Source Rocks and Paleogeography, Austral Basin, Argentina

Jorge F. Rodriguez and Marcelo J. Cagnolatti
Exploration, Petrobras Energia S.A., Neuquen, Argentina

The Austral Basin, located in the extreme south of South America, produces oil and gas both onshore and offshore. The fill of the basin includes rocks ranging in age from Jurassic to Quaternary.

The objective of this work is the characterization of the source rocks in relation to the paleoenvironmental and paleogeographic changes of the basin, based on geological and geochemical information.

Several successions have source potential, however with different quality. Basin development began with a rift phase represented by the rocks of the “Serie Tobífera” (Jurassic), including shales with some source potential. After that, a regionally extended thick succession mainly of marine fine sediments was deposited in a sagging stage. Two main source sections are present, informally known as “Lower Inoceramus” (Tithonian-Aptian) and “Margas Verdes” (Aptian-Albian). Deposition occurred in an offshore ramp, probably under dysaerobic to anaerobic conditions. Organic rich sediments in the “Lower Inoceramus” seem to be related to the presence of physical barriers that restricted the basin and produced stages of water stratification. During that period a general transgression occurred, associated to warm and humid climatic conditions. The restricted sea developed during the tectonic instability related to the opening of the South Atlantic Ocean. The “Lower Inoceramus”, the main source rock, is composed of black shales with TOC ranging from 0.6 to 2% and kerogen type II to III. The Springhill Formation was deposited partially contemporaneously with “Lower Inoceramus” in marginal paleoenvironments, ranging from continental to shallow marine. In the Springhill Formation, continental and marine black shales are interbeded with the main reservoir sandstones, and have some source potential.

Additional possible source levels in the basin are of lower potential.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas