The name “Sespe formation” is applied to the non-marine redbed facies of a group of sedimentary rocks up to 7,500 feet thick that range in age from upper Eocene into lower Miocene in the southern and eastern parts of the Ventura basin, California, but are probably restricted to the Oligocene in most of the northern and western part of the basin.

The lower part of the Sespe becomes progressively marine westward beginning about 25 miles west of Santa Barbara, but this marine Oligocene is mainly sandstone or silty shale, low in organic material that does not suggest a source rock.

The bulk of the evidence, which is discussed in detail, suggests that the larger part of the oil was derived from Eocene shales. Vertical migration across the bedding of several hundred to a few thousand feet of predominantly sandy strata seems to be required. Countless minor joints and cracks in the shaly interbeds are suggested as the principal channels of vertical migration. In the southeastern part of the Ventura basin some of the oil may have reached the lower, or Eocene part of the Sespe by lateral migration from upper Eocene shales, into which the lower Sespe may grade, followed by upward migration within the anticlines to the shalier middle Sespe, where most of it is trapped.

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