The Montalvo oil field, discovered in 1947 by the Standard Oil Company of California, is located at the mouth of the Santa Clara River on the Oxnard Plain, Ventura County, California.

The accumulation of oil is due to a pinch-out of lower Pliocene (Repetto) sands on the north flank of a faulted, easterly plunging anticline. The producing sands are known as the McGrath zone, which is highly lenticular and in places is divided into two parts by an intermediate shale. The proved acreage is currently limited to this McGrath zone and covers about 375 acres.

In the vicinity of the Montalvo oil field, two distinct Miocene sedimentary provinces meet. The northern province, in which the field is located, contains a Miocene section, the whole of which is as yet unexplored. The southern province, which is separated from the northern by a large fault, contains a Miocene section similar to that of the Oakridge uplift and in one place has been completely penetrated by a test well. That well proves that the lower Miocene and Sespe formations contain reservoirs of value.

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