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In the Lower Saxony basin of northwest Germany several deep seated batholithic intrusions are indicated by pronounced gravity and magnetic anomalies. At considerable distance from these intrusions, the igneous heat has significantly advanced the rank of coaly particles finely dispersed in shales and siltstones of Upper Carboniferous to Lower Cretaceous-age strata, and isorank patterns have been documented in the literature. The results of a detailed geochemical study of kerogens and extractable low and high molecular weight hydrocarbons in Lower Jurassic Pliensbachian-age source bed-type shales involving a multitude of analytical methods (solvent extraction, MPLC, HPLC, capillary gas chromatography of saturated and polycyclic aromatic hydrocarbons, GC/MS of steranes and t iterpanes, hydrogen stripping/thermovaporization of light hydrocarbons, Rock-Eval pyrolysis and reflectance microscopy) are reported in this paper. A reconstruction of the pre-intrusive burial and maturation history based on a modified Lopatin method allowed, for this sourcerock unit, estimation of temperature and maturity increase associated with the intrusion.
The main conclusion from this study concerns a significant influence which the high heating rate exercised, during the period of cooling of the intrusions, on the evolution of hydrocarbon generation processes in these Lower Jurassic source beds. The established relationship between stages of petroleum hydrocarbon generation and maturity progress (expressed, e.g., as vitrinite reflectance), as it has been documented for many case histories worldwide and referred to as "liquid window," cannot be seen in this rapidly heated source rock series. The liquid window is shifted toward higher maturity stages and extends up to 1.75% Rm. This is documented on the basis of maturity trends for the evolution of yields and compositional patterns of the extractable hydrocarbons. The most li ely explanation for these observations is that the reaction rate causing vitrinite reflectance to increase is more temperature-dependent than the rates of the hydrocarbon generation reactions. In kinetic terms this means that, although the processes are highly complex, the effective activation energy of the vitrinite reaction is higher than that of the hydrocarbon generation reactions.
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