empirical relationships among seismic velocity effective pressure porosity and clay content in sandstone we use a multivariate analysis to investigate the influence of effective pressure pe porosity and clay content c on the compressional velocity vp and shear velocity vs of sandstones laboratory measurements on water saturated samples of 64 different sandstones provide a large data set that was analyzed statistically for each sample relationships between effective pressure and vp and vs have been determined all samples were well fit by relationships that have an exponential increase in velocity at low pe tapering to a linear increase with pe for pe greater than 0 2 kbar there are differences in the pressure dependences of velocity for different rocks particularly at very low pressures however the differences cannot be attributed to or c for the combined set of measurements from all samples the best fitting formulations are vp 5 77 6 94 1 73c 0 446 pe e 16 7pe and vs 3 70 4 94 1 57c 0 361 pe e 16 7pe while this is admittedly a very simplified parameterization it is remarkable how well

oil and gas developments in western canada in 1989 there were 5513 wells drilled in western canada during 1989 30 fewer than in 1988 total meterage drilled was 6 459 797 m for an average well depth of 1172 m exploratory drilling in 1989 dropped by 13 and development drilling by 40 from 1988 levels british columbia was the only area in western canada with increased activity the industry continued to acquire more acreage for less total bonus at provincial crown sales throughout western canada during 1989 3 564 827 ha were acquired for a total bonus of 484 872 000 footnote 5 although the total disposition of crown lands decreased only 4 the average bonus dropped to 136 ha down 26 from 1988 in alberta the most active exploration areas were the southern and eastern mesozoic gas play the western deep devonian gas play the oil plays on and adjacent to the peace river arch and the middle devonian carbonate oil plays in the northwest in british columbia the most active exploration areas were the mesozoic and paleozoic gas plays in the northeastern portion of the province for the first time in 5 years crown drilling rights were issued within the fraser valley in the southwest there was a focus on gas exploration in the province as pipeline reversal was implemented permitting access to canadian and california markets through the alberta transportation network in manitoba exploration and development activity continued to decline in northern canada exploratory drilling success offering of crown lands for exploration approval of gas exports and indications that many of the native lands claims may be approaching settlement all provided exploration encouragement for the next decade exploration activities occurred throughout the northwest territories from the offshore beaufort to the southern border in saskatchewan active exploration areas were shallow mesozoic gas plays in the western portions of the province and deep paleozoic oil plays in the south

brittle ductile transition shear failure and leakage in shales and mudrocks abstract this paper presents an experimental study on brittle ductile transition shear behaviour the formation of shear fractures in shales and mudrocks and the hydraulic properties of shales and mudrocks with shear induced fractures the experimental study is based on triaxial tests on different mudrocks and shales from north sea reservoirs and adjacent areas sheared at different effective confining stresses it is shown that during burial mudrocks behave as normally consolidated materials and exhibit ductile response to increased load however several processes like uplift chemical diagenesis and overpressure build up turn mudrocks into overconsolidated materials which exhibit brittle behaviour during loading triaxial tests with different loading conditions have been performed to establish the brittle to ductile transition of various mudrocks based on the results of the tests it is shown that the brittle to ductile transition can be related to the overconsolidation ratio a relationship between normalized undrained shear strength and overconsolidation ratio is established in combination with this relationship a correlation between compressional wave velocity and apparent pre consolidation stress which accounts for both mechanical and chemical diagenesis may be used as a tool to evaluate possible leakage of hydrocarbon seals

the structural development of the wessex basin the mesozoic and cenozoic structures of southern england form part of a system of disturbances which extends across the channel into northern france they are reviewed in the light of published data from south dorset basin development started with permo triassic rifting and is believed to have continued through the jurassic and early cretaceous with intermittent growth on deep seated listric normal faults probably associated with roll over anticlines this regime had ceased by the aptian and the basin passed through a period of relative stability in the late cretaceous followed by tectonic inversion and in the miocene by northwards compressive movement along the former normal faults this partially restored the earlier separative displacement at depth and probably accentuated pre existing roll over anticlines it was expressed in the surficial upper cretaceous palaeogene sequence as monoclinal flexuring the early rifting and the change from overall n s crustal stretching to compression may reflect plate tectonic events in the north atlantic region

co2 permeability of indian bituminous coals implications for carbon sequestration abstract the permeability of coal is one of the most critical parameters in estimating the performance of coal seams as methane reservoirs or carbon dioxide sinks the reservoir behaviour of coal offers many challenges for the long term planning of gas production performance and the drainage of water from reservoirs moreover heterogeneity in coal from basin to basin from seam to seam and within parts of the same seam makes the process more complex reservoir specific understanding is needed for the best possible estimation of its behaviour factors such as shrinkage swelling of coal matrix gas flow and slippage geomechanical changes cleat anisotropy and effective stresses control the flow behaviour of gases in coal studies of these aspects have gathered momentum in recent decades for different basins across the world however no research is available on swelling induced permeability characteristics or the role of effective stresses for gondwana coals in india considering the fairly high co 2 storage potential of indian coals bituminous coals of the barakar formation in jharia basin india were tested using gas phase co 2 under triaxial conditions this paper presents the results of changing effective stresses in terms of confining stresses and pore pressure incorporating the role of coal swelling with time on the flow pattern of gaseous co 2 corresponding to different depths of target coal seams coal was subjected to different confining stresses in a newly developed triaxial cell for each confinement co 2 was injected at different injection pressures to minimize the effects of temperature on the characteristics of the gas phase it was kept constant at 26 c throughout while use of the drained method of testing maintained the downstream pressure equal to one atmosphere the results showed that at low confinement increase in gas injection pressures leads to an initial decrease in permeability of coal which then becomes almost constant however at higher confinements the permeability increases with increasing injection pressure swelling in coal contributes to reduction of permeability the volumetric expansion in coal reduces the permeability with changing gas injection pressure during the initial confinement as the confining pressure is increased coal permeability reduces by one to two orders of magnitude finally an empirical relation accounting for the role of effective stresses and swelling on coal permeability is proposed for indian coals at lower effective stresses the logarithmic decline model is better suited while the exponential decline relationship fits the data better at effective stresses 12 mpa

the shale gas sweet window the cracked and unbroken state of shale and its depth range abstract to assist in the progression of shale gas exploration in southern china we aimed to clarify the burial depth of the sweet window of shale gas reservoirs in this study we statistically explored the relationship between burial depths and shale gas production of the wufeng longmaxi formations in the sichuan basin southwest china analyzed the development morphology of fractures under different stress fields and fracturing effects of high yield strata conducted a series of triaxial mechanical experiments on shales under the formation conditions and summarized the characteristics of the shale gas sweet window as well as its main affecting factors the results show some interesting conclusions first a sweet window of shale gas reservoir burial depths does exist second it is found that the bedding cracks and artificial fracture network formations are affected by burial depth third shale mechanical properties in the sweet windows generally have the characteristics of transitioning from brittleness to ductility based on the above results a method for determining the top and bottom depth of the shale gas sweet window is proposed the top boundary of the sweet window i e the depth at which shale suffers from brittle fractures can be determined using the shale s over consolidation ratio ocr the bottom boundary of the sweet window i e the depth at which shale transitions to a ductile state can be estimated using triaxial stress experiments the depth range of the sweet window top boundary is 2000 2700 m for the longmaxi shale in the sichuan basin and the depth range of the bottom boundary is 4200 4400 m in areas with tectonic stability and low geo stress differences the top boundary of the sweet window can be shallower than the brittle boundary depth as determined by ocr and the shale gas buried shallower than 2000 m may be preserved effectively and allow for higher production capacity in areas with higher brittle mineral content and greater geo stress differences the bottom boundary of the sweet window may reach 4500 5000 m the area of the wufeng longmaxi shale burial depth between 3500 m and 4500 m is up to 5 5 104 km2 in the sichuan basin which is a potential area for shale gas exploration and development in the future

the 3 d structural model of an out of sequence earthquake in china implication for the reactivation of positive inversion structures along the northern tianshan fold and thrust belt

stress dependent permeability of a de mineralised fracture in shale abstract the paper presents the results of laboratory direct shear tests on the hydro mechanical behaviour of an extensional fracture in shale the tests were conducted on fracture specimens obtained by mechanically splitting blocks of a naturally fractured shale along the cemented fracture and dissolving the calcite fracture filling with a strong acid the experiments consisted of radial fluid flow tests from a single fluid source along the fracture surface fracture samples were tested under different levels of effective normal stress and shear displacement the experimental results show significant reduction of fracture permeability during increasing contact normal stress across the fracture and after shearing of the fracture under constant high normal stress however the tested fractures never completely closed even under normal stresses close to or higher than the unconfined compressive strength of the intact shale the fracture permeability remained much higher than the intact shale or matrix permeability even when the fracture surface has undergone some gouge formation from local asperity failure during shearing the results indicate that fractures once created are difficult to close by mechanical loading in tight formations like shales fractures will always be conduits for fluid flow unless closed by cementation currently it is widely assumed that fractures in sedimentary basins stay open only when there is an overpressure which could keep the fractures hydraulically parted however the results presented here indicate that given a sufficient hydraulic gradient fluid can still flow along a fracture even in the absence of large overpressures

distribution of resistive and conductive structures in nankai accretionary wedge reveals contrasting stress paths in this article we study the characteristics and spatial distribution of the deformation structures along the kumano transect of the nankai accretionary wedge and use this information to interpret the stress path followed by the sediments deformation structures are identified from logging while drilling lwd resistivity images of the materials surrounding the drill hole and from 3 dimensional x ray ct images of cores acquired during the iodp nantroseize project the relative resistivity of the structures identified on logs and the strike dip and density of structures identified on ct scan images are measured the analysis of dip and strike of structures indicates that most of the resistive structures identified on logging data correspond to compactive shear bands results also indicate that conductive structures predominate at the toe of the prism and above the main out of sequence thrust in locations where past and recent erosion occurred we propose several mechanisms that could explain the relation between erosion and the absence of compactive shear bands we conclude that sediments followed different stress paths depending on their location within the wedge and that those differences explain the distribution of deformation structures within the wedge we also show the coexistence of dilatant and compactant structures in fault zones including the frontal thrust and mega splay fault and we interpret the coexistence of these structures as a possible consequence of a transient fluid pressure

load redistribution rules for progressive failure in shallow landslides threshold mechanical models rainfall induced landslides are often preceded by progressive failures that culminate in abrupt mass release local failure progression is captured by a landslide hydro mechanical triggering lht model that represents the soil mantle as interacting columns linked by tensile and compressive mechanical bonds mechanical bonds may fail at a prescribed threshold leaving a modeling challenge of how to redistribute their load to neighboring intact soil columns we employed an elastic spring block model to analytically derive redistribution rules defined by the stiffness ratio of compressive to tensile bonds these linear elastic rules were generalized to real soil using measurable young s modulus and poisson s ratio results indicate that local failure characteristics of ductile like soils e g clay are reproduced by low stiffness ratios whereas global failure of brittle sandy soils corresponds to large stiffness ratios systematic analyses yield guidelines for selecting load redistribution rules for failure of geological materials and mass movement phenomena represented by discrete threshold mechanics

a diagenesis model for geomechanical simulations formulation and implications for pore pressure and development of geological structures forward basin modelling is routinely used in many geological applications with the critical limitation that chemical diagenetic reactions are often neglected or poorly represented here a new temperature dependent kinetic diagenesis model is formulated and implemented within a hydromechanical framework the model simulates the macroscopic effects of diagenesis on 1 porosity loss 2 sediment strength 3 sediment stiffness and compressibility 4 change in elastic properties 5 increase in tensile strength due to cementation and 6 overpressure generation a brief overview of the main diagenetic reactions relevant to basin modelling is presented and the model calibration procedure is demonstrated using published data for the kimmeridge clay formation the calibrated model is used to show the implications of diagenesis on prediction of overpressure development and structural deformation the incorporation of diagenesis in a uniaxial burial model results in an increase in overpressure of up to 9 mpa due to both stress independent porosity loss and overpressure generated by disequilibrium compaction caused by a reduction in permeability finally a compressional model is used to show that the incorporation of diagenesis within geomechanical models allows the transition from ductile to brittle behaviour to be captured due to the increase in strength that results in an over consolidated stress state this is illustrated by comparison of the present day structures predicted by a geomechanical only model where a ductile fold forms and a geomechanical model accounting for diagenesis in which a brittle thrust structure is predicted

estimation of the highest potential transmissivity of discrete shear fractures using the ductility index abstract previous studies indicated that the highest potential transmissivities of fractures in fault zones might be well estimated by a mechanical indicator the ductility index di the di is defined as the effective mean stress normalized to the tensile strength of the intact rock mass the mechanism of formation and preservation of pore structures in fault zone fractures can be explained by local shear induced tensile stresses and pore structures in discrete shear fractures may form in the same way i investigated therefore whether the di model can correctly predict the highest transmissivities of shear fractures recorded in fractured neogene diatomaceous mudstone koetoi formation where fault zones and joints are rare but discrete shear fractures without mineral fillings are abundant analyses of the shear fractures reveal that the detected states of paleostress are nearly equal to the current stress state implying the shear fractures have potentially been tectonically active thus the highest transmissivities detected at fractures by flowing fluids electric conductivity ffec borehole logging may potentially be treated as the highest potential transmissivities in terms of the given dis relationships between the transmissivities of flow anomalies and dis fit the existing di model well which suggests the model is applicable to discrete shear fracture systems as well as fault zones

seismic anisotropy and subduction induced mantle fabrics beneath the arabian and nubian plates adjacent to the red sea for most continental areas the mechanisms leading to mantle fabrics responsible for the observed anisotropy remain ambiguous partially due to the lack of sufficient spatial coverage of reliable seismological observations here we report the first joint analysis of shear wave splitting measurements obtained at stations on the arabian and nubian plates adjacent to the red sea more than 1100 pairs of high quality splitting parameters show dominantly n s fast orientations at all 47 stations and larger than normal splitting times beneath the afro arabian dome aad the uniformly n s fast orientations and large splitting times up to 1 5 s are inconsistent with significant contributions from the lithosphere which is about 50 80 km thick beneath the aad and even thinner beneath the red sea the results can best be explained by simple shear between the lithosphere and the asthenosphere associated with northward subduction of the african arabian plates over the past 150 ma

compaction behavior of argillaceous sediments as function of diagenesis the paper presents the results of laboratory compaction tests on samples of kimmeridge clay taken from two different locations in the uk the main objective of the laboratory tests is to determine how diagenesis affects the hydro mechanical properties and compaction behavior of argillaceous sediments the two kimmeridge clays used in the tests are essentially from the same parent material but have undergone different degrees of mechanical and chemical diagenesis the laboratory tests are designed to determine the effects of mechanical loading and chemical processes on changes in porosity permeability compressibility strength and effective horizontal stress in argillaceous materials another objective of the study is to investigate the adequacy of soil mechanical constitutive models in predicting the response of argillaceous materials to conditions corresponding to burial depths in sedimentary basins the tests on kimmeridge clay are carried out to as high as 150 mpa effective vertical stress corresponding to about 9 km in burial the results indicate that mechanical compaction and burial depth cannot explain the differences in the hydro mechanical behavior of the tested materials the differences in hydro mechanical response are attributed to chemical diagenesis by precipitation of minerals in the pores of the sediment other observed effects of chemical diagenesis on hydro mechanical behavior of lithified sediments are 1 increase in apparent pre consolidation stress 2 decrease in compressibility 3 decrease in permeability and 4 decrease in effective horizontal stress implications of the results to fluid flow and overpressure development in sedimentary basins are discussed

hydrofracturing and episodic fluid flow in shale rich basins a numerical study low permeability sedimentary rocks commonly are fractured direct examination of exposed rock faces and drill cores shows evidence of hydrofracturing the mechanism for hydrofracturing its effects on fluid migration and its dependence on sediment permeability sedimentation rate and sedimentary sequences have not been explored in this study we carry out systematic numerical experiments to study the compaction induced hydrofracturing we show that the compaction induced hydrofracturing commonly may occur in shale rich basins and in sand shale sequences the frequency of such hydrofracturing depends on sediment permeability sedimentation rate and sedimentary sequence an important result is that compaction induced hydrofracturing may occur at relatively shallow depths in shale rich basins but it may mobilize enhanced fluid flow throughout the sedimentary basin over 60 of the total compaction induced fluid flow in the basin may be expelled during hydrofracturing we test the model against field data from the yinggehai basin of the south china margin where abundant hydrofractures in the uppermost quaternary marine mud have been recently detected by seismic imaging we suggest that the compaction induced hydrofracturing may mobilize fluid flow at great depths and affect hydrocarbon migration in shale rich basins

shale permeability and its significance in hydrocarbon exploration a complex dynamic series of chemical physical and geologic processes influence the creation sediment infilling and fluid quality history of sedimentary basins deposits of sands and muds for example through progressive burial and compaction become exposed to increased temperatures and pressures this exposure precipitates a series of interrelated diagenetic and mechanical processes that in turn alter the physical and chemical properties of the sediments and their pore fluids hydrocarbon generation from kerogens organic material is one important consequence of sediment diagenesis such processes also control the capacity of a particular sediment to transport and store fluids which in some cases results in hydrocarbon bearing reservoirs

episodic fluid expulsion from geopressured sediments numerical simulation of fluid explusion from geopressured sediments using a one dimensional finite difference model shows that large volumes of fluid can be transported through a fractured seal during short lived 100 year expulsion events in this model abnormal fluid pressures are maintained by an impermeable seal when the fluid pressure exceeds 85 of the lithostatic pressure a vertical fracture opens in the seal providing a permeable pathway from the geopressured sediments into the overlying section fluid pressure in the seal decreases as fluid flows through the fracture closing the fracture as sediments in the seal compact once the fracture closes the seal remains impermeable until fluid flowing from deeper in the section increases fluid pressure to the fracture criteria a process that takes 10 000 500 000 years fractures emplaced in relatively incompressible seals such as compacted and or cemented sediments remain open for 20 50 years fluid fluxes through the seal integrated over the life of these events are 1 5 107 kg m2 and temperatures in the overlying sediments are increased by 5 20 c these temperature anomalies decay to less than half their original value in 300 years suggesting that the thermal anomalies observed in wells that are attributed to fluid expulsion formed in response to a very recent or currently active event

effect of phycosiphoniform burrows on shale hydrocarbon reservoir quality unconventional gas tight gas coalbed methane and shale gas has become an increasingly significant source of energy economic production from such low permeability reservoirs relies upon identifying regions of the reservoir that will yield the highest gas production rates currently available gas recovery technologies are highly dependent on the fracturability of the reservoir zones of enhanced brittleness and permeability within shale gas reservoir horizons are a prerequisite for successful shale gas recovery such brittle zones are directly linked with increased quartz and or carbonate content within the mudstone in mudstones with high clay mineral content quartz may be concentrated and redistributed as a result of burrowing activities of infaunal organisms high quality porosity and permeability zones in shale petroleum reservoirs may be present in the form of silty and sandy tortuous strips of selectively concentrated grains of quartz that constitute burrow halos grain selective burrows therefore can improve reservoir capacity permeability and fracturability and thus control the storativity of the shale petroleum reservoir this study presents three dimensional reconstructions of three different types of phycosiphon like burrows and investigates the possible fluid flow paths caused by the ichnofabric the volumetric approach to the bioturbation generated by phycosiphoniform burrow makers used herein shows that the volume of sediment that becomes more porous and more permeable media within such bioturbated interval can range from 13 to 26 of the total volume the quartzose strips of sediment caused by bioturbation are highly tortuous and interconnected vertically and horizontally thereby increasing both horizontal and vertical permeability additionally the quartz frameworks created by the burrows may locally increase fracturability within otherwise nonbrittle mudstones

stress fields a key to oil migration a coherent oil migration model based on geomechanical considerations includes both the high molecular kerogen structure and the capillary properties of source rocks oil is squeezed from kerogen by compaction following oil generation this squeezing effect should be created by the differential stress maximum compressive stress minus least compressive stress acting on the kerogen which has been chemically broken up by oil formation in sedimentary bodies whose water is at hydrostatic pressure the migration of oil seems to involve two processes 1 lateral transfer by channeling into the more coarsely microporous layers of the source rocks from the oil generation site toward the geologic structure or lower pressured zone and 2 vertical transfer from source rock to eservoir by the opening or reopening of vertical fractures in the few areas such as structural tops where the least compressive stress is slightly greater than or equal to the pore pressure and where the capillary pressure increment 2 ggr r of oil in the microporosity exceeds the tensile strength of the rock in sedimentary bodies whose water is overpressured the pore pressure should be governed by the least compressive stress and thus migration should begin by oil transfers in a system of small open fractures and eventually in larger fractures the theory demonstrates the impossibility of oil being transferred to the reservoir under true tensile conditions negative effective compressive stress and thus explains the large asphaltic veins of southeastern turkey and the well known bitumen veins of the uinta basin

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

comparison of evolutionary and static modeling of stresses around a salt diapir abstract we compare an evolutionary with a static approach for modeling stress and deformation around a salt diapir we show that the two approaches predict different stress histories and very different strains within adjacent wall rocks near the base of a rising salt diapir significantly higher shear stresses develop when the evolutionary analysis is used in addition the static approach is not able to capture the decrease in the hoop stress caused by the circumferential diapir expansion nor the increase in the horizontal stress caused by the rise of the diapir hence only the evolutionary approach is able to predict a sudden decrease in the fracture gradient and identify areas of borehole instability near salt furthermore the evolutionary model predicts strains an order of magnitude higher than the strains within the static model more importantly the evolutionary model shows significant shearing in the horizontal plane as a result of radial shortening accompanied by an almost equivalent hoop extension the evolutionary analysis is performed with elfen and the static analysis with abaqus we model the sediments using a poro elastoplastic model overall our results highlight the ability of forward evolutionary modeling to capture the stress history of mudrocks close to salt diapirs which is essential for estimating the present strength and anisotropic characteristics of these sediments

factors controlling the thermo mechanical deformation of oil shales implications for compaction of mudstones and exploitation abstract six permian to miocene organic matter rich mudstones were subjected to unconfined compression tests under three thermo mechanical regimes the aims of this study were to assess the role of factors that control deformation and highlight implications for mudstone compaction and oil shale exploitation axial strain ranged from 1 9 to 23 at 25 c 5 3 70 mpa 12 79 at 25 310 c 31 42 mpa and 1 38 40 at 25 350 c 0 8 mpa the axial strain data showed that the principal factor controlling mudstone compaction is temperature with a secondary role for effective stress during high temperature tests dehydration of smectite in the 94 150 c interval resulted in a distinct axial strain effect decomposition of organic matter and other minerals caused visible strain only above 280 c tests up to 350 c resulted in mass loss of 4 54 43 8 wt with 1 1 8 wt due to organic matter a strong positive correlation was found between the petroleum generation indices 4 6 55 8 and expulsion efficiencies 38 6 96 2 while compaction 0 68 51 4 correlated with axial strain 1 38 40 solid volume loss 2 55 12 15 due to petroleum generation from kerogen resulted in an increase in porosity 1 56 6 36 low rate heating and low retorting temperature are necessary to optimise yield from oil shale exploitation

tensile and shear fracturing in predominantly compressive stress fields a review abstract many rock engineering geologic problems can be better solved if the genesis of rock fractures rock joints is understood this paper concentrates on tensile fractures and shear fractures created in predominately compressive stress fields such fractures prevail in many areas stress based and fracture mechanics based concepts are introduced they are used to show how tensile fractures are created how shear fractures can be produced through coalescence of tensile fractures and how shear fractures and tensile fractures can be created simultaneously which of these mechanisms acts depends on the particular combination of stress field and material properties laboratory experiments and observations in nature obtained from an extensive literature review strongly support the theoretical predictions they also show that actual fracture patterns are often produced by a combination of mechanisms acting simultaneously or in sequence the better understanding of fracture genesis which should result from this paper is not only interesting by itself but is useful when interpreting exploration results

experimental investigation into the sealing capability of naturally fractured shale caprocks to supercritical carbon dioxide flow geological storage of co2 is considered a solution for reducing the excess co2 released into the atmosphere low permeability caprocks physically trap co2 injected into underlying porous reservoirs injection leads to increasing pore pressure and reduced effective stress increasing the likelihood of exceeding the capillary entry pressure of the caprocks and of caprock fracturing assessing on how the different phases of co2 flow through caprock matrix and fractures is important for assessing co2 storage security fractures are considered to represent preferential flow paths in the caprock for the escape of co2 here we present a new experimental rig which allows 38 mm diameter fractured caprock samples recovered from depths of up to 4 km to be exposed to supercritical co2 scco2 under in situ conditions of pressure temperature and geochemistry in contrast to expectations the results indicate that scco2 will not flow through tight natural caprock fractures even with a differential pressure across the fractured sample in excess of 51 mpa however below the critical point where co2 enters its gas phase the co2 flows readily through the caprock fractures this indicates the possibility of a critical threshold of fracture aperture size which controls co2 flow along the fracture

models for dynamic evaluation of mudstone shale cap rocks and their applications in the lower paleozoic sequences sichuan basin sw china abstract the estimation of the sealing capability of cap rocks dynamically is one of the fundamental geological problems that must be resolved in petroleum exploration in this paper a porosity capillary pressure p c method was used to estimate the sealing capacity during cap rock burial and a permeability capillary pressure k p c plus over consolidation ratio ocr method to estimate the sealing capacity during uplift based on 120 capillary pressure porosity measurements a p c mathematical model has been established for the burial phase by analyzing the permeability data measured from 12 mudstone shale samples from different stratigraphic intervals under step loading conditions a permeability confining pressure k p mathematical model has been constructed moreover capillary pressure permeability data measured from 141 mudstone or shale cap rocks have been used to establish a k p c mathematical model for seal capacity evaluation during uplift additionally the ocr of mudstone or shale was calculated from the results of reconstructed burial and uplift histories these mathematical models were used in case studies in the sichuan basin south china the results indicate that capillary pressure p c is inadequate in evaluating sealing capacity of cap rocks but the combination of parameters p c and ocr works well the agreement between calculated and measured values shows the effectiveness and reliability of the models for dynamic estimation of the sealing capacity of mudstone shale cap rocks established in this study

modeling stress evolution around a rising salt diapir abstract we model the evolution of a salt diapir during sedimentation and study how deposition and salt movement affect stresses close to the diapir we model the salt as a solid visco plastic material and the sediments as a poro elastoplastic material using a generalized modified cam clay model the salt flows because ongoing sedimentation increases the average density within the overburden sediments pressurizing the salt stresses rotate near a salt diapir such that the maximum principal stress is perpendicular to the contact with the salt the minimum principal stress is in the circumferential direction and drops near the salt the mean stress increases near the upper parts of the diapir leading to a porosity that is lower than predicted for uniaxial burial at the same depth we built this axisymmetric model within the large strain finite element program elfen our results highlight the fact that forward modeling can provide a detailed understanding of the stress history of mudrocks close to salt diapirs such an understanding is critical for predicting stress porosity and pore pressure in salt systems

evidence for compressionally induced high subsidence rates in the kurile basin okhotsk sea a combined volcanological geochemical paleo oceanological geochronological and geophysical study was undertaken on the kurile basin in order to constrain the origin and evolution of this basin very high rates of subsidence were determined for the northeastern floor and margin of the kurile basin dredged volcanic samples from the geophysicist seamount which were formed under subaerial or shallow water conditions but are presently located at depths in excess of 2300 m were dated at 0 84 0 06 and 1 07 0 04 ma with the laser 40ar 39ar single crystal method yielding a minimum average subsidence rate of 1 6 mm year for the northeast basin floor in the quaternary trace element and sr nd pb isotope data from the volcanic rocks show evidence for contamination within lower continental crust and or the subcontinental lithospheric mantle indicating that the basement presently at 6 km depth is likely to represent thinned continental crust average subsidence rates of 0 5 2 0 mm year were estimated for the northeastern slope of the kurile basin during the pliocene and quaternary through the determination of the age and paleo environment depth of formation of sediments from a canyon wall taken together the data from the northeastern part of the kurile basin indicate that subsidence began in or prior to the early pliocene and that subsidence rates have increased in the quaternary similar rates of subsidence have been obtained from published studies on the sakhalin shelf and slope and from volcanoes in the rear of the kurile arc the recent stress field of the kurile basin is inferred from the analysis of seismic activity focal mechanism solutions and from the structure of the sedimentary cover and of the alaid back arc volcano integration of these results suggests that compression is responsible for the rapid subsidence of the kurile basin and that subsidence may be an important step in the transition from basin formation to its destruction the compression of the kurile basin results from squeezing of the okhotsk plate between four major plates the pacific north american eurasian and amur we predict that continued compression could lead to subduction of the kurile basin floor beneath hokkaido and the kurile arc in the future and thus to basin closure

pleistocene glaciolacustrine breccias of seismic origin in an active graben central poland varved layers in the partly consolidated glaciolacustrine early saalian deposits exposed in the belchatow mine central poland show specific types of deformations two types dominate half varve deformations constituting the uppermost clayey part of a single varve and multi varve deformations affecting a few varves the latter are found either as clast supported breccia beds with varve fragments in a quasi horizontal position within a clayey matrix or as deformed beds resulting from subsequent plastic deformation and folding the deformed beds are interpreted as seismites derived from earthquakes related to the deep graben structure the magnitudes of the earthquakes have determined the type of deformations the seismites are thought to have originated intraformationally

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

geomechanical analysis of a welding salt layer and its effects on adjacent sediments abstract we simulate welding of the source layer of a salt diapir with a forward finite element model and study stresses and deformation in the salt layer and the diapir as well as in their adjacent sediments welded salt layers are abundant in mature salt basins where most or all of the salt has withdrawn into diapirs however there is little understanding of the stress field in these layers and their adjacent sediments we show that salt flow along the source layer leads to significant stress anomalies inside the layer and in adjacent sediments in the source layer salt pressure becomes higher than overburden stress in nearly welded areas and becomes lower than overburden stress in adjacent thicker areas when the source layer welds stresses increase significantly in sediments near the weld tip which helps compaction of these sediments and possibly their fracturing and faulting our model illustrates that all sediments overlying the weld experience this stress increase and the associated material changes as the weld tip propagates along the weld we present natural examples fitting our predictions and discuss the importance of our results for the exploration characterization and production of reservoirs near welded salt layers

shale tectonic processes field evidence from the parras basin north eastern mexico abstract major decollements located within buried overpressured shale commonly develop in thrust fronts accretionary prisms and sedimentary deltas controlled by gravity tectonics in seismic data it is possible to observe only large scale deformation of what is commonly designed as mobile shale but the precise geometry and the dynamic evolution of these bodies remains poorly understood it is often difficult to define if we are dealing with ductile or brittle deformation and to understand the role of the fluids in time and space during deformation for this reason large scale outcrops were studied in the parras basin mexico which makes possible a direct observation of the shale tectonic processes this work suggests changes in space and time of the deformation processes which occurred within the shale formation hosting the decollement distributed deformation was observed within the shale formation hosting the decollement compared to more localized deformation above also a change of the rheology of the shale over time occurred progressively toward brittle processes in the whole sedimentary pile xrd and microscopic studies have shown that diagenetic processes are favored in the shear zones of penetrative deformation leading notably to reverse gradient of illitization the isotopic analysis of cements in veins and the study of associated fluid inclusions have shown that fluid dynamics also evolved during time showing notably evidence for widespread fluid migration issued from rocks located below the decollement during the beginning of the deformation progressively the tectonic system located above the decollement tends to be preserved from fluid migration coming from below the decollement and to be influenced only by local fluid migration closed system