martian north polar cap and circumpolar clouds 1975 1980 telescopic observations abstract international planetary patrol pictures from 1975 1980 have been used to derive partial regression curves for mars north polar cap during the 3 martian years included in the study the results confirm the existence of a surface cap during late winter and early spring which extends to at least 55 n lat and the mid spring plateau in cap recession noted by previous studies comparison of images acquired using different filters reveals that a substantial increase in circumpolar clouds accompanies the resumption of cap sublimation at the end of the plateau period

predicting fracture localization in folded strata from mechanical stratigraphy and fold shape case study of east kaibab monocline utah abstract joints within folds are often localized within zones these joint clusters may influence the flow of fluids in the subsurface but are often undetectable on conventional seismic lines joints develop in folded strata in response to fold shape and mechanical stratigraphy mechanical stratigraphy describes the distribution of layers layer thicknesses and the nature of the layer contacts within folded strata joints may localize at regions of high curvature within the outer arc of the fold or at regions of interlayer slip along fold limbs this study compares the observed fracture pattern within the navajo formation at east kaibab monocline utah with those predicted by numerical models in order to validate the modeling technique the boundary element method model simulates folding of the navajo formation fold shape and mechanical stratigraphy are estimated from field observations the numerical model matches the observed fracture pattern while the details of fracture localization will be controlled by heterogeneous interlayer material the technique presented in this study may be used to predict the location and extent of subsurface joint clusters if fold shape can be resolved from seismic lines and mechanical stratigraphy layer thickness and contact friction determined from borehole data

dependence of joint spacing on rock properties in carbonate strata joint density is studied in relation to petrographic and petrophysical parameters in two sedimentary carbonate formations characterized by different diagenetic histories the kimmeridgian limestones of the chay peninsula western france with a mean joint density of 6 37 fractures per meter fr m 1 94 fr ft and the bathonian limestones of the bouye outcrop western france with a mean joint density of 1 9 fr m 0 58 fr ft the chay carbonates are characterized by a lower caco3 content a higher average porosity and a lower sound velocity than values recorded in the bouye limestones the compressive strength and young s modulus of the bouye carbonates are respectively 10 and 3 times higher than in the chay carbonates a statistical analysis was used to identify relationships between joint density and carbonate rock properties when facies variations are marked the joint density at outcrop scale is related to the mean bed thickness the facies descriptors and the young s modulus when textural variations are more limited the joint density is controlled by the porosity at the scale of a sedimentary basin 63 8 of the variation in joint density may be accounted for by young s modulus and the sparite micrite ratio the decrease in the sparite micrite ratio reflects an increased number of grain boundaries in the carbonate rock which limits grain deformation and enhances joint density the variations in young s modulus depend essentially on the porosity and mineralogy of the studied rocks any increase in caco3 content or decrease in porosity is associated with an increase in the elastic properties of the rock and a reduction of joint density

practical approaches to identifying sealed and open fractures for one essential ingredient of permeable fracture networks degree of fracture pore space preservation in large fractures i show how the characterization challenge presented by sparse fracture sampling can be overcome by measuring a surrogate the abundance of rock mass cement that precipitated after fractures ceased opening sampling limitations are overcome because the surrogate is readily measured in small rock samples including sidewall cores and cuttings permitting site specific diagnosis of the capacity of fractures to transmit fluid over a wider range of sample depths than conventional methods allow a diverse core database shows that this surrogate correctly predicts where large fractures are sealed information on timing of fracture opening relative to cement sequence can be obtained in two ways first evidence of fracture movement history and cement sequences in sparse large fractures can be extrapolated to areas having only cement data alternately evidence of fracture timing can be acquired from sealed micrometer scale fractures distribution of porosity reducing cement is commonly heterogeneous from bed to bed and location to location in siliciclastic and carbonate rocks however because patterns of sealed or open fractures cannot be delineated using fracture observations alone surrogates have practical value for production fairway mapping and other applications in which identifying open fractures is essential this study highlights the vital interplay among structural and diagenetic processes for fracture porosity preservation or destruction

predicting rock mechanical properties of carbonates from wireline logs a case study arab d reservoir ghawar field saudi arabia abstract four hundred plug samples from the arab d carbonate reservoir ghawar field were tested for acoustic and mechanical properties at increasing triaxial stress the results show that the rock mechanical parameters are primarily functions of porosity and to a lesser degree of mineralogy texture and pore fabric in order of degree of impact from higher to lower the rock mechanical parameters of the intact matrix rock show no significant changes with stress under reservoir condition the study enabled the generation of general and layer specific correlation formulae of porosity with p wave velocity s wave velocity static and dynamic constants and the angle of internal friction the formulae were then applied to a key well in the ghawar using the compressional wave slowness from a multipole array acoustic log mac baker atlas to derive rock mechanical pseudo logs on reservoir scale referred to here as general and on individual reservoir zone scale referred to here as layer specific comparison made between the general and the layer specific pseudo logs shows good agreement for each of the elastic constants with matching peaks and troughs throughout the logs in addition the laboratory derived rock mechanical constants show a good agreement with the pseudo logs where there is some difference between the general and layer specific pseudo logs the layer specific log shows better correlation with the laboratory derived constants it can be concluded that the porosity correlation is an accurate representative and cost effective method of obtaining a rock mechanical profile of the arab d reservoir the derived formulae have been implemented as predictive tools in reservoir development and management e g hydrofracturing and underbalanced drilling and new prospect evaluation the rock mechanical layering scheme shows higher resolution in the prolific part of the arab d reservoir than the saudi aramco conventionally used zonation each of zones 2b and 3a consists of two rock mechanical layers furthermore the least prolific zones lower part of 3a the whole of 3b and 4 form one rock mechanical layer

martian north polar cap 1979 1980 a regression curve for the north polar cap of mars is extracted from the photographs obtained at kwasan observatory and at hida observatory during the 1979 1980 apparition the north polar cap was first observed at ls 26 and the latitude of the edge of the cap was about 66 n the retreat of the cap stopped until mars reached ls 57 when the cap started to shrink the edge reached a latitude around 80 n at ls 85 and then became almost stationary the roughly constant dimension of the north polar cap before mid spring appears to be a regular phenomenon but the onset time when the cap starts to shrink after the temporary stop appears to be different from year to year the measurements of the edge of the permanent cap showed an asymmetric shape in relation to the pole the cap extends farther to the south for longitudes near 0 than for those near 180 the shape of the permanent cap for ls 84 101 in 1980 agrees very well with that for ls 97 in 1972 obtained by mariner 9

structures and mechanisms associated with development of a fold in the cantabrian zone thrust belt nw spain a detailed history of progressive fold development based on mesostructural analysis is provided for a metre scale fold in the cantabrian fold and thrust belt of northwestern spain the well exposed asymmetric syncline displays a wide variety of structures including extensional faults thrusts veins tectonic stylolites bedding plane faults drag folds and a main limb thrust that transects the entire fold important constraints on fold development are provided by the spatial distribution and style of deformation angular relations and cross cutting relations among the structures fold mechanisms varied as a function of structural position while the northwest limb was subjected to pure shear shortening in the form of contractional faults the southeast limb was bent under tangential longitudinal strain resulting in a combination of outer arc extensional faulting in

regression of martian north polar cap 1990 1997 hubble space telescope observations abstract hubble space telescope imaging of mars from december 13 1990 through april 17 1997 has resulted in the synoptic monitoring of parts of four consecutive martian north polar cap regressional cycles spanningls 335 65 tols 144 56 the 1990 1993 observations were obtained with the wide field planetary camera 1 wfpc1 instrument while subsequent observations from 1994 to 1997 were obtained with the improved optics of the wide field planetary camera 2 wfpc2 mars observations were obtained from the ultraviolet 218 nm to the near infrared 1042 nm which has allowed us to constrain the effects of clouds and dust particulates on cap edge measurements hst observations of the polar cap regressions afterls 0 are fairly consistent from year to year with some variations possible beforels 0 the 1991 observations atls 35 25 andls 59 88 in the longitude range 270 280 possibly support a standstill in cap retreat at a latitude 70 n the standstill in cap retreat reported in previous years at a latitude 67 n was not observed in 1992 1997 comparison with previous ground based and spacecraft observations suggests that interannual variations may be significant they may be due to interannual atmospheric changes such as global dust storm activity lambert albedos of the north polar cap in red and violet bandpasses are similar to those reported before

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

microfractures in the middle carboniferous carbonate rocks and their control on reservoir quality in the zanaral oilfield abstract fractures are considered an important controlling factor for hydrocarbon accumulation in the pre salt carbonate reservoirs of the zanaral oilfield they are also a precondition for the high and stable production of oil and gas based on core observations and descriptions systematic sampling analysis including rock mineral composition analysis petrographic thin sections stained thin sections porosity and permeability the study about the characteristics and major controlling factors of microfractures and their influence on the reservoir seepage capacity was performed on wells 2399a and 2092 the results suggest that although there are few macrofractures microfractures are common in the study area there are several types of microfractures principally including micro tectonic fractures grain breakage fractures microstylolites and micro corrosion fractures the micro tectonic fractures especially those with relatively large apertures are mostly completely filled with calcite the formation of micro tectonic fractures is related to calcite filled macrofractures but unrelated to bitumen filled fractures the distribution of grain breakage fractures microstylolites and micro corrosion fractures differs substantially among the various wells and layers most of the grain breakage fractures are partially filled with calcite or barren and the major corrosion fractures are partially filled with calcite the non tectonic factors controlling the distribution of microfractures are complicated and variable and include facies and differential diagenesis micro tectonic fractures formation is promoted by the local enrichment of brittle minerals small grain size and strong cementation grain breakage fractures are dominant in pure limestones with coarse grains and abundant intergranular pores microstylolites mainly distribute in tight limestones with more impurities and fine grained micro corrosion fractures are associated to porosity density dolomite and acid insoluble residues contents of the limestones intermediate between those of grain breakage fractures and microstylolites dolomite and acid insoluble residues were probably concentrated by the formation of stylolites by contrast low acid insoluble residue content facilitates the formation of corrosion fractures fractures could significantly enhance the seepage capacity of the reservoirs in the study area the contribution of microfractures to reservoir seepage is given by stylolites tectonic fractures corrosion fractures and grain breakage fractures in this order high degrees of filling of the tectonic fractures greatly affect their seepage capacity

natural fractures in the spraberry formation midland basin texas the effects of mechanical stratigraphy on fracture variability and reservoir behavior horizontal cores from sandstone siltstone reservoirs in the spraberry formation midland basin west texas have documented two systems of dramatically different yet dynamically compatible natural fractures in reservoirs separated vertically by only 145 ft 44 m each system is capable of producing a different degree of the northeast trending permeability anisotropy recognized in spraberry reservoirs one fracture system consists of two vertical fracture sets with an apparent conjugate geometry striking north northeast and east northeast the other system consists of evenly spaced northeast striking vertical fractures nearly bisecting the acute angle of the first system although lithologically similar differences in quartz overgrowth and clay content in the layers resulted in a yield strength of the lower bed that is only half of that of the upper layer producing different fracture systems in the two reservoirs despite their proximity such differences in the mechanical properties due to variations in diagenetic and depositional histories of the strata are probably widespread within the formation they have the potential to cause significant vertical and lateral variation in the spraberry fracture system across the basin low present day in situ stresses in the reservoirs allow the fractures to open to become more conductive and even to propagate under very low injection pressures

integrating borehole image logs with core a method to enhance subsurface fracture characterization image logs provide critical data for characterizing fractured reservoirs however the nature of such logs coupled with borehole modified stresses creates challenges that may lead to erroneous fracture interpretations we developed a methodology to integrate image logs core and geomechanical models to obtain high confidence fracture data sets derived from image logs the methodology starts with a detailed comparison of overlapping image logs and core first to properly shift them into alignment core to log shift and second to determine which features interpreted as fractures on image logs picks can be validated as fractures in core then we derive guidelines for interpreting image logs without core control to reduce the likelihood of false fracture picks geomechanical modeling is used to assist with quality control and interpretation of image logs where drilling induced tensile fractures occur we use a fractured carbonate reservoir to demonstrate the application of this methodology comparison of image log fracture picks with core led to an average rejection rate of greater than 50 of fractures picked on the image logs these high rejection rates are mostly caused by overinterpretation of discontinuous sinusoids and misidentification of sedimentary features and induced fractures we derived rules for interpreting high quality fractures 1 only sinusoids with continuity greater than 50 should be interpreted and 2 sinusoids with dip angles less than 55 should be excluded from the interpretation application of this methodology has provided much higher quality fracture orientation and frequency data than that resulting from interpretation of borehole images alone the improved fracture data set provides a more reliable basis for testing hypotheses and deriving models of fracture distribution

the relationship between joint aperture spacing distribution vertical dimension and carbonate stratification an example from the kimmeridgian limestones of pointe du chay france joint aperture and joint development have been studied in the kimmeridgian limestones of the pointe du chay at the northern boundary of the jurassic biscay basin france at belette outcrop in some layers the mean joint spacing of the n120 joint set is close to mean layer thickness there the classical spacing to thickness relationship appears to be valid in the competent carbonate layers that are included in a more argillaceous matrix at pillar outcrop the n10 joint set is characterised by a high level of joint density and a non saturated spacing distribution as indicated by the mode mean ratio values and the cv values cv is the ratio of standard deviation to mean fracture spacing the classical relationship between layer thickness and fracture spacing has not been observed at the pillar outcrop joint aperture reaches larger values at the pillar outcrop than at the belette outcrop where aperture is more homogeneously distributed almost all the joints are opened with moderate aperture values at belette outcrop whereas most joints with large vertical dimension have large apertures at pillar outcrop and smaller fractures are closed or poorly opened from two outcrops that have been subjected to the same geological conditions apertures of non stratabound joints appear to be controlled by the vertical dimension of the joints whereas stratabound joints are more regularly spaced and opened

log derived thickness and porosity of the barnett shale fort worth basin texas implications for assessment of gas shale resources this study estimates reservoir quality and free gas storage capacity of the barnett shale in the main natural gas producing area of the fort worth basin by mapping log derived thickness porosity and porosity feet in the barnett shale the density porosity dphi log curve is a very useful tool to quantitatively assess shale gas resources and gamma ray gr and neutron porosity log curves are important factors in identifying the shale gas reservoir the key data were digital logs from 146 wells selected based on the availability of gr and density log curves log quality and good spatial distribution the barnett shale pay zone was determined on the basis of 1 dphi 5 2 high gr values commonly 90 api units 3 no significant intercalated carbonate rich beds and 4 individual pay zones being thick enough to be commercially successful for the current design of horizontal wells in the study area the barnett shale pay zone varies from about 165 ft 50 m to 420 ft 128 m in thickness h average dphi values of individual wells for the pay zone vary from 8 5 to 14 0 porosity feet maps of the pay zone show that areas of high dphi h values coincide with areas of high natural gas production indicating that log derived porosity feet maps are a good method for evaluating reservoir quality and assessing natural gas resource in the barnett shale play a limitation to this method is shown in the northwestern corner of the study area which is located in the liquids rich window with lower thermal maturity

mechanical stratigraphy and fault fold interaction absaroka thrust sheet salt river range wyoming abstract a distinct contrast in deformation style from tightly folded and faulted cambrian through ordovician strata to broadly folded and faulted mississippian and younger strata is displayed in the hanging wall of the absaroka thrust salt river range wyoming based on style the paleozoic section can be divided into two main structural lithic units each unit is characterized by a relatively weak ductile anisotropic lower section and a relatively strong brittle more isotropic upper section where the two units were stacked within the thrust sheet inverted fault propagation folds formed in the center of each unit and the overall transition upward from close to wide spaced folds and imbricate faults developed in the multilayer where the upper unit was isolated deformation was dominated by imbricate faulting with little associated folding and inverted fault propagation folds did not form observations illustrate that the mechanical interaction between the two units and boundary conditions imposed on them were significant to defining the deformation response it is note worthy that the boundary between the two structural lithic unts in part defined by the roof thrust of the stewart passive roof duplex occurs within a dolostone rather than in overlying shale of the darby formation i propose that the roof thrust and other detachments in dolostone reflect the relatively low frictional strength of dolostone under shallow crustal conditions cross cutting relations suggest that the absaroka sheet was cut by large imbricate faults during late stage movement over a major dip ramp a high deviatoric and low mean compressive stress state just beyond the upper ramp flat corner promoted break back imbrication and allowed tectonic thickening of the thrust wedge necessary to reestablish critical taper and for forward translation

a scale independent approach to fracture intensity and average spacing measurement fracture intensity the number of fractures per unit length along a sample line is an important attribute of fracture systems that can be problematic to establish in the subsurface lack of adequate constraints on fracture intensity may limit the economic exploitation of fractured reservoirs because intensity describes the abundance of fractures potentially available for fluid flow and the probability of encountering fractures in a borehole traditional methods of fracture intensity measurement are inadequate because they ignore the wide spectrum of fracture sizes found in many fracture systems and the consequent scale dependence of fracture intensity an alternative approach makes use of fracture size distributions which allow more meaningful comparisons between different locations and allow microfractures in subsurface samples to be used for fracture intensity measurement comparisons are more meaningful because sampling artifacts can be recognized and avoided and because common thresholds of fracture size can be enforced for counting in different locations additionally quantification of the fracture size distribution provides a mechanism for evaluation of uncertainties estimates of fracture intensity using this approach for two carbonate beds in the sierra madre oriental mexico illustrate how size cognizant measurements cast new light on widely accepted interpretation of geologic controls of fracture intensity

fold generated imbricates examples from the caledonides of southern norway abstract the cambro silurian sequence of the oslo region contains fold trains and closely spaced imbricate thrust faults the imbricates could be interpreted as having ramped off the sole detachment and generated fault propagation folds several imbricate thrusts however appear to die out both downwards and upwards into folds this suggests that the thrusts were generated within the cambro silurian sequence and propagated down to the sole thrust rather than propagating upwards from the sole thrust the mechanical stratigraphy is apparently responsible for this style where during folding relatively brittle limestone members faulted in the highly strained fold forelimbs prior to through going faulting developing in the shales understanding the origin of such imbricates is important when choosing a method of section restoration

compositional controls on early diagenetic pathways in fine grained sedimentary rocks implications for predicting unconventional reservoir attributes of mudstones diagenesis significantly impacts mudstone lithofacies processes operating to control diagenetic pathways in mudstones are poorly known compared to analogous processes occurring in other sedimentary rocks selected organic carbon rich mudstones from the kimmeridge clay and monterey formations have been investigated to determine how varying starting compositions influence diagenesis the sampled kimmeridge clay formation mudstones are organized into thin homogenous beds composed mainly of siliciclastic detritus with some constituents derived from water column production e g coccoliths s depleted type ii kerogen as much as 52 6 total organic carbon toc and others from diagenesis e g pyrite carbonate and kaolinite the sampled monterey formation mudstones are organized into thin beds that exhibit pelleted wavy lamination and are predominantly composed of production derived components including diatoms coccoliths and foraminifera in addition to type iis kerogen as much as 16 5 toc and apatite and silica cements during early burial of the studied kimmeridge clay formation mudstones the availability of detrital fe iii and reactive clay minerals caused carbonate and silicate buffering reactions to operate effectively and the pore waters to be fe ii rich these conditions led to pyrite iron poor carbonates and kaolinite cements precipitating preserved organic carbon being s depleted and sweet hydrocarbons being generated in contrast during the diagenesis of the sampled monterey formation mudstones sulfide oxidation coupled with opal dissolution and the reduced availability of both fe iii and reactive siliciclastic detritus meant that the pore waters were poorly buffered and locally acidic these conditions resulted in local carbonate dissolution apatite and silica cements precipitation natural kerogen sulfurization and sour hydrocarbons generation differences in mud composition at deposition significantly influence subsequent diagenesis these differences impact their source rock attributes and mechanical properties

subsurface fractures and their relationship to stress history in east texas basin sandstone abstract subsurface fractures were sampled with more than 400 m of core from the lower cretaceous travis peak formation a tabular sandstone and shale unit approximately 600 m thick that was deposited in the gradually subsiding gulf coast basin depth to the top of the formation in east texas now ranges between 1800 and 2900 m fractures in sandstone exhibit features that are due to intermittent fracturing and sealing crack seal which suggest that the fractures are the result of episodes of fluid pressure in excess of hydrostatic correlation of the vein mineral precipitation sequence with the diagenetic history and the results of previous 18 o studies of quartz cement indicates that fractures propagated at depths of between 900 and 1500 m during the migration of quartz precipitating fluids fracture permeability provides a mechanism for the passage of large fluid volumes recorded by the precipitation of extensive quartz cement in the formation fractures stopped propagating before maximum burial depth was achieved during approximately 1500 m of subsequent burial fractures either remained partly open or were sealed by postkinematic ankerite and clay minerals

albedo of the south pole on mars determined by topographic forcing of atmosphere dynamics the nature of the martian south polar cap has remained enigmatic since the first spacecraft observations in particular the presence of a perennial carbon dioxide ice cap the formation of a vast area of black slab ice known as the cryptic region and the asymmetric springtime retreat of the cap have eluded explanation here we present observations and climate modelling that indicate the south pole of mars is characterized by two distinct regional climates that are the result of dynamical forcing by the largest southern impact basins argyre and hellas the style of surface frost deposition is controlled by these regional climates in the cold and stormy conditions that exist poleward of 60 degrees s and extend 180 degrees in longitude west from the mountains of mitchel approximately 30 degrees w surface frost accumulation is dominated by precipitation in the opposite hemisphere the polar atmosphere is relatively warm and clear and frost accumulation is dominated by direct vapour deposition it is the differences in these deposition styles that determine the cap albedo

characteristics and origins of coal cleat a review abstract cleats are natural opening mode fractures in coal beds they account for most of the permeability and much of the porosity of coalbed gas reservoirs and can have a significant effect on the success of engineering procedures such as cavity stimulations because permeability and stimulation success are commonly limiting factors in gas well performance knowledge of cleat characteristics and origins is essential for successful exploration and production although the coal cleat literature spans at least 160 years mining issues have been the principal focus and quantitative data are almost exclusively limited to orientation and spacing information few data are available on apertures heights lengths connectivity and the relation of cleat formation to diagenesis characteristics that are critical to permeability moreover recent studies of cleat orientation patterns and fracture style suggest that new investigations of even these well studied parameters can yield insight into coal permeability more effective predictions of cleat patterns will come from advances in understanding cleat origins although cleat formation has been speculatively attributed to diagenetic and or tectonic processes a viable mechanical process for creating cleats has yet to be demonstrated progress in this area may come from recent developments in fracture mechanics and in coal geochemistry

stratigraphic controls on eformation patterns in fault related folds a detachment fold example from the sierra madre oriental northeast mexico abstract deformation patterns are described by the style intensity and distribution of structures throughout a volume of rock in fault related folds presently observed deformation patterns are controlled by structural factors like kinematic history environmental factors like burial history and stratigraphic factors like layer thickness and competence to demonstrate the role of stratigraphy in controlling fold related deformation patterns we document the stratigraphic variability of mesostructures at one structural position on a map scale detachment fold unique assemblages of faults fractures and cleavages in certain stratigraphic intervals define a mechanical stratigraphy comprised of several units each exhibiting a characteristic deformation response to the folding process the varying response of each mechanical unit suggests that folding mechanisms such as orthogonal flexure flexural slip or flexural flow were sequentially or coevally operative in each of the units using the simple theoretical strain distributions associated with each folding mechanism and the fact that each mechanical unit accommodates fold related strain by developing a characteristic set of mesostructures we are able to predict general deformation patterns elsewhere in the fold the significance of this approach is that it recognizes that even in a single structure fold kinematics and deformation response may be dramatically different in various parts of a stratigraphic section because of this the quality and character of reservoir rocks in anticlinal traps may change markedly over short stratigraphic distances predictions of fold related deformation based solely on fold kinematics are unlikely to resolve these variations

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

foreland basement involved structures basement involved structures commonly occur as long irregular chains of uplifts in foreland basins these structures commonly contain significant hydrocarbon accumulations with most major fields located on the broad crests of these structures the search for complex traps in deeper targets and in subthrust structures requires an improved understanding of the geometry and evolution of these structures characteristic features of basement structures include deformation zones within the sedimentary cover that dissipate significant fault slip and gently dipping frontlimbs and backlimbs fault slip in the basement is usually accommodated in the cover by a triangular widening upward deformation zone on the forelimb with the nature of deformation controlled primarily by the mechanical stratigraphy if the cover contains interlayered competent and incompetent units the incompetent units are characterized by significant penetrative deformation whereas the competent units are faulted after a relatively small amount of penetrative deformation depending on the competency contrast between the basement and cover the nature of basement and the physical conditions of deformation the deformation zone may also propagate downward into the basement gently dipping backlimb and frontlimb panels are related to movement of the hanging wall over synclinal and anticlinal bends in the major fault respectively many basement faults are characterized by a number of synclinal fault bends within the basement which result in long and gently dipping backlimbs forelimb panels are related to anticlinal bends that typically occur at the basement cover interface as well as at one or more locations in the sedimentary cover case studies of well constrained examples of structures from the bighorn and uinta basins and the central basin platform demonstrate the development of these characteristic features and their strong dependence on the mechanical stratigraphy these models and case studies will be useful in interpreting foreland basement structures in areas with poor or limited data

stratigraphic controls on vertical fracture patterns in silurian dolomite northeastern wisconsin vertical opening mode fractures are mapped on quarry walls to assess the stratigraphic controls on fracture patterns in the relatively undeformed silurian dolomite of northeastern wisconsin our two stage study uses maps of vertical fractures to assess the effectiveness of various types of stratigraphic horizons e g organic partings or cycle bounding mud horizons in terminating opening mode fractures first the mechanical stratigraphy of the exposures is interpreted from the observed fracture pattern both visual inspection and a newly developed quantitative method are employed to identify effective mechanical interfaces the two methods show similar results confirming the validity of qualitative visual inspection the second stage of our study stochastically predicts mechanical stratigraphy and subsequent fracture pattern from empirical relationships between the observed sedimentary stratigraphy and the interpreted mechanical stratigraphy for example 63 of cycle bounding mud horizons within the inner middle and middle shelf facies associations serve as mechanical interfaces these empirical percentages are input to a monte carlo analysis of 50 stochastic realizations of mechanical stratigraphy comparisons of the stochastically predicted and interpreted mechanical stratigraphy yield errors ranging from 13 to 33 this method yields far better results than assuming that all stratigraphic horizons act as mechanical interfaces the methodology presented in this article demonstrates an improved prediction of fracture pattern within relatively undeformed strata from both complete characterization of sedimentary stratigraphy and understanding mechanical controls on fracturing

martian south polar cap 1988 the regression of the martian south polar cap during the 1988 apparition is examined with the measurements of the photographs of mars obtained at the bosscha observatory at the hida observatory and at the kwasan observatory around ls 230 degree the regression of the south polar cap in 1988 appears similar to those in other years after ls 240 degree the regression of the south polar cap in 1988 seems slower than those in 1956 in 1971 in 1973 or in 1977 however in the period ls 240 degree 250 degree it seems faster than that in 1939 which is the slowest reported or than that in 1986 which is also very slow the 1988 regression seems to be one of the slowest in the period ls 240 degree 290 degree like the case of the south polar cap in 1986 the regression of the south polar cap in 1988 supports interannual difference of the regression of the south polar cap the data from 1988 also support the proposition that the size of the south polar cap around ls 250 degree becomes large in the year when global dust storms do not occur

application of acoustic full wavetrains for the determination of lithology reservoir and mechanical parameters of formation1 the significant development in acoustic full waveform logging during the last ten years has made it increasingly possible for log analysts to determine the physical properties of a rock formation in situ parallel to the methods applied to a single wavetrain during seismic processing the new techniques used for sets of wavetrains have been successfully tested with acoustic full waveforms instantaneous characteristics analysis is included in this group of methods this approach leading to qualitative and quantitative interpretation reveals the influence of small changes in physical properties on acoustic full wavetrains applications of complex acoustic waveform analysis for the determination of inhomogeneous zones are presented colour diagrams of instantaneous characteristics are used for the detection of fractured regions and slow formations with increased attenuation of acoustic waves results of the interpretation of individual acoustic full waveforms based on cross correlation and spectral analysis using the idnp and idns computer programs e g velocities of compressional waves shear and stoneley waves are presented since the bulk density of the rocks was known it was possible using the velocities of p and s waves obtained to calculate the dynamic elastic moduli we used the interpretation of acoustic full wavetrains to calculate porosity the sonic porosity is compared to the porosity obtained from other logs and to that obtained from core sample analysis the examples of acoustic full wavetrains were recorded in the miocene sulphur bearing limestones in central poland field measurements were made using the domestic prototype equipment for well log recordings in shallow boreholes

characterizing carbonate reservoir fracturing from borehole data a case study of the visean in northern belgium abstract fractured carbonate rocks are widely used as hydrocarbon or geothermal reservoirs to provide a better understanding of the distribution and characteristics of such fractures a general workflow will be presented for the characterization of fracturing from borehole data by exploration and statistical analyses of integrated datasets in a case study of visean limestones in a belgian borehole both cores and geophysical well logs were used to investigate which factors control the characteristics of partially open veins which contribute to permeability relationships between multiple variables were tested statistically lithology geochemistry and geophysical well log values were taken into account as well as quantified veins vugs and stylolites from cores although natural joint frequencies appeared hard to quantify from the available data partially open vein characteristics could be well quantified the results show that differential compaction controlled the development of fractures fracturing occurred preferentially in massive reefal buildup boundstones in contrast to layered wacke to grainstones layer parallel slip along bedding surfaces could also have reduced fracture development in the latter limestones frequencies of cemented veins and partially open veins are positively correlated which suggests that the partially open veins result from either re opening by dissolution or re opening due to a later fracturation phase in summary this multi source study provides a workflow for fracture characterization from boreholes as well as insights into the factors controlling the distribution and characteristics of partially open veins which enhance reservoir permeability

quantitative prediction of fractures using the finite element method a case study of the lower silurian longmaxi formation in northern guizhou south china abstract natural fractures have long been considered important factors in the production of gas from shale reservoirs because they can connect pore spaces and enlarge transport channels thereby influencing the migration accumulation and preservation of shale gas industrial level shale gas production has been initiated in the lower silurian longmaxi formation in northern guizhou south china however it is important to quantitatively predict the distribution of natural fractures in the lower silurian shale reservoirs to locate additional sweet spots in northern guizhou in this study data obtained from outcrops cores thin sections field emission scanning electron microscope fe sem images and x ray diffraction xrd were used to determine the developmental characteristics and controlling factors of these fractures correlation analysis indicated that the mechanical parameters of the longmaxi shale are mainly related to the total organic carbon toc quartz clay calcite and dolomite contents the spatial variations in the mechanical parameters of the longmaxi shale were determined based on the spatial variations in the toc and mineral contents then a heterogeneous geomechanical model of the study area was established based on interpretations of the fault systems derived from seismic data and acoustic emission ae experiments performed on samples of the relevant rocks the paleotectonic stress fields during the yanshanian period were obtained using the finite element method fem finally a fracture density calculation model was established to analyze the quantitative development of fractures and the effects of faults and mechanical parameters on the development of fractures were determined the results suggest that the main developmental period of tectonic fractures in the longmaxi formation was the early yanshanian period during this time the horizontal principal stress conditions were dominated by a se nw trending 135 315 compressional stress field and the longmaxi formation experienced a maximum tectonic stress of 110 120 mpa this simulated paleotectonic stress field was mainly controlled by faults and the contents of toc quartz clay calcite and dolomite at different positions along the same fault the degree of fracture development varies significantly overall the distribution of fractures in the longmaxi formation can be used to optimize well deployment and provides a basis for the future exploration of shale gas

non linear growth kinematics of opening mode fractures abstract fracture aperture is commonly assumed to be a linear function of fracture length stress and elastic material properties under constant stress fracture opening displacement may change without concurrent length or height growth if the material effectively weakens after initial linear elastic fracture growth by changes in elastic properties or by non elastic deformation processes to investigate the kinematics of fracture opening and its dependence on length and height growth we reconstructed the opening history of three opening mode fractures that are bridged by crack seal quartz cement similar crack seal cement bridges had been interpreted to form by repeated incremental fracture opening and subsequent precipitation of quartz cement using scanning electron microscope cathodoluminescence imaging we determined the thickness and number of crack seal cement layers as a function of position along fracture length and height observed trends in crack seal cement layer thickness and number of fracture opening increments are consistent with non linear fracture growth kinematics consisting of an initial stage of fast fracture propagation relative to aperture growth followed by a stage of slow propagation and pronounced aperture growth consistent with earlier fluid inclusion observations indicating fracture opening and propagation occurring over 40 50 m y we interpret the second stage of pronounced aperture growth and slow propagation to result from fracture opening strain accommodated by solution precipitation creep and concurrent slow possibly subcritical fracture propagation

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

groundwater derived land subsidence in the north china plain land subsidence usually bears strong relationship to abstraction of underground fluid land subsidence occurs commonly in the north china plain ncp and has become a major environmental factor hindering regional sustainable development this paper focuses on issues associated with mechanism of land subsidence in the ncp the analysis shows that multi layer aquifer systems with deep confined aquifers and the thick normally consolidated or unconsolidated compressible clay layers are the key of geological and hydrogeological conditions favorable for the development of land subsidence in the ncp groundwater withdrawal results in an increase in the distribution of effective stress within the strata and the compression of the aquifers and the confining layers and then triggers land subsidence in the middle east plain of the ncp the land subsidence volume approximately represents the amount of water released from compression of deep aquifers and aquitards in that land subsidence is primarily caused by excessive groundwater withdrawal in deep aquifer system the percentage of water released from compression of aquifers and aquitards in deep groundwater abstraction is significant but distinct in cangzhou city and in the whole middle east plain this is due to the difference in local lithological structure and recharge and discharge conditions of deep groundwater system the hysteresis of land subsidence is also discussed in typical areas and the results reveal that the time for completing the primary consolidation ranges from less than one year to tens of years and that the rate of secondary compression tends to increase with the moisture content

multi scale fracture network analysis from an outcrop analogue a case study from the cambro ordovician clastic succession in petra jordan abstract a regional multi scale fracture network is prominently exposed in the cambro ordovician sedimentary succession in south western jordan this succession consists of marine deposits overlain by heterogeneous deltaic sediments culminating in 500 m of massive to cross bedded fluvial sandstones a data set acquired from high resolution satellite imagery and field measurements enables the quantification of the geometry and spatial characteristics of the fracture network in three dimensions five directional fracture sets are identified from the satellite data with mean strikes of 000 040 065 120 and 145 large fracture zones longer than 600 m composed of tightly spaced joints confined in fracture swarms exhibit a log normal trace length distribution and the fracture swarms are regularly spaced the trace length distribution of intermediate fractures 40 600 m long exhibits power law characteristics and the fractures are clustered reflecting different fracture system styles and stratigraphic controls as a function of scale the small scale fracture network is strongly dependent on the distribution of bedding plane discontinuities throughout different parts of the succession the multi scale fracture network is categorized in a three tier hierarchical fracture model that reflects underlying stratigraphic controls in the study area this type of model can be used to improve the representation of fracture networks in fluid flow simulations of fractured reservoirs with hybrid models that combine elements of discrete fracture networks and continuum models

lithological control on fracture cementation in the keuper marl triassic north somerset uk the spatial arrangement of gypsum veins as preserved natural hydraulic fractures have been characterized in the triassic keuper marl formation uk a caprock for hydrocarbon reservoirs and co 2 sequestration the marls cropping out are subdivided into five discrete fracture units based on the presence and abundance of gypsum veins the nodular gypsum in evaporite horizons provides excess gypsum for nodule rooted horizontal gypsum veins our petrographic observations demonstrate that the development of gypsum veins in beds lacking macroscopic evaporites is closely associated with disseminated gypsum cement in the marls we interpret that the gypsum veins in marl are sourced from disseminated gypsum cements in the host rocks based on stratigraphic correlations and much lower sr concentrations than gypsum nodules gypsum was transported to adjacent veins mainly through diffusion in the low permeability marls the localization of gypsum veins and varied sr concentrations of veins and nodules indicate that the diagenetic fluids are a mix of connate water with meteoric water rather than brines transported from evaporite beds along faults to non evaporite beds this results in the absence of gypsum fillings in fractures in rocks without primary gypsum cements the study implies that the cementation of natural fractures in low permeability rocks can highly depend on the presence of cement minerals in the host rock

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

arcuate thrust systems in sandbox experiments a comparison to the external arcs of the western alps in the external western alps two regional structural arcs were generated during tertiary northwest directed collision between the apulian indenter and the european passive margin these arcs distinguished by their geographic position and their age are examined by using a new compilation of structural data and comparing these to the results of sandbox analogue experiments the principal western alpine arc comprises two orthogonal synchronous thrust systems in the late eocene early miocene major shortening 105 km was toward the northwest to west northwest and minor shortening 11 km was toward the southwest shortening in each branch decreased toward the core of the arc during the late miocene and pliocene the jura arc accommodated 35 km of northwest directed shortening while 10 5 km of southwest to south southwest directed shortening was accommodated on the digne thrust system sandbox experiments were used to investigate the role of the motion vector of a rigid rectangular indenter orthogonal diagonal curved or rotational paths and the mechanical stratigraphy of the foreland in the evolution of upper crustal arcuate systems e g presence of a basal easy slip silicone horizon comparison of experimental results with the external alpine arc suggests that the indenter followed a slightly diagonal path with respect to the european margin from the eocene to the early miocene and curved counterclockwise by 10 15 in the middle miocene mechanical stratigraphy experiments support the hypothesis that thick triassic evaporites played a primary role in the evolution of the jura arc the influence of mechanical stratigraphy was most prominent during weak deformation at the external boundaries of the alpine orogen jura fold and thrust belt digne thrust

the structure of the helvetic nappes and its relation to the mechanical stratigraphy abstract this paper discusses structure and variations in structural style due to changes in mechanical stratigraphy of the helvetic nappes based on five cross sections the ratio n of the thickness of mechanically weak to the thickness of adjacent mechanically strong layers is of particular importance a low value of n favours imbricate thrusting and harmonic folding a high value favours detachment and disharmonic folding the transition at around n 0 5 is characterized by polyharmonic folding stratigraphic thickness variations due to local basin subsidence resulted in arcuate fold hinges and important plunges upon basin inversion lateral variations in the overall structure of the helvetic nappes is largely controlled by the initial irregular arrangements of the various small basins one more or less coherent thrust sheet the cretaceous stockwerk of the wildhorn drusberg santis nappe can be traced along the entire fold and thrust belt

the tectonic development of albania explanation and prediction of structural styles abstract this paper summarises the results of a study of the regional geological development of albania carried out with the objective to develop a better understanding of the complicated geometry and kinematics within the framework of the tectonic development the available geological information has been integrated into a model that is consistent with all the data the data included the geological map of albania well data interpreted satellite imagery existing and new shell seismic offshore and onshore published paleomagnetic data and radiometric ages and own field work the model offers a geomechanical explanation for the tectonic style and kinematics of the albanian fold and thrust belt this includes an explanation for out of sequence thrusting as a function of the mechanical stratigraphy brittle limestones and viscous salt and the structural style in the peri adriatic depression the timing of the tectonic events is consistent with plate tectonic movements as modelled for the eastern mediterranean the result of the study demonstrates the value of regional geology for a better understanding of the local geology and the importance of an integrated approach

diagenesis in porosity evolution of opening mode fractures middle triassic to lower jurassic la boca formation ne mexico abstract opening mode fractures joints in middle triassic to lower jurassic la boca formation sandstones northeastern mexico have patterns of fracture porosity mineral fill structures and size distributions not previously described from outcrop patterns match those found in cores from many basins we used aperture measurements along lines of observation scanlines fracture trace maps petrography high resolution scanning electron microscope sem based cathodoluminescence and fluid inclusions to characterize fracture populations open fractures are lined by quartz that precipitated while fractures were opening whereas sealed fractures additionally contain calcite deposited after fractures ceased opening large fractures and arrays of contemporaneous microfractures have consistent power law aperture size scaling over approximately three orders of magnitude our results imply that open fractures and fracture sizes depend on diagenetic state the interplay of fracture mechanics and diagenetic history is a determinant on effective porosity within fractures and thus open fracture persistence connectivity and fluid flow

structural lithic units in external orogenic zones abstract changes in types of thrust fault structures are typical of external orogenic zones and are inferred here to be related to thickness and facies changes in stratigraphie sequences obvious deformation style changes occur from external to intermediate thrust sheets similar changes in style are observed and most easily interpreted along strike within individual thrust sheets to correlate stratigraphic and structural changes stratigraphic sections are divided into structural lithic units the subdivisions are based on the dominant deformation styles vertically in individual cross sections the major variables in determining deformation style are controlling stratigraphic members within individual stratigraphic structural units and the stacking sequence of lithologie types into different structural units in many areas the critical taper of one major dominant member regionally controls where structural style changes will occur areal change in either internal composition or in stacking sequence causes lateral change in deformation style lithological changes in structural units are coincident with changes in frontal thrust ramp positions connected by major lateral ramps and with changes in thrust zone geometries imbricate thrusts in one area versus a duplex zone along strike they are also associated with changes in thrust shapes from listric to stepped and changes from fault bend fold to fault propagation fold structures controlling members may be either strong or weak fold dominated regions and thrust faulted regions without major shales or evaporites are usually controlled by strong controlling members weak controlling members are major evaporite horizons or thick shales a thorough understanding of stratigraphic facies realms therefore may allow predictions of types and changes in structural styles it can also be very useful in the interpretation of less well exposed thrust terranes

toward a quantitative definition of mechanical units new techniques and results from an outcropping deep water turbidite succession tanqua karoo basin south africa the physical properties of reservoirs are strongly influenced by distributed fracture fields outcrop studies are commonly used to determine them but have provided unsatisfactory results because the definition of mechanical units i e groups of layers displaying homogeneous fracture patterns is difficult and typically conducted in a qualitative manner we have developed a systematic methodology to acquire and process fracture patterns in outcrops and to define their relation with stratigraphy we dedicate particular attention to the vertical distribution of fractures in a sedimentary succession commonly composed of layers of different thicknesses and compositions the method makes full use of geographic information system technologies and allows for direct digital acquisition in the field leading to time efficient acquisition data are processed with a newly developed routine that permits an objective description of the changes of fracture characteristics along the stratigraphy of the outcrop the operator is then able to define the most suitable fracture stratigraphy the integration of results from different outcrops is thought to provide a tool for predicting fracture distributions in subsurface target areas

hoods mists frosts and ice caps at the poles of mars the sequence of seasonal events that occurs each martian year around the two poles of mars is more precisely described due to the input of new telescopic surveys and polarimetric measurements combined with available orbital photographs viking lander images and atmospheric pressure recordings the interplays between vaporization and condensation of both carbon dioxide and water products seasonally at work around both poles produce clouds and mists in the atmosphere and frosts and ices at the surface the succession of occurrences of prepolar hood frost depositions polar hood formations and evolutions and surface cap depositions and vaporizations is described for both poles as a function of the seasonal evolution parameter ls planetocentric longitude of the sun

evolving mechanical stratigraphy during detachment folding abstract the combination of a complex lithostratigraphy and shortening can lead to a mechanical stratigraphy that changes character and mode of deformation during detachment folding in the detachment folded carbonates of the carboniferous lisburne group of northern alaska a complex lithostragraphy led to a mechanical stratigraphy that evolved during folding with mechanical unit thickness decreasing as deformation progressed and the fold tightened newly forming mechanical boundaries changed the dominant deformation behavior and subsequent geometry during folding of a given horizon during early folding flexural slip dominated in units comprised of thick bedded competent lithologies while flexural flow dominated in units composed of thinner bedded incompetent lithologies after fold lock up homogeneous flattening dominated in the less competent mechanical units the final result was a fold with a complex geometry and a suite of fractures and other structures indicative of both flexural slip flexural flow folding and homogeneous flattening

primary migration by oil generation microfracturing in low permeability source rocks application to the austin chalk texas fracturing of low permeability source rocks is induced by pore pressure changes caused by the conversion of organic matter to less dense fluids oil and gas these fractures increase the permeability and provide pathways for hydrocarbon migration an equation for the pressure change is derived using four major assumptions 1 the permeability of the source rock is negligibly small 0 01 d 10 20 m2 so that the pore pressure buildup by the conversion is much faster than its dissipation by pore fluid flow 2 the stress state is isotropic so that horizontal and vertical stresses are equal the source rock fails when the pore pressure equals the overburden pressure 3 the properties of the rock organic matter and fluids remain constant during oil generation this assumption is valid when the change in depth i e pressure and temperature is small 4 only two reaction rates are required for the conversions a low temperature reaction rate for the kerogen oil conversion e approx 24 kcal mol a approx 1014 m y and a high temperature reaction rate for oil gas conversion e approx 52 kcal mol a approx 5 5 1026 m y the equations for generation rate and pressure change are applied to the austin source rock by adjusting the several variables to fit geochemical data core saturations and observed levels of oil and gas production this application demonstrates that the equations are easily applied in calculating depths of primary migration for low permeability source rocks

the hemispherical asymmetry in the martian polar caps an energy balance model is used to study the behavior of co2 ice on mars the effect of the solar zenith angle dependence of albedo is to lengthen co2 ice lifetimes at the poles hemispherical asymmetries in cloud and dust abundance could result in the survival of seasonal co2 ice through summer in the south and not in the north in agreement with observations co2 ice observed in the summertime polar cap in the south could be of recent origin although a permanent co2 polar cap cannot be ruled out

combining high resolution vertical gradients and sequence stratigraphy to delineate hydrogeologic units for a contaminated sedimentary rock aquifer system summary hydrogeologic units hgus representing subsurface contrasts in hydraulic conductivity form the basis for all conceptual and numerical models of groundwater flow however conventionally delineation of these units relies heavily on data sets indirect with respect to hydraulic properties here we use the spatial and temporal characteristics of the vertical component of hydraulic gradient i e vertical gradient as the primary line of evidence for delineating hgus for cambrian ordovician sedimentary rocks at a site in dane county wisconsin the site includes a 16 km2 area encompassing a 3 km long mixed organic contaminants plume the vertical gradients are derived from hydraulic head profiles obtained using high resolution westbay multilevel systems installed at 7 locations along two orthogonal 4 km long cross sections and monitoring to depths between 90 and 146 m with an average of 3 4 monitoring zones per 10 m these vertical gradient cross sections reveal 11 laterally extensive hgus with contrasting vertical hydraulic conductivity kv the position and thickness of the kv contrasts are consistently associated with sequence stratigraphic features maximum flooding intervals and sequence boundaries distinguished at the site using cores and borehole geophysical logs the same sequence stratigraphic features are also traceable across much of the cambrian ordovician aquifer system of the midwest us the vertical gradients and sequence stratigraphy were arrived at independently and when combined provide a hydraulically calibrated sequence stratigraphic framework for the site this framework provides increased confidence in the precise delineation and description of the nature of hgu contacts in each borehole reduced uncertainty in interpolation of the hgus between boreholes and some capability to predict hgu boundaries and thickness in offsite areas where high resolution hydraulic data sets are not available consequently this hgu conceptual model will serve as a better basis for numerical simulations of groundwater flow and contaminant transport needed for continued risk assessment and to evaluate potential remedial technologies for the site more broadly the association between kv contrasts and sequence stratigraphy demonstrated at this site provides valuable insight into the relationship between geology and hydraulic contrasts that can be transferred to other areas of the upper midwest us and perhaps elsewhere to strata deposited in similar settings

mechanical stratigraphic controls on fracture patterns within carbonates and implications for groundwater flow abstract groundwater flow in low matrix permeability carbonate rocks is largely controlled by fracture networks the stratigraphic features that control fracture initiation and termination within a sequence of sedimentary rock strata define the mechanical stratigraphy of the sequence we investigate the effectiveness of various types of stratigraphic horizons in terminating opening mode fractures in two different carbonate rock sequences a relatively homogeneous dolomite sequence in door county wi and an interbedded chalk and marl sequence within the austin chalk tx additionally we present analog and numerical modeling results that delineate the specific mechanisms that facilitate fracture termination the combination of model results and empirical relationships between observed sedimentary features and mechanical stratigraphy shows 1 fractures terminate at weak contacts e g thin organic layers shallowly buried contacts or thick fine grained units adjacent to thin fractured beds 2 fractures propagate across strong contacts e g intracycle contacts between different lithology and thin fine grained units adjacent to thick fractured beds and 3 fractures step over at moderate strength contacts we use these guidelines to predict fracture network from sedimentary stratigraphy by qualitatively assessing the mechanical stratigraphy of a portion of the relatively complex cretaceous shelf margin sequence at sant corneli spain this predictive demonstration illustrates the utility of assessing the mechanical stratigraphy of subsurface strata within which fractures are not directly observable we conclude that for a variety of carbonate mechanical stratigraphic sequences dominant fluid flow characteristics such as horizontal high flow zones and flow compartmentalization can be evaluated using fracture spacing and connectivity within fracture networks that is predicted from sedimentary stratigraphy although the resulting heterogeneous flow networks do not rely on every fracture present they are highly dependent on the mechanical stratigraphy

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

controls on fracture distribution in cretaceous sedimentary rocks from the isfahan region iran in this study relationships between fracture patterns lithology thickness diagenetic processes and grain size are evaluated within cretaceous sediments in two sections of dizlu and kolah ghazi of isfahan this study area was selected based on its outcrops of different rock units and its well developed tectonic fractures the fracture patterns within stratigraphic units of these sections are studied using geometrical and statistical analyses this study finds that variable fracture spacing and fracture spacing ratios can be affected by lithology thickness grain size of sediments and diagenetic processes a study of fracture stratigraphy based on fracture pattern evaluation within different cropped out sedimentary rocks can be used to improve understanding of the same types of sedimentary rock units below the surface or throughout other sedimentary basins consequently this could improve information regarding storage and fluid flow pattern throughout sedimentary rocks in different regions even for subsurface purposes

mariner 9 observations of the south polar cap of mars evidence for residual co2 frost the first spacecraft observations of the south residual polar cap of mars were obtained by the mariner 9 orbiter during the martian southern summer season 1971 1972 analyses of viking orbiter observations obtained 3 mars years later have shown that residual carbon dioxide frost was present at the south polar cap in 1977 in this study mariner 9 infrared interferometer spectrometer spectra and television camera images are used in conjunction with multispectral thermal emission models to constrain the temperatures of dark bare ground and bright frost regions within the south residual cap the results provide strong evidence that carbon dioxide frost was present throughout the summer season despite the fact that the residual frost deposits observed by mariner 9 were less extensive than those observed by viking

influence of mechanical stratigraphy on folding vs faulting abstract the competing processes of folding and faulting during shortening of a layered sequence are investigated using elastic plastic models in order to determine the influence of mechanical stratigraphy on the development of fold belts vs thrust belts the models consist of a strong middle layer and weaker decollement and cover layers the mechanical behaviour of each layer is determined by its plastic hardening modulus h and its elastic shear modulus g with bonded contacts fold amplification at low shortening strain relatively large h is greatest for a thin cover layer and a thick decollement layer at higher shortening strain h g the middle layer may reach its peak strength before the other layers causing it to become relatively weaker in shortening and causing fold amplification to be greatest for a thick middle layer free slip contacts between the layers enhance folding and fold amplification is greatest if the slip surfaces are roughly equally distributed throughout the sequence a relatively weak decollement layer enhances folding consistent with the fact that fold belts usually include a salt decollement layer dominant wavelengths increase with increasing strength contrasts between the layers and with increasing thicknesses of the decollement and strong layers

a method to detect natural fracture strike in sandstones in siliciclastic hydrocarbon reservoir rocks economic gas and oil production may depend on the attributes of natural fractures and with the advent of horizontal drilling fractures are increasingly exploration and development targets yet reliable information on such key fracture attributes as orientation strike is sparse because few fractures intersect vertical well bores this paper describes how fracture strike can be documented on a bed by bed basis even in well bores where few or no visible fractures are directly sampled quartz lined opening mode microfractures lengths of microns to millimeters in quartz cemented sandstones commonly are not visible using standard petrographic methods but systematic mapping of these microfractures is possible using photomultiplier based electron beam induced luminescence scanned cathodoluminescence imaging as shown by observations primarily from three natural gas plays and one oil play in the united states microfracture strike is a good guide to the strike of large fractures macrofractures that formed concurrently because microfractures are widespread and small specimens can be used to get accurate fracture strike data this approach can be applied to samples obtained from wireline conveyed rotary drilled sidewall coring devices as well as to samples from full diameter core

modeling fracture porosity evolution in dolostone abstract opening mode fractures in dolostones buried to depths of 1 5 km contain synkinematic dolomite cement the amount and internal structure of which has a systematic relationship to fracture size narrow fractures we developed a geometric crystal growth model for synkinematic dolomite fracture fill in fractured dolostones where periodic incremental fracture opening events are introduced with concurrent cement growth we assumed constant temperature and supersaturation with respect to dolomite a key assumption in the model is that rapid dolomite accumulation within bridges is governed by high cement growth rates on repeatedly broken grain surfaces during the process of crack seal slower cement growth rates occur on euhedral crystals this assumption is made on the basis of a comparison with quartz cement growth in fractured sandstones simulations with different fracture opening rates mimic bridge and lining cement morphologies including characteristic rhombic shapes of dolomite bridges

the mechanical contrast between layers controls fracture containment in layered rocks abstract open natural fractures allow fluids to flow which is necessary for the production of low permeable geothermal and petroleum reservoirs these reservoirs often consist of lithological layers with significant variation in rock strength which makes it difficult to predict fracture containment within the rock succession in this paper fractures are classified as contained when they do not cross the layer interface of the adjacent layer so that formation and growth are inhibited in the adjacent layer this study concerns the impact of the differences in rock strength i e mechanical contrast between two adjacent brittle layers on the fracture containment in finely layered rocks laboratory deformation tests and 2d finite element modelling were performed on three layered samples with varying mechanical contrasts to examine the behaviour of fractures at different stress conditions fractures initiate as shear fractures in weak layers and propagate with a steeper angle tensile fracture through the adjacent stronger layer the mechanical contrast within a layered rock does not always act as a containment barrier meaning that fractures do not cross the layer interface of the adjacent layer this is due to differences in differential stress between the weakest layer and strongest layer the mechanical contrast combined with the magnitude of the confining pressure has a significant influence on fracture containment an increase in mechanical contrast and confining pressure prevents fractures from propagating into stronger layers the results contribute to predict natural fracture containment in brittle sequences at shallow depth in the subsurface

fracture density in the deep subsurface techniques with application to point arguello oil field because monterey formation reservoirs rely on fractures joints for permeability quantitative information on fracture spacing is important to exploration strategies and for understanding reservoir behavior density of joints in cores of four wells from the point arguello reservoir has been determined with a new probability based method and these subsurface joint densities are compared with joint densities measured at nearby outcrops my measure of joint density is the fracture spacing index which is the slope of the trend of layer thickness to joint spacing a single set of extension joints that strike parallel to bedding dip predominates at outcrops likewise the cores contain a single set of extension joints perpendicular to the anticline axis and parallel to present day maximum horizontal compressive stress in core and at outcrop the only lithologic control on joint density is between nonjointed mudstone and harder more brittle jointed rocks within each well the fracture spacing index is the same for all hard rocks though it varies between wells in the reservoir joint density relates to structural position the most densely jointed strata fracture spacing index 0 45 are from the hinge of a minor anticline where its plunge steepens a less steeply plunging anticlinal nose has lower joint density fracture spacing index 0 30 the lowest joint density fracture spacing index 0 095 and 0 12 is in cores from gently dipping areas at outcrops in various structural settings the fracture spacing index is the same approximately 1 29 in chert dolostone and porcelanite and siliceous shale these rocks may be saturated with joints so that differences in brittle strain due to local structural variations have been overwhelmed as joints continued to form during unroofing of these strata chert looks more fractured than other lithologies because of thin bedding fracture spacing index is used to compute such parameters as fracture porosity and volume of fractures that directly contact the well bore these parameters may be important when trying to model the behavior of a petroleum reservoir or when trying to assess the feasibility of strongly deviating wells to improve the performance of a fracture reservoir

complex rotation of maximum horizontal stress in the wufeng longmaxi shale on the eastern margin of the sichuan basin china implications for predicting natural fractures abstract the in situ stress state of the eastern margin of the sichuan basin where there is a complicated fault zone is poorly understood this study offers vital insights into the in situ stress state based on information from 17 wellbores in the wufeng longmaxi shale near the eastern boundary fault i e the qiyueshan fault of the sichuan basin china drilling induced tensile fractures and borehole breakouts revealed a decrease in the angle between the shmax maximum horizontal stress orientation and fault strike when the wellbores are close to the damage zone of the qiyueshan fault this stress rotation may be caused by the lateral contrast in the poisson s ratio between the country rock and damage zone of the qiyueshan fault a high angle between the shmax orientation and fault strike also appeared in the individual fault blocks of the qiyueshan fault as a result of local stress concentration the higher magnitudes of horizontal stresses indicate stronger lateral compression in the qiyueshan fault zone owing to the lateral compression of a sandwich consisting of a high e young s modulus roof and low e shale intervals natural fractures mainly appeared in the low e shale intervals when the wellbore was close to the fault damage zone the low fracture intensity in the low e shale intervals of the wellbores located at the boundaries of the stress concentration blocks may be associated with an extensional stepover process

effect of pore structure on the producibility of tight gas sandstones the estimated ultimate recovery eur is one of the most significant properties of tight gas sandstone reservoirs but it remains difficult to predict estimated ultimate recovery is dependent on the success of stimulation by hydraulic fracturing the existence and connectivity of natural fractures and as illustrated in this article the pore structure of the matrix here we analyze the lab measurements that are indicative of the pore structure and then we predict the effect of pore structure on producibility we develop a relationship between the eur of tight gas sandstones and their petrophysical properties measured by drainage and imbibition tests mercury intrusion withdrawal and porous plate and by resistivity analyses we use the ratio of residual mercury saturation after mercury withdrawal s gr to initial mercury saturation s gi which is the saturation at the start of withdrawal as a measure of gas likely to be trapped in the matrix during production and hence a proxy for eur a multitype pore space model is required to explain mercury intrusion capillary pressures in these rocks implications of this model are supported by other available laboratory measurements the model comprises a conventional network model and a treelike pore structure an acyclic network that mimic the intergranular and intragranular void spaces respectively the notion of the treelike pore structure is introduced here for the first time in the context of tight gas sandstones applying the multitype model to porous plate data we classify the pore spaces of rocks into intergranular dominant intermediate and intragranular dominant this pore space classification is topological and is not based on scale or size these classes have progressively less drainage and imbibition hysteresis which leads to the prediction that significantly more hydrocarbon is recoverable from intragranular porosity than intergranular porosity available field data production logs corroborate the higher producibility of intervals with intragranular porosity although the data are not sufficient to eliminate the possible contribution of other factors such as size and shape of the volume contacted by hydraulic fractures or the presence and attributes of natural fractures the superior recovery of hydrocarbon from intragranular dominant pore structures is despite its inferior initial production rate

martian south polar cap boundary 1971 and 1973 data abstract international planetary patrol photographs of mars obtained during the 1971 and 1973 apparitions have been used to produce regression curves for the martian south polar cap for those years useful data span the range in areocentric longitude l s 186 261 in 1971 and l s 255 301 in 1973 these regression curves are compared to the 1977 regression curve obtained from viking observations and to the results of 1956 observations there is remarkable consistency between the 1971 ipp and 1977 viking data points and the 1973 regression does not appear to have differed significantly from those of 1971 or 1977

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

what controls diffuse fractures in platform carbonates insights from provence france and apulia italy abstract fractures are widespread in rocks and regional opening mode arrays are commonly ascribed to major tectonic events however fractures occur in otherwise undeformed rocks some of these are early developed features independent of tectonics and forming a background network at regional scale to overcome this lack of understanding two hydrocarbon reservoir analogues from platform carbonates have been targeted the provence se france and the apulian platform se italy in both areas an early fracturing stage has been observed made of high angle to bedding opening mode fractures and bed parallel stylolites these features developed synchronously during the first burial stages and prior to major tectonic events the fracture sets are not genetically related to the present day layering contrarily fractures developed in a brittle media where facies transitions were not sharp and did not act as mechanical discontinuities carbonate facies distribution and early diagenetic imprint constrained the mechanical stratigraphy when fractures occurred in addition we observed that fractures related to late tectonic inversion were partly inhibited indeed rock mechanical properties change through time characterizing the temporal evolution of carbonate rocks has revealed that diagenesis and sedimentary facies are the prime actors for brittleness and mechanical layering in carbonates

insights into fold growth using fold related joint patterns and mechanical stratigraphy despite how common folds are as structural features along strike fold propagation has proven elusive to document however if a fold grows laterally along its axis early formed fold related joints may differ significantly in orientation from joints that form later in this paper we integrate mechanical stratigraphy with joint pattern analysis to determine relative timing of jointing additionally we demonstrate that joint patterns can be related to stresses on both the top and bottom of the bed during flexure we present joint data from eight sedimentary beds on the fold terminus at sheep mountain anticline wyoming usa the joint patterns around the terminus show two distinct patterns joints in six of the beds show a radial pattern around the terminus whereas joint patterns in the two remaining beds differ from proximal units despite being in the same structural position fracture resistance calculations confirm that the beds with mis oriented fractures are less resistant to fracturing than other units in the study and therefore would have fractured earlier in fold growth history we present a plate bending model that illustrates potential joint patterns around a plunging fold nose from stresses along both the top and bottom of the bed the joint strike predictions for the area in front of the inflection line on the fold nose match the orientations in our less resistant beds which are now positioned behind the inflection line suggesting that the fold grew laterally the combined analysis of fracture pattern and mechanical stratigraphy provides a new way to investigate fold evolution

mars the regolith atmosphere cap system and climate change abstract a new model for predicting the behavior of the martian regolith atmosphere cap co 2 regime and describing its role in climate change is derived the model describes the time temperature histories of 90 regolith chunks on a latitude depth grid accounting for co 2 exchange by means of laboratory derived expressions relating temperature co 2 pressure p co 2 and adsorbed co 2 mass co 2 polar cap formation and sublimation is accounted for subject to the constraint of constant total co 2 in the atmosphere regolith cap system the influence of differences in regolith adsorption laws for basalt and clay and the influence of variations in regolith depth with latitude regolith thermal diffusivity and total exchangeable co 2 inventory on predicted variations in atmospheric pressure and cap mass are examined the following results are indicated 1 the atmosphere acts as a low capacity conduit through which flows between 10 and 100 times the current atmospheric mass of co 2 between two reservoirs a regolith ocean of adsorbed co 2 and a polar cap cryosphere 2 exchange between these reservoirs is driven by variations of obliquity with the polar cap the dominant co 2 sink at low and the regolith dominating at high atmospheric pressure ranges between 0 1 low and 15 mbar high pressures of up to 25 mbar could result from pre tharsis variations pressure up to 40 mbarare achieved only in models which also call for a huge quasi permanent present co 2 cap which is not apparent 3 at high the atmospheric pressure is buffered by the regolith and satisfies adsorption equilibrium for the 90 chunks as well as conservation of co 2 mass at low a potetially large co 2 cap appears and the atmospheric pressure is buffered by the cap not the regolith 4 the periodic reservoir flushing can account for the observation of simultaneous nonenrichment of 18 o and enrichment of 13 n 5 mars climate history may be divided into three parts a post tharsis formation history as described above b pre tharsis formation with qualitatively similar processes but larger oscillations between reservoirs and larger atmospheric pressure and cap mass variation and c the very early period of channel formation 6 the nature of the response of the three part regolith atmosphere polar cap martian climate system to surface insolation variations may be analogous to that of earth s ocean atmosphere cryosphere system

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

how facies and diagenesis affect fracturing of limestone beds and reservoir permeability in limestone marl alternations abstract facies and diagenetic heterogeneities in carbonate reservoir rocks affect both fracture distribution and fracture permeability many studies focussed on fracture patterns in limestone marl alternations as e g fluid flow models are based on laterally continuous beds here we examine 4010 fractures in multiple layers of limestone marl alternations using a modified scan line method the studied successions belong to the blue lias formation hettangian sinemurian exposed on the coast of the bristol channel united kingdom we combine methods of sedimentology and structural geology with rock physics to gain a better understanding of the role of facies diagenesis and petrophysical properties tensile and compressive strength hardness porosity on the distribution of fractures fracture orientation density spacing and height fracture distribution varies significantly despite similar bed thicknesses indicating that planar bedding planes i e well bedded limestones wbl and beds with bedding plane irregularities i e semi nodular limestones snl must be distinguished snl show higher percentages of non stratabound fractures 67 while they are more stratabound in wbl 57 additionally beds with variable bed thicknesses in scale of 15 m long beds exhibit a wide range of fracture spacing whereas fractures in beds with more continuous bed thicknesses are more regularly spaced considering all lithologies the percentage of non stratabound fractures increases proportionally with caco 3 content three subsections studied in detail reveal different main sedimentological and diagenetic features from early lithified over differentially compacted to physically compacted all of them are characterised by dissimilar percentages of stratabound and non stratabound fractures in limestone beds and marl interbeds our findings demonstrate that the distribution of fractures in individual well bedded limestones is not necessarily representative for successions of limestone marl alternations multiple layers should therefore be studied in outcrop analogues as basis for fluid flow models of reservoirs composed of such lithologies

seasonal recession of mars south polar cap in 1986 photographs of mars obtained during the 1986 opposition of the planet have been used to derive the regression curve for the south polar cap between ls 190 and ls 255 the 1986 regression appears to have been unexceptional until after ls 230 when it becomes retarded relative to the normal established by the 1971 and 1977 regressions a study of the data as a function of the filter used suggests that circumpolar clouds were present in early spring unlike the case in the 1977 recession

influence of palaeogeography on thrust system geometries an analogue modelling approach for the abruzzi molise italy case history abstract the abruzzi molise sector in the central apennines is a part of a fold and thrust belt that has been deforming since the late cretaceous as a result of collision tectonics between the european and adriatic plates the superposition of different deformational styles highly reworked the originally complex palaeogeography of this portion of the southern tethyan margin analogue modelling has been performed on thrusting mechanisms in the abruzzi molise area in order to 1 reduce the number of admissible hypotheses regarding palaeogeographic setting and 2 define thrusting mechanics both of these goals are crucial for hydrocarbon exploration purposes the sandbox apparatus used to simulate the undeformed passive margin consisted of a thin compartment juxtaposed against a thick one along a linear boundary having variable geometries and mechanical stratigraphy a rigid but mobile backstop was used to deform the stratigraphy in a coulomb thrust wedge results from six experiments show that the geometric relationships between different structural units depend on the distribution of palaeogeographic domains these domains are defined by mechanical and or geometrical parameters such as the orientation between the maximum compression direction and the palaeogeographic boundary the mechanical stratigraphy and the thickness of the successions reproduced in the models the present day tectonic styles and meso cenozoic palaeogeography of the abruzzi molise area are discussed in terms of the mechanisms and structures analysed through the models

paleostress directions from the preferred orientation of closed microfractures fluid inclusion planes in sandstone east texas basin u s a abstract closed extensional microfractures are useful indicators of paleostress directions in shallowly buried quartz cemented quartz arenites of the lower cretaceous travis peak formation a tabular sandstone and shale unit approximately 670 m thick that was deposited in the gradually subsiding northern gulf of mexico basin the closed microfractures are syndiagenetic fluid inclusion rich planes that have an ene strike that is subparallel to contemporary quartz filled extension veins and regional normal faults indicating that closed microfractures contain the maximum paleostress direction these microstructures are potentially useful for mapping paleostress trajectories in passive margin and platform settings particularly where samples are obtained from core previous experimental studies of microfracture closure suggest that microfractures evolved into fluid inclusion planes by dendritic quartz precipitation in east texas microfracture closure may have occurred at temperatures as low as 85 c and depths as shallow as 1500 m

evidence for temporally changing mechanical stratigraphy and effects on joint network architecture previous studies of jointing in sedimentary rocks have shown that joint network archi tecture is controlled by mechanical stratigraphy which is described by 1 the thickness and rigidity of stratigraphic units and 2 the nature of interfaces between beds using joint pattern to infer rigidity contrast between beds is complicated by the possibility that material properties of the beds may change between fracturing events observation of both an early bed contained joint set and a differently oriented late throughgoing joint set in the same beds of growth strata in the oliana anticline in the spanish pyrenees may reflect changing mechanical properties i e via diagenesis during the folding process using a schmidt hammer we assess the rigidity contrast between the individual units and show that the present day contrast is not great enough to terminate joints at interfaces this result is consistent with an interpretation that the late stage throughgoing joints formed in strata with conditions similar to those of the present day and that the early bed contained joints formed when the rigidity contrast between beds was significantly greater than that of the present day for example differential diagenesis rates between layers of differing grain size would produce temporally changing mechanical stratigraphy we propose that changes in mechanical stratigraphy may have previously been unrec ognized in other stratigraphic sections and that these changes affect joint network archi tecture and subsequent fluid flow pathways