an experimental study of the relationship between joint spacing and layer thickness abstract two methods for measuring joint spacing are described and compared the area method is a constant for a given outcrop area and is not affected by joint distribution within that area in contrast the line method depends on the location of the linear traverse two kinds of joint sets are distinguished on bedding surfaces 1 a poorly developed set represents the early stages of development when typical joint lengths are less than typical spacing 2 a well developed set represents later stages when lengths are much greater than spacing the area method is applicable to both poorly and well developed sets whereas the line method produces inconsistent results for poorly developed joint sets surface textures on many joints indicate point fracture origins and propagation parallel to bedding whereas most numerical model studies of spacing assume linear origins and propagation perpendicular to bedding the laboratory experiments described in this paper do not suffer from these restrictions an important concept confirmed during these experiments is fracture saturation when the applied strain reaches a certain value fracture spacing stops evolving and remains nearly constant these fracture sets are well developed spacing at saturation is a function of layer thickness but is independent of strain whereas spacing before saturation varies strongly with applied strain thus plotting spacing vs thickness and comparing the slopes of lines fit to such data for poorly developed joint sets in different layers is unlikely to be a diagnostic test for differences in material properties on the other hand spacing may be a sensitive indicator of strain for layers with poorly developed joint sets assessing fracture saturation is a lirst order consideration when gathering spacing data

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

dependence of joint spacing on rock properties in carbonate strata joint density is studied in relation to petrographic and petrophysical parameters in two sedimentary carbonate formations characterized by different diagenetic histories the kimmeridgian limestones of the chay peninsula western france with a mean joint density of 6 37 fractures per meter fr m 1 94 fr ft and the bathonian limestones of the bouye outcrop western france with a mean joint density of 1 9 fr m 0 58 fr ft the chay carbonates are characterized by a lower caco3 content a higher average porosity and a lower sound velocity than values recorded in the bouye limestones the compressive strength and young s modulus of the bouye carbonates are respectively 10 and 3 times higher than in the chay carbonates a statistical analysis was used to identify relationships between joint density and carbonate rock properties when facies variations are marked the joint density at outcrop scale is related to the mean bed thickness the facies descriptors and the young s modulus when textural variations are more limited the joint density is controlled by the porosity at the scale of a sedimentary basin 63 8 of the variation in joint density may be accounted for by young s modulus and the sparite micrite ratio the decrease in the sparite micrite ratio reflects an increased number of grain boundaries in the carbonate rock which limits grain deformation and enhances joint density the variations in young s modulus depend essentially on the porosity and mineralogy of the studied rocks any increase in caco3 content or decrease in porosity is associated with an increase in the elastic properties of the rock and a reduction of joint density

late paleoproterozoic extension and a paleostress field reconstruction of the north china craton abstract mafic dyke swarms and aulacogens are major anorogenic extensional events in the late paleoproterozoic north china craton ncc the n nnw mafic dyke swarms are widespread in the ncc whose ages span between 1 83 and 1 77 ga the similar ages and orientations of 1 8 ga dyke swarms in the ncc demonstrate that the amalgamated ncc experienced widespread extension at this time based on the width statistics of dyke swarms on ten survey lines an average crustal extension ratio of 0 35 was found for the ncc the small magnitude of overall extension suggests that the mafic dyke swarms were emplaced into the elastic fractures and indicates that the ncc had become a brittle plate prior to the emplacement of the mafic dyke swarms precisely dated mafic dyke swarms when used as paleostress indicators can be employed in the paleostress field reconstruction of precambrian cratons two dimensional finite element modeling 2 d fem of the ncc in which the various blocks were assigned densities and elastic constants shows that north south compression favors dyke intrusion along generally n nw lines and that deviations in dyke trends can be explained by the effects of boundary constraints and the physical properties of the crust the best fitting model can be considered a plausible representation of the tectonic force acting on the ncc that produces the intraplate stress field that is most consistent with the observed orientation of dyke swarms the results of modeling of the late paleoproterozoic stress field suggest a common tectonic setting for the emplacement of mafic dyke swarms in the central orogenic zone western and east blocks of the ncc the results also show that the north south tectonic forces play an important role in determining the paleostress field in the ncc the widespread extension of the ncc resulted from the north south tectonic forces which may be related to the break up of the late paleoproterozoic supercontinent the paleostress field modeling provides a possible approach to consider the supercontinent paleostress reconstruction and to reveal the mechanisms of the supercontinent break up

effects of internal structure and local stresses on fracture propagation deflection and arrest in fault zones the way that faults transport crustal fluids is important in many fields of earth sciences such as petroleum geology geothermal research volcanology seismology and hydrogeology for understanding the permeability evolution and maintenance in a fault zone its internal structure and associated local stresses and mechanical properties must be known this follows because the permeability is primarily related to fracture propagation and their linking up into interconnected clusters in the fault zone here we show that a fault zone can be regarded as an elastic inclusion with mechanical properties that differ from those of the host rock as a consequence the fault zone modifies the associated regional stress field and develops its own local stress field which normally differs significantly both as regard magnitude and orientation of the principal stresses from the regional field the local stress field together with fault rock heterogeneities and interfaces discontinuities fractures contacts determine fracture propagation deflection along discontinuities interfaces and arrest in the fault zone and thereby its permeability development we provide new data on the internal structure of fault zones in particular the fracture frequency in the damage zone as a function of distance from the fault core new numerical models show that the local stress field inside a fault zone modelled as an inclusion differ significantly from those of the host rock both as regards the magnitude and the directions of the principal stresses also when the mechanical layering of the damage zone due to variation in its fracture frequency is considered the numerical models show abrupt changes in local stresses not only between the core and the damage zone but also within the damage zone itself abrupt changes in local stresses within the fault zone generate barriers to fracture propagation and contribute to fracture deflection and or arrest also analytical solutions of the effects of material toughness the critical energy release rate of layers and their interfaces show that propagating fractures commonly become deflected into and often arrested at the interfaces generally fractures propagating from a compliant soft layer towards a stiffer one tend to become deflected and arrested at the contact between the layers whereas fractures propagating from a stiff layer towards a softer one tend to penetrate the contact thus it is normally easier for fractures to propagate from the host rock into the damage zone than vice versa similarly it is easier for fractures to propagate from the outer stiffer parts of the damage zone to the inner softer parts and from the stiff host rock to the outer damage zone than in the opposite directions these conclusions contribute to increased understanding as to how fractures propagate and become arrested within fault zones and how the fault zone thickness is confined at any particular time during its evolution

constraining tectonic compression processes by reservoir pressure evolution overpressure generation and evolution in the kelasu thrust belt of kuqa foreland basin nw china abstract the kuqa foreland basin kfb immediately south of the south tianshan mountains is a major hydrocarbon producing basin in west china the kelasu thrust belt in the basin is the most favorable zone for hydrocarbon accumulations widespread overpressures are present in both the cretaceous and paleogene reservoirs with pressure coefficients up to 2 1 the tectonic compression process in kfb resulted from the south tianshan mountains uplift is examined from the viewpoint of the overpressure generation and evolution in the kelasu thrust belt the overpressure evolution in the reservoir sandstones were reconstructed through fluid inclusion analysis combined with pvt and basin modeling overpressures at present day in the mudstone units in the kelasu thrust belt and reservoir sandstones of the dabei gas field and the keshen zone are believed to have been generated by horizontal tectonic compression both disequilibrium compaction and horizontal tectonic compression are thought to contribute to the overpressure development at present day in the reservoir of the kela 2 gas field with the reservoir sandstones showing anomalously high primary porosities and low densities from wireline log and core data the overpressure evolution for the cretaceous reservoir sandstone in the kelasu thrust belt evolved through four stages a normal hydrostatic pressure 12 5 ma a rapidly increasing overpressure 5 3 ma an overpressure release 3 1 64 ma and overpressure preservation 1 64 0 ma overpressure developed in the second stage 5 3 ma was generated by disequilibrium compaction as tectonic compression due to the uplift of the tianshan mountains acted at the northern monocline of kfb from 5 ma to 3 ma which provided abundant sediments for the kfb and caused the anomalously high sedimentation rate during the n2k deposition from 3 ma to 1 64 ma the action of tectonic compression extended from the northern monocline to the kelasu thrust belt and returned to the northern monocline of kfb from 1 64 ma to present day therefore the horizontal tectonic compression was the dominant overpressure mechanism for the overpressure generation in the third stage 3 1 64 ma and overpressure caused by disequilibrium compaction from 5 ma to 3 ma was only preserved in the kela 2 gas field until present day

characteristics and deformation mechanism of salt related structures in the western kuqa depression tarim basin insights from scaled sandbox modeling abstract three scaled analog sandbox models are constructed to simulate the evolution of thick skinned shortening on broad and narrow salt basins these models simulate the evolution of the cenozoic salt salt related structures in the westernmost kuqa salt bearing depression quele area tarim basin the formation mechanism and the controlling factors of structural deformation in this area are investigated and discussed systematically model results show that the variable depositional width of the salt basin has a major controlling influence on the differential structural deformation in different longitudinal segmentations from east broad to west narrow in the western kuqa depression quele area model results also demonstrate that the early salt structure salt diapir has an important controlling role on the subsequent compressional structural deformation the pre existing thin overburden quele diapir which acts as a stress concentration zone during the subsequent shortening is deformed to the quele salt nappe on the basis of the model results it is suggested that the formation of the long distance quele allochthonous salt sheet is related with the pre existing quele diapir which displaces up paralleling the hanging wall and flows to the surface during the subsequent shortening

mechanics of the giant radiating mackenzie dyke swarm a paleostress field modeling 1 the 1 27 ga mackenzie dyke swarm of the canadian shield is a giant radiating dyke swarm that gradually swings in orientation from n s in the focal area to nw se trends in peripheral areas in this paper we propose a new model the plug model that accounts for the paleostress contribution to the mechanism of emplacement for the mackenzie dyke swarm in the canadian shield the 1 27 ga stress field on the canadian shield calculated by the plug model explains the radiating nature of the mackenzie dyke swarm around the coppermine river lava field by local stress concentrations the parallel nature of the dyke swarm at distance more than 1000 km from the focal source can be explained by the existence of a regional tectonic stress field created by ridge push acting on the southeast margin of the canadian shield from the grenville ocean the thin elastic plate and two dimensional cross section modeling suggest that the interaction between stresses from a mantle upwelling and the grenville ocean spreading play an important role in the intrusion mechanism of the mackenzie dyke swarm the change in dyke orientation from n s trending to nw se trending is caused by coupling between resistance from the focal area plug area and a grenville ocean ridge push

open shear fractures at depth in anticlines insights from the kuqa foreland thrust belt tarim basin shear fractures that close under compressive stress regime were described by previous workers here we show the existence of hinge parallel shear opening mode fractures at a certain depth within an anticline structure through core observations formation micro imager fmi log interpretations and production data studied reservoir is a tight sandstone body of cretaceous age which is emplaced in ks2 anticline with depth ranging between 6000 and 8100 m we explore the mechanism of hinge parallel shear fractures by stereographic technique and backtilting as well as show field evidences for opening displacements these shear fractures mainly consist of vertical tension set moderate dipping shear set steeply dipping shear set and sub horizontal shear set crosscutting relationships and surface textures suggest that although the moderate dipping shear set is older and maybe sealed by cements it may have been opened by local stress due to outer arc extension through folding calcite bridges with rhombohedral structures may support fracture walls helping them to remain open in case of compressive stress our work documents shear fracture openings in the anticline at depth and fluid flow in subsurface reservoirs

the relationship between joint aperture spacing distribution vertical dimension and carbonate stratification an example from the kimmeridgian limestones of pointe du chay france joint aperture and joint development have been studied in the kimmeridgian limestones of the pointe du chay at the northern boundary of the jurassic biscay basin france at belette outcrop in some layers the mean joint spacing of the n120 joint set is close to mean layer thickness there the classical spacing to thickness relationship appears to be valid in the competent carbonate layers that are included in a more argillaceous matrix at pillar outcrop the n10 joint set is characterised by a high level of joint density and a non saturated spacing distribution as indicated by the mode mean ratio values and the cv values cv is the ratio of standard deviation to mean fracture spacing the classical relationship between layer thickness and fracture spacing has not been observed at the pillar outcrop joint aperture reaches larger values at the pillar outcrop than at the belette outcrop where aperture is more homogeneously distributed almost all the joints are opened with moderate aperture values at belette outcrop whereas most joints with large vertical dimension have large apertures at pillar outcrop and smaller fractures are closed or poorly opened from two outcrops that have been subjected to the same geological conditions apertures of non stratabound joints appear to be controlled by the vertical dimension of the joints whereas stratabound joints are more regularly spaced and opened

fracture zones constrained by neutral surfaces in a fault related fold insights from the kelasu tectonic zone kuqa depression abstract stress variation associated with folding is one of the controlling factors in the development of tectonic fractures however little attention has been paid to the influence of neutral surfaces during folding on fracture distribution in a fault related fold in this study we take the cretaceous bashijiqike formation in the kuqa depression as an example and analyze the distribution of tectonic fractures in fault related folds by core observation and logging data analysis three fracture zones are identified in a fault related fold a tensile zone a transition zone and a compressive zone which may be constrained by two neutral surfaces of fold well correlation reveals that the tensile zone and the transition zone reach the maximum thickness at the fold hinge and get thinner in the fold limbs a 2d viscoelastic stress field model of a fault related fold was constructed to further investigate the mechanism of fracturing statistical and numerical analysis reveal that the tensile zone and the transition zone become thicker with decreasing interlimb angle stress variation associated with folding is the first level of control over the general pattern of fracture distribution while faulting is a secondary control over the development of local fractures in a fault related fold

some considerations on the kinematics of chevron folds geometrical modelling and field analysis of chevron folds suggest that these structures are a result of the combination of several kinematical mechanisms whose magnitude and order of application vary within certain limits a possible mechanism operating early in the fold growth is homogeneous layer shortening whose contribution is restricted by the high competence contrast that the multilayers developing chevron folds usually exhibit in the early stages of folding when the curvature is small equiareal tangential longitudinal strain etls is an essential mechanism since the operation of parallel tangential longitudinal strain ptls or flexural flow ff would give rise respectively to area changes or strain in the limbs of the final fold that are too high to be geologically realistic after folding by etls probable mechanisms are ptls and ff which can operate in this order or simultaneously in general ff is necessary at the last stage of buckling although the increment of folding due to this mechanism can be very small high values of slip between layers and area change produced in the later stages of chevron folding can bring an end to buckling probably at an interlimb angle value of 60 70 and induce the onset of homogeneous strain hs this strain is not coaxial in many cases with simple shear playing an important role and gives rise to asymmetrical folds

coupling and transition of meso cenozoic intracontinental deformation between the taihang and qinling mountains abstract during the mesozoic and cenozoic the qinling mountains formed a nearly e w striking uplifted mountain range separating the north china and south china blocks while the taihang mountains formed a s n striking uplifted mountain range within the central north china block thus the strikes of both are perpendicular to each other in order to investigate the exhumation uplift history and transition of tectonic stress fields between these two mountain ranges within the eurasian plate by systematic comparison and analysis of their regional compressive and extensional tectonics and contrasting the transitions of tectonic regimes during the mesozoic and cenozoic interval it is concluded that the transition period of the two tectonic regimes occurred around the late early cretaceous through analysis and comparison of coeval stress fields in different tectonic units it is revealed that their shortening and compressive deformation is related to intraplate heterogeneity of lithospheric structures caused by the formation of ancient orogenic belts and their subsequent extension is closely related to penetrative intraplate rifting in east china therefore the extensional tectonics displays the formation of wide rift valleys whose overall tectonic framework is inherited from its pre existing basement tectonic framework in summary the difference between intraplate deformations is controlled by pre existing basement tectonics and the responses of complex pre existing boundaries of different intraplate blocks to the geodynamics at plate margins after removing the far field effect of plate subduction and collision the intraplate deformation mechanism itself may be closely associated with the heterogeneity of intraplate thermal mechanical structures

insights into the damage zones in fault bend folds from geomechanical models and field data abstract understanding the rock mass deformation and stress states the fracture development and distribution are critical to a range of endeavors including oil and gas exploration and development and geothermal reservoir characterization and management geomechanical modeling can be used to simulate the forming processes of faults and folds and predict the onset of failure and the type and abundance of deformation features along with the orientations and magnitudes of stresses this approach enables the development of forward models that incorporate realistic mechanical stratigraphy e g the bed thickness bedding planes and competence contrasts include faults and bedding slip surfaces as frictional sliding interfaces reproduce the geometry of the fold structures and allow tracking strain and stress through the whole deformation process in this present study we combine field observations and finite element models to calibrate the development and distribution of fractures in the fault bend folds and discuss the mechanical controls e g the slip displacement ramp cutoff angle frictional coefficient of interlayers and faults that are able to influence the development and distribution of fractures during fault bend folding a linear relationship between the slip displacement and the fracture damage zone the ramp cutoff angle and the fracture damage zone and the frictional coefficient of interlayers and faults and the fracture damage zone was established respectively based on the geomechanical modeling results these mechanical controls mentioned above altogether contribute to influence and control the development and distribution of fractures in the fault bend folds

impacts of the tectonic stress field on natural gas migration and accumulation a case study of the kuqa depression in the tarim basin china stress fluid and temperature are three of the major factors that impact natural gas migration and accumulation in order to study the influences of tectonic stress field on natural gas migration and accumulation in low permeability rocks we take the kuqa depression as an example and analyze the evolution of the structure and tectonic stress field at first then we study the influences of tectonic stress field at different tectonic episodes on fractures and fluid potentials through the numerical simulation method on the section across the kl2 gas field we summarize two aspects of the impact of the tectonic stress field on natural gas migration and accumulation firstly under the effects of the tectonic stress field the rock dilation increases with the added stress and strain and when the shear stress of rock exceeds its shear strength the shear fractures are well developed on one hand the faults which communicate with the hydrocarbon source rocks become the main pathways for natural gas migration on the other hand these positions where fractures are well developed near faults can become good reservoirs for natural gas accumulation secondly because fluid potentials decrease in these places near the faults where fractures are well developed natural gas can migrate rapidly along the faults and accumulates the impact of tectonic stress fields on natural gas migration and accumulation allows for hydrocarbon migration and accumulation in the low permeability rocks in an active tectonic compressive setting

stress orientation and fracturing during three dimensional buckling numerical simulation and application to chocolate tablet structures in folded turbidites sw portugal abstract two orthogonal sets of veins both orthogonal to bedding form chocolate tablet structures on the limbs of folded quartzwackes of carboniferous turbidites in sw portugal structural observations suggest that 1 mode 1 fractures transverse to the fold axes formed while fold amplitudes were small and limbs were under layer subparallel compression and 2 mode 1 fractures parallel to the fold axes formed while fold amplitudes were large and limbs were brought to be under layer subparallel tension we performed two and three dimensional numerical simulations investigating the evolution of stress orientations during viscous folding to test whether and how these two successive sets of fractures were related to folding we employed ellipses and ellipsoids for the visualization and quantification of the local stress field the numerical simulations show a change in the orientation of the local 1 direction by almost 90 with respect to the bedding plane in the fold limbs the coeval 3 direction rotates from parallel to the fold axis at low fold amplitudes to orthogonal to the fold axis at high fold amplitudes the stress orientation changes faster in multilayers than in single layers the numerical simulations are consistent with observation and provide a mechanical interpretation for the formation of the chocolate tablet structures through consecutive sets of fractures on rotating limbs of folded competent layers

the kuqa late cenozoic fold thrust belt on the southern flank of the tian shan mountains the kuqa fold thrust belt kftb a late cenozoic fold thrust belt on the southern flank of the tian shan mountains consists of several deformation zones trending nearly w e the main decollement fault of the kftb gradually rises southwards there are three regional main decollement faults in the triassic dark mudstone paleogene gypsum salt kumugeliemu formation and neogene gypsum salt jidike formation respectively and possibly a fourth in the jurassic coalbed laterally thin skinned structures are developed in the main segments of the kftb whereas thick skinned structures are in the root zone vertically the structural deformation above the cenozoic gypsum salt layers paleogene gypsum salt in the middle segment of the kftb and neogene gypsum salt in the eastern segment is characterized by decollement folding whereas that below is characterized by thrusting the kftb was resulted from the late cenozoic intra continental orogeny in the tian shan area under the far field effect of the india asia collision the deformation of kftb began folding and thrusting ca 23 ma when the far field effect of the india asia collision reached the tian shan area the deformation of kftb accelerated ca 10 5 2 and 1 0 ma in general the evolution of the kftb is forward propagating and the hinter parts of the kftb continue to deform while its front propagates southwards

the problem of area change in tangential longitudinal strain folding this paper deals with some problems with the concept and properties of the folding mechanism named tangential longitudinal strain a general two dimensional mathematical description of this mechanism in terms of displacements and finite strains is presented in the analysis of this mechanism of folding two geologically reasonable variants are considered the first of these referred to as parallel tangential longitudinal strain folding involves no finite elongation of lines perpendicular to the layer and produces class 1b parallel folds the second variant is characterized by the conservation of area across the fold profile and is therefore termed equiareal tangential longitudinal strain folding it produces folds ranging from class 1b to more complex shapes with the development of a bulge in the hinge zone inner arc when amplitude and curvature are high using the computer program foldmodeler which incorporates the derived equations for displacements and finite strains the geometrical features of idealized folds produced by these two variants have been studied together with those arising from their successive or simultaneous combination the implications of the operation of these two deformation mechanisms in natural folds are then considered and a discussion is presented about the features that can be diagnostic of their operation in nature it is suggested that the two mechanisms operate together in the formation of natural folds in a way that deformation probably begins with equiareal tangential longitudinal strain but subsequently gives way to parallel tangential longitudinal strain when strain concentration in some parts of the folded layer makes area change probable

predicting the orientation of joints from fold shape results of pseudo three dimensional modeling and curvature analysis we treat layers of sedimentary rock as elastic plates and predict the orientations of joints by assuming that they open parallel to the maximum instantaneous stretch of a layer because the direction of maximum instantaneous stretch is parallel to the maximum curvature of a surface we hypothesize that joints will trend parallel to the minimum curvature of an elastically deformed layer after constructing pseudo three dimensional trishear models of laramide style uplifts that grow self similarly we calculated the direction of minimum curvature axes during the evolution of the fold our analysis of minimum curvature axes in evolving folds suggests several important characteristics for fold related joint sets 1 joints that are neither parallel nor perpendicular to the fold axis may be induced by local fold related strains 2 at any time during folding joint orientations may vary according to the structural position on a fold 3 at any location on a fold joint orientation may depend on when a joint forms during the evolution of the fold and 4 joint patterns in trishear folds may vary with stratigraphic position natural folds that evolve along simple geometric pathways may develop fold related joint sets the orientation dominance abutting relationships spacing and continuity of which will vary systematically throughout the structure this variation in joint system architecture may reflect the history of fold growth

stress evolution during 3d single layer visco elastic buckle folding implications for the initiation of fractures abstract buckle folds of sedimentary strata commonly feature a variety of different fracture sets some fracture sets including outer arc tensile fractures and inner arc shear fractures at the fold hinge zones are well understood by the extensional and compressional strain stress pattern however other commonly observed fracture sets including tensile fractures parallel to the fold axis tensile fractures cutting through the limb extensional faults at the fold hinge and other shear fractures of various orientations in the fold limb fail to be intuitively explained by the strain stress regimes during the buckling process to obtain a better understanding of the conditions for the initiation of the various fractures sets associated with single layer cylindrical buckle folds a 3d finite element modeling approach using a maxwell visco elastic rheology is utilized the influences of three model parameters with significant influence on fracture initiation are considered burial depth viscosity and permeability it is concluded that these parameters are critical for the initiation of major fracture sets at the hinge zone with varying degrees the numerical simulation results further show that the buckling process fails to explain most of the fracture sets occurring in the limb unless the process of erosional unloading as a post fold phenomenon is considered for fracture sets that only develop under unrealistic boundary conditions the results demonstrate that their development is realistic for a perclinal fold geometry in summary a more thorough understanding of fractures sets associated with buckle folds is obtained based on the simulation of in situ stress conditions during the structural development of buckle folds

the model of fracture development in the faulted folds the role of folding and faulting abstract tectonic fractures are otherwise known as joints or fissures the density of tectonic fractures is governed by the lithology layer thickness and structures the distribution and density of tectonic fractures in the kuqa depression in the northern tarim basin of northwest china are dominantly controlled by the regional faulted fold belt our purpose of this paper is to analyze how folding and faulting control the development of tectonic fractures based on outcrop and logs of drill data in the folded tight sandstone reservoirs in the kuqa depression the extensional area transition area and shortening area have been separated by two neutral lines which control the distribution and density of tectonic fractures in the buckle folds from the twelve sites of field measurements six logs of drill core data and analysis of the micro texture of tectonic fractures it can be deduced that the fracture development in the sub salt faulted fold belt is related to the neutral lines and related faults the high density tensile fractures in the vicinity of fold hinges are more developed than in the limbs of the fold folding is thus the first level of control over the general pattern of fracture distribution the fractures in the hanging wall of the thrust faults are more developed than those in the footwall further more the fractures in the hinge zone and steeply tilted limbs of folds close to thrust faults are more developed than the gently tilted limbs which are far from faulting this suggests that faulting is a secondary control over the development of local fractures within the faulted fold belt the development of fractures related to faulted folds in the kuqa depression can be also divided into three episodes pre folding syn folding and post folding which are identified by field observation

petroleum geology of kela 2 the most productive gas field in china abstract the kela 2 gas field located in the kuqa depression of tarim basin is the most productive gas field in china based on per unit surface acreage the gas trap is a subsurface anticline formed along an e w trending thrust belt in front of the southern tianshan mountains the gas pay zones are located in fine grained sandstones of a fan delta front facies in the lower cretaceous bashijiqik formation k l bs and eogene evaporitic rocks serve as regional cap rocks the gases were derived primarily from over mature jurassic coal measures peak gas generation and final shaping of the kela 2 structure occurred relatively late in the late himalayan orogeny 5 0 ma thrust faulting appears to have played a significant role in the hydrocarbon generation migration accumulation and preservation in this foreland basin detailed delineation of the kela 2 gas field indicates that high quality regional cap rocks are essential for the formation of the super rich abnormally pressured giant gas fields while buried anticlines under the overthrust zone are the favorable gas habitat late gas accumulation is considered to be decisive for the conservation of the giant gas fields

fractures in sandstone reservoirs with ultra low permeability a case study of the upper triassic yanchang formation in the ordos basin china the upper triassic yanchang formation in the ordos basin central china is a typical sandstone reservoir with an ultra low permeability high angle tectonic fractures and diagenetic fractures such as near horizontal bedding fractures intragranular fractures and grain boundary fractures are abundant fractures are major pathways and enhance fluid flow in sandstone reservoirs with ultra low permeability because of their weak lateral continuity and their small apertures under lithostatic pressure bedding fractures make a relatively small contribution to the overall permeability of reservoirs as they are both of small size and low permeability intragranular fractures and grain boundary fractures mainly improve the connectivity of reservoirs by connecting the matrix pores high angle tectonic fractures control the fluid movement in the ultra low permeability reservoirs under the effect of different successive tectonic stress fields four assemblages of high angle tectonic fractures developed in the sandstone reservoirs under the present day stress differently oriented fractures have different connectivities apertures and permeabilities the northeast southwest fractures parallel to the maximum principal stress have a good connectivity large apertures and a high permeability forming the dominant flow paths knowledge of these paths can be used for optimizing well placement because of their sensitivity to pressure fractures in different directions will show varying apertures as the formation pressure decreases therefore the permeability of the fractures of different orientations and its impact on reservoirs vary at different developmental stages

analysis of the developmental characteristics and major regulating factors of fractures in marine continental transitional shale gas reservoirs a case study of the carboniferous permian strata in the southeastern ordos basin central china abstract fractures play an important role in the formation of shale gas reservoirs because they can enlarge the transport channels and aggregation spaces and increase the specific surface area of the gas shale for artificial hydraulic fracturing of these reservoirs the natural fracture system must be fully integrated with the artificial fracture system to form an intact fracture system in this study we first comprehensively examined the fractures in 42 shale gas wells using several approaches including a systematic examination and description of the cores and the casting of thin sections a compilation of the statistics of fracture feature parameters and observation of various analytical and test data such as the mineral composition the organic carbon contents and the rock mechanics properties for specimens from the corresponding fractured intervals the data enabled us to thoroughly explore the developmental features and major factors affecting organic rich shale fractures in the upper paleozoic carboniferous permian marine continental transitional coal bearing formation which is located in the southeastern ordos basin our results reveal that in comparison with the paleozoic marine shale in the united states and southern china as well as the paleozoic basin transitional shale in northern china the upper paleozoic black shale in the ordos basin is primarily characterized by a relatively low content of brittle minerals and a high content of clay ingredients the total content of brittle minerals e g quartz feldspar and siderite was approximately 33 which included 27 quartz and 0 3 k feldspar but did not include carbonate the total content of clay minerals reached 64 and was dominated by mixed layer illite smectite i s which accounted for more than 41 of the total clay ingredients the shale accommodated the widespread development of various types of macro and microfractures in the core specimens medium angle slip fractures and horizontal bedding cracks were the most common types of fractures whereas vertical and high angle fractures and horizontal bedding cracks were underdeveloped in the thin sections microfractures arising in organic matter laminations or at their edges as well as those of tectonic origin were the predominant type of fractures and they were mainly short narrow and open overall the surface fracture ratios of the thin sections were concentrated in a range of approximately 0 1 0 3 the developmental level of the fracture was influenced by various factors including tectonism lithology rock mechanics and organic matter and mineral content thus increased developmental level of fractures was correlated with higher paleostructural elevation and increased sand content whereas the developmental level of microfractures was correlated with high lamellation development high level of organic carbon leading to more pronounced laminations and high contents of quartz mixed layer i s and illite leading to low levels of kaolinite these findings were corroborated by other data generated in this study including the rock mechanics results for the upper paleozoic black shale and silty shale the observation results from the cores and from the thin sections sampled from more than 40 shale gas wells and the anomalies of gas logging

information from folds a review abstract folds are spectacular geological structures that are seen in layered rock on many different scales to mark 30 years of the journal of structural geology we review the information that can be gained from studies of folds in theory experiment and nature we first review theoretical considerations and modeling from classical approaches to current developments the subject is dominated by single layer fold theory with the assumption of perfect layer parallel shortening but we also review multilayer fold theory and modeling and folding of layers that are oblique to principal stresses and strains this work demonstrates that viscosity ratio degree of non linearity of the flow law anisotropy and the thickness and spacing distribution of layers of different competence are all important in determining the nature and strength of the folding instability theory and modeling provide the basis for obtaining rheological information from natural folds through analysis of wavelength thickness ratios of single layer folds and fold shapes they also provide a basis for estimating the bulk strain from folded layers information about folding mechanisms can be obtained by analysis of cleavage and fabric patterns in folded rocks and the history of deformation can be revealed by understanding how asymmetry can develop in folds by how folds develop in shear zones and how folds develop in more complex three dimensional deformations

fracture systems and petrophysical properties of tight sandstone undergoing regional folding a case study of the cretaceous reservoirs in the kuqa foreland thrust belt tarim basin abstract the kuqa foreland thrust belt of the tarim basin hosts a prolific hydrocarbon system the main reservoirs are characterized by tight sandstone with low porosity and ultra low permeability buried presently at more than 5000 m depth natural fracture systems have a significant impact on reservoir performance and it may form paths for fluid flow in tight sandstone reservoirs and therefore plays an essential role in controlling high production rates with the aim of analyzing the distribution of favorable reservoir we studied subsurface fracture patterns and petrophysical properties of the bashijiqike formation with data collected from dabei structure and keshen structure we document the presence of four fracture systems whose distribution is related to far field stress thrusting and folding in the keshen anticlines higher fracture intensities are observed in the limbs with principal east west hinge parallel fracture system direction the hinge parallel fracture system includes a moderate dipping fracture set and a vertical fracture set crosscutting relationships stress field inversion and fracture surface textures suggest that the moderate dipping fracture set consists of shear fractures and can be explained by intra layer shear caused by thrusting the vertical fracture set may be formed due to the local outer arc extension during folding the oblique fracture system shows scattering in strike and is supposed to be related to a shear event of locally left lateral strike slip whereas in the dabei anticlines the major system is hinge perpendicular fracture system nw se showing a trending of increasing fracture intensity from limbs to hinge the hinge perpendicular fractures system in the dabei anticlines consist of shear fractures and tension fractures stress inversion result shows it corresponds to northwest southeast orientation compression stage besides we analyzed petrophysical properties include porosity permeability young s module poisson ratio and compressive strength of the bashijiqike formation concerning their structural position within folds it indicates that the highest porosity and permeability are associated with dissolution and fracturing the compressive strength of different structural positions also restrict the variations of mechanical properties of rocks the best reservoir quality is suggested in the hinge zone and areas close to thrust the findings have given maybe helpful for providing favorable reservoir targets for prospecting in the compressional basin

a review of deformation pattern templates in foreland basin systems and fold and thrust belts implications for the state of stress in the frontal regions of thrust wedges aesthetically appealing thrust systems and related large scale anticlines in both active and fossil foreland fold and thrust belts and the economic potential associated with the m have captured the interest of structural geologists for many decades as a consequence a large amount of data on sub seismic deformation patterns from thrust related an ticlines is available in the literature we provide a review of deformation pattern templates from fi eld data in foreland fold and thrust belts and show that the most frequent tre nds of sub seismic syn orogenic deformation structures hosted in km scale thrust related folds frequently and pa radoxically indicate a syn th rusting strike slip stress fi eld con fi guration with a near vertical 2 and a sub horizontal 3 rather than a contractional one where the latter is expected to be the vertical principal axis of the stress ellipsoid this a pparent inconsistency between sub seismic syn orogenic deformation structures and stress fi eld orientation is here named the 2 paradox field data support a possible expla nation of the paradox provided by the major role played by i nherited early orogenic extensional deformation struc tures on thrust fault nucleation nucleation of major thru sts and their propagation is facilitated and driven by the positive inversion and linkage of the early orogenic sub s eismic extensional inheritance developed in the foreland basin this process eventually leads to the development of l argereversefaultzonesandcanoccurbothincontractional and strike slip stress fi eld con fi gurations

magnetostratigraphic study of the kuche depression tarim basin and cenozoic uplift of the tian shan range western china abstract magnetostratigraphic and rock magnetic results from 1006 horizons in paleogene and neogene sediments between the upper kumugeliemu formation and the base of the kuche formation within the kuche depression of the tarim basin nw china are used to evaluate postcollisional uplift and deformation in the tian shan range characteristic remanences with both normal and reversed polarity were isolated by thermal demagnetization generally between 300 and 690 c and a composite magnetostratigraphy is constructed from data at 969 accepted levels correlating with polarity chrons c12n to c3r dated between 31 0 and 5 5 ma on the ck95 geomagnetic polarity time scale observed height dependent changes of rock magnetic parameters km and ams ellipsoid parameters indicate that these sediments were influenced by weak deformation with the succession accumulated before 15 ma recording the effects of compressive deformation this finding together with two substantial increases in accumulation rate at 16 17 and 7 ma define a framework for cenozoic uplift and deformation of the tian shan range we argue that thrusting in the south of the range was probably initiated at 20 ma and followed by rapid uplift at 16 17 ma on the assumption that sediment accumulation in western china was still predominately tectonically controlled at 7 ma another later pulse of rapid uplift with a much larger regional influence is also recognized in the tian shan the kuche depression was subjected to two postcollisional phases of rotational deformation with clockwise rotation prior to 12 ma 1 36 ma succeeded by counterclockwise rotation at 1 64 ma after 12 ma and possibly continuing to the present day this rotational deformation is not temporally related to the history of basin formation resolved from the magnetostratigraphy and may be linked to block interactions with adjoining terranes

magnetostratigraphy of the yaha section tarim basin china 11 ma acceleration in erosion and uplift of the tian shan mountains we report a magnetostratigraphic and rock magnetic study of the yaha section located on the southern flank of the central tian shan mountains asia our results show a two fold increase in sedimentation rate as well as marked changes in rock magnetic characteristics ca 11 ma after 11 ma sedimentation rate remained remarkably constant until at least 5 2 ma these findings are consistent with sedimentary records from other sections surrounding the tian shan we conclude that uplift and erosion of the tian shan accelerated ca 11 ma long after the onset of the collision between india and asia and that the range rapidly evolved toward a steady state geometry via a balance between tectonic and erosion processes

structural controls of fracture orientations intensity and connectivity teton anticline sawtooth range montana the teton anticline is a multiple hinge anticline containing fractured mississippian devonian carbonates in the frontal part of the sawtooth range in montana the structure serves as a good surface analog for fracture patterns and connectivities within subsurface folded carbonate reservoirs the primary fracture sets are longitudinal and transverse relative to the axis of the fold although two additional oblique sets are also present the length and density of the longitudinal fracture sets are strongly controlled by position relative to multiple hinges the transverse fractures are related to changes in fold plunge and exhibit less variation in fracture density fracture connectivity is dependent on the number of fracture sets their orientations and dispersions and the densities of the fracture sets the connectivity is measured using two parameters the fractional connected area fca which represents the fraction of the total sample area that is connected by fractures and the distribution of clusters of different sizes in any given area because the longitudinal fractures represent the dominant fracture set and also show the most variation with structural position the fracture connectivity as measured by both the fcas and the distribution of cluster sizes is greater in the vicinity of the fold hinges the results and approaches used in the study have some important implications for subsurface folded fractured carbonate reservoirs the analysis of sparsely distributed fracture data from wells must be integrated with an understanding of the controls of the macroscopic structure on fracture parameters to effectively simulate fracture patterns and connectivities around subsurface structures

cenozoic detachment folding in the southern tianshan foreland nw china shortening distances and rates abstract intracontinental foreland basins with fold and thrust belts on the southern periphery of the tianshan orogenic belt in china resulted from still active contractional deformation ultimately cased by the india asia collision to quantify the amounts of shortening distance and the rates of deformation and to decipher the architectural framework we mapped the stratigraphy and structure of four anticlines in the kuqa and baicheng foreland thrust belts in the central southern tianshan in the baicheng foreland thrust belts lower cretaceous baxigai and bashijiqike formations located in the core of the kumugeliemu anticline are overlain by the paleocene to eocene kumugeliemu formation above which are conformable oligocene through pleistocene sediments a disharmonic transition from parallel to unconformable bedding at the boundary of the miocene kangcun and pliocene kuqa formations suggests a change from pre detachment folded strata to beds deposited on top of a growing anticline most of the anticlines have steep limbs 70 90 and are box to isoclinal folds suggestive of detachment folding or faulted detachment folding faults that transect a fold core or limb shortening estimates calculated from the cross sections by the excess area method indicate that the total shortening for the kelasu kuchetawu kezile and yaken sections are 6 3 km 6 4 km 5 8 km and 0 6 km respectively and the respective depths of the detachment zones are 2 3 km and 6 9 km 2 3 km 2 5 km and 3 4 km time estimates derived from a paleomagnetic study indicate that the transition to syn folding strata occurred at 6 5 ma at the kuchetawu section along the kuqa river in addition according to our field observations and previous sedimentary rate studies the initial time of folding of the yaken anticline was at 0 15 0 21 ma therefore the average shortening rate that began at 6 ma was 2 mm a for the kelasu kuchetawu and kezile sections at 0 15 0 21 ma the average shortening rate increased to 3 4 mm a in the yaken section combined with the recent gps data the shortening rate in the central southern tianshan area increased to 4 7 1 5 mm a at present we suggest that there was a linear increase in shortening rate in the southern tianshan foreland basin which also indicates that the far field stress increased considerably from the late miocene to present in response to the india asia collision

large scale mechanical buckle fold development and the initiation of tensile fractures tensile failure associated with buckle folding is commonly associated to the distribution of outer arc extension but has also been observed on fold limbs this study investigates whether tensile stresses and associated failure can be explained by the process of buckling under realistic in situ stress conditions a 2 d plane strain finite element modeling approach is used to study single layer buckle folds with a maxwell viscoelastic rheology a variety of material parameters are considered and their influence on the initiation of tensile stresses during the various stages of deformation is analyzed it is concluded that the buckling process determines the strain distribution within the fold layer but is not solely responsible for the initiation of tensile stresses the modeling results show that tensile stresses are most dependent on the permeability viscosity and overburden thickness low permeability 10 19 m2 high viscosity 1021 pa s and low overburden pressure can explain tensile failure at the fold hinge tensile stresses in the limb of the fold cannot in general be explained by buckling rather it develops due to a combination of compression and erosional unloading the modeling results show that erosion of high permeability rocks can explain the generation of tensile stresses at significant depths 2 km both at the hinge of the fold and throughout the limb of the fold this study shows that tensile stresses and associated failure within buckle folds is directly dependent on the distribution of material parameters but moreover to the strain history of the geologic system

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

the neutral lines in buckle folds abstract the neutral line in a buckle fold dividing areas of outer arc extension from areas of inner arc shortening is a fundamental concept in structural geology in the past folds have been constructed kinematically from a given neutral line geometry using the tangential longitudinal strain pattern in this study a mechanical finite element model is used to numerically buckle single layer folds with newtonian and power law viscous rheology two neutral lines can be distinguished the incremental neutral line zero layer parallel strain rate and the finite neutral line zero finite layer parallel strain the former develops first and migrates through the layer from the outer towards the inner arc ahead of the latter both neutral lines are discontinuous along the fold and terminate either at the bottom or top interface of the layer for decreasing viscosity ratio between layer and matrix and for decreasing initial amplitude the neutral lines develop later during folding and for some cases no neutral line develops the dynamical behaviour of the neutral lines is similar for newtonian and power law viscous rheology if the viscosity ratio is large but substantially different for small viscosity ratios the results are discussed in light of fold related structures such as outer arc extension structures and inner arc shortening structures