The Napo Formation, Oriente Basin, Ecuador: Hydrocarbon Source Potential and Paleoenvironmental Assessment

Abstract

The Napo Formation of the Oriente basin, Ecuador, contains one of the most prolific hydrocarbon source rocks in South America. Its organic-rich succession, mainly composed of calcareous black shales and marls tones (TOC up to 13 wt.% and S2 from Rock Eval up to 78 kg HC/t of rock) ranging in age from Albian to early Campanian, was deposited in dysoxic-anoxic, paralic-neritic environments with variable amounts of algal and terrestrial plant input. The facies with the highest hydrocarbon source potential were deposited during a paleobathymetric maximum at the Cenomanian-Turonian boundary, with salinity-stratified, bottom-water conditions. Such an event is also recorded in the stratigraphic sequences of most Subandean basins of South America and corresponds to a global tectonic-eustatic, sea-level rise recorded in the latest Cenomanian-earliest Turonian and to the worldwide Oceanic Anoxic Event 2 (OAE-2). Biological marker data of the oils discovered up to now in the Oriente basin show very good correlation with the ones presented by the organic extracts from the organic-rich facies of the Napo Fm. (Cenomanian-Turonian), therefore confirming the oils’ origin.