earth stress and reservoir quality evaluation in high and steep structure the lower cretaceous in the kuqa depression tarim basin china abstract knowledge of the state of the in situ stress field provides important insights into the petroleum engineering and geology the in situ stress orientation and magnitudes in the high and steep structure of lower cretaceous bashijiqike formation in kuqa depression were analyzed based on well log calculations and the impacts of in situ stress field on reservoir quality macroscopic reservoir properties and microscopic pore structure as well as fracture aperture were investigated the maximum horizontal stress shmax orientation was determined as nearly north south according to the image log derived drilling induced fractures and borehole breakouts the vertical stress sv shmax and minimum horizontal stress shmin magnitudes were calculated via well logs the result reveals a dominant strike slip stress regime shmax sv shmin in the bashijiqike formation in the kuqa depression the sv at the depth ranging from 5000 to 7100 m is 150 180 mpa and horizontal stresses shmax and shmin vary significantly from 130 to 210 mpa the horizontal stress difference is in the range from 40 to 70 mpa and fluid overpressures are commonly encountered the effective stress e has a positive relationship with the when the is less than 50 mpa intergranular pores can be preserved in layers with low values the microscopic pore structure will be complex and macroscopic reservoir quality will be poor with the increasing values in addition fractures are mainly corresponding with the layers where the value is low and the fracture aperture and porosity decreased significantly with the effective stress e by determining the magnitude and orientation of in situ stresses around the wellbore insights can be provided into the wellbore instability optimum drilling trajectories and reservoir quality evaluation

comparison of smectite and illite rich gouge frictional properties application to the updip limit of the seismogenic zone along subduction megathrusts along plate boundary subduction thrusts the transformation of smectite to illite within fault gouge at temperatures of 150 c is one of the key mineralogical changes thought to control the updip limit of seismicity if correct this hypothesis requires illite rich gouges to exhibit frictionally unstable velocity weakening behavior here we report on laboratory experiments designed to investigate the frictional behavior of natural and synthetic clay rich gouges we sheared 5 mm thick layers of commercially obtained pure ca smectite a suite of smectite quartz mixtures and natural illite shale grain size ranging from 2 to 500 m in the double direct shear geometry to shear strains of 7 30 at room humidity and temperature xrd analyses show that the illite shale contains dominantly clay minerals and quartz within the clay sized fraction 40 mpa our data specifically the velocity strengthening behavior of illite shale under a wide range of conditions do not support the hypothesis that the smectite illite transition is responsible for the seismic aseismic transition in subduction zones we suggest that other depth and temperature dependent processes such as cementation consolidation and slip localization with increased shearing may play an important role in changing the frictional properties of subduction zone faults and that these processes in addition to clay mineralogy should be the focus of future investigation

crustal stress determination from boreholes and rock cores fundamental principles abstract obtaining quantitative information on the state of stress in the crust can only be accomplished by accessing the zone of interest by drilling the borehole cavity however disrupts the virgin stress state by concentrating stresses in predictable patterns with a number of consequences at sufficiently low stress magnitudes the concentrated stresses amplify the elastic anisotropy azimuthally around the borehole because the elastic properties of most rocks depend nonlinearly on stress at higher levels the stresses damage then fracture the rock near the wellbore wall or within a growing core stub indeed almost all of the borehole techniques indirectly measure stress through these manifestations of the concentrated stresses this contribution reviews at a fundamental level the concentration of stresses by the borehole the effects on the materials in the borehole s vicinity and how these are used to infer stress states stress concentrations applied to rocks which are generally nonlinear elastic materials induce both radial and azimuthal variations in elastic wave speeds near the borehole that can be used to infer stress directions from advanced acoustic logging methods hydraulic fractures initiate propagate and remain propped open by measured pressures that can be interpreted with knowledge of the stress concentrations to obtain quantitative stress magnitudes at higher stress levels the rock fails producing borehole breakouts or drilling induced tensile fractures both of which are indicative of stress directions and can be used to constrain stress magnitudes similarly the various styles of drilling induced core fractures indicate both stress directions and the faulting environment encountered unfortunately no method currently allows for complete determination of the stress tensor using a number of different but complementary techniques is necessary to best constrain in situ stress states

implications of the pore pressure and in situ stress for the coalbed methane exploration in the southern junggar basin china abstract the southern junggar basin hosts abundant coalbed methane cbm resources and is expected to be an important cbm production base in china the pore pressure and in situ stress are key factors that influence the exploration and development of cbm however these factors are poorly understood in this area based on extended leak off test data mathematical models were established for pore pressure and in situ stress prediction using regression analysis subsequently the correlations between the pore pressure in situ stress and permeability were analyzed and the stress ratios were investigated finally the forming mechanism of stress zonation in the study area was discussed the results show that cbm reservoirs are generally underpressured and partly overpressured at depths greater than 800 m in the study area the study area is characterized by notable stress zoning the stress regime varies from reverse to strike slip to normal faulting with increasing depth significantly positive correlations exist between the pore pressure maximum principal stress minimum principal stress vertical stress and principal stress difference the minimum horizontal stress is approximately 70 of the vertical stress in the study area a mud weight window varying from the pore pressure to the most likely breakdown pressure is suggested at depths greater than 600 m a relatively high principal stress difference is observed which indicates that deep coals have a relatively high mechanical strength and their permeability can be enhanced by hydraulic fracturing the depth dependent stress zonation is mainly due to the influence of thrusting on the in situ stress which diminishes with increasing depth the compressional stress regime in the shallow area benefits the sealing of faults and the preservation of cbm resources in the study area the deep cbm resources have a development potential the outcomes of this study contribute to the cbm exploration and development in the study area

mid crustal ramping of the main himalayan thrust in nepal to bhutan himalaya new insights from analogue and numerical experiments abstract the mid crustal ramp on the main himalayan thrust mht in the lesser himalaya sequence lhs is the most critical geometrical asperity to trigger major seismic events in the himalaya including the 2015 mw 7 8 gorkha nepal earthquake however it is still not well understood what caused the mht to ramp up at laterally varying locations in central to eastern himalaya during the southward propagation of the himalayan tectonic wedge based on laboratory and numerical model experiments approximated to the central nepal and eastern bhutan himalayan geological settings we show that the coal bearing gondwana horizon in the stratigraphic sequence has played a crucial role in producing the mid crustal ramp on the mht during the southward propagation of the himalayan wedge the mechanically weak gondwana sequence resulted in strain localization at its northern edge and it eventually caused the mht to ramp from a depth of 20 km to 6 7 km combining our experimental and field observations with the geophysical sections we interpret that a strike wise non uniform occurrence of the gondwana horizon is responsible for the mht to ramp at different locations separated by n s trending natural barriers like tear faults the weak zone model also explains the rock uplift versus erosion pattern recorded in central nepal the ramp induced surface uplift rates calculated from our model are in good agreement with those reported from central nepal u 0 28 normalized to the convergent rate

development of polygonal fault systems a test of hypotheses polygonal networks of normal faults in layer bound sequences of fine grained mudstones have formed without regional tectonic extension the two leading hypotheses concerning the generic mechanism responsible for their development are 1 horizontal stresses are reduced by syneresis 2 coefficients of residual friction are very low to discriminate between these hypotheses the ratios between the minimum horizontal and the vertical effective stresses have been estimated in four north sea wells penetrating oligocene and miocene sequences that contain polygonal fault networks the effective stress ratios are c 0 8 consistent with very low coefficients of friction but not with syneresis

isrm suggested methods for rock stress estimation part 3 hydraulic fracturing hf and or hydraulic testing of pre existing fractures htpf contents1 introduction 10112 summaryoftestmethods 10123 assumptionsandlimitations 10123 1 hydraulicfracturing hf 10123 2 hydraulictestsonpre existingfractures htpf 10124 apparatus 10134 1 fractureorientationdetectiondevices 10145 personnelquali cation 10146 equipmentperformance 10147 hydraulicfracturingtestprocedure 10148 hydraulic fracturedelineation 10168 1 impressionpackertechnique 10168 2 geophysicalimagingtechnique 10179 obtainingcriticalpressureparametersfromthepressure timerecords 101710 stresscalculations 101810 1 hydraulicfracturing hf 101810 2 hydraulictestsonpre existingfractures 101911 reportingofresults 1019references 1020

computer simulations of california tectonics confirm very low strength of major faults recent improvements in the technique of modeling fault networks with thin plate finite elements yield full convergence of the solutions without any compromise in the representation of the frictional rheology of the upper crust or the transition to dislocation creep in the lower crust we apply these techniques to model the california region incorporating all faults with estimated slip rate over 1 mm yr as well as variations in elevation heat flow and crustal thickness velocity boundary conditions on the model sides are based on the nuvel 1 plate model and an approximation of deformation in the great basin the frictional and dislocation creep rheologic constants of the crust are calibrated to reproduce the observed variations in the maximum depth of seismicity which occurs at model temperatures of 350 410 c this leaves two free parameters the time averaged coefficient of friction on faults and the apparent activation energy for creep in the lower crust these parameters are systematically varied in three sets of 10 81 and 64 experiments respectively the predictions of each model are tested against three published data sets a set of 79 geologic limits on average slip rate on faults a set of 221 principal stress directions and a set of 841 secular rates of geodetic baselines both trilateration and vlbi very long baseline interferometry the patterns of model scores indicate that the time averaged friction coefficient of major faults is only 0 17 0 25 or only 20 30 of the value 0 85 that is assumed for the friction in the intervening blocks the laser trilateration data taken alone would indicate a preference for a high stress model but this model is one with very little fault slip and seismicity which can be rejected on other grounds in the final three parameter set of 64 models we test whether the anomalous weakness of faults is uniform or proportional to net slip with slip dependent weakening there is a general reduction in prediction errors which supports the hypothesis the geologic data and geodetic data agree in implying that fault weakness that is friction of 0 17 is general with an additional 30 slip dependent weakening of the san andreas to friction of 0 12 or 14 of normal the stress data however are fit best if all weakness is slip dependent effectively only the san andreas is weak thus the hypothesis that all weakening is slip dependent should not be rejected the best models predict all available data with rms root mean square mismatch of as little as 3 mm yr 6 of the relative plate velocity so their predictions may be useful for seismic hazard estimation at least along faults where no data are available this study extends the finding of very low friction from subduction zones where it was previously documented to a dominantly strike slip system in which underthrusting of wet sediment is not widespread return of previously subducted water from the mantle and its chemical or physical binding in fine grained gouge are possible explanations for the very low friction

determination of three dimensional in situ stresses by anelastic strain recovery in wenchuan earthquake fault scientific drilling project hole 1 wfsd 1 abstract the wenchuan earthquake fault scientific drilling project was implemented rapidly after the great 12 may 2008 earthquake mw 7 9 to better understand rupture mechanisms of the seismic faults the first borehole of the project wfsd 1 was located in hongkou township dujiangyan city sichuan province china on the hanging wall of the yingxiu beichuan fault which underwent large dextral and vertical displacement during the earthquake the near vertical borehole was 1201 m long in situ stresses were measured in rock samples from depths between 424 and 1173 m drilling depth by the anelastic strain recovery asr method the average trend of the maximum principal stress 1 was n309 ranging from n291 to n325 rotated with increasing borehole depth from nw se to wnw ese the magnitude of the in situ maximum principal stress was estimated to be 35 3 mpa at the depth of 1173 m the relations between horizontal and vertical stresses are vertical stress v maximum horizontal stress h minimum horizontal stress h at depths above 424 m h h v at depths from 424 m to 800 m and h v h below the depth of 800 m these indicate that the stress states are in a normal faulting stress regime above 424 m a reverse faulting regime from 424 m to 800 m and a dextral strike slip regime below 800 m the stress measurements from wfsd 1 are consistent with the focal mechanism solution in which the mainshock was dominated by thrusting accompanied by dextral strike slip motion the orientations of 1 are roughly consistent with the tectonic displacement direction of the longmenshan area which provides further evidence of nw se movements representing compression of the songpan ganzi block toward the sichuan basin

vegetation responses to abrupt climatic changes during the last interglacial complex marine isotope stage 5 at tenaghi philippon ne greece the discovery that climate variability during the last glacial shifted rapidly between climate states has intensified efforts to understand the distribution timing and impact of abrupt climate change under a wide range of boundary conditions in contribution to this we investigate the nature of abrupt environmental changes in terrestrial settings of the mediterranean region during the last interglacial complex marine isotope stage mis 5 and explore the relationships of these changes to high latitude climate events we present a new temporally highly resolved mean 170 years pollen record for the last interglacial complex from tenaghi philippon north east greece the new pollen record which spans the interval from 130 000 to 65 000 years ago forms part of an exceptionally long polleniferous sediment archive covering the last 1 35 million years the pollen data reveal an interglacial followed by alternating forest and steppe phases representing the interstadials and stadials of the early glacial superimposed on these millennial scale changes is evidence of persistent sub millennial scale variability we identify ten high amplitude abrupt events in the pollen record characterised by rapid contractions of closed forest to open steppe environment and interpreted to indicate major changes in moisture availability and temperature the contractions in forest cover on millennial timescales appear associated with cooling events in the mediterranean sea north atlantic and greenland regions linked to the dansgaard oeschger do cycles of the early glacial on sub millennial timescales the pattern of changes in forest cover at tenaghi philippon display a structure similar to the pattern of short lived precursor and rebound type events detected in the greenland ice core record our findings indicate that persistent high amplitude environmental variability occurred throughout the early glacial on both millennial and submillennial timescales furthermore the similarity of the pattern of change between tenaghi philippon and greenland on sub millennial timescales suggests that teleconnections between the high latitudes and the mediterranean region operate on sub millennial timescales and that some terrestrial archives such as tenaghi philippon are particularly sensitive recorders of these abrupt climate changes

rapid environmental changes in southern europe during the last glacial period oxygen isotope records from greenland ice cores1 2 indicate numerous rapid climate fluctuations during the last glacial period north atlantic marine sediment cores show comparable variability in sea surface temperature and the deposition of ice rafted debris3 4 5 in contrast very few continental records of this time period provide the temporal resolution and environmental sensitivity necessary to reveal the extent and effects of these environmental fluctuations on the continents here we present high resolution geochemical physical and pollen data from lake sediments in italy and from a mediterranean sediment core linked by a common tephrochronology our lacustrine sequence extends to the past 102 000 years many of its features correlate well with the greenland ice core records demonstrating that the closely coupled ocean atmosphere system of the northern hemisphere during the last glacial4 extended its influence at least as far as the central mediterranean region numerous vegetation changes were rapid frequently occurring in less than 200 years showing that the terrestrial biosphere participated fully in last glacial climate variability earlier than 65 000 years ago our record shows more climate fluctuations than are apparent in the greenland ice cores together the multi proxy data from the continental and marine records reveal differences in the seasonal character of climate during successive interstadials and provide a step towards determining the underlying mechanisms of the centennial millennial scale variability

effect of clay content and mineralogy on frictional sliding behavior of simulated gouges binary and ternary mixtures of quartz illite and montmorillonite 1 we investigated the frictional sliding behavior of simulated quartz clay gouges under stress conditions relevant to seismogenic depths conventional triaxial compression tests were conducted at 40 mpa effective normal stress on saturated saw cut samples containing binary and ternary mixtures of quartz montmorillonite and illite in all cases frictional strengths of mixtures fall between the end members of pure quartz strongest and clay weakest the overall trend was a decrease in strength with increasing clay content in the illite quartz mixture the trend was nearly linear while in the montmorillonite mixtures a sigmoidal trend with three strength regimes was noted microstructural observations were performed on the deformed samples to characterize the geometric attributes of shear localization within the gouge layers two micromechanical models were used to analyze the critical clay fractions for the two regime transitions on the basis of clay porosity and packing of the quartz grains the transition from regime 1 high strength to 2 intermediate strength is associated with the shift from a stress supporting framework of quartz grains to a clay matrix embedded with disperse quartz grains manifested by the development of p foliation and reduction in riedel shear angle the transition from regime 2 intermediate strength to 3 low strength is attributed to the development of shear localization in the clay matrix occurring only when the neighboring layers of quartz grains are separated by a critical clay thickness our mixture data relating strength degradation to clay content agree well with strengths of natural shear zone materials obtained from scientific deep drilling projects

lithology dependent minimum horizontal stress and in situ stress estimate abstract based on the generalized hooke s law with coupling stresses and pore pressure the minimum horizontal stress is solved with assumption that the vertical minimum and maximum horizontal stresses are in equilibrium in the subsurface formations from this derivation we find that the uniaxial strain method is the minimum value or lower bound of the minimum stress using anderson s faulting theory and this lower bound of the minimum horizontal stress the coefficient of friction of the fault is derived it shows that the coefficient of friction may have a much smaller value than what it is commonly assumed e g f 0 6 0 7 for in situ stress estimate using the derived coefficient of friction an improved stress polygon is drawn which can reduce the uncertainty of in situ stress calculation by narrowing the area of the conventional stress polygon it also shows that the coefficient of friction of the fault is dependent on lithology for example if the formation in the fault is composed of weak shales then the coefficient of friction of the fault may be small as low as f 0 2 this implies that this fault is weaker and more likely to have shear failures than the fault composed of sandstones to avoid the weak fault from shear sliding it needs to have a higher minimum stress and a lower shear stress that is the critically stressed weak fault maintains a higher minimum stress which explains why a low shear stress appears in the frictionally weak fault

borehole stability analysis accounting for anisotropies in drilling to weak bedding planes abstract borehole instabilities pose significant challenges to drilling and completion operations particularly in regions with weak bedding planes and pre existing fractures where formations have strong anisotropies the bedding planes rock anisotropy and their impacts on horizontal stresses are considered in the proposed model to improve borehole stability modeling this improved model enables to calculate borehole failures and minimum mud weight along borehole trajectories with various drilling orientations versus bedding directions laboratory test data of rock compressive strengths are analyzed and a new correlation is developed to allow for predicting uniaxial compressive strengths in weak rocks from sonic velocities time dependent rock compressive strength is also examined to analyze the wellbore failure evolution with time the slip failure gradient in the weak planes is derived which can be used to model wellbore sliding shear failure in the planes of weakness the mud weight applied to prevent borehole shear failures in both intact rocks and ones with weak bedding planes can be obtained from the proposed model

the niger delta basin fracture pressure prediction accurate knowledge of formation fracture pressure is essential to optimize well design at all stages of the field development however erroneous prediction of formation fracture pressure can lead to process safety incidents such as surface and underground blowouts while fracture pressure prediction models have been developed for some sedimentary basins it is difficult to transfer these models to areas beyond the regions of study in the niger delta basin few fracture pressure prediction models have been developed however these models were developed primarily from leak off test data acquired from the normally pressured intervals basically the existing niger delta fracture pressure prediction models lack the leak off test measurements in the overpressure intervals because such data are not available in this paper a new fracture pressure prediction model that can be applied to normally pressured intervals and overpressure zones is being proposed model development is based on establishing a relationship between fracture pressure true vertical depth and magnitude of overpressure using several leak off test data acquired from over 100 wells in various fields scattered across the basin unlike the previous models the newly developed model incorporates leak off test measurements from the overpressure intervals in the basin in general the newly proposed model can be used with a high degree of confidence to predict the fracture pressure required for safe and economic well planning across the entire basin

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

enhanced seasonality of precipitation in the mediterranean during the early part of the last interglacial the deposition of sapropels in the eastern mediterranean sea is thought to occur during intervals of intensified african monsoon and increased precipitation in the mediterranean borderlands speleothem and pollen records however reveal conflicting evidence for a mediterranean wide precipitation increase suggesting that seasonal changes in the hydrological regime may be important using a multiproxy record we present the first independent evidence for seasonality of precipitation during the early last interglacial ca 130 119 ka from the tenaghi philippon peatland in northeast greece during the early part of the interglacial mineralogical macrofossil and pollen records from the same core show a shift from mire to lacustrine conditions simultaneous with an expansion of sclerophyllous vegetation and the presence of acicular aragonite indicating the onset of highly evaporative summer conditions this indicates enhanced seasonality of precipitation and reconciles the apparent incongruity between mediterranean pollen and speleothem records it also provides evidence for significantly increased winter precipitation coeval with the deposition of sapropel s5 one of the most prominent sapropels of the pleistocene we suggest that in addition to the summer african monsoon component increased winter precipitation from the northern mediterranean borderlands may have contributed to maintaining reduced surface water salinities in the mediterranean sea over the entire year

fault structure frictional properties and mixed mode fault slip behavior abstract recent high resolution gps and seismological data reveal that tectonic faults exhibit complex multi mode slip behavior including earthquakes creep events slow and silent earthquakes low frequency events and earthquake afterslip the physical processes responsible for this range of behavior and the mechanisms that dictate fault slip rate or rupture propagation velocity are poorly understood one avenue for improving knowledge of these mechanisms involves coupling direct observations of ancient faults exhumed at the earth s surface with laboratory experiments on the frictional properties of the fault rocks here we show that fault zone structure has an important influence on mixed mode fault slip behavior our field studies depict a complex fault zone structure where foliated horizons surround meter to decameter sized lenses of competent material the foliated rocks are composed of weak mineral phases possess low frictional strength and exhibit inherently stable velocity strengthening frictional behavior in contrast the competent lenses are made of strong minerals possess high frictional strength and exhibit potentially unstable velocity weakening frictional behavior tectonic loading of this heterogeneous fault zone may initially result in fault creep along the weak and frictionally stable foliated horizons with continued deformation fault creep will concentrate stress within and around the strong and potentially unstable competent lenses which may lead to earthquake nucleation our studies provide field and mechanical constraints for complex mixed mode fault slip behavior ranging from repeating earthquakes to transient slip episodic slow slip and creep events

skarn mineralization cr fe au in an archean greenstone belt white river property hemlo area ontario three prominent mineralized calc silicate occurrences cr rich skarn fe rich skarn and ree enriched calc silicates are present in close association with regional planar structures at the white river property of lac minerals ltd in the archean hemlo heron bay greenstone belt of the superior province ontario canada the cr and fe rich skarns were metasomatically derived respectively from ultramafic mafic host rocks at 400 degrees or 50 degrees c 3 kbars and an aqueous fluid x sub co 2 2

last interglacial arctic warmth confirms polar amplification of climate change the warmest millennia of at least the past 250 000 years occurred during the last interglaciation when global ice volumes were similar to or smaller than today and systematic variations in earth s orbital parameters aligned to produce a strong positive summer insolation anomaly throughout the northern hemisphere the average insolation during the key summer months m j j was ca 11 above present across the northern hemisphere between 130 000 and 127 000 years ago with a slightly greater anomaly 13 over the arctic greater summer insolation early penultimate deglaciation and intensification of the north atlantic drift combined to reduce arctic ocean sea ice allow expansion of boreal forest to the arctic ocean shore across vast regions reduce permafrost and melt almost all glaciers in the northern hemisphere insolation amplified by key boundary condition feedbacks collectively produced last interglacial summer temperature anomalies 4 5 c above present over most arctic lands significantly above the average northern hemisphere anomaly the last interglaciation demonstrates the strength of positive feedbacks on arctic warming and provides a potentially conservative analogue for anticipated future greenhouse warming

dual porosity poroelastic analyses of wellbore stability this paper presents a numerical solution of dual porosity poroelastic formulations that couple solid deformations with fluid flow in both matrix and fracture systems of naturally fractured reservoirs the finite element numerical method was applied to solve the inclined wellbore problem in which mud weight was considered for both permeable and impermeable boundary conditions failure criteria relevant to tensile collapse and shear failure modes were introduced into the numerical model and resulting failure areas around boreholes were subsequently defined several related applications including the stability of inclined and horizontal wellbores under various in situ stress regimes were evaluated upper and lower mud weight bounds and the most stable borehole orientations were determined the best trajectory selections for horizontal boreholes were also investigated

abrupt temperature changes in the western mediterranean over the past 250 000 years a continuous high resolution western mediterranean sea surface temperature sst alkenone record spanning the past 250 000 years shows that abrupt changes were more common at warming than at cooling during marine isotope stage mis 6 sst oscillated following a stadial interstadial pattern but at lower intensities and rates of change than in the dansgaard oeschger events of mis 3 some of the most prominent events occurred over miss 5 and 7 after prolonged warm periods of high stability climate during the whole period was predominantly maintained in interglacial interstadial conditions whereas the duration of stadials was much shorter

discussion on integrating borehole breakout dimensions strength criteria and leak off test results to constrain the state of stress across the chelungpu fault taiwan abstract the in situ stress computations presented by haimson et al 2010 overestimated the maximum horizontal stress in the vicinity of the chelungpu fault taiwan our analyses from extended leak off tests drilling induced tensile fractures and anderson s faulting theory show considerable lower values for the maximum horizontal stress our results suggest that closer to the ruptured fault more s h is released implying a strong earth crest in the shallower section which is far away from the ruptured fault however it would require more detailed and intensive work in order to achieve an accurate interpretation of the in situ stress in this region

in situ stresses abnormal pore pressures and their impacts on the triassic xujiahe reservoirs in tectonically active western sichuan basin abstract pore pressures in the xujiahe gas bearing formations are highly overpressured in the western sichuan basin the abnormally high overpressure and strong tectonic stress cause very high horizontal stresses the high in situ stresses tectonic stress regimes and tight formations severely impact development and production of the xujiahe tight sandstone reservoirs overpressure generation mechanisms are analyzed based on measured data in several dozen wells in the studied gas fields it is found that the sonic or seismic transit time can be used to predict pore pressure two major reservoirs xu2 and xu4 belong to different fluid compartments with distinct pore pressure gradients the xu4 has a much higher pore pressure gradient 20 mpa km than the xu2 15 6 mpa km measured data from hydraulic fracturing tests and borehole image logs in vertical wells are analyzed the results show that the minimum horizontal stress gradient in the xu4 reservoir reaches as high as 24 mpa km close to the overburden gradient the xu2 reservoir has a slightly lower minimum horizontal stress gradient the high minimum horizontal stress and high rock strength cause abnormally high formation breakdown pressure gradient 24 8 34 mpa km the high breakdown pressure high propagation pressure and unfavorable stress regime result in a great difficulty for the reservoir stimulation one of the major impacts is that hydraulic fracturing operations can only create very short hydraulic fractures which markedly limits reservoir productivity based on field measurements in situ stress determination and assessment are given and a new method for horizontal stress estimation is proposed accordingly reliable prediction of formation breakdown and propagation pressures are proposed which are more suitable for the tight reservoirs recommendations are given for effective development of the tight reservoirs with impacts of high in situ stresses

the strength of faults in the crust in the western united states abstract the strength of secondary faults within plate boundary zones and of master faults like the san andreas has been controversial for decades we use a global finite element code with a variable resolution grid and global plate driving forces to determine whether the effective friction on the san andreas fault is high 0 6 1 intermediate 0 3 0 5 or low 0 2 whether a single value of can be used for all mapped faults within california and whether weakening of the ductile lower crust associated with faulting is important we compare our model results with existing data on fault slip rates gps velocities stress field and earthquake depth distribution the comparison indicates that all faults are weak 0 2 and that additional weakening of major faults is important all viable solutions also indicate that weakening of the lower crust below major faults is necessary the strongest faults in the region have in the range 0 2 0 05 the san andreas fault is a very weak fault among weak faults with

layering in shales controls microfracturing at the onset of primary migration in source rocks the process of primary migration which controls the transfer of hydrocarbons from source to reservoir rocks necessitates the existence of fluid pathways in low permeability sedimentary formations primary migration starts with the maturation of organic matter which produces fluids that increase the effective stress locally the interactions between local fluid production microfracturing stress conditions and transport remain difficult to apprehend in shale source rocks here we analyze these interactions using a coupled hydro mechanical numerical model based on the discrete element method the model is used to simulate the effects of fluid production emanating from kerogen patches contained within a shale rock alternating kerogen poor and kerogen rich layers we identify two microfracturing mechanisms that control fluid migration i propagation of hydraulically driven fractures induced by kerogen maturation in kerogen rich layers and ii compression induced fracturing in kerogen poor layers caused by fluid overpressurization of the surrounding kerogen rich layers the relative importance of these two mechanisms is discussed considering different elastic properties contrasts between the shale layers as well as various stress conditions encountered in sedimentary basins from normal to reverse faulting regimes the layering in shales causes local stress redistribution that controls the prevalence of each mechanism over the other and the onset of microfracturing during kerogen maturation the approach is applied to the draupne formation a major source rock in the norwegian continental shelf in the north sea

in situ stress pore pressure and stress dependent permeability in the southern qinshui basin this study focuses on the in situ stress pore pressure and permeability in the southern qinshui basin one of largest coalbed methane basins in china well tests show that permeability in this basin is higher than other coalbed methane reservoirs this is because it is located in an extensional basin where the normal faulting stress regime is dominated this in situ stress regime is advantageous to keep coal cleats open hydraulic fracturing tests indicate that the fracture gradient or minimum horizontal stress is much lower than the shales in the gulf of mexico and other oil basins the minimum horizontal stress model is proposed with consideration of the stress coefficient based on the uniaxial strain method this model provides a fairly good prediction on the minimum stress permeability data show that the effective stress dependent permeability is pronounced in the coalbed methane reservoir this is significant for the dual porosity and dual permeability coal reservoirs which consist of coal porous matrices and cleats the reason is that a rapid increase in effective stress can induce the closure of cleats which may cause a permanent loss of permeability in the cleats this reduces the connectivity between the cleats and coal matrices hence the coal matrices cannot deliver gas pressure to the cleats for supporting the cleat space therefore slowing down the effective stress change during production e g slowing reservoir drawdown can decelerate the permeability reduction this is particularly important for the reservoir in which the pore pressure is not significantly overpressured such that in the southern qinshui basin

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

on the timing and mechanism of millennial scale climate variability during the last glacial cycle the demonstration that natural climate variability during the last glacial cycle shifted rapidly between remarkable extremes has dramatically revised the understanding of climate change to further advance our understanding research continues into the timings geographic distribution and nature of the millennial scale climate extremes and into the mechanisms for intra and inter hemispheric transmission of variability through the climate ocean system complementing the traditional definition of the timings of millennial scale climate variability from ice core 18o records we here further narrow down the temporal constraints by determining statistically significant anomalies in the major ion series of the gisp2 ice core this exercise offers an objective definition of the timing of climatic anomalies in northern hemisphere palaeoclimate proxy records of the last 110 000 years that significantly improves the potential for inter calibration of ice core tuned chronostratigraphies we then present a process oriented synthesis of proxy records from the northern hemisphere this leads to a conclusion that the dansgaard oeschger d o style fluctuations in these records are virtually in phase since all fall within a clear atmospheric pattern of concerted relative dominance shifts between polar westerly dominated winter type conditions and tropical monsoon dominated summer type conditions finally we speculate on a monsoon related mechanism that could help explain the anomalously long duration of d o interstadials 12 8 and 1 which coincided with cooling trends in antarctic records

character of vegetational and environmental changes in southern europe during the last glacial period evidence from lesvos island greece this paper presents high resolution results of palynological and sedimentological analyses undertaken on two sediment cores from the megali limni ml basin an area characterised by serpentine soils in the southeastern part of lesvos island greece six tephra horizons and multiple radiocarbon dates provide independent controls towards the development of a chronological framework the composite pollen record spans the period from 22 to 62 thousand years ago ka bp and shows a number of oscillations between steppe forest steppe and forest in concert with north atlantic millennial scale variability vegetation during the late marine isotope stage mis 4 was grassland steppe indicating cold and arid conditions while sediment composition suggests increased erosion rates arboreal populations dominated by pinus and to a lesser extent deciduous quercus expanded during mis 3 interstadials suggesting increases in precipitation and temperature within the course of the longer interstadials changes in vegetation composition point to a trend towards increased aridity and sometimes decreasing winter temperatures during intervening stadials vegetation was composed mainly of artemisia chenopodiaceae and gramineae indicating reversals to arid and cold conditions with most extreme conditions recorded during stadials corresponding to heinrich events during the course of mis 3 the basin was progressively infilled with sediments only a small portion of mis 2 is represented in the sequence showing a short lived expansion of arboreal populations comparisons with other pollen sequences from southern europe underscore the important role of pinus throughout the last glacial period a reflection of the serpentine soils of the megali limni area where pinus brutia dominates today

influence of poroelastic behavior on the magnitude of minimum horizontal stress sh in overpressured parts of sedimentary basins in many sedimentary basins of the world the minimum hori zontal stress s h is greater in overpressured zones than in normally pressured zones at equivalent depths a common explanation is that the frictional slip on listric normal faults keeps the difference between vertical stress s v and s h within certain bounds and the difference is smaller under lower effective stress i e higher pore pressure p p however in the overpressured parts of the central north sea graben united kingdom and the sable subbasin of the scotian shelf canada conventional friction envelopes underestimate the magnitude of s h these data instead indicate that s h increases at a rate proportional to but less than the rate of increase of p p a condition consistent with a p p induced deformation of the rock called poroelastic behavior this paper argues that whereas friction may govern s h in normally pressured basins poroelastic behavior is responsible for the unusually high s h in the overpres sured parts of these same basins data on the p p and s h gradients from these basins suggest that s h p p 0 7

the strength of large scale plate boundaries constraints from the dynamics of the philippine sea plate since 5ma abstract friction controls most of the strength of tectonic plate boundaries and thus the force mutually exchanged between plates estimates of the plate boundary friction coefficient are therefore of paramount importance to our understanding of the lithosphere torque balance however several lines of evidence indicate that the friction coefficient of plate margins is significantly lower than is measured in laboratory experiments on fractured rocks this poses a significant limit to extrapolation from the laboratory scale to the large scale of earth s lithosphere the record of past and present day plate motions represents an efficient probe into the torque balance of plate tectonics because plate motions readjust virtually instantaneously to temporal changes in driving and resisting forces here i derive inferences on the strength of large scale plate boundaries by focusing on the convergence of the fast moving philippine sea plate towards eurasia since subduction initiation 5 myr ago because at the present day the philippine slab reaches depths shallower than the 410 km transition zone in the upper mantle its weight is unlikely to provide sufficient driving force to shear the trailing plate over the viscous mantle at the observed rates using global models of the coupled mantle lithosphere system i argue that frictional coupling with the pacific and eurasia plates is the main driver for the dynamics of the philippine sea plate this allows me to infer that the friction coefficient of large scale plate boundaries is in range 0 01 0 07 and that the amount of sediments entering plate margins efficiently modulates the friction coefficient contributing significantly to its lateral variations importantly i corroborate my conclusions by looking at the present day vertical deformation of the philippine sea eurasia margin evident from observed trench parallel gravity anomalies

stress inversion and basement cover stress transmission across weak layers in the paris basin france we investigate the source of non purely gravitational horizontal stresses in the paris basin a nowadays tectonically quiet intracratonic basin in its eastern border of which outstandingly dense stress measurements are available based on a synthesis of published data the stress state in the basin is first shown to be very close to the one that may be extrapolated for the underlying basement in terms of principal stress orientations and horizontal to vertical stress ratios this is in favour of a mechanical coupling between the basement and its sedimentary cover which may seem contradictory to the presence of several weak rock layers in the basin fill e g an argillite layer that was shown to bear low deviatoric stresses and salt layers that are implicated in a major decollement elsewhere to unravel this apparent contradiction a 3d numerical modelling is performed following a rigorous inverse problem approach to determine the long term elastic properties of both the basement and the basin rocks the objective is to find the set of elastic constants that provides the best fit between the calculated stress state in the basin and the in situ data by assuming that the stress state in the basement is known this methodology provides a realistic set of mechanical parameters in agreement with previous studies which leads to the conclusion that the horizontal stresses in the basin constitute its mechanical response to the stresses that developed in the underlying basement during and since the last tectonic event alpine phase the fact that horizontal stresses could be transmitted across the weak horizons contrary to what may be expected at first glance is explained both by the geometry of the basin and the fact that over the long term the stiffnesses of the various sedimentary rocks are only slightly different from each other

european climatic response to millennial scale changes in the atmosphere ocean system during the last glacial period abstract pollen foraminifer dinocyst and coarse lithic high resolution analyses and 18 o measurements have been carried out for the last glacial section of marine core md95 2042 located near the southwestern margin of the iberian peninsula the pollen data indicate a high frequency of vegetational changes on the adjacent continent during this period suggesting a climatic variability very similar to that of the dansgaard oeschger cycles recorded by the greenland ice cores the detailed direct correlation of the terrestrial and marine proxy data from core md95 2042 indicates a three phase pattern of heinrich events in land and ocean environments the first and last phases of the h5 and h4 events are characterized by a mild and humid climate in southwestern europe probably associated with the european origin of the ice rafted detritus the middle phase exhibits a cold and dry climate in iberia linked with the maximum input of ice rafted detritus this phase seems to correspond with the laurentide ice sheet surges between the heinrich events several cold and dry periods on land are correlated with stades of the dansgaard oeschger cycles the impact of the dansgaard oeschger stades in southwestern europe seems to be preferentially connected to the cold winter air masses reaching this mid latitude region