seismic petrophysics and isotropic anisotropic avo methods for unconventional gas exploration exploration and drilling for natural gas in north america has moved radically away from conventional reservoirs to focus on unconventional reservoirs such as tight gas sands and shales these reservoirs have low porosity and near zero permeability with gas stored in natural fractures and within the matrix porosity economic gas production requires hydraulic fracture stimulation to open connections to existing natural fractures or matrix porosity and successful stimulation depends on the formation s geomechanical brittleness being capable of supporting extensive induced fractures however despite adequate stimulation significant variations exist between wells in expected ultimate recovery eur due to the heterogeneity of these resource plays consequently predicting natural fractures or fracture prone sweet spots is essential to optimize development of such plays

seismic inversion for organic richness and fracture gradient in unconventional reservoirs eagle ford shale texas abstract a method using eagle ford organic shale from texas predicts key unconventional reservoir properties total organic carbon toc and fracture pressure gradient fg applying petrophysical and rock physics models from previous work to the available well log data generates the required logs for calibration i e shear sonic toc organic porosity and saturation prestack seismic data can be evaluated using synthetic forward modeling and can be conditioned further to improve avo response simultaneous prestack inversion to derive acoustic impedance ai and shear impedance si volumes can be run and toc can be derived from those volumes by linear interpolation fracture gradient which also can be thought of as frackability is related directly to poisson9s ratio and effective stress and can be estimated from elastic properties the fracture gradient is related nonlinearly and inversely to toc i e assuming laterally and vertically homogeneous pore pressure distribution inside the spatially limited block of the eagle ford shale higher toc results in lower fg and hence better frackability

fracture mapping using seismic amplitude variation with offset and azimuth analysis at the weyburn co2 storage site abstractcap rock integrity is an essential characteristic of any reservoir to be used for long term co2 storage seismic avoa amplitude variation with offset and azimuth techniques have been applied to map hti anisotropy near the cap rock of the weyburn field in southeast saskatchewan canada with the purpose of identifying potential fracture zones that may compromise seal integrity this analysis supported by modeling observes the top of the regional seal watrous formation to have low levels of hti anisotropy whereas the reservoir cap rock composite midale evaporite and ratcliffe beds contains isolated areas of high intensity anisotropy which may be fracture related properties of the fracture fill and hydraulic conductivity within the inferred fracture zones are not constrained using this technique the predominant orientations of the observed anisotropy are parallel and normal to the direction of maximum horizontal stress northeast southwest and agree closely with previous fracture studies

valuable lessons from acquiring 3d seismic for coal seam gas coalbed methane cbm or coal seam gas csg is being developed on a basinwide scale primarily as feedstock for three new liquefied natural gas lng facilities on the east coast of australia a recent 3d seismic survey acquired specifically for csg in the surat and bowen basins shows that although structural detail of the reservoir is a valuable tool deriving other information for reservoir characterization from thinly bedded coals requires a thorough investigation of the effects of high reflectivity thin beds although work continues to progress the results so far suggest that 3d seismic surveys can provide information for improved reservoir characterization

astronomical tuning of the aptian stage from italian reference sections a high resolution grayscale series of the pelagic fucoid marls piobbico core central italy shows strong pervasive lithological rhythms throughout the aptian interval a hierarchy of centimeter to meter scale cycles characterizes the rhythms when calibrating 1 m cycles to earth s 405 k y orbital eccentricity cycle these rhythms correspond to the periods of the eccentricity obliquity and precession index tuning to orbital eccentricity cycles provides a high resolution time scale for the aptian correlation to the cismon core northern italy extends the tuning to the aptian barremian boundary the tuning indicates a minimum duration of 13 42 m y for the aptian stage where previous estimates range from 6 4 to 13 8 m y the combined aptian albian astronomical tuning of the entire 77 m long piobbico core and part of the cismon core provides a 25 85 m y long astronomically calibrated time scale for earth history

science capabilities unlock australia s oil and gas potential abstract australia is on the way to become the world s largest exporter of liquefied natural gas lng with first shipments leaving the newly established export terminal on the east coast and four more development projects on the north west shelf to be completed by 2017 despite australia being a key supplier of natural gas to the ever expanding energy markets in the asia pacific region the declining oil production is of concern in this context the australian government actively supports the acquisition of precompetitive data that are used to enhance the understanding of hydrocarbon prospectivity in underexplored regions modern geophysical techniques such as potential field data interpretation that allow the evaluation of basin configurations at regional scale are tools of ever increasing importance organic geochemistry has been applied to group the existing hydrocarbon accumulations according to oil families and tie those to the related source rock intervals such work identifies oil and gas accu

a primary magnetization fingerprint from the cretaceous laytonville limestone further evidence for rapid oceanic plate velocities paleomagnetic data from the cretaceous 101 88 ma laytonville limestone of northern california are of geodynamic interest because they suggest a rate of absolute plate motion 15 cm yr that exceeds those of all present day plates such findings assume that a primary magnetization is preserved in these accreted pelagic limestone outcrops this assumption is supported by a trend in the paleomagnetic inclination data that matches in magnitude and sign the expected sense of motion predicted by comparing the mean paleolatitude value with the north american apparent polar wander path the pervasiveness of remagnetizations seen in shallow water carbonates of north america and elsewhere however raises questions as to whether the laytonville limestone is remagnetized regardless of tests indicating a primary magnetization the remagnetized carbonates have a distinctive wasp waisted magnetic hysteresis signature thought to reflect a bimodal distribution of magnetite grain sizes this signature can be quantified by the relationship between saturation remanence mr saturation magnetization ms coercivity hc and the coercivity of remanence hcr mr ms 0 89 hcr hc 06 this power law is compared with reference data from pacific deep sea carbonates deep sea drilling project sites 167 317 and 463 ocean drilling program site 806 and tethyan pelagic carbonates valle del mis section southern alps which together define a different curve mr ms 0 53 hcr hc 1 0 the laytonville limestone falls in two categories on the basis of unblocking temperature spectra resistance to alternating field demagnetization and behavior during the acquisition of isothermal remanent magnetization red laytonville limestone is thought to contain pigmentary hematite in addition to magnetite hysteresis curves from red laytonville limestone are wasp waisted but because such curves also typify magnetite hematite mixtures these data do not provide an unambiguous test for remagnetization some white laytonville limestone however appears to contain negligible amounts of hematite and can be directly compared with the remagnetized carbonates hysteresis parameters derived from white samples are indistinguishable from those of the reference pelagic limestones such samples have a stratigraphic significance occurring where paleolatitude data indicate an equatorial crossing these samples may have depleted pigmentary hematite due to increased magnetic dissolution as the limestone entered the equatorial belt of high productivity the lack of a remagnetization fingerprint in these white samples together with paleomagnetic lithologic and paleontologic data form a consistent data set supporting the primary nature of the laytonville limestone magnetization and rapid oceanic plate velocities a small plate size coupled with the unusually vigorous mantle plume volcanism of the cretaceous pacific basin could have combined to reduce the effectiveness of asthenospheric drag accounting for the rapid motion

magnetostratigraphic calibration of the late valanginian carbon isotope event in pelagic limestones from northern italy and switzerland abstract magnetostratigraphic biostratigraphic and carbon isotope data are presented from five lower cretaceous pelagic limestone sections in the southern alps a positive carbon isotope event is recorded in bulk carbonate from the upper valanginian of all five sections and the magnetostratigraphy and biostratigraphy provide the age model the record of the isotope event in absolute time is closely replicated in all five sections as the sections are located in two paleogeographic basins and are separated by up to 300 km the synchronous isotope event cannot be interpreted as an artifact of diagenesis a reduction in magnetization intensity coincident with the 13 c peak may be due to diagenetic magnetite dissolution enhanced by increased organic carbon burial according to the age model which uses gts89 timescale 1 background 13 c values of 1 5 are recorded from 145 to 137 ma between 137 and 136 ma 13 c increases to a maximum value of about 3 values begin to decrease from 136 ma and reach background values at about 132 ma the rate of change of 13 c for the onset of the event about 1 5 m y is greater than for the decay about 0 5 m y suggesting an abrupt perturbation of the carbon system from equilibrium followed by a more gradual return to background values

the winds of change anisotropic rocks their preferred direction of fluid flow and their associated seismic signatures part 1 editor s note this article is the first of two parts that together form an abbreviated version of the fall 2004 seg aapg distinguished lecture a powerpoint presentation of this material is posted on the seg web site the winds of change refer to much more than just anisotropic rocks whose seismic anisotropy provides insight into horizontal permeability anisotropy the way we view our planet is changing the planet is now being studied as the interaction of four interlocking systems the hydro sphere the atmosphere the lithosphere and the biosphere as all absorb energy from our sun and change the planet and all its subsystems like geology or climate are explicitly evaluated as interdependent and interacting within the context of the local community the heterogeneity at the surface of our planet and the recycling of compounds and elements water nitrogen sulfur etc enable our planet to support life my earth science career has spanned two revolutions the plate tectonic revolution during the 1970s and today s profound reevaluation of how our planet works and our roles as planet dwellers the rocks that are the primary focus of most of our studies are best understood as firstly the result of the interactions of the climate the water conditions the depositional setting the biosphere and the provenance of the sediments and secondly the lithification and geologic history they have undergone the biosphere shows up in our rocks both as fossils a key means of dating rocks and as liquid or solid organic remains we understand any given stratigraphic facies in the context of its spatial neighbors its community and its plate tectonic setting the climate includes the actions of gravity sunlight and atmosphere but it is affected by where the land masses are and how they are linked together oil industry geoscientists are uniquely placed via access to important data and sophisticated tools

palaeoceanography of organic rich rocks of the loma chumico formation of costa rica late cretaceous eastern pacific the late cretaceous late cenomanian campanian loma chumico formation consists of laminated to massive bedded siliceous shales cherts ash fall tuffs volcanic glass beds and volcaniclastic sandstones the loma chumico formation is at least 630 m thick core data in the tempisque basin and can be subdivided in the subsurface into organic rich and organic lean total organic carbon 1 facies rock eval total organic carbon toc data show that toc in the organic rich facies averages 15 5 with values as high as 33 inorganic geochemical petrological and sedimentological analyses indicate that all the lithologies of the loma chumico formation were deposited in middle shelf to outer slope environments adjacent to an active subaerial island arc the organic geochemistry and palaeontology of the loma chumico formation help constrain the factors that controlled biological productivity in the region during the late cretaceous biomarkers identified in organic rich facies by gas chromatography mass spectrometry gc ms show a high abundance of total steranes relative to hopanes in the morote 1 core indicating algal dominated organic input the hopane to sterane ratio of the extract from the manzanillo 1 core indicates a relatively high bacterial component to the organic matter upwelling and extrusive volcanism were the most important factors that controlled nutrient supply including silica flux and biological productivity within the loma chumico formation depositional system episodic radiolarian dinoflagellate or diatom blooms instigated by rapid infusions of volcanogenic silica may have resulted in expansion of the oxygen minimum zone omz enhancing the preservation of organic matter productivity appears to have been uniform even during deposition of the organic lean facies the findings of this study suggest that ocean wide anoxia in the eastern pacific was unlikely during the late cretaceous high toc contents of organic rich radiolarites from central caribbean deep sea drilling project dsdp sites may be related to upwelling produced in the eastern pacific by convergence of the easterly trade winds eastern pacific equatorial upwelling zone or may be part of a more complex pacific atlantic ocean interaction the end of deposition of organic rich rocks began near the end of the latest santonian earliest campanian in the central caribbean and costa rica palaeoceanographic models suggest that improved circulation and ventilation of bottom waters due to early opening of the north atlantic south atlantic seaway may explain this observation this may represent the influx of a previously unrecognized source of colder oxygenated intermediate bottom water proto antarctic intermediate water into the late cretaceous central atlantic and eastern pacific oceans

uncertainty in discriminating lithology in different data domains a major goal of geophysical analysis is to predict the lithologic and fluid content of the subsurface of necessity most of the measurements are conducted remotely using seismic or electromagnetic equipment these remote data should be calibrated with the expected properties of the subsurface principally from well control as a result rock physics is a key component of the process to ensure that the appropriate geophysical response is measured and obtained

using avo for fracture detection analytic basis and practical solutions tight low porosity reservoirs can produce significant amounts of hydrocarbons if fracture networks provide the necessary permeability characterization of naturally fractured reservoirs using surface seismic data is an important exploration problem that has attracted much attention in the literature tle for example had an entire issue august 1996 on this subject while the data and modeling in existing publications demonstrate that fracturing causes significant azimuthal variation in recorded seismic signatures the problem is how to infer fracture parameters from these data

towards a more practical two feldspar geothermometer the thermodynamic basis of several recent attempts to formulate a simple two feldspar geothermometer is discussed together with a review of earlier empirical geothermometers and ones based on experimental studies in the ternary feldspar system it is shown that double binary thermometers which involve the combination of regular solution mixing models for the binary alkali feldspar system with ideal mixing in plagioclases do not give a satisfactory representation of two feldspar relations especially for albite rich compositions where a critical point exists thermometers based on mixing parameters for ordered alkali feldspar frameworks are even more unjustified both because low plagioclases are certainly non ideal and because of uncertainty in knowing the degree of al si order in the alkali feldspar when exchange equilibrium was achieved a thermodynamic thermometer requires knowledge of ternary activities which are at present unknown experimental determinations of relationships in the ternary feldspar system are reviewed and the correct general form of the thermometer constructed using mainly the experimental data of seck 1971a and smith and parsons 1974 chemographic tests for equilibrium between feldspar pairs are suggested and petrographie features discussed in an appendix new values are given of margules parameters calculated for binary disordered alkali feldspars from recent solvus data up to 15 kbars and their physico chemical basis examined we suggest that accurate representations of the mixing properties of disordered alkali feldspars using margules parameters are at present premature

the skaergaard layered series part iii non dynamic layering layering in the skaergaard intrusion has been divided into two introduction general types one produced by magmatic flow and another by recent studies of the structural and textural features of processes resulting from variations of rates of nucleation and crysthe skaergaard intrusion mcbirney nicolas 1997 tallization and in the case of the layered series by compactionhave distinguished two broad types of layering one related processes modal variations caused by shifts of cotectic produced by the dynamic effects of magmatic flow and proportions produce thick layers which in the layered and upper another by processes that operate in situ such as varied border series are diffuse and normally lack strong foliation and nucleation and growth of crystals recrystallization or by lineation in the marginal border series the layers are thinner and compaction related mechanisms we refer to layering sharper possibly because the rate of accumulation was slower formed by these latter processes as non dynamic to oscillatory nucleation may have played a role in producing fineemphasize that it is not the result of fluid dynamic scale cyclic layers but it was less important than solution and processes although most layering combines elements of reprecipitation during slow cooling and ostwald ripening evidence more than one process the contribution of each mechfor compaction is found in deformed plagioclase laths and a relative anism can usually be recognized from its distinctive form deficiency of incompatible elements in rocks formed on the floor and setting layering related to compaction becomes sharper with increasing the distinguishing features of dynamic magmatic layheight in the layered series until it suddenly disappears above the ering have been described in part ii of this series mcbirtrough horizon near the base of upper zone b mechanical sorting ney nicolas 1997 they are best seen near the steep during compaction may have produced crude layering but if it did margins of the layered series in what wager brown the evidence has long since been destroyed by the superimposed effects 1968 referred to as the cross bedded zone where the of solution and reprecipitation when interstitial liquid rose through layering is disrupted by slumping and channeling and the overlying crystals and re equilibrated with them numerical the rocks have a marked foliation and lineation simulations illustrate how small differences of surface energy caused layering not associated with magmatic flow takes a by variations of grain size textural dependence of solubility and variety of forms but we can distinguish two general pressure solution can cause segregation of minerals into layers during end members each of which has a distinct origin and solution and reprecipitation characteristics the first results from varied rates of

interpreting seismic data in the presence of azimuthal anisotropy or azimuthal anisotropy in the presence of the seismic interpretation measuring p wave azimuthal anisotropy has been in the recent past an elusive task therefore the interpreter ignored this attribute of the seismic data and left the subject to the research and technology group geophysicists conversely the interpreting geophysicist knew that when measured the anisotropy could yield important reservoir properties related to fractures and stress fields however little did we suspect that p wave azimuthal anisotropy both in velocity and avo would change our perception of data interpretation as is happening now

fluctuations in biosiliceous production and the generation of early cretaceous oceanic anoxic events in the pacific ocean shatsky rise ocean drilling program leg 198 integration of geophysical logs with sedimentological biostratigraphic and physical properties data from poorly recovered chert rich sequences at ocean drilling program leg 198 sites 1207 and 1213 allows us to propose an aptian albian early cretaceous depositional history for shatsky rise north pacific ocean the logs indicate that the lower aptian and lower albian deposits are relatively lithified and unporous compared to contiguous sediments these characteristics are best explained by increased siliceous cementation that resulted from an elevated flux of radiolarians to the seafloor the intervals of increased biosiliceous production may record a biotic response to enhanced nutrient delivery coeval with but longer in duration than oceanic anoxic events oaes 1a early aptian and 1b early albian on shatsky rise oae 1a is represented by organic carbon rich sediments but oae 1b is not thus black shale deposition in the pacific ocean was only possible when a carbon burial threshold was crossed we speculate that this threshold was related to the balance between productivity and carbon oxidation and that ultimately the supply of biolimiting nutrients through hydrothermal alteration of basalt may have been the key to the development of black shale in the pacific basin

nannofossils and superplumes the early aptian nannoconid crisis a group of calcareous nannoplankton named nannoconids experienced a crisis in the early aptian and recovered only later in the late aptian after a period of virtual absence although no extinctions occurred the widespread nature of the nannoconid crisis suggests a global causal factor this crisis is recorded within the chiastozygus litterarius nannofossil and globigerinelloides blowi planktonic foraminiferal zones postdates magnetic chronozone m0 by approximately 300 kyr and precedes the oceanic anoxic subevent 1a and associated 13c anomaly by some 40 100 kyr selective dissolution and anoxia cannot explain the crisis because nannoconids are dissolution resistant forms and their crisis clearly precedes the deposition of anoxic sediments at least 1 m y prior to the nannoconid crisis the onset of a nannoplankton speciation event may be the response of nannofloras to a major rise in relative sea level the nannoconid crisis seems to be synchronous with the early aptian volcanic eruptions in the pacific ocean hence calcareous nannoplankton were severely affected by the superplume volcanic episode the coccolithophorid bloom nannoconid crisis was possibly induced by the excessive co2 levels in the atmosphere and or caused by changes in nutrient content of oceanic surface waters fertility was enhanced by rapid turnover of nutrients due to the abnormal volcanic activity and accelerated transfer of nutrients from the continents into the oceans under warm and humid conditions of the mid cretaceous greenhouse climate the nannoconid crisis may represent a competition between phytoplankton groups for nutrients or more likely competition between different calcareous nannoplankton the biologic affinity and mode of life of nannoconus are unknown because there is no modern analog of this genus however comparison of lower cretaceous nannofossil assemblages with modern nannoplankton cummunities suggests that nannoconids like extant florisphaera profunda possibly inhabited the lower photic zone concentrations of nutrients in the upper euphotic zone may have triggered blooms of coccolithophorids and nannoconid depletion this model implies that the nannoconid crisis is the result of an abrupt major change in the structure of surface waters caused directly or indirectly by the superplume the adjustments of the biosphere to the new paleoceanographic and climatic conditions required some 40 100 kyr before changing into abnormally high primary productivity and deposition of organic carbon rich sediments with dinoflagellates outcompeting nannoplankton

seismic anisotropy of fractured rock a simple method for including the effects of geologically realistic fractures on the seismic propagation through fractured rocks can be obtained by writing the effective compliance tensor of the fractured rock as the sum of the compliance tensor of the unfractured background rock and the compliance tensors for each set of parallel fractures or aligned fractures the compliance tensor of each fracture set is derivable from a second rank fracture compliance tensor for a rotationally symmetric set of fractures the fracture compliance tensor depends on only two fracture compliances one controlling fracture compliance normal the other tangential to the plane of the fractures the stiffness tensor which is more useful in the consideration of elastic wave propagation through rocks can then be obtained by inversion the components of the excess fracture compliance tensor represent the maximum amount of information that can be obtained from seismic data if the background rock is isotropic and the normal a

effects of sand shale anisotropy on amplitude variation with angle ava modelling the sawan gas field pakistan as a key case study for south asia s sedimentary basins amplitude variation with angle ava is a technique widely used in the characterisation of hydrocarbon reservoirs and assumes the earth s crust to be an isotropic medium yet anisotropy is ubiquitous in stratigraphic sequences and has first order effects on seismic ava responses when investigating subsurface prospects this work analyses the effects of anisotropic strata on ava responses using the lower goru formation middle indus basin pakistan as a case study in the study area shale intervals are interbedded with reservoir sands of the sawan gas field shales in this field form laminae or are dispersed within reservoir sands making the lower goru formation an example of a vertically transversely isotropic vti medium in this work we calculate the effective saturated mechanical properties of the lower goru formation based on rock physics templates the backus 1962 average typically designed for layered media combined with the empirical relations of brown and korringa 1975 and wood 1955 the input data used in our rock physics modelling is based on detailed petrophysical analyses of well data using the saturated effective mechanical properties of the lower goru formation we generate angle dependent reflection coefficient curves and seismic ava responses based on exact and approximate solutions for both isotropic and anisotropic reservoir scenarios our results suggest that the effects of lithological anisotropy are more pronounced in places with thick shale beds within reservoir sands conversely angle dependent reflection curves and seismic ava responses based on isotropic or anisotropic cases give similar solutions in the presence of thin shale beds as a corollary of this work we present a bayesian inversion method for the estimation of porosity in vti media

palaeomagnetic time and space constraints of the early cretaceous rhenodanubian flysch zone eastern alps the rhenodanubian flysch zone rdf is a lower cretaceous lower palaeocene turbidite succession extending for similar to 500 km from the danube at vienna to the rhine valley eastern alps it consists of calcareous and siliciclastic turbidite systems deposited in a trench abyssal plain the age of deposition has been estimated through micropalaeontologic dating however palaeomagnetic studies constraining the age and the palaeolatitude of deposition of the rdf are still missing here we present palaeomagnetic data from the early cretaceous tristel and rehbreingraben formations of the rdf from two localities in the bavarian alps rehbrein creek and lainbach valley southern germany and from the stratigraphic equivalent of the falknis nappe liechtenstein the quality of the palaeomagnetic signal has been assessed by either fold test ft or reversal test rt sediments from the falknis nappe are characterized by a pervasive syntectonic magnetic overprint as tested by negative ft and are thus excluded from the study the sediments of the rehbreingraben formation at rehbrein creek with positive rt straddle magnetic polarity chron m0r and the younger m 1r reverse event with an age of similar to 127 123 ma late barremian early aptian at lainbach valley no polarity reversals have been observed but a positive ft gives confidence on the reliability of the data the primary palaeomagnetic directions after correction for inclination shallowing allow to precisely constrain the depositional palaeolatitude of the tristel and rehbreingraben formations around similar to 28 degrees n in a palaeogeographic reconstruction of the alpine tethys at the barremian aptian boundary the rdf is located on the western margin of the brianconnais terrain which was separated from the european continent by the narrow valais ocean

alteration and self reversal in oceanic basalts 1 we report on magnetic properties and compositions of titanomaghemites from moderately to highly oxidized basalts recovered at nine drilling sites in the pacific ocean these new data together with similar data sets allow us to test compositional fields of titanomaghemite that could carry self reversed components of natural remanent magnetization above room temperature these compositions have been proposed on the basis of theoretical considerations of cation distributions during ionic reordering associated with low temperature oxidation maghemitization the data indicate that very high oxidation states z 0 9 and relatively high ti contents x 0 6 are needed to produce natural self reversed components this compositional range is much more limited than predicted the severe restrictions on compositions and oxidation state strongly suggest that self reversed magnetizations should not be a general feature of the magnetization carried by oceanic basalt instead the formation of self reversed components may signify unusual conditions such as extreme fluid flow and iron removal which could result in the required oxidation states while maintaining stability of the cation deficient titanomaghemite lattice structure

on the magnetostratigraphic age of nauru basin basalts of the western pacific ocean and timing of ontong java volcanism abstract cretaceous lava flows overlie jurassic to early cretaceous oceanic crust in the nauru basin of the western equatorial pacific but their exact age and origin is controversial in one model they are generically related to volcanism forming the ontong java plateau however paleomagnetic data from basalts recovered by ocean drilling in the nauru basin have been interpreted as recording numerous geomagnetic reversals suggesting the nauru basin basalts are older than the early aptian flows on the ontong java plateau and the correlative volcanism seen in the western equatorial and southwestern pacific ocean basin here we examine the magnetic fidelity of the nauru basin basalts through rock magnetic and paleomagnetic approaches we find the magnetic carriers in the lavas are unlike most basaltic units recovered by oceanic drilling in that they are magnetically soft this quality makes the rocks especially prone to the acquisition of secondary magnetic components induced during drilling we demonstrate that the reversed polarity intervals are illusory and instead record subtle changes in magnetic hardness that result in partial and complete overprinting by the magnetic field associated with the drill string e g the core barrel drill pipe and bit the recognition of these magnetic overprints the identification of only normal polarity in the nauru basin basalts and a critical consideration of the available radiometric and biostratigraphic age data lead us to conclude that coeval formation of the nauru basin basalts and ontong java plateau in aptian times remains a viable hypothesis

integration of hydraulically induced microseismic event locations with active seismic attributes a north texas barnett shale case study abstracthydraulic fractures are delineated by induced microseismic event distributions and typically propagate perpendicular to the regional minimum stress direction however at a smaller scale varying mineralogical composition and existing fault and fracture networks can influence developing fracture networks we integrated microseismic event locations with seismic attributes from multichannel seismic reflection data including inversion results for impedance and lame parameters and seismic curvature attributes we found that microseismic event locations consistently correlate to zones of low seismic impedance and low and values describing characteristic material properties of fracture prone zones within the north texas lower and upper barnett shale additionally event locations showed a weak correlation with anticlinal structures as defined by volumetric curvature attributes we suggest that the low impedance low and zones were related to the boundary between calcite filled fractures a

geomechanical lithology based analysis of microseismicity in organic shale sequences a pennsylvania marcellus shale example abstract using an innovative workflow incorporating microseismic attributes and geomechanical well logs we have defined major geomechanical drivers of microseismic expression to understand reservoir stimulation response in an engineering geologic context microseismic data from a hydraulically fractured marcellus well in the appalachian basin central pennsylvania were sampled vertically through the event cloud crossing shale limestone sandstone and chert we focused our analysis on the devonian organic shale and created pseudologs of moment magnitude mw b value and event count the vertical moving average sampling of microseismic data was completed such that the sample interval matched that of the geophysical well logs this technique creates robust high resolution microseismic logs that show subtle changes in microseismic properties and allow direct crossplotting of microseismic versus geophysical logs we chose five geomechanical properties to form the framework against which to interrogate

size variations of coccoliths in cretaceous oceans a result of preservation genetics and ecology abstract biometric studies of coccoliths the remains of coccolithophores offer the opportunity to survey single species instead of entire assemblages we obtained and analyzed size data of three common species biscutum constans zeugrhabdotus erectus and watznaueria barnesiae in a stratigraphically very well defined interval of early aptian age 126 ma cretaceous material is derived from four sites lower saxony basin north sea western tethys mid pacific covering nearshore to open oceanic paleosettings length and width measurements of 1986 specimens were evaluated the recorded size patterns show a larger data spread for b constans and w barnesiae in the western tethys and the mid pacific than in the north sea and the lower saxony basin the latter two sites are dominated by small coccoliths of b constans while coccoliths of w barnesiae show similar sizes at all four sites solely small specimens of z erectus characterize the samples from the north sea and the lower saxony basin while only large ones are present in the samples of the western tethys and mid pacific for explaining the recorded size patterns three theories are discussed in detail these include 1 preservation of nannofossils 2 genetics and 3 paleoecology 1 intense dissolution or overgrowth of the nannofossils may have altered the original coccolith sizes particularly when biometric data from different sites with potentially varying states of preservation are compared due to its delicate morphology z erectus appears most prone to dissolution probably explaining its size pattern 2 if the recorded size data of the remaining two species represent original patterns these can be interpreted by new findings in recent coccolithophore genetics it has been shown that size variations within a single cryptic species are only minor shifts of coccolith sizes both in recent and fossil taxa can be related to genotypic variation the varying size ranges recorded for b constans and w barnesiae may therefore reflect diversity changes of cryptic species at the different sites 3 these cryptic species may have preferred different depth habitats depending on geographically controlled factors such as sea surface temperatures light availability or trophic load

new evaluation methods using conventional logging data for gas shale in southern china abstract because of characteristics of compactness rich in organic matter and gas adsorption the parameters that decide shale quality are not only porosity and permeability but also the abundance of organic matter mineral composition crack and mechanical properties logging evaluation methods of conventional reservoirs are not valid in gas shale advanced technology of core measurements can obtain more accurate parameters of gas shale however because it is restricted by heterogeneity and the high price of core this method cannot be used widely to develop a continuous section based on conventional wireline logging data the natural properties of rocks underground are calculated using the improved evaluation model including mineral composition clay constituents total organic carbon toc content porosity permeability crack porosity and mineral brittleness index moreover based on the rock physics model with rich organic matter elastic properties are achieved to calculate mechanical propert

the barremian and aptian stepwise development of the oceanic anoxic event 1a oae 1a crisis integrated benthic and planktic high resolution palaeoecology along the gorgo a cerbara stratotype section umbria marche basin italy abstract the barremian and aptian were times of global plate reconfiguration and profound changes in the ocean climate system culminating in the worldwide deposition of lower aptian black shale layers oae 1a based on high resolution lithostratigraphic and micropaleontological analyses precursor conditions and timing of the anoxia are here reconstructed along a 33 06 m thick section in the proposed barremian aptian boundary gssp stratotype at gorgo a cerbara umbria marche basin italy a non uniform history of benthic foraminiferal diversification is interrupted by the selli level oae 1a and by three turnover points for both benthic and planktic organisms each highlighted by prominent breaks in lithology and outcrop morphology the first two points at 1 1 myr and 20 50 kyr prior to the onset of the oae 1a correspond respectively to the nannoconid bathypelagic calcareous nannofossils decline and crisis events and separate a pelagic limestone chert claystone lithological cycles of the uppermost maiolica formation showing frequencies reminiscent of orbital eccentricity and containing an oligotrophic barremian benthic and planktic assemblage dominated by k selected nannoconids and rhizammina b greenish grey cherty marls of the lowermost marne a fucoidi formation associated to declining sedimentation rates and eutrophic early aptian assemblages dominated by radiolarians and lituolid benthic foraminifera and c anoxic radiolarites and shales of the carbonate free selli level high magnitude and frequency assemblage fluctuations occur between the nannoconid crisis 25 cm below the lowermost oae 1a black shale layer and the selli level base with suggestions of bottom water acidification at 800 kyr after the oae 1a end a third turnover point in the basal reddish member of the marne a fucoidi highlights middle aptian assemblages characterized by a bloom of early macroperforate planktic foraminifera hedbergella spp and appearances of new benthic species marking a shift towards better oxygenation oae 1a oxygen and pelagic carbonate factory crises are here suggested to have been step wise and linked to global warming conditions

assimilation and fractionation in adjacent parts of the same magme chamber vandfaldsdalen macrodike east greenland the vandfaldsdalen macrodike which lies in the skaergaard region of east greenland is a remarkably zoned fossil magma chamber with a granophyric cap overlying cumulate gabboros the intrusion is distinctly bimodal with a large compositional discontinuity at the contact between the gabbro and granophyre although the exposed part of the macrodike is in contact with tertiary basalts and sediments the granophyre originated by assimilation of xenoliths derived from the underlying archean basement sr and nd isotopic ratios throughout the cumulate sequence are remarkably similar indicating insignificant contamination of the gabbro by the granophyre modelling of the compositional effects of cooling and crystallization indicate that the cumulate pile resulted from fractional crystallization with the complicating effects of trapped liquid and post cumulus fractionation the uppermost rocks in the mafic part of the chamber sio2 62 feo 12 4 resulted from about 85 fractional crystallization a transgressive sill of strongly fractionated magma sio2 67 feo 8 8 formed from extracted intercumulus liquid that was the result of 90 fractional crystallization of the original magma mass balance indicates that typical granophyre is made up of about 75 dissolved xenoliths by weight and 25 mantle derived basaltic magma the magmas were not measurably affected by material exchange across the interface between the gabbro and granophyre this magma chamber evolved by both assimilation and fractional crystallization but the residual liquids formed by fractional crystallization were unaffected by assimilation heat exchange between were unaffected by assimilation heat exchange between the two parts of the chamber was obviously important but there was insignificant material exchange the inability of fractional crystallization and assimilation to affect the same liquid is related to the dynamic behavior of this particular magma chamber particularly the buoyancy of granophyre relative to evolving tholeiitic magma

causal fracture prediction curvature stress and geomechanics we propose scaling volume curvature measurements with material property estimates to produce a superior prediction of natural fractures curvature is one of many indirect fracture inferring attributes it does not detect fractures but is causally related to them through the assumption that increasing curvature relates to increasing strain there are many other variables that are causally related to fractures we propose that it would be advantageous to create combinations of these causal variables with curvature some of the most well known and important causes of variations in natural fracture density are material properties relating to brittleness material properties are critical geologically at all scales from large scale regional studies to prospect level inquiries because the properties may vary significantly within individual formations and between formations these vertical and lateral changes in material properties may be important and should be considered in fracture estimation along with c

elastic anisotropy due to aligned cracks in porous rock all theoretical expressions which relate the characteristics of saturated aligned cracks to the associated elastic anisotropy are restricted in some important way for example to the case of stiff pore fluids or of the absence of equant porosity or of a moderately high frequency band because of these restrictions previous theory is not suitable for application to the upper crust where the pore fluid is brine kf k820 the equant porosity is often substantial p 0 1 and the frequency band is sonic to seismic this work removes these particular restrictions recognizing in the process an important mechanism of dispersion a notable feature of these more general expressions is their insensitivity at low frequency to the aspect ratio of the cracks only the crack density is critical an important conclusion of this more general model is that many insights previously achieved concerning the shear wave splitting due to vertical aligned saturated cracks are sustained however conclusions on crack orientation or crack aspect ratio which were derived from p wave data or from shear wave critical angles may need to be reconsidered further the non linear coupling between pores and cracks due to pressure equalization effects means that the linear schoenberg muir calculus may not be applied to such systems the theory receives strong support from recent data by rathore et al on artificial samples with controlled crack geometry

tuning effect and interference reflections from thin beds and coal seams the ricker and widess criteria of resolving thin beds are established and widely accepted axioms analysis of tuning effects from an isolated thinning bed is commonly used in estimating the thickness of a thin bed however frequency and amplitude tuning effects may also result from a sequence of thin beds depending on the thicknesses acoustic properties and the relative location of thin beds in space tuning effects from interference reflections can sometimes deform the seismic wavelet and cause complex waveforms coal seams as thin as 1 20 of the predominant wavelength can produce reflections and these reflections may be a composite of primary and multiple reflections interference reflections from two closely spaced coal seams were recorded and used to interpret a seam anomaly roll that may create adverse conditions for longwall mining the seismic data and computer model results correlated with findings of several boreholes

m sequence reversals recorded in dsdp sediment cores from the western mid pacific mountains and magellan rise new paleomagnetic and paleontologic data from pacific dsdp sites 463 and 167 define the magnetic reversals that predate the cretaceous normal polarity superchron k n data from mid pacific mountain site 463 provide the first definition of polarity chron m0 in the pacific deep sea sedimentary record foraminiferal biostratigraphy suggests that polarity chron m0 is contained entirely within the lower aptian hedbergella similis zone in agreement with foraminiferal data from the italian southern alps and atlantic ocean nannofossil assemblages also suggest an early aptian age for polarity chron m0 contrary to results from the italian umbrian apennines and southern alps which place polarity chron m0 on the barremian aptian boundary biostratigraphic dating discrepancies caused by the time transgressive preservational or provincial nature of paleontological species might be reconciled by the use of magnetostratigraphy specifically polarity chron m0 which lies close to the barremian aptian boundary at magellan rise site 167 five reversed polarity zones are recorded in hauterivian to aptian sediments correlation with m anomalies is complicated by synsedimentary and postsedimentary sliding about 25 m y after basement formation producing gaps in and duplications of the stratigraphic sequence the magnitude and timing of such sliding must be addressed when evaluating the stratigraphy of these oceanic rise environments

reversed polarity characteristic magnetizations in the albian contessa section umbrian apennines italy implications for the existence of a mid cretaceous mixed polarity interval paleomagnetic and paleontologic data from the valle della contessa umbrian apennines italy span a 14 m y gap in previous magnetostratigraphic sections and reveal several intervals of reversed characteristic remanent magnetization chrm in limestones and marls of the albian marne a fucoidi rock magnetic data indicate that the reversed chrm is carried by hematite having high unblocking temperatures 625 690 c hematite with these unblocking temperatures also carries a normal polarity chrm in other intervals a potential bias towards the identification of normal polarity is present since at every horizon measured a normal polarity component carried by magnetite and hematite is isolated at lower unblocking temperatures 300 600 c bulk magnetic properties vary throughout the measured section in the portion containing most of the reversed chrm intervals hematite is the dominant carrier of natural remanent magnetization while magnetite is the dominant carrier stratigraphically above and below these changes correspond approximately with lithology the zone of dominant hematite magnetization is marked by reddish bands pigmentary hematite which can be correlated throughout umbria and are thought to record changes in the oxidation state of the seafloor during deposition two models may account for the observed magnetization directions the reversed magnetizations may record a remagnetization which occurred at least 18 m y after deposition while the beds were still flat lying in reversed polarity chron 33r 83 79 ma or later if the reversed chrm represents a remagnetization the process is potentially of great importance since it can produce magnetization patterns which resemble polarity intervals alternatively the reversed magnetizations may have been acquired during intense seafloor oxidation episodes during the mid albian 107 104 ma and may record unrecognized intervals of reversed geomagnetic field polarity milankovitch like bedding cyclicity can be used to tune the sedimentation rate and obtain estimates of the duration of the potential reversed intervals using these estimates two of the potential reversed polarity intervals are of sufficient duration 100 kyr to be recognizable in both detailed stratigraphic sections and marine magnetic anomaly surveys a primary magnetization model predicts that several intervals of reversed magnetization should be found near the boundary of the biticinella breggiensis and ticinella primula foraminiferal zones and within the prediscosphaera cretacea nannofossil zone on a worldwide basis

mid cretaceous strontium isotope stratigraphy of deep sea sections large variations exist between published mid cretaceous late barremian to early turonian stages seawater sr isotope stratigraphies this has resulted in disparate interpretations of crustal production rates we report on a detailed investigation of seawater sr isotope stratigraphy based on foraminifers and where available on inoceramid bivalves from 12 mid cretaceous deep sea drilling project and ocean drilling program sections the effects of diagenesis are assessed using scanning electron microscope observations and traceelemental analyses but are best distinguished by comparing the 87 sr 86 sr values of similarage samples from different sites strontiumisotope analyses compiled from 9 of 12 sites that have detailed age control define one band of common values this band is used as a composite curve which presumably represents seawater 87 sr 86 sr values the composite curve shows a trough of markedly lower 87 sr 86 sr values in the aptian and early albian stages higher but constant values for the middle albian cenomanian stages followed by a decrease in 87 sr 86 sr values in the early turonian

new constraints upon isotope variation during the early cretaceous barremian cenomanian from the pacific ocean carbon and oxygen isotope data from a succession of cretaceous barremian cenomanian age recovered from the pacific ocean dsdp site 463 are presented the carbon isotope curve reveals a large isotope excursion within the early aptian where 1 3 c values reach 4 8 in the l cabri g ferreolensis foraminifera zone a decrease in 1 3 c values is observed at the base g algerianus zone before a return to more positive values at the top of the g algerianus t bejaouaensis zone the pronounced early aptian positive event is preceded by a large negative isotope excursion confined to the g blowi zone synchronous with this excursion are increased total organic carbon values and increases in mn and fe concentrations integrated biostratigraphic and magnetostratigraphic data together with the carbon isotope profile suggest that the organic rich units of site 463 are correlatable with oceanic anoxic event la the input of isotopically light volcanic co 2 in concert with the intensification and upwelling of intermediate water enriched in 1 2 c could account for the observed trends a potential trigger may have been the destabilization of the water column and the prodigious co 2 emissions associated with hydrothermal activity and the emplacement of the ontong java plateau coupled with faunal evidence the subsequent positive carbon isotope excursion is interpreted to be resulting from high but decreasing productivity and possibly increasing ocean stratification resulting in strong carbon isotopic gradients and 1 3 c enriched surface waters the decrease in 1 3 c within the g algerianus zone is coincident with more positive 1 8 o values if these are interpreted in terms of temperature this interval may be characterized by a period of cooling and possibly a waning of c o r g cycling a return to lower 1 3 c values during the middle albian is considered to be related to the increased influence of upwelling as opposed to a waning of c o r g cycling upwelling introduced isotopically light carbon to the surface arresting the stratified oceanic conditions

detection of gas in sandstone reservoirs using avo analysis a 3 d seismic case history using the geostack technique the geostack technique is a method of analyzing seismic amplitude variation with offset avo information one of the outputs of the analysis is a set of direct hydrocarbon indicator traces called fluid factor traces the fluid factor trace is designed to be low amplitude for all reflectors in a clastic sedimentary sequence except for rocks that lie off the mudrock line the mudrock line is the line on a crossplot of p wave velocity against s wave velocity on which water saturated sandstones shales and siltstones lie some of the rock types that lie off the mudrock line are gas saturated sandstones carbonates and igneous rocks in the absence of carbonates and igneous rocks high amplitude reflections on fluid factor traces would be expected to represent gas saturated sandstones of course this relationship does not apply exactly in nature and the extent to which the mudrock line model applies varies from area to area however it is a useful model in many basins of the world including the one

absolute inclination values from deep sea sediments a reexamination of the cretaceous pacific record new data are used to examine anomalously shallow paleomagnetic inclinations derived from previous analyses of cretaceous dsdp sediments of the central western pacific ocean while viscous remanent magnetizations vrms are shown to produce a similar effect the bias toward shallow values is still present with thoroughly demagnetized data superimposed upon this bias is a trend towards maximum flattening at an expected inclination of approximately 55 degree with the flattening falling to zero at the equator and pole flattening due to sediment compaction or inclination error is proposed to explain these observations

anisotropy estimate for the horn river basin from sonic logs in vertical and deviated wells abstract shale anisotropy must be quantified to obtain reliable information about reservoir fluids lithology and pore pressure from seismic data failure to account for anisotropy in seismic processing can lead to errors in normal moveout nmo dip moveout dmo migration time to depth conversion and amplitude variation with offset avo analysis kriged estimates of density vertical compressional wave and shear wave velocities were derived from vertical wells in an area of interest in the horn river resource play in northeastern british columbia the kriged predictions were compared with measured logs along the trajectory of a deviated well whereas the density comparison provided a blind test of kriging accuracy because density is scalar and independent of well deviation the same comparison for sonic velocities revealed that they are systematically higher than the kriged vertical velocities this difference was used to estimate anisotropy parameters at the location of the deviated well the fac

metasomatism and self organization of dolerite dyke marble contacts beinn an dubhaich skye dolerite dykes which intrude dolomitic marble in the aureole of the beinn an dubhaich granite on the isle of skye have developed metasomatized margins close to the contact with the younger granite in the southern part of torrin quarry these margins display well defined mineralogical zones which can be divided into two groups on the basis of composition chlorite bearing margins display the sequence dolerite diopside spinel chlorite calcite magnetite olivine calcite spinel dolomite whereas phlogopite bearing sequences have a phlogopite calcite fluorapatite layer in place of the chlorite bearing layer no spatial pattern is seen in the distribution of the two margin types but textural and compositional evidence points to an importance of metasomatism by localized infiltration of magmatic fluids in the development of the phlogopite bearing sequences extensive patterning of the phlogopite bearing margins by a process of self organization during grain growth has resulted in the formation of alternating silicate only calcite only layers parallel to the dyke margin on a millimetre to centimetre scale early development of chlorite bearing sequences at the dolerite dolomite contact during contact metamorphism was followed by metasomatism controlled growth of phlogopite and patterning at or after the peak of metamorphism associated with the expulsion of aqueous fluids from the crystallizing pluton

more stable yet bimodal geodynamo during the cretaceous superchron we report palaeomagnetic and ar 40 ar 39 dating results from two sequences of basaltic lava flows deposited at the same locality in western china yet separated in time by similar to 50 myr one set lies within the cretaceous normal superchron at 112 115 ma and a second at 59 70 ma spanning the cretaceous palaeogene boundary we find that magnetic field directions during the superchron exhibit bimodal populations one with inclinations representative of a dipolar field and another with shallow inclinations that could reflect a more complex multipolar field however the time dependent variability in field directions was 50 lower during the superchron than after which implies greater field stability during the superchron our results suggest that episodes of less dipolar field behavior occurred within the cretaceous superchron and raise the question whether a second more multipolar field state is more persistent than previously thought

geomagnetic paleointensity and direct age determination of the isea m0r chron abstract a combined paleomagnetic and geochronologic study has been conducted on an andesite lava sequence at jianguo liaoning province northeastern china thermal demagnetization and thermomagnetic analysis revealed that natural remanent magnetization is carried by both magnetite and hematite stepwise thermal demagnetization up to 675 c isolated well defined reverse characteristic remanent magnetization chrm in three time independent lava flows with a mean direction of d i 179 2 59 7 with 95 3 0 it also showed that a high temperature component 585 c has the same chrm direction as that of the low to medium temperature ca 170 300 585 c components a modified version of the thellier thellier paleointensity method coe j geophys res 72 1967 3247 3262 with systematic partial thermoremanent magnetization checks was used for paleointensity determinations twelve out of 39 samples yielded reliable results in the temperature interval of 170 550 c virtual dipole moment values range from 3 9 to 4 7 1022 am2 with an average of 4 2 0 1 1022 am2 40ar 39ar age determination on one lava flow is 116 8 3 0 myr 2 error relative to fish canyon sanidine 28 02 0 28 myr the magnetic reversed polarity with well defined age could correspond to the isea within the older part of the cretaceous normal superchron cns but we cannot rule out the possibility that this reversal interval could also correspond to the m0r due to many ambiguities on previous published ages on isea and m0r intervals as well as the new monitor age correction we applied it also suggests that a weak magnetic field nature already documented before the cns extended at least into the very beginning of the cns

timing and magnitude of early aptian extreme warming unraveling primary δ18o variation in indurated pelagic carbonates at deep sea drilling project site 463 central pacific ocean abstract in order to elucidate early aptian marine paleotemperature evolution across the period of enhanced organic carbon corg burial oceanic anoxic event oae 1a stable isotope analyses were performed on pelagic limestones at deep sea drilling project site 463 central pacific ocean the 18o data exhibit a distinct anomaly by 2 spanning the oae 1a interval i e a 6 m thick phytoplanktonic corg rich unit constrained by magneto bio and 13c stratigraphy elucidation of paleotemperature significance of the 18o shift is made by taking account of recent sr ca evidence at the same section which revealed that geochemical signals in carbonate poor lithologies are relatively unaltered against burial diagenesis by discriminating 18o values from carbonate poor samples caco3 contents 5 30 wt it appears that an abrupt rise in sea surface temperatures ssts by 8 c 1 7 shift in 18o occurred immediately before oae 1a whereas a cooling mode likely prevailed during the peak corg burial in terms of its stratigraphic relationship as to the corg rich interval and to a pronounced negative 13c excursion as well as its timescale the observed sst rise resembles those associated with the paleocene eocene thermal maximum and more strikingly jurassic toarcian oae this observation is consistent with the hypothesis that these paleoenvironmental events were driven by a common causal mechanism which was likely initiated by the greenhouse effect via massive release of ch4 or co2 from the isotopically light carbon reservoir and terminated by a negative productivity feedback

paleomagnetic calibration of milankovitch cyclicity in lower cretaceous sediments abstract bedded pelagic limestones of early to mid cretaceous age may record cyclic climate variability driven by changes in the earth s orbit milankovitch cycles if properly calibrated the lithologic cycles could provide sedimentary chronometry at far shorter time scales than biostratigraphy and magnetic stratigraphy the lower cretaceous maiolica formation of northern and central italy displays rythmic bedding in sections whose magnetic reversal sequence is clearly correlated to the upper m series marine anomalies the magnetic reversals divide measured sections into 0 5 2 myr increments in which sedimentation rate assuming continous deposition can be measured to 15 25 accuracy paleomagnetically estimated sedimentation rates over a number of polarity zones establish with one exception a narrow range in periods of the carbonate cycles carbonate couplets have an estimated mean period of 23 5 kyr and prominent modulations in bedding thickness bundling have a mean period of 117 kyr kent and gradstein 1985 time scale the close match of sedimentary periodicities to orbital repeat times implies that the carbonate cycles reflect a combination of professional and eccentricity climatic forcing comparison of measured sections from different locations shows that bed to bed correlations are possible regionally cosistent with the milankovitch hypothesis episodes of barremian black shale deposition occur in the troughs of the short eccentricity cycle as roughly one half of the carbonate marl couplet or about 10 kyr duration it is possible to estimate both the timing relative to the top of reversed chron m 0 and duration of a period of unusual organic carbon rich deposition selli horizon in the early part of the cretaceous long normal magnetic polarity interval by tracing the sedimentary cycles up from the barremian strata

spontaneous development of concentric layering in a solidified siliceous dike east greenland abstract a meter wide rhyolitic dike near the eastern margin of the skaergaard intrusion developed fine concentric layering in a small section where a fault appears to have fractured the dike and enhanced its interaction with meteoric water layers two or three millimeters thick consist of quartz alternating with low temperature albite and potassium feldspar around a core of radiating clots of the same minerals the layering seems to have resulted from late stage crystallization and self organization during devitrification in a warm hydrous environment the development of fine scale layering can be modeled quantitatively using the model for competitive particle growth cpg of ortoleva and co workers owing to the small differences of surface energy of grains of differing sizes large crystals tend to grow at the expense of smaller ones and a repetitive pattern of layering develops from a small initial perturbation of grain size the spacing of layers predicted by a numerical model using reasonable estimates for the physical parameters such as diffusivity and the rate of cooling is in good agreement with the observed geometry and dimensions

sr ca ratios as indicators of varying modes of pelagic carbonate diagenesis in the ooze chalk and limestone realms abstract there has been an interest in the application of sr ca ratios in pelagic carbonates to reconstruct paleoceanographic conditions such as changes in the oceanic ca and sr budgets or calcareous nannoplankton productivity but the effect of burial diagenesis on primary sr ca records has not been sufficiently understood in order to clearly document sr ca records of pelagic carbonates relative to original paleoceanography or secondary diagenetic processes sr ca analyses were performed on bulk sediments from an 822 5 m thick ooze chalk limestone sequence at the mid pacific mountains deep sea drilling project site 463 the sr ca ratios exponentially decrease from 2 3 to 1 1 mmol mol with increasing burial depth in the ooze chalk interval 0 452 0 m below seafloor mbsf the sr ca ratios further decline to as low as 0 4 mmol mol in the limestone interval 452 0 mbsf with relatively high values of 1 1 mmol mol at carbonate poor lithology around 620 mbsf the stratigraphic sr ca trends appear to be consistent with porosity profile and similar sr ca porosity covariation in association with diagnetic changes in lithologies is also common in ontong java plateau sections by adopting a recent diagenetic model from backscattered electron image analysis of ontong java plateau sections it is postulated for site 463 that exponential sr ca decrease in the ooze chalk interval is due to the release of sr into interstitial water via recrystallization i e purification effect and that further decrease in sr ca ratios in the limestone interval is most likely due to acquisition of sr depleted calcite cement at the chalk limestone transition newly proposed as the dilution effect specifically sr ca ratios of deeply buried pelagic limestones are sensitive to the amount of calcite cement that is acquired in proportion to caco3 content i e carbonate rich lithology facilitates precipitation of secondary calcite at the chalk limestone transition therefore paleoceanographic interpretation by means of sr ca ratios in pelagic carbonates will become valid only when these diagenetic effects are appropriately constrained

amplitude versus offset variations in gas sands seismic reflections from gas sands exhibit a wide range of amplitude versus offset avo characteristics the two factors that most strongly determine the avo behavior of a gas sand reflection are the normal incidence reflection coefficient r0 and the contrast in poisson s ratio at the reflector of these two factors r0 is the least constrained based on their avo characteristics gas sand reflectors can be grouped into three classes defined in terms of r0 at the top of the gas sand class 1 gas sands have higher impedance than the encasing shale with relatively large positive values for r0 class 2 gas sands have nearly the same impedance as the encasing shale and are characterized by values of r0 near zero class 3 sands have lower impedance than the encasing shale with negative large magnitude values for r0 each of these sand classes has a distinct avo characteristic an example of a gas sand from each of the three classes is presented in the paper the class 1 example involves a hartshorn channel s