mechanical properties of shale gas reservoir rocks part 2 ductile creep brittle strength and their relation to the elastic modulus we studied the elastic moduli ductile creep behavior and brittle strength of shale gas reservoir rocks from barnett haynesville eagle ford and fortst john shale in a series of triaxial laboratory experiments we found a strong correlation between the shale compositions in particular the volume of clay plus kerogen and intact rock strength frictional strength and viscoplastic creep viscoplastic creep strain was approximately linear with the applied differential stress the reduction in sample volume during creep suggested that the creep was accommodated by slight pore compaction in a manner similar to instantaneous strain there was more viscoplastic creep in samples deformed perpendicular to the bedding than parallel to the bedding the tendency to creep also correlated well with the static young s modulus we explained this apparent correlation between creep behavior and elastic modulus by appealing to the stress partitioning that occurs between the soft components of the shales clay and kerogen and the stiff components quartz feldspar pyrite and carbonates through a simple 1d analysis we found that a unique relation between the creep compliance and elastic modulus independent of composition and orientation can be established by considering the individual creep behavior of the soft and stiff components that arises from the stress partitioning within the rock this appears to provide a mechanical explanation for why long term ductile deformational properties can appear to correlate with short term elastic properties in shale gas reservoir rocks

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

mesozoic geology of southwestern china indosinian foreland overthrusting and subsequent deformation abstract the southwestern part of the south china block scb records triassic and subsequent deformations and is a key region that provides evidence of the post amalgamation tectonic evolution of southeast asia here we outline the tectonothermal evolution of early mesozoic orogenesis in this region using structural analysis and fission track thermochronology this region is divided into three tectonic units south to north the youjiang fold and thrust belt yftb the qianzhong massif qzm and the thin skinned thrust belt ttb of the southeastern sichuan basin all three of which record three deformation events here termed d 1 d 2 and d 3 d 1 deformation is represented by triassic top to the north thrusts f 1 and fault related folds f 1 that are indicative of n s shortening the intensity of deformation decreases toward the north the subsequent d 2 and d 3 deformations are marked by cretaceous top to the nw thrusting and fault propagation folding and cenozoic ne sw trending normal faults f 3 respectively the temperature time t t path obtained by apatite fission track modeling of samples from the yftb qzm and ttb areas provides evidence of uplift and denudation the d 1 deformation at 230 210 ma is characterized by buried thrusts within the relatively stable yftb and qzm and an increase in depth of the ttb this was coeval with the late triassic northward migration of uplift and denudation that is evidenced by a late triassic stratigraphic gap within the yftb a thin layer of coarse grained continental clastics within the qzm and a thick layer of clastic rocks within the ttb the study area records a triassic thrust system d 1 that progressively migrated northwards and represents a foreland fold and thrust belt that formed during the indochina scb collision this initial deformation was subsequently overprinted by late jurassic to cretaceous nw se thrusting d 2 and the development of cenozoic nw se extensional structures d 3

two episodes of structural fractures numerical simulation of yanchang oilfield in the ordos basin northern china abstract the tight oil sand reservoirs of the yanchang oilfield in the upper triassic yanchang 6 7 and 8 formations t3yc6 7 8 fms in the ordos basin are in the exploration stage two episodes of fractures during the yanshanian and himalayan periods observed in outcrops and cores of the t3yc6 7 8 fms are thought to be the controlling factors for the tight oil sweet spot finite element numerical simulation fes technology is used to perform a three dimensional 3d fracture simulation for a large region during the exploration stage to serve in tight oil reconnaissance for the yanchang oilfield the innovations of our work are mainly shown in the following simulation process 1 three controlling factors including the paleotopography layer thickness and rock mechanical faces are used to construct the 3d geologic model in the simulation 2 the rock mechanical environment characterized by the an gou qiliha zhangjiatan and dongjiahe outcrops the y1011 well cores and the corresponding mechanical experimental parameters is divided into three types as follows the delta front tractive current sand faces df tr s the lacustrine turbidity current sand faces l tu s and the mud faces m 3 the spatial boundary stress conditions are calculated based on the newberry formula for tensional and compressional stress fields considering the synthetical effects of the overburden pressure the pore fluid pressure and the tectonic stress 4 the modified griffith s criterion is used to fit the fracture index by simultaneously taking into consideration the tension and shear fracture mechanisms for the simulation results the predicted fracture index distribution map agrees well with the measured fracture density the yanshanian fractures mainly propagated in the dingbian and an bian areas adjacent to the western thrust belt while the himalayan fractures mainly propagated in the zhidan fuxian and huangling areas adjacent to the weihe graben the simulated fracture index has a positive correlation with the stratigraphic dip and a negative correlation with the layer thickness additionally structural fractures are prone to propagate near the mechanical interface in either the upward or downward penetration direction and are assumed to produce a fracture type sweet spot section in the vertical direction the reservoir oiliness also increases with the simulation fracture index indicating the fracture type tight oil sweet spot of the yanchang oilfield moreover the critical components used in the 3d fracture fes including the 3d geologic model the spatial boundary stress conditions and the fracture index fitted by the modified griffith criterion can be of great significance to produce more accurate numerical simulations during the oil and gas exploration stage in large regions

joint development in perturbed stress fields near faults abstract field evidence is presented for complex spatial and temporal perturbations of an otherwise systematic joint pattern around faults from well exposed faulted rock platforms joints propagating in perturbed stress fields will curve to follow the directions of the stress field trajectories a progressive change in joint direction is observed from unperturbed regions away from faults to strongly perturbed zones adjacent to faults this indicates that the joint pattern can reflect perturbations of the regional stress field around faults in the examples the stress field perturbations are probably due to points of high friction on the fault plane which concentrate stress and distort the stress field in the surrounding rock the corresponding joints converge at these points and are sub parallel to the fault along the remainder of the fault plane the possibility that a fault plane acts as a free surface contained within an elastic body is considered in this situation the fault plane induces a rotation of the principal stress axes to become either perpendicular or parallel to the fault the free surface model seems to explain the metre scale curvature of joints in the vicinity of existing joints but at the kilometre scale of a large fault plane the model becomes unrealistic unless the fault is open at the earth s surface two examples are investigated from the lias of great britain at nash point and robin hood s bay both comprise sub horizontal strata of relatively homogeneous lithology and bed thickness which provide striking examples of joints developed near faults

closely spaced fractures in layered rocks initiation mechanism and propagation kinematics spacings of opening mode fractures joints and veins in layered sedimentary rocks often scale with the layer thickness field observations reveal that the ratio of fracture spacing to the thickness of the fractured layer s tf ranges from less than 0 1 to greater than 10 there is a critical spacing to layer thickness ratio that defines the condition of fracture saturation and explains the observed spacing ratios between 0 8 and 1 2 values of s tf 1 2 are explained as the results of the fracturing process having not reached the saturation level to explain ratios of s tf 0 8 we study the possibility for further fracture infilling by considering flaw distributions between adjacent fractures loaded by extension of the layer results show that infilling fractures grow more easily from flaws located near the interface than from those in the middle of the fractured layer the propagation of a flaw located in the middle of the fractured layer is unstable but for the flaw to propagate toward the interfaces its height has to be greater than a critical size this critical size decreases with increasing s tf the propagation behavior of a flaw with one of its tips at the interface depends on s tf the propagation is unstable when s tf is greater than a critical value when s tf is less than this critical value the propagation is first unstable then stable and then unstable again an infilling fracture can cut through the fractured layer only if s tf is greater than another critical value otherwise the infilling fracture can only partially cut the fractured layer for models with the same elastic constants for the fractured layer and the neighboring layers this critical value is 0 546 and the minimum spacing to layer thickness ratio of fractures formed by the in filling process under extension is 0 273

pore structure of the graptolite derived om in the longmaxi shale southeastern upper yangtze region china abstract the lower silurian longmaxi shale in the southeastern upper yangtze region which has been the main target for shale gas exploration and production in china is black marine organic rich shale and rich in graptolites graptolites usually only periderms preserved in shales are important organic component of the longmaxi shale however the pore structure of graptolite periderms and its contribution to gas storage has not yet been studied before a combination of optical microscopy for identification and mark of graptolite and scanning electron microscope sem for pore observations were conducted for the longamxi shale samples results show that pores are anisotropic developed in the longmaxi graptolite periderms and greatly associated with their fine structure micrometer sized fractures and spindle shaped pores between cortical fibrils in the cortical bandage are greatly developed at section parallel to the bedding while they are rare at section perpendicular to the bedding besides numerous sapropel detritus rich in nanometer sized pores are discretely distributed in the shale though graptolite periderms are low porosity from sem image analysis microfractures and elongated pores along the graptolite periderm wall may still make the graptolite an interconnected system together with the discrete porous sapropel detritus in shale these graptolite derived organic matter om may form an interconnected organic pore system in the shale the difference of pore development observed in graptolite periderms and sapropel detritus also give us new insight for the organic pore heterogeneity study the om composition their fine structure and orientation in the rock may be important factors controlling om pore development the combination of identifying om type under optical microscopy and pores observation under sem for may be an effective method to study the om pore development especially in shale that contain more om

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

2 5 b y of punctuated earth history as recorded in a single rock deciphering the absolute chronology of ancient crustal rocks is often problematical because of isotopic reequilibration and or mixing during multiple geologic events ion probe u pb age data from texturally distinct zircon crystal domains in a single sample of high grade gneiss from the nimrod group in antarctica reflect successive resorption and crystallization under disequilibrium conditions our data indicate that archean crust of the east antarctic shield initially formed from juvenile magmas ca 3000 ma extends to the central transantarctic mountains successive zircon growth ca 2955 1720 and 530 ma records a sequence of distinct widely spaced high temperature metamorphic and or anatectic events related to archean paleoproterozoic and eocambrian orogenesis

joint sets that enhance production from middle and upper devonian gas shales of the appalachian basin the marine middle and upper devonian section of the appalachian basin includes several black shale units that carry two regional joint sets j1 and j2 sets as observed in outcrop core and borehole images these joints formed close to or at peak burial depth as natural hydraulic fractures induced by abnormal fluid pressures generated during thermal maturation of organic matter when present together earlier j1 joints are crosscut by later j2 joints in outcrops of black shale on the foreland northwest side of the appalachian basin the east northeast trending j1 set is more closely spaced than the northwest striking j2 set however j2 joints are far more pervasive throughout the exposed devonian marine clastic section on both sides of the basin by geological coincidence the j1 set is nearly parallel the maximum compressive normal stress of the contemporary tectonic stress field shmax because the contemporary tectonic stress field favors the propagation of hydraulic fracture completions to the east northeast fracture stimulation from vertical wells intersects and drains j2 joints horizontal drilling and subsequent stimulation benefit from both joint sets by drilling in the north northwest south southeast directions horizontal wells cross and drain j1 joints whenever present then staged hydraulic fracture stimulations if necessary run east northeast i e parallel to the j1 set under the influence of the contemporary tectonic stress field thereby crosscutting and draining j2 joints

fracture development in paleozoic shale of chongqing area south china part two numerical simulation of tectonic stress field and prediction of fractures distribution abstract a tectonics sedimentation evolution has been researched in southeast chongqing and the reasonable longmaxi shale highstand system tract hst and transgressive system tract tst geological model were built respectively based on the rock mechanical test and acoustic emission experiment which the samples are from field outcrop and the yuye 1 well the longmaxi shale two dimension tectonic stress field during the cenozoic was simulated by the finite element method and the distribution of fractures was predicted the research results show that the tectonic stress field and the distribution of fractures were controlled by lithology and structure as a result of cretaceous movement there are trough like folds wide spaced synclines battlement like folds similar spaces between synclines and anticlines and ejective folds wide spaced anticlines which are regularly distributed from southeast to northwest in the study area since the strain rate and other physical factors such as the viscosity are not taken into account and the stress intensity is the main factor that determines the tectonic strength therefore the stronger tectonic strength leads the higher stress intensity in the eastern and southeastern study area than in the northwest the fracture zones are mainly concentrated in the fold axis transition locations of faults and folds the regions where are adjacent to faults the fragile mineral contents such as siliceous rock carbonate rock and feldspar in the shelf facies shale from south of the study area are higher than in the bathyal facies and abyssal facies shale from center of the study area the shales characterized by low poisson s ratio and high elastic modulus from south of the study area are easily broken during cenozoic orogenic movement

a scale independent approach to fracture intensity and average spacing measurement fracture intensity the number of fractures per unit length along a sample line is an important attribute of fracture systems that can be problematic to establish in the subsurface lack of adequate constraints on fracture intensity may limit the economic exploitation of fractured reservoirs because intensity describes the abundance of fractures potentially available for fluid flow and the probability of encountering fractures in a borehole traditional methods of fracture intensity measurement are inadequate because they ignore the wide spectrum of fracture sizes found in many fracture systems and the consequent scale dependence of fracture intensity an alternative approach makes use of fracture size distributions which allow more meaningful comparisons between different locations and allow microfractures in subsurface samples to be used for fracture intensity measurement comparisons are more meaningful because sampling artifacts can be recognized and avoided and because common thresholds of fracture size can be enforced for counting in different locations additionally quantification of the fracture size distribution provides a mechanism for evaluation of uncertainties estimates of fracture intensity using this approach for two carbonate beds in the sierra madre oriental mexico illustrate how size cognizant measurements cast new light on widely accepted interpretation of geologic controls of fracture intensity

fracture aperture size frequency spatial distribution and growth processes in strata bounded and non strata bounded fractures cambrian meson group nw argentina abstract in cambrian meson group nw argentina small faults and three opening mode fracture sets defined by orientation and cement texture sets 1 3 formed sequentially in sandstone that most likely had constant mechanical properties throughout deformation yet the opening mode sets display contrasting fracture aperture size distributions spacing patterns and tendency to be bed bounded set 1 fractures are quartz filled or lined opening mode fractures with crack seal texture having a wide range of opening displacement kinematic aperture sizes they are irregularly spaced and non strata bounded fractures set 1 macro and microfracture opening displacement sizes are well described by a power law with slope 0 8 set 2 fractures are microscopic mostly quartz filled and have characteristic aperture sizes are probably not bed bounded and have either a near random or clustered spatial distribution set 3 fractures are quartz lined opening mode fractures with extensive open pore space having a narrow characteristic opening displacement size distribution they are regularly spaced and stratabounded differences between sets 1 and 3 can be accounted for by quartz deposition resisting fracture reopening to a greater extent for set 1 during repeated episodic growth where crack seal texture is present in fracture spanning quartz in contrast set 3 fractures are nearly barren with only trace cement deposits that did not resist opening power law opening displacement size distributions may be favored in cases where fracture growth is unequally partitioned amongst variably cemented fractures whereas a characteristic size is favored where growth is unaffected by cementation results imply that thermal history and diagenesis are important for fracture size distribution patterning

unconventional shale gas systems the mississippian barnett shale of north central texas as one model for thermogenic shale gas assessment shale gas resource plays can be distinguished by gas type and system characteristics the newark east gas field located in the fort worth basin texas is defined by thermogenic gas production from low porosity and low permeability barnett shale the barnett shale gas system a self contained source reservoir system has generated large amounts of gas in the key productive areas because of various characteristics and processes including 1 excellent original organic richness and generation potential 2 primary and secondary cracking of kerogen and retained oil respectively 3 retention of oil for cracking to gas by adsorption 4 porosity resulting from organic matter decomposition and 5 brittle mineralogical composition the calculated total gas in place gip based on estimated ultimate recovery that is based on production profiles and operator estimates is about 204 bcf section 5 78 10 9 m 3 1 73 10 4 m 3 we estimate that the barnett shale has a total generation potential of about 609 bbl of oil equivalent ac ft or the equivalent of 3657 mcf ac ft 84 0 m 3 m 3 assuming a thickness of 350 ft 107 m and only sufficient hydrogen for partial cracking of retained oil to gas a total generation potential of 820 bcf section is estimated of this potential approximately 60 was expelled and the balance was retained for secondary cracking of oil to gas if sufficient thermal maturity was reached gas storage capacity of the barnett shale at typical reservoir pressure volume and temperature conditions and 6 porosity shows a maximum storage capacity of 540 mcf ac ft or 159 scf ton

partitioning of the cretaceous pan yangtze basin in the central south china block by exhumation of the xuefeng mountains during a transition from extensional to compressional tectonics abstract the formation of a series of intermountain basins is likely to indicate a geodynamic transition especially in the case of such basins within the central south china block cscb determining whether or not these numerous intermountain basins represent a division of the cretaceous pan yangtze basin by exhumation of xuefeng mountains is key to understanding the late mesozoic to early cenozoic tectonics of the south china block scb here we present apatite fission track aft data and time temperature modeling in order to reconstruct the evolution history of the pan yangtze basin fourteen rock samples were taken from a ne sw trending mountain basin system within the cscb including from west to east the wuling mountains wuling shan the south and north mayang basins the xuefeng mountains xuefeng shan and the hengyang basin cretaceous lacustrine sequences are well preserved in the south and north mayang and hengyang basins and sporadically crop out in the xuefeng mountains whereas paleogene piedmont proluvial lacustrine sequences are only found in the south mayang and hengyang basins aft results indicate that the wuling and xuefeng mountains underwent rapid denudation post 84 ma whereas the south and north mayang basins were more slowly uplifted from 67 and 84 ma respectively following a quiescent period from 32 to 19 ma both the mountains and basins have been rapidly denuded since 19 ma both the aft data and sedimentary facies changes suggest that the cretaceous deposits that cover the south north mayang and hengyang basins through to the xuefeng mountains define the cretaceous pan yangtze basin integrating our results with tectonic background for the scb we propose that rollback subduction of the paleo pacific plate produced the pan yangtze basin which was divided into the south north mayang and hengyang basins by the abrupt uplift and exhumation of the xuefeng mountains from 84 ma to present apart from a period of tectonic inactivity from 32 to 19 ma this late late cretaceous to paleogene denudation resulted from movement on the ziluo strike slip fault which formed due to intra continental compression most likely associated with the eurasia indian plate subduction and collision sinistral transpression along the ailao shan red river fault at 34 17 ma probably transformed this compression to the extrusion of the indochina block and produced the quiescent window period from 32 to 19 ma for the mountain basin system in the cscb therefore the initiation of exhumation of the xuefeng mountains at 84 ma indicates a switch in tectonic regime from cretaceous extension to late late cretaceous and cenozoic compression

quantitative prediction of fractures using the finite element method a case study of the lower silurian longmaxi formation in northern guizhou south china abstract natural fractures have long been considered important factors in the production of gas from shale reservoirs because they can connect pore spaces and enlarge transport channels thereby influencing the migration accumulation and preservation of shale gas industrial level shale gas production has been initiated in the lower silurian longmaxi formation in northern guizhou south china however it is important to quantitatively predict the distribution of natural fractures in the lower silurian shale reservoirs to locate additional sweet spots in northern guizhou in this study data obtained from outcrops cores thin sections field emission scanning electron microscope fe sem images and x ray diffraction xrd were used to determine the developmental characteristics and controlling factors of these fractures correlation analysis indicated that the mechanical parameters of the longmaxi shale are mainly related to the total organic carbon toc quartz clay calcite and dolomite contents the spatial variations in the mechanical parameters of the longmaxi shale were determined based on the spatial variations in the toc and mineral contents then a heterogeneous geomechanical model of the study area was established based on interpretations of the fault systems derived from seismic data and acoustic emission ae experiments performed on samples of the relevant rocks the paleotectonic stress fields during the yanshanian period were obtained using the finite element method fem finally a fracture density calculation model was established to analyze the quantitative development of fractures and the effects of faults and mechanical parameters on the development of fractures were determined the results suggest that the main developmental period of tectonic fractures in the longmaxi formation was the early yanshanian period during this time the horizontal principal stress conditions were dominated by a se nw trending 135 315 compressional stress field and the longmaxi formation experienced a maximum tectonic stress of 110 120 mpa this simulated paleotectonic stress field was mainly controlled by faults and the contents of toc quartz clay calcite and dolomite at different positions along the same fault the degree of fracture development varies significantly overall the distribution of fractures in the longmaxi formation can be used to optimize well deployment and provides a basis for the future exploration of shale gas

residual stress measurements on a quartz vein a constraint on paleostress magnitude 1 the residual stress in a quartz vein from a metamorphic province is evaluated by an x ray diffraction technique diffraction analyses at multiple incident angles yield residual stress magnitudes for the specified directions on the plane parallel to the sample surface residual stress formation in the vein is found to be determined by the platy geometry with respect to the host rock the results indicate a compressive residual stress of 15 to 20 mpa parallel to the long axis of the vein the compressive stress magnitude parallel to the quartz vein is found to increase with sample temperature indicating that the quartz vein has a greater thermal expansion coefficient than that of the host rock a two dimensional model explaining residual stress formation in the vein in response to tectonic exhumation is proposed and the paleostress magnitude at the time of vein formation is estimated from the residual stress magnitude and fluid inclusion microthermometric data the quartz vein is concluded to have formed under a horizontal compressive stress of 550 660 mpa in the direction parallel to the long axis of the vein

natural fractures in the barnett shale and their importance for hydraulic fracture treatments gas production from the barnett shale relies on hydraulic fracture stimulation natural opening mode fractures reactivate during stimulation and enhance efficiency by widening the treatment zone knowledge of both the present day maximum horizontal stress which controls the direction of hydraulic fracture propagation and the geometry of the natural fracture system which we discuss here is therefore necessary for effective hydraulic fracture treatment design we characterized natural fractures in four barnett shale cores in terms of orientation size and sealing properties we measured a mechanical rock property the subcritical crack index which governs fracture pattern development natural fractures are common narrow 0 05 mm 0 002 in sealed with calcite and present in en echelon arrays individual fractures have high length width aspect ratios 1000 1 they are steep 75 and the dominant trend is west northwest other sets trend north south the narrow fractures are sealed and cannot contribute to reservoir storage or enhance permeability but the population may follow a power law size distribution where the largest fractures are open the subcritical crack index for the barnett shale is high indicating fracture clustering and we suggest that large open fractures exist in clusters spaced several hundred feet apart these fracture clusters may enhance permeability locally but they may be problematic for hydraulic fracture treatments the smaller sealed fractures act as planes of weakness and reactivate during hydraulic fracture treatments because the maximum horizontal stress trends northeast southwest and is nearly normal to the dominant natural fractures reactivation widens the treatment zone along multiple strands

early jointing in coal and black shale evidence for an appalachian wide stress field as a prelude to the alleghanian orogeny early ene striking joints present coordinates within both pennsylvanian coal and devonian black shale of the central and southern appalachians reflect an approximately rectilinear stress field with a dimension 1500 km this appalachian wide stress field awsf dates from the time of joint propagation when both the coal and shale were buried to the oil window during the 10 15 m y period straddling the pennsylvanian permian boundary the awsf was generated during the final assembly of pangea as a consequence of plate boundary tractions arising from late stage oblique convergence where maximum horizontal stress s h of the awsf was parallel to the direction of closure between gondwana and laurentia after closure the awsf persisted during dextral slip of peri gondwanan microcontinents when sh appears to have crosscut plate scale trans current faults at around 30 sup o following 10 m y of dextral slip during tightening of gondwana against laurentia the awsf was disrupted by local stress fields associated with thrusting on master basement decollements to produce the local orocline shaped alleghanian map pattern seen today

present day stress orientation in the molasse basin abstract the present day state of stress in western europe is considered to be controlled by forces acting at the plate boundaries it is assumed that the alpine orogen only influence the regional pattern of present day stress in western europe within the alps themselves we examine the present day maximum horizontal stress orientation in the molasse basin in the alpine foreland in order to investigate the possible influence of the alps on the far field stress pattern of western europe four arm caliper and image logs were analysed in 137 wells in which a total of 1348 borehole breakouts and 59 drilling induced fractures were observed in 98 wells in the german molasse basin the borehole breakouts and drilling induced fractures reveal that stress orientations are highly consistent within the molasse basin and that the present day maximum horizontal stress orientation rotates from n s in southeast germany 002 n 19 to approximately nnw sse in southwest germany and the swiss molasse basin 150 n 24 the present day maximum horizontal stress orientation in the molasse basin is broadly perpendicular to the strike of the alpine front indicating that the stress pattern is probably controlled by gravitational potential energy of alpine topography rather than by plate boundary forces the present day maximum horizontal stress orientations determined herein have important implications for the production of hydrocarbons and geothermal energy in the german molasse basin in particular that hydraulically induced fractures are likely to propagate n s and that wells deviated to the north or south may have reduced wellbore instability problems

age provinces in the antarctic craton evidence from detrital zircons in permian strata from the beardmore glacier region antarctica abstract the antarctic gondwana sequence crops out along the transantarctic mountains permian strata at mt bowers and mt weeks in the central part of the range were mainly derived from the ice covered east antarctic craton for which the geology is inferred from coastal outcrops and the miller range in the transantarctic mountains analysis of detrital zircons from permian sandstones at those two localities demonstrate a slowly changing zircon age probability distribution for four samples from the lower part of the sequence with an abrupt change for the upper buckley formation sandstone the stratigraphically lowest sample from a unit informally designated the mt bowers sandstone is dominated by zircons in the age range 610 540 ma with minor components at 1 0 ga and 2 7 ga upward the pagoda formation sandstone is likewise dominated by zircons of 600 550 ma age together with a significant grenville age cluster 1 3 1 0 ga and a minor 1 5 1 4 ga cluster the fairchild formation sandstone has a preponderance of grains in the range 600 530 ma with a scatter of older ages zircon grains in the lower buckley formation sandstone are principally in the range 575 520 ma with another cluster at 470 450 ma and a scattering of older ages most zircons in the upper buckley formation sandstone are permian in age but with a very small component in the range 650 475 ma and a trace of older ages archean to early mesoproterozoic zircon grains are few in number and may have been recycled from distal outcrops the 1 5 1 4 ga grains suggest an unexposed mesoproterozoic source within the east antarctic craton which could have been linked to laurentian north america the grenville age source is considered to be similarly a sub glacial belt in the interior of east antarctica zircons with a late neoproterozoic to ordovician signature 600 460 ma were derived from a ross orogen foreland sequence and or an early and inland extension of the orogen itself permian zircons in the upper buckley sandstone reflect the development of a magmatic arc on the gondwana pacific margin

natural fractures in shale a review and new observations natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low porosity and low permeability shale host rock many shale outcrops cores and image logs contain fractures or fracture traces and microseismic event patterns associated with hydraulic fracture stimulation have been ascribed to natural fracture reactivation here we review previous work and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization orientation and size patterns a wide range of shales have a common suite of types and configurations of fractures those at high angle to bedding faults bed parallel fractures early compacted fractures and fractures associated with concretions these fractures differ markedly in their prevalence and arrangement within each shale play however constituting different fracture stratigraphies differences that depend on interface and mechanical properties governed by depositional diagenetic and structural setting several mechanisms may act independently or in combination to cause fracture growth including differential compaction local and regional stress changes associated with tectonic events strain accommodation around large structures catagenesis and uplift fracture systems in shales are heterogeneous they can enhance or detract from producibility augment or reduce rock strength and the propensity to interact with hydraulic fracture stimulation burial history and fracture diagenesis influence fracture attributes and may provide more information for fracture prediction than is commonly appreciated the role of microfractures in production from shale is currently poorly understood yet potentially critical we identify a need for further work in this field and on the role of natural fractures generally

numerical modelling of stress fields and earthquakes jointly controlled by ne and nw trending fault zones in the central north china block abstract the stress field of the central north china block cncb has been affected by the interaction among the indian eurasian and pacific ocean plates and the effect is manifested by the occurrence of large earthquakes in this region here using numerical simulation of the finite element method attempts were made to evaluate ese directed compression on three tectonic zones namely the shanxi graben zone the zhangjiakou penglai fault zone and the taihangshan fault zone in these zones the structural geometry and viscoelastic property of the model were determined as well as 49 main faults identified by means of a comprehensive structural analysis of seismic profiles gravity data and tomography this study shows that the stress concentration areas in the three zones are all located in the transition zone between ne trending and nw trending faults numerical simulation analysis of the xingtai and tangshan earthquakes revealed that the ne and nw trending faults had played an important role in controlling the distribution of the stress field triggering aftershocks and or stopping stress propagation during earthquakes the differences of the stress fields and earthquake distributions among the three tectonic zones resulted from not only the fault combination patterns but also the geometry of the main faults

simulation of paleotectonic stress fields and quantitative prediction of multi period fractures in shale reservoirs a case study of the niutitang formation in the lower cambrian in the cen gong block south china abstract reservoirs where tectonic fractures significantly impact fluid flow are widespread industrial level shale gas production has been established from the lower cambrian niutitang formation in the cen gong block south china the practice of exploration and development of shale gas in the cen gong block shows that the abundance of gas in different layers and wells is closely related to the degree of development of fractures in this study the data obtained from outcrop cores and logs were used to determine the developmental characteristics of such tectonic fractures by doing an analysis of structural evolution acoustic emission burial history logging evaluation seismic inversion and rock mechanics tests 3 d heterogeneous geomechanical models were established by using a finite element method fem stress analysis approach to simulate paleotectonic stress fields during the late hercynian early indo chinese and middle late yanshanian periods the effects of faulting folding and variations of mechanical parameters on the development of fractures could then be identified a fracture density calculation model was established to determine the quantitative development of fractures in different stages and layers favorable areas for shale gas exploration were determined by examining the relationship between fracture density and gas content of three wells the simulation results indicate the magnitude of minimum principal stress during the late hercynian early indo chinese period within the cen gong block is 100 110 mpa with a direction of se nw 140 320 and the magnitude of the maximum principal stress during the middle late yanshanian period within the cen gong block is 150 170 mpa with a direction of nnw sse 345 165 during the late hercynian early indo chinese period the mechanical parameters and faults play an important role in the development of fractures and fractures at the downthrown side of the fault are more developed than those at the uplifted side folding plays an important role in the development of fractures in the middle late yanshanian period and faulting is a secondary control this 3 d heterogeneous geomechanical modelling method and fracture density calculation modelling are not only significant for prediction of shale fractures in complex structural areas but also have a practical significance for the prediction of other reservoir fractures

3 d simulation of tectonic loading at convergent plate boundary zones internal stress fields in northeast japan the evidence of east west compression in northeast japan has been reported by many investigators on the basis of geodetic geologic and geomorphic data but its origin still remains far from understood in the present study we have proposed a mechanical model of tectonic loading at convergent plate boundary zones and demonstrated its validity through the numerical simulation of internal stress fields in northeast japan with realistic 3 d geometry of plate interfaces at convergent plate boundary zones in general a part of plate convergence is consumed by steady slip along plate interfaces and the remaining part by inelastic deformation seismic faulting aseismic faulting and active folding of overriding plates such a plate boundary process to be called partial collision can be quantitatively described by introducing a collision rate defined as c 1 steady slip rate at plate interfaces plate convergence rate by this definition we can simply represent the mechanical process of partial collision which includes total subduction c 0 and total collision c 1 as two extreme cases in terms of steady slip rates at plate interfaces on the basis of elastic dislocation theory first we numerically computed the internal stress fields in northeast japan produced by the total subduction of the pacific plate beneath the north american plate however the computed stress pattern was opposite in sense to observations then we computed the internal stress fields by taking c 0 1 on average and succeeded in reproducing the observed east west compression in northeast japan this indicates that the concept of partial collision is essential to understand the mechanism of intraplate tectonic loading

proterozoic crustal evolution of central east antarctica age and isotopic evidence from glacial igneous clasts and links with australia and laurentia abstract rock clasts entrained in glacial deposits sourced from the continental interior of antarctica provide an innovative means to determine the age and composition of ice covered crust zircon u pb ages from a suite of granitoid clasts collected in glacial catchments draining central east antarctica through the transantarctic mountains show that crust in this region was formed by a series of magmatic events at 2 01 1 88 1 85 1 79 1 57 1 50 1 41 and 1 20 1 06 ga the dominant granitoid populations are ca 1 85 1 45 and 1 20 1 06 ga none of these igneous ages are known from limited outcrop in the region in addition to defining a previously unrecognized geologic history zircon o and hf isotopic compositions from this suite have 1 mantle like 18o signatures 4 0 4 5 and near chondritic hf isotope compositions ehf 1 5 for granitoids of 2 0 ga age 2 mostly crustal 18o 6 0 8 5 and variable hf isotope compositions ehf 6 to 5 in rocks with ages of 1 88 1 85 1 79 and 1 57 ga in which the 1 88 1 79 ga granitoids require involvement of older crust 3 mostly juvenile isotopic signatures with low mantle like 18o 4 5 and radiogenic hf isotope signatures ehf 6 to 10 in rocks of 1 50 1 41 ga age with some showing crustal sources or evidence of alteration and 4 mixed crustal and mantle 18o signatures 6 0 7 5 and radiogenic hf isotopes ehf 3 to 4 in rocks of 1 2 ga age together these age and isotopic data indicate the presence in cratonic east antarctica of a large composite igneous province that formed through a punctuated sequence of relatively juvenile proterozoic magmatic events further they provide direct support for geological correlation of crust in east antarctica with both the gawler craton of present day australia and proterozoic provinces in western laurentia prominent clast ages of 2 0 1 85 1 57 and 1 45 ga together with sediment source linkages provide evidence for the temporal and spatial association of these cratonic elements in the columbia supercontinent abundant 1 2 1 1 ga igneous and metamorphic clasts may sample crust underlying the gamburtsev subglacial mountains indicating the presence of a mesoproterozoic orogenic belt in the interior of east antarctica that formed during final assembly of rodinia

quantifying and predicting naturally fractured reservoir behavior with continuous fracture models this article describes the workflow used in continuous fracture modeling cfm and its successful application to several projects our cfm workflow consists of four basic steps 1 interpreting key seismic horizons and generating prestack and poststack seismic attributes 2 using these attributes along with log and core data to build seismically constrained geocellular models of lithology porosity water saturation etc 3 combining the derived geocellular models with prestack and poststack seismic attributes and additional geomechanical models to derive high resolution three dimensional 3 d fracture models and 4 validating the 3 d fracture models in a dynamic reservoir simulator by testing their ability to match well performance our cfm workflow uses a neural network approach to integrate all of the available static and dynamic data this results in a model that is better able to identify fractured areas and quantify their impact on well and reservoir flow behavior this technique has been successfully applied in numerous sandstone and carbonate reservoirs to both understand reservoir behavior and determine where to drill additional wells three field case studies are used to illustrate the capabilities of the cfm approach

developing a model discrete fracture network drilling and enhanced oil recovery strategy in an unconventional naturally fractured reservoir using integrated field image log and three dimensional seismic data in this study we develop a model discrete fracture network dfn for the unconventional naturally fractured tensleep sandstone oil reservoir at teapot dome wyoming reservoir characterization is based on three dimensional 3d seismic data fracture image logs from teapot dome and field observations of the tensleep exposure in the alcova anticline and fremont canyon areas image logs reveal that the dominant reservoir fracture set trends parallel to the present day maximum horizontal compressive stress inferred from drilling induced fractures analog field studies of the alcova anticline and fremont canyon suggest fracture heights and lengths are power law distributed while the fracture spacing distribution is best described as log normal image log derived fracture apertures are also log normally distributed these properties are incorporated into a model dfn we assume subseismic folds faults and fracture zones control fracture intensity distribution and use composite 3d seismic attributes to locate subtle changes in seismic response interpreted to result from subseismic structure directional curvature defines aperture opening strain normal to the dominant reservoir fracture set seismic attributes are scaled and combined to control fracture intensity variations in the model grid cell porosity and permeability distributions derived from the dfn suggest the presence of northeast southwest trending reservoir compartments we suggest that enhanced oil recovery operations may be optimized using lateral injection and production wells oriented along interpreted compartment boundaries at high angles to this combination of injection and production laterals could help maximize storage and hydrocarbon recovery in depleted reservoirs and in down dip residual oil zones

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

analogue models of second order faults genetically linked to a circular strike slip system abstract sandbox experiments with sand and sand silicone models were performed above a circular strike slip fault to understand the influence of curvature on the development of second order faults riedel r faults appear already at low displacements in the concave side of the circular plate where they always form more numerous they are followed by r faults in the convex side and eventually by throughgoing d faults that join the r faults and that develop parallel to the underlying circular fault the angle between the riedel faults and the trace of the main circular fault at the surface of the models is c 26 on the concave side and 15 on the convex side we infer a larger obliquity of 1 in the concave side of the circular plate which corresponds to a larger transpressional component than in the convex side of the main wrench fault due to the more confined volume of deforming sand in sand experiments most of the faults root into the underlying strike slip fault in sand silicone experiments instead the faults form close to the displacement discontinuity in case of a high displacement rate only uplifted areas are located all along the main fault in sand experiments in sand silicone experiments they are mostly located in wedges defined by the riedel faults and the main wrench fault and the width of these uplifted areas appear to be related to the length and activity of the riedel faults our results differ significantly from those of experiments with straight strike slip faults where strain and second order faults are symmetrically arranged on both sides of the main fault

paleotectonic stress field modeling and prediction of natural fractures in the lower silurian longmaxi shale reservoirs nanchuan region south china abstract natural fractures serve as significant storage spaces for hydrocarbons and are favorable for fluid flow in shale gas reservoirs therefore predicting the intensity and distribution of natural fractures within reservoirs are of extreme significance for shale gas exploration and development natural fractures in the nanchuan region are generally in high dip angles and the dominant fracture strikes are in the ne sw and nw se directions most natural fractures within the longmaxi formation are formed in the late yanshanian period therefore in this study the late yanshanian paleotectonic stress field was numerically investigated the comprehensive rupture rate crr was defined and calculated as an indicator to quantitatively predict the intensity and distribution of natural fractures within the longmaxi formation the results indicated that regions with well developed fractures were mainly located in around fault zones and fold zones among fault fold orientations change and at fault tips the intensity and distribution of natural fractures in the longmaxi formation of nanchuan region were influenced by many factors including tectonic activities mineral compositions etc the present study provides preliminary insights into natural fractures within the longmaxi formation of nanchuan region south china which can guide and support shale gas exploration and development

40 ar 39 ar mineral age constraints on the paleozoic tectonothermal evolution of high grade basement rocks within the ross orogen central transantarctic mountains the precambrian to early paleozoic orogenic history of the central transantarctic mountains is obscured by uncertainties in the regional extent relative intensities and tectonic significance of three or more events the nimrod beardmore and ross orogenies significant problems exist in correlation of structural styles structural geometry and lithotectonic units across major tectonic boundaries as well as in conflicting geochronological results in an effort to resolve these disparities 40 ar 39 ar ages were determined for hornblende and muscovite from several lithotectonic subunits of the nimrod group exposed in the miller range including amphibolitic gneiss ca 1 7 ga orthogneiss and metasedimentary mica schist these rocks display ductile deformation fabrics that formed prior to intrusion of ca 500 ma post kinematic granitic plutons hornblende concentrates from eight gneiss samples in the miller range display internally discordant 40 ar 39 ar apparent age spectra due to va

eclogite facies metamorphism preserved in tectonic blocks from a lower crustal shear zone central transantarctic mountains antarctica in the geologists and miller ranges of the central transantarctic mountains mafic and ultramafic tectonic blocks occur in an early cambrian amphibolite to granulite facies lower crustal transpressional shear zone mafic blocks consist of garnet clinopyroxene symplectite hornblende plagioclase quartz ilmenite orthopyroxene biotite mineral compositions in these blocks generally reflect equilibration under upper amphibolite to lower granulite facies conditions t 700 c p 8 12 kbar these conditions are the same as those indicated by peak syn kinematic assemblages in the enclosing shear zone tectonites mineral and textural evidence for an earlier higher pressure p 12 25 kbar eclogite facies metamorphism includes 1 plagioclase reaction rims around anhedral garnet 2 complex clinopyroxene intergrowths consisting of augite plagioclase quartz hornblende orthopyroxene interpreted as exsolved na rich clinopyroxene omphacite and 3 unusual magnesian staurolite atomic mg mg fe 0 58 inclusions in pyrope almandine garnet we suggest that the relict eclogite facies metamorphism records an earlier period of crustal thickening that occurred prior to formation of the transpressional shear zone perhaps as an early stage in ross orogenesis

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

characterization of natural fracture systems analysis of uncertainty effects in linear scanline results the exploitation of hydrocarbon reserves in naturally fractured reservoirs composed of different types of rocks has drawn considerable attention from the fracture characterization research community because of the importance of fractures to the prediction of fluid flow one of the most common methods for rapidly analyzing fracture features is the scanline technique which provides an estimate of fracture density and frequency despite the confidence provided by the systematic use of this method errors and uncertainties caused by sampling biases exist the problems caused by these uncertainties can detrimentally affect the construction of a computational model due to misleading trends this study evaluated the uncertainty caused by sampling biases in the scanline data of opening mode fractures in outcrops of naturally fractured aptian laminated limestone from the crato formation araripe basin northeastern brazil the monte carlo method was chosen to introduce random values into the sampled values which enabled us to verify the importance of errors in the accuracy of the method of representing the fracture network in this study errors and uncertainties were grouped into one parameter termed the coefficient of uncertainty which was defined as the ratio between the uncertainties created by the errors and artifacts introduced artificially and the original scanline data the propagation of errors and uncertainties in the scanline data to the coefficients of the corresponding power law were determined this evaluation can be applied in the construction of more reliable geomechanical models using analog geological models for naturally fractured reservoirs

determining the stress and convergence at beypazari trona field by three dimensional elastic plastic finite element analysis a case study abstract in this paper deformation and failure characteristics of the main drift t 2000 and a roadway t 3000 excavated for beypazari trona natural soda underground mine which is located in ankara turkey were investigated by in situ measurements and numerical analysis t 2000 was planed as a main drift with a length of 965 m 14 inclined and excavated with an area of 18 m 2 the t 3000 roadway was driven in the trona ore body for the first stope load and convergence measurements were carried out at seven stations installed in main drift t2000 and roadway t3000 measurement results indicate that the total convergences at all the stations were below 1 of the total gallery height and deformation rates were reduced significantly three dimensional numerical modeling was performed to analyze the deformation and failure behavior of the drift for three different models using an elasto plastic finite element method fem approach considering drucker prager failure criterion the results of the numerical analyses were found to be comparable with in situ measurements

analysis of antarctic glacigenic sediment provenance through geochemical and petrologic applications abstract the number of provenance studies of glacigenic sediments in antarctica has increased dramatically over the past decade providing an enhanced understanding of ice sheet history and dynamics along with the broader geologic history such data have been used to assess glacial erosion patterns at the catchment scale flow path reconstructions over a wide range of scales and ice sheet fluctuations indicated by iceberg rafted debris in circumantarctic glacial marine sediments it is notable that even though most of the bedrock of the continent is ice covered and inaccessible provenance data can provide such valuable information about antarctic ice and can even be used to infer buried rock types along with their geo and thermochronologic history glacigenic sediments provide a broader array of provenance analysis opportunities than any other sediment type because of their wide range of grain sizes and in this paper we review methods and examples from all size fractions that have been applied to the antarctic glacigenic sedimentary record interpretations of these records must take careful consideration of the choice of analytical methods uneven patterns of erosion and spatial variability in sediment transport and rock types which all may lead to a preferential identification of different elements of sources in the provenance analyses because of this we advocate a multi proxy approach and highlight studies that demonstrate the value of selecting complementary provenance methods

effects of rock mechanical properties on local stress field of the mahanadi basin india results from finite element modelling 1 two dimensional 2 d finite element models fem are generated using two available seismic sections across the mahanadi basin mb at the east coast of india this permits to study the interactive effect between applied stress in the direction of maximum horizontal compressive stress sh and variations in rock mechanical properties of several litho units present in the basin the sh orientation for six offshore wells in the mb known from our previous studies chatterjee and mukhopadhyay 2000 have been used as constraints for fem local stress modifications due to contrast in the young s modulus between sedimentary in fill and the crust down to 20 km depth are investigated for two more orthogonal seismic profiles traversing the basin from onland to offshore fem derived stress vectors clearly respond to variations in elastic properties of rocks corresponding to lithological interfaces in the basin and its underlying crust rotation of the principal stress is prominent where the stress concentration is high further the stress concentration reflects the basinal configuration

u pb data on granulite facies rocks from fold island kemp coast east antarctica abstract the northern tip of fold island 67 16 s 59 20 e on the kemp coast is composed largely of quartzofeldspathic gneisses charnockitic and enderbitic and pyroxene granulite garnet biotite sillimanite gneiss garnet clinopyroxene plagioclase calc silicate rock magnetite bearing rocks and ultramafic rocks are very subordinate the gneisses were deformed by the two phases of isoclinal folding and were subsequently intruded by mafic dikes a second stage of deformation involving tight ductile folding post dates the dikes this deformation culminated with the emplacement of hypersthene bearing pegmatites in discordant planar veins and irregular masses and with the hornblende granulite facies metamorphism that resulted in the present day mineral assemblages u pb isotope data on zircon and monazite from two pegmatites define a chord intersecting concordia at 0 94 0 08 ga and 0 21 0 21 ga the 0 94 ga age dates deformation granulite facies metamorphism and the pegmatite emplacement of stage ii u pb data on a total of five fractions of highly rounded zircons from two quartzofeldspathic gneisses define a chord with intersections at 3 08 0 17 ga and 0 82 0 48 ga our results suggest that the gneiss zircons are xenocrysts and do not provide any direct indication of the age of the host gneisses the fold island gneisses are interpreted to be older napier complex rocks that were reworked during the late proterozoic 1000 ma rayner event a similar sequence of events has been reported for a coastal portion of the eastern ghats province of south india suggesting that these two areas were part of the same metamorphic complex in gondwanaland

mechanical and fracture stratigraphy using examples from core studies this article shows that separate identification of mechanical stratigraphy and fracture stratigraphy leads to a clearer understanding of fracture patterns and more accurate prediction of fracture attributes away from the wellbore mechanical stratigraphy subdivides stratified rock into discrete mechanical units defined by properties such as tensile strength elastic stiffness brittleness and fracture mechanics properties fracture stratigraphy subdivides rock into fracture units according to extent intensity or some other observed fracture attribute mechanical stratigraphy is the by product of depositional composition and structure and chemical and mechanical changes superimposed on rock composition texture and interfaces after deposition fracture stratigraphy reflects a specific loading history and mechanical stratigraphy during failure because mechanical property changes reflect diagenesis and fractures evolve with loading history mechanical stratigraphy and fracture stratigraphy need not coincide in subsurface studies current mechanical stratigraphy is generally measurable but because of inherent limitations of sampling fracture stratigraphy is commonly incompletely known to accurately predict fractures in diagenetically and structurally complex settings we need to use evidence of loading and mechanical property history as well as current mechanical states

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

joint development and tectonic stress field evolution in the southeastern mesozoic ordos basin west part of north china abstract major joint sets trending e w j1 ene wsw j2 ne sw j3 n s j4 nnw sse j5 nne ssw j6 nw se j7 and wnw ese j8 respectively are recognized in mesozoic strata within the southeast of ordos basin among them the j1 j2 and j3 joint sets are systematic joints while the other five joint sets j4 j5 j6 j7 j8 are nonsystematic joints there are three groups of orthogonal joint systems i e j1 and j4 sets j2 and j5 sets and j6 and j8 sets and two groups of conjugate shear fractures ene wsw and nne ssw ene wsw and ese wnw in the study area joint spacing analysis indicates that 1 layer thickness has an effect on the joint spacing but the correlation of joint spacing and layer thickness is low 2 joint density of systematic joints is greater than nonsystematic joints and the joint density of a thin layer is also greater than that of a thick layer and 3 the joints of mesozoic strata in the basin are the result of tectonic events affected by multiple stress fields all these joints in the mesozoic strata are formed in the two main tectonic events since late mesozoic times one is the westward subduction of the pacific plate beneath the eurasia plate which formed the approximately e w trending compressive stress field in the china continent the trends of the j1 joint set e w and the bisector of conjugate shear fractures composed of ene wsw and ese wnw fractures are all parallel to the trend of maximum compressive stress e w the other stress field is related to the collision of the indian and eurasian plates which formed the ne sw trending compressive stress field in the china continent the trends of the j3 joint set and bisector of conjugate shear fractures composed of ene wsw and nne ssw fractures are all parallel to the trend of maximum compressive stress ne sw finally we conclude that the j1 and j4 sets are formed in the e w trending compressive stress field and the j2 j3 j5 j6 j7 and j8 sets are formed in the ne sw trending stress field

fractured shale gas systems the first commercial united states natural gas production 1821 came from an organic rich devonian shale in the appalachian basin understanding the geological and geochemical nature of organic shale formations and improving their gas producibility have subsequently been the challenge of millions of dollars worth of research since the 1970s shale gas systems essentially are continuous type biogenic predominant thermogenic or combined biogenic thermogenic gas accumulations characterized by widespread gas saturation subtle trapping mechanisms seals of variable lithology and relatively short hydrocarbon migration distances shale gas may be stored as free gas in natural fractures and intergranular porosity as gas sorbed onto kerogen and clay particle surfaces or as gas dissolved in kerogen and bitumen five united states shale formations that presently produce gas commercially exhibit an unexpectedly wide variation in the values of five key parameters thermal maturity expressed as vitrinite reflectance sorbed gas fraction reservoir thickness total organic carbon content and volume of gas in place the degree of natural fracture development in an otherwise low matrix permeability shale reservoir is a controlling factor in gas producibility to date unstimulated commercial production has been achievable in only a small proportion of shale wells those that intercept natural fracture networks in most other cases a successful shale gas well requires hydraulic stimulation together the devonian antrim shale of the michigan basin and devonian ohio shale of the appalachian basin accounted for about 84 of the total 380 bcf of shale gas produced in 1999 however annual gas production is steadily increasing from three other major organic shale formations that subsequently have been explored and developed the devonian new albany shale in the illinois basin the mississippian barnett shale in the fort worth basin and the cretaceous lewis shale in the san juan basin in the basins for which estimates have been made shale gas resources are substantial with in place volumes of 497 783 tcf the estimated technically recoverable resource exclusive of the lewis shale ranges from 31 to 76 tcf in both cases the ohio shale accounts for the largest share

experimental rapid alteration of basaltic glass implications for the origins of atmospheric particulates glassy microspherules collected from the plume of etna volcano show a surprising compositional variability with sio 2 from 48 to 90 wt despite a very homogeneous basaltic source magma sio 2 48 this same variability is also observed in individual microspherules from their inner to outer parts the most plausible hypothesis concerning these compositional variabilities is the interaction of the microspherules with the volcanic gases inside the crater prior to their atmospheric dispersal in order to study this compositional variability we experimentally exposed basaltic glass to volcanic gases at etna the resulting glass composition documents a fundamental alteration mechanism involving an apparent leaching process that can rapidly form nearly pure silica because microspherules erupted directly into the air from lava fountains remain unaltered the duration of microspherules within a gas plume can explain the chemical variability of glass alteration a major implication for the origin of glassy atmospheric particulates is that chemical compositions cannot be used as a reliable indicator of their source e g meteoric volcanic anthropogenic

quantitative prediction of lower order faults based on the finite element method a case study of the m35 fault block in the western hanliu fault zone in the gaoyou sag east china

simulation of three dimensional tectonic stress fields and quantitative prediction of tectonic fracture within the damintun depression liaohe basin northeast china abstract tectonic fractures are important factors that influence oil and natural gas migration and accumulation within buried hill reservoirs to obtain a quantitative forecast of the development and distribution of reservoir fractures in the damintun depression we analyzed the characteristics of regional structural evolution and paleotectonic stress field setting a reasonable geological model of the research area was built based on an interpretation of the geological structure a test for rock mechanics and experiment on acoustic emission thereafter a three dimensional paleotectonic stress field during the yanshan movement was simulated by the finite element method rock failure criterion and comprehensive evaluation coefficient of fractures were used to determine the quantitative development of fractures and predict zones that are prone to fracture development under an intense yanshan movement high stress strength is distributed in the south and northeast parts of the study area where stress is extremely high the fracture development zones are mainly controlled by the tectonic stress field and typically located in the same areas as those of high maximum principal and shear stresses the predicted areas with developed fractures are consistent with the wells with high fracture linear density and in locations with high producing oil and gas wells

numerical study of multi period palaeotectonic stress fields in lower cambrian shale reservoirs and the prediction of fractures distribution a case study of the niutitang formation in feng gang no 3 block south china abstract fractures not only control the distribution of oil and gas reservoirs but also are key points in the research of oil and gas reservoir development programmes the tectonic fractures in the lower cambrian shale reservoirs in the feng gang no 3 block are effective reservoir spaces for hydrocarbon accumulation and these fractures are controlled by palaeotectonic stress fields therefore quantitatively predicting the development and distribution of tectonic fractures in the lower cambrian shale reservoir is important for the exploration and exploitation of shale gas in the feng gang no 3 block in the present study a reasonable geological mechanical and mathematical model of the study area was established based on the faults systems interpreted from seismic data fracture characteristics from drilling data uniaxial and triaxial compression tests and experiments on the acoustic emissions ae of rocks then a three dimensional 3 d finite element method is applied to simulate the palaeotectonic stress field with the superposition of the yanshan and himalayan movements and used to predict the fracture distribution the simulation results indicate that the maximum principal stress value within the study area ranged from 269 97 mpa to 281 18 mpa the minimum principal stress ranged from 58 29 mpa to 79 64 mpa and the shear stress value ranged from 91 05 mpa to 106 21 mpa the palaeotectonic stress field is controlled by the fault zone locations the fracture development zones are mainly controlled by the tectonic stress fields and are located around the faults at the end of the fault zones at the inflection point and at the intersection of the fault zones

magnetic fabric and paleomagnetism of the middle triassic siliciclastic rocks from the nanpanjiang basin south china implications for sediment provenance and tectonic process a combined magnetic fabric and paleomagnetic study has been carried out on the siliciclastic rocks gathered from a stratigraphic cross section through the nanpanjiang basin south china in an attempt to extract the paleoflow information preserved in and thus constrain the possible origins of these clastic rocks the sediments used for this study were formed by sediment gravity flows along the southern margin of the south china block in the middle triassic time ca 245 228 ma the results show a normal distribution of both low field magnetic susceptibility values and natural remanent magnetization intensities which along with the monotonic detrital framework mode mainly comprising quartz and lithic particles may suggest a single provenance involved in deposition of these clastic deposits anisotropy of magnetic susceptibility ams analysis acquires primarily the sedimentary magnetic fabrics which in this study reveal paleoflow directions ranging from nnw to ene with an overall mean orientation of ne demagnetization on a part of samples isolates a characteristic remanent component averaged at d 44 8 degrees i 16 9 degrees k 9 7 alpha 95 6 5 degrees n 55 corresponding to a paleolatitude n8 6 degrees and a clockwise rotation of ca 45 degrees since the middle triassic for the studied cross section this mean direction passes fold tests and is consistent with the reference direction expected from the south china block at the 95 confidence level restoring this similar to 45 degrees declination renders an overall northward paleoflow which combined with other evidence suggests a southern provenance for these sediments during deposition in the middle triassic time in terms of the early mesozoic plate framework of southeastern asia a tectonic scenario is proposed here whereby the nearly n s convergence of the indochina and south china blocks and its related indosinian orogeny in the middle triassic caused the formation of the nanpanjiang foreland basin which was filled by voluminous detritus shed from the uplifted orogenic belt on its southern side c 2013 elsevier ltd all rights reserved