Smectite-illite Transformation Research Articles

Chemical composition of smectites formed in clastic sediments. Implications for the smectite-illite transformation

AbstractAnalytical electron microscopy of representative smectites from soils and sediments revealed that K was present in significant proportions. It was the major interlayer cation in soils from pelitic rocks, continental and marine sediments, independent of their diagenetic grade. Sodium was predominant only in soils from basic rock. Fluvial sediments contained smectites with both kinds of interlayer compositions. The octahedral composition of each sample ranged widely, covering various fields of dioctahedral smectites. The most important trend was the substitution of Al by Fe and Mg; the chemistry of each smectite particle was determined by the parent mineral from which it formed. The real interlayer composition has important implications for the diagenetic smectite–illite transformation. When considering a typical K content, the smectite–illite reaction, with chlorite and quartz as subproducts, needs only 0.21 K atoms. For more K-rich compositions, a reaction is possible without an external supply of K.

Carbonate cement stratigraphy and timing of diagenesis in a Miocene mixed carbonate-clastic sequence, offshore Sabah, Malaysia: constraints from cathodoluminescence, geochemistry, and isotope studies

A mixed carbonate-clastic sequence is one of several Miocene hydrocarbon-bearing reservoirs in the northwestern Sabah Basin. This sequence consists of bioclast-rich sandstones interpreted as prograding storm shoal deposits. The sequence contains a complex variety of carbonate nodules.The sequence has undergone nine stages of cementation that completely occluded pores. Each stage represents a distinctive cement texture, precipitating at specific temperatures and burial conditions. The diagenetic evolution began with a syndepositional iron-free marine calcite (Ca1), followed by an early methane-derived dolomite (Do1), bladed calcite (Ca2), blocky, vein-filling Fe-calcites (Ca3Ca4, clay mineral-associated dolomite (Do2), Fe-dolomite and ankerite (Do3Do4, and late iron-rich calcite (Ca5). Each stage of cement has distinctive Sr2+, Mn2+, Mg2+, Fe2+, Ca2+, and Na+ contents, reflecting changes in the palaeo-porefluid systems with time. The cements are correlatable over the whole study area.Precipitation occurred from very early to late stages of diagenesis at near surface to 2.0 km depths. With progressive burial and temperature increase, oxygen isotope values become strongly negative (stage Ca4: δ13C = −0.5%. PDB, δ18O = −5.7%. PDB). The cements were precipitated at 20–75°C. The source of carbon varies from marine dissolved carbonates to fermentation of organic matter, methane, meteoric waters, and possibly hydrothermal fluids.Strontium isotope dating of Do1 (with δ13C = −32.5%. PDB, δ18O = +2.60%. PDB) and Do2 (δ13C = 3.0%. PDB; δ18O = −2.5%. PDB) dolomites indicates that these dolomitization events took place at 10.5 Ma and 8.9 Ma, respectively, assuming precipitation within a completely open seawater system. The Do1 dolomite was precipitated from a methane-derived brine at the boundary between the Serravallian and Tortonian stages after major uplift, and induced faulting, at a sea-level lowstand. The Do2 dolomite was precipitated during intermediate to deep burial diagenesis during smectite-illite transformation before hydrocarbon emplacement.

Alterations in the Non-Clay-Mineral Fraction of Pelitic Rocks Across the Diagenetic to Low-Grade Metamorphic Transition, Ouachita Mountains, Oklahoma and Arkansas

ABSTRACT The transformation of smectite to illite has been cited by many authors as a source of silica during diagenesis of mudrocks. Illites themselves, however, undergo chemical changes as they recrystallize into micas during high-grade diagenesis/low-grade metamorphism. Average compositions of phyllosilicates from the literature suggest that an equivalent amount of silica is available from transformation of illite to muscovite as from illitization of smectites. The fate of silica released by this process has not been reported, but could be a major contributor to the silt-size quartz population. The quartz and feldspar fraction of pelites from the Stanley Shale (Mississippian) in the Ouachita Mountains of Oklahoma and Arkansas was separated using sodium bisulfate fusions. The mineralogy and the grainsize distribution of this fraction were determined using standard petrographic and X-ray diffraction (XRD) techniques. Bulk rock samples were analyzed using X-ray fluorescence (XRF) and instrumental neutron activation analysis (INAA) methods. The data obtained were related to illite crystallinity and vitrinite reflectance as reported by Guthrie et al. (1986) and Houseknecht and Matthews (1985). Both the percentage of quartz and the mean grain size of the quartz and feldspar fraction increase with greater illite crystallinity values. The growth in quartz is especially apparent in the finest size fractions. A corresponding decrease in the silica content of the clay-mineral fraction is also observed. Development of quartz polycrystallinity occurs across the same interval. Whole-rock chemical abundances show no statistical correlation with thermal maturity. Relative to titanium, both the major-element and the trace-element concentrations show little variation. Rare-earth element ratios do not correlate with thermal maturity and remain essentially constant. Our results are consistent with reported differences between quartz in schists and their shale precursors, and suggest that release of silica during diagenesis of phyllosilicates continues after the smectite-illite transformation. This silica precipitates as quartz within the pelite, consistent with the suggestion by Blatt (1987) that metapelites are the source of abundant silt-size quartz. The lack of whole-rock chemical variation with thermal maturity implies closed-system behavior across much of the pelite-to-metapelite transition.

Water budgets in accretionary wedges: a comparison

Direct or indirect measurements of fluid flow out of the toe of accretionary wedges have now been made in the Barbados, Central Oregon, Northern Cascadia and Nankai subduction zones. The steady-state local compaction model predicts velocities of fractions of a millimetre per year and total outflows from the toe of a few cubic metres per year per metre of length along strike of the subduction zone (m2 a-1). Sea bottom measurements reveal channellized flows at velocities of hundreds of metres per year and total outflows from the toes of a few hundreds of square metres per year. Thermal arguments show in the Nankai area that all of this large surface flow cannot come from as deep as the decollement and that consequently a significant dilution by shallow sea water convection must be present. We propose that this convection is driven by the reduced density of less saline fluid of deep origin. Thus the outflow of water of deep origin may be only a few tens of square metres per year. We note, however, that there are indications in the Barbados area of massive flow of low salinity water at depth along the decollement. These flows imply the existence of large scale non-steady-state lateral transport and require the existence of sources of low salinity water. These might include the smectite-illite transformation and the fluid contained in the oceanic crust. However, these sources are limited and the possibility exist that other more important sources may be required such as long distance transport of fresh water from adjacent sedimentary basins and (or) recharge mechanisms such as the seismic pumping one.

First steps of smectite-illite transformation with humectation and desiccation cycles

Recent studies (Mamy and Gaultier, 1975; Eberl, 1984) have shown that the collapse of smectite layers associated with the development of a three-dimensional arrangement from the initially turbostratic smectite structure can be obtained by wetting-drying cycles. The purpose of the present report is to better the understanding and characterization of these changes using high-resolution transmission electron microscopy (HRTEM) and small-angle X-R diffusion (SAXRD). Low-charge montmorillonite (Wyoming) and high-charge 2 : 1 soil-clay samples, saturated with potassium, were investigated during 40 wetting-drying cycles (desiccation at 60°C and rewetting by water saturation). Results obtained show that structural reorganization does occur with a decrease of the layer thickness, which tends toward 10 A and to the evolution of a three-dimensional order. This evolution is more easily obtained with high-charge soil smectite than with Wyoming montmorillonite. A certain three-dimensional order can exist for small particles, but always there is a total layer disorder along the c-axis. The most original result concerns the “textural reorganization” of the clays. Microaggregates made up of packets of particles with several layers, can be formed by K fixation and wetting-drying cycles.

Smectite Dehydration--Its Relation to Structural Development and Hydrocarbon Accumulation: ABSTRACT

A comparison of clay diagenesis data obtained from a study of Tertiary shales from wells drilled in the Brazos-Colorado River system of Texas, the Mississippi River system of Louisiana, and the Niger River system of Nigeria illustrates significant differences in temperature intervals over which smectite (expandable clay) diagenesis occurs. The age of the shales studied ranges from 1 to 50 m.y., and the threshold temperature required to initiate diagenesis ranges from about 160°F (71°C) in Mississippi River sediments to more than 300°F (150°C) in the Niger delta. Water expelled from smectite into the pore system of the host shale during the process of diagenesis may migrate out of the shale early, or it may be totally or partially trapped and released slowly through time. In either situation, the water can act as a vehicle for hydrocarbon migration provided hydrocarbons are present in a form and in sufficient quantities to be transported. Observations from the northern Gulf of Mexico basin indicate a close relation between buildup of high fluid pressure and the smectite-illite transformation process. Abnormal pressures exert partial control on the type and quantity of hydrocarbons accumulated because pressure potential determines the direction of fluid flow, and overpressuring partly controls the geometry of growth faults and other related faults and folds in the basin. The depths to which growth faults can penetrate and the angle of dip that these faults assume at depth are largely dependent on fluid pressure in the sedimentary section at the time of faulting. Dips of some faults in Texas have been observed to change abruptly within the interval of smectite diagenesis, and some faults formed in the overpressured Miocene and younger sections become bedding-plane types at depths above the temperature level required for thermal generation of petroleum. Although these faults are important for fluid redistribution in the shallow sandstone-shale section, they play a minor role in moving hydrocarbons out of shales below the faults in much of the Texas offshore area. Fluid movement upward along fault systems in the lower Tertiary section, which overlies fault trends in the sub-Tertiary section, is proposed as a mechanism for flushing hydrocarbons from the deeper portion of the northern Gulf of Mexico basin. These fault systems would have maximum development immediately above the basement faults, with displacement decreasing progressively upward. Seismic data indicate that, in the upper (younger) Tertiary section, these deep fault systems are represented by near-vertical (high-angle) fracture systems that cut across the low-angle growth faults. Fluid movement within these deep fault and fracture systems would be enhanced by smectite diagenesis because water derived from smectite that was trapped during basin subsidence would cause the flushing pr cess to continue for longer periods of time and to extend to greater depths than could be attained if only remnants of original pore water were present. Based on data obtained from both the Brazos-Colorado and Mississippi River systems, it is concluded that smectite dehydration in shale is a major factor in both hydrocarbon migration and accumulation in basins where expandable clays are present. Concepts developed here can be applied to any basin that has had or now contains expandable clay shales. The effect of smectite diagenesis and the time of fluid release out of the shales must be considered with all other stratigraphic, structural, and geochemical parameters considered in basin evaluation. End_of_Article - Last_Page 2047------------

Smectite Dehydration--Its Relation to Structural Development and Hydrocarbon Accumulation in Northern Gulf of Mexico Basin

A comparison of clay diagenesis data obtained from a study of Tertiary shales from the Brazos-Colorado River system of Texas, the Mississippi River system of Louisiana, and the Niger River system of Nigeria illustrates significant differences in temperature intervals over which smectite diagenesis occurs. The threshold temperature required to initiate diagenesis ranges from about 160°F (71°C) in Mississippi River sediments to more than 300°F (150°C) in the Niger delta. Water expelled from smectite into the pore system of the host shale during the process of diagenesis may migrate out of the shale early or may be totally or partially trapped and released slowly through time. In either situation, the water can act as a vehicle for hydrocarbon migration p ovided hydrocarbons are present in a form and in sufficient quantities to be transported. Observations from the northern Gulf of Mexico basin indicate a close relation between buildup of high fluid pressure and the smectite-illite transformation process. Abnormal pressures exert partial control on the type and quantity of hydrocarbons accumulated because pressure potential determines the direction of fluid flow, and overpressuring partly controls the geometry of growth faults and related folds in basins where shale structures are the dominant type formed. The depths to which growth faults can penetrate and the angle of dip that these faults assume at depth are largely dependent on fluid pressure in the sedimentary section at the time of faulting. Dips of some faults in Texas have been observed to change abruptly within the interval of smectite diagenesis, and some faults formed in the overpressured Miocene and younger sections become beddingplane types at depths where the temperature is near that required for thermal generation of petroleum. Although these faults may be important for fluid redistribution in shallow sandstone-shale sections, they are a minor factor in moving hydrocarbons out of shale below the faults in much of the Texas offshore area. Fluid movement upward through microfracture systems in the deeply buried overpressured section overlying and extending upward from fault trends in the sub-Tertiary section is proposed as a mechanism for flushing hydrocarbons from the deeper portion of the northern Gulf of Mexico basin. This flushing process would be enhanced by smectite diagenesis because water derived from smectite that was trapped during basin subsidence would cause the flushing process to continue for longer periods of time and to extend to greater depths than could be attained if only remnants of original pore water were present. Shale tectonism is also the primary mechanism for structural development in the Tertiary section of the Niger delta; however, seismic data indicate that the rate of dip change of seaward-dipping listric growth faults is commonly less than that observed in Texas where dips as low as 10°-15° can occur at depths as shallow as 10,000-15,000 ft (3,048-4,572 m). Syndepositional faults in Nigeria are formed in sandstone-shale sections where the clay composition of shale is primarily kaolinite and where little water of smectite diagenesis has been added to the pore system of the host sedimentary section. Subtle differences in structural styles in the Tertiary sections of Texas and Nigeria are probably the result of differences in clay composition of the shaly sections being deformed

Application of Solids MAS Nuclear Magnetic Resonance to Study of Diagenetic Processes: ABSTRACT

Magic angle spinning-nuclear magnetic resonance spectroscopy (MAS-NMR) provides the opportunity to probe composition of and ordering in minerals involved in the formation and alteration of sediments. MAS-NMR has the capability to detect a large number of elements, including aluminum, silicon, boron, oxygen, and magnesium. The chemical state, structural location, and with cross polarization, hydration character and surface proximity can also be determined using this method. Although MAS-NMR is relatively new and quantitative methodology is still being developed, a variety of geologic processes have been clarified through its application. Use of 27Al NMR allows detailed determination of the smectite-illite transformation by monitoring the movements of aluminum in o tetrahedral positions and resultant cation ordering. Because 27Al is detectable to low ppm levels, clay mineral components can be determined well below XRD detection levels. The 29Si and 27Al MAS-NMR have sufficient resolution to discriminate between minerals in a natural assemblage but not with the resolution of XRD. Quadrupolar nuclei such as 27Al have relatively poor spectral resolution as compared to nonquadrupolar nuclei such as 29Si. However, modern high field instrumentation can discriminate between most aluminum-containing minerals including aluminum oxides, hydroxides, oxyhydroxides, clays, and feldspars, as well as trace aluminum levels in quartz, cristobalite, and tridymite. The combination of 27Al and 29 /SUP>Si NMR (and availability of other nuclei) provide a powerful aid to the resolution of exploration and production problems including determination of minor to trace amorphous components, hydration state of elements in cherts and clays, and formation damage. End_of_Article - Last_Page 530------------

Relation of Smectite-Illite Transformation and Development of Abnormal Fluid Pressure and Structure in Northern Gulf of Mexico Basin: ABSTRACT

Water expelled from smectite into the pore system of the host shale during the process of diagenesis may migrate out of the shale early, or may be totally or partially trapped and released slowly through time. In areas such as the northern Gulf of Mexico basin, where much of the water is partially trapped, clay diagenesis data indicate a close relation between high fluid pressure build-up and the smectite-illite transformation process. Abnormal pressures affect, in part, the type and quantity of hydrocarbons accumulated, as pressure controls the direction of fluid flow and partially controls the geometry of structures formed in basins where shale tectonism is the primary mechanism for structural development. In basins of these types, contemporaneous faults and related anticlines are the most common types of productive structures found. The depth to which faults can penetrate and the angle of dip that faults assume at depth is dependent largely on fluid pressure in the sedimentary section at the time of faulting. Some faults, formed in the overpressured Tertiary section of Texas, have been observed to flatten and become bedding plane types at End_Page 1425------------------------------ depths near or above the temperature level required for thermal generation of hydrocarbons. This observation suggests faults of these types are minor factors in draining hydrocarbons from deep shales within basins where thick overpressured sedimentary sections are present at shallow depths and where shale tectonism is the primary mechanism for structural development. Microfracturing resulting from increased fluid pressure is indicated to be a primary mechanism for flushing fluids from deep basins where thick abnormally pressured sedimentary sections are present. This flushing process would be enhanced by clay diagenesis since water supplied from smectite would cause the processes to continue for longer periods of time and to extend to greater depths than could be attained if only remnants of the original pore water were present in the section. Large volumes of diagenetic water present within the microfracturing interval could also act as a vehicle for primary hydrocarbon migration, provided hydrocarbons are present in sufficient quantities to be transported. End_of_Article - Last_Page 1426------------

Relation of Illite/Smectite Diagenesis and Development of Structure in the Northern Gulf of Mexico Basin: ABSTRACT

Water expelled from smectite into the pore system of the host shale during the process of diagenesis may migrate out of the shale early or may be totally or partly trapped and released slowly through time. In areas such as the northern Gulf of Mexico basin, where much of the water is partly trapped, clay diagenesis data indicate a close relation between high fluid pressure buildup and the smectite-illite transformation process. Abnormal pressures affect, in part, the type and quantity of hydrocarbons accumulated since pressure controls the direction of fluid flow and partly controls the geometry of structures formed in basins where shale tectonism is the primary mechanism for structural development. In basins of these types, contemporaneous faults and related anticlines are the most common types of productive structures found. The depth to which faults can penetrate and the angle of dip that faults assume at depth is dependent largely upon fluid pressure in the sedimentary section at the time of faulting. Some faults formed in the overpressured Tertiary section of Texas have been observed to flatten and become bedding plane types at depths near of above the temperature level required for thermal generation o hydrocarbons. This observation suggests faults of these types play a minor role in draining hydrocarbons from deep shales within basins where thick overpressured sedimentary sections are present at shallow depths and where shale tectonism is the primary mechanism for structural development. Microfracturing resulting from increased fluid pressure is indicated to be a primary mechanism for flushing fluids from deep basins where thick abnormally pressured sedimentary sections are present. This flushing process would be enhanced by clay diagenesis since water supplied from smectite would cause the process to continue for longer periods of time and to extend to greater depths than could be attained if only remnants of the original pore water were present in the section. Large volumes of diagenetic water present within the microfracturing interval could also act as a vehicle for primary hydrocarbon migration provided hydrocarbons are present in a form and in sufficient quantities to be transported. End_of_Article - Last_Page 432------------

Smectite-Illite Transformation--Role in Generating and Maintaining Geopressure: ABSTRACT

Mixed layer smectite-illite clays comprise a significant fraction of fine-grained clastic sediments in many basins around the world. Abnormally high fluid pressure (geopressure) is associated with parts of these basins. The presence of smectite-illite is a necessary but not sufficient criterion for the existence of geopressure. Smectite reacts with potassium feldspar producing illite, silica, sodium-calcium feldspar, and releasing loosely bound water. Observations in the northern Gulf of Mexico support an equilibrium model for the reaction, the shift in the logarithm of the equilibrium coefficient with depth being proportional to the product of the reaction enthalpy and the geothermal gradient. Reaction enthalpies range from 2,000 to 26,000 cal/mole, highest reaction enthalpies occurring along the south Texas coast, lowest in the Mississippi delta. Abrupt diagenesis-depth profiles are associated with geopressure, gradual reaction with depth associated with near hydro-static fluid pressure gradients. Sediments compact over intervals where the effective pressure increases. Geopressure is associated with porosity increase and effective pressure decrease with depth. The top of the zone of sediment under-compaction coincides with change in sign of the effective pressure gradient from positive to negative. In this interval, fluid pressure increases with depth faster than does the overburden pressure. Very high fluid-pressure gradients are associated with the combination of low shale permeability, high shale porosity, and rapid basement subsidence. Because of the close connection between high fluid-pressure gradient and abrupt conversion of smectite to illite, we conclude that this reaction is responsible for abnormal loss of permeability, probably a result of the finely divided silica hat is produced. End_of_Article - Last_Page 708------------

Smectite-Illite Transformation--Its Role in Generating and Maintaining Geopressure: ABSTRACT

The effects of the smectite-illite transformation are incorporated into compaction and fluid mechanical models for shales by connecting the extent of this reaction to their elastic rigidity and permeability coefficents. In this way the reaction partly determines both the thermo-elastic response of a shale body and the equation of motion of its fluid phase. Computer simulation of these model equations reproduces the spectra of geopressure phenomena encountered in the northern Gulf of Mexico. End_of_Article - Last_Page 1424------------