Organic matter in the Franciscan and Cedros subduction complexes: the problems of ‘instantaneous maturation’ and ‘missing petroleum’ in accretionary prisms

Most accretionary complexes are submarine mountain belts consisting predominantly of sedimentary rock. Past studies of accretionary complexes have concluded that they have minimal petroleum potential, although they do contain potential source rocks. Owing to their great bulk of sedimentary rock, large amounts of petroleum must be generated in accretionary prisms, and these hydrocarbons must be trapped in the complexes, or vented into the ocean or on land. To understand the nature and abundance of organic material in accretionary complexes, organic matter in the shales, limestones and melange matrix of the Franciscan and Cedros subduction complexes was studied, mostly using Rock-Eval pyrolysis. The largest amounts of total organic carbon (TOC) were found in the Calera Limestone from the San Francisco area (1–6% TOC). Terrigenous sedimentary rocks of probable trench or trench-slope origin contain from