permian triassic red stained albitized profiles in the granitic basement of ne spain evidence for deep alteration related to the triassic palaeosurface extensive areas of the variscan granitic basement in ne spain display profiles of red stained albitized facies characterized by albitization of ca plagioclase chloritization of biotite and microclinization of orthoclase along with the alteration of igneous quartz to secondary cl dark quartz these profiles have a geopetal structure beneath the triassic unconformity with a very intense and pervasive alteration in the upper part that progressively decreases with depth to 150 200 m where the alteration is restricted to the walls of fractures the red albitized facies contains secondary maghemite and hematite that indicate oxidizing conditions dating of microclinized orthoclase and secondary monazite that have formed in the red stained albitized facies yielded k ar and u th pbtotal ages of 240 and 250 ma respectively suggesting that the alteration developed during the permian triassic period the geopetal disposition of the red albitized profile with respect to the triassic unconformity its large regional extent and the fracture controlled alteration in the lower part of the profile indicate groundwater interaction the 18o values of albitized plagioclase 11 microclinized orthoclase 13 and secondary cl dark quartz 12 suggest that the alteration temperature was about 55 c this low temperature suggests that the alteration occurred during interaction of the granitic rocks with na rich fluids below a surficial weathering mantle on the permian triassic palaeosurface the latter is possibly related to triassic evaporitic environments in long lasting stable landscapes in which na rich solutions infiltrated deep regional groundwaters

reconstructing the formation and land use history of the mound 2 depression at rio viejo oaxaca mexico archaeological and geoarchaeological work in the lower rio verde valley oaxaca mexico reveal a varied spatial relationship between the prehistoric inhabitants and this dynamic floodplain region over the last 3000 years significant demographic expansion occurred by the late formative period following floodplain aggradation resulting from a change in stream hydrology and morphology during the classic and postclassic periods major shifts in settlement and land use occurred primarily from the productive floodplain to the piedmont the archaeological site of rio viejo has played a pivotal role in the political and social history of the valley archaeological data indicate the site was occupied from the middle formative to the early postclassic and was an urban center and the political seat of the region during the terminal formative and again in the late classic four large depressions are associated with mounded architecture at the site we collected paleoenvironmental and geoarchaeological data from the mound 2 feature a synthesis of this multi proxy data has enhanced our understanding of site formation occupation and land use from the late formative to postclassic times the analysis indicates that the depression associated with mound 2 was originally a borrow pit on the floodplain that was enhanced and augmented by monumental mound building the area around mound 2 was the focus of domestic and agricultural use during the terminal formative an apparent cessation of activity around mound 2 occurred during the early classic which is consistent with a significant contraction in population at the site at this time activity resumed at mound 2 in the late classic and continued until at least the early postclassic these changes in occupation history appear to have resulted from political developments within the valley

petrology and ion microprobe u pb chronology applied to a metabasic intrusion in southern sweden a study on zircon formation during metamorphism and deformation the aker metabasite occupies a key position in a major tectonic lineament in southernmost sweden the protogine zone which coincides closely with the eastern boundary of the late mesoproterozoic sveconorwegian orogen of southwest scandinavia metamorphic reactions associated with the transformation from isotropic gabbro to foliated garnet amphibolite were identified from disequilibrium textures of which some involved release of zirconium zr and growth of metamorphic zircon ion microprobe dating of igneous zircon gave 1562 6 ma whereas metamorphic zircons yielded ages of 1437 21 1217 75 and 1006 68 ma the presence of baddeleyite pseudomorphs made up of saccharoidal zircon and a higher abundance of older rather than younger metamorphic zircons suggest redistribution of zr into new zircon first by the breakdown of baddeleyite zro2 and later by the consumption of igneous phases containing trace amounts of zr several generations of metamorphic zircon and the presence of 1 56 and 1 22 ga mafic intrusions along the protogine zone call for a complex tectonic history probably reaching back to at least similar to1 56 ga growth of metamorphic zircon at similar to1 44 ga may relate to a regional compressional event the wnw trending deformational structures on both sides of the protogine zone may possibly relate to that event the similar to1 22 ga metamorphic zircons are coeval with the emplacement of numerous granitic syenitic and mafic intrusions along and parallel to the protogine zone the age around 1 0 ga finally marks sveconorwegian metamorphism for which thermobarometry of the aker garnet amphibolite suggests 1000 1200 mpa at 600degreesc 630degreesc thereafter significant relative uplift of the rocks to the west of the protogine zone occurred on nearly vertical north south trending deformation zones less

simulation of potassium feldspar dissolution and illitization in the statfjord formation north sea prior work has suggested that potassium mobility is an important control on the formation of late stage illite in deep reservoirs of the north sea we developed a one dimensional finite difference reactive transport model to simulate the concurrent dissolution of k feldspar and illitization of kaolinite in statfjord formation reservoirs the kinetics of k feldspar dissolution is set to be faster than diffusion consistent with the observed behavior and the proposition that the reaction was transport limited k feldspar dissolution produced dissolved potassium and silica that diffused and then precipitated to form illite and quartz areas that had higher initial reactant k feldspar produced higher solute concentrations and formed chemical gradients having areas that had lower initial feldspar content in typical north sea cases this means mass transfer would be from sandstone to shale the diffusive transport moved significant amounts of potassium and silica over distances of several meters and created observable diagenetic alteration simulations of an oil filled reservoir showed that oil emplacement did not stop illitization but did limit potassium mobility and maximized illite formation within the sandstone preservation of preillitization permeability was favored in water filled reservoirs that had slower illite formation where diffusion could export potassium from the sandstone before precipitation similar patterns of transport and precipitation were predicted for silica from k feldspar dissolution the results of the model showed good agreement with field data from other north sea reservoirs

relationship between diagenesis and reservoir quality in sandstones of the garn formation haltenbanken mid norwegian continental shelf a quantitative model is proposed for the dependence of porosity and permeability on diagenesis in subarkosic arenites of the middle jurassic garn formation porosity variation is controlled by the combined effects of compaction and quartz cementation in general more porosity has been lost by compaction than by quartz cementation but compactional porosity loss appears to have occurred relatively early in the burial history before burial below 2 km below the sea floor thus progressive reduction of the total porosity with increasing burial below 2 km results mainly from quartz cementation permeability is a function of the abundance of intergranular macroporosity which is interpreted to be a measure of effective porosity at depths less than 3 5 km below the sea fl or intergranular macroporosity generally comprises more than 50 percent of the total porosity but at greater depth as total porosity decreases below about 16 percent the ratio of effective to ineffective porosity drops below one this depth corresponds to the onset of extensive illitization and grain dissolution which results in major reorganization of the pore system generation of secondary porosity by feldspar dissolution appears not to have been beneficial to reservoir quality in the garn formation but it is regarded as an intrinsic part of the overall process of porosity destruction with increasing diagenesis

near surface k feldspar precipitation in cretaceous sandstones from the potiguar basin northeastern brazil abstract the cretaceous fluvial sandstones of the acu formation potiguar basin northeastern brazil contain abundant very early authigenic k feldspar overgrowths around detrital orthoclase and microcline grains detailed optical and cathodoluminescence petrography scanning electron microscopy and electron microprobe analyses provided information about the conditions and timing of precipitation of these overgrowths as well as of the associated diagenetic phases the overgrowths are optically discontinuous with the detrital grains untwined nonluminescing and were formed clear on grain surfaces under phreatic conditions the overgrowths commonly show a rhombohedral sanidine habit range in thickness from about 0 4 to 1 0 mm are chemically pure kalsi3o8 and constitute up to 6 of the whole rock the extremely early origin of the k feldspar overgrowths is indicated by their coating of mechanically infiltrated smectitic clays authigenic smectite rims booklet and vermicular aggregates of kaolinit

thermal hydraulic mechanical chemical coupling with damage mechanics using escriptrt and abaqus abstract we present a method to couple the thermal t hydraulic h and fluid rock chemical c interaction capabilities of escript rt with a rate dependent mechanical m formulation of the finite element method including continuum damage mechanics for geomaterials implemented using abaqus standard the resulting code builds on the modular architecture of escript rt to solve sequentially these coupled thmc mechanisms and a number of feedback mechanisms including shear heating and damage damage in the host rock is linked to porosity evolution which in turn affects permeability and this permeability damage dependency is validated against published results the motivation behind the development of this code is to improve our understanding of the formation of hydrothermal ore deposits and we illustrate the use of this numerical tool with a simple geologic scenario involving shear zone development and albitisation this application example focuses on the effects of damage on permeability enhancement and resultant fluid flow using damage mechanics allows for the localisation of narrower shear zones and reproduces the logarithmic pattern of rock pulverisation observed in nature on the sides of shear zones the damage induced permeability evolution creates fluid pathways which may play a major role in forming hydrothermal ore deposits

reservoir characterization of a co2 storage aquifer the upper triassic stuttgart formation in the northeast german basin abstract ketzin in the northeast german basin negb is the site for pilot injection of co 2 co 2 sink project into a saline aquifer the upper triassic stuttgart formation situated at a depth of about 630 700 m this paper reports the baseline characterization of the reservoir formation based on new core material and well logs obtained from one injection well and two observations wells drilled at a distance from 50 m to 100 m from each other the reservoir is lithologically heterogeneous and made up by fluvial sandstones and siltstones interbedded with mudstones showing remarkable differences in porosity the thickest sandstone units are associated with channel sandstone whose thickness varies over short lateral distances in depth petrographic mineralogical mineral chemical and whole rock geochemical analysis were performed focusing on the sandstone intervals which display the best reservoir properties for co 2 injection the dominantly fine grained and well to moderately well sorted immature sandstones classify as feldspathic litharenites and lithic arkoses quartz 22 43 wt plagioclase 19 32 wt and k feldspar 5 13 wt predominate mineralogically muscovite plus illite and mixed layer minerals are omnipresent 4 13 wt quartz feldspar as well as meta sedimentary and volcanic rock fragments comprise the most abundant detrital components which often are rimmed by thin early diagenetic coatings of ferric oxides and locally of clay minerals feldspar grains may be unaltered and optically clear partially to completely dissolved partially altered to sheet silicates mainly illite or albitized analcime and anhydrite constitute the most widespread often spatially associated pore filling cement minerals authigenic dolomite barite and coelestine is minor the percentage of cements ranges in total from about 5 vol to 32 vol except of samples intensely cemented by anhydrite and analcime total porosities of the sandstones range from 13 to 26 the fraction of intergranular porosity varies between 12 and 21 about 1 5 porosity has been generated by dissolution of detrital plagioclase k feldspar and volcanic rock fragments the comparatively large modal abundance of feldspars micas chlorite clay minerals fe ti oxides and analcime account for the richness in ti al fe mg na and k and the paucity in si of the stuttgart sandstones relative to mature sandstones altogether these sandstones are comparatively rich in minerals that may potentially react with the injected co 2

diagenesis of namurian fluvio deltaic sandstones from the trumfleet field south yorkshire abstract the namurian sequence of the trumfleet field south yorkshire uk contains four sandstone units between the gastrioceras subcrenatum and reticuloceras gracile marine bands each deposited in a period between glacio eustatic marine transgressions these medium to coarse grained sandstone units up to 25 m thick were deposited as multi channel upwards fining distributary channel fills within a predominantly argillaceous deltaic interdistributary and swamp complex associated sandstones are crevasse splay and overbank deposits four stages of diagenetic alteration can be identified early diagenesis eogenesis was controlled by the oxidation of organic matter causing reducing conditions to be established pyrite siderite and calcite are characteristic of this phase burial diagenesis mesogenesis can be divided into three stages the first early burial was characterized by dolomite and quartz cements the second intermediate burial was controlled by the generation of acidic pore fluids by the thermal maturation of organic matter in adjacent and down dip shales and was manifested by dissolution of feldspar and carbonate with the precipitation of kaolinite in secondary pores the third stage late burial was characterized by the precipitation of ankerite albite and illite cements integrating the diagenetic data with the burial history and modelling the source rock geochemistry of the wells allows the timing of diagenetic events to be predicted within absolute ranges early diagenesis took place very near the surface and early burial diagenesis occurred during the carboniferous from depths of only a few centimetres below the sediment water interface to up to 1500 m burial depth at approximately 140 my intermediate burial diagenesis took place when the rocks passed through the 80 c isotherm at approximately 140 my late burial diagenesis took place during maximum burial prior to tertiary uplift when temperatures in excess of 100 c were reached the present day porosity is secondary after carbonate and feldspar dissolution primary textural factors have little effect on the present reservoir properties which are controlled by diagenesis

potassic and sodic metasomatism in the southern province of the canadian shield evidence from the paleoproterozoic serpent formation huronian supergroup canada abstract major element geochemistry petrographic observations and microprobe analyses of the paleoproterozoic serpent formation huronian supergroup canada suggest that these strata experienced both potassium and sodium metasomatism major element data plotted in al2o3 cao na2o k2o compositional space form an array nearly identical to data from sub huronian paleosols reported elsewhere suggesting that serpent fluvial systems captured materials from entire weathering profiles potassium metasomatism was concentrated in shales and represents the illitization of kaolinite which is a residual product of plagioclase weathering sodium metasomatism principally affected silt and sand sized particles microprobe analyses of plagioclase grains indicate that they are nearly pure albite ab96 corresponding whole rock major element data plot in bulk albite space on an al2o3 k2o cao na2o ternary diagram whereas basement rocks plot in bulk andesine and oligoclase space end member compositions and the absence of luminescence when bombarded with an electron beam indicate that the albites are of secondary origin extremely pure albite cements ab99 commonly fringe detrital grains there has been little replacement of detrital k feldspars despite the complete albitization of plagioclase grains the timing of metasomatism at least the last stage is constrained to the period between 1700 and 1750 ma based on geochronologic data reported elsewhere when the southern province of the canadian shield experienced considerable felsic plutonism possibly related to arc collision this plutonism may have resulted in the heating of deep basinal brines stresses associated with orogenic build up or gravity driven groundwater circulation working in concert with the plutonism expelled these brines through the serpent formation and the entire huronian supergroup thoroughly metasomatizing the region such processes associated with collisional tectonics are akin to the development of mississippi valley type pb zn mineralization in the mid continent of the united states

closed system burial diagenesis in reservoir sandstones examples from the garn formation at haltenbanken area offshore mid norway abstract the middle jurassic garn formation of the haltenbanken area has been studied using mineralogical and geochemical data from 21 wells ranging in burial depths from 2 0 to 4 1 km relative to seafloor rsf k feldspar and plagioclase contents show variations on a regional scale both laterally and as a function of burial depth the content of pore filling authigenic illite increases sharply and the content of k feldspar and kaolinite decreases in garn sandstones presently at depths greater than 3 6 3 7 km rsf 120 130 c the depletion in k feldspar below 3 7 km rsf is not accompanied by lower potassium values in the bulk chemical composition wt k2o this suggests that the potassium released during k feldspar dissolution is retained in the sandstones and is precipitated as illite the variations in bulk contents of potassium and sodium are therefore considered to be related principally to primary variations in sandstone mineralogy the shallower sandstones 1 3 have a k feldspar kaolinite ratio greater than one the deeply buried 3 7 km rsf sandstones with similar potassium contents contain excess k feldspar and most of the kaolinite is illitized however deeply buried sandstones containing an average of 0 38 wt k2o k al molar ratio 1 4 contain a significant amount of kaolinite but negligible k feldspar this suggests that the k feldspar kaolinite ratio before the onset of illitization was less than one and hence that the kaolinite illite reaction has been restricted by an insufficient supply of potassium absence of k feldspar this illustrates how illitization of kaolinite depends upon k feldspar as a local source of potassium prediction of illitization in sandstones therefore must be based on integration of models for provenance facies and early diagenesis in addition to burial and thermal history the formation of pore filling authigenic illite in these sandstones is an important influence on the total reservoir quality

major factors controlling lamina induced fractures in the upper triassic yanchang formation tight oil reservoir ordos basin china abstract lamina induced fractures lf are abundant in lacustrine formations playing an important role for the tight oil resource exploration studies of controlling factors of the lf are still lacking especially for the heterogeneous formations characterized with complex combinations of the lamina and surrounding rock therefore 28 long column samples for triaxial compression tests 37 short column samples for brazil split tests and 12 powder samples for mineral component analysis and isotopes analysis carbon and oxygen are made from 6 cores in 4 petrophysical faces the experiments are designed to study the lf propagation process considering the controlling factors in terms of mineral component the lamina angle and the petrophysical faces our results show that clay content and dolomite cement degree more significantly control the lf propagation than the other minerals the lf propagation is not only dependent on the mineral component in the lamina but even more on the mineral component difference between the lamina and the surrounding rock i e the l s values where l represents the lamina and s represents the surrounding rock adjacent to the lamina the lf tended to propagate in a middle lamina angle range approximately 10 60 in shear properties and characterized with lower compression strength values all the lf observed in the thin sections were associated with oil although there was no obvious oil shown in the study cores the 13c and 18o values of lamina samples varied with the lamina mineral component for the complex organic matter oxidation and degradation reactions additionally the hydrocarbon flow parameter phf has been proposed on the carbon and oxidation isotope trace lines to indicate the oil rock reactions during the lf open period

diagenetic history and provenance of devonian terrestrial sandstones at the margin of gondwana padeha formation eastern alborz iran abstract the northern margin of gondwana in iran rifted in the devonian forming a passive continental margin to the paleotethys ocean the details of which are obscured by later collision a record of the early phase of opening of paleotethys is preserved in the 600 m thick padeha formation a middle devonian terrestrial rift basin succession the sandstones are quartz arenite sub arkose and arkose and contain minimal detrital matrix 1 bulk rock geochemistry according to major elements suggests a source to the sandstones that was dominated by i type granite but also based on trace elements included small amounts of intermediate or mafic rocks low zr and hree contents suggest little or no sandstone in the source area and thus low abundance of fine grained heavy minerals

authigenesis of amphibole and its relationship to the diagenetic evolution of lower cretaceous sandstones of the potiguar rift basin northeastern brazil abstract authigenic amphibole occurs in the fan delta front arkoses and lithic sandstones of the neocomian pendencia formation in the potiguar rift basin northeastern brazil as overgrowths fracture healing and replacement of detrital amphibole the amphibole which is closely associated with diagenetic albite chlorite and sphene has precipitated from k na cl brines enriched in ti fe mg at temperatures 100 c these ions were derived from the diagenetic alteration of detrital amphibole and pyroxene chloritization of volcaniclastic sediments at the base of the formation and from brines that ascended along faults from the underlying basement

the effects of oil emplacement on diagenetic processes examples from the fulmar reservoir sandstones central north sea geologic note 1 sedimentary petrography and petroleum geochemistry studies have been done on fulmar formation upper jurassic cores from fulmar field central north sea the extent of quartz overgrowths and albitization of k feldspars is lower in oil saturated zones compared with water saturated zones however microthermometric studies of aqueous fluid inclusions in quartz overgrowths show similar temperature ranges for both oil and water saturated zones 85 125 degrees c with maximum recorded temperatures close to the current reservoir temperature about 130 degrees c this suggests that quartz overgrowth development in the oil saturated zones continued after oil emplacement but was retarded compared with development in the water saturated zones the quartz cement therefore mus have been essentially locally derived by pressure solution albitization of k feldspars is significantly less about 50 in the oil saturated zones compared with albitization in the water saturated zones because albitization of k feldspars depends on a supply of sodium and removal of potassium this process is more likely to be terminated by oil emplacement the observed difference in the albitization has been used to suggest that the fulmar reservoir probably was filled during the neogene this age estimate agrees broadly with timing determined independently by modeling of oil generation in adjacent source basins the fulmar sandstones contain little or no kaolinite and show no evidence of substantial leaching of feldspars or carbonates this is probably because the fulmar sandstones being partly turbiditic offshore sand deposits were not exposed to meteoric water their close proximity to the marginally mature to mature kimmeridge clay formation seems to have little effect in terms of developing secondary porosity suggesting that the role of organic acids or co 2 generated in these source rocks is of minor importance

strontium isotopic compositions and potassium and rubidium contents of formation waters in sedimentary basins clues to the origin of the solutes abstract strontium isotopic compositions of formation waters in sedimentary basins relate primarily to sr from dissolution replacement of alkali feldspar minerals dissolution recrystallization precipitation of marine carbonate and sulfate minerals and illitization formation waters in sedimentary basins are often depleted in k but enriched in rb relative to evaporated seawater and have k rb ratios commonly between 200 and 2000 the k and rb characteristics of formation waters suggest that deep burial conversion of smectite to illite has a major influence on the evolutionary history of these solutes

a hybrid origin for lachlan s type granites the murrumbidgee batholith example the murrumbidgee batholith is typical of other composite s type granite batholiths in the lachlan fold belt lfb of southeastern australia consisting of discrete peraluminous cogenetic granite suites however it contains a unique suite murrumbucka at its southern extremity that has chemical and petrological features transitional with metaluminous i type granites previous detailed structural and metamorphic studies have shown that the batholith is tilted northward exposing subvolcanic plutons in the north and root zones in the south originally located at depths of not vert similar10 km a transition from mafic foliated sheet like granites in the south to felsic generally less foliated homogeneous granites in the north is consistent with magma ascent via subvertical structurally controlled sheets to emplacement in an overlying magma chamber in the inferred root zones the murrumbucka suite hosts migmatitic metasedimentary and gabbroic rocks both of which have transitional contacts and show evidence for mafic felsic interaction with the murrumbucka suite the migmatites extend southward to become part of the high t low p cooma metamorphic complex which contains a core of heterogeneous remobilised diatexitic granite cooma suite lenses of which also occur throughout the southern deeper parts of the murrumbidgee batholith the composition of the most mafic rocks from the batholith lies on a chemical tie line between cooma suite granites and the gabbros for almost all elements this chemical coincidence is interpreted to reflect derivation of parental murrumbidgee s type granite magmas by bulk mixing between a felsic crustal and mafic mantle derived component consistent with sr and nd isotopic results and field observations based on the tie lines and simple numerical modelling the murrumbucka suite is estimated to be a 50 50 mix of mantle and crustal derived magmas whereas the more peraluminous and widespread clear range suite which is very typical of lachlan s type granites is modelled as a 40 60 mix chemical variation trends diverge from the mixing lines and suggest that much of the fractionation process operated after mixing mixing occurred within sheets during the ascent of the magmas whereas fractionation was the dominant process generating chemical diversity during emplacement in the magma chamber generation of the s type granites probably occurred in an extensional tectonic environment

dislocation formation and albitization in alkali feldspars from the shap granite orthoclase rich alkali feldspars in the lower devonian shap granite northwest england contain two generations of albite rich feldspar these have partially replaced earlier exsolution microtextures consisting of albite lamellae coarse semicoherent albite films and fine coherent albite platelets in tweed orthoclase the earlier generation of replacive albiterich feldspar ab90an9ori occurs together with orthoclase rich feldspar ab10or90 i n veins that crosscut exsolution microtextures throughout grain interiors this episode of recrystallization was mediated by magmatic fluids at 410 8c estimated from two feldspar geothermometry and was driven by stored elastic strain energy which was relatively homogeneously distributed throughout the microtextures the later generation of replacive albite rich feldspar which is restricted to grain margins and is compositionally pure ab 99 was produced by magmatic hydrothermal fluids at 370 8c this generation of albite rich feldspar does not crosscut exsolution microtextures and has selectively replaced volumes of highly elastically strained feldspar surrounding edge dislocations along semicoherent albite films marked differences in controls of the localization of the two generations of replacive albite rich feldspar by pre existing exsolution microtextures indicate that significant numbers of edge dislocations developed along albite films after the first phase of fluid feldspar interaction and associated albitization but before the second phase this relation indicates that edge dislocations formed between 410 and 370 8c these observations have important implications for understanding the factors that control the interaction of alkali feldspars with fluids both in cooling igneous rocks and in clastic sedimentary rocks during diagenesis

impact of structural setting on diagenesis of fluvial and tidal sandstones the bahi formation upper cretaceous nw sirt basin north central libya abstract this petrographic mineral chemical fluid inclusion and stable isotopic study shows that the distribution of diagenetic modifications and their influence on reservoir quality and heterogeneity in tidal and fluvial sandstones of the upper cretaceous bahi sandstones in the rift sirt basin nw libya varies systematically along a series of closely spaced dominantly normal faults between the basin margin and more basinward located areas shallow eogenetic modifications resulting from the percolation of meteoric waters include infiltration of grain coating clays kaolinitization of detrital silicates and cementation by dolomite and k feldspar overgrowths mesogenetic alterations 70 c 2 km include feldspar albitization illitization of infiltrated clay and kaolinite conversion of kaolinite into dickite and cementation by quartz overgrowths t h 112 c 134 c barite t h 145 c 158 c and fe carbonates the restriction of barite and fe carbonate cements to the basinward located sandstones suggests formation by hydrothermal fluids along the faults extensive feldspar dissolution and formation of moldic pores in sandstones from the basin margin were probably caused by deep percolation of meteoric waters results from this study regarding the structural control on the spatial distribution of diagenetic alterations have implications for constraining the flux of pore fluids and by extension reservoir quality in analogous epicratonic rift basins

authigenic albite formation due to water rock interactions case study magnus oilfield uk northern north sea abstract it is the aim of this contribution to test whether organic inorganic interactions could induce the formation of authigenic albite this concept and related results are being compared with modelling scenarios which are purely based on inorganic geochemical reactions in order to unravel the pathway of authigenic albite formation this paper presents results of a multidisciplinary study from imaging geochemistry mineralogy and hydrogeochemical modelling the jurassic reservoir sandstones of the magnus oilfield uk north sea were chosen as a test site albite occurs with 4 18 wt in the magnus sandstones and its contents vary with depth however albite contents increase with increasing k feldspar contents and decreasing grain size it occurs in three forms 1 as lamellae in perthite 2 as overgrowth on in corroded feldspar and 3 as cloudy replacing albite patches in k feldspar the albite overgrowth has the highest chemical purity 100 albite whilst albite lamellae and replacing albite patches are slightly less pure containing 1 4 anorthite albite appears non altered and has a euhedral morphology and dull cathodoluminescence it commonly co occurs with corroded k feldspar grains the precipitation of diagenetic albite in the magnus sandstones is attributed to deep burial 80 ma ago and may have continued until today at temperatures between 90 120 c the results of hydrogeochemical modelling offer two possible pathways for the authigenic albite formation 1 dissolution of unstable minerals such as kaolinite and chalcedony coupled to reduction of ferric iron minerals by products generated during oil generation migration and degradation 2 dissolution of non end member feldspar such as k feldspar with 10 albite coupled to illite formation can account for trace amounts of albite due to an elevated na k activity ratio in the pore water

composition porosity and reservoir potential of the middle jurassic kashafrud formation northeast iran abstract in the kopet dagh basin of iran deep sea sandstones and shales of the middle jurassic kashafrud formation are disconformably overlain by hydrocarbon bearing carbonates of upper jurassic and cretaceous age to explore the reservoir potential of the sandstones we studied their burial history using more than 500 thin sections supplemented by heavy mineral analysis microprobe analysis porosity and permeability determination and vitrinite reflectance the sandstones are arkosic and lithic arenites rich in sedimentary and volcanic rock fragments quartz overgrowths and pore filling carbonate cements calcite dolomite siderite and ankerite occluded most of the porosity during early to deep burial assisted by early compaction that improved packing and fractured quartz grains iron oxides are prominent as alteration products of framework grains probably reflecting source area weathering prior to deposition and locally as pore fills minor cements include pore filling clays pyrite authigenic albite and k feldspar and barite existing porosity is secondary resulting largely from dissolution of feldspars micas and rock fragments with some fracture porosity porosity and permeability of six samples averages 3 2 and 0 0023 md respectively and 150 thin section point counts averaged 2 7 porosity reflectance of vitrinite in eight sandstone samples yielded values of 0 64 0 83 in the early mature to mature stage of hydrocarbon generation within the oil window kashafrud formation petrographic trends were compared with trends from first cycle basins elsewhere in the world inferred burial conditions accord with the maturation data suggesting only a moderate thermal regime during burial some fractures iron oxide cements and dissolution may reflect cenozoic tectonism and uplift that created the kopet dagh mountains the low porosity and permeability levels of kashafrud formation sandstones suggest only a modest reservoir potential for such tight sandstones fractures may enhance the reservoir potential

diagenetic albitization in the tera group cameros basin ne spain recorded by trace elements and spectral cathodoluminescence abstract this paper deals with the diagenetic albitization of both plagioclases and k feldspars in the tithonian fluvial sandstones of a rift basin cameros basin the sandstones in the lower part of the rift record have not suffered this albitization process a clear relationship is observed between sodium contents as the main element of some feldspars and their cathodoluminescence cl color the higher the sodium content the lower is their cl intensity in conclusion albitization processes are detectable by decreased cl intensities and changes in the cl spectra in addition very different trace element compositions are obtained by laser ablation when comparing trace elements of non albitized feldspars in sandstones of the lower part of the rift record with those of albitized feldspars in sandstones of the infill top non albitized k feldspars show rb sr ba and pb contents of up to 1000 ppm in contrast very flat profiles of trace element contents cl spectra from non albitized and albitized k feldspars and plagioclases revealed marked differences non albitized k feldspars present blue main emission band at 460 nm and brownish cl colors 590 nm sometimes in the same grain brownish colors are related to weathering processes the primary blue emission is related to al o al centers enhanced probably by al incorporation due to the coupled substitution of ba2 al3 m si4 weathered k feldspars present 4 8 times lower ba content than fresh blue luminescent ones the brownish colors are related to the external border or fractured grain zones altered by weathering processes therefore the observed 590 nm emission is assumed to be caused by structural defects resulting from weathering and alteration albitized k feldspars are usually weak luminescent with a typical cl emission band at 620 nm sometimes relicts of the original blue luminescence 460 nm band are still present the leaching of probably both al and ba can be responsible for the decrease in the blue band the characteristic 620 nm band is also dominant in albitized weak luminescent plagioclases two additional emission bands at 440 nm al o al center and 565 nm mn2 occur when albitized plagioclases preserved their original cl characteristics green cl color another spectral peak at ca 720 nm can be explained by fe3 activation due to fe3 al3 substitution the spectral cl measurements indicate that changes in luminescence due to albitization 620 nm emission seem to be more related to structural defects than to trace element activation or quenching

diagenetic evolution and porosity destruction of turbiditic hybrid arenites and siliciclastic sandstones of foreland basins evidence from the eocene hecho group pyrenees spain abstract this study aims to unravel the impact of diagenetic alterations on porosity loss of foreland basin turbiditic hybrid arenites and associated siliciclastic sandstones of the eocene hecho group south central pyrenees spain in this succession hybrid arenites and calclithites are extensively cemented by mesogenetic calcite cement 18ovpdb 10 0 to 5 8 th mode 80 c salinity mode 18 8 wt eq nacl fe dolomite 18ovpdb 8 5 to 6 3 and trace amounts of siderite the extent of carbonate cementation is interpreted to be related to the amounts of extrabasinal and intrabasinal carbonate grains which provided nuclei and sources for the precipitation and growth of carbonate cements other diagenetic alterations such as pyrite and albitization had no impact on reservoir quality scarce early diagenetic cements coupled with abundant ductile carbonate and siliciclastic framework grains have led to rapid porosity loss owing to compaction conversely abundant quartz in the sandstones prevented rapid loss of porosity by mechanical compaction reservoir quality was affected by mesogenetic cementation by quartz overgrowths calcite and dolomite intergranular pressure dissolution of quartz grains and formation of fracture filling calcite cement 18ov pdb values from 10 4 to 7 8 th temperatures of 150 c which are attributed to deep circulation of hot meteoric waters during extensional stages of tectonism the results of this study illustrate that diagenetic evolution pathways of the arenites and sandstones are closely linked to the variation in detrital composition particularly the proportion and types of extrabasinal noncarbonates extrabasinal carbonates and intrabasinal carbonate grains these insights suggest that marine turbiditic hybrid arenites and calclithites of foreland basins are subjected to more rapid and extensive porosity loss owing to compaction and cementation than associated siliciclastic sandstones degradation of reservoir quality makes these hybrid arenites calclithites and sandstones suitable as tight gas reservoirs but only if fracture porosity and permeability develop during tectonic deformation

correlation of carbon dioxide abundance with temperature in clastic hydrocarbon reservoirs relationship to inorganic chemical equilibrium the mol co2 of gas reservoired in sandstones tends to increase with increasing temperature along both the us gulf coast and the norwegian continental shelf each of these provinces displays identical linear correlations between temperature and the log of the partial pressure of co2 i e pco2 in the reservoir fluid that is the product of the mol co2 the reservoir pressure this correlation is the same both in gulf coast reservoirs younger than 17 ma and in norwegian reservoirs of jurassic to triassic age 144 245 ma indicating that it is not dependent on reaction kinetics norwegian shelf data indicate that the same correlation also applies for gas condensate and oil reservoirs we suggest that the log pco2versus temperature correlation is the result of inorganic chemical equilibria between feldspar clay and carbonate minerals a trend of decreasing ph with increasing temperature is fixed by equilibria between aluminosilicate minerals feldspar kaolinite at lower temperatures followed by various reactions between kaolinite illite chlorite and feldspar as temperatures exceed 110 140 c responding to the ph set by the aluminosilicate system co2 activity in the formation water and thereby co2 partial pressure in any gas phase present is controlled by equilibrium with carbonate minerals this model predicts that co2 released by organic maturation will in general result in precipitation of carbonate minerals below 100 120 c and that there should be widespread carbonate dissolution at higher temperatures

climatic depositional and burial controls on diagenesis of appalachian carboniferous sandstones qualitative and quantitative methods abstract carboniferous quartz lithic and feldspar rich sandstones from different stratigraphic intervals within the central appalachian basin contain evidence for eogenetic and mesogenetic diagenesis important eogenetic products include fe oxide oxyhydroxide siderite kaolinite chlorite and calcite whereas quartz albite illite sericite fe carbonate and secondary porosity are the most important mesogenetic components correspondence and detrended correspondence analysis of the framework grain and diagenetic mineral data set has guided diagenetic interpretations and leads to two major conclusions first minerals that formed early appear to have been influenced by stratigraphic position the distribution of siderite and iron oxide oxyhydroxide is considered to reflect a second order paleoclimatic signature recognized throughout the carboniferous where siderite formed during everwet periods and iron oxide oxyhydroxide during semi arid conditions reflecting differences in redox second framework grain composition controlled the distribution of diagenetic products in the burial environment lithic arenites are deficient in authigenic quartz yet have undergone various degrees of illitization quartz deficiency is attributed to compaction related loss of primary porosity relatively early which inhibited sufficient fluid flow to account for much higher concentrations of quartz cement in the more competent quartz rich sandstones

systematic destruction of k feldspar in deeply buried rift and passive margin sandstones systematic patterns of detrital k feldspar decline with increasing burial depth are revealed during deep burial 1 5 4 5 km of sandstones in diverse sedimentary basins from three rift and two passive margin settings in the north sea and usa gulf coast k feldspar destruction is predominantly by dissolution though in situ alteration to albite also occurs dissolution can remove up to 15 solid volume of the rock by 4 5 km so that primary arkosic sandstones become diagenetic quartzarenites at depth with increased secondary primary porosity ratios the quantity of k feldspar destroyed during temperature increase is very variable 0 05 1 0 solid volume c compaction continues during deep burial and petrographic textures diagnostic of secondary porosity are destroyed so that many sandstones preserve exceptions to this scenario of detrital k feldspar dissolution are noted proximity of k rich evaporites high sio 2 and k activity due to the breakdown of volcanic clasts or very low p co 2 can stabilize both k feldspar and a suite of k rich cements

peristeritic plagioclase in north sea hydrocarbon reservoir rocks implications for diagenesis provenance and stratigraphic correlation reservoir sandstones from well 30 16 7 in the upper jurassic humber group uk north sea contain plagioclase feldspars with a range of chemical compositions microtextures and origins the chemical composition of feldspar grains in samples from three different depths in the core ranges from 0 to 18 mol an two of the samples have a well defined gap in the compositional range between an 4 and an 9 this gap reflects the absence of plagioclase feldspars with those compositions in the ultimate source area of the humber group sandstones examination of an bearing feldspars on either side of the gap by tem shows that they have lamellar and or tweed exsolution microtextures that are characteristic of plagioclase feldspars having bulk compositions within the peristerite miscibility gap this is the first time that peristerites have been found in a sedimentary rock and they suggest a low grade metamorphic provenance for the plagioclase population compositional gaps have been commonly observed in ab rich plagioclase feldspars from metamorphic rocks although the precise reasons why feldspars do not form within the gap remain poorly understood in addition to the an bearing peristerites 30 16 7 contains a varied suite of an poor feldspars the microtexture of some of these grains is consistent with a low grade metamorphic provenance whereas others are inferred to be diagenetic in origin and to have formed by albitization of detrital alkali or plagioclase feldspar stratigraphic differences in the presence and size of the compositional gap within the humber group suggests that plagioclase feldspars may be a powerful new tool for high resolution stratigraphic correlation and may also be very sensitive indicators of changes in provenance

the role of detrital composition and climate on the diagenetic evolution of continental molasses evidence from the cambro ordovician guaritas sequence southern brazil abstract the cambro ordovician molassic guaritas sequence camaqua basin southern brazil comprises alluvial fan and braided fluvial sandstones and conglomerates with intercalated aeolian and lacustrine deltaic deposits and andesitic lava flows the sediments display a complex detrital composition derived from plutonic gneissic acidic volcanic and low grade metamorphic source rocks this detrital assemblage was strongly modified by semi arid continental near surface diagenesis early cementation by hematite smectite quartz and calcite and the relatively limited burial prevented strong compaction and preserved some primary macroporosity in most of the sandstones whereas the absence of early cements and or abundance of ductile grains promoted substantial porosity destruction by compaction and the inhibition of further diagenetic modifications the diagenetic dissolution and replacement of volcanic rock fragments and detrital feldspars by clays and albite changed the original framework composition as well as the tectonic provenance classification of the sandstones detailed quantitative petrographic study allowed the reconstruction of the original detrital compositions and the distinction of six different pathways of diagenetic evolution of the sandstones attesting to the efficiency of this method for diagenetic modelling and provenance analyses of ancient sandstones

albitized microcline grains of post depositional and probable detrital origins in brottum formation sandstones upper proterozoic sparagmite region of southern norway untwinned albite ab 99 4 an 0 4 or 0 2 grains forming during diagenesis and very low grade metamorphism are common in sandstones of the brottum formation upper proterozoic in the sparagmite region of southern norway these untwinned albite grains are usually clouded by vacuoles and tiny inclusions which are mostly made of carbonaceous material although some albite grains also contain some combination of carbonates quartz phengite chlorite pyrite halite and anatase the presence of microcline grains replaced partly by albite indicates that the albite grains are pseudomorphs after microcline chessboard albite which occurs in the brottum formation is suggested to be detrital in origin

alkali feldspar microtextures as provenance indicators in siliciclastic rocks and their role in feldspar dissolution during transport and diagenesis abstract alkali feldspars usually exhibit optical or sub optical intergrowths of albite and k feldspar known generically as perthite which have morphologies and crystallographic characteristics that are distinctive of the igneous or metamorphic environment in which they grew and cooled to surface temperatures we review the current terminology and understanding of how the microtextures form and discuss their role in feldspar dissolution during weathering transport and diagenesis we show how microtextures can be used as provenance indicators in arkosic siliciclastic rocks using examples from the fulmar formation a reservoir rock in the upper jurassic humber group in the central north sea in which feldspar dissolution is a major source of secondary porosity we describe the most effective techniques for routine characterization of which the most practical is back scattered electron imaging in a scanning electron microscope often coupled with cathodoluminescence we provide an atlas of the main types of perthitic intergrowth likely to be encountered in igneous and metamorphic rocks worldwide and show how many of the microtextures can be matched in clastic grains in the fulmar formation grains which are preferentially preserved have microtextures that make them relatively unreactive in aqueous fluids at low temperatures and we conclude that provenance has a major impact on reservoir quality detrital grains in the fulmar exhibit authigenic albitization k feldspar overgrowths and replacement and we discuss how diagenetic features can be distinguished from replacive features developed in the protolith

melt generation and fluid flow in the thermal aureole of the bushveld complex granite sheets emplaced into the migmatite zone of the eastern contact aureole of the bushveld complex resulted from fluid enhanced incongruent biotite melting of the underlying silverton formation shales during prograde metamorphism ba concentrations are extreme in both the sheets 1000 ppm and the hornfels 800 ppm into which they have been emplaced we conclude that a ba rich hydrothermal fluid induced melting in the aureole and that fluid transport of ba2 and to a lesser extent sr2 and eu2 persisted in the melt zones under subsolidus conditions sr isotope systematics from high ba localities define an errorchron of 2161 106 ma with an initial 87sr 86sr ratio of 0 705 0 001 metasedimentary rocks unaffected by fluid infiltration were homogenized at the same time but with an increased initial ratio suggesting that whereas isotope homogenization was achieved between outcrops permeated by fluids there is no evidence of regional homogenization oxygen isotope compositions of psammitic metasediments in the aureole are uncorrelated with distance from the contact suggesting the infiltrating fluid equilibrated isotopically with the metasediments their elevated 18o values 11 3 12 1 are consistent with a fluid source from devolatilization of the sedimentary lithologies textural analysis at both outcrop and thin section scale of arkose and psammites in the aureole lakenvalei and magaliesberg formations shows that the extent of melting was highly heterogeneous even on the grain scale and resulted from heterogeneously distributed infiltration of aqueous fluid on dilatant cracks and grain boundaries cathodoluminescence imaging of quartz shows a marked difference in the amount of fine structure with that in the melted rocks having uniform luminescence in contrast to that in rocks containing little or no melt which preserve textures inherited from the regionally metamorphosed protolith simple finite difference thermal modelling of the aureole suggests that the width of the melt zone 500 m is inconsistent with conductive heat transfer and hence that the thermal structure has been modified by fluid advection

ion microprobe study of intragrain micropermeability in alkali feldspars alkali feldspar cleavage fragments from the klokken layered syenite south greenland were heated to 700 c at 0 1 gpa in 99 h218o for 75 h these samples were then polished and imaged by ion microprobe for 18o the feldspars were known to contain areas of pristine braid micro perthite which were not turbid and areas of deuteric patch perthite which were turbid turbidity is related to the presence of micropores in the feldspars on imaging the grain it was found that the 18o had penetrated into the parts of the grain which were microporous and not into the pristine areas micropores are therefore responsible for rendering the feldspars permeable as well as porous the implications of micropermeable feldspars in several areas of geology are discussed

microtextures of authigenic or rich feldspar in the upper jurassic humber group uk north sea detrital alkali feldspars currently at burial depths of 3 2 3 5 km in the upper jurassic humber group of the fulmar oilfield uk north sea are overgrown and have been partially replaced by authigenic or rich feldspar intracrystal microtextures suggest several different provenances for the detrital grains the overgrowths are uniformly non cathodoluminescent and have occasional celsian rich zones transmission electron microscopy shows that they are composed of a microporous mosaic of sub m to m sized sub grains associated with barite illite and pyrite the subgrains are somewhat rounded but have an approximate 110 adularia habit and display a faint modulated microtexture on the nanometre scale they have triclinic symmetry but the lattice angles depart only slightly from monoclinic symmetry these features are characteristic of k feldspar precipitated relatively rapidly and at low temperature authigenic or rich feldspar has also partially replaced microcline and perthitic albite within the detrital grains often at a suboptical scale although like diagenetic albitization replacement by k feldspar is probably a very common diagenetic reaction it has rarely been reported owing to difficulties in imaging the diagnostic textures with the scanning electron microscopy techniques used by most workers the permeability of the subgrain microtexture may significantly hinder the use of feldspar overgrowths for k ar and 4 0 ar 3 9 ar dating of diagenesis and the existence of suboptical replacive authigenic k feldspar within detrital grains may significantly modify the apparent ar ages of detrital grains similar subgrain microtextures in optically featureless quartz overgrowths are also illustrated

diagenesis and reservoir quality evolution of palaeocene deep water marine sandstones the shetland faroes basin british continental shelf abstract the palaeocene deep water marine sandstones recovered from six wells in the shetland faroes basin represent lowstand transgressive and highstand systems tract turbiditic sediments mineralogic petrographic and geochemical analyses of these siliciclastics are used to decipher and discuss the diagenetic alterations and subsequent reservoir quality evolution the middle upper palaeocene sandstones subarkoses to arkoses from the shetland faroes basin british continental shelf are submarine turbiditic deposits that are cemented predominantly by carbonates quartz and clay minerals carbonate cements intergranular and grain replacive calcite siderite ferroan dolomite and ankerite are of eogenetic and mesogenetic origins the eogenetic alterations have been mediated by marine meteoric and mixed marine meteoric porewaters and resulted mainly in the precipitation of calcite 18 o v pdb 10 9 and 3 8 trace amounts of non ferroan dolomite siderite 18 o v pdb 14 4 to 0 6 as well as smectite and kaolinite in the lowstand systems tract lst and highstand systems tract hst turbiditic sandstone below the sequence boundary minor eogenetic siderite has precipitated between expanded and kaolinitized micas primarily biotite the mesogenetic alterations are interpreted to have been mediated by evolved marine porewaters and resulted in the precipitation of calcite 18 o v pdb 12 9 to 7 8 and fe dolomite ankerite 18 o v pdb 12 1 to 6 3 at temperatures of 50 140 and 60 140 c respectively quartz overgrowths and outgrowth which post and pre date the mesogenetic carbonate cements is more common in the lst and tst of distal turbiditic sandstone discrete quartz cement which is closely associated with illite and chlorite is the final diagenetic phase the clay minerals include intergranular and grain replacive eogenetic kaolinite smectite and mesogenetic illite and chlorite kaolinite has been subjected to mesogenetic replacement by dickite the k feldspar and plagioclase grains have been albitized dissolution of calcite cement and of framework grain feldspar volcanic fragments and mud intraclasts has resulted in a considerable enhancement of reservoir quality

diagenesis of titaniferous minerals in jurassic sandstones from the norwegian sea abstract ti oxides anatase and in some cases rutile that attain various textures and modes of occurrence are minor yet very common authigenic minerals in the jurassic sandstones of the haltenbanken oil field offshore central norway these ti oxides pyrite commonly occur as pseudomorphs after detrital fe ti oxides usually ilmenite titanomagnetite and titanohematite pseudomorphs comprised of ti oxides as well as quartz and or calcite are believed to result upon alteration of detrital sphene ti oxides have also resulted upon kaolinization of detrital biotite and less significantly muscovite authigenic sphene occurs in very small amounts in some of the jurassic sandstones at burial depths 2000 m it displays three major crystallization habits 1 as almond shaped crystals that have most commonly crystallized on authigenic calcite 2 as discrete rhombic crystals that have crystallized adjacent to altered fe ti oxides and biotite and 3 as aggregates of euhedral and subhedral crystals these crystallization habits suggest that sphene has been formed by reaction of si and ti ions with calcite crystallization from pore fluids and replacement of fe ti oxides respectively microprobe analyses of authigenic sphene revealed that al and fe 3 substitute ti in appreciable amounts average 0 08 and 0 04 moles per 5 oxygen atoms respectively whereas substitutions of ca by mn k and na and of ti by mg are negligible

diagenetic influence on reservoir quality evolution examples from triassic conglomerates arenites in the edvard grieg field norwegian north sea abstract the utsira high southern norwegian north sea has been a petroleum exploration target since the late 1960s the southwestern part of the utsira high complex haugaland high attracted new interest when in 2007 lundin norway as and partners revus energy asa and rwe dea norge as discovered commercial hydrocarbon reservoirs in triassic to early cretaceous formations edvard grieg field in the present study four siliciclastic cores from the edvard grieg field have been investigated sedimentologically including core logging optical mineralogy x ray powder diffraction xrd and scanning electron microscope sem quantification and characterization of the different mineralogical phases in thin section have been executed to better understand porosity evolution through time intergranular volume igv and cement volume were also compared in order to estimate the most important factors effecting porosity loss within the studied samples the results show that through time the various diagenetic events and phases such as dissolution of unstable grains and precipitation of clay minerals and cements strongly influenced reservoir quality in the edvard grieg field two main lithology groups are identified within the cored intervals conglomerates and sandstones these two groups can further be sub divided into four units 1 sandy conglomerate 2 silty conglomerate 3 interbedded sandstone 4 large scale cross bedded sandstone most of the reservoir quality reduction in the conglomerate matrix can be related to poor sorting carbonate cement and pore filling pore lining authigenic clay minerals better reservoir quality characterizes the sandstone dominated alluvial fan intervals which generally are associated with a greater degree of sorting that led to higher intergranular porosities large scale cross stratified sandstone units interpreted to be of aeolian origin have the highest reservoir quality these deposits are moderately to well sorted with only minor occurrences of patchy carbonate cement and generally low proportions of authigenic minerals however locally abundant kaolinite has been observed filling the pore space and reducing the porosity predominantly by blocking interconnected primary pores

albitisation related to the triassic unconformity in igneous rocks of the morvan massif france the carboniferous morvan massif in the northern part of the french massif central consists of granite and some rhyolite a triassic erosional unconformity has developed on the massif which is covered by mesozoic sediments of the paris basin the igneous rocks of the morvan massif show a strong alteration with pseudomorphic replacement of the primary plagioclases into albite pseudomorphic replacement of primary biotite into chlorite and minor precipitation of neogenic minerals like albite chlorite apatite haematite calcite and titanite the geometry and arrangement of these alterations give significant constraints about their development some of the altered facies develop in a pervasive manner others are restricted to centimetric to metric wide joints that imply fluid flow phenomena moreover the alteration facies are arranged in a clear succession with strongly altered facies at the top and weakly altered facies towards the depth which point to a genetic relationship with the triassic unconformity regional distribution of the alterations which affect the carboniferous igneous and volcanic formations beneath the jurassic sedimentary cover also leads to associate these alterations with the triassic unconformity dating of the alterations provides even a further constraint alterations are of triassic age that means the same age as the unconformity taking into account all these geological constraints it is proposed that albitisation of the morvan massif was developed under low temperature subsurface conditions in relation to the triassic palaeosurface

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

diagenesis and quartz cement distribution of low permeability upper triassic middle jurassic reservoir sandstones longyearbyen co2 lab well site in svalbard norway reservoir properties of upper triassic middle jurassic sandstones spitsbergen are studied as part of a co2 storage pilot project in longyearbyen the reservoir formations show large contrasts in sandstone compositions with unexpected low permeability despite moderate porosity values petrographic analyses were performed to investigate the influence and distribution of diagenesis it is concluded that because of various compaction cementation and dissolution processes the sandstone porosity is mainly isolated molds and micropores and associated with fibrous illite and chamosite explaining the low permeability diagenesis and the distribution of quartz cement is influenced by lithofacies and detrital compositions mineralogically immature sandstones de geerdalen formation show a homogeneous distribution of quartz cement overgrowths on quartz grains distributed interstitial to labile grains and other cements e g late calcite the main silica source was from the dissolution of adjacent feldspar and labile grains as part of the chemical compaction in contrast quartz dominated sandstones knorringfjellet formation show a heterogeneous patchy distribution of quartz cement influenced by the sedimentary bioturbation pattern with silica sourced also from dissolution at clay rich microstylolites phosphatic beds at the base and top of the formation are strongly influenced by marine eogenesis and reworking processes and associated with concentration of iron rich authigenic minerals the highest porosity appears in sand supported conglomerate where moldic clay mineral ooids contributed to reduce quartz cementation the stratigraphic change from mineralogical immature triassic to mature uppermost triassic jurassic sandstone compositions is detected in wide areas of the barents shelf and has considerable implications for the distribution of sandstone reservoir properties

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

role of sandstone provenance in the diagenetic albitization of feldspars a case study of the jurassic tera group sandstones cameros basin ne spain abstract the cameros basin iberian chain ne spain formed during the latest jurassic early cretaceous rifting stage in an extensional regime characterized by high subsidence rates its sedimentary infill thicker than 6000 m has been subdivided into eight depositional sequences ds mainly composed of continental sediments ds 1 and ds 2 represent the first rifting stage tera group tithonian mainly formed by fluvial and lacustrine sediments sandstone petrofacies evolved from quartz sedimentolithic in ds 1 to quartz feldspathic in ds 2 due to the rifting process in ds 2 three different types of detrital feldspars k feldspars albites and polysynthetic plagioclases with similar sodium rich compositions mean ab94 0 an4 5 or1 5 can be recognized chemically pure nonluminescent albites ab 99 are common in ds 2 diagenetic albitization of both plagioclases and k feldspars is inferred from conventional microscopy observations cathodoluminescence and electron microprobe analyses ds 1 contains few plagioclase grains which show no evidence of transformation into albite although the albitization is characterized as diagenetic it seems to be provenance controlled since it affects the units showing higher original plagioclase k feldspar ratio ds 2 due to the greater influence of plutonic and metamorphic source areas in ds 2 possible na sources are 1 the percolation of moderate to high salinity residual brines from related alkaline lakes developed at top of ds 2 in the eastern sector of the basin 2 clay mineral reactions sodium smectite to illite and chlorite indicated by mudstone composition in the interlayered mudstones and 3 the replacement of detrital sodium plagioclases by carbonate these three sources can be complementary

destruction of a fluvial reservoir by hydrothermal activity cameros basin spain abstract this study provides an example of a high quality fluvial hydrocarbon reservoir that was completely destroyed by hydrothermal processes the reservoir unit was deposited in the cameros basin located in the nw sector of the iberian chain spain the basin was filled with clastic fluvial deposits sandstones and conglomerates between late berriasian and early aptian times provenance of sands was mainly from coarse crystalline rocks a humid tropical climate produced intense weathering of k feldspar during transport from source to basin thus a mineralogically mature rigid framework with high porosity existed at the time of deposition which would have constituted a high quality hydrocarbon reservoir at present however the porosity of the reservoir is negligible porosity was reduced by a sequence of diagenetic processes 1 mechanical compaction i e crushing of metamorphic lithic grains and chemical compaction 2 kaolinite and siderite cementation and 3 early quartz cementation hydrocarbon emplacement probably occurred between phases 2 and 3 a low grade metamorphic hydrothermal event reaching greenschist facies took place during the late cenomanian it dramatically reduced the remaining porosity of the reservoir and destroyed the hydrocarbon charge hydrothermal processes which affected the sandstones include 1 re compaction 2 late quartz cementation and silicification of remaining feldspars 3 carbonate cementation 4 chloritization of feldspars metamorphic lithic fragments and intrabasinal argillaceous grains and 5 growth of pyrite and chloritoid crystals on argillaceous material of intrabasinal extrabasinal or even diagenetic origin hydrocarbons that migrated to the margins of the basin escaped these hydrothermal modifications and were preserved the results of this study may be used to predict the diagenetic and hydrothermal evolution of other potential reservoirs in similar tectonic settings

the provenance of triassic continental sandstones from the beryl field northern north sea mineralogical geochemical and sedimentological constraints a geochemical study of the heavy mineral suite in triassic strata from two wells in the beryl embayment was undertaken in order to identify changes in provenance through the lewis formation detailed provenance studies also allow the reconstruction of paleodrainage patterns and aid in predicting facies distributions the heavy mineral assemblage in the lewis formation is dominated by almandine pyrope garnets which are typically derived from pelitic metasediments together with zircon apatite tourmaline and rutile some of which are rich in niobium nb these latter phases could have been sourced either from metasediments or granitic protoliths although nb rich rutiles have a more restricted paragenesis being associated with alkaline plutonic rocks and granite pegmatites in the uppermost members of the lewis formation influxes of more exotic provenance specific heavy minerals such as cr spinel staurolite and chloritoid become more common the chemistry of the detrital cr spinel implies that it was derived from an ophiolitic source gross grain size and facies changes in the embayment suggest a nw to se dispersal of detritus although hydraulic sorting of the heavy mineral suite between relatively distal and proximal locations has resulted in subtle variations in whole rock trace element geochemical signatures the complete overlap in the geochemical data from distal and proximal wells requires that the sediments were essentially derived from a single source area the combination of mineralogical sedimentological and geochemical data implies that the dominant source for these sediments was the devonian old red sandstone ors the shetland ophiolite and the underlying dalradian metasedimentary rocks of the shetland isles and the east shetland platform provenance studies such as this are central to the reconstruction of paleo drainage patterns which in turn lead to improved prediction of potential reservoir quality distribution and connectivity

mass transfer and porosity evolution during low temperature water rock interaction in gneisses of the simano nappe arvigo val calanca swiss alps late alpine fissures and fractures in amphibolite facies basement gneisses at arvigo val calanca swiss alps show distinct cm sized reaction selvages parallel to the fracture walls that composed of subgreenschist facies assemblages produced by the interaction of water present in the fracture porosity with the old high grade gneiss assemblages the process of selvage or reaction vein formation occurred in the brittle deformation regime and at temperatures characteristic of first the prehnite pumpellyite facies and then later of the zeolite facies the vein formation occurred during uplift and cooling at very late stages of the alpine orogeny the reaction veins are composed of a selvage of altered gneiss on both sides of the central fracture and a central zone with fissure minerals that have been growing in the open fracture pore space the central zone of the arvigo veins contains an early assemblage with epidote prehnite and chlorite and a late succession sequence of various species of zeolite the veins of the arvigo quarry are convincing evidence that fracture fluids in gneiss and granite have the potential to precipitate ca zeolite this is an important find because many fluids recovered from deep continental drill holes and from geothermal energy exploration are found to be oversaturated in respect to a number of ca zeolite species vein formation during late uplift and cooling of the alps occurred at continuously decreasing t and at hydrostatic pressure 1 coexisting prehnite epidote records temperatures of 330 380 c 2 chlorite formation at temperature of 333 32 c and 3 formation of zeolites 250 c in the selvages the prime reaction is the replacement of plagioclase by albite along a sharp reaction front that separates the selvage from unaltered gneiss in addition to albitisation chloritisation of biotite is the second important reaction in the alteration process the reactions release components for the formation of ca al silicates the water rock interaction is associated with a depletion of al si ca fe and k in the altered wall rock the overall reaction is associated with an increase in porosity of up to 14 2 2 2 in the selvage zone altered wall rock caused by the volume decrease during albitisation and the removal of biotite the propagation of the sharp reaction front through the gneiss matrix occurred via a dissolution reprecipitation mechanism zeolite formation is tied to the plagioclase alteration reaction in the rock matrix which releases components for zeolite formation to a co2 poor aqueous liquid

geochemical constraints from formation water analyses from the north sea and the gulf coast basins on quartz feldspar and illite precipitation in reservoir rocks

sandstone diagenesis in a halite deposit from surface to high grade diagenesis haselgebirge formation eastern alps abstract the evaporitic haselgebirge formation of the northern calcareous alps was deposited in an evolving rift during the upper permian the jurassic to cretaceous alpine orogenesis caused an overprint of 200 c the aim of the study was to investigate the diagenetic evolution of these evaporite dominated sediments from intact sandstone lenses field work thin section analysis sem and xrd were completed by measurements of organic material and age dating in this paper we show that all authigenic minerals developed step by step on a prograding path but under low temperatures of

the impact of diagenesis on the heterogeneity of sandstone reservoirs a review of the role of depositional facies and sequence stratigraphy diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability linking the types and distribution of diagenetic processes to the depositional facies and sequence stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diagenetic alterations controlling quality and heterogeneity the heterogeneity patterns of sandstone reservoirs which determine the volumes flow rates and recovery of hydrocarbons are controlled by geometry and internal structures of sand bodies grain size sorting degree of bioturbation provenance and by the types volumes and distribution of diagenetic alterations variations in the pathways of diagenetic evolution are linked to 1 depositional facies hence pore water chemistry depositional porosity and permeability types and amounts of intrabasinal grains and extent of bioturbation 2 detrital sand composition 3 rate of deposition controlling residence time of sediments at specific near surface geochemical conditions and 4 burial thermal history of the basin the amounts and types of intrabasinal grains are also controlled by changes in the relative sea level and therefore can be predicted in the context of sequence stratigraphy particularly in paralic and shallow marine environments changes in the relative sea level exert significant control on the types and extent of near surface shallow burial diagenetic alterations which in turn influence the pathways of burial diagenetic and reservoir quality evolution of clastic reservoirs carbonate cementation is more extensive in transgressive systems tract tst sandstones particularly below parasequence boundaries transgressive surface and maximum flooding surface because of the abundance of carbonate bioclasts and organic matter bioturbation and prolonged residence time of the sediments at and immediately below the sea floor caused by low sedimentation rates which also enhance the formation of glaucony eogenetic grain coating berthierine odinite and smectite formed mostly in tst and early highstand systems tract deltaic and estuarine sandstones are transformed into ferrous chlorite during mesodiagenesis helping preserve reservoir quality through the inhibition of quartz cementation the infiltration of grain coating smectitic clays is more extensive in braided than in meandering fluvial sandstones forming flow barriers in braided amalgamated reservoirs and may either help preserve porosity during burial because of quartz overgrowth inhibition or reduce it by enhancing intergranular pressure dissolution diagenetic modifications along sequence boundaries are characterized by considerable dissolution and kaolinization of feldspars micas and mud intraclasts under wet and warm climates whereas a semiarid climate may lead to the formation of calcrete dolocrete cemented layers turbidite sandstones are typically cemented by carbonate along the contacts with interbedded mudrocks or carbonate mudstones and marls as well as along layers of concentration of carbonate bioclasts and intraclasts commonly hybrid carbonate turbidite arenites are pervasively cemented proximal massive turbidites normally show only scattered spherical or ovoid carbonate concretions improved geologic models based on the connections among diagenesis depositional facies and sequence stratigraphic surfaces and intervals may not only contribute to optimized production through the design of appropriate simulation models for improved or enhanced oil recovery strategies as well as for co2 geologic sequestration but also support more effective hydrocarbon exploration through reservoir quality prediction

the non glacial and non cratonic origin of an early archean felsic volcaniclastic unit barberton greenstone belt south africa abstract recent studies have suggested that 3 445 ga felsic volcaniclastic rocks of member h6 of the hooggeneoeg formation in the barberton greenstone belt south africa show evidence for deposition above a major erosional unconformity under glacial to periglacial conditions and contain debris eroded from older granitic continental crust these conclusions represent fundamental re interpretations of this sequence its crustal setting and processes of deposition we here present evidence that these felsic strata represent terrestrial to shallow water mass flows deposits storm influenced shallow marine units and turbidity current like storm driven submarine flows all of which lack any evidence of a glacial influence during sedimentation they accumulated as part of a conformable stratigraphic sequence and show no evidence of mantling an erosional surface the debris consists entirely of volcanic derived materials no clasts of plutonic rock are found in the conglomerates low temperature diagenesis and potash metasomatism have altered primary volcanic feldspar in the volcaniclastic debris to tartan twinned microcline previously thought to be characteristic of granitic plutonic rocks this potassium feldspar is quite distinctive and unlike the microcline found in plutons surrounding the greenstone belt this succession is part of a classic felsic volcaniclastic sequence deposited as an apron flanking a major felsic volcanic center

petrography and major element geochemistry of the permo triassic sandstones central india implications for provenance in an intracratonic pull apart basin abstract detrital mode composition of feldspars and heavy minerals and major element chemistry of sandstones from the permo triassic succession in the intracratonic satpura gondwana basin central india have been used to investigate provenance the talchir formation the lowermost unit of the succession comprises glacio marine and glacio fluvial deposits the rest of the succession base to top comprising the barakar motur bijori pachmarhi and denwa formations largely represent variety of fluvial depositional systems with minor fluvio deltaic and fluvio lacustrine sedimentation under a variety of climatic conditions including cold warm arid sub humid and semi arid qfl compositions of the sandstones indicate a predominantly continental block provenance and stable cratonic to fault bounded basement uplift tectonic setting compositional maturity of sandstones gradually increases upwards from the early permian talchir to the middle triassic denwa but is punctuated by a sharp peak of increased maturity in the barakar sandstones this temporal change in maturity was primarily controlled by temporal variation in fault induced basement uplift in the craton and was also influenced by climatic factors plots of different quartz types suggest plutonic source rocks for the talchir sandstones and medium to high rank metamorphic plus plutonic source rocks for the younger sandstones composition of alkali feldspars in the permo triassic sandstones and in different precambrian rocks suggests sediment derivation from felsic igneous and metasedimentary rocks compositions of plagioclase in the talchir and bijori sandstones are comparable with those of granite acid volcanic and metasedimentary rocks of the precambrian basement suggesting the latter as possible source rare presence of high k plagioclase in the talchir sandstones however indicates minor contribution from volcanic source rock exclusively plagioclase bearing metasedimentary rock tonalite gneiss and mafic rocks are the probable sources of plagioclase in the upper denwa sandstones quartz rich nature of the sandstones predominance of k feldspar over plagioclase and albite rich character of plagioclase in the sandstones is consistent with deposition in an intracratonic pull apart basin like the satpura gondwana basin composition of garnet and its comparison with that from the precambrian basement rocks suggests mica schist and amphibolite as possible sources predominance of dravite variety of tourmaline in the permian sandstones suggests sediment supply from metasedimentary rocks presence of both dravite and schorl variety of tourmaline in subequal amount in the triassic sandstones indicates sediment derivation from granitic and metasedimentary rocks however schorl bearing rocks are absent in the basement complex of the study area a cn k plot suggests granites acid volcanic rock and meta sediments of the basement as possible sources of the talchir sandstones and metasedimentary rocks for the barakar to pachmarhi sandstones the denwa sandstones were possibly derived from k feldspar free plagioclase bearing metasediments mafic rocks and tonalite gneiss chemical index of alteration cia values suggest low intensity source rock weathering for the talchir sandstones and higher intensity source rock weathering for the others various bivariate plots of major oxides composition of the sandstones suggest passive to active continental margin setting and even arc tectonic setting for a few samples

magnetite in alh 84001 an origin by shock induced thermal decomposition of iron carbonate in martian orthopyroxenite alh 84001 pockets of feldspathic glass frequently contain carbonate masses that have been disrupted and dispersed within feldspathic shock melt as a result of impact s transmission electron microscope studies of carbonate fragments embedded within feldspathic glass show that the fragments contain myriad nanometer sized magnetite particles with cuboid irregular and teardrop morphologies frequently associated with voids the fragments of carbonate must have been incorporated into the melt at temperatures of 900 c well above the upper thermal stability of siderite feco3 which decomposes to produce magnetite and co2 below 450 c these observations suggest that most if not all of the fine grained magnetite associated with fe bearing carbonate in alh 84001 could have been formed as result of the thermal decomposition of the siderite feco3 component of the carbonate and is not due to biological activity

diagenetic albitization of detrital k feldspar in jurassic lower cretaceous and tertiary clastic reservoir rocks from offshore norway i textures and origin abstract the origin of albitized grains in sandstones has been the topic of considerable discussion in recent years some workers believe that the albitization of feldspar can occur in situ during burial diagenesis but others argue that albitization is a feature inherited from source clastic grains our petrological and chemical studies of subsurface samples of the jurassic lower cretaceous and tertiary reservoir sandstones from offshore norway show that detrital grains of potassium feldspar have been albitized during burial diagenesis criteria suggesting that the albitization has occurred during burial diagenesis include 1 euhedral habit of albite crystals with sharp edges and comers and markedly smooth crystal faces 2 generally untwinned albitized grains mostly riddled with minute br wnish inclusions 3 lack of cathodoluminescence in albite 4 homogeneous and pure albite composition 99 ab 5 absence of albitized grains in carbonate cemented zones and 6 increase in the percentage and degree partial to complete of albitized k feldspar with depth under sem albitized grains display two types of fabrics type i albitized grains are composed of numerous tiny albite crystals 1 30 m growing along cleavage planes in parent grains and they have abundant dissolution porosity while type ii albitized grains show blocky albite crystals 5 80 m forming pseudomorphs of detrital k feldspar and they lack any dissolution porosity type i albitized grains are found commonly at shallower depths between 2 2 and 3 km and temperatures varying from 65 to 90 c whereas type ii albitized grains dominate at greater depths 3 5 km and at temperatures 90 c albitization proceeds by a dissolution precipitation mechanism at shallower depth dissolution of the parent k feldspar is more rapid than the precipitation of albite and rates of albite precipitation increase with depth favoring pseudomorphic replacement of k feldspars other factors that influence albitization processes are ana ak permeability reduction and surface dissolution kinetics of the parent grain

geochemical modelling of sandstone diagenesis and its consequences on the evolution of porosity abstract geochemical modelling of fluid rock interaction is one theoretical approach that can be used for understanding diagenetic processes and their effects on reservoir quality results of successive modelling of dissolution and heating using two specific path calculation codes dissol and thermal from 25 to 160 c are presented if one considers one volume of reacting solution the diagenetic effects are very important for fluid chemistry while mass transfers have little effect on porosity only cumulative effects due to the circulation of fluids in reservoirs may explain the extent of diagenetic processes on minerals comparison between the computed data this study and available data from the north sea from gulf coast sandstones from pattani basin gulf of thailand from san joaquin basin california u s a and from a triassic sandstone reservoir in the rhine graben cronenbourg france show interesting similarities in the chemical evolution of the fluid the modelling has interesting implications for the relations between ph p co 2 conditions and the diagenetic stability of calcic plagioclase

geochronological and geochemical constraints on the petrogenesis of middle paleozoic kwangsian massive granites in the eastern south china block abstract to achieve a better understanding of the kwangsian orogenic event of the eastern south china block this paper documents a set of new zircon u pb geochronological and hf isotopic data and whole rock elemental and sr nd isotopic analytical results for the representative massive granite intrusions across the jiangshan shaoxing fault the studied samples are classified into two groups representing the rocks from the cathaysia block to the east of the jiangshan shaoxing fault group 1 and those from the eastern yangtze block between the anhua luocheng and jiangshan shaoxing faults group 2 the group 1 samples gave the zircon u pb ages of 405 454 ma and e hf t values of 3 6 to 15 2 with hf model ages of 1 6 2 4 ga group 2 yielded the zircon u pb ages of 400 432 ma and e hf t values of 0 2 to 12 7 with hf model ages of 1 3 2 2 ga geochemically the group 1 samples a cnk 1 02 1 43 have relatively lower al 2 o 3 mgo cao p 2 o 5 and e nd t but higher k 2 o na 2 o than those of group 2 a cnk 0 93 1 44 both groups show similar chondrite normalized patterns of rare earth elements with eu eu values of 0 15 0 92 and strongly negative ba sr nb p and ti anomalies in primitive mantle normalized spider diagrams their e nd t values range from 11 1 to 8 0 for group 1 and 8 9 to 5 0 for group 2 generally similar to those of precambrian paragneiss and contemporaneous gneissoid granites in the eastern south china block our geochronological results indicate that the kwangsian massive granites in the eastern south china block were crystallized between 400 ma and 454 ma synchronous to the kwangsian gneissoid granites along the wugong and wuyi baiyun yunkai domains in the eastern south china block the synthesis of these whole rock geochemical and in situ zircon hf isotopic data suggests that both the group 1 and 2 granites across the jiangshan shaoxing fault were predominantly derived from a crustal source with some proportional metapelitic and metaigneous components from the precambrian basement the input of juvenile mantle derived magma is insignificant taking into account other available evidences it is proposed that the jiangshan shaoxing fault might be a pre existing boundary between the yangtze and cathaysia blocks which was reactivated during the middle paleozoic kwangsian orogenic event the massive granitic magmatism was probably resulted from the doubly crustal thickening and subsequent isostatic readjustment in an intracontinental tectonic regime

modelling of thermal convection in sedimentary basins and its relevance to diagenetic reactions abstract mathematical calculations of thermal convection have been carried out using a porous three player model to simulate pore water flow in a sedimentary basin with layers of different permeabilities the calculated flow lines demonstrate that even very thin layers eg around salt domes and igneous or hydrothermal intrusions analyses of formation waters from sedimentary basins like the north sea often show evidence of a crude stratification of the pore water with respect to salinity and isotopic composition this is evidence suggesting that large scale convection or other types of mixing does not take place and positive salinity depth gradient may help to physically stabilize the formation water the fact that many of the shallow reservoir rocks from the north sea have formation water with low salinity and negative 18 o values suggest that this is modified meteoric water and that migration of petroleum from deeper parts of the basin occurred as a separate phase along restricted migration pathways and was not associated with a high flux of pore water in the absence of thermal convection the total pore water flux through sandstones will be rather small except locally where we may have focused compactional flow through small cross sections diagenetic reactions will therefore normally be relatively isochemical during deeper burial

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