Clay Mineral Diagenesis Research Articles

Clay mineral diagenesis in two East African lake sediments

AbstractThe vertical clay mineral distribution pattern in two sediment cores taken from two East African lakes has been studied in detail. In the core from Lake Albert (Mobutu Sese Seko) a clay assemblage consisting principally of smectite with some illite and kaolinite in the uppermost segment of the core changed with depth into first a mixed-layer illite-smectite dominated clay and then at 6·60 m into an iltite dominated clay. Lower in the core there was again a transition towards interlayer mixtures, and in the bottom segment towards smectite. These changes are interpreted as a diagenetic transformation of smectite into illite, effected by saline palaeo-lake water with an unusually high K/Na ratio. Three smectite→illite-smectite→illite transition cycles were recognized in a 56 m deep sediment core from Lake Manyara. The sections in which illite dominated contained silt-sized analcime, while those with interlayer mixtures contained alkaline zeolites. The diagenetic illitization of smectite appeared to parallel the process of analcime formation, the K necessary for illitization being released during K,Na-zeolite→analcime transformation. These occurrences suggest that high temperature and pressure may not constitute absolute prerequisites for the diagenetic illitization of smectite.

Thermal Maturation of Lower Paleozoic Shales in Northern Appalachian Nappe Structures Around Quebec City, Canada: ABSTRACT

End_Page 504------------------------------Progressive changes in organic matter maturation and clay mineral diagenesis have been studied in transverse and longitudinal profiles in the external domain of the Quebec Appalachians. Reflectance measurements (90 samples) on dispersed organic matter, probably asphaltic pyrobitumen, and illite crystallinity values reveal an increasing degree of diagenesis from northwest to southeast. In the northwest, reflectance in oil ranges from 1.70 to 2.30 in the Cambrian Chaudiere nappe, from 1.53 to 1.90 in Cambrian to Ordovician Bacchus nappe, from 1.08 to 1.27 in Lower Ordovician Pointe de Levy nappe, and from 0.99 to 1.21 in Middle Ordovician Quebec Promontory nappe. Illite crystallinity ranges from 4 to 8 mm (0.53 to 1.05° 2^Thgr). In individual nappes, reflectance increases with stratigraphic depth but may not have been affected significantly by tectonic resulting from the superposition of nappes. Predeformational diagenesis is therefore suggested as the principal agent of thermal maturation. In the southeast, the Middle Ordovician St. Henedine nappe, representing the anchimetamorphic zone, has reflectance values from 2.82 to 3.79% Ro and illite crystallinity (< 3 mm = 0.40°2^Thgr) lower than in the northwest. In this region, high reflectance in conjunction with low illite crystallinity may be related to elevated temperatures resulting from syndeformational or postdeformational regional metamorphism. For hydrocarbon exploration in the Quebec Appalachians it appears significant that the trend of decreasing thermal maturation with increasing tectonic burial depth may continue into the little deformed platform sequence of the St. Lawrence Lowlands underlying the nappes of the external domain. End_of_Article - Last_Page 505------------

Clay mineral diagenesis and oil migration in the Middle Jurassic Brent Sand Formation

Authigenic clay minerals, in an almost complete sequence of oil-saturated Brent Sand from an oilfield in the Viking Graben in the northern North Sea, show a transition from kaolinite in the upper part into illite in the basal part of the reservoir; replacement textures indicate that the illite has replaced kaolinite. After early kaolinite formation, it is suggested that illite diagenesis took place synchronously with oil migration, and that the progressive extension of oil saturation down through the reservoir terminated diagenetic processes. Early diagenetic stages are thus 'frozen' and preserved in the upper part of the reservoir.

Diagenesis of Clay Minerals from Lower Cretaceous Shales of North Eastern British Columbia

AbstractClay minerals from shale outcrops of the Lower Cretaceous Buckinghorse Formation (4250 ft thick) were investigated in order to assess their degree of diagenesis and their oil-generating potential. Crystallinity index, sharpness ratio, per cent of illite which is the 2M polymorph and presence of discrete minerals have been studied in the whole clay fraction, while the very fine clay fraction has been subjected to X-ray diffraction, differential thermal, thermogravimetric, differential thermogravimetric, i.r. spectroscopy, surface area and chemical analyses. With information derived from these studies and from published data, a classification scheme was devised which relates variation of clay mineralogy to diagenetic stages and burial depth.Data on the &lt; 2 μm size fraction show that the crystallinity index decreases while the sharpness ratio and per cent of illite which is the 2M polymorph increase with burial depth. Results on the &lt;0·08 μm fraction reveal that a three-component interstratified clay mineral exists. In addition, Fourier transform calculations and chemical and physicochemical analyses indicate that both the ratio of the amounts of non-hydrated clays (illite) to hydrated clays and the K2O content of clays increase with burial depth; cation exchange capacity and surface area decrease with burial depth.Based upon a classification scheme, which was devised by combining criteria and data derived from the studies of Weaver (1961a), Kubier (1966), Burst (1969) and Dunoyer de Seconzac (1970), the upper and middle parts of the formation (upper 3250 ft) fall within the middle stage of diagenesis whereas the lower part (1000 ft) is allocated to the beginning of late diagenesis. In terms of Burst’s (1969) work, the upper 3250 ft are transitional between the stability and dehydration zones indicating that, prior to uplift, hydrocarbons may have been in the process of migration. The lower 10000 ft of the formation are in the restricted dehydration zone, indicating that hydrocarbon migration should have been completed.

Mineralogy of Deep-Sea Sediments from Cretaceous te Holocene: ABSTRACT

The mineralogy of the Cretaceous to Holocene sediments of the Pacific and Atlantic basins has close affinities to present sedimentation patterns. Carbonate sediments dominate equatorial and shallow oceanic areas. Turbidites are common close to continental margins. Siliceous radiolarian and diatomaceous sediments are abundant in zones close to the carbonate compensation depth. One of the more striking features of the deep sea is the common occurrence of an amorphous metal-oxide basal facies. Many areas of both the Atlantic and Pacific show high iron- and manganese-content sediment facies at, or close to, the contact with basement basalt. This basal facies grades upward into the biogenous and detrital lithogenous overlying sediments. Some areas of the basal facies have high copper and zinc contents, and in other places manganese is prominent. In a few places, this facies is essentially hematitic. The carbonates present in the deep sea include, in addition to calcite and aragonite, dolomite, siderite, rhodochrosite, and ankeritic dolomite. An unusual palygorskite and sepiolite and bentonite associated with dolomite is well developed in the vicinity of the Cap Verde Islands in the eastern Atlantic. Basaltic volcanic glass usually alters to montmorillonite plus a zeolite which is usually phillipsite or clinoptilolite. Erionite has been discovered in the western Pacific for the first time in the deep sea. Biogenous opaline silica dissolves and reprecipitates to form crystobalite cherts. These in turn are recrystallized to form quartz cherts in pre-Cenozoic sediments. The range of minerals facies available suggests that clay mineral diagenesis is slight but the in situ formation of zeolites and clays from recrystallization of volcanic ash is important. End_of_Article - Last_Page 647------------

Early Diagenesis of Clay Minerals, Rio Ameca Basin, Mexico

ABSTRACT Chemical analyses of Recent sediments from Banderas Bay, Mexico, show that the non-exchangeable Mg content of the clay fraction is higher, and the Fe content is lower, in sediments from strongly reducing environments than in similar sediments from less reducing environments. Clay mineralogy (52% montmorillonite, 28% kaolinite, 20% illite) does not vary significantly among the different environments. The total cation-exchange capacity of the clays does not differ in the different environments although exchangeable Na is markedly lower in clays from the sulfide-rich sediments. A possible explanation for these effects is that in a strongly reducing environment, Fe leaves the structure to form a sulfide, and Mg enters the same sites from the surrounding water, so that gros clay mineralogy is unchanged. Mg is depleted in the interstitial water of the anoxic sediments and Mg must be diffusing into the sediment from the overlying sea-water to account for the increased Mg content of the clay. Analysis of clay minerals separated by continuous particle electrophoresis indicates that the montmorillonite contains approximately 1.2% K2O, and that the non-exchangeable Mg content of the in sediment from 1 m below the surface is 10% higher than in surface sediment. Alkali-soluble Al2O3 and SiO2 in the clay fraction of the sediments (amorphous material) decreases with depth in the cores, but there is no evidence that this represents a reaction which removes cations from sea water. Apart from initial ion exchange and glauconite formation, there is no evidence for uptake of K or Na by land-derived clay minerals.

Clay Mineral Diagenesis in Atoka (Pennsylvanian) Sandstones, Crawford County, Arkansas

ABSTRACT This study concerns the origin and composition of clay minerals in the lower part of the Atoka Formation (Pennsylvanian) of Crawford County, Arkansas. Samples studied are from a 237' core which penetrated a complex interval of interbedded sandstones and shales. Chemical and mineralogical variations in these rocks are primarily related to the post-depositional formation of iron-rich chlorite and destruction of kaolinite. Such effects increase with sandiness of the rocks and are thought to be related to the preferential circulation of formation waters through these relatively porous and permeable sandy sediments. Associated diagenetic phenomena include the formation of authigenic siderite rhombohedra and silica cementation; the latter being related to proximity to shale. The overall pattern of diagenesis is compatible with reducing alkaline conditions. Constancy of inter-element ratios suggests that chemical and mineralogical changes have occurred without gain or loss of a number of elements (Al, B, Ti, Ga, K, Rb) on a whole-rock basis. The most important chemical variation is a reciprocal relationship between iron and aluminum. Detrital interpretations of clay minerals should he made with caution; possible diagenetic effects must also he considered. Clay minerals in sandstone are particularly likely to be of diagenic origin.

Diagenesis of Gulf Coast Clayey Sediments and Its Possible Relation to Petroleum Migration

The writer has established reference intervals in the subsurface on the basis of apparent systematic interlayer water loss of swelling clay minerals. The intervals are used in much the same manner as the familiar indicators for metamorphism, but are present at sufficiently shallow depths to be evident within oil-bearing strata of the Gulf Coast. The resulting conclusion is that clay-mineral diagenesis indicators may prove to be important petroleum-evaluation markers as well as fundamental properties of sedimentary basins. Sedimentary basins are viewed as combinations of gases, liquids, and semisolids distributed through a solid matrix. During geologic development the interstitial components segregate by migration and produce various commercially exploitable concentrations. Water, the principal fluid component of the sedimentary section, is thought to migrate in three separate stages. Initially, pore water and excessive (more than two) clay-water interlayers are removed by the action of overburden pressure. This initial water flow (which is essentially completed after the first few thousand feet of burial) reduces the water content of the sediment to about 30 percent, most of which is in the semi-solid interlayer form. A second stage of dehydration is thought to occur when the heat absorbed by the burie sediment becomes sufficiently great to mobilize the next-to-last water interlayer in an M(H2O)x + ^DgrHr = 1 + XH2O fashion. The final stage of sediment dehydration which extracts the last remaining water monolayer from clay lattices is apparently very slow, even by geologic standards, requiring tens or possibly hundreds of millions of years depending upon the geothermal and burial history of the sediment. Petroleum hydrocarbons which are distributed throughout the matrices of potential source beds in normal frequencies of 300-3,000 ppm are thought to be too sparse to initiate continuous fluid flow. In normal marine sediments, however, the water associated with clay minerals is present to a considerable depth in the order of 200,000 ppm, and therefore it is reasoned that this phase forms the connection between petroleum source and reservoir beds. The first and last dehydration stages are probably unimportant in Gulf Coast oil migration, inasmuch as they occur, respectively, at levels too shallow and too deep to intersect the interval of maximum liquid petroleum availability. The amount of water in movement during the second stage, at a level which does intersect this interval, is 10-15 percent of the compacted bulk volume and represents a significant fluid displacement capable of redistributing any mobile subsurface component. A measure of the degree to which the second-stage interlayer water has been discharged into the system can be noted on X-ray diffractograms. The movement appears to occur in a relatively restricted, depth-dependent temperature zone in which the average dehydration temperature of the points measured is 22 °F. With the use of an empirically derived P/T curve and a geothermal-gradient map, a set of regional subsurface dehydration contours can be constructed. A plot of 5,368 liquid petroleum production depths referenced to this dehydration surface shows an almost perfect Gaussian distribution. It seems significant that, although the dehydration depths range from 4,000 to 16,000 ft, hydrocarbon production depths are distributed in a statistically consistent relation to the calculated clay-dehydration contour surface.

Clay-Mineral Diagenesis in Atoka (Pennsylvanian) Sandstone, Crawford County, Arkansas: ABSTRACT

Chemical and mineralogic evidence indicates significant clay-mineral diagenesis in a shallow (237 ft) core of Atoka (Pennsylvania) sandstone from Crawford County, Arkansas. This core spanned a complex sequence of interbedded sandstone and shale that is at least partly marine. Clay shales are dominated by illite with significant amounts of kaolinite and minor amounts of chlorite. Increasingly sandy shales contain progressively more chlorite and less kaolinite; illite remains the dominant clay-mineral component. The clay fractions (< 2µ) of sandstones are dominated by chlorite with lesser amounts of illite and no kaolinite. The strong contrast between the clay-mineral contents of these closely interbedded sandstones and shales suggests a diagenetic change that occurred primarily in the sandstones because of their greater permeability. The chlorite of the sandstones most likely has been formed authigenically with concurrent destruction of kaolinite. Elimination of kaolinite from sandstones by differential sedimentation seems unlikely because (1) it is doubtful that natural mechanical processes could produce the perfect separation of kaolinite found in these rocks, and (2) numerous studies of Recent and ancient sediments indicate that kaolinite commonly is concentrated in sandstones rather than eliminated from them. Clay-mineralogical changes in the sandstones may be part of an overall pattern of diagenesis in an alkaline, reducing environment that also included the formation of siderite rhombohedra and significant solution of silica (as indicated by straight or embayed contacts of quartz grains). Much of the discussion on the origin of clayey sediments in modern deposits has been devoted to the relative importance of provenance versus environmental End_Page 552------------------------------ alteration or segregation. With respect to ancient deposits, however, approaches confined to these factors are inadequate where diagenesis of clay minerals has occurred, as it apparently has in the Atoka sandstone and sandy shale. End_of_Article - Last_Page 553------------

Clay Minerals and Oceanic Evolution

AbstractA review of mineralogical and geochemical studies on Recent sediments indicates that the clay fraction of marine sediments is not in isotopic or chemical equilibrium with the oceanic reservoir and will not reflect the chemical environment of deposition. Consideration of the sedimentary geochemistry of the alkali metals suggests that fractionation of these elements, which may be one of the major features of the chemical evolution of ocean water, occurs in the terrestrial weathering environment during the formation of clay minerals and in the subsurface environment during clay mineral diagenesis. It is proposed that during the initial stages of oceanic development the Na/K ratio of ocean water was adjusted to a value of forty to fifty by the extensive diagenesis of the existing natural water. With an increase in oceanic volume and a major change in the hydrologic cycling of natural waters the chemical evolution of alkali metals in ocean water has now become primarily controlled by mixing with continental drainage water.

Diagenesis in Clay Minerals—A Review

AbstractDiagenesis refers to the process, and the changes (usually excluding cation exchange) that take place in sediments after deposition, but definitions of the process vary widely over which changes, and what part (or all) of the time between the transportation of the sediment and the time that the sediment(ary rock) is studied, should be included in diagenesis. Furthermore, Weaver, Griffin, and others, have favored alternative explanations to account for many clay mineral distributions that previously have been interpreted as representing changes in clay minerals in both recent and ancient sediments, and which were referred to diagenesis.Although agreeing that such alternative explanations account best for many distributions of clay minerals, additional evidence that presumably is convincing of diagenesis is presented herein to account for the formation of certain other occurrences of glauconitic mica, glauconite, illite, corrensite, kaolinite, and vanadiferous micaceous clay. A two-stage mechanism is proposed for a possible process by which illite and probably corrensite are formed: (1) cation sorption driven by energy (activities) of the ions in solution, and binding energy of the clay minerals, and (2) subsequent rearrangement of ions in the clay mineral to produce illite. It is finally proposed that diagenesis of clay minerals is characterized by the addition to and incorporation into them of Me ions, such as Mg, K, Ca, Na, Al, Fe, V, SiO2, and others, driven mainly by chemical energy and aided by mechanical and thermal energy, but as the latter two become more effective the process is designated metamorphism (anamorphism).Long-time, diagenetic reactions after burial of sediments are probably less important, volume-wise, than provenance and terrestrial weathering in determining the mineralogy of argillaceous sedimentary rocks in the geologic column.

Clay Mineral Diagenesis in the Rappahannock Estuary: An Explanation

Mineralogical changes observed in bottom sediments along the Rappahannock salinity gradient include: (1) decrease in kaolinite, (2) decrease in dioctahedral vermiculite, (3) increase in illite, (4) appearance of thermally stable chlorite, and (5) appearance of clay-sized feldspar. All of these changes are real. They correlate with increase in bottom water salinity from 0 to 20%, increase in bottom sediment pH from 5.5 to 7.8, change in grain size distribution, and change in adsorbed ion populations. In the critical area the bottom sediments lose clay by selective sorting and become more silty. The adsorbed ions are Ca and Al in fresh water, Ca and Na with minor K at low salinities, and Na and Ca with major fractions of exchangeable K and Mg at high salinites. Studies of the suspended sediments over a period of years reveal that illite, vermiculite, kaolinite, and clay-sized feldspar are the normal constituents in the load of the fresh water stream. No mineralogical segregation, by differential flocculation for example, occurs during transport of this suspension along the salinity gradient. The increase in illite and feldspar content of the bottom sediments is due to normal processes of sediment deposition. The high kaolinite and dioctahedral vermiculite contents of fresh water bottom samples is due to two possible causes. (1) Samples are obtained from drowned, Pleistocene-weathered, terrace materials that possess this basic mineralogy. (2) Bedload sediment with kaolinite-dioctahedral vermiculite mineralogy is deposited in the fresh water region. Thermally stable chlorite is generated in the saline environment, probably from vermiculite. Simple ion exchange is insufficient to generate the chlorite. It forms by diagenesis.

Clay Mineral Diagenesis as a Possible Source of Silica Cement in Sedimentary Rocks

ABSTRACT The post-depositional transformation of montmorillonite and/or mixed-layer illite-montmorillonite to illite is suggested as a source of silica cement in some sedimentary rocks. Depth of burial appears to be an important factor in effecting this change and although supporting data are lacking, the changes in mineralogy dictate that silicon should be released to the general geologic environment.