Compartmentalisation of fluid migration pathways in the sub-Andean Zone, Bolivia

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 °C, 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).

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 displacement (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 (Travé et al., 1998) and thrust fault zones Larroque et al., 1996, Labaume et al., 1997. In the Barbados accretionary prism, direct monitoring of the décollement level also demonstrated current fluid circulation along the fault plane (Moller et al., 1995) and seismic parameters have been interpreted as the mark of overpressure in another part of the fault (Shipley et al., 1997). All these examples strongly suggest that the hydraulic behaviour of faults may change with time. In the case of sand/shale intercalations, some predictive models have been proposed based on the clay smearing approach Weber et al., 1978, Lehner and Pilaar, 1997. Following this theory, shaly beds undergo plastic deformation when sandstones undergo brittle faulting. The shale smears the fault plane which becomes impermeable if the offset is small enough to permit the continuity of the clay smearing. When the offset increases, the hydraulic behaviour changes when the clay smear is disrupted along the fault plane. This model has been developed in deltaic series (e.g. Niger delta) for small-scale normal faults (offsets of a few meters to hundreds of meters). In this work on sandstone series, we propose another predictive model for the behaviour of fluids based on a multidisciplinary study of the Andean thrust front in southeastern Bolivia.

In petroleum-rich compressional provinces such as the Bolivian Andean foothills, oil seeps are numerous along reverse fault planes. However, in the Bolivian foredeep, various fields are closed by reverse faults. Working from the regional to thin-section scales, we have tried to understand this apparent discrepancy between fault behaviour in the foreland, below the syntectonic deposits, and that of the outcropping faults in the foothills. The structure of fault zones in the foothills and their paleo-hydraulic behaviour are deduced from the distribution of cements and carbon and oxygen isotope geochemistry of carbonate cements that are presented in detail in Labaume and Moretti (2001) and Labaume et al. (2001). In this paper, we summarise these results obtained on paleo-fluids, then we describe the present fluid flow systems (hydrocarbons and water) and focus the discussion on the resulting large-scale fluid flow pattern and its implication for exploration in the foothills.