prediction of lithofacies and reservoir quality using well logs late cretaceous williams fork formation mamm creek field piceance basin colorado understanding the controls and distribution of reservoir quality is important for the economic success of tight gas reservoirs in which diagenesis interacts with primary depositional variations in environment and texture to exert a strong control on pore networks rock mechanical properties and natural fractures in the upper cretaceous williams fork formation of the piceance basin framework grain composition is a major control on compaction and the occurrence of authigenic phases alteration of volcanic grains in the upper williams fork led to grain coating clay precipitation ferroan dolomite cement is found only in the deeper marine influenced intervals in which dolostone fragments are present this study shows that careful petrographic assessment of lithofacies heterogeneity can be upscaled by correlation with log properties to yield tools for field scale reservoir quality prediction twelve lithofacies are identified based on sand grain populations cement types and clay matrix content sandstones of the highest reservoir quality are those with grain coating clays that inhibit quartz cementation these sandstones can be identified based on high density porosity log values sandstones with the poorest reservoir qualities are tightly cemented with carbonate and quartz cement or are rich in clay matrix carbonate cemented intervals are identified by low density porosity clay matrix rich samples have high gamma ray and low density porosity values the presence of abundant potassium feldspar in the upper intervals results in high gamma ray readings even in the clean clay matrix free sandstone

fracture distribution on the swift reservoir anticline montana implications for structural and lithological controls on fracture intensity title of special publication folding and fracturing of rocks 50 years of research since the seminal text book of j g ramsay this research was funded by oil search ltd santos ltd and interoil through the university of aberdeen fold thrust research group electron microscopy was performed in the acemac facility at the university of aberdeen with assistance from john still joyce neilson is thanked for advice on the use of imagej software midland valley are thanked for the use of their move software for field data collection and model building we thank alfred lacazette and stefano tavani for reviewing the manuscript and providing constructive comments

the co diagenesis of sulfur and iron in acid lake sediments of southwestern quebec reduced inorganic sulfur species constitute the major fraction of total sulfur in the recent sediments of eight acid lakes of southwestern quebec nearly all of the anthropogenically derived excess s present in the sediments has accumulated as reduced inorganic forms showing that organic s formation plays a very minor role in long term anthropogenic s retention in these lakes these results appear different from most other published studies on s fixation in acid lake sediments respectively 74 and 76 of the variance in total s and reduced inorganic s burden in the recent sediments can be explained by their fe content alone indicating that the reduced s is associated with iron and that the capacity of the sediments to retain s may be limited by available fe supply when indexes of organic c supply winter o2 deficit and sediment c are included with fe in a multiple regression analysis respectively 97 and 98 of the variance in total s and reduced inorganic s accumulation can be explained the origin of the pronounced subsurface fe maximum observed in most cores is attributed to the reduction and recent burial as iron sulfides of a surficial diagenetic fe oh 3 reserve in some sediments this fe iii reserve presently appears to be nearly or completely exhausted and the lakes may have lost their capacity to generate permanent alkalinity via so2 4 reduction the present rate of alkalinity production due to so2 4 reduction and fixation of reduced s in the sediments of one lake were estimated from several 210pb dated s profiles and averaged to 163 eq m 2 d 1 this value is comparable to those reported for other acid lakes

lithologic and structural controls on natural fracture distribution and behavior within the lisburne group northeastern brooks range and north slope subsurface alaska the carboniferous lisburne group of northern alaska has been deformed into a variety of map scale structures in both compressional and extensional structural settings thus providing a series of natural experiments for observing the formation distribution and behavior of fractures in this thick carbonate unit two fracture sets dominate the lisburne group carbonates of the north slope subsurface and the nearby northeastern brooks range fold and thrust belt north northwest striking regional extension fractures probably formed in front of the northeastern brooks range fold and thrust belt in the north slope subsurface this fracture set overprints east northeast striking fractures related to earlier extensional deformation in contrast in the fold and thrust belt the north northwest striking fracture set is overprinted by younger east northeast striking fractures related to subsequent contractional deformation lithology is the primary control on the fracture density of both sets in mildly deformed lisburne copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received february 20 1996 revised manuscript received november 25 1996 final acceptance june 5 1997 2geophysical institute university of alaska fairbanks fairbanks alaska 99775 3sandia national laboratories ms 0705 albuquerque new mexico 87185 4department of petroleum natural gas engineering new mexico institute of mining technology socorro new mexico 87801 and sandla national laboratories ms 0705 albuquerque new mexico 87185 this study was supported by a department of energy subcontract administered by sandia national laboratories additional support was provided by arco alaska bp alaska chevron exxon mobil and japan national oil company we would like to thank arco alaska and bp alaska for giving us permission to view selected lisburne group cores w wallace for helpful discussions on deformational styles of detachment folds a j mansure for help in interpreting the interference tests and w wallace k biddle w belfield n hurley and j kelley for helpful reviews of the manuscript

the influence of fracture anisotropy on co2 flow 1 greenhouse gas mitigation through geological storage of carbon dioxide is dependent on rock formations storing co2 effectively secure containment for periods of 100 105 years nee7ds to be verifiable the effectiveness of geological storage is reliant on the chemical and physical properties of the geological storage complex and its ability to inhibit migration of co2 petroleum reservoir data and field evidence show that fracture networks often act as pathways for fluid movement potentially allowing fluids to migrate to the surface within the time scale of interest we demonstrate the importance of predicting the effects of fracture networks on flow using a case study from the in salah co2 storage site and show how fracture permeability is closely controlled by the stress regime determining the conductive fracture network our results demonstrate that fracture network prediction combined with present day stress analysis can be used to successfully predict co2 movement in the subsurface

mechanical constraints on the chronology of fracture activation in folded devonian sandstone of the western moroccan anti atlas the three dimensional meter scale fracture networks observed on exposed folds between the towns of tata and akka western moroccan anti atlas consist mostly of planar discontinuities which are sub perpendicular to the bedding and partitioned in three main sets the chronology of their activation is proposed in five stages since the hercynian orogeny stage 1 predates folding and involves the horizontal compression of the emsian sandstone it involves fracture set i composed of systematic joints parallel to the direction of compression stages 2 4 correspond to the folding and are marked in the outer arc by the activation of fracture set ii composed mainly of joints parallel to the fold axial plane stage 5 is a regional shear event during which sets i and iii separated by an angle close to 608 are activated in a conjugate manner to throw light on the recurrent difficulty in discriminating between activation of inherited and new fractures an elasto plastic model is used to construct a stress path in the pervasively fractured medium idealized as a continuum each fracture set obeys the mohr coulomb criterion truncated in tension to describe both sliding and opening activations finite element simulations of a simple buckling event accounting for the field fracture sets are presented it is shown that set i cannot be generated by folding and thus does belong to stage 1 set ii is activated at a later stage of folding than expected from the field interpretation set iii cannot be activated during stage 2 confirming its role in stage 5 the advantages and limitations of the proposed modeling are finally discussed q 2002 elsevier science ltd all rights reserved

non steady state diagenesis of organic and inorganic sulfur in lake sediments abstract sulfur controls the fate of many geochemical elements in lake sediments including iron phosphorus and environmentally important trace elements we measured the speciation of pore water and sediment bound sulfur aqueous sulfate and sulfides elemental sulfur iron monosulfide pyrite organic sulfur and supporting geochemical variables carbon oxygen iron in the sediments of a perennially oxygenated and a seasonally anoxic basin of an oligotrophic lake in quebec using a combination of pore water analyses sequential extractions and x ray absorption near edge structure a non steady state early diagenetic model was developed and calibrated against this extensive dataset to help unravel the pathways and quantify the rates of s transformations results suggest that the main source of s to the sediments is the settling of organic ester sulfate r o so3 h hydrolysis of these compounds provides an additional source of sulfate for anaerobic microbial oxidation of sedimentary organic matter releasing sulfide to the pore water reduced solid bound s species accumulate as thiols r sh and iron sulfides in the perennially oxygenated and seasonally anoxic basin respectively the model estimated rate constant for r sh formation is lower than previously estimated for this particular lacustrine site but similar to that proposed for marine shelf sediments the solid sediment s profiles however carry the imprint of the time dependent sulfate input to the lake iron sulfide enrichments formed during past decades of elevated atmospheric so4 deposition are presently dissolving in the sediments of the perennially oxygenated basin this reaction hampers the build up of fe iii oxy hydroxide near the sediment water interface

a model for sulfur accumulation in soft water lake sediments a geochemical model for sulfur in soft water lake sediments of the adirondack mountains new york is developed to gain insight into the timing and magnitude of historical changes in sulfur loading surficial sulfur concentrations range from 3500 to 10 000 mu g g minus sup 1 increase to 6000 to 20 000 mu g g minus sup 1 at a depth of 2 6 cm and then decrease downcore to background levels of 2500 7500 mu g g minus sup 1 sulfate concentrations are about 50 80 mu m in the overlying water and decrease rapidly below the sediment water interface a mathematical model is constructed assuming sulfur is incorporated into sediments by burial of detrital organic matter and by diffusion of dissolved sulfate from the overlying water with subsequent fixation into the sediment solids several scenarios of historical sulfate deposition were examined as model boundary conditions the observed sediment sulfur profiles are best modeled using boundary conditions showing loading increases of about a factor of 8 since about 1940 assuming as much as a decade of retention of sulfate within the terrestrial portion of the ecosystem this suggests that significant increases in sulfate deposition rates began sometime about 1930 in this area 34 refs 7 figs 2 tabs

are open fractures necessarily aligned with maximum horizontal stress abstract fluid flow in fractured rock is an increasingly central issue in recovering water and hydrocarbon supplies and geothermal energy in predicting flow of pollutants underground in engineering structures and in understanding large scale crustal behaviour conventional wisdom assumes that fluids prefer to flow along fractures oriented parallel or nearly parallel to modern day maximum horizontal compressive stress or s hmax the reasoning is that these fractures have the lowest normal stresses across them and therefore provide the least resistance to flow for example this view governs how geophysicists design and interpret seismic experiments to probe fracture fluid pathways in the deep subsurface contrary to these widely held views here we use core stress measurement and fluid flow data to show that s hmax does not necessarily coincide with the direction of open natural fractures in the subsurface 3 km depth consequently in situ stress direction cannot be considered to predict or control the direction of maximum permeability in rock where effective stress is compressive and fractures are expected to be closed chemical alteration dictates location of open conduits either preserving or destroying fracture flow pathways no matter their orientation

a 48 m y history of fracture opening temperature and fluid pressure cretaceous travis peak formation east texas basin quartz cement bridges across opening mode fractures of the cretaceous travis peak formation provide a textural and fluid inclusion record of incremental fracture opening during the burial evolution of this low porosity sandstone incremental crack seal fracture opening is inferred based on the banded structure of quartz cement bridges consisting of up to 700 cement bands averaging 5 m in thickness as observed with scanning electron microscope cathodoluminescence crack seal layers contain assemblages of aqueous two phase fluid inclusions based on fluid inclusion microthermometry and raman microprobe analyses we determined that these inclusions contain methane saturated brine trapped over temperatures ranging from 130 c to 154 c using textural crosscutting relations of quartz growth increments to infer the sequence of cement growth we reconstructed the fluid temperature and pore fluid pressure evolution during fracture opening in combination with published burial evolution models this reconstruction indicates that fracture opening started at ca 48 ma and above hydrostatic pore fluid pressure conditions and continued under steadily declining pore fluid pressure during partial exhumation until present times individual fractures opened over an 48 m y time span at rates of 16 23 m m y these rates suggest that fractures can remain hydraulically active over geologically long times in deep basinal settings

the connectivity of two dimensional networks of spatially correlated fractures 1 continuum percolation thresholds for fracture systems are based on models with random fracture locations resulting in uniformly clustered systems in which all connections are fracture intersections fractures in natural systems can be either clustered or anticlustered and a significant portion of the connectivity is achieved through fracture abutments or splays analytical and numerical methods are combined to define critical properties of isotropic networks of constant length lines as a function of fracture clustering and of the ratio of abutments to intersections a second suite of numerical results indicates that the influence of these parameters varies as a function of the fracture length distribution a preliminary predictor of critical density of anisotropic nonrandom systems with lognormal length distributions is derived and applied to a suite of natural fracture systems predicted connectivities are closer to the observed connectivities when the predictor is used than when connectivity is estimated from the random threshold

from hydroplastic to brittle deformation controls on fluid flow in fold and thrust belts insights from the lower pedraforca thrust sheet se pyrenees abstract we present a multidisciplinary study to decipher the controls of deformation on fluid flow regime in fold and thrust belts using the lower pedraforca thrust sheet in the se pyrenees as an example we integrate field based and petrographic observations and geochemical and geochronological data to differentiate seven types of fractures eight types of calcite cement cc1 to cc8 and two sets of stylolites during the deformation stretching the studied thrust sheet during syn sedimentary hydroplastic normal faulting affecting poorly consolidated upper cretaceous and eocene syn orogenic sediments calcite cement did not precipitate during burial bed parallel stylolites formed and cc1 and cc2 precipitated from formation waters in a closed palaeohydrological system during the layer parallel shortening cc3 precipitated from formation waters 5 4 vsmow with 87sr 86sr ratios of 0 707922 and at 70 c cc4 precipitated from formation waters recording different burial conditions as the up to 4 dispersion in 18o of this cement suggests contrarily during folding and thrusting cc5 to cc7 precipitated in an open palaeohydrological system cc6 precipitated from formation waters 5 vsmow with 87sr 86sr ratios of 0 707817 and at 75 c these fluids carried hydrocarbons and probably interacted with upper triassic evaporites an 87sr 86sr ratio of 0 708230 for cc5 indicates that formation waters also interacted with clays within continental deposits during this period stylolites formed in relation to faulting and previous hydroplastic normal faults reactivated as reverse and strike slip faults allowing fluid flow cc7 precipitated after cc6 also from fluids in isotopic disequilibrium with their adjacent host rock the fluid system continued open during the oligocene when cc8 precipitated in normal faults affecting syn orogenic conglomerates deposited during the reactivation of the lower pedraforca thrust sheet the influence of deformation on fluid flow observed in the lower pedraforca thrust sheet is similar to that observed in other fractured areas worldwide

thermal effects and timing of thrusting in the moine thrust zone illite crystallinity studies in the foreland to the moine thrust belt show a widespread heating event which is interpreted as a thermal response to thrust sheet emplacement temperatures reached 275 c approximately 24 ma after the main thrusting events based on the relationships between syn thrusting intrusions and the thrust planes combined with radiometric dates from the moine sheet it is argued that movement on the moine thrust started between 440 and 430 ma ago contractional slip in the moine thrust belt ceased at about 425 ma ago and it is proposed that illite metamorphism attained temperatures of 275 c at about 408 ma ago

influence of provenance and burial history on diagenesis of lower cretaceous frontier formation sandstones green river basin wyoming abstract the upper cretaceous frontier formation on the moxa arch in the western green river basin wyoming has had a varied diagenetic history that was controlled in part by differences in composition of detrital framework grains and in burial history petrographic examination of 247 thin sections from 13 cores from the south plunging arch and adjacent deep basin is the basis for diagenetic investigation of sandstones ranging in depth from 2 km to almost 5 km major diagenetic events were 1 mechanical compaction by grain rearrangement and deformation of ductile grains 2 formation of illite and mixed layer illite smectite rims 3 precipitation of quartz overgrowths 4 precipitation of calcite cement 5 generation of secondary porosity by dissolution of feldspar chert biotite and mudstone grains and calcite cement 6 precipitation of kaolinite in primary and secondary pores and 7 chemical compaction by intergranular pressure solution and stylolitization and additional precipitation of quartz cement the northern and southern ends of the moxa arch differ in the magnitude of each of these diagenetic events provenance differences caused more abundant ductile rock fragments and feldspar to be deposited at the northern end of the moxa arch as a result frontier sandstones from the northern moxa arch underwent more extensive mechanical compaction in addition feldspar dissolution and albitization buffered acid rich basinal fluids at the northern end resulting in greater development of secondary porosity and precipitation of calcite cement than at the southern end deeply buried sandstones at the southern end of the arch and in the basin contain the most abundant quartz cement because intergranular pressure solution and stylolitization liberated silica for overgrowths

circulation in enewetak atoll lagoon1 currents at enewetak atoll marshall islands were measured on the reef margins in the channels and in the lagoon lagoon circulation is dominated by wind driven downwind surface flow and an upwind middepth return flow this wind driven flow has the characteristics of an ekman spiral in an enclosed sea lagoon flushing is accomplished primarily by surf driven water input over the windward eastern reefs and southerly drift out the south channel mean water residence time is 1 month while water entering the northern portion of the atoll takes about 4 months to exit

natural hydraulic fracturing of tight gas sandstone reservoirs piceance basin colorado natural fractures form preferred pathways for basinal fluid flow and associated heat and mass transport in gas sandstone reservoirs with low matrix permeability fractures provide flow pathways between organic rich source and reservoir layers during gas charge and between matrix pores hydraulic fractures and the well bore during production while the formation of natural fractures has previously been associated with gas generation and pore fluid pressure increase through a process referred to as natural hydraulic fracturing other driving mechanisms such as stress changes by tectonic or exhumation processes remained viable alternatives to test whether these mechanisms contributed to fracture development we investigated the spatial and temporal distribution of fracture formation and its relationship to gas generation migration and charge in sandstone of the cretaceous mesaverde group across the entire production interval on a basinwide scale using fluid inclusion microthermometry of crack seal fracture cement formed concurrently with fracture opening we observed temperature trends that when compared with temperature evolution models of the formation date fracture formation between 41 and 6 ma in the northern and between 39 and 6 ma in the southern piceance basin the onset of fracture formation 20 30 m y prior to maximum burial eliminates changes in stress state associated with exhumation as a mechanism for triggering the onset of fracture formation instead calculated paleo pore fluid pressures of 40 90 mpa 5800 13 000 psi during fracture opening and the presence of methane rich inclusions in fracture cement suggest that fracture formation was aided by high pore fluid pressures during gas generation in organic rich shales and coals and associated charging of adjacent and interlayered sandstone reservoirs a 10 20 m y age progression in the onset of fracture formation from deeper to shallower horizons of the mesaverde group is consistent with gas generation and onset of fracture formation activated by burial temperature with limited upward migration of gas at this stage of reservoir evolution this age progression with depth is inconsistent with fracture formation triggered by changes in stress conditions associated with tectonic or structural processes expected to affect the entire formation synchronously our observations are thus most consistent with fracture formation by natural hydraulic fracturing in response to gas generation in interbedded source layers and reservoir charge based on widespread observations of fractures with similar structural and diagenetic attributes we consider natural hydraulic fracture formation in response to thermocatalytic gas generation to be a fundamental mode of brittle failure in otherwise structurally quiescent basins

linking onshore and offshore structures mesozoic extension in the scottish highlands geological relationships in the offshore basins flanking northern scotland suggest that a regional caledonian control on the structure of these basins is far from proven but conversely mesozoic faults of the offshore set may extend onshore the strathconnon fault long held to be late caledonian may be mesozoic in age forming the principal kinematic link between the east and west coast basins of northern scotland the numerous outcrops of precambrian basement within the minch hebrides basin nearly all lie in the uplifted footwalls of mesozoic faults similar fault controls on the distribution of onshore outcrop within the precambrian may exist

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

quantifying diagenetic and stratigraphic controls on fracture intensity in platform carbonates an example from the sierra madre oriental northeast mexico abstract normalized fracture intensity measured along 1d scanlines was used to compare fracture intensity for different sedimentary facies stratigraphic position bed thickness and degree of dolomitization in carbonate beds of the cupido and tamaulipas formations northeast mexico we calculated the fracture intensity for individual beds for single and combined fracture sets and the statistical significance of relationships using bivariate weighted regressions and multivariate methods results suggest that the degree of dolomitization is positively correlated with fracture intensity and has the strongest correlation followed by the position of the bed in the stratigraphic cycle and the mud content dolomite content normalized position in the parasequence environment of deposition and mud content show a significant degree of correlation among themselves and should be considered dependent variables for statistical purposes geological observations suggest that dolomite precipitation and fracturing occurred at least in part synchronously in these rocks a fracture intensity distribution model integrating sequence stratigraphic and diagenetic history is proposed for the cupido and tamaulipas formations with potential use in similar carbonate successions our analysis does not support the classic bed thickness fracture spacing relationship

quantifying opening mode fracture spatial organization in horizontal wellbore image logs core and outcrop application to upper cretaceous frontier formation tight gas sandstones usa abstract the upper cretaceous frontier formation is a naturally fractured gas producing sandstone in wyoming regionally random and statistically more clustered than random patterns exist in the same upper to lower shoreface depositional facies east west and north south striking regional fractures sampled using image logs and cores from three horizontal wells exhibit clustered patterns whereas data collected from east west striking fractures in outcrop have patterns that are indistinguishable from random image log data analyzed with the correlation count method shows clusters 35 m wide and spaced 50 to 90 m apart as well as clusters up to 12 m wide with periodic inter cluster spacings a hierarchy of cluster sizes exists organization within clusters is likely fractal these rocks have markedly different structural and burial histories so regional differences in degree of clustering are unsurprising clustered patterns correspond to fractures having core quartz deposition contemporaneous with fracture opening circumstances that some models suggest might affect spacing patterns by interfering with fracture growth our results show that quantifying and identifying patterns as statistically more or less clustered than random delineates differences in fracture patterns that are not otherwise apparent but that may influence gas and water production and therefore may be economically important

the khaviz anticline an outcrop analogue to giant fractured asmari formation reservoirs in sw iran the carbonate reservoirs in the late oligocene early miocene asmari formation in the dezful embayment of sw iran are characterized by low matrix permeability and effective drainage is dependent on the occurrence of open fractures limited information on fracture orien tation and fracture density is available from core and borehole image data and high quality high resolution three dimensional seismic is often lacking in this area well and core data do not contain information on important fracture parameters like length distribution crosscutting relationships fracture density v lithology and bed thickness the understanding of fracture distri bution and formation in the region and their effects on fluid flow has been greatly improved by the use of outcrop analogue data exposures of the asmari formation in the khaviz anticline are in close vicinity to the giant hydrocarbon fields the khaviz anticline has a similar geometry and structural history to the major hydrocarbon fields in the area and represents an excellent analogue for these two types of fracture features were observed diffuse fracturing and fracture swarms the diffuse fractures form networks and comprise structures grouped into four fractures sets which are the typical for this type of anticline two orthogonal fracture sets are oriented parallel and perpendicular to the fold axis and two conjugate fracture sets are oblique to the fold axis with their obtuse angle intersecting the trend of the fold axis the fractures are typically stratabound sub perpendicular to bedding and commonly about the bounding stratigraphic surfaces to a large extent the density and height of fractures in the asmari formation are controlled by the mechan ical stratigraphy which is controlled by the depositional environment and cycles these outcrop data have been essential in the generation of discrete fracture network dfn models and the population of the fracture properties in the reservoir models

anthropogenic inventories and historical and present accumulation rates of copper in great lakes sediments abstract sediment cores were collected from depositional basins in lakes michigan ontario and superior to assess spatial and temporal variations in cu accumulation rates and inventories sediment chronologies were determined via 210 pb dating and 210 pb inventories were used to correct for the effects of sediment focusing among the lakes studied cu loading histories generally decrease to the present signifying a regional reduction in the anthropogenic release of cu to the environment focusing corrected cu accumulation rates in surficial sediments vary significantly within lakes michigan and ontario suggesting that recent inputs of cu to these 2 lakes may have been dominated by localized sources variations in cu accumulation rates within lake superior are interpreted to be insignificant suggesting either that lake superior is well mixed with respect to copper inputs or that the lake is dominated by a single source for cu recent measurements of atmospheric fluxes of cu within the great lakes region generally indicate that the atmosphere accounts for less than 50 of the cu accumulating in surficial sediments historical cu accumulation rates from lake superior are poorly correlated with cu production associated with local smelters but favorably agree with national production rates trends in historical cu accumulation rates within all 3 lakes may indicate that recent declines in cu loading can be attributed to the enactment of stricter environmental regulations as well as changes in the use of cu focusing corrected cu inventories in lakes michigan ontario and superior are generally invariant within a given lake indicating that historically these lakes have been dominated either by a single source or were well mixed with respect to cu inputs the disparity between the results from recent accumulation rates and historical inventories may reflect differences in how the great lakes respond to contaminant loading on short e g years versus long e g decades timescales

fracture porosity creation and persistence in a basement involved laramide fold upper cretaceous frontier formation green river basin usa fracture hosted porosity and quartz distribution along with crack seal texture and fluid inclusion assemblage sequences in isolated bridging quartz deposits show that open fractures can persist through protracted burial and uplift in foreland basins fractures oriented at a high angle to current maximum compressive stress remain open and were weak mechanical discontinuities for millions of years even at great depth upper cretaceous frontier formation sandstones in the basement involved laramide table rock anticline eastern greater green river basin wyoming sampled by two horizontal wells cut parallel or nearly parallel to bedding and at a high angle to steeply dipping fractures have 41 5 m of rock in four cores at depths of 4538 4547 m cores intersect older e striking set 1 fractures are abutted by or locally cross cut by n striking set 2 fractures both sets contain quartz and porosity sequenced using quartz crack seal cement texture maps set 1 fluid inclusion assemblage fia trapping temperatures increase progressively from 140 to 165 c then decrease to c 150 c compatible with fracture opening over c 15 ma during rapid burial followed by uplift in eocene oligocene time set 2 opened at c 160 c probably near maximum burial after a period of quiescence set 2 reopened at c 5 ma at c 140 c on a cooling trajectory intermittent set 2 movement could reflect local basement involved fault movement followed after a pause by further set 2 reactivation in the modern stress field during uplift interpretations are sensitive to available burial thermal histories which have considerable uncertainty

fracture abundance and patterns in the subandean fold and thrust belt devonian huamampampa formation petroleum reservoirs and outcrops argentina and bolivia abstract opening mode fractures in devonian sandstones in outcrop and in several fields in the subandean ranges of southern bolivia and northern argentina compose two fracture sets i and ii that strike west northwest and north northeast respectively abundant set i fractures are at a high angle to local andean structural trends and set ii is aligned with fold axes crosscutting relations and quartz textures in fractures suggest that although set i is locally older these sets may have opened partly contemporaneously sets comprise both macroscopic fractures and more abundant millimeter scale microfractures fractures with opening displacement of less than 0 1 mm are typically sealed with quartz but wider fractures are lined with quartz and contain connected fracture porosity microfractures are more abundant than macrofractures and size distributions can be interpreted to approximate power laws microfracture strain is an efficient method of quantifying fracture abundance both sets record small fracture strains of 0 00016 0 0083 in backlimbs set i strain is higher in brittle quartz rich sandstones set ii strain varies markedly and is generally high in hinges and steep forelimbs of asymmetric anticlines for individual samples set i ii strains in fold hinges and forelimbs are comparable consistent with concurrent set i ii growth open fractures that could augment permeability are present both on and off structure but microfractures document fracture abundance that varies with structural position and sandstone quartz content that could account for variations in production outcomes

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

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

geological constraints structural evolution and deep geology of the northwest scottish caledonides the front of the caledonian orogenic belt in nw scotland is marked by a zone of foreland thrusting which contains imbricates of lewisian basement proterozoic cover and cambrian shelf sediments faults within this moine thrust belt in most localities propagated in a foreland directed sequence therefore the earliest of these thrusts was the moine thrust which carried a basement assemblage of moine and lewisian rocks over the cambrian shelf the development of large duplex and imbricate structures in the footwall postadted movement on this thrust hence a restoration of these imbricates provides a minimum estimate of the eastern extent of the cambrian shelf no synsedimentary or prethrust extensional features are recognised within the cambrian succession which displays a remarkable stratigraphic and thickness consistency this implies a corresponding consistency in crustal structure beneath these cover rocks immediately prior to caledonian thrusting a balanced cross section constructed across the northern part of the moine thrust belt has given a restored width for the cambrian shelf of 54 km geophysical data suggest that the present foreland crustal thickness is 28 km if this is taken to be the crustal thickness beneath the cambrian shelf during caledonian thrusting a minimum cross sectional area of 1512 km of lewisian rocks must remain beneath the present outcrop of the moine thrust sheet this foreland basement wedge may have been imbricated subsequent to the development of the cambrian imbricates caledonian structures within the moine thrust sheet are interpreted as forming an imbricate stack with the moine thrust acting as a floor thrust the ductile imbricate thrusts possibly roofed into the naver slide late open folds which warp foliation slides and metamorphic isograds within the moine sheet may represent culminations in the foreland lewisian imbricates beneath the moine thrust these postulated culminations may have been sufficient to initiate gravity driven structures this may explain late movements on high level faults within the moine thrust belt

laurentian palaeostress trajectories and ephemeral fracture permeability cambrian eriboll formation sandstones west of the moine thrust zone nw scotland abstract cambrian eriboll formation sandstones of the ardvreck group that crop out west of the moine thrust zone contain joints and quartz filled or quartz lined fractures that resemble cemented joints of the fractures containing quartz five sets strike north nw to wnw ne west and north according to crosscutting relations this is a progression from the oldest to the youngest set sets include opening mode microfractures partly visible in transmitted light as fluid inclusion planes and sharply defined as microveins using sem based cathodoluminescence cl dating the oldest north striking set using inferred quartz accumulation rates fluid inclusions and burial history suggests that these fractures mark a palaeozoic east west least horizontal stress trajectory in laurentia the youngest two sets of porous fractures are associated with faults that cut and postdate the moine thrust zone data indicate that at depth in basins pervasive fracture systems arising from discrete loading events are ephemeral owing to fracture porosity destruction by cementation

fracpaq a matlab toolbox for the quantification of fracture patterns abstract the patterns of fractures in deformed rocks are rarely uniform or random fracture orientations sizes and spatial distributions often exhibit some kind of order in detail relationships may exist among the different fracture attributes e g small fractures dominated by one orientation larger fractures by another these relationships are important because the mechanical e g strength anisotropy and transport e g fluids heat properties of rock depend on these fracture attributes and patterns this paper describes fracpaq a new open source cross platform toolbox to quantify fracture patterns including distributions in fracture attributes and their spatial variation software has been developed to quantify fracture patterns from 2 d digital images such as thin section micrographs geological maps outcrop or aerial photographs or satellite images the toolbox comprises a suite of matlab scripts based on previously published quantitative methods for the analysis of fracture attributes orientations lengths intensity density and connectivity an estimate of permeability in 2 d is made using a parallel plate model the software provides an objective and consistent methodology for quantifying fracture patterns and their variations in 2 d across a wide range of length scales rock types and tectonic settings the implemented methods presented are inherently scale independent and a key task where applicable is analysing and integrating quantitative fracture pattern data from micro to macro scales the toolbox was developed in matlab and the source code is publicly available on github and the mathworks fileexchange the code runs on any computer with matlab installed including pcs with microsoft windows apple macs with mac os x and machines running different flavours of linux the application source code and sample input files are available in open repositories in the hope that other developers and researchers will optimise and extend the functionality for the benefit of the wider community

sedimentary and diagenetic controls on the multiscale fracturing pattern of a carbonate reservoir the madison formation sheep mountain wyoming usa a multidisciplinary approach has been applied to characterize the madison formation in the sheep mountain anticline at different scales in order to illustrate the relationship between sedimentology diagenesis and fracturing pattern sedimentary facies are organized into three types of elementary facies sequences showing different vertical evolutions of their petrophysical properties this is controlled by the double influence of the initial sedimentary facies and the diagenetic evolution three main sets of fractures related to the sevier and laramide compressive phases and to the folding of sheep mountain have been described at three orders of scale at small scale the sevier related set i and folding related set iii are the most abundant set ii dated from the early stage of the laramide compression exhibits the highest fracture intensity there is also a clear faciologic control on the fracture intensity small scale mechanical units correspond to entire facies sequences when no or little petrophysical property contrasts between beds are observed conversely small scale mechanical units correspond to each bed of a facies sequence in the case of high contrasts of petrophysical properties between beds at intermediate scale set ii is not present and sets i and iii show the highest fracture intensities the latter are partly controlled by the mechanical unit thickness intermediate and large scale mechanical unit distribution is mainly controlled by the overall faciologic vertical stacking pattern and by the position across the sheep mountain anticline forelimb or backlimb sheep mountain outcrops provide a good spatial representation of the carbonate reservoir heterogeneities from the micro to the field scale illustrating the complexity of fractured carbonate reservoirs

distribution of joints in the hinge line culmination of foreland verging overturned anticlines an example from the montagna dei fiori structure in the central apennines of italym francioni et al distribution of joints in overturned anticlines the distribution of joints veins and shear fractures in the montagna dei fiori overturned anticline is here reported two joint sets longitudinal and transverse to the nnw sse anticline axial trend and a conjugate system of shear fractures are recognized the transverse and longitudinal joint sets have a pre folding development likely related to the foreland foredeep flexure the transverse joints were reopened during layer parallel shortening along with the development of the shear fractures then during syn folding outer arc extension and hinge line culmination fold axial parallel extension both the longitudinal and transverse joints were enhanced and reactivated respectively

spatial arrangement of faults and opening mode fractures spatial arrangement is a fundamental characteristic of fracture arrays the pattern of fault and opening mode fracture positions in space defines structural heterogeneity and anisotropy in a rock volume governs how faults and fractures affect fluid flow and impacts our understanding of the initiation propagation and interactions during the formation of fracture patterns this special issue highlights recent progress with respect to characterizing and understanding the spatial arrangements of fault and fracture patterns providing examples over a wide range of scales and structural settings five papers describe new methods and improvements of existing techniques to quantify spatial arrangement one study unravels the time evolution of opening mode fracture spatial arrangement which are data needed to compare natural patterns with progressive fracture growth in kinematic and mechanical models three papers investigate the role of evolving diagenesis in localizing fractures by mechanical stratigraphy and nine discuss opening mode fracture spatial arrangement two papers show the relevance of complex cluster patterns to unconventional reservoirs through examples of fractures in tight gas sandstone horizontal wells and a study of fracture arrangement in shale four papers demonstrate the roles of folds in fracture localization and the development spatial patterns one paper models along fault friction and fluid pressure and their effects on fault related fracture arrangement contributions address deformation band patterns in carbonate rocks and fault size and arrangement above a detachment fault three papers describe fault and fracture arrangements in basement terrains and three document fracture patterns in shale this collection of papers points toward improvement in field methods continuing improvements in computer based data analysis and creation of synthetic fracture patterns and opportunities for further understanding fault and fracture attributes in the subsurface through coupled spatial size and pattern analysis

fault and fracture systems in a fold and thrust belt an example from bolivia this paper presents an outcrop analog study of fractures in low porosity sandstones in the subandean thrust belt we analyze the evolution of fault and joint systems in these sandstones quantify their density along the structural trend and identify the main factors controlling their variability we show that faults and joints occur at different scales in a hierarchical fashion as a consequence of progressive shearing the first generation is an orthogonal set of joints one parallel and the other perpendicular to the bedding azimuth shearing along these joints transformed them into small faults and created new sets of fractures oblique to the bedding attitude linkage of these small faults facilitated the formation of larger faults with significant strike slip offset shearing along bedding planes created subvertical splay joints that induced the formation of conjugate normal faults in this thrust belt subordinate strike slip and normal faults are concomitant products of compressive deformation this study documents a hierarchical correspondence between spacing of structural heterogeneities and stratigraphic architecture we measured spacings of joints and outcrop scale faults along the backlimb of the abra del condor anticline we subdivided the structural discontinuities into four main groups joints small faults intermediate faults and fault zones spacing of joints small faults and intermediate faults has a lognormal distribution whereas spacing of fault zones shows a normal distribution the mean of these distributions is about the same as the thickness of the confining stratigraphic intervals therefore spacing and dimensions of joints and faults have a first order relationship to the thickness of the confining stratigraphic sequences

numerical experiments on permeability tensor and its application to jointed granite at stripa mine sweden rock masses containing a large number of geological discontinuities called joints are treated as homogeneous anisotropic porous media a permeability tensor hydraulically equivalent to a flow network formed by joints is formulated in terms of two tensors and a nondimensional scalar both depending only on the geometrical aspects of joints such as the spacing size orientation and aperture two dimensional numerical analyses on seepage flow through the networks support the validity of the present theory the information about the geometry of joints which is obtained from the ventilation drift at stripa mine sweden is interpreted to give the corresponding permeability tensor the permeability tensor is compared with the result of the large scale hydraulic conductivity test with the conclusion that it provides a reasonable prediction for the hydraulic properties of stripa jointed granite

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

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

topological characterization of a polyphased gold bearing vein network abstract it is challenging to evaluate the connectivity of metal bearing hydrothermal vein systems i e the degree to which fractures or veins are connected within a network moreover many ancient ore bearing vein networks have been deformed and metamorphosed which hinders the reconstruction and quantification of their primary characteristics here we explore the reliability and limitations of the network distribution and topologic classification of vein terminations into i y and x nodes approaches to study deformed and metamorphosed mineral deposits we focus on the cheechoo gold deposit located in the eeyou istchee james bay area of quebec canada archean superior province this gold deposit shows a complex hydrothermal and magmatic polyphased gold bearing vein system hosted in a granodioritic tonalitic intrusion the cheechoo deposit is composed of quartz feldspar diopside qtz fsp di and quartz qtz veins that underwent ductile deformation and amphibolite facies metamorphism the high grade metamorphism led to the modification of vein shape relative position and spacing making these parameters unreliable for the characterization of the cheechoo vein system and similar metamorphosed networks however the number of veins per length unit and topology are reliable tools to characterize such networks in the gold bearing cheechoo system the gold mineralization appears to be mainly controlled by i the number of qtz and qtz fsp di veins per length unit and by ii the connectivity of these networks as shown by the increase of both parameters with the gold grade furthermore we introduce the y i ratio as a simple topological parameter to quantify the connectivity of two dimensional i y dominated systems this parameter is positively correlated with the number of connections per branch cb used to quantify network connectivity and ultimately represent the development of a fault or fracture zone

identifying multiple detachment horizons and an evolving thrust history through cross section restoration and appraisal in the moine thrust belt nw scotland abstract many thrust systems including parts of the moine thrust belt are commonly interpreted as rather simple imbricate fans splaying from a master detachment floor thrust at depth we use field observations and geological map data to construct cross sections through the achnashellach culmination southern moine thrust belt northwest scotland to test this interpretation initially cross sections are constructed by assuming a single lower detachment line length imbalances and thrust trajectory mismatches between deformed and restored state sections indicate an invalid model significant differences in horizon lengths between two rock units are seen indicating the position of a second detachment which when incorporated into the deformed state cross section creates a valid structural model the presence of this second detachment accounts for complex geometries seen at outcrop and indicates that the achnashellach culmination is likely to have formed by the sequential activation of two detachment horizons this new structural model has been derived using an iterative workflow involving cross section construction section balancing and integration of field observations from across the study area ensuring model validity in three dimensions this workflow is applicable to other systems in general

fault zone deformation controlled by carbonate mechanical stratigraphy balcones fault system texas normal faults in cretaceous carbonates in the balcones fault system provide important analogs for fault zone architecture and deformation in carbonate reservoirs worldwide mechanical layering is a fundamental control on carbonate fault zones relatively planar faults with low displacement gradients develop in massive strong clay poor limestones and dolomites in less competent clay rich strata shale beds impede fault propagation resulting in fault related folding and locally steep bedding dips faults in clay poor massive limestones and dolomites tend to be steep 70 or more whereas weaker clay rich limestones develop faults with shallower dips 60 or less fault zone rocks show evidence of cataclasis cementation deformation of cement by mechanical twinning and pressure solution and multiple generations of cement with differing degrees of deformation indicating contemporaneous cementation and fault slip in stratigraphic sequences consisting of both competent and incompetent strata the ratio of incompetent to competent strata by thickness is a useful guide for inferring the relative rates of fault displacement and propagation low displacement to propagation ratios associated with competent strata generate low displacement gradients inhibiting fault related folding conversely high displacement to propagation ratios associated with incompetent strata promote high displacement gradients and fault related folding

lateral variations and linkages in thrust geometry the traligill transverse zone assynt culmination moine thrust belt nw scotland abrupt lateral changes in thrust geometry occur in many fold and thrust belts along so called transverse zones commonly related to pre existing basement faults however the causative structures are usually concealed we analyse here the traligill transverse zone in the assynt culmination of the caledonian moine thrust belt nw scotland this transverse zone trends sub parallel to the wnw transport direction and is associated with en echelon faults cutting thrusts discontinuity of thrust architecture and oblique fold and thrust structures thick thrust sheets north of the transverse zone contain thick basement slices thrust sheets to the south are thin and involve a thin bedded sequence the traligill transverse zone developed above the loch assynt fault a basement cross fault and reactivated proterozoic ductile shearzone piercing point analysis shows that the cross fault was active both before and after thrusting thrusting thus affected strata that were already disrupted by steep faults the amplitude of the disturbance in fold and thrust architecture along the traligill transverse zone is much greater than the vertical displacement along the fault this is attributed to localised transpressional thrust stacking other basement cross faults and their relationship with lateral variations within the moine thrust belt and in other thrust belts are discussed

a structural analysis of the moine thrust zone between loch eriboll and foinaven nw scotland abstract the northern part of the moine thrust zone as exposed around the valley of srath beag sutherland was developed by thrusts propagating in the tectonic transport direction deformation on any particular thrust surface evolved from dominantly ductile to dominantly brittle with time the foreland has been progressively accreted onto the overriding moine thrust sheet by duplex formation a process which has continuously folded the roof thrust and the rocks above its hanging wall fold culminations and depression can be related to lateral ramps which may give the rocks above the hanging wall a complex history of extensional and compressional strains normal to the transport direction folds within the thrust zone are laterally independent because they are controlled by short lived variations in deformation style on an evolving thrust footwall topography therefore there may be no correlation between structures across or along the thrust zone this variation limits the construction of balanced cross sections as structure cannot be projected onto particular section lines

appraisal of fracture sampling methods and a new workflow to characterise heterogeneous fracture networks at outcrop abstract characterising fractures at outcrop for use as analogues to fractured reservoirs can use several methods four important fracture data collection methods are linear scanline sampling areal sampling window sampling and circular scanline sampling in regions of homogeneous fracture networks these methods are adequate to characterise fracture patterns for use as outcrop analogues however where fractures are heterogeneous it is more difficult to characterise fracture networks and a different approach is needed we develop a workflow for fracture data collection in a region of heterogeneous fractures in a fold and thrust belt which we believe has applicability to a wide variety of fracture networks in different tectonic settings we use an augmented circular scanline method along with areal sampling to collect a range of fracture attribute data including orientation length aperture spatial distribution and intensity this augmented circular scanline method more than halves the time taken for data collection provides accurate unbiased data that is representative of local fracture network attributes and involves data collection of a wider range of fracture attributes than other sampling techniques alone

an equivalent continuum model for coupled stress and fluid flow analysis in jointed rock masses to provide a set of governing equations for solving the coupled stress and fluid flow a rock mass which commonly contains a large number of geological discontinuities is treated as an anisotropic elastic porous medium with the corresponding elastic compliance and permeability tensors hydro mechanical equivalents to the discontinuous mass the hydromechanical equivalents with the special emphasis on the permeability tensor are formulated on the assumption that any crack can be replaced by a set of parallel planar plates connected by two springs two dimensional numerical analyses on seepage flow networks support the validity of the permeability tensor some field evidence is analyzed to examine the applicability of the present model for the practical purpose of rock hydraulics