As with shales, the source potential of carbonate rocks depends primarily upon the organic facies rather than the mineral matrix. Where the depositional and early diagenetic environment is highly oxygenated, the total-organic-carbon (TOC) content is low. The remaining kerogen is highly oxygenated, with a negligible generative capacity for hydrocarbons, despite a relatively high hydrocarbon/TOC ratio in the immature state. An anoxic depositional early diagenetic environment can result in the deposition of organic-rich, fine-grained carbonate sediments that are excellent potential source rocks.
Excellent oil-prone source rocks, whether with carbonate- or clay-mineral matrices, have many characteristics in common. Both form in anoxic environments, are generally laminated and heterogeneous, have moderate to high TOC, and contain high-quality organic matter (OM). The latter is exemplified by atomic H/C ratios ≥ 1.2 near a vitrinite reflectance of 0.50% Ro. Although they constitute a small percentage of all carbonate rocks, organic-rich, fine-grained carbonate rocks are widespread in both time and space and are the probable source of 30–40% or more of the petroleum reserves of the world.
Gas-prone organic facies are rare in carbonate rocks because they are usually dominated by terrestrial organic matter deposited in a dominantiy clay matrix. However, gas-prone organic facies may occur in carbonate rocks as a result of turbidite deposition or by a mixture of kerogen types II and IV. Most carbonate rocks contain nongenerative organic facies, as do most siliceous rocks. Oxygen-rich depositional environments for carbonates are found from sea level (reefs) to the ocean depths (Globigerina ooze).
Despite the basic relationship between organic-rich oil-prone carbonate and shale source rocks, some significant differences exist. Oils derived from carbonate rocks are often richer in cyclic hydrocarbons and sulfur compounds than oils derived from shales, owing to the dearth of terrestrial-plant waxes in the OM and less iron in the pore water. In addition, the generally earlier decrease of porosity and permeability and the greater contrast between the physical properties of the OM and the rock matrix in carbonate source rocks often result in different primary migration characteristics.
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.