fluid flow equations governing primary oil migration as a separate phase abstract a mathematical model of primary oil migration as a separate phase out of compacting shales is presented during burial and oil generation source rock porosity decreases and oil saturation increases until residual oil saturation is reached at this stage oil is expelled out by capillary and excess fluid pressure gradients the model is a system of differential equations which relate changes in oil and water saturation in time to water and oil flow out of the source rock during burial an additional set of equations for periods of erosion of overburden are also provided the equations can be numerically solved by finite difference method if oil and water flow is to be simulated during oil generation then at each time step changes by oil generation in oil and water saturations and porosity must be calculated the solution procedure is briefly outlined

establishing resolution limits for tectonic subsidence curves by forward basin modelling abstract a forward model was used to study the relationship between gravitational compaction and tectonic subsidence in an idealized one dimensional sedimentary basin gravitational compaction was modelled by a non linear partial differential equation that gives the sediment porosity as a function of time and burial depth the sediments were described in terms of soil mechanics laws for mechanical compressibility and permeability the sedimentation rate appeared as a boundary condition at the depositional surface tectonic subsidence was coupled to the compaction model via an isostatic balance based on airy s hypothesis parameter studies showed that changes in the sedimentation rate and in the sediment compressibility can lead to variations from 454 to 781 m in the subsidence for sediment thicknesses in the range 6 9 7 9 km the forward model was used to assess how well standard backstripping methods reconstructed subsidence history from present day porosity versus depth data standard backstripping can bias the tectonic subsidence by 100 200 m comparable with inferred eustatic variations this bias results because the porosity depends not only on burial depth but also on time by using deterministic models to describe gravitational compaction mechanical properties can be used to better constrain compaction history thus improving the accuracy of reconstructed tectonic subsidence curves

a two dimensional model of overpressure development and gas accumulation in venture field eastern canada 1 we used a basin scale model to reconstruct the development of overpressures in a sedimentary basin the area of application is the venture gas field nova scotia canada it is in an overpressured zone in which sedimentation rates have been relatively low since 90 ma and where large horizontal stresses have been evidenced we investigated the importance of overpressure mechanisms using the institut francais du petrole temispack model which integrates compaction hydraulic fracturing fluid flow heat transfer and the formation and migration of hydrocarbons we first constructed a reference model using geological geochemical and physical data second we studied sensitivity of the model to several input parameters and from this deduced additional constraints the modeled permeability along faults and of shaly beds interlayered in the reservoirs must be very low to enable both the local and regional distributions of overpressure and gas to be reproduced several runs of the reference model studying the relative importance of possible overpressuring mechanisms and the relative contribution of plausible source rocks to gas accumulations showed that gas generation and accumulation and lateral compression contribute little to overpressuring which is mainly accounted for by compaction disequilibrium in spite of low sedimentation rates faults and shaly beds play an important role in maintaining the overpressure gas generation in the reservoir units or close to them does not contribute significantly to the field gas accumulations gas sources are likely to be found in the underlying formations

evolution of structural and physical parameters of clays during experimental compaction abstract to provide a better understanding of sedimentary basin geological history it is important to describe correctly the evolution of the various physical mechanical and hydraulic properties of clayey rocks as a function of burial depth as a contribution to this field a programme of experimental studies on reworked clay samples compacted under various load pressures in oedometric conditions has been set up the evolution of samples under compaction was followed with microscopic and macroscopic measurements in a specially designed oedometric cell samples are compacted under different total stresses from 0 1 to 50 mpa in this cell cylindrical cores are submitted to progressive loading from both ends under controlled pore pressure conditions the symmetrical loading allows more even deformation about the midplane of the sample this device allows the evolution of hydraulic pressure radial stress displacement and expelled pore fluid to be followed as a function of time in a first step kaolinite was chosen because it retains a high permeability compared with other clays such as illite or smectite which allows compaction tests to be performed within a few days a complete set of measurements was performed after the tests these measurements are 1 micro structures investigated by means of transmission electron microscopy tem mercury porosimetry water removal under low water vapour pressure granulometry and specific area measured by ethylene glycol adsorption and 2 various physical parameters measured including hydraulic conductivity and thermal conductivity tem gives an understanding of the arrangement of particles it was found that each particle is composed of several crystal units each unit formed by 25 individual kaolinite layers during compaction these particles remain undeformed but are rotated the angular distribution of grain orientation is a function of the applied effective stress this reorientation is in agreement with the observed decrease in porosity and pore size it also explains the occurrence of a strong anisotropy in the thermal conductivity and hydraulic permeability the combination of these experimental results allows a qualitative and quantitative understanding of the behaviour of kaolinite with respect to parameters such as permeability porosity mechanical and thermal properties the knowledge of which are necessary for basin modelling

the characterisation of cohesive sediment properties abstract this paper describes apparatus techniques and methods used by participants in mast g6m project 4 cohesive sediments for determining cohesive sediment properties this comparison of methods aims to stimulate a more general discussion on standardisation of techniques which will lead to characterisation of muds in terms of physical parameters such characterisation would allow inter comparison of muds from different sources methods are given for sediment properties of grain size distribution settling velocity and rheological parameters and for water bed exchange properties of permeability effective stress and critical shear stress for erosion and deposition accuracy and repeatability are discussed a typical range of values is indicated for each of these parameters

instruments for particle size and settling velocity observations in sediment transport in this paper we describe two sensors for measurement of particle size distribution and settling velocity distribution these measurements are critical to the correct estimation of the true sediment concentration in the field as well as to evaluating models for transport rates of sediments a multi angle measurement of laser scattering is made and inverted to obtain the particle size distribution since small angle scattering is relatively insensitive to particle composition the size distribution measurements are robust and do not require particle refractive index it is shown that with a knowledge of the size distribution true particulate volume concentration can be obtained unaffected in calibration by changes in particle size distribution the data from bottom boundary layer experiments using the instrument show the presence of temporal variability in size distribution associated with the strength of forcing of the boundary layer the importance of these observations lies in the implication that historical data acquired with single parameter optical or other sensors needs to be revisited in the second instrument analyzing the observation of size distributions during settling in a settling column produces settling velocity estimates in this case the history of concentration of each size class is examined to determine the settling velocity without invoking any assumptions of settling regime settling velocity data from a field experiment off the new jersey coast fit the model wf na 0 45 10 23 a 1 2 n

the development of polygonal fault systems by syneresis of colloidal sediments abstract polygonal fault systems occur in numerous sedimentary basins worldwide are generally located on passive margins in onlap fill units and tend to comprise the finest grained sediments in this geological setting these fault systems have been most thoroughly described in the central north sea basin and the detailed structure shows a significant correlation with lithological variations both vertically and laterally extension measured in stacked decoupled tiers of polygonal faults correlates positively with both clay fraction and smectite content lateral facies variations are also observed and indicate that time equivalent sequences upslope from the smectite rich polygonally faulted sediments are coarse grained clay poor and undeformed this leads us to believe that the structure and geometry of the fault system are controlled by the colloidal nature of the sediments and that the volumetric contraction measured on seismic sections can be accounted for by syneresis of colloidal smectitic gels during early compaction syneresis results from the spontaneous contraction of a sedimentary gel without evaporation of the constituent pore fluid this process occurs due to the domination of interparticle attractive forces in marine clays dependent on environment and is governed by the change of gel permeability and viscosity with progressive compaction the process of syneresis can account for a number of structural features observed within the fault systems such as tiers of faults the location of maximum fault throw and growth components at upper fault tips as such this paper represents the first attempt to correlate microscale properties of clay rich sediments to their macroscale seismic character

disequilibrium compaction overpressure in shales of the bavarian foreland molasse basin results and geographical distribution from velocity based analyses abstract shale velocity data from sonic logs and vertical seismic profiles drilling data and in situ pressure measurements from a total of 116 wells have been analyzed to gain an improved understanding of the lateral and vertical distribution and formation of overpressure in the bavarian foreland molasse basin pore pressure from sonic and vertical seismic profile velocities has been analyzed by establishing a normal compaction trend for cenozoic and mesozoic shales combined with a classical eaton approach the study demonstrates that a single shale normal compaction trend for the bavarian foreland molasse basin is sufficient to estimate pore pressure and thus overpressure from sonic and vertical seismic profile velocity maximum overpressure develops at depths between 1500 and 2500 m and increases with depth at a constant effective stress of 20 mpa the strong dependency of overpressure on burial depth constant effective stress and restriction to shale units that are overlain by sequences with very high sedimentation rates indicates that disequilibrium compaction is the main cause for overpressure also variable presence of cretaceous shales is a key control of overpressure occurrence in oligocene shales in the bavarian foreland molasse basin since cretaceous shales likely act as a pressure buffer to underpressured mesozoic carbonates successful detection and prediction of overpressure from vertical seismic profile data is encouraging for future pre drill prediction of overpressure from velocity data of seismic surveys in the bavarian foreland molasse basin resulting in improved well planning reliable calculation of project costs and improved safety during drilling activities in the bavarian foreland molasse basin the presented overpressure distributions will be a key input for future geomechanical basin and reservoir modelling studies in the bavarian foreland molasse basin

aquathermal pressuring and geopressure evaluation geologic notes evaluation of the aquathermal mechanism proposed by barker indicates that this process cannot produce geopressures in clays or sediments with even negligible permeabilities it is suggested that because the amount of fluid expansion is very small with temperature increase typical hydraulic conductivities of clay rocks can accommodate the removal of this excess fluid in the time span available thus it would appear that expansion of waters cannot contribute to the geopressure generating process lithologies of effectively zero permeabilities however may provide adequate sealing of contained pore fluids so that pressures increase with increasing temperature the subsequent rupturing of the matrix as pore pressures reach overburden pressures particularly rapidly at shallow depths produces fractures which may later provide routes for hydrocarbon migration as a pressure transition zone cannot exist due to the necessary isolation of pore fluids current geopressure evaluation techniques are limited in their application such that only flowline temperature interpretation may indicate subsequent penetration of an aquathermally generated geopressure if a transition zone does exist above a geopressured interval then some process other than aquathermal pressuring must have been instrumental

a pretreatment method for grain size analysis of red mudstones article i nfo traditional sediment disaggregation methods work well for loose mud sediments but not for tightly cemen ted mudstones by ferric oxide minerals in this paper a new pretreatment method for analyzing the grain size of red mudstones is presented the experimental samples are eocene red mudstones from the dongying de pression bohai bay basin the red mudstones are composed mainly of clay minerals clastic sediments and ferric oxides that make the mudstones red and tightly compacted the procedure of the method is as follows firstly samples of the red mudstones were crushed into fragments with a diameter of 0 6 0 8 mm in size secondly the cbd citrate bicarbonate dithionite treatment was used to remove ferric oxides so that the cementation of intra aggregates and inter aggregates became weakened and then 5 dilute hydrochloric acid was added to further remove the cements thirdly the fragments were further ground with a rubber pes tle lastly an ultrasonicator was used to disaggregate the samples after the treatment the samples could then be used for grain size analysis or for other geological analyses of sedimentary grains compared with other pretreatment methods for size analysis of mudstones this proposed method is more effective and has higher repeatability

formation of the zeebrugge coastal turbidity maximum the role of uncertainty in near bed exchange processes abstract despite availability of a large amount of observational data and modelling studies the mechanisms maintaining the turbidity maximum in the belgian dutch coastal zone around the port of zeebrugge belgium are insufficiently understood in order to better understand the dynamics of this turbidity maximum we examine the role of baroclinic salinity and sediment induced processes and local sediment sources on the formation and persistence of the turbidity maximum through two different numerical model approaches one model approach allows erosion of the highly compacted muddy seabed serving as a sediment source in line with observations of bed level change over several decades the other approach reduces the exchange between the bed and the water column to mimic the formation of highly concentrated near bed suspensions with concentrations of several g l observed around the port of zeebrugge both model approaches are calibrated to various sources of available data in situ sediment concentration observations satellite image bed level changes mud content and dredging data which they reproduce comparably well however reducing the water bed exchange strengthens sediment convergence in the turbidity maximum whereas the sediment source leads to sediment export with the available data it is difficult to determine which of the approaches is more realistic apparently the lack of knowledge on near bed exchange processes introduces an important source of uncertainty which cannot be adequately addressed with currently available observations this work therefore shows that more quantitative knowledge on water bed exchange processes in turbid marine environments is needed it is further hypothesized that the large scale erosion of the muddy seabed following the extension the port of zeebrugge in the early 1980 s brought such a large amount of sediment in suspension 50 100 million ton that sediment convergence was strengthened this increasing sediment convergence introduces a positive feedback mechanism that maintains sediment in the turbidity maximum or even strengthens it the high sediment concentrations observed today may therefore be a long term effect of port construction carried out decades earlier

lithospheric flexure and the evolution of sedimentary basins the sediments that have accumulated in sedimentary basins during geological time represent a load on the lithosphere that should respond by flexure simple elastic and viscoelastic maxwell plate models have been used to examine quantitatively the contribution of flexure to basin formation the models have been used to predict the stratigraphy and gravity anomalies associated with basins for different thermal and loading histories the predictions of the models have been compared with observed stratigraphy and free air gravity anomalies from interior and cratonic basins the best overall fit to the observations is for an elastic plate model in which the flexural strength of the lithosphere increases with age a similar model has recently been used successfully to explain observations from continental margin basins oceanic islands and seamounts and deep sea trench outer rise systems this model explains the increase in the overall width of basins during their evolution as well as the stratigraphy of the basin edges the apparent decrease in the widths of some basins through time can be explained by the model if sediment deposition is followed by erosion of the basin and its edges the models results suggest that the flexural properties of continental and oceanic lithosphere are generally similar and that flexure is an important factor to consider in backstripping studies in which the tectonic subsidence of a basin is isolated and stratigraphic studies in which relative changes of sea level are estimated

modeling of fluid flow and pore pressure at the toe of oregon and barbados accretionary wedges fluid venting was recently found on the central oregon accretionary wedge and evidence of fluid flow along the decollement were obtained in the barbados wedge the chemistry of hydrocarbons contained in the migrating fluids is different in these two areas and indicates that the main source is deep for the barbados decollement and shallow for the oregon vents we apply a finite element model with imposed thrusting kinematics and nonlinearities in the compaction equation to the toe of both wedges no deep fluid source is included thus the main source is the footwall just below the frontal thrust the model indicates the following 1 pore pressure build up would be essentially controlled by kn the equivalent permeability in the direction normal to the strata in spite of very different sediment lithologies kn is of the order of 10 17 m2 for both oregon turbidites horath 1989 and barbados muds taylor and leonard 1990 and no high pore pressures can be obtained inside the offscraped sediments 2 permeability anisotropy greatly modifies fluid flow paths a fault with a permeability anisotropy of 103 would channel fluids efficiently but leave the footwall pressure almost unchanged a stratigraphic permeability anisotropy of 102 as documented for the oregon turbidites would redirect the flow of pressurized fluids in the footwall along strata into the trench if clogging of the frontal thrust occured fluids expelled from the footwall would move across the frontal thrust migrate along permeable strata in the hanging wall and then be vented near the top of the eroded anticline as observed in oregon 3 comparison of published estimates from data of fluid explusion fluxes with the modeling results suggests that a deeper and larger fluid source is needed at the barbados site but is not required for the oregon site

permeability of illite bearing shale 2 influence of fluid chemistry on flow and functionally connected pores 1 bedding parallel permeability of illite rich shale of the wilcox formation has been investigated using distilled water and 1 m solutions of nacl kcl and cacl2 as pore fluids despite low modal concentrations of swelling clays specimens expand upon fluid saturation and permeabilities depend on fluid composition permeabilities to flow of 1 m cacl2 are 3 5 times greater than values measured for the other pore fluids suggesting sensitivity to exchange of divalent cations for monovalent cations at clay mineral surfaces permeabilities of individual samples exhibit nonrecoverable changes with sequential changes in composition of incoming fluid permeabilities k at varying effective pressure pe fit a cubic law k k0 1 pe p1 m 3 where m and p1 are independent of fluid composition and k0 is greater for transport of 1 m cacl2 than that for transport of the other pore fluids assuming that fluid conduits have crack like dimensions the lack of sensitivity of m and p1 to fluid composition suggests that surface roughness and asperity stiffness of conduits are unaffected by cation exchange while changes in k0 reflect changes in the clay fluid interfaces of the connected pore space index terms 5114 physical properties of rocks permeability and porosity 5139 physical properties of rocks transport properties 1832 hydrology groundwater transport 3947 mineral physics surfaces and interfaces 1045 geochemistry low temperature geochemistry keywords permeability shale fluid chemistry citation kwon o b e herbert and a k kronenberg 2004 permeability of illite bearing shale 2 influence of fluid chemistry on flow and functionally connected pores j geophys res 109 b10206 doi 10 1029 2004jb003055

a laboratory evaluation of the laser in situ scattering and transmissometery instrument using natural sediments abstract the lisst laser in situ scattering and transmissometery instrument was designed by sequoia scientific to measure the particle size distributions and concentrations of sediment suspensions in the field environment to understand the lisst s performance with natural sediments from a variety of marine sources several experiments were performed to compare the lisst s results to traditional sieving filtering and weighing techniques the lisst was able to correctly locate the peak of a unimodal particle size distribution and resolve the two peaks of a bimodal distribution if they are separated by at least 1 for sediment sizes of 5 to 250 m unlike a single frequency scattering sensor for sediment concentration which require sediment size specific calibration constants the lisst is able to accurately measure sediment concentration with a single calibration constant for varying size distributions within the size range of 5 to 250 m the lisst was also found to adequately represent the particle volumetric size distribution for two different samples from marine environments

permeability controls on aquathermal pressuring aquathermal pressuring refers to fluid pressures in excess of the hydrostatic which have been created by fluid thermal expansion against a less expansive sediment matrix two commonly stated objections to this mechanism are 1 hydraulic gradients and the finite but very low permeabilities now existing in the gulf of mexico are sufficient to dissipate aquathermal pressures and 2 the increasing temperatures decrease fluid viscosity at a rate so that hydraulic conductivity increases fast enough to dissipate these pressures evaluation of the momentum transport equations in the eulerian frame of reference indicates the first objection is not applicable because it treats the problem as steady flow and not transient flow furthermore existing data indicate that under no mal diagenetic conditions the decrease in intrinsic permeability caused by consolidation is greater than the viscosity effect therefore the two commonly stated objections to aquathermal pressuring do not by themselves prove the mechanism is ineffective in settings such as the gulf of mexico permeability increases by fracturing and faulting may however be sufficient to bleed off aquathermally produced excess pressures the measured very low permeabilities of gulf coast fine grained sediments indicate that aquathermal pressuring is a potentially important mechanism if the fine grained sediments do control the hydraulic response

geological controls on matrix permeability of devonian gas shales in the horn river and liard basins northeastern british columbia canada abstract controls of matrix permeability are investigated for devonian gas shales from the horn river and liard basins in northeastern british columbia canada mineralogy is varied with high carbonate high quartz and moderate quartz carbonate and clay rich strata quartz content varies between 2 and 73 carbonate varies between 1 and 93 and clay varies between 3 and 33 the toc content ranges between 0 3 and 6 wt and porosity varies between about 1 and 7 for horn river basin samples quartz is mainly biogenic in origin derived from radiolarians toc content increases with the quartz content suggesting the toc and quartz both are derived from siliceous phytoplankton a positive relationship between porosity and quartz content is due to the positive relationship between quartz and toc matrix permeability parallel to bedding varies between 7 5e 02 and 7 1e 07 md at an effective stress of 15 mpa variation in permeability is due to a complex combination of factors that includes origin and distribution of minerals pore size distribution and fabric mercury intrusion capillary curves indicate that the higher matrix permeability values 2e 03 md occurs in samples that contain interconnected pore apertures greater than 16 m even when these samples may contain less macropores than low permeability samples the fabric of high permeability samples can be either isotropic or anisotropic however permeability of anisotropic samples is more sensitive to changes in effective stress than isotropic samples more highly anisotropic samples contain moderate amounts of quartz carbonate and in some clay high permeability samples that contain a more balanced ratio between micro meso and macroporosity would not only have faster flow rates but also greater access to sorbed gas within the microporosity compared to samples that lack mesopores several muskwa samples compared to evie and besa river samples contain higher quartz moderate clay and high toc content coupled with high permeability less sensitivity to effective stress and balanced ratios between micro meso and macroporosity would be a lower exploration risk due a greater propensity to fracture the ability to produce and store hydrocarbons due to higher toc contents and greater communication between macropores and micropores in the organic and clay fractions

the calibration of optical backscatter sensors for suspended sediment of varying darkness levels a series of calibration experiments were run in order to determine the effect of the colour of suspended sediment on the response of optical backscatter sensors obs munsell notation describes the properties of soil and sediment colour using the three parameters hue chroma and value which represent respectively the spectral position lightness level and amount of pure chromatic colour present the exposure of the obs to munsell soil chart patches revealed that the obs is sensitive to variations in munsell value and not munsell hue or chroma since obs output decreased with darkness level and increased with lightness level and munsell value it is suggested that the level of blackness of particles acts to absorb the near infrared ir signal of the obs modifying its output a strong correlation r2 0 984 was found between the observed and predicted obs outputs of varying concentrations and ratios of black and white suspended sediment the obs output was predicted using a linear mixture model incorporating end member calibrations of black and white sediment similar to green and boon green m o boon j d iii 1993 the measurement of constituent concentrations in nonhomogeneous sediment suspensions using optical backscatter sensors marine geology 110 pp 73 81 a strong correlation r2 0 907 was also observed between luminance factor and ir reflectance for various minerals suggesting a link between munsell notation and obs output formulae were developed to predict the ir reflectance of sediment of known munsell value concentration and size models predicting sediment transport should consider the effect of the level of blackness of particles on the obs response

fabric development and the smectite to illite transition in upper cretaceous mudstones from the north sea an image analysis approach in this study upper cretaceous shetland group mudstone cuttings from a range of depths in the northern north sea have been studied using x ray diffraction mercury porosimetry and electron microscopy millimetre to micrometre mudstone textures have been quantified using image analysis of backscattered electron microscope images relatively shallow samples 1615 m have isotropic mudstone fabric no alignment of clay minerals are dominated by smectite and have porosity values of approximately 35 in contrast more deeply buried samples 3300 m have developed an anisotropic fabric distinct alignment of clay minerals are dominated by illite and have porosity values of approximately 22 the change in mineralogy is due to smectite replacement by illite which occurs simultaneously with porosity loss and fabric development during progressive burial image analysis of differentially buried mudstones has proved to be a rapid flexible and quantitative method for characterizing mudstone textures the coincidence of mineralogical evolution with textural development and compaction implies that the transformation of smectite to illite occurs by dissolution and precipitation and that chemically facilitated compaction may contribute to porosity loss

mudstone compaction and its influence on overpressure generation elucidated by a 3d case study in the north sea mudstones are one of the least permeable rocks in most sedimentary sequences accordingly they can act as seals for fluid flow leading to abnormal overpressures nevertheless mudstone compaction and related permeability and porosity decrease are not adequately described in current basin modelling software because only mechanical compaction is taken into account in reality however clay minerals undergo severe chemical diagenesis which certainly influences petrophysical properties and compaction in this context a mathematical approach which has been originally developed in soil mechanics has been adapted to basin modelling the underlying mathematical equations are carefully explained in the text in the basic equation the compression coefficient is a function of void ratio and effective stress using these equations overpressure can be predicted by using petroleum systems modelling techniques this is shown for a real 3d case study in the north sea in which strong overpressure occurs a compaction model for mudstones that depends strongly on the clay content of the individual stratigraphic units is used for the calibration of porosities in the 3d case study in addition a chemical compaction model that reduces porosities by using a kinetic reaction is used for the deeper part of the basin where mechanical compaction processes are less important the pressure generation process depends strongly on permeability and compressibility of the porous medium therefore the use of mudstone compaction and permeability models is sufficient to produce pore overpressures in the case studied abnormal overpressures are generated during burial together with the petroleum generation process the mechanical and chemical compaction mechanisms ensure that the pressures are preserved in the deeper part of the basin

sediment tracing and environmental history for two small catchments karoo uplands south africa abstract as part of an investigation of land degradation in the south african karoo three types of fine sediment signatures radionuclides mineral magnetism and sediment geochemistry were used to trace the origins of fine particulate sediment accumulating in two reservoirs that were constructed around the mid 1930s both reservoirs are fed by gullies that are known to have been in existence before their construction robust chronologies were derived from the 137 cs profiles in the reservoir sediments from aerial photographic records and from an analysis of extreme storm events that have left stratigraphic markers in the reservoir sediments extreme daily rainfalls have become more frequent in the last 30 years but sediment accumulation rates in the two reservoirs differ significantly one reservoir has high sediment accumulation rates between 1958 and 1964 that correlates with an expansion in the area of cereal cultivation the second catchment has no cultivated land but sedimentation rates in the reservoir declined after 1941 when livestock grazing densities decreased by a factor of around three despite the increase in the frequency of high magnitude daily rainfalls after 1970 sediment availability as controlled by land use appears to be the most important limit on changes in sedimentation rates differences in lithology and land use provided an opportunity to evaluate the number of sources that could successfully be discriminated in each catchment at different timescales 137 cs and unsupported 210 pb can be used to trace sources of actively transported or very recently deposited sediment and to discriminate sources of reservoir sediments between topsoils cultivated and uncultivated and subsoils or gully sidewalls lack of current 137 cs inputs means that it will become less useful as a future discriminator of topsoil subsoil sources because it only has a half life of ca 30 years and will eventually be undetectable the potential for unsupported 210 pb to provide an alternative method for discriminating these sources is demonstrated using principal component and discriminant function analysis and the three groups of sediment fingerprint signatures it was possible to discriminate doleritic soils from other potential sources in the catchments including valley fill and soils underlying vegetation that had been recently burned as part of the rangeland management system results suggest that fire was an important contributor to destabilising soil which was subsequently delivered to the reservoir during high magnitude rainfalls it is concluded that 1 changes in the extent of cereal cultivation and livestock grazing density are most closely related to temporal changes in sedimentation rates in each of the two reservoirs 2 temporal changes in the magnitude and frequency of extreme daily rainfalls do not correlate with reconstructed patterns of sediment accumulation through time 3 the gully systems provide connectivity between the dominant hillslope sources and the reservoirs but have not contributed significantly to the sediment deposited in either reservoir and 4 there is no evidence to suggest that sources of sediment accumulating in the reservoirs have changed significantly over the last ca 70 years

interactive aggregation and sedimentation of diatoms and clay sized lithogenic material monospecific diatom cultures thalassiosira punctigera and skeletonema costatum were incubated in rotating cylinders together with clay suspensions present in a range of concentrations 5 100 mg kaolinite l 21 and as different minerals 50 mg l 21 kaolinite smectite illite and clay sized quartz powder the addition of lithogenic suspensions to diatom cultures accelerated the formation of visible aggregates in the roller tanks by a factor of 3 aggregate size decreased and density increased proportionally to the amount of kaolinite added to the diatom cultures in the presence of kaolinite and illite aggregate sizes were smaller and sinking rates lower than in the presence of smectite and quartz the influence of lithogenic matter on the sinking velocities of aggregates was ambiguous compound aggregates sank faster with increasing amounts of lithogenic matter present in cultures of t punctigera until a certain ratio between lithogenic and biogenic material was reached further increasing the amount of lithogenic matter did not increase sinking rates significantly in contrast increasing the concentration of kaolinite added to cultures of s costatum could decrease sinking velocities of the evolving compound aggregates this nonlinear behavior is argued to be primarily a function of aggregate composition on aggregate sizes and excess densities although the possibility of a mutual acceleration of vertical flux of algae and clay is confirmed the results show that the presence of lithogenic material could also decrease the downward flux of phytoplankton biomass rapid sinking of phytoplankton derived organic carbon via sedimentation of aggregates contributes significantly to the vertical transport of marine particulate organic carbon poc a process known as the biological pump volk and hoffert 1985 rapid sinking of clay sized lithogenic material can be inferred from sediment trap data wefer 1989 laboratory studies on marine aggregate formation and properties have focused on pure phytoplankton aggregates kiorboe et al 1990 riebesell et al 1991 kiorboe and hansen 1993 or on monomineralic clay aggregates kranck 1973 however coaggregation of phytoplankton and clay is a familiar phenomenon in aquatic environments first drawings of marine snow show sundry marine organisms and mineral grains embedded in a mucous matrix which dominates the aggregates by volume and appears to hold the solid compounds together tsujita 1953 avnimelech et al 1982 described enhanced floc formation and sedimentation of freshwater algae in the presence of clay and vice versa the fact that lithogenic and biogenic particles are often found in deep sediment traps during the same time interval in spite of independent occurrence in the surface water also suggests coaggregation and cosedimentation of these particles deuser et al 1983 wefer 1989 ittekkot 1993 while small amounts of both biogenic and lithogenic particles are present in all marine environments areas with elevated concentrations of lithogenic particles often show a high phytoplankton abundance at the same time and are thus particularly interesting in terms of interaction between phytoplankton and lithogenic material examples for marine en

inverse modelling and seismic data constraints on overpressure generation by disequilibrium compaction and aquathermal pressuring application to the eastern black sea basin summary pore pressure above the hydrostatic overpressure is common in deep basins it plays an important role in pore fluid migration represent a significant drilling hazard and is one of the factors controlling slope stability and deformation in seismically active areas here we present an inverse model to calculate overpressure due to disequilibrium compaction and aquathermal pressuring we minimize a function that contains the misfits between estimates from our forward model and observed values using a non linear least squares approach the inverse model allows the introduction of observed seismic and geological constraints such as p wave velocity vp and density data and depth of the layer boundaries for a better pore pressure prediction the model output also provides estimates of 1 surface porosity 2 compaction factor 3 intrinsic permeability at surface conditions 4 a parameter controlling the evolution of the intrinsic permeability with porosity 5 the ratio between horizontal and vertical permeability and 6 uncompacted thickness so sedimentation rate assuming known time intervals for each sedimentary layer we apply our inverse approach to the centre of the eastern black sea basin ebsb where the vp structure has been inferred from wide angle seismic data first we present results from a 1 d inverse model and an uncertainty analysis based on the monte carlo error propagation technique to represent the observed rapid change from low vp to normal vp below the maikop formation we impose a zero overpressure bottom boundary and subdivide the layer below the maikop formation into two sublayers an upper layer where the rapid change is located and a lower layer where the vp is normal secondly we present the results from a 2 d inverse model for the same layers using two alternative bottom boundary conditions zero overpressure and zero flow we are able to simulatetheobserved vp suggestingthatthelowvelocityzone lvz at 3500 6500mdepth below the seabed mbsf can be explained by overpressure generated due to disequilibrium compaction 90 per cent and to aquathermal pressuring 10 per cent our results suggest thattheuppersublayer belowthemaikopformation behavesasasealduetoitslowpermeability 0 3 2 10 14 ms 1 this seal layer does not allow the fluids to escape downwards and hence overpressure develops in the maikop formation and not in the layers below this overpressure was mainly generated by the relatively high sedimentation rate of 0 29 m ka 1 of the maikop formation at 33 9 20 5ma and an even higher sedimentation rate of 0 93 m ka 1 at 13 11 ma we estimate a maximum ratio of overpressure to vertical effective stress in hydrostatic conditions o f 0 62 at 5200 mbsf associated with an overpressure

turbidite pathways pore fluid pressures and productivity in the central wales orefield abstract lodes in the central wales orefield are mineralized oblique slip normal faults that postdate the regional joints they developed shortly after the acadian deformation of the welsh basin in the late early devonian and many were reactivated with further mineralization in the early carboniferous although orefield extent is clearly centred on the structural culmination around the plynlimon dome the major areas of lead zinc production are strongly off centered these areas lie on the depositional pathways of three horizons of late ordovician early silurian massive turbidite sandstones it is argued that these basin floor sand bodies are preferential sites of lode nucleation and that their distribution controls the size and abundance of the lodes and hence their chances to link laterally and at depth this in turn controls their ability to capture and transmit mineralizing fluids and thus their chances of hosting large ore deposits the prime cause of high fluid pressure gradients during the first phase of mineralization was rapid uplift of tight9 rock with dehydration fluids from acadian metamorphism second phase fluids probably originated as seawater and were driven by convective overturn at essentially hydrostatic pressures the stratigraphic control of lode productivity has implications for mineral exploration in such terranes

some new developments for modelling the geological compaction of fine grained sediments introduction abstract from 1992 1995 the european union supported a programme of research entitled interdisciplinary basin studies one objective of the programme was to develop a better understanding of the compaction of fine grained sediments some of the results of the research are presented in the subsequent four papers which are introduced and summarised in this contribution

venture gas field offshore nova scotia case study of overpressuring in region of low sedimentation rate the venture gas field is a deep overpressured gas reservoir on the scotian shelf off eastern canada overpressuring occurs at depths below 4 5 km and is associated with significant quantities of gaseous hydrocarbons the overpressured zone is composed of high porosity sandstones interbedded with low porosity low permeability shales which have nearly normal levels of compaction pore pressures in this zone show a step wise increase with depth where pressure jumps correlate with low permeability 10 20m2 where 10 12m2 1 01 d shale beds petrophysical and lithological data for sandstones and shales in the overpressured zone are used to constrain a one dimensional single phase model of pore pressure development the con ersion of kerogen to hydrocarbons is simulated by simple kinetic reaction theory estimates of the effects on overpressuring of sediment compaction temperature changes and hydrocarbon generation were obtained from this model using the above data 2 disequilibrium compaction apparently is responsible for much of the overpressuring in the venture gas field in our simple single phase model the contribution of hydrocarbon generation to overpressuring was slight

geopressured shale and sealing effect of smectite to illite transition ten shale samples of frio and vicksburg tertiary overpressured shales from brazoria and hidalgo counties texas were examined by transmission electron microscopy tem techniques initially illite and smectite with some mixed layer component are present in subparallel domains with smectite packets dominating and providing abundant dislocations for ion transport supplying avenues that contribute to local permeability as the transition proceeds illite packets grow within a shrinking matrix of smectite dislocations decrease and pathways for ion transport are restricted we suggest this development of illite packets and diminished ion transport causes loss of local permeability and a rise in the fluid pressure gradient as illite packets coalesce local permeabil ty is increasingly lost hydraulic continuity with the surface is further restricted and a more efficient geopressure seal is formed in both hidalgo and brazoria counties the base of normal hydropressure 0 465 psi ft or 10 5 kpa m coincides with the depth of onset of the smectite to illite transition geopressure greater than 0 7 psi ft 15 8 kpa m occurs at depths corresponding to 50 illite packets in brazoria county and 70 in hidalgo county in both cases 0 7 psi ft 15 8 kpa m geopressure occurs prior to the depth at which the transition ceases these relationships suggest that if the smectite to illite transition begins and proceeds to completion the development of coalescing illite packets during diagenesis will lead to increasing geopressure with the actual values of illite proportion and corresponding fluid pressure gradient determined by the local geologic setting

oil generation as the dominant overpressure mechanism in the cenozoic dongying depression bohai bay basin china the dongying depression in the bohai bay basin is a young prolific oil producing basin in china the gray to black mudstones calcareous mudstones and oil shales of the eocene shahejie formation es3 and es4 are the major source rocks that are primarily dominated by type i kerogens with a high total organic carbon of up to 18 6 the es3 interval is characterized by a sedimentation rate of up to 500 m m y widespread overpressures are present in the eocene es3 and es4 intervals in the depression with pressure coefficients up to 1 99 from drillstem tests among the sonic resistivity and density logs only the sonic log displays an obvious response to the overpressure from which the top of the overpressure can be clearly identified acoustic traveltime versus effective vertical stress analysis of more than 300 wells in the dongying depression suggests that the acoustic traveltime of the normally pressured and overpressured mudstones is reduced with increasing vertical effective stress pore pressures are accurately predicted in the dongying depression using an eaton 1972 exponent of 2 0 by comparing the predicted pressure coefficients with measured pressure coefficients disequilibrium compaction has been previously proposed as the sole cause of the high magnitude overpressures in the eocene strata of the dongying depression because of rapid deposition of fine grained sediments however our data indicate that the overpressures are caused by oil generation from the source rocks within the es3 and es4 intervals the overpressured sediments display a normal compaction as evidenced from the overpressured mudstones exhibiting no anomalously low density the apparent lack of correlation between mudstone densities and effective vertical stress and the overpressured reservoir sandstones showing no anomalous high matrix porosities or anomalous geothermal gradient the depths to the top of the overpressure intervals range from 2000 to 3000 m 6562 9843 ft following closely source rock depths all the overpressured reservoirs and source rocks have a minimum temperature of approximately 87c and overpressured source rocks generally have vitrinite reflectance ro values of 0 6 or higher overpressures are not found in the strata within which the ro values are less than 0 5 the overpressured reservoirs in the es3 and es4 intervals are predominantly oil saturated or oil bearing organic rich source rocks with overpressures are capable of generating hydrocarbons and thus can maintain an abnormally high pressure the calcite precipitation in the calcareous mudstones may have caused a significant reduction in porosity and permeability to form an effective pressure seal the presence of widespread microfractures in the source rocks may relate to episodic expulsion of hydrocarbons or overpressure dissipation overpressures in the reservoir rocks are generated by the fluid transmission from the overpressured source rocks through active faulting and fracturing

suspended particulate matter dynamics and aggregate sizes in a high turbidity area measurements of aggregate size suspended particulate matter spm concentration and current velocity have been carried out in the belgian coastal zone southern north sea two stations were situated in the coastal turbidity maximum zone another station was located more offshore at the edge of this turbidity maximum the data have been collected using a lisst 100 obs sensors water samples and a bottom mounted adcp turbulence kolmogorov microscale has been modelled for the same period using a 3d numerical model the results show that the size of the aggregates is significantly smaller in the coastal turbidity maximum area the processes responsible for the occurrence of smaller aggregate size in the coastal zone compared with the more offshore location are the higher turbulence the smaller time available for the aggregates to grow up to an equilibrium size the higher deposition of mud resulting in a break up of the floes and the lower availability of organic matter which may limit the size of the floes c 2006 elsevier b v all rights reserved

time and temperature in petroleum formation application of lopatin s method to petroleum exploration n v lopatin in the soviet union has developed a method for taking both time and temperature into account as factors in thermal maturation of kerogen lopatin s time temperature index of maturity tti values correlate with the thermal regimes corresponding to generation and preservation of hydrocarbons because such information is potentially of great interest for oil exploration a calibration and evaluation have been made of lopatin s method within the limitations of the data presently available the following statements can be made 1 the rate of the chemical reactions involved in thermal maturation of organic material appears to double with every 10 c 18 f rise in temperature 2 threshold values of lopatin s time temperature index of maturity tti are 15 onset of oil generation 75 peak oil generation 160 end oil generation 500 40 oil preservation deadline 1 000 50 oil preservation deadline 1 500 wet gas preservation deadline 65 000 dry gas preservation deadline 3 tti values calculated from lopatin reconstructions consistently agree with other maturation parameters commonly used by petroleum geochemists potential applications of lopatin s method for oil exploration include timing of oil generation calculation of volume of hydrocarbons generated within a basin and determination of economic deadlines

the lack of relationship between overpressure and porosity in north sea and haltenbanken shales abstract disequilibrium compaction undercompaction is probably the most accepted mechanism for explaining overpressured formations this mechanism relies on the hypothesis that overpressured shales have higher porosity than normally pressured shales for a given depth converted to porosity units log response data from two jurassic shales at haltenbanken and nine cretaceous and tertiary shales in the norwegian sector of the north sea have been investigated to see whether such a relationship exists in these areas the depth of the studied shale formations varies between 0 5 5 km the degree of overpressuring within one formation typically varies by 20 mpa the log response data show that neither the porosity of massive north sea shales derived from sonic and density logs nor the porosity of haltenbanken intra reservoir shales derived from density and neutron logs differ significantly between the overpressured and normally pressured stratigraphic units distinct differences between the pressure regimes are however apparent in the resistivity and sonic log data from deeply buried shales at haltenbanken these differences do not reflect porosity variations and are interpreted to reflect textural changes resulting from the overpressuring these observations demonstrate that the haltenbanken and north sea shales do not conform to an undercompaction mechanism for overpressuring as the overpressured wells fail to show elevated porosity on the contrary each formation seems to have been individually compacted according to burial depth independent of pressure regimes

quantitative estimation of overpressure caused by oil generation in petroliferous basins abstract oil generation can be the dominant overpressure mechanism in low permeability source rocks because of the volume increase upon conversion of organic matter to less dense oil during the petroleum generation process to quantitatively express the relationship between oil generation and overpressure in source rocks an equation for the pressure change due to oil generation was derived to quantitatively estimate the overpressure generated in the source rock by considering of the episodic fluid expulsion and parameters such as the residual oil coefficient source rock porosity transformation ratio toc total organic carbon hydrogen index the compressibility of type i kerogen oil and pore water the equation was then calibrated with the results from physical simulation experiments in a closed pyrolysis system sensitivity analyses on the residual oil coefficient toc and hydrogen index were performed to investigate the dominant parameters controlling overpressure due to oil generation a good correlation was achieved between measured overpressure from the physical simulation experiments and the calculated data using the derived equations for the pressure change due to oil generation suggesting that the equation can be used to quantitatively estimate overpressure caused by oil generation in source rocks many parameters can affect overpressure caused by oil generation including residual oil coefficient source rock porosity toc hydrogen index and transformation ratio sensitivity analyses indicate that the most effective parameter on the pressure change due to oil generation in the source rocks is the residual oil coefficient implying that the internal pressure seal for the source rock is the most important parameter for overpressure development caused by oil generation overpressure cannot be generated from oil generation if the residual oil coefficient is less than 0 75 source rock toc and hydrogen index have moderate effects on the pressure change caused by oil generation overpressure can be generated when the toc content is as low as 0 5 provided the source rocks have a good internal pressure seal

contributions of compaction and aquathermal pressuring to geopressure and the influence of environmental conditions reply luo and vasseur 1992 presented a useful methodology of simplifying the complex equations that describe basin compaction and thereby directly assessing the contribution of aquathermal effects in the development of abnormal pressures their methodology leads to the conclusion that aquathermal pressuring plays only a small role in the development of overpressures in sedimentary basins although this conclusion follows from the cases studied it is possible to conclude otherwise using 1 a more general model of sediment compaction and 2 material properties that are more representative of conditions at depth the primary factor behind objection to the conclusion in luo and vasseur 1992 is that is fundamental equations which formed the basis for the simplifying methodology do not account fully for conditions in which a sediment s effective stresses are reduced we emphasize here that we do not claim that thermal effects are the dominant cause of overpressures rather our calculations suggest that they are not negligible in all cases there is a complex interplay of the various mechanisms that cause excess pore water pressures no one mechanism of overpressuring dominates all others for all situations because thermal effects not only cause higher excess pore pressures but also reduce effective stresses thermal effectsmore will be minimized by default in calculations that do not account for effective stress reduction the resulting reduced effective stresses also have important implications for geological questions such as acoustic velocities and fracture potential less

velocity depth trends in mesozoic and cenozoic sediments from the norwegian shelf sonic velocity density and resistivity log data from 60 wells on the norwegian shelf have been used to investigate velocity depth trends in sedimentary rocks as a function of sediment composition porosity pore pressure burial history and compaction processes a first order linear velocity depth trend line has been estimated from published velocity data the data analyzed in this study however show significant variations from this trend line indicating that no general velocity depth function can be used when performing more accurate analyses like depth conversion of seismic data pore pressure prediction or basin modeling lower tertiary smectitic sediments from the northern north sea and haltenbanken are characterized by relatively low velocities compared to the overlying pliocene and pleistocene sediments causing a distinct velocity inversion a significant velocity increase at a burial depth corresponding to 70 100c was found and may reflect the alteration of smectite to illite and the initial precipitation of quartz cement in both sandstones and shales overpressured jurassic sediments from haltenbanken have lower velocities than equivalent hydrostatically pressured sequences but no significant porosity difference the reduced velocities may be a direct response to lower effective stresses and thus reduced elastic compaction low velocities in source rocks are mainly attributed to the relatively soft kerogen and resulting velocity anisotropy the high velocity depth ratio of barents sea sediments after correcting for tertiary exhumation is explained by the burial history of the area the subsequent thermal exposure of the sediments over time and thus the amount of quartz cementation

quantitative analysis of north sea subsidence stratigraphic and geophysical maps of the northern and southern north sea basin are used to study mechanisms of intracratonic basin formation these maps include 1 tectonic subsidence for ten stratigraphic units 2 predicted and observed sediment overpressure 3 geographic variation of compaction constants 4 net basement fault extension 5 free air gravity 6 flexural rigidity 7 heat flow and 8 predicted hydrocarbon maturation in northern areas of the north sea observed basement fault extension crustal thinning present day flexural rigidity and tectonic subsidence are predicted reasonably well by a simple stretching model of basin formation with rifting phases during the triassic and mid jurassic early cretaceous in southern areas of the north sea however a similar analysis suggests that other non extensional rift related processes dominated basin evolution the effects of anomalously active aesthenospheric convection coupled with basalt eclogite and deep crustal dewatering reactions are considered as possible additional mechanisms the troughlike form of quaternary tectonic subsidence suggests a new phase of rifting may be starting in the north sea the importance of such thermal rejuvenation is borne out by the association of areas of differential subsidence with high heat flow and low flexural rigidity one component of tectonic reactivation during this period may be due to thermally induced isostatic reequilibration of older tectonic loads

origin of the permian triassic iberian basin central eastern spain abstract the iberian basin was an intracratonic rift basin in central eastern spain developed since early permian times the basin boundary faults were normal listric faults controlling an asymmetric extension propagating northeast with time hercynian or older lineaments controlled the orientation of the iberian basin and extension was accommodated basically in the hanging wall block by the formation of secondary grabens and a central high the basin was related with the coeval ebro catalan and cuenca mancha basins and their connections are discussed subsidence curves show that the early permian early jurassic period of extension can be subdivided into three rifting episodes and a flexural one extension factor increases from 1 17 in the northwest to 1 29 near the mediterranean coast the increasing extension rate was accommodated by transfer faults trending nne ssw more important in the levante area the rift evolution is intermittent and seems to reflect distinct stress fields the collapse of the late hercynian orogen and related increased heat flux extension and rifting is the most probable origin of the iberian basin and related basins the origin of the catalan and the valencia prebetic basins is related to the southwards migration of the hesse burgundy rift along the eastern margin of the iberian microplate

introduction to special issue on integrated basin studies ibs a european commission dgxii project this issue contains papers issued from the integrated basin studies project ibs which is part of a larger program of the european commission dgxii entitled geosciences ii the ibs project ec contract jou2 ct92 0110 originated in geosciences consultation meetings the first of them being held in strasbourg in june 1989 under the auspices of mr hubert curien former french minister for research and technology and the subsequent meeting being held in brussels these consultation meetings have resulted in the conclusion that a main axis of future research in geosciences should be the study of sedimentary basins upstream of industrial activities mainly the oil and gas industry but also water resources management the mining industry the storage of undesirable products and coastal engineering they also resulted in the conclusion that future research on sedimentary basins should fulfill two requirements integration of disciplines and modelling such a conclusion may now seem trivial but was not so generally accepted at that time in this spirit the ibs project was designed to provide the oil and gas industry with a new generation of models for basin formation basin evolution and basin fill architecture from basin scale to reservoir scale its main objectives are the linkage of crust thermomechanical behaviour to basin formation and deformation mechanics and the corresponding modelling the quantification of tectonic control on the sedimentary record and the analysis of compaction and mass and heat transfer the ibs strategy is to link subsurface data to field data and an important part of the project is devoted to thematic field studies in european sedimentary basins of different tectonic styles taken as natural laboratories basin research has gone through a rapid evolution during the last decade in particular through the development of new acquisition and processing methods for seismic data and advances in drilling technology modelling provides an important tool to analyse different aspects of basin formation processes and find their tectonic expression in the basin fill the need for 3 d modelling techniques and models capable of linking processes operating on basinwide to subbasin scales is increasingly recognized and a significant effort has been made in the ibs project to develop a new generation of basin formation models coping with these needs see also cloetingh et al 1993a van wees and cloetingh 1994 as many data required to test these models reside in industrial companies ibs has promoted a closer link between academic research and basin studies carried out by industry the participation of the petroleum industry has been vital in this project and ibs has been able to establish a well functioning system of cooperation that not only facilitated data access but most important guaranteed the suitability of the end products of the project this has led to an intensive exchange of modelling concepts and data sets between industry and academia the ibs project has also used extensively the international lithosphere program task force origin of sedimentary basins see cloetingh sassi and task force team 1994 cloetingh et al 1993a b 1994 to implement planning and to discuss preliminary results it also builds partly on joint research through the european commission tempus and peco programs on pannonian basin studies horvath 1993 van balen and cloetingh 1995 van balen et al 1995 horvath and cloetingh 1995 a stimulating aspect of the activity of the ibs group is the creation of a research network in the corresponding field of knowledge which comprises now 30 teams belonging to eight european countries england france germany hungary norway spain switzerland and the netherlands this demonstrates the capacity of the procedures used by the dgxii of the european community to formulate a really european research space phd students and young researchers form an important component of the research groups

simple quantitative evaluation of porosity of argillaceous sediments at various depths of burial abstract relationship of porosity to depth of burial d in m geologic age a in millions of years and lithology ratio of thickness of shales total thickness of terrigenous deposits r has been developed by the writers which gives satisfactory results 0 exp 0 014 13 3 log a 83 25 log r 2 79 10 3 d where 0 initial porosity of the argillaceous sediment a nomogram has been prepared for an easy solution of the above equation

one dimensional normal compression laws for resedimented mudrocks abstract four different empirical models used to describe the one dimensional normal compression behavior of mudrocks have been evaluated by investigating the behavior of 15 different resedimented mudrocks these mudrocks originate from a wide variety of geologic origins and were tested in the laboratory over an effective stress range of 0 1 100 mpa the normal compression of silt rich low liquid limit mudrocks is best described by assuming a log linear relationship between vertical effective stress v and void ratio for smectite rich higher liquid limit mudrocks their behavior is better described by assuming a log linear relationship between v and porosity the assumption of a log log relationship between 1 void ratio and v is not the most appropriate for any particular mudrock type but gives a reasonably good description of compression behavior for all mudrocks and is therefore ideal for situations in which the general composition of a sediment is unknown for all mudrocks the assumption of an exponential relationship between porosity and v gives a very poor fit to the experimental data as mudrocks undergo compression to high effective stresses high liquid limit mudrocks display a much greater compressibility and a corresponding larger loss in porosity compared to more silt rich mudrocks the porosities of all materials tend to converge into a much narrower range above about 10 mpa where all mudrocks display a similar compression behavior and a constant value of compression index of 0 21 regardless of their composition at low effective stresses though their compression behavior is strongly controlled by composition

hydrofracturing and episodic fluid flow in shale rich basins a numerical study low permeability sedimentary rocks commonly are fractured direct examination of exposed rock faces and drill cores shows evidence of hydrofracturing the mechanism for hydrofracturing its effects on fluid migration and its dependence on sediment permeability sedimentation rate and sedimentary sequences have not been explored in this study we carry out systematic numerical experiments to study the compaction induced hydrofracturing we show that the compaction induced hydrofracturing commonly may occur in shale rich basins and in sand shale sequences the frequency of such hydrofracturing depends on sediment permeability sedimentation rate and sedimentary sequence an important result is that compaction induced hydrofracturing may occur at relatively shallow depths in shale rich basins but it may mobilize enhanced fluid flow throughout the sedimentary basin over 60 of the total compaction induced fluid flow in the basin may be expelled during hydrofracturing we test the model against field data from the yinggehai basin of the south china margin where abundant hydrofractures in the uppermost quaternary marine mud have been recently detected by seismic imaging we suggest that the compaction induced hydrofracturing may mobilize fluid flow at great depths and affect hydrocarbon migration in shale rich basins

from sedimentary records to sediment budgets multiple approaches to catchment sediment flux this paper reviews the traditional approach to sediment budget studies in geomorphology new approaches and more specifically the potential impact of new methodological advances each component of the budget is discussed including the spatial and volumetric estimations of erosion deposition and storage and the precision and accuracy of flux rate estimation examples are used from recent work in germany the netherlands and the uk and include the pedogenic approach to erosion estimation remote sensing lidar geophysics and sediment based dating techniques for flux rate estimation the precision and accuracy of catchment sediment flux estimates based upon sediment storage is not only dependant upon volumetric accuracy but also on the precision and accuracy of sediment dating in this area there has been a revolution with direct sediment dating techniques tl osl esr freeing budget studies from the constraints and biases of radiocarbon of particular importance is the use of cosmogenic nuclides for dating but which can also be used to derive long term erosion rates but only using a steady state assumption finally a tentative initial application of the sediment budget approach to pleistocene terrace staircase in unglaciated basins is discussed it is argued that only now do we have the techniques available to be able to produce accurate sediment budget estimations at spatial scales greater than that of zero order basins and over time periods greater than that covered by direct observations

top seal development in the shale dominated upper devonian catskill delta complex western new york state the preferential generation of vertical natural hydraulic fractures at the contact of the upper devonian hanover gray shale and overlying dunkirk black shale of the catskill delta complex western new york state suggests that the latter served as a hydraulic top seal to formation fluids migrating upward from deeper in the sediment pile petrophysical properties and small scale textural characteristics of these siliceous fine grained rocks confirm the crucial role of depositional environment and sequence stratigraphic position of a shale lithotype in determining its sealing capacity the especially high sealing capacity of the basal interval of the dunkirk shale inferred early high stand systems tract hst strata reflects the anoxic depositional environment of these deposits that favored the preservation of their abundant organic matter and finely laminated depositional texture the absence of bioturbation enabled the undisrupted sediment notably carbonaceous clay rich laminae to undergo rapid mechanical compaction platy grain reorientation and porosity reduction compaction induced squeezing of ductile organic matter into void spaces further reduced pore throat diameters immediately underlying heavily bioturbated deposits of the organic lean hanover shale inferred upper hst or low stand wedge sediments accumulated in a dysoxic depositional environment disruption of layering and homogenization of sediment by burrowing organisms produced a more porous and permeable microfabric through which formation fluids moved only to be arrested by the high capillary entry pressures at the base of the dunkirk shale natural hydraulic fractures some of which propagated into the dunkirk shale formed when fluid pressure at the top of the hanover shale reached the fracture gradient the high sealing capacity of the basal dunkirk shale was probably enhanced by its finely laminated nature and the generation of biogenic methane both of which contributed to the formation of a near impermeable gas capillary seal

compaction driven evolution of porosity and permeability in natural mudstones an experimental study this paper describes a series of experiments designed to investigate the influence of lithology on the compactional loss of porosity and permeability in mudstones two intact samples of london clay with clay fractions of 40 and 67 were compacted to 33 mpa effective stress clay fraction permeability porosity pore size distribution and specific surface area were measured and their evolution was monitored throughout the compaction process electron microscopy was combined with mercury porosimetry to trace the collapse of the pore structure with increasing effective stress in both cases porosity loss occurred primarily by the collapse of large pores this process is more obvious in the coarser grained sample because throughout the compaction process it has a much broader range of pore radii and a much greater mean pore radius consistent with the pore size distributions the permeability of the coarser sample ranges from 10 10 m s 1 to 10 12 m s 1 while that of the finer grained sample ranges from 4 10 12 m s 1 to 5 10 14 m s 1 during progressive compaction from 2 to 33 mpa the compressibility of the finer grained sample is greater than that of the coarser grained sample 0 15 as opposed to 0 07 however in both cases the compressibility is much lower than that inferred for lithologically similar samples compacted over geological timescales the demonstration that both porosity and lithology clay fraction influence the permeability of mudstones should allow the development of more realistic porosity permeability relationships which take into account lithological variations exhibited by mudstones

thermal subsidence and compaction in sedimentary basins application to baltimore canyon trough cooling with the subsidence of continental crust is likely to be an important mechanism for the formation of sedimentary basins if the lithosphere was initially very thin and sedimentation continued to fill the basin to the original depth of basement a similarity solution is possible in this paper the similarity solution is extended to include sediment compaction and a depth dependent thermal conductivity the similarity solution is also used to determine the thermal evolution and petroleum potential of sedimentary basins experience with similarity solutions in many branches of applied physics indicates that they commonly give good results even when the required assumptions are rather poor approximations an empirical subsidence relation valid for older basins is al o extended to include sediment compaction these results are applied to the evolution of the baltimore canyon trough on the outer continental margin of the eastern united states as given by the cost b 2 well good agreement is obtained with both the subsidence record and the present thermal gradient it is predicted that optimal thermal conditions for the conversion of kerogens to petroleum occur between depths of 4 and 6 km

mechanical versus thermal cause of abnormally high pore pressures in shales the formation of a seal to shale pore fluids is essential to both the thermal and the mechanical processes proposed for the generation of abnormally high pore fluid pressures the thermal hypothesis requires a more perfect seal than the mechanical inferred gradients of pore fluid pressure with depth in shales are commonly greater than the overburden pressure gradient such gradients are unstable and can only be maintained with a perfect seal if the shale is also impermeable shales have measurable permeability it is therefore concluded that these pore pressure gradients are the result of pore fluid flow and an imperfect seal and that the thermal contribution is minor this conclusion is important for the understanding of the petroleum geology of regressive sequences which is the stratigraphic context of most abnormally pressured shales

episodic fluid expulsion from geopressured sediments numerical simulation of fluid explusion from geopressured sediments using a one dimensional finite difference model shows that large volumes of fluid can be transported through a fractured seal during short lived 100 year expulsion events in this model abnormal fluid pressures are maintained by an impermeable seal when the fluid pressure exceeds 85 of the lithostatic pressure a vertical fracture opens in the seal providing a permeable pathway from the geopressured sediments into the overlying section fluid pressure in the seal decreases as fluid flows through the fracture closing the fracture as sediments in the seal compact once the fracture closes the seal remains impermeable until fluid flowing from deeper in the section increases fluid pressure to the fracture criteria a process that takes 10 000 500 000 years fractures emplaced in relatively incompressible seals such as compacted and or cemented sediments remain open for 20 50 years fluid fluxes through the seal integrated over the life of these events are 1 5 107 kg m2 and temperatures in the overlying sediments are increased by 5 20 c these temperature anomalies decay to less than half their original value in 300 years suggesting that the thermal anomalies observed in wells that are attributed to fluid expulsion formed in response to a very recent or currently active event

laboratory and field evaluations of the lisst 100 instrument for suspended particle size determinations abstract advances in technology have resulted in a new instrument that is designed for in situ determination of particle size spectra such an instrument that can measure undisturbed particle size distributions is much needed for sediment transport studies the lisst 100 laser in situ scattering and transmissometry uses the principle of laser diffraction to obtain the size distribution and volume concentration of suspended material in 32 size classes logarithmically spaced between 1 25 and 250 m this paper describes a laboratory evaluation of the ability of lisst 100 to determine particle sizes using suspensions of single size artificial particles findings show the instrument is able to determine particle size to within about 10 with increasing error as particle size increases the instrument determines volume or mass concentration using a volume conversion factor cv this volume conversion factor is theoretically a constant in the laboratory evaluation cv is found to vary by a factor of about three over the particle size range between 5 and 200 m results from field studies in south san francisco bay show that values of mass concentration of suspended marine sediments estimated by lisst 100 agree favorably with estimates from optical backscatterance sensors if an appropriate value of cv according to mean size is used and the assumed average particle aggregate density is carefully chosen analyses of size distribution of suspended materials in south san francisco bay over multiple tide cycles suggest the likelihood of different sources of sediment because of different size characteristics during flood and ebb cycles

pore pressure estimation from mudrock porosities in tertiary basins southeast asia porosity reduction during mechanical compaction of a sediment generally has been assumed to be controlled by the increase in vertical effective stress which is convenient because vertical stress profiles may be readily calculated from density logs poroelasticity theory shows however that mean effective stress controls porosity reduction according to published data horizontal stresses increase with overpressure as well as with depth so mean stress and vertical stress profiles are poorly correlated in overpressured sections we have used wireline logs to compare the pore pressures estimated in mudrocks by relating porosity to mean effective stress and to vertical effective stress for overpressured tertiary sections in southeast asia wells from three different basins were studied mudrock porosities were estimated from the sonic log response and sorted by lithology according to the natural gamma log response two sets of normal compaction curves relating porosity to mean effective stress and to vertical effective stress were determined empirically by fitting data points where the pore pressure was thought to be hydrostatic these curves were then used to estimate the minimum pore pressure corresponding to mudrock porosity values in the overpressured sections the pore pressures inferred using the mean effective stress are consistent with direct measurements of pore pressure in the adjacent sands in contrast pore pressures inferred in mudrocks using the vertical effective stress are significantly lower for the overpressured sections implying discontinuities in the pore pressure profiles at lithological boundaries which cannot readily be explained we conclude that the pore pressures estimated using the vertical effective stress are wrong and that empirical relationships between porosity and vertical effective stress should not be used for estimating pore pressures porosity should be empirically related to mean effective stress instead

influence of tectonic exhumation on porosity of wufeng longmaxi shale in the fuling gas field of the eastern sichuan basin china thermal cracking of organic matter om can enhance gas shale porosity however thermally matured shales are generally characterized by low porosity with high om content in tectonic exhumation settings the wufeng longmaxi shale in the fuling gas field china was investigated via bulk analysis and scanning electron microscopy to reveal the influence of tectonic exhumation on the porosity of om rich shale thermal maturity and organic and mineralogical compositions are regionally homogeneous in the fuling gas field shale porosity is positively correlated with total organic carbon toc and clay mineral content in the gentle and open anticlines respectively the om hosted pores were preserved with spherical morphologies because of overpressure associated overburden stress unloading in the gentle anticlines whereas om hosted pores in the open anticlines are characterized by high aspect ratios and are regularly aligned perpendicular to lateral compression stress a simple simulation confirmed that tectonic folding and compaction during exhumation caused the loss of porosity in the inner arcs of the open anticlines only pressure shadows formed by assemblages of clay mineral platelets and mechanically competent grains inhibited tectonic compaction of some om hosted pores in the open anticlines tectonic compaction of porosity generally increased the mean value of water saturation in the open anticlines the extremely high water saturation 60 in the high toc interval 2 wt was generally associated with natural fractures suggesting the occurrence of gas leakage

tectonic evolution of the north sea basin crustal stretching and subsidence summary the lithospheric stretching model for the formation of sedimentary basins was tested in the central north sea by a combined study of crustal thinning and basement subsidence patterns a profile of crustal structure was obtained by shooting a long range seismic experiment across the central graben the main axis of subsidence a seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin the data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure and the final model showed substantial crustal thinning beneath the graben subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central north sea since jurassic times changes in water depth were quantified using foraminiferal assemblages where possible and observed basement subsidence paths were corrected for sediment loading compaction and changes in water depth through time the seismic model is shown to be compatible with the observed gravity field and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the central graben during the mid jurassic to early cretaceous extensional event this extension appears to have occurred over crust already slightly thinned beneath the graben and the seismic data suggest that total extension since the early permian may have been more than 100km the data presented here may all be explained using a simple model of uniform extension of the lithosphere

biological controls on resuspension and the relationship between particle size and the kolmogorov length scale in a shallow coastal sea abstract measurements of suspended particle matter spm and turbulence have been obtained over five tidal surveys during spring and summer 2010 at station l4 50 25 n 04 22 w depth 50 m in the western english channel the relationship between turbulence intensity and bed stress is explored with an in line holographic imaging system evaluating the extent to which material is resuspended image analysis allows for the identification of spm above a size threshold of 200 m capturing particle variability across tidal cycles and the two seasons dissipation of turbulent kinetic energy which exceeds 10 5 w kg 1 yields maximum values of bed stress of between 0 17 and 0 20 n m 2 frequently resulting in the resuspension of material from the bed resuspension is shown to promote aggregation of spm into flocs where the size of such particles is theoretically determined by the kolmogorov microscale lk during the spring surveys flocs of a size larger than lk were observed though this was not repeated during summer it is proposed that the presence of gelatinous biological material in spring allows flocculated particles to exceed lk this suggests that under specific circumstances the limiting factor on the growth of flocculated spm is not only turbulence as previously thought but the presence or absence of certain types of biological particle

on changes in porosity and volume during burial of argillaceous sediments abstract the porosity and hence volume of argillaceous sediments is determined by 1 the magnitude of the effective stress acting within the sediment 2 the previous stress history of the sediment and 3 at shallow depths of burial by features such as the mineralogy and the nature of the depositional environment stress paths and the critical state diagrams for a number of clays are used to investigate the range of porosities possible in argillaceous sediments as the effective stresses increase it is found that all porosity effective stress curves converge at large stresses the change in porosity is strongly dependent on the mean effective stress but largely independent of the deviatoric stress and thus is largely independent of the nature of the stress field acting on the basin compressional extensional etc because of the dependence of porosity on the mean effective stress no simple relationship exists between porosity and depth of burial but in the absence of overpressured pore fluids and assuming the sediment is not overconsolidated it is possible to contour the porosity effective stress diagram in terms of burial depths these data should assist in recalculating stratigraphic thicknesses for basin reconstruction and stratigraphic correlation studies

sediment yield in africa abstract several studies have compiled and analysed measured contemporary catchment sediment yield sy t km 2 y 1 values for various regions of the world although this has significantly contributed to our understanding of sy africa remains severely underrepresented in these studies the objective of this article is therefore 1 to review and compile available sy data for africa 2 to explore the spatial variability of these sy data and 3 to examine which environmental factors explain this spatial variability a literature review resulted in a dataset of sy measurements for 682 african catchments from 84 publications and reports representing more than 8340 catchment years of observations these catchments span eight orders of magnitude in size and are relatively well spread across the continent a description of this dataset and comparison with other sy datasets in terms of spatial and temporal distribution and measurement quality is provided sy values vary between 0 2 and 15 699 t km 2 y 1 median 160 t km 2 y 1 average 634 t km 2 y 1 the highest sy values occur in the atlas region with sy values frequently exceeding 1000 t km 2 y 1 also the rift region is generally characterised by relatively high sy values while rivers in western and central africa have generally low sy values spatial variation in sy at the continental scale is mainly explained by differences in seismic activity topography vegetation cover and annual runoff depth other factors such as lithology catchment area or reservoir impacts showed less clear correlations the results of these analyses are discussed and compared with findings from other studies based on our results we propose a simple regression model to simulate sy in africa although this model has a relatively low predictive accuracy 40 it simulates the overall patterns of the observed sy values well potential explanations for the unexplained variance are discussed and suggestions for further research that may contribute to a better understanding of sy in africa are made

magnitude mechanisms and prediction of abnormal pore pressure using well data in the krishna godavari basin east coast of india abnormal pressures are encountered during exploration drilling in different parts of krishna godavari basin on the east coast of india the nature and stratigraphic occurrences of the overpressure zones vary across the basin three different clusters of wells covering a large part of the basin encompassing both onshore and offshore drilled wells are analyzed to capture this variation a wide range of pore pressure gradients from normal to as high as 18 mpa km was observed in the present data set the tops of overpressure zones demonstrate a large range from 2200 to 3000 m 6562 to 9842 ft these depths generally correspond to either a miocene deltaic sequence or cretaceous synrift and postrift sequences available well data reveal two main reasons for the development of overpressure considerably high pore pressure regimes in the cretaceous sequence in the eastern corner of the basin are found to be mainly caused by gas generation whereas disequilibrium compaction is proposed as the main cause for overpressure in the other parts of the basin the outcome of this analysis provides a fair idea of the nature magnitude and distribution of the overpressure and this will also help to strategize further exploration activities in the basin

flow asymmetry associated with astronomical tides implications for the residual transport of sediment 1 tidal current asymmetry is usually associated to nonlinear tidal interactions in shallow water generating compound tides and overtides the present paper demonstrates that tidal asymmetry is not only caused by nonlinear tidal interaction but is also generated by astronomical tides in absence of shallow water effects in particular the k1 o1 and m2 constituents give an asymmetrical periodic flow pattern with the largest peak velocities persistently in the same direction a theoretical explanation for this phenomenon is presented and an inventory is made of other subordinate constituents that may contribute to repetitive asymmetric flow patterns tidal current asymmetry may induce residual transport of sediment analytical expressions are derived which quantify the residual transport of sediment due to the k1 o1 and m2 tides as a function of the phases and amplitudes of these constituents and the time lag between variation in suspended sediment transport and flow velocity the residual transport induced by the k1 o1 and m2 tides is compared to the residual transport due to the asymmetry associated with the m2 and m4 tides the former mechanism is particularly relevant in diurnal or mixed mainly diurnal tidal regimes where the k1 and o1 tides are dominant

introduction to proceedings from 6th international conference on tidal sedimentology this volume of marine geology is the outgrowth of the 6th international conference on tidal sedimentology tidalites 2004 held at department of geography university of copenhagen in august 2004 this series of conferences on tidal sedimentology is organized with the purpose of bringing together sedimentologists who work with tide dominated depositional systems the conferences are planned with an interval of four years and deal with various aspects of modern and ancient tidal or tide influenced sedimentation and related physical geomorphological biological geochemical and geotechnical studies in clastic carbonate and or mixed settings the first meeting was held in utrecht the netherlands in august 1985 de boer et al 1988 the second in calgary canada in august 1989 smith et al 1991 the third in wilhelmshaven germany in august 1992 flemming and barthloma 1995 the fourth in savannah georgia usa in may 1996 alexander et al 1998 and the fifth in seoul south korea in june 2000 park and davis 2001

claystone porosity and mechanical behavior vs geologic burial stress abstract a dataset was assembled from studies on 35 preserved claystone cores from different locations around the world and burial depths from

assessment of β the compression coefficient of mudstones and its relationship with detailed lithology abstract the mechanical compaction of soils and fine grained sediments is commonly described in soil mechanics by a relationship between void ratio and effective stress the relationship is defined by two parameters derived from the slope of the line relating void ratio and log effective stress and e the void ratio at a chosen effective stress e g 100 kpa and e are strongly correlated and are related to the atterburg liquid limit which is itself related in a general way to the grain size and mineralogy of the sediment in general finer grained sediments have higher initial void ratios and compact faster than coarser sediments porosity depth trends for mudstones in sedimentary basins are therefore strongly influenced by lithology steps or changes in the slope of porosity depth trends can be caused by lithological change rather than changes in the pressure regime a method is presented to evaluate for mechanically compacted mudstones using porosities determined from the density log fdc and based on the relationship between effective stress and void ratio developed in soil mechanics the method allows one to obtain rapidly approximate values for materials for which it is essentially impossible to obtain compaction parameters experimentally calculated values for mudstones in wells 8 3 2 and 30 2 1 on the norwegian margin vary between 0 2 and 0 65 the values correlate positively with the clay particle content of the mudstones which we have determined for 50 mudstone samples from the same wells the strong link between mudstone lithology and suggests that a downhole log may be a useful way to visualize downhole variations in mudstone lithology the log also shows how lithology affects the rate of porosity loss with depth or effective stress for example porosity in mudstones within both wells 8 3 2 and 30 2 1 is essentially constant over 700 m sections of normally pressured mudstones the log and grain size data indicate that this reflects changes in the lithological character and of the muds and not as commonly inferred a change in the pressure regime

calibration of the mudrock compaction curve by eliminating the effect of organic matter in organic rich shales application to the southern ordos basin china abstract the mudrock log derived compaction curve is a significant tool for investigating the primary migration of hydrocarbon predicting fluid overpressure estimating formation erosion thicknesses and restoring the buried history and paleo structure of a basin however the presence of kerogen in organic rich shales can create typically high logging values of the acoustic transit time thus the abnormally high values of the acoustic transit time for organic rich rocks may not truly reflect the porosity variations of subsurface rocks leading to great uncertainties in the understanding of the mudstone compaction and a certain amount of error in the abnormal fluid pressure estimation when using the mudrock log derived compaction curve therefore it is necessary to recalibrate the mudstone compaction curve by eliminating the increment of the acoustic transit time caused by the kerogen content of organic rich mudstones taking the southwest ordos basin as an example this paper presents a new equivalent volume model based on the composition of organic rich shale in which the kerogen content is also considered based on the quantitative relationship between the rock composition and the acoustic transit time a quantitative formula for calculating the acoustic transit time increment caused by the kerogen is derived this formula shows that the increment depends not only on the organic content but also on the occurrence state pore size pore fluid composition and other factors x ray diffraction xrd data were used to determine the main mineral composition of the mudstone and to calculate the acoustic transit time of the rock skeleton then the mudstone compaction curve in the zhenjing area was calibrated by combining the measured porosity and total organic carbon toc of the mudstone based on the correction formula the compaction characteristics varied significantly between before and after the calibration the slope of the normal compaction trend nct line decreased by 30 55 and the acoustic transit time deviation from the nct in the undercompaction interval decreased significantly the overpressure at the maximum burial depth estimated by the equivalent depth method is in better agreement with the results obtained by numerical simulation after the calibration and the porosity determined from the well log after the calibration is also closer to the true measured value the method proposed in this paper is of great significance for improving the reliability and accuracy of compaction research on organic rich mudstones especially for the accurate estimation of abnormal pressure in the source rock layer additionally it can be used as an effective reference for mudstone compaction studies in similar geological settings areas or basins

overpressure generation and evolution in lower paleozoic gas shales of the jiaoshiba region china implications for shale gas accumulation abstract overpressured shale gas plays have been found in the lower paleozoic marine shales of jiaoshiba region in the sichuan basin china the black shales from the upper ordovician wufeng group o3w and the lower part of lower silurian longmaxi group s1l are the producing reservoirs overpressure is widespread in the o3w s1l shales with the pressure coefficient up to 1 74 fluid inclusion microthermometric and raman spectroscopic techniques pvt and basin modeling are used to investigate overpressure generation and evolution in the o3w s1l shales the results show that thermocatalytic dry gas generation is the main genetic mechanism for overpressure development in the o3w s1l shales with specific characteristics as follows 1 the maximum burial depth of overpressured shales is more than 6200 m without anomalously low density and high primary porosity according to wireline log and core data 2 the over mature o3w s1l shales with the mean toc values ranging from 3 36 wt to 4 48 wt and the vitrinite reflectance values of 2 7 2 9 ro determined by kerogen ftir fourier transform infrared spectroscopy characteristics has excellent potential to produce dry gas 3 overpressured reservoirs are characterized by high gas saturations and the pressure coefficients positively correlate with gas saturation 4 the abnormally high pressures recorded in the fluid inclusions are coincident with high homogenization temperatures which is supportive of overpressure occurring during dry gas generation and provides convincing in situ evidence for gas generation overpressuring the calculated pore fluid pressures for methane inclusions within the maximum buried o3w s1l shales are approximately 90 3 139 4 mpa suggesting that gas generation produced a maximum pressure gradient of 23 1 mpa km which is equivalent to the lithostatic magnitudes the overpressure evolution in the o3w s1l shales is noticed to have evolved through four stages a rapidly increasing overpressure caused by thermocatalytic gas generation 145 85 ma a rapid pressure release 85 61ma overpressure preservation 61 20 ma and a relatively slow pressure release 20 ma present during post generation evolution 85 ma present of the o3w s1l shales both the overpressure and pressure coefficients decreased with time due to the intensive tectonic uplifts and denudation and approximately 19 0 28 5 of the initial total gas content was continuously released until the present day overpressure state the study of overpressure evolution in the o3w s1l shales provides basic information for the further study of the conversion between adsorbed gas and free gas and the method and technique used in this study to stress the overpressure generation and evolution may be generally applied to other overpressured or normally pressured shale gas systems

observations of distribution and flocculation of suspended particulate matter in the minjiang river estuary china abstract in this study we described the distribution pattern of suspended particulate matter spm and its influencing factors in the waters around the minjiang river estuary further we investigated the flocculation process of spm and its underlying mechanism in the study area the objectives were achieved using water temperature salinity turbidity fluorescence and particle size distribution of spm data measured in august 2009 as influenced by dilution from the minjiang river water the water column structure was significantly stratified in the estuary with an obvious thermocline and halocline the turbidity was relatively low in the surface water ssc 15 mg l fluorescence was high up to 10 l l in the surface water which was markedly lower and more uniform in the lower water layer the total volume concentration and particle size of the spm in the upper water layer were significantly higher than those of the lower water layer the in situ particle size frequency distribution of spm markedly changed in the different water layers whereas the dispersed particles were fine with little difference in frequency distribution the mean effective density of spm was generally 3 in the surface and benthic water and the values were slightly higher in the benthic water layer than in the surface water layer with a significant increase in the middle water layer the significant differences between the distribution trends of the total volume concentration and water turbidity were controlled by the size distribution of spm the concentration distribution of spm was mainly influenced by source re suspension and thermocline among other factors flocs widely occurred in the study area and the flocculation mechanism of spm varied in the different water layers in the upper water layer intense biological activity promoted the formation of stable macroflocs in the benthic water layer the biological activity was limited the flocculation of sediment particles was mainly caused by physico chemical attraction of inorganic particles and the hydrodynamic conditions and high settling velocity were also unfavourable to the formation of macroflocs

the effects of flocculation and bed erodibility on modeling cohesive sediment resuspension 1 flocculation and bed erodibility are two main processes causing the transport of cohesive sediments to be more complicated than typical noncohesive sediments earlier flocculation models assume a constant fractal dimension and or a constant floc yield strength however recent studies have shown that considering both the fractal dimension and the floc yield strength to be variable is critical to the prediction of temporal evolution of floc size due to consolidation it is also well established that critical bed shear stress of a mud bed cannot be parameterized as a constant this study further investigates how flocculation models with different degrees of complexity and bed erodibility can affect the resulting cohesive sediment resuspension driven by tidal flows a one dimensional vertical numerical model for sediment transport is revised to incorporate modules for flocculation and bed erodibility model results are compared with data measured in the ems dollard estuary model study suggests that it is important to incorporate variable critical shear stress in order to properly model the supply of sediment from the bed when flocculation is neglected or incorporated incompletely numerical model predicts nearly zero sediment concentration during slack water and very steep concentration gradient which are inconsistent with the observed data when the fractal dimension and the floc yield strength are both considered to be variable the numerical model predicts much smaller settling velocity and hence captures the more well mixed condition consistent with field observations

pore pressure generation in sedimentary basins overloading versus aquathermal the principal mechanism for abnormally high pore pressure generation has been an important matter of debate for more than a decade among the various mechanisms proposed mechanical overloading and aquathermal pressuring have probably received the greatest attention most previous discussions however have been based upon qualitative conceptual models here we propose a quantitative analysis by first examining the basic principles of porous flow and the fundamental properties of sediments and second by comparing the relative importance of the overloading versus the aquathermal mechanisms the coupling between pore pressure and compaction as well as the nonlinear and path dependent material properties are found to be critical in the present analysis we find that although changes in stress and temperature both contribute to pressure generation mechanical overloading is the main mechanism under normal geological conditions and is capable of producing the observed pore pressure profiles

experimental mechanical compaction of clay mineral aggregates changes in physical properties of mudstones during burial dry and brine saturated clay aggregates ranging from pure smectite to pure kaolinite were compacted in the laboratory experiments were conducted by increasing vertical effective stress up to 50 mpa to study the changes in physical properties of mudstones during burial the results suggest that the physical properties porosity density acoustic velocity etc of mudstones vary greatly with increasing effective stress clay mineralogy and fluid content kaolinite aggregates are much more compressible than those composed of smectite brine saturated clay mixtures are much more compressible than dry clay mixtures brine may soften and lubricate the clay matrix compared to dry clays resulting in higher compressibility in the brine saturated state the lesser degree of compaction of dry smectite compared to dry kaolinite aggregates can be explained by the differences in grain size as kaolinitic clays have much larger grains than smectitic clays the extremely fine grained nature of smectite implies that the imposed vertical stress is distributed over a very large number of grain contacts so that the force per contact area is very low in smectite compared to that in coarse grained kaolinite at 20 mpa effective stress corresponding to about 2 km burial depth with hydrostatic pore pressure brine saturated pure kaolinite compacted to 20 porosity while pure smectite retained 41 porosity brinesaturated clay mixtures show higher vp and lower vs than dry clay mixtures a pronounced difference in vs was observed among the brinesaturated clay mixtures the lowest vs as a function of vertical effective stress was found for pure smectite for a given porosity value smectitic clays have higher velocities and elastic moduli than the kaolinitic clays the vp vs ratio is significantly higher in smectite than in kaolinite aggregates treating mudstones as a uniform class of rocks introduces significant errors in estimates of physical properties which are strongly affected by the clay mineralogy grain size the amount of other minerals present and fluid content our results have implications for well log interpretation and mudstone and shale property determinations from seismic data at shallow burial depth o 2k m 801c above depths where significant chemical compaction takes place r 2007 elsevier ltd all rights reserved

modeling the occurrence of overpressures on the scotian shelf offshore eastern canada many wells on the scotian shelf offshore from nova scotia have drilled into overpressured zones in particular the wells comprising the venture gas field have all encountered overpressures at depths below 4 5 km the characteristics of the overpressure zones in the venture field vary in some important details from overpressured zones on the u s gulf coast in the venture field the shales in the overpressured zones are observed to be normally compacted and interbedded with layers of overpressured sandstones this is in contrast to the gulf coast where the shales in the overpressure zone are undercompacted resistivity logs sonic logs and bulk density logs all provide some indications of overpressures in the venture field but are not as unequivocal indicators of overpressure as they are on the gulf coast theoretical investigations of the causes of overpressures in an environment such as the venture field show that overpressures are unlikely to be caused by loading of sediments the most probable cause of overpressures is recent and continuing fluid generation which is related to the transformation of kerogen to hydrocarbons limits are placed on the average permeabilities and fluid generation rates which are required if overpressures are to be produced by fluid generation

a mathematical model for compaction in sedimentary basins summary a mathematical model for the non equilibrium compaction of clay rocks in sedimentary basins is formulated the model generalizes those of earlier authors the simplest assumptions are made concerning the rheology and diagenesis is neglected in this case we show that the model reduces to a generalized consolidation equation which for the classical darcy flow is a non linear diffusion equation for the porosity with a free boundary the model is non dimensionalized and it is shown that solutions depend on one significant dimensionless parameter a which is the ratio of the darcy flow rate and the sedimentation rate the model is solved numerically and asymptotic descriptions of the solutions are given for the cases of large and small a

cenozoic differential subsidence of north sea the structural evolution of the central north sea has been dominated by subsidence throughout the last 65 m y which occurred at different rates in various segments of the basin the bulk of the tertiary sediments accumulated in a relatively narrow elongate basin in the central north sea the base of the tertiary today is deeper than 3 250 m in the axis of the central graben the overall shape of the basin is well defined by contours decreasing in depth northward to the norwegian sea and landward in all other directions different rates of subsidence of the various subbasins are evident by comparison of isopach maps during the paleocene and eocene the main depocenter was located in the northwest part of the north sea the viking graben whereas the oligocene and miocene depocenters are observed to have been in the southern part of the central graben in addition two ancillary depocenters have been identified for other intervals of the tertiary the moray firth and the central jutland portion of the danish subbasin

tectonic subsidence of the early paleozoic passive continental margin in eastern california and southern nevada quantitative analysis of tectonic subsidence in cambrian and ordovician platform carbonates and associated strata exposed in the spring mountains nevada and the nopah funeral and inyo ranges california indicates that subsidence associated with this segment of the early paleozoic passive continental margin is exponential in form consistent with thermal contraction of the lithosphere following extension as in other parts of the north american cordillera continental separation in the southern great basin appears to have taken place between 590 and 545 ma these results are not sensitive to uncertainties in stratigraphic thickness biostratigraphic age control or paleobathymetry uncertainties in the cambrian time scale lead to predictable variations in the inferred time of onset of thermal subsidence but they have no effect on the inferred stratigraphic position of the rift to post rift transition a younger age for the base of the middle cambrian results in a younger inferred age of onset of thermal subsidence accompanied by greater rates of subsidence during the cambrian whereas a significantly older estimate of the onset of thermal subsidence can be obtained only if the base of the middle cambrian is substantially older than 540 ma a possibility that is inconsistent with available data results of the subsidence analysis are particularly significant because this is one of the few regions along the length of the north american cordillera where they can be compared directly to the geologic evidence for syn rift and post rift deposition basement involved faulting associated with the amargosa basin aulacogen ceased during deposition of the noonday dolomite which is thought to be older than 700 680 ma on the basis of stromatolites of late riphean affinity the overlying johnnie formation contains supposed vendian stromatolites younger than 700 680 ma if it is assumed that our results indicate the timing of the final rift to post rift transition then either the ages inferred from stromatolites are incorrect or the lithosphere was thinned regionally after deposition of the noonday the latter possibility is supported by limited geologic evidence for extension in latest proterozoic and early cambrian time the lack of appreciable physical evidence for crustal extension after deposition of the noonday however may imply that 1 a uniform extension model for lithospheric thinning is inappropriate for this part of the margin or that 2 some or all of the localities studied are continentward of the hinge zone and that the observed subsidence is exaggerated by flexural loading in a deeper basin to the west

factors controlling floc settling velocity along a longitudinal estuarine transect a 147 km longitudinal transect of flocculated cohesive sediment properties in san francisco bay sfb was conducted on june 17th 2008 our aim was to determine the factors that control floc settling velocity along the longitudinal axis of the estuary the inssev lf video system was used to measure floc diameters and settling velocities at 30 stations at a distance of 0 7 m above the estuary bed floc sizes d ranged from 22 m to 639 m and settling velocities ws ranged between 0 04 mm s 1 and 15 8 mm s 1 during the longitudinal transect nearbed turbulent shear stresses throughout the transect duration were within the 0 2 0 5 pa range which typically stimulates flocculation growth the individual d ws floc density plots suggest the suspended sediments encountered throughout sfb were composed of both muddy cohesive sediment and mixed sediments flocs mass weighted population mean settling velocity wsmass ranged from 0 5 mm s 1 to 10 mm s 1 the macrofloc and microfloc demarcation at 160 m sub populations demonstrated parameterised settling velocities which spanned nearly double the range of the sample mean settling velocities wsmean the macroflocs tended to dominate the suspended mass up to 77 of the ambient suspended solid concentration ssc from san pablo bay to carquinez strait the vicinity of the turbidity maximum zone microfloc mass was particularly significant typically 60 100 of the ssc in the northern section of south bay and most of central bay the transect took eleven hours to complete and was not fully synoptic during slack tide larger and faster settling flocs deposited accounting for most of the longitudinal variability the best single predictor of settling velocity was water velocity 39 min prior to sampling not suspended sediment concentration or salinity resuspension and settling lags are likely responsible for the lagged response of settling velocity to water velocity the distribution of individual floc diameters and settling velocities indicates that floc density for a given floc diameter varies greatly a small portion a few percent of suspended sediment mass in sfb is sand sized and inclusion of sand in flocs appears likely fractal theory for cohesive sediment assumes that there is a single primary particle size that flocculates which is not the case for these types of mixed sediment flocs the wide variability in the physical biological and chemical processes which contribute to flocculation within sfb means that spatial floc data is required in order to accurately represent the diverse floc dynamics present in the bay system the importance in determining accurate estimates of floc density has been highlighted by the sfb data as these provide the basis for realistic distributions of floc dry mass and the mass settling flux across a floc population however although video floc sampling devices can produce the various floc property trends observed in sfb good survey practice is still paramount one can see that if the sampling coverage i e data collection frequency is poor this could lead to potential mis interpretations of the data and only limited conclusions may be drawn from such a restricted survey for example a limited survey i e only 3 stations compared to the 10 stations in the full survey in south bay produces an under estimate in both the macrofloc sscmacro distribution by a factor of four and the wsmacro by a factor of two to develop sediment transport numerical models for sfb high quality floc size and settling data are needed to understand and simulate the depositional qualities of both suspended cohesive sediment and mixed sediments in san francisco bay this study has shown that the most pragmatic solution is a physically based approach whereby the detailed flocs d vs ws spectra are parameterised in terms of their macrofloc and microfloc properties this aids in model calibration whilst retaining more of the dynamical aspects of the floc populations all forms of flocculation are dynamically active processes therefore it is important to also include both ssc and turbulence functions together with the floc data

effects of early petroleum charge and overpressure on reservoir porosity preservation in the giant kela 2 gas field kuqa depression tarim basin northwest china kela 2 is a giant gas field with a proven reserve of 597 tcf in the kuqa depression northern tarim basin widespread overpressures have been encountered in the eocene and cretaceous sandstone reservoirs of the field with pressure coefficients up to 2 1 from drill stem tests and well log data analysis disequilibrium compaction associated with horizontal tectonic compression may be the dominant overpressure mechanism in the sandstone reservoirs because the overpressured sandstone with a maximum burial depth over 6000 m 19 685 ft displays anomalously high porosity and low density the causes for sandstone reservoirs with anomalously high porosity in the kela 2 gas field were studied based on an integrated investigation of sandstone reservoir characteristics paleo oil water contact petroleum charge history and overpressure evolution collective evidence indicates that early oil charge had retarded the porosity reduction of the reservoir sandstone and resulted in disequilibrium compaction from overburden rocks and overpressure from disequilibrium compaction and horizontal tectonic compression at the beginning of the rapid subsidence and deposition in the kela 2 gas field again contributed to the preservation of the reservoir porosity 1 overpressured mudstones in the kela 2 gas field are characteristic of normal compaction and overpressure was generated by horizontal tectonic compression instead of disequilibrium compaction 2 the reservoir sandstones with high porosity and permeability are associated with high paleo oil saturation as indicated by quantitative grain fluorescence qgf responses and anomalous qgf on extract intensity 3 sandstone units below the paleo oil water contact have very low porosity and permeability and 4 three episodes of oil and one episode of gas charge are identified in the sandstone reservoirs of the kela 2 gas field and the later two episodes of oil charge occurred circa 5 5 4 5 ma which corresponds to the beginning of the rapid tectonic subsidence and deposition in the kuqa depression the initially charged oil in the sandstone reservoirs was subsequently displaced by gas at circa 3 2 ma through fault activation at the edge of the anticline trap the overpressure evolution for the k1bs reservoir sandstone in the kela 2 gas field indicates that the apparent overpressure development in the sandstone reservoir began at 5 ma following the major oil charge and has been maintained to the present overpressure development from 5 ma in the sandstone reservoirs of the kela 2 gas field is believed to be the dominant cause of the porosity preservation

effective energy controls on flocculation under various wave current regimes transport of sediments is a critical process in the coastal zone because of its relation with coastal erosion productivity and pollution of particular interest are the dynamics of suspended cohesive sediments known as flocs which can aggregate and break up during the flocculation process this changes their size density settling velocity and overall transport even though turbulence is widely accepted to be an important control on floc aggregation and break up specific and detailed floc behaviour is still not fully understood the present study seeks to help in the understanding of the intra tidal turbulence induced flocculation under different current wave regimes observations of floc size and currents at high sample rates are used to investigate the changes throughout a fortnightly cycle the occurrence of waves at different stages during the sampling period enabled determination of three regimes of currents dominant combined waves and currents and wave dominant the first two regimes showed quarter diurnal floc size variability with aggregation during low turbulence slack waters and higher floc aggregation magnitude on low water slack break up occurred with high turbulence flood and ebb with higher magnitude after ebb during the currents waves regime waves were tidally modulated and led to enhanced aggregation and break up with larger floc size range than during the current dominant regime wave tidal modulation and quarter diurnal variability of floc size were lost when waves were dominant flocs sizes exhibited a low range related to wave height inverse relationships between turbulent properties and median floc size were found for the three regimes with higher scatter of data for the kolmogorov microscale and shear rate due to different floc behaviour during flood and ebb phases effective kinetic energy obtained from the combined effect of both currents and waves seems to have a better relationship with floc size which suggests its use as a floc size predictor instead of shear stress