image log analysis of in situ stress orientation breakout growth and natural geologic structures to 2 5 km depth in central scandinavian caledonides results from the cosc 1 borehole stress induced borehole deformation analysis in the collisional orogeny in the scandinavian caledonide deep scientific borehole establishes in situ stress orientation in a poorly characterized regi

integrated computer aided basin modeling applied to analysis of hydrocarbon generation history in a northern italian oil field abstract the application of reaction kinetics to the analysis of hydrocarbon hc generation processes requires a detailed temperature history of potential source rocks in order to obtain optimum results the necessary accuracy can be obtained by employing integrated computer aided basin modeling techniques to simulate the depositional history resulting in a continuous record of temperatures throughout the study area the technique has been applied to the analysis of an on off structure hc generation problem in a northern italian oil field the simulation of the geologic processes shows that low sedimentation rates and more importantly the high thermal conductivities of the carbonate rocks resulted in low temperatures throughout most of the geologic history of the study area which were followed by a sudden rapid temperature increase in the off structure area during the last 6 5 ma the calculated temperature history enables an accurate determination of the hc generation history using a kinetic approach which shows how hc generation is closely controlled by the sedimentation history and structural development and which also describes its relation to trap formation it also shows why hcs are generated at very low vitrinite reflectance levels i e peak oil generation at approximately 0 6 r o determination of the hc generation rate gives additional critical information as even source rocks located in apparently good potential zones at present i e in the on structure area can be of minor importance for a trap due to slow generation rates and consequent low expulsion efficiencies

development of microporosity in a micritic limestone reservoir lower cretaceous middle east abstract abundant intercrystalline microporosity is developed within the matrix of micritic limestones over several hundreds of meters of section in the thamama group lower cretaceous at sajaa field eastern u a e sediments in the upper shuaiba formation uppermost tamama group include muddy rudist algal coralgal and orbitolina dominated lithofacies matrix is composed of euhedral to subhedral micro rhombic low mg calcite crystals with average diameters of about 5 m microporosity development was not related to leaching chalkification of lithified micrite during post depositional emergence of the shuaiba formation features diagnostic of emergence are lacking in core below the upper contact of the unit matrix microporosity is believed to have been an early developed property of micritic limestones trace element compositions of matrix reflect a diagenetic process that involved conversion of a calcitic lime mud into micro rhombic micrite and microspar using a closed system geochemical model the observed matrix oxygen isotope compositions 18 o of 4 to 5 pdb are compatible with dissolution of aptian lime mud 18 o of 2 pdb and reprecipitation of calcite in fluids of marine composition under conditions corresponding to burial depths of 550 800 m porosity of 30 50 at 40 50 c closed system conditions are consistent with textural constraints for limited or arrested cementation in the matrix chemical compaction was responsible for partial occlusion of matrix microporosity as microporosity is decreased in zones of intense stylolitization slight corrosion of matrix crystals and leaching of peloids and orbitolina postdated cementation and chemical compaction during deep burial

coalification in the northern wildhorn nappe and adjacent units western switzerland implications for tectonic burial histories abstract vitrinite reflectance of fourteen eocene coal samples from the northern wildhorn nappe in the kander valley have been analyzed and compared to nine agewise similar coals from adjacent tectonic units the degree of coalification decreases regularly from semi anthracite in the kander valley to lignite in the plateau molasse the degree of coalification in the tertiary coals appears to be solely dependent on their tectonic history and indicates and estimated 6 km of burial by higher pennine nappes for the kander valley the rank at this locality low volatile bituminous to semi anthracite is significantly higher than in the border chain at beatenberg high volatile a bituminous the latter locality therefore was never buried deeper than about 4 km a further decrease in rank is found in the subalpine flysch and molasse high volatile b bituminous which in turn is significantly more mature than lignites from the folded plateau molasse at linden two samples of dogger coal from the pennine prealpes medianes are medium to low volatile bituminous that is intermediate between the kander valley and beatenberg their rank however was achieved before their final emplacement north of the wildhorn nappe by comparing calculated tti time temperature indices values for time depth burial curves with measured vitrinite reflectances maximum burial temperatures were estimated for the various tectonic units for the wildhorn nappe 160 180 c were obtained in the kander valley and 100 130 c in the border chain as a result of tectonic burial due to the thrusting by the pennine prealps sedimentary burial led to temperatures of 100 120 c in the subalpine molasse below the border chain prior to its thrusting

static fatigue of granite at 200 c cylinders of barre and westerly granites have been compressed to failure in static fatigue tests under dry and wet conditions at 1 kb confining pressure and temperatures of 24 sup 0 and 200 sup 0 c the effect of increasing temperature and water content is as expected to weaken the rocks appreciably and to reduce the time to failure at constant stress by about two orders of magnitude the activation energy estimated for dry barre granite deformed in temperature range 24 sup 0 to 200 sup 0 c is 10 2 kcal mol which seems reasonable and is consistent with other work at higher temperatures extrapolation of the static fatique equations obtained for specimens deformed at 200 sup 0 c both wet and dry to 10 sup 8 years yields maximum stress differences near 3 kb while such extrapolations may be reasonable for intact material over time spans of interest to national concerns the stresses obtained seem unreasonably high for tectonic deformations the discrepancy probably results from the very short durations of the tests which do not permit effective operation at these low temperatures of the several thermally activated weakening effects of h sub 2 o on silicates

time dependent cracking and brittle creep in crustal rocks a review abstract rock fracture under upper crustal conditions is driven not only by applied stresses but also by time dependent chemically activated subcritical cracking processes these subcritical processes are of great importance for the understanding of the mechanical behaviour of rocks over geological timescales a macroscopic manifestation of time dependency in the brittle field is the observation that rocks can deform and fail at constant applied stresses a phenomenon known as brittle creep here we review the available experimental evidence for brittle creep in crustal rocks and the various models developed to explain the observations laboratory experiments have shown that brittle creep occurs in all major rock types and that creep strain rates are extremely sensitive to the environmental conditions differential stress confining pressure temperature and pore fluid composition even small changes in any of these parameters produce order of magnitude changes in creep strain rates and times to failure three main classes of brittle creep model have been proposed to explain these observations phenomenological statistical and micromechanical statistical and micromechanical models explain qualitatively how the increasing influence of microcrack interactions and or the increasing accumulated damage produces the observed evolution of macroscopic deformation during brittle creep however no current model can predict quantitatively all of the observed features of brittle creep experimental data are limited by the timescale over which experiments are realistically feasible clearly an extension of the range of available laboratory data to lower strain rates and the development of new modelling approaches are needed to further improve our current understanding of time dependent brittle deformation in rocks

determination of critical parameters for evaluating coal measure shale gas in china abstract coal measure shale is one of the most abundant types of marine continental shales in china two sets of typical coal measure shales in china are developed in the shanxi formation ordos basin and in the longtan formation guizhou province in this study thermal simulation rock pyrolysis and x ray diffraction experiments were used to analyze four coal measure shale samples collected from typical areas of china based on the study of hydrocarbon generation process of the samples the lower limits of thermal maturity ro of favorable and target areas for coal measure shale were 1 8 and 2 5 respectively the threshold ro value of oil generation in coal measure shale was preliminarily inferred to be 0 6 the gaseous hydrocarbon production per unit of s1 s2 after the gas generation threshold was linearly and positively correlated with ro the lower limits of total organic carbon toc corresponding to the favorable and target areas were 2 45 wt and 2 84 wt respectively combined with x ray diffraction experiments the diagenetic stage and the hydrocarbon generation model of coal measure shale were established and could be divided into four evolutionary stages the results of this study have direct scientific and practical significance for the evaluation of gas content and selection of areas in china for the extraction of coal measure shale

time dependent brittle creep in darley dale sandstone 1 the characterization of time dependent brittle rock deformation is fundamental to understanding the long term evolution and dynamics of the earth s crust the chemical influence of pore water promotes time dependent deformation through stress corrosion cracking that allows rocks to deform at stresses far below their short term failure strength here we report results from a study of time dependent brittle creep in water saturated samples of darley dale sandstone initial porosity 13 under triaxial stress conditions results from conventional creep experiments show that axial strain rate is heavily dependent on the applied differential stress a reduction of only 10 in differential stress results in a decrease in strain rate of more than two orders of magnitude however natural sample variability means that multiple experiments must be performed to yield consistent results hence we also demonstrate that the use of stress stepping creep experiments can successfully overcome this issue we have used the stress stepping technique to investigate the influence of confining pressure at effective confining pressures of 10 30 and 50 mpa while maintaining a constant 20 mpa pore fluid pressure our results demonstrate that the stress corrosion process appears to be significantly inhibited at higher effective pressures with the creep strain rate reduced by multiple orders of magnitude the influence of doubling the pore fluid pressure however while maintaining a constant effective confining pressure is shown to influence the rate of stress corrosion within the range expected from sample variability we discuss these results in the context of microstructural analysis acoustic emission hypocenter locations and fits to proposed macroscopic creep laws

seismicity changes associated with the 2000 earthquake swarm in the izu island region a statistical method taking into account the information of magnitude occurrence time and location of earthquakes was applied to the earthquake data 1977 2000 of the japan meteorological agency jma to investigate seismicity changes in the izu island region the analysis indicated that a quiescence anomaly started about 1 5 years before the occurrence of the largest swarm in the summer of 2000 in the izu island region close investigations of the possible artifacts due to the selection of model parameters and the improvement of the seismological network lead to the conclusion that the above quiescence anomaly is unlikely a man made change the further stochastic test using 1000 random earthquake catalogs supports the idea that the above anomaly is significant the spatial distribution of the seismic quiescence which is quantified by a newly developed q parameter revealed a clear anomalous region around the epicentral area of the above earthquake swarm the seismicity revealed by the normalized parameter tends to increase just before the swarm as the first test of the above statistical method for investigating seismicity changes of earthquake swarms this study indicates that the increased rtl parameter would be a new potentially useful index for the risk alarm of earthquake swarms

thermal strain in a water saturated limestone under hydrostatic and deviatoric stress states abstract the present study is aimed at investigating the evolution of thermally induced bulk strain in a water saturated limestone blaubeuren at three different stress states three cylindrical rock samples are respectively loaded to a constant stress state 1 3 15 mpa 1 45 mpa 3 15 mpa 1 75 mpa 3 15 mpa at drained conditions in a conventional triaxial rock deformation apparatus before the sample temperature is cycled between 30 c and defined levels up to 120 c strain measurements are performed by one circumferential and two axial extensometers irreversible strain in both the lateral and axial sample directions are measured after each temperature cycle indicating permanent increases in diameter and in length the measured bulk strain is separated into different strain components related to 1 initial stress loading 2 reversible thermal expansion and contraction of rock matrix and 3 some residual strain the magnitudes of the residual strain increase with increasing deviatoric stress 1 3 in the lateral direction but decrease with increasing deviatoric stress in the axial direction the derived matrix thermal expansion coefficients range from 6 10 6 c 1 to 1 8 10 5 c 1 and from 9 10 6 c 1 to 1 5 10 5 c 1 respectively in the axial and lateral directions and present lower values at higher deviatoric stresses microstructural analyses evidence tensile cracks which are interpreted to have been induced during the temperature cycles these cracks have the potential to offset matrix thermal expansion yielding lower matrix thermal expansion coefficients at higher deviatoric stresses

effects of water on rock strength in a brittle regime abstract failure strengths of granite and andesite have been measured under various conditions of strain rate and confining pressure both in the dry and wet states constant stress creep tests for granitic samples have been also conducted all specimens show that the failure strength decreased linearly as the logarithm of the strain rate decreased the strain rate dependence of the failure strength is increased at higher confining pressures the strain rate effect is more apparent on the failure strengths of wet samples than dry samples in lower confining pressure ranges in the constant stress creep experiments the creep failure strength decreased as the logarithm of the time to failure increased time dependent failure strength of all rocks observed in the regime of the present study may be interpreted by subcritical crack growth assisted by the stress corrosion mechanism

time and temperature in petroleum formation application of lopatin s method to petroleum exploration n v lopatin in the soviet union has developed a method for taking both time and temperature into account as factors in thermal maturation of kerogen lopatin s time temperature index of maturity tti values correlate with the thermal regimes corresponding to generation and preservation of hydrocarbons because such information is potentially of great interest for oil exploration a calibration and evaluation have been made of lopatin s method within the limitations of the data presently available the following statements can be made 1 the rate of the chemical reactions involved in thermal maturation of organic material appears to double with every 10 c 18 f rise in temperature 2 threshold values of lopatin s time temperature index of maturity tti are 15 onset of oil generation 75 peak oil generation 160 end oil generation 500 40 oil preservation deadline 1 000 50 oil preservation deadline 1 500 wet gas preservation deadline 65 000 dry gas preservation deadline 3 tti values calculated from lopatin reconstructions consistently agree with other maturation parameters commonly used by petroleum geochemists potential applications of lopatin s method for oil exploration include timing of oil generation calculation of volume of hydrocarbons generated within a basin and determination of economic deadlines

growth of borehole breakouts with time after drilling implications for state of stress nantroseize transect sw japan resistivity at the bit tools typically provide images of wellbore breakouts only a few minutes after the hole is drilled in certain cases images are taken tens of minutes to days after drilling of the borehole the sonic caliper can also image borehole geometry we present four examples comparing imaging a few minutes after drilling to imaging from about 30 min to 3 days after drilling in all cases the borehole breakouts widen with time the tendency to widen with time is most pronounced within a few hundred meters below the seafloor mbsf but may occur at depths greater than 600 mbsf in one example the widening may be due to reduced borehole fluid pressure that would enhance borehole failure in the three other cases significant decreases in fluid pressure during temporal evolution of breakouts are unlikely the latter examples may be explained by time dependent failure of porous sediments that are in an overconsolidated state due to drilling of the borehole this time dependent failure could be a consequence of dilational deformation decrease of pore fluid pressure and maintenance of sediment strength until migrating pore fluids weaken shear surfaces and allow spallation into the borehole breakout orientations and thus estimates of stress orientations remain consistent during widening in all four cases in vertical boreholes breakouts wider than those initially estimated by resistivity imaging would result in higher estimates of horizontal stress magnitudes because the vertical overburden stress is fixed higher estimated horizontal stresses would favor strike slip or thrust faulting over normal faulting

palynofacies analysis of sediments from the cote d ivoire ghana transform margin preliminary correlation with some regional events in the equatorial atlantic abstract analyses of the palynofacies and sporomorph thermal alteration indices tai of sediments from ocean drilling program odp sites 959 to 962 in the cote d ivoire ghana transform margin west africa were undertaken to 1 determine the source and depositional conditions of the organic matter in the sediments 2 refine a paleobathymetric curve derived from other data for site 959 which drilled the most continuous sedimentary sequence from pleistocene to albian and 3 interpret the paleothermal history of the area twelve types of dispersed organic matter were identified amorphous organic matter aom marine palynomorphs algae resins black debris yellow brown fragments black brown fragments cuticles plant tissue wood sporomorphs and fungi the relative abundances of these organic matter components at each site were analyzed using cluster analysis resulting in the identification of seven palynofacies assemblages at site 959 five each at sites 960 and 961 and four at site 962 amorphous organic matter which is chiefly marine derived black debris and wood have played the most significant role in defining palynofacies assemblages the palynofacies assemblages show some correlation with lithologic units sediment sources and depositional environments previous palynofacies studies in passive margins have demonstrated that changes in the ratio of aom to terrestrial organic matter are related primarily to proximal distal positions of depositional environments relative to the shoreline however this assumption does not always hold true for a transform margin where tectonic factors play an important role in the organic matter distribution at least in the early stages of evolution lithofacies ccd paleodepths for the north atlantic trace fossil association benthic foraminifera and palynofacies data were the criteria used for reconstructing a paleobathymetric curve for site 959 a cyclicity in the organic matter distribution of the upper miocene to lower pliocene pelagic sediments could be related to fluctuations in productivity of biosiliceous and calcareous organisms and sedimentation rates a drastic increase in the amount of aom and a decrease in black debris and wood in the carbonate and clastic rocks lithologic unit iv overlying the tectonized albian sediments lithologic unit v at sites 959 and 960 coincide with the presence of an unconformity qualitative color analysis of palynomorphs was undertaken for all sites although the main focus was on site 959 where detailed organic geochemical data were available at site 959 tai values indicate an immature stage of organic maturation 2 except for the claystone samples between 1012 52 and 1036 5 mbsf and one limestone sample at 1043 4 mbsf reaching peak levels of 2 58 to 3 0 in the tectonized albian sediments below the unconformity these tai values show a positive correlation with the tmax values derived from rock eval pyrolysis data the highest values recorded in the basal tectonized units at all the sites sites 960 962 have mean values between 2 25 and 3 13 may be related to high heat flow during the intracontinental to syntransform basin stage in the region

brittle creep in basalt and its application to time dependent volcano deformation time dependent brittle deformation is a fundamental and pervasive process operating in the earth s upper crust its characterization is a pre requisite to understanding and unraveling the complexities of crustal evolution and dynamics the preferential chemical interaction between pore fluids and strained atomic bonds at crack tips a mechanism known as stress corrosion allows rock to fail under a constant stress that is well below its short term strength over an extended period of time a process known as brittle creep here we present the first experimental measurements of brittle creep in a basic igneous rock a basalt from mt etna volcano under triaxial stress conditions results from conventional creep experiments show that creep strain rates are highly dependent on the level of applied stress and can be equally well fit by a power law or an exponential law with a 20 increase in stress producing close to three orders of magnitude increase in creep strain rate results from stress stepping creep experiments show that creep strain rates are also influenced by the imposed effective confining pressure we show that only part of this change can be attributed to the purely mechanical influence of an increase in effective pressure with the remainder interpreted as due to a reduction in stress corrosion reactions the result of a reduction in crack aperture that restricts the rate of transport of reactive species to crack tips overall our results also suggest that a critical level of crack damage is required before the deformation starts to accelerate to failure regardless of the level of applied stress and the time taken to reach this point the experimental results are discussed in terms of microstructural observations and fits to a macroscopic creep law and compared with the observed deformation history at mt etna volcano c 2011 elsevier b v all rights reserved

influence of temperature on brittle creep in sandstones the characterization of time dependent brittle creep promoted by chemically active pore fluids is fundamental to our understanding of the long term evolution and dynamics of the earth s crust here we report results from a study of the influence of temperature on both short term strength and time dependent brittle creep in three sandstones under triaxial stress conditions we show that an increase in temperature from 20 degrees to 75 degrees c significantly enhances stress corrosion cracking in all three sandstones leading to 1 a systematic reduction in strength during constant strain rate experiments and 2 an increase by several orders of magnitude in brittle creep strain rates during stress stepping creep experiments we also show that a conventional creep experiment performed at 75 degrees c exhibits a qualitatively similar three stage brittle creep curve as that observed at ambient temperature extrapolation of our results suggests that temperature is likely to be the dominant influence on the evolution of creep strain rate with depth in the shallow crust citation heap m j p baud and p g meredith 2009 influence of temperature on brittle creep in sandstones geophys res lett 36 l19305 doi 10 1029 2009gl039373

a combined approach for estimating horizontal principal stress magnitudes from borehole breakout data via artificial neural network and rock failure criterion abstract in this paper a newly proposed approach on horizontal stress estimation from borehole breakout data is presented in a previous study a machine learning model was developed capable of estimating maximum horizontal stress h from breakout data accurately however due to the limitation in experimental data it was difficult to obtain the minimum horizontal stress h reliably in this study a series of breakout tests on hydrostone tb specimens was carried out to investigate the influence of h and vertical stress v on breakout geometries as these two parameters were rarely studied previously results revealed that both breakout angular span and depth decrease with increasing h or v although the influence of h is more significant based on experimental results from this paper as well as the literature nine failure criteria were examined on the prediction accuracy of h providing the magnitude of h except for one model all the other eight failure criteria consider the influence of v as indicated in experimental findings however none of the failure criteria yielded reasonable h estimations to overcome this problem an artificial neural network ann model was developed from the experimental dataset once the model was constructed it was examined against twenty three field data and yielded an acceptable average error rate of 15 88 on h considering the easily accessible breakout data then a comparative analysis on h estimation was performed via a number of approaches namely kriging ann and constitutive modeling results revealed that the use of the mogi coulomb failure criterion is the most reliable approach for h estimation with an average error rate of 6 82 overall this newly presented ann mogi coulomb approach to horizontal stress estimation shows reasonable prediction results which is expected to be improved in future studies by including additional data

basin analysis predictions of known hydrocarbon occurrences the north sea viking graben as a test case abstract basin analysis can assimilate structural burial and thermal histories to predict hydrocarbon prone zones in speculative areas the validity of the modelling approach was field tested in the north sea viking graben an area for which good data are available a recent birps deep seismic reflection profile across tectonic strike of the graben was interpreted and depth converted with good well control the burial and structural histories were determined by backstripping the chronostratigraphic layers using a one dimensional fluid flow compaction model on pseudo wells after removal of each sequence the section was manually reconstructed the thermal history was examined using thermal indicator tomography extensional models and calculations of depth to detachment lack of resolution required that three different paleo heat flow trends be tested to encompass the range in variability a gradual warming trend a constant heat flow through time and a low heat flow until the cretaceous tertiary boundary where a heat pulse occurred basement subsidence extension depth to detachment calculations and thermal indicator inversions all suggest that the uk side of the graben has developed differently to the norwegian section the generation modelling predicts the position of the oil window in the cross section the areas where this window enveloped the draupne formation kimmeridge clay source rock broadly correlate with the positions of the gullfaks statfjord and thistle oilfields of the uk sector however in the norwegian sector the draupne formation was generally overmature except in the area around the troll oilfield predicted middle to late cretaceous generation in the rift axis may have caused updip oil migration towards the horda platform the correct prediction of oil prone zones validates the use of quantitative basin analysis as a tool for evaluating frontier areas where data are limited

time dependent brittle creep as a mechanism for time delayed wellbore failure abstract by drilling a wellbore cavity high stresses arise at the wellbore wall leading to the formation of breakouts which enlarge the hole to an elongated shape oriented along the direction of the minimum principal stress if formation of breakouts is delayed rock debris falls on the drill bit which may lead to stuck pipe problems or even abandonment of the drill string reasons for such time delayed failure of the wellbore may be due to chemical fluid rock interaction especially in swelling clays however such delayed instabilities have also been observed e g in gneiss formations at the ktb borehole germany that are not known to exhibit a swelling behavior we propose to explain observations of delayed wellbore failure by time dependent brittle creep which has been observed for many types of rocks following this approach rock fails under loads less than their short time strength but after a long enough time span this time is in exponential relation to the load applied to the rock we implement a model developed for the creation of shear bands on the basis of time dependent brittle creep by amitrano and helmstetter 2006 here progressive damage of the formation is captured by a damage parameter d and the time to failure ttf young s modulus e is decreased by a factor every time ttf is expired i e when failure is reached subsequently stresses are redistributed according to the new distribution of e in the formation using this approach we obtain closure of the well with primary and secondary creep phases wellbore breakouts are formed progressively with deepening and widening of the initially damaged zone after a certain time the formation of breakouts comes to an end with an omori like decay of failure approaching a steady state

a generalized omori s law for earthquake aftershock decay 1 earthquake aftershock sequences have been found to approximately satisfy three empirical scaling relations i the gutenberg richter frequency magnitude scaling ii bath s law for the difference in the magnitude of a mainshock and its largest aftershock and iii the modified omori s law for the temporal decay of aftershock rates the three laws are incorporated to give a generalized omori s law for aftershock decay rates that depend on several parameters specific for each given seismogenic region it is shown that the characteristic time c first introduced in the modified omori s law is no longer a constant but scales with a lower magnitude cutoff and a mainshock magnitude the generalized omori s law is tested against earthquake catalogs for the aftershock sequences of the landers northridge hector mine and san simeon earthquakes

oil generation and migration in the gifhorn trough nw germany abstract the origin and migration of oils from selected oil fields in the gifhorn trough nw germany was investigated by geochemical analysis of crude oils and possible lower and middle jurassic source rocks productive reservoir sandstones in the gifhorn trough occur in the dogger beta zone of the middle jurassic bajocian and are interbedded with shales that contain 1 2 toc mostly type iii kerogen in the lower jurassic toarcian organic matter rich shales 9 12 toc whose kerogen has a type ii composition form the best quality oil prone source rocks biomarker and stable isotope analyses of oils and source rock extracts reveal that all oils analysed are derived from the lower toarcian shales lias epsilon the timing of oil generation and migration was investigated by reconstructing the geothermal history of the source rock in specific drainage areas and determining the availability of migration pathways through geological time hydrocarbon generation started in the central part of the gifhorn trough in upper cretaceous time differences in the maturity of the oils display separate small drainage areas close to the reservoirs variations in oil composition within deep buried reservoirs indicate the preservation of different migration phases

hydrocarbon prospects across narmada tapti rift in deccan trap central india inferences from integrated interpretation of magnetotelluric and geochemical prospecting studies abstract as the mesozoic sediments contribute most of the oil and gas reserves of the world we present an integrated interpretation approach using magnetotellurics mt and surface geochemical prospecting studies to demarcate hydrocarbon prospective gondwana mesozoic formations underneath the deccan flood basalts of late cretaceous age across narmada tapti rift between bhusawal and barwah in central india the mt interpretation shows deep 5 km basement structure between southern and central part of the mt profile however it gradually becomes shallower to either ends of the profile with a predominant basement depth reduction in the northern end compared to the southern end the geophysical results suggest thick 2 3 5 km mesozoic sediments in the area characterized by deep basement structure the geochemical analysis of the near surface soil samples indicate higher concentrations of light gaseous hydrocarbons constituents over the area marked with thick sub basalt mesozoic formations analyses of the geochemical data imply that these hydrocarbons are genetically related generated from a thermogenic source and these samples fall in the oil producing zone the temperature depth estimations in the region supports favorable temperature conditions 80 120 c for oil generation at basement depths

the aspo pillar stability experiment part ii rock mass response to coupled excavation induced and thermal induced stresses a 1 m thick pillar was subject to coupled excavation and thermal induced stresses to induce brittle rock mass yielding the yielding strength of the heterogeneous and fractured rock mass consisting of aspo diorite was evaluated at eighteen discrete locations using data from the displacement acoustic emission and thermal monitoring systems the average rock mass yielding strength was determined to be 0 59 of the uniaxial compressive strength the onset of dilation in uniaxial laboratory tests determined from strain gauge data was found to occur at approximately 0 45 of the uniaxial compressive strength it was shown that that the onset of acoustic emission events in situ also occurred when the tangential stress exceeded 0 43 of the uniaxial compressive strength for sites with absence of in situ data it is recommended that this lower bound value determined from laboratory data may be used for assessing the in situ rock mass yielding strength visual observation and displacement monitoring showed that extent of rock mass yielding is sensitive to small changes in the tangential stress magnitudes it was determined using three dimensional modelling that changes in the tangential stress magnitude of approximately 1 mpa was sufficient to cause yielding of the pillar to propagate in what appeared to be intact rock observations suggest that without this small stress change yielding of the rock mass would not occur in other words there appeared to be a well defined boundary and if the stresses reached this boundary yielding was observed however if stresses were only slightly below this boundary yielding or time dependant processes were not observed over the monitoring period used in the experiment

well bore breakouts and in situ stress the detailed cross sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data we show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression this work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ

poroelastic response of a borehole in a non hydrostatic stress field abstract this paper deals with the solution of the stress displacement and pore pressure field induced by the drilling and or the pressurization of a vertical borehole the rock which is permeated by fluid is assumed to behave as a poroelastic material with compressible constituents following the biot theory the analytical solution is derived in the laplace transform space and is transformed to the time domain using an approximate numerical inversion technique the solution reveals a full range of coupled poroelastic effects which provide potential mechanisms for delayed borehole instability and shear fracture initiation inside the rock

elastic moduli evolution and accompanying stress changes with increasing crack damage implications for stress changes around fault zones and volcanoes during deformation summary the elastic moduli of rock in areas susceptible to crack damage such as within fault zones or volcanic edifices can be subject to large modifications knowledge of how elastic moduli may vary in such situations is important for both the reliable modelling of volcano deformation and stability and for linear and non linear elastic crack models for earthquake rupture furthermore it has previously been shown that changes in elastic moduli can induce changes in the stress field surrounding faults here we report both uniaxial experimental measurements of changes in elastic moduli during increasing amplitude cyclic stressing experiments on a range of different rock types basalts sandstones and granite and the results of modelled stress modifications the trend in elastic moduli evolution with increasing damage was remarkably similar for each rock type with the exception of essentially crack free intrusive basalt that exhibited very minor changes in general young s modulus decreased by between 11 and 32 per cent and poisson s ratio increased by between 72 and 600 per cent over the total sequence of loading cycles these changes are attributed to an increasing level of anisotropic crack damage within the samples our results also show that acoustic emission ae output during any loading cycle only commenced when new crack damage was generated this corresponded to the level of stress where ae ceased during the unloading portion of the previous cycle using the multilayer elastic model of faulkner et al we demonstrate that the damage induced changes in elastic moduli also result in significant decreases in differential stress increases in mean stress and rotation of the applied greatest principal stress relative to the orientation of the mechanical layering the similar trend in the evolution of the elastic moduli of all the rocks tested suggests that stress modification in the damage zone of faults might take the same form regardless of the lithology through which the fault runs these observations are discussed in terms of their applicability to both fault zones and deformation at volcanoes

the use of free radicals of organic matter to determine paleogeothermal gradient abstract free radicals in 60 kerogen samples from 5 wells in the qiongdongnan tarim and liaohe basins in china were analyzed by electron spin resonance esr for use in geothermometry based on esr data for 44 kerogen samples from the tertiary qiongdongnan basin a regression curve was shown to fit free radical densities n g with burial depth of samples for use in geothermal gradient estimation using this curve the highest geothermal gradients experienced by carboniferous permian strata well tmc1 in the tarim basin and oligocene eocene strata well lr16 in the liaohe basin were estimated concurrently five samples from liaohe basin were examined by apatite fission track analysis to assess the reliability of the esr kerogen method for geothermal gradient determination results of esr analyses match those of vitrinite reflectance and apatite fission track analysis indicating that maximum geothermal gradients obtained from the esr curve are reliable this paper presents a useful esr kerogen method for estimating paleogeothermal gradients which is especially useful where vitrinite and apatite are not present

theory of time dependent rupture in the earth at a velocity x which is a well defined function of k this propagation is stable and quasi static and is referred to as subcritical crack growth we shall use the terms stable quasistatic and subcritical to mean propagation at velocities much less than the sonic velocities of the medium this behavior results from stress induced corrosion at the crack tip the principal corrodent for the present application being h20 this behavior has been firmly established in the laboratory for mode i tensile cracks in a wide variety of materials ineluding silicates and silicate glasses see cholz 1968a 1972a martin 1972 wiederhorn and bolz 1970 atkinson 1979 lawn and wilshaw 1975 knott 1973 the form of the relationship between k and x does not vary significantly with the material only the parameters in the law vary as an example we show in figure 1 data on subcritical crack growth in quartz after atkinson 1979 the empirical relationship is found to take the form