Abstract

The Bolivar Coastal Fields (BCF) are located on the eastern margin of Lake Maracaibo, Venezuela. They contain mostly heavy oil with a gravity less than 22 degrees API. Tectonic movement continues today. In the Cretaceous, the area was part of the platform of a large geosyncline, but by the Eocene it was near a coast where a series of large sandy deltas was deposited, with terrestrial sediments on the south and thick marine shales on the north. Conditions for oil generation in the shales and migration to the sands were established, but the subsequent Oligocene faulting, uplift, and erosion may have allowed meteoric water to penetrate into reservoirs. During the Miocene and Pliocene, the basin was tilted Tilting continues and oil is moving up along the Oligocene unconformity, forming surface seeps. Most oil fields are located in sands above the unconformity or in fault blocks immediately below it. The oils show the classic sequence of biodegradation and range from green, 40 degrees API oils with a full suite of n-alkanes and isoprenoids, to black, heavy oils with a gas chromatogram that is an unresolved hump. The oils are chemically similar and possibly share the same source rock. The Miocene L-5 reservoir contained two oil types which did not appear to fit the main trend. One type is depleted in n-alkanes in the range C 8 -C 149 whereas the other type is depleted in n-alkanes above C 17 . Benzene and toluene values for these oils were normal. Formation waters associated with the degraded oils are typically meteoric in chemical composition with condiserable bicarbonate (20 to 90 meq/L), small quantities of chloride (2 to 25 meq/L), and extremely low amounts of magnesium and calcium (mostly less than 1 meq/L). The more meteoric the water, the more degraded the oil. There is no through-going flow at present. There was large scale secondary migration of oil from south to north, probably along the unconformity surface, which still leaks oil where it is exposed. In the shallower Miocene reservoirs along the northeast margin of the field, heavy asphaltic oil overlies lighter oil downdip to the west and south. Oil became more degraded the farther it moved, finally becomming viscous and asphaltic, possilby even gelled. In this immobilized form it acted as a seal trapping the less degraded oil which followed.--Modified journal abstract.

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