ecologic interpretation of deep sea trace fossil communities abstract trace fossil types and their vertical arrangement tiering are closely related to the biotope and hence can provide information on ecological conditions especially on oxygenation benthic food content sedimentation rate and substrate consistency however to use the full potential of trace fossils for environmental analyses various ichnologic criteria are necessary in addition to trace fossil taxonomy based on a taxonomically oriented ichnologic evaluation of modern deep sea environments six ichnologic aspects to ecologic changes have been found to be of major importance 1 1 penetration depth of burrows is influenced by oxygenation benthic food availability and substrate consistency 2 2 the number and vertical extension of tiers varies due to environmental changes a decrease in the vertical extension of tiers telescoping indicates a decrease in oxygenation benthic food content and or sedimentation rate if shallow penetrating traces disappear oxygen may be depleted and if the deepest tiers disappear the benthic food content may be lowered 3 3 the size diameter of specific trace fossils can indicate oxygen and benthic food availability 4 4 degree of bioturbation is related to sedimentation rate and benthic food content and the diversity of the whole ichnocoenoses or within a specific tier is affected by oxygenation benthic food content sedimentation rate and substrate consistency 5 5 the type of burrow wall unlined mucus lined pellet lined mainly refers to substrate consistency and subordinately to pore water oxygenation 6 6 the type of sediment displacement by body appendages used for excavation ingestion sideward pressing or outside pumping is related to substrate consistency and grain size

yield envelope assessment as a preliminary screening tool to determine carbon capture and storage viability in depleted southern north sea hydrocarbon reservoirs abstract this paper describes a laboratory study of the hydro mechanical properties of samples from the sherwood sandstone group ssg an onshore analogue of the finer grained lower porosity portions that make up the bunter sandstone formation bsf the study provides a yield envelope for this sandstone and demonstrates that it is a competent sandstone at relevant reservoir depths a theoretical yield envelope has been calculated based on the anticipated in situ stress induced by depletion and reinjection showing that only the high porosity 35 large grain diameter 290 m end member of the bsf is likely to result in deformation of the reservoir rock stress analysis of four fields within the southern north sea suggest that depletion of 10 mpa will not result in permanent deformation of the reservoirs assuming similar porosity and grainsize characteristics to the ssg tested furthermore re inflation is unlikely to result in permanent deformation should the injection pressure not exceed the initial pre production reservoir pore pressure

co2 sequestration by mineral trapping in natural analogues in the yinggehai basin south china sea abstract mineral trapping of co2 by precipitation of carbonate minerals is seen as the most permanent and secure mechanism of co2 storage we have investigated mineral trapping in co2 rich siliciclastic reservoirs of the upper miocene age in the yinggehai basin south china sea and used nearby co2 poor reservoirs of similar age as benchmarks for the analysis within the reservoir the co2 has triggered the reaction from calcite plus chlorite to ankerite plus kaolinite which traps 5 mol of co2 per mole of chlorite geochemical modelling shows that the total amount of permanently trapped co2 is approximately one half of the co2 in the newly formed ankerite caprock mineralogy shows that co2 leakage has occurred and co2 has migrated into the shale rich caprock but without loss of caprock integrity

source and distribution of pollen and spores in surface sediments of a plateau lake in southeastern amazonia abstract in the past two decades several paleobotanical studies on lakes have indicated a drier climatic condition in amazonia during the last glacial maximum lgm and early mid holocene which likely resulted in the contraction of the tropical forest and expansion of savanna however these investigations in the eastern amazon are scarce and the reliability and precision of the inference remains largely limited in this study palynological analyses were carried out in surficial sediments of violao lake southeastern amazonia to assess the distribution and sources of pollen and spores from montane savanna and forest based on surrounding vegetation types catchment basin and wind patterns and also to identify modern pollen rain characteristics of the ecosystems developed under edaphic conditions considering the spatial distribution of pollen and spores in the lake basin the montane savanna pollen are predominant in the eastern and western borders and subordinate in the central portion of the lake whereas those of forest formations are mainly present in its western margin pteridophytes and algae spores show a distinct surface distribution compared to montane savanna pollen while the algae spores have a good correlation with forest formation pollen the correlation of these data along with vegetation types of the catchment basin indicates that the absence of pollen grains from species that present large occurrence along the catchment basin may be partially related to phenological characteristics that avoided pollen losses however in general montane savanna pollen in the bottom sediments tends to represent a signal of the regional vegetation developed over ferruginous duricrusts in the serra do carajas while forest formation pollen may be used to identify changes in forest structure of the catchment basin of violao lake as montane savanna is developed under edaphic conditions in hydric stress substrate a systematic description of pollen grains clearly demonstrates that only changes in forest vegetation can be used as a reliable indicator of paleoclimate changes

in situ carbonation of peridotite for co2 storage the rate of natural carbonation of tectonically exposed mantle peridotite during weathering and low temperature alteration can be enhanced to develop a significant sink for atmospheric co2 natural carbonation of peridotite in the samail ophiolite an uplifted slice of oceanic crust and upper mantle in the sultanate of oman is surprisingly rapid carbonate veins in mantle peridotite in oman have an average 14c age of 26 000 years and are not 30 95 million years old as previously believed these data and reconnaissance mapping show that 104 to 105 tons per year of atmospheric co2 are converted to solid carbonate minerals via peridotite weathering in oman peridotite carbonation can be accelerated via drilling hydraulic fracture input of purified co2 at elevated pressure and in particular increased temperature at depth after an initial heating step co2 pumped at 25 or 30 c can be heated by exothermic carbonation reactions that sustain high temperature and rapid reaction rates at depth with little expenditure of energy in situ carbonation of peridotite could consume 1 billion tons of co2 per year in oman alone affording a low cost safe and permanent method to capture and store atmospheric co2

evaluation of the impact of co2 co contaminant gas aqueous fluid and reservoir rock interactions on the geologic sequestration of co2 lowering the costs of front end processes e g separation in the geologic sequestration of co2 can dramatically lower the overall costs one possible approach is to sequester less pure co2 waste streams that are less expensive or require less energy to separate from flue gas or a coal gasification process etc the objective of this research is to evaluate the impacts of co2 itself as well as an impure co2 waste stream on geologic sequestration using reaction progress models reactive transport simulators and analogous reactive transport experiments run in a plug flow reactor specifically we are investigating the potential for co injecting the sox nox and h2s present in coal fired waste streams along with the co2 we present here the results of reactive transport simulations made to investigate the long term impact of dissolved co2 h2s and so2 on carbon sequestration in the frio fm tx the results suggest that addition of relatively large amounts of h2s to a co2 injection should not adversely impact injectivity or sequestration compared to the injection of co2 alone the co injection of so2 however could produce significantly different results if conditions are such that so2 can oxidize to sulfate in the long term the simulations suggest that carbonate mineral formation can sequester a significant fraction of the injected carbon through the formation of calcite magnesite and dawsonite

microbial and meiofaunal abundance redox potential ph and shear strength profiles in deep sea pacific sediments vertical profiles of microbial and meiofaunal abundances and of redox potential ph water content and shear strength have been measured in sediment subcores from seven box cores collected in the southern central pacific and the findings have been related to potential effects on pelagic fluxes and sedimentation microbial and meiofaunal abundance decreased exponentially with sediment depth micro organisms were found throughout the depth of the core to 40 cm meiofauna were only present in the top 5 cm to 10 cm meiofaunal metazoans decreased more rapidly than foraminiferans redox potential was always positive decreased below the surface and often rose again deeper in the sediment ph showed no clear cut trend with depth shear strength increased with sediment depth and showed occasional anomalous fluctuations water content decreased progressively downwards with the widest scatter in the top 5 10 cm the most important implications of these results for the water column above are likely to be biologically mediated organic mineralization and nutrient regeneration into the water column and biodeposition of heavy metals and the binding of particulates at and above the sediment water interface by extracellular polymeric material which will in turn lead to differential patterns of sedimentation

long term performance of a mudrock seal in natural co2 storage the ability of mudrock seals to prevent co 2 leakage is a major concern for geological storage of anthropogenic co 2 the long term performance of a mudrock seal which provides a natural analogue in the north sea miller oil fi eld has been evaluated this mudrock seal is immediately above a natural co 2 rich reservoir the paper reports the stable isotopes of carbon from carbonate minerals in the mudrock that have precipitated in contact with co 2 during 4 km of burial a well defi ned linear trend of upward decreasing 13 c traces the progressive penetration of free phase co 2 causing dissolution and reprecipitation of carbonate minerals the co 2 was emplaced ca 70 80 ma and has only penetrated 12 m vertically in this case the infi ltration rate is estimated as 9 8 10 7 g cm 2 yr 1

modelling carbon dioxide accumulation at sleipner implications for underground carbon storage an analytical solution to the equations describing the flow of a buoyant fluid released into a porous medium below a horizontal impermeable boundary is used to model the growth of co2 accumulations beneath thin mudstone beds in the utsira sand reservoir at sleipner in the north sea here supercritical co2 has been injected at a rate of 1 mt yr since 1996 and imaged by time lapse seismic data in 1999 2001 and 2002 the co2 rises as a narrow plume and is partially trapped by a number of thin mudstones before reaching the caprock to the reservoir the radii of the individual layers of trapped co2 increase as the square root of time since initiation as predicted by the modelling for constant input flux however apparent negative initiation times for horizons low in the reservoir suggests that net input fluxes for these layers have decreased with time most probably as the spreading layers have increased their leakage rates accumulation of co2 in the layers higher in the reservoir was initiated up to 3 yr after injection started modelling of the thickness profiles across three of the higher layers suggests that their net input fluxes have increased with time the observation that the central thicknesses of the deeper layers have remained approximately constant or have slightly decreased since first imaged in 1999 is consistent with the model predictions that the central thickness is directly proportional to net input flux however estimates of the permeability of the reservoir from the rate of increase of the radii of the co2 accumulations are an order of magnitude less than measured permeabilities on the reservoir sandstone permeabilities estimated from the modelling of layer thickness changes scatter in the same range these discrepancies may arise from 1 approximations in the model not being valid 2 the measured permeabilities not being representative of the permeability for two phase flow on the scale of the reservoir or considered less likely 3 that much less co2 is being stored in the imaged co2 accumulations than estimated from the seismic reflection profiles the most probable cause of the discrepancy is that the relative permeability for the co2 phase is significantly reduced at lower co2 saturations

in situ geotechnical characterization of sediments on the nova scotian slope eastern canadian continental margin abstract a seabed 2 m long cone penetrometer and coring system geotechnical module has been used at 17 stations in four transects on the scotian slope to characterise in situ shear strength and induced pore pressure on several different types of late pleistocene and early holocene failure study sites were selected using the sar high resolution deep towed acoustic system equipped with a digital 160 190 khz sidescan sonar and a 3 5 khz subbottom profiler several distinctive types of geotechnical signature were recognised from plots of cone resistance and induced pore pressure with depth in the sediment normally consolidated sediments show a progressive increase in cone resistance with depth to about 75 kpa at 2 m subbottom holocene surficial muds show spectacular apparent overconsolidation reaching a peak of 250 kpa at about 50 cm subbottom and then decreasing down to 1 5 m this overconsolidation is associated with zoophycos burrows late pleistocene sediments exhumed by bedding plane slides show strong true overconsolidation consistent with the original depth of burial inferred from high resolution seismic stratigraphy debris flows show only a slight shear stress gradient with depth 40 45 kpa over 0 5 1 m subbottom with under consolidation due to remoulding of sediment

biological modifiers of marine benthic seascapes their role as ecosystem engineers benthic organisms in marine ecosystems modify the environment on different spatial and temporal scales these modifications many of which are initially at a microscale are likely to have large scale effects on benthic seascapes this is especially so if the species are ecosystem engineers most species of infaunal and epifaunal invertebrates and macrophytes contribute at a geophysical or geochemical level microorganisms also play a key but currently neglected role in the intertidal and immediately sublittoral zone algae and seagrasses and mussels in mussel beds have received considerable attention a substantial fossil record also exists mathematical modelling of these systems is still in its infancy although several sophisticated mathematical tools have been applied the effects of bioturbation and of microorganisms have been less studied and little is known about the activities of benthic organisms in the deep sea this paper addresses all these effects and places them in the context of large scale benthic seascapes and of the extensive literature on species defined as ecosystem engineers in the sea

geochemical monitoring for potential environmental impacts of geologic sequestration of co2 carbon dioxide sequestration is now considered an important component of the portfolio of options for reducing greenhouse gas emissions to stabilize their atmospheric levels at values that would limit global temperature increases to the target of 2 c by the end of the century pacala and socolow 2004 ipcc 2005 2007 benson and cook 2005 benson and cole 2008 iea 2012 romanak et al 2013 increased anthropogenic emissions of co2 have raised its atmospheric concentrations from about 280 ppmv during pre industrial times to 400 ppmv today and based on several defined scenarios co2 concentrations are projected to increase to values as high as 1100 ppmv by 2100 white et al 2003 ipcc 2005 2007 eia 2012 global ccs institute 2012 an atmospheric co2 concentration of 450 ppmv is generally the accepted level that is needed to limit global temperature increases to the target of 2 c by the end of the century this temperature limit likely would moderate the adverse effects related to climate change that could include sea level rise from the melting of alpine glaciers and continental ice sheets and from the ocean warming increased frequency and intensity of wildfires floods droughts and tropical storms and changes in the amount timing and distribution of rain snow and runoff ipcc 2007 sundquist et al 2009 iea 2012 rising atmospheric co2 concentrations are also increasing the amount of co2 dissolved in ocean water lowering its ph from 8 1 to 8 0 with potentially disruptive effects on coral reefs plankton and marine ecosystems adams and caldeira 2008 schrag 2009 sundquist et al 2009 sedimentary basins in general and deep saline aquifers in particular are being investigated as possible repositories for the large volumes of

geological carbon storage storage of the carbon dioxide that is produced by burning fossil fuels is one way to avoid the damaging consequences of climate change a range of observations suggests that geological carbon storage is much less risky than unabated carbon emissions to the atmosphere

the effect of vertically varying permeability on tracer dispersion we study the migration of a tracer within an injection driven flow in a horizontal aquifer in which the permeability varies with depth the permeability gradient produces a shear and this leads to lateral dispersion of the tracer in the high permeability regions the tracer moves substantially faster than the mean flow and eventually enters the nose region of the flow where the depth of the current is less than the depth of the aquifer depending on the influence of i the viscosity contrast between the injected fluid and the original fluid and ii the vertical permeability gradient the nose of the current may be of fixed shape or may gradually lengthen with time this leads to a variety of patterns of dispersal of the tracer which may either remain in the nose or cycle through the nose and be left behind our results illustrate the complexity of the migration of a tracer in a heterogeneous aquifer which has important implications for interpreting the results of tracer tests as may be proposed for monitoring text co 2 or gas injected into subsurface reservoirs

rapid reactions between co2 brine and silicate minerals during geological carbon storage modelling based on a field co2 injection experiment the dissolution of co2 into formation brines and the subsequent reactions of the co2 charged brines with reservoir minerals are two key processes likely to increase the security of geological carbon dioxide storage these processes will be dependent on the permeability structure and mineral compositions of the reservoirs but there is limited observational data on their rates in this paper we report the cation and anion concentrations and sr oxygen and carbon isotopic compositions of formation waters from four extraction wells sampled at surface over 6 months after commencement of co2 injection in a five spot pattern for enhanced oil recovery at the salt creek field wyoming sampled fluids separated from the minor oil component exhibit near monotonic increases in alkalinity and concentrations of cations but little change in cl and br concentrations and oxygen and deuterium isotope ratios the increases in alkalinity are modelled in terms of reaction with reservoir calcite and silicate minerals as the changes in fluid chemistry and sr isotopic compositions are inconsistent with simple addition of injected fluids sampled over the course of the experiment the reservoir mineral chemical and isotopic compositions are characterised by sampling core as well as surface exposures of the frontier formation elsewhere in wyoming the evolution of the fluid chemistries reflects extensive dissolution of both carbonate and silicate minerals over the course of the six months sampling implying rapid dissolution of co2 in the formation waters and reaction of co2 bearing brines with formation minerals rates of co2 diffusion into the brines and advection of co2 charged brines in the reservoir are sufficiently slow that if present calcite should react to be close to equilibrium with the fluids this allows estimation of the co2 partial pressures in the sampled fluids and comparison with the thermodynamic driving force for the relatively rapid average plagioclase dissolution rates of 10 12 mol m 2 s 1

biogeomorphological implications of microscale interactions between sediment geotechnics and marine benthos a review abstract at the foundations of biogeomorphological processes in the sea lie interactions between the activities of marine benthic animals and the geotechnical properties of their sedimentary environments the potential significance of these interactions which take place at a microscale level of millimetres to metres for the large scale geomorphology of the seabed has rarely been appreciated in the context of this review large scale is defined as greater than 50 m to hundreds of kilometres the present review addresses this link drawing examples from a wide range of marine environments including estuaries the intertidal zone continental shelves and slopes and the deep sea it firstly considers sediment stabilisation slope failure sediment mixing biodeposition sediment compaction and hydrodynamic effects this is followed by a consideration of two extremes of the ecological pyramid the effects of marine meiofauna and marine vertebrates the final section draws attention to the central role of faunal mucus and extracellular polymeric material ecpm in many of the microscale interactions that we describe the implications of these microscale biological processes and features are discussed in terms of their influence on and control of the large scale geomorphology of the seabed

bioturbation in deep sea pacific sediments bioturbation structures have been recorded in sediment cores from 5000 m to 5300 m water depth collected from the pacific ocean using subcores from spade box cores shear strength and eh profiles were also taken the results have been related to pelagic fluxes and sedimentation bioturba tion structures consist of holes and chambers of various sizes running vertically and horizontally whose abundance size and shapes have been measured values did not decrease exponentially with sediment depth but end abruptly at depths ranging from 30 cm to 45 cm in many instances there were subsurface peaks of vertical and horizontal burrows there was also evidence of lateral galleries large deep in filled burrows were observed these consisted of light and dark brown sediment having different shear strengths eh profiles showed that the sediment was aerobic with slightly lower values near the sediment surface possibly associated with meiofaunal and bacterial activity the most important implications of this work for pelagic processes are likely to be increased transfer of dissolved and particulate material between the sediment and the water column due to the physical network of burrows within the sediment and increased surface roughness caused by burrow openings mucus binding of particulates and faecal pelletization are also likely to be important all of these effects will be most noticeable at high burrow densities

pseudobreccias revealed as calcrete mottling and bioturbation in the late dinantian of the southern lake district uk two types of pseudobreccia one with grey and the other with brown mottle fabrics occur in shoaling upward cycles of the urswick limestone formation of asbian late dinantian carboniferous age in the southern lake district uk the grey mottle pseudobreccia occurs in cycle base packstones and developed after backfilling and abandonment of thalassinoides burrow systems burrow infills consist of a fine to coarse crystalline microspar that has dull brown to moderate orange colours under cathodoluminescence mottling formed when an early diagenetic aerobic decay clock operating on buried organic material was stopped and sediment entered the sulphate reduction zone this probably occurred during progradation of grainstone shoal facies after which there was initial exposure to meteoric water microspar calcites then formed rapidly as a result of aragonite stabilization the precipitation of the main meteoric cements and aragonite bioclast dissolution post date this stabilisation event the brown mottle pseudobreccia fabrics are intimately associated with rhizocretions and calcrete which developed beneath palaeokarstic surfaces capping cycle top grainstones and post date all depositional fabrics although they may also follow primary depositional heterogeneities such as burrows they consist of coarse inclusion rich microspar calcites that are always very dull to non luminescent under cathodoluminescence sometimes with some thin bright zones these are interpreted as capillary rise and pedogenic calcrete precipitates the d 18 o values 5 to 8 pdb and the d 13 c values 2 to 3 pdb of the pseudobreccias are lower than the estimated d 18 o values 3 to 1 pdb and d 13 c values of 2 to 4 pdb of normal marine calcite precipitated from late dinantian sea water reflecting the influence of meteoric waters and the input of organic carbon

gas water rock interactions in frio formation following co2 injection implications for the storage of greenhouse gases in sedimentary basins to investigate the potential for the geologic storage of co2 in saline sedimentary aquifers 1600 t of co2 were injected at 1500 m depth into a 24 m thick sandstone section of the frio formation a regional brine and oil reservoir in the u s gulf coast fluid samples obtained from the injection and observation wells before co2 injection showed a na cacl type brine with 93 000 mg l total dissolved solids tds at near saturation with ch4 at reservoir conditions following co2 breakthrough samples showed sharp drops in ph 6 5 5 7 pronounced increases in alkalinity 100 3000 mg l as hco3 and fe 30 1100 mg l and significant shifts in the isotopic compositions of h2o dissolved inorganic carbon dic and ch4 geochemical modeling indicates that brine ph would have dropped lower but for the buffering by dissolution of carbonate and iron oxyhydroxides this rapid dissolution of carbonate and other minerals could ultimately create pathways in the rock seals or well cements for co2 and brine leakage dissolution of minerals especially iron oxyhydroxides could mobilize toxic trace metals and where residual oil or suitable organics are present the injected co2 could also mobilize toxic organic compounds environmental impacts could be major if large brine volumes with mobilized toxic metals and organics migrated into potable groundwater the d 18 o values for brine and co2 samples indicate that supercritical co2 comprises 50 of pore fluid volume 6 mo after the end of injection postinjection sampling coupled with geochemical modeling indicates that the brine gradually will return to its preinjection composition

fluid transport in geological reservoirs with background flow this paper presents fundamental analysis of the injection and release of fluid into porous media or geological reservoirs saturated by a different fluid undergoing a background flow and tests the predictions using analogue laboratory experiments the study reveals new results important for an understanding of the transport of hazardous contaminants through aquifers and the long term fate of carbon dioxide in geological sequestration using numerical and asymptotic analysis we describe a variety of flow regimes that arise and demonstrate an almost instantaneous control of injected fluid by the far field conditions in geological reservoirs for a continuous input the flow develops a horizontal interface between the injected and ambient fluids the background flow thereby effectively caps the height of the injected fluid into a shallower region of vertical confinement for a released parcel of fluid gravitational spreading is found to become negligible after a short time a dominant control of the interface by the background pressure gradient arises and stems from the different velocities at which it drives the injected and ambient fluids individually similarity solutions describing these dynamics show that the parcel approaches a slender triangular profile that grows horizontally as where is time a rate faster than relaxation under gravity shock layers develop at the front or back of the parcel depending on whether it is more or less viscous than the ambient fluid new analytical results describing the long term effects of residual trapping due to capillary retention are developed which yield explicit predictions for the time and length scales on which a parcel of becomes retained we end by applying our results to geological contexts concluding that even slight background motion can have considerable implications for long term transport through the subsurface

the noble gas geochemistry of natural co2 gas reservoirs from the colorado plateau and rocky mountain provinces usa identification of the source of co2 in natural reservoirs and development of physical models to account for the migration and interaction of this co2 with the groundwater is essential for developing a quantitative understanding of the long term storage potential of co2 in the subsurface we present the results of 57 noble gas determinations in co2 rich fields 82 from three natural reservoirs to the east of the colorado plateau uplift province usa bravo dome nm sheep mountain co and mccallum dome co and from two reservoirs from within the uplift area st john s dome az and mcelmo dome co we demonstrate that all fields have co2 3he ratios consistent with a dominantly magmatic source the most recent volcanics in the province date from 8 to 10 ka and are associated with the bravo dome field the oldest magmatic activity dates from 42 to 70 ma and is associated with the mcelmo dome field located in the tectonically stable centre of the colorado plateau co2 can be stored within the subsurface on a millennia timescale the manner and extent of contact of the co2 phase with the groundwater system is a critical parameter in using these systems as natural analogues for geological storage of anthropogenic co2 we show that coherent fractionation of groundwater 20ne 36ar with crustal radiogenic noble gases 4he 21ne 40ar is explained by a two stage re dissolution model stage 1 magmatic co2 injection into the groundwater system strips dissolved air derived noble gases asw and accumulated crustal radiogenic noble gas by co2 water phase partitioning the co2 containing the groundwater stripped gases provides the first reservoir fluid charge subsequent charges of co2 provide no more asw or crustal noble gases and serve only to dilute the original asw and crustal noble gas rich co2 reservoir scale preservation of concentration gradients in asw derived noble gases thus provide co2 filling direction this is seen in the bravo dome and st john s dome fields stage 2 the noble gases re dissolve into any available gas stripped groundwater this is modeled as a rayleigh distillation process and enables us to quantify for each sample 1 the volume of groundwater originally stripped on reservoir filling and 2 the volume of groundwater involved in subsequent interaction the original water volume that is gas stripped varies from as low as 0 0005 cm3 groundwater cm3 gas stp in one bravo dome sample to 2 56 cm3 groundwater cm3 gas stp in a st john s dome sample subsequent gas groundwater equilibration varies within all fields each showing a similar range from zero to 100 cm3 water cm3 gas at reservoir pressure and temperature