Compaction of Modern Carbonate Sediments: Implications for Generation and Expulsion of Hydrocarbons
Eugene A. Shinn, Daniel M. Robbin, George E. Claypool, 1984. "Compaction of Modern Carbonate Sediments: Implications for Generation and Expulsion of Hydrocarbons", Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks, James G. Palacas
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Combined compaction and heating experiments conducted with in-situ modern carbonate sediments produced dissolved and undissolved tarry hydrocarbons. During heating at temperatures ranging from 100° to 200°C (212°-392°F) and under pressures of up to the equivalent of 13,000 ft (4,000 m) of burial, hydrocarbons with an immature, biologically related, bimodal carbon distribution were converted to hydrocarbons with a relatively more mature unimodal distribution. The new hydrocarbon distribution resembles those typical of Upper Cretaceous carbonate-rock extracts in south Florida. The data from these short-term experiments lend support to the idea that limestones can serve as hydrocarbon source beds and that compaction, both mechanical and chemical (i.e., pressure dissolution), could provide the "driving force" for expulsion into a carrier bed or directly into a reservoir. The suggestion is made that, at depths and temperatures at and within the oil-generation window, liquid hydrocarbons together with compaction waters escape during chemical compaction (akin to stylolitization). As temperature and pressure increase, liquid hydrocarbons escape through hydraulically formed microfractures. Finally, with continued burial and increased temperature, below the oil-generation window, the residual hydrocarbons and other hydrogen-rich materials (kerogen) are converted to and escape as gas condensates and gases.
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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.