coagulation and transport of sediments in the gironde estuary the distribution of suspended particle size and concentration were measured along the gironde estuary france from the river seaward to the ocean the suspended particle size and volume concentration were measured using in situ holography and onboard optical techniques utilizing special procedures in order to avoid floc breakage sediments discharged by the rivers coagulate upon encountering the very low salinities 0 2 of the upper estuary confirmed with laboratory experiments and are then transported and deposited by currents in the remainder of the estuary this coagulation coupled with estuarine circulation produces a turbidity maximum which is offset between the surface and bottom waters the floc size maximum is oceanward of the turbidity maximum and is likewise offset along the estuary by about 30 km the estuary can be subdivided into the following zones 1 coagulation 2 hydrodynamic landward of the null point and 3 hydrodynamic seaward of the null point initial coagulation appears to be completed in coagulation zone 1 and particles are transported and settled with very little floc breakage and recoagulation in zones 2 and 3 only the floc settling velocities coupled with estuarine circulation control the concentration and size distributions of flocs in the water column and eventually control the deposition of sediments

three dimensional reservoir characterization and flow simulation of heterolithic tidal sandstones tidal sandstone reservoirs contain significant intervals of hydrocarbon bearing heterolithic facies characterized by the presence of tide generated sedimentary structures such as flaser wavy and lenticular bedding millimeter to centimeter sand mud alternations we have characterized the reservoir properties sandstone connectivity effective permeability and displacement efficiency of these facies using three dimensional 3 d models reconstructed directly from large rock specimens the models are significantly larger than a core plug but smaller than a typical reservoir model grid block we find that the key control on reservoir quality is the connectivity and continuity of the sandstone and mudstone layers if the sandstone layers form a connected network they are likely to be productive even at low values of net to gross about 0 30 5 this may explain why the productivity of low net to gross heterolithic tidal sandstones is commonly underestimated or overlooked connectivity is the dominant control on the transition between productive pay and nonproductive nonpay heterolithic facies however connectivity is difficult to characterize because core plugs sampled from the subsurface are too small to capture connectivity whereas two dimensional outcrop measurements can significantly underestimate the true 3 d value our results suggest that core plug measurements of permeability and displacement efficiency are unlikely to yield representative values at the scale of a reservoir model grid block because the connectivity and continuity of sandstone and mudstone layers varies significantly with length scale

estuarine clay mineral distribution modern analogue for ancient sandstone reservoir quality prediction the spatial distribution of clay minerals in sandstones which may both enhance or degrade reservoir quality is poorly understood to address this clay mineral distribution patterns and host sediment properties grain size sorting clay fraction abundance and bioturbation intensity have for the first time been determined and mapped at an unprecedentedly high resolution in a modern estuarine setting ravenglass estuary uk results show that the estuary sediment is dominated by illite with subordinate chlorite and kaolinite although the rivers supply sediment with less illite and significantly more chlorite than found in the estuary fluvial supplied sediment has been locally diluted by sediment derived from glaciogenic drift deposits on the margins of the estuary detailed clay mineral maps and statistical analyses reveal that the estuary has a heterogeneous distribution of illite chlorite and kaolinite chlorite is relatively most abundant on the northern foreshore and backshore and is concentrated in coarse grained inner estuary dunes and tidal bars illite is relatively most abundant as well as most crystalline and most fe mg rich in fine grained inner estuary and central basin mud and mixed flats kaolinite has the highest abundance in fluvial sediment and is relatively homogenous in tidally influenced environments clay mineral distribution patterns in the ravenglass estuary have been strongly influenced by sediment supply residence time and subsequently modified by hydrodynamic processes there is no relationship between macro faunal bioturbation intensity and the abundance of chlorite illite or kaolinite based on this modern analogue study outer estuarine sediments are likely to be heavily quartz cemented in deeply buried burial temperatures exceeding 80 to 100 c sandstone reservoirs due to a paucity of clay grade material 0 5 to form complete grain coats in contrast chlorite enriched tidal bars and dunes in the inner estuary with their well developed detrital clay coats are likely to have quartz cement inhibiting authigenic clay coats in deeply buried sandstones

early diagenesis of inner shelf phosphorite and iron silicate minerals lower cretaceous of the orpheus graben southeastern canada implications for the origin of chlorite rims wells in the orpheus graben encountered the most proximal part of the deltaic lower cretaceous rocks of the scotian basin more distal sandstones are important gas reservoir rocks with good reservoir quality where fe rich chlorite chamosite rims on framework grains have inhibited quartz cementation cutting samples from the orpheus graben show the presence of fe rich sheet silicates berthierine or chamosite and early diagenetic phosphorite these minerals were analyzed by electron microprobe and their textures were mapped with backscattered electron images studies in the north sea have shown a relationship between high phosphorus and the presence of good chlorite rims in reservoir rocks the mineralization of pore water phosphorus instead of its return to seawater is favored by fe rich sediments and sorption on iron oxides during shallow sea floor diagenesis the fe ti and p contents are uncommonly high in scotian basin shales compared with global average shale compositions the uncommon occurrence of inner shelf phosphorite in this study is interpreted to be a consequence of the same high fe content of the sediment that also favors the formation of fe rich sheet silicates in rapidly deposited deltaic sandstones of the offshore reservoirs the dominance of type 3 kerogen led to sulfate depletion occurring at depths of tens of meters and a corresponding great thickness for overlying eh oxidation potential controlled diagenetic zones the thick fe reduction zone allowed the formation of early diagenetic berthierine which on burial formed the chamosite rims that resulted in the improved reservoir quality in sandstones the distribution of phosphorus minerals may be an indicator of conditions suitable for berthierine formation

clay coated sand grains in petroleum reservoirs understanding their distribution via a modern analogue abstract clay coated grains can inhibit ubiquitous porosity occluding quartz cement in deeply buried sandstones and thus lead to anomalously high porosity a moderate amount of clay that is distributed as grain coats is good for reservoir quality in deeply buried sandstones being able to predict the distribution of clay coated sand grains in petroleum reservoirs is thus important to help find and exploit anomalously good reservoir quality here we have adopted a high resolution analogue approach using the ravenglass estuary marginal shallow marine system in nw england u k extensive geomorphic mapping grain size analysis and bioturbation intensity counts were linked to a range of scanning electron microscopy techniques to characterize the distribution and origin of clay coated sand grains in surface sediment our work shows that grain coats are common in this marginal shallow marine system but they are heterogeneously distributed as a function of grain size clay fraction and depositional facies the distribution and characteristics of detrital clay coated grains can be predicted with knowledge of specific depositional environment clay fraction percentage and grain size the most extensive detrital clay coated grains are found in sediment composed of fine grained sand containing 3 5 to 13 0 clay fraction associated with inner estuary tidal flat facies thus against common convention the work presented here suggests that in deeply buried prospects the best porosity might be found in fine grained clay bearing inner tidal flat facies sands and not in coarse clean channel fill and bar facies

identification of subglacial lakes using ers 1 radar altimeter radar altimeter data from ers 1 allow detailed topographic mapping of antarctica to 82 degrees s to be carried out revealing several notable surface features among these is the large subglacial lake near vostok which is mapped here in detail the central part of the lake is found to have an exceptionally smooth surface with r m s elevation variations of less than 0 2 m a search for other large antarctic lakes in the data is made based on the smoothness and low gradient of the surface a number of other flat areas are identified with lake locations previously determined from radio echo sounding observations however radar altimeter observations show that a minimum lake size of 20 km is required for a surface above a lake to become flat numerous bowl like features can be seen in the surface topography and these may be associated with intermediate sized subglacial lakes it is determined that high spatial resolution radar altimetery could be used to identify subglacial lakes greater than 10 km in lateral extent flat regions of the ice sheet are particularly useful as they may be used as height reference surfaces to help fix the orbits of radar altimeter satellites

facies and architecture of an estuarine tidal bar the trompeloup bar gironde estuary sw france abstract this study describes the sedimentary facies and internal architecture of a modern estuarine tidal bar present in the bay head delta of a wave and tide dominated estuary the gironde estuary in southwest france based on 51 cores 2 to 5 m long this work demonstrates that such bars are composite sedimentary bodies made up of individual sigmoidal sand units isolated from one another by thick muddy layers the bar appeared in the study area around 1874 since then its evolution has been characterized by phases of rapid downstream bar migration alternating with periods of bar abandonment during the periods of active bar progradation which commonly last between 10 and 20 years the bar progrades up to 7 km seaward and forms a narrow 200 800 m wide sand ridge up to 6 m thick during abandonment phases which also last around 10 20 years the bar is starved of sand and a mud layer 1 to 3 m thick is deposited the vertical succession formed by the seaward progradation of the tidal bar records these alternating phases of growth and abandonment a typical vertical succession in the bar system is 8 m thick and comprises several superimposed upward coarsening units of cross stratified sand deposited during phases of seaward progradation separated by a thick layer of thinner bedded mud and sand laminae deposited during phases of bar abandonment when a tidal bar builds up to the supratidal zone marshes accumulate and the bar becomes a permanent island in the estuary the large scale architecture of the bar is thus made of an alternation of potential sandy reservoir units and muddy internal seals the reservoir units are characterized by a sigmoidal shape the muddy layer which is deposited between the two sigmoidal units is a thick continuous potential internal seal this complex internal architecture is due to the fact that there is a considerable amount of mud present in the bay head delta of the gironde estuary it is thought to be representative of the architecture of tidal bars in the inner part of wave and tide dominated estuaries

reservoir quality and rock properties modeling triassic and jurassic sandstones greater shearwater area uk central north sea abstract the complex burial and diagenetic histories of the jurassic fulmar and triassic skagerrak sandstones in the uk central north sea present significant challenges with regard to reservoir quality and rock property prediction commercial reservoir quality is retained despite deep burial and associated high temperatures and pressures shallow marine fulmar sands are normally compacted mean igv 26 3 yet have porosities of 21 33 porosity was preserved through inhibition of quartz cementation by clay and microquartz coatings and enhanced by dissolution of framework grains 5 skagerrak fluvial sands are more compacted mean igv 23 2 exhibit minor feldspar dissolution we modeled reservoir quality evolution using the forward diagenetic model touchstone which simulates porosity loss due to compaction and quartz cementation quantitative petrographic analyses and burial history data were used to calibrate touchstone model parameters the results were applied to deeper prospects for pre drill prediction of porosity and permeability in parallel petrophysical data were used to characterize the elastic properties of the sandstones to provide a basis for quantitative seismic forward modeling experimental data and core calibrated petrophysical results reflecting variable in situ fluids and saturations were used to build an elastic properties model the model is robust and was used to generate fluid filled sandstone properties incorporating touchstone results for prospect specific seismic attribute modeling well results from exploration wells are in good agreement with pre drill touchstone and elastic properties model predictions

tidal and fluvial controls on the internal architecture and sedimentary facies of a lobate estuarine tidal bar the plassac tidal bar in the gironde estuary france abstract estuarine tidal bars emplaced in estuaries are complex sediment bodies composed of sand and mud provided by rivers this study focuses on the plassac tidal bar which is the unique lobate shape tidal bar of the gironde estuary located at the very upstream extremity of the bay head delta because no detailed sedimentological studies have been made on such geomorphological category of tidal bar its stratigraphy was investigated using a dense grid of 65 very high resolution seismic profiles ground truthed by 6 to 9 meter long cores and time controlled by historical bathymetric surveys the base of the tidal bar consists of a tidal erosional surface this surface has eroded muddy tidalites corresponding to the maximum flooding surface of the gironde estuary the bar is composed of two vertical sequences of facies composed of thickening up cross stratified fine to medium grained sand beds at the top and decimeter thick mud layers at the base those mud layers constitute the master bedding of the bar and are dipping in a channelward direction perpendicularly to the tidal flows and to the sediment transport directions one phase of lateral accretion of the plassac tidal bar is correlated with variations in fluvial discharge of the last decades and suggests a climate control on the internal architecture of this sandbar low water periods lead to the upstream migration of the turbidity maximum and to the deposition and consolidation of the muddy master bedding planes during the subsequent flood periods the muddy master bedding planes are partially eroded and then buried by the fluvially sourced sandy mini flood lobes that merge with the tidal bar the internal architecture of the plassac tidal bar is controlled by a lateral accretion process its master bedding surfaces which record this process are dipping perpendicularly to the axis of the tidal flows all the other tidal bars of the bay head delta around a dozen exhibit an elongated morphology and prograde longitudinally by seaward frontal accretion occurring in the axis of the tidal flows the master bedding surfaces which record this process are seaward dipping along the axis of the tidal flows those variations in internal accretion surfaces orientation of tidal bars are of considerable importance and must be taken into account for studies of ancient tidal deposits paleoenvironment reconstructions and for heterogeneities characterization and quantification of reservoir properties in these types of sandstones

infilling stratigraphy of macrotidal tide dominated estuaries controlling mechanisms sea level fluctuations bedrock morphology sediment supply and climate changes the examples of the seine estuary and the mont saint michel bay english channel nw france this article presents a synthesis of recent works performed on two macrotidal tide dominated estuaries located along the southern coast of the english channel the seine estuary and the mont saint michel bay nw france on the basis of very high resolution seismic data combined with sediment cores and ams dating these works allow reconstructing the holocene in ll of such singular estuaries yet rarely documented in the literature rate in sea level rise bedrock morphology sediment supply are the main controlling factors of the general in ll pattern both in lls are composed of two units the transgressive systems tract tst and highstand systems tract hst the maximum ooding surface mfs is dated around 6500 cal bp when the holocene sea level rise slowed down and corresponds to the main architectural change the tst is poorly preserved compared with the hst that constitutes the main depositional unit making this incised valley ll deviating from the classical models the hst is composed above the axis of the incised valley of a highly wandering estuarine channel belt characterized by deep tidal reworking throughout its building on the margin of this high energy estuarine system the hst is made of wave dominated barriers tidal ats and offshore tidal banks they constitute dominantly aggradational systems the aggradation rate of which is in accordance with the rate of sea level rise from 6500 cal bp onwards consequently the impacts of the rapid climate changesthat characterized mid to late holocene times can be preserved in these marginal successions various sedimentary and geometrical signatures indicate that periodic enhanced storminess episodes probably related to the bond cold events 1500 years periodicity are responsible for the most drastic environmental changes in these macrotidal tide dominated settings the role of long term tidal cycles as well as the impact of human activities are also considered

high frequency turbulence and suspended sediment concentration measurements in the garonne river tidal bore the study details new sediment concentration measurements associated with some turbulence characterisation conducted at high frequency in the undular tidal bore of the garonne river france acoustic doppler velocimetry was used and the suspended sediment concentration was deduced from the acoustic backscatter intensity the field data set demonstrated some unique flow features of the tidal bore including some large and rapid turbulent velocity fluctuations during and after the bore passage some unusually high suspended sediment concentration was observed about 100 s after the tidal bore front lasting for more than 10 min it is thought that the tidal bore passage scoured the bed and convected upwards the bed material reaching the free surface after the bore passage behind the tidal bore the net sediment flux magnitude was 30 times larger than the ebb tide net flux and directed upstream a striking feature of the data set was the intense mixing and suspended sediment motion during the tidal bore and following flood tide this feature has been rarely documented

elevation and volume changes on the harding icefield alaska airborne surface elevation profiles of the harding icefield south central alaska were made in 1994 and 1996 thirteen glaciers were profiled along with the upper region of the icefield the profiles were compared to u s geological survey topographic maps made in the 1950s to obtain elevation and volume changes comparison of the changes for the different glaciers shows no significant correlation between volume change and the type of glacier or characteristics such as location aspect size slope or terminus changes estimated total volume change for this 43 year period is about 34 km 3 which corresponds to an area average elevation change of 21 m the estimated error in this elevation change of 5 m is mainly due to errors in the maps at higher elevations our measurements provide an accurate baseline against which future determinations of volume change can be made

ams 14c chronology of 40 0 cal ka bp continuous deposits from a crater lake lake massoko tanzania abstract lake massoko tanzania 8 20 s 33 45 e 870 m a s l is a freshwater maar lake belonging to the rungwe volcanic area 15 km northwest of the livingstone mountains and lake malawi the rainfall regime is driven by the yearly oscillation of the intertropical convergence zone which determines a wet summer and a dry and windy winter the presence of lake malawi to the south and high ranges to the north result in a higher annual precipitation than that observed at a regional scale air masses overloaded with humidity bypassing lake malawi are submitted to ascending currents producing rainfall up to 2400 mm yr 1 in the lake massoko area important exchanges between the lake and groundwater reservoirs have been demonstrated from hydrological and isotopic balances of lake massoko on a regional scale these exchanges are superimposed on groundwater circulation the hydrological setting of lake massoko is also influenced by the tectonics of the region dominated by the northwest southeast tilt of tectonic blocks the sedimentary sequences of lake massoko cored in 1995 and 1996 under the eec project rukwa rukwa project final report 1997 likely register the palaeoenvironmental changes on the catchment the sediment record is nearly continuous due to the near constant high water level of the lake in contrast with many other lacustrine sequences in which reduced deposition rates discrepancies and or gaps characterise arid periods such as the last glacial maximum and the younger dryas event all the ams 14c dates on authigenic organic samples fit with the stratigraphy and the palaoenvironmental record of lake massoko exhibits a constant mean sedimentation rate of 0 628 mm yr 1 for the last ca 40 cal ka bp

a sequence stratigraphic model for an intensely bioturbated shallow marine sandstone the bridport sand formation wessex basin uk the bridport sand formation is an intensely bioturbated sandstone that represents part of a mixed siliciclastic carbonate shallow marine depositional system at outcrop and in subsurface cores conventional facies analysis was combined with ichnofabric analysis to identify facies successions bounded by a hierarchy of key stratigraphic surfaces the geometry of these surfaces and the lateral relationships between the facies successions that they bound have been constrained locally using 3d seismic data facies analysis suggests that the bridport sand formation represents progradation of a low energy siliciclastic shoreface dominated by storm event beds reworked by bioturbation the shoreface sandstones form the upper part of a thick up to 200 m steep 2 3 mud dominated slope that extends into the underlying down cliff clay clinoform surfaces representing the shoreface slope system are grouped into progradational sets each set contains clinoform surfaces arranged in a downstepping offlapping manner that indicates forced regressive progradation which was punctuated by flooding surfaces that are expressed in core and well log data in proximal locations progradational shoreface sandstones corresponding to a clinoform set are truncated by conglomerate lags containing clasts of bored reworked shoreface sandstones which are interpreted as marking sequence boundaries in medial locations progradational clinoform sets are overlain across an erosion surface by thin 10 m and overlie conglomerate lags at sequence boundaries sequence boundaries are thus interpreted as being amalgamated with overlying transgressive surfaces to form composite erosion surfaces in distal locations oolitic ironstones that formed under conditions of extended physical reworking overlie composite sequence boundaries and transgressive surfaces over most of the wessex basin clinoform sets corresponding to high frequency sequences are laterally offset thus defining a low frequency sequence architecture characterized by high net siliciclastic sediment input and low net accommodation aggradational stacking of high frequency sequences occurs in fault bounded depocentres which had higher rates of localized tectonic subsidence

detrital clay grain coats in estuarine clastic deposits origin and spatial distribution within a modern sedimentary system the gironde estuary south west france

measurement of ice sheet topography using satellite radar interferometry detailed digital elevation models dems do not exist for much of the greenland and antarctic ice sheets radar altimetry is at present the primary in many cases the only source of topographic data over the ice sheets but the horizontal resolution of such data is coarse satellite radar interferometry uses the phase difference between pairs of synthetic aperture radar sar images to measure both ice sheet topography and surface displacement we have applied this technique using ers 1 sar data to make detailed i e 80 m horizontal resolution maps of surface topography in a 100 km by 300 km strip in west greenland extending northward from just above jakobshavns isbrae comparison with a 76 km long line of airborne laser altimeter data shows that we have achieved a relative accuracy of 2 5m along the profile these observations provide a detailed view of dynamically supported topography near the margin of an ice sheet in the final section we compare our estimate of topography with phase contours due to motion and confirm our earlier analysis concerning vertical ice sheet motion and complexity in ers 1 sar interferograms

facies associations detrital clay grain coats and mineralogical characterization of the gironde estuary tidal bars a modern analogue for deeply buried estuarine sandstone reservoirs abstract estuarine tidal bar sandstones are complex reservoir geobodies commonly exploited by the oil and gas industry in order to better predict the reservoir potential of these geobodies this study provides a modern day reservoir analogue describing tidal bars in the inner and outer gironde estuary from the microscopic to the macroscopic scale the originality of this work lies in the multi scale study of modern estuarine tidal bars based on numerous piston cores extracted in a high energy environment this work demonstrates that these tidal bars are composite sedimentary bodies made up of individual reservoir sand units separated by thick muddy layers their vertical facies associations and internal architectures are controlled by local hydrodynamic variations and seasonal river influxes detrital clay grain coats are notably characterized using a portable and handheld mineral spectrometer from the base to the top of the tidal bars x ray diffraction and electron microscopes reveal that these coats are mainly composed of di octahedral smectite illite chlorite and kaolinite associated with other components such as diatoms or pyrite the best reservoir geobodies are those with the minimum clay permeability barriers at the macro and mesoscale an optimum coated grain content and clay fraction volume is also needed for generating authigenic clay coatings and inhibiting quartz overgrowth these conditions are met within the tidal sand bars of the outer estuary funnel that are expected to be the best reservoir geobodies in deeply buried sandstones

the effect of grain coating microquartz on preservation of reservoir porosity clay coatings have been widely accepted by many workers as an explanation for preserving high porosity in deeply buried sandstones but few workers have realized that similar effects can be produced by microcrystalline quartz coatings this phenomenon can be expected only under special circumstances but in such cases it can have profound consequences for exploration in the central graben area of the southern north sea unusually high porosity 20 27 and permeability 100 1000 md are found in certain zones in upper jurassic sandstones at depths of 3 4 4 4 km the porosity in these zones is 5 15 higher than expected based on average porosity depth trends from brent and haltenbanken sandstones we propose that the high porosity is due to continuous grain coatings of euhedral microcrystalline quartz crystals that are 0 1 2 m thick the distribution of microcrystalline quartz coatings is controlled by the presence of siliceous sponge spicules rhaxella which implies a sedimentological control on the reservoir quality we present a thermodynamic model showing how continuous microcrystalline quartz coatings inhibit development of no mal macrocrystalline quartz overgrowths sourced mainly from stylolites high porosities in parts of various upper jurassic oil fields ula and gyda have previously been explained by inhibition of quartz cementation by early hydrocarbon charge we suggest that the microcrystalline quartz coatings provide a more plausible explanation

effect of estuarine sediment resuspension on early diagenesis sulfide oxidation and dissolved molybdenum and uranium distribution in the gironde estuary france abstract vertical distributions of dissolved major redox parameters mo and u from the water to the sediment were sampled in the salinity gradient of the gironde estuary in the water sulfate and mo are conservatively transported throughout the salinity gradient in contrast u shows a significant removal rate of 85 mol m 2 a 1 related to reductive sequestration of u in the anoxic sediment early diagenesis is highly transient and characterized by a three zone regime related to three density layers functioning at different timescales excess of sulfate up to 1 mm is observed in the soft mud layer and is assigned to sulfide oxidation by both abiotic mn and fe oxide reduction and bacterial nitrate reduction during early diagenesis mo and u are released in the sediment porewater from mn and fe oxides in not dredged anoxic sediments authigenic precipitation of mo and u with iron sulfides is proposed to explain their removal from solution in contrast in dredged sediments authigenic precipitation of u is inhibited probably owing to the less effective reduction of soluble u vi to insoluble u iv due to oxidizing transient conditions mixing of anoxic sediment porewater with oxygenated water due to i cyclical resuspension of the soft mud layer and ii dredging operations in the navigation channel induces addition of dissolved sulfate produced from diagenetic sulfide oxidation mo and u to the water this addition is equivalent to 2 sulfate 5 5 mo and 0 5 u of the dissolved fluvial inputs into the estuary for the natural resuspension and 0 4 mo less than 0 2 sulfate u for dredge induced resuspension addition of dissolved mo and u produced by oxidation of sulfide phases in the dredged sediments redeposited in dynamic environments are equivalent to 12 mo and 0 8 u of their respective fluvial inputs additional mo and u inputs into the water originate from diffusive outflow from muddy sediments and are equivalent to 2 mo and 0 3 u of their respective fluvial inputs however we show that all these dissolved inputs are scavenged by solid phases

anomalously high porosity and permeability in deeply buried sandstone reservoirs origin and predictability porosity and permeability generally decrease with increasing depth thermal exposure and effective pressure however a significant number of deep 4 km approximately 13 000 ft sandstone reservoirs worldwide are characterized by anomalously high porosity and permeability anomalous porosity and permeability can be defined as being statistically higher than the porosity and permeability values occurring in typical sandstone reservoirs of a given lithology composition and texture age and burial temperature history in sandstones containing anomalously high porosities such porosities exceed the maximum porosity of the typical sandstone subpopulation major causes of anomalous porosity and permeability were identified decades ago however quantification of the effect of processes responsible for anomalous porosity and permeability and the assessment of the predictability of anomalous porosity and permeability occurrence in subsurface sandstones have rarely been addressed in published literature the focus of this article is on quantification and predictability of three major causes of anomalously high porosity 1 grain coats and grain rims 2 early emplacement of hydrocarbons and 3 shallow development of fluid overpressure grain coats and grain rims retard quartz cementation and concomitant porosity and permeability reduction by inhibiting precipitation of quartz overgrowths on detrital quartz grains currently prediction of anomalous porosity associated with grain coats and grain rims is dependent on the availability of empirical data sets in the absence of adequate empirical data sedimentologic and diagenetic models can be helpful in assessing risk due to reservoir quality such models provide a means to evaluate the effect of geologic constraints on coating occurrence and coating completeness required to preserve economically viable porosity and permeability begin page 302 in a given play or prospect these constraints include thermal history and sandstone grain size and composition the overall effect of hydrocarbon emplacement on reservoir quality is controversial it appears that at least some cements quartz and illite may continue to precipitate following emplacement of hydrocarbons into the reservoir our work indicates that integration of basin modeling with reservoir quality modeling can be used to quantify prior to drilling the potential impact of hydrocarbon emplacement on porosity and permeability the best case scenario for significant reservoir quality preservation due to fluid overpressure development is in rapidly deposited tertiary or quaternary sandstones our models suggest that significant porosity can be preserved in sandstones that have experienced continuous high fluid overpressures from shallow burial depths the models also indicate that the potential for porosity preservation is greatest in ductile grain rich sandstones because compaction tends to be the dominant control on reservoir quality in such rocks the case for significant porosity preservation associated with fluid overpressures in pre tertiary basins however is more problematic because of the complexities in the history of fluid overpressure and the greater significance of quartz cementation as a potential mechanism of porosity loss

evolution of glacier dammed lakes through space and time brady glacier alaska usa abstract glacier dammed lakes and their associated jokulhlaups cause severe flooding in downstream areas and substantially influence glacier dynamics brady glacier in southeast alaska is well suited for a study of these phenomena because it presently dams 10 large 1 km 2 lakes our objectives are to demonstrate how brady glacier and its lakes have co evolved in the past and to apply this knowledge to predict how the glacier and its lakes will likely evolve in the future to accomplish these objectives we georeferenced a variety of maps airphotos and optical satellite imagery to characterize the evolution of the glacier and lakes we also collected bathymetry data and created bathymetric maps of select lakes despite small advances and retreats the main terminus of brady glacier has changed little since 1880 however it downwasted at rates of 2 3 m y between 1948 and 2000 more than the regional average the most dramatic retreat 2 km and downwasting 120 m have occurred adjacent to glacier dammed lakes and are primarily the result of calving brady glacier is a former tidewater glacier with continued downwasting brady glacier may return to a tidewater regime and enter into a phase of catastrophic retreat the situation at brady glacier is not unique and the lessons learned here can be applied elsewhere to identify future glacier dammed lakes jokulhlaups and glacier instability

does englacial water storage drive temperate glacier surges hydrological studies of surge type and steady flow glaciers combined with recent space borne synthetic aperture radar interferometry measurements of the motion of bagley ice valley alaska u s a during its 1993 95 surge suggest a temperate glacier surge hypothesis that is consistent with observational evidence and appears capable of shedding light on several aspects of surge behavior we propose that the fundamental driver of temperate glacier surges is englacial storage of water combined with gravity driven movement of stored water to the bed during winter whether a given glacier is surge type is a matter not of whether but of the degree to which these processes occur a surge type glacier must have sufficient storage capacity for continued downward movement of englacially stored water during winter to finally overwhelm the constricted basal drainage system thereby forcing pervasive failure of the subglacial till or alternatively widespread and rapid basal sliding thus initiating a surge we further propose that the sufficient storage capacity requirement is most easily met by glaciers with large thickness which are therefore likely to be long and to have on average low surface slopes the average length of the surge cycle in a given region appears to be a function of the mass balances which after each surge determine the time required to restore glaciers to their pre surge geometries we suggest that the stochastic timing of surge onset for a particular glacier however is a result of the uncertainty of the meteorological conditions required to cause englacial storage of a sufficiently large volume of water

icelandic jokulhlaup impacts implications for ice sheet hydrology sediment transfer and geomorphology abstract glaciers and ice sheets erode entrain and deposit massive quantities of debris fluxes of subglacial meltwater exert a fundamental control on ice dynamics and sediment transport budgets within many glacial systems outburst floods jokulhlaups constitute high magnitude high frequency fluxes of meltwater relative to normal ablation controlled discharge this paper presents a synthesis of research on recent icelandic jokulhlaups and their geomorphological and sedimentary impact we identify jokulhlaup impacts within subglacial englacial and proglacial settings and discuss their wider significance for ice sheet hydrology sediment transfer and geomorphology because jokulhlaups erode deposit and re work sediment simultaneously they usually cause significant glaciological and sedimentological impacts jokulhlaups that propagate as subglacial flood waves often produce widespread hydromechanical disruption at the glacier base recent icelandic jokulhlaups have been recognised as highly efficient agents of reworking subglacial sediment and glacial sediment entrainment models of jokulhlaup impact therefore need to encompass the sub and englacial environment in addition to the proglacial zone where research has traditionally been focussed most jokulhlaups transport sediment to proglacial sandar and often directly to oceans where preservation potential of the impact is greater proglacial jokulhlaup deposits form distinctive sedimentary assemblages coupled with suites of high energy erosional landforms this study of modern jokulhlaup processes and sedimentary products may be useful for the interpretation of meltwater processes associated with quaternary ice sheets

pre mesozoic iawsonite in anglesey northern wales preservation of ancient blueschists the existence of lawsonite blueschists in the mona complex of anglesey northern wales has been confirmed using optical and microprobe data the blueschists occur locally along the eastern margin of one of three belts of fine grained mylonitic schists known as the penmynydd zone of metamorphism the penmynydd zone is interpreted as representing remnants of deep seated ductile shear zones produced by the movement of sialic basement up into low grade melange the lawsonite schists display complex textures resulting from the overprint of blueschist conditions on greenschist facies rocks followed by a subgreenschist dynamic metamorphism the blueschist minerals are interpreted as a late product of shear zone metamorphism under conditions of decreasing temperature and high fluid pressure the schists are no younger than cambrian probably middle cambrian lawsonite of this age is exceptionally rare older examples are reported only from russia rather than invoking a change in crustal isotherms through time the paucity of pre mesozoic blueschists may simply be a result of the rapid exhumation needed to expose such rocks the appearance of a blueschist at earth s surface will be transient unless the uplift responsible for its exposure either slows or ceases

coagulation rates of clay minerals and natural sediments abstract coagulation experiments conducted using illite kaolinite montmorillonite and four natural sediment samples in blade and couette reactors with solutions of various salinities yielded collision efficiency factors significantly higher than those in the previous literature indicating that coagulation begins at lower salinities 1 2 1 the natural sediments do not generally behave as might be predicted for a mixture of the standards of the minerals of which the natural sediment samples were comprised the half times for coagulation of the particles indicate that in nature the coagulation process would probably be completed in the first few parts per thousand salinity

tidal bore dynamics in funnel shaped estuaries the formation and dynamics of tidal bores in funnel shaped estuaries is investigated from both a global tidal wave scaling analysis and new quantitative field observations we show that tidal bore occurrence in convergent estuaries can be estimated from a dimensionless scaling parameter characterizing the relative intensity of nonlinear friction versus local inertia in the momentum equation a detailed analysis of tidal bore formation and secondary wave structure is presented from a unique long term database observations of more than 200 tides acquired during four campaigns in the two main french tidal bore estuaries the seine and gironde garonne estuaries we describe the effect of freshwater discharge on the global tidal wave transformation at the estuarine scale and on local tidal bore occurrence in the upper estuary our field data suggest that the tidal bore intensity is mainly governed by the dimensionless tidal range which characterizes the local tidal wave nonlinearity we also show that the secondary wavefield associated with tidal bore propagating in natural estuaries differs significantly from those associated to undular bores in rectangular channels in particular we observe an abrupt decrease of the whelp steepness when the froude number goes below 1 1 this secondary field transition can explain why tidal bore occurrence in worldwide estuaries is certainly underestimated

a portable coring device for use in the intertidal environment abstract the construction of a portable coring unit with a telescopic tube and stationary piston mounted on a tripod frame is described using the procedure outlined it is possible to core unconsolidated sediments clay and sands in intertidal environments with the minimum of disturbance down to depths of 6 m

rising ela and expanding proglacial lakes indicate impending rapid retreat of brady glacier alaska brady glacier is a large alaskan tidewater glacier that is beginning a period of substantial retreat examination of 27 landsat and modis images from the period 2003 to 2011 indicates that brady glacier has a mean equilibrium line altitude ela of 745 m and accumulation area ratio aar of 0 40 the zero balance ela is 600 m and equilibrium aar 0 65 the negative mass balance associated with the increased ela has triggered thinning of 20 100 m over most of the glacier below the ela from 1948 to 2010 the thinning has caused substantial retreat of seven calving distributary termini of the glacier thinning and retreat have led to an increase in the width of and water depth at the calving fronts in contrast the main terminus has undergone only minor retreat since 1948 in 2010 several small proglacial lakes were evident at the terminus by 2000 a permanent outlet river issuing from trick lake had developed along the western glacier margin initial lake development at the terminus combined with continued mass losses will lead to expansion of the lakes at the main terminus and retreat by calving the glacier bed is likely below sea level along the main axis of brady glacier to the glacier divide retreat of the main terminus in the lake will likely lead to a rapid calving retreat similar to bear excelsior norris portage and yakutat glaciers copyright 2013 john wiley sons ltd

residual transport model in correlation with sedimentary dynamics over an elongate tidal sandbar in the gironde estuary southwestern france abstract this study qualitatively describes by a multidisciplinary approach hydrodynamic and residual sediment transport over a sandbar in a macrotidal environment the sandbar is elongated and attached to a headland that is located in the gironde estuary southwestern france bathymetric surveys side scan sonar sonograms grain size trends of surficial sediment and 2dh numerical simulations of current patterns were used to define a conceptual model for the dynamics of this sandbar the sandbar is oriented anticlockwise with respect to the main flow the channel oriented bar face is ebb dominated whereas the opposite face is flood dominated resulting in a clockwise residual transport the line of sediment flux convergence corresponds to the crest of the bar the residual flow over the bar appears to originate from the regional estuarine flow the dominant process responsible for the maintenance of the sandbar seems to be the depth averaged main tidal flow rather than secondary circulation due to headlands or the interaction between the tidal flow and the seabed to generalize this regional conceptual model a qualitative comparison is made with the theories of sandbar dynamics due to tidal currents pingree and maddock 1979 zimmerman 1981 huthnance 1982 and others the seabed stability approach developed by huthnance 1982 seems to be the most reliable theory to explain the saint georges bank dynamics even though many natural factors are not taken into account

relationship of diagenetic chlorite rims to depositional facies in lower cretaceous reservoir sandstones of the scotian basin the relationship between diagenetic chlorite rims and depositional facies in deltaic strata of the lower cretaceous missisauga formation was investigated using a combination of electron microprobe bulk geochemistry and x ray diffraction data the succession studied comprises several stacked parasequences the delta progradational facies association includes i fluvial or distributary channel sandstones some with tidal influence ii thick bedded delta front graded beds of sandstone interpreted as resulting from fluvial hyperpycnal flow during floods and storms and iii more distal muddier delta front and prodeltaic facies the transgressive facies association includes lag conglomerate siderite cemented muddy sandstone and mudstone and bioclastic sandy limestone chlorite rims are absent in the fluvial facies and best developed in thick sandstones lacking mudstone baffles good quality chlorite rims are well correlated with ti in bulk geochemistry ti is a proxy for fe availability principally from the breakdown of abundant detrital ilmenite fetio3 under conditions of sea floor diagenesis the abrupt decrease in sedimentation rate at transgressive surfaces caused progressive shallowing of the sulphate depletion level and of the overlying eh controlled diagenetic zones resulting in conditions suitable for diagenetic formation of berthierine to migrate upwards through the packet of reservoir sandstones this early diagenetic berthierine suppressed silica cementation and later recrystallized to chlorite thick euhedral outer chlorite rims were precipitated from formation water in sandstone lacking muddy baffles on this chlorite substrate and inhibited late carbonate cementation this study thus shows that the preservation of porosity by chlorite rims is a two stage process rapidly deposited delta front turbidite facies create early diagenetic conditions that eventually lead to the formation of chlorite rims but the best quality chlorite rims are restricted to sandstones with high permeability during burial diagenesis

the impact of diagenesis on the heterogeneity of sandstone reservoirs a review of the role of depositional facies and sequence stratigraphy diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability linking the types and distribution of diagenetic processes to the depositional facies and sequence stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diagenetic alterations controlling quality and heterogeneity the heterogeneity patterns of sandstone reservoirs which determine the volumes flow rates and recovery of hydrocarbons are controlled by geometry and internal structures of sand bodies grain size sorting degree of bioturbation provenance and by the types volumes and distribution of diagenetic alterations variations in the pathways of diagenetic evolution are linked to 1 depositional facies hence pore water chemistry depositional porosity and permeability types and amounts of intrabasinal grains and extent of bioturbation 2 detrital sand composition 3 rate of deposition controlling residence time of sediments at specific near surface geochemical conditions and 4 burial thermal history of the basin the amounts and types of intrabasinal grains are also controlled by changes in the relative sea level and therefore can be predicted in the context of sequence stratigraphy particularly in paralic and shallow marine environments changes in the relative sea level exert significant control on the types and extent of near surface shallow burial diagenetic alterations which in turn influence the pathways of burial diagenetic and reservoir quality evolution of clastic reservoirs carbonate cementation is more extensive in transgressive systems tract tst sandstones particularly below parasequence boundaries transgressive surface and maximum flooding surface because of the abundance of carbonate bioclasts and organic matter bioturbation and prolonged residence time of the sediments at and immediately below the sea floor caused by low sedimentation rates which also enhance the formation of glaucony eogenetic grain coating berthierine odinite and smectite formed mostly in tst and early highstand systems tract deltaic and estuarine sandstones are transformed into ferrous chlorite during mesodiagenesis helping preserve reservoir quality through the inhibition of quartz cementation the infiltration of grain coating smectitic clays is more extensive in braided than in meandering fluvial sandstones forming flow barriers in braided amalgamated reservoirs and may either help preserve porosity during burial because of quartz overgrowth inhibition or reduce it by enhancing intergranular pressure dissolution diagenetic modifications along sequence boundaries are characterized by considerable dissolution and kaolinization of feldspars micas and mud intraclasts under wet and warm climates whereas a semiarid climate may lead to the formation of calcrete dolocrete cemented layers turbidite sandstones are typically cemented by carbonate along the contacts with interbedded mudrocks or carbonate mudstones and marls as well as along layers of concentration of carbonate bioclasts and intraclasts commonly hybrid carbonate turbidite arenites are pervasively cemented proximal massive turbidites normally show only scattered spherical or ovoid carbonate concretions improved geologic models based on the connections among diagenesis depositional facies and sequence stratigraphic surfaces and intervals may not only contribute to optimized production through the design of appropriate simulation models for improved or enhanced oil recovery strategies as well as for co2 geologic sequestration but also support more effective hydrocarbon exploration through reservoir quality prediction

sedimentary controls on modern sand grain coat formation abstract coated sand grains can influence reservoir quality evolution during sandstone diagenesis porosity can be reduced and fluid flow restricted where grain coats encroach into pore space conversely pore lining grain coats can restrict the growth of pore filling quartz cement in deeply buried sandstones and thus can result in unusually high porosity in deeply buried sandstones being able to predict the distribution of coated sand grains within petroleum reservoirs is thus important to help find good reservoir quality here we report a modern analogue study of 12 sediment cores from the anllons estuary galicia nw spain collected from a range of sub environments to help develop an understanding of the occurrence and distribution of coated grains the cores were described for grain size bioturbation and sedimentary structures and then sub sampled for electron and light microscopy laser granulometry and x ray diffraction analysis the anllons estuary is sand dominated with intertidal sand flats and saltmarsh environments at the margins there is a shallowing fining upwards trend in the estuary fill succession grain coats are present in nearly every sample analysed they are between 1 m and 100 m thick and typically lack internal organisation the extent of grain coat coverage can exceed 25 in some samples with coverage highest in the top 20 cm of cores samples from muddy intertidal flat and the muddy saltmarsh environments close to the margins of the estuary have the highest coat coverage mean coat coverage of 20 2 and 21 3 respectively the lowest mean coat coverage occurs in the sandy saltmarsh 10 4 beyond the upper tidal limit and sandy intertidal flat environments 8 4 close to the main estuary channel mean coat coverage correlates with the concentration of clay fraction the primary controls on the distribution of fine grained sediment and therefore grain coat distribution are primary sediment transport and deposition processes that concentrate the clay fraction in the sediment towards the margins of the estuary bioturbation and clay illuviation mechanical infiltration are secondary processes that may redistribute fine grained sediment and produce grain coats here we have shown that detrital grain coats are more likely in marginal environments of ancient estuary fills which are typically found in the fining upward part of progradational successions

integrated hydrologic and hydrochemical observations of hidden creek lake jokulhlaups kennicott glacier alaska 1 hidden creek lake hcl an ice marginal lake impounded by kennicott glacier wrangell mountains alaska fills annually to 20 to 30 106 m3 and then drains subglacially within 2 to 3 days during the 1999 and 2000 jokulhlaups we carried out a series of planned observations around the lake and in the kennicott river which drains the glacier approximately 20 of the lake volume was contained within a subglacial water wedge beneath the ice dam the entire volume of the lake drains through the wedge hydraulic head loss through this constriction may be responsible for the fairly symmetrical shape of the hcl outflow hydrographs deduced from lake level records basin hypsometry and collapse of the ice dam the flood hydrographs in the kennicott river are similar in shape to the outflow hydrographs and within error lake volume matched the river flood volume in both years up to 12 106 m3 of water was temporarily stored within the glacier during the 2000 jokulhlaup during the 2000 jokulhlaup the background flow in the kennicott river shifted to a dilute chemical composition as the hcl jokulhlaup progressed donoho falls lake filled with water whose chemistry was closer to that of the background flow in kennicott river than to hcl water comparison of these chemical signals with typical summer variations in kennicott river chemistry suggests that the jokulhlaup created high subglacial water pressure that impeded normal drainage of solute rich water from a distributed drainage system into a conduit system at the glacier bed and even caused flow direction locally to reverse

biofilm origin of clay coated sand grains the presence of clay sized particles and clay minerals in modern sands and ancient sandstones has long presented an interesting problem because primary depositional processes tend to lead to physical separation of fine and coarse grained materials numerous processes have been invoked to explain the common presence of clay minerals in sandstones including infiltration the codeposition of flocculated muds and bioturbation induced sediment mixing how and why clay minerals form as grain coats at the site of deposition remains uncertain despite clay coated sand grains being of paramount importance for subsequent diagenetic sandstone properties we have identified a new biofilm mechanism that explains clay material attachment to sand grain surfaces that leads to the production of detrital clay coats this study focuses on a modern estuary using a combination of field work scanning electron microscopy petrography biomarker analysis and raman spectroscopy to provide evidence of the pivotal role that biofilms play in the formation of clay coated sand grains this study shows that within modern marginal marine systems clay coats primarily result from adhesive biofilms this bio mineral interaction potentially revolutionizes the understanding of clay coated sand grains and offers a first step to enhanced reservoir quality prediction in ancient and deeply buried sandstones

sedimentological control on the diagenesis and reservoir quality of tidal sandstones of the upper cape hay formation permian bonaparte basin australia abstract the deep siliciclastic reservoir 3500 m of the upper cape hay formation of the bonaparte basin petrel gas field petrel sub basin permian exhibits wide heterogeneity in porosity 2 26 and permeability 0 001 2500 md to investigate this variability 42 samples were taken from five wells drilled through this formation six facies were identified from core descriptions and microscopic study of the sandstones these facies are typical of a tide dominated estuary and include 1 mud flat 2 sand flat 3 top of tidal sand bar 4 middle of tidal sand bar 5 bottom of tidal sand bar and 6 outer estuary facies the paragenetic sequence comprises the emplacement of early aggregates of ferrous clay mineral precursors mechanical compaction recrystallization of those ferrous clay mineral precursors to fe rich chlorite and crystallization of fe rich chlorite forming coatings around detrital grains chemical compaction development of quartz overgrowth feldspar alteration crystallization of dickite and illite rich illite smectite i s mixed layers and ferrous calcite cementation the middle and top of the tidal bars generally exhibit the highest porosity 10 and permeability values k 1 md feldspar alteration released silica and aluminium into the reservoir promoting the development of dickite and illite rich i s mixed layers which tended to destroy porosity and permeability as calcite cements and quartz overgrowths diagenetic chlorite coatings around detrital grains are restricted to the sand bar facies deposited at the end of the last third order transgressive systems tract of the cape hay formation the formation and conservation of ferrous clay precursors seems to be possible in an estuarine environment where seawater and fresh water are mixed and tidal sand bars are formed these ferrous clay precursors recrystallized to fe rich chlorite coating after mechanical compaction these coatings inhibited quartz cementation and prove to be the key parameter behind good reservoir qualities