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

the northeast brazilian rift system the northeast brazilian rift basins provide important data critical to the understanding of continental rifting processes associated with the opening of the south atlantic these basins represent the locus of intersection of the southern and equatorial branches and some basins yield substantial chronostratigraphic data that constrain the temporal and spatial interaction of the rift phases similar data are not found in its counterpart in africa especially for the neocomian these early cretaceous rift basins of northeast brazil illustrate key three dimensional geometries of intracontinental rift systems mainly controlled by the basement structural framework during the main rift phase syn rift ii neocomian early barremian extensional deformation was distributed over three main rift axes 1 the gabon sergipe alagoas gsa trend 2 the reconcavo tucano jatoba rtj trend and 3 the cariri potiguar cp trend during this phase extensional deformation jumped west from the easternmost basins gsa trend to a series of ne trending intracratonic basins rtj and cp trends characterized by a set of asymmetric half grabens separated by basement highs transfer faults and or accommodation zones these basins are typically a few tens of kilometers wide and trend ne sw roughly perpendicular to the main extension direction during the neocomian preexisting upper crustal weakness zones like the dominantly ne sw trending shear zones of the brazilian pan african orogeny controlled the development of intracrustal listric normal faults internal transverse structures such as transfer faults and accommodation zones were also controlled by the local basement structural framework the megashear zones of pernambuco brazil and ngaundere africa seem to have behaved like a huge accommodation zone accommodating extensional deformation along the rtj gsa trends with simultaneous extension along the cp trend during the late barremian syn rift phase iii a significant change in rifting kinematics occurred when the cp trend was aborted and major rifting initiated at the equatorial branch during the aptian while the equatorial branch and benue trough africa experienced the main rift phase the rtj trend was aborted and the gsa trend developed a transitional phase between the rift and drift stage the gsa trend and the offshore potiguar basin represent the site of continued evolution into passive margin basins following the main rift episode

mechanical and petrophysical properties of north sea shales one of the problems when evaluating potential borehole stability problems in the drilling of hydrocarbon exploration and production wells is the lack of relevant data to describe the lithologies this relates in particular to the shales since these constitute most of the overburden and are also the most troublesome this paper presents a set of data from a variety of deeply cored shales from the north sea the shales have been extensively characterised and span a wide range of both petrophysical and mechanical properties depending on for example burial history and diagenesis special characteristics of shales have been tested and described including pore size distribution permeability anisotropy and temperature dependence appropriate procedures for sample handling testing and interpretation have been developed or modified from other related sciences

evidence for transform margin evolution from the ivory coast ghana continental margin the relative motions between lithospheric plates can be reduced to three major types convergence divergence and transform motion convergence leads to an active margin and eventual collision divergence results in the construction of a rifted passive margin transform motion generates a transform or sheared margin although passive margins have been extensively studied1 2 and many models have been proposed for their origin and subsequent evolution3 5 little is known about transform margins with the exception of a few studies of their morphologies6 7 shallow structures or crustal sections8 9 here we present the main conclusions derived from a recent study of the northern gulf of guinea margins particularly off the eastern ivory coast and ghana where the continental margin is one of the best preserved examples of an extinct transform margin10 12 our observations support a four stage model for transform margin evolution technically active transform contacts first between normal continental crusts and then between thinned margins induce characteristic structures such as pull apart grabens and shear folds the next stage in which thermal exchange between oceanic and continental lithos pheres controls a complex subsidence is followed by the transition to a true intra oceanic fracture zone

continental break up along strike slip fault zones observations from the equatorial atlantic the study focuses on equatorial atlantic margins and draws from seismic well gravimetric and magnetic data combined with thermo mechanical numerical modelling our data and numerical modelling indicates that early drift along strike slip originated margins is frequently characterized by up to 10 20 spreading vector adjustments in combination with the warm thinned crust of the continental margin these adjustments control localized transpression our observations indicate that early drift margin slopes are too steep to hold sedimentary cover which results in their inability to develop a moderately steep slope undergoing cycles of gravitational instability resulting in cyclic gravity gliding these slopes either never develop such conditions or gain them at later development stages our modelling suggests that the continental margin undergoing strike slip controlled break up experiences warming due to thinning along pull apart basin systems pull apart basins eventually develop sea floor spreading ridges margins bounded by strike slip faults located among pull apart basins with these ridges first undergo cooling however spreading ridges leaving the break up trace along its strike eventually pass by these cooling margins warming them again before the final cooling proceeds as a result the structural highs surrounded by several source rock kitchens witness a sequential expulsion onset in different kitchens along the trajectory of spreading ridges

on isostasy at atlantic type continental margins cross spectral techniques have been used to analyze the relationship between free air gravity anomalies and topography of approximately 100 profiles over the continental margins of eastern north america southwest africa and the coral sea lord howe rise the resulting admittance functions for these margins have been interpreted in terms of the flexure model of isostasy the relatively young coral sea lord howe rise margin is associated with the lowest value of the effective elastic thickness te 5 km while the relatively old eastern north american margin is associated with the highest value 10 te 20 km thus te appears to increase with the age of the margin these results which are consistent with flexure studies from the major ocean basins have been interpreted in terms of a simple model in which the flexural strength of the lithosphere increases with age the model suggests that a low rigidity plate or airy type model is most applicable early in rifting history while a high rigidity plate is more applicable later in margin evolution thus sediments at a margin initially load a relatively thin and hot plate while sediments later in margin evolution load a relatively thick and cold plate the model explains a number of features of the tectonic stratigraphic evolution of margins these include a change in flexural style from a narrow and deep basin to a broad and shallow basin coastal plain onlap and uplift in the continental interior furthermore the flexure model of isostasy contributes in a major way to observed free air gravity anomalies at continental margins in particular the model explains the existance of airy isostatic gravity anomalies at margins the amplitude and wavelength of these anomalies are a function of the sediment supply the type of mechanisms controlling sediment deposition up building and or out building and the thermal structure history and age of the margin

thermal consequences of lithosphere extension over continental margins the initial stretching phase summary we compute the thermal evolution of a lithosphere submitted to stretching during a finite duration of time in order to discuss the initial stretching phase of future continental margins the numerical method developed can handle 2 d laterally variable stretching as well as sedimentation it is shown that lateral conduction is more important than vertical conduction over most continental margins during their formation by stretching a simple way to evaluate the relative importance of lateral and vertical conduction effects at the axis of the zone of rifting just prior to oceanization is proposed a simple way to evaluate the amplitude of the thermal uplift on the edges of the zone of rifting at the end of the stretching phase is also presented for small width zones of rifting 70 100 km lateral cooling becomes so large as to prevent large scale melting and presumably prevent the transition to oceanization the effect of highsedimentation rates 100 500 m myr 1 is to increase the surface temperature of the lithosphere and consequently significantly decrease the surface heat flow

velocity anisotropy of upper jurassic organic rich shales norwegian continental shelf abstractthis study investigates the seismic velocity anisotropy of two organic rich shales from the norwegian continental shelf the tested organic rich shale samples were from the upper jurassic draupne and hekkingen formations collected from two wells 16 8 3s and 7125 1 1 drilled in the central north sea and western barents sea respectively the two tested shales are different in organic matter richness and thermal maturation and they have experienced different burial histories the shale core plugs were tested in a triaxial cell under controlled pore pressure seismic velocities vp and vs were measured along different orientations with respect to layering to identify the complete tensor of the rock elastic moduli and to investigate the velocity anisotropy as a function of increasing effective stress the measured velocity values exhibit strong anisotropy for the two tested organic rich shales the anisotropy for both shales is strongest for vs seismic velocities follow an increasing trend as th

early cretaceous rifts of western and central africa an overview abstract the structure and evolution of early cretaceous rift basins in western and central africa are described two stages of rift development and fracturing have been identified 1 from neocomian to early aptian roughly e w and nw trending troughs upper benue n cameroon s chad sudan etc opened in response to a submeridian extensional regime in central africa while in western africa the n s trending transsaharian fault zone acted as a sinistral wrench 2 from middle aptian to late albian large northwest trending troughs e niger sudan sirte etc opened in response to a northeast extensional regime while the central african fault zone from benue to sudan exhibited strike slip movements generating pull apart basins these rift and fracture systems delimit three large blocks within the african plate a western block an arabian nubian block and an austral block the arabian nubian block tends to separate from the two other blocks migrating towards the north during the first stage of basin development and then towards the ne during the second stage the opening of the atlantic ocean was the dominant driving force for the western and austral blocks while the arabian nubian block probably moved in response to the opening of the indian ocean and to the evolution of the tethyan margin

differential opening of the central and south atlantic oceans and the opening of the west african rift system abstract plate tectonic studies of the development of the central and south atlantic oceans using seasat and geosat altimeter and magnetic anomaly isochron data now provide quantitative models of seafloor spreading through time such models enable an initial assessment of the differential opening between these two oceanic basins to be determined the equatorial atlantic is an integral part of this oceanic rifting process allowing stresses arising from the differential opening to be dissipated into both the caribbean and africa along its northern and southern boundaries respectively the tectonic model for the west african rift system based on geological and geophysical studies shows a series of strike slip fault zones diverging into africa from the gulf of guinea and dissipating their shear movement into the development of extensional basins orientated perpendicular to these faults zones the development of the west african rift system was contemporaneous with the early opening of the south atlantic continued to develop well after the final breakup of south america from africa and did not cease until the late cretaceous when there was a major phase of basin inversion and deformation santonian 80 ma deformation across the benue trough nigeria is broadly contemporaneous with dextral shear reactivation of the central african fracture system which in turn resulted in renewed extension in the sudan basins during the late cretaceous and early tertiary this paper illustrates the close linkage in both time and space between the history of the african rift basins and the opening of the atlantic both exhibit distinct phases of evolution with the rift basins developing in direct response to the differential opening between the central and south atlantic in order to dissipate stresses generated by this opening the mesozoic tectonic model proposed is therefore one of an intimate interaction between oceanic and continental tectonics

uplift and extension at the gulf of suez indications of induced mantle convection reconstruction of tectonic movements since the initiation of the gulf of suez rift indicates that there has been 25 27 km of extension the uplift bordering rift indicates that lithosphere heating greatly exceeds that producible by uniform lithosphere extension i propose that small scale convection induced by the rifting augments the heating introduced by extensional rifting and produces the broad uplifts flanking the rift

sediment loading on the western platform of the new zealand continent implications for the strength of a continental margin abstract mechanical properties of the new zealand lithosphere are determined by forward modelling the gravity and isostatic response of a plio pleistocene sediment loaded shelf margin the giant foreset formation on the western platform of new zealand is a 2 km thick 250 km long and 100 km wide sediment package both the downward flexed pliocene miocene boundary beneath the package and a 35 50 mgal peak to trough gravity anomaly over the sediment package are best matched with a flexural rigidity in the region of 5 6 10 22 1 5 10 23 n m effective elastic thickness t e 20 28 km the giant foresets are a particularly important load to model because unlike most sediment loaded passive margins there is a time gap of about 70 80 ma between rifting and subsequent deposition of the sedimentary load furthermore the width of the giant foreset load is sufficiently small to afford a reasonable resolution of t e

microfracturing and microporosity in shales shales are ubiquitous rocks in sedimentary basins where their low permeability makes them efficient seals for conventional oil and gas reservoirs and underground waste storage repositories waste waters co2 nuclear fuels moreover when they contain organic matter they form source rocks for hydrocarbons that may escape towards a more porous reservoir during burial a process referred to as primary migration and when the hydrocarbons cannot escape these rocks can be exploited as oil or shale gas reservoirs while the presence of fractures at the outcrop scale has been described the existence of fractures at smaller scales their link with microporosity the mechanisms that created them their persistence over geological times and their effect on the petrophysical properties of shales represent scientific challenges for which drillings in various sedimentary basins over the past decades may hold timely key data here we review and synthetize the current knowledge on how microfractures and micropores in shales can be imaged and characterized and how they control their anisotropic mechanical properties and permeability one question is whether such microfractures when observed in outcrops or in drilled core samples extracted from boreholes are related to decompaction and do not exist at depth another question is whether veins observed in shales represent microfractures that were open long enough to have acted as flow paths across the formation the mechanisms of microfracture development are described some have an internal origin fracturing by maturation of organic matter dehydration of clays while others are caused by external factors tectonic loading importantly the amount of microfracturing in shales is shown to depend strongly on the content in 1 organic matter and 2 strong minerals the nucleation of microfractures depends on the existence of mechanical heterogeneities down to the nanometer scale their propagation and linkage to create a percolating network will depend on the presence of heterogeneities at the meso to macro scales such percolating microfracture networks could control both the long term sealing capabilities of cap rocks and the further propagation of hydraulic fracturing cracks finally possible areas of research for describing the mechanism of microfracture formation in greater detail and how this impacts the transport and mechanical properties of shales are also discussed

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

4d imaging of fracturing in organic rich shales during heating to better understand the mechanisms of fracture pattern development and fluid escape in low permeability rocks we performed time resolved in situ x ray tomography imaging to investigate the processes that occur during the slow heating from 60 to 400 c of organic rich green river shale at about 350 c cracks nucleated in the sample and as the temperature continued to increase these cracks propagated parallel to shale bedding and coalesced thus cutting across the sample thermogravimetry and gas chromatography revealed that the fracturing occurring at approx 350 c was associated with significant mass loss and release of light hydrocarbons generated by the decomposition of immature organic matter kerogen decomposition is thought to cause an internal pressure build up sufficient to form cracks in the shale thus providing pathways for the outgoing hydrocarbons we show that a 2d numerical model based on this idea qualitatively reproduces the experimentally observed dynamics of crack nucleation growth and coalescence as well as the irregular outlines of the cracks our results provide a new description of fracture pattern formation in low permeability shales

the effect of rift shoulder erosion on stratal patterns at passive margins implications for sequence stratigraphy abstract numerical modelling indicates that the erosion of uplifted rift flanks at passive margins has a profound effect on offshore stratigraphic patterns flexural uplift due to isostatic rebound in response to erosion extends far into the basin and causes uplift of the shelf as a result the contemporaneously deposited sedimentary wedge displays a characteristic offlap pattern when the rift shoulder is largely eroded onlap promoting mechanisms related to cooling of the lithosphere enable sediments to onlap onto the basin margin the initial offlapping and subsequent onlapping strata form one complete second order depositional sequence comprising a shelf margin transgressive and highstand systems tract the modelling inferences are in broad agreement with stratal patterns and basin geometries observed on the u s east coast the southeastern brazilian and southeastern australian passive margins and the transantarctic mountains ross sea shelf system the initial offlaps caused by erosion of rift shoulders have important implications for the deriviation of eustatic signals from coastal onlap patterns

mesozoic plate tectonic reconstructions of the central south atlantic ocean the role of the west and central african rift system abstract the mesozoic opening history of the central equatorial and south atlantic oceans is closely linked in time and space to the development of the cretaceous rift system in west and central africa geological and geophysical studies of the west and central african rift system indicate it to be dominated by a set of sinistral and dextral strike slip zones diverging from the gulf of guinea and terminating as extensional rift basins in niger and sudan plate tectonic studies of the opening history of the atlantic basins indicate four stages of development stage 1 the jurassic opening of the central atlantic stage 2 the early cretaceous 130 119 ma opening of the south atlantic with rifting propagating deep into africa via the benue trough stage 3 the opening of the equatorial atlantic 119 105 ma and stage 4 the linkage of these oceanic basins and development of a single opening mid oceanic rift system as seen today the west and central african rift system continued to develop until the end of the cretaceous i e into stage 4 when rifting was terminated by a compressional shear deformation event causing the folding of sediments in the benue trough and reactivation of the central african shear zone and extensional tectonics in the basins of southern sudan this event correlates with an important change in the relative opening of the atlantic basins during tertiary recent times the west african rift system has not been active however during this period domal uplift and volcanism have occurred in central cameroon and western sudan and a very active phase of volcanism has occurred within the last 10 ma along the present day cameroon volcanic line although these uplift and volcanic events indicate thermal processes in the upper mantle there is as yet no clear link between their development and plate tectonic processes

role of a ductile decollement in the development of pull apart basins experimental results and natural examples abstract traditional models describe pull apart basins as graben or half graben basins with normal or normal oblique slip master faults analogous to death valley california yet many pull aparts are characterized by asymmetric basins with strike slip master faults indicating that not all pull apart basins conform to the simple death valley models we present analogue modelling results that show developmental sequences and structural styles of pull aparts are dramatically different when overburden rides over a ductile horizon and that thickness of the ductile horizon exerts control on basin development in our models synthetic and antithetic strike slip faults control basin geometries while localized normal faulting and local oblique slip on strike slip faults accommodate basin subsidence faults evolve from initial strike slip to normal oblique and normal dip slip to form a system of either isolated sub basins in the case of thick ductile layers or coalescing sub basins in the case of thin ductile layers these results demonstrate distinct differences between non ductile and ductile decollement pull apart structures basin boundaries dominated by normal faults suggest a decollement within or at the base of a non ductile layer similar to death valley california basin bounding faults dominated by strike slip and oblique slip faults indicate basin formation over a ductile layer similar to the gulf of elat aqaba or gulf of paria venezuela and trinidad

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

shallow structures and evolution of the ivory coast and ghana transform margin abstract seismic profiles across the transform continental margin off the ivory coast and ghana western africa illustrate the structural style resulting from the early cretaceous phase of shear stress which leads to the final separation between the african and brazilian continental margins in this area most of the characteristic tectonic features observed along this portion of margin asymmetric grabens on the ghanean platform folds of the deep ivory coast basin the ivory coast ghana marginal ridge are believed to result from progressive transform contacts between the african and brazilian continents as their margins were created during early cretaceous time a major tectonic unconformity inferred to be of upper albian lower cenomanian age may be a direct consequence of the final separation of the continental margins the later evolution of the transform margin is chiefly explained by thermal subsidence

definition of groundwater flow patterns by environmental tracers in the multiple aquifer system of southern arava valley israel abstract the present paper describes a model for identification and semi quantitative evaluation of the different water bodies replenishing groundwater in the southern arava rift valley the potential sources of recharge into the alluvial aquifer of the valley were assessed by multivariable cluster analysis of environmental tracers such as dissolved ions and stable isotopes this data was also used to determine the flow pattern to the valley and within its alluvial aquifer to the natural outlet of the red sea spatial variations in the chemical compositions of groundwater and of their isotopic ratios are explained as the result of differential rates of replenishment of various water bodies flowing to the arava the alluvial aquifer of the southern arava valley is recharged by the following water bodies 1 lateral subsurface flow caused by infiltration of water from flash floods occurring in the neighbouring mountain massifs 2 upward and lateral groundwater flow from sandstone and carbonate aquifers in the negev and edom mountains and 3 leakage from deep seated brine reservoirs a complete groundwater flow pattern based upon hydrological hydrochemical and isotopic data is presented in a compartmental configuration

crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin implications for thermal exchanges across the palaeotransform boundary summary crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin is determined by 2 d modelling of wide angle seismic data and gravity data the resulting models together with geological data recently collected by drilling on the margin are compared with available thermomechanical models of transform margins in particular we test the predicted effects on the transform margin crust of continental heating by the adjacent oceanic lithosphere these effects include i ductile flow of the lower continental crust either parallel to the transform direction or perpendicular to it from the continent towards the ocean and ii thermal uplift of the transform margin in the region of the cote d ivoire ghana transform margin the seismic and gravity data indicate that the continental crust remains uniform in thickness as the continent ocean boundary is approached the interpreted models lack a transitional domain between continental and oceanic crusts and abnormally thin oceanic crust is found along the margin these results exclude large scale ductile flow of lower continental crust during the margin formation the geological data are moreover not consistent with thermal uplift of the margin during the period of contact between the transform margin and the spreading centre our geophysical and geological data thus infer only moderate heating of the transform continental margin by the adjacent oceanic lithosphere moderate heating of the cote d ivoire ghana transform margin during margin formation implies that the thermal exchange between the transform margin and the adjacent spreading centre were lower than predicted by available thermal models two main factors may have contributed to reduce thermal exchanges across the continent ocean boundary the first factor is that the oceanic edge may have been colder than assumed by thermal models an abnormally cold spreading axis along the transform margin when the oceanic crust was emplaced is supported by the atypical structure of the oceanic crust along the cote d ivoire ghana transform margin the oceanic crustal structure is similar to that found along oceanic fracture zones where oceanic crust is also emplaced near a ridge transform intersection in both cases abnormally low thermal gradients at the spreading centre along the transform margin may be explained by the cumulative effect of i a cold edge effect of the adjacent continental lithosphere ii accretion at the end of a ridge segment away from the main magma supply point along the spreading axis and iii an efficient upward cooling of the oceanic lithosphere owing to deep hydrothermal circulation within the faulted lithosphere the second factor causing moderate thermal exchanges is the possibility of large amounts of water in oceanic lithospheric faults which reduces the lateral thermal conduction in the lithosphere since water is much less conductive than rock during the margin formation the oceanic edge thus seems to have acted as a thermal buffer between the cold continental margin and the hot spreading centre we propose that this thermal buffer hypothesis may be generalized to most transform margins implying a lower amount of continental heating than assumed by the available thermomechanical models

petrophysical implications of source rock microfracturing abstract hydrocarbon generation induced microfracturing in source rocks is important affecting both primary migration of hydrocarbons and unconventional shale plays in this study petrographic observations rock eval and total organic carbon content toc data and log derived physical properties were analyzed in order to evaluate eventual petrophysical implications of microfracturing induced by hydrocarbon generation petrographically evidences of microfracturing both normal and parallel to bedding have been found horizontal microfractures typically cross cut each other at an angle between 30 and 90 horizontal microfractures are readily imaged compared to vertical microfractures vertical microfractures were clearly observed only in rare cases in samples with a high degree of microfracturing in the case of moderate microfracturing the vertical component of microfractures may be too small to be detectable using conventional microscopy techniques longer vertical components of the microfractures may be facilitated by higher toc and particularly dense laminated organic matter fabric as well as higher thermal maturity levels clay minerals particularly smectites are also playing a role in reducing permeability and thereby increasing the local pressure generated by hydrocarbon generation eventually resulting in microfracturing log derived petrophysical properties particularly p wave velocity vp and formation resistivity can give information on microfracturing potential even though the techniques can t resolve the microfractures directly vp has been postulated to be sensitive to organic matter quantity and fabric whereas formation resistivity is largely governed by fluid content and clay minerals a reverse trend of increasing resistivity with decreasing vp was found and is linked to higher hydrocarbon saturation and toc and or dense laminated fabric of organic matter favoring microfractures the implications of the log derived petrophysical results are in agreement with the direct petrographic observations formation resistivities in the source rocks however are lower than those observed in non clay dominated hydrocarbon saturated intervals this indicate that low permeable organic rich source rocks with a high clay content particularly smectites may facilitate formation of the hydrocarbon filled vertical microfractures even at relatively low thermal maturities

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

morphotectonic evolution of rifted continental margins inferences from a coupled tectonic surface processes model and fission track thermochronology we use a numerical model to study the topographic evolution and erosional history of rifted continental margins the model combines a kinematic description of lithospheric extension tectonic model with a surface processes model that includes short range hillslope and long range fluvial transport the tectonic model predicts the evolution of the lithospheric temperature and strength elastic thickness distribution as well as the tectonic uplift through time this is input into the surface processes model which calculates the degradation of topography and associated isostatic rebound modeled denudation histories across the margin are used to predict apatite fission track age and length patterns modeling results indicate that depending on the adopted parameters an uplifted rift flank should degrade by erosion within 50 100 m y without significant retreat of the topographic elevation maximum the development of an escarpment system at rifted continental margins is in itself not an indication of tectonic rift flank uplift but results from the existence of a high elevation interior plateau erosional base level lowering as a consequence of rifting and regional isostatic response to erosion of the margin however apatite fission track thermochronology reveals that the areas seaward of the escarpment at a number of rifted margins have been exhumed from several kilometers depth such amounts of denudation cannot be accommodated with isostatic rebound alone and require additional tectonic uplift of the rift flanks modeling of apatite fission track patterns suggests that fission track thermochronology dates the onset of rapid erosion which coincides with the initiation of strong relief i e initiation of rifting fission track ages which are younger than the age of rifting thus cannot be unambiguously interpreted as excluding prerift uplift the timing of margin uplift can be established only by careful track length analysis and integration with regional stratigraphic data the model is applied to the saudi arabian red sea margin and the southeastern australian highlands where it is constrained by present day topography and apatite fission track data as well as seismic and gravity data for the saudi arabian red sea margin synrift regional plateau uplift with a magnitude of approximately 1 km is inferred possibly as a result of asthenospheric upwelling flexurally induced tectonic uplift of the rift flank with a magnitude of 2 km is superimposed on this regional uplift the relatively high elevation of the southeastern australian highlands their steep front and the relatively high amounts of erosion suggest that apart from mesozoic synrift flexural uplift tertiary rejuvenation of topography has occurred possibly as a result of renewed lithospheric thinning and underplating the low elevation of the australian interior would inhibit the evolution of a major escarpment in the absence of such renewed uplift

modelling progressive failure in fractured rock masses using a 3d discrete element method instabilities in rock structures involve coupled mechanisms related to both deformations along existing discontinuities and brittle fracture of intact rock conventional kinematic and limit equilibrium techniques used to study rock slope stability suffer from oversimplifications mass strength degradation and progressive failure mechanisms in rock bridges cannot be ignored and must be considered to predict the overall slope behaviour a 3d numerical model based on the discrete element method has been developed to overcome these limitations pre existing discontinuities as a discrete fracture network dfn can be initially plugged into a set of discrete elements combined with the use of a modified contact logic which provides an explicit representation of joints both fracturing of intact material and yielding within discontinuities can therefore be reproduced depending on the loading conditions and material strength simulations of referenced experimental tests are presented here to show the capabilities of the model in tackling the failure mechanisms of intact rock in the presence of pre existing discrete fractures with an emphasis on the initiation and propagation processes this model proves to be a promising tool in understanding and predicting instabilities that could lead to the failure of fractured rock slopes

the shallow structures of the guinea and ivory coast ghana transform margins their bearing on the equatorial atlantic mesozoic evolution abstract recent geological and geophysical data from a marine survey conducted off guinea and the ivory coast ghana region equatorial atlantic are discussed we interpret the southern guinean margin firstly as an area linked to the central jurassic atlantic and secondly as an area affected by the progressive transform opening of the equatorial atlantic in the early cretaceous off the ivory coast and ghana we have demonstrated distinct structural domains interpreted as consequences of the progressive transform contact between the west african and brazilian cratons also during the early cretaceous on both margins we observe a clear tectonic unconformity of probable late albian early cenomanian age we propose that this unconformity results from the final disruption between the two continental west african and brazilian crusts the geological structures generated along transform margins can be considered as good geological tools to constrain early phases of plate motions particularly around the equatorial atlantic

hydrocarbon generation induced microcracking of source rocks laboratory measurements of ultrasonic velocity and anisotropy in kerogen rich black shales of varying maturity suggest that extensive bedding parallel microcracks exist in situ in most mature source rocks undergoing the major stage of hydrocarbon generation and migration given the normal faulting regime with the vertical stress being the maximum principal stress typical of most sedimentary basins this microcrack alignment cannot be accounted for using simplified fracture mechanics concepts this subhorizontal microcrack alignment is consistent with 1 a model of local principal stress rotation and deviatoric stress reduction within an overpressured formation undergoing hydrocarbon generation and with 2 a strong mechanical strength anisotropy of kerogen rich shales caused by bedding parallel alignment of kerogen microlayers microcracks originate within kerogen or at kerogen illite interfaces when pore pressure exceeds the bedding normal total stress by only a few mpa due to the extremely low fractu

mechanical controls on horizontal stresses and fracture behaviour in layered rocks a numerical sensitivity analysis in layered materials the deformation style orientation confinement and 3d connectivity of natural fractures is generally impacted by changes in sedimentary facies and alternations in mechanical properties in this study we address this effect and perform a numerical sensitivity analysis mechanical properties confining pressures and interfacial frictions are varied for a three layered model to investigate and quantify the relation between contrasting material properties the principal horizontal stresses and fracture behaviour i e deformation style and orientation firstly the results show that tensile stresses develop in the stiffer layers due to the contrasting elastic parameters the magnitude of these stresses is dependent on the ratio between the elastic parameters of stiffer and softer layers i e estiff esoft and stiff soft there are no horizontal tensile stresses when applying a compressive horizontal confining pressure approx 1 5 of the applied vertical stress implementing an interfacial friction lower than 0 2 will result in decoupling of the layers resulting in slip on the layer boundaries and no tensile stresses within the stiffer layers further the acquired numerical results are in good agreement with previously conducted laboratory work finally we discuss whether the presented results can be used for better relating contrasting mechanical properties to potential fracture deformation styles and orientations in layered outcrops or subsurface reservoirs

modelling petroleum migration through microcrack propagation in transversely isotropic source rocks summary to investigate primary petroleum migration through microfracturing of source rocks we develop a theoretical multiphysics model incorporating simultaneous generation of oil and gas from kerogen elastic anisotropy of the source rock and propagation of microcracks filled with oil and gas the variations of excess fluid pressure in the crack and crack propagation distance with time are determined a detailed parametric analysis is performed to study the sensitivity of petroleum migration behaviour to changes in the input parameters including kerogen type chemistry of kerogen to oil gas conversion elastic anisotropy of source rocks geothermal gradient and burial rate numerical results show that a microcrack in type iii kerogen bearing source rocks can attain greater length than in rocks containing type i and ii kerogen which have higher oil potentials and transform to oil gas much faster than type iii kerogen elastic anisotropy of source rocks has a profound influence on the crack propagation distance but only a marginal effect on the duration of crack propagation and the excess fluid pressure the simulation also shows that higher geothermal gradients and fast burial reduce the crack propagation duration significantly but excess pressure and final crack length are not sensitive to the variations of geothermal gradient and burial rate

basement heat flow in the western canada sedimentary basin abstract the effective thermal conductivity of the entire sedimentary succession is calculated at 487 locations in the western canada sedimentary basin based on net rock analyses and thermal conductivity values corrected for temperature and porosity effects an easterly trend of increasing values is caused by shallower burial depth and by a reduced proportion of low conductivity shales geothermal gradients calculated previously and corrected for pleistocene glaciations are used in estimating the basement heat flow after subtracting the heat generated by radioactive decay in the sedimentary strata the distribution of basement heat flow with values between 30 and 80 mw m 2 is linked with the complex structure of the basement which is comprised of archean blocks accreted terranes orogenic belts and magmatic arcs heat flow heat generation relations in alberta show that the reduced heat flow and the characteristic length scale for the uppercrust thermal layer are of the order of 35 mw m 2 and 6 km respectively

continental break up mechanism lessons from intermediate and fast extension settings continental break up mechanisms vary systematically between slow and fast extension systems slow extension break up has been established from studies of the central atlantic european and adria margins this study focuses on the intermediate and fast cases from gabon and east india and draws from the interpretation of reflection seismic gravimetric and magnetic data interpretation indicates continental break up via continental mantle unroofing in all systems with modifications produced by magmatism in faster extension systems break up of the intermediate extension gabon system involves partial upper continental crustal decoupling from continental mantle whereas in the fast east coast india system decoupled and lower crustal regimes underwent upwarping in soggy zones in the footwalls of major normal faults usually upper crustal break up is affected by pre existing anisotropies which form systems of constraining rails for extending continental crust this modifies the local stress regimes they regain a regional character as the function of constraining rails vanishes during progressive unroofing of the upper mantle different regions attain different amounts of upper crustal stretching prior to the break up the break up location is then controlled by the upper crustal energy balance principle of wound linkage by which the minimum physical work is performed for linking upper crustal wounds leading to successful upper crustal break up

the ivory coast ghana transform margin a marginal ridge structure deduced from seismic data abstract the ivory coast ghana icg marginal ridge is a prominent feature of the icg transform margin and includes a fossil ridge partially buried by a thick undeformed sedimentary cover the fossil icg ridge is 130 km long and 25 km wide and towers over the adjacent rifted basin deep ivorian basin dib and the oceanic crust by 1 3 km and more than 4 km respectively it formed in three successive stages 1 1 during the rifting of the dib both vertical and horizontal motions between the dib and the south american plate varied along the plate boundary this relative motion occurred in an accommodation zone that tilted the northern slope of the icg ridge along en echelon mainly strike slip faults 2 2 after the rifting of the dib the relative motion remained constant along the transform plate boundary at this time strike slip deformation was localized into a narrow and highly deformed belt that truncated the accommodation zone 3 3 finally the transform motion occurred between the dib and an occanic plate thermal exchanges between the two adjacent plates induced thermal uplift of the icg ridge that amplified previous tectonic tilting

cote d ivoire ghana margin seismic imaging of passive rifted crust adjacent to a transform continental margin i i summary during may 1990 and january february 1991 an extensive geophysical data set was collected over the cote d ivoire ghana continental margin located along the equatorial coast of west africa the ghana margin is a transform continental margin running subparallel to the romanche fracture zone and its associated marginal ridge the cote d ivoire ghana ridge from this data set an explosive refraction line running 150 km ene wsw between 3 55 n 3 21 w and 4 23 n 2 4 w has been modelled together with wide angle airgun profiles and seismic reflection and gravity data this study is centred on the cote d ivoire basin located just to the north of the cote d ivoire ghana ridge where bathymetric data suggest that a component of normal rifting occurred rafher than the transform motion observed along the majority of the equatorial west african margin traveltime and amplitude modelling of the ocean bottom seismometer data shows that the continental moho beneath the margin rises in an oceanward direction from 24 km below sea level to 17 km in the centre of the line where the crust thins most rapidly there exists a region of anomalously high velocity at the base of the crust reaching some 8km in thickness this higher velocity region is thought to represent an area of localized underplating related to rifting modelling of marine gravity data collected coincident meth the seismic line has been used to test the bestfitting seismic model this modelling has shown that the observed free air anomaly is dominated by the effects of crustal thickness and that a region of higher density is required at the base of the crust to fit the observed data this higher density region is consistent in size and location with the high velocities required to fit the seismic data

late aptian to maastrichtian foraminiferal biogeography and palaeoceanography of the sergipe basin brazil abstract the oldest marine deposits of the sergipe basin are late aptian in age the foraminifera recovered from these strata reveal a characteristic tethyan affinity such as the assemblages of favusellids other rugose and or non rugose hedbergellids ticinellids globigerinelloidids vaginulinids nodosariids polymorphinids gavelinellids eggerellids textulariids lituolids ammosphaeroidinids haplophragmiieds trochamminids and cyclamminids the microfaunal evidence suggests that this area had at least surface water exchange with low latitude central north atlantic western tethyan biogeographic provinces even possibly at intermediate epi to mesopelagic water depths contribution of microfaunal elements from high latitude northern boreal realm and or southern austral regions is also apparent although of lesser significance s similarity index of simpson about 10 20 the occurrence of deep water planktonic morphotypes biticinella ticinella rotalipora dicarinella marginotruncana throughout the mid cretaceous succession confirms a well developed open connection with low latitude western tethyan regions at intermediate water depths since the mid to late albian low diversity gabonita assemblages occur in the uppermost cenomanian middle turonian onshore deposits of the basin they have a more conspicuous occurrence in the lowermost turonian strata similar assemblages have been reported from the cenomanian turonian of gabon and upper cenomanian lower turonian outcrops of morocco and israel west african endemic species of gabonita have not been found in the coniacian maastrichtian of sergipe however on the whole the high diversity foraminiferal microfauna bears striking affinities s about 80 95 with those reported from the gulf coast caribbean and guinean provinces attesting to similar palaeoceanographic conditions at low latitudes on both sides of the atlantic during that time interval the biogeographic distribution of late cretaceous slope assemblages of calcareous and agglutinated benthonic foraminifera was pandemic

denudation of the cote d ivoire ghana transform continental margin from apatite fission tracks apatite fission track analysis of samples from the shoulder marginal ridge of the cote d ivoire ghana transform contin ental margin reveal a cooling of the margin between 85 and 65 ma for the central and eastern parts of the ridge all samples were heated in situ during sedimentary burial with a temperature 120 c except for two samples located in the eastern part which were heated between 105 and 120 c for the first time age depth diagram along a transform margin shows a shape involving erosion starting at the bottom of the continental slope then stepping backwards towards the edge of the slope this retrogressive erosion can result from the deepening of the lithospheric plate sliding along the transform margin from thick continental crust to thin continental crust and finally to oceanic crust this process could be at the origin of the shoulder uplift by flexural response to the important crustal discharge 2 km

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

the dynamics of rifting deformation of the lithosphere by active and passive driving forces summary the thermal and mechanical behaviour of the lithosphere during rifting is investigated using a two layer model for its rheology and different driving forces for the rift process the uppermost crustal layer is assumed to deform by brittle failure while the lower lithosphere including the lower crust is assumed to behave as a newtonian viscous fluid results of numerical modelling of the deformation flow and heat transport in the viscous layer are presented in which viscosity is a strong function of depth and temperature deformation of the upper brittle layer is complex and difficult to quantify thus deformation of this upper layer was approximated by a simple kinematic model in which the layer is thinned by an amount proportional to the amount of extension it experiences two fundamentally different driving forces were investigated they correspond to active rifting in which the litho sphere is thinned by an upwelling limb of a convection cell or mantle plume and to passive rifting or stretching of the lithosphere the results show that for zones of stretching which are wider than the thickness of the viscous layer thinning at all levels in the lithosphere can be described by a single parameter as in earlier stretching models this is not always true when the rift zone is narrow or near the edges of the rift zone initial subsidence due to isostatic and thermal changes during stretching slows as stretching progresses giving an exponential character to the rift stage subsidence active rifting is associated with thermal uplift in this case lithospheric deformation is confined to a narrow zone near the base of the lithosphere and thinning of the crust does not occur until the base of the lithosphere approaches that level the rate of lithospheric thinning and width of the rift zone depend on the convection velocities in the underlying asthenosphere and on the viscosity depth gradient near the lithosphere asthenosphere boundary also contributing to the elevation changes during rifting are dynamic stresses which result from the flow these dynamic affects may significantly modify the uplift or subsidence of the rift zone another important process which results from viscous flow is the generation of secondary convection in the upwelling asthenosphere ths will further thin the lithosphere and can cause depth dependent stretching in which the lower lithosphere is thinned more than the upper lithosphere both of these properties of the flow dynamic stresses and secondary convection critically depend on absolute values of viscosity and so are not well constrained at present the model results presented here exhibit the degree of complexity which results from viscous flow in the lower lithosphere and which is matched by the wide variety of observations on rifting

microfracturing during primary migration in shales abstract in several geological environments chemical reactions are coupled to rock deformation and the associated stresses induced locally interact with the far field loading this is the case in immature shales that undergo burial and diagenesis where the organic matter evolves with temperature into hydrocarbons which induces local volume expansion at large scale this mechanism is responsible for the transport of hydrocarbons from source to reservoir rocks a process referred to as primary migration however how the interactions between local fluid production microfracturing and transport are coupled remain to be understood here we analyze this coupling phenomenon by developing a discrete element model where the generation of local overpressures occurring in kerogen patches is simulated while the surrounding rock is subjected to external loading it is shown that due to local fluid overpressure microfracturing occurs and brings the fluids to migrate through the medium the numerical results are confirmed by laboratory experiments where the network of microfractures induced in an immature green river shale sample heated under small differential stress was imaged in three dimensions using x ray microtomography moreover the numerical simulations identify that the state of differential stress and the initial kerogen distribution constitute two key parameters that control the formation of the three dimensional percolating microfracture network and could thus explain primary migration in shale rocks

the mechanical contrast between layers controls fracture containment in layered rocks abstract open natural fractures allow fluids to flow which is necessary for the production of low permeable geothermal and petroleum reservoirs these reservoirs often consist of lithological layers with significant variation in rock strength which makes it difficult to predict fracture containment within the rock succession in this paper fractures are classified as contained when they do not cross the layer interface of the adjacent layer so that formation and growth are inhibited in the adjacent layer this study concerns the impact of the differences in rock strength i e mechanical contrast between two adjacent brittle layers on the fracture containment in finely layered rocks laboratory deformation tests and 2d finite element modelling were performed on three layered samples with varying mechanical contrasts to examine the behaviour of fractures at different stress conditions fractures initiate as shear fractures in weak layers and propagate with a steeper angle tensile fracture through the adjacent stronger layer the mechanical contrast within a layered rock does not always act as a containment barrier meaning that fractures do not cross the layer interface of the adjacent layer this is due to differences in differential stress between the weakest layer and strongest layer the mechanical contrast combined with the magnitude of the confining pressure has a significant influence on fracture containment an increase in mechanical contrast and confining pressure prevents fractures from propagating into stronger layers the results contribute to predict natural fracture containment in brittle sequences at shallow depth in the subsurface

controls of hydrocarbon generation on the development of expulsion fractures in organic rich shale based on the paleogene shahejie formation in the jiyang depression bohai bay basin east china abstract the development of expulsion fractures in organic rich shale is closely related to hydrocarbon generation and expulsion from kerogen organic rich shales from the upper part of the fourth member and the lower part of the third member of the paleogene shahejie formation in the jiyang depression bohai bay basin east china are used as an example based on thin sections sem and thermal simulation experiments the characteristics of hydrocarbon generation and the conditions supporting the development of expulsion fractures were explored the key factors influencing these fractures include the presence of kerogens their distribution along laminae and around particle boundaries their exposure to heat and the build up in pressure due to confinement by low permeability the development of excess pore fluid pressures and intrinsic low rock fracture strength are the main influencing factors pressurization by rapid generation of hydrocarbon provides impetus for fracture initiation and cause bitumen to migrate quickly the shale laminae results in distinctly lower fracture strength laminae parallel than laminae normal and this directs the formation of new fractures in the direction of weakness when pore fluid pressure increases maximum and minimum principal effective stresses decrease by different proportions with a larger reduction in the maximum principal effective stress this increases the deviatoric stress and reduces the mean stress thus driving the rock towards failure moreover the tabular shape of the kerogen aids the generation of hydrocarbon and the initiation of expulsion fractures from the tip and edge the resulting fractures extend along the laminae when the tensile strength is lower in the vertical direction than in the horizontal direction particle contact boundaries are weak and allow fractures to expand around particles and to curve as the stress strength regime changes when pore fluid pressure fields at different fracture tips overlap fractures will propagate and interconnect forming a network this paper could provide us more detailed understanding of the forming processes of expulsion fractures and better comprehension about hydrocarbon expulsion primary migration in source rocks

fracture toughness anisotropy in shale the use of hydraulic fracturing to recover shale gas has focused attention on the fundamental fracture properties of gas bearing shales but there remains a paucity of available experimental data on their mechanical and physical properties such shales are strongly anisotropic so that their fracture propagation trajectories depend on the interaction between their anisotropic mechanical properties and the anisotropic in situ stress field in the shallow crust here we report fracture toughness measurements on mancos shale determined in all three principal fracture orientations divider short transverse and arrester using a modified short rod methodology experimental results for a range of other sedimentary and carbonate rocks are also reported for comparison purposes significant anisotropy is observed in shale fracture toughness measurements at ambient conditions with values as high as 0 72mpam1 2 where the crack plane is normal to the bedding and values as low as 0 21mpam1 2 where the crack plane is parallel to the bedding for cracks propagating non parallel to bedding we observe a tendency for deviation towards the bedding parallel orientation applying a maximum energy release rate criterion we determined the conditions under which such deviations are more or less likely to occur under more generalized mixed mode loading conditions we find for mancos shale that the fracture should deviate towards the plane with lowest toughness regardless of the loading conditions

in situ imaging of fracture development during maturation of an organic rich shale effects of heating rate and confinement abstract the statistics and dynamics of fractures formed during the accelerated maturation of kerogen rich shale was investigated by heating green river shale r 8 unit piceance basin northwestern colorado core samples while 3d x ray microtomographic images were acquired previous studies have shown that when there was no confining stress fractures formed while the kerogen contained in the shale matured and the produced hydrocarbon was expelled through these fractures in the present study x ray tomographic scans at multiple voxel sizes were conducted on similar samples during heating in one experiment the shale sample was tightly fitted in a non porous ceramic tube to confine it while it was heated three unconfined samples were heated and then held at different final temperatures to investigate the effects of the gas production rate on fracturing 3d image processing was used to survey fracture network development and time lapse 2d digital image correlation analysis was used to monitor the development of the displacement and strain fields the results revealed that fracturing is strongly dependent on the heating rate and the final heating temperature while most of the fractures were oriented more or less parallel to the bedding plane some were strongly inclined relative to the bedding plane the formation of inclined fractures is attributed primarily to the shape and orientation of a minority of the flake like kerogen patches and also to the effective stress field and fractured zones of weakness a conceptual model is proposed to explain the dynamics of fluid expulsion and the associate fracturing behavior

flexural unloading and uplift along the cote d ivoire ghana transform margin equatorial atlantic recent ocean drilling program sampling of the cote d ivoire ghana margin of west africa provides for the first time the opportunity to study the development of a marginal ridge that formed along a sheared passive margin adjacent to the continent ocean transition after the end of intracontinental wrenching we model its evolution using a two dimensional flexural backstripping technique the model is constrained by existing seismic refraction and reflection data on the crustal structure and stratigraphy and paleobathymetric evidence from the cores following rifting at 120 ma aptian intracontinental wrenching continued until 105 ma mid albian when south america and africa separated along this transform while the possible presence of thicker crust under the marginal ridge compared to the deep ivorian basin may explain some of the ridge s topography the entire uplift can be readily modeled as a flexural response to unloading along a shallow dipping 25 fault at the time of continental separation forward subsidence modeling of the marginal ridge suggests that an effective elastic thickness of 2 5 km at 105 ma is most appropriate to match the observed structure conduction of heat from the oceanic plate across the continent ocean transition drove temporary uplift of at least 1200 m peaking just before 89 ma when sedimentary data show deposition within the photic zone 0 50 m this study shows that significant transient thermal uplift can be found in sheared continental passive margin settings even where magmatism is insignificant

geopressuring mechanism of organic matter cracking numerical modeling since the 1950s hydrocarbon generation has been considered as one of the most important abnormal pressuring mechanisms hydrocarbon generation effects on pore pressure evolution 1 it increases the pore fluid volume through the conversion of solid kerogen into liquid or gaseous hydrocarbon and 2 it decreases the sediment permeability because hydrocarbons remain an immiscible phase in pore water in this paper the two products of organic matter decomposition liquid hydrocarbon oil and gaseous hydrocarbon methane are assumed to occur in sequence as a supplementary phase in pores as a basis for evaluating the effect of this pressuring mechanism a simplified three stage first order kinetic reaction model is applied to simulate the generation of oil and gas we u e a numerical basin model with a two phase hydrodynamic formulation to calculate the effect of organic maturation on geopressure evolution oil generation does not play an important role except when the organic matter content of rocks is relatively large 5 however the generation of gas has a large influence the results show that beneath a certain depth oil to gas cracking strongly influences overpressuring the effect of this mechanism becomes more important with increasing organic matter content of rocks several environmental conditions such as kerogen type temperature gradient and lithological characteristics influence to various degrees the effect of organic matter maturation on overpressure development

pyrolysis induced p wave velocity anisotropy in organic rich shales abstractthe sources of elastic anisotropy in organic rich shale and their relative contribution therein remain poorly understood in the rock physics literature given the importance of organic rich shale as source rocks and unconventional reservoirs it is imperative that a thorough understanding of shale rock physics is developed we made a first attempt at establishing cause and effect relationships between geochemical parameters and microstructure rock physics as organic rich shales thermally mature to minimize auxiliary effects e g mineralogical variations among samples we studied the induced evolution of three pairs of vertical and horizontal shale plugs through dry pyrolysis experiments in lieu of traditional samples from a range of in situ thermal maturities the sensitivity of p wave velocity to pressure showed a significant increase post pyrolysis indicating the development of considerable soft porosity e g microcracks time lapse high resolution backscattered electron scanning electron

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