Dunlin Group

Fig. 4. Seismic and geoseismic cross-section through the 30/9-3A and -4S wells, illustrating the sedimentary growth across faults in the Oseberg Syd area. Note the dramatic thickness increase of the Brent Group of ca. 100% from the Gamma to the Omega North structure, and an -i-additional 30 ms increase to the westernmost fault block. The growth observed in the Dunlin Group and StatQord Fm indicate that most of the faults were active through most of the Jurassic period.

Petrophysical analysis of the well data was used to define the shale fraction in each stratigraphic unit. CPI logs were used to derive explicit shale percentages within both sandstone units (e.g., 5% shale in the Lower Ness Sandstone) and shale units (e.g., 65% shale in the upper part of the Dunlin Group). This information was then compiled geographically to estimate likely compositions between the wells, i.e., at the fault locations. Example profiles of shale-volume fractions are shown in Fig. 9. 

There is no discernible pattern to the spatial distribution of siderite other than its sporadic occurrence within wells and within the field. Ankerite is also sporadically distributed and restricted to samples located at or close to the boundary with the underlying shales of the Drake Formation (Dunlin Group Fig. 4). Systematic distribution of diagenetic ankerite in the vicinity of mudstone units has been noted by Macaulay et al. (1993) in Jurassic sandstones of the Magnus field. North Sea. Siderite and ankerite represent minor cements in terms of total volume precipitated at field scale. 

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