Authigenic Illite Research Articles

Timing of Hydrocarbon Fluid Emplacement in Sandstone Reservoirs in Neogene in Huizhou Sag, Southern China Sea, by Authigenic Illite 40Ar- 39Ar Laser Stepwise Heating

Timing of oil or gas emplacements is a new subject in isotopic geochronology and petroleum geology. Hamilton et al. expounded the principle of the illite K-Ar age: Illite is often the last or one of the latest mineral cements to form prior to hydrocarbon accumulation. Since the displacement of formation water by hydrocarbons will cause silicate diagenesis to cease, K-Ar ages for illite will constrain the timing of this event, and also constrain the maximum age of formation of the trap structure. In this study, the possibility of authigenic illites 40Ar- 39Ar dating has been investigated. The illite samples were separated from the Tertiary sandstones in three rich oil reservoir belts within the Huizhou sag by cleaning, fracturing by cycled cooling–heating, soxhlet-extraction with solvents of benzene and methanol and separating with centrifugal machine. If oil is present in the separated samples, ionized organic fragments with m/e ratios of 36 to 40 covering the argon isotopes will be yielded by the ion source of a mass spectrometer, resulting in wrong argon isotopic analyses and wrong 40Ar- 39Ar ages. The preliminary experiments of illite by heating did show the presence of ionized organic fragments with m/e ratios of 36 to 44. In order to clean up the organic gases completely and obtain reliable analysis results, a special purification apparatus has been established by Qiu et al. and proved valid by the sequent illite analyses. All the illite samples by 40Ar- 39Ar IR-laser stepwise heating yield stair-up age spectra in lower laser steps and plateaux in higher laser steps. The youngest apparent ages corresponding to the beginning steps are reasonable to be interpreted for the hydrocarbon accumulation ages. The weighted mean ages of the illites from the Zhuhai and Zhujiang Formations are (12.1 ± 1.1) Ma and (9.9 ± 1.2) Ma, respectively. Therefore, the critical emplacement of petroleum accumulation in Zhujiang Formation in Huizhou sag took place in ca 10 Ma. Late fault activity strengthened the entrance of hydrocarbon fluids into the oil systems based on the data of the studies of the fault evolvement history, petroleum system combinations, and homogeneous temperatures of fluid inclusions.

Clay Mineral Assemblages and Sandstone Compositions of the Mesozoic Longobucco Group, Northeastern Calabria: Implications for Burial History and Diagenetic Evolution

Mesozoic mudrocks and sandstones from the Longobucco Group (Sila Unit, North Calabria) were mineralogically, chemically, and petrographically analyzed for their burial, diagenesis, and sourcearea weathering histories. These sediments mark an important phase of the regional geological evolution along internal domains of the circum-Mediterranean chains from the Gibraltar Arc (Spain and Morocco) to the Calabria-Peloritani orogen (Italy). The effects of diagenesis on quartzose sandstone-mudstone lithotypes in the Mesozoic Longobucco Group succession are quantified. The basal Longobucco Group consists of continental clastic redbeds deposited in a rapidly subsiding rift-valley basin, overlain by shallow-marine to deep-marine carbonate and clastic strata. These sediments were analyzed using thin-section petrography, SEM/EDS, X-ray diffraction (XRD), and fluorescence. Quartzarenites display heterogeneous distributions of authigenic quartz, kaolin, illite, feldspar, and minor carbonate cementation. Authigenic zoned syntaxial overgrowths on detrital quartz represent the principal cement, and interstitial clay is an important component of these lithotypes. The mineralogical assemblage of mudrocks is dominated by illite and illite/smectite mixed layers. This observation, coupled with the CIA index and the A-CN-K plot, suggests post-depositional K-enrichment. Paleoweathering indices (CIW and PIA ratios) suggest that the source experienced intense weathering and that mudrocks likely record recycling from metasedimentary basement rocks. Clay-mineral distributions of sandstone/mudstone lithotypes indicate that mixed-layer illite/smectite with ordered interstratification (from R = 1 to R = 3) dominates mudstone mineralogy. Authigenic clays in sandstone are kaolinite, illite, chlorite, and kaolinite illitization as pore filling and lining. These authigenic clays show a distinct distribution, reflecting differences in burial/temperature history. The illite crystallinity index, the illitization of kaolinite, and the occurrence of high-illite I/S mixed-layer clay minerals suggest burial depths of at least 4-6 km and a temperature typical of the boundary between late diagenesis to low anchizone.

Direct dating of carbonate remagnetization by 40Ar/ 39Ar analysis of the smectite–illite transformation

The occurrence of remagnetizations in carbonate rocks is a longstanding issue, the result of a cryptic process that affects rocks in both fold-thrust belts and undisturbed continental interiors. This paper examines the clay mineralogy of insoluble material from remagnetized carbonates taken from the Cantabrian–Asturian belt, northern Spain. Detailed X-ray diffraction patterns indicate that mixed-layer clays from these rocks are mostly illitic, with an increasing proportion of authigenic illite polytype 1M d in finer grain size fractions. The 40Ar/ 39Ar age data from the different grain size fractions, coupled with quantitative polytype modeling, indicates an authigenic age that is coeval with the established late Paleozoic remagnetization age of these rocks. We propose that the transformation of Fe-rich smectite to Fe-poor illite created a population of authigenic magnetite that is responsible for the remagnetization of these rocks. The irreversible, kinetic dependence of clay mineral transformations appears to account for the irreproducibility and unpredictability of carbonate remagnetization, and may explain such vexing phenomena as synfolding magnetizations. This technique provides an independent assessment of the presence and timing of rock remagnetization with numerous potential applications in future studies of carbonate sequences.

On the origin of mixed-layered clay minerals from the San Andreas Fault at 2.5–3 km vertical depth (SAFOD drillhole at Parkfield, California)

A detailed mineralogical study is presented of the matrix of mudrocks sampled from spot coring at three key locations along the San Andreas Fault Observatory at depth (SAFOD) drill hole. The characteristics of authigenic illite–smectite (I–S) and chlorite–smectite (C–S) mixed-layer mineral clays indicate a deep diagenetic origin. A randomly ordered I–S mineral with ca. 20–25% smectite layers is one of the dominant authigenic clay species across the San Andreas Fault zone (sampled at 3,066 and 3,436 m measured depths/MD), whereas an authigenic illite with ca. 2–5% smectite layers is the dominant phase beneath the fault (sampled at 3,992 m MD). The most smectite-rich mixed-layered assemblage with the highest water content occurs in the actively deforming creep zone at ca. 3,300–3,353 m (true vertical depth of ca. 2.7 km), with I–S (70:30) and C–S (50:50). The matrix of all mudrock samples show extensive quartz and feldspar (both plagioclase and K-feldspar) dissolution associated with the crystallization of pore-filling clay minerals. However, the effect of rock deformation in the matrix appears only minor, with weak flattening fabrics defined largely by kinked and fractured mica grains. Adopting available kinetic models for the crystallization of I–S in burial sedimentary environments and the current borehole depths and thermal structure, the conditions and timing of I–S growth can be evaluated. Assuming a typical K+ concentration of 100–200 ppm for sedimentary brines, a present-day geothermal gradient of 35°C/km and a borehole temperature of ca. 112°C for the sampled depths, most of the I–S minerals can be predicted to have formed over the last 4–11 Ma and are probably still in equilibrium with circulating fluids. The exception to this simple burial pattern is the occurrence of the mixed layered phases with higher smectite content than predicted by the burial model. These minerals, which characterize the actively creeping section of the fault and local thin film clay coating on polished brittle slip surfaces, can be explained by the influence of either cooler fluids circulating along this segment of the fault or the flow of K+-depleted brines.

Response of clastic sediments to episodic hydrothermal fluid flows in intramontane troughs: a case study from Black Forest, Germany

Permo-Triassic sandstones of the Breisgau and Schramberg troughs from the central Black Forest and the adjoining Rhine Graben were hydrothermally illitized during post-Variscan times. Illite K-Ar dating outlines a multi-episodic activity during Late Jurassic (about 145 Ma ago), Early Cretaceous (about 130 Ma ago) and Tertiary (about 60 Ma ago). The last episode appears to be restricted to the foothill zone along the Rhine Graben boundary fault. Detrital feldspars, micas, illite and authigenic kaolinite are the percursors of the hydrothermal illite that is Al-rich when derived from feldspars and authigenic kaolinite, or Al-depleted when formed from micas and detrital illite. The chemical composition of authigenic illite is therefore not an appropriate geothermometer to be used on immature sandstones. Furthermore, the occurrence of mixed-layered illite/smectite and the limited extent of intensive alteration suggest low fluid-rock ratios. Illitization was induced by hot, low saline, metamorphic and/or formation fluids (150– 250 °C, eq ). A pulse of high saline and low temperature fluids (about 100 °C, 20–25 wt% NaCl eq ) derived from mixtures of deep seated saline basement brines and meteoric waters caused quartz overgrowths only in the Triassic sandstones. In addition, kaolinite deriving from detrital feldspars in the Rhine Graben area, could have resulted from interaction with thermal, acidic and low-saline fluids. The results indicate tectonically controlled fluid transport, mainly along reactivated NE and NNE trending faults, due to large-scale tectonic processes, namely: (1) rifting in the central Atlantic and western Tethys oceans, (2) Alpine orogenesis, and (3) rifting of the Rhine Graben.

Clay quantification and Ar–Ar dating of synthetic and natural gouge: Application to the Miocene Sierra Mazatán detachment fault, Sonora, Mexico

Direct dating of brittle fault rocks has been predicated on the assumption that illite in fault gouge is a mixture of two populations of clays: one detrital, derived from the wall rock, and the other authigenic, forming in the brittle fault zone during faulting. Owing to complex diagenetic histories of wallrock shales in previous fault-dating studies, this assumption has remained largely untested. We demonstrate the validity of our clay quantification technique using calculated WILDFIRE© patterns to accurately model both artificial mixtures of 2M 1 and 1M d illite, and natural mixtures in young fault gouges. Using our quantification approach, we obtain well-defined detrital and authigenic ages from the Sierra Mazatán metamorphic core complex in Sonora, Mexico. Because the age population of the wall rock is well constrained by cooling ages, we confirm that the detrital component in the gouge is the age of the footwall granite, while the age of the authigenic component is the time at which faulting ceased, which is in agreement with field constraints. Based on the Ar–Ar age and the thermal conditions for the growth of the authigenic illite we also conclude that the Sierra Mazatán detachment continued slipping in the brittle regime at strain rates similar to those for the plastic regime (on the order of 10 −14 s −1).

Diagenesis, Porosity Evolution, and Petroleum Emplacement in Tight Gas Reservoirs, Taranaki Basin, New Zealand

Abstract Tight gas reservoirs have become popular targets in petroleum exploration in recent years, due largely to the increasing market demand for gas and also to technology advances used in extraction. Reservoir quality is typically poor due to deep-burial diagenesis, resulting in significant compaction, cementation, and illitization. However, analysis of tight reservoirs using integrated techniques can improve our understanding about the controls on reservoir quality, and these results can potentially be used to help predict reservoir quality at other sites. The use of integrated analysis of diagenetic and burial history is here shown for the Eocene, K3E Kapuni Group reservoir in Cardiff-1, Taranaki Basin, New Zealand. The K3E belongs to a deep gas play that was considered uncommercial in the early 1990's due to poor reservoir quality associated with extensive authigenic illite. Potassium-argon dating has shown that destruction of the reservoir through illitization occurred in the Pliocene, associated with a late-stage heating and/or fluid-flow event. Timing of illite authigenesis in the K3E reservoir postdates the likely circulation of CO2-rich fluids that caused reaction of feldspar to form kaolinite and quartz. Homogenization temperatures from two-phase aqueous fluid inclusions within quartz cements, together with 1D basin models of CO2 expulsion from coaly source rocks, indicate that feldspar reactions may have started in the mid Miocene. The CO2-rich fluids are thought to have been generated by thermal decarboxylation of intraformational Paleocene–Eocene coals. At the same time, oil was being expelled from older, Cretaceous coals and migrated into the mid-late Eocene reservoirs; evidence for oil migration is demonstrated by the presence of residual oil and by the local occurrence of abundant oil-bearing fluid inclusions. The products of feldspar dissolution appear to be in reasonable balance with the amount of dissolved feldspar, suggesting a relatively closed diagenetic system with little improvement in reservoir quality despite the large volume of feldspar dissolved. Petrographic evidence of reservoir sandstones at Cardiff-1 demonstrates the presence of some illite-free secondary macropores lined by residual oil. This local preservation of porosity may be replicating the original distribution of oil-saturated pores in the Pliocene, at the time when authigenic kaolinite underwent reaction to form illite. Remobilization of the oil probably occurred by gas flushing after the main phase of illitization at c. 5 Ma. The data presented in this study are consistent with redistributed liquid petroleum into up-dip traps and represent the potential for a new exploration play. The Eocene reservoir at Cardiff-1 is currently gas-charged, with the pore system composed of approximately one third large macropores connected by a tortuous, microporous network. However, it is possible that, with the currently buoyant gas market and modern hydraulic fracturing techniques, it may now be economic to flow gas from the locally large pores identified in the K3E reservoir.

Intracrystalline boron isotope partitioning in illite-smectite: Testing the geothermometer

Intracrystalline B-isotope differences of ~40‰, are observed between the interlayer and tetrahedral crystallographic sites of interstratified illite-smectite (I-S). We tested the hypothesis that partitioning of B-isotopes between these sites could provide a low-temperature, single-mineral geothermometer. Samples studied include a metabentonite transected by a dike in the Cretaceous Pierre Shale (200-500 °C), buried mudstones from the Eocene Wilcox Formation (60-125 °C), and I-S products from hydrothermal experiments (300-350 °C). Different reaction kinetics are represented by these different sample sets, therefore results test the equilibrium partitioning of B in the interlayer vs. tetrahedral sites. In all samples, interlayer δ 11 B values are isotopically heavier than the tetrahedral δ 11 B. Because 11 B prefers trigonal coordination, we infer that B(OH) 3 dominates the interlayer sites. Within each sample set, the intracrystalline differences are greatest (20-40‰) in the most expanded I-S (i.e., smectite-rich), and approach 0 as illitization increases. There is good correlation (R = 0.84) between the interlayer δ 11 B (calculated by mass balance) and water δ 11 B indicated by the established maximum temperature of each sample. These results suggest that the interlayer sites of I-S preserve the B isotopic composition of water at the temperature that produced the authigenic illite. Direct measurements of interlayer δ 11 B equilibrated with water of known δ 11 B are needed to refine the relationship with temperature, but the existing data indicate the following temperature dependent relationship: T (°C) = (δ 11 B tetrahedral - δ 11 B interlayer + 30)/0.05.

Radioactive production and diffusional loss of radiogenic 40Ar in clays in relation to its flux to the atmosphere

This paper addresses the release mechanisms of radiogenic 40Ar from clays, for the purpose of estimating the 40Ar flux from the sedimentary crust to the atmosphere. Clays from sedimentary sequences of Cambrian to Neogene age show discrepancies between their depositional stratigraphic ages and the K–Ar apparent ages or dates of the different size fractions. The possible geochemical mechanisms that may be responsible for the K–Ar dates being either higher or lower than the stratigraphic age are discussed. In general, the known observation of a decrease in the K–Ar apparent age of clays with decreasing particle size is interpreted as due to a faster escape of radiogenic 40Ar from the smaller particles. In the younger, Neogene clays, the mass fractions of 40Ar lost are in a range from 13 to 34% in the fine-size particles that are smaller by a factor of 10 in their linear dimension than the bigger particles. In the older formation samples, Cambrian to Triassic, the 40Ar loss is smaller, from 7 to 19%, for the same particle-size range. This smaller decrease in the 40Ar/ 40K ratio in the stratigraphically older samples may be, at least in part, accounted for by the closure of the mineral system after the process of 40Ar escape went on for some time. In all sediments, a lowering of the K–Ar apparent age of the different size fractions, mainly in the smaller, is possibly due to authigenesis of mixed-layered illite–smectite and illite as a new generation of particles that result from, and are therefore better adapted to, the new diagenetic environment. In a Neogene basin sediments of depositional age < 18 Ma, the amount of the authigenic illite fraction increases with an increase of its crystallization age from about 22 to 38%. If the rate of new crystallization is linear with time, this change corresponds to about 1.5%/Ma of authigenic particles added to the smaller-size fractions. As an overall model, we present a mechanism that explains the lowering of the 40Ar/ 40K ratio and K–Ar apparent age in fine-clay size fractions based on the production of 40Ar in a closed system for some period of time, followed subsequently by the diffusional escape of 40Ar with its continuing production from 40K in the particles. This analysis gives 40Ar diffusion coefficients consistently in a range from 10 − 28 to 10 − 27 cm 2/s. From the results in this paper, the flux of radiogenic 40Ar from the sedimentary crust to the atmosphere is estimated as 5 to 13 × 10 6 mol/year, which is about 15 to 40% of the flux from the whole continental crust, as reported by other investigators.

Determination of layer stacking microstructures and intralayer transition of illite polytypes by high-resolution transmission electron microscopy (HRTEM)

A detailed investigation of microstructures of authigenic illite has been carried out using highresolution TEM. Near-atomic images of 1M and 2M1 polytypes of illite with incidence [100] and [110] were observed. We unambiguously determine 1M illite in both individual particles and microtwinned domains. Newly formed 2M1 domains are surrounded by former packets of 1Md or 1M illite. Such intergrowth of different illite-polytypic domains on a scale of several layers suggests that they are metastable. The lateral-coherent 1M and 2M1 illite domains are aligned along the same orientation and crystallographically continuous normal to [001]*. This is a new phenomenon observed for polytypic intergrowth and transition. The intralayer-stepwise change of stagger direction in a series of successive layers and consequently the interlayer-stacking angle change resulting in polytypic transition are hypothesized as an “intralayer-stepwise transition” model. It explains the process of illite polytypic transition in the solid state.

Isotopic evidence on the diagenetic evolution of coastal sabkha reservoirs from the Solimões Basin, northern Brazil

The isotopic composition of diagenetic carbonates, sulfates, illites and associated diagenetic constituents of the Carboniferous Juruá sandstones, Solimões Basin, largest hydrocarbon reservoirs of northern Brazil, was analyzed in conjunction with petrographic characterization and chemical composition. Eolian Juruá sandstones, deposited in a coastal sabkha of a wide cratonic sag, are the best reservoirs, but diagenetic processes introduced strong and complex heterogeneities, which affect hydrocarbon production. Dolomite and anhydrite cementation, together with compaction, exerts the main control on the quality of the reservoirs. Early dolomite cements precipitated under strong evaporation, while late dolomites are related to thermal decarboxylation. Late anhydrite cements are related to the interbedded evaporites and possibly to the oxidation of dissolved sulfide. Authigenic illite K–Ar ages are related to the voluminous Triassic basic magmatism (around 200 Ma), and to the Jurassic–Cretaceous tectonism (around 150 Ma), which may have also affected the late anhydrite and dolomite–ankerite cementation. This isotopic study revealed the major conditions of the diagenetic processes that strongly affected the Juruá reservoirs, which are essential for the development of models that will optimize the production, as well as decrease the risks in the exploration for new Juruá reservoirs.

MINERALOGICAL, PORE AND PETROPHYSICAL CHARACTERISTICS OF THE DEVONIAN JAUF SANDSTONE RESERVOIR, HAWIYAH FIELD, EASTERN SAUDI ARABIA

Throughout the subsurface of the Arabian Peninsula, the approximately 460ft thick, Devonian Jauf Formation generally consists of well-compacted, low-porosity sandstones and shales, but it also includes friable and highly porous sandstones which form significant gas and condensate reservoir intervals. The mineralogy and pore properties of these reservoir intervals at the Hawiyah field (part of the giant Ghawar structure) were studied by integrating petrographic data with petrophysical measurements of reservoir sandstone samples. The reservoir sandstones are mainly composed of quartz arenites containing small amounts of altered potassium feldspar grains, authigenic illite and chlorite. Based on the pore types, which reflect the habits of the intergranular clays, three reservoir sandstone types have been defined: Type A, characterized by macroporosity; Type B, with microporosity; and Type C, with combined laminations of Types A and B. The dominance of pore-lining clay (as in Type A) or pore-filling clay (as in Type B) is the principal factor controlling the petrophysical properties of the samples. Types A and C sandstones contain macro pores, but irreducible water saturation is high (25 to 45%) compared to clean samples elsewhere, because of the presence of micropores associated with clay. In Type B sandstones the irreducible water saturation is commonly greater than 40% because all the pores spaces are in the microporosity range. The irreducible water saturation in Type B sandstones increases rapidly as porosity decreases. When porosity is less than 10%, the corresponding permeability is 0.2 mD, but no economic production can be expected because water saturation is as high as 100%. In the producing intervals, authigenic clays result in low electrical resistivity due to high water saturation; however, water-free gas is produced.

Low-Porosity Haloes at Stylolites in the Feldspathic Upper Jurassic Ula Sandstone, Norwegian North Sea: An Integrated Petrographic and Chemical Mass-Balance Approach

Abstract Although stylolites in quartz-rich sandstones have been described and modeled, more feldspathic sandstones have not been similarly investigated. The Upper Jurassic Ula sandstone, Norwegian North Sea, a subarkose with 15 to 25% feldspar, displays stylolites with prominent low-porosity haloes up to 10 cm wide. Detailed mineralogical and chemical profiles across several stylolites and haloes are used to investigate the role of feldspar in mineral reactions at stylolites and to constrain the transport of chemical components. Quartz and feldspar are intensely dissolved at the margins of stylolites. The surrounding haloes are characterized by: dissolution of quartz and feldspar, intense compaction, anomalously high feldspar content, high concentrations of clay, and oxide cement volumes that increase away from the stylolite. Chemical mass-balance calculations demonstrate that silica and elements associated with feldspar (Al2O3, Na2O, K2O) were exported from the haloes. Grain-size data and evidence for aluminum loss from the halo indicate that primary concentrations of clay and mica were not higher near the stylolites and that the petrographic and geochemical patterns reflect the diagenetic concentration of clay. The analysis suggests that K-feldspar dissolution at and near the stylolite resulted in precipitation of authigenic illite near the site of K-feldspar dissolution, due to the low solubility of Al3+. This in turn catalyzed chemical dissolution of quartz in the low-porosity halo. Transport of CaO, MgO, and MnO into the haloes suggests that the haloes were zones of higher pH. The increased pH, perhaps due to the presence of clays, would have also increased quartz solubility, consistent with evidence for silica export from the halo. The dissolution of K-feldspar in the absence of kaolinite would also have raised pH around the site of K-feldspar dissolution and illite precipitation, favoring carbonate precipitation. This interpretation of stylolite diagenesis differs from conventional models based on quartz-rich sandstones, in which quartz cement volumes decrease away from stylolites. Models for compaction in arkosic sandstones need to account for the enhanced dissolution of quartz that is driven by K-feldspar dissolution and for catalysis by the resulting illite.

Jurassic and Cretaceous clays of the northern and central North Sea hydrocarbon reservoirs reviewed

AbstractThe principal clays of the northern and central North Sea are illite (sometimes with interlayered smectite) and kaolin. Chlorite is only locally important. Although it has been proposed that kaolin within North Sea sandstones is detrital in origin, the majority of workers have concluded that it is authigenic, largely the product of feldspar alteration. Kaolin is found within a wide range of sedimentary settings (and within shales) apparently defying the notion that kaolin is an indicator of meteoric water deposition. Within sandstones, the earliest authigenic kaolin has a vermiform morphology, the distribution of which is controlled by the availability of detrital mica to act as a nucleus, and the composition of the post-depositional porewaters. This vermiform kaolin formed in meteoric water, the presence of which is easily accounted for below sub-aerial exposure surfaces in non-marine formations, and below unconformities over marine units. In fully marine sands, and even marine shale units, kaolin still occurs. It has therefore been suggested that even these locations have been flushed with meteoric water.Early vermiform kaolin recrystallizes to a more blocky morphology as burial proceeds, at least in the Brent Group. Blocky kaolin has been reported as growing before, synchronously with, and after the formation of quartz overgrowths, though oxygen isotope studies support low-temperature growth, pre-quartz. Blocky kaolin may form during meteoric flushing associated with lower Cretaceous uplift and erosion, though it is found in fault blocks that are thought to have remained below sea level. Here, the kaolin may form in stagnant meteoric water, relics of the post-depositional porewater. It has also been proposed that the blocky kaolin grew in ascending basinal waters charged with carboxylic acids and CO2, though this hypothesis is not supported by stable oxygen isotope data. Some of the blocky kaolin is dickite, the stable polymorph above ∼100°C.Fibrous illite occurs almost ubiquitously within the clastic sediments of the North Sea. An early pore-lining phase has been interpreted as both infiltrated clastic clay, and as an early diagenetic phase. Early clays may have been quite smectite-rich illites, or even discrete smectites. Later, fibrous illite is undoubtedly neoformed, and can degrade reservoir quality significantly. Both within sandstones and shales, there is an apparent increase in the K content deeper than 4 km of burial, which could be due to dilution of the early smectite-rich phase by new growth illite, or to the progressive illitization of existing I-S. Much of the ‘illite’ that has been dated by the K-Ar method may therefore actually be I-S.The factors that control the formation of fibrous illite are only poorly known, though temperature must play a role. Illite growth has been proposed for almost the entire range of diagenetic temperatures (e.g. 15–20°C, Brent Group; 35–40°C, Oxfordian Sand, Inner Moray Firth; 50–90°C, Brae formation; 100–110°C, Brent Group; 130–140°C, Haltenbanken). It seems unlikely that there is a threshold temperature below which illite growth is impossible (or too slow to be significant), though this is a recurring hypothesis in the literature. Instead, illite growth seems to be an event, commonly triggered by oil emplacement or another change in the physiochemical conditions within the sandstone, such as an episode of overpressure release. Hence fibrous illite can grow at any temperature encountered during diagenesis.Although there is an extensive dataset of K-Ar ages of authigenic illites from the Jurassic of the North Sea, there is no consensus as to whether the data are meaningful, or whether the purified illite samples prepared for analysis are so contaminated with detrital phases as to render the age data meaningless. At present it is unclear about how to resolve this problem, though there is some indication that chemical micro-analysis could help. It is a common belief that illite ages record the timing of oil charge, and so can be used to calibrate basin models.Grain-coating Fe-rich chlorite cements can preserve exceptional porosity during burial. They are found in marginal marine sandstones, formed during diagenesis from precursor Fe-rich clays such as berthierine or verdine.

Origin and migration timing of hydrothermal fluids in sedimentary rocks of the Paraná Basin, South America

Clay authigenesis induced by migrating hydrothermal fluids related to the Early Cretaceous Serra Geral magmatism in the Paraná Basin was investigated by mineralogical, geochemical and isotopic means. Stratigraphically equivalent sedimentary samples from the Permian Rio Bonito (in southeastern Brazil) and San Miguel (in eastern Paraguay) formations were collected close to and far from major intrusive bodies. The rocks are generally cemented, in addition to the clay material, by a variety of minerals, such as albite, K-feldspar, calcite, dolomite, zeolite and quartz. The authigenic clay minerals mainly consist of different types of illite-rich mixed layers illite–smectite and chlorite. Corrensite and smectite were also observed. Measured and estimated K–Ar ages indicate that the varied authigenic illite assemblages were most probably generated 140 to 130 Ma ago, during the Serra Geral magmatic event. Hydrothermal fluids depleted in LREE and sometimes enriched in Eu promoted the formation of the well-crystallized illite minerals in rocks characterized by high water / rock ratios. Additional authigenic assemblages with PAAS-type REE patterns probably formed in less porous rocks with low water / rock ratios. Also, some of the authigenic clays yield HREE-depleted patterns, which point to the occurrence of organic matter during crystallization.

Losses of Radiogenic 40Ar in the Fine-Clay Size Fractions of Sediments

AbstractThe common observation that smaller particle-size fractions of sedimentary rocks yield younger K-Ar apparent ages than the larger particle-size fractions of the same stratigraphic age was analyzed with the aid of the 40Ar/40K ratio from 14 stratigraphically and regionally different sections. Estimation of the loss of radiogenic 40Ar from varied clay-rich size fractions was based on two models: a relationship between particle size and the 40Ar/40K ratio, and a theoretical diffusional loss from spherical particles. The differences between the two models and reconciliation of their results are discussed. For the smallest fractions (up to &lt;0.5 μm), percent-wise losses of 40Ar from the spherical particles model increase from Upper Carboniferous and Permian (38±10%), to Late Triassic (47±10%), and to Miocene and Late Neogene (65±8%). This trend suggests that escape of 40Ar from the smaller particles in older sediments decreased or even stopped after deposition of the sedimentary sections.The large 40Ar losses derived from small 40Ar/40K ratios in the younger Tertiary sediments, indicate that addition of K to the small fractions is, at least in part, responsible for the young K-Ar apparent ages in geologically different settings. In several 102–103 m thick sections, authigenic illite in the &lt;0.1 to &lt;2 μm fractions yields young K-Ar apparent ages resulting from simultaneous 40Ar production and release during clay authigenesis. In a production and loss model, a first-order escape-rate parameter (e) was estimated at 0.2 × 10−8 to 4 × 10−8 y−1, depending on the K-Ar apparent age of the size fractions and the stratigraphic age of the section. The limitations and uncertainties of the methods of evaluating diagenetic 40Ar losses from fine clay particles are discussed.

A km-scale illite alteration zone in sedimentary wall rocks adjacent to a hydrothermal fluorite vein deposit

AbstractIn a study of wall-rock alteration in the 1.4 km long Würmtal adit at the Käfersteige hydrothermal vein deposit (northern Black Forest, Germany) illite was found to be the only clay mineral within the Bunter sandstone and intercalated claystone. Illite occurs mainly as a detrital mineral in the claystone, whereas it is hydrothermally neoformed in the sandstone, either in the pores or as an alteration product of K-feldspar. The extensive occurrence of authigenic illite along the entire 1.4 km long profile confirms that the fluids migrated far into the sandstone.The authigenic illite formed during a first pulse of high-temperature fluids (Th of ~220°C) with a low salinity (~1 wt.% NaCleq). These fluids also dissolved Sr and Rb from detrital illite of the claystones at the vein. A later hydrothermal pulse with a lower temperature (~70–150°C) and higher salinity (21–28 wt.% NaCleq) silicified the sandstone adjacent to the vein and caused partial substitution of OH- by F- in the structure of the detrital and neoformed illite along the profile.Within analytical error, the K-Ar dates for the neoformed illite of the &lt;2 µm fraction are the same along the profile (~150 Ma). During this hydrothermal process, the age of the detrital illite within the claystone was reset from 310 to 190 Ma. The illite-rich &lt;0.2 µm fractions yield ages of ~142 Ma (sandstone) indicating a Jurassic origin. The uniform age data for illite in the sandstone and in the claystone are probably due to extensive migration of hot fluids through the wall rocks.The hydrothermal fluids are attributed to recycled meteoric water and brines that ascended from the basement into the cover rocks during the opening of the North Atlantic and/or the nearby Tethys area.

Postdepositional Evolution of Platform Claystones Based on a Simulation of Thermally Induced Diffusion of Radiogenic 40Ar from Diagenetic Illite

ABSTRACT The Lower Cambrian Lontova claystones contain well crystallized authigenic 1M illite mixed with detrital 2M illite. Part of the authigenic illite probably precipitated during sedimentation and subsequently experienced a temperature up to 140°C, despite a limited burial to 500-1000 m. The difference between the Rb-Sr and K-Ar dates of this authigenic illite are attributed to preferential loss of radiogenic 40Ar induced by a short-lasting thermal pulse. This interpretation is based on an Ar-diffusion model suggesting that the Lontova blue clays were heated to around 140°C for 2-5 Myr. This thermal pulse could have occurred in relation with hot-fluid migration induced by either tectonic dislocations in the basement rocks during the Late Silurian-Early Devonian, or rifting conditions during the Permian. By comparing Rb-Sr and K-Ar data, evaluation of radiogenic 40 Ar losses by small authigenic illite particles at low temperatures could be a powerful tool for reconstruction of evolutionary histories of sedimentary successions that were either shallowly buried, or percolated by migrating fluids released during tectono-thermal activities.

Timing and history of marine hydrocarbon accumulation in Tarim craton

Based on the analyses of hydrocarbon inclusions, K-Ar dating of authigenic illites and oil/gas-water contact retrospection as well as other methods, the marine primary reservoirs in the cratonic region of the Tarim Basin are found to have been formed in the late Caledonian to early Hercynian, late Hercynian, and Himalayan, and oil and gas adjusted and redistributed in the Yanshanian (Jurassic to Cretaceous) and Himalayan (Tertiary). The analyses also show that the remaining primary oil accumulations are mostly formed in the late Hercynian, and the secondary oil reservoirs resulted from the adjustment and redistribution of the earlier accumulations during the Yanshanian and Himalayan (especially late Himalayan), of which the Himalayan is also the major stage of gas accumulation. The primary oil reservoirs formed in the late Hercynian mostly occur within the Paleozoic formation, whereas the secondary reservoirs formed in the Himalayan in the Mesozoic. The late adjustment and redistribution of reservoirs in the craton region of the Tarim Basin is ubiquitous because of the intensified tectonic movements since the Mesozoic, especially Cenozoic and intrusion of the high-to overmature gas. Furthermore, the lower amplitude of reservoirs in the craton region also makes them easier to be adjusted and redistributed. Therefore, the remaining paleouplift and palaeoslope developed in relatively stable tectonic regions are the main potential areas for the middlelarge marine reservoirs in the Tarim Basin.

Rare earth element fractionation in authigenic illite–smectite from Late Permian clastic rocks, Bowen Basin, Australia: implications for physico-chemical environments of fluids during illitization

REE analyses were performed on authigenic illitic clay minerals from Late Permian mudrocks, sandstones and bentonites from the Bowen Basin (Australia). The mixed-layer illite–smectite exhibit REE patterns with an obvious fractionation of the HREE from the LREE and MREE, which is an apparent function of degree of illitization reaction. The highly illitic ( R≥3) illite–smectite from the northern Bowen Basin show a depletion of LREE relative to the less illitic ( R=0 and 1) clays. In contrast, an enrichment of HREE for the illite-rich clays relative to less illitic clays is evident for the southern Bowen Basin samples. The North American Shale Composite-normalized (La/Lu) sn ratios show negative correlations with the illite content in illite–smectite and positive correlations with the δ 18O values of the clays for both the northern and southern Bowen Basin samples. These correlations indicate that the increasing depletion of LREE in hydrothermal fluids is a function of increasing water/rock ratios in the northern Bowen Basin. Good negative correlations between (La/Lu) sn ratios and illite content in illite–smectite from the southern Bowen Basin suggest the involvement of fluids with higher alkalinity and higher pH in low water/rock ratio conditions. Increasing HREE enrichment with δ 18O decrease indicates the effect of increasing temperature at low water/rock ratios in the southern Bowen Basin. Results of the present study confirm the conclusions of some earlier studies suggesting that REE in illitic clay minerals are mobile and fractionated during illitization and that this fact should be considered in studies of sedimentary processes and in identifying provenance. Moreover, our results show that REE systematic of illitic clay minerals can be applied as an useful technique to gain information about physico-chemical conditions during thermal and fluid flow events in certain sedimentary basins.