oil sand - Graciosa coarse-grained member of the Careaga sandstone impregnated with oil from the underlying Monterey Formation - Unit of 5 student specimens

In the Solomon Hills, on Graciosa Ridge, in Santa Barbara County, California, the prolific oil producing Monterey Formation is fractured, with oil seeping up into the overlying Careaga sandstone. These oil sand specimens were collected in the Orcutt Oil Field not far from the location of the Hartnell well #1 shown here. The Careaga is lower Pliocene in age.

The Monterey has been richly productive for over 100 years in California. Its oil originated as the oil in diatoms, which, when they die, rain to the sea floor by the billions. The diatomaceous shales of the Monterey are difficult to develop as a direct petroleum source, but fractures often allow the oil to migrate upward into overlying sandstones.

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