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Abstract
Chapter from: M
62: Petroleum Basins of South America
Edited by
A. J. Tankard, R. Suarez Soruco, and H. J. WelsinkAuthors:
J. F. Dunn, K. G. Hartshorn, and P. W. Hartshorn Basin and Aerial Analysis/Evaluation
Published 1995 as
part of Memoir 62
Copyright © 1995 The American Association of Petroleum
Geologists. All Rights Reserved. |
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Structural
Styles and Hydrocarbon Potential of the Sub-Andean Thrust Belt of Southern
Bolivia
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John F. Dunn
Kelly G. Hartshorn
Peter W. Hartshorn
Chevron International
Limited (Bolivia)
Santa Cruz, 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. |
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