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The AAPG/Datapages Combined Publications Database
AAPG Special Volumes
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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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