In 1941, the Prado Dam was built in Riverside County, California, near the City of Corona. In 1965, one federal and one private lease were granted for oil and gas exploration behind the dam. The federal lease stipulated that "…all rights under this lease are subordinate to the rights of the United States to flood and submerge the lands, permanently or intermittently in connection with the operation and maintenance of the Prado Flood Control Basin Project."

Thirteen oil wells were drilled and completed on these two leases and oil and gas were produced in the field from 1965 through 1993. During this period, the wells were submerged numerous times and several oil spills occurred. After an oil spill in 1993, the wells were shut in by a court order. In addition, California Department of Conservation's Division of Oil, Gas, and Geothermal Resources (Division) issued a formal order outlining corrective action to be taken by the operator to prevent another spill. The operator failed to comply with this order. To prevent another spill, the Division undertook remedial action over a three-year period. Finally in 1996, the wells were plugged and abandoned when the operator failed to comply with the formal order and it was determined the wells were an environmental hazard.

Site Description

Wetlands. Rain runoff from the San Gabriel Mountains and the Chino Hills form the Santa Aria River. Several branches of this river meander through the Prado Basin and then merge into a main channel which flows through densely populated cities in Orange County. Numerous floods have occurred along this river with some causing significant losses of property and life. For this reason, the Flood Control Act of 1936 authorized the construction of the Prado Dam, which was completed in 1941.

Behind the Prado Dam, the Prado Basin contains one of the largest remaining wetlands in southern California. It is the home of more than 200 species of migratory birds, including five endangered species. Most notable of these is the least Bell's vireo, a bird placed on the federal and state endangered species lists in 1986. The least Bell's vireo nests in trees and brush behind the dam from March through September. Leasing and Land Use. In 1965, the Bureau of Land Management (BLM) leased the mineral rights for 1961 acres in the Prado Basin. The Army Corps of Engineers (CORPS) maintained the surface rights. At the same time, 672 acres of private land in the Prado Basin that was adjacent to the federal property was leased by the Santa Ana River Development Company (SARDCO). In 1967, the surface rights for the private property were acquired by the Orange County Water District (OCWD) to help augment Orange County water supplies. SARDCO retained the mineral rights.

The Prado Dam is operated by the CORPS, which is directed by the OCWD to slowly release water stored behind the dam into downstream catch basins along the Santa Ana River. The catch basins divert excess water from the river to vast underground aquifers, which are pumped by local water districts. The underground reservoirs supply water to about two million Orange County residents.

Oil Field Description and History

Geology. The Prado-Corona field is in a two-mile-wide portion of the narrow, trough-like Chino basin. The field is comprised of two separate producing areas called the Sardco and Goedhart, which are faulted anticlines. The Sardco Area contains both the private "Sardco" and federal "Gov. 165" lease. (The Goedhart Area, about 100 feet higher in elevation than the Sardco Area, has not been subjected to flooding and is not discussed in this paper.) The productive interval in the Sardco Area, found at an average depth of 3,000 feet, is divided into two zones, the Upper and Lower Hunter (Late Miocene in age). A series of northwest-trending normal faults form a group of petroleum traps within the fault blocks.

Field Development and Flooding. The field discovery well, "Sardco" 1, was drilled in 1966 and initially produced 168 b/d of 18.5 API gravity oil. A follow-up well, "Sardco" 2, was completed the same year and produced 160 b/d of 15.5 API gravity oil. In December 1966, both wells were covered by flood waters from the Santa Ana River, which had backed up behind the Prado Dam.

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