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AAPG Bulletin

Abstract


Volume: 67 (1983)

Issue: 2. (February)

First Page: 242

Last Page: 270

Title: Geology and Geochemistry of Crude Oils, Bolivar Coastal Fields, Venezuela

Author(s): Harry Bockmeulen (2), Colin Barker (3), Parke A. Dickey (4)

Abstract:

The Bolivar Coastal Fields (BCF) are located on the eastern margin of Lake Maracaibo, Venezuela. They form the largest oil field outside of the Middle East and contain oil which is mostly heavy with a gravity less than 22° API. Lake Maracaibo is now in an intermontane basin enclosed on three sides by the Andes Mountains. The area has a complex history and 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. At this time, conditions for oil generation in the shales and migration to the sands were established, but the subsequent Oligocene faultin , uplift, and erosion may have allowed meteoric water to penetrate into reservoirs. During the Miocene and Pliocene, the basin was tilted first west and then south, and filled with continental sediments from the rising Andes. Tilting is still continuing 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.

Thirty crude oils from the BCF were collected along two parallel and generally southwest-northeast trends. These oils were characterized by their API gravity, percent saturates, aromatics, NSO and asphaltic compounds, gas chromatograms for whole oils, C4-C7 fractions, and aromatics. Also, 24 associated waters were sampled and analyzed for Ca++, Mg++, Na+, HCO3-, CO3=, SO4=, pH, and total dissolved solids (TDS). The oils show the classic sequence of biodegradation and range from green, 40° API oils with a full suite of n-alkanes and isoprenoids, to black, heavy oils with a gas chromatogram that is an unresolved hump. In many respects the oils are chemically simi ar and appear related, possibly sharing 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 C8-C14, whereas the other type is depleted in n-alkanes above C17. Benzene and toluene values for these oils were normal. In general, oils in the Eocene reservoirs below the unconformity are less degraded than those in the Miocene sands above it.

The formation waters range from very salty (62,000 mg/L), to quite dilute (3,000 mg/L). Those associated with the degraded oils are typically meteoric in chemical composition with considerable 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). If the amount of bicarbonate is taken as an indicator of the meteoric character of the water, then the more meteoric the water, the more degraded the oil. The presence of at least four classes of waters with different compositions in the area of the BCF suggests that there is no through-going flow at present. Many of the fields have oil-water contacts descending toward the south, showing that continued tilting southward occurred during the Mio ene and Pliocene after the oil was emplaced in the reservoir. It seems likely that 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. This suggests that the oil became more degraded the farther it moved, finally becoming viscous and asphaltic, possibly even gelled. In this immobilized form it acted as a seal trapping the less degraded oil which followed.

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