188 
OIL DISTRICTS OF SOUTHERN CALIFORNIA. 
layer is highly charged with sulphur and other minerals, which are 
probably derived from the underlying shale. These surface waters 
are usually shut off in the sandy or clayey shale at depths of about 
150 feet. Brea and heavy oil are also occasionally encountered at 
the base of the Pleistocene, having accumulated at the top of local 
fracture zones which penetrate the underlying oil-bearing strata. In 
fact in some parts of the field, especially in the vicinity of the sup¬ 
posed flexure, oil appears to impregnate the soil and rocks “from the 
grass roots down.” 
From the base of the Pleistocene to the first important oil sand, 
which is struck at 1,000 to 3,000 feet, the rocks penetrated are essen¬ 
tially clayey and sandy shale (the latter known locally as “adobe”) 
interbedded toward the base with a few 1- to 5-foot layers of hard 
siliceous or calcareous shale (the “shell” of the drillers). The great 
bulk of this shale probably belongs to the Fernando formation. The 
sandy and clayey facies of the shale appear to grade into each other 
both laterally and vertically, so that the personal equation of the 
driller enters largely into their differentiation. Gravel and coarse- 
sand lenses are also encountered in some of the wells, but these are 
usually only local in extent and of little importance. 
Gas and oil, increasing in quantity downward, are found in many of 
the beds of the formation, the most important accumulations occur¬ 
ring as a rule just beneath the hard, impervious “shell” layers. 
Some of these gas accumulations or pockets are confined under great 
pressure and when penetrated by the drill have been known to clean 
out the well with considerable force. The shale beds near the flexure 
or anticline appear to be more petroliferous than the same layers 
farther away, to the northwest of the flexure. This is doubtless due 
to the more or less fractured condition of the rocks in the vicinitv of 
the disturbance, which allows the oil and gas to penetrate many of 
the beds of the shale which otherwise would be impervious. A per¬ 
sistent stratum of salt water occurs beneath a “shell”' layer at about 
950 to 1,000 feet above the top of the first important oil sand in the 
area northwest of the flexure, but does not appear in any of the wells 
southeast of it. (See fig. 17.) Salt water is also encountered in some 
of the wells at horizons 150 to 200 feet and 650 feet above the oil zone. 
The oil zone proper varies in thickness from 150 to 500 feet and 
consists of fine to coarse sand interstratified with clayey shale and 
“shell.’’ The logs of a few wells near the flexure show no well-defined 
oil sand, but rather a series of thin productive sands interbedded with 
clayey and sandy shales. Whether the sand occurs as persistent 
layers or as lenses is problematical, although from the evidence in 
hand it appears highly probable that it is present in both forms 
within the area under discussion. It is known, however, that the 
uppermost important oil sand in the wells over a large part of the 
