run up of internal waves on a gentle slope in a two layered system this paper describes the dissipative phase of internal wave run up on uniform slopes of 0 030 and 0 054 rad as observed in a series of laboratory experiments the waves were generated continuously at the interface of two miscible layers of differing density as each wave in the perodic train propagated onto the slope it steepened and developed into a solitary like wave before finally overturning surrounding fluid was engulfed into the wave as it overturned and the resulting gravitational instability produced considerable turbulence and mixing the broken wave took the form of a discrete bolus of dense fluid which propagated for some distance up the slope bulk parameters which characterize the nature of the bolus were defined and the dependence of these on the incident wave parameters and their behaviour during the run up phase were examined

ankerite cementation in deeply buried jurassic sandstone reservoirs of the central north sea abstract upper jurassic fulmar formation shelf sandstones of the high temperature high pressure franklin and elgin fields south central graben north sea contain abundant disseminated and concretionary ankerite in contrast most jurassic north sea reservoirs contain only minor amounts of dispersed ankerite disseminated ankerite cement in the franklin and elgin fields has a fairly uniform isotopic composition 18o 10 to 12 5o oo pdb 13c 3 to 5o oo pdb ankerite concretions have 18o values similar to disseminated cements but a wider range of 13c values 1 to 5 5o oo pdb they also have highly variable intergranular volumes which together with the 13c data are interpreted as a combination of pore filling cementation and in situ replacement of comminuted bioclastic debris by ankerite fluid inclusion 18o and paragenetic evidence suggests that ankerite formed during deep burial c 3 5 to 4 5 km 140 170 c after the onset of overpressuring but before hydrocarbon emplacement in the reservoirs the regionally consistent 18o data suggest that ankerite formed via a temperature influenced mechanism and the relatively uniform 13c cement value indicates that organic matter and marine bioclastic carbonate contributed to the dissolved carbon reservoir in constant proportion this can be explained by calcite dissolution in response to ph decrease during thermal breakdown of organic acids such acids were derived from adjacent mudrocks undergoing hydrocarbon maturation and clay mineral transformations and are likely to have been transported in pore fluids with mg2 and fe2 the presence of these cations in solution upon thermal decarboxylation is inferred to have stabilized ankerite at the expense of calcite a relative paucity of ankerite in other fulmar formation reservoirs may reflect different sedimentological compositions less bioclastic debris and or lower burial temperatures less advanced decarboxylation

diagenesis in north sea hpht clastic reservoirs consequences for porosity and overpressure prediction upper jurassic clastic reservoirs of the fulmar formation in the central north sea possess anomalously high porosities for their present day depth of burial reservoirs with the highest overpressures have the highest porosities possess less macroquartz cement and have significant secondary porosity quartz cementation in hpht high pressure high temperature reservoirs has been inhibited by a combination of factors high overpressure limited fluid movement presence of early grain coating cements high pore fluid salinity and possibly petroleum migration secondary porosity has contributed to reservoir quality with an average of 4vol extra porosity created quantitative prediction of porosity would require an improved depositional model for the fulmar accurate thermal and pressure modelling and detailed knowledge of field filling and leakage histories theoretical calculations indicate that diagenetic reactions occurring in the fulmar formation smectite illitisation and quartz cementation did not generate significant overpressure because seal permeabilities were too high and the rate of volume increase associated with the reactions too small therefore diagenetic reactions can effectively be ignored when modelling overpressure generation in the central north sea although cementation will affect rock permeability and rates of fluid dissipation

hydrocarbon composition of authigenic inclusions application to elucidation of petroleum reservoir filling history abstract geochemical analysis of petroleum inclusions trapped in authigenic feldspar and quartz in the ula formation in the north sea ula oil field revealed a petroleum of markedly different composition than the oil presently in the reservoir using microthermometry and the burial history as a dating tools it is concluded that the petroleum in the k feldspar inclusions was present in the more porous and permeable parts of the ula formation as early as 45 75 my bp when the field was at a depth of about 1 0 1 5 km as compared with the current depth of 3 4 km this early petroleum which was trapped as inclusions in authigenic k feldspar shows a distinctly different distribution of tricyclic terpanes and pentacyclic triterpanes from that of the current petroleum charge in the ula formation which was derived from the mandal formation source rock in late neogene time molecular parameters show that the oil in the k feldspar inclusions is significantly less mature than the crude oil in the present reservoir the approximate 90 c temperature increase occurring after entrapment of the early petroleum in kfeldspar the field is currently at 143 c appears not to have reset the low maturity signature of the oil in the k feldspar inclusions this could suggest that the temperature in the inclusions is too low for isomerization selective thermal degradation to occur lack of catalysts or that there are other controls on the ratio of some of these parameters still parameters like the ratio of c21 to c28 triaromatic steroids and those based on dimethyl and trimethyl naphthalenes are comparatively similar in both the inclusions and in the reservoir crude the oil inclusions in authigenic quartz and albite formed from about 10 my bp burial depth 2 5 km until the present burial depth 3 4 km are interpreted as representing a palaeo petroleum charge having a composition intermediate between the oil found in k feldspar inclusions and the oil charge in the present reservoir our data suggest strongly that the petroleum trapped in the inclusions in quartz and plagioclase represents progressive dilution of the early reservoired oil now found only in k feldspar inclusions with oil from the mandal formation analysis of solution gas in these inclusions suggests that the first petroleum associated gas to arrive at the trap from the progressively maturing mandal formation had a much wetter composition than the current reservoir solution gas to explain the present comparatively drier and isotopically lighter reservoir solution gas composition the inference is made that there has been a recent during formation of k feldspar inclusions 45 75 my bp maturation of the upper jurassic mandal formation the main source rock in this region was limited possibly to the graben axis this suggests that the petroleum found in the k feldspar inclusions was most likely derived from a deeper post caledonian palaeozoic formation

evidence for reduced quartz cementation rates in oil filled sandstones when quartz rich sands are buried and heated pore space is gradually filled by precipitation of quartz cement from aqueous formation fluids here we examine whether the presence of oil in the pore space can retard or halt this loss of porosity by slowing or stopping quartz cementation the effect of oil fill on quartz cementation is examined by using the distribution of quartz cement in the brae formation deep water sandstone reservoir of the miller oil field north sea petrographic data demonstrate that sandstones from the oil zone have much less quartz cement and more porosity than sandstones from the water zone sandstones in both oil and water zones are compositionally and texturally identical and have been affected by a similar burial history kinetic modeling of the cementation process suggests that progressive oil charging has slowed quartz cement growth rates by at least two orders of magnitude halting it completely in the most extreme cases our data demonstrate that early oil charging in the crestal part of an anticline can preserve porosity in deeply buried sandstones this knowledge is especially relevant to porosity prediction for petroleum exploration in deeply buried sandstones

sandstone vs carbonate petroleum reservoirs a global perspective on porosity depth and porosity permeability relationships plots are presented comparing average porosity vs depth for 30 122 siliciclastic petroleum reservoirs and 10 481 carbonate petroleum reservoirs covering all petroleum producing countries except canada however separate plots cover 5534 siliciclastic and 2830 carbonate reservoirs of the alberta basin in canada average permeability vs average porosity is shown for the non canadian reservoirs key similarities and differences between sandstones and carbonates are noted and implications are discussed regarding the dominant factors controlling reservoir quality in each lithology trends of steadily decreasing median and maximum porosity with increasing depth reflect burial diagenetic porosity loss in response to increasing thermal exposure with depth these trends seem inconsistent with the suggestions that both sandstones and carbonates commonly increase in porosity by dissolution during deeper burial carbonate reservoirs have lower values of median and maximum porosity for a given burial depth probably because of greater chemical reactivity of carbonate minerals relative to quartz and the resulting lower resistance to chemical compaction and associated cementation relative paucity of low porosity 0 8 siliciclastic reservoirs at all depths compared with carbonates may reflect the more common occurrence of fractures in carbonates and the effectiveness of these fractures for facilitating economic flow rates in low porosity rock overall carbonate reservoirs do not have lower permeability for a given porosity compared with sandstones but do have strikingly lower proportions of both high porosity and high permeability values the data presented can serve as a general guide for the distribution of reservoir quality that can reasonably be expected in exploration wells drilled to any given depth in the absence of detailed geologic information such as burial and thermal history

on the breaking of internal solitary waves at a ridge an experimental laboratory study has been carried out to investigate the propagation of an internal solitary wave of depression and its distortion by a bottom ridge in a two layer stratified fluid system wave profiles density fields and velocity fields have been measured at three reference locations namely upstream downstream and over the ridge experiments have been performed with wave amplitudes in the range 0 2 1 9 times the depth of the upper layer and a ratio between the lower and the upper layer in the range 3 0 8 5 the ridge slope was varied from 0 1 to 0 33 and the maximum ridge height was two thirds of the thicker fluid layer over the ridge the flow has been classified into i cases when the bottom ridge has little influence on the propagation and spatial structure of the internal solitary wave ii cases where the internal solitary wave is significantly distorted by the blocking effect of the ridge though no wave breaking occurs and iii cases for which the internal solitary wave is broken as it encounters and passes over the bottom ridge a detailed description of the processes leading to wave breaking is given breaking has been found to take place when the fluid velocity in the lower layer exceeds 0 7 of a local nonlinear wave speed defined at the top of the ridge the breaking condition is also expressed in terms of the amplitude of the incident wave the layer thickness ratio and the relative height of the ridge the wave breaking can be determined from the input parameters of the experiment the transmitted waves have been found to always consist of a leading pulse solitary wave followed by a dispersive wavetrain the solitary wave amplitude is significantly reduced only when breaking takes place at the ridge internal waves of mode two are generated in cases with strong breaking

secondary porosity generation during deep burial associated with overpressure leak off fulmar formation united kingdom central graben the overpressure history of a sandstone can be estimated using a numerical model if the burial curve and geological setting are known from the resulting effective stress the maximum potential porosity mpp can be calculated the mpp is the maximum porosity the rock could theoretically hold open at the modeled burial depth and pore pressure measured rock porosities should be at or below the mpp we have determined the mpp history for the fulmar formation sandstones upper jurassic of the central graben north sea and have compared the predictions to measured core data we conclude that for the majority of the fulmar formation sandstones the porosity evolution is a simple pattern of reduction during burial caused by compaction and cementation however in wells sited close to regional overpressure leak off points the porosity has been significantly increased from an end of oligocene low mean 21 to the present day values mean 31 this porosity increase occurred by feldspar dissolution with the reaction products being removed from the sandstones secondary porosity generation and the export of solute occurred while the sandstone was highly overpressured although still part of an open hydrogeological system the generation of porosity within deeply buried sandstones is of commercial importance and potentially can be predicted

hydrocarbon filling and leakage history of a deep geopressured sandstone fulmar formation united kingdom north sea diagenetic minerals in a water filled borehole from the jurassic fulmar formation united kingdom central north sea record two phases of hydrocarbon filling and emptying initial oil charge was during the late cretaceous at shallow burial depths of about 1 5 km 0 9 mi as we consider that hydrocarbon has preserved porosity during burial this has implications for the understanding of the porosity evolution of the fulmar formation which in other locations is an important hydrocarbon reservoir the early oil charge as recorded by illite k ar ages progressively filled the structure from 84 to 59 ma and possibly precipitated bitumen because of biodegradation the first oil predated many of the burial diagenetic reactions within the sandstone after leak off at ca 60 ma diagenetic reactions continued in an open geochemical system with possible import of co2 products of these reactions include ankerite and quartz overgrowth cements hydrocarbon staining postdates these phases and provides evidence of a hydrocarbon charge probably gas condensate the second hydrocarbon charge also leaked off and the sandstone is now water bearing previous work on the fulmar formation has incorrectly placed all the diagenetic reactions as predating the first arrival of hydrocarbon the present day pore fluids are high salinity high 18o fluids derived from the underlying permian zechstein evaporates these fluids entered the reservoir during a phase of overpressure release that caused fracturing of the framework quartz grains possibly coincident with the second phase of hydrocarbon leak off at 2 5 ma mark wilkinson studied sandstone hosted calcite concretions for his ph d and has subsequently tried to take a more holistic view of the diagenesis of sedimentary basins his career has been funded by a variety of oil companies although he is currently involved in geological disposal of some of the co2 that he was partly responsible for producing stuart haszeldine is a professor at edinburgh university he received his ph d on clastic and coal sedimentology in 1981 from the university of strathclyde after an attempted reeducation in the oil industry he has collaborated with many companies to research on hydrocarbon reservoir quality radioactive waste disposal and coal resources he currently heads the united kingdom s largest group investigating carbon dioxide sequestration tony fallick was educated at the university of glasgow scotland in physics b sc and chemistry ph d he did research at mcmaster university ontario canada and cambridge england before moving in 1980 to the scottish universities research and reactor center where he is currently the director

oil charge preserves exceptional porosity in deeply buried overpressured sandstones central north sea uk abstract exceptionally high porosity sandstones are reported from oil provinces worldwide yet the mechanism of porosity preservation remains controversial we present strong evidence that the exceptional porosities in the jurassic fulmar formation within the north sea are the result of early oil charging preventing cementation and chemical compaction in sands with relatively low detrital clay contents although overpressure and grain coatings do have some effect the most dramatic evidence that reservoir quality is related to early oil charge is the spatial distribution of the porosity the maximum porosity of the fulmar formation is systematically highest immediately below the top seal and decreases below this despite uniform sedimentology hence porosity is preferentially preserved where oil first accumulated in these unusually homogeneous overpressured reservoir sandstones at the top oil charge prevents or significantly retards chemical compaction and cementation of a reservoir preserving porosity there is independent evidence for early oil charging of reservoirs within the central graben from fluid inclusion studies and k ar age dates of authigenic illite micro quartz grain coatings which have been proposed to preserve porosity during burial are associated with only 5 extra porosity in sands that exceed the worldwide average sandstone porosity by almost 20

spatial evolution of young wind waves numerical modelling verified by experiments a numerical model that allows one to study numerically the evolution of waves along the test section of a wind wave tank is offered the simulations are directly related to wind wave tank experiments carried out for a range of steady wind velocities at each wind forcing condition the evolving wind wave field is strongly non homogeneous with wave energy growth along the test section accompanied by frequency downshifting the wave parameters measured at a short fetch serve as a basis for generating numerous realizations of the initial conditions in the monte carlo numerical simulations the computations are based on a modified unidirectional spatial version of the zakharov equation that accounts for wind input and dissipation and is applicable for the whole range of wind velocities employed the model contains two empirical parameters that are selected by comparison of the experimental and numerical results the same values of those parameters are applied for all wind forcing conditions the availability of an experimentally verified numerical model allows one to study the contributions of nonlinear wave wave interactions dissipation and wind input separately special attention is given to accounting for the three dimensional and random nature of wind waves as observed in experiments the suggested model combines approaches adopted in the wind wave growth theories by miles and phillips

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

statistical analysis of the spatial evolution of the stationary wind wave field detailed investigation of wind generated water waves in a 5 m long wind wave flume facility is reported careful measurements were carried out at a large number of locations along the test section and at numerous airflow rates the evolution of the wind wave field was investigated using appropriate dimensionless parameters when possible quantitative comparison with the results accumulated in field measurements and in larger laboratory facilities was performed particular attention was given to the evolution of wave frequency spectra along the tank distinguishing between the frequency domain around the spectral peak and the highfrequency tail of the spectrum notable similarity between the parameters of the evolving wind wave field in the present facility and in field measurements was observed

aluminium loss during sandstone diagenesis petrographical observations from a sandstone buried to between 3 2 and 5 7 km in the north sea central graben suggest a significant change in bulk chemistry during burial the sand contained 35 or more feldspar upon deposition but due to extensive dissolution only 5 remains in the deep sandstones this has released much more al than is currently contained as authigenic minerals up to 1460 mol cm 3 of al have been exported this deep secondary porosity is economically important but its recognition by petrographic textures is unreliable as are bulk chemical provenance studies of deeply buried sedimentary and metamorphic strata

x band sea surface coherence time inferred from bistatic sar interferometry coherence time plays an important role in ocean surface observation by means of synthetic aperture radar sar for x band the autocorrelation function acf of the backscatter from the sea surface has been predicted using models up to now there exist only few experimental data and an empirical analysis of the sea surface coherence time based on a larger amount of data is missing in this paper we show how space based bistatic sar along track interferometry is used for the first time to measure the acf we achieve this by statistical analysis of the interferometric scene coherence as a function of the along track interferometric time lag instead of indirect estimation from the doppler bandwidth we use many data acquisitions of opportunity for that purpose thereby exploiting the diversity of along track baselines between the two sar sensors of the tandem x satellite formation the acf is related to the temporal decorrelation which is not directly observable we extract this component by quantifying and compensating for all other sources of decorrelation that contribute to the observed coherence

boundary layer intrusions from a sloping bottom a mechanism for generating intermediate nepheloid layers 1 laboratory experiments were used to investigate the growth of intrusions due to internal wave reflection from a sloping boundary when normalized by the incident energy density flux the average intrusion spreading velocity was found to be a linear function of the frequency ratio c where is the frequency of the incident wave and c is the critical frequency at which the wave characteristic has the same angle as the bottom slope evenly spaced layers indicating thin perturbations in the background density gradient developed within the mixing region and spread into the tank interior the vertical spacing of these layers also bore a linear relationship to c a linear model of internal wave reflection suggests that these layers may be related to an isopycnal displacement or overturn scale intrusion growth occurred at a range around the critical frequency and was strongest at slightly supercritical conditions a balance relating the spreading rate of intrusions to the divergence of energy density flux across the boundary layer is derived fitting the laboratory results to this theoretical prediction suggested a weak net buoyancy flux this balance might be of use in predicting spreading rates of intermediate nepheloid layers generated by internal wave mixing at oceanic margins

the effects of oil emplacement on diagenetic processes examples from the fulmar reservoir sandstones central north sea geologic note 1 sedimentary petrography and petroleum geochemistry studies have been done on fulmar formation upper jurassic cores from fulmar field central north sea the extent of quartz overgrowths and albitization of k feldspars is lower in oil saturated zones compared with water saturated zones however microthermometric studies of aqueous fluid inclusions in quartz overgrowths show similar temperature ranges for both oil and water saturated zones 85 125 degrees c with maximum recorded temperatures close to the current reservoir temperature about 130 degrees c this suggests that quartz overgrowth development in the oil saturated zones continued after oil emplacement but was retarded compared with development in the water saturated zones the quartz cement therefore mus have been essentially locally derived by pressure solution albitization of k feldspars is significantly less about 50 in the oil saturated zones compared with albitization in the water saturated zones because albitization of k feldspars depends on a supply of sodium and removal of potassium this process is more likely to be terminated by oil emplacement the observed difference in the albitization has been used to suggest that the fulmar reservoir probably was filled during the neogene this age estimate agrees broadly with timing determined independently by modeling of oil generation in adjacent source basins the fulmar sandstones contain little or no kaolinite and show no evidence of substantial leaching of feldspars or carbonates this is probably because the fulmar sandstones being partly turbiditic offshore sand deposits were not exposed to meteoric water their close proximity to the marginally mature to mature kimmeridge clay formation seems to have little effect in terms of developing secondary porosity suggesting that the role of organic acids or co 2 generated in these source rocks is of minor importance

petroleum emplacement inhibits quartz cementation and feldspar dissolution in a deeply buried sandstone abstract whether the emplacement of petroleum in sandstone reservoirs can preserve porosity during burial remains controversial in the kessog field uk central north sea average porosities of the crestal sections of the fluvial deltaic pentland formation reservoir can exceed 25 despite burial to 4 km or more the predicted porosity for the reservoir at this depth is only around 14 based on regional data oil saturation data thin section point counts grain size and sorting measurements reservoir pressure and sem images were combined to analyze the cause of the high reservoir porosity petroleum emplacement preventing cementation is the most likely mechanism for porosity preservation facies variation is not responsible as the high porosity sandstones from the crestal well are in terms of average grain size fine grained and sorting coefficient moderately well sorted nearly the same as the lower porosity sandstones from the flanks of the field average porosity 13 15 other potential porosity preservation mechanisms such as overpressure grain coats and feldspar dissolution can be discounted the sandstones with high oil saturations are characterized by 1 most porosity being primary as opposed to secondary 2 there being 2 5 less quartz cement than in the water saturated sandstones 3 there being 2 3 more k feldspar and 2 6 less kaolin than the water saturated counterparts this study demonstrates that petroleum emplacement can effectively inhibit quartz cementation and k feldspar transformation to kaolin in sandstone reservoirs

regional porosity trends of the upper jurassic norphlet formation in southwestern alabama and vicinity with comparisons to formations of other basins sandstone porosity of the upper jurassic norphlet formation in southwestern alabama and vicinity decreases systematically as depth and thermal maturity increase over a wide range median porosity is about 25 where equivalent vitrinite reflectance ro is slightly over 0 7 in the northern part of the study area clarke county mississippi median porosity is reduced to 8 where ro approaches 2 7 in the southern part of the study area state waters of mobile bay porosity of the cemented tight zone at the top of the norphlet in downdip locations is roughly 10 lower than porosities of facies underlying the tight zone but nevertheless is slightly above the norm for other sandstones at similar ro levels porosity of dune facies is consistently 2 5 higher than that of interdune facies other factors being equal our data show 3 6 higher porosity in chlorite dominated intervals relative to intervals where illite is the dominant clay mineral norphlet porosity has little or no correlation with position relative to the present day hydrocarbon water contact based on comparisons at similar ro levels median 50th percentile norphlet porosity exceeds porosities of typical sandstones in other basins by more than a factor of two throughout the study area even the lower 10th percentile norphlet porosities are higher than median porosities of sandstones in general

application of the wigner ville distribution to temperature gradient microstructure a new technique to study small scale variations the wigner ville distribution a new tool in the time frequency analysis of signals is applied to temperature gradient microstructure records in particular the wigner ville distribution is used to compute the local instantaneous and maximum frequencies of the signal as a function of depth and these frequencies are then related to the dissipation of turbulent kinetic energy the method is applied to two temperature gradient microstructure records from the wellington reservoir it is shown that a high resolution estimate of the dissipation is obtained that provides insight into the patchiness the wavenumber content and the reynolds froude number variability of the integral scales of motion in a strongly stratified water column

deep burial dissolution of k feldspars in a fluvial sandstone pentland formation uk central north sea the pentland formation of the uk central north sea is a quartzite in a sequence of more arkosic sandstones provenance sensitive heavy mineral indices are interpreted to indicate that it was probably derived by erosion of arkosic sediments which also supplied the arkosic marine sandstones of the contemporaneous fulmar formation a lack of apatite within the pentland formation suggests that the feldspars could have dissolved during exposure to groundwaters during very shallow burial however petrographic and geochemical evidence supports survival of at least some of the feldspar to burial depths in excess of at least 2 km when the sandstones had been stabilized against compaction by quartz cements or overpressure abundant kaolin which has not reacted to form illite is preserved at burial depths in excess of 4000 m suggesting an unusual lack of available potassium there is a strong contrast in diagenetic pathways with the overlying marine fulmar formation which most probably had the same initial composition but in which kaolin is absent and fibrous illite is present it is suggested that a lower concentration of potassium and aluminium within the pentland formation compared with the probable source and the comparably sourced fulmar formation represents export of k from the sands on a metre scale

vertical effective stress as a control on quartz cementation in sandstones abstract temperature controlled precipitation kinetics has become the overwhelmingly dominant hypothesis for the control of quartz cementation in sandstones here we integrate quantitative petrographic data high spatial resolution oxygen isotope analyses of quartz cement basin modelling and a kinetic model for quartz precipitation to suggest that the supply of silica from stress sensitive intergranular pressure dissolution at grain contacts is in fact a key control on quartz cementation in sandstones we present data from highly overpressured sandstones in which despite the current burial temperature of 190 c quartz cement occurs in low amounts 4 6 1 2 of bulk volume in situ oxygen isotope data across quartz overgrowths suggest that cementation occurred over 100 ma and a temperature range of 80 150 c during which time high fluid overpressures resulted in consistently low vertical effective stress we argue that the very low amounts of quartz cement can only be explained by the low vertical effective stress which occurred throughout the burial history and which restricted silica supply as a result of a low rate of intergranular pressure dissolution at grain contacts

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

disseminated jigsaw piece dolomite in upper jurassic shelf sandstones central north sea an example of cement growth during bioturbation unusual textural and chemical characteristics of disseminated dolomite in upper jurassic shelf sediments of the north sea have provided the basis for a proposed new interpretation of early diagenetic dolomite authigenesis in highly bioturbated marine sandstones the dolomite is present throughout the franklin sandstone formation of the franklin and elgin fields as discrete non ferroan generally unzoned subhedral to highly anhedral jigsaw piece crystals these are of a similar size to the detrital silicate grains and typically account for 5 of the rock volume the dolomite crystals are never seen to form polycrystalline aggregates or concretions or ever to envelop the adjacent silicate grains they are uniformly dispersed throughout the sandstones irrespective of detrital grain size or clay content dolomite authigenesis predated all the other significant diagenetic events visible in thin section the dolomite is overgrown by late diagenetic ankerite and bulk samples display stable isotope compositions that lie on a mixing trend between these components extrapolation of this trend suggests that the dolomite has near marine delta18o values and low positive delta13c values the unusual textural and chemical characteristics of this dolomite can all be reconciled if it formed in the near surface zone of active bioturbation sea water provided a plentiful reservoir of mg and a pore fluid of regionally consistent delta18o labile bioclastic debris e g aragonite mg calcite supplied isotopically positive carbon to the pore fluids during shallow burial dissolution such dissolution took place in response to the ambient calcite sea conditions but may have been catalysed by organic matter oxidation reactions bioturbation not only ensured that the dissolving carbonate was dispersed throughout the sandstones but also prohibited coalescence of the dolomite crystals and consequent cementation of the grain framework continued exchange of mg2 and ca2 with the sea water reservoir maintained a sufficient mg ca ratio for dolomite rather than calcite to form irregular crystal shapes resulted from dissolution of both the dolomite and the enclosed fine calcitic shell debris before ankerite precipitation during deep burial diagenesis

depositional and diagenetic controls on deeply buried eocene sublacustrine fan reservoirs in the dongying depression bohai bay basin china abstract this paper investigates the reservoir potential of deeply buried eocene sublacustrine fan sandstones in the bohai bay basin china by evaluating the link between depositional lithofacies that controlled primary sediment compositions and diagenetic processes that involved dissolution precipitation and transformation of minerals this petrographic mineralogical and geochemical study recognizes a complex diagenetic history which reflects both the depositional and burial history of the sandstones eogenetic alterations of the sandstones include 1 mechanical compaction and 2 partial to extensive non ferroan carbonate and gypsum cementation typical mesogenetic alterations include 1 dissolution of feldspar non ferroan carbonate cements gypsum and anhydrite 2 precipitation of quartz kaolinite and ferroan carbonate cements 3 transformation of smectite and kaolinite to illite and conversion of gypsum to anhydrite this study demonstrates that 1 depositional lithofacies critically influenced diagenesis which resulted in good reservoir quality of the better sorted middle fan but poor reservoir quality in the inner and outer fan lithofacies 2 formation of secondary porosity was spatially associated with other mineral reactions that caused precipitation of cements within sandstone reservoirs and did not greatly enhance reservoir quality and 3 oil emplacement during early mesodiagenesis temperatures 70 c protected reservoirs from cementation and compaction

low porosity haloes at stylolites in the feldspathic upper jurassic ula sandstone norwegian north sea an integrated petrographic and chemical mass balance approach abstract although stylolites in quartz rich sandstones have been described and modeled more feldspathic sandstones have not been similarly investigated the upper jurassic ula sandstone norwegian north sea a subarkose with 15 to 25 feldspar displays stylolites with prominent low porosity haloes up to 10 cm wide detailed mineralogical and chemical profiles across several stylolites and haloes are used to investigate the role of feldspar in mineral reactions at stylolites and to constrain the transport of chemical components quartz and feldspar are intensely dissolved at the margins of stylolites the surrounding haloes are characterized by dissolution of quartz and feldspar intense compaction anomalously high feldspar content high concentrations of clay and oxide cement volumes that increase away from the stylolite chemical mass balance calculations demonstrate that silica and elements associated with feldspar al2o3 na2o k2o were exported from the haloes grain size data and evidence for aluminum loss from the halo indicate that primary concentrations of clay and mica were not higher near the stylolites and that the petrographic and geochemical patterns reflect the diagenetic concentration of clay the analysis suggests that k feldspar dissolution at and near the stylolite resulted in precipitation of authigenic illite near the site of k feldspar dissolution due to the low solubility of al3 this in turn catalyzed chemical dissolution of quartz in the low porosity halo transport of cao mgo and mno into the haloes suggests that the haloes were zones of higher ph the increased ph perhaps due to the presence of clays would have also increased quartz solubility consistent with evidence for silica export from the halo the dissolution of k feldspar in the absence of kaolinite would also have raised ph around the site of k feldspar dissolution and illite precipitation favoring carbonate precipitation this interpretation of stylolite diagenesis differs from conventional models based on quartz rich sandstones in which quartz cement volumes decrease away from stylolites models for compaction in arkosic sandstones need to account for the enhanced dissolution of quartz that is driven by k feldspar dissolution and for catalysis by the resulting illite

a fluid inclusion study of quartz cemented sandstones from offshore mid norway possible evidence for continued quartz cementation during oil emplacement abstract jurassic sandstones from the oil zone in a well from the haltenbanken area offshore mid norway contain both aqueous and hydrocarbon fluid inclusions within quartz overgrowths homogenization temperatures for the inclusions range from 92 c up to the present formation temperature of approximately 118 c since the inclusions mostly are located at the boundaries between quartz clasts and quartz overgrowths the homogenization temperatures indicate that quartz cementation started at temperatures slightly below 92 c poikilotopic ferroan calcite cement usually predates formation of quartz overgrowths and most calcite cement must be precipitated at temperatures below 90 c although in one calcite cemented zone ferroan calcite encloses quartz overgrowths with inclusions trapped at temperatures of up to 105 c none of the examined hydrocarbon inclusions homogenized below 102 c and this suggests that oil started entering the reservoir at temperatures of around 100 c the burial history of the examined sandstones indicates that temperatures of 100 c were not reached before the late pliocene and that filling of the reservoir has taken place during the last few million years homogenization temperatures approximately equal to present formation temperature suggest that quartz cementation may still be in progress and that emplacement of oil did not necessarily stop precipitation of quartz cement possible continued quartz cementation after oil filled the reservoir could be explained by dissolution of quartz at stylolites and grain contacts as transport of dissolved quartz from these sources might possibly take place by diffusion through irreducible water

a nonlinear model of internal tide transformation on the australian north west shelf a numerical solution to the generalized korteweg de vries k dv equation including horizontal variability and dissipation is used to model the evolution of an initially sinusoidal long internal wave representing an internal tide the model shows the development of the waveform to the formation of shocks and solitons as it propagates shoreward over the continental slope and shelf the model is run using observed hydrographic conditions from the australian north west shelf and results are compared to current meter and thermistor observations from the shelf break region it is found from observations that the coefficient of nonlinearity in the k dv equation changes sign from negative in deep water to positive in shallow water and this plays a major role in determining the form of the internal tide transformation on the shelf there is strong temporal variability in the nonlinear coefficient due to both background shear flow and the large amplitude of the internal tide which distorts the density profile over a wave period both the model and observations show the formation of an initial shock on the leading face of the internal tide in shallow water the change in sign of the coefficient of nonlinearity causes the shock to evolve into a tail of short period sinusoidal waves after further propagation a second shock forms on the back face of the wave followed by a packet of solitons the inclusion of bottom friction in the model is investigated along with the dependance on initial wave amplitude and variability in the coefficients of nonlinearity and dispersion friction is found to be important in limiting the amplitudes of the evolving waves

internal solitons on the pycnocline generation propagation and shoaling and breaking over a slope in part 1 a study is made of the internal solitary wave on the pycnocline of a continuously stratified fluid a korteweg de vries kdv equation for the interfacial displacement is developed following benney s method for long nonlinear waves experiments were conducted in a long wave tank with the pycnocline at several different depths below the free surface while keeping the total depth approximately constant a step like pool of light water trapped behind a sliding gate served as the initial disturbance condition the number of solitons generated was verified to satisfy the prediction of inverse scattering theory the fully developed soliton was found to satisfy the kdv theory for all ratios of upper layer thickness to total depth in part 2 of this study we investigate experimentally the evolution and breaking of an internal solitary wave as it shoals on a sloping bottom connecting the deeper region where the waves were generated to a shallower shelf region it is found through quantitative measurements that the onset of wave breaking was governed by shear instability which was initiated when the local gradient richardson number became less than the internal solitary wave of depression was found to steepen at the back of the wave before breaking in contrast with waves of elevation two slopes were used with ratios 1 16 and 1 9 and the fluid was a boussinesq fluid with weak stratification using brine solutions

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

characterization of turbulent airflow over evolving water waves in a wind wave tank 1 results of an experimental investigation of the turbulent boundary layer in airflow over evolving young wind waves are presented the experiments were conducted in a laboratory wind wave flume consisting of a closed loop wind tunnel capable of generating wind speed that may exceed 15 m s atop of a 5 m long wave tank simultaneous measurements of mean wind velocity and of instantaneous fluctuations of the horizontal and vertical air velocity components were carried out along the test section at different airflow rates and at numerous heights above the highest wave instantaneous surface elevation at the air sensors location was simultaneously recorded the friction velocities at all locations and for all airflow rates were determined by two independent methods by fitting the logarithmic velocity profiles and by extrapolating the measured reynolds shear stresses to mean water surface level the variation with height and along the test section of the fluctuations of two velocity components in the mean flow and in the vertical directions was also studied and the results compared with flow behavior over rough and smooth plates wave induced airflow parameters were then investigated by application of cross spectral analysis results on the vertical extent of wave induced boundary layer on the phase relation between the wave induced velocity fluctuations and the surface elevation as well as on the wave induced reynolds shear stress are reported

fluid overpressure as a control on sandstone reservoir quality in a mechanical compaction dominated setting magnolia field gulf of mexico the reservoir quality porosity and permeability of deeply buried hydrocarbon reservoir sandstones in sedimentary basins is significantly affected by burial diagenesis many deep reservoirs develop anomalous fluid overpressures during burial previous studies on the effect of fluid overpressure on reservoir quality in these deep reservoirs have been inconclusive because of the difficulty in constraining the individual contributions of various porosity preserving factors that are simultaneously active in these reservoirs owing to its rapid burial and low burial temperatures the neogene gravity flow sandstone reservoirs from the magnolia field gulf of mexico offers a unique opportunity to investigate in isolation the effect of fluid overpressure on reservoir quality examination of petrography pore pressure and routine core analysis datasets showed a positive correlation between high fluid overpressure and enhanced reservoir quality this study confirms that fluid overpressure preserves reservoir quality in deeply buried sandstone reservoirs in compaction dominated high sedimentation basin settings

water waves excited by near impulsive wind forcing only limited information is currently available on the evolution of waves generated by wind that varies in time and in particular on the initial stages of wind wave growth from rest under a suddenly applied wind forcing the emerging wind wave field varies in time as well as in space detailed knowledge of wave parameter distributions under those conditions contributes to a better understanding of the mechanisms of wind wave generation in the present study the instantaneous surface elevation and two components of the instantaneous surface slope were recorded at various fetches in a small scale experimental facility under nearly impulsive wind forcing numerous independent realizations have been recorded for each selection of operational conditions sufficient data at a number of fetches were accumulated to calculate reliable ensemble averaged statistical parameters of the evolving random wind wave field as a function of the time elapsed from activation of wind forcing distinct stages in the wave evolution process from appearance of initial ripples to emergence of a quasi steady wind wave field were identified the experimental results during each stage of evolution were analysed in view of the viscous instability theory by kawai j fluid mech vol 93 1979 pp 661 703 and the resonance model by phillips j fluid mech vol 2 1957 pp 417 445

the degeneration of large scale interfacial gravity waves in lakes mechanisms for the degeneration of large scale interfacial gravity waves are identified for lakes in which the effects of the earth s rotation can be neglected by assuming a simple two layer model and comparing the timescales over which each of these degeneration mechanisms act regimes are defined in which particular processes are expected to dominate the boundaries of these regimes are expressed in terms of two lengthscale ratios the ratio of the amplitude of the initial wave to the depth of the thermocline and the ratio of the depth of the thermocline to the overall depth of the lake comparison of the predictions of this timescale analysis with the results from both laboratory experiments and field observations confirms its applicability the results suggest that for small to medium sized lakes subject to a relatively uniform windstress an important mechanism for the degeneration of large scale internal waves is the generation of solitons by nonlinear steepening since solitons are likely to break at the sloping boundaries leading to localized turbulent mixing and enhanced dissipation the transfer of energy from an initial basin scale seiche to shorter solitons has important implications for the lake ecology

transport by breaking internal gravity waves on slopes we use the results of a direct numerical simulation dns with a particle tracking model to investigate three dimensional transport by breaking internal gravity waves on slopes onshore transport occurs within an upslope surge of dense fluid after breaking offshore transport occurs due to an intrusion of mixed fluid that propagates offshore and resembles an intermediate nepheloid layer inl entrainment of particles into the inl is related to irreversible mixing of the density field during wave breaking maximum onshore and offshore transport are calculated as a function of initial particle position and can be of the order of the initial wave length scale for particles initialized within the breaking region an effective cross shore dispersion coefficient is also calculated and is roughly three orders of magnitude larger than the molecular diffusivity within the breaking region particles are transported laterally due to turbulence that develops during wave breaking and this lateral spreading is quantified with a lateral turbulent diffusivity lateral turbulent diffusivity values calculated using particles are elevated by more than one order of magnitude above the molecular diffusivity and are shown to agree well with turbulent diffusivities estimated using a generic length scale turbulence closure model based on a favourable comparison of dns results with those of a similar two dimensional case we use two dimensional simulations to extend our cross shore transport results to additional wave amplitude and bathymetric slope conditions

confocal microscopy of fluid inclusions reveals fluid pressure histories of sediments and an unexpected origin of gas condensate we present two examples of how fluid inclusion data can be used to determine geologic pressure histories and to quantify the compositional evolution of petroleum in oil reservoirs volumetric liquid vapor ratios generated with a confocal laser scanning microscope are used along with pressure vapor temperature p v t modeling software to estimate the composition p t phase envelope and isochore of single petroleum inclusions in the north sea9s judy and alwyn fields in both cases the gas condensates currently in the reservoirs formed by the emplacement of gas into preexisting oil accumulations pressure histories of individual units in each field are also revealed providing the kind of data needed to determine the permeability and fluid flow histories of sedimentary basins

genesis and distribution pattern of carbonate cements in lacustrine deep water gravity flow sandstone reservoirs in the third member of the shahejie formation in the dongying sag jiyang depression eastern china abstract the lacustrine deep water gravity flow sandstone reservoirs in the third member of the shahejie formation are the main exploration target for hydrocarbons in the dongying sag eastern china carbonate cementation is responsible for much of the porosity and permeability reduction in the lacustrine deep water gravity flow sandstone reservoirs the sandstones are mainly lithic arkose with an average framework composition of q43f33l24 the carbonate cements are dominated by calcite ferroan calcite ankerite and a small amount of dolomite the calcite and ferroan calcite are mainly poikilotopic blocky crystals while the dolomite and ankerite are mainly euhedral rhombohedra crystals filling intergranular and intragranular pores the relatively positive 13c values 2 to 3 9 of the carbonate cements in the sandstone reflect a mainly inorganically sourced carbon from 32 ma to 25 ma the pore water was rich in bicarbonate and ca2 due to carbonate dissolution in mudstone and which were transported with the pore water from mudstone to sandstone via advection and precipitated calcite cementation in thinly bedded sandstones and some high permeability zones in the middle of medium to thick sandstone beds from 12 ma to present abundant ca2 fe3 fe2 mg2 and bicarbonate had been transported from mudstone to sandstone via diffusion to form tight ferroan calcite cementation in the upper and lower parts of the medium to thick bedded sandstones ankerite is mainly distributed in the reservoirs associated with oil migration or charge because change of fe3 to fe2 from oil charge may supply sufficient fe2 for ankerite precipitation the center of sandstone beds 0 6 m is with potential of high quality reservoirs in the research area carbonate cementation appears to be an important factor that controls the accumulation of oil in deep water gravity flow sandstone reservoirs in the study area

internal wave induced dispersion and mixing on a sloping boundary 1 a fluorescent dye tracer was released in a stratified water column above a sloping boundary revealing profound dispersive asymmetry initially the dye spread symmetrically in the vertical until the deeper portion became influenced by the bottom boundary layer bbl the shallower portion then spread rapidly up slope in a thin boundary layer influenced by highly non linear isopycnal distortions the deeper down slope portion of dye subsequently became detached from the boundary and intruded into the basin interior in three distinct layers along isopycnal surfaces the intrusions result from the gravitational collapse of mixing patches created by baroclinic flow over an irregular boundary the vertical separation of the layers matches the inverse topographic wavenumber up slope displacement of the dye is consistent with an advective diffusive balance near the sloping boundary controlled by slope angle and roughness horizontal diffusivities increased from 1 4 m2s 1 to 18 4 m2s 1 during the 4 day experiment

effects of oil cracking on fluorescence color homogenization temperature and trapping pressure reconstruction of oil inclusions from deeply buried reservoirs in the northern dongying depression bohai bay basin china the effects of oil cracking on fluorescence color homogenization temperature tho and trapping pressure pt of oil inclusions from deeply buried reservoirs dbrs 3672 4359 m in the northern dongying depression were determined based on fluorescence spectroscopy and homogenization temperatures of oil inclusions kinetic modeling of crude oil cracking and petroleum inclusion thermodynamics modeling the modeling results demonstrate that fluorescence color tho and predicted pt have strong relationships with the transformation rate via cracking of oil to gas tr and the formation temperature tf that the inclusions experienced the fluorescence color is hardly influenced at all during the initial stages of oil cracking tr 13 tf 160 c but fluorescence color begins to shift toward shorter wavelengths blue shift during progressive oil cracking tr 24 tf 190 c with further oil cracking the fluorescence color may either experience no change or continue its blue shift eventually the fluorescence color will disappear as the aromatic compounds are completely cracked the tho increases at first tr 24 tf 190 c but then decreases or even becomes negative during major oil cracking the reconstructed pt values show a corresponding reverse trend oil inclusions from dbrs and other shallow reservoirs in the dongying depression show an obvious blue shift in fluorescence color at a depth of approximate 4000 m tf 160 c and generally contain solid bitumen below 4000 m supporting the effect of oil cracking on fluorescence variation consistent with the modeling result the tho from dbrs in the minfeng area increases with increasing burial depth tf 190 c which is also consistent with the modeling results however the tho of oil inclusions with blue white fluorescence from dbrs in the shengtuo area did not show such a trend recent trapping high trapping pressure and higher maturity oil may have led to a low degree of oil cracking and thus less modification of tho in the shengtuo area oil cracking results in consistent volume ratios of pyrobitumen to oil inclusions fvpy in the same fluid inclusion assemblage and the fvpy value increases with oil cracking level which can be used to recognize if oil cracking has occurred in oil inclusions and what level of oil cracking they have experienced as the oil cracking model used in this study did not account for the role of pressure it is more applicable for oil inclusions that were trapped under normally pressured conditions oil inclusions trapped under overpressured conditions will be less influenced by oil cracking

the seasonal evolution of wind internal wave resonance in lake kinneret data from a thermistor chain and wind sensor collected over an annual stratification cycle in lake kinneret israel during 2000 were used to investigate the seasonal evolution of wind internal wave resonance internal wave periods determined from an analytical model were compared with observations and we show that resonance during 2000 occurred during three distinct times of the year at the onset of stratification march during the heating phase june and during the cooling phase november in all cases resonance was between the wind and the dominant radial azimuthal and vertical mode one cyclonic kelvin wave previously observed in lake kinneret internal waves are a ubiquitous feature of lakes existing because of wind forcing acting on a stratified water column where the stratification is generally due to temperature wind applied to the surface can cause a surface setup of water at the downwind end this pressure force is balanced by a tilting of the metalimnion in the opposite sense that is downward at the downwind end spigel and imberger 1980 when the wind forcing relaxes the water surface oscillates a barotropic wave as does the metalimnion a baroclinic wave the two motions can effectively be considered to be decoupled monismith 1985 because they form orthogonal solutions to the wave equation it is the internal baroclinic waves that are of interest in the present study because they provide much of the energy and motion for biogeochemical processes in lakes in large lakes the picture becomes complicated because the effects of the earth s rotation begin to influence the frequency and structure of the internal waves internal gravity waves in large lakes take the form of cyclonic kelvin and or poincare or anticyclonic poincare waves antenucci et al 2000 antenucci and imberger 2001 where the wave crests rotate around the perimeter of the lake basin antenucci and imberger 2001 showed that the frequency as well as the ratio of potential kinetic energy of internal gravity waves in approximately circular or elliptic lakes was dependent on the aspect ratio radial mode azimuthal mode and burger number the burger number is defined asc lf where c is the phase speed of internal waves in a nonrotating system l is some length scale characterizing the basin width and f is the coriolis parameter for a small burger number rotation dominates over gravity and waves approach the oceanic case known as inertial oscillations in which the motion is dominated by the kinetic energy signal for a large acknowledgments we acknowledge the field support and technical staff from the kinneret limnological laboratory kll for providing assistance particularly tzahi rosenberg for maintaining the lake station and helping in the transfer of data from kll to the centre for water research and peter yeates who provided the time series of the lake number

dissolution and its impacts on reservoir formation in moderately to deeply buried strata of mixed siliciclastic carbonate sediments northwestern qaidam basin northwest china abstract petroleum exploration is increasingly extending from shallowly to moderately to deeply buried strata with reservoir quality being a key the formation mechanism of conventional clastic and carbonate reservoirs under such conditions has been widely investigated however the mechanism of mixed siliciclastic carbonate reservoirs has not been well studied in this paper we address this issue using a case study in the northwestern qaidam basin northwest china n1 e1 2 age 2500 4500 m dissolution pores and fractures dominate the reservoirs however only the fractures were the focus of previous studies thus here we investigate dissolution and its impacts on reservoir formation providing a complementary understanding of the reservoir formation in the basin using core observations and examining thin section it was discovered that dissolution was both random and followed bedding the dissolved components primarily included calcareous and gypsum minerals with fingerprints that are characteristic of burial dissolution further electron probe analysis on authigenic minerals revealed that the dissolution fluid might have originated from acidic formation fluids associated with hydrocarbon generation the fluids passed through faults and fractures dissolution pores were an important component of the reservoir providing approximately 60 of the porosity in addition segments of high porosity generally above 5 are associated with dissolution based on these observations a schematic model was established to explain the impacts of dissolution on reservoir formation specifically organic acidic formation fluids enter tectonic fractures resulting in dissolution as they pass through them this dissolution enhances the size of the pore space and the reservoir properties of the rocks eventually developing a fracture dissolution reservoir

seasonal evolution of the basin scale internal wave field in a large stratified lake the response of the water column to varying conditions of stratification and wind forcing was investigated in lake kinneret israel using data collected from thermistor chains and acoustic doppler current profilers during 1997 and 1998 the strong daily sea breeze was found to generate a vertical mode 1 internal kelvin wave and basin scale internal poincarewaves of vertical modes 1 2 and 3 the kelvin wave the dominant component of the internal wave field was responsible for alongshore velocities in the nearshore regions in the upwind nearshore regions velocities were dominated by the forced response to the wind and were cross shore in nature in the lake interior the kelvin wave effect on the horizontal velocity field was minor compared to the higher vertical mode poincarewaves the kelvin wave is shown to exist in resonant and nonforced states with the wind whereas the vertical mode 1 poincarewave energy remained relatively constant despite large variability in the forcing condi tions the energy in the higher mode poincarewaves varied greatly both on daily and seasonal timescales the results demonstrate that the wind energy forces multiple basin scale internal wave modes and that prior motion in the water column must be considered when determining the subsequent internal wave response in periodically forced systems lake kinneret israel has long been an important regional resource providing a substantial supply of drinking water as well as supporting a commercial fishery the monomictic lake is forced daily during the summer by a powerful sea breeze with stratification and wind patterns showing little variation from year to year these factors along with a fa vorable bathymetric pattern combine to make lake kinneret an ideal place for the field investigation of large scale inter nal waves and related phenomena such as the benthic bound ary layer horizontal transport and vertical mixing processes in addition to providing substantial insight into the local pro cesses and hence influencing local management study of the lake also provides fundamental knowledge of the dynam ics of enclosed basins on a geophysical scale during summer the internal rossby radius in lake kin neret is typically one half the basin width hence large scale motion in the lake will be affected by the earth s rotation and basin scale internal waves may be of topographic ross by kelvin or poincareform topographic rossby waves are sub inertial frequency oscillations that exist only in rotating fluids with variable bathymetry mysak et al 1985 stocker and hutter 1987 and these waves result from changes in depth affecting the relative vorticity of fluid elements in the absence of vertical boundaries wavelike solutions to the lin ear inviscid equations of motion for a flat bottomed region of homogeneous fluid with a constant rate of rotation are 1

residual circulation due to internal waves shoaling on a slope we describe the residual circulation induced in a two layer system when internal waves with a wave length equal to or longer than the slope length propagate over a sloping bottom and either reflect or break for the case of no wave breaking scaling is used to derive boussinesq type equations the solution of which leads to an expression for the residual circulation adjacent to the sloping bottom this residual circulation is maintained by a weak slope on the density interface for breaking waves laboratory experiments and numerical modeling were used to ascertain the details of the residual circulation arising from the formation of gravitational intrusions results from these three investigative tools were used to determine as a function of bottom slope and incident wave amplitude the internal wave reflection coefficient and the strength of the residual circulation pattern with and without wave breaking the ocean and most lakes are thermally stratified for at least part of the year in stratified fluids the density anomaly associated with the thermal stratification supports internal waves with a range of vertical modal structures antenucci et al 2000 antenucci and imberger 2001 antenucci and imberger 2003 the gravest or first vertical mode induces a large current over the lake or ocean bottom and when the bottom slopes the waves shoal causing them to break and lose energy to dissipation the associated currents and turbulence can often lead to sediment resuspension and mixing of the water column gloor et al 1994 pierson and weyhenmeyer 1994 steinman et al 1997 the gravitational motions in turn induced by the mixing may combine with secondary internal wave motions to drive a residual circulation that may transport resuspended and dissolved material to the central area of a lake hakanson 1982 ostrovsky et al 1996 inall 2009 also demonstrated mass transport due to intrusion caused by internal wave breaking in the fjord loch etive by using a fluorescent dye tracer such intrusions have been observed but their dynamics do not seem to have received any attention in this paper the processes leading to residual circulation are described when a continuous train of internal waves with an individual wavelength equal to or longer than the slope length supported by a two layer stratification impinges perpendicularly onto a sloping bottom two cases were investigated when the waves break and when they are reflected perfectly without breaking internal waves and their associated mass transport in lakes fjords and coastal oceans are reviewed in ostrovsky and stepanyants 1989 and huthnance 1989 whereas solitary internal waves are reviewed by brown and christie 1998 lamb 2002 and grue et al 2000 regarding mass transport lamb 2002 investigated shoaling of largeamplitude internal waves and demonstrated that trapped

vertical effective stress and temperature as controls of quartz cementation in sandstones evidence from north sea fulmar and gulf of mexico wilcox sandstones abstract we present quantitative petrographic data high spatial resolution oxygen isotope analyses of quartz cement basin modelling and a kinetic model for quartz precipitation for two paleocene eocene wilcox group sandstones from texas and two jurassic fulmar formation sandstones from the central north sea at each location one sandstone has been buried to ca 145 c and one to ca 185 c a key difference between the wilcox and fulmar burial histories is that the wilcox sandstones are currently at higher vertical effective stresses and from basin modelling studies have been subjected to generally higher vertical effective stresses through their burial history the amounts of quartz cement in the wilcox sandstones are between 12 and 18 and between 2 and 6 in the fulmar sandstones high spatial resolution oxygen isotope data obtained from the quartz cements suggest temperature ranges for quartz precipitation from 60 to 80 c to values approaching maximum burial temperature factors such as grain coatings or the timing of petroleum emplacement cannot explain the differences in the amounts of quartz cement petrographic data show that most of the silica for quartz cement can be derived from intergranular pressure dissolution although the sample set is small we interpret the results to suggest that the differences in quartz cementation in fulmar and wilcox sandstones can be explained better by differences in their vertical effective stress history than their temperature history in this case the supply of silica rather than the precipitation of quartz becomes an important control on the rate and extent of cementation

the degeneration of internal waves in lakes with sloping topography in a laboratory study we quantified the temporal energy flux associated with the degeneration of basin scale internal waves in closed basins the system is two layer stratified and subjected to a single forcing event creating available potential energy at time zero a downscale energy transfer was observed from the wind forced basin scale motions to the turbulent motions where energy was lost due to high frequency internal wave breaking along sloping topography under moderate forcing conditions steepening of nonlinear basin scale wave components was found to produce a high frequency solitary wave packet that contained as much as 20 of the available potential energy introduced by the initial condition the characteristic lengthscale of a particular solitary wave was less than the characteristic slope length leading to wave breaking along the sloping boundary the ratio of the steepening timescale required for the evolution of the solitary waves to the travel time until the waves shoaled controlled their development and degeneration within the domain the energy loss along the slope the mixing efficiency and the breaker type were modeled using appropriate forms of an internal iribarren number defined as the ratio of the boundary slope to the wave slope amplitude wavelength this parameter allows generalization to the oceanographic context analysis of field data shows the portion of the internal wave spectrum for lakes between motions at the basin and buoyancy scales to be composed of progressive waves both weakly nonlinear waves sinusoidal profile with frequencies near 10 24 hz and strongly nonlinear waves hyperbolic secant squared profile with frequencies near 10 23 hz the results suggest that a periodically forced system may sustain a quasi steady flux of 20 of the potential energy introduced by the surface wind stress to the benthic boundary layer at the depth of the pycnocline

the fate of co2 derived from thermochemical sulfate reduction tsr and effect of tsr on carbonate porosity and permeability sichuan basin china abstract this article discusses the role of methane in thermochemical sulfate reduction tsr the fate of tsr derived co2 and the effect of tsr on reservoir porosity and permeability and the causes of the anomalously high porosity and permeability in the lower triassic soured carbonate gas reservoirs in the northeast sichuan basin southwest china the lower triassic carbonate reservoirs were buried to a depth of about 7000 m and experienced maximum temperatures up to 220 c before having been uplifted to the present day depths of 4800 to 5500 m but they still possess porosities up to 28 9 and permeabilities up to 3360 md the present day dry gas reservoirs evolved from a paleo oil accumulation and experienced varying degrees of tsr alteration as evidenced from the abundant sulfur rich solid bitumens and varying h2s and co2 concentrations tsr occurred mainly within the oil and condensate wet gas windows with liquid hydrocarbons and wet hydrocarbon gases acting as the dominant reducing agents responsible for sulfate reduction sulfur rich solid bitumen and h2s generation and calcite precipitation methane dominated tsr was a rather late event and had played a less significant role in altering the reservoirs intensive h2s and co2 generation during tsr resulted in calcite cementation rather than carbonate dissolution which implies that the amount of water generated during tsr was volumetrically insignificant 13c depleted co2 derived from hydrocarbon oxidation preferentially reacted with ca2 to form isotopically light calcite cements and the remaining co2 re equilibrated with the 13c enriched water rock systems with its 13c rapidly approaching the values for the host rocks which accounted for the observed heavy and relatively constant co2 13c values the carbonate reservoirs suffered from differential porosity loss by tsr involved solid bitumen generation and tsr induced calcite and pyrite precipitation intensive tsr significantly reduced the porosity and permeability of the intervals expected to have relatively high sulfate contents the evaporative platform dolostones and the platform margin shoal dolostones immediately underlying the evaporative facies early oil charge and limited intensity of tsr alteration together with very low phyllosilicate content and early dolomitization accounted for the preservation of anomalously high porosities in the reservoirs above the paleo oil water contact a closed system seems to have played a special role in preserving the high porosity in the gas zone reservoirs below the paleo oil water contact the closed system which is unfavorable for deep burial carbonate dissolution and secondary porosity generation was favorable for the preservation of early formed porosity in deeply buried carbonates especially sucrosic and vuggy dolostones have a high potential to preserve such porosity

the effect of oil emplacement on quartz cementation in a deeply buried sandstone reservoir quartz is an important porosity occluding cement in sandstone reservoirs that have been subjected to elevated temperature 80 c to 100 c for a substantial period of time the effect of oil emplacement on quartz cementation in reservoir sandstones is controversial some studies have concluded that early oil emplacement can inhibit quartz cementation leading to the preservation of porosity whereas other studies have concluded that quartz cementation appears largely unaffected by oil emplacement here we have studied shallow marine upper jurassic sandstones from ula field norwegian north sea with reservoir temperatures of approximately 150 c to determine whether oil emplacement had a significant impact on diagenesis with particular attention to quartz cementation following sedimentological description of cores samples above and below the oil water contact have been collected adjacent to core analysis plug points these samples then underwent a series of studies including petrographic point counting with a transmitted light microscope scanning electron microscopy sem backscattered electron microscopy sem cathodoluminescence microscopy and fluid inclusion studies these data were integrated with routine core analysis and petrophysical log data density and resistivity log data have been used to determine the precise oil saturation of each sample studied the distributions of all potential controls on porosity and permeability such as grain size sorting matrix clay content and degree of bioturbation the presence of grain coatings and dolomite cement and the amount of quartz cement have been assessed the presence of primary oil inclusions within quartz cement shows that oil ingress into the ula reservoir commenced prior to the onset of quartz cementation very fine grained matrix rich bioturbated microquartz cemented sandstones have uniformly low quartz cement contents irrespective of oil saturation medium grained graded matrix poor microquartz poor sandstones have quartz cement ranging from 1 to greater than 17 associated with core porosities of approximately 22 and 7 respectively higher oil saturations equate to higher porosities and permeabilities in the medium grained graded matrix poor microquartz poor sandstones which cannot be explained by any control other than the amount of quartz cement as a function of pore fluid type oil emplacement therefore appears to have inhibited quartz cementation at high oil saturations and can be viewed as an important control on reservoir quality the significance of this study is that the presence of oil in a sandstone reservoir at the time that quartz cement was growing can have a considerable impact on reservoir quality models that seek to predict quartz cement and reservoir quality in sandstones need to account for the timing of oil emplacement compared with other diagenetic processes

sedimentological aspects of the humber group upper jurassic of the south central graben uk north sea complex structure poorly understood sedimentology and poor biostratigraphic control make the upper jurassic humber group of the south central graben one of the least understood and most complex hydrocarbon reservoirs of the north sea detailed logging of available core from 19 exploration wells has been combined with an improved understanding of the relevance of trace fossils and a recognition of important base level variations to provide a greatly enhanced understanding of the depositional system active within the area at that time a new sedimentological model based upon the distribution of facies and facies associations illustrates that upper jurassic structure and consequent basin geometry were the principal controls upon the distribution of depositional environments rifting and second order transgression controlled the back stepping onlap patterns observed and higher frequency base level fluctuations controlled the internal architecture of individual sandbodies the model presented accounts for features of these deposits that were previously considered anomalous such as the thickness of bioturbated sandstones paucity of foreshore deposits and complex age relationships of sands

composition of pliocene to quaternary mixed terrigenous and calcareous sandy beds in contourite drift deposits at odp site 1119 off new zealand insights into sandy drift development and drift petroleum reservoir characterization abstract odp ocean drilling program site 1119 was drilled in drift successions deposited by northward flowing currents on the canterbury slope south island new zealand pliocene 3 9 ma to pleistocene drift accumulation was concomitant with tectonic uplift and sediment supply from the alpine fault plate boundary to the west glacial development and eustatic sealevel change that created cyclicity in sandy input sand detrital and biogenic modes of very fine and fine sand fractions from 24 unconsolidated core samples 44 thin sections were determined fine sand fractions contain an average of 52 bioclastic debris with bioclasts making up 23 of the very fine sand fraction foraminifers are the dominant bioclasts with pelagic more common than benthic varieties the average proportions of monocrystalline quartz plagioclase and potassium feldspar are similar in both size fractions average qmkp of 25 12 63 in contrast the very fine sand fractions have on average higher lithic qfl l of 16 vs 3 mica totalgrain m of 10 vs 6 and dense mineral totalgrain d of 8 vs 5 content as compared with the fine sand fractions younger clinoform drape unit i and older mounded drift unit ii seismic facies showed no distinct detrital compositional differences in their sand fractions though bioclast content ranges higher in the fine sand fraction of unit ii albite feldspar metamorphic rock fragment and chlorite components in the sand are consistent with a schist provenance other components such as biotite plagioclase and k feldspar require a contribution from metasedimentary and volcanic rocks thus there is some along slope mixing during sediment transport by south to north flowing currents cemented microporous sandstone at the base of the section provides some insight into contourite burial diagenesis this work provides a better understanding of the compositional variability of sand components of drift successions which are potential hydrocarbon reservoirs elsewhere