Showing papers on "Ankerite published in 1987"

Burial dolomitization and porosity development in a mixed carbonate‐clastic sequence: an example from the Bowland Basin, northern England

Abstract: Widespread dolomitization and leaching occur in the Asbian to Brigantian (Dinantian) sequence of the Bowland Basin. Within this mudrock-dominated succession, dolomite is developed in calcarenites and limestone breccia/conglomerates deposited in a carbonate slope environment (Pendleside Limestone) and also within graded quartz wackes deposited by density currents in a generally ‘starved’ basin environment (Pendleside Sandstone). The dolomitized intervals range in thickness from less than one metre to several tens of metres and have a stratabound nature. All stages of calcite cement pre-date dolomitization and calcite veins are dolomitized. Dolomite crystals replace neomorphic spar and may also contain insoluble residues that were concentrated along stylolites. Thus dolomitization was a late stage process within the carbonate diagenetic sequence. A late-stage diagenetic origin is also indicated within the sandstones, with dolomite post-dating the development of quartz overgrowths. Six main textural styles of dolomite are observed: (1) scattered; (2) mosaic; (3) subhedral to euhedral rhombic; (4) microcrystalline; (5) single crystal and (6) saddle. The style of dolomite developed is dependent on the host rock mineralogy, on whether it is space-filling or replacive and also on temperature. Chemically the dolomite varies from near stoichiometric compositions to ankeritic varieties containing up to 20 mole % FeCO3. Generally the dolomites have isotopic compositions depleted in δ18O compared to the host limestone, with similar or lighter δ13C values. Initial dolomite was of the scattered type, but with progressive replacement of the host a mosaic dolostone with a sucrosic texture was produced. There was a general increase in the Fe and Mn content and reduction in δ18O ratio of the crystals during dolomitization. Leaching is restricted to partly dolomitized horizons, where calcite, feldspars, micas, clays and, to some extent, dolomite have been leached. This has produced biomouldic and vuggy secondary porosity within the carbonates, whereas in the sandstones honeycombed, corroded and floating grains associated with oversized pores occur. Porosity within both carbonates and sandstones is reduced by ferroan dolomite/ankerite cements. Field, petrographic and chemical characteristics indicate that dolomitizing solutions were predominantly derived from the enclosing mudrocks (Bowland Shales) during intermediate/deep burial. Fluid migration out of the mudrocks would have been sided by dehydration reactions and overpressure, the fluids migrating along the most permeable horizons—the coarse grained carbonates and sandstones that are now dolomitized and contain secondary porosity.

Composition-Induced Microstructures in Rhombohedral Carbonates

Abstract: Recent TEM observations of two-phase microstructures and associated crystal defects in selected, rare dolomites have been extended to calcite-structured (R3c) carbonates and to other natural and synthetic carbonates that crystallize with the dolomite (R3) structure. The samples included siderites (FeCO3), smithsonites (ZnCO3), ankerites (Ca[Mg,Fe](CO3)2), and kutnahorites (Ca[Mn,Fe](CO3)2). TEM methods show that the forms of second phases which result from the presence of common, divalent, metallic impurities are morphologically similar in R3c and R3 carbonates and occur more widely than hitherto realized. The most common form consists of thin ribbons of second phase which are coherent with and have the same crystallographic orientation as the host carbonate. Another form of microstructure, manifest as modulations in diffraction contrast, appears to be associated with incipient breakdown of single-phase carbonate. The results of extensive TEM/EDS microanalyses show that in siderite and ankerite the formation of ribbons is promoted by Ca impurity or Ca excess (with respect to R3c stoichiometry). In smithsonite, Cu and Ca impurities can play similar roles in relation to modulated microstructures. In kutnahorites, the perfection of grains and the absence of second-phase effects is strongly dependent on the ratio of Fe to Mn but is also affected by Ca in excess of the stoichiometric requirement. Electron diffraction results from several of the minerals show c-type spots, which can be interpreted as indicating ordering within basal layers of cations. The results show that, by correlating analytical TEM data with the study of second phases and incipient two-phase microstructures, it should be possible to determine the limits of solid solubility in carbonate systems.

The relationship of ferroan dolomite aggregate to rapid concrete deterioration

Abstract: Some of Iowa's 13,200 miles of portland cement concrete (pcc) pavement have remained structurally sound for more than 50 years, while others have suffered premature deterioration. Research has shown that the type of coarse aggregate used in the pcc is the major cause of this premature deterioration. Some coarse aggregates for concrete exhibit a nonuniform performance history. They contribute to premature deterioration on heavily salted primary roadways while providing long maintenance-free life on unsalted secondary pavements. This inconsistency supports the premise that there are at least two mechanisms that contribute to the deterioration. Previous research has shown that one of these mechanisms is a bad pore system. The other is apparently a chemical reaction. The objective of this research is to develop simple rapid test methods to predict the durability of carbonate aggregate in pcc pavement. X-ray diffraction analyses of aggregate samples have been conducted on various beds from numerous quarries producing diffraction plots for more than 200 samples of dolomitic or dolomite aggregates. The crystalline structures of these dolomitic aggregates show maximum-intensity dolomite/ankerite peaks ranging from a d-spacing of 2.884 angstroms for good aggregates to a d-spacing of 2.914 angstroms for nondurable aggregates. If coarse aggregates with known bad pore systems are removed from this summary, the d-spacing values of the remaining aggregates correlate well with expected service life. This may indicate that the iron substitution for magnesium in the dolomite crystal is associated with the instability of the ferroan dolomite aggregates in pcc pavement.

Hydrochemistry, origin and evolution of sedimentary subsurface fluids. III: Early diagenetic mineralization reactions in high sedimentation-rate basins

Abstract: The early diagenetic evolution of pore-water chemistry is closely linked to mineralization reactions which consume significant portions of the metabolites released by bacterial organic matter decomposition. These reactions are most intense in high-sedimentation rate basins and include the precipitation of iron-sulfides and various carbonates leading to concretion growth. Early diagenetic pyrite is typically framboidal attesting to its recrystallization from precursor mackinawite, greigite or amorphous FeS which are the favored phases at high supersaturation levels during the initial sulfate reduction stages. The sulfur isotopic composition of early diagnetic pyrite can be used to differentiate diffusion-controlled, open-system conditions with isotopically light sulfide (δ 34S = − 35 to − 20‰) from closed system conditions, under which Raleigh distillation produces increasingly heaver sulfide (δ 34 S = − 35 to + 18‰). Alabandite (Mn-sulfide) is a rare authigenic sulfide in Mn-rich environments such as certain restricted, semi-stagnant basins (Baltic Sea). pH-buffering by hydrogen sulfide and hydrogen ion uptake by the reduction of manganese and iron oxides and hydroxides in the nitrate and sulfate reduction zones raise the pH sufficiently to cause supersaturation of the porewaters with respect to Ca-, Mg-, Fe- and Mn-carbonates and complex solid solutions of them. Fe-carbonates cannot form in the sulfate reduction zone in the presence of dissolved sulfide which competes for the dissolved iron. Likewise, dolomite formation appears to be inhibited or slowed down in the presence of substantial dissolved sulfate. The appearance of siderite and ankerite therefore signals carbonate precipitation below the sulfate reduction zone. Supporting evidence for the early diagenetic origin of many carbonate concertions is provided by their high carbonate contents (70 to 90% reflecting the porosity existing at the time of precipitation, called “minus-cement porosity”), isotopic composition, clay fabrics, and preservation of original bedding features including the shapes of fossils and fecal pellets. In these environments increasing carbon isotope ratios (δ 13 C = − 20 to + 15‰) indicate concretion growth below the sulfate reduction zone, i.e., in the methane generation zones. Continuing concretion growth at greater burial depth explains pore water profiles with constantly low Ca and downward decreasing Mg concentrations. Dissolved ammonia and phosphate profiles reguire adsorption and ion-exchange reactions as additional removal machanisms (besides apatite precipitation) in order to explain their downward decrease after they have reached maximum concentrations below the alkalinity maximum. Classification of early diagnetic environments into oxic and anoxic and further subdivision of the latter into sulfidic and non-sulfidic (with suboxic or post-oxic and methanic as further subcategories of the non-sulfidic environment) according to Berner (1981) is preferred over the previous classification in terms of pH/Eh fields. The temperature range of the early diagenetic stage extends from O to about 75°C, at which temperature thermocatalytic organic matter decomposition replaces the earlier bacterially mediated reactions and causes a whole set of new diagenetic reactions (such as feldspar dissolution, smectite to illite transformation) to start.

Hydrochemistry, origin and evolution of sedimentary subsurface fluids: III. Early diagenetic mineralization reactions in high sedimentation—rate basins

Abstract: The early diagenetic evolution of pore-water chemistry is closely linked to mineralization reactions which consume significant portions of the metabolites released by bacterial organic matter decomposition. These reactions are most intense in high-sedimentation rate basins and include the precipitation of iron-sulfides and various carbonates leading to concretion growth. Early diagenetic pyrite is typically framboidal attesting to its recrystallization from precursor mackinawite, greigite or amorphous FeS which are the favored phases at high supersaturation levels during the initial sulfate reduction stages. The sulfur isotopic composition of early diagnetic pyrite can be used to differentiate diffusion-controlled, open-system conditions with isotopically light sulfide (δ 34S = − 35 to − 20‰) from closed system conditions, under which Raleigh distillation produces increasingly heaver sulfide (δ 34 S = − 35 to + 18‰). Alabandite (Mn-sulfide) is a rare authigenic sulfide in Mn-rich environments such as certain restricted, semi-stagnant basins (Baltic Sea). pH-buffering by hydrogen sulfide and hydrogen ion uptake by the reduction of manganese and iron oxides and hydroxides in the nitrate and sulfate reduction zones raise the pH sufficiently to cause supersaturation of the porewaters with respect to Ca-, Mg-, Fe- and Mn-carbonates and complex solid solutions of them. Fe-carbonates cannot form in the sulfate reduction zone in the presence of dissolved sulfide which competes for the dissolved iron. Likewise, dolomite formation appears to be inhibited or slowed down in the presence of substantial dissolved sulfate. The appearance of siderite and ankerite therefore signals carbonate precipitation below the sulfate reduction zone. Supporting evidence for the early diagenetic origin of many carbonate concertions is provided by their high carbonate contents (70 to 90% reflecting the porosity existing at the time of precipitation, called “minus-cement porosity”), isotopic composition, clay fabrics, and preservation of original bedding features including the shapes of fossils and fecal pellets. In these environments increasing carbon isotope ratios (δ 13 C = − 20 to + 15‰) indicate concretion growth below the sulfate reduction zone, i.e., in the methane generation zones. Continuing concretion growth at greater burial depth explains pore water profiles with constantly low Ca and downward decreasing Mg concentrations. Dissolved ammonia and phosphate profiles reguire adsorption and ion-exchange reactions as additional removal machanisms (besides apatite precipitation) in order to explain their downward decrease after they have reached maximum concentrations below the alkalinity maximum. Classification of early diagnetic environments into oxic and anoxic and further subdivision of the latter into sulfidic and non-sulfidic (with suboxic or post-oxic and methanic as further subcategories of the non-sulfidic environment) according to Berner (1981) is preferred over the previous classification in terms of pH/Eh fields. The temperature range of the early diagenetic stage extends from O to about 75°C, at which temperature thermocatalytic organic matter decomposition replaces the earlier bacterially mediated reactions and causes a whole set of new diagenetic reactions (such as feldspar dissolution, smectite to illite transformation) to start.

Preliminary Geologic Analysis of the Tar Sands Near Sunnyside, Utah: Section VI. Recovery

Abstract: Sandstones that crop out along the Roan Cliffs near Sunnyside, Utah, are estimated to contain approximately 6 billion barrels of bitumen, making it one of the largest deposits in the United States. Little is known of the geologic aspects, particularly sedimentology, mineralogy (especially clays), and diagenesis, of these sandstones. These aspects of tar sands must be characterized before the feasibility of bitumen recovery can be assessed. Preliminary results of this ongoing study indicate that the sandstones were deposited in fluvial and marginal lacustrine environments. Sandstone geometry is dependent on depositional environment: Fluvial sandstones tend to be more extensive basinward but are less laterally extensive than the marginal lacustrine sandstones. The sandstones are mainly feldspathic arenites, containing both plagioclase and K-feldspar. Authigenic cements include calcite, dolomite, ankerite, siderite, and analcime. Authigenic clay minerals include kaolinite, illite, and smectite. The percentages of clays and cements vary considerably from sample to sample, particularly between bitumen-bearing sandstones and adjacent lithologies. Some of these authigenic minerals have replaced framework grains. Dissolution ha produced secondary porosity, and the bitumen appears to occupy secondary porosity, preferentially in the coarser grained sandstones.

Diagenesis and burial history of Lower Cretaceous Travis Peak Sandstone, east Texas

Abstract: Fine-grained quartzarenite and subarkose in the Lower Cretaceous Travis Peak Formation have been extensively modified during burial diagenesis Permeability in much of this gas-bearing formation has been reduced to less than 01 md as a result of compaction, extensive precipitation of authigenic minerals, and minor pressure solution Timing of the diagenetic events can be constrained by combining petrographic and geochemical data with subsidence and thermal history The Travis Peak Formation in east Texas is approximately 600 m thick Depth to the top of the formation ranges from 1800 to 2900 m now, but maximum burial depth was probably 450 m deeper during the Eocene Variable amounts of uplift of the formation also occurred during mid-Cretaceous movement of the Sabine uplift The geothermal gradient is 38/sup 0/C/km now, but it may have been as high as 44/sup 0/C/km when the Travis Peak was deposited because of elevated heat flow caused by crustal stretching associated with rifting of the Gulf of Mexico Illite rims and dolomite were the first authigenic minerals to precipitate in Travis Peak sandstone Dolomite probably formed soon after deposition at about 25/sup 0/C from water with a delta/sup 18/O composition of 0 per thousand (SMOW) Next, extensive quartzmore » cement, averaging 17% of the rock volume in well-sorted sandstone, occluded much of the primary porosity The delta/sup 18/O composition of quartz overgrowths indicates they precipitated from meteoric fluids at temperatures of 55/sup 0/ to 75/sup 0/C, which equate to depths of 900 to 1500 m Dissolution of orthoclase and albitization of plagioclase followed quartz cementation and occurred prior to mid-Cretaceous movement of the Sabine uplift Illite, chlorite, and ankerite precipitated after feldspar diagenesis« less

Abstract: Sandstone Petrology, Diagenesis and Reservoir Quality, Lower Cretaceous Kuparuk River Formation Kuparuk River Field, North Slope, Alaska

Abstract: The Kuparuk River Formation consists of upper and lower members separated by an intraformational unconformity. Marine sandstone in each is distinct in terms of depositional environments, sandbody geometry, texture, composition, diagenesis, and reservoir quality. Sandstone in the upper member is very fine to very coarse grained sublitharenite to lithic arenite with an average Q-F-L of 75-1-24. Glauconite constitutes 10-50 percent of framework grains. Chert, muscovite, heavy minerals and mudstone, limestone, siderite, and metasedimentary rock fragments are less abundant. The diagenetic sequence is: aragonite or high-Mg calcite-collophane-pyrite-siderite-ankerite-calcite-fdissolution of carbonate cements and glauconite)-quartz-kaoliniteillite/smectite-pyrite. Sandstone in the lower member is very-fine to fine-grained quartz arenite to subarkose with an average Q-F-L of 92-5-3. Mudstone fragments, chert, muscovite, heavy minerals and glauconite are less abundant. The diagenetic sequence is: pyrite-siderite-ankerite-calcite-(dissolution of ankerite and feldspar)-quartz-kaolinite-illite/smectite-pyrite. Early diagenesis in upper and lower member sandstones is different, whereas burial diagenesis is similar. Early siderite cemented sandstones in the upper member, but did not significantly affect sandstones in the lower member. Subsequent changes in pore fluid chemistry during burial resulted in precipitation of the cement sequence siderite-ankerite-calcite in both upper and lower member sandstones. Stable isotope trends in carbonate cements parallel those of cement texture and composition. Upper member porosity (mostly secondary) and permeability average 23 percent (%) and 130 millidarcies (md) with upper limits of 28-33 percent and 500-1500 md. Reservoir quality is heterogeneous and controlled by grain size, distribution of primary and secondary porosity, and fractures. Both horizontal and vertical permeability are similar except where fractures enhance horizontal permeability. Lower member porosity (mostly primary) and permeability average 23 % and 100 md, with upper limits of 28-30 % and 400-500 md. Reservoir quality is homogeneous. Ankerite locally eliminates porosity, and shale beds and laminations reduce vertical permeability. *Graphic presentation of this paper received the Best Poster Award at the 1985 AAPG/SEPM/SEG Pacif ic Sections Meeting,