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The accelerated diagenesis, maturation, and catagenesis of organic matter to hydrothermal petroleum was studied in sediments from Ocean Drilling Program Leg 139 in Middle Valley, northern Juan de Fuca Ridge. Sediments at Sites 855 and 856 have experienced high heat flow resulting in accelerated diagenesis of the immature organic matter without product migration. Ruid migration through these sediments has resulted in an influx of mature organic components, yielding diverse bitumen mixtures. At Site 857 the alteration has occurred in situ to the catagenetic stage and at Site 858, the hottest holes, the hydrothermal petroleums have migrated after generation. Maturation for the rc-alkanes proceeds from high CPI values to <l .0 (i.e., strong even-carbon-number preference), and due to the low TOC these sediments do not generate high amounts of hydrocarbons from kerogen, thus revealing this unique carbon number preference. The biomarkers are converted from the biological precursors to the geological equivalents with various maturities. The organic matter is principally marine in origin and contains a significant terrigenous component, as confirmed by the alkane and biomarker distributions. High-temperature alteration yields polynuclear aromatic hydrocarbons, which in these sediments are concentrated in the high molecular weight range (>250 dalton).

Lipid/Bitumen Maturation by Hydrothermal Activity in Sediments of Middle Valley, Leg 139