Compartmentalisation of fluid migration pathways in the sub-Andean Zone, Bolivia Isabelle Moretti a, * , Pierre Labaume b , Simon M.F. Sheppard c , Jacques Boule `gue d a Division Geologie – Geochimie, Institut Franc ßais de Pe ´trole, 1-4 avenue de Bois-Pre ´au, 92852 Rueil-Malmaison, France b LGIT, Universite ´ Joseph Fourier– CNRS, BP 53, 38041 Grenoble Cedex 9, France c Sciences de la Terre, ENS de Lyon– CNRS, 46 Alle ´e d’Italie, 69364 Lyon, France d Institut de Physique du Globe de Paris, 4 Place Jussieu, 75005 Paris, France Received 15 December 2000; accepted 19 July 2001 Abstract Numerous observations indicate that faults play a major role on the migration pathways of fluids in the Bolivian sub-Andean Zone. Most oil seeps in the foothills are located on faults, but oil fields in the foredeep are closed by faults. In the foothills, analysis of cements in fractures inside and around fault zones indicates that the faults act as barriers for transversal migration but can be preferential lateral migration pathways for both hydrocarbons and water. A detailed study of these apparent contradictions suggests that the hydraulic behaviour of faults changes with depth in relation with sandstone diagenesis, but it is strain-independent. From microstructural analyses of fault zones, we suggest that the main controlling factor is temperature, which facilitates or inhibits quartz precipitation. This result implies that the same fault is a barrier for lateral and transversal migration at depths > 3 km, due to sealing of fractures by authigenic quartz at T >80 jC, and is a lateral drain in its shallower parts. As a result, the various thrust sheets are isolated from a hydraulic point of view, whereas migration in the foreland may take place over long distances (>100 km). D 2002 Elsevier Science B.V. All rights reserved. Keywords: Fault; Fluid flow; Hydrocarbon migration; Cement (quartz, carbonate); Bolivia 1. Introduction The role of faults and fractures along the migration pathways of fluids has been mainly studied at the reservoir scale and after fault activity. On a basin scale, however, the number of studies is much more limited. Some papers suggest that normal faults act as drains, especially when they are seismically active, and that reverse faults act as barriers (Muir Wood and King, 1993). Seismic studies in fault zones have proven the existence of overpressures which facilitate displace- ment (Evans, 1992; and many others) and imply, at least temporarily, impermeable fault zones. On the other hand, geological data show evidence for paleo- fluid circulation both in normal (Trave ´ et al., 1998) and thrust fault zones (Larroque et al., 1996; Labaume et al., 1997). In the Barbados accretionary prism, direct monitoring of the de ´collement level also demonstrated current fluid circulation along the fault plane (Moller et al., 1995) and seismic parameters have been inter- preted as the mark of overpressure in another part of the fault (Shipley et al., 1997). All these examples 0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0040-1951(01)00246-3 * Corresponding author. Tel.: +33-1-4752-6191; fax: +33-1- 4752-7067. E-mail addresses: isabelle.moretti@ifp.fr (I. Moretti), pierre.labaume@dstu.univ-montp2.fr (P. Labaume), Simon.Sheppard@geologie.ens-lyon.fr (S.M.F. Sheppard), jacques.boule `gue@ipgp.fr (J. Boule `gue). www.elsevier.com/locate/tecto Tectonophysics 348 (2002) 5 –24