Chlorite In Sandstones Research Articles

Formation mechanism of authigenic chlorite in sandstones of the Middle Jurassic Sha 1 Member in the northeastern central Sichuan Basin, China

Formation mechanism of authigenic chlorite in sandstones of the Middle Jurassic Sha 1 Member in the northeastern central Sichuan Basin, China

Improved imaging and analysis of chlorite in reservoirs and modern day analogues: new insights for reservoir quality and provenance

Abstract Chlorite is a key mineral in the control of reservoir quality in many siliciclastic rocks. In deeply buried reservoirs, chlorite coats on sand grains prevent the growth of quartz cements and lead to anomalously good reservoir quality. By contrast, an excess of chlorite – for example, in clay-rich siltstone and sandstone – leads to blocked pore throats and very low permeability. Determining which compositional type is present, how it occurs spatially, and quantifying the many and varied habits of chlorite that are of commercial importance remains a challenge. With the advent of automated techniques based on scanning electron microscopy (SEM), it is possible to provide instant phase identification and mapping of entire thin sections of rock. The resulting quantitative mineralogy and rock fabric data can be compared with well logs and core analysis data. We present here a completely novel Quantitative Evaluation of Minerals by SCANning electron microscopy (QEMSCAN®) SEM–energy-dispersive spectrometry (EDS) methodology to differentiate, quantify and image 11 different compositional types of chlorite based on Fe : Mg ratios using thin sections of rocks and grain mounts of cuttings or loose sediment. No other analytical technique, or combination of techniques, is capable of easily quantifying and imaging different compositional types of chlorite. Here we present examples of chlorite from seven different geological settings analysed using QEMSCAN® SEM–EDS. By illustrating the reliability of identification under automated analysis, and the ability to capture realistic textures in a fully digital format, we can clearly visualize the various forms of chlorite. This new approach has led to the creation of a digital chlorite library, in which we have co-registered optical and SEM-based images, and validated the mineral identification with complimentary techniques such as X-ray diffraction. This new methodology will be of interest and use to all those concerned with the identification and formation of chlorite in sandstones and the effects that diagenetic chlorite growth may have had on reservoir quality. The same approach may be adopted for other minerals (e.g. carbonates) with major element compositional variability that may influence the porosity and permeability of sandstone reservoirs.

Further research on chlorite rims in sandstones: evidence from the Triassic Yanchang Formation in the Ordos basin, China

Chlorite rims are the most common type of authigenic chlorite in sandstones, and attract particular attention because they are always found in high-quality sandstone reservoirs. This study researches chlorite rims in sandstones of the Chang 9 oil-bearing member of the Triassic Yanchang Formation in the Ordos basin, China. Using casting thin sections, a scanning electron microscope and porosity–permeability data, the following issues were addressed: (1) the formation stage of chlorite rims, (2) the relationship of chlorite rim formation to other diagenetic features, and (3) the relationship of chlorite rims with sandstone reservoir quality. From the results, the following conclusions can be drawn: (1) chlorite rims form during the A–B phase of eodiagenesis, (2) chlorite rims cannot enhance the mechanical strength of rocks and their ability to resist compaction, (3) the reason that chlorite rims inhibit quartz secondary enlargement is a change in pH and silica concentration leading to the cessation of quartz growth. However, chlorite rims have no relationship with the formation of later idiomorphic authigenic quartz; (4) chlorite rims cannot enhance the reservoir quality of sandstone, but can be an indicator of high primary intergranular porosity.

Chlorite cement and its effect on the reservoir quality of sandstones from the Panyu low-uplift, Pearl River Mouth Basin

Based on porosity and permeability measurements, mercury porosimetry measurements, thin section analyses, SEM observations, X-ray diffraction (XRD) analysis and granulometric analyses, diagenetic features of reservoir sandstones taken from the Zhuhai formation in the Panyu low-uplift of the Pear River Mouth Basin were examined. This study shows that chlorite cements are one of the most important diagenetic features of reservoir sandstones. The precipitation of chlorite was controlled by multiple factors and its development occurred early in eo-diagenesis and continued till Stage A of middle diagenesis. The precipitation of chlorite at the early stage was mainly affected by the sedimentary environment and provenance. Abundant Fe- and Mg-rich materials were supplied during the deposition of distributary channel sediments in the deltaic front setting and mainly in alkaline conditions. With the burial depth increasing, smectite and kaolinite tended to be transformed into chlorite. Smectite cements were completely transformed into chlorite in sandstones of the studied area. Volcanic lithics rich in Fe and Mg materials were dissolved and released Fe2+ and Mg2+ into the pore water. These cations precipitated as chlorite cements in middle diagenesis in an alkaline diagenetic environment. Chlorite coatings acted as porosity and permeability, thus helping preserve cements in the chlorite cemented sandstones. The reservoir quality of chlorite cemented sandstones is much better than sandstones without chlorite cements. Chlorite cements play an important role in the reservoir evolution that was mainly characterized by preserving intergranular porosity and forming better pore-throat structures of sandstones.

Origin and Growth Mechanism of Authigenic Chlorite in Sandstones of the Lower Vicksburg Formation, South Texas

ABSTRACT Authigenic chlorite, occurring as grain coatings, pore fillings, and rosettes, is common in deltaic sandstones of the lower Vicksburg Formation in South Texas. Chlorite rosettes are isolated and occur late in the diagenetic history, whereas grain-coating and pore-filling chlorite occur early, predating the development of secondary porosity and the precipitation of quartz overgrowths and Fe-poor calcite. Grain-coating and pore-filling chlorite, which are interstratified with approximately 15 percent 7 A layers, are Fe-rich with a mean Fe/(Fe+Mg) ratio of 0.68. Stable-isotope data (18O averages 13.5o/oo (SMOW)) indicate that the grain-coating and pore-filling chlorite began precipitating between 20°C to 40°C within the upper 1.8 km (6000 ft) of burial. Sandstones of the lower Vicksburg Formation were deposited in a relatively shallow deltaic environment as part of a large fluvial system draining the volcanics of Sierra Madre Occidental, Mexico and Trans Pecos, Texas. Weathering of these volcanics brought large amounts of particulate Fe to the river mouth. An increase in salinity in the distributary channel complex caused Fe flocculation and deposition. During periods of high river discharge, or when coastal currents acted to concentrate particulate Fe, large amounts of particulate Fe were deposited in the delta system. This resulted in thick layers of Fe-rich clay (odinite?) around grains. During periods of low river discharge, or when coastal currents caused wide distribution of particulate Fe, less particulate Fe was deposited. It is proposed that the grain-coating and pore-filling chlorite began forming through the transformation of an Fe-rich clay precursor in the formation-water hydropressure zone following Ostwald processes. The presence of this synsedimentary Fe-rich clay, and its recrystallization to Fe-chlorite, strongly influenced fluid flow and played a large role in developing the diagenetic heterogeneity seen in lower Vicksburg sandstones today.

Formation of grain-coating chlorite in sandstones. Laboratory synthesized vs. natural occurrences

AbstractGrain-coating chlorites in clastic quartz-rich sandstones have long been recognized as an important porosity-preserving constituent in medium- to deep-burial diagenesis. As little is known about the occurrence and origin of chlorite coatings, chlorite synthesis experiments were performed to study how grain-coating chlorites form in certain sandstones during burial. The starting material was naturally-occuring sandstones from the Oseberg and the Veslefrikk fields offshore Norway, where the same sandstone formation is buried to different depths due to faulting. Grain-coating chlorites exist below ~3000 m burial depth only. At shallower burial (2400 m), an X-ray amorphous iron containing thin clay coating is present.The samples were heated to 200 and 250°C (at water vapour pressure) in a hydrothermal bomb for 2–4 weeks. Both starting material and end-products were studied (electron-) optically in both scanning and transmission microscopes. The TEM showed the Fe-rich precursor material to consist of a fine-grained berthierine-dominated mixed-layer. The neoformed grain coatings in the reacted samples were similar in appearance to naturally-occurring chlorite coatings. The TEM analyses of individual grains documented an Fe-rich chloritic phase with an average composition of Mg0.41Fe3.52Mn0.10Al1.51(Al0.58Si3.42)O10(OH)8. The reacted waters were found to be close to saturation with the newly formed chlorites.Grain-coating chlorite thus appears to form in the natural environment from Fe-rich berthierine precursors at a burial depth corresponding to a temperature around 90°C.

Authigenic Chlorites in Sandstones as Indicators of High-temperature Diagenesis, Arkoma Foreland Basin, USA

ABSTRACT Abundant authigenic chamosite associated with higher-than-average porosities is present in a deep gas reservoir (Spiro sandstone) of the Arkoma Basin, east-central Oklahoma. Three types of chlorite can be distinguished petrographically, all of which appear texturally to have formed early: chlorite peloids, diffuse matrix, and grain coatings. X-ray diffraction, scanning electron microscopy, and electron microprobe studies show that most chlorites are composed of two distinct polytypes, Ib (s = 90°) and IIb. A third structure, Ia polytype, was identified in only one sample. The relative percentage of the high-temperature IIb structure increases gradually with increasing thermal maturity, from = 40% at 3.5% R SUB>o. IIb chlorite forms rather thick, blocky crystals distinct from the thin, pseudohexagonal plates typical of low-temperature Ib chlorite. Temperature estimates based on data on vitrinite reflectance and fluid inclusions suggest that IIb chlorite formed at burial temperatures >= 150-180°C. Higher contents of tetrahedral Al3+ and slightly higher Fe/[Fe+Mg] ratios in IIb chlorite are consistent with precipitation temperatures higher than those of the Ib structure. A higher-temperature origin for the IIb structure is also consistent with oxygen isotope data.

Approach to chemical equilibrium in diagenetic chlorites

Electron microprobe analyses were made on diagenetic chlorites in sandstones and mudstones from two deep wells according to the petrographic character of the chlorite occurrence: as pseudomorphic phases, rims on quartz or glauconite or as distinct phases in the clay matrix. Chlorite compositions do not depend upon crystallization site (reacting phases) making it apparent that new chlorites can form in an approach to chemical equilibrium at or near the surface (40° C, 1 km depth). Comparison of this data with that for late diagenetic and early metamorphic chlorites indicates that the compositional range for different grains in the same thin section is similar for the samples throughout the 40°–270° C temperature span. Compositional range decreases upon further metamorphism. Al content appears to be a more reliable indicator of temperature variations than other substitutions in the chlorite structure. The clay mineral assemblage which indicates sedimentary facies affects the trends in composition (Al increase or decrease) as a function of temperature. The octahedral site occupancy show a general increase in going from diagenesis to metamorphic conditions in pelitic rocks. The range of Fe-Mg ratios seems to depend more on the chemistry of each sample than the temperature of formation of the minerals.

Diagenetic chloritization of feldspars in sandstones

Detrital microcline and albite are commonly replaced diagenetically by chlorite in sandstones of the Visingsö Group (Upper Proterozoic, southern Sweden). Authigenic quartz and, in some cases, albite and illite are associated with the chlorite. The SEM examinations suggest that chloritization of feldspar has usually been preceded by dissolution of the replaced feldspar grain. Fe 2+ and Mg 2+, which are needed for the chloritization, have partly been derived from the detrital Fe-Mg silicates (mainly biotite) Fe-oxide pigment and Fe-Ti oxide grains, whereas Si 4+ and Al 3+ are supplied mostly by the altered feldspars. The K + released from microcline and Na − from albite were mostly incorporated in authigenic illite and albite, respectively.