Diagenetic Carbonate Research Articles

Evolution of Network of Clastic Petroleum Source Rock During Catagenesis (Lower Toarcian Posidonia Shale, Hils Syncline, Northwestern Germany): ABSTRACT

The Posidonia shale represents the most prolific petroleum source rock of central Europe. Completely cored sections of this lower Toarcian oil shale interval from six shallow boreholes were studied in order to investigate the influences of lithofacies, diagenesis, hydrocarbon content, and compaction on the pore network at various catagenetic stages. Based on geochemistry, petrophysics, and basin modeling, the following conclusions can be drawn. (1) Of the two lithofacies penetrated, the pore network of the marlstone is more permeable than that of the claystone. Therefore, according to this criterion, the marlstones ought to have expelled hydrocarbons earlier than did the claystones. (2) Early diagenetic carbonate cementation has reduced the size and volume of the pores at specific stratigraphic levels. However, around the contact of the two lithofacies, updip-streaming formation waters created secondary porosity which can be considered a migration avenue for generated hydrocarbons from the source rock. (3) Before hydrocarbon expulsion, soluble organic matter, which is evenly distributed within the pore network, occupied up to 60% of the available pore space. (4) With progressive maturation of the organic matter, analyzed pore size and porosity values became continuously smaller. Using the method of computer-aided integrated basin modeling, they can explain the observedmore » porosity reduction trend only partly by mechanical compaction. Mechanical compaction is computed considering the changes in effective stress by taking into account several parameters controlling water flow and pressure buildup. As a consequence, the diminution of the pore network during catagenesis seems to be tied to the collapse of the alginite structures.« less

Silica diagenesis: origin of inorganic and replacement cherts

Silicification of originally non-siliceous sediments affects a wide variety of rock-types and materials and ranges from minor to pervasive. Partial and minor chertification occur mostly in Phanerozoic carbonates, carbonate-bearing sandstones, evaporites, and fossil wood. The source of the silica is predominantly biogenic. In petrified wood the silicification mechanism is a permeation or void-filling process, not a replacement. In this example, the sequence of silica-phase transformation is the same as that in deep-sea siliceous sediments. In many silicified rocks, particularly in certain carbonates, the transformation sequence is different from that in radiolarites or diatomites. The chemical environment and conditions of early diagenetic chert formation in shallow water carbonates are delineated by the general mixing model of Knauth (1979), but remain unknown for most other types. An exception are the flint nodules and bands of the English Chalk. A detailed geochemical study of the paramoudra flint structures by Clayton (1986) provided remarkable insight into the replacement process. Seven different recurring silica fabrics have been recognized in chert-replaced carbonates including equigranular (microcrystalline quartz or microquartz and megaquartz) and fibrous types (chalcedony, quartzine or length-slow chalcedony, lutecite, zebraic chalcedony and microflamboyant quartz). Among the latter, quartzine and microflamboyant quartz are common in, but by no means restricted to chert-replaced evaporites, for which Milliken (1979) recognized a sequence of seven quartz-fabrics. As a single criterion, only anhydrite inclusions in megaquartz, quartzine or microflamboyant quartz provide unequivocal evidence for an evaporite precursor. The relative timing between silicification and well-established diagenetic carbonate reactions shows that virtually all theoretically possible sequences occur. Chertification of carbonate host sediment thus may take place during early, intermediate or late diagenesis, and even during anchimetamorphism. Pervasive to complete silicification has been described in lacustrine, pedogenic and hydrothermal volcanogenic rocks and occurs on the scale of individual beds, members or entire formations. It may affect some of the aforementioned rock types too, for example carbonates. The source of the silica is predominantly inorganic. Chert formation in these environments includes direct chemical silica precipitation from solution through a gel stage. Magadi-type cherts result from the conversion of the hydrous sodium silicate magadiite into microcrystalline quartz which has occurred in East African rift valley lakes such as Lake Magadi in Late Pleistocene time. They are closely related to directly precipitated inorganic cherts which have been observed in alkaline environments of playa lakes. Silcretes originate from weathering and soil-forming processes under climatic and environmental conditions conducive to the formation of duricrusts and laterites. In more humid climates, however, non-weathering-profile silcretes occur which have been distinguished from weathering-profile silcretes. Four to five different silcrete fabric types have been established, including the (1) grain-supported (or ‘quartzitic’), (2) floating (terrazzo), (3) matrix (Albertinia and opaline), and (4) conglomeratic types. In volcanic edifices, silicification related to hydrothermal activity occurs (1) along the ascent-routes of the fluids (vents) within the volcanic complexes, (2) in isolated ponds and depressions on the sea-floor where the fluids discharge, mostly in the median rift valley of mid-ocean ridges, and (3) in geothermal areas on land associated with spreading lineaments, volcanic islands arcs, transform faults or continental hot spots. Attention has focussed on the associated base-metal concentrations and less on the silicification and accompanying iron enrichment processes. The Precambrian Banded Iron Formations may be indirectly or directly related to hydrothermal-volcanic processes. The origin of these cherty iron formations certainly requires more than one genetic model and at present remains a major unresolved problem despite massive efforts to tackle it.

Carbonate Replacements After Sulfate Evaporites in the Middle Miocene of Egypt

ABSTRACT Massive zones of diagenetic carbonate which replace sulfate deposits are observed in the Middle Miocene evaporites in several outcrops along the Egyptian coast of the Gulf of Suez. These carbonates display marked similarities of morphology, lithofacies, and stable isotope composition to diagenetic carbonates described in some portions of Messinian Gessoso Solfifera Formation of Sicily and in the Castiles of the Upper Permian Castile Formation of the Delaware basin. The mechanism of mineral transformation is basically bacterial sulfate reduction because the low 13C values of these carbonates argue for the organic origin of carbon. The 18O values fall i to two distinct groupings, indicating that the carbonates were crystallized during two different diagenetic stages. The early diagenetic stage occurred soon after deposition, in pore waters of marine origin and at different levels of evaporation; organic matter used by bacteria was produced in situ. The late diagenetic stage occurred when the tectonic or halokinetic uplift of the sequence caused flushing of the sediments by groundwaters of the phreatic aquifer and hydrocarbon migration. Furthermore, to explain the very low 18O values of these carbonates, it is inferred that locally ambient temperatures reached more than 35°C due to the large quantities of energy released during the bacterial sulfate reduction.

Chemical model for the origin of minor limestone-shale cycles by anaerobic methane oxidation

Chemical and isotopic studies of Jurassic carbonate concretions indicate an origin by anaerobic methane oxidation. Concretion growth occurred within the top 1 m of sediment in a thin zone where methane was consumed to stimulate a late, rejuvenated phase of sulfate reduction. Similar chemical and isotopic characteristics are shown by diagenetic carbonate in several limestone-shale sequences, and an origin by anaerobic methane oxidation is proposed for these sequences also. The evolution of isolated concretions into diagenetic limestones depends on the extent of, and depth variations in, carbonate supersaturation arising from anaerobic methane oxidation and the persistence of a reduced sedimentation rate or a depositional hiatus. Diagenetic limestones formed in this way can often be recognized by their association with a later pyrite phase that is more abundant and has more positive /sup 34/S values than the adjacent shales.

Rare Earth Element and Nd Isotopic Variations in Regionally Extensive Dolomites From the Burlington-Keokuk Formation (Mississippian): Implications for Ree Mobility During Carbonate Diagenesis

ABSTRACT Marine carbonates of the Burlington-Keokuk Fm. (Miss.) of the midcontinent region have been subject to two major episodes of regionally extensive (100,000 km2) dolomitization. Previous studies of the cathodoluminescent petrography, Sr, C, and O isotopic compositions, and Sr, Fe, Mn, and Ca concentrations of Burlington-Keokuk dolostones outline a diagenetic history in which lime mud was pervasively dolomitized by a seawater-dominated fluid. At many localities, this early dolomite generation (dolomite I) was recrystallized by nonmarine diagenetic fluids at shallow-burial depths to produce dolomite II. Dolomites I and II have similar rare earth element (REE) patterns and abundances and a narrow range of initial 143Nd/144Nd isotope ratios of 0.511906 ± 0.000025 (1 ). This consistency in the Nd isotope ratios and REE patterns, along with the presence of anomalously low abundances of cerium in the dolomites, cherts, glauconite pellets, fish apatite, and crinoids, suggests that 1) seawater-derived REE dominated the system, and 2) the extensive water-rock interaction that produced profound textural and compositional changes during the formation of dolomite II did not significantly alter the REE patterns or Nd isotopic signatures of the dolomites. The results of quantitative modeling of isotopic and trace lement exchange during water-rock interaction in carbonate systems indicate that extremely large water: rock ratios are required to alter the REE signatures of diagenetic carbonates. Such large water: rock ratios can reflect either 1) large extents of fluid flow during carbonate recrystallization, or 2) minimal interaction between diagenetic fluids and host rocks during carbonate cement crystallization. Late vug-filling calcite and dolomite have low Sm/Nd and high (Ce/Nd)n ratios relative to the dolostones. Such distinct REE signatures probably reflect limited interaction between late diagenetic fluids and Burlington-Keokuk host rocks. Penecontemporaneous clastic sediments have similar initial 143Nd/144Nd isotope ratios (0.511857-0.511877) and lower Sm/Nd ratios compared to the dolomites, suggesting 1) a common crustal source of REE for the clastics and carbonates, and 2) REE fractionation during the formation of carbonate, apatite, glauconite, and other authigenic phases in the marine environment.

Ocean Drilling Program Leg 112, Peru continental margin: Part 2, Sedimentary history and diagenesis in a coastal upwelling environment

Research Article| October 01, 1988 Ocean Drilling Program Leg 112, Peru continental margin: Part 2, Sedimentary history and diagenesis in a coastal upwelling environment E. Suess; E. Suess 1College of Oceanography, Oregon State University, Corvallis, Oregon 97331-5503 Search for other works by this author on: GSW Google Scholar R. von Huene R. von Huene 2U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Geology (1988) 16 (10): 939–943. https://doi.org/10.1130/0091-7613(1988)016<0939:ODPLPC>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation E. Suess, R. von Huene; Ocean Drilling Program Leg 112, Peru continental margin: Part 2, Sedimentary history and diagenesis in a coastal upwelling environment. Geology 1988;; 16 (10): 939–943. doi: https://doi.org/10.1130/0091-7613(1988)016<0939:ODPLPC>2.3.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 On the shelf and upper slope off Peru the signal of coastal upwelling productivity and bottom-water oxygen is well preserved in alternately laminated and bioturbated diatomaceous Quaternary sediments. Global sea-level fluctuations are the ultimate cause for these cyclic facies changes. During late Miocene time, coastal upwelling was about 100 km west of the present centers, along the edge of an emergent structure that subsequently subsided to form the modern slope. The sediments are rich in organic carbon, and intense microbially mediated decomposition of organic matter is evident in sulfate reduction and methanogenesis. These processes are accompanied by the formation of diagenetic carbonates, mostly Ca-rich dolomites and Mg-calcites. The downhole isotopic signatures of these carbonate cements display distinct successions that reflect the vertical evolution of the pore fluid environment. From the association of methane gas hydrates, burial depth, and low-chloride interstitial fluids, we suggest an additional process that could contribute to the characteristic chloride depletion in pore fluids of active margins: release of interlayer water from clays without a mineral phase change. The shelf sediments also contain a subsurface brine that stretches for more than 500 km from north to south over the area drilled. The source of the brine remains uncertain, although the composition of the oxygen isotopes suggests dissolution of evaporites by seawater. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Phosphatic Hardgrounds and Hiatus Concretions in Neogene Marine Sequences of California Coastal Ranges: ABSTRACT

Distinctive phosphatic intervals mark hiatuses of varying durations at several localities along the central California coast. At Capitola in the La Honda basin, a complex series of carbonate-cemented hardgrounds containing authigenic phosphatic nodules and marine vertebrate bones occurs within 50 cm of the upper Purisima Formation (Pliocene). Paleomagnetic measurements and diatom stratigraphy indicate that this 50-cm interval represents a hiatus of about 1 m.y. (4.5-3.5 Ma). At Naples Beach in the Santa Barbara basin, a severely condensed section of about 7 m of interbedded nodular phosphatic hardgrounds, phosphatic marlstones, and dolomites appears to represent as much as 5 m.y. (14.5-9.5 Ma). In the Lompoc Quarry of the Santa Maria basin, a thin phosphatic hardground at the contact between the Monterey and the Sisquoc Formations represents about 0.5-0.7 m.y. (between 7.0 and 6.0 Ma). At Mussel Rock in the Santa Maria basin, a nodular phosphatic conglomerate occurs at the boundary between the Monterey and Sisquoc Formations; the age of the conglomerate determined from diatom biostratigraphy is approximately 6.2 Ma (Messinian), but no time gap can be recognized given the resolution of diatom age dating. The phosphatic hardgrounds of Capitola, Lompoc Quarry, and Naples Beach appear to correspond to high sea levelmore » stands, whereas the conglomerate at Mussel Rock coincides with a sea level lowstand. During high sea level stands, depocenters shifted landward, leading to clastic-sediment starvation possibly accompanied by current-induced erosion and winnowing, which in term led to condensation and early diagenetic carbonate and phosphatic cementation.« less

Biodegradation of petroleum as a source of 13C-enriched carbon dioxide in the formation of carbonate cement

The biodegraded oils of the Lower Cretaceous Mannville Group of Alberta, Canada, are associated with high- 13C secondary carbonate mineralizations (δ 13C-values up to +14.3‰ PDB). These cements are genetically linked to biological degradation of the petroleums by a two-step process. Oxidation of the paraffinic components by microorganisms produced alcohols, organic acids and ketones. Later anoxic conditions favored anaerobic fermentation of the above products, leading to the formation of 14C-enriched CO 2 which precipitated as carbonate cement. The phenomenon observed in north Alberta is not universal. If anoxic conditions are not achieved oxidation will persist leading to the formation of 13C-depleted CO 2 and consequently the precipitation of isotopically light carbonate cements. This difference is observed in the heavy oil deposits of Zaire and Malagasy (δ 13C ratios as low as −20.6‰). Certain diagenetic carbonates postdating oil emplacement are characterized by an enrichment in 13C, a lack of coexisting 13C-depleted carbonate cements and a negative δ 13C-δ 18O correlation. It is suggested that these features positively indicate that petroleums are degraded by the activity of microorganisms that utilize in part a fermentation metabolic pathway.

Diagenetic Carbonate in Miocene Sandstone Reservoir, San Joaquin Basin, California

Stevens arkosic sandstones, deposited about 7 Ma in a deep marine environment, form the North Coles Levee reservoir. Dolomite and calcite cements occur as zones averaging 1 ft thick and having less than 300 ft of lateral continuity. Based on ^dgr18O measurements and estimated pore fluids between 0 and 2.8, the dolomite formed at 30°-40°C, an early calcite cement formed at 50°-60°C, and a late pore-filling and plagioclase-replacing calcite formed at 70°-80°C. Corresponding to these temperature estimates, dolomite formed in the zone of methanogenesis (^dgr13C = 4 to 8) and calcites in the zone of decarboxylation (^dgr13C = -3 to -12). Dolomite has between 4 and 12 mole % Fe, and calcites contain up to 8 mole % o Fe + Mg + Mn with a characteristic (Fe + Mn)/Mg ratio of about 3:1. The latest calcite is the most Ca rich, due in part to Ca sourced from plagioclase dissolution. Calcite cements in nonmarine sections have markedly high Mn contents (to 5 mole %) compared with the marine section at North Coles Levee. The volume of cement systematically decreases from about 40-20% over the cementation temperature interval, suggesting progressive compaction of the sandstones. Present-day mean porosity of the reservoir (15%) appears to be largely a result of mechanical compaction. A very late dolomite cement with high Fe content (13 mole %) and light carbon (^dgr13C = -14) is found in the cleavage flakes of crushed biotite. The dolomite is calculated to have formed from a fluid with a ^dgr18O of 0.9 at present reservoir temperatures of 105°C, suggesting precipitation in the past 20 years during release of the gas cap and injection of isotopically light water. Calcite also forms thick scales in production tubing primarily as a result of pressure drops from 4,900 to 400 psi in the tubing string. The calcites have high Mg contents (7 mole %) due to the composition of the injection water. Calculated distribution coefficients for Fe and Mn are similar to published values, but Mg is two times higher than published values at 90°C. Similar results are obtained for late calcite cements. A time-temperature plot for North Coles Levee constrains the timing of dolomite cementation between 5 and 6 Ma and the calcite cementation between 4 and 2.5 Ma. Plagioclase dissolution and hydrocarbon emplacement occurred within the past few million years, suggesting that the presence of the hydrocarbons and associated byproducts of kerogen maturation enhance porosity. Despite the narrow window for diagenesis, minimum fluid fluxes of only 0.02 cm3/yr per 100 cm3 rock are required to account for the volume of dissolved plagioclase.

Late Burial Diagenesis of Niagaran (Middle Silurian) Pinnacle Reefs in Michigan Basin

Core samples from Middle Silurian pinnacle reefs in northern Michigan exhibit a regionally consistent assemblage of late diagenetic phases including geopetal diagenetic sediment, disseminated pyrite, pyrobitumen, rhombic dolomite cement, and equant calcite spar. High (> 80°C) fluid inclusion homogenization temperatures and inclusions of Silurian-sourced oil in these diagenetic phases indicate that they formed between the Mississippian and Jurassic, when pinnacle reefs resided in the deep subsurface. Fluid inclusion salinites and stable carbon/oxygen isotopic ratios suggest that late diagenetic carbonates did not precipitate from connate fluids but from basinal brines that were conducted to pinnacle reefs by two regional carbonate aquifers. These data confirm that ate burial diagenesis can affect carbonate rocks residing in high-salinity, low-permeability fluid environments.

Diagenetic carbonate and evaporite minerals in Rotliegend aeolian sandstones of the Southern North Sea: their nature and relationship to secondary porosity development

AbstractDeeply buried (&gt; 3·5 km) Rotliegend aeolian sandstones in the Southern North Sea Basin display a number of interesting diagenetic features including (i) zoned iron-rich carbonate cements, (ii) anhydrite, halite and baryte cements, (iii) at least two generations of authigenic illite, and (iv) significant secondary porosity created by cement and framework-grain dissolution. The creation and destruction of secondary porosity is the result of changes in porewater chemistry during burial and subsequent uplift. Three pore-fluid regimes can be identified: (1) alkaline, oxidizing conditions during shallow to intermediate burial; (2) acid, reducing conditions during intermediate to deep burial; (3) alkaline, reducing conditions during deep burial and uplift. The transition from stage 1 to stage 2 was probably caused by expulsion of waters from the underlying Carboniferous shales. The transition to stage 3 probably began when faulting associated with uplift allowed invasion by alkaline fluids derived from Zechstein sediments.

Primary and diagenetic carbonates in the anoxic sediments of Lake Bosumtwi, Ghana

Cores of organic-rich muds from the tropical meteorite crater Lake Bosumtwi, Ghana, contain laminae of authigenic calcite polyhedra and aragonite needles, as well as scattered diagenetic calcite, Mg-calcite spherulites, and aggregates of dolomite crystals. Their respective origins are traced by analyses of oxygen and carbon stable isotopes. Anoxic dolomite and Mg-calcite formed in pore waters reflecting highest degrees of evaporative concentration and high rates of methanogenesis. Carbon dioxide reduction must have been the dominant methanogenic pathway, producing diagenetic carbonates with ..delta../sup 13/C values up to + 27% PDB. This example shows the inherent problems of interpreting bulk samples with progressive mineralogical sequences of calcite to Mg-calcite to aragonite to dolomite merely in terms of Mg/sup 2 +//Ca/sup 2 +/ in lake waters. It provides a model for the interpretation of ancient lacustrine carbonates from organic-rich environments.

Pleasants Sandstone Member, Williams Formation, Santa Ana Mountains: A Southern California Upper Cretaceous Shelf Sandstone: ABSTRACT

The Williams Formation consists of two distinct members: a medium to coarse-grained sandstone and conglomeratic unit (Schulz Ranch Sandstone Member), and a fine to medium-grained sandstone unit (Pleasants Sandstone member). This study of the Pleasants Sandstone Member focused on strata in a linear outcrop belt from Modjeska Grade (southeast) to Baker Canyon to Black Star Canyon (northwest). The dominantly fine-grained clayey sandstones of the Pleasants are interbedded with numerous medium to coarse-grained sandstone beds. The lack of clay minerals, the medium to coarse-grained texture, rarity of microfossils, primary sedimentary structures, transported shell materials, numerous escape burrows, and the thickness of individual units suggest that these strata were deposited rapidly, as tempestites on a narrow shelf. The intensity and duration of storm activity determined the thickness of units, as well as their preservability. Most of this sediment influx came from the vicinity of Modjeska Grade, the study section interpreted as having been in the shallowest part of the Pleasants paleoenvironment. The fine-grained clayey sandstones become progressively more dominant toward Black Star Canyon. These strata represent sediments deposited by waning storm activity, as well as hemipelagic sedimentation. These rocks exhibit a complete range of biologically to physically dominated sedimentologic fabrics, production ofmore » which depended on sedimentation rates, infaunal organism densities, and dissolved oxygen levels. Diagenetic carbonate concretions are abundant and are useful in determining the initial sediment fabric of the deposits. Carbonate concretions formed through the decomposition of abundant organic matter (plant and animal) in porous water-saturated sediments.« less

Some chemical aspects of diagenetic concretions from the Westphalian of Yorkshire, England

Chemical analyses of acid-soluble material and XRD study of both concretionary and disseminated diagenetic carbonates in fine-grained sediments provide evidence of time and origin of growth.Siderite occurs both as segregations (nodules and continuous sheets) and as disseminated cement in freshwater/brackish mudstones. Substitution by Mg and Ca in siderite is very variable but a later generation of siderite cement with higher Mg/Ca ratio is identified in one mudstone and towards the margins of a sectioned nodule. A change in pore-fluid composition is implied which is probably related to diagenetic modification of silicates.“Bullion” concretions in carbonaceous marine shales began growth very early around centres of decaying organic matter. Either dolomite or calcite is interspersed in these complex zoned bodies with pyrite derived by bacterial reduction of seawater sulphate. Gypsum and in one concretion later dolomite formed by reaction of carbonate and pyrite in an oxidising groundwater regime.

Diagenesis and Primary Migration in Upper Jurassic Claystone Source Rocks in North Sea: ERRATUM

Carbonate bands are common in Central graben Kimmeridgian (Upper Jurassic) claystone that is mature for oil generation. X-ray diffraction, thermal analysis, Mossbauer spectroscopy, thin-section studies, scanning microscopy, and electron microprobe investigations showed that the bands are composed of claystone fragments, coarse grains of dolomite, quartz, and siderite, and diagenetic layer-silicates, all cemented to a certain degree by ankerite. The diagenetic layer-silicates are kaolinite being neoformed into muscovite. In claystones adjacent to the carbonate bands, ankerite fills pores about 50 µm in diameter; however, claystone further away from the bands contains little ankerite. Gas adsorption with N2, Kr, and H2O together with microscopy showed porosity in the claystone is composed of a large amount of micropores and mesopores, possibly with inlet constrictions, and a few macropores with diameters of about 50 µm. In the carbonate bands, microporosity and mesoporosity are probably located in the claystone fragments, whereas several pores with diameters of about 50 µm are present in the carbonate matrix. The presence of claystone fragments, fluid release structures, and large amounts of diagenetic carbonates indicates that carbonate bands formed through fracturing, possibly as a result of overpressuring. Porosity parameters indicate that fluid transport occurred by diffusion in the claystone matrix to claystone macropores and further through macropores in the carbonate bands, out of the source rock. The diagenesis of carbonates and clay minerals in the carbonate bands and in the claystone points to two migration stages: an early migration of fresh, neutral fluids, and a later migration in connection with hydrocarbon migration of saline-alkaline fluids.

Isotopic Variations in Dolomite Concretions from the Monterey Formation, California

ABSTRACT Isotopic analysis of dolomite concretions from the Miocene Monterey Formation of California yields a covariance of 18O and 18C within every concretion analyzed. The total range for all concretions in 18O (SMOW) is +27.3 to +33.6. The total range in 13C (PDB) is - 10.2 to + 10.8. Variations of up to 3.6 in oxygen and 12.2 in carbon are encountered within a single concretion. Decreasing 18O is interpreted in terms of increasing temperatures of formation as the concretion grows during burial. The Claypool and Kaplan (1974) model can be successfully used to interpret carbon isotope variations as the result of progressive concretion growth in the zones of microbial sulfate reduction, microbial methane production, and thermocatalytic decarboxylation. Monterey dolomite concretions are too rich in carbonate to have originated by simple cementation of initially porous sediment. Much of the originally diatomaceous sediment appears to have been replaced by dolomite. Isotopic zonation within one concretion is found to be complex, suggesting nucleation at several sites which then grew together. Complex growth patterns may result from local variations in lithology and porosity during precipitation. Multiple sampling of several concretions at a single outcrop yields a more complete early diagenetic history than by sampling only one concretion. Isotopic diagenetic profiles inferred from the dolomite concretions are similar to those seen in Deep Sea Drilling Project cores where deep-sea dolomitization is occurring today. Single samples from a concretion cannot reveal the range or the trend of the diagenetic reactions that have occurred. Multiple sampling of diagenetic carbonate concretions is clearly essential in determining the early diagenetic history from isotopic measurements.

C13-Rich Diagenetic Carbonates Associated with Heavy-Oil Deposits: ABSTRACT

The isotopic composition of secondary calcites in 120 samples from biodegraded heavy-oil deposits of the Lower Cretaceous Mannville Group of Alberta ranged in ^dgr13C from -1.3 to +14.1 ^pmil PDB. The calcites postdate oil emplacement and are anomalously enriched in C13 in comparison with common sedimentary and diagenetic carbonates associated with conventional oil pools and authigenic carbonates of other shallow Cretaceous sediments of Alberta. In contrast to previously reported occurrences of heavy carbonates (Moneray Formation, Kimeridge Clay of Dorset, Caspian syncline), the heavy-oil calcites are not related to organic-rich clayey sediments and no coexisting low 13C-carbonates were found. The API gravities of the biodegraded oil corre ate with the ^dgr13C values of the carbonate, suggesting a relationship between the biodegradation process and C13-enriched cements. Different isotopic systematics were observed in 25 samples from other heavy-oil deposits (Indonesia, Malagasy, Brazil, Sicily, Zaire, California, Kentucky, Utah) whose ^dgr13C ratios ranged from -21.6^pmil to +1.1^pmil PDB. An explanation may involve oxidation of the paraffinic oil components by microorganisms producing alcohols, organic acids, and ketones. A change to anoxic conditions causes fermentation of these biodegradation products, leading to the formation of 13C-rich CO2, which precipitates as heavy carbonate. If anoxic conditions are not achieved, oxidation will persist and lead to the formation of light carbonate cements. The detection and understanding of the formation mechanism of anomalously heavy carbonates can be useful in both future exploration strategy and exploitation of heavy oil deposits, particularly to enhanced recovery schemes based on CO2 flooding. End_of_Article - Last_Page 847------------

Diagenesis and Primary Migration in Upper Jurassic Claystone Source Rocks in North Sea

Carbonate bands are common in Central graben Kimmeridgian (Upper Jurassic) claystone that is mature for oil generation. X-ray diffraction, thermal analysis, Mossbauer spectroscopy, thin-section studies, scanning microscopy, and electron microprobe investigations showed that the bands are composed of claystone fragments, coarse grains of dolomite, quartz, and siderite, and diagenetic layer-silicates, all cemented to a certain degree by ankerite. The diagenetic layer-silicates are kaolinite being neoformed into muscovite. In claystones adjacent to the carbonate bands, ankerite fills pores about 50 µm in diameter; however, claystone further away from the bands contains little ankerite. Gas adsorption with N2, Kr, and H2O together with microscopy showed porosity in the claystone is composed of a large amount of micropores and mesopores, possibly with inlet constrictions, and a few macropores with diameters of about 50 µm. In the carbonate bands, microporosity and mesoporosity are probably located in the claystone fragments, whereas several pores with diameters of about 50 µm are present in the carbonate matrix. The presence of claystone fragments, fluid release structures, and large amounts of diagenetic carbonates indicates that carbonate bands formed through fracturing, possibly as a result of overpressuring. Porosity parameters indicate that fluid transport occurred by diffusion in the claystone matrix to claystone macropores and further through macropores in the carbonate bands, out of the source rock. The diagenesis of carbonates and clay minerals in the carbonate bands and in the claystone points to two migration stages: an early migration of fresh, neutral fluids, and a later migration in connection with hydrocarbon migration of saline-alkaline fluids.

Genesis of late Proterozoic copper mineralization, Copper Claim, South Australia

Bedded disseminations. No evidence for hydrothermal alteration. The disseminated sulfides have a delta 34 S range of -17.3 to 15.8 per mil, consistent with bacterial reduction of sulfate. Veinlet sulfides have a similar but smaller delta 34 S range, in accord with formation by remobilization of the disseminated sulfides. No significant delta 13 C differences between bedded and veinlet carbonates; marked variations in delta 18 O values, with magnesite values being generally higher than dolomite, and the median value for bedded carbonates being around 23 per mil, some 3 per mil higher than the median value for veinlet carbonates. Low-temperature oxygen isotope equilibration during early diagenetic carbonate reactions. Low-grade regional metamorphism caused minor recrystallization and formation of additional veinlets but did not extensively overprint the diagenetic isotopic trends. Isotopic similarities between the cupriferous and the unmineralized lithologies support introduction of the copper during sedimentation and/or early diagenesis. Veinlet carbonates contain small NaCl-rich fluid inclusions, formed at around 300 degrees C and 4 km depth, during the metamorphism. General trend of increasing mean delta 34 S values with decreasing age of the late Proterozoic strata in the Adelaide geosyncline and Stuart shelf. This trend may be the product of local environmental conditions which resulted in sulfate reduction outweighing sulfate replenishment; alternatively, it could be reflecting global processes which caused major 34 S enrichment of seawater sulfate.--Modified journal abstract.

Influence of mica surfaces on pore-water pH

The close spatial association of detrital biotite with diagenetic carbonate, prehnite, or laumontite is attributed to attraction of H + to the mica surface, resulting in an increase in pore-water pH and stabilization of carbonate or calcium aluminosilicates in the adjacent pore space. Experimental results at 25°C with biotite and muscovite show that in the pH range of 10-4 biotite adsorbs H + whereas muscovite does not adsorb H + until pH is less than ∼6.5 (for 0.01 M NaCl solutions) or less than ∼5.8 (for 0.5 M NaCl solutions). Increasing salinity reduces the H + affinity of the micas. At seawater-type salinities (0.5 M NaCl), biotite is still capable of adsorbing from ∼1· 10 −5 mol H + g (pH = 7) to 2 · 10 −5 mol H + g (pH = 6). During late-stage diagenetic compaction, if iotite/pore fluid ratios are high, this effect can substantially raise pore-fluid pH. Biotite may also have an inherited net negative charge in the framework, or develop such a charge by incongruent loss of K + from cleavage surfaces. The theoretical cation distribution adjacent to such a charged surface has been evaluated by the Guoy—Chapman double-layer theory. Calculations show that within 10 Å of the biotite surface pH can be several orders of magnitude lower than in the bulk fluid, and [ Na +] [ Ca 2+] ratios may be more than three times higher than the bulk fluid.