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

Abstract

Numerous observations indicate that faults play a major role on the migration pathways 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 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 hydraulic behaviour of faults changes with depth. It also indicates that, in the studied area where the series consist mainly of sandstone, the fault behaviour is strain independent. Based on the microstructural analyses of fault zones, we suggest that the controlling factor is temperature that facilitates or inhibits silica precipitation. These results imply that faults are a barrier for lateral and transverse migration in the foredeep below the >3km-thick Tertiary deposits at T>100°C, due to sealing by authigenic quartz. The same faults are lateral drains in their shallow parts (<2.5–3km) since the fracture created by the deformation remain open. Due to the increase in compression, the initially deep impermeable part of a fault becomes a lateral drain if the fault is reactivated after an uplift and erosive event.