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

how are diagenesis and reservoir quality linked to depositional facies a deltaic succession edgeoya svalbard abstract middle to late triassic strata exposed at edgeoya eastern svalbard represents an uplifted part of the northwestern corner of the barents sea this interval is characterized by a predominantly mud dominated deltaic depositional system where rocks with petroleum reservoir potential are expected both in delta front and channelized sandstone deposits recent drilling campaigns into time equivalent rocks in the barents sea have however been disappointing with porosity and permeability below expectation this study improves the current understanding of reservoir potential within depositional systems of this kind by examining the link between different depositional facies diagenesis and their impact on reservoir quality five depositional facies were mapped and correlated channel floodplain shallow marine prodelta and offshore our study suggests that diagenetic signatures that control the quality of reservoir rocks vary systematically with these depositional facies mechanical compaction was the main cause of porosity destruction in the channel shallow marine and floodplain samples while early carbonate cementation occludes the intergranular volume in the prodelta samples moreover quartz cement reduced the porosity in the shallow marine depositional facies the estimated maximum burial temperature of 124 c indicates burial to several km depths depositional settings within a deltaic to shallow marine system do not exert a strong control on the abundance of chlorite but it does impact whether the chlorite is pore filling or grain coating and porosity preserving efficient chlorite coating have aided in preserving porosities up to 32 in the very well well sorted medium to fine grained sandstones deposited by fluvial dominated channels

experimental modelling of ground deformation associated with shallow magma intrusions abstract active volcanoes experience ground deformation as a response to the dynamics of underground magmatic systems the analysis of ground deformation patterns may provide important constraints on the dynamics and shape of the underlying volcanic plumbing systems nevertheless these analyses usually take into account simplistic shapes sphere dykes sills and the results cannot be verified as the modelled systems are buried in this paper i present new results from experimental models of magma intrusion in which both the evolution of ground deformation during intrusion and the shape of the underlying intrusion are monitored the models consisted of a molten vegetable oil simulating low viscosity magma injected into cohesive fine grained silica flour simulating the brittle upper crust oil injection resulted is sheet intrusions dykes sills and cone sheets the initial topography in the models was flat while the oil was intruding the surface of the models slightly lifted up to form a smooth relief which was mapped through time after an initial symmetrical development the uplifted area developed asymmetrically at the end of the experiments the oil always erupted at the steepest edge of the uplifted area after the experiment the oil solidified the intrusion was excavated and the shape of its top surface mapped the comparison between the uplifted zone and the underlying intrusions showed that 1 the complex shapes of the uplifted areas reflected the complex shapes of the underlying intrusions 2 the time evolution of the uplifted zone was correlated with the evolution of the underlying intrusion and 3 the early asymmetrical evolution of the uplifted areas can be used to predict the location of the eruption of the oil the experimental results also suggest that complex intrusion shapes inclined sheet cone sheet complex sill may have to be considered more systematically in the analyses of ground deformation patterns on volcanoes

oil origin and accumulation in the paleozoic chepaizi xinguang field junggar basin china abstract the chepaizi xinguang area is a petroliferous region in the western junggar basin that contains several permian tight reservoirs and has a complex petroleum system and accumulation mechanism the oil sources and accumulation mechanisms are investigated by an integrated analysis of biomarker assemblages in reservoir oil samples microscopic analysis of rock samples study of fluid inclusions and simulation of the thermal evolution the results show that the permian oil in the xinguang bulge is mainly generated from the permian fengcheng formation with tricyclic terpane ratios c19 tt c23 tt lower than 0 15 c23 tt c21 tt higher than 1 0 and gammacerane c30 hopane higher than 0 6 the permian oil in the chepaizi area is mostly derived from permian wuerhe formation with c19 tt c23 tt higher than 0 15 c23 tt c21 tt lower than 1 0 and gammacerane c30 hopane lower than 0 4 the permian oils from the xinguang bulge show higher maturity compared to the permian oils from the chepaizi area possibly as a result of later oil filling the fluid inclusions and fluorescence record the main oil charging movements during the late triassic to early jurassic and the late jurassic to early cretaceous in this area and most of the generated oils in late permian are lost the unconformities between permian strata show higher porosity and more oil and gas enrichment comparatively and are the main pathways for lateral oil migration laumontite has had a considerable effect on reservoir properties laumontite dissolution creates pores that provide good oil reservoir space and favorable exploration targets

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

evaluating the mechanical compaction of quartzarenites the importance of sorting llanos foreland basin colombia abstract mechanical compaction is the main porosity reducing process in sandstones including high reservoir quality rigid grain sandstones for such sandstones the extrapolation of theoretical or experimental compaction algorithms needs calibration with rocks having well constrained burial histories evaluating the compaction of these rocks is achieved by comparing current intergranular volume igv with depositional igv which is strongly dependent on sorting however because sandstone sorting is difficult to measure accurately its impact on depositional porosity and compaction state is largely underestimated we use the quartzarenites of the oligocene carbonera formation in the subsurface of the hydrostatically pressured llanos basin to illustrate the importance of sorting when evaluating the compaction of rigid grain sandstones igv and sorting were measured in core samples using a combination of transmitted light and cathodoluminescence images resulting in improved accuracy over standard procedures the compaction state of clean quartzarenites at given depths is best described using igv versus sorting plots which are used to derive compaction curves for specified sorting values the igv versus sorting trends are displaced to lower igv values with increasing burial depth the differences in igv caused by differences in sorting exceed the differences in igv resulting from 1000 m of burial illustrating the high impact of sorting when evaluating compaction contrasting with published experimental results the compaction of the llanos basin ductile grain poor quartzarenites is independent of grain size and grain rearrangement is the main compaction mechanism during the first 1 6 km of burial based on the llanos data we have generated igv versus depth curves for clean pure quartzarenites of specific sorting which can be used to predict their maximum primary porosity up to moderate burial depths differences with other published burial curves are probably related to unaccounted variations in sorting ductile grain content and framework strengthening cements however the llanos basin quartzarenites contain virtually no cements explaining their high degree of compaction relative to other rigid grain sandstones and making them ideal to isolate the effects of compaction on the igv of quartzarenites the llanos basin data suggest that below 2 5 km of depth clean well to moderately well sorted quartzarenites continue reducing their igv by mechanical compaction below the 26 limit which should apply only to extremely well sorted rigid grain uncemented sandstones

evidence for reduced quartz cementation rates in oil filled sandstones when quartz rich sands are buried and heated pore space is gradually filled by precipitation of quartz cement from aqueous formation fluids here we examine whether the presence of oil in the pore space can retard or halt this loss of porosity by slowing or stopping quartz cementation the effect of oil fill on quartz cementation is examined by using the distribution of quartz cement in the brae formation deep water sandstone reservoir of the miller oil field north sea petrographic data demonstrate that sandstones from the oil zone have much less quartz cement and more porosity than sandstones from the water zone sandstones in both oil and water zones are compositionally and texturally identical and have been affected by a similar burial history kinetic modeling of the cementation process suggests that progressive oil charging has slowed quartz cement growth rates by at least two orders of magnitude halting it completely in the most extreme cases our data demonstrate that early oil charging in the crestal part of an anticline can preserve porosity in deeply buried sandstones this knowledge is especially relevant to porosity prediction for petroleum exploration in deeply buried sandstones

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

origin and significance of redistributional secondary porosity abstract redistributional secondary rds porosity is created by the dissolution of framework grains and cements and the simultaneous precipitation of reaction products in the nearby pore space aluminosilicate minerals and in particular feldspars are the usual precursors of rds porosity in sandstones common product minerals are authigenic quartz and clay minerals usually kaolinite this type of secondary porosity is almost ubiquitous in sandstones rds porosity has little effect on the total porosity of reservoir rocks and contrary to popular opinion cannot give rise to higher than expected porosities at depth petrographic criteria alone are an insufficient measure that the rock has undergone leaching resulting in porosity enhancement to demonstrate porosity enhancement due to leaching it is necessary to show that the porosity of the rock is statistically higher than would otherwise be the case for a rock of similar characteristics that has not been leached the dissolution precipitation reactions that give rise to rds arise from the inherent thermodynamic instability of minerals such as the feldspars and other aluminosilicates which formed under magmatic or metamorphic conditions when these minerals dissolve they add aluminium and silica to the pore fluid however the concentration of aluminium and silica in the pore fluid is buffered by clay minerals and quartz respectively during burial diagenesis thus any increase in the concentration of these components leads to the precipitation of an authigenic clay mineral and authigenic quartz carboxylic acids are claimed to have the capacity to complex aluminium and so increase the solubility of aluminium and eliminate the precipitation of a clay phase however they are unlikely to have any major effect on aluminium solubility in natural i e complex pore fluids in fact dissolution will occur in acid neutral or alkaline conditions at a rate that is kinetically controlled reaction rates speed up exponentially with increasing temperature and therefore depth resulting in the destruction of more and more unstable components this increase in the rate of destruction of unstable components at depth can give rise to what appears to be a late leaching event what is seen in fact is the natural culmination of the destruction of thermodynamically unstable phases at an ever increasing rate during burial thus for a rock of a given composition the fraction of the total porosity that is secondary will increase with burial depth until all labile components are consumed the behaviour of mineral mixtures in the presence of a moving pore fluid is an additional contributing factor to the ubiquity of rds porosity where two or more minerals add the same ion to the pore fluid then the steady state concentration of that ion which is reached at some distance down the flow path theoretically an infinite distance is a weighted average of the equilibrium solubilities of that ion with respect to the minerals that contribute the ion thus as the pore fluid approaches a steady state concentration of the ion it will be undersaturated with respect to the least stable minerals and oversaturated with respect to the most stable mineral hence rds porosity must form as a result of the dissolution of the least stable mineral and the precipitation of the most stable mineral although kaolinite is the most common dissolution product of feldspars with increasing temperature its stability field shrinks with respect to other clay minerals such as illite thus it is possible for feldspars and other aluminosilicates to decompose at depth to give illite and authigenic quartz the growth of fibrous illite together with the change from a dominantly primary to a dominantly secondary pore structure can give rise to a rapid loss of permeability with depth where changes in the clay mineral type do not occur the impact of rds porosity on the porosity permeability behaviour of sandstones is minimal up to secondary porosities of about 10 beyond this figure there is at present too little data to make a clear statement

petroleum generation and charge history of the northern dongying depression bohai bay basin china insight from integrated fluid inclusion analysis and basin modelling abstract the petroleum generation and charge history of the northern dongying depression bohai bay basin was investigated using an integrated fluid inclusion analysis workflow and geohistory modelling one and two dimensional basin modelling was performed to unravel the oil generation history of the eocene shahejie formation es3 and es4 source rocks based on the reconstruction of the burial thermal and maturity history calibration of the model with thermal maturity and borehole temperature data using a rift basin heat flow model indicates that the upper interval of the es4 source rocks began to generate oil at around 35 ma reached a maturity level of 0 7 ro at 31 30 ma and a peak hydrocarbon generation at 24 23 ma the lower interval of the es3 source rocks began to generate oil at around 33 32 ma and reached a maturity of 0 7 ro at about 27 26 ma oil generation from the lower es3 and upper es4 source rocks occurred in three phases with the first phase from approximately 30 20 ma the second phase from approximately 20 5 ma and the third phase from 5 ma to the present day the first and third phases were the two predominant phases of intense oil generation samples from the es3 and es4 reservoir intervals in 12 wells at depth intervals between 2677 7 m and 4323 0 m were investigated using an integrated fluid inclusion workflow including petrography fluorescence spectroscopy and microthermometry to determine the petroleum charge history in the northern dongying depression abundant oil inclusions with a range of fluorescence colours from near yellow to near blue were observed and were interpreted to represent two episodes of hydrocarbon charge based on the fluid inclusion petrography fluorescence spectroscopy and microthermometry data two episodes of oil charge were determined at 24 20 ma and 4 3 ma respectively with the second episode being the predominant period for the oil accumulation in the northern dongying depression the oil charge occurred during or immediately after the modelled intense oil generation and coincided with a regional uplift and a rapid subsidence suggesting that the hydrocarbon migration from the already overpressured source rocks may have been triggered by the regional uplift and rapid subsidence the expelled oil was then charged to the already established traps in the northern dongying depression the proximal locations of the reservoirs to the generative kitchens and the short oil migration distance facilitate the intimate relationship between oil generation migration and accumulation

investigations of soil cracking and preferential flow in a weighing lysimeter filled with cracking clay soil an improved understanding of deep drainage processes in irrigated cracking soils is needed for sustainable irrigation management to investigate the effect of crack dynamics and macropore flow on drainage in cracking soils a series of irrigation experiments was carried out in a weighing lysimeter subsurface soil cracks of the initially very dry soil were investigated with a videoscope and changes in the surface expression of cracks in response to irrigation events were monitored by time lapse photography a bromide tracer was applied to one irrigation event variations in the combined soil and moisture mass and the volume of drainage out of the soil column was logged and the drainage ec and bromide content were determined no drainage occurred out of the soil column during the first 3 out of 6 irrigation events even though substantial surface runoff into the cracks occurred and at least initially soil cracks provided an uninterrupted flow path through the profile the breakthrough of the bromide tracer as well as an initially low ec of the drainage water indicate that preferential flow accounted for a substantial part of the first of the two drainage events even though the soil cracks were sealed on the surface at the onset of the irrigation causing the drainage the results show that lateral infiltration of macropore flow into the soil matrix can be substantial and should not be neglected while simulating macropore flow and deep drainage in cracking soils the results also indicate that soil cracks can remain pathways for preferential flow even after they are closed at the soil surface the type of water application appears to have an impact on the location of crack formation with flood irrigation favouring reappearance of cracks at previous crack locations and simulated rainfall resulting in shifting crack locations

carbon dioxide injection and associated hydraulic fracturing of reservoir formations the storage potential of subsurface geological systems makes them viable candidates for long term disposal of significant quantities of co2 the geo mechanical responses of these systems as a result of injection processes as well as the protracted storage of co2 are aspects that require sufficient understanding a hypothetical model has been developed that conceptualises a typical well reservoir system comprising an injection well where the fluid co2 is introduced and a production abandoned well sited at a distant location this was accomplished by adopting a numerical methodology discrete element method specifically designed to investigate the geo mechanical phenomena whereby the various processes are monitored at the inter particle scale fracturing events were simulated in addition the influence of certain operating variables such as injection flow rate and fluid pressure was studied with particular interest in the nature of occurring fractures and trend of propagation the pattern and magnitude of pressure build up at the well vicinity pressure distribution between well regions and pore velocity distribution between well regions modelling results generally show an initiation of fracturing caused by tensile failure of the rock material at the region of fluid injection however fracturing caused by shear failure becomes more dominant at the later stage of injection furthermore isolated fracturing events were observed to occur at the production abandoned wells that were not propagated from the injection point this highlights the potential of co2 introduced through an injection well which could be used to enhance oil gas recovery at a distant production well the rate and magnitude of fracture development are directly influenced by the fluid injection rate likewise the magnitude of pressure build up is greatly affected by the fluid injection rate and the distance from the point of injection the dem modelling technique illustrated provides an effective procedure that allows for more specific investigations of geo mechanical mechanisms occurring at subsurface systems the application of this methodology to the injection and storage of co2 facilitates the understanding of the fracturing phenomenon as well as the various factors governing the process

doppler radar analysis of a low precipitation severe storm abstract this is a detailed study of the structure and evolution of a convective storm that formed along the dryline in oklahoma on 26 april 1984 and developed into a supercell dual doppler wind analyses analyses of surface mesonetwork and instrumented tower data storm intercept photographs analyses of raingage data from a dense network and storm reports are presented and discussed the storm exhibited the following low precipitation lp characteristics the absence of any strong evaporatively cooled storm scale downdraft near the surface a paucity of rain reaching the ground large hail long lived rotating updrafts and a strong relatively small in areal extent unicellular radar echo the storm split and its right moving member developed into an isolated high precipitation supercell it is suggested that the region under cloud base was associated with strong radar echo caused by widely spaced large water droplets and hailstones separate cyclonically and anticyclonically rotating updrafts

reply to the discussion by au and yeung on forms and sand transport in shallow hydraulic fractures in residual soil appears in canadian geotechnical journal 44 1474 1475 au and young have studied our paper carefully and we appreciate both their interest in our work and the opportunity to address some of the questions that arose from their study we would like to thank them for pointing out their papers and dissertations and describing results that resemble those from our work a significant body of research over the last 50 years describes studies of hydraulic fractures in the laboratory hubbert and willis 1957 were among the first and the papers by au and yeung and their colleagues are solid contributions to this effort however our paper focused on a field study and we decided to restrict the discussion of laboratory studies to maintain focus on the field problem the effects of external boundaries must be carefully considered when evaluating the form of hydraulic fractures created in the laboratory for example our results along with those of many other investigators suggest that hydraulic fractures curve upward when they interact mechanically with an overlying stress free surface the apparatus used by au et al e g fig 2 au et al 2003 shows rigid plates confining the top and bottom of the sample along with rigid walls around it no stress free surface appears to be present all of the papers cited by au and yeung seem to use this same apparatus their apparatus is nicely designed and certainly well suited for studying some aspects of hydraulic fractures and we do not dispute the importance of the findings described in their papers however the boundary conditions imposed by the upper and lower plates were different than the stress free conditions assumed to influence fracture form at shallow depths in the field the difference in boundary conditions made it difficult for us to assess the relevance of the forms of the fractures they created in the laboratory to the ones we created in the field we avoided this comparison in the paper as a result it is worth noting that experimental techniques were described by galland et al 2003 for evaluating interactions between hydraulic fractures and free surfaces in silt sized material with high lateral stresses we only recently learned about this work and regrettably did not cite it in our paper au and yeung raised several other good points some of their questions can be answered by referring to the theses that form the basis of our paper we are aware that it can be difficult to access the theses and dissertations through normal channels so to address this issue we have posted the theses written by tan 2003 richardson 2003 and malin 2005 at http www ces clemson edu hydro murdoch student 20project index htm that site also has photographs and other supporting material for the purposes of clarity the comments of au and yueng will be summarized into several questions before presenting a reply why did we create hydraulic fractures in the b horizon of the soil the hydraulic fractures were created in the b horizon primarily because this material is relatively homogeneous and isotropic compared with the underlying saprolite which has a banded fabric as a weathered relic from the original gneiss the gneiss is strongly folded and the orientation of the mineral banding in the saprolite can be spatially variable we wanted to conduct experiments in material with relatively uniform properties because we were interested in evaluating the forms of the fractures independent from the influence of major heterogeneities and complex structures which could possibly dominate the fracture form this

variations in cloud to ground lightning characteristics among three adjacent tornadic supercell storms over the tennessee valley region the contrasting behavior of cloud to ground cg lightning associated with three adjacent supercell thunderstorms observed on 18 may 1995 is examined thunderstorm characteristics and anvil interactions are related to north south variations in cg lightning properties while tornadic activity was not consistently related to variations in cg properties radar reflectivity factor area greater than 65 db z was generally inversely related to cg frequency it is hypothesized that suppression of cg activity was produced by reduction of large number concentrations of precipitation sized particles i e presence of large hail in the particle interaction mixed phase region it is further hypothesized that seeding from upstream storm anvil ice was associated with nearly coincident enhancement of cg activity in downstream storms likewise it is hypothesized that the reduction in 65 dbz echo area inferred suppression of hail is related to this inferred anvil seeding process

muddy and dolomitic rip up clasts in triassic fluvial sandstones origin and impact on potential reservoir properties argana basin morocco abstract the significance of rip up clasts as sandstone framework grains is frequently neglected in the literature being considered as accessory components in bulk sandstone composition however this study highlights the great value of muddy and dolomitic rip up clast occurrence as a information source about low preservation potential from floodplain deposits and b key element controlling host sandstone diagenetic evolution and thus ultimate reservoir quality high resolution petrographic analysis on triassic fluvial sandstones from argana basin t6 and t7 t8 units highlights the significance of different types of rip up clasts as intrabasinal framework components of continental sediments from arid climates on the basis of their composition and ductility three main types are distinguished a muddy rip up clasts b dolomitic muddy rip up clasts and c dolomite crystalline rip up clasts spatial distribution of different types is strongly facies related according to grain size origin of rip up clasts is related to erosion of coeval phreatic dolocretes in different development stages and associated muddy floodplain sediments cloudy cores with abundant inclusions and clear outer rims of dolomite crystals suggest a first replacive and a subsequent displacive growth respectively dolomite crystals are almost stoichiometric this composition is very similar to that of early sandstone dolomite cement supporting phreatic dolocretes as dolomite origin in both situations sandstone diagenesis is dominated by mechanical compaction and dolomite cementation a direct correlation exists between 1 muddy rip up clast abundance and early reduction of primary porosity by compaction with irreversible loss of intergranular volume igv and 2 occurrence of dolomitic rip up clasts and dolomite cement nucleation in host sandstone occluding adjacent pores but preserving igv both processes affect reservoir quality by generation of vertical and 3d fluid flow baffles and barriers that compartmentalize the reservoir these findings may provide quantitative useful data for the better understanding of reservoir quality in analogous hydrocarbon bearing basins such as the bay of fundy nova scotia canada

impacts of lithologic characteristics and diagenesis on reservoir quality of the 4th member of the upper triassic xujiahe formation tight gas sandstones in the western sichuan basin southwest china abstract the 4th member of the upper triassic xujiahe formation has become a primary target for tight gas exploitation in the western sichuan basin in past decades finding relatively high porosity reservoirs in tight sandstones is significant for successful tight gas exploration in this study the xu4 sandstone was studied by a variety of experimental methods including core observation thin section and cathodoluminescence observation porosity and permeability measurement mercury intrusion scanning electron microscopy x ray diffraction electron probe micro analyzer and fluid inclusion to determine the reservoir characteristics and also diagenesis evolution and to discuss their impacts on the reservoirs the xu4 sandstones are mainly litharenites feldspathic litharenites and sublitharenites among these calcarenaceous sandstone cs sandstone and sandstone with a high kaolinite content ks sandstone are classified by their special compositions the reservoir quality of the xu4 sandstone is poor and among the five types of sandstone the reservoir quality of the ks sandstone is generally better than that of the other types the diagenetic variations of the xu4 sandstone are complicated by compaction kaolinite and quartz cementation two stages of carbonate cementation and two stages of dissolution of feldspar and some unstable rock fragments the ks sandstone is the product of feldspathic litharenite that underwent early stage dissolution of feldspar during eodiagenesis because of the acidic fluids in the sandstone this process resulted in a low k h ratio of the pore fluids i e acidic fluids derived from atmospheric fresh water and fluids of adjacent strata bearing coal seams the determination of the diagenetic evolution indicated that the poor reservoir quality can be attributed to strong compaction or extensive cementation mainly calcite cementation however the occurrence of relatively high porosity reservoirs varies among the sandstones for cs sandstone weak early carbonate cementation is essential for sublitharenite feldspathic litharenite and litharenite late dissolution of feldspar and less cement are necessary for the formation of relatively high quality reservoirs due to their high porosity almost all of the ks sandstones are relatively high quality reservoirs

experimental mechanical compaction of reconstituted shale and mudstone aggregates investigation of petrophysical and acoustic properties of sw barents sea cap rock sequences abstract this study investigates petrophysical and acoustic properties of experimentally compacted reconstituted samples of seal sequences from the southwestern barents sea the aggregates were collected from drill cuttings of mudstone and shale formations of two exploration wells 7220 10 1 salina discovery and 7122 7 3 goliat field the washed and freeze dried samples were characterized for grain size distributions geochemical analyses and mineralogical compositions a total of 25 compaction tests 12 dry and 13 brine saturated were performed with a maximum effective vertical stress of 50 mpa the laboratory measurements demonstrated that petrophysical and acoustic properties of argillaceous sediments can change within a sedimentary basin and even within a given formation the results show that the collected aggregates from goliat field are compacted more compared to salina discovery the maximum and minimum compaction are measured in samples collected from snadd and fuglen formations respectively the final porosity of brine saturated specimens varies between 5 and 22 the ultrasonic velocity measurements depict that samples with the same porosity values can have a broad range of velocity values the resulting compaction trends in this study were compared to published compaction curves for synthetic mixtures of quartz and clay all compaction trends show higher porosity reduction than the silt fraction with 100 quartz comparison of experimental compaction result of each mudstone and shale aggregate with its corresponding acquired well log data helps to delineate the burial history and exhumation in the study area a net exhumation of 950 m and 800 m is estimated at salina and goliat wells respectively the outcomes of this study can provide insights for hydrocarbon prospect discovery in a pre mature sedimentary basin in terms of exploration and production and also for geological co2 storage sites the experimental results may provide information for well log and seismic interpretation basin modeling and seal integrity of investigated horizons

diagenetic controls on evolution of porosity and permeability in lower tertiary wilcox sandstones from shallow to ultradeep 200 6700 m burial gulf of mexico basin u s a abstract reservoir quality is a critical risk factor in deep to ultradeep reservoirs at depths 4 5 km analysis of paleogene wilcox sandstones on the upper texas gulf coast provides insight into the evolution of reservoir quality during shallow to ultradeep burial diagenesis reduction of porosity and permeability with burial in wilcox sandstones was evaluated using subsurface samples from 200 to 6700 m at temperatures of 25 230 c diagenesis and petrophysical properties were interpreted from petrographic data and core analyses wilcox sandstones are mostly lithic arkoses and feldspathic litharenites having an average composition of q59f22r19 provenance did not change significantly during wilcox deposition in this area nor does average sandstone composition vary among lower middle and upper wilcox sandstones however composition does vary with sequence stratigraphic position lowstand slope fan deposits contain more rock fragments than do deposits from highstand or transgressive systems tracts given observations from this onshore dataset wilcox sandstones deposited in deepwater environments in the gulf of mexico are likely to contain more rock fragments than their linked highstand equivalents pore types in wilcox sandstones change from a mix of primary and secondary pores and micropores at shallower depths to predominantly secondary pores and micropores in deeper sandstones wilcox sandstones show steady porosity reduction from an average of 33 at 38 c to 12 at 132 c with little additional loss at higher temperatures by 132 c most primary pores have been lost by mechanical compaction or occluded by quartz cementation volume of secondary pores remains nearly constant despite dissolution of 3 5 average potassium feldspar during deep burial volume of this late dissolution of potassium feldspar is offset by precipitation of ankerite albite illite and minor quartz in secondary pores the proportion of microporosity which occurs in authigenic clays altered grains and matrix increases in the deepest sandstones change in proportion of pore types and reduction in total porosity with increasing temperature changes porosity permeability transforms because most deep to ultradeep wilcox sandstones are dominated by secondary pores and micropores porosity permeability transforms have lower slopes than those for shallower sandstones

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

genetic mechanism of high quality reservoirs in permian tight fan delta conglomerates at the northwestern margin of the junggar basin northwestern china fan delta conglomerate reservoirs in the permian jiamuhe formation in the zhongguai area at the northwestern margin of the junggar basin northwestern china are reservoirs for large accumulations of natural tight gas the tight conglomerates and sandstones are mainly litharenites with a large amount of texturally and compositionally immature volcanic clastic materials core porosities demonstrate the development of anomalously high porosities at depths of 3200 4200 m 10 500 13 800 ft and 4500 4900 m 14 800 16 100 ft the permian reservoirs experienced initial rapid subsidence uplift and further subsidence and diagenetic reactions occurred in these reservoirs involving compaction precipitation of chlorite clays calcites zeolites iron oxides kaolinite and quartz cements and dissolution of unstable minerals including analcite laumontite feldspars and rock fragments low porosity reservoirs with extensive compaction and or cementation of calcite and heulandite ca consist of only few visible secondary pores formed by dissolution of feldspars and rock fragments reservoirs with anomalously high porosity however experienced relatively weak compaction and contain significant amounts of secondary pores formed by dissolution of mainly laumontites some feldspars and rock fragments comprehensive studies of sedimentary features and diagenesis of the reservoirs indicate that these anomalously high porosities originate from chronological coupling of four important geological processes 1 sedimentary facies and detrital compositions controlled distribution of various zeolites in reservoirs with abundant laumonites developed in reservoirs in the fan delta front subfacies 2 early precipitation of the laumontites inhibited compaction during deep burial however they provided unstable minerals for secondary porosity development 3 the permian reservoirs experienced subaerial exposure erosion during the uplift stage and meteoric freshwater leached laumontite cements and aluminosilicate grains to form secondary pores in reservoirs beneath the unconformity 4 hydrocarbon emplacement at relative shallow depth during a second subsidence period preserved the secondary pores in reservoirs by retarding late carbonate cementation

airborne doppler radar analysis of supercells during cops 91 abstract on 26 may 1991 noaa p 3 airborne doppler radar data were collected near two tornadic supercells in the southern plains during the cooperative oklahoma profiler studies cops 91 field program the 3 cm radar mounted in the tail of the aircraft was operated using the fore aft scanning technique fast both storms were sampled just minutes after each had produced a tornado the cops 91 storms are the first tornadic supercells to be sampled extensively by airborne doppler radar using the fast methodology pseudo dual doppler analyses of a dissipating storm in southwest kansas show no remaining low level circulation even though the storm had just produced a tornado the analyses of a storm in northwest oklahoma reveal better defined features in the wind field near the surface in contrast to what has been previously observed in post tornadic supercells the cyclonic vorticity in both storms was greater aloft than at low levels the 26 may 1991 storms provide further evidence that supercells often

analytical model of surface uplift above axisymmetric flat lying magma intrusions implications for sill emplacement and geodesy abstract in this paper we develop a new axisymmetric analytic model of surface uplift upon sills and laccoliths based on the formulation of a thin bending plate lying on an elastic foundation in contrast to most former models also based on thin bending plate formulation our model accounts for i axi symmetrical uplift ii both upon and outside the intrusion the model accounts for shallow intrusions i e the ratio a h 5 where a and h are the radius and depth of the intrusion respectively the main parameter of the model is the elastic length l which is a function of the elastic properties of the bending plate and of the elastic foundation the model exhibits two regimes depending on the ratio a l when a l a l 5 the uplift is mostly restricted upon the intrusion when the elastic foundation is very stiff our model converges towards that of a clamped plate we provide as supplementary material a matlab function that calculates the surface uplift from the set of system and control parameters we discuss three possible applications of our model i the model can be used to describe sill propagation by introducing a propagation criterion for realistic values our model reproduces well the behavior of horizontal intrusions simulated in experiments ii the model can also be used to compute the critical size of saucer shaped sills it shows for instance that a soft elastic foundation favors the horizontal spreading of sills before they form inclined sheets iii we show that the classical mogi point source model cannot be used to constrain sill properties from the surface uplift we thus propose that our model can be used as a valuable alternative to both simple analytical models like mogi s and more complex numerical models used to analyze ground deformation resulting from sill intrusions in active volcanoes

evolution of cloud to ground lightning characteristics and storm structure in the spearman texas tornadic supercells of 31 may 1990 abstract on 31 may 1990 four tornadic supercell storms formed sequentially near the intersection of a dryline and an outflow boundary in the northern texas panhandle staccato lightning flashes which have been hypothesized to be positive ground flashes were observed beneath the anvil of one storm during the most violent tornado that the storm produced evidence was found from a lightning mapping system that at least some of the staccato flashes were negative ground flashes although the four supercell storms on this day formed in approximately the same area traveled over roughly the same region and produced tornadoes and large hail the relationship between the genesis and evolution of tornadoes and the polarity and flash rates of ground flashes varied widely as in previous studies the second of the supercell storms had low precipitation supercell characteristics the third and fourth did not in previously studied storms ground flash activity in low precipitation supercell storms has always bee

interaction mechanism between deep mixing column and surrounding clay during installation laboratory and field investigation has shown that the shear strength of the surrounding clay increased in the vicinity of a deep mixed dm column in the short period following installation field observations indicated that significant excess pore pressure which may be higher than the hydraulic fracture pressure was generated during installation the behaviour between a dm column interacting with the surrounding clay during installation can be simulated as the shearing expanding process of a cylindrical cavity a simple approach is proposed to calculate the excess pore pressure around a dm column during installation the proposed method of calculation considers the effect of a shearing force caused by rotating blades during mixing in the proposed approach the excess pore pressure is expressed in terms of undrained strength of clay shearing force injection pressure and a pore pressure parameter an approach is developed to analyze clay fracturing using a tensile failure mechanism analytical results

relationship of temperature to reservoir quality for feldspathic sandstones of southern california abstract the diagenetic alteration of a sandstone results from the combined effects of many factors in order to ascertain the role played by a particular diagenetic factor the remaining diagenetic factors must be held constant for the sandstones of the basins of southern california we have found that the effects of almost all of the principal diagenetic factors are essentially uniform the notable exception is thermal history because of this we have been able to evaluate the diagenetic imprint of temperature upon the sandstones measured reservoir property data taken on core samples of reservoir sandstones from 16 fields in the los angeles ventura and san joaquin basins were used to determine the average rate of porosity and permeability loss with depth for each field a s raight line appears to be the proper representation for the porosity versus depth profiles for the interval of interest the slope of this line is defined here as the porosity gradient porosity gradients for the fields investigated range from 1 1 to 5 8 per 1 000 ft a direct relationship exists between the porosity gradient and the present geothermal gradient for the 16 fields that have been examined as geothermal gradient increases porosity gradient increases the correlation coefficient between these variables is 0 916 for geothermal gradients between 1 6 f and 2 2 f 100 ft a similar relationship also exists between the rate of permeability loss with depth and the geothermal gradient but the average deviation from the mean permeability value is so great the relations ip has little practical significance end of article last page 250

occurrence composition and origin of analcime in sedimentary rocks of non marine petroliferous basins in china abstract natural analcime a highly brittle aluminosilicate mineral that dissolves easily during diagenesis occurs in the sedimentary rocks of eight main non marine petroliferous basins in china this mineral has been well documented for the past 30 years and has proved to be significant for the formation of high quality hydrocarbon reservoirs the present study fully reviews analcime occurrences chemical compositions si al ratios origin and controlling factors and influence on the hydrocarbon reservoir quality in permian to paleogene sedimentary rocks previous research has shown that analcime is commonly distributed within argillaceous laminae l type occurs as intergranular cement c type replacement r type and crack filling f type forms or is closely intergrown with other authigenic exhalative minerals a type analcime in lacustrine basins is slightly al rich and na poor and its si al ratios range from 1 95 to 2 80 the geochemical compositions of the analcime and ankerite bearing exhalites are characterised by enrichment in light rare earth elements and 13c and depletion in heavy rare earth elements and 18o with negative eu and positive ce anomalies diagenetic analcime forms by the alteration of volcanic glass clays or zeolite precursors in saline alkaline lakes or during buried diagenetic stages in sedimentary rocks in addition it coexists with other authigenic zeolite carbonate and clay minerals as well as albite hydrothermal analcime presumably precipitates from hydrothermal fluids and is not dependent on a local precursor mineral or volcanic glass this type is commonly related to exhalative mineralisation of hydrothermal springs the formation of natural analcime is controlled by various factors such as the composition of the parent material temperature prevalent during crystallisation chemistry of the fluid phase and thermodynamic characteristics of the geological environment as zeolites are formed in alkaline fluid phases they become unstable in acidic pore water secondary pores created by analcime mineral dissolution and intercrystalline micropores among analcime minerals are highly significant in hydrocarbon reservoirs analcime bearing mudstone siltstone dolostone and exhalite are favourable lithofacies for unconventional hydrocarbon accumulation owing to its high brittleness and selective dissolution of analcime and other associated minerals

forms of hydraulic fractures in shallow fine grained formations hydraulic fractures have been created in fine grained formations at depths of 2 10 m to improve the performance of environmental remediation projects at dozens of locations and in a wide range of geologic conditions the effectiveness of a hydraulic fracture during remediation will depend primarily on its form that is its shape thickness orientation length width and location with respect to the borehole the forms of many hydraulic fractures have been determined by mapping exposures in excavations and by compiling split spoon sampling data these observations indicate that a typical hydraulic fracture at shallow depths is gently dipping slightly elongate in plan and slightly asymmetric with respect to the parent borehole shallow hydraulic fractures lift the ground surface to produce gentle domes and the pattern of uplift reflects the location and thickness of the fracture at depth the forms of hydraulic fractures created over a narrow range of depths at the same site are similar but the forms

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

reservoir quality modeling of tight gas sands in wamsutter field integration of diagenesis petroleum systems and production data the late cretaceous almond formation is a lithic rich tight gas sandstone reservoir in wamsutter field wyoming the almond has measured porosities of less than 12 and permeabilities that are generally less than 0 1 md this is the result of significant mechanical and chemical compaction precipitation of carbonate cements and authigenic clays and deep burial cementation by quartz despite the advanced diagenesis and poor reservoir quality of this tight gas sandstone touchstone diagenetic modeling was used successfully to simulate rock properties during burial basin modeling and fluid inclusion data were integrated with touchstone simulations to constrain the timing of petroleum charge into the reservoir model results indicate that the almond was initially charged by liquid petroleum when the reservoir retained high porosity and permeability subsequent gas charging flushed most of the liquids from the system as the reservoir continued to experience porosity and permeability reduction from continued quartz cementation the almond touchstone model was applied to 15 well locations throughout the field ranging from areas of low thermal stress to high thermal stress a comparison of touchstone predicted permeability and standard thermal stress yielded a correlation that was used to build a pseudopermeability reservoir quality risk map across the wamsutter area a comparison of this map with historical production data demonstrates that the map successfully predicts areas of enhanced well performance the method used to build this risk map may be applied elsewhere as a quick means of high grading areas of risk during field development

forms and sand transport in shallow hydraulic fractures in residual soil four sand filled hydraulic fractures were created at a depth of 1 5 m and the vicinities of the fractures were excavated and mapped in detail all the fractures were shaped like slightly asymmetric saucers between 4 5 and 7 0 m across that were roughly flat lying in their centers and curved upward to dip between 10 and 15 along their peripher ies three different colors of sand were injected in sequence to trace the relative ages of the sand in the fracture the first sand to be injected remained in the vicinity of the injection casing whereas the last sand moved rapidly to the leading edge sand transport occurred through localized channel like pathways that extended from the injection casing and then branched into multiple paths as they approached the leading edge at least four branching pathways of differ ent ages were identified in one fracture suggesting that this represents a fundamental mechanism of sand transport in these shallow fractures a theoretical model was developed by adapting franc2d a code well known in structural me chanics to predict the propagation paths of curved hydraulic fractures at shallow depths the model predicts fracture forms that are remarkably similar to those in field exposures when properties typical of field conditions are used

compaction of lithic sands experimental results and applications compaction experiments showed that the degree of physical compaction of a lithic sand is related to the amount and type of lithic material present these experiments produced a product that is texturally indistinguishable from natural lithic sandstones generally sedimentary lithic fragments are more ductile than metamorphic lithic fragments volcanic lithic fragments when altered to phyllosilicate minerals by weathering or diagenesis are extremely ductile mineralogy and intragranular microporosity influence ductility ratios based on weight percent of soft ductile minerals clay and micas to hard brittle minerals quartz and feldspar within the lithic fragments provides a good indication of ductility a ratio greater than 1 indicates the lithic material will be h ghly ductile microporous lithic fragments are more ductile than dense lithic fragments physical compaction models were developed based on experimental data from well sorted lithic sandstones for which the percentage of preserved porosities were functions of the effective stress the type of lithic material and the quantity of lithic material these models are derived from laboratory sandstones with at least 25 lithic grains in the following categories moderately ductile metamorphic lithic fragments highly ductile shale lithic fragments and extremely ductile altered volcanic lithic fragments application of the compaction models of the experimental compaction techniques to modern subsurface sand samples or outcrop equivalents can provide valuable estimates of the preserved porosities i e reservoir potential for lithic sandstones that may be exploratory objecti es in frontier basins examples using the models and experimental compaction techniques for exploration are given experimental data showed that precompaction cement and overpressure may be beneficial to porosity preservation an early formed partial cement can retard compaction and preserve porosity by stabilizing the sand pack a later cement does not have this beneficial effect overpressure when developed early retards compaction by reducing the effective stress of course a late developing overpressure is not effective at preserving porosity because compaction is an irreversible process

secondary porosity formed by deep meteoric leaching botucatu eolianite southern south america the botucatu sandstone of the parana basin of brazil a jurassic eolianite exhibits unusually deep basinwide penetration of meteoric water this flow of dilute groundwater has removed calcite cement for an average distance of about 140 km from the outcrop and down to a burial depth of about 250 m feldspar has been leached from the framework for about 30 km downdip transforming what was deposited as a feldspathic sandstone into a diagenetic quartz arenite on outcrop these two dissolution processes have generated appreciable secondary porosity by weathering reactions at sites far removed from contact with the atmosphere between 100 and 500 pore volumes of water have passed through the aquifer system to produce this leaching making this an end member case of extensive meteoric diagenesis of sandstone consideration of other basins with meteoric incursion suggests that basin scale leaching is favored when uplift of one margin provides a steep gradient and there is a downdip escape route via faults or outcrop of the aquifer cratonic basins close to rifted continental margins seem to provide the best chance of meeting these conditions

diagenesis and provenance of silurian quartz arenites in south eastern new york state abstract combined scanning electron microscopy cathodoluminescence and optical microscopy sem cl om of single quartz grains was used to determine the diagenetic history and provenance of framework grains in quartz arenites of the silurian shawangunk conglomerate and binnewater sandstone in south eastern new york state most sand sized sedimentary rocks of these two units experienced compaction and quartz cementation after deposition except for those with high matrix or pseudomatrix content no primary porosity is left today and secondary porosity is insignificant leaving the shawangunk conglomerate and binnewater sandstone with poor reservoir qualities the medium to coarse sand fraction of the shawangunk conglomerate and binnewater sandstone is dominated by quartz of plutonic origin of these usually sub rounded to rounded grains 10 15 carry remnants of inherited quartz cement indicating recycling from a sedimentary source such as quartz rich sedimentary rocks of the former iapetus passive margin in contrast polycrystalline vein quartz pebbles in the shawangunk conglomerate were most likely derived from a proximal crystalline source in the taconic orogen to the east formation of the shawangunk conglomerate and binnewater sandstone quartz arenites was likely supported by subtropical climatic conditions given the latitudinal position of eastern north america during the silurian with relatively high mean annual temperatures and high precipitation rates which accelerated the breakdown of unstable grains

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

sem petrography of eastern mediterranean sapropels analogue data for assessing organic matter in oil and gas shales abstract examination of organic matter hosted pore systems in unconventional reservoir rocks has drawn attention to the appearance of sedimentary organic matter om as seen in high resolution sem images field emission sem imaging of eastern mediterranean sapropels pliocene pleistocene age from ocean drilling program drill sites was performed on samples prepared by ar ion cross section polishing the immaturity of marine kerogen in these rocks allows inspection of petrographic textures without overprints related to thermal maturation hydrocarbon generation kerogen in these samples includes discrete particles that in some cases contain primary intragranular pores the most abundant organic matter is in the form of microns thick flaky or stringy material largely nonporous and internally amorphous and lacking the well defined shapes of discrete particles this dominant om has behaved in a highly ductile manner in compaction and is now highly pervaded into intergranular spaces between silt and clay size grains a portion of the larger mineral hosted pores remain open unfilled by ductile om minor nanometer scale pores of uncertain origin occur in the dominant om silt and clay size mineral crystals are distributed in organic matter of most types and it is difficult to fully ascertain if the om mineral mixing has occurred during compaction versus during aggregation during gravity settling marine snow sedimentation or through sediment accumulation in om rich microbial mats results indicate that sediments rich in marine kerogen are subject to substantial compactional porosity loss in early burial these observations of marine detrital organic matter in its early diagenetic state provide a useful baseline for interpretation of petrographic features in more mature om rich mudrocks it is clear that petrographic discrimination of diagenetically generated organic matter bitumen and related om types from ductile dispersed amorphous detrital om kerogen using sem data will be challenging in the absence of compositional information

microscopic modelling of the hydraulic fracturing process a microscopic perspective is introduced in this study which offers a detailed insight at the inter particle level to the geo mechanical responses caused by fluid injection and the resulting pressure build up this was achieved by employing the discrete element method dem to model the pressure development and the subsequent fracturing and or cavity propagation this technique represents the formation material as an assembly of discrete particles linked to each other through contacts numerical experiments were carried out on two sample materials for the first instance tests were carried out on a bulk material representative of a generic intact rock with the breakage of inter particle bonds indicating the formation of cracks the second series of tests was carried out on granular type materials such as sand where particle separation signified cavity initiation and separation it was observed from the dem modelling results that the intact rock material showed a predominance of mode ii fracturing at high fluid velocities however when the fluid velocity is reduced considerably the fracturing behaviour tended towards more of mode i also records of the pressure development were taken from the numerical results and were used to monitor the fracturing events the outcome of this study highlights important aspects of the hydraulic fracturing process especially at the particle particle scale and thus provides a strong basis for more exhaustive studies involving larger scale reservoir modelling and more complex fracturing scenarios

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

insights from petrography mineralogy and u pb zircon geochronology into the provenance and reservoir potential of cenozoic siliciclastic depositional systems supplying the northern margin of the eastern black sea the eastern black sea is one of the few remaining underexplored hydrocarbon basins in europe reservoir quality is a key risk factor in the basin the analysis of up dip portions of oligocene to pliocene depositional systems exposed around its northern and eastern margins enable the composition and provenance of sandstones within the basin to be predicted two major sources of quartz rich sandstone are present the russian western greater caucasus which became a subaerial sediment source in the oligocene and the east european craton scythian platform these are predicted to form better quality reservoirs at shallow and moderate burial depths than their rock fragment rich counterparts due to lesser amounts of compactional porosity loss oligo miocene sandstones from the russian western greater caucasus were deposited as turbidites in the tuapse trough sandstones from the east european craton scythian platform were largely trapped within the indolo kuban basin north of the greater caucasus they only entered the eastern black sea in large volumes from latest miocene or pliocene time in the region of the kerch taman shelf at the western tip of the greater caucasus however it is possible that a precursor to this system supplied the oligocene to early miocene reservoir sandstones within the subbotina field the dziruli massif in western georgia also generated quartz rich sandstones but these are not thought to have entered the black sea

note on the importance of hydrocarbon fill for reservoir quality prediction in sandstones oil emplacement retarded the rate of quartz cementation in the brae formation deep water sandstone reservoirs of the miller and kingfisher fields united kingdom north sea thus preserving porosity despite the rocks being buried to depths of 4 km and 120 degreesc quartz precipitation rates were reduced by at least two orders of magnitude in the oil legs relative to the water legs important contrasts in quartz cement abundances and porosities have emerged between the oil and water legs where reservoirs have filled with hydrocarbons gradually over a prolonged period of time 15 m y the earlier the hydrocarbon fill the greater is the degree of porosity preservation failure to consider this phenomenon during field development could lead to overestimation of porosity and permeability in the water leg potentially leading in turn to poor decisions about the need for and placement of downflank water injectors during exploration the retarding effect of oil on quartz cementation could lead to the presence of viable reservoirs situated deeper than the perceived regional economic basement

hydraulic fracturing of soil during laboratory experiments part 2 propagation the propagation of hydraulic fractures in soil was studied in the laboratory by glycerine injected at a constant rate into rectangular samples of silty clay confined in a triaxial cell during propagation the trace of a hydraulic fracture appeared on the side of the cell as a nearly straight narrow line that tapered from 0 05 mm to negligible aperture at the tip pressure of the glycerine was recorded as a function of time and is characterized by a period of nearly linear increase a break in slope followed by a period of increasing pressure but decreasing slope a maximum and then a period of decreasing pressure tests terminated early in that sequence of events indicated that the break in slope marks the onset of hydraulic fracturing the pressure at the onset of fracturing was predicted from a determination of the fracture toughness of the silty clay the forms of the pressure records were markedly affected by the water content of the soil increasing water content decreased the pressure required to

geometric models of porosity reduction by ductile grain compaction and cementation pore volume reduction of sediments by plastic deformation during compaction and by cementation of grains has been evaluated for different proportions of ductile and hard grains we represent the compaction behavior of grains with a purely geometric model which uses the cooperative rearrangement algorithm to produce dense random packings of partly interpenetrating spheres we varied the fraction of grains assumed to be ductile and the radius of the rigid core of the ductile grains the predicted relationship between the fraction of ductile grains in the sediment and the porosity after compaction agrees well with previously published experimental data in the literature the radius of the rigid core of the ductile grains is an effective way to represent different kinds of ductile material ranging from brittle rigid radius 0 9 to extremely ductile rigid radius 0 7 we simulated quartz cementation in our compacted rock by adding isopachous cement cement thickness was reduced on the smaller grains and increased on the larger grains to account for presumed export of pressure dissolved material from finer grained regions and the import of material into coarser grained regions these simulations yield descriptions of pore scale geometry resulting from processes common in sandstones modeled pore geometry provides insight into transport properties of such rocks for example the models predict to within a factor of five the permeability of samples of tight gas sandstones having little intragranular porosity

structural controls on mechanical compaction within sandstones an example from the apsheron peninsula azerbaijan abstract mechanical compaction of sand rich reservoirs usually occurs during shallow burial and involves the rearrangement of framework grains and the ductile deformation of soft lithoclasts the reservoir quality porosity and permeability of some neogene sandstones of the south caspian basin has however been dramatically reduced by mechanical compaction involving extensive grain fracturing i e porosity collapse these sandstones were probably susceptible to pervasive grain fracturing because they were buried rapidly and experienced compressional deformation prior to reaching 80 c consequently they did not undergo quartz cementation and were therefore exposed to high stresses while they were extremely weak grain size and structural position are also important controls on the distribution of grain fracturing in the onshore analogue in the apsheron peninsula microstructural analysis confirms that susceptibility to distributed grain fracturing increases with increasing grain size structural position has also an important impact on the distribution of porosity collapse in particular sandstones within the hinges of folded sections have undergone much more extensive grain fracturing than within the surrounding area the increased stresses in this structural position have enhanced distributed grain fracturing and subsequent deformation band development

controls on reservoir quality in fan deltaic conglomerates insight from the lower triassic baikouquan formation junggar basin china abstract fan delta sandy conglomerates form important oil and gas reservoirs recently a new giant oilfield was discovered in the lower triassic baikouquan formation t1b in the mahu sag of the junggar basin northwestern china however the fan delta sandy conglomerate reservoirs are strongly heterogeneous most have poor permeability and host little hydrocarbons while a subset at the delta front exhibit high porosity and permeability forming high quality reservoirs to explain this disparity the sedimentary facies diagenesis and reservoir quality of the sandy conglomerate strata and their interactions were studied detailed logging of 253m cores indicates that the t1b was deposited in a lacustrine fan delta complex comprising ten lithofacies from coarse matrix supported pebbly conglomerate gm to fine massive mudstone fm in four facies associations interpretation of optical microscope backscattered and scanning electron microscope imaging show that these ten lithofacies underwent differential diagenesis especially dissolution of minerals such as orthoclase dissolution was particularly prevalent in the subaqueous distributary channel deposits of sandstones st hyperpycnal flow massive sandstones sm and grain flow deposits of sandy gravelly conglomerates gcs at the delta front forming abundant secondary pores these pores enhanced the reservoir performance and led to high porosity and permeability within these three lithofacies for these reservoirs continuous winnowing at the delta front led to relatively high primary porosities and subsequent mineral dissolution during burial diagenesis closely related to acidic mn rich hydrocarbon bearing fluids significantly improved the porosities further migration of hydrocarbon bearing fluids was facilitated by faults cutting through the reservoir and underlying source rocks this led to the development of belts of high quality reservoir at the delta front near the fault conduits this model for reservoir development and charge suggests that further reservoirs can be expected within the t1b of the mahu sag

diagenesis porosity evolution and petroleum emplacement in tight gas reservoirs taranaki basin new zealand abstract tight gas reservoirs have become popular targets in petroleum exploration in recent years due largely to the increasing market demand for gas and also to technology advances used in extraction reservoir quality is typically poor due to deep burial diagenesis resulting in significant compaction cementation and illitization however analysis of tight reservoirs using integrated techniques can improve our understanding about the controls on reservoir quality and these results can potentially be used to help predict reservoir quality at other sites the use of integrated analysis of diagenetic and burial history is here shown for the eocene k3e kapuni group reservoir in cardiff 1 taranaki basin new zealand the k3e belongs to a deep gas play that was considered uncommercial in the early 1990 s due to poor reservoir quality associated with extensive authigenic illite potassium argon dating has shown that destruction of the reservoir through illitization occurred in the pliocene associated with a late stage heating and or fluid flow event timing of illite authigenesis in the k3e reservoir postdates the likely circulation of co2 rich fluids that caused reaction of feldspar to form kaolinite and quartz homogenization temperatures from two phase aqueous fluid inclusions within quartz cements together with 1d basin models of co2 expulsion from coaly source rocks indicate that feldspar reactions may have started in the mid miocene the co2 rich fluids are thought to have been generated by thermal decarboxylation of intraformational paleocene eocene coals at the same time oil was being expelled from older cretaceous coals and migrated into the mid late eocene reservoirs evidence for oil migration is demonstrated by the presence of residual oil and by the local occurrence of abundant oil bearing fluid inclusions the products of feldspar dissolution appear to be in reasonable balance with the amount of dissolved feldspar suggesting a relatively closed diagenetic system with little improvement in reservoir quality despite the large volume of feldspar dissolved petrographic evidence of reservoir sandstones at cardiff 1 demonstrates the presence of some illite free secondary macropores lined by residual oil this local preservation of porosity may be replicating the original distribution of oil saturated pores in the pliocene at the time when authigenic kaolinite underwent reaction to form illite remobilization of the oil probably occurred by gas flushing after the main phase of illitization at c 5 ma the data presented in this study are consistent with redistributed liquid petroleum into up dip traps and represent the potential for a new exploration play the eocene reservoir at cardiff 1 is currently gas charged with the pore system composed of approximately one third large macropores connected by a tortuous microporous network however it is possible that with the currently buoyant gas market and modern hydraulic fracturing techniques it may now be economic to flow gas from the locally large pores identified in the k3e reservoir

experimental modelling of shallow magma emplacement application to saucer shaped intrusions abstract saucer shaped dolerite and sandstone intrusions are common in sedimentary basins world wide we have conducted a series of scaled experiments simulating the process of magma emplacement in sedimentary basins with particular attention on the formation of saucer shaped sills the model materials were 1 cohesive fine grained silica flour representing brittle crust and 2 molten low viscosity oil representing magma the experiments were performed in both homogeneous and layered models in all the experiments oil injection resulted in doming of the surface in the homogeneous models the injected oil formed cone sheets and sub vertical dykes cone sheets formed for shallow injection 1 3 cm and vertical dykes formed for deeper injection 4 5 cm in layered models the injected oil always formed saucer shaped intrusions our experimental results show that 1 sill intrusion results in the formation of a dome with melt erupting at the rim 2 layering controls the formation of sills and saucer shaped sills 3 saucer shaped sills are fed from the bottom and the fluid flows upward and outward and 4 the diameter of saucer shaped sills increase with increasing emplacement depth the systematic relation between domes and sills and the depth dependence of sill diameters show that saucer shaped intrusions result from the interaction between a growing flat lying shallow sill and doming of the free surface we conclude that saucer shaped intrusions represent fundamental geometries formed by shallow magma intrusion in stratified basins

forensic numerical analysis of gas venting in southwestern ontario abstract a forensic modeling study examines the potential causes of gas and water venting observed during the excavation of a landfill in a 40 m thick clayey till deposit in southwestern ontario canada the clayey till is known to be underlain by permeable natural gas bearing rock and gas has been diffusing through the clayey deposit over about the last 13 000 15 000 years attention is focused on 2d modeling of hydrofracturing as well as gassy soil behavior the model examines the predicted location of venting with and without the presence of gassy sand lenses and a discontinuous layer of basal till between the bedrock and the low permeability till the modeling shows that hydrofracturing alone can explain the occurrence and location of the observed venting in one of the sub cells in the other sub cell hydrofracturing explains the occurrence but not the location of the observed venting or the presence of the sand in the venting water at this location modeling of appropriately placed gassy sand lenses is shown to explain the occurrence and location of the observed venting as well as the presence of both waters with different geochemistry from the bedrock and the till in the venting waters

diagenesis and reservoir quality evolution of the eocene sandstones in the northern dongying sag bohai bay basin east china abstract the eocene sandstones in the northern dongying sag bohai bay basin china are reservoirs for large accumulations of hydrocarbons the sandstones are mainly lithic arkoses and feldspathic litharenites texturally and compositionally immature these sandstones have a wide range of porosity 0 4 37 and permeability 0 004 6969 md and show an overall decrease in reservoir quality from 1500 m to 5000 m below sea level the reduction in reservoir quality is a product of several digenetic processes these include compaction precipitation of dolomite and calcite in eodiagenetic stage compaction feldspar dissolution precipitation of quartz cements and clays kaolin and illite and precipitation of ferrocalcite and ankerite in mesodiagenetic stage mineral distribution pattern and isotopic composition suggest carbonate cements in sandstones originate from sources outside the sandstones carbonate cementation together with compaction reduced the sandstones porosity and permeability significantly in a sandstone bed marginal sandstones with distance to sandstone mudstone interface less than one meter always have lower porosity than central sandstones as burial depth exceeds 4000 m marginal sandstones have very low porosities 2 m can be potential effective reservoirs feldspar dissolution and precipitation of clays and quartz cements have little impact on absolute porosity mineral distribution pattern and quantitative data show that leached feldspars are the internal source of authigenic quartz and clays in sandstones and the volume difference between feldspar secondary porosity and related authigenic cements is generally less than 0 25 however although there is little or no net import of matter to the sandstones the pore architecture changes dramatically primary macropores are lost as clays and quartz precipitate while the proportion of microporosity increases occurring mainly between clay crystals the overall result is that permeability is significantly degraded

diagenesis and reservoir potential of permian triassic fluvial lacustrine sandstones in the southern junggar basin northwestern china the junggar basin is one of the largest oil producing areas in china and contains upper permian lacustrine oil shales with some of the greatest hydrocarbon potental in the world in this study we present the diagenetic characteristics of permian triassic sandstones from the southern junggar basin and evaluate their reservoir potential the uppermost permian and lower triassic cangfanggou group in the southern junggar basin is characterized by alternating fluvial and lacustrine deposits whereas the middle upper triassic xiaoquangou group was deposited predominantly in a lacustrine environment fluvial and deltaic sedimentation was subordinate the sandstones of the cangfanggou and xiaoquangou groups are volcanic litharenites their detrital modes and textures of volcanic fragments suggest a primarily andesitic basaltic volcanic arc provenance early diagenesis of the sandstones is characterized by nonferroan calcite cementation grain coating pore lining clay minerals and the initial dissolution of detrital grains authigenic quartz pore filling phyllosilicates pore filling grain replacive zeolites albitized detrital plagioclase authigenic k feldspar copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received april 15 1996 revised manuscript received november 4 1996 final acceptance may 2 1997 2department of earth sciences university of western ontario london ontario n6a 5b7 canada 3school of geosciences queen s university of belfast belfast bt7 1nn united kingdom this work was done at the queen s university of belfast with funding from exxon bhp mobil and itotal which we gratefully acknowledge the karamay institute of petroleum exploration and development generously provided borehole samples geng ansong at the institute of geochemistry guangzhou academia sinica and zhou yisheng at the xinjiang institute of petroleum are thanked for assistance with field work and for valuable discussions special thanks go to the staff of the electron microscopy unit at the queen s university of belfast we also thank a h ruffell and s r lawrence for their constructive comments on an earlier version of the manuscript stable isotope analysis and ongoing research concerning the junggar basin at the university of western ontario have been supported by the natural science and engineering research council of canada the manuscript benefited substantially from critical commentary by kevin t biddle suzanne kairo w r kaiser and an anonymous reviewer

impact of diagenesis on reservoir quality and heterogeneity of the upper triassic chang 8 tight oil sandstones in the zhenjing area ordos basin china abstract reservoir quality and heterogeneity are critical risk factors in tight oil exploration the integrated analysis of the petrographic characteristics and the types and distribution of diagenetic alterations in the chang 8 sandstones from the zhenjing area using core log thin section sem petrophysical and stable isotopic data provides insight into the factors responsible for variations in porosity and permeability in tight sandstones the results indicate that the chang 8 sandstones mainly from subaqueous distributary channel facies are mostly moderately well to well sorted fine grained feldspathic litharenites and lithic arkose the sandstones have ultra low permeabilities that are commonly less than 1 md a wide range of porosities from 0 3 to 18 1 and two distinct porosity permeability trends with a boundary of approximately 10 porosity these petrophysical features are closely related to the types and distribution of the diagenetic alterations compaction is a regional porosity reducing process that was responsible for a loss of more than half of the original porosity in nearly all of the samples the wide range of porosity is attributed to variations in calcite cementation and chlorite coatings the relatively high porosity reservoirs formed due to preservation of the primary intergranular pores by chlorite coatings rather than burial dissolution however the chlorites also obstruct pore throats which lead to the development of reservoirs with high porosity but low permeability in contrast calcite cementation is the dominant factor in the formation of low porosity ultra low permeability reservoirs by filling both the primary pores and the pore throats in the sandstones the eogenetic calcites are commonly concentrated in tightly cemented concretions or layers adjacent to sandstone mudstone contacts while the mesogenetic calcites were deposited in all of the intervals and led to further heterogeneity this study can be used as an analogue to understand the variations in the pathways of diagenetic evolution and their impacts on the reservoir quality and heterogeneity of sandstones and is useful for predicting the distribution of potential high quality reservoirs in similar geological settings

variations in supercell morphology part i observations of the role of upper level storm relative flow abstract it is hypothesized that the precipitation intensity beneath a supercell updraft is strongly influenced by the amount of hydrometeors that are reingested into the updraft after being transported away in the divergent upper level flow of the anvil this paper presents the results of a climatological analysis of soundings associated with three types of isolated supercells having distinctive precipitation distributions the so called classic low precipitation lp and high precipitation hp storms it is shown that storm relative flow at 9 10 km above the ground is strongest in the environments of lp storms and relatively weak in the environments of hp storms with classic storms occurring in environments with intermediate magnitudes of upper storm relative flow it is plausible that comparatively strong flow in the anvil bearing levels of lp storms transports hydrometeors far enough from the updraft that they are relatively unlikely to be reingested into the updraft leading to greatly diminishe

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

volcano load control on dyke propagation and vent distribution insights from analogue modeling 1 the spatial distribution of eruptive vents around volcanoes can be complex and evolve as a volcano grows observations of vent distribution at contrasting volcanoes from scoria cones to large shields show that peripheral eruptive vents concentrate close to the volcano base we use analogue experiments to explore the control of volcano load on magma ascent and on vent location results show that the local loading stress field favors eruption of rising magma away from the volcano summit if a central conduit is not established or is blocked two sets of scaled experiments are developed with contrasting rheological properties to analyze similarities and differences in simulated magma rise below a volcano 1 golden syrup magma analogue is injected into a sand plaster mixed layer crust analogue under a cone 2 water or air magma analogues is injected into gelatin under a sand cone rising dykes approaching the cone stress field are stopped by the load compressive stress with continued intrusion dyke overpressure builds up dykes extend laterally until their tips are able to rise vertically again and to erupt in the flank or at the base of the volcano lateral offset of the extrusion point relative to the edifice summit depends on substratum thickness volcano slope and dyke overpressure the 3d geometry of golden syrup intrusions varies with experimental parameters from cylindrical conduits to dyke and sill complexes experimental results are compared with illustrative field cases and with previously published numerical models this comparison enables applications and limitations of the analogue models to be highlighted and allows us to propose a conceptual model for the evolution of vent distribution with volcano growth

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