the east, as well as the Rincon, San Miguelito, and Ventura fields onshore. Oil is found in two formations, the Pliocene-age Pico Formation and the underlying Miocene Santa Margarita Formation. By far the most productive unit is the Repetto Sands portion of the Pico Formation. Only one well has produced from the Santa Margarita; all the others are in the Repetto Sands. The Repetto Sands unit consists of layers of mudstone, siltstone, and shale, and due to its depositional environment the general grain size and porosity increase towards the east. It is the same formation which is richly productive in the oil fields of the Los Angeles Basin, such as the Salt Lake and Beverly Hills fields, where it is also folded into anticlinal traps. In the Dos Cuadras field, the oil-bearing strata are at depths ranging from 500 to 4,200 feet below the sea floor, and individual strata are separated by impermeable layers of shaly material. Oil from the field averages API gravity of 25, with a range from 18 to 34, which classifies as medium-grade crude. Reservoir pressure began at 750 psi, sufficient for easy pumping in the early years of field development. History and production Oil has been known in the Santa Barbara Channel since prehistoric times; the native Chumash people used tar from the numerous natural seeps as a sealant, and tar regularly washes up on the beaches from the offshore seeps. The world's first offshore oil drilling took place at the Summerland Oil Field in 1896, only five miles north of the Dos Cuadras field. Those wells were put in from piers in shallow water. Technology for drilling in deeper water from platforms did not come about until the middle of the 20th century. While the existence of the field was suspected in the 1950s, the field was not discovered until 1968. Unocal and several other oil companies took out leases on the field in February 1968, and put in the first well, and the first platform, that same year. Data regarding the four platforms are as follows: Unocal succeeded in installing the first four wells from its Platform A by January 1969, but their attempt to install the fifth well was catastrophic, and resulted in one of the most notorious environmental disasters in United States history. Because the drillers were using an insufficient length of protective casing, when the well hit a high-pressure zone