Colour alteration, thermal maturity, and burial diagenesis in fossil foraminifers

Abstract

Foraminifers are potentially sensitive indicators of thermal maturity. Changes in colour of the organic cement in the agglutinated test, and mineralogical changes in agglutinated and calcareous benthic foraminifers, provide evidence of thermal alteration. Empirical data from fossil specimens, and pyrolysis experiments, both indicate that darker colouration is related to thermal alteration. Colour changes can be measured accurately and quantitatively by comparison with the standard Munsell Colour Chart. An index consisting of ten categories has been established and is named the Foraminiferal Colouration Index (FCI). Examination of FCI in exploration wells in the Beaufort-Mackenzie Basin of arctic Canada indicate that FCI is most sensitive at levels of maturation equivalent to the early stages of petroleum generation. It also indicates that FCI may be retarded, or even reversed, by the influence of pore fluids in overpressured zones. Furthermore, FCI appears to be sensitive to heating rate, with the most rapidly deposited sediments showing the least increase in FCI with temperature. Mineralogical changes within the tests of both agglutinated and calcareous benthic foraminifers can be caused by diagenetic alteration. Based on data from exploration wells in the Beaufort-Mackenzie Basin, four zones of burial diagenesis (A-D) have been recognized. Zone A is characteristic of burial depths less that 2400 m (less than 75 C), wherein fossil foraminifers show little or no alteration. Zone B, at burial depths of 2400 to 3500 m (75 to 110 C), is characterized primarily by quartz mineralization (silicification). Zone C, at burial depths of 3500 to 5000 m (110-140 C), is characterized by further silicification and precipitation of kaolin and smectite in foraminiferal tests. Zone D, at estimated burial depths of 6 to 8 km (150-250 C), is recognized by intense silicification, recrystallized calcite, and precipitation of illite and chlorite. Calcareous specimens in Zone D are particularly unstable and may be replaced entirely by chloritic clay (chloritized).