Petroleo

Cuenca de Tarija

159 Sistemas Petroleros de las Cuencas Andinas Carlos E. Cruz - Jorge F. Rodríguez - Jorge J. Hechem - Héctor J. Villar, eds.

LOS SISTEMAS PETROLEROS DEVÓNICOS DEL SUBANDINO SUR Y PIE DE MONTE DE LA CUENCA DE TARIJA. BOLIVIA
Carlos E. Cruz1, Jaime Oller Veramendi2, Matías Di Benedetto3, Martín Pereira4 y Héctor J. Villar5
Pluspetrol S.A., Buenos Aires, Argentina,ccruz@pluspetrol.net Petrobras Bolivia, Santa Cruz de la Sierra, Bolivia, joller@petrobras.com.bo 3 Repsol YPF, Buenos Aires, Argentina, mdibenedettor@repsolypf.com 4 Pluspetrol S.A., Buenos Aires, Argentina, mpereira@pluspetrol.net 5 GeoLab Sur S.A., Buenos Aires, Argentina, lqvillar@fibertel.com.ar
1 2

Abstract. The Devonian petroleum systems of the South Subandean and Foothill of the Tarija Basin.Bolivia. The charge system of the Tarija Basin involves Devonian sources that have poor to moderate –occasionally good- quality for hydrocarbon generation. Thermal maturity of the source intervals show wide thermal maturity variations from the initial stages of the oil window to very advanced phases of gas generation, depending on the stratigraphic position and geologic setting. Effectiveexpulsion of hydrocarbons is believed to have begun at relatively high levels of thermal maturity (VRE>0.9-1.0%), with prevalence of gassy hydrocarbons. Modeling of hydrocarbon generation of the Devonian source rocks demonstrates that the main episodes of expulsion and charge occurred coupled to the Andean tectonics. A generally low thermal heat flow linked to the Tertiary foreland deposits constrainedthe maturation process. Main generation areas are related to the synclines in the South Subandean Thrust Belt and two main depocenters in the Santa Cruz and Pilcomayo areas. Preferred migration pathways are interpreted to be the main thrusts and faults. GAE estimations points to low efficiency petroleum systems. The main trapping mechanism is structural, both in the South Subandean and in theFoothills. The predominance of condensate accumulations precludes assessing definite oilsource correlation patterns. Nevertheless, genetic relations were proved for the Eifelian sources. Isotope data, arranged by reservoir and geographic location, is consistent with significant variations in thermal maturity, allowing for distinct kitchens of a same source rock and/or co-sourcing. Three petroleum zonesare recognized in the Tarija Basin: Santa Cruz, Pilcomayo (extending up to northern Argentina) and the South Subandean Thrust Belt. In addition to the classical Los Monos-Huamampampa (!), a second petroleum system is proposed with the name Sección Lochkoviano-Carbónico(.). 2P EUR reserves were allocated to these Devonian systems; yet to find resources were estimated in the range 17.5-56.0 x 1012cfg. Contributions of a Silurian petroleum system have a hypothetical significance for deep gas in the Subandean Thrust Belt.

INTRODUCCIÓN
La región del Subandino Sur y Pie de Monte de la Cuenca de Tarija, en Bolivia, es una provincia gasífera que abarca un área cercana a los 100,000 km2. Sus reservas recuperables finales probadas más probables (EUR P1 + P2) de petróleo y gas al año 2005, segúnYacimientos Petrolíferos Fiscales Bolivianos, ascienden a 956 x 106 bo y 52 x 1012 cfg (CBH, 2008). El sector analizado comprende, desde el punto de vista geológico, la Faja Corrida Externa y el Pie de Monte ubicado entre el Codo de Santa Cruz al Norte y la región del límite político Bolivia – Argentina hacia el Sur (Figura 1). Se excluye de este análisis la zona de la cuenca denominadaBoomerang, situada al Norte del Codo de Santa Cruz. Este trabajo pretende caracterizar el hábitat de hidrocarburos del área y su entrampamiento, diferenciando regiones con patrones determinados de generación y tipo de hidrocarburos. Sobre esta base, se definen zonas donde las evidencias apuntan a un sistema petrolero único o, alternativamente, sugieren la interacción de sistemas petroleros múltiples....