diagenesis and hydrocarbon accumulation brent sandstone jurassic bergen high area north sea diagenesis of brent group sandstones in the bergen high area was studied and related to the history of hydrocarbon maturation and pore fluid environment calcite and kaolinite are low temperature diagenetic precipitates that formed as meteoric pore water entered the brent at the end of brent deposition and later during late cimmerian erosion a major period of feldspar dissolution associated with quartz and illite cementation occurred at huldra field precipitation of diagenetic illite in brent sandstones was closely linked with hydrocarbon maturation in the adjacent viking graben and accumulation of generated fluids in traps at structural highs at huldra field illite k ar ages suggest onset of illite diagenesis in the latest cretaceous initial reservoiring of hydroca bons during the latest paleocene to early eocene 58 ma and filling of the reservoir by early oligocene 38 ma pore water character during illite diagenesis interpreted from oxygen isotope and quartz overgrowth fluid inclusion data was warmer than predicted by burial history analysis and one and a half to two times seawater salinity the incursion of saline isotopically evolved fluids at huldra suggests replacement of paleometeoric water by basin compaction fluid illite k ar ages from veslefrikk field indicate a minimum age of hydrocarbon accumulation at about 30 ma modern pore water chemistry in the brent at veslefrikk is unlike the suggested water chemistry at huldra and indicates that migrating compaction water did not completely displace older fresh connate water this sugge ts that hydrocarbons migrated into veslefrikk as a discrete phase without great volumes of associated compaction water

the reporoa caldera taupo volcanic zone source of the kaingaroa ignimbrites the reporoa caldera occupies the northern end of the reporoa depression previously described as a tectonic fault angle depression earlier confirmation of the topographic basin as a caldera had been hindered by the lack of an associated young pyroclastic flow deposit of large enough volume to have caused caldera collapse new exposures on the eastern margin of the reporoa basin reveal thick lithic lag breccias 30 m interbedded within the 0 24 ma kaingaroa ignimbrites these ignimbrites were previously attributed to the adjacent okataina volcanic centre lag breccia thicknesses and maximum clast sizes decrease rapidly outward from the caldera rim and discrete breccias are absent from ignimbrite sections more than 3 km from the rim the lithic lag breccias together with structural and geophysical evidence confirm reporoa caldera as the source of the c 100 km3 kaingaroa ignimbrites adding another major rhyolitic volcanic centre to the seven previously recognized in the taupo volcanic zone other older calderas may also be present in the reporoa depression

improved imaging and analysis of chlorite in reservoirs and modern day analogues new insights for reservoir quality and provenance abstract chlorite is a key mineral in the control of reservoir quality in many siliciclastic rocks in deeply buried reservoirs chlorite coats on sand grains prevent the growth of quartz cements and lead to anomalously good reservoir quality by contrast an excess of chlorite for example in clay rich siltstone and sandstone leads to blocked pore throats and very low permeability determining which compositional type is present how it occurs spatially and quantifying the many and varied habits of chlorite that are of commercial importance remains a challenge with the advent of automated techniques based on scanning electron microscopy sem it is possible to provide instant phase identification and mapping of entire thin sections of rock the resulting quantitative mineralogy and rock fabric data can be compared with well logs and core analysis data we present here a completely novel quantitative evaluation of minerals by scanning electron microscopy qemscan sem energy dispersive spectrometry eds methodology to differentiate quantify and image 11 different compositional types of chlorite based on fe mg ratios using thin sections of rocks and grain mounts of cuttings or loose sediment no other analytical technique or combination of techniques is capable of easily quantifying and imaging different compositional types of chlorite here we present examples of chlorite from seven different geological settings analysed using qemscan sem eds by illustrating the reliability of identification under automated analysis and the ability to capture realistic textures in a fully digital format we can clearly visualize the various forms of chlorite this new approach has led to the creation of a digital chlorite library in which we have co registered optical and sem based images and validated the mineral identification with complimentary techniques such as x ray diffraction this new methodology will be of interest and use to all those concerned with the identification and formation of chlorite in sandstones and the effects that diagenetic chlorite growth may have had on reservoir quality the same approach may be adopted for other minerals e g carbonates with major element compositional variability that may influence the porosity and permeability of sandstone reservoirs

4d imaging of fracturing in organic rich shales during heating to better understand the mechanisms of fracture pattern development and fluid escape in low permeability rocks we performed time resolved in situ x ray tomography imaging to investigate the processes that occur during the slow heating from 60 to 400 c of organic rich green river shale at about 350 c cracks nucleated in the sample and as the temperature continued to increase these cracks propagated parallel to shale bedding and coalesced thus cutting across the sample thermogravimetry and gas chromatography revealed that the fracturing occurring at approx 350 c was associated with significant mass loss and release of light hydrocarbons generated by the decomposition of immature organic matter kerogen decomposition is thought to cause an internal pressure build up sufficient to form cracks in the shale thus providing pathways for the outgoing hydrocarbons we show that a 2d numerical model based on this idea qualitatively reproduces the experimentally observed dynamics of crack nucleation growth and coalescence as well as the irregular outlines of the cracks our results provide a new description of fracture pattern formation in low permeability shales

workflow model for the digitization of mudrocks abstract mudrocks are highly heterogeneous in a range of physical and chemical properties including porosity and permeability fissility colour particle composition size orientation carbon loading degree of compaction and diagenetic overprint it is therefore important that the maximum information be extracted as efficiently and completely as possible this can be accomplished through high resolution analysis of polished thin sections by scanning electron microscopy sem with the collection of large area images and x ray elemental map montages and the application of targeted particle analysis a workflow model based on these techniques for the digitization of mudrocks is presented herein a range of the data that can be collected and the variety of analyses that can be achieved are also illustrated data collection is discussed in terms of inherent problems with acquisition storage transfer and manipulation which can be time consuming and non trivial similar information and resolutions can be achieved through other techniques such as qemscan and infra red ir raman spectroscopic mapping these can be seen as complementary to the workflow described herein supplementary material supplementary materials on typical workflows based on the specific software packages used herein are available at https doi org 10 6084 m9 figshare c 4191140

a renewed look at calcite cement in marine deltaic sandstones the brent reservoir heather field northern north sea uk wireline and seismic acoustic impedance imaging show that the marine part of the clastic brent group reservoir in the heather field northern north sea contains much calcite cement in the flank parts of the structure the non marine ness formation and crest parts of the structure contain negligible calcite cement this localised calcite cement has led to relatively poor reservoir performance since first oil in 1978 although a new suite of wells has boosted production with plans to keep the field active until 2030 understanding the origin and distribution of calcite cement would help the development of more realistic reservoir models and boost production rates through optimum well location we have thus used a suite of techniques including standard point counting sem eds mineralogy bse microscopy fluid inclusion thermometry and stable isotope analysis to develop new and improved models of calcite distribution calcite seems to have attributes of both early and late diagenetic cement 30 to 40 intergranular volume in calcite cemented beds seems to support pre compactional growth but high temperature fluid inclusions and the presence of primary oil inclusions suggests late growth much calcite may have developed early but it seems to have recrystallized and possibly undergone redistribution at close to maximum burial or had a late growth event calcite cement probably originated as marine derived micrite bioclasts or early marine cement but adopted the isotopic characteristics of high temperature growth as it recrystallized quartz grains have corroded outlines in calcite cemented areas with one sample with 79 calcite cement displaying signs of nearly total replacement of quartz grains by calcite the flank localisation of calcite remains to be explained although it could be due to primary depositional factors early diagenetic loss of calcite from crestal regions or late diagenetic loss of calcite from crestal regions controversially the growth of calcite seems to be associated with quartz dissolution although the geochemical and petrophysical cause of this remains obscure diagenetic loss of quartz from sandstones cannot easily be explained by conventional modelling approaches and yet seems to be an important phenomenon in heather sandstones

an experimental study of wind abrasion rocks were abraded by a sand laden air jet in the laboratory to examine the quantitative relationships between wind abrasion and wind velocity quantity and grain size of windblown sand particles and physical properties of the target rocks the results of 101 runs are expressed by an experimental equation with dimensionless constant which indicates that the rate of abrasion is directly related to the kinetic energy of the sand projectiles and inversely proportional to the compressive strength of the target rocks a tentative extension of the experimental equation to field condition leads to an equation for the wind abrasion in fields an application of the latter equation to the natural settings is preliminarily examined by comparing it with sharp s 1964 field experiments and a plausible result is obtained

synchrotron tomographic quantification of strain and fracture during simulated thermal maturation of an organic rich shale uk kimmeridge clay analyzing the development of fracture networks in shale is important to understand both hydrocarbon migration pathways within and from source rocks and the effectiveness of hydraulic stimulation upon shale reservoirs here we use time resolved synchrotron x ray tomography to quantify in four dimensions 3 d plus time the development of fractures during the accelerated maturation of an organic rich mudstone the uk kimmeridge clay with the aim of determining the nature and timing of crack initiation electron microscopy em both scanning backscattered and energy dispersive was used to correlatively characterize the microstructure of the sample preheating and postheating the tomographic data were analyzed by using digital volume correlation dvc to measure the three dimensional displacements between subsequent time heating steps allowing the strain fields surrounding each crack to be calculated enabling crack opening modes to be determined quantification of the strain eigenvectors just before crack propagation suggests that the main mode driving crack initiation is the opening displacement perpendicular to the bedding mode i further detailed investigation of the dvc measured strain evolution revealed the complex interaction of the laminar clay matrix and the maximum principal strain on incipient crack nucleation full field dvc also allowed accurate calculation of the coefficients of thermal expansion 8 10 5 c perpendicular and 6 2 10 5 c parallel to the bedding plane these results demonstrate how correlative imaging using synchrotron tomography dvc and em can be used to elucidate the influence of shale microstructure on its anisotropic mechanical behavior

mercia mudstone formation caprock to carbon capture and storage sites petrology and petrophysical characteristics porous rocks in the subsurface are now used for carbon capture and storage ccs to help ameliorate the effects of greenhouse gas emissions these porous reservoir rocks require a caprock to retain the co 2 it is important to characterize caprock quality and its stability in the presence of elevated partial pressures of co 2 lower triassic sandstones are common in the uk and nw europe and are being considered for future ccs projects the caprock to these sandstones is the middle and upper triassic mercia mudstone group we have studied the mercia mudstone using mineralogy petrology and mercury injection porosimetry to assess its caprock quality detrital minerals are dominated by quartz k feldspar illite and chlorite diagenetic pore filling minerals are dominated by calcite dolomite and gypsum in samples with abundant clay minerals there are only small quantities of pore filling diagenetic cements porosity is broadly uniform for both clay rich and clay poor samples the cleaner clay poor samples had their initial pore spaces filled with early diagenetic pore filling carbonates and gypsum despite the broadly uniform porosity mean pore throat diameter displays a strong inverse correlation with clay content whereas threshold capillary entry pressure shows a strong positive correlation with clay content the more clay rich samples represent much better caprock than the coarser grained samples that contain abundant pore filling cement the samples could support potential column heights of supercritical co 2 between 70 and 540 m and have calculated permeabilities between 10 20 and 10 19 m 2 leakage would occur only on geological time scales assuming that these samples are representative of the whole caprock because caprock quality correlates with illite content and illite will be relatively immune to elevated partial pressures of co 2 the mercia mudstone probably represents a durable caprock for future lower triassic sandstone ccs projects

fracture toughness and subcritical crack growth during high temperature tensile deformation of westerly granite and black gabbro abstract the double torsion testing method has been used to determine catastrophic and subcritical crack propagation parameters for pre cracked specimens of westerly granite and black gabbro under a number of environmental conditions the critical stress intensity factor for catastrophic crack propagation fracture toughness of granite and gabbro has been measured at temperatures from 20 to 400 c in a vacuum at 20 c the fracture toughness of westerly granite was 1 79 0 02 mpa m 1 2 and for two blocks of black gabbro it was 3 03 0 08 mpa m 1 2 and 2 71 0 15 mpa m 1 2 respectively these values are very close to those reported by other investigators for tests conducted in air of ambient humidity at room temperature for both rocks fracture toughness at first increased slightly and then decreased steadily on raising the temperature above ambient conditions this behaviour is explained in terms of the density and distribution of thermally induced microcracks as determined by quantitative optical microscopy subcritical crack growth behaviour has been studied at temperatures up to 300 c and under water vapour at pressures of 0 6 to 15 kpa both the load relaxation and incremental constant displacement rate forms of the double torsion testing method were utilised to generate stress intensity factor crack velocity diagrams crack growth was measured over the velocity range 5 10 3 to 10 7 m s 1 increasing both temperature and water vapour pressure resulted in substantially higher crack growth rates the overall effect of raising the temperature over the range studied here 20 300 c was to increase the crack growth rate in granite and gabbro by 5 and 7 orders of magnitude respectively at constant stress intensity factor and vapour pressure of water for both rocks the slopes of stress intensity factor crack velocity curves were sensitive to changes in both temperature and water vapour pressure at low values of the latter parameter slopes fell substantially on raising the water vapour pressure but were relatively insensitive to changes in temperature at these higher pressures no subcritical crack growth limit was encountered estimates of the uncertainty in our experimental data are given from the results of multiple load relaxation experiments on westerly granite specimens we estimate the uncertainty in position of stress intensity factor crack velocity curves along the stress intensity axis to be c 10 of the fracture toughness and the uncertainty in slope of such curves to be c 12 problems associated with the extrapolation of our experimental data to regions of higher effective confining pressure in the earth s crust are discussed

thermomechanical milling of accessory lithics in volcanic conduits accessory lithic clasts recovered from pyroclastic deposits commonly result from the failure of conduit wall rocks and represent an underutilized resource for constraining conduit processes during explosive volcanic eruptions the morphological features of lithic clasts provide distinctive textural fingerprints of processes that have reshaped them during transport in the conduit here we present the first study focused on accessory lithic clast morphology and show how the shapes and surfaces of these accessory pyroclasts can inform on conduit processes we use two main types of accessory lithic clasts from pyroclastic fallout deposits of the 2360 b p subplinian eruption of mount meager british columbia as a case study i rough and subangular dacite clasts and ii variably rounded and smoothed monzogranite clasts the quantitative morphological data collected on these lithics include mass volume density 2 d image analysis of convexity c and 3 d laser scans for sphericity and smoothness s shaping and comminution i e milling of clasts within the conduit are ascribed to three processes 1 disruptive fragmentation due to high energy impacts between clasts or between clasts and conduit walls 2 ash blasting of clasts suspended within the volcanic flux and 3 thermal effects we use a simplified conduit eruption model to predict ash blasting velocities and lithic residence times as a function of clast size and source depth thereby constraining the lithic milling processes the extent of shape and surface modification i e rounding and honing is directly proportional to clast residence times within the conduit prior to evacuation we postulate that the shallow seated dacite clasts remain subangular and rough due to short 2 min residence times whereas monzogranite clasts are much more rounded and smoothed due to deeper source depths and consequently longer residence times up to 1 h larger monzogranite clasts are smoother than smaller clasts due to longer residence times and to greater differential velocities within the ash laden jet lastly our model residence times and mass loss estimates for rounded clasts are used to estimate minimum attrition rates due to volcanic ash blasting within the conduit e g 12 cm3 s 1 for 25 cm clasts sourced at 2500 m depth

long term observations of transport eddies and rossby waves in the mozambique channel data from an array of current meter moorings covering a period of two and a half years are used to estimate the varying transport through the mozambique channel the total transport during this period is small below 1200 m the transport is weak but a prominent deep western boundary undercurrent with cores at 1700 and 2200 m is found that transports 1 5 sv to the north the transport shows a large temporal variability and neither a continuous upper layer western boundary current nor a continuous deep undercurrent is found the variability in the upper layer is dominated by a period of 68 days and results mainly from eddies that migrate southward through the mozambique channel in addition to this southward propagation a westward propagating signal is evident from a space time diagram of the throughflow the signal is interpreted as a mozambique channel rossby normal mode this interpretation is consistent with results from a principal oscillation pattern analysis that estimates normal modes from the data and a quasi geostrophic channel model a detailed inspection of a single eddy event shows that a precursor of an anticyclone is a strong southward current along the madagascar coast that propagates westward to the center of the channel during the westward propagation the current becomes unstable inducing an anticyclone this scenario connects the westward propagating mode with the eddy growth and explains the coincidence of the eddy and rossby mode frequency

bib sem characterization of pore space morphology and distribution in postmature to overmature samples from the haynesville and bossier shales abstract four haynesville shale and four bossier shale samples were investigated using a combination of scanning electron microscopy sem and broad ion beam bib polishing this approach enables the microstructure and porosity to be studied down to the mesopore size r in the haynesville shale to 1 79 2 26 vr r in the bossier shale this variety within the samples enabled us to study controls on the porosity distribution in these shales visible pores exist as intraparticle pores mainly in carbonate grains and pyrite framboids and as interparticle pores mainly in the clay rich matrix pores in organic matter show a characteristic porosity with respect to the type of organic matter which mainly consists of mixtures of amorphous organic matter and minerals organic laminae and discrete macerals a clear positive trend of organic matter porosity with maturity was found pore sizes are power law distributed in the range of 4 4 m to at least 36 nm in equivalent diameter the differences in power law exponents suggest that a more grain supported coarse grained matrix may prevent pores from mechanical compaction porosities measured in the bib cross sections were significantly lower in comparison to porosities obtained by mercury intrusion porosimetry mip this difference is mainly attributed to the different resolution achieved with bib sem and mip and type of pore network extrapolation of pore size distributions psds enables the bib sem porosity to be estimated down to the resolution of the mip and thus to upscale microstructural observation at the confined space of the bib sem method to bulk porosity measurement these inferred porosities are in good agreement with the mip determined porosities which underpins the assumption that pores segmented in bib sem mosaics are representative of the mip methodology

effects of sandstone provenance on reservoir quality preservation in the deep subsurface experimental modelling of deep water sand in the gulf of mexico deep water turbidite sandstone reservoirs in the gulf of mexico have been sourced from a variety of provenance terrains as a result the framework composition of each reservoir in the basin varies widely including volcanic rich litharenite oligocene vicksburg feldspathicrich lithic arkose oligocene frio metamorphic rich feldspathic litharenite palaeogene wilcox lithic poor quartzarenite miocene quartz rich sublitharenite cretaceous and quartzand feldspar rich subarkose norphlet provenance driven differences in composition have a complex but critical influence on how each of these reservoirs responds to burial induced changes in depth fluid pressure effective stress and temperature a combination of petromod and touchstone modelling programs are used in this study to simulate the influence of provenance on compaction and cementation of the main reservoir types in the gulf of mexico for example modelling results predict that at higher levels of thermal exposure some lithic rich sands although more ductile and highly compacted will experience less quartz cementation than less ductile quartz rich sands thereby preserving a higher range of porosity and permeability furthermore modelling results predict that temperature effective stress depth windows for optimal reservoir quality preservation vary widely depending on sandstone provenance the evaluation of sandstone grain composition and subsequent interpretation of sand provenance has proven to be a useful methodology for oil and gas exploration for many years specific applications of provenance studies are varied and have included the following assistance in the interpretation of basin evolution and tectonic setting dickinson 1985 help in discriminating sandstone petrofacies and source areas ingersoll 1990 interpretation of relief and climate of source areas folk 1980 help with the interpretation of palaeo drainage and sediment dispersal patterns lowe et al 2011 and help with improving stratigraphic correlation morton hurst 1995 recent workers have used provenance as a means of discriminating potential areas of basins with higher reservoir quality potential vincent et al 2010 2013 provenance reconstruction has also become a critical step in selecting the most appropriate analogues for predictive reservoir quality modelling using diagenetic modelling software such as exemplarw lander walderhaug 1999 and touchstone lander et al 2008 what has not been reported as yet is the degree to which provenance affects predictive modelling results with this in mind we pose a question could provenance related differences in sandstone composition within a single basin be large enough to significantly affect predictive modelling results this study is intended to address that question by comparing a variety of sandstone compositions across a wide range of simulated burial histories specifically sandstone compositions representing the mesozoic through tertiary gulf of mexico reservoirs of the deep water basin are used in a series of test cases this basin provides a natural laboratory for testing the effects of provenance on reservoir quality prediction and the findings from this study are intended to be used for global application provenance and reservoir quality modelling provenance transport distance climate and depositional setting all affect the initial detrital composition and to some extent the textural attributes of sandstone reservoirs at the onset of burial depositional texture and composition greatly control the rate and style of compaction and other diagenetic processes that occur during burial specific subsurface processes that are affected by initial sand composition include the following 1 mechanical compaction 2 chemical compaction 3 dissolution of unstable minerals 4 precipitation of quartz cement 5 precipitation of other cements e g carbonates zeolites sulphates from scott r a smyth h r morton a c richardson n eds 2014 sediment provenance studies in hydrocarbon exploration and production geological society london special publications 386 27 47 first published online september 12 2013 http dx doi org 10 1144 sp386 17 the geological society of london 2014 publishing disclaimer www geolsoc org uk pub ethics at conocophillips on august 28 2014 http sp lyellcollection org downloaded from

bib sem study of the pore space morphology in early mature posidonia shale from the hils area germany abstract this contribution reports on the study of the pore space morphology in two early mature vr r 0 59 and 0 61 samples of posidonia shale from the hils syncline in germany using broad ion beam bib polishing and high resolution scanning electron microscopy sem this allows imaging pores with resolution down to 10 nm in equivalent diameter d eq and quantitative estimation of porosity using a combination of bse and se detectors and semi automatic segmentation of the gigapixel images the representative elementary area of the samples on the scale of a few mm is inferred to be about 140 140 m 2 pore morphologies and pore sizes are clearly related to the mineral fabric with large differences very large typically several microns pores with internal faceted crystal morphology in recrystallized calcite clasts and smaller pores d eq cf schizosphaerella nanofossils typically 200 nm pores are less common in organic material and in pyrite aggregates pore characteristics are very similar for both samples and porosity resolvable by bib sem is 2 75 and 2 74 pore size distribution can be described by a power law with an exponent about 2 0 and 2 2 respectively for the pore population excluding the fossils pores in the carbonate fossils show dual power law distribution with power law exponents of about 1 and 3 by extrapolating the power law distribution for each sample total porosity is estimated to be 4 8 0 9 1 7 and 6 5 2 7 7 2 respectively this estimate can be compared with 3 4 3 7 and 3 3 3 6 as measured by mercury injection porosimetry we interpret this difference to reflect the unconnected for mercury part of the porosity comparison of imaged pores and mercury injection porosimetry suggest a very high pore body to pore throat ratio this results in a pore model where large pores represented mainly by pores in fossils and calcite grains are connected via a low porous and low permeable clay rich matrix with pore throats below 10 nm in both samples

making oil from magma volume title application of analytical techniques to petroleum systems acknowledgements the authors would like to thank w meredith and f bebb for constructive reviews that greatly improved this manuscript s bowden is thanked for useful discussions on earlier drafts finally we would like to thank p dowey for his editorial management of this volume funding m duffy is supported by a school of geosciences scholarship and n mark is supported by jx nippon exploration production uk as part of the volcanic margin research consortium phase 2

electrical resistivity structure of the eastern moroccan atlas system and its tectonic implications the electrical resistivity structure of the crust and upper mantle of the atlas mountain system was studied using magnetotelluric and geomagnetic deep soundings field experiments were done in eastern morocco along a traverse from the anti atlas to the rides rifaines in two campaigns in 1983 and 1988