Early Diagenetic Phases Research Articles

Acid saline fluid inclusions: examples from modern and Permian extreme lake systems

AbstractSome of the Earth's most extreme aqueous chemistries are found at acid saline lakes and associated shallow groundwaters. Modern lakes and groundwaters in Western Australia have pHs as low as 1.5, total dissolved solids as high as 32%, unusually high concentrations of Al, Si, Fe, As, and Sb, and many other chemical characteristics atypical of natural waters. Analogous Permian red bed and evaporite lake deposits in the US midcontinent also had extreme water compositions, including negative pH. Fluid inclusions in chemical sediments from modern and Permian acid saline lakes record specific physical, chemical, and biological conditions. Primary inclusions in this bedded halite and gypsum are trapped shallow lake water, air bubbles, crystals of other minerals, and microorganisms and organic compounds. Isolated fluid inclusions in early diagenetic phases of halite are remnants of shallow groundwaters and can be analyzed to determine chemical compositions of past groundwaters. A variety of traditional methods, including petrography, freezing–melting microthermometry and laser Raman spectroscopy, and innovative variations, such as homogenization of artificially nucleated vapor bubbles and UV‐vis petrography, produce data detailing environmental conditions. Other traditionally used methods, such as laser ablation ICP‐MS, are not effective due to the inclusions' high and complex salinity, extremely low pH and/or low pressure. Making synthetic solutions that match the complexity of natural acid saline inclusions is another challenge. Fluid inclusions in acid‐precipitated halite have proven to be excellent repositories for past surface chemical, physical, and biological conditions. Although the methods used and data obtained from acid‐precipitated halite and gypsum are not exactly the same as for neutral‐alkaline evaporites, acid inclusions provide high‐resolution environmental data for modern and ancient extreme environments. These extreme fluids may also have similarities to other past extreme environments, to ore mineralizing fluids, and to past liquids on Mars.

An Early Jurassic brachiopod-dominated seep deposit enclosed by serpentinite, eastern Oregon, USA

Many Paleozoic and Mesozoic hydrocarbon-seep deposits are typified by superabundant dimerelloid brachiopods. By contrast, Cenozoic and modern seeps have been and still are dominated by mollusks and tube worms. To date, five genera of the rhynchonellide superfamily Dimerelloidea have been confirmed to have lived at hydrocarbon seeps. Here, we look into the paleoecology of another dimerelloid genus, studying an unusual limestone body in the Blue Mountains Province of eastern Oregon, USA. This limestone block, found in the vicinity of the town of Seneca, is characterized by a mass occurrence of a dimerelloid brachiopod assigned to the species Sulcirostra paronai. The above block, along with two other sizeable exotic blocks of sedimentary rock scattered in its immediate periphery, and small pods of Sulcirostra limestone scattered in its wider periphery are enclosed in serpentinite. The brachiopod limestone predominantly consists of early diagenetic phases including clotted micrite and fibrous, banded and botryoidal cement. Negative δ13C values of these carbonate phases as low as −23.5‰ agree with precipitation resulting from the oxidation of hydrocarbons. Petrographic and isotopic evidence reveals that the dimerelloids lived at a seep on the Early Jurassic seafloor. The age assignment is based on Sinemurian ammonoids associated with Sulcirostra in one of the exotic blocks from the direct periphery of the main deposit. The exotic blocks are attributed to the Lower Jurassic Keller Creek Formation. The occurrence of the Sulcirostra limestone within serpentinite calls for an evaluation of the possibility that the Jurassic chemosynthesis-based ecosystem was driven by serpentinization, given the presence of chemosymbiotic communities on the modern seafloor where hydrogen and methane anomalies are generated by serpentinization. The nature of the contacts between the blocks of sedimentary rock including the Sulcirostra limestone and the surrounding serpentinite reveals that these lithologies were juxtaposed by tectonic processes, probably thrusting of the Keller Creek Formation strata, as well as other Aldrich Mountains Group rocks, over the serpentinites. No unambiguous evidence has been found that supports a derivation of seepage fluids from serpentinization, yet it is obvious that Sulcirostra lived at a hydrocarbon seep, confirming the association of the superfamily Dimerelloidea with seep environments for one more genus.

Stochastic Joint Simulation of Facies and Diagenesis: A Case Study on Early Diagenesis of the Madison Formation (Wyoming, USA)

The aim of this paper is to propose an integrated approach to reproduce both facies and diagenetic trends in a static reservoir model based on an outcrop case study. In Wyoming (USA), the Madison Formation (Mississippian) is a thick (up to 350 m) carbonate series, outcropping in several locations of the Bighorn foreland basin.Within these series, nine sedimentary facies have been identified. Based on their vertical stacking pattern, they are organized in small-scale facies sequences: 1) intertidal to supratidal facies sequence; 2) shallow subtidal to intertidal facies sequence; 3) deep subtidal facies sequence. These facies associations have been integrated in a synthetic depositional model, which corresponds to a carbonate ramp progressively evolving towards the most inner part of a platform. This enables to propose a sequence stratigraphy framework for the studied series, that represents at least six third-order sequences (some of them being locally eroded).The diagenetic study has been focused on the identification of the early diagenetic phases. Results from these analyses show the occurrence of several successive early diagenetic phases (micritization, marine calcite cementation, dolomitization, etc.). For modeling purposes, seven “diagenetic imprints” have been defined, each of them corresponding to a succession of diagenetic phases that can coexist in the same sedimentary facies. Moreover, as each sedimentary facies may be affected by several diagenetic imprints, a quantification of these imprints has been realized.A 3D gridded model designed for geostatistical modeling has been constructed in order to reproduce the facies organization of the three first third-order sequences (that are the best documented). The gridding is then based on the four sequence boundaries which have been recognized on every section. The relationships between sedimentary facies and diagenesis have been used to define lithofacies simulation rules. The simulations are based on the plurigaussian and nested algorithms. Finally, a discussion on the distribution of the potential reservoir heterogeneities is proposed, taking into account the sedimentary characteristics (facies, architectures, etc.) and the diagenetic impact.

Mineralogical constraints on the paleoenvironments of the Ediacaran Doushantuo Formation

Assemblages of clay minerals are routinely used as proxies for paleoclimatic change and paleoenvironmental conditions in Phanerozoic rocks. However, this tool is rarely applied in older sedimentary units. In this paper, the clay mineralogy of the Doushantuo Formation in South China is documented, providing constraints on depositional conditions of the Ediacaran Yangtze platform that host the earliest animal fossils in the geological record. In multiple sections from the Yangtze Gorges area, trioctahedral smectite (saponite) and its diagenetic products (mixed-layer chlorite/smectite, corrensite, and chlorite) are the dominant clays through the lower 80 m of the formation and constitute up to 30 wt% of the bulk rock. Saponite is interpreted as an in situ early diagenetic phase that formed in alkaline conditions (pH > or = 9). The absence of saponite in stratigraphically equivalent basin sections, 200-400 km to the south, indicates that alkaline conditions were localized in a nonmarine basin near the Yangtze Gorges region. This interpretation is consistent with crustal abundances of redox-sensitive trace elements in saponitic mudstones deposited under anoxic conditions, as well as a 10 per thousand difference in the carbon isotope record between Yangtze Gorges and basin sections. Our findings suggest that nonmarine environments may have been hospitable for the fauna preserved in the Yangtze Gorges, which includes the oldest examples of animal embryo fossils and acanthomorphic acritarchs.

TAPHONOMY OF VERTEBRATE FOSSIL ASSEMBLAGES FROM SWAMPY CIRCUM-LAKE ENVIRONMENTS: AN EXAMPLE FROM THE LATE EOCENE OF ZAMBRANA (IBERIAN PENINSULA)

Biostratinomic and bone-diagenesis parameters are used to evaluate the effects of abiotic and biotic processes on the final composition of two late Eocene vertebrate assemblages preserved in a swampy circum-lake environment from the Zambrana site (Basque-Cantabrian Region, northern Iberian Peninsula). No significant transport or bone sorting by fluvial action is observed, and complete and fragmentary bones in the assemblages (Z4 and Z6 beds) show the same biostratinomic features. The attritional bone accumulation in both excavated beds was caused mostly by biotic factors related to routine ecological deaths of population members, probably with some input from predators on the most vulnerable ungulate individuals (autochthonous), although a weak input of small remains by superficial water currents from nearby areas (parautochthonous) cannot be rejected. The vegetation and wet conditions of the swampy environment, together with possible predator and scavenger activity, could have caused the disarticulation and dispersion of some vertebrate remains, mainly those of ungulates. The bones were buried relatively quickly in the phreatic zone under reducing conditions and suffered considerable crushing and fragmentation because of lithostratigraphic compaction. Mineralogic similarities between sedimentary fillings in the fossils and the host sediment, as well as the homogeneous rare earth element (REE) trends of the fossils, are indicative of a uniform and unique diagenetic history and the absence of reworked elements. The fossil bone mineral is well-crystallized francolite (carbonate fluorapatite). The REE enrichment of bones and their calcite and pyrite crusts and fillings were formed during early diagenetic phases. The black coloration of the bones may be explained by their high hydrocarbon (n-alkane components) contents.

Expression of the Early Toarcian negative carbon-isotope excursion in separated carbonate microfractions (Jurassic, Paris Basin)

The causes of the pronounced negative excursion in carbon-isotope values that was recorded during the Early Toarcian Oceanic Anoxic Event (T-OAE) are still under debate, particularly with regard to the local versus global pattern of the excursion, and the extent to which recorded signals are under a diagenetic control. In this study we employ a novel microseparation technique in order to investigate the isotopic and mineralogical characteristics of different size fractions of the carbonate content from a Toarcian section recovered from the Sancerre–Couy borehole, southern Paris Basin. Beyond the recognition of a − 6‰ δ 13C excursion in the bulk carbonate content, our data also demonstrate that biogenic particles (such as coccoliths) and inorganic grains precipitated as early diagenetic phases (including dolomite) both record the excursion with the same magnitude. Although several black shales occur through the Paris Basin Toarcian section, it is only that associated with the onset of the OAE that coincides with a large negative carbon-isotope excursion. Taken together these observations indicate that during this event, the entire water column was characterized by homogeneous carbon-isotope values; such a pattern is incompatible with the idea that the negative excursion was generated simply through the upwelling of bottom waters enriched in re-mineralized organic carbon (cf. “the Küspert model”), since this would have required a strong vertical gradient in the water column. Additionally, the Paris Basin data show that the decrease in carbonate δ 13C values during the OAE occurred in several discrete steps (each of some − 2‰), as has previously been found for organic carbon substrates in other European sections. The stepped nature of the isotopic profile, which is part of a stratigraphic signature previously ascribed to Milankovitch forcing, is compatible with regular pulsed input of light carbon into the whole atmosphere–ocean system from a climatically sensitive source such as gas hydrate, or from thermal methanogenesis of organic-rich sediments in the Karoo–Ferrar large igneous province. Contrasts in the amplitude of the negative carbon-isotope excursion on a regional scale remain an important unexplained aspect of the Toarcian record.

Anaerobic and Aerobic Oxidation of Methane at Late Cretaceous Seeps in the Western Interior Seaway, USA

The Late Cretaceous (Campanian) Tepee Buttes represent a series of conical, fossiliferous limestone deposits embedded in marine shales that deposited in the Western Interior Seaway. The previously suggested origin of the Tepee Buttes at methane-seeps was confirmed by this study. δ13C values as low as −50‰ of early diagenetic carbonate phases of two Tepee Buttes near Pueblo (Colorado) reveal that methane was the major carbon source. Molecular fossils released from a methane-seep limestone contain abundant 13C-depleted archaeal lipids (PMI, biphytane; δ 13C: −118 and −102‰), derived from anaerobic methanotrophs. A suite of 13C-depleted bacterial biomarkers (branched fatty acids; −73 to −51‰) reflects the former presence of sulfate-reducing bacteria, corroborating that a syntrophic consortium of archaea and bacteria mediating anaerobic oxidation of methane already existed in Cretaceous times. Molecular fossils also suggest that methane was not exclusively oxidized in an anaerobic process. A series of unusual C34/C35-8,14-secohexahydrobenzohopanes with low δ13C values (−110 and −107‰) points to the presence of aerobic methanotrophic bacteria at the ancient seep site.

Jurassic and Cretaceous clays of the northern and central North Sea hydrocarbon reservoirs reviewed

AbstractThe principal clays of the northern and central North Sea are illite (sometimes with interlayered smectite) and kaolin. Chlorite is only locally important. Although it has been proposed that kaolin within North Sea sandstones is detrital in origin, the majority of workers have concluded that it is authigenic, largely the product of feldspar alteration. Kaolin is found within a wide range of sedimentary settings (and within shales) apparently defying the notion that kaolin is an indicator of meteoric water deposition. Within sandstones, the earliest authigenic kaolin has a vermiform morphology, the distribution of which is controlled by the availability of detrital mica to act as a nucleus, and the composition of the post-depositional porewaters. This vermiform kaolin formed in meteoric water, the presence of which is easily accounted for below sub-aerial exposure surfaces in non-marine formations, and below unconformities over marine units. In fully marine sands, and even marine shale units, kaolin still occurs. It has therefore been suggested that even these locations have been flushed with meteoric water.Early vermiform kaolin recrystallizes to a more blocky morphology as burial proceeds, at least in the Brent Group. Blocky kaolin has been reported as growing before, synchronously with, and after the formation of quartz overgrowths, though oxygen isotope studies support low-temperature growth, pre-quartz. Blocky kaolin may form during meteoric flushing associated with lower Cretaceous uplift and erosion, though it is found in fault blocks that are thought to have remained below sea level. Here, the kaolin may form in stagnant meteoric water, relics of the post-depositional porewater. It has also been proposed that the blocky kaolin grew in ascending basinal waters charged with carboxylic acids and CO2, though this hypothesis is not supported by stable oxygen isotope data. Some of the blocky kaolin is dickite, the stable polymorph above ∼100°C.Fibrous illite occurs almost ubiquitously within the clastic sediments of the North Sea. An early pore-lining phase has been interpreted as both infiltrated clastic clay, and as an early diagenetic phase. Early clays may have been quite smectite-rich illites, or even discrete smectites. Later, fibrous illite is undoubtedly neoformed, and can degrade reservoir quality significantly. Both within sandstones and shales, there is an apparent increase in the K content deeper than 4 km of burial, which could be due to dilution of the early smectite-rich phase by new growth illite, or to the progressive illitization of existing I-S. Much of the ‘illite’ that has been dated by the K-Ar method may therefore actually be I-S.The factors that control the formation of fibrous illite are only poorly known, though temperature must play a role. Illite growth has been proposed for almost the entire range of diagenetic temperatures (e.g. 15–20°C, Brent Group; 35–40°C, Oxfordian Sand, Inner Moray Firth; 50–90°C, Brae formation; 100–110°C, Brent Group; 130–140°C, Haltenbanken). It seems unlikely that there is a threshold temperature below which illite growth is impossible (or too slow to be significant), though this is a recurring hypothesis in the literature. Instead, illite growth seems to be an event, commonly triggered by oil emplacement or another change in the physiochemical conditions within the sandstone, such as an episode of overpressure release. Hence fibrous illite can grow at any temperature encountered during diagenesis.Although there is an extensive dataset of K-Ar ages of authigenic illites from the Jurassic of the North Sea, there is no consensus as to whether the data are meaningful, or whether the purified illite samples prepared for analysis are so contaminated with detrital phases as to render the age data meaningless. At present it is unclear about how to resolve this problem, though there is some indication that chemical micro-analysis could help. It is a common belief that illite ages record the timing of oil charge, and so can be used to calibrate basin models.Grain-coating Fe-rich chlorite cements can preserve exceptional porosity during burial. They are found in marginal marine sandstones, formed during diagenesis from precursor Fe-rich clays such as berthierine or verdine.

Chemostratigraphy of the Jurassic System: applications, limitations and implications for palaeoceanography

Current chemostratigraphical studies of the Jurassic System primarily involve the use of one sedimentary component (marine organic carbon), one divalent transition metal substituted in carbonate (manganese), and two isotopic tracers: strontium-isotope ratios ( 87 Sr/ 86 Sr) and carbon-isotope ratios (δ 13 C carb and δ 13 C org ) in carbonate and in organic matter. Other parameters such as Mg/Ca and Sr/Ca ratios in calcite, oxygen-isotope ratios (δ 18 O) in carbonate, sulphur-isotope ratios (δ 34 S) in carbonate-hosted sulphate, nitrogen-isotope ratios (δ 15 N org ) in organic matter, osmium-isotope ratios ( 187 Os/ 188 Os) in black shales and neodymium-isotope ratios ( 143 Nd/ 144 Nd) in various mineral phases are also useful but at present give poor resolution because the database is incomplete or compromised by various factors. Stratigraphical patterns in total organic carbon (TOC) can be of either local or regional significance, depending on the lateral extent of the former nutrient-rich and productive water mass. Divalent manganese follows a similar pattern, being concentrated, most probably as a very early diagenetic phase, only in oxygen-depleted waters that typically underlie zones of elevated organic productivity. Shifts in Mg/Ca and Sr/Ca ratios on the time scale of ammonite subzones seem largely to reflect temperature changes. Strontium-isotope ratios from pristine skeletal calcite provide a global signal; δ 13 C values from carbonates with minimal diagenetic overprint potentially do the same, although small spatial differences in palaeo-water-mass composition may have been locally significant. Oxygen-isotope determinations on carbonate rocks and fossils generally yield values that are too scattered to be stratigraphically useful, because they reflect palaeotemperature, the evaporation–precipitation balance in sea water and the impact of any diagenesis involving an aqueous phase. Nitrogen-isotope ratios in organic matter reflect the chemistry of ancient water masses as affected by nitrate utilization and denitrification, and the stratigraphical pattern of this parameter is more likely to correlate only on a regional basis. Neodymium-isotope ratios in sea water are also water mass dependent and greatly affected by regional sources and oceanic current systems. Preliminary data on sulphur-isotope ratios in carbonates and osmium-isotope ratios in organic-rich shales, both potentially offering global correlation, indicate that these tracers may be valuable, although the records at present are not sufficiently well established to allow high-resolution regional correlation. In all cases, biostratigraphically well-dated reference sections, against which the relevant geochemical data have been calibrated, are required in the first instance. To date, studies on the stratigraphical distribution of organic carbon have been principally carried out in both northern (Boreal) and southern (Tethyan) Europe; carbon-isotope stratigraphy has been undertaken primarily, but not exclusively, on bulk pelagic sediments from the Alpine–Mediterranean or Tethyan domain; and strontium-isotope stratigraphy has been undertaken largely on calcitic skeletal material (belemnites and oysters) from northern and southern Europe. In many sections, including those containing ammonites, multi-parameter chemostratigraphy can give resolution that exceeds that attainable by classic biostratigraphical means. Strontium-isotope ratios in skeletal calcite are a particularly powerful tool for illustrating changes in sedimentary rate and revealing gaps in the stratigraphical record.

Contradictory magnetic polarities in sediments and variable timing of neoformation of authigenic greigite

In several recent published studies, paleomagnetic results from greigite-bearing sediments reveal characteristic remanences that are anti-parallel to those carried by coexisting detrital magnetic minerals and polarities that are opposite to those expected for the age of the rock unit. These observations have important implications for the reliability of paleomagnetic data from greigite-bearing sediments. We have investigated the origin of such contradictory magnetic polarities by studying the formation mechanisms of greigite in mudstones from the Lower Gutingkeng Formation, southwestern Taiwan. Scanning electron microscope observations indicate that the Gutingkeng greigite has three modes of occurrence, including nodular, framboidal and matrix greigite. Microtextural observations, including transection of bedding by iron-sulfide nodules with no deviation of sediment textures, the presence of partially dissolved edges around detrital and early diagenetic phases, and neoformation of greigite and Fe-rich clays around detrital phyllosilicates, indicate that all three types of greigite have a diagenetic origin that post-dates early diagenetic pyrite. In addition, paleomagnetic data yield contradictory polarities even for greigite-bearing sister samples from the same stratigraphic horizon. The data are collectively interpreted to indicate that neoformation of the Gutingkeng greigite occurred after partial dissolution of syngenetic or early diagenetic pyrite. The timing of greigite formation can apparently vary enough to give contradictory polarities for different greigite components even within a single stratigraphic horizon. Direct petrographic observation of authigenic magnetic iron-sulfide phases, as carried out in this study, can provide important constraints on formation mechanisms and timing of remanence acquisition for these minerals and suggests that care should be taken when interpreting magnetostratigraphic data from greigite-bearing sediments.

Siderite in the Ivishak Sandstone, Prudhoe Bay, Alaska: Is It an Indicator of an Early Burial Environment?: ABSTRACT

Petrographic, elemental, and isotope data suggest that siderite in the Ivishalk Sandstone is not entirely an early diagenetic phase. Previous workers have used siderite cement in the Triassic Ivishak Sandstone as an example of early diagenetic siderite from a fresh-water depositional environment. Abundant and ubiquitous siderite cement includes: porefilling, siderite replacement of framework chert grains, and wheat seed morphologies that include a later diagenetic phase. Petrographic data show that some siderite cement follows compaction. Burial history curves indicate that the Ivishalk was not buried to more than 2,000 ft. before it was uplifted and exposed in the Cretaceous. It was subsequently buried to its present burial depths of 8,000-1 0,000 ft. A siderite phase following compaction is probably post-Triassic in age. Elemental and isotopic analyses indicate distinct generations of siderite cement. Siderite compositions fall into a non-marine field with high Fe and low Mg and Ca concentrations. Zoned siderite shows lower Fe concentrations. Siderite cement in mudstones contain higher Fe concentrations than in sandstones and conglomerates. The isotopic compositions of siderite varies widely with [delta][sup 13]C and [delta][sup 18]O of -1 3.7 to +1 7.4 PDB and -8.8 to 0.0 PDB, respectively. The [delta][sup 13]C and [delta][sup 18]O of sideritemore » is more enriched in the mudstones (0.0 to +1 7.4 and -5.0 to 0.0, respectively) than in the sand- stones and conglomerates (-1 3.7 to -1.6 and -8.8 to -5.0, respectively). Siderite in mudstones document precipitation under methanogenic conditions, whereas siderite in sandstones and conglomerates record precipitation in the suboxic environment.« less

U-Series Dating by the TIMS Technique of Land Snails from Paleosols in the Canary Islands

A sequence of seven superimposed paleosols developed on eolian calcarenites and alluvium was sampled on the island of Lanzarote in order to examine the possibility of dating land snail shells by the U-series method, using a TIMS technique allowing measurement of U and Th isotopes in very small samples. In the lower six units, the fossil shells yielded D-allo/L-isoleucine (A/I) ratios of about 0.5 and apparent AMS 14C ages ranging from 41,000 to 34,000 yr B.P., indicating that most paleosols formed during a relatively short mid-Würm humid episode. The upper unit (paleosol 7) yielded more variable A/I ratios (ranging from 0.6 to 0.2) and a younger 14C age ∼27,000 yr B.P. Most samples contained enough U to allow the calculation of U-series ages, after correction for the presence of a detrital component. In samples containing a few tens of ppb of U (paleosols 1, 2, 3, 6, and 7), the ages are strongly dependent upon the model used for the correction. In samples containing more than 300 ppb of U (paleosols 4 and 5), concordant ages of ∼31,000 ± 1000 yr were obtained regardless of the correction model used. U uptake in these shells occurred during one single early diagenetic phase, soon after burial, since shells of modern snails do not contain any significant amount of U. The arid conditions subsequent to the mid-Würm humid episode have likely ensured since then a fair closure of the radioactive system.

Authigenic K-NH4-Feldspar in Sandstones: A Fingerprint of the Diagenesis of Organic Matter

ABSTRACT In arkosic sandstones of the San Joaquin and Los Angeles Basins presently at temperatures between 35°C and 174°C, trace amounts of authigenic K-NH4-feldspar are present as microfracture fillings and overgrowths on detrital K-feldspar. Microchemical analyses of this authigenic phase reveal up to 80 mole % buddingtonite. The largest ammonium concentrations are observed in a sandy interval of the Antelope shale (80 mole %) and in the Stevens Sands ( 50 mole %) of the San Joaquin Basin. This latter unit was deposited as a turbidite in the organic-rich Fruitvale Shale, an equivalent of the Antelope Shale. The lowest ammonium contents (0-16 mole %) are present in the shallow-marine Vedder Sands and the marginal marin San Joaquin Formation in the same basin. Petrographic, 18O, 13C, and 87Sr/86Sr analyses of a dolomite cement that postdates authigenic K-NH4-feldspar indicate that the feldspar precipitated below 28°C in the zone of methanogenesis, from pore waters with the same Sr signature as sea water at the time of sedimentation. Authigenic K-NH4-feldspar is thus an early-diagenetic phase that crystallized prior to oil migration, under anoxic conditions when organic matter releases ammonium. The source of ammonium is bacterial decay of organic matter in the sandstones themselves and/or in contemporaneous shales.

Diagenesis of the Sandflat and Mudflat Facies of the Upper Queen Formation, Midland Basin, Texas

The upper Queen Formation (Permian, Guadalupian) of the Midland basin, Texas, consists of cyclically interbedded clastics and evaporites that were deposited in a fluvial-dominated continental sabkha environment. Fluvial sandflat deposits, consisting of thin units (0.2-5.0 m) of very fine grained arkosic sandstones constitute reservoir horizons, whereas 0.1-1.3 m thick units of coarse siltstones and evaporites of playa-mudflat deposits are nonproductive. This study addresses the diagenetic histories of the reservoir and nonreservoir clastics. The primary porosity of the clastics was first reduced by pore-filling hematitic smectite clay, anhydrite, and dolomite during an early diagenetic phase. Subsequent dissolution of the anhydrite and dolomite by acidic pore-waters created high porosities (mean = 15%) and permeabilities (mean = 70 md) in the sandflat deposits, porosities which were only slightly occluded by later dissolution and reprecipitation of grain-lining smectite. Pore-water movement and subsequent hydrocarbon migration were both controlled by the coarser grain size and lower clay-matrix and silt content of these sandflat deposits. In contrast, the finer grain size and higher clay-matrix and silt content prevented similar dissolution of cements within the mudflat facies, which have significantly lower porosities (mean{lt}10%) and permeabilities (mean{lt}0.1 md). Fluids and gases used in enhanced recovery techniques will follow pathwaysmore » created by dissolution of anhydrite and dolomite cements within the sandflat facies. However, caution must be used with fluids that can cause swelling of the grain-lining smectite.« less

Sandstone diagenesis in the Pattani Basin (Gulf of Thailand): history of water-rock interaction and comparison with the Gulf of Mexico

In the Pattani Basin, a failed-rift basin, extensive water-rock interaction has occurred between subquartzose alluvial sandstones of Miocene age and their pore fluids. Diagenetic rates and pathways have been strongly influenced by high geothermal gradients, high CO 2 fugacities, and low pore water salinities. Depositional pore water was fresh to brackish, depending on the depositional environment of the sediments. Chloride concentrations in modern formation water are believed primarily to reflect the proportions of river and sea water in the depositional environment. However, the concentration of other important solutes and the isotopic composition of the formation waters can not be explained by roportional mixing of these two end-member waters. Dissolution of detrital plagioclase (An = 3) and K- feldspar are reactions of major significance that are reflected chemically in the Na/Cl and K/Cl ratios of the formation water. Despite the high temperature of the sandstones (120–200°C), diagenetic albite does not occur. Geochemical calculations indicate the formation water is undersaturated with respect to both orthoclase and albite. This style of feldspar diagenesis differs significantly from that of sandstones of similar composition in other basins, and has probably influenced other aspects of silicate diagenesis. Important authigenic minerals are: 1. locally abundant calcite cement (δ 13C= −12.8, δ 18O= −17.3 PDB), an early diagenetic phase that formed at about 60°C; 2. pore-filling kaolinite (δ 18O= 9.9, δD= −83.5SMOW) that was closely associated with feldspar dissolution and formed over a range of temperatures; and 3. fibrous pore-lining and pore-bridging illite (δ 18O = 9.8, δD = − 86.7 SMOW, the last significant cement, formed at temperatures of 120 to 150°C. Potassium/argon dates on illite indicate that sandstone diagenesis took place during a period of rapid sedimentation in the first two-thirds of the burial history. Comparison of Pattani Basin diagenesis with diagenesis of sandstones of similar age in other sedimentary basins demonstrates that chemical diagenesis, relative to mechanical compaction, has been especially rapid in the Pattani Basin. This reflects the effect of high temperatures on reaction rates. The net effect is a high average rate of porosity loss with burial (11% km).

Diagenetic Alteration of Perlite in the Guryongpo Area, Republic of Korea

AbstractPerlite, which occurs at the contacts between dacite and lacustrine tuffs of Miocene age in the Guryongpo area, Korea, has undergone more sluggish and incomplete diagenetic alteration than the surrounding zeolitic tuffs. Alkali-clinoptilolite, mordenite, smectite, K-rich gel-like glass, low-cristobalite, and K-feldspar are characteristic diagenetic phases in the altered perlite.Hydration of the glass to form perlite resulted in the expulsion of cations, mainly Na, from the glass into the pore fluid and in the relative enrichment of K in the perlite. Hydration of the glass also resulted in increased alkalinity of the pore fluid, which, in turn, affected the nature and behavior of subsequent glass dissolution. Textural observations and chemical data on the early diagenetic phases indicate a sequence of incongruent dissolution reactions, which depended on silica activity and alkalinity of the ambient pore fluid: Reaction (1) 12.5 perlitic glass + 3.88 K+ + 0.65 H+ + 15.4 H20 = smectite + 9.5 gellike glass + 4.03 Na+ + 0.25 Ca2+ + 10.55 H4SiO4 released Ca and silica which were consumed by reaction (2) perlitic glass + 0.1 Ca2+ + 0.1 H4SiO4 + 0.1 H+ + H20 = clinoptilolite + 0.1 K+ + 0.2 Na+. The paragenesis from glass via smectite to alkali zeolites in most glass-bearing rocks may be explained by a sequence of such dissolution reactions. Still later reactions involved the transition from less-silicic clinoptilolite to an assemblage of silicic Na-clinoptilolite + mordenite and the crystallization of the gel-like glass to K-feldspar.

Diagenesis of the Gully Oolite (Lower Carboniferous), South Wales

AbstractDeposition of the Gully Oolite was locally interrupted by emergence and a regionally extensive palaeosol is present at the top of the unit. Early diagenetic phases include isopachous, fibrous submarine cements, nonluminescent vadose cements, and mixing zone dolomite. Subsequent nonferroan phreatic cements are non‐ to dully luminescent and in restricted vertical intervals predate significant compaction. More usually, however, phreatic cements postdate extensive overpacking of allochems. Ooid isotopic composition (δ18O=‐7·80° to ‐3·10° and δ13C = ‐2·38° to +3·28°) is similar to that of associated phreatic cements and the data suggest that the bulk of ooid stabilization and cementation occurred within meteoric groundwaters. The extensive allochem overpacking appears to have occurred during the first few tens of metres of burial and intergranular macroporosity was eliminated prior to deep burial. Fracturing of the Gully Oolite during the Hercynian Orogeny and subsequent post‐orogenic uplift led to localized dolomitization, several generations of calcite veins, and the restricted occurrence of 18O depleted cements in inter‐ and intragranular microporosity. Some of the veins clearly relate to Triassic exhumation of the Carboniferous Limestone, but others may be related to post‐Mesozoic uplift and erosion of South Wales. Fracture‐associated dolomitization may have occurred within a large‐scale post‐orogenic groundwater system, with Mg2+ being supplied through the release of deeply buried diagenetic brines.

Oxygen and carbon isotope compositions of Upper Cretaceous chalk from the Danish sub‐basin and the North Sea Central Graben

ABSTRACTThe oxygen and carbon isotopic compositions of Upper Cretaceous chalk have been studied from a large number of outcrops and boreholes in the Danish sub‐basin and the North Sea Central Graben. The carbon isotopic compositions vary from +0.50%o to +3.00%oδ13C which correspond to the expected carbon isotopic composition of carbonate precipitated from normal sea water. The oxygen isotopic compositions of samples from outcrops and near‐surface drilled sections fall in the range from —0.50%o to — 2.00%oδ18O, which is close to the expected values for carbonate produced by coccoliths in the Late Cretaceous sea. A geographic distribution exists in the Danish sub‐basin showing relatively heavy oxygen isotope values in the centre of the basin and slightly more negative values in the eastern part of the basin. Primarily the phenomenon is thought to reflect minor variations in the oceanographic parameters in the Late Cretaceous sea.The deep subsurface sections represent a depth interval of 0–3100 m. Mechanical compaction in the uppermost part of the sequences apparently has no influence on the isotopic composition. Chemical compaction dominates at greater depth, leading to temperature‐induced isotopic re‐equilibration in the water‐rock system. In these sequences the oxygen isotope values range from − 1.50%o to — 7.50%oδ18O and are significantly correlated with depth of burial, cementation and porosity. The data indicate that pressure‐dissolution, recrystallization, reprecipitation and ion‐exchange between solid carbonate phase and pore water, are all actively involved in the oxygen isotope re‐equilibration process. This process is believed to take place in a diagenetically closed system and is tentatively divided into two phases: 1) an early diagenetic phase which takes place at porosities down to approximately 20% in which the chalk possesses a pore‐water controlled isotopic re‐equilibration system and 2) a late diagenetic phase at porosities below 20% in which the re‐equilibration process increasingly becomes influenced by the rock‐introduced isotopic change in the composition of the formation water.

Diagenesis of a deeply buried sandstone reservoir: Hild Field, Northern North Sea

AbstractCalcite cementation and extensive dissolution of feldspar with formation of authigenic kaolinite, quartz cement and secondary porosity are the main diagenetic processes in the deeply buried Hild Field. Mineralogical and isotopic analyses, reservoir pressure and depositional environment suggest that these diagenetic processes occurred prior to burial at a depth of 1·5–2 km. The timing of the diagenetic sequence suggests that feldspar dissolution is related to meteoric water flow. Calcite occurs as an early diagenetic iron-poor cement, and as two types of later diagenetic (<120°C) ferroan calcite cements. The ferroan calcites are mainly an in situ dissolution-reprecipitation product of the early diagenetic phase. Extensive local dissolution of calcite was important for forming secondary porosity which is closely associated with a prominent ‘gas chimney’ in the area studied. A high CO2 content in the natural gases of the reservoir suggests that the solvent was carbonic acid formed from CO2 generated during thermal maturation of organic matter. Calcite dissolution probably occurred between 70° and 100°C.

EARLY DIAGENESIS AND THE PRESERVATION OF POROSITY IN JURASSIC LIMESTONES

The study of Quaternary sediments has shown that carbonates are susceptible to early lithification, or to dissolution, in many different environments. Most of these surface effects, although reducing primary porosity, rarely destroy it completely. Although similar diagenetic phenomena also occur in ancient rocks, the great majority of limestones do not exhibit early diagenetic fabrics; much sparitic cement is possibly related to carbonate liberated by pressure‐solution within the limestone mass. In spite of the fact that carbonate sediments frequently have high initial porosities, most of it is destroyed by sparitic cement. The exceptional circumstances favouring the preservation of effective porosity may be related to the presence of an early diagenetic phase. A number of reservoirs exhibit early diagenetic cements, dissolution and dedolomitisation fabrics. It is concluded that early lithification tends to retard or prevent subsequent pressure‐compaction and thus reduces the volume of carbonate available for porosity destruction. The recognition of early diagenetic phenomena in ancient limestones therefore has important economic implications.