Composition Of Authigenic Minerals Research Articles

Diagenesis of the Paleogene Sandstones in the DN2 Gas Field, Kuqa Foreland Basin and Its Link with Tectonics

AbstractWe investigated the diagenesis of sandstones from DN2 Gas Field of Kuqa Foreland Basin (KFB) to infer timing of fluid migration and discuss link between fluids and tectonics. Textures and chemical composition of authigenic minerals, fluid evidence from fluid inclusions and formation water measurements were used to fulfill the aim. Eodiagenesis occurred with the participation of meteoric waters and connate waters. Mesodiagenesis are related to high salinity fluids, which were attributed to originate from overlying Neogene Jidike Formation evaporite (main minerals including halite, anhydrite, glauberite, carnallite and thenardite). The onset of high salinity fluid migration was inferred to occur during the late Miocene (12.4–9.2 Ma) by using integration of Th measured in the present study and K‐Ar dating of authigenetic illites from previous study. The period is consistent with the crucial phase (13–10 Ma) witnessing the rapid uplift of southern Tianshan Mts and the stage when calcite and anhydrite veins formed in the studied strata. We thus argue that diagenesis related to high salinity fluids occurred as a response to Tianshan Mts's rapid uplift and related tectonic processes. The flow of high salinity fluids was probably driven by density gradient and channeled and focused by fractures formed contemporaneously.

Evidence for methane seepage in South Atlantic from the occurrence of authigenic gypsum and framboidal pyrite in deep-sea sediments

An association of authigenic pyrite and gypsum has been found in deep-sea sediments from a core retrieved on the continental slope of the Santos Basin, South Atlantic. Pyrite occur mainly as tubular concretions formed by aggregates of framboids while gypsum show a rosette and acicular microcrystals habit. The concomitant presence of these minerals indicates the occurrence of biogeochemical reactions in marine environments that can be associated with methane seepage environments. Sulphur isotopic composition of authigenic minerals, magnetic susceptibility, and redox sensitive trace elements concentrations in bulk sediments are also used to characterize redox conditions and delimit a sulphate-methane transition zone (SMTZ) in this site. In this region, a pockmark field with methane release activity has already been recognized in sediments deposited during the Last Glacial Maximum. The conditions for the formation of these minerals, as well as their crystal habits, indicate the effect of biogeochemical processes consistent with methane seep environments. Therefore, our data provides new evidence about the occurrence of a past methane activity in this area.

Pore structure evaluation of low permeability clastic reservoirs based on sedimentation diagenesis: A case study of the Chang 8 reservoirs in the Zhenbei region, Ordos Basin

Sedimentary diagenesis is one of the most significant factors in the development of low permeability sandstone reservoirs due to its important influence on the formation of pore structure. This study takes Group 8 of the Yanchang Formation in the Zhenbei region of the Ordos Basin as the research object. Methods including casting thin-sections, mercury injection, and scanning electron microscopy were used to analyse core and casting thin-sections to examine the sedimentary diagenetic processes, authigenic mineral composition, pore structure, and physical properties of the reservoirs in the study area. The influence of sedimentary diagenesis on the physical properties of the reservoirs was analysed, and the pore structures in the study area were divided into four types. By determining the development mechanism and distribution characteristics of each type of pore structure, the favourable reservoir areas were also predicted. The results showed that type I (large mesopores with medium throats) and type II (mesopores with fine throats) were the favourable pore structures in the research area. These were mainly formed by the rim cementation of chlorite, the dissolution of feldspar and other soluble components, and a weak to mid compaction. Areas with type I and type II pore structures are considered to have a high reservoir quality, with a porosity of between 10% and 25% and a permeability of between 1 mD and 50 mD. These areas are suggested as regions for optimised exploration and development.

Mudstone/sandstone ratio control on carbonate cementation and reservoir quality in Upper Permian Rotliegend sandstones, offshore the Netherlands

Abstract The eolian-fluvial sandstones of the Upper Permian Rotliegend formation, which were deposited in the Southern Permian Basin, are today deeply buried (~3–4 km) and constitute important gas reservoirs in the Netherlands and the southern North Sea. The reservoir properties of the sandstones have been documented to be strongly affected by diagenesis, but the primary diagenetic factors impairing reservoir quality and their cause remain variably interpreted in the literature. Here, we present the results of a detailed investigation on the diagenetic processes controlling reservoir quality in the Lower Slochteren formation in the L and K blocks offshore the Netherlands, where fluvial and aeolian sandstones intercalate with playa lake muds in a delta setting. Quantitative analysis of the diagenetic mineral phases occurring in all main depositional facies (eolian, fluvial, playa-lake) was carried out on more than 200 samples from 21 wells, with the authigenic mineral composition of an additional 500 samples being evaluated qualitatively. The integration of petrographical observations with log data and core descriptions reveals that cement distribution/abundance is not dominantly driven by depositional facies, nor are reservoir properties. Early pore-filling dolomite cement can be as high as 40% and represents the main control on reservoir properties. Detailed analysis of its spatial distribution shows it to be distinctly related to mudstone proximity and mudstone/sandstone (M/S) ratio. Sandstones occurring as thin beds in mudstone-rich depositional sequences (high M/S ratio) typically exhibit strong pervasive carbonate cement regardless of sedimentary facies. In contrast, sandstones forming thick beds in mudstone-poor sequences (low M/S ratio) are commonly free/low in dolomite cement. Our results demonstrate, for the first time, that reservoir quality in the Rotliegend sandstones in the delta setting of the Netherlands is primarily controlled by early dolomite cement. The latter is most developed in areas with high (>70%) vertical M/S ratio, where it may be a devastating factor for reservoir quality. Best reservoir sandstones should be expected where the depositional stacking pattern is poor in shaly deposits (playa-lake and distal sheet-flood sediments), such as in the southern/south-eastern part of the studied area.

Diagenesis and reservoir quality of sandstones with ancient “deep” incursion of meteoric freshwater——An example in the Nanpu Sag, Bohai Bay Basin, East China

An example of diagenesis and reservoir quality of buried sandstones with ancient incursion of meteoric freshwater is presented in this study. The interpretation is based on information including porosity and permeability, petrography, stable isotopic composition of authigenic minerals, homogenization temperatures (Th) of aqueous fluid inclusions (AFIs), and pore water chemistry. These sandstones, closely beneath or far from the regional unconformity formed during the late Paleogene period, are located in the thick Shahejie Formation in the Gaoliu area of Nanpu Sag, Bohai Bay Basin, East China. Early-diagenetic calcite cements were leached to form intergranular secondary pores without precipitation of late-diagenetic calcite cements in most sandstones. Feldspars were leached to form abundant intragranular secondary pores, but with small amounts of concomitant secondary minerals including authigenic quartz and kaolinite. The mass imbalance between the amount of leached minerals and associated secondary minerals suggests that mineral leaching reactions occurred most likely in an open geochemical system, and diagenetic petrography textures suggest that advective flow dominated the transfer of solutes from leached feldspars and calcites. Low salinity and ion concentrations of present pore waters, and extensive water rock interactions suggest significant incursion of meteoric freshwater flux in the sandstones. Distances of the sandstones to the regional unconformity can reach up to 1800 m, while with significant uplift in the Gaoliu area, the burial depth of such sandstones (below sea level) can be less than 800–1000 m during the uplift and initial reburial stage. Significant uplift during the Oligocene period provided substantial hydraulic drive and widely developed faults served as favorable conduits for downward penetration of meteoric freshwater from the earth's surface (unconformity) to these sandstone beds. Extensive feldspar leaching has been occurring since the uplift period. Coupled high Th (95∼115 °C) of AFI and low δ18O(SMOW) values (+17∼+20‰) within the quartz overgrowths show that quartz cementation occurred in the presence of diagenetic modified meteoric freshwater with δ18O(SMOW) values of −7∼−2‰, indicating that authigenic quartz only have been formed during the late reburial stage when meteoric fresh water penetration slowed down. Secondary pores in thin sections and tested porosity suggest that meteoric freshwater leaching of feldspars and calcite minerals generated approximately 7–10% enhanced secondary porosity in these sandstones. Meteoric freshwater leaching reactions cannot be ignored in similar sandstones that located deep beneath the unconformity, with great uplift moving these sandstones above or close to sea level and with faults connecting the earth's surface with the sandstone beds.

Mineral and chemostratigraphy of a Toarcian black shale hosting Mn-carbonate microbialites (Úrkút, Hungary)

Toarcian black shale that hosts Mn-carbonate microbialites at Úrkút, Hungary was investigated by mineralogical, inorganic, and organic geochemical methods for characterization and comparison with other European black shales representative of the Toarcian Oceanic Anoxic Event. Based on the authigenic mineral composition, calculations were made to estimate environmental conditions during sediment accumulation and early diagenesis. Geochemical and petrographic results of organic, carbonate, and REE multiple-proxy analyses revealed a strong congruence between the host black shale and the Mn-carbonate ore beds. The Úrkút black shale is really a gray shale with moderate to low TOC contents that accumulated in a starved basin. The organic matter and anoxic characteristics resulted from rapid accumulation of organic matter from microbial booms, accompanied by a geothermally generated hydrothermal circulation system, and a high rate of authigenic mineral formation (clay minerals and proto-ore minerals). The inferred enzymatic Mn and Fe oxidation blocked carbonate formation by decreasing the pH. The system remained suboxic via syngenetic mineral accumulation (Fe-rich biomats), and became anoxic during diagenesis in conjunction with pyrite generation. The separation of black shale beds and Mn-ore beds is not distinct through the section. Instead, a distal hydrothermally induced clay-rich authigenic assemblage (marlstone) best describes the black shale, in which Mn-oxide proto-ore beds (Mn-rich laminae) formed from the beginning of black shale deposition, when the oxygen supply in the sedimentary basin was insufficient for enzymatic Mn(II) oxidation. Mn-oxide proto-ore was transformed to Mn-carbonate ore during microbially mediated processes during early diagenesis. The drivers for Mn-bearing organic matter-rich marlstones were most probably a combination of regional and local processes, with generation of a tectonic rift system that promoted geothermally generated hydrothermal fluids, which initiated microbial blooms. Black shale mineralogy, geochemistry, and organic matter at Úrkút differ from those of the epicontinental shelf black shales of the Tethyan Ocean.

ALTERACE DETRITICKÉHO TITANITU V KULMU DRAHANSKÉ VRCHOVINY

Titanite is a common accessory mineral of magmatic rocks, low- and medium-grade metamorphic rocks and it occurs as a detrital mineral in clastic sediments. Our paper deals with titanite alterations as a result of interaction of diagenetic fluids with detrital titanite during deep burial of sedimentary rocks of Drahany Upland, Protivanov Formation. Detrital titanites and their alteration products were examined by electron microprobe. Titanite dissolution was observed from the rim and along the cracks, accompanied by the formation of an authigenic calcite, TiO2 phase, synchysite-(Ce), and rarely chlorite. With an increasing degree of alteration, detrital titanite occurs as small irregular relics inside a mixture of authigenic minerals. Based on the composition of authigenic minerals, we can suppose diagenetic fluid rich in CO2, F and Ca responsible for alteration of detrital titanite. The maximum temperatures in this area during deep burial of sedimentary rocks reached 250–300 °C, corresponding to the conditions of very low-grademetamorphism. Contents of REE (0.67–3.55 wt. % REE2O3) in detrital titanite are rather high and vary significantly. Authigenic synchysite-(Ce) compared with detrital titanite strongly prefers LREE over HREE, indicating that LREE released during dissolution of detrital titanite were preferentially incorporated into the structure of synchysite-(Ce), whereas HREE were mobilized and removed away by diagenetic fluids.

Distribution and composition of authigenic minerals in surface sediments of the western Gulf of Thailand

Generation, morphology, and distribution of authigenic minerals directly reflect sedimentary environment and material sources. Surface sediments were collected from the western Gulf of Thailand during 2011–2012, and 159 samples were analyzed to determine detrital minerals. Authigenic minerals, including siderite, pyrite, and glauconite, are abundant whereas secondary minerals, such as chlorite and limonite, are distributed widely in the study area. Siderite has a maximum content of 19.98 g/kg and appears in three types from nearshore to continental shelf, showing the process of forming-maturity-oxidation. In this process, the MnO content in siderite decreases, but Fe2O3 and MgO content increase. Colorless or transparent siderite pellets are fresh grains generated within a short time and widely distributed throughout the region; high content appears in coastal area where river inputs are discharged. Translucent cemented double pellets appearing light yellow to red are mature grains; high content is observed in the central shelf. Red-brown opaque granular pellets are oxidized grains, which are concentrated in the eastern gulf. Pyrite is mostly distributed in the central continental shelf with an approximately north–south strip. Pyrite are mainly observed in foraminifera shell and distributed in clayey silt sediments, which is similar to that in the Yangtze River mouth and the Yellow Sea. The pyrite in the gulf is deduced from genetic types associated with sulfate reduction and organic matter decomposition. Majority of glauconite are granular with few laminar. Glauconite is concentrated in the northern and southern parts within the boundary of 9.5° to 10.5°N and is affected by river input diffusion. The distribution of glauconite is closely correlated with that of chlorite and plagioclase, indicating that glauconite is possibly derived from altered products of chlorite and plagioclase. The K2O content of glauconite is low or absent, indicating its short formation time.

Tracing the composition, fluid source and formation conditions of the methane-derived authigenic carbonates in the Gulf of Cadiz with rare earth elements and stable isotopes

The mineral composition, stable carbon and oxygen isotopes, and rare earth element (REE) concentrations in methane-derived authigenic carbonates collected from the Gulf of Cadiz were used to trace fluid sources and examine the controlling factors on REE composition during carbonate formation. These authigenic carbonate samples are composed of a detrital fraction (mainly quartz and clay) cemented by authigenic carbonate minerals: aragonite and Mg-calcite, pure Mg-calcite, or dolomite and Mg-calcite. The δ13C values of the whole rock samples vary between −45.8‰ and −9.7‰ VPDB and δ18O values range from 3.7‰ to 6.9‰ VPDB. The δ13C values are indicative of a methane-derived source for these carbonates, most likely resulting from thermogenic gas or a mixture of thermogenic and biogenic gas. The total REE content (ΣREE) of these authigenic carbonate minerals range from 13 to 31 ppm; all but four of the subsamples are lower than a typical marine carbonates (∼28 ppm). This suggests that the REE composition of methane-derived authigenic carbonates is controlled primarily by the methane-rich fluids. The results also show that the REE contents in carbonate mineral subsamples containing dolomite and Mg-calcite is higher than those subsamples with aragonite, suggesting that the authigenic mineral composition and the formation environment are important factors. The shale-normalized REE patterns of the seep carbonates show no or slightly positive Ce anomalies, suggesting that the formation of these seep carbonates occurred under anoxic conditions and associated with rapid fluid seepage. The correlation results of Ce/Ce* and LaN/SmN, Ce/Ce* and DyN/SmN, and Ce/Ce* and ΣREE suggest that the REE characteristics of most seep carbonate samples preserve the original redox conditions of their formation. Late diagenesis or post-oxidation of the samples by their exhumation from the sediments into seawater has little effect on REEs. Therefore, it can be assumed these methane-derived authigenic carbonates represent primary carbonate phases; they have not undergone significant diagenesis and their geochemical and isotopic signatures mainly reflect their formation processes.

Revisiting silicate authigenesis in the Pliocene–Pleistocene Lake Tecopa beds, southeastern California: Depositional and hydrological controls

The Pliocene–Pleistocene Lake Tecopa beds present a well-documented example of authigenic silicate diagenesis in an ancient saline, alkaline lake environment. Controls on authigenic mineral formation and distributions were investigated in nine stratigraphic sections aligned along a north-south transect in the Tecopa basin. Specifically, potential depositional and hydrologic controls on mineral assemblages and distributions were addressed by correlating detailed sedimentological data and basin hydrology with authigenic mineral facies distributions. Deposition occurred within the Lake Tecopa basin in environments ranging from alluvial and eolian around the basin margin to lake margin, mudflat, and shallow and perennial lacustrine in the basin center. The authigenic silicate minerals include trioctahedral smectite, phillipsite, clinoptilolite, opal C-T (cristobalite-tridymite), potassium feldspar, illite, albite, and searlesite, as well as many other minor or less commonly observed phases. Authigenic mineral distributions along the margin of the basin are strongly controlled by sediment composition (primarily tuffaceous component) and lake-level variations. Authigenic mineral compositions in the center of the basin are dominated by feldspar, illite, and searlesite, and are less influenced by sediment composition or short-term changes in lake level. The authigenic silicate mineral composition in the central part of the basin is interpreted to be a result of chemical interaction with a saline, alkaline brine that moved in accord with lake-level changes and induced density-driven circulation. The results suggest that distributions of authigenic silicate minerals in saline, alkaline lake deposits are complexly related to depositional and hydrologic processes and may be of limited utility in resolving lake-level changes in ancient lacustrine systems.

Reconstructing the paleotopography of mountain belts from the isotopic composition of authigenic minerals

Reconstructing the paleotopography of mountain belts from the isotopic composition of authigenic minerals

Reconstructing the paleotopography of mountain belts from the isotopic composition of authigenic minerals

Research Article| February 01, 2000 Reconstructing the paleotopography of mountain belts from the isotopic composition of authigenic minerals C. Page Chamberlain; C. Page Chamberlain 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA Search for other works by this author on: GSW Google Scholar M. A. Poage M. A. Poage 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA Search for other works by this author on: GSW Google Scholar Geology (2000) 28 (2): 115–118. https://doi.org/10.1130/0091-7613(2000)28<115:RTPOMB>2.0.CO;2 Article history received: 09 Jun 1999 rev-recd: 27 Sep 1999 accepted: 05 Oct 1999 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation C. Page Chamberlain, M. A. Poage; Reconstructing the paleotopography of mountain belts from the isotopic composition of authigenic minerals. Geology 2000;; 28 (2): 115–118. doi: https://doi.org/10.1130/0091-7613(2000)28<115:RTPOMB>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The paleorelief of mountain belts can be estimated from the δ18O value of authigenic minerals. Development of relief during mountain building often creates lee-side rain shadows in which precipitation is depleted in 18O and D. The magnitude of this rain-shadow effect is strongly correlated to relief. A compilation of δ18O data from surface waters throughout the globe shows a linear relationship between net elevation change and Δδ18O (R2 = 0.79). Through the use of this relationship, we investigated the timing and magnitude of elevation change in the Southern Alps of New Zealand and the Sierra Nevada of California. The δ18O values of kaolinites from New Zealand show an ∼6‰ decrease in the early Pliocene that corresponds to an ∼2 km elevation change in the Southern Alps. The δ18O of smectites from the Sierra Nevada show little change since 16 Ma, suggesting that these mountains have been a long-standing topographic feature. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Meromixis origin and recent trophic evolution in the Spanish mountain lake La Cruz

A sediment core from a Mediterranean karstic lake was studied through its pollen, diatom, chydorid, ostracod, charcoal and authigenic mineral composition. Information about environmental history recorded in the sediment sequence since the Middle Ages is presented. The main fluctuations of water volume and trophic status of the lake occurred during periods of great change in land management and during climatic cold phases. The synergetic effect of these two factors led to a high water level phase and triggered a rising of the trophic level which produced meromixis. The onset of meromictic conditions at about 1700 AD coincides with the Maunder minimum in the Little Ice Age as well as with a period of increasing human population, woodland clearance and agricultural expansion to the detriment of the nomadic livestock-breeding or transhumance ("Mesta").

Geochemical Evidence for the History of Diagenesis and Fluid Migration: Brent Sandstone, Heather Field, North Sea

The diagenesis of Brent Group sandstones at Heather Field was studied to reconstruct time-dependent variations in reservoir quality, hydrodynamic history, and oil emplacement. Depositional facies, isotopic and trace-element composition of authigenic minerals, and present-day formation-water chemistry indicate several major changes in porewater chemistry related to both gravitational and compactional flow systems that significantly impacted diagenesis. Early cementation by calcite was related to influx of meteoric water and completely occludes porosity in certain areas of the Field, especially in lower reservoir zones. Geochemical, petrographic, and structural evidence indicate that average calcite precipitated at low to moderate temperature from reducing isotopically-depleted water having high levels of radiogenic Sr (40°–50°C, δ 18O = −4 to −6‰, 87Sr/86Sr > 0·71). A major period of kaolinite precipitation and feldspar dissolution followed calcite cementation. The isotopic composition of pore-filling kaolinite shows Field-wide uniformity (δ 18O average 13·8‰, δD average −53·2‰), suggesting thorough flushing of the reservoir by meteoric water and precipitation at low to moderate temperature (45°–60°C). Tectonic, burial, and thermal histories suggest that meteoric flushing occurred during the late-Cimmerian sea-level low, possibly in response to gravitational flow of meteoric water from exposed parts of the adjacent East Shetlands Platform. Illite and quartz diagenesis post-date kaolinite cementation, with illite K-Ar ages indicating precipitation through much of the Paleogene (55–27 Ma), coincident with migration of hydrocarbons from neighbouring sub-basins of the East Shetlands Basin. Illite stable isotopic data indicate precipitation in a system resulting from partial mixing of trapped meteoric pore-fluids with saline compaction water. The intensity of sandstone diagenesis is influenced by differences in the fluid migration history, content of detrital K-feldspar, and the time of hydrocarbon emplacement and results in spatial and temporal variations in reservoir quality.