Organic-rich, Cretaceous black shales from DSDP Site 41-368 on the Cape Verde Rise in the eastern Atlantic were penetrated by hot diabase sills during Miocene time. Programmed pyrolysis and pyrolysis-gas chromatography were conducted on organic matter from core samples taken at various distances from a major sill. These methods show the types of hydrocarbons generated, the remaining generative potential, and the thermal maturity of the kerogen in the shales.

Systematic changes in kerogen elemental compositions are a consequence of the thermal cracking of volatile organic products from the kerogen. Loss of these products causes progressive aromatization of the residual kerogen closer to the sill. This conclusion is supported by an increase in the ratio of thermally distilled hydrocarbons to total hydrocarbons generated by pyrolysis, and an increase in the temperature required for maximum generation of hydrocarbons from the kerogen.

Although the major sill was 15 m thick, solvent extraction and pyrolysis results show that hydrocarbon generation was restricted to within about 10 m of the shale/sill contacts. At a given distance, the temperature appears to have been higher above than below the sill.

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