Geochemistry of Crude Oils and Crude-Oil-Source-Rock Correlations in Four Carbonate Basins
Jacques Connan, Georges Hussler, Pierre Albrecht, 1984. "Geochemistry of Crude Oils and Crude-Oil-Source-Rock Correlations in Four Carbonate Basins", Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks, James G. Palacas
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Crude oils and rock samples from carbonate basins in France (Aquitaine), Guatemala, Iraq, and Tunisia/Libya were analyzed by the usual methods of organic geochemistry. The studies were based mainly on computerized gas chromatography-mass spectrometry, with special emphasis on mass fragmentography. Several classes of biological markers were used for the purpose of characterizing the environments of deposition and for correlating crude oils with source rocks. The following specific features commonly were observed in the oil and rock samples from these carbonate formations: (1) the predominance of normal alkanes with even carbon numbers, usually more pronounced in the rocks than in the oils; (2) a ratio of pristane to phytane mostly below 1; (3) the presence in significant concentrations of the higher homologs of the hopane triterpanes, up to C35 (more important in rocks than in oils); (4) usually small amounts of steranes (C21-C23 and C27-C29 ranges), with predominance of non-rearranged structures; and (5) commonly an unusually high concentration of individual isoalkanes (C23) and cyclohexylalkanes (C21).
These molecular criteria partly reflect an important microbial contribution, which is usually more pronounced in highly anoxic environments. Furthermore, they have been used successfully in correlations among crude oils and between crude oils and their source rocks. The results agree with other geochemical and geological data.
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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.