The use of burial diagenetic calcite cements to determine the controls upon hydrocarbon emplacement and mineralization on a carbonate platform, Derbyshire, England
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Abstract Late diagenetic calcite cements in the Upper Dinantian limestones of the Derbyshire Platform are contemporaneous with both hydrocarbon emplacement and Mississippi Valley-type (MVT) mineralization. Calcite cementation began during the progressive burial of the Derbyshire Platform and the surrounding basins, principally within fractures generated during the waning effects of Upper Carboniferous extension. Six burial calcite cements can be recognized in dilational vein systems. Successive veins contain progressively more mature hydrocarbon inclusions, and calcite cements are intergrown with fluorite, baryte, galena and sphalerite in increasing quantities. Compacting Dinantian-Namurian shales in basins adjacent to the platform offer the most likely sources of fluids, trace elements and hydrocarbons. Fluids entered the platform along major fault systems, and circulated using smaller fracture systems, precipitating calcite. The final phases of calcite cementation and the main phase of MVT mineralization coincided with the onset of the Variscan Orogeny. A model is now established relating fluid flow to Variscan tectonic events in northern Britain.Carbonate diagenesis is an important branch in carbonate sedimentology,especially in carbonate reservoir sedimentology.Recently,there have been remarkable advances on carbonate diagenesis in the following aspects: 1) the geometry and petrogenesis of dolomite hydrocarbon reservoir,particularly referring to the review and reappraisal on dolomitization models cited by many sedimentologists for a long time;2) the mechanism of structurally controlled hydrothermal alteration of carbonate reservoirs,concerning the origin of hydrothermal dolomite and its effects on hydrocarbon reservoir,especially the influences of hydrothermal fluids on carbonate diagenesis in deep-burial setting;3) renewals of classic diagenesis theories of carbonate and challenges to past diagenesis theories that people have been using until now,some new sights of diagenesis in marine-meteoric mixing-zone and marine-burial environments should make us to reappraisal the understanding of carbonate diagenesis;4) wide application of strontium isotope composition to marine carbonate sedimentology,including isochronous correlation of sedimentary sequences,directly dating marine sediments,and interaction of water-rock during diagenesis.However,some following questions existing in the carbonate diagenesis need us to pay more attentions to: 1) the most important dissolving fluids to carbonates are probably H2S and CO2 as byproducts of sulfate reduction in deep-buried setting with sulfate minerals,but carbonates are more soluble in relatively low temperature,which is socalled retrograde solubility;2) a temperature difference(ΔT) for cooling water to dissolve carbonates can be implemented by following two geological processes: ① fluids flowing upward along faults,showing the importance of faults in carbonate diagenesis;② tectonic uplift,suggesting the significance of burial history related to tectonics in carbonate diagenesis;3) the first thing controlling Sr content in dolomite is the crystalline chemistry habits,lower distribution coefficient of Sr into dolomite results in pervasively lower Sr content in dolomite,thus,people can not only depend upon contents of Sr in dolomite to evaluate the characteristics of dolomitization fluids,and a lower content of Sr in dolomite can not indicate that dolomitization fluid is irrespective to seawater;4) remove of SO4~(2-) related to sulphate reduction may place a premium on the dolomitiztion or precipitation of dolomite,and the presence of SO4~(2-) in fluid may increase the rate of dolomite dissolution.
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This research is to assess the influence of regional versus global controls on the diagenesis and reservoir quality of Tertiary carbonate platforms. The three main objectives are: (1) to evaluate controlling influences on diagenesis, fracturing and potential reservoir quality of a Cenozoic syntectonic carbonate platform (Tonasa Formation, Indonesia). (2) To contribute towards understanding diagenetic alteration of volcanogenic covered and/or influenced carbonate platforms. (3) To evaluate controlling influences on diagenetic variability of slope and basinal deposits.
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The compressibility of calcite to 40 kbar has been remeasured by using a piston-cylinder apparatus. Calcite 1 is found to transform to calcite 2 at 14.5 kbar with a volume change of 0.00483 cm3/g, and calcite 2 is found to change to calcite 3 at 17.4 kbar with a volume change of 0.01291 cm3/g. The volume compression data for the three phases are described by the following quadratic relations: Calcite 1 Calcite 2 Calcite 3 where P is pressure in kilobars. The compression data for calcite 1 and calcite 3 are in good agreement with those available in the literature. The data exhibiting an abnormally high compression of calcite 2 have been reported for the first time. The compression data for calcite 2 have been used to explain quantitatively the abnormal drop near 15 kbar observed in the ultrasonic sound velocity in calcite.
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The various (low-Mg calcite, intermediate-Mg calcite, high-Mg calcite, and aragonite) carbonate components from the Pennsylvanian Brush Creek Formation of Pennsylvania are preserved in different stages of diagenetic alteration. In general, these components follow the predicted diagenetic changes in structure, mineralogy, and chemistry deduced from theoretical considerations for progressively altered carbonates. The low-Mg calcite brachiopods show no signs of either structural or chemical alteration. The shell material is preserved as low-Mg calcite fibers with no apparent dissolution and/or infilling by diagenetic calcite. Also, the average Sr2+ content of the brachiopods is 820 ppm, which is in agreement with the chemical content of their Holocene counterparts. In contrast, the intermediate-Mg calcite rugose corals show signs of structural aggrading neomorphism. The trabecular fibers are in part recrystallized to small mosaic calcite grains. This structural alteration is concomitant with chemical changes in the Brush Creek rugose corals. The least-altered components contain about 1,770 ppm Sr2+, whereas the most altered components contain only about 1,030 ppm Sr2+ /SUP>. For the high-Mg calcite crinoids, diagenetic alteration is mostly a cementation process with minor mineralogical alteration. The open meshwork structure typical of the Echinodermata is infilled in the Brush Creek crinoids by diagenetic cement. This infilling cement has decreased the average Sr2+ content of 2,140 ppm of unaltered crinoids to that of 1,090 ppm Sr2+ for the most-altered Brush Creek crinoids. Scanning electron microscope analysis of the originally aragonitic mollusks (gastropods, pelecypods, and cephalopods) shows a complete structural diagenetic transition series. The original and least-altered mollusk material is preserved as nacre, which is always aragonite. The second phase of the transition series in the mollusks is represented by the aggrad ng neomorphism of the nacreous tablets into small, coarse mosaic calcite crystals. The structural transition is completed by the replacement of the mosaic calcite by coarse calcite spar. Mineralogically, the mollusk material changes from aragonite to aragonite-calcite to calcite, relative to the least- and to the most-altered specimens, respectively. The structural and mineralogical changes of the originally aragonitic mollusks are also confirmed by changes in their overall chemical composition. Average Sr2+ values measured for the least-altered Brush Creek mollusk material is 4,470 ppm. Mollusk material of the second phase of the diagenetic transition series contains on average about 2,170 ppm Sr2+. The most-altered mollusks, which are calcite, contain on average a out 1,110 ppm Sr2+. Similarily, the Na+ values follow the diagenetic trend of strontium. The least-altered material contains 750 ppm Na+, the intermediate-altered material contains 420 ppm Na+, and the most-altered material contains 250 ppm Na+. Thus the diagenetic alteration process and rate proceed in accordance with mineralogical stability. This sequence is aragonite, high-Mg calcite, intermediate-Mg calcite, and low-Mg calcite relative to fastest to slowest reaction, respectively. The diagenetic alteration and preservation process of the Brush Creek carbonate components is probably a two-stage event. The first stage occurs in the marine phreatic zone, and the second stage occurs in the meteoric phreatic zone. End_of_Article - Last_Page 520------------
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This chapter contains sections titled: Introduction Carbonate diagenesis Sequence stratigraphy Carbonate diagenesis and relative sea-level falls (LST) Carbonate diagenesis during relative sea-level rises (TST) Carbonate diagenesis during relative sea-level highstands (HST) Carbonate diagenesis on the parasequence scale Carbonate diagenesis and sequence stacking patterns Carbonate diagenesis and role of associated siliciclastics Carbonate diagenesis through the Phanerozoic: broad trends and the first-order sea-level curve Carbonate diagenesis and sequences through the Phanerozoic Summary Acknowledgements References
Sequence Stratigraphy
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ABSTRACT Cultures of known species of fungi placed on crystals of Iceland spar calcite resulted in extensive dissolution of the calcite. This organically mediated dissolution produced large patches of spiky calcite within a period of 253 days. The dissolution of the calcite occurred via surface-reaction-controlled kinetic processes that were mediated by the fungi. This occurred despite the lack of vast quantities of fluids undersaturated with respect to calcite. Locally, at least 10 µm of calcite was removed from the original crystal surface.
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ABSTRACT Mineralogical analysis of calcite and Mg‐calcite by X‐ray diffraction requires that the samples be ground to a powder. Such grinding determines the particle size of the powder and the structural damage of the minerals. Both of these in turn affect the peak intensities recorded by the X‐ray machine. Most carbonate sediments are inhomogeneous; they contain both calcite and Mg‐calcite which are affected differently by grinding. Such differences cause quantitative analytical results to be inconsistent with the true mineralogical abundance. The two acceptable methods of analysis—(1) measurement of peak height from the base and (2) measurement of the area under the peak—were compared to determine if sample preparation affects the quantitative results. In samples with variable and relatively small amounts of calcite and Mg‐calcite the measurement of peak height yields more reproducible results than does the measurement of peak areas. Different proportions of particle size of the mineralogical components in a sample powder, affect proportionally more the peak areas than the peak heights. Extensive grinding causes structural damage of the component minerals which affects much more the peak areas than the peak heights. Thus for quantitative analyses of calcite and Mg‐calcite in inhomogeneous carbonate samples which require differing grinding times and have greatly variable amounts of calcite and Mg‐calcite, the peak height measurement seems to be a better method than peak area measurement.
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