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Compartmentalisation of fluid flow by thrust faults, Sub-Andean Zone, Bolivia
Journal of Geochemical Exploration 69–70 (2000) 493–497 www.elsevier.nl/locate/jgeoexp
Compartmentalisation of fluid flow by thrust faults, Sub-Andean Zone, Bolivia I. Moretti a,*, P. Labaume b, S. Sheppard c, J. Boule`gue d a
Institut Franc¸ais 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-CNRS, 46 Alle´e d’Italie, 69364 Lyon, France d Institut de Physique du Globe de Paris, 4 Place Jussieu, 75005 Paris, France
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 ⬎3 km-thick Tertiary deposits at T ⬎ 100⬚C, due to sealing by authigenic quartz. The same faults are lateral drains in their shallow parts (⬍2.5–3 km) 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. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: fault; fluid flow; hydrocarbon migration; cement (quartz, carbonate); Bolivia
1. Introduction In the literature, the role of faults and fractures along the migration pathway has been studied mainly at a reservoir scale, and after fault activity. On a basin scale, however, the number of studies is much more restricted. Some papers suggest that normal faults act as drains, especially when they are seismically active,
* Corresponding author. E-mail addresses: [email protected] (I. Moretti), pierre. [email protected] (P. Labaume), simon.sheppard@ geologie.ens-lyon.fr (S. Sheppard), [email protected] (J. Boule`gue).
and that reverse faults act as barriers (Wood and King, 1993). Seismologists have proven the existence of overpressures in fault zones that facilitate displacement (Evans, 1992 and many others), implying at least temporarily impermeable fault zones. On the other hand, geological data show palaeo fluid circulation both in normal (Trave´ et al., 1998) and thrust zones (Larroque et al., 1995; Labaume et al., 1997). In the Barbados accretionnary prism, direct monitoring of the de´collement level has also proven recent fluid circulation along the fault plane (Moore et al., 1995. These data strongly suggest that fault hydraulic behaviour changes with time. In the case of sand/shale intercalation some predictive models have been
0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0375-674 2(00)00103-5
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I. Moretti et al. / Journal of Geochemical Exploration 69–70 (2000) 493–497
Fig. 1. Structural cross section of the Bolivian SAZ, with the location of six of the studied faults (four others are on a parallel section, located 120 km northward). Inset: outline of Bolivia with the location of the section (thick line).
proposed based on the clay smearing approach (Weber et al., 1978). In this work, we propose another predictive model for the case of sandstone series. In petroleum-rich compressional provinces such as the Andean foothills in Bolivia, oil seeps are numerous along fault planes that are all reverse. However, in the Bolivian foredeep, various fields are closed on reverse faults. Working at different scales, from regional fluid flow to thin sections, we tried to understand this apparent discrepancy between the fault behaviour in the foreland, below the syntectonic deposits, and that of the outcropping faults in the foothills. Details of fault zone structure and hydraulic behaviour are presented in a companion paper (Labaume et al., this volume). 2. Geological setting The Sub-Andean Zone (SAZ) is a Neogene East verging thrust system that constitutes the eastern
border of the Andes (Fig. 1). The thrust sheets are comprised of 10 km-thick, mainly siliciclastic succession with Palaeozoic–Mesozoic, platform sediments at the base and up to 3 km-thick Neogene continental foreland deposits above (Baby et al., 1992). The main de´collement is located in Silurian shales. The main thrusts have an average spacing of 25 km with displacements up to 20 km. The ramp anticlines often feature minor reverse faults related to detachments in Devonian and Carboniferous shaly intervals (Moretti et al., 1996). The source rocks are Palaeozoic but the current petroleum system is very recent. The traps are the thrust anticlines, which are usually not older than 6 My, in the HC rich zone and the reservoirs are all sandstone from the Lower Devonian to the Miocene (Moretti et al., 1995). Faults play a key role in the migration pathway and the compartmentalisation of the drainage area, since all the traps are structural. They also influence the retaining capacity of the stratigraphic seals. We studied the hydrocarbons (HC) from more than 50 oil fields, various oil seeps, 6 hydrothermal springs and 10 fault zones, outcropping in the foothills and featuring rocks buried between a few hundred meters (Tertiary) and about 8 km (Silurian) at the beginning of thrusting.
3. Fault permeability
Fig. 2. Fault zone anisotropy (flat position).
The permeability of a rock is usually anisotropic, especially in a siliciclastic platform displaying sand/ shale intercalation. Due to sedimentary heritage, the
I. Moretti et al. / Journal of Geochemical Exploration 69–70 (2000) 493–497
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synclines and the currently eroded adjacent anticlines. The very high porosity of the Mesozoic sandstone, possibly increased by fracturation in the anticline hinges, permits such a fast water flow parallel to the beds, which are steeply dipping in the anticline limbs. The data do not indicate any largescale fluid flow between the Andes and the foothills. Each spring may have a local source in the closest syncline. Fig. 3. Oil seep classification.
5. Oil seeps horizontal (bedding-parallel) permeability is usually 10–100 times higher than the vertical one (beddingnormal), when looked at on a plurimetric scale (10– 500 m). In addition to this bulk permeability, fractures may create a second permeability, also strongly anisotropic. In a fault zone, one may define a longitudinal and a transverse permeability in each domain (the core zone where the stratigraphic bedding is no longer visible, the damage zone which is affected by high fracture density, and the external zone not disturbed by the fault proximity, see Fig. 2). Important questions for the oil industry are to define the characteristics of each domain. We already proved that with “good” longitudinal permeability in a fault zone, the fault zone thickness is not crucial, and that if the surrounding material has a low permeability, the value of the fault permeability itself is not crucial as soon as it is greater than a few millidarcies (Moretti, 1998).
More than 200 seeps have been reported in the Bolivian SAZ. The oil seep location can be classified in relation to its structural position: 60% leak from large thrust faults, 30% from anticline hinges and only 10% from monoclines (Fig. 3). This classification emphasises the capacity of faults to focus oil migration. Most oil seeps are located in the Upper Devonian, i.e. very close to the source rock, the required migration along the fault never exceeding a few hundred meters. These features prove the low transverse permeability of the fault zones, i.e. HC cannot ignore the fault during vertical migration, but they do not prove a high longitudinal permeability of the fault zone. Usually, the oil seeps are located inside or below the fault zone and the HC may have migrated in the sandstone of the footwall.
6. Hydrocarbon accumulations 4. Hot water springs Hot waters have been sampled and analysed in various structures. The springs are systematically located in anticline hinges and leak from Mesozoic sandstone. The discharge temperature is generally in the range of 60–70⬚C. Chemical analyses show that the water is not, or almost not, contaminated by recent meteoric water and that the deeper sources of the various waters are not hydraulically connected. The estimated source temperature is 100/120⬚C; i.e. the reservoirs are located between 3 and 4 km deep. We interpret these springs as flow of Mesozoic formation water due to the pressure change between the buried
The frontal compressive structures, below the Tertiary deposits are all charged in HC (Nupuco, Villamontes, San Roque). Some of the largest accumulations have been found there, e.g. Rio Grande in the Santa Cruz area. In the foothills, various accumulations have been found in the anticlines in the Upper Devonian and younger reservoirs and recently, very large gas reserves have been found in deep wells in Lower Devonian reservoirs (Humampampa and Santa Rosa Fms). These discoveries prove the efficiency of the lower petroleum systems (Silurian/SantaRosa and Icla/Humampampa Fms) and the good seal capacity of both the shaly intervals (especially the Los Monos Fm) and the faults.
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I. Moretti et al. / Journal of Geochemical Exploration 69–70 (2000) 493–497
7. Summary of the current fluid flows To summarise, the current fluid flows suggest that faults act as barriers in the foredeep and as preferential migration pathways in the foothills. Since each fault is formed in the foredeep and is then integrated in the foothills, this may imply an evolution, possibly controlled by various key factors. (1) Stress: in the foothills anticlines, the stress regime is extensive due to the relief, whereas it is compressive in the foredeep (Sassi and Faure, 1997). (2) Strain: at the beginning, faults are barriers and when strain and thus fracturation in the fault zone increase, they become preferential migration pathways. (3) Burial: the faults are barriers below a certain depth, due to P– T conditions, and lateral migration pathway above. 8. Palaeofluid flow 8.1. Evidence of palaeofluid flow Carbonate cements (calcite, locally dolomite or Mg-siderite) occur both inside and outside the outcropping fault zones in all formations, although they are in small amounts and have an irregular distribution. Textural relationships show that carbonates precipitated after sandstone compaction. These cements required a source external to the sandstone bodies, since the latter do not contain sedimentary carbonates or Ca-bearing minerals. This source was most likely fossils in the shale layers, a hypothesis supported by the carbon isotopic ratios of carbonates (d 13C between ⫹1.7 and ⫺26.1‰ PDB), which show that fluid flow, from which the carbonate precipitated, was contemporaneous with the CO2/CH4 leakage from the source rocks (the Silurian and Devonian shales). This implies a transfer of carbonate by fluid flow at least at the kilometre scale in most cases. 8.2. Hydraulic behaviour of fault zones The fault zones correspond to a few metres- to tens of metres-thick intervals of fractured rocks on each side of the fault surface (Fig. 2). We assessed their hydraulic behaviour from the analysis of deformation structures and cement distribution. Details of the results are presented in Labaume et al. (this volume). Characteristic structures are cataclastic slip bands and
fractures whose sealing properties are controlled by temperature, i.e. depth. Below 2.5–3 km, they are sealed by authigenic quartz, due to local silica transfer through pressure solution-precipitation activated at T ⬎ 80⬚; whereas quartz sealing is absent at shallower depths. Hence, the deep part of faults are potential barriers to transverse fluid flow, whereas the shallow parts have remained open and were permeable to allochthonous fluid flow, as shown by the frequent occurrence of carbonate cements. At the centimetre scale, cements occur both in the cataclastic slip bands/ fractures and in their host-sandstone; mineralogical, textural, cathodoluminescence and O–C isotope data show that they were precipitated coevally at both sites. This shows that fluid percolated during/after fault formation, sometimes in successive pulses. At the fault-zone scale, cements are concentrated in fault zones whereas they are absent or scarce in the close external zones. This shows that the fault zones acted as preferential pathways for lateral fluid migration, due to the high density of connected fractures, but had a low transverse permeability. The latter conclusion is corroborated by the isotopic analyses of carbonate cements outside the fault zones, which show that the faults separate compartments with different fluid histories.
9. Conclusions on hydraulic fault behaviour and fluid flow pattern All the approaches suggest that each syncline/anticline couple is isolated from a hydraulic point of view. Since the Los Monos Fm is also a seal, the resulting drainage areas are rather compartmentalised. Already, this compartmentalisation of the drainage area has been suggested as the cause of the small scale of the upper prospects in the south SAZ. The faults may be considered as transverse seals below 2.5–3 km, and as lateral migration pathways above. A segment of fault may thus change its hydraulic behaviour if deformation is still active while burial decreases. The oil seeps are located in the fault zones due to the secondary porosity created by the uncemented fractures in the first kilometres, but this migration occurs over a short distance. Compared to other thrust belts where veins are numerous and very large, we note that the evidence
I. Moretti et al. / Journal of Geochemical Exploration 69–70 (2000) 493–497
for fluid circulation is limited in the SAZ. In particular, we did not find any evidence of large-scale fast circulation that may induce thermal discrepancies between the fluids and the host-rocks. The hot springs are very local and do not require more than few kilometres, on the same thrust sheet, of water flow between recharge and discharge. This is coherent with the fact that the drainage distance along the fault is short. Acknowledgements We thank Yacimientos Petroliferos Fiscales Bolivianos for access to data and support during field work, and J Oller and G Montemuro for helpful discussions. This study was funded by Elf, Maxus and Pluspetrol, and partially by Repsol, Mobil and Exxon. References Baby, P., He´rail, G., Salinas, R., Sempere, T., 1992. Geometry and kinematic evolution of passive roof duplexes deduced from cross section balancing: example from the foreland thrust system of the southern Bolivian Sub Andean Zone. Tectonophysics 11 (3), 523–536. Evans, B., 1992. Greasing the fault. Nature 358, 544–545.
497
Labaume P., Kastner, M., Trave´, A., Henry, P., 1997. Carbonate veins from the de´collement zone at the toe of the northern Barbados accretionnary prism. Proceeding of the ODP, Scientifics results, vol. 156, pp. 79–96. Larroque, C., Guilhaumou, N., Stephan, J.F., Roure, F., 1995. Advection of fluids at the front of the Sicilian Neogene subduction complex. Tectonophysics 254, 41–55. Moore, J.C., ODP Leg 156 team, 1995. Abnormal fluid pressures and fault zone dilation in the Barbados accretionary prism: evidence from logging while drilling. Geology 23 (7), 605–608. Moretti, I., 1998. The role of faults in hydrocarbon migration. Petrol. Geosci. 4, 81–94. Moretti, I., Martinez, E.D., Montemurro, G., Aguilera, E., Perez, E., 1995. The Bolivian source rocks: Sub Andean Zone, Madre de Dios, Chaco. Revue de l’IFP 50 (6), 753–777. Moretti, I., Baby, P., Mendez, E., Zubieta, D., 1996. Hydrocarbon generation in relation to thrusting in the Sub-Andean Zone from 18 to 22⬚S, Bolivia. Petrol. Geosci. 2, 17–28. Wood, R.M., King, G., 1993. Hydrological signatures of earthquake strain. JGR 98, 22,035–22,068. Sassi, W., Faure, J.L., 1997. Role of faults and layer interfaces on the spatial variation of stress regimes in basins: inferences from numerical modeling. Tectonophysics 266, 101–119. Trave´, A., Calvet, F., Soler, A., Labaume, P., 1998. Fracturing and fluid migration during Palaeogene compression and Neogene extension in the Catalan coastal ranges, Spain. Sedimentology 45, 1063–1082. Weber, K., Mandl, G., Pilaar, W., Lehner, F., Precious, R., 1978. The role of faults in hydrocarbon migration and trapping in the Nigerian growth fault structures: 10th Annual offshore Technology Conference proceedings, vol. 4, pp. 2643–2653.
Compartmentalisation of fluid flow by thrust faults, Sub-Andean Zone, Bolivia I. Moretti a,*, P. Labaume b, S. Sheppard c, J. Boule`gue d a
Institut Franc¸ais 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-CNRS, 46 Alle´e d’Italie, 69364 Lyon, France d Institut de Physique du Globe de Paris, 4 Place Jussieu, 75005 Paris, France
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 ⬎3 km-thick Tertiary deposits at T ⬎ 100⬚C, due to sealing by authigenic quartz. The same faults are lateral drains in their shallow parts (⬍2.5–3 km) 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. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: fault; fluid flow; hydrocarbon migration; cement (quartz, carbonate); Bolivia
1. Introduction In the literature, the role of faults and fractures along the migration pathway has been studied mainly at a reservoir scale, and after fault activity. On a basin scale, however, the number of studies is much more restricted. Some papers suggest that normal faults act as drains, especially when they are seismically active,
* Corresponding author. E-mail addresses: [email protected] (I. Moretti), pierre. [email protected] (P. Labaume), simon.sheppard@ geologie.ens-lyon.fr (S. Sheppard), [email protected] (J. Boule`gue).
and that reverse faults act as barriers (Wood and King, 1993). Seismologists have proven the existence of overpressures in fault zones that facilitate displacement (Evans, 1992 and many others), implying at least temporarily impermeable fault zones. On the other hand, geological data show palaeo fluid circulation both in normal (Trave´ et al., 1998) and thrust zones (Larroque et al., 1995; Labaume et al., 1997). In the Barbados accretionnary prism, direct monitoring of the de´collement level has also proven recent fluid circulation along the fault plane (Moore et al., 1995. These data strongly suggest that fault hydraulic behaviour changes with time. In the case of sand/shale intercalation some predictive models have been
0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0375-674 2(00)00103-5
494
I. Moretti et al. / Journal of Geochemical Exploration 69–70 (2000) 493–497
Fig. 1. Structural cross section of the Bolivian SAZ, with the location of six of the studied faults (four others are on a parallel section, located 120 km northward). Inset: outline of Bolivia with the location of the section (thick line).
proposed based on the clay smearing approach (Weber et al., 1978). In this work, we propose another predictive model for the case of sandstone series. In petroleum-rich compressional provinces such as the Andean foothills in Bolivia, oil seeps are numerous along fault planes that are all reverse. However, in the Bolivian foredeep, various fields are closed on reverse faults. Working at different scales, from regional fluid flow to thin sections, we tried to understand this apparent discrepancy between the fault behaviour in the foreland, below the syntectonic deposits, and that of the outcropping faults in the foothills. Details of fault zone structure and hydraulic behaviour are presented in a companion paper (Labaume et al., this volume). 2. Geological setting The Sub-Andean Zone (SAZ) is a Neogene East verging thrust system that constitutes the eastern
border of the Andes (Fig. 1). The thrust sheets are comprised of 10 km-thick, mainly siliciclastic succession with Palaeozoic–Mesozoic, platform sediments at the base and up to 3 km-thick Neogene continental foreland deposits above (Baby et al., 1992). The main de´collement is located in Silurian shales. The main thrusts have an average spacing of 25 km with displacements up to 20 km. The ramp anticlines often feature minor reverse faults related to detachments in Devonian and Carboniferous shaly intervals (Moretti et al., 1996). The source rocks are Palaeozoic but the current petroleum system is very recent. The traps are the thrust anticlines, which are usually not older than 6 My, in the HC rich zone and the reservoirs are all sandstone from the Lower Devonian to the Miocene (Moretti et al., 1995). Faults play a key role in the migration pathway and the compartmentalisation of the drainage area, since all the traps are structural. They also influence the retaining capacity of the stratigraphic seals. We studied the hydrocarbons (HC) from more than 50 oil fields, various oil seeps, 6 hydrothermal springs and 10 fault zones, outcropping in the foothills and featuring rocks buried between a few hundred meters (Tertiary) and about 8 km (Silurian) at the beginning of thrusting.
3. Fault permeability
Fig. 2. Fault zone anisotropy (flat position).
The permeability of a rock is usually anisotropic, especially in a siliciclastic platform displaying sand/ shale intercalation. Due to sedimentary heritage, the
I. Moretti et al. / Journal of Geochemical Exploration 69–70 (2000) 493–497
495
synclines and the currently eroded adjacent anticlines. The very high porosity of the Mesozoic sandstone, possibly increased by fracturation in the anticline hinges, permits such a fast water flow parallel to the beds, which are steeply dipping in the anticline limbs. The data do not indicate any largescale fluid flow between the Andes and the foothills. Each spring may have a local source in the closest syncline. Fig. 3. Oil seep classification.
5. Oil seeps horizontal (bedding-parallel) permeability is usually 10–100 times higher than the vertical one (beddingnormal), when looked at on a plurimetric scale (10– 500 m). In addition to this bulk permeability, fractures may create a second permeability, also strongly anisotropic. In a fault zone, one may define a longitudinal and a transverse permeability in each domain (the core zone where the stratigraphic bedding is no longer visible, the damage zone which is affected by high fracture density, and the external zone not disturbed by the fault proximity, see Fig. 2). Important questions for the oil industry are to define the characteristics of each domain. We already proved that with “good” longitudinal permeability in a fault zone, the fault zone thickness is not crucial, and that if the surrounding material has a low permeability, the value of the fault permeability itself is not crucial as soon as it is greater than a few millidarcies (Moretti, 1998).
More than 200 seeps have been reported in the Bolivian SAZ. The oil seep location can be classified in relation to its structural position: 60% leak from large thrust faults, 30% from anticline hinges and only 10% from monoclines (Fig. 3). This classification emphasises the capacity of faults to focus oil migration. Most oil seeps are located in the Upper Devonian, i.e. very close to the source rock, the required migration along the fault never exceeding a few hundred meters. These features prove the low transverse permeability of the fault zones, i.e. HC cannot ignore the fault during vertical migration, but they do not prove a high longitudinal permeability of the fault zone. Usually, the oil seeps are located inside or below the fault zone and the HC may have migrated in the sandstone of the footwall.
6. Hydrocarbon accumulations 4. Hot water springs Hot waters have been sampled and analysed in various structures. The springs are systematically located in anticline hinges and leak from Mesozoic sandstone. The discharge temperature is generally in the range of 60–70⬚C. Chemical analyses show that the water is not, or almost not, contaminated by recent meteoric water and that the deeper sources of the various waters are not hydraulically connected. The estimated source temperature is 100/120⬚C; i.e. the reservoirs are located between 3 and 4 km deep. We interpret these springs as flow of Mesozoic formation water due to the pressure change between the buried
The frontal compressive structures, below the Tertiary deposits are all charged in HC (Nupuco, Villamontes, San Roque). Some of the largest accumulations have been found there, e.g. Rio Grande in the Santa Cruz area. In the foothills, various accumulations have been found in the anticlines in the Upper Devonian and younger reservoirs and recently, very large gas reserves have been found in deep wells in Lower Devonian reservoirs (Humampampa and Santa Rosa Fms). These discoveries prove the efficiency of the lower petroleum systems (Silurian/SantaRosa and Icla/Humampampa Fms) and the good seal capacity of both the shaly intervals (especially the Los Monos Fm) and the faults.
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I. Moretti et al. / Journal of Geochemical Exploration 69–70 (2000) 493–497
7. Summary of the current fluid flows To summarise, the current fluid flows suggest that faults act as barriers in the foredeep and as preferential migration pathways in the foothills. Since each fault is formed in the foredeep and is then integrated in the foothills, this may imply an evolution, possibly controlled by various key factors. (1) Stress: in the foothills anticlines, the stress regime is extensive due to the relief, whereas it is compressive in the foredeep (Sassi and Faure, 1997). (2) Strain: at the beginning, faults are barriers and when strain and thus fracturation in the fault zone increase, they become preferential migration pathways. (3) Burial: the faults are barriers below a certain depth, due to P– T conditions, and lateral migration pathway above. 8. Palaeofluid flow 8.1. Evidence of palaeofluid flow Carbonate cements (calcite, locally dolomite or Mg-siderite) occur both inside and outside the outcropping fault zones in all formations, although they are in small amounts and have an irregular distribution. Textural relationships show that carbonates precipitated after sandstone compaction. These cements required a source external to the sandstone bodies, since the latter do not contain sedimentary carbonates or Ca-bearing minerals. This source was most likely fossils in the shale layers, a hypothesis supported by the carbon isotopic ratios of carbonates (d 13C between ⫹1.7 and ⫺26.1‰ PDB), which show that fluid flow, from which the carbonate precipitated, was contemporaneous with the CO2/CH4 leakage from the source rocks (the Silurian and Devonian shales). This implies a transfer of carbonate by fluid flow at least at the kilometre scale in most cases. 8.2. Hydraulic behaviour of fault zones The fault zones correspond to a few metres- to tens of metres-thick intervals of fractured rocks on each side of the fault surface (Fig. 2). We assessed their hydraulic behaviour from the analysis of deformation structures and cement distribution. Details of the results are presented in Labaume et al. (this volume). Characteristic structures are cataclastic slip bands and
fractures whose sealing properties are controlled by temperature, i.e. depth. Below 2.5–3 km, they are sealed by authigenic quartz, due to local silica transfer through pressure solution-precipitation activated at T ⬎ 80⬚; whereas quartz sealing is absent at shallower depths. Hence, the deep part of faults are potential barriers to transverse fluid flow, whereas the shallow parts have remained open and were permeable to allochthonous fluid flow, as shown by the frequent occurrence of carbonate cements. At the centimetre scale, cements occur both in the cataclastic slip bands/ fractures and in their host-sandstone; mineralogical, textural, cathodoluminescence and O–C isotope data show that they were precipitated coevally at both sites. This shows that fluid percolated during/after fault formation, sometimes in successive pulses. At the fault-zone scale, cements are concentrated in fault zones whereas they are absent or scarce in the close external zones. This shows that the fault zones acted as preferential pathways for lateral fluid migration, due to the high density of connected fractures, but had a low transverse permeability. The latter conclusion is corroborated by the isotopic analyses of carbonate cements outside the fault zones, which show that the faults separate compartments with different fluid histories.
9. Conclusions on hydraulic fault behaviour and fluid flow pattern All the approaches suggest that each syncline/anticline couple is isolated from a hydraulic point of view. Since the Los Monos Fm is also a seal, the resulting drainage areas are rather compartmentalised. Already, this compartmentalisation of the drainage area has been suggested as the cause of the small scale of the upper prospects in the south SAZ. The faults may be considered as transverse seals below 2.5–3 km, and as lateral migration pathways above. A segment of fault may thus change its hydraulic behaviour if deformation is still active while burial decreases. The oil seeps are located in the fault zones due to the secondary porosity created by the uncemented fractures in the first kilometres, but this migration occurs over a short distance. Compared to other thrust belts where veins are numerous and very large, we note that the evidence
I. Moretti et al. / Journal of Geochemical Exploration 69–70 (2000) 493–497
for fluid circulation is limited in the SAZ. In particular, we did not find any evidence of large-scale fast circulation that may induce thermal discrepancies between the fluids and the host-rocks. The hot springs are very local and do not require more than few kilometres, on the same thrust sheet, of water flow between recharge and discharge. This is coherent with the fact that the drainage distance along the fault is short. Acknowledgements We thank Yacimientos Petroliferos Fiscales Bolivianos for access to data and support during field work, and J Oller and G Montemuro for helpful discussions. This study was funded by Elf, Maxus and Pluspetrol, and partially by Repsol, Mobil and Exxon. References Baby, P., He´rail, G., Salinas, R., Sempere, T., 1992. Geometry and kinematic evolution of passive roof duplexes deduced from cross section balancing: example from the foreland thrust system of the southern Bolivian Sub Andean Zone. Tectonophysics 11 (3), 523–536. Evans, B., 1992. Greasing the fault. Nature 358, 544–545.
497
Labaume P., Kastner, M., Trave´, A., Henry, P., 1997. Carbonate veins from the de´collement zone at the toe of the northern Barbados accretionnary prism. Proceeding of the ODP, Scientifics results, vol. 156, pp. 79–96. Larroque, C., Guilhaumou, N., Stephan, J.F., Roure, F., 1995. Advection of fluids at the front of the Sicilian Neogene subduction complex. Tectonophysics 254, 41–55. Moore, J.C., ODP Leg 156 team, 1995. Abnormal fluid pressures and fault zone dilation in the Barbados accretionary prism: evidence from logging while drilling. Geology 23 (7), 605–608. Moretti, I., 1998. The role of faults in hydrocarbon migration. Petrol. Geosci. 4, 81–94. Moretti, I., Martinez, E.D., Montemurro, G., Aguilera, E., Perez, E., 1995. The Bolivian source rocks: Sub Andean Zone, Madre de Dios, Chaco. Revue de l’IFP 50 (6), 753–777. Moretti, I., Baby, P., Mendez, E., Zubieta, D., 1996. Hydrocarbon generation in relation to thrusting in the Sub-Andean Zone from 18 to 22⬚S, Bolivia. Petrol. Geosci. 2, 17–28. Wood, R.M., King, G., 1993. Hydrological signatures of earthquake strain. JGR 98, 22,035–22,068. Sassi, W., Faure, J.L., 1997. Role of faults and layer interfaces on the spatial variation of stress regimes in basins: inferences from numerical modeling. Tectonophysics 266, 101–119. Trave´, A., Calvet, F., Soler, A., Labaume, P., 1998. Fracturing and fluid migration during Palaeogene compression and Neogene extension in the Catalan coastal ranges, Spain. Sedimentology 45, 1063–1082. Weber, K., Mandl, G., Pilaar, W., Lehner, F., Precious, R., 1978. The role of faults in hydrocarbon migration and trapping in the Nigerian growth fault structures: 10th Annual offshore Technology Conference proceedings, vol. 4, pp. 2643–2653.