Compartmentalisation of fluid flow by thrust faults, Sub-Andean Zone, Bolivia

Numerous observations indicate that faults play a major role on the migrati on pathways in the Bolivian Sub-Andean Zone. Most oil seeps in the foothill s are located on faults, but oil fields in the foredeep are closed by fault s. In the foothills, analysis of cements in fractures inside and around fau lt zones indicate that the faults act as barriers for transverse migration but can be preferential lateral (i.e. fault parallel) migration pathways. A detailed study of these apparent contradictions suggests that the hydrauli c behaviour of faults changes with depth. It also indicates that, in the st udied area where the series consist mainly of sandstone, the fault behaviou r is strain independent. Based on the microstructural analyses of fault zon es, we suggest that the controlling factor is temperature that facilitates or inhibits silica precipitation. These results imply that faults are a bar rier for lateral and transverse migration in the foredeep below the >3 km-t hick Tertiary deposits at T > 100 degrees C, due to sealing by authigenic q uartz. The same faults are lateral drains in their shallow parts (<2.5-3 km ) since the fracture created by the deformation remain open. Due to the inc rease in compression, the initially deep impermeable part of a fault become s a lateral drain if the fault is reactivated after an uplift and erosive e vent. (C) 2000 Elsevier Science B.V. All rights reserved.