hydrocarbon generation migration alteration entrapment and mixing in the central and northern north sea abstract there is little doubt that sediments of upper jurassic to lowermost lower cretaceous age particularly those of the kimmeridge clay formation and its equivalents are the source of the vast bulk of the oil in the central and northern north sea this is true both of oil in conventional mesozoic plays and in tertiary clastic reservoirs the timing of oil generation and migration ranges from the late cretaceous through to the present the oil productivity of the kimmeridge clay formation in the central graben moray firth and south viking graben areas of the north sea is estimated to be in excess of 250 billion barrels of which about 25 is accounted for in accumulations discovered to date there is strong areal differentiation between oil trapped in mesozoic reservoirs as opposed to upper cretaceous chalk and tertiary reservoirs this paper is particularly concerned with hydrocarbons in tertiary reservoirs which have a wide range of compositions and appear to have complex accumulation and degradation histories migration to chalk and younger tertiary reservoirs occurs often through thick shale sequences the evidence for the timing and mechanism of this process is considered and a preferred model for the vertical migration and subsequent lateral migration mixing and degradation is proposed

ankerite cementation in deeply buried jurassic sandstone reservoirs of the central north sea abstract upper jurassic fulmar formation shelf sandstones of the high temperature high pressure franklin and elgin fields south central graben north sea contain abundant disseminated and concretionary ankerite in contrast most jurassic north sea reservoirs contain only minor amounts of dispersed ankerite disseminated ankerite cement in the franklin and elgin fields has a fairly uniform isotopic composition 18o 10 to 12 5o oo pdb 13c 3 to 5o oo pdb ankerite concretions have 18o values similar to disseminated cements but a wider range of 13c values 1 to 5 5o oo pdb they also have highly variable intergranular volumes which together with the 13c data are interpreted as a combination of pore filling cementation and in situ replacement of comminuted bioclastic debris by ankerite fluid inclusion 18o and paragenetic evidence suggests that ankerite formed during deep burial c 3 5 to 4 5 km 140 170 c after the onset of overpressuring but before hydrocarbon emplacement in the reservoirs the regionally consistent 18o data suggest that ankerite formed via a temperature influenced mechanism and the relatively uniform 13c cement value indicates that organic matter and marine bioclastic carbonate contributed to the dissolved carbon reservoir in constant proportion this can be explained by calcite dissolution in response to ph decrease during thermal breakdown of organic acids such acids were derived from adjacent mudrocks undergoing hydrocarbon maturation and clay mineral transformations and are likely to have been transported in pore fluids with mg2 and fe2 the presence of these cations in solution upon thermal decarboxylation is inferred to have stabilized ankerite at the expense of calcite a relative paucity of ankerite in other fulmar formation reservoirs may reflect different sedimentological compositions less bioclastic debris and or lower burial temperatures less advanced decarboxylation

comparison of covalently bound aliphatic biomarkers released via hydropyrolysis with their solvent extractable counterparts for a suite of kimmeridge clays abstract fixed bed hydropyrolysis at pressures greater than 10 mpa has the unique ability to maximise the yields of covalently bound alkanes without adversely affecting their stereochemistries in order to compare the yields and compositions of the solvent extractable and kerogen bound hopanes and steranes for source rocks of varying maturity six kimmeridge clay samples jurassic type ii kerogen have been subjected to successive dichloromethane extraction and hydropyrolysis the ratios of free to kerogen bound aliphatic biomarkers increase markedly at relatively low vitrinite reflectance ca 0 45 0 50 with large reductions occurring in the concentrations of bound hopanes and steranes however the fact that the total concentrations of observable hopanes and steranes i e free plus kerogen bound do not decrease until the vitrinite reflectance is greater than ca 0 50 provides evidence that neither oil migration nor aromatisation has begun to occur to a significant extent below this maturity level to the authors knowledge this represents the first study to observe a systematic change in epimer ratios with increasing maturity for hopanes and steranes covalently bound in kerogens indeed the kerogen bound hopanes and steranes undergo the same epimerisation reaction pathways as their free counterparts in the bitumen but they are generally less mature in terms of isomerisation at both ring and side chain chiral centres indeed the range of the moretane hopane index for the hydropyrolysis oils is much larger than that for the bitumens ca 5 times indicating a much greater sensitivity to relatively small changes in maturity the covalently bound hopanes and steranes released via hydropyrolysis have enabled the maturity of a contaminated core from the north sea central graben maturity to be assessed and have showed that it is compatible with the kimmeridge formation

origin of tar mats in petroleum reservoirs part ii formation mechanisms for tar mats abstract tar mats and oil legs from several marine source rock derived oil fields including the ula and oseberg fields in the north sea and an unidentified north american example have been analysed geochemically in all cases the tar mats were derived from the same petroleum system as the oil leg oils and were found in the most porous and permeable portions of the reservoir comparing case history data with published data on the subsurface solubility characteristics of asphaltenes in petroleums it is possible to eliminate some potential tar mat formation mechanisms and suggest the most likely causative mechanisms thus the adsorption of asphaltenes on reservoir clays and light to moderate levels of biodegradation are considered unlikely to contribute to tar mat formation in the ula field thermal degradation of the oil in the carrier and reservoir causing asphaltene precipitation may be a major contributing factor to the tar mats there in the oseberg field increased gas solution in the oil leg may have contributed to deasphaltation tentative mass balance calculations the extreme pressure dependence of asphaltene solubility in subsurface crude oils the occurrence of tar mats in zones of elevated permeability plus a simple transport model suggest that during secondary migration an asphaltene rich petroleum phase may be produced in dipping carrier beds and may be carried into the reservoir contributing in some instances to tar mat formation

high pressure water pyrolysis of coal to evaluate the role of pressure on hydrocarbon generation and source rock maturation at high maturities under geological conditions this study investigates the effect of water pressure on hydrocarbon generation and source rock maturation at high maturities for a perhydrous tertiary arctic coal svalbard using a 25 ml hastalloy vessel the coal was pyrolysed under low water pressure 230 300 bar and high water pressure 500 700 and 900 bar conditions between 380 c and 420 c for 24 h at 380 c and 420 c gas yields were not affected by pressure up to 700 bar but were reduced slightly at 900 bar at 380 c the expelled oil yield was highest at 230 bar but reduced significantly at 900 bar at 420 c cracking of expelled oil to gas was retarded at 700 and 900 bar as well as direct cracking of the coal the main source of gas generation at high pressure at both 380 c and 420 c is from bitumen trapped in the coal indicating that this is a key mechanism in high pressure geological basins vitrinite reflectance vr was reduced by 0 16 ro at 380 c and by 0 27 ro at 420 c at 900 bar compared to the low pressure runs indicating that source rock maturation will be more retarded at higher maturities in high pressure geological basins

secondary porosity generation during deep burial associated with overpressure leak off fulmar formation united kingdom central graben the overpressure history of a sandstone can be estimated using a numerical model if the burial curve and geological setting are known from the resulting effective stress the maximum potential porosity mpp can be calculated the mpp is the maximum porosity the rock could theoretically hold open at the modeled burial depth and pore pressure measured rock porosities should be at or below the mpp we have determined the mpp history for the fulmar formation sandstones upper jurassic of the central graben north sea and have compared the predictions to measured core data we conclude that for the majority of the fulmar formation sandstones the porosity evolution is a simple pattern of reduction during burial caused by compaction and cementation however in wells sited close to regional overpressure leak off points the porosity has been significantly increased from an end of oligocene low mean 21 to the present day values mean 31 this porosity increase occurred by feldspar dissolution with the reaction products being removed from the sandstones secondary porosity generation and the export of solute occurred while the sandstone was highly overpressured although still part of an open hydrogeological system the generation of porosity within deeply buried sandstones is of commercial importance and potentially can be predicted

hydrocarbon filling and leakage history of a deep geopressured sandstone fulmar formation united kingdom north sea diagenetic minerals in a water filled borehole from the jurassic fulmar formation united kingdom central north sea record two phases of hydrocarbon filling and emptying initial oil charge was during the late cretaceous at shallow burial depths of about 1 5 km 0 9 mi as we consider that hydrocarbon has preserved porosity during burial this has implications for the understanding of the porosity evolution of the fulmar formation which in other locations is an important hydrocarbon reservoir the early oil charge as recorded by illite k ar ages progressively filled the structure from 84 to 59 ma and possibly precipitated bitumen because of biodegradation the first oil predated many of the burial diagenetic reactions within the sandstone after leak off at ca 60 ma diagenetic reactions continued in an open geochemical system with possible import of co2 products of these reactions include ankerite and quartz overgrowth cements hydrocarbon staining postdates these phases and provides evidence of a hydrocarbon charge probably gas condensate the second hydrocarbon charge also leaked off and the sandstone is now water bearing previous work on the fulmar formation has incorrectly placed all the diagenetic reactions as predating the first arrival of hydrocarbon the present day pore fluids are high salinity high 18o fluids derived from the underlying permian zechstein evaporates these fluids entered the reservoir during a phase of overpressure release that caused fracturing of the framework quartz grains possibly coincident with the second phase of hydrocarbon leak off at 2 5 ma mark wilkinson studied sandstone hosted calcite concretions for his ph d and has subsequently tried to take a more holistic view of the diagenesis of sedimentary basins his career has been funded by a variety of oil companies although he is currently involved in geological disposal of some of the co2 that he was partly responsible for producing stuart haszeldine is a professor at edinburgh university he received his ph d on clastic and coal sedimentology in 1981 from the university of strathclyde after an attempted reeducation in the oil industry he has collaborated with many companies to research on hydrocarbon reservoir quality radioactive waste disposal and coal resources he currently heads the united kingdom s largest group investigating carbon dioxide sequestration tony fallick was educated at the university of glasgow scotland in physics b sc and chemistry ph d he did research at mcmaster university ontario canada and cambridge england before moving in 1980 to the scottish universities research and reactor center where he is currently the director

oil charge preserves exceptional porosity in deeply buried overpressured sandstones central north sea uk abstract exceptionally high porosity sandstones are reported from oil provinces worldwide yet the mechanism of porosity preservation remains controversial we present strong evidence that the exceptional porosities in the jurassic fulmar formation within the north sea are the result of early oil charging preventing cementation and chemical compaction in sands with relatively low detrital clay contents although overpressure and grain coatings do have some effect the most dramatic evidence that reservoir quality is related to early oil charge is the spatial distribution of the porosity the maximum porosity of the fulmar formation is systematically highest immediately below the top seal and decreases below this despite uniform sedimentology hence porosity is preferentially preserved where oil first accumulated in these unusually homogeneous overpressured reservoir sandstones at the top oil charge prevents or significantly retards chemical compaction and cementation of a reservoir preserving porosity there is independent evidence for early oil charging of reservoirs within the central graben from fluid inclusion studies and k ar age dates of authigenic illite micro quartz grain coatings which have been proposed to preserve porosity during burial are associated with only 5 extra porosity in sands that exceed the worldwide average sandstone porosity by almost 20

overpressure retardation of organic matter maturation and petroleum generation a case study from the yinggehai and qiongdongnan basins south china sea three superimposed pressure systems developed in the yinggehai basin south china sea as indicated by seismic data well logs and direct pressure measurements the organic maturation profile is nonlinear with three nonparallel segments that correspond to the shallow normal pressured system the intermediate overpressured system and the deep strongly overpressured system respectively the intermediate and deep overpressured systems have abnormally low r0 gradients the organic maturity of these overpressured rocks is significantly lower than the maturity of normal pressured source rocks in nearby wells with similar thermal histories and does not match the thermal histories of the rocks such an organic maturity anomaly is distinctly different from those aused by variation in activation energies conductivity contrasts and hydrologic effects and is confirmed to be the result of overpressure retardation the degree to which the organic matter maturation is retarded expressed as the difference between predicted and measured vitrinite reflectance increases exponentially with increasing fluid pressure confirming that pressure increases the activation energies of organic matter maturation reactions overpressure retardation has been proven to be conditional and quite important for clearly understanding petroleum generation migration and accumulation in overpressured sedimentary basins

kinetic modelling of petroleum formation and cracking implications from the high pressure high temperature elgin field uk north sea in this paper we use a case study to test the simulation of generation migration and cracking of petroleum fluids by a 2d compositional basin simulator using kinetic models based on chemical classes of compounds the selected area is the high pressure 1100 bar high temperature 190 c elgin field in the north sea this study is mainly focused on kinetic modelling of secondary cracking because in this zone paraffinic condensates with almost 50 c6 alkanes were recently discovered whereas only gas c5 was expected given the high temperature conditions the detailed hydrocarbon composition observed today in the fulmar sands reservoir of the elgin field is compared with the results of several simulations aimed at testing the influence of jurassic source rocks retention properties kinetic parameters and temperature variations on the predicted composition of fluids the influence of pressure was neglected in the modelling on the basis of results published in the literature results show that in this north sea area primary cracking of the kerogen into oil is completed whichever source rock is considered and that secondary cracking of accumulated petroleum compounds in the reservoir is presently in progress under these conditions source rock type and retention during primary and secondary migration have little influence on the present oil composition the occurrence of a monophasic paraffinic fluid in the elgin reservoir despite the high temperature is due to its very recent increase from 160 c to over 180 c in the last million years the fluid composition evolves rapidly and our compositional kinetic scheme accounts correctly for the presence of c6 hydrocarbons in these geological conditions first results using the compositional kinetic scheme of behar et al behar f kressman s rudkiewicz j l vandenbroucke m 1991 experimental simulation in a confined system and kinetic modelling of kerogen and oil cracking organic geochemistry 19 1 3 173 189 for secondary cracking showed a reasonably good fit between calculated and observed detailed fluid composition except for methylated aromatics which are overestimated and methane which is underestimated a recent comparison of kinetic parameters obtained from pyrolysis experiments on model compounds showed that it is not realistic to assume as done previously the same frequency factor for all chemical classes of compounds the higher stability of methylated aromatics relative to saturates observed under laboratory conditions would be extended to geological conditions a second set of simulations using frequency factors and activation energies obtained on these model compounds gave a better fit for methylated aromatics however methane is still strongly underestimated and reasons which could explain this discrepancy are discussed

the effects of oil emplacement on diagenetic processes examples from the fulmar reservoir sandstones central north sea geologic note 1 sedimentary petrography and petroleum geochemistry studies have been done on fulmar formation upper jurassic cores from fulmar field central north sea the extent of quartz overgrowths and albitization of k feldspars is lower in oil saturated zones compared with water saturated zones however microthermometric studies of aqueous fluid inclusions in quartz overgrowths show similar temperature ranges for both oil and water saturated zones 85 125 degrees c with maximum recorded temperatures close to the current reservoir temperature about 130 degrees c this suggests that quartz overgrowth development in the oil saturated zones continued after oil emplacement but was retarded compared with development in the water saturated zones the quartz cement therefore mus have been essentially locally derived by pressure solution albitization of k feldspars is significantly less about 50 in the oil saturated zones compared with albitization in the water saturated zones because albitization of k feldspars depends on a supply of sodium and removal of potassium this process is more likely to be terminated by oil emplacement the observed difference in the albitization has been used to suggest that the fulmar reservoir probably was filled during the neogene this age estimate agrees broadly with timing determined independently by modeling of oil generation in adjacent source basins the fulmar sandstones contain little or no kaolinite and show no evidence of substantial leaching of feldspars or carbonates this is probably because the fulmar sandstones being partly turbiditic offshore sand deposits were not exposed to meteoric water their close proximity to the marginally mature to mature kimmeridge clay formation seems to have little effect in terms of developing secondary porosity suggesting that the role of organic acids or co 2 generated in these source rocks is of minor importance

vitrinite reflectance gradients of deep wells with thick chalk sections and high pressure implications for source rock maturation danish norwegian central graben north sea abstract nine deep wells td from 4793 to 5903 m drilled with jurassic syn rift targets in the danish norwegian central graben show abnormal vitrinite reflectance vr profiles they all penetrated thick up to 950 m post rift upper cretaceous chalk sections and about 3000 m of cenozoic mudstone dominated sediments the penetrated strata in the wells are further characterised by high fluid pressures overpressure in the thick sections of upper jurassic lowermost cretaceous mudstones forming the principal source rocks in the area the overpressure in the jurassic sandstone reservoir target may attain a value of about 50 mpa the wells display overall the same vr profiles characterised by three vr segments i a more or less well defined increase in vr through the cenozoic to the top of the upper cretaceous chalk ii an interpolated lower vr gradient through the chalk section and iii a likewise well defined increase in vr from the base of the chalk and through the upper jurassic lowermost cretaceous mudstones but with a higher gradient than determined for the cenozoic strata the vr values at the top of the source rock section are much lower than expected indicating that maturation is significantly retarded the lower vr values are determined by two factors i the high thermal conductivity of chalk accounts for the interpolated lower vr gradients in the thick chalk sections while the higher vr gradients and the higher geothermal gradients in the upper jurassic lowermost cretaceous source rock mudstones agree with the low thermal conductivity of mudstone and ii the thick sections of overpressured mudstones serve as insulator for the overlying chalk and retard source rock maturation as excessive pressure can prevent liquid and gaseous generation products to escape the source rock and thereby preventing the condensation and reorganisation of the organic matter which is necessary to increase maturity indirect evidence of the effect of excessive pressure comes from maturity modelling of the measured vr values in the danish hejre 1 well using the easy ro and presro models the latter incorporating pressure calibrated with measured present day pressures the presro model is able to predict the measured vr values and thus the maturity gradient and predicts the source rock interval in the well to be in the middle of the oil window even at a depth of 5000 m hence as a consequence of the irregular maturity trends the upper jurassic lowermost cretaceous source rocks are of lower thermal maturity than expected and if the top of the oil window is set at a vr of 0 6 ro the decreased maturation results in a downward displacement of the top of the oil window of up to approximately 200 350 m

deep burial dissolution of k feldspars in a fluvial sandstone pentland formation uk central north sea the pentland formation of the uk central north sea is a quartzite in a sequence of more arkosic sandstones provenance sensitive heavy mineral indices are interpreted to indicate that it was probably derived by erosion of arkosic sediments which also supplied the arkosic marine sandstones of the contemporaneous fulmar formation a lack of apatite within the pentland formation suggests that the feldspars could have dissolved during exposure to groundwaters during very shallow burial however petrographic and geochemical evidence supports survival of at least some of the feldspar to burial depths in excess of at least 2 km when the sandstones had been stabilized against compaction by quartz cements or overpressure abundant kaolin which has not reacted to form illite is preserved at burial depths in excess of 4000 m suggesting an unusual lack of available potassium there is a strong contrast in diagenetic pathways with the overlying marine fulmar formation which most probably had the same initial composition but in which kaolin is absent and fibrous illite is present it is suggested that a lower concentration of potassium and aluminium within the pentland formation compared with the probable source and the comparably sourced fulmar formation represents export of k from the sands on a metre scale

experimental study on the pressure dependence of vitrinite maturation we conducted the first systematic experiments over a pressure range of 0 5 to 20 0 kbar at temperatures of 200 to 350 c and run durations of 2 4 and 7 days to investigate the influence of pressure on vitrinite reflectance the data indicate that applied pressure lowers vitrinite reflectance the suppression appears to be stronger under wet than under dry conditions in addition high pressure favors liptinite preservation at temperatures 300 c whereas at the same temperature range yet at low pressure liptinite is decomposed our data fit an empirical rate equation which expresses rr as a function of pressure temperature and time the derived equation satisfactorily describes our experiments and enables extrapolations to geological timescales values of rr calculated for a pressure of 0 5 kbar are remarkably consistant with previous numerical pressure independent models a literature review shows that previous experimental data and field observations are inconsistent regarding the effect of pressure on vitrinite maturation this and some previous studies suggest that vitrinite maturation is suppressed by increased pressure whereas other workers regarded pressure as having little influence on the rate of coalification we suggest that original virinite composition experimental design and natural environment tectonic regime play important roles regarding the effects of pressure on vitrinite maturation interpretation of geothermal histories of sedimentary rocks based on vitrinite reflectance data must therefore include evaluation of the pressure otherwise vitrinite reflectance data from high pressure terranes should be interpreted with caution if used as a quantitative paleogeothermometer

the relationship between aromaticity vitrinite reflectance and maceral composition of coals implications for the use of vitrinite reflectance as a maturation parameter abstract previous studies on the aromaticity f a of coals have either limited themselves to the effects of rank or to differences between the macerals and generally conclude that f a and rank increase simultaneously although not necessarily linearly this study contains 13 c cp mas nmr analysis data of more than 70 coals obtained from different coal basins these are combined with published data to produce a data base with more than 130 analyses the aim is to determine the influence of rank and maceral composition on the measured f a of coals a plot of f a vs r o vitrinite reflectance shows a non linear relationship that can be described by a polynomial equation the shape of the curve produced by this equation is determined by the processes that occur during maturation only approx 50 of the coals in the data base have f a values that are accurately predicted by the vitrinite reflectance this is an indication of the large spread of values observed in the plot of f a vs r o as a result of the processes that occur during maturation the influence of the maceral composition on the f a was restricted to coals with r omax values 0 70 r o the equation generated shows that more than 80 of the coals tested with this level of rank have f a values that can be accurately predicted by an equation containing the r omax value the total vitrinite content and the contents of fusinite semifusinite and macrinite in the coals comparison of the f a values obtained from vitrinitic type iii kerogen with the f a values obtained from liptinitic sporinites and type ii kerogens reveals a pattern which illustrates the structural changes that occur during maturation both vintrinites and liptinites show a change in the slope in an f a vs r o plot although the reflectance level at which the change in slope occurs differs between the two kerogen types the changes in molecular structure that occur within the two kerogen types during maturation are discussed the implications of the f a r o relationship of vitrinites on the use of vitrinite reflectance as a maturation parameter are also discussed

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

disseminated jigsaw piece dolomite in upper jurassic shelf sandstones central north sea an example of cement growth during bioturbation unusual textural and chemical characteristics of disseminated dolomite in upper jurassic shelf sediments of the north sea have provided the basis for a proposed new interpretation of early diagenetic dolomite authigenesis in highly bioturbated marine sandstones the dolomite is present throughout the franklin sandstone formation of the franklin and elgin fields as discrete non ferroan generally unzoned subhedral to highly anhedral jigsaw piece crystals these are of a similar size to the detrital silicate grains and typically account for 5 of the rock volume the dolomite crystals are never seen to form polycrystalline aggregates or concretions or ever to envelop the adjacent silicate grains they are uniformly dispersed throughout the sandstones irrespective of detrital grain size or clay content dolomite authigenesis predated all the other significant diagenetic events visible in thin section the dolomite is overgrown by late diagenetic ankerite and bulk samples display stable isotope compositions that lie on a mixing trend between these components extrapolation of this trend suggests that the dolomite has near marine delta18o values and low positive delta13c values the unusual textural and chemical characteristics of this dolomite can all be reconciled if it formed in the near surface zone of active bioturbation sea water provided a plentiful reservoir of mg and a pore fluid of regionally consistent delta18o labile bioclastic debris e g aragonite mg calcite supplied isotopically positive carbon to the pore fluids during shallow burial dissolution such dissolution took place in response to the ambient calcite sea conditions but may have been catalysed by organic matter oxidation reactions bioturbation not only ensured that the dissolving carbonate was dispersed throughout the sandstones but also prohibited coalescence of the dolomite crystals and consequent cementation of the grain framework continued exchange of mg2 and ca2 with the sea water reservoir maintained a sufficient mg ca ratio for dolomite rather than calcite to form irregular crystal shapes resulted from dissolution of both the dolomite and the enclosed fine calcitic shell debris before ankerite precipitation during deep burial diagenesis

impact of modern deepwater drilling and testing fluids on geochemical evaluations evaluation of petroleum fluid properties hydrocarbon shows and source rock characteristics requires new tools to properly recognize and correct for drilling and test induced contamination which is increasingly common in modern deepwater field operations oil exploration development and now production are more frequently conducted in deeper water environments where the challenges faced by drilling and operations can severely impact the evaluation of oil and rock geochemistry and fluid properties poorly consolidated sediments swelling clay minerals and responses to evolving environmental regulations regarding offshore disposal of drill cuttings have resulted in the widespread use of enhanced mineral oil or synthetic based muds also water based drilling fluids used in some deepwater operations contain additives that may impact fluid and rock geochemistry for example asphalt based shale stabilizers are added to aid well bore competency and prevent sticking drill pipe and polyalkylated glycols are added to depress freezing temperatures and prevent the formation of gas hydrates in the drilling mud because these and other additives are often a significant component of water based muds they may affect the geochemical signature of fluids and rocks and alter fluid properties highly saline brines are another important source of contamination as they are used in completion fluids water wet muds and are emulsified in oil wet muds brine components impact metal contents of petroleum fluid tests and complicate the determination of formation water compositions despite potential problems introduced by these additives successful strategies can be devised to accurately access key geochemical and engineering parameters

tectonic evolution of the north sea basin crustal stretching and subsidence summary the lithospheric stretching model for the formation of sedimentary basins was tested in the central north sea by a combined study of crustal thinning and basement subsidence patterns a profile of crustal structure was obtained by shooting a long range seismic experiment across the central graben the main axis of subsidence a seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin the data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure and the final model showed substantial crustal thinning beneath the graben subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central north sea since jurassic times changes in water depth were quantified using foraminiferal assemblages where possible and observed basement subsidence paths were corrected for sediment loading compaction and changes in water depth through time the seismic model is shown to be compatible with the observed gravity field and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the central graben during the mid jurassic to early cretaceous extensional event this extension appears to have occurred over crust already slightly thinned beneath the graben and the seismic data suggest that total extension since the early permian may have been more than 100km the data presented here may all be explained using a simple model of uniform extension of the lithosphere

tidal effects on intermediate waters a case study in the east japan sea although tides are believed to be the most important source for diapycnal mixing in the ocean few studies have directly simulated open ocean circulation including tides because the east japan sea ejs has been considered to be a miniature ocean tidal effects on the intermediate water of the ejs are investigated by using an eddy resolving ocean general circulation model that can take account of m2 and k1 tides as well as oceanic flows the simulated temperature and salinity in the intermediate layer are significantly improved by including tides the improvement results from the combined effect of two internal tides the m2 internal tide propagating into the interior of the ejs enhances vertical mixing and brings watermass characteristics closer to those observed the k1 internal tide trapped along the coast induces southwestward flow along the russian coast in the upper layer and thereby enhances the so called liman current which transports fresh waters with density conducive to the ventilation of intermediate waters in the ejs

sequential extraction a useful tool for reservoir geochemistry abstract little is known about the microscale distribution and potential molecular fractionation of bitumen residing in reservoir rocks few data exist on the comparative organic geochemistry of hydrocarbons in authigenic inclusions formed in reservoir rocks and the bitumen in the pore space a three step sequential extraction technique has been applied to clastic petroleum reservoir samples using a suite of solvents with increasing polarity in order to address these questions in the analyzed samples the following main trends were observed i the first extract represents 95 of the total extract ii the relative polar compound content increases going from the first to the last extract iii the last extract has generally a slightly lower apparent maturity as assessed using biomarker parameters additionally a reduction in the degree of biodegradation from the first to the last extracts was observed implying tentatively that biodegradation takes place dominantly in the center of the pores fluid inclusion extracts show distinct source characteristics however the last extract appears to approach the fluid inclusion extract signature in some aspects the use of sequential extraction as a routine tool in the reservoir geochemical context is judged to be limited however it may help to solve specific problems

application of organic petrography in north american shale petroleum systems a review abstract organic petrography via incident light microscopy has broad application to shale petroleum systems including delineation of thermal maturity windows and determination of organo facies incident light microscopy allows practitioners the ability to identify various types of organic components and demonstrates that solid bitumen is the dominant organic matter occurring in shale plays of peak oil and gas window thermal maturity whereas oil prone type i ii kerogens have converted to hydrocarbons and are not present high magnification sem observation of an interconnected organic porosity occurring in the solid bitumen of thermally mature shale reservoirs has enabled major advances in our understanding of hydrocarbon migration and storage in shale but suffers from inability to confirm the type of organic matter present herein we review organic petrography applications in the north american shale plays through discussion of incident light photographic examples in the first part of the manuscript we provide basic practical information on the measurement of organic reflectance and outline fluorescence microscopy and other petrographic approaches to the determination of thermal maturity in the second half of the paper we discuss applications of organic petrography and sem in all of the major shale petroleum systems in north america including tight oil plays such as the bakken eagle ford and niobrara and shale gas and condensate plays including the barnett duvernay haynesville bossier marcellus utica and woodford among others our review suggests systematic research employing correlative high resolution imaging techniques and in situ geochemical probing is needed to better document hydrocarbon storage migration and wettability properties of solid bitumen at the pressure and temperature conditions of shale reservoirs

kerogen origin evolution and structure kerogen commonly defined as the insoluble macromolecular organic matter om dispersed in sedimentary rocks is by far the most abundant form of om on earth this fossil material is of prime importance as the source of oil and natural gas moreover kerogen can provide essential information on major topics such as past environments climates and biota this review reports the main advances in kerogen studies since the comprehensive synthesis edited by durand durand b ed kerogen insoluble organic matter from sedimentary rocks editions technip paris 1980 it is organized into eight sections the first two are concerned with the successive definitions of kerogen and the definition used here the different techniques used for kerogen isolation without loss or degradation and basic kerogen analysis the third and fourth focus on sedimentary om sources and preservation processes in relation to depositional environment including sedimentation conditions favourable for kerogen accumulation and extrapolation to past geological time great strides have been made in the latter topics over the last 25 years owing to a combination of classical studies in organic geochemistry and studies in other domains such as biogeochemistry oceanography hydrology and soil science along with the development of powerful analytical tools the next two sections deal with the different kerogen classifications by type and kerogen evolution and maturation upon burial in sediments structural modelling of coal and kerogen based on physical and or chemical structural analysis is described in the following section although only statistical the models thus derived provide a synthetic view of the main structural resemblances and differences among various samples in relation to source maturity or physicochemical properties finally the last section explores some of the advances in kerogen understanding expected for the near future the review includes a list containing about 500 references

a semi empirical method to calculate the permeability of homogeneously fluidized pyroclastic material this work postulates that highly polydisperse materials have an effective size distribution that controls permeability existence of such effective distribution implies that not all clasts participate to the permeable network resisting to gas flow and that clasts smaller than the minimal effective size are elutriated when this concept is coupled to a generalized blake kozeny equation the resulting semi empirical law links permeability to material properties only bed void fraction clast sizes and densities after calibration of an experimental constant it is able to replicate within 0 6 log unit experimentally measured permeabilities of both loosely packed and expanded beds made of highly polydisperse from 1 m to 4 mm pyroclastic deposits that were resampled so as to ensure homogeneous fluidization the presence of an experimentally calibrated constant and the necessary absence of segregation during fluidization limit the extrapolation of the proposed law to any pyroclastic bed satisfactory fitting of the experimental values however confirms that the permeability of homogeneously fluidized beds is controlled by a balance between settling and elutriation this balance suggests a first order link between permeability and bed expansion which has implications on the kinetics of dense pyroclastic flows

improving spatial predictability of petroleum resources within the central tertiary basin spitsbergen a geochemical and petrographic study of coals from the eastern and western coalfields central tertiary basin ctb coals from a variety of palaeogeographic conditions within the longyear and verkhnij seams were sampled to assess the relationship between the petroleum present the remaining generation potential and coal geochemistry in order to improve the spatial predictability of petroleum resources within the basin vitrinite reflectance vr values from the ctb coals have been shown to be suppressed marshall et al 2015b this study attempts to quantify and correct for this suppression effect by applying the lo 1993 method lovr which uses hydrogen index hi values to modify vr data and the coal rank sr scale of suggate 2000 2002 a technique not affected by suppression in addition the oil generation and expulsion thresholds for the ctb coals were investigated with discussions on oil potential versus marine influence upon the mires in which the coals formed a pseudo van krevelen diagram shows that the majority of the coals plot on the type ii kerogen line while the remainder plot between the types ii and iii kerogen lines with hi between 151 and 410 mg hc g toc however maceral analysis shows that type iii kerogen predominates this is attributed to the presence of abundant fluorescing oil prone vitrinites the lovr tmax and rank sr parameters all show that maturity increases from basin margins towards basin centre i e from bassen to lunckefjellet to breinosa and colesdalen and indicate that all the coals are within the oil generation window the marginal samples at bassen are within the early mature stage of the oil window i e 0 7 ro meaning the threshold for oil generation in the basin could not be clearly defined however the observed maturation trend somewhat parallels the maturation pathway of the new zealand coal band nz coal band and the envelope of the sykes and snowdon 2002 nz coal data set therefore it is considered that the oil generation threshold for the ctb coals is likely at rank sr 9 10 tmax 420 430 c in line with the observed rise in bitumen index bi some of the lunckefjellet coals and all the breinosa and colesdalen coals have either reached or progressed beyond the threshold for oil expulsion as indicated by the peak in hi at rank sr 11 12 lovr 0 75 0 85 ro tmax 430 440 c the peak in bi at rank sr 12 5 13 5 suggests that some of the lunckefjellet and breinosa coals and all the colesdalen coals have reached the effective oil window total sulphur st contents range between 0 46 and 12 05 indicating non marine to strong marine influence upon precursor peats with st contents of the longyear seam appearing to record instances of coastal retreat associated with base level rise marine deposition seems to significantly control the distribution of oil prone coals within seams and across the ctb the levels of marine influence as indicated by st content show clear positive relationships between bi and hi within the bassen samples because they have not started expelling oil conversely the levels of marine influence show clear negative relationships with bi and hi within the colesdalen samples because they have commenced oil expulsion and probably reached the effective oil window the more marine influenced coals appear to have commenced petroleum generation relatively earlier which is a plausible explanation why the coals from the lunckefjellet locality appear to be at different stages within the oil window

overpressure retardation of organic matter maturation a kinetic model and its application abstract a kinetic model t p ro has been developed to address the issue that overpressure increases the activation energies during organic matter maturation and that the retardation of organic matter maturation by overpressure is due to the fact that organic matter is hindered from progressive molecular ordering under overpressured conditions the mean effective stress i e mean stress minus pore pressure is much lower and the pore pressure is usually higher than under hydrostatic pressure conditions as a result the progressive molecular ordering of the vitrinite such as the aromatization is retarded and the vitrinite reflectance can hardly be increased the reflectance values predicted by t p ro model as it has been applied to the well ld3011 at yinggehai basin in south china sea are similar to the measured ones the kinetic model predicts that overpressure can cause the oil window floor and the peak oil generation to shift to greater burial depth in the case of yinggehai basin the local geological conditions are prone to generate light hydrocarbons and natural gas instead of oil hence they constitute the predominant products in this high temperature heating rate basin

two case studies of thermal maturity and thermal modelling within the overpressured jurassic rocks of the barrow sub basin north west shelf of australia abstract thermal maturity within the jurassic thick overpressured source rocks of the bambra 2 and barrow deep 1 wells barrow sub basin western australia was investigated and modelled using a temperature time related kinetic model the aim of this study is to investigate whether overpressure has a determinable influence on thermal maturation repeat formation tests rfts drill stem tests dsts and mud weights indicate that the jurassic section in the barrow sub basin is highly overpressured it appears that the overpressured zone is associated with high sonic transit times and low resistivity values in the jurassic fine grained rocks anomalously low measured vitrinite reflectance r o values in the overpressured zone appear to be due to either r o suppression or inaccurate identification of vitrinite anomalously low t max values are found to be caused by contamination from drilling mud additives a good correlation is found between the reliable maturity data including biomarker maturity and predicted maturity values r o between 0 8 and 2 2 and suggests that the anomalously low r o and t max values are probably not related to the overpressuring fluid pressure about 40 80 mpa at 3000 4600 m these results demonstrate that the true maturity level in the jurassic overpressured section probably depends on temperature and time alone we therefore conclude that overpressure is not an important factor in thermal maturity and hydrocarbon generation in this region

integrated study of the judy field block 30 7a an overpressured central north sea oil gas field the triassic reservoirs of the judy field an overpressured petroleum accumulation in the central north sea have been studied to determine their pressure and petroleum filling history the magnitude of overpressure in the pre cretaceous aquifer is similar across the field at about 24 mpa 3500 psi but with some higher pressure laterally in areas closest to the major depocentres pressure modelling shows that the magnitude of the overpressure can be attributed almost entirely to disequilibrium compaction and largely due to late rapid tertiary burial although modelling does require nanodarcy i e shale like permeability in the cretaceous chalk section the contributions from other mechanisms which may be relevant in this setting i e gas generation and lateral transfer appear to be small the existence of both aqueous and petroleum phases in secondary fluid inclusions in microfractures in quartz and feldspar grains allows estimation of palaeopressure the data show fracture healing from 115 to 136 c during a time when reservoir palaeopressures were above hydrostatic reconstruction of the temperature history of the reservoir places the timing of the fluid inclusion formation during fracture healing at between 3 and 1 my ago coincident with a period of rapid burial the palaeopressure estimation using fluid inclusions validates the pressure modelling studies which are derived independently from commercial basin modelling software the fluid inclusions also revealed a low gor oil over the whole field prior to late stage flooding by gas leading to the variable low to high gor fluids plus gas condensates within the field today

pyrobitumen occurrence and formation in a cambro ordovician sandstone reservoir fahud salt basin north oman the cambro ordovician barik sandstone reservoirs in the fahud salt basin in oman contain bitumen which may fill up to 40 of the porosity in well jaleel 1 this bitumen was isolated according to kerogen procedure and typed by nmr elemental analysis and density measurements the isolated bitumen is characterized by 1 a highly aromatic character nmr 75 caro h c atomic ratio 0 65 2 a very high sulphur content 4 2 and 3 a relatively high density 1 3 1 4 g cm3 the insolubility and the reflectivity of the bitumen 1 2 vr qualify it as a low mature pyrobitumen the combination of rock eval and density data was used to calculate the actual volume of the pyrobitumen in the rock as a percentage of porosity it was found that the pyrobitumen volume shows a negative correlation with total porosity indicating that small pores are more invaded by bitumen than larger ones finally closed system pyrolysis experiments performed on oils with different nso contents indicate that an in situ oil with a very high content of nso compounds is required to generate such large amounts of pyrobitumen in the pore system these observations suggest that the precursor oil of the current pyrobitumen was a very heavy oil tentatively assumed to be the result of a severe biodegradation basin modeling shows that the reservoir was charged already in devonian times a major uplift brought the oil accumulation near the surface during the carboniferous and a rather regular burial to the present day position 4500 m 140 c loosveld et al 1996 this scenario involving a residence time at shallow depth strengthens the biodegradation hypothesis the numerical modeling which involves the ifp kinetic model for secondary oil cracking suggests that pyrobitumen formation is a very recent event inclusion of pyrobitumen particles within quartz overgrowth containing fluid inclusions provides an upper temperature limit for the beginning of pyrobitumen formation which comforts the result of kinetic modelling

release of bound aliphatic biomarkers via hydropyrolysis from type ii kerogen at high maturity abstract at relatively low maturity kerogen bound aliphatic biomarkers invariably display retarded maturity compared to their free counterparts as a result of the protection afforded by the macromolecular structure this study investigates the release of hopanes and steranes at elevated post oil window maturity from type ii kerogen results for a suite of north sea petroleum source rocks kimmeridge clay formation indicate that below a hydrogen index hi value of ca 300 differences between the free and bound phases for hopane and sterane side chain isomerisation are small however ring isomerisation within the kerogen bound phase is still more retarded for a corresponding series of kerogen samples artificially matured via hydrous pyrolysis both free and bound phase side chain isomerisation reactions are significantly retarded relative to the naturally occurring samples this is consistent with earlier studies but the extent of retardation for ring isomerisation in the bound phase is even more pronounced than for the free phase

geochemical characterization of a tar mat in the oseberg field norwegian sector north sea abstract concentrations of heavy oil usually at or near the oil water contact in reservoirs are called tar mats this phenomenon can arise from reservoir oil alteration processes such as deasphalting biodegradation or maturation effects of tar mats include reduction in water drive efficiency and lower oil quality in the bottom section of a reservoir the study of tar mats can be greatly improved by the systematic application of petroleum geochemical techniques and a new classification scheme is proposed based on these methods application of these general concepts to a tar mat occurrence in the oseberg field showed that although some biodegradation probably occurred in the early stages of field formation there was little evidence to indicate that this process was a significant factor in the formation of the main tar mat deasphalting was considered to be the major if not the only process leading to tar mat formation in this field