Illite Layers Research Articles

Structural investigations of mixed-layer smectite-illite clay minerals from North Sea oil source rocks

The purpose of the present investigation is to identify mixed-layering between illite and smectite in oil source rocks with HREM and electron diffraction. Smectite and mica are both groups within the 2:1 type of layer silicates. Illite is a commonly used term for micas in the fraction less than 2 microns. Illite has a fixed d(001) spacing of 10A. Smectite is able to swell from 10A to more than 18A under uptake of water or organic polar molecules. Mixed-layer illite-smectite is built of alternating layers of illite and smectite. Smectite and illite layers are recognized by x-ray diffraction (XRD) from their swelling characteristics, but cannot be distinguished in this way in HREM, because smectite layers tend to contract to 10A by evaporation of water in the vacuum of the microscope. Illites are identified by HREM as small packets showing very regular lattice fringes and smectites as highly imperfect, anastomosing layers showing very irregular lattice fringes (1).

MINERALOGICAL AND GEOCHEMICAL STUDIES OF TERTIARY SEDIMENTS FROM THE EASTERN NIGER DELTA AND THEIR RELATIONSHIP TO PETROLEUM OCCURRENCE

Mineralogical and chemical studies have been made of shale cuttings from the Akiri‐1 well in Oligocene — Miocene sediments of the Niger Delta. The < 2μm fraction from the 2,183 to 3,720m stratigraphic interval was analyzed by X‐ray diffraction. Kaolinite and chlorite are more abundant than other clay minerals. The illiteh mectite composition changes from a phase with 32% illite layers, to one with 82% illite layers over the 3,250 to 3,720m stratigraphic interval. Kaolinite decreased in shallow shales from 22 to 16% and increases to 29% in deep shales. Quartz increases in abundance with depth, while chlorite and plagioclase feldspar show no systematic trend with depth. Calcite is present in small amounts in all the samples.Variations in the bulk chemical composition of true shale with depth only show significant changes in the K2O and Fe2O3 content: other major chemical constituents show no systematic variations. The potassium appears to be derived from the pore water. The loss of smectite and abundance of Fe coincided with hydrocarbon occurrence. On the basis of the apparent systematic transformation of smectites to illites through mixed layers, the illite/smectite reference interval in the subsurface has been established: this interval coincided with hydrocarbon occurrence.Mineralogical and chemical changes observed are dependent on the pore system chemistry, the temperature, and possibly compaction effects.

Compositional Variation in Component Layers in Natural Illite/Smectite

AbstractPublished chemical data for suites of mixed-layer minerals from diagenetic sedimentary rocks, hydrothermally altered tufts, and a metasomatic bentonite bed indicate that the layer charge and composition of the different components of illite/smectite (I/S) differ from one geological environment to another. It appears that the composition of the elemental smectite and illite layers in the I/S is more or less constant for samples within each geologic setting. In the examples considered, the smectite layers are predominantly montmorillonitic in character (i.e., the charge is in the octahedral site), whereas the illite layers show different types of charge sites, depending upon the suite studied. Illite layers appear to have about the same charge in all three suites studied, slightly more than 0.7 per O10(OH)2 unit, whereas the smectite layers in the different suites range in charge from about 0.3 to 0.7 per O10(OH)2 unit. Cation-exchange capacities reflect these differences in charge, although not ideally. The differences in the composition of the component layers in each geologic suite of mixed-layer clays are probably due either to differences in the bulk chemistry of the rocks in the different suites or to differences in intensive variables, such as temperature and pressure, of the regime under which they have formed.

Chemistry of Detrital Biotites and Their Phyllosilicate Intergrowths in Sandstones

AbstractMicroprobe analyses of optically homogeneous detrital biotites from sandstones of the Visingsö Group and the Dala Sandstone, Sweden, revealed a consistently low K content (generally <0.75 atom/formula unit) and variable amounts of Fe, Mg, Al, and Si. Electron probe profiles of some biotite grains indicated two major types of interstratification, one consisting of mainly illite layers and the other apparently consisting of chlorite layers. The layer thicknesses commonly ranged between 0.5 and 3 μm. Microprobe analyses of some thick (∼5 μm) illitic layers indicated a phengitic composition, wherein the mica was relatively rich in octahederal Fe and Mg. The chloritic layers appeared to be Fe-Mg-rich and generally had octahederal totals of <6 atom/formula unit. Variations in the chemical composition of the biotite and some of the illite and chlorite were probably due to an uneven distribution of small amounts of the interstratified phases. The illite and chlorite layers were apparently formed by pseudomorphic replacement of detrital biotites, i.e., gradual replacement of one biotite layer by one layer(s) of illite and/ or chlorite during diagenesis.

Chemistry of Illite/Smectite and End-Member Illite

AbstractChemical data from three different series of diagenetic illite/smectites (I/S), analyzed statistically by two regresion techniques, indicate that the content of fixed-K per illite layer is not constant, but ranges from ~0.55 per O10(OH)2 for illite layers in randomly interstratified I/S (R=0; >50% smectite layers) to ~ 1.0 per O10(OH)2 for illite layers formed in ordered I/S (R>0; <50% smectite layers). By extrapolation of the experimental data, the following chemical characteristics were obtained for end-member illite derived from the alteration of smectite in bentonite: average fixed-K per illite layer = 0.75 per O10(OH)2; total charge = about -0.8; cation-exchange capacity = 15 meq/100 g; surface area (EGME) = 150 m2/g.

Pathways of Smectite Illitization

AbstractJunction probability diagrams show variation in both composition and layer arrangement in mixed-layer clay minerals. These diagrams can represent short-range and long-range ordered, random, and segregated interstratifications. Mineralogical analyses of illite/smectite from shale cuttings, bentonites, and hydrothermally altered tuffs define characteristic reaction pathways through these diagrams. Shale and bentonite analyses fall along pathways joining smectite and illite on diagrams showing nearest-neighbor (R1) layer arrangements. Transition from random to R1-ordered interstratifications occurs in shale samples containing 60–70% illite layers, and in bentonites containing 55–67% illite layers. Analyses of alteration products, however, fall near a line connecting rectorite and illite, which represents the maximum degree of R1 layer ordering. No mineralogical evidence is available to suggest that these alteration samples formed from a smectite precursor. All samples develop next-nearest (R2) and thrice-removed (R3) neighbor ordering along similar pathways. Transition to R2 ordering occurs gradually in samples composed of 65–80% illite layers, and samples containing more than 85% illite layers may show strong R3 ordering.

Morphology of Clay Minerals in the Smectite-to-Illite Conversion Series by Scanning Electron Microscopy

AbstractThe morphology of illite/smectite (I/S) from deeply buried bentonites and hydrothermally altered Tertiary volcanic rocks from Japan changes in parallel with the proportion of expandable layers in the I/S. As viewed by scanning electron microscopy, the morphologies range from the typical “cornflake,” “maple leaf,” or “honeycomb” habit of smectite to the typical platy or scalloped (with curled points) habit of illite. Although the changes are more subtle near either end member, at a composition of 60-70% illite layers, the morphology changes from sponge-like or cellular to platy or ribbon-like. The change of morphology at this composition correlates with a change in layer stacking from turbostratic to rotational ordering of the 1Md type. Turbostratic stacking can be thought of as randomly distributed translations of successive layers by any magnitude and in any direction. The rotationally ordered structure, which allows nearly precise juxtaposition of quasihexagonal oxygen surfaces from adjacent layers, probably permits more crystalline regularity in the a-b plane, which promotes a more plate-like or sheet-like habit.

Microstructure in Mixed-Layer Illite/Smectite and Its Relationship to the Reaction of Smectite to Illite

AbstractThe reaction of smectite to illite in shale from the COST 1 well in the south Texas Gulf Coast and from altered ash-fall tuffs from the Morrison Formation in New Mexico was investigated using X-ray powder diffraction in conjunction with transmission electron microscopy. In the COST 1 well, the bulk of the detrital clay was originally a K+-deficient mixed-layer illite/smectite (I/S). As the I/S adsorbed K+ released by the dissolution of K-feldspar during burial, the proportion of expandable layers decreased with depth from ~65% near the top of the well to ~25% at 4500 m depth. In contrast, the proportion of low-charged structural planes [<0.8 eq per (Al,Si)4O10 unit] in the I/S decreased gradually from ~40% near the top of the well to ~15% near the bottom. Authigenic smectite with 100% expandable layers from the Morrison Formation tuffs is an alteration product of vitric ash. Where these tuffs have been buried to ~ 1400 m the smectite has reacted to form I/S with ~ 15% expandable layers.Direct lattice images of I/S crystallites from both locations reveal a correspondence between edge dislocations and the interface between illite layers and smectite layers. Al, Si, Fe, Ca, Mg, and Na were apparently mobile along these dislocations as the reaction of smectite to illite proceeded. Al was probably retained within the crystallite when illite layers replaced the smectite layers; however, some of the remaining cations were expelled. Lateral replacement of smectite layers by illite appears to have been the principal growth mechanism.

Transmission and Analytical Electron Microscopy of the Smectite-To-Illite Transition

AbstractThe smectite to illite reaction was studied by transmission and analytical electron microscopy (TEM/AEM) in argillaceous sediments from depths of 1750, 2450, and 5500 m in a Gulf Coast well. Smectite was texturally characterized as having wavy 10- to 13-Å layers with a high density of edge-dislocations, and illite, as having relatively defect-free straight 10-Å layers. The structures of smectite and illite were not continuous parallel to (001) at smectite-illite interfaces. AEM data showed that the smectite and illite were chemically distinct although smectite had a more variable composition. Illite formation appeared to have initiated with the growth of small packets of illite layers within subparallel layers of smectite matrix. With increasing depth, ubiquitous thin packets of illite layers increased in size until they coalesced.A model for the transition requires that the structure of smectite was largely disrupted at the illite-smectite interface and reconstituted as illite, with concomitant changes in the chemistry of octahedral and tetrahedral sites. At least partial Na-K exchange of smectite preceded illite formation. Transport of reactants (K, Al) and products (Na, Si, Fe, Mg, H20) through the surrounding smectite matrix may have taken place along dislocations.The smectite-to-illite conversion process for the studied samples does not necessarily appear to have required mixed-layer illite/smectite as an intermediate phase, and TEM and AEM data from unexpanded samples were found to be incompatible with the existence of mixed-layer illite/smectite in specimens whose XRD patterns indicated its presence.

Evidence for syntectonic crystallization for the mudstone to slate transition at Lehigh gap, Pennsylvania, U.S.A.

Data primarily for phyllosilicates have been obtained for the continuous transitional sequence from mudstone to slate with well-developed slaty cleavage at Lehigh Gap, Pennsylvania, and for slates from quarries in the same area. Samples were studied by optical microscopy, powder X-ray diffraction and electron microprobe analysis, with emphasis on transmission and analytical electron microscopy. Mineral grains are virtually free of deformation-induced strain. Concomitant with the gradual development of cleavage normal to bedding the following changes are observed or confirmed: (1) the orientation of phyllosilicate grains changes discontinuously from being preferentially parallel to bedding to being parallel to cleavage; (2) crystal imperfections as expressed in layer terminations, low angle grain boundary-like features and other defects decrease in density; (3) complex mixed layering is replaced by homogeneous packets of layers of single phases and (4) illite transforms to muscovite, with increase in K + Al and change from a 1M d to 2M polytype. Slaty cleavage apparently develops due in part to pressure solution of phyllosilicates oriented parallel to bedding, mass transport of components, and crystallization to form new grains parallel to cleavage. It reflects transitions from imperfect, metastable phases toward ordered stable phases in a low temperature (∼225°C) metamorphic environment.

Oxygen and hydrogen isotope geochemistry of Cretaceous bentonites and shales from the Disturbed Belt, Montana

The mineralogy, δO 18, and δD of the <0.1 μm fraction of 22 Cretaceous bentonites and the mineralogy and δO 18 of the < 0.1 μm fraction of 14 adjacent shales collected from outcrops in the Sweetgrass Arch and Disturbed Belt, Montana, have been determined. Mixed-layer illite/smectite (I/S) is the dominant mineral in the bulk bentonite and usually the only mineral in the < 0.1 μm fraction. I/S is also the major clay mineral in the shales. The diagenetic grade in bentonite is qualitatively given by the percentage of illite layers in I/S, which varies from 2 to 25 (Sweetgrass Arch) to as high as 95 (Disturbed Belt). δO 18 of < 0.1 μm bentonite generally decreases from about +20%. to about +13%. with increasing diagenetic grade. On a plot of δD versus δO 18, data for the < 0.1 μm bentonite define a field that generally parallels, but falls on the meteoric water line side of the smectite-water line (Savin and Epstein, 1970). δO 18 of bulk bentonite is 1 to 3%. more negative than the δO 18 of the < 0.1 μm fraction, due to the presence of volcanic quartz and feldspar. δO 18 of several size fractions of clay-sized quartz separated from the bentonite varies from +11%. to +24%., and, in a given bentonite, generally increases with decreasing grain size. Among the different bentonites, the δO 18 range of the different grain sizes decreases as the percentage of illite layers in the coexisting I/S increases. The δO 18 of 0.1–0.5 μm shale quartz is generally 1 to 4%. more positive than clay-sized quartz from an adjacent bentonite, and the δO 18 of < 0.1 μm I/S concentrate of shales is generally < 1 to 4%. more negative than the < 0.1 μm I/S from an adjacent bentonite. Isotopic temperatures, interpreted to be maximum burial temperatures, range between about 160°C (shale), to about 250°C (bentonite). The isotopic data can be interpreted using the stages: 1. 1) deposition of volcanic glassy ash containing some quartz and feldspar; 2. 2) devitrification into mostly 100% expandable smectite with a δO 18 of about 20%. and δD of −60 to −80%.; 3. 3) tectonic burial beneath thrust sheets in the Disturbed Belt region which induced elevated temperatures that allowed oxygen and hydrogen isotope reequilibration as the percentage of illite layers in I/S increased and diagenetic quartz formed; and 4. 4) exhumation of the strata with preservation of O 18 O 16 and D/H attained during burial.

The physical dimensions of fundamental clay particles

AbstractThe thickness, length, width, area and perimeter of 575 particles from 16 aqueously dispersed samples of a variety of interstratified clays, smectites and illite have been recorded using TEM techniques. Complete dispersion of the clay material was achieved by saturating the clay with either Na+or Li+, removal of excess ions by dialysis, and isolation of the &lt;0·1 or &lt;0·2µm fraction by centrifugation. The samples have mean maximum dimensions of 1900 to 90 nm and the dispersed system can be considered as colloidal in nature. The mean thickness of the clays is about 1 nm for smectites, corresponding to that of elementary 2:1 silicate layers, from 1·9 to 4·9 nm for the interstratified clays, and 9 nm for illite. From these data the volume, total surface area and other parameters have been calculated and compared with independent determinations of surface area and CEC. The total surface area by TEM, assuming a density of 2·6 g/cm3, varies from ∼675 m2/g for smectites to 86 m2/g for illite, and is inversely proportional to the mean particle thickness. The charge density of monovalent cation exchange sites on the surface of the particles as determined for nine of the samples varies from 0·54 to 1·16 nm2/site. The particle-thickness distribution data can be used to calculate interstratified XRD layer-sequence probabilities and composition parameters, and agree with XRD data for interstratified clay with &lt;60% illite layers. The thickness data also provide a rationale for the interpretation of TEM lattice-fringe images. Relationships between the particle area, length, thickness and volume are shown to be potentially useful in assessing the mechanism(s) of crystal growth of these extremely small phyllosilicate particles.

Determination of layer-charge density of expandable 2:1 clay minerals in soils and loess sediments using the alkylammonium method

AbstractThe alkylammonium adsorption method for the determination of layer-charge density was slightly modified and applied to mixtures of expandable clay minerals (i.e. predominantly 18 Å minerals after glycerol sorption) from two loess samples and two soils (Haplaquept and Aquentic Chromudert) having different K-adsorption properties. The layer-charge density of the so-called 18 Å minerals from these sediments and soil samples varied between −0·23 and −0·85 per formula unit, which suggested the presence of different amounts of vermiculite within the 18 Å minerals. The amounts of these vermiculites were related to K-fixing and K-buffering properties of the different samples. High-resolution electron micrographs of vermiculites saturated with tetradecylammonium exhibited a basal spacing of 25 Å. It was confirmed, that the alkylammonium ions in the interlayers of the vermiculites formed a paraffin-type structure. In the fine clay from the loess samples an interstratification of vermiculite and illitic layers was observed.

Mineralogical and Morphological Evidence for the Formation of Illite at the Expense of Illite/Smectite

AbstractThe conversion of smectite to illite by way of a mixed-layer illite/smectite (I/S) series was found to be the major depth-related reaction in clay-mineral assemblages from two cored sedimentary sequences in the Rocky Mountains. The I/S reaction occurred in both interbedded sandstone and shale of Upper Cretaceous and lower Tertiary age in the Green River basin, Wyoming, and in chalk and chalky shale of the Upper Cretaceous Niobrara Formation, Denver basin, Colorado. As the proportion of illite layers in I/S increased with depth in these rocks, the amount of I/S in the clay fraction decreased, and the amount of discrete illite increased. Scanning electron microscopy revealed that the morphologies of highly expansible, randomly interstratified I/S clay (samples from shallow cores) exhibited no distinctive intergrowth or overgrowth textures. In deeply buried rocks containing highly illitic, ordered I/S and abundant discrete illite, however, fibers or laths of illite were formed on earlier I/S substrates. Less commonly, I/S of low expandability shows morphological features of both smectite and illite whereby rigid laths of illite appear to have formed diagenetically from the wall surfaces of I/S honeycombs. This combined morphology suggests some dissolution and reprecipitation (or some reorganization) of materials from the I/S substrate as the substrate was transformed into a more illitic mixed-layer clay.These data suggest that some I/S clay was destroyed by the selective cannibalization of smectite layers in I/S to provide the components needed to make a more illitic I/S. Moreover, discrete illite and other minerals apparently were formed during the reaction. Also, coarser mineral phases, such as potassium feldspar and detrital micas, may not have been required to supply the chemical components in the reaction. The observations provide an explanation for late diagenetic I/S reactions that occurred in restricted or relatively closed geochemical systems, as in early cemented rocks having extremely low permeability and little or no potassium feldspar.

The conversion of smectite to illite during diagenesis: evidence from some illitic clays from bentonites and sandstones

AbstractDiagenetic illitic clays from seven North American bentonites of Ordovician, Devonian, and Cretaceous ages and from three subsurface North Sea sandstones of Permian and Jurassic ages have been examined by X-ray diffraction (XRD) and transmission and scanning electron microscopy (TEM and SEM). XRD indicates that the clays from the bentonites are randomly and regularly interstratified illite/smectites (I/S) with 30–90% illite layers, whereas the clays from the Jurassic and Permian sandstones are regularly interstratified I/S, with 80–90% illite layers, and illite respectively. TEM of shadowed materials shows that randomly interstratified I/S consists primarily of mixtures of elementary smectite and ‘illite’ particles (10 and 20Å thick respectively) and that regularly interstratified I/S and illite consist mainly of ‘illite’ particles 20–50 Å thick and &gt; 50 Å thick respectively. Regularly interstratified I/S from bentonites and sandstones are similar with regard to XRD character and particle thickness distribution. These observations can be rationalized if the interstratified XRD character arises from an interparticle diffraction effect, where the smectite interlayers perceived by XRD, result from adsorption of exchangeable cations and water or organic molecules at the interfaces of particles generally &lt; 50Å thick. A neoformation mechanism is proposed by which smectite is converted to illite with increasing depth of burial in sedimentary rocks, based on dissolution of smectite particles and the precipitation/growth of ‘illite’ particles occurring within a population of thin phyllosilicate crystals.

Thermal History of Sandstones and Shales: Oxygen Isotope and K/Ar Evidence: ABSTRACT

The combined use of oxygen isotope and K/Ar dating can yield information about the diagenetic history of clay minerals in shales and sandstones. In the Tertiary shale sequences of the United States Gulf Coast, in which the dominant detrital clay mineral is mixed-layer illite/smectite, the progressive conversion of expandable clay layers to illite layers can be monitored by the isotope systematics. Oxygen isotopes of fine-grained clay and quartz approach equilibrium with one another, raising the (as yet unrealized) possibility of an O-isotope geothermometer. Fine-grained quartz becomes isotopically zoned as detrital grains are overgrown by diagenetic quartz that forms, accompanying the smectite-to-illite transformation. The K/Ar clock of the diagenetically formed illite la ers is set to zero age at the time of diagenesis (although that of existing illite layers within the crystals remains unaffected), and it is therefore possible to estimate the mean time of diagenesis in such shale sequences. Illite is a common cement in sandstones. The time of cementation by illite can be estimated from K/Ar systematics in cases, such as that of the Permian Rotliegendes of the North Sea, where the clay-sized fraction of the original detritus was relatively free of illite or other K-bearing phases. Conditions of cementation can be inferred from oxygen isotope measurements, augmented by knowledge of the geologic/tectonic history of the sandstone. In the case of the Rotliegendes, the timing and conditions of illite cementation were relatively uniform within fault blocks but varied from block to block. Oxygen isotope measurements indicate that meteoric water components were important in illite formation at times when nearby sections of the Rotliegendes were exposed to the surface by uplift an erosion. End_of_Article - Last_Page 304------------

X-Ray Powder Diffraction Identification of Illitic Materials

AbstractThe 10-Å clay components of sedimentary rocks (“illites”) are commonly mixtures of 100% nonexpandable illite and an ordered illite/smectite mixed-layer mineral. If the proportion of the illite/ smectite in a mixture is sufficient to produce a measurable reflection between 33–35°2θ (CuKα radiation) that is noncoincident with an illite reflection, the ratio of component layers and type of interstratification for the mixed-layer mineral can be determined. The identification technique developed in this study rests upon the following experimental findings for ordered illite/smectites of diagenetic origin: (1) the thickness of the illite layer in illite/smectites is 9.97 Å; (2) the thickness of smectite-ethylene glycol complex ranges from 16.7 to 16.9 Å; (3) illite/smectites form a continuous sequence of interstratification types—random, random/IS, IS, IS/ISII, ISII—and each type is related to a specific range of expandability.The new technique broadens the computer simulation method developed by R. C. Reynolds and J. Hower to include those sedimentary materials which are dominated by the presence of discrete illite, are low in illite/smectite, and, as such, have been described previously only by an “illite crystallinity index.”

Complete Dehydration of Illite/Smectite in Gulf Coast Overpressured Shales: ABSTRACT

Twelve samples of Frio and Vicksburg (Tertiary) overpressured shales from Brazoria and Hidalgo Counties, Texas, were examined by both x-ray and transmission electron microscopy (TEM) techniques. TEM lattice fringes from shallower samples show mixed-phase illite/smectite (I/S) layers in random orientations relative to each other. Electron diffraction patterns of these shallower I/S layers show very diffuse basal reflections together with very pronounced turbostratic structure and streaking along z*. Electron diffraction patterns of I/S layers from intermediate depths still show z* streaking and turbostratic structure, but the basal reflections are more distinct. TEM lattice fringe images of intermediate depth samples show I/S layers arranged in a subp rallel orientation. Electron diffraction patterns of I/S layers from deeper samples generally show well-defined basal reflections, and both turbostratic structure and z* streaking are less pronounced. An electron diffraction pattern of I/S layers from one deep sample (12,490 ft, 3,800 m, calculated equilibrium temperature of 168°C, 334°F, and pore-pressure gradient > 0.7 psi/ft, 15.8 kPa/m) shows an illite 2M1 pattern, indicating complete dehydration of original I/S layers. The presence of pronounced z* streaking is thought to be due to disordered stacking of layers, which in turn is caused by diagenesis of original mixed-phase I/S layers. TEM lattice fringe images for this sample show parallel illite layers with basal spacing of approximatel 20 A. End_of_Article - Last_Page 478------------

Tarasovite, a mixed-layer illite-smectite which approaches an ordered 3:1 layer ratio

AbstractBasal X-ray peaks from normal and ethylene glycol-saturated tarasovite have been measured to 0.0.36 and 0.0.38, respectively; the mean basal spacings were 43·73 ± 0·44 and 46·05 ± 0·33 Å, withCVvalues of 1·31 and 0·97. The spacings indicate a near approach to a 3 : 1 illite-smectite but theCVvalues indicate a greater variability than is normally consistent with the allocation of a special name. Comparison of the observed spacings with those for illite showed that the more deviant values were always adjacent to positions of illite reflections. A modified Mering treatment of diffraction by mixed-layer sequences was used to estimate the proportion of illite layers; the average result was 79·9 ± 2·0%. The mineral is considered to have 3 : 1 illite-smectite units with randomly added illite layers, bringing the ratio from 75% for the ideally ordered sequence to near 80%.

Clay Mineral Transformations in Tertiary and Mesozoic Sediments from North Sea

Clay mineral transformations of 10 North Sea wells have been studied, and several transformation stages picked out and correlated with geothermal variations: smectite disappears at 149 to 167°F (65 to 75°C), whereas transformations in the mixed-layer illite-smectite clay minerals are observed between 113 and about 212°F (45 and about 100°C). At temperatures above 113 to 149°F (45 to 65°C), more than 30% illitic layers are found in the mixed-layered clay minerals, whereas temperatures from 140 to 167°F (60 to 75°C) indicate the transition zone toward more than 50% dehydrated lattice positions. Temperatures above 176 to 212°F (80 to 100°C) mark the lower limit for mixed-layer clay minerals containing more than 70% illitic la ers. The major part of the clay mineral transformations apparently takes place in the mesodiagenetic zone, in which most of the hydrocarbons are also formed.