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Abstract: Geochemistry and Organic Facies of La Luna-Tres Esquinas Cycle: Maturity, Biomarkers and Kerogen Issues

Olivares Carolina, Maria A. Lorente, Fernado Cassani

Four surface sections from the Venezuelan Andes were choosen for this study. The results show interesting trends for exploration of the Andean Belt.

In the Eastern Andes (Trujillo), sections San Lazaro and Chejende yield thick, post-mature, highly tectonically disturbed La Luna Formation.

San Lazaro section has a fault contact showing La Luna post-mature, inertinitic shales in contact with gray shales, ftanites and carbonates bearing marginally mature, highly fluorescent organic gels. Biomarkers show a high level of hopanes, predominance of C27/C29, and S/R ratio=64% characteristic of marine, moderate mature organic matter. Chejende section has almost the same pattern of marine organic matter (COT=9%) but post-mature.

In the Central Andes (Merida), El Valle and San Javier sections yield extremely rich source rocks with very different organic matter.

El Valle section (Tres Esquinas Member) has very rich structured algal matter (COT=8%), marginally mature, which is correlated with a short term carbon isotope (^dgr13C) fluctuation found in the Campanian-Santonian (anoxic?) cycle. The abundance of C27/C29, and high levels of hopanes are related to marine anoxic conditions.

The San Javier section shows evidence of a very rich type I/II kerogen, bearing algal-bacterial amorphous masses, marginally mature and rich (COT=3%); this pattern matches with the abundance of C27/C29 as well as with the ratio S/R=64%, which means moderate maturity. From these results, two provinces can be separated today: a highly tectonized, post-mature, Eastern Andes Province and a very rich, marginally mature, Central Andes Province.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela