Quartz Precipitation Research Articles

Botryoidal quartz as an abiotic signature in Palaeoarchean cherts of the Pilbara Supergroup, Western Australia

Chemically oscillating reactions (COR) are abiotic processes that generate self-repeating circularly concentric morphologies during decarboxylation of organic acids. The geometry and millimetre to centimetre size dimensions of laminated quartz botryoids recorded in the Palaeoarchean Apex and Strelley Pool chert horizons in Western Australia simulate the self-similar fractal patterns arising in vitro from COR using classical and modified reactants of the Belousov-Zhabotinsky (B-Z) reaction. The botryoidal patterns akin to those developed by the COR include circular concentric laminae, open-book features, microbialite- and stromatolite-like laminations, wavy and domal structures, and rosettes. The mineral composition and organic matter (OM) distribution of these objects indicates an origin from the early diagenetic decarboxylation of carboxylic acids. Pore-water alkalinity decrease is likely due to the decomposition of OM, the generation of carbonic acid, and the cessation of chemical wave diffusion. The chemical waves developed from primary circular oxidation spots, under solubility equilibrium with respect to silica could trigger diagenetic precipitation of quartz. The presence of Fe2+-bearing hematite in various botryoidal geometries in the Apex chert is an analogue of ferroin-derived Fe in B-Z solutions, whereas the presence of Fe and Zn sulphides in the Strelley Pool quartz botryoids is akin to sulphur redox intermediates in B-Z experiments. Correlated microscopy and Raman spectral analysis corroborate that the metamorphosed OM associated with these Palaeoarchean botryoids is indigenous and syngenetic with the host chert. OM in botryoidal quartz displays circular concentric laminations as well as gradients of its density. Accordingly, an abiotic model of COR for the diagenetic growth of the studied Apex and Strelley Pool quartz botryoids is proposed. Comparisons with the chemical compositions, geometric morphology and range of size dimensions of self-similar patterns have been the criteria for development of this model expression. The explored ancient botryoids can thus represent abiotic sedimentological signatures of carbon cycling.

Genesis of the Loma Galena Pb-Ag Deposit, Navidad District, Patagonia, Argentina: A Unique Epithermal System Capped by an Anoxic Lake

Abstract Loma Galena (978,852 t Pb, 206 Moz Ag) is one of eight epithermal deposits in the world-class Navidad Pb + Ag ± (Zn, Cu) district located in the Cañadón Asfalto continental foreland basin, northern Patagonia, Argentina. This basin formed during the Jurassic in an extensional tectonic regime during the breakup of Gondwana. Host rocks comprise major listric faulted and tilted blocks of K-rich andesite to dacite lava flows (173.9–170.8 Ma; U-Pb ages for zircon) unconformably overlain by mudstone interbedded with stromatolitic and pisolitic limestones, sandstone, coal, and an Sr-rich evaporite layer deposited in a lacustrine environment. The mineralization occurs as disseminations in the organic-rich sedimentary rocks, in veins and hydrothermal breccia dikes in the hanging walls and footwalls of NW- and NE-striking normal faults, in volcanic autobreccias, and in a phreatic breccia at the contact of volcanic and sedimentary rocks. The earliest hydrothermal minerals consist of veins of colloform, crustiform, and cockade calcite 1 (δ13Cfluid –4.7 to 0.8‰; δ18Ofluid 4.8–11.6‰) and siderite. The precipitating fluids were likely basement-exchanged basinal brines having salinities of 9.5 to 16.4 wt % NaCl equiv and temperatures of 154.7° to 212°C. The interaction of these fluids with the host volcanic rocks formed calcite, albite, adularia, and celadonite-glauconite-group minerals followed by chlorite and siderite as fO2 decreased. Fluids intermittently boiled, as evidenced by bladed (platy) texture in calcite 1. Subsequent mineralizing stages contributed to the metal endowment of Loma Galena. The abundance of organic-rich mudstone and δ34S from –15.4 to 12.9‰ for sulfides suggests that the bottom waters of the lake were anoxic and the loci of microbial sulfate reduction (evaporites have δ34S 35‰). Mixing of upflowing metal-rich basinal fluids carrying some S from depth with this H2S-rich connate water efficiently precipitated Ag-bearing framboidal pyrite, colloform pyrite-marcasite, chalcopyrite, bornite, tennantite-tetrahedrite, sphalerite, and galena as veins, breccias, and disseminations in host rocks. The highest grade and tonnage of the ores are found in autobreccias at the junction of the uppermost lava flow and in the overlying mudstone, where the addition of a strong microbial signature is recorded in sulfides. This event also led to partial dissolution of magmatic and hydrothermal feldspar and calcite 1 in the altered volcanic rocks. Mineralization was followed by hydrothermal brecciation and successive precipitation of chalcedony (δ18Ofluid 2.6–4.8‰), barite (δ34S 15.7–22‰; 160.9°–183.8°C; 7.7–9.7 wt % NaCl equiv), calcite 2 (δ18Ofluid –10.2 to –3.7‰, 58°–95°C; 1.9–7.0 wt % NaCl equiv), strontianite, and quartz in brecciated veins and breccias; kaolinite (δ18Ofluid 2–6.2‰), illite-smectite, smectite, and carbonates with minor chalcedony and barite in the volcanic rocks; and calcite, chalcedony, and barite in the sedimentary rocks. A trend of decreasing salinity with decreasing temperature and lowering δ18O of the fluids with time suggests dilution of the basinal fluids by mixing with Jurassic meteoric water (δ18O −9 to −5.2‰). Loma Galena is a unique example of a polymetallic epithermal system formed in a sublacustrine anoxic environment that promoted the efficient deposition and preservation of Ag-bearing sulfides, thereby contributing to the large size and relatively high grade of the deposit.

Illitization in the Mt. Simon Sandstone, Illinois Basin, USA: Implications for carbon dioxide storage

Clay mineral cementation is one of the most important controls on sandstone reservoir properties. In this paper, the diagenetic history of the Mt. Simon reservoir complex is studied to reveal the origin, timing, and controls of illitization. Samples of sandstone and shale from Mt. Simon reservoir complex were acquired from the Illinois Basin–Decatur Project (IBDP), a CO2 storage demonstration project in the central Illinois Basin. Petrographic, SEM, TEM, XRD analyses and K/Ar age dating were completed to identify the major detrital and diagenetic components of the samples and reveal illite to be the major clay component in all samples. Illitic clay coatings in the lower Mt. Simon reservoir are identified as a major control on reservoir properties by inhibiting major precipitation of authigenic quartz during illitization, resulting in highly permeable sandstone, essential for CO2 storage. The coating box-work morphology and mineralogy are indicative of a detrital smectite origin with subsequent illite growth associated with feldspar dissolution and kaolinite alteration. The mineralogy of bulk material and separate grain size fractions (2–0.6 μm; 0.6–0.2 μm; < 0.2 μm) with illite polytypes 2M1, 1M, and 1Md were quantified and age dated via 40K-40Ar methods. The shales or reservoir seals contain the highest proportions of detrital illite with illite in the lower Mt. Simon Sandstone reservoir identified as solely diagenetic. Two major events of illitization are identified throughout the Mt. Simon with more porous reservoir rock exhibiting the older event from approximately 360 to 315 Ma and tighter sandstone units with reservoir properties too low to be considered reservoir exhibiting illite dates from 250 to 220 Ma. This partitioning of illitization is attributed to varied mineralogy controlled by depositional changes and evolution of the greater basin.

Permeability and Porosity Changes in Sandstone Reservoir by Geothermal Fluid Reinjection: Insights from a Laboratory Study

Geothermal energy is a clean and environmentally friendly energy source that can be used sustainably; however, attention towards geothermal energy has been intermittent during the last 40 years as a function of the crisis of oil price. However, geothermal reinjection and clogging has been a challenge limiting geothermal development and utilization. In China, widely distributed sandstone geothermal reservoirs have reduced production due to technical constraints such as excessive reinjection pressure and blockage. In this paper, we took the Binzhou sandstone geothermal field in North China as an example and conducted displacement experiments under different temperature and flow rate conditions by collecting in situ geothermal fluid and core rock to obtain changes in sandstone permeability. By comparing the variation in geochemical and mineral composition of geothermal fluids and cores before and after the experiments, combined with a water–rock interaction simulation, we investigated the reasons for the changes in permeability and porosity. The results show that high temperature and low flow rate have relatively minimal displacement pressure, and a flow rate of 1.0 mL/min at 45 °C shows a minimal effect on permeability, while 1.0 mL/min at 55 °C and 0.5 mL/min at 45 °C show a minimal effect on porosity. Flow rate is the main factor controlling permeability, while temperature demonstrated a relatively minor effect. The shift in permeability and porosity is mainly caused by the precipitation of quartz and the conversion of albite to montmorillonite. The injection of fluids at 55 °C may have dissolved additional minerals with a minimal change in porosity. However, the permeability reduction at 55 °C is greater than that at 45 °C, indicating that the blockage, which led to the permeability reduction, contains multiple causes, such as chemical and physical blockages. From the laboratory studies, we recommended that reinjected geothermal water be cooled or kept below the reservoir temperature before reinjection and at moderate flow conditions.

Fluid Pressure Changes Recorded by Trace Elements in Quartz

AbstractFluid pressure is a key parameter in earthquake mechanics, controlling seismic failure and plate coupling in convergent zones. Yet fluid pressure is also extremely difficult to quantify at seismogenic depth, which limits our knowledge of the stress state in accretionary prisms. Here, we show that the geochemical record of exhumed hydrothermal quartz veins may be used to place quantitative bounds on fluid pressure variations in subduction zones. The studied veins come from sediments accreted and exhumed by plate convergence in southwestern Japan. Quartz in veins displays growth rims of contrasted bright blue/dark brown cathodoluminescence (CL) colors, high/low Al concentrations, and low/high fluid inclusion abundance. Because Si‐Al substitution (and charge compensation by Li) strongly depends on the rate of quartz precipitation and Si solubility, Al‐Li concentrations must be sensitive to fluid pressure. This is confirmed by fluid inclusions, the density of which, converted into trapping pressures, record fluid pressure drops by up to ∼70 MPa from CL‐brown, Al‐Li‐poor rims to CL‐blue, Al‐Li‐rich quartz rims. CL‐blue rims grow at a fast rate, high Si supersaturation and low fluid pressure whereas CL‐brown rims grow at a slower pace, lower Si supersaturation, and higher fluid pressure. Quartz trace element chemistry thus offers a promising tool to quantify deep fluid pressure variations and their relationships to earthquakes.

Quartz Vein Formation and Deformation during Porphyry Cu Deposit Formation: A Microstructural and Geochemical Analysis of the Butte, Montana, Ore Deposit

Abstract Hydrothermal quartz veins from the Butte deposit display euhedral and mottled cathodoluminescent (CL) textures that reflect the growth and deformation history of quartz crystals. A CL-euhedral texture consists of oscillatory dark-light zonations that record primary precipitation from an aqueous fluid. The origin of a CL-mottled texture, which consists of irregularly distributed dark and light portions, is less clear. Previous work showed that in some veins, CL-euhedral and CL-mottled crystals coexist, but the processes leading to their formation and coexistence were unknown. We find that CL-mottled crystals occur predominantly along the wall rock fracture surface and in vein centers and that CL-euhedral cockscomb quartz protrudes from the mottled layers along the wall rock. We infer that the mottled crystals formed by strain-induced recrystallization that was preferentially accommodated by the rheologically weaker layers of noncockscomb quartz because cockscomb crystals are in hard glide orientations relative to adjacent noncockscomb layers. During strain, crystals in noncockscomb layers that are not initially susceptible to slip can rotate in their deforming matrix until they deform plastically. Some of the CL-mottled crystals exhibit a relict CL-euhedral texture (“ghost bands”) whereby bright bands have been blurred and deformed owing to Ti redistribution facilitated by grain boundary migration. The edges of some CL-euhedral crystals become CL-mottled by localized grain boundary migration along adjacent crystals that do not align perfectly. Throughout the veins, CL-mottled crystals are randomly oriented, indicating that small deviatoric stresses were sufficient to drive recrystallization and mobilization of trace elements. Ti concentrations in CL-mottled crystals (23-47 ppm Ti; mean of 31 ppm) overlap those of CL-euhedral dark growth bands (16-40 ppm Ti; mean of 25 ppm Ti) in neighboring CL-euhedral crystals. Average Ti concentrations in CL-mottled quartz and CL-euhedral dark growth bands correspond to temperature estimates of 600°C (31 ppm Ti; CL-mottled) and 619°C (25 ppm Ti; dark bands), which are in good agreement with previous quartz precipitation temperature estimates based on independent thermobarometers. We conclude that recrystallization resets CL-mottled Ti concentrations close to the equilibrium value for the conditions of deformation and that CL-dark growth bands record near-equilibrium Ti concentrations. Recognition of widespread quartz recrystallization in porphyry Cu deposits underscores the significant role that strain plays in deposit formation. Individual veins host crystals that preserve conditions of primary growth and other crystals that preserve conditions of deformation and thermal overprint. Textural information is key to accurately interpreting trace element data and identifying different stages of vein formation. Our suggestion that CL-dark bands are the best candidates for near-equilibrium growth will aid the interpretation of trace element zoning in other hydrothermal systems.

Characteristics and Consequences of Red Bed Bleaching by Hydrocarbon Migration: A Natural Example From the Entrada Sandstone, Southern Utah

AbstractExtensive regions of yellow and white (“bleached”) sandstones within the terrestrial Jurassic red bed deposits of the Colorado Plateau reflect widespread interaction with subsurface reduced fluids which resulted in the dissolution of iron‐oxide grain coatings. Reduced fluids such as hydrocarbons, CO2, and organic acids have been proposed as bleaching agents. In this study, we characterize an altered section of the Slick Rock member of the Jurassic Entrada Sandstone that exposes bleached sandstone with bitumen‐saturated pore spaces. We observe differences in texture, porosity, mineralogy, and geochemistry between red, pink, yellow, and gray facies. In the bleached yellow facies we observe quartz overgrowths, partially dissolved K‐feldspar, calcite cement, fine‐grained illite, TiO2‐minerals, and pyrite concretions. Clay mineral content is highest at the margins of the bleached section. Fe2O3 concentrations are reduced up to 3× from the red to gray facies but enriched up to 50× in iron‐oxide concretions. Metals such as Zn, Pb, and rare‐earth elements are significantly enriched in the concretions. Supported by a batch geochemical model, we conclude the interaction of red sandstones with reduced hydrocarbon‐bearing fluids caused iron‐oxide and K‐feldspar dissolution, and precipitation of quartz, calcite, clay, and pyrite. Localized redistribution of iron into concretions can account for most of the iron removed during bleaching. Pyrite and carbonate stable isotopic data suggest the hydrocarbons were sourced from the Pennsylvanian Paradox Formation. Bitumen in pore spaces and pyrite precipitation formed a reductant trap required to produce Cu, U, and V enrichment in all altered facies by younger, oxidized saline brines.

Interactions between mineral evolution and organic acids dissolved in bitumen in hybrid shale system

Lamina, a common lithologic unit in shale, is the smallest unit for organic–inorganic fluid–rock interactions. The effects of lamina-scale diagenetic processes are superimposed to exert on lithofacies-scale diagenetic sequences, which further impose on the entire diagenetic system of a shale reservoir. Organic acids from kerogen maturation are indispensable components in diagenetic reaction. However, previous studies mainly focused on low molecular weight (LMW) water soluble acids, few studies have paid attention to the role of high molecular weight (HMW) carboxylic acids dissolved in fluid–rock reactions. Herein, by taking the shale of the seventh member of the Upper Triassic Yanchang Formation in the Ordos Basin, China, as a case study, we investigate the mutual impacts between mineral alteration and HMW organic acid degradation. The characterization is carried out at two scales: lamina-scale (organic-rich clay-rich lamina and organic-lean silty lamina) and lithofacies-scale (shale and sandstone). Integrated analytical techniques (microdrill technique, thin section observation, X-ray diffraction, advanced mineral identification and characterization system, Rock-Eval pyrolysis, Fourier transform ion cyclotron resonance mass spectrometry) are performed. Results show that, in reservoirs of both silty lamina and sandstone, consumption of LMW aqueous acids through mineral transformation reactions promotes the forward evolution of HMW carboxylic acids in the bitumen phase toward decarboxylation or dehydration. During diagenesis, short chain fatty acids (SCFAs) are more easily broken down into small acids and, thus, more susceptible to be consumed. The lamina-scale reservoirs (silty lamina) have the privilege to receive abundant aqueous organic acids to maintain the micro-geochemical environment and progressively boost organic–inorganic fluid–rock interactions. The resultant dissolution pores not only accumulate plenty of bitumen but also an appreciable amount of bitumen dissolved acids. Because of the strong storage capacity of silty lamina within lithofacies-scale source rock (shale), fluids are less readily to expel out to lithofacies-scale reservoir (sandstone). As a result, the sandstone may not receive enough fluids from shale to maintain a consistent diagenetic environment, resulting in the observed discontinuous diagenesis of quartz precipitation transition to quartz dissolution.

Simultaneous Measurements of Elastic Wave Velocity and Porosity of Epidosites Collected From the Oman Ophiolite: Implication for Low VP/VS Anomaly in the Oceanic Crust

AbstractGeophysical surveys of the oceanic crust indicate that hydrothermal circulation universally occurs in seismic layer 2, which results in a low seismic velocity and high VP/VS due to the occurrence of cracks. However, the anomalously low VP/VS observed at the layer 2/3 transition cannot be explained by the crack model, because the effective medium theory predicts an increase in VP/VS due to crack development. In this study, we present the first evidence that shows the low VP/VS in the oceanic crust is caused by epidotization due to upward fluid flow in hydrothermal systems. Simultaneous measurements of elastic wave velocity and porosity of epidosites collected by the Oman Drilling Project show that quartz precipitation and spheroidal pores results in low VP/VS, in contrast to diabases that contain thin cracks. The presence of spheroidal pores in epidosites is supported by CT imaging, and is consistent with predictions from the effective medium model.

Grain assemblages and diagenesis in Ordovician-Silurian transition shale deposits of the Upper Yangtze Platform, South China

• Grain assemblages in shale establish a key starting condition for diagenetic pathway. • There are four origins of quartz and two origins of organic matter in marine shale. • Cementation and mechanical compaction make a significant reduction of porosity. • Shale with different lithfacies has differences in pore types. • Authigenic quartz cementation made porosity reduction but enhanced the brittleness . This paper considers possible impacts of mineralogy and organic matter contents and assemblages on the diagenetic history, pore structure, and brittleness of the Ordovician-Silurian Wufeng Formation and Longmaxi Formation (Northwestern Hunan, upper Yangtze Platform) of South China. Multiple analytical approaches, including X-ray diffraction, scanning electron microscopy, cathodoluminescence, and X-ray mapping, were utilized to identify favorable shale gas intervals. Analyzed shale samples of the Wufeng-Longmaxi succession are dominated by such extrabasinal components as quartz, K-feldspar, albite, lithic fragments, and K-rich and Mg-rich clay minerals. Subordinate components include siliceous tests and organic particles. Authigenic minerals present as grain replacements and pore-fillings include quartz, illite, pyrite, and carbonate minerals. Depositional and diagenetic organic matter, which are discriminated petrographically, are present in the studied deposits. Wufeng Formation samples are enriched in organic matter (mainly of diagenetic origin) and authigenic quartz. The dominant types of porosity of the Wufeng samples include pores hosted by organic particles and interparticle pores between authigenic quartz crystals. The precipitation of authigenic quartz appears to have been the principal means of porosity reduction and enhanced the brittleness of the shale. Analyzed samples of the Longmaxi Formation are dominated by extrabasinal grains and lesser authigenic minerals. Reduction of depositional porosity of these deposits appears to have been largely the result of physical compaction. Thus, extensive authigenic quartz cementation and thermal maturation of organic matter of the Wufeng Formation likely enhanced the reservoir capacity and fracturing ability of these deposits relative to the overlying Longmaxi Formation. This understanding is especially relevant to shale gas exploration and production considerations of organic-rich fine-grained sediments of similar sedimentological background.

Influences of SO2 contamination in long term supercritical CO2 treatment on the physical and structural characteristics of the Zululand Basin caprock and reservoir core samples

Understanding the mineral and microstructure changes in sandstone samples during supercritical CO2 gas treatment is an important aspect of geological CO2 sequestration. To gain insight into the effects of SO2 contamination in CO2 in South Africa, two core samples extracted from the reservoir lateral seal (ZC) and Cenomanian sandstone aquifer (ZG) within the Zululand Basin were studied. The samples were treated with CO2 and CO2/SO2 gas streams at typical reservoir temperature and pressures (10 MPa and 316 K, and 17,5 MPa and 346 K) under supercritical conditions. High pressure Parr reactors were used for storing the rock-gas-water mixtures for up to two months using a water:rock ratio of 23:1. Tests were conducted using pure CO2 and a 99% by weight (wt.) CO2 and 1% (wt.) SO2 mixed gas stream. Due to the prohibitive costs associated with CO2 purification, the knowledge of the consequences of key impurities relevant to geological sequestration is critical. Pre-and post- CO2/CO2–SO2 treatment characterisation was conducted using X-ray Diffraction (XRD) Analyses, Fourier transform infrared spectroscopy (FTIR), and low-pressure gas adsorption (LPGA). Varying mineral alterations were observed in the CO2 treated samples, mainly comprising of calcite, plagioclase and smectite dissolution and the precipitation of quartz, plagioclase, calcite and smectite. Dissolution pores and pore clogging were observed and increased microstructure heterogeneity was reported. Increases in the adsorption capacity, surface area and pore volume were observed in all samples. After treatment with CO2–SO2 gas mixture, increases in mineral reactivity were observed in the ZC sample along with gypsum precipitation, indicating potential improvement in the lateral seal's self-sealing capacity. The introduction of SO2 lead to the increase in quartz and plagioclase precipitation and increased dissolution of smectite and stilbite in the ZG sample. The study presents a novel investigation of the changes expected to take place during CO2 injection in sandstone basins.

Geochemical and physical alteration of clay-rich shales under supercritical CO2 conditions

The injection of Carbon Dioxide (CO2) into shale gas formations is a promising approach that not only reduces the impact of greenhouse gas on climate change but also enhances the gas recovery due to geochemical interactions between CO2 and clay minerals. However, these geochemical interactions have not been fully explored and changes in the petrophysical properties of shales have been reported over time. This paper evaluates the geochemical reactions and physical changes caused by the injection of supercritical carbon dioxide (ScCO2) on the properties of clay-rich shales. Samples from the Eagle Ford formation were collected and exposed to ScCO2 at different pressures ranging from 10 to 24 MPa at 70 °C. Analytical methods such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thin-section microscopy were used to characterize the ScCO2-treated samples. The results showed dissolution of clays and precipitation of quartz with increasing ScCO2 treatment pressure. The content of carbonate minerals was also reduced at high pressure, which can be attributed to the reactions of the dolomite with H+ to form magnesium carbonate. The percentage of absorption of aromatic hydrocarbons and oxygenated groups gradually increased with increasing pressure, which can be attributed to the increase in CO2 adsorption. On the other hand, the absorption of aliphatic and hydroxyl groups decreased after treatment. ScCO2 treatment pressure is an important factor to evaluate the CO2 adsorption capacity of clay-rich shales. The presented results enrich the understanding of the interactions between CO2 and shale under different pressures, which may be helpful to determine the feasibility of long-term injection and storage in shale.

Diagenetic evolution of the lower Yaojia Formation of Songliao Basin, China: Impact on reservoir quality

The oil-bearing layer in the first member of the Upper Cretaceous Yaojia Formation (K2y1) in the middle of the Changling Sag of the southern Songliao Basin, northeast China, has low-permeability and is deemed to be an unconventional reservoir. Therefore, better evaluation of reservoir quality and related controlling factors are crucial for the reduction in exploration risk and sustainable development. The current study integrates diagenesis, sequence stratigraphy, and sedimentary facies to decipher their influence on reservoir quality using core and thin section observations, X-ray diffraction, scanning electron microscopy, fluid inclusion, and carbon and oxygen isotope analyses. The K2y1 is composed of shallow delta sediments of transgressive system (TST) and lowstand system (LST) tracts. The depositional microfacies of the sandstones include TST deltaic front underwater distributary channels and sheet sands, and LST deltaic plain distributary channels, distributary channel intersections, and crevasse splays. The sandstones are dominantly fine-grained moderately sorted feldspathic litharenites and lithic arkoses with silt. Early diagenetic events included compaction, development of smectite, and alteration of feldspar and kaolinite whereas mesogenetic alteration included feldspar dissolution, precipitation of quartz and carbonate cements, and clay mineral alteration. The reservoir quality of the K2y1 sandstones is generally poor, exhibiting variations among different deltaic depositional facies within the LST and TST. The (underwater) distributary channel (intersection) sandstones, particularly those of the LST, have relatively higher reservoir quality owing to their larger grain size, better sorting, lower carbonate cement content, mixed-layer I/S and illite, and greater occurrence of chlorite.

The Role of Quartz Cementation in the Seismic Cycle: A Critical Review

AbstractBecause quartz veins are common in fault zones exhumed from earthquake nucleation temperatures (150°C–350°C), quartz cementation may be an important mechanism of strength recovery between earthquakes. This interpretation requires that cementation occurs within a single interseismic period. We review slip‐related processes that have been argued to allow rapid quartz precipitation in faults, including: advection of silica‐saturated fluids, coseismic pore‐fluid pressure drops, frictional heating, dissolution‐precipitation creep, precipitation of amorphous phases, and variations in fluid and mineral‐surface chemistry. We assess the rate and magnitude of quartz growth that may result from each of the examined mechanisms. We find limitations to the kinetics and mass balance of silica precipitation that emphasize two end‐member regimes. First, the mechanisms we explore, given current kinetic constraints, cannot explain mesoscale fault‐fracture vein networks developing, even incrementally, on interseismic timescales. On the other hand, some mechanisms appear capable, isolated or in combination, of cementing micrometer‐to‐millimeter thick principal slip surfaces in days to years. This does not explain extensive vein networks in fault damage zones, but allows the involvement of quartz cements in fault healing. These end‐members lead us to hypothesize that high flux scenarios, although more important for voluminous hydrothermal mineralization, may be of subsidiary importance to local, diffusive mass transport in low fluid‐flux faults when discussing the mechanical implications of quartz cements. A renewed emphasis on the controls on quartz cementation rates in fault zones will, however, be integral to developing a more complete understanding of strength recovery following earthquake rupture.

The genesis of authigenic minerals and the porosity evolution of various lithologies and their implications for identifying high-quality reservoirs in the fourth member of Xujiahe Formation (Northeast-Sichuan Basin, China)

With petroleum exploration extending to deeper reservoirs, it becomes more and more difficult to find relatively high-quality reservoirs in heterogeneous sandstones. This research integrates fluid inclusion analysis, petrographic, mineralogical, and geochemical data to determine the origin of authigenic minerals, establish the porosity evolution model and discuss diagenetic effects on reservoir quality of the fourth member of the Upper Triassic Xujiahe Formation. The main rock types of studied sandstones are identified by thin-section, SEM and XRD analysis-feldspathic litharenites, litharenites, calcareous sandstone and chlorite-rich sandstone. Of the four types of sandstone, reservoir porosity of the chlorite-rich sandstone is generally the highest. Target sandstones have experienced complicated diagenetic histories including mechanical compaction, three stages of carbonate cementation, the dissolution of K-feldspar or rock fragments, three kinds of authigenic chlorite precipitation, and two stages of quartz precipitation. Two reaction pathways between K-feldspar and kaolinite were determined and limited dissolution was generated in the closed diagenetic system. Diagenetic sequence and porosity evolution suggest that the critical factors in reducing reservoir quality can be attributed to early intense compaction and widespread calcite cementation. The genesis of high-quality reservoirs among the various lithologies is different. In feldspathic litharenites and litharenites, the key factors for the development of relatively high-quality reservoirs is the acidic dissolution of K-feldspar and less cement being present in the rock. Because chlorite helps retain primary pores by inhibiting quartz overgrowths, almost all chlorite-rich sandstone are relatively high-quality reservoirs. In general, the reservoir quality of calcareous sandstone is poor unless fractures exist that can improve permeability. Determining the porosity evolution of various lithologies is an effective method to predict relatively high-quality reservoirs, which can provide a useful reference for deep petroleum exploration in other areas.

Origin of Quartz Cement in the Paleogene Sandstone Reservoir of Shahejie Formation, Bohai Bay Basin, China

AbstractThe precipitation of authigenic quartz plays a significant role to reduce the reservoir characteristics and enhance the stiffness of the rock. The Es1 sandstone of Shahejie Formation is acting as a significant hydrocarbon producing rock in the Nanpu Sag. Various methods like thin section petrography, cathodoluminescence (CL), scanning electron microscope (SEM, with EDS), and electron microprobe analysis has been used to reveal the origin of quartz cement as well as to evaluate the effect of quartz cement on reservoir quality. The studied sandstone is classified as immature to mature feldspathic litharenite and lithic arkose and consists of quartz, feldspar, rock fragments and micas. Petrographic studies and SEM analysis shows that the authigenic quartz is acting a significant cement that reduces the reservoir quality. Whereas clay minerals (kaolinite and mixed layer illite to smectite) are dominant in the Es1 sandstone, that can reduce the reservoir quality. SEM, CL and thin section analysis reveal that there are two stages of quartz cement in the studied samples; that are pore filling authigenic cement and quartz overgrowth cement. Fluid inclusion homogenization temperatures shows that stages of quartz cement were developed with continuous process from 70°C to 130°C. Quartz cements were generally originated from I/S reaction, feldspar dissolution, conversion of rock fragments and pressure solution. Feldspar dissolution (K‐feldspar) and kaolinite to illite reaction is an insignificant silica source for the silica cement which is internally precipitated in a close system with diffusion transporting mechanism. Overall, quartz cement significantly enhance the rock strengthen and brittleness effectively as well as it reduce the overall reservoir quality.

Occurrence and origin of chlorite and associated impact on tight sandstone reservoir quality: A case study of the Xujiahe Formation (NE Sichuan Basin, China)

Chlorite is beneficial to the preservation of primary pores, and its formation mechanism in marine sandstone has been widely reported. However, few studies have focused on the genetic mechanism of various types of chlorite in lacustrine deltaic sandstone. Therefore, the occurrence, diagenetic time and origin of chlorite in Xujiahe tight sandstone were investigated using microscopic images and mineralogical chemical analysis, and three stages of chlorite were identified, namely, grain-coating (Ch1), pore-lining (Ch2), and pore-filling (Ch3) chlorite, which belong to the Fe chlorite, Al chlorite and transitional types, respectively. The variations in morphology and chemical composition reflect differences in the sources and formation mechanisms. Chlorite sandstones are mainly distributed in the underwater distributary channel, which has strong hydrodynamic conditions. The formation of Ch1 is controlled by the sedimentary environment and flocculation. Salinity differences resulted in a larger amount of flocculation deposits in the estuaries during the syngenetic period. In the shallow burial stage, the clay coating underwent in situ transformation under thermal impacts and formed the Ch1. Xujiahe sandstones experienced complex diagenetic processes, which include early compaction, three stages of chlorite cementation, two stages of quartz precipitation, dissolution and two stages of carbonate cementation. Organic acid dissolution of volcanic rock fragments, phyllite, and biotite likely supplied sufficient Fe 2+ for the growth of Ch2 in a relatively closed diagenetic system. As the pore water became weakly alkaline, diagenetic fluid migration originating from the clay mineral transformation in mudstone mainly provided Fe 2+ and Mg 2+ for Ch3 growth in sandstones. Chlorite is considered to have “composite” impacts on deep tight sandstones, with grain-coating and pore-lining chlorites protecting the primary pores by restricting the silica supply and occupying potential quartz nucleation sites; however, grain-coating chlorite caused throat obstruction or even closure under strong mechanical compaction, which reduced the permeability of the reservoir. • Three different types of chlorite have been found in the Xujiahe tight sandstones. • It has been clear that the formation time of chlorite and the paragenetic sequence of authigenic minerals. • The origin and formation mechanism of authigenic chlorite in three types have been investigated. • We considerd that authigenic chlorite has a “composite” impact on deep tight sandstone: preserve primary porosity but reduce the permeability.

Origin of authigenic kaolinite with implications for Permian tight gas sandstone reservoirs in the northern Ordos Basin, central China

This study deciphers the origin of authigenic kaolinite in the lower unit of Permian Shihezi Formation (P2x) tight gas sandstones in the Hangjinqi gas field, northern Ordos Basin, China, to determine its implications for reservoir quality. Authigenic kaolinite is the most abundant diagenetic cement and two occurrences of authigenic kaolinite are identified in the P2x sandstones: 1) in-situ replacive kaolinite; and 2) pore-filling kaolinite. Petrographic and geochemical evidence reveals that authigenic kaolinite is mainly derived from dissolution of K-feldspar via flushing with organic fluids from underlying source rocks. This evidence includes: 1) spatial association of partially dissolved K-feldspar and authigenic kaolinite; 2) the amount of K-feldspar dissolution is mostly equal to that of kaolinite precipitation within sandstones; 3) a homogenization temperature of 110–120 °C for aqueous inclusions in quartz overgrowths; 4) negative stable carbon isotopic compositions of calcite cements (δ13CPDB from −16.05‰ to −9.35‰). Dissolution of K-feldspar, coupled with precipitation of kaolinite and quartz, is interconnected fluid-rock interactions within sandstones that are inferred to simultaneously occur in a relatively closed system. Intra-formational diffusive transport is inferred to be the dominant transport mechanism during these fluid-rock interactions, which may result in precipitation of in-situ replacive kaolinite or pore-filling kaolinite. This transport pattern may have been localized (several centimeters to meters). The fluid-rock interactions lead to redistributional secondary porosity and would not significantly enhance total reservoir porosity. The occurrence of authigenic kaolinite greatly reduced reservoir permeability and gas saturation by occluding the pore-throats in the P2x sandstones.

Influence of CO2 migration from geological storage on the chemical composition of groundwater and monitoring indicators

In the process of CO2 geological sequestration, there is a risk that CO2 will leak into groundwater, resulting in a series of physical and chemical reactions, with influence on chemical compositions of groundwater. In this work, numerical simulation is conducted to study the influence of CO2 migration on the chemical composition of groundwater. The modeling results indicated that when CO2 leaks into groundwater, gaseous CO2 will migrate upward and diffuse laterally under the effect of buoyancy. The acidity of groundwater is enhanced and the pH is significantly reduced, leading to dissolution of calcite, illite, oligoclase, K-feldspar, chlorite and hematite, and precipitation of quartz, kaolinite, smectite-Na, siderite and smectite-Ca. The porosity of the formation increases. The concentrations of K+ and Fe2+ in groundwater gradually increase, the concentrations of Ca2+ and HCO3− basically remain unchanged after increasing to a certain value, the concentrations of Mg2+ and AlO2− first increase and then decrease, and the concentrations of Na+, Cl− and SO42− first decrease slightly and then gradually increase, but the change in concentration is small. Therefore, Fe2+, Mg2+, Ca2+ and pH can be used as important monitoring indicators of whether CO2 leakage into groundwater during geological storage.

Diagenesis and reservoir quality of Cambrian carbonates in the Tarim Basin, northwestern China

Quantitative petrographic analysis revealed the diagenetic controls on the reservoir quality of deep (>5,000 m) carbonate reservoirs in the Tarim Basin. Dolostones were formed by the extensive dolomitization of originally low-energy (siliciclastic mudrocks, calcimudstones, and wackestones) and moderate-to-high-energy (peloidal, intraclastic, and/or bioclastic packstones to grainstones, rarely microbialites) deposits. Replacement of primary and diagenetic constituents by dolomite occurred in multiple phases. Eogenetic dolomitization precipitated microcrystalline to finely crystalline dolomite, followed by dolomite overgrowth during shallow mesogenesis. Saddle, coarse, and radial undulose dolomite precipitated during deep mesogenesis under increasing temperature and pressure and were locally replaced by microcrystalline quartz. Primary porosity is absent, but these carbonates can display vuggy, intercrystalline, and fracture porosity due to dolomite dissolution and local fracturing. Secondary porosity can be partially or totally filled by saddle dolomite and/or quartz. Petrological and fluid-inclusion data indicated that three ascending fluids considerably influenced the diagenetic evolution and quality of the Cambrian reservoirs, namely, organic acid-rich fluids from the maturation of source rocks, SO2-rich fluids derived from the dissolution of calcium sulfate and hydrothermal fluids. The structurally controlled percolation of SO2-rich fluids was the main process responsible for creating porosity in deep carbonate reservoirs in the Tarim Basin. The influence of hydrothermal fluids is not ubiquitous, and it had a dual impact on reservoir quality, i.e., the hydrothermal fluids enhanced the porosity through dolomite dissolution and decreased the porosity through dolomite and quartz precipitation.