MORE ON GIANT FIELDS OF NORTHERN S. AMERICA

Bernard C. Duval, Carlos Cramez
Total Exploration Production
Paris

Gustavo E. Valdes
Petroleos de Venezuela SA
Caracas

The first part of this article profiled Cano Limon-Guafita, Ceuta-Tomoporo (Centro), and El Furrial-Carito-Tejero oil fields in Venezuela. In this concluding part we will take a look at the Colombian foothills.

FOOTHILLS, RELATED AREA

Unfortunately we lack the data to fully incorporate Cano Limon-Guafita field into the analysis and have to rely mainly on published material. In spite of the well-marked Andean tectonism, the cross section C-C' from central Colombia through Cusiana field (Figs. 2, 10) is relatively similar, in some features, to the previous regional cross section B-B' through the Maracaibo basin, as discussed and shown below. We interpret the eastern Cordillera, the Colombian foothills, and the Maracaibo Lake as corresponding to Neogene tectonic inversions of the old Ceno-Mesozoic back-arc basin by reactivation of old normal Jurassic or older faults like the fault illustrated on the seismic line (Fig. 11)(93306102.gif). Cusiana field, located in the Colombia foothills, corresponds to a large asymmetrical antiform feature striking north 30-40, related to a major westward dipping reverse fault with secondary accommodation faults dipping to the east near the apex of the structure (Fig. 12). The closed area of the structure is greater than 100 sq km with a proven hydrocarbon column of 500 m. The high angle faulting involving the basement is clearly seen as consequence of inversion of a rift basin (Fig. 13). This tectonic mechanism is very different from the mechanism occurring in the Furrial area. We postulate the existence of pre-compressional structural highs would have trapped hydrocarbons from an early migration, a long time before compressional traps were created. This hypothesis is consistent with the geological reconstruction (Fig. 14), where prior to the compression, highs appear between the "rifts" during this phase of extension. Even though Cusiana's structural expression looks more complex because of the most recent Andean tectonism, it is not very different from other structures formed in a back-arc context, as described earlier in the text. Thus the petroleum parameters of the Cusiana area are significantly similar to those described for Maracaibo. Rather than repeat the full story, we will only summarize a few points of particular interest (Table 3)(4718 bytes):

• The seal efficiency of the thick Oligo-Miocene Carbonera and Leon formations;
• Biomarkers and other crude characteristics confirm at least two periods of charging, early and late;
• The multiphase shortening that took place during the Paleogene-Neogene is supported by observed stratigraphic hiatus. This point is not as well controlled as it is in Maracaibo because of the low density of existing data. More detailed correlations will become available as work progresses in the area.

• If the majority of the fields discovered in the Andean foothills are associated with traps of compressional origin, our observations recounted above suggest that large reserves are directly related to the proximity of positive areas in existence before compression.

SIMILARITIES, DIFFERENCES

Although the four fields mentioned are recognized (as a regional approximation) as being sourced from the same formation-studies in progress show a contribution from Tertiary source rocks in the foothills of Colombia-three of them are associated with a back-arc basin, whereas one, Furrial, is associated with a divergent margin. This regional geological context (Figs. 15, 16), reconstructed before compression took place, explains the similarities and differences, mainly from a structural standpoint, between the new giants of Venezuela and Columbia. Two families of petroleum systems are clearly defined and can be useful to select criteria for the location of other similar deposits. These criteria include the regional context, the type of faulting involved in the trapping mechanism, and the possibility of multiphase shortening (Table 4)(8606 bytes). For the reason given above, some criteria may be missing in the case of Cano-Limon-Guafita.

CONCLUSIONS

We have drawn four major conclusions from our studies.

1. Criteria should not be defined only in terms of "thrust belt" vs. "Platform" contexts as is the conventional wisdom but also in terms of "back-arc" vs. "divergent margin" contexts.

2. Key factors for back-arcs are:

• The existence of old high angle rejuvenated faults;
• The proximity of old arches such as the Maracaibo high for Ceuta-Tomoporo and the Arauca arch for Cano Limon, and why not a similar arch for Cusiana?
• The integrity of trapping and relatively moderate dips should be preserved, particularly in foothills areas, to preserve retention;
• We should add the important Upper Eocene shortening and the related structural configuration, particularly visible in Cano Limon and Ceuta.

3. Key factors for divergent margins are:

• efficient lateral ramps;
• overpressured seals; and
• high structural position at the present time is sufficient to increase the likelihood of an hydrocarbon accumulation.

4. The tools of choice are:

• New and better seismic of excellent resolution. This was the key in all cases discussed in the article, for good geometrical definition of the traps, as well as an analysis of fault behavior;
• Regional geochemistry anti modeling studies (particularly for better prediction of the types of hydrocarbons). Such studies are more difficult in the foredeep and folded belts because the inner part of the Cordillera is part of the picture and the original sedimentation (prior to the Andean erosion) must be reconstructed;
• Detailed sedimentological and reservoir analysis, to understand the evolution and preservation of petrophysical characteristics (horizontal and vertical);
• Seal studies and recognition of overpressured intervals;
• Revised structural interpretations, particularly incorporating the complete history of faults leading to the generation of migration path maps.

In summary, the dangers and limits of big magic classifications and simple paradigms like "thrust belt hydrocarbons" are evident! The understanding of a single field, although it is a much needed step, is not sufficient to get the full picture. Petroleum systems are dynamic, discontinuous, and heterogeneous. Any seemingly slight difference in the initial conditions can hive a disproportionate impact on key parameters, and recognizing them is very much what makes a "lucky" explorer.

ACKNOWLEDGMENTS

We thank all subsidiaries of Petroleos de Venezuela SA, as well as Total, for permission to publish this article.

BIBLIOGRAPHY

McCullough, C., and Carver, J.A., The giant Cano Limon field, Llanos basin, Colombia, in Giant oil and gas fields of 1978-98, AAPG Memoir 54, 1992, pp. 175-195. Prieto, R., and Valdes, G., El Furrial oil field, a new giant in an old basin, in Giant oil and gas fields of 1978-88, AAPG Memoir 54, 1992, pp. 155-161. Roberto, M., Mompart, L., Puche, E., and Scherer, F., New oil discoveries in the Centro area, S.E. Lake Maracaibo, Venezuela, in Classic Petroleum Provinces, Geol. Sec. of London Spec. Pub. No. 50, 1990, pp. 77-88. Copyright 1995 Oil & Gas Journal. All Rights Reserved.