Carbon isotope composition of macerals separated from various kerogens by density separation method.

Source rock-oil correlation has often been inferred from carbon isotope ratios o f insoluble organic matter (kerogen) contained in sediments, since each kerogen has a peculiar value controlled by maceral composition. The objective of this study is to characterize δ13C values of each maceral separated from various kerogens and to examine whether each maceral has a peculiar value or not.Firstly, maceral groups of kerogens were separated from the Green River shale, Kimmeridge Clay, Sarufutsu coal and other typical samples by the density separation method (Kinghorn and Rahman, 1983). Amorphous kerogen can be subdivided into three fractions, namely fluorescent (FA), weakly fluorescent (WFA) and non-fluorescent (NFA) amorphous kerogens by visual kerogen method using their fluorescent characters under a high magnification microscope. FA, WFA and NFA are considered to be derived from sporinite and cutinite, alginite of marine planktons, and vitrinite, respectively.In the next step δ13C values of maceral groups, separated from the Middle to Upper Miocene Onnagawa shales of a petroleum exploration well in Akita City, were measured. The WFA fraction has different δ13C values ranging within 2‰ in different stratigraphic horizons, while sporinite and vitrinite group macerals have their own constant values throughout different horizons. Carbon isoptope composition of whole kerogen is controlled by the WFA fraction which makes up the bulk of the kerogen. It is, therefore, concluded that the δ13C value of WFA is much more important than that of total kerogens for source rock-oil correlation since WFA is considered to be the main source matter of petroleum.