The Jurassic reservoir rocks at Haltenbanken are currently buried to depths ranging from 2 to 5.5km and modelling of reservoir quality retention at depth is a critical issue. In general at depths exceeding 3.7-4.0km the volume of pore-filling authigenic illite increases sharply and is concomitant with a decline in the overall volume of K-feldspar and kaolinite.

However, both the total feldspar content and the plagioclase/K-feldspar ratio can vary greatly. For example in a central area around the Smorbukk and Heidrun Fields the feldspar content of the Garn Formation is dominated by K-feldspar whereas in some wells to the west the feldspars are almost exclusively plagioclase. These diferences in feldspar composition occur over a wide depth range and the variations are difficult to explain as resulting from diagenesis alone. They are therefore interpreted to reflect very different depositional feldspar distribution patterns and hence provenance areas.

When significant quantities of kaolinite are found at depths of greater than 4km then K-feldspars occur in very low concentrations or are absent altogether. This suggests that kaolinite has not been replaced by illite because the main source of potassium (K-feldspar) was not available. Petrographic and geochemical data indicate that kaolinite may remain unillitised when K-feldspar is unavailable locally even when excess K-feldspar occurs a few metres away. This has implications for, and constrains the effectiveness of, advective or diffusive transport of potassium within sedimentary basins. It also brings into question the effectiveness of shales as sinks for the potassium released from sandstones during burial diagenesis.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah