permeability prediction from geologic models permeability is a critical parameter for the petroleum geologist by simulating the processes of compaction and cementation in a model porous medium we have gained a new understanding of how permeability is controlled in reservoir sandstones this understanding can be used predictively for simple sandstones the geometry of the model pore system is completely defined so permeability can be calculated directly using a network model for flow the calculation is based on first principles and is physically rigorous in contrast to many previous efforts to predict permeability there are no adjustable parameters in the calculations and no additional measurements or correlations e g capillary pressure data or pore system data from thin sections are required the model derived porosity permeability trend for a compacted or quartz cemented sandstone or a sandstone having a combination of these processes closely matches measurements on fontainebleau sandstone samples whose permeabilities span nearly five orders of magnitude the model also correctly predicts mercury injection measurements of pore throat size distribution for the fontainebleau sandstone pore scale geometric features of the model are found to be spatially correlated and this departure from randomness significantly affects macroscopic properties such as permeability the agreement between predictions and measurements suggests that spatial correlation is inherent in granular porous media and consequently uncorrelated or arbitrarily correlated models of transport in such media are unlikely to be physically representative we also discuss extending the model to predict properties of more complicated rocks

core based evidence for sandy slump and sandy debris flow facies in the pliocene and pleistocene of the gulf of mexico implications for submarine fan models examination of nearly 3 500 feet of conventional core from pliocene and pleistocene deep water reservoirs cored in 25 wells in 8 different areas covering the eastern central and western gulf of mexico reveals that the reservoirs are predominantly composed of mass transport deposits mainly sandy slumps and sandy debris flows 60 100 of cored intervals bottom current reworked sands are common 10 50 of importance to existing submarine fan models is that turbidities are extremely rare 1 of all cores sedimentary features indicative of slump and debris flow origin include sand units with sharp upper contacts slump folds discordant steeply dipping layers up to 60 degrees glide planes shear zones brecciated clasts rafted mudstone clasts planar clast fabric inverse grading of clasts and moderate to high matrix content 5 20 these reservoirs have been interpreted by others to represent turbidite dominated basin floor fans and slope fans of the often used sequence stratigraphic model however our core data do not show a dominance of turbidities sandy debris flows exhibit a variety of log motifs e g blocky fining up and coarsening up due to changes in concentration of midstone clasts and a variety of internal seismic facies e g parallel continuous irregular discontinuous chaotic discontinuous and lateral pinch out perhaps due to changes in stacking patternsmore of debris flows and slumps classic submarine fan models commonly advocated for these reservoirs may not be appropriate we propose a slump and debris flow dominated slope model in which sea floor topography and depositional freezing i e plastic flows control sand distribution and geometry contrary to popular belief sandy debris flows can be thick areally extensive and excellent reservoirs less

petrography and reservoir physics ii relating thin section porosity to capillary pressure the association between pore types and throat size porosity in reservoir rocks is configured into a few types of pores whose size and shape are controlled by depositional fabric and postdepositional processes the size shape and abundance of each pore type can be objectively determined from thin section using image analysis and pattern recognition procedures each pore type tends to be associated with a limited range of throat sizes the association between pore type and throat size can be determined using regression procedures linking pore type data obtained from thin section with capillary pressure data to do so a set of samples is required wherein the association between pore type and throat size is fixed but where pore type proportions vary between samples this condition is met by a sample suite representing res rvoir facies from a single core or in many cases from a single field the relationship between pore type and throat size is an effective means to relate reservoirs in terms of the efficiency of the porous microstructure to multiphase flow parameters derived from the relationship can be used to construct accurate physical models that subdivide physical response in terms of the contributions of each pore type

evidence for episodic cementation and diagenetic recording of seismic pumping events north coles levee california u s a abstract bands of calcite and dolomite cements alternating with zones of nearly carbonate free sand occur in the stevens sandston aat north coles levee san joaquin valley california temperatures calculated from o isotopes suggest that the calcite cement bands were emplaced episodically as a result of repeated injections of hot water from deeper in the section burial analysis suggests that these cements precipitated from 7 ma to the present over the temperature range of 45 to 95 c carbon isotope data suggest that the c in the cements is a mixture derived from two sources detrital shell material 13c pdb and co2 liberated from maturing kerogen 13c 24 plots of 13c vs time and depth of crystallization show that the cementation sequence was 1 dolomite cements possibly concretionary precipitated at depths a log of 13c vs calculated depth of crystallization correlates with the stratigraphic column at north coles levee if the correlation is valid the light 13 in each cement sample can be tied to its source a model based on this interpretation suggests that the early light c was derived from maturing kerogen in the kreyenhagen formation eocene as it passed through the oil window between 4 and 5 ma the subsequent passage of younger sediments with less organic material produced correspondingly smaller amounts of light co2 which was reflected in the relatively heavier c isotopes in the later cements it is suggested that the epidsodic injections of hot water carried dissolved gases and minerals principally calcite upward from rocks as deep as 2 3 km below the stevens sandstone and reprecipitated the calcite in more permeable zones in the rock degassing of co2 from rising pore waters likely triggered the precipitation and accounts for the relatively large volumes of cement the sibson model for seismic pumping of pore fluids is considered a likely explanation for the observed cementation

recent developments in dating ancient crustal fluid flow several isotopic techniques have recently been developed or substantially improved allowing the accurate determination of the timing of fluid related processes these include k ar rb sr and sm nd dating of diagenetic minerals in clastic rocks u pb dating of carbonates sm nd dating of fluorite and uraninite re os and rb sr dating of sulfides 40ar 39ar and rb sr dating of fluid inclusions and u pb dating of ilmenite magnetite sphene and rutile while many of these techniques require further evaluation some have already resulted in new constraints on a variety of crustal fluid flow models

putting transport into water rock interaction models new quantitative models for water rock interaction are needed that explicitly couple fluid flow and chemical reactions one such model is based on the finite difference method and is applicable to both one and two dimensional reactive flow in applying the model we find that ignoring the coupling between hydrodynamic dispersion and chemical reactions leads to incorrect predictions of the rates of reactions along fluid flow paths and in cases of which reactions will occur

preservation of primary porosity in deeply buried sandstones a new play concept from the cretaceous tuscaloosa sandstone of louisiana abstract

role of microbial processes in linking sandstone diagenesis with organic rich clays abstract numerous chemical reactions within clay sequences have been proposed to produce dissolved material for diagenesis of deeply buried sands however reactions responsible for solubilizing inorganic and organic constituents in clays at intermediate depths and their importance to sandstone diagenesis have not been evaluated results from this study show that the processes of microbial organic acid production via fermentation in clays and microbial organic acid consumption via sulfate reduction in sands effectively link organic rich clays to sandstone diagenesis in the black creek formation of south carolina diagenetic processes have resulted in the formation of 10 volume percent calcite cement 0 1 volume percent authigenic pyrite and 1 5 volume percent secondary porosity in black creek sands however the distribution of these diagenetic processes is not uniform resulting in net destruction of porosity in some parts of the sand and net porosity enhancement in other parts mass balance derived rates of microbial organic acid production 10 5 mmole carbon 1 1 yr 1 and microbial co2 production 4 10 5 mmole l 1 yr 1 show that microbial processes can account for all organic carbon in the calcite cements at least 11 of carbonate carbon based on isotope balance calculations all observed authigenic pyrite and all observed secondary porosity these findings show that microbial processes can serve to link organic rich clays with sandsto e diagenesis at intermediate depths

geothermometry from brine analyses lessons from the gulf coast u s a abstract geothermometers based on mineral water equilibria are of limited use at temperatures characteristic of hydrocarbon production from sedimentary basins metastable detrital phases such as alkali feldspars and plagioclase persist to temperatures characteristic of the deepest wells sasmpled approaching 200 c and equilibria involving stable phases such as albite illite and quartz are not commonly achieved empirical relations such as that between mg and li are more successful in predicting subsurface formation water temperatures than are equilibrium relations involving quartz carbonates or feldspars the concentrations of most components in saline formation waters seem to be governed by kinetic rather than by equilibrium considerations

numerical models of fluid flow in carbonate platforms implications for dolomitization abstract the role of fluid flow in dolomitization is evaluated through hydrologic simulations designed to capture and test numerically the essential elements of some common dolomitization models a two dimensional finite difference code bethke 1985 was used to simulate single phase flow of variable salinity fluids driven by compaction convection and topography in carbonate platforms and adjacent basinal sediments a simple hydrostratigraphic model consisting of a platform reef complex surrounded by basin filling shale was constructed to examine the magnitude and direction of compaction flow during the initial phase of subsidence most pore fluids in the shale migrate to the surface at velocities less than 0 1 mm yr and basin derived fluids are focused into the reef from only a limited distance of 10 km relatively porous and permeable basinal limestones and sandstones may act as conduits for updip fluid flow into the platform reef complex magnesium mass balance constraints coupled with hydrologic simulations suggest that burial compaction is not an efficient mechanism for regional dolomitization two layer hydrostratigraphic models involving variable salinity fluids show that evaporation of seawater results in downward and seaward fluid migration the magnitude of flow is a function of fluid density even fluids at salinities marginally elevated above normal seawater values are capable of penetration several hundreds of meters into the underlying sediments albeit at rates of less than 1 cm yr paleohydrologic simulations coupled with thermodynamic arguments support that reflux of surface derived waters evaporated past gypsum saturation is a viable dolomitization mechanism calculations utilizing rates of mg mass flux show that dolomitization of large carbonate platforms 10 km3 by fluids below halite saturation requires millions of years of reflux driven flow p thermal convection of seawater through carbonate platforms has been simulated by varying the basal heat flow the thermal pump produces a convection cell in which lateral and vertical flow achieve rates of 75 0 and 13 0 mm yr respectively the potential for dolomitization is high because a large reservoir of mg seawater is tapped elevated temperatures drive fluids further into the stability field for dolomite and kinetic inhibition of dolomite nucleation and growth is decreased simulations of high frequency glacio eustatic sea level oscillations suggest that a dynamic hydrologic regime may be established a sea level fall of 2 m lasting for 10 000 yr can drive lateral flow at rates up to 0 15 m yr dolomitization is possible because marine waters migrate through the carbonate platform in response to gradients of hydraulic head created by eustatic sea level fluctuations

illite smectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the texas gulf coast paleocene eocene abstract mudrock samples from a well penetrating eocene and paleocene strata 1235 4455 m of the texas gulf coast have been studied to assess geochemical redistribution in mudrocks during burial diagenesis and the mechanism by which detrital smectitic illite smectite i s is transformed into diagenetic illitic i s idealized smectite and illite end member compositions for interstratified i s are estimated to be k0 00x 10 56mg0 39fe0 57al1 13si3 90o10 oh 2 and k0 53x 10 18mg0 17fe0 16al2 28 si3 40o10 oh 2 respectively there is no obvious difference in the amount of k addition to i s in random and ordered interstratified i s aluminum substitution into both tetrahedral and octahedral sites suggests that the smectite to illite reaction is a complete dissolution precipitation reaction rather than a solid state k 1 al 3 for si 4 substitution reaction depth relat d mineral trends and mass balance calculations suggest that illitic i s may form from kaolinite and possibly illite or mica as well as from smectitic i s although most whole rock element abundances are invariant over the sampled depth interval k2o content increases from ca 2 0 weight percent at 1500 m of burial to ca 3 8 weight percent at 4000 m some of this increase is probably caused by import of k into the shale via fluids derived from interbedded sandstones implying that gulf coast mudrocks behave as open chemical systems during burial diagenesis if this is the case a minimum of 103 pore volumes of fluid must have passed through the most k enriched shales to have introduced the added potassium provided that the k content of present day formation water is representative of that of ancient formation water

differential mobility of elements in burial diagenesis of siliciclastic rocks abstract mass balance calculations involving smectite rich siliciclastic mudstones sampled from boreholes show that significant k2o addition and cao loss correlate strongly with the progress of illitization of smectite and with the dissolution of calcite the calculations are based on the immobility of al2o3 and require a chemically homogeneous sediment at deposition such that the shallower samples represent protolith compositions of deeper samples constancy of ratios of tio2 al2o3 are used to 1 establish that both tio2 and al2o3 have similarly low differential mobilities and 2 identify stratigraphic sections available in the literature that are sufficiently homogeneous to allow the calcu ations our calculations show that most major oxide components are immobile at the 5 limit of resolution of the calculations exceptions are k2o which is almost universally added to mudstones and cao which is commonly removed from mudstones during diagenesis the addition of up to 50 k2o correlates with percent illite in mixed layer illite smectite indicating that illitization of smectite involves fixation of k2o from diagenetic fluids removal of 40 cao from some sections correlates with decreasing calcite mode and is attributed to dissolution of calcite together these reactions lead to a 10 15 loss of mineral volume a volume loss that must be added to porosity loss when modeling compaction illitization and carbonate dissolution may be linked because k fixation during illitization decreases the ph of the local fluid a reverse weathering reaction which enhances carbonate dissolution addition of up to 20 na2o in some rocks may be caused by albitization of plagioclase but na2o addition is not pervasive the apparent lack of mass transfer associated with dissolution of kaolinite and feldspars or precipitation of chlorite implies that these reactions are balanced by other reactions such that their oxide components in mudstones are conserved limited data from sandstone mudstone pairs are consistent with transfer of k2o from sandstones to mudstone via rising diagenetic fluids except for cao and the alkalis diagenesis of siliciclastic mudstones appears isochemical 5 relative for most major oxide components the much larger changes recorded in some reservoir sandstones are clearly exceptions and may not reflect changes occurring at the scale of kilometers

evidence for vertical movement of fluids gulf coast sedimentary basin sediments between 10 and 15 km thick have prograded approximately 400 km into the small oceanic basin which formed along the southern margin of north america subsequent to early mesozoic rifting redistribution of mass during burial diagenesis and the chemistry of present day formation waters in jurassic through pleistocene formations document local kilometer scale and in cases basin scale vertical fluid transport within the sedimentary wedge fluid flow is in part driven by porosity reduction which takes place by both physical and chemical processes during burial vertical and horizontal transport also occurs as the result of thermally and salinity generated buoyancy effects input of fluids into the sedimentary wedge from the underlying crust is predicted but not documented

sem imaging of bacteria and nannobacteria in carbonate sediments and rocks abstract bacteria and nannobacteria 0 1 m dwarfed forms are abundant in both non etched and gently etched travertines deposited by hot springs in viterbo italy they are also plainly visible in bahama ooids and hardgrounds and in great salt lake aragonite ooids and cements in addition nannobacteria are found in paleozoic and mesozoic rocks because of their extraordinary abundance in some specimens they probably play a prominent role in catalyzing the precipitation of carbonate materials

quantification of quartz cements using combined sem cl and image analysis quartz cement content is often difficult to quantify using conventional petrography developments in equipment and technique reported here show how excellent high contrast images of quartz cements can be produced using combined scanning electron microscopy and cathodoluminescence sem cl in most cases quartz cement can be differentiated readily from detrital quartz image analysis allows the percentage of authigenic quartz to be quantified advantages of this method are 1 it can be readily automated 2 backscatter electron bse and cl images can be compared directly and 3 images can be improved through acquiring and integrating multiple images comparison of results from conventional optical petrography with those from sem cl suggests that the percentages of quartz cement determined by conventional methods are overestimated where quartz cement is present in high proportions and are underestimated where quartz cement is less abundant by adopting a statistically rigorous approach to image collection the abundance of quartz cement can be determined by sem cl with a known accuracy

carboxylic acid anions in formation waters san joaquin basin and louisiana gulf coast u s a implications for clastic diagenesis reply to discussion by p d lundegard and l s land abstract macgowan and s urdam 1990a suggested some modifications to the model of lundegard and land 1989 to make it more geologically and geochemically reasonable the predictive power of such a geochemical model is wholly dependent on the species modeled and the constants used any model that excludes important species or important thermodynamic data or one that couples certain reactions in an unrealistic way may produce results which are not geologically or geochemically reasonable w k harrison pers commun 1988 y k kharaka pers commun 1991 we have long recognized that under early burial diagenetic conditions aluminosilicate hydrolysis generally controls formation water ph surdam and eugster 1976 mariner and surdam 1970 taylor and surdam 1981 and that during intermediate burial either aqueous co2 or caa species in the absence of aqueous s species or other weak conjugate acid base pairs will dominate formation water alkalinity and control ph surdam et al 1989c we reassert that the model of lundegard and land 1989 does not take into account the relative importance of pco2 and of concentrations of both ca2 and caa and their relative organic metal complexes to carbonate mineral stability in sandstones in the zone of intermediate burial clastic diagenesis cf the models of surdam et al 1984 and surdam and crossey 1985 the usefulness of such models is predicted on the completeness of the model and the use of the best most accurate thermodynamic data also geologically realistic concentrations of critical species are required for reasonable modeling to be done although their model is vigorously defended in the discussion of lundegard and land 1989 lundegard and land 1993 we continue to disagree that their analysis of their model conditions are either geologically or geochemically satisfying we agree with the fundamental approach and philosophy of lundegard and land 1989 lundegard and land 1993 it is of the utmost importance to determine from experimental geochemical petrographic and geological data what the controls on ph and alkalinity in formation waters are as well as the exact thermodynamic speciation of aqueous moieties and the stability of detrital and authigenic minerals lundegard and land 1993 raise an additional point about caa reaction with carbonate minerals in shales although fisher and lewan 1989 lewan 1989 and macgowan and surdam 1990b have demonstrated that caa generated in shale likely migrate in the oil phase along incipient shale microfractures to the sandstone reservoir and thus are likely to not react much with the shale finally we agree with lundegard and land that these areas require much additional experimental and field analysis and petrographic study

physical and chemical controls on the composition of waters in sedimentary basins abstract the composition of subsurface waters is determined not only by diagenetic reactions but also by physical processes of fluid convection and hydrodynamic dispersion the south west louisiana gulf coast provides an instructive field example of the net effects of diagenetic reaction and solute transport on pore water compositions in a regional siliciclastic sequence most formation waters here have compositions totally unlike the compositions of the connate meteoric and marine fluids that were buried with their host sediments at the time of deposition dissolved chloride has been generated by the subsurface dissolution of salt domes and has been pervasively transported by fluid flow throughout most of the upper 3 km of the sedimentary section the simultaneous systematic variation in dissolved na k mg ca and alkalinity with chloride in these waters supports the hypothesis that metastable thermodynamic buffering by silicate carbonate mineral assemblages is a first order control on fluid compositions the chemical potential of chloride or alternatively total anionic charge appears to be a master variable which ranks in importance with such other variables as pressure and temperature in driving diagenesis in this region this variable is controlled largely by physical processes of advection and dispersion in the upper 3 km of the section and by dehydration reactions in deeper mudstone dominated sediments where the composition of the fluid is largely rock buffered the ultimate origin of the fluid and its pathway of chemical evolution may be obscured at least in terms of major solute composition non buffered components such as cl and br or isotopic compositions are more likely to retain information on original end member fluid compositions and reaction pathways

salinity variations in north sea formation waters implications for large scale fluid movements abstract sodium chloride is the major salt dissolved in formation waters even at a considerable distance away from any evaporites chloride anions are not consumed to any significant extent by diagenetic reactions outside the salt deposits chloride ions are therefore preserved in the pore waters and the distribution of pore water salinities in sedimentary basins may help to constrain pore water flow and the transport of dissolved species of silica and carbonate minerals in the north sea basin and haltenbanken mid norway salinity gradients can be inferred from formation water samples measurements of pore water salinity from wireline logs are less accurate than formation water samples but have the advantage that a much larger database can be obtained data from 150 wells in the north sea and haltenbanken show that the salinities measured from well logs range from 20000 to 300000 ppm in the southern part of the norwegian north sea and haltenbanken there is a clear trend towards higher salinities with greater burial depth there is a pronounced salinity increase in the 500 m of section closest to the evaporites in the northern north sea where underlying evaporites are not known there is only a slight increase in the measured salinities with depth there are however no measurements of high salinity pore water 100000 ppm at depths shallower than 2200 m this pattern of salinity distribution in the pore water precludes large scale convection of the pore water during diagenesis it also indicates limited compaction driven flow log derived salinity data from the north sea suggest that the pore waters in upper jurassic and tertiary reservoirs are generally more saline than those of middle and lower jurassic reservoirs at the same depth the constraints on pore water flow that can be deduced from salinity variations puts severe limitations on the mechanisms of transport of dissolved solids in sedimentary basins and thus also on aspects of diagenetic models

evidence for resetting of fluid inclusion temperatures from quartz cements in oilfields abstract quartz is a major pore occluding cement in north sea oilfield reservoir sandstones fluid inclusions yield homogenization temperatures t h which have been thought to represent growth temperatures for the host quartz however early diagenetic inclusions from the detrital grain overgrowth boundary yield excessively high palaeotemperatures which do not match the growth temperatures calculated from the depth of cementation the temperatures of these inclusions increase with present day depth of burial and approach present day reservoir temperatures this suggests that the inclusions have begun to reset towards present day pressure temperature conditions the t h of the inclusions hence represent burial temperatures rather than growth temperatures for the quartz experimental studies indicate that the amount of resetting undergone by an inclusion varies depending on its size shape and fluid composition a relationship between inclusion t h and these variables has been observed in inclusions from deeply buried quartz cements in the north sea

methyl compound use and slow growth characterize microbial life in 2 km deep subseafloor coal and shale beds the past decade of scientific ocean drilling has revealed seemingly ubiquitous slow growing microbial life within a range of deep biosphere habitats integrated ocean drilling program expedition 337 expanded these studies by successfully coring miocene aged coal beds 2 km below the seafloor hypothesized to be hot spots for microbial life to characterize the activity of coal associated microorganisms from this site a series of stable isotope probing sip experiments were conducted using intact pieces of coal and overlying shale incubated at in situ temperatures 45 c the 30 month sip incubations were amended with deuterated water as a passive tracer for growth and different combinations of 13 c or 15 n labeled methanol methylamine and ammonium added at low micromolar concentrations to investigate methylotrophy in the deep subseafloor biosphere although the cell densities were low 50 2 000 cells per cubic centimeter bulk geochemical measurements and single cell targeted nanometer scale secondary ion mass spectrometry demonstrated active metabolism of methylated substrates by the thermally adapted microbial assemblage with differing substrate utilization profiles between coal and shale incubations the conversion of labeled methylamine and methanol was predominantly through heterotrophic processes with only minor stimulation of methanogenesis these findings were consistent with in situ and incubation 16s rrna gene surveys microbial growth estimates in the incubations ranged from several months to over 100 y representing some of the slowest direct measurements of environmental microbial biosynthesis rates collectively these data highlight a small but viable deep coal bed biosphere characterized by extremely slow growing heterotrophs that can utilize a diverse range of carbon and nitrogen substrates

material flux and porosity changes during sediment diagenesis understanding the changes in porosity that occur during burial of sediments is of importance both in modelling fluid flow in sedimentary basins and for prediction of petroleum reservoir quality when deposited up to 50 of the volume of sands is intergranular pore space porosity generally decreases with depth because of compaction and precipitation of diagenetic minerals but some sandstones retain an anomalously high porosity whether this is due to dissolution and removal of material or to its local redistribution is not clear we originally intended to quantify losses of major chemical components during burial of a complete sedimentary unit we have investigated ten reservoir sandstones of permian to tertiary age from oilfields worldwide these show consistent results and here we present detailed data from four of them we found that the silica content of these sandstones actually increased by 220 to 350 kg m 3 following compaction no statistically significant changes were observed for aluminium potassium or sodium the flux of silica cannot be modelled satisfactorily using currently accepted values of permeability silica solubility and flow rates posing a challenge for existing models of fluid flow and ore emplacement

40ar39ar dating of very fine grained samples an encapsulated vial procedure to overcome the problem of 39ar recoil loss 40ar39ar dating of very fine grained materials is compromised by the loss of 39ar by recoil and also sometimes other ar isotopes during irradiation an encapsulated sample 40ar39ar procedure is described which overcomes this problem and which produces a k ar equivalent date measurements indicate that the procedure yields reliable and reproducible results it offers the potential for dating small amounts of material of many types for example very fine grained authigenic sheet silicates from rock cores which may suffer ar loss during irradiation

compaction of lithic sands experimental results and applications compaction experiments showed that the degree of physical compaction of a lithic sand is related to the amount and type of lithic material present these experiments produced a product that is texturally indistinguishable from natural lithic sandstones generally sedimentary lithic fragments are more ductile than metamorphic lithic fragments volcanic lithic fragments when altered to phyllosilicate minerals by weathering or diagenesis are extremely ductile mineralogy and intragranular microporosity influence ductility ratios based on weight percent of soft ductile minerals clay and micas to hard brittle minerals quartz and feldspar within the lithic fragments provides a good indication of ductility a ratio greater than 1 indicates the lithic material will be h ghly ductile microporous lithic fragments are more ductile than dense lithic fragments physical compaction models were developed based on experimental data from well sorted lithic sandstones for which the percentage of preserved porosities were functions of the effective stress the type of lithic material and the quantity of lithic material these models are derived from laboratory sandstones with at least 25 lithic grains in the following categories moderately ductile metamorphic lithic fragments highly ductile shale lithic fragments and extremely ductile altered volcanic lithic fragments application of the compaction models of the experimental compaction techniques to modern subsurface sand samples or outcrop equivalents can provide valuable estimates of the preserved porosities i e reservoir potential for lithic sandstones that may be exploratory objecti es in frontier basins examples using the models and experimental compaction techniques for exploration are given experimental data showed that precompaction cement and overpressure may be beneficial to porosity preservation an early formed partial cement can retard compaction and preserve porosity by stabilizing the sand pack a later cement does not have this beneficial effect overpressure when developed early retards compaction by reducing the effective stress of course a late developing overpressure is not effective at preserving porosity because compaction is an irreversible process

experimental simulation of clay precipitation within reservoir sandstones 1 techniques and examples experiments have been performed in which cores of clay free sand and clay forming mineral phases placed beneath the cores have been treated hydrothermally with various solutions using experimental conditions in the range 150 500 degrees c and 0 75 1 0 kbar 75 100 mpa illite illite smectite kaolinite 7 a chlorite and trioctahedral smectite have been precipitated in the sand pore space with morphologies similar to those of authigenic clay minerals in sandstone petroleum reservoirs this study demonstrates that the mechanism rate of clay precipitation and controls on clay morphology and mineral stability can all be determined by experimental studies these results have direct application to the interpretation of clay mineral paragenesis in reservoir sandstones the study demonstrates 1 authigenic clays in sandstones are direct precipitates from solution and do not require a preexisting clay coating 2 grain surface effects such as abrasion and detrital clay coatings do promote nucleation of clays and 3 clay morphology is controlled by precipitation mechanism specifically the formation of metastable clays which are subsequently modified the synthesis of authigenic type illite and illite smectite is by direct precipitation from solution following dissolution of mineral phases such as detrital illite or feldspar both platy and fibrous illite morphotypes are synthesized platy illites are more readily formed the compositions of synthetic illite and illite smectite clays are controlled by temperature and a sub k super a sub h super fluid composition ratio platy illites show subtle changes in morphology with compositional variation illite smectite and illite with low k content forms a crenulated boxwork texture while more k rich illites approaching muscovite composition form larger discrete plates no correlation between illite composition and fibrous formation is recognized kaolinite and chlorites were also precipitated in experiments vermiform kaolinite appears to be a direct precipitate kaolinite morphology however is controlled by precipitation mechanism isolated discrete kaolinite plates are associated with early formation of a mixed layered smectitic phase chlorite synthesis proceeds via the formation of metastable 7 a chlorite and expandable phases which have previously been recognized as possible precursors to 14 a chlorites in reservoir sandstones

kinetics of the smectite to illite transformation in the denver basin clay mineral k ar data and mathematical model results clay mineral data potassium argon k ar ages and mathematical model simulations indicate the important kinetic factors controlling the smectite to illite transformation illitization in upper cretaceous mowry and niobrara core and outcrop bentonites from the denver basin the data also provide additional information on the thermal history of the basin data show that both the percentages of illite layers and the k ar ages of mixed layer illite smectite i s increase with increasing depths of burial and this trend was simulated mathematically employing a very simplified burial model with a fifth order overall kinetic expression for the formation of illite for most of the basin the data is compatible with i s having been formed in response to increased temperature from progressive burial because the oldest k ar ages of i s are from the deepest buried i s along the basin axis in addition these data qualitatively suggest that i s even as small as 0 1 micrometer retains radiogenic argon under denver basin burial temperatures i s separated from drill core bentonites from the wattenberg gas field astride the basin axis and known to have been subjected to an anomalously high temperature history are the most illitic and among the oldest measured from core in this study the ages of illitization about 60 ma are coincident with the laramide time of structural development of the basin and uplift of the front range the reconstructions of the burial history and the modelling of the timing and extent of illitization suggest that the area of the wattenberg gas field was an upper cretaceous depocenter an area of relatively deeper burial along the basin axis however the data can not rule out the possibility that these sediments at and near the wattenberg gas field were heated not only by burial but also by i neous and related hydrothermal activity associated with the colorado mineral belt during the laramide uplift 70 45 ma of the front range likewise the anomalously illitic i s in outcrop bentonites along the front range adjacent to the wattenberg gas field may have been formed in response either to deep burial and or by laramide age secondary heating superimposed on burial

carboxylic acid anions in formation waters san joaquin basin and louisiana gulf coast u s a implications for clastic diagenesis critical comment the analyses of carboxylic acid anions caas in san joaquin basin and louisiana gulf coast formation waters published by macgowan and scrdam 1990 do little to resolve the important question of whether caas are of great significance to clastic diagenesis over the temperature range in which they dominate fluid alkalinity as advocated by these two authors comparison of their analyses with analyses of formation waters from the same wells previously published by three independent research groups hanor and workman 1986 land et al 1988 and land and macpherson 1989 and fisher and boles 1990 a body of data not discussed in their article reveals that most of the macgowan and surdam values are grossly inflated relative to the previous analyses table 1 fig 1 cedures of llco et al 1982 while field procedures for caas now normally include the addition of a bactericide to the sample an independent check on the upper limit of total caas which could have been present in this water is provided by the results of an alkalinity titration to ph 2 5 also published by hanor and workman 1986 which show that the total concentration of weak acid anions bicarbonate plus caas could not have exceeded approximately 428 mg 1 the alkalinity determinations were done shortly after collection on separate untreated aliquots stored in gas tight glass containers further details are given in workman 1985

carbon helium neon and argon isotopes in a po basin northern italy natural gas field the abundance and isotopic composition of he ne and ar have been determined in 19 gas samples taken from the dosso degli angeli hydrocarbon gas field between 3000 and 3750 m depth in the po basin carbon isotope ratios in these samples vary between 13c ch4 67 5 and 73 4 indicating a predominantly biogenic source for the methane c2 c2h6 c3h8 concentrations increase systematically from 0 04 to 0 17 by volume with depth suggesting that there is a small additional thermogenic or diagenetic hydrocarbon contribution to these gases ch4 36ar ratios in the dosso degli angeli gases are in the range predicted for a groundwater which has derived its rare gas by equilibration with the atmosphere and is ch4 saturated under reservoir conditions there is also up to 10 excess of ch4 which is not associated with the groundwater these results are consistent with 90 of the ch4 in the dosso degli angeli reservoir originating from ch4 saturated groundwater measured 20ne 36ar ratios depart from the predicted ratio in air equilibrated groundwater this fractionation is consistent with water gas phase equilibration under reservoir conditions and is similar to that observed in gas reservoirs in the vienna and pannonian basins austria and hungary respectively the he in the dosso degli angeli gases is almost entirely radiogenic and derived from the crust the concentration of 4he in the reservoir gases requires that he is mostly sourced from outside the reservoir volume radiogenic 4he 40ar ratios input into the reservoir gas must be much higher than averaged crustal production ratios as no shift from the atmospheric 40ar 36ar ratio is observed preferential release of 4he over 40ar from the sites of their production is the preferred explanation this observation contrasts with previous studies from active extensional basins in which radiogenic 4he and 40ar appear to have been stored released and transported at near production ratio on a regional scale the most obvious difference between these studies is the comparatively low thermal gradient of the po basin

mapping diagenetic fluid flow within a reservoir kar dating in the alwyn area uk north sea abstract the alwyn area demonstrates that variation in the timing of diagenesis on the field scale depends on the structure and the proximity to the sources of diagenetic fluids variation in reservoir quality within the structure is the result of facies control on fluid migration illite in alwyn south formed in the middle palaeocene to middle oligocene between 60 and 38 ma at this time the brent group was at depths of between 2400 and 3000 m and reservoir temperatures ranged from 110 to 120 c extensive k ar dating shows that illitization pre oil emplacement proceeded from the deepest reservoir panels along and up structure fluid migration may have occurred along the deep seated alwyn south ninian fault system where the reservoir is lithologically homogeneous illite precipitation proceeded systematically base upwards at rates of 5 5 m ma 1 the duration of illite formation at any one level was between 2 and 8 ma in contrast in heterolithic reservoirs fluid migration occurred initially along high permeability fairways and consequently illite ages do not vary systematically with depth

sulfidization and magnetization above hydrocarbon reservoirs geochemical and rock magnetic studies of strata over cement oil field anadarko basin oklahoma simpson oil field north slope basin alaska and the edwards deep gas trend south texas coastal plain document changes in original magnetizations caused by postdepositional iron sulfide minerals that are or may be related to hydrocarbon seepage at cement ferrimagnetic pyrrhotite fe sub 7 s sub 8 formed with pyrite and marcasite in permian red beds the fe s minerals contain isotopically heavy abiogenic sulfur derived from thermal degradation of petroleum and or isotopically light sulfur derived from sulfate reducing bacteria fed by leaking hydrocarbons at simpson ferrimagnetic greigite fe sub 3 s sub 4 dominates magnetizations in upper cretaceous nonmarine beds that contain biodegraded oil sulfur isotopic data are consistent with but do not prove a genetic link between the greigite delta sup 34 s gt 20 per mil and seepage in middle tertiary sandstones of southeast texas pyrite and marcasite formed when abiogenic h sub 2 s migrated upward from deep reservoirs or when h sub 2 s was produced at shallow depths by bacteria that utilized organic material dissolved in migrating water from depth the sulfide minerals replaced detrital magnetite to result in a systematic decrease in magnetic susceptibility toward faults that connect deep petroleum reservoirs tomore shallow sandstone the authors results show that abiologic and biologic mechanisms can generate magnetic sulfide minerals in some sulfidic zones of hydrocarbon seepage the magnetizations in such zones are diminished most commonly by replacement of detrital magnetic minerals with nonmagnetic sulfide minerals or are unchanged if such detrital minerals were originally absent less

the composition of permian seawater forty nine brine inclusions in marine halite from the ochoan salado formation in the delaware basin and fifteen inclusions in halite from the leonardian wellington formation in the kansas basin were extracted and their chemical compositions were determined the brines are of the na k mg cl so4 type their compositions resemble those of evaporated modern seawater the values of mcl mna mbr and mmg2 mca2 mso42 12 mmhco3 mbr of the inclusion brine from the two formations are equal to or slightly higher than those of modern seawater the original mna mbr and mcl mbr ratios of the inclusion brines were probably equal to or slightly larger than those of modern seawater the values of mmg2 mbr of the inclusion brines from the salado formation are very close to that of modern seawater the ratios of inclusion brines from the wellington formation are slightly lower probably due to the formation of dolomite magnesite the mmg2 mbr ratio in the initial seawater was probably close to the parent seawater of the salado brines the values of mso42 mca2 12mhco3 mbr of the inclusion brines appear to be reduced by the formation of dolomite magnesite and the value of this ratio in permian seawater was probably similar to that of modern seawater the mk mbr ratios of the inclusion brines are variable but the original ratios are probably close to or slightly larger than that of modern seawater if the br concentration of permian seawater was equal to that of modern seawater the composition of permian seawater can be narrowly constrained in mmol kg h2o 460 mna 630 550 mcl 730 mmg2 54 6 mk 11 mso2 4 mca2 12mmhco3 17 20 mso42 45 5 mca2 20 and 0 15 mhco3 5 the composition of permian seawater was therefore quite similar to that of modern seawater

approaches to predicting reservoir quality in sandstones despite limited understanding of the details of many diagenetic processes empirical techniques can be used effectively to predict reservoir quality prior to drilling the predictive approach depends mainly on the availability of empirical data in the area of interest in frontier basins mean or maximum porosity of a potential sandstone reservoir can be estimated for a given composition and level of thermal exposure or burial depth the reliability of the estimates is constrained by input data approximate values of input parameters can be obtained from seismic data in combination with geological analysis of the area in basins with sufficient information to generate calibration data sets the predictive technique uses regression analysis the applicability of this approach is constrained by the limits imposed by the calibration data set and is generally limited to samples containing less than 10 pore filling cement quartz rich sandstones 85 framework quartz cemented with quartz are an exception to the 10 limit because quartz cementation commonly is related to burial history and rock texture in weakly cemented sandstones the critical variables controlling porosity are detrital composition sorting and burial history permeability can be predicted independently of porosity using the same variables plus grain size these variables can be evaluated from seismic data and facies models t is approach is best suited either for sandstones in which compaction is the main porosity reducing process or for quartz rich sandstones an adequate predictive model for sandstone suites with a wide range of cement content can be obtained by dividing the calibration data set into two or more subsets and developing a predictive model for each for example one subset can be limited to samples with less than 10 cement whereas the second subset will consist of samples with more than 10 nonquartz cement porosity and permeability in the first subset are then expressed by linear regression whereas controls on reservoir quality in the more heavily cemented sandstones are determined independently based on understanding of cement distribution patterns

the origin and fate of silt sized carbonate in subsurface miocene oligocene mudstones south texas gulf coast combined petrographic and geochemical data document several kinds of detrital carbonate in subsurface miocene oligocene mudstones of the texas gulf coast in the extreme south of texas in muddy sediments deposited by ancient precursors to the modern rio grande mudstone carbonate is dominated by extrabasinal detritus derived from cretaceous limestones of the edwards plateau further north oligocene mudstones contain carbonate that is mostly syndepositional skeletal material minor amounts of authigenic carbonate are found as replacements of silt sized feldspars in all the mudstones depth related shifts in 18ocalcite and 13ocalcite suggest that small amounts of authigenic carbonate also form by replacement of detrital carbonate recrystallization although this cannot be demonstrated petrographically pervasive carbonate cementation in primary pores in mudstones is not generally observed textural evidence of carbonate loss through pressure dissolution is widespread providing a mechanism for the depth related decline in carbonate content observed in many gulf coast wells carbonate dissolution in mudstones and export of the caco3 implies a massive acid source probably resulting from silicate reactions within the mudstones during burial diagenesis

sandstone porosity loss a big picture view of the importance of compaction proper assessment of the relative importance of porosity loss by compaction and cementation is important to the prediction of porosity in sandstones studies of mass transport during diagenesis and the modeling of basin fluid flow quantitative estimation of the amounts of porosity loss by compaction and cementation can be made from standard point count data on cement and pore space abundance this method is discussed and its sources of error are evaluated error in the method arises mainly from uncertainty in initial sandstone porosities and the amount of local grain dissolution during burial data on the porosity of modern sands indicate that assumed initial porosities of 40 to 45 will produce small average errors in the calculation of compactional porosity loss for clean sandstones grain dissolution whether resulting in local porosity enhancement by export in solution of former grain mass or local precipitation of authigenic cement will cause an underestimation of the significance of compaction a large database of point count data from diverse sandstones is used to provide a broad perspective on the importance of compaction and cementation to sandstone porosity loss it is shown that the global importance of compactional porosity loss has generally been under appreciated in fact compaction mechanical and chemical is probably the dominant mechanism of porosity loss in most sandstones the extent of compactional porosity loss is related to sand grain composition and basin thermal structure

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

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

integration of petroleum engineering studies of producing brent group fields to predict reservoir properties in the pelican field uk north sea abstract diagenetic models and the results of laboratory experiments on cores from producing brent group fields cormorant and tern have been combined to aid prediction of reservoir properties in a deeply buried prospect pelican in cormorant and tern illite particles are thin 20 40 a lath shaped and liable to damage during drying before air permeability measurement leading to erroneously high permeabilities the magnitude of this drying effect k air conventional drying k air critical point drying varies with illite morphology and the timing of hydrocarbon emplacement in tern pore filling illites occur above and below the oil water contact and there is no significant difference in drying effect in cormorant the drying effect is higher below the oil water contact where a diversity of late diagenetic pore filling illite morphologies occur injectivity tests on cores from cormorant indicate that sea water injection will not lead to impairment of reservoir quality as a result of clay dispersal or other rock related phenomena impairment occurred when flooding cores with produced water and was avoided by introducing divalent cations saline brines or approximating in situ equilibrium conditions ph 6 4 this equilibrium point is strongly influenced by the surface chemistry of feldspar and illite rather than the more abundant kaolinite and quartz burial diagenetic replacement of kaolinite by illite will increase this equilibrium towards ph 7 but should not cause impairment during sea water injection into deeper reservoirs pelican contains abundant pore filling illite k ar age dates 44 25 ma suggest illite growth started after burial to over 8000 ft and palaeoburial temperatures of over 80 90 c compared with nearby reservoirs variations in the nature and abundance of illite result from illitisation being arrested by hydrocarbon charge at different times optimal reservoir and aquifer properties occur in shallower structures charged from more deeply buried source rocks good reservoir properties occur with charge after short lived illite growth although later burial may reduce aquifer properties the more prolonged illite growth experienced by deeply buried reservoirs such as pelican charged late in the tertiary has resulted in poor reservoir properties and poor aquifer permeabilities may be expected

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

origin of magnetite responsible for remagnetization of early paleozoic limestones of new york state scanning electron microscope and scanning transmission electron microscope observations of thin sections and separates of the devonian onondaga and helderberg limestones and ordovician trenton limestone in new york state allow us to identify three types of magnetite pseudoframboids nonspherical magnetite and fine grained magnetite magnetite was observed replacing pyrite occurring as crystals in spherical pseudoframboids or with nonspherical shapes fine grained magnetite consisting of aggregates of one or more rounded single crystals approximately 2000 a 200 run in diameter could not be observed in thin sections of new york carbonates due to its small size but was found in magnetic extracts hysteresis measurements of magnetic extracts verify that fine grained magnetite is capable of carrying remanent magnetizations however pseudoframboidal magnetite and nonspherical magnetite are polycrystalline and consist of assemblages of single to pseudosingle domain sized crystals that also can be carriers of the remagnetization these data in combination with observations from other localities collectively imply that the alleghenian remagnetization is carried by the fine grained magnetite although pseudoframboidal and nonspherical magnetite may also contribute thus the alleghenian remagnetization is a chemical remanent magnetization due to authigenic magnetite crystallization of magnetite was mediated by fluids with dissolution and crystallization activated by stress during the alleghenian orogeny the origin of such fluids is unknown but they may have originated through crustal scale fluid migration tectonically induced brine migration due to emplacement of thrust sheets is preferred over a meteoric source of the fluids

spatial 87sr 86sr variations in formation water and calcite from the ekofisk chalk oil field implications for reservoir connectivity and fluid composition abstract the 87 sr 86 sr ratios of formation waters from the chalk of the ekofisk oilfield were measured using a new technique extracting salt residues from the interstices of core samples in the danian ekofisk formation the chalk matrix itself has 87 sr 86 sr and 13 c values characteristic of danian seawater even though 18 o data indicate the presence of varying quantities of diagenetic carbonate cement in the ekofisk formation porewater 87 sr 86 sr values are very constant range 0 70786 0 70810 the formation water probably contains locally derived sr from carbonate plus a small contribution from noncarbonate minerals with higher 87 sr 86 sr in the underlying tor formation chalk samples containing a mixture of primary chalk and later cement have 87 sr 86 sr values that rise with depth to a maximum of 0 70865 much higher than contemporaneous seawater porewater 87 sr 86 sr values increase even more dramatically with depth to a maximum of 0 70988 both cement and waters have been influenced by sr with a high 87 sr 86 sr probably derived from underlying sedimentary rocks comparison with waters brought to the surface together with produced oil indicates that the depthwise increase in 87 sr 86 sr reflects an increase in salinity from 50 000to 140 000 mg l total solute content this suggests that zechstein evaporites may be the source of the radiogenic sr in the tor formation the difference in isotope composition between carbonates and waters in the ekofisk and tor formations indicates that their hydrochemical systems have evolved differently during diagenesis this is due to the barrier effect of the ekofisk tight zone a hardground at the base of the ekofisk formation which effectively splits the reservoir vertically into two this study thus provides a new way of determining the positions of permeability barriers in oil reservoirs also demonstrated for the first time is the existence of vertical variations in salinity of water in oil such variations could have important implications for the calibration of resistivity logs and calculation of the amount of oil in place

petrography and reservoir physics i objective classification of reservoir porosity porosity observed in thin section can be objectively classified using a combination of digital acquisition procedures and pattern recognition algorithms pore types are derived from the frequency distributions of sizes and shapes of patches of porosity exposed in thin section each pore type is represented by a characteristic distribution of sizes and shapes found in thin section most sandstone reservoirs contain fewer than six pore types much of the intersample variability in reservoir physics is associated with changes in pore type abundance the advantages of this approach to porosity classification are 1 the criteria for classification are objectively defined 2 classification procedure is rapid accurate and precise 3 pore types are understood easily in ter s of conventional genetic classification schemes and 4 pore type data are related strongly to petrophysical properties

experimental simulation of clay precipitation within reservoir sandstones 2 mechanism of illite formation and controls on morphology illite and illite smectite with various morphologies have been formed on quartz grains by precipitation from solution following dissolution of amorphous al si gels at 150 350 degrees c and 0 75 1 0 kbar 75 100 mpa initial solutions include deionized water chloride and oxalate the experimental results have application to the formation of authigenic illite in sandstones from pore waters containing carboxylic acid anions a series of experiments monitored the effects of reaction temperature and time on the composition and morphology of illitic minerals precipitated following the dissolution of an amorphous gel of illite composition in deionized water a progressive increase of 70 to 100 in the illite content of illite smectite occurs between 150 and 350 degrees c but there is no change in illite smectite composition for reaction times of 24 hours to 37 days these results indicate that illite and illite rich illite smectite form by direct precipitation from solution and do not require a smectitic precursor the compositional variation in illite smectite and illite has little or no effect on clay morphology all experiments produced a platy boxwork morphology experiments performed at 250 degrees c and initially containing oxalate in both acidic and alkaline k bearing solutions strongly promote illite precipitation following the dissolution of an amorphous al si oxide starting material the very rapid precipitation that occurs under these conditions is associated with the formation of fibrous lath type illites rather than platy types which are characteristic of slower precipitation in the absence of oxalate precipitation of fibrous illite is related to the breakdown of oxalate which lowers illite solubility by either ph variation or by breakdown of an al oxalate complex oxalate is not a direct requirement for fibrous illite formation although it controls fluid composition which favors fibrous illite formation the association of illite precipitation with carboxylate anion breakdown suggests that the formation of fibrous illite cements in petroleum reservoirs is related to fluids released from source rocks which contain both aqueous organic acids and inorganic solutes