The Foraminiferal Colouration Index (FCI) is a tool for assessing the thermal alteration

Metamorphism

Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...

 of organic matter buried in sedimentary rock

Sedimentary rock

Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

. It uses temperature-controlled colour changes in the organic cement of agglutinated foraminifera

Foraminifera

The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...

 (microfossils) to estimate thermal alteration. The method is empirical and based on determination of colour by visual comparison of fossil specimens to the Geological Society of America

Geological Society of America

The Geological Society of America is a nonprofit organization dedicated to the advancement of the geosciences. The society was founded in New York in 1888 by Alexander Winchell, John J. Stevenson, Charles H. Hitchcock, John R. Proctor and Edward Orton and has been headquartered at 3300 Penrose...

 Rock-Color Chart (Munsell colour system) under a binocular microscope.

The FCI values 0 to 10 are indicated below:

FCI	Munsell Notation	Colour Description
0	7.5YR6/6	reddish yellow
1	10YR8/1, 10YR8/2	white
2	10YR6/1, 10YR7/2	light grey
3	10YR5/1, 10YR6/2	light brownish grey to grey
4	10YR4/1, 10YR5/2	grey to greyish brown
5	10YR3/1, 10YR/4/2	dark grey to dark greyish brown
6	10YR2/1, 10YR3/2	very dark grey to very dark greyish brown
7	10YR2/1, 10YR2/2	very dark brown to brownish black
8	10YR2/1, N2/0	very brownish black
9	N2/0	black (partially translucent)
10	N2/0	black

Agglutinated foraminifera have a long geological history spanning the Paleozoic

Paleozoic

The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

, Mesozoic

Mesozoic

The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

, and Cenozoic

Cenozoic

The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

. The FCI has many potential applications especially in the analysis of samples from hydrocarbon exploration wells. Significant colour changes (FCI 2 to 6) occur through a temperature range of ~60 °C to ~110 °C. The FCI is therefore an indicator of temperatures required to initiate petroleum generation (oil window). The onset of oil generation correlates approximately to FCI 5-6.

The FCI methodology is similar to the Conodont Alteration Index

Conodont Alteration Index

The Conodont Alteration Index is used to estimate the maximum temperature reached by a sedimentary rock using thermal alteration of conodont fossils. Conodonts in fossiliferous carbonates are prepared by dissolving the matrix with acid, since the conodonts are composed of apatite and thus do not...

(CAI), but the colour/temperature relationships differ in that FCI is more sensitive to colour change at lower temperatures.