characterization of land subsidence induced by groundwater withdrawals in su xi chang area china su xi chang area is one of the typical regions in china which suffers from severe land subsidence various tools of field monitoring were integrated to study the characteristics and mechanisms of land subsidence in this region the occurrence and the development of the land subsidence in this region are strongly related to the groundwater pumping both in time and space the main consolidation layers are the soft mud layers however the compressibility of the confined sandy layers should not be ignored the second and third confined aquifers contributed more than 30 of total subsidence meanwhile irrecoverable deformations were also observed in the sandy layers different sandy layers deform diversely under different stress conditions some have the elastic feature but the soil strata including both sandy layers and clayey layers located in the center of the groundwater level depression cone exhibited obvious viscous mechanical behavior which caused the common lag phenomenon the sand composition mingled with small clay particles or interbeds and sand rheology are the two main reasons for the lag phenomena in sandy layers a series of laboratory tests for modeling the effective stress changes due to groundwater withdrawals were conducted to investigate the mechanism of the lag phenomenon based on the test results the relationship of stress strain time for saturated sands is obtained and it could be expressed as power functions the results also showed that the compression of the sandy layers was time dependent and its deformation could be remarkable when establishing land subsidence model the deformation for the similar soil formation could be elastic visco elastic and even visco elastic plastic because of the different groundwater level fluctuation experienced

porosity and hydrocarbon distribution in the north sea chalk reservoirs abstract since 1970 more than 25 billion barrels of hydrocarbons in place at reservoir conditions have been discovered in the north sea chalk reservoirs for accurate development drilling satisfactory reserve estimate and optimal recovery a precise description of the porosity and hydrocarbon distribution has proved necessary because such reservoirs were unknown it took some time to fully understand their local and regional variation based on a 12 year experience of the north sea chalks and the data from 180 wells a basic geological model was established and tested on the other fields in the area and is described in this paper thickness variations are due to differential diagenesis as well as depositional factors porosity generally decreases downsection and towards the flank as a result of the progressive invasion of rock by oil and subsequent inhibition of chemical compaction layering in porosity reflects differences in initial porosity hydrocarbon saturation varies with structural elevation and rock type i e depositional facies mineral content differential behaviour to early and late diagenesis pore and pore throat size and morphology tortuosity this report is divided into two parts development of the model and comments on the individual fields

dinoflagellate cyst concentration as an independent reference for monitoring mineral mobilization in stylolites abstract the absolute abundance of dinoflagellate cysts in chalk samples can be used as an independent reference for analysis of mineral mobilization in stylolites the mineralogy of eleven samples from chalks from the dan field danish north sea was studied by electron microscopy bse x ray diffraction and atomic absorbtion spectrometry aas and palynologically using the abundance of dinoflagellate cysts as a reference it can be demonstrated that diagenetic mobilization of pyrite has occurred along stylolites and that primary anomalies in composition have influenced stylolite genesis

significance of fracture sets associated with stylolite zones geologic notes stylolite zones consist of an irregular seam and often a variety of associated fracture sets the geometry of this zone can be used to determine paleostress directions of the fracture sets commonly observed in close proximity to stylolites one is generally induced by the coring procedure unloading fractures and another is natural and possibly associated with stylolites of tectonic origin so called tension gashes within a unit possessing abundant stylolites the stylolites are most common in zones of relatively low porosity in addition because of the relatively compact poorly sorted fine grained nature of the seam itself stylolite zones commonly have much reduced permeability compared to the undeformed rock above and below laboratory measured permeability associated with stylolite zones can however be misleadingly large owing to the presence of unloading fractures and or tension gashes these fractures may dramatically increase laboratory permeability but commonly are either not present in the reservoir or are relevant only to the size of the laboratory measurement sample it is critical therefore to understand the origin and scale of associated fracture development in interpreting su h data

regional land subsidence simulation in su xi chang area and shanghai city china su xi chang area and shanghai city located in the south of yangtze delta china has subsided due to groundwater overpumping because of the regional scale of the groundwater exploitation cone of depression and land subsidence at present su xi chang area and shanghai city are treated as a single area for land subsidence study to avoid the uncertainty of boundary condition due to the regionalism the characteristics of aquifer system compaction are complex because of the difference in the types compositions and structures of the soils that the hydrostratigraphic units are composed of and in the histories of groundwater level change the hydrostratigraphic units have experienced considering the fact that different hydrostratigraphic units have different kinds of deformation and that an identical unit may also present different deformation characteristics such as elasticity elasto plasticity and visco elasto plasticity at different sites of the cone of depression or in different periods corresponding constitutive laws have been adopted this avoids the shortcomings of the previous research that the same constitutive law was adopted in all the hydrostratigraphic units during the entire time period a coupled flow and subsidence model which includes a three dimensional flow model with variable coefficients and a one dimensional vertical subsidence model is built according to the complicated hydrological condition in the region the simulation model is calibrated using observed data which include compression of individual strata from groups of extensometers and groundwater levels from observation wells from 1995 to 2002 the model reproduced that the primary subsidence layer in shanghai shifts from the shallow aquitard to the fourth confined aquifer because of the groundwater yield variations and the change of exploitation aquifers however the third aquitard was the primary subsidence layer in su xi chang area and the compaction deformation of the sandy aquifers was remarkable the simulation results could provide some reasonable advice about groundwater exploitation in the future

laboratory study on the relationship between soilmass deformation and water seepage in north china the study on the relationship between the soilmass deformation and water seepage under certain stress in north china was conducted through combined tests of the consolidation deformation and water seepage the results showed that the actual deformation of the clay soil under stress is greater than the deformation induced by the seepage on average the deformation induced by seepage was about 60 of the actual clay deformation under stress the actual deformation of sandy soil however is less than the deformation induced by seepage on average the seepage induced deformation was approximately 125 of the actual sandy soil deformation these results indicated that the sandy soil did not have complete plasticity clay had the highest amount of water seepage and deformation while silty soil calcareous highly cemented clayed soil and sandy soil had less water seepage and deformation the underlying mechanisms were also discussed the results of this study contribute to the understanding of land subsidence mechanism and rough estimation of groundwater exploitation

quantification of tertiary erosion in the inner moray firth using sonic velocity data from the chalk and the kimmeridge clay abstract sonic velocities for the chalk and kimmeridge clay were analysed to determine the importance and magnitude of tertiary erosion for 26 wells in the inner moray firth apparent erosion height above maximum burial depth was derived by the displacement along the depth axis of a given velocity depth trend from the normal undisturbed trend the similarity in apparent erosion results derived from the cretaceous chalk and late jurassic kimmeridge clay successions of the inner moray firth suggests that burial beyond present depths caused the observed overcompaction rather than any sedimentological and or diagenetic process resultant apparent erosion estimates are variable across the inner moray firth such values reach approximately 1 km in the western part of the basin and decrease progressively to zero in the eastern inner moray firth however the actual magnitude of erosion equal to the sum of apparent erosion and post erosional tertiary burial was approximately 1 km throughout the whole inner moray firth area indicating that early tertiary erosion was regional and not simply restricted to the inverted basin margins however a well defined mechanism sufficient to have generated the regional event is still sought

the geology and geophysics of the ekofisk field waterflood abstract successful waterflooding of ekofisk field will enhance recovery of the total oil in place producing an extra 170 mm barrels of oil monitoring matching and predicting the reservoir performance will be performed with a complex reservoir model of 7700 cells the chalk reservoir sequence at ekofisk is multilayered with each layer exhibiting distinct thickness porosity permeability and water saturation trends the upper part of the tor formation is one of those layers with imbibition characteristics suitable for waterflooding overlying the tor formation is the ekofisk tight zone a thin layer of low but variable permeability and porosity which generally acts as an hydraulic barrier this geological and geophysical model provides a data set for the required complex reservoir model

triassic evaporites and plio quaternary diapirism in western greece wide triassic evaporite outcrops run parallel to the strike of hellenide thrust faults in western greece they crop out along the leading edges of thrust sheets emplaced as part of the foreland migrating fold and thrust belt the evaporites acted as the plane of decollement for thrust sheet emplacement over much of the external hellenides plio quaternary diapirs occur locally within the broad outcrops of triassic evaporite they are the result of local mobilization of evaporite after its emplacement their rise may have been initially triggered by hellenide thrust sheet loading and subsequently accentuated by plio quaternary subsidence geophysical records and onland exposures show that diapiric rise controlled local sedimentation patterns above the sites of earlier hellenide thrusts the subsequent intrusion of diapirs into these cover sediments caused localized extensional structures to develop this is best seen in se zakinthos where strain hardened conjugate faults developed in friable pliocene sandstones early in the intrusion history principal strains calculated from these sets demonstrate patterns of extension sub orthogonal to intrusion margins they are comparable with patterns known from other field and experimental examples but contrast with some recent studies on diapir related strains

sedimentology and hydrogeology of two braided stream deposits abstract two approaches were used to quantify the spatial distribution of hydrofacies in braided stream deposits one approach involved mapping a 50 by 60 by 3 3 m section of a proximal braided stream deposit in a second study we generated a 400 by 400 by 2 6 m section of a medial braided stream deposit using a computer model in both cases we produced three dimensional images showing connected hydrofacies with high permeabilities that form preferential flow paths this information was input to a groundwater flow model and flow paths were analyzed by following the transport of imaginary particles in both systems particles that were uniformly distributed at the up gradient end of the model clustered along preferential flow paths during transport showing that connection among high permeability facies is a critical factor in hydrogeological investigations involving assessment of contaminant movement and remediation

inverse modeling of interbed storage parameters using land subsidence observations antelope valley california 1 we use land subsidence observations from repeatedly surveyed benchmarks and interferometric synthetic aperture radar insar in antelope valley california to estimate spatially varying compaction time constants and inelastic specific skeletal storage coefficients s kv in a previously calibrated regional groundwater flow and subsidence model the observed subsidence patterns reflect both the spatial distribution of head declines and the spatially variable inelastic skeletal storage coefficient using the nonlinear parameter estimation program ucode we estimate compaction time constants between 3 8 and 285 years the s kv values are estimated by linear estimation and range from 0 to almost 0 09 we find that subsidence observations over long time periods are necessary to constrain estimates of the large compaction time constants in antelope valley the insar data used in this study cover only a three year period limiting their usefulness in constraining these time constants this problem will be alleviated as more sar data become available in the future or where time constants are small by incorporating the resulting parameter estimates in the previously calibrated regional model of groundwater flow and land subsidence we can significantly improve the agreement between simulated and observed land subsidence both in terms of magnitude and spatial extent the sum of weighted squared subsidence residuals a common measure of model fit was reduced by 73 with respect to the original model however the ability of the model to adequately reproduce the subsidence observed over only a few years is impaired by the fact that the simulated hydraulic heads over small time periods are often not representative of the actual aquifer hydraulic heads errors in the simulated hydraulic aquifer heads constitute the primary limitation of the approach presented here

geomechanics of subsurface water withdrawal and injection land subsidence and uplift ground ruptures and induced seismicity are the principal geomechanic effects of groundwater withdrawal and injection the major environmental consequence of groundwater pumping is anthropogenic land subsidence the first observation concerning land settlement linked to subsurface processes was made in 1926 by the american geologists pratt and johnson who wrote that the cause of subsidence is to be found in the extensive extraction of fluid from beneath the affected area since then impressive progress has been made in terms of a recognizing the basic hydrologic and geomechanic principles underlying the occurrence b measuring aquifer compaction and ground displacements both vertical and horizontal c modeling and predicting the past and future event and d mitigating environmental impact through aquifer recharge and or surface water injection the first milestone in the theory of pumped aquifer consolidation was reached in 1923 by terzaghi who introduced the principle of effective intergranular stress in the early 1970s the emerging computer technology facilitated development of the first mathematical model of the subsidence of venice made by gambolati and freeze since then the comprehension measuring and simulation of the occurrence have improved dramatically more challenging today are the issues of ground ruptures and induced triggered seismicity which call for a shift from the classical continuum approach to discontinuous mechanics although well known for decades anthropogenic land subsidence is still threatening large urban centers and deltaic areas worldwide such as bangkok jakarta and mexico city at rates in the order of 10 cm yr

mechanisms of natural fracturing in a sedimentary basin mechanisms of natural fracture generation in a sedimentary basin may be studied by the use of physical properties of sedimentary rocks and of pore fluid pressure the physical properties of rocks such as poisson s ratio and young s modulus under laboratory conditions are available but only a few of these are reported under subsurface conditions in this paper in situ values of poisson s ratio derived from analysis of artificial hydrofracturing data in the gulf coast are used for estimating horizontal stresses in both hydrostatically and abnormally pressured zones there in a tectonically relaxed basin such as the gulf coast the horizontal stress can be a function of vertical overburden stress fluid pressure and poisson s ratio the horizontal stress increases with depth in the hydrostatic pressure zone but is abnormally low in the abnormally pressured zone fracture pressure and its gradient given as a sum of least horizontal stress and fluid pressure is however not abnormally low in the abnormally pressured zone because the fluid pressure there is abnormally high this paper describes the in situ fracture pressure gradient estimated for the gulf coast area on the basis of models of vertical fluid charging through an aquifer and a fault system during continuous burial the depth at which the charged fluid pressure gradient exceeds the fracture gradient or the point of natural hydrofracturing is estimated after an erosional event the most significant decline of fracture pressure gradient is expected in the uppermost part of the original abnormal pressure zone hydrofracturing there could occur if an effective fluid is charging from deep to shallow through an aquifer or a fault system if the erosion continues to a point at which approximately two thirds of the total sediment thickness is removed the horizontal stress may become almost zero if such a stress condition is developed the sedimentary rocks become greatly relaxed possibly causing fractures

the valhall field a geological overview abstract the valhall field is located within the central graben in the southern part of the norwegian north sea the discovery well 2 8 6 was drilled in 1975 and the field went on production in 1982 production is currently 55 000 barrels of oil per day the valhall field structure trends nw se and has an areal closure in excess of 60 000 acres the hydrocarbon column measures 200 m the structure is located on an elongate anticlinal feature called the lindesnes ridge which developed during late cenomanian to oligocene tectonic movements the geophysical evaluation of the field is complicated by the existence of a gas charged tertiary section which obscures the deeper seismic reflections vertical seismic profiles vsps have been utilized in an attempt to resolve the reservoir configuration reservoirs occur both in the tor formation maastrichtian and in the underlying hod formation turonian coniacian the tor formation is represented by a largely allochthonous chalk facies which thins regionally to the southwest a crestal graben is developed in which the tor formation is locally thick autochthonous chalks are characteristic of the hod formation primary porosity of the chalk varies between 36 to 50 on the crest of structure preservation of such high porosity is primarily due to formation overpressure inhibiting mechanical compaction of the chalk the field contains paraffinic oil of 36 api gravity it originated from the upper jurassic kimmeridge clay formation which is at peak oil generating capacity under the field at present day

land subsidence in central mexico detected by alos insar time series abstract massive groundwater extraction is common throughout mexico and is well known to result in land subsidence however most land subsidence surveys focus on one single city mainly mexico city and thus fail to reveal the regional extent of the problem here we use 2007 2011 interferometric synthetic aperture radar insar time series analysis of alos data to resolve land subsidence in the entire central mexico region we identify land subsidence in 21 areas including 17 cities linear vertical rates over 30 cm yr are observed in mexico city while in the other locations rates of 5 10 cm yr are detected we define 3 main categories of subsidence using the averaged velocity maps in conjunction with previously published structural surface geology and land use mapping 1 rapid large scale subsidence 2 rapid local scale subsidence and 3 slow patchy subsidence the correlation between subsidence and land use confirms that groundwater extraction mainly for agricultural and urban activities is the main cause of land subsidence we observe that the boundaries of the subsiding areas are typically characterized by high velocity gradients often coinciding with pre existing faults motion on these faults being driven by water extraction rather than by tectonic activity regional surveys of this type are necessary to understand the spatial and temporal evolution of land subsidence to constrain the distribution and connectivity of water bearing units and ultimately to reach better hazard mitigation plans

numerical simulation and prediction of regional land subsidence caused by groundwater exploitation in the southwest plain of tehran iran abstract this study characterizes land subsidence in southwest plain of tehran using numerical modeling and predicts the trend through 2018 excessive groundwater withdrawal has caused severe land subsidence in tehran in the past 28 years 1984 2012 groundwater level has decreased 11 65 m the multi layered aquifer system in the southwestern plain of tehran contains three aquifers and three aquitard units the present model was developed simulation using pmwin modflow for windows first groundwater level and land subsidence were simulated for the end of 2004 the model was calibrated using hydraulic head measurements and insar data the simulation results were in fairly good agreement with the measurement results the calibrated and evaluated model was then used to assess the future evolution of land subsidence and for prediction of subsidence through the end of 2018 numerical results show that assuming a constant rate of pumping in the future land subsidence in the southwestern plain of tehran will reach 33 cm by 2018 the study confirmed that land subsidence caused by groundwater pumping is a serious threat to southwest tehran

numerical simulation of groundwater flow and aquifer system compaction using simulation and insar technique saveh basin iran in this research a multidisciplinary approach has been presented to evaluate land subsidence due to heavy groundwater withdrawal land subsidence in aquifers with complex stratigraphy was predicted using the terzaghi s 1d instantaneous compaction principle and was incorporated into a 3d groundwater flow model modflow the integrated model was then calibrated for saveh aquifer located in iran to simulate observed hydraulic heads and compaction in order to control the model results interferometric synthetic aperture radar insar a generated 3d geological model monitoring wells and available literature were used to predict land subsidence in saveh aquifer and apply the results in the developed model the results showed that the insar extensometers and numerical simulations closely agree in predicting the land subsidence the simulation results show that the regional subsidence began in the mid 1990s and that the area has experienced up to 70 cm of subsidence where heavy pumping and thick clay layers are found the calibrated model indicates that if the pumping rate remains at the current level the subsidence will reach as high as 170 cm over the next 5 years the results show that the proposed approach which integrates various sources of data is useful in estimating land subsidence especially in areas where field measurements are scarce and satellite radar images are available

concretion morphology classification and genesis abstract a discussion of the most relevant morphological features of concretionary bodies and the different classifications and the criteria involved in these classifications is presented together with suggestions for improvements to the various classification schemes the meaning of syngenetic diagenetic and epigenetic related to the relative timing and environment of growth of concretionary bodies is also reviewed and discussed the replacement of mixed morphogenetic classifications which lead to conflicting results by classification categories based on textural features is proposed because the identification of the genetic environment of a concretionary body tells little about its history i recommend defining growth patterns in which the successive steps associated with changes in composition and or texture and the development of new structures are recorded a new path is presented that accounts for the contradiction between textural and isotope features which suggests respectively syngenetic and diagenetic or even epigenetic signatures this new path is characterized by the preservation of large porosities over very long time spans and down to depths that are somewhat greater than expected due to the inhibition of normal compaction caused by early overpressuring this overpressuring is the result of the development of hydraulic seals since approximately the syngenetic phase times the concepts of force of crystallization and displacive growth are also reviewed it is being suggested to discard some controversial interpretations of their actual importance for true concretionary displacive growth under epigenetic conditions in accordance with other authors the conclusion is reached that displacive growth is only possible if the shear strength of the host is very low and the stress field within the host is almost hydrostatic a new model the symcompactional concretionary growth model is proposed which explains how a concretionary body generally a nodule can grow while its host thins down by compaction this overpressured undercompacted model will be useful for the interpretation of an assemblage of features such as injection dikes hydraulic breccias cone in cone structures among others that are thought to be representative of the former presence of overpressured horizons these overpressured horizons could serve as detachment planes in paleotectonically active basins seals that could have controlled the movement of brines and hydrocarbons during the diagenetic evolution of the basin can also be assessed this is relevant for the identification of maturation conditions the potential development of secondary fracture permeability due to hydraulic fracturing in buried areas of the basin can also be evaluated based on the identification of formerly overpressured horizons in outcrops the origin of concretions is generally a geological puzzle clifton 1957

microscopic pore structures of tight sandstone reservoirs and their diagenetic controls a case study of the upper triassic xujiahe formation of the western sichuan depression china abstract the study of microscopic pore structures is of central significance to the evaluation of the reservoir quality and well productivity of tight gas sandstone the second and fourth members abbreviated as t3x2 and t3x4 respectively of the upper triassic xujiahe formation of the western sichuan depression were used as cases to analyze microscopic pore structures and diagenetic controls in consideration of the disparities in reservoir quality and well productivity optical microscopy of thin sections micro ct scanning scanning electron microscopy sem high pressure mercury injection hpmi and apex methods based on the hpmi results were used to determine the pore and throat geometries and the types and pore size distributions in terms of the quantity and levels of pore connectivity tight sandstone reservoirs of the study area are characterized by poor physical properties with the t3x2 reservoirs presenting relatively higher levels of permeability and lower porosity levels than those of the t3x4 reservoirs the pore spacing configurations sizes and distributions and throat geometries reveal significant disparities primary pores and micro fractures were found to be more developed in the t3x2 reservoirs with bending and narrow pore throats measured as smaller than 0 5 m compared to those of the t3x2 reservoirs secondary pores including moldic and intracrystalline pores were more developed in the t3x4 reservoirs with relatively larger straighter and wider throats of 0 5 1 0 m an evaluation of the pore connectivity levels shows that only 26 86 and 28 90 pores are closely connected to the whole for t3x2 and t3x4 reservoirs respectively four major diagenetic processes were found to control reservoir quality and the microscopic pore structure mechanical compaction is the primary cause of the densification of reservoirs in the study area compared to that observed in the t3x4 reservoirs more intensive mechanical compaction was observed in the t3x2 reservoirs changing the configuration of the throats present reducing the pore throat radii and ultimately lowering the porosity of t3x2 reservoirs as a whole quartz overgrowth and late period carbonate cements were observed in partly filled pore spaces narrowing throats and reducing pore connectivity but eogenetic cementation and chlorite cements can increase the pressure resistance of sandstone and inhibit compaction quartz overgrowth thus t3x4 reservoirs are relatively more porous as a result of eogenetic calcite cementation and primary pores are more developed in t3x2 reservoirs with chlorite cements dissolution is a major factor that improves the tight sandstone reservoir quality because it generates a large volume of secondary pores the more intensive levels of dissolution in t3x4 reservoirs not only creates more intergranular intragranular moldic and intracrystalline pores but also further increases the porosity of t3x4 reservoirs fracturing effects have developed in the study area to improve pore connectivity levels and thus the permeability of tight sandstone reservoirs potentially explaining the pore connectivity and permeability of certain t3x2 reservoirs this work may be able to be used as a useful reference for the reservoir quality evaluation and exploration of tight gas sandstone reservoirs for similar oil fields worldwide

magnetostratigraphy of the sherwood sandstone group lower and middle triassic south devon uk detailed correlation of the marine and non marine anisian the sherwood sandstone group in south devon consists of the budleigh salterton pebble beds and the otter sandstone formation the otter sandstone formation comprises four lithostratigraphic sub units from the base labelled a d and contains tetrapods corresponding to the perovkan land vertebrate faunachron and is consequently a key european non marine anisian succession a total of 181 palaeomagnetic specimens from 92 horizons were measured to determine the palaeomagnetic signal the magnetic properties and characteristic remanence is predominantly carried by haematite most of which probably formed early in the depositional and diagenetic history the lower part of the sherwood sandstone group budleigh salterton pebble beds and otter sandstone formation unit a lower part of unit b has predominantly normal polarity magnetozones bs1 bs2 and bs3n with three thin reversed polarity magnetozones the lower part of unit b to the lower part of unit c of the otter sandstone formation is predominantly reverse polarity magnetozones bs3r bs4 and bs5 but includes three short normal polarity intervals the upper ca 50 m of unit c shows a mix of normal and reverse polarity bs6 bs7 with normal dominating the ca 15 m thick unit d is entirely of normal polarity bs8n the magnetostratigraphy allows unambiguous comparison with conodont and ammonite calibrated magnetostratigraphies from elsewhere in europe these show that the bs2 and bs3n magnetozones of the otter sandstone formation are lower aegean to middle pelsonian lower to middle anisian the bs3r to bs5r magnetozone interval is middle pelsonian to lower illyrian middle to upper anisian and the bs6 to bs8n magnetozone interval is middle illyrian to lower fassanian the top of unit d is correlated to an interval in the middle of the nevadites zone close to the anisian ladinian boundary consequently the otter sandstone formation correlates with the muschelkalk in the polish basin but is equivalent to only part of the germanic basin muschelkalk the continuous magnetostratigraphy from the sherwood sandstone group provides additional details of the polarity pattern through the pelsonian to illyrian interval which appears to be lacking in discontinuous marine sections a synthesis of adjacent european sedimentary basins in combination with the magnetostratigraphy suggests that the budleigh salterton pebble beds are probably late spathian to lower aegean

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

fault patterns associated with domes an experimental and analytical study experimental clay and analytical models suggest that regional strain either extension or compression significantly affects the fault patterns produced by doming our models specifically simulate the shallow deformation produced by gentle doming of a homogeneous material with and without a simultaneously applied regional horizontal strain the models of circular domes show that without regional strain normal faults develop on the crests and flanks on the flanks the normal faults have radial trends with regional extension normal faults on the crests of circular domes trend perpendicularly to the regional extension direction whereas many normal faults on the flanks trend obliquely to it strike slip faults trending 60 from the regional extension direction fo m near the peripheries with regional compression many normal faults on the crests and flanks of circular domes strike parallel with the regional compression direction strike slip faults trending 30 from the regional compression direction also form on the flanks and reverse faults striking perpendicularly to the regional compression direction develop on the peripheries our models show that regional strain affects the fault patterns produced by elliptical doming similarly the fault patterns of our models resemble fault patterns of domed strata above salt diapirs and above uplifted blocks of basement

dips of normal faults in british coal measures and other sedimentary sequences fault dip measurements n 3291 from 122 faults in british coalfields are presented two classes of normal fault are recognized a gently dipping 55 post coalification tectonic faults formed at depths of about 2 5 km which comprise 90 of the faults analysed dip data from the steeply dipping faults are variable standard deviation 9 both on single faults and between faults and 70 is a better approximation of fault dip than the commonly accepted value of 60 data on tectonic normal faults with comparable depths of formation using the modified griffith fracture criterion the dips of faults at varying depths up to 4 km are calculated for both unsaturated dry and saturated rocks the dips of normal faults are expected to vary from 90 at the surface to 60 at 3 4 km and this variation is consistent with the limited field data available the coalfield data are consistent with fault formation at standard hydrostatic pore fluid pressures at depths of about 2 5 km below depths of a few kilometres normal faults are expected to flatten out gradually to a dip of 45 but such flattening is not necessarily an indication of an underlying detachment

carbonate porosity creation by mesogenetic dissolution reality or illusion many authors have proposed that significant volumes of porosity are created by deep burial dissolution in carbonate reservoirs we argue however that this model is unsupported by empirical data and violates important chemical constraints on mass transport because of the ubiquitous presence and rapid kinetics of dissolution of carbonate minerals the mesogenetic pore waters in sedimentary basins can be expected to be always saturated and buffered by carbonates providing little opportunity for the preservation of significantly undersaturated water chemistry during upward flow even if the initial generation of such undersaturated pore water could occur a review of the literature where this model has been advanced reveals a consistent lack of quantitative treatment in consequence the presumption of mesogenetic dissolution producing a net increase in secondary porosity should not be used in the prediction of carbonate reservoir quality

calcretes in semi arid alluvial systems formative pathways and sinks late quaternary deposits in gujarat western india show an abundant development of calcretes three major sinks of carbonate in the alluvial deposits are recognized 1 groundwater calcretes 2 pedogenic calcretes and 3 calcrete conglomerates groundwater calcretes originate from carbonate saturated waters travelling preferentially along stratification planes pedogenic calcretes form through soil forming processes typically in extra channel areas calcrete conglomerates occur as ribbons sheets and lenses due to the reworking of both pedogenic as well as groundwater calcretes as a result a pathway of calcretization develops that has the route groundwater calcrete to pedogenic calcrete to calcrete conglomerate the formation of pedogenic calcretes over sediments containing groundwater calcretes demonstrates that 1 apart from aeolian dust river waters are also a major source of carbonate and 2 pedogenic carbonates may attain large sizes at accelerated rates due to the presence of pre existing groundwater calcretes consequently the maturity of a soil may be overestimated if determined by following established morphogenetic sequences

microfractures in chalks of albuskjell field norwegian sector north sea possible origin and distribution production data from many north sea chalk fields have indicated moderate to considerable contribution to production from natural fractures this paper illustrates a detailed study of natural fracturing in the albuskjell field where gas condensate hydrocarbons are contained in upper cretaceous maastrichtian and lower tertiary danian chalks the field is a large halokinetically induced dome located on the northern limits of the productive chalk region known as the greater ekofisk area examination of core material from wells 1 6 3 2 4 f10 and 1 6a 10 albuskjell has revealed the presence of two types of fractures the first appear to be early are commonly compacted predominantly healed and resemble conjugate shear fractures the second type is related mainly to the tips of stylolites these are vertical preferentially open and are interpreted as tension fractures tension fractures form when the minimum effective stress is reduced to the tensile strength of the chalk as a result of increased pore fluid pressure and or decreased total confining stress due to relative extension construction of effective stress versus progressive burial depth profiles using three models of overpressure generation suggests that high pore fluid pressures alone could not have formed the tension fractures in albuskjell measurements and calculations from depth converted seismic sections have shown that halokinesis occurred throughout chalk deposition and continued in a series of pulses until early miocene time incremental stress values associated with halokinesis predict optimum conditions for tension fracture formation at 4 500 ft 1 370 m burial owing to a combination of halokinetic doming and overpressuring with an important contribution from source rock maturation these fractures possibly formed therefore post hydrocarbon emplacement 3 000 ft 1 000 m burial identity mid oligocene and have been preserved by hydrocarbon invasion the healed shear fractures formed prior to significant pressure solution and hydrocarbon emplacement and are thought to be associated with extra dome processes possibly involving graben tectonics or halokinetic reactivation of earlier northwest southeast trending major faults their distribution is expected to be fairly uniform over the field with broadly northwest southeast orientations shear fracture density is moderately constant between the danian and maastrichtian chalk sequences but is significantly reduced to absent in the argillaceous base danian model studies predict that tension fractures should be concentrated in crestal areas of the field where near random orientations may occur preferentially radial with some concentric orientations relative to the structural dome may occur off the crest with radial fractures persisting farther downflank predicted tension fracture orientations agree well with those measured from the oriented cores of well 1 6 a10 open tension fractures are preferentially absent from tight early cemented or argillaceous chalks they are also poorly developed in soft lithologies and predominantly occur in chalks with moderate to high porosities with stylolites the spacing of open tension fractures is assumed to be constant over the field whereas fracture width decreases from crest to flank this is in line with the concept of lower horizontal stresses in the structural crest a plot of optimum fracture spacing versus fracture width has been constructed using strain values calculated from the halokinetic models analysis of production data supports the proposed model and agrees well with a strong halokinetic influence on fracture development although significant halokinesis ceased during the early miocene residual strain may still exist in albuskjell with larger horizontal effective stresses occurring in the flanks of the structure calculations suggest that tension fractures are probably open under initial field conditions to a potentia depth of 11 500 ft 3 500 m

fracture stresses at shallow depths during burial abstract single layer joints in lower eocene chalks near beer sheva israel developed at shallow depths during burial prior to folding and uplift fracturing occurred in two stages initially extensional joints marked by circular fracture surface morphology formed at about 100 m depth when pore pressure p 1 5 mpa horizontal maximum effective stress gs y 1 0 mpa vertical effective stress gs z 1 0 mpa and horizontal minimum effective stress gs x 0 5 mpa subsequently hybrid joints that produced a dihedral angle of 35 and were marked by axial horizontal plumes formed at a burial depth of about 150 m where p 2 0 mpa gs x 0 5 mpa gs y 1 0 mpa and gs z 1 8 mpa fracturing continued during further burial to depths of perhaps 200 400 m

estimating rates of calcrete formation and sediment accretion in ancient alluvial deposits the use of calcrete horizons as a means of assessing rates of sediment accretion in ancient alluvial sequences is reviewed any attempt to use such horizons in quantifying ancient alluvial deposition must appreciate that accurate data on rates of formation of quaternary calcretes are limited such data show considerable variations in the rates and no reliable data exist at all for some calcrete types the abundance of calcretes in the geological record warrants a new initiative to improve our understanding oftheir origins and rates of formation

interbed storage changes and compaction in models of regional groundwater flow water released from permanent compaction of compressible fine grained interbeds within confined aquifers may be a significant source of pumped water permanent or inelastic compaction of the interbeds occurs when head declines from pumping cause the effective stress or grain to grain load to exceed the elastic limits of the interbeds as a result the grains in the interbeds rearrange and excess water is expelled the amount of inelastic compaction generally is proportional to the increase in effective stress and the decrease in head a common approach for the simulation of elastic and inelastic compaction in groundwater flow models is to assume that head changes in the aquifer result in instantaneous storage changes in the compressible interbeds a term is added to the groundwater flow equation to account for the storage changes changes in specific storage may be computed explicitly from the results at the previous time step in the finite difference formulation of the groundwater flow equation a better approach is to implicitly apportion storage changes between elastic and inelastic components within a time step if storage changes cannot be considered to occur instantaneously with change in head in the aquifer another approach is taken equations for horizontal flow in the aquifer are coupled with equations for vertical flow in the compressible interbeds to simulate flow in half a representative doubly draining interbed storage changes and compaction are calculated for the representative half thickness and are extrapolated to the entire thickness of all interbeds within each cell this approach was applied to an existing model of groundwater flow in the central valley of california the simulation demonstrates that solving coupled systems of equations is feasible for a regional flow model

geodetic evidence for subsidence due to groundwater withdrawal in many parts of the united states of america abstract analysis of repeated levelings conducted by the national geodetic survey indicates many locations of relative subsidence in the u s a specific criteria are used to identify those subsidence features which most likely result from sediment compaction due to lowering of water levels in underground aquifer systems previously reported areas of such subsidence that seem to be confirmed by this study include south central arizona savannah georgia pecos and houston galveston texas denver colorado san joaquin valley santa clara valley saugus basin los angeles basin and bunker hill san timoteo california milford utah raft river valley idaho baton rouge and new orleans louisiana and las vegas nevada in addition our analysis indicates that such subsidence is more widespread than previously reported occurring in a number of locations within the atlantic and gulf of mexico coastal plains and along the mississippi valley as well as in many other unconsolidated sedimentary basins from which groundwater has been extracted and for which repeated leveling measurements are available more than 30 new locations of subsidence possibly due to water withdrawal have been identified the best documented of these include ventura ontario and san pedro santa monica california monroe and alexandria louisiana and jackson mississippi subsidence in newly identified locations ranges from several tens of millimeters to a few hundred millimeters over distances of 10 60 km and times of 2 48 yr subsidence may also have occurred in other groundwater basins where water levels have been drawn down and remained undetected because of a lack of repeated levelings it is important to be aware of and to continue to monitor such effects because of increasing rates of utilization of groundwater resources and the potential for significant engineering problems due to surface movements in addition identifying cases of subsidence due to water withdrawal is essential in order to use effectively releveling observations for investigating tectonic deformation along these lines our analysis suggests that some of the releveling observations used to define a region of tectonic uplift in southern california more likely reflect subsidence due to water level declines within sedimentary basins around the periphery of the presumed zone of uplift

chalk porosity and tertiary uplift western approaches trough sw uk and nw french continental shelves porosities have been derived from sonic logs for the cenomanian maastrichtian chalk of 22 hydrocarbon exploration wells in the western approaches trough although the burial depth of the chalk studied ranges from 40 1500 m below sea bed porosities are all within the range 0 45 the most deeply buried chalk sequences exhibit close to normal porosity depth relations for offshore chalks abnormally low porosities in the remainder of the wells result from burial at greater depth than that presently observed low porosities were preserved during tertiary alpine related uplift of the area for a given well the magnitude of uplift not reversed by subsequent subsidence apparent uplift equals the displacement of its porosity depth relation from the normal relation along the depth axis apparent uplift in the western approaches trough ranges from 0 1 3 km in the western approaches trough chalk porosity data provide evidence of more widespread tertiary uplift than do seismic reflection records alpine related compression and uplift fault bounded basin inversion is apparent only on seismic records in the southern part of the trough brittany and south west channel basins whereas chalk porosity data suggest regional uplift occurred throughout the trough and adjacent highs chalk porosity data were also used to quantify tertiary uplift in the cleethorpes l well of the east midlands shelf the resultant value of 1 1km is considerably greater than that previously derived from shale sonic velocities approximately 0 3 km and slightly less than that based on apatite fission track analysis 1 3 1 7 km

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

a numerical model to calculate land subsidence applied at hangu in china abstract the deep unconsolidated sedimentary aquifers of eastern china are heavily over exploited and this causes aquifer compaction 1 which results in land subsidence the geological formation contains significant thicknesses of clay which introduces time delays between the extraction of the groundwater and the resulting subsidence a new computer code called the interbed drainage package has been developed for the popular modflow model to address this behaviour it has been applied in a simulation of the port of hangu and the results are being used to manage the land subsidence problem

groundwater pumping and land subsidence in the emilia romagna coastland italy modeling the past occurrence and the future trend 1 the emilia romagna coastland south of the po river delta italy has experienced a dramatic land settlement mainly due to the large groundwater withdrawal related to the local economic and tourist development started in the early 1950s although the use of surface water has reduced the settlement rate over the last three decades anthropogenic land subsidence still continues in a few kilometer wide coastal strip at a rate larger than the natural one the occurrence is reconstructed since 1946 with the aid of advanced finite element flow and poromechanical models implemented with a realistically detailed geology of the regional shallow multiaquifer system the models have been calibrated against the piezometric leveling and extensometer records observed over the last 50 years and a land subsidence prediction in 2016 is performed the results show that the extensive groundwater pumping that occurred in the past is most likely the main cause of the recent land settlement as well because of the delayed compaction of the clay aquitards comprised between the depleted aquifers however the available pumping data do not allow for a thorough understanding of the current local settlement process along the coastline which is the most vulnerable area of the emilia romagna region from an environmental viewpoint if the planned scenario of groundwater resource management will be implemented anthropogenic land subsidence is bound to become a marginal problem for the central and northern portion of the emilia romagna coastland

calcrete as a source of heterogeneity in triassic fluvial sandstone aquifers otter sandstone formation sw england carbonate is not generally considered as a potential source of large scale heterogeneity in permo triassic sandstone aquifers this study shows that carbonate in the form of early diagenetic calcrete forms an abundant component of the triassic otter sandstone formation in south devon three types of calcrete are described from the outcrop of this fluvial sandstone dominated aquifer rhizocretions calcrete sheets and calcrete conglomerates data obtained from core plugs show that calcrete reduces sandstone permeability to less than 10 15 m2 and porosity to less than 12 calcrete can be sufficiently abundant to produce a significant up to 30 reduction in the total effective porosity of the aquifer moreover calcrete conglomerates can form laterally extensive up to 1 km low permeability sheets that will represent major baffles to vertical flow

strain and scaling of faults in the chalk at flamborough head u k analysis has been made of the orientations displacements and spacings of 1340 extensional faults with displacements of up to 6 m along an almost completely exposed 6 km length of cliff this data set has been used to study how fault populations account for strain in a region and to study relationships between different scales of fault strains have been estimated the maximum and intermediate extensions are sub horizontal with approximately equal extension e 0 01 in all horizontal directions it can be inferred that the minimum extension maximum compression was sub vertical but that the wide variety of fault orientations and cross cutting relationships resulted from variable horizontal extensions some faults have oblique slip slickenside lineations which imply a period of later dominantly nnw sse extension which possibly developed as the exposure scale faults linked up e w striking larger scale normal faults effectively forming a single wide fault zone graphs of displacement per unit distance are used to illustrate variations in displacement the scaling of fault displacement appears to follow a power law relationship the differences in orientation between the small scale and large scale faults precludes a simple estimation of the total strain over all scales

petrology and geochemistry of rhizoliths from plio pleistocene fluvial and marginal lacustrine deposits east lake turkana kenya abstract recent studies of rhizoliths from the koobi fora formation plio pleistocene of east lake turkana kenya indicate that their shapes vary with the depositional environment of their host sediment vertical rhizoliths are associated with channel bar and overbank deposits of fluvial origin these rhizoliths represent the tap roots of phreatophytes that lived in well drained upland settings horizontal rhizoliths predominate in facies ascribed to beaches lagoons and floodplains associated with ancient lake turkana they are produced by water emergent aquatic macrophytes and coastal grasses that grow in saturated or poorly drained soil environments the rhizoliths consist of sparry and micritic calcite cements whose petrography and geochemistry reflect differences in diagenesis between the two types of root systems the vertical roots contain little organic debris and have poorly developed meniscus and microstalactitic cements these attributes reflect precipitation of the calcite in well oxygenated well drained vadose conditions in contrast the cements of the horizontal root systems lack textures indicative of vadose conditions contain abundant clay and plant debris and display mn concentrations as high as 4 5 cation percent these features confirm that the horizontal rhizoliths developed in chemically reduced water saturated conditions bacterial decay of the plant debris maintained reduced eh conditions while also supply ng abundant mn2 to the pore solutions groundwater that had moved through volcanic strata may have also contributed mn2 the mn may have been incorporated in the calcite cements through a combined process of chemisorption and ionic substitution of mn2 as well as by inclusion of manganese oxides

preconsolidation stress of aquifer systems in areas of induced land subsidence aquifer systems in the eloy picacho area arizona the houston galveston area texas and the tulare wasco area and santa clara valley california appear to have been overconsolidated by an amount that ranged approximately from 1 6 to 6 2 bars 16 to 63 m of water before man began to withdraw groundwater from them the relation between land subsidence and water level decline in these areas consists of two linear segments in these areas subsidence per unit water level decline was approximately constant until water levels had declined an amount that ranged from 16 to 63 m when water levels declined past these values subsidence per unit water level decline increased to larger constant values although slow drainage from aquitards may have contributed to this response it is interpreted here to be caused primarily by natural overconsolidation of the compacting part of the aquifer system the water level decline at which the ratio of subsidence to unit water level decline changed indicates approximately the amount by which the preconsolidation stress exceeded the overburden stress on the aquifer system that existed before groundwater withdrawals began

parameter estimation of an overconsolidated aquitard subjected to periodic hydraulic head variations within adjacent aquifers abstract extensometers provide long term records of deformation over a depth interval of interest an overconsolidated aquitard usually undergoes elastic deformation if the hydraulic head within adjacent aquifers remains higher than the preconsolidation head within the aquitard the vertical hydraulic conductivity k v and elastic skeletal specific storage s ske are two critical parameters that control the deformation behavior of an overconsolidated aquitard and they could be interpreted from valuable extensometer data an analytical solution for the elastic aquitard deformation caused by periodic hydraulic head variations phhv within adjacent aquifers is derived using the separation of variables method based on euler s formula the phase difference in phhv between adjacent aquifers is found to reduce the amplitude of periodic aquitard deformation meanwhile the phase difference is found to cause no influence on the growth pattern of the phase shift which is between the periodic aquitard deformation and phhv versus the period a new parameter estimation method is further proposed to interpret aquitard k v and s ske from periodic aquitard deformation data this new method yields robust k v and s ske estimates of a moderately thick aquitard whose deformation history has been recorded by an extensometer located in shanghai china the field study confirms that the phase difference in phhv between the aquifers adjacent to the overconsolidated aquitard of interest cannot be ignored

deformation characteristics of a clayey interbed during fluid injection surface deformation due to fluid withdrawal has long been observed at the surface above aquifers and oil reservoirs uplift associated with fluid injection has also been observed although the processes of subsidence and uplift are reversible in a poroelastic setting the presence of clayey interbeds can result in ground deformation behavior non reversible because of their low permeability and nonlinear behavior in this investigation a cam clay model is used in conjunction with a poroelastic model to simulate the presence of a laterally extensive lens shaped clayey interbed in a sandstone aquifer during fluid injection spatial and temporal changes associated with this interbed during aquifer pressurization are captured and can be clearly differentiated from the deformation of the surrounding poroelastic aquifer formation results for pore pressures and ground surface deformation patterns can be categorized into three distinct time intervals an early time interval where the aquifer is pressurized but not the interbed leading to a lower region above the interbed an intermediate time interval in which the interbed pressures slowly increase approaching the pressure of the adjacent aquifer leading to a potentially large surface uplift above the interbed and a late time interval in which pressure equilibration is achieved between the interbed and aquifer and a highly non symmetrical irregular surface deformation pattern results which may be higher or lower than the background depending on the interbed characteristics reservoir configuration is found to be an important factor influencing ground deformation behavior with more obvious deformation always occurring in a laterally confined aquifer as opposed to a laterally infinite aquifer

phase difference between groundwater storage changes and groundwater level fluctuations due to compaction of an aquifer aquitard system abstract drainage and deformation of intercalated confining layers due to internal stress change discharge recharge cycle in an aquifer aquitard system not only can have great effects on the groundwater storage gws but also can cause unsynchronized water level wl fluctuations the phase relationship between gws and wl is crucial for the accuracy and attribution of gws changes we identify the dominating episodic components of gws and wl through the singular spectrum analysis ssa analysis and harmonic analysis first we analyzed a generic aquifer aquitard system using numerical simulations which showed that the dissipation of overpressure from the aquitard and flow from it is the inherent cause of a phase shift between the gws and wl observations water released from confining layers with compaction time constant of zero no delay to century time scale results in detected phase shift between total gws and wl in the aquifer then we analyzed the complex and varied phase relationship between gws derived using the gravity recovery and climate experiment grace data and measured wl time series in the subsiding north china plain ncp aquifer the spatially varied phase relationship between gws and measured wl is reasonably related to varied land subsidence development features and may be related with the compaction time constant and thickness of the confining layers results of the generic numerical model and the ncp observations suggest that elastic inelastic response need to be considered in the interpretation and correction of gws changes for a compacted aquifer aquitard system

calcretes related to phreatophytic vegetation from the middle triassic otter sandstone of south west england carbonate cementation in the triassic otter sandstone of budleigh salterton sw england occurs in two distinct forms large vertical concentrically zoned cyclinders and thin subhorizontal sheets the former represent rhizocretions of the tap roots of phreatophytic plants which colonized bars and abandoned channels on a large braidplain the sheets represent cementation around ancient water tables the precipitation of the rhizocretions took place at least in part during the life of the plants and 13c and 18o values support the view that evapo transpiration induced carbonate precipitation palaeosol profiles are rare in the otter sandstone reflecting the geomorphological instability of the braidplain surfaces in contrast rhizocretionary and sheet calcretes which formed several metres below the active depositional surface are abundant because they had a high preservation potential

compaction of aquifer units under complex patterns of changing groundwater level long term groundwater withdrawal has caused severe land subsidence in shanghai in which compaction of aquifer units has taken an important role the compaction characteristics of aquifer units are closely related to the changing patterns of groundwater levels the units have experienced they may be elastic elasto plastic visco elastic or visco elasto plastic several compaction models are discussed and used to calculate the compaction of the aquifer units in shanghai compared with other models it is found that double yielding visco elasto plastic model can capture the compaction characteristics of aquifer units and more reasonably simulate the compaction of aquifer unit under complex changing patterns of groundwater level in particular the double yielding model can describe the plastic compaction when the average groundwater level declines but is higher than the historically lowest level and the obvious visco plastic compaction when the groundwater level is lower than the historically lowest level which is a popular deformation characteristic for aquifer units in shanghai

reworked calcretes their significance in the reconstruction of alluvial sequences permian and triassic minorca balearic islands spain the permian and triassic of minorca balearic islands consists of a 670 m thick red alluvial succession that includes in situ calcrete profiles and reworked calcrete material in the permian succession the calcretes vary from laminar forms developed on the carboniferous basement to weakly developed nodular calcretes in fluvial sediments the palaeosols in the triassic are mostly dolomitic and the profiles reach up to stage iii of soil development spec pap geol surv am 203 1995 1 the clasts formed through reworking of the palaeosol profiles are about 0 5 10 cm across and include mosaics of calcite dolomite crystals brecciated clasts rhizolith fragments and aggregates of clay and or silt these clasts appear in three different types of deposits type 1 corresponds to lenticular bodies that fill small scour surfaces and consists only of intraformational conglomerates these deposits are interpreted as ephemeral channels and sheet floods that represent the interfluvial drainage systems that captured only the precipitation falling on the alluvial plain type 2 includes sand dune 3 d bodies with flat bottoms and convex tops these bodies are about 20 cm high and 2 m wide and were formed by floodwaters that flowed down the levees of the major streams type 3 channel deposits contain reworked calcretes and extrabasinal clasts which overlie erosive surfaces and are found in layers within cross bedded sandstones and conglomerates these are interpreted as channel floor lag deposits of major channels that entered from distant uplands and drained the alluvial plain variations in the aggradation rates of the floodplain resulted in five different infill stages in the lowstand to early transgressive interval as in stages i p1 and iv b1 the fluvial deposits filled palaeovalleys calcretes and reworked calcrete deposits were of difficult formation apart from terraces and preservation accommodation space was at its greatest in the transgressive stages ii p2 and v b2 this caused the greatest aggradation of the floodplains which are formed of thick sequences of fine grained sediments isolated meandering channels weakly developed calcretes compound and reworked calcrete deposits mostly of types 1 and 2 the density of channels notably increased in stage iii p3 highstand interval because of the reduction of accommodation space this could favour the formation of composite or even cumulative palaeosols but of difficult preservation reworked calcrete deposits are mostly of type 3 but types 1 and 2 are also recognised the reworked calcrete deposits are an important part of the permian and triassic fluvial sediments and their occurrence and characteristics are important in order to interpret the infill of terrestrial basins and the construction of floodplains

origin of fracture porosity example from altamont field utah the occurrence of natural fracture systems in subsurface rock can be predicted if careful evaluation is made of the geologic processes that affect sedimentary strata during their cycle of burial diagenesis uplift and erosional unloading variations in the state of stress within rock arise for example from changes in temperature pore pressure weight of overburden or tectonic loading hence geologic processes acting on a sedimentary unit should be analyzed for their several contributions to the state of stress and this information used to compute a stress history from this stress history predictions may be made as to when in the burial cycle to expect fracture joint formation what type of fractures extension or shear may occur and which geologic factors are most favorable to development of fractures a stress history is computed for strata of the naturally fractured altamont oil field in utah s uinta basin calculations suggest that fractures formed in extension that the well cemented rocks are those most likely to be fractured that fractures began to develop only after strata were buried to great depth and that the fracture system continued to develop as strata were uplifted and denuded of overburden geologic evidence on fracture genesis and development is in accord with the stress history prediction stress history can be useful in evaluating a sedimentary basin for naturally fractured reservoir exploration plays

evolution of solution structures in a deformed quartz arenite geometric changes related to permeability changes abstract quartz arenite of the silurian keefer sandstone at a site in west virginia u s a records an early pervasive solution related deformation by grain interpenetration volume losses measured by the pss method show shortening directions from both diagenetic compaction and early tectonic layer parallel shortening later solution related tectonic deformation during folding formed transgranular stylolites along pre existing faults or in rock volumes with abundant microfractures thus the geometry of solution related deformation changed progressively from pervasive at grain scale to focused along transgranular surfaces we propose that this geometric change resulted from intense porosity and permeability reduction during diagenetic cementation and early layer parallel shortening which required fluids to exploit tectonically created pathways along faults or networks of microfractures during later deformation

dynamic model of the genesis of calcretes replacing silicate rocks in semi arid regions abstract in both pedogenic and groundwater calcretes calcium carbonate precipitates in voids or displacing other grains or replacing underlying parent silicates replacement textures are widespread in pedogenic calcrete many calcretes also contain magnesium layer silicates and minor chert we present a reaction transport model that accounts for the genesis of replacement in calcretes and for their mineralogy replacement is difficult to account for geochemically because it requires simultaneous removal of large amounts of silicates and import of also large amounts of caco 3 in the model the genesis of replacement is directly related to seasonally alternating dry wet climates and to appropriate groundwater or circulating soil water compositions in a dry season water evaporation causes caco 3 and sepiolite or attapulgite to precipitate if groundwater contains enough mg 2 sepiolite precipitation by the chemical divide mechanism depletes sio 2 aq resulting in the dissolution of parent silicates in the following wet season sepiolite dissolves fast and silica and cations are flushed away by rainwater making room for caco 3 precipitation in the next dry season as climate cycles repeat caco 3 is accumulated and silicates are removed the sepiolite or attapulgite or mg smectite serves as a temporary storage of silica between seasons if the groundwater contains too little aqueous mg then the model predicts growth of calcium carbonate without removing silicates thus producing void filling and or displacive textures instead of replacement the model consists of a set of nonlinear partial differential equations taking account of mass conservation dispersion advection rainwater infiltration evaporation and the kinetics of mineral reactions the hydrodynamics of unsaturated media is applied in determining water flow in calcrete profiles wet dry seasonal changes are incorporated by alternating the upper boundary conditions the model successfully produces the mineral and textural zonation observed in many calcretes namely at the bottom of the profile the parent rock is first replaced by sepiolite only part of which is in turn replaced by calcite whereas at the top of the calcrete the sepiolite is itself completely replaced by the calcite which appears to directly replace the parent rock the model can produce calcrete near the earth s surface well above the water table low p co 2 intensive evaporation and long dry seasons all are predicted to produce thicker calcretes calcretes constitute an effective geochemical tool that via replacement removes parent silicate rocks and shapes the landscape of semi arid countries the model herein provides a mechanism that accounts for the efficiency of replacement in removing silicates

late jurassic half graben control on the siting and structure of hydrocarbon accumulations uk norwegian central graben abstract the late jurassic structure of the central graben is discussed both in terms of the extensional features developed at this time and their subsequent controls on younger inversion structures the regional kinematics of the north sea rift suggest that during the late jurassic the central graben opened approximately orthogonally to the main nnw sse fault trend within the basin some evidence that triassic extensional faults were reactivated at this time is seen towards the basin margins and a significant triassic extensional history is inferred the late jurassic basin comprises the axial c 100 km of the central graben s l the basin axis is offset progressively to the se by both transfer zones and discrete transfer faults footwall uplift of the basin margins during extension provided the source for the upper jurassic reservoir sands encountered by exploration drilling these sands were probably preferentially fed into the basin at transfer zones in the margins salt structures are ubiquitous within the central graben and their siting is everywhere controlled by the underlying fault block topography salt structures form preferentially towards the crests of half graben or buttressed against their boundary faults inversion structures in the norwegian sector reactivated with a reverse sense of displacement late jurassic extensional faults a model is proposed which shows the two main inversion structures feda graben norway tail end graben denmark to be detached on antithetic reverse faults riding in the salt these faults terminate at tip folds within major salt walls the geometry of the inversion structures can be understood only when considered together with the geometry of the underlying late jurassic half graben

western greece and ionian sea petroleum systems in western greece the ionian and pre apulian zones represent respectively the basin and the transitional zone slope to the apulian platform the apulian platform constitutes the weakly deformed foreland of the external hellenides the pre apulian zone appears in the ionian islands and the eastern ionian sea whereas the apulian platform is exclusively found in the ionian sea the ionian zone consists of triassic evaporites jurassic upper eocene mostly pelagic carbonates minor cherts and shales overlainbytheoligoceneflysch organic richsourcerocks occur within triassic evaporites and jurassic cretaceous pelagic argillaceous siliceous rocks the pre apulian zone consists of triassic to miocene deposits mainly mixed neritic pelagic carbonates hydrocarbon source rocks include pelagic and hemipelagic deposits rich in marine organic material although terrigenous organic matter is also found in siliciclastic layers apulian platformsourcerocksaremainlytheorganic richshaleswithinthe triassic burano evaporites western greece contains major petroleum systems which extend into the ionian sea ionian pre apulian and apulian petroleum systems contribute to the probable hydrocarbon accumulations within the big offshore ionian sea anticlines western greece contains important oil and gas shale reservoirs with a potential of unconventional exploration promising areas for hydrocarbons need systematic and detailed threedimensional seismic data exploration for conventional petroleum reservoirs through the interpretation of seismic profiles and the abundant surface geologic data will provide the subsurface geometric characteristics of the unconventional reservoirs their exploitation should follow that of conventional hydrocarbons to benefit from the anticipated technological advances eliminating environmental repercussions

an organodiagenetic model for marinoan age cap carbonates abstract the nuccaleena formation cap carbonate is the global stratotype for the marinoan glaciation and ediacaran gssp a designation that emphasises the importance of cap carbonates to our current understanding of global neoproterozoic biogeochemistry sedimentology and stratigraphy however to date there is no agreed depositional model for cap carbonates and there remains minimal detailed paragenetic data for developing process based depositional models here an early diagenetic organodiagenetic dolomite cementation model for marinoan cap carbonate formation is hypothesised and explored it is demonstrated how this process based model can explain the three main atypical sedimentary structures that are the basis for correlation of marinoan age cap carbonates giant tepee like structures sheet veins and tubestones these features are interpreted to be products of fluid overpressure deformation induced by organodiagenetic expansive cementation giant tepee like structures the hallmark of marinoan cap carbonates conform to a simple structural analysis that is consistent with the predicted stress field induced by expansive cementation driven fluid overpressure the undeformed to highly deformed gradient of deformation features in cap carbonates is consistent with the range of rheologies in a shallowly buried cementing carbonate modification of matrix textures and bedding surfaces also conforms to the range of expected textures within this model overall it is hypothesised that the aggradational condensed section architecture of cap carbonates was the primary control over the generation of these atypical sedimentary features the available paragenetic analysis suggests that the geochemical data particularly the c isotopic data can be reinterpreted as supportive of the organodiagenetic model and that the neoproterozoic ocean was perhaps similar to that of today the proposed model indicates that the current criteria for the placement of the ediacaran gssp are non unique and potentially non isochronous perhaps the most important aspect of this model is that it is testable and is a call for focused research on the much overlooked paragenesis of cap carbonates

adaptive explicit implicit quasi three dimensional finite element model of flow and subsidence in multiaquifer systems a quasi three dimensional finite element model is presented for the analysis of groundwater flow and land subsidence due to pumpage in multiaquifer systems in the model aquifers are simulated with the aid of two dimensional horizontal finite element grids each aquifer is connected to its neighbors above and below by one dimensional vertical finite element strings which allow leakage to take place across aquitards and aquicludes land subsidence is modeled by varying the void ratio of each vertical element according to a nonlinear version of terzaghi s one dimensional consolidation theory a major feature of the new method is its ability to solve the finite element equations explicitly in one part of the mesh and implicitly in another part depending on the ratio between the stability limit of each node and any given time step size since the hydraulic diffusivity of aquitards is usually very small compared to that of aquifers it is often possible to solve the finite element equations explicitly for most and sometimes all aquitard nodes this leads to a virtual decoupling of the aquifer equations during a given time step which results in a significant saving of computer time and storage

numerical simulation of land subsidence induced by groundwater overexploitation in su xi chang area china su xi chang area is one of the typical regions in china which suffers from severe land subsidence various field monitoring records were integrated to study the characteristics and mechanisms of land subsidence in this region the development of the land subsidence in this region shows a tight spatial and temporal correlation with the groundwater pumping based on the analysis of the field data it is found that the deformation patterns of the hydrogeologic units are greatly related to the hydrogeologic properties and groundwater level variations some have an elastic behavior others may have an elastic plastic rheology hence a 3d finite element numerical model considering the rheological properties of the soil was developed to simulate the groundwater level and land subsidence both hydraulic conductivity and specific storage were expected to vary with the porosity during the process of consolidation multiscale finite element method msfem was applied to solve the model during the period from 1996 to 2004 after calibrating the model with the observed groundwater level and subsidence data the parameters of the multi layers system were estimated the calibrated model outputs fit reasonably well with the observed data consequently the model can be applied to predict groundwater level and land subsidence in future pumping scenarios the model predictive results show that land subsidence rate can be controlled and even rebound may occur after the implementation of the groundwater exploitation prohibition

north sea chalk diagenesis cementation of chalks and healing of fractures abstract petrological and geochemical studies have been carried out on the chalk matrix and the fracture filling calcite from several oil fields in the great ekofisk area of the central graben norwegian north sea in order to understand the mechanism of chalk cementation and fracture healing a hypothesis involving an episode of ascending hot water from the underlying permian evaporites has been postulated by some workers to explain the healing of similar fractures in the chalk reservoirs of the norwegian and danish north sea and the danish sub basin however the results presented in this paper including cathodoluminescence microscopy major and trace element geochemistry c o and sr isotope composition and modelling of heating and compactional water flow rates suggest that the ascending hot water hypothesis does not satisfactorily explain the required water budget rate of flow and the source of the calcite cement heat flow simulations demonstrate that a fluid flow rate of 7 5 cm yr is required by the ascending hot water hypothesis this is much higher than the flow rates calculated for this and other sedimentary basins an alternative model is proposed which can account for the observed 18o values of the fracture filling calcite 12 7 to 8 5 pdb the chalk matrix 7 4 to 4 1 pdb and the formation water 4 smow in terms of closed to semi closed system diagenesis in this model calcite from the chalk matrix dissolves congruently and cement precipitates with an isotope ratio governed by the geothermal gradient similarity in the cathodoluminescence trace element composition and carbon isotope values between the fracture filling calcite and the chalk matrix also suggests that the cements in the fractures and the chalk matrix are similar and were probably derived locally by pressure solution processes

high temperature diagenesis in shallow chalk reservoir skjold oil field danish north sea evidence from fluid inclusions and oxygen isotopes samples of calcite filled fractures from the chalk reservoir of the skjold oil field were studied by fluorescence microscopy cathodoluminescence microscopy fluid inclusion microthermometry and oxygen and carbon stable isotope and trace element analyses two generations of calcite infilling are tentatively related to two major phases of faulting caused by salt movements owing to recrystallization of the first generation of infillings the chemical isotopic and fluid inclusion data only apply to the later generation results of the fluid inclusion studies suggest a precipitation temperature of the infilling calcite significantly above what it should be applying today s geothermal gradient 52 c km or 2 85 f 100 ft to the past this higher temperature indic tes that hot water ascending from deeper strata may have flushed the reservoir apparently during a late miocene to pliocene faulting phase when ring faults guided the water to the reservoir this feature may be typical of most chalk reservoirs in the danish central graben where diapirs have penetrated the chalk

multi temporal insar evidence of ground subsidence induced by groundwater withdrawal the montellano aquifer sw spain this study uses the insar technique to analyse ground subsidence due to intensive exploitation of an aquifer for agricultural and urban purposes in the montellano town sw spain the detailed deformation maps clearly show that the spatial and temporal extent of subsidence is controlled by piezometric level fluctuations and the thickness of compressible sediments the total vertical displacement measured with multi temporal insar between 1992 and 2010 is 33 mm that corresponds with a decrease of 43 m in the groundwater level this technique allows monitoring the evolution of settlement related to water level fall in an area where subsidence has not yet been reported by population or authorities through infrastructure damages and to discuss the effect of the aquifer recovery this information is therefore valuable for implementing effective groundwater management schemes and land use planning and to propose new building regulations in the most affected areas

origin and evolution of formation waters from oil fields on the norwegian shelf abstract stable isotopes of h and o and chemical analyses of formation waters from upper triassic jurassic clastic reservoirs and cretaceous carbonate reservoirs offshore norway are employed to interpret the origin and evolution of these waters in both types of reservoir the formation waters become progressively enriched in total dissolved solids and the heavy isotope of o with increasing depth whereas deuterium varies erratically between 20 and 40 smow bromine analyses and stable isotope data indicate that the formation waters are mixtures of a brine end member formed by evaporation of sea water beyond the point of halite saturation and meteoric water the chemical and isotopic composition of the formation waters and general geological considerations suggest that the brines originated as connate waters in permian evaporites and were later expelled into overlying strata there is a disequilibrium stable isotope distribution between the bulk chalk formations and the formation waters and only minor diagenetic modification of the formation waters appears to have taken place in these reservoirs the formation waters from the clastic reservoirs on the other hand are modified by diagenetic reactions