shear wave splitting in northeast venezuela trinidad and the eastern caribbean abstract we present new measurements of splitting of core shear wave phases sks skks pks recorded at seven stations in the eastern caribbean region six stations in northeastern venezuela and trinidad constituted the secasa92 temporary broadband array the seventh station sjg is installed on puerto rico we also attempted to measure source side splitting of s waves from earthquakes in the lesser antilles subducted slab but with little success mean splitting parameters fast polarization direction and delay time t at the stations are consistently east west trending and over 1 s respectively delay times at the secasa92 array are amongst the highest yet measured reaching 2 1 s and generally over 1 5 s we note an approximately linear decrease in delay times here from maxima at the northernmost stations to a minimum at the southernmost the secasa92 array is situated in the wide south america caribbean sa ca plate boundary zone and sjg is within the north america caribbean na ca plate boundary zone in both cases trends are parallel to the plate boundary zones locally we interpret these results to indicate that the sa ca plate boundary deformation extends to depths of at least 200 km and that south american crust lithospheric mantle and asthenosphere are coherently deformed the extreme delay times we measure are probably the result of transpression in the sa ca plate boundary and overriding of the lesser antilles slab by south america compressional flattening results in extremes of foliation development and dextral shearing yields a strong lineation in upper mantle olivine by contrast the delay time at sjg 1 2 s is near the global average and reflects the fact that the na ca plate boundary zone is purely sinistral or includes minor transtension we note that recent counterclockwise rotation of puerto rico does not apparently affect upper mantle deformation beneath the island we suggest that this rotation is crustal driven by far field sinistral shear and partly decoupled from the mantle shear our results are consistent with the trench parallel mantle flow hypothesis russo and silver 1994 science 263 1105 1111 and indicate that asthenospheric mantle flow beneath the caribbean plate is eastward and probably drives the plate s eastward motion

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

geometry and kinematics of the north monagas thrust belt venezuela abstract the complex geometry and structural evolution of the north monagas thrust belt were investigated and new exploration trends in this part of eastern venezuela considered the pirital thrust and related structures of the maturin basin are used to discuss the sequence of thrusting and to illustrate the constant balance between erosion and thrust reactivation in a recent fold and thrust belt a careful analysis of the chronostratigraphy and the internal geometry of the foredeep and piggy back syntectonic deposits has been used to constrain balanced cross sections and to establish the kinematics of each individual structure basement involvement is postulated for the pirital thrust which remobilizes previously emplaced cover structures such as the el furrial or orocual structural units it thus probably relates to the pinch out of a shallow decollement level coal measures of the barranquin formation older late jurassic to lower cretaceous evaporites or other mesozoic plastic horizons and to the reactivation of a deeper basement fault the westward emergence of the pirital and el furrial thrusts in a complex transfer area the urica fault is also analysed according to transverse profiles the surficial urica fault does not connect directly to a basement fault but instead acts as a lateral ramp for the el furrial cover unit and as a tear fault for the pirital thrust reactivation of ancient thrust faults also occurs at the rear of the pirital thrust in the west in areas where the topographic profile of the eastern venezuelan ranges has recently been modified by the pirital thrust emplacement these out of sequence thrusts resulting from fault reactivation in the inner parts of the prism are discussed in terms of the preservation of the critical taper they effectively relate directly to erosional processes which significantly decrease the load applied on this portion of the wedge the influence of the sequence of thrusting on the potential plays is considered

stress concentrations and failure modes in horizontal wells accounting for elastic anisotropy of shale formations abstract this study presents a systematic analysis of stress changes near the horizontal well sections drilled in shale and clay formations using a new approach most of the prior analyses of stress concentrations and wellbore stability in anisotropic shale formations use the traditional lekhnitskii amadei solutions we present a faster set of analytical stress functions based on largely overlooked solutions by green and taylor first the full theoretical range of possible compliances for transverse isotropic materials is evaluated using data for 5 shale clay samples and 12 additional test materials contour plots show the principal stress trajectory patterns and variation in the spatial stress concentration due to deviation from isotropic elasticity next we show how uni axial far field stress solutions can be mutually superposed and combined with stress function solutions for local stress concentration due to internal pressure on the wellbore the reappraised set of green and taylor equations is then applied to horizontal wells in three andersonian stress states for compressional strike slip and extensional basins the stress concentrations in the horizontal wellbore are evaluated for well depths between 0 and 10 000 ft using the elastic stiffnesses from five shale and clay samples to show the stress deviations from the isotropic reference case finally a full wellbore stability evaluation model combines the stress concentrations around the wellbore with failure criteria to determine the safe drilling window at each well depth for each andersonian stress state practical graphs and supporting equations are provided to assess wellbore stability in transverse isotropic shale

extent of power law scaling for natural fractures in rock new data sets from natural faults and extension fractures exhibit simple power law scaling across 3 4 4 9 orders of magnitude regardless of rock type or movement mode the data show no evidence of natural gaps or scaling changes each data set consists of independent measurements made at different observational scales a power law regression to the subset of smaller fractures in each case provides an extrapolation that accurately predicts associated larger fractures consequently data representing a limited range of fracture sizes may be used to characterize a much broader spectrum of fracture sizes

historical seismicity of the southeastern caribbean and tectonic implications we have relocated the twenty eight largest magnitude 4 3 ms 7 3 historical 1922 1963 earthquakes of the southeastern caribbean we also present new focal mechanisms for seven of these events the relocations are based on reported issp ands arrival times that we analyzed using generalized linear inversion techniques the new focal mechanisms were constrained by first motionp polarities as reported by the iss and as picked by us where records were available and by the polarities and ratios ofsh andssh andsv andssv arrivals that we determined from seismograms the results of the relocations are commensurate with the distribution of seismicity observed in the recent era hypocenters are shallow and intermediate in depth 0 200 km and the events occur almost exclusively in areas known to be currently seismic the frequent seismic activity in the vicinity of the paria peninsula venezuela is clearly a persistent feature of the regional earthquake pattern intermediate depth earthquakes indicative of subduction beneath the caribbean plate occur here and along the lesser antilles arc the grenadines seismic gap is confirmed as an area of low seismic moment release throughout the historical era trinidad and the eastern gulf of paria were also largely quiescent

regional scale strain variations in banded iron formations of eastern india results from anisotropy of magnetic susceptibility studies anisotropy of magnetic susceptibility ams data are used as a tool to determine strain variations in different parts of the banded iron formations bifs of the bonai synclinorium eastern india ams data of 88 cylindrical cores drilled from 29 samples collected from the limb and hinge parts of mesoscopic scale folds as well as different parts of the entire synclinorium are presented it is found that the samples from limbs of small scale folds and also from limbs of the regional scale synclinorium have higher degrees of anisotropy than the hinges this is inferred to indicate that the limbs accommodated higher strain than the hinges ams orientation data are analysed in conjunction with field data it is concluded that the magnetic fabric developed in the limbs as well as hinges of the bifs of the study area is related to deformation and is not a manifestation of sedimentary fabric

estimating average fracture spacing in subsurface rock knowledge of the spacing of fractures in reservoir rocks i e the distance between parallel fractures in a subsurface joint set can lead to a better understanding of the production characteristics of a reservoir and serve to quantify the relative degree of deformation in subsurface rocks in this paper i present a new method for estimating the spacing of subsurface fractures this new method is easy to use from the standpoint of both data collection and data analysis the average fracture spacing method can be applied with boreholes of any orientation relative to a fracture set the method is especially powerful when it is used for the relatively common case of a borehole nearly parallel to a fracture set e g vertical borehole intersecting vertical fractures average fracture spacing is estimated from an analytical solution based on observed borehole fracture intersections and observed fracture porosity the only data required are the dimensions of the core or imaged borehole and the total height of all sampled fractures because the likelihood of intersecting fractures increases when a well is deviated perpendicular to the fractures of a set fracture reservoirs commonly are candidates for deviated boreholes an informed decision on borehole deviation requires predicting the fracture intersection frequency as a function of both deviation magnitude and direction a new method based on probabilities of borehole fracture intersections uses spacing and height data from subsurface joint like fractures and the borehole diameter to predict fracture intersection frequencies for all possible well deviations fracture intersection frequency solutions are presented with respect to a conventional geographic reference frame thus simplifying even the most complex three dimentional situations

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

palaeomagnetic and rock magnetic study of a deeply exposed continental section in the charnockite belt of southern india implications to crustal magnetisation and palaeoproterozoic continental nucleii abstract this study has investigated palaeomagnetism and magnetic properties of a tract of continental basement in south india metamorphosed to granulite facies close to the archaean proterozoic boundary 2600 ma at former depths of 20 35 km the metamorphism is characterised by extensive development of charnockite produced by flushing of co2 rich fluids through older crust rock magnetic studies thermomagnetic high and low temperature susceptibility hysteresis and irm show that very low ti metamorphic magnetite is the ubiquitous ferromagnet in all rock types it has predominantly multidomain characteristics but is of high magnetic stability probably due to exsolution precipitation and microfracture filling producing anisotropic grain shapes magnetic susceptibility shows little increase with temperature falling to zero at 550 c implying that induced magnetisation drops to a weak paramagnetic effect near this isotherm susceptibilities and intensities of magnetisation increase from amphibolite to granulite facies and by an order of magnitude within charnockites in the lower dehydrated crust induced magnetisations are able to explain 50 of observed magnetic anomalies of deep origin intensities of magnetisation and qn values are high especially in charnockite where metasomatism has resulted in new magnetite formation and it is concluded this facies is able to explain observed magnetic anomalies provided that viscous remnant magnetisation is enhanced at depth to contribute the remaining magnetisation the magnitude of anisotropy of magnetic susceptibility ams is also increased by charnockite formation as are degrees of lineation and foliation however directions of ams in these anhydrous rocks are essentially random and reflect the static nature of the charnockite metasomatism no appreciable phanerozoic overprinting is evident in this terrane and the dominant feature of the palaeomagnetic record is a west to north shallow swathe of component directions with a minor antiparallel sequence between means summarised by a1 d i 280 8 a2 d i 319 11 and a3 d i 1 10 this swathe is interpreted as an uplift related cooling acquisition following charnockite formation at 2600 ma and a remanence age of 2600 2400 ma is implied by the absence of most of the swathe in early palaeoproterozoic and younger dyke swarms cutting the region a second group of component directions b mean d i 238 87 is similar to magnetisations found in dykes linked to an age of 2370 ma and interpreted as somewhat younger than the a swathe we argue that the entire magnetic record was acquired before final basement uplift prior to 2000 1800 ma collective late archaean early proterozoic palaeopoles from india define a low to high latitude movement of the shield at 2600 2300 ma which is replicated in contemporaneous results from the other ancient gondwana nucleii of southern africa and australia the cratonic nucleii of these three shields have the oldest laterally extensive supracrustal covers and comprise the core of the protocontinent of ur the 2900 2200 ma palaeopoles are consistent with close proximities and subsequent movements between them were apparently limited because continental proximities but not configurations were similar within the palaeozoic supercontinent of gondwana

experimental determination of subcritical crack growth parameters in sedimentary rock mounting evidence suggests that subcritical crack growth is an important mechanism for the development of natural fractures numerical simulations of fracture patterns are sensitive to the subcritical crack growth index the exponent used to describe the power law dependence of crack velocity on stress intensity few measured values of subcritical indices in sedimentary rocks have been published however and no systematic studies of diagenetic controls of subcritical crack index behavior are available subcritical crack index measurements are especially difficult in weak porous rock primarily because non elastic material response can obscure fracture related behavior we have carried out numerous subcritical crack index measurements with the widely used dual torsion beam test configuration and developed a modified test methodology for subcritical crack index measurements on porous rocks the principal element of the methodology is a specimen pre loading procedure to minimize transient material response measurement reliability is documented in the study by documentation of reproducibility in three sets of sandstone and carbonate specimens

impact of drilling fluids on friction coefficient of brittle gas shale abstract the stability of the horizontal long section of a wellbore is the main restriction of efficient development of shale gas the use of oil based drilling fluids in the initial stage of longmaxi shale gas exploration in the southern sichuan basin china still causes wellbore collapses for reasons not well understood based on the geological characteristics and engineering conditions of brittle shale a significant factor influencing shale wellbore stability friction coefficient is proposed due to the developed facture networks of brittle shale in this region frictional sliding along the fracture surface is the main source of movement within the longmaxi formation here we determined the effects of drilling fluids on the longmaxi formation shale in southern sichuan basin natural fractures slide due to compression shear slip and the friction coefficient of the fracture surfaces is a controllable factor of shear and slip in natural fractures in shales the typical experimental apparatus of rock friction sliding was modified to accommodate for drilling fluids and typical in situ temperatures results show that the friction coefficient decreases induced by infiltration of drilling fluids filtrate and the friction coefficient is more sensitive to oil based drilling fluid filtrate than water based the high ph oil based drilling fluid filtrate shortens the length of time taken by static friction stage and smooths the time vs friction coefficient curve which creates an easier situation for frictional sliding high viscosity and good wettability lead to the strong lubricity of oil based drilling fluid the surface properties of the shale samples changed after the high alkalinity fluid immersion such as fracture surface planarization and floc like particle generation causing an obvious reduction of friction coefficient between fracture surfaces as a result of rock strength decreasing after immersion into drilling fluids filtrate asperities on fracture surface were prone to shear failure in the process of frictional sliding and their failure products migrated with the fracture surface movement this not only decreased the roughness shale surface but also acted as a friction reduction agent between the sliding surfaces this study provides theoretical guidance for drilling fluid selection and optimization during brittle shale well drilling

a palaeomagnetic study of charnockites from madras block southern granulite terrain india abstract the south indian craton is composed of low grade and high grade metamorphic rocks across different tectonic blocks between the moyar bhavani and palghat cauvery shear zones and an elongated belt of eastern margin of the peninsular shield the madras block north of the moyar bhavani shear zone which evolved throughout the precambrian period mainly consists of high grade metamorphic rocks in order to constrain the evolution of the charnockitic region of the pallavaram area in the madras block we have undertaken palaeomagnetic investigation at 12 sites chrm directions in 61 oriented block samples were investigated by alternating field af and thermal demagnetization titanomagnetite in cation deficient cd and multi domain md states is the remanence carrier the samples exhibit a chrm with reverse magnetization of d m 148 1 i m 48 6 k 22 2 95 9 0 and a palaeomagnetic pole at 37 5 n 295 6 e dp dm 7 8 11 8 this pole plots at a late archaean location on the indian apparent polar wander path apwp suggesting an age of magnetization in the pallavaram charnockites as 2600 ma the nearby st thomas mount charnockites indicate a period of emplacement at 1650 ma mesoproterozoic thus the results of madras block granulites also reveal crustal evolution similar to those in the eastern ghats belt with identical palaeopoles from both the areas

superposed deformation fabrics in the precambrian metabasic rocks of the iron ore group singhbhum craton eastern india evidences from anisotropy of magnetic susceptibility studies abstract anisotropy of magnetic susceptibility ams data have been utilized to build up the multiple deformation sequences and strain variation from an apparently undeformed metabasic lava sequence of the basal part of the paleo mesoarchean greenstone succession of the iron ore group rocks in the western iron ore basin singhbhum craton eastern india the basal lava crops out bordering the regional horseshoe shaped outcrop the horseshoe synclinorium structure of the overlying bif rocks ams data of 892 cylindrical specimens drilled from 83 samples collected from different structural domains within the synclinorium structure are presented in the analysis ams orientation data have been analyzed in the light of superposed fold type 1 interference fabrics developed within bif as a result of successive deformation episodes d1 and d2 it is found that the lava samples ubiquitously present two consistent magnetic fabrics homogeneous over smaller sub areas which have been successfully correlated with the d1 and d2 deformation fabrics from the bif rocks moreover a regional scale variation in intensity and nature of strain is noticed from the ams data which have been utilized to decipher folding mechanism as well as the changing deformation pattern during successive folding events

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

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

aggregate properties of fracture populations abstract empirical studies indicate that the individual attributes of both faults and extension fractures follow power law scaling aggregate properties of fracture populations are important in a variety of problems and can be specified in terms of the scaling parameters of individual fracture attributes development of an expression for an aggregate property requires consideration of a number of independent factors including the topologic dimension of the aggregate property the topologic dimension of sampling and the possibility of scaling changes for fractures that span some mechanically significant layer the riemann zeta function provides an alternative to integration for the analytical and numerical solution of aggregate problems previous work regarding aggregate properties of fracture populations has focused on the strain due to faulting new expressions are developed here for other aggregate properties of interest fracture surface area fracture porosity fracture permeability and shear wave anisotropy a general characteristic of these aggregate properties is that for most values of scaling exponents the aggregate properties are dependent on the size of the sampling domain this implies that the aggregate properties are scale dependent additionally it appears that fracture surface area is concentrated in the smallest fractures of many populations fracture porosity is concentrated in the largest fractures of most populations but not as strongly as fracture permeability which probably derives almost entirely from the largest fractures in populations

stress corrosion cracking of quartz a note on the influence of chemical environment abstract tensile mode i stress corrosion growth of cracks on the a plane of quartz in a direction normal to z has been studied in de ionized water in 2n hcl and in 2n naoh solutions at 20 c the double torsion testing method was used to obtain crack velocity v stress intensity factor ki curves the stress corrosion index n where v kin was 19 3 for 2n hcl 12 8 for deionized water and 9 5 for 2n naoh the degree of influence of reagent chemistry on crack velocity was most marked at low ki values at 0 5 kic 3 orders of magnitude increase in crack velocity results from substituting 2n naoh for 2n hcl as the corrosive medium but decreased monotonically on raising ki so that close to kic there was no clearly discernible influence on crack growth rates ca 10 3 m s 1 these observations are explained in terms of a model of crack propagation in which at low ki values the chemistry of the bulk fluid environment controls the corrosion reaction the rate of crack growth is enhanced by increasing environmental oh concentration at high ki or velocity however the chemical composition of the quartz surface may control crack propagation rates through its influence on the chemistry of the crack tip fluid as the ph of natural pore fluids is likely to vary widely in different geological environments these results should be noted when invoking stress corrosion to explain geophysical phenomena

cenozoic plate tectonic history of the northern venezuela trinidad area geological and geophysical data coupled with recent plate tectonic reconstructions suggest that the cenozoic geologic history of the northern venezuela trinidad area has been dominated by strike slip displacement of discrete crustal blocks allochthonous terranes within the area include metavolcanic rocks of the cretaceous villa de cura group and metamorphic rocks of the precambrian to cretaceous cordillera de la costa a relatively competent crustal block margarita block is defined by an outline around the metamorphic basement of margarita island the araya paria peninsula the northern range of trinidad and tobago island reconstruction of the margarita block to its original position requires at least partial closure of the falcon basin closure of the bonaire and cariaco basins and restoration of about 50 km of motion on both the oca and bocono faults post middle eocene eastward translation of the caribbean plate caused eastward motion of the margarita block a minor change in relative plate motion during the late oligocene or early miocene produced a right step in the moron fault forming the cariaco pull apart basin and el pilar fault zone maximum offset on el pilar fault is estimated to be no more than 125 km though displacement along the entire fault zone may have been greater transpressional stresses between the caribbean plate and northern south america caused folding of the serrania del interior of venezuela and the central range of trinidad eastward migration of transpressional stresses at the southeastern corner of the caribbean south american plate boundary is being accommodated by formation of oblique thrusts transpressive anticlines and downwarping of the crust bouguer gravity data suggest that jurassic aged atlantic oceanic crust is being depressed as the caribbean plate expands into the demerara plateau area this study suggests that the faults and transtensional transpressional compressional structures identified in this study are the result of stresses produced during the large eastward translation of the caribbean plate since the paleocene and are not the product of a shear couple

use of anisotropy to determine the origin of characteristic remanence in the siwalik red beds of northern pakistan it is often difficult or impossible to determine the origin of the characteristic remanent magnetization of red beds from the bulk remanence alone however anisotropy of remanence or susceptibility is strongly controlled by the statistical alignment of hematite grains this in turn may reflect the development of the magnetic fabric of the sediment over time so the shape of the anisotropy ellipsoid may provide clues to the origin of remanence in this work we make a study of the anisotropy of magnetic susceptibility ams in siwalik red beds of miocene age from northern pakistan comparison of the results with detailed petrographic studies and other information suggests that advanced soil development leads to the destruction of primary fabrics and often with it a coherent magnetization furthermore it should be possible to use ams fabric information to quantify the degree of pedogenesis in these miocene soils we attempted to determine the anisotropy of isothermal remanence air but found ams to be the technique of choice because of apparent changes in coercivity during air experiments we interpret the air data as resulting from metastable domains in hematite grains which change domain state during the air experiment

palaeomagnetic study of archaean banded hematite jasper rocks from the singhbhum orissa craton india abstract the banded hematite jasper formation within the iron ore supergroup of the singhbhum craton in eastern india comprises fine alternating layers of jasper and specularite it was deposited at 3000 ma and deformed by a mobile episode at 2700 ma hematite pigment 10 m is chiefly responsible for red to brown rhythmic bands in the hematite jasper facies although thermomagnetic study also shows that minor amounts 1 2 of magnetite are present palaeomagnetic study identifies a dual polarity remanence resident in hematite d i 283 60 95 12 which predates deformation studies of the fabric of magnetic susceptibility and rock magnetic results suggest a diagenetic origin for this magnetisation with the hematite formed from oxidation of primary magnetite the palaeopole 32 e 24 n dp dm 14 18 records the earliest post metamorphic magnetisation event in the orissa craton a minimum apparent polar wander motion of the orissa singhbhum craton of through 80 is identified during late archaean times 2900 2600 ma

kinematics of compressional fold development in convergent wrench terranes abstract kinematic models are presented for compressional fold development in wrench and convergent wrench terranes that relate fold shortening axial rotation and axial extension fold shortening may be derived from final fold geometry existing fold geometry and axial orientation two readily measurable quantities provide the data needed to determine the relative components of shearing and convergence within the fold system analyses utilizing these kinematic models indicate that folds developed in sedimentary rocks in the wrench borderlands of both the rineonada and san andreas wrench faults in central california are the product of strongly convergent wrenching the axes of these folds have been rotated no more than a few degrees during the course of their development in contrast folds developed in the alpine schists along the alpine fault in new zealand and in pleistocene sediments along the southern limit of the san andreas fault suggest an almost pure wrench setting and large 25 axial rotations significant axial extension is inherent in wrench related compressional folds this axial extension is commonly manifest in the form of normal and strike slip faults that are internal to the folds and trend at high angles to the fold axes the relative amount of axial extension diminishes as the degree of convergence increases this axial extension and the associated extensional features can be a diagnostic indication of the influence of wrenching

role of shale thickness on vertical connectivity of fractures application of crack bridging theory to the austin chalk texas abstract contrasting material properties of alternating chalk and shale layers control vertical connectivity of fractures our field observations within the austin chalk texas indicate that 1 the majority of vertical fractures occur in chalk layers and abut against contacts with shale layers 2 thicker shale layers have greater resistance to fracture propagation than thinner shale layers from these observations we hypothesize both the resistance of shale to fracturing and the thickness of shale layers may inhibit fracture propagation across the shale and into the next chalk layer we model crack propagation within a three layered system brittle chalk fracture resistant shale brittle chalk the modeled crack extends across the shale but closing tractions applied to the crack segment within the shale layer simulate resistance of shale to fracturing the crack tip lies a short distance within the unfractured chalk layer simulating a coplanar flaw with potential to propagate if the stress intensity factor at the flaw exceeds the chalk fracture toughness the crack propagates thereby bridging and eventually rupturing the shale layer for any chalk thickness there is a critical shale thickness above which fractures cross the shale layer and below which fractures arrest at shale finite element method fem analysis evaluates the influence of shale ductility within the chalk shale chalk system although remote and fluid pressure driven fractures produce identical stress intensity factors in elastic chalk shale systems lower driving stresses are required to propagate fluid pressure driven fractures through a system ductile shale layers than fractures under remote tension

biogenic magnetite and the magnetization of sediments biogenic magnetites are produced through the reduction of ferric iron by both biologically induced extracellular and biologically controlled intracellular processes with few exceptions all are ultra fine grained single domain magnetite biogenic magnetites formed by magnetotactic bacteria biologically controlled have been shown to contribute significantly to the natural remanent magnetization of carbonates and limestones hemipelagic and deep sea marine sediments the input into sediments of ultra fine grained magnetite produced by dissimilatory iron reducing bacteria biologically induced has yet to be firmly established but may be even more significant whether either type of authigenic biomagnetite is preserved is determined by postdepositional factors including oxidation reduction by substitution and dissolution unconsolidated and lithified sediments can be screened for putative biogenic magnetite by rock magnetic techniques it is not generally appreciated that magnetotactic bacteria and their magnetofossils can be identified by the unusual and in some cases unique morphologies size range and composition of the magnetite crystals magnetite produced by dissimilatory iron reducing bacteria have a distinctive morphology and size range but it is currently controversial as to whether these can be distinguished from certain chemically precipitated magnetites the presence of dissimilatory iron reducing bacteria however can be detected using microbiological techniques and sediment geochemistry biogenic magnetites are trace fossils and potentially useful environmental indicators and are considered to have significant input to the magnetization of most sediments both modern and ancient

3 d seismic evidence of the effects of carbonate karst collapse on overlying clastic stratigraphy and reservoir compartmentalization a multidisciplinary team composed of stratigraphers petrophysicists reservoir engineers and geophysicists studied a portion of boonsville gas field in the fort worth basin of north central texas to determine how modern geophysical geological and engineering techniques can be combined to understand the mechanisms by which fluvio deltaic depositional processes create reservoir compartmentalization in a low to moderate accommodation basin an extensive database involving well logs cores production and pressure data from more than 200 wells 26mi2 67km2 of 3 d seismic data vertical seismic profiles vsps and checkshots was assembled to support this investigation we found the most important geologic influence on stratigraphy and reservoir compartmentalization in this basin to be the existence of numerous karst collapse chimneys over the 26 mi2 67km2 area covered by the 3 d seismic grid these near vertical karst collapses originated in or near the deep ordovician age ellenburger carbonate

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

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

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

microstructures in a banded iron formation gua mine india this paper provides a detailed documentation of microstructures developed in the banded iron formation bif of gua mine located in the bonai synclinorium eastern india where the rocks have been subjected to three deformations d 1 to d 3 folded iron ores quartz strain fringes around rigid core objects and folded iron ore layers and refracted quartz veins are described from samples taken from d 2 folds in the banded iron formation orientations of microstructures are compared with mesoscopic structures to interpret the generations of ore minerals planar structures and the time relationship between deformation and development of different microstructures the mechanism of d 2 folding is worked out and its bearing on microstructure development is discussed the d 2 folds are inferred to have developed by a combination of tangential longitudinal strain in the competent layer flexural flow in the incompetent layers and flexural slip at the interface between layers of differing competence homogeneous flattening strain superposed the earlier strain which led to modification of the folds in the competent layer from class 1b to 1c this strain is quantified and is found to be higher in the limb than the hinge of a fold diffusive mass transfer by solution and bulging dynamic recrystallization in quartz are inferred as the dominant deformation processes during folding moreover based on comparison with published deformation microstructure maps the microstructures of the present study are estimated to have developed between 300 and 350 c temperatures at a strain rate of 10 14 10 12 s 1 which are geologically realistic conditions for naturally deformed rocks

palaeomagnetism of sukinda chromites and their geological implications summary thirty eight oriented samples were collected from seven sites over the precambrian sukinda chromite ore between 85 43 e 21 01 n and 85 50 e 21 03 n which is in the form of a plunging syncline on isolating the primary component of magnetization and affecting the necessary tectonic correction four sites a b c f were found to possess a reverse magnetic polarity d 210 i 22 with k 122 and 95 6 and one site e was found to bear a normal polarity d 49 i 16 with k 474 and 95 2 the remaining two sites d and g were rejected the former because of scatter in directions and the latter due to overprinting by a dominant secondary magnetization induced by lateritization on comparison of vgps evaluated for these chromite deposits with those of other precambrian rocks almost of a similar age it is seen that these chromites are probably older than the gwalior traps whose age is 1830 200 ma from the rock magnetic and microscopic examination it is deduced that the massive chromites of the sukinda valley have not been affected magnetically to any great extent by serpentinization though their host rocks are serpentinized ultramafics consistent directions of remanent magnetism point to the fact that these deposits crystallized in fairly stable conditions characteristic of the stratiform type of chromite deposits therefore leading us to infer the sukinda chromites to be of that type these palaeomagnetic data when subjected to the fold test show the primary remanence to pre date the tectonic disturbance and hence prove these chromites to have consolidated prior to the regional tectonic activity of the area

seismicity and tectonics of the southeastern caribbean we present 33 new focal mechanisms for se caribbean earthquakes 1963 1988 we use these mechanisms in conjunction with 28 previously available mechanisms to distinguish between two models of plate boundary zone interaction in the se caribbean the trench trench transform and hinge faulting model and the right oblique collision model shallow 0 70 km and intermediate 70 200 km depth earthquakes occur in the study region we focus on the tectonic causes of these events and the motions they delineate the shallow earthquakes are in a broad linear zone which trends ne from the paria peninsula of venezuela towards barbados intermediate depth earthquakes cluster beneath and nw of the peninsula and deepen to the nw perpendicular to the ne trending shallow events the vertical distribution of the earthquakes suggests a slab with steep nw dip shallow dextral strike slip on e striking faults is restricted to a 60 km wide linear zone between the gulf of cariaco and the western margin of the gulf of paria dextral strike slip is active only as far east as the gulf of paria and not within or east of trinidad shallow thrust events with ene striking planes distributed between the araya peninsula and the gulf of paria indicate collision at crustal levels between south america and caribbean and that folding and thrusting are still active over a 60 km interval south of the araya paria isthmus active thrusting in venezuela corroborates predictions of transpression between caribbean and south america and discounts transtensional motions between the two plates in the se caribbean the conjunction of shallow thrust strike slip and normal earthquakes in the gulf of paria at around 62 3 may be the expression of unpartitioned oblique compressive deformation in the plate boundary zone intermediate 165 km h 70 km depth thrust and dip slip events within the nw dipping slab indicate that oceanic lithosphere probably originally attached to south america subducts to the nw beneath the caribbean plate shallow normal faulting events e and ne of trinidad are expressions of plate bending about near horizontal axes parallel to the lesser antilles subduction zone we conclude that the earthquake mechanisms provide strong support for the right oblique collision model of caribbean south american plate interaction

oil and gas geochemistry and petroleum systems of the fort worth basin detailed biomarker and light hydrocarbon geochemistry confirm that the marine mississippian barnett shale is the primary source rock for petroleum in the fort worth basin north central texas although contributions from other sources are possible biomarker data indicate that the main oil generating barnett shale facies is marine and was deposited under dysoxic strong upwelling normal salinity conditions the analysis of two outcrop samples and cuttings from seven wells indicates variability in the barnett shale organic facies and a possibility of other oil subfamilies being present light hydrocarbon analyses reveal significant terrigenous sourced condensate input to some reservoirs resulting in terrigenous and mixed marine terrigenous light hydrocarbon signatures for many oils the light hydrocarbon data suggest a secondary condensate generating source facies containing terrigenous or mixed terrigenous marine organic matter this indication of a secondary source rock that is not revealed by biomarker analysis emphasizes the importance of integrating biomarker and light hydrocarbon data to define petroleum source rocks gases in the fort worth basin are thermogenic in origin and appear to be cogenerated with oil from the barnett shale although some gas may also originate by oil cracking isotope data indicate minor contribution of biogenic gas except for reservoirs in the pennsylvanian bend group which contain gases spanning the complete range of observed maturities the gases appear to be stratigraphically segregated younger reservoirs contain less mature gas and older reservoirs contain more mature gas we cannot rule out the possibility that other source units within the fort worth basin such as the smithwick shale are locally important petroleum sources