been recognized as possible precursors to 14 a chlorites in reservoir sandstones

kinetics of the smectite to illite transformation in the denver basin clay mineral k ar data and mathematical model results clay mineral data potassium argon k ar ages and mathematical model simulations indicate the important kinetic factors controlling the smectite to illite transformation illitization in upper cretaceous mowry and niobrara core and outcrop bentonites from the denver basin the data also provide additional information on the thermal history of the basin data show that both the percentages of illite layers and the k ar ages of mixed layer illite smectite i s increase with increasing depths of burial and this trend was simulated mathematically employing a very simplified burial model with a fifth order overall kinetic expression for the formation of illite for most of the basin the data is compatible with i s having been formed in response to increased temperature from progressive burial because the oldest k ar ages of i s are from the deepest buried i s along the basin axis in addition these data qualitatively suggest that i s even as small as 0 1 micrometer retains radiogenic argon under denver basin burial temperatures i s separated from drill core bentonites from the wattenberg gas field astride the basin axis and known to have been subjected to an anomalously high temperature history are the most illitic and among the oldest measured from core in this study the ages of illitization about 60 ma are coincident with the laramide time of structural development of the basin and uplift of the front range the reconstructions of the burial history and the modelling of the timing and extent of illitization suggest that the area of the wattenberg gas field was an upper cretaceous depocenter an area of relatively deeper burial along the basin axis however the data can not rule out the possibility that these sediments at and near the wattenberg gas field were heated not only by burial but also by i neous and related hydrothermal activity associated with the colorado mineral belt during the laramide uplift 70 45 ma of the front range likewise the anomalously illitic i s in outcrop bentonites along the front range adjacent to the wattenberg gas field may have been formed in response either to deep burial and or by laramide age secondary heating superimposed on burial

northern north sea benthic ostracoda modern distribution and palaeoenvironmental significance the ostracod faunas of the northern north sea exhibit a depth related distribution that is controlled by substrate and hydrographic conditions four biofacies are defined on the basis of total live dead counts sandy substrates shallower than 110 m are characterized by robertsonites tuberculatus and cytheropteron latissimum these species are replaced by acanthocythereis dunelmensis and palmenella limicola on sandy muds in the fladen ground 110 150 m muellerina abyssicola dominates from 175 250 m in the norwegian channel giving way to krithe praetexta at greater depths on soft bottom substrates the results are compared with data from the skagerrak and the western european shelf area temperature ranges for the commonest taxa show that north sea faunas contain endemic western european species temperature range on average 2 20 c some southern species 5 26 c and taxa that range to high latitudes 2 to 14 c southern species are confined to the southern and western margins of the north sea

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

organic facies of surface sediments in the north sea abstract surface sediments of the north sea were investigated by organic geochemical and sedimentological methods including organic carbon and nitrogen determination rock eval pyrolysis protokerogen microscopy gas chromatography of aliphatic hydrocarbons grain size fractionation and clay mineral analysis the total organic matter was found to be a mixture of mostly reworked or oxidized marine and terrestrial constituents with a generally strong influence of the latter throughout the platform locally improved hydrodynamic conditions yielded an incorporation of more freshly produced marine organic matter differences in water current velocities and turbulence also appeared to determine whether this supplement is incorporated into the fine or coarse fractions of the sediments a general constancy of the organic carbon contents within the individual size fractions was recognized independently of sediment type and age i e in recent and pre recent deposits this suggested that the latter have been effectively overprinted by the shelf processes operating at present or that these processes have been of comparable intensity since about the beginning of the holocene transgression the major factors controlling the spatial variations in the organic facies are the sediment grain size and the present day hydraulic regime

carbon helium neon and argon isotopes in a po basin northern italy natural gas field the abundance and isotopic composition of he ne and ar have been determined in 19 gas samples taken from the dosso degli angeli hydrocarbon gas field between 3000 and 3750 m depth in the po basin carbon isotope ratios in these samples vary between 13c ch4 67 5 and 73 4 indicating a predominantly biogenic source for the methane c2 c2h6 c3h8 concentrations increase systematically from 0 04 to 0 17 by volume with depth suggesting that there is a small additional thermogenic or diagenetic hydrocarbon contribution to these gases ch4 36ar ratios in the dosso degli angeli gases are in the range predicted for a groundwater which has derived its rare gas by equilibration with the atmosphere and is ch4 saturated under reservoir conditions there is also up to 10 excess of ch4 which is not associated with the groundwater these results are consistent with 90 of the ch4 in the dosso degli angeli reservoir originating from ch4 saturated groundwater measured 20ne 36ar ratios depart from the predicted ratio in air equilibrated groundwater this fractionation is consistent with water gas phase equilibration under reservoir conditions and is similar to that observed in gas reservoirs in the vienna and pannonian basins austria and hungary respectively the he in the dosso degli angeli gases is almost entirely radiogenic and derived from the crust the concentration of 4he in the reservoir gases requires that he is mostly sourced from outside the reservoir volume radiogenic 4he 40ar ratios input into the reservoir gas must be much higher than averaged crustal production ratios as no shift from the atmospheric 40ar 36ar ratio is observed preferential release of 4he over 40ar from the sites of their production is the preferred explanation this observation contrasts with previous studies from active extensional basins in which radiogenic 4he and 40ar appear to have been stored released and transported at near production ratio on a regional scale the most obvious difference between these studies is the comparatively low thermal gradient of the po basin

recent sediment accumulation organic carbon burial and transport in the northeastern north sea abstract organic carbon and nitrogen and sedimentation rate determinations were made of boxcores from the norwegian channel north sea the geographical distribution of recent sedimentation areas were defined by analysis of published data and 3 5 khz penetrating echo sounder data the annual dry bulk sediment accumulation in the northeastern north sea is established at 74 10 6 tons as the average organic carbon content for the norwegian channel and skagerrak kattegat sediments is 0 6 and 1 8 respectively the organic carbon accumulation rate in the northeastern north sea could be calculated the total organic carbon accumulation in the norwegian channel and skagerrak northern kattegat is 0 17 10 6 tons yr 1 and 0 83 10 6 tons yr 1 respectively less than 10 of this is accounted for by local primary production this means that more than 90 of the organic carbon buried in the sediments of the northeastern north sea must have been supplied from different sources terrigenous sources supply 20 of the organic carbon the remainder is marine organic matter produced elsewhere in the north sea or imported from the atlantic ocean norwegian sea and the baltic sea storm wave and current induced near bottom nepheloid layers are the main likely mechanism to transport fine grained sediments and associated organic matter from the north sea plateau into the kattegat skagerrak and norwegian channel

sediment resuspension characteristics in baltimore harbor maryland abstract critical bed shear stress for sediment resuspension and sediment erosion rate were measured in situ at sites from inner to outer baltimore harbor using the vims sea carousel clay mineral contents and biological conditions were almost the same at the four study sites the experimental results indicated that the erosion rate increased from the outer harbor toward the inner harbor with a maximum difference of about 10 times at an excess bed shear stress of 0 1 pa the measured critical bed shear stress strongly depended on the existence of a fluff layer it was approximately 0 05 pa if a fluff layer existed and increases to about 0 1 pa in the absence of a fluff layer

sulfidization and magnetization above hydrocarbon reservoirs geochemical and rock magnetic studies of strata over cement oil field anadarko basin oklahoma simpson oil field north slope basin alaska and the edwards deep gas trend south texas coastal plain document changes in original magnetizations caused by postdepositional iron sulfide minerals that are or may be related to hydrocarbon seepage at cement ferrimagnetic pyrrhotite fe sub 7 s sub 8 formed with pyrite and marcasite in permian red beds the fe s minerals contain isotopically heavy abiogenic sulfur derived from thermal degradation of petroleum and or isotopically light sulfur derived from sulfate reducing bacteria fed by leaking hydrocarbons at simpson ferrimagnetic greigite fe sub 3 s sub 4 dominates magnetizations in upper cretaceous nonmarine beds that contain biodegraded oil sulfur isotopic data are consistent with but do not prove a genetic link between the greigite delta sup 34 s gt 20 per mil and seepage in middle tertiary sandstones of southeast texas pyrite and marcasite formed when abiogenic h sub 2 s migrated upward from deep reservoirs or when h sub 2 s was produced at shallow depths by bacteria that utilized organic material dissolved in migrating water from depth the sulfide minerals replaced detrital magnetite to result in a systematic decrease in magnetic susceptibility toward faults that connect deep petroleum reservoirs tomore shallow sandstone the authors results show that abiologic and biologic mechanisms can generate magnetic sulfide minerals in some sulfidic zones of hydrocarbon seepage the magnetizations in such zones are diminished most commonly by replacement of detrital magnetic minerals with nonmagnetic sulfide minerals or are unchanged if such detrital minerals were originally absent less

the composition of permian seawater forty nine brine inclusions in marine halite from the ochoan salado formation in the delaware basin and fifteen inclusions in halite from the leonardian wellington formation in the kansas basin were extracted and their chemical compositions were determined the brines are of the na k mg cl so4 type their compositions resemble those of evaporated modern seawater the values of mcl mna mbr and mmg2 mca2 mso42 12 mmhco3 mbr of the inclusion brine from the two formations are equal to or slightly higher than those of modern seawater the original mna mbr and mcl mbr ratios of the inclusion brines were probably equal to or slightly larger than those of modern seawater the values of mmg2 mbr of the inclusion brines from the salado formation are very close to that of modern seawater the ratios of inclusion brines from the wellington formation are slightly lower probably due to the formation of dolomite magnesite the mmg2 mbr ratio in the initial seawater was probably close to the parent seawater of the salado brines the values of mso42 mca2 12mhco3 mbr of the inclusion brines appear to be reduced by the formation of dolomite magnesite and the value of this ratio in permian seawater was probably similar to that of modern seawater the mk mbr ratios of the inclusion brines are variable but the original ratios are probably close to or slightly larger than that of modern seawater if the br concentration of permian seawater was equal to that of modern seawater the composition of permian seawater can be narrowly constrained in mmol kg h2o 460 mna 630 550 mcl 730 mmg2 54 6 mk 11 mso2 4 mca2 12mmhco3 17 20 mso42 45 5 mca2 20 and 0 15 mhco3 5 the composition of permian seawater was therefore quite similar to that of modern seawater

approaches to predicting reservoir quality in sandstones despite limited understanding of the details of many diagenetic processes empirical techniques can be used effectively to predict reservoir quality prior to drilling the predictive approach depends mainly on the availability of empirical data in the area of interest in frontier basins mean or maximum porosity of a potential sandstone reservoir can be estimated for a given composition and level of thermal exposure or burial depth the reliability of the estimates is constrained by input data approximate values of input parameters can be obtained from seismic data in combination with geological analysis of the area in basins with sufficient information to generate calibration data sets the predictive technique uses regression analysis the applicability of this approach is constrained by the limits imposed by the calibration data set and is generally limited to samples containing less than 10 pore filling cement quartz rich sandstones 85 framework quartz cemented with quartz are an exception to the 10 limit because quartz cementation commonly is related to burial history and rock texture in weakly cemented sandstones the critical variables controlling porosity are detrital composition sorting and burial history permeability can be predicted independently of porosity using the same variables plus grain size these variables can be evaluated from seismic data and facies models t is approach is best suited either for sandstones in which compaction is the main porosity reducing process or for quartz rich sandstones an adequate predictive model for sandstone suites with a wide range of cement content can be obtained by dividing the calibration data set into two or more subsets and developing a predictive model for each for example one subset can be limited to samples with less than 10 cement whereas the second subset will consist of samples with more than 10 nonquartz cement porosity and permeability in the first subset are then expressed by linear regression whereas controls on reservoir quality in the more heavily cemented sandstones are determined independently based on understanding of cement distribution patterns

noble gas tracing of groundwater coalbed methane interaction in the san juan basin usa the san juan basin natural gas field located in northwestern new mexico and southwestern colorado in the usa is a case type coalbed methane system groundwater is thought to play a key role in both biogenic methane generation and the co2 sequestration potential of coalbed systems we show here how noble gases can be used to construct a physical model that describes the interaction between the groundwater system and the produced gas we collected 28 gas samples from producing wells in the artesian overpressured high production region of the basin together with 8 gas samples from the underpressured low production zone as a control stable isotope and major species determination clearly characterize the gas in the high production region as dominantly biogenic in origin and the underpressured low producing region as having a significant admix of thermogenic coal gas 3he 4he ratios increase from 0 0836ra at the basin margin to 0 318ra towards the center indicating a clear but small mantle he signature in all gases coherent fractionation of water derived 20ne 36ar and crustal 4he 40ar are explained by a simple rayleigh fractionation model of open system groundwater degassing low 20ne concentrations compared to the model predicted values are accounted for by dilution of the groundwater associated gas by desorbed coalbed methane this rayleigh fractionation and dilution model together with the gas production history allows us to quantify the amount of water involved in gas production at each well the quantified water volumes in both underpressured and overpressured zones range from 1 7 103 m3 to 4 2 105 m3 with no clear distinction between over and underpressured production zones these results conclusively show that the volume of groundwater seen by coal does not play a role in determining the volume of methane produced by secondary biodegradation of these coalbeds there is no requirement of continuous groundwater flow for renewing the microbes or nutrient components we furthermore observe strong mass related isotopic fractionation of 20ne 22ne and 38ar 36ar isotopic ratios this can be explained by a noble gas concentration gradient in the groundwater during gas production which causes diffusive partial re equilibration of the noble gas isotopes it is important for the study of other systems in which extensive groundwater degassing may have occurred to recognize that severe isotopic fractionation of air derived noble gases can occur when such concentration gradients are established during gas production excess air derived xe and kr in our samples are shown to be related to the diluting coalbed methane and can only be accounted for if xe and kr are preferentially and volumetrically trapped within the coal matrix and released during biodegradation to form ch4

seismic stratigraphy and quaternary sedimentation in the skagerrak northeastern north sea abstract ice movement sea level fluctuations and currents are the controlling factors of the late pleistocene and holocene sedimentation processes in the skagerrak the erosional truncation at the top of a mesozoic depositional sequence defines the extent of the norwegian channel and functions as the initial depositional surface for quaternary sedimentation on this surface in shallow waters off the danish coast weichselian glacial sediments are deposited an end morain ridge marks a temporary stillstand of the retreating ice front at the beginning of ice free conditions some 15 000 yrs ago when the sea was about 70 m below the present level the skagerrak was a fjord like forebasin of the northern atlantic and constituted a favorable outlet for rivers draining the subaerial glacial deposits of the southern skagerrak and the present southern north sea fluvial erosion released large amounts of detrital material which was laid down in the form of a progradational delta prodelta complex slumping and mass flow processes led to high sedimentation rates and a rapid infilling of the norwegian channel in front of the delta complex with the rapid transgression of the southern north sea at the beginning of the holocene a dramatic change in the depoenvironment of the skagerrak took place the modern current pattern became established whereby sediment transport and redeposition became the rule along the danish coast sandwaves and megaripples with an apparent northeasterly migration direction were formed as a result of the wind forced jutland current fine suspended particles bypass the coastal zones to settle out in the deep norwegian channel

late glacial and holocene paleoceanography in the skagerrak from high resolution grain size records abstract high resolution grain size analyses of the ams 14c dated 32 m long core md99 2286 from the northeastern skagerrak were performed in order to study late glacial and holocene paleoceanographic and sedimentary changes all ages in this study are given in calibrated thousand years before present ad 1950 abbreviated kyr unless otherwise noted the distinct ending of ird ice rafted debris in core md99 2286 which was retrieved from a location down current from the final calving ice margin in the region indicates that iceberg calving in the skagerrak ended between 10 6 and 10 2 kyr a clay rich sequence in core md99 2286 deposited between 11 3 and 10 3 kyr is attributed to outflow from the baltic basin across south central sweden the sequence is correlated to similar units from cores along the swedish west coast the onset of this clay rich deposition occurs progressively later in cores further south along the coast supporting a previous hypothesis that differential glacio isostatic uplift caused a southward migration of the baltic outflow from the otteid stenselva to the gota alv outlet a distinct coarsening towards younger sediments in core md99 2286 indicates a hydrographic shift at 8 5 kyr which is correlated to a shift previously reported in the skagerrak kattegat and the norwegian channel this shift reflects the establishment of the modern circulation system in the eastern north sea as a consequence of the opening of the english channel and the danish straits and increased atlantic water inflow and the subsequent development of the south jutland current a general trend of coarsening poorer sorting and increasing variability from 8 5 kyr until the present indicates increasing strength and influence of the variable south jutland current a series of changes from ca 6 3 to ca 3 8 kyr in core md99 2286 reflects strengthening of the jutland current towards the present day sedimentation system in the skagerrak kattegat these changes are correlated to previously reported hydrographic shifts at 5 5 14c years bp in the skagerrak and at 4 0 14c years bp in the kattegat it is suggested that these shifts were separate features of a transitional period related to strengthening of the current system the resulting changes are differently manifested in different parts of the skagerrak kattegat due to the complex circulation system the last 800 years are characterised by poorly sorted sediments with a relatively high and variable proportion of coarse material reflecting a circulation system significantly modified by regional climatic conditions especially the general wind directions and storm frequency over the southern north sea

the origin and fate of silt sized carbonate in subsurface miocene oligocene mudstones south texas gulf coast combined petrographic and geochemical data document several kinds of detrital carbonate in subsurface miocene oligocene mudstones of the texas gulf coast in the extreme south of texas in muddy sediments deposited by ancient precursors to the modern rio grande mudstone carbonate is dominated by extrabasinal detritus derived from cretaceous limestones of the edwards plateau further north oligocene mudstones contain carbonate that is mostly syndepositional skeletal material minor amounts of authigenic carbonate are found as replacements of silt sized feldspars in all the mudstones depth related shifts in 18ocalcite and 13ocalcite suggest that small amounts of authigenic carbonate also form by replacement of detrital carbonate recrystallization although this cannot be demonstrated petrographically pervasive carbonate cementation in primary pores in mudstones is not generally observed textural evidence of carbonate loss through pressure dissolution is widespread providing a mechanism for the depth related decline in carbonate content observed in many gulf coast wells carbonate dissolution in mudstones and export of the caco3 implies a massive acid source probably resulting from silicate reactions within the mudstones during burial diagenesis

early diagnesis of carbohydrates in the marine environment ii composition and origin of carbohydrates in skagerrak sediments abstract the distribution and composition of carbohydrates were determined for the upper 15 cm of four cores from the southern slope of the skagerrak high concentrations were found in the upper bioturbated layers rapidly decreasing at sediment depths 3 cm total contents appear to be controlled by bioturbation application of various biomarker criteria suggests that carbohydrates originated from terrestrial macroalgal and planktonic sources input from these sources might be by episodic events rather than by continuous sedimentation as can be hypothesised from the irregular occurrence of marine and terrestrial signals

the dynamics of oceanic fronts part ii shelf water structure due to freshwater discharge abstract in part i of this series a framework was introduced for the study of oceanic frontal dynamics the dynamics was studied by posing an initial value problem starting with a near surface discharge of buoyant water with a prescribed density deficit into an ambient stationary fluid of uniform density an essential aspect of the framework was the identification of the proper length scales an inertial length scale l0 a buoyancy length scale h0 and a diffusive length scale hv in part i the horizontal and vertical dimensions were scaled by l0 and h0 respectively and two dimensionless parameters were formed viz ro l0 h0 and e hv h0 2 it was shown in part i that under this scaling the normalized equations depended on e only for ro sufficiently large the solution for e small i e for the almost inviscid case was given in part i and the equilibrium state was discussed in a frame of reference in which the front was stationary in this paper we present the solution for large e it will b

experimental simulation of clay precipitation within reservoir sandstones 2 mechanism of illite formation and controls on morphology illite and illite smectite with various morphologies have been formed on quartz grains by precipitation from solution following dissolution of amorphous al si gels at 150 350 degrees c and 0 75 1 0 kbar 75 100 mpa initial solutions include deionized water chloride and oxalate the experimental results have application to the formation of authigenic illite in sandstones from pore waters containing carboxylic acid anions a series of experiments monitored the effects of reaction temperature and time on the composition and morphology of illitic minerals precipitated following the dissolution of an amorphous gel of illite composition in deionized water a progressive increase of 70 to 100 in the illite content of illite smectite occurs between 150 and 350 degrees c but there is no change in illite smectite composition for reaction times of 24 hours to 37 days these results indicate that illite and illite rich illite smectite form by direct precipitation from solution and do not require a smectitic precursor the compositional variation in illite smectite and illite has little or no effect on clay morphology all experiments produced a platy boxwork morphology experiments performed at 250 degrees c and initially containing oxalate in both acidic and alkaline k bearing solutions strongly promote illite precipitation following the dissolution of an amorphous al si oxide starting material the very rapid precipitation that occurs under these conditions is associated with the formation of fibrous lath type illites rather than platy types which are characteristic of slower precipitation in the absence of oxalate precipitation of fibrous illite is related to the breakdown of oxalate which lowers illite solubility by either ph variation or by breakdown of an al oxalate complex oxalate is not a direct requirement for fibrous illite formation although it controls fluid composition which favors fibrous illite formation the association of illite precipitation with carboxylate anion breakdown suggests that the formation of fibrous illite cements in petroleum reservoirs is related to fluids released from source rocks which contain both aqueous organic acids and inorganic solutes

sediment deposition in offshore deeps of the western north sea questions for models a three dimensional hydrodynamic model is used to investigate the potential for offshore bathymetric deeps in the north sea to act as fine sediment traps simple conditions are established representative of the observed conditions and the response of the model investigated two deeps are considered here inner silver pit and sole pit the model study suggests that in inner silver pit deposition of fine sediment is only possible under weak tidal neap flows under all other conditions the flow is too energetic to allow deposition a transport path for fine sediment across inner silver pit is established but does not significantly modify the sediment dynamics in the pit experiments including wave action show that 50 year return period waves induce bed shear velocities sufficiently large to remove coarse sediment for sole pit a cycle of deposition followed by erosion at spring tides will inhibit deposition in the pit long term accretion of fine sediment does not seem possible in either pit

duration of quartz cementation in sandstones north sea and haltenbanken basins abstract evidence from fluid inclusions petrography and burial history modelling is used to estimate the duration of quartz cementation in sandstones the homogenization temperatures of primary fluid inclusions in quartz cements from samples from the brent group north sea and haltenbanken offshore norway show variations 4 of 17 4 and 20 2 c these are interpreted to reflect durations of less than approximately 9 and 17 ma low variances are general features of such data sets and suggest that quartz cementation usually takes place as events with durations of the order of 10 ma or possibly less confining the age of quartz cement growth using a combination of geological petrographical and radiometric dating methods that are independent of fluid inclusions also suggests that quartz cements grow over periods substantially shorter than the age of the sandstone it is not easy to reconcile this deduction with the suggestion that burial itself causes cementation

sandstone porosity loss a big picture view of the importance of compaction proper assessment of the relative importance of porosity loss by compaction and cementation is important to the prediction of porosity in sandstones studies of mass transport during diagenesis and the modeling of basin fluid flow quantitative estimation of the amounts of porosity loss by compaction and cementation can be made from standard point count data on cement and pore space abundance this method is discussed and its sources of error are evaluated error in the method arises mainly from uncertainty in initial sandstone porosities and the amount of local grain dissolution during burial data on the porosity of modern sands indicate that assumed initial porosities of 40 to 45 will produce small average errors in the calculation of compactional porosity loss for clean sandstones grain dissolution whether resulting in local porosity enhancement by export in solution of former grain mass or local precipitation of authigenic cement will cause an underestimation of the significance of compaction a large database of point count data from diverse sandstones is used to provide a broad perspective on the importance of compaction and cementation to sandstone porosity loss it is shown that the global importance of compactional porosity loss has generally been under appreciated in fact compaction mechanical and chemical is probably the dominant mechanism of porosity loss in most sandstones the extent of compactional porosity loss is related to sand grain composition and basin thermal structure

recent sediments and foraminiferal distribution in the skagerrak northeastern north sea abstract a number of bottom sediment samples from the skagerrak have been studied the surface sediment samples reflect the presence of three main sediment types with a fourth which occurs more scattered a selection of 50 samples was used for the study of the foraminifers of the 125 m fraction benthic foraminifers dominate the assemblages using factor analysis and the computer program cabfac the benthic foraminiferal data were reduced to five factors eight species have proven to be significant the factors benthic foraminiferal assemblages are dominated by bolivina cf b robusta elphidium excavatum cassidulina laevigata bulimina marginata melanis barleeanum globobulimina turgida and hyalines balthica respectively the first factors are clearly related to known hydrographic and bottom conditions

accumulation of barium in recent skagerrak sediments sources and distribution controls abstract geochemical and mineralogical methods have been applied to document the ba and barite distributions in the skagerrak sediments barite released in the central north sea during hydrocarbon drilling as a component of the drilling mud is a principal carrier of ba in the recent skagerrak sediments the systematic pattern of the surface ba contents displaying a generally decreasing trend from south to north is consistent with the barite transport from the north sea towards the skagerrak the predominant barite grain size ranges between 10 and 20 m the sub surface ba distribution is characterised by the elevated contents within the top 5 10 cm which downward is followed by uniformly low contents considering the sedimentation rate the start of sub surface ba enrichment generally corresponds to the late 1960s coinciding with the beginning of off shore drilling in the north sea in addition to the primary depositional factors controlling the shallow stratigraphic ba distribution the influence of diagenetic processes is suggested by the dissolution features on the barite grain surfaces and the profile relationships to the interpreted diagenetic zones

erosion of soft cohesive sediment deposits erosion behavior of soft cohesive sediment deposits has been investigated in laboratory experiments such deposits are representative of the top active layer of estuarial beds an experimental procedure involving layer by layer erosion under a range of bed shear stresses b of successively increasing magnitude was utilized interpretation of the resulting concentrationtime data together with bed density profiles yielded a description of the variation of the bed shear strength s with depth as well as an expression for the rate of surface erosion in general s increased with depth and was also influenced by the type of sediment bed consolidation period and salinity the rate of erosion was found to vary exponentially with b s 1 2 in modeling estuarial bed erosion it is essential to take these characteristics of s and the rate of erosion into account

evidence against natural deformation of fluid inclusions in diagenetic quartz abstract this paper describes evidence for and against burial induced deformation of fluid inclusions in diagenetic quartz fluid inclusions in diagenetic minerals are prone to non elastic deformation stretching or leakage if subjected to progressive burial in a sedimentary basin if they deform they cannot record mineral growth conditions correctly a correlation between homogenization temperature and depth may be interpreted as indicating wholesale leakage or stretching experimental work and theoretical studies suggest that heating 200 c higher than the homogenization temperature is required to produce significant deformation but it may not be possible to extend the results to natural conditions the experiments further predict a correlation between homogenization temperature and inclusion size in a sample of inclusions that have undergone natural deformation this is not observed in natural examples but the absence of a correlation is not proof that leakage has not occurred several features of inclusion data in quartz cements do however argue against stretching or leakage homogenization temperatures commonly show narrow distributions and are compatible with independent data such as illite potassium argon ages and petrography different generations of inclusions in a single sample set also retain homogenization temperature distributions compatible with their sequential origin taken as a whole the body of evidence argues against widespread stretching or leakage of fluid inclusions in diagenetic quartz

tracking dynamical changes in mud bed due to waves the time variation of the phase velocity of a high frequency shear wave was measured using a miniature rheometer in a laboratory mud bed subjected to monochromatic waves the fall in the velocity starting with an undisturbed bed is shown to correlate with a corresponding decrease in bed rigidity as the bed becomes fluidized the in situ rheometry may be an effective means to track mud fluidization in the prototype environment

integration of petroleum engineering studies of producing brent group fields to predict reservoir properties in the pelican field uk north sea abstract diagenetic models and the results of laboratory experiments on cores from producing brent group fields cormorant and tern have been combined to aid prediction of reservoir properties in a deeply buried prospect pelican in cormorant and tern illite particles are thin 20 40 a lath shaped and liable to damage during drying before air permeability measurement leading to erroneously high permeabilities the magnitude of this drying effect k air conventional drying k air critical point drying varies with illite morphology and the timing of hydrocarbon emplacement in tern pore filling illites occur above and below the oil water contact and there is no significant difference in drying effect in cormorant the drying effect is higher below the oil water contact where a diversity of late diagenetic pore filling illite morphologies occur injectivity tests on cores from cormorant indicate that sea water injection will not lead to impairment of reservoir quality as a result of clay dispersal or other rock related phenomena impairment occurred when flooding cores with produced water and was avoided by introducing divalent cations saline brines or approximating in situ equilibrium conditions ph 6 4 this equilibrium point is strongly influenced by the surface chemistry of feldspar and illite rather than the more abundant kaolinite and quartz burial diagenetic replacement of kaolinite by illite will increase this equilibrium towards ph 7 but should not cause impairment during sea water injection into deeper reservoirs pelican contains abundant pore filling illite k ar age dates 44 25 ma suggest illite growth started after burial to over 8000 ft and palaeoburial temperatures of over 80 90 c compared with nearby reservoirs variations in the nature and abundance of illite result from illitisation being arrested by hydrocarbon charge at different times optimal reservoir and aquifer properties occur in shallower structures charged from more deeply buried source rocks good reservoir properties occur with charge after short lived illite growth although later burial may reduce aquifer properties the more prolonged illite growth experienced by deeply buried reservoirs such as pelican charged late in the tertiary has resulted in poor reservoir properties and poor aquifer permeabilities may be expected

origin of magnetite responsible for remagnetization of early paleozoic limestones of new york state scanning electron microscope and scanning transmission electron microscope observations of thin sections and separates of the devonian onondaga and helderberg limestones and ordovician trenton limestone in new york state allow us to identify three types of magnetite pseudoframboids nonspherical magnetite and fine grained magnetite magnetite was observed replacing pyrite occurring as crystals in spherical pseudoframboids or with nonspherical shapes fine grained magnetite consisting of aggregates of one or more rounded single crystals approximately 2000 a 200 run in diameter could not be observed in thin sections of new york carbonates due to its small size but was found in magnetic extracts hysteresis measurements of magnetic extracts verify that fine grained magnetite is capable of carrying remanent magnetizations however pseudoframboidal magnetite and nonspherical magnetite are polycrystalline and consist of assemblages of single to pseudosingle domain sized crystals that also can be carriers of the remagnetization these data in combination with observations from other localities collectively imply that the alleghenian remagnetization is carried by the fine grained magnetite although pseudoframboidal and nonspherical magnetite may also contribute thus the alleghenian remagnetization is a chemical remanent magnetization due to authigenic magnetite crystallization of magnetite was mediated by fluids with dissolution and crystallization activated by stress during the alleghenian orogeny the origin of such fluids is unknown but they may have originated through crustal scale fluid migration tectonically induced brine migration due to emplacement of thrust sheets is preferred over a meteoric source of the fluids

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

spatial 87sr 86sr variations in formation water and calcite from the ekofisk chalk oil field implications for reservoir connectivity and fluid composition abstract the 87 sr 86 sr ratios of formation waters from the chalk of the ekofisk oilfield were measured using a new technique extracting salt residues from the interstices of core samples in the danian ekofisk formation the chalk matrix itself has 87 sr 86 sr and 13 c values characteristic of danian seawater even though 18 o data indicate the presence of varying quantities of diagenetic carbonate cement in the ekofisk formation porewater 87 sr 86 sr values are very constant range 0 70786 0 70810 the formation water probably contains locally derived sr from carbonate plus a small contribution from noncarbonate minerals with higher 87 sr 86 sr in the underlying tor formation chalk samples containing a mixture of primary chalk and later cement have 87 sr 86 sr values that rise with depth to a maximum of 0 70865 much higher than contemporaneous seawater porewater 87 sr 86 sr values increase even more dramatically with depth to a maximum of 0 70988 both cement and waters have been influenced by sr with a high 87 sr 86 sr probably derived from underlying sedimentary rocks comparison with waters brought to the surface together with produced oil indicates that the depthwise increase in 87 sr 86 sr reflects an increase in salinity from 50 000to 140 000 mg l total solute content this suggests that zechstein evaporites may be the source of the radiogenic sr in the tor formation the difference in isotope composition between carbonates and waters in the ekofisk and tor formations indicates that their hydrochemical systems have evolved differently during diagenesis this is due to the barrier effect of the ekofisk tight zone a hardground at the base of the ekofisk formation which effectively splits the reservoir vertically into two this study thus provides a new way of determining the positions of permeability barriers in oil reservoirs also demonstrated for the first time is the existence of vertical variations in salinity of water in oil such variations could have important implications for the calibration of resistivity logs and calculation of the amount of oil in place

petrography and reservoir physics i objective classification of reservoir porosity porosity observed in thin section can be objectively classified using a combination of digital acquisition procedures and pattern recognition algorithms pore types are derived from the frequency distributions of sizes and shapes of patches of porosity exposed in thin section each pore type is represented by a characteristic distribution of sizes and shapes found in thin section most sandstone reservoirs contain fewer than six pore types much of the intersample variability in reservoir physics is associated with changes in pore type abundance the advantages of this approach to porosity classification are 1 the criteria for classification are objectively defined 2 classification procedure is rapid accurate and precise 3 pore types are understood easily in ter s of conventional genetic classification schemes and 4 pore type data are related strongly to petrophysical properties

biomarkers in upper holocene eastern north sea and wadden sea sediments total extracts of sediment cores from five different stations in the north sea and wadden sea were analysed for their biomarker composition only sediments of the skagerrak contained significant amounts of marine biomarkers mainly alkenones other sites contained predominantly terrestrial biomarkers bioturbation in the skagerrak is however far too high to determine sea surface temperature sst changes within short time intervals these results indicate that biomarkers contained in these sediments are not useful to reconstruct climate fluctuations during the upper holocene high amounts of and hydroxy fatty acids as well as small amounts of dihydroxy fatty acids were released from the insoluble organic material of the sediments from the wadden sea station indicating a significant input of the eelgrass zostera marina this was confirmed by microscopic observations this is the first time the dihydroxy fatty acids have been found in a sediment core and they have proven to be potential biomarkers for these seagrass species

carboxylic acid anions in formation waters san joaquin basin and louisiana gulf coast u s a implications for clastic diagenesis critical comment the analyses of carboxylic acid anions caas in san joaquin basin and louisiana gulf coast formation waters published by macgowan and scrdam 1990 do little to resolve the important question of whether caas are of great significance to clastic diagenesis over the temperature range in which they dominate fluid alkalinity as advocated by these two authors comparison of their analyses with analyses of formation waters from the same wells previously published by three independent research groups hanor and workman 1986 land et al 1988 and land and macpherson 1989 and fisher and boles 1990 a body of data not discussed in their article reveals that most of the macgowan and surdam values are grossly inflated relative to the previous analyses table 1 fig 1 cedures of llco et al 1982 while field procedures for caas now normally include the addition of a bactericide to the sample an independent check on the upper limit of total caas which could have been present in this water is provided by the results of an alkalinity titration to ph 2 5 also published by hanor and workman 1986 which show that the total concentration of weak acid anions bicarbonate plus caas could not have exceeded approximately 428 mg 1 the alkalinity determinations were done shortly after collection on separate untreated aliquots stored in gas tight glass containers further details are given in workman 1985

diagnostic criteria for reconstruction of tidal continental shelf regimes changing the paradigm abstract a biogeochemical flux paradigm is presented which provides a conceptual and numerical framework for reconstructing the dynamical and biogeochemical regimes of ancient tide driven continental shelves the paradigm links turbulence primary production and suspended particulate matter flux in the water column to microbiological and isotopic proxies in the sediment record and identifies the diagnostic signatures of sediments deposited in stratified frontal and mixed dynamic regimes of tidal shelves the critical governing processes in the water column and at the sediment water interface are temperature particulate organic carbon supply and benthic oxygen consumption which exhibit strong gradients across tidal mixing fronts the diagnostic proxies in the sediment record are microplanktonic e g dinoflagellate cysts and microbenthic e g foraminifera and their associated stable isotopic properties advanced numerical models are available which incorporate biophysical coupling in the water column and benthic boundary layer and which simulate biogeochemical and ecological processes and organic fluxes to the seabed these models offer potential advances for interpreting microbiological and isotopic proxies of biogeochemical regime preserved in the fine sediment record quaternary shelf deposits provide the best potential validation of the biogeochemical flux paradigm since most quaternary species are living today but the paradigm is applicable to ancient tidal shelf deposits the paradigm is particularly applicable to the fine sediment record which potentially preserves the most complete history of shelf evolution