isolation of diagenetic overgrowths on quartz sand grains for oxygen isotopic analysis abstract diagenetic overgrowths on detrital quartz sand grains in sandstones can be isolated for direct measurement of their oxygen isotope ratios interfacial materials between detrital cores and diagenetic overgrowths are dissolved by etching with dilute hf the weakened interface breaks when grains are agitated ultrasonically subsequent particle size separation is an effective means of concentrating the diagenetic component

diagenesis in north sea hpht clastic reservoirs consequences for porosity and overpressure prediction upper jurassic clastic reservoirs of the fulmar formation in the central north sea possess anomalously high porosities for their present day depth of burial reservoirs with the highest overpressures have the highest porosities possess less macroquartz cement and have significant secondary porosity quartz cementation in hpht high pressure high temperature reservoirs has been inhibited by a combination of factors high overpressure limited fluid movement presence of early grain coating cements high pore fluid salinity and possibly petroleum migration secondary porosity has contributed to reservoir quality with an average of 4vol extra porosity created quantitative prediction of porosity would require an improved depositional model for the fulmar accurate thermal and pressure modelling and detailed knowledge of field filling and leakage histories theoretical calculations indicate that diagenetic reactions occurring in the fulmar formation smectite illitisation and quartz cementation did not generate significant overpressure because seal permeabilities were too high and the rate of volume increase associated with the reactions too small therefore diagenetic reactions can effectively be ignored when modelling overpressure generation in the central north sea although cementation will affect rock permeability and rates of fluid dissipation

geochemistry and zircon geochronology of the permian a type hasanrobat granite sanandaj sirjan belt a new record of the gondwana break up in iran the sanandaj sirjan metamorphic plutonic belt ssb in west central iran is a polyphase metamorphic terrain composed of dominantly greenschist grade metasedimentary and metavolcanic rocks and felsic to mafic plutons of neoproterozoic phanerozoic ages the hasanrobat granite in central ssb occurs as a single pluton 20 km 2 surface area with relatively consistent mineralogy and chemistry quartz alkali feldspars microcline and perthite sodic plagioclases and biotite are the main constituents commonly associated with minor amphibole accessory phases include zircon allanite apatite and magnetite the country rocks are upper carboniferous lower permian sandstones and dolomitic limestones scattered patches of skarn type assemblages dominated by tremolite and talc occur in the dolomitic limestones and sandstones are recrystallized to a coarse grained quartzite at contact with the granite the granite is metaluminous to slightly peraluminous and is distinguished by high feo t mgo ratios typical of ferroan a type granites the a type affinity is also reflected by high na 2o k 2o high ga al ratios high contents of large ion lithophile elements lile high field strength elements hfse and rare earth elements ree as well as low contents of sr and distinct negative eu anomalies the biotites are aluminous fe rich and plot near the siderophyllite corner in the quadrilateral biotite diagram they are further distinguished by high fluorine contents 0 61 to 1 33 wt amphibole is ferrohastingsite in composition ion microprobe analyses of zircon grains separated from a representative granite sample yielded concordant u pb ages with weighted mean 206 pb 238 u age of 288 3 3 6 ma the granite and the country rocks are cut by a set of mafic dykes with asthenosphere like geochemical signatures such association suggests anorogenic intraplate magmatism in lower permian in the region this provides further evidence for and significantly constrains timing of a major extension in upper paleozoic in iran previously inferred from the rock record the extension led to the gondwana break up and the opening of neotethys ocean between sanandaj sirjan and zagros in permian

exceptional reservoir quality in hpht reservoir settings examples from the skagerrak formation of the heron cluster north sea uk abstract as the exploration of hydrocarbons moves into more complex and deeper basinal settings the need to understand the effect of high temperatures and high pressures on reservoir quality and rock properties becomes even more important the fluvial channel sandstone reservoirs of the skagerrak formation in the central north sea exhibit anomalously high porosities and permeabilities considering their depth of burial 3500 m below sea floor despite the complex depositional setting diagenetic history high overpressure and temperatures encountered in the skagerrak formation hydrocarbons are currently being produced the skagerrak formation reservoirs used in this study have encountered overpressures of 40 mpa and temperatures up to 185 c at present day maximum burial to identify the role played by the high pressure and high temperature encountered in the reservoir sandstones a multidisciplinary approach involving petrographic fluid inclusion and burial history modelling studies has been adopted our interpretation of the results is that the generation of shallow overpressure in these fields limited mechanical compaction and also played an important role in minimizing pressure solution in the chemical compaction regime as evidenced by reduced quartz cementation fluid inclusions found in the quartz overgrowths indicate late stage development with temperatures of formation in the range 130 to 170 c coincident with late stage deeper burial hydrocarbon emplacement occurred after quartz cementation and has had little to no effect on porosity preservation the formation of well developed authigenic chlorite 70 surface coating and to a lesser extent illite clay coats with burial had a positive effect on porosity preservation even though permeability was marginally reduced in the illite rich sandstones a schematic porosity and quartz cement evolution model has been developed which allows for pre drill prediction of reservoir quality in the heron cluster and provide valuable insights for other complex high pressure high temperature reservoirs

the sanandaj sirjan zone in the neo tethyan suture western iran zircon u pb evidence of late palaeozoic rifting of northern gondwana and mid jurassic orogenesis abstract the zagros orogen marking the closure of the neo tethyan ocean formed by continental collision beginning in the late eocene to early miocene collision was preceded by a complicated tectonic history involving pan african orogenesis late palaeozoic rifting forming neo tethys followed by mesozoic convergence on the ocean s northern margin and ophiolite obduction on its southern margin the sanandaj sirjan zone is a metamorphic belt in the zagros orogen of gondwanan provenance zircon ages have established pan african basement igneous and metamorphic complexes in addition to uncommon late palaeozoic plutons and abundant jurassic plutonic rocks we have determined zircon ages from units in the northwestern sanandaj sirjan zone golpaygan region a sample of quartzite from the june complex has detrital zircons with u pb ages mainly in 800 1050 ma with a maximum depositional age of 547 32 ma latest neoproterozoic earliest cambrian a shrimp u pb zircon age of 336 9 ma from gabbro in the june complex indicates a carboniferous plutonic event that is also recorded in the far northwestern sanandaj sirjan zone together with the permian hasanrobat granite near golpaygan they all are considered related to rifting marking formation of neo tethys scarce detrital zircons from an extensive package of metasedimentary rocks hamadan phyllite have ages consistent with the triassic to early jurassic age previously determined from fossils these ages confirm that an orogenic episode affected the sanandaj sirjan zone in the early to middle jurassic cimmerian orogeny although the cimmerian orogeny in northern iran reflects late triassic to jurassic collision of the turan platform southern eurasia and the cimmerian microcontinent we consider that in the sanandaj sirjan zone a tectonothermal event coeval with the cimmerian orogeny resulted from initiation of subduction and closure of rift basins along the northern margin of neo tethys

amorphous silica nanofilms result in growth of misoriented microcrystalline quartz cement maintaining porosity in deeply buried sandstones deeply buried sandstones in sedimentary basins typically have low porosity due to cementation and compaction there are several known causes of anomalously high porosity in sandstones one of which is microquartz coatings on sand grains that seem to inhibit growth of quartz cement however there has been no mechanistic understanding of why or how microquartz grows or why it maintains high porosity in sandstones here we have used high resolution scanning electron microscopy electron backscattered diffraction and transmission electron microscopy to study the microquartz cemented late cretaceous heidelberg formation germany we have revealed that a nanofilm of amorphous silica 50 100 nm and a layer of chalcedony are between the detrital grain and microquartz cement the amorphous silica insulates the detrital quartz grains and prevents syntaxial growth while microquartz adopts the orientation of the underlying chalcedony with its fast growth c axis parallel to the grain surface thus preventing growth into the pore now that we know what controls microquartz growth and why it preserves porosity it can be used to help identify rank and appraise deeply buried petroleum accumulations

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

inversion of field data in fault tectonics to obtain the regional stress iii a new rapid direct inversion method by analytical means summary a new method for determining the reduced stress tensor with four degrees of freedom the orientations of the three principal stress axes as well as the ratio of principal stress differences using fault slip data or focal mechanisms of earthquakes is presented from a computational point of view the inversion of fault slip data is made in a direct way by purely analytical means as a result the determination process is extremely fast and adaptable on small microcomputers from a physical point of view the method aims at simultaneously i minimizing the angles between theoretical shear stress and actual slip vector and ii having relative magnitudes of shear stress large enough to induce slip despite rock cohesion and friction examples of application to actual fault slip data sets with good or poor variety of fault slip orientations are shown the double significance of the basic criterion adopted results in a more realistic solution of the inverse problem than the single minimization of the shear stria angle

the relation between regionally consistent stretching lineations and plate motions abstract regional stretching lineations in the himalayas and southern tibet coincide with the known relative motions of the converging plates this relation is used to infer plate motions from the stretching lineations in the variscan and caledonian belts of europe and in the late proterozoic belts of northeast and east africa in each region high angle convergence of plates produced transverse stretching lineations and high angle collision was followed by or combined with large displacements first by ductile shear and later by brittle fractures parallel to the plate boundaries apparent stretching as a result of volume loss ramsay wood 1973 is common but recognisable as such reorientation by later deformations must be allowed for ramsay 1967 in ductile shear zones simple shearing leads to the progressive approach towards the shear plane of the xy plane of the strain ellipsoid and progressive approach of x the extension axis to the shear line ramsay graham 1970 thus large extensions approximate to the shear line large extensions tend to produce sheath folds whose acutely curved axes approach the stretching direction quinquis et al 1978 also the pre existence of a strong linear fabric predisposes the development of later folds with axes parallel to the earlier linear fabric for all of these reasons the extension axis x rather than the fold axis is of prime significance

systematic changes in orientation of linear mylonitic fabrics an example of strain partitioning during transpressional deformation in north golpaygan sanandaj sirjan zone iran abstract kilometer scale shallowly dipping nw striking top to the ne reverse and dextral strike slip shear zones occur in metamorphic rocks of north golpaygan these metamorphic rocks are exposed at the ne margin of the central part of the sanandaj sirjan zone in the hinterland of the zagros orogen nw striking top to the ne normal shear zones were also found in a small part of the study area structural evidence of three deformation stages were found pre mylonitization metamorphic mineral growth happened during d1 the main mylonitization event was during the d2 deformational event following coaxial refolding synchronous to retrograde metamorphism of amphibolite to greenschist facies in the late cretaceous paleocene and before d3 folding and related mylonitization we documented the systematic changes in the orientations of d2 linear fabrics especially stretching lineations and superimposition relations of structures it is concluded that the dextral strike slip and dip slip shear zones were coeval kinematic domains of partitioned dextral transpression the shallowly dipping reverse and strike slip shear zones are compatible with partitioning in a very inclined transpressional model fabric relations reflect that the top to the ne normal shear zones were not produced during deformation partitioning of inclined dextral transpression the late cretaceous paleocene strain partitioning was followed by later n s shortening and ne extension in the north golpaygan area

gondwanan eoarchean neoproterozoic ancient crustal material in iran and turkey zircon u pb hf isotopic evidence the sanandaj sirjan zone is a basement culmination northeast of the neo tethys suture in iran in this zone near azna granite has a magmatic zircon u pb age of 568 11 ma with 900 800 ca 2400 and ca 3600 ma inherited cores the ca 3600 ma inherited zircon is the oldest crustal component yet detected in iran near chadegan orthogneiss has a magmatic zircon u pb age of 637 15 ma and carries ca 1000 and 2000 ma inherited zircons inherited 900 1000 ma zircons have juvenile initial ehf values of ca 8 to 9 whereas the younger 630 and 568 ma magmatic zircons show lower initial ehf values however the 3600 ma core has initial ehf 0 0 a neoproterozoic rim on the inherited 3600 ma core has the most extreme initial ehf value of 18 the hf isotopic data indicates generation of the magmatic protoliths from a mixture of juvenile neoproterozoic and archean sources previous studies showed that in turkey the central anatolian crystalline complex is underlain by neo eoarchean rocks the menderes mas

late palaeozoic hydrocarbon migration through the clair field west of shetland uk atlantic margin geochemical analysis of bitumen and hydrocarbon bearing fluid inclusions from the devonian carboniferous clair field indicates that the reservoirs contain a mixture of oils from different marine and lacustrine sources reconstruction of the clair field oil charge history using fluid inclusion petrography show that oil charging occurred at times of k feldspar quartz and calcite cementation temperature composition time data yielded from the integration of fluid inclusion microthermometry with high resolution ar ar dating date hydrocarbon bearing k feldspar overgrowths at 247 3 3 ma these data show that in order for oil to be trapped within primary fluid inclusions in k feldspar overgrowths hydrocarbon migration throughout the uk atlantic margin must have been taking place during the late palaeozoic and as such current industry oil play models based solely on oil charging from jurassic cretaceous marine sources are clearly incomplete and need revision apatite fission track analysis and vitrinite reflectance data were used to reconstruct thermal burial histories and assess potential oil generation from middle devonian lacustrine source rocks thermal history data from wells along the rona ridge adjacent to the clair field show that the palaeozoic section was heated to greater than 100 c at some time between 270 and 230 ma confirming that devonian source rocks were mature and expelling oil during the late palaeozoic at the time that authigenic k feldspar overgrowths were growing in the clair field

cenozoic evolution of neotethys and implications for the causes of plate motions africa north america eurasia plate circuit rotations combined with red sea rotations and new estimates of crustal shortening in iran define the cenozoic history of the neotethyan ocean between arabia and eurasia the new constraints indicate that arabia eurasia convergence has been fairly constant at 2 to 3 cm yr since 56 ma with slowing of africa eurasia motion to 1 cm yr near 25 ma coeval with the opening of the red sea ocean closure occurred no later than 10 ma and could have occurred prior to this time only if a large amount of continental lithosphere was subducted suggesting that slowing of africa significantly predated the arabia eurasia collision these kinematics imply that africa s disconnection with the negative buoyancy of the downgoing slab of lithosphere beneath southern eurasia slowed its motion the slow steady rate of northward subduction since 56 ma contrasts with strongly variable rates of magma production in the urumieh dokhtar arc implying magma production rate in continental arcs is not linked to subduction rate

diagenetic controls on reservoir quality in middle to upper jurassic sandstones in the south viking graben north sea the deeply buried synrift play of the south viking graben is characterized by highly variable reservoir quality an integrated approach incorporating petrophysics petrography and one dimensional basin modeling methods was applied to investigate these variations analysis shows that average porosities below 4000 m 13 123 ft vertical depth below sea floor range from approximately 5 to as much as about 25 in comparable quartz arenitic sandstones from porosity depth trends three porosity categories can be recognized normal low and high porosity sandstones normal porosity sandstones fall along the regional average porosity depth trend low porosity sandstones have been subject to extensive quartz cementation as a consequence of a higher degree of thermal maturity and plot below the regional porosity depth trend high porosity sandstones plot above the regional porosity depth trend here quartz cementation has been inhibited by grain coating microquartz and thus porosity has been preserved hydrocarbon emplacement has previously been thought to have inhibited quartz cementation in the study area but this study concludes that the reservoirs are mainly water wet allowing for continued quartz cementation despite the presence of hydrocarbon pore fluids predicting the distribution of microquartz coated sandstones and the degree of thermal maturity is therefore fundamental for successful exploration in the deeply buried parts of the synrift play this study presents a regional and stratigraphic framework for such predictions that may be incorporated into play models in the area

heterogeneous packing and quartz cementation of the eureka quartzarenite middle ordovician utah and nevada u s a abstract the eureka sandstone is a quartzarenite 200 m thick that was deposited on the eastern shelf of the cordilleran foreland basin from canada to california it has many characteristics of other lower paleozoic quartzarenites in terms of its sheet like geometry mineralogical and textural maturity 99 5 detrital quartz almost free of detrital clay shallow marine environment of deposition and abundance of quartz cement 10 26 however it displays wide variations in compaction as measured by intergranular volume igv range of 16 that are not all related to differences in burial depth plus igv values as low as 10 sutured grain contacts and fractured grains cannot account for such abnormally low igv values which suggests that dissolution of the edges of grains where they overlapped their neighbors the fuchtbauer effect leaving minimal evidence was a major process of compaction the normal evolution of microquartz overgrowths 10 mm with time was retarded consequently microquartz and mesoquartz cement 80 dominate over macroquartz 20 and are the only cements in the poorest cemented beds and laminations illite co precipitated with micro and mesoquartz and impeded quartz cementation by coating quartz crystal faces typical macrocrystalline quartz was the final cement only in illite poor samples heterogeneous illite distribution may reflect heterogeneous abundance of k feldspar or detrital illite that possibly served as seed crystals for authigenic illite in spite of reaching temperatures 135 c for 100 my many eureka sandstones especially in utah remain incompletely and unevenly cemented consequently beds range from weakly friable to pervasively quartz cemented quartzite in the same outcrop porosity was generally reduced to less than 2 except in rare laminations where it reaches 22 grain size had some influence on the amount of quartz cement locally but the chief cause of cement heterogeneity appears to have been illite abundance the most likely major source of silica for quartz cement was quartz grain dissolution at shale beds and clay drapes although documentation is absent intergranular grain dissolution was a minor source of silica overlap quartz averages less than 4 oxygen isotopic data are compatible with the precipitation of quartz either from diagenetically evolved meteoric water that invaded the sandstones from tectonic highlands to the west or evaporative brines

origin of quartz cements in some sandstones from the jurassic of the inner moray firth uk the extent of quartz cementation in shallow marine sandstones of the brora arenaceous formation oxfordian is closely related to the occurrence and abundance of rhaxella perforata sponge spicules three cement morphologies are identified chalcedonic quartz microquartz and mesoquartz chalcedonic quartz forms matrix supported cements which preserve moulds of rhaxella spicules chalcedonic quartz crystals have inequant development of crystal faces on average 0 1 m in diameter and are the first formed cement and reveal homogeneous dark grey tones on the sem cl bei microquartz forms 5 10 m diameter crystals which commonly grow on chalcedonic quartz substrates and show various grey tones under sem cl bei mesoquartz crystals grow in optical continuity with their host grains have 20 m a axial diameter crystals and exhibit distinctly zoned luminescence although no opaline silica is preserved the quartz cement is interpreted to have formed from an opaline precursor detrital quartz has an average 18o composition of 12 2 and mesoquartz syntaxial overgrowth has an average 18o composition of 20 0 estimates of the 18o compositions of microquartz and chalcedonic quartz are complicated by the problem of isolating the two textural types mixtures of the two give consistently higher 18o compositions than mesoquartz the higher estimate being 39 2 from oxygen isotope data the formation of quartz microquartz and chalcedonic quartz is interpreted to have taken place between 35 and 71 c in marine derived pore waters organic and inorganic maturation data constrain the upper temperature limit to less than 60 c

extensive authigenic quartz overgrowths in the gas bearing haynesville bossier shale usa abstract in sandstone reservoirs despite grain rearrangement during compaction significant pore volumes may be retained prior to the onset of late diagenetic quartz cementation in mudstone reservoirs grain rearrangement during compaction results in significant pore volume reduction prior to late diagenesis where quartz overgrowths have been previously reported in post compaction mudstones they have been in volumetrically low concentrations and interpreted as anomalous occurrences quartz cementation alters rock brittleness resulting in changes to mechanical fracture properties quartz overgrowths reduce reservoir porosity and permeability we present petrographic evidence of two phases of quartz cement in the haynesville bossier shale i grain replacive and ii quartz overgrowths carbonate grain replacement is volumetrically low petrographic evidence indicates three processes in the development of quartz overgrowths mica and pyrite are i engulfed and ii displaced by quartz overgrowth cement the absence of a supportive primary granular framework surrounding engulfed detrital and early authigenic minerals would indicate that quartz overgrowths are also iii replacive pressure dissolution of detrital quartz silt grains and smectite to illite transformation are likely sources of silica for quartz cement this study is the first to document large scale replacive authigenic quartz overgrowth development within a producing mudstone

strain partitioning in transpression zones abstract transprcssional strain acting upon structurally anisotropic rocks can be partitioned into separate deformational domains of pure shear and simple shear this contrasts with homogeneous transpression in which both the pure shear and the simple shear strain components are uniformly distributed across the zone of deformation structural weaknesses capable of partially or fully accommodating one component of deformation include lithological contacts rheological heterogeneities and faults or shear zones situated within the deformation zone or lying along its boundaries partitioning of transprcssional strain can occur when stress is applied oblique to pre existing structural weaknesses or can occur during later stages of progressive strain when the early deformation of isotropic rocks imparts sufficient anisotropy to allow subsequent strain to be partitioned partitioning of transpressional strain into domains lying parallel to the deformation zone boundaries can be distinguished from fault stepped transpression in which strain is partitioned along the length of a segmented fault zone mesofracture analysis of rocks affected by mid devonian deformation on both sides of the highland boundary fault zone hbfz in central scotland shows that strain was not homogeneous the mesofracture data suggest that regional north south compression orientated oblique to the pre existing ne sw trending hbfz was partitioned into separate deformational domains the hbfz accommodated most of the simple shear component whilst the rocks flanking the zone were deformed predominantly by pure shear a contemporary example is the san andreas fault in central california where analyses of neo tectonic stresses show that the direction of principal compression is perpendicular to the fault zone many examples of the partitioning of transpressional strain have also been recognised at destructive plate margins where the direction of plate motion is oblique to the edge of the over riding plate

structural evolution of sanandaj sirjan ranges near esfahan iran the sanandaj sirjan ranges constitute the southwestern border of the central iranian plateau and lie between the main zagros fault and zagros mountains on the southwest and the urmia dokhtar volcanic belt on the northeast the ranges occupy a northwest trending belt in which the zagros structural grain is overprinted on the typical central iran structural and stratigraphic framework detailed investigation of the tectonic evolution of part of the belt revealed four major northwest striking high angle crustal fault zones in which most deformation was concentrated throughout cenozoic convergence main mechanisms of crustal shortening on these faults were high angle reverse faulting and dextral translation upper cretaceous northeast southwest convergence resulted in interblock underthrusting of the persian plate along these major faults within or bordering the sanandaj sirjan ranges and the formation of intrablock folds and thrusts continued convergence during the tertiary resulted in the episodic subsidence of elongate northwest oriented troughs southwest of the high angle reverse faults and the parallel uplift of the adjacent blocks on the northeast side of the faults the depressions served as the spatially restricted depocenters for the eocene miocene and pliocene pleistocene sediments pre miocene northwest striking normal faults on the northeastern flank of one of the uplifted blocks are thought to have resulted from post paleocene uplift due to intrablock arching post miocene northeast striking normal faults are structurally spatially and temporally related to at least one of the northwest trending depressions e g the gavkhuni depression and appear to be important in its formation and evolution continued compression has resulted in the pervasive development of near vertical fractures in pliocene pleistocene cemented gravels these fractures indicate holocene reactivation of the northwest and northeast fault zones as do focal mechanism solutions of shallow seismic events right lateral strike slip faulting on large northwest faults was previously thought to be entirely post miocene however the presence of older second order structures on interblock dextral faults and of older intrablock strike slip faults suggests that this style of crustal shortening was also active in the early tertiary

late cretaceous and early miocene andean type plutonic activity in northern makran and central iran two calc alkaline plutonic complexes bazman and natanz intruded through the south eastern active and south western ancient continental margins of central iran have been dated by the rb sr whole rock isochron method at 74 2 and 24 4 5 ma respectively detailed trace element studies together with low 87 sr 86 sr initial ratios 87 sr 86 sr ratios and the gabbros at least were intruded some 10 ma earlier

coeval blueschist exhumation along thousands of kilometers implications for subduction channel processes we herein focus on understanding what controls the detachment and migration of oceanic crustal slices along subduction zones we provide evidence for the coeval middle to late cretaceous exhumation of oceanic blueschists in the same neotethyan subduction zone across more than 3000 km from depths around 30 40 km over a short time interval similar to 25 ma compared to the duration of subduction 100 ma we stress the importance of such a geodynamic process and suggest that this exhumation was promoted by a regional scale change in the long term interplate mechanical coupling on the basis of geological and geophysical evidence we propose that the upward migration of oceanic fragments in subduction zones is normally inhibited and only occurs discontinuously as the subduction channel opens up in response to a major change in plate geodynamics slab geometry and dynamics mantle wedge and slab hydration or a combination of these

eocene gold ore formation at muteh sanandaj sirjan tectonic zone western iran a result of late stage extension and exhumation of metamorphic basement rocks within the zagros orogen a multidisciplinary study including field geology microstructure analysis 40ar 39ar dating and fluid inclusion microthermometry and raman spectrometry has been carried out on the muteh gold deposit located in a greenschist to amphibolite facies metamorphic rock complex of the sanandaj sirjan tectonic zone zagros orogen iran the muteh gold deposit has been previously interpreted as genetically related to precambrian granitic intrusions as an exhalative hot spring deposit related to paleozoic rhyolitic acidic tuffs and to local metamorphic processes host rocks of the gold deposit are predominantly schist and gneiss and subsidiary amphibolite and quartzite and are intruded by leucogranites in the vicinity of the gold deposit the metamorphic and granitic rocks display a subhorizontal mylonitic foliation containing a northeast oriented stretching lineation field investigations in one of the producing open pits show that the gold orebodies are controlled by northwest oriented normal faults and joints dipping to the northeast and the southwest and crosscutting the ductile fabric of the host rocks both the ductile fabric of the host rocks and the gold ore controlling brittle structures are interpreted to have formed within a single continuous extensional event which started with ductile deformation and gradually changed into brittle deformation hydrothermal alteration associated with ore formation consists of quartz muscovite pyrite dolomite ankerite and albite which crosscuts the ductile fabric and overprints the metamorphic minerals of the host rocks pyrite is the dominant opaque mineral and is the major phase associated with gold chalcopyrite marcasite bismuth galena sphalerite and pyrrhotite are subsidiary to rare phases 40ar 39ar incremental heating and in situ laser ablation age data reveal a coherent sequence of cooling and hydrothermal events in the metamorphic complex hosting the muteh gold deposit muscovite samples from the alteration zone and from one quartz vein from the orebodies yield 40ar 39ar ages between 55 7 and 38 5 ma and show that gold mineralization is the youngest among the different dated events this eocene age is consistent with the young structural setting of the gold orebodies revealing an emplacement along northwestoriented normal faults which can be correlated with tertiary extensional tectonic events reported by previous field investigations the data also indicate that gold ore formation is coeval with magmatism in the adjacent tertiary urumieh dokhtar magmatic belt and with intrusive activity within the sanandaj sirjan zone with a 40ar 39ar age of 54 ma near muteh according to our study the remaining 40ar 39ar data reveal a cretaceous to early tertiary metamorphic magmatic and deformation history of the complex hosting the muteh gold deposit the fluid inclusion study showed that distinct fluids were present during the regional geologic evolution predating ore formation and later eocene gold ore formation early regional fluids consist of an h2o co2 nacl fluid with subsidiary n2 and ch4 that underwent local unmixing the salinities of the liquid rich inclusions resulting from unmixing range between 2 2 and 17 5 wt percent nacl equiv and those of the co2 rich vaporrich inclusions range between 2 8 and 7 2 wt percent nacl equiv the high salinity of the regional liquid rich fluids is attributed to metamorphism of evaporite bearing sedimentary rocks magmatic intrusions retrograde hydration reactions metamorphism of impure marbles and migmatization the fluid inclusions spatially associated with the gold veins postdating regional metamorphism and ductile deformation of the host rocks contain a low salinity aqueous fluid with a dilute low density co2 component and maximum salinities between 2 6 and 5 8 wt percent nacl equiv gold deposition at muteh occurred during or after late stages of eocene brittle extension as a result of exhumation of the host metamorphic complex possibly during magmatic activity in the sanandaj sirjam zone it is most likely linked to the introduction of a distinct low salinity low co2 content aqueous fluid during crustal extension these fluids are younger than and therefore unrelated to the highly saline and co2 rich regional metamorphic magmatic fluids our results refute previous models advocating precambrian to paleozoic ages of the muteh gold deposit

tectonics of the zagros orogenic belt of iran new data and interpretations abstract the zagros orogenic belt of iran is the result of the opening and closure of the neo tethys oceanic realm and consists from northeast to southwest of three parallel tectonic subdivisions 1 1 the urumieh dokhtar magmatic assemblage 2 2 the sanandaj sirjan zone and 3 3 the zagros simply folded belt recent detailed and reconnaissance mapping and stratigraphie structural analyses in various parts of the belt indicate that the tectonics of the sanandaj sirjan zone is dominated by either large scale composite duplex structures or low and high angle ne dipping imbricate systems these thrust systems have transported numerous horses and sheets nappes of metamorphosed and non metamorphosed phanerozoic rocks for tens or possibly hundreds of kilometers stratigraphic evidence and synorogenic conglomerates indicate that thrusting was initiated in the late cretaceous and has continued episodically since that time shear sense indicators in various parts of the zone reveal a ne sw direction of transport thrust systems generated in each episode are superimposed upon the previous structures cutting across and displacing them and producing complicated stacks of thrust sheets southwestward transport and stacking of the thrust sheets has resulted in an increase of about 10 15 km in the thickness of the continental crust in the southwestern part of this zone the so called z main zagros thrust which has been traditionally considered as the boundary between the sanandaj sirjan zone and the zagros simply folded belt is by no means a single high angle reverse fault nor is it a narrow zone of crushed rocks but a series of low angle thrusts locally cut by normal faults these thrusts are indistinguishable from those mapped in the sanandaj sirjan zone and are geometrically part of the sanandaj sirjan zone structures there is no single thrust among them that can be singled out and referred to as the main one the boundary of the sanandaj sirjan zone with the urumieh dokhtar magmatic assemblage to the northeast is characterized by a series of structural depressions formed by compression rock sequences including ophiolite complexes exposed along this boundary are strongly dismembered sheared and mylonitized shearing planes and foliations are either subvertical or dipping at high angles to the northeast this zone is suggested to be the suture between the afro arabian and the iranian plates overall the intensity of deformation increases northeastward from the persian gulf mesopotamian basin where strata are undeformed and horizontal through the zagros simply folded belt which is characterized by en echelon doubly plunging flexural slip folds gentle to open in the southwest and close to locally sw overturned in the northeast the intensity of deformation continues to increase further northeastward through the sanandaj sirjan zone where thrust faulting is dominant this pattern suggests that the deformational front has migrated through time from the northeastern part of the sanandaj sirjan zone i e the suture zone to its present position in the central portion of the persian gulf mesopotamian foreland basin

the effect of grain coating microquartz on preservation of reservoir porosity clay coatings have been widely accepted by many workers as an explanation for preserving high porosity in deeply buried sandstones but few workers have realized that similar effects can be produced by microcrystalline quartz coatings this phenomenon can be expected only under special circumstances but in such cases it can have profound consequences for exploration in the central graben area of the southern north sea unusually high porosity 20 27 and permeability 100 1000 md are found in certain zones in upper jurassic sandstones at depths of 3 4 4 4 km the porosity in these zones is 5 15 higher than expected based on average porosity depth trends from brent and haltenbanken sandstones we propose that the high porosity is due to continuous grain coatings of euhedral microcrystalline quartz crystals that are 0 1 2 m thick the distribution of microcrystalline quartz coatings is controlled by the presence of siliceous sponge spicules rhaxella which implies a sedimentological control on the reservoir quality we present a thermodynamic model showing how continuous microcrystalline quartz coatings inhibit development of no mal macrocrystalline quartz overgrowths sourced mainly from stylolites high porosities in parts of various upper jurassic oil fields ula and gyda have previously been explained by inhibition of quartz cementation by early hydrocarbon charge we suggest that the microcrystalline quartz coatings provide a more plausible explanation

transpressional deformations within the sanandaj sirjan metamorphic belt zagros mountains iran abstract polyphase deformation structures of the sanandaj sirjan metamorphic belt in the neyriz area zagros mountains iran are consistent with dextral transpressional deformation which is related to the oblique collision between the african arabian continent and the iranian microcontinent the collision started in the late cretaceous and is continuing to the present day quantitative kinematic analyses quartz textures suggest localized shear zones deformed with a significant pure shear component spatial orientation of lineation and semi quantitative kinematic indicators indicate shear zone parallel stretching and shear zone perpendicular shortening the occurrence of a horizontal stretching component parallel to the deformation zone boundary allows a kinematic model of combined transpression and lateral extrusion for this part of the zagros orogeny

zagros orogeny a subduction dominated process this paper presents a synthetic view of the geodynamic evolution of the zagros orogen within the frame of the arabia eurasia collision the zagros orogen and the iranian plateau preserve a record of the long standing convergence history between eurasia and arabia across the neo tethys from subduction obduction processes to present day collision from 150 to 0 ma we herein combine the results obtained on several geodynamic issues namely the location of the oceanic suture zone the age of oceanic closure and collision the magmatic and geochemical evolution of the eurasian upper plate during convergence as testified by the successive sanandaj sirjan kermanshah and urumieh dokhtar magmatic arcs the p t t history of the few zagros blueschists the convergence characteristics across the neo tethys kinematic velocities tomographic constraints subduction zones and obduction processes together with a survey of recent results gathered by others we provide lithospheric scale reconstructions of the zagros orogen from 150 to 0 ma across two sw ne transects the evolution of the zagros orogen is also compared to those of the nearby turkish and himalayan orogens in our geotectonic scenario for the zagros convergence we outline three main periods regimes 1 the mid to late cretaceous 115 85 ma corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper plate fragmentation 2 the paleocene eocene 60 40 ma witnesses slab break off major shifts in arc magmatism and distributed extension within the upper plate and 3 from the oligocene onwards 30 0 ma collision develops with a progressive sw migration of deformation and topographic build up sanandaj sirjan zone 20 15 ma high zagros 12 8 ma simply folded belt 5 0 ma and with partial slab tear at depths 10 ma to present our reconstructions underline the key role played by subduction throughout the whole convergence history we finally stress that such a long lasting subduction system with changing boundary conditions also makes the zagros orogen an ideal natural laboratory for subduction processes

detrital quartz and quartz cement in upper triassic reservoir sandstones of the sichuan basin characteristics and mechanisms of formation based on cathodoluminescence and electron backscatter diffraction analysis abstract the relationship between detrital quartz grains and silica cement is significant because cement is often a major cause of porosity reduction in oil and gas reservoirs thus degrading hydrocarbon reservoir quality on the other hand the attributes of grain surfaces may inhibit cementation thus preserving porosity in the reservoirs in this study we focus on the microstructures in quartz grains and secondary overgrowths quartz cement in the heavily cemented sandstones from uplifted upper triassic siliciclastic rocks in the southern sichuan basin china electron backscatter diffraction ebsd optical and cathodoluminescence cl imaging methods have been used to address the microstructural and crystallographic attributes of the detrital grains and their overgrowths this study confirms that 1 the overgrowth zones of quartz are clearly visible in optical and cl images 2 all silica overgrowths are perfectly crystalline quartz 3 quartz cements have a syntaxial relationship with the host detrital grain indicating crystallographic continuity between the detrital quartz and its overgrowth and 4 crystallographic maps confirm that dauphine twins commonly occur both in the detrital grains and in the quartz cement most dauphine twins are developed at grain grain contacts including the margins of detrital grains where they are in contact with quartz cement some twins are limited to a single grain whereas others cross grain cement boundaries based on these twin distributions we can distinguish inherited twins from twins that formed during after burial compaction this study documents the grain deformation structures induced by sedimentary compaction and these structures can be accurately illustrated by ebsd data

the role played by carbonate cementation in controlling reservoir quality of the triassic skagerrak formation norway anomalously high porosities up to 30 at burial depth of 3000 m along with varying amounts and types of carbonate cements occur in the fluvial channel sandstone facies of the triassic skagerrak formation central graben norway however porosities of the skagerrak formation are lower in the norwegian sector than in the uk sector in this study petrographic analysis core examination scanning electron microscopy elemental mapping carbon and oxygen isotope fluid inclusion and microgeometry analysis are performed to determine the diagenesis and direct influence on reservoir quality with particular focus on the role played by carbonate cementation the sandstones are mainly fine grained lithic arkosic to sub arkosic arenites and display a wide range of intergranular volumes 2 3 43 7 with an average of 23 6 porosity loss is mainly due to compaction av 26 6 with minor contribution from cementation av 12 1 the carbonate cements are patchy in distribution from trace to 20 7 and appear as various types e g calcretes i e calcareous concreted gravels poikilitic sparite and sparry ferroan dolomite and euhedral or and aggregated ankerite ferroan dolomite crystals this study highlights the association of carbonate precipitation with the remobilisation of carbonate from intra skagerrak calcretes during early burial stage i e 500 m during deeper burial compaction is inhibited by carbonate cements resulting high intergranular volume of up to 32 and 29 for fine and medium grained sandstones respectively carbonate cement dissolution probably results from both meteoric water flow with co2 during shallow burial and organic co2 and carboxylic acid during deep burial the maximum intergranular volume enhanced by dissolution of early carbonate cements is calculated to 8 and 5 for fine and medium grained sandstones respectively compaction continues to exert influence after dissolution of carbonate cements which results in a loss of 6 intergranular volume for fine and medium grained sandstones reservoir quality of the norwegian sector is poorer than that of the uk sector due to a lower coverage of clay mineral coats e g chlorite later and deeper onset of pore fluid overpressure lower solubility of carbonate compared to halite and a higher matrix content

development of kinematic partitioning within a pure shear dominated dextral transpression zone the southern ellsworth mountains antarctica the heritage range of the southern ellsworth mountains west antarctica is composed of cambrian to permian sedimentary and volcanic rocks which were deformed during the permo triassic gondwanian orogeny the structural grain of the heritage range exhibits a previously unrecognized 18 swing from 333 153 in the north to 315 135 in the south a change in structural and kinematic style accompanies this strike swing with a structure consistent with near orthogonal shortening present in the south and a structural style consistent with dextral transpression within the central and northern heritage range kinematic partitioning is present within the central heritage range where strikeparallel contemporaneous domains of dextral and reverse shear have developed simultaneously with the regional cleavage comparison of the structure and kinematics within both structural domains suggests that the central and northern heritage range experienced pure shear dominated dextral transpression with an approximate angle of relative shortening of 65 70 results derived by integrating field data into a published kinematic partitioning model suggest relatively efficient kinematic partitioning has occurred however such efficient partitioning cannot be explained by strain partitioning models based purely on plate boundary conditions therefore it is proposed that pre existing weak structures were present within the unexposed basement facilitating the apparent high percentage of kinematic partitioning

orogen parallel extension and oblique tectonics the relation between stretching lineations and relative plate motions many collisional orogens exhibit two distinct deformation phases the first and relatively early phase forms within an obliquely subducting continent and is characterized by regionally consistent stretching lineations subparallel to the orogen and to associated isoclinal and sheath folds second phase folds and stretching lineations develop during and after imbrication of the subducting plate into the overriding plate and they become progressively disharmonic with stretching lineations oriented perpendicular to the orogen structural morphology and stretching lineations indicate that a fundamental kinematic transition occurs as material is imbricated from footwall to hanging wall kinematic modeling based on modern oblique convergent boundaries and considerations of buoyancy forces suggest that only the relatively early structures reflect the relative plate motion but that the correspondence is not simple in particular the orientation of stretching lineations is a function of finite strain kinematics of deformation and angle of subduction and convergence and it will consistently underestimate the angle of oblique convergence second phase structures reflect isostatic emergence of the hanging wall and are independent of relative plate convergence examples are drawn from the south central canadian rockies the western alps the variscides of ireland and france and the himalaya

the geotectonic history of the makran and adjacent areas of southern iran abstract the makran mountain range in southern iran has been entirely covered by regional mapping on a scale of 1 250 000 but in contrast to the mapping of oman on the other side of the gulf the recent mapping is little known the cenozoic accretionary prism in the south fronts onto a mesozoic melange zone representing earlier subduction and to the north of this is an eastwards tapering microcontinental sliver which is followed by a zone of largely intact mesozoic ophiolite complexes to the north of this lies the jaz murian depression a desert expanse with no bedrock outcrop the sanandaj sirjan bajgan dur kan microcontinental sliver which extends from turkey through the zagros to the makran is one of a number of microcontinents which detached from gondwana no later than the early triassic nw iran yazd tabas lut birk helmand farah during the mesozoic these were enclosed by tracts of ocean now represented by ophiolites and the sectors closed at different times ranging from early cretaceous to end palaeogene an analogy is drawn with the present day mediterranean the sistan ocean in the east was infilled with an immense thickness of abyssal turbidites during the eocene before it closed the field evidence indicates that there was a second cenozoic accretionary prism here but sedimentation ceased after the eocene on its collision with the back of the main makran prism this eastern sistan trough now completely occluded is compared with the still open tract separating the makran from oman at the present time and its northern extension which has been similarly occluded by collision beneath the zagros some residual very shallow inland seas persisted inland from the makran through the neogene the occurrence of blueschists in the inner ophiolite belt of the makran is discussed abyssal turbidite sedimentation continued in the main southern zone of the makran into the early miocene to be succeeded by thick shallow water sequences including reefal limestones intense tectonic dislocation at the end of the miocene was accompanied by a shift of the subduction front to its present position out in the indian ocean and uplift leading to the deposition of spectacular pliocene pleistocene fanglomerates

tectonic evolution of the hinterland of the zagros orogen revealed from the deformation of the golpaygan metamorphic complex iran in the southwestern part of the central iran zone the golpaygan metamorphic complex gmc was evolved during the zagros orogeny this complex is composed of different metamorphic rocks including schist marble slate gneiss and amphibolite the metamorphic complex is overlain by a cover sequence including permian carbonates jurassic clastic rocks cretaceous carbonates and eocene clastic rocks the internal structure of the gmc is characterized by four deformational stages the first stage d1 developed contemporaneously with the prograde regional metamorphism and resulted in formation of the first generation folds and prevailing axial plane foliation the second stage of deformation d2 was accompanied by deformation partitioning and formation of contractional shear zones these structures were overprinted by different scale folds of the third stage d3 the fourth stage of deformation d4 is characterized by generation of the extensional shear zones which gradually transformed into normal faults d1 and accompanied metamorphism may have resulted from steep subduction of the neotethys ocean beneath the active margin of the central iran zone during jurassic times d2 d3 structures and retrograde metamorphism may have been formed by a regional shortening and transpression deformation event which resulted in exposure of the metamorphic rocks in shallower levels during cretaceous flat slab subduction this condition lasted until paleocene time when the slab roll back caused extension and thinning of the overriding central iran plate the subsequent decompression and regional extension resulted in intrusion of magmatic rocks and generation of d4 which formed the gmc and related magmatism in paleocene eocene time

provenance of gravity flow sandstones from the upper jurassic lower cretaceous farsund formation danish central graben north sea abstract through the upper jurassic and lower cretaceous gravity flow sandstones were deposited in the danish central graben these gravity flow sandstones were deposited within mudstones of the farsund formation which is the main source rock in the area and they are thus potential oil and gas reservoirs the petrography and geochemistry of the sandstones show good intra well relations and several local geographical relations which may indicate that they were derived from multiple locally sourced flows and not merely by one basin axial flow system the gravity flow sandstones deposited in the eastern part of the basin are less mature and richer in especially carbonates and albite compared to previously deposited jurassic sandstones and simultaneously deposited sandstones in the western part of the danish area this may indicate a division of the basin in an eastern and a western provenance province this study addresses the provenance of the gravity flow sandstones by petrographical and geochemical analyses in order to derive possible shared detrital sources as a means of understanding the transport system s of the gravity flows

formation of abnormally high porosity permeability in deltaic sandstones oligocene bozhong depression offshore bohai bay basin china abstract the porosity enhancing capability of secondary porosity in a closed sandstone geochemical system has been suggested to be small in past studies however the mechanisms responsible for the formation of abnormally high porosity permeability remain unclear the oligocene dongying formation in the bozhong depression offshore bohai bay basin preserves abnormally high porosity i e 36 4 average or 42 1 median higher than background values this work selects samples with abnormally high porosity ahp samples with normal porosity np and samples with abnormally low porosity alp and compares their differences in physical properties porosities permeabilities and thin section porosities diagenetic minerals pore structures compaction and cementation detrital compositions and grain texture in an attempt to find the real factors controlling this heterogeneity an effective export of dissolution by products al3 and si4 from ahp to np and alp leads to 2 01 average or 2 11 median newly created porosity in ahp which is still significantly lower than the porosity difference between ahp and np in comparison compaction reduction and mechanisms beneficial for compaction reduction should be the major factors controlling ahp formation the content and distribution pattern of early carbonate cements compared to the sedimentary factors grain size and rigid grains have significant influences on compaction reduction only samples with moderate contents 0 5 3 0 of uniformly point distributed early carbonate cements and with effective export of dissolution by products can form ahp three formation models for alp and two formation models for np are also proposed in summary the evolution of porosity permeability is controlled by early carbonate cements and induced compaction reduction major causes and by feldspar dissolution and redistribution of by products minor causes this work provides new constraints on the formation of ahp and heterogeneity and is therefore important for sandstone reservoir quality prediction

illite distribution and morphology explaining basinal variations in reservoir properties of upper jurassic sandstones danish north sea abstract growth of fibrous illite may destroy an otherwise promising sandstone reservoir even sandstones characterized by microquartz coatings therefore detailed petrographical and diagenetic investigations were performed in order to understand the factors that control illite precipitation and other factors affecting the reservoir properties in the upper jurassic heno formation the heno formation is encountered in large parts of the danish central graben where its lower part the gert member was mainly deposited in back barrier environments and its upper part the ravn member was deposited in a shoreface environment in total 215 core samples from 15 wells were investigated petrographically by optical and scanning electron microscopy this was supplemented by plug porosity and permeability measurements and x ray diffraction of bulk and clay fraction the investigations show among other things that microquartz coatings may help to preserve porosity and permeability whereas the permeability is reduced when microquartz occurs combined with fibrous illite illite precipitation is largely governed by the amounts of k feldspar present during burial consequently the initial abundance of k feldspar in the sediment source area and its alteration during and immediately after deposition in particular during subaerial exposure at sequence boundaries has a major influence on the final reservoir properties of the sandstones the risk of illite precipitation can largely be predicted from the available sediment source material depositional environment and burial depth

diagenetic silica enrichment and late stage groundwater activity in gale crater mars diagenetic silica enrichment in fracture associated halos that crosscut lacustrine and unconformably overlying aeolian sedimentary bedrock is observed on the lower north slope of aeolis mons in gale crater mars the diagenetic silica enrichment is colocated with detrital silica enrichment observed in the lacustrine bedrock yet extends into a considerably younger unconformably draping aeolian sandstone implying that diagenetic silica enrichment postdates the detrital silica enrichment a causal connection between the detrital and diagenetic silica enrichment implies that water was present in the subsurface of gale crater long after deposition of the lacustrine sediments and that it mobilized detrital amorphous silica and precipitated it along fractures in the overlying bedrock although absolute timing is uncertain the observed diagenesis likely represents some of the most recent groundwater activity in gale crater and suggests that the timescale of potential habitability extended considerably beyond the time that the lacustrine sediments of aeolis mons were deposited

inclined transpression in the neka valley eastern alborz iran three major nappes in the neka valley in the eastern alborz mountains of iran allow the cimmerian to present convergence following the oblique collision between iran and the southern margin of eurasia this work reports the identification of an inclined transpression zone recognized by field investigations and strain analyses of the geometries of formations and detailed mesoscopic structural analyses of multiple faults folds and a cleavage the main structures encountered include refolded recumbent asymmetric fold nappes highly curved fold hinges in a transpression zone that dips 37 to the nw between boundaries thrusts striking from n050 to n060 the angle the angle between the zone boundary and direction of horizontal far field shortening is about 80 the north west and south east boundaries of this zone coincide with the haji abad thrust and the shah kuh thrust respectively fold axes generally trend ne sw and step to both right and left as a result of strike slip components of fault displacements strain analyses using fry s method on macroscopic ooids and fusulina deformed into oblate ellipsoids indicate that the natural strain varies between 2 1 and 3 14 the estimated angle between the maximum instantaneous strain axis isamax and the transpression zone boundary is between 6 and 20 the estimated oblique convergence angle therefore ranges between 31 and 43 the average kinematic vorticity number wk is 0 6 in a zone of sinistral pure shear dominated inclined triclinic transpression these results support the applicability of kinematic models of triclinic transpression to natural brittle ductile shear zones

quartz cement and its origin in tight sandstone reservoirs of the cretaceous quantou formation in the southern songliao basin china abstract the tight sandstones of the cretaceous quantou formation are the main exploration target for hydrocarbons in the southern songliao basin authigenic quartz is a significant cementing material in these sandstones significantly reducing porosity and permeability for efficient predicting and extrapolating the petrophysical properties within these tight sandstones the quartz cement and its origin need to be better understood the tight sandstones have been examined by a variety of methods the sandstones are mostly lithic arkoses and feldspathic litharenites compositionally immature with an average framework composition of q43f26l31 which are characterized by abundant volcanic rock fragments mixed layer illite smectite i s ordered interstratified with r 1 and r 3 is the dominating clay mineral in the studied sandstone reservoirs two different types of quartz cementation modes namely quartz grain overgrowth and pore filling authigenic quartz have been identified through petrographic observations cl and sem analysis homogenization temperatures of the aqueous fluid inclusions indicate that both quartz overgrowths and pore filling authigenic quartz formed with a continuous process from about 70 c to 130 c sources for quartz cement produced are the conversion of volcanic fragments smectite to illite reaction and pressure solution at micro stylolites potassium for the illitization of smectite has been sourced from k feldspar dissolution and albitization silica sourced from k feldspars dissolution and kaolinite to illite conversion is probably only minor amount and volumetrically insignificant the internal supplied silica precipitate within a closed system where the transport mechanism is diffusion the quartz cementation can destroy both porosity and permeability but strengthen the rock framework and increase the rock brittleness effectively at the same time

convergence history across zagros iran constraints from collisional and earlier deformation the zagros orogen provides a unique opportunity within the alpine system to evaluate the interplay between a young tertiary collision and earlier subduction obduction processes within the crush zone and the sanandaj sirjan internal zone separating the zagros fold belt from central iran we document several major tectonic events taking place at the end of the cretaceous of the eocene and from the mio pliocene onwards ca 20 15 ma contrary to recent interpretations our data cross sections and description of the overall deformation style strongly suggest that the main zagros thrust mzt is deeply rooted possibly to moho depths and that the suture zone effectively runs along the mzt field observations show that the final resorption of the oceanic domain took place slightly after 35 ma and that collision must have started before ca 23 25 ma in northern zagros the shortening rate across the crush zone since the mid miocene 20 15 ma is estimated at a minimum 3 4 mm year shear movements in the crush zone during the eocene oligocene period and extensional strike slip movements in the internal zones during the late cretaceous point to an oblique setting early in the convergence history a geotectonic scenario for convergence from the time of obduction to the present is finally proposed

diagenetic trends in the siliceous facies of the monterey shale in the santa maria region california x ray diffraction and oxygen isotopic analyses of outcrop and subsurface samples of siliceous rocks were used to reconstruct thermal and diagenetic histories of the miocene monterey shale near santa maria california within many stratigraphic sections soft porous diatomaceous rocks change gradationally to underlying hard and brittle chert porcellanite and siliceous shale the accompanying silica mineral zones are in descending stratigraphic order 1 biogenic silica opal a 2 cristobalitic silica opal ct and 3 microcrystalline quartz boundaries between silica mineral zones and stratigraphic horizons are often discordant within the opal ct zone the d 101 spacing of opal ct decreases in a smooth non linear fashion from about 4 10 a to 4 04 a in the santa maria valley and bradley oil field areas the thicknesses of the opal ct zones are greater and the present thermal gradients less than in the adjacent orcutt oil field thin opal ct zones at shallow maximum burial depths apparently correlate with higher thermal gradients using present thermal gradients and reconstructed maximum burial depths from well data in the santa maria region the ranges in temperatures for the top and base of the opal ct zone are 38 54 c and 55 110 c respectively the temperature difference between these two boundaries ranges from 17 to 60 c in comparison temperature ranges for these two boundaries computed from oxygen isotopic compositions of opal ct and quartz extrapolated experimental quartz water fractionations and assuming o18 0 o for the isotopic composition of the equilibrating fluid are 18 56 c and 31 80 c for the top and base of the opal ct zone respectively the temperature difference between these boundaries is 11 36 c using this method thermal gradients and sedimentation rates strongly influence rates of silica transformations reconstructed thermal and diagenetic histories of siliceous rocks of the monterey shale at four well sites in the santa maria region demonstrate that most silica conversions probably occurred during the last 3 4 myr in response to accelerated rates of sedimentation and therefore burial heating during the pliocene

development of early diagenetic silica and quartz morphologies examples from the siri canyon danish north sea abstract the siri canyon has proved to be a perfect area for investigating various morphologies of diagenetic silica in sandstones the development in silica morphologies can be observed from very shallow 1700 m to increased burial depth 3000 m and increased proximity to the central graben distance from 0 to 65 km hydrocarbons and pore fluids now found in the siri fairway have at least partly originated from the central graben the siri canyon is a submarine canyon system eroded into the uppermost chalk deposits and filled with palaeogene hemipelagic and turbiditic marls and mudstones interbedded with sandstone units deposited from sandy mass flows and sandy turbidites which originated on the stavanger platform several hydrocarbon exploration and production wells have been drilled in the siri canyon seven of which are included in this study nini 3 nini 1 na 2p sofie 1 siri 4 celilie 1a and augusta 1 the reservoir sandstones in these wells all contain authigenic silica of various morphologies identified with a combination of traditional optical microscopy and scanning electron microscopy the silica morphologies in some places are classic and well documented in the literature whereas others at least to our knowledge have never previously been described some of the silica morphologies presented here show gradual transition from one to another and others are stand alone forms without clear relationships to other forms the silica morphologies can be expressed in the following way 1 opal rims characteristic of the initial phase of the silica diagenesis in most sandstone units in the siri canyon thick opal rims characterise the sandstone parts adjacent to the mudstone units in the stine segment of the siri field 2 microquartz quartz crystals with a size of 1 5 m seen as coatings on the opal rims both ordered and random 3 cavity overgrowth found as quartz outgrowths in circular and angular cavities formed by dissolution of early authigenic phases angular cavities in the microquartz coatings origin from dissolution of clinoptilolite possibly with a source in volcanic ash whereas dissolution of lepispheres probably resulted in circular cavities 4 quartz ridges syntaxial quartz overgrowths a few micro millimetre in width in parallel lines defined in this paper 5 quartz mountains syntaxial irregular 5 10 m large quartz crystals defined in this paper developed in the water filled sandstone intervals where fluids from the central graben were introduced possibly together with the hydrocarbon and where the continued growth was not retarded by the presence of hydrocarbons in the pore fluids 6 macroquartz syntaxial quartz overgrowths 20 m seen as continued growths from quartz mountains this takes place in the deeper reservoir sandstones promoted by fluids originating from the central graben and increased burial depth 7 quartz with sutured grain contacts occurring only in the deepest well 3000 m

reservoir quality in the jurassic sandstone reservoirs located in the central graben north sea abstract this study investigates the diagenesis and reservoir quality of upper jurassic sandstones of ula formation from the central graben petrophysical and petrographical studies have been done on cored interval from well 2 1 6 of ula field precipitation of quartz cement is the main porosity destroying process in deeply buried quartz rich sandstone reservoirs of the north sea quartz cement precipitates in the form of syntaxial overgrowth over detrital grain of quartz grain coatings like micro quartz and illite are the main reasons for preservation of porosity in sandstones as they cover the grain and inhibit the quartz overgrowth petrographical data in this study clearly indicates that grain coatings are present in the studied samples micro quartz grain coating is the most common grain coat in the upper jurassic sandstones of ula which is generated from the transformation of siliceous sponge spicules known as rhaxella perforata clay grain coats like illite and chlorite are also present relation between intergranular volume igv versus matrix and quartz cementation versus porosity have also been studied igv is strongly affected by mechanical compaction grain size grain shape quartz and carbonate cements sandstones with high amount of matrix and fine grains have high igv as compared to coarse grains because coarse grains are compacted more when they are subjected to mechanical compaction grain shape also has a pronounced effect on porosity angular grains tend to lose porosity easier as they are subjected to stress

quartz cement volumes across oil water contacts in oil fields from petrography and wireline logs preliminary results from the magnus field northern north sea

quartz cementation history of sandstones revealed by high resolution sims oxygen isotope analysis abstract evaluating the timing and origin of quartz cement is central to understanding how porosity is lost in sandstones during burial kinetic models of quartz cementation have been calibrated using large scale datasets but have never been tested at the microscopic level at which cement forms here we use high precision in situ oxygen isotope analyses on sandstone from the jurassic ness formation from the north sea to reveal the growth history of single quartz overgrowths to a resolution of 2 m measured 18o cement range from 28 to 20 v smow in early to late cement and are consistent with quartz cementation models that propose the bulk of quartz precipitates as a continuous process beginning at 60 70 c quantitative x ray diffraction analyses and clay mineralogical analysis of interbedded shales are inconsistent with a silica source from shale implying that silica for the cement is sourced internally to the sand these isotope data are broadly consistent with predictive conceptual quartz cementation models and provide a critical link from micron scale measurements to basin scale predictions and observations

offset on the main recent fault of nw iran and implications for the late cenozoic tectonics of the arabia eurasia collision zone summary we use drainage patterns geological markers and geomorphological features to determine a right lateral offset of 50 km and possibly as much as 70 km on the main recent fault in nw iran this fault trends nw se and forms the ne border of the zagros mountains it accommodates the strike slip component of the n s convergence between arabia and eurasia with the ne sw shortening component being accommodated in the zagros fold belt its 50 km strike slip offset implies a shortening of 50 km in the fold belt and 70 km total n s convergence accommodated in the nw zagros this is a substantial fraction of the 85 140 km overall arabia eurasia convergence expected over the last 3 5 ma if the main recent fault initiated at that time as seems likely from geological arguments it has a horizontal slip rate of at least 10 17 mm yr 1 and should be the source of frequent earthquakes of ms 6 7 as has been seen in the 20th century and the earlier historical record the similarity of the offsets and probable ages of the north anatolian and main recent faults suggests that they have been active as an almost continuous zone of right lateral shear on the north edge of the arabian and anatolian plates since the early pliocene

deformation path partitioning within the transpressional white mountain shear zone california and nevada most numerical simulations of transpression zones predict a change in finite stretching direction from subhorizontal to subvertical for simple shear dominated zones we provide a detailed description of a dextral transpression zone the white mountain shear zone wmsz with a range of lineation orientations and compare these natural data to numerical models the wmsz is characterized by steeply dipping foliations with dominant shallowly plunging lineations and coeval subordinate domains of steeply plunging lineations within shallowly lineated domains foliation geometry shear sense indicators and quartz c axis fabrics indicate a large component of simple shear while microstructural and quartz c axis fabric data from steeply lineated domains indicate a large component of pure shear geometric relationships between foliations and lineations and quartz c axis fabrics demonstrate that lineation orientation has remained constant during much of the deformation history comparison of numerical models with the data collected from wmsz shows that the shear zone geometry and the observed strain path partitioning do not match any of these models we propose a conceptual kinematic model for the wmsz involving stable segregated coeval kinematic domains of simple shear dominated fabrics and pure shear dominated fabrics that accommodate the transcurrent and contractional components of deformation respectively

cretaceous tertiary convergence and continental collision sanandaj sirjan zone western iran abstract the sanandaj sirjan zone contains the metamorphic core of the zagros continental collision zone in western iran the zone has been subdivided into the following from southwest to northeast an outer belt of imbricate thrust slices radiolarite bisotun ophiolite and marginal sub zones which consist of mesozoic deep marine sediments shallow marine carbonates oceanic crust and volcanic arc respectively and an inner complexly deformed sub zone late palaeozoic mesozoic passive margin succession rifting and sea floor spreading of tethys occurred in the permian to triassic but in the sanandaj sirjan zone extension related successions are mainly of late triassic age subduction of tethyan sea floor in the late jurassic to cretaceous produced deformation metamorphism and unconformities in the marginal and complexly deformed sub zones deformation climaxed in the late cretaceous when a major southwest vergent fold belt formed associated with greenschist facies metamorphism and post dated by abundant palaeogene granitic plutons in the southwest of the zone a late cretaceous island arc passive margin collision occurred with ophiolite emplacement onto the northern arabian margin similar to that in oman final closure of tethys was not completed until the miocene when central iran collided with the northeast arabian margin

slip partitioning and inclined dextral transpression along the zagros thrust system iran abstract flexural duplex structures and shear zones are well developed in the heneshk area in southwestern iran the zagros thrust system in this area consists of eight sheets of nw striking ne dipping dextral strike slip duplexes that are linked with imbricate fans and oblique slip thrusts the zagros thrust system is a portion of the internal zone of the zagros orogenic belt this internal zone is characterized by penetrative plastic deformation and metamorphism based on the kinematic vorticity number w k 0 73 0 02 calculated from quartz porphyroclasts of the goshti shear zones along the zagros thrust system the estimated angle between the maximum instantaneous strain axis isa and the transpressional zone boundary is 33 the estimated angle of the plate convergence or the flow apophysis of the displacement field is 25 the presence of dominantly dextral shear sense indicators in the area is consistent with dextral inclined transpressional convergence the mean estimated finite deformation w m value indicates relative contributions of 47 pure shear and 53 simple shear for the deformation which involved both strike slip and oblique slip displacements in this inclined transpression about 40 strike slip partitioning was required to accommodate the finite strain and re orientation of instantaneous strain

the effect of oil emplacement on quartz cementation in a deeply buried sandstone reservoir quartz is an important porosity occluding cement in sandstone reservoirs that have been subjected to elevated temperature 80 c to 100 c for a substantial period of time the effect of oil emplacement on quartz cementation in reservoir sandstones is controversial some studies have concluded that early oil emplacement can inhibit quartz cementation leading to the preservation of porosity whereas other studies have concluded that quartz cementation appears largely unaffected by oil emplacement here we have studied shallow marine upper jurassic sandstones from ula field norwegian north sea with reservoir temperatures of approximately 150 c to determine whether oil emplacement had a significant impact on diagenesis with particular attention to quartz cementation following sedimentological description of cores samples above and below the oil water contact have been collected adjacent to core analysis plug points these samples then underwent a series of studies including petrographic point counting with a transmitted light microscope scanning electron microscopy sem backscattered electron microscopy sem cathodoluminescence microscopy and fluid inclusion studies these data were integrated with routine core analysis and petrophysical log data density and resistivity log data have been used to determine the precise oil saturation of each sample studied the distributions of all potential controls on porosity and permeability such as grain size sorting matrix clay content and degree of bioturbation the presence of grain coatings and dolomite cement and the amount of quartz cement have been assessed the presence of primary oil inclusions within quartz cement shows that oil ingress into the ula reservoir commenced prior to the onset of quartz cementation very fine grained matrix rich bioturbated microquartz cemented sandstones have uniformly low quartz cement contents irrespective of oil saturation medium grained graded matrix poor microquartz poor sandstones have quartz cement ranging from 1 to greater than 17 associated with core porosities of approximately 22 and 7 respectively higher oil saturations equate to higher porosities and permeabilities in the medium grained graded matrix poor microquartz poor sandstones which cannot be explained by any control other than the amount of quartz cement as a function of pore fluid type oil emplacement therefore appears to have inhibited quartz cementation at high oil saturations and can be viewed as an important control on reservoir quality the significance of this study is that the presence of oil in a sandstone reservoir at the time that quartz cement was growing can have a considerable impact on reservoir quality models that seek to predict quartz cement and reservoir quality in sandstones need to account for the timing of oil emplacement compared with other diagenetic processes

deciphering multiple controls on reservoir quality and inhibition of quartz cement in a complex reservoir ordovician glacial sandstones illizi basin algeria

the effect of systematic diagenetic changes on the mechanical behavior of a quartz cemented sandstone abstracta key goal of petrophysical studies of sandstones is to relate common field measurements particularly seismic or sonic velocities to parameters defining the rock s mechanical and hydrologic characteristics these include elastic and inelastic mechanical properties porosity and permeability we explored relationships among these properties in variably quartz cemented mature arenites of the st peter sandstone with porosities ranging from 9 to 25 in a previous paper we described microstructural changes accompanying progressive quartz cementation and related porosity and permeability reduction in this sample suite here we report ultrasonic velocities ultrasonic velocities dynamic and static elastic properties confined compressive strength and tensile strength analyses of these data demonstrated that factors controlling permeability also fundamentally determined the elastic and inelastic mechanical properties we found that the number of grain contacts or bonds per number of grains vi

compaction microstructures in quartz grains and quartz cement in deeply buried reservoir sandstones using combined petrography and ebsd analysis abstract sandstone diagenesis involves porosity loss by mechanical and chemical compaction in this study we have examined relations of diagenesis and microstructures in quartz grains and quartz cement in sandstones from relatively deep burial depths offshore mid norway electron backscatter diffraction analysis ebsd has been combined with optical and cathodoluminescence petrography study of samples with different degrees of compaction and quartz cementation quartz cement is shown to be syntaxial to the nearest host domain grain or subgrain both for monogranular polygranular undeformed and deformed quartz the quartz cement growth may have been initiated at different subgrain surfaces and preserving pre depositional deformation structures ebsd inverse pole figure imaging shows dauphine twins to be common in all samples both in quartz grains and quartz cement the dauphine twins appear in grain grain contacts and in cement crystal boundaries and commonly crossing grain cement boundaries on basis of twin distributions we suggest that both inherited twins from the source area and twins formed by compaction induced grain boundary deformation are present a general problem in sedimentary rocks is to distinguish inherited from sedimentary compaction induced intra grain deformation structures this study shows that the role of ductile deformation mechanisms e g twinning in sediment compaction can be clarified by more comprehensive following up studies using ebsd analysis in combination with other techniques

origin and evolution of formation waters from oil fields on the norwegian shelf abstract stable isotopes of h and o and chemical analyses of formation waters from upper triassic jurassic clastic reservoirs and cretaceous carbonate reservoirs offshore norway are employed to interpret the origin and evolution of these waters in both types of reservoir the formation waters become progressively enriched in total dissolved solids and the heavy isotope of o with increasing depth whereas deuterium varies erratically between 20 and 40 smow bromine analyses and stable isotope data indicate that the formation waters are mixtures of a brine end member formed by evaporation of sea water beyond the point of halite saturation and meteoric water the chemical and isotopic composition of the formation waters and general geological considerations suggest that the brines originated as connate waters in permian evaporites and were later expelled into overlying strata there is a disequilibrium stable isotope distribution between the bulk chalk formations and the formation waters and only minor diagenetic modification of the formation waters appears to have taken place in these reservoirs the formation waters from the clastic reservoirs on the other hand are modified by diagenetic reactions