(Table 1) Summary of low-molecular-weight hydrocarbon concentrations, organic carbon contents, Rock-Eval pyrolysis and lithology at DSDP Leg 79 Holes

A series of core samples taken during Cruise 79 of Glomar Challenger, drilling offshore Morocco (Mazagan Plateau), is analyzed for their low-molecular-weight hydrocarbon contents. Fifty-four samples from DSDP Holes 544A, 545, 547A, and 547B, deep frozen on board immediately after recovery, are studied by a hydrogen-stripping/thermovaporization technique combined with capillary gas chromatography. Thirty-eight compounds in the C2-C8 molecular range, including saturated, olefinic, and aromatic hydrocarbons, are identified.Because of large differences in organic carbon contents, the total C2-C8 hydrocarbon concentrations vary from about 20 to 1500 ng/g dry sediment weight in the whole sample series. Organic-carbon normalized values are about 3.2 x 104 ng/g Corg for Lithologic Subunits IIIA and IIIB at Site 545 (Cenomanian to Aptian) and 1.0 x 105 ng/g Corg for Unit V at Site 547 (Cenomanian to Albian) reflecting the slightly more advanced maturity stage at the latter site. Values exceeding 105 ng/g Corg (Site 545) and 2 x 105 ng/g Corg (Site 547) are associated with samples that are very lean in organic carbon and are generally rich in carbonate. These samples are enriched by small amounts of gaseous hydrocarbons.A detailed study of individual hydrocarbon concentrations, plotted against depth, reveal additional indications for migration phenomena. At Site 547, for instance, the most mobile hydrocarbons studied (e.g., ethane) appear to migrate by diffusion or a related process from more than 700 m depth toward the surface.