Illite In Sandstones Research Articles

Uranium mineralization in the Thrace Basin, NW Türkiye: Evidence from radiation-induced defects in detrital quartz and synchrotron XRF/XANES analysis

The Paleogene-Neogene Thrace Basin in northwestern Türkiye has long been known to host economic gas and oil resources and has recently been reported to potentially host sandstone-type uranium deposits in the Oligocene Süloğlu Formation. The latter discovery raises questions about the source and deposition mechanism of uranium mineralization in the basin. This contribution reports on the results of a detailed electron paramagnetic resonance (EPR) spectroscopic study of detrital quartz from four sandstone and one mudstone samples in the Süloğlu Formation and documents the distribution and speciation of uranium using combined microbeam synchrotron X-ray fluorescence maps (μsXRF) and microbeam X-ray near edge structure spectroscopy (μsXANES). The EPR spectra of quartz separates are characterized by the presence of diagnostic radiation-induced defects (i.e., silicon-vacancy hole centers H′3, H′4, and H′7 with gmax = 2.049, 2.034, and 2.018, respectively, and the oxygen-vacancy electron center E′1), formed by the bombardment of alpha particles emitted from uranium, thorium, and their unstable progenies. Moreover, notable decreases in the intensity of silicon-vacancy hole centers in the EPR spectra of quartz separates after partial dissolution with hydrofluoric acid, provide compelling evidence for the circulation of uranium-bearing fluids in the Thrace Basin. The μsXRF and μsXANES data reveal the occurrences of mixed U6+ and U4+ species in hematite partially replacing pyrite aggregates but dominantly U4+ in disseminated pyrite and illite in sandstones of the Süloğlu Formation. These results provide new insights into uranium transport, reduction, and deposition mechanisms, with important implications for better understanding sandstone-type uranium deposits in general and further exploration in the Thrace Basin.

Thermal profiles inferred from fluid inclusion and illite geothermometry from sandstones of the Athabasca basin: Implications for fluid flow and unconformity-related uranium mineralization

The Proterozoic Athabasca basin and underlying basement host numerous unconformity-related uranium deposits that were formed from extensive fluid circulation near the basement-cover interface. Although it is generally agreed that the mineralizing fluids were basinal brines, it is still unclear what driving forces were responsible for the circulation of the basinal fluids. Because different fluid flow driving forces are associated with different thermal profiles, knowing the basin-scale distribution of paleo-fluid temperatures can help constrain the fluid flow mechanism. This study uses fluid inclusions entrapped in quartz overgrowths and authigenic illite in sandstones from three drill cores (WC-79-1, BL-08-01, and DV10-001) in the central part of the Athabasca basin as thermal indicators of paleo-fluids in the basin. A total of 342 fluid inclusions in quartz overgrowths were studied for microthermometry. The homogenization temperatures (Th) range from 50° to 235°C, recording the minimum temperatures in various diagenetic stages. Temperatures estimated from illite geothermometry (121 points) range from 212° to 298°C, which are systematically higher than (partly overlapping) the Th values, suggesting that illite was precipitated in hotter fluids following the formation of quartz overgrowths. Neither the fluid inclusion Th values nor the illite temperatures show systematic increase with depth in individual drill cores. This, together with the high illite temperatures that cannot be explained by burial at a normal geothermal gradient (35°C/km), is interpreted to indicate that basin-scale fluid convection took place during the diagenetic history of the basin. Prolonged fluid convection is inferred to be responsible for delivering uranium (extracted from the basin or the upper part of the basement) to the unconformity, where uranium mineralization took place due to redox reactions associated with fluid-rock interaction or structurally controlled fluid mixing.

Illitization of Kaolinite: The Effect of Pressure on the Reaction Rate

AbstractStudies of the paragenesis of authigenic illite in arkosic sandstones of various regions and ages have revealed that the illitization of kaolinite is an important reaction accounting for the formation of authigenic illite in sandstones during burial diagenesis. The illitization of kaolinite takes place at an intermediate burial depth of 3–4 km, where pressure can reach values of 100 MPa (≈ 1000 bars). The purpose of the present study was to analyze the effect of pressure on the rate of kaolinite illitization in alkaline conditions. Hydrothermal reactions were conducted on KGa-1b kaolinite in KOH solution at 300°C and under pressures of 500, 1000, and 3000 bars for 1 to 24 h. The visual examination of the X-ray diffraction (XRD) patterns indicated a notable influence of pressure on the reaction rate. Molar percentages of muscovite/illite formed at each time interval were calculated from the analysis of two diagnostic XRD peaks, representing the 060 reflections of kaolinite and muscovite/illite. The data were modeled to obtain the initial rate of conversion at each pressure. The results indicated that the initial rate of kaolinite to muscovite/illite conversion is one order of magnitude greater at 3000 bars than at 500 or 1000 bars. Comparison of these data with those in the literature show a faster conversion rate (several orders of magnitude) in an initially high-alkaline solution than in a near-neutral solution.

A model for fibrous illite nucleation and growth in sandstones

We have developed a model for the formation of fibrous illite in sandstones where kaolinite is a primary reactant and potassium is derived from in-situ K-feldspar grain dissolution or imported into the model reference frame. Illite fiber nucleation and growth are modeled using Arrhenius expressions that consider saturation state in addition to temperature and time. Nucleation occurs on pore walls, and muscovite and detrital illite may be defined as energetically favorable substrates. The model is integrated with other Touchstone™ models to account for the influence of other diagenetic processes on surface area and reactant volumes and to provide input for permeability simulations. We evaluated the illite model performance on two data sets: (1) Jurassic quartzose samples from offshore mid-Norway with maximum temperatures ranging from 108 to 173C (226 to 343F) and (2) Miocene lithic samples from offshore Southeast Asia that have maximum temperatures ranging from 157 to 182C (315 to 360F). The model matches measured abundances of illite, kaolinite, and K-feldspar in both data sets using identical kinetic parameters. Predicted K-Ar ages are consistent with available data given uncertainties associated with detrital contaminants. Although no illite particle-size data are available from the analyzed samples, modeled crystallite thicknesses from the Norway data set are comparable to published measurements of 0.004 to 0.012 m from North Sea samples with similar temperature histories.

Experimental Determination of the Rates of Precipitation of Authigenic Illite and Kaolinite in the Presence of Aqueous Oxalate and Comparison to the K/Ar Ages of Authigenic Illite in Reservoir Sandstones

AbstractThe importance of precipitation rate as an effective control on illite and kaolanite formation during diagenesis has been examined by measuring precipitation rates, from Al fluid concentration, in a Dickson fluid-sampling vessel at 160°-250° and 500 bars (50 MPa). These experiments are considered to be analogues of the precipitation of clays in sandstones from porewaters containing dissolved carboxylic acids, which have a transient stability and may influence aluminosilicate solubility. Precipitated illite had a lath-shaped morphology and its composition was consistent with authigenic illite in sandstones. Kaolinite formed tabular rather than vermicular shaped crystals. Kaolinite precipitation rate was two orders of magnitude faster than illite precipitation and was rate-limited by the decomposition of oxalate; kaolinite formation should be equilibrium-controlled at virtually all stages of burial. Extrapolation of illite precipitation rate to burial temperatures indicates that the first appearance of illite in a burial sequence may be kinetically controlled. A model of illite precipitation based on these experimental results has been used to predict the time required to precipitate illite during burial of a sandstone, taking into account temperature changes during burial. For northern North Sea examples, a predicted illitization threshold of -60°C occurring at 60–80 Ma corresponds to the observed initiation of authigenic illite precipitation. Times of around 2–5 Ma would be required to reach a 98% approach to equilibrium at this threshold. The main phase of illite precipitation in the northern North Sea basin is a later, hydrologically controlled event (30-50 Ma). Equilibrium would be approached in around 0.1 Ma during this phase, which is consistent with the narrow illite K/Ar age range (1-5 Ma) recorded for some sequences.

Experimental simulation of clay precipitation within reservoir sandstones; 2, Mechanism of illite formation and controls on morphology

Illite and illite-smectite, with various morphologies, have been formed on quartz grains by precipitation from solution following dissolution of amorphous Al/Si gels at 150-350 degrees C and 0.75-1.0 kbar (75-100 MPa). Initial solutions include deionized water, chloride and oxalate. The experimental results have application to the formation of authigenic illite in sandstones, from pore waters containing carboxylic acid anions. A series of experiments monitored the effects of reaction temperature and time on the composition and morphology of illitic minerals precipitated following the dissolution of an amorphous gel of illite composition in deionized water. A progressive increase of 70 to 100% in the illite content of illite-smectite occurs between 150 and 350 degrees C, but there is no change in illite-smectite composition for reaction times of 24 hours to 37 days. These results indicate that illite and illite-rich illite-smectite form by direct precipitation from solution and do not require a smectitic precursor. The compositional variation in illite-smectite and illite has little or no effect on clay morphology; all experiments produced a platy-boxwork morphology. Experiments performed at 250 degrees C and initially containing oxalate, in both acidic and alkaline K-bearing solutions, strongly promote illite precipitation following the dissolution of an amorphous Al/Si oxide starting material. The very rapid precipitation that occurs under these conditions is associated with the formation of fibrous/lath-type illites rather than platy types, which are characteristic of slower precipitation in the absence of oxalate. Precipitation of fibrous illite is related to the breakdown of oxalate, which lowers illite solubility by either pH variation or by breakdown of an Al-oxalate complex. Oxalate is not a direct requirement for fibrous illite formation, although it controls fluid composition which favors fibrous illite formation. The association of illite precipitation with carboxylate anion breakdown suggests that the formation of fibrous illite cements in petroleum reservoirs is related to fluids released from source rocks, which contain both aqueous organic acids and inorganic solutes.

Formation of Diagenetic Illite in Sandstones of the Garn Formation, Haltenbanken Area, Mid-Norwegian Continental Shelf

Subarkosic arenites of the Middle Jurassic Garn Formation are important hydrocarbon reservoirs in the Haltenbanken area. XRD and petrographic analyses of core samples from 11 wells show that a transition from incipient to extensive illitization in these sandstones is associated with a present burial depth of 3·7 km below the sea floor, corresponding to a formation temperature of 140°C. Illite has formed by reaction between K-feldspar and earlier diagenetic kaolinite and probably also by alteration of mica. Although several of the cores are filled with hydrocarbons that probably were in place before base-Eocene (57 Ma), there are no indications that illitization was inhibited by the presence of hydrocarbons in pore spaces. It is therefore suggested that 20–30% residual water saturation is sufficient for extensive illitization to occur by short-range diffusive transport in a compositionally closed system. There is an unresolved problem regarding both the timing and cause of illitization. On the one hand, the extent of illitization in the 11 wells correlates with variations in both present formation temperature and thermal maturity, implying that the present burial depth of 3·7 km below the seafloor coincides with a critical thermal threshold for illitization. According to this interpretation, the times of illitization in the different wells should correlate with variations in burial history and should in several cases be as young as 3 Ma, when deposition of one kilometer of glacially-derived sediment sharply increased temperatures throughout the Haltenbanken area. On the other hand, conventional K-Ar analyses of illite separates give dates ranging from 31–55 Ma. In general, these dates appear to be too old to fit the interpretation that the degree of illitization is a simple function of present temperature or thermal maturity. This inconsistency may reflect errors in the K-Ar dates due to a contamination problem.