trishear kinematic modeling of extensional fault propagation folding we present a new non linear 2 d numerical trishear model that is capable of modeling both symmetrical and asymmetrical trishear zones and a strain prediction approach based on the velocity field within the trishear zone a simple relationship exists between the shape factor r of the velocity field the hanging wall apical angle and footwall apical angle the implication of this relationship is that when dealing with a real deformed state cross section one can measure the hanging wall and footwall apical angles to determine the r value to specify the velocity field within the trishear zone for either the forward modeling or restoration the new trishear model can accurately reproduce the geometry of two experimental clay models and an extensional fault propagation fold from the gulf of suez predicted strain is greatest directly above the fault tip and decreases with distance from the fault tip as in the experimental models greater strain occurs in the hanging wall because of the greater velocity gradient there whereas the footwall is much less intensely deformed consequently secondary faulting and fracturing if present should be better developed along the fault tip line or in the hanging wall the orientation of fractures or minor faults can be predicted from the strain ellipses calculated from the trishear velocity field

crustal accretion beneath the koyna coastal region india and late cretaceous geodynamics abstract in 1967 a major earthquake in the koyna region attracted attention to the hitherto considered stable indian shield the region is covered by a thick pile of deccan lava flows and characterized by several hidden tectonic features and complex geophysical signatures although deep seismic sounding studies have provided vital information regarding the crustal structure of the koyna region much remains unknown the two available dss profiles in the region have been combined along the trend of bouguer gravity anomalies unified 2 d density modelling of the koyna crust mantle suggests a ca 3 km thick and 40 km wide high velocity high density anomalous layer at the base of the crust along the coastline the thickness of this anomalous layer decreases gradually towards the east and ahead of the koyna gravity low the layer ceases to be visible based on the seismic and gravity data interpretation in the geodynamical rheological boundary conditions the anomalous layer is attributed to igneous crustal accretion at the base of the crust it is suggested that the underplated layer is the imprint of the magmatism caused by the deep mantle plume when the northward migrating indian plate passed over the reunion hotspot

volume based curvature computations illuminate fracture orientations early to mid paleozoic central basin platform west texas volumetric curvature analysis is a simple but computationally intensive procedure that provides insight into fracture orientation and regional stresses until recently curvature analysis has been limited to computation along horizon surfaces that may be affected by unintentional bias and picking errors introduced during the interpretation process volumetric curvature is best estimated in a two step process in the first step we use a moving analysis subvolume to estimate volumetric reflector dip and azimuth for the best fit tangent plane for each sample in the full volume in the second step we calculate curvature from adjacent measures of dip and azimuth we use larger curvature analysis windows to estimate longer wavelength curvatures such a technique allows us to output full 3d volumes of curvature values for one or more scales of analysis we apply these techniques to a data set from the central basin platform of west texas and find lineaments not observable with other seismic attributes these lineaments indicate that in the lower paleozoic interval a left lateral shear couple oriented due east west controls the local stress regime such a model predicts that extension faulting and fractures will be oriented northeast southwest the example demonstrates the potential of this new technology to determine stress regimes and predict azimuths of open fractures

the role of compressive stresses in jointing on vancouver island british columbia this study demonstrates that joint parallel compressive stresses were integral to the development of joint sets on vancouver island british columbia the mapped study area contains paleostress indicators in the form of calcareous concretions which have multiple internal fractures in precisely the same orientation as one of the surrounding joint sets in the sandstone matrix field and laboratory tests indicate that the stiffest concretions are the most likely to be fractured however results from numerical simulations using measured rock properties preclude an origin for the concretion fractures from either a far field uniaxial tensile or compressive stress fracturing is only found to be possible if the concretions possessed a lower poisson s ratio than the sandstone at the time of fracturing in the latter case a far field uniaxial compressive stress may have generated tensile effective stresses in the vicinity of high modulus concretions seeding the field site with an initial population of concretion fractures and joints given the close spacing of some joints their extension cannot be satisfactorily explained without invoking grain scale compression driven tensile fracturing mechanisms

3 d seismic horizon based approaches to fracture swarm sweet spot definition in tight gas reservoirs in this article we will discuss some horizon attribute based methods that we have been exploring to help identify productive fracture swarms in tight gas reservoirs

natural fractures in the spraberry formation midland basin texas the effects of mechanical stratigraphy on fracture variability and reservoir behavior horizontal cores from sandstone siltstone reservoirs in the spraberry formation midland basin west texas have documented two systems of dramatically different yet dynamically compatible natural fractures in reservoirs separated vertically by only 145 ft 44 m each system is capable of producing a different degree of the northeast trending permeability anisotropy recognized in spraberry reservoirs one fracture system consists of two vertical fracture sets with an apparent conjugate geometry striking north northeast and east northeast the other system consists of evenly spaced northeast striking vertical fractures nearly bisecting the acute angle of the first system although lithologically similar differences in quartz overgrowth and clay content in the layers resulted in a yield strength of the lower bed that is only half of that of the upper layer producing different fracture systems in the two reservoirs despite their proximity such differences in the mechanical properties due to variations in diagenetic and depositional histories of the strata are probably widespread within the formation they have the potential to cause significant vertical and lateral variation in the spraberry fracture system across the basin low present day in situ stresses in the reservoirs allow the fractures to open to become more conductive and even to propagate under very low injection pressures

geological modeling of the new champ magnetic anomaly maps using a geographical information system technique 1 reliable global crustal field anomaly maps produced from magnetic measurements of the champ satellite mission now allow for quantitative geological studies of crustal structure and composition we have developed a gis based forward modeling technique to model these anomaly maps on the basis of the geologic and tectonic maps of the world laboratory susceptibility values of the occurring rock types and the seismic thickness of the crust a vertically integrated susceptibility grid is generated in the gis system in addition a remanent magnetization grid is computed for the oceanic crust using a digital isochron map of the ocean floor and rotation models of the paleoplates combining the global vis and remanent magnetization grids the vertical magnetic field anomaly is computed at satellite altitude and compared with the corresponding champ magnetic anomaly map over the oceans induced and remanent magnetization explains well the prominent observed anomalies over the cretaceous quiet zones we also find a good agreement between predicted and observed anomalies over the continents remaining discrepancies between the predicted and observed anomalies can be used to adjust poorly known boundaries and the composition of the buried precambrian provinces until the recomputed anomalies fit the observed anomalies the feasibility of this approach is demonstrated on greenland the west african craton bangui in central africa and the kolyma omolon block in siberia we conclude that quantitative information on the lateral extent the composition and the thickness of the lower crust within a precambrian province can thus be inferred from the new satellite magnetic anomaly maps

magnetic mineralogy of the lower continental crust rocks that were once present in the deep continental crust can be found in surface exposure as xenoliths as parts of high grade metamorphic and plutonic terrains and as obducted lower crustal cross sections a review of magnetic and mineralogic studies of samples representing all such occurrences shows that magnetite is the only magnetic phase present in significant amounts in these rocks thermodynamic considerations indicate that tipoor magnetite is the expected magnetic oxide under equilibrium conditions that exist in the lower crust regardless of the primary magmatic composition of the spinel phase furthermore it seems unlikely that serpentinites with metal alloys contribute significantly to the magnetization of the lower crust or mantle thus magnetite in the deep crust is the most likely candidate for the source of the long wavelength magnetic anomalies observed in regional aeromagnetic and satellite magnetometer surveys if so the entire crust should be magnetic in most continental areas including shields in areas of high heat flow the bottom of the magnetic crust may lie considerably above the moho

fracture system evolution within the cardium sandstone central alberta foothills folds the natural fracture system developed in the cardium sandstone is examined in four outcropping structures that represent different stages of fold development at the incipient stage of folding the fracture system is dominated by large widely spaced hybrid fractures that have very small displacements and are aligned in the regional shortening direction type i orientation these fractures are naturally propped open by asperities along the fracture surfaces a lesser number of small thrust faults type iii orientation are also developed extension fractures aligned parallel to the fold axis type ii orientation begin to develop in the early stage of folding through the intermediate stage of folding there is a progressive increase in the intensity of both type i and type ii orientation fractures incremental increases in shear displacement on new or reactivated fractures create a gouge of comminuted sandstone grains along the fracture interface as folding progresses to an advanced stage there is major increase in the amount of shear displacement on both type i and type ii orientation fractures many existing fractures coalesce into connected fracture zones and small faults that have shear offsets ranging from several centimeters to several meters a breccia can result from intense fracturing in the rock within and marginal to these shear features slickensides on type i orientation features consistently indicate slip in a subhorizontal direction even as bed dip increases multiple slickenside patterns record reactivation of these features type ii orientation fractures and small faults consistently undergo bed perpendicular slip type i and type ii features both serve to stretch the cardium sandstone beds but in different directions only type iii features which are a minor component of the fracture population result in bed thickening

contribution of lithospheric remanent magnetization to satellite magnetic anomalies over the world s oceans regional studies have shown that remanent magnetization of the cretaceous quiet zones cqzs created during the long period of geomagnetic normal polarity at 118 84 ma produce well defined magnetic anomalies at satellite altitudes we investigate the effects of the remanent magnetization of the oceanic lithosphere on satellite magnetic anomalies on a global scale we consider entire oceanic areas because oceanic lithosphere formed during periods other than cqz but with a predominant polarity also have appreciable contributions to the satellite anomalies the magnetic anomalies of the world s oceans are calculated from a distribution of vertically integrated remanent magnetization that is computed using age map plate relative motions and the apparent polar wander path of africa three magnetization models are examined thermoremanent magnetization confined to extrusive layer 2a and thermoviscous remanent magnetization of the crust and uppermost mantle down to a maximum depth of 12 km or 30 km although all models lead to rather similar anomaly distributions the amount of magnetization required suggests the need for deeper sources all models produce the satellite anomalies associated with the cqz in the north atlantic and parts of the indian and pacific oceans with slight differences in location depending on the models low amplitude observed anomalies associated with areas created at fast and intermediate spreading rates in the indian and pacific oceans are successfully modeled a major difference between models and observation is the north south elongated model anomalies associated for instance with the cqzs in the south atlantic and the fast spreading east pacific rise these elongated anomalies are systematically absent on the observed map probably because they were removed by along track filtering in the early stages of processing the satellite data a careful comparison of the model anomalies with observation favors model with thermoviscous magnetization down to 12 km and saturation magnetizations of 4 0 1 and 0 6 a m for the extrusive basalts intrusive basalts gabbros and peridotites respectively

beginning of foreland subsidence in the columbian sevier belts southern canada and northwest montana subsidence analysis and geometry of jurassic cretaceous foreland strata in northwestern montana and southern alberta and british columbia suggest that loading by the fold thrust belt in canada began as much as 40 m y earlier than in montana in canada early foreland basin deposits are late jurassic age thicken rapidly westward and are restricted to a narrow belt within 30 km of the thrust belt in western montana contemporaneous deposits are widespread and do not increase markedly in thickness toward the thrust belt the unconformity overlying these deposits also changes from canada where it is angular to a disconformity in western montana near great falls between these two areas foreland geometry is transitional over a distance of 250 km beyond the transition zone early foreland basin geometries are broadly consistent showing late jurassic foreland subsidence in southern canada and early cretaceous initial subsidence in the united states

cyclic ramp to basin carbonate deposits lower mississippian wyoming and montana a combined field and computer modeling study abstract the lower mississippian lodgepole lower madison formations 20 225 m thick developed along a broad 700 km storm dominated pericratonic ramp three types of fifth order upward shallowing cycles are recognized across the ramp to basin transition peritidal cycles consist of very shallow subtidal facies overlain by algal laminated tidal flat deposits which are rarely capped by paleosol breccia layers shallow subtidal cycles consist of stacked ooid grainstone shoal deposits or deeper subtidal facies overlain by ooid skeletal grainstone caps deep subtidal cycles located along the outer ramp consist of basal sub storm wave base limestone argillite overlain by storm deposited limestone which are capped by hummocky stratified to massive skeletal ooid grainstone deep subtidal c cles pass downslope into ramp slope facies composed to rhythmically interbedded limestone and argillite with local deep water mud mounds no upward shallowing cycles occur within the ramp slope facies average cycle durations calculated along the outer ramp are between 30 110 ky the fifth order cycles are stacked to form three third to fourth order depositional sequences which are recognized by regional transgressive regressive facies trends and cycles stacking patterns ramp margin wedges rmw developed during long term sea level fall and lowstand and consist of cyclic crinoidal bank and oolitic shoal facies which pass downdip into deep subtidal cycles transgressive systems tracts tst which onlapped the ramp during long term sea level rise include thick shallow and deep subtidal cycles peritidal cycles are restricted to the inner ramp highstand systems tracts hst developed during long term sea level highstand and fall and along the ramp are composed of early hst shallow subtidal cycles overlain by late hst peritidal cycles shallow through deep subtidal cycles characterize the hst along the ramp slope two dimensional computer modeling of the cyclic sequences suggests that for the assumed water depths of facies fifth order sea level oscillations of at least 20 25 m were required to generate deep subtidal cycles along the ramp slope synthetic sequences run with fifth order sea level amplitudes end page 1194 and tst deposition a feature not observed in the actual sequences other factors in addition to fifth order sea level oscillations likely played a role in generating synchronous peritidal and deep subtidal cycles during hst deposition these factors may include short term climatic changes which influenced the depths to storm wave reworking the moderate amplitude sea level oscillations suggested by the cyclic sequences may reflect the initial effects of carboniferous glaciation that was occurring in gondwana

modes and timing of fracture network development in poly deformed carbonate reservoir analogues mt chianello southern italy abstract structural and paleostress analyses carried out on a kilometre sized outcrop of allochthonous shallow water carbonate units of the southern apennines allowed us to unravel a superposed deformation pattern associated with plate convergence the reconstructed tectonic evolution involves i early extensional faulting and fracturing associated with bending of the foreland lithosphere during forebulge and foredeep stages including the development of both tangential and radial normal fault and tensile fractures early middle miocene ii large scale thrusting and folding late miocene iii transcurrent faulting including two distinct sub stages characterized by different remote stress fields pliocene early pleistocene and iv extensional faulting late quaternary stage i normal faults generally occurring as conjugate sets and related fractures and veins are variably deformed and overprinted by later horizontal shortening despite having experienced such a long and complex structural history the studied carbonates are characterized by a background fracture network including two joint vein sets orthogonal to each other and to bedding that appears to be associated with the early fault sets that formed during the first foredeep forebulge related deformation stage therefore away from younger late miocene to quaternary fault zones the permeability structure of the studied carbonates appears to be essentially controlled by the early inherited fracture network as a similar fracture network is likely to characterize also the buried apulian platform carbonates representing the reservoir units for major oil fields in southern italy our results also bear possible implications for a better understanding of fluid flow in the subsurface and related hydrocarbon production

mechanical models of fracture reactivation and slip on bedding surfaces during folding of the asymmetric anticline at sheep mountain wyoming abstract we use finite element methods to investigate the reactivation of fractures opening and shearing and the development of bedding surface slip during the deformation of the asymmetric anticline at sheep mountain wyoming a series of numerical simulations were run to show the effect of mechanical stratigraphy layer thickness slip on bedding surfaces and tectonic shortening of the fold on the response of a representative three layer ductile brittle ductile two dimensional system the model uses large deformation frictional contact mechanics to capture the response of existing fractures and slip along bedding surfaces and considers both elastic and elastoplastic layer properties the computational results demonstrate the relationships among overall configuration of the multilayer slip on bedding surfaces and the sequence and mode of deformation opening versus shearing of bed perpendicular fractures we show that fractures located in the hinge are mainly reactivated as joints and that those in the forelimb are predominantly reactivated as thrust faults a flexural slip mechanism develops during folding when the layers bounded by frictional bedding surfaces have similar stiffnesses in contrast when the difference in the layer stiffnesses is significant softer outer layers the deformation is accommodated within the softer units without exceeding the frictional strength of the bedding surfaces a reduction of the middle layer thickness from 100 m to 10 m has a minor quantitative effect on the slip along the bedding surfaces we compare the numerical results with fracture data collected at sheep mountain anticline and discuss the similarities and differences between the field observations and the model results

multivariate fracture intensity prediction application to oil mountain anticline wyoming the geometric characteristics of natural fractures significantly impact the hydraulic behavior of fractured reservoirs prediction of fracture geometry is therefore important for reservoir development decisions and production forecasting although many geometric kinematic mechanical geomechanical petrophysical sedimentary and geophysical attributes correlate to fracture intensity typically only the attribute with the highest absolute value correlation is chosen to be carried forward to influence prediction we employ a geostatistical bayesian updating approach that quantitatively accounts for multiple important attributes together impacting fracture geometry prediction the resulting models are more representative of the true geological complexity this methodology is applied to the oil mountain anticline outcrop near casper wyoming

crustal structure along mehmadabad billimora profile in the cambay basin india from deep seismic soundings abstract deep seismic sounding dss investigations were carried out along a 220 km long n s profile from mehmadabad 22 50 n 72 46 e to billimora 20 46 n 72 58 e in the cambay basin india for studying the detailed crustal structure in the region seismic refraction as well as narrow and wide angle reflection travel time data were obtained for various crustal boundaries starting from the sedimentary section down to the moho by continuous profiling analysis of reversed reflection and refraction travel time data has yielded crustal velocity depth functions for the mehmadabad broach and broach navsari sections along the profile which are significantly different from each other the average velocities at depths in the magmadabad broach section have been found to be considerably lower than those in the broach navsari section by about 25 at 5 km 17 at 10 km 7 at 20 km and 2 at the moho depth and even deeper down in the upper mantle the presence of a relatively thicker sedimentary section especially in the jambusar broach area as revealed by the reversed refraction data analysis is responsible for this gross difference in the average velocity functions for the two parts of the profile and the same have therefore been used for obtaining the detailed crustal depth along the profile the cambay basin which is well known to be bounded by step faults on the eastern and western margins is also dissected into seven major crustal blocks in the n s direction from mehmadabad to billimora the various crustal blocks identified are from the north 1 i mehmadabad block 2 ii tarapur block 3 iii petlad katana block 4 iv jambusar broach block 5 v kosamba block 6 vi surat block and 7 vii navsari billimora block these blocks are displaced up or down relative to the adjacent blocks due to movements along the faults bounding them most of the faults have been found to extend quite deep and to affect the moho boundary considerably as is quite evident from the relief of the moho found along the profile further the seismic activity observed in the region during historical as well as modern times suggests that these deep faults may even be currently active the jambusar broach block which is well known as a broad regional syncline with the maximum thickness of cenozoic sediments has been found to be a major graben revealing a maximum crustal thickness of 38 40 km the granitic basement in this graben is also the deepest at 6 0 6 5 km depth towards the north and south of this major graben the moho as well as the granitic basement are found to be at relatively shallower depths in various crustal blocks the boundary velocity at the moho has been found to be 8 1 8 2 km sec whereas at the granitic basement level there seems to be a lateral velocity variation from 5 7 to 6 3 km sec the crustal depth section obtained in the present study reveals a close relationship between the deep and shallow structural features in this region the deccan traps which form the basement of the overlying tertiary sediments over most part of the profile vary in thickness from 1 0 to 1 8 km however the trap surface rises to very shallow depths on the southern part of the profile the boundary velocity in the deccan traps has been found to vary between 4 8 and 5 2 km sec in the anklesvar kosamba area there is also evidence for the presence of a 1 2 km thick layer of mesozoic sediments with a velocity of about 4 0 km sec underlying the 1 1 km thick deccan traps layer

the moho as a magnetic boundary revisited we have re examined the question of whether it is generally true that in continental regions a relatively magnetic crust overlies a relatively non magnetic mantle whereas the original conclusion was based on limited data the present study is based on about 400 globally distributed xenolith samples while it is now clear that the moho represents a maximum velocity gradient in a complex transition from crust to mantle and while crustal magnetization is highly variable the original concept of a magnetic crust overlying a non magnetic mantle is strongly supported by the new data a large upper mantle xenolith suite of ultramafic composition is characterized by non magnetic chrome spinels and magnesian ilmenites such rocks are magnetic only where demonstrably altered the most significantly magnetic rocks within the crust are mafic types prograded to granulite grade or mafic melts of basaltic composition which crystallized deep in the crust at high pressure and temperature prograded rocks have characteristic magnetite curie points and are strongly magnetic especially the more mafic compositions while the mafic rocks formed by crystallization at depth have curie points between 400 570 c and are strongly magnetic induced magnetizations are commonly several a m and can readily account for long wavelength magnetic anomalies measured by satellite and aircraft thus there is no need to search for exotic sources for missing crustal magnetization no metal or other peculiar magnetic species have been observed by us in any of our unaltered samples mineral assemblages including magnetic components are consistent with oxidizing conditions close to the fmq buffer

a transverse conductive structure in the northwest himalaya abstract data from a linear profile of magnetometers running parallel to the strike of the northwest himalaya are analysed to bring out more information on the major conductivity structure delineated through the 1979 magnetometer array examination of data in the form of induction arrows and pseudosection of z h has permitted a precise estimate of the position and orientation of the conductor numerical modelling of the data suggests that a 45 km wide ridge rising from asthenospheric depths to a depth of 15 km and having an electrical resistivity of 2 m accounts for the observations fairly well the coincidence of the conductivity anomaly with a zone of high seismicity suggests that stresses associated with underthrusting of the indian plate at its collision with the eurasian plate may cause fracture in the indian shield resulting in the upflow of asthenospheric material in the form of a ridge certain signatures indicative of an electrical discontinuity at the main boundary thrust are also identified

fracture development and mechanical stratigraphy of austin chalk texas the mechanical stratigraphy of the upper cretaceous austin chalk is established from study of fracture intensity along its outcrop trend from dallas to san antonio and westward to langtry texas and in the subsurface from study of cores and or fracture identification logs from 30 wells three mechanical stratigraphic units are recognized 1 an upper fractured massive chalk corresponding to the big house chalk member 2 a middle ductile chalk marl corresponding to the dessau chalk and burditt marl members and 3 a lower fractured massive chalk corresponding to the atco chalk member representative samples from the three mechanical stratigraphic units were experimentally shortened dry at 10 17 34 and 70 mpa confining pressure at 24 c and at a strain rate of 2 5 10 4 sec 1 to determine if the relative mechanical behavior observed at the surface could be extrapolated into the subsurface at different simulated burial depths the experimentally determined ductilities parallel those determined from outcrop and subsurface studies multiple linear regression analysis indicates that porosity is most strongly correlated with fracture strength smectite content has the second strongest correlation for low porosity specimens 9 13 5 the strength of specimens with 4 smectite is reduced 30 42 compared to specimens with no smectite the coefficient of internal friction at 17 mpa confining pressure decreases from 1 66 to 0 61 as smectite content increases from 0 to 4 sem photomicrographs of undeformed specimens show that smectite and other clays are distributed as large 30 m discrete amorphous concentrated masses throughout the chalk they are comminuted along the induced fracture surfaces where their grain size is 0 5 m or less these observations suggest that smectite acts as a soft inclusion localizing shear failure and correspondingly weakening the material

fracture initiation development and reactivation in folded sedimentary rocks at raplee ridge ut abstract fractures at raplee ridge a 14 km long monoclinal fold of pennsylvanian sedimentary rock in southeast utah are examined to determine their sequential development and to explain the mechanical relationships among the three primary fracture sets and the fold folding is attributed to slip on an unexposed thrust fault of laramide age fracture orientations were measured at 98 sites and fracture patterns were mapped at 8 sites on two sandstone and two limestone units of the rico formation revealing at least four stages of fracture formation stages 1 and 2 occurred pre folding and are defined respectively by e w trending fracture set i and n s trending fracture set ii set i fractures are characterized by a unimodal distribution of attitudes an intermediately developed outcrop pattern and continuous linear traces on bedding surfaces set ii fractures have a greater dispersion of orientations and are poorly to intermediately developed abutting relationships confirm that set i fractures are older than set ii stage 3 occurred during folding when nw se trending set iii fractures initiated both from isolated flaws and as wing cracks due to right lateral shearing along reactivated set i fractures set iii fractures increase in density as the fore limb dip increases from 10 to 30 later stages of deformation are characterized by the formation of fracture subsets iv and v structurally associated with fore limb dips greater than 30 fracture sets i and ii are found throughout the surrounding monument upwarp and are attributed to that structure whereas set iii is attributed to local stresses induced by monoclinal folding and or thrust faulting

joint interaction with embedded concretions joint loading configurations inferred from propagation paths abstract the interaction between propagating joints and embedded concretions in a devonian black shale near seneca lake ny permits identification of the loading configurations responsible for two joint sets of different ages striking at nearly the same orientation the earlier set consists of systematic joints cut by later alleghanian joints of the appalachian plateau the later set consists of non systematic curving cross joints that abut these same alleghanian joints field evidence shows that concretions functioned as stiff inclusions in a compliant black shale as a consequence of this elastic contrast local perturbations in the remote stress field persisted around the concretions during burial tectonic deformation and exhumation these stress perturbations influenced joint propagation paths of both joint sets our conclusions about loading configurations are based on finite element modeling of the effect of the local stress perturbation on concretion modified joint propagation modeling shows that the local stress perturbation from a thermoelastic loading was responsible for deflecting cross joints away from concretions in a curved propagation path near the concretion this load configuration also led to arrest of cross joints before they penetrated the shale concretion interface at greater distances from the concretion the propagation path of cross joints was controlled by the contemporary tectonic stress field the interface between concretions and the surrounding shale was strongly bonded as indicated by the crossing of the interface by some of the systematic ene joints higher compressive stress levels within the concretions relative to the shale suppressed joint development in the concretion causing the arrest of those joints once they had driven across the interface and a short distance into the concretion numerical modeling shows that interface penetration by the systematic ene joints is consistent with a fluid load the same loading configuration postulated for the subsequent alleghanian joints the traces of the systematic ene joints align on opposite sides of concretions rather than curving toward the concretion as predicted by two dimensional models of the fluid load co planar traces are indicative of large planar joints propagating in plane around the concretion making it energetically inefficient for the crack front to curve as it enters the local stress perturbation near the concretion a fluid load for the systematic ene joints came from high pore pressure during the pre alleghanian stages of burial of the devonian catskill delta complex

curie isotherm map of indian subcontinent from satellite and aeromagnetic data abstract estimating depth to the bottom of the magnetic crust synonymously called the curie isotherm on a regional scale from long wavelength magnetic anomalies requires that large areas of survey data be used for the calculations there is still no consensus on the minimum survey area required to arrive at a reliable estimate of the curie isotherm depth from the available aeromagnetic data over india depth to the curie isotherm is estimated using spectral techniques we found that the significant spectral maxima required to calculate the depth to the curie isotherm existed only for 4 4 blocks in southern peninsular india up to 18 n latitude and for 5 5 blocks for the rest of the region in central india as aeromagnetic data coverage over india is incomplete we also calculate the magnetic crustal thickness from the lithospheric model of the champ satellite data for the whole country using an iterative forward modeling approach the calculated curie isotherm is shallow in the mobile belts and deeper in the cratons in both the derivations as the curie isotherms calculated from the two data sets collected at very different altitudes using totally different techniques match reasonably well it lends credence to the methodology adopted the derived curie isotherm depth map is in accordance with the basic structural trend of the major tectonic units within the indian subcontinent a comparison is made of the calculated curie isotherm depth with moho depths along available dss profiles over india and it is found that the curie depth is generally shallower than the moho depth implying that it possibly represents a thermal boundary rather than a compositional change further we find that high magnitude earthquakes are associated with high gradients in curie depth

impact of interlayer slip on fracture prediction from geomechanical models of fault related folds understanding and interpreting the timing location orientation and intensity of natural fractures within a geologic structure are commonly important to both exploration and production planning activities here we explore the application of finite element based geomechanical models to fracture prediction our approach is based on the idea that natural fractures can be interpreted or inferred from the geomechanical model derived permanent strains for this analysis we model an extensional fault tip monocline developed in a mechanically stratified limestone and shale sequence because field data exist that can be directly compared with model results the approach and our conclusions however are independent of the specific structural geometry the presence or absence of interlayer slip is shown to strongly control the distribution and evolution of strain and this control has important implications for interpreting fractures from geomechanical models

the influence of the great falls tectonic zone on the thrust sheet geometry of the southern sawtooth range montana usa the reactivation potential of pre existing deep seated structures influences deformation structures produced in subsequent compression this contribution investigates thrust geometries produced in surface thrust sheets of the sawtooth range montana usa deforming over a previously faulted sedimentary section surface thrust fault patterns were picked using existing maps and remote sensing thrust location and regional transport direction was also verified in the field these observations were used to design a series of analogue models involving deformation of a brittle cover sequence over a lower section with varying numbers of vertical faults a final model tested the effect of decoupling the upper cover and lower section with a ductile detachment in a scenario closer to that of the sawtooth range results demonstrate that complexity in surface thrust sheets can be related to heterogeneity within the lower sedimentary section even when there is a detachment between this section and the rest of the cover this complexity is best observed in the map view as the models do not show the deep seated faults propagating into the cover these results were then used to predict specific locations of discrete basement fault strands in the study area associated with what is generally mapped as the scapegoat bannatyne trend the deep seated faults are more likely to be reactivated as strike slip features in nature given the small obliquity between the ene directed compression direction and the ne oriented basement faults more generally these results can be used to govern evaluation of thrust belts deforming over faulted basement and to predict the locations of specific fault strands in a region where this information is unknown

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

combining outcrop data and three dimensional structural models to characterize fractured reservoirs an example from wyoming combining a detailed outcrop characterization of fracture and fault occurrence with attributes from a three dimensional model of an anticlinally folded clastic reservoir body we determine which characteristics of the structural form and evolution are most closely related to the development of important reservoir scale structures our example reservoir body studied is the frontier formation 1 sandstone in oil mountain an asymmetric anticline on the western flank of casper arch in central wyoming the three dimensional model of the structure was constructed using an iterative scheme designed to maximize interpretation accuracy and precision the model was analyzed to determine the spatial variance in morphologic and kinematic attributes using a quantitative testing approach we found that the intensity of tectonically produced fractures is closely related spatially to rate of dip change and total curvature with the former having the strongest correlation this folding is a low strain process compared to tear faulting which has the strongest spatial correlation to larger strains the location and magnitude of these higher strain areas can be adequately predicted by three dimensional restoration and forward modeling of the upper bounding surface of the reservoir body we use these results to build a predictive model for fault and fracture distribution at oil mountain and to discuss how this approach can aid in the exploitation of analogous producing reservoirs

potential field power spectrum inversion for scaling geology we propose a method for inverting the power spectrum of gravity and magnetic data the method is demonstrated on aeromagnetic and bore well data from the german continental deep drilling project ktb density and susceptibility distributions in the earth s crust exhibit scaling behavior with power spectra proportional to f where f is the wavenumber and is the scaling exponent of the source distribution we model the sources of the potential field by a random function with scaling properties defined on a half space with its top at a specified depth beneath the observation plane comparing the theoretical power spectrum for this model with the power spectrum of the measured data we obtain the best values for the depth to source and the scaling exponent as a global minimum of the misfit function despite the simplicity of the model it offers a new understanding of the factors influencing the shape of the potential field power spectrum in particular the low wavenumber part of the power spectrum can be dominated by the scaling properties of the source distribution and not by the depth to some kind of basement the scaling exponent of the field varies with the type of surface geology the question of whether the scaling exponent can actually be used to identify different types of geology gives an interesting new aspect to power spectrum inversion

kinematic evolution and fracture prediction of the valle morado structure inferred from 3 d seismic data salta province northwest argentina the valle morado structure located in northwest argentina is an approximately 7 km long and 4 km wide north northeast south southwest trending anticline cut by faults that involve paleozoic basement and mesozoic cenozoic sedimentary cover geological interpretation of a three dimensional seismic survey that covers the structure plus a well and two dimensional regional scale seismic lines indicates that the valle morado is an andean pop up structure formed by foreland directed faults resulting from reverse reactivation of cretaceous extensional faults dipping westward and newly formed hinterland directed reverse faults dipping eastward the orientation of the preexisting extensional faults is nearly perpendicular to the andean tectonic transport direction in the northern part of the structure and oblique toward the south resulting in almost pure contraction in the north and transpression southward curvature analysis performed on two interpreted reservoir seismic horizons suggests that the maximum density of open fractures is likely to occur along the anticline axis in the northern portion of the fold and along the northeast southwest striking foreland directed reactivated faults to the south the direction of the open fractures interpreted from ultrasonic borehole image data along the well coincide with that of the open fractures predicted from the curvature analysis

deformation of a foreland carbonate thrust system sawtooth range montana an analysis of mesoscale faulting associated with thrust belt development was completed along a transect crossing several foreland thrust sheets of the central sawtooth range montana at a regional scale a balanced cross section of the central sawtooth range constrained by well seismic and surface data indicates a minimum of 60 horizontal shortening of the upper paleozoic carbonate bank sequence accommodated by a forward developing thrust system subsurface data delineates a 4 westward dipping precambrian basement and westward thickening of the precambrian belt supergroup major decollements within the central sawtooth range are present at the base of the devonian jefferson formation in the lower mississippian allan mountain limestone and in the cretaceous colorado group within the diversion french norwegian and beaver thrust sheets of the central sawtooth range deformation has been partitioned into arrays of meter to decimetersized faults that constitute brittle deformation zones bdzs bdz thickness is directly related to fault displacement but is not related to fault trajectories or subsequent foreland imbrication the ratio of bdz thickness to displacement is nearly constant with bdz thicknesses ranging from 65 m to 200 m for faults whose displacements range from 2 km to 5 km the bdzs of each thrust sheet can be separated into three regions defined by differences in fault diversity fault intensity and fault kinematics proportionally the locations of boundaries between regions within the bdzs are remarkably constant for each of the four thrust sheets studied indicating that the bdzs widen uniformly during thrust sheet emplacement bdz bases are characterized by an interlocking network of faults that exhibit great diversity and accommodate mesoscale cataclastic flow bdz bases are dominated by oblique slip faults whereas the middle and upper regions of bdzs are dominantly dip or strike slip faults in all regions of a bdz localized zones of contractional extensional or more commonly transport parallel strike slip faults bounded by bedding plane detachments are present fault length m2 of outcrop decreases logarithmically to zero from a maximum of 4 m m2 across the french and norwegian bdzs zones of intense faulting alter this general pattern of decreasing fault intensity in the diversion and beaver thrust sheets

the role of fractures in the structural interpretation of sheep mountain anticline wyoming the development of fractures in the sedimentary layers of sheep mountain anticline a laramide asymmetric fault cored fold of the bighorn basin is documented and interpreted to constrain the kinematic evolution of the fold the fracture pattern is interpreted to identify five main fracture sets a fracture set striking 110 set i oblique to the future fold trend is interpreted as a regional fracture set that predates laramide compression a joint set set ii striking 045 and present in the hinge and backlimb is related to the ne oriented laramide compression just prior to and during initial anticline growth joints striking 135 set iii parallel to the fold trend are found within the hinge and are interpreted to have developed in response to the bending of layers the two youngest fracture sets are attributed to a late stage of fold growth a joint set set iv in the backlimb striking parallel to the set i fractures but vertical and a fracture set in the forelimb consisting of set i fractures reactivated as reverse faults the relative chronology mode of formation opening vs shearing and structural locations of these fractures suggest the following kinematics of folding at sheep mountain anticline little or no lateral fold propagation no hinge migration and limb rotation with only minor limb flexure and stretching

structural geology of the swift reservoir culmination sawtooth range montana northwest trending thrusted and folded rocks of paleozoic and mesozoic age comprise the sawtooth range of northwestern montana the sawtooth range is a part of the northern disturbed belt and is characterized by thin skinned deformation stratigraphy plays an important role in the location and character of the various thrust sheets major decollement surfaces include the upper cambrian the top of the devonian and several horizons within the mississippian in the swift reservoir area a broad culmination exposes cambrian through cretaceous strata in a series of imbricately stacked west dipping thrust sheets the structural configuration of the culmination appears to be a compound duplex zone with structures north and south of swift reservoir dipping away from the culmination mapping of the culmination reveals vertical stacking of thrust sheets and a lateral ramp that forms the southern boundary of the duplex a structural high in the basement may have resulted in ramping of the thrust sheets and formation of the culmination deformation within the thrust sheets is controlled by structural position and by ductility contrasts between the stratigraphic units deformation varies widely ranging from tight overturned folds in the cambrian units to broad open folds and fractures within the mississippian because of the variationmore in lithology and ductility the most intense deformation is observed within the cambrian units this deformation is characterized by overturning of folds in the direction of thrust transport and the development of small scale kink folds cleavage pencil structures and boundinage less

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

fifth generation lithospheric magnetic field model from champ satellite measurements 1 six years of low orbit champ satellite magnetic measurements have provided an exceptionally high quality data resource for lithospheric magnetic field modeling and interpretation here we describe the fifth generation satellite only magnetic field model mf5 the model extends to spherical harmonic degree 100 as a result of careful data selection extensive corrections filtering and line leveling the model has low noise levels even if evaluated at the earth s surface the model is particularly suited for inferring large scale structure and composition of the lithosphere it is also meant to serve as the long wavelength part of continental and global scale marine and aeromagnetic anomaly maps

crack propagation sequence and pore fluid conditions during fault bend folding in the appalachian valley and ridge central pennsylvania bedding parallel bpv strike sv and cross fold cfv veins represent a sequence of polyphase fracturing during the development of a fault bend fold in lower paleozoic carbonate beds in the appalachian valley and ridge province pennsylvania brecciation and layer parallel shearing played important roles in the development of the earliest vein set bpv which propagated prior to folding fluid inclusions in calcite bpv are highly saline brines 23 4 wt naci trapped at conditions close to lithostatic p 180 mpa t 267 c while passing through the lower kink plane of a fault bend fold strike joints propagated in dolomitic beds located on the extensional side of neutral surfaces stylolitization of strike joint surfaces accompanied slip of the hanging wall up a ramp renewed extension upon passing through a second kink plane led to propagation of antitaxial sv along the stylolitized joints by the crack seal process slightly less saline fluids 22 4 wt naci were trapped in sv at fluid pressures 144 mpa and temperatures 217 c with the concurrent formation of lateral ramps the carbonates moved to the upper flat and were subjected to strike parallel extension as manifested by the propagation of antitaxial cfv due to further mixing of fresh waters the salinity of fluids forming cfv decreased 20 5 wt nacl with trapping conditions at p 116 mpa and t 179 c the fluid evolution path from bpv to cfv through sv shows a modest decrease in salinity with a sharp decrease in possible trapping pressures

fractured reservoirs in the eastern foothills colombia and their relationship with fold kinematics analysis of fracture systems in subsurface structures is limited by the amount and uncertainty of available data with the aim of analyzing the distribution of fracture systems we studied surface structures as analogs for oil fields in the fractured reservoirs of the llanos foothills of colombia here we document the presence of four widespread fracture systems whose distribution is related to fold geometry and folding mechanism at surface in the tierranegra and silbadero anticlines the principal fracture systems are symmetrical with respect to northeast and northwest trending fold axes showing higher fracture intensities in the forelimbs of the structures in the guavio anticline higher fracture intensities are located in the backlimb with principal east west and northwest southeast directions in contrast we document northeast southwest fractures near the hinge zones in the adjacent synclines this distribution suggests that in the guavio anticline fractures respond to movement of the hanging wall above a ramp consistent with a fault bend fold model whereas in the tierranegra and silbadero anticlines fractures respond to limb rotation and hinge migration consistent with detachment fold models comparing these with subsurface structures we identified that el morro anticline has fracture distributions like those in the tierranegra and silbadero anticlines but have higher fracture intensities in the case of the cusiana structure fracture intensities are higher in the crest but not in the limbs and intensities differ from the ones found in the guavio anticline showing that these structures are not appropriate analogs the results show how fracture distribution depends on structural position and fold evolution and is controlled in part by folding mechanism this suggests that models based on holocene fold geometry cannot accurately predict the observed fracture distributions and should not be used to construct discrete fracture network models instead the patterns we describe can be used as a guide for similar structures our work illustrates the possibility of having different fracture patterns and fracture abundances in adjacent folds in the same fold thrust belt

influence of structural position on fracture networks in the torridon group achnashellach fold and thrust belt nw scotland abstract in fold and thrust belts rocks undergo deformation as fold geometries evolve deformation may be accommodated by brittle fracturing which can vary depending on structural position we use 2d forward modelling and 3d restorations to determine strain distributions throughout folds of the achnashellach culmination moine thrust belt nw scotland fracture data is taken from the torridon group a thick coarse grained fluviatile sandstone deposited during the proterozoic modelling infers a correlation between strain and simple curvature we use simple curvature to infer how structural position and strain control fracture attribute variations in a fold and thrust belt in high curvature regions such as forelimbs fracture intensities are high and fractures are short and oriented parallel to fold hinges in low curvature regions fractures have variable intensities and are longer fracture orientations in these regions are scattered and vary over short distances these variations do not relate to strain data suggests lithology may influence fracturing the strain history of fold structures also influences fracturing structures with longer deformation histories exhibit consistent fracture attributes due to moderate high strain during folding despite present day low curvature this is in contrast to younger folds with similar curvatures but shorter deformation histories we suggest in high strain regions fracturing is influenced by structural controls whereas in low strain regions lithology becomes more important in influencing fracturing

the relative importance of supraglacial versus subglacial meltwater escape in basaltic subglacial tuya eruptions an important unresolved conundrum the hydraulic behaviour of meltwater during subglacial basaltic eruptions in temperate ice is of paramount importance in understanding the eruptive processes lithofacies and architecture of the edifices formed hydraulics also determines the timing location and volume of meltwater discharge which may be sudden and catastrophic and via subglacial and or supraglacial routes increasing our knowledge of meltwater hydraulics is therefore important for understanding predicting and mitigating the impact of meltwater release on vulnerable human communities new observations about eruption related meltwater hydraulics are presented for well exposed glaciovolcanic lava fed deltas on james ross island antarctica and accounts of historical eruptions are also re examined to identify the major meltwater discharge routes the study provides the first conceptual model for how meltwater escapes supraglacially in the absence of a crevassed layer which will dominate any meltwater flow overflowing may be initiated by enhanced rates of seepage despite the intrinsically low hydraulic conductivities of snow and firn once overflowing is established the rate of spillway incision is a likely overriding control on the evolution of the system and whether the discharge is unstable fast or stable slower the james ross island sequences demonstrate that meltwater discharge is highly dynamic and may have involved both subglacial and supraglacial escape subglacial discharge probably occurs throughout basaltic tuya eruptions but some periods may be dominated by concurrent overflowing it is still unclear if overflowing systems are sufficiently stable to enable the growth of laterally extensive glaciovolcanic lava fed deltas

regional fractures i a mechanism for the formation of regional fractures at depth in flat lying reservoirs 1 regional fractures may form slowly at depth in extension and parallel to regional horizontal tectonic compression such fracture sets can be common in reservoirs despite the absence of flexure of the strata and are commonly important contributors to reservoir permeability such fractures make good targets for deviated wells high formation pore pressure is crucial to creating low effective confining stresses and brittle rock properties in the reservoirs conditions conducive to fracturing despite deep burial but pore pressure does not exceed the least principal stress and does not cause tensile fracturing the mechanism of natural hydraulic fracturing is incompatible with the characteristics of most regional fracture sets rather many regional fractures form duri g far field compression initiate at locally induced tensile stresses caused by flaws in the rock and propagate in the plane of the maximum and intermediate compressive stresses the differential stress required for initiation and propagation of regional fractures is well below that necessary for shear failure in the presence of high pore pressure fractures can be held open at depth by a tectonic basinwide dilatancy of the strata and the open void space commonly becomes mineralized

fracture patterns in the zagros simply folded belt fars iran constraints on early collisional tectonic history and role of basement faults pre early folding fracture patterns were recognized in several anticlines from three structural domains in the simply folded belt of the fars arc these fracture sets were characterized in terms of opening mode and relative chronology and used to reconstruct the main compressional trends related to the early zagros collisional history the palaeostress reconstructions based on these fracture sets were further refined by combination with newly collected or already available fault slip and calcite twin data as an alternative to previous models of rigid block rotations or regional stress rotation we propose that the complex pattern of pre folding fractures and the contrasting palaeostress orientations through time in the different domains investigated are related to the presence of basement faults with n s and wnw trends above which basement and cover were variably coupled during stress build up and early deformation of the arabian margin beyond regional implications this study draws attention to the need to carefully consider pre existing fractures possibly unrelated to folding to build more realistic conceptual fold fracture models

mixed sediment deposition in a retro arc foreland basin lower ellis group m jurassic wyoming and montana u s a abstract the lower ellis group m jurassic of northern wyoming and southern montana affords an excellent opportunity to examine the influence of tectonics sea level change and incipient topography on facies dynamics and the evolution of mixed sediment ramp deposits the sawtooth piper and gypsum spring formations bajocian to callovian represent sedimentation along the forebulge of a retro arc foreland basin the lower ellis group records deposition during two transgressive regressive cycles 1 a bajocian age cycle dominated by evaporites and red shales and 2 a bathonian age cycle characterized by carbonates evaporites and red shales these cycles are capped by a callovian age cycle distinguished by carbonates and red shales that is represented by the lower sundance and rierdon formations transgressive episodes favored intensified chemical sediment production resulting in thick units deposited in subtidal to peritidal environments regressive periods are characterized by supratidal redbed progradation and subsequent shallowing upward cycles the depositional cycles in the lower ellis group developed due the interplay between sea level change and tectonic subsidence related to the evolution of a retro arc foreland basin differential subsidence before during and after deposition created paleohighs that locally influenced accommodation space and thereby complicated depositional and erosional patterns this paper provides a regional framework for further analysis of the depositional history of the lower ellis group by addressing the stratigraphic relationships between the sawtooth piper and gypsum spring formations

deep seismic sounding in the godavari graben and godavari coastal basin india abstract deep seismic sounding investigations along two profiles one along and the other across the godavari graben delineate the northeastern extension of the bapatla ridge which separates the godavari graben from the godavari coastal basin the godavari graben is filled with a maximum of 2 8 km thick lower gondwana upper permian to lower triassic sediments it is inferred on the basis of p wave velocities that the upper precambrian pakhal formation does not continue south of paloncha the upper gondwana upper jurassic to lower cretaceous rocks with marine incursions appear to continue towards the southeast for about 15 km from the exposures below the rajahmundry sandstone mio pliocene two dimensional velocity modelling of the upper crust indicates a major interface at 3 5 km depth in the upper crust where the velocity increases from 5 4 5 5 km s 1 to 6 2 6 4 km s 1 this interface shows a dpmal upwarp across the godavari graben which may be an indication of ne sw crustal extension in this region the deeper part of the crust consists of a large number of almost horizontal small reflector segments two dimensional crustal velocity modelling for some correlatable wide angle reflections yielded a three layered structure below 10 km depth with the moho at about 41 to 43 km

integrating 3d seismic curvature and curvature gradient attributes for fracture characterization methodologies and interpretational implications abstractin 3d seismic interpretation curvature is a popular attribute that depicts the geometry of seismic reflectors and has been widely used to detect faults in the subsurface however it provides only part of the solutions to subsurface structure analysis this study extends the curvature algorithm to a new curvature gradient algorithm and integrates both algorithms for fracture detection using a 3d seismic test data set over teapot dome wyoming in fractured reservoirs at teapot dome formed by tectonic folding and faulting curvature helps define the crestal portion of reservoirs that is associated with strong seismic amplitude and high oil productivity in contrast curvature gradient helps define the regional northwest trending and the cross regional northeast trending lineaments that are associated with weak seismic amplitude and low oil productivity in concert with previous reports from image logs cores and outcrops an integrated seismic curvature and curvature gradient analysis suggests t

fringe cracks key structures for the interpretation of the progressive alleghanian deformation of the appalachian plateau vertical joints in devonian clastic sedimentary rocks of the finger lakes area of new york state are ornamented with arrays of fringe cracks that reveal the complex deformational history of the appalachian plateau detachment sheet during the alleghanian orogeny three types of fringe cracks were mapped gradual twist hackles abrupt twist hackles and kinks gradual twist hackles are curviplanar en echelon fringe cracks that propagate with an overall vertical direction within the bed hosting the parent crack and are found in all clastic lithologies of the detachment sheet abrupt twist hackles propagate as planar features in thick shale beds above or below the siltstone beds hosting parent joints kinks propagate horizontally as planar surfaces from the tips of parent joints in siltstone beds the breakdown of the parent joint into either gradual or abrupt twist hackles depends on the orientation and magnitude of the remote stress field internal fluid pressure and the elastic properties of the bed the twist angle of gradual twist hackles is larger in coarser clastic beds indicating that stress and internal pressure are more important parameters than elastic properties in controlling breakdown assuming that the vertical stress axis s v equals 78 mpa at 3 km burial depth the difference in twist angle between sandstone and shale beds is used to estimate the maximum horizontal stress difference in the shale beds as s h s h 2 5 mpa when s h s h 12 mpa in sandstone beds the twist angle of the fringe cracks and the abutting relationships of parent joints give an indication of the overall change in stress field orientation within the detachment sheet during alleghanian tectonics these parent joints indicate a regional clockwise stress rotation of alleghanian age concordant with the twist angle of fringe cracks throughout the western part of the study area a counterclockwise twist angle in the eastern portion indicates a local stress attributed to drag where no salt was available to detach the eastern edge of the plateau sheet the clockwise change in stress orientation is consistent with the rotation in stress orientation found in the anthracite belt of the pennsylvania valley and ridge but is opposite to the sense of rotation in the southwestern portion of the detachment sheet western pennsylvania and west virginia the two regional rotation domains are separated by the juniata culmination

implications of heterogeneous fracture distribution on reservoir quality an analogue from the torridon group sandstone moine thrust belt nw scotland abstract understanding fracture network variation is fundamental in characterising fractured reservoirs simple relationships between fractures stress and strain are commonly assumed in fold thrust structures inferring relatively homogeneous fracture patterns in reality fractures are more complex commonly appearing as heterogeneous networks at outcrop we use the achnashellach culmination nw scotland as an outcrop analogue to a folded tight sandstone reservoir in a thrust belt we present fracture data is collected from four fold thrust structures to determine how fracture connectivity orientation permeability anisotropy and fill vary at different structural positions we use a 3d model of the field area constructed using field observations and bedding data and geomechanically restored using move software to determine how factors such as fold curvature and strain influence fracture variation fracture patterns in the torridon group are consistent and predictable in high strain forelimbs however in low strain backlimbs fracture patterns are inconsistent heterogeneities in fracture connectivity and orientation in low strain regions do not correspond to fluctuations in strain or fold curvature we infer that where strain is low other factors such as lithology have a greater control on fracture formation despite unpredictable fracture attributes in low strain regions fractured reservoir quality would be highest here because fractures in high strain forelimbs are infilled with quartz heterogeneities in fracture attribute data on fold backlimbs mean that fractured reservoir quality and reservoir potential is difficult to predict