Formation Of Kaolinite Research Articles

The effect of fluid/rock ratio on feldspar dissolution and illite formation under reservoir conditions

AbstractExperiments on the conversion of feldspar to illite reveal that variation of the fluid/rock ratio (flow rate) has a significant effect on the kinetics of feldspar dissolution and illite formation. Dissolution of albite in near-neutral KCl solution at 200°C and 500 bar shows that the Si and Na release rate per unit surface area of albite is faster with higher fluid/rock ratio. These results have application to the interpretation of secondary porosity formation. The experiments also reveal a special set of conditions under which rapid illite formation can occur. In contrast to the slow process of illite formation in neutral solution, there can be mass nucleation and growth of illite platelets on albite surfaces in an initially acidic solution with low fluid/rock ratio. In similar experiments with higher fluid/rock ratio, kaolinite forms instead of illite because there is insufficient solid to titrate the large volume of solution. This study suggests that illitization of feldspar or kaolinite may be triggered by a decreased rate of acidic fluid-influx during burial diagenesis.

P-T evolution of metasediments from the Eclogite Zone, south-central Tauern Window, Austria

Petrologic data on the paragenesis of (I) kyanite-zoisite marbles and (II) garnet-chloritoid quartz-mica schists are presented with the goal of providing constraints on the pressure-temperature evolution of the Eclogite Zone, Tauern Window, Austria. The peak metamorphic assemblages in the two rock types are: (I) kyanite + zoisite + dolomite + quartz; zoisite + muscovite + dolomite + calcite + quartz; and (II) garnet + chloritoid + kyanite + muscovite + quartz + epidote ± dolomite ± Zn-staurolite. The estimated peak metamorphic conditions are 19 ± 2 kbar, 590 ± 20°C. Secondary alteration of the kyanite-zoisite marbles was accomplished in two stages. The early stage resulted in the production of margarite, paragonite, secondary muscovite and chlorite and the later stage resulted in the formation of sudoite (a di/trioctahedral MgAl layer silicate) and kaolinite. The early alteration is bracketed at conditions between 3 and 10 kbar, 450–550°C and the later alteration between 200 and 350°C, P ⩽ 3 kbar. The P- T path is characterized by maximum burial to approximately 19 kbar (60–70 km) (at≈590°C), followed by nearly isothermal decompression to approximately 10 kbar (30 km), and then more gradual decompression with cooling to approximately 3 kbar (10 km). Alteration was apparently accomplished by the influx of H 2O-rich fluids, with the composition of the fluid locally buffered by the mineral assemblage.

Diagenesis of a deeply buried sandstone reservoir: Hild Field, Northern North Sea

AbstractCalcite cementation and extensive dissolution of feldspar with formation of authigenic kaolinite, quartz cement and secondary porosity are the main diagenetic processes in the deeply buried Hild Field. Mineralogical and isotopic analyses, reservoir pressure and depositional environment suggest that these diagenetic processes occurred prior to burial at a depth of 1·5–2 km. The timing of the diagenetic sequence suggests that feldspar dissolution is related to meteoric water flow. Calcite occurs as an early diagenetic iron-poor cement, and as two types of later diagenetic (<120°C) ferroan calcite cements. The ferroan calcites are mainly an in situ dissolution-reprecipitation product of the early diagenetic phase. Extensive local dissolution of calcite was important for forming secondary porosity which is closely associated with a prominent ‘gas chimney’ in the area studied. A high CO2 content in the natural gases of the reservoir suggests that the solvent was carbonic acid formed from CO2 generated during thermal maturation of organic matter. Calcite dissolution probably occurred between 70° and 100°C.

Alluvial Sandstone Composition and Paleoclimate, II. Authigenic Mineralogy

ABSTRACT Various empirical and theoretical arguments suggest that early formed cement in sandstone is a product of meteoric water whose chemistry is controlled by climate. The nature and distribution of early silicate cement in Gondwana, Fountain, and Cutler sandstone support this supposition. Sandstones from all three units have experienced a broadly similar diagenetic history characterized by two stages of authigenesis. Stage I was typified by neomorphic development of kaolinite, chlorite, smectite, quartz, and, additionally in the Cutler Formation, laumontite. Stage II was dominated by replacement reactions involving the production mostly of illite, iron oxide, carbonate minerals, and, mostly in the Gondwana sandstone, a second generation of quartz. Thermodynamic relations, hydrologic constraints, and radiometric dating of the late-stage illite indicate that early authigenesis took place within a few million years of deposition at burial depths measured in hundreds of meters. Consequently, the nature of the silicate cement was a function of the groundwater chemistry, as controlled by climate. During times of relative aridity, ionic concentration of the groundwater was high, and high proportions of smectite, chlorite, and, more rarely, laumontite formed. Such was the case in Petrofacies I, IV, V of the Gondwana Supergroup (for a definition of Gondwana petrofacies see part I of this study) in the Cutler Fm., and in the upper 150 m of the Fountain Fm. During times of high precipitation, pore water was dilute, which promoted authigen c formation of kaolinite and quartz. This is represented by sandstones in petrofacies II and VI of the Gondwana and by sandstones in the lower 200 m of the Fountain Fm. Illite is especially abundant as a pseudomorphic replacement of kaolinite in the older Gondwana sandstones. Illitization apparently began at burial depths of about 1,600 m at a temperature of about 75°C. Illite is not common in the Cutler Formation and is present only in subordinate amounts in the Fountain Formation from the study area. The 18O values of early clay cement in the Gondwana sandstones range systematically from base to top from 5.00 to 13.20. The systematic variation in 18O value of Gondwana clay reflects the gradual migratory drift of the Gondwana harm toward lower latitudes through time. Average 18O values for Cutler cement (13.23) and Fountain cement (19.82) are greater than the analogous values for Gondwana cements. This is consistent with the lower-latitude setting of deposition for the Cutler and Fountain relative to the latitudinal location of the Gondwana basins. However, the values are anomalously lower than what have been observed in modern-day neoformed clays in weathering profiles from low latitudes.

Facies and Diagenesis of Grayburg-San Andres Dolomite Reservoirs, Central Basin Platform, Permian Basin: ABSTRACT

Porous dolomites of the Grayburg and San Andres Formations (Permian, Guadalupian) form major hydrocarbon reservoirs on the Central Basin platform of the Permian basin. Wireline-log and core studies reveal a shallowing-upward carbonate shelf sequence, from 250 to 300 ft thick, in each of five fields that lie along 55 mi of the eastern edge of the platform. The sequence consists of (1) a basal unit of open shelf dolomudstones and bioturbated fusulinid dolowackestones/packstones; (2) a middle unit of shallow shelf dolomudstones and non-skeletal dolopackstones/grainstones; and (3) an upper unit of arid coastline dolomudstones, quartz siltstones, and evaporites that grade into the overlying Queen Formation. Reservoir zones averaging greater than 9% intercrystalline and moldic porosity and 10-md permeability are best developed in the middle unit of the depositional sequence. Diagenesis occurred in two stages. Stage 1, during deposition and earliest burial, involved submarine cementation, pervasive dolomitization, leaching of grains, minor dolomite cementation, and sulfate cementation and replacement; and stage 2, near maximum burial depth and during subsequent uplift, included fracturing, anhydrite cementation, minor calcite replacement of anhydrite, and gypsum and kaolinite formation.

THE DIAGENESIS OF RESERVOIR SANDS OF THE ZUBAIR FORMATION (EARLY CRETACEOUS), NORTHERN KUWAIT ‐ ARABIAN GULF

A total of ten conventional core samples of the Lower Cretaceous Zubair Sandstone Formation, as encountered in the Raudhatain sabriyah oilfields of Kuwait have been studied by grain‐size analysis and scanning electron microscope (SEM) examination. Diagenesis of the sandstone members, in the form of silica overgrowths in optical continuity with the detrital quartz grains and the formation of authigenic kaolinite, is indicated. Further studies will be required in order to establish a possible correlation among the reservoir properties, diagenetic changes and oil accumulation in the Zubair Formation examined in these two fields.

Diagenesis of Lower Tuscaloosa as Seen in du Pont de Nemours 1 Lester Earnest, Harrison County, Mississippi: ABSTRACT

The lower Tuscaloosa occurs at depths between 9,010 and 9,650 ft in the du Pont de Nemours 1 Lester Earnest well, Sec. 4, T8S, R13W, Harrison County, Mississippi. The diagenetic history of the section has been determined by means of thin-section, SEM, and XRD studies. The diagenetic sequence can be resolved into the following stages: (1) mechanical compaction; (2) early pyrite and nodular siderite development; (3) relatively early dissolution of feldspars and lithic fragments; (4) authigenic chlorite formation; (5) wheat seed and rhombic siderite formation, probably early; (6) syntaxial quartz cementation; (7) formation of authigenetic vermiform kaolinite, and precipitation of poikilotopic ferroan calcite and ankerite cement; and (8) carbonate cement dissolution, possib y accompanied by further silicate dissolution and minor amounts of additional mechanical compaction. The ferroan calcite and ankerite cements are both characterized by sweeping extinction, more pronounced in the ankerite. The semiquantitatively analyzed calcites contain approximately 6% total iron plus manganese. Minus cement values (sum of volume of open pore spaces plus volume of cement) suggest very early cementation at shallow burial depths. However, the cement composition indicates a much deeper site of origin, and close examination of cement-grain contacts suggests that much grain-surface etching and grain replacement have occurred. The general applicability of minus cement porosity curves for determining depth of cementation (as used by some authors) is therefore thrown into doubt, unless replacement carbonate can clearly be separated from true (pore-filling) cement . End_of_Article - Last_Page 1423------------

Depositional Environments, Diagenesis, and Hydrocarbon Potential of Nonmarine Upper Cretaceous and Lower Tertiary rocks, Eastern Uinta Basin, Utah: ABSTRACT

Core studies of nonmarine rocks from the Natural Buttes field, Utah, indicate that depositional environment and diagenetic alteration control the geometry and quality of low-permeability gas reservoirs in the eastern part of the Uinta basin. The Tuscher Formation (Upper Cretaceous) is composed of fine to medium-grained, moderately to well-sorted sandstones and less abundant carbonaceous and coaly shale that formed on the lower part of an alluvial braidplain. The Wasatch Formation (Paleocene and Eocene) unconformably overlies Cretaceous rocks and consists of fine-grained lenticular cross-bedded sandstones, argillaceous siltstones, and variegated mudstones, which were deposited in lower delta-plain settings along the margin of Lake Uinta. Cretaceous and Tertiary sandstones ave been modified by minor quartz overgrowths, by the precipitation and subsequent dissolution of ferroan and nonferroan calcite, by poikilotopic anhydrite, and by the formation of authigenic illite, mixed-layer illite-smectite, kaolinite, chlorite, and corrensite. Most authigenic carbonate and anhydrite formed during early burial, before significant compaction. During later stages of diagenesis, precipitation of authigenic clay in secondary pores created by carbonate dissolution reduced porosity and permeability. Large amounts of natural gas generated in situ are stratigraphically trapped in these lenticular, diagenetically modified sandstones. Source rocks in the Tuscher Formation have reached the advanced stages of thermogenic gas generation (0.7% Ro) but are only moderatel mature with respect to liquid hydrocarbon generation. Interbeds of lacustrine Green River shale are in the early stages of gas generation (0.5% Ro) and are source rocks for gas produced from the Wasatch Formation. End_of_Article - Last_Page 860------------

Hydrothermal synthesis of kaolinite illustrated by X-ray diffractograms and scanning electron micrographs, part II

The hydrothermal synthesis of kaolinite from various alumina and silica starting materials reacted at 200°C in aqueous solutions of oxalic acid and H2SO4 with the corresponding saturated water vapour pressure, in periods of time from 10 to 25 days, has been followed by X-ray diffraction measurements, scanning electron microscopy, and energy dispersive analysis of the products. The temperature and time periods of reactions were chosen on the basis of previously collected data on the effects of H4EDTA, a complex-forming agent, and of H2SO4, an inorganic acid, on the rate of formation of kaolinite. Aluminium-complexing acids increase the rate of formation or decrease the temperature of formation of kaolinite as shown by evidence from X-ray diffraction patterns. The influence of the starting materials as sources of aluminum, amorphous aluminum hydroxides, pseudoboehmite, allophane or gibbsite, has also been examined by X-ray diffraction, scanning electron microscopy, and by energy dispersive analysis. Scanning electron micrographs indicated that silica is a more mobile phase than alumina, based on the progressive formation of a thin coating on gibbsite during early reaction, then by growth of clusters of platy crystals of kaolin, and finally by production of pellets of stacked crystal flakes. Where gibbsite was used in the reaction mixture, boehmite was commonly observed as an intermediate phase.

The soil clays of Great Britain: II. Scotland

AbstractThe soils of Scotland are relatively young, being developed mainly on glacial drift deposited some 10 000 years ago. Only a small number of genetic soil types are represented, but this is more than compensated for by the wide variety of parent materials from which the drift ultimately derives. The major parent materials include granite and granitic gneiss, gabbro, basalt/andesite, mica-schist and related metamorphic rock types, Lower Palaeozoic greywackes and shales, Old Red Sandstone sediments, Carboniferous sediments, fluvioglacial sands and gravels, and estuarine silts and clays. The clay mineralogy of the soil associations developed on these parent materials is described and the origin of the clay minerals is interpreted. It is concluded that the influence of inheritance is predominant, but that the effects of pre-glacial weathering and Holocene pedogenesis can also be discerned. Inheritance has contributed a wide variety of clay minerals to the soils, including illite, kaolinite, chlorite, smectite and a number of interstratified minerals, pre-glacial weathering has resulted in the widespread formation of kaolinite and halloysite, irrespective of soil parent material or drainage class, and recent pedogenesis has brought about the transformation of inherited layer silicates by vermiculitization processes, with concomitant interlayer alumination, particularly in surface horizons.

Mineralogy and diagenesis of Lower Jurassic sediments of the Lossiemouth borehole, north-east Scotland

SUMMARY Sandstones in the Lossiemouth borehole have subarkosic and sublitharenitic compositions, and are thought to have been derived from a southerly source. Early diagenesis in sandstones took place in low ionic concentration waters which generated dissolution secondary porosity and formed kaolinite. Grain-coating illite/smectite and calcite are locally abundant early diagenetic cements. Illite and K-feldspar authigenesis post-dates kaolinite formation and may be associated with mineralisation. Quartz overgrowth is the final stage of diagenesis observed. Shales have an illite + chlorite mineralogy interpreted to be of detrital origin.

Mineralverwitterung, tonmineralbildung und rubefizierung in Terrae calcis der slowakei Ein beitrag zum paläoklimatischen aussagewert von kalkstein-Rotlehmen in Mitteleuropa

Mineralverwitterung, tonmineralbildung und rubefizierung in Terrae calcis der slowakei Ein beitrag zum paläoklimatischen aussagewert von kalkstein-Rotlehmen in Mitteleuropa

Clay mineralogy of a soil sequence in slope deposits derived from hauptdolomit (dolomite) in the bavarian alps

Clay mineralogy was determined in a sequence of soils formed in slope deposits derived from Hauptdolomit and ranging from a Mullrendzina (Rendoll) to a Terra fusca (Eutrochrept) and a Parabraunerde-Terra fusca (Hapludalf). Results indicated an alteration of illite to hydroxyl-Al interlayered clay in the course of soil formation. No evidence was found of formation of kaolinite in these soils. This clearly confirms all soils to have been formed under a recent cold humid climate. Furthermore clay mineral weathering starts even before carbonates are dissolved and completely removed from these soils.

Mineralverwitterung, Tonmineralbildung Und Rubefizierung In Terrae Calcis Der Slowakei: Ein Beitrag Zum Paläoklimatischen Aussagewert Von Kalkstein-Rotlehmen In Mitteleuropa

Mineralverwitterung, Tonmineralbildung Und Rubefizierung In Terrae Calcis Der Slowakei: Ein Beitrag Zum Paläoklimatischen Aussagewert Von Kalkstein-Rotlehmen In Mitteleuropa

Rules of Sandstone Diagenesis Related to Reservoir Quality: ABSTRACT

The reservoir quality of sandstone is almost entirely controlled by diagenetic events. The chemical and physical processes responsible for diagenesis are complex and they influence sand during all stages of burial and, in some basins, during subsequent uplift. Petrographic studies by many workers in the past 10 years provide the basis for formulating rules of sandstone diagenesis that help in predicting reservoir quality in different End_Page 1215------------------------------ formations. Most of the rules listed below are empirical, and causative factors are still poorly understood. The list is also not complete. 1. The detrital mineral composition of a sand predetermines 50-80% of its diagenetic history. 2. Porosity lost by compaction cannot be regenerated during subsequent diagenetic events. Sands with abundant ductile grains (clay clasts, fecal pellets, shale clasts, micaceous rock fragments) can lose much primary porosity from the mashing of these grains during compaction. 3. The loss of primary porosity through compactional deformation of clays and ductile grains takes place during burial of 8,000-10,000 ft (2,450-3,050 m); loss of porosity by compaction at greater depths is by pressure solution of detrital grains. 4. Pressure solution of quartz grains is enhanced by the presence of grain coatings of illite and to a lesser degree by chlorite. 5. Quartz cement has an affinity for the coarser, more permeable sands in a formation. However, it rarely fills all pores in a sandstone except in some coarser grained laminae or in quartzarenites. 6. Carbonate cement may have a patchy distribution in a bed, but it fills all pores to produce a nonporous rock where it is present. 7. Carbonate cement is the dominant or only cement in sands with abundant carbonate fossil fragments or carbonate rock fragments; the carbonate cement is derived from the sand. 8. Poikilotopic calcite cement is the result of cementation that progressed from widely spaced nucleation sites. 9. Kaolinite forms by the replacement of feldspar and to a lesser degree of muscovite and also by free-standing growth in both primary and secondary pores. 10. Kaolinite should be expected as a diagenetic mineral in feldspar-rich sands (arkose and subarkose) that are poor in volcanic rock fragments. 11. Chlorite and/or mixed-layer clay should be expected as a diagenetic mineral in sandstone with more than 10% volcanic rock fragments. 12. Chlorite and mixed-layer clay, because of their tendency to bridge pores and produce baffles in pores, can seriously reduce permeability. 13. Early formed illite, typically present as grain coatings, does not seriously reduce permeability; late-formed illite tends to bridge pores and produce baffles and seriously reduce permeability. 14. Micrite cement is formed in continental sands above the water table. 15. Opal cement is formed in continental sands above the water table in stratigraphic sections where volcanic ash beds are intercalated with sands. 16. Some degree of secondary porosity is to be expected in a sandstone. However, there are few clues at present to predict the degree of secondary porosity or the cleanliness of secondary pores produced. Many secondary pores are micropores within incompletely dissolved feldspar or rock fragments. The best secondary pores are produced by dissolution of carbonate cement and evaporite cement. 17. Secondary porosity develops chiefly at depths greater than 6,000 ft (1,830 m) in the Gulf Coast, but can persist to depths of 20,000 ft (6,100 m). After their generation, secondary pores undergo some destruction during subsequent deeper burial by infilling with late-diagenetic ferroan carbonate and/or kaolinite. 18. Sands that had the greatest permeability at the time of deposition will develop the best secondary porosity. The best permeability will be in the coarsest, well-sorted sandstones (except in quartzarenites, where the coarsest beds selectively may undergo complete cementation by quartz). End_of_Article - Last_Page 1216------------

Mineral-groundwater interactions and the formation of authigenic kaolinite within the southeastern intake beds of the Great Australian (Artesian) Basin, New South Wales, Australia

Petrographic studies undertaken on samples from outcrops of the Jurassic Pilliga Sandstone intake beds in the extreme southeastern portion of the Great Australian Basin reveal an abundance of low birefringent clay material filling pore spaces and detrital grain fractures. Thin-section petrography and scanning electron microscopy indicate that much of this material is authigenic and X-ray diffraction studies show it to be essentially monomineralic, consisting of well-ordered kaolinite. Although the Pilliga Sandstone is quartz-rich, micrometric analysis indicates that potassium feldspar is present in quantities up to 20% of the total detrital material. Sporadic biotite mica is also present, constituting up to 3% of the detrital volume. Mechanisms for the formation of authigenic kaolinite are discussed in terms of chemical equilibrium and detrital mineral stability in aqueous systems. Chemical data from bore water and surface waters from this stratigraphic unit indicate that kaolinite is the major stable mineral phase in contact with these natural waters and that minerals such as potassium feldspar or mica would chemically alter to kaolinite. Such alteration of detrital mineral grains is supported by thin-section petrography and this mechanism is considered to be the source for the majority of the authigenic kaolinite observed. The hydrogeological characteristics of the Pilliga Sandstone intake beds are related to the extent of development of authigenic kaolinite.

Reddening of tropical coastal dune sands

Summary This paper describes the pedological processes, including that of reddening, in latosol formation in dune sands on humid tropical coastlines. A time-scale is established for these processes in South-east India, Sri Lanka and the Natal Province of the Republic of South Africa. Weathering in situ of the dune sands has occurred since their deposition in all three locations. Evidence from thin section and TEM analyses shows that detrital addition of clay minerals and iron oxides has been negligible. The major pedogenic processes (as shown by thin section, SEM, TEM, XRD and chemical analyses) in the formation of the latosols in the field areas are: weathering of garnet, feldspar, hornblende and opaque minerals; formation of kaolinite and illite clay minerals; oxidation of iron to haematite, and loss of carbonate and partial removal of silica from weathered horizons, followed by their deposition lower down the profile in some cases. Dating of the red latosols (2.5 YR 4/6-10R 4/8), radiometrically and using archaeological evidence, indicates that formation to depths of 10 m has taken place in under 20,000 years in the coastal dune sands in the field areas. It is suggested that reddening of desert sands by mineral breakdown could take place on time-scales that are much shorter than has previously been assumed.

Geologic Factors Influencing Reservoir Performance at Texaco's Salem Tertiary Recovery Project, Marion County, Illinois: ABSTRACT

A detailed lithologic reservoir study was conducted to aid Texaco personnel in designing and monitoring an experimental surfactant-polymer flood in the Mississippian Benoist Sandstone, one of several producing formations in the Salem field of south-central Illinois. Twelve elongated five-spot patterns are distributed over the 60-acre (24 ha.) project area. The Benoist Sandstone averages about 49 ft (15 m) of net pay at a depth of about 1,800 ft (550 m). Cores from eight wells were studied in detail. Particular attention was paid to variations in sedimentary structures, lithology, and mineralogy that could influence reservoir performance. Techniques employed in this study included examination of slabbed cores, thin-section petrography, X-ray diffraction (XRD) mineralogy, a d scanning electron microscopy/energy dispersive spectrometry (SEM/EDS). The Benoist Sandstone is one of several Late Mississippian deltaic sandstone units deposited in the subsiding Illinois basin. These sandstones are bounded above and below by fossiliferous marine limestone and shale. Delta-front sandstones, hereafter referred to as bar-finger sandstones, comprise the bulk of the formation. Channel-fill deposits are found near the base of the unit and nonreservoir, tidal-flat deposits near the top. The bar-finger deposits are moderately to well-sorted, fine to medium-grained sandstones with horizontal to inclined planar bedding and some ripple and planar cross-bedding. The planar bedding is accentuated by clay and mica-rich layers, one millimeter to several centimeters thick. These shale layers increase and thicken upward, and separate the bar-finger sa ds into several reservoir units. The Benoist sandstones are quartzose, containing 70 to 98% monocrystalline and polycrystalline quartz and small amounts of detrital feldspar and shale clasts. Cement is predominantly quartz in the form of syntaxial overgrowths, with minor calcite. Small amounts of clay occur as detrital laminae, authigenic pore fillings, and sand-grain coatings. The percentage of detrital and authigenic clay increases near the top of the bar-finger sandstone and significantly reduces permeability. Illite, the dominant clay, End_Page 445------------------------------ occurs as both grain coats and pore bridgings, kaolinite forms scattered pore fills, and Fe-chlorite coats a few grains. The channel-fill deposits are well to poorly sorted, very fine to medium-grained sandstones, commonly conglomeratic, and contain calcite-cemented zones. These deposits are mineralogically similar to the bar-finger deposits but contain abundant shale and fossil fragments at the base. Noncalcareous channel sandstones are characterized by scattered Fe-chlorite grain coatings and pore-filling illite. Intergranular porosity is well developed and has not been severely reduced by the pervasive quartz overgrowth cementation. The eastern part of the project area contains a higher quality reservoir section because of the sparsity of clay zones in the bar-finger sandstone and the thicker channel-fill deposits in this area. Secondary porosity, produced by the dissolution of feldspar grains, has slightly enhanced the overall quality of the Benoist Sandstone. End_of_Article - Last_Page 446------------

Sandstone Diagenesis as Function of Depositional Environment and Plate Tectonic Setting--Comparison Between Jurassic Sandstones from North Sea Basin and Some Oligocene Sandstones from Coast Ranges of California: ABSTRACT

Jurassic sandstones from the North Sea and Oligocene sandstones from the Coast Ranges of California are characterized by very different diagenetic mineral composition, which can be explained by differences in source rocks, sedimentary facies, climate, and ultimately in terms of plate-tectonic setting. The Jurassic sandstones from the North Sea were deposited in a rifted basin and were mostly derived from uplifted Precambrian terrane. A high content of diagenetic kaolinite in these sandstones can be attributed to meteoric water flushing through fluvial and deltaic sediments, causing the breakdown of feldspar and mica and the formation of kaolinite. Montmorillonite and zeolites are rare as authigenic minerals in these sandstones. In California, the rapid subsidence of the b sins in this subduction regime caused marine sandstone facies to predominate. These sandstones contain smaller amounts of kaolinite, probably because they did not experience any strong flushing by meteoric water after deposition. In addition, a drier climate in this region explains why fluvial sandstones such as the Sespe Formation show less evidence of meteoric water diagenesis than the Jurassic North Sea sandstones. Basin subsidence and marine transgression after the deposition of the Sespe Formation also limited the time these sandstones were exposed to meteoric water and thereby the formation of kaolinite. A higher content of mafic clastic minerals and chert, which formed unstable mineral assemblages in the California sandstones, favored extensive growth of diagenetic montmorillonite End_of_Article - Last_Page 1685------------

Clay Mineralogy and Depositional History of Frio Formation in Two Geopressured Wells, Brazoria County, Texas: ABSTRACT

Twenty-three shale samples, ranging in depth from 5,194 to 13,246 ft (1,583 to 4,037 m), from Gulf Oil Corp. 2 Texas State Lease 53034 well, and 33 shale samples, ranging in depth from 2,185 to 15,592 ft (666 to 4,752 m), from General Crude Oil Co./Department of Energy 1 Pleasant Bayou well were examined by X-ray techniques to determine the mineralogy of the geopressured zone in the Brazoria fairway. Both wells have similar weight-percent trends with depth for a portion of the mineralogy. Calcite decreases, whereas plagioclase, quartz, and total clay increase slightly. Within the clays, illite in mixed-layer illite-smectite increases and smectite in mixed-layer illite-smectite decreases. Four minerals have distinctly different trends with depth for each well. In the 2 Texas State Lease 53034 well, potassium feldspar and mixed-layer illite-smectite decrease, kaolinite increases, and discrete illite is constant. In the 1 Pleasant Bayou well, potassium feldspar and kaolinite are constant, mixed-layer illite-smectite increases, and discrete illite decreases. The most important diagenetic change in each well is the transformation of smectite to illite within the mixed-layer phase which occurs according to the reaction suggested by J. R. Boles and S. G. Franks with Al3+ acting as an immobile component. This change begins at calculated equilibrium temperatures of 89 to 92°C. The decrease in calcite and the lack of chlorite in the shales suggest that carbonate, iron, and magnesium migrate out of the shale in each well. In the 2 Texas State Lease 53034 well, the Boles and Franks reaction is consistent with a steady supply of original mixed-layer illite-smectite during deposition. Potassium feldspar provides K+ for the smectite to illite transformation. The breakdown of potassium feldspar also results in the formation of kaolinite and the increase of plagioclase feldspar, which is due to the reaction with Na+ and Ca2+, provided by the smectite to illite change. In the 1 Pleasant Bayou well, the Boles and Franks reaction is consistent with an unusually high mixed-layer illite-smectite content in the early depositional stages. The source of K+ for the smectite to illite reaction is discrete illite. The breakdown of discrete illite results in both the formation of kaolinite and the increase in plagioclase feldspar. End_of_Article - Last_Page 1430------------