Use of Thiophenic Organosulfur Compounds in Characterizing Crude Oils Derived from Carbonate Versus Siliciclastic Sources
William B. Hughes, 1984. "Use of Thiophenic Organosulfur Compounds in Characterizing Crude Oils Derived from Carbonate Versus Siliciclastic Sources", Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks, James G. Palacas
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Several families of thiophenic organosulfur compounds (primarily benzo- and dibenzothiophenes) show promise for distinguishing oils derived from carbonate sources versus siliciclastic sources. This observation is based on analyses, using high-resolution gas chromatography coupled with a flame photometric detector, of several representative oils derived from carbonate sources (Smackover and Sunniland Formations, southeastern United States; Burgan and Minagish, Kuwait; and the Khatiyah Formation, Dubai) and, for comparison, of oils from siliciclastic sources (Ekofisk, North Sea; McKee, Texas; Muddy/Dakota, Wyoming; Beaverhill Lake, Alberta; and Klasaman, Indonesia). Some of the distinguishing features of carbonate-derived oils are an abundance of benzothiophenes, a fairly equal distribution among the dibenzothiophenes, and a distinctive distribution of methyldibenzothiophene isomers. Conversely, oils derived from siliciclastic sources commonly show low concentrations of benzothiophenes, decreasing amounts of dibenzothiophenes, and a different pattern in methyldibenzothiophene isomers.
The distinguishing features of carbonate-derived oils are applicable through the main phase of oil generation. Additional thermal maturity results in a significant reduction in the amounts of benzothiophenes and a redistribution of methyldibenzothiophene isomers, making the differences between carbonate- and siliciclastic-derived oils less distinctive. However, in these cases the distribution of mono-, di-, and trimethyl substituted dibenzothiophenes is generally relatively uniform in carbonate oils, while siliciclastic oils show decreasing amounts of dimethyl- and trimethyldibenzothiophenes relative to methyldibenzothiophenes.
Used in conjunction with such other geochemical parameters as total sulfur content, n-alkane compositions, and sterane/triterpane compositions, thiophenic organosulfur analysis provides a useful tool for oil-oil and oil-source-rock correlations as exemplified by a study of 20 oils from the Salawati basin, Indonesia.
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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.