Isabelle_Moretti_06_05

Structural style and decollement levels in the Llanos Orientales basin (Colombia) Isabelle Moretti, (Cepsa and IFP), Juan Carlos Mondragon (Cepcolsa), Julio Cesar Garzon (Cepsa), Gabriella Bosio (Cepcolsa), Jean Marc Daniel (IFP) especially eastward where they are infilled by the Cretaceous © Copyright 2009 ACGGP. or Oligocene series depending on the position. This paper was prepared for presentation at the X Simposio Bolivariano Exploración Petrolera en Cuencas Subandinas held in Cartagena, Colombia, July 2009. This paper was selected for presentation by the X symposium Technical Committee following review of information contained in an abstract submitted by the author(s).

NEOGENE

Mioc.

SEAL

León Fm.

Middle

Eocene Olig. Paleoc.

Carbonera Fm. Priabonian Bartonian Lutenian Ypresian Selandian

CRETACEOUS

Upper

Campanian

Mirador Fm.

Los Cuervos Fm. Barco Fm. Guadalupe Fm.

Santonian Coniacian Turonian

Gacheta Fm.

PERM

Lower

Cenomanian Une Fm. Albian

Upper PERMIAN

CARB

CARBONIFEROUS

DEVON

MESOZOIC

Guayabo Fm.

Upper

Maastrichtian

PALEOZOIC

Recent Deposits.

Lower

PALEOG

CENOZOIC

Holocene Pleistocene Pliocene

PREDOMINANT LITHOLOGY

SOURCE

STRATIGRAPHIC NOMENCLATURE

AGE

Introduction The so called Llanos, the northern part of the Andes current foreland of Colombia, is affected by light but complex deformation. In addition of the eastward pinchout of the series above the crystaline basement, the seismic data shows numerous normal faults, often east-dipping, compressive features affecting the Tertiary sequence near the thrust belt and a lot of faults within the Paleozoic. The shaly sequences are numerous and thick enough to likely produce disharmonies. The relationship between the Paleozoic structures and the more recent ones is open to discussion, as well as the age of the normal faults, their eventual strike slip component and the effect of the current compression. Since hydrocarbon discoveries are related to structural traps, excepting Rubiales, the understanding of this deformation has a crucial exploration impact. Based on our extensive new 3D seismic data and a large review of the existing 2D lines we will illustrate our current knowledge. Comparison with analogue models of transtension followed by compression has been done especially to illustrate the kinematic evolution of the normal fault pattern.

RESERVOIR

P. SYSTEM ELEMENTS

DEVONIAN SILURIAN ORDOVICIAN CAMBRIAN UPPER SILURIAN

PRECAMBRIAN

Fig 2: Simplified stratigraphic collumn Cauchos Fi g

3

Caracara

Rubiales

Fig 1: Location of the study area and of the cross section shown figure 3

Geological Setting The basement of the Llanos is covered by Paleozoic series, not all very well dated but including silico-clastic SiluroDevonian deposits. Strong unconformities ususaly separate these locally highly deformed series from the Cretaceous series, and significant paleoreliefs can also be observed,

The Lower Cretaceous sediments present in the Eastern Cordillera and deposited in an extensional context are absent in the Llanos. In westernmost Colombia the compression that leads to the formation of the Andes started during the Cretaceous with a first accretion occurring during the Aptian, Amaine terrain, now Western Cordillera. Subsidence continued eastward with the deposition of the Une, Gacheta and Guadalupe Fms which pinchout in the Llanos Orientales (Casero et al., 1997; Sarmiento, 2001). Subsequent accretion took place in the west at the end of the Maastrichtian resulting in the onset of uplift for the Central Cordillera and the first inversion in the area of present day Magdalena Valley. In the Llanos subsidence continued in a very weakly tectonized basin (Barco, Cuervos and Mirador Fms deposits – Cooper et al., 1995, Bayona et al., 2008). A compressive context predominated thereafter in the Eastern Cordillera the Eocene onwards. After the inversion of the Mesozoic extensional

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grabens, a more classical thrust regime developed, the Llanos basin became a classical, but rather external, foreland (Carbonera and Leon Fms deposits). The Eastern Cordillera is a double verging mountain belt: the western flank in the Magdalena Valley was initially the more active one but the compressive front has now shifted to the Llanos where the Miocene foreland is currently affected by the compression that propagates eastward. As a result, the Guayabo Fm deposits (late Miocene to quaternary) are deformed and partially eroded westward (Parra et al., 2008). The easternmost prominent thrust is the Yopal one in the central Llanos and the structure that corresponds to the former Early Cretaceous basin edge is the Guaicaramo thrust (Bayona et al., 2008). Petroleum system Numerous source rocks are present, most of them shales, (Moretti et al., 2009). There are also numerous reservoirs, all of them sandstone, the most productive ones being the Une, the Guadalupe, the Mirador and the Carbonera C7 Fms. The producing structures are the noses of subtle faulted blocks that affect the Upper Cretaceous and Tertiary series (Ramon et al., 2006). The small offsets on these normal faults result in small size fields. On the contrary, near or within the foothills, large compressive structures have been successfully drilled and host almost all the large fields of the area (e.g. Cusiana, Cupiagua, Volcanera). Eastward, over the wedge of the Oligocene-early Miocene basin, non structural traps have also been successfully drilled; they may contain large reserves (e.g. Rubiales). Structural style The main features of the foreland such as the progressive thinning of the formations eastward have been known from sometime and have been described in the regional synthesis by Ecopetrol and Beicip (1995). The cross-section in figure 3 is extracted from this report and highlighted the pinchout of the series. At that time, only 2D seismic data was available. The display of this long cross section on a regular paper sheet could only be done with a very large vertical exaggeration (more than 5 here), that may give an erroneous idea of the fault dip. In addition 2D data are inadequate to study fault pattern. This paper aims to discuss few facts that the recent 3D seismic acquisition in the Llanos has put in front line. Fig 3: Cross-section modified from Ecopetrol-Beicip (1995), see location in figure 1

Before discussing the kinematics of the fault network let us see first how the faults look like in 3D. The seismic images shown are extracted from the CEPSA 3D seismic data on two areas, one near the foothills that we will called here Cauchos and one central, Caracara (see figure 1). Fault trend Three main trends of faults are visible almost everywhere in the Llanos. The faults for Cauchos and Caracara study areas are shown in figure 4. The seismic data were in time when interpreted and the time to depth conversion is approximate, so the dip of the faults on the Stereo diagram is also approximate (in addition, as usual in this type of representation, what is represented is a plane that averages the fault). The N50 (±10) trend corresponds to the normal faults in this central area, than can be either east or west-dipping. They are called often in the literature antithetic and synthetic, however in this article we will call these families F1E and F1W. The second trend observed regionaly has a N-S direction; it corresponds to faults within the Paleozoic that affect the Tertiary. The two other azimuths (E-W and NWSE) are less numerous, they correspond to faults within the Paleozoic, only some of them extend to the Cretaceous layers. Caracara

F1E Main normal faults, east-dipping

F1w: West-dipping normal faults

South Caucho

Fig 4: Stereo diagram of the faults in the sedimentary cover

The first exploration targets in the Llanos Orientales are the footwalls of the F1-family normal faults, i.e. prospect with structural closure and lateral seal along the F1E faults. Fig 5 shows one of these faults bordering a producing field in the Caracara block. In this specific case the offset is about 30 ms at the top of the C7. The time slice in the coherency cube allows arresting the horizontal extension of the fault. One may note that the top basement (strong reflector below the

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[Structural style and decollement levels in the Llanos basin (Colombia)]

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time slice) is not affected by the main normal fault (F1E family) and that a small anthithetic fault, against the main one is visible. Together they border a small graben and merge more or less at the top of the Paleozoic.

Guayabo Fm. Leon Fm. Carbonera Fm. Une Fm. Paleozoic

Fig 5: Example of a F1E normal fault. Seismic data PSTM type. Time slice in the coherency cube near top Paleozoic.

The Fig 6 illustrates the problems faced by the interpreters in the area. The seismic data shown is an inline through a Pre Stack Depth Migrated cube. The display is done without vertical exageration. From top to bottom, the different seismic facies of the area may be observed: • the Guayabo Fm., foreland deposit with a lot of conglomerate resulting in variable seismic characters. • the Leon shale, rather transparent, obvious at large scale but with a top and a bottom difficult to pick in detail (the dark horizon is a good intra-Leon Shale seismic marker), • the Carbonera Fms, the beige horizon, corresponds to the C7, one of the main reservoirs in the area; below, the Mirador is a better reservoir but not a good seismic marker. • The Paleocene (Barco Cuervo Fms) is not pickable without well calibration. • The Cretaceous layers end with the Une sand that marks well on the seismic, it is, in this case, very near the top Paleozoic. • The base of the Paleozoic, so called basement, is well marked by the onlaping of the overlaying series eastward. Two F1_family normal faults are visible on this line, the larger one, about 140 m of offset at the level of the Une Fm, belongs to the F1E family, and affects more or less all the sedimentary pile (even it could be discussed how). The small one which belongs the F1w family, affects the Cretaceous and the Paleocene but not the Oligo-Miocene Carbonera Fms

Fig 6: Resume of the problems arised by the normal fault interpretation. Inline of a 3D seismic PSDM, display 1x1. Depth value in meters (x10), datum is not 0.

On the non-interpreted image on Fig 6, the reader may convince it self that the link between the normal faults in the Mid-Cretaceous-Mid-Miocene sequence (Une to top Carbonera Fms) with the accidents within the Paleozoic and the faults above the Leon Shale is far from trivial. In the upper part of the section, the top Leon Shale is not affected by the normal faulting and a slight fold may be interpreted. Below the Leon reflector, a single fault connecting the normal fault segments could be draw. This single fault may terminate within the upper part of the Paleozoic and an inverse fault may be drawn from the basement up to the Paleozoic, although other possibilities are feasible. In general the fact that the larger normal faults are linked to something below is systematic in the Llanos. Disharmony within the Leon Shale are commun, above it fault extension may be unclear; However some normal faults, as shown on figure 5, also extend through and affect the Guayabo Fm. Near the Cordillerra, the deformation could be more clearly compressive above the Leon shale, the Guayabo deposit slightly folded and eroded in surface. Westward, but East of the Yopal thrust, inverse faults through the Carbonera exist, they usually detach on the C6 and C8 shales. Fault dip Faults constructed with interpretation as the dashed line figure 6 are highly segmented, that is in line with their

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small offset, and present dip values changing with depth. The mapping of the local dip values on the fault segments surface for the same South Cauchos area is shown figure 7. The lowest values (between 35 and 45º) correspond to the Leon Shale interval. Values around 60º (dark blue) are rather systematic in the Carbonera Fm. Dip values increase downward in the Cretaceous; on Fig 7, the displayed horizon is near (above) top Paleozoic. Low values across shale are in compliance with the theory, which relates the fault dip to the internal shear angle of the material. The regular 60º in the Carbonera Fms is to be memorized since explorationist may have higher numbers in mind. The higher angle in the deepest series could be due to the higher level of compaction of the oldest series but result often, as in the case shown figure 6, of the choice of the interpreter. The seismic shows a zigzag shape in the upper part of the Paleozoic, it has been here voluntarily dismiss and the signal has been smoothed to get a single normal fault with a high angle in its deepest part. One may either draw two normal faults or prolongate the inverse faults. The Paleozoic faults will not be discussed in this short paper. Let us focus on the fault characteristic in the area of current economic interest (Cretaceous and Tertiary).

rather a string of fault segments, yet or not yet connected. Fig 8a and 8b show an example of such segments, on the time slice three segments are clearly visible, two ends on a preexisting transverse accident, the other tip is just a relay without any fault as shown on the 3D view.

a

b

F

c

3 km 0º

North 20º

40º

60º

80º

Dip

Fig 7: Dip along the fault plane (6 segments on the image) of the F1E family in the South Cauchos area. Interpretation of a PSDM, the inline is the one partially displayed Fig 6. Yellow horizon near top Paleozoic.

Fault segment horizontal extension Field works, in other places of the word, as 3D seismic data here, have the pecularity to result in fault segmentation. Taking into account the small offset of the faults in the Llanos it is not a suprise. The growth of faults by linkage of small fault segments as well as the relationship between offset and fault segment length is well established (see for instance Schlische et al., 1996). Therefore a piece of fault with an offset of 100m can’t have a length of 20 km. However, by optimism (it definitively enlarges the prospects) and in need of additional data, long faults have been initialy drawn in the Llanos. 3D seimic data doesn’t show these long fault but

Fig 8: Example of fault segmentation, with relay and with preexisting accident. Time seismic example from the Caracara area (a) time slice in a coherence cube (b) Geometry in the relay at the top C7 (c) Sketch of the theory left the growth of fault by leakage of small segments and an oblique view of a relay between two segments.

Very often when the decision to acquire an exploration block has to be taken, the explorationists only have 2D seismic data with a rather large spacing. In order to have guide line for fault segment length extrapolation the Llanos data have been reported on the general diagram of the worldwide compilation done by Schlische et al., 1996, the result is shown figure 9. The orange stars correspond to fault segments in the Cauchos area (the fault segments displayed figure 7). The Llanos faults appear to have very similar characteristic to any other fault network. This relationship offset-segment length does not allow defining accurately a fault length knowing an offset. The display of the values is on a log-log scale and therefore the cloud of points is rather large. However it may help to avoid

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[Structural style and decollement levels in the Llanos basin (Colombia)]

unrealistic values. The orange line highlights the 10 km long length for an individual segment. One may note that nowhere a fault longer than 10km has been found with an offset of less than 200 meters.

different sand bodies. Fault activity during the deposition of the reservoirs has implication on the sandy channel location, likely to be found in the relays between the fault segments for instance. Some of the scenarios we may evaluate are. (1) the faults develop during the full Cenozoic due to local relative vertical movements (even differential compaction may create small normal faults) or (2) the faulting starts during Cretaceous and continues up to Late Miocene (remaiming active during the phase of slow subsidence) and is influenced by a farfield stress (3) the faulting only started when the burial became larger during the Guayabo deposit (current “real” foreland). Fig 5 and 6 show that some normal faults affect up to the Upper Miocene series while other ones affect only the Cretaceous and Lower Tertiary Fms. The Leon Fm is a rather thick shale likely to act as a level of dysharmony (see figure 6). Therefore the fact that a fault doesn’t cross the Leon Fm (or any other plastic layers) does not mean that the fault is sealed by this serie either that the serie post date the fault activity. The fact that a fault plane cut the Leon Shale doesn’t means either that the initial fault is post Miocene. Compression propagates eastward since the Cretaceous and affects effectively the area for the last 5 Myr, so inversion of the normal fault and therefore upward propagation of their dips could also take place.

Fig 9: Length of fault segment versus offset, data from the Llanos, orange starts, at the top of the worldwide compilation published by Schlische et al., 1996.

Age of the faults The faults have a normal offset but alternatively, or in addition, by extrapolation of the oblique stress field documented in the Cordillera (Sarmiento et al., 2006) authors have proposed an important strike slip component for the normal faults. Clearly the lack of outcrop doesn’t allow us measuring slickenside and the small amount of deformation in the Llanos complicates all the usual reasoning; however one may note that one of the argument, the high angle of the fault planes, appear to be erroneous. With a dip of 60º, any offset, means a non negligible extension. When (and why) did this extension take place? Estimating the age of normal faulting is the Llanos Orientales is not easy. During the Lower Cretaceous, the Llanos were the external border of the rift and therefore not really affected by the extension (no deposition), then at the end of the Cretaceous, the subsiding area extended eastward and weak normal faulting may have happened without any farfield extensional force (differential vertical movement, gravity gliding...). Since then the Llanos are more or less a foreland, but large subsidence rates only occurred during the last 5 Myr when the compressive front propagates eastward. Knowing the age and cause of faulting has implications for predicting fault density and direction but also to go further in the description of the facies change. The Carbonera reservoirs for instance are channeled, so that even rather close wells may found

Conclusive arguments for the activity of a fault may be deduced from the thickness change between its hangingwall and the footwall. Such a map has been done in the Cauchos area (Fig 10). The results show that: - Some fault directions are already present in the Paleozoic. - During the Late Cretaceous, there is no major tectonic activity neither thickness change in this area - During the Paleocene-Eocene (Barco-Cuervo-Mirador deposit) at least two of the normal faults (segmented as described before) of the F1 family were active. - During the Oligocene, the trend of thickness change rotated to E-W. None of the faults looks particularly active except maybe the north-south one during the second part of the Carbonera (northern part of the block) - Middle Miocene, general N130 thinning of the serie, i.e. current foreland geometry. Reactivation of the N50 fault (F1w and F1E) active during the Paleocene. This analysis of the fault-thickness relationship therefore allows us to conclude that preexisting accidents have been reactivated, some time more than once, during the Tertiary evolution of the Llanos: - Some Paleozoic trend, inactive during the early Cenozoic has been reactivated recently - Some of the F1 family faults, active during the Paleocene, have been reactivated since the Leon shale deposit. We have already published in Moretti et al. (2009) that the Llanos subsidence history varies from North to South (the Guayabo deposits are twice thicker in the North whereas

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the Carbonera Fm is thicker southward). In addition, the Cretaceous series (Une-Gacheta-Guadelupe Fms) are much thicker in the northern part, defining a through oriented more or less N30 that is not visible at the scale of the Cauchos block. So this analysis of the faults done for the Cauchos block in the central Llanos can not be directly extrapoled to the full area and to complete the analysis, regional maps have also be carried out. The regional maps were done at a completely different scale. The mesh on the 3D seismic base maps is 25 m, on the regional maps, it is 1 km. They are in depth and mainly based on well data, so only general trends could be discussed. We have used them mainly to test a hypothesis often suggested in the literature without too much detail (as in Cooper et al., 19995 for instance): the fact that the normal faulting is due to flexure. The why of the normal faults in the Llanos Orientales? In general, normal faults in areas where the farfield stresses are compressive are described as due either to gravity gliding or to bending (extrado). In case of gliding, the vergence of the fault is toward the slope, and so in a foreland to the frontbelt. In case of extrado, in theory the faults are perpendicular to the bedding (case of a bended beam), that may means with a dipping direction at the opposite of the bed dip. In nature, due to heterogeneity of the material, existence of decollement levels and evolution through time, things are not so simple and so geologists often speak about “normal fault due to flexure” without taking decision on the main driving force. Some forelands display mainly normal faults verging to the front belt (Apennines, Bolivia) when others have more faults with the opposite vergence (Cuba). Anyway, if the faults are due to the flexure, they must increase in number (or exhibit larger offset) when the dip of the bed increases and they are expected to be oriented as the beds. So to test the idea of a F1 fault family related to flexure, comparisons have been done between dip values and fault density and between isobath and fault azimuth. On the regional maps, the flexure has been computed today and through time. A multisurfacic 3D restoration has been done using a flexural slip hypothesis but without decompaction (see Moretti et al., 2008 for the algorithms and concepts behind this kind of restauration). Figure 11 shows the results and a comparison with the known fault trend and HC discovery in the Llanos Orientales. The North West limit is the front belt border and dips lower than 1º have been put transparent. Fig 12 shows a zoom at the top of the main base Carbonera reservoirs (C7). Dip values have been higher in the past in the western central and southern part, near the current compressive front, but northward the burial and therefore the increasing flexure is only due to the Guayabo deposit, indeed much thicker in the North of the foreland. Slightly different values could be obtained if a no-horizontal paleobathymetry was taken into account

during the restoration however we are confident on the fact that these restorations show that normal faults exist in all the eastern area that have never been bent North

Intra Leon - C1

C1-Mirador

Gacheta-Top Paleozoic

C1-C4

Mirador-Gacheta

Top Paleozoic-basal uncoformity

Fig 10: Kinematic of fault based on thickness (isochore in ms) change in the Cauchos area. Size of the seismic cube: 23kmx25km. The borders of the maps are badly defined due to the quality of the information and the lack of well calibration. They do not have to be analyzed.

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[Structural style and decollement levels in the Llanos basin (Colombia)]

Dip values End of the Leon Fm deposit

To Day

7

confirm the observation made on the Cauchos Southern block, that some of the faults were active during the Carbonera Fm deposition.

C4

C7 Dip at C1 time

N

Gacheta

N 100 km

Top Paleozoic

Fig 11: Dip of various layer today (rigth) and at the end of the Leon Shale deposits (left). Dip values lower than 1º are transparent.

In terms of fault density, we do not have 3D seismic data over all the Llanos and the number of faults is influenced by the density of the seismic data but even considering the map as qualitative, one may note that there is no relationship between the dip values and the density of large faults. This lack of correlation suggests that the density of faults is due to “some thing else”. Since different stress tensors seem very unlikely, pre-existing heterogeneities are the more natural answer. Another criterion that may help to confirm or weaken the hypotheses of normal fault due to flexural bending is the fault direction. Fig 13 shows fault trend compared to the current isobath of the C7 and to the thickness of the Carbornera Fms. One may note that if the fault trend is often more or less parallel to the isobaths, this is not systematic. The comparison between the fault trend and the Carbonera Fms thickness isovalues show that in the northern part of this central area, both are parallel (and North-South oriented). At that regional scale, one may so

C7 Dip at Leon time

N 100 km

C7 Dip today

N 100 km

Fig 12: Dip versus fault density and direction. The points are the HC Discoveries.

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MORETTI I., MONDRAGON J:C:, GARZON, J:C: and DANIEL J.M.

The conclusions of this analysis on the normal faulting in the Llanos are: - dip values due to the flexure remain low in the Colombian foreland. - the bending could have created some normal faults but have mainly reactivated accidents - the normal fault density is not linked to the dip acquired during the Tertiary subsidence - some normal faults have been active during the Paleocene-Eocene-early Miocene, period of slow sedimentation without any relief, and may have influenced the deposition of the reservoirs. Usually normal faults exist in foreland but there are never so numerous and so crucial for exploration than in Colombia. What are the particularities of the Llanos Orientales as foreland? At least three elements are candidates to play a key role: (1) the pinch out of the sedimentary layers eastward, (2) the fact that the deformation is oblique, (3) the high proportions of shale in the series. Intend to test their influences has been done through analogue modeling.

C7 Depth

Analogue model of transtension-compression in a shaly foreland. Analogue models are known to be an useful tool to test concepts in structural geology. The use of X-ray scanner allows getting a complete 4D view of the deformation (Colletta et al., 1991). 3D scans can be done quickly and therefore are an interesting alternative to numerical models. They have limitation on the materials, roughly only silicone and sand are used but the contrast of mechanical behavior between these two materials is rather adequate to model viscous layers (evaporite, shale...) versus brittle layers. Limitations also exist in term of size of the box when using a Xray scanner (designed for human body) and so a model for the full Llanos will result, due to the scaling factor, in a too thin model to be carried out. We designed an experiment to observe the fault network that may result from an oblique extension followed by a compression in a rather plastic inhomogeneous series (i.e. with couple of silicone levels in a sand box model). It is not a model of a flexure, imposing strain at the border of the model, one considers that farfield horizontal stresses influence the deformation. Heterogeneities in the basement have been introduced through an initial paleogeography. Using the stereo diagram of Fig 4, we have chosen to impose an initial angle of 30º between the basement accident and the final compression. The amount of extension is of 7%, the transtension is dextral (same amount than the extension). The compression is without strike slip component as observed today in the central Llanos (Dimate et al., 2003)

a

N

b

100 km

c

Carbonera Fm thickness

d

e

N 100 km Fig 13: Current geometry and Carbonera Fms thickness versus fault azimuth

Fig 14: Sandbox model evolution versus time

Fig 14 shows the evolution of a cross-section through time and figure 15 a time-slice. The box at the beginning contains at the bottom a level of silicone that allows

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[Structural style and decollement levels in the Llanos basin (Colombia)]

decoupling with the box and the transmission of the strain. The sand and corundum (= sand with another absorption to the X-ray to create apparent layering). A small amount of trantension has been imposed. Then another layer of silicone and a layer of sand have been added (step b), transtension continue (Carbonera time). The extension has been stopped and 2 more layers (silicone + sand) have been added (Leon - Base Guayabo Fms), then compression was imposed (step e). Dark layers are the ones in silicone, the white ones in sand. In the Llanos the intercalation are even more numerous but getting too thin layers of silicone is impossible. The observed deformation is due to the imposed strain at the border of the box and to compaction. The 3D view on figure 13 is taken at stage d, after a small amount of compression. The fault network is to be compared with a time slice near the base of the Carbonera (C7).

Fig 15: 3D view within the sand box. Color code based X-ray absortion; faults are visible due to dilatancy. In the cross-section the white layer consists on silicone, its position is equivalent to one of the Leon Fms in the Llanos.

In figures 14 and 15, one may observe the development of a fault network with two families of opposite vergence. The faults have a “regular” angle and not a high one as observed in the pure shear modeling. The faults are segmented and get connected with curvature of the fault plane at the opposite of the dip, they area locally concave. The connection, or the relay zones, between the segments is different from the one described for the pure extensional context (Fig 8c) but similar to the observations done in the Llanos (Fig 7). On figures 14d and 14e, the disharmony at the level of the upper silicone layer is clear, the compression below is absorbed by the beginning of the inversion of the normal faults whereas folds appear above. In the step d on figure 14, “new” normal fault or increase of normal offset may be noted, there is no extension at this step of the experiment, they are due to differential

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compaction. Comparison between Fig14e and 14d show that the compression changes the fault geometry sometime without clear expression, an interpreter may understate its role when working on a small zone (with the scale factor the cross-sections on figure 14 are on the range of 250 km). Going too far in the comparison between a sand box model and a real geological case study is not legitimate. But the numerous similarity between the observed Llanos fault network and this sand box suggest that the hypothesis of a transtensional regime during the Paleogene will explain rather well the Llanos fault network. Conclusions: The various elements of this structural synthesis of the Llanos Orientales allow us concluding that: - The normal faults of the Llanos are not high angle fault but rather have a very classical .dip of 60º within the Mirador and Carbonera Fms, which decreases in the Leon shale. - The faults are segmented; individual segments are only few kilometers long (range of value 5km, for an offset of 150m) - The N50 oriented faults are common in the central Llanos Orientales, this direction is close to a trend inherited from the Paleozoic. - Many of the faults have a long history that may include activity during the Paleocene-Eocene, quiescence during the Carbonera, reactivation during the Middle Miocene and eventually inversion during the quaternary. - In the central Llanos Orientales, the density and direction of the faults are compatible with a transtensional regime during the Paleogene and a pure compression today. At the opposite there is no clear link between the fault network characteristic and the flexure through time. - Due to the high shale content of the layers, the current compression may propagate eastward and affect a large part of the Llanos resulting in the deformation of the former normal faults even if the first thrust is located westward Disharmony, due to the high shale content, reactivation of faults through time and impact of the Paleozoic inheritance are so the common role in the Llanos Orientales. They should be taken into account when interpreting. Acknowledgments. We thank CEPSA and all the colleagues who worked on the Llanos for their support. We thank also IFP for providing us the analog model. The seismic interpretation was done with SISMAGE (Total) and the structural models with GOCAD-KINE3D (Paradigm); 3D restoration has been done with KINE3D-2. The images recorded by the Xray scanner are processed to be eventually loaded and displayed as a 3D seismic data set within GOCAD.

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MORETTI I., MONDRAGON J:C:, GARZON, J:C: and DANIEL J.M.

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