Environment of Deposition of Middle Miocene (Alcanar) Carbonate Source Beds, Casablanca Field, Tarragona Basin, Offshore Spain
Gerard Demaison, F. T. Bourgeois, 1984. "Environment of Deposition of Middle Miocene (Alcanar) Carbonate Source Beds, Casablanca Field, Tarragona Basin, Offshore Spain", Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks, James G. Palacas
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The oil in the Casablanca field, offshore Spain, is present in Jurassic limestones unconformably overlain by thick Tertiary sediments that provide both source and seal. The source beds for the Casablanca oil have been identified as parts, but not the whole, of a carbonate unit at the very base of the Tertiary transgressive sequence, the Alcanar Formation of middle Miocene age. The Alcanar oil-source beds are dark-brownish-gray, well-lithified, pelagic marlstones to limestones containing foraminifers. The mineral carbonate content ranges from 38 to 75%. The organic-carbon content in the source facies ranges from 3.2 to 4.7%. The hydrocarbon yield from whole-rock pyrolysis is high (5 kg/ton or more than 1 gal/ton) and is compatible with the identified kerogen type (type II).
Oriented thin sections from conventional cores in the Alcanar source facies show no evidence of large burrowing infauna and only sparse benthic foraminifers. Rare microscopic, horizontal worm burrows are present. Most, but not all, thin sections in the source facies show microscopic laminations. These combined characteristics are indicative of past low oxygen concentrations above sea bottom typical of dysaerobic environments. These conditions, when combined with high sedimentation rates, favor oil-source-bed deposition.