Fluid inclusions cement

Walderhaug O. (1994) Temperatures of quartz cementation in Jurassic sandstones from the Norwegian continental shelf— evidence from fluid inclusions. J. Sedim. Petrol. 64,311-323. 

Walderhaug, O. 1997. Precipitation rates for quartz cement in sandstones determined by fluid-inclusion microthermometry and temperature-history modeling. J. Sedim. Res., in press. 

Such cements occur in weakly compacted sediments and are characterized by low 6 0 values and fluid inclusions with high homogenization temperatures. This mechanism imposes difficulties in recognizing these cements from those formed by recrystallization at increased temperatures, as both mechanisms preserve a high, pre-cement porosity. A few workers (Giroir et al., 1989 Souza et al., 1995) argued that the early emplacement of calcite cement in sandstones of rift basins may take place... 

The fluid inclusions in the microcrystalline and coarse blocky Fe-dolomite and ankerite cement are extremely rare and too small (< 2 im) to be used for reliable determination of homogenization temperatures. 

Diagenetic conditions of carbonate cementation constraints from isotopes and fluid inclusions... 

Fluid inclusion microthermometry indicates the presence of moderately saline waters in the calcite cements of both units. 

Calcite cement typically pre-dates quartz cementation, as indicated by the lack of quartz overgrowths within calcite-cemented zones (Table 1). As abundant fluid inclusion evidence (see review in Walderhaug, 1994) strongly indicates that quartz cementation typically becomes significant at temperatures of 70-80°C, this is strong confirmation that most calcite cementation was completed at temperatures below around 0 C. 

Few fluid inclusion data have been published for calcite cement in shallow marine sandstones, and homogenization temperatures in calcite may possibly be reset (Barker Goldstein, 1990). However, the measurements reported by Saigal Bjorlykke (1987) are in the range 56-68 C and thus compatible with calcite precipitation prior to deep burial. Finally, the high intergranular volumes found in many calcite-cemented sandstone samples (Table 1) also indicate relatively early calcite precipitation. 

Walderhaug, O. (1990) A fluid inclusion study of quartz-cemented sandstones from offshore mid-Norway— possible evidence for continued quartz cementation during oil emplacement. J. sediment. Petrol., 60, 203-210. 

Diagenetic carbonate cement in reservoir sandstones of the Oseberg Formation (Brent Group) in the Oseberg field, Norwegian North Sea, occurs as disseminated siderite and ankerite, and as massively calcite-cemented intervals. Other diagenetic features include extensive feldspar dissolution and K-feldspar, quartz, kaolinite and dickite cements. Conditions of carbonate cementation are constrained on the basis of textural, geochemical and fluid inclusion evidence. 

Rare two-phase fluid inclusions e found in anker-ite and in calcite (Fig. 6H). They occur as isolated inclusions or in groups of a few inclusions with a random distribution. Owing to the scarcity of inclusions and to the lack of visible intracrystalline growth features in the carbonate cements, relationships between fluid inclusions and carbonate growth could not be evaluated. As a result, the primary or secondary origin of the inclusions studied cannot be ascertained. Microthermometric measurements are summarized in Table 2 and Fig. 7. 

The timing and conditions of carbonate cementation in the Oseberg Formation in the Oseberg field have been discussed on the basis of petrological, fluid inclusion and geochemical evidence. Samples from structural blocks and from a major fault zone were investigated. 

Ankerite cement is very scarce in the Oseberg reservoir, except in one sample located at the boundary with the underlying shales. In this sample, fluid inclusion analysis indicates that ankerite formed at temperatures of 70-80 °C, i.e. during the latest Cretaceous or Early Tertiary. The Sr/ Sr ratios and REE patterns of ankerite are consistent with a significant influence of fluids and/or trace elements derived from shales. The 5 0 values indicate a marked contribution of meteoric water during ankerite formation. Ankerite from the fault zone and from the structural blocks have similar 5 0-5 C values, suggesting similar conditions of formation. 

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