a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

fast and slow spreading ridges structure and hydrothermal activity ultramafic topographic highs and ch4 output different parts of the world ridge system have quite different morphologies which reflect different constructional processes it appears that hydrothermal circulation at all spreading centers is an important exchange process between the ocean and the newly formed oceanic crust this hydrothermal circulation may vary according to morphology and crustal composition and may also affect ridge constructional processes the tag 26 n and snake pit 23 n hydrothermal sites on the mid atlantic ridge mar display mn ch4 ratios in overlying seawater similar to those of nonsedimented east pacific rise sites i e about 0 2 mol l in contrast large methane anomalies with very low mn ch4 ratios of 0 005 mol l are associated with ultramafic topographic highs near 15 n close to the axis and on ultramafic walls of the rift valley the association of ultramafic bodies and ch4 anomalies in seawater indicates active serpentinization processes ch4 is produced during serpentinization according to the fisher tropsch reaction charlou et al 1991 many ultramafic rock assemblages have been sampled on the mar away from fracture zones in particular by the deep sea drilling project changes of mechanical properties and of density of uplifted deep material in the accreting plate boundary zone caused by serpentinization may play an important role in the construction of slow spreading ridges

inversion for seismic slip rate history and distribution with stabilizing constraints application to the 1986 andreanof islands earthquake we develop and present a method of inverting seismograms including their absolute amplitudes to obtain the complete seismic slip time history and distribution using the method of linear programming a rectangular fault is discretized into cells and the source duration into time steps the inversion for the slip rate distribution on the fault as a function of time reduces to the solution of a system of linear equations the influence on the solution of constraining different source parameters such as the sign of the slip rate and the seismic moment is investigated it is found that the constraint of positiveness of slip rate and the constraint that the seismic moment determined from the solution should be close to that obtained from the centroid moment tensor cmt solution are very strong constraints and are essential to obtaining a physically reasonable solution since the inverse problem is unstable instead of presenting only one particular solution we examine several solutions equally well fitting the data for all practical purposes we find that when both constraints are applied the solution becomes quite stable the method is applied to a data set consisting of the vertical component of available digitally recorded teleseismic p waves at stations within a chosen distance window for the 1986 andreanof islands earthquake mw 8 0 the preferred solution is the one in which the slip rate is constrained to be positive and the moment is set to the cmt value the average rupture speed in the 90 km segment to the east of the hypocenter was 3 2 km s and in the 190 km section to the west was 2 km s the maximum slip in the area of large moment release to the west was 5 m and the maximum slip rate which occurs in the same place as the maximum slip was 6 5 m s the slip distribution on the fault shows a corrugated pattern with the long axis in the direction of the plate convergence rather than normal to the arc which was the slip vector direction for the earthquake with most of the slip occurring pn a region of the fault to the west of the rupture nucleation zone this region continued to slip for about 55 s the total source duration being about 80 s the results of this analysis suggest that the long term motion on this portion of the aleutian arc is indeed in the direction of plate convergence

migration of mid ocean ridge volcanic segments the propagation of small offset volcanic spreading centre segments along mid ocean ridge crests may reflect absolute motion of the plate boundary relative to the underlying mesospheric frame such a relationship could be caused by a purely vertical flow of the mantle under spreading centres and would have value in constraining past plate motions from non transform trends generated during along ridge propagation and in linking the major element variability of oceanic crust and upper mantle to the bulk composition and temperatures of mantle ascending under mid ocean ridges

joint spacing distribution of granites in hong kong abstract the structural stability of rock masses is dictated by the inherent geological structures present most notably joints joint spacings are one of crucial parameters to control the block volume permeability deformability as well as strength of rock masses while extensive rock engineering projects have been carried out in hong kong in the last century the distribution models of joint spacings in hong kong rocks have still rarely been studied systematically and published this research examines true joint spacings of granites in fourteen different slope outcrops at various localities of hong kong a total number of 1323 joint spacing measurements belonging to 44 joint sets spanning from 0 005 m to 4 5 m are obtained the mean joint spacings among 44 joint sets studied are computed in a range of 0 11 m to 1 33 m lognormal distribution is found to be the dominant joint spacing distribution of granites in hong kong a further in depth understanding of joint spacing distributions of hong kong granitic rocks which covers about 35 of the total land area is of profound practical value to the assessment of rock mass excavatability and numerical modelling of rock fracture network development

observations on source time functions of earthquakes obtained from inversion of teleseismic body waveforms summary an interesting aspect of the nucleation of large eastern mediterranean earthquakes is the emergent nature of their source time functions several large and shallow normal strike slip and thrusting events which occurred in other parts of the world and broke the surface for which the source time functions have been determined using the same inversion procedure exhibit similar characteristics these emergent source time functions may reflect a difficulty in breaking barriers the localized region of the fault which remains unbroken at the completion of the dynamic fault process at which ruptures nucleated or some particular physical process e g creep instability that may commonly occur during the rupture of large earthquakes earthquakes in similar tectonic settings with similar source mechanisms reflect common patterns

evaluation indexes of coalbed methane accumulation in the strong deformed strike slip fault zone considering tectonics and fractures a 3d geomechanical simulation study both the deformation and rupture characteristics of rocks are related to geomechanics in this paper we identify the evaluation indexes related to coalbed methane cbm accumulation in strongly deformed strike slip fault zones considering tectonics and fractures we found that fault scale the fault combination the tectonic stress the preservation conditions and fractures all have important effects on the cbm distribution areas near the large scale opening faults are unfavourable to the preservation of coalbed methane the distribution of gas wells with different capacities is influenced by tectonic extension and convergence a 3d geomechanical method was used to analyse the influence of the ribbon effect of strike slip faults on the cbm distribution due to the influence of the ribbon effect the tectonic stress presents a plane in situ stress heterogeneity which in turn will affect the gas well productivity we also calculated the integrated rupture rate if to characterize the degree of tectonic fracture development in the target coal reservoir the appropriate fracture development degree can improve the petrophysical properties of the coal reservoirs while maintaining good storage conditions such that the gas wells can achieve a higher production capacity this study is of great significance for the enrichment of the geomechanical theory of oil and gas exploration

present day stress of the central persian gulf implications for drilling and well performance abstract the present day state of stress in the persian gulf is poorly understood but has significant impacts on well drilling and performance the upper permian to lower triassic formation of kangan dalan persian gulf exhibits a complex structural context in the neighborhood of the oman mountains and the zagros orogenies this formation is divided into four reservoir layers k1 to k4 where three main lithologies limestone dolomite and anhydrite are alternating we conduct an analysis of the present day stress and natural fractures at the wellbore using full bore fmi logs leak off test and density logs for this purpose borehole breakout and tensile fracture data are used to determine orientation of s h furthermore density log leak off test and kirsch equation for tensile fracture formation in the wellbores are used to calculate the magnitude of s v s h and s h respectively vertical stress s v gradient at 3100 m depth approximates 20 mpa km 2 9 psi m indicating a bulk density of 2 04 g cm 3 a total of 131 drilling induced tensile fractures and 21 breakouts with an overall length of 262 m are observed in two wells indicating a mean maximum horizontal stress s h orientation of n53 18 45 for drilling induced tensile fracture ditf data and n50 10 79 for breakout data the mean orientation of s h rotates counterclockwise with depth from k2 n70 4 2 to k4 n40 5 1 reservoirs noticed correlation between these data and stress orientations from earthquake focal mechanism solution first of all indicates that the stresses are linked to the resistance forces generated by the arabia eurasia collision at the zagros orogeny and secondly confirms the reliability of focal mechanism solution data near continental collision zones in the kangan dalan formation the nw se main open fracture direction is found as a common regional direction which is sub perpendicular to the present day maximum horizontal stress minimum horizontal stress s h gradient in reservoir sections is estimated to be equal to 17 mpa km 2 5 psi m the concluded strike slip stress regime s h s v s h in the study area is consistent with the compressive regime in the zagros thrust fold belt the present day stress in the kangan dalan formation has implications for wellbore stability lost circulation and well inflow performance relationship ipr wells are more unstable if deviated toward the s v direction whereas well productivity and mud loss increase in wells deviated toward s h which conveys the idea of a strike slip faulting effect tends to keep the natural fractures open in that direction

fractures in the low porosity and ultra low permeability glutenite reservoirs a case study of the late eocene hetaoyuan formation in the anpeng oilfield nanxiang basin china the naxiang basin located within the qin and dabie mountains orogenic belt in central china is a small intermountain faulted basin in which the late eocene hetaoyuan formation in the anpeng oilfield is the low porosity and ultra low permeability glutenite reservoirs the large scale fractures are mainly developed in the thick bedded conglomerates and gravel bearing gritstones the small scale jointed fractures bounded by layer are mainly developed in the medium granular and fine sandstones between the conglomerates or gravel bearing gritstones there also developed three kinds of micro fractures namely intergranular fractures intragranular fractures and transgranular fractures in the tight glutenites the fractures in the glutenite reservoirs are of chiefly high dip angles they assume chiefly the e w strikes next are the ne sw and nw se strikes on the log curve fractures usually show such responses as a decrease of the deep and shallow lateral resistivities an increase of the acoustic time difference a reduction of the density an increase of the compensation neutrons an occurrence of middle and high gamma ray and an expansion of the calipers fractures make contributions to the low porosity and ultra low permeability glutenite reservoirs by improving the permeability and then porosity micro fractures are the important channels connecting intergranular and intragranular solution pores thereby making better the connectivity within the tight glutenite reservoirs the small and medium scale fractures serve as the flow channels within the layers of fine and medium granular sandstones while the large scale fractures serve as the main flow channels for the whole reservoir under the impact of the present day stress the ne sw fractures perpendicular to the minimum principal stress assume a tensile state and have good connectivity large apertures and high permeability and are the major direction for fluid flow thereby providing a major basis for a further development

permeability of sub critical carbon dioxide in naturally fractured indian bituminous coal at a range of down hole stress conditions abstract understanding reservoir dynamics from a geotechnical and petrophysical perspective is essential for safe and steady fluid extraction transport or injection in the reservoir the recent approach on storage of anthropogenic greenhouse gases ghg for mitigation of climate change in the deep geologic reservoirs has led to fast paced research on resolving storage related issues a coal seam is one such target reservoir where co 2 may be injected for storage this may be coupled with the enhanced extraction of coalbed methane cbm it is known that the interaction of co 2 with coal induces hydro mechanical changes however detailed investigation is mandatory for site specific operation the lack of such studies on indian coal is the motivation for this research a naturally fractured coal specimen was chosen and the subcritical co 2 flow was maintained by monitoring the injection pressures and ambient temperature the variations in permeability under various scenarios were also investigated the results indicate that initial co 2 injection caused coal matrix swelling which took nearly 5 6 h of flow related sorption for maximum closure of the natural fractures linear variation in flow rate with the pressure gradient across the sample indicated laminar or darcian flow of co 2 through the coal fractures deeper coal seams have high confinements the co 2 permeability results for such in situ conditions indicated an exponential decline in naturally fractured coal due to increasing confining pressures the observed reduction of co 2 permeability with increase in gas inlet pressure is debated and may be attributed fully to matrix swelling or partly to the klinkenberg effect this study is vital for successful ghg sequestration in coal seams and for better understanding of the gas flow behaviour in the seams

moment tensor solutions for the 24 november 1987 superstition hills california earthquakes the teleseismic long period waveforms recorded by the global digital seismograph network from the two largest superstition hills earthquakes are inverted using an algorithm based on optimal filter theory these solutions differ slightly from those published in the preliminary determination of epicenters monthly listing because a somewhat different improved data set was used in the inversions and a time dependent moment tensor algorithm was used to investigate the complexity of the main shock the foreshock origin time 01 54 14 5 m b 5 7 m s 6 2 had a scalar moment of 2 3 1025 dyne cm a depth of 8 km and a mechanism of strike 217 dip 79 rake 4 the main shock origin time 13 15 56 4 m b 6 0 m s 6 6 was a complex event consisting of at least two subevents with a combined scalar moment of 1 0 1026 dyne cm a depth of 10 km and a mechanism of strike 303 dip 89 rake 180

characteristics and evolution of the segmentation of the mid atlantic ridge between 20 n and 24 n during the last 10 million years abstract high resolution bathymetry and geophysical data collected along the slow spreading axis and flanks of the mid atlantic ridge between 20 n and 24 n reveal the characteristics and history of different wavelengths of segmentation during the last 10 m y the bathymetric data exhibit a morphotectonic pattern dominated by ridge normal and oblique bathymetric lows that partition the ridge flanks into rhomb shaped areas of relatively high elevation at least four different types of oblique bathymetric lows have been identified which represent the off axis traces of axial discontinuities and suggest a complex and ongoing evolution of ridge axis segmentation one group of oblique structures is represented by two deep ridge normal depressions with typical fracture zone characteristics that are connected to the present active transform by oblique depressions near the ridge axis these oblique traces correspond to the southward shift of axial discontinuities associated with the propagation of the ridge axis while maintaining a constant offset of the latter two other types of oblique structures correspond to elongate bathymetric lows and oblique alignments of ridge parallel bathymetric lows symmetric about the ridge axis both types of oblique structures frequently change their orientation from normal to subparallel to the ridge axis and appear to merge and diverge off axis these oblique depressions are characterized by positive filtered mantle bouguer anomalies high magnetizations complex magnetic anomaly patterns and possible exposure of mantle lithologies the ridge segments defined by these oblique depressions lengthen or shorten along the ridge axis with propagation rates varying from 0 to 25 km m y 1 the last and smallest discontinuities observed in this area correspond to small ridge axis offsets and off axis traces identified by alignments of the terminations of abutting abyssal hills the ridge flank morphotectonic patterns produced by the evolution of these elementary segments of accretion may represent temporally variable upwelling volumes of melt the centres of the rhomb shaped areas correspond to maximum crust production and thin lithosphere and the discontinuities correspond to a thick lithosphere with very thin crust and possible outcrops of peridotites we propose a model which accounts for the punctuated injection of magma and the evolution of elementary segments of accretion over periods of several million years

characterization of strain burst rock fragments under a scanning electron microscope an illustrative study abstract fractures around excavations in hard fine grained brittle rock sometimes display unique patterns from which an interpretation of the manner of failure fracture propagation can be made igneous rock fragments from a strain burst event at a site in western australia have been studied under a scanning electron microscope sem in order to characterize their surficial features sem image analysis indicates that anisotropy a contrast in geomechanical properties geometry and contact patterns present at the micro scale as they do at the large mine drive scale it is proposed that these micro scale features can lead to anisotropic material behaviour and stress concentrations that manifest as strain burst events

destruction of the eastern north china craton in a backarc setting evidence from crustal deformation kinematics abstract the eastern north china craton ncc west of the pacific ocean has a long history of rifting from early cretaceous to the end of paleogene when the craton experienced destruction and significant thinning of the lithosphere the cause and geotectonic environment for this destruction remain controversial we use a set of field observation including basin patterns fault geometry and kinematics fault plane slip data dike distribution and fabric data from metamorphic core complexes to determine the kinematic evolution of the deformation in the eastern ncc during this period we show that the principal extension direction in the eastern ncc evolved from wnw ese in the earliest middle early cretaceous via nw se in the latest early cretaceous to nearly n s in the late cretaceous paleogene the movement history from late mesozoic to early cenozoic of the oceanic plates in the pacific ocean with respect to the eastern ncc is available from previous studies the izanagi plate first subducted nearly orthogonally wnw wards during earliest middle early cretaceous time and then moved obliquely nnw wards in the latest early cretaceous while the relaying pacific plate moved generally northwards from the late cretaceous to paleogene both the movement direction of oceanic plates and the principal extension direction of the continental deformation rotated clockwise we suggest that such a correlation can be explained by the eastern ncc being in a backarc setting in the cretaceous paleogene period the results support the backarc extensional model for the destruction and significant lithospheric thinning of the eastern ncc

timing and nature of the partial melting processes during the exhumation of the garnet bearing biotite gneiss in the southern altyn tagh hp uhp belt western china abstract partial melting of the garnet grt bearing biotite gneiss from the jianggalesayi area in the southern altyn tagh high pressure hp ultra high pressure uhp metamorphic belt has been identified and reported here felsic veins were distributed along the foliation plane and deformed together with the grt bearing biotite gneiss which dominated the main outcrop and preserved diagnostic features of anataxis highly cuspate and irregular quartz qz melt and plagioclase pl melt surrounded or stretched into biotite and or quartz grains with obvious eroded boundaries further provided the microstructural evidences for partial melting of the gneiss cl images showed that zircons from the grt bearing biotite gneiss were characterized by multizone structures of core rim i rim ii rim iii the different zircon zones from the gneiss had integrated recordings of the protolith ages 836 1613 ma the peak metamorphic age of 503 5 ma the retrograded age of 456 4 ma and the anatectic age of 418 3 ma zircons from felsic veins also exhibited core rim structures on cl images the felsic veins yielded similar protolith ages 1099 1072 ma and anatectic age 417 2 ma as the host gneiss the zircons from the host gneiss and felsic veins had plotted identical crustal evolution curves in the ehf t vs u pb age diagram with identical tdm ages the u pb ages and lu hf isotope both suggested that the felsic veins were internally derived by the partial melting of the host gneiss moreover the high content of al2o3 and rb in felsic veins indicated that the decomposition of biotite accounted for the partial melting of the gneiss the temperatures calculated by ti in zircon thermometer for the anatectic zircons from the felsic veins 582 694 c were identical to the conditions of the amphibolite facies stage of the grt bearing biotite gneiss t 619 646 c which indicated that the crystallization of the melt had taken place during the amphibolite facies stage of the exhumation the anatectic ages 418 3 ma and 417 2 ma of the gneiss and felsic veins were determined to be within the range of the fourth episode of the early paleozoic magmatism 420 385 ma which had formed in a post collisional extensional setting therefore the combination of the magmatic heat events and stress release may have played an important role in the partial melting of the gneiss

along axis variations in seafloor spreading in the mark area recent investigations with the manned submersible alvin and the angus deep towed camera sled greatly extended the known range of variations in the style of seafloor spreading along the axis of the mid atlantic ridge five transects of the spreading centre at intervals of10 20 km south of the kane fracture zone at 24 n latitude demonstrate dramatic changes in the style and magnitude of tectonic extension development of the neovolcanic zone expression of hydrothermal venting types oflithologic exposures and morphology of the median valley

geochemical and geochronologic constraints for paleozoic magmatism related to the orogenic collapse in the qimantagh south altyn region northwestern china abstract voluminous and discrete early paleozoic bimodal magmatic suites are thought to be the result of post collisional extension following the amalgamation of east kunlun and altyn tagh in this paper four representative magmatic units were studied for their geochemical fingerprint in conjunction with geochronological studies the 467 445 ma mangya mafic suite shows e morb type rare earth element ree patterns that are the result of asthenospheric interaction with a metasomatized subcontinental lithosphere high k calc alkaline granodiorites intruded at ca 450 ma are characterized by high mg the least fractionated ree pattern without an eu anomaly as well as high sr and low rb sr ratio we interpret these geochemical signals to result from lower crustal melting of garnet amphibolite at pressures between 16 and 22 kbar a 430 420 ma a type granite is interpreted to result from the melting of metaigneous rocks at middle to lower crustal depths lastly a late magmatic pulse occurs between 400 and 380 ma and is represented by the alk granite the alk granite is interpreted to be a product of metapelite melts and is associated with a smaller volume of mafic melts u pb zircon geochronology of paleozoic igneous rocks of the qimantagh south altyn reveals that most of the magmatic episodes are either coeval with or post extensional deformation this phase of extension is supported by the exhumation of hp uhp metamorphic rocks and crustal anatexis collectively the evolutionary stages documented in this study correspond to a succession of post collisional postorogenic and ultimately within plate magmatic episodes the overall features support orogenic collapse via removal of a thickened lithospheric root beneath the east kunlun altyn tagh collisional orogen during early paleozoic

geomechanical simulation of low order fracture of tight sandstone abstract the core factors in the study of fractures within tight sandstone reservoirs are tectonics geomechanics and fluid mobility which are not independent but are coupled to each other in this paper taking the upper paleozoic gas reservoir within the low amplitude tectonic zone in the sulige gas field of the ordos basin as an example the developmental characteristics and geomechanical model of fractures and their relationships with the productivity of tight gas sandstone reservoirs were systematically studied the results show that this low amplitude structural zone develops mainly low order ruptures the paleotectonic stress field of the target layer in the yanshanian period was restored using a 3d finite element method the horizontal principal stresses exhibited stress diffusion and were affected by local low amplitude structures there was a positive correlation between h and h and h h was distributed primarily between 1 35 and 1 95 with an average of 1 62 the strain energy density u was calculated to quantitatively characterize the microfracture distribution of the target layer based on the principle of energy conservation there are certain positive correlations between u and the horizontal principal stresses the geostresses of high u regions underwent a certain degree of stress torsion which manifested as a stress deflection reaching up to 30 during stress propagation finally we proposed the concept of u value geomechanical sweet spots in a tight gas sandstone based on this criterion commercial gas wells can be effectively identified

review of diagenetic facies in tight sandstones diagenesis diagenetic minerals and prediction via well logs abstract the tight sandstones are characterized by low porosity low permeability complex pore structure and strong heterogeneity due to the extensive diagenetic modifications they experienced understanding of the impact of diagenetic alterations on reservoir quality is crucial to the hydrocarbon exploration and production in tight sandstones diagenetic facies which is the comprehensive description of the diagenesis and diagenetic minerals determines the formation and distribution of sweet spot by correlating the diagenetic facies to well log responses the subsurface distribution of porosity and permeability can be predicted however the prediction of diagenetic facies and reservoir quality via well logs in tight sandstones remains a challenging task this paper critically reviews the impact of diagenesis and diagenetic minerals on reservoir quality in tight sandstones and establishes a model for prediction of diagenetic facies via well logs as assessed from peer reviewed papers in the literature as well as from the authors personal experiences this review begins with reviewing the impacts of compaction cementation dissolution and various types of diagenetic minerals on reservoir quality evolution the definition and classification schemes of diagenetic facies are then discussed and the reservoir quality as well as diagenetic evolution sequence of various diagenetic facies is summarized the same diagenetic facies commonly display similar compositional and textural attributes matrix and cement as well as porosity systems the well log responses gr ac den cnl and rt of various diagenetic facies are summarized by the calibration of log values with cores and related thin sections by translating the diagenetic facies to conventional well logs a predictable model which can be used for subsurface reservoir quality prediction is established then the theory of ecs logs is reviewed and the application of ecs logs in diagenetic facies evaluation is discussed at last the quantitative characterization for various type and degree of diagenesis is reviewed and the subsurface diagenetic facies is predicted by quantitative calculation of the compactional porosity loss cementational porosity loss and dissolution porosity content via well logs correlating the diagenetic facies to well logs provides a powerful tool to predict the distribution of high quality reservoirs in tight sandstones this review will provide insights into the reservoir quality evaluation and sweet spot prediction via well logs in tight sandstones

3d geomechanical modeling and numerical simulation of in situ stress fields in shale reservoirs a case study of the lower cambrian niutitang formation in the cen gong block south china abstract an analysis of the in situ state of stress in a shale reservoir was performed based on comprehensive information about the subsurface properties from wellbores established during the development of an oil and gas field industrial level shale gas production has occurred in the niutitang formation of the lower cambrian cen gong block south china in this study data obtained from hydraulic fracturing drilling induced fractures borehole breakout global positioning system gps and well deviation statistics have been used to determine the orientation of the maximum horizontal principal stress additionally hydraulic fracturing and multi pole array acoustic logging xmac were used to determine the vertical variations in the in situ stress magnitude based on logging interpretation and mechanical experiments the spatial distributions of mechanical parameters were obtained by seismic inversion and a 3d heterogeneous geomechanical model was established using a finite element stress analysis approach to simulate the in situ stress fields the effects of depth faults rock mechanics and layer variations on the principal stresses horizontal stress difference horizontal stress difference coefficient k h and stress type coefficient s p were determined the results show that the direction of the maximum principal stress is ese 120 additionally the development zones of natural fractures appear to correlate with regions with high principal stress differences at depths shallower than 375 m the stress type is mainly a thrust faulting stress regime at depths ranging from 375 to 950 m the stress type is mainly a strike slip faulting stress regime when the depth is 950 m the stress type is mainly a normal faulting stress regime depth fault orientation and rock mechanics all affect the type of stress the knowledge regarding the cen gong block is reliable and can improve borehole stability casing set point determination well deployment optimization and fracturing area selection

fracture detection in oil based drilling mud using a combination of borehole image and sonic logs abstract the deeply buried 5 5 7 1 km lower cretaceous bashijiqike sandstones are important fractured reservoirs in the kuqa depression in these high temperature high pressure drilling environments oil based muds are used to limit drilling risks and improve efficiency making facture detection using resistivity based image logs difficult the maximum horizontal stress sh max trends close to north south 0 10 based on orientations of the drilling induced fractures and borehole breakouts core descriptions thin sections and image logs used to describe the fractures in the bashijiqike sandstone reservoirs show that opening mode natural fractures are common and the fractures range from fully open to sealed by calcite and clay minerals the open fractures are dominantly steeply dipping or vertical and primarily strike north south locally horizontal fractures or fractures with low angle dip are detected we compare fracture visibility on image logs acquired in the same ks 2 2 12 key well in oil based mud and water based mud fmi hd logs successfully detect large 0 1 mm aperture vertical fractures in oil based mud better than obmi image logs and the two imaging tools have a similar ability in detecting the high dip angle fractures since the ubi tool is not affected by oil based muds most of the fractures can be picked out on ubi images minor negative deviations of bulk density could be observed in fractured zones and the presence of fractures obviously causes an increase in sonic transit time full waveforms from dsi tools combined with electrical image logs detect fractured zones both the velocity and amplitude of the sonic waves are sensitive to fractures and the fractured zones could be identified according to the attenuation of stoneley waves the combination of cores image logs and full wave sonic logs offers comprehensive detection of fractures in oil based mud environments

charging of carboniferous volcanic reservoirs in the eastern chepaizi uplift junggar basin northwestern china constrained by oil geochemistry and fluid inclusion substantial amounts of petroleum were recently discovered in the carboniferous andesite tuff breccia and basalt reservoirs of the chepaizi uplift in the western junggar basin however the charging history of the carboniferous petroleum reservoir is poorly understood oil oil correlation studies indicate that all of the oils were mainly derived from the middle permian wuerhe formation source rocks possibly mixed with a small contribution from carboniferous baogutu formation source rocks in the neighboring changji sag based on the petrographic and microthermometry of fluid inclusions two hydrocarbon charging episodes are defined these episodes were characterized by a low peak range homogenization temperature th distribution 80 c 90 c and high salinity 13 22 13 42 wt nacl and a high peak range th distribution 120 c 130 c and low salinity 4 89 11 72 wt nacl respectively through one dimensional basin modeling and pressure volume temperature composition simulation the burial thermal histories for wells p61 p66 p668 and p663 were reconstructed and their trapping temperatures of the hydrocarbon inclusions were calculated to be higher than their corresponding highest paleotemperature i e 56 8 c 53 7 c 60 9 c and 58 1 c respectively implying fast hydrocarbon charging processes promoted by deep hydrothermal fluids associated with the hydrocarbon generation history sealing process of the hongche fault and regional tectonic evolution these two hydrocarbon charging events were deduced as the adjustments of oils previously accumulated along the hongche fault zone because of the tectonic extension in the paleogene and regional tilting in the neogene respectively the general direction of oil charging was traced from south to north and from east to west as indicated by the molecular parameters of nitrogen bearing compounds and c20 c21 triaromatic steroids c20 c21 c26 c28 triaromatic steroids ta i ta i ii which roughly coincided with the active fracturing

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

joint development during fluctuation of the regional stress field in southern israel four trends of joint sets wnw ese nw se nnw sse and ne sw are found in upper turonian carbonate rocks within the neqarot syncline of south central israel the two most predominant sets strike parallel to the trend of maximum compressive stress directions sh associated with the plate related syrian arc stress field sas wnw ese active during the cretaceous to present and the perturbed regional stress field nnw sse related to stress accumulation on the dead sea transform during the miocene to the present eighty two percent of the beds in this study contain joints parallel with the latter trend whereas 42 contain joints parallel to the former trend all beds with layer thickness to spacing ratio fsr 1 5 have nnw sse joint sets compatible with the dead sea transform stress field dss whereas all joints sets that are not compatible with the dss stress field fall beneath this value for fsr considering lithology joints in five of six chalky limestone beds and all marly limestone beds are compatible with the dss whereas joints compatible with the sas do not develop in these marly and chalky limestone beds in the study area the joint sets lack a consistent formation sequence where more than one set is found in a single bed we use these observations to conclude that all studied joints are miocene or younger that the regional stress field from the miocene to the present fluctuated between dss and sas states and that the higher fsrs correspond to a greater amount of joint normal strain in response to the dss q 2001 elsevier science ltd all rights reserved

methodology for predicting reservoir breakdown pressure and fracture opening pressure in low permeability reservoirs based on an in situ stress simulation abstract using numerical simulations of the paleostress field during the time of fracture formation we predict the occurrence of fractures in the reservoirs of the second member of the funing formation in the tongcheng fault zone subei basin east china we use sonic velocity measurements from rock cores and microseismic monitoring to determine the in situ stress which is combined with fracture data to calculate the magnitude of the in situ stress at the well site by determining the mechanical parameters of the rock we use the finite element method fem to develop a geomechanical model and conduct numerical simulations of the in situ stress field based on the occurrence of fractures and the numerical simulation results of the in situ stress field we predict the opening pressure and opening sequence of natural fractures in the reservoirs the results indicate that the maximum horizontal principal stress in the second member of the funing formation in the tongcheng fault zone is oriented ene during the injection process the ene trending fractures open first and the ese trending fractures open later the opening pressure of the fractures increases as the angle between the fracture strike and the maximum horizontal principal stress increases and the burial depth of the fractures also has a relatively positive correlation with the opening pressure at the structurally high site 1850 m 2350 m the opening pressure of the fractures is in the range of 25 mpa 50 mpa and at the structurally low site 3300 m 4000 m the opening pressure of the fractures is in the range of 45 mpa 75 mpa by calculating the actual fracture pressure of the reservoirs we propose using different injection pressures in different blocks to ensure the high and stable production of oil from wells

influence of fracture induced weakening on coal mine gateroad stability abstract the ground stability of gateroads is a major concern in underground coal mines especially when the surrounding strata are weak and fractured this paper presents a novel numerical modelling technique for gateroad stability analysis based on a case study conducted at zhaogu no 2 mine china considering the occurrence of fracturing and its weakening effect on the stiffness of the rock mass a tension weakening model is implemented into flac3d whereby the stiffness of the rock mass is progressively decreased with the failure state a relationship between the intensity of fractures and the residual strength of the rockmass is built in the numerical model a parametric study of the tension weakening model is carried out and the results are compared to elastic perfectly plastic and strain softening models it is shown that the tension weakening model exhibits noticeable effects on ground deformation and rock support loading which are observed from field measurements thus the tension weakening model offers a more realistic simulation of the gateroad behavior than elastic plastic and strain softening models the proposed model can be utilized for other applications involving rock reinforcement of mine openings under similar geotechnical conditions

diagenetic constraints on the heterogeneity of tight sandstone reservoirs a case study on the upper triassic xujiahe formation in the sichuan basin southwest china abstract heterogeneity is an essential feature of tight sandstones because it is directly related to reservoir quality the upper triassic xujiahe formation in the sichuan basin was used as a case study to analyse the diagenetic constraints on the heterogeneity of tight sandstone reservoirs the three main diagenetic processes of interest are mechanical compaction cementation and dissolution a dissolution facies quartz cement facies and carbonate cement facies present sequentially from the centre to the margin of sand bodies abundant feldspars and rock fragments were dissolved to form secondary porosity or filled by carbonate cement authigenic chlorite coatings generated from eogenesis to the deep burial stage are thickest in high porosity samples the 13 c of carbonate cement ranges from 10 56 to 1 15 pdb suggesting multiple sources of carbon including carbonate fragments early inorganic carbonate cement and organically derived co 2 based on 18 o carbonate cementation occurred from 33 c to 110 c microthermometric data of aqueous inclusions indicate that quartz cement precipitated at a temperature range of 120 c 140 c a summary model of the diagenetic constraints on the heterogeneity of sandstones was constructed the spatial distribution of digenetic facies is controlled by differences in fluid flow velocity and ph fluids especially hydrocarbons migrated preferentially within the centre of sand bodies and caused dissolution dissolution generated secondary porosity and provided materials for authigenic mineral precipitation carbonate fragments were better preserved and superimposed with later carbonate cement along the sandstone mudstone boundary quartz overgrowths formed after carbonate cementation and developed in the transition zone between the intervals of carbonate cementation and dissolution consequently the reservoir quality of the xujiahe sandstones decreases gradually moving from the centre to the margin of the sand bodies

multi scale modeling of shale laminas and fracture networks in the yanchang formation southern ordos basin china abstract outcrops and core observations show that sandy and tuffaceous laminas and natural fractures are well developed in the continental shale gas reservoir this study reports the distribution of laminas and natural fracture networks at different scales in cases of outcrops wells and cores from yanchang formation southern ordos basin firstly based on two dimensional fracture field investigations a 3 d joint network model of the study area was built using the monte carlo simulation method then the laminations were identified from macroscopic scales to microscopic scales with multiple probe techniques statistical analysis of multi scale layer thickness suggested that the average thickness of layers at multiple scales showed a fractal feature and that the layer thickness at each scale followed with an exponential probability distribution whose exponential coefficient was 3 38 1 91 1 65 1 21 corresponding to meter scale decimeter scale centimeter scale and millimeter scale respectively the fitting curves indicated that the majority of laminas at each scale were relatively thin and the thicker the layer the less the number of layers contained in shale finally the lamination models were constructed and superposed over the joint network model to generate 3 d geological structure models at various scales the models were validated by reproducing fracture and lamina parameters fairly close to those obtained from realistic geological bodies in the study area the findings of this work could provide a more reliable geological model for the numerical simulation and physical model test of hydraulic fracturing in shale gas reservoir

coal seam porosity and fracture heterogeneity of macrolithotypes in the hancheng block eastern margin ordos basin china abstract heterogeneity in cleats and pore size distribution within coal macrolithotypes bright semi bright semi dull and dull coals determined from the overall relative luster and percentage of bright components impact coalbed methane cbm extraction however few studies have investigated the characteristics of macrolithotypes across multiple length scales here the macrolithotype description was performed for the no 3 5 and 11 coal seams in the hancheng block of ordos basin the cleats were characterized by optical microscopy and x ray computed tomography ct while pores and their connectivity were determined from mercury intrusion porosimetry n 2 and co 2 adsorption and low field nuclear magnetic resonance nmr of water saturated cores evaluated both cleats and pores the ch 4 adsorption capacity of coal samples scraped from the surface of the macrolithotype layers was also determined dull lithotypes semi dull and dull were dominant followed by bright lithotypes bright and semi bright in these seams the bright lithotype coal has a higher microfracture density averaging 339 microcleats per 9 cm 2 m mm better connectivity and a larger fractal dimension averaging 1 7 from optical microscopy the dull lithotype has a lower average microfracture density 272 microcleats per 9 cm 2 and a smaller fractal dimension 1 6 this higher connectivity observation was consistent with x ray ct observations and low field nmr of the water saturated cores and the transition upon water removal from the fractures and larger pores also the ease of mercury injection from cores also supported higher connectivity in the bright lithotypes both the semi bright and semi dull lithotypes have equivalent pore proportions micro to macropores and good apparent connectivity at the nm m scale despite mineral occlusions based on the ch 4 adsorption capacities of the macrolithotypes the bright coal has the greatest gas adsorption capacity due to the largest smaller micropores

in situ stress state in the linxing region eastern ordos basin china implications for unconventional gas exploration and production abstract the carboniferous and permian sedimentary rocks mainly the shanxi and taiyuan formations in the linxing region eastern ordos basin china host a significant volume of unconventional gas resources coalbed methane shale gas and tight sandstone gas currently the in situ stress state is poorly understood but knowledge of this is extremely important for a range of applications such as gas exploration and production fracture stimulation and wellbore stability the maximum horizontal stress s hmax minimum horizontal stress s hmin and vertical stress s v magnitudes and the s hmax orientation in the linxing region were systematically analyzed for the first time in the present study which can provide a reference for subsequent numerical simulation and hydraulic fracturing design based on borehole breakouts and drilling induced tensile fractures interpreted from borehole imaging logs the s hmax orientation rotates from nee sww trending in the southern part to nww see trending in the northern part of the linxing region both conventional logs and extended leak off tests were used for stress magnitude determination the results revealed three types of in situ stress fields s v s hmax s hmin s hmax s v s hmin and s hmax s v s hmin and a dominant strike slip stress regime s hmax s v s hmin was found for the entire well section in the target shanxi formation and taiyuan formation in the linxing region in addition differential stress increased with depth in the linxing region which indicates that wellbore instability might be a potentially significant problem when drilling wells that are vertical or n s trending

alteration of the oceanic crust implications for geochemical cycles of lithium and boron abstract fresh tholeiitic basalt glass has been reacted with seawater at 150 c water rock mass ratio of 10 and fresh diabase has been reacted with a na k ca cl fluid at 375 c water rock mass ratios of 1 2 and 5 to understand better the role of temperature basalt composition and water rock mass ratio on the direction and magnitude of b and li exchange during basalt alteration at 150 c slight but nevertheless significant amounts of b and li were removed from seawater and incorporated into a dominantly smectite alteration phase at 375 c however b and li were leached from basalt b behaved as a soluble element and attained concentrations in solution limited only by the b concentration in basalt and the water rock mass ratio li however was less mobile for example at water rock mass ratios of 1 2 and 5 the percent of li leached from basalt was 58 70 and 92 respectively this suggests some mineralogic control on li mobility during hydrothermal alteration of basalt especially at low water rock mass ratios in general these results as well as those for b are consistent with the temperature dependent chemistry of altered seafloor basalt and the chemistry of ridge crest hydrothermal fluids based on the distribution and chemistry of products of seafloor weathering low 150 c and high temperature hydrothermal alteration of basalt and the chemistry of ridge crest hydrothermal fluids it was estimated that alteration of the oceanic crust is a li source for seawater this is not true for b however since the hot spring flux estimated for b is balanced by low temperature basalt alteration these data coupled with b and li flux estimates for other processes e g continental weathering clay mineral adsorption authigenic silicate formation and formation of siliceous skeletal material yield new insight into the b and li geochemical cycles calculations performed here indicate relatively good agreement between the magnitude of b and li sources and sinks the geochemical cycle of b however may be affected by serpentinization of mantle derived peridotite in oceanic fracture zones serpentinites are conspicuously enriched in b and if the b source for these rocks is seawater then an additional b sink exists which must be integrated into the b geochemical cycle however until more data are available in terms of areal extent of serpentinization serpentite chemistry and isotopic composition the importance of b in these rocks with respect to the b geochemical cycle remains speculative at best

fractures in continental shale reservoirs a case study of the upper triassic strata in the se ordos basin central china fractures are important for shale gas reservoirs with low matrix porosity because they increase the effective reservoir space and migration pathways for shale gas thus favouring an increased volume of free gas and the adsorption of gases in shale reservoirs and they increase the specific surface area of gas bearing shales which improves the adsorption capacity we discuss the characteristics and dominant factors of fracture development in a continental organic matter rich shale reservoir bed in the yanchang formation based on observations and descriptions of fracture systems in outcrops drilling cores cast thin sections and polished sections of black shale from the upper triassic yanchang formation in the se ordos basin detailed characteristics and parameters of fractures analyses and tests of corresponding fracture segment samples and the identification of fracture segments with normal logging the results indicate that the mineral composition of the continental organic matter rich shale in the yanchang formation is clearly characterized by a low brittle mineral content and high clay mineral content relative to marine shale in the united states and china and mesozoic continental shale in other basins the total content of brittle minerals such as quartz and feldspar is c 41 with quartz and feldspar accounting for 22 and 19 respectively and mainly occurring as plagioclase with small amounts of carbonate rocks the total content of clay minerals is high at up to 52 and mainly occurs as a mixed layer of illite smectite i s which accounts for more than 58 of the total clay mineral content the upper triassic yanchang formation developed two groups of fracture joint systems a nw se trending system and near e w trending system multiple types of fractures are observed and they are mainly horizontal bedding seams and low dip angle structural fractures micro fractures are primarily observed in or along organic matter bands shale fractures were mainly formed during late jurassic late early cretaceous time under superimposed stress caused by regional wnw ese trending horizontal compressive stress and deep burial effects the extent of fracture development was mainly influenced by multiple factors tectonic factors and non tectonic factors such as the lithology rock mechanical properties organic matter abundance and brittle mineral composition and content specifically higher sand content has been observed to correspond to more rapid lithological changes and more extensive fracture development in addition higher organic matter content has been observed to correspond to greater fracture development and higher quartz feldspar and mixed layer i s contents have been observed to correspond to more extensive micro fracture development these results are consistent with the measured mechanical properties of the shale and silty shale the observations of fractures in cores and thin sections from more than 20 shale gas drilling wells and the registered anomalies from gas logging

impact of diagenesis on reservoir quality and heterogeneity of the upper triassic chang 8 tight oil sandstones in the zhenjing area ordos basin china abstract reservoir quality and heterogeneity are critical risk factors in tight oil exploration the integrated analysis of the petrographic characteristics and the types and distribution of diagenetic alterations in the chang 8 sandstones from the zhenjing area using core log thin section sem petrophysical and stable isotopic data provides insight into the factors responsible for variations in porosity and permeability in tight sandstones the results indicate that the chang 8 sandstones mainly from subaqueous distributary channel facies are mostly moderately well to well sorted fine grained feldspathic litharenites and lithic arkose the sandstones have ultra low permeabilities that are commonly less than 1 md a wide range of porosities from 0 3 to 18 1 and two distinct porosity permeability trends with a boundary of approximately 10 porosity these petrophysical features are closely related to the types and distribution of the diagenetic alterations compaction is a regional porosity reducing process that was responsible for a loss of more than half of the original porosity in nearly all of the samples the wide range of porosity is attributed to variations in calcite cementation and chlorite coatings the relatively high porosity reservoirs formed due to preservation of the primary intergranular pores by chlorite coatings rather than burial dissolution however the chlorites also obstruct pore throats which lead to the development of reservoirs with high porosity but low permeability in contrast calcite cementation is the dominant factor in the formation of low porosity ultra low permeability reservoirs by filling both the primary pores and the pore throats in the sandstones the eogenetic calcites are commonly concentrated in tightly cemented concretions or layers adjacent to sandstone mudstone contacts while the mesogenetic calcites were deposited in all of the intervals and led to further heterogeneity this study can be used as an analogue to understand the variations in the pathways of diagenetic evolution and their impacts on the reservoir quality and heterogeneity of sandstones and is useful for predicting the distribution of potential high quality reservoirs in similar geological settings

crenulated seafloor evidence for spreading rate dependent structure of mantle upwelling and melting beneath a mid oceanic spreading center abstract the availability of dense geosat gm satellite altimetry data south of 30 s 1 provides the first opportunity to compare at 25 km resolution seafloor generated at ridges spreading with a wide range of spreading rates the axis of the slow spreading southwest indian ridge as well as the seafloor that it has generated has a distinctly crenulated appearance this crenulated seafloor shows regularly spaced gravity lineations in the spreading direction crenulations that record stationary centers of mantle upwelling over the life of the southwest indian ridge a similar pattern of crenulation is seen along sections of the slow spreading northern mid atlantic ridge but along the faster intermediate spreading southern mid atlantic ridge while regularly spaced highs are apparent along sections of the spreading axis stationary crenulation does not persist off the axis this pattern implies that the faster spreading southern mid atlantic ridge has variable centers of time dependent upwelling that migrate along the ridge crenulations are not visible near the axis of the fast spreading pacific antarctic ridge crenulation amplitudes decrease strongly with increasing spreading rate the absence of crenulation at high spreading rate could be due to a 3d 2d transition in flow structure or the effect of mechanisms to redistribute crust along axis while crenulations are not ubiquitous even at slowly spreading ridges where they exist they provide a time record to observe the characteristics of mid ocean ridge upwelling melting and melt migration not only beneath active spreading centers but through a much longer spreading history