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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:
P. Baby, I. Moretti, B. Guillier, R. Limachi, E. Mendez,
J. Oller, and M. Specht 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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Petroleum
System of the Northern and Central Bolivian Sub-Andean Zone
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P. Baby
ORSTOM
Grenoble, France
I. Moretti
IFP
Rueil Malmaison, France
B. Guillier
ORSTOM
Quito, Ecuador
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R. Limachi
E. Mendez
YPFB
Santa Cruz, Bolivia
J. Oller
Petrolex
Santa Cruz, Bolivia
M. Specht
Total
Paris-LaDefense, France
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Abstract
A coupled study of the kinematics
of thrusting and hydrocarbon  maturation has been carried out in the northern
and central sub-Andean belt of Bolivia to define the petroleum potential
of the area. In addition to the classic Devonian source rock (Tomachi-Tequeje
formations to the north and Iquiri-Limoncito formations in the central
area), two other source rock intervals are recognized: the Retama Formation
(Upper Devonian-Lower Carboniferous) and the Copacabana Formation (Upper
Carboniferous-Lower Permian). These are the most prospective units in northern
Bolivia and are of marine origin. The structural style varies from north
to south due to variations in the sedimentary column involved in the thrusts.
The orogenic front was guided by the northern boundary of a Paleozoic sedimentary
wedge. In the Boomerang area, this boundary is oriented obliquely to the
regional shortening and controlled the development of a prominent transfer
zone. To the north, the thrusts are wider and the amount of shortening
increases. The western part of the northern sub-Andean zone is characterized
by a very thick Tertiary piggyback basin fill.
Two phases of hydrocarbon
maturation are recognized. The first began in Early Carboniferous and affected
mostly Devonian strata. Formation of structural traps during this period
occurred rarely. The entire basin was then deeply eroded in Permian-Jurassic
time, causing any hydrocarbons that may have formed to be lost. The second
phase of maturation was contemporaneous with Andean deformation and with
the resulting burial under the Tertiary cover in the foreland basin and
in piggyback basins on thrust structures. The hydrocarbon expelled during
this period may fill the Andean anticlines. The known source rocks are
not proven to be gas prone, but current discoveries indicate a high gas
to oil ratio that may be due to secondary cracking in the source rock.
Because the initial potential of the source rocks is low, expulsion of
heavy compounds is expected to be weak.
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