chemical and mechanical processes during burial diagenesis of chalk an interpretation based on specific surface data of deep sea sediments burial diagenesis of chalk is a combination of mechanical compaction and chemical recrystallization as well as cementation we have predicted the characteristic trends in specific surface resulting from these processes the specific surface is normally measured by nitrogen adsorption but is here measured by image analysis of scanning electron micrographs this method concentrates on the micritic matrix alone deep sea sediments are ideally suited to the study of burial diagenesis because they accumulate in a relatively conservative tectonic setting we used material from the ontong java plateau in the pacific where a 1 km thick package of chalk facies sediments accumulated from the cretaceous to the present in the upper 200 300 m the sediment is unconsolidated carbonate ooze throughout this depth interval compaction is the principal porosity reducing agent but recrystallization has an equal or larger influence on the textural development in the chalk interval below compaction is not the only porosity reducing agent but it has a larger influence on texture than concurrent recrystallization below 850 m grain bridging cementation becomes important resulting in a lithified limestone below 1100 m this interpretation is based on specific surface data alone and modifies current diagenetic models

deciphering the hydrothermal evolution of a vms system by la icp ms using trace elements in pyrite an example from the bracemac mcleod deposits abitibi canada and implications for exploration textural and chemical characterization of pyrite was used to reconstruct the hydrothermal evolution of the bracemac mcleod archean volcanogenic massive sulfide vms deposits 6 mt in the matagami district abitibi canada the mineralization hosted in a bimodal volcanic sequence is divided into a 1 zn rich zone concordant to the key tuffite a marker horizon at the district scale 2 a cu rich zone which locally crosscuts the stratigraphic pile and 3 some localized magnetite rich zones replacing sphalerite and pyrite detailed petrographic and laser ablation inductively coupled plasma mass spectrometry la icp ms studies of whole pyrite grains including inclusions and zonation from the key tuffite and from the different ore zones of the deposits has revealed five pyrite types nodular pyrites i in the key tuffite and subidiomorphic pyrites from the low temperature 250 c zn rich zone ii have the same chemical signature suggesting that both precipitated under similar physicochemical conditions a wide range of trace elements cu zn ag sn sb te au pb bi tl are present in both but compared to the other types of pyrite they are significantly enriched in sb and tl subidiomorphic pyrites from the copper rich zone iii have a different signature enriched only in se co in pb and bi these pyrites commonly overgrow the nodular pyrites and are related to later higher temperature fluids 300 c pyrite preserved during a later magnetite replacement 350 c stage iv also are enriched in se but are depleted in all other elements a similar signature is found in idiomorphic metamorphic pyrite v which occurs at the district scale during high temperature recrystallization of pyrite iv and v only the contents of ni co as and se are mostly preserved whereas other base metals are expelled from the pyrite structure consequently only ni co as se sb and tl were useful to reconstruct the hydrothermal evolution of the vms system and used to build a suite of discrimination diagrams specifically the ratio se tl can discriminate pyrites from mineralized zn rich zone se tl 10 000 the innovative approach of this study based on la icp ms acquisition of the chemistry of entire pyrite grains makes our results directly applicable for exploration consequently pyrite concentrate samples analyzed for as tl and se by more classical mass spectrometric techniques have the potential to be used for vectoring to ore by using the proposed discrimination diagrams

microstructure and poro mechanical performance of haubourdin chalk abstract large chalk deposits are to be found in the north of france and as a result developing chalk as a load bearing aggregate is considered by french road building companies due to its limited mechanical performance chalk is subjected to a medium heat treatment process up to 500 c in this context microstructure and poro mechanical performance of haubourdin chalk are characterized before and after heat treatment the investigation of the so called water weakening effect is associated to that of heat treatment mainly to show to what extent the strength of haubourdin chalk decreases at a given water saturation state prior to or after heat treatment in terms of microstructure analysis sem and fib sem observations show the initial weak cementation of haubourdin chalk in direct relation with its moderate mechanical performance after heat treatment minor re crystallisation of calcium carbonate is observed which should be confirmed further the specific behavior of haubourdin chalk is highlighted under partial water saturation and after heat treatment by using uniaxial compressive tests and triaxial drained and undrained compressive tests pore collapse is quantified directly under increasing hydrostatic stress by measuring coupled gas permeability and porosity change a heat hardening effect is identified which is counter balanced by the water weakening effect the latter makes natural haubourdin chalk unsuitable as load bearing road aggregate without further binding matter

a review of the nature and origin of limestone microporosity abstract limestone microporosity is ubiquitous and extensively developed in most phanerozoic limestones from an economic perspective microporosity is important because it contributes substantially to the carbonate pore system which can host significant volumes of water and hydrocarbons therefore determining the presence and distribution of limestone micropores is necessary for accurate hydrocarbon estimations reservoir characterization and fluid flow simulations from an academic standpoint microporosity is important because its genesis is intimately linked with the mineralogical stabilization of metastable sediments a fundamental process in carbonate diagenesis many types of micropores contribute to what has been referred to as microporosity but the vast majority is hosted among low magnesium calcite lmc microcrystals that are present in limestone matrix and allochems geochemical textural and mineralogical data from natural settings and laboratory experiments indicate that lmc microcrystals are diagenetic in origin more specifically these data support a diagenetic model of mineralogical stabilization that involves dissolution of precursor sediments dominated by aragonite and high magnesium calcite hmc minerals and precipitation of lmc microcrystal cements the stabilization process is inferred to take place in the meteoric marine and burial diagenetic realms although it has not been directly observed carbon and oxygen isotopes as well as trace element data suggest that lmc microcrystals form during burial diagenesis in marine like fluids evidence suggests that porosity is not generated during this dissolution precipitation process but rather inherited from the precursor sediments the final arrangement of the micropores in a limestone however depends on the precise diagenetic pathway lmc microcrystals exhibit a range of microcrystalline textures that are classified on the basis of crystal morphology and size the three main textural classes granular framework fitted mosaic and clustered have been recognized across a wide range of ages depositional settings burial depths and precursor types and are characterized by distinct petrophysical properties such as porosity permeability and pore throat size observations from modern sediments also support the hypothesis that lmc microcrystals develop from aragonite and hmc dominated lime mud the origin of lime mud has been extensively studied but still highly debated of particular interest to the discussion of microporosity are proposed secular variations in the dominant mineralogy of carbonate sediments through the phanerozoic microporous limestones comprised of lmc microcrystals are equally abundant during times of aragonite seas and calcite seas which suggests that no special mineral precursor is required microporous textures are also observed in deep marine chalks where micropores are hosted between chalk constituents unlike shallow marine limestones deep marine sediments start out as mostly lmc therefore mineralogical stabilization is not a significant process in chalk diagenesis

how burial diagenesis of chalk sediments controls sonic velocity and porosity based on p wave velocity and density data a new elastic model for chalk sediments is established the model allows the construction of a series of isoframe if curves each representing a constant part of the mineral phase contributing to the solid frame the if curves can be related to the progress of burial diagenesis of chalk which is revised as follows newly deposited carbonate ooze and mixed sediments range in porosity from 60 to 80 depending on the prevalence of hollow microfossils despite the high porosity these sediments are not in suspension as reflected in ifs of 0 1 or higher upon burial the sediments lose porosity by mechanical compaction and concurrently the calcite particles recrystallize into progressively more equant shapes high compaction rates may keep the particles in relative motion whereas low compaction rates allow the formation of contact cement whereby if increases and chalk forms rock mechanical tests show that when compaction requires more than in situ stress porosity reduction is arrested during subsequent burial crystals and pores grow in size as a consequence of the continuing recrystallization the lack of porosity loss during this process testifies to the absence of chemical compaction by calcite calcite pressure dissolution as well as to the porosity preserving effect of contact cementation at sufficient burial stress the presence of stylolites indicates that pressure dissolution takes place between calcite and silicates and depending on pore water chemistry and temperature pore filling cementation may occur over a relatively short depth interval limestone and mixed sedimentary rock form and porosity may be reduced to less than 20 isoframe increases to more than 0 6 in hydrocarbon reservoirs in north sea chalk relatively high porosity and high ifs are found the reason may be that recrystallization and porosity preserving contact cementation progress whereas pore filling cementation is small probably because pressure dissolution along stylolites is arrested pressure dissolution may be arrested for two reasons 1 the introduction of hydrocarbons causes a fall in effective burial stress and 2 adsorption of polar hydrocarbons on the silicates may shield calcite from the silicates

chalk porosity and sonic velocity versus burial depth influence of fluid pressure hydrocarbons and mineralogy seventy chalk samples from four formations in the overpressured danish central north sea have been analyzed to investigate how correlations of porosity and sonic velocity with burial depth are affected by varying mineralogy fluid pressure and early introduction of petroleum the results show that porosity and sonic velocity follow the most consistent depth trends when fluid pressure and pore volume compressibility are considered quartz content up to 10 has no marked effect but more than 5 clay causes lower porosity and velocity the mineralogical effect differs between p wave and shear velocity so that smectite bearing chalk has a high poisson s ratio in the water saturated case but a low value in the dry case oil bearing chalk has up to 25 units higher porosity than water saturated chalk at similar depth but similar velocity probably because hydrocarbons prevent pore filling cementation but not pore structure stiffening cementation in this presumably water wet chalk these results should improve the modeling of chalk background velocity for seismic inversion analysis when describing the porosity reducing process pore volume compressibility should probably be disregarded when correcting for fluid pressure because the cementing ions originate from stylolites which are mechanically similar to fractures we find that cementation occurs over a relatively short depth interval

permeability prediction in chalks the velocity of elastic waves is the primary datum available for acquiring information about subsurface characteristics such as lithology and porosity cheap and quick spatial coverage ease of measurement information of permeability can be achieved if sonic velocity is used for permeability prediction so we have investigated the use of velocity data to predict permeability the compressional velocity from wireline logs and core plugs of the chalk reservoir in the south arne field north sea has been used for this study we compared various methods of permeability prediction from velocities the relationships between permeability and porosity from core data were first examined using kozeny s equation the data were analyzed for any correlations to the specific surface of the grain sg and to the hydraulic property defined as the flow zone indicator fzi these two methods use two different approaches to enhance permeability prediction from kozeny s equation the fzi is based on a concept of a tortuous flow path in a granular bed the sg concept considers the pore space that is exposed to fluid flow and models permeability resulting from effective flow parallel to pressure drop the porosity permeability relationships were replaced by relationships between velocity of elastic waves and permeability using laboratory data and the relationships were then applied to well log data we found that the permeability prediction in chalk and possibly other sediments with large surface areas could be improved significantly using the effective specific surface as the fluid flow concept the fzi unit is appropriate for highly permeable sedimentary rocks such as sandstones and limestones that have small surface areas

gravitational trapping of carbon dioxide in deep sea sediments permeability buoyancy and geomechanical analysis 1 liquid carbon dioxide injected in deep sea sediments at km depths and near freezing temperatures is denser than surrounding pore water and will be trapped by gravitational forces storage capacity for co2 in such formations below the ocean floor is shown to vary with seafloor depth geothermal gradient porosity and pore water salinity the formation permeability or the successful engineering of such permeability through hydraulic fracturing will determine the capacity for gravitational trapping in deep sea geological formations we conclude that most ocean sediments at appropriate depth will lack the required permeability and that conventional hydraulic fracturing would only be possible in carefully selected sites

prediction of archie s cementation factor from porosity and permeability through specific surface based on archie s cementation factor as measured on north sea chalk and on published data we explore how the cementation factor depends on other physical properties of the chalk a relationship between cementation factor and specific surface with respect to bulk volume is obtained for chalk this leads to how archie s cementation factor may be predicted from porosity and permeability for chalk as well as for sandstone a common relationship between cementation factor and specific surface with respect to bulk volume calculated from porosity and permeability is obtained as an alternative the a factor in archie s equation is related to specific surface with respect to bulk volume but the relationship is less clear than the simple relationship between cementation factor and specific surface

new insight into the microtexture of chalks from nmr analysis an integrated petrographical and petrophysical study was carried out on a set of 35 outcrop chalk samples covering a wide range of lithologies and textures in this study various chalk rock types have been characterized in terms of microtextures and porous network by integrating both geological sediment petrological and petrophysical data including porosity permeability low field nmr nuclear magnetic resonance micp and specific surface area bet measurements the data allow an in depth understanding of the nmr signal of chalks with a focus on tight chalks including all low reservoir quality chalks independently of their sedimentological and or diagenetic history the study aims to develop an nmr based approach to characterize a broad range of chalk samples the provided laboratory low field nmr chalk classification can be used as a guide to interpret nmr logging data based on the petrographical and petrophysical analysis 6 groups of samples were identified each of them characterized by a unique nmr signature 1 micritic chalks 2 grainy chalks 3 cemented chalks 4 marl seam chalks 5 argillaceous chalks and 6 silicified chalk nmr t2 distributions were linked to pore body size and t2 logarithmic t2lm was calculated it is apparent that tight chalks whether their characteristics are sedimentological or diagenetic yield smaller pore body sizes t2lm 60 ms and pore throats average radius 290 nm and higher permeabilities up to 13 md the marl seam chalk samples yield bimodal t2 distributions with a first peak related to the micritic matrix pores and a second peak related to intraparticle pores within fossils for all samples permeability was inferred from nmr spectra using sdr schlumberger doll research model

a universal classification scheme for the microcrystals that host limestone microporosity abstract microporosity is recognized as a significant concern in phanerozoic age limestone reservoirs throughout the world because its presence can severely complicate hydrocarbon evaluation and production numerous studies have documented the occurrence and abundance of micropores their physical appearance and diagenetic origins in specific reservoirs but no published study has been systematic and global in its approach presented here is a global assessment of limestone microporosity that is based on scanning electron microscope crystal size distributions helium porosimetry and mercury injection capillary pressure data collected from 12 microporous limestone reservoirs spanning a broad range of geologic ages depositional facies and burial depths results indicate that the vast majority of limestone microporosity is hosted within a framework of low magnesium calcite microcrystals in general crystal size distributions are narrow and positively skewed compiled data indicate that 99 of all microcrystals measure between 0 5 and 9 0 m in diameter with a mode at ca 2 0 m based on microcrystal shape packing and edge density microporous limestones can be classified into three major textural classes which we term granular clustered and fitted granular textures which include subhedral and euhedral subclasses are characterized by a framework of unconsolidated rhombic to polyhedral microcrystals that exhibit straight to curvilinear boundaries and intermediate crystal edge densities clustered textures which include loose and fused subclasses are characterized by a framework of irregularly shaped microcrystals with relatively high densities of discontinuous curvilinear crystal edges fitted textures which include partial and fused subclasses are characterized by a dense mosaic of slightly larger microcrystals with mostly curvilinear boundaries and relatively low edge densities these textural classes have unique mean crystal sizes average pore throat radii bulk porosities and permeabilities furthermore textural classes and their characteristic pore throat radii fall along a single log normal transform in porosity permeability space based on these relationships three petrophysical microporosity types are distinguished type 1 is associated with granular subhedral crystal fabrics and typically exhibit relatively large average pore throat radii ca 0 7 m as well as high porosity 20 and permeability 1 20 md type 2 includes granular euhedral and clustered fabrics which most commonly exhibit intermediate average pore throat radii ca 0 2 m porosity 10 20 and permeability 0 1 1 md type 3 includes fitted crystal fabrics which are generally characterized by smaller average pore throat radii ca 0 06 m and relatively low porosity 10 and permeability 0 1 md

modelling of porosity evolution and mechanical compaction of calcareous sediments a continuum mechanics model for the gravitational compaction of sediments is derived by assuming that the sediments are normally pressured and in a one dimensional state of stress sediment strength is characterized in terms of effective stress laws adopted from soil mechanics the model is a relatively simple mathematical formula that gives the porosity as a function of burial depth the shape of the porosity profile is controlled by two mechanical parameters the compression index and the void ratio at an effective stress of 100 kpa the model was verified by analysing the porosity depth data of oozes and chalk from the ontong java plateau gathered during leg 130 of the ocean drilling program the mechanical parameters of the sediments were estimated using a least squares method to fit the theoretical profile to the porosity data the theoretical profile described accurately the ooze porosity data over depth ranges of 100 m or more however over smaller length scales of 10 50 m there were systematic deviations between the theoretical porosity values and the ooze porosity data the porosity deviations correlated with variations in the mean grain size of the sediments due in part to changes in the foraminifera abundance in the case of the oozes the estimated mechanical parameters were consistent with published values obtained from one dimensional compression tests in contrast the estimated mechanical properties for the chalks differed from published values the chalk porosities were lower than could be explained by mechanical compaction this explanation is supported by the compressional p wave velocity data in the chalk sections the p wave velocity increases more rapidly with burial depth than it does in the ooze sections suggesting that sediment elastic properties are increasing due to interparticle binding

modeling the effects of diagenesis on carbonate clumped isotope values in deep and shallow water settings the measurement of multiply isotopically substituted clumped isotope carbonate groups provides a way to reconstruct past mineral formation temperatures however dissolution reprecipitation i e recrystallization reactions which commonly occur during sedimentary burial can alter a sample s clumped isotope composition such that it partially or wholly reflects deeper burial temperatures here we derive a quantitative model of diagenesis to explore how diagenesis alters carbonate clumped isotope values we apply the model to a new dataset from deep sea sediments taken from ocean drilling project site 807 in the equatorial pacific this dataset is used to ground truth the model we demonstrate that the use of the model with accompanying carbonate clumped isotope and carbonate 18 o values provides new constraints on both the diagenetic history of deep sea settings as well as past equatorial sea surface temperatures specifically the combination of the diagenetic model and data support previous work that indicates equatorial sea surface temperatures were warmer in the paleogene as compared to today we then explore whether the model is applicable to shallow water settings commonly preserved in the rock record using a previously published dataset from the bahamas we demonstrate that the model captures the main trends of the data as a function of burial depth and thus appears applicable to a range of depositional settings

diagenetic origins of the calcite microcrystals that host microporosity in limestone reservoirs abstract micron size pores are common in limestones and they host significant hydrocarbon reserves in conventional and unconventional plays worldwide previous studies have shown that the vast majority of micropores in limestones occur in the interstices between micron size calcite crystals that are present in nearly all depositional environments and burial depths throughout the phanerozoic in an effort to elucidate the genetic history of these calcite microcrystals a geochemical study was undertaken to investigate a global dataset collected from seven new subsurface microporous reservoirs as well as data from 21 previous studies bulk elemental analyses indicate that the vast majority of microcrystals are composed of low mg calcite average 14 mmol mol range 1 6 90 mg ca microcrystal sr mg ratios are consistent with abiotic calcite cements though overall mg ca and sr ca are lower than modern abiotic calcite cements reported in the literature higher sr ca observed in depositional chalks implies a biotic though probably not aragonitic precursor most calcite microcrystals have oxygen isotope compositions that are 0 to 4 more negative and carbon isotope compositions just slightly more negative 1 than contemporaneous abiotic marine calcite most stable oxygen and carbon isotopic data plot either along a burial trend anchored at its most positive end in the age equivalent marine calcite composition or parallel to that trend with an offset in 13c assuming that 13c offsets are due to marine carbon isotopic excursions the isotopic data are consistent with shallow burial diagenesis as the dominant diagenetic pathway for most calcite microcrystals for samples with information on porosity and depth more negative 18o ca 1 4 is associated with increased depth and reduced porosity implying that progressive overgrowth of calcite microcrystals is modifying bulk chemistry and rock properties of the reservoir during burial diagenesis a meteoric calcite line was evident only in two datasets this demonstrates that evidence of meteoric diagenesis can be preserved in the chemistry of calcite microcrystals but that its occurrence is rare in comparison with evidence for diagenesis during shallow burial geochemical data from calcite microcrystals are bulk samples i e measurements averaged over many many calcite microcrystals which themselves may have zoned chemistries as such microcrystal data require additional interpretation compared to samples from calcite macrocrystals where superposition puts samples into clear chronological order and produces diagenetic trends on for example 18o 13c plots bulk samples of calcite microcrystals should show mixing between phases on diagenetic trends with this model in mind the assembled database suggests that most samples are mixes of primary marine and burial calcite

chalk dolomitization beneath localized subsiding tertiary depressions in a marginal marine setting in the paris basin france abstract dolomitization of the late cretaceous chalk in the southeastern paris basin forms kilometer size bodies that crosscut bedding these diagenetic structures were first identified during petroleum exploration of intra chalk seismic discordances and then they were cored down to 700 m the high velocity chalk forms kilometer size hill shaped structures between 150 and 600 m depth similarly the top of the chalk shows kilometer size domes and depressions with an average amplitude of 50 m and slopes over 3 whereas the base of the chalk shows only very gentle dip two drill cores 2 km apart intersecting the seismic discordances allowed us to establish that these structures correspond to dolomitized chalk and porosity reduction detailed biostratigraphic correlations between the two drill cores show that 1 the seismic discordances are not related to the stratigraphy and 2 dolomitization of the chalk coincides with an apparent compaction of the whole unit the overlying early and middle eocene formations vary spatially between 20 and 100 m thickness the distribution of their sedimentary facies indicates that the dome and depression structures at the top of the chalk developed during the middle eocene the depression structures and localized middle eocene depocenters directly overlie the dolomitized diagenetic structures in the chalk furthermore the depressions commonly correspond to zones where sand channels are cut through the early eocene claystone down to chalk groundwater circulation within the chalk may have been linked to the tertiary sand channels such circulation might have been responsible for the dolomitization of the chalk and concurrent dissolution and or compaction leading to differential subsidence in the tertiary deposits the link between tertiary localized depocenters and dolomitization constrains the chalk diagenesis to the eocene and to nearsurface environments stable isotope compositions of the dolomites indicate that sea water of normal salinity was likely the dolomitizing fluid dolomitization was apparently triggered by recharge of the chalk groundwater with sea water entering the chalk in depression areas lastly the diagenetic processes result in sharp seismic reflectors that cut across stratigraphy these seismic reflectors first interpreted as deep intra chalk erosional surfaces do not follow any depositional stratigraphy at all consequently such structures cannot be interpreted in terms of sequence stratigraphy

burial stress and elastic strain of carbonate rocks burial stress on a sediment or sedimentary rock is relevant for predicting compaction or failure caused by changes in e g pore pressure in the subsurface for this purpose the stress is conventionally expressed in terms of its effect the effective stress defined as the consequent elastic strain multiplied by the rock frame modulus we cannot measure the strain directly in the subsurface but from the data on bulk density and p wave velocity we can estimate the rock frame modulus and biot s coefficient and then calculate the effective vertical stress as the total vertical stress minus the product of pore pressure and biot s coefficient we can now calculate the elastic strain by dividing effective stress with the rock frame modulus by this procedure the degree of elastic deformation at a given time and depth can be directly expressed this facilitates the discussion of the deformation mechanisms the principle is illustrated by comparing carbonate sediments and sedimentary rocks from the north sea basin and three oceanic settings a relatively shallow water setting dominated by coarse carbonate packstones and grainstones and two deep water settings dominated by fine grained carbonate mudstones and wackestones

regional velocity depth anomalies north sea chalk a record of overpressure and neogene uplift and erosion a normal velocity depth trend for the upper cretaceous danian chalk group is determined by identifying interval velocity data that represent maximum burial in areas unaffected by overpressuring these data are derived from 845 wells throughout the north sea basin data from pelagic carbonate deposits on a stable plateau constrain the trend for shallow depths positive velocity anomalies relative to the trend are mapped along the western and eastern margins of the north sea basin and reflect regional neogene uplift and erosion of up to 1 km along the present day limit of the chalk a hiatus at the base of the quaternary increases in magnitude away from the basin center where a complete cenozoic succession is found this hiatus is consistent in size with the missing section estimated from chalk velocities when allowance is made for the quaternary reburial of the chalk negative velocity anomalies in the central and southern parts of the basin outline an area within which overpressures in the chalk exceed 10 mpa equivalent to a burial anomaly greater than 1 km relative to the normal trend the chalk pressure system is primarily dependent on overburden properties because retention of overpressure generated by the load of the upper overburden depends on the thickness and sealing quality of the lower overburden therefore the chalk is considered to represent a regional aquitard and the hydrodynamic model of long distance migration within the chalk is rejected the neogene uplift and erosion of the margins of the north sea basin and the rapid late cenozoic subsidence of its center fit into a pattern of late cenozoic vertical movements around the north atlantic

experimental observation on laterally loaded pile in unsaturated silty soil an experimental study was carried out to investigate the effects of soil partial saturation on the behaviour of laterally loaded piles the proposed study was conducted by means of centrifuge tests

elastic behaviour of north sea chalk a well log study we present two different elastic models for respectively cemented and uncemented north sea chalk well log data we find that low biot coefficients correlate with anomalously low cementation factors from resistivity measurements at low porosity and we interpret this as an indication of cementation in contrast higher biot coefficients and correspondingly higher cementation factors characterize uncemented chalk for the same low porosity accordingly the poisson s ratio porosity relationship for cemented chalk is different from that of uncemented chalk we have tested the application of the self consistent approximation which here represents the unrelaxed scenario where the pore spaces of the rock are assumed to be isolated and the gassmann theory which assumes that pore spaces are connected as tools for predicting the effect of hydrocarbons from the elastic properties of brine saturated north sea reservoir chalk in the acoustic impedance poisson s ratio plane we forecast variations in porosity and hydrocarbon saturation from their influence on the elastic behaviour of the chalk the gassmann model and the self consistent approximation give roughly similar predictions of the effect of fluid on acoustic impedance and poisson s ratio but we find that the high frequency self consistent approach gives a somewhat smaller predicted fluid saturation effect on poisson s ratio than the low frequency gassmann model the gassmann prediction for the near and potentially invaded zone corresponds more closely to logging data than the gassmann prediction for the far virgin zone we thus conclude that the gassmann approach predicts hydrocarbons accurately in chalk in the sonic frequency domain but the fluid effects as recorded by the acoustic tool are significantly affected by invasion of mud filtrate the amplitude versus angle ava response for the general north sea sequence of shale overlying chalk is predicted as a function of porosity and pore fill the ava response of both cemented and uncemented chalk generally shows a declining reflectivity coefficient versus offset and a decreasing normal incidence reflectivity with increasing porosity however for the uncemented model a phase reversal will appear at a relatively lower porosity compared to the cemented model

required but disguised environmental signals in limestone marl alternations the nature of rhythmic carbonate rich successions such as limestone marl alternations has been and still is subject to controversy the possibility of an entirely diagenetic origin for the rhythmic calcareous alternations is discarded by most authors one problem with an entirely diagenetic self organized development of limestone marl alternations is the fact that limestone and marl beds in many examples are laterally continuous over hundreds of meters or even kilometers in an entirely self organized system lateral coupling would be very limited thus one would expect that rather than laterally continuous beds randomly distributed elongate nodules would form we address the origin of limestone marl alternations using a computer model that simulates differential diagenesis of rhythmic calcareous successions the setup uses a cellular automaton model to test whether laterally extensive rhythmic calcareous alternations could develop from homogeneous sediments in a process of self organization our model is a strong simplification of early diagenesis in fine grained partly calcareous sediments it includes the relevant key mechanisms to the question whether an external trigger is required in order to obtain laterally extensive limestone marl alternations our model shows that diagenetic self organization alone is not sufficient to produce laterally extensive correlatable beds although an external control on bedding formation could be considered to have solved the problem as commonly assumed we here suggest an interesting third possibility the rhythmic alternations were formed through the interaction of both an external trigger and diagenetic self organization in particular we observe that a very limited external trigger either in time or amplitude readily forms correlatable beds in our otherwise diagenetic model remarkably the resulting rhythmites often do not mirror the external trigger in a one to one fashion and may differ in phase frequency and number of couplets therefore the interpretation of calcareous rhythmites as a one to one archive of climate fluctuations may be misleading parameters independent of diagenetic alteration should be considered for unequivocal interpretation

diagenetic processes in carbonate mound sediments at the south west rockall trough margin cold water coral covered carbonate mounds at the south west margin of the rockall trough form ridges several kilometres long and up to 380 m high piston cores obtained at three mound crests reveal the complex internal structure of the mound build up with alternating unlithified coral dominated intervals and lithified intervals the most recent lithified interval is covered by corals embedded in a fine grained matrix comprising ca 11 000 years of continuous mound evolution before this time 230th u dating shows the presence of several hiatuses in mound build up aragonitic coral material is absent or only present as mouldic porosity in the lithified intervals and coccoliths display widespread overgrowth downcore x ray fluorescence scanning computer tomography scan images and petrographic observations indicate different degrees of diagenetic alteration the upper boundary of the most recent lithified interval shows some erosional features but petrographic observations indicate that initial lithification of the sediments is not related to this erosive event or to long term non sedimentation but to earlier sub surface diagenesis organic matter oxidation and the subsequent lowering of the saturation state of the carbonate system drives dissolution of the unstable aragonitic coral skeletons depending on the openness of the system this can lead to precipitation of a more stable low magnesium carbonate a model is presented describing the sedimentary and diagenetic processes leading to the formation of lithified intervals

stylolites porosity depositional texture and silicates in chalk facies sediments ontong java plateau gorm and tyra fields north sea comparison of chalk on the ontong java plateau and chalk in the central north sea indicates that whereas pressure dissolution is controlled by effective burial stress pore filling cementation is controlled by temperature effective burial stress is caused by the weight of all overlying water and sediments as counteracted by the pressure in the pore fluid so the regional overpressure in the central north sea is one reason why the two localities have different relationships between temperature and effective burial stress in the chalk of the ontong java plateau the onset of calcite silicate pressure dissolution around 490 m below sea floor bsf corresponds to an interval of waning porosity decline and even the occurrence of proper stylolites from 830 m bsf is accompanied by only minor porosity reduction because opal is present the pore water is relatively rich in si which through the formation of ca silica complexes causes an apparent super saturation of ca and retards cementation the onset of massive pore filling cementation at 1100 m bsf may be controlled by the temperature dependent transition from opal ct to quartz in the stylolite bearing chalk of two wells in the gorm and tyra fields the nannofossil matrix shows recrystallization but only minor pore filling cement whereas microfossils are cemented cementation in gorm and tyra is thus partial and has apparently not been retarded by opal controlled pore water a possible explanation is that due to the relatively high temperature silica has equilibrated to quartz before the onset of pressure dissolution and thus in this case dissolution and precipitation of calcite have no lag this temperature versus effective burial stress induced difference in diagenetic history is of particular relevance when exploring for hydrocarbons in normally pressured chalk while most experience has been accumulated in the over pressured chalk of the central north sea

timing and magnitude of early aptian extreme warming unraveling primary δ18o variation in indurated pelagic carbonates at deep sea drilling project site 463 central pacific ocean abstract in order to elucidate early aptian marine paleotemperature evolution across the period of enhanced organic carbon corg burial oceanic anoxic event oae 1a stable isotope analyses were performed on pelagic limestones at deep sea drilling project site 463 central pacific ocean the 18o data exhibit a distinct anomaly by 2 spanning the oae 1a interval i e a 6 m thick phytoplanktonic corg rich unit constrained by magneto bio and 13c stratigraphy elucidation of paleotemperature significance of the 18o shift is made by taking account of recent sr ca evidence at the same section which revealed that geochemical signals in carbonate poor lithologies are relatively unaltered against burial diagenesis by discriminating 18o values from carbonate poor samples caco3 contents 5 30 wt it appears that an abrupt rise in sea surface temperatures ssts by 8 c 1 7 shift in 18o occurred immediately before oae 1a whereas a cooling mode likely prevailed during the peak corg burial in terms of its stratigraphic relationship as to the corg rich interval and to a pronounced negative 13c excursion as well as its timescale the observed sst rise resembles those associated with the paleocene eocene thermal maximum and more strikingly jurassic toarcian oae this observation is consistent with the hypothesis that these paleoenvironmental events were driven by a common causal mechanism which was likely initiated by the greenhouse effect via massive release of ch4 or co2 from the isotopically light carbon reservoir and terminated by a negative productivity feedback

ultrasonic velocities of north sea chalk samples influence of porosity fluid content and texture we have studied 56 unfractured chalk samples of the upper cretaceous tor formation of the dan south arne and gorm fields danish north sea the samples have porosities of between 14 and 45 and calcite content of over 95 the ultrasonic compressional and shear wave velocities vp and vs for dry and water saturated samples were measured at up to 75 bar confining hydrostatic pressure corresponding to effective stress in the reservoir the porosity is the main control of the ultrasonic velocities and therefore of the elastic moduli the elastic moduli are slightly higher for samples from the south arne field than from the dan field for identical porosities this difference may be due to textural differences between the chalk at the two locations because we observe that large grains i e filled microfossils and fossil fragments that occur more frequently in samples from the dan field have a porosity reducing effect and that samples rich in large grains have a relatively low porosity for a given p wave modulus the clay content in the samples is low and is mainly represented by either kaolinite or smectite samples with smectite have a lower p wave modulus than samples with kaolinite at equal porosity we find that ultrasonic vp and vs of dry chalk samples can be satisfactorily estimated with gassmann s relationships from data for water saturated samples a pronounced difference between the vp vs ratios for dry and water saturated chalk samples indicates promising results for seismic amplitude versus offset analyses

sr ca ratios as indicators of varying modes of pelagic carbonate diagenesis in the ooze chalk and limestone realms abstract there has been an interest in the application of sr ca ratios in pelagic carbonates to reconstruct paleoceanographic conditions such as changes in the oceanic ca and sr budgets or calcareous nannoplankton productivity but the effect of burial diagenesis on primary sr ca records has not been sufficiently understood in order to clearly document sr ca records of pelagic carbonates relative to original paleoceanography or secondary diagenetic processes sr ca analyses were performed on bulk sediments from an 822 5 m thick ooze chalk limestone sequence at the mid pacific mountains deep sea drilling project site 463 the sr ca ratios exponentially decrease from 2 3 to 1 1 mmol mol with increasing burial depth in the ooze chalk interval 0 452 0 m below seafloor mbsf the sr ca ratios further decline to as low as 0 4 mmol mol in the limestone interval 452 0 mbsf with relatively high values of 1 1 mmol mol at carbonate poor lithology around 620 mbsf the stratigraphic sr ca trends appear to be consistent with porosity profile and similar sr ca porosity covariation in association with diagnetic changes in lithologies is also common in ontong java plateau sections by adopting a recent diagenetic model from backscattered electron image analysis of ontong java plateau sections it is postulated for site 463 that exponential sr ca decrease in the ooze chalk interval is due to the release of sr into interstitial water via recrystallization i e purification effect and that further decrease in sr ca ratios in the limestone interval is most likely due to acquisition of sr depleted calcite cement at the chalk limestone transition newly proposed as the dilution effect specifically sr ca ratios of deeply buried pelagic limestones are sensitive to the amount of calcite cement that is acquired in proportion to caco3 content i e carbonate rich lithology facilitates precipitation of secondary calcite at the chalk limestone transition therefore paleoceanographic interpretation by means of sr ca ratios in pelagic carbonates will become valid only when these diagenetic effects are appropriately constrained

the effect on chalk recrystallization after treatment with oxidizing agents abstract chalk consists of the fossilized remains of the hard parts from some species of unicellular algae these algae use polysaccharides to control the morphology of their shields called coccoliths which are made of from 20 to 60 individual sub micrometer size calcite crystals this biogenic calcite has resisted recrystallization during the more than 65 million years of diagenesis in this study we examined the effect of oxidizing agents on the ability of the chalk to recrystallize samples from a north sea drill core that had never been exposed to oil and rordal chalk from the alborg portland quarry north jutland denmark were treated with oxidizing agents to remove organic material and the recrystallization rates were determined we treated the chalk with solutions of naocl and h2o2 with and without 0 1 m nacl and monitored recrystallization in caco3 saturated 0 1 m nacl solutions using h14co3 as a tracer x ray photoelectron spectroscopy xps showed that initially the north sea chalk had a higher surface concentration of organic carbon than the alborg sample but it decreased significantly after the oxidizing treatment loss of c o correlated with higher recrystallization rates rate increase was highest 50 after h2o2 treatment our results demonstrate that through geological time miniscule amounts of organic material remain associated with the chalk surface and act as an inhibitor for chalk recrystallization this implies that the polysaccharides originally made by the algae to control coccolith morphology have contributed to hindering chalk diagenesis to limestone keeping the biogenic particles much smaller even after 65 million years than inorganic calcite grown in the lab for a few days