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Exploration Research and Services Division, Conoco Inc., Ponca City, Oklahoma 74603, USA
Brent sandstone compositions vary systematically with increasing depth and temperature in three North Sea fields. At Statfjord field (c. 2500 m), sandstones are arkoses, lithic arkoses and subarkoses. At Hutton field (c. 3050 m), sandstones are subarkoses and quartzarenites, whereas at Lyell field (c. 3500 m), sandstones are mostly quartzarenites. Compositions are transformed by pervasive dissolution of plagioclase at relatively shallow depth, dissolution of K-feldspar at greater depth, and alteration of detrital mica to kaolinite or illite. Quartz cement is a minor constituent at Statfjord, mostly forming early during meteoric water flushing. Quartz cement abundance increases significantly at Hutton and particularly at Lyell, where quartz cement comprises up to 130f rock volume; the increase in quartz cement coincides with dissolution of detrital quartz along stylolites and microstylolites at temperatures greater than 90–100°C as well as the dissolution of feldspar.
The dominant clay at Statfjord is kaolinite; at Hutton, illite forms as a pore-lining precipitate and, at Lyell, also by alteration of previously formed kaolinite. The transition from a kaolinite-dominated to an illite-dominated sandstone is probably controlled by several factors, among these the dissolution of K-feldspar, which may be due either to an influx of organic acids from the overlying Heather and Kimmeridge Clay Formations or to thermodynamic instability of the assemblage kaolinite-illite-K-feldspar at temperatures greater than 100°C. Elevated silica activity due to quartz dissolution on styloites and microstylolites may also induce illite formation.
Chemical analysis of sandstone samples shows the Brent undergoes systematic loss of sodium and, to a lesser extent, potassium during burial diagenesis. The sodium loss is consistent with observed plagioclase dissolution. The potassium loss indicates that the potassium in illite is derived from the Brent itself, probably from dissolved K-feldspar, and not from an external source such as a basinal brine.
