episodic and simultaneous illitization in oil bearing brent group and fulmar formation sandstones from the northern and southern north sea based on illite k ar dating size fractions 0 4 and 0 4 1 0 m of brent group sandstones from the northern north sea contain mostly illite smectite mixed layers with kaolinite whereas the same size fractions of fulmar formation sandstones from the south central north sea consist of illite smectite mixed layers with minor chlorite transmission electron microscope observations show elongated illite laths or agglomerates consisting of small laths when larger individual laths are lacking the k ar data of the fractions less than 0 4 m of brent group samples plot on two arrays in a 40ar 36ar vs 40k 36ar diagram that have isochron characteristics with ages of 76 5 4 2 and 40 0 1 5 ma and initial 40ar 36ar ratios of 253 16 and 301 18 respectively for the fulmar formation samples the data points of the fractions less than 0 2 and less than 0 4 m also fit two isochrons with ages of 76 6 1 4 and 47 9 0 5 ma and initial 40ar 36ar ratios of 359 52 and 304 2 respectively some of the coarser 0 4 1 0 m fractions also plot on the two isochrons but most plot above indicating the presence of detrital components more than 0 4 m the almost identical ages obtained from illite type crystals of sandstones with different deposition ages that are located about 600 km 373 mi apart record two simultaneous illitization episodes these events were not induced by local burial conditions but are related to episodic pressure and or temperature increases in the studied reservoirs probably induced by hydrocarbon injection this interpretation is indirectly supported by notably different k ar illite ages from cores of a nearby reservoir at hydrostatic pressure illite is not as well crystallized as expected for potential crystallization temperatures above 160c measured by fluid inclusion determinations in both the northern and south central north sea the two illite generations remain unaffected after crystallization despite continued burial suggesting notably higher crystallization temperatures than those estimated from geothermal gradients no recent illite crystallization or alteration is recorded in the k ar ages despite a dramatic regional acceleration of the subsidence in the southern north sea

kinetic constraints on illitization reactions and the effects of organic diagenesis in sandstone shale sequences abstract based on water rock interaction modelling and kinetic considerations the present study is aimed at testing the impact of organic matter maturation on two kinds of diagenetic reactions leading to the alteration of the petrophysical properties of sandstones 1 clay mineral conversion to illite with k feldspar being a local potential source of k and 2 dissolution precipitation processes produced in sandstones by the action of water soluble organic species derived from adjacent shales assuming firstly that the chemical reactions take place in closed systems the nature and timing of diagenetic reactions in marine sandstone shale formations were modelled for a 50 120 c temperature range in order to improve understanding of the factors that control the illitization reaction with k feldspar coexisting with aluminous clay illite is modelled here as a muscovite type mineral we tested the effects of an energy barrier on illite growth by allowing or preventing the muscovite illite precipitation reaction to occur while using several illitization reaction rates we also compared the stable mineral parageneses predicted for organic material free systems with those predicted in the case of organic diagenesis release of co 2 ch 4 acetic and oxalic acids similarities and discrepancies between numerical results and natural mineral assemblages suggest that the illitization reaction depends on the nature of the reacting clays kaolinite conversion to end member illite involves high energy conditions 2 kcal mol 1 which are not met when the pore water equilibrates with the mineral matrix from undersaturated conditions in a closed system to overcome this barrier the fluid should be oversaturated with respect to k feldspar an external source of k or a ph increase in an open system is necessary for this reaction no particular effect of organic diagenesis on this reaction was found in the present study on the other hand smectite to illite conversion involves a lower energy barrier and can operate in closed systems where k feldspars are the source of k the maturation of organic matter may speed up the smectite to illite conversion rate by increasing the gibbs free energy of illite growth interactions between sandstone and shale were examined by modelling the transfer of aqueous species of organic origin from shale to sandstone fluid expulsion from shale had little or no effect on diagenesis in adjacent sandstones during each flushing cycle mainly because organic protons and ligands were neutralized in the source rock however the diffusion of shale derived cations through sandstone as organometallic complexes mg ca fe al appears to be an efficient process during carbonate cementation in sandstones where oxidizing conditions enhance the decomposition of such complexes

sedimentological and diagenetic control on the reservoir quality of deep lacustrine sedimentary gravity flow sand reservoirs of the upper triassic yanchang formation in southern ordos basin china abstract the upper triassic deep lacustrine sedimentary gravity flow sandstones yanchang formation southern ordos basin form an important oil reservoir however it is considered as a low permeability to tight reservoir this study combines petrographic mineralogical and geochemical data in order to better constrain the controls on reservoir quality two major lithofacies types lithofacies 1 sandy debrite lithofacies 2 turbidite were identified and their origin relating to deep lacustrine gravity flows have been addressed the gravity flow sandstones are significantly diagenetically altered including mechanical compaction authigenic clay formation carbonate and quartz cementation and dissolution in addition volcanic tuffaceous dust alteration is one of the important processes during burial which provided silica for quartz cementation and that easily transformed to authigenic smectite it also formed poorly crystallised kaolinite and random mixed layer i s clays the relatively negative 13c with an average of 2 3 and 18o values with an average of 17 0 of carbonate cements in the gravity flow sandstones reflect decarboxylation of organic matter as one of the important carbonate sources the factors controlling reservoir quality relate to depositional and diagenetic processes depositional factors e g grain size grain sorting had a stronger influence on lithofacies 2 sandstones compared to lithofacies 1 which made lithofacies 2 sandstones more easily compact and lose most of its primary porosity samples with less intergranular volume igv indicate that mechanical compaction plays an important role in reducing reservoir quality however cementation clay minerals and carbonate cementation affected sandstones of lithofacies 1 to a stronger degree heavily destroying reservoir quality while in this lithofacies dissolution e g feldspar volcanic fragments and minor quartz significantly improved reservoir characteristics in contrast to lithofacies 2 sandstones quartz cements could stabilize the framework and preserve the open porosity however they are present with relatively low contents and have limited effect in influencing the reservoir quality in summary lithofacies 1 sandstones possess a slightly better reservoir potential in the study area all these factors have great implications for tight oil exploration and production of the yanchang formation in the southern ordos basin which can serve as an example for other tight sandstone gravity flow reservoirs elsewhere in the world

mass transfer and problems of secondary porosity creation in deeply buried hydrocarbon reservoirs abstract secondary porosity created by the post depositional leaching of detrital grains and cements can be of great economic importance the term secondary porosity is only descriptive and does not give any information on whether the porosity of the rock has been enhanced in reality there are two end member volumetric outcomes to the leaching process firstly the mineral can be completely dissolved and its components transported out of the system in such an instance the porosity of the rock will be enhanced secondly it is possible that a mineral will decompose and by product minerals will be precipitated which will occupy approximately the same volume as the original mineral the latter case is more likely where the pore waters are close to equilibrium with the majority of the minerals of the rock this will occur at elevated temperatures and or where the pore waters have been in contact with the rock for a long time these conditions are commonly met during intermediate to deep burial many metastable minerals such as high temperature feldspars probably decompose in this way a consequence of the decomposition of such metastable minerals is that primary porosity will be swapped for secondary and that older and more deeply buried rocks will contain a higher proportion of secondary porosity the mass transport equation may be used to examine the physico chemical environment where enhanced porosities most likely occur during leaching by using a simple non equilibrium mass transport model based on the silica system it is possible to demonstrate the relative importance of flow rate reaction rates specific surfaces and temperature in controlling the areal extent of leaching wide leaching zones are related to high flow rates which in turn promote a high throughput of undersaturated fluid and dispersion leaching over a wide area is also favoured by low reaction rates which will occur at low temperatures although it is true to say that less of a given silicate mineral can be dissolved by an undersaturated pore water at low temperature than at high temperature it is probable that pore fluids from deeply buried sediments are more likely to be close to equilibrium with a given mineral than for instance meteoric water thus the most likely environment for the production of widespread enhanced porosity is in the shallow subsurface the lower flow rates and higher temperatures that are encountered during deep burial diagenesis will result in far narrower more intense leaching zones as a result providing sufficient reactive mineral is present any undersaturated pore water will approach equilibrium in a short distance in order to transport undersaturated pore waters appreciable distances in the deep subsurface it is necessary to minimize their interaction with the host rock this could occur if the pore fluid were to move through a fracture system where it would interact with a far smaller surface area of reactive mineral than if it moved through the pore network

grain rimming kaolinite in permian rotliegend reservoir rocks upper rotliegend sediments of permian age from the northeast netherlands show moderate to good reservoir

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

link between aluminum mobility and destruction of secondary porosity geologic notes acid pore waters generated within organic rich muds destabilize carbonates and aluminosilicates and take metals including aluminum into solution sediment compaction forces some of these modified pore waters into sandstones once isolated from the principal source of acidity kerogen maturation the ph of these solutions must rise as further mineral dissolution occurs the inevitable consequence is precipitation of kaolinite even though solutions may still be acid this explains why kaolinite is commonly the first precipitate to reduce secondary pore space in sandstones and can form even as other minerals are being dissolved away

maastrichtian oil shale deposition on the southern tethys margin egypt insights into greenhouse climate and paleoceanography abstract a comprehensive set of organic and inorganic geochemical proxies clay mineralogy and molecular fossils are presented from two biostratigraphically well dated oil shale horizons of egypt within the upper cretaceous duwi and dakhla formations the studied oil shales were deposited within intracratonic sedimentary basins in a broad northern african epeiric sea calcareous nannofossil biostratigraphy indicates that the oil shales range from early to late maastrichtian in age covering nannofossil zones uc18 uc19 and uc20 the oil shales contain smectite and kaolinite as the main clay minerals that formed by weathering of precursor basement rocks the relative abundances of clay minerals and several geochemical proxies e g c value cia ga rb rb sr and sr cu along with the occurrence of warm water nannofossil taxa demonstrate that the maastrichtian oil shales accumulated under a warm arid to humid greenhouse climate during a general maastrichtian cooling trend this episode of oil shale deposition in egypt coincided with the first potential imprint of the global warming that has been recorded during early to late maastrichtian times 70 6 67 7 ma elevated eolian terrigenous input was recorded within the duwi formation oil shales shown by high ti al si al zr al and low la lu ratios the duwi formation environment was characterized by enhanced salinity and a stratified water column compared to that of the dakhla formation shown by elevated values of the gammacerane index tetracyclic terpane carotene and sr ba ratio the caco3 content and carbonate siliciclastic ratio indicate that the dakhla formation oil shales were deposited during a relative sea level rise in a deeper marine setting than the duwi formation oil shales the data further imply a wind driven upwelling scenario of nutrient input s that simulated primary production and increased organic matter fluxes thus paleoclimate and paleoceanography had a considerable impact on organic matter enrichment

geochemistry of coal mine drainage groundwater and brines from the ibbenburen mine germany a coupled elemental isotopic approach abstract a coupled elemental isotopic approach is applied to reconstruct the origin and chemical evolution of mine drainage groundwater and brines from the carboniferous anthracite coal mine in ibbenburen germany all solutions are characterized by an increase in salinity with depth as well as by an increase in 34s 32s isotopic ratios of dissolved so42 br cl and na cl ratios in deep na cl type water indicate halite dissolution as the common source of salinity 34sso4 values increase up to 21 1 vcdt linking the salinity to the migration of groundwater from the surrounding mesozoic sediments 87sr 86sr ratios between 0 7108 and 0 7135 and elevated alkali concentrations indicate ongoing water rock interaction of the evaporite derived brines with the carboniferous siliciclastic rocks of the mine a positive correlation of 87sr 86sr ratios with 2hh2o and 18oh2o values suggests mixing of the brines with isotopically heavy formation water within the carboniferous bedrock the oxidation of pyrite is the dominant sulfate source in shallow mine drainage and groundwater with a relatively low ionic strength i

reservoir quality controls on rotliegend fluvio aeolian wells in germany and the netherlands southern permian basin impact of grain coatings and cements abstract reservoir quality in sandstones can be a function of temperature pressure and chemical alterations over time some intrinsic properties of sandstones can additionally impact the evolution of reservoir quality the detrital composition depositional environment and properties controlled thereby dictate some of the diagenetic changes extrinsic processes like external fluids e g from hydrothermal events or dewatering of clay minerals can additionally result in changes of reservoir quality one of these properties are grain coatings on quartz grains which can strongly affect pore cementation by quartz overgrowths to predict and infer the quality of undrilled reservoirs constraints are needed to predict their occurrence we analyse two reservoir settings in the southern permian basin and delineate the impact of depositional systems lithologies detrital and authigenic composition on reservoir quality and with regard to grain coatings results highlight the importance of diagenetic alterations and the source area in regards to burial development in different parts of the southern permian basin samples from the rotliegend bahnsen sandstone member from germany and the slochteren sandstone formation from the netherlands have been petrographically and petrophysically analysed and linked to depositional environments our data shows that the emplacement and presence of grain coating minerals cannot be simply linked to the depositional environment or processes controlled thereby e g grain size and sorting neither on a reservoir nor on a basin scale the dissolution of alumosilicates additionally has no consistent impact on the authigenesis of chloritic grain coatings on the reservoir or basin scale in this study this implies the necessity of sample specific analyses and importance of the assessment of diagenetic alterations when analysing modelling or predicting reservoir quality in similar settings worldwide

volatile organic acids produced during kerogen maturation amounts composition and role in migration of oil abstract maturation dependent generation of organic compounds from north sea source rocks has been investigated by hydrous pyrolysis of powdered source rock samples the generation of water soluble organic acids is emphasised and their composition compared with oil field water samples similar compositions are found matrix adsorption effects are not found to be quantitatively important timing of acid generation relative to oil generation is discussed effects of realistic concentrations of a volatile organic acid in the water phase on interfacial tension is determined and effects on fluid flow of crude oil through water saturated sandstone is investigated using a long core flow rig

potassium distribution and metasomatism in pelites and schists how and when relation to postdepositional events a large global database for over 9000 archean to holocene pelites in numerous basins has been developed to evaluate diagenetically unaltered and altered compositions the mineralogy and geochemistry of quaternary muds and ancient mudstones provide first order controls on diagenetic potassium metasomatism in pelites clay mineralogy of quaternary muds is dominated by smectite and kaolinite with minor detrital mica and illite thus k2o is 4 k al is 0 25 and k rb is 220 in most quaternary muds with diagenetic alteration of clays to illite smectite and illite k2o content increases to 4 8 and k al rises to 0 2 0 5 or more in extreme potassium metasomatism authigenic k feldspar develops sources of potassium are internal to shale sandstone units wherein detrital k feldspar is dissolved during diagenesis providing k for clay illitization in closed system isochemical alteration as in some gulf coast and north sea basin cases potassium in amounts greater than that available from closed system alteration is provided from extraformational circulating brines in open system diagenesis k rb with diagenesis varies according to whether the metasomatizing fluid was relatively poor in rb seawater derived brines or richer in rb as in many hydrothermal waters diagenetic fluids may originate from seawater or meteoric water enriched in cations by water rock interactions along their subsurface migration paths and or by dissolution of evaporites potassium metasomatizing fluids penetrate low permeability pelites through faults fractures and microfractures and by diffusion in the mineral matrix typically k metasomatism possibly with pb zn cu ba and f mineralization occurs long after sediment deposition and burial at temperatures of 50 to 150 c related to tectonic activity and fluid movement different modes of metasomatism are recognized and correlated with geological setting in the passive margin gulf coast tertiary basin open system k metasomatism where present is limited to areas adjacent to steeply dipping growth faults which act as conduits for episodic pulses of altering fluids migrated upward from deeper basin sources in contrast widespread pervasive open system k metasomatism produced by continuous long term gravity advective and diffusive flow of brine from the late paleozoic appalachian ouachita wichita mountains highland to altered earlier paleozoic shales plus sandstones bentonites and carbonates in the illinois and anadarko ardmore foreland basins this mode of k alteration following major mountain building events is characteristic of many precambrian and paleozoic collision orogens and associated foreland basins another mode is in rift basins with no external drainage in which indigenous waters are evaporated to brines these brines cause albitization of k feldspar to yield potassium for illitization of detrital clays

diagenesis and reservoir quality evolution of incised valley sandstones evidence from the abu madi gas reservoirs upper miocene the nile delta basin egypt abstract elucidation of the distribution of diagenetic alterations in incised valley sandstones within a sequence stratigraphic framework allows a better understanding of their reservoir quality evolution during burial gas reservoirs of the upper miocene abu madi formation present day depth 3350 m temperature 116 the nile delta basin egypt consist of lowstand systems tract lst fluvial and transgressive systems tract tst estuarine sandstones deposited in an incised valley these sandstones have a wide range of porosity 2 to 29 and permeability 0 01 to 6071 md which reflect both depositional facies and diagenetic controls diagenetic events that influenced the reservoir quality evolution include mainly the formation of extensive grain rimming fe mg chlorite mechanical compaction of ductile grains pressure dissolution of quartz and quartz cementation the chlorite rims distributions of which are not clearly related to sequence stratigraphy have preserved reservoir quality by inhibiting quartz cementation but have increased microporosity values in the reservoirs calcite cementation 18o 13 5 to 6 0 13c 14 3 to 1 0 and 87sr 86sr 0 707124 to 0 708181 has not influenced the overall reservoir quality evolution of the sandstones because of its localized occurrence other diagenetic events that have little influence on reservoir quality include the more frequent formation of kaolin in the lst fluvial sandstones and formation of pyrite in the tst estuarine sandstones this study demonstrates the possibility of constraining and evaluating the impact of diagenetic alterations on reservoir quality evolution in incised valley deposits and their sequence stratigraphic distribution and thus has an important impact on hydrocarbon exploration in such settings

constraints on the development of secondary porosity in the subsurface re evaluation of processes abstract secondary porosity formed by the dissolution of grains and cements is of great economic importance in both clastic and carbonate reservoirs a review of the commonly proposed mechanisms for the production of secondary porosity indicates that meteoric water penetration and mixing corrosion may be able to create significant volumes of secondary porosity in the shallow subsurface leaching by acidic fluids generated by solution of co 2 or monocarboxylic acids both derived during the thermal maturation of organic matter or by acidic fluids created by clay mineral reactions in shales are the most commonly proposed mechanisms by which secondary porosity is said to form in the deep subsurface however these mechanisms suffer from great difficulties in producing significant volumes of secondary porosity once mass balance and fluid migration problems are taken into account it is highly likely that the correct mechanism for the creation of economically significant amounts of secondary porosity in the deep subsurface is yet to be found

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

potassium leached zones at the contacts of the garn formation mid norwegian continental shelf evidence for the role of shale in sandstone diagenesis abstract

time reverse location of seismic long period events recorded on mt etna we present the first application of a time reverse location method in a volcanic setting for a family of long period lp events recorded on mt etna results are compared with locations determined using a full moment tensor grid search inversion and cross correlation method from 2008 june 18 to july 3 50 broad band seismic stations were deployed on mt etna italy in close proximity to the summit two families of lp events were detected with dominant spectral peaks around 0 9 hz the large number of stations close to the summit allowed us to locate all events in both families using a time reversal location method the method involves taking the seismic signal reversing it in time and using it as a seismic source in a numerical seismic wave simulator where the reversed signals propagate through the numerical model interfere constructively and destructively and focus on the original source location the source location is the computational cell with the largest displacement magnitude at the time of maximum energy current density inside the grid before we located the two lp families we first applied the method to two synthetic data sets and found a good fit between the time reverse location and true synthetic location for a known velocity model the time reverse location results of the two families show a shallow seismic region close to the summit in agreement with the locations using a moment tensor full waveform inversion method and a cross correlation location method

siderite mineralization of the gemericum superunit western carpathians slovakia review and a revised genetic model abstract the gemericum is a segment of the variscan orogen subsequently deformed by the alpine carpathian orogeny the unit contains abundant siderite sulphide and quartz antimony veins together with stratabound siderite replacement deposits in limestones and stratiform sulphide mineralization in volcano sedimentary sequences the siderite sulphide veins and siderite replacement deposits of the gemericum represent one of the largest accumulations of siderite in the world with about 160 million tonnes of mineable feco 3 more than 1200 steeply dipping hydrothermal veins are arranged in a regional tectonic and compositional pattern reflecting the distribution of regional metamorphic zones siderite sulphide veins are typically contained in low grade chlorite zone sedimentary volcano sedimentary or volcanic lower and upper paleozoic rocks quartz antimony veins are hosted by higher grade units biotite zone siderite sulphide veins are dominated by early siderite followed by a complex set of stages including quartz sulphide chalcopyrite tetrahedrite barite tourmaline quartz and sulphide remobilization stages the temporal evolution of these stages is difficult to study because of the widespread and repeated tectonic processes within vein replacement and recrystallization siderite sulphide veins show considerable vertical up to 1200 m and lateral up to 15 km extent and a thickness typically reaching several metres carbonate replacement siderite deposits of the gemericum are hosted by a silurian limestone belt and are similar to stratabound siderite deposits of the eastern alps e g erzberg austria based on a review of geological petrological and geochronological data for the gemericum and extensive stable and radiogenic isotope data and fluid inclusion data on hydrothermal minerals the siderite sulphide veins and siderite replacement deposits are classified as metamorphogenic in a broad sense the deposits were formed during several stages of regional crustal scale fluid flow isotope s c sr pb fingerprinting identifies the metamorphosed rock complexes of the gemericum as a source of most components of hydrothermal fluids fluid inclusion and stable isotope data evidence the participation of several contrasting fluid types and the existence of contrasting p t conditions during vein evolution a high 1 8 o medium to high salinity h 2 o type fluid is the most important component during siderite deposition whereas h 2 o co 2 type fluid inclusion containing dense liquid co 2 and corresponding to minimal pressures between 1 and 3 kbar were found in a younger tourmaline quartz stage younger quartz ankerite siderite sulphide stages are characterized by high salinity 17 to 35 wt nacl equivalent and low temperature t h 90 to 180 c h 2 o type fluids the vein deposits are interpreted as a result of multistage hydrothermal circulation with variscan and alpine mineralization phases based on available indirect data the most important mineralization phase was related to regional fluid flow during the uplift of a variscan metamorphic core complex producing siderite sulphide barite mineralization while tourmaline quartz stage and sulphide remobilization stages are related to alpine processes two phases of vein evolution are evident from two groups of 87 sr 86 sr isotope ratios of sr rich rb poor hydrothermal minerals 0 71042 0 71541 in older barite and 0 7190 0 7220 in late stage celestine and strontianite

alteration mineralogy lithochemistry and stable isotope geochemistry of the murgul artvin ne turkey volcanic hosted massive sulfide deposit implications for the alteration age and ore forming fluids abstract the murgul artvin ne turkey massive sulfide deposit is hosted dominantly by late cretaceous calc alkaline to transitional felsic volcanics the footwall rocks are represented by dacitic flows and pyroclastics whereas the hanging wall rocks consist of epiclastic rocks chemical exhalative rocks gypsum bearing vitric tuff purple vitric tuff and dacitic flows multi element variation diagrams of the hanging wall and footwall rocks exhibit similar patterns with considerable enrichment in k rb and ba and depletion in nb sr ti and p the chondrite normalized rare earth element rees patterns of all the rocks are characterized by pronounced positive negative eu anomalies as a result of different degrees of hydrothermal alteration and the semi protected effects of plagioclase fractionation mineralogical results suggest illite illite smectite chlorite kaolinite and chlorite in the footwall rocks and illite smectite kaolinite and chlorite illite in the hanging wall rocks overall the alteration pattern is represented by silica sericite chlorite and chlorite carbonate epidote sericite and quartz albite zones increments of ishikawa alteration indexes resulting from gains in k 2 o and losses in na 2 o and the chlorite carbonate pyrite index towards to the center of the stringer zone indicate the inner parts of the alteration zones calculations of the changes in the chemical mass imply a general volume increase in the footwall rocks abnormal volume increases are explained by silica and iron enrichments and a total depletion of alkalis in silica zone relative k increments are linked to the sericitization of plagioclase and glass shards and the formation of illite smectite in the sericite zone in addition fe enrichment is always met by pyrite formation accompanied by quartz and chlorite illite is favored over chlorite smectite and kaolinite in the central part of the ore body due to the increase in the al k na ca ratio although the rees were enriched in the silicification zone light rees show depletion in the silicification zone and enrichment in the other zones in contrast to the heavy rees behavior hydrothermal alteration within the hanging wall rocks apart from the gypsum bearing vitric tuffs is primarily controlled by chloritization with proportional fe and mg enrichments and sericitization the 18 o and d values of clay minerals systematically change with increasing formation temperature from 6 6 to 8 7 and 42 to 50 for illites and 8 6 and 52 for chlorite respectively the o and h stable isotopic data imply that hydrothermal alteration processes occurred at 253 332 c for illites and 136 c for chlorite with a temperature decrease outward from the center of the deposit the positive 34 s values 20 3 to 20 4 for gypsum suggest contributions from seawater sulfate reduced by fe oxide hydroxide phases within altered volcanic units thus the hydrothermal alteration possibly formed via a dissolution precipitation mechanism that operated under acidic conditions the k ar dating 73 62 ma of the illites indicates an illitization process from the maastrichtian to early danian period

asphaltene rich residues in sandstone reservoirs of haltenbanken province mid norwegian continental shelf abstract thin zones of asphaltene rich residue are present near the crests of two adjacent oil and gas filled structures in one structure the smorbukk sor field multiple thin 5 50 cm dark coloured zones are observed within the lower 20 m of the 88 m thick garn formation these dark coloured zones tend to occur immediately above stylolitized mica clay laminations and become gradually lighter upwards apparently reflecting gradual upward decrease in residue abundance in the other structure the smorbukk field the lowermost 1 m of the formation is dark coloured and the sandstone gradually becomes lighter in colour upwards for a distance of about 3 m similar but more diffuse residues are also present throughout the oil zone in two other fields nearby organic geochemical analyses show that the residue rich dark coloured zones differ from the sorrounding lighter coloured sandstone mainly in having a higher content of asphaltenes whereas the total content of extractable organic matter is often similar thus the darker rocks might be regarded more as zones of conspicuous oil staining due to asphaltene deposition rather than as small tar mats the residues typically appear to have accumulated under the influence of gravity at points where the downward flow was hindered by a lithological discontinuity present day zonation upward decreasing gas to oil ratio in the oil column of the smorbukk sor reservoir is consistent with a model of asphaltene precipitation in the lower part of the reservoir in response to upward leakage of gas from an underlying gas condensate reservoir in the ile formation this is also consistent with the available organic geochemical data in thin section the asphaltene rich material is seen to preferentially coat feldspar and clay but not quartz surfaces thus illustrating why sandstone wettability should be expected to depend on both mineralogy and the asphaltene content of the petroleum involved

kaolinized potassium leached zones at the contacts of the garn formation haltenbanken mid norwegian continental shelf abstract the middle jurassic garn formation consists mainly of subarkosic arenite in which potassium feldspar is several times as abundant as plagioclase in cores from eight wells the top of the formation upper 6 12 m appears to have undergone relatively intense alteration of potassium feldspar to kaolinite with the resulting loss of bulk postassium content this could be explained as a result of subaerial weathering of the top of the sandstone body except for the fact that in two of the wells the same pattern of alteration is observed at the base of the formation an alternative explanation is that alteration resulted from the introduction of acid released by the maturation of organic matter in adjacent shales in all instances the altered zones display a gradual increase in kaolinite content and a decrease in the ratio of bulk rock potassium aluminium over a distance of several metres approaching the formation boundary this pattern indicates that the active species responsible for the alteration entered vertically from the formation contacts rather than being introduced by lateral flow along the sandstone bed the cores studied are tentatively interpreted as recording different stages in the following depth related sequence of diagenetic events 1 early kaolinization throughout the formation by meteoric water flushing 2 formation of additional kaolinite near the formation contacts by the influx of acid from surrounding shales at approximately 2 7 km depth below the sea floor and 3 extensive alteration of kaolinite to illite at approximately 3 7 km depth illitization has been impeded in the leached zones near the formation contacts due to a local deficiency in reactive potassium here kaolinite survives unaltered whereas kaolinite throughout the rest of the formation has been extensively replaced by illite this implies that the illitization of sandstone can occur by short range diffusion in a chemically closed system solely in response to increasing thermal exposure at greater depth

low porosity haloes at stylolites in the feldspathic upper jurassic ula sandstone norwegian north sea an integrated petrographic and chemical mass balance approach abstract although stylolites in quartz rich sandstones have been described and modeled more feldspathic sandstones have not been similarly investigated the upper jurassic ula sandstone norwegian north sea a subarkose with 15 to 25 feldspar displays stylolites with prominent low porosity haloes up to 10 cm wide detailed mineralogical and chemical profiles across several stylolites and haloes are used to investigate the role of feldspar in mineral reactions at stylolites and to constrain the transport of chemical components quartz and feldspar are intensely dissolved at the margins of stylolites the surrounding haloes are characterized by dissolution of quartz and feldspar intense compaction anomalously high feldspar content high concentrations of clay and oxide cement volumes that increase away from the stylolite chemical mass balance calculations demonstrate that silica and elements associated with feldspar al2o3 na2o k2o were exported from the haloes grain size data and evidence for aluminum loss from the halo indicate that primary concentrations of clay and mica were not higher near the stylolites and that the petrographic and geochemical patterns reflect the diagenetic concentration of clay the analysis suggests that k feldspar dissolution at and near the stylolite resulted in precipitation of authigenic illite near the site of k feldspar dissolution due to the low solubility of al3 this in turn catalyzed chemical dissolution of quartz in the low porosity halo transport of cao mgo and mno into the haloes suggests that the haloes were zones of higher ph the increased ph perhaps due to the presence of clays would have also increased quartz solubility consistent with evidence for silica export from the halo the dissolution of k feldspar in the absence of kaolinite would also have raised ph around the site of k feldspar dissolution and illite precipitation favoring carbonate precipitation this interpretation of stylolite diagenesis differs from conventional models based on quartz rich sandstones in which quartz cement volumes decrease away from stylolites models for compaction in arkosic sandstones need to account for the enhanced dissolution of quartz that is driven by k feldspar dissolution and for catalysis by the resulting illite

origin of the fluids associated with granodiorite hosted sb as au w mineralisation at dubrava nizke tatry mts western carpathians mineral parageneses of the polymetallic sbrich deposit at dubrava has been formed during five separated stages a fluid inclusion study demonstrates that the earliest stages with scheelite molybdenite and arsenopyrite have been associated with immiscible co2 ch4 rich low saline fluids at temperatures between 300 and 400 c and pressures as much as 2 kbar deposition of the main superimposed ores stibnite and zinckenite has been intimately connected with circulation of aqeuous moderately saline fluids 15 5 23 5 wt nacl equiv upon epithermal conditions salinity of the aqueous fluids associated with tetrahedrite is clustered around 10 wt nacl equiv quartz from the latest barite stage has precipitated from aqueous fluids enriched in divalent cations these fluids are believed to be genetically linked with triassic evaporite formations preserved in the region temperature salinity diagrams constructed from microthermometry data indicate influx of diluted meteorite water in the stibnite tetrahedrite and barite stages isotopic data are in accordance with model the 18o values between 9 3 and 1 5 have been derived for water in equilibrium with quartz coexisting with sphalerite tetrahedrite and barite thus confirming the participation of isotopically lighter meteoric waters in the mineral forming solutions the 18o values between 3 3 and 8 5 estimated for the water associated with the scheelite and arsenopyrite stages are suggestive for the majority of metamorphic and or magmatic water in the mineral forming co2 rich solutions

evolution of uranogenic and thorogenic lead 1 a dynamic model of continuous isotopic evolution a dynamic model of the continuous isotopic evolution of uranogenic and thorogenic lead based on the minimum possible number of assumptions is proposed here the model is based on the simple assumption that the rate of change of the ratios 238 u 204 pb and w 232 th 204 pb present day values increased initially and after attaining a maximum started decreasing because of surface cooling of the earth the time t m when the and w rates of change attained their maximums is a model parameter depending on the age of formation of the different layers of the crust or in mixing processes on the participation of materials from the older or younger crust and mantle mathematically this is suitably described by a two parameter function which is the product of a linear and an exponential function the deduced equations of the model are calibrated according to the isotopic data on ore lead from the isotopic ratios of a lead sample it is possible to determine the two model ages uranogenic and thorogenic and the model parameter t m or another parameter t related to t m the values of the model parameters t m or t indicate the time of the enrichment ancient or younger in u pb which is an extremely important index for identifying the pb source type the use of these model parameters provide the possibility of a better theoretical description of the pb isotopic characteristics in the different types of sources

model for k bentonite formation evidence from zoned k bentonites in the disturbed belt montana thirty four k bentonites from the marias river formation upper cretaceous in the montana disturbed belt have been studied in detail a 2 5 m thick k bentonite is zoned mineralogically and chemically the upper and lower contacts of the bed being more illitic and k rich than the middle of the bentonite k ar dates of illite smectite separated from the bed are also zoned yielding older ages at the contacts than at the center a model for the formation of k bentonites enclosed by k rich shale is presented in which k bentonites form from smectite bentonites during a thermal event t 100 200 c potassium is derived from mineralogic breakdown probably mica and k feldspar in the shale and migrates by diffusion into the bentonite

interactions between organic acids anions in formation waters and reservoir mineral phases abstract formation waters from the norwegian continental shelf have been analysed for short chain organic acid anions acetic acid is the major component and the observed levels are in the range 0 3 20 mm in uncontaminated formation water samples studies of individual reservoirs and formations over time and area show a quite uniform distribution of the acids within each unit for large volumes of fluid the organic acid anion levels are generally low in reservoirs containing biodegraded oil statistical analysis has been used to find correlations between the organic and inorganic ion compositions and the reservoir conditions and shows a dominating negative correlation between the organic acid anion levels and the levels of inorganic ions indicating that the main effect of the organic ions is to increase the stability of mineral phases laboratory experiments have also been performed and show similar effects for sandstone samples under oxidizing conditions the buffering capacity of the organic acid species is also important as it prevents the development of extreme ph conditions caused by pyrite oxidation in carbonate systems few systematic effects were observed the results both for the natural systems and the laboratory experiments show that the presence of organic acids in the formation waters does not contribute to the development of secondary porosity in the reservoir rock the observed relationships rather indicate decreasing concentration levels of inorganic ions in the formation waters with increasing levels of organic anions and thus an increase in mineral stability

diagenetic heterogeneity pore throats characteristic and their effects on reservoir quality of the upper triassic tight sandstones of yanchang formation in ordos basin china abstract upper triassic tight sandstones in ordos basin are typical tight oil reservoirs in china using a combination of thin sections sem bse eds and mercury injection 48 sandstone core samples from 17 wells in the study area were collected to investigate the diagenetic heterogeneity pore throat characteristics and their correlations with reservoir quality five diagenetic types were identified according to their specific diagenetic evolution pathways type a sandstones are characterized by abundant authigenic chlorite rims rare quartz cement and high porosity type b sandstones underwent moderate compaction moderate dissolution and moderate cementation samples from type c are characterized by intense dissolution and large amounts of carbonate cements they can be subdivided into two categories according to if pore throats in larger size were still reserved type c2 or not type c1 after late stage cementation type d sandstones underwent intense compaction moderate dissolution and minor carbonate cementation type e sandstones are characterized by the most intense compaction depending on the pore throat size distributions from micp analysis three sandstone types with their own specific pore throat characteristics were identified type sandstones generally contain a certain amount of mesopores including a few macropores and pore spaces are dominant intergranular pores intragranular pores and even oversized pores these sandstones generally correspond to type a b c2 and d sandstones and thus represent good quality reservoir type sandstones usually contain dominant transitional pores 77 in average and their pore systems mianly consist of clay intercrystal microporosity and a few isolated intergranular or intragranular pores type sandstones are representative by relatively abundant micropores 17 7 in average and pore spaces are dominant clay intercrystal microporosity type and type pore structures generally correspond to type c1 and e and thus represent poor quality reservoir this study gives insights into the link between different diagenetic features pore structures and their impact on reservoir quality and this will be helpful for reservoir prediction

ancient subaerial exposure and freshwater leaching in sandstones the authors present an example of leaching by meteoric water in the jurassic magnus sandstone member of the northern north sea the interpretation is based on information from several independent techniques including downhole natural gamma ray spectrometry detailed petrographic analysis and cross sectional time slice reconstructions from seismic data the initial lines of evidence that prompted further investigation into freshwater involvement in the magnus sandstone member were brackish formation water compositions and a potassium depletion shown by the gamma ray spectrometry log in sandstone beneath the unconformity at the base of the cretaceous section subsequent detailed petrographic study confirmed that the potassium depletion had been brought about by potassium feldspar dissolution resulting in precipitation of kaolinite and creation of secondary porosity the latter also revealed by conventional core analysis porosity and permeability measurements finally cross sectional time slice reconstructions from a regional seismic line across the magnus oilfield strongly suggested subaerial exposure of the magnus sandstone member in early cretaceous time and consequent freshwater leaching of the magnus sandstone member and generation of the unconformity at the base of the cretaceous the combination of several lines of evidence from the magnus field supports the process of subaerial exposure and freshwater leaching to create secondary porosity in sandstones

cretaceous evolution of a metamorphic core complex the veporic unit western carpathians slovakia p t conditions and in situ40ar 39ar uv laser probe dating of metapelites alpine metamorphism related to the development of a metamorphic core complex during cretaceous orogenic events has been recognized in the veporic unit western carpathians slovakia three metamorphic zones have been distinguished in the metapelites 1 chloritoid chlorite garnet 2 garnet staurolite chlorite 3 staurolite biotite kyanite the isograds separating the metamorphic zones have been modelled by discontinuous reactions in the system k2o feo mgo al2o3 sio2 h2o kfmash the isograds are roughly parallel to the north east dipping foliation related to extensional updoming along low angle normal faults thermobarometric data document increasing p t conditions from c 500 c and 7 8 kbar to c 620 c and 9 10 kbar reflecting a coherent metamorphic field gradient from greenschist to middle amphibolite facies 40ar 39ar data obtained by high spatial resolution in situ ultraviolet uv laser ablation of white micas from the rock slabs constrain the timing of cooling and exhumation in the late cretaceous mean dates are between 77 and 72 ma however individual white mica grains record a range of apparent 40ar 39ar ages indicating that cooling below the blocking temperature for argon diffusion was not instantaneous the reconstructed metamorphic p t t path is clockwise reflecting post burial decompression and cooling during a single alpine orogenic cycle the presented data suggest that the veporic unit evolved as a metamorphic core complex during the cretaceous growth of the western carpathian orogenic wedge metamorphism was related to collisional crustal shortening and stacking following closure of the meliata ocean exhumation was accomplished by synorogenic orogen parallel extension and unroofing in an overall compressive regime

nature of secondary porosity created by dissolution of aluminum silicates abstract porosity enhancement in sandstone reservoirs cannot be due to dissolution of aluminum silicates without implying aluminum mobility however aluminum has yet to be reported in reservoir fluids at greater than trace concentrations predicted aqueous aluminum concentrations in subsurface fluids approach a level that curtis has termed a good approximation of zero several studies of secondary pore systems formed by aluminum silicate dissolution demonstrate that aluminum is conserved on a scale of a few inches in the form of diagenetic clay minerals notably kaolinite our examinations indicate that the products of dissolution of aluminum silicate grains which include feldspar and many rock fragments can be accounted for in thin section primarily by the presence of adjacent pore filling kaolinite and possibly authigenic quartz and albite at high temperatures the diagenetic mineral is often illite secondary porosity formed by dissolution of aluminum silicate grains should not necessarily be construed as adding to total porosity but should rather be thought of as a rearrangement of porosity already present indeed the process may actually be harmful inasmuch as the newly formed clay minerals can greatly reduce permeability calcite resorption remains the principal means of adding to total porosity through a dissolution process end of article last page 311

distribution of diagenetic alterations in glaciogenic sandstones within a depositional facies and sequence stratigraphic framework evidence from the upper ordovician of the murzuq basin sw libya abstract the spatial and temporal distribution of diagenetic alterations has been constrained in relationship to depositional facies and sequence stratigraphy of the upper ordovician glaciogenic quartzarenite sandstones in the murzuq basin sw libya which were deposited during the haritanian glaciation when the basin was laying along the continental margin of gondwana eogenetic alterations encountered include i replacement of detrital silicates mud matrix and pseudomatrix by kaolinite in paraglacial tide dominated deltaic in foreshore to shoreface highstand systems tract hst and in post glacial gilbert type deltaic lowstand systems tract lst sandstones particularly below the sequence boundaries sb kaolinite formation is attributed to the influx of meteoric water during relative sea level fall and basinward shift of the shoreline ii cementation by calcite 18 o vpdb 3 1 to 1 1 and 13 c vpdb 1 7 to 3 5 and mg rich siderite in the paraglacial tide dominated deltaic and foreshore to shoreface hst sandstones in the glacial tide dominated estuarine transgressive systems tract tst sandstones and in the post glacial shoreface tst sandstones is interpreted to have occurred from marine pore waters iii cementation by mg poor siderite which occurs in the post glacial gilbert type deltaic lst sandstones and in the paraglacial tide dominated deltaic and foreshore to shoreface hst sandstones is interpreted to have occurred from meteoric waters during relative sea level fall and basinward shift of the shoreline iv pervasive cementation by iron oxides has occurred in the glacial shoreface offshore tst sandstones and post glacial shoreface tst sandstones immediately below the maximum flooding surfaces mfs which was presumably enhanced by prolonged residence time of the sediments under oxic diagenetic conditions at the seafloor v formation of grain coating infiltrated clays mainly in the glacial fluvial incised valley lst sandstones and in the post glacial gilbert type deltaic lst sandstones as well as less commonly in the paraglacial foreshore to shoreface hst sandstones and in the tide dominated deltaic hst sandstones below the sbs mesogenetic alterations include mainly the formation of abundant quartz overgrowths in the glacial fluvial incised valley lst sandstones post glacial gilbert type deltaic lst sandstones and glacial shoreface tst sandstones in which early carbonate cements are lacking illite chlorite and albitized feldspars which occur in small amounts are most common in the glacial tide dominated estuarine tst sandstones and paraglacial shoreface hst sandstones this study demonstrates that the spatial and temporal distribution of diagenetic alterations and their impact on reservoir quality evolution in glacial paraglacial and post glacial sandstones can be better elucidated when linked to the depositional facies and sequence stratigraphic framework

petroleum migration faults and overpressure part ii case history the haltenbanken petroleum province offshore norway abstract petroleum inclusion and geochemical data from core extracts were applied to deduce a model for oil migration overpressure development and palaeo leakage of oil from currently dry structures in the haltenbanken vest area the existence of fluorescent oil type inclusions in quartz in the smorbukk asgard 2 field suggest that oil migrated into this structure 70 50 million years before present ma bp this is also the case for the dry structures 6506 12 4 6506 11 3 and 6506 11 1 west of the main smorbukk fault zone black oil inclusions with medium gas oil ratio gor occur in these fields together with condensate type petroleum inclusions this suggests that the dry structures transformed from containing oil to condensate before leakage petroleum extracted from inclusions in these structures and in nearby fields have identical marine type ii kerogen signatures source rocks at the spekk formation level in the current drainage area of smorbukk and these dry structures were immature 70 50 ma bp and the smorbukk sor asgard 3 field did not fill at this early time thus oil must initially have entered into smorbukk from areas to the w sw through the currently pressure sealing smorbukk fault zone which today marks the westward limit of the smorbukk field diagenesis in this fault zone caused the much later overpressure development and petroleum was lost from the 6506 12 4 6506 11 3 and 6506 11 1 structures as overpressure built up regionally petroleum loss from these structures with their often thick seals must have occurred via self propagating open fracture induced mechanisms lack of petroleum in the cretaceous strata above these structures suggests that leakage occurred to even shallower strata this could imply that the cretaceous strata in halten vest were overpressured at the time of leakage in contrast the oil in the cretaceous lysing and lange formation above the jurassic reservoirs in smorbukk and smorbukk sor most likely originated based on geochemistry and gors from the jurassic reservoirs below and not from cretaceous strata this migration event would have been facilitated if it occurred before these sands became overpressured as they are today modelling suggests that the spekk formation became mature in the smorbukk sor region

measuring sandstone compaction from modal analyses of thin sections how to do it and what the results mean abstract calculation of compactional porosity loss from modal analyses of thin sections is a useful technique for characterizing the diagenesis and porosity evolution of sandstone reservoirs to obtain meaningful results however it is essential to assume an appropriate value for the original porosity and to use point counting categories that clearly distinguish between grains and intergranular materials because of the latter requirement it is commonly not appropriate to make calculations of compactional porosity loss from modal analyses that were not performed for the specific objective of measuring intergranular volume suites of modal analyses from deeply buried sandstone units typically show wide variations in the relative importance of compaction versus quartz cementation this heter geneity must reflect subtle variations in depositional sand characteristics in particular the distribution of detrital intergranular clays that tend to promote both mechanical grain rearrangement and stylolitic grain dissolution as well as locally inhibiting quartz cement growth despite wide variation in the mechanism of porosity loss total helium porosity tends to be relatively constant at individual well locations suggesting a close interdependence between compaction and quartz cementation with total porosity loss dependent mainly upon thermal exposure either maximum temperature or time temperature integral

kaolin coating of stylolites effect on quartz cementation and general implications for dissolution at mineral interfaces abstract the deeply buried 4 km 160 c quartz rich sandstones of the middle jurassic garn formation in well 6507 7 1 offshore mid norway typically contain 20 30 quartz cement and only 3 9 porosity the exception is a less than 1 5 m thick interval at the top of the formation where quartz cement volumes typically drop to 8 16 and porosities are in the range 7 19 the uppermost part of the garn formation is not isolated from the rest of the formation by any type of impermeable barrier the sandstones are water filled indicating that the change in cementation does not correspond to a hydrocarbon contact and there are no significant changes in grain size grain coating abundance grain mineralogy or other factors that could give a lower quartz surface area for development of quartz overgrowths within the uppermost 1 5 m of the sandstones however in the samples from the less cemented interval illitization of authigenic kaolin is either not detectable or very restricted compared to deeper down in the cores where illitization of kaolin is usually complete we propose that the cause of reduced quartz cementation in the top of the garn formation may be the coating of stylolites developed from primary illitic clay rich and micaceous laminae with authigenic kaolin as quartz cementation proceeds and the authigenic kaolin rich rock adjacent to the stylolites is dissolved although we do not have evidence indicating that this may be a common phenomenon of major importance for preservation of reservoir quality our observations may help to unravel the mechanisms of quartz dissolution at stylolites specifically we suggest that the lack of significant surface charge on kaolin compared to illite and mica may lead to a reduced thermodynamic drive for quartz dissolution at kaolin quartz contacts similarly lack of dissolution at clean quartz quartz surfaces may be due to smaller and equal surface charges setting up oppositely directed electric fields that cancel each other whereas the positive surface charge of carbonate minerals may actually enhance the electric fields from negatively charged illitic clay and mica which may explain the extensive dissolution often seen at stylolites in limestones

compaction model for quartzose sandstones application to the garn formation haltenbanken mid norwegian continental shelf abstract the compaction of quartzose sandstones is described by a macroscopical visco elasto plastic model derived from microscopical considerations this model considers that the reduction of total porosity the relative pore volume is related to the changes in the relative cemented volume and the relative intergrain volume furthermore we consider that the variations of these two petrophysical parameters result from two physical mechanisms mechanical compaction and closed system chemical compaction the model has been calibrated for the middle jurassic garn formation from the haltenbanken area of the mid norwegian continental shelf around 300 data points from 17 wells have been used in this study the viscous parameters have been optimised against the present day data with total porosity data the best fit is obtained for an activation energy between 14 and 16 kj mole and a macroscopical viscosity at 15 c between 39 and 46 gpa ma 1 210 24 1 410 24 pa s if quartz cement data are used the best fit is obtained for an activation energy between 16 and 18 kj mole and a macroscopical viscosity at 15 c between 47 and 56 gpa ma 1 510 24 17 10 24 pa s the visco elasto plastic compaction model has been tested in the haltenbanken area of mid norway with the result that observed porosity trends and pore pressures can be modelled using multiple 2 d basin modelling simulations the results of this model and its implications for fluid flow and petroleum systems analysis can be seen the compaction method has significant impact on the simulated fluid flow and petroleum migration pattern

a model for diagenetic mass transfer between adjacent sandstone and shale abstract there are numerous suggestions that mass transfer between sandstone and shale is an important factor in the diagenesis of sandstone reservoirs however advective transport models for mass transport have not produced results that support these proposals a model is developed that overcomes the problems of advective transport models by assuming that diffusive mass transfer is the principle transport mechanism the process occurs when the rate of dissolution of reactive detrital components exceeds the local rate of diffusive transport at that point in the burial history units with greater amounts of a reactive component will generate more solute per unit time than units with less reactive component the process forms chemical gradients that can rapidly transfer mass equivalent to several vol of rock over distances of at least 5 m the observed transport of potassium and silica between adjacent sandstone and shale in the north sea during late stage illitization was correctly predicted using this conceptual approach the conceptual framework from the illite model can be extended to other diagenetic reactions and suggests that the burial of adjacent sandstone shale units could produce episodic diffusion gradients that transfer mass and form multiple generations of cement in sandstone reservoirs

uranium depletion across the permian triassic boundary in middle east carbonates signature of oceanic anoxia we present a geochemical profile through a 445 m 1459 9 ft section of shallow water carbonate platform strata in the upper part of the khuff formation the permian triassic boundary ptb is recognized in this section based on the immediately preceding negative shift in bulk rock carbonate carbon isotope composition equivalent to the end permian extinction horizon combined with biostratigraphic control from nearby wells these strata show an abrupt and long lasting decrease in bulk rock uranium u content coincident with the carbon isotope shift because of low siliciclastic content and the consequently low potassium and thorium of these carbonates the decrease in u is clearly reflected in the total gamma ray gr profile recorded by wire line logging published log curves show similar distinctive gr profiles throughout a large area of the middle east indicating that u depletion across the ptb is a regional characteristic this feature cannot be explained as diagenetic and is not related to the organic matter content of the host sediments but it is suggested to reflect the global depletion of u in earliest triassic seawater caused by the abrupt onset of deep ocean anoxia and the resulting increase in u precipitation in oxygen poor sediments this explanation carries the implication that similar u depletion should be characteristic of lowermost triassic carbonates from shallow water oxygenated settings worldwide analogous signatures of u depletion should also have developed in shallow water carbonates deposited contemporaneously with episodes of deep ocean anoxia during other periods of geological time these predictions can be tested by high accuracy u profiling of other well characterized carbonate successions potentially yielding a new approach for tracking the degree of oceanic circulation throughout earth s history

influence of detrital composition on the diagenetic history of tight sandstones with implications for reservoir quality examples from the permian xiashihezi formation and carboniferous taiyuan formation daniudi gas field ordos basin china abstract tight gas reservoirs characterized by low porosity and low permeability are widely considered to be the product of post depositional diagenetic processes associated with progressive burial this study utilizes a combination of thin section petrography scanning electron microscopy microprobe and back scatter electron analysis stable isotope geochemistry and fluid inclusion analysis to compare the diagenetic history including porosity formation within sandstones of the second member of carboniferous taiyuan formation c3t2 and the first member of permian xiashihezi formation p1x1 in the ordos basin in central china in the p1x1 member relatively high abundances of metamorphic rock fragments coupled with a braided river and lacustrine delta environment of deposition produced more smectite for transformation to illite 50 120 c this reaction was driven by dissolution of unstable minerals k feldspar and rock fragments during the early to middle stages of mesodiagenesis and consumed all k feldspar abundant intragranular porosity average values of 2 8 and microporosity in kaolinite average values of 1 5 formed at these burial depths with chlorite and calcite developed as by products in the c3t2 member relatively low abundances of metamorphic rock fragments coupled with an incised valley coastal plain environment of deposition resulted in less smectite for transformation to illite high k h ratios in the early pore waters related to a marine sedimentary environment of deposition promoted this reaction under these conditions k feldspar was partially preserved during the middle to late stages of mesodiagenesis k feldspar breakdown produced secondary intragranular average values of 1 4 and intergranular pores average values of 1 2 release of k ions promoted illitization of kaolinite with quartz overgrowths and ferrous carbonates developed as by products this study has demonstrated that whereas both members are typical tight gas sandstones they are characterized by quite different diagenetic histories controlled by the primary detrital composition especially during mesodiagenesis types of secondary porosity vary between the two members and developed at different stages of progressive burial the content of unstable detrital components notably feldspar was the key factor that determined the abundance of secondary porosity

diagenetic model of the bajo barreal formation cretaceous in the southwestern flank of the golfo de san jorge basin patagonia argentina abstract this paper analyses the diagenetic evolution of sandstones belonging to the bajo barreal formation cretaceous in the golfo de san jorge basin patagonia argentina the bajo barreal formation includes the main reservoirs which are located along the western area of the basin and is composed of sandstones conglomerates mudstones tuffaceous mudstones and some layers of tuffs the principal reservoirs comprise medium to coarse grained sandstones which are dominated by feldspathic litharenites and contain minor amounts of litharenites and lithic arkoses the authigenic minerals include kaolinite smectite chlorite quartz overgrowths microquartz and calcite with minor proportions of megaquartz siderite analcime laumontite feldspar overgrowths and illite smectite and chlorite smectite mixed layers secondary porosity is much more important than primary porosity and is produced by the dissolution of feldspar lithic clasts and clay cements the diagenetic history of the bajo barreal sandstones can be divided into seven diagenetic stages each of which is characterized by a specific assemblage of authigenic minerals and diagenetic processes eogenetic conditions occur in stages 1 2 3 and 4 stage 1 corresponds to shallow burial characterized by the physical reduction of primary porosity by compaction during stage 2 rim clay cements of chlorite smectite and clinoptilolite as well as thin quartz overgrowths were formed the precipitation of pore filling cements of kaolinite chlorite and smectite occurred during stage 3 while stage 4 records the intense dissolution of feldspar lithic fragments and kaolinite cements mesogenesis occurs in diagenetic stages 5 and 6 the former corresponds to a new phase of authigenic kaolinite while the latter records the significant dissolution of feldspar lithic clasts and previous cements which produced the highest values of secondary porosity finally stage 7 corresponds to the highest degree of diagenesis in the bajo barreal formation mesogenesis which resulted in the precipitation of cements of zeolites and calcite as well as quartz and plagioclase overgrowths

diagenetic history and reservoir quality of tight sandstones a case study from shiqianfeng sandstones in upper permian of dongpu depression bohai bay basin eastern china abstract the upper permian shiqianfeng sandstones located in the dongpu depression bohai bay basin china are fluvio lacustrine deposits and have high content of pore filling volcanic dust average 7 93 vol this study focuses on diagenetic processes and reservoir quality of the tight sandstones based on different lithofacies an integrated approach such as core observation x ray diffraction scanning electron microscopy cathode luminescence stable isotope analysis of carbon and oxygen compositions electron microprobe analysis and fluid inclusion analysis were utilized the tight sandstones consist of subarkose and lithic subarkose and have undergone significant diagenetic events including compaction quartz cementation carbonate cementation mainly calcite and ferrocalcite and formation of authigenic clay minerals i e chlorite rims kaolinite and mixed layer illite smectite the volcanic dust altered with increase of burial depth resulting in the production of abundant silicic materials and depletion of metallic ions i e fe 2 mg 2 ca 2 and mn 2 this process formed subhedral micro quartz pore filling authigenic quartz smectite and chlorite rims during the early diagenetic stage early phase carbonate cements calcite ferrocalcite occurred in several samples they were also influenced by volcanic dust alteration resulting in intrusive of fe 2 and high contents of mn 2 average 12 5 mol further alteration of volcanic dust at the mesogenetic stage can result in cementation of kaolinite and euhedral micro quartz quartz overgrowth developed because of smectite to illite reactions and the alteration of volcanic dust late phase carbonate cements dolomite and ankerite are rare the present low porosity average 6 17 vol and permeability average 0 198 md of the shiqianfeng sandstones mainly resulted from intense compaction diverse cementation and pore filling of volcanic dust as strong hydrodynamic conditions of pore water modification of reservoir quality in the conglomeratic sandstone and gritstone lithofacies mainly distribute in braided channel occurred by alteration of volcanic dust thick braided channel at the high structural position should be the focal point for the future exploration

preservation of shallow plagioclase dissolution porosity during burial implications for porosity prediction and aluminum mass balance the abundance and distribution patterns of secondary porosity formed by dissolution of plagioclase were examined in 216 samples of middle eocene oligocene shallow marine to nonmarine arkosic sandstones from the southern san joaquin basin california the proportion of plagioclase intragranular porosity and plagioclase moldic porosity is depth dependent for purposes of this study pores containing less than approximately 40 of remnant plagioclase were counted as moldic plagioclase porosity pores within skeletal plagioclase grains or mostly enclosed by skeletal grains were counted as intragranular plagioclase porosity moldic porosity is most significant at shallow 5000 ft or 1500 m depths whereas the relative importance of intragranular porosity increases belo approximately 5000 ft 1500 m these trends are most likely caused by gradual preferential collapse with increasing burial of moldic porosity generated predominantly at shallow depths intragranular porosity in more deeply buried sandstones is partly inherited from shallow depths feldspar dissolution porosity formed in shallow open system settings typically is not accompanied by aluminosilicate e g kaolinite by products this porosity either moldic or intragranular can be preserved during burial particularly in sandstones with high percentages of nonductile detrital grains removal of much of the aluminum released by feldspar dissolution in shallow open system settings coupled with incorrect petrographic interpretations placing extensive feldspar dissolution at deep late burial conditions may account for the controversial aluminum source sink imbalance the presence of plagioclase dissolution porosity moldic and intragranular is important but it does not have a significant impact on the accuracy of empirical porosity predictions such predictions are based on an approach that uses a calibration data set and multiple regression analysis to determine the relationship between known geologically significant variables and reservoir quality high accuracy of porosity predictions within 2 porosity of the mean measured porosity in sandstones containing dissolved plagioclase is possible because the presence of secondary porosity is implicitly accounted for by independent variables e g composition grain size sorting burial history in the calibration data set that provides the basis for empirical predictions

a physical model of cementation and its effects on single phase permeability in this paper we present a mathematical model to quantify the effects of cements on the single phase permeability estimate of clastic rocks the model represents pore filling pore lining and pore bridging cements by treating the cement particles as rectangular crystals attached radially to detrital grains these clusters influence permeability through an increase in effective specific surface area and tortuosity because the model is based on physical principles it is largely devoid of empiricism and should be applicable to a wide range of rock types permeability estimates by our model match experimental data from two sets of sandstone samples of five hydrocarbon bearing formations in the north sea the model also confirms the well known result that the effect of pore bridging cement is a factor of ten greater in reducing the single phase permeability compared to pore lining and pore filling cements