Structural Styles and Hydrocarbon Potential of the Sub-Andean Thrust Belt of Southern Bolivia

Abstract

The sub-Andean fold and thrust belt is a thin-skinned, generally in-sequence system with two major linked detachment horizons--one in the Silurian Kirusillas Formation and the other in the Devonian Los Monos Formation. We have developed a new structural model by integrating field work and seismic data with balanced cross sections. Sequential restoration of balanced cross sections indicates about 100 km (43-51%) of shortening in the sub-Andean zone. Two structural styles predominate. In the foreland of the Chaco basin and the eastern sub-Andean foothills, fault-propagation and fault-bend folds from the basal Kirusillas detachment are wedged beneath small, steep, surface anticlines detached in the higher level Los Monos Formation. Most of the faults ramp up gradually through the entire section. West of the Mandiyuti thrust in the western sub-Andean zone, ramp-flat-ramp fault-bend fold geometries are more predominant due to a stratigraphic change that yields an additional higher level detachment. This higher level detachment may allow more prospective Carboniferous section to be preserved in the footwall structures, which is a new untested play type in the sub-Andean zone. In general, many of the deep folds imaged on seismic data do not match the idealized geometry expected of a fault-bend fold. We suggest either fault-bend folding with breakthrough at the top of the ramp or fault-propagation folding with detachment breakthrough as possible mechanisms for the development of such folds. Geochemical analyses and modeling provide a means to assess the quality, distribution, and maturity of the primary source rock in the southern sub-Andean belt and the overall hydrocarbon potential of the area. The primary oil-generative area is in the central and eastern parts of the sub-Andean foothills where surface oil seeps are abundant and corridors of more oil-prone kerogen are found. Biomarker analyses and burial history modeling show that the primary source rock entered the peak oil window during thrusting in the sub-Andean belt. Variations in source rock facies and heat flow, combined with different burial depths of the source rock section, may explain the distribution of oil and gas/condensate in southern Bolivia.