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

geochemical and mineralogical control of different genetic types of deep sea nodules from the pacific ocean deep sea nodules from the northeast pacific nodule belt and the southeast pacific sonne basin being formed in areas bordering the equatorial zone of high biological productivity accumulate by two basically different growth processes a early diagenetic growth by supply from pore water and b hydrogenetic growth by supply from near bottom sea water these growth processes lead to different genetic types of nodules early diagenetic type a hydrogenetic type b and mixed type ab a further type ac very rich in mn is being formed by increasing influence of early diagenesis these types can clearly be distinguished by their shapes surface textures mineral constituents of oxide fraction internal microstructures and geochemistry a genetical classification is being proposed on the basis of statistically computed interelement relationships todorokite very poor in fe is the main mn phase in the early diagenetic substance mno2 intimately intergrown with feooh xh2o is the main phase in the hydrogenetic substance consequently an important difference can be pointed out the metal supply for the growth of the early diagenetic nodules is based on an ionic solution of me2 e g mn2 ni2 cu2 zn2 whereas the supply for the hydrogenetic nodules is caused by transport of colloidal particles mobilization of mn2 and fractionation from fe is controlled by the amount of decomposing organic matter in the peneliquid layer of the sediments the main factor controlling the intensity of early diagenesis is the biological productivity in surface waters the crucial point of reversal at a mn fe ratio of about 5 obtained by hyperbolical regression of the analyses of nodules from the southeast pacific represents best concentrations in ni and cu mn fe quotients greater than 5 cause a decrease of ni and cu content nodules from the northeast pacific nodule belt generally contain higher concentrations in cu than nodules from the southeast pacific this can be explained by an additional supply of cu transported below ccd by siliceous plankton

linear coupling of carbon and strontium isotopes in rotliegend sandstone north sea evidence for cross formational fluid flow isotopic analyses of carbon and strontium in diagenetic dolomite and ankerite present in the eolian lower permian rotliegend sandstone southern north sea show an excellent linear correlation between delta sup 13 c and sup 87 sr sup 86 sr r 0 93 suggesting that the sources of these elements were interlinked the isotopic data indicate that late permian zechstein seas flooded the basin and displaced the interstitial meteoric rotliegent pore fluids producing early dolomite with predominantly marine bicarbonate delta sup 13 c minus 15 per thousand and strontium sup 87 sr sup 86 sr 0 7076 isotopic signatures the isotopic values of this dolomite form a trend to merge with the diagenetically later ankerite which has lower sup 13 c sup 12 c delta sup 13 c minus 4 per thousand and more radiogenic strontium sup 87 sr sup 86 sr 0 7112 the obvious source of low delta sup 13 c carbon was thermal decarboxylation of organic matter in the underlying upper carboniferous mudstones the authors also suggest that the increasing quantities of radiogenic strontium sup 87 sr sup 86 sr 0 720 were released when organic acids dissolved silicates feldspar in the same underlying mudstones the upward movement of fluid or diffusion of ions across formation boundaries therefore progressively became the dominant control on carbonate precipitation in the rotliegend sandstone

foraminiferal ooze from the galapagos rift area hydrothermal impact and diagenetic mobilization of elements abstract chemical changes during diagenesis in foraminiferal oozes have been studied in five cores from the hydrothermally active spreading centre of the galapagos rift shifts of concentrations of elements such as mn co cu ni zn and ba are related to changes of eh potential in the range from 0 33 to 0 07 v mn co and cu are strongly mobilized in the reduction zone and reprecipitated as hydroxides near the sediment water interface the ree distribution patterns of these oxide rich sediments are very similar to those in the base metal poor diagenetically formed mn nodules during diagenesis several elements ree co sb ba and zn were redistributed between carbonate and non carbonate phases whereas others were not fe sc th and cr although the composition of these sediments may be attributed to diverse sources convincing evidence supporting a hydrothermal impact comes from the sb concentrations

patterns of early post depositional and burial cementation in distal shallow marine sandstones upper cretaceous kenilworth member book cliffs utah usa there is a limited understanding of the spatial linkage between cementation processes in shoreface coastal plain successions and those operating in distal offshore successions to address this an integrated sedimentological sequence stratigraphic and diagenetic study was undertaken on a distal shallow marine succession the upper cretaceous kenilworth member book cliffs utah usa laterally extensive carbonate cement is the most volumetrically significant diagenetic feature it constitutes up to 55 vol of the distal lower shoreface sandstone and sandy siltstone units beneath major flooding surfaces during rising relative sea level major flooding events resulted in low sedimentation which extended residence time of sediments in early diagenetic zones and promoted enhanced cementation beneath these boundaries carbonate for early calcite cements was probably derived from sulphate reduction marine water and oxidation of organic matter stable isotopic composition of later ankerite 18o 11 9 to 6 0 vdpb 13c 4 5 to 0 3 vdpb suggests precipitation during progressive burial c 1 7 to 3 4 km 72 to 122 c from evolved marine fluids the carbonate and other mineralizing solutes were derived increasingly with burial from decarboxylation of organic matter and clay mineral transformations in adjacent siltstones and shales these interbedded overlying and underlying facies provided an important source of mineralizing solutes that influenced the extent of pervasive cementation during burial the combination of stratigraphic and lithofacies control resulted in laterally extensive cements that differ to shoreface successions although are similar in form to those described for many offshore mudstone dominated successions limited early concretionary ferroan dolomite beneath the expression of the sequence boundary and some flooding surfaces however suggests cementation by meteoric fluids at times of relative sea level lowstand similar processes have been demonstrated in more proximal successions in the book cliffs

dolomitization of shallow water mixed silicilastic carbonate sequences the lower triassic ramp succession of the transdanubian range hungary abstract the lower triassic succession of the transdanubian range in hungary comprises limestones dolomite marl sandstones and siltstones deposited in tidal flat lagoon and ooid shoal environments on the marginal ramp of the western neotethys seven cores were chosen for petrographic and stable isotope investigations aiming to reconstruct the paragenetic sequence with special regard to the dolomitization and hydrothermal events five lithotypes were differentiated i dolomite ii sandy silty clayey dolomite and dolomarl iii dolomitic siltstone and sandstone iv dolomitic limestone and v limestone in these lithotypes three types of dolomites are present non ferroan replacive ferroan replacive and ferroan cement fabric retentive and fabric destructive non ferroan replacive dolomitization are interpreted to have occurred by seepage reflux supporting evidence includes the presence gypsum and anhydrite in the lower triassic beds stable isotope values of the ferroan dolomite 18o of 10 7 to 4 2 and 13c of 4 8 to 4 7 suggest dolomitization by fluids of relatively high temperature the similar stable isotope values 18o of 9 3 to 5 8 and 13c of 1 9 to 2 5 of the non ferroan dolomite phase suggest that the reflux dolomite was overprinted by this second dolomitization event traces of exotic minerals such as barite chalcopyrite galena and sulphosalts were found as fillings of vugs and fractures in the dolomite cemented sandstone the metals could have been sourced from the underlying permian red sandstone beds the heterogeneous sediment composition had profound impact on the diagenesis of these sedimentary successions

classification of dolomite rock textures abstract dolomite rock textures can be classified according to crystal size distribution and crystal boundary shape the classification scheme presented here is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics and crystal boundary shape is controlled by growth kinetics size distributions are classified as unimodal or polymodal crystal boundary shapes are classified as planar or nonplanar if the evidence permits a complete classification includes a description of recognizable allochems matrix and void filling allochems and preexisting cements may be unreplaced partially replaced replaced mimically or replaced nonmimically allochems may be dissolved leaving molds matrix can be unreplace partially replaced or replaced by a unimodal or polymodal size dolomite unimodal size distributions generally indicated a single nucleation event on a unimodal substrate polymodal sizes can be formed by multiple nucleation events on a unimodal or polymodal substrate or differential nucleation on an originally polymodal substrate planar crystal boundaries develop when crystals undergo faceted growth and nonplanar boundaries develop when crystals undergo nonfaceted growth nonplanar boundaries are characteristic of growth at elevated temperature 50 c and or high supersaturation both planar and nonplanar dolomite can form as a cement replacement of caco3 or neomorphism of a precursor dolomite

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

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

distribution chemistry isotopic composition and origin of diagenetic carbonates magnus sandstone north sea abstract diagenetic ferroan carbonates grew in the upper jurassic reservoir sandstones of the magnus oilfield in porewaters which differed in composition across the field these porewaters remained compositionally different and stratified for at least 35 m y variations in carbonate chemistry across the field are attributable to these porewater variations which resulted from displacement of marine depositional water from the crest of the field by meteoric water during late cimmerian subaerial exposure original depositional facies and detrital mineralogy strongly influenced diagenetic carbonate distribution rare diagenetic calcite occurs as discrete rhombic crystals diagenetically late magnesian siderites have developed throughout the reservoir sandstone and are commonly intimately associated with altered detrital biotite grains poikilotopic ankerite cement postdates calcite and siderite and occurs only adjacent to mudstones and in thin sandstones within mudstones three compositional growth zones in siderite crystals are observed across the field from crest to flank in all three wells studied a similar trend of compositional evolution through time is observed in both two and three zoned rhombs first formed siderite is relatively magnesian intermediate zones are more ferroan and outer zones are at least as magnesian as the first stage these individual grain variations overprint a fieldwide variation where siderite is more ferroan in the crestal samples up to 87 mol fe mn and more magnesian downdip up to 58 mol mg this reflects the greater influence of relatively fe rich meteoric derived water in the crest and the greater influence of marine derived mg rich porewater downdip ankerite shows a similar variation in fe and mg abunda ces across the field crest and flank maximum 25 and 17 mol fe mn respectively 27 and 43 tool mg and developed due to release of mg fe ca and hco3 ions from mudstones into adjacent sandstones following dissolution of detrital minerals and organic decarboxylation reactions both siderite and ankerite have lower 18o at the crest of the oilfield than downdip respectively siderite 16 0 and 17 6 smow ankerite 17 7 and 21 4 these differences in 18o reflect the retention during burial diagenesis of a larger component of meteoric water in the crest of the field below the unconformity whereas downdip porefluid contained a larger marine derived component strong organic influence on 13c 8 0 to 14 6 pdb for magnesian siderite 7 7 to 13 6 for ankerite closed system 34s values up to 15 7 for late cubic pyrite and stratified 18o from crest to flank of the field argue against large scale porewater movements diagenetic porewater stratification is strongly supported by the parallel but distinct geochemical fingerprints of siderite and ankerite cements from the crest to the flank of the field

nw african hydrology and vegetation during the last glacial cycle reflected in plant wax specific hydrogen and carbon isotopes abstract we present a hydrologic reconstruction of the sahara sahel transition covering the complete last glacial cycle 130 ka based on a combination of plant wax specific hydrogen d and carbon isotopes 13c the d and 13c signatures of long chain n alkanes from odp site 659 off nw africa reveal a significant anti correlation complementary to published pollen data we infer that this plant wax signal reflects sensitive responses of the vegetation cover to precipitation changes in the sahel region as well as varying contributions from biomes north of the sahara c3 domain by north east trade winds netw during arid phases especially the northern parts of the sahel likely experienced crucial water stress which resulted in a pronounced contraction of the vegetation cover thus reducing the amount of c4 plant waxes from the region the increase in netw strength during dry periods further promoted a more pronounced c3 plant wax signal derived from the north african c3 plant domain during humid periods the c4 dominated sahelian environments spread northward into the saharan realm in association with lower netw inputs of c3 plant waxes arid humid cycles deduced from plant wax d are in accordance with concomitant changes in weathering intensity reflected in varying major element distributions environmental shifts are generally linked to periods with large fluctuations in northern hemisphere summer insolation during marine isotope stages 2 and 3 when insolation variability was low coupling of the hydrologic regime to alkenone based estimates of ne atlantic sea surface temperatures becomes apparent

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

fluid inclusion technique for determining maximum temperature in calcite and its comparison to the vitrinite reflectance geothermometer theory laboratory experiments and empirical observation suggest that many aqueous fluid inclusions in calcite reequilibrate during overheating and therefore some homogenization temperatures t h record a temperature close to the maximum reached by the rock this characteristic suggests that aqueous fluid inclusions in calcite can be used to establish maximum temperature t peak to test this hypothesis we have compiled fluid inclusion t peak mean random vitrinite reflectance r m and present day t peak from 46 diverse geologic systems that have been at t peak from 10 4 to 10 6 yr present t peak ranged from 65 to 345 c t h modes and means ranged from 59 to 350 c and r m data ranged from 0 4 to 4 6 spanning the temperature and thermal maturity range associated with burial diagenesis hydrothermal alteration and low grade metamorphism plots of t h and t peak data for systems thought to be currently at maximum temperature demonstrate close agreement between t h and present t peak in sedimentary basins although caution should be applied the relation suggests that t h of aqueous fluid inclusions in calcite may be a useful measure of maximum temperature this study also compares t h to mean random vitrinite reflectance r m to offer further support for the use of t h as a measure of t peak and to provide a better understanding of r m t h correlates well with r m and results in a curve similar to r m vs t peak calibrations determined by other workers the strong correlation correlation coefficient r 0 93 between t peak and r m in these systems suggests that maximum temperature is the major control on thermal maturation

reservoir quality and rock properties modeling triassic and jurassic sandstones greater shearwater area uk central north sea abstract the complex burial and diagenetic histories of the jurassic fulmar and triassic skagerrak sandstones in the uk central north sea present significant challenges with regard to reservoir quality and rock property prediction commercial reservoir quality is retained despite deep burial and associated high temperatures and pressures shallow marine fulmar sands are normally compacted mean igv 26 3 yet have porosities of 21 33 porosity was preserved through inhibition of quartz cementation by clay and microquartz coatings and enhanced by dissolution of framework grains 5 skagerrak fluvial sands are more compacted mean igv 23 2 exhibit minor feldspar dissolution we modeled reservoir quality evolution using the forward diagenetic model touchstone which simulates porosity loss due to compaction and quartz cementation quantitative petrographic analyses and burial history data were used to calibrate touchstone model parameters the results were applied to deeper prospects for pre drill prediction of porosity and permeability in parallel petrophysical data were used to characterize the elastic properties of the sandstones to provide a basis for quantitative seismic forward modeling experimental data and core calibrated petrophysical results reflecting variable in situ fluids and saturations were used to build an elastic properties model the model is robust and was used to generate fluid filled sandstone properties incorporating touchstone results for prospect specific seismic attribute modeling well results from exploration wells are in good agreement with pre drill touchstone and elastic properties model predictions

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

aluminium loss during sandstone diagenesis petrographical observations from a sandstone buried to between 3 2 and 5 7 km in the north sea central graben suggest a significant change in bulk chemistry during burial the sand contained 35 or more feldspar upon deposition but due to extensive dissolution only 5 remains in the deep sandstones this has released much more al than is currently contained as authigenic minerals up to 1460 mol cm 3 of al have been exported this deep secondary porosity is economically important but its recognition by petrographic textures is unreliable as are bulk chemical provenance studies of deeply buried sedimentary and metamorphic strata

fluid flow and related diagenetic processes in a rift basin evidence from the fourth member of the eocene shahejie formation interval dongying depression bohai bay basin china the purpose of this paper is to relate diagenetic processes in deeply buried sandstones in the fourth member of the eocene shahejie formation interval bohai bay basin china to pore fluid flow changes with progressive burial based on petrographic mineralogical and geochemical analysis distribution patterns of authigenic minerals are recognized that reflect 1 the sources and patterns of fluid flow and 2 fluid flow in an evolving open to closed system partial to extensive precipitation of calcite and dolomite at or near mudstone sandstone contacts during eogenesis was a result of large scale mass transfer between sandstones and adjacent mudstones this process was driven by steep diffusion gradients from adjacent mudstones in a relatively open geochemical system on the local scale support for this model is provided by large sulfur isotope fractionation between framboidal pyrite and precursor gypsum dissolution of feldspar grains and dissolution of nonferroan carbonate cements during early mesogenesis are spatially associated with quartz and ferroan carbonate cementation respectively this process was related to organic carbon dioxide expelled from adjacent source rocks and indicates a relatively open system during late mesogenesis dissolution of evaporitic cements related to thermochemical sulfate reduction tsr generated ankerite and nodular pyrite cements in adjacent pores a lack of sulfur isotope fractionation between parent anhydrite and late stage nodular pyrite during tsr supports a relatively closed fluid flow system because the velocities of pore fluid flow were low during mesogenesis large scale thermal convection and advection probably did not occur instead diffusion over short distances is inferred as the predominant transport mechanism for dissolved solids that were precipitated as other phases either in situ or in adjacent pores

a model for the effect of illitization on porosity and quartz cementation of sandstones abstract quartz cementation is commonly the most important porosity reducing process in deeply buried quartzose sandstones on the norwegian continental shelf the quartz cement is mostly sourced from stylolites but silica may also be derived from illitization of kaolinite illitization can proceed at silica activities several times higher than quartz saturation whereas dissolution of quartz at stylolites is very sensitive to silica supersaturations and the illitization reaction may consequently stop or retard dissolution of quartz at stylolites since the illitization reaction does not cause a net porosity loss porosity loss due to precipitation of quartz cement is reduced or totally prevented over the depth temperature range where illitization takes place illitization affects albitizatio of k feldspar because both reactions consume k feldspar oil replacement during illitization may conceivably cause greater porosity loss than if oil were not present since oil emplacement might stop or retard illitization but not quartz cementation sourced from stylolites

formation of clay minerals during early diagenesis of a calcareous ooze abstract deep sea sediments from a basin in the southeast pacific are described in which strong carbonate dissolution and strong diagenetic migration of metals can be observed clay minerals of smectite type are formed in large quantities during the early diagenesis of this sediment in a first stage as an x ray amorphous phase which later crystallizes to smectite rb sc na and k become enriched during the process of clay mineral formation probably due to uptake of these elements from sea water to interlayer positions of smectite the unsolved question is what sort of mechanism connects the calcite dissolution with the formation of smectite the source of the al necessary for the formation of smectite is also unknown

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

significance of molluscan shell beds in sequence stratigraphy an example from the lower cretaceous mannville group of canada1 detailed study of marine shales the ostracod zone within a cretaceous third order transgressive regressive sequence in the alberta foreland basin reveals a systematic association between shell beds and parasequence scale flooding surfaces including surfaces of maximum flooding the ostracod zone a subsurface lithostratigraphic unit known as the calcareous member in outcrop consists of 10 20 m of black shale and bioturbated sandstones with many thin fossiliferous limestones parasequences shallowing up cycles 2 3 m thick were delineated within this transgressive unit based on lithology sedimentary structures degree of bioturbation dinoflagellate diversity total organic carbon and carbon sulphur ratios many flooding surfaces are firmgrounds or hardgrounds shell rich limestones occur in three different positions relative to these flooding surfaces and each has a distinctive bioclastic fabric and origin i base of parasequence shell beds bops lie on or just above flooding surfaces in the deepest water part of a parasequence they are thin up to a few centimetres graded or amalgamated skeletal packstones wackestones composed of well sorted granular shell and are interpreted as hydraulic event concentrations of exotic shell debris ii top of parasequence shell beds tops are capped by flooding surfaces at the top shallowest water part of a parasequence they typically are several decimetres thick are physically amalgamated packstones grainstones or bioturbated wackestones and contain abundant whole as well as comminuted shells these are composite multiple event concentrations of local shells iii mid sequence shell beds rest on as well as are capped by firmgrounds or hardgrounds and are intercalated between parasequences in the deepest water part of the larger sequence they are laterally extensive lime mudstones a few decimetres thick with sparse shells in various states of dissolution recrystallization and replacement these beds are terrigenous starved hiatal concentrations and record maximum flooding within the ostracod zone offshore sections of the ostracod zone typically contain several starved mid sequence shell beds underscoring the difficulty of identifying a single maximum flooding surface within a third order sequence

the role of fluid pressure and diagenetic cements for porosity preservation in triassic fluvial reservoirs of the central graben north sea anomalously high porosities and permeabilities are commonly found in the fluvial channel sandstone facies of the triassic skagerrak formation in the central north sea at burial depths greater than 3200 m 10 499 ft from which hydrocarbons are currently being produced the aim of our study was to improve understanding of sandstone diagenesis in the skagerrak formation to help predict whether the facies with high porosity may be found at even greater depths the skagerrak sandstones comprise fine to medium grained arkosic to lithic arkosic arenites we have used scanning electron microscopy petrographic analysis pressure history modeling and core analysis to assess the timing of growth and origin of mineral cements with generation and the impact of high fluid pressure on reservoir quality our interpretation is that the anomalously high porosities in the skagerrak sandstones were maintained by a history of overpressure generation and maintenance from the late triassic onward in combination with early microquartz cementation and subsequent precipitation of robust chlorite grain coats increasing salinity of pore fluids during burial diagenesis led to pore filling halite cements in sustained phreatic conditions the halite pore filling cements removed most of the remaining porosity and limited the precipitation of other diagenetic phases fluid flow associated with the migration of hydrocarbons during the neogene is inferred to have dissolved the halite locally dissolution of halite cements in the channel sands has given rise to megapores and porosities of as much as 35 at current production depths

pore fluid evolution in the kimmeridge clay formation of the uk outer moray firth implications for sandstone diagenesis abstract carbonate concretions in upper jurassic kimmeridge clay formation from three overpressured wells provide a detailed record of pore fluid evolution in the outer moray firth northern central graben the concretions contain multiple generations of septarian cements with morphologies ranging from simple cracks to complex fractures discrepancy exists between diagenetic studies which indicate these concretions formed during the initial 1 1 5 km of burial and palaeotemperature predictions based on a thermal history calibrated from the vitrinite reflectance kinetic model which indicate formation at much greater depths of between 1 5 and 2 5 km modelling undertaken for this study indicate that the concretions formed during the initial stages of burial under high heat flows fitting the early diagenetic model for their formation these conclusions have important implications for understanding the cementation of adjacent sandstone reservoirs where cement sequences have similar mineralogy and isotopic compositions with precipitation in the same temperature range from mudrock derived pore fluids early cementation of these sandstones is implied by analogy with the mudrock concretions

basin scale dolomite cementation of shoreface sandstones in response to sea level fall this study documents a link between sea level fall and basin scale dolomite cementation of shoreface sandstones in the upper cretaceous desert member book cliffs utah by integrating sedimentology sequence stratigraphy and sedimentary geochemistry within the desert member shoreface sandstones a number of high frequency sequence boundaries formed as a result of relative sea level falls that led to localized incised valley formation subaerial exposure and meteoric leaching of upper shoreface sandstones shoreface sandstones up to 10 km downdip from these subaerial exposure surfaces contain ferroan dolomite concretion bodies as much as 8 m thick basinward of this smaller as much as 1 m in size less abundant dolomite concretions can be traced at the same horizons some 1 8 o data suggest that cement precipitated from porewater with a significant meteoric component although reequilibration and recyrstallization cannot be eliminated some 1 3 c data suggest carbonate derivation from detrital dolomite leached from beneath organic rich coastal plain strata deposited during high frequency sequence boundary development we propose that during high frequency sequence boundary formation meteoric fluids migrated basinward into shoreface sandstones as a result of the relative fall in sea level the precipitation of dolomite cement in the shoreface sandstones was promoted by the updip dissolution and remobilization of detrital dolomite by the meteroic fluids mixing with marine pore fluids and the presence of detrital dolomite in the shoreface sandstones which acted as nucleation sites for dolomite precipitation this study illustrates the impact of stratigraphic development upon basin scale early diagenetic cementation of siliciclastic succcesions

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

vertical effective stress as a control on quartz cementation in sandstones abstract temperature controlled precipitation kinetics has become the overwhelmingly dominant hypothesis for the control of quartz cementation in sandstones here we integrate quantitative petrographic data high spatial resolution oxygen isotope analyses of quartz cement basin modelling and a kinetic model for quartz precipitation to suggest that the supply of silica from stress sensitive intergranular pressure dissolution at grain contacts is in fact a key control on quartz cementation in sandstones we present data from highly overpressured sandstones in which despite the current burial temperature of 190 c quartz cement occurs in low amounts 4 6 1 2 of bulk volume in situ oxygen isotope data across quartz overgrowths suggest that cementation occurred over 100 ma and a temperature range of 80 150 c during which time high fluid overpressures resulted in consistently low vertical effective stress we argue that the very low amounts of quartz cement can only be explained by the low vertical effective stress which occurred throughout the burial history and which restricted silica supply as a result of a low rate of intergranular pressure dissolution at grain contacts

sources of ca2 in the major carbonate cements in eocene sandstones and conglomerates evidence from sr isotopes sr ca ratios and rare earth elements abstract different sources of calcium have important effects on the formation and distribution of carbonate cements on the basis of identifying all possible sources of calcium comprehensive quantitative analyses of sr c and o isotopic values and trace elements including rare earth elements were conducted to determine the ca2 sources and their changes in the same type of carbonate cements at different vertical positions in the same reservoirs ferroan calcite and ankerite were the major types of carbonate cements ca2 in ferroan carbonate cements in the sandstones near the mudstone sandstone m s interfaces was derived primarily from diagenetic fluids mixed with ca2 originating from the transformation of smectite to illite and from original sedimentary water in the adjacent mudstones however ca2 in the ferroan carbonate cements in the sandstones away from the m s interfaces originated from the transformation of smectite to illite the dissolution of carbonate minerals and original sedimentary water in source rocks the gradual change of ca source of the same type of ferroan carbonate cements indicates the diversity of migration direction of diagenetic paleo fluid in the same period during the transformation of smectite to illite concentration gradients were created from mudstones to sandstones ca2 mg2 fe2 and other cations in the mudstones diffused to the adjacent sandstones and provided sources of ferroan carbonate cements at the edge of the sandstones pore water with ca2 mg2 fe2 co32 and other cations in the source rocks was collectively charged into sandstones during oil emplacement and promoted the precipitation of ferroan carbonate cements in residual pore water due to oil emplacement and material supplied by the adjacent mudstones the content of ferroan carbonate cements in the sandstones was lower and the reservoir porosity was higher at a greater distance from the m s interfaces however in thick bedded sandstones due to lack of material supply from the adjacent mudstones ca2 mg2 fe2 and co32 in the ferroan carbonate cements were mainly derived from source rocks as a result no significant changes were observed in the content of ferroan carbonate cements and reservoir porosity in the vertical direction

impact of structural setting on diagenesis of fluvial and tidal sandstones the bahi formation upper cretaceous nw sirt basin north central libya abstract this petrographic mineral chemical fluid inclusion and stable isotopic study shows that the distribution of diagenetic modifications and their influence on reservoir quality and heterogeneity in tidal and fluvial sandstones of the upper cretaceous bahi sandstones in the rift sirt basin nw libya varies systematically along a series of closely spaced dominantly normal faults between the basin margin and more basinward located areas shallow eogenetic modifications resulting from the percolation of meteoric waters include infiltration of grain coating clays kaolinitization of detrital silicates and cementation by dolomite and k feldspar overgrowths mesogenetic alterations 70 c 2 km include feldspar albitization illitization of infiltrated clay and kaolinite conversion of kaolinite into dickite and cementation by quartz overgrowths t h 112 c 134 c barite t h 145 c 158 c and fe carbonates the restriction of barite and fe carbonate cements to the basinward located sandstones suggests formation by hydrothermal fluids along the faults extensive feldspar dissolution and formation of moldic pores in sandstones from the basin margin were probably caused by deep percolation of meteoric waters results from this study regarding the structural control on the spatial distribution of diagenetic alterations have implications for constraining the flux of pore fluids and by extension reservoir quality in analogous epicratonic rift basins

diagenetic signatures of stratal surfaces in the upper jurassic fulmar formation central north sea ukcs sedimentation rate and changes in relative sea level affect early diagenetic cementation along key stratal surfaces within the deeply buried upper jurassic fulmar formation south central graben ukcs as a result of the bioturbated nature of the shallow marine strata variations in ichnofabrics and cross cutting relationships between trace fossils are important in identifying and correlating stratal surfaces two transgressive and two regressive surfaces sequence boundaries were studied using petrographical mineralogical and geochemical techniques cements including early diagenetic pyrite framboids grain rimming apatite microcrystalline dolomite and ankerite occur at above and immediately below both transgressive and forced regressive surfaces sequence boundaries breaks or subdued rates of sedimentation associated with both types of surfaces meant that the sediment package resided within early diagenetic zones for prolonged periods of time enhancing diagenetic reactions at and below the surfaces the distribution of ankerite despite being a deep burial cement was primarily controlled by concentrations of bioclasts that are particularly abundant at transgressive surfaces the diagenetic character of the forced regressive surfaces sequence boundaries is more complex than that of the transgressive surfaces both in terms of mineralogy and paragenesis this is attributed to the superimposition of diagenetic assemblages as a result of erosion re sedimentation and sediment by passing although the diagenetic signature of the surfaces has been modified by dissolution replacement reactions during deep burial early diagenetic signatures can still be distinguished

meteoric burial diagenesis of pennsylvanian arkosic sandstones southwestern anadarko basin texas pennsylvanian arkosic sandstones at mobeetie field were deposited in fan deltas that prograded onto a shallow shelf in the southern anadarko basin the detrital minerals of the sandstones reflect the composition of the precambrian granites and granodiorites that were exposed in the nearby amarillo uplift distal margins of some fan delta lobes were reworked by marine processes and carbonate fossil fragments and oolites were mixed with terrigenous clastics the diagenetic history and hence reservoir quality of the distal marine reworked sandstones differ from those of the more proximal nonreworked sandstones the earliest diagenetic events formation of chlorite ooids and precipitation of fibrous submarine mg calcite cement took place in the depositional environment of the marine reworked sandstones on the basis of isotopic data the remainder of the diagenesis evidently occurred in fluids of meteoric origin shallow early meteoric diagenesis induced precipitation of iron poor calcite spar in reworked sandstones nonreworked sandstones lack calcite spar but contain early pore lining chlorite cement dissolution of aragonite by fresh meteoric ground water created moldic porosity within reworked sandstones as burial increased porosity in both the reworked and nonreworked fan delta sandstones was reduced by precipitation of authigenic quartz feldspar kaolinite fe calcite and ankerite oxygen isotope data suggest that these cements also precipitated from fluids of meteoric origin that became increasingly hot but remained relatively constant in isotopic composition the last diagenetic events were influenced by the overlying permian evaporites sodium rich fluids caused partial albitization of detrital plagioclase and sulfate derived from the evaporites precipitated as anhydrite and celestite

formation of zoned ankerite in gravity flow sandstones in the linnan sag bohai bay basin eastern china evidence of episodic fluid flow revealed from in situ trace elemental analysis abstract it has been shown that ankerite in clastic reservoirs can preserve an extended record of diagenesis and thus the fluid flow history in sedimentary basins ankerite cements with distinct zoning textures are well developed in gravity flow sandstones associated with magmatic intrusions in the linnan sag eastern china this example offering a rare opportunity to investigate the history of complex diagenetic fluid flow events in a lacustrine rift basin an integrated study including thin section petrography fluid inclusion micro thermometry carbon and oxygen isotope sem eds xrd cl bse epma la icp ms analyses was undertaken the aim is to resolve the mineralogical geochemical and isotopic composition of the sandstone and constitutional minerals the sandstones are mainly of lithic arkoses q43f34l23 with an average carbonate cements of approximately 8 2 wt the carbonate cements are dominated by zoned ankerite appearing as rhombohedral and poikilotopic blocky crystals calcite ferroan calcite dolomite and siderite cements are also present non carbonate cements including barite authigenic clay minerals and quartz overgrowth are common in the sandstone the 13 c values of ferroan calcite range from 4 2 to 1 2 pdb ferroan calcite was re precipitated at a temperature of 93 1 c at the expense of the dissolution of eodiagenesis carbonate cement in the sandstone ankerite was precipitated at temperature ranges from 107 8 to 140 9 c accompanied with the formation of pore filling barite the 13c values of ankerite range from 1 8 to 1 1 and 18o values range from 14 8 to 10 5 indicating an involvement of hydrothermal fluids in the formation of the zoned ankerite the hydrothermal fluids were probably related to the mafic magmatic activities and migrated into the sandstones along faults these fluids are rich in iron and magnesium and also contains abundant inorganic carbon dioxide and sulphate episodic charging into the reservoir the abundances of most of the elements including magnesium iron rees and trace elements in the zoned ankerite consistently decrease away from the magma intrusion centres we therefore conclude that multiple magmatic activities were primarily responsible for the zoned ankerite precipitation in the research area the intimate relationship between magmatic activity and carbonate cementation in sandstone reservoirs documented in this paper may have significant implications for studying fluid flow in lacustrine rift basins

the role of trace fossil ichnofabric analysis in the development of depositional models for the upper jurassic fulmar formation of the kittiwake field quadrant 21 ukcs

world energy developments 1981 the papers in this issue review significant petroleum and natural exploration and production activities throughout the world with the exception of the soviet union in addition papers on other energy minerals have been included to prepare these papers well data land status production statistics and geophysical activity information was accumulated from numerous sources such as foreign and domestic scouting services foreign and domestic governmental agencies at all levels industry organizations oil companies individual consultants and local representatives 1 fig 1 tab

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

iron mineralization and taphonomy of microfossils of the 2 45 2 21 ga turee creek group western australia abstract we report a new assemblage of carbonaceous microfossils intimately associated with siderite and fe silicates from a black chert nodule included in iron formation of the ca 2 45 2 21 ga turee creek group western australia this chert comprises microbial fabrics dominated by filaments preserved in matrix of nano to micrometric quartz filaments occur in clumps and in a cobweb like fabric interspersed with coarse crystalline void filling quartz granules we studied this chert with optical microscopy combined with scanning transmission electron microscope observations of focused ion beam sections of microfossils this distinguished three types of well preserved fossil interpreted as polysaccharide sheaths that usually do not preserve chains of cells trichomes type 1 comprises narrow filaments with thin continuous kerogen sheaths type 2 comprises narrow filaments with thick granular sheaths and type 3 comprises broad filaments with thin sheaths type 4 filaments are poorly preserved as granular kerogen organic ultrastructures of type 2 4 microfossils are variably replaced by siderite crystals associated with minor fe silicates iron isotope analyses on bulk powder and reactive iron fraction show indistinguishable and highly positive 56 fe values 1 45 relative to the reference irmm 014 indicating that the bulk of siderite derives from reduction of fe iii oxides this provides indirect evidence that the microbial community was originally associated with fe iii oxides siderite and fe silicates are found with type 2 4 but not type 1 filaments suggesting that only the former were encrusted by fe iii oxides which may have been reduced in situ siderite and fe silicates could result from oxidation of organic matter in filaments coupled with microbial and or thermal reduction of fe iii bio minerals the increasing abundance of siderite correlated with decreasing organic matter preservation in filaments supports that this reaction occurred to variable extents in situ on each microfossil type 2 4 microfossils may thus represent iron oxidizing bacteria these microbial mats display strong similarities with those associated with immediately overlying carbonate rocks of the turee creek group where filaments were interpreted as sulfur oxidizing bacteria some filamentous bacteria can oxidize both iron and sulfur such metabolic versatility could have enabled benthic microbial mats to thrive in the drastically changing chemical conditions of the great oxidation event

depositional and diagenetic controls on deeply buried eocene sublacustrine fan reservoirs in the dongying depression bohai bay basin china abstract this paper investigates the reservoir potential of deeply buried eocene sublacustrine fan sandstones in the bohai bay basin china by evaluating the link between depositional lithofacies that controlled primary sediment compositions and diagenetic processes that involved dissolution precipitation and transformation of minerals this petrographic mineralogical and geochemical study recognizes a complex diagenetic history which reflects both the depositional and burial history of the sandstones eogenetic alterations of the sandstones include 1 mechanical compaction and 2 partial to extensive non ferroan carbonate and gypsum cementation typical mesogenetic alterations include 1 dissolution of feldspar non ferroan carbonate cements gypsum and anhydrite 2 precipitation of quartz kaolinite and ferroan carbonate cements 3 transformation of smectite and kaolinite to illite and conversion of gypsum to anhydrite this study demonstrates that 1 depositional lithofacies critically influenced diagenesis which resulted in good reservoir quality of the better sorted middle fan but poor reservoir quality in the inner and outer fan lithofacies 2 formation of secondary porosity was spatially associated with other mineral reactions that caused precipitation of cements within sandstone reservoirs and did not greatly enhance reservoir quality and 3 oil emplacement during early mesodiagenesis temperatures 70 c protected reservoirs from cementation and compaction

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

diagenetic mobilization of manganese in peru basin sediments abstract the mn contents and sedimentation rates of two cores from different areas of the peru basin have been determined the southern core is associated with mn nodules of conventional slow accretion rates mm 106 yr while the northern one accompanies nodules with very high growth rates 102 mm 106 yr the depletion of mn observed within the top 40 cm of the cores is interpreted as resulting from a diagenetic remobilization of mn 2 due to the oxidative characteristics of the sedimentary column in both cores the calculated flux of mn provided by the sediment is higher than the total mn recovered in the overlying nodules this indicates that the mn content of the peru basin nodules can be supplied by diagenetic processes from the sediment not only for the slowly growing nodules but also for the extremely fast growing concretions found in the north of the basin

geochemical trends and palaeohydrological significance of shallow burial calcite and ankerite cements in middle jurassic strata on the east midlands shelf onshore uk this paper discusses isotopic and geochemical trends in shallow burial 1500 m carbonate cements in limestones from a middle jurassic shale dominated marine to paralic succession situated on the east midlands shelf the results are used to investigate carbonate shale interactions between the near surface realm of microbially mediated diagenesis and the deep thermobaric regime of clay mineral transformations and hydrocarbon generation they are also combined with published data from cements in stratigraphically and geographically adjacent strata to better understand the late mesozoic cenozoic palaeohydrology of central eastern england east midlands shelf midland platform shallow burial cements sampled from the blisworth limestone formation consist of ferroan calcite and calcic ankerite which are associated with ferroan neomorphic calcite and beef cone in cone calcite veins calcite cements are petrographically complex and variable yet record consistent compositional trends carbon isotope values 13c 1 2 to 1 7 pdb show that organic matter breakdown did not contribute significant carbon to the shallow burial pore fluids oxygen isotope data 18o 4 5 to 11 2 pdb are consistent with precipitation from modified marine pore fluids that displaced the original brackish depositional fluids from the limestone during palaeocene burial the same fluids accompanied cementation in underlying bajocian limestones ferroan calcite cements neomorphic spar and beef record a surprising correlation in fe2 and mg2 concentration that suggests a common source for the ions in the adjacent shales this trend is not present in the bajocian limestones and indicates that ionic mass transfer was on a smaller scale than the regional plumbing system the results suggest that middle jurassic limestone palaeoaquifers on the east midlands shelf hosted a westerly directed up dip flow from compacting upper and lower jurassic marine mudrocks in the sole pit basin during the cretaceous and early tertiary however early cretaceous meteoric groundwater palaeoaquifers recorded from cements in laterally equivalent middle jurassic limestones on the midlands platform and in the weald basin appear not to have extended northward into the east midlands shelf

facies characteristics and depositional models of highly bioturbated shallow marine siliciclastic strata an example from the fulmar formation late jurassic uk central graben abstract an extensive core database from the late jurassic fulmar formation illustrates the practicalities of facies analysis and depositional modelling in highly bioturbated heterolithic shallow marine siliciclastic strata twelve major sedimentary facies 1 12 are defined on the basis of lithology grain size visually estimated sand silt content and the presence or absence of primary sedimentary structures ichnofabric characteristics are also defined and have proved to be not always facies specific for purposes of depositional modelling the 12 facies types are resolved into six facies associations a f each representative of distinct shoreline shallow marine environments from these associations three broad depositional models have been constructed storm influenced shoreface model 1 bioturbated shoreface model 2 and a speculative bioturbated shelf model model 3 stratigraphical and palaeontological criteria suggest that models 1 and 2 are shoreline attached sedimentological data indicate that they occur as end members to a spectrum of shoreline settings constrained by incident wave energy bands for shorefaces subject to significant storm influence well preserved event beds occur seaward of the upper shoreface in the bioturbated shoreface model however the lower shoreface and adjacent shelf deposits are characterized by intense infaunal reworking the bioturbated shelf model displays no clear evidence of shoreline connection with the resultant facies often exhibiting highly abundant siliceous sponge spicules together with other open marine indicators such as belemnites and rare ammonites unlike the shoreface sand bodies which received sediment from basin margin fluvial systems shelf sand bodies in the fulmar formation are more likely to have been intrabasinally sourced along relay ramps or other forms of transfer zone this sediment may have undergone a long transit residence period on the shelf prior to eventual burial in hanging wall depocentres strongly influenced by zechstein salt withdrawal this study demonstrates that an ichnofabric approach as opposed to one of simple ichnodiversity is highly significant with respect to the description and modelling of highly bioturbated shallow marine siliciclastic strata in the fulmar formation ichnofabric analysis is capable of providing sensitive information on fluctuations in depositional energy levels water depth sedimentation rates erosion rates substrate consistency and dissolved oxygen levels evidence of these fluctuations is often apparent at intra facies scale but would largely go unnoticed using a conventional sedimentological approach to data gathering based on ichnodiversity at best depositionally significant changes in ichnofabric may occur without any significant change in ichnodiversity ichnofabric analysis also plays an important role in the hierarchical assessment of key stratal surfaces for purposes of sequence stratigraphic modelling

organic acid metastability in sedimentary basins thermodynamic calculations indicate that co sub 2 in sedimentary basins may be in stable redox equilibrium with organic acids found in basinal brines but that ch sub 4 is not these calculations also show that the high concentrations of carboxylic acids are from equilibrium with decarboxylation reactions a compilation of recent analyses of carboxylic acids from aqueous solutions associated with petroleum reservoirs shows that there is no trend in acid concentrations with temperature however inspection of these data at approximately 100 c indicates that acetic and propanoic acids are in homogeneous metastable equilibrium oxygen fugacities indicated by this metastable assemblage are consistently lower than those set by iron oxide assemblages this is not surprising because both hematite and magnetite are unstable relative to siderite in the presence of detectable concentrations of acetic acid

relationship between reservoir diagenetic evolution and petroleum emplacement in the ula field north sea abstract the diagenetic and petroleum filling histories of the ula field have been studied by analysing aqueous and petroleum inclusions occurring in authigenic cements this study shows that diagenesis continues actively after the arrival of petroleum in the sandstones although the reaction rates and petroleum saturation remain obscure microthermometric measurements of fluid inclusions in authigenic quartz suggest an onset of extensive quartz cementation at temperatures around 110 c i e 2 5 km and that this cementation has continued to the present day synchronously with quartz cementation authigenic albite was formed as inter and intragranular cements large amounts of petroleum inclusions locally occur in the albite and quartz cements these inclusions were trapped in the cements as the main charge of petroleum arrived at the structure diffusion through thin water layers between the petroleum and the mineral surface would probably have dominated mass transfer of solutes for the precipitation of authigenic quartz and albite it is evident from analysis of the petroleum inclusions in early formed k feldspar overgrowths that some secondary migrated petroleum probably arrived in the reservoir at a shallow burial depth before the rapid subsidence in the last 25 my compared with petroleum in the free porosity of the ula formation the petroleum in the k feldspar inclusions is evidently sourced from a different source facies and most maturity parameters testify to the latter being less mature later trapped petroleum inclusions in quartz and albite have characteristics found both in k feldspar and in the ula formation dst oil and thus is likely to reflect the progressive change in the ula field petroleum charge which occurred during the time period of quartz diagenesis

the kamoa copper deposit democratic republic of congo stratigraphy diagenetic and hydrothermal alteration and mineralization the kamoa copper deposit is located in the katanga province of the southern democratic republic of congo outside of the area previously considered prospective for large copper deposits with a 1 cu cutoff the hypogene ore zone of kamoa spans over 81 km2 varying in thickness from 2 to 15 m and is currently laterally unconstrained the deposit is hosted at the stratigraphic contact between hematitic and locally pyritic mwashya subgroup sandstones and overlying fine grained pyritic diamictites of the grand conglomerat unit of the nguba group the mwashya sandstones appear to have been deposited in a marginal marine to fluvial environment the grand conglomerat diamictite contains glacially derived mass transport and sediment gravity flow deposits the unit appears to have been deposited in a tectonically active locally anoxic marine environment the contact between the mwashya and grand conglomerat units represents a major change in depositional style during what was probably a period of rapid subsidence or sea level rise the majority of the kamoa orebody occurs within the lowermost portion of the grand conglomerat unit that contains siltstones with high concentrations of diagenetic framboidal and later cubic pyrite that may be indicative of early hydrothermal activity later hydrothermal alteration mineral assemblages within the grand conglomerat are stratigraphically zoned changing from a potassic alteration and silicification assemblage in the lowermost stratigraphic units to a dominantly magnesian alteration assemblage higher in the stratigraphy ore stage sulfide minerals are zoned vertically away from the mwashya grand conglomerat contact changing from chalcocite to bornite to chalcopyrite to pyrite with increased stratigraphic height copper sulfide minerals occur as fine grained disseminations within the diamictite matrix that probably represent replaced diagenetic pyrite as well as in coarse grained mineral rims on diamictite clasts the rims are best developed in the lowermost stratigraphic units and gradually lessen in size vertical elongation and abundance up stratigraphic section sulfur isotope studies indicate that most of the sulfur in the copper sulfides was derived from earlier formed diagenetic iron sulfide fluid inclusion analyses indicate that the ore forming fluid was saline 23 to 26 wt nacl wt equiv and had homogenization temperatures t h ranging from 210 to 240 c

dolomite from clay in argillaceous or shale associated marine carbonates abstract postcompactional late stage ferroan dolomite commonly occurs as 1 finely disseminated rhombic microspar in argillaceous lime mudstone 2 sparry cement within voids in argillaceous carbonates and 3 sparry cement within carbonates adjacent to marine shales the distribution and abundance of these dolomites correspond with that of clay deposited in the marine environment and cannot easily be explained by alternative models of dolomite formation conversion of smectite to illite during burial is a common diagenetic process which is capable of releasing large amounts of fe mg ca na and si sandstone petrologists have emphasized the significance of this process in generating quartz cements in sandstone shale sequences however the potential of this conversion for producing postcompactional dolomite cement in limestone shale sequences or in argillaceous limestones has been neglected smectite illite conversion is here invoked as the source of ions that formed late stage postcompactional dolomites in examples of the pennsylvanian of oklahoma kansas missouri and iowa the devonian of iowa and the ordovician of oklahoma the smectite illite conversion model for generating dolomite may be widely applicable to many other ex mples of late stage dolomitization it should prove useful in predicting the distribution of late stage dolomite cement that reduces reservoir quality and may prove useful for estimating the timing of hydrocarbon migration relative to carbonate cementation

palaeo climate controlled diagenesis of the westphalian c d fluvial sandstones in the campine basin north east belgium the westphalian c and d fluvial sandstones in the campine basin north east belgium are potential reservoirs for the sequestration of co2 and interesting analogues of the hydrocarbon reservoirs in the southern north sea although these sandstones were deposited in a relatively short period of time their reservoir properties and mineralogical compositions are very different a petrographic study complemented with stable isotope analyses fluid inclusion microthermometry and x ray diffraction analyses of the clay fractions of the sandstones which were sampled from deep boreholes 1000 m in the campine basin revealed that these differences are related mainly to the climate at the time of deposition the most important eogenetic processes affecting the westphalian sandstones were the generation of a pseudomatrix by physical compaction of al silicates and lithic fragments that were strongly altered by extensive meteoric leaching kaolinitization of unstable silicates and precipitation of siderite these processes had a detrimental influence on the reservoir properties of westphalian c sandstones but their impact on the westphalian d sandstones was minimal the difference is assumed to be related to the climate at the time of deposition which changed from tropical humid in the westphalian c to semi arid arid during the late westphalian d both the westphalian c and d sandstones were affected by similar mesogenetic processes mesogenetic quartz cementation resulted from chemical compaction and illitization of kaolinite k feldspar and smectitic clays illitization of kaolinite was controlled by the available quantities of co existing kaolinite and k feldspar and mainly affected the westphalian d sandstones illitization of k feldspar was controlled by the k feldspar content it had a much larger impact on the reservoir properties of the westphalian d as in these sandstones k feldspar was less affected by eogenetic alteration the illitization of smectitic clays resulted in illite quartz and ankerite cementation in both reservoirs this process had a more important impact on the westphalian c reservoir since cementation here also resulted from smectite to illite conversion in the interbedded and underlying shales the effect of mesogenetic alterations on the reservoir properties was much less drastic than the impact of eodiagenesis mesogenetic alterations do exert a significant control on the properties of the westphalian d the vast impact of eodiagenesis on the westphalian c sandstones made them less susceptible to mesogenetic alteration the effect of telogenetic processes on the porosity and permeability of the westphalian sandstones was small and restricted to the top reservoir intervals that directly underlie the cimmerian unconformity no significant telogenetic alterations related to the variscan unconformity were observed

illite distribution and morphology explaining basinal variations in reservoir properties of upper jurassic sandstones danish north sea abstract growth of fibrous illite may destroy an otherwise promising sandstone reservoir even sandstones characterized by microquartz coatings therefore detailed petrographical and diagenetic investigations were performed in order to understand the factors that control illite precipitation and other factors affecting the reservoir properties in the upper jurassic heno formation the heno formation is encountered in large parts of the danish central graben where its lower part the gert member was mainly deposited in back barrier environments and its upper part the ravn member was deposited in a shoreface environment in total 215 core samples from 15 wells were investigated petrographically by optical and scanning electron microscopy this was supplemented by plug porosity and permeability measurements and x ray diffraction of bulk and clay fraction the investigations show among other things that microquartz coatings may help to preserve porosity and permeability whereas the permeability is reduced when microquartz occurs combined with fibrous illite illite precipitation is largely governed by the amounts of k feldspar present during burial consequently the initial abundance of k feldspar in the sediment source area and its alteration during and immediately after deposition in particular during subaerial exposure at sequence boundaries has a major influence on the final reservoir properties of the sandstones the risk of illite precipitation can largely be predicted from the available sediment source material depositional environment and burial depth

sediments and micronodules in the northern and central peru basin the mineralogy and composition of sediments and manganese micronodules from the northern and central sectors of the peru basin are discussed because of the proximity of the basin to the carbonate compensation depth c c d surface sediments vary between calcareous oozes and siliceous muds besides biogenic components clay minerals are important by far the most abundant clay mineral is smectite which is thought to be of diagenetic origin on a carbonate free basis the surface sediments are rather uniform in composition throughout the basin and are similar in composition to those of the equatorial pacific sedimentation rates for the uppermost core sections are in the range of 3 to 5 mm 1000 yr

quartz and fe dolomite cements record shifts in formation water chemistry and hydrocarbon migration in devonian shoreface sandstones ghadamis basin libya cementation by quartz overgrowths and subsequently by carbonates is a very common global paragenetic phenomenon in sandstones of many basins that is poorly constrained in the literature integrated

isotopic evidence on the diagenetic evolution of coastal sabkha reservoirs from the solimoes basin northern brazil abstract the isotopic composition of diagenetic carbonates sulfates illites and associated diagenetic constituents of the carboniferous jurua sandstones solimoes basin largest hydrocarbon reservoirs of northern brazil was analyzed in conjunction with petrographic characterization and chemical composition eolian jurua sandstones deposited in a coastal sabkha of a wide cratonic sag are the best reservoirs but diagenetic processes introduced strong and complex heterogeneities which affect hydrocarbon production dolomite and anhydrite cementation together with compaction exerts the main control on the quality of the reservoirs early dolomite cements precipitated under strong evaporation while late dolomites are related to thermal decarboxylation late anhydrite cements are related to the interbedded evaporites and possibly to the oxidation of dissolved sulfide authigenic illite k ar ages are related to the voluminous triassic basic magmatism around 200 ma and to the jurassic cretaceous tectonism around 150 ma which may have also affected the late anhydrite and dolomite ankerite cementation this isotopic study revealed the major conditions of the diagenetic processes that strongly affected the jurua reservoirs which are essential for the development of models that will optimize the production as well as decrease the risks in the exploration for new jurua reservoirs

low mg calcite marine cement in cretaceous turbidites origin spatial distribution and relationship to seawater chemistry lower cretaceous hauterivian bioclastic sandstone turbidites in the scapa member north sea basin were extensively cemented by low mg calcite spars initially as rim cements and subsequently as concretions five petrographically distinct cement stages form a consistent paragenetic sequence across the scapa field the dominant and pervasive second cement stage accounts for the majority of concretions and is the focus of this study stable isotope characterization of the cement is hampered by the presence of calcitic bioclasts and of later cements in sponge spicule moulds throughout the concretions nevertheless trends from whole rock data augmented by cement separates from synlithification fractures indicate an early calcite 18o value of 0 5 to 1 5 pdb as such the calcite probably precipitated from marine pore fluids shortly after turbidite deposition carbon isotopes 13c 0 to 2 pdb and petrographic data indicate that calcite formed as a consequence of bioclastic aragonite dissolution textural integrity of calcitic nannoplankton in the sandstones demonstrates that pore fluids remained at or above calcite saturation as expected for a mineral controlled transformation electron probe microanalyses demonstrate that early calcite cement contains 2 mol mgco3 despite its marine parentage production of this cement is ascribed to a combination of an elevated aragonite saturation depth and a lowered marine mg2 ca2 ratio in early cretaceous calcite seas relative to modern oceans scapa cement compositions concur with published models in suggesting that hauterivian ocean water had a mg2 ca2 ratio of 1 this is also supported by consideration of the spatial distribution of early calcite cement in terms of concretion growth kinetics in contrast to the dominant early cement late stage ferroan 18o depleted calcites were sourced outwith the scapa member and precipitated after 1 2 km of burial our results emphasize that bioclast dissolution and low mg calcite cementation in sandstone reservoirs should not automatically be regarded as evidence for uplift and meteoric diagenesis

geochemistry and diagenesis of stratabound calcite cement layers within the rannoch formation of the brent group murchison field north viking graben northern north sea abstract up to four calcite cemented horizons doggers form impermeable barriers to fluid flow within the middle jurassic rannoch formation and are correlatable across the murchison field calcite precipitated during early diagenesis within high porosity permeability sandstones at the top of coarsening shoaling upward shoreface cycles calcite 13c and 18o compositions range from 4 1 to 13 4 pdb and 6 6 to 16 7 pdb respectively sr concentrations of up to 1334 ppm are consistent with marine carbonate sources probably shell fragments but no viable intraformational carbonate source has been identified in the murchison field area initial 87sr 86sr compositions 0 71109 0 71266 are higher than middle jurassic seawater 0 7073 and consistent with precipitation from modified porewaters containing significant proportions of continentally derived meteoric fluids enriched in 87sr as a result of basement weathering or percolation through hinterland soils unconsolidated detritus an internal source of 87sr is not considered viable in view of the high proportion up to 25 clastic constituents of unaltered detrital alkali feldspar and mica within the rannoch formation geochemical and isotope data indicate correlations between increasing 18o composition and increasing iron and magnesium content within calcite calcium concentrations decrease with increasing 18o for calcites geochemical data trends can be interpreted differently in terms of either static or evolving 18o porewater models static 18o porewater models using jurassic early cretaceous meteoric water 18 o 6 smow predict cement precipitation temperatures of 17 77 c however 13c compositions are more depleted than those typical of carbon derived from shell debris and correlation between decreasing 18o and decreasing 13c suggests modification of a meteoric derived porewater system during burial via contribution of low 13c carbon derived from a deep basinal source however it is difficult to constrain accurately the degree to which porewater 18o may have evolved during burial the degree to which porewater 18o could have evolved from that of a jurassic early cretaceous meteoric water can be loosely constrained as from 18 o 6 smow to 18 o 2 smow if the temperature at which the most 18o depleted cements precipitated is limited by present day bottom hole temperatures 110 c for wells currently at maximum burial depth

compaction processes and rock properties in uplifted clay dominated units the egersund basin norwegian north sea abstract a thick succession of fine grained sediments in any sedimentary basin can serve as source if organic rich and top seals both sealing and petroleum generation capacity of any fine grained succession is dependent on compaction history mechanical and chemical compactions of the sediments in the egersund basin a thick succession of fine grained sediments of upper jurassic lower cretaceous age overly middle jurassic reservoir sands the egersund basin however has experienced a complex depositional and tectonic history particularly late exhumation neogene which complicates the overall burial thermal and pressure history in this study petrophysical and acoustic properties bulk density and p wave velocity of 10 exploration wells from two neighboring blocks in the egersund basin were utilized to investigate compaction processes in the area in addition x ray diffraction xrd and scanning electron microscopy sem analyses from selected wells and available total organic carbon toc data were used as guide to interpret the compaction processes and resulting rock properties the results show that the transition from mechanical to chemical compaction is not sharp the onset of chemical compaction imposes a measurable deviation in the burial depth trends of the petrophysical properties expected from purely mechanical compaction the continuation of the depth related trends however indicates that chemical compaction may occur in concurrent with continuing mechanical compaction the observed compaction varies between the different clay dominated stratigraphic units indicating that the major control on both mechanical and chemical compaction is the primary sediment composition

diagenesis and reservoir properties of middle jurassic sandstones traill o east greenland the influence of magmatism and faulting the c 500 m thick middle jurassic sandstones of the fluvial bristol elv and marine pelion formations of the east greenland basin are evaluated here in order to improve the understanding of the processes that influenced the diagenetic evolution the study may help to predict the reservoir properties of sandstones affected by magmatism and faulting both in general and specifically in undrilled areas on and offshore east greenland and in the voring basin on the mid norwegian shelf the study shows a variety of authigenic mineral phases dominated by quartz cement carbonate cement illite and iron oxide one of the clear differences between the two formations is the presence of early carbonate cemented horizons in the marine sandstones these horizons are inferred to reflect a primary concentration of biogenic clasts and fossil shells intense quartz cementation occurs primarily in the fluvial sandstones but the marine sandstones are also highly quartz cemented two episodes of burial and uplift are recorded in the diagenetic sequence and widespread grain crushing in coarse grained intervals is believed to result from overpressure and subsequent compaction due to sudden pressure release along major faults maximum burial depths may only have been around 2000 2500 m cathodoluminescence analyses show that grain crushing was followed by intense quartz cementation the quartz cement is to a great deal believed to have formed due to increased surface area from crushing of detrital quartz grains creating fresh nucleation sites for the quartz cathodoluminescence investigations also show that only minor pressure dissolution has taken place between detrital quartz grains and that the ubiquitous quartz cementation displays several growth zones and was thus in part the result of the introduction of silica rich extra formational fluids related to the flow of hot fluids along reactivated faults and increased heat flow and temperature due to magmatism this interpretation is supported by fluid inclusion homogenization temperatures between 117 and 158 c in quartz cements in one of the two study areas the development of macroscopic stylolites has significantly enhanced quartz cementation probably in connection with thermal convection flow as a result of the magmatic and fault related quartz cementation and illitization the reservoir quality of the sandstone formations deteriorated and changed drastically

vertical effective stress and temperature as controls of quartz cementation in sandstones evidence from north sea fulmar and gulf of mexico wilcox sandstones abstract we present quantitative petrographic data high spatial resolution oxygen isotope analyses of quartz cement basin modelling and a kinetic model for quartz precipitation for two paleocene eocene wilcox group sandstones from texas and two jurassic fulmar formation sandstones from the central north sea at each location one sandstone has been buried to ca 145 c and one to ca 185 c a key difference between the wilcox and fulmar burial histories is that the wilcox sandstones are currently at higher vertical effective stresses and from basin modelling studies have been subjected to generally higher vertical effective stresses through their burial history the amounts of quartz cement in the wilcox sandstones are between 12 and 18 and between 2 and 6 in the fulmar sandstones high spatial resolution oxygen isotope data obtained from the quartz cements suggest temperature ranges for quartz precipitation from 60 to 80 c to values approaching maximum burial temperature factors such as grain coatings or the timing of petroleum emplacement cannot explain the differences in the amounts of quartz cement petrographic data show that most of the silica for quartz cement can be derived from intergranular pressure dissolution although the sample set is small we interpret the results to suggest that the differences in quartz cementation in fulmar and wilcox sandstones can be explained better by differences in their vertical effective stress history than their temperature history in this case the supply of silica rather than the precipitation of quartz becomes an important control on the rate and extent of cementation

the origin of authigenic ankerite from the ninian field uk north sea the carbonate minerals ankerite and ferroan dolomite are widely reported in studies of concretions1 2 and of sandstone hydrocarbon reservoirs3 8 these minerals are often thought to have formed relatively late in the sandstones diagenetic history and in the gulf coast their occurrence has been linked to clay mineral diagenesis in the surrounding shales9 i describe here authigenic ankerite from the ninian field10 ankerite formation was probably synchronous with hydrocarbon emplacement and is related to clay mineral diagenesis in the hydrocarbon source rocks of the viking graben it occurs throughout the reservoir sandstones but is particularly abundant below the oil water contact this distribution reflects hydrocarbon emplacement arresting diagenesis at progressively deeper levels of the reservoir the resulting preferential cementation may adversely affect transmissibility between the aquifer and the oil column and consequently could reduce hydrocarbon recovery

exceptional preservation of fossil plant spores in high pressure metamorphic rocks it is commonly believed that high grade metamorphism erases all traces of fossils and more generally of biogenicity in rocks to address this issue we report in this study x ray spectromicroscopy raman microspectroscopy and electron microscopy observations at the nm scale of fossil lycophytes megaspores found in triassic metasedimentary rocks from the french alps these rocks have undergone high pressure metamorphic conditions not vert similar 14 kbar not vert similar 360 c during the alpine orogeny corresponding to a burial of not vert similar 35 km during subduction despite such metamorphism the fossil megaspores are remarkably well preserved texturally an unusual mineralogical zonation consisting of magnetite ankerite and calcite is superposed on the structures of the spore walls in parallel chemical heterogeneities of the organic matter om composing the spore walls were detected we discuss different interpretations for our observations in particular a scenario in which these features are remnants of the original structural heterogeneities in the spores and provide insights on the fossilization processes under high grade metamorphic conditions whatever the processes leading to such heterogeneities our observations oppose the usual assumption that high grade metamorphism erases all indications of biogenicity in rocks and open new avenues for investigation at the nm scale of the evolution of organic matter during diagenesis and metamorphism

dawsonite and ankerite formation in the ldx 1 structure yinggehai basin south china sea an analogy for carbon mineralization in subsurface sandstone aquifers abstract the geochemistry and petrology of the ldx 1 structure of the yinggehai basin a natural analog of a sedimentary carbon storage site was investigated to understand the consequences of the charging of co2 gas in this system the rocks in this structure are dominated by subarkose and sublitharenite sandstones the authigenic minerals formed after co2 injection are dawsonite microcrystalline quartz kaolinite and ankerite dawsonite and ankerite are formed just beneath a co2 bearing anticlinal structure due to the reactions between silicate minerals feldspars and clay minerals and the fluid phase carbon and oxygen isotopic analyses indicate that the main carbon source for dawsonite and ankerite formation was mantle magmatic co2 the aqueous activities of sodium and calcium the partial pressure of co2 ph and temperature are the key factors influencing the stability of the dawsonite and ankerite the presence of the anticlinal structure maintaining a locally high co2 partial pressure in the waters beneath this structure is likely responsible for the observed long term persistence of dawsonite and ankerite in this system

sedimentological aspects of the humber group upper jurassic of the south central graben uk north sea complex structure poorly understood sedimentology and poor biostratigraphic control make the upper jurassic humber group of the south central graben one of the least understood and most complex hydrocarbon reservoirs of the north sea detailed logging of available core from 19 exploration wells has been combined with an improved understanding of the relevance of trace fossils and a recognition of important base level variations to provide a greatly enhanced understanding of the depositional system active within the area at that time a new sedimentological model based upon the distribution of facies and facies associations illustrates that upper jurassic structure and consequent basin geometry were the principal controls upon the distribution of depositional environments rifting and second order transgression controlled the back stepping onlap patterns observed and higher frequency base level fluctuations controlled the internal architecture of individual sandbodies the model presented accounts for features of these deposits that were previously considered anomalous such as the thickness of bioturbated sandstones paucity of foreshore deposits and complex age relationships of sands

climatic depositional and burial controls on diagenesis of appalachian carboniferous sandstones qualitative and quantitative methods abstract carboniferous quartz lithic and feldspar rich sandstones from different stratigraphic intervals within the central appalachian basin contain evidence for eogenetic and mesogenetic diagenesis important eogenetic products include fe oxide oxyhydroxide siderite kaolinite chlorite and calcite whereas quartz albite illite sericite fe carbonate and secondary porosity are the most important mesogenetic components correspondence and detrended correspondence analysis of the framework grain and diagenetic mineral data set has guided diagenetic interpretations and leads to two major conclusions first minerals that formed early appear to have been influenced by stratigraphic position the distribution of siderite and iron oxide oxyhydroxide is considered to reflect a second order paleoclimatic signature recognized throughout the carboniferous where siderite formed during everwet periods and iron oxide oxyhydroxide during semi arid conditions reflecting differences in redox second framework grain composition controlled the distribution of diagenetic products in the burial environment lithic arenites are deficient in authigenic quartz yet have undergone various degrees of illitization quartz deficiency is attributed to compaction related loss of primary porosity relatively early which inhibited sufficient fluid flow to account for much higher concentrations of quartz cement in the more competent quartz rich sandstones

burial evolution of evaporites with implications for sublacustrine fan reservoir quality a case study from the eocene es4x interval dongying depression bohai bay basin china abstract eocene sublacustrine fan sandstones that developed in a rift basin are important tight gas reservoirs in the dongying depression bohai bay basin northeastern china two units of evaporites developed at the top and bottom of the lower unit of the es4 interval es4x consist predominantly of anhydrite with subordinate gypsum evaporite and related diagenetic processes greatly influenced reservoir quality 13 c values for micritic dolomite cements in es4x are depleted 7 45 to 2 57 due to microbial sulfate reduction msr under shallow burial conditions and this interpretation is supported by large 34 s fractionation between anhydrite and framboidal pyrite precipitation temperatures for micritic dolomite are calculated as 57 5 72 8 c anhydritization of gypsum probably occurred at 100 5 145 2 c during progressive burial as evidenced by homogenization temperatures of aqueous inclusions within anhydrite cements this process resulted in dehydration fractures within anhydrite cements that increased reservoir permeability by connecting isolated pores thermochemical sulfate reduction tsr probably resulted in dissolution of gypsum and anhydrite cements under relatively deep burial conditions ankerite cements are replaced by anhydrite cements and are enclosed by solid bitumen in es4x ankerite cements likely were derived from tsr as reflected in negative 13 c values 7 12 to 3 70 and high calculated temperatures 121 3 185 1 c dissolution by products e g saddle dolomite ankerite nodular pyrite related to tsr precipitated in adjacent pores a lack of significant 34 s fractionation between parent sulfate and nodular pyrite indicates that tsr occurred in a relatively closed system therefore dissolution of gypsum and anhydrite related to tsr contributed little to reservoir quality middle fan lithofacies with better sorting porosity and permeability than inner and outer fan lithofacies constitute high quality reservoirs in es4x

stratigraphic control on laterally persistent cementation book cliffs utah field data from the upper cretaceous blackhawk formation in the book clitfs utah colorado demonstrate the presence of laterally extensive carbonate cements beneath major marine flooding surfaces the occurrence of firmgrounds compaction around concretions and high minus cement porosities indicate that cementation occurred during early diagenesis it is proposed that the early cementation associated with these marine flooding surfaces occurred as a result of reduced sediment accumulation rates or non depositional hiatuses linked to abrupt deepening during marine flooding these reduced sediment accumulation rates extended sediment residence time within early diagenetic zones leading to enhanced early cementation in shallow marine sandstones and mudstones