seismic petrophysics and isotropic anisotropic avo methods for unconventional gas exploration exploration and drilling for natural gas in north america has moved radically away from conventional reservoirs to focus on unconventional reservoirs such as tight gas sands and shales these reservoirs have low porosity and near zero permeability with gas stored in natural fractures and within the matrix porosity economic gas production requires hydraulic fracture stimulation to open connections to existing natural fractures or matrix porosity and successful stimulation depends on the formation s geomechanical brittleness being capable of supporting extensive induced fractures however despite adequate stimulation significant variations exist between wells in expected ultimate recovery eur due to the heterogeneity of these resource plays consequently predicting natural fractures or fracture prone sweet spots is essential to optimize development of such plays

seismic inversion for organic richness and fracture gradient in unconventional reservoirs eagle ford shale texas abstract a method using eagle ford organic shale from texas predicts key unconventional reservoir properties total organic carbon toc and fracture pressure gradient fg applying petrophysical and rock physics models from previous work to the available well log data generates the required logs for calibration i e shear sonic toc organic porosity and saturation prestack seismic data can be evaluated using synthetic forward modeling and can be conditioned further to improve avo response simultaneous prestack inversion to derive acoustic impedance ai and shear impedance si volumes can be run and toc can be derived from those volumes by linear interpolation fracture gradient which also can be thought of as frackability is related directly to poisson9s ratio and effective stress and can be estimated from elastic properties the fracture gradient is related nonlinearly and inversely to toc i e assuming laterally and vertically homogeneous pore pressure distribution inside the spatially limited block of the eagle ford shale higher toc results in lower fg and hence better frackability

berriasian aptian black shales of the maiolica formation lombardian basin southern alps northern italy local to global events the lower cretaceous maiolica formation represents the classic pelagic succession of the apulian margin of the tethys ocean in the southern alps it consists of white calcilutites with minor marly layers and black shales a multidisciplinary study including lithostratigraphy nannofossil assemblage composition and organic geochemistry was performed on two composite sections in order to characterise the marl shale layers that record production and storage of organic matter om and terrigenous input in the berriasian aptian interval such layers are associated with turbidite deposition and bottom current activity in the late berriasian earliest valanginian late valanginian and early aptian the uppermost hauterivian barremian black shales alternate with pelagic calcilutites forming an interval barren of redeposited layers the quantity of organic carbon increases from the berriasian to the barremian the background composition of the kerogen is continental dominated with varying proportions of marine derived om preservation of om did not necessarily occur under anoxic conditions on the contrary storage of om probably resulted from enhanced terrestrial input of nutrients siliciclastics woody and herbaceous fragments inducing mesotrophic conditions and increased primary productivity higher fertility of surface waters during the late valanginian and early aptian is reflected by distinctive decreases in abundance of oligotrophic nannoconids and increase in abundance of mesotrophic diazomatholithus in the valanginian during the barremian the rhythmic establishment of eutrophic conditions documented by the highest amounts of marine om determined high frequency fluctuations in nannoconid abundance the early aptian nannoconid crisis is interpreted as the result of a major climate change and enhanced primary productivity variations of calcareous nannofloras as well as changes of lithology and om which we describe in the lower cretaceous maiolica formation of the lombardian basin can also be recognised in coeval units from other sites of the apulian side of the tethys similar episodes of siliciclastic and terrestrial om accumulation and major changes in calcareous nannofossils are also recorded in the upper valanginian and lower aptian pelagic sediments of the tethys atlantic and partly pacific oceans such nannofossil variations are coeval with positive excursions of the 13c curve and suggest perturbations of the bio geochemical cycles at a global scale additional minor changes in siliciclastic and terrestrial om accumulation as well as in nannofloras are probably the result of local events strictly related to the physiography and evolution of the tethyan margin

fracture mapping using seismic amplitude variation with offset and azimuth analysis at the weyburn co2 storage site abstractcap rock integrity is an essential characteristic of any reservoir to be used for long term co2 storage seismic avoa amplitude variation with offset and azimuth techniques have been applied to map hti anisotropy near the cap rock of the weyburn field in southeast saskatchewan canada with the purpose of identifying potential fracture zones that may compromise seal integrity this analysis supported by modeling observes the top of the regional seal watrous formation to have low levels of hti anisotropy whereas the reservoir cap rock composite midale evaporite and ratcliffe beds contains isolated areas of high intensity anisotropy which may be fracture related properties of the fracture fill and hydraulic conductivity within the inferred fracture zones are not constrained using this technique the predominant orientations of the observed anisotropy are parallel and normal to the direction of maximum horizontal stress northeast southwest and agree closely with previous fracture studies

correlation of basinal carbonate cycles to nearshore parasequences in the late cretaceous greenhorn seaway western interior u s a upper cretaceous limestone shale couplets developed within the late transgressive stage of the greenhorn cyclothem may be correlated from carbonate dominated basinal sequences in central kansas and colorado westward to clastic cycles in southern utah six such basinal couplets have been traced to corresponding upward coarsening progradational cycles developed on the western margin of the western interior basin in the central basin in colorado and kansas these sedimentary cycles are represented by limestone shale and marlstone shale couplets 0 5 1 0 m in thickness more calcareous parts of these couplets may be correlated westward into condensed fossiliferous concretion and shell beds in proximal offshore lithofacies of arizona and utah these concretion and shell beds are physically traceable farther landward westward into bioturbated fossil rich transgressive lag deposits that bound 10 to 20 m thick coarsening upward progradational strand plain deposits parasequences in southwestern utah thus the progra dational phase of parasequence deposition correlates with accumulation of clay rich sediment in the central basin and the transgressive phase is characterized by reduced terrigenous input and deposition of carbonate rich sediment we consider milankovitch style orbital forcing of climate and tectonically induced fluctuations in rates of foredeep basin subsidence as possible forcing mechanisms for these basinwide events based on the widespread distribution of the limestone shale couplets as well as on estimated sedimentation rates and geochronology it has been widely speculated that these carbonate cycles reflect milankovitch cycles with periodicities on the order of 20 k y to 100 k y if so then stratigraphic data suggest that orbital forcing of climate affected eustasy and or sediment input and biogenic production in the western interior basin alternatively thrusting events in the sevier orogenic belt may have produced episodic changes in the rates of foredeep basin subsidence and consequent changes in base level which could have controlled the deposition of the greenhorn parasequences and carbonate cycles in either case correlation of these units demonstrates a consistent basinwide sedimentary response to high frequency base level or sediment input changes in the western interior epicontinental basin

valuable lessons from acquiring 3d seismic for coal seam gas coalbed methane cbm or coal seam gas csg is being developed on a basinwide scale primarily as feedstock for three new liquefied natural gas lng facilities on the east coast of australia a recent 3d seismic survey acquired specifically for csg in the surat and bowen basins shows that although structural detail of the reservoir is a valuable tool deriving other information for reservoir characterization from thinly bedded coals requires a thorough investigation of the effects of high reflectivity thin beds although work continues to progress the results so far suggest that 3d seismic surveys can provide information for improved reservoir characterization

correlation of early cretaceous carbon isotope stratigraphy and platform drowning events a possible link the early cretaceous carbonate carbon isotope record is marked by three positive high amplitude 1 5 excursions each covering time spans of more than 106 years they are of late valanginian hauterivian and early and late aptian age in a case study along a transect across the western tethys ocean we identified a coincidence between 13c excursions black shale formation and widespread carbonate platform drowning events we conclude that the 13c excursions reflect a change in partitioning of carbon between the organic and carbonate carbon sinks which was triggered by climate induced ecological changes in cretaceous pelagic and neritic environments episodes of intensified greenhouse climate conditions led to an increase in weathering erosion and runoff rates and to elevated nutrient transfer rates from continents into oceans the resulting increase in oceanic nutrient levels favoured marine phytoplankton production and black shale deposition while conditions for carbonate producing biotas became unfavourable partial choking of carbonate production along river influenced coasts resulted in widespread carbonate platform drowning during times of sea level rise in the valanginian and aptian widespread contemporaneous black shale deposits and drowned carbonate platforms therefore reflect the contrasting response of the marine organic and carbonate carbon pumps to nutrient enhanced phytoplankton productivity the change in marine carbon partitioning is mirrored in a shift of the 13c record towards more positive values the transition to the 13c peak values lasted up to several hundred thousand years the peaks of the excursions also covering a time span of up to several hundred thousand years reflect a new stabilisation of the carbon partitioning between carbonate and organic carbon sinks a renewed intensification of carbonate sedimentation under mesotrophic conditions was facilitated by stabilisation of the sea level rise at a high level decreasing 13c values record increasing carbonate carbon burial rates at constant or decreasing organic carbon accumulation rates these changes contributed to the stabilisation of the marine carbon budget and the global carbon cycle up to millions of years after its initial perturbation

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

volcanism co2 and palaeoclimate a late jurassic early cretaceous carbon and oxygen isotope record a composite tethyan late jurassic early cretaceous carbon and oxygen isotope curve is presented c isotope data provide information on the evolution and perturbation of the global carbon cycle o isotope data are used as a palaeotemperature proxy in combination with palaeontological information the resulting trends in climate and in palaeoceanography are compared with biocalcification trends and oceanographic conditions favouring or inhibiting biocalcification positive c isotope anomalies in the valanginian and aptian correlate with episodes of increased volcanic activity regarded as a source of excess atmospheric carbon dioxide a major warming pulse accompanies the aptian but not the valanginian c isotope event the observed change in early aptian temperatures could have triggered the destabilization of sedimentary gas hydrates and the sudden release of methane to the biosphere as recorded as a distinct negative carbon isotope pulse preceding the positive excursion both c isotope anomalies are accompanied by biocalcification crises that may have been triggered by p co 2 induced changes in climate and in surface water chemistry elevated nutrient levels in river influenced coastal waters and in upwelling regions further weakened marine calcification these conditions contrast with normal trophic conditions prevailing in the latest jurassic and favouring biocalcification the c and o isotope curves record a stable mode of carbon cycling and stable temperatures we conclude that biocalcification is mostly triggered and inhibited by co 2 conditions in the atmosphere ocean system

integrated workflows for shale gas and case study results for the horn river basin british columbia canada the variation in well performance observed between various shale gas plays and indeed within individual basins and on individual pads has gone some way to dispelling myths regarding the perceived homogeneity of shale gas targets with increased quantities of data and more determined analysis we show that understanding the micro and mesoscale heterogeneity can be advanced through interdisciplinary studies that incorporate traditional and advanced geophysical data and methods with geological understanding and engineering measurements this understanding is critical in optimizing well placement the spacing and length of horizontal wells and hydraulic fracturing effort to maximize recovery specifically we illustrate that in the muskwa formation and the otter park klua and evie members of the horn river formation reservoir quality can be predicted using lambda rho and mu rho data extracted from avo inversion studies from log data we show that the most prospective reservoir intervals are characterized by decreasing lambda increasing mu and or a lambda mu ratio less than one

ammonoid and nannoplankton abundance in valanginian early cretaceous limestone marl successions from the southeast france basin carbonate dilution or productivity a quantification of calcareous nannofossil and ammonoid absolute abundance was carried out in the valanginian limestone marl alternations of the vergol section drome located in the epicontinental vocontian basin southeast france this section is characterised by four centimetric organic carbon rich layers barrande layers b1 b4 the aim of this work is to reconstruct changes in the distribution of nutrients in the water column on the base of the analysis of fossil organisms at different levels of the food chain calcareous nannofossil abundance correlates negatively with carbonate content thus calcareous nannoplankton probably did not produce the bulk of carbonates in the valanginian of the studied section nannofossil abundance was more likely controlled by carbonate mud produced and exported from shallow platform environments to the basin variations of carbonate export controlled by climate and hydrodynamics are responsible for the limestone marl alternations the levels of highest abundance of nannoplankton are interpreted as mainly linked to a reduced accumulation rate and to a minor extent to variations in primary production a dilution by allochthonous carbonate mud is further evidenced by the relationship between total organic carbon toc and calcium carbonate caco3 toc content decreases with increasing caco3 content except for one of the organic carbon rich layers b4 both taphonomy and dilution factors may have partly controlled the variations in macrofauna abundance these can be more likely interpreted as a response to palaeoenvironmental changes of the various taxa with different palaeoecological affinities the high abundance and dominance of bochianitids orthocone ammonoids in the layer b4 can be explained by their opportunistic behaviour and their deep nektonic possibly nektobenthic mode of life their great capacity for vertical mobility may have permitted brief feeding incursions to the bottom waters when the trophic resources were higher in depth than in the surface waters high trophic levels at the sea bottom may have also benefited the bivalves and gastropods which are very abundant in layer b4 the mode of life of different ammonoid families is also discussed

magnetostratigraphic calibration of the late valanginian carbon isotope event in pelagic limestones from northern italy and switzerland abstract magnetostratigraphic biostratigraphic and carbon isotope data are presented from five lower cretaceous pelagic limestone sections in the southern alps a positive carbon isotope event is recorded in bulk carbonate from the upper valanginian of all five sections and the magnetostratigraphy and biostratigraphy provide the age model the record of the isotope event in absolute time is closely replicated in all five sections as the sections are located in two paleogeographic basins and are separated by up to 300 km the synchronous isotope event cannot be interpreted as an artifact of diagenesis a reduction in magnetization intensity coincident with the 13 c peak may be due to diagenetic magnetite dissolution enhanced by increased organic carbon burial according to the age model which uses gts89 timescale 1 background 13 c values of 1 5 are recorded from 145 to 137 ma between 137 and 136 ma 13 c increases to a maximum value of about 3 values begin to decrease from 136 ma and reach background values at about 132 ma the rate of change of 13 c for the onset of the event about 1 5 m y is greater than for the decay about 0 5 m y suggesting an abrupt perturbation of the carbon system from equilibrium followed by a more gradual return to background values

palaeoenvironmental control on sponge microbialite reefs and contemporaneous deep shelf marl limestone deposition late oxfordian southern germany abstract the late oxfordian bimammatum zone deep shelf deposits of southern germany are characterised by abundant reefs that laterally pass into marl limestone alternations the bioconstructions are made from three main components sponges microbialites and allochthonous sediment a detailed morphological analysis of sponge microbialite reefs in the plettenberg section swabian alb southern germany is combined with investigation of calcareous nannofossil assemblages in laterally correspondent marl limestone alternations in order to trace the evolution of palaeoenvironmental conditions during reef development fluctuations of caco3 content in deep shelf deposits provides evidence of a succession of small scale lithological units generally formed by five to six marl limestone couplets within a small scale lithological unit a close link is observed between growth patterns of sponge microbialite reefs and the changes in the nannofossil assemblage composition phases of expansion of the build ups correspond to carbonate deposits characterised by a relative dominance of large oligotrophic nannoplankton species large watznaueria britannica watznaueria manivitiae and schizosphaerella spp marly intervals reveal a high relative abundance of small sized taxa mainly lotharingius hauffii small w britannica and subordinated biscutum dorsetensis and bioconstructions show a net reduction in size or a total demise this suggests that the same palaeoenvironmental parameters controlled at the same time marl versus limestone deposition reef growth phases and calcareous nannofossil assemblage composition successive reef growth phases recognized within a bioherm are correlated to the limestone hemi couplets each reef growth phase is composed of several stacked elementary sequences comprising the following succession i sponge ii microbialites and iii carbonate mud in turn the microbialitic crust is formed of three successive microbial layers 1 a fine crust of dense to clotted micrite 2 a centimetre scale to multicentimetric columnar layer of clotted to peloidal micrite 3 a centimetre scale stromatolitic crust the passage from an initial dense microbial layer with numerous microencrusters to a final stromatolitic crust is interpreted as resulting from an increase of the microbial growth rate mainly driven by changes in the accumulation rate deep shelf deposits resulted from changes in platform carbonate production and export basinwards controlled both by sea level and climate changes climate controlled the intensity of carbonate production in shallow platform environments and export towards the deep shelf as well as the trophic conditions of the deep shelf marine waters low trophic conditions favoured platform carbonate production and export basinwards and a large development of the sponge microbialite reefs in the deep shelf however a high carbonate accumulation rate in the deep shelf was also commonly responsible for the demise of sponge microbialite reefs during unfavourable and higher trophic conditions during marl deposition reduced development and frequent demise of the sponge microbialite reefs is observed

long distance correlations by sequence stratigraphy and cyclostratigraphy examples and implications oxfordian from the swiss jura spain and normandy the oxfordian sedimentary successions studied in the swiss jura in normandy and in the soria and cazorla regions of spain display complex facies evolution and stacking patterns based on bio stratigraphy and absolute age dating it is suggested that the shallow water depositional settings in the jura normandy and the soria region as well as the deeper water environments in the cazorla region recorded climatic and sea level fluctuations in the milankovitch frequency band beds and bedsets corresponding to 20 100 and 400 ka cyclicities can be identified facies evolution inside such small scale sequences and also in the larger sequences of million year scale is interpreted in terms of sequence stratigraphy superposition of high frequency cyclicity on a longer term sea level trend led to multiplication of diagnostic surfaces se quence boundary and maximum flooding zones in the large scale sequences can thus be defined these zones are correlated between closely spaced sections but also from the swiss jura to normandy and to spain the narrow time lines given by milankovitch cyclicity then allow comparison of facies evolution in the di erent regions on a scale of 100 ka or less by filtering out local e ects of di erential subsidence and sediment supply a long term sea level curve valid for the northwestern margin of the tethys ocean can be reconstructed for the middle to late oxfordian di erential subsidence is implied from varying thicknesses of the sequences as well as from the distribution of siliciclastics which have been channelized through depressions tilted blocks reduced sedimentation or increased input of siliciclas tics appearing at the same time in all study areas point to a widespread regional tectonic event distribution through the sequences of climate dependent facies components such as corals ooids palynomorphs and siliciclastics indicates that climate changes were depen dent on atmospheric circulation patterns and thus on paleolatitude rainy periods and related increase of siliciclastics in the swiss jura were more abundant during low sea level stands whereas in the soria region they coincided with sea level highs through the com bination of high resolution sequence stratigraphy and cyclostratigraphy and supported by biostratigraphy and absolute dating it becomes possible to analyze paleoenvironmental changes in a very narrow time framework

relationship between the sedimentary microfacies and geomechanical behavior of the asmari formation carbonates southwestern iran abstract the objective of this paper is to address the impact of texture and thus the related sedimentary environments and facies and diagenetic overprint on the geomechanical behavior of the asmari formation carbonates three sets of data were utilized in this study i sedimentological data including thin section petrography porosity density xrf and sem ii rock mechanical data comprising uniaxial compressive strength ucs cohesion c friction angle tensile strength brittleness and drillability index and iii ultrasonic data including s and p wave velocity to study a possible relationship between the sedimentological features and rock mechanical properties a facies index was defined based on the textural and diagenetic features allowing to quantify a sedimentary microfacies based on facies index porosity and ucs five different geomechanical facies gmf were determined for the asmari formation where gmf1 to gmf5 represent very ductile to very brittle material respectively the results showed that mechanical behavior are controlled by the depositional environment open marine to supratidal and diagenetic overprint dissolution cementation dolomitization micritization mechanical and chemical compaction and authigenic clay formation by quantifying the sedimentological and diagenetic features the rock strength may be predicted based on petrographical observations results of this study provide insights for creating a relationship between sedimentological features and rock mechanical behavior based on the gmf concept presented here such studies are helpful for reducing the costs time for acquiring geomechanical datasets

the winds of change anisotropic rocks their preferred direction of fluid flow and their associated seismic signatures part 1 editor s note this article is the first of two parts that together form an abbreviated version of the fall 2004 seg aapg distinguished lecture a powerpoint presentation of this material is posted on the seg web site the winds of change refer to much more than just anisotropic rocks whose seismic anisotropy provides insight into horizontal permeability anisotropy the way we view our planet is changing the planet is now being studied as the interaction of four interlocking systems the hydro sphere the atmosphere the lithosphere and the biosphere as all absorb energy from our sun and change the planet and all its subsystems like geology or climate are explicitly evaluated as interdependent and interacting within the context of the local community the heterogeneity at the surface of our planet and the recycling of compounds and elements water nitrogen sulfur etc enable our planet to support life my earth science career has spanned two revolutions the plate tectonic revolution during the 1970s and today s profound reevaluation of how our planet works and our roles as planet dwellers the rocks that are the primary focus of most of our studies are best understood as firstly the result of the interactions of the climate the water conditions the depositional setting the biosphere and the provenance of the sediments and secondly the lithification and geologic history they have undergone the biosphere shows up in our rocks both as fossils a key means of dating rocks and as liquid or solid organic remains we understand any given stratigraphic facies in the context of its spatial neighbors its community and its plate tectonic setting the climate includes the actions of gravity sunlight and atmosphere but it is affected by where the land masses are and how they are linked together oil industry geoscientists are uniquely placed via access to important data and sophisticated tools

sr ca variations in cretaceous carbonates relation to productivity and sea level changes abstract we present high resolution sr ca measurements on coccolith dominated pelagic carbonates of berriasian valanginian albian santonian and campanian maastrichtian age from dsdp sites and tethyan land sections carbonate sr ca varies by up to 80 over both long and short timescales and may reflect either temporal variations in seawater sr ca or variable sr partitioning in biogenic carbonates numerical models of seawater sr and ca budgets indicate that while sea level changes can cause comparably large variations in the seawater sr ca ratio the timing and pattern of predicted sr ca variations agrees poorly with the carbonate record reef crises unrelated to sea level changes may also elevate seawater sr ca and in some cases the record of reef crises corresponds to increasing carbonate sr ca changes in sr partitioning likely due to changes in calcareous nannoplankton productivity also appear to control carbonate sr ca in these records cretaceous carbonate sr ca broadly covaries with indicators of productivity derived from microfossil ecology recent studies in sediment core tops and cultures have shown that sr partitioning in coccolith carbonate is strongly influenced by changes in coccolithophorid growth and calcification rate where the effects of changing seawater sr ca can be constrained by independent geological data carbonate sr ca measurements could provide a valuable perspective on changes in the productivity of calcareous nannoplankton and variations in the carbonate and carbon cycle in the cretaceous

influencing factors and fracability of lacustrine shale oil reservoirs abstract shale oil reservoirs need fracturing in order to achieve commercial productivity a key factor influencing the fracturing process is shale fracability which is strictly dependent on rock fabric in this study we focused on the lacustrine shale oil reservoirs located in dongying sag and in the northern jiangsu basin through x ray mineral diffraction rock mechanics and p and s wave velocity tests we analyzed the mineral components and mechanics of different shale lithofacies as well as the influence of these factors on shale fracability based on the results of these analyses we proposed a new method for the evaluation of lake shale oil reservoir fracability the experimental results showed that the macrostructure mineral content lithology and the presence of microfractures in the shale are the main factors affecting its brittleness shales with lamellar and sheet like structures have relatively high young s modulous or lower poisson s ratios under these conditions netted fractures are easily formed during fracturing the fracture toughness of carbonate minerals in shales is strong an appropriate amount of carbonate minerals seemed to increase the shale brittleness however an excessive amount increased its cohesiveness hindering the initiation and propagation of fractures felsic minerals are characterized by high brittleness a characteristic that favors shale fracability on the other hand clay minerals and organic matter are highly plastic a characteristic that hinders fracturing transformations due to their macrostructure and composition lamellar clayey limestones dolomite lamellar clayey siltstones layered calcareous dolomitic siltstones sheet like clay containing siltstones and flaggy carbonate containing siltstones are expected to be easily fractured in addition the presence of microfractures seemed to reduce the overall fracture toughness of the rock they penetrated artificial fractures enhancing rock fracability taking into account the fracture toughness of carbonate minerals and the plastic effect of organic matter we proposed a new formula for the calculation of the brittleness index based on the mineral component method the brittleness indices obtained through the proposed formula were linearly correlated with those calculated through the rock mechanics parameters method overall our results indicate that the silty lithological successions occurring in the study area should be the easiest to fracture followed by the calcareous dolomitic and clayey or organic rich lithological successions this study is of guiding significance for evaluating the fracability of different lithofacies in lacustrine shale oil reservoirs especially the new proposed brittleness index containing the plastic effect of organic matter can correctly evaluate the actual brittleness of organic rich shale

the valanginian isotope event a complex suite of palaeoenvironmental perturbations abstract the valanginian records a severe crisis of carbonate systems both on platforms and in the pelagic realm this crisis is roughly concomitant with the weissert event characterized by a positive 13c excursion of about 2 in marine carbonates however it is unclear if the response of these two carbonate systems to the global perturbations is contemporaneous or if they react differently for this purpose accumulation rates of pelagic carbonates produced by nannofossils and of platform derived carbonates have been quantified in a hemipelagic environment the vocontian basin se france that has the potential to record the reaction of both shallow water and pelagic carbonate production to a major perturbation of the carbon cycle testified by the isotope excursion also changes through time of clay input have been measured in order to evaluate variations in continental weathering rates and thus in continent derived nutrient input and climatic conditions in terms of humidity aridity accumulation rates of pelagic carbonates of platform derived carbonates and of clays were calculated on the basis of nannofossil absolute abundance and species specific masses of weight percent carbonate content of rocks and a high resolution timescale given by cyclostratigraphy the onset of the positive 13c excursion and the demise of the platforms testify for a synchronous but inverse response of the shallow water and pelagic carbonate systems to an increase in nutrient influx to marine waters as carbonate production by nannofossils decreased during the late valanginian in time of still low platform production the low temperature of marine waters coeval to a still high nutrient input recorded in the latest late valanginian possibly could explain that both platform and pelagic carbonate production contemporaneously decreased

controls on grain size patterns in periplatform carbonates marginal setting versus glacio eustacy the importance of grain size as a fundamental property controlling other physically derived properties is well known recent grain size studies on periplatform sediments of the caribbean and western north atlantic have shown that grain size variability is coupled with changes in carbonate platform productivity and export in response to pleistocene holocene glacio eustacy these findings showed that periplatform sediments are dominated by fine grained material during sea level highstands and coarser material during sea level lowstands this paper addresses the question of grain size patterns in periplatform settings and their relationship to glacio eustacy furthermore it considers whether these patterns are applicable to different types of margin settings accretionary erosional here existing data pleistocene holocene from the accretionary western leeward margin of great bahama bank odp leg 166 and new data from the erosional eastern windward margin odp leg 101 are used the results show that the grain size patterns of both margins are controlled by late neogene sea level fluctuations the accretionary leeward margin sediments indicate the same grain size pattern as previously described in the literature however the erosional windward margin shows a reverse pattern with coarser sediments in interglacials and finer deposits in glacials this proposes that the leeward margin grain size pattern is controlled by the productivity export mode of the platform this is indicated by the platform top derived fine muds transported off bank to form a thick sediment wedge on the low angled slope during interglacials in contrast on the windward margin the sediment flux pattern is controlled by the impact of mass transport processes this is indicated by the increased occurrence of turbidites during interglacials that might be more frequent at this time because the steep angled erosional slope is less apt for the deposition of excess fine material this material is eroded and or bypasses the slopes and is redeposited in the periplatform basin setting

phosphogenesis carbon isotope stratigraphy and carbonate platform evolution along the lower cretaceous northern tethyan margin we distinguish three different stages in the evolution of the tithonian late jurassic to aptian early cretaceous northern tethyan carbonate platform 1 carbonate production in the coral oolite mode late tithonian to early valanginian early barremian to early aptian 2 carbonate production in the crinoid bryozoan mode early valanginian haute rivian late aptian and 3 platform retrogradation and destruction condensation and phosphogenesis that is platform drowning early valanginian to early hauterivian middle hauterivian late hauterivian to early barremian early to early late aptian and latest aptian to earliest albian transitions from the coral oolite mode to the crinoid bryozoan mode and consequently to platform drowning may have been driven by increases in nutrient levels on the shelf phases of relative sea level rise in times of carbonate production in the coral oolite mode are named constructive because of the observed platform regeneration following phases of widespread emersion during late sea level highstands in contrast phases of relative sea level rise in times of platform drowning are termed destructive the 13 c stratigraphies obtained from valanginian hauterivian and aptian albian hemipelagic successions beyond the platform correlate well with the early cretaceous pelagic 13 c record positive excursions in the pelagic 13 c record correspond in time to episodes of platform drowning this suggests the existence of a coupling mechanism between changes in the global carbon cycle and platform drowning in our view the early cretaceous crises in carbonate platform growth were the consequence of reinforced greenhouse conditions which may have been triggered by episodes of extensive flood basalt volcanism strong greenhouse conditions may have induced the following chain of feedback mechanisms enabling the biosphere to return to normal conditions climate warming sea level rise accelerated water cycle intensified weathering nutrient mobilization platform destruction increased productivity increased phosphogenesis and carbon burial weakened greenhouse conditions

uncertainty in discriminating lithology in different data domains a major goal of geophysical analysis is to predict the lithologic and fluid content of the subsurface of necessity most of the measurements are conducted remotely using seismic or electromagnetic equipment these remote data should be calibrated with the expected properties of the subsurface principally from well control as a result rock physics is a key component of the process to ensure that the appropriate geophysical response is measured and obtained

using avo for fracture detection analytic basis and practical solutions tight low porosity reservoirs can produce significant amounts of hydrocarbons if fracture networks provide the necessary permeability characterization of naturally fractured reservoirs using surface seismic data is an important exploration problem that has attracted much attention in the literature tle for example had an entire issue august 1996 on this subject while the data and modeling in existing publications demonstrate that fracturing causes significant azimuthal variation in recorded seismic signatures the problem is how to infer fracture parameters from these data

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

interpreting seismic data in the presence of azimuthal anisotropy or azimuthal anisotropy in the presence of the seismic interpretation measuring p wave azimuthal anisotropy has been in the recent past an elusive task therefore the interpreter ignored this attribute of the seismic data and left the subject to the research and technology group geophysicists conversely the interpreting geophysicist knew that when measured the anisotropy could yield important reservoir properties related to fractures and stress fields however little did we suspect that p wave azimuthal anisotropy both in velocity and avo would change our perception of data interpretation as is happening now

complex human vegetation climate interactions in the late holocene and their significance for paleotemperature reconstructions using 3 980 14c ages and 43 669 pollen data from 632 globally distributed lakes 88 of them in north america and europe fig 1 a jenny et al 1 demonstrate that increased soil erosion rates fig 1 b and decreased tree coverage fig 1 c have occurred in lake watersheds across a substantial portion of earth s surface since 4 000 y bp besides using climate model results fig 1 d they propose that the late holocene tree coverage decreases were caused mainly by anthropogenic deforestation 1 notably the model deduced late holocene slight cooling trends in north america and europe 1 fig 1 d are the opposite of the slight global warming trends indicated by other models 2 fig 1 e arguably it is problematic to use model results to determine a climate vegetation relationship moreover the model deduced slight cooling trends since 4 000 y bp 1 fig 1 d are supported by a synthesized global mean temperature record 3 fig 1 f however only 4 out of the 73 selected records fig 1 a comprising the synthesized record are terrestrial records 3 fig 1 a locations of the study sites cited herein the blue squares represent the 1 1to whom correspondence may be addressed email raozhg at hunnu edu cn 1 xref corresp 1 1

reply to li et al human societies began to play a significant role in global sediment transfer 4 000 years ago in a recent study 1 we conducted a global synthesis of long term records of reconstructed sediment accumulation rates sars in lakes and diagnosed that a dominant anthropogenic imprint on soil erosion rates started 4 000 y ago our approach was based on the assumption that lake sars are watershed integrators of terrestrial biogeochemical responses to atmospheric climate and land surface dynamics spanning several millennia 2 4 our statistical analysis was based on sar reconstructions for the last 12 000 y 1 global pollen records spanning the same period to infer past vegetation changes 5 and model based climate reconstructions from transient holocene simulation with 1 1to whom correspondence may be addressed email jean philippe jenny at inrae fr 1 xref corresp 1 1

effects of sand shale anisotropy on amplitude variation with angle ava modelling the sawan gas field pakistan as a key case study for south asia s sedimentary basins amplitude variation with angle ava is a technique widely used in the characterisation of hydrocarbon reservoirs and assumes the earth s crust to be an isotropic medium yet anisotropy is ubiquitous in stratigraphic sequences and has first order effects on seismic ava responses when investigating subsurface prospects this work analyses the effects of anisotropic strata on ava responses using the lower goru formation middle indus basin pakistan as a case study in the study area shale intervals are interbedded with reservoir sands of the sawan gas field shales in this field form laminae or are dispersed within reservoir sands making the lower goru formation an example of a vertically transversely isotropic vti medium in this work we calculate the effective saturated mechanical properties of the lower goru formation based on rock physics templates the backus 1962 average typically designed for layered media combined with the empirical relations of brown and korringa 1975 and wood 1955 the input data used in our rock physics modelling is based on detailed petrophysical analyses of well data using the saturated effective mechanical properties of the lower goru formation we generate angle dependent reflection coefficient curves and seismic ava responses based on exact and approximate solutions for both isotropic and anisotropic reservoir scenarios our results suggest that the effects of lithological anisotropy are more pronounced in places with thick shale beds within reservoir sands conversely angle dependent reflection curves and seismic ava responses based on isotropic or anisotropic cases give similar solutions in the presence of thin shale beds as a corollary of this work we present a bayesian inversion method for the estimation of porosity in vti media

seismic anisotropy of fractured rock a simple method for including the effects of geologically realistic fractures on the seismic propagation through fractured rocks can be obtained by writing the effective compliance tensor of the fractured rock as the sum of the compliance tensor of the unfractured background rock and the compliance tensors for each set of parallel fractures or aligned fractures the compliance tensor of each fracture set is derivable from a second rank fracture compliance tensor for a rotationally symmetric set of fractures the fracture compliance tensor depends on only two fracture compliances one controlling fracture compliance normal the other tangential to the plane of the fractures the stiffness tensor which is more useful in the consideration of elastic wave propagation through rocks can then be obtained by inversion the components of the excess fracture compliance tensor represent the maximum amount of information that can be obtained from seismic data if the background rock is isotropic and the normal a

integration of hydraulically induced microseismic event locations with active seismic attributes a north texas barnett shale case study abstracthydraulic fractures are delineated by induced microseismic event distributions and typically propagate perpendicular to the regional minimum stress direction however at a smaller scale varying mineralogical composition and existing fault and fracture networks can influence developing fracture networks we integrated microseismic event locations with seismic attributes from multichannel seismic reflection data including inversion results for impedance and lame parameters and seismic curvature attributes we found that microseismic event locations consistently correlate to zones of low seismic impedance and low and values describing characteristic material properties of fracture prone zones within the north texas lower and upper barnett shale additionally event locations showed a weak correlation with anticlinal structures as defined by volumetric curvature attributes we suggest that the low impedance low and zones were related to the boundary between calcite filled fractures a

geomechanical lithology based analysis of microseismicity in organic shale sequences a pennsylvania marcellus shale example abstract using an innovative workflow incorporating microseismic attributes and geomechanical well logs we have defined major geomechanical drivers of microseismic expression to understand reservoir stimulation response in an engineering geologic context microseismic data from a hydraulically fractured marcellus well in the appalachian basin central pennsylvania were sampled vertically through the event cloud crossing shale limestone sandstone and chert we focused our analysis on the devonian organic shale and created pseudologs of moment magnitude mw b value and event count the vertical moving average sampling of microseismic data was completed such that the sample interval matched that of the geophysical well logs this technique creates robust high resolution microseismic logs that show subtle changes in microseismic properties and allow direct crossplotting of microseismic versus geophysical logs we chose five geomechanical properties to form the framework against which to interrogate

sedimentary patterns and geometries of the bahamian outer carbonate ramp miocene lower pliocene great bahama bank core logging and high resolution seismic data from odp leg 166 were used to analyse deposits of the neogene miocene lower pliocene bahamian outer carbonate ramp ramp sediments are cyclic alternations of light and dark grey wackestones packstones with interbedded calciturbidite packages and minor slumps cyclicity was driven by high frequency sea level changes light grey layers containing shallow water bioclasts were formed when the ramp exported material whereas the dark grey layers are dominantly pelagic calciturbidites are arranged into mounded lobes with feeder channels internal bedding of the lobes shows a north directed shingling as a result of the asymmetrical growth of these bodies calciturbidite packages occur below and above sequence boundaries indicating that turbidite shedding occurred during third order sea level highstands and lowstands highstand turbidites contain shallow water components such as green algal debris and epiphytic foraminifera whereas lowstand turbidites are dominated by abraded bioclastic detritus gravity flow depocentres shifted from an outer ramp position during the early miocene to a basin floor setting during the late miocene to early pliocene this change was triggered by an intensification of the strength of bottom currents during the tortonian which was also responsible for shaping the convex morphology of the outer ramp the miocene and lower pliocene of the leeward flank of great bahama bank provides an example of the poorly known depositional setting of the outer part of distally steepened carbonate ramps the contrast between its sedimentary patterns and the well known upper pliocene quaternary slope facies associations of the flat topped great bahama bank shows the strong control that the morphology of a carbonate platform exerts on the depositional architecture of the adjacent slope and base of slope successions

decoupling of p and corg burial following early cretaceous valanginian hauterivian platform drowning along the nw tethyan margin during the hauterivian three important phases of platform drowning phosphogenesis and mesotrophic carbonate deposition along the northern margin of the tethys are not mirrored by positive n 13 ccarb excursions such as during the valanginian and aptian but rather by decreasing to stable trends the aim of this study is to understand the decoupling of organic carbon and phosphorus burial during the hauterivian for this purpose po4 concentrations were determined from biogenic limestones from eight sections along a platform to basin transect integration with a previously obtained dataset for the valanginian leads to a biostratigraphically calibrated high resolution phosphorus accumulation rate mg cm 2 kyr curve based on 575 data points from the late valanginian to early hauterivian phosphorus accumulation rates increased nearly threefold from 0 75 mg cm 2 kyr to 2 1 mg cm 2 kyr the phosphorus accumulation rate increase obtained in this study correlates well with a compilation based on odp and dsdp records indicating that phosphorus accumulation rates along the northern margin of the tethys reflect global changes in p in and output a global rise in continental p input is thought to have resulted from intensified greenhouse climate conditions leading to increased riverine runoff coeval sea level rise led to re arrangement of circulation and oxygenation of bottom waters which meant that p was increasingly well retained by means of fe and mnoxyhydroxides furthermore the nw margin of the tethys may have been especially susceptible to phosphogenesis as it was influenced by cold water exchange with the boreal realm and prone to coastal upwelling the late hauterivian witnessed sea level fall more sluggish circulation along the nw tethyan margin and also globally lower weathering and erosion rates leading to generally lower phosphorus accumulation rates increased regeneration of phosphorus might have occurred as circulation stagnated and bottom water oxygen levels decreased increased phosphorus input into early hauterivian oceans did not lead to the production of organic carbon rich rocks but rather to increased carbonate carbon burial also seen in a decoupling between the n 13 ccarb record decreasing and phosphorus accumulation rates increasing decoupling of these records reflects the ecological recovery of the carbonate system after a prolonged phase of reef destruction during the late early and late valanginian carbonate production in the

the phosphorus cycle phosphogenesis and marine phosphate rich deposits abstract phosphorus in the form of phosphate is an essential nutrient and energy carrier on many different levels of life and a key element in mediating between living and lifeless parts of the biosphere one of the most important aspects of the phosphorus cycle is its vital role in governing productivity thereby interacting with the exogenic part of the carbon cycle which in turn is important in regulating earth s climate phosphorus is a prime element to be traced in earth s history because it allows for the reconstruction of long term feedback mechanisms between climate environment and ecology and of global change as such marine sedimentary phosphate deposits are particularly suited to study aspects of the phosphorus cycle because in the case of ubiquity their origin may result from a general acceleration of the global phosphorus cycle sources of sedimentary phosphate are microbial breakdown of buried organic matter and redox driven phosphate desorption from iron and manganese oxyhydroxides dissolved sea water phosphate represents an additional source which may become important in the formation of phosphatic hardgrounds the main locus of phosphogenesis is near the sediment water interface but phosphogenesis also occurs at greater sediment depths current induced winnowing and transport processes along the sea floor concentrate phosphate precipitates into deposits which exhibit internal stratification patterns typical for the prevailing hydraulic energy regime in a sequence stratigraphic context phosphate deposits preferentially occur along marine or maximum flooding surfaces consequent sedimentary reworking may result in the transfer of phosphates to highstand or lowstand deposits bio chemical weathering on continents represents the most significant source of bioavailable phosphorus this implies that long term changes in marine phosphorus levels and with these changes in marine ecology productivity rates and ratios of exported carbonate carbon and organic carbon are a response to changes in continental weathering rates a compilation of marine sedimentary phosphorus burial rates for the last 160 myr suggests that natural variations have occurred that span one order of magnitude for the late jurassic cretaceous and most of the paleogene the phosphorus cycle appears to have been accelerated in times of climate warming which was most likely due to the spreading of zones of humid climate and more intense continental weathering in the neogene the phosphorus cycle appears to have responded to changes in glacially induced weathering this suggests that uniform interpretations with respect to the emplacement of major phosphorite deposits should be treated with caution integrated analyses of the sedimentary and biogeochemical context of phosphorite occurrences may help to identify paleoenvironmental conditions as well as to improve our understanding of periods of enhanced phosphate accumulation periods which were usually characterized by steep gradients in the development of climate and environment with regard to the complexity of feedback mechanisms between the phosphorus cycle and the biosphere the present day input rates of phosphate into the world s oceans should be of great concern they are more than doubled by anthropogenic means and affect ecological systems on a rapidly increasing scale

lake sediments erosion and landscape change during the holocene in britain and ireland abstract lakes are excellent repositories of air borne and especially stream borne materials it has long been recognised that lake sediments contain a strong record of catchment soils via the inputs of minerogenic and chemical erosional products to these may be added a variety of palaeoecological indicators including pollen fungi sphagnum leaves and faunal remains pollen analysis in particular enables land use change to be assessed over many millennia and demonstrates the landscape impacts of woodland clearance grazing and crop cultivation radiocarbon dating provides a chronology for environmental and agricultural change and acts as an indicator of erosion in its own right the use of such indicators to reconstruct past instances and patterns of erosion is demonstrated with reference mainly to 50 british and irish lake sites and especially those which have produced signals of erosion in the form of accelerated sediment accumulation and inversed or reversed sequences of radiocarbon dates during the mid and late holocene sites displaying even or decreased sedimentation through time are concentrated in northern and northwestern scotland and have either no clear signs of human impact or only so towards the latter part of the holocene this pattern probably reflects a combination of factors including low population densities thin soils and the sealing effect of blanket peat in contrast all sites with accelerated erosion have indications of anthropogenic impact this is often prior to levels of increased sedimentation showing a delay in system response while the spread of dates for the start of the rise in sediment accumulation spans 6200 940 bp three clusters are evident at 5295 4970 4530 4235 and 2980 2810 bp radiocarbon inversions mostly reversals are evident for 18 sites no obvious causal patterning is indicated and they probably demonstrate no more than the fact that catchment soils including peats around a particular site were pushed beyond an erosional threshold sufficient to age the sediments deposited within the lakes it is hoped that improvements in data quality will allow the refinement of such exercises in the future and that lakes will be valued as repositories of long term landscape sensitivity

the cenomanian turonian oceanic anoxic event ii palaeoceanographic controls on organic matter production and preservation correlation of the 13c spike with the well dated occurrences of strata rich in organic carbon detailed in schlanger et al this volume indicates that a global episode of intense organic carbon orgc burial took place during the latest cenomanian earliest turonian oceanic anoxic event oae a plenus through i labiatus macrofossil zones and upper r cushmani trz through w archecretacea prz foraminiferal zones over a period of no more than 1 million years m y the shape of the 13c curve indicates that rates of orgc burial gradually increased in the early part of the late cenomanian increased more rapidly in the later cenomanian and levelled off at peak values in latest cenomanian early turonian time during the maximum rate of orgc burial the 13c values decreased nearly to pre late cenomanian levels in the early to middle turonian the decrease in 13c reflects decreasing rates of orgc burial following the cenomanian turonian oceanic anoxic event as well as the probable oxidation and return of significant amounts of orgc to the oceans following regression and re oxygenation of much of the deeper water masses in contact with the seafloor

new evaluation methods using conventional logging data for gas shale in southern china abstract because of characteristics of compactness rich in organic matter and gas adsorption the parameters that decide shale quality are not only porosity and permeability but also the abundance of organic matter mineral composition crack and mechanical properties logging evaluation methods of conventional reservoirs are not valid in gas shale advanced technology of core measurements can obtain more accurate parameters of gas shale however because it is restricted by heterogeneity and the high price of core this method cannot be used widely to develop a continuous section based on conventional wireline logging data the natural properties of rocks underground are calculated using the improved evaluation model including mineral composition clay constituents total organic carbon toc content porosity permeability crack porosity and mineral brittleness index moreover based on the rock physics model with rich organic matter elastic properties are achieved to calculate mechanical propert

combined microseismic and 4d to calibrate and confirm surface 3d azimuthal avo lmr predictions of completions performance and well production in the horn river gas shales of nebc the ability to high grade gas shales is essential to optimizing completions and maximizing stimulated rock volume srv in the capital intensive development of the horn river resource play in northeast british columbia nebc to assist in optimizing stimulation efforts seismic data are used to estimate and map four parameters that influence hydraulic fracture effectiveness rock properties in situ stress natural fractures and reservoir geometry

causal fracture prediction curvature stress and geomechanics we propose scaling volume curvature measurements with material property estimates to produce a superior prediction of natural fractures curvature is one of many indirect fracture inferring attributes it does not detect fractures but is causally related to them through the assumption that increasing curvature relates to increasing strain there are many other variables that are causally related to fractures we propose that it would be advantageous to create combinations of these causal variables with curvature some of the most well known and important causes of variations in natural fracture density are material properties relating to brittleness material properties are critical geologically at all scales from large scale regional studies to prospect level inquiries because the properties may vary significantly within individual formations and between formations these vertical and lateral changes in material properties may be important and should be considered in fracture estimation along with c

the impact of calcareous nannofossils on the pelagic carbonate accumulation across the jurassic cretaceous boundary abstract calcareous nannofossils were important producers of pelagic carbonates in mesozoic oceans in order to better understand the origin of mesozoic pelagic carbonates we studied upper jurassic to lower cretaceous sediments in the central atlantic ocean dsdp sites 105 367 534a with respect to their content of calcareous nannofossils the interval under investigation is characterized by a significant increase in the deposition of carbonate rich sediments a trend that goes along with the rapid radiation of calcareous nannofossils the assemblage composition and the size variation of common taxa were analyzed and subsequently the contribution of this phytoplankton group to the pelagic carbonate accumulation was calculated results reveal two long term changes of the nannofossil assemblage composition 1 the early tithonian coccolith assemblages are of low diversity watznaueria spp cyclagelosphaera spp zeugrhabdotus spp whilst the mid to late tithonian assemblage is dominated by nannoliths conusphaera mexicana polycostella beckmannii nannoconus spp and large sized watznaueria 2 the early berriasian is characterized by a shift from the late tithonian nannolith rich assemblage to a highly diverse coccolith assemblage morphometric studies of the placolith genus watznaueria show for all three dsdp sites large sized forms of this genus in the mid and late tithonian followed by a decrease of up to 2 m for the mean size in the earliest berriasian at dsdp site 105 the studied nannolith taxa show an increase in size during the mid tithonian to berriasian interval the records of both nannofossil carbonate estimates and the measured bulk rock carbonate reveal two periods of increase in the nannofossil carbonate record of dsdp site 105 a first significant increase of the carbonate accumulation is observed in the mid and late tithonian probably caused by mass occurrences of strongly calcified taxa c mexicana p beckmannii nannoconus spp watznaueria cf manivitae this interval is here named nannofossil calcification event nce the second carbonate maximum in the late berriasian is related to a rise in absolute abundances of nannofossils this peak is amplified by an overall increase of the sedimentation rate the calculated accumulation rates of nannofossils nannofossil carbonate and bulk rock carbonate for the late berriasian are on the same scale as values from recent ocean surface sediments a comparison of nannofossil carbonate values with the bulk rock carbonate content shows that on average only 27 of the total carbonate can be explained by our nannofossil carbonate estimates this discrepancy is most likely caused by the high amount of unidentifiable micrite and fragments of calcareous nannofossils other factors contributing to the error are possible inaccuracies in the determination of absolute abundances and nannofossil volume calculations the nce occurs during a long term sea level fall dry climate and presumed low pco 2 levels a decline in abundance of strongly calcified nannofossils coincides with the opening of the pacific atlantic seaway via central america which may have had a substantial impact on the palaeoceanographic situation in the central atlantic ocean these changes are here considered to have at least partly caused the shifts in abundance size and assemblage composition of calcareous nannofossils observed across the jurassic cretaceous boundary interval

the valanginian δ13c excursion may not be an expression of a global oceanic anoxic event abstract marine and terrestrial sediments of the valanginian age display a distinct positive 13 c excursion which has recently been interpreted as the expression of an oceanic anoxic episode oae of global importance here we evaluate the extent of anaerobic conditions in marine bottom waters and explore the mechanisms involved in changing carbon storage on a global scale during this time interval we asses redox sensitive trace element distributions rste uranium vanadium cobalt arsenic and molybdenum and the quality and quantity of preserved organic matter om in representative sections along a shelf basin transect in the western tethys in the polish basin and on shatsky rise om rich layers corresponding in time to the 13 c shift are generally rare in the tethyan sections and if present the layers are not thicker than several centimetres and total organic carbon toc contents do not surpass 1 68 wt palynological observations and geochemical properties of the preserved om suggest a mixed marine and terrestrial origin and deposition in an oxic environment in the polish basin om rich layers show evidence for an important continental component rste exhibit no major enrichments during the 13 c excursion in all studied tethyan sections rste enrichments are however observed in the pre 13 c excursion om rich barrande levels of the vocontian trough in addition all tethyan sections record higher mn contents during the 13 c shift indicating rather well oxygenated bottom waters in the western tethys and the presence of anoxic basins elsewhere such as the restricted basins of the north atlantic and weddell sea we propose that the valanginian 13 c shift is the consequence of a combination of increased om storage in marginal seas and on continents as a sink of 12 c enriched organic carbon coupled with the demise of shallow water carbonate platforms diminishing the storage capacity of 13 c enriched carbonate carbon as such the valanginian provides a more faithful natural analogue to present day environmental change than most other mesozoic oaes which are characterized by the development of ocean wide dysaerobic to anaerobic conditions

elastic anisotropy due to aligned cracks in porous rock all theoretical expressions which relate the characteristics of saturated aligned cracks to the associated elastic anisotropy are restricted in some important way for example to the case of stiff pore fluids or of the absence of equant porosity or of a moderately high frequency band because of these restrictions previous theory is not suitable for application to the upper crust where the pore fluid is brine kf k820 the equant porosity is often substantial p 0 1 and the frequency band is sonic to seismic this work removes these particular restrictions recognizing in the process an important mechanism of dispersion a notable feature of these more general expressions is their insensitivity at low frequency to the aspect ratio of the cracks only the crack density is critical an important conclusion of this more general model is that many insights previously achieved concerning the shear wave splitting due to vertical aligned saturated cracks are sustained however conclusions on crack orientation or crack aspect ratio which were derived from p wave data or from shear wave critical angles may need to be reconsidered further the non linear coupling between pores and cracks due to pressure equalization effects means that the linear schoenberg muir calculus may not be applied to such systems the theory receives strong support from recent data by rathore et al on artificial samples with controlled crack geometry

depositional sequences in shallow carbonate dominated sedimentary systems concepts for a high resolution analysis abstract oxfordian and berriasian sections representing shallow water carbonate dominated sedimentary systems have been studied in the swiss and french jura in spain and in normandy they all display a hierarchical stacking of depositional sequences facies evolution and stacking pattern allow to define elementary small scale medium scale and large scale sequences some depositional sequences display well marked sequence boundaries others are limited by transgressive or maximum flooding surfaces the hierarchical organisation of such sequence stratigraphic elements implies that sea level fluctuations were an important factor in their formation and that these fluctuations had different frequencies the superposition of high frequency sea level changes on a long term sea level trend leads to repetition of diagnostic surfaces defining sequence boundary and maximum flooding zones wherein the corresponding high frequency surfaces are well developed chronostratigraphic tie points permit us to estimate the duration of large scale sequences this time control and the observed hierarchical stacking suggest that the high frequency sea level changes were controlled by climatic cycles in the milankovitch frequency band the variability of stacking pattern and facies evolution between sections illustrates the complexity of the studied environments furthermore because of the minimal accommodation space available in these shallow water settings much of the geologic time is not recorded nevertheless detailed analysis of the depositional sequences allows the interpretation of the evolution of the sedimentary system with a high time resolution thus there is a potential to monitor sedimentological ecological and diagenetic processes on a time scale of 20 to 100 ka

orbital tuning of a lower cretaceous composite record maiolica formation central italy 1 a high resolution pelagic bulk carbonate stable isotope record from a central tethyan lower cretaceous composite section is presented three well exposed sedimentary sequences chiaserna monte acuto bosso and gorgo a cerbara sections central italy cropping out throughout the maiolica formation were correlated by a detailed magnetostratigraphy lithostratigraphy and calcareous plankton biostratigraphy in order to reconstruct a continuous composite record from the middle berriasian to the lower aptian the integrated stratigraphy of the three sequences provided an accurate time framework for the new high resolution c isotope curve which is presented in this study the composite 13c signal recorded in the depth domain was analyzed by combined lomb scargle periodogram and weighted wavelet z transform wwz weighted wavelet amplitudes wwa foster wavelet spectral methodologies both appropriate for unevenly sampled curves these tools allowed us to unravel the main frequencies modulating the record and their hypothetical shift in depth respectively the long term 400 000 and 2 400 000 years eccentricity cycles were consistently recorded throughout all the composite record once band pass filtered in these two periodicity bands and compared to the lithologic pattern cycles identified throughout the composite sequence the 13c signal was used as a valuable proxy record for a reliable construction of an orbital tuning of the early cretaceous an estimated age for all the different stratigraphic events recognized throughout the composite record was reported in particular the reestimated ages of the paleomagnetic chrons documented in the upper part of the record show differences with those reported by gradstein et al 2004 up to 2 m y the valanginian carbon shift present in the middle part of the composite sequence was estimated to be 2 3 m y long the good agreement between the estimated age of the base of this positive carbon isotope excursion 136 34 m y and the predominant volcanic phase associated to the parana etendeka large igneous province intrusion confirmed a possible cause effect link between the two events

preservation of delta 18 o and delta 13 c in belemnite rostra from the jurassic early cretaceous successions stratigraphic collection of 263 belemnite rostra guards from jurassic to lower cretaceous sediments in the moscow area of russia northern germany england new zealand and morocco were analyzed to determine temporal variations in their delta 18 o and delta 13 c as a proxy for isotopic composition of surface seawater internal structures of the rostra were studied using cathodoluminescence and sem techniques their trace element fe mn mg and sr and the stable carbon and oxygen isotope compositions the average stratigraphic resolution is about 1 ma these samples represent a broad range of paleolatitudes and paleoceanographic settings ranging from 42 degrees n to 82 degrees s covering boreal to subtropical brackish to open marine shelf paleoenvironments the rostra originally contained domains enriched in organic matter that were later replaced by luminescent diagenetic calcite thereby generating an impression of seasonal growth rings diagenetic calcite accounts for up to 6 8 percent of the rostrum the trace element content of the rostra range from 227 to 3935 ppm for mg 379 to 1514 ppm for sr and up to 305 and 3662 ppm for mn and fe respectively however selective drilling of only primary calcitic domains resulted in 90 percent of all analyzed samples to fall within the range of trace element values typical for modern low mg calcite shells the chemical and textural data suggest that the primary calcite of the rostra is well preserved as is its isotopic signature the delta 13 c and delta 18 o measurements define oscillating temporal bands of about 3 to 4 per mil width around more or less modern marine values of 2 and 1 per mil respectively oxygen isotopes in the well preserved rostra most likely reflect temperature variations within a approximately 14 or 8 degrees c range the observed 3 to 4 per mil spread of values between contemporaneous specimens which is comparable to the scatter of data reported for modern low mg calcitic brachiopod shells probably reflects the diversity of local ecosystems although some of the spread may be due to real short term temporal variations within single biozones the delta 13 c values of the rostra define a band similar to that of the oxygen isotopes and the temporal oscillations of both isotope curves are commonly in phase the above dataset complemented by the results of jones jenkyns and hesselbo 1994 and jones and others 1994 yields a composite delta 18 o and delta 13 c variations in belemnite guards for the entire jurassic early cretaceous interval for delta 18 o the oscillating band indicates that the most negative values occur at times of independently postulated global and or regional anoxic events such as those in the toarcian and kimmeridgian as well as the aptian these oaes appear to be characterized by up to 10 degrees c higher seawater temperatures than the intervening background times the delta 13 c record shows 13 c depletion for the kimmeridgian event but enrichment for the other two suggesting that the delta 13 c temporal oscillations and oaes are not simply a reflection of surficial productivity due to operation of the biological pump but rather of changing oceanic circulation patterns due to variations in latitudinal surface temperature gradients

benthic carbonate factories of the phanerozoic marine carbonate precipitation occurs in three basic modes abiotic or quasi abiotic biotically induced and biotically controlled on a geologic scale these precipitation modes combine to form three carbonate production systems or factories in the benthic environment 1 tropical shallow water factory dominated by biotically controlled mainly photo autotrophic and abiotic precipitates 2 cool water factory dominated by biotically controlled mainly heterotrophic precipitates and 3 mud mound factory dominated by biotically induced mainly microbial and abiotic precipitates sediment accumulations of the factories differ in composition geometry and facies patterns and some of these differences appear prominently in seismic data thus facilitating subsurface prediction the characteristic accumulation of the tropical factory is the flat topped often reef rimmed platform in cool water systems reefs in high energy settings are scarce and hydrodynamic influence dominates producing seaward sloping shelves and deep water sediment drifts often armored by skeletal framework the typical accumulation of the mud mound factory is groups of mounds in deeper water where the mud mound factory expands into shallow water it forms rimmed platforms similar to the tropical factory the tropical factory is most productive the mud mound factory reaches 80 90 and the cool water factory 20 30 of the tropical growth rate the three factories represent end members connected by transitions in space transitions in time are linked to biotic evolution

detection of gas in sandstone reservoirs using avo analysis a 3 d seismic case history using the geostack technique the geostack technique is a method of analyzing seismic amplitude variation with offset avo information one of the outputs of the analysis is a set of direct hydrocarbon indicator traces called fluid factor traces the fluid factor trace is designed to be low amplitude for all reflectors in a clastic sedimentary sequence except for rocks that lie off the mudrock line the mudrock line is the line on a crossplot of p wave velocity against s wave velocity on which water saturated sandstones shales and siltstones lie some of the rock types that lie off the mudrock line are gas saturated sandstones carbonates and igneous rocks in the absence of carbonates and igneous rocks high amplitude reflections on fluid factor traces would be expected to represent gas saturated sandstones of course this relationship does not apply exactly in nature and the extent to which the mudrock line model applies varies from area to area however it is a useful model in many basins of the world including the one

the mineralogical composition of precursor sediments of calcareous rhythmites a new approach previous studies in silurian carbonates from gotland sweden have led to a model for the development of limestone marl alternations this model postulates that early diagenesis of precursor sediments without strong primary differences can result in a differentiation by selective dissolution of aragonite in marl beds and reprecipitation of calcite cement in limestone beds this model is described as a set of mathematical equations that quantify the diagenetic processes aragonite dissolution and calcite reprecipitation that occur during the formation of limestone marl interbeds from a hypothetical homogeneous precursor sediment the calculations demonstrate that resulting hypothetical limestone marl alternations show characteristic mathematical relationships between the ratios of the bed thicknesses of limestones and marls on one side and the carbonate contents on the other by reversing this model the original mineralogical composition of the precursor sediment of real world rhythmic successions can be determined in this study alternations from the silurian of gotland the cambrian devonian and mississippian of north america the jurassic of france and germany and the cretaceous of france are shown to exhibit mathematical relationships similar to those calculated for hypothetical precursor sediments without primary differences therefore the mineralogical composition of their precursor sediments can be estimated in contrast the clear mismatch shown by the lower jurassic belemnite marls from dorset indicates that these rhythms did not suffer an early diagenetic overprint our model helps to differentiate between rhythmites with strong depositional variations and those without however it cannot indicate whether a given alternation is the product of rhythmic diagenesis of a homogeneous precursor sediment or the result of diagenetic enhancement of subtle underlying sedimentary rhythms for horizontally correlated patterns such as laterally extensive beds and layers of nodules an a priori unknown external signal has to be assumed

accumulation of organic carbon rich sediments in the late jurassic and cretaceous atlantic ocean a synthesis abstract organic carbon rich sediments black shales were deposited at some 40 deep sea drilling project dsdp sites in the atlantic ocean during late jurassic and cretaceous times in order to reconstruct the depositional environments of black shale formation we have studied in detail changes in the occurrence of black shales as well as the type and amount of organic matter in relation to changes in paleogeography paleooceanic circulation and paleoclimate we have calculated accumulation rates of marine and terrigenous organic carbon and combined these with organic carbon content sedimentation rate relationships to obtain information about paleoenvironments main factors causing the preservation and accumulation of organic carbon are 1 the supply of terrestrial organic matter 2 the supply of marine organic matter 3 the oxygenation of the deep water and 4 the bulk sedimentation rate changes in the importance of these different factors and changes in the interrelationships among them determine the complex system of black shale formation and their cyclic variations increased supply of terrestrial organic matter due to increased river discharge and humid climate in northeast america southwest europe and south africa were important in the hauterivian albian northwest and northern northeast atlantic and in the late jurassic aptian south atlantic in the latter anoxic deep water conditions due to restricted circulation in the nascent largely isolated basin as well as higher oceanic productivity may have reinforced the high preservation rates of marine organic matter during that time off northwest africa higher flux rates of marine organic matter were recorded in areas of higher productivity which may have been caused by fluvial nutrient supply during the hauterivian and by intensified upwelling during albian turonian times in general the depositional environment was oxygenated and the content of marine organic matter was a function of the sedimentation rate furthermore at some places redeposition by turbidity currents or slumps may have been important for the accumulation of organic matter in the deep sea basins the only episode of widespread anoxia in the western and eastern basins of the north atlantic may have occurred in the cenomanian turonian when high amounts of marine organic matter were also preserved in areas of low productivity and low sedimentation rates

the temperature of europe during the holocene reconstructed from pollen data we present the first area average time series reconstructions of warmest month coldest month and mean annual surface air temperatures across europe during the last 12 000 years these series are based on quantitative pollen climate reconstructions from over 500 pollen sites assimilated using an innovative four dimensional gridding procedure this approach combines three dimensional spatial gridding with a fourth dimension represented by time allowing data from irregular time series to be focussed onto a regular time step we provide six regional reconstructed temperature time series as well as summary time series for the whole of europe the results suggest major spatial and seasonal differences in holocene temperature trends within a remarkably balanced regional and annual energy budget the traditional mid holocene thermal maximum is observed only over northern europe and principally during the summer this warming was balanced by a mid holocene cooling over southern europe whilst central europe occupied an intermediary position changes in annual mean temperatures for europe as a whole suggest an almost linear increase in thermal budget up to 7800 bp followed by stable conditions for the remainder of the holocene this early holocene warming and later equilibrium has been mainly modulated by increasing winter temperatures in the west which have continued to rise at a progressively decreasing rate up to the present day

anisotropy estimate for the horn river basin from sonic logs in vertical and deviated wells abstract shale anisotropy must be quantified to obtain reliable information about reservoir fluids lithology and pore pressure from seismic data failure to account for anisotropy in seismic processing can lead to errors in normal moveout nmo dip moveout dmo migration time to depth conversion and amplitude variation with offset avo analysis kriged estimates of density vertical compressional wave and shear wave velocities were derived from vertical wells in an area of interest in the horn river resource play in northeastern british columbia the kriged predictions were compared with measured logs along the trajectory of a deviated well whereas the density comparison provided a blind test of kriging accuracy because density is scalar and independent of well deviation the same comparison for sonic velocities revealed that they are systematically higher than the kriged vertical velocities this difference was used to estimate anisotropy parameters at the location of the deviated well the fac

fertilization of the northwestern tethys vocontian basin se france during the valanginian carbon isotope perturbation evidence from calcareous nannofossils and trace element data a high resolution calcareous nannofossil study associated with a geochemical analysis major trace elements and carbon and oxygen isotope stratigraphies was carried out in the angles section hemipelagic setting of the vocontian basin se france during the valanginian positive carbon isotope excursion the behaviour of calcareous nannofossil taxa in relation to fertility conditions was studied to elaborate new nutrient indices in this environment a high nutrient index based on biscutum spp discorhabdus rotatorius zeugrhabdotus fissus high fertility indicators and watznaueria barnesae low fertility indicator and a medium nutrient index based on lithraphidites carniolensis medium fertility indicator and w barnesae low fertility indicator these two indices show a major fertilization from the stephanophorus ammonite zone to the trinodosum ammonite zone with a maximum during the positive carbon isotope excursion since high values of the nutrient indices are in phase with high values of chemical elements related to terrigenous material and low values of the coccolith total abundance it is proposed that pulses of detrital inputs into the basin triggered the nutrification which in turn caused a biocalcification crisis of the calcareous nannofossils nutrification is also responsible for the reef demise in the surrounded platforms as indicated by the increased sr ca seawater ratio at that time the intensification of the parana etendeka volcanic activity triggering co2 excess in the atmosphere is probably responsible for an acceleration of the hydrological cycle the increased weathering and the subsequent higher terrigenous and nutrient transfer from continents to the vocontian basin in such a scenario nutrification is a dominant factor controlling neritic and hemipelagic biocalcification however one cannot exclude that the global increase of atmospheric co2 could generate chemical changes of the sea surface waters acting with the nutrification to modify the biocalcification of the carbonate producers

paleoceanographic changes during the early cretaceous valanginian hauterivian evidence from oxygen and carbon stable isotopes we investigated valanginian hauterivian bulk rock and belemnite samples from vocontian basin sections in southeastern france for their stable carbon and oxygen isotope signature firstly because these sections permit calibration with high resolution biostratigraphy based on ammonites and secondly because detailed isotope studies for the hauterivian are lacking the results show that 13c values for bulk rock decrease during the late valanginian early hauterivian with 1 and increase again during the late hauterivian with 1 the 18o signal for bulk rock samples is mostly disturbed by diagenesis the belemnites show carbon and oxygen isotope values that are lower and higher than bulk rock samples respectively we explain this as the result of the belemnites recording a deeper water signal with lighter 13c values and heavier 18o values implicating colder water at greater depth the overall preservation of the belemnites is very good and permits the construction of a paleo temperature trend this trend shows warm deeper water temperatures during the late early valanginian 15 c and progressively cooler temperatures during the late valanginian and early hauterivian 11 c during the late hauterivian temperatures increase again 13 c we relate this cooling trend for deeper water to a second order sea level rise which allowed for the exchange of cold boreal and warm tethyan water masses the influx of cold nutrient rich water had a profound effect on carbonate producing biota along the northern margin of the tethys during the hauterivian leading to prolonged phases of condensation and platform destruction during the early hauterivian the carbonate system along the northern tethyan margin shifted into a green water mode of carbonate production high rates of carbonate production under mesotrophic conditions also observed in other parts of the world meant that the global carbon cycle became buffered shown by the stable trends in carbon isotopes

interactions between environmental change and shallow water carbonate buildup along the northern tethyan margin and their impact on the early cretaceous carbon isotope record 1 the evolution of the early cretaceous northern tethyan carbonate platform was not only influenced by changes in sea level detrital influx and surface water temperature but also by changes in trophic levels we distinguish between phases of carbonate production dominated by oligotrophic photozoan communities and by mesotrophic and eventually colder water heterozoan communities superimposed on this bimodal trend in platform evolution were phases of platform demise for which we provide improved age control based on ammonite biostratigraphy the initial phase of these episodes of platform demise corresponds in time to episodes of oceanic anoxic events and environmental change in general on the basis of a comparison between the temporal changes in an early cretaceous ammonite calibrated d 13 c record from southeastern france and coeval changes in the platform record we suggest that the history of carbon fractionation along the northern tethyan margin was not only influenced by changes in the oceanic carbon cycle such as in the rate of production and preservation of organic and carbonate carbon and in the size of the oceanic dissolved inorganic carbon reservoir but it was also influenced by the above mentioned changes in the ecology and geometry of the adjacent carbonate platform phases of photozoan carbonate production induced positive trends in the hemipelagic carbonate d 13 c record phases of heterozoan carbonate production pushed the d 13 c system toward more negative values platform drowning episodes implied an initial increase in d 13 c values followed by a longerterm decrease in d 13 c values

thermal evolution of cretaceous tethyan marine waters inferred from oxygen isotope composition of fish tooth enamels 1 the evolution of subtropical 30 35 n upper ocean temperatures through the cretaceous is inferred from the oxygen isotope compositions of 64 fish teeth enamel coming from the western tethyan platform mean 18o values of 22 at the berriasian valanginian boundary decrease with oscillations to 18 5 around the cenomanian turonian boundary and progressively increase to 21 5 by the end of the cretaceous the similarity of this oxygen isotope curve for bioapatites from platform environments with those for foraminifera and bulk carbonates that were deposited in deeper waters and at other paleolatitudes indicates that they record global climatic signals major cooling events at the million year scale can be distinguished 1 at the berriasian valanginian boundary and 2 during the earliest late valanginian a third cooling event is detected during the earliest aptian these events already proposed as icehouse interludes during the lower cretaceous are also recorded at subtropical latitudes a progressive warming is identified from the aptian to the cenomanian turonian interval that corresponds to a thermal optimum and then upper ocean temperatures decreased to the maastrichtian minimum isotopic temperatures range from 15 c to 28 c assuming a 18oseawater of 1 for an ice free world taking more realistic 18oseawater values of 0 for tropical waters during glacial periods within the berriasian valanginian interval and earliest aptian or with above average salinities possibly the maastrichtian temperatures are increased by 4 5 c temperature differences between climatic extremes of the valanginian and cenomanian turonian are estimated to have been 10 c latitudinal thermal gradients for the albian cenomanian turonian and maastrichtian were 0 2 0 3 c latitude and thus weaker than modern oceanic values at about 0 4 c latitude

160 m y record of marine sedimentary phosphorus burial coupling of climate and continental weathering under greenhouse and icehouse conditions a compilation of all meaningful phosphate data published in the proceedings of the deep sea drilling project and the ocean drilling program is used here for the extraction of a 160 m y marine burial record of bulk phosphorus all sediment types 5648 measurements and a 100 m y marine burial record of biogenic phosphorus pelagic biogenic sediment types 1754 data these records serve as estimates for total and dissolved bioavailable phosphorus flux rates which in turn are highly dependent on total and chemical continental weathering rates respectively the similarity of both records indicates that in the past 100 m y changes in total weathering rates may have been tracked by changes in chemical weathering rates prior to 32 ma phosphorus burial and long term sea level change are positively correlated from 32 ma to the present this correlation is inverse this points to a fundamental change in feedback mechanisms between continental weathering phosphorus and climate which was probably linked to the onset of major glaciation

amplitude versus offset variations in gas sands seismic reflections from gas sands exhibit a wide range of amplitude versus offset avo characteristics the two factors that most strongly determine the avo behavior of a gas sand reflection are the normal incidence reflection coefficient r0 and the contrast in poisson s ratio at the reflector of these two factors r0 is the least constrained based on their avo characteristics gas sand reflectors can be grouped into three classes defined in terms of r0 at the top of the gas sand class 1 gas sands have higher impedance than the encasing shale with relatively large positive values for r0 class 2 gas sands have nearly the same impedance as the encasing shale and are characterized by values of r0 near zero class 3 sands have lower impedance than the encasing shale with negative large magnitude values for r0 each of these sand classes has a distinct avo characteristic an example of a gas sand from each of the three classes is presented in the paper the class 1 example involves a hartshorn channel s