critically stressed fractures analysis of the shaikan field kurdistan region of iraq the shaikan field is a large producing oil field in the kurdistan region of iraq it consists of multiple fractured reservoirs consisting of limestones calcareous sandstones and mudstones the surrounding tectonic terrane is situated in the seismically active zagros taurus orogenic zone where present day stresses are high the regional stresses are found to impose conditions that satisfy failure along reservoir bound fractures suggesting that a significant proportion of fractures are likely to be critically stressed the in situ maximum principal stress magnitudes are estimated using three methods namely the tensile and compressive strengths of reservoir rock and leak off test lot data stress field orientations are determined from wellbore image log data which are used to interpret wellbore breakouts and the associated induced tensile fractures reservoir pressure has declined since production started and poroelastic responses have been assessed and used to estimate the present day stress state and the criticality of those fractures that are most likely to fail or slip although a conventional approach has been used the present authors argue that a new approach to stress response with changing pore pressure should be taken unlike the previous theory of criticality in which a reduction in pore pressure is considered to lead to a stabilization of the fracture network the present study suggests that a system may remain critically stressed regardless of pressure decline thematic collection this article is part of the the geology of fractured reservoirs collection available at https www lyellcollection org cc the geology of fractured reservoirs

on the movement and low level structure of cold fronts abstract this paper presents a review of theoretical and observational studies relating to the low level structure of cold fronts and explores the factors that are pertinent to frontal motion observational studies have shown that in some cases surface cold fronts move at speeds faster than the normal component of the wind at all levels in the lower troposphere and therefore propagate other case studies have shown that the low level flow immediately behind the front and normal to it is faster than the front and that the front has the local structure of a gravity current its speed of movement being well determined by the gravity current speed equation them different types of behavior are related to results of recent theoretical studies and the mechanism by which fronts can propagate is elucidated it is shown that a necessary requirement for propagation is the existence of an alongfront temperature gradient we question the relevance of the gravity current speed equation in general despite its appar

the structure and evolution of cold surges east of the rocky mountains abstract northerly surges of cold air often move southward along the eastern side of the rockies from southern canada into mexico the strongest surges which generally develop in midwinter are associated with temperature decreases and pressure rises of 20 30 c and 15 30 mb respectively within 24 h surges are usually accompanied by a meridionally elongated pressure ridge and strong low level ageostrophic winds that parallel the terrain the width of the pressure ridging is approximately 1000 km over the southern plains but decreases to only a few hundred kilometers when the surge enters mexico this paper provides a detailed description of a northerly surge to the east of the rocky mountains that occurred on 12 14 november 1986 using both observational and model data the structural evolution of the surge is analyzed in addition the dynamics of the event is explored by diagnosing the momentum thermodynamic energy and vorticity equations to determine the typical synoptic scale evolution of these

geochemical variations among eastern saudi arabian paleozoic condensates related to different source kitchen areas abstract large reserves of gas and condensate are found in paleozoic reservoirs khuff unayzah and jauf in the ghawar area of eastern saudi arabia the source rock for these hydrocarbons is the basal organic rich hot shale of the qusaiba member of the silurian qalibah formation this is the same prolific source rock that generated the sweet light crude oil and condensate found in the unayzah reservoirs in central saudi arabia thermal maturity modeling of the source rock identifies two kitchen areas to the east and west of the ghawar structure the west kitchen was well into the oil window by 210 ma while the east kitchen was just entering the oil window large areas of both kitchens were in the wet gas window by 125 ma and by 90 ma most of the area north of ghawar was generating dry gas in eastern saudi arabia paleozoic oil and condensate are found in early formed structures carboniferous and early triassic growth whereas late cretaceous and tertiary structures contain only dry gas the geochemistry of the oils and condensates indicates two groups each generated from a different hydrocarbon kitchen suggesting that the two kitchens differ slightly in organic facies within each group variations among hydrocarbons are related to the thermal maturity of the source rock during generation within the khuff reservoir gas souring is caused by thermochemical sulfate reduction

kilometre scale sheath folds in the western betics south of spain the three dimensional exposures of the sierra blanca marbles and neighbouring rocks in the western betic cordillera of the south of spain show tight to isoclinal f3 folds re folded by a larger sheath like structure the relationships between both structures are as follows 1 the mean axial surface for sierra blanca sheath fold can be defined as the great circle of best fit through the s axes of sub cylindrical f3 folding domains 2 the asymmetric f3 folds indicate a single mainly eastward normal sense of shear 3 the mean attitude of the axial plane crenulation foliation sc is also sub parallel to the mean axial surface for sierra blanca sheath fold the regional context of the sierra blanca sheath fold is finally discussed and a model of a heterogeneous high strain extensional zone proposed

surface features of winter monsoon surges over south china abstract the surface features associated with two kinds of winter monsoon surges over south china are studied the easterly surge es and the northerly surge ns surface meteorological parameters over the region 15 50 n 90 130 e for the surges that occurred in the three winters october march from 1988 to 1991 are analyzed for the northerly surge the surface features found are 1 an abrupt temperature drop and wind direction turning from easterly to northerly which can be related to the passage of a cold front 2 an increase in the dewpoint depression and 3 a large north south pressure gradient on the other hand the easterly surge is found to be associated with strong easterly winds up to approximately 40 km h 1 little temperature or pressure change and a southeastward motion of a high pressure center from dahinggangling to the yellow sea together with a sharp pressure ridge along the east china coast furthermore an es and an ns are associated with different perturbations anomalies in

new scaling of fractures in a giant carbonate platform from outcrops and subsurface abstract we scale properties of fractures in the late kimmeridgian carbonate platform of the arabian shelf from outcrop and subsurface strata the platform is flat lying undeformed except from low relief anticlines and domes at which the strata hosts the world s most prolific petroleum reserves the strata exhibit stylolites and veins besides the dominant fractures the fractures are opening mode steep and forming major sets of nw se and ne sw orientations faults are uncommon fracture abundance varies significantly across the platform but it is constrained spatially by inherent variation in mechanical stratigraphy and temporally by anticlinal growth and isotope geochemistry in early burial stages the platform strata were slightly deformed by bed parallel stylolites and mutually crosscutting veins the strata fracturing increased by the anticlinal growth in the late cretaceous fracture spacing is an average of 2 m in the horizontal bedded outcrops in central arabia and 0 30 m in the equivalent subsurface in a dome in eastern arabia the dome is an end member in the deformation spectrum of the platform interior with up to c 5 structural dip and 2 dilational strain expressed in the doming and a few major faults this finding indicates that carbonate platforms can reach fracture oversaturation under such a subtle tectonic extension

fracture apertures from electrical borehole scans three dimensional finite element modeling was performed to investigate the response to fractures of the formation microscanner mark of schlumberger which records high resolution electrical scans of the borehole wall it is found that the equation w carmbrx0l describes over two orders of magnitude of resistivity contrasts between borehole mud and the formation the relationship between fracture width w in mm formation sensitivity rx0 mud resistivity rm and the additional current flow a caused by the presence of the fracture a is the additional current which can be injected into the formation divided by the voltage integrated along a line perpendicular across the fracture trace coefficient c and exponent b are obtained numerically from forward modeling tool standoffs of up to 2 5 mm and fracture dips in the range from 0 to 40 were found to have an insignificant effect on the above relation a three step approach to detect trace and quantify fractures is used potential fractures in formation

active deformation within the zagros mountains deduced from gps measurements we present and interpret the results of global positioning system gps measurements at 35 stations in and beside the zagros mountain belt sw iran for three campaigns ending march 1998 december 1999 and june 2001 preliminary motion estimates show clearly the change in character along the strike of the belt stations to the se move at 13 22 3 mm a 1 towards n 7 5 e with respect to eurasia most of the shortening indicated by the gps velocities seems to occur in the se zagros along two major seismic zones and along the zagros front to the nw stations move oblique to the trend of the belt towards n 12 8 w at 14 19 3 mm a 1 most of the shortening in the nw zagros seems to occur along the mountain front fault with its major earthquakes as well as along the zagros front the change in direction and magnitude of the velocity vectors across the north south trending kazerun and karebas faults involves extension of up to 4 mm a 1 along the strike of the zagros belt

superposed and refolded metamorphic isograds and superposed directions of shear during late orogenic extension in the alboran domain southern spain the timing kinematics and mode of exhumation within the alpujarride complex the largest tectonic unit of the alboran domain are controversial alternative tectonic models include synorogenic exhumation during pre miocene crustal thickening intraorogenic exhumation during repeated phases of contraction and extension and late orogenic exhumation during early miocene crustal thinning new structural and petrographic relationships documented in this study support the latter model subhorizontal foliations stretching lineations and recumbent folds are associated with early miocene high temperature decompressional metamorphism and vertical condensation of the metamorphic sequence relict isograds associated with high pressure metamorphism and burial are systematically overprinted by isograds associated with high temperature metamorphism and rapid decompression during the early stages of extensional exhumation and growth of sillimanite ene directed shear d2 overprinted thrusts and foliations associated with crustal thickening d1 during the later stages of extensional exhumation and growth of andalusite from ductile to semibrittle conditions north directed shear d3 produced kilometer scale recumbent folds that refold isograds stretching lineations characterize superposed directions of orogen parallel l2 and later orogen perpendicular l3 shear associated with a rapid temporal transition in the kinematics of extensional exhumation during the final stages of crustal thinning and cooling ductile structures d2 d3 were cut by brittle low angle normal faults d4

eclogites of the ojen nappe a record of subduction in the alpujarride complex betic cordilleras southern spain eclogites interlayered with amphibolites and migmatites occur in an upper unit ojcn nappe of the alpujarride complex betic cor dilleras estimates of p t conditions reveal a retrograde meta morphic path from an edogitic stage at 15 kbar and c 730 c to amphibolite facies conditions at 5 8 kbar or less and c 650 c the ronda peridotites belonging to the uppermost alpujarride nappe los reales nappe structurally overlie the eclogite bearing unit it is suggested that eclogitic metamorphism and overthrusting of the peridotites reflect dfierent stages of the same convergent episode

winter monsoon convection in the vicinity of north borneo part i structure and time variation of the clouds and precipitation abstract radar and satellite observations in the vicinity of northern borneo obtained during the international winter monsoon experiment wmonex showed that the convection in that region underwent an extremely regular diurnal cycle over the sea to the north of borneo the general level of convective activity was increased during monsoon surges and during the passages of westward propagating near equatorial disturbances convective activity was decreased during monsoon lulls the diurnal cycle was well defined regardless of whether the general level of convective activity was enhanced or suppressed by synoptic scale events the cycle of convection over the sea was especially well documented it was typically initiated at about midnight when an offshore low level wind began where this wind met the monsoonal northeasterly flow usually just off the coast convective cells formed after midnight the convection continued to develop and by 0800 lst it had evolved into an organized mesoscale system with a

eye to eye with a mega sheath fold a case study from wadi mayh northern oman mountains sheath folds are highly curvilinear folds typically considered to develop by hinge rotation toward the transport direction during intense deformation most sheath folds are recognized on the centi meter to meter scale with very few detailed studies of larger kilometer scale structures we here present evidence for a megascale sheath fold superbly exposed in the northern oman mountains the saih hatat culmination consists of an exhumed highpressure subduction zone with deep level eclogites and garnet blueschists as sifah unit epidote blueschists hulw unit and carpholite bearing metasediments part of the upper plate sheath folds are present at a variety of scales throughout the as sifah hulw and upper plate tectonic packages curvilinear fold hinges along a 15 km long profi le outline the wadi mayh mega sheath fold which closes to the ssw and faces to the nne it consists of at least four secondary sheath folds with cross sectional y z eye folds measur ing 200 m stacked on top of one another within permian and triassic shelf car bonates these eye folds display highly elliptical geometries ryz 5 and cat s eye fold patterns consistent with simple general shear the sheath fold is enveloped by ordovician and older rocks that do not show equal amounts of internal deformation the upper bounding envelope is a detachment that has been subsequently sheath folded the lower boundary of the sheath fold is the upper plate lower plate detachment which forms a major high strain shear zone showing little vertical displacement but signifi cant horizontal displacement the sheath fold displays a ssw elongation x axis of at least 15 km and possibly up to 25 km coupled with signifi cant amounts of vertical fl attening and wnw ese shortening

gap winds in a fjord part i observations and numerical simulation abstract gap winds in howe sound british columbia are described and placed in context by reviewing studies of similar phenomena in other locations an observational program consisting of a surface mesonetwork and vertical soundings shows that gap winds vary considerably along and across the channel as well as vertically wind strength generally increases down channel and strongest winds are found below 1000 m depth results from application of a 3d mesoscale numerical model to a gap wind case compare reasonably well with observations model output reveals more details of horizontal and especially vertical flow structure than is possible from observations model vertical cross sections and froude number output indicate similarity with hydraulic flow this is further substantiated by a force balance analysis of model output

winds in the strait of gibraltar summary automated meteorological observations were made from october 1985 to september 1986 in the narrows of the strait of gibraltar additional surface and upper air measurements were made in the strait from may to july 1986 the main wind events in summer were mostly mesoscale levanters winds from the east through the strait and a lesser number of synoptic scale levanters and westerlies during mesoscale levanters the wind speed increased and the pressure decreased from gibraltar downstream through the narrows as far as tangier a mesoscale low and the highest winds were found over the axis of the strait north of tangier high velocity 20 m s winds occurred only under 1 km height the air temperature profile became progressively warmer moving from gibraltar towards tangier a weak air temperature inversion decreased in height to the west of the narrows venturi flow as an explanation for the downstream increase of the low level winds during a levanter is inconsistent with observations a new model is proposed to explain the wind field associated with levanter winds in the strait that includes some orographic blocking and low level convergence into the narrows some west side divergence due to drag over water and leeside heating that generates a leeside mesoscale low pressure the surface low accelerates the surface winds to their maximum strength well downstream of the narrows a similar flow regime has been observed recently in the sea level channel between hawaii and maui

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

interaction of potential vorticity anomalies in extratropical cyclogenesis part ii sensitivity to initial perturbations in part i of this series of papers the static potential vorticity pv inversion diagnostics are performed to examine the lateral and vertical interactions between two upper level northern and southern troughs and their possible influences on the development of the march 1993 superstorm this study continues the investigation of the relative importance of two pv anomalies associated with the short wave troughs and their interaction in the surface cyclogenesis by considering their individual roles as an initial value problem the piecewise pv inversion technique is first used to isolate the upper level disturbances with a balanced vortex for each the vortex is then either removed or its intensity doubled by subtracting the balanced vortex from or adding to the control initial conditions with the modified initial conditions a mesoscale model is integrated for 36 h to examine how each of the upper level disturbances contributes to the surface development it is found that the northern and southern troughs play different roles in the surface cyclogenesis without the northern trough the eastward propagation of the southern trough slows down and the surface development is severely impeded more rapid cyclogenesis occurs when the northern trough s intensity is doubled on the other hand the deepening rate increases considerably when the southern trough is removed doubling the southern trough intensity slows the eastward progression of the northern trough and leads to a reduced deepening rate in conclusion the northern trough is crucial for the rapid development of the superstorm whereas the southern trough is important only during the incipient stage of the cyclone in particular a much stronger surface cyclone could be spawned in the absence of the southern trough this finding appears to contradict the previous work on the roles of trough mergers in extratropical cyclogenesis

a windstorm in the lee of a gap in a coastal mountain barrier abstract this paper describes a localized windstorm that struck some areas of northwest washington state on 28 december 1990 with winds exceeding 45 m s 1 resulting in extensive property damage treefalls and power outages arctic air originating within the interior of british columbia descended into a mesoscale gap in the coast cascade mountains and then accelerated ageostrophically to the west this gap acceleration is explained quantitatively by a three way balance among the pressure gradient force friction and inertia the flow maintained its integrity as a narrow current of high speed air as it exited the gap and subsequently accelerated over water troughing in the lee of the cascade mountains enhanced the horizontal pressure gradient over northwest washington this pressure gradient approximately balanced frictional drag resulting in only minimal acceleration farther south the flow decelerated as the current spread out horizontally

stress orientations from oil well fractures in alberta and texas many oil wells in alberta exhibit spalling of the walls known as breakouts which elongates the holes with the longer axis aligned northwest southeast this alignment is observed over an area in excess of 4 105 km2 in siltstones sandstones carbonate sediments and one shale formation through the stratigraphic column from devonian to cretaceous it is unrelated to dip of the beds we have elsewhere suggested that these breakouts are produced through stress concentration near the hole walls in a stress field having large unequal horizontal principal stresses and with the larger compression oriented northeast southwest it is probable that this northeast southwest principal stress is 1 this paper adds new data from oil wells in alberta and northern british columbia and shows that the breakouts and by inference the stress orientations are consistent through much of the western canadian sedimentary basin it also uses evidence from hydraulic fracturing in west central alberta and from steam injec

an application of potential vorticity inversion to improving the numerical prediction of the march 1993 superstorm in this study a methodology is proposed to improve the model initial conditions based on available surface temperature observations from ships buoys and drifters it is tested with the numerical prediction of the 12 14 march 1993 superstorm that is initialized at its incipient stage over the gulf of mexico in this methodology the authors make use of the piecewise potential vorticity pv inversion technique and treat the temperature errors at the lowest level as a surrogate pv anomaly after inverting the wind and mass perturbations from the surface thermal anomaly and its pertinent interior pv anomaly a three dimensional dynamically consistent set of errors are obtained and added to the model initial conditions to improve the representation of the lower troposphere over the data sparse ocean it is found that the numerical model prediction initialized with the modified initial conditions exhibits significant improvements in the early rapid deepening and the track of the superstorm over ocean the development of a prefrontal squall line and the central sea level pressure traces during the life cycle of the cyclone as verified against observations these results show that the methodology proposed is promising in improving the representation of lower tropospheric meteorological variables in the model initial conditions based on available surface observations over data sparse regions

diverse fracture properties and their impact on performance in conventional and tight gas reservoirs saudi arabia the unayzah south haradh case study the upper permian carboniferous unayzah formation in south haradh saudi arabia includes two major mechanical and petrophysical layers that are separated by shale rich zones open tectonic fracture clusters are rare and not essential for fluid flow in the unayzah a zone which has high porosity and permeability however such fracture clusters are essential to and impact the production performance in the unayzah b c tight gas reservoir the occurrence of the tectonic fractures in the unayzah formation is linked to the rock mechanical properties which vary with porosity shale volume cement type and texture the b c unit is more fractured than the a unit but its layers vary in the degree of fracturing the variation in fracture development within the b c unit results in differences in fracture enhanced permeability based on production profiles where flow is restricted to preferentially fractured mechanical layers that lack effective vertical fluid communication with other layers we identify two tectonic fracture systems an older subordinate fully mineralized system and a younger primary mostly open system early extensional fractures including joints and faults developed parallel to the basement faults during the opening of the neotethys these are fully mineralized and have little or no function as fluid conduits the younger system includes open fracture clusters that are predominantly parallel or nearly parallel to the regional east northeast west southwest maximum horizontal stress of the zagros that has been active since the late cretaceous and is independent of local structures therefore these fractures are controlled by remote stresses instead of the basement rooted forced folds and faults in this article we demonstrate that in the unayzah b c natural fractures are essential to permeability and in some areas to porosity and thence to reservoir performance the results of this study are being implemented in well placement and completion design to optimize the intersection of open fracture clusters with positive preliminary results

a new structural and kinematic framework for the alboran domain betic rif arc western mediterranean orogenic system the processes that drastically thinned and rapidly exhumed the alboran domain in southern spain are poorly understood geological maps cross sections and a synthesis of previous work in the alpujarride complex provide a new structural framework for the alboran domain with major revision of the previously defined lithotectonic units alboran continental lithosphere was buried and thickened in the paleogene d 1 and subsequently thinned during late orogenic extension in the early miocene regional patterns of stretching lineations and late recumbent folds record important spatiotemporal changes in the kinematics of extension east directed shear d 2 is preserved in deep orogenic levels and thinned the crustal sequence during rapid exhumation and high temperature metamorphism north directed shear d 3 d 4 is preserved in upper orogenic levels and reworked earlier structures during the later stages of exhumation and cooling this large scale switch in the direction of finite stretching resulted from the interaction between components of orogen parallel d 2 and orogen perpendicular d 3 d 4 extension and associated vertical and lateral gradients in strain we provide a novel kinematic model for gravity driven extensional collapse of the alboran domain compatible with both westward motion of the alboran domain relative to the iberian margin and subsequent northward extensional flow and emplacement of alboran lithosphere onto the iberian margin supplementary material a list of sample names coordinates and descriptions plus a google earth kmz file of sample locations is available at https doi org 10 6084 m9 figshare 5508643 a geological sketch map of the sierra de las estancias indicating the trend of stretching lineations in the alpujarride complex and location of major geological boundaries is available at https doi org 10 6084 m9 figshare 5513398

constraining coalbed methane reservoir petrophysical and mechanical properties through a new coal structure index in the southern qinshui basin northern china implications for hydraulic fracturing structurally deformed coal sdc influences not only gas outbursts during coal mining but also coalbed methane cbm production different sdcs have different mechanical strengths and physical properties the structurally constrained petrophysical and mechanical properties of a cbm reservoir from the southern qinshui basin sqb northern china have been investigated by using a coal structure index csi the csi was established as a quantitative criterion for the deformation of sdcs based on surface conditions and blockiness using the data of 783 coring samples from 109 cbm wells the results show that undeformed coal and slight brittle sdcs are dominant in the sqb with ductile sdcs distributed locally both compressive strength and tensile strength decline exponentially with increasing csi average avg coal structure is a crucial factor for coal seam fracturing with an increasing csi avg the major fracture length will first increase to the maximum of 266 m at a csi avg of 45 and then decrease rapidly however branch fractures are independent of the csi avg the major fracture height attained a minimum of 4 39 m at a csi avg of 40 and then slowly increased therefore slightly brittle sdcs should be beneficial for fracturing at least for high rank coals the presence of variable sdcs can have a considerable impact on cbm reservoir petrophysical and mechanical properties finally a csi model that strives to establish the relation between structure deformation and cbm reservoir physical and mechanical properties in coal is expected to effectively guide hydraulic fracturing

a model for stratigraphic and metamorphic inversions at sulitjelma central scandes the relationship between disposition of metamorphic isograds and early strain history is examined for the sulitjelma fold nappe of the central scandes evidence for stratigraphic inversion and the widespread inversion of the garnet isograd is reviewed and new evidence for the nature of early scandian deformation is presented a two stage model is presented which explains the inversions by i an initial phase of horizontal compression resulting in crustal thickening by vertical stretching during closure of a marginal basin followed by ii continued horizontal compression and crustal thickening by eastward directed nappe emplacement the emplacement of the sulitjelma fold nappe during the latter phase of progressive simple shear dominated deformation was accompanied by the development of sheath fold geometries and the overturning of isotherms to produce the widespread inversion of the garnet isograd

a diagnostic analysis of the superstorm of march 1993 abstract in this study the synoptic evolution of the march 1993 superstorm is documented using conventional observations and the predictability and deepening mechanisms of the storm are investigated using a newly implemented mesoscale version of the canadian regional finite element rfe mode it is found that although the rfe model can predict well the large scale background flow associated with the storm it fails to predict various important mesoscale elements when it is initialized with fields that contain weak signals of the low level circulations however when the storm is located close to the data rich region the model has considerable skill in predicting those mesoscale elements such as a prefrontal squall line upper and low level jets as well as the quantitative aspects of their associated precipitation observational analysis reveals that the storm developed in a convectively unstable prestorm environment with ample moisture content it first experienced an antecedent surface vorticity g

the function of fractures and in situ stresses in the khuff reservoir performance onshore fields saudi arabia the highly variable performance of the permian triassic khuff reservoir in onshore saudi arabia has been attributed to the presence of natural fractures similar preproduction pressure profile and hydrocarbons in the different reservoir units in some fields have been attributed to vertical communication through large faults to validate these assumptions we studied the static and dynamic data from the khuff reservoir in 19 major structural traps we identified two distinctive fracture domains based on fracture orientation and density fracture evolution is mainly controlled by the extensional and consequent compressional plate tectonics instead of local structures in situ stresses are dominated by the zagros plate tectonics and affect fracture aperture differently in the two fracture domains the fracture impact on the khuff reservoir performance is mostly subtle because of the nature and distribution of the fractures high fracture enhanced productivity occurs locally in some of the producing wells and it results from high density fracture clusters including mesoscopic faults with channel type apertures the following findings challenge the perceived major functions of fractures in the khuff reservoir performance in onshore fields 1 individual fractures are dominantly tensile and small mesoscopic and microscopic 2 individual faults are small and not readily resolvable at seismic scale 3 the depth and carbonate nature of the khuff reservoir make the fractures highly susceptible to fast healing unless preserved within the hydrocarbon column 4 initial vertical pressure gradient changes with production indicate a lack of present day communication across the anhydrite sealing layers between the different khuff reservoir units 5 horizontal well direction does not generally have an impact on productivity and 6 sustained and heavy losses of circulation are rarely encountered in the khuff reservoir wells mohammed s ameen received his ph d and diploma of imperial college in structural geology and geomechanics from imperial college london 1988 he has more than 20 years of academic and industrial experience and has patented a new method for the characterization of microfractured reservoirs he has published 25 articles on fractures and folds and has edited three special publications for the geological society london he conducted the first classic work on the fractures and folds across the taurus zagros range iraq covering 30 major fold traps the work is published in the aapg bulletin the geological magazine and the journal of petroleum geology he joined the reservoir characterization department at saudi aramco in 1998 since 2004 he has been leading the structural geology and rock mechanics group in the geological technical services division saudi aramco he is an active member of the aapg society of petroleum engineers european association of geoscientists and engineers and the geological society london ismail m buhidma is a petroleum engineering consultant in the gas reservoir management division of saudi aramco with 32 years of diverse industrial experience for the last 12 years he has been actively involved in aramco s nonassociated gas development program his areas of interest include reservoir management well performance analysis well test analysis phase behavior geomechanics reservoir simulation reservoir characterization and well stimulation prior to joining aramco ismail worked for exxon in libya schlumberger and atlantic richfield in the usa and qatar petroleum in qatar ismail holds b s and m s degrees in petroleum engineering from the university of tulsa he is a member of society of petroleum engineers spe and has published numerous spe appears zillur rahim is a petroleum engineering consultant with saudi aramco working in gas field development he received his b s degree from algerian petroleum institute and m s degree and ph d from texas a m university college station all in petroleum engineering he has 25 years of industry experience has published more than 50 technical papers and has taught numerous industry courses previously he worked with holditch and associates petroleum consultants and with schlumberger reservoir technology group his area of expertise includes reservoir engineering hydraulic and acid fracturing and reservoir management

vergence an evaluation abstract fold asymmetry is an independent concept distinct from facing fold vergence is a direction relating to the sense of asymmetry of folds the complementary concept of cleavage vergence is defined allowing asymmetry to be used to locate major fold traces where no minor folds exist cleavage vergence is a valuable tool in complex structural terrains both concepts can be used singly or in combination to identify refolding vergence can define structural way up where sedimentological or stratigraphic way up cannot be recognised

a review on the applications of image logs in structural analysis and sedimentary characterization abstract image logs provide important information on lithology sedimentary textures paleoflow directions fractures and in situ stress analysis lacking of criteria used for establishing the image log facies limits the applications of image logs in sedimentary reservoir interpretation this paper provides a thoroughgoing review focusing on the recent applications of borehole image logs for sedimentological and structural description and interpretation and aims to establish image log facies which can provide guidelines in sedimentary reservoir interpretation this paper firstly summarizes the principles and basic characteristics of various imaging logging tools and then briefly introduces the pre processing workflow of the image log data then the generated images are used for depth and orientation shifts of cores by calibrating individual sedimentary and structural features descriptive and concise image log facies are established based on combinations of image textures including dip type dip pattern and color scheme and the characteristics as well as physical criteria for each individual image facies are summarized the established image log facies are then interpreted in terms of structural and sedimentological features such as lithology sedimentary structures vugs fractures and faults the image log facies and its stacking patterns are then used to interpret the lithofacies associations by calibrating with cores and conventional logs natural fractures and induced fractures are recognized by image logs and the principles of breakouts and drilling induced fractures for in situ stress analysis are reviewed then the applications of image logs in investigation of fracture attitudes and states as well as in computation of fracture parameters are summarized the procedures to evaluate fracture effectiveness through image logs are discussed at last the application of image logs for structural dip analysis is reviewed and image logs are used for recognizing faults fracture sets and attitudes of stratum through the dip patterns the basic procedures for paleocurrent reconstruction which includes dip picking of cross beddings structural dip determination and structural dip removal are reviewed then the paleocurrent directions of the lower cretaceous bashijiqike formation in the kuqa depression were reconstructed which could help further understanding of the depositional systems this review will help extend the utility of image logs in interpreting small to large scale sedimentary and structural features and bridges the gaps between well log analysis and sedimentary and structural interpretation

australian southerly busters part i analysis of a numerically simulated case study abstract a diagnostic study of the australian southerly buster is performed using a numerical weather prediction model it is demonstrated that the model run at 50 km resolution is capable of capturing the transition of a cold front into a southerly buster as it traverses the cut coast of the australian continent the main features of the modified front am identified through comparison of the simulation with an adiabatic simulation from which precipitation and surface processes including orography have been removed from this comparison the southerly buster effect is defined in terms of the deformation of the front from its adiabatic ostentation and in terms of enhancement of temperature gradients and southerly winds along the coastal strip sensitivity experiments are then analyzed with a view to objectively determining the relative roles of mountains precipitation and land sea contrasts of heating and friction in bringing about these perturbations the major conclusions are that i the primary e

contemporary tectonic stress pattern of the persian gulf basin iran abstract the contemporary crustal stress pattern is important to several aspects of earth sciences such as reservoir geomechanics neotectonic deformation and geodynamic modelling the persian gulf basin is a petroliferous region in southern iran that hosts hundreds of petroleum wells and hence the knowledge of the present day stress is critical for any exploration and production in this region in addition to geo resources this region has a great economic importance and the in situ stresses provide key insights on neotectonic and seismic hazard of the basin however there is limited in situ stress information in the persian gulf region in this study we analyse 8 km of borehole image logs in 20 wellbores in the persian gulf in order to provide a clear picture of the stress pattern in this region our wellbore stress analyses provide 35 quality ranked maximum horizontal stress orientations shmax data records in addition we compile 27 earthquake focal mechanisms in the study region from different seismic catalogues and published literature and present the first comprehensive quality ranked in situ stress map of the persian gulf basin with 62 data records which can be used as a valuable source of information for future exploration and seismic hazard assessment the resulting database suggests that the shmax azimuth in central and western persian gulf is ne sw parallel to the arabia eurasia tectonic plate convergence which demonstrate the key role of relative plate motion in the present day stress pattern of these areas however the shmax azimuth in the eastern part of the study area is nw se which is different from the central and western persian gulf the nw se shmax azimuth is consistent with other geological and geophysical observations that suggest an indentor model for musandam peninsula oman in the zagros makran syntaxis

doppler radar observation of a land breeze cold front abstract during the early morning hours of ii january 1981 a very narrow intense band of snow which had formed along the leading edge of a land breeze cold front was viewed by cp 4 radar near muskegon michigan the front formed well inland and moved through the area from the northeast shortly after dawn the driving force for the land breeze was the large temperature contrast that existed between radiationally cooled air over the northern highlands of lower michigan and the relatively warm lake modified air near the shore of lake michigan the possibility that the observed snowband was associated with a mesoscale feature other than the land breeze is investigated an analysis of minimum temperatures in the great lakes region for 11 january 1981 shows that a strong temperature gradient existed over western lower michigan this gradient reached a maximum value well inland rather than at the lake shore radar observations show the band to be over 80 km long and only 4 km wide oriented roughly parallel t

observations and modeling of a mesoscale cold surge during wispit a transient feature marked primarily by a sudden increase in wind speed and brief period of heavy snow was observed over northeastern colorado on 11 12 march 1993 little if any abrupt changes in surface pressure or temperature were noted such as might accompany a gravity current gradual temperature falls and pressure rises were observed behind the velocity surge however there was no cyclonic wind shift nor did the feature last for more than a few hours suggesting that it was not likely a classical front data indicate that the surge originated near the peak of the cheyenne ridge a 400 m east west elongated hill near the border between colorado and wyoming simulations using an idealized two dimensional isentropic model show that a transient resembling observed surge results from impulsively started flow over heated terrain heating creates convergence that is in phase with the velocity surge at the leading edge of the topographic wave the result is a coherent disturbance in velocity which moves downstream at nearly the mean flow speed simulations with the pennsylvania state university national center for atmospheric research nonhydrostatic model mm5 using a horizontal resolution of 6 7 km capture the evolution of the observed surge these simulations support the identification of the surge as a topographic wave modified by heating but they also indicate an important role played by latent heating in the updraft at the leading edge of the surge because the latent heating occurs mainly downstream from the cheyenne ridge the formation of the topographic wave is unaltered by it however the updraft at the leading edge of the surge it greatly intensified leading to a narrow propagating band of heavy snow

kilometre scale sheath fold at mattmark and implications for transport direction in the alps folds were the first tectonic features of rocks to be studied and as such are taken as a reference for the interpretation of other structures intuitively fold axes were assumed to be subperpendicular to the direction of transport nevertheless many studies of metamorphic belts showing intense ductile deformation describe folds with axes close to the transport direction in particular sheath folds1 9 are highly curvilinear folds elongated parallel to the direction of transport we discuss here a spectacular large sheath fold situated in a crustal shear zone in the monte rosa nappe swiss alps our analysis implies that both the geometry and significance of folds in the internal alps must be re examined we further demonstrate the existence of crustal ductile shear zones in an area of the alps where previously emphasis was generally placed on fold nappes the movements on these shear zones is often tens of kilometres

a family of frontal cyclones over the western atlantic ocean part i a 60 h simulation despite marked improvements in the predictability of rapidly deepening extratropical cyclones many operational models still have great difficulties in predicting frontal cyclogenesis that often begins as a mesoscale vortex embedded in a large scale parent cyclone system in this paper a 60 h simulation and analysis of a family of frontal cyclones that were generated over the western atlantic ocean during 13 15 march 1992 are performed using the pennsylvania state university national center for atmospheric research mesoscale model with a fine mesh grid size of 30 km although it is initialized with conventional observations the model reproduces well the genesis track and intensity of the frontal cyclones their associated thermal structure and precipitation pattern as well as their surface circulations as verified against the canadian meteorological centre analysis and other available observations it is shown that each frontal cyclone is initiated successively to the southwest of its predecessor in the cold sector first appearing as a pressure trough superposed on a baroclinically unstable basic state in the lowest 150 300 hpa then it derives kinetic energy from the low level available potential energy as it moves over an underlying warm ocean surface with weak static stability toward a leading large scale frontal zone and deepens rapidly by release of latent heat occurring in its own circulations one of the frontal cyclones originating in the cold air mass deepens 44 hpa in 42 h and overwhelms the parent cyclone after passing over the warm gulf stream water into the leading frontal zone these cyclones have diameters ranging from 500 to 1100 km as denoted by the last closed isobar and are spaced 1000 1400 km apart between their circulation centers during the mature stage they begin to establish their own cold warm frontal circulations once their first closed isobars appear thus distorting the leading large scale frontal structures and altering the distribution and type convective versus stratiform of precipitation it is found that the frontal cyclones accelerate and experience their central pressure drops as they move from high to low pressure regions toward the parent cyclone center and then they decelerate and fill as they travel away from the parent cyclone their spatial and temporal scales vertical structures as well as deepening mechanisms are shown to differ significantly from those typical extratropical cyclones as previously studied

possible forced folding in the taurus zagros belt of northern iraq the folds in the taurus zagros belt of northern iraq have generally been considered to be decollement buckle folds this implies the presence of a decollement horizon at or near the base of the sedimentary cover the infra cambrian hormuz salt and a passive role of the precambrian basement in the tectonic evolution of the folded belt structural stratigraphic geophysical and remote sensing evidence suggests that forced folding due to faulting in the basement has played a significant role in the development of many of the folds in this region this is clear from the substantial evidence of basement faulting the lack of any convincing evidence for the presence of an extensive and regional decollement horizon i e the hormuz salt between the basement and the cover rocks and the geometries of the folds and related mesostructures the study shows that in the taurus foothills zone the folds are short oval in plan shape and arranged in an en echelon pattern along two sets of dislocation zones in the basement however the folds in the zagros foothills zone are longer linear and arcuate in plan shape many of the studied folds show similar features to those observed in the rocky mountains u s a

comparison of carbonate reservoir fractures from core and modern electrical borehole images abstract several methods exist for collecting data about opening mode fractures hereinafter fractures in subsurface reservoir rocks the datasets with the exception of core data are indirect and subject to limited resolutions this paper includes new results on the detection of fractures in carbonate reservoirs by modern electrical borehole imagers barren fractures and calcite filled fractures calcite veins were described and compared from whole core and borehole images these datasets were obtained from directional drilling into the arab petroleum reservoirs in a low relief dome structure in eastern arabia the comparison of fracture abundance resulted in a significant mismatch because many reservoir fractures in the core could not be detected from corresponding borehole images most calcite veins were undetected because of their small widths below 2 mm exceptions include a few calcite veins of 0 5 mm that were detected because of the strong contrast between resistive veins and conductive host rock beds the barren fractures that are aligned sub parallel to the maximum horizontal principal stress shmax were detected more abundantly than other non aligned barren fractures the present day shmax is roughly trending between 60 and 100 azimuth in eastern arabia the aligned fractures have larger apertures in the borehole images than the non aligned fractures the apertures diminish in size as the fractures gradually deviate in orientation from the shmax most of the non aligned barren fractures were undetected except the fractures with rough and mismatching walls this indicates that the walls of the non aligned fractures are mechanically closed by the normal shmax and that the remaining apertures were dependent on the availability of roughness mismatches between the closed fracture walls electrical borehole images are biased in measuring fracture abundance but indispensable to detect and measure the orientations and apertures of fractures especially the large i e more conductive fractures in reservoir rocks

active tectonics of the alpine himalayan belt between western turkey and pakistan summary over 80 new fault plane solutions combined with satellite imagery as well as both modern and historical observations of earthquake faulting are used to investigate the active tectonics of the middle east between western turkey and pakistan the deformation of the western part of this region is dominated by the movement of continental material laterally away from the lake van region in eastern turkey this movement helps to avoid crustal thickening in the van region and allows some of the shortening between arabia and eurasia to be taken up by the thrusting of continental material over oceanic type basement in the southern caspian mediterranean makran and black sea thus central turkey bounded by the north and east anatolian strike slip faults is moving west from the van region and overrides the eastern mediterranean at two intermediate depth seismic zones one extending between antalya bay and southern cyprus and the other further west in the hellenic trench the motion of northern iran eastwards from the van region is achieved mainly by a conjugate system of strike slip faults and leads to the low angle thrusting of iran over the southern caspian sea the seismicity of the caucasus shows predominantly shortening perpendicular to the regional strike but there is also some minor elongation along the strike of the belt as the causcasus overrides the caspian and black seas the deformation of the eastern part of this region is dominated by the shortening of iran against the stable borders of turkmenistan and afghanistan the north east direction of compression seen in zagros is also seen in north east iran and the kopet dag where the shortening is taken up by a combination of strike slip and thrust faulting large structural as well as palaeomagnetic rotations are likely to have occurred in ne iran as a result of this style of deformation north south strike slip faults in southern iran allow some movement of material away from the collision zone in ne iran towards the makran subduction zone where genuinely intermediate depth seismicity is seen within this broad deforming belt large areas such as central turkey nw iran azerbaijan central iran and the southern caspian appear to be almost aseismic and therefore to behave as relatively rigid blocks surrounded by active belts 200 300 km wide the motion of these blocks can usefully be described by poles of rotation the poles presented in this paper predict motions consistent with those observed and also predict the opening of the gulf of iskenderun ne of cyprus the change within the zagros mountains from strike slip faulting in the nw to intense thrusting in the se and the relatively feeble seismicity in se iran baluchistan this description also explains why the north south structures along the iran afghanistan border do not cut the east west ranges of the makran within the active belts surrounding the relatively aseismic blocks a continuum approach is needed for a description of the deformation even though motions at the surface may be concentrated on faults the evolution of fault systems within the active zones is controlled by geometric constraints such as the requirement that simultaneously active faults do not in general intersect many of the active processes discussed in this paper particularly large scale rotations and lateral movement along the regional strike are likely to have caused substantial complexities in older mountain belts and should be accounted for in any reconstructions of them

development of large north facing folds and their relation to crustal extension in the alboran domain alpujarras region betic cordilleras spain detailed research in the eastern part of the alpujarras region betic cordilleras has revealed the existence of large recumbent fold structures involving lithological sequences previously considered to belong to different tectonic units we have reconstructed a large complex fold structure which can be followed axially for over 50 km based on a careful revision of the boundaries between lithological formations a structural analysis of the carbonate and metapelite sequences and the use of mineralogical techniques such as measurements of d001 basal spacing of phengite paragonite and chlorite determinations of the illite crystallinity index ic and b0 parameters of mica and chlorite structural field data described in this paper include gently dipping axial plane cleavage surfaces common occurrence of gently dipping boudinage and pinch and swell structures and associated sub orthogonal stretching lineation similarity in orientation of this stretching lineation with the slip direction of the low angle normal faults and close association between north facing recumbent folds and top to the north low angle normal faults the slip direction of the faults being broadly perpendicular to the fold axes these features clearly indicate that extensional processes played a decisive role in the generation of the alpujarras large recumbent folds the development of these recumbent folds probably occurred during the early to middle miocene extensional tectonics that affected the betic cordilleras the formation of large scale folds in association with normal faulting is consistent with the p t paths and geochronological data established for the alpujarride region

gap winds in the strait of juan de fuca abstract gap winds can be defined as a flow of air in a sea level channel which accelerates under the influence of a pressure gradient parallel to the axis of the channel in february 1980 two distinct cases of gap winds were observed in the strait of juan de fuca between western washington state and british columbia during a study that measured spatial variation of low level marine winds and other parameters from the noaa p 3 research aircraft and a dense network of surface stations which included eight meteorological buoys these two cases were a high pressure region over central british columbia and a low pressure system propagating northward seaward of the washington coast both cases produced strong easterly winds of 13 15 m s 1 at the western end of the strait of juan de fuea the high pressure region provided a drainage air mass from the interior of british columbia which flowed through the straits of georgia and juan de fuca and eventually into the pacific ocean this air mass remained nearly hom

the influence of mesoscale features of the sea surface temperature distribution on marine boundary layer winds off the scotian shelf during the superstorm of march 1993 abstract this paper studies the mesoscale wind field during the blizzard of march 1993 off the east coast of north america and examines the influence of the sea surface temperature distribution on surface winds can the gulf stream and its meanders by its strong influence on the marine boundary layer generate mesoscale features in the wind field numerical simulations of the storm are carried out using the mc2 a fully elastic nonhydrostatic model simulations are conducted at different resolutions 50 25 10 5 and 2 km with both detailed and smoothed sst fields so as to examine the influence of these parameters on the marine boundary layer winds results from these numerical simulations are compared with surface observations from buoys the study reveals some mesoscale features in the wind field caused by the gulf stream s meanders and the warm eddies of the sst field in a stable boundary layer the meanders shaped a 50 55 kt 26 28 m s 1 band of winds in a general 40 45 kt 21 23 m s 1 wind fi

vergence and facing patterns in large scale sheath folds the careful geometric analysis of minor structural detail elucidates the relationships and evolution of associated large scale curvilinear hinge geometries developed during wnw directed caledonian thrusting exposed in neoproterozoic moine psammites of the moine nappe reversals in the polarity of structural facing associated with minor folding mark the position of major sheath folds which parallel transport upwardly convex sheaths closing in the direction of thrust transport cored by older gneissose basement inliers are termed culminations whilst those opening in the transport direction and cored by moine psammites are termed depressions sheath folds are bisected by transport parallel and foliation normal culmination depression surfaces which separate not only the reversals in facing but also delineate zones of minor fold hinge obliquity into clockwise and anticlockwise domains relative to the transport direction the sense of obliquity of minor z and s folds is thus dependent on position with respect to the surfaces of culmination and depression and not the fold axial surfaces surfaces of culmination and depression may be superimposed on original overturned antiformal and synformal folds to produce a variety of dome culmination on antiform saddle depression on antiform inverted saddle culmination on synform and basin depression on synform configurations the curvilinear hinges of minor folds may also be asymmetrical about the transport direction and within the plane of the regional foliation to define patterns of fold hinge line vergence classical concepts of fold limb vergence may thus relate to larger antiformal and synformal hinges whilst the fold hinge line vergence defines major curvilinear hinges associated with culminations and depressions major sheath folds may therefore be interpreted in terms of both minor fold hinge line and limb vergence coupled with fold axis obliquity and reversals in the polarity of structural facing the ability to recognise consistent and reliable structural relationships between facing and hinge obliquity at small scales indicates that the regional deformation process forms a linked and coherent system through several orders of magnitude 1999 elsevier science ltd all rights reserved

a new technology for the characterization of microfractured reservoirs test case unayzah reservoir wudayhi field saudi arabia this article presents a test case of a new technology using artificially enhanced anisotropy of magnetic susceptibility referred to here as eams for the characterization of microfractured reservoirs these are reservoirs in which microfractures are essential to porosity and or permeability a conventional geological characterization is costly time consuming and difficult to quantify in terms of assessing fracture impact on porosity and permeability therefore an efficient and effective method is required to characterize these microfractures and to determine their contribution to porosity and permeability the eams technology which we developed and tested allows rapid analysis that bridges reservoir geology and engineering using petrography the margin of error to detect microfractures that impact porosity and or permeability is 43 however it requires three times the sampling rate of the new eams technology the lower part of the unayzah reservoir unayzah b c in the wudayhi field saudi arabia where fractures were studied and microfractures are known to impact reservoir performance is used to develop and verify the eams technology the results show that eams derived microfracture fabric strikes east northeast west southwest consistent with that obtained by geological means the effective porosity profile obtained from eams tests is similar to that of the conventionally acquired porosity open microfractures in tested samples increase mean values of reservoir effective porosity by 36 50 in unayzah b c the occurrence of connected microfractures is estimated to cause an increase in average permeability of 75 in unayzah b c this is in agreement with the fact that wells in microfractured unayzah b c have 4 5 14 times the productivity of wells as nonfractured sections of this reservoir a maximum permeability trend of northeast southwest permeability anisotropy is detected the implementation of the eams technology in other fields with microfractured reservoirs will directly impact operational and simulation effort

the ohio valley wave merger cyclogenesis event of 25 26 january 1978 part i multiscale case study abstract the long standing observational view of cyclogenesis involves an interaction between tropopause and surface based finite amplitude disturbances there are however instances where more than one upper level disturbance contributes to low level development a subset of these events involves wave or trough merger which has been defined as the amalgamation of two or more distinct 500 hpa vorticity maxima an example of this phenomenon involving a case of very intense continental cyclogenesis 25 26 january 1978 over the eastern united states is selected to elucidate the origin and evolution of precursor disturbances from the planetary scale to the mesoscale the analysis of this event reveals that the two well defined tropopause based disturbances that contribute to cyclogenesis have distinctly different origins and are brought together by confluent planetary scale flow one of these disturbances originates over the western north pacific ocean 10 days prior to cyclogenesis and tracks eastward

variation in the orientation of the maximum horizontal stress in thick channel fill sandstones with low permeability a case of the bonan oilfield in the bohai bay basin eastern china abstract measurements of in situ stress in a single well from borehole breakouts hydraulic fracturing multipole acoustic logs and differential strain analysis of core indicate that the orientation of the maximum horizontal stress is northwest southeast in thick channel fill sandstones in the central western bonan oilfield while the orientation is northeast southwest in the thin interbed siltstones and mudstones of the floodplain in the eastern bonan oilfield this result shows that the orientation of the maximum horizontal stress in the thin interbed of siltstones and mudstones is in accordance with the regional stress orientation obtained by the focal mechanism of a natural earthquake but has a 30 40 variation with the thick channel fill sandstones finite element simulation suggests that the main reason for the variation in the orientation of the maximum horizontal stress is the diversity of overall mechanical properties especially modulus of elasticity of the thick channel fill sandstones and the interbedded thin layer of siltstones and mudstones the average young s modulus over the stratigraphic column governs stress rotations the transformation of the orientation of the maximum horizontal stress from the central western to the eastern locations is at the channel boundary this research provides a case in which the variation in the orientation of the maximum horizontal stress is affected by thick channel fill sandstones it shows that lithological changes can cause variations in the direction of in situ stress

a technique for diagnosing three dimensional ageostrophic circulations in baroclinic disturbances on limited area domains abstract the methodology developed by keyser et al for representing and diagnosing three dimensional vertical circulations in baroclinic disturbances using a two dimensional vector streamfunction referred to as the psi vector is restricted to f plane channel model geometry the vertical circulation described by the psi vector consists of the irrotational or divergent part of the ageostrophic wind and the vertical velocity a key property of the psi vector is that its projections onto arbitrarily oriented orthogonal vertical planes yield independent vertical circulations allowing separation of a three dimensional vertical circulation into two two dimensional components and thus objective assessment of the extent to which a three dimensional vertical circulation is oriented in a preferred direction here the methodology for determining the psi vector is modified to be suitable for real data applications the modifications consists of reformulating the diagnostic equations to apply to conformal map pr

fracture detection in oil based drilling mud using a combination of borehole image and sonic logs abstract the deeply buried 5 5 7 1 km lower cretaceous bashijiqike sandstones are important fractured reservoirs in the kuqa depression in these high temperature high pressure drilling environments oil based muds are used to limit drilling risks and improve efficiency making facture detection using resistivity based image logs difficult the maximum horizontal stress sh max trends close to north south 0 10 based on orientations of the drilling induced fractures and borehole breakouts core descriptions thin sections and image logs used to describe the fractures in the bashijiqike sandstone reservoirs show that opening mode natural fractures are common and the fractures range from fully open to sealed by calcite and clay minerals the open fractures are dominantly steeply dipping or vertical and primarily strike north south locally horizontal fractures or fractures with low angle dip are detected we compare fracture visibility on image logs acquired in the same ks 2 2 12 key well in oil based mud and water based mud fmi hd logs successfully detect large 0 1 mm aperture vertical fractures in oil based mud better than obmi image logs and the two imaging tools have a similar ability in detecting the high dip angle fractures since the ubi tool is not affected by oil based muds most of the fractures can be picked out on ubi images minor negative deviations of bulk density could be observed in fractured zones and the presence of fractures obviously causes an increase in sonic transit time full waveforms from dsi tools combined with electrical image logs detect fractured zones both the velocity and amplitude of the sonic waves are sensitive to fractures and the fractured zones could be identified according to the attenuation of stoneley waves the combination of cores image logs and full wave sonic logs offers comprehensive detection of fractures in oil based mud environments

relationships between distributed and localized shear in the tectonic evolution of a caledonian fold and thrust zone northwest ireland broad zones of distributed shear operating through mid crustal regions of orogenic belts may incorporate narrow horizons of intense localized deformation culminating in discrete large magnitude displacements the relative importance and relationship between distributed and localized shear are influenced by a variety of factors including lithological variation pre existing structural anisotropy strain rate and migration of fluids rigorous structural analysis of lower amphibolite facies dalradian metasediments in northwestern ireland demonstrates that an early d 1 discrete ductile detachment was subsequently reactivated during distributed non coaxial d 2 deformation operating in a broad zone through the structural pile regional shear was directed towards the southeast and resulted in the generation and translation of kilometre scale isoclinal recumbent sheath folds which close and face towards the transport direction the d 1 detachment is clearly folded around the hinges of these major folds whilst on fold limbs it was reactivated and acted as a local decollement within the zone of distributed shear shear criteria along the detachment indicate a southeast directed translation of the major folds in sympathy with regional shear a broad zone of d 3 translation operating through the nappe pile resulted in coaxial refolding of large scale f 2 folds by the d 3 ballybofey nappe producing a complex fold interference pattern non coaxial d 3 deformation resulted in continued reactivation of local decollements together with the initiation of east southeast directed oblique thrusts and partial dismemberment of d 2 folds detailed structural investigation allows concepts of distributed and localized shear to be evaluated and models of crustal deformation to be assessed

attenuation and excision of a crustal section during extensional exhumation the carratraca massif betic cordillera southern spain extensional dismemberment of the betic cordillera during late oligocene to early miocene time caused attenuation and excision of a crustal section overlying mantle peridotite an integrated study of metapelites overlying the peridotite sheet in the carratraca area shows truncation of a typical orogenic cycle of burial and heating by rapid exhumation and late to post tectonic low pressure metamorphism attenuation by a factor of five or more condensed a greenschist to granulite section to 4 km structural thickness initially coaxial attenuation of the sequence involving formation of a flat lying foliation and conjugate shear bands was superseded by non coaxial deformation concentrated along low angle top to the ne extensional fault zones these excised parts of the metamorphic section a 2 km loss was calculated across one fault zone thermobarometry throughout this section demonstrates that the structural attenuation was related to isothermal decompression to low pressures 2 4 kbar even at the base of the crustal section a late thermal event largely postdates the ductile stage of attenuation particularly in the upper part of the section thermal modelling implies that this thermal event could not have been produced by the hot peridotite sheet alone rapid tectonic exhumation and asthenospheric upwelling are the probable causes following detachment of material from the base of the lithosphere

interaction of potential vorticity anomalies in extratropical cyclogenesis part i static piecewise inversion the relative importance of various potential vorticity pv perturbations and their mutual interactions associated with the superstorm of 12 14 march 1993 are investigated by applying a piecewise pv inversion diagnostic system to a 36 h simulation of the storm it is shown that the contributions from all pv anomalies to the surface development increase with time although their relative significance varies during the rapid deepening stage in general the upper level dry pv anomalies contribute the most to the rapid deepening of the storm followed in order by the lower level thermal anomaly and latent heat release comparing the pv anomalies and their inverted circulations reveals that there exists a favorable phase tilt between the upper and lower level anomalies that allows lower and upper level mutual interactions in which the circulations associated with the upper level pv anomalies enhance the lower level anomalies and vice versa in addition to the vertical interactions lateral interactions are also present among the upper level pv anomalies and the background flow it is also found that the background flow advection dominates the vortex vortex and vortex background flow interactions in the deepening of the storm the vortex vortex interactions of the two upper level positive pv anomalies cause the negative tilt of the main upper level trough during the rapid deepening period

the structure and evolution of gap outflow over the gulf of tehuantepec mexico abstract mesoscale model simulations are used to examine the structure and dynamics of a gap outflow event over the gulf of tehuantepec mexico that was associated with a surge of cold air along the eastern slopes of the sierra madre the simulated gap outflow winds emerged from chivela pass reached a maximum speed of 25 m s 1 and turned anticyclonically as they fanned out over the gulf northerly winds were also able to ascend the mountains east and to a lesser extent west of chivela pass indicating that the movement of cold air across the sierra madre was not confined to the pass a mesoscale pressure ridge was aligned along the axis of the gap outflow jet which was flanked to the west by an anticyclonic eddy and to the east by a weaker cyclonic eddy a model derived trajectory along the axis of the outflow jet traced an inertial path with anticyclonic curvature produced primarily by the coriolis acceleration the cross flow pressure gradient acceleration along this trajectory was negligible be

alternating contractional and extensional events in the alpujarride nappes of the alboran domain betics gibraltar arc in the western alpine system neogene extensional tectonics triggered the development of marine basins on the concave side of tight orogenic arcs as happened within the alboran crustal domain the hinterland of the gibraltar arc a detailed analysis of the structural and metamorphic records of one of the main alboran domain complexes however plainly reveals a complex tectonic evolution prior to the development of the miocene arc back arc system which includes a major intraorogenic extensional event this large scale subvertical shortening that can be assessed from the pt paths of representative tectonic units was subsequent to the continental crust subduction inferred from high pressure low temperature mineral asssemblages the crustal section was thinned in nearly isothermal conditions its thickness being reducted to at least 1 3 of the initial value yet still before the miocene a second contractional event led to the overthrusting of high grade metamorphic rocks over other low grade rocks accompanied by subordinate overturning of the metamorphic zones since migration of the gibraltar arc is roughly balanced by crustal spreading in the back arc available data concerning miocene extension suggest that the alboran domain can be restored to its appropriate position several hundred kilometers to the east thus a collision belt that underwent significant intraorogenic extension must have existed in what is now the western south balearic basin

on the relationship between kilometer scale sheath folds ductile thrusts and minor structures in the basal high pressure units of the cabo ortegal complex nw spain abstract large sheath folds in the basal high pressure nappes of the cabo ortegal complex are described and a kinematic interpretation provided the principal penetrative and map structures relate to regional d2 deformation which produced foliations s2 bearing mineral and stretching lineations l2 and several types of folds a type sheath like and folded folds the latter structures are subparallel to the trend of the orogen their attitude suggests that the units involved shared a common tectonic evolution during progressive ductile deformation of an anisotropic medium reconstruction of major geological structures was accomplished through projection of map scale features onto the ductile flow plane and the plane perpendicular to the ductile flow direction the structures reconstructed illustrate their development in the deeper structural levels of an orogenic channel subjected to high pressure metamorphism during the early phases of the hercynian orogeny in nw iberia we argue that orogen normal tectonic displacements of up to a few hundreds of kilometers represent the minor components of the transpression with possibly thousands of kilometers along strike dextral displacement between the intervening plates during subduction collision

extensional shearing deformation and kilometer scale a type folds in a cordilleran metamorphic core complex raft river mountains northwestern utah cordilleran metamorphic core complexes are characterized by ductile mylonitic deformation and regional stretching lineations in the raft river mountains a mylonitic zone and a late superimposed detachment fault separate precambrian crystalline and metasedi mentary basement rocks from an upper plate made of allochthonous paleozoic rocks which suffered a brittle extensive deformation we suggest that the mylonitic ductile deformation is the result of tertiary extensional tectonics strain analysis shows that 1 strain results from large scale progressive shearing deformation in a non coaxial regime 2 e w regional stretching lineations represent the direction of shearing and transport 3 shear criteria indicate an east directed movement 4 strain magnitude is increasing eastward 5 kilometer scale folds parallel to the lineations are interpreted as a type folds formed by eastward shearing of a previously northward dipping layering measurement of striated faults in the upper plate gives an e w direction of extension parallel to the lineations thus ductile and brittle deformations are closely related to the same regional scale extensional tectonics a model is proposed to explain the observed geometry and kinematics of the structures the mesozoic to paleocene cordilleran compressional tectonics induced an overthickening of the crust in the hinterland belt at the future emplacement of the complexes important uplift abnormal heating and granite emplacement occurred later during the collapse of the former belt the combined effects of thermal and gravitational instabilities during tertiary extensional tectonics enabled the development of a ductile shear zone localized at the precambrian basement paleozoic cover interface the brittle extension of the upper crust is displaced laterally along this flat shear zone and transformed farther east into the extended regions of the middle and lower crust

meteorological tower measurements of a surface cold front abstract measurements from the boulder atmospheric observatory meteorological research tower are used to describe the structure and physical processes of a strong surface cold front analysis reveals that the horizontal gradients in temperature and wind velocity at the front are concentrated within a 200 m distance and measured vertical velocities exceed 5 m s 1 at the leading edge of the front calculations with the miller frontogenesis equation show the magnitude and relative importance of cross frontal confluence versus differential vertical motion tilting in forcing frontogenesis

characterization and impact on reservoir quality of fractures in the cretaceous qamchuqa formation zagros folded belt abstract reservoir quality in tight fractured carbonate rocks is controlled by the fracture properties the tectonic origin of fractures the relationship of fractures to other sedimentary features within the rockmass and diagenesis in this study fracture analysis porosity and permeability of the qamchuqa formation in the miran west block of the zagros folded belt in kurdistan has been studied this was undertaken using core analysis micro resistivity image logs drill stem tests dst mud logging data repeat formation test rft drilled cutting samples and wireline log data the integrated analysis shows that the qamchuqa formation is characterized by heterogeneous sedimentary and tectonic features the sedimentary features include burial stylolites dissolution seams and some fractures which formed at early stages of burial and diagenesis phase 1 open fractures partially open mineralized fractures veins and tectonic stylolites were formed by later tectonic activity phase 2 the finding of this study highlight that the fracture properties including aperture length frequency and origin play an important role in controlling the reservoir s quality the open fractures and partially open mineralized fractures enhance permeability providing an inter connected fracture network leading to a high fracture permeability that controls fluid flow in this tight carbonate reservoir rock the fracture network had little impact on the volume of hydrocarbon storage which is controlled by the pore matrix in contrast structures including stylolites dissolution seams and veins generally have a negative impact on the reservoir quality by blocking the fluid flow this work emphasizes the importance of understanding the regional geological history and the role of sedimentary and tectonic features when investigating in tight carbonate reservoirs

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

thermal evolution rate of exhumation and tectonic significance of metamorphic rocks from the floor of the alboran extensional basin western mediterranean high grade metamorphic rocks drilled at ocean drilling program site 976 in the alboran sea show a pt path characterized by decompression from about 1050 mpa 40 km depth to 350 mpa 13 km depth accompanied by an increase in temperature from about 550 50 c to 675 25 c the ar ar dating on muscovite and apatite fission track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20 5 18 ma which coincides with the initiation of sedimentation in the alboran sea basin the alboran sea formed by miocene extension on the site of a late cretaceous to paleogene contractional orogen and extension coincided with thrusting in the peripheral parts of the betic rif arc which surrounds the basin on three sides thermal modeling of the pt path was carried out with the aim of constraining geodynamic models for the formation of the basin variables considered in the modeling included 1 the thickness and thermal gradient of the postorogenic lithosphere 2 the radiogenic heat production in the thickened crust 3 the time gap pause between the end of contractional tectonics and the start of extension 4 removal of lithospheric mantle below 125 75 or 62 5 km and 5 the rate of extension the only combinations of variables that produce modeled pt paths with the observed characteristics involve high radiogenic heat production combined with a significant postcontractional pause to produce high temperatures in rocks initially at 40 km depth removal of lithosphere below 62 5 km to produce further heating during decompression extension by a factor of 3 in 6 m y to delay the attainment of the maximum temperature until the rocks reached shallow depths and final exhumation and cooling in about 3 3 m y to satisfy radiometric and petrological constraints this gives a maximum of about 9 m y for exhumation from 40 km depth to the surface lithospheric stretching in response to plate boundary forces such as trench rollback without removal of lithosphere cannot explain the late onset of heating and the high temperatures reached by these rocks removal of lithosphere at depths significantly greater than 62 5 km cannot explain the combination of high temperatures reached by these rocks and the shallow depth at which they attained the maximum temperature only a combination of significant postcollisional radiogenic heating then wholesale removal of lithospheric mantle below the orogenic crust followed by rapid stretching can explain the observed pt path these results appear to support some form of lithospheric delamination as the primary cause for the formation of the alboran sea basin

utilization of observations of well bore failure to constrain the orientation and magnitude of crustal stresses application to continental deep sea drilling project and ocean drilling program boreholes

two severe freezes in brazil precursors and synoptic evolution abstract two freezes with repercussions on world coffee markets struck brazil in recent years one during fgge in 1979 the other in 1981 data from multiple sources including satellites and drifting buoys were analyzed for early warning signs and synoptic evolution of the freeze events two important precursors were found 1 a slowly moving long wave pattern in the central south pacific ocean amplified greatly one to two days before frontogenesis in south america and four to five days before the freezes in brazil 2 the long wave ridge when at maximum amplitude was located near the southern andes and the next long wave trough downstream was located in the south atlantic at the longitude of brazil this configuration channeled subantarctic air northward entirely over land into subtropical latitudes frontogenesis in low latitudes in the mid pacific was the first sign of wave amplification in the 1979 case subsequent frontogenesis in northern argentina was accelerated by the opportune approach of a s

intense non coaxial shear and the development of mega scale sheath folds in the arunta block central australia abstract the strangways orogenic belt of 1800 ma granulites in the central arunta block was completely reworked at granulite facies grades during the middle proterozoic 1400 1500 ma the bulk strain regime of the initial episodes of this reworking is presented ductile reworking involved non coaxial deformation on a regional scale and produced map scale sheath folds high bulk shear strains are inferred on microscopic and macroscopic scales and the spatial variation in bulk shear strain throughout the region is discussed the high bulk shear strains were accommodated by east over west ductile ramping and fold repetition resulting in easterly inclined structures crustal shortening and crustal over thickening

fracture and in situ stress patterns and impact on performance in the khuff structural prospects eastern offshore saudi arabia abstract we characterized natural fractures and in situ stresses for exploration and prospect evaluation in nine periclinal structural traps in the eastern province of saudi arabia where several major gas discoveries were made in the deeply buried permian triassic khuff formation borehole image logs oriented cores seismic gravity magnetic data and dynamic observations were used in the study two fracture systems were identified a younger major system which enhances reservoir permeability and an older minor fully mineralized system the older system consists of subordinate northerly striking extensional mesofractures including joints and faults which are fully mineralized by anhydrite and calcite this mineralization occurred during an early diagenetic phase this system acted as paleo fluid conduits facilitating the occlusion of matrix porosity and deteriorating the reservoir quality in the immediate vicinity of the fractures the younger system is regionally dominant and includes mesofractures with persistent strike ranging from ne sw to ese wnw irrespective of local structure these younger fractures are nearly parallel to the present day maximum horizontal in situ stress and perpendicular to the minimum horizontal in situ stress which are dominated by the zagros plate tectonics the development of this system commenced during the convergence of the arabian and eurasian plates late cretaceous to cenozoic and culminated during the continental collision the fractures are predominantly extension joints and hybrid extensional shear fractures and were facilitated by increases in pore pressure due to the oil placement and the subsequent cracking of this oil into gas hydrocarbon migration into the khuff reservoirs was crucial in slowing down diagenesis and preserving both fracture apertures and matrix porosity therefore most of the fractures in this system tend to be partly mineralized mainly by carbonates and or coated with hydrocarbons these fractures show channel type apertures that enhance permeability and productivity of the khuff by up to two fold the channel apertures can endure operational changes in reservoir pressure with little or no reduction of their permeability critically stressed open fractures have no major role geomechanical analyses show that they are estimated to occur under the upper limits of differential stresses within the margin of error of stress estimates the static and dynamic observations show the permeability and productivity enhancement follows mechanical layering patterns production and pressure profiles in individual wells indicate lack of vertical communication seal breach across the different reservoir units similar pre production pressure and hydrocarbons across the khuff reservoir units is the result of normalization over geological time hydrocarbon migration across the anhydrite seals happened via episodic paleoseismic pumping along faults with no sufficient vertical offset to permanently breach the reservoir seals

the 1993 superstorm cold surge frontal structure gap flow and tropical impact in the wake of the eastern united states cyclone of 12 14 march 1993 a cold surge originating over alaska and western canada brought northerlies exceeding 20 m s21 and temperature decreases up to 158c over 24 h into mexico and central america this paper addresses the multiscale aspects of the surge from the planetary scale to the mesoscale focusing on 1 the structure and evolution of the leading edge of the cold surge 2 the reasons for its extraordinary intensity and equatorward extent and 3 the impact of the surge on the tropics specifically on the strength of the trade winds and on the sea surface temperature in the eastern pacific the cold surge was initiated as a developing cyclone over the gulf of mexico and an upstream anticyclone east of the rockies caused an along barrier pressure gradient to form forcing topographically channeled northerlies along the rocky and sierra madre mountains to transport cold air equatorward on the mesoscale the leading edge of the cold surge possessed nonclassical frontal structure for example as the cold surge entered mexico the coldest air and the strongest wind arrived at about 900 hpa before affecting the surface suggestive of a tipped forward leading edge to the surge also satellite imagery and surface observations indicate that the leading edge appeared to be successively regenerated in the warm presurge air the cold surge had characteristics reminiscent of a kelvin wave a tipped forward cold front a pressure jump line a bore and a gravity current but none of these conceptual dynamical models was fully applicable associated with the cold surge gap winds up to 25 m s21 were observed in the gulfs of tehuantepec a tehuantepecer fonseca papagayo and panama owing to the strong cross mountain pressure gradient in the case of the tehuantepecer a rope cloud emanated from the isthmus of tehuantepec and turned anticyclonically consistent with an inertial oscillation on the synoptic and planetary scales the extraordinary equatorward extent of the cold surge was aided by topographic channeling similar to cold air damming by a low latitude upper tropospheric trough and by the lower branch of the secondary circulation associated with a confluent jet entrance region aloft the cold surge also impacted the tropical atmosphere and ocean by contributing to the strengthening of the northeast trade winds over the eastern pacific ocean and by inducing local cooling of the sea surface temperature in the gulfs of tehuantepec and papagayo by about 48 88c

atmospheric structure and momentum balance during a gap wind event in shelikof strait alaska abstract gap winds occur in topographically restricted channels when a component of the pressure gradient is parallel to the channel axis aircraft flight level data are used to examine atmospheric structure and momentum balance during an early spring gap wind event in shelikof strait alaska alongshore sea level pressure ridging was observed vertical cross sections show that across strait gradients of boundary layer temperature and depth accounted for the pressure distribution geostrophic adjustment of the mass field to the along strait wind component contributed to development of the observed pressure pattern boundary layer structure and force balance during this event was similar to that often observed along isolated barriers however the rossby radius was lager than the strait width and atmospheric structure in the strait exit region indicates transition of the flow to open coastline conditions two across strait momentum budgets show that the coriolis force and across strait pressure gradient w