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

distributed subglacial discharge drives significant submarine melt at a greenland tidewater glacier this is the publisher s final pdf the article is copyrighted by american geophysical union and published by john wiley sons inc it can be found at http agupubs onlinelibrary wiley com agu journal 10 1002 28issn 291944 8007

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

geochemical anomalies in the lower silurian shale gas from the sichuan basin china insights from a rayleigh type fractionation model abstract to verify recent single well data from the sichuan basin we analyzed the molecular and isotopic composition of late mature longmaxi shale gases from the southwestern portion of the fuling block the gases are highly enriched in methane 96 1 and have low c2 hydrocarbon contents

deep water depositional mechanisms and significance for unconventional hydrocarbon exploration a case study from the lower silurian longmaxi shale in the southeastern sichuan basin the purpose of this work was to study the depositional mechanisms and significance of the longmaxi shale in the sichuan basin in southern china seven lithofacies were identified based on the detailed observation of outcrops and cores using petrographic and scanning electron microscope examination of thin sections and other data analyses 1 laminated calcareous mudstone 2 laminated carbonaceous mudstone 3 laminated silty mudstone 4 laminated claystone 5 laminated siliceous shale 6 siltstone and 7 massive mudstone the laminated mudstone and laminated claystone originated from suspension deposition and siliceous shale is associated with ocean upwelling whereas massive mudstone and siltstone were primarily deposited by turbidity currents the depositional mechanisms have a great effect on the source rock and reservoir properties suspension deposition near oceanic upwelling zones can provide favorable conditions for the production and preservation of organic matter and are thus conducive to the formation of high quality source rocks total organic carbon content up to 5 4 the reservoir storage spaces are primarily interlaminated fractures and organic pores with good physical reservoir properties high porosity permeability and brittle mineral content turbidity currents may carry a large quantity of oxygen to the seafloor resulting in the oxidation of organic matter which is unfavorable for its preservation the lithofacies formed by turbidity currents have relatively low total organic carbon contents average 1 structural fractures and intergranular pores are the primary storage spaces that are present in the reservoir in summary organic rich shale and siliceous shale that was deposited from suspension near upwelling zones are key exploration targets for shale oil and gas the widely distributed multilayer tight sandstone is important in the exploration for tight oil a better understanding of the deposition mechanism and its effect on oil reservoirs may assist in identification of favorable areas for exploration

deeply incised submarine glacial valleys beneath the greenland ice sheet the bed topography beneath the greenland ice sheet controls the flow of ice and its discharge into the ocean a combination of sparse radar soundings of ice thickness and high resolution ice motion data suggest that many submarine ice covered valleys extend significantly deeper below sea level and farther inland than thought

channelized bottom melting and stability of floating ice shelves 1 the floating ice shelf in front of petermann glacier in northwest greenland experiences massive bottom melting that removes 80 of its ice before calving into the arctic ocean detailed surveys of the ice shelf reveal the presence of 1 2 km wide 200 400 m deep sub ice shelf channels aligned with the flow direction and spaced by 5 km we attribute their formation to the bottom melting of ice from warm ocean waters underneath drilling at the center of one of channel only 8 m above sea level confirms the presence of ice shelf melt water in the channel these deep incisions in ice shelf thickness imply a vulnerability to mechanical break up and climate warming of ice shelves that has not been considered previously

differential enrichment of shale gas in upper ordovician and lower silurian controlled by the plate tectonics of the middle upper yangtze south china abstract the upper ordovician wufeng formation and lower silurian longmaxi formation marine shale are the major shale gas exploration and development target in the middle upper yangtze region although three national shale gas demonstration areas have been built and several high yield shale gas wells have been discovered in the wufeng formation and longmaxi formation shale gas in these areas has not been fully commercialized because the enrichment degree of shale gas varies greatly in different tectonic settings by analyzing the formation processes and distribution characteristics of the organic rich shale reconstructing the burial history and hydrocarbon generation history and comparing the shale gas enrichment and preservation conditions in different tectonic settings in south china this paper points out that 1 the craton depression and foreland basin on the cratonic margin are favorable zones for the development of organic rich shale because the yangtze plate is small the foreland basin is greatly affected by the input of terrigenous clastic and the thickness and area of organic rich shale in the craton basin is larger than that in the foreland basin the organic rich shale is mainly distributed in south sichuan deep water shelf and west hubei east chongqing deep water shelf and the enrichment interval is the dillcellograptus complexus cystograptus vesicolus graptolite zone from the wufeng formation to the bottom of longmaxi formation are favorable intervals for shale gas enrichment 2 since the late jurassic significant differential intracontinental tectonic deformation uplift and denudation occurred in the middle upper yangtze region moderate compression and uplift were beneficial to the development of micro fractures and bedding fractures in shale resulting in good preservation conditions of shale gas reservoirs with a high pressure coefficient in contrast strongly deformed areas where faults and high angle large fractures were developed are not conducive to the preservation of shale gas reservoirs 3 the craton depression of the middle upper yangtze region where organic rich shale was deposited is favorable to shale gas enrichment because it provides a good material basis and also a relatively stable and compression resistant tectonic environment for later tectonic deformation and uplift derived destruction based on the analysis and evaluation of tectonic construction transformation conditions under the macro control of plate tectonics it can well explain the existing commercial shale gas discoveries and failure cases and can guide the optimization of future exploration targets

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

over consolidation effect on shear behavior of rock joints abstract although many researchers have studied the normal and shear behavior of fractures under stresses the over consolidation effect on the slip shear behavior of discontinuities has not been considered the over consolidation behavior of non planar rock fractures should be considered when deposition consolidation erosion or excavation sequences occur plaster replicas of representative natural rock joint surfaces were prepared for this study in this case the surface roughness and other geometrical properties remain constant during the laboratory direct shear tests it was observed that the shear strength within a large range of roughness joint wall strength and normal stress values significantly increases with increasing over consolidation ratio according to the test results a new model is developed as an extended form of barton s shear failure criterion for rock joints this model considers the effect of various paths of normal loading unloading before shearing and over consolidation ratio in a fracture a new joint over closure joc parameter is also introduced as the ratio of closure in over closed to normally closed conditions

differential deformation on two sides of qiyueshan fault along the eastern margin of sichuan basin china and its influence on shale gas preservation abstract structural deformation is one of the main factors controlling shale gas preservation in silurian longmaxi formation in the sichuan basin the exploration practices demonstrate that the shale gas preservation condition in the areas to the west of the qiyueshan fault qysf along the eastern margin of the sichuan basin is better than that to the east in this study we synthesize seismic interpretation apatite fission track aft and triaxial rock mechanics experiment to determine the structural segmentation deformation time and shale fracturability of the qysf belt seismic interpretation shows that the qysf can be divided into three segments among which the central segment is transitional deformation zone the burial depth and deformation structure difference of silurian strata on the two sides of the fault are not variable obviously and shale gas preservation conditions are good aft simulation shows that the initial deformation time of chevron syncline thrust structure to the east of the qysf was 160 120 ma while that of the chevron anticline thrust structure deformation between the qiyueshan and the huayingshan was 100 80 ma and it gradually changes from the central to the north and south rock mechanics test reveals that under the same stress field the difficulty of fracture is influenced the dip angle of the shale bedding plane shale is most likely to produce fracture when the dip angle of bedding plane 34 37 but horizontal formation is difficult to rupture this proves that gentle anticline and syncline are more conducive to shale gas preservation therefore shale gas is mainly enriched in the core of gentle anticline and synclinal combined with the analysis of typical wells we believed that the two sides of the qysf are different in deformation time uplift amplitude and later multi directional stress reworking these cause differences in the direction scale and density of fractures in the strata thus affecting shale gas preservation on two sides of the qysf

u pb geochronology of detrital zircons from a continental margin assemblage in the northern coast mountains southeastern alaska metamorphic rocks within and west of the northern coast mountains in southeastern alaska consist of an upper proterozoic to upper paleozoic continental margin assemblage that we interpret to belong to the yukon tanana terrane u pb geochronologic analyses of single detrital zircon grains from four samples of quartzite suggest that the zircons were shed from source regions containing rocks of 495 ma 750 ma 1 05 1 40 ga 1 75 2 00 ga 2 3 ga 2 5 2 7 ga and 3 0 ga multigrain fractions from two samples yield upper intercepts between 2 0 and 2 3 ga but the scarcity of single grains of similar age suggests that these fractions comprise a mixture of 2 3 ga grains zircons in these rocks generally overlap in age with i detrital zircons in metasedimentary rocks of the yukon tanana terrane in eastern alaska and yukon ii detrital zircons in strata of the cordilleran miogeocline and iii plutonic and gneissic rocks that intrude or are overlain by miogeoclinal strata in addition the p

north atlantic warming and the retreat of greenland s outlet glaciers mass loss from the greenland ice sheet quadrupled over the past two decades contributing a quarter of the observed global sea level rise increased submarine melting is thought to have triggered the retreat of greenland s outlet glaciers which is partly responsible for the ice loss however the chain of events and physical processes remain elusive recent evidence suggests that an anomalous inflow of subtropical waters driven by atmospheric changes multidecadal natural ocean variability and a long term increase in the north atlantic s upper ocean heat content since the 1950s all contributed to a warming of the subpolar north atlantic this led in conjunction with increased runoff to enhanced submarine glacier melting future climate projections raise the potential for continued increases in warming and ice mass loss with implications for sea level and climate

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

ice flow in greenland for the international polar year 2008 2009 a digital representation of ice surface velocity is essential for a variety of glaciological geologic and geophysical analyses and modeling here we present a new reference comprehensive high resolution digital mosaic of ice motion in greenland assembled from satellite radar interferometry data acquired during the international polar year 2008 to 2009 by the envisat advanced synthetic aperture radar asar the advanced land observation system alos s phase array l band sar palsar and the radarsat 1 sar that covers 99 of the ice sheet in area the best mapping performance is obtained using alos palsar data due to higher levels of temporal coherence at the l band frequency but c band frequency sar data are less affected by the ionosphere the ice motion map reveals various flow regimes ranging from patterned enhanced flow into a few large glaciers in the cold low precipitation areas of north greenland to diffuse enhanced flow into numerous narrow fast moving glaciers in the warmer high precipitation sectors of northwest and southeast greenland we find that the 100 fastest glaciers v 800m yr drain 66 of the ice sheet in area marine terminating glaciers drain 88 of greenland and basal sliding motion dominates internal deformation over more than 50 of the ice sheet this view of ice sheet motion provides significant new constraints on ice flow modeling copyright 2012 by the american geophysical union

natural fractures in shale a review and new observations natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low porosity and low permeability shale host rock many shale outcrops cores and image logs contain fractures or fracture traces and microseismic event patterns associated with hydraulic fracture stimulation have been ascribed to natural fracture reactivation here we review previous work and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization orientation and size patterns a wide range of shales have a common suite of types and configurations of fractures those at high angle to bedding faults bed parallel fractures early compacted fractures and fractures associated with concretions these fractures differ markedly in their prevalence and arrangement within each shale play however constituting different fracture stratigraphies differences that depend on interface and mechanical properties governed by depositional diagenetic and structural setting several mechanisms may act independently or in combination to cause fracture growth including differential compaction local and regional stress changes associated with tectonic events strain accommodation around large structures catagenesis and uplift fracture systems in shales are heterogeneous they can enhance or detract from producibility augment or reduce rock strength and the propensity to interact with hydraulic fracture stimulation burial history and fracture diagenesis influence fracture attributes and may provide more information for fracture prediction than is commonly appreciated the role of microfractures in production from shale is currently poorly understood yet potentially critical we identify a need for further work in this field and on the role of natural fractures generally

observations and modeling of ocean induced melt beneath petermann glacier ice shelf in northwestern greenland we update observationally based estimates of subaqueous melt qm beneath petermann glacier ice shelf pgis greenland and model its sensitivity to oceanic thermal forcing tf and subglacial runoff qsg using the massachusetts institute of technology general circulation model mitgcm in a two dimensional domain with 20 m vertical and 40 m horizontal resolution at the grounding line we adjust the drag coefficient to match the observationally based qm with the inclusion of qsg the maximum melt rate qmmax is two times larger in summer and 1 3 larger annually than in winter qmmax increases above linear with tf and below linear with qsg we estimate that qmmax increased by 24 8 1 m yr beneath pgis from the 1990s to the 2000s from a 0 21 c warming in ocean temperature and a doubling in qsg hence contributing to its thinning if the pgis is removed we estimate that the modeled melt rate near the grounding line will increase 13 16 times

dependence of century scale projections of the greenland ice sheet on its thermal regime observations show that the greenland ice sheet has been losing mass at an increasing rate over the past few decades which makes it a major contributor to sea level rise here we use a threedimensional higher order ice flow model adaptive mesh refinement and inverse methods to accurately reproduce the present day ice flow of the greenland ice sheet we investigate the effect of the ice thermal regime on 1 basal sliding inversion and 2 projections over the next 100 years we show that steady state temperatures based on present day conditions allow a reasonable representation of the thermal regime and that both basal conditions and century scale projections are weakly sensitive to small changes in the initial temperature field compared with changes in atmospheric conditions or basal sliding we conclude that although more englacial temperature measurements should be acquired to validate the models and a better estimation of geothermal heat flux is needed it is reasonable to use steady state temperature profiles for short term projections as external forcings remain the main drivers of the changes occurring in greenland 2014 publishing technology

an enthalpy formulation for glaciers and ice sheets polythermal conditions are ubiquitous among glaciers from small valley glaciers to ice sheets conventional temperature based cold ice models of such ice masses cannot account for that portion of the internal energy which is latent heat of liquid water within temperate ice so such schemes are not energy conserving when temperate ice is present temperature and liquid water fraction are however functions of a single enthalpy variable a small enthalpy change in cold ice is a change in temperature while a small enthalpy change in temperate ice is a change in liquid water fraction the unified enthalpy formulation described here models the mass and energy balance for the three dimensional ice fluid for the surface runoff layer and for the subglacial hydrology layer together in a single energy conserving theoretical framework it is implemented in the parallel ice sheet model results for the greenland ice sheet are compared with those from a cold ice scheme this paper is intended to be an accessible foundation for enthalpy formulations in glaciology

modeling ice shelf cavities in a z coordinate ocean general circulation model 1 processes at the ice shelf ocean interface and in particular in ice shelf cavities around antarctica have an observable effect on the solutions of basin scale to global coupled ice ocean models despite this these processes are not routinely represented in global ocean and climate models it is shown that a new ice shelf cavity model for z coordinate models can reproduce results from an intercomparison project of earlier approaches with vertical or isopycnic coordinates as a proof of concept ice shelves are incorporated in a 100 year global integration of a z coordinate model in this simulation glacial meltwater can be traced as far as north as 15 s the observed effects of processes in the ice shelf cavities agree with previous results from a coordinate model notably the increase in sea ice thickness however melt rates are overestimated probably because the parameterization of basal melting does not suit the low resolution of this configuration

numerical investigation of hydraulic fracture network propagation in naturally fractured shale formations abstract hydraulic fracture network hfn propagation in naturally fractured shale formations is investigated numerically using a 3d complex fracturing model based on the discrete element method to account for the plastic deformation behavior of shales the drucker prager plasticity model is incorporated into the fracturing model parametric studies are then conducted for different young s moduli horizontal differential stresses natural fracture nf properties injection rates and number and spacing of perforation clusters numerical results show that horizontal differential stress primarily determines the generation of a complex hfn the plastic deformation of shale can reduce the stimulated reservoir volume this is more obvious with young s modulus of less than 20 gpa in addition a higher injection rate could largely increase the fracture complexity index fci moreover increasing perforation cluster numbers per fracturing stage is beneficial for increasing the fci but it also increases the potential merging of neighboring fractures which may lead to non uniform development of hfn in far wellbore regions to achieve uniform development of hfn within a fracturing stage the distribution of nfs should be fully considered the results presented here may provide improved understanding of hfn generation and are favorable for optimizing fracturing treatment designs for shale formations

undercutting of marine terminating glaciers in west greenland marine terminating glaciers control most of greenland s ice discharge into the ocean but little is known about the geometry of their frontal regions here we use side looking multibeam echo sounding observations to reveal that their frontal ice cliffs are grounded deeper below sea level than previously measured and their ice faces are neither vertical nor smooth but often undercut by the ocean and rough deep glacier grounding enables contact with subsurface warm salty atlantic waters aw which melts ice at rates of meters per day we detect cavities undercutting the base of the calving faces at the sites of subglacial water sgw discharge predicted by a hydrological model the observed pattern of undercutting is consistent with numerical simulations of ice melt in which buoyant plumes of sgw transport warm aw to the ice faces glacier undercutting likely enhances iceberg calving impacting ice front stability and in turn the glacier mass balance

performance of multiple fractured horizontal wells in shale gas reservoirs with consideration of multiple mechanisms summary gas flow in shales is believed to result from a combination of several mechanisms including desorption diffusion viscous flow and the effect of stress sensitivity of reservoir permeability however little work has been done in literature to simultaneously incorporate all these mechanisms in well testing models for shale gas reservoirs this paper presents a new well testing model for multiple fractured horizontal wells mfhw in shale gas reservoirs with consideration of desorption diffusive flow viscous flow and stress sensitivity of reservoir permeability comparing with current well testing models for mfhw the model presented here takes into consideration more mechanisms controlling shale gas flow which is more in line with the actual reservoir situation laplace transformation point source function perturbation method numerical discrete method and gaussian elimination method are employed to solve the well testing model the pressure transient responses are then inverted into real time space with stehfest numerical inversion algorithm type curves are plotted and different flow regimes in shale gas reservoirs are identified the effects of relevant parameters are analyzed as well the presented model can be used to interpret pressure data more accurately for shale gas reservoirs and provide more accurate dynamic parameters which are important for efficient reservoir development

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

ice shelf basal channels in a coupled ice ocean model a numerical model for an interacting ice shelf and ocean is presented in which the iceshelf base exhibits a channelized morphology similar to that observed beneath petermann gletscher s greenland floating ice shelf channels are initiated by irregularities in the ice along the grounding line and then enlarged by ocean melting to a first approximation spatially variable basal melting seaward of the grounding line acts as a steel rule die or a stencil imparting a channelized form to the ice base as it passes by ocean circulation in the region of high melt is inertial in the along channel direction and geostrophically balanced in the transverse direction melt rates depend on the wavelength of imposed variations in ice thickness where it enters the shelf with shorter wavelengths reducing overall melting petermann gletscher s narrow basal channels may therefore act to preserve the ice shelf against excessive melting overall melting in the model increases for a warming of the subsurface water the same sensitivity holds for very slight cooling but for cooling of a few tenths of a degree a reorganization of the spatial pattern of melting leads surprisingly to catastrophic thinning of the ice shelf 12 km from the grounding line subglacial discharge of fresh water along the grounding line increases overall melting the eventual steady state depends on when discharge is initiated in the transient history of the ice showing that multiple steady states of the coupled system exist in general