BONNELL, L. M.¹, C. J. LOWREY², and A. A. BRAY^3
1Geologica AS, Stavanger, Norway
²University of Oslo, Oslo, Norway
³Neste Petroleum, Oslo, Norway

Abstract: The Timing of Illitization, Haltenbanken, Mid-Norway

In Haltenbanken illite is restricted to sandstones whose present day burial depths are greater than 3500 m. This has led several workers to propose that illitization occurred when temperatures reached 140°C, generally within the last 2 million years (Bjorlykke et al, 1986, Ehrenberg & Nadeau, 1989).

Petrographic studies from a Haltenbanken well, however, suggest that either: (1) illitization in this well occurred earlier in the burial history at temperatures lower than 140°C, or (2) temperatures of 140°C were reached earlier in the burial history. Thin-section studies show illite enclosed within large (50µ) quartz overgrowths. We used the Exemplar model to calculate the timing and extent of quartz cementation for these samples using continuous burial with maximum temperatures at the present day. The model shows that 2 million years is insufficient time to precipitate the amount of post-illitization quartz cement observed. Illitization earlier in the burial history at lower temperatures would allow time for sufficient post-illitization quartz cement to form but is difficult to reconcile with the lack of illite in wells shallower than 3500 m.

Higher temperatures in the past are consistent with both high temperature of illitization and sufficient time for quartz cementation. Based on stratigraphic evidence, we propose a burial history with episodes of deep burial, uplift and erosion during the Cretaceous, Eocene and Pliocene. Eocene deep burial resulted in temperatures near 140°C. Illitization could have occurred at ~40Ma allowing time for significant quartz cementation. Further support for this burial history is given by K/Ar dates averaging 39 Ma for illites from this well.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas