Oseberg Formation fluid inclusions

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. 

Fig. 7. Histograms of homogenization temperatures of fluid inclusions in ankerite and calcite in the Oseberg Formation. Salinities derived from limited Tm data are indicated. Vertical black arrows represent present-day reservoir temperature.

Table 2. Summary of fluid inclusion data for diagenetic carbonates in the Oseberg Formation...

If heated to temperatures significantly higher than entrapment temperature, fluid inclusions in carbonates may stretch or leak, resulting in partial or complete resetting of Th values (Goldstein Reynolds, 1995). In the Oseberg Formation there is reasonable evidence that inclusions in ankerite and calcite did not reset. 

If it is assumed that fluid inclusions in ankerite are primary, measured Th values would represent minimum trapping temperatures (the presence of dissolved CH4 or hydrocarbons in these inclusions could not be ascertained). However, as ankerite pre-dates calcite and quartz in the diagenetic sequence, the lowest Th yielded by fluid inclusions in calcite and quartz, i.e. 85-90°C, may represent a maximum formation temperature for ankerite. This implies that diagenetic ankerite in the Oseberg must have formed at temperatures between 65 and 85° C, i.e. essentially identical to measured Th values (Table 2). 

The formation temperatures of ankerite and calcite in the Oseberg reservoir derived from fluid inclusions are in good agreement with petrographic observations indicating a late diagenetic precipitation. Combined with the thermal history of the reservoir, these temperatures imply that ankerite... 

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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