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Organic-rich Cretaceous black shales from the Cape Verde Rise in the eastern Atlantic were penetrated by hot diabase sills during the Miocene. Programmed pyrolysis and pyrolysis-gas chromatography of organic matter from core samples taken at various distances from a major sill were used to show: (1) the type of kerogen, (2) its relative level of thermal maturity, (3) the fraction of pyrolyzable organic compounds which are free, and (4) the compositions of the bitumen and kerogen.
The dominant factor affecting these sediments appears to be thermal maturity rather than differences in the quality of the organic matter. Systematic changes in the pyrolyzate resulted from thermal cracking of volatile organics from the oil-prone Type II kerogen. Generation of these products caused progressive aromatization of the residual kerogen closer to the sill. These conclusions are supported by changes in kerogen elemental compositions, quantity of bitumen extract, and vitrinite reflectance.
Although the major sill is 15 m (~ 50 ft) thick, solvent extraction and pyrolysis results show that hydrocarbon generation was restricted to within about 10 m (~ 33 ft) of the shale/sill contacts. At equal distances, the maximum temperature reached by the shales was higher above than below the sill. The reflectance of vitrinite responds more rapidly than bitumen composition to high temperatures imposed over a short time.
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