Authigenic Illite Research Articles

Fluid Phase Modeling and Evolution of Complex Reservoirs in the Halahatang Depression of the Tabei Uplift, Tarim Basin.

The diversity of fluid phases in the oil-gas system and complexity of petroleum genesis bring difficulties to the exploration and development of oil and gas. The phase state and evolution of the complex reservoirs in the Halahatang Depression, Tabei Uplift of the Tarim Basin remains unresolved. In this paper, we simulated the phase characteristic of reservoirs in different blocks and layers including Xinken (O), Ha6 (C), Repu (K), and Yueman (O) distributed from north to south of this area using PVTsim software; rebuilt the burial, temperature, and pressure histories of different blocks and layers by using the PetroMod (1D) software; and recovered the fluid phase evolution process by combined basin modeling, PVT simulation, and fluid inclusion thermal metrics results. The phase modeling results show that the Xinken (O), Ha6 (C), and Yueman (O) reservoirs are confirmed to be oil reservoirs, and the Repu (K) reservoir is in the condensate gas phase currently. The vital time points and temperature and pressure conditions for the three oil reservoirs of Xinken (O), Ha6 (C), and Yueman (O) that transited from the gas–liquid phase to the liquid phase are 356 Ma (57.45 °C, 12.93 MPa), 331 Ma (35.67 °C, 4.03 MPa), and 454 Ma (63.63 °C, 13.27 MPa), respectively. The Ordovician reservoir in the Xinken block underwent three stages of accumulation, which occurred at 400–379 Ma (Devonian), 282–256 Ma (Permian), and 18–16 Ma (Neogene), respectively, and after final accumulation, it remained in a single oil phase state. The Ordovician reservoir in the Yueman block underwent two stages of accumulation in the 294–290 Ma (Permian) and 25–12 Ma (Paleogene–Neogene) and remained in a single oil phase state until now. The Carboniferous reservoir in the Ha6 block was deduced to be charged in the 94–86 Ma (Cretaceous) according to the published authigenic illite K–Ar isotope dating results and then stayed in a single oil phase state unalterably. As for the Cretaceous reservoir in the Repu block, the time point of 11 Ma (98.86 °C, 35.56 MPa) is vital for changing from the gas–liquid coexistence phase state to the condensate gas phase one. In contrast with the Ordovician (ZG7–5) and Cambrian reservoirs (ZS1, ZS5) in the Tazhong Uplift, the oil and condensate gas reservoirs in the Tabei Uplift enjoy a lower pressure range, lower GOR, and a heavier oil density and viscosity. This study provides a quantitative way to rebuild the geologic evolutionary process, phase characteristics, and phase evolution process in complex reservoirs.

Characteristics and Genetic Mechanism of Chang Eight Low Permeability and Tight Reservoir of Triassic Yanchang Formation in Central-East Ordos Basin

In this article, the characteristics of Chang 8 reservoir of Triassic Yanchang Formation in northern Shaanxi are studied by using polarizing microscope, field emission scanning electron microscope, image particle size, X-ray diffraction analysis of clay, and constant pressure mercury intrusion. The study shows that the target layer is in a relatively stable and uniform sinking burial period after deposition, and the lithology composition in the area is relatively complex, mainly composed of debris–feldspar sandstone and feldspar sandstone, with the characteristics of fine grain and high content of interstitial material. The porosity of the reservoir is generally between 4% and 12%, with an average of 8.05%. The permeability is generally between 0.03 × 10−3 and 0.5 × 10−3 μm2, with an average of 0.16 × 10−3 μm2. Strong compaction and well-developed cementation of calcareous, siliceous, and authigenic illite are important reasons for the formation of extra-low porosity and extra-low permeability reservoirs. But at the same time, because of the protective effect of chlorite film, some residual intergranular pores are preserved, which makes the some reservoirs with relatively good physical property, forming a local relatively high-porosity and high-permeability section of the “highway.”

Lateral variations of detrital, authigenic and petrophysical properties in an outcrop analog of the fluvial Plattensandstein, Lower Triassic, Central S-Germany

The lateral heterogeneity of reservoir rocks is important to assess storage potential and permeability. This case study focusses on a Lower Triassic (upper Olenekian) outcrop of fluvial sandstones of the Plattensandstein Member (Rot Formation) near Rottbach in the southern part of the Germanic Basin, southern Germany. The Buntsandstein Group is considered a suitable reservoir for thermal storage or exploration in southern Germany. In the Rottbach Quarry, the >10 m thick lithic arkoses are exposed over 35 m laterally in three adjacent walls and consist of two units separated by an erosive surface. They are covered by several metres of the Lower Rot Claystone Member. Using sedimentological logging, spectral gamma ray-, porosity- and permeability measurements as well as petrographic analyses calculating compaction parameters, lateral reservoir quality differences are studied. Sedimentary structures in the multi-storey channels suggest a point bar deposit. The migration direction of the channels correlates with deteriorating reservoir quality. This reduction in porosity and permeability correlates with an increased detrital mica and authigenic illite content, the occurrence of rip-up clasts and a lateral increase of gamma ray signatures by 19 API (avg. 5.0 vol% K, 2.7 ppm U, 11.9 ppm Th). The occurrence of larger amounts of mica and clay minerals is linked to decreasing flow velocities throughout the evolution of a meandering system. They enhance compaction and reduce available intergranular pore space. Permeability laterally deteriorates by up to three orders of magnitude (48 to 0.02 mD, avg. 9 mD) while measured porosities show minor variability (11 to 19 %, avg. 16 %). The deterioration of reservoir quality is related to compactional (COPL 11 to 28 %, avg. 18 %) and cementational porosity loss (CEPL 13 to 29 %, avg. 22 %) and is mainly controlled by detrital mica and authigenic illite.

The pore structures of different lithofacies in low-permeability sandy conglomerate reservoirs and their diagenetic impacts: A case study from the Es4 member of the northern steep slope in Dongying Depression, Bohai Bay Basin, NE China

Sandy conglomerates are an important reservoir type in the Bohai Bay Basin, China. These reservoirs are characterized by complex lithofacies distribution, low permeability and heterogeneous pore structures that significantly impact reservoir quality and field development strategies. This study evaluated the Es4 member of the Eocene Shahejie Formation based on cores from three wells in the Dongying Depression. The analytical methods include thin section analyses, scanning electron microscopy (SEM), pressure-controlled mercury injection (PMI), rate-controlled mercury injection (RMI) and X-ray computed tomography (CT). The results show that the microscopic pore structures among various lithofacies of the Es4 sandy conglomerate reservoirs are controlled by different levels of diagenetic alteration, resulting in the heterogeneity of macroscopic reservoir quality. The high content of matrix in Mc (matrix-supported conglomerate) reservoirs leads to low primary porosity. Due to the poor grain sorting, Gc (grain-supported fine conglomerate) and Ps (pebbly sandstone) reservoirs lose a great number of primary pores during mechanical compaction process, whereas Ms (massive medium sandstone) reservoirs preserves abundant intergranular pores. Highly mineralized pore fluids, resulting from massive dissolution of volcanic rock fragments and carbonate fragments, contributes to extensive ferroan carbonate cementation during the late diagenetic stage. Powder crystal ankerite, with a grain radius of about 0.02 mm, blocks most pore throats within Gc reservoirs. Authigenic illite and mixed-layer illite/smectite bridges the throats and reduces the pore throat radius exponentially. Moderate dissolution of rock fragments and feldspars generates a number of secondary pores in Gc, Ps and Ms reservoirs, slightly enhancing permeability. With greater residual primary intergranular porosity and more large-size throats, Ms reservoirs possess the best reservoir quality, followed by Ps reservoirs. These research results are crucial to reservoir evaluation and predication in similar sandy conglomerate reservoirs.

Authigenic Phyllosilicates in Sand Layers from the Mudflats of Saline Lakes in the Northern Great Prairies, Saskatchewan

ABSTRACT The mudflats of saline lakes are amenable to authigenic clay formation due to the high ionic strength of the solutions driven by evaporative concentration and due to the fluctuating wet/dry cycles. However, the mudflats of saline lakes have received relatively little study given the challenges in sampling unstable sediments coupled with post-depositional alterations that make direct relationships to the climate difficult. In an effort to gain a better understanding of the authigenic phyllosilicates present, the mudflats of 17 sulfate-rich saline lake basins across southern Saskatchewan were sampled. The <2 μm fraction was separated from the sediments and analyzed utilizing X-ray diffraction (XRD), scanning electron microscopy, bulk chemical analysis via digestion and inductively coupled optical emission spectroscopy, and visible and near-infrared reflectance spectroscopy. The mudflat sediments were characterized as highly variable and were classified based on particle size into sediment classes A (clay-rich), B (unsorted till), and C (sand). Despite the high variability in sorting and thickness of the sedimentary layers, the phyllosilicates were distinctive within each class independent of the basin. Phyllosilicates in sediment class A were characterized by well-crystalline dioctahedral (Al) clays similar to the surrounding soils with smectite > illite > kaolinite > chlorite. Phyllosilicates from sediment class B displayed highly variable characteristics ranging between classes A and C. Clays from sediment class C were dominated by illite with decreasing proportions of smectite, kaolinite, and chlorite. The illite in the sand lenses was poorly formed, with broad reflections in the XRD patterns indicative of small crystallite size or high disorder, which is consistent with an authigenic nature. The clays in class C were rich in iron (Fe) and magnesium (Mg) and displayed lath-like morphologies common with authigenic illite forming in sandy porous sediments. The sand lenses of mudflats represent viable targets for finding authigenic clay minerals in detrital-rich sediments to use in understanding past climates on Earth and Mars.

Eocene fault-controlled fluid flow and mineralization in the Paradox Basin, United States

Abstract Sedimentary rocks of the Paradox Basin of the Colorado Plateau (southwestern USA) record widespread manifestations of paleo–fluid flow and fluid-rock reactions including Cu, U-V, and Fe-Mn mineral deposits, Si and Ca metasomatism, hydrocarbon accumulations, and bleached sandstones. Many of these are spatially associated with faults. Here we show evidence for a widespread phase of fault-related fluid migration and mineralization at 41–48 Ma in the Paradox Basin. We measured K-Ar dates of multiple size fractions of clay-rich fault gouge, yielding statistically overlapping dates of authigenic (1Md) illite for the Salt Valley (47.0 ± 3.0 Ma), Kane Springs (47.7 ± 3.8 Ma), Cliffdweller (43.4 ± 4.6 Ma), Courthouse (41.9 ± 2.3 Ma), Lisbon Valley (45.3 ± 0.9 Ma), and GTO (48.1 ± 2.6 Ma) faults. The latter two have an illite Rb-Sr isochron age of 50.9 ± 3.5 Ma, and fault-adjacent bornite has a Re-Os isochron age of 47.5 ± 1.5 Ma. Authigenic illite from a paleo–oil reservoir near the Courthouse fault formed from the interaction of reduced fluids with oxidized red-bed sandstones at 41.1 ± 2.5 Ma. The Moab and Keystone faults have older authigenic illite ages of 59.1 ± 5.7 Ma and 65.2 ± 1.0 Ma, respectively. Our results show a close temporal relationship between fault gouge formation, red-bed bleaching, and Cu mineralization during an enigmatic time interval, raising questions about drivers of Eocene fluid flow.

The effect of thermal fluid derived from mud diapir on sandstone reservoirs in the Yinggehai Basin, South China Sea

The underground thermal fluid is one of the significant factors controlling the formation and quality of reservoirs. The Huangliu Formation (N1h) in YF1 area of the central depression belt in the Yinggehai Basin, South China Sea, is characterized by intense thermal fluid activities related to mud diapir and large-scale shallow-water gravity flow deposits. The multi-episodic invasion of high-temperature and CO2-rich thermal fluid into the formation induces complex water-rock reaction during diagenetic process, providing a preferable investigation opportunity for revealing how thermal fluid affects reservoir quality. This study characterizes the reservoirs in the formation through core and thin section analyses as well as physical property test. The reservoirs are dominated by fine-grained sandstone characterized by medium porosity (15.2–21.3%) and lower permeability (0.56–15.75mD). Based on an analysis of casting thin section, cathode luminescence (CL), scanning electron microscope (SEM), carbon and oxygen isotope, inclusion test, and electron-probe microanalysis (EPMA), we systematically investigate the diagenetic patterns and pore evolution process for the reservoirs in the formation. The episodic invasion of thermal fluid occurred approximately 0.4 Ma ago plays an important role in controlling reservoir development: The CO2-rich formation water induces massive late-stage dissolution, resulting in a higher proportion of dissolved pores (38.7–46.4%), which improves the porosity of reservoirs at a depth of 2600–3100 m. Nevertheless, the late dissolution together with carbonate cementation occurred in closed diagenetic system blocks most seepage channels. Furthermore, the relatively high level clay mineral transformation in the YF1 area leads to a higher content of authigenic illite (44–62%) in the formation. Massive authigenic illite severely blocks the pore throats in fine sandstone, reducing permeability. This study offers an insight to the understanding of mud diapir-derived thermal fluid affecting and controlling the quality of reservoirs in some areas of the Yinggehai Basin.

Did the eruption of the Tarim LIP control the formation of Paleozoic hydrocarbon reservoirs in the Tarim basin, China?

The Tarim basin not only is one of the main targets for hydrocarbon exploration in China, but also hosts the Early Permian LIP, thereby providing an ideal laboratory to investigate how the LIP eruptions interact with the petroleum system in the volcanic-affected basins. Six major second-order domes were identified in the first-order dome that involves the entire Tarim LIP, based on seismic probing and lithofacies and stratigraphic observations in boreholes and the stratigraphic sections. These second-order domes are characterized by abundant Permian basaltic dykes and volcanic craters, indicative of the origin from the LIP eruption. They also are the sites of the almost all Paleozoic oil and gas reservoirs in the Tarim basin. Dating on authigenic illites collected from 20 Silurian bituminous and oil-saturated sandstone samples in the Tarim basin, along with literature data, indicates that hydrocarbon charge and emplacement in the Paleozoic reservoirs prevalently occurred during 300–255 Ma, approximately concurrent with the eruption of the Tarim LIP lavas (300–262 Ma). Therefore, we propose that the LIP eruption not only created the second-order domes that provided space for the oil and gas charge but also triggered the rapid release of hydrocarbon through the volcanic vents, thereby controlling the formation of Paleozoic oil and gas fields in the Tarim basin.

A new K-Ar illite dating application to constrain the timing of subduction in West Sarawak, Borneo

Abstract The timing of subduction is a fundamental tectonic problem for tectonic models, yet there are few direct geological proxies for constraining it. However, the matrix of a tectonic mélange formed in a subduction-accretion setting archives the physical/chemical attributes at the time of deformation during the subduction-accretion process. Thus, the deformation age of the matrix offers the possibility to directly constrain the period of the subduction-accretion process. Here we date the Lubok Antu tectonic mélange and the overlying Lupar Formation in West Sarawak, Borneo by K-Ar analysis of illite. The ages of authigenic illite cluster around 60 Ma and 36 Ma. The maximum temperatures calculated by vitrinite reflectance values suggest that our dating results were not affected by external heating. Thus, the ages of authigenic illite represent the deformation age of the mélange matrix and the timing of the Rajang Unconformity, indicating that the subduction in Sarawak could have continued until ca. 60 Ma and the thermal and/or fluid flow events triggered by a major uplift of the Rajang Group occurred at ca. 36 Ma. Furthermore, this study highlights the potential of using the tectonic mélange to extract the timeframe of subduction zone episodic evolution directly.

Direct dating of brittle extensional deformation contemporaneous of Neogene exhumation of the internal zones of the Rif Chain

The brittle deformation of internal zones of the Rif chain, corresponding to the southern margin of the West Alboran Basin, is not or poorly dated because of the lack of stratigraphic constraints. We provide for the first time a direct dating of brittle deformation of internal zones of the Rif chain, in the Ceuta peninsula located in the westernmost part of the Gibraltar arc. 40Ar/39Ar illite ages from fault gouges in the footwall and hanging wall of the Ceuta Shear Zone range from 11.9 ± 2.5 Ma to 15.7 ± 1.7 Ma and from 14.7 ± 0.1 Ma to 18.9 ± 0.8 Ma, respectively. In addition, (UTh)/He apatite ages from both units have been measured. They are younger in the hangingwall (~20–21 Ma) than in the footwall (14–19 Ma), which highlights slightly different cooling histories. Thus, we interpret the illite ages to date growth of authigenic illite (1M polytype) during exhumation through the top few kilometers of the earth's surface.This study of brittle deformation within the internal zones of the Rif chain reveals that an E-W directed extension occurred between ~18–15 Ma during the low temperature exhumation of Sebtide-Alpujárride units. From 15 to 11 Ma, the direction of extension shifted to NNW-SSE and only minor steep normal faults occurred. Brittle normal faulting occurred during a stage of slow cooling (~1–2 °C/myr) starting at ~19 Ma and coeval with exhumation processes within the uppermost levels of the continental crust. These new data emphasize a continuum of extensional deformation during the Burdigalian–Serravalian interval with an early E-W directed phase and then a NNW-SSE directed phase that match well the main subsidence pulses identified offshore in the neighboring West Alboran Basin.

Mesozoic Hydrothermal Overprint on Carboniferous Bauxite in China

Abstract Bauxite is the world’s main source of aluminum and typically consists of gibbsite, boehmite, and minor amounts of diaspore. However, bauxite deposits from the North and South China blocks consist mostly of diaspore and associated minerals, including anatase and illite. Much of this illite is authigenic and occurs as three polytypes (1M, 1Md, and 2M1), with Kübler indices ranging from 0.23 to 0.47 indicating precipitation temperatures of 175° to 300°C. The Raman spectra of anatase show an intensity ratio of <1.5 for G (~1,600 cm–1) and D bands (~1,350 cm–1) diagnostic of organic matter, suggesting its presence during bauxite sedimentation followed by heating (165° to 270°C). The K-Ar ages of authigenic illite from the South China block (178–137 Ma) and the North China block (214–203 Ma) are synchronous with known regional Mesozoic tectono-thermal events generating hydrothermal overprints resulting in (1) the formation of illite, (2) the conversion of some diaspores from thermal of gibbsite and boehmite, and (3) the heating of anatase postsedimentation of the Carboniferous bauxites.

Timing and characterization of multiple fluid flow events in the Beibuwan Basin, northern South China Sea: Implications for hydrocarbon maturation

The history of thermal and fluid flow events is of crucial importance to understand the mechanism of hydrocarbon maturation. The northern South China Sea (SCS) contains abundant hydrocarbon resources with high heat flow anomalies, but the mechanisms responsible for modification of thermal regimes are poorly understood. Here we conducted the first high-precision Rb-Sr isochron dating and geochemical analyses of authigenic illites separated from the Palaeogene sandstones in the Beibuwan Basin, northern SCS. Eight high-precision Rb-Sr isochron ages reveal three major thermal and fluid flow events in the Beibuwan Basin, northern SCS. The 34.5 ± 0.9 Ma and 23.6 ± 0.8 Ma events occurred in the western area are probably related to the Eocene-Oligocene transition characterised by a significant regional unconformity resulted from intensive uplift of Hainan Uplift and Oligocene-Miocene transition marked by a post-rift unconformity, respectively. By contrast, the 31.2 ± 0.6 Ma event affecting the eastern area appears to be closely associated with the igneous intrusion dated by zircon U/Pb to 32.3 ± 0.7 Ma. REE and stable isotope characteristics indicate that the illites from the western area formed within a meteoric-hydrothermal system, whilst those from the eastern area were likely precipitated from fluids heated by the large-scale igneous intrusion at higher water/rock ratios. Moreover, our data in conjunction with palaeo-temperature reconstructions show that the fluid flow events had profound effects on the rapid organic maturation in the northern SCS.

K-Ar and Rb-Sr Dating of Nanometer-Sized Smectite-Rich Mixed Layers From Bentonite Beds of the Campos Basin (Rio De Janeiro State, Brazil)

AbstractK-Ar isotopic dating has been applied to alkali-rich nanometer-sized illite separates of bentonites since the late 1990s. In the present study, K-Ar and Rb-Sr isotopic analyses were based on mineralogical determinations and morphological observations of similarly nm-sized separates (<0.02, 0.02–0.05, and 0.05–0.1 μm) depleted in alkalis and recovered from Santonian (85.8–83.5 Ma) bentonites of the Campos Basin located offshore the southeastern Atlantic coast (Rio de Janeiro State, Brazil). On the basis of XRD analyses and geochemical/mineralogical simulations, the size fractions consist essentially of the smectite-rich end-member of the smectite-to-illite trend with no more than 9% authigenic illite layers. High K-Ar values from 42.6 ± 3.2 to 70.2 ± 2.1 Ma confirm the occurrence of detrital illite in one sample at least, the age data being meaningless. A second group of K-Ar values ranges from 15.5 ± 10.7 to 41.3 ± 10.8 Ma, while the smallest (<0.02 and 0.02–0.05 μm) fractions with <0.42% K2O lack detectable radiogenic 40Ar and yield analytically 0 Ma ages. Two samples including that with the detrital illite were leached with dilute acid and the Rb-Sr method was applied to the untreated, leachate, and residual fractions of the different separates. The combined isotopic data suggest that illitization started at ~44 ± 4 Ma when the bentonites were subjected to a temperature of ~60°C. The leachable Sr yielded 87Sr/86Sr ratios of 0.7106–0.7108, which is greater than those of seawater either during deposition or recently, and of the initial ashes. They do not correspond to the chemical signature of pore fluids, but more likely to removals from fragile edges of the illite-smectite layers probably impacted by the rough initial chemical treatment applied to empty the smectite interlayers. Illitization was either a side effect of a major contemporaneous smectitization or an independent process that occurred later, in the progressively buried bentonite beds. Of variable duration, it was probably dependent on burial-induced temperature increase in the bentonites, which monitored the fixation of K in the smectite layering with or without a changing fluid chemistry. On the basis of the combined K-Ar and Rb-Sr isotopic data, illitization lasted either until ~15 Ma or even 0 Ma for some of the finest size fractions.

Precambrian faulting episodes and insights into the tectonothermal history of north Australia: microstructural evidence and K–Ar, <sup>40</sup>Ar–<sup>39</sup>Ar, and Rb–Sr dating of syntectonic illite from the intracratonic Millungera Basin

Abstract. Australian terranes concealed beneath Mesozoic cover record complex Precambrian tectonic histories involving a successive development of several Proterozoic to Palaeozoic orogenic systems. This study presents an integrated approach combining K–Ar, 40Ar–39Ar, and Rb–Sr geochronologies of Precambrian authigenic illites from the recently discovered Millungera Basin in north-central Australia. Brittle deformation and repeated fault activity are evident from the sampled cores and their microstructures, probably associated with the large-scale faults inferred from interpretations of seismic surveys. Rb–Sr isochron, 40Ar–39Ar total gas, and K–Ar ages are largely consistent in indicating late Mesoproterozoic and early Proterozoic episodes (∼1115±26, ∼ 1070±25, ∼1040±24, ∼1000±23, and ∼905±21 Ma) of active tectonics in north-central Australia. K–Ar results show that illites from fault gouges and authigenic matrix illites in undeformed adjacent sandstones precipitated contemporaneously, indicating that advection of tectonically mobilized fluids extended into the undeformed wall rocks above or below the fracture and shear (fault gouge) zones. Isotopic age data clearly indicate a Mesoproterozoic minimum age for the Millungera Basin and thus previously unrecorded late Mesoproterozoic–early Neoproterozoic tectonic events in north-central Australia. This study provides insight into the enigmatic time–space distribution of Precambrian tectonic zones in central Australia, which are responsible for the formation of a number of sedimentary basins with significant energy and mineral resources.

Hydrogeochemistry of authigenic mineral formation in Upper Jurassic sediments (the Nadym-Taz interfluve area, Arctic regions of Western Siberia)

This paper presents a novel numerical modelling approach to physicochemical evolution of water – rock interactions in Upper Jurassic sediments in the Nadym–Taz interfluve (Arctic regions of Western Siberia). The results show that the authigenic mineral formation proceeds continuously and strictly successively (kaolinite – montmorillonite – illite – mica – chlorite – albite – microcline), involving definite geochemical parameters of the medium (pH, concentrations of SiO2, Al, Na, K, Ca, Mg in solution). For example, the widely spread kaolinite cement forms in the medium with near neutral pH (on average, рH = 7.2) with silica concentration in solution 15–20 mg/dm3, while the formation of authigenic illite begins at silica concentration of 25–40 mg/dm3 and pH about 7.3–7.4. Despite rather low salinity (up to 63 g/dm3) of the investigated groundwater and long-term interaction of water with the rocks (spanning ca. 165 Ma), the equilibrium with primary (endogenous) minerals such as albite, anorthite, microcline is practically unobserved. At the same time, the groundwater is in equilibrium with mica, illite, Ca–, Na– and Mg–montmorillonites, kaolinite, more rarely with Mg–chlorite. The state of groundwater equilibrium with primary aluminosilicate minerals is always challenged by a carbonate barrier, so rocks carbonatization in various degree is observed almost everywhere.

Syncing fault rock clocks: Direct comparison of U-Pb carbonate and K-Ar illite fault dating methods

Abstract The timing of slip on brittle faults in Earth’s upper crust is difficult to constrain, and direct radiometric dating of fault-generated materials is the most explicit approach. Here we make a direct comparison between K-Ar dating of fault gouge clay (authigenic illite) and U-Pb dating of carbonate slickenfibers and veins from the same fault. We have dated fault generated materials from the Big Creek fault, a northwest-striking, dextral strike-slip fault system in Yukon Territory, Canadian Cordillera. Both methods yielded dates at ca. 73 Ma and ca. 60–57 Ma, representing at least two periods of fault slip that form part of a complex fault and fluid-flow history. The Cretaceous result lies within previous indirect estimates for major slip on the fault. The Paleocene–Eocene result coincides with the estimated timing of slip of the nearby Tintina and Denali faults, which are crustal-scale, northwest-striking dextral faults, indicating Big Creek fault reactivation during regional faulting. The coincidence of periods of carbonate-crystallizing fracturing and fluid flow with intervals of seismic, gouge-generating slip supports the fault valve model, where fault strength is mediated by fluid pressures, and fluid emplacement requires seismic pumping in otherwise impermeable aseismic fault zones. The reproducibility of slip periods for distinct fault-generated materials using different decay systems indicates that these methods provide complimentary results and can be reliably applied to date brittle fault slip, opening new opportunities for investigating fault conditions with associated mineralizing fluid events.

Evidence of Hydrocarbon-Rich Fluid Interaction with Clays: Clay Mineralogy and Boron Isotope Data from Gulf of Cádiz Mud Volcano Sediments

Clay dehydration at great depth generates fluids and overpressures in organic-rich sediments that can release isotopically light boron from mature organic matter, producing 10B-rich fluids. The B can be incorporated into the tetrahedral sites of authigenic illite during the illitization of smectite. Therefore, the crystal-chemical and geochemical characterization of illite, smectite or interlayered illite–smectite clay minerals can be an indicator of depth (temperature) and reactions with the basin fluids. The aim of this study was to determine the detailed clay mineralogy, B-content and isotopic composition in illite–smectite rich samples of mud volcanoes from the Gulf of Cádiz, in order to evaluate interactions of hydrocarbon-rich fluids with clays. Molecular modeling of the illite structure was performed, using electron density functional theory (DFT) methods to examine the phenomenon of B incorporation into illite at the atomic level. We found that it is energetically preferable for B to reside in the tetrahedral sites replacing Si atoms than in the interlayer of expandable clays. The B abundances in this study are high and consistent with previous results of B data on interstitial fluids, suggesting that hydrocarbon-related fluids approaching temperatures of methane generation (150 °C) are the likely source of B-rich illite in the studied samples.

Long‐distance fluid migration defines the diagenetic history of unique Ediacaran sediments in the East European Craton

AbstractThe diagenetic history of the Ediacaran sedimentary rocks in the East European Craton (EEC) over the area extending from Arkhangelsk (Russia) in the north to Podolia (Ukraine) in the south was revealed by means of the XRD characterization and K–Ar dating of clay fractions, mudstone porosity measurements and organic geochemistry investigations. Mudstone porosity measurements produced direct evidence of shallow maximum burial of the Ediacaran sediments on the craton (Russia, Lithuania, Belarus, Volyn), not exceeding 1.5 km, and much deeper burial at the cratonic margin, in Podolia and Poland. In general, illitization of smectite and biomarker indices indicates more advanced diagenesis at the cratonic margin.K–Ar dating of authigenic illite–smectite and aluminoceladonite revealed the Palaeozoic age of mineral diagenesis (ca. 450–300 Ma) both on the craton and its margin, with older ages generally observed in the north. When the maximum palaeotemperatures were evaluated from illite–smectite and biomarkers, based on the calibrations from the conventional burial diagenetic sections, a major mismatch was detected for the cratonic area: 100°C–130°C from illite––smectite and tens of oC lower from the lipid biomarkers. This diagenetic pattern was interpreted as the result of short‐lasting (in ky scale) pulses of potassium‐bearing hot fluids migrating from the Caledonian and Variscan orogens deep in the craton interior, effectively promoting illitization in porous rocks without altering the organic matter. Analogous short pulses of fluids were responsible for numerous diagenetic phenomena, including Mississippi Valley‐Type ore deposits, in the American Midwest, in front of the Appalachians. K–Ar dating indicates that the entire Proterozoic sedimentary cover of the Great Unconformity on the EEC remained untouched by measureable post‐sedimentary changes until the early Palaeozoic, thus for over 1000 My, which is an unprecedented finding.

Locally Derived, Meteoric Fluid Infiltration Was Responsible for Widespread Late Paleozoic Illite Authigenesis in the Appalachian Basin

AbstractIsotopic and geochronologic investigation of authigenic, K‐bearing clays in the Appalachian Plateau of the northeastern U.S. Midcontinent yields new insights about the tectonic and diagenetic history of the North American sedimentary cover sequence. In situ texture analysis by High Resolution X‐ray Texture Goniometry indicates preservation of bedding‐parallel diagenetic fabrics with burial depths of 2–5 km, and illite mineralization temperatures are spatially variable, ranging from 80 to 190 °C, correlating to similar depths of 3–6 km. The mineralizing geofluid is surface derived, with δD values ranging from −48‰ to −72‰ (in the range of predicted Pangea meteoric fluid). In addition, we find that mineralizing fluid δD values increase away from the orogenic front, consistent with a rain shadow effect from the high elevation Appalachian orogen. The age of authigenic illite is constrained by 40Ar/39Ar geochronology to 308–318 Ma, reflecting Upper Carboniferous diagenesis. We postulate that far‐field stress transmission from continent‐continent collision created regional permeability pathways for surface fluids, altering the hydrologic architecture of the brittle crust and allowing meteoric fluid infiltration into upper crustal rocks. This interpretation challenges the popular view of tectonically forced, lateral fluid flow from the Appalachian orogen (squeegee hypothesis).

Controls of diagenetic alteration on the reservoir quality of tight sandstone reservoirs in the Triassic Yanchang formation of the Ordos Basin, China

The Ordos Basin is rich in tight oil resources, and the Upper Triassic Yanchang Formation tight oil reservoir is one of the most important exploration targets. In the study area, the Chang 63 tight sandstone reservoir exhibits lacustrine deep-water lobe-shaped delta deposits with low porosity, low permeability and a complex diagenetic alteration history. The reservoir quality is extremely heterogeneous because of the heterogeneity of the depositional lithofacies and diagenetic alterations. This research integrates multiple techniques to investigate the depositional lithofacies, the characteristics and distributions of various diagenetic alterations, and their controls on reservoir quality. Four lithofacies are identified, and the various lithofacies have different diagenetic characteristics and diverse reservoir qualities. The Chang 63 tight sandstone reservoir has experienced significant diagenetic alteration. Mechanical compaction and authigenic illite are the most important causes of the decrease in reservoir quality. Carbonate cementation preferentially occurs in sandstones of lithofacies “i“ and “ii” near the mudstone-sandstone contact surfaces. Sandstones within 1 m of adjacent mudstone-sandstone contact surfaces are usually tightly cemented by carbonate cement. Illite can be divided into two types according to its morphology. Both types of illite lead to a decrease in reservoir quality. Ultimately, by combining the relationship between depositional facies and lithofacies with the distribution of different diagenetic processes, five evolutionary patterns are identified for reservoir quality in the lacustrine deep-water lobe-shaped delta deposits can be summarized by ratings of best, moderate, relatively poor, poor and worst.