earth stress and reservoir quality evaluation in high and steep structure the lower cretaceous in the kuqa depression tarim basin china abstract knowledge of the state of the in situ stress field provides important insights into the petroleum engineering and geology the in situ stress orientation and magnitudes in the high and steep structure of lower cretaceous bashijiqike formation in kuqa depression were analyzed based on well log calculations and the impacts of in situ stress field on reservoir quality macroscopic reservoir properties and microscopic pore structure as well as fracture aperture were investigated the maximum horizontal stress shmax orientation was determined as nearly north south according to the image log derived drilling induced fractures and borehole breakouts the vertical stress sv shmax and minimum horizontal stress shmin magnitudes were calculated via well logs the result reveals a dominant strike slip stress regime shmax sv shmin in the bashijiqike formation in the kuqa depression the sv at the depth ranging from 5000 to 7100 m is 150 180 mpa and horizontal stresses shmax and shmin vary significantly from 130 to 210 mpa the horizontal stress difference is in the range from 40 to 70 mpa and fluid overpressures are commonly encountered the effective stress e has a positive relationship with the when the is less than 50 mpa intergranular pores can be preserved in layers with low values the microscopic pore structure will be complex and macroscopic reservoir quality will be poor with the increasing values in addition fractures are mainly corresponding with the layers where the value is low and the fracture aperture and porosity decreased significantly with the effective stress e by determining the magnitude and orientation of in situ stresses around the wellbore insights can be provided into the wellbore instability optimum drilling trajectories and reservoir quality evaluation

prediction of tensile fractures in ks2 trap kuqa depression nw china abstract tectonic fractures that generally controlled by tectonic stress field especially tensile fractures are crucial for tight reservoirs that are characterized by low porosity and low permeability prediction of fracture distribution is extremely important for further exploration and development of such reservoirs tensile fractures are developed in the upper bashijiqike formation in kuqa depression in northwest china the subject of this study was the ks2 faulted fold and the characteristics of the tensile fractures were analyzed in detail a three dimensional finite element elastic model of the faulted fold was constructed and the two factor method was applied to predict the distribution of tensile fractures in the upper bashijiqike formation in the faulted fold distributions of predicted tensile fractures reveal that positions relative to the fold axis and faults in the hinge zone both have significant influence on the distribution of tensile fractures the predicted tensile fractures are developed in the hinge zone in the faulted fold particularly in the area around fault tips which is the region with the most potential for oil and gas exploration in faulted folds

propagation velocity of a natural hydraulic fracture in a poroelastic medium the propagation rate of a natural hydraulic fracture is limited by the rate that fluid flows from the saturated rock into the void space created by fracture growth unlike induced hydraulic fractures natural hydraulic fractures can not be modeled by specifying rates of fracture growth or fluid flow into the fracture as boundary conditions numerical solutions of the governing equations for natural hydraulic fracture growth in a poroelastic medium indicate that growth rates are primarily controlled by the hydraulic conductivity the storage sp and the initial flaw length 2 o computationally more efficient models which partially and completely decouple material stresses from fluid pressures give similar results results for several rock types indicate that although the rate of fracture propagation is limited by fluid flow fracture growth still accelerates results are generalized in dimensionless plots of fracture length versus time for various values of the dimensionless parameter 1 gsp where g is the shear modulus and is the drained poisson s ratio

natural hydraulic fractures in the wessex basin sw england widespread distribution composition and history bedding parallel veins of fibrous calcite beef are historical in the wessex basin the veins are common in mesozoic mudstones and shales especially of liassic to mid cretaceous ages cone in cone structures which consist of multiple nested cones are also well developed within the beef to investigate the distribution and the context of formation of beef in the basin we have made several field studies and analysed numerous samples the veins are widespread vertically and horizontally in the sedimentary sequence but they are especially common near or within potential source rocks for petroleum or near major tectonic faults the internal structures of some beef veins have revealed that they formed during late cretaceous to tertiary compressional inversion of the basin the typical composition for beef is of calcite with some pyrite and fragments of shale however inclusions or patches of hydrocarbons liquid or solid occur within calcite crystals or between fibres respectively according to some previous studies as well as ours beef veins of the wessex basin represent natural hydraulic fractures which formed as a result of fluid overpressure this may have resulted in part from chemical compaction of petroleum source rocks during late cretaceous to tertiary times indeed source rocks at outcrop in the wessex basin could be more mature than previously thought and the beef veins may be good markers of maturation

geological and industrial implications of extensive dilatancy anisotropy extensive dilatancy anisotropy is the hypothesized distribution of stress aligned fluid filled microcracks pervading most rocks in the earth s crust the geometry of the cracks and the aligning stress field can be monitored by analysing the waveforms of shear waves propagating through the rockmass the ability to estimate some of the parameters of the crack and stress geometry by analysing shear wave particle displacements has widespread implications

open shear fractures at depth in anticlines insights from the kuqa foreland thrust belt tarim basin shear fractures that close under compressive stress regime were described by previous workers here we show the existence of hinge parallel shear opening mode fractures at a certain depth within an anticline structure through core observations formation micro imager fmi log interpretations and production data studied reservoir is a tight sandstone body of cretaceous age which is emplaced in ks2 anticline with depth ranging between 6000 and 8100 m we explore the mechanism of hinge parallel shear fractures by stereographic technique and backtilting as well as show field evidences for opening displacements these shear fractures mainly consist of vertical tension set moderate dipping shear set steeply dipping shear set and sub horizontal shear set crosscutting relationships and surface textures suggest that although the moderate dipping shear set is older and maybe sealed by cements it may have been opened by local stress due to outer arc extension through folding calcite bridges with rhombohedral structures may support fracture walls helping them to remain open in case of compressive stress our work documents shear fracture openings in the anticline at depth and fluid flow in subsurface reservoirs

late cenozoic tectonic evolution of the southern chinese tian shan structural sedimentological magnetostratigraphic and 40ar 39ar thermochronological investigations were conducted in the southern chinese tian shan on the basis of our own mapping and earlier investigations in the area the late cenozoic southern tian shan thrust belt may be divided into four segments based on their style of deformation from west to east they are 1 kashi aksu imbricate thrust system 2 the baicheng kuche fold and thrust system 3 the korla right slip transfer system and 4 the lop nor thrust system the westernmost kashi aksu system is characterized by the occurrence of evenly spaced 12 15 km imbricate thrusts the baicheng kuche and korla systems are expressed by a major north dipping thrust the kuche thrust that changes its strike eastward to become a nw striking oblique thrust ramp the korla transfer zone the lop nor system in the eastern most part of the southern chinese tian shan consists of widely spaced thrusts all involved with basement rocks geologic mapping and cross section construction suggest that at least 20 40 km of crustal shortening with a horizontal shortening strain of 20 30 has occurred in the southern chinese tian shan during the late cenozoic these estimates are minimum because of both conservative extrapolation of the thrust geometries and partial coverage of the thrust belt by the cross sections the timing of initial thrusting is best constrained in the kuche basin where crustal shortening may have occurred at 21 24 ma the time of a major facies transition between lacustrine and braided fluvial sequences constrained in general by biostratigraphy and in detail by magnetostratigraphy this estimate represents only a minimum age as development of thrusts in the southern chinese tian shan may have propagated southward toward the foreland thus the sedimentary record only represents the southernmost and therefore youngest phase of thrusting if our estimate of timing for the thrust initiation 21 24 ma is correct using the estimated magnitude of shortening 20 40 km and shortening strain 20 30 the averaged rates of late cenozoic horizontal slip and shortening strain are 1 1 9 mm yr 1 and 2 9 4 5 10 16 s 1 respectively our reconnaissance 40ar 39ar thermochronological analysis in conjunction with earlier published results of apatite fission track analysis by other workers in the chinese tian shan suggests that the magnitude of cenozoic denudation is no more than 10 km most likely less than 5 km we demonstrate via a simple airy isostasy model that when the thermal effect on changes in surface elevation is negligible determination of the spatial distribution and temporal variation of both horizontal shortening strain and denudation becomes a key to reconstructing the elevation history of the tian shan using this simple model the loosely constrained magnitude of crustal shortening strain and denudation in the southern chinese tian shan implies that it may have been elevated 1 0 2 0 km since the onset of cenozoic thrusting

paleozoic tectonic evolution of the northern xinjiang china geochemical and geochronological constraints from the ophiolites 1 the northern xinjiang of china is a composite orogenic belt in the south central part of the altaids formed by subduction accretion mainly during the paleozoic geochronologic and geochemical data from the paleozoic ophiolites and associated assemblages are reported here to place constrains on the tectonic evolution of this region and to reconstruct its history using a model of subduction accretion plus large scale map view strike slip faulting geologic and geochronologic review indicates that the paleozoic ophiolites and associated assemblages of the western and eastern junggar regions are tectonically related occurring as a pattern with the early paleozoic tangbale mayile hongguleleng aermantai complexes in the center the late paleozoic darbut karamaili and the northern tianshan complexes in the south and the late paleozoic kekesentao qiaoxiahala kuerti complexes in the north available geochemical data suggest that the paleozoic ophiolites of the northern xinjiang are mainly oceanic fragments of these three types mid ocean ridge basalt oceanic island basalts and island arc basalts the paleozoic subduction accretion complexes grew mainly along different segments of the kipchak arc and in associated with migration of magmatic arc the paleozoic ophiolite associated sedimentary sequences and subduction related volcanic rocks exhibit an evolution history from intraoceanic early paleozoic to marginal settings late paleozoic implying coeval strike slip stacking with subduction accretion processes and by which to transport the respective complexes together and further to amalgamate with neighboring units of the altaids subduction accretion processes terminated in the mid carboniferous in this region in the late permian early triassic a counterclockwise rotation of fault blocks in a giant sinistral strike slip and extension domain between altay and tianshan lastly generated present configuration of the paleozoic subduction accretion complexes in the northern xinjiang

fracture zones constrained by neutral surfaces in a fault related fold insights from the kelasu tectonic zone kuqa depression abstract stress variation associated with folding is one of the controlling factors in the development of tectonic fractures however little attention has been paid to the influence of neutral surfaces during folding on fracture distribution in a fault related fold in this study we take the cretaceous bashijiqike formation in the kuqa depression as an example and analyze the distribution of tectonic fractures in fault related folds by core observation and logging data analysis three fracture zones are identified in a fault related fold a tensile zone a transition zone and a compressive zone which may be constrained by two neutral surfaces of fold well correlation reveals that the tensile zone and the transition zone reach the maximum thickness at the fold hinge and get thinner in the fold limbs a 2d viscoelastic stress field model of a fault related fold was constructed to further investigate the mechanism of fracturing statistical and numerical analysis reveal that the tensile zone and the transition zone become thicker with decreasing interlimb angle stress variation associated with folding is the first level of control over the general pattern of fracture distribution while faulting is a secondary control over the development of local fractures in a fault related fold

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

late carboniferous high end t ehf t granitoids enclaves and dikes in western junggar nw china ridge subduction related magmatism and crustal growth abstract we report results of petrologic geochronological and geochemical investigation of the late carboniferous diorites granodiorites amphibole am bearing granites and associated dioritic and monzonitic enclaves and mafic and granitic dikes in the keramay area of the western junggar region of central asian orogenic belt caob zircon u pb dating suggests that they were generated in the late carboniferous 316 304 ma the diorite and granodiorite compositions extend over a wide range of sio2 53 70 wt sr 240 602 ppm and mg 41 58 values and are characterized by moderately fractionated rare earth element ree patterns nb ta depletion and relatively low y and yb contents the mafic dikes consist of dolerites diorite porphyries and minor granodiorite porphyries and have variable sio2 51 59 wt and high mg cr and ni values with the exception of two samples with relatively high heavy ree hree contents the mafic dikes exhibit trace element characteristics similar to diorites and granodiorites the am bearing granites and a granite porphyry dike sample have high levels of sio2 73 77 wt hrees e g yb 3 46 15 7 ppm and low mg cr and ni contents along with clearly negative eu ba and sr anomalies similar to typical a type granites all granitoids enclaves and dikes in this region have high positive end t 7 13 to 9 74 and zircon ehf t 10 to 16 values and moderate initial 87sr 87sr ratios 0 7004 0 7049 mineral composition data suggest that the parental magmas for mafic dikes are similar to cenozoic sanukitoids in the setouchi arc area japan and were possibly generated under water rich and high oxygen fugacity nno 1 5 to nno 2 7 conditions they most likely originated from partial melting of a mantle source variably modified by subducted oceanic crust derived melts and minor fluids and subsequently underwent fractional crystallization the diorites and granodiorites were possibly generated by magma mixing between enriched lithospheric mantle and juvenile lower crust derived magmas coupled with minor crystal fractionation the am bearing granites and granite porphyry dike were produced by partial melting of juvenile crustal materials at shallow crustal levels taking into account widespread contemporaneous magmatism including morb type basalts and slab derived adakites in western junggar we suggest that the keramay intrusive rocks were generated in a special arc setting related to ridge subduction and resultant slab window which played an important role in the crustal growth of the caob

evidence in cerro pampa volcanic rocks for slab melting prior to ridge trench collision in southern south america late miocene approx 12 ma hornblende bearing andesitic to dacitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage wasysym pagestyle empty declaremathsizes 10 9 7 6 begin document mathrm 63 to 68 sio 2 1 2 to 1 9 k 2 o end document adakite flows at the small cerro pampa center in patagonia 47 55 s 71 25 have some of the clearest slab melt geochemical signatures yet seen in a phanerozoic center on continental crust these magmas formed in response to melting of the hot thin slab that was subducting beneath south america prior to the collision of the chile rise at approx 6 ma or at approx 10 ma their n morb like 87 sr 86 sr 0 70285 0 70309 and low 206 pb 204 pb 18 44 18 59 ratios show that they could have been generated by

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

a decade of shear wave splitting in the earth s crust what does it mean what use can we make of it and what should we do next it is 10 years since shear wave splitting throught to be diagnostic of some form of seismic anisotropy was first positively identified in the earth s crust from the beginning it was argued that the splitting was probably associated with the presence of stress aligned cracks inclusions in the crust and that this would provide the opportunity for monitoring the in situ geometry of cracks and stress in a variety of different circumstances and in a variety of different applications

early silurian 440 ma adakitic andesitic and nb enriched basaltic lavas in the southern altay range northern xinjiang western china slab melting and implications for crustal growth in the central asian orogenic belt abstract as an important part of the central asian orogenic belt caob the altay range contains large scale paleozoic magmatic rocks however owing to the lack of precise age constraints the tectonic setting and petrogenesis of the magmatic rocks in this area have been controversial which has led to the debate on phanerozoic crustal growth mechanisms and accretionary orogenic processes in the caob herein we report geochronological and geochemical data of the suoerkuduke adakitic andesitic and nb enriched basaltic neb lavas in the southern margin of the southern altay range la icp ms zircon u pb analyses for five adakitic andesitic and neb samples indicate that they were coevally generated in the early silurian 440 ma the adakites and basaltic andesites are geochemically characterized by high na 2 o k 2 o sr y al 2 o 3 sr e nd t and zircon e hf t values and relatively low 87 sr 86 sr i ratios the nebs are sodium rich and have higher tio 2 p 2 o 5 zr nb and nb u values than those of typical arc basalts they also have positive e nd t values and positive and variable zircon e hf t values we suggest that the suoerkuduke adakites were derived by a partial melting of the subducted oceanic crust with minor overlying sediments and the continuous compositional variations between adakites and basaltic andesites confirm that the interaction between slab melts and mantle peridotite played an important role in the formation of basaltic andesites the associated nebs were possibly generated by a partial melting of mantle wedge peridotites metasomatized by slab derived adakitic melts and minor fluids in combination with the occurrence of voluminous silurian devonian granitoids coeval ophiolite melanges and a series of intra arc basins a slab window model triggered by slab tearing is proposed to account for the formation of the suoerkuduke adakite basaltic andesite neb suites the upwelling of the asthenospheric mantle through the slab window probably caused significant extension in the overlying lithosphere extensive melting of the subducted oceanic crust mantle and juvenile lower crust and intense interaction between slab derived melts or fluids and the mantle wedge slab tearing and subsequent asthenospheric upwelling possibly played an important role in the crustal growth of the caob during the phanerozoic

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

investigation of pore structure and petrophysical property in tight sandstones abstract laboratory measurements include porosity permeability high pressure mercury intrusion hpmi nuclear magnetic resonance nmr measurements and microscopic analysis of thin sections and scanning electron microscope sem were used to gain insight into the nature of the pore throat structure including pore geometry pore size distribution and pore connectivity of bashijiqike tight gas sandstones in keshen 5 keshen 6 keshen 8 keshen 9 and keshen 13 wellblock of kuqa depression the relationships between microscopic pore structure parameters and macroscopic petrophysical behaviors were investigated by regression analysis the results show that various types of pores with wide ranges of pore radius are observed the pore throats are very small commonly rapex controls permeability in tight gas sandstones the nmr pore size distribution is broader than the hpmi pore throat distribution in the nmr analysis only the pore systems connected by the relatively large pore throats should be accounted as movable and some portion of large pores which are not connected by effective large pore throats are not effectively movable the effective movable porosity is calculated by excluding those pores restricted by the tiny pore throats the content of effective movable porosity from nmr measurements shows good correlation relationship with the porosity connected by large pore throats in hpmi analysis three typical types of pore structures are recognized through thin section sem analysis combined with the capillary curve and nmr t2 spectrum and the microscopic and macroscopic characteristics of the three pore structures are investigated the pore structure were comprehensively evaluated and characterized by linking nmr t2 spectrum with hpmi analysis

nd sr isotopic mapping of the chinese altai and implications for continental growth in the central asian orogenic belt article i nfo the chinese altai orogen is a typical domain of the central asian orogenic belt and in previous syntheses was divided into six tectonic units or terranes in this paper 41 new and 36 published whole rock nd and sr isotopic data for granitoid intrusions and mafic dykes within units 2 and 3 of the central altai and units 4 to 6 of the southern altai were used for isotopic mapping the age of the samples used is known from u pb zircon dating end t values range from 4 to 2 in the central altai units 2 and 3 and from 1 4 to 6 in the southern altai units 4 and 5 and up to 8 4 in unit 6 correspondingly nd model ages tdm decrease from 1 6 1 1 ga in the central altai to 1 0 0 5 ga in the southern altai except for a continental fragment in unit 4 isr is elevated in the central units decreasing to b0 705 in the southern units 5 and 6 these results demonstrate that the central altai contains widespread old crustal components in its basement whereas the southern altai comprises a higher proportion of juvenile crust when integrated with published u pb zircon ages our data show that new crust grew southwards in the chinese altai in addition our data suggest that units 2 and 3 are a single tectonic unit whereas the southern altai unit 4 which previously was regarded as a single young accretionary terrane should be subdivided into two units a younger arc and an older continental fragment the granitoids in the chinese altai can be divided into syn and post non orogenic groups the syn orogenic granitoids have ca 50 80 mantle derived component whereas the post non orogenic granitoids contain ca 70 90 the generation of syn and post non orogenic granitoids represents horizontal lateral and vertical crustal growth which contributed ca 18 28 and 7 8 respectively to the area of the chinese altai this work shows that isotopic mapping is an effective approach to elucidate terrane composition in the central asian orogenic belt and thereby better constrain palaeozoic crustal growth

fracture systems and petrophysical properties of tight sandstone undergoing regional folding a case study of the cretaceous reservoirs in the kuqa foreland thrust belt tarim basin abstract the kuqa foreland thrust belt of the tarim basin hosts a prolific hydrocarbon system the main reservoirs are characterized by tight sandstone with low porosity and ultra low permeability buried presently at more than 5000 m depth natural fracture systems have a significant impact on reservoir performance and it may form paths for fluid flow in tight sandstone reservoirs and therefore plays an essential role in controlling high production rates with the aim of analyzing the distribution of favorable reservoir we studied subsurface fracture patterns and petrophysical properties of the bashijiqike formation with data collected from dabei structure and keshen structure we document the presence of four fracture systems whose distribution is related to far field stress thrusting and folding in the keshen anticlines higher fracture intensities are observed in the limbs with principal east west hinge parallel fracture system direction the hinge parallel fracture system includes a moderate dipping fracture set and a vertical fracture set crosscutting relationships stress field inversion and fracture surface textures suggest that the moderate dipping fracture set consists of shear fractures and can be explained by intra layer shear caused by thrusting the vertical fracture set may be formed due to the local outer arc extension during folding the oblique fracture system shows scattering in strike and is supposed to be related to a shear event of locally left lateral strike slip whereas in the dabei anticlines the major system is hinge perpendicular fracture system nw se showing a trending of increasing fracture intensity from limbs to hinge the hinge perpendicular fractures system in the dabei anticlines consist of shear fractures and tension fractures stress inversion result shows it corresponds to northwest southeast orientation compression stage besides we analyzed petrophysical properties include porosity permeability young s module poisson ratio and compressive strength of the bashijiqike formation concerning their structural position within folds it indicates that the highest porosity and permeability are associated with dissolution and fracturing the compressive strength of different structural positions also restrict the variations of mechanical properties of rocks the best reservoir quality is suggested in the hinge zone and areas close to thrust the findings have given maybe helpful for providing favorable reservoir targets for prospecting in the compressional basin

review of diagenetic facies in tight sandstones diagenesis diagenetic minerals and prediction via well logs abstract the tight sandstones are characterized by low porosity low permeability complex pore structure and strong heterogeneity due to the extensive diagenetic modifications they experienced understanding of the impact of diagenetic alterations on reservoir quality is crucial to the hydrocarbon exploration and production in tight sandstones diagenetic facies which is the comprehensive description of the diagenesis and diagenetic minerals determines the formation and distribution of sweet spot by correlating the diagenetic facies to well log responses the subsurface distribution of porosity and permeability can be predicted however the prediction of diagenetic facies and reservoir quality via well logs in tight sandstones remains a challenging task this paper critically reviews the impact of diagenesis and diagenetic minerals on reservoir quality in tight sandstones and establishes a model for prediction of diagenetic facies via well logs as assessed from peer reviewed papers in the literature as well as from the authors personal experiences this review begins with reviewing the impacts of compaction cementation dissolution and various types of diagenetic minerals on reservoir quality evolution the definition and classification schemes of diagenetic facies are then discussed and the reservoir quality as well as diagenetic evolution sequence of various diagenetic facies is summarized the same diagenetic facies commonly display similar compositional and textural attributes matrix and cement as well as porosity systems the well log responses gr ac den cnl and rt of various diagenetic facies are summarized by the calibration of log values with cores and related thin sections by translating the diagenetic facies to conventional well logs a predictable model which can be used for subsurface reservoir quality prediction is established then the theory of ecs logs is reviewed and the application of ecs logs in diagenetic facies evaluation is discussed at last the quantitative characterization for various type and degree of diagenesis is reviewed and the subsurface diagenetic facies is predicted by quantitative calculation of the compactional porosity loss cementational porosity loss and dissolution porosity content via well logs correlating the diagenetic facies to well logs provides a powerful tool to predict the distribution of high quality reservoirs in tight sandstones this review will provide insights into the reservoir quality evaluation and sweet spot prediction via well logs in tight sandstones

accretionary orogenesis of the chinese altai insights from paleozoic granitoids zircon u pb dating and whole rock major oxide trace element and nd sr isotope compositions have been determined for four representative granitic intrusions in the sw chinese altai in order to understand the orogenesis and history of crustal growth in the central asian orogenic belt caob the ashile and halatas bodiesare relatively small intrusions formed in the late carboniferous 318 6ma andpermian 267 4ma respectively thelargertarlang tb andkeketuohaibatholiths kb havemagmaticagesof 359 to 412 ma the granitoids consist chiefly of tonalite granodiorite and granite and are metaluminous to weakly peraluminous in composition asi 0 8 1 1 the ashile pluton is characterized by relatively high mgo 2 73 3 54 wt and sr 395 456 ppm contents with low k2o na2o ratios 0 31 0 42 and mantle like nd sr isotopic compositions endt 2 85 to 3 26 initial 87 sr 86 sr 0 7047 0 7051 it probably formed by fractional crystallization of a mantle derived magma with limited assimilation of oldcrustalmaterial theotherintrusionshavenear zeroornegative endtvalues 0 67to 4 41 andhighersrisotopiccompositions initial 87 sr 86 sr 0 7067 0 7092 indicating a mixture of juvenile and old components in their genesis these intrusions are characterized by relatively low k rb cs contents and low rb sr ratios mostly b0 6 and generally exhibit moderate to high heavy rare earth element hree concentrations ybn 8 26 and weak hree fractionation dy ybnb1 5 with or without negative europium anomalies eu eu 0 5 1 0 these characteristics are consistent with an origin by dehydration melting of a hornblendebearing mid crustal source above the garnet stability field dominated by mafic to intermediate rocks this suggests that the deep crust of the chinese altai may contain a considerable proportion of juvenile material mantle derived magmatism probably in an active extensional continental margin setting played an important role in the formation of the chinese altai acting not only as a heat sourceforcrustalmeltingbutasasourceofjuvenilecomponents therelativelysmallvolumesanddistinctgeochemicalcompositions of the ashile and halatas plutons may imply a substantial change in the geodynamic setting in the late paleozoic 2007 elsevier b v all rights reserved

the geochemistry of young volcanism throughout western panama and southeastern costa rica an overview oblique aseismic subduction below western panama and southeastern costa rica has produced recent arc related volcanism the aseismicity is probably related to the subduction of relatively hot oceanic lithosphere the volcanism throughout this region over the past 2 ma has been quite distinct consisting of felsic magmas andesites to rhyolites but mainly dacites with geochemical signatures suggesting a metamorphosed basaltic source it is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge rocks with slab melt geochemistries and associated with young subducted crust have been termed adakites elsewhere the young adakite melts are sometimes associated with a few rare young high nb basalts but there is no obvious genetic link between them through differentiation high nb basalts may also be derived from the partial melting of the subducted oceanic crust high nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the catalina schist california alternatively the high nb basalts may be partial melts of phlogopite rich mantle that has previously reacted with adakite magmas eruption of adakites and high nb basalts was preceded by a 2 3 ma period of relative quiescence prior to this there was a 7 ma period of calc alkaline volcanism typical of the present day magmatism associated with a distinct benioff zone found throughout the central american arc the abrupt transition in volcanism with time from an early calc alkaline sequence to a later adakite high nb basalt sequence may record a change in the tectonic setting of western panama and southeastern costa rica over the past 12 ma

genesis of post collisional granitoids and basement nature of the junggar terrane nw china nd sr isotope and trace element evidence the post collisional magmatism of the junggar terrane is characterized by intrusion of large amounts of granitoids and minor basic ultrabasic rocks the granitoids comprise two magmatic suites calc alkaline and alkaline which were emplaced contemporaneously at about 294 ma the calc alkaline rocks are typically sodium rich na2o k2o 1 1 3 5 and metaluminous a nk 1 0 a cnk 0 78 1 04 they show mildly fractionated ree patterns la yb n 15 and spidergrams with strong depletion of nb ti and to a lesser extent sr the alkaline granites have high contents of sio2 75 78 alkalis nb hree y sn f and high feo mgo ratios and huge sr and eu depletion in the spidergrams moreover they display characteristic tetrad ree patterns and non charac trace element behaviour the two rock suites have similar initial nd and sr isotopic compositions with end t in the range 5 2 to 7 1 and isr mostly in the range 0 7031 0 7041 this points to a predominance of juvenile components in their sources the calc alkaline rocks are most probably derived by dehydration melting of a basic lower crust leaving behind a granulite residue the process was probably triggered by underplating of mantle derived basic magmas in an extensional regime the alkaline granites are considered to have formed by differentiation of the calc alkaline granitoids our study argues for a juvenile continental crust for the basement of the junggar terrane which is likely dominated by early paleozoic oceanic crust and arc complex that were deeply buried during the late paleozoic subduction and accretion

geochemistry of bimodal amphibolitic felsic gneiss complexes from eastern massif central france high grade basic and acidic meta igneous rocks are widespread in the bimodal amphibolitic felsic gneiss complexes which are characteristic formations of the middle allochthonous unit from eastern and southern french massif central the metabasites from the lyonnais and doux complexes are chemically diverse and range from n morb type tholeiitic to transitional types the two populations are not related by fractional crystallization or crustal contamination processes and their chemical characteristics reflect differences in their mantle sources an ensialic setting is supported by the crustally derived character of some of the associated felsic rocks but the presence of n morb type metabasites argues for an extensional environment this bimodal association compares well with the magmatism of rifted continental margins and may reflect a transitional stage between continental rifting and oceanic crust formation during the cambro ordovician spreading event

slab tearing following ridge trench collision evidence from miocene volcanism in baja california mexico abstract neogene magmatic activity in central baja california underwent a major change at ca 12 5 ma when the pacific farallon active oceanic ridge collided with the trench east of vizcaino peninsula the calc alkaline magmatism which built the comondu volcanic arc vanished and was replaced by unusual volcanic associations which were erupted within six late miocene to quaternary volcanic fields jaraguay san borja san ignacio santa rosalia santa clara la purisima delineating a 600 km array along the baja california peninsula new fieldwork k ar datings and geochemical analyses on jaraguay and san borja lavas combined with previous data allow us to show that these associations include 1 adakites emplaced between 12 5 and 8 2 ma in the santa clara santa rosalia and jaraguay volcanic fields 2 niobium enriched basalts neb in santa clara and santa rosalia 11 2 to 7 4 ma 3 tholeiitic basalts and basaltic andesites 11 3 to 7 2 ma displaying a very weak subduction imprint in la purisima and san ignacio 4 alkali trachybasalts in mesas san carlos and santa catarina northwest of jaraguay 9 3 to 7 5 ma and finally 5 basalts and associated magnesian basaltic andesites and andesites locally referred to as bajaites the latter lavas display very specific geochemical characteristics including very high sr and ba contents they were emplaced in all the above mentioned volcanic fields between 14 6 and 5 3 ma the origin of these late miocene volcanics has been considered linked either to the opening of an asthenospheric window through which they ascended tholeiites and alkali trachybasalts or to the melting of its edges due to thermal erosion adakites and the subsequent reaction between adakitic melts and the supraslab mantle neb and bajaites however the identification of the remnants of the pacific farallon fossil ridge south of vizcaino peninsula indicates that active ridge subduction was not responsible for the opening of the asthenospheric window we propose that the emplacement of slab window related volcanic rocks between 13 and 7 ma from ca 25 n to 30 n was due to a process of slab tearing which started when the active ridge collided with the trench east of vizcaino peninsula the oldest part of the downgoing plate sunk into the mantle leading to the opening of a tear in the slab which likely started from the southern limit of the already existing southern california slab window it propagated parallel to the trench over about 600 km and developed into an asthenospheric window

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out

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