Discrete Illite Research Articles

Mineralogy and Geochemistry of the Subcrenatum Marine Band and Associated Coal-Bearing Sediments, Langsett, South Yorkshire

A detailed description is given of the stratotype marking the Namurian-Westphalian boundary in the Carboniferous. Major element analyses are presented for twelve representative samples, including, in upward sequence, underclay, coal, nonmarine shale, and marine shale. The clay mineralogy is described based on powder and smear-mounts of whole-rock samples and clay-sized fractions subjected to standard treatments and cation saturations. Ascending the sequence there is a decrease in the proportion of kaolinite and increases both in the ratio of discrete illite to mixed-layer illite/smectite and in their sum. Chlorite and vermiculite are absent in the underclay but appear higher in the sequence. The upward changes are attributed to progressive flooding associated with a eustatic rise in sea level leading to reworking of mature soils in the flood-plain environment. With time this source decreased in importance compared with the detrital clay input from the primary source area. The underclay in the section has undergone extensive alteration indicative of long subaerial exposure, possibly due to eustatic fall in sea-level.

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.”

Silica Cement Source and Porosity Preservation for Ohio's Lower Silurian Sandstones: ABSTRACT

The Lower Silurian sandstone sequence of Ohio is comprised of sandstone, siltstone, and shale, commonly recognized as three more-or-less distinct informal subunits. The lowermost is called white Clinton, the intermediate is red Clinton, and the uppermost is stray Clinton. These subunits have been correlated by others, respectively, as the Cabot Head Shale, the Grimsby Sandstone, and the Thorold Sandstone of western New York. The sandstones and siltstones are generally tight and highly cemented by pervasive secondary silica. Original depositional clay was probably mixed-layer illite-smectite which, as indicated by x-ray diffraction, was transformed to the present discrete illite and very minor iron-chlorite. Remnant expandable illite-smectite is present as less than 5% tot l clay. The released silica of this transformation probably provided much, if not most, of the silica cement. The remnant mixed-layer illite-smectite occurs within the sandstones where apparently it was preserved by the protective framework of detrital sand grains. Other probable minor sources of silica cement were pressure solution of original quartz grains and dissolution of allogenic feldspar, as exhibited in this section and by scanning electron microscopy. Porosity increase toward the middle of some sandstone intervals suggests migration of silica from transformed overlying and underlying shales. Porosity was preserved, in part, by collophane grain coatings inhibiting authigenic silica growth, as exhibited by petrography and scanning electron microscopy, and by clay overgrowths, as emonstrated in earlier study. End_of_Article - Last_Page 1928------------

Clay-Mineral Relationships in Some Low-Permeability Hydrocarbon Reservoirs and Their Use as Predictive Resource Tools: ABSTRACT

Detailed mineralogical characterizations, using X-ray diffraction and scanning electron microscopy of sand-shale and carbonate-shale-bentonite sequences, of some classic low permeability hydrocarbon reservoirs reveal basic clay-mineral relationships that are helpful in evaluating the extent of diagenesis and petroleum resource potential. Gas-potential units of the El Paso Natural Gas 1 Wagon Wheel and BELCO 3-38 (formerly INEXCO 1-A WASP) wells, Green River basin, Wyoming, contain shales composed primarily of altered detrital clay suites of various origins and compositions that are consistently different than authigenic clays formed within adjacent sandstones. Commonly, discrete illite is abundant in the shales as a major detrital component; the percentage of illite is lower in the sandstones. Chlorite typically comprises a high percentage of the clay-size (< 2µm) material of sandstones and is primarily authigenic, however, little chlorite (< 8%) is inherent in the shales. Authigenic interstratified illite-smectite (I/S) formed in relatively clean sandstones is typically less illitic and more restricted in omposition range than I/S clay of various origins within shales or shale-laminated sandstones. Therefore, because of these primary differences in clay mineral assemblages between sandstones and shales from these units, log interpretations of low permeability sand reservoirs should not be extrapolated from the log response of adjacent shales. The composition of I/S clay from relatively cleaner sands also may give a better indication of diagenetic minimums. Indigenous gas and oil are produced from low-permeability chalk beds of the Upper Cretaceous Niobrara Formation within the Denver basin of eastern Colorado and western Kansas. Clay-mineral studies of I/S clay in insoluble residues of shaly chalk sampled throughout the basin show that the starting composition of the I/S clay in these strata is highly variable due to a complex mixing of I/S clays from many different sources at the time of deposition. However, the composition of I/S clay from thin bentonites in these strata is quite consistent throughout a section from any one specific location and/or depth, and these clays become progressively more illitic and ordered with increasing temperature due to increased burial. At approximately 60% illite layers in I/S, the I/S of bentonites pr ceed from random (R = 0) to short-range ordered (R = 1) interstratification. I/S clay formed in thin, discrete bentonite beds within Niobrara strata, where original starting compositions were nearly fixed and uniform and there was little or no contribution from detrital clays, is the best indicator of the extent of diagenetic reactions. The presence (or absence) of regular interstratified clay minerals in thin bentonite beds of the Niobrara can, therefore, be used as relative geothermometers for constructing predictable petroleum resource maturation maps of the Denver basin and adjacent areas. End_of_Article - Last_Page 536------------

Origin of an Underclay as Revealed by Vertical Variations in Mineralogy and Chemistry

AbstractRegular vertical variations in mineralogy and chemistry indicate that underclay beneath the Herrin (No. 6) coal in southwestern Illinois has undergone in situ alteration. Alteration resulted from the downward movement of hydrogen ions, as indicated by the progressive leaching of acid-sensitive minerals adjacent to the coal. Mineralogical trends observed in the underclay with increasing depth below the coal include: (1) a decrease in the expandability of mixed-layer illite/smectite (I/S); (2) an increase in the amount of ordered I/S with respect to randomly interstratified I/S; (3) an increase in the amount of discrete illite with respect to expandable clays; and (4) an increase in chlorite and calcite. Ordered I/S is the dominant mixed-layer clay where calcite is present, but randomly interstratified I/S dominates where calcite is absent. The pH of the underclay also increases with depth. These trends are consistent with an origin by acid leaching of a preexisting mineral assemblage that included illite, chlorite, and calcite. Other acid-alteration trends may be expected for different precursor minerals and for different leaching intensities and durations.

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------------

K/Ar Dating of Illitic Clay in Sandstone Reservoirs and Timing of Petroleum Migration: ABSTRACT

Illite and illite/smectite, which occur as authigenic pore-fill in some sandstones, are excellent argon-retentive clocks. Dating their mean time of growth in conjunction with other basin information may add to predictive understanding of illitic cement distribution. Paragenesis and distribution of authigenic clay in the sandstone body with respect to hydrocarbon occurrence may allow the measured age to be related to time of hydrocarbon emplacement. For certain petroleum-source shales, the mean age of illite/smectite burial metamorphism, and thus petroleum maturation, can be measured directly and then compared to the age of clay authigenesis in reservoir sandstones to see if the two phenomena are related. Three sandstones have been examined: the Cretaceous Muddy on the eastern flank of the Powder River basin; the Jurassic Nugget of the Wyoming Overthrust belt; and the Permian Rotliegendes of northern Europe. For the regressive poorly sorted Muddy, logistic problems arise because of the low abundance of authigenic illite/smectite compared to kaolinite and the common occurrence of old detrital illite. The age of illite/smectite authigenesis thus is asymptotically derived from analyses of a series of progressively finer (and authigenically purer) clay size fractions down to < 0.05µ. Over a broad region, this derived age is about 40 m.y., excluding Bell Creek which is anomalous. This age may associate illite/smectite growth in the reservoir with the early Tertiary deep burial of th basin interior. Because similar ages occur in both oil and water zones, illite/smectite growth may either have preceded or accompanied oil emplacement. Logistic problems are minimized for the abundant, high-K2O, discrete illite cements of the well-sorted aeolian Nugget and Rotliegendes. For both of these, precise sets of ages have been obtained for individual fields in both water and hydrocarbon zones, implying cementation may have been a relatively short-lived event. The Nugget samples, all from the Absaroka sheet, give late Mesozoic ages. In the Rotliegendes, complex block faulting has led to unique post-late Jurassic burial and gas generation histories for adjacent fault blocks. However, thus far the three areas of the Rotliegendes examined indicate illite growth largely preceded this history. End_of_Article - Last_Page 1442------------

Volcanic components in pelitic sediments

The origin of the clay minerals present in pelitic sediments has been widely debated. Weaver1 suggests that illite and interstratified illite/smectite (I/S) dominate. The association of illite with marine sediments2 and the proposal3,4 that illite forms from smectite (an expandable clay mineral sometimes referred to as montmorillonite) in the marine environment has also been discussed. Weaver5 argues for a continental origin of illite, produced by weathering of K-feldspar. Others6–9 have shown that smectite and I/S (20–50% illite) are converted to I/S of higher illite content (60–80%) in pelitic sediments by low-grade metamorphism. Per cent illite will always refer to the per cent illite in I/S. The designation should not be confused with the content of discrete illite or mica commonly present in these sediments. Here we present clay mineral and K–Ar data for Cretaceous marine shales from the Western Interior of North America demonstrating a volcanic origin for much of these sediments. This observation is correlated with eustatic sea-level rise, increased volcanic activity, sedimentary preservation of organic matter, and incorporated into a petrogenetic/tectonic model for pelitic sediments.

X-ray identification of randomly interstratified illite-smectite in mixtures with discrete illite

AbstractProportions of the component layers in randomly interstratified illite-smectites can be determined precisely in the presence of discrete illite if two reflections (at 15–16 and 31–32°2θ Cu Kα) are strong enough to be measured accurately. The method takes into account the variable thickness of the smectite-ethylene glycol complex and involves a correction for the interference from illite reflections.

Probable Burial Diagenetic and Provenance Effects on Dakota Group Clay Mineralogy, Denver Basin

ABSTRACT Illite-smectites in cored Dakota Group (Cretaceous) shales and bentonites of the northern Denver Basin, identified using a modification of Reynolds' and Hower's (1970) X-ray diffraction criteria, show decreasing sample-sample expandability ranges, lower modal expandability, and a change from random to ordered mixed-layering with increasing burial depths and temperatures. These are results characteristic of burial diagenesis as described by Perry and Hower (1970). Ordered illite-smectites from low-temperature wells and much of the kaolinite (± chlorite, vermiculite) and discrete illite from all wells are interpreted as detrital. Episodic but frequent admixture of highly-expandable bentonitic illite-smectite with detrital clays is reflected by fluctuating expandabilities within individual cores and the lack of samples containing both highly-expandable illite-smectite and high proportions of other clay minerals. Differential diagenesis and/or differential sedimentation rates do not satisfactorily account for lower modal expandabilities and increased proportions of ordered illite-smectite observed in shales from D and J sand-stones compared with those from Huntsman and Skull Creek shales of equivalent temperatures. Instead, they are attributed to provenance differences which gave lower-expandability illite-smectites--and a head start on diagenetic trends--to sand-associated shales. Eastern and western provenances identified from heavy mineral studies (e.g., MacKenzie and Poole, 1962) for D and J sands were not matched by similar clay mineral provenances affecting shales. However, decreasing proportions of kaolinite (± chlorite, vermiculite) encountered from northeastern to southeastern D locales are interpreted as the results of differing sources.

Clay Minerals as Indicators for Depositional Environment in South Halletsville Field, Lavaca County, Texas: ABSTRACT

The South Halletsville field, Lavaca County, Texas, has gas and condensate production from lower Wilcox sandstones and shales which have been interpreted as either channel turbidite deposits in outer-shelf to slope locations or deltaic and strand-plain sands and muds. Twenty-four core samples from the General Crude Oil Co. 1 A. G. Henkes Gas Unit were analyzed by X-ray diffraction methods to determine whether a semiquantitative estimate of clay mineral content would aid in determining the depositional environment. Discrete illite and chlorite are of particular interest because the presence of these minerals is interpreted as being due to original deposition. Three shale samples, from 10,194 to 10,206 ft (3,107 to 3,111 m) and 11 sandstone samples, 10,180 to 10,194 ft (3,103 to 3,107 m) were selected from one core section. This sequence of samples is particularly important because it contains a shale and the overlying sandstone. In addition, a deeper sandstone was sampled in the interval of 11,032 to 11,072 ft (3,363 to 3,375 m). If a turbidity-type event had occurred, the weight percent of non-diagenetic clays should (1) decrease significantly as the boundary is crossed between the shale and the overlying sandstone, and (2) gradually increase in progressively shallow samples within the sandstones. However, the weight percent for chlorite does not vary significantly regardless of a change in lithology, shale to sandstone, or a change in depth. The illite content gradually decreases with shallower depths in both core intervals. This sequence is more compatible with a transgressive deltaic environment. End_of_Article - Last_Page 1557------------

Authigenic Illites in Sandstone Reservoirs: ABSTRACT

Authigenic illites have been found in pores of many sandstones which are known to be hydrocarbon reservoirs. Typical samples are from the Norphlet Formation in southern Mississippi, the Wilcox Formation in south Texas, and the Lance Formation in Wyoming. Illites in these sandstones are mainly in the form of laths with perfectly developed morphologies. Laths have widths of 0.1 to 0.3µ and lengths ranging up to 30µ. Scanning electron (SEM) images show that these hair like illites fill many of the pores of these sandstones, and cause a serious reduction in permeability. Elemental analysis of the laths with EDAX energy dispersive X-ray analyzer shows that Si, Al, and K are the major constituents and Mg is the minor constituent in the chemical composition of the la hs. Transmission electron images show that illite laths have grown from an amorphous core which is rather similar to an irregular smectite aggregate. X-ray diffraction patterns display the coexistence of discrete illite (9.9A) with an illite/smectite mixed layer with a distinct reflection varying from 10.5 to 11.0A. In the sample from the Wilcox Formation the laths form regular arrangements which lead to the development of platelets. End_of_Article - Last_Page 460------------

Mixed-Layer Clays in Eocene Interlaminated Shales and Sandstones: ABSTRACT

Kaolinite, illite, and mixed-layer montmorillonite-illite are present in interlaminated sandstones and shales from the Eocene Wilcox Formation of the Texas Gulf Coast at depths of 2,000 to 4,000 m. Ratios of kaolinite to discrete illite are higher in the sandstones than in the interlaminated shales. Mixed-layer clays in the sandstones are 5 to 20% more expansible and less ordered than those in interlaminated shales. These mineralogic differences are interpreted to reflect a significant difference in the solution chemistry of pore waters in the sandstones from that of pore waters in the interlaminated shales. At the relatively shallow depth of 2,000 m, mixed-layer montmorillonite-illite from the shales is roughly 30% expansible; this figure is significantly less than the 7 % expansibility reported by J. Hower et al for Miocene age mixed-layer clays at similar depths. Assuming similar source and geothermal gradient for the two sets of samples, two explanations may account for these differences. The most plausible explanation is that the expansibility of the mixed-layer clay is controlled by the montmorillonite-to-illite transformation rate. Because the Eocene sediments have had an additional 25 m.y. to react, the montmorillonite-to-illite transformation is more complete in these samples. An alternative explanation is that the chemistry of pore waters in the Wilcox Formation is significantly different from that of pore waters in sediments studied by Hower. End_of_Article - Last_Page 469------------

Early Diagenesis: Expansible Soil Clay-sea Water Reactions

ABSTRACT Two clay-sized vermiculites, one montmorillonite, and one Al-hydroxy-vermiculite, all of which were extracted from four different soil samples, were Ca++-saturated and suspended in sea water for periods up to 10 months. Chemical analyses of the sea water immersed clays show that the adsorbed cation composition of the clays changes very little after only a few days of contact with sea water. A large proportion (up to 40%) of the interlayer cations become non-exchangeable after prolonged contact with sea water. After contact with sea water for 11 weeks, the ratio of adsorbed Mg++ to adsorbed K+ varies from about 1:1 to 5:1 for these four samples, and the amount of adsorbed Na+ is less than 20% of the total adsorbed cation content for all these samples. X-ray diffraction patterns of sea water-immersed samples show that discrete illite does not form as had been expected. Instead, a phase develops. This phase is a random interstratification of illite-like and expansible layers. The 001 diffraction maximum produced by this phase is extremely broad. Because this material gives diffraction over such a large 2 range, this phase when mixed with other clay minerals may have been undetected in many previous studies of Recent marine clays. The suggestion is offered that burial diagenesis would tend to transform this mixed-layer clay into discrete illite and/or chlorite. Computations show that expansible soil clays delivered to the oceans as part of the su pended load of rivers can adsorb up to 50% or more of all the K+ and Mg++ delivered as dissolved load to the oceans.

Burial Diagenesis in Gulf Coast Pelitic Sediments

AbstractSubsurface samples of shales ranging in age from Pleistocene to Eocene obtained from five Gulf Coast oil wells were examined mineralogically and chemically to determine the nature and extent of burial diagenesis in pelitic sediments. Illite/montmorillonite dominates the mineralogy and undergoes a monotonic decrease in expandability from about 80 to a limit of 20 per cent montmorillonite layers with increasing depth. The interstratification changes from random to ordered at about 35 per cent expanded layers. Discrete illite and kaolinite phases are ubiquitous and judged detrital. The discrete illite (mica) content of the whole rock decreases with depth, while the kaolinite shows no systematic variation. Chlorite occurs in appreciable amounts in only one well and there only in samples from the shallow water facies. This chlorite is also considered detrital.Whole rock chemical analyses show no systematic variation with depth except for a decrease in calcium and magnesium caused by solution of carbonate. In contrast, potassium increases progressively with depth in the clay-size fraction, indicating a redistribution of potassium within the rock. Detrital illite (mica) seems to break down with increasing depth, thereby supplying potassium for interlayer fixation in illite/montmorillonite as the proportion of illite layers increases. The diagenetic reaction is independent of the geologic age of the sediments and of stratigraphic boundaries. Temperature seems to be more important than pressure in governing the reaction.