R. Burwood, 1984. "Carbonate Source Rocks for Six Million Barrels of Oil per Day—Zagros Fold Belt, Southwestern Iran", Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks, James G. Palacas
Download citation file:
The giant fields of the Zagros fold belt of southwestern Iran fall within the greater context of the Arabian-Iraq-Persian basin and contain cumulative recoverable reserves estimated at 87 billion barrels of oil and 514 trillion cubic feet of gas.
The regional geology of the area comprises a wedge of Paleozoic to Holocene sediments, 10–15 km (6–9 mi) or more in thickness, supported on a mobile Eocambrian salt economic basement. Forming a classic carbonate–evaporite sequence, the succession contains prolific source-reservoir combinations and effective seals of integrity. Located on the eastern subducting boundary of the Arabian plate, hydrocarbon generation has been largely controlled by the Neogene Zagros orogenic event. A number of potential source sequences have been recognized. With one exception, all are typically of an organic-rich, argillaceous, lime-mudstone litho-textural type.
Most of the giant oil accumulations in Asmari (Oligocene-Miocene) and Bangestan Group (Upper Cretaceous) reservoirs have a common provenance in the Kazhdumi Formation of Early Cretaceous (Albian) age. The Pabdeh (Paleogene), lower Garau–Gadvan (Lower Cretaceous, Neocomian), and Sargelu (Middle Jurassic) formations contribute less to overall reserves, having been the source for selected subordinate Asmari, Bangestan, and Khami Group (Upper Jurassic-Lower Cretaceous) reservoirs only. Gas in Permian-Triassic reservoirs has a provenance in Ordovician-Silurian siliciclastics.
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.