stylolites in limestone magnitude of contractional strain accommodated and scaling relationships abstract the amount of contractional strain accommodated by a set of stylolites is analyzed by comparing the amplitudes of stylolitic teeth and spikes to independent measures of thinning of the enclosing layer the scaling relations between along strike trace length l and the maximum d max and average d avg amplitudes of stylolitic topography are also investigated the studied stylolites occur in a 212 mm thick limestone layer dragged into the damage zone of the gubbio normal fault zone in central italy layer thinning was assessed independently from the layer geometry with a maximum value of 23 nearest the fault a total of 28 mm of thinning are related to 24 stylolites whose lengths range from 6 4 to 146 mm with average amplitudes from 0 1 to 1 3 mm the average and maximum amplitudes of stylolitic topography increase with stylolite length with d avg 0 0011 l 0 17 r 2 0 57 implying propagation to greater lengths as contractional strains increase along them the stylolites increase in number and amplitude into the most thinned area correlating with increasing contractional strain accommodated by the layer the average amplitude of stylolites visible in outcrop provides a measure of the minimum magnitude of contractional strain in the rock although other mechanisms such as grain scale dissolution appear necessary to account for the remaining layer thinning

spectrum of pore types and networks in mudrocks and a descriptive classification for matrix related mudrock pores matrix related pore networks in mudrocks are composed of nanometer to micrometer size pores in shale gas systems these pores along with natural fractures form the flow path permeability network that allows flow of gas from the mudrock to induced fractures during production a pore classification consisting of three major matrix related pore types is presented that can be used to quantify matrix related pores and relate them to pore networks two pore types are associated with the mineral matrix the third pore type is associated with organic matter om fracture pores are not controlled by individual matrix particles and are not part of this classification pores associated with mineral particles can be subdivided into interparticle interp pores that are found between particles and crystals and intraparticle intrap pores that are located within particles organic matter pores are intrap pores located within om interparticle mineral pores have a higher probability of being part of an effective pore network than intrap mineral pores because they are more likely to be interconnected although they are intrap om pores are also likely to be part of an interconnected network because of the interconnectivity of om particles in unlithifed near surface muds pores consist of interp and intrap pores and as the muds are buried they compact and lithify during the compaction process a large number of interp and intrap pores are destroyed especially in ductile grain rich muds compaction can decrease the pore volume up to 88 by several kilometers of burial at the onset of hydrocarbon thermal maturation om pores are created in kerogen at depth dissolution of chemically unstable particles can create additional moldic intrap pores

practical approaches to identifying sealed and open fractures for one essential ingredient of permeable fracture networks degree of fracture pore space preservation in large fractures i show how the characterization challenge presented by sparse fracture sampling can be overcome by measuring a surrogate the abundance of rock mass cement that precipitated after fractures ceased opening sampling limitations are overcome because the surrogate is readily measured in small rock samples including sidewall cores and cuttings permitting site specific diagnosis of the capacity of fractures to transmit fluid over a wider range of sample depths than conventional methods allow a diverse core database shows that this surrogate correctly predicts where large fractures are sealed information on timing of fracture opening relative to cement sequence can be obtained in two ways first evidence of fracture movement history and cement sequences in sparse large fractures can be extrapolated to areas having only cement data alternately evidence of fracture timing can be acquired from sealed micrometer scale fractures distribution of porosity reducing cement is commonly heterogeneous from bed to bed and location to location in siliciclastic and carbonate rocks however because patterns of sealed or open fractures cannot be delineated using fracture observations alone surrogates have practical value for production fairway mapping and other applications in which identifying open fractures is essential this study highlights the vital interplay among structural and diagenetic processes for fracture porosity preservation or destruction

diagenetic history of tight sandstones and gas entrapment in the yulin gas field in the central area of the ordos basin china abstract the ordos basin is a large cratonic basin with an area of 250 000 km2 in central china upper paleozoic coals and shales serve as gas source rocks with peak generation and migration at the end of the early cretaceous recent exploration has verified the huge gas potential in the basin centered gas accumulation system bcgas however the mechanism for the gas accumulation is controversial with an integrated approach of thin section petrography ultra violet fluorescence microscopy fluid inclusion microthermometry raman microspectrometry scanning electron microscopy and x ray diffractometry we identified diagenetic trapping and evaluated the diagenetic history of sandstone reservoirs in the yulin gas field in the central area where structural stratigraphic and or sedimentary lithologic traps have not been found it was revealed that three phases of diagenesis and hydrocarbon charging occurred respectively in the late triassic late jurassic and at the end of the early cretaceous in the first two phases acidic water entered the reservoir and caused dissolution and cementation resulting in porosity increase however further subsidence and diagenesis including compaction and cementation markedly reduced the pore space at the end of the early cretaceous the bulk of the gas migrated into the tight reservoirs and the bcgas trap was formed in the updip portion of this system cementation continued to occur due to low gas saturation and has provided effective seals to retain gas for a longer period of time than water block in the bcgas the mechanism for the gas entrapment was changed from water block by capillary pressure in the bcgas to diagenetic sealing the diagenetic seals in the updip portion of the sand body were formed after gas charging which indicates that there is a large hydrocarbon exploration potential at the basin centered area

sandstone vs carbonate petroleum reservoirs a global perspective on porosity depth and porosity permeability relationships plots are presented comparing average porosity vs depth for 30 122 siliciclastic petroleum reservoirs and 10 481 carbonate petroleum reservoirs covering all petroleum producing countries except canada however separate plots cover 5534 siliciclastic and 2830 carbonate reservoirs of the alberta basin in canada average permeability vs average porosity is shown for the non canadian reservoirs key similarities and differences between sandstones and carbonates are noted and implications are discussed regarding the dominant factors controlling reservoir quality in each lithology trends of steadily decreasing median and maximum porosity with increasing depth reflect burial diagenetic porosity loss in response to increasing thermal exposure with depth these trends seem inconsistent with the suggestions that both sandstones and carbonates commonly increase in porosity by dissolution during deeper burial carbonate reservoirs have lower values of median and maximum porosity for a given burial depth probably because of greater chemical reactivity of carbonate minerals relative to quartz and the resulting lower resistance to chemical compaction and associated cementation relative paucity of low porosity 0 8 siliciclastic reservoirs at all depths compared with carbonates may reflect the more common occurrence of fractures in carbonates and the effectiveness of these fractures for facilitating economic flow rates in low porosity rock overall carbonate reservoirs do not have lower permeability for a given porosity compared with sandstones but do have strikingly lower proportions of both high porosity and high permeability values the data presented can serve as a general guide for the distribution of reservoir quality that can reasonably be expected in exploration wells drilled to any given depth in the absence of detailed geologic information such as burial and thermal history

sedimentary association of alternated mudstones and tight sandstones in china s oil and gas bearing basins and its natural gas accumulation abstract oil and gas resources are abundant in china s continental sedimentary basins where the main task of exploration has been finding oil for many years in recent years however new discoveries of large scale natural gas have been successively obtained the natural gas mainly exists in the sedimentary association of alternated mudstones and tight sandstones and is dominantly low permeability tight sand gas through the in depth study on gas reservoir analysis diagenetic evolution source rock distribution and hydrocarbon generating behavior natural gas generation and accumulation it is concluded that during the major subsiding stage of large scale lake basins the multicyclic subsiding process of the lake surface controls the development of high quality source rocks the wide distribution of sands and the superimposition of the two types of rocks in the vertical direction the lacustrine muddy source rocks are developed including mud shale carbonaceous mudstone and coal bed which are in the medium high evolution stage and produce mainly gas and the gas generation intensity is high through the analysis of the subsidence evolution processes of the carboniferous permian systems transitional marine continental facies in the ordos basin and the triassic xujiahe formation continental facies in the sichuan basin it is concluded that the widely distributed sandbodies of delta facies although with tight properties are interbedded with source rocks and easy to accumulate natural gases the natural gas is migrated and accumulated within small distance and is characterized by large area accumulation because of the strong hydrocarbon generation capacity big thickness and stable distribution of the underlying mud shale the potential of gas resources should not be underestimated the geocyclicity of china s continental sedimentary basins controls the sedimentary association of alternated mudstones and tight sandstones resulting in superimposed accumulations of shale gas and tight sand gas

sea level reconstructions from the peri vocontian zone south east france point to valanginian glacio eustasy the valanginian is a period of global environmental change as illustrated by sedimentary palaeontological geochemical and climatic perturbations a production crisis in most of the carbonate platforms suggests important changes in palaeoenvironmental conditions during the same time interval a major positive excursion in 13c the weissert event suggests perturbations of the carbon cycle from the latest early valanginian to the early hauterivian in order to better understand the link between these changes sea level fluctuations have been reconstructed in detail from the middle berriasian to the earliest hauterivian sections from the peri vocontian zone south east france have been investigated because of the good quality of outcrops on the carbonate platforms their margins and in the vocontian basin sections ranging from the most proximal zone swiss jura to the basin were interpreted in terms of sequence stratigraphy and cyclostratigraphy and correlated at high resolutions using the identified small medium and large scale sequences as well as depositional geometries sea level fluctuations were reconstructed two main trends are evidenced during the studied interval i the peak amplitude magnitude of the sea level fluctuations increased gradually from the middle berriasian to the early valanginian and reached a maximum more than 50 m from the middle early valanginian to the valanginian hauterivian boundary and ii sea level variations were quite symmetrical during the late berriasian slightly asymmetrical during the early valanginian and strongly asymmetrical fast sea level rise slow fall from the latest early valanginian to the earliest hauterivian moreover three orders of sea level fluctuations were recognized in the sedimentary rocks of the peri vocontian zone platform basin correlations and cyclostratigraphic interpretations of the basinal sections evidence an astronomical control on the sea level variations mainly by the two eccentricity cycles of 100 and 400 kyr the increase in the amplitude of the sea level fluctuations and their change from symmetrical to asymmetrical can be related to the onset of a major cooling event in the early valanginian fast transgressions followed by slower regressions would correspond to waxing and waning of high latitudinal ice during most of the valanginian especially from the latest early valanginian to the latest late valanginian glacio eustatic sea level fluctuations in tune with the 100 and 400 kyr eccentricity cycles are in agreement with glaciations during the valanginian

the importance of shale composition and pore structure upon gas storage potential of shale gas reservoirs abstract the effect of shale composition and fabric upon pore structure and ch 4 sorption is investigated for potential shale gas reservoirs in the western canadian sedimentary basin wcsb devonian mississippian d m and jurassic shales have complex heterogeneous pore volume distributions as identified by low pressure co 2 and n 2 sorption and high pressure hg porosimetry thermally mature d m shales 1 6 2 5 vro have dubinin radushkevich d r co 2 micropore volumes ranging between 0 3 and 1 2 cc 100 g and n 2 bet surface areas of 5 31 m 2 g jurassic shales which are invariably of lower thermal maturity ranging from 0 9 to 1 3 vro than d m shales have smaller d r co 2 micropore volumes and n 2 bet surface areas typically in the range of 0 23 0 63 cc 100 g co 2 and 1 9 m 2 g n 2 high pressure ch 4 isotherms on dried and moisture equilibrated shales show a general increase of gas sorption with total organic carbon toc content methane sorption in d m shales increases with increasing toc and micropore volume indicating that microporosity associated with the organic fraction is a primary control upon ch 4 sorption sorption capacities for jurassic shales however can be in part unrelated to micropore volume the large sorbed gas capacities of organic rich jurassic shales independent of surface area imply a portion of ch 4 is stored by solution in matrix bituminite solute ch 4 is not an important contributor to gas storage in d m shales structural transformation of d m organic matter has occurred during thermal diagenesis creating and or opening up microporosity onto which gas can sorb as such d m shales sorb more ch 4 per weight percent wt toc than jurassic shales inorganic material influences modal pore size total porosity and sorption characteristics of shales clay minerals are capable of sorbing gas to their internal structure the amount of which is dependent on clay type illite and montmorillonite have co 2 micropore volumes of 0 78 and 0 79 cc 100 g n 2 bet surface areas of 25 and 30 m 2 g and sorb 2 9 and 2 1 cc g of ch 4 respectively dry basis a reflection of microporosity between irregular surfaces of clay platelets and possibly related to the size of the clay crystals themselves mercury porosimetry analyses show that total porosities are larger in clay rich shales compared to silica rich shales due to open porosity associated with the aluminosilicate fraction clay rich sediments low si al ratios have unimodal pore size distributions

matrix hydrogeological properties of devonian carbonate rocks of olkusz southern poland abstract the limestone matrix of the olkusz region s poland does not present good conditions for water transmission and storage according to a laboratory study of 165 core samples from 4 boreholes the mean values obtained for open porosity specific yield and hydraulic conductivity for limestone were 0 015 0 00006 and 1 94 10 10 m s respectively for dolomites they were 0 011 0 0013 and 2 77 10 10 m s respectively no clear relationship between hydraulic conductivity and depth was identified nevertheless it was established that the matrix can retain small quantities of water with the dolomite matrix exhibiting more favourable conditions for storing and transmitting water

quantitative analyses of porosity evolution in tight grainstones a case study of the triassic feixianguan formation in the jiannan gas field sichuan basin china abstract tight grainstones although widespread throughout the lower triassic feixianguan formation in the sichuan basin have received little attention in part due to their lower porosity and greater heterogeneity relative to their dolostone counterparts based on data from cores and thin sections as well as petrophysical properties the feixianguan grainstones representing a major gas reservoir in the jiannan gas field were systemically analysed to better understand porosity evolution in tight carbonates that have experienced original oil accumulation and subsequent thermal cracking during progressive burial the grainstones were divided into two types according to whether pyrobitumen was present and their porosity evolutions were quantitatively reconstructed taking 40 as the original porosity the grainstones without pyrobitumen which were ineffective palaeo oil reservoirs lost 21 94 and 3 13 of their porosities through marine and burial calcite cementation respectively and 13 34 by compaction and have a current porosity of 1 59 thus allowing them to serve as major present day gas reservoirs comparatively pyrobitumen bearing grainstones which were once palaeo oil reservoirs lost 23 96 and 2 36 of their porosities through marine and burial calcite cementation respectively 11 4 by compaction and 1 44 by pyrobitumen and have a current porosity of 0 84 thus making them ineffective gas reservoirs this study provides a quantitative understanding of the close association between porosity evolution and reservoir effectiveness for the palaeo oil charge and present day gas accumulation with respect to diagenetic history which is useful for the future exploration in tight gas limestone reservoirs

contourite porosity grain size and reservoir characteristics abstract contourites are now recognised as having a significant potential as hydrocarbon reservoirs in the subsurface and several fields have been interpreted as comprising bottom current reworked turbidite sands however very little has been published on the porosity characteristics of contourites this study documents porosity data from iodp expedition 339 sites in the gulf of cadiz we use grain size analyses porosity depth plots and exponential models to yield a better understanding of grain size characteristics and facies porosity characteristics and the reservoir potential of contourites in the subsurface new grain size data for over 350 samples from the cadiz contourites is presented building on earlier work these data confirm the distinctive trends in textural properties linked to depositional processes under the action of bottom currents the finest muddy contourites porosity depth relationships from four cadiz sites show a moderately high initial porosity for both sand and mud facies 50 60 and a systematic decrease with depth to around 35 40 near 500 m burial depth according to the exponential models of porosity with depth contourite porosity should exceed 10 at 2500 m burial depth we compare the data from the gulf of cadiz contourite depositional system with those of the eirik drift newfoundland drift gardar drift and canterbury slope drifts similar depth trends are observed and all show anomalies linked to interbedded sandy and muddy facies composition carbonate vs siliciclastic and the presence of hiatuses in the sediment record these data provide good insight into the likely reservoir characteristics of contourites for both conventional and unconventional reservoirs they are comparable with those of existing contourite fields most of which are mixed turbidite contourite systems

transport of suspended particulate matter in an antarctic fjord abstract vertical profiles of temperature salinity and light attenuation were used in conjunction with underwater photography and water samples to characterize the nature and dynamics of particle transport in an antarctic fjord most of the particles are sand sized 0 1 2 mm floccules comprised of individual grains in the 5 to 50 m size range the quartz silt grains are derived from basal debris laden meltwater originating from beneath the submerged valley glacier the meltwater buoyantly rises to form cold water interflows at mid water depths turbulent mixing near the seafloor also plays a role in the transport and break up of floccules together the midwater cold tongues and near bottom turbidity account for 87 of the total sediment load thus attesting to the importance of non surface transport mechanisms in polar fjords

fractures in the low porosity and ultra low permeability glutenite reservoirs a case study of the late eocene hetaoyuan formation in the anpeng oilfield nanxiang basin china the naxiang basin located within the qin and dabie mountains orogenic belt in central china is a small intermountain faulted basin in which the late eocene hetaoyuan formation in the anpeng oilfield is the low porosity and ultra low permeability glutenite reservoirs the large scale fractures are mainly developed in the thick bedded conglomerates and gravel bearing gritstones the small scale jointed fractures bounded by layer are mainly developed in the medium granular and fine sandstones between the conglomerates or gravel bearing gritstones there also developed three kinds of micro fractures namely intergranular fractures intragranular fractures and transgranular fractures in the tight glutenites the fractures in the glutenite reservoirs are of chiefly high dip angles they assume chiefly the e w strikes next are the ne sw and nw se strikes on the log curve fractures usually show such responses as a decrease of the deep and shallow lateral resistivities an increase of the acoustic time difference a reduction of the density an increase of the compensation neutrons an occurrence of middle and high gamma ray and an expansion of the calipers fractures make contributions to the low porosity and ultra low permeability glutenite reservoirs by improving the permeability and then porosity micro fractures are the important channels connecting intergranular and intragranular solution pores thereby making better the connectivity within the tight glutenite reservoirs the small and medium scale fractures serve as the flow channels within the layers of fine and medium granular sandstones while the large scale fractures serve as the main flow channels for the whole reservoir under the impact of the present day stress the ne sw fractures perpendicular to the minimum principal stress assume a tensile state and have good connectivity large apertures and high permeability and are the major direction for fluid flow thereby providing a major basis for a further development

quantifying uncertainties in multi scale studies of fractured reservoir analogues implemented statistical analysis of scan line data from carbonate rocks in this study we performed a fracture analysis on a cretaceous bedded carbonate succession well exposed in the sorrento peninsula the studied succession includes stratigraphic units that are very similar to the productive units of the buried apulian platform reservoir rocks in southern italy we analyzed eight carbonate beds including both limestones and dolomites the basic technique used in this study consisted of measuring fractures along bedding parallel scan lines for one limestone bed a microscale scan line about 15 cm long was also analyzed using a digital microcamera provided the cumulative distribution of fracture apertures is well described by a power law our analysis shows how the uncertainty in the estimate of fracture aperture cumulative frequencies grows for large aperture values this feature results in a large uncertainty in the estimate of the slope of the least squares line in a bi logarithmic diagram approximating the data distribution which is the exponent of the power law as the latter represents a fundamental parameter characterizing a fracture set and fracture distribution over different scales reducing the uncertainty in the estimate of the slope of the curve represents an important objective of quantitative fracture analysis this is obtained in this study by the application of multi scale analysis and by integrating micro scan line data with classic outcrop based scan line analysis the quantification of uncertainties in the cumulative distribution estimates of fracture apertures is performed by analyzing in detail the spacing distribution and consequently fracture density distribution for each aperture value our results suggests that a meaningful statistical analysis of fracture attributes such as aperture or opening displacement may be effectively carried out by using properly determined confidence intervals and by the integration of outcrop based and micro scan line data sets

burial dissolution of micrite in middle east carbonate reservoirs jurassic cretaceous keys for recognition and timing abstract this paper discusses burial diagenesis and especially a burial dissolution phase and its effects on micrite microtexture in reservoirs in the middle east three microporous reservoirs were selected the cenomanian mishrif formation in iraq the kimmeridgian tithonian arab d and the barremian kharaib formations both in abu dhabi staining cathodoluminescence and scanning electron microscopy were used for petrographic observations in the selected reservoirs three typical micrite microtextures are distinguished micro rhombic compact anhedral and rounded the chronology of diagenesis based on petrographic observations shows that mineralogical stabilisation through high magnesian calcite replacement by low magnesian calcite led to the formation of micro rhombic micrites non reservoir facies such as seals present compact anhedral micrites correlation between these micrites and high aluminium content suggests alumino silicate minerals clays as a catalyst for pressure dissolution and thus for reservoir degradation through cementation micrites from the upper part of oil reservoirs are dissolved during burial diagenesis dissolution is responsible for rounded shape and for reduction of crystal size it also implies pore diameter and pore throat size enlargement comparison of porosity and permeability with scanning electron microscope data confirms that rounded micrites are better reservoirs than micro rhombic micrites

morphometry of micrite particles in cretaceous microporous limestones of the middle east influence on reservoir properties abstract microporosity may account for a significant part of the total porosity of cretaceous limestone reservoirs of the middle east in these microporous facies porosity is moderate to excellent up to 35 while permeability is poor to moderate up to 190 md micritic limestones also may form dense layers with very low porosity and permeability values micritic samples were collected from three fields of the habshan and mishrif formations to examine the spatial relationship with their porosity permeability and pore throat radius distributions two key parameters of the micritic particles are studied using scanning electron microscopy their morphology shape and inter crystal contacts and their crystallometry results reveal that micrite matrixes can be subdivided into three petrophysical classes class c strictly microporous limestones with coarse punctic to partially coalescent micrites is made up of coarse 2 m polyhedral to rounded micritic crystals it has good to excellent porosity 8 28 poor to moderate permeability 0 2 190 md and a mean pore threshold radius of more than 0 5 m the class c is usually observed in rudist shoal facies where relatively high hydrodynamic energy disfavoured deposition of the finer micritic crystals it also developed within meteoric leaching intervals below exposure surfaces class f strictly microporous limestones with fine punctic to partially coalescent micrites is composed of fine

microporous and tight limestones in the urgonian formation late hauterivian to early aptian of the french jura mountains focus on the factors controlling the formation of microporous facies microporous and tight limestones with contrasting porosity and permeability values directly related to the microfabric of the micritic matrix outcrop in the urgonian formation of the french jura mountains this study investigates the factors controlling the differentiation between the microporous and tight facies and proposes a diagenetic model for the development and preservation of the microporosity in these limestones the petrophysical properties are not related to the depositional texture the petrographical content or the mineralogical composition however the tight layers contain indications of emersion e g bird eyes keystone vugs and desiccation cracks the sedimentation in very shallow conditions up to emersion is confirmed by the covariant more positive values of oxygen and carbon isotopes the microporous intervals systematically occur a few meters below the tight layers affected by emersion this position strongly suggests the importance of meteoric water input rapidly after sedimentation in the differentiation between tight and microporous limestones the diagenetic model proposed for the development and preservation of the microporous facies involves partial early cementation of the interstitial mud mainly composed of low mg calcite crystals sedimentation during a calcite sea period inside a meteoric phreatic lens by in situ dissolution reprecipitation processes hybrid ostwald ripening this early cementation partly preserves the original microfabric and intercrystalline microporosity and allows the carbonate sediment to resist compaction during burial the identification of the conditions favorable to the development of microporosity in these urgonian limestones may improve the knowledge and modeling of some microporous carbonate reservoir rocks

influence of depositional setting and sedimentary fabric on mechanical layer evolution in carbonate aquifers abstract carbonate aquifers in fold thrust belt settings often have low matrix porosity and permeability and thus groundwater flow pathways depend on high porosity and permeability fracture and fault zones methods from sedimentology and structural geology are combined to understand the evolution of fracture controlled flow pathways and determine their spatial distribution through this process bed parallel pressure solution surfaces ps1 are identified as a fracture type which influences fragmentation in peritidal and basinal carbonate and upon shearing provides a major flow pathway in fold thrust belt carbonate aquifers through stratigraphic analysis and fracture mapping depositional setting is determined to play a critical role in ps1 localization and spacing where peritidal strata have closer spaced and less laterally continuous ps1 than basinal strata in the peritidal platform facies units with planar lamination have bed parallel pressure solution seams along mudstone laminae in contrast burrowed units of peritidal strata have solution seams with irregular and anastamosing geometries laminated units with closely spaced bed parallel solution seams are more fragmented than bioturbated units with anastamosing solution seams in the deeper water depositional environment pelagic settling and turbidity currents are the dominant sedimentation processes resulting in laterally continuous deposits relative to the peritidal platform environment to quantify the fracture patterns in the basinal environment mechanical layer thickness values were measured from regions of low to high bed dip the results define a trend in which mechanical layer thickness decreases as layer dip increases a conceptual model is presented that emphasizes the link between sedimentary and structural fabric for the peritidal and basinal environments where solution seams localize in mud rich intervals and the resulting pressure solution surface geometry is influenced by sedimentary geometry i e stacked fining upward cycles burrows planar laminations in both facies types laterally continuous ps1 can behave as mechanical layer boundaries as layer parallel slip increases to accommodate shear strain in the fold thrust belt more ps1 behave as mechanical layer boundaries

relating background fractures to diagenesis and rock physical properties in a platform slope transect example of the maiella mountain central italy abstract fractured reservoirs are of prime interest as fracture networks control most of the fluid flow and or accumulation however characterizing 3d fracture patterns from subsurface data remains challenging studying fractures on outcrops is a good substitution to 1d data from subsurface exploration tools in addition outcrops allow deciphering the nature origin and conditions for fracture formation through the geodynamic history in this paper we aim at characterizing the true 3d fracture patterns and determining the genetic role of facies diagenesis and rock physical properties we targeted a platform slope transect within a carbonate reservoir analog the maiella mountain in central italy where implications for analog hydrocarbon reservoir can be discussed fracture patterns are sorted based on geometric and kinematic criteria from field measurements and petrographic analyzes on thin sections sedimentary facies pore types and rock physical properties have been characterized in order to establish the impact of early diagenesis on rock evolution diagenetic sequences have been unraveled and correlated to the fractures fracture sequences have been determined considering the cross cutting relationships and compared with burial uplift history in the two studied formations platform and slope carbonates we interpret a stage of fracturing perpendicular to bedding formed at shallow depth and occurring prior to major regional tectonic events the studied carbonates have undergone early diagenesis during fast and shallow burial conferring early brittle behavior the amount of stylolites is not correlated to burial depth but to fracture density porosity and free air p wave velocity it means that fracture development mechanical and petrophysical properties are acquired during early diagenesis both studied formations have undergone the same geodynamic history and their brittle response is different and not related to folding but to burial and early cementation deciphering the close relationship between sedimentary facies diagenetic and geodynamic history has allowed unraveling the controling factors on rock properties and therefore on fracture pattern

diagenesis and porosity preservation in eocene microporous limestones south florida usa abstract microporous limestones may contain immobile capillary bound irreducible water that is only in diffusional contact with mobile pore waters or in some reservoirs may contain producible hydrocarbons the preservation and distribution of microporosity impact both subsurface fluid flow and solute transport the diagenesis of microporous limestones has received relatively little attention because their very fine grains and cements are not amenable to standard analytical methods the ocala limestone upper eocene and upper avon park formation middle eocene in south florida contain microporous micritic limestones mudstones to packstones that are at an intermediate stage of diagenesis the limestones have been exposed to the active near surface environment but have not yet reached a burial depth sufficient for intense chemical compaction and associated porosity reduction nuclear magnetic resonance nmr logging allowed for the quantification of total porosity pore size distribution and permeability the ocala limestone and avon park formation have different predominant microfacies and porosity size distributions but yet both retain total porosities predominantly between 35 and 37 estimated microporosities range mostly between 12 and 45 the mudstones and wackestones of the ocala limestone have significantly lower permeabilities mostly 3 to 12 md than the wackestones to grainstones of the avon park formation commonly in the 100 to 3000 md range which have more mixed and overall coarser pore sizes computer modeling using carbon and oxygen stable data indicates that the studied microporous limestones underwent only a low degree of chemical diagenetic alteration despite likely experiencing episodes of freshwater flushing associated with post depositional sea level lowstands the ocala limestone and avon park formation limestones illustrate the general concept that total porosity is often largely preserved through early diagenesis although may undergo intra formational redistribution and that confined aquifers are diagenetic quiescent environments

characterization of reservoirs fluids and productions from the ordovician carbonate condensate field in the tarim basin northwestern china the tz no1 gas field in the tarim basin is the largest condensate field in china however it has not developed efficiently because of low and unpredictable production during the past 20 yr cores logging interpretation seismic descriptions fluid properties and production data indicate that this ordovician carbonate field is different from conventional stratigraphic oil and gas fields as follows 1 the hydrocarbon bearing area covers a large region of 2000 km2 2 matrix reservoirs have low porosity 20 per year an examination of the carbonate reservoir and hydrocarbon accumulation history suggests that this large scale stratigraphic accumulation formed during hydrocarbon emplacement in the early paleozoic with the reservoirs gradually evolving into tight reservoirs as a result of intense diagenesis variable amounts of neogene gas charged the tight carbonate reservoirs and formed unconventional accumulations the tz no1 gas field is characterized by a strongly heterogeneous tight matrix reservoir with a superimposed fracture cave reservoir and a complicated unconventional fluid distribution which provides insights into the exploitation challenges of unconventional carbonate resources

marine sedimentation at a calving glacier margin a 2 month study at a subpolar tidewater glacier was conducted at anvers island antarctic peninsula lat 64 s to improve understanding of sedimentary processes and associated deposits in glacial marine settings data collected in the ice proximal environment were 1 brash ice size distribution and melt rates to assess sediment source and transport mechanisms 2 conductivity temperature turbidity depth cttd profiles water samples and sediment trap catches to study processes and patterns of sediment dispersal 3 bottom cores and grabs to document recent glacial marine sedimentation and 4 remotely operated vehicle video surveys of the ice terminus and sea bottom to obtain images of physical and biological processes the water column is stratified and a halocline occurs at 20 30 m water temperature ranged from 1 to 1 6 c salinity was 32 34 and suspended solids were 3 5 mg l well above the bottom ephemeral turbid horizons contain 8 15 mg l suspended solids rare surface plumes and a quasipermanent turbid layer produced by resuspension during calving events reach 25 35 mg l meltwater drains from the glacier surface but no subglacial meltwater was detected directly entering the sea the most important sources of sediment appear to be the direct melting of the submerged ice front and melting of calved glacial ice most fragments of floating ice melt within 1 km of the ice front some sediment may be contributed as low density 1 1 g cm 3 algae bound sediment pellets by meltwater streams draining the ablating ice surface sediment enters the stratified water column and is moved by tidal and wind driven circulation marine sedimentation rates are low 3 mm yr relative to depth of bioturbation resulting in unstratified sandy mud with dispersed clasts

sulige field in the ordos basin geological setting field discovery and tight gas reservoirs abstract the sulige gas field in central ordos basin is one of the largest gas fields in china with about 860 7 billion cubic meters bcm of proven gas reserve this field covers an area of about 500 km 2 with largely tight clastic reservoirs the upper carboniferous lower permian deltaic sandstones and upper permian lacustrine mudstones deposited in a stable cratonic tectonic setting form an effective petroleum system that is favorable for gas accumulation in large scale stratigraphic lithologic traps based on wire line log response drilling stem test and core data a generalized classification system was developed for the gas reservoirs of the sulige field thus type i and ii rocks are capable of gas production without natural and or artificial fracturing type iii rocks are capable of gas production after artificial fracturing and type iv rocks are too tight to produce at commercial rates even with artificial fracturing the discovery of the sulige field not only adds large gas reserve in the ordos basin but strongly indicates the overlook potential of both tight sandstone reservoir rock and relatively porous and permeable sweet spots in it

glaciomarine sedimentation in disraeli fiord high arctic canada abstract disraeli fiord represents a glaciomarine environment unique to the north coast of ellesmere island featuring floating glacier tongues multi year landfast sea ice and a strongly stratified water body which results from enclosure of the fiord by ward hunt ice shelf it is a very low energy depositional environment even at sites proximal to glaciers sediment enters the fiord by interflow and is deposited through suspension settling ice rafting is precluded by the year round sea ice cover sedimentation rates in the inner fiord since about 7000 yrs b p are estimated to be 5 10 cm ka a reflection of the limited sediment influx from the cold based glaciers which contact the fiord changes in sediment structure through the holocene reflect changes in the rate and process of sediment input as well as changes in fiord circulation all of which relate to regional climate during deglaciation early holocene sedimentation rates were orders of magnitude greater than at present subglacial meltwater was the dominant source of inflow with sediment transported in turbid plumes as over or interflow and deposited through suspension settling this is supportive of other studies indicating that a pronounced climatic amelioration led to warm based glaciers at this time

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

the glacimarine sedimentary environment of expedition fiord canadian high arctic abstract expedition fiord is a small shallow inlet on the west coast of axel heiberg island near 80 n latitude it receives runoff and sediment at its head from a 1079 km 2 drainage basin 72 of which is glacier covered subbottom acoustic survey and cores from the fiord floor were used to assess the sedimentary environment most of the sediment is deposited within 3 km of the inflow from suspension in the overflowing cap and by gravity flows on the foreset beds of the delta occasionally weak turbidity currents reach the mid fiord where they deposit fine grained sediments icebergs from a large calving glacier in an adjacent fiord raft additional sediment especially to the outer part of the fiord they also scour the seafloor although the persistent ice cover and slow currents in the fiord restrict this process except near the inflow the total sediment accumulation since deglaciation is less than 20 m and the rates of 0 5 1 mm yr have not varied significantly to the present a thicker deposit in the outer fiord is probably related to an early holocene glacier margin near that location

coupling fracture facies with in situ permeability measurements to generate stochastic simulations of tight carbonate aquifer properties example from the lower cretaceous aquifer northern provence se france summary the relationships between fracture facies and permeability distribution have been explored using a well constrained 3d implicit structural model of hemipelagic low porosity low permeability carbonate from the northern provence region se france fracture and permeability facies were determined using an extensive dataset of 99 hydrogeological wells data processing was undertaken using a step by step approach involving i identification of the fracture facies based on well logs detailing structure and stratigraphy ii determination of permeability facies from the a priori correlation between the dimension of the hydraulic radius of influence deduced from slug test analyses and the type of reservoir heterogeneity fissure fracture fault zone etc iii three dimensional plot of fracture and permeability facies in the geological model using a variographic analysis of data thirty three sequential indicator simulations sis based on geostatistic analyses were realised on both fracture and permeability facies finally a connectivity algorithm was developed to compute the probability of connection between selected infiltration areas and the major aquifer springs via moderate to high permeability geological bodies key results are i fault zones have a greater role in controlling permeability facies distribution than on fracture facies repartition ii there is little correlation between permeability and fracture facies distributions iii connectivity results highlight the compartmentalisation of aquifers in the cadarache area the extensions of permeable geological bodies being limited by the n130 faults

innovative methods for flow unit and pore structure analyses in a tight siltstone and shale gas reservoir tight gas reservoirs are notoriously difficult to characterize routine methods developed for conventional reservoirs are not appropriate for tight gas reservoirs in this article we investigate the use of nonroutine methods to characterize permeability heterogeneity and pore structure of a tight gas reservoir for use in flow unit identification profile permeability is used to characterize fine scale 1 in 2 5 cm vertical heterogeneity in a tight gas core more than 500 measurements were made profile permeability although useful for characterizing heterogeneity will not provide in situ estimates of permeability furthermore the scale of measurement is much smaller than log scale pulse decay permeability measurements collected on core plugs under confining pressure were used to correct the profile permeability measurements to in situ stress conditions and 13 point averages of profile permeability were used to relate to log derived porosity measurements finally n2 adsorption a new method for tight gas was used to estimate the pore size distribution of several tight gas samples a unimodal or bimodal distribution was observed for the samples with the larger peak corresponding to the dominant pore throat size as confirmed by independent methods furthermore the adsorption desorption hysteresis loop shape was used to interpret the dominant pore shape as slot shaped pores which is typical of many tight gas reservoirs the n2 adsorption method provides rapid analysis and does not suffer from some of the same limitations of hg injection in the future we hope that the n2 adsorption method may prove useful for flow unit characterization based on dominant pore size of fine grained siltstone shale tight gas reservoirs

oceanographic and physiographic controls on modern sedimentation within antarctic fjords physical oceanographic data and modern surface sediments were collected from eleven fjords along the western side of the antarctic peninsula and south shetland islands surface sediment samples 62 were analyzed for texture and total organic carbon content the distribution of biogenic and terrigenous facies within the fjords is controlled by bay geometry and oceanographic regime climate plays a secondary role but along with ice drainage basin size controls the rate of terrigenous supply to the glacial marine environment specifically fjords along the danco coast and palmer archipelago with a high length to width ratio tend to have bottom sediments that are arenaceous where ice rafted sediment is released preferentially at the head of the fjord biogenic facies are favored where the bay geometry is complex where such complexity exists separate oceanographic regimes develop that lead to separation of terrigenous and biogenic sediments processes of interflow mid and deep water turbid cold tongues and coriolis deflection produce terrigenous facies along the inner fjord and western edges of a fjord system warm outer bay waters tend to develop a stable eddy circulation pattern that favors the productivity of phytoplankton in the surface layers outer bays are therefore floored with organic rich siliceous muds and ice rafted material only in the south shetland islands is melt water input significant enough to generate estuarine circulation within the fjord but here strong bottom currents result in arenaceous bottom sediments with no biogenic facies ice rafted diamictons are produced proximal to the edges of small tide water glaciers in the south shetlands the facies relationships established in this study provide a strong reference for paleoclimatic studies that utilize downcore measurements of texture and organic carbon

successive episodes of normal faulting and fracturing resulting from progressive extension during the uplift of the holy cross mountains poland conjugate normal faults extension fractures and mesh fracture structures were investigated in devonian carbonate rocks from the southern part of the holy cross mountains hcm central poland strata folded during variscan deformations were later subject to uplift resulting in increasing extension in the upper part of the rock mass at a relatively shallow depth faults and fractures developed in an orderly vertical succession first mesh fracture structures and conjugate normal fault sets enclosing acute dihedral angles 2 of over 45 were formed next conjugate normal faults with 2 less than 45 and sub vertical extension fractures developed the occurrences of conjugate normal sets enclosing different dihedral acute angles and extension fractures with similar strikes juxtaposed with each other at the same stratigraphic level point to the fact that the uplifted rock mass underwent successive changes in a stress regime leading to the formation of these structures the first sets of conjugate normal faults and fractures developed when the hcm were uplifted during a late stage of variscan deformations the next sets of extension fractures and conjugate normal faults developed during the following uplift events interrupted by periods of sedimentation of the mesozoic and younger strata

stylolites as open fluid conduits abstract stylolites have hitherto been regarded as a direct result of dissolution in response to stresses applied during compaction zones of cementation and dolomitization and in some rocks associated internal sediments indicate that they were pathways to fluid flow yet clay residues and impermeable zones suggest that they are now less able to transmit fluids it is proposed that some stylolites represent former open fluid pathways developed in an over pressured system and that the familiar morphologies were only imposed during compaction as formation pore water pressures declined

glacigenic and associated marine sediments in the weddell sea fjords of spitsbergen and the barents sea a review abstract studies of glacigenic and associated marine sediments in the weddell sea fjords of spitsbergen and barents sea show that during periods of limited glacial coverage interstadials large amounts of material may be brought into the sea mainly by meltwater the major load is deposited relatively close to the icefront 1 at times of maximum glacial extent grounded ice extends onto the shelf with the shelf edge as the outer limit and land and inner shelf areas are eroded main glacial depositional sites are then the outer shelf and shelf edge the sedimentary environment outside an ice shelf is characterized by low clastic supply mainly deposited from icebergs due to low turbidity and a supply of nutrient rich water biogenic matter accumulates leading to a glaciomarine sequence rich in bioclastics sea level lowering results in grounding and seaward expansion of the ice shelf subsequent eustatic rise causes the grounded ice to float deglaciation and sea level rise reduces clastic input to the shelves glacial sediments are reworked and carbonates may accumulate forming tillite carbonate sequence e g late precambrian break up of marine ice sheets such as the barents sea ice sheet seems to have taken place stepwise in response to eustatic rise and climatic changes

biogenic facies in the antarctic glacimarine environment basis for a polar glacimarine summary abstract the association of glacial deposits diamicts or tillites with biogenic and or chemically derived sedimentary rocks in the ancient record is one which continues to challenge interpretation an analogous association can be found on portions of the east antarctic continental shelf where relict glacial and glacimarine diamictons are overlain by a varied and complex suite of modern biogenic deposits carbonate bioclastic material can be found in two distinct zones and its deposition may have been contemporaneous with partial glacial expansion onto the shelf a nearshore carbonate facies consists of in situ and partially reworked barnacles bryozoans ostracods pelycepods gastropods planktonic and benthonic foraminifera a siliceous biogenic component is intermixed within the shallow water 500 m siliceous facies of pelagic hemipelagic and sediment gravity flow origin is found within inner shelf depressions and consists predominantly of diatom frustules and radiolaria these accumulations of siliceous ooze and mud clearly post date glacial recession from the shelf and contain laminated cross laminated and massive interbeds which are accumulating at rates of 0 3 cm yr the extreme polar climate inhibits meltwater production and terrigenous influx into marine basins this allows for the accumulation of biogenic rich material which is also favored by the high seasonal productivity inherent in the south polar oceans ancient analogs to these modern deposits exist and their association with glacimarine strata could be used to infer polar climatic settings and glacial retreat that is related to eustatic sea level rather than regional warming

pore system characterisation in heterogeneous carbonates an alternative approach to widely used rock typing methodologies abstract rock typing is used in carbonate reservoir characterisation to group petrophysically similar rocks for reservoir modelling purposes the focus is often upon bringing together samples with similar porosity permeability and capillary pressure to achieve this rock typing studies are usually conducted primarily on core plugs using routine core analysis data and thin sections however such an approach results in a tendency to focus on grouping samples with consistent petrophysical properties ignoring the geological controls on pore evolution this means that modelling rock types predicatively using 3d geocellular models is not easily accomplished furthermore where geological observations are made they tend to consider only depositional rock properties and not the effects of diagenesis in doing so a key control on the evolution of the carbonate pore network is ignored this paper addresses these issues using a giant carbonate reservoir in northern oman rock types were defined on the basis of pore geometry whilst retaining distinct geological descriptors the aim was to ensure that each rock type could be defined on the basis of both its petrophysical properties and behaviour during hydrocarbon recovery the results show that characterisation of pore heterogeneity is critical to the prediction of flow behaviour under reservoir conditions and that routine petrophysical parameters are often not good indicators of sweep efficiency

porosity gain and loss in unconventional reservoirs example of rock typing in lower cretaceous hemipelagic limestones se france provence abstract we characterize porosity evolution in hemipelagic limestones of the lower cretaceous vocontian basin northern provence se france which are surface analogs for deep unconventional reservoirs firstly a structural and sedimentologic analysis was conducted on outcrops and on borehole cores of berriasian to lower valanginian rocks secondly 330 plugs were analyzed for facies pore typing porosity and acoustic vp velocity measurements this petrophysical dataset was compared to the burial uplift history of the host rock hemipelagic carbonates include four sedimentary formations comprising nine facies splitted into mud and grain dominated formations which display 0 8 porosity range depending on the initial grain to mud ratio of the sediments and on basin stress regime evolution during burial uplift history initial porosity was better preserved in grain dominated formations where early micritic cements built bridges between grains generating a solid skeleton that prevented inhibited the burial compaction effects in mud dominated facies micrite was initially diffuse within the rock matrix but was progressively rearranged under burial stress while the generation of an extensive amount of stylolites occurred the differential response of compacted grain and mud dominated facies to a late regional 1 km uplift resulted in large porosity generation should range from 15 to 35 localized in the mud dominated facies through the dilatancy of stylolite and pressure solution seams this study highlights how important is the combination of classical approaches including facies pore type and diagenesis characterization with the structural analyses one key result applicable to several types of unconventional reservoirs is the effective porosity generated by reactivation of compaction structures

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out

glacimarine processes and inductive lithofacies modelling of ice shelf and tidewater glacier sediments based on quaternary examples abstract three variables control the type of glacial debris source and where and when debris is introduced to an ocean 1 type of glacial ice source ice sheet or valley glacier 2 condition of a grounded glacier melting freezing or frozen base and cold or warm internal ice and 3 type of glacier front ice shelf or tidewater using these variables in different combinations eight glacimarine regimes are postulated to exist by integrating information from holocene sediment and pleistocene glacimarine sequences with the eight inductive regimes lithofacies relationships are predicted and modelled the models show a complexity of glacimarine lithofacies the importance of recognizing this complexity lies in realistic interpretations of ancient sequences ice shelf sediment may be limited in the geologic record because of tectonic and climatic constraints required for ice shelf formation alternatively in some tidewater glacier regimes larger volumes of glacimarine sediment are produced than in ice shelf regimes ice proximal sediment is distinctive especially near subglacial meltwater stream effluxes ice shelves and tidewater glaciers may produce similar ice proximal lithofacies e g morainal bank associations and laminites in certain regimes however details of lithofacies may aid in distinguishing ice shelf from tidewater regimes ice shelf and tidewater iceberg zone sediment can be distinguished from each other if basal and high level englacial debris can be distinguished ice shelf zone sediment out from the grounding line has a basal debris source high level englacial debris is deposited in more distal ice shelf zones resulting in sediment by pass in corresponding positions of a tidewater glacier regime iceberg rafted debris comprises mixed basal and high level englacial debris deeper water zones receive large volumes of sediment when a melting freezing base grounding line is at the edge of a continental shelf when the grounding line retreats sediment supply may be drastically reduced because of a back sloping continental shelf and trapping by morainal banks cool water bioclastic carbonates may accumulate once a grounding line has retreated although ice shelf iceberg zones are more extensive than those in tidewater regimes ice shelf icebergs have low debris content and resulting ice rafted debris may be scarce the smallest volume of sediment is produced by a frozen base ice sheet ending as a tidewater front and the largest volume by a melting freezing base valley tidewater glacier glacimarine sediment has a relatively high preservation potential near continental interiors or margins however at continental margins mass flow olistostrome and tectonic melange deposits may be preserved with glacimarine diamictites these deposits may be difficult to distinguish from each other especially if metamorphosed such as at a convergent margin the models presented do not fully represent the glacimarine environment because many other variables are omitted however the principal concepts of generating glacimarine lithofacies associations are considered continued research will verify if each regime exists and will provide data on those regimes that exist today to compare and modify these models

factors controlling elastic properties in carbonate sediments and rocks carbonate sediments are prone to rapid and pervasive diagenetic alterations that change the mineralogy and pore structure within carbonate rocks in particular cementation and dissolution processes continuously modify the pore structure to create or destroy porosity in extreme cases these modifications can completely change the mineralogy from aragonite calcite to dolomite or reverse the pore distribution whereby original grains are dissolved to produce pores as the original pore space is filled with cement to form the rock figure 1 all these modifications alter the elastic properties of the rock and therefore the sonic velocity the result is a dynamic relationship among diagenesis porosity pore type and sonic velocity the result is a wide range of sonic velocity in carbonates in which compressional wave velocity vp ranges from 1700 to 6600 m s and shear wave velocity vs from 600 to 3500 m s

biogenic sedimentation in mcmurdo sound antarctica surface sediments from eastern mcmurdo sound and fjord basins of the victoria land coast are enriched in organic carbon and opal at depths below 600 m opal contents commonly exceed 30 organic carbon contents average 1 5 and are as high as 3 5 opal and organic carbon are supplied by local production within and below sea ice and by advection from open water areas of the southwestern ross sea the distribution of organic carbon and opal at the sea floor is consistent with cyclonic clockwise water circulation in mcmurdo sound advective transport from the ross sea supplies biogenic sediment to eastern and northern mcmurdo sound the southwestern shelf is bathed by waters derived in part from beneath the ross ice shelf which transport very little allochthonous carbon the supply of biogenic debris in southwestern mcmurdo sound is further curtailed by sea ice conditions e g more prevalent multi year sea ice which reduces photosynthesis and the absence of summer basal melting and ice breakout which restricts the flux of sea ice and open water production to the sea floor consistent with this hypothesis organic carbon fluxes measured via sediment trapping beneath fast ice are one to two orders of magnitude higher in eastern versus western mcmurdo sound pronounced cross sound gradients in shallow water benthic biomass and species diversity have previously been attributed to east west variations in productivity our surface sediment data suggest that similar or even more dramatic contrasts in benthic community structure may exist in the deeper water areas of mcmurdo sound pervasive resuspension is documented by pronounced near bottom flux increases measured by sediment trapping resuspension events which may be more common during the austral winter act to winnow and transport biogenic debris and fine terrigenous sediment from shallow regions 400 m to protected shelf basins despite oxidative loss within the water column the accumulation rate of organic carbon in the deep basins of mcmurdo sound averages 45 mg c m 2 day 1 more than an order of magnitude higher than the world average organic carbon flux to continental margins and equivalent to accumulation rates observed in many anoxic settings preservation efficiency of opal during transport and deposition appears anomalously high and the most opal rich sediments of mcmurdo sound are depleted in organic carbon relative to mid and low latitude siliceous deposits despite this observation if similar patterns of biogenic sedimentation exist on many other parts of the shelf the antarctic continental margin is an important sink for sedimentary organic carbon as has been previously suggested for the silica system

heterogeneities and diagenetic control on the spatial distribution of carbonate rocks acoustic properties at the outcrop scale abstract carbonate rocks are characterized by a high heterogeneity of their properties at the outcrop scale that results from complex interactions between sedimentary diagenetic and fracturing processes here we show acoustic p wave velocity measurements conducted step by step on three decameter scale outcrop surfaces in cretaceous carbonate rocks with contrasted heterogeneities two shallow water highly fractured cassis and low fractured calissanne carbonates and one basin highly compacted grignantes p wave measurements were compared to outcrop geology and with plug porosity p wave velocity laboratory measurements and a thin section study results show a strong scale dependence of p wave velocities which always are lower at the decimeter scale than at the plug scale vp in situ versus vp plug large discrepancies 2000 m s highlight the signature of large heterogeneities open fractures stylolithes and lower discrepancies in situ distribution follows the dip of the ripple foresets which are controlling grain sorting and porosity in calissanne outcrop the near isotropic vp are explained by an early intense fracturing and cementation in cassis outcrop and the strong horizontal anisotropy with a hole effect behavior on variograms highlights the late re opening of multiple intervals of compaction bands in grignantes these results illustrate that the early diagenesis coupled to fracture or stylolithe reactivation opening are two key processes that explain the outcrop scale p wave velocity distribution anisotropy in carbonates

are stylolites fluid flow efficient features abstract it sounds counter intuitive to consider contraction features such as stylolites as potential conduits for flow however this idea has grown since 1980 with geoscientists finding many examples principally in carbonate reservoirs where stylolites can be considered as fluid efficient features among others these features can be reactivated stylolites can generate positive porosity and permeability anomalies can drive corrosive fluids or can remain open in an overpressured system conversely stylolites can also be closed forever these impermeable stylolites can generate permeability anisotropy that may impact fluid movements stylolites require particular attention to evaluate whether they act as drains or as barriers to flow compartmentalisation we review some of the key studies of the past thirty years with a special attention to the most recent ones we end up considering their mechanical origin their nucleation and growth their past and present impact on reservoir properties and performances as key factors influencing the flow efficiency differentiation of these features this short review presents the latest theories and observations about stylolites with respect to the key factors aforementioned the authors support herein that a distinction should be made between processes occurring in the past and the present day impact the stylolite had on reservoir properties

sedimentary and diagenetic controls on the multiscale fracturing pattern of a carbonate reservoir the madison formation sheep mountain wyoming usa a multidisciplinary approach has been applied to characterize the madison formation in the sheep mountain anticline at different scales in order to illustrate the relationship between sedimentology diagenesis and fracturing pattern sedimentary facies are organized into three types of elementary facies sequences showing different vertical evolutions of their petrophysical properties this is controlled by the double influence of the initial sedimentary facies and the diagenetic evolution three main sets of fractures related to the sevier and laramide compressive phases and to the folding of sheep mountain have been described at three orders of scale at small scale the sevier related set i and folding related set iii are the most abundant set ii dated from the early stage of the laramide compression exhibits the highest fracture intensity there is also a clear faciologic control on the fracture intensity small scale mechanical units correspond to entire facies sequences when no or little petrophysical property contrasts between beds are observed conversely small scale mechanical units correspond to each bed of a facies sequence in the case of high contrasts of petrophysical properties between beds at intermediate scale set ii is not present and sets i and iii show the highest fracture intensities the latter are partly controlled by the mechanical unit thickness intermediate and large scale mechanical unit distribution is mainly controlled by the overall faciologic vertical stacking pattern and by the position across the sheep mountain anticline forelimb or backlimb sheep mountain outcrops provide a good spatial representation of the carbonate reservoir heterogeneities from the micro to the field scale illustrating the complexity of fractured carbonate reservoirs

influence of a major exposure surface on the development of microporous micritic limestones example of the upper mishrif formation cenomanian of the middle east microporous platform top limestones of the cenomanian mishrif formation offshore qatar were studied in order to investigate the diagenetic processes associated with the top mishrif subaerial unconformity and its influence on the development of microporosity in underlying carbonates petrographical and stable isotope results indicate that complex diagenetic changes occurred during subaerial exposure of the mishrif formation including pervasive dissolution and meteoric cementation as well as neomorphism of the micritic matrix micrites at the top of the mishrif formation are coarse i e 2 m sub rounded and very dull luminescent under cathodoluminescence in this uppermost part of the studied interval the limestone matrix first underwent dissolution of unstable grains in the vadose zone with subsequent precipitation of low magnesium calcite lmc overgrowths within an oxidising phreatic setting this process explains the poor luminescence of the micrite crystals and their relatively coarse crystallometry which results in the present day in relatively good reservoir properties 13c ratios within the microporous limestones are negative up to 4 v pdb due to the incorporation of isotopically light carbon derived from palaeosols which developed during exposure by contrast fine i e 2 m polyhedral and bright luminescent micritic matrix with higher 13c values up to 4 v pdb and lower permeability are dominant within the underlying carbonate interval these fine micrites are likely to be the product of early neomorphic recrystallisation in the lowermost part of the palaeo aquifer associated with the top mishrif unconformity and were precipitated from meteoric or mixed dysoxic waters which were slightly supersaturated with respect to calcite

recent atmospheric warming and retreat of ice shelves on the antarctic peninsula in 1978 mercer1 discussed the probable effects of climate warming on the antarctic ice sheet predicting that one sign of a warming trend in this region would be the retreat of ice shelves on the antarctic peninsula analyses of 50 year meteorological records have since revealed atmospheric warming on the antarctic peninsula2 3 and a number of ice shelves have retreated4 8 here we present time series of observations of the areal extent of nine ice shelves on the antarctic peninsula showing that five northerly ones have retreated dramatically in the past fifty years while those further south show no clear trend comparison with airtemperature data shows that the pattern and magnitude of ice shelf retreat is consistent with the existence of an abrupt thermal limit on iceshelf viability the isotherm associated with this limit having been driven south by the atmospheric warming ice shelves therefore appear to be sensitive indicators of climate change

a model for sedimentation by tidewater glaciers sampling of sediment from the fjord floor in front of tidewater glaciers in glacier bay alaska has provided information about processes in this restricted glacimarine dreimanis 1979 setting sediment sampling in conjunction with oceanographic and glacial dynamics data has also enabled the discrimination of sediment types and their facies associations deposits are strongly controlled by sea water characteristics position and sediment discharge of melt water streams iceberg calving and rate of glacierfront retreat five distinct facies associations have been found to reflect glacier fjord regimes the facies associations and ice fjord conditions responsible for them form the basis for constructing a preliminary model for glacimarine sedimentation by tidewater glaciers the model can be used to predict i rapid retreat of an actively calving ice front ii slow retreat or stabilization of a calving ice front at a channel constriction iii stabilization of a melting very rarely calving ice front when the glacier base is near tidewater elevation and iv large outwash delta progradation into a fjord when the ice front retreats onto land this model can be used to interpret facies associations found in a stratigraphic record

scaling relationships of joint and vein arrays from the burren co clare ireland abstract we present a study of the systematics of veins and joints in carboniferous limestones of the burren ireland scaling relationships were established for fracture arrays mapped from low elevation aerial photographs that image fractures on numerous limestone pavements for areas up to ca 1 km2 the veins and joints occur in the same sequence but have contrasting scaling properties the veins strike north south and cut many beds to form vertically persistent non stratabound arrays they are strongly clustered and have scale invariant geometric properties vein geometries suggest they grew sub critically under relatively high differential stresses during north south directed variscan compression the joints form stratabound arrays with regular spacings that scale with bed thickness they show greater strike variation than the veins and have lognormal length distributions the joints formed during uplift under low differential stress conditions the contrasting scaling properties of the joints and veins are attributed to different overburden stresses at the time of formation the veins formed at greater depths than the joints in conditions that favoured fracture propagation across mechanical discontinuities resulting in the development of non stratabound scaling properties

calciclastic submarine fans an integrated overview abstract calciclastic submarine fans are rare in the stratigraphic record and no bona fide present day analogue has been described to date possibly because of that and although calciclastic submarine fans have long intrigued deep water carbonate sedimentologists they have largely been overlooked by the academic and industrial communities to fill this gap we have compiled and critically reviewed the existing sedimentological literature on calciclastic submarine fans thus offering an updated view of this type of carbonate slope sedimentary system calciclastic submarine fans range in length from just a few to more than 100 km three different types can be distinguished 1 coarse grained small sized most calciclastic submarine fans were formed on low angle slopes and were sourced from distally steepened carbonate ramps subjected to high energy currents under these conditions shallow water loose grainy sediments were transferred to the ramp slope and eventually funnelled into the submarine fan by sediment gravity flows these conditions seem to have been more easily met on leeward margins in which the formation of reefs was hampered by cool waters nutrient enrichment or oligophoty another circumstance that contributes to the transfer of shallow water sediments to the distal ramp slope is a low sea level forcing the carbonate factory closer to the slope break and destabilizing sediments by increased pore water pressure however the most important factor controlling the development of calciclastic submarine fans was the existence of an efficient funnelling mechanism forcing sediment gravity flows to merge downslope and build up a point sourced sedimentary accumulation in most cases this occurred through a major slope depression associated with tectonic structures an inherited topography or large scale mass failures

geometry and petrophysical parameters of a calcarenitic turbidite lobe barremian aptian pas de la cluse france abstract several bioclastic resedimentation systems late barremian to early aptian in age outcrop in southeastern france the upslope portion of a calcarenitic turbidite lobe outcrops as a cliff 2 3 km wide and up to 45 m high oriented roughly perpendicular to the direction of palaeoflow it has been studied through 31 sections the studied body is point sourced and corresponds to a channel lobe transition located at the toe of slope of a urgonian carbonate platform on the western edge of the vocontian trough french subalpine chains the internal geometry of the lobe is the result of repeated erosion and filling of channels creating well defined stacked units filling phases result in lobe shaped morphology units evolve from base to top in an ordered way first the sediments exported from the platform and deposited in the basin are represented by decimetre thick sheet like and fine grained bouma a e and ab e sequences in this first unit erosion is limited and represented by shallow and symmetrical channels this initial phase of deposition corresponds to the formation of a lobe with a positive morphology the next phase of system development is more erosional and this is dominated by migrating sinuous channels several metres deep filled with coarse grained poorly graded turbidites these channels are first filled through lateral accretion and then sediment spills over to create a sub lobe the mixture of channel and lobe features could be considered as a carbonate equivalent of the channel levee system of siliciclastic deposits finally the system ends with the deposition of a flat based unit this unit extends for more than 2 km and is composed of the southwards lateral accretion of hundreds of metres wide sequences each composed of a debris flow deposit capped by a coarse grained calcarenite bed these sigmoidal bodies could be the ultimate forms of a spreading out phenomenon of the lobe overall the pas de la cluse system is a coarsening upward body as the grain size is related to the nature of the material this coarsening upward implies either an evolution in the nature of the carbonate material produced on the platform from grainy to bioconstructed a displacement of the source sediment from inner platform to margin or a displacement of the channel head through time from distal to proximal the upward evolution from classic turbidites to debris flow deposits can be explained either by the way the platform facies evolved with time in a depositional sequence or through retrogressive erosion by successive slope failures that cut deeper and deeper into the platform slope although its present day porosity is low the size of the system and the fact that it is encased in marls make it an interesting outcrop analogue for hydrocarbon exploration

hemipelagic and turbiditic mudstone facies associations in the upper cretaceous gosau group of the northern calcareous alps austria abstract turbiditic and hemipelagic mudstones from the upper cretaceous to lower paleocene nierental formation of the northern calcareous alps austria can be differentiated by employing relatively simole laboratory and higher hemipelagic mudstones turn out to be characterized by finer grain sizes of the non carbonate fraction and higher carbonate contents sediment colour bioturbation distribution of microfossils weathering characteristics and bed sequence analysis were used as additional criteria to recognize the genetically different mudstone types for acquiring the non carbonate grain size data the samples were leached in hydrochloric acid and the remaining siliciclastic material was used for analysis with a sedigraph the median values of the grain size distributions of the turbidite mudstones reach up to 9 7 whereas the hemipelagic median values lie between 9 5 and 10 8 carbonate contents of the turbidite mudstones range between 22 and 68 those of the hemipelagic mudstones between 58 and 82 single bed profiles show distinct vertical trends of both grain size and carbonate content values for the turbidite mudstone division no such trends exist in the hemipelagic mudstone beds five basic types of turbiditic and or hemipelagic mudstones can be recognized 1 turbidite hemipelagite couplets 2 pure hemipelagic mudstone sequences 3 hemipelagic mudstone sequences with very thin turbiditic interacalations 4 a thorouthly mixed turbiditic hemipelagic mudstone type which is characterized by intense bioturbations and 5 mud turbidites without any hemipelagic intervals most of these types occur simultaneously at different localities of the nierental formation

thermohaline steps induced by melting of the erebus glacier tongue a vertically stable step like thermohaline structure is observed throughout a continuous 400 m conductivity temperature depth ctd profile taken near the erebus glacier tongue mcmurdo sound antarctica the pattern is best developed between the sea surface and 250 m depth the interval corresponding to that of the irregular underwater profile of the glacier tongue the steps average 17 m in thickness and typically display discontinuities of 0 1 c in temperature 0 04 in salinity and 3 5 10 4 g cm 3 in density the observations are compared with theory and laboratory experiments of cell development and lateral flow near ice melting into vertically stratified salt water at this location subsurface seawater is inferred to remain above the in situ freezing point year round and contains sufficient heat to account for much of the glacier tongue thinning by basal melting an adequate volume of meltwater would result to produce the measured salinity steps we discuss related observations and some implications of this process for ocean circulation and biological productivity in the antarctic

subglacial meltwater discharge in the open marine tidewater glacier environment observations from nordaustlandet svalbard archipelago the ice proximal environment of the nordaustlandet tidewater ice cap svalbard archipelago is one of the best analogues for understanding glacial geologic processes of northern continental shelves during initial pleistocene deglaciation investigations of the proglacial region in 1980 1983 showed that the sedimentary environment is dominated by numerous meltwater outflows which discharge sediment laden water from subglacial meltwater streams during the summer two large stable meltwater outflows were observed in embayments along the southern part of the ice front landsat images show that both outflows have been in approximately the same position since at least 1976 they are located at the intersection of glacial drainage basins and centered over depressions in the underlying bedrock an outflow valley extending away from the ice front was observed in front of the western meltwater outflow sidescan sonar profiling along the glacier front showed a 200 m wide gap in acoustic reflection at the base of the western meltwater outflow probably caused by meltwater effluence enhanced sediment accumulations in this region observed as a 3 ms sediment drape in front of the outflow and large arcuate ridges in the outflow valley testify to the transport efficiency of the subglacial meltwater stream several mounds up to about 25 m high and 200 m wide are observed on sidescan and 3 5 khz profiles directly in front of the outflow although samples from these structures are absent they are most likely composed of sediment and are similar to beaded eskers observed in pleistocene glacimarine sequences indicating locally very high sedimentation rates fine grained components of the subglacial discharge incorporated in the buoyant meltwater plume are usually entrained in a westerly coastal current elevated suspended particulate material concentrations are observed within the coastal waters in a region extending about 15 km perpendicular to the glacier front and at least 60 km along the ice front extending into the northwestern barents sea

an apparatus to experimentally model the dynamics of ductile shear zones abstract we present a ring shear apparatus designed to deform soft ductile rock analogues to arbitraily high shear strains in simple shear in experiments attempting to model processes that occur in natural ductile shear zones the sintered specimen is deformed between two unsupported cylinders by two rotating grips under a normal and shear stress up to 5 mpa and at temperatures up to 600 k first results of experiments with octachloropropane ocp c 3 cl 8 and camphor c 10 h 16 o are presented experiments with pure ocp samples show that a steady state strain rate is achieved at a shear strain of around 4 when a steady state grain shape and crystallographic preferred orientation is established mechanical properties compare well with those of cylindrical samples deformed in triaxial compression shear localisation is relatively minor samples of pure camphor on the other hand show very strong shear localisation behaviour and mechanical properties which do not compare well with those of cylindrical samples deformed in triaxial compression experiments with two phase samples ocp containing stronger camphor inclusions show an initial softening of the composite followed by hardening this is associated with the development of a foliation defined by camphor inclusions that stretched to lenses and layers and the subsequent folding of this foliation at shear strains above 50 a steady state strain rate is not reached even at strains of 100 the experiments produce results closely resembling the kinematics observed in natural shear zones and give a first look into the complex changes in mechanical properties which occur at very high shear strains they show how this technique in conjunction with transparent deformation cells can be used to study both microstructural and rheological development during long simple shear deformation histories

marine controls on modern sedimentation on the antarctic continental shelf abstract the extreme depth and rugged topography of the antarctic continental shelf its seasonal to perennial sea ice cover and its large slowly calving ice shelves exert major influences on the adjacent ocean and sediments regionally variable exchanges of heat and momentum between the atmosphere and ocean result in net off shelf transport of sea ice and net downward salt flux this buoyancy partitioning drives a vertical convection that is followed by westward and northward flows of ventilated dense water across the continental shelves shelf waters meet mix with and modify circumpolar deep water at a dynamic slope front near the continental shelf break locally or periodically forming antarctic bottom water nutrient rich deep water upwells along this front regionally intruding onto the continental shelf in long thermally attenuated filaments these intrusions are frequently linked to mass balance processes at the base of ice tongues and ice shelves melting freezing and tidal currents beneath glacial ice are related to the larger scale circulation and to the fate of basal debris in the ice sheet the ocean circulation near ice fronts and decadal variability in ice front positions may account for several features of the sediment record time series 1 yr current measurements in the ross and weddell seas have revealed moderate to strong currents that are seasonally variable vertically coherent and dominated by the diurnal tide and longer term events few long term current measurements exist in the shelf and slope bottom boundary layers that are most important to sediment deposition and erosion icebergs may frequent certain routes e g in association with currents and fronts near the edge of the continental shelf and are capable of scouring and resuspending sediments down to shelf break depths sea ice plays a role in shelf sedimentation by rafting eolian and bottom debris away from the nearshore regions in addition generally offshore winds produce numerous leads and polynyas along the coastline and within the sea ice fields higher biological productivity in these polynyas may be linked to higher local sedimentation rates biogenic marine sediments apparently comprise a major percentage of the regionally variable modern shelf deposits despite the impressive glacial setting and polar environment