tricyclic terpane maturity parameters response to heating by an igneous intrusion the effects of thermal maturation upon the abundance and composition of the tricyclic terpanes have been investigated within a single sedimentary horizon within the dun caan shale member isle of skye scotland intruded by a 0 9 m thick tertiary dolerite dyke heating by the igneous body caused the concentrations of all 13 h 14 h and 13 h 14 h tricyclic terpanes to increase towards or within the effective oil window recording generation from abundant non hydrocarbon precursors such as kerogen and asphaltene bound components progressive changes in tricyclic terpane composition accompany these concentration changes an increase in the c23 maturity parameter towards the dyke resulted from the relatively greater rate of generation of the isomer combined with the earlier decline in concentration by degradation or isomerisation of the component the tricyclics tricyclics hopanes parameter shows a dramatic increase within the latter part of the oil window due to the greater relative thermal stability of the tricyclic terpanes the quantitative data demonstrate the importance of the processes of generation from non hydrocarbon fractions of the sedimentary organic matter and thermal degradation in the operation of tricyclic terpane maturity parameters

how are diagenesis and reservoir quality linked to depositional facies a deltaic succession edgeoya svalbard abstract middle to late triassic strata exposed at edgeoya eastern svalbard represents an uplifted part of the northwestern corner of the barents sea this interval is characterized by a predominantly mud dominated deltaic depositional system where rocks with petroleum reservoir potential are expected both in delta front and channelized sandstone deposits recent drilling campaigns into time equivalent rocks in the barents sea have however been disappointing with porosity and permeability below expectation this study improves the current understanding of reservoir potential within depositional systems of this kind by examining the link between different depositional facies diagenesis and their impact on reservoir quality five depositional facies were mapped and correlated channel floodplain shallow marine prodelta and offshore our study suggests that diagenetic signatures that control the quality of reservoir rocks vary systematically with these depositional facies mechanical compaction was the main cause of porosity destruction in the channel shallow marine and floodplain samples while early carbonate cementation occludes the intergranular volume in the prodelta samples moreover quartz cement reduced the porosity in the shallow marine depositional facies the estimated maximum burial temperature of 124 c indicates burial to several km depths depositional settings within a deltaic to shallow marine system do not exert a strong control on the abundance of chlorite but it does impact whether the chlorite is pore filling or grain coating and porosity preserving efficient chlorite coating have aided in preserving porosities up to 32 in the very well well sorted medium to fine grained sandstones deposited by fluvial dominated channels

distribution and evolution of tricyclic terpanes in lacustrine carbonates abstract abundant and peculiar pairs of c20 to c25 tricyclic terpanes were discovered in the tertiary lacustrine carbonates of the biyang depression though all organic matter contained in the entire carbonate section are of type i kerogen the distribution of the tricyclic terpanes varies regularly with the burial depth of samples the relative concentration of 13 h 14 h isomers in the paired tricyclic terpanes declines sharply with increasing burial depth on the basis of this variation new tricyclic terpane maturity parameters for immature and early mature carbonate source rocks are proposed

simulation of potassium feldspar dissolution and illitization in the statfjord formation north sea prior work has suggested that potassium mobility is an important control on the formation of late stage illite in deep reservoirs of the north sea we developed a one dimensional finite difference reactive transport model to simulate the concurrent dissolution of k feldspar and illitization of kaolinite in statfjord formation reservoirs the kinetics of k feldspar dissolution is set to be faster than diffusion consistent with the observed behavior and the proposition that the reaction was transport limited k feldspar dissolution produced dissolved potassium and silica that diffused and then precipitated to form illite and quartz areas that had higher initial reactant k feldspar produced higher solute concentrations and formed chemical gradients having areas that had lower initial feldspar content in typical north sea cases this means mass transfer would be from sandstone to shale the diffusive transport moved significant amounts of potassium and silica over distances of several meters and created observable diagenetic alteration simulations of an oil filled reservoir showed that oil emplacement did not stop illitization but did limit potassium mobility and maximized illite formation within the sandstone preservation of preillitization permeability was favored in water filled reservoirs that had slower illite formation where diffusion could export potassium from the sandstone before precipitation similar patterns of transport and precipitation were predicted for silica from k feldspar dissolution the results of the model showed good agreement with field data from other north sea reservoirs

relationship between diagenesis and reservoir quality in sandstones of the garn formation haltenbanken mid norwegian continental shelf a quantitative model is proposed for the dependence of porosity and permeability on diagenesis in subarkosic arenites of the middle jurassic garn formation porosity variation is controlled by the combined effects of compaction and quartz cementation in general more porosity has been lost by compaction than by quartz cementation but compactional porosity loss appears to have occurred relatively early in the burial history before burial below 2 km below the sea floor thus progressive reduction of the total porosity with increasing burial below 2 km results mainly from quartz cementation permeability is a function of the abundance of intergranular macroporosity which is interpreted to be a measure of effective porosity at depths less than 3 5 km below the sea fl or intergranular macroporosity generally comprises more than 50 percent of the total porosity but at greater depth as total porosity decreases below about 16 percent the ratio of effective to ineffective porosity drops below one this depth corresponds to the onset of extensive illitization and grain dissolution which results in major reorganization of the pore system generation of secondary porosity by feldspar dissolution appears not to have been beneficial to reservoir quality in the garn formation but it is regarded as an intrinsic part of the overall process of porosity destruction with increasing diagenesis

the structure of the rockall trough imaged by deep seismic reflection profiling westline is a 450 km long near normal incidence seismic reflection profile crossing the rockall trough from the rockall bank to the irish coast beneath rockall bank and the irish shelf unreflective upper crust overlies reflective lower crust and upper mantle crustal reflectors beneath the irish shelf are interpreted as caledonian the nw slope of the trough is dissected by a series of east dipping faults forming small sediment filled half grabens the steep se slope is formed by the erris ridge the floor of the trough is formed from rotated blocks separated by mainly east dipping faults wedges of sediment fill half grabens formed by this faulting there is no evidence for oceanic crust or a 9reflection moho9 within the trough the floor and slopes of the trough are blanketed by c 4 km of parallel bedded sediments the lower part of the parallel bedded sequence is intruded by sills and dykes that do not extend above a minor unconformity it is suggested that the bulk of the sediment preserved in the rockall trough accumulated during and following a major rifting event in the early or mid cretaceous

clay coating preserving high porosities in deeply buried intervals of the sto formation abstract the sto formation is the most important reservoir interval in the norwegian barents sea however the reservoir quality can be highly affected by the detrimental effects of quartz cement where there have been extensive post depositional burial core plug data from well 7219 8 2 in the southwestern barents sea shows abnormally high porosity and permeability values in certain units of the deeply buried and otherwise highly quartz cemented sto formation the amount of quartz cement in the samples is inversely proportional to the porosity samples with high and low porosities are similar texturally and mineralogically but the high porosity samples have a layer of illitic clay coating the majority of the detrital quartz grains illitic clay coating present at grain contacts can result in a lowered igv given they aid in the dissolution of quartz at interfaces also creating a source of dissolved silica clay induced dissolution means that silica saturation is not a limiting factor in quartz cementation in these samples the results show that the illitic clay coating is capable of limiting the amount of authigenic quartz overgrowth from 20 to 23 in samples with negligible grain coating to 5 11 in the intervals with high coating coverage the illitic clay coating inhibits quartz overgrowth by limiting the surface area available for nucleation on detrital grains the sto formation comprises mainly shallow marine deposits of highly reworked clean sandstone abnormally high porosities appear to be linked to settings where sediments of a more proximal location are preserved without extensive reworking the grain coating clay is illitic and most likely originates from clay infiltration processes prior to final deposition the difference in extent of clay coating in similar facies can mostly be correlated with varying amount of post depositional reworking this study suggests that there is a potential for considerable porosity and permeability to be preserved in deeply buried sandstones in the barents sea this study could be important in the future exploration activity of deeply buried structures in the area

newly proposed methyltricyclic and methyltetracyclic terpane series in the shuicheng boghead coal china abstract two newly proposed series of methyltricyclic and methyltetracyclic terpanes have been detected and tentatively identified from an upper permian boghead coal in the dahebian mine shuicheng mining district guizhou province southwest china based on gc ms analyses and mass spectrum interpretation the boghead coal is characterized by microbial biomarkers which account for 52 4 of the aliphatic hydrocarbon fraction in the coal sample with higher plant biomarkers amounting to 32 2 of the same fraction on this basis a microbial source is considered to be the most likely source of both the methyltricyclic and methyltetracyclic terpane series

artificial maturation of an immature sulfur and organic matter rich limestone from the ghareb formation jordan abstract an immature r o 0 39 s rich s org c 0 07 organic matter rich 19 6 wt toc limestone from the ghareb formation upper cretaceous in jordan was artificially matured by hydrous pyrolysis 200 220 300 c 72 h to study the effect of progressive diagenesis and early catagenesis on the amounts and distributions of hydrocarbons organic sulfur compounds and s rich geomacromolecules the use of internal standards allowed the determination of absolute amounts with increasing thermal maturation large amounts of alkanes and alkylthiophenes with predominantly linear carbon skeletons are generated from the kerogen the alkylthiophene isomer distributions do not change significantly with increasing thermal maturation indicating the applicability of alkylthiophenes as biomarkers at relatively high levels of thermal maturity for a given carbon skeleton the saturated hydrocarbon alkylthiophenes and alkylbenzo b thiophenes are stable forms at relatively high temperatures whereas the alkylsulfides are not stable the large amount of alkylthiophenes produced relative to the alkanes may be explained by the large number of monosulfide links per carbon skeleton these results are in good agreement with those obtained previously for an artificial maturation series of an immature s rich sample from the gessoso solfifera formation

porosity reduction in sandstone by quartz overgrowth a mathematical model for predicting porosity histories due to quartz overgrowth in quartz rich sandstone reservoirs has been developed the model neglects secondary dissolution porosity and other diagenetic processes such as compaction and precipitation of carbonate and clay minerals nevertheless the model is straightforward and easy to use to calculate porosity the calculated porosity corresponds to measured porosity of simple quartz cemented sandstones in 27 wells in north and south america the model is based on fluid flow through initially porous unlithified sands in which the fluid phase is saturated with si oh 4 and is always in equilibrium with quartz as the continuously circulating fluid migrates updip toward the basin edge it cools and quartz precipitates into the pore spaces causing a loss of porosity basinal fluid velocities may be calculated assuming 1 thermal convection and 2 hydrostatic pressure due to recharge at the edge of the basin porosity diagrams relating porosity to geothermal gradient burial rate and depth of burial are compared to thermal maturation models of source rocks fluid flow history and grain size distribution the rate of porosity reduction depends on the following variables in decreasing order of significance buria rate age initial porosity basin size dip angle fluid dynamics initial permeability and geothermal gradient

diagenetic evolution and its impact on reservoir quality of tight sandstones a case study of the triassic chang 6 member ordos basin northwest china abstract the triassic chang 6 member sandstones are important oil reservoirs in the ordos basin of north western china and are actually considered to be typical tight sandstone reservoirs diagenetic alterations play a prominent role in considerably impacting the reservoir quality in this study an integrated approach of thin sections scanning electron microscopy sem x ray diffraction xrd cathodoluminescence cl fluid inclusions laser raman spectrographic analysis and stable isotope analysis was applied to examine the diagenetic history recorded in the chang 6 tight sandstones and investigate the influences of diagenesis on reservoir quality the sandstones consist of fine grained moderated to well sorted arkoses q48 5f40 9r10 6 and lithic arkoses q54 3f29 1r16 6 with an average porosity and permeability of 9 10 and 0 39 md respectively the sandstones have undergone complex diagenetic processes dominant eogenetic features include mechanical compaction leaching of feldspar in meteoric water chlorite coats originating from the transformation of precursor clays and precipitating directly from interstitial water calcites filling intergranular pores as poikilotopic cements and quartz and albite overgrowths on surfaces of grains subsequent mesogenetic events include ankerite cements filling porosity and replacing framework grains extensive dissolution of feldspars and precipitation of kaolinites due to emplacement of organic acids and secondary quartz overgrowths in the late period of mesogenesis early calcite is characterized by 13c 13cpdb ranging from 3 45 to 0 30 mean 2 00 and 18o 18opdb varying from 16 70 to 5 80 mean 10 50 while late ankerite is rich in 12c 13cpdb ranging from 10 71 to 0 80 mean 3 58 and 16o 18opdb varying from 24 30 to 15 80 mean 19 93 moreover calcite hosts single liquid fluid inclusions two phase fluid inclusions are contained in ankerite cements with mean homogenization temperature th of 96 0 c and salinity estimates from 11 47 to 18 96 eq wt nacl these data indicate that the calcite originated from meteoric water and the ankerite was associated with organic matter and recrystallization of the early calcite cements furthermore the metallic ions such as fe2 mg2 and ca2 required for ankerite precipitation were produced through alterations of volcanic fragments feldspars and biotites in comparison compaction has more important role than cementation in the loss of original porosity although cementation exerts enormous influence on reducing porosity quantitatively the compactional porosity loss reaches 54 9 while the cementation porosity loss accounts for up to 42 8 carbonate cements and the other authigenic products quartz and albite overgrowth and kaolinite mostly fill intergranular pores thus reduce porosity chlorite coats have effects in inhibiting compaction and restraining the development of quartz overgrowth thereby protect the intergranular porosity to some extent early secondary porosity was enhanced by leaching and dissolution of feldspar in the eogenetic period however these processes did not increase the net porosity in the mesogenetic stage

geochemical characterization of the upper mississippian goddard formation noble ranch group and related oils in the anadarko basin of oklahoma the woodford shale is anecdotally assumed to be the major source of oil for plays in the anadarko basin including the stacked oil and condensate play in the southcentral oklahoma oil province scoop however there is little published geochemical work to confirm this assumption this study set out to identify a characteristic geochemical fingerprint for the goddard formation another potential source rock in the scoop to determine if the goddard is contributing to oil accumulations within the scoop the upper mississippian goddard formation has been a target for unconventional production in scoop since 2012 published geochemical interpretations for the goddard formation are still limited and this study represents the first detailed oil source rock correlation for the goddard in the scoop area the most characteristic geochemical signature of the goddard extracts is a predominance of tricyclic terpanes relative to the hopanes in the mass to charge ratio 191 chromatogram possibly related to enrichment via thermal stress or a unique source signature additional distinctive signatures were also identified for the triaromatic steroid hydrocarbons sesquiterpenoids and steranes the biomarker fingerprints of scoop oils are nearly identical with those of the goddard extracts in this study oils in the scoop therefore appear to originate from a mississippian source such as the goddard rather than the woodford this observation demonstrates the value of oil source rock correlation studies and suggests that there may be effective source rocks in the anadarko basin that have been overlooked in the past and should be re evaluated in detail

extended ketones of the tricyclic terpane series in a tasmanian tasmanite bitumen abstract the following novel ketones of the extended tricyclic terpane series cleistanthane skeleton were identified in the ketone fraction of a permian tasmanite bitumen from tasmania australia by hrgc ms c and hrgc ms ms c srm c20 the c23 13 14 stereoisomers the c28 and c33 diastereomers r s and r s and higher homologs in lower relative abundance these tricyclic terpane ketones may be derived from oxidation of the side chain of a tricyclooctaisoprene the proposed biological precursor for these series is tricyclooctaprenol c40 derived from cyclization of octaprenol in the tasmanites algae

mixing and biodegradation of hydrocarbons in the daerqi oilfield baiyinchagan depression northern china abstract crude oils in the daerqi oilfield vary greatly in viscosities including normal viscous and highly viscous oils they are also dramatically different in thermal maturity from immature to mature based on biomarker parameters according to bulk and molecular geochemical characteristics crude oils can be classified into three groups in this oilfield group i oils are mature and derived from the deeply buried and relatively old source rocks within the tengger formation and the second member of the arshan formation of early cretaceous age group ii oils are immature to marginally mature and derived from the relatively young source rocks that experienced shallow burial within the first member of the duhongmu formation of earlier cretaceous age group iii oils are mixtures of group i and group ii oils with dominance of the group i oils 70 the viscosity is low for the non biodegraded group i and group iii oils 7 1 24 mpa s but very high for the non biodegraded group ii oils e g 433 104 mpa s for the representative oil da12o1 the group iii oils are similar to the group i oils in gross properties e g gross composition and viscosity but are similar to group ii oils in terms of their molecular maturity parameters the oil reservoirs are currently at a shallow depth

productivity control on oil shale formation mae sot basin thailand abstract neogene oil shales from the freshwater lacustrine intermontane mae sot basin western thailand are examined in order to define a oil source rock potential b variations in molecular and isotopic geochemical characteristics and c the influence on source potential of organic matter productivity and preservation twelve samples spanning 7 m of section were examined using conventional isotopic molecular and organic petrographic methods of geochemical analysis temporal resolution is 2700 yr per sample because thermal maturity differences and fluid migration within the sample set are absent and organic matter type input variation is either minor or absent based on visual kerogen analysis we propose that organic matter differences among the samples are caused by variability in surface water productivity all samples are excellent potential source rocks vr 0 4 0 5 ro containing types i and ii organic matter modified van krevelen plot determination consisting of almost 100 fluorescent amorphous kerogen visual kerogen determination numerous diagnostic molecular markers and characteristics are present including perhydro carotene dammaranes norsterane s and novel distributions of tricyclic terpanes stable carbon isotope ratios of kerogens and extract fractions range over 5 and co vary with hydrogen index this covariance is attributed to temporal increases in surface water productivity leading to co2 limiting conditions and consequent increase in 13c of the organic matter in the photic zone during times of elevated productivity increased amounts of 13c enriched organic matter reached the water sediment interface thus times of greater productivity are accompanied by higher 13c and hydrogen index values with 13c related to productivity under co2 limiting conditions and hi related to the relative amount of the water column that was anoxic during deposition our findings suggest that in petroleum geochemical studies involving lacustrine source rocks 13c values must be used with caution as oil oil and oil source rock correlation parameters

burial dolomitization and porosity development in a mixed carbonate clastic sequence an example from the bowland basin northern england widespread dolomitization and leaching occur in the asbian to brigantian dinantian sequence of the bowland basin within this mudrock dominated succession dolomite is developed in calcarenites and limestone breccia conglomerates deposited in a carbonate slope environment pendleside limestone and also within graded quartz wackes deposited by density currents in a generally starved basin environment pendleside sandstone the dolomitized intervals range in thickness from less than one metre to several tens of metres and have a stratabound nature all stages of calcite cement pre date dolomitization and calcite veins are dolomitized dolomite crystals replace neomorphic spar and may also contain insoluble residues that were concentrated along stylolites thus dolomitization was a late stage process within the carbonate diagenetic sequence a late stage diagenetic origin is also indicated within the sandstones with dolomite post dating the development of quartz overgrowths six main textural styles of dolomite are observed 1 scattered 2 mosaic 3 subhedral to euhedral rhombic 4 microcrystalline 5 single crystal and 6 saddle the style of dolomite developed is dependent on the host rock mineralogy on whether it is space filling or replacive and also on temperature chemically the dolomite varies from near stoichiometric compositions to ankeritic varieties containing up to 20 mole feco3 generally the dolomites have isotopic compositions depleted in 18o compared to the host limestone with similar or lighter 13c values initial dolomite was of the scattered type but with progressive replacement of the host a mosaic dolostone with a sucrosic texture was produced there was a general increase in the fe and mn content and reduction in 18o ratio of the crystals during dolomitization leaching is restricted to partly dolomitized horizons where calcite feldspars micas clays and to some extent dolomite have been leached this has produced biomouldic and vuggy secondary porosity within the carbonates whereas in the sandstones honeycombed corroded and floating grains associated with oversized pores occur porosity within both carbonates and sandstones is reduced by ferroan dolomite ankerite cements field petrographic and chemical characteristics indicate that dolomitizing solutions were predominantly derived from the enclosing mudrocks bowland shales during intermediate deep burial fluid migration out of the mudrocks would have been sided by dehydration reactions and overpressure the fluids migrating along the most permeable horizons the coarse grained carbonates and sandstones that are now dolomitized and contain secondary porosity

prediction of permeability in quartz rich sandstones examples from the norwegian continental shelf and the fontainebleau sandstone abstract correctly predicting sandstone permeability from variables such as porosity composition and texture is one of the major problems of petroleum geology and hydrology using data sets comprising composition texture porosity and permeability for sandstone samples from the norwegian continental shelf and the fontainebleau sandstone we have tested several previously suggested methods for permeability prediction and also suggest a new method for sandstones containing significant volumes of clay our results indicate that permeability is successfully predicted from porosity and mean grain size by the kozeny equation for most sandstones with clay contents less than 3 when porosities exceed 6 14 with the lowest threshold values of porosity tending to occur in the sandstones containing least clay the kozeny equation also seems to enable calculation of the probable maximum possible permeabilities for sandstones with given porosities and grain sizes for clay contents above 3 the kozeny equation typically overpredicts permeability including a percolation threshold in the kozeny equation using estimated macroporosity rather than total porosity or applying a form of the equation where internal surface area is used as input instead of grain size does not provide generally acceptable permeability estimates however a modified kozeny equation which includes a parameter reflecting the type of pore system often provides useful permeability estimates even for clay rich samples this parameter can often be regarded as a constant for a specific formation or sandstone unit when porosity variations are not excessive mercury injection data suggest that its value is controlled by the width and connectivity of the dominant flow paths through the system which in turn are probably determined by factors such as volume and distribution of clay

asphaltene rich residues in sandstone reservoirs of haltenbanken province mid norwegian continental shelf abstract thin zones of asphaltene rich residue are present near the crests of two adjacent oil and gas filled structures in one structure the smorbukk sor field multiple thin 5 50 cm dark coloured zones are observed within the lower 20 m of the 88 m thick garn formation these dark coloured zones tend to occur immediately above stylolitized mica clay laminations and become gradually lighter upwards apparently reflecting gradual upward decrease in residue abundance in the other structure the smorbukk field the lowermost 1 m of the formation is dark coloured and the sandstone gradually becomes lighter in colour upwards for a distance of about 3 m similar but more diffuse residues are also present throughout the oil zone in two other fields nearby organic geochemical analyses show that the residue rich dark coloured zones differ from the sorrounding lighter coloured sandstone mainly in having a higher content of asphaltenes whereas the total content of extractable organic matter is often similar thus the darker rocks might be regarded more as zones of conspicuous oil staining due to asphaltene deposition rather than as small tar mats the residues typically appear to have accumulated under the influence of gravity at points where the downward flow was hindered by a lithological discontinuity present day zonation upward decreasing gas to oil ratio in the oil column of the smorbukk sor reservoir is consistent with a model of asphaltene precipitation in the lower part of the reservoir in response to upward leakage of gas from an underlying gas condensate reservoir in the ile formation this is also consistent with the available organic geochemical data in thin section the asphaltene rich material is seen to preferentially coat feldspar and clay but not quartz surfaces thus illustrating why sandstone wettability should be expected to depend on both mineralogy and the asphaltene content of the petroleum involved

the effect of grain coating microquartz on preservation of reservoir porosity clay coatings have been widely accepted by many workers as an explanation for preserving high porosity in deeply buried sandstones but few workers have realized that similar effects can be produced by microcrystalline quartz coatings this phenomenon can be expected only under special circumstances but in such cases it can have profound consequences for exploration in the central graben area of the southern north sea unusually high porosity 20 27 and permeability 100 1000 md are found in certain zones in upper jurassic sandstones at depths of 3 4 4 4 km the porosity in these zones is 5 15 higher than expected based on average porosity depth trends from brent and haltenbanken sandstones we propose that the high porosity is due to continuous grain coatings of euhedral microcrystalline quartz crystals that are 0 1 2 m thick the distribution of microcrystalline quartz coatings is controlled by the presence of siliceous sponge spicules rhaxella which implies a sedimentological control on the reservoir quality we present a thermodynamic model showing how continuous microcrystalline quartz coatings inhibit development of no mal macrocrystalline quartz overgrowths sourced mainly from stylolites high porosities in parts of various upper jurassic oil fields ula and gyda have previously been explained by inhibition of quartz cementation by early hydrocarbon charge we suggest that the microcrystalline quartz coatings provide a more plausible explanation

pore fluid evolution distribution and water rock interactions of carbonate cements in red bed sandstone reservoirs in the dongying depression china the compositions distribution and its interaction with rocks of the evolving pore fluids controls the distribution of carbonate cements and reservoir storage spaces the reservoir quality of the red bed sandstone reservoirs in the dongying depression was investigated by an integrated and systematic analysis including carbonate cement petrology mineralogy carbon and oxygen isotope ratios and fluid inclusions the investigation was also facilitated by probing the mineral origins precipitation mechanisms pore fluid evolution and distribution and water rock interaction of carbonate cements and their influences on reservoir quality diagenetic evolving fluids in the interbedded mudstones are the main source for the precipitation of calcite cements that completely fill the intergranular volume cfiv calcite with heavier oxygen and carbon isotopes the ferro carbonate cements in the reservoir sandstone are enriched in lighter carbon and oxygen isotopes in addition to the cations released by the conversion of clay minerals in reservoirs products of organic acid decarboxylation and the associated feldspar dissolution process provide important sources for such carbonate cementation the carbon isotopes of co2 and the oxygen isotopic composition of fluids equilibrated with the cfiv calcite ferro calcite dolomite and ankerite cements indicate that the pore in the red bed reservoirs experienced high salinity fluids which evolved from the early formed interbedded mudstones through organic acid input and to organic acid decarboxylation pore fluids from nearby mudstones migrated from the edge to the centre of sandbodies causing strong calcite cementation along the sandbody boundaries and forming tight cementation zones pore fluids associated with organic co2 and acids and organic acid decarboxylation are mainly distributed in the internal portion of sandbodies causing feldspar dissolution and precipitation of ferro carbonate cements the distribution of pore fluids caused the zonal distribution of carbonate cements in sandbodies during different periods this may be advantageous to preserve the porosity of reservoirs as exemplified by the distribution of high quality reservoirs in the red bed sandbodies

a fluid inclusion study of quartz cemented sandstones from offshore mid norway possible evidence for continued quartz cementation during oil emplacement abstract jurassic sandstones from the oil zone in a well from the haltenbanken area offshore mid norway contain both aqueous and hydrocarbon fluid inclusions within quartz overgrowths homogenization temperatures for the inclusions range from 92 c up to the present formation temperature of approximately 118 c since the inclusions mostly are located at the boundaries between quartz clasts and quartz overgrowths the homogenization temperatures indicate that quartz cementation started at temperatures slightly below 92 c poikilotopic ferroan calcite cement usually predates formation of quartz overgrowths and most calcite cement must be precipitated at temperatures below 90 c although in one calcite cemented zone ferroan calcite encloses quartz overgrowths with inclusions trapped at temperatures of up to 105 c none of the examined hydrocarbon inclusions homogenized below 102 c and this suggests that oil started entering the reservoir at temperatures of around 100 c the burial history of the examined sandstones indicates that temperatures of 100 c were not reached before the late pliocene and that filling of the reservoir has taken place during the last few million years homogenization temperatures approximately equal to present formation temperature suggest that quartz cementation may still be in progress and that emplacement of oil did not necessarily stop precipitation of quartz cement possible continued quartz cementation after oil filled the reservoir could be explained by dissolution of quartz at stylolites and grain contacts as transport of dissolved quartz from these sources might possibly take place by diffusion through irreducible water

the effect of stylolite spacing on quartz cementation in the lower jurassic sto formation southern barents sea abstract the shallow marine lower jurassic quartz arenites of the sto formation in the southern barents sea comprise 1 intervals where dispersed detrital clay is absent and where the spacing between clay rich laminae that evolved into stylolites upon burial is exceptionally large up to several meters and 2 intervals where minor detrital clay matrix occurs clay laminae are very common and stylolite spacing is typically less than a centimeter point counting of thin sections and cathodoluminescence micrographs shows that quartz cement contents are far lower in the intervals where stylolite spacing is exceptionally large 4 11 versus 10 20 outside these intervals there is also a correlation between distance to nearest stylolite and volume of quartz cement samples located a centimeter or less from a stylolite contain 10 20 quartz overgrowths for distances of 3 20 cm quartz cement content is 4 10 and only 3 8 when the closest stylolite is more than 20 cm distant modeling of quartz cementation with the exemplartm diagenetic modeling program indicates that the observed trend of decreasing quartz cement abundance outwards from stylolites is not caused by variations in grain size degree of grain coating or content of quartz grains i e the trend is not due to more quartz surface area being available for overgrowth formation close to stylolites on the contrary the modeling suggests that the samples situated more than 20 cm from stylolites contain 5 8 less quartz cement than what would have been the case given a more normal stylolite abundance this study indicates that sandstones with exceptionally few clay rich or micaceous laminae and without clay or mica at individual grain contacts will be significantly less quartz cemented and more porous than other sandstones with similar temperature histories however such sandstones seem to be highly unusual on the norwegian continental shelf this suggests that exceptionally low abundance of stylolite precursors may be of only local importance for preserving reservoir quality at elevated temperatures and that it is normally not necessary to include stylolite spacing and distance to the nearest stylolite as variables in quantitative models of quartz cementation

compaction and porosity evolution of pliocene sandstones ventura basin california sandstones of the pico and repetto formations were studied to quantify the processes involved in compaction and to determine the porosity evolution of the beds during burial the sandstones are mineralogically immature and except for local carbonate cemented beds have insufficient cement to have prevented compaction stratigraphic studies and graphic porosity depth plots suggest that the samples maximum burial depths were approximately 10 000 ft 3 048 m deeper than the present day depths of 850 13 000 ft 259 3 962 m compaction produced closer grain packing as measured by the contact index ci average number of contacts per grain and tight packing index tpi average number of long sutured and embayed contacts per grain for samples with less than 10 cement the ci depth gradient is 0 21 1 000 ft 0 69 1 000 m and tpi depth gradient is 0 27 1 000 ft 0 89 1 000 m assuming sands had 40 initial porosity the sandstones that are at present most deeply buried lost a total absolute porosity of 26 by all compactional processes which includes 14 by grain rearrangement and 6 each by pressure solution and ductile grain deformation porosity loss by nondeformational grain rearrangement was complete by a present day depth of 5 000 ft 1 524 m but in uncemented sandstones pressure solution plus associated grain rearrangement and ductile grain deformation continued to the maximum depths sampled compaction is most abrupt in sands with abundant micas and ductile rock fragments the ci gradient is 0 14 1 000 ft 0 46 1 000 m for samples with ess than 10 ductile grains but 0 22 1 000 ft 0 72 1 000 m for samples with more than 10 ductile grains compaction was almost entirely the result of overburden pressure and was not noticeably retarded in overpressured zones suggesting that overpressures developed after maximum burial was achieved for samples with less than 10 cement the ci is a better depth predictor than the tpi r2 0 58 vs 0 45 but the tpi is a better predictor of pre cement porosity than the ci r2 0 54 vs 0 31 secondary porosity averages 20 40 of the total thin section porosity secondary porosity is present at all depths but makes up the greatest percentage of total porosity between present depths of 6 000 and 9 500 ft 1 829 and 2 896 m except for grain fracturing secondary porosity formed only by dissolution of framework grains fractured grain porosity contributes up to 29 of total porosity 5 absolute porosity in some samples fractures formed during pleistocene tectonic episodes

new evidence for evaporitic middle devonian lacustrine sediments with hydrocarbon source potential on the east shetland platform north sea the ukcs united kingdom continental shelf well 9 16 3 drilled on the western flank of the beryl embayment encountered a middle devonian sequence interpreted as part of the lacustrine orcadian basin an anhydrite layer found within the sequence provides confirmation of the local development of hypersaline environments within the orcadian basin detailed geochemical analysis of a representative source interval equivalent to the achanarras sandwick fish bed has revealed a close affinity with contemporaneous middle devonian source rocks found in the inner moray firth and crude oil from the beatrice oil field

evolution of tricyclic alkanes in the espirito santo basin brazil abstract the distributions of tricyclic isoprenoid alkanes were characterized in the neocomian sequence from the espirito santo basin brazil by high resolution gas chromatography mass spectrometry and co injection with synthetic standards where available a suite of homologous components ranging from c 20 to c 28 were present throughout the sequence as a mixture of stereoisomers whose relative abundance changed with depth the epimerization rates were similar for the various tricyclic compounds irrespective of side chain length achieving equilibrium ratios before the main stage of oil generation this new maturity parameter should be useful as a supplement to sterane hopane measurements

the structure and subsidence of rockall trough from two ship seismic experiments coincident multichannel seismic reflection and refraction profiles acquired northwest of britain during a two ship seismic experiment in the southern rockall trough provide evidence for a thin 6 km syn rift crust below rockall trough the variation of seismic velocity with depth is consistent with either oceanic crust similar to that found in other parts of the north atlantic or with thinned continental crust heavily intruded by syn rift igneous rocks the syn rift crust beneath rockall trough is now buried by nearly 5 km of sediments that are intruded by tertiary volcanics varying in extent from isolated sills to a large sill complex extending 100 km laterally the basement imaged on seismic profiles exhibits a band of subhorizontal reflectors that extend laterally over distances of up to 40 km which they interpret as sediments intercalated with submarine lava flow generated during the opening of the trough estimates of stretching from crustal thinning and from subsidence indicate that rockall trough has undergone extensive rifting beta 6 upwelling asthenosphere beneath the thinned lithosphere generated at least 1 3 km thickness of melt as it decompressed the molten rock rose upward from the mantle until it was in part extruded as lava flows and in partmore intruded into the crust less

hydrocarbon biomarkers thermal maturity and depositional setting of tasmanite oil shales from tasmania australia this study represents the first geological and organic geochemical investigation of samples of tasmanite oil shale representing different thermal maturities from three separate locations in tasmania australia the most abundant aliphatic hydrocarbon in the immature oil shale from latrobe is a c19 tricyclic alkane whereas in the more mature samples from oonah and douglas river low molecular weight n alkanes dominate the extractable hydrocarbon distribution the aromatic hydrocarbons are predominantly derivatives of tricyclic compounds with 1 2 8 trimethylphenanthrene increasing in relative abundance with increasing maturity geological and geochemical evidence suggests that the sediments were deposited in a marine environment of high latitude with associated cold waters and seasonal seaice it is proposed that the organism contributing the bulk of the kerogen tasmanites occupied an environmental niche similar to that of modern sea ice diatoms and that bloom conditions coupled with physical isolation from atmospheric co2 led to the distinctive isotopically heavy 13c values 13 5 to 11 7 for the kerogen 13c data from modern sea ice diatoms 7 supports this hypothesis isotopic analysis of n alkanes in the bitumen 13 5 to 31 suggest a multiple source from bacteria and algae on the other hand the n alkanes generated from closed system pyrolysis of the kerogen 15 are mainly derived from the preserved tasmanites biopolymer algaenan the tricyclic compounds mean 8 both in the bitumen and pyrolysate have a common precursor they are consistently enriched in 13c compared with the kerogen and probably have a different source from the n alkanes the identification of a location where the maturity of the tasmanite oil shale approaches the oil window raises the possibility that it may be a viable petroleum source rock

petrophysical heterogeneity in a lower cretaceous limestone reservoir onshore abu dhabi united arab emirates the upper zone of the lower cretaceous kharaib formation 151 177 ft 46 54 m thick in the studied wells is a major oil reservoir in several giant oil fields wide variations in porosity and permeability of this zone have been shown to result from both the inhibition of burial cementation by oil in the crest of each field and localized cementation adjacent to stylolites combined with the more subtle influence of widely varying depositional mud content and grain size the present study examines these relationships in closer detail using core and petrographic observations from two wells on the oil filled crest and two wells on the water filled flanks of a giant domal oil field although porosities are higher overall in the crestal cores each well shows wide variations within each of seven main groupings of the samples by depositional texture this heterogeneity results mainly from the distribution of clay which is concentrated along depositional laminations and causes widely varying porosity losses in all textures by promoting stylolite development and associated calcite cementation higher clay abundance and lower porosity within the upper and lower 12 17 ft 4 5 m of the reservoir reflects increased influx of siliciclastic fines across the epeiric barremian carbonate platform immediately following and preceding respectively third order falls in global sea level most 95 of porosity permeability data from the studied wells lie within lucia rock fabric class 3 showing distinct but relatively subtle differences between texture groups whereas a subordinate part of the data from the upper relatively mud poor third of the reservoir plot at higher permeabilities development of a predictive model for the petrophysical heterogeneity of this example requires a combination of the following 1 a diagenetic model for porosity controls 2 the use of a modestly higher porosity permeability transform upper class 3 in the upper part of the reservoir than in the lower reservoir lower class 3 and 3 a recognition of the scattered and widely varying occurrences of exceptionally high permeabilities in the upper reservoir

the petroleum geochemistry of the termit basin eastern niger abstract sixty crude oils from the termit basin eastern niger were analysed using biomarker distributions and bulk stable carbon isotopic compositions comprehensive oil to oil correlation indicates that there are two distinct families in the termit basin the majority of the oils are geochemically similar and characterized by low pr ph pristane to phytane ratios and high gammacerane c 30 hopane ratios small amounts of c 24 tetracyclic terpanes but abundant c 23 tricyclic terpane and lower 13 c values for saturated and aromatic hydrocarbon fractions all of these geochemical characteristics indicate possible marine sources with saline and reducing depositional environments in contrast oils from well dd 1 have different geochemical features they are characterized by relatively higher pr ph and lower gammacerane c 30 hopane ratios higher amounts of c 24 tetracyclic terpane but a low content of c 23 tricyclic terpane and relatively higher 13 c values for saturated and aromatic hydrocarbon fractions these geochemical signatures indicate possible lacustrine sources deposited under freshwater suboxic oxic conditions this oil family also has a unique biomarker signature in that there are large amounts of c 30 4 methylsteranes indicating a freshwater lacustrine depositional environment the maturity of the termit oils is assessed using a number of maturity indicators based on biomarkers alkyl naphthalenes alkyl phenanthrenes and alkyl dibenzothiophenes all parameters indicate that all of the oils are generated by source rocks within the main phase of the oil generation stage with equivalent vitrinite reflectance of 0 58 0 87

mesozoic rocks from the hebrides shelf and implications for hydrocarbon prospectivity in the northern rockall trough abstract shallow boreholes drilled in the west lewis and west flannan basins outer hebrides shelf have proved rocks of probable latest bajocian bathonian to barremian age where previously a mainly permo triassic infill has been assumed the full geological succession of the basins remains unproved although the permo triassic history of the basins may be similar the rocks encountered suggest that the basins may have dissimilar geological histories during the jurassic and early cretaceous lithologies are mainly fine grained poorly sorted sandstones or organic rich mudstones some correlation can be made with the isle of skye and the west shetland area the sandstones are poor hydrocarbon reservoirs but the middle jurassic and ryazanian mudstones although immature where drilled have excellent oil source rock potential and may have generated large amounts of hydrocarbons if they are present at depth in the adjacent northern rockall trough

unusual carbon isotope compositions of biomarker hydrocarbons in a permian tasmanite abstract the permian oil shale sample of this study is from a deposit in tasmania in which the only recognizable structures are large thick walled unicellular specimens of the green alga tasmanites the fossils are so densely packed that this shale is termed tasmanite the carbon isotopic composition of bulk kerogen carbon c 13 c org 16 6 vs pdb is unusually enriched in 13 c compared to marine permian organic carbon 25 to 30 this 13 c enrichment suggests specific environmental conditions co 2 draw down and or physiological specialties cell size and growth rate of tasmanites which in modern environments are known to cause 13 c enrichment in marine phytoplankton isotope signatures of extractable organic species unlike in normal immature oil shales are considerably enriched in 13 c compared to the kerogen the bitumen 10 9 is composed predominantly of extended saturated c 18 c 25 tricyclic terpane hydrocarbons 9 9 to 12 2 total range with an average of 10 8 these tricyclics are not only found as free lipids in the extract but also in pyrolysates with similar isotopic signatures and are therefore likely biosynthesis products of tasmanites the monoaromatic and diaromatic tricyclic terpane hydrocarbons c 17 c 19 have similar isotopic signatures 9 3 to 12 8 total range with an average of 10 6 two monoaromatic tetracyclic hydrocarbons c 23 and c 24 a quantitatively minor but biogenetically interesting group average at 10 2 in contrast pristane and phytane as well as n alkanes 18 to 22 are depleted in 13 c compared to kerogen carbon the isotopic similarity of all the cyclic terpanes in this sample together with their occurrence in the free lipids and kerogen suggests that they have a common origin and are biosynthesis products of the marine unicellular green algae tasmanites the tetracyclic terpanes therefore are not des a oleananes derived from land plants but are possibly monoaromatic des a gammaceranes or other des a triterpenoids derived from marine sources the unusual enrichment in 13 c in the cyclic hydrocarbons is hypothesized to result from the special growth conditions of the algae isoprenoids and n alkanes are also likely biosynthesis products of tasmanites possibly during the spore formation stage when physiological and environmental conditions were different than during planktonic biosynthesis

lithologic characteristics and diagenesis of the devonian jauf sandstone at ghawar field eastern saudi arabia abstract the lower devonian jauf formation in saudi arabia is an important hydrocarbon reservoir however in spite of its importance as a reservoir published studies on the jauf formation more specifically on the reservoir quality including diagenesis are very few this study which is based on core samples from two wells in the ghawar field northeastern saudi arabia reports the lithologic and diagenetic characteristics of this reservoir the jauf reservoir is a fine to medium grained moderate to well sorted quartz arenite the diagenetic processes recognized include compaction cementation calcite clay minerals quartz overgrowths and a minor amount of pyrite and dissolution of the calcite cements and of feldspar grains the widespread occurrences of early calcite cement suggest that the jauf reservoir lost a significant amount of primary porosity at a very early stage of its diagenetic history early calcite cement however prevented the later compaction of the sandstone thus preserving an unfilled part of the primary porosity based on the framework grain cement relationships precipitation of the early calcite cement was either accompanied or followed by the development of part of the pore lining and pore bridging clay cement secondary porosity development occurred due to partial to complete dissolution of early calcite cements and feldspar late calcite cement occurs as isolated patches and has little impact on reservoir quality of the sandstones in addition to calcite several different clay minerals including illite and chlorite occur as pore filling and pore lining cements while the pore filling illite and chlorite resulted in a considerable loss of porosity the pore lining chlorite may have helped in retaining the porosity by preventing the precipitation of syntaxial quartz overgrowths illite which largely occurs as hair like rims around the grains and bridges on the pore throats caused a substantial deterioration to permeability of the reservoir diagenetic history of the jauf formation as established here is expected to help better understanding and exploitation of this reservoir

petroleum migration faults and overpressure part ii case history the haltenbanken petroleum province offshore norway abstract petroleum inclusion and geochemical data from core extracts were applied to deduce a model for oil migration overpressure development and palaeo leakage of oil from currently dry structures in the haltenbanken vest area the existence of fluorescent oil type inclusions in quartz in the smorbukk asgard 2 field suggest that oil migrated into this structure 70 50 million years before present ma bp this is also the case for the dry structures 6506 12 4 6506 11 3 and 6506 11 1 west of the main smorbukk fault zone black oil inclusions with medium gas oil ratio gor occur in these fields together with condensate type petroleum inclusions this suggests that the dry structures transformed from containing oil to condensate before leakage petroleum extracted from inclusions in these structures and in nearby fields have identical marine type ii kerogen signatures source rocks at the spekk formation level in the current drainage area of smorbukk and these dry structures were immature 70 50 ma bp and the smorbukk sor asgard 3 field did not fill at this early time thus oil must initially have entered into smorbukk from areas to the w sw through the currently pressure sealing smorbukk fault zone which today marks the westward limit of the smorbukk field diagenesis in this fault zone caused the much later overpressure development and petroleum was lost from the 6506 12 4 6506 11 3 and 6506 11 1 structures as overpressure built up regionally petroleum loss from these structures with their often thick seals must have occurred via self propagating open fracture induced mechanisms lack of petroleum in the cretaceous strata above these structures suggests that leakage occurred to even shallower strata this could imply that the cretaceous strata in halten vest were overpressured at the time of leakage in contrast the oil in the cretaceous lysing and lange formation above the jurassic reservoirs in smorbukk and smorbukk sor most likely originated based on geochemistry and gors from the jurassic reservoirs below and not from cretaceous strata this migration event would have been facilitated if it occurred before these sands became overpressured as they are today modelling suggests that the spekk formation became mature in the smorbukk sor region

geometric models of porosity reduction by ductile grain compaction and cementation pore volume reduction of sediments by plastic deformation during compaction and by cementation of grains has been evaluated for different proportions of ductile and hard grains we represent the compaction behavior of grains with a purely geometric model which uses the cooperative rearrangement algorithm to produce dense random packings of partly interpenetrating spheres we varied the fraction of grains assumed to be ductile and the radius of the rigid core of the ductile grains the predicted relationship between the fraction of ductile grains in the sediment and the porosity after compaction agrees well with previously published experimental data in the literature the radius of the rigid core of the ductile grains is an effective way to represent different kinds of ductile material ranging from brittle rigid radius 0 9 to extremely ductile rigid radius 0 7 we simulated quartz cementation in our compacted rock by adding isopachous cement cement thickness was reduced on the smaller grains and increased on the larger grains to account for presumed export of pressure dissolved material from finer grained regions and the import of material into coarser grained regions these simulations yield descriptions of pore scale geometry resulting from processes common in sandstones modeled pore geometry provides insight into transport properties of such rocks for example the models predict to within a factor of five the permeability of samples of tight gas sandstones having little intragranular porosity

predicting lithology and transport properties from acoustic velocities based on petrophysical classification of siliciclastics based on the recently developed petrophysical classification of siliciclastics which takes into account the amount of the volumetric clay content c and textural position of clay it is shown that acoustic velocities can be fairly accurate tools in predicting lithology porosity and ultimately transport properties of these rocks four major petrophysical groups of carbonate and organic poor siliciclastics are distinguished 1 clean arenites c 35 percent the compressional velocity versus porosity relation for consolidated rocks in each of these groups is found to be linear with very high correlation coefficients this allows for remarkably accurate porosity estimates or lithology prediction in consolidated siliciclastics from acoustic velocities compared to the widely used time average wyllie equation or its improved modification raymer equations both of which neglect textural factors

kaolin coating of stylolites effect on quartz cementation and general implications for dissolution at mineral interfaces abstract the deeply buried 4 km 160 c quartz rich sandstones of the middle jurassic garn formation in well 6507 7 1 offshore mid norway typically contain 20 30 quartz cement and only 3 9 porosity the exception is a less than 1 5 m thick interval at the top of the formation where quartz cement volumes typically drop to 8 16 and porosities are in the range 7 19 the uppermost part of the garn formation is not isolated from the rest of the formation by any type of impermeable barrier the sandstones are water filled indicating that the change in cementation does not correspond to a hydrocarbon contact and there are no significant changes in grain size grain coating abundance grain mineralogy or other factors that could give a lower quartz surface area for development of quartz overgrowths within the uppermost 1 5 m of the sandstones however in the samples from the less cemented interval illitization of authigenic kaolin is either not detectable or very restricted compared to deeper down in the cores where illitization of kaolin is usually complete we propose that the cause of reduced quartz cementation in the top of the garn formation may be the coating of stylolites developed from primary illitic clay rich and micaceous laminae with authigenic kaolin as quartz cementation proceeds and the authigenic kaolin rich rock adjacent to the stylolites is dissolved although we do not have evidence indicating that this may be a common phenomenon of major importance for preservation of reservoir quality our observations may help to unravel the mechanisms of quartz dissolution at stylolites specifically we suggest that the lack of significant surface charge on kaolin compared to illite and mica may lead to a reduced thermodynamic drive for quartz dissolution at kaolin quartz contacts similarly lack of dissolution at clean quartz quartz surfaces may be due to smaller and equal surface charges setting up oppositely directed electric fields that cancel each other whereas the positive surface charge of carbonate minerals may actually enhance the electric fields from negatively charged illitic clay and mica which may explain the extensive dissolution often seen at stylolites in limestones

porosity permeability relationships in miocene carbonate platforms and slopes seaward of the great barrier reef australia odp leg 194 marion plateau this paper reports a series of 700 porosity permeability analyses and supporting petrographic and sedimentologic descriptions from early to late miocene carbonate strata cored on the marion plateau offshore from north eastern australia during ocean drilling program odp leg 194 the samples analysed are not only mainly coarse bioclastic limestones and dolomitized equivalents from platform top facies but also include 79 plugs from deeper water slope to hemipelagic drift facies outstanding characteristics of this data set are the wide ranges of porosity and permeability in both limestones and dolostones the large degree of short range heterogeneity typical of these strata and the better porosity permeability correlation of dolostones than limestones the platforms have experienced widely varying calcite cementation dolomitization and dissolution but show little clear evidence of meteoric diagenesis suggesting that subaerial exposure may have played little role in porosity permeability evolution permeability for given porosity is controlled by grain size and calcite cement content in grainstones and by occurrence of larger shelter pores and vugs in mud rich samples dolomitization tends to reduce the variation of permeability for given porosity by recrystallizing mud matrix to form intercrystalline macroporosity that connects vugs and moulds to become integrated with the effective pore system as a result there are no differences in permeability porosity trends for different dolostone textures whether dominated by intercrystalline vuggy or preserved intergranular pore types two platform top sites separated by only 5 km display a major lateral variation in dolostone porosity permeability characteristics within the youngest dolostone units this difference is interpreted as reflecting a relatively windward current facing setting of the site with the overall higher permeability for given porosity site 1199 that led to less muddy depositional facies greater cementation and lesser grain dissolution pore geometry parameters measured by petrographic image analysis confirm that the windward dolostones have pores that are both larger and less intricate than dolostones comprising the more current protected location

compaction model for quartzose sandstones application to the garn formation haltenbanken mid norwegian continental shelf abstract the compaction of quartzose sandstones is described by a macroscopical visco elasto plastic model derived from microscopical considerations this model considers that the reduction of total porosity the relative pore volume is related to the changes in the relative cemented volume and the relative intergrain volume furthermore we consider that the variations of these two petrophysical parameters result from two physical mechanisms mechanical compaction and closed system chemical compaction the model has been calibrated for the middle jurassic garn formation from the haltenbanken area of the mid norwegian continental shelf around 300 data points from 17 wells have been used in this study the viscous parameters have been optimised against the present day data with total porosity data the best fit is obtained for an activation energy between 14 and 16 kj mole and a macroscopical viscosity at 15 c between 39 and 46 gpa ma 1 210 24 1 410 24 pa s if quartz cement data are used the best fit is obtained for an activation energy between 16 and 18 kj mole and a macroscopical viscosity at 15 c between 47 and 56 gpa ma 1 510 24 17 10 24 pa s the visco elasto plastic compaction model has been tested in the haltenbanken area of mid norway with the result that observed porosity trends and pore pressures can be modelled using multiple 2 d basin modelling simulations the results of this model and its implications for fluid flow and petroleum systems analysis can be seen the compaction method has significant impact on the simulated fluid flow and petroleum migration pattern

development of early diagenetic silica and quartz morphologies examples from the siri canyon danish north sea abstract the siri canyon has proved to be a perfect area for investigating various morphologies of diagenetic silica in sandstones the development in silica morphologies can be observed from very shallow 1700 m to increased burial depth 3000 m and increased proximity to the central graben distance from 0 to 65 km hydrocarbons and pore fluids now found in the siri fairway have at least partly originated from the central graben the siri canyon is a submarine canyon system eroded into the uppermost chalk deposits and filled with palaeogene hemipelagic and turbiditic marls and mudstones interbedded with sandstone units deposited from sandy mass flows and sandy turbidites which originated on the stavanger platform several hydrocarbon exploration and production wells have been drilled in the siri canyon seven of which are included in this study nini 3 nini 1 na 2p sofie 1 siri 4 celilie 1a and augusta 1 the reservoir sandstones in these wells all contain authigenic silica of various morphologies identified with a combination of traditional optical microscopy and scanning electron microscopy the silica morphologies in some places are classic and well documented in the literature whereas others at least to our knowledge have never previously been described some of the silica morphologies presented here show gradual transition from one to another and others are stand alone forms without clear relationships to other forms the silica morphologies can be expressed in the following way 1 opal rims characteristic of the initial phase of the silica diagenesis in most sandstone units in the siri canyon thick opal rims characterise the sandstone parts adjacent to the mudstone units in the stine segment of the siri field 2 microquartz quartz crystals with a size of 1 5 m seen as coatings on the opal rims both ordered and random 3 cavity overgrowth found as quartz outgrowths in circular and angular cavities formed by dissolution of early authigenic phases angular cavities in the microquartz coatings origin from dissolution of clinoptilolite possibly with a source in volcanic ash whereas dissolution of lepispheres probably resulted in circular cavities 4 quartz ridges syntaxial quartz overgrowths a few micro millimetre in width in parallel lines defined in this paper 5 quartz mountains syntaxial irregular 5 10 m large quartz crystals defined in this paper developed in the water filled sandstone intervals where fluids from the central graben were introduced possibly together with the hydrocarbon and where the continued growth was not retarded by the presence of hydrocarbons in the pore fluids 6 macroquartz syntaxial quartz overgrowths 20 m seen as continued growths from quartz mountains this takes place in the deeper reservoir sandstones promoted by fluids originating from the central graben and increased burial depth 7 quartz with sutured grain contacts occurring only in the deepest well 3000 m

a model for diagenetic mass transfer between adjacent sandstone and shale abstract there are numerous suggestions that mass transfer between sandstone and shale is an important factor in the diagenesis of sandstone reservoirs however advective transport models for mass transport have not produced results that support these proposals a model is developed that overcomes the problems of advective transport models by assuming that diffusive mass transfer is the principle transport mechanism the process occurs when the rate of dissolution of reactive detrital components exceeds the local rate of diffusive transport at that point in the burial history units with greater amounts of a reactive component will generate more solute per unit time than units with less reactive component the process forms chemical gradients that can rapidly transfer mass equivalent to several vol of rock over distances of at least 5 m the observed transport of potassium and silica between adjacent sandstone and shale in the north sea during late stage illitization was correctly predicted using this conceptual approach the conceptual framework from the illite model can be extended to other diagenetic reactions and suggests that the burial of adjacent sandstone shale units could produce episodic diffusion gradients that transfer mass and form multiple generations of cement in sandstone reservoirs

stability of detrital heavy minerals on the norwegian continental shelf as a function of depth and temperature abstract petrographic analysis of 1931 thin sections from sandstones of pliocene to devonian age on the norwegian continental shelf indicate that the heavy minerals zircon tourmaline rutile apatite and spinel survive to depths exceeding 5000 m and to temperatures of around 200 c without showing signs of intrastratal dissolution on the other hand the maximum depth and temperature of occurrence for amphibole are 841 m and 40 c around 1500 m and 55 c for sphene when 0 5 km of probable uplift is taken into account 2231 m and 92 c for kyanite 2559 m and 95 c for epidote 2810 m and 109 c for staurolite and 4589 m and 175 c for garnet initial signs of garnet dissolution occur at around 2000 m depth and all garnets are partially dissolved at depths greater than 3500 m monazite is present down to 4858 m and 182 c and chloritoid to 3811 m and 149 c it is however uncertain whether this is due to the low number of samples deeper than 4858 m and to the rare occurrence of chloritoid or to intrastratal dissolution the same sequence of heavy mineral stability is found when volume of quartz cement is used as a proxy for time temperature exposure quartz cement is not found in samples containing amphibole or detrital sphene a maximum of a fraction of a percent quartz cement is found in samples containing kyanite up to 1 together with epidote up to 6 with staurolite up to 24 with chloritoid up to 25 with garnet whereas maximum values of 25 35 are present together with zircon tourmaline rutile patite spinel and monazite the sequence of persistence of heavy minerals versus depth found in this study is very similar to results from several previous studies although unstable heavy minerals survive to greater depths in very young and rapidly buried sandstones and there are some differences regarding the stability of sphene and epidote heavy mineral dissolution has created secondary pores and possibly has contributed to precipitation of kaolinite illite quartz and carbonates in the studied sandstones but these reactions are of very limited volumetric importance

nearshore mesozoic basins off nordland norway structure age and sedimentary environment abstract seismic data acquisition along the nordland coast norway has resulted in the discovery of two downfaulted sedimentary basins of mesozoic age in an area of precambrian lower palaeozoic metamorphic rocks the stabbfjorden basin informal name west of meloya has a sedimentary rock succession that is up to 800 m thick while the lyngvaerfjorden basin informal name northeast of traena is at least 350 m deep microfossil dating of ice transported erratic blocks thought to be eroded from these basins and found on nearby skerries range in age from barremian to triassic the majority of the samples are of middle late jurassic age the sedimentary blocks comprise conglomerates sandstones siltstones and mudstones and frequently contain shells and coal fragments lithofacies analyses show that they reflect shallow marine deposition but it is possible that the oldest samples were deposited in a continental environment the lithofacies reflect the well known tectonic phases and sea level changes that have been reported for the middle jurassic early cretaceous in the nordland offshore region vitrinite reflectance measurements show that organic material in the samples can be classified as immature and the newly discovered basins have no hydrocarbon exploration potential

thermogenetic degradation of early zeolite cement an important process for generating anomalously high porosity and permeability in deeply buried sandstone reservoirs abstract diagenetic explanations to anomalously high porosities and permeabilities in deeply buried sandstones have been presented for many reservoirs for example grain coatings of microquartz or chlorite can preserve porosity by preventing succeeding quartz overgrowths and dissolution of grains or cement can generate secondary porosity anomalously high porosity and permeability up to 25 and 500 md respectively characterize the deeply buried 5 km upper jurassic shoreface sandstones of the gert member heno formation in the hejre field in the danish central graben optical cathodoluminescence and scanning electron microscopy were used to characterize the petrography and diagenetic changes that resulted in this high pressure and high temperature hpht reservoir this was supplemented by chemical analyses icp oes and icp ms mineralogical composition xrd and oxygen isotope composition of quartz and calcite cement sims the petrographic investigation shows that none of the known explanations for anomalously high porosity and permeability can be applied hence a new hypothesis for deeply buried sandstones in contact with lavas and volcaniclastic lithologies is proposed the new hypothesis includes reducing mechanical compaction due to early patchy zeolite cement which was sourced from adjacent lavas and volcaniclastic rocks locally stylolites developed where zeolite cement was less effective hence the present overpressure formed at a later stage increased temperatures during burial resulted in thermal instability of the zeolite cement which was dissolved and authigenic k feldspar and quartz precipitated instead this resulted in a reduction of the cement volume of up to 70 if the zeolite was k heulandite and a corresponding relative porosity increase thus a reduction in cement volume was generated by replacing zeolite of an open crystal structure with mineral phases k feldspar and quartz of denser crystal structures the thermogenetic breakdown of zeolite was forced by increased temperatures during burial this process may explain previously enigmatic highly porous and permeable deeply buried sandstone reservoirs

controls of diagenetic alteration on the reservoir quality of tight sandstone reservoirs in the triassic yanchang formation of the ordos basin china abstract the ordos basin is rich in tight oil resources and the upper triassic yanchang formation tight oil reservoir is one of the most important exploration targets in the study area the chang 63 tight sandstone reservoir exhibits lacustrine deep water lobe shaped delta deposits with low porosity low permeability and a complex diagenetic alteration history the reservoir quality is extremely heterogeneous because of the heterogeneity of the depositional lithofacies and diagenetic alterations this research integrates multiple techniques to investigate the depositional lithofacies the characteristics and distributions of various diagenetic alterations and their controls on reservoir quality four lithofacies are identified and the various lithofacies have different diagenetic characteristics and diverse reservoir qualities the chang 63 tight sandstone reservoir has experienced significant diagenetic alteration mechanical compaction and authigenic illite are the most important causes of the decrease in reservoir quality carbonate cementation preferentially occurs in sandstones of lithofacies i and ii near the mudstone sandstone contact surfaces sandstones within 1 m of adjacent mudstone sandstone contact surfaces are usually tightly cemented by carbonate cement illite can be divided into two types according to its morphology both types of illite lead to a decrease in reservoir quality ultimately by combining the relationship between depositional facies and lithofacies with the distribution of different diagenetic processes five evolutionary patterns are identified for reservoir quality in the lacustrine deep water lobe shaped delta deposits can be summarized by ratings of best moderate relatively poor poor and worst

a physical model of cementation and its effects on single phase permeability in this paper we present a mathematical model to quantify the effects of cements on the single phase permeability estimate of clastic rocks the model represents pore filling pore lining and pore bridging cements by treating the cement particles as rectangular crystals attached radially to detrital grains these clusters influence permeability through an increase in effective specific surface area and tortuosity because the model is based on physical principles it is largely devoid of empiricism and should be applicable to a wide range of rock types permeability estimates by our model match experimental data from two sets of sandstone samples of five hydrocarbon bearing formations in the north sea the model also confirms the well known result that the effect of pore bridging cement is a factor of ten greater in reducing the single phase permeability compared to pore lining and pore filling cements

occurrence of four stereoisomeric tricyclic terpane series in immature brazilian shales we report the presence in an immature brazilian oil shale irati of c19 c26 tricyclic triterpanes present as mixtures of stereoisomers the c20 components have been identified by comparison with synthetic standards as 13 h 14 h 13 h 14 h 13 h 14 h and 13 h 14 h isomers by extrapolation all fourc21 homologues and the c23 c26 14 h and 13 h 14 h isomers are also shown to occur the geochemical significance of these findings is discussed

reservoir geology of fangst group middle jurassic heidrun field offshore mid norway heidrun field the largest oil discovery in the mid norway area known as haltenbanken contains an estimated 755 million recoverable bbl of oil and more than 1 tcf of gas the reserves are contained mostly in two clastic sequences the middle jurassic fangst group described in this paper and the lower jurassic tilje formation highly favorable reservoir characteristics in the fangst will result in the bulk of the production coming from that unit the fangst tested at rates of up to 31 million standard ft3 of gas 890 000 m3 per day and 15 491 bopd in the discovery and subsequent appraisal wells the fangst sandstones were deposited during a lowstand of global sea level also marked by deposition of the brent group in the north sea the fangst group can be divided into four clearly recognizable stratigraphic units that serve as basic reservoir units for modeling purposes in ascending stratigraphic order these are f1 a coarsening upward sequence deposited on a tidally dominated shallow marine shelf f2 a coarsening upward sequence deposited on a wave dominated shallow marine shelf that culminated in upper shoreface deposition f3 a coarsening upward sequence deposited in deeper water on a marine shelf and f4 a fluvial sequence the fangst group consists almost entirely of sandstone with only a thin shale occurring at the base of f3 fangst sandstones are largely unaffected by diagenesis sandstone permeability is largely controlled by depositional factors such as grain size and detrital clay content porosities and permeabilities are extremely high porosities commonly exceed 30 and permeabilities exceed 500 md

the effect of oil emplacement on quartz cementation in a deeply buried sandstone reservoir quartz is an important porosity occluding cement in sandstone reservoirs that have been subjected to elevated temperature 80 c to 100 c for a substantial period of time the effect of oil emplacement on quartz cementation in reservoir sandstones is controversial some studies have concluded that early oil emplacement can inhibit quartz cementation leading to the preservation of porosity whereas other studies have concluded that quartz cementation appears largely unaffected by oil emplacement here we have studied shallow marine upper jurassic sandstones from ula field norwegian north sea with reservoir temperatures of approximately 150 c to determine whether oil emplacement had a significant impact on diagenesis with particular attention to quartz cementation following sedimentological description of cores samples above and below the oil water contact have been collected adjacent to core analysis plug points these samples then underwent a series of studies including petrographic point counting with a transmitted light microscope scanning electron microscopy sem backscattered electron microscopy sem cathodoluminescence microscopy and fluid inclusion studies these data were integrated with routine core analysis and petrophysical log data density and resistivity log data have been used to determine the precise oil saturation of each sample studied the distributions of all potential controls on porosity and permeability such as grain size sorting matrix clay content and degree of bioturbation the presence of grain coatings and dolomite cement and the amount of quartz cement have been assessed the presence of primary oil inclusions within quartz cement shows that oil ingress into the ula reservoir commenced prior to the onset of quartz cementation very fine grained matrix rich bioturbated microquartz cemented sandstones have uniformly low quartz cement contents irrespective of oil saturation medium grained graded matrix poor microquartz poor sandstones have quartz cement ranging from 1 to greater than 17 associated with core porosities of approximately 22 and 7 respectively higher oil saturations equate to higher porosities and permeabilities in the medium grained graded matrix poor microquartz poor sandstones which cannot be explained by any control other than the amount of quartz cement as a function of pore fluid type oil emplacement therefore appears to have inhibited quartz cementation at high oil saturations and can be viewed as an important control on reservoir quality the significance of this study is that the presence of oil in a sandstone reservoir at the time that quartz cement was growing can have a considerable impact on reservoir quality models that seek to predict quartz cement and reservoir quality in sandstones need to account for the timing of oil emplacement compared with other diagenetic processes

diagenetic albitization of detrital k feldspar in jurassic lower cretaceous and tertiary clastic reservoir rocks from offshore norway i textures and origin abstract the origin of albitized grains in sandstones has been the topic of considerable discussion in recent years some workers believe that the albitization of feldspar can occur in situ during burial diagenesis but others argue that albitization is a feature inherited from source clastic grains our petrological and chemical studies of subsurface samples of the jurassic lower cretaceous and tertiary reservoir sandstones from offshore norway show that detrital grains of potassium feldspar have been albitized during burial diagenesis criteria suggesting that the albitization has occurred during burial diagenesis include 1 euhedral habit of albite crystals with sharp edges and comers and markedly smooth crystal faces 2 generally untwinned albitized grains mostly riddled with minute br wnish inclusions 3 lack of cathodoluminescence in albite 4 homogeneous and pure albite composition 99 ab 5 absence of albitized grains in carbonate cemented zones and 6 increase in the percentage and degree partial to complete of albitized k feldspar with depth under sem albitized grains display two types of fabrics type i albitized grains are composed of numerous tiny albite crystals 1 30 m growing along cleavage planes in parent grains and they have abundant dissolution porosity while type ii albitized grains show blocky albite crystals 5 80 m forming pseudomorphs of detrital k feldspar and they lack any dissolution porosity type i albitized grains are found commonly at shallower depths between 2 2 and 3 km and temperatures varying from 65 to 90 c whereas type ii albitized grains dominate at greater depths 3 5 km and at temperatures 90 c albitization proceeds by a dissolution precipitation mechanism at shallower depth dissolution of the parent k feldspar is more rapid than the precipitation of albite and rates of albite precipitation increase with depth favoring pseudomorphic replacement of k feldspars other factors that influence albitization processes are ana ak permeability reduction and surface dissolution kinetics of the parent grain

compaction microstructures in quartz grains and quartz cement in deeply buried reservoir sandstones using combined petrography and ebsd analysis abstract sandstone diagenesis involves porosity loss by mechanical and chemical compaction in this study we have examined relations of diagenesis and microstructures in quartz grains and quartz cement in sandstones from relatively deep burial depths offshore mid norway electron backscatter diffraction analysis ebsd has been combined with optical and cathodoluminescence petrography study of samples with different degrees of compaction and quartz cementation quartz cement is shown to be syntaxial to the nearest host domain grain or subgrain both for monogranular polygranular undeformed and deformed quartz the quartz cement growth may have been initiated at different subgrain surfaces and preserving pre depositional deformation structures ebsd inverse pole figure imaging shows dauphine twins to be common in all samples both in quartz grains and quartz cement the dauphine twins appear in grain grain contacts and in cement crystal boundaries and commonly crossing grain cement boundaries on basis of twin distributions we suggest that both inherited twins from the source area and twins formed by compaction induced grain boundary deformation are present a general problem in sedimentary rocks is to distinguish inherited from sedimentary compaction induced intra grain deformation structures this study shows that the role of ductile deformation mechanisms e g twinning in sediment compaction can be clarified by more comprehensive following up studies using ebsd analysis in combination with other techniques

porosity depth trends in reservoir sandstones theoretical models related to jurassic sandstones offshore norway abstract theoretical models of compaction and porosity modification in sandstones during burial are discussed theory predicts that mechanical compaction is the dominant porosity reducing mechanism during burial from 0 to 2 5 3 km porosity depth trends are at this stage mainly controlled by the stability of the grain framework and the effective stress time and temperature also influence the porosity depth gradients at shallow to moderate depths but are probably less important than effective stress and framework grain composition at greater burial depths below 2 5 3 km chemical compaction including pressure solution can explain most of the porosity reduction in well sorted clean arenites theoretical modelling of the intergranular pressure solution suggests that sandstones may lose porosity rapidly by this mechanism below 2 5 3 km and that factors capable of retarding pressure solution and quartz cementation are required to maintain prospective porosities below 4 km this theory and comparisons with empirical data from jurassic sandstones from offshore norway indicate that high porosity is favoured by 1 overpressures approaching the lithostatic pressure gradient 2 a short residence time below 3 km burial 3 coatings on grains of clay or microcrystalline quartz which prevent quartz dissolution and precipitation and 4 a high content of feldspar and early diagenetic secondary pores early hydrocarbon emplacement may retard quartz cementation and preserve porosity but chemical compaction probably continues after oil emplacement except at very high hydrocarbon saturations