Showing papers on "Ankerite published in 1993"

Distribution, chemistry, isotopic composition and origin of diagenetic carbonates; Magnus Sandstone, North Sea

Abstract: Diagenetic ferroan carbonates grew in the Upper Jurassic reservoir sandstones of the Magnus oilfield in porewaters which differed in composition across the field. These porewaters remained compositionally different and stratified for at least 35 M.y. Variations in carbonate chemistry across the field are attributable to these porewater variations, which resulted from displacement of marine depositional water from the crest of the field by meteoric water during late Cimmerian subaerial exposure. Original depositional facies and detrital mineralogy strongly influenced diagenetic carbonate distribution. Rare diagenetic calcite occurs as discrete rhombic crystals. Diagenetically late magnesian siderites have developed throughout the reservoir sandstone and are commonly intimately associated with altered detrital biotite grains. Poikilotopic ankerite cement postdates calcite and siderite and occurs only adjacent to mudstones and in thin sandstones within mudstones. Three compositional growth zones in siderite crystals are observed across the field from crest to flank. In all three wells studied, a similar trend of compositional evolution through time is observed in both two and three zoned rhombs. First-formed siderite is relatively magnesian, intermediate zones are more ferroan, and outer zones are at least as magnesian as the first stage. These individual grain variations overprint a fieldwide variation where siderite is more ferroan in the crestal samples (up to 87 mol % Fe + Mn) and more magnesian downdip (up to 58 mol % Mg). This reflects the greater influence of relatively Fe-rich meteoric-derived water in the crest and the greater influence of marine-derived Mg-rich porewater downdip. Ankerite shows a similar variation in Fe and Mg abunda ces across the field (crest and flank maximum 25 and 17 mol % Fe + Mn respectively, 27 and 43 tool % Mg) and developed due to release of Mg, Fe, Ca and HCO3- ions from mudstones into adjacent sandstones following dissolution of detrital minerals and organic decarboxylation reactions. Both siderite and ankerite have lower 18O at the crest of the oilfield than downdip (respectively siderite 16.0 and 17.6 SMOW; ankerite 17.7 and 21.4). These differences in 18O reflect the retention during burial diagenesis of a larger component of meteoric water in the crest of the field below the unconformity, whereas downdip porefluid contained a larger marine-derived component. Strong organic influence on 13C (-8.0 to -14.6 PDB for magnesian siderite; -7.7 to -13.6 for ankerite), closed system 34S values (up to 15.7 for late cubic pyrite), and stratified 18O from crest to flank of the field argue against large scale porewater movements. Diagenetic porewater stratification is strongly supported by the parallel, but distinct, geochemical fingerprints of siderite and ankerite cements from the crest to the flank of the field.

Diagnostic magmatic features in carbonatites: implications for the origins of dolomite- and ankerite-rich carbonatites

Abstract: Carbonatites are especially susceptible to subsolidus modifications, because of the low strength and high ductility of calcite and the solubility of many carbonatite minerals in aqueous fluids. As a result, magmatic textures and mineral assemblages of carbonatites are commonly altered and even obliterated, and then secondary features may be misinterpreted as magmatic. Furthermore, non-magmatic carbonate-rich rocks may be misidentified as carbonatites. Isotope ratios and trace-element concentrations can provide evidence of magmatic origin that may endure through recrystallization and alteration, but such proof by no means implies that all textural and mineralogical features now visible were formed in the presence of a carbonate-rich magmatic liquid

Carbonate rhombohedra in nodular cherts; Mesozoic of the West Carpathians

Abstract: Carbonate rhombohedra have been Found in a majority of chert nodules examined In limestones, they are calcite, sometimes accompanied by a small quantity of dolomite, Fe-dolomite and ankerite; in dolostones only dolomite was found within the cherts The presence of syntaxial rims on the corroded cores of rhombohedra having a different composition and rhombic pores after dissolved rhomdohedra and silica pseudomorphs suggests changing conditions during the growth of rhombohedra Syntaxial overgrowth on echinoderm ossicles can be older or younger than silicification Rhombohedra were formed during the early stages of silica diagenesis They preserved relics of microspherules (lepispheres ?) which are totally absent in other parts of the chert nodules

The Effect of Differing Hydrogeological Regimes on Sandstone Diagenesis: Brent Group Oilfields, U.K. North Sea

Abstract: Deltaic sandstones of the Middle Jurassic Brent Group, northern North Sea, have a complex diagenetic history. The paragenetic sequence can be simplified to; siderite - feldspar dissolution - vermiform kaolinite - calcite - feldspar dissolution - blocky kaolinite - quartz - ankerite - illite. Petrographic and stable isotopic studies indicate that Fe-rich siderite precipitated from meteoric water which flushed through the Brent Group during the northward progradation of the delta system. Later Ca and Mg rich siderites precipitated from sea water during the subsequent marine transgression which eventually drowned the delta. Later diagenesis is related to depth of burial and hence depends upon the subsidence history of individual oilfields. In shallow buried oilfields ( 3.0km; >80°C), continued subsidence meant that the reservoirs were sealed-off from the influx of surface-derived meteoric fluids. Illite cement, and greater volumes of quartz cement (>5%) then precipitated in a closed geochemical system. The unusually high "palaeotemperatures" of the fluid inclusions in the quartz overgrowths are false and are a product of resetting during continued subsidence, and do not indicate that influx of hot fluids occured. Finally pore-waters evolved to present day isotopic compositions (delta18O= 0 to +2‰) due to water-rock interaction.