rainy winters, and dry summers during which the heat is greatly diminished by prevailing winds from the cold waters of the Pacific Ocean, thirty miles to the west. Approximately of rain falls in a typical winter, with the rainy season lasting from around November to April. Geology The overall structure of the Zaca field is an anticline, with the oil-bearing units capped by impermeable beds covering them like the gabled roof on a house. The productive unit in the Zaca field is fractured shale of the Monterey Formation. This is the same unit which is productive in the nearby Orcutt and Lompoc fields. As in those fields, oil has pooled in the abundant pore space provided by the fractures and along bedding planes in the unit. The Monterey Formation in this area dips to the south, and oil migrating updip has pooled against a vertical fault which marks the northeastern boundary of the field. The depth to the oil reservoir below ground surface varies from 4,000 feet at the northwestern extent of the field to about 1,000 feet at the southeastern end, with an average thickness of around 1,700 feet. Above the reservoir are the non-oil-bearing Sisquoc, Foxen, and Paso Robles formations. Oil in the reservoir is in the "extra heavy" range, with API gravity ranging from 6 to 10.5, and averaging 8.0. In these low ranges, oil has difficulty flowing and usually requires assistance from a diluent, steam, water flooding, or another mechanism. Steam and water flooding have both been used on the Zaca field. Sulfur content of oil from the field is among the highest from any California reservoir, ranging from 6.76 to 8.00 percent by weight. Even in the earliest days of production, operators injected diluent in order to mobilize the heavy oil. History Tidewater Associated Oil Company drilled the discovery well for the Zaca field in November 1942, during a period of rapid exploration for oil to aid the World War II war effort. The Davis No. 1 well produced from the fractured shale of the Monterey Formation, initially at 94 barrels a day, and settling at 150. Tidewater followed on this modest success by drilling other wells into the reservoir, and by 1948 there were ten wells producing, and it had extracted 348,000 barrels of oil. In 1953, J. Paul Getty's Getty Oil acquired Tidewater, commencing a waterflooding operation to boost production, and by 1954