Questions concerning the origin and constitution of so-called heavy crude oils have multiplied immensely with escalating interest in recovery of these oils. Although defining the term heavy crude oils presents a major and as-yet-unsolved problem, substances such as naturally occurring tars, asphalts, and bitumens are all considered to be expressions of heavy crude oils and are so treated in this presentation. Much evidence indicates that some heavy oils are residues of biodegraded conventional oils or are conventional oils from which the light ends have been stripped. On the other hand, such observations do not readily explain the origin of all heavy oils, among which are those found at great depth where neither biodegradation nor evaporative stripping is a likely mechanism.
The association of vanadium and sulfur with heavy crude oils also suggests that many such oils may have been formed by mechanisms other than those thought to be basic to the origin of conventional oils. In particular, evidence has recently been accumulated to show that carbonate rocks may be source beds for many heavy crude oils and that mechanisms leading to the direct formation of heavy oils are probably unlike those by which conventional oils are formed. Moreover, oil formed in carbonate sequences may have accumulated in source-bed reservoirs if conditions of porosity and permeability were appropriate.
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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.