Abstract: The Petroleum System in Barinas-Southern Andes, Venezuela: a Stratigraphic and Geochemical Model

Anka, Zahie; Angel Callejón; Vicente Hernández - PDVSA; James Lacey - HARC; Oswaldo Gallango and Angel González - PDVSA

The main objective of this study was to determine the factors controlling the distribution of gas and oil in the foreland Barinas Basin and the southern flank of the Venezuelan Andes, in order to develop a predictive model capable of identifying those structures most likely to contain oil rather than gas. This work was based upon geological and geochemical data from wells, and the stratigraphic and structural analysis of several NW-SE and NE-SW seismic lines which allowed us to elaborate a tectono-stratigraphic reconstruction and hydrocarbon generation/ expulsion modeling of 45 wells in the whole area. The area was divided into 3 different sectors according to their structural styles (Fig.1). Sector I, located to the North of the Caribbean thrusting front, includes both pre-Andean and Andean structures, and the overthrusting within the Paguey Formation. Sector II, located between sector I and the neogene foothill synclinal, includes pre-Andean and Andean structures but no thrusting into the Paguey Fm., and Sector III, located south of the Neogene synclinal, which comprises mainly pre-Andean normal faulting.

La Morita member of the Navay Formation, which is the lateral equivalent to the upper part of the Cretaceous La Luna Fm., has a marine organic matter content between 70-90%. It is thermally mature all over the area, with ranges of 0.97-1.20% Ro in sectors I and II, and 0.74-0.86% Ro in sector III. The present day TOC content is around 2-2.2%, which represents an equivalent to 1-3% original TOC content. The effective thickness of La Morita member is around 30% of the total thickness. All these parameters make La Morita a good candidate for potential local source rock, although the most important constraint seems to be the low values of the effective thickness (ª20?-30?).

Through the 1-D modeling of hydrocarbon generation and expulsion we noticed that oil generation in sector I started 40 m.y. ago due to deposition of the significantly thick Paguey Fm. Expulsion in this area was 39 m.y. ago and it is related to the thrusting within the Paguey Fm. In sector II, both generation and expulsion of hydrocarbon were due to the deposition of Paguey Fm. In sector III, the deposition of thick molasses triggered the hydrocarbon generation and expulsion 7 to 5 m.y. ago. These results suggest that the thickness of the Paguey Fm. and its subsequent overthrusting are the main factors in the timing of generation and expulsion of hydrocarbons prior to the Andes uplifting. Although La Morita member actually expulsed hydrocarbons, the expulsed volumes can not explain the hydrocarbon reserves estimated in the Barias basin. Further modeling of hydrocarbon generation and expulsion, considering the La Luna Fm. as a source rock located to the North of the area, showed that this formation could have easily expulsed the volume of hydrocarbons found in the Barinas Basin. In consequence, we propose that the petroleum system that has operated in the Barinas Basin and the southern Andes foothills is defined by the La Luna Formation as a source rock (Fig.2).

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