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Webster, Robert E.1 
(1) Hunt Oil Company, Dallas, TX

ABSTRACT: Tropical versus Temperate Zone Lacustrine Source Rocks: Examples from Takutu Basin, Guyana, and General Levalle Basin, Argentina

Rift basin lacustrine source rocks have generated 5.7 % of global reserves, and some of today’s most active exploration plays are driven by lacustrine-shale-based petroleum systems. However, paleolatitude and paleoclimate play major roles in determining organic richness of lake sediments, and not all restricted lacustrine basins contain commercial petroleum systems. Two Mesozoic continental-interior, restricted saline rift basins in South America display dramatic differences. 
In the Takutu basin, at a tropical paleolatitude of 2°N, deposition in an Early to Middle Jurassic rift produced organic-rich shale with 1.5 to over 4% TOC, hydrogen indices over 360, and pyrolysis yields up to 9300 ppm. Terrestrial kerogen is significant, but Type I/II non-marine algal amorphous kerogen is dominant in richer intervals. Significant oil and gas were generated but mostly lost during Miocene(?) tectonic reactivation. 
In contrast, the Lower Cretaceous General Levalle rift basin formed at a temperate zone paleolatitude of 45°S. Lacustrine shale is organic-lean, with maximum TOC just over 1%. Kerogen is Type III/IV, with little generative capacity, and derived mostly from pollen from nearby upland forests. Here the annual seasonal cycle probably caused lake turnover and prevented long-term establishment of an anoxic bottom layer to preserve organic matter, which is common in more tropical settings such as the Takutu basin. 


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.