prediction of lateral variability in fracture intensity using multicomponent shear wave surface seismic as a precursor to horizontal drilling in the austin chalk summary a method of multicomponent multisource shear wave interpretation utilizing the output of the alford rotation for separating the fast and slow particle motion stacks in azimuthally anisotropic media yielded a qualitative prediction of lateral fracture intensity variation prior to horizontal drilling the method exploits a fundamental difference between the two particle motion stacks when vertical aligned fractures are the source of the shear wave splitting the fast shear wave s1 section is insensitive to the lateral variation in fracture intensity since its particle motion is parallel to the fractures hence reflectivity on the s1 cdp stacked section should be likewise insensitive to fractures the slow shear wave s2 section however possesses particle motion perpendicular to the fracture planes so variation in lateral fracture intensity will affect velocity and reflectivity examination of the reflectivity on the two sections for a seismic line reveals laterally consistent reflector strength on the s1 section for the austin chalk the horizon of interest but laterally inconsistent reflector strength for the same horizon on the s2 section in the study area reflector dimming on the s2 section was interpreted as indicating fracture intensity maxima horizontal drilling confirmed both fracture azimuth and intensity predictions and resulted in significantly higher initial hydrocarbon production than for surrounding wells

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

new scaling of fractures in a giant carbonate platform from outcrops and subsurface abstract we scale properties of fractures in the late kimmeridgian carbonate platform of the arabian shelf from outcrop and subsurface strata the platform is flat lying undeformed except from low relief anticlines and domes at which the strata hosts the world s most prolific petroleum reserves the strata exhibit stylolites and veins besides the dominant fractures the fractures are opening mode steep and forming major sets of nw se and ne sw orientations faults are uncommon fracture abundance varies significantly across the platform but it is constrained spatially by inherent variation in mechanical stratigraphy and temporally by anticlinal growth and isotope geochemistry in early burial stages the platform strata were slightly deformed by bed parallel stylolites and mutually crosscutting veins the strata fracturing increased by the anticlinal growth in the late cretaceous fracture spacing is an average of 2 m in the horizontal bedded outcrops in central arabia and 0 30 m in the equivalent subsurface in a dome in eastern arabia the dome is an end member in the deformation spectrum of the platform interior with up to c 5 structural dip and 2 dilational strain expressed in the doming and a few major faults this finding indicates that carbonate platforms can reach fracture oversaturation under such a subtle tectonic extension

thermo chronological approach to reservoir diagenesis in the offshore angola basin a fluid inclusion 40 ar 39 ar and k ar investigation 1 microthermometric analyses of fluid inclusions and geochronological surveys of authigenic silicates in neocomian sandstones and albian dolomitic reservoirs from the offshore angola basin have allowed determination of the age and the physical chemical conditions for fluid migration in a late diagenetic stage the 40 ar 39 ar laser probe dating of adularia overgrowths from the albian reservoirs shows that the trapping of primary aqueous and oil inclusions occurred during the early miocene 25 6 or 3 2 ma the k ar age of authigenic illites from the neocomian sandstones is slightly younger 19 or 1 ma and the crystallization of regular illite smectite mixed layers is related to a previous eocene to paleocene diagenetic stage 51 58 ma there appears to have bee both a regional and vertical variation in the salinity of aqueous fluids during the miocene the fluid inclusion trapping temperatures were estimated by correcting the homogenization temperatures of aqueous fluids assuming that pressures during the miocene were close to the present reservoir pressures and by intersecting the isochores of aqueous and oil inclusions assuming they were trapped at strictly similar pressure and temperature conditions the fluid inclusion temperatures are consistent with those derived from the tetrahedral aluminum content aliv of authigenic chlorites 168 206 degrees c in the albian reservoirs at depths between 2350 and 3580 m and 175 237 degrees c in the neocomian sandstones at depths between 2770 and 4470 m in both reservoir formations miocene tempe atures were higher than the present ones and related either to deeper burial or higher geothermal gradient

transport and deposition of plutonium in the ocean evidence from gulf of mexico sediments abstract a study of sediments in the gulf of mexico shows dramatic gradients in pu content and isotope ratios from the continental shelf to the sigsbee abyssal plain in terms of predicted direct fallout inventory of pu one shelf core contains 745 of the predicted inventory while abyssal plain sediments contain only 15 20 of the predicted value absolute pu concentrations of shelf sediments are also conspicuously high up to 110 dpm kg compared to 13 5 dpm kg in mississippi river suspended sediment there is no evidence of pu remobilization in gulf of mexico shelf sediments based on comparison of pu profiles with mn al and fe al profiles horizontal transport of fallout nuclides from the open ocean to removal sites in ocean margin sediments is concluded to be the source of both the high concentrations and high inventories of pu reported here the shelf sediments show 240 pu 239 pu ratios close to 0 179 the average stratospheric fallout value but the ratios decrease progressively across the gulf to low values of 0 06 in abyssal plain sediments the source of low ratio pu in deep water sediments may be debris from low yield tests transported in the troposphere alternatively it may represent a fraction of the pu from global stratospheric fallout which has been separated in the water column from the remainder of the pu in the ocean in either case the low ratio material must have been removed rapidly to the sea floor where it composes a major fraction of the pu in abyssal plain sediments pu delivered by global atmospheric fallout from the stratosphere has apparently remained for the most part in the water or has been transported horizontally and removed into shallow water sediments

fluid inclusion evidence of petroleum migration through a buried metamorphic dome in the pannonian basin hungary some of the petroleum currently produced in the pannonian basin in hungary stems from buried metamorphic basement domes that form structural highs between neogene sedimentary sub basins in the bekes sedimentary sub basin the source rocks of the petroleum are known to be of middle to late miocene age whereas at least part of the petroleum in the adjacent szeghalom basement dome appears to be of lower miocene age or even older virtually nothing is known about the compositions distribution sources and migration history of this older petroleum although such variables are crucial to future exploration strategies this study addresses the first two of these gaps in knowledge by providing a basic characterization of the older petroleum trapped as fluid inclusions in fracture hosted quartz crystals within the szeghalom dome szd fluid inclusion samples from 9 wells at depths of 1900 to 2200 m were analysed by visible and cie calibrated uv microscopy microthermometry laser raman and 1 h nmr spectroscopy two major types of early miocene oils were found distributed over 5 distinct temporal generations an earlier immature brown oil succeeded by a light gas condensate the early petroleum was undersaturated with respect to methane gas and it migrated through the fracture network at temperatures between about 160 to 130 c entraining blebs of bitumen and flushed by weakly saline water the later gas condensate migration consisted of a heterogeneous mixture of volatile oil methane rich gas weakly saline water at slightly lower temperatures between 135 and 120 c the temperature evolution was caused by cooling of the metamorphic wall rocks during basement uplift culminating in middle miocene exhumation the various hydrocarbons are unevenly distributed across the szeghalom dome showing that pre middle miocene fluid migration through the fracture network was compartmentalized at the km scale 2007 elsevier b v all rights reserved

hydrodynamic effects on mission canyon mississippian oil accumulations billings nose area north dakota mission canyon oil production on the south flank of the williston basin provides an example of an area in the mature stage of exploration that shows significant hydrodynamic effects on oil accumulations related to stratigraphic traps the effects are illustrated by the billings nose fields and the elkhorn ranch field the reservoirs have low hydraulic gradients of about 2 m km 10 ft mi tilted oil water contacts with gradients of 5 m km 25 ft mi and variable formation water salinities that range from brackish to highly saline oil accumulations in some zones are displaced off structure and downdip to the northeast parallel to porosity pinch outs other zones are pure hydrodynamic traps lacking both structural and stratigraphic closure future success in exploration and development in the play will depend on recognizing the hydrodynamic effects and predicting oil displacement

is there a genetic relationship between selected regional joints and contemporary stress within the lithosphere of north america on the appalachian plateau of new york ene striking joints parallel the mid continent contemporary stress field and do not correlate with other structures joints within ohio indiana michigan and west virginia also parallel the mid continent maximum horizontal compression as indicated by hydraulic fracture measurements in northern indiana where the contemporary stress field deviates significantly from the orientation of the mid continent stress the attitude of the joints deviates accordingly in addition to joints structures on several scales from subcontinent size fracture systems to microfabrics in shales apparently parallel the mid continent stress field from this i conclude that these joints are mode i cracks whose orientation corresponds to the contemporary tectonic stress field in itself the formation of these structures constitutes a tectonic event that has often been overlooked in favor of tectonic events that produce the more spectacular but less pervasive faults and folds

petroleum inclusions in sedimentary basins systematics analytical methods and applications abstract fluid dynamics in sedimentary basins is of tremendous interest both from a scientific and an economic point of view integration between fluid inclusion and present day fluid data provides the time aspect necessary for reconstruction of fluid flow paths and can be used for mapping fluid dynamics both on a regional basin scale or on the more local scale of petroleum reservoirs this paper presents a review of analytical and modelling methods for petroleum in fluid inclusions essentially four types of data for petroleum inclusions can be established through analysis and modelling 1 textures which give indirect information of the time aspect 2 fluid composition 3 fluid properties and 4 pressure temperature of trapping during the last decade development of analytical methods for determination of inclusion fluid compositions has taken place traditional correlation studies for characterisation of maturity and source facies may now therefore include fluid inclusion data the development has also been directed towards an improved understanding of the physical properties of petroleum inclusions although these methods are in their early stages concerning precision and accuracy data necessary for fluid flow modelling such as fluid densities viscosities pressure and temperature can be estimated

successive episodes of normal faulting and fracturing resulting from progressive extension during the uplift of the holy cross mountains poland conjugate normal faults extension fractures and mesh fracture structures were investigated in devonian carbonate rocks from the southern part of the holy cross mountains hcm central poland strata folded during variscan deformations were later subject to uplift resulting in increasing extension in the upper part of the rock mass at a relatively shallow depth faults and fractures developed in an orderly vertical succession first mesh fracture structures and conjugate normal fault sets enclosing acute dihedral angles 2 of over 45 were formed next conjugate normal faults with 2 less than 45 and sub vertical extension fractures developed the occurrences of conjugate normal sets enclosing different dihedral acute angles and extension fractures with similar strikes juxtaposed with each other at the same stratigraphic level point to the fact that the uplifted rock mass underwent successive changes in a stress regime leading to the formation of these structures the first sets of conjugate normal faults and fractures developed when the hcm were uplifted during a late stage of variscan deformations the next sets of extension fractures and conjugate normal faults developed during the following uplift events interrupted by periods of sedimentation of the mesozoic and younger strata

origin of reservoir fractures in little knife field north dakota thin vertical planar fractures observed in the mission canyon formation at the little knife field are naturally occurring and appear to be extension fractures the predominant east west trend of the fractures measured in oriented core from six wells parallels the contemporary maximum horizontal compressive stress in the williston basin the fractures occur only in carbonate units but within the carbonates their occurrence is not lithology dependent fracture density measured in cores of the reservoir carbonates averages 1 ft 3 m of fracture per 2 3 ft 7 m of core the formation and mineralization of reservoir fractures were the most recent diagenetic events in the mission canyon formation at little knife study of aqueous and hydrocarbon fluid inclusions associated with the fractures reveals 1 fractures formed after the strata were buried to at least their present depth of 9 800 ft 2 987 m which indicates their age is post mesozoic 2 the pore fluid pressure gradient was normal hydrostatic immediately after if not during fracture system development 3 formation water salinity has remained fairly constant since fracture initiation 4 migration of hydrocarbons into the reservoir probably preceded fracture genesis and 5 methane concentration may have decreased since fracture initiation

synsedimentary faulting in a mesozoic deltaic sequence svalbard arctic norway fault geometries faulting mechanisms and sealing properties distributary channel sandstones of the lower cretaceous helvetiafjellet formation and underlying prodelta shales and thin bedded sandstones of the upper jurassic janusfjellet formation exposed along the east coast of spitsbergen are cut by syndepositional planar and listric faults forming collapse scars with depths of 35 90 m and widths up to 1 5 km the fault zones mostly have a dip of 40 60 a width of up to 2 3 m and contain fault parallel 3 5 m long overlapping sandstone sheets with widths of 5 40 cm and up to 2 m thick fault parallel sandy mudstones the intrafault sandstones show fault parallel banding resulting from differences in detrital clay content and grain size the banding has been enhanced by selective late diagenetic quartz cementation of the clay poor bands thin clay laminae now developed into fault parallel stylolites occur along the margins of the intrafault sandstones the clay laminae do not emerge from clay layers in the fault blocks and are not clay smears the laminae probably formed during faulting when fluidization within the fault zones allowed clay particles to move laterally and accumulate along the margins of the fault zones there is no enhanced cementation or cataclastic deformation within the fault zones the ability of the fault zones to act as capillary seals or barriers to fluid flow is therefore mostly determined by the clay laminae rimming the intrafault sandstones

fluid inclusion constraints on petroleum pvt and compositional history of the greater alwyn south brent petroleum system northern north sea abstract pressure volume temperature pvt simulation of individual petroleum inclusions that are trapped in source rocks migration pathways and reservoirs is when combined with standard inclusion analytical techniques a useful tool for the quantitative estimation of paleopressures paleotemperatures and changes in petroleum compositions over time this study presents the results from the application of these techniques to determine the evolution of the petroleum trapped through time as maturation and migration progress within the greater alwyn south brent petroleum system of the northern north sea the results for the brent reservoir from the six study fields indicate that the petroleum inclusions recorded a wide range of compositions during the filling history of the reservoir the methane concentrations and gas oil ratios of included petroleum are lower than those presently within the reservoir the fluid inclusion data suggest that changes in petroleum compositions through time can be interpreted in terms of gradual mixing of earlier reservoired oil with later migrated more mature and gas rich fluids fields nearest the kitchen area appear to have been more severely affected by the emplacement of increasingly mature petroleum phase separation during vertical migration may also occur but is thought to be a volumetrically minor process the estimated paleopressures mostly correspond to the current pressure regime which is slightly higher than hydrostatic and lower than lithostatic combining fluid inclusion results with thermal burial models suggests that petroleum migration began at approximately 55 64 ma in the area this study provides quantitative limits on and confirms the value of fluid inclusion technology in such applications as evaluating oil compositions migration timing trap fill history paleopressures and in constraining the outputs of basin modeling

a method for estimating subsurface fracture density in core quantitative information on fracture density may be used to delineate areas structures or lithologies that have a relatively high density of subsurface fractures a quantitative estimate of subsurface fracture density can be obtained from core data previously no unequivocal technique existed for estimating the degree of subsurface fracturing from oil well core data the spacing of joints at outcrops is commonly observed to be a direct linear function of the thickness of the jointed bed our method for estimating subsurface fracture density is a statistical estimate based on this empirical relation the probability of a given number of core fracture intersections is determined for a range of possible fracture densities after accounting for core diameter variations bed thickness geometric corrections necessitated by bedding dip and variations in fracture pitch the true fracture density is then estimated by finding the fracture density value that yields a calculated number of fractured beds equal to the observed number of fractured beds the technique was tested by measuring fracture density in the field at 2 outcrops hypothetical cores were constructed on photographs of the outcrops and estimates of fracture density from the core data yielded values in agreement with those obtained by field measurement the core data required for estimation of fracture density are core diameter bed thickness number of beds fractured and unfractured and the angles between core axis bedding and fractures the mathematics should be programmed on a computer for ease of use

the geochemistry of fallout plutonium in the north atlantic ii ratios and their significance a systematic decrease in the 240pu239pu ratio in marine sediments is found with increasing water depth along a transect of cores between woods hole and bermuda the 240pu239pu atom ratios range from o 18 on the shelf to o 10 at 5000 m but do not change with depth in individual cores a model is presented which can account for the range of 240pu239pu ratios found in this and other similar studies noshkin and gatrousis 1974 scottet al 1983 we propose that there have been at least two distinct sources of fallout pu to this region the major source of pu is global stratospheric fallout characterized by a 240pu239pu ratio of 0 18 and a relatively long residence time in seawater the second source is characterized by a much lower 240pu239pu ratio and relative to global fallout it must have been much more efficiently removed from the water column to deep sea sediments we suggest that surface based low yield testing at the nevada test site is the only source of low ratio fallout pu which could account for the timing inventories and refractory characteristics of this second component of fallout pu inputs to the north atlantic

why joints are more abundant than faults a conceptual model to estimate their ratio in layered carbonate rocks it is a commonplace field observation that extension fractures are more abundant than shear fractures the questions of how much more abundant and why are posed in this paper and qualitative estimates of their ratio within a rock volume are made on the basis of field observations and mechanical considerations a conceptual model is also proposed to explain the common range of ratios between extension and shear fractures here called the j f ratio the model considers three major genetic stress components originated from overburden pore fluid pressure and tectonics and assumes that some of the remote genetic stress components vary with time i e stress rates are included other important assumptions of the numerical model are that i the strength of the sub volumes is randomly attributed following a weibull probabilistic distribution ii all fractures heal after a given time thus simulating the cementation process and therefore iii both extensional jointing and shear fracturing could be recurrent events within the same sub volume as a direct consequence of these assumptions the stress tensor at any point varies continuously in time and these variations are caused by both remote stresses and local stress drops associated with in situ and neighbouring fracturing events the conceptual model is implemented in a computer program to simulate layered carbonate rock bodies undergoing brittle deformation the numerical results are obtained by varying the principal parameters like depth viz confining pressure tensile strength pore fluid pressure and shape of the weibull distribution function in a wide range of values therefore simulating a broad spectrum of possible mechanical and lithological conditions the quantitative estimates of the j f ratio confirm the general predominance of extensional failure events during brittle deformation in shallow crustal rocks and provide useful insights for better understanding the role played by the different parameters for example as a general trend it is observed that the j f ratio is inversely proportional to depth viz confining pressure and directly proportional to pore fluid pressure while the stronger is the rock the wider is the range of depths showing a finite value of the j f ratio and in general the deeper are the conditions where extension fractures can form moreover the wider is the strength variability of rocks i e the lower is the m parameter of the weibull probabilistic distribution function the wider is the depth range where both fractures can form providing a finite value of the j f ratio natural case studies from different geological and tectonic settings are also used to test the conceptual model and the numerical results showing a good agreement between measured and predicted j f ratios

petroleum and aqueous inclusions from deeply buried reservoirs experimental simulations and consequences for overpressure estimates abstract synthetic hydrocarbon and aqueous inclusions have been created in the laboratory batch reactors in order to mimic inclusion formation or re equilibration in deeply buried reservoirs inclusions were synthesized in quartz and calcite using pure water and mexican dead oil or n tetradecane c14h30 at a temperature and pressure of 150 c and 1 kbar one phase hydrocarbon inclusions are frequently observed at standard laboratory conditions leading to homogenization temperatures between 0 and 60 c uv epifluorescence of mexican oil inclusions is not uniform blue and green yellow colored inclusions coexist however no clear evidence of variations in fluid chemistry were observed homogenization temperatures were recorded and the maxima of th plotted on histograms are in good agreement with expected th in a range of 6 c broad histograms were reconstructed showing non symmetrical th distributions over a 20 c temperature range centered on the expected th this histogram broadening is due to the fragility of the fluid inclusions that were created by re filling of pre existing microcavities such th histograms are similar to th histograms recorded on natural samples from deeply buried carbonate reservoirs th values lower than those expected were measured for hydrocarbon inclusions in quartz and calcite and for aqueous inclusions in calcite however the results confirm the ability of fluid inclusions containing two immiscible fluids to lead to pt reconstructions even in overpressured environments

near conservative behavior of 129i in the orange county aquifer system california abstract iodine is a biophilic element with one stable isotope 127 i and one long lived radioisotope 129 i radioiodine originates in the surface environment almost entirely from anthropogenic activities such as nuclear fuel reprocessing in europe and thus provides a unique point source tracer very few studies have evaluated the geochemical behavior of i isotopes in the subsurface in this study the concentrations of 129 i and 127 i were measured in wells fed by a series of artificial recharge ponds in the forebay area of the orange county ground water basin california usa to evaluate their potential use as hydrological tracers to substantiate interpretation of 129 i and 127 i concentration data the aquifer system was evaluated using the literature values of aquifer water mass age based on 3 h 3 he xe and 18 o tracer data the aquifer data demonstrate the nearly conservative behavior of 129 i with 129 i 127 i ratios likely reflecting variations in source functions as well as climatic conditions and with inferred particle water partition coefficients k d of 0 1 cm 3 g 1 or less

129i in gulf of mexico waters abstract nuclear weaponry and power generation have released far more 129 i onto the earth s surface than was formerly present the effects of this release are clearly visible in the waters of the gulf of mexico far from the major sources new 129 i is found down to at least 500 m and probably to 1000 m surface values are approximately 60 times the pre nuclear era levels the 129 i penetration depth of 470 m agrees well with that determined from bomb fallout isotopes 3 h and 14 c during the 1970s and 1980s in the gulf of mexico and in the western north atlantic at similar latitudes the observed levels exceed amounts expected from globally distributed nuclear test fallout the observed surplus must come primarily from spent fuel reprocessing plants in the eastern north atlantic followed by rapid circulation of the north atlantic gyre and subsequent delivery into the caribbean sea and thence into the gulf of mexico airborne transport also appears significant

individual characterization of petroleum fluid inclusions composition and p t trapping conditions by microthermometry and confocal laser scanning microscopy inferences from applied thermodynamics of oils abstract this paper proposes a new method to characterize individual oil bearing fluid inclusions it uses both the homogenisation temperatures measured by microthermometry and the degree of gas bubble filling measured by confocal laser scanning microscopy in conjunction with thermodynamic modelling for describing liquid gas phase transitions and the volumetric behaviour of hydrocarbon mixtures it is associated with a two parameter compositional model that describes the wide range of compositions of petroleums we show that this method can give 1 useful estimations of the compositions and pressure temperature entrapment conditions of oils in fluid inclusions and 2 insights into the various processes that have affected these fluids either before entrapment liquid gas unmixing gas leaching mixing etc or after leakage etc

paleotemperatures from fluid inclusions advances in theory and technique fluid inclusions in diagenetic minerals can be used to determine paleotemperatures three sets of observations are necessary to make accurate interpretations 1 detailed petrography to establish the relative time of formation of the inclusions 2 careful analysis of the burial and tectonic history of the host rocks to tie diagenesis to the geologic history of the basin and finally 3 analysis of the phase behavior and chemical composition of individual inclusions to define the pressure volume temperature pvt properties of the trapped fluids

determination of natural and anthropogenic 129i in marine sediments sup 129 i concentrations in two sediment cores from the continental slope near cape hatteras were determined with tandem accelerator mass spectrometry tams the natural sup 129 i sup 127 i ratios in these cores are approximately 10 sup 12 the input of anthropogenic sup 129 i has increased this ratio to values close to 10 sup 10 at the surfacee of the sediments the observed natural level of sup 129 i is about two orders to magnitude above the current detection limit with tams which opens up to possibility for applications of sup 129 i in the marine system such as dating of sediments and crude oils and tracing of slow water movement in sediments

determining contemporary stress directions from neotectonic joint systems a neotectonic joint is a crack which propagated in a tectonic stress field that has persisted with little or no change of orientation until the present day investigating neotectonic joints is of value because the approximate orientation of the contemporary stress field can be inferred from them although exposed neotectonic joints in the flat lying sedimentary rocks of some cratons are related to regional stress fields their initiation and propagation occurred close to the earth s surface for example neotectonic joints in the centre of the ebro basin n spain preferentially developed in a thin near surface channel sited within a sequence of weak miocene limestones underlying the upper levels of plateaux the ebro basin joints strike uniformly nnw sse throughout an area of at least 10000 km2 and they are parallel or subparallel to the direction of greatest horizontal stress extrapolated from in situ stress measurements and fault plane solutions of earthquakes 1 preamble this paper has three aims firstly to review the general attributes of exposed neotectonic joints secondly to demonstrate that in s england n france and the ebro basin spain the directions of greatest horizontal principal stress that were inferred from neotectonic joints are parallel to those determined later from geophysical observations and thirdly to discuss why neotectonic joints in the ebro basin are better developed in weak limestones cropping out in the upper parts of plateaux in this paper the word neotectonic is used to indicate that a fracture was propagated in a tectonic stress field that has persisted with little or no change of orientation until the present day hancock engelder 1989 a fracture is called a joint if at the scale of observation possible in the field it is barren and there is no measurable offset related to shear dilation or pressure solution because joints are the most abundant of non penetrative geological structures they are of great potential value for tracking the orientations of principal stress axes at the time of failure confidence in the value of joints as stress indicators is greatest where they are uniformly arranged throughout a large area of flat lying rocks

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

variations in 129i 127i ratios in recent marine sediments evidence for a fossil organic component abstract the value of the pre bomb 129 i 127 i ratio in the hydrosphere is important for the use of this isotope system for dating and tracing purposes in order to determine this value and to examine spatial variability 129 i 127 i ratios were measured in 25 sediment samples from five cores taken at geographically distinct coastal areas the results indicate a pre anthropogenic 129 i 127 i ratio of 1500 10 15 in good agreement with values derived from previous sediment and groundwater studies results from shallow sediments show that the layer of bioturbation is dominated by the addition of anthropogenic 129 i the good agreement between results for this layer from different cores supports the understanding that marine iodine is isotopically homogeneous and associated with organic material which is actively remineralized at the sediment water interface lower ratios down to 354 10 15 corresponding to an age of 32 5 ma and larger variations than expected were found in sections of the cores below the layer of bioturbation these observations can be explained by the presence of refractory fossil organic material transported by rivers to the sediments from uplifted continental shales the results suggest that marine iodine is present in two forms a labile component which is isotopically homogeneous and a refractory component which is associated with kerogen or other fossil organic compounds whose 129 i 127 i ratio reflects the age of these compounds 129 i 127 i ratios can be used to identify and date fossil organic material in recent sediments the presence of which can alter the assumptions underlying models of the global carbon cycle

scaling relationships of joint and vein arrays from the burren co clare ireland abstract we present a study of the systematics of veins and joints in carboniferous limestones of the burren ireland scaling relationships were established for fracture arrays mapped from low elevation aerial photographs that image fractures on numerous limestone pavements for areas up to ca 1 km2 the veins and joints occur in the same sequence but have contrasting scaling properties the veins strike north south and cut many beds to form vertically persistent non stratabound arrays they are strongly clustered and have scale invariant geometric properties vein geometries suggest they grew sub critically under relatively high differential stresses during north south directed variscan compression the joints form stratabound arrays with regular spacings that scale with bed thickness they show greater strike variation than the veins and have lognormal length distributions the joints formed during uplift under low differential stress conditions the contrasting scaling properties of the joints and veins are attributed to different overburden stresses at the time of formation the veins formed at greater depths than the joints in conditions that favoured fracture propagation across mechanical discontinuities resulting in the development of non stratabound scaling properties

regional fractures i a mechanism for the formation of regional fractures at depth in flat lying reservoirs 1 regional fractures may form slowly at depth in extension and parallel to regional horizontal tectonic compression such fracture sets can be common in reservoirs despite the absence of flexure of the strata and are commonly important contributors to reservoir permeability such fractures make good targets for deviated wells high formation pore pressure is crucial to creating low effective confining stresses and brittle rock properties in the reservoirs conditions conducive to fracturing despite deep burial but pore pressure does not exceed the least principal stress and does not cause tensile fracturing the mechanism of natural hydraulic fracturing is incompatible with the characteristics of most regional fracture sets rather many regional fractures form duri g far field compression initiate at locally induced tensile stresses caused by flaws in the rock and propagate in the plane of the maximum and intermediate compressive stresses the differential stress required for initiation and propagation of regional fractures is well below that necessary for shear failure in the presence of high pore pressure fractures can be held open at depth by a tectonic basinwide dilatancy of the strata and the open void space commonly becomes mineralized

the application of fluid inclusions to migration of oil and diagenesis in petroleum reservoirs abstract fluid inclusions can be used to interpret thermal history and petroleum maturation and migration relative to burial history temperature pressure and composition data collected from fluid inclusions are used to determine the environment of diagenesis and the timing of cementation and migration cements in petroleum reservoirs contain both oil and aqueous fluid inclusions fluorescence spectroscopy is used to identify oil inclusions and to determine the maturity of entrapped oil the lifetime of fluorescence induced by a pulsed laser is related to the api gravity of entrapped oil interpretation of fluid inclusion data depends on knowing the origin of fluid inclusions and the probability that they survive in the burial environment those aspects of fluid inclusion study are investigated by synthesizing oil and aqueous inclusions in calcite crystals in laboratory experiments examples of how fluid inclusions are used to determine the physico chemical environments of diagenesis in petroleum reservoirs and the timing of cementation and migration are given for the wealden basin england the mishrif formation dubai the smackover formation gulf coast u s a and jurassic sandstones offshore norway in the wealden basin temperature data from fluid inclusions are used to determine that oil migration occurred in the cretaceous and that the reservoir rocks have been uplifted to varying degrees at a time after migration distribution of oil inclusions indicates that generation and migration of oil was principally in the western part of the basin the geochemistry of oil inclusions in calcite cements from the mishrif formation dubai are used to determine the type and maturity of entrapped oil temperature data from oil and water inclusions are used to relate reservoir diagenesis to burial history and the migration of oil in deep smackover reservoirs oil contains h 2 s the origin of the h 2 s is examined by study of fluid inclusions containing h 2 s in jurassic sandstones offshore norway fluid inclusion studies show that silica cementation is related to burial depth whereas a later calcite cementation originated from invasion of a hot fluid

combined use of confocal laser scanning microscopyand pvt simulation for estimating the composition andphysical properties of petroleum in fluid inclusions we present a method to determine the composition and pvt properties of petroleum inindividual petroleum fluid inclusions confocal laser scanning microscopy is used to generatethree dimensional images of single petroleum inclusions because liquid petroleum fluorescesunder the laser the images readily distinguish the liquid and vapour within the inclusion and canbe used to determine the inclusions volumetric liquid vapour ratio using pvt modellingsoftware the liquid vapour ratio is used along with the homogenisation temperature to determinethe bulk composition phase envelope isochore and a range of physical properties of the includedpetroleum this is done using an iterative series of pvt calculations which match twoparameters 1 the molar volume of the petroleum at room and homogenisation temperatures 2 the liquid vapour ratio of the inclusion at room temperature key uncertainties in the methodare explored including the accuracy with which the liquid vapour ratio can be determined thecomposition of the titrant gas used in the iterative procedure and the composition of thepetroleum chosen to model the physical properties of the included petroleum data from coevalinclusions suggest that the saturation pressure gas oil ratio viscosity molar volume densityand surface tension of included petroleum are determined with a precision of a few percent confirmation of the accuracy of the method awaits tests using inclusions grown in the laboratoryunder carefully controlled pvtx conditions however it is likely that the physical properties ofincluded petroleums are more accurately modelled by fluids which are genetically related tothem if proven to be accurate the method will provide a routine method for determiningpalaeopressure in petroleum systems finally we present data from a central north sea examplewhich are geologically realistic and which for the first time record the evolution of fluid pressureand petroleum composition in a petroleum reservoir

the 129iodine bomb pulse recorded in mississippi river delta sediments results from isotopes of i pu cs pb and c 129i t1 2 1 56 107 yr has both natural as well as anthropogenic sources anthropogenic sources from nuclear reprocessing discharges and bomb test fallout have completely overwhelmed the natural signal on the surface of the earth in the last 50 years however the transfer functions in and out of environmental compartments are not well known due to temporal variations in the sources of 129i and to a lack of knowledge regarding the forms of iodine from a vertical profile of 129i 127i ratios in sediments located in the mississippi delta region in approximately 60 meters water depth the 129i input function to this region was reconstructed dates in the core were assigned based on the plutonium peak at 20 cm depth assumed to have been deposited in 1963 and the excess 210pb profile in the same depth interval and below that based on the steadily decreasing 240pu 239pu ratios from a ratio of 0 18 at 22 cm to 0 05 at 57 cm depth the 1953 horizon these low 240pu 239pu values are attributed to low yield close in tropospherically transported bomb fallout produced from the nevada test site in the early 1950s which had a value of about 0 035 and strongly suggest a terrestrial source for pu isotopes 129i 127i ratios increased from 2 10 10 at 3 cm to 4 10 10 at 20 cm and from there decreased monotonously to pre anthropogenic values at 53 cm and below 129i concentrations ranged from 8 13 106 atoms g in the top 20 cm and decreased to values of less than 1 106 atoms g below 50 cm atom ratios of 129i 137cs decay corrected to 1962 the year of maximum radionuclide production are about 0 3 very close to the production ratios of about 0 2 during atomic bomb tests this evidence combined with other observations strongly suggests that 129i in mississippi river delta sediments originates from atomic bomb fallout eroded from soils of the mississippi river drainage basin with little alteration of the isotopic ratios during transport from watershed to coastal deposits this is further corroborated by a close correspondence between 129i 127i ratios and other bomb fallout nuclides in this core based on these observations and on laboratory evidence we propose a conceptual model which explains this correspondence and the low 129i 127i ratios found in our sediment core as caused by geochemical i isotope fractionation processes during organic carbon leaching from erodible soils in the mississippi river watershed differences in mobilities of the different chemical forms of 129i and 127i as well as the variances in chemical forms of 129i from nuclear bomb fallout versus nuclear fuel reprocessing are proposed to have created such a correspondence between i isotope ratios and bomb fallout nuclides without revealing recent inputs from nuclear fuel reprocessing releases to the northern hemisphere observed in watersheds of the usa and europe

129i and 36cl concentrations in waters of the eastern clear lake area california residence times and source ages of hydrothermal fluids abstract the clear lake area of northern california is the location of hot spring activity some of which is associated with the formation of au and hg deposits we measured 129 i i and 36 cl cl ratios in nine warm springs formation springs six cold springs with elevated levels of cl and i mineral springs and five springs of recent meteoric origin dating of the i in the formation waters indicated that sources of i in these waters are formations with minimum ages between 60 and 80 ma this age range is in good agreement with the ages of the franciscan complex and the great valley sequence the dominant formations in this area because the mineral waters are essentially formation waters diluted with meteoric water i in these waters is of the same origin residence times of the waters were calculated based on the build up of129i and 36cl as a consequence of the presence of u and th in the crust the residence time of the formation waters in the great valley sequence the location for most of these springs was probably not longer than 84 000 yr the concentrations found for 36cl and 129i in the mineral waters indicate that these waters have residence times of similar magnitude in formations such as the franciscan complex or the clear lake volcanics which have slightly higher levels of u and th than the great valley sequence