Authigenic Quartz Overgrowths Research Articles

Diagenetic controls on the quality of shallow marine sandstones: An example from the Cambro-Ordovician Saq Formation, central Saudi Arabia

The Saq Formation is the most important Paleozoic aquifer in central and northern Saudi Arabia. However, owing to the continued depletion of the aquifers and the great depths to which the aquifers are buried in undrilled/underexplored regions, the aquifers porosity and permeability are increasingly becoming an issue of concern. Here, we employed thin-section petrography, petrophysical measurements, X-ray diffraction and scanning electron microscopic analyses, and fluid inclusion microthermometry to investigate the composition and diagenetic evolution of the Saq sandstones. The results of this study indicate that the sandstones are mainly quartz arenite in composition, mostly well sorted, and ranged in grain size from silt to coarse-grained sandstones. We observed three diagenetic stages (early, middle, and late) that impacted the porosity evolution of the Saq sandstones. Mechanical compaction, formation of authigenic kaolinite, and infiltration of smectitic clays occurred during early shallow burial diagenesis. The middle (deep-burial) diagenesis was dominated by mechanical and chemical compaction, formation of authigenic quartz overgrowths, transformation of kaolinite and smectite into illite, and precipitation of late calcite. The late (uplift-related) diagenesis resulted in the formation of authigenic kaolinite, dissolution of the late calcite, and the formation of predominantly pore-filling Fe-oxide (goethite) cement. Fluid inclusion results of this study show that quartz cement mostly formed at temperatures ranging between 151 °C and 175 °C and reached up to 272 °C, with, at least, two phases of overgrowth development. The overall high amounts of rigid grain contents (>65 %), low detrital matrix (<10 %), and coarser grain size have maintained the Saq sandstone quality (average permeability = 2100 mD) despite having been buried to great depths. The findings of this study provide useful insights into the diagenetic evolution of the Saq Formation, which has provided important input data for forward diagenetic modelling of their subsurface equivalent in Saudi Arabia and sandstones of similar depositional settings elsewhere.

Ezeaku Formation Calcarenaceous Sandstone Facies at Abini, Cross River State, Nigeria

Mineralogical and paleocurrent studies of Amasiri Sandstone, a member of Ezeaku Group outcropping as a ridge in Abini were done to determine the quantitative values of component minerals for proper designation of the facies of sandstone and the direction of paleocurrent. Sampling, paleocurrent measurements, petrographic and X-ray diffraction (XRD) analyses were employed to achieve results. Three highly indurated beds of white fine – medium, angular - subrounded grained sandstones were observed. The basal and second beds had thin laminations, whereas the third was massive. The sandstone is found to be hybrid facies, it is not a quartz arenite neither a calcareous sandstone. Calcite (CaCo3) is the most abundant mineral ranging from 43% - 66%, with an average of 51.39%. Calcite is seen occurring as skeletal grains and sparry calcite, corroding, and replacing quartz grains. Quartz (SiO2) ranges from 3% - 37.6% with an average of 17.22%, occurring as detrital quartz and authigenic quartz overgrowth. Monocrystalline and polycrystalline quartz occur, with monocrystalline dominating. Feldspars such as albite, plagioclase, sanidine and orthoclase occur, with the potasssic feldspars dominating. Silica, calcite, and iron oxide were the observed cementing materials. The paleocurrent direction is in the SE. The angularity of the grains implies a source area with close proximity. Therefore, the source area could be inferred to be the Oban Massif or and the Cameroun Mountains.

Poro-perm evolution in Oligo-Miocene coastal sandstones: Constraining the relative influence of sedimentary facies, mineralogy, and diagenesis on analogue reservoir quality of the Nyalau Formation, Borneo

The application of enhanced recovery techniques to improve oil recovery requires a critical understanding of the depositional and post-depositional factors that impact fluid flow in sandstone reservoirs. This study combined facies, petrographic, mineralogical, and helium porosimetry analyses to examine the quality and producibility of the subsurface reservoir equivalents (Cycles I-III) of the Nyalau Formation in the underexplored Onshore Central Sarawak, Borneo. The Nyalau Formation consists of medium-to fine-grained, moderate-to well-sorted, and sub-rounded to sub-angular sublitharenite to lithic subarkose sandstones. Three facies associations and eight reservoir facies interpreted to have been deposited within a coast-to-coastal barrier depositional environment were delineated. The sandstones have undergone a wide range of diagenetic alterations, with the primary and secondary pore spaces predominantly occluded by authigenic quartz overgrowths and clay mineral cement. The helium porosity (ɸH) and Klinkenberg permeability (KK) in the reservoir facies widely vary, ranging from 3.0 to 22.14% and 0.003 to 8.283 mD, respectively. However, the weak correlation between ɸH and KK (R2 = 0.35) and petrographic datasets indicate limited diagenetic and mineralogical controls on the porosity-permeability (poro-perm) parameters, consistent with relatively high heterogeneous reservoirs. We inferred that the heterogeneous reservoir properties and fluid flow in the studied analogue reservoir sandstones are dominantly influenced by depositional processes, as shoreface sandstones typically yield better reservoir quality than those deposited within the intertidal reaches. Our data indicate that diagenetic modification and bioturbation exerted lesser control on fluid-flow quality. Nevertheless, the disparity of poro-perm values in samples of the same facies suggest that localised post-depositional modifications can disproportionately affect reservoir quality, regardless of uniformity or variations in depositional conditions.

Petrography and authigenic chlorite in the Siegenian reservoir rocks, Berkine Basin, eastern Algerian Sahara

In Berkine Basin, the Siegenian stage is constituted by clayey sandstones and it contains several reservoir levels especially in the perimeter of Zemlet El Arbi, where the sandstone levels can reach 30 m of thickness. The Siegenian reservoir shows a tidal deposit environment, it is located at depths greater than 3200 m, its average thickness is 200 m in the south and it decreases to 30 m because it is strongly affected by Hercynian orogeny mainly in the northwest of the studied area. Petrographic and X-ray diffraction studies indicate the presence of authigenic chlorite, which is the most important cement in Siegenian sandstones. Chlorite appears in different forms: grain coating, pore lining, pore filling and rarely in the form of oolite. It often has a great impact on petrophysical reservoir qualities by inhibiting authigenic quartz overgrowth in porous space and preserving porosity. Other diagenetic phenomena observed are mechanical compaction, alteration and dissolution of feldspars and biotite, precipitation of clay minerals (smectite, berthierine, chlorite, kaolinite and illite) and precipitation of secondary silica, carbonates, pyrite and haematite.

Clay coating preserving high porosities in deeply buried intervals of the Stø Formation

The Stø Formation is the most important reservoir interval in the Norwegian Barents Sea, however the reservoir quality can be highly affected by the detrimental effects of quartz cement where there have been extensive post depositional burial. Core plug data from well 7219/8-2 in the Southwestern Barents Sea shows abnormally high porosity and permeability values in certain units of the deeply buried and otherwise highly quartz cemented Stø Formation. The amount of quartz cement in the samples is inversely proportional to the porosity. Samples with high and low porosities are similar texturally and mineralogically, but the high porosity samples have a layer of illitic clay coating the majority of the detrital quartz grains. Illitic clay coating present at grain contacts can result in a lowered IGV given they aid in the dissolution of quartz at interfaces, also creating a source of dissolved silica. Clay induced dissolution means that silica saturation is not a limiting factor in quartz cementation in these samples. The results show that the illitic clay coating is capable of limiting the amount of authigenic quartz overgrowth from 20 to 23% in samples with negligible grain coating to 5–11% in the intervals with high coating coverage. The illitic clay coating inhibits quartz overgrowth by limiting the surface area available for nucleation on detrital grains. The Stø Formation comprises mainly shallow marine deposits of highly reworked clean sandstone. Abnormally high porosities appear to be linked to settings where sediments of a more proximal location are preserved without extensive reworking. The grain coating clay is illitic and most likely originates from clay infiltration processes prior to final deposition. The difference in extent of clay coating in similar facies can mostly be correlated with varying amount of post depositional reworking. This study suggests that there is a potential for considerable porosity and permeability to be preserved in deeply buried sandstones in the Barents Sea. This study could be important in the future exploration activity of deeply buried structures in the area.

Artificially induced clay mineral authigenesis in an underground gas storage field, North Alpine Foreland Basin, Austria

ABSTRACT Secondary processes within reservoir sandstones during and after hydrocarbon production are poorly understood. This study looks at the effect of secondary water fill on a sandstone reservoir within a time span of 8 yr. The reservoir rocks consist of medium-grained litharenites with large clasts of shales and carbonates. They originate from a depleted gas reservoir that has been converted into an underground storage field for natural gas. Gas production resulted in a rise of the gas–water contact of approximately 30 m (98 ft). Based on their initial and final gas and water saturations, four zones can be identified. Observed diagenetic changes in all four zones include carbonate cementation, K-feldspar overgrowths, authigenic quartz overgrowths, pyrite formation, and poorly crystallized authigenic clay minerals. However, the authigenic clay mineral fraction differs significantly within the zones. Total clay mineral content and crystallinities of smectite, chlorite, kaolinite, and illite increase from the gas-bearing to the initial water zone. Additionally, expandable clay minerals and kaolinite were not identified in the gas-bearing zone. This is different in the secondary watered zones, where smectites and kaolinites are developing. The study shows that within a maximum of 8 yr from the start of water influx into the gas zone, new clay minerals are forming. The porosity and permeability reduction caused by this artificially induced process might continue and could also be of relevance within producing reservoirs, where water saturation increases during production.

Extensive authigenic quartz overgrowths in the gas-bearing Haynesville-Bossier Shale, USA

In sandstone reservoirs, despite grain rearrangement during compaction, significant pore volumes may be retained prior to the onset of late diagenetic quartz cementation. In mudstone reservoirs, grain rearrangement during compaction results in significant pore volume reduction prior to late diagenesis. Where quartz overgrowths have been previously reported in post-compaction mudstones they have been in volumetrically low concentrations and interpreted as anomalous occurrences. Quartz cementation alters rock brittleness resulting in changes to mechanical fracture properties. Quartz overgrowths reduce reservoir porosity and permeability.We present petrographic evidence of two phases of quartz cement in the Haynesville-Bossier Shale: (i) grain replacive and (ii) quartz overgrowths. Carbonate grain replacement is volumetrically low (<1%). Quartz overgrowths identified from SEM-CL imaging are volumetrically more significant (8–13%). Quartz overgrowths were most commonly observed in the sandy and coarse mudstone microfacies, but are present in both medium and fine mudstone microfacies.Petrographic evidence indicates three processes in the development of quartz overgrowths. Mica and pyrite are (i) engulfed and (ii) displaced by quartz overgrowth cement. The absence of a supportive, primary granular framework surrounding engulfed detrital and early authigenic minerals would indicate that quartz overgrowths are also (iii) replacive. Pressure dissolution of detrital quartz silt grains and smectite-to-illite transformation are likely sources of silica for quartz cement. This study is the first to document large-scale, replacive, authigenic quartz overgrowth development within a producing mudstone.

Isotopes (&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C and &amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O) Geochemistry of Lower Triassic Montney Formation, Northeastern British Columbia, Western Canada

Oxygen isotope (δ18O) serves as paleothermometer, and provides paleotemperature for carbonates. δ18O signature was used to estimate the temperature of fractionation of dolomite and calcite in Montney Formation, empirically calculated to have precipitated, between approximately 13°C to ±33°C during Triassic time in northeastern British Columbia, Western Canada Sedimentary Basin (WCSB). Measurements of stable isotopes (δ13C and δ18O) fractionation, supported by quantitative X-ray diffraction evidence, and whole-rock geochemical characterization of the Triassic Montney Formation indicates the presence of calcite, dolomite, magnesium, carbon and other elements. Results from isotopic signature obtained from bulk calcite and bulk dolomite from this study indicates depleted δ13CPDB (-2.18‰ to -8.46‰) and depleted δ18OPDB (-3.54‰ to -16.15‰), which is interpreted in relation to oxidation of organic matter during diagenesis. Diagenetic modification of dolomitized very fine-grained, silty-sandstone of the Montney Formation may have occurred in stages of progressive oxidation and reduction reactions involving chemical elements such as Fe, which manifest in mineral form as pyrite, particularly, during early burial diagenesis. Such mineralogical changes evident in this study from petrography and SEM, includes cementation, authigenic quartz overgrowth and mineral replacement involving calcite and dolomite, which are typical of diagenesis. High concentration of chemical elements in the Montney Formation -Ca and Mg indicates dolomitization. It is interpreted herein, that calcite may have been precipitated into the interstitial pore space of the intergranular matrix of very fine-grained silty-sandstone of the Montney Formation as cement by a complex mechanism resulting in the interlocking of grains.

The sandstone-hosted stratiform copper mineralization at Mwitapile and its relation to the mineralization at Lufukwe, Lufilian foreland, Democratic Republic of Congo

The Lufilian foreland is a triangular-shaped area located in the SE of the Democratic Republic of Congo and to the NE of the Lufilian arc, which hosts the well-known Central African Copperbelt. The Lufilian foreland recently became an interesting area with several vein-type (e.g., Dikulushi) and stratiform (e.g., Lufukwe and Mwitapile) copper occurrences. The Lufilian foreland stratiform Cu mineralization is, to date, observed in sandstone rock units belonging to the Nguba and Kundelungu Groups (Katanga Supergroup). The Mwitapile sandstone-hosted stratiform Cu prospect is located in the north eastern part of the Lufilian foreland. The host rock for the Cu mineralization is the Sonta Sandstone of the Ngule Subgroup (Kundelungu Group). A combined remote sensing, petrographic and fluid inclusion microthermometric analysis was performed at Mwitapile and compared with similar analysis previously carried out at Lufukwe to present a metallogenic model for the Mwitapile- and Lufukwe-type stratiform copper deposits. Interpretation of ETM+ satellite images for the Mwitapile prospect and the surrounding areas indicate the absence of NE–SW or ENE–WSW faults, similar to those observed controlling the mineralization at Lufukwe. Faults with these orientations are, however, present to the NW, W, SW and E of the Mwitapile prospect. At Mwitapile, the Sonta Sandstone host rock is intensely compacted, arkosic to calcareous with high silica cementation (first generation of authigenic quartz overgrowths). In the Sonta Sandstone, feldspar and calcite are present in disseminated, banded and nodular forms. Intense dissolution of these minerals caused the presence of disseminated rectangular, pipe-like and nodular dissolution cavities. Sulfide mineralization is mainly concentrated in these cavities. The hypogene sulfide minerals consist of two generations of pyrite, chalcopyrite, bornite and chalcocite, separated by a second generation of authigenic quartz overgrowth. The hypogene sulfide minerals are replaced by supergene digenite and covellite. Fluid inclusion microthermometry on the first authigenic quartz phase indicates silica precipitation from an H 2O–NaCl–CaCl 2 fluid with a minimum temperature between 111 and 182 °C and a salinity between 22.0 and 25.5 wt.% CaCl 2 equiv. Microthermometry on the second authigenic quartz overgrowths and in secondary trails related to the mineralization indicate that the mineralizing fluid is characterized by variable temperatures (Th = 120 to 280 °C) and salinities (2.4 to 19.8 wt.% NaCl equiv.) and by a general trend of increasing temperatures with increasing salinities. Comparison between Mwitapile and Lufukwe indicates that the stratiform Cu mineralization in the two deposits is controlled by similar sedimentary, diagenetic and structural factors and likely formed from a similar mineralizing fluid. A post-orogenic timing is proposed for the mineralization in both deposits. The main mineralization controlling factors are grain size, clay and pyrobitumen content, the amount and degree of feldspar and/or calcite dissolution and the presence of NE–SW to ENE–WSW faults. The data support a post-orogenic fluid-mixing model for the Mwitapile- and Lufukwe-type sandstone-hosted stratiform Cu deposits, in which the mineralization is related to the mixing between a Cu-rich hydrothermal fluid, with a temperature up to 280 °C and a maximum salinity of 19.8 wt.% NaCl equiv., with a colder low salinity reducing fluid present in the sandstone host rock. The mineralizing fluid likely migrated upwards to the sandstone source rocks along NE–SW to ENE–WSW orientated faults. At Lufukwe, the highest copper grades at surface outcrops and boreholes were found along and near to these faults. At Mwitapile, where such faults are 2 to 3 km away, the Cu grades are much lower than at Lufukwe. Copper precipitation was possibly promoted by reduction from pre-existing hydrocarbons and non-copper sulfides and by the decrease in fluid salinity and temperature during mixing. Based on this research, new Cu prospects were proposed at Lufukwe and Mwitapile and a set of recommendations for further Cu exploration in the Lufilian foreland is presented.

Postorogenic Origin of the Stratiform Cu Mineralization at Lufukwe, Lufilian Foreland, Democratic Republic of Congo

The Central African Copperbelt, which stretches across the border between Zambia and the Democratic Republic of Congo, is one of the largest sediment-hosted stratiform Cu-Co provinces in the world. The triangular-shaped Lufilian foreland is located to the northeast of the Copperbelt. Recently sediment-hosted stratiform copper occurrences have been explored in the foreland, including the Kinkumbi prospect in the Lufukwe anticline. Although the stratiform copper mineralization in the Copperbelt is mainly concentrated in the Roan Group sedimentary rocks, where it is commonly associated with cobalt, the Lufilian foreland copper mineralization occurs in sedimentary rock units of the Nguba and Kundelungu Groups where it is mainly associated with silver. This paper examines the metallogenesis of the stratiform copper mineralization at Lufukwe. The Lufukwe anticline is situated in the eastern part of the Lufilian foreland and is composed of Neoproterozoic sedimentary rocks belonging to the Katanga Supergroup. The Kinkumbi prospect, a sediment-hosted stratiform copper-silver occurrence, is located in the northern part of the western flank of the anticline. This part of the anticline is characterized by disseminated copper-silver mineralization hosted in the lower 10 to 15 m of the Monwezi Sandstone (Nguba Group). A comparison between the location of high copper grades in surface samples and boreholes and the location of structural lineaments visible on ASTER images indicates that the mineralization is spatially related to northeast-southwest to east-northeast‐west-southwest strike-slip faults. These faults are nearly perpendicular to the strike of the host rock and postdate both the Lufilian folding and deposition of the entire Katanga Supergroup. The Monwezi Sandstone was subjected to strong compaction and silica cementation (authigenic quartz overgrowths), followed by intense feldspar dissolution, which resulted in a well-developed secondary porosity represented by dissolution cavities. Copper sulfides are mainly concentrated in these cavities and partially replace the detrital grains. The copper mineralization is both hypogene (chalcopyrite, bornite, and chalcocite) and supergene (digenite, covellite, and minor native copper), with malachite and chrysocolla as the main oxidation products. Point counting and grain size measurements demonstrate that the sandstone horizons with high copper content (>1.25% Cu) are those with a detrital grain size larger than 175 µm, more than 35 percent altered feldspars and little or no fine-grained matrix. Microthermometry of fluid inclusions indicates that the authigenic quartz overgrowths precipitated from a moderate-temperature (80°‐130°C), high-salinity (18.8‐23.4 wt % CaCl2 equiv) H2O-NaCl-CaCl2 fluid. The hypogene copper-silver mineralization was deposited from a hot (120°‐180°C) and low- to moderate-salinity (1.9‐7.7 wt % NaCl equiv) H2O-NaCl fluid with a general trend of increasing homogenization temperatures with increasing salinities. The interpretation of the available structural, stratigraphic, petrographic, and fluid inclusion microthermometric data constrain the timing of the mineralization to a time after the Lufilian folding and deposition of the entire Katanga Supergroup. The data presented support a postorogenic fluid-mixing model in which the mineralization is related to the mixing of a copper-rich mineralizing fluid with a temperature ≥180°C and salinity ≥7.7 wt percent NaCl equiv, likely migrating upward along northeast-southwest‐ to east-northeast‐west-southwest‐oriented strike-slip faults, with a colder, low-salinity, reducing fluid in the Monwezi Sandstone. The location and distance between the northeast-southwest to east-northeast‐west-southwest faults strongly influenced the spatial distribution of the copper mineralization in the anticline, and the variability in grain size and composition of the Monwezi Sandstone caused the preferential lateral migration of the mineralizing fluid through the lower 10 to 15 m. Copper precipitation was possibly induced by reduction by preexisting noncopper sulfides (pyrite and arsenopyrite) and hydrocarbons and by the drop in fluid temperature and salinity. Based on this research the areas with dominant northeast-southwest to east-northeast‐west-southwest structural lineaments are considered favorable sites for further copper exploration in the Lufilian foreland, especially where these lineaments cut previously deformed Katanga sediments. Coarse-grained sandstones with no fine matrix that underwent intense feldspar dissolution are the most promising host rocks for the late disseminated stratiform copper mineralization in the Lufilian foreland. Airborne gamma-ray spectrometric surveys could be used as a powerful exploration tool for targeting ore-related, K-depleted zones in these sandstones.

Optically continuous silcrete quartz cements of the St. Peter Sandstone: High precision oxygen isotope analysis by ion microprobe

A detailed oxygen isotope study of detrital quartz and authigenic quartz overgrowths from shallowly buried (<1 km) quartz arenites of the St. Peter Sandstone (in SW Wisconsin) constrains temperature and fluid sources during diagenesis. Quartz overgrowths are syntaxial (optically continuous) and show complex luminescent zonation by cathodoluminescence. Detrital quartz grains were separated from 53 rocks and analyzed for oxygen isotope ratio by laser fluorination, resulting in an average δ 18O of 10.0 ± 0.2‰ (1SD, n = 109). Twelve thin sections were analyzed by CAMECA-1280 ion microprobe (6–10 μm spot size, analytical precision better than ±0.2‰, 1SD). Detrital quartz grains have an average δ 18O of 10.0 ± 1.4‰ (1SD, n = 91) identical to the data obtained by laser fluorination. The ion microprobe data reveal true variability that is otherwise lost by homogenization of powdered samples necessary for laser fluorination. Laser fluorination uses samples that are one million times larger than the ion microprobe. Whole rock (WR) samples from the 53 rocks were analyzed by laser fluorination, giving δ 18O between 9.8‰ and 16.7‰ ( n = 110). Quartz overgrowths in thin sections from 10 rocks were analyzed by ion microprobe and average δ 18O = 29.3 ± 1.0‰ (1SD, n = 161). Given the similarity, on average, of δ 18O for all detrital quartz grains and for all quartz overgrowths, samples with higher δ 18O(WR) values can be shown to have more cement. The quartz cement in the 53 rocks, calculated by mass balance, varies from <1 to 21 vol.% cement, with one outlier at 33 vol.% cement. Eolian samples have an average of 11% cement compared to marine samples, which average 4% cement. Two models for quartz cementation have been investigated: high temperature (50–110 °C) formation from ore-forming brines related to Mississippi Valley Type (MVT) mineralization and formation as silcretes at low temperature (10–30 °C). The homogeneity of δ 18O for quartz overgrowths determined by ion microprobe rules out a systematic regional variation of temperature as predicted for MVT brines and there are no other known heating events in these sediments that were never buried to depths >1 km. The data in this study suggest that quartz overgrowths formed as silcretes in the St. Peter Sandstone from meteoric water with δ 18O values of −10‰ to −5‰ at 10–30 °C. This interpretation runs counter to conventional wisdom based on fibrous or opaline silica cements suggesting that the formation of syntaxial quartz overgrowths requires higher temperatures. While metastable silica cements commonly form at high degrees of silica oversaturation following rapid break-down reactions of materials such as of feldspars or glass, the weathering of a clean quartz arenite is slower facilitating chemical equilibrium and precipitation of crystallographically oriented overgrowths of α-quartz.

Quartz Cement in the Fontainebleau Sandstone, Paris Basin, France: Crystallography and Implications for Mechanisms of Cement Growth

Abstract Authigenic quartz cement is the most abundant form of diagenetic cement in clastic sedimentary rocks. Despite this, there are many unknowns relating to mechanisms of growth of quartz cement and the crystallography of quartz cement. The key focus of this paper is to investigate further the issues of crystallography and quartz cement growth mechanisms, using the shallowly buried Oligo-Miocene Fontainebleau sandstone, France, as a case study. We address the following points: (1) are authigenic quartz overgrowths really in crystallographic continuity with their substrate grains? (2) What is the crystallographic inter-relationship between zones of quartz cement growth? (3) Are all quartz overgrowths entirely quartz, or do other silica polymorphs exist within overgrowths? The study combines an array of techniques to answer these questions, including transmitted-light optics, cathodoluminescence (CL), and electron backscatter diffraction (EBSD), the latter of the two being performed with the use of a scanning electron microscope (SEM). The use of EBSD to this study is crucial because it provides essential crystallographic information on the grains and their overgrowths. The data revealed: (1) quartz overgrowths comprise several zones visible in optical and CL images as parallel, isopachous, alternating bright and dark bands; (2) these bands represent areas of poorly crystalline silica and fully crystalline quartz; (3) one entire zone consists only of poorly crystalline quartz; (4) the final growth stage occurred as prismatic microcrystalline quartz into the remaining porosity; (5) the crystallographic orientation across most of the overgrowth, as far as the microcrystalline quartz layer, is the same as that of the detrital grain (i.e., it is syntaxial); and (6) the microcrystalline quartz layer has crystals with different and variable orientations relative to the detrital grain. This indicates that part of the quartz cement is not in crystallographic continuity with the substrate grain and displays an epitaxial relationship. Detailed analysis of the orientation data shows that there is a rational crystallographic rotation around a variety of axes, which indicates that the orientation of the final growth stages was not random.

DIAGENESIS AND MASS TRANSFER BETWEEN PERMO-TRIASSIC SANDSTONES AND INTERBEDDED MUDSTONES, ULSTER BASIN

Beds of siltstone, mudstone and shale are interbedded in the Permo-Triassic sandstones of the Ulster Basin, UK, at different stratigraphic levels. The paragenetic sequences of authigenic minerals both in the sandy and fine-grained sediments (mudstones and siltstones) indicate red bed diagenetic trend. Abundant authigenic quartz overgrowths and carbonate crystals observed near the sandstone-siltstone and sandstone-mudstone contacts, indicating that pore water and mass transfer from the fine-grained sediments to sandstone facies. The fine-grained sediments are the main source of ions for the authigenesis of quartz and feldspar overgrowths and carbonate precipitation. Mechanical compaction of fine-grained sediments, influx of gravity-driven acidic meteoric pore water, dehydration water, diffusion and convection are the most important means of mass transfer and pore water circulation. Pore water migration depends upon permeability distribution and pressure difference. (Received December 8, 2000; revised April 26, 2002) Bull. Chem. Soc. Ethiop. 2002, 16(1), 9-35.

Abstract: Depositional Environments and Sandstone Diagenesis in the Tyler Formation (Pennsylvanian), Western North Dakota&nbsp;

Thirty-three oil well cores were examined and approximately 3,100 oil well logs were used to determine the depositional environments of the Tyler Formation (Pennsylvanian) in northwestern North Dakota. Ninety sandstone thin-sections were described and twenty-nine sandstone samples were analyzed by scanning electron microscope/X-ray microanalysis techniques to characterize sandstone diagenesis. The Tyler Formation in northwestern North Dakota is correlative with the lower unit of the Tyler Formation in southwestern North Dakota. The upper unit of the Tyler Formation in southwestern North Dakota is not present in northwestern North Dakota, contrary to previous workers suggestions. The Tyler Formation in northwestern North Dakota consists of varicolored mudstone, medium-gray claystone, thin bituminous coal beds, dark-gray shale and limestone, and gray sandstone. The upper unit of the Tyler Formation in southwestern North Dakota consists of gray sandstone, dark-gray limestone and shale, and varicolored mudstone. Lenticular sandstones present in the middle and rarely at the base of the formation, which occur in northeast-southwest linear trends, indicate that the Tyler Formation in northwestern North Dakota was deposited as a river channel system on a low-lying, prograding coastal plain. Depositional environments associated with the coastal plain include river channels, flood plains, lakes, estuaries, caliche paleosols, and backswamps. Throughout deposition of the Tyler Formation, several major river channels flowed in a predominantly southwest direction across a low-lying coastal plain and transported quartz sand toward a delta or shoreline, not identified in this study, at the margin of a shallow epeiric sea. Initially, the presence of detrital clay and infiltration clay and early hematite and chlorite coatings on quartz grains was the most important factor influencing porosity reduction in Tyler sandstones. The first major diagenetic event responsible for porosity reduction was the development of authigenic quartz overgrowths. Other cements that have contributed to porosity reduction in Tyler sandstones include amorphous silica (opal), calcite, ankerite, siderite, anhydrite, barite, hematite, and pyrite, In addition, authigenic kaolinite, illite, and chlorite have reduced porosity in Tyler sandstones. Dissolution of carbonate cement resulted in the formation of secondary porosity. Sandstone diagenesis took place shortly after sand was deposited in a marine environment, while sediments were being flushed with meteoric ground water, and while sands were being flushed with chemically evolved pore water released after diagenesis and compaction of interbedded mud-rich sequences. River-channel sandstones in northwestern and southwestern North Dakota and barrier-island sandstones present only in southwestern North Dakota contain considerable amounts of intergranular porosity. Petroliferous sandstones in southwestern North Dakota are fine- to medium-grained, well-sorted, texturally mature…

Authigenic fluid inclusions in lithic sandstone: A case study from the Permo‐Triassic Gunnedah Basin, New South Wales

The Gunnedah Basin is a structural subdivision of the Permo‐Triassic Sydney‐Bowen Basin, a major Permian coal province in eastern Australia. Coarse‐grained sediments of the Permian Maules Creek, Porcupine and Watermark Formations within the basin are predominantly volcanolithic to quartz‐lithic conglomerate and sandstone that were deposited in alluvial, marine fan delta and marine shelf environments. Triassic strata of the Digby and Napperby Formations were deposited in alluvial and lacustrine environments respectively. Fluid inclusions in authigenic quartz overgrowths and healed grain fractures identified in sandstone from these rocks comprise four distinct types (Types 1–4) generated during three discrete diagenetic episodes (GI [oldest]‐GIII [youngest]). Two of the inclusion types (Types 3 and 4) are hydrocarbon‐bearing and their distribution indicates that migration of hydrocarbons occurred late in the diagenetic history of the rocks, although homogenisation temperatures imply that emplacement of these inclusions pre‐dated maximum burial temperature. Microthermometric analysis of 57 aqueous inclusions gave homogenisation temperatures ranging from 62°C to 140°C. The majority of samples are now located at depths shallower than those at which the fluids were entrapped and data are used to constrain the palaeothermal and tectonic history of the basin. Freezing runs yielded first melts at between −65°C and −31°C suggesting the presence of a Ca‐Na‐Cl system. Ice melts ranged from −8.5°C to −0.2°C and are interpreted to represent much lower salinities than would have been present in the original marine depositional environment of the Porcupine and Watermark Formations.

High-precision, in situ oxygen isotope ratio measurements obtained from geological and extra-terrestrial materials using an Isolab 54 ion microprobe

We have developed an ion probe technique for the in situ measurement of oxygen isotope ratios, 18O/16O and 17O/16O, with high spatial resolution in polished thin sections of silicate minerals. This technique allows the isotopic analysis of samples as small as picomoles of material which represents only 10–7 of the sample size required by conventional fluorination oxygen isotope measurement methods and thus has wide application to the study of many terrestrial and extra-terrestrial samples that have heterogeneous oxygen isotope compositions on a scale of tens of micrometres. To illustrate the breadth of studies that are made possible by the ion probe, we report here oxygen isotope measurements from an authigenic quartz overgrowth obtained from the Penrith sandstone, UK, and measurements of magnetite grains from the Orgueil and Yamato 82162 carbonaceous chondrites which may be used to constrain possible formation mechanisms of the magnetite.

High Spatial Resolution Measurements of δ18O in Authigenic Quartz Overgrowths

High Spatial Resolution Measurements of δ18O in Authigenic Quartz Overgrowths

Oxygen Isotopic Analysis of Diagenetic Quartz Overgrowths from the Brent Sands: A Comparison of Two Preparation Methods

ABSTRACT Two preparation methods used for determining the 18O of quartz overgrowths are compared from the sandstones of the Middle Jurassic Brent Group, Northern North Sea. Method one uses dilute HF to preferentially attack interfacial materials present between the overgrowth and the detrital core, with subsequent ultrasonic agitation used to break the overgrowths off at the weakened contacts. Size fractionation by sieving should concentrate overgrowths into the smaller grain size separates. In the Brent sands studied, however, little interfacial material is present between overgrowth and core, and the overgrowths are thinly developed (average width 20-50 microns). Concentration of the diagenetic overgrowths is therefore difficult. This difficulty is compounded by large amounts of detrital quartz in the size fractions, as a result of internal fracturing of the grains. The 18O (SMOW) values for the finest size fraction (supposedly overgrowth rich), ranging from 9 to 13.3, are unusually low for diagenetic quartz overgrowths, and result from the high detrital component. The second method derives the 18O values for the overgrowths by the difference in 18O for the quartz grains plus overgrowths and quartz grains from which the overgrowths have been removed by HF dissolution. The mass balance used requires the overgrowth abundance to be known from point counting. The calculated 18O values obtained are typical for authigenic quartz overgrowths and range from 17.1 to 20.9. Direct isolation of quartz overgrowths for 18O measurement is suitable where interfacial material exists between the detrital grain and the overgrowth and where overgrowths are well developed. Otherwise, preferential HF dissolution of the overgrowths and a 18O mass balance calculation is the preferred method.

Genetic studies of red bed mineralization in the Triassic of the Cheshire Basin, northwest England

The Triassic sediments of the Cheshire Basin are host to a number of small mineral deposits characteristic of red beds. The mineralization comprises Cu and Pb with minor Zn, Ag, CO, V, Ni, As, Sb and Mn which, together with widespread barite mineralization, is closely associated with basin margin faults. The Wilmslow Sandstone Formation is cemented by authigenic quartz overgrowths enclosing iron sulphides, which predate the main sulphide cements of tennantite, bornite and sulpharsenides. Studies on late calcite-hosted fluid inclusions show homogenization temperatures ranging between 60–70 o C (regarded as the minimum temperature of mineralization) and salinities of 9–22 wt% equivalent NaCl (mean = 17wt%), indicating that the mineralizing fluids were warm and saline. Consideration of the basin history suggests that fluid temperatures in the Permo-Triassic sequence may have reached 150 °C at the time of mineralization. New data significantly constrain the genetic hypothesis for the deposits, refuting a magmatic origin. A basin brine expulsion model is favoured. The sulphur isotope data from the barite mineralization (mean δ 34 S = +17‰) suggest that the bulk of the barite sulphur was derived directly from the Upper Triassic evaporites (mean δ 34 S = +19‰), whilst the distribution of δ 34 S values for sulphides (–1.8‰ to +16.2‰) is consistent with ultimate derivation of sulphur from the evaporites by closed system reduction of a sulphate-bearing brine. It seems likely that the reducing agent was organic material (methane) derived from the underlying Coal Measures, but it is uncertain at present whether the principal mechanism was thermochemical or bacterial reduction. The pattern of sulphide δ 34 S favours the former process.

Cathodoluminescence Colours of α-Quartz

AbstractA new cathodoluminescence-microscope has been developed with a considerably improved detection limit. Time-dependent luminescence intensity changes observed during electron bombardment enabled the recognition of short-lived, long-lived, and brown luminescence colour types in α-quartz.Short-lived bottle-green or blue luminescence colours with zones of non-luminescing bands are very common in authigenic quartz overgrowths, fracture fillings or idiomorphic vein crystals. Dark brown, short-lived yellow or pink colours are often found in quartz replacing sulphate minerals. Quartz from tectonically active regions commonly exhibits a brown luminescence colour. A red luminescence colour is typical for quartz crystallized close to a volcanic dyke or sill.The causes of these different and previously poorly understood luminescence colours were investigated using heat treatment, electron bombardment and electrodiffusion. Both natural and induced brown luminescence colours reflect the presence of lattice defects (nonbonding Si-O) due to twinning, mechanical deformation, particle bombardment or extremely rapid growth. The bottle-green and blue linearly polarized luminescence colour, characterized by a plane of polarization parallel to the c-axis, both depend on the presence of interstitial cations. The yellow and red luminescence colours in α-quartz both exhibit a plane of polarization perpendicular to the c-axis and appear to be related to the presence of trace elements in an oxidizing solution and to ferric iron respectively.