ankerite cementation in deeply buried jurassic sandstone reservoirs of the central north sea abstract upper jurassic fulmar formation shelf sandstones of the high temperature high pressure franklin and elgin fields south central graben north sea contain abundant disseminated and concretionary ankerite in contrast most jurassic north sea reservoirs contain only minor amounts of dispersed ankerite disseminated ankerite cement in the franklin and elgin fields has a fairly uniform isotopic composition 18o 10 to 12 5o oo pdb 13c 3 to 5o oo pdb ankerite concretions have 18o values similar to disseminated cements but a wider range of 13c values 1 to 5 5o oo pdb they also have highly variable intergranular volumes which together with the 13c data are interpreted as a combination of pore filling cementation and in situ replacement of comminuted bioclastic debris by ankerite fluid inclusion 18o and paragenetic evidence suggests that ankerite formed during deep burial c 3 5 to 4 5 km 140 170 c after the onset of overpressuring but before hydrocarbon emplacement in the reservoirs the regionally consistent 18o data suggest that ankerite formed via a temperature influenced mechanism and the relatively uniform 13c cement value indicates that organic matter and marine bioclastic carbonate contributed to the dissolved carbon reservoir in constant proportion this can be explained by calcite dissolution in response to ph decrease during thermal breakdown of organic acids such acids were derived from adjacent mudrocks undergoing hydrocarbon maturation and clay mineral transformations and are likely to have been transported in pore fluids with mg2 and fe2 the presence of these cations in solution upon thermal decarboxylation is inferred to have stabilized ankerite at the expense of calcite a relative paucity of ankerite in other fulmar formation reservoirs may reflect different sedimentological compositions less bioclastic debris and or lower burial temperatures less advanced decarboxylation

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

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

porosity trends in the skagerrak formation central graben united kingdom continental shelf the role of compaction and pore pressure history this paper describes reservoir properties in the triassic skagerrak formation in the central north sea this prolific sandstone reservoir often possesses anomalously high porosity for its depth of burial simple statistical analysis of wire line log derived porosity data is used to derive empirical trends as a function of both depth and vertical effective stress that show variations between neighboring hydrocarbon fields and between different parts of the basin porosity data from the josephine j ridge quadrant 30 of the united kingdom continental shelf ukcs show a marked degradation with depth but the porosities are significantly higher than in similarly deeply buried areas such as the puffin high to the west quadrant 29 or the forties montrose high to the north quadrant 22 to understand the porosity patterns better the data have been analyzed by plotting against vertical effective stress this allows a better comparison to be made between fields and wells within the high pressure high temperature hpht realm high pressure here refers to fluid pressures above 10 000 psi whereas high temperatures are above 300 f 149 c results show that porosity and fractional effective reservoir the proportion of net sandstone with a porosity greater than a predetermined cutoff decrease systematically with increasing vertical effective stress data from the different j ridge fields fall on a common compaction trend even though they are derived from structures with marked variations in present day depth of burial and static formation overpressure trends from the other areas of the central graben the puffin and forties montrose highs indicate more indurate reservoir states the observed porosity trends are independent of fluid type within the reservoir and the absolute magnitude of overpressure the main observed hydrocarbon effect is the result of buoyancy forces the analysis supports the contention that after accounting for facies related grain size variations compaction controls average reservoir properties differences in compaction state between areas are postulated to relate primarily to structurally controlled timing of overpressure development relative to burial and how these affect the resultant vertical effective stress history both the puffin and forties montrose highs are directly attached to the basin margins across stepped faults these marginal terraces were open to lateral fluid flow for longer probably because across fault seals were only established late in the burial history when higher temperatures promoted cementation and the destruction of permeability within fault cores as a result they developed overpressures in the last 5 10 m y or so and are largely normally compacted the j ridge horst block is hydrologically more isolated within the basin center by across fault juxtaposition seals here overpressure development appears to have started earlier possibly between 50 and 60 ma retarding compaction and allowing preservation of higher porosities compaction continues to present day driven by the large static vertical effective stress gradients in these deeply buried reservoirs the observed empirical trends offer a means of predicting average reservoir properties in deep untested exploration targets

a model for fibrous illite nucleation and growth in sandstones we have developed a model for the formation of fibrous illite in sandstones where kaolinite is a primary reactant and potassium is derived from in situ k feldspar grain dissolution or imported into the model reference frame illite fiber nucleation and growth are modeled using arrhenius expressions that consider saturation state in addition to temperature and time nucleation occurs on pore walls and muscovite and detrital illite may be defined as energetically favorable substrates the model is integrated with other touchstone models to account for the influence of other diagenetic processes on surface area and reactant volumes and to provide input for permeability simulations we evaluated the illite model performance on two data sets 1 jurassic quartzose samples from offshore mid norway with maximum temperatures ranging from 108 to 173c 226 to 343f and 2 miocene lithic samples from offshore southeast asia that have maximum temperatures ranging from 157 to 182c 315 to 360f the model matches measured abundances of illite kaolinite and k feldspar in both data sets using identical kinetic parameters predicted k ar ages are consistent with available data given uncertainties associated with detrital contaminants although no illite particle size data are available from the analyzed samples modeled crystallite thicknesses from the norway data set are comparable to published measurements of 0 004 to 0 012 m from north sea samples with similar temperature histories

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

sandstone diagenesis and reservoir quality prediction models myths and reality models and concepts of sandstone diagenesis developed over the past two decades are currently employed with variable success to predict reservoir quality in hydrocarbon exploration not all of these are equally supported by quantitative data observations and rigorous hypothesis testing simple plots of sandstone porosity versus extrinsic parameters such as current subsurface depth or temperature are commonly extrapolated but rarely yield accurate predictions for lithified sandstones calibrated numerical models that simulate compaction and quartz cementation when linked to basin models have proven successful in predicting sandstone porosity and permeability where sufficient analog information regarding sandstone texture composition and quartz surface area is available analysis of global regional and local data sets indicates the following regarding contemporary diagenetic models used to predict reservoir quality 1 the effectiveness of grain coatings on quartz grains e g chlorite microquartz as an inhibitor of quartz cementation is supported by abundant empirical data and recent experimental results 2 vertical effective stress although a fundamental factor in compaction cannot be used alone as an accurate predictor of porosity for lithified sandstones 3 secondary porosity related to dissolution of framework grains and or cements is most commonly volumetrically minor 2 exceptions are rare and not easily predicted with current models 4 the hypothesis and widely held belief that hydrocarbon pore fluids suppress porosity loss due to quartz cementation is not supported by detailed data and does not represent a viable predictive model 5 heat flow perturbations associated with allochthonous salt bodies can result in suppressed thermal exposure thereby slowing the rate of quartz cementation in some subsalt sands

direct dating of hydrothermal tungsten mineralization using in situ wolframite u pb chronology by laser ablation icp ms abstract the wolframite series may contain relatively high u and low common pb contents and thus has been used for direct u pb dating of hydrothermal w mineralization in this paper we present in situ laser ablation inductively coupled plasma mass spectrometry la icpms analysis of u pb isotopes and trace elements of wolframite crystals from the giant yaogangxian and piaotang w deposits in the extensive nanling metallogenic belt south china the objective of this work is to directly constrain the timing of hydrothermal w mineralization in these two deposits wolframite crystals from both deposits show textural evidence for two generations early wolframite wol 1 has higher fe and lower mn contents relative to those in the late wolframite wol 2 uranium is correlated positively with nb5 tetravalent ti sn zr hf and trivalent sc v y rees cations in all wolframite samples suggesting that the incorporation of u into wolframite is controlled by the coupled substitution mechanisms the early and late wolframite domains from the yaogangxian deposit yielded u pb ages of 159 1 2 0 ma 2 mswd 0 7 and 153 7 0 7 ma 2 mswd 0 5 respectively similarly two wolframite generations from the piaotang deposits yielded u pb ages of 159 5 1 3 ma 2 mswd 0 3 and 152 1 0 9 ma 2 mswd 0 5 the obtained u pb ages confirm two successive episodes of hydrothermal w mineralization at 159 ma and 153 ma in the yaogangxian and piaotang deposits this study demonstrates the potential of u pb dating of wolframite by la icpms and highlights its importance for directly dating hydrothermal ore forming processes

progressive 18o depletion during co2 separation from a carbon dioxide rich hydrothermal fluid evidence from the grey river tungsten deposit newfoundland oxygen isotope data for the grey river tungsten prospect newfoundland canada indicate a progressive depletion in 18ofluid during mineralization early veins with pegmatitic affinities were deposited at 470 c and pressures greater than 1 kbar 100 mpa from a fluid with a 18o composition of 7 4 presumed to be of magmatic origin successive vein deposition at progressively lower temperatures and pressures culminated in the precipitation of wolframite bearing veins at a temperature of 300 c and pressures of 150 320 bar 15 32 mpa from a low salinity fluid with a 18o composition in the range 3 2 1 6 low values of 18o and dfluid are recorded in many vein tungsten deposits and are normally interpreted as reflecting mixing of isotopically light meteoric fluids or formation brines with magmatic fluids however fluid inclusion evidence for the grey river mineralization indicates that a 40 mol co2 loss occurred by immiscibility and retrograde boiling of the hydrothermal fluid between 420 and 3

secondary porosity generation during deep burial associated with overpressure leak off fulmar formation united kingdom central graben the overpressure history of a sandstone can be estimated using a numerical model if the burial curve and geological setting are known from the resulting effective stress the maximum potential porosity mpp can be calculated the mpp is the maximum porosity the rock could theoretically hold open at the modeled burial depth and pore pressure measured rock porosities should be at or below the mpp we have determined the mpp history for the fulmar formation sandstones upper jurassic of the central graben north sea and have compared the predictions to measured core data we conclude that for the majority of the fulmar formation sandstones the porosity evolution is a simple pattern of reduction during burial caused by compaction and cementation however in wells sited close to regional overpressure leak off points the porosity has been significantly increased from an end of oligocene low mean 21 to the present day values mean 31 this porosity increase occurred by feldspar dissolution with the reaction products being removed from the sandstones secondary porosity generation and the export of solute occurred while the sandstone was highly overpressured although still part of an open hydrogeological system the generation of porosity within deeply buried sandstones is of commercial importance and potentially can be predicted

hydrocarbon filling and leakage history of a deep geopressured sandstone fulmar formation united kingdom north sea diagenetic minerals in a water filled borehole from the jurassic fulmar formation united kingdom central north sea record two phases of hydrocarbon filling and emptying initial oil charge was during the late cretaceous at shallow burial depths of about 1 5 km 0 9 mi as we consider that hydrocarbon has preserved porosity during burial this has implications for the understanding of the porosity evolution of the fulmar formation which in other locations is an important hydrocarbon reservoir the early oil charge as recorded by illite k ar ages progressively filled the structure from 84 to 59 ma and possibly precipitated bitumen because of biodegradation the first oil predated many of the burial diagenetic reactions within the sandstone after leak off at ca 60 ma diagenetic reactions continued in an open geochemical system with possible import of co2 products of these reactions include ankerite and quartz overgrowth cements hydrocarbon staining postdates these phases and provides evidence of a hydrocarbon charge probably gas condensate the second hydrocarbon charge also leaked off and the sandstone is now water bearing previous work on the fulmar formation has incorrectly placed all the diagenetic reactions as predating the first arrival of hydrocarbon the present day pore fluids are high salinity high 18o fluids derived from the underlying permian zechstein evaporates these fluids entered the reservoir during a phase of overpressure release that caused fracturing of the framework quartz grains possibly coincident with the second phase of hydrocarbon leak off at 2 5 ma mark wilkinson studied sandstone hosted calcite concretions for his ph d and has subsequently tried to take a more holistic view of the diagenesis of sedimentary basins his career has been funded by a variety of oil companies although he is currently involved in geological disposal of some of the co2 that he was partly responsible for producing stuart haszeldine is a professor at edinburgh university he received his ph d on clastic and coal sedimentology in 1981 from the university of strathclyde after an attempted reeducation in the oil industry he has collaborated with many companies to research on hydrocarbon reservoir quality radioactive waste disposal and coal resources he currently heads the united kingdom s largest group investigating carbon dioxide sequestration tony fallick was educated at the university of glasgow scotland in physics b sc and chemistry ph d he did research at mcmaster university ontario canada and cambridge england before moving in 1980 to the scottish universities research and reactor center where he is currently the director

petrogenesis of nb ta aplo pegmatites and fine grained granites from the early cretaceous huangshan rare metal granite suite northeast jiangxi province southeast china abstract the early cretaceous ca 130 ma nb rich and ta poor huangshan suite south china includes a series of medium grained granites that contain nb rich mica and very low columbite contents which are indicative of a potential nb resource in addition the huangshan suite contains fine grained granites and aplo pegmatites in which disseminated nb ta columbite is an ore two types of fine grained microcline bearing and albite rich leucogranite sills and dikes were identified sills and dikes of leucogranites and aplo pegmatites intrude the medium grained granites and surrounding hornfels the aplo pegmatites consist of large microcline and quartz crystals that are typically broken and embedded within a foliated albite rich matrix niobium rich and mn poor columbite with ta ta ta nb 0 5 and mn mn mn fe

oil charge preserves exceptional porosity in deeply buried overpressured sandstones central north sea uk abstract exceptionally high porosity sandstones are reported from oil provinces worldwide yet the mechanism of porosity preservation remains controversial we present strong evidence that the exceptional porosities in the jurassic fulmar formation within the north sea are the result of early oil charging preventing cementation and chemical compaction in sands with relatively low detrital clay contents although overpressure and grain coatings do have some effect the most dramatic evidence that reservoir quality is related to early oil charge is the spatial distribution of the porosity the maximum porosity of the fulmar formation is systematically highest immediately below the top seal and decreases below this despite uniform sedimentology hence porosity is preferentially preserved where oil first accumulated in these unusually homogeneous overpressured reservoir sandstones at the top oil charge prevents or significantly retards chemical compaction and cementation of a reservoir preserving porosity there is independent evidence for early oil charging of reservoirs within the central graben from fluid inclusion studies and k ar age dates of authigenic illite micro quartz grain coatings which have been proposed to preserve porosity during burial are associated with only 5 extra porosity in sands that exceed the worldwide average sandstone porosity by almost 20

quartz veins record vertical flow at a graben edge fulmar oil field central north sea the fulmar oil field lies at the faulted western margin of the central graben in the central north sea chalcedony and quartz and later dolomite occur as diagenetic cements in veins within the upper jurassic fulmar formation quartz in one particular vein contains primary aqueous fluid inclusions with homogenization temperatures ranging from 85 degrees to 140 degrees c with a mean of 117 degrees c the measured d18o of the authigenic quartz ranges from 25 1 to 27 1o oo smow the d18o of the water from which most of the quartz precipitated is calculated to have been around 8o oo which is significantly more positive than the measured present day formation water d18o of 4o oo quartz and chalcedony cements in the vein are postdated texturally by dolomite that has a d18o of 23 5o oo smow high salinities are recorded in the fluid inclusions in the quartz the majority ranging from 15 to 19 wt nacl equivalent also the quartz contains inclusions of anhydrite high salinity aqueous inclusions combined with anhydrite inclusions in the quartz and the 18o enriched calculated water composition suggest that the quartz precipitated from an evaporite influenced fluid the same fluid contributed mg for dolomite precipitation more deeply buried sections including the permian zechstein evaporites underlying the fulmar field are the most obvious source of an 18o enriched saline fluid mineralization of the vein resulted from cross formational flow of warm fluids from depth through the zechstein and into the jurassic succession over vertical distances of 600 2000 m fluids most probably migrated up faults and fractures during episodes of salt movement or overpressure release

reservoir quality and rock properties modeling triassic and jurassic sandstones greater shearwater area uk central north sea abstract the complex burial and diagenetic histories of the jurassic fulmar and triassic skagerrak sandstones in the uk central north sea present significant challenges with regard to reservoir quality and rock property prediction commercial reservoir quality is retained despite deep burial and associated high temperatures and pressures shallow marine fulmar sands are normally compacted mean igv 26 3 yet have porosities of 21 33 porosity was preserved through inhibition of quartz cementation by clay and microquartz coatings and enhanced by dissolution of framework grains 5 skagerrak fluvial sands are more compacted mean igv 23 2 exhibit minor feldspar dissolution we modeled reservoir quality evolution using the forward diagenetic model touchstone which simulates porosity loss due to compaction and quartz cementation quantitative petrographic analyses and burial history data were used to calibrate touchstone model parameters the results were applied to deeper prospects for pre drill prediction of porosity and permeability in parallel petrophysical data were used to characterize the elastic properties of the sandstones to provide a basis for quantitative seismic forward modeling experimental data and core calibrated petrophysical results reflecting variable in situ fluids and saturations were used to build an elastic properties model the model is robust and was used to generate fluid filled sandstone properties incorporating touchstone results for prospect specific seismic attribute modeling well results from exploration wells are in good agreement with pre drill touchstone and elastic properties model predictions

zoning in columbite tantalite crystals from the granitic pegmatites of the erajarvi area southern finland abstract electron microprobe analyses of zoned columbite tantalite crystals from the granitic pegmatites of the erajarvi area in orivesi southern finland indicate wide compositional variation within the series fenb2o6 feta2o6 mnnb2o6 mnta2o6 especially in specimens from thin pegmatite dikes most crystals show gentle progressive zoning characterized by small scale variations in the major elements where the compositional variation is large backscattered electron images indicate oscillatory or patchy zoning or various replacement textures the zones in oscillatory zoned crystals are usually 1 50 m in width exceptions reaching 50 120 m the wider zones often consist of a group of very narrow subzones of only slightly different composition zoning is due mainly to the compositional variation in ta and nb in two of these crystals the oscillations in mn follow the strongest oscillations in nb content patchy zoned crystals exhibit corroded cores of early columbite tantalite surrounded by later zones enriched in ta the mottled appearance of such crystals results from two or more successive replacements replacement tongues or network like replacement textures are typical in the rims of some crystals the zoning of the columbite tantalite is related to the complex crystallization history of the pegmatite dikes the main factors controlling oscillatory zoning are considered to be the growth dynamics of the crystals the concentration and diffusion of the main elements and the successive flows of the magma in an intrusion channel the generation of a corrosive supercritical vapor phase at the end of magmatic crystallization caused resorption patchy zoning and the replacement of the columbite tantalite

the role of fluid pressure and diagenetic cements for porosity preservation in triassic fluvial reservoirs of the central graben north sea anomalously high porosities and permeabilities are commonly found in the fluvial channel sandstone facies of the triassic skagerrak formation in the central north sea at burial depths greater than 3200 m 10 499 ft from which hydrocarbons are currently being produced the aim of our study was to improve understanding of sandstone diagenesis in the skagerrak formation to help predict whether the facies with high porosity may be found at even greater depths the skagerrak sandstones comprise fine to medium grained arkosic to lithic arkosic arenites we have used scanning electron microscopy petrographic analysis pressure history modeling and core analysis to assess the timing of growth and origin of mineral cements with generation and the impact of high fluid pressure on reservoir quality our interpretation is that the anomalously high porosities in the skagerrak sandstones were maintained by a history of overpressure generation and maintenance from the late triassic onward in combination with early microquartz cementation and subsequent precipitation of robust chlorite grain coats increasing salinity of pore fluids during burial diagenesis led to pore filling halite cements in sustained phreatic conditions the halite pore filling cements removed most of the remaining porosity and limited the precipitation of other diagenetic phases fluid flow associated with the migration of hydrocarbons during the neogene is inferred to have dissolved the halite locally dissolution of halite cements in the channel sands has given rise to megapores and porosities of as much as 35 at current production depths

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

reservoir quality and burial model evaluation by kinetic quartz and illite cementation modeling case study of rotliegendes north germany silicate reaction kinetics provide a complementary means to other established paleothermal indicators such as organic maturation for evaluating thermal reconstructions in this study we combine the use of an organic maturation model with kinetic models for quartz and illite cementation to evaluate burial history scenarios for five subsalt wells in lithologically and structurally complex rotliegendes reservoirs models for organic maturation are most sensitive to maximum temperature and provide no direct evidence for the time of peak temperature or the overall duration of high temperatures by contrast the kinetics of quartz cementation are much more strongly influenced by the duration of exposure to high temperatures compared with organic indicators kinetic models for fibrous illite formation similarly are sensitive to time and temperature and provide predictions for the time of illite formation that can be compared with radiometric dates used collectively these organic and inorganic paleothermal indicators provide improved constraints on thermal evolution compared with conventional approaches in this study we use these indicators to evaluate two alternative burial history scenarios scenario 1 incorporates a hypothesized jurassic heat flow peak together with significant late jurassic deposition and subsequent erosion scenario 2 omits the jurassic heat flow peak and omits the deposition and erosion of the upper jurassic although both of these scenarios are consistent with organic maturation data scenario 2 leads to a far better match with quartz cement volumes and fibrous illite k ar dates

a sulphur isotopic investigation of the potential sulphur sources for lower palaeozoic hosted vein mineralization in the english lake district current models for cu fe as devonian pb zn carboniferous and barite upper carhoniferous permian vein mineralization in the lake district invoke a variety of sources for fluids and metals including the skiddaw group sediments the borrowdale volcanic group the lake district batholith meteoric water and carboniferous seawater this study isotopically characterizes the sulphur reservoirs within the lower palaeozoic sedimentary and igneous sequences and late caledonian intrusives and assesses their importance as sources of sulphur for the vein mineralization the ordovician skiddaw group is a reservoir of isotopically heavy sulphur with a range of 34 s from 11 to 29 x 1 18 53 6 7 n 10 four analyses of borrowdale volcanic group rocks yielded values of 1 to 15 such positive values are thought to be the result of closed system bacteriogenic reduction of ordovician seawater sulphate in the case of the skiddaw group and mixing of mantle derived magmatic sulphur with ordovician seawater sulphate for the borrowdale volcanic rocks the sulphides in the vein systems hosted by the skiddaw group in the vale of newlands formed from fluids with 34 s h 2 s signatures ranging from 16 7 to 22 5 partially homogenized skiddaw group sulphur is considered to be the source mineralization at coniston hosted by the borrowdale volcanic group shows similar sulphur isotopic characteristics and skiddaw group at depth is considered to be the predominant source of sulphur with a possible minor input from the wallrock volcanic rocks at the site of deposition although the lake district batholith provided a heat source for the cu fe as vein mineralization the granite did not supply any sulphur to those systems however magmatic sulphur was an important constituent in mineralization associated with the shap and skiddaw granites the 34 s values for barite mineralization at force crag greenside skiddaw and shap ranges from 13 to 18 and carboniferous seawater provides the most reasonable source for sulphate

vertical effective stress as a control on quartz cementation in sandstones abstract temperature controlled precipitation kinetics has become the overwhelmingly dominant hypothesis for the control of quartz cementation in sandstones here we integrate quantitative petrographic data high spatial resolution oxygen isotope analyses of quartz cement basin modelling and a kinetic model for quartz precipitation to suggest that the supply of silica from stress sensitive intergranular pressure dissolution at grain contacts is in fact a key control on quartz cementation in sandstones we present data from highly overpressured sandstones in which despite the current burial temperature of 190 c quartz cement occurs in low amounts 4 6 1 2 of bulk volume in situ oxygen isotope data across quartz overgrowths suggest that cementation occurred over 100 ma and a temperature range of 80 150 c during which time high fluid overpressures resulted in consistently low vertical effective stress we argue that the very low amounts of quartz cement can only be explained by the low vertical effective stress which occurred throughout the burial history and which restricted silica supply as a result of a low rate of intergranular pressure dissolution at grain contacts

small scale diagenetic facies heterogeneity controls porosity and permeability pattern in reservoir sandstones the fluvial aeolian upper rotliegend sandstones from the bebertal outcrop flechtingen high germany are the famous reservoir analog for the deeply buried upper rotliegend gas reservoirs of the southern permian basin while most diagenetic and reservoir quality investigations are conducted on a meter scale there is an emerging consensus that significant reservoir heterogeneity is inherited from diagenetic complexity at smaller scales in this study we utilize information about diagenetic products and processes at the pore and plug scale and analyze their impact on the heterogeneity of porosity permeability and cement patterns eodiagenetic poikilitic calcite cements illite iron oxide grain coatings and the amount of infiltrated clay are responsible for mm to cm scale reservoir heterogeneities in the parchim formation of the upper rotliegend sandstones using the petrel e p software platform spatial fluctuations and spatial variations of permeability porosity and calcite cements are modeled and compared offering opportunities for predicting small scale reservoir rock properties based on diagenetic constraints

diagenetic signatures of stratal surfaces in the upper jurassic fulmar formation central north sea ukcs sedimentation rate and changes in relative sea level affect early diagenetic cementation along key stratal surfaces within the deeply buried upper jurassic fulmar formation south central graben ukcs as a result of the bioturbated nature of the shallow marine strata variations in ichnofabrics and cross cutting relationships between trace fossils are important in identifying and correlating stratal surfaces two transgressive and two regressive surfaces sequence boundaries were studied using petrographical mineralogical and geochemical techniques cements including early diagenetic pyrite framboids grain rimming apatite microcrystalline dolomite and ankerite occur at above and immediately below both transgressive and forced regressive surfaces sequence boundaries breaks or subdued rates of sedimentation associated with both types of surfaces meant that the sediment package resided within early diagenetic zones for prolonged periods of time enhancing diagenetic reactions at and below the surfaces the distribution of ankerite despite being a deep burial cement was primarily controlled by concentrations of bioclasts that are particularly abundant at transgressive surfaces the diagenetic character of the forced regressive surfaces sequence boundaries is more complex than that of the transgressive surfaces both in terms of mineralogy and paragenesis this is attributed to the superimposition of diagenetic assemblages as a result of erosion re sedimentation and sediment by passing although the diagenetic signature of the surfaces has been modified by dissolution replacement reactions during deep burial early diagenetic signatures can still be distinguished

mineralogy and geochemistry of pegmatite hosted sn ta nb and zr hf bearing minerals from the southeastern part of the bastar malkangiri pegmatite belt central india abstract primary tin and rare metal mineralization in the bastar malkangiri pegmatite belt central india is associated with granitic pegmatites hosted by metabasic and metasedimentary rocks the rare metal minerals identified from six mineralized pegmatites at govindpal mundaguda mundval dhuramagurah haladikunda and bacheli include cassiterite columbite tantalite wodginite microlite fersmite and hf zircon ta nb poor cassiterite is ubiquitous and the most abundant ore mineral in all the mineralized pegmatites subtle differences are noted in the ore mineral assemblages and in the compositions of individual minerals in different pegmatites ta nb oxides are represented by microlite wodginite manganotantalite in li f p bearing lct pegmatites govindpal mundaguda and mundval in contrast least evolved members of the lct type pegmatites are characterized by the presence of ferrocolumbite dhuramgurah haladikunda and rare ferrotantalite bacheli the compositional variations of columbite tantalite in terms of ta ta nb and mn mn fe ratios and zircon in terms of hf contents from different pegmatites correlate with the degree of fractionation of the pegmatitic melt in the govindpal pegmatite ta nb poor cassiterite occurs as aggregates of grains small pockets and veinlets in the intermediate and core zone the ta nb oxides are restricted to the core margin texturally and compositionally distinct ta nb rich cassiterite along with ta nb oxides and hf zircon occur as irregular inclusions in ta nb poor cassiterite at the core margin the field evidence textural relations and compositions of the ore minerals suggest that the main mineralizing event was associated with hydrothermal precipitation of ta nb poor cassiterite which postdated precipitation of manganotantalite wodginite microlite hf zircon and ta nb rich cassiterite in the govindpal pegmatite the crystallization of ta nb oxides was facilitated possibly by the removal of li and f from the melt and that of ta nb rich cassiterite by increasing oxygen fugacity calculated higher fe 3 fe 2 in wodginite compared to manganotantalite suggests such a possible increase in oxygen fugacity anastomosing veins of microlite in ta nb poor cassiterite was the last mineralizing event in this pegmatite the consistently high ti contents of this subsolidus vein microlite compared to primary ta nb oxides manganotantalite wodginite and microlite might suggest contamination by ingress of metamorphic fluid during its formation

wolframite solubility and precipitation in hydrothermal fluids insight from thermodynamic modeling abstract wolframite a solid solution series between hubnerite mnwo4 and ferberite fewo4 is the main tungsten bearing ore mineral in vein type tungsten deposits much progress has been made on characterizing the mineralizing fluids precipitating wolframite but it remains poorly understood what are the mechanisms depositing wolframite effectively and the variables controlling mn fe ratios in wolframite this study examines chemical controls on wolframite solubility in hydrothermal fluids and mn fe ratios in wolframite using thermodynamic models for fluids in the systems of w mn cl na o h and w fe cl na o h the fluid buffered models are established based on the characteristics of vein type tungsten deposits the modeling results indicate that solubilities of hubnerite and ferberite in hydrothermal fluids at typical w mineralizing conditions 300 400 c 500 1500 bars 5 15 wt nacl equivalent ph 4 6 reach up to hundreds of ppm the predominant tungsten manganese and iron species in fluids are h w o 4 m n c l 2 0 and f e c l 4 2 respectively an increase in ph and a decrease in fluid temperature and salinity are efficient mechanisms precipitating hubnerite and ferberite from acidic hydrothermal fluids hubnerite and ferberite are also significantly dissolved in alkaline hydrothermal fluids but their solubilities in alkaline fluids are insensitive to fluid temperature and salinity this may affect w mineralization at stages with different ph levels e g the greisen stage and the alkaline stage the mn fe ratio of mineralizing fluids is the principal variable controlling mn fe ratios in wolframite more supply of fe than mn from the fertile magma tends to produce fe rich mineralizing fluids this may be one of reasons why ferberite is more common than hubnerite in tungsten deposits and why hubnerite is more likely to precipitate at an early stage

magmatic hydrothermal rare element mineralization in the songshugang granite northeastern jiangxi china insights from an electron microprobe study of nb ta zr minerals abstract the songshugang granite hidden in the sinian metasedimentary stratum is a highly evolved rare element granite in northeastern jiangxi province south china the samples were systematically taken from the ck 102 drill hole at the depth of 171 423 m four types of rocks were divided from the bottom upwards topaz albite granite as the main body greisen nodules topaz k feldspar granite and pegmatite layer electron microprobe study reveals that the rare element minerals of the songshugang granite are very different from those of other rare element granites mn mn fe mn and ta ta nb ta of columbite group minerals and hf hf zr hf of zircon are nearly constant within each type of rocks however back scattered electron imaging revealed that nb ta oxides and zircon of the songshugang granite especially those of topaz albite granite topaz k feldspar granite and greisen are commonly characterized by a specific two stage texture on the crystal scale the early stage nb ta oxide is simply subhedral shaped columbite fe cgm i with low mn 0 16 0 37 and ta 0 05 0 29 columbite fe is penetrated by the later stage tantalite veinlets cgm ii or surrounded by complex nb ta sn w mineral assemblages including tantalite fe wodginite sl cassiterite and ferberite tantalite has wide range of mn values 0 15 0 88 from fe dominance to mn dominance wodginite with ta nb has large variable concentrations of w sn and ti cassiterite and ferberite are all enriched in nb and ta nb 2 o 5 ta 2 o 5 up to 20 12 wt and 31 42 wt respectively with high ta 0 5 similar to nb ta oxides and nb ta sn w mineral assemblages the early stage zircon is commonly included by the later stage zircon with sharply boundary they have contrasting hf contents and hfo 2 of the later stage zircon is up to 28 13 wt petrographic features indicate that the early stage of columbite and zircon were formed in magmatic environment however the later stage of rare element minerals were influenced by fluxes enriched fluids tantalite together with wodginite cassiterite and ferberite implies a ta dominant media an interstitial fluid rich melt enriched in ta and flux at the magmatic hydrothermal transitional stage is currently a favored model for explaining the later stage of rare element mineralization

facies associations detrital clay grain coats and mineralogical characterization of the gironde estuary tidal bars a modern analogue for deeply buried estuarine sandstone reservoirs abstract estuarine tidal bar sandstones are complex reservoir geobodies commonly exploited by the oil and gas industry in order to better predict the reservoir potential of these geobodies this study provides a modern day reservoir analogue describing tidal bars in the inner and outer gironde estuary from the microscopic to the macroscopic scale the originality of this work lies in the multi scale study of modern estuarine tidal bars based on numerous piston cores extracted in a high energy environment this work demonstrates that these tidal bars are composite sedimentary bodies made up of individual reservoir sand units separated by thick muddy layers their vertical facies associations and internal architectures are controlled by local hydrodynamic variations and seasonal river influxes detrital clay grain coats are notably characterized using a portable and handheld mineral spectrometer from the base to the top of the tidal bars x ray diffraction and electron microscopes reveal that these coats are mainly composed of di octahedral smectite illite chlorite and kaolinite associated with other components such as diatoms or pyrite the best reservoir geobodies are those with the minimum clay permeability barriers at the macro and mesoscale an optimum coated grain content and clay fraction volume is also needed for generating authigenic clay coatings and inhibiting quartz overgrowth these conditions are met within the tidal sand bars of the outer estuary funnel that are expected to be the best reservoir geobodies in deeply buried sandstones

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

controls on reservoir quality in exhumed basins an example from the ordovician sandstone illizi basin algeria abstract petroleum exploration in many north african intracratonic basins targets early paleozoic sandstones as the primary reservoir objective these sandstones are often characterized by highly variable reservoir quality 0 0001 1000 md and the ability to predict and selectively target areas of enhanced porosity and permeability is crucial to unlock the hydrocarbon potential the objective of this study is to characterize the primary controls on reservoir quality in an ordovician field in the illizi basin of algeria through detailed core and petrographic analysis and establish if variations in thermal history across the field have a material impact on reservoir quality the best reservoir quality is observed in facies where primary intergranular porosity has been preserved in fine to coarse grained quartzarenites with less than 1 fibrous illite these lithologies are most commonly found within the high energy tidally reworked post glacial facies sandstones of the uppermost ordovician succession observed differences in quartz cement volume within compositionally and texturally similar samples from the southern and northern parts of the field are interpreted to reflect variations in thermal exposure due to deeper burial this interpretation is supported by field wide numerical modelling of sandstone diagenesis this study indicates that subtle variations in thermal history can have a material impact on the spatial trends in reservoir permeability thermal history therefore is an important consideration in reservoir quality studies in exhumed basins where variations in present day burial depth will be a poor guide for evaluating reservoir quality risk across a basin or play

primary geochemical characteristics of mineral deposits implications for exploration extract the old adage mineral exploration is like finding a needle in a haystack recaps the difficulty of finding economic mineral deposits near or beneath the surface in general two main approaches can be used to detect the presence of mineral deposits near or beneath the ground surface one employs a mineral deposit model and the other aims at the identification of a primary halo about a mineral deposit a mineral deposit model provides systematic empirical descriptive and or theoretical genetic descriptions of the essential geological geochemical and geophysical features of a type of mineral deposits cf cox and singer 1986 and roberts et al 1988 the primary halo of a mineral deposit as defined originally by safronov 1936 is an area including rock surrounding mineral deposit ore bodies and enriched elements that make up that deposit thus descriptions of primary haloes may form part of a mineral deposit model but both deposit model and primary halo approaches to mineral exploration are based on studies of primary geochemical characteristics of mineral deposits because chemical processes during mineralization are the ones that ultimately bring about metal precipitation or mineral formation however a deposit model approach to mineral exploration is apt for identifying which haystacks contain a needle whereas a primary halo approach to mineral exploration is proper for deducing where in a needle bearing haystack is the needle located

mineral chemistry of columbite tantalite from spodumene pegmatites of kolmozero kola peninsula russia abstract compositional variation results of electron microprobe analyses and mass spectrometry analyses of columbite group minerals cgm from fully differentiated albite spodumene pegmatites at kolmozero in the kola peninsula is evaluated concentric zoning typical of rare metal pegmatites was not observed in the kolmozero pegmatites columbite group minerals occur in all main parageneses of the pegmatites and form four generations reflecting the sequence of pegmatite formation these minerals demonstrate wide variations in the content of major and trace elements the composition of cgm ranges from columbite fe to tantalite mn fractionation trends were observed in mn mn fe versus ta ta nb diagrams and trace element abundances plotted versus xta and xmn the early cgm paragenesis is characterized by homogeneous oscillatory and progressive oscillatory zoning and corresponds to a primary magmatic type late generation cgm show patchy irregular internal textures replacing earlier regular patterns of zoning the irregular zoning points to metasomatic replacement processes for the first time it is shown that distributions of rare earth elements ree in cgm reflect the evolution of a pegmatite forming system at kolmozero the main trend of ree variation from early to late generations of cgm involves decreasing total ree contents due to a decrease in heavy ree and y decreasing negative eu anomaly and decreasing magnitude of m shape tetrad effect between gd and ho these changes are accompanied by gradual flattening of the bird like patterns of chondrite normalized ree distribution all these features are typical for late differentiates of granitic volatile rich magma late metasomatic tantalite mn is characterized by sharp changes in its ree distribution pattern decreasing total ree contents changing shape of the ree distribution pattern the absence of eu anomaly and tetrad effects and the appearance of a negative ce anomaly the textural characteristics and mineral chemistry of cgm indicate that the pegmatite forming system underwent several stages of evolution the earliest magmatic stage can be divided into two sub stages involving direct crystallization and collective recrystallization respectively and was succeeded by a late hydrothermal metasomatic post magmatic stage variations in chemical composition among the different generations of cgm are explained by the interplay of several processes fractional crystallization competitive crystallization of main rock forming feldspar muscovite spodumene and accessory triphylyte lithiophilite spessartine fluorapatite zircon microlite minerals and evolution of the mineral forming environment from a melt to a hydrothermal metasomatic fluid

autoradiography by x ray excited optical luminescence xeol application to scheelite and fluorite mineralisation this paper investigates the practical application of xeol as an autoradiography technique of use in characterising the mineralogy of geological samples a simple apparatus is described in which thin sections are excited with a low power x ray tube operated at 19 kv 0 9 ma luminescence is detected on photographic film 400 asa using a camera focused through a leaded glass window onto the rear of the thin section xeol autoradiographs recorded for exposure times varying from 3 to 120 min are presented for zinc sulphide powder a test sample scheelite in a wolframite bearing quartz vein and fluorite plus calcite in a fluorite calcite quartz vein the latter two samples being collected from mineralised localities in the eastern highlands of scotland u k these xeol autoradiographs show not only the distribution of the specified phases but also delineate textural information of value in interpreting mineralisation processes

anomalously high sc contents in ore minerals from sn w deposits possible economic significance and genetic implications abstract an investigation on sn w and w deposits and associated granites in the erzgebirge region central kazakhstan mongolian altai and central mongolia reveals anomalously high sc contents up to several thousands of parts per million in wolframite and cassiterite from sn w deposits in the eastern erzgebirge even though whole rock samples of granites from these localities have low sc contents typical of high silica granitic rocks it is possible to trace the distribution of sc in rocks and veins using data on accessory and ore minerals enriched in sc columbite rutile cassiterite wolframite zircon scheelite and mica there are no indications for an enrichment of sc during the magmatic evolution of the granite intrusions magmatic fractionation causes rather low sc contents in evolved granites on the other hand sc enrichment in zircon found in these rocks is clearly related to processes of secondary alteration leading to high u ca and fe besides sc enrichment in the mineral enhanced sc contents were also found in whole rock samples from the greisen the occurrence of sc rich wolframite and cassiterite is mainly restricted to endocontact orebodies greisen and veins outside the rare metal bearing granites the sc content in wolframite drops off if host rocks are formed by schist and gneiss correlation between sc and nb concentrations in wolframite found in endocontact orebodies also disappears in the exocontact veins except if the vein system provides a focused fluid flow sc enrichment in sn w mineralisation is accompanied by strong y sc y yb and th u fractionation in sc bearing minerals as indicated by trace element contents in wolframite and scheelite y sc ratios in wolframite below 0 1 are typical of sn deposits it is suggested that certain conditions of complexation ph and eh in f rich fluids are necessary prerequisites for sc enrichment and deposition our results hint for a sc input into the ore forming system by an external f rich fluid carrying also u and heavy ree and possibly derived from a mantle related source although whole rock samples of rare metal bearing topaz granite have low sc contents the related sn w ores may represent an interesting source for sc extraction because of their enrichment in sc

oxygen isotope evidence for the mixing of magmatic and meteoric waters during tin mineralization in the mole granite new south wales australia oxygen isotope measurements of quartz in the tin mineralized permian mole granite new south wales support a model of progressive mixing of magmatic water and surface derived water of meteoric origin in mineralizing hydrothermal circulation systems the magmatic water observed as multiphase fluid inclusions in hydrothermal quartz biotite orthoclase topaz bearing pegmatites had a high temperature 550 degrees c high salinity 55 75 wt and a delta 18 o smow value of about 8 7 per mil the initial magmatic delta 18 o smow value of the granite is estimated to be about 9 per mil the meteoric water observed as liquid rich fluid inclusions in quartz from late veins of quartz sphalerite galena had a low temperature 250 degrees c low salinity 3 5 wt and a delta 18 o smow value of 15 per mil permian meteoric water is expected to be light isotopically owing to a near polar 70 degrees palcolatitude for the new england fold belt quartz from cassiterite quartz chlorite sericite ores has a wide range of delta 18 o values from 2 8 to 6 7 per mil this is intermediate between the values for quartz deposited from magmatic and from meteoric waters the fluid inclusions in these samples have homogenization temperatures of about 300 degrees c and a wide range of salinities 25 35 wt the variation in isotopic composition observed for quartz is attributed to a fluctuating isotopic composition of the hydrothermal solutions resulting from variable degrees of mixing of magmatic and meteoric water cassiterite has a narrow range of delta 18 o 5 9 7 2ppm which suggests precipitation of cassiterite over a narrow range of mixing of the hydrothermal fluids three cassiterite quartz mineral pairs with textures indicating synchronous crystallization imply cassiterite water fractionations of 2 2 per mil at 300 degrees c and 4 1 per mil at 370 degrees c hydrogen isotope data are less systematic due to postcrystallization isotopic exchange of hydrous minerals and the presence of younger fluids in secondary fluid inclusions in quartz

mineralisation at the carrock fell tungsten mine n england paragenetic fluid inclusion and geochemical study tungsten ore at carrock fell mine comprises wolframite and scheelite in polyminerallic quartz veins which traverse the grainsgill granite cupola and surrounding country rocks in the veins a wolframite scheelite apatite assemblage pre dates a scheelite arsenopyrite pyrite plus other sulphides assemblages temperatures of mineralisation declined from a peak near 350 c to 170 c and the hydrothermal fluid contained about 6 weight nacl and 3 wt nahco3 contemporaneous greisenisation involved loss of na cr ca and ba from granite but si and k were retained while b be and al increased slightly sn also increased but is always a trace constituent and f appears to have decreased zones of intense alteration contain high concentrations of quartzhosted fluid inclusions resulting from penetration of the granite by fluid chemically similar to that in the vein quartz the w rich sn poor nature of the mineralisation may relate to the weakly saline f deficient but co2 rich fluid chemistry the alteration and mineralisation processes took place during late cooling of the lower devonian skiddaw granite cross cutting quartz ankerite veins and argillitic zones which may be considerably younger than those producing the tungsten ore have a distinct mineral suite lacking w and as and including major pb and zn temperatures at this late stage were below 150 c and the fluid is estimated to have contained approximately 12 wt nacl and 15 wt cacl2

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

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

contribution of infrared microscopy to studies of fluid inclusions hosted in some opaque ore minerals possibilities limitations and perspectives during the past two decades several studies of fluid inclusions hosted in some opaque ore minerals using near infrared microscopy have been performed results indicated that this method can be applied to several sulfidic ores and metal oxides depending on their electronic band structures and infrared active vibration modes infrared transmittance of individual ore minerals can be best characterized using fourier transform infrared spectroscopy infrared microscopic observations are limited to the near infrared region to about 2 3 m depending on the ir sensitivity of the ir camera the trace element content in ore minerals can be another limiting factor for optical observations in near infrared light still ir transmittance gradually decreases upon heating caused by shifting of ir absorption edges for higher wavelengths possibilities and limitations of studying fluid inclusions hosted in opaque minerals by near infrared light microthermometry and laser ablation inductively coupled plasma mass spectrometry la icp ms are discussed

anomalously high porosity and permeability in deeply buried sandstone reservoirs origin and predictability porosity and permeability generally decrease with increasing depth thermal exposure and effective pressure however a significant number of deep 4 km approximately 13 000 ft sandstone reservoirs worldwide are characterized by anomalously high porosity and permeability anomalous porosity and permeability can be defined as being statistically higher than the porosity and permeability values occurring in typical sandstone reservoirs of a given lithology composition and texture age and burial temperature history in sandstones containing anomalously high porosities such porosities exceed the maximum porosity of the typical sandstone subpopulation major causes of anomalous porosity and permeability were identified decades ago however quantification of the effect of processes responsible for anomalous porosity and permeability and the assessment of the predictability of anomalous porosity and permeability occurrence in subsurface sandstones have rarely been addressed in published literature the focus of this article is on quantification and predictability of three major causes of anomalously high porosity 1 grain coats and grain rims 2 early emplacement of hydrocarbons and 3 shallow development of fluid overpressure grain coats and grain rims retard quartz cementation and concomitant porosity and permeability reduction by inhibiting precipitation of quartz overgrowths on detrital quartz grains currently prediction of anomalous porosity associated with grain coats and grain rims is dependent on the availability of empirical data sets in the absence of adequate empirical data sedimentologic and diagenetic models can be helpful in assessing risk due to reservoir quality such models provide a means to evaluate the effect of geologic constraints on coating occurrence and coating completeness required to preserve economically viable porosity and permeability begin page 302 in a given play or prospect these constraints include thermal history and sandstone grain size and composition the overall effect of hydrocarbon emplacement on reservoir quality is controversial it appears that at least some cements quartz and illite may continue to precipitate following emplacement of hydrocarbons into the reservoir our work indicates that integration of basin modeling with reservoir quality modeling can be used to quantify prior to drilling the potential impact of hydrocarbon emplacement on porosity and permeability the best case scenario for significant reservoir quality preservation due to fluid overpressure development is in rapidly deposited tertiary or quaternary sandstones our models suggest that significant porosity can be preserved in sandstones that have experienced continuous high fluid overpressures from shallow burial depths the models also indicate that the potential for porosity preservation is greatest in ductile grain rich sandstones because compaction tends to be the dominant control on reservoir quality in such rocks the case for significant porosity preservation associated with fluid overpressures in pre tertiary basins however is more problematic because of the complexities in the history of fluid overpressure and the greater significance of quartz cementation as a potential mechanism of porosity loss

40 ma of hydrothermal w mineralization during the variscan orogenic evolution of the french massif central revealed by u pb dating of wolframite we present u pb thermal ionization mass spectrometer tims ages of wolframite from several granite related hydrothermal w sn deposits in the french massif central fmc located in the internal zone of the variscan belt the studied wolframite samples are characterized by variable u and pb contents typically 10 ppm and show significant variations in their radiogenic pb isotopic compositions the obtained u pb ages define three distinct geochronological groups related to three contrasting geodynamic settings i visean to namurian mineralization 333 327 ma coeval with syn orogenic compression and emplacement of large peraluminous leucogranites ca 335 325 ma ii namurian to westphalian mineralization 317 315 ma synchronous with the onset of late orogenic extension and emplacement of syn tectonic granites ca 315 310 ma and iii stephanian to permian mineralization 298 274 ma formed during post orogenic extension contemporaneous with the permian volcanism in the entire variscan belt the youngest ages 276 274 ma likely reflect the reopening of the u pb isotopic system after wolframite crystallization and may correspond to late hydrothermal alteration e g ferberitization our results demonstrate that w sn mineralization in the fmc formed during at least three distinct hydrothermal events in different tectono metamorphic settings over a time range of 40 ma

facies characteristics and depositional models of highly bioturbated shallow marine siliciclastic strata an example from the fulmar formation late jurassic uk central graben abstract an extensive core database from the late jurassic fulmar formation illustrates the practicalities of facies analysis and depositional modelling in highly bioturbated heterolithic shallow marine siliciclastic strata twelve major sedimentary facies 1 12 are defined on the basis of lithology grain size visually estimated sand silt content and the presence or absence of primary sedimentary structures ichnofabric characteristics are also defined and have proved to be not always facies specific for purposes of depositional modelling the 12 facies types are resolved into six facies associations a f each representative of distinct shoreline shallow marine environments from these associations three broad depositional models have been constructed storm influenced shoreface model 1 bioturbated shoreface model 2 and a speculative bioturbated shelf model model 3 stratigraphical and palaeontological criteria suggest that models 1 and 2 are shoreline attached sedimentological data indicate that they occur as end members to a spectrum of shoreline settings constrained by incident wave energy bands for shorefaces subject to significant storm influence well preserved event beds occur seaward of the upper shoreface in the bioturbated shoreface model however the lower shoreface and adjacent shelf deposits are characterized by intense infaunal reworking the bioturbated shelf model displays no clear evidence of shoreline connection with the resultant facies often exhibiting highly abundant siliceous sponge spicules together with other open marine indicators such as belemnites and rare ammonites unlike the shoreface sand bodies which received sediment from basin margin fluvial systems shelf sand bodies in the fulmar formation are more likely to have been intrabasinally sourced along relay ramps or other forms of transfer zone this sediment may have undergone a long transit residence period on the shelf prior to eventual burial in hanging wall depocentres strongly influenced by zechstein salt withdrawal this study demonstrates that an ichnofabric approach as opposed to one of simple ichnodiversity is highly significant with respect to the description and modelling of highly bioturbated shallow marine siliciclastic strata in the fulmar formation ichnofabric analysis is capable of providing sensitive information on fluctuations in depositional energy levels water depth sedimentation rates erosion rates substrate consistency and dissolved oxygen levels evidence of these fluctuations is often apparent at intra facies scale but would largely go unnoticed using a conventional sedimentological approach to data gathering based on ichnodiversity at best depositionally significant changes in ichnofabric may occur without any significant change in ichnodiversity ichnofabric analysis also plays an important role in the hierarchical assessment of key stratal surfaces for purposes of sequence stratigraphic modelling

predictive permeability variations in a hermod sand reservoir stine segments siri field danish north sea abstract the hermod sand of the stine segments siri field danish north sea displays large permeability variations 1 600 md these permeability variations represent horizontal layering in the reservoir largely formed by diagenetic processes they have a significant impact on reservoir performance because horizontal producers in fairly thin reservoirs 30 m are protected against bottom water influx and can produce water free for long periods of time this paper presents the results of a detailed multidisciplinary study where the observed variation in permeability can be explained by two main diagenetic phases silica as opal cement and microquartz and berthierine a serpentine mineral that is closely related to chlorite opal microquartz and berthierine cements dominate in separate parts of the reservoir bodies opal microquartz cemented sands have high permeability typically 500 600 md berthierine cement has a different growth pattern in the oil and water zones resulting in intermediate permeability in the oil zone typically 50 100 md and very low permeability in the water zone typically 1 10 md

new mineral names the following new minerals are discussed in this issue langbanshyttanite pseudolyonsite and yangzhumingite these new minerals have been described in the european journal of mineralogy and are reported here n v chukanov i v pekov e jonsson n v zubkova y e filinchuk d i belakovskiy and d yu pushcharovsky 2011 langbanshyttanite a new low temperature arsenate mineral with a novel structure from langban sweden eur j mineral 23 675 681 the new mineral langbanshyttanite was discovered in a specimen from the langban mine filipstad district varmland county bergslagen ore province sweden the carbonate hosted langban deposit 59 86 n 14 27 e is together with related occurrences in the western part of the palaeoproterozoic bergslagen ore province in south central sweden characterized by anomalously pb as ba be sb rich metamorphosed stratabound fe and mn oxide ores with associated skarn units sulfide assemblages as well as abundant vein and fissure hosted mineral assemblages the name is for langbanshyttan which is the old name of both the mine mining village and smelter that in more recent times has been known plainly as langban langbanshyttan actually means langban s smelter or langban s furnace langbanshyttanite occurs as a very late low temperature species in the deposit it crystallized during the last stage of hydrothermal activity involving metal remobilization in the langban system post dating regional metamorphism of the volcanic hydrothermal ores and their host rocks based on existing evidence these minerals formed at very low temperature and pressure in this case most likely below 70 c the mineral formed in fractures and corrosion veinlets post dating calcite in a mn oxide bearing mn silicate rich rock associated late stage minerals are calcite antigorite sparse euhedral trigonite and a few microscopic aggregates of a so far unidentified as bearing mineral typically both calcite and antigorite exhibit evidence of being partly corroded away prior to langbanshyttanite formation trigonite coeval with langbanshyttanite occurs as sparse euhedral crystals the groundmass to the veinlets or fractures mainly consists

modification of a rare earth element deposit by low temperature partial melting during metamorphic overprinting norra karr alkaline complex southern sweden abstract rare earth elements play a crucial role in modern technologies and are necessary for a transition to a green economy potentially economic deposits of these elements are typically hosted in minerals such as monazite bastnasite and eudialyte a complex na ca fe zr silicate mineral with cl making these prime targets for geological research globally rare earth mineral deposits commonly show evidence of polyphase development and mineralisation processes which need to be better understood to improve exploration strategies the norra karr alkaline complex sweden contains a globally significant deposit of rare earth elements hosted in the mineral eudialyte in this study we focussed on eudialyte crystals in undeformed cross cutting pegmatoid veins from norra karr in order to determine their age we refined an established micromilling method to enable sampling of minerals rich in rare earth elements for precise analysis of major and trace elements nd isotope ratios and sm nd geochronology down to a scale of

quantitative evaluation of the reservoir potential and controlling factors of semi deep lacustrine tempestites in the eocene lijin sag of the bohai bay basin east china abstract ancient lacustrine storm deposits that act as petroleum reservoirs are seldom reported the lijin sag which is located in the southeastern corner of the bohai bay basin in east china is a ne sw trending cenozoic half graben basin some of its eocene deposits bindong deposits were interpreted as lacustrine tempestites the bindong tempestites which developed in the lower part of the upper fourth member of the shahejie formation es41 constitute a new kind of petroleum reservoir and are novel petroleum exploration targets in the bindong area however the characteristics of the es41 bindong tempestite reservoirs and their controlling factors are poorly understood point count analyses of thin sections scanning electron microscope image analyses x ray diffractometry data and the petrophysical parameters of the bindong tempestite reservoirs were utilized to estimate the reservoir quality the reservoirs have undergone significant diagenetic alteration which can be divided into negative and positive aspects the negative alteration includes compaction authigenic minerals and cementation such as carbonates clay minerals and overgrowths of quartz and feldspar the uneven distribution of carbonate cement increased the reservoir s heterogeneity with carbonate cement commonly precipitating along the sandstone mudstone contacts the primary porosity was severely reduced because of compaction and extensive carbonate cementation positive alteration includes dissolution carbonate cementation undercompaction and fractures carbonate cementation exhibited both positive and negative effects on the reservoir quality overall the objective reservoir quality is quite poor a quantitative evaluation of the reservoirs potential was conducted the cutoff values of several of the reservoir s parameters were calculated the lower limits of the porosity and permeability are 8 35 5 85 and 1 2587 0 2753 10 3 m2 respectively depending on the depth the upper limits of the carbonate and mud content are approximately 18 5 and 9 10 respectively a fundamental understanding of these characteristics will provide necessary information for extracting hydrocarbons from analogous subsurface reservoirs

textural features and chemical evolution in tantalum oxides magmatic versus hydrothermal origins for ta mineralization in the tanco lower pegmatite manitoba canada tantalum a key element in the electronics industry is produced mainly from rare element granitic pegmatites although their internal structure mineralogy and petrogenesis have been extensively investigated the processes that control tantalum mineralization remain poorly understood in particular the role of fluids in the crystallization of tantalum ore one of the major problems arises from the difficulty in distinguishing primary magmatic from secondary hydrothermal textures in such complex rocks in the tanco pegmatite manitoba canada tantalum mineralization shows a complexity that reflects the complex petrogenesis of its host pegmatite eight different families of tantalum oxides occur in various associations and compositions the tanco lower pegmatite is an isolated body beneath the main pegmatite body that contains abundant tantalum associated with mica alteration tantalum mineralization in the tanco lower pegmatite occurs as three different styles facies 1 is hosted by the wall zone and hosts primary magmatic ta oxides with simple textures progressive and oscillatory zoning facies 2 is hosted by the lower and upper intermediate zones where most mineralization occurs with dendritic amblygonite facies 3 is hosted by the central zone which is affected by mica alteration in this latter facies the oxides show particularly complex textures evident from x ray mapping by combining information obtained from textural observations and chemical analyses we are able to determine the paragenesis for each ta oxide bearing mineral assemblage and hence to evaluate the relative contributions of magmatic versus hydrothermal processes in complex associations we observe relics of primary ta phases that are replaced or overgrown by secondary ta phases we propose the paragenetic sequence columbite group minerals microlite early primary magmatic columbite wodginite group minerals microlite late primary magmatic wodginite group minerals microlite secondary magmatic ferrotapiolite secondary magmatic in addition chemical variations were identified in the columbite and wodginite group minerals both at the crystal scale and at the pegmatite scale columbite and wodginite group minerals show the typical ta ta ta nb and mn mn mn fe atomic ratios increase from earlier to later zones at the crystal scale the increase in ta and mn with fractionation in the columbite group is explained by the higher solubility of the ta end member relative to the nb end member and the crystallization of other minerals that shift the melt composition toward mn enrichment the fractionation trend in the wodginite group shows fe enrichment which is consistent with experimental results that show a higher solubility of the fe end member relative to the mn end member in columbite group minerals considering the available experimental data as well as the intimate association of ta oxides with zircon observed in this study we conclude that tantalum mineralization is a product of direct crystallization from the melt rather than hydrothermal in origin fluids are attributed an indirect role only as they could have brought minor elements fe mn or ca into the melt which resulted in the crystallization of secondary ta phases textural evidence shows that the late secondary phases were formed at the same time as mica alteration we suggest that mica alteration was due to a late melt rather than a hydrothermal fluid that reacted with the blocky k feldspar of the central zone during the alteration event tantalum was remobilized from primary phases and incorporated in new secondary ta oxide species

a physical model for porosity reduction in sandstones the experimental elastic moduli porosity trends for clean sandstones can be described by the modified upper hashin shtrikman muhs bound one geometrical but not necessarily geological realization is as porosity decreases the number of the pores stays the same and each pore shrinks while maintaining its shape this concept of uniform porosity reduction implies that permeability is proportional to the effective porosity squared and that formation factor is proportional to the inverse of the effective porosity the effective porosity here refers to the part of the pore space that dominates fluid flow the proposed relations for permeability and formation factor agree well with the experimentally observed values these laws are different from the often used forms of the kozeny carman equation and archie s law where permeability is proportional to the total porosity cubed and formation factor is proportional to the inverse of the total porosity squared respectively we suggest that the uniform porosit

stable isotope studies of the dae hwa tungsten molybdenum mine republic of korea evidence of progressive meteoric water interaction in a tungsten bearing hydrothermal system the dae hwa w mo mine is located approximately 100 km southeast of seoul within the precambrian metamorphic belt of the southern korean peninsula the major ore minerals wolframite scheelite and molybdenite together with minor cassiterite chalcopyrite and bismuthinite occur within fissure filling quartz veins contained within precambrian granitic gneiss adjacent to a granitic stock of cretaceous age k ar dates of 88 or 2 m y for vein muscovite suggest a late cretaceous age for w mo mineralization the ore mineral paragenesis can be divided into three distinct stages a molybdenite wolframite stage 400 degrees 315 degrees c an iron sulfide scheelite stage 315 degrees 230 degrees c and a late carbonate stage 230 degrees 150 degrees c the delta 18 o values of vein minerals are quartz 10 6 to 11 4 per mil muscovite 8 2 per mil cassiterite 2 9 per mil wolframite 4 0 to 4 3 per mil color zoned scheelite 7 3 to 2 2 per mil cores 0 8 2 2ppm edges 7 3 2 7ppm siderite 11 8 per mil dolomite 7 7 to 10 0 per mil calcite 5 7 to 9 3 per mil there is a nearly monotonic decrease in calculated delta 18 o values of hydrothermal waters with decreasing temperature in the dae hwa hydrothermal system from values of approximately 6 per mil for quartz muscovite molybdenite cassiterite wolframite deposition to approximately 3 5 to 0 0 per mil for iron sulfide early scheelite mineralization to 3 5 to 7 0 per mil for late scheelite carbonate deposition we believe the decrease of delta 18 o water values with paragenetic time and decreasing temperature represents the progressively increasing importance of meteoric water interaction in the dae hwa tungsten molybdenum hydrothermal system the measured and calculated ranges of delta d values of inclusion fluids and hydrous minerals are 48 to 78 per mil for quartz muscovite molybdenite wolframite deposition 60 to 67 per mil for early scheelite deposition and 71 to 101 per mil for late scheelite carbonate deposition the hydrogen and oxygen isotope compositions of hydrothermal fluids at dae hwa are consistent with progressive mixing of a magmatic or highly exchanged meteoric water exchanged with granitic rocks over a series of temperatures at variable but low water to rock ratios with an unexchanged meteoric water delta d 140ppm the stable isotope composition of the tungsten molybdenum mineralizing fluid is controlled predominately by the host granitic rocks sulfur isotope analyses of sulfide vein minerals suggest an igneous source of sulfur with a delta 34 s sub sigma s value near 3 0 per mil together with the hydrogen and oxygen isotope data they indicate that deposition of molybdenite wolframite and scheelite occurred during a period of declining temperatures from 400 degrees to 230 degrees c in response to inundation of an original magmatic system with low temperature waters of meteoric origin differences in mineralogy paragenesis and delta 18 o values of hydrothermal fluids between w mo scheelite rich and w base metal wolframite rich vein type deposits may reflect differences in their postwolframite deposition interactions with meteoric waters the hydrothermal system at dae hwa may have experienced more pervasive fracturing and a higher degree of meteoric water interaction resulting in remobilization of tungsten and a higher scheelite to wolframite ratio than lower water to rock ratio systems such as pasto bueno and san cristobal peru and panasqueira portugal

tin tantalum niobium mineralization in the penouta deposit nw spain textural features and mineral chemistry to unravel the genesis and evolution of cassiterite and columbite group minerals in a peraluminous system abstract the sn ta nb penouta granite is a highly evolved rare metal post kinematic leucogranite located in north western spain which was intermittently mined from roman times until 1985 when falling metal prices led to the closure of most mines in the country mineralization consists mainly of cassiterite cassiterite i and columbite group minerals cgm disseminated in the leucogranite that increase in abundance and crystal size towards the upper part of the granitic cupola especially in banded pegmatite like dikes upwards quartz veins and greisen zones associated with the wall rock mainly augen gneisses were developed containing coarse grained cassiterite cassiterite ii in contrast to nearly homogenous cassiterite chemistry cgm may show complex compositional zonation patterns in relation with compositional variations of the leucogranite taking into account the mineral chemistry and textural features of cgm two main groups were distinguished i the first one group i follows a general main trend from columbite fe to columbite mn and finally to tantalite mn which is in accordance with the evolution of the magma observed from the less to the more evolved zones of the leucogranite and from bottom to top of the body and responds to common fractionation patterns of decreasing nb ta ratios in the melt and ii a second group group ii with intermediate values of mn mn fe and ta ta nb due to local supersaturation effects of the melt during crystallization resulting in complex oscillatory zoned cgm crystals textural and chemical features of cassiterite and cgm support the magmatic origin for both minerals although cassiterite crystallized later in the sequence in both generations mineral chemistry evolution of cgm is in good agreement with a greater solubility of tantalite in the melt at the upper levels where besides the fluorine contents of apatite and muscovite were increased and the onset of fluorite crystallization occurred so the fluorine enrichment in the upper levels seems likely to be an important component to explain the mineral chemistry of the penouta granite although the same mineral chemistry scheme could also be explained in terms of white mica fractionation upwards furthermore the absence of typical textures originated by fluids in the cgm as well as the lack of these minerals in the quartz veins point to a low influence of the hydrothermal fluids in dissolution and recrystallization processes of the primary oxides

influence of grain size and temperature on intergranular pressure solution quartz cementation and porosity in a quartzose sandstone abstract cathodoluminescence petrography and scanning electron microscopy sem reveal that grain size and temperature significantly influenced intergranular pressure solution quartz cementation and porosity evolution in the quartzose hartshorne sandstone middle pennsylvanian of the arkoma basin among samples from each locality a negative linear relationship exists between mean grain size and volume of quartz dissolved by intergranular pressure solution in contrast a generally positive relationship exists between mean grain size and volume of quartz cement although considerable variability exists in this relationship a positive relationship also exists between mean grain size and porosity intergranular pressure solution directly reduced minus cement porosity by causing tighter pack ng of detrital grains it thereby controlled volumes of both cement and primary porosity for this reason little primary porosity is preserved in pressolved finer grained sandstones even though they contain small absolute volumes of quartz cement appreciably more primary porosity may be retained by unpressolved coarser grained sandstones even though they contain larger absolute volumes of quartz cement these relationships also vary with thermal maturity which increases eastward across the basin this thermal maturity trend is the result of elevated temperatures associated with mesozoic rifting and intrusive events in the mississippi embayment among samples of equal grain size more intergranular pressure solution less quartz cement and less porosity are evident in areas of higher thermal maturity as is the case with grain size the influence that temperature exerted on intergranular pressure solution is in turn reflected in volumes of quartz cement and porosity thus primary porosity tends to be preserved preferentially in areas of lower thermal maturity even though the absolute volume of quartz cement is generally higher in areas of higher thermal maturity more quartz has been dissolved by intergranular pressure solution than is present as cement whereas the opposite is true in areas of lower thermal maturity this suggests that intergranular pressure solution was an important agent of mass transfer during diagenesis of these quartzose sandstones

niobian wolframite from glen gairn in the eastern highlands of scotland a microprobe investigation electron microprobe data are presented from zoned niobian wolframite crystals in a quartz wolframite cassiterite vein and wall rock from a zinnwaldite bearing granite pluton that forms part of the late caledonian glen gairn granitic complex in the eastern highlands of scotland compositions range from nb and ta rich wolframite fe mn w nb ta o 4 forming strongly zoned difficult to polish regions of crystals to almost pure hubnerite mnwo 4 forming relatively unzoned easily polished regions nb and ta substitution is very restricted in mn rich 80 mol hubnerite wolframites but increases regularly in more fe rich compositions through a coupled substitution mechanism between mn 2 w 6 and fe 3 nb ta 5 most of the wolframite crystals contain two forms of zonation a physical zonation observed optically and by electron imaging and a compositional zonation revealed by x ray traverses compositional zoning is progressive involving gradual changes in mineral chemistry from core to rim on which is superimposed a form of oscillatory zoning involving repeated resetting of compositions physical zones are layers of uniform width separated by distinct boundaries they are arranged parallel to crystal faces and reflect intermittent crystal growth these zones vary in width from less than 5 m in the niobium rich wolframite to over 200 m in regions of niobium poor hubneritic wolframite hydrothermal alteration particularly of the intensely zoned niobium rich regions has resulted in assemblages of altered nb and ta rich wolframite tungstite secondary hubneritic wolframite and scheelite nb rich inclusions 5 10 m across are found sporadically in the larger wolframite crystals progressive variation in wolframite chemistry is readily explained in terms of gradually changing physiochemical conditions such as ph f o 2 temperature pressure and fluid composition during crystallization the oscillatory zonation reflects cyclicity of those conditions plausible models which could explain such effects include 1 a intermittent mixing of chemically distinct magmatic and meteoric fluids 2 b repeated hydraulic fracturing and injection of fluids leading to fluctuating pressure and temperature of the hydrothermal fluid volatile species f cl co 2 may have been effective as complexing agents thus affecting the supply of nb from the fluid deposition of nb and ta rich compositions were probably favoured by high f o 2 low ph conditions

estimation of single phase permeability from parameters of particle size distribution the single phase permeability of a permeable medium is determined by both the bulk physical properties of the interconnected pore system e g porosity and tortuosity and the statistics of its particle size distribution psd an expression of the relation between permeability and pore level properties is the carman kozeny ck equation this equation has historically been used to explain the fundamental causes of permeability because it provides a link between media attributes and flow resistance because the ck equation has not been applied to media consisting of mixed particle sizes we modify the ck equation to express the single phase permeability of unconsolidated media in terms of both the psd statistics and the bulk physical properties we find that we can relate permeability to the parameters of a psd for unconsolidated media this relationship matches published data it generally fails for permeabilities less than 1 m2 1 darcy probably because not all of the pore space is supporting flow in this region with the model partially verified by experimental data we use it to investigate the nature of permeability porosity relationships and of the origin of variability in permeability the latter capability will explain many of the permeability differences observed in outcrop studies

tracing metal sources in peribatholitic hydrothermal w deposits based on the chemical composition of wolframite the example of the variscan french massif central abstract we present a complete dataset for major minor and trace elements in wolframite fe mn wo4 based on the example of several peribatholitic hydrothermal w deposits located in the variscan french massif central fmc the wolframite samples were characterized down to the micrometer scale by combining scanning electron microscopy observations and in situ chemical analysis by electron probe microanalyzer and laser ablation inductively coupled plasma mass spectrometry most samples are fe dominated wolframites and are characterized by variable fe fe mn values for the studied deposits from 0 36 to 0 86 without significant alteration features wolframites present intracrystalline variations for concentrations of several minor and trace elements principally for nb ta and sn which reflect growth zoning the minor and trace element compositions of the different wolframite samples show at first order similar enrichments in ta nb sn zn and in as well as depletions in lree li th ti zr hf pb v co and mg compared to the upper continental crust second order compositional variations are observed between the studied deposits which can be separated in four main regional geochemical groups this work highlights that the chemical composition of wolframite is controlled by both the crystallochemical parameters and the composition of the primary hydrothermal fluid the regional variations observed in the geochemical signatures of wolframite reflect local variations in the composition of the hydrothermal fluid i e variable metal sources therefore this suggests that the minor and trace element contents in wolframite likely represent direct markers of the source of the hydrothermal fluid and ultimately of the metals the comparison between the trace element compositions of wolframite with the compositions of regional granitic rocks and worldwide shales suggests that evolved peraluminous granites represent the main source of metals in peribatholitic hydrothermal w deposits from the fmc whereas metasedimentary rocks have a limited to no influence on the metal budget

geology and gravity modeling of the logrosan sn w ore deposits central iberian zone spain abstract sn w granite hosted ore deposits are characterized by the occurrence of pegmatites stockworks and quartz veins within a granitic cupola and its surrounding host rocks a regional exploration tool for these deposits consists of targeting syn to late orogenic specialized s type hidden shallow granites by geophysical methods however gravity as a tool for brownfield type exploration in a mature granite related ore setting has not been widely used the logrosan area central iberian zone provides a perfect case scenario with sn w endo and exogranitic ore deposits within a relatively small scale realm we suggest that the understanding of the subsurface of the logrosan area provides a key tool to develop a more comprehensive geological and metallogenic model for the ore deposits in addition a better explanation for the mineral evolution and the sn w ore precipitation can be achieved by comparing the chemical and textural features of ores from several of the logrosan deposits field gravity and geologic data allowed the modeling of the hidden granite bodies combining this information with detailed mineralogical geochemical data and ore deposit modeling has allowed a definition of targets for probable new sn w deposits all within a few kilometers of the main intrusion center

geothermal energy reconnaissance of southeastern nigeria from analysis of aeromagnetic and gravity data given the daunting energy challenges currently bedeviling nigeria with demand for electrical power exceeding availability alternative energy sources preferentially renewable are vigorously considered this is the first study to examine geothermal energy prospects in southeastern nigeria with the ultimate aim of locating and mapping regions of shallow curie point depths cpds crustal thickness and geologic structures supportive of a geothermal system we estimated cpd using the spectral analysis technique on magnetic field anomalies gravity anomalies were subjected to a series of procedures to assess geological structures result reveals deeper cpds within central and southern regions of okposi afikpo and biase towns in an approximate range of 18 4 to 19 3 km shallow cpds 9 8 17 4 km have also been obtained within obubra and abakaliki regions estimated crustal thickness moho ranges between 26 5 and 35 8 km part of the region with shallow cpd correlates with regions of shallow moho depths particularly in abakaliki and deeper moho depths coincide with deeper cpd estimates at the afikpo location the estimated geothermal gradient and heat flow values range between 29 0 and 45 0 oc and 52 2 101 5 mw m2 respectively and fall within values evaluated from deep wells drilled for oil exploration within the adjoining anambra basin region this study recommends the regions with shallow depths for possible geothermal power plant location gravity data evaluations also reveal significant subsurface intrusions within ugbodo obubra and abakaliki locations overall this study is a step forward toward a better evaluation of the geothermal energy resources potential of southeastern nigeria

sedimentological aspects of the humber group upper jurassic of the south central graben uk north sea complex structure poorly understood sedimentology and poor biostratigraphic control make the upper jurassic humber group of the south central graben one of the least understood and most complex hydrocarbon reservoirs of the north sea detailed logging of available core from 19 exploration wells has been combined with an improved understanding of the relevance of trace fossils and a recognition of important base level variations to provide a greatly enhanced understanding of the depositional system active within the area at that time a new sedimentological model based upon the distribution of facies and facies associations illustrates that upper jurassic structure and consequent basin geometry were the principal controls upon the distribution of depositional environments rifting and second order transgression controlled the back stepping onlap patterns observed and higher frequency base level fluctuations controlled the internal architecture of individual sandbodies the model presented accounts for features of these deposits that were previously considered anomalous such as the thickness of bioturbated sandstones paucity of foreshore deposits and complex age relationships of sands

importance of vertical effective stress for reservoir quality in the skagerrak formation central graben north sea abstract the complex fluvial sandstones of the triassic skagerrak formation are the host reservoir for a number of high pressure high temperature hpht fields in the central graben north sea all the reservoir sandstones in this study comprise of fine grained to medium grained sub arkosic to arkosic sandstones that have experienced broadly similar burial and diagenetic histories to their present day maximum burial depths despite similar diagenetic histories the fluvial reservoirs show major variations in reservoir quality and preserved porosity reservoir quality varies from excellent with anomalously high porosities of up to 35 at burial depth of 3500 m below seafloor to non economic with porosities this study has combined detailed petrographic analyses core analysis and pressure history modelling to assess the impact of differing vertical effective stresses ves and high pore fluid pressures up to 80 mpa on reservoir quality it has been recognised that fluvial channel sandstones of the skagerrak formation in the uk sector have experienced significantly less mechanical compaction than their equivalents in the norwegian sector this difference in mechanical compaction has had a significant impact upon reservoir quality even though the presence of chlorite grain coatings inhibited macroquartz cement overgrowths across all skagerrak formation reservoirs the onset of overpressure started once the overlying chalk seal was buried deeply enough to form a permeability barrier to fluid escape it is the cumulative effect of varying amounts of overpressure and its effect on the ves history that is key to determining the reservoir quality of these channelised sandstone units the results are consistent with a model where vertical effective stress affects both the compaction state and subsequent quartz cementation of the reservoirs

coupled sm nd and u pb systematics in late caledonian granites and the basement under northern britain the british late caledonian granites display a close correspondence between inherited u pb systematics in zircons and initial end indicating that both were controlled by the age of a major component of continental crust the data confirm and refine previous models of a basement comprising young crust in the south which includes arc and oceanic crust scrapings and progressively older continental crust northwestwards across scotland

dike intrusion into unconsolidated sandstone and the development of quartzite contact zones abstract shallow near surface magma emplacement into a porous unlithified sandstone occurred by forceful dike dilation and was accompanied by a localized hydrothermal event petrographic fabric authigenic mineralogy and petrophysical data grade smoothly through the quartzite contact zone outward into the friable inmar sandstone of makhtesh ramon israel quartz grain deformation fracturing and rehealing of grains and pressure solution constitute overwhelming evidence of a compressive environment adjacent to dike margins albeit with temperatures and pressures insufficient to fully recrystallize detrital grains dikes were accommodated primarily by the repacking of the sandstone with both brittle and elastic grain interaction post emplacement heating together with elastic compressive stress dissipation lead to fracture healing and pressure solution the occasional columnar jointing must be related to thermal contraction of the quartzite during later cooling and resulting mode i cracks the alteration of both contact zone and dike material occurred via a transitory and confined hydrothermal seepage up dike margins elsewhere around the world highly altered intrusions bordered by indurated sometimes uniformly jointed sediments may also be the result of forceful magma emplacement and hydrothermal alteration at near surface levels

apatite in the ilimaussaq alkaline complex occurrence zonation and compositional variation abstract apatite from the augite syenite and the roof sequence of layered nepheline syenites pulaskite foyaite sodalite foyaite and naujaite of the ilimaussaq alkaline complex has been analysed by electron microprobe for ca p na si selected rare earth elements ree f and cl in the augite syenite chilled facies and middle zone euhedral apatite crystals are relatively abundant as inclusions in other liquidus phases the ree content is ca 1 wt ree 2 o 3 the same level is found in interstitial apatite in the coarse grained innermost facies ree enriched intercumulus apatite overgrowths are common and can have up to ca 18 wt ree 2 o 3 apatite is a common phase in the pulaskite and foyaite whereas it is rare in the sodalite foyaite and naujaite based on compositional variation and zoning it is possible to distinguish four generations of apatite in the layered roof sequence 1 primary apatite belonging to the high temperature mineral assemblage this apatite occurs in the pulaskite foyaite and sodalite foyaite as oscillatory zoned crystals whereas only remnants are found in the naujaite the ree enrichment is dominated by the ca 2 p 5 ree 3 si 4 substitution and maximum ree 2 o 3 exceeds 20 wt 2 irregularly zoned apatite occurs as rim zones on the primary apatite in the foyaite and as the dominant type of apatite in the sodalite foyaite and naujaite most irregularly zoned grains are altered primary apatite during the dissolution and recrystallisation process the composition gradually changed towards a lower ree content 3 overgrowths of homogeneous apatite on the irregularly zoned apatite in the naujaite 4 na and ree rich apatites occur in the foyaite sodalite foyaite and naujaite either as overgrowths on earlier generations of apatite or as individual crystals calculated chondrite normalised patterns show that the later generations of apatite are relatively enriched in lree and depleted in hree the most pronounced evolution is found in na and ree rich apatite apatite melt partition coefficients for la ce nd sm dy y and yb are calculated for the augite syenite and nepheline syenites of the roof sequence the coefficients show the characteristic convex shape with a maximum around nd and sm using the calculated partition coefficients and the chemical composition of the na and ree rich apatite the ree content of a late aqpaitic melt in the roof sequence is estimated the shape of the chondrite normalised pattern is similar to that of a late arfvedsonite lujavrite dyke and steeper than the patterns of the primary magma

geotechnical properties of weak sandstones weak sandstones are here defined as having a saturated uniaxial compression strength of 0 5 20 mpa the mineralogy texture density porosity and saturated moisture content of these materials have

compositional variation and structural state of columbite tantalite in rare element granitic pegmatites of the preissac lacorne batholith quebec canada

late jurassic half graben control on the siting and structure of hydrocarbon accumulations uk norwegian central graben abstract the late jurassic structure of the central graben is discussed both in terms of the extensional features developed at this time and their subsequent controls on younger inversion structures the regional kinematics of the north sea rift suggest that during the late jurassic the central graben opened approximately orthogonally to the main nnw sse fault trend within the basin some evidence that triassic extensional faults were reactivated at this time is seen towards the basin margins and a significant triassic extensional history is inferred the late jurassic basin comprises the axial c 100 km of the central graben s l the basin axis is offset progressively to the se by both transfer zones and discrete transfer faults footwall uplift of the basin margins during extension provided the source for the upper jurassic reservoir sands encountered by exploration drilling these sands were probably preferentially fed into the basin at transfer zones in the margins salt structures are ubiquitous within the central graben and their siting is everywhere controlled by the underlying fault block topography salt structures form preferentially towards the crests of half graben or buttressed against their boundary faults inversion structures in the norwegian sector reactivated with a reverse sense of displacement late jurassic extensional faults a model is proposed which shows the two main inversion structures feda graben norway tail end graben denmark to be detached on antithetic reverse faults riding in the salt these faults terminate at tip folds within major salt walls the geometry of the inversion structures can be understood only when considered together with the geometry of the underlying late jurassic half graben

fluid inclusion gas studies carrock fell tungsten deposit england implications for regional exploration a fluid inclusion investigation of the carrock fell tungsten deposit northern england confirms that the quartz wolframite scheelite veins associated with the caledonian skiddaw granite are almost exclusively related to an exocontact hydrothermal system developed at the margin of a local cupola fluid circulation as defined by the spatial variation in temperature and h2o co2 ratios for inclusions in vein quartz reveals a strong structural control the zone of maximum flow which extends 0 400 m out from the granite contact is characterised by high h2o co2 ratios and corresponds closely with the known distribution of high grade oreshoots based on the fluid inclusion gas signature for the carrock fell deposit a distinction can be made between potentially tungstaniferous quartz veins and those related to cu pb zn deposits in the absence of diagnostic ore minerals also a regional survey of quartz veins in the lake district suggests that at several localities the fluids have a close affinity with those at carrock fell this is interpreted as the high level distal expression of tungsten mineralisation at depth evidence for similar mineralisation elsewhere in the british caledonides favours those granites in the paratectonic zones of ireland and southern scotland

vertical effective stress and temperature as controls of quartz cementation in sandstones evidence from north sea fulmar and gulf of mexico wilcox sandstones abstract we present quantitative petrographic data high spatial resolution oxygen isotope analyses of quartz cement basin modelling and a kinetic model for quartz precipitation for two paleocene eocene wilcox group sandstones from texas and two jurassic fulmar formation sandstones from the central north sea at each location one sandstone has been buried to ca 145 c and one to ca 185 c a key difference between the wilcox and fulmar burial histories is that the wilcox sandstones are currently at higher vertical effective stresses and from basin modelling studies have been subjected to generally higher vertical effective stresses through their burial history the amounts of quartz cement in the wilcox sandstones are between 12 and 18 and between 2 and 6 in the fulmar sandstones high spatial resolution oxygen isotope data obtained from the quartz cements suggest temperature ranges for quartz precipitation from 60 to 80 c to values approaching maximum burial temperature factors such as grain coatings or the timing of petroleum emplacement cannot explain the differences in the amounts of quartz cement petrographic data show that most of the silica for quartz cement can be derived from intergranular pressure dissolution although the sample set is small we interpret the results to suggest that the differences in quartz cementation in fulmar and wilcox sandstones can be explained better by differences in their vertical effective stress history than their temperature history in this case the supply of silica rather than the precipitation of quartz becomes an important control on the rate and extent of cementation

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

diagenetic modeling to assess the relative timing of quartz cementation and brittle grain processes during compaction this study describes porosity reduction by brittle deformation and the application of touchstone sandstone diagenesis modeling software to assess the relative timing and interactions between grain fracturing and cement formation during burial compaction two examples from a previous study of compactional fracturing are used the oligocene frio formation gulf of mexico basin and the cambrian mount simon formation illinois basin united states grain fracturing during compaction creates intragranular fracture surfaces that are favorable sites for quartz nucleation compared to external grain surfaces that may bear coatings that inhibit the nucleation and growth of quartz cement thus the progress of brittle fracture processes during diagenesis affects quartz cementation in turn modeling of the quartz cementation process can serve to place fracturing into its proper context in burial history in the mount simon formation the extent of brittle deformation of quartz grains correlates with reconstructed effective stress at the onset of quartz cementation for frio formation samples however the extent of brittle deformation does not correlate well with reconstructed effective stress obtained using a one dimensional basin model that uses compaction disequilibrium as the dominant mechanism for overpressure generation judging from the observed degree of grain fracturing significant fluid overpressures in the frio may not have developed at the shallow depths indicated by our basin models the degree of compactional fracturing in sandstones constitutes observable evidence that can be used to decipher the complexities of pressure history

integrated study of the judy field block 30 7a an overpressured central north sea oil gas field the triassic reservoirs of the judy field an overpressured petroleum accumulation in the central north sea have been studied to determine their pressure and petroleum filling history the magnitude of overpressure in the pre cretaceous aquifer is similar across the field at about 24 mpa 3500 psi but with some higher pressure laterally in areas closest to the major depocentres pressure modelling shows that the magnitude of the overpressure can be attributed almost entirely to disequilibrium compaction and largely due to late rapid tertiary burial although modelling does require nanodarcy i e shale like permeability in the cretaceous chalk section the contributions from other mechanisms which may be relevant in this setting i e gas generation and lateral transfer appear to be small the existence of both aqueous and petroleum phases in secondary fluid inclusions in microfractures in quartz and feldspar grains allows estimation of palaeopressure the data show fracture healing from 115 to 136 c during a time when reservoir palaeopressures were above hydrostatic reconstruction of the temperature history of the reservoir places the timing of the fluid inclusion formation during fracture healing at between 3 and 1 my ago coincident with a period of rapid burial the palaeopressure estimation using fluid inclusions validates the pressure modelling studies which are derived independently from commercial basin modelling software the fluid inclusions also revealed a low gor oil over the whole field prior to late stage flooding by gas leading to the variable low to high gor fluids plus gas condensates within the field today

experimental production of clay rims by macrobiotic sediment ingestion and excretion processes using controlled experiments that simulate interlayered sand and mud in marine conditions it is shown that clay rims develop on sand grains during the digestive processes of the polychaete worm arenicola marina the clay size minerals also undergo accelerated weathering and mineral authigenesis synthetic mud crushed slate and clean eolian sand of known composition and texture were horizontally layered in experimental and control tanks natural seawater was added to these tanks along with several a marina fecal casts were collected at regular time intervals and these were analyzed texturally using electron microscopy and mineralogically using x ray diffraction xrd total bioturbation of the sand and mud layering by a marina was achieved after only eight weeks of feeding activity sand grains in the fecal casts developed clay mineral rims a feature absent in the original material and not found in the control tanks thin composite curvilinear structures assumed to be detached clay rims were also produced over a period of many months the synthetic mud proved to be unchanged in the control tank but it was significantly different in the fecal casts from the experimental tank that contained the worm a marina the original minerals were degraded and new authigenic clay minerals formed it is postulated that a combination of low ph and the biologically active microenvironment in the guts of annelid worms may radically accelerate mineral dissolution and clay mineral precipitation processes during digestion it is likely that mucous produced by the worms not only led to the aggregation of the fine grained material into fecal casts but also adhered the fine sediment onto sand grains thus creating clay mineral rims in this case the solid part of the clay rim will be a combination of the primary material crushed slate and the neoformed clay minerals

imaging pores in sedimentary rocks foundation of porosity prediction abstract this review examines the history and current practice of technologies for imaging of pores in sedimentary rocks pores are that portion of the rock volume occupied by components of relative mobility such as water of various salinities microbial life petroleum liquids gases and supercritical fluids through a rock s history the contained pores evolve as the primary detrital assemblage responds chemically and mechanically to changing conditions in the subsurface description and classification of pores based upon their paragenesis depends upon inspection by imaging that allows the pores to be seen and interpreted in the context of historical processes responsible for formation of the pore walls since the late 1960s sample preparation and imaging methodologies that clarified our understanding of pores in their correct historical context have been at the root of important advances in prediction of porosity and related bulk properties in the subsurface these older approaches are now joined by new technologies that serve current efforts to extend predictive capabilities to nm scale pores in the fine grained low permeability rocks that dominate in the deeper parts of sedimentary basins and indeed in the sedimentary record as a whole placing the smallest pores into their proper paragenetic context presents a challenge in the quest to develop predictive models for porosity evolution in fine grained rocks

packing states of ideal reservoir sands insights from simulation of porosity reduction by grain rearrangement abstract the question being tackled in this study is to which extent grain rearrangement contributes to porosity reduction in very well sorted quartzose sands ideal reservoir sands a numerical model rampage an acronym of random packing generator has been developed to address this long standing problem rampage represents a synthesis of various algorithms designed to simulate packing of equal sized spheres which have been used to represent ideal solids liquids and gases as well as natural porous media the results of rampage simulations compare favourably to theoretical and experimental data from various disciplines and allow delineation of the field of gravitationally stable random packing of equal sized spheres in the 2 d state space of porosity p versus mean coordination number n three end member packing states have been identified random loose packing rlp p 45 4 n 5 2 random close packing rcp p 36 3 n 7 0 and bridged random close packing bridged rcp p 39 5 n 5 2 unlike previously proposed models rampage can simulate the transition from rlp to any other point in the stability field the rlp state is fully consistent with wet packed porosities of synthetic sands with lognormal mass size distributions reported in the literature the much higher in situ porosity values reported for modern air packed sands are unlikely to be preserved at depth on geological time scales data on the relation between intergranular volume and burial depth indicate that the observed intergranular volume reduction in the upper 800 m of the sediment column corresponds to the evolution of rlp to rcp and is thus fully explained by non destructive grain rearrangement

a fluid inclusion model for the genesis of the ores of the dolgellau gold belt north wales the results of a detailed fluid inclusion study of a zone of auriferous quartz vein in the clogau st davids goldmine north wales are presented these show that both aqueous and an immiscible pair of h 2 o ch 4 fluids were present during the mineralizing event compositions of the immiscible fluid pair are estimated from laser raman and bulk inclusion volatile analysis data thermometry and barometry using these fluid compositions shows that the ores were formed at c 300 320 c and c 1 8 kbar a model is developed which is consistent with both the fluid inclusion data and other considerations this involves the introduction of aqueous fluid into the graphitic shales hosting the mineralized veins which proceeds to react with graphite in the wall rocks to produce h 2 o ch 4 fluids as h 2 o is consumed to produce ch 4 an increase in salinity is observed in the residual aqueous inclusion fluids deposition of gold is ascribed to destabilization of either aucl 2 or au hs 2 complexes in the incoming fluid as a result of interaction with graphite auriferous zones within the veins are defined by high ch 4 values and low co 2 ch 4 ratios in bulk inclusion analyses these effects are thought to be the result of increased fluid rock interaction in these zones related to structural features in the wall rocks

porosity depth trends in reservoir sandstones theoretical models related to jurassic sandstones offshore norway abstract theoretical models of compaction and porosity modification in sandstones during burial are discussed theory predicts that mechanical compaction is the dominant porosity reducing mechanism during burial from 0 to 2 5 3 km porosity depth trends are at this stage mainly controlled by the stability of the grain framework and the effective stress time and temperature also influence the porosity depth gradients at shallow to moderate depths but are probably less important than effective stress and framework grain composition at greater burial depths below 2 5 3 km chemical compaction including pressure solution can explain most of the porosity reduction in well sorted clean arenites theoretical modelling of the intergranular pressure solution suggests that sandstones may lose porosity rapidly by this mechanism below 2 5 3 km and that factors capable of retarding pressure solution and quartz cementation are required to maintain prospective porosities below 4 km this theory and comparisons with empirical data from jurassic sandstones from offshore norway indicate that high porosity is favoured by 1 overpressures approaching the lithostatic pressure gradient 2 a short residence time below 3 km burial 3 coatings on grains of clay or microcrystalline quartz which prevent quartz dissolution and precipitation and 4 a high content of feldspar and early diagenetic secondary pores early hydrocarbon emplacement may retard quartz cementation and preserve porosity but chemical compaction probably continues after oil emplacement except at very high hydrocarbon saturations