182 
OIL DISTRICTS OF SOUTHERN CALIFORNIA. 
by the gas pressure, which is rather strong in this area. West of the 
line of disturbance water is encountered in the wells at a depth of 
400 feet, but no oil was struck, although one of the wells was con¬ 
tinued to 580 feet. 
AREA SOUTH OF COLEGROVE. 
The wells in and near the SE. 1 sec. 15, T. 1 S., R. 14 W., about a 
mile south of Colegrove, penetrate the same strata that underlie the 
Salt Lake field, a mile and a half to the southwest. The wells reach 
the oil zone near the surface, where it is unproductive and almost 
dry, while those of the Salt Lake field encounter it at depths of 1,000 
to 3,000 feet, where it is exceedingly rich in oil and gas. The two 
areas offer an excellent illustration of the differences in saturation 
of a single zone at different points, and also of the fact that within 
reasonable depths oil-bearing strata which outcrop at the surface or 
whose truncated ends are overlain near the surface by comparatively 
thin deposits of porous material are deprived of most of the petro¬ 
liferous contents of their upper portions either by slow distillation 
or by some other process. 
The wells of the Colegrove area penetrate Pleistocene sand, usually 
water bearing at the base for about 50 feet, below which they enter 
thin-bedded clayey shale, sandstone, and “shell.” The northeast- 
ernmost wells strike the oil sand at a depth of a little more than 100 
feet, and from this point down for at least 300 feet pass through 
alternating layers of oil sand, clayey shale, and “shell,” with here 
and there one of conglomerate. From the northeastern edge of the 
Colegrove area the oil sands dip toward the southwest at an angle of 
about 22°, being encountered at greater and greater depths in the 
wells as the Salt Lake held is approached. The wells farthest north¬ 
east start down in beds which underlie what has been called the 
“first" of “150-foot” oil sand, but those situated southwest of the 
middle point of the line dividing the SW. I from the SE. I of sec. 15 
penetrate this sand at depths varying from 100 feet down. A well 
located about one-eighth of a mile west of the middle of the south line 
of sec. 15 strikes this first oil sand at 722 feet and from this depth 
down to 1,532 feet penetrates an almost continuous series of oil-and 
gas-bearing sandstone and sandy shale interbedded with clayey shale 
and thin layers of hard siliceous shale or “shell.” For some reason 
the sands here are not very productive and no important wells have 
been developed over this part of the area. 
Northeast of the Colegrove area the' oil-bearing shale and sand¬ 
stone pass over the Los Angeles anticline and extend in low folds 
toward Prospect Park. Traces of oil and gas have been found in these 
beds in nearly all the wells drilled between the productive belt and 
the Prospect Park region, but no accumulations of consequence have 
been encountered. 
