<P> Because of their diversity, abundance, and complex morphology, fossil foraminiferal assemblages are useful for biostratigraphy, and can accurately give relative dates to sedimentary rocks, as was discovered by Alva C. Ellisor in 1920 . The oil industry relies heavily on microfossils such as forams to find potential hydrocarbon deposits . </P> <P> Calcareous fossil Foraminifera are formed from elements found in the ancient seas where they lived . Thus, they are very useful in paleoclimatology and paleoceanography . They can be used, as a climate proxy, to reconstruct past climate by examining the stable isotope ratios and trace element content of the shells (tests). Global temperature and ice volume can be revealed by the isotopes of oxygen, and the history of the carbon cycle and oceanic productivity by examining the stable isotope ratios of carbon; see δ18O and δ13C . The concentration of trace elements, like magnesium (Mg), lithium (Li) and boron (B), also hold a wealth of information about global temperature cycles, continental weathering, and the role of the ocean in the global carbon cycle . Geographic patterns seen in the fossil records of planktonic forams are also used to reconstruct ancient ocean currents . Because certain types of Foraminifera are found only in certain environments, they can be used to figure out the kind of environment under which ancient marine sediments were deposited . </P> <P> For the same reasons they make useful biostratigraphic markers, living foraminiferal assemblages have been used as bioindicators in coastal environments, including indicators of coral reef health . Because calcium carbonate is susceptible to dissolution in acidic conditions, Foraminifera may be particularly affected by changing climate and ocean acidification . </P> <P> Foraminifera have many uses in petroleum exploration and are used routinely to interpret the ages and paleoenvironments of sedimentary strata in oil wells . Agglutinated fossil Foraminifera buried deeply in sedimentary basins can be used to estimate thermal maturity, which is a key factor for petroleum generation . The Foraminiferal Colouration Index (FCI) is used to quantify colour changes and estimate burial temperature . FCI data is particularly useful in the early stages of petroleum generation (about 100 ° C). </P>

What kind of rock do foraminifera shells produce