Stratigraphic Relations of Arabian Jurassic Oil1
Jurassic rocks over much of the central and northeastern parts of Saudi Arabia constitute a major cycle of carbonate deposition closing with evaporites. Lower, Middle, and lower Upper Jurassic rocks apparently represent shallow-water shelf deposits in the west, passing to the east into rocks presumably originally lime muds deposited in water of unknown but greater depth. Two of the units (Tuwaiq Mountain and Hanifa) prominent in the shallow-water sequence of central Arabia seem to be lacking in the east. It is reasonable to assume that oil accumulations in these rocks (Dhruma and Jubaila formations) of coastal Arabia, originated in and near the rocks in which the oil is now found.
The later Jurassic Arab formation (middle or upper Upper Jurassic) shows increased paleogeographic differentiation. During the time of lower Arab deposition an extensive evaporite-depositing lagoon covered much of the western part of the area in which sediments of this unit survive. This lagoon was apparently bounded to the east by a bar, or group of bars, composed predominantly of cleanly washed calcarenite. Farther east are granular, fine-grained limestones, no doubt originally silt sized, with minor calcarenite (generally with a lime mud matrix). Still farther, lithographic limestones which must have been laid down as essentially pure lime mud, become prominent. In the area of the oil fields, the upper Arab evaporites periodically expand from west to east and the general cycle finally closed with the Hith anhydrite apparently blanketing the whole area.
The largest Arabian Jurassic oil accumulations occur in the lowest member of the Arab ("D member") in the area where the dominant rocks are clean-washed calcarenites, and other more or less porous fine-grained carbonate rocks, apparently as a belt between the evaporite-depositing lagoon on the west and a lime-mud-depositing area on the east. Oil is generally present in the upper Jubaila in this same general area when porosity is sufficient.
Oil of the upper Jubaila and "D member" of the Arab may have originated either 1. In the general stratigraphic units in which it is now found, or 2. In the lime muds of the underlying Jubaila from which it was forced upward during compaction. Oil in higher members of the Arab formation probably was generated in the members in which it is now found.
The preferred hypothesis for the geological history of upper Jubaila and Arab oil accumulations is that permeability barriers of stratigraphic type surrounding the main calcarenite lenses and other bodies of porous rock maintained the oil in the general area until the gentle folding of Middle Cretaceous to Eocene times, the main period of growth of the structures, concentrated the oil into the fields as they are now known.
Definitions of the main sedimentary rock units currently used in Saudi Arabia are included.
1 The material of this paper was originally read in two parts under titles—"Mesozoic Rocks of Eastern Saudi Arabia," by Max Steineke and R. A. Bramkamp, before the Association at Los Angeles, March 25, 1952; and "Stratigraphic Relations of Arabian Jurassic Oil," by R. A. Bramkamp and N. J. Sander, before the Association at New York, March 30, 1955. The late Max Steineke has been retained as senior author because of his extensive contributions to the stratigraphy presented, and his role in the 1952 paper. Published by permission of the Arabian American Oil Company. Manuscript received, September 26, 1955.
Many geologists have had parts in the study of the geology of Saudi Arabia during the 22 years of exploration work of the Arabian American Oil Company. It is not possible to give credit here for even the more important individual contributions, but the large amounts of excellent field and laboratory work by these men form the basis for the summary which is presented here.