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.
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Carbonate rocks have diverse characteristics. They can be excellent reservoirs as well as prolific source rocks for oil. Oils from carbonate rocks commonly have distinctive bulk chemical and molecular characteristics that reveal their origin. The lack of widespread appreciation for these facts in the geological community was one reason that a symposium entitled “Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks” was organized and held at the Geological Society of America annual meeting in Atlanta, Georgia, in October 1980. The symposium was sponsored by the Organic Geochemistry Division of the Geochemical Society during my term as chairman of the division. Of the 18 papers given in the symposium, 12 papers and four abstracts are included herein. Also included in this volume are two papers that were prepared later.
I hope that this collection of original papers, which synthesize data from about 20 different sedimentary basins, will help to correct any lingering misconceptions concerning the effectiveness of carbonate rocks as major sources of petroleum. I also believe that the information presented herein, including the references, will serve as a valuable resource for evaluating petroleum occurrence in other carbonate sequences and for locating petroleum reserves in unexplored, partially explored, and even maturely explored basins where possible carbonate-generated oil and gas may have been overlooked.
The first 11 papers, arranged in geo-chronological order, are descriptions and interpretations (that is, case histories) of specific carbonate source rocks that range in age from Precambrian to Miocene. Some of the highlights of these papers are summarized below.
The paper by Fu Jia Mo, Dai Yong Ding, Liu De Han, and Jia Rong Fen, in addition to describing the geochemistry of petroleum accumulations and source rocks ranging in age from Precambrian to Triassic, points out some interesting differences in thermal histories of Precambrian carbonate-rock sequences in eastern China. In one basin, Precambrian carbonate rocks are surprisingly thermally immature and have yielded heavy oils and asphalts. In another basin, on the other hand, Precambrian carbonate rocks are definitely overmature and have generated methane-rich gas.
The paper by McKirdy, Kantsler, Emmett, and Aldridge on the Eastern Officer basin, South Australia, includes the first reported examples of nonmarine carbonate rocks and oils of Cambrian age that are similar to those of the Eocene Green River Formation, Utah.
In their study of crude oils in the Michigan basin, Gardner and Bray indicate that the interreef, laminated carbonate rocks of Silurian age are the primary source of commercial oil accumulations in the Silurian pinnacle reefs.