Hydrocarbon Genesis and Organic Facies in Cambrian Carbonates of the Eastern Officer Basin, South Australia
David M. McKirdy, Agu J. Kantsler, John K. Emmett, Alan K. Aldridge, 1984. "Hydrocarbon Genesis and Organic Facies in Cambrian Carbonates of the Eastern Officer Basin, South Australia", Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks, James G. Palacas
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In mid-1979, a stratigraphic well drilled by the South Australian Department of Mines and Energy (SADME), Byilkaoora-1, encountered shows of aromatic-naphthenic and naphthenic oils while coring Cambrian carbonates in the northeastern Officer basin, South Australia. The oils occupy vugs and partly healed fractures in an alkaline playa-lake sequence of algal-plate dolomite mudstone, dolomite mudstone with Magadi-type chert, and organic-rich dolomitic argillaceous mudstone containing calcite pseudomorphs of sodium carbonate-bicarbonate minerals, notably trona and shortite. Oil-source-rock correlations confirm that the oils originated within those facies drilled. The Byilkaoora oils are thus the first reported examples of nonmarine Cambrian petroleum. They are unusually rich in the C15-C25 and C30 acyclic isoprenoid alkanes, except where severely biodegraded. Sterane and hopane distributions indicate that the oils were expelled from marginally mature source rocks. On the southeastern flank of the Officer basin, micritic carbonates deposited in a marine-sabkha environment are marginally mature to mature and have good oil-source potential, although minor staining is the only evidence of oil generation and migration in these rocks.
The primary organic facies of the Officer basin carbonates determines whether they are oil or gas prone. Kerogen in the oil-prone carbonates is either Type I (deposited in an alkaline playa–lacustrine environment) or Type II (deposited in a marine-sabkha or lacustrine environment). Marine lagoonal carbonates contain gas-prone Type III kerogen derived from algal (including cyanobacterial) mucilage. Petrographically, the major components of the oil-prone kerogens are lamellar alginite and bituminite. Likely precursors include the lipids of cyanobacteria (blue-green algae) and various heterotrophic bacteria. The sesterterpanes and squalane present in high concentrations in the Byilkaoora oils and their source rocks may be biological markers of halophilic and/or methanogenic archaebacteria.
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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.