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Implications of Reservoir Quality Development and Distribution from Initial Petrographic Investigations of Lacustrine Microbial Carbonates of the Yacoraite Formation, Northwestern Argentina

Eleson, Jason W.; Kaczmarek, Stephen E.

The proliferation of hydrocarbon discoveries within the South Atlantic presalt has led to an increase in research of microbial lacustrine carbonate analogs. The Maastrichtian/Danian Yacoraite Formation of northwestern Argentina is a candidate presalt depositional analog due to similarities in depositional and tectonic settings, scale of outcrops, and abundance, morphology and distribution of microbial facies. ExxonMobil conducted fieldwork in the Yacoraite Formation from the Salta/Jujuy provinces of Argentina in June, 2011 in order to evaluate proprietary and published lacustrine depositional models and provide a high resolution sequence stratigraphic framework for the area. Standard thin section petrography from 74 hand samples was used to evaluate reservoir quality potential in this emerging reservoir type.

Most of the carbonate-prone facies exhibited fair to poor reservoir quality, though good reservoir quality was developed locally. The best reservoir quality occurs in microbial boundstones and intercalated grainstones and packstones, with visual porosity estimates of 15-20%. The boundstones exhibit stromatolitic laminations within biostromal heads, and commonly display framework, micro-, and local vuggy porosity. The grain-prone intervals contain abundant coated grains and intraclasts with interparticle/intercrystalline porosity and local vuggy porosity. Better reservoir quality occurs in samples with higher abundances of early dolomite that may facilitate porosity preservation during burial. Samples with poor reservoir quality contain relatively little dolomite, and are degraded by a combination of calcite cements, mechanical compaction, and/or poor sorting. These samples generally exhibit <5% porosity, and are dominated by micro- and intercrystalline porosity.

Fieldwork and petrographic studies show that the Yacoraite carbonates are similar to the South Atlantic presalt deposits in carbonate sedimentology, stratigraphy, and overall reservoir quality. However, different mechanisms are responsible for reservoir quality variation between the two locales, and are likely a function of differing diagenetic histories. For example, authigenic/detrital clays and burial dissolution are key factors that affect reservoir quality within the South Atlantic presalt, versus early dolomite formation within the Yacoraite Formation. Thus, further investigations of these key factors are required before more direct comparisons can be made.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013