Abstract: The Chaco-Tarija Basin: Are the Petroleum Systems of NW Argentina Different From the Bolivian Ones?

Starck, Daniel and Estanislao Kozlowski - Pan American Energy

The Chaco-Tarija Basin, located in the west-central part of South America contains over 10 km of Paleozoic to Recent sediments. It covers an area of 170,000 km² in Bolivia, Argentina and Paraguay. Its western part was deformed during the Upper Tertiary, and is now part of the Andean Foothills, while its eastern part remains an undeformed foreland. Nearly all proven reserves (500 MMm³ of oil equivalent) are in structural traps in the thrust belt.

The stratigraphic column in this basin can be divided into three major packages. The lower package (Silurian to Devonian, > 3000m) is composed of alternating shales and quartzites deposited in a shallow shelf environment. From older to younger, the formations are: Kirusillas, Santa Rosa, Icla, Huamampampa, Los Monos and Iquiri. The thick Los Monos shales are the proven source rock, whereas the Santa Rosa and Huamampampa quartzites are the main reservoirs for fields in the southern tip of the basin in Argentina. The middle package (Carboniferous to Jurassic), up to 2500m of thick glacial and periglacial facies and units deposited in arid environments, is separated by a regional unconformity from the older one. This section is composed of an alternating sequence of sand-rich paleovalleys (reservoirs) and shaly, diamictic units (seals). The upper package consists of > 5000m of Neogene continental synorogenic deposits. It is marked by an erosional unconformity at its base, and it overlies the Silurian in the south.

The Los Monos shales entered the generation window during the Mesozoic, but the last maturation pulse occurred during the Neogene. The expulsion is related to Andean tectonics. The expulsed hydrocarbons followed two migration paths, resulting in two main petroleum systems: one characterized by downward migration to the fractured reservoirs of the Huamampampa and Santa Rosa formations, and the other by upward migration charging Carboniferous to Tertiary reservoirs. Hydrocarbons in the latter migration pattern reached the younger reservoirs by following complex paths through the paleovalley fills and faults.

In Bolivia, the paleovalleys in the Carboniferous provide an important pathway to upward migration, allowing hydrocarbons to charge the several Carboniferous to Tertiary reservoirs. In this case, main regional sealing units control field locations (e.g., the Upper San Telmo shales at Rio Grande field). In Argentina, shales (diamictites) of the Tarija Formation, in a lower stratigraphic position within the Carboniferous, have prevented widespread upward migration. In this area, the major fields are located below this unit (e.g., Campo Duran field).

There are exceptions to these two migration/entrapment patterns. That is, fields do exist in lower levels of the Carboniferous in Bolivia, and in higher Carboniferous levels in Argentina, but these represent relatively minor accumulations.

Fields related to downward migration from the Los Monos represent the major discoveries in the Argentinean portion of the basin, whereas in Bolivia, pre-Carboniferous reservoirs have, so far, not yielded major reserves. Current exploration efforts will show us, in a few years, if giant accumulations in pre-Carboniferous reservoirs are also present in Bolivia.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil