Authigenic Quartz Crystals Research Articles

Authigenic quartz reveals the triple oxygen isotope composition and diagenetic temperature of saline brines

Triple oxygen isotope compositions of authigenic minerals are an emerging tool for the reconstruction of fluid temperatures and the fluid isotopic composition. In this study, we analyzed euhedral authigenic quartz crystals from a halite deposit of the Upper Permian Zechstein evaporitic basin for their triple oxygen isotope composition using laser fluorination isotope ratio mass spectrometry. In the triple oxygen isotope space, additional information, such as the triple oxygen isotope composition of the ambient water, provides insights into equilibrium conditions during quartz formation that cannot be identified from δ18O alone. The triple oxygen isotope composition of authigenic quartz shows, that single oxygen isotope (δ18O) thermometry is not applicable for samples from evaporitic environments, where the fluids' δ18O deviates from dynamic oxygen isotope equilibrium. By combining the Craig and Gordon evaporation model with H2O – SiO2 triple oxygen isotope equilibrium fractionation, we reconstruct the formation temperature of authigenic quartz and the triple oxygen isotope composition of the marine-derived saline brine pore fluid in equilibrium with the quartz. We obtain temperatures of 90 to 130 °C that is interpreted as crystallization temperatures of microcrystalline quartz in pore fluid of halite during burial diagenesis. The triple oxygen isotope composition of authigenic quartz from evaporitic environments is suited as a geochemical tracer for fluid oxygen isotope compositions and ancient fluid or diagenetic temperatures.

Organic Matter of the Wufeng–Longmaxi Formation Shales Using Scanning Electron Microscopy

Fine-grained organic matter (OM) particles are commonly widely dispersed in shale deposits. However, carrying out investigations of pores hosted by OM particles and the nature of grain interactions in OM particles and associated detrital grains using optical microscopy is difficult at best. Scanning electron microscopy (SEM) is much better suited for characterizing the microstructure of dispersed OM particles and has found wide application in the study of unconventional oil and gas systems. Scanning electron microscopy was used to define the types of OM contained in marine shale deposits of the Wufeng and Longmaxi Formations spanning the Ordovician–Silurian transition in South China. Of particular interest was the development of OM-hosted pores and the identification of the factors that controlled their formation. The dominant OM type contained in the studied deposits is pyrobitumen, with subordinate graptolitic OM and sparse OM of unknown origin. Pyrobitumen is present in four forms, including pore fillings among authigenic quartz grains, within framboidal pyrite, and between authigenic clay grains and massive material. Diagenetic alteration has given rise to OM pores of differing morphology, size, and time of formation. Common small, equisized circular or oval OM pores are most developed and appear to have formed in association with the generation of hydrocarbons. Shale deposits containing abundant pyrobitumen filling interparticle pores among authigenic quartz crystals display robust reservoir and fracturing capacity. A sedimentary environment appears to have been the main factor affecting the type of OM and the nature of its association with detrital and authigenic minerals. Results of this study demonstrate that a sedimentary environment is a primary requisite for the formation of highly prospective/high-yielding hydrocarbon shale reservoir deposits.

Mineral diagenesis in a carbonate-rich mudstone: the Lower Carboniferous Hodder Mudstone, UK

Abstract Mineral authigenesis in mudstones responds differently across the compositional spectrum of fine-grained rocks, resulting in a complex array of possible rock fabrics that affect the mechanical capacity of mudstones. Within carbonate-rich mudstones, geochemical controls on silicate and carbonate mineral reactions are variable and poorly understood. Here we analyse the diagenetic minerals of the carbonate-rich Hodder Mudstone from the Carboniferous Bowland Basin, UK, using SEM petrography combined with X-ray-based inorganic geochemical data. Our findings show that up to 90% of quartz cements in the samples are diagenetic, originating mainly from biogenic silica dissolution and clay mineral reactions. It is also evident that due to the varying diagenetic silica-yielding reactions, authigenic silica overgrowths and isolated authigenic quartz crystals are localized in argillaceous samples, while calcareous samples are characterized by silica replacement textures and quartz/calcite intergrowths. Moreover, euhedral dolomite crystals are concentrated within argillaceous units, whereas calcareous units are characterized by anhedral dolomite precipitation and replacement textures. These finding presents a case for facies-selective cementation, as both early and burial diagenetic alterations were observed to be controlled by primary depositional components of biogenic debris and extrabasinal silicate detritus that resulted in complex and variable precipitation of authigenic minerals.

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.

Fine-grained volcanic-hydrothermal sedimentary rocks in Permian Lucaogou Formation, Santanghu Basin, NW China: Implications on hydrocarbon source rocks and accumulation in lacustrine rift basins

Abstract Subaqueous volcanic-hydrothermal field is a complex hybrid geological system, where sediments originated from volcanic, hydrothermal, lacustrine, terrestrial, and biotic sources are mixed. Recent petrological studies on such sediments have indicated a potential relationship between volcanic-hydrothermal activities and oil generation and accumulation. Here, we explore the mechanisms of sedimentation and oil accumulation in well-laminated organic-rich fine-grained sedimentary rocks in Permian Lucaogou Formation deposited in the lacustrine Santanghu rift basin, NW China. We conducted petrographic, carbon and oxygen isotopic, and organic geochemical analyses of fifty-four core samples from three wells (forty-eight samples from Well W1, two from W2, four from W3) designed for tight oil exploration. Four main observations and interpretations are: 1) The fine-grained sediments are a mixture of volcanic-hydrothermal detrital feldspar grains, authigenic quartz crystals, and primary lacustrine dolomite, forming interbedded tuffaceous shale and dolostone; 2) authigenic quartz crystals were precipitated from acidic hydrothermal fluids, whereas fine-grained angular alkaline feldspars were derived from subaqueous volcanic eruption; 3) the TOC content and oil generation potential increase with increased mixing of feldspars and carbonate minerals. Hence, the dolomite-bearing tuffaceous shale and tuffaceous dolostone are the best source rocks; and 4) the fine-grained sedimentary rocks are interpreted as a subaqueous volcanic-hydrothermal deposit that controls the source potential. The sedimentary processes of an ancient sediment-hosted geothermal system and its implications on hydrocarbon source rocks and accumulation were first identified. Algal bloom induced by subaqueous eruptions is the main cause for episodic deposition of organic matter and extreme heterogeneity in both lithofacies and source-rock distribution. The results suggest that cone-shaped buildups on seismic sections, which were formed by volcanic-hydrothermal deposits and contain highly-mixed feldspar and carbonate sediments, are the best hydrocarbon reservoirs, and demonstrate a strong relationship between volcanic-hydrothermal activities and hydrocarbon accumulation in an extensional tectonic setting.

Raman spectroscopic and microthermometric studies of authigenic quartz (the Pepper Mts., Central Poland) as an indicator of fluids circulation

Differently colored authigenic quartz crystals were found as the druses compound within mudstone heteroliths from the Pepper Mts. Shale Formation (Cambrian unit of the Holy Cross Mts., Central Poland). The genesis of this mineral was established on the basis of fluid inclusion study. Raman microspectroscopy was the key instrumental technique to identify the nature of the compounds trapped in the fluid inclusions. Methane (2917cm−1) or water vapor (broad band ~2500–3000cm−1) occur within two-phased primary inclusion assemblages, while nitrogen (2329cm−1) associated with methane and trace amount of carbon dioxide (1285, 1388cm−1) occur within secondary fluid inclusion assemblage. Temperatures of homogenization of primary fluid inclusions was obtained on the basis of heating experiments and ranged from 171° to 266°C. These values are much higher than expected for the diagenetic system without metamorphic changes what may imply hydrothermal origin of quartz crystals. The source of fluids is uncertain as in the Holy Cross Mts. there was no volcanic activity to the end of Late Devonian. However, fluids originated in metamorphic basin could use deep faults as the migration paths.

Silicon isotope compositions of euhedral authigenic quartz crystals: Implications for abiotic fractionation at surface temperatures

Silicon (Si) isotopes have been demonstrated as proxies for the paleoenvironmental conditions of various silica deposits. In an effort to investigate the petrogenesis and paleoenvironments of Phanerozoic silica deposits, a suite of euhedral megaquartz crystals in the Cretaceous Edwards Formation from central Texas was petrographically analyzed and their Si isotopic compositions analyzed by bulk and in situ techniques. Petrographic analysis shows a close relationship between megaquartz and evaporite minerals. The lithologic association of silica with evaporite-bearing dolomitized carbonate strata suggests that the silicification probably developed in a back-reef tidal-flat environment in which quartz crystals formed after the primary calcite cementation and partial dissolution of evaporites. Silicon isotopic mapping across the megaquartz crystals show that δ30Si(NBS28) values range from −2.90 to +2.94‰. These values span the majority of published silicon isotopic values observed in nature and indicate complex growth histories. The negative δ30Si values are attributed to the dissolution of sponge spicules, which likely act as the primary source of the authigenic megaquartz. The observed range of δ30Si values in megaquartz crystals is interpreted using a two stage model in which amorphous silica from sponge spicules is dissolved and re-precipitated as megaquartz in a closed system during diagenesis. This Rayleigh-type fractionation model also adds a new level of insight into the abiotic fractionation factor between dissolved and precipitated silica. Based on temperature estimates of 20 to 50°C for megaquartz precipitation, the fractionation factor was determined to be between −1.8 and −2.1‰. The estimated average δ30Si value of Early Cretaceous seawater is +2.7 to +3.0‰, significantly higher than modern seawater.

Timing of post-depositional events in the Burano Formation of the Secchia valley (Upper Triassic, Northern Apennines), clues from gypsum–anhydrite transitions and carbonate metasomatism

Abstract The Burano Evaporite Formation from the Secchia River Valley is an up to 2200 m-thick sequence composed of meter-to decameter-scale interbeds of gypsum–anhydrite and dolostones with minor halite. The deposit has been affected by a complex array of post-depositional modifications, thermal events and large-scale evaporite dissolution, preventing a satisfactory reconstruction of the environment of deposition. The modifications are intense because these rocks were the main decollement horizon during the formation of the Northern Apennines chain. The carbonate rocks are massive and-or laminated dolomitic mudstone, wackestone, oolitic packstones and oolitic, peloidal, bioclastic grainstones, which commonly appear as mega-boudins within a sulfate groundmass. The dolostones (δ 18 O =−5.7 to −3.7‰; δ 13 C =+1.3 to +3.0‰; PDB) have been affected by Mg-metasomatic replacement by magnesite (δ 18 O =−14.0 to −2.6‰; δ 13 C =−2.6 to +1.4‰; PDB) induced by hydrothermal circulation. Total homogenization temperatures of fluid inclusions in hydrothermal magnesite range from 275 to 310°C. The anhydrite rocks are characterized by flow structures such as centimeter-scale pseudo-lamination composed of aligned prismatic crystals with transposed isoclinal folds outlined by dolostones fragments. Homogenization temperatures of fluid inclusions in authigenic quartz incorporated into sulfate rocks range from 260 to 305°C (Emilia) and from 230 to 315°C (Tuscany). The gypsum rocks are composed of xenotopic irregular cloudy crystals and, more rarely, by centimeter-scale idiotopic crystals showing the same structures as the anhydrite rocks. The origin of the gypsum rocks is due to late alteration of anhydrite by migration of sharp hydration fronts. The hydration is a two step process and is revealed by the presence in the gypsum rocks of corroded anhydrite micro-relics and authigenic quartz crystals which include anhydrite. The role of the Burano Evaporites during the Apennines tectogenesis can be depicted as follows: (a) prevalent deposition of gypsum in the Upper Triassic; (b) gypsum dehydration at burial conditions to form anhydrite (Cretaceous?); (c) syn-tectonic flow of anhydrite rocks, brecciation of dolostones; syn-tectonic growth stage of quartz euhedra at deep burial conditions possibly related to the development of the Oligocene–Miocene greenschist facies Apuane metamorphic complex; (d) hydrothermal deposition of sparry magnesite and partial Mg-metasomatic replacement of dolostones by magnesite; (e) sub-surface dissolution of halite to form thick matrix-supported residual caprock-like anhydrite mega-breccias; (f) complete gypsification of anhydrite at sub-surface conditions; and (g) evaporite dissolution at surface exposure producing dolostone breccias with partial calcitization and removal of most clasts (“Calcare cavernoso”).

Epigenetic dolomitization of the Přaídolí formation (Upper Silurian), the Barrandian basin, Czech Republic: implications for burial history of Lower Paleozoic strata

Stratabound epigenetic dolomite occurs in carbonate facies of the Barrandian basin (Silurian and Devonian), Czech Republic. The most intense dolomitization is developed in bioclastic calcarenites within the transition between micritic limestone and shaledominated Přidoli and Lochkov formations deposited on a carbonate slope. Medium-crystalline (100–400 µm), inclusion-rich, xenotopic matrix dolomite (δ18O=−4.64 to −3.40‰ PDB;δ13C=+1.05 to +1.85‰ PDB) which selectively replaced most of the bioclastic precursor is volumetrically the most important dolomite type. Coarse crystalline saddle dolomite (δ18O=−8.04 to −5.14‰ PDB;δ18C=+0.49 to +1.49 PDB) which precipitated in fractures and vugs within the matrix dolomite represents a later diagenetic dolomitization event. In some vugs, saddle dolomite coprecipitated with petroleum inclusion-rich authigenic quartz crystals and minor sulfides which, in turn, were post-dated by semisolid asphaltic bitumen. The interpretation of the dolomitization remains equivocal. Massive xenotopic dolomite, although generally characteristic of a deeper burial setting, may have been formed by a recrystallization of an earlier, possibly shallow burial dolomite. Deeper burial recrystallization by reactive basinal pore fluids that presumably migrated through the more permeable upper portion of the Přidoli sequence appears as a viable explanation for this dolomitization overprint. Saddle dolomite cement of the matrix dolomite is interpreted as the last dolomitization event that occurred during deep burial at the depth of the oil window zone. The presence of saddle dolomite, the fluid inclusion composition of associated quartz crystals, and vitrinite paleogeothermometry of adjacent sediments imply diagenetic burial temperatures as high as 160°C. Although high geothermal gradients in the past or the involvement of hydrothermally influenced basinal fluids can account for these elevated temperatures, burial heating beneath approximately 3-km-thick sedimentary overburden of presumably post-Givetian strata, no longer preserved in the basin, appears to be the most likely interpretation. This interpretaion may imply that the magnitude of post-Variscan erosion in the Barrandian area was substantially greater than previously thought.

The origin of authigenic quartz in the Helvetic Siliceous Limestone (Helvetischer Kieselkalk), Switzerland

ABSTRACTHelvetic Siliceous Limestone (Lower Cretaceous) is characterized by up to 40% of very fine (1‐10 μm), evenly distributed authigenic quartz crystals. Early diagenetic dissolution of opaline sponge spicules led to silica enrichment of interstitial waters, which reprecipitated silica in the overlying horizons, forming tiny quartz crystals in pore spaces.

A Mineralogical Study of Some Lower Devonian (Helderberg) Rocks of the Central Hudson Valley, New York

ABSTRACT The mineralogy of three lithologies (carbonate, argillaceous limestone, mudstone) has been characterized for formational units of the Helderberg outcrop belt between Ravena and Kingston, New York. In the mudstones, throughout the geologic time interval from Rondout through Becraft, the percentage range of clay minerals and calcite has remained relatively constant whereas there has beeen a continuous increase in quartz and decrease in dolomite. In the carbonate rocks, the quartz and clay mineral content is low and relatively constant; the carbonate mineralogy of the Manlius is more complex than that of other formations. The amount of each mineral in the argillaceous limestones is intermediate between the amounts of the mudstones and limestones. A clay mineral suite unvarying in type (illite and chlorite) and with little variation in relative amounts was available to Helderberg depositional environments. Quartz clasticity values suggest that detrital quartz in all lithologies at both Ravena and Kingston come from the same source. Textural information derived from thin section study and mineralogical data reinforce the ideas of earlier workers concerning the nature of depositional environments for Helderberg carbonate sediments. Mudstones formed in the same physical environments as the carbonate rocks with which they occur. A sensitive balance governed the change from mudstone to limestone sedimentation. The immediate control for this change existed in the region of sedimentation--not in the source area--although the data of this study does not generally permit this control to be identified. Trace element analysis suggests a continuity of New Scotland sedimentary conditions in time, as well as in space. Textures indicate that shortly after deposition of shallow water carbonate sediments, a process of leaching and infilling produced a coarse calcite mosaic in ooids and skeletal material. Concomitant was the increase in grain size of calcite micrite to a coarser spar (micrite enlargement). Dolomitization followed these changes. Grain mount studies show that authigenic quartz crystals formed at some unknown time during this post-depositional sequence of events.

Evaporite Solution Breccias from the Mississippian of Southwest Montana

ABSTRACT Zones of breccia within the Madison Group of southwest Montana consist of very poorly sorted angular fragments of calcilutite or silty limestone, which are set in a fine grained matrix of crushed rock-flour and cemented by calcite. In places where the matrix was silty or sandy it weathers yellow-brown and contains abundant authigenic quartz crystals. The breccias are confined stratigraphically to zones which may be correlated with evaporites in subsurface sections. The upper contacts of the breccia zones are indistinct but the lower contacts are well defined; in some localities, brecciation occurred in place with little displacement of the fragments. The brecciation resulted from the collapse and crushing of non-soluble strata after the solution by ground water of beds of evaporite.

The Origin of Stylolites in the Light of a Petrofabric Study

ABSTRACT Petrofabric analysis of authigenic quartz crystals in stylolites from Carboniferous limestone in North Wales has been undertaken. The results obtained support Stockdale's Pressure-solution theory.