the elastic properties of carbonates although carbonates contain about 60 of the world s oil reserves the characterization of carbonate reservoirs remains challenging because of their heterogeneity andcomplex microstructure central to the seismic characterization of carbonates is an understanding of the effect of porosity on the elastic moduli and seismic wave velocities figure 1 shows the compressional and shear wave velocities measured by rafavich et al 1984 for limestones and dolomites from the williston basin north dakota measured on air saturated samples it is seen that although the velocities decrease with increasing porosity there is considerable scatter of the data in this article the possible contribution of variations in pore shape to the elastic moduli of carbonates is discussed the role of microcracks and fractures is assessed and differences between the static and dynamic moduli are highlighted

interannual variability of the south pacific convergence zone and implications for tropical cyclone genesis the interannual variability of the south pacific convergence zone spcz and its influence on tropical cyclone tc genesis in the south pacific are investigated using observations and era40 reanalysis over the 1979 2002 period in austral summer the spcz displays four typical structures at interannual timescales the first three are characterized by a diagonal orientation of the spcz and account for 85 of the summer seasons one is close to climatology and the other two exhibit a 3 northward or southward departure from the spcz climatological position in contrast the fourth one that only encompasses three austral summer seasons the extreme 1982 1983 and 1997 1998 el nino events and the moderate 1991 1992 el nino event displays very peculiar behaviour where the spcz largely departs from its climatological position and is zonally oriented variability of the western central pacific equatorial sea surface temperature sst is shown to modulate moisture transport south of the equator thereby strongly constraining the location of the spcz the spcz location is also shown to strongly modulate the atmospheric circulation variability in the south pacific with specific patterns for each class however independently of its wide year to year excursions the spcz is always collocated with the zero relative vorticity at low levels while the maximum vorticity axis lies 6 to the south of the spcz position this coherent atmospheric organisation in the spcz region is shown to constrain tropical cyclogenesis to occur preferentially within 10 south of the spcz location as this region combines all the large scale atmospheric conditions that favour the breeding of tcs this analysis also reveals that cyclogenesis in the central pacific in the vicinity of french polynesia only occurs when the spcz displays a zonal orientation while this observation was previously attributed to el nino years in general different characteristics of el nino southern oscillation enso related pacific equatorial warming are shown to impact differently on the spcz position suggesting that for regional scales such as the south pacific the spcz classification is more appropriate than a simple enso index to characterize tc interannual variability these findings suggest that forecasting the strength of el nino through sst variations in the eastern pacific may not be sufficient to accurately predict cyclogenesis in the south pacific especially east of the dateline

revisiting the wyllie time average equation in the case of near spherical pores we suggest a generalized form of the wyllie equation to better describe acoustic velocity variation with porosity in rocks with a mixture of intercrystalline and near spherical pores introducing a pore shape factor in the wyllie equation allows different weights to be applied to the solid phase p wave velocity to account for rock frame stiffness this is demonstrated in carbonate and igneous rocks having variable amounts of spherical porosity ranging from 0 to 80 of the bulk porosity the generalized equation describes a velocity porosity envelope instead of the simple line given by the wyllie equation the lower limit of the velocity envelope is for rocks that have only interparticle or intercrystalline porosity wyllie equation behavior and the upper limit is when all pores are spherical or near spherical this upper limit corresponds to the hashin shtrikman upper bound for porosities up to 30 the realization portrays wyllie s velocity porosity relationship as one particular case in which all por

measurement and modelling of diffusion porosity and other pore level characteristics of sandstones abstract the diffusion coefficients of methane iso butane and n butane through dry and water saturated sandstones and their dependence on relative molar mass are discussed the diffusion of iso butane is anomalously fast in dry sandstones and that of methane is anomalously low in water saturated sandstones a three dimensional computer model of the pores and throats in the sandstone has been developed simulations have been made of the sandstone s porosity surface area pore and throat size distribution tortuosity mercury porosimetry primary drainage curve average pore connectivity and diffusion coefficient for mixed gaseous alkanes

the relationship between acoustic properties and the petrographic character of carbonate rocks laboratory studies of the detailed relationships between acoustic properties and the petrographic character of brine and air saturated carbonate rocks with a wide range of facies porosities lithologies and rock fabrics indicate that porosity is the major factor influencing both p and s wave impedance and velocity primary lithology and secondary mineralogy have only a small influence on impedance and velocity combined use of p and s wave velocity data discriminates porosity changes from lithologic changes all other variables including pore fluid type and petrographic fabric have no significant influence on velocities laboratory measurements of p wave velocity under simulated in situ conditions reproduce well log velocity values reliably laboratory porosity velocity trends agree with the time average equation when the correct matrix velocities are used rock property results were used to interpret porosity lithology variations for an inverted seismic section from the williston basin where well

second peak in the far eastern pacific sea surface temperature anomaly following strong el nino events 1 the eastern pacific sea surface temperature anomaly ssta associated with el nino usually peaks in december january and february and decays in the ensuing months sometimes however after a strong el nino a second peak in ssta is observed in the far eastern equatorial pacific it is found that an enhanced westerly wind stress anomaly over the equatorial pacific basin usually associated with a meridional swing of south pacific convergence zone spcz precedes the occurrence of the second peak as the equatorial thermocline slope balances with the zonally integrated zonal wind stress the westerly wind stress anomaly induces deepened thermocline and suppresses upwelling in the eastern pacific for the double peak cases on the other hand enhanced latent heat release and reduced shortwave radiation cause a pause in warming between the two peaks as a result a strong el nino usually accompanied by a zonal spcz exhibits distinct double peak characteristics

a new velocity model for clay sand mixtu res1 none of the standard porosity velocity models e g the time average equation raymer s equations is satisfactory for interpreting well logging data over a broad depth range clays in the section are the usual source of the difficulty through the bias and scatter that they introduce into the relationship between porosity and p wave transit time because clays are composed of fine sheet like particles they normally form pores with much smaller aspect ratios than those associated with sand grains this difference in pore geometry provides the key to obtaining more consistent resistivity and sonic log interpretations a velocity model for clay sand mixtures has been developed in terms of the kuster and toksoz effective medium and gassmann theories in this model the total pore space is assumed to consist of two parts 1 pores associated with sand grains and 2 pores associated with clays including bound water the essential feature of the model is the assumption that the geometry of pores associated with sand grains is significantly different from that associated with clays because of this porosity in shales affects elastic compliance differently from porosity in sand stones the predictive power of the model is demonstrated by the agreement between its predictions and laboratory measurements and by its ability to predict sonic logs from other logs over large depth intervals where formations vary from unconsolidated to consolidated sandstones and shales

velocities of compressional and shear waves in limestones carbonate rocks are important hydrocarbon reservoir rocks with complex textures and petrophysical properties porosity and permeability mainly resulting from various diagenetic processes compaction dissolution precipitation cementation etc these complexities make prediction of reservoir characteristics e g porosity and permeability from their seismic properties very difficult to explore the relationship between the seismic petrophysical and geological properties ultrasonic compressional and shear wave velocity measurements were made under a simulated in situ condition of pressure 50 mpa hydrostatic effective pressure at frequencies of approximately 0 85 mhz and 0 7 mhz respectively using a pulse echo method the measurements were made both in vacuum dry and fully saturated conditions in oolitic limestones of the great oolite formation of southern england some of the rocks were fully saturated with oil the acoustic measurements were supplemented by porosity and permeability measurements petrological and pore geometry studies of resin impregnated polished thin sections x ray diffraction analyses and scanning electron microscope studies to investigate submicroscopic textures and micropores it is shown that the compressional and shear wave velocities vp and vs respectively decrease with increasing porosity and that vp decreases approximately twice as fast as vs the systematic differences in pore structures e g the aspect ratio of the limestones produce large residuals in the velocity versus porosity relationship it is demonstrated that the velocity versus porosity relationship can be improved by removing the pore structure dependent variations from the residuals the introduction of water into the pore space decreases the shear moduli of the rocks by about 2 gpa suggesting that there exists a fluid matrix interaction at grain contacts which reduces the rigidity the predicted biot gassmann velocity values are greater than the measured velocity values due to the rock fluid interaction this is not accounted for in the biot gassmann velocity models and velocity dispersion due to a local flow mechanism the velocities predicted by the raymer and time average relationships overestimated the measured velocities even more than the biot model

pore system characterisation in heterogeneous carbonates an alternative approach to widely used rock typing methodologies abstract rock typing is used in carbonate reservoir characterisation to group petrophysically similar rocks for reservoir modelling purposes the focus is often upon bringing together samples with similar porosity permeability and capillary pressure to achieve this rock typing studies are usually conducted primarily on core plugs using routine core analysis data and thin sections however such an approach results in a tendency to focus on grouping samples with consistent petrophysical properties ignoring the geological controls on pore evolution this means that modelling rock types predicatively using 3d geocellular models is not easily accomplished furthermore where geological observations are made they tend to consider only depositional rock properties and not the effects of diagenesis in doing so a key control on the evolution of the carbonate pore network is ignored this paper addresses these issues using a giant carbonate reservoir in northern oman rock types were defined on the basis of pore geometry whilst retaining distinct geological descriptors the aim was to ensure that each rock type could be defined on the basis of both its petrophysical properties and behaviour during hydrocarbon recovery the results show that characterisation of pore heterogeneity is critical to the prediction of flow behaviour under reservoir conditions and that routine petrophysical parameters are often not good indicators of sweep efficiency

estimating permeability of carbonate rocks from porosity and vp vs we present a method for predicting permeability from sonic and density data the method removes the porosity effect on the ratio vp vs of dry rock and it addresses the specific surface as an indirect measure of permeability we look at ultrasonic data porosity and the permeability of 114 carbonate core plugs in doing so we establish an empirical relationship between the specific surface of the solid phase as calculated by kozeny s equation and vp vs linearly transformed to remove the porosity effect one must view the specific surface derived by using kozeny s equation as an effective specific surface because kozeny s equation only holds for homogeneous rock with interconnected pores the ratio vp vs of dry rocks on the other hand seems to be controlled by the true specific surface pointing to an inherent limitation in the method the 114 carbonate plugs originate in three geological settings and comprise 83 calcitic and 31 dolomitic samples their depositional texture varies from mud do

factors controlling elastic properties in carbonate sediments and rocks carbonate sediments are prone to rapid and pervasive diagenetic alterations that change the mineralogy and pore structure within carbonate rocks in particular cementation and dissolution processes continuously modify the pore structure to create or destroy porosity in extreme cases these modifications can completely change the mineralogy from aragonite calcite to dolomite or reverse the pore distribution whereby original grains are dissolved to produce pores as the original pore space is filled with cement to form the rock figure 1 all these modifications alter the elastic properties of the rock and therefore the sonic velocity the result is a dynamic relationship among diagenesis porosity pore type and sonic velocity the result is a wide range of sonic velocity in carbonates in which compressional wave velocity vp ranges from 1700 to 6600 m s and shear wave velocity vs from 600 to 3500 m s

two types of el nino events cold tongue el nino and warm pool el nino abstract in this study two types of el nino events are classified based on spatial patterns of the sea surface temperature sst anomaly one is the cold tongue ct el nino which can be regarded as the conventional el nino and the other the warm pool wp el nino the ct el nino is characterized by relatively large sst anomalies in the nino 3 region 5 s 5 n 150 90 w while the wp el nino is associated with sst anomalies mostly confined to the nino 4 region 5 s 5 n 160 e 150 w in addition spatial patterns of many atmospheric and oceanic variables are also distinctively different for the two types of el nino events furthermore the difference in the transition mechanism between the two types of el nino is clearly identified that is the discharge process of the equatorial heat content associated with the wp el nino is not efficient owing to the spatial structure of sst anomaly as a result it cannot trigger a cold event it is also demonstrated that zonal advective feedback i e zonal adve

evaluation of the south pacific convergence zone in ipcc ar4 climate model simulations of the twentieth century abstract understanding how the south pacific convergence zone spcz may change in the future requires the use of global coupled atmosphere ocean models it is therefore important to evaluate the ability of such models to realistically simulate the spcz the simulation of the spcz in 24 coupled model simulations of the twentieth century is examined the models and simulations are those used for the fourth assessment report ar4 of the intergovernmental panel on climate change ipcc the seasonal climatology and interannual variability of the spcz is evaluated using observed and model precipitation twenty models simulate a distinct spcz while four models merge intertropical convergence zone and spcz precipitation the majority of models simulate an spcz with an overly zonal orientation rather than extending in a diagonal band into the southeast pacific as observed two thirds of models capture the observed meridional displacement of the spcz during el nino and la nina events the four models that use

thickening of young pacific lithosphere from high resolution rayleigh wave tomography a test of the conductive cooling model direct seismic measurements of the thickening of oceanic lithosphere away from the spreading axis are rare due to the remoteness of mid ocean ridges on the east pacific rise at 17 s there were two long term broadband ocean bottom seismometer deployments the melt and glimpse experiments these arrays spanned young pacific plate seafloor ranging in age from 0 to 8 ma combining these two data sets we describe the increase in rayleigh wave phase velocities for 16 100 s period as function of distance from the ridge with two parameterizations an arbitrary function of seafloor age and a simple polynomial function of age on the pacific and nazca plates although resolution analysis shows that 10 to 15 independent pieces of information about the age variation of phase velocity on the pacific plate can be resolved at periods of 50 s the three parameter polynomial model fits the data almost as well indicating a simple pattern of the evolution of structure with age to compare our observations to the predictions for the conductive cooling of the lithosphere we convert a thermal half space cooling model to shear velocity using anharmonic and anelastic contributions the patterns in the velocities are not consistent with simple conductive cooling the conductive cooling model under predicts the changes in phase velocity with age observed in the 20 to 78 s period range we observe significant changes in shear velocity in the asthenosphere at depths greater than conductive cooling should extend the asthenospheric low velocity zone is asymmetric dipping to the west with the lowest velocities beneath the pacific plate west of the spreading center the conductive cooling model also over predicts the shear velocities in the lithosphere by 0 2 km s these anomalies indicate that more complicated mantle flow and significant mantle heterogeneities such as melt are required

abrupt termination of the 1997 98 el nino in response to a madden julian oscillation the role of the madden julian oscillation a global atmospheric wave in the tropics that is associated with convective activity and propagates eastwards with a period of about 30 60 days refs 1 2 in triggering el nino events has been discussed before3 4 5 6 7 8 but its possible connection with a termination of el nino has yet to be investigated despite the difficulty in explaining the timing of el nino terminations by the basic wind induced oceanic wave processes9 10 for the extreme 1997 98 event the mechanism of both onset and termination have been investigated3 but the reason for the abruptness of the termination has yet to be resolved here we present global data of precipitation sea surface temperatures and wind speeds that show a precipitation system associated with an exceptionally strong madden julian oscillation travelling around the equator in may 1998 the propagation of this atmospheric system was associated with an abrupt intensification of the easterly trade winds over the eastern equatorial pacific ocean combined with the already shallow equatorial thermocline in the central and eastern pacific ocean at that time3 these strong winds provided the triggering mechanism for the observed accelerated ending of the 1997 98 el nino event

contrasting the termination of moderate and extreme el nino events in coupled general circulation models as in the observed record the termination of el nino in the coupled ipcc ar4 climate models involves meridional processes tied to the seasonal cycle these meridional processes both precondition the termination of el nino events in general and lead to a peculiar termination of extreme el nino events such as those of 1982 83 and 1997 98 in which the eastern equatorial pacific warm sea surface temperature anomalies ssta persist well into boreal spring early summer the mechanisms controlling the peculiar termination of extreme el nino events which involves to the development of an equatorially centred intertropical convergence zone are consistent across the four models that exhibit extreme el ninos and observational record suggesting that this peculiar termination represents a general feature of extreme el ninos further due to their unusual termination extreme el ninos exhibit an apparent eastward propagation of their ssta which can strongly influence estimates of the apparent propagation of enso over multi decadal periods interpreting these propagation changes as evidence of changes in the underlying dynamical feedbacks behind el nino could therefore be misleading given the strong influence of a single extreme event

quantifying flow in variably wet microporous carbonates using object based geological modeling and both lattice boltzmann and pore network fluid flow simulations abstract micropores can constitute up to 100 of the total porosity within carbonate hosted hydrocarbon reservoirs usually existing within micritic fabrics here three dimensional computational representations of end point micritic fabrics are produced using a flexible object based algorithm to further our understanding of the contribution that micropores make to flow by methodically altering model parameters we explore the state space of microporous carbonates quantifying single and multiphase flow using lattice boltzmann and network models in purely micritic fabrics average pore radius ranging from 0 26 to 0 44 m was found to have a positive correlation with single phase permeability 1 7 to 2 7 md respectively similarly increasing average pore size resulted in decreasing residual oil saturation under both water wet and 50 fractionally oil wet states permeability was found to increase by an order of magnitude from 0 6 to 7 5 md within fabrics of varying total matrix porosity from 18 to 35 because of increasing pore size 0 37 to 0 56 m respectively but minimal effect on multiphase flow was observed increased pore size due to micrite rounding notably increases permeability in comparison with original rhombic fabrics with the same porosity but multiphase flow properties are unaffected finally when moldic mesopores are added to a micritic matrix they impact flow when directly connected otherwise micropores control single phase permeability magnitude importantly recovery is dependent on both wetting scenario and pore network homogeneity under water wet imbibition increasing proportions of microporosity yield lower residual oil saturations together these results quantify the importance of micropores in contributing to or controlling overall flow and sweep characteristics in such fabrics

quantification of pore structure and its effect on sonic velocity and permeability in carbonates carbonate rocks commonly contain a variety of pore types that can vary in size over several orders of magnitude traditional pore type classifications describe these pore structures but are inadequate for correlations to the rock s physical properties we introduce a digital image analysis dia method that produces quantitative pore space parameters which can be linked to physical properties in carbonates in particular sonic velocity and permeability the dia parameters derived from thin sections capture two dimensional pore size domsize roundness aspect ratio ar and pore network complexity poa comparing these dia parameters to porosity permeability and p wave velocity shows that in addition to porosity the combined effect of microporosity the pore network complexity and pore size of the macropores is most influential for the acoustic behavior combining these parameters with porosity improves the coefficient of determination r2 velocity estimates from 0 542 to 0 840 the analysis shows that samples with large simple pores and a small amount of microporosity display higher acoustic velocity at a given porosity than samples with small complicated pores estimates of permeability from porosity alone are very ineffective r2 0 143 but can be improved when pore geometry information poa r2 0 415 and domsize r2 0 383 are incorporated furthermore results from the correlation of dia parameters to acoustic data reveal that 1 intergrain and or intercrystalline and separate vug porosity cannot always be separated using sonic logs 2 p wave velocity is not solely controlled by the percentage of spherical porosity and 3 quantitative pore geometry characteristics can be estimated from acoustic data and used to improve permeability estimates

modelled rainfall response to strong el nino sea surface temperature anomalies in the tropical pacific abstractel nino southern oscillation strongly influences the interannual variability of rainfall over the pacific shifting the position and orientation of the south pacific convergence zone spcz and intertropical convergence zone itcz in 1982 83 and 1997 98 very strong el nino events occurred during which time the spcz and itcz merged into a single zonal convergence zone szcz extending across the pacific at approximately 5 s the sea surface temperature anomalies sstas reached very large values and peaked farther east compared to other el nino events previous work shows that tropical pacific precipitation responds nonlinearly to changing the amplitude of the el nino ssta even if the structure of the ssta remains unchanged but large canonical el nino sstas cannot reproduce the szcz precipitation pattern this study conducts idealized sst forced experiments starting with a large amplitude canonical el nino ssta and gradually adding a residual pattern until the full 1982 83 and 1997 98 me

permeability and acoustic velocity controlling factors determined from x ray tomography images of carbonate rocks carbonate reservoir rocks exhibit a great variability in texture that directly impacts petrophysical parameters many exhibit bi and multimodal pore networks with pores ranging from less than 1 m to several millimeters in diameter furthermore many pore systems are too large to be captured by routine core analysis and well logs average total porosity over different volumes consequently prediction of carbonate properties from seismic data and log interpretation is still a challenge in particular amplitude versus offset classification systems developed for clastic rocks which are dominated by connected intergranular unimodal pore networks are not applicable to carbonate rocks pore geometrical parameters derived from digital image analysis dia of thin sections were recently used to improve the coefficient of determination of velocity and permeability versus porosity although this substantially improved the coefficient of determination no spatial information of the pore space was considered because dia parameters were obtained from two dimensional analyses here we propose a methodology to link local and global pore space parameters obtained from three dimensional 3 d images to experimental physical properties of carbonate rocks to improve p wave velocity and permeability predictions results show that applying a combination of porosity microporosity and 3 d geometrical parameters to p wave velocity significantly improves the adjusted coefficient of determination from 0 490 to 0 962 a substantial improvement is also observed in permeability prediction from 0 668 to 0 948 both results can be interpreted to reflect a pore geometrical control and pore size control on p wave velocity and permeability

the velocity deviation log a tool to predict pore type and permeability trends in carbonate drill holes from sonic and porosity or density logs the velocity deviation log which is calculated by combining the sonic log with the neutron porosity or density log provides a tool to obtain downhole information on the predominant pore type in carbonates the log can be used to trace the downhole distribution of diagenetic processes and to estimate trends in permeability laboratory measurements on over 300 discrete carbonate samples reveal that sonic velocity is a function not only of total porosity but also of the predominant pore type in general there is an inverse porosity velocity correlation but significant deviations occur from this relationship for certain pore types frame forming pore types such as moldic or intrafossil porosity result in significantly higher velocity values at equal total porosities than do pore types that are not embedded in a rigid rock frame such as interparticle porosity or microporosity the results of the laboratory measurements can be applied to expand interpretations of standard wireline log data as shown in this study on two drill holes through neogene carbonates from the great bahama bank the velocity deviation log is calculated by first converting porosity log data to a synthetic velocity log using a time average equation the difference between the real sonic log and the synthetic sonic log can then be plotted as a velocity deviation log because deviations are the result of the variability of velocity at a certain porosity the deviation log reflects the different rock physical signatures of the different pore types positive velocity deviations mark zones where velocity is higher than expected from the porosity values such as zones where frame forming pore types dominate zero deviations show intervals where the rock lacks a rigid frame such as in carbonates with high interparticle porosity or microporosity negative deviations mark zones in which sonic log velocities are unusually low caused for instance by a cavernous bore hole wall fracturing or possibly by a high content of free gas by tracing the velocity deviations continuously downhole one can identify diagenetic zones that are characterized by these different pore types in addition this method can be used to observe permeability trends because pore types influence the permeability of the rock