Los Angeles City Oil Field, Los Angeles, California, United States

Structurally the field is a faulted anticline which trends generally east to west, with oil accumulations trapped in sand units dipping south, ending to the north either at a fault – in the eastern part of the field – or at the surface as tar seeps, in the western area. Mechanisms of entrapment include pinchouts and local changes of permeability – forms of stratigraphic traps – and structural traps such as oil-bearing units blockaded by unrelated, impermeable units put there by motion along faults. Three separate producing horizons, or vertical zones, are present in the Puente Formation, and are given ordinal numbers: First, Second, and Third zones. In addition to these zones, small pockets of oil have been found throughout the upper part of the Puente. The average depth of the three zones from top to bottom is 900, 1,100, and 1,500 feet. Although wells have been drilled to much greater depths – for example, Seaboard Oil Company of Delaware drilled over 7,500 feet (2,300 m) into the Topanga Formation, of Miocene age – no commercial quantities of oil have been found at these great depths.
The field is split into three geographic zones, unrelated to the three vertical zones. The Western Area contains seeps that were known prehistorically; it is separated from the Central zone by a fault. The Central Area, the first to be exploited, extends from the fault to approximately the intersection of the Hollywood and Pasadena Freeways, and the Eastern Area extends northeast from that intersection.
Oil in the field is generally heavy, with API gravity averaging about 14, and ranging overall from 12 to 20. An early assessment by Paul Prutzman (1913) rated the quality of the oil from the field as low, due to the high sulfur content and absence of light fractions suitable for refining. The main use for oil taken from the field during the first decades of the 20th century was fuel oil, and it was also sprayed onto the young city's dirt roads to settle the dust.