Some Aspects of the Hydrocarbon Geochemistry of a Middle Devonian Barrier-Reef Complex, Western Canada
T. G. Powell, 1984. "Some Aspects of the Hydrocarbon Geochemistry of a Middle Devonian Barrier-Reef Complex, Western Canada", Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks, James G. Palacas
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Organic geochemical data have been obtained on potential source rocks, bitumens, and oils from within and in the vicinity of a Middle Devonian carbonate-barrier complex in western Canada. Organic-carbon values in carbonate rocks vary widely (0.11 to 30.1%) according to facies type. Extract yields from immature samples from the basinal facies can be extremely high and appear to be a consequence of the highly reducing conditions in which the organic matter was deposited. Characteristic features of these high-yield extracts are a low proportion of hydrocarbon, low saturate-to-aromatic ratios, pristane-to-phytane ratios less than 1.0, a high content of acyclic isoprenoids relative to n-alkanes, and an even-to-odd predominance in the n-alkanes. The hydrocarbon yields at low levels of maturity are sufficiently high for these carbonates to be source rocks. The main migration product is heavy oil/bitumen, which shows all the characteristics of the parent-rock extracts. Locally these bitumens have been altered by heat during dolomitization to form an insoluble hydrogen-rich pyrobitumen classified as epi-impsonite. This pyrobitumen probably was formed by polymerization of the NSO/asphaltenes fraction. With increasing depth the normal rock extracts show a decline in NSO (nitrogen, sulfur, and oxygen) compounds and asphaltenes yield owing to polymerization under normal conditions of maturation but with essentially no change in hydrocarbon yield. The oils in pinnacle-reef reservoirs in the Rainbow and Zama fields south of the barrier complex also show features inherited from a source rock deposited under highly reducing conditions. These oils are more mature than heavy oils encountered in the low-maturity area.
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Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks
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.