2-D Modelling of the Petroleum System in a Structurally Complex Area: The Madrejones Structure, South Bolivia

Carlos Cruz², Frederic Schneider¹, Laurent Sage¹, Frederic Dartois¹, Patrick Wojciak¹, and Daniel Herrera^3
1Beicip-Franlab, Rueil-Malmaison, France
²Pluspetrol, Buenos Aires, Argentina
³Beicip Inc, Houston, TX

The Madrejones structure is located in the Yacuiba Block, in South Bolivia. The objective of the study is to analyse the reasons of the quite limited infilling of the main structure of the field. Especially, a quantification of the overpressure in the area is done, and its influence over the hydrocarbon expulsion, migration and entrapment is studied.

Madrejones field produces gas in Devonian sandstones. The trap is a thrust fold being part of a larger system of over-thrusted tectonic sheets. In term of rock volume, potential for gas and condensate accumulations is important (taking into account the generally high source maturity in the area).

The workflow of the study is: (1) detailed structural interpretation of a representative seismic section, and 2D restoration; (2) analysis of well and field data; (3) 2D complex basin modelling with Ceres®.

The study shows that:
-the Madrejones structure started to grow 5 Ma ago;
-the small potential of the source rocks and their low permeability implied hydrocarbon retention within the source layers, and therefore a low expulsion rate. Expulsion had already started before the trap formation, and went on during structural growth;
-the migration efficiency was very poor. Hydrocarbons migrated updip and per descensum toward the main reservoir layer, located between the two main source rocks. The upper source rock is the regional seal and is at the origin of the overpressure observed in the reservoir;
-the quite high maturity level at the age of trap formation explains the presence of gas and condensates;
-the order of magnitude of modelled trapped hydrocarbon volumes, and their type, are compatible with field estimation.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas