--> The Monterey(!) is a Significant Undercharged Petroleum System in the Onshore Santa Maria Basin, California, U.S.A.

AAPG ACE 2018

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The Monterey(!) is a Significant Undercharged Petroleum System in the Onshore Santa Maria Basin, California, U.S.A.

Abstract

The Onshore Santa Maria Basin is triangular area containing a sequence of sedimentary rock deposited over the last 18.5 m.y. located north of the west trending Santa Ynez fault, south of the northwest trending Foxen Canyon-Little Pine faults. Fourteen oil fields are divided into three tiers; northern, central and southern. Sixty-six percent of the recoverable oil is in the northern tier, 27 percent in the central tier and 7 percent in the southern tier. The Monterey Formation is the source rock and the major reservoir rock. Carbon isotopic data of the kerogen and oil indicate that the oil originated from the source rock, thus the name and level of certainty of this petroleum system is Monterey(!).

A billion barrels of oil equivalent from these 14 fields indicate that the Monterey(!) is a significant petroleum system. Oil gravities range from 36 to 7 API degrees with a gas-to-oil ratio of 2200 to 100 cubic feet of gas for each barrel of oil. The largest three fields (>60%) are stratigraphic traps and the other five large fields (~35%) are structural traps. The highest API gravities correspond to the smallest and deepest accumulations. None of the fields are fully charged and the oil-water contact of each pool fails to follow structural contours of the reservoir rock. Except for the Guadalupe field, the Monterey Formation is a fractured shale reservoir rock.

Early work suggested that the large volume of low gravity oil in the basin with unusually high sulfur content were expelled and accumulated at a very low thermal maturity. However, atomic H/C data suggest that the threshold of maturity is at a present-day burial of 9,000 feet and the kinetic profile of the kerogen indicates that maturity is only slightly lower than that of a low-sulfur kerogen. A 100 m.y. unconformity occurs between the Franciscan basement and the Miocene and younger sedimentary rocks. A major unconformity occurs in the Orcutt oil field where the Careaga Formation unconformally overlies the Sisquoc Formation. In addition, the surface outcrops indicate the basin is inverted. This information indicates a complicated burial history for the source rock. The present-day location of the pod of active source rock and the oil fields indicate that petroleum migration was up to ten miles. The API gravity of the expelled oil is approximately 25 to 35 degrees with up to 2-wt. percent sulfur. After emplacement, the API gravity decreased because of partial to complete biodegradation of the alkanes.