SECTIONAL TRANSACTIONS.—C. 393 
present-day brackish lagoons, or organic material may be carried in, forming source 
material for thin oil or bituminous shales, but it is unlikely that such beds are the 
source of appreciable amounts of oil. 
The Miocene of Egypt, Persia and Iraq has often been cited as instances of a salt- 
gypsum petroliferous series, but in all three cases there is much stronger evidence 
pointing to an origin in marine foraminiferal marls. 
The most constant bituminous formation in Sinai, Palestine, Transjordania, Syria, 
Iraq, and Persia is the foraminiferal marls of the Upper Cretaceous. Locally similar 
lithological conditions extend upward into the Eocene or downward into the Lower 
Cretaceous and Jurassic, and where this has occurred these higher and lower forma- 
tions are also bituminous. The Senonian is, however, the richest horizon and the 
most constantly so. The rocks range from maris to chalky marls and marly lime- 
stones. They are very rich in smaller foraminifera, in particular globigerina, but are 
poor in macro fossils, except locally at one or two definite horizons. 
In the oilfields belt of S.W. Persia, the Upper Cretaceous at one time lay under a 
cover of 15,000 to 20,000 feet of later rocks, and with a pressure of as many pounds to 
the square inch and a temperature of about 200° C. the original organic material has 
been completely transformed into oil or bitumen. Subsequently, upward migration 
took place, the oil in many places reaching as high as the Upper Fars or Bakhtiari beds 
of Pliocene age. The principal oil reservoir rock in the Persian oilfields is the Asmari 
limestone of Lower Miocene and Oligocene age. 
Dr. J. V. Harrison. 
For the development of an oil pool two conditions seem to be necessary. Firstly, 
a petroleum mother rock must be laid down, and secondly, forces must act on this 
potential oil supply to make the oil available. 
A petroleum mother rock is a deposit laid down in stagnant, sulphur-polluted 
water in which consequently a normal fauna cannot live. Itisa fine-grained, ferrous 
sulphide-bearing sediment, rich in organic matter, often of animal origin. Decay of 
this organic matter proceeds slowly anzrobically, usually with the operation of 
sulphur-loving bacteria. In general, such deposits appear to be of marine origin, 
although lagunal, estuarine, and perhaps even fresh water conditions can also be 
favourable. The conditions of stagnation may be temporarily disturbed by local 
currents which may give rise to a layer of grit before further slime is deposited when 
stagnation recurs. 
Tn the fossil condition, a mother rock is found as a marcasite or ferrous sulphide 
rich bituminous shale. Fossils are confined to the remains of organisms which have 
fallen into the original slime, such as the scales, teeth, and excrement of fish, tests of 
foraminifera, ammonites, or belemnites and rarely reptilian and mammalian 
skeletons. The calcareous remains have frequently been replaced by marcasite or 
pyrite, and phosphatic residues such as glauconite are commonly encountered. When 
variable conditions have occurred, a thin bedded shale results, the layers being 
separated by thin partings of calcareous sands. 
The stagnation occurs in seas of a comparatively restricted area such as the Black 
Sea, or in some oceanic deeps where the disturbing influence of ocean currents does 
not obtain. In lagoons the stagnation may be accompanied by settling crystals of 
salt and anhydrite, or, where abnormally large amounts of organic matter are available, 
mother rock may develop. East of the Rio Unare, near Piritu in E. Venezuela, a 
present-day example of such a saline slime occurs, where abundant organic matter is 
furnished by the bodies of fish which are killed annually by the progressive desiccation 
of the lagoon during the dry season. 
From a petroleum mother rock an oil pool can be produced, given proper tectonic 
conditions to drive out the potential supply and a reservoir rock to contain the oil 
generated. I olding or loading may supply the necessary force, and a reservoir may 
_ be encountered either above or below. 
Tn Persia, a mother rock answering closely to this definition occurs in the Eocene 
Cretaceous as globigerina, sulphide, rich and rarely glauconitic marls. The Pliocene 
orogenesis was probably effective in driving out the oil from the mother rock into the 
Miocene-Oligocene limestones, in which it is now found. 
In England, the Oxford and Kimmeridge Clays are mother rocks, but the treatment 
they have received since deposition has not been favourable to concentration of the 
potential oil into a workable oil pool. 
