Recurrent Appearance of Source-Rock Facies in Cretaceous to Eocene Carbonate Series of Tunisia
B. Tissot, R. Pelet, B. B. Furollet, J. L. Oudin, 1984. "Recurrent Appearance of Source-Rock Facies in Cretaceous to Eocene Carbonate Series of Tunisia", Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks, James G. Palacas
Download citation file:
Oil production in Tunisia is obtained from a carbonate series of mainly shallow-water facies comprising Middle to Upper Cretaceous, Paleocene and Eocene formations interbedded with the reservoirs. Several organic-rich beds with good petroleum potential appear in the series; however, they are of limited geographic extent, which results in poor chronostratigraphic correlation from well to well. The deposition of these beds seems to have depended on the local appearance of anoxic conditions in an otherwise constant environment. The most plausible model, consequently, is a shallow, consistently oxygen-depleted basin with eutrophic planktonic growth in which anoxic conditions could occur in slightly depressed topographic hollows that varied both in place and time.
The character of the organic matter of these beds exhibits features that correspond well with the model suggested. The chloroform extract, rich in NSO (nitrogen, sulfur, and oxygen) compounds and especially in resins, generally shows a strong, even predominant, n-alkane distribution and a high phytane-pristane ratio. In some interesting but uncommon cases this picture is reversed, however. The chemically isolated, chloroform-insoluble fraction, studied by Rock-Eval pyrolysis, exhibits characteristics typical of type II organic matter that originates from marine planktonic remains. All these features show little variation with geologic age. Correspondingly, the oils in the reservoirs are all of similar composition independent of reservoir age. The composition of the oil is compatible with the concept of generation from the organic beds previously considered. Consequently, a single oil type appears to have been generated by distinct source beds corresponding to the recurrent appearance of the same organic facies.
Figures & Tables
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.