geological controls on gas saturation in the yanchuannan coalbed methane field southeastern ordos basin china abstract the paper investigates lateral variation of gas saturation and its geological controls of the no 2 coal seam in the permian shanxi formation based on coal samples from 21 exploration wells in the yanchuannan coalbed methane cbm field southeastern ordos basin china the data set reveals that gas saturation of the no 2 coal seam shows a high lateral variation from 32 44 to 89 69 and that there is an overall trend in gas saturation with depth overprinted by four other factors 1 influence of faults 2 secondary biogenic gas generation 3 local top seal conditions and 4 variations in coal properties the reverse faults f1 and f2 are effective barriers horizontally for the hydraulic communication and divide our study area into relatively closed and recharge zones gas saturation of the relatively closed zone is generally higher than that in the recharge zone in the recharge gas saturation increases generally along the flow pathway of groundwater with the exceptions of the areas affected by secondary biogenic gas the stable isotope compositions suggest that late stage biogenic methane was generated via co 2 reduction associated with meteoric water recharge which significantly contributes to higher gas saturation than expected meanwhile the biogenic gas generation was is restricted to shallow depths the apparent variations in local areas of lithology of the overlying stratum next to the coal seam suggest that the top seal conditions are deteriorative which can cause gas dissipation and reduce gas saturation remarkably integrally for the variations in coal properties gas saturation has a general positive relationship with coalification and weak negative correlations with moisture content and ash yield but the relationships lacks statistical significances gas saturation is not associated with coal maceral in the study area

simulation of oil expulsion by 1 d and 2 d basin modelling saturation threshold and relative permeabilities of source rocks abstract in order for oil to accumulate in economic quantities it first has to be generated and expelled from source rocks in sufficient quantities in spite of long term efforts the mechanism of oil expulsion from the source rocks is not completely understood for modelling expulsion the adoption of pressure driven multiple phase fluid flow governed by darcy s law is widely accepted however relative permeabilities for fine grained source rocks which is an essential parameter for this model are very difficult to specify the conventional reservoir rock curve is obliged to be used for the modelling simplification of the relative permeability model is generally used for one dimensional basin modelling the 1 d model also requires substantial optimization of the key parameter saturation threshold laboratory measurement of relative permeabilities for fine grained rocks is very difficult therefore we carefully interpreted the analysed laboratory data on various sandstones to establish a relationship between relative permeability curves and pore geometry parameters with a view to extrapolate the relationships developed in sandstones to fine grained source rocks it was found that the relative permeability curves or irreducible water saturation are controlled by two factors grain size and clay content since the total surface area of the pore system becomes greater as the grain size decreases we consider surface water adsorbed on the grain surface as a part of irreducible water since clay contains micro porosity which cannot be displaced by oil due to high capillary pressure we regard this to also play a role prediction of relative permeability curves for fine grained rocks by both processes results in the curve with high irreducible water saturation the new relative permeability curves were tested by both one dimensional and two dimensional basin modelling test results indicate that the new curves can reproduce expulsion efficiency locations of accumulations and leaking through cap rock which is consistent with actual observations

mesozoic and cenozoic stratigraphy eastern scotian shelf new interpretations a 320 km long deep seismic reflection line across the central part of the scotian basin provides new insight on the deep stratigraphy of the basin confirms the existence of up to 18 km of mesozoic and cenozoic sedimentary strata and provides additional control on the relationship of proximal and distal stratigraphic units within the basin detailed biostratigraphic and seismic correlation from three outer shelf upper slope wells yields important new information on late cretaceous and tertiary unconformities and identifies a number of third order sequence boundaries the dating of prominent unconformities and correlation with the oceanic seismic markers ac and au results in the reidentification of these horizons in the sohm abyssal plain

geology and diagenetic history of overpressured sandstone reservoirs venture gas field offshore nova scotia canada deep exploratory wells in the scotian basin offshore nova scotia canada have encountered overpressured formations with pressures 1 9 x the normal hydrostatic gradient the overpressures occur over an area of approximately 10 000 sq km in the venture field the abnormal pressures are confined below a depth of 4500 m and are associated with upper jurassic lower cretaceous gas and condensate bearing sandstone reservoirs the overpressures occur within normally compacted shales containing numerous overpressured sandstone reservoir beds the development of overpressures seals and secondary reservoirs are all diagenetically driven the volumetric increase achieved by kerogen to petroleum gas conversion and hydrocarbon expulsion are believed to be the main driving forces or venture overpressures three secondary porosity depth levels which top at 2500 m 65 degrees c 3700 m 95 degrees c and 4600 m 130 degrees c correlate with major steps in the organic matter maturation in the basin secondary porosity is initially achieved by alumosilicate dissolution with ferroan sparry calcite cement dissolution dominating below 4000 m porosity enhancement and preservation is not the result of a single diagenetic event but instead the result of a series of diagenetic events that overlapped in time formation of dynamic diagenetic barriers within the zone of peak gas generation helps retard the diffusive migration of hydrocarbons and other fluids expelled during shale diagenesis resulting in pressure build up the preservation of up to 32 percent porosity under 500 1000 atm of pressure could not be achieved without simultaneous pressuring of developing voids thus the expansion of secondary reservoirs in the overpressured zone also had to be late 50 ma or younger and about synchronous with pressurizing of the reservoirs significant for hydrocarbon exploration is that venture type diagenetic overpressures are not associated with undercompacted sediments and hence they cannot be predicted from compaction trends during drilling petrographic diagenetic and lithofacies studies can be instrumental in predicting potential areas of deep subsurface secondary reservoirs development

simultaneous inversion of prestack seismic data for rock properties using simulated annealing a new prestack inversion algorithm has been developed to simultaneously estimate acoustic and shear impedances from p wave reflection seismic data the algorithm uses a global optimization procedure in the form of simulated annealing the goal of optimization is to find a global minimum of the objective function which includes the misfit between synthetic and observed prestack seismic data during the iterative inversion process the acoustic and shear impedance models are randomly perturbed and the synthetic seismic data are calculated and compared with the observed seismic data to increase stability constraints have been built into the inversion algorithm using the low frequency impedance and background vs vp models the inversion method has been successfully applied to synthetic and field data examples to produce acoustic and shear impedances comparable to log data of similar bandwidth the estimated acoustic and shear impedances can be combined to derive other elastic parameters which may be use

oil source rock correlation in the north viking graben abstract based on evaluation of geochemical properties of nearly 100 oils from the north viking graben we have identified three hydrocarbon populations one population is derived from marine shales of the draupne fm one from marine shales of the heather fm that are rich in land plant detritus and one from a mixture of the two while the draupne fm is the dominant source rock the heather fm lower callovian is an important source on the northern horda platform furthermore on the northern horda platform heather fm shales play a major role in controlling migration oil from the draupne fm migrates only into upper jurassic reservoirs as no large titled fault blocks permit juxtaposition of draupne fm and brent gp sands and the heather fm shales are thick and extensive oil from the heather fm migrates into both upper and middle jurassic reservoirs variations in properties of the hydrocarbons in the north viking graben petroleum system are due to a complex interplay of organic source maturity migration pressure distribution and biodegradation integration of geochemical and geological data on a regional scale has enhanced our ability to evaluate prospects in this important hydrocarbon system

preliminary hydrogeological assessment of late cretaceous tertiary ardley coals in part of the alberta basin alberta canada abstract a preliminary hydrogeological evaluation was undertaken on the gas potential of shallow coals in the pembina warburg exploration area in the alberta basin alberta canada regional data for the late cretaceous tertiary ardley coal zone acz were compiled and supplemented with site specific data collected from a key test well drilled as part of a regional exploration program limited regional pressure data suggest hydraulic communication between the uppermost ardley with the overlying paskapoo formation a comparison between hydraulic head and topography suggests that flow at least in part of the ardley paskapoo is gravity driven however a decoupling of the hydraulic regime appears evident from pressure test data in beds stratigraphically below the uppermost scollard ardley and above the base of scollard at least in the eastern part of the study area where the test well was drilled the decoupling is evident in regional pressure data but the precise stratigraphic position may vary regionally formation waters typically are na hco 3 type with salinities as tds of less than 2000 mg l anomalously high bicarbonate dissolved inorganic carbon or dic concentrations exceeding 1500 mg l with 13 c dic 22 50 and dissolved methane identified in formation waters collected directly from ardley coal in test well 103 point to the presence of secondary biogenic gas the 13 c isotopes for dic coupled with 18 o and 2 h isotopes for associated groundwater and regional hydraulic data suggest that the uppermost ardley coal zone in the eastern part of the study area is part of a regional topographically driven dynamic flow system in which methanogenic processes are modifying groundwater chemistry and gas charging parts of the area whether or not biogenic gas charging in the ardley is pervasive is uncertain the relatively small coal data set requires that further exploration in the study area should consider the presence of microbial gas and the potential for hydrogeological controls on its distribution however further detailed testing will be necessary to develop a consistent and useful database for exploration and development

layer thickness determination and stratigraphic interpretation using spectral inversion theory and application spectralinversionisaseismicmethodthatusesaprioriinformation and spectral decomposition to improve images of thinlayerswhosethicknessesarebelowthetuningthickness we formulate a method to invert frequency spectra for layer thickness and apply it to synthetic and real data using complexspectralanalysis absolutelayerthicknessessignificantly below the seismic tuning thickness can be determined robustly in this manner without amplitude calibration we extend our method to encompass a generalized reflectivity series represented by a summation of impulse pairs application of our spectral inversion to seismic data sets from the gulfofmexicoresultsinreliablewelltiestoseismicdata accurate prediction of layer thickness to less than half the tuning thickness and improved imaging of subtle stratigraphic features comparisons between well ties for spectrally inverted data and ties for conventional seismic data illustrate the superior resolution of the former several stratigraphic examples illustrate the various destructive effects of the wavelet including creating illusory geologic information suchasfalsestratigraphictruncationsthatarerelatedtolateralchangesinrockproperties andmaskinggeologicinformation suchasupdiplimitsofthinlayers weconcludethatdata that are inverted spectrally on a trace by trace basis show greater bedding continuity than do the original seismic data suggestingthatwaveletside lobeinterferenceproducesfalse beddingdiscontinuities

constraints on the development of secondary porosity in the subsurface re evaluation of processes abstract secondary porosity formed by the dissolution of grains and cements is of great economic importance in both clastic and carbonate reservoirs a review of the commonly proposed mechanisms for the production of secondary porosity indicates that meteoric water penetration and mixing corrosion may be able to create significant volumes of secondary porosity in the shallow subsurface leaching by acidic fluids generated by solution of co 2 or monocarboxylic acids both derived during the thermal maturation of organic matter or by acidic fluids created by clay mineral reactions in shales are the most commonly proposed mechanisms by which secondary porosity is said to form in the deep subsurface however these mechanisms suffer from great difficulties in producing significant volumes of secondary porosity once mass balance and fluid migration problems are taken into account it is highly likely that the correct mechanism for the creation of economically significant amounts of secondary porosity in the deep subsurface is yet to be found

hierarchies of overpressure retardation of organic matter maturation case studies from petroleum basins in china the effects of overpressure on different aspects of organic matter maturation have been examined in three basins using multiple parameters overpressure in the yinggehai basin has retarded kerogen maturation hydrocarbon generation and thermal cracking of long chain normal hydrocarbons as well as evolution of isoprenoid hydrocarbons overpressure in the dongpu depression bohai bay basin seems to have retarded the maturation of bulk kerogens reflected by rock eval tmax the thermal cracking of long chain normal hydrocarbons and the evolution of isoprenoid hydrocarbons but seems to have had no detectable effects on vitrinite reflectance overpressure in the qiongdongnan basin has had no detectable retardation effects on all aspects of organic matter maturation the observed phenomena suggest differential retardation of organic matter maturation that is different organic matter maturation reactions and different maturity parameters have been retarded by overpressure to different degrees in the same overpressured system and four hierarchies of overpressure retardation hierarchy i retardation is defined as the state where the overpressure has retarded all aspects of organic matter maturation and is proposed to have been caused by the combination of high formation pressure and excess pressure a high water organic matter ratio and strong retention of reaction products in an early developed strong persistent overpressured system hierarchy ii retardation refers to the case in which all aspects of organic matter maturation except the maturation of hydrogen poor kerogens especially vitrinite have been retarded and seem to have been caused by early developed moderate to strong persistent overpressure hierarchy iii retardation refers to the situation where overpressure has retarded only the thermal cracking of liquid hydrocarbons but has had no detectable effect on the maturation of both hydrogen rich and hydrogen poor kerogens and is proposed to have been caused by moderate to strong persistent overpressures that began to develop around peak oil generation hierarchy iv retardation is defined as the situation where overpressure has had no detectable retardation on all aspects of organic matter maturation and may have been caused by cases wherein the overpressure developed too late the intensity of the overpressure was too low the duration of the overpressure was too short or overpressured fluids had been expelled frequently resulting from both the diversities of organic matter maturation reactions and the complexities of overpressure generation and development the hierarchies of overpressure retardation in different basins may be quite different and it is necessary to use multiple parameters to understand the evolution of the petroleum systems in overpressured basins

multiple origins of methane in the earth methane occurrences in the earth s crust are predominantly of biogenic origin i e their ultimate source is biologically formed organic matter methane can also form through inorganic reactions and is consequently termed abiogenic biogenic methanes can either form through bacterial or thermogenic processes bacterial processes follow a co2 reduction and or fermentation pathway the fermentation processes are quantitatively more important in recent fresh sediments and swamps methane formed by co2 reduction however is most common in older sediments and commercial gasfields temperature organic substrate and age may be the major factors controlling the relative importance of the two pathways stable isotope concentrations in thermogenic methanes seem to be controlled by the extent of conversion of organic matter the timing of gas expulsion and trapping the different character of methane in individual sedimentary basins may be a result of the geologic history geothermal methanes are most likely derived from pyrolysis of organic matter abiogenic methane occurs in hydrothermal vents and ophiolite complexes inorganic reactions either surficial or deep seated are the likely source of such methanes a uniform mantle origin of methane is not supported by the observed isotope variations in naturally occurring methanes

an alternative early opening scenario for the central atlantic ocean abstract the opening of the central atlantic ocean basin that separated north america from northwest africa is well documented and assumed to have started during the late jurassic however the early evolution and the initial breakup history of pangaea are still debated most of the existing models are based on one or multiple ridge jumps at the middle jurassic leaving the oldest crust on the american side between the east coast magnetic anomaly ecma and the blake spur magnetic anomaly bsma according to these hypotheses the bsma represents the limit of the initial basin and the footprint subsequent to the ridge jump consequently the evolution of the northwest african margin is widely different from the northeast american margin however this setting is in contradiction with the existing observations in this paper we propose an alternative scenario for the continental breakup and the mesozoic spreading history of the central atlantic ocean the new model is based on an analysis of geophysical data including new seismic lines an interpretation of the newly compiled magnetic data and satellite derived gravimetry and recently published results which demonstrate that the opening of the central atlantic ocean started already during the late sinemurian 190 ma based on a new identification of the african conjugate to the ecma and on the extent of salt provinces off morocco and nova scotia the identification of an african conjugate magnetic anomaly to bsma the african blake spur magnetic anomaly absma together with the significant change in basement topography are in good agreement with that initial reconstruction the early opening history for the central atlantic ocean is described in four distinct phases during the first 20 myr after the initial breakup 190 170 ma from late sinemurian to early bajocian oceanic accretion was extremely slow 0 8 cm y at the time of blake spur 170 ma early bajocian a drastic change occurred both in the relative plate motion direction from nnw sse to nw se and in the spreading rate an increase to 1 7 cm y after a small increase between chron m25 154 ma kimmeridgian and chron m22 150 ma tithonian the spreading rate slowed down to about 1 3 cm y and remained fairly constant until chron m0 125 ma barremian aptian boundary in addition kinematic reconstructions illustrate a significant spreading asymmetry during the early history of the central atlantic ocean the accretion rates were higher on the american side and led to the formation of more oceanic crust on this plate we infer that this asymmetry could be related to the fact that the thermal anomaly responsible for the significant magmatism of the central atlantic magmatic province camp was preferentially located below the african plate

basin modelling a new comprehensive method in petroleum geology abstract integrated basin modelling is used to understand and reconstruct the geological and thermal evolution of a sedimentary basin to quantify this information and hence to derive among other results the history of generation migration and accumulation of petroleum this new methodical approach is rapidly gaining world wide recognition throughout the geo scientific community because it offers two distinct advantages when addressing geological problems firstly it enables a logical quantitative treatment of very complex interrelated geological processes and secondly it permits these processes to be examined on the basis of an absolute geological time scale the prerequisites for basin modelling are 1 the translation of logical and consistent geological concepts into numerical form and then into suitable computer programs and 2 the application of these programs using high speed computers with large memories using such an approach is an important part of modern petroleum exploration because 1 1 provision is made for total coverage of the basin by allowing extrapolation to non drilled often deeper parts of the basin accessible only by using seismic data 2 2 a three dimensional view of the basin can be constructed and followed through geological time 3 3 an internal logic is utilised which is consistent with physical and chemical laws 4 4 geological geophysical and geochemical data are integrated and modelled iteratively and therefore control each other this paper shows the rapidly widening scope of basin modelling and points out some of the present day limitations and necessary research directions for the future it describes concepts for modelling of tectonic processes and for simulating expulsion of liquid petroleum

experimental simulation of primary migration abstract around fifty expulsion experiments were performed in a high pressure vessel specifically designed for the study of primary migration on natural source rock cores the results indicate that temperature pressure source rock richness and source rock architecture are the main parameters controlling the amount of petroleum expelled during the simulation in particular the control exerted by the pressure gradient and the source rock bulk permeability is well demonstrated another relevant result is the observation of the need for the production of a sufficient quantity of petroleum in the source rock before expulsion can start all these results have been compared with migration phenomena observed in the natural system and we discuss herein the similarities observed

organic matter maturation under the influence of a deep intrusive heat source a natural experiment for quantitation of hydrocarbon generation and expulsion from a petroleum source rock toarcian shale northern germany four shallow boreholes were drilled in the hils syncline northern germany in order to determine quantitatively the amount of hydrocarbons generated and expelled during maturation of a typical kerogen type ii bearing source rock the holes penetrated the carbonceous lias shales posidonia shale lower toarcian and part of the adjacent dogger and lias mudstones the maturity of the organic matter in the cores recovered ranges from immature 0 48 r0 to overmature 1 45 r0 due to location of the hils syncline in the vicinity of the vlotho massif which is deep seated intrusive body facies variations of the lias within the short geographical distances in the study area are negligible organic matter mass balance calculations were based on detailed organic geochemical analyses of residual material in the lias shales kerogen bitumen etc and on the evidence of a uniform initial composition of these sediments in the study area dead carbon determinations supported this latter criterion but were not used as a parameter in the calculations about 50 of the initial kerogen was transformed into oil gas and inorganic compounds during the vitrinite reflectance increase from 0 48 to 0 88 ro and only marginally more during the maturity increase from 0 88 to 1 45 ro only a small portion of the generated material remained in the source rock even at a relatively early stage of generation 0 68 ro expulsion efficiency of oil plus gas reached a value of 86 at the end of the main generation stage 0 88 ro

quantitative modeling of organic matter connectivity in source rocks using fractal geostatistical analysis abstract numerical modeling of hydrocarbon expulsion requires accurate assessment of the 3 d geometry and connectivity of hydrocarbon pathways such as pore systems organic matter networks fractures and stylolites in source rocks in this paper a method combining scanning electron microscopy sem digital image analysis fractal geostatistical analysis and a renormalization algorithm is developed to determine the 3 d geometrical properties defined as organic matter connectivity factor ocf of these pathways in source rocks the method is applied to derive fractal geostatistical models and the ocfs of organic matter networks omn of fourteen source rocks with wide variation in total organic carbon toc content and texture the results show that the connectivity of the omns is affected by both the amount and distribution of organic matter in source rocks uniformly distributed omns lose 3 d connectivity when the toc content is below approx 6 whereas omns with preferred orientation may maintain lateral connectivity at 4 and perhaps lower toc content thus in shales such as the bakken in which vertical omn connectivity may not exist the fracture systems may provide not only primary migration pathway but hydrocarbon reservoirs that are recharged through the laterally connected omns the method developed in this paper can be used to quantify geological models and average geological properties over a wide range of scales and thereby provide reliable geometric parameters for basin modeling using a numerical technique

geochemical effects of primary migration of petroleum in kimmeridge source rocks from brae field area north sea i gross composition of c15 soluble organic matter and molecular composition of c15 saturated hydrocarbons in mature oil prone source rocks of the upper jurassic kimmeridge clay in the brae field area north sea the yield and the gross composition of solvent extractabte pctroleum likc compounds as well as the molecular proportions of c15 saturated hydrocarbons are primarily controlled by the effects of hydrocarbon expulsion during primary migration within sample series extending from shale centres towards adjacent reservoir sandstones regular and pronounced yield decreases have been documented as well as marked compositional changes for c15 soluble organic matter and its compound class fractions saturated hydrocarbons aromatic hydrocarbons nso compounds and for normal and isoprenoid alkanes relative migration losses indicating relative expulsion efficiencies were determined for the c15 alkanes throughout the outer five meters of thick shales relative expulsion efficiencies increase regularly and drastically for all compounds considered e g exceeding 90 for the c15 n alkanes in the outermost shale samples samples from thin 5 50 cm shale layers interbedded between reservoir sandstones reveal similarly high expulsion efficiencies for n alkanes the degree of depletion remains uniform throughout the molecular range c15 to c30i e no compositional fractionation occurred during primary migration which is interpreted to reflect expulsion of oil as a single phase fluid a simple conceptual model is proposed to explain the mechanisms and relationships of generation and expulsion processes in time and space a geochemically relevant conclusion is that the c15 soluble organic matter recovered by solvent extraction of mature source rocks represents only a portion of the total non volatile petroleum like material originally generated

plate tectonic reconstructions and paleogeographic maps of the central and north atlantic oceans1 2 1this article is one of a series of papers published in this cjes special issue on the theme of mesozoic cenozoic geology of the scotian basin 2earth sciences sector contribution 20120172 we have established a new plate kinematic model of the central and north atlantic oceans between north america africa meseta iberia flemish cap and galicia bank from late triassic to late cretaceous to better understand the nature and timing of rifting of nova scotia and morocco conjugate continental margins since late triassic the maps of salt distributions at the sinemurian pliensbachian limit 190 ma after salt deposition and in middle bajocian 170 ma show that an area of the nova scotia margin is devoid of allochthonous salt and that an area of similar size located oceanward of the west african coast magnetic anomaly shows salt deposits suggesting that a portion of the nova scotia margin with its overlying salt deposits could have been transferred onto the moroccan side right after the formation of the conjugate east coast magnetic anomaly and west african coast magnetic anomaly seven paleogeographic maps from late triassic to late cretaceous are presented with structural elements and ma

understanding of hc migration in sedimentary basins present state of knowledge abstract great advances were made during the decade of 1975 1985 in the understanding of hc migration by establishing that the main mechanisms were polyphasic flow whatever its stage and that migration in solution in water and or by molecular diffusion was insignificant except for methane ethane and benzene research is now focusing on points which still have not been satisfactorily answered such as 1 1 behaviour of source rocks during hc expulsion the main driving force for fluid expulsion has been recognized to be excess fluid pressure within the source rock with respect to its surrounding environment of permeable beds the pressure build up is due mainly to accommodation by the fluids of part of the mechanical stresses which are exerted on the rock mainly sedimentary load but also stresses developed by tectonics to thermal expansion of water and to hc formation from kerogen in response to increasing temperatures however it is not clear whether or not petroleum expulsion proceeds preferentially along oil wet networks kerogen or oil wet mineral surfaces nor it is not clear whether or not microfracturing provoked by an internal fluid pressure higher than allowed by the mechanical strength of source rocks is necessary for hc expulsion 2 2 modifications of the compositions of hc phases e g oil versus gas or the distribution of biomarkers during migration and losses of hydrocarbons along the migration pathways although careful observations of natural sample series and also experimentation have been done no clear picture has emerged for the moment however compositional effects seem to be due more to changes in thermodynamic conditions along migration pathways which provoke inter alia hydrocarbon phase separation retrograde condensation than to differential retention by mineral surfaces it is likely that while careful observations on a large variety of source rock samples are needed to more clearly understand source rocks functioning progress in understanding hc migration will come increasingly from modeling and experimentation models which mathematically simulate natural processes are necessary to help the human mind in handling such complex phenomena and therefore are of precious help to focus research on the most important points they replace migration in its geological and historical dimensions with numerical or conceptual tools while observation only describes the present state models are also the tools by which knowledge on migration can be applied to exploration problems however quantitative applications of models require numerous and precise geological data that are difficult to gather at early stages of exploration in the field of experimentation it will be easier to design crucial experiments now that hc migration mechanisms are better understood thanks to experimentation it should also be possible to validate models however so as to reach such aims substantial progress has to be made in the technology of experimental systems

authigenesis of titania minerals and the mobility of ti new evidence from pro deltaic sandstones cretaceous scotian basin canada titania tio2 minerals are minor diagenetic phases in many sandstone shale successions but the conditions of ti mobility and titania authigenesis are poorly known here new modes of occurrence of authigenic titania minerals filling pores and replacing phytodetritus are documented from lower cretaceous prodeltaic sandstones in the offshore scotian basin of southeastern canada textural relationships suggest that titania minerals formed at the same time as kaolinite most commonly below lowstand sequence boundaries related to flux of meteoric water such authigenic titania is most abundant in thin bedded sandstones interbedded with thick prodeltaic mudstone in which fine grained detrital ti rich minerals are concentrated textures in titania that has replaced uncompacted phytodetritus suggest microbial mediation in near surface eodiagenesis eogenetic titania is enriched in phosphorus 0 6 p2o5 and may co occur with authigenic fluorapatite and crandallite crystals such titania resulted from prolonged flow of meteoric water that dissolved p from apatite and feldspar thereby enhancing ti solubility in basins with supply of immature sediment from granitic terranes and important diagenesis by meteoric water the assumption that ti is an immobile element and that rutile or other titania minerals are exclusively of detrital origin may not be valid probable mesodiagenetic titania has low p content suggesting that ti mobility under these conditions was facilitated by some other agent probably organic acids the results of this study illustrate that even in cases where titania minerals are volumetrically insignificant their microtextures and trace element chemistry can provide important insights into the character of fluid flow during diagenesis of siliciclastic rocks

enhanced structural interpretation from 3d seismic data using hybrid attributes new insights into fault visualization and displacement in cretaceous formations of the scotian basin offshore nova scotia abstract meta or hybrid attributes extracted from seismic data through artificial neural networks enhance structural details in subsurface strata cretaceous units within the penobscot prospect are devoid of such interpretations although these units are plausible zones for hydrocarbon generation and accumulation the role of geologic structures responsible for such entrapment is poorly understood the present study enlightens these latter geological structures through a case study that uses 3d time migrated seismic data from the penobscot prospect in the scotian basin a modern data conditioning approach used together with multi attribute analyses and a supervised neural network have brought out the minute details of subsurface geologic structures through a hybrid attribute called the fault probability cube fpc the present fault displacement analysis reveals that the major fault of the prospect has evolved by the coalescence of different fault segments grown over space and time and has demonstrated a predominant nnw sse structural trend such a geometry has structurally modulated the cretaceous units the logan canyon the dawson canyon and the wyandot formations giving rise to different horst graben structures and fracture networks as shown by the fpc attribute this generates complicated structural deformation and accommodation space thereby enabling different trapping zones for hydrocarbon accumulation within these units such a detailed structural understanding will provide significant input to the future exploration and development programme in the penobscot prospect

coalbed methane potential of the paleocene fort union coals in south central wyoming usa abstract the paleocene age wasatch and fort union formations in the study area contain numerous coal seams that are interbedded with shale sandstone and conglomerates individual seam thickness ranges from 2 to 15 ft 0 6 4 6 m and aggregate thickness ranges from 64 to 163 ft 19 5 49 7 m the coals are of sub bituminous to high volatile c b bituminous rank ro ran 0 47 to 0 59 and are found at depths from 3 g arb which is higher than expected for the coal rank the coals appear to be close to full saturation based on a comparison of adsorption isotherms with desorption data the coals pinch out updip in the subsurface forming a stratigraphic trap which may explain the high gas content and the mixed biogenic thermogenic origin supported by isotopic analysis gas composition is 97 methane with 3 inerts and the gas has a heating value of 990 btu ft 3 the coals are relatively clean average ash content is 15 wt and have fast diffusivity 63 of total gas desorbs in 3 with wells drilled on 80 acre spacing which translates to a gross eur of 1 4 bcf per well for the 500 well locations identified in the developable acreage based on numerical simulation using the simplistic fekete tm model peak coal gas rate is expected to be 320 mcf d 9014 m 3 per day after 36 months and production should remain at that level for 12 months before declining at a rate of 10 per year conventional gas potential also exists in numerous intervals within the upper fort union and wasatch formations sandstones at depths shallower than 2000 ft 526 m few wells drilled decades ago having perforations in these sands showed ip initial production rates ranging from 500 mcf d 14 084 m 3 per day to 12 mmcf d 338 028 m 3 per day a 24 inch 60 cm diameter pipeline that has an operating pressure of 800 1000 psig a tap and a capacity of 275 mmcf d 7 74 million m 3 per day crosses the study area the pipeline is connected to the colorado interstate gas mainline which provides gas to markets in the eastern united states

coupling accumulation model with gas bearing features to evaluate low rank coalbed methane resource potential in the southern junggar basin china to reveal the resource potential of low rank coalbed methane lrcbm and determine the favorable area from six different zones in the southern junggar basin in china a new lrcbm assessment methodology was developed this methodology comprises three steps developing the regional accumulation model determining the zone with gas bearing features and exploring for the favorable targets first the coalbed methane cbm generating characteristics the sealing properties of the over and underlying rocks in the cbm reservoir and the hydrogeological characteristics preserved in the reservoir were analyzed by combining these characteristics with the tectonic burial history geothermal history hydrocarbon evolution and cbm gas saturation changes the lrcbm regional accumulation model that illustrates the lrcbm enrichment characteristics of the middle lower jurassic series in the southern junggar basin was derived second the lrcbm gas bearing features of each zone were comprehensively analyzed using a large data set acquired from a coalfield the lrcbm bearing features and the variation of their related parameters in six different zones of the southern junggar basin were presented these parameters relate to features such as coal seam distribution petrology reservoir property coal thermal evolution and coal and gas content characteristics third the favorable targets are explored by calculating the lrcbm resources and intensively evaluating each zone the favorable targets are candidates for further exploration in the southern junggar basin at depths shallower than 2000 m 7000 ft the lrcbm resources in the southern junggar basin are 3054 7 108 m3 10 8 1012 ft3 with resource abundance of 0 42 11 65 108 m3 km2 0 06 1 66 108 ft3 ac thereby demonstrating a significantly good exploitation prospect moreover the favorable and relatively favorable areas for further exploration in the southern junggar basin are zone 4 and 5 respectively

widespread erosion and mass failure from the 51 ma montagnais marine bolide impact off southwestern nova scotia canada12 1this article is one of a series of papers published in this cjes special issue on the theme of mesozoic cenozoic geology of the scotian basin 2earth sciences sector contribution 20120241 the 51 ma montagnais impact crater on the outer scotian shelf is well known but the potential effects from the impact event on the slope and rise seaward of the crater have until now remained poorly understood through detailed seismic stratigraphic correlation and ties to available wells we define a threefold seismic stratigraphic subdivision for upper cretaceous to eocene strata on the shelf and slope calibrated to the most recent biostratigraphic results using this framework we identify a number of depositional and erosional products that are temporally consistent with a late ypresian impact event within the limits of seismic and biostratigraphic resolution we link a series of prominent failure scarps on the outer shelf and upper slope to a single widespread mass transport deposit mtd on the lower continental slope rise and abyssal plain failed material amassed in a large debris field referred to here as the montagnais mtd it covers an area of 93 000 km2 and travelled up to 580 km f

pore pressure variation within the tuscaloosa trend morganza and moore sams fields louisiana gulf coast repeat formation tester rft mark of schlumberger pore pressure data from the lower tuscaloosa formation upper cretaceous are analyzed across a pressure transition zone in two gas fields of the tuscaloosa trend louisiana documenting a pressure seal within a sandstone dominated interval that maintains a pressure anomaly of 20 mpa pressure measurements within the formation span a depth interval of 5500 6060 m and are limited to the crests of rollover structures associated with growth faults the onset of overpressure occurs at a depth between 5620 m and 5690 m in two fault blocks while the strata are displaced across the fault by 100 to 120 m suggesting that the pressure seal may be near horizontal crosscutting stratigraphic boundaries the pressure seal zone is as thin as 38 m and occurs within interbedded sandstones and shales overpressures in all wells of the moore sams field and in two wells of the morganza field follow local hydrostatic gradients indicating that portions of the overpressured zone are in fluid communication though isolated from shallower normally pressured fluids in other wells overpressures increase with depth in a stair step manner to 117 mpa at depths of 5 9 km the occurrence of the pressure seal in interbedded sandstones and shales where higher permeability is expected suggests that the sandstones of the seal zone are unusually tight a companion petrographic study of sandstones near the pressure seal suggests that extreme compaction of the sandstones after dissolution of grain supporting cements may have contributed to the low permeability of the seal zone

abnormal formation pressure and shale porosity an abnormally pressured jurassic shale from the north sea viking graben has a fluid pressure over burden ration close to 0 8 at a depth of more than 4 000 m and is associated with low porosity and high density the porosity and fluid pressure history of the shale involves three successive stages 1 normal gravitational compaction prevailed during the first few hundred meters of burial 2 the shale porosity was severely reduced by carbonate cementation in conjunction with a major break in sedimentation but the fluid pressure remained hydrostatic and finally 3 the porosity was further reduced by compaction and possibly cementation and the fluid pressure increased to above hydrostatic in response to renewed subsidence and the cretaceous and tertiary overburden lo d

diagenetic sequences of continuously deposited tight sandstones in various environments a case study from upper paleozoic sandstones in the linxing area eastern ordos basin china an integrated analysis of the petrographic characteristics and types and distribution of diagenetic alteration in the upper paleozoic benxi taiyuan shanxi and xiashihezi formations provides insights into the controlling factors on variations in porosity and permeability in tight sandstones 85 of the sandstone samples display porosity values 10 and 90 of the samples exhibit permeability 1 md diagenetic alteration includes mesogenetic compaction cementation by dolomite ankerite and quartz dissolution of feldspar and illitization of smectite eodiagenesis includes compaction development of smectite cementation by pore filling quartz and disordered kaolinite and precipitation of calcite and fe calcite chlorite and quartz preserve primary pores against damage whereas kaolinite illite smectite i s mixed layer and illite significantly diminish reservoir quality via permeability reduction chlorite and i s content decrease abruptly as depth increases whereas the kaolinite content remains elevated at depth because of the complete destruction of k feldspar the transformation from smectite to illite provides silica ions for the widely distributed quartz overgrowths as the depositional environment transformed from fluvial xiashihezi to deltaic shanxi and to epicontinental taiyuan and benxi the dissolution effect increased monotonically feldspar dissolution is dominant in the shanxi formation whereas the benxi and taiyuan formations commonly contain quartz dissolution pores the taiyuan formation has markedly higher porosities than in the overlying and underlying formations caused by strong dissolution and high silica content the decrease in porosity in the benxi formation results from the extensive formation of clay minerals caused by high frequency transgressions in a transitional environment

interpretation of the penobscot 3d seismic volume using constrained sparse spike inversion sable sub basin offshore nova scotia abstract in this study we investigate the utility of constrained sparse spike inversion cssi applied to a small 87 km 2 public domain 3d seismic volume at penobscot in the sable sub basin offshore nova scotia the objective is to establish the effectiveness of this technique at sable in mapping absolute acoustic impedance that can be directly and quantitatively related to lithology the intent is to subsequently apply the workflow to large 3d seismic volumes sub regionally with the goal of identifying and mapping source seal reservoir and overpressured intervals either not known or partially known from well control absolute acoustic impedance is an incremental parameter that is not explicit in the reflection amplitude cube and is the primary driver for undertaking this inversion it has quantitative advantages over relative acoustic impedance that can be derived using fast track inversion techniques cssi at penobscot has been completed and provides a valid geological model in the upper jurassic to lower cenozoic interval that is constrained by wireline logs at two well penetrations one of which has 12 m net pay this acoustic impedance volume facilitates interpretation of 1 low impedance cretaceous reservoir sandstones in both complex confined channel systems and extensive unconstrained marginal marine systems 2 polygonal fault systems pfs in a high impedance late cretaceous chalk 3 interfingering of low impedance shales and high impedance carbonates at the margin of the jurassic abenaki carbonate bank three other inversion studies have been reported at penobscot ahmad 2013 sayers 2013 qayyum et al 2014 relative acoustic impedance is derived in each study using recursive spectral and coloured inversion algorithms respectively none of these studies documented the pfs and cretaceous channel systems described here and there are significant interpretation differences in the jurassic section which is only partly penetrated by wells at penobscot based on this work at penobscot cssi is the optimum technique for progressively building large sub regional scale inversions at sable where we need to balance work effort signal to noise ratio spectral inversion results are very broadband but appear very noisy reliable quantitative mapping of absolute acoustic impedance and the identification of stratigraphic features on horizon slices for which fast track inversions are also be suitable

simple kinetic models of petroleum formation part iii modelling an open system abstract the movement of petroleum from organic rich source beds to potential carrier beds is the net result of two processes expulsion the release of generated petroleum from kerogen within the source bed and primary migration flow of a petroleum phase through the inorganic void networks of the source bed and all other intervening fine grained rocks expulsion is by far the more important process within the source bed orgas a simple zero dimensional model of the source rock as an open system explicitly models expulsion concurrent with oil and gas generation and oi lgas cracking as a storage threshold problem oil and gas are retained adsorbed absorbed within kerogen until their concentrations exceed the respective sorptive capacity of the residual organic carbon 0 1 and 0 02 g g ck 1 the initial hydrogen index hl 0 controls the important balance between potential sorbate petroleum and sorbant residual carbon using simple geochemical measurements the initial organic carbon is divided into initial oil inert kerogen and oil and gas generative reactive kerogen components reactive kerogens are assigned to one of five global kinetic organofacies with pre determined kinetic parameter sets governing separately rates of oil and gas generation kinetic parameters governing the rates of oil cracking within the source rock are predicted from hl 0 the two most important determinants of source rock behaviour are organofacies and hl 0 at a reference constant heating rate of 2 c ma 1 the projected oil expulsion threshold for typical organofacies members increases in the order a de 100 110 120 135 c f typically expels no oil an order of magnitude increase decrease in heating rate increases decreases these thresholds by ca 15 c typical cumulative gas mass fractions in their ultimate expulsion products are a f 0 25 0 3 0 25 0 65 and 1 0 g g 1 a value of 200 mg hc g c 1 is an approximate minimum hl 0 prerequisite for oil expulsion irrespective of absolute petroleum potential p 0 as a family coals vary dramatically in their expulsion behaviour as their hl 0 ranges both above and velow this value primary migration is efficient for expulsive source rocks movement within the source bed is limited only at low p 0 a source rocks total genetic potential p 0 thickness becomes important during migration outside the source bed it potentially restricts the primary migration distance and the excess of petroleum available for secondary migration and entrapment we evaluate primary migration approximately using a small migration loss factor no more than about 10 saturation of porosity or 1 mg g 1 coupled with an understanding of fluid potential gradients which determine migration direction within the stratigraphic architecture ultimately kerogen degradation and oil to gas cracking in the source rock are the rate limiting processes in petroleum accumulation

a revised biostratigraphic and well log sequence stratigraphic framework for the scotian margin offshore eastern canada12 1this article is one of a series of papers published in this cjes special issue on the theme of mesozoic cenozoic geology of the scotian basin 2earth sciences sector contribution 20120137 in this study we build on a foundation of previous biostratigraphic studies for the scotian basin offshore eastern canada by conducting new quantitative multidisciplinary biostratigraphic studies of the mesozoic sections in eight wells bonnet p 23 chebucto k 90 cohasset l 97 glenelg j 48 glooscap c 63 mohican i 100 south desbarres o 76 and south griffin j 13 these wells were chosen to provide good spatial coverage stratigraphic penetration and correlation with the seismic grid we have also evaluated pre existing biostratigraphic data and undertaken the well log sequence stratigraphic interpretation of upper triassic to cenozoic sediments in 16 additional wells using a consistent multidisciplinary event scheme derived from the new wells key to the dating of some horizons has been integration of the palynology and micropaleontology disciplines mostly used previously on the scotian margin with new nannofossil data simultaneously we have closely integrated the biostratigraphic results with

study of petroleum generation by compaction pyrolysis i construction of a novel pyrolysis system with compaction and expulsion of pyrolyzate from source rock abstract a compaction pyrolysis system was developed to simulate changes of source rocks during the course of burial and subsidence in a sedimentary basin this system can reproduce a number of phenomena that previous methods could not achieve with programmed temperature and pressure controllers the system can simulate the following facets of sediment diagenesis in the natural system 1 a increase in temperature overburden pressure and fluid pressure during burial and subsidence of a formation 2 b petroleum generation from thermal degradation of kerogen 3 c expulsion of interstitial and bound water during compaction of sedimentary rocks 4 d expulsion of oil and gas generated 5 e phase changes of clay minerals zeolites and silica minerals due to increases in temperature and pressure in addition comparison of compaction pyrolysis with non compacting hydrous pyrolysis shows the following 1 1 the composition of gas from compaction pyrolysis was similar to that of natural gases containing only small proportions of unsaturated hydro carbons in contrast gas from hydrous pyrolysis contain 5 10 unsaturated hydrocarbons 2 2 both compaction and hydrous pyrolysis produced oils resembling crude oil in the natural system 3 3 while macerals after hydrous pyrolysis showed abundant holes by evaporation of volatile products by pyrolysis macerals after compaction pyrolysis showed fewer holes however in samples from compaction pyrolysis deformation from compaction was observed 4 4 during the compaction pyrolysis experiment expulsion of oil and gas from source rock compaction cell was observed with the increase in fluid pressure and fluid volume in summary compaction pyrolysis permits a more realistic reproduction of natural phenomena as a result of which a more detailed study of petroleum generation and migration is possible

state of the art of research in kinetic modelling of oil formation and expulsion abstract the last ten years have seen a rapid development in the understanding of petroleum generation and expulsion by a combined use of natural series and experimental simulation this knowledge is now used to develop mathematical models which reproduce the kinetics of petroleum generation and migration in the purpose of basin evaluation by a combination of observation of homogeneous geochemical series experimental simulation and mathematical modelling it has been now established that parameters governing the quantity and composition of petroleum are temperature type of organic matter time or heating rate catalysis by clay minerals which would produce important amounts of isoalkanes is not necessary to produce petroleum the role of minerals although still controversial is rather to retain the heavy fraction of petroleum pressure seems to have a limited influence within the range for effective source rocks kerogen is a tremendously complex macromolecule which is now insufficiently characterized to develop fundamental predictive models of thermal cracking i e involving intermediates such as radicals and olefins such models presently apply only to the cracking of simple molecules hexane hexadecane at limited degrees of cracking for this reason kinetic models are still empirically calibrated on pyrolysis experiments and or on natural maturation trends the most popular models involve a series or a continuous distribution of parallel reactions to describe primary cracking i e direct formation of mobile compounds from kerogen these are generally calibrated against open system pyrolysis with various temperature programs consideration of natural maturation trends i e decrease in petroleum potential may be used also for calibration but discrepancies of organic matter properties availability of deep samples and uncertainties on thermal history limit the effectiveness of such methods whatever the method main activation energies are found in the range 45 60 kcal mol secondary cracking which refers generally to cracking of mobile compounds cannot be modelled from open system pyrolysis nor from natural series unknown residence times but only from closed system pyrolysis although models of primary and secondary cracking still need experimental improvements and more realistic kinetic schemes their ability to predict quantities and composition of petroleum depends primarily on migration which is poorly known indeed migration controls the residence time of petroleum in hot zones of sedimentary basins and thus determines the extent of oil conversion into gas as reproduced by recent numerical models preliminary studies documenting the interactions between thermal cracking and expulsion are shown from haltenbanken area and paris basin in the last few years a kind of agreement has been reached that primary migration was most likely occuring in a separate phase from water and that a sufficient concentration of petroleum must be generated for expulsion to start indeed high expulsion efficiencies over 50 are systematically found for rich and mature source rocks a further understanding of primary migration requires observations of natural series petrophysical studies of source rocks porosities under 10 permeabilities in the nanodardy range thermodynamic investigation of unconventional multiphase organic systems kerogen liquid s gas and experimental simulation under stress this on going research should enable models to track correctly migration paths and thermal history of petroleum fluids as necessary to model their composition

modeling the occurrence of overpressures on the scotian shelf offshore eastern canada many wells on the scotian shelf offshore from nova scotia have drilled into overpressured zones in particular the wells comprising the venture gas field have all encountered overpressures at depths below 4 5 km the characteristics of the overpressure zones in the venture field vary in some important details from overpressured zones on the u s gulf coast in the venture field the shales in the overpressured zones are observed to be normally compacted and interbedded with layers of overpressured sandstones this is in contrast to the gulf coast where the shales in the overpressure zone are undercompacted resistivity logs sonic logs and bulk density logs all provide some indications of overpressures in the venture field but are not as unequivocal indicators of overpressure as they are on the gulf coast theoretical investigations of the causes of overpressures in an environment such as the venture field show that overpressures are unlikely to be caused by loading of sediments the most probable cause of overpressures is recent and continuing fluid generation which is related to the transformation of kerogen to hydrocarbons limits are placed on the average permeabilities and fluid generation rates which are required if overpressures are to be produced by fluid generation

relationship of diagenetic chlorite rims to depositional facies in lower cretaceous reservoir sandstones of the scotian basin the relationship between diagenetic chlorite rims and depositional facies in deltaic strata of the lower cretaceous missisauga formation was investigated using a combination of electron microprobe bulk geochemistry and x ray diffraction data the succession studied comprises several stacked parasequences the delta progradational facies association includes i fluvial or distributary channel sandstones some with tidal influence ii thick bedded delta front graded beds of sandstone interpreted as resulting from fluvial hyperpycnal flow during floods and storms and iii more distal muddier delta front and prodeltaic facies the transgressive facies association includes lag conglomerate siderite cemented muddy sandstone and mudstone and bioclastic sandy limestone chlorite rims are absent in the fluvial facies and best developed in thick sandstones lacking mudstone baffles good quality chlorite rims are well correlated with ti in bulk geochemistry ti is a proxy for fe availability principally from the breakdown of abundant detrital ilmenite fetio3 under conditions of sea floor diagenesis the abrupt decrease in sedimentation rate at transgressive surfaces caused progressive shallowing of the sulphate depletion level and of the overlying eh controlled diagenetic zones resulting in conditions suitable for diagenetic formation of berthierine to migrate upwards through the packet of reservoir sandstones this early diagenetic berthierine suppressed silica cementation and later recrystallized to chlorite thick euhedral outer chlorite rims were precipitated from formation water in sandstone lacking muddy baffles on this chlorite substrate and inhibited late carbonate cementation this study thus shows that the preservation of porosity by chlorite rims is a two stage process rapidly deposited delta front turbidite facies create early diagenetic conditions that eventually lead to the formation of chlorite rims but the best quality chlorite rims are restricted to sandstones with high permeability during burial diagenesis

a 1 ma record of sediment flux south of the grand banks used to infer the development of glaciation in southeastern canada abstract a 20 m long piston core from a basement ridge on the continental rise off southeastern canada provides a 0 9 ma record of distal continental margin sedimentation sediment sources are distinguished by sand and clay sized mineralogy and detrital palynomorphs they include detritus eroded from a northern canadian shield and lower paleozoic rocks transported southward through the labrador sea b the grand banks and c the gulf of st lawrence and newfoundland isotopic stratigraphy allows correlation of glacial and interglacial stages with changes in sources and flux rates via different transport processes nepheloid layer ice rafting and distal turbidity currents the glacial history of southeastern canada is interpreted from variation in the flux and type of detrital material and correlation with other data from the continental margin from stages 27 to 15 the grand banks formed a forested coastal plain limited local glaciation may have occurred in stages 18 and 16 tertiary regolith and cretaceous clays were stripped from land areas in stages 14 and 12 culminating in the excavation of the laurentian channel late in stage 12 extensive cold based ice sheets are inferred for glacial stages 10 8 and 6 with less severe glaciation in stages 4 and 2 extent of this local appalachian ice correlates poorly with global ice volumes

calculation of petroleum masses generated and expelled from source rocks abstract an algebraic scheme for calculating the masses of petroleum formed in mature source rocks has been constructed the scheme exploits routine geochemical measurements and a basic understanding of the behaviour of sedimentary organic matter at elevated temperatures under both laboratory and subsurface conditions application of these techniques to data from a wide range of source rock types has led to the following general conclusions 1 1 major oil and gas generation from kerogen occurs between 120 and 150 c in the subsurface gas generation continues to temperatures 160 c 2 2 oil prone source rocks with average initial potential 5 kgt 1 of rock and average organic carbon contents 1 5 expel 60 90 of the total oil generated 3 3 oil expulsion from leaner source rocks is less efficient most of the oil generated remains in the source rock it is subsequently converted to lighter components at higher temperatures and expelled in the vapour state 4 4 the major loss of organic carbon from rich oil prone source rocks during generation and expulsion means that the petroleum potential and organic carbon content of mature source rocks are much less than their initial immature values hence the algebraic scheme must be used to calculate the initial values before expelled petroleum masses can be determined