early diagenetic silica deposition in algal cysts and spores a source of sand in black shales abstract devonian black shales deposited on the north american craton contain abundant tasmanites cysts although these are typically flattened because of compaction a small proportion are filled with diagenetic silica the latter are spherical to ellipsoidal 0 1 0 5 mm and filled with chalcedony microquartz and megaquartz and single quartz crystals chalcedonic cyst fillings are preserved best in cheat and phosphate nodules whereas megaquartz and single quartz crystals are most common in shale matrix together with colloform textures this suggests that the various silica types originated from recrystallization of early diagenetic silica deposits thin sandstone beds in the chattanooga shale e g bransford sandstone contain abundant quartz sand that is much coarser than the detrital quartz component of the underlying black shales because of this their quartz component is thought to have been transported over considerable distances from the basin margin however because certain shale horizons contain as much as 10 silicified cysts that upon reworking could have yielded quartz grains of fine to coarse sand size the quartz component of these sandstone beds may actually have formed in situ indeed petrographic examination of the sandstone beds shows them to contain quartz grains with morphological and textural features of cyst quartz e g rounding sphericity chalcedony pyrite inclusions lobate grain margins thus silica deposition in algal cysts may provide a significant component of intrabasinal quartz sand in shale sequences distinction of this type of quartz from extrabasinal detrital quartz is important to the reconstruction of the depositional history of shale sequences

tectono sedimentary evolution of the bowland basin n england during the dinantian the bowland basin is a ne sw trending sedimentary basin lying between the askrigg bowland high to the n and nw and the central pennine high to the se the basin was initiated in late devonian tournaisian times sediment thickness and facies variations within the dinantian succession show marked asymmetry on both a basin wide and intra basin scale as a result of approximate nw se to n s extension across normal faults thickening of the sedimentary sequence from ne to sw over much of the basin resulted from fault controlled extension along the se margin of the basin with downthrow to the nw giving the basin a half graben character extension occurred in two distinct episodes marked by basin wide debris flow and sedimentary slide slump deposits during the late chadian early arundian the simple half graben became modified by a series of ne sw trending antithetic and nw se trending transfer faults associated with local unconformities and marked lateral thickness and facies variations this interval marks the transition from carbonate ramp to slope sedimentation and a temporary halt in carbonate supply to the basin the second episode of extension the late asbian early brigantian marks the break up of the northern basin margin ending major carbonate deposition within the basin reactivation of these structures at the end of the carboniferous created the ribblesdale fold belt as a result of nw se to n s compression transpression application of this research to areas surrounding the bowland basin has led to the recognition of additional shelf basins the orientation of which suggests control by normal and transfer fault systems discussion and

novel 3d centimetre to nano scale quantification of an organic rich mudstone the carboniferous bowland shale northern england x ray computed tomography and serial block face scanning electron microscopy imaging techniques were used to produce 3d images with a resolution spanning three orders of magnitude from 7 7 m to 7 nm for one typical bowland shale sample from northern england identified as the largest potential shale gas reservoir in the uk these images were used to quantitatively assess the size geometry and connectivity of pores and organic matter the data revealed four types of porosity intra organic pores organic interface pores intra and inter mineral pores pore sizes are bimodal with peaks at 0 2 m and 0 04 m corresponding to pores located at organic mineral interfaces and within organic matter respectively these pore size distributions were validated by nitrogen adsorption data the multi scale imaging of the four pore types shows that there is no connected visible porosity at these scales with equivalent diameter of 20 nm or larger in this sample however organic matter and clay minerals are connected and so the meso porosity 20 nm within these phases provides possible diffusion transport pathways for gas this work confirms multi scale 3d imaging as a powerful quantification method for shale reservoir characterisation allowing the representative volumes of pores organic and mineral phases to be defined to model shale systems the absence of connected porosity at scales greater than 20 nm indicates the likely importance of the organic matter network and associated smaller scale pores in controlling hydrocarbon transport the application of these techniques to shale gas plays more widely should lead to a greater understanding of properties in the low permeability systems

mechanical properties of shale gas reservoir rocks part 1 static and dynamic elastic properties and anisotropy abstractunderstanding the controls on the elastic properties of reservoir rocks is crucial for exploration and successful production from hydrocarbon reservoirs we studied the static and dynamic elastic properties of shale gas reservoir rocks from barnett haynesville eagle ford and fort st john shales through laboratory experiments the elastic properties of these rocks vary significantly between reservoirs and within a reservoir due to the wide variety of material composition and microstructures exhibited by these organic rich shales the static young s modulus and dynamic p and s wave moduli elastic parameters generally decrease monotonically with the clay plus kerogen content the variation of the elastic moduli can be explained in terms of the voigt and reuss limits predicted by end member components however the elastic properties of the shales are strongly anisotropic and the degree of anisotropy was found to correlate with the amount of clay and organic content as well as the shale fab

role of fluids in the earthquake occurrence around aswan reservoir egypt 1 continuing seismicity for about 30 years near a large western embayment of the lake nasser about 50 km from the aswan high dam in egypt has led to a debate about the possibility of its relation with the reservoir impoundment the largest event in the region occurred on 14 november 1981 m 5 3 20 km beneath the wadi kalabsha embayment a westward extension of the lake aswan since then continuous monitoring of seismic activity has given an excellent opportunity to study the spatiotemporal distribution of seismicity in the area most of the immediate aftershocks of the 1981 main shock were located in the gebel marawa area at depths between 15 and 30 km depths of almost all earthquakes away from this zone were shallower than 12 km to quantify the effect of the reservoir impoundment on the seismicity of the aswan area we calculated changes in stress and pore pressure due to the reservoir impoundment using green s function approach the change in coulomb stress s is calculated on the fault planes responsible for majority of the seismicity of the region we found that for all the seismogenic faults s is negative i e stabilizing when we consider the effect of the reservoir load only whereas it is positive i e destabilizing when we include pore pressure for example at the hypocenter of the main earthquake shear stress normal stress and pore pressure due to reservoir operation are estimated as 5 5 13 2 and 13 5 kpa respectively which suggest that s is 3 1 kpa when we do not consider the effect of pore pressure and 5 7 kpa when contribution from pore pressure is considered hence the seismicity in the aswan lake region is driven by the pore pressure due to reservoir impoundment

the application of sequence stratigraphy to upper carboniferous fluvio deltaic strata of the onshore uk and ireland implications for the southern north sea stratigraphical correlations and fades interpretations of upper carboniferous fluvio deltaic strata have been based traditionally on cyclothems bound by marine flooding surfaces marine bands the recent recognition of major regionally extensive erosional unconformities exxon style sequence boundaries within selected cyclothems questions their validity as units of genetically related strata u sing examples from the carboniferous of the onshore uk and ireland we present sedimentological criteria for the recognition of sequence boundaries placing particular emphasis on the regional context of these surfaces sequence boundaries comprise widespread deeply eroded surfaces at the base of major fluvial sandstone complexes and laterally equivalent palaeosols developed on interfluves at the margins of the fluvial complexes these sequence boundaries define units of genetically related strata sequences which contain other key surfaces of time stratigraphic significance including marine bands and regionally extensive coals the recognition of key surfaces enables the construction of a high resolution stratigraphic framework within which coeval facies relationships can be interpreted sequence boundaries can be correlated between individual basins in the onshore uk by reference to their position in relation to a particular marine band for example the sequence boundary at the base of the farewell rock in the south wales basin can be correlated with that at the base of the rough rock in the pennine basin northern england since both these sandstone bodies are directly overlain by the subcrenatum marine band interbasinal correlations of this nature imply that potential fluvial sandstone reservoirs within major incised valley fills in the upper carboniferous strata of the southern north sea can be predicted by correlation with the onshore uk the stratigraphical framework can be extended and tested using core and well log data particularly spectral gamma ray data which are able to identify key sequence stratigraphic surfaces

normalized undrained shear strength of clay shales this paper examines the possibility of normalizing the undrained shear strength behavior of clay shales shales are formed in sedimentary basins by diagenetic processes which turn young clay sediments into compacted and lithified shales cementation at grain contacts affects many properties of shales including the apparent preconsolidation and shear strength based on a study of 25 different types of materials the normalized undrained shear strength of clay shales was found to correlate satisfactorily with overconsolidation ratio following the shansep stress history and normalized soil engineering properties normalization procedure for clays the normalized undrained shear strength correlation appears valid regardless of the cause of the apparent preconsolidation correlations of the apparent preconsolidation stress with unconfined compressive strength and compressional wave velocity are proposed to predict the normalized undrained shear strength of clay shales

deposition and diagenesis of upper miocene arkoses yowlumne and rio viejo fields kern county california upper miocene arkoses of the yowlumne and rio viejo fields southern san joaquin valley california are channel fill turbidites characterized by the dominance of incomplete massive a or ab turbidite sequences they occur in a depth range of 11 250 to 14 300 ft 3 430 to 4 360 m quartz and feldspar constitute more than 90 of the detrital mineralogy however in the yowlumne and stevens sandstones k feldspars dominate over plagioclase by more than a 2 1 ratio and the plagioclase has an average composition of an14 or less in the 10 4 sandstone this ratio is 1 1 and the average plagioclase composition is an32 diagenesis during deep burial results in extensive feldspar alteration and clay authigenesis the clay minerals vary in mineralogy abundance and sequence of formation in the different units these variations were considered geochemically and we concluded that the effect of detrital feldspar composition on the pore fluid chemistry is the dominant control porosity and permeability of these arkoses are dependent on the abundance and type of authigenic clays the overall averages are 14 7 and 3 8 md for the 10 4 sandstone compared to 16 9 and 38 md for the yowlumne and stevens sandstones thus development of reservoir quality is strongly influenced by detrital feldspar composition and diagenesis

evidence of fluid flow in microfractures in geopressured shales petrographic and microprobe analyses of geopressured shales from the oligocene frio formation brazoria county texas provide direct evidence of hydraulic fracturing and significant fluid flow in the shales the optical continuity of the fracture margins and their higher al si ratios indicate that the fracture margins have undergone more complete alteration of smectite to illite than the interiors of the fracture blocks crosscutting relationships between calcium sulfate and the organic material that fills the fractures indicate that calcium sulfate was emplaced before the organic material and thus was precipitated in the subsurface the calculated fracture permeabilities of the shales on the order of 10 13 m2 are similar in magnitude to that of he frio sandstone reservoirs which suggests that these fractured shales could support flow comparable to that in the sandstones fracture fill deposits or collapse could in turn reduce this flow in addition the more extensive alteration of the fracture margins suggests that fluid in these shales flowed preferentially through the microfractures rather than through the matrix

radiocarbon constraints on antarctic peninsula ice sheet retreat following the last glacial maximum lgm abstract we present marine sedimentologic and radiocarbon data for the timing of retreat of the largely marine based antarctic peninsula ice sheet since the last glacial maximum lgm our findings indicate minimum estimates of deglaciation between 18 000 and 9000 calibrated years before present cal yr bp roughly in phase with the northern hemisphere deglaciation and eustatic sea level rise our findings show this retreat occurred progressively from the outer middle and inner continental shelf regions as well as progressively from the north to the south retreat initiated on the outer shelf of the northern peninsula by 18 000 cal yr bp and continued southward by 14 000 cal yr bp on the outer shelf of marguerite bay several thousand years earlier than estimated by numeric models while individual cores yield estimates of glacial retreat that may vary up to 1100 years we note steps in the data occur at 14 000 and possibly 11 000 cal yr bp coincidental to rapidly rising eustatic sea level including the well documented melt water pulses mwp 1a and 1b these data support the hypothesis that rapidly rising sea level is associated with marine ice sheet destabilization although additional dates are necessary to substantiate this finding this study highlights problems with radiocarbon dating acid insoluble organic aio matter in proximal lateglacial sediments as well as the need for more accurate dating techniques

effect of heterogeneity of brittle rock on micromechanical extensile behavior during compression loading a grain based universal distinct element code model was developed to generate a deformable polygonal grain like structure to simulate the microstructure of brittle rock it takes into account grain scale heterogeneity including microgeometric heterogeneity grain scale elastic heterogeneity and microcontact heterogeneity the microgeometric heterogeneity can be used to match the grain size distribution of the rock the discrete element approach is able to simulate the microheterogeneity caused by elastic variation and contact stiffness anisotropy the modeling approach was evaluated using lac du bonnet granite and a aspo diorite the microheterogeneity played an important role in controlling both the micromechanical behavior and the macroscopic response when subjected to uniaxial compression loading the crack initiation stress was found to be controlled primarily by the microscale geometric heterogeneity while the microcontact heterogeneity controlled the strength characteristics the effect of heterogeneity on the distribution and evolution of tensile stresses and associated extension cracks was also examined

diagenesis provenance and reservoir quality of triassic tagi sandstones from ourhoud field berkine ghadames basin algeria the triassic tagi trias argilo greseux inferieur fluvial sandstones are the main oil reservoirs in the berkine basin algeria nonetheless their provenance and diagenesis and their impact on reservoir quality are virtually unknown samples from the ourhoud field representing the lower middle and upper tagi subunits were studied using a combination of petrographic mineralogical and geochemical techniques the lower tagi sandstones have an average framework composition of q98 3f0 6r1 1 and 95 of the quartz grains are monocrystalline by contrast the middle upper tagi sandstones have an average framework composition of q88 3f9 8r1 9 and 79 7 of the quartz grains are monocrystalline the lower tagi quartz arenites derived from paleozoic siliciclastic rocks whereas the middle upper tagi subarkoses originated mainly from metamorphic terrains this change in provenance is a potential criterion for correlation within the tagi also this change has contributed to the significantly different diagenetic paths followed by the lower tagi quartz arenites and the middle upper tagi subarkoses grain coating illitic clays are abundant in the lower tagi where they exert a critical control on reservoir quality these clays are interpreted as pedogenic and or infiltrated in origin and to have had in part smectitic precursors shallow burial fe dolomite cementation was favored in the downthrown block of the field bounding fault where it contributed to the poor reservoir quality magnesite siderite cements are multiphase the earliest generation is composed of fe rich magnesite that precipitated during shallow burial from hypersaline fluids with high mg ca ratios probably refluxed residual brines associated with the liassic evaporites later magnesite siderite generations precipitated during deeper burial from waters with progressively higher fe mg ratios authigenic vermicular kaolin largely consists of dickite that replaced previously formed kaolinite dickitization was followed by late stage illitization related to the dissolution of detrital and authigenic k feldspar quartz the most abundant cement was mainly sourced by the pressure or clay induced dissolution of detrital quartz and is a critical factor controlling the reservoir quality overall quartz cement is more abundant in the lower tagi than in the middle upper tagi and this increase correlates with a decrease in average porosity within the lower tagi quartz cement abundance is stratigraphically very variable which is in part related to facies controlled variations in grain coating clay resulting in major vertical variations in reservoir quality anhydrite and barite cements postdate quartz overgrowth the sulfate necessary for their formation was likely sourced by deep subsurface dissolution of late triassic liassic evaporites

organic geochemistry of mississippian shales bowland shale formation in central britain implications for depositional environment source rock and gas shale potential abstract marine carboniferous shales are proven hydrocarbon source rocks in central britain in this contribution the depositional environment and shale gas liquid potential of the lower namurian part of the bowland shale formation is studied using 77 thermally immature samples from the duffield borehole the bowland shale formation comprises mudstone and turibidite lithofacies reflecting a pronounced sea level controlled cyclicity the total organic carbon toc content of the mudstones lithofacies including marine bands and of fine grained rocks within the turibidite lithofacies varies between 1 3 and 9 1 hydrogen index hi values imply the presence of kerogen type iii ii according to biomarker ratios and bulk geochemical parameters marine bands maximum flooding surfaces mfs were deposited in deep water with slightly enhanced normal or slightly reduced salinity mudstones of the highstand systems tract hst were deposited in environments with normal to reduced salinity whereas photic zone anoxia favoured the preservation of marine organic matter during deposition of the mfs and the hst the supply of landplant debris increased during the hst turbidites and their non calcareous mudstone equivalents represent lowstand systems tracts deposited in low salinity environments terrestrial organic matter dominates in turbiditic sediments marine organisms prevail in time equivalent mudstones mudstone beneath marine bands represents transgressive systems tracts when normal marine conditions and photic zone anoxia were re established the mudstone lithofacies exhibits a very good to excellent potential to generate conventional mixed oil and gas toc content of fine grained rocks in the turbidite lithofacies depends on the amount of detrital minerals supplied from the south moreover their organic matter is gas prone high toc contents and large thicknesses of the mudstone lithofacies show that the bowland shale formation holds a significant shale gas liquid potential in areas with appropriate maturity a relatively low average hi and high clay contents may have negative effects on the shale gas potential

an enhanced understanding of the basinal bowland shale in lancashire uk through microtextural and mineralogical observations abstract variability in the lower bowland shale microstructure is investigated here for the first time from the centimetre to the micrometre scale using optical and scanning electron microscopy om sem x ray diffraction xrd and total organic carbon content toc measurements a significant range of microtextures organic matter particles and fracture styles was observed in rocks of the lower bowland shale together with the underlying pendleside limestone and worston shale formations encountered the preese hall 1 borehole lancashire uk four micro texture types were identified unlaminated quartz rich mudstone interlaminated quartz and pyrite rich mudstone laminated quartz and pyrite rich mudstone and weakly interlaminated calcite rich mudstone organic matter particles are classified into four types depending on their size shape and location multi micrometre particles with and without macropores micrometre size particles in cement and between clay minerals multi micrometre layers and organic matter in large pores fractures are categorized into carbonate sealed fractures bitumen bearing fractures resin filled fractures and empty fractures we propose that during thermal maturation horizontal bitumen fractures were formed by overpressuring stress relaxation compaction and erosional offloading whereas vertical bitumen bearing resin filled and empty fractures may have been influenced by weak vertical joints generated during the previous period of veining for the majority of samples the high toc 2 wt low clay content 50 wt and the presence of a multi scale fracture network support the increasing interest in the bowland shale as a potentially exploitable oil and gas source the microtextural observations made in this study highlight preliminary evidence of fluid passage or circulation in the bowland shale sequence during burial

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 interaction of propagating opening mode fractures with preexisting discontinuities in shale field observations show that hydraulic fracture growth in naturally fractured formations like shale is complex preexisting discontinuities in shale including natural fractures and bedding act as planes of weakness that divert fracture propagation to investigate the influence of weak planes on hydraulic fracture propagation we performed semicircular bend tests on marcellus shale core samples containing calcite filled natural fractures veins the approach angle of the induced fracture to the veins and the thickness of the veins have a strong influence on propagation as the approach angle becomes more oblique to the induced fracture plane and as the vein gets thicker the induced fracture is more likely to divert into the vein microstructural analysis of tested samples shows that the induced fracture propagates in the middle of the vein but not at the interface between the vein and the rock matrix cleavage planes and fluid inclusion trails in the vein cements exert some control on the fracture path combining the experimental results with theoretical fracture mechanics arguments the fracture toughness of the calcite veins was estimated to range from 0 24 mpa m1 2 to 0 83 mpa m1 2 depending on the value used for the young s modulus of the calcite vein material measured fracture toughness of unfractured marcellus shale was 0 47 mpa m1 2

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

microfractures due to overpressures caused by thermal cracking in well sealed upper devonian reservoirs deep alberta basin microfractures 3 this condition is favored by high pore pressures but failure occurs before pore fluid pressures reach the lithostatic pressure radial and random microfractures are rare and of uncertain origin

nature and timing of late mississippian to mid pennsylvanian glacio eustatic sea level changes of the pennine basin uk the pennine basin of northern england contains a comparatively complete serpukhovian moscovian succession characterized by high resolution ammonoid zonation and cyclic paralic sedimentation two new isotope dilution thermal ionization mass spectrometry zircon ages from a bentonite deposited during the arnsbergian mid serpukhovian regional substage and tonstein of earliest bolsovian early moscovian regional substage have been determined the weighted mean 206 pb 238 u ages of 328 34 0 55 and 314 37 0 53 ma total uncertainty respectively require modification of the time scale for the western europe regional chronostratigraphy the areal extent of acme ammonoid facies is used as a proxy for the magnitude of 47 discrete flooding events incised valleys major sequence boundaries are used as a proxy for the magnitude of sea level falls the frequency of these events in the light of the new radiometric dating indicates the following 1 there is temporal coincidence between major glaciations in gondwana and phases of increased frequency of sequence boundaries in the pennine basin 2 high amplitude flooding surfaces have an average frequency of c 400 ka 3 average cycle durations during the pendleian early arnsbergian and chokierian bolsovian of c 111 and c 150 ka respectively reflect short duration eccentricities 4 multiple flooding surfaces with the same ammonoid assemblages may equate with sub 100 ka precession or obliquity frequencies supplementary material u pb method description and data procedure for the calculation of the areal extent of marine bands and tables showing a full listing of biostratigraphical data used in the study are available at www geolsoc org uk sup18505

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

geologic analysis of the upper jurassic haynesville shale in east texas and west louisiana the upper jurassic haynesville shale is currently regarded as one of the most prolific emerging shale gas plays in the continental united states it has estimated play resources of several hundred trillion cubic feet and per well reserves estimated as much as 7 5 bcf the reservoir spans more than 16 counties along the boundary of eastern texas and western louisiana although this basin has a long history of exploration and analysis of its mesozoic section a comprehensive subsurface study characterizing the haynesville shale has not been conducted this article is the first to address the structural setting stratigraphy depositional environment and facies fracturing and production challenges of the haynesville shale gas play basement structures and salt movement influenced carbonate and siliciclastic sedimentation associated with the opening of the gulf of mexico the haynesville shale is an organic and carbonate rich mudrock that was deposited in a deep partly euxinic and anoxic basin during the kimmeridgian to the early tithonian related to a second order transgression that deposited organic rich black shales worldwide the haynesville basin was surrounded by carbonate shelves of the smackover and haynesville lime louark sequence in the north and west several rivers supplied sand and mud from the northwest north and northeast into the basin haynesville mudrocks contain a spectrum of facies ranging from bioturbated calcareous mudstone laminated calcareous mudstone and silty peloidal siliceous mudstone to unlaminated siliceous organic rich mudstone framboidal to colloidal pyrite is variably present in the form of concretions laminae and individual framboids and replaces calcite cement and mollusk shells haynesville reservoirs are characterized by overpressuring porosity averaging 8 to 12 sw of 20 to 30 nanodarcy permeabilities reservoir thickness of 200 to 300 ft 70 to100 m and initial production of as much as 30 mmcf day reservoir depth ranges from 9000 to 14 000 ft 3000 to 4700 m and lateral drilling distances are 3000 to 5000 ft 1000 to 1700 m typical haynesville wells exhibit a steeper decline curve 80 in the first year than other shale gas plays which is attributed to a very high overpressure

characterizing the shale gas resource potential of devonian mississippian strata in the western canada sedimentary basin application of an integrated formation evaluation devonian mississippian strata in the northwestern region of the western canada sedimentary basin wcsb were investigated for shale gas potential in the subsurface thermally mature strata of the besa river horn river muskwa and fort simpson formations attain thicknesses of more than 1 km 0 6 mi encompassing an area of approximately 125 000 km2 48 300 mi2 and represent an enormous potential gas resource total gas capacity estimates range between 60 and 600 bcf section of particular exploration interest are shales and mudrocks of the horn river formation including the laterally equivalent lower besa river mudrocks muskwa formation and upper besa river formation which yield total organic carbon toc contents of up to 5 7 wt fort simpson shales seldom have toc contents above 1 wt horn river and muskwa formations have excellent shale gas potential in a region between longitudes 122w and 123w and latitudes 59n and 60n national topographic system nts 94o08 to 94o15 in this area which covers an areal extent of 6250 km2 2404 mi2 average toc contents are higher 3 wt as determined by wire line log calibrations and have a stratal thickness of more than 200 m 656 ft gas capacities are estimated to be between 100 and 240 bcf section and possibly greater than 400 tcf gas in place a substantial percentage of the gas capacity is free gas caused by high reservoir temperatures and pressures muskwa shales have adsorbed gas capacities ranging between 0 3 and 0 5 cm3 g 9 6 16 scf t at reservoir temperatures of 60 80c 140 176f whereas besa river mudrocks and shales have low adsorbed gas capacities of less than 0 01 cm3 g 0 32 scf t liard basin region because reservoir temperatures exceed 130c 266f potential free gas capacities range from 1 2 to 9 5 cm3 g 38 4 to 304 scf t when total pore volumes 0 4 6 9 are saturated with gas the mineralogy has a major influence on total gas capacity carbonate rich samples indicative of adjacent carbonate platform and embayment successions commonly have lower organic carbon content and porosity and corresponding lower gas capacity 1 toc and 1 porosity seaward of the carbonate slave point edge muskwa and lower besa river mudrocks can be both silica and toc rich up to 92 quartz and 5 wt toc and most favorable for shale gas reservoir exploration because of possible fracture enhancement of the brittle organic and siliceous rich facies however an inverse relation between silica and porosity in some regions implies that zones with the best propensity for fracture completion may not provide optimal gas capacity and a balance between favorable reservoir characteristics needs to be sought

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

the diagenetic destruction of porosity in plutoniclastic petrofacies the miocene diligencia and eocene maniobra formations orocopia mountains southern california usa

the effect of interbedding on shale reservoir properties north sea oil is overwhelmingly generated in shales of the upper jurassic basal cretaceous kimmeridge clay formation once generated the oil is expelled and ultimately migrates to accumulate in sandstone or carbonate reservoirs the source rock shales however still contain the portion of the oil that was not expelled as a consequence such shales and juxtaposed non source lithofacies can form the targets for the exploration of unconventional oil in this paper we examine part of the kimmeridge clay formation as a hybrid shale resource system within which hot shale and organic lean sandstone and siltstone intervals are intimately interbedded this hybrid system can contain a greater volume of oil because of the increased storage capacity due to larger matrix porosities of the sand silt interbeds together with a lower adsorptive affinity in the interbedded sandstone the relationship between the estimated volume percentages of sand and mudstone and free oil determined from rock eval s1 yields is used to place limits on the drainage of oil from source mudstone to reservoir sand at the decimeter scale these data are used to determine oil saturations in interbedded sand mudstone sequences at peak oil maturity higher values of free hydrocarbon as evidenced by the s1 value in mudstone suggest that more oil is being retained in the mudstone while higher s1 values in the interbedded sands suggest the oil is being drained to saturate the larger pore spaces high silica content in the interbeds confirms the brittleness in this mudstone sandstone lithofacies an important factor to be considered for fracture stimulation to successfully work in a hybrid system the key points of this hybrid unconventional system are the thickness storage capacity and the possibility to capture a portion of the expelled as well as retained oil

geochemistry and mineralogy of marine and non marine namurian black shales from the tansley borehole derbyshire the namurian black shales studied have a good palaeontological control pyrite illite and organic matter are higher in the marine shales and siderite and kaolinite are higher in the non marine shales the distribution of the diagenetic iron minerals is attributed to anionic activities controlled by salinity the same control could account for the observed clay mineral distribution by differential flocculation the major element geochemistry closely reflects the above mineral variation mn and zr are higher in the non marine shales due to siderite and zircon respectively co cr rb y and ba are not significantly different and a detrital source mainly clay minerals is suggested in the marine shales the elements pb cu v ni sr and zn are significantly higher using a discriminant function analysis the palaeosalinity groups are separated and the contribution of each element to the separation is calculated the present element locations are thought from correlation analysis to be pyrite and organic matter by analogy with experimental work and modern environments the element enrichment is mainly attributed to reactions involving organic matter and oxyhydroxide material in environments in which salinity and slow rates of sedimentation were important factors the element enrichment cannot be related directly to seawater concentrations unlike some black shales and the accumulation rates for pb and cu are thought to be unusually high

experimental investigation of the effect of temperature on the mechanical behavior of tournemire shale this paper is devoted to the experimental investigation of the effect of temperature on the mechanical behavior of a typical anisotropic clayey rock the tournemire shale hydrostatic and conventional triaxial compression tests are first performed under room temperature for two principal loading orientations respectively parallel and perpendicular to the bedding planes the obtained results confirm an anisotropic mechanical behavior observed by the previous works further hydrostatic and triaxial compression tests with different confining pressures are carried out under different temperatures up to 250 c with the increase of temperature there is a significant decrease of young s modulus and the compression failure strength but an increase of the overall deformability of material the temperature change also affects the anisotropic response related to the deformation of bedding planes

the organic matter distribution and methane capacity of the lower cretaceous strata of northeastern british columbia canada abstract the methane sorption capacity of a succession of sandstone siltstone shale and coal from the lower cretaceous fort st john group of northeastern british columbia was investigated average organic matter om content for all formations is 2 3 wt with a minimum of 0 6 wt and maximum of 10 1 wt the methane sorption capacity ranges between 0 19 and 2 74 cm3 g at 6 mpa and at 30 c total gas capacity free and sorbed gas ranges between 2 2 and 16 6 cm3 g at 6 mpa micropore volumes range between 0 43 and 1 69 cm3 100 g a positive correlation exists between the om content micropore volume and the methane capacity of shales shales with high methane capacities have either high contents of inertodetrinite or vitrinite moisture content shows a positive relationship with om content micropore volume and methane capacity the negative relationship between moisture and surface area derived from n2 adsorption suggests the moisture is located within the microporosity of the om and also explains the positive relationship between methane capacity and moisture the high inertodetrinite and vitrinite contents correlate with sea level regressions that delivered terrestrial om from coastal plains located south of the study area into the basin

upper palaeozoic basins of the british isles caledonide inheritance versus hercynian plate margin processes hypotheses to explain the origin and location of upper palaeozoic sedimentary basins are presented group a basins were those old red sandstone basins whose location and subsidence were controlled by caledonide features including active and remnant fore arc basins midland valley and major transcurrent movements orcadian basin newer granite plutonic magmas in the southern uplands subduction complex post dated subduction and resulted from post collision cratonization processes as yet little understood group b basins were those whose initial positions in space were controlled by palaeo plate factors in particular the location of granitoid plutons but whose subsidence was controlled by active hercynian neoplate processes some group a basins were regenerated by this process midland valley a new hypothesis is presented to account for the origin of a carboniferous rift to sag trend similar to that proven in modern continental margin basins such as the north sea the hypothesis is an adaptation of mckenzie9s lithospheric stretching model some of whose major features are well shown in the carboniferous history of the north and central british isles group b processes were controlled by ligurian bretonic plate consumption in the s brittany massif central vosges areas group c basins in the rheno hercynian zone also owe their location and origin to back arc rifting and continental crustal oceanization processes caused by liguro bretonic subduction

diagenetic origin of quartz silt in mudstones and implications for silica cycling mudstone the most abundant sedimentary rock type1 composed primarily of clay or silt sized particles contains most of the quartz found in sedimentary rocks2 these quartz grains which are chemically and mechanically resistant and therefore preserve their characteristics well have long been considered to be derived from the continental crust1 here we analyse quartz silt from black shales in the eastern usa dating back to the late devonian period about 370 million years ago using backscattered electron and cathodoluminescence imaging and measure oxygen isotopes with an ion probe our results indicate that up to 100 of the quartz silt in our samples does not originate from the continental crust instead it appears to have precipitated early in diagenesis in algal cysts and other pore spaces3 with silica derived from the dissolution of opaline skeletons of planktonic organisms such as radiolaria and diatoms transformation of early diatoms into in situ quartz silt might explain the time gap between the earliest fossil occurrences of diatoms about 120 myr ago4 and molecular evidence for a much earlier appearance between 266 or even 500 myr ago5 6 moreover if many other mudstone successions show similarly high proportions of in situ precipitated rather than detrital quartz silt the sedimentary record in mudstones may have been misinterpreted in the past with consequences for our estimates of palaeoproductivity as well as our perceptions of the dynamics and magnitude of global biogeochemical cycling of silica

mississippian barnett shale fort worth basin north central texas gas shale play with multi trillion cubic foot potential the mississippian barnett shale serves as source seal and reservoir to a world class unconventional natural gas accumulation in the fort worth basin of north central texas the formation is a lithologically complex interval of low permeability that requires artificial stimulation to produce at present production is mainly confined to a limited portion of the northern basin where the barnett shale is relatively thick 300 ft 92 m organic rich presentday total organic carbon 3 0 thermally mature vitrinite reflectance 1 1 and enclosed by dense limestone units able to contain induced fractures the most actively drilled area is newark east field currently the largest gas field in texas newark east is 400 mi 2 1036 km 2 in extent with more than 2340 producing wells and about 2 7 tcf of booked gas reserves cumulative gas production from barnett shale wells through 2003 was about 0 8 tcf wells in newark east field typically produce from depths of 7500 ft 2285 m at rates ranging from 0 5 to more than 4 mmcf day estimated ultimate recoveries per well range from 0 75 to as high as 7 0 bcf efforts to extend the current barnett play beyond the field limits have encountered several challenges including westward and northward increases in oil saturation and the absence of lithologic barriers to induced fracture growth patterns of oil and gas occurrence in the barnett in conjunction with maturation and burial history data indicate a complex multiphased thermal evolution with episodic expulsion of hydrocarbons and secondary cracking of primary oils to gas

fibrous calcite veins overpressures and primary oil migration the fibrous calcite veins found parallel to the bedding of certain shales may have been formed during burial as a consequence of overpressuring their presence could even be indicative of former overpressures this suggestion if it can be upheld carries implications for theories of primary oil migration

an analysis of horizontal microcracking during catagenesis example from the catskill delta complex horizontal bitumen filled microcracks are common within clay laminae of the finely laminated organic carbon rich shale in the lower half of the heavily jointed upper devonian dunkirk shale western new york state such cracks are not found higher in the dunkirk shale where moderate bioturbation resulted in a relatively porous and permeable microfabric horizontal microcracks in a hydrocarbon source rock that carries regional vertical joints indicating a horizontal least principal stress owe their presence to material properties of the fractured shale and the magnitude and orientation of the crack driving stress during kerogen maturation three material properties favored the horizontal initiation of microcracks in the dunkirk shale 1 the abundance of flat kerogen grains oriented parallel to layering 2 a marked strength anisotropy in large part caused by the laminated nature of the rock and 3 the tight strongly oriented planar clay grain fabric produced by gravitational compaction of flocculated clay at shallow burial depth the latter was especially important to sustaining elevated pore pressure the crack driving stress which was generated by the conversion of kerogen to bitumen poroelastic deformation of the low permeability laminated shale pressurized by catagenesis perhaps enhanced by compaction disequilibrium prior to kerogen conversion elevated the in situ horizontal stress in excess of the vertical stress which remained constant during pore pressure buildup thereby favoring the propagation of microcracks in the horizontal plane

hydrocarbon generation as a mechanism for overpressuring in rocky mountain region overpressured reservoirs are present in the deeper parts of most basins in the rocky mountain region a variety of conditions can cause overpressuring however most high pressures in the region are caused by present day or recently active oil and gas generation or generation in the last few million years in low permeability rock successions that still contain organic matter capable of yielding thermally generated hydrocarbons rocks with above normal pressure range from devonian to tertiary and commonly are associated with low permeability tight reservoirs most of the overpressuring occurs in cretaceous and tertiary rocks overpressuring in rocks older than cretaceous is rare and where present only occurs in strata with very rich hydrocarbon source beds such as the d vonian mississippian bakken formation most organically lean pre cretaceous source beds are no longer capable of yielding enough hydrocarbons to maintain abnormal pressure nearly all overpressured reservoirs and source rocks have temperatures of approximately 200 f 93 c or higher in addition available data indicate that hydrocarbon related overpressuring does not generally occur if vitrinite reflectance ro values are less than 0 6 in oil prone strata or less than 0 7 in gas prone strata overpressured gas bearing strata generally have ro values of 0 8 or higher maximum reservoir pressures are about equal to the fracture gradient pressure in the reservoir rocks some vertical fractures in reservoir rocks are probably caused by rapid expulsion of hydrocarbons from the source beds high pressures in the rocky mountain region means that the rock succession is gas or oil saturated and discrete hydrocarbon water contacts are generally not present classical concepts of hydrodynamics do not appear to be applicable to these overpressured successions the optimum depths for exploration are generally the upper 1 000 2 000 ft 305 610 m of the overpressured succession

characterization of gas shale pore systems by porosimetry pycnometry surface area and field emission scanning electron microscopy transmission electron microscopy image analyses examples from the barnett woodford haynesville marcellus and doig units the nanometer scaled pore systems of gas shale reservoirs were investigated from the barnett marcellus woodford and haynesville gas shales in the united states and the doig formation of northeastern british columbia canada the purpose of this article is to provide awareness of the nature and variability in pore structures within gas shales and not to provide a representative evaluation on the previously mentioned north american reservoirs to understand the pore system of these rocks the total porosity pore size distribution surface area organic geochemistry mineralogy and image analyses by electron microscopy were performed total porosity from helium pycnometry ranges between 2 5 and 6 6 total organic carbon content ranges between 0 7 and 6 8 wt and vitrinite reflectance measured between 1 45 and 2 37 the gas shales in the united states are clay and quartz rich with the doig formation samples being quartz and carbonate rich and clay poor higher porosity samples have higher values because of a greater abundance of mesopores compared with lower porosity samples with decreasing total porosity micropore volumes relatively increase whereas the sum of mesopores and macropore volumes decrease focused ion beam milling field emission scanning electron microscopy and transmission electron microscopy provide high resolution 5 nm images of pore distribution and geometries image analysis provides a visual appreciation of pore systems in gas shale reservoirs but is not a statistically valid method to evaluate gas shale reservoirs macropores and mesopores are observed as either intergranular porosity or are confined to kerogen rich aggregates and show no preferred orientation or align parallel with the laminae of the shale networks of mesopores are observed to connect with the larger macropores within the kerogen rich aggregates

organic rich radioactive marine shale a case study of a shallow water condensed section cretaceous shaftesbury formation alberta canada abstract organic rich radioactive shales are a common regional feature resting on cretaceous transgressive surfaces in western canada the basal shale in the shaftesbury formation late albian from the peace river area of northern alberta is characterized by high gamma ray high resistivity and low neutron wireline log signatures three facies in ascending order are present within the basal shaftesbury formation 1 a brackish water estuarine shale 2 a restricted marginal marine shale which is radioactive and 3 an open marine normal salinity shale the radioactive shale contains an abundance of large lenticular algal cysts cf lancettopsis lanceolata madler 1963 which are rare in overlying and underlying shale the algal cysts and high organic content may be the locus of th radioactivity the total organic carbon content 6 and sulphur content 3 4 of the radioactive shale also are higher than the shale above and below with a different mineralogy as well the radioactive portion of the basal shaftesbury shale has the characteristics of a condensed section it is directly above a ravinement surface and transgressive lag deposit which in turn locally overlie estuarine sediments deposited within an incised valley other characteristics include evidence of low oxygen values low concentrations of benthonic foraminifera and evidence of a slow sedimentation rate palynological micropaleontological and geochemical results indicate that the radioactive s ale was deposited in restricted marginal marine conditions and that overlying shale shows a progressive deepening to nearshore open marine conditions this radioactive shale does not represent the deepest water sediments of the transgression but is a condensed section deposited in relatively shallow water

a simple analysis of oil induced fracturing in sedimentary rocks abstract geometric form of kerogen patches control oil induced fracturing in impermeable source rocks thin elongate kerogen flakes cause lateral fracturing kerogen has to occur in spherical or cylindrical forms to induce vertical fractures if length to width ratio of kerogen flakes is sufficiently large then lateral fractures are definitely initiated during oil generation in impermeable source rock oil and rock compressibilities and lateral to vertical stress ratio are decisive factors in vertical fracturing in impermeable rocks lateral to vertical stress ratio is generally higher than the maximum for oil induced vertical fracturing in tectonically relaxed sedimentary basins oil generation cannot initiate vertical fractures except when the lateral to vertical stress ratio is reduced below critical level either by tectonic tension or erosion of overburden when the source rock is permeable then the rate of oil seepage from kerogen into the surrounding rock is crucial in fracture initiation oil induced fractures cannot form unless permeability and hence rate of oil seepage from kerogen into the surrounding rock is non existent or negligible most source rocks have some permeability therefore oil induced fracture initiation should be a rare phenomenon

felt seismicity associated with shale gas hydraulic fracturing the first documented example in europe we describe the origin of felt seismicity during the hydraulic fracturing of the carboniferous bowland shale at the preese hall 1 exploration well near blackpool in the uk during 2011 the seismicity resulted from the interaction of hydraulic fracturing and a fault the location of which was unknown at the time but has subsequently been located and does not intersect the well borehole waveform cross correlation is used to detect 50 events in the sequence a representative hypocenter and strike slip focal mechanism is calculated using the best recorded seismic event the hypocenter is calculated to lie 300 400 m east and 330 360 m below the injection point and shown to lie on a fault imaged using 3 d seismic at a depth of about 2930 m the 3 d survey shows that not only the event hypocenter but also the focal mechanism correlates strongly with a subsequently identifiable transpressional fault formed during the late carboniferous variscan basin inversion

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

structural synthesis of s staffordshire uk implications for the variscan evolution of the pennine basin the presence of strata of westphalian a to possibly stephanian age in south staffordshire located at the southern margin of the pennine basin provides a rare control on the timing of late carboniferous deformation in central england during westphalian a c times sedimentation in south staffordshire was controlled by thermal subsidence with a superimposed influence of synsedimentary normal and reverse faulting alkaline basic igneous activity occurred during westphalian c times synchronous with uplift of the southern margin of the pennine basin uplift culminated during westphalian c d times in a phase of variscan inversion along basement lineaments and the development of a regional unconformity renewed subsidence and deposition of a relatively thick sedimentary sequence of westphalian d to possibly stephanian age was followed by a final phase of variscan deformation comprising possibly two pulses of uplift tilting and erosion observations presented for south staffordshire in conjunction with studies from other parts of the pennine basin indicate a complex interrelationship between rifting thermal subsidence and variscan basin inversion pulses of n s extension and e w to nw se compression are interpreted as the product of regional strike slip deformation superimposed upon a regime of thermal subsidence alkaline igneous activity is considered to be the product of mantle partial melting in response to limited decompression due to transtensional displacements on crustal thickness faults

fracture controlled paleohydrology in a map scale detachment fold insights from the analysis of fluid inclusions in calcite and quartz veins abstract this study uses fluid inclusions in quartz and calcite veins to characterize the fracture controlled paleohydrology of a map scale evaporite cored detachment fold in the sierra madre oriental of northeastern mexico field observations indicate that the veins are tectonic in origin and that they formed in a general sequence that corresponds to four broad stages of progressive folding we collected samples from each vein stage in various structural and stratigraphic positions across the fold after determining the mineral paragenesis and origin of inclusions in each sample we used standard microthermometric techniques to measure the homogenization temperatures t h salinities and eutectic temperatures of the available two and three phase aqueous inclusions neither t h salinity nor eutectic temperature varied systematically with inclusion origin or mineral type the data are also not significantly correlated with structural position or vein type but are strongly partitioned by stratigraphy suggesting that the area was a vertically stratified hydrologic system consisting of three regional paleohydrostratigraphic units an upper unit comprised the indidura formation through difunta group and is characterized by t h near 150 c and salinity t h near 150 c and salinities near 12 or 22 wt nacl equivalent the lower unit comprised the zuloaga and la casita formations and is characterized by an average t h near 225 c and salinities in three groups near 12 22 or 36 wt nacl equivalent the preservation of multiple distinct fluid types in many veins suggests that fracture development during folding and uplift created a second paleohydrologic system that overprinted the vertically stratified system fracture controlled fluid migration in this second system occurred in periodic repeated and spatially variable pulses

structural evolution of underthrusted sediments kodiak islands alaska the late cretaceous to early tertiary sediments of the kodiak accretionary complex preserve the earliest deformation associated with underthrusting along a thickly sedimented convergent margin because of the thick trench fill this early deformation resulted in the formation of two distinct structural terranes 1 coherent terranes of layered turbidites and 2 melange terranes the earliest deformation d1 in the coherent terranes is marked by normal faults mineralized layer perpendicular veins in sandstones and microfaults in shales these features suggest that 1 was vertical and that 3 was horizontal and nw trending during d1 we interpret d1 structures as hydrofractures that formed in flat lying sediments with high fluid pressures caused by rapid tectonic loading in contrast the melanges appear to record a more protracted and complex history that culminated in structures similar to the d1 structures above we have recognized three successive transitional stages in the melanges i breakup of sand beds along cataclastic shear zones with dispersal of sand inclusions within a relatively ductile shale matrix resulting in the blocks in matrix character of the melange ii development of ductile shear bands that crosscut the melange fabric at a low angle and iii hydrofracturing with the formation of calcite and quartz filled veins in sandstone inclusions and microfaults in the shales the sequence of microstructures associated with stages i iii records increased cohesion of the sediments during deformation dominated by layer parallel shear intense mineralization and microfaulting during stage iii suggest that the melanges are zones of concentrated fluid flow with a fracture dominated permeability stage iii structures are texturally indistinguishable and contemporaneous with the less pervasive d1 structures in the coherent terranes based on these observations we believe that the melanges are major shear zones that formed at the top of the subducting pile and immediately below a major decollement the coherent terranes represent the structurally and stratigraphically lowest sediments and they bypassed the stage i and ii deformations of the melanges because they were the greatest distance from the decollement when they were deformed moreover both the melange and coherent terranes appear to have bypassed the shallow zone of intense horizontal shortening that dominates the toe of the overriding accretionary prism