diagenesis and hydrocarbon accumulation brent sandstone jurassic bergen high area north sea diagenesis of brent group sandstones in the bergen high area was studied and related to the history of hydrocarbon maturation and pore fluid environment calcite and kaolinite are low temperature diagenetic precipitates that formed as meteoric pore water entered the brent at the end of brent deposition and later during late cimmerian erosion a major period of feldspar dissolution associated with quartz and illite cementation occurred at huldra field precipitation of diagenetic illite in brent sandstones was closely linked with hydrocarbon maturation in the adjacent viking graben and accumulation of generated fluids in traps at structural highs at huldra field illite k ar ages suggest onset of illite diagenesis in the latest cretaceous initial reservoiring of hydroca bons during the latest paleocene to early eocene 58 ma and filling of the reservoir by early oligocene 38 ma pore water character during illite diagenesis interpreted from oxygen isotope and quartz overgrowth fluid inclusion data was warmer than predicted by burial history analysis and one and a half to two times seawater salinity the incursion of saline isotopically evolved fluids at huldra suggests replacement of paleometeoric water by basin compaction fluid illite k ar ages from veslefrikk field indicate a minimum age of hydrocarbon accumulation at about 30 ma modern pore water chemistry in the brent at veslefrikk is unlike the suggested water chemistry at huldra and indicates that migrating compaction water did not completely displace older fresh connate water this sugge ts that hydrocarbons migrated into veslefrikk as a discrete phase without great volumes of associated compaction water

diagenesis associated with cooling during burial an example from lower cretaceous reservoir sandstones sirt basin libya fluid inclusions in quartz and dolomite cements were used to determine temperature timing origin and function of fluids in the diagenetic evolution of the lower cretaceous upper sarir sandstone sirt basin libya transmitted light backscattered electron and scanning electron microscope sem cathodoluminescence analyses were carried out to determine the paragenesis in quartz overgrowths fluid inclusions along the grain overgrowth boundary yield homogenization temperatures from 130 to 141 degreesc and salinities between seawater and freshwater aqueous fluid inclusions in later quartz overgrowths have homogenization temperatures from 114 to 129 degreesc and salinities between 18 and 21 wt nacl equivalent fluid inclusions in dolomite have variable homogenization temperatures 120 141 degreesc and high salinities 20 22 wt nacl equivalent this trend in homogenization temperatures of fluid inclusions is interpreted as the result of quartz cement precipitation in the presence of hot low salinity connate fluids connected with an increased heat flow related to regional rifting later quartz overgrowths precipitated during regional cooling concurrent with the end of the rifting and after invasion of saline fluids possibly brought in from adjacent areas or refluxed downward during or after times of evaporite deposition this diagenetic history linked to a decreasing heat flow could be common in other aborted rift basins which are important targets for oil exploration

making diagenesis obey thermodynamics and kinetics the case of quartz cementation in sandstones from offshore mid norway calculation of the quantity and distribution of quartz cement as a function of time and temperature depth in quartzose sandstones is performed using a coupled dissolution diffusional transport precipitation model this model is based on the assumptions that the source of the silica cement is quartz surfaces adjoining mica and or clay grains at stylolite interfaces within the sandstones and the quantity of silica transport into and out of the sandstone by advecting fluids is negligible integration of the coupled mass transfer transport equations over geologically relevant time frames is performed using the quasi stationary state approximation results of calculations performed using quartz dissolution rate constants and aqueous diffusion coefficients generated from laboratory data are in close agreement with both the overall porosity and the distribution of quartz cement in the middle jurassic garn formation only after optimizing the product of the effective surface area and quartz precipitation rate constants with the field data when quartz precipitation rate constants are fixed to equal corresponding dissolution rate constants the effective surface area required to match field data depends on the choice of laboratory generated quartz rate constant algorithm and ranges from 0 008 cm 1 to 0 34 cm 1 in either case these reactive surface areas are 2 to 4 orders of magnitude lower than that computed using geometric models

synthetic fluid inclusions vii re equilibration of fluid inclusions in quartz during laboratory simulated metamorphic burial and uplift p t conditions inferred from fluid inclusions in metamorphic rocks often disagree with the values predicted from mineral equilibria calculations these observations suggest that inclusions formed during early stages of regional metamorphism continue to re equilibrate during burial and subsequent uplift in response to differential pressure p t conditions accompanying burial and uplift were experimentally simulated by initially forming pure h2o inclusions in quartz at elevated temperatures and pressures and then re equilibrating the inclusions in the presence of a 20 wt nacl solution such that final confining pressures ranged from 5 kbar above to 4 kbar below the initial internal pressure of the inclusions at the temperature of re equilibration in all samples re equilibrated at confining pressures below the internal pressure some inclusions were formed that had compositions of 20 wt nacl and densities in accord with the final p t conditions additionally some inclusions were observed to contain fluids of intermediate salinities between 0 and 20 wt nacl densities of these inclusions were also consistent with formation at the re equilibration p t conditions the remainder of the fluid inclusions observed in these samples contained pure h2o and their homogenization temperatures corresponded to densities intermediate between the initial and final p t conditions in short term experiments 7 days where the initial internal overpressure exceeded 1 kbar no inclusions were found that contained the original density and none were found to have totally re equilibrated instead most h2o inclusions re equilibrated until their internal pressures were between 750 and 1500 bars above the confining pressure regardless of the initial pressure differential in a long term experiment 52 days inclusions re equilibrated at a lower confining pressure than the initial internal pressure displayed homogenization temperatures corresponding to a range in final internal pressures between 0 kbar i e total re equilibration and 1 2 kbar above the confining pressure in experiments where the confining pressure during re equilibration exceeded the initial internal pressure densities of pure h2o inclusions increased to values intermediate between the initial and final p t conditions additionally these inclusions were generally surrounded by a three dimensional halo of smaller inclusions also of intermediate density resulting in a texture similar to that previously ascribed to decrepitation from internal overpressure in extreme cases where confining pressures were 4 5 kbar above the initial pressure the parent inclusion almost completely closed leaving only the three dimensional array of small 5 m inclusions the outline of which may be several times the volume of the original inclusion groups of such inclusions closely resemble textures commonly observed in medium to high grade metamorphic rocks inclusions containing 10 and 42 wt nacl solutions trapped at 600 c and 3 kbar were re equilibrated at 600 c and 1 kbar for 5 days in dry argon to evaluate the importance of h2o diffusion as a mechanism of lowering the inclusion bulk density salinities of re equilibrated inclusions obtained from freezing point depressions and halite dissolution temperatures indicate that original compositions were preserved density changes similar to those previously described were noted in these experiments in inclusions showing no visible microfractures therefore density variations observed in inclusions in this study re equilibrated under rapid deformation conditions are considered to result from a change in the inclusion volume without significant loss of contents by diffusion or leakage

sandstone porosity as a function of thermal maturity sandstone porosity decreases in the subsurface as a power function of thermal maturity phi a m sup b where phi is porosity and m is a measure of thermal maturity representing integrated time temperature history a and b are constants for a given sandstone of homogeneous properties but vary between data sets the commonly observed exponential dependence of sandstone porosity upon depth follows as a special case from this power function relation when temperature increases linearly with depth the consideration of sandstone porosity in terms of time temperature exposure offers advantages in the comparison of porosity data from diverse geologic settings the recognition of unusual porosity within a sandstone sequence and the prediction of porosity ahead of the drill and at times in the geologic past

diagenesis and reservoir quality evolution of fluvial sandstones during progressive burial and uplift evidence from the upper jurassic boipeba member reconcavo basin northeastern brazil the reservoir quality of fluvial sandstones of the upper jurassic boipeba member reconcavo basin northeastern brazil is highly heterogeneous and controlled by eodiagenesis under semiarid climate mesodiagenesis during burial to a depth of 3500 m and telodiagenesis due to local uplift eodiagenesis resulted in mechanical compaction calcite cementation clay infiltration and limited grain dissolution whereas mesodiagenesis resulted in the precipitation of calcite cement and quartz over growths intergranular quartz grain dissolution chloritization and illitization of smectite and albitization of feldspars sandstones continuously buried at maximum burial depths of about 1600 m t 65 c since 125 ma display a relatively greater degree of mesogenetic modifications and on average poorer reservoir quality than sandstones that were buried deeper 2100 m t 75 c prior to uplift but only since 13 ma uplift which affected the sequence along the western border of the basin has resulted in telogenetic dissolution of framework silicates and formation of kaolinite relatively good reservoir quality in the deeply buried sandstones occurs when 1 the grains are coated with a thin layer of chloritized infiltrated smectite 2 there is little or no pseudomatrix and 3 there are widely scattered patches of eogenetic calcite cement that supported the framework of sandstones against compaction

assessment of mesozoic and upper paleozoic source rocks in the south yellow sea basin based on the continuous borehole csdp 2 abstract being lack of core data and measured data although the marine oil gas exploration in the south yellow sea basin has been going on for 57 years no commercial hydrocarbon reservoirs have been found so far the quantitative source rocks evaluation based on geochemical data and fluid inclusions containing hydrocarbon benefits from the completion of borehole csdp 2 in 2016 a continuous coring well of the chinese continental shelf drilling program csdp which indicates that the marine origin oil gas exploration in the south yellow sea basin enters into a new stage as revealed by the well csdp 2 three sets of effective hydrocarbon source rocks in the mesozoic and upper paleozoic have been identified which are distributed in the lower triassic qinglong formation t1q upper permian longtan dalong formation p3l d and middle permian gufeng qixia formation p2g q respectively among them organic rich source rocks mainly concentrate in the lower part of the p3l d and the middle upper part of the p2g q and the p3d mudstone also works as good source rock the p3l d mudstone with high organic abundance is the chief source rock in the south yellow sea basin whose maximum hydrocarbon generating intensity may reaches up to 795 09 104 t km2 and hydrocarbon expulsing intensity remains at the level of 165 03 104 t km2 a noteworthy feature of the p2g q source rocks is the strong vertical heterogeneity with the hydrocarbon expulsing intensity of the upper part being 34 14 104 t km2 while that of the lower part being 18 07 104 t km2 in contrast the organic abundance of argillaceous limestone in the t1q is mainly poor to medium and its hydrocarbon expulsing intensity is the weakest one in the three sets of hydrocarbon source rocks only about 12 7 104 t km2 taking into consideration the joint influence of depositional environment organic matter parent source and biological productivity the overall quality of source rocks in the south yellow sea basin is at the similar level to the lower yangtze subei basin but lower than that of the upper yangtze sichuan basin the most significant oil gas information is that multiple direct oil indications and high value abnormalities of gas logging have been discovered in the well csdp 2 moreover the mesozoic and upper paleozoic marine strata in the south yellow sea basin have experienced three hydrocarbon charging stages indicating a broad prospect of marine oil gas exploration

the timing of quartz cementation in mesozoic sandstones from haltenbanken offshore mid norway fluid inclusion evidence the temperature of quartz cementation in mesozoic sandstones from the haltenbanken area offshore mid norway has been evaluated by micro thermometry of primary aqueous fluid inclusions in quartz overgrowths samples were collected from a wide range of burial depths 1 6 to 4 4 km and large area 7500 km 2 there is a systematic increase in homogenization temperatures t h of approximately 75 c between the shallowest and deepest wells when converted to time using burial histories the t h data indicate that cementation was short lived and started roughly synchronously throughout haltenbanken within the last 5 ma it therefore seems that depth and or temperature alone are not enough to bring about quartz cementation some other factor s must also be involved

diagenetic and porosity evolution of conglomerate sandstones in bayingebi formation of the lower cretaceous chagan sag china mongolia frontier area abstract the fan delta conglomerate sandstones of bayingebi formation including conglomerates and pebbly sandstones in the chagan sag are reservoir for large accumulations of hydrocarbons based on observation of cores thin sections sem and cl combined with application of fluid inclusion and carbon and oxygen isotopes it indicates that the conglomerate sandstones are mainly feldspathic litharenites with immature texture it has considerable heterogeneity with a wide range of porosity from 0 3 to 15 3 and the permeability from 0 004 md to 1168 md this study discusses conglomerate sandstones diagenesis and its controls on reservoir quality its eodiagenesis is primary compaction and the minor of calcite cementation the subsequent mesodiagenesis includes weak compaction precipitation of quartz overgrowth and feldspar overgrowth albitization dissolution of early carbonate cement and metastable framework grains late carbonate cementation and illitization a comparison between porosity depth data and normal compaction curve of conglomerate sandstones shows two groups group a is mainly composed of tight conglomerate sandstones with permeability less than 1 md and porosity less than 10 petrology studies show that two types of the tight conglomerate sandstones developed in this group one type mainly depositing in the thinly bedded subaqueous channels of fan delta front is characterized by abundant clay minerals occluding intergranular pores the other type mainly depositing in the thinly bedded subaqueous channels or along sandstone mudstone interface of thickly bedded subaqueous channels is characterized by abundant carbonate cements occluding intergranular and intragranular pores group b is mainly composed of conglomerate sandstones with higher permeability more than 1 md and porosity ranging from 5 to 16 corresponding to the burial depth from 2200 to 3400 m it mainly deposited within the thickly bedded subaqueous channels of fan delta front significant dissolution of carbonate cement feldspars and other metastable framework grains by organic acids and co 2 generated from organic maturation are the major cause of abnormally high porosity

the age of illite cement growth village fields area southern north sea evidence from k ar ages and 18o 16o ratios in this paper we describe k ar radiometric ages and oxygen isotope ratio measurements of illite cements from eolian sandstone samples of the permian lower leman sandstone formation rotliegendes group the samples come from a large about 100 100 km part of the southern north sea basin village fields area and from a range of burial depths 2 8 3 4 km subsea bed mean illite ages in the gas fields of ravenspurn north 164 ma 20 1 2 sgr n 11 hyde 161 ma 12 3 n 7 cleeton 155ma 20 2 n 3 and hoton 152 ma 14 0 n 14 fields and in two wells located in block 47 5a 162 ma 16 1 n 4 are all in the middle to late jurassic apart from two early cretaceous ages considered to represent unreliable analyses the m an for all ages is 158 ma 18 6 2 sgr n 39 we largely avoided contamination with old detrital k bearing minerals by careful sample selection and monitoring to a first approximation the assumptions of k ar dating appear to be fulfilled so the measured ages are likely to date illite cement precipitation oxygen isotope ratios measured on the dated illite samples show small but significant variations among fields and a weak correlation with depth during the middle jurassic when illite grew both k ar dating and oxygen isotope ratios are compatible with a regional middle to late jurassic phase of illite growth during which the mineral precipitated over a large area and over a range of temperatures governed by the depth of the formation at that time the age of illite cementa ion coincides with rifting which ultimately may have caused illite growth by driving pore water flow

reequilibration of fluid inclusions in low temperature calcium carbonate cement calcium carbonate cements precipitated in low temperature near surface vadose environments contain fluid inclusions of variable vapor to liquid ratios that yield variable homogenization temperatures cements precipitated in low temperature phreatic environments contain one phase all liquid fluid inclusions neomorphism of unstable calcium carbonate phases may cause reequilibration of fluid inclusions stable calcium carbonate cements of low temperature origin which have been deeply buried contain fluid inclusions of variable homogenization temperature and variable salt composition most inclusion fluids are not representative of the fluids present during cement growth and are more indicative of burial pore fluids therefore low temperature fluid inclusions probably reequilibrate with burial fluids during progressive burial reequilibration is likely caused by high internal pressures in inclusions which result in hydrofracturing the resulting fluid inclusions population could contain a nearly complete record of burial fluids in which a particular rock has been bathed

geochemical constraints on the origins of calcite cements and their impacts on reservoir heterogeneities a case study on tight oil sandstones of the upper triassic yanchang formation southwestern ordos basin china calcite cementation has been identified as an active process in the upper triassic yanchang formation throughout its burial history and as a major diagenetic factor causing strong reservoir heterogeneities the origins of calcite cements and their relevance to reservoir heterogeneities were investigated using a suite of petrographic and geochemical methods including optical microscopy with fluorescence and cathodoluminescence scanning and backscattered electron microscopy with energy dispersive spectrometry x ray diffraction x ray fluorescence electron probe microanalysis quantitative evaluation of minerals by scanning electron microscopy fluid inclusion analysis and carbon and oxygen stable isotope analyses the sandstones are compositionally immature with relatively high amounts of volcanic rock fragments the two generations of calcite cements are ca i and ca ii the ca i calcites are distributed along the interface of sandstone and mudstone units and were formed during the late triassic to early jurassic at formation temperatures of approximately 90 c the ca ii calcite mainly developed in the lower part of the fining upward sandstone units and was formed in the late jurassic at higher temperatures of approximately 110 c the origins of calcite cements were constrained by geochemical and isotope measurements fluid inclusion homogenization temperature and in situ element analysis the ca i calcite cement originated from dissolution of the lacustrine depositional carbonates in the interbedded mudstones and reprecipitation in the adjacent sandstones the ca ii calcite was mainly related to organic matter decarboxylation with ca2 having been provided internally by volcanic fragment alteration and plagioclase dissolution calcite cementation had caused strong reservoir heterogeneities in the yanchang formation tight sandstones the ca i calcite cementation destroyed reservoir properties along the interface of sandstones and mudstones the lower parts of the fining upward sandstone units were tightly cemented by ca ii calcite although they originally had high porosity and permeability the middle upper parts of the fining upward sandstone units contain less calcite cements and thus have better preserved reservoir pores because of oil emplacement inhibiting the calcite cementation processes

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

absolute ages of multiple generations of brittle structures by u pb dating of calcite direct dating of brittle structures is challenging especially absolute dating of diagenesis followed by a series of superimposed brittle deformation events we report 22 calcite u pb ages from tec

synthetic fluid inclusions vi quantitative evaluation of the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure has been evaluated using pure h2o synthetic inclusions formed by healing fractures in natural quartz three different modes of non elastic deformation referred to as stretching leakage or partial decrepitation and total decrepitation have been observed the internal pressure required to initiate non elastic deformation is inversely related to inclusion size according to the equation internal pressure kbar 4 26 d 0 423 where d is the inclusion diameter in microns regularly shaped inclusions require a higher internal pressure to initiate non elastic deformation than do irregularly shaped inclusions of similar size heating inclusions through the quartz inversion results in mechanical instability in the quartz crystal and leads to mass decrepitation of inclusions owing to structural mismatches generated by pressure gradients in the quartz around each inclusion long term heating experiments 2 years suggest that the internal pressure required to initiate non elastic deformation does not decrease significantly with time and indicates that short lived thermal fluctuations in natural systems should not alter the inclusion density and homogenization temperature inclusions that do exhibit decreased density higher homogenization temperature are however always accompanied by a change in shape from irregular to that of a negative crystal observations of this study are consistent with elasticity theory related to fracture generation and propagation around inclusions in minerals these results indicate that an inclusion will not be influenced by a neighbouring inclusion or other defect in the host phase as long as the distance between the two is 2 4 diameters of the larger of the two inclusions

fluid inclusions and pvtx modelling examples from the garn formation in well 6507 2 2 haltenbanken mid norway abstract fluid inclusions study and multi techniques quantitative analyses have been coupled to reconstitute pvtx conditions of aqueous and petroleum fluid entrapment of a local haltenbanken area well 6507 2 2 from the garn formation unusual low temperature behaviour showing the liquid portion separating into two liquids has been described this behaviour is typical of a gas condensate volatile mixture with high methane content and heavy alkanes the variable liquid vapour ratio compositions and morphologies of all petroleum inclusions can be interpreted as a result of a combination of heterogeneous trapping and necking down thus all petroleum inclusions can be related to a single petroleum source three main episodes of fluid entrapment can be distinguished the first one corresponds to the water trapped within detrital quartz microfractures at around 50 c and 50 bar the second to the petroleum fluid arrival in the biphase field of a critical system around 100 120 c and 190 230 bar the third one to the main trapping of present day petroleum inclusions during the recrystallisation of quartz cements around 110 160 c and 230 280 bar within the last few million years of a short and relatively rapid burial

experimental stretching of fluid inclusions in calcite implications for diagenetic studies homogenization temperatures of fluid inclusions trapped in calcite crystals may change with increased heating beyond the homogenization temperature fluid inclusions trapped in calcite during basin subsidence will tend to yield homogenization temperatures approaching maximum burial temperatures diagenetic studies that depend on these data will be biased coarse crystalline calcite containing saline fluid inclusions was used to examine the influence of increased temperatures and pressures on measured homogenization temperatures stepped heating experiments were conducted at 1 atm over a temperature range of 90 to 245 c along with hydrothermal experiments at confining pressures of 173 and 691 bar and temperatures of 160 250 and 350 c in all cases significant increases in the homogenization temperatures of the inclusion population were recorded presumably stresses generated during heating of the fluid inclusions exceeded the combined strength of the calcite crystal and the external pore pressure and thus caused fluid inclusion stretching size is a secondary factor that controls fluid inclusion reequilibration

volcanological study of the great tambora eruption of 1815 the 1815 eruption of tambora is one of the largest explosive volcanic events of the past 10 000 yr by a conservative estimate 175 km3 of nepheline normative trachyandesitic pyroclastic material equivalent to about 50 km3 of dense rock was erupted in 24 h major activity began with a brief plinian phase followed by pyroclastic flows much of the ignimbrite that was produced shows evidence of deposition by turbulent flows a 6 7 km caldera about 1 km deep formed at the end of the pyroclastic flow phase caldera volume closely matches that of magma ejected plinian and co ignimbrite ash fall 1 cm thick covered 500 000 km2 of the java sea and surrounding islands accounting for more than two thirds of the magma volume erupted this distal tephra is known only from 1815 reports indicating that some great eruptions especially those from island volcanoes may be virtually undetectable in the geologic record without supporting information such as deep sea core data land based volume estimates may be insufficient to characterize eruption magnitude in many cases

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

fluid overpressure as a control on sandstone reservoir quality in a mechanical compaction dominated setting magnolia field gulf of mexico the reservoir quality porosity and permeability of deeply buried hydrocarbon reservoir sandstones in sedimentary basins is significantly affected by burial diagenesis many deep reservoirs develop anomalous fluid overpressures during burial previous studies on the effect of fluid overpressure on reservoir quality in these deep reservoirs have been inconclusive because of the difficulty in constraining the individual contributions of various porosity preserving factors that are simultaneously active in these reservoirs owing to its rapid burial and low burial temperatures the neogene gravity flow sandstone reservoirs from the magnolia field gulf of mexico offers a unique opportunity to investigate in isolation the effect of fluid overpressure on reservoir quality examination of petrography pore pressure and routine core analysis datasets showed a positive correlation between high fluid overpressure and enhanced reservoir quality this study confirms that fluid overpressure preserves reservoir quality in deeply buried sandstone reservoirs in compaction dominated high sedimentation basin settings

mass transfer between sandstones and interbedded mudstones impact on petroleum charge bohai bay basin china abstract the permo carboniferous sandstones in the gubei area bohai bay basin are reservoirs for large accumulations of natural gas the natural gas accumulations only occur in the sandstone beds thicker than 1 9 m moreover the maximum porosity of every bed correlates positively with bed thickness up to 2 0 m and for thicker beds porosity is uniform porosity transitional zones developed at the top and bottom of each bed with mean thicknesses of 1 25 m and 0 75 m respectively porosity shows a positive correlation with the distance to the sandstone mudstone contact in the zones interpretations based upon an extensive petrographic and geochemical database indicate that the sandstones experienced a mass fluid input from the adjacent mudstones during the early diagenesis the infiltration resulted in extensive clay coats and pore filling cements in sandstones which were effective for inhibiting dissolution of grains during subsequent diagenesis process the mass introduction only affected the marginal parts of the sandstones within 1 25 m of the top and 0 75 m of the bottom of every bed causing thin sandstone beds with thickness of approximately 2 m to be tightly cemented totally thus the central parts of each thicker beds became preferential sites for the natural gas accumulation the differential levels of cementation in thin and thick sandstone beds observed here has significant implications for exploration of similar interbedded sandstone and shale sequences elsewhere including shale gas targets

petrography fluid inclusion isotope and trace element constraints on the origin of quartz cementation and feldspar dissolution and the associated fluid evolution in arkosic sandstones feldspar dissolution quartz cementation and clay cementation are significant diagenetic processes in deep water fan feldspar rich sandstones in the shengtuo area dongying sag east china the timing and origin of these water rock interactions as well as the paleofluids in which relevant chemical reactions occurred were deduced using data from microscopy fluid inclusion microthermometry oxygen isotope microanalysis and trace element microanalysis three distinct separate episodes of quartz overgrowths referred to as q1 q2 and q3 were distinguished by cathodoluminescence microscopy the q1 quartz identified in all porous sandstones from 2500 to 3600 m 8200 to 11 800 ft was formed at approximately 100 c 115 c 212 f 239 f before oil filled the reservoirs the q2 quartz was mainly precipitated at 115 c 130 c 239 f 275 f accompanying or slightly postdating the main phase of oil filling and was identified in samples from 2800 to 3600 m 9200 to 11 800 ft the q3 quartz was only identified in sandstones buried deeper than 3500 m 11 400 ft and was likely precipitated in the quaternary when temperature exceeded 130 c 135 c 266 f 275 f secondary ion mass spectrometry oxygen isotope microanalyses yielded 18ovsmow vienna standard mean ocean water values ranging from 21 42 to 24 35 for q1 quartz from 22 03 to 24 99 for q2 quartz and from 21 72 to 22 91 for q3 quartz a mass balance calculation and quantitative petrography data of the amount of leached feldspars and associated secondary minerals suggest that the internal feldspar dissolution is likely the primary source for the authigenic clays and quartz cements in these sandstones positive 18o water values 0 5 to 4 5 vsmow of quartz forming waters indicate that quartz cementation and feldspar dissolution occurred in a geochemical system with a limited volume of diagenetically modified connate water the variations of 18o cements and trace element compositions from q1 quartz to q2 quartz in individual overgrowth suggest that hydrocarbon filling changed the chemistry of the pore fluid significantly further 18o water values of the pore water increased by approximately 2 vsmow after oil filling meteoric freshwater did not contribute to quartz cementation and simultaneous feldspar dissolution the detected co2 and hydrocarbons in fluid inclusions in the quartz cements the existence of co2 in hydrocarbon rich natural gas and the pyrobitumen in the feldspar hosted pores suggest that acids derived from organic matter kerogen in source rocks and hydrocarbons in reservoirs probably have promoted the extensive subsurface leaching reactions of feldspars in these sandstones the ongoing development of q2 and q3 quartz suggests that quartz precipitation did not cease after oil filling further diagenetic reactions likely have proceeded from water rock interactions to hydrocarbon water rock interactions

a fluid inclusion study of cement and vein minerals from the celtic sea basins offshore ireland abstract fluid inclusion petrography and microthermometry have been carried out on jurassic sandstone cements quartz calcite and dolomite and later fracture filling calcite quartz veins from the north celtic sea and the fastnet basins fluid inclusions in the sandstone cements are small 1 8 m monophase liquid only or liquid rich two phase liquid vapour aqueous inclusions all display irregular to elliptical morphologies the two phase aqueous inclusions homogenise to the liquid phase between 60 122 c and have salinities ranging from 7 0 16 0 wt nacl equivalent indicating precipitation of the cements by moderate salinity waters in contrast the veins host both aqueous and petroleum inclusions the aqueous inclusions are liquid rich two phase liquid vapour inclusions 2 30 m long that are typically primary and show irregular and negative crystal shape morphologies their homogenisation temperatures to liquid range from 50 145 c and salinities from 6 20 wt nacl equivalent indicating broadly similar salinities coupled with a wider range of minimum trapping temperatures to those recorded from the cements yellow brown two phase liquid vapour petroleum inclusions are hosted by vein calcite from several wells in the central north celtic sea basin some of these inclusions appear to be primary and all are spatially associated with the aqueous inclusions the two phase liquid vapour petroleum inclusions homogenise mostly to the liquid between 50 130 c petroleum inclusions in the vein calcites indicate that petroleum was transported via fractures at a late stage through the jurassic and cretaceous strata of the celtic sea basins and apparently not through the sandstone matrix

quartz cement and its origin in tight sandstone reservoirs of the cretaceous quantou formation in the southern songliao basin china abstract the tight sandstones of the cretaceous quantou formation are the main exploration target for hydrocarbons in the southern songliao basin authigenic quartz is a significant cementing material in these sandstones significantly reducing porosity and permeability for efficient predicting and extrapolating the petrophysical properties within these tight sandstones the quartz cement and its origin need to be better understood the tight sandstones have been examined by a variety of methods the sandstones are mostly lithic arkoses and feldspathic litharenites compositionally immature with an average framework composition of q43f26l31 which are characterized by abundant volcanic rock fragments mixed layer illite smectite i s ordered interstratified with r 1 and r 3 is the dominating clay mineral in the studied sandstone reservoirs two different types of quartz cementation modes namely quartz grain overgrowth and pore filling authigenic quartz have been identified through petrographic observations cl and sem analysis homogenization temperatures of the aqueous fluid inclusions indicate that both quartz overgrowths and pore filling authigenic quartz formed with a continuous process from about 70 c to 130 c sources for quartz cement produced are the conversion of volcanic fragments smectite to illite reaction and pressure solution at micro stylolites potassium for the illitization of smectite has been sourced from k feldspar dissolution and albitization silica sourced from k feldspars dissolution and kaolinite to illite conversion is probably only minor amount and volumetrically insignificant the internal supplied silica precipitate within a closed system where the transport mechanism is diffusion the quartz cementation can destroy both porosity and permeability but strengthen the rock framework and increase the rock brittleness effectively at the same time

quartz cement volumes across oil water contacts in oil fields from petrography and wireline logs preliminary results from the magnus field northern north sea

geochemical constraints from formation water analyses from the north sea and the gulf coast basins on quartz feldspar and illite precipitation in reservoir rocks

duration of quartz cementation in sandstones north sea and haltenbanken basins abstract evidence from fluid inclusions petrography and burial history modelling is used to estimate the duration of quartz cementation in sandstones the homogenization temperatures of primary fluid inclusions in quartz cements from samples from the brent group north sea and haltenbanken offshore norway show variations 4 of 17 4 and 20 2 c these are interpreted to reflect durations of less than approximately 9 and 17 ma low variances are general features of such data sets and suggest that quartz cementation usually takes place as events with durations of the order of 10 ma or possibly less confining the age of quartz cement growth using a combination of geological petrographical and radiometric dating methods that are independent of fluid inclusions also suggests that quartz cements grow over periods substantially shorter than the age of the sandstone it is not easy to reconcile this deduction with the suggestion that burial itself causes cementation

dating diagenesis in a petroleum basin a new fluid inclusion method the final porosity and permeability of sandstone petroleum reservoirs is greatly affected by the diagenetic growth of minerals after deposition for example a sand may be deposited with a porosity of 25 and a permeability of 5 000 mdarcy md 1 diagenetic growth of quartz around detrital sand grains may leave a rock with only 10 porosity and later growth of clays may partly fill these remaining pores and block inter pore connections reducing permeability to 100 md ref 2 if the depth and timing of such diagenetic alteration can be measured and the extent of diagenesis estimated then prediction of the diagenetic state of undrilled sandstones may become possible and diagenesis related more closely to the timing of hydrocarbon migration and the formation of hydrocarbon traps we present an example of a new method for estimating the date of quartz diagenesis using a combination of techniques from thin section petrography fluid inclusion thermometry organic geochemical thermometry and sedimentary basin stratigraphic analysis these results suggest that quartz in the beatrice oilfield was precipitated from moving and cooling pore fluids at a temperature between 68 c and 94 c in the late jurassic

diagenesis and reservoir quality evolution of the eocene sandstones in the northern dongying sag bohai bay basin east china abstract the eocene sandstones in the northern dongying sag bohai bay basin china are reservoirs for large accumulations of hydrocarbons the sandstones are mainly lithic arkoses and feldspathic litharenites texturally and compositionally immature these sandstones have a wide range of porosity 0 4 37 and permeability 0 004 6969 md and show an overall decrease in reservoir quality from 1500 m to 5000 m below sea level the reduction in reservoir quality is a product of several digenetic processes these include compaction precipitation of dolomite and calcite in eodiagenetic stage compaction feldspar dissolution precipitation of quartz cements and clays kaolin and illite and precipitation of ferrocalcite and ankerite in mesodiagenetic stage mineral distribution pattern and isotopic composition suggest carbonate cements in sandstones originate from sources outside the sandstones carbonate cementation together with compaction reduced the sandstones porosity and permeability significantly in a sandstone bed marginal sandstones with distance to sandstone mudstone interface less than one meter always have lower porosity than central sandstones as burial depth exceeds 4000 m marginal sandstones have very low porosities 2 m can be potential effective reservoirs feldspar dissolution and precipitation of clays and quartz cements have little impact on absolute porosity mineral distribution pattern and quantitative data show that leached feldspars are the internal source of authigenic quartz and clays in sandstones and the volume difference between feldspar secondary porosity and related authigenic cements is generally less than 0 25 however although there is little or no net import of matter to the sandstones the pore architecture changes dramatically primary macropores are lost as clays and quartz precipitate while the proportion of microporosity increases occurring mainly between clay crystals the overall result is that permeability is significantly degraded

evidence against natural deformation of fluid inclusions in diagenetic quartz abstract this paper describes evidence for and against burial induced deformation of fluid inclusions in diagenetic quartz fluid inclusions in diagenetic minerals are prone to non elastic deformation stretching or leakage if subjected to progressive burial in a sedimentary basin if they deform they cannot record mineral growth conditions correctly a correlation between homogenization temperature and depth may be interpreted as indicating wholesale leakage or stretching experimental work and theoretical studies suggest that heating 200 c higher than the homogenization temperature is required to produce significant deformation but it may not be possible to extend the results to natural conditions the experiments further predict a correlation between homogenization temperature and inclusion size in a sample of inclusions that have undergone natural deformation this is not observed in natural examples but the absence of a correlation is not proof that leakage has not occurred several features of inclusion data in quartz cements do however argue against stretching or leakage homogenization temperatures commonly show narrow distributions and are compatible with independent data such as illite potassium argon ages and petrography different generations of inclusions in a single sample set also retain homogenization temperature distributions compatible with their sequential origin taken as a whole the body of evidence argues against widespread stretching or leakage of fluid inclusions in diagenetic quartz

diagenesis and reservoir quality prediction the modern concept of diagenesis was introduced by walther one hundred years ago to encompass the postdepositional processes that affect sediments from deposition to low grade metamorphism since walther 1893 diagenetic studies have been concerned with an extraordinary variety of chemical and physical interactions each of which has the potential to significantly affect the final composition and texture of a sedimentary rock the issues to which those studies have been applied are equally varied and include 1 improved techniques to predict the producibility of hydrocarbon reservoirs 2 improved techniques to predict subsurface fluid pressures 3 better constraints on the timing of fluid flow events 4 reconstruction of global climate change and 5 evaluation of groundwater contamination commercial applications of diagenetic studies have traditionally focused on the producibility of hydrocarbon reservoirs as such a review of this topic serves as an effective illustration of how basic research has been commercialized and a useful focus for the following dicussion however it is worth noting that decreased industrial research funding coupled with increased government funding for environmental research have recently resulted in a gradual evolution of diagenetic publications away from the producibility of hydrocarbon reservoirs toward the producibility of uncontaminated groundwater and other environmental issues despite the shift in emphasis many of the basic research issues remain unchanged whether one is focused on deep hydrocarbon reservoirs or shallow aquifers

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