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Measurements of dispersed vitrinite in several exploration wells within the Tertiary and Quaternary rift sediments of the northern Upper Rhine Graben indicate a complex thermal history. While most wells show “normal” increasing maturity trends with depth, some lack any obvious trend. One of the investigated wells, Nordheim-1, even features a bell-shaped downhole anomaly having an inverse maturation trend. Abnormal maturation effects, such as oxidation, reworking, or deposition of previously coalified material from the hinterland, are ruled out as a result of the relative position of the well with respect to the graben shoulders or active fault systems. Thus, secondary maturation caused by focused, lateral hydrothermal fluid flow has been proposed.

To verify the measured maturity anomaly organic (Rock-Eval pyrolysis, methylphenanthrene distribution), geochemical analyses were used to obtain independent data for comparison. Tmax values from Rock-Eval pyrolysis and various ratios of methylphenanthrene isomers indicate abnormally high maturities at shallow depth. Like the vitrinite reflectance values, these techniques show maturities that are inconsistent with their present burial depth but support enhanced convective hydrothermal heat flow in these strata. The consistency of results obtained using independent techniques verifies a hydrothermal origin of the maturity anomaly and excludes other modes of alteration.

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