Oils from the North Andean Foreland Basins: Geochemistry, Origin and Distribution R.Baudino, W

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Oils from the North Andean Foreland Basins: Geochemistry, Origin and Distribution R.Baudino, W. Hermoza and R.Tocco (Repsol Exploration S.A., Madrid, Spain)

This paper was selected for presentation by an ALAGO Technical Committee following review of information contained in an abstract submitted by the author(s). ______In this study we will focus on the Colombian, Ecuadorian and All along the eastern border of the Andes exist foreland basins North Peruvian Foreland Basins where source rocks of that are among the most prolific hydrocarbon provinces of the different ages and organic facies generated large amounts of world. hydrocarbons in different geodynamical contexts (Figure 1).

The evolution of these basins was greatly controlled by the 24.11 BBOE have been discovered in total from the Barinas- Andean uplift and its consequences on deformation and Apure (South-Western Venezuela), Llanos and Putumayo sedimentation. (Colombia), Oriente (Ecuador) and Marañon (Northern Peru) basins (Figure 2).

Venezuela

Guyana Shield Cret. & Tert. PS 11.32 BBOE Colombia

Ecuador Carnegie Cret. PS Peru 11.55 BBOE

Brazil Paleoz. PS

Jurassic PS Tert. Ps 0.34 BBOE

Guapore Shield

Peru Bolivia Paleozoic PS 16.95 BBOE

Chile

Argentina

Figure 1: Morpho-tectonic map of the Northern and Central Figure 2: Main Petroleum Systems of the Northern and Andes with study area outlines in red. Triangles: volcanoes, Central Foreland Andean basins with related discovered arrows indicate direction of subduction with associated volumes of hydrocarbons. velocity.

2 R.BAUDINO, W. HERMOZA AND R.TOCCO The accumulation of sediments with the contemporary - Oils analysed from the North Peruvian Marañon Basin were development of structural and stratigraphic traps led to the sourced by Cretaceous and Jurassic-Triassic rocks. activation of several petroleum systems during the Tertiary 1 and particularly the Neogene. 0,9

0,8 Central and Southern 0,7 The controlling factors on the generation of hydrocarbons in Northern Area Foothills 0,6 such foreland basins are related to the pre-Andean 0,5 paleogeography and on the other hand, to the Late Cretaceous Central and Southern - Cainozoic Andean uplift. 0,4 0,3 Areas

0,2 Oleanane/C30 HopaneOleanane/C30 Pre-Andean paleogeography allowed the deposition of thick 0,1 Central Platform organic rich sediments during Jurassic, Cretaceous and 0 0 2 4 6 Tertiary times. Most of these events are related to global : ncreasing terrestrial input transgressive and anoxic events but the peculiar dynamics of Pristane/Phytane this portion of the Andes controlled the distribution of subsiding areas that were reached by these global phenomena. Figure 3: Example of biomarkers analysis interpretation of For example, the distribution of the Jurassic source layers is different oil samples from the Eastern Llanos Basin of conditioned by the occurrence of small basins created during Colombia rifting events that makes prediction of presence and quality hard to realize. On the same way, the existence of Andean Proprietary data complemented with published information relieves during the Tertiary conditioned the extension of helped defining different oil families and associating them inland transgressions as well as the detrital input affecting the with possible sources. organic richness and preservation. Finally, a regional distribution of crude oils according to their The Late Cretaceous-Cainozoic Andean uplift was driven by origin can be drawn in an attempt to understand any the subduction of the Nazca oceanic plate underneath the geodynamic control on occurrence. South American continental plate. This led to the burial of source rock layers by accumulation of eroded clastics into a series of subsiding basins to the East of the mountain chain. Most of the producing structural traps are also the result of the Tertiary basin deformation. In addition to foreland subsidence Acknowledgments and sediments accumulation, subduction processes also include thermal consequences related to more localized events The authors would like to thank Repsol Exploration S.A., the such as volcanism or aseismic ridges subduction. directors of D.G. Upstream and D. Geology, the Regional Study Group in Madrid as well as the Colombia and Peru Filiates to authorize the publication of these data. Among generation controlling factors of the hydrocarbons discovered in the Northern and Central Andean Foreland basins we can thus mention: - Source rocks distribution and facies changes, - Basins burial history, - Thermal regime and anomalies.

The main part of this work consisted in the study of oil samples based on biomarkers and isotopic analysis (Figure 3). Published results were also integrated.

Our geochemical characterization indicates different origins for the oils: - Cretaceous and Tertiary source rocks in the SW Venezuelan and Colombian foreland; - In Ecuador, oils produced from the Oriente basin have a Cretaceous origin and according to published data a Jurassic one in some cases;