Abstract: Synthesis of OCS Well Information, Offshore Central and Northern California, Oregon, and Washington

D. L. Zieglar, J. K. Cassell

Well histories and records from the 20 exploratory wells drilled on OCS leases offshore central and northern California, the 8 wells drilled offshore Oregon, and the 4 wells drilled offshore Washington were made public by the U.S. Geological Survey December 1, 1974. Analysis and synthesis of data related to drilling times, mud weights, fracture gradients, rock types, and shows of petroleum are of value in evaluating geologic hazards, environmental risks, petroleum resources, and the programming of future offshore exploration. OCS sale 53 offshore central and northern California is scheduled for February 1981.

The 20 central and northern California wells were drilled in the period from September 1963 to September 1967. Weather interrupted the drilling of two of the five wells drilled from shipshape vessels but did not delay those drilled using semisubmersible drilling equipment. As no excessive mud weights were used in drilling the wells, abnormal formation pressures are not present in the wells to the depths drilled. Mud-weight gradients were only 65 to 70% of the fracture gradient. Shows of oil were found in the wells drilled in the offshore Santa Maria, Outer Santa Cruz, Bodega, and Point Arena basins in sandstone zones and fractured siliceous shale and chert intervals.

The 12 tests drilled offshore Oregon and Washington were drilled during the period from April 1965 to August 1967. Weather-related delays affected each of the two wells drilled from shipshape vessels. One test using a semisubmersible was drilled during the winter season from November through March, and its operations were interrupted for periods of several days by severe winter storms. Some rigs, when occupying their locations, had difficulties setting anchors because of the nature of the seafloor sediments and had to have holes drilled within which to secure the anchors. One well had to drill 3 surface holes before having one stable enough to run conductor casing. With the site investigating and positioning techniques currently used before drilling, such problems can now be recognize and mitigated. All the wells, except the Soco-Union 103, encountered sloughing shales and abnormal subsurface pressures requiring the usage of heavy drilling muds with mud-weight gradients of 85 to 90% of the fracture gradient. Only minor shows of oil and gas were found. In response to two Department of Interior inquiries, industry has ranked offshore Oregon and Washington as a low-priority area of interest.

AAPG Search and Discovery Article #90963©1978 AAPG/SEG/SEPM Pacific Section Meeting, Sacramento, California