The distribution of thermogenic, bacterial and ...

Marine and Petroleum Geology 71 (2016) 391e403

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Research paper

The distribution of thermogenic, bacterial and inorganic fluid sources in the petroleum systems of the Llanos Basin (Colombia) e Insights from the noble gases and carbon stable isotopes Felipe Gonzalez-Penagos a, b, d, Virgile Rouchon a, *, Xavier Guichet a, Isabelle Moretti c IFP Energies Nouvelles, 1-4 avenue de Bois-Pr eau, 92852 Rueil-Malmaison, France UPMC, Universit e Paris 6, UMR 7193, ISTEP, 75005 Paris, France c ENGIE-EPI, 1 Place Samuel de Champlain, 92930 Paris La Defense Cedex, France d , Colombia Ecopetrol SA, Cra 7 # 32-42, piso 10, 110311 Bogota a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 October 2015 Accepted 13 November 2015 Available online 20 December 2015

The Colombian Andean foreland is a rich petroleum province, where various source rocks and an active circulation of fresh water are present in the subsurface, resulting in a complex fluid mixture within the reservoirs. Moreover, some of the traps are shallow, and low API gravity oils are found. Massive meteoric water infiltration and local biodegradation have been proposed to explain these observations, but require some in-depth investigation to be confirmed. In order to provide some new insights into the Llanos Basin petroleum system, we performed a natural gas geochemical survey over different areas of the basin, including stable isotopes of hydrocarbon, non-hydrocarbon and noble gases. Results show that the influence of meteoric water infiltration is dominant in the shallow area eastward, but decreases towards the deepest part of the basin westward. A general gas/oil phase separation differentiates gas-depleted biodegraded, shallow reservoirs from deep gas-rich less-altered reservoirs. Data suggest that there is a shallow biodegradation associated with meteoric water circulation in the Carbonera Fm, while some of the deeper heavy oils (Mirador, Une fms) were more likely produced by an early mature source rock. However, biodegraded oil later mixed with less altered oil and associated gas are plausible. This study also indicates a contribution of mantle fluids in the deepest parts of the basin near the contact with the Guaycaramo Fault System. There, older rift sequences may have recorded mantle fluid fluxes, or alternately the presence of diffuse mantle fluxing along the deep-rooted thrust front during the formation of the Llanos Basin. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Llanos basin Gas geochemistry Noble gases Petroleum system Meteoric fluids Biodegradation

1. Introduction The Llanos Basin of Colombia (LBC) is the foreland basin of the Andes, locally represented by the Eastern Cordillera (Fig. 1). Seventy per cent of the Colombian oil is produced in this basin. Traps are both structural, with thrust anticlines in the west (Casero et al., 1997) and normal faults in the foredeep (Moretti et al., 2009a) and stratigraphic such as the eastward pinchout of the series (Gomez et al., 2009). The produced oil presents a wide range of quality (from 10 to 40 API gravity) without any direct correlation with depth except eastward and southward to the Macarena range, where shallow reservoirs contain only heavy oil. These reservoirs

* Corresponding author. E-mail address: [email protected] (V. Rouchon). http://dx.doi.org/10.1016/j.marpetgeo.2015.11.007 0264-8172/© 2015 Elsevier Ltd. All rights reserved.

have been targeted for large stratigraphic traps since the Rubiales field discovery (Gomez et al., 2009). Despite being a rather mature basin (more than 3000 wells have been drilled) with a petroleum system generally well understood (Mora, 2000, Moretti et al., 2009b, Garcia, 2008), the processes of fluid mixing (water, oil and gas interactions), meteoric water infiltration and oil degradation remain poorly understood. The aim of this paper is to provide some new geochemical data and their interpretations that may help to describe these various fluids and discriminate some hypothesis about their respective origins. The geochemistry of natural gas has become a powerful tool for exploration and oil and gas prospect development. Molecular composition, carbon isotopic composition, abundance and isotopic ratios of noble gases (He, Ne, Ar, Kr) accumulated in hydrocarbon reservoirs have been studied to determine the origins, mixtures

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F. Gonzalez-Penagos et al. / Marine and Petroleum Geology 71 (2016) 391e403

Fig. 1. Localization of the Llanos Foreland Basin of Colombia (boundary is the black line), Eastern Cordillera, MV: Magdalena Valley, Guaycaramo Thrust Front (thick red) and oil fields (green). Some other elements referenced in the paper are also shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

and fluid flow history of the LBC. Non-hydrocarbon N2, CO2 and He origins (Rice and Claypool, 1981; Wycherley et al., 1999; Ballentine and Burnard, 2002; Kennedy, 2003; Prinzhofer, 2013), hydrocarbon provenance, organic matter degradation (Lorant et al., 1998), migration and accumulation (Battani et al., 2000; Prinzhofer and Huc, 1995; Prinzhofer et al., 2000), bacterial activity (Rice and Claypool, 1981; Whiticar, 1999; Shurr and Ridgley, 2002) and leakage are processes that can be highlighted by the stable isotopes and the noble gas analyses on natural gas samples. We have characterized the natural gas of several reservoirs, at different depths and representing different oil qualities. The geochemical analyses allow us to track the origins of hydrocarbon and non-hydrocarbon natural gas. The distribution of samples collected allows us to discriminate hypothesis about migration, accumulation, leakage and post accumulation processes such as biodegradation at the basin scale. 2. Context 2.1. Geological setting The tectonic evolution of the northern part of South America is controlled by a complex dynamic interaction among the Nazca, Caribbean and South American Plates. The two last major tectonic phases are the Mesozoic extension and the Tertiary compression, which is still active. During the Early Cretaceous, a regional extensive regime resulted in a deep extensional basin bordered by large normal faults, with synrift sedimentation mainly in marine depositional environments. The eastern edge of sedimentation was limited by the Guaycaramo Fault System, which borders the deeper

depocenter to the current Eastern Cordillera area (Villamil, 1999). Upper Cretaceous post-rift sediments (sag period) were also deposited mainly in a marine environment that extended westward and eastward. In the LBC, these formations lie with an angular unconformity over the folded and eroded Paleozoic sedimentary units (Une, Gacheta and Guadalupe fms) and the pinchout progressively migrates eastward. Since the Maastrichtian, a compressive regime started and caused the progressive emergence of the Central and later the Eastern Cordillera. The formal basin was then inverted with the reactivation of Cretaceous normal faults, followed by the propagation of thrusts westward (above the Magdalena Valley) and eastward toward the Llanos (Colletta et al., 1990; Cooper et al., 1995; Gomez et al., 2005; Cortes et al., 2006; Sarmiento-Rojas et al., 2006; Egbue and Kellogg, 2012). Although the LBC is the current foreland of the Eastern Cordillera, the area was only weakly deformed during the Eocene-Oligocene and recorded at the time complex strike slip tectonics due to the far field Caribbean tectonic evolution. A more classical foreland setting has been ongoing since the Middle Miocene, with a depocentre toward the Cordillera and a progressive eastward thrust front propagation (Moretti et al., 2009a, 2009b). During the Tertiary the sedimentary regime was controlled by continental fluvial to coastal marsh sedimentary environments, with sediments supplied by the Eastern Cordillera from the west and by the Guyana Shield from the east. The depocentre axis was oriented NEeSW, parallel to the Eastern Cordillera (Cooper et al., 1995; Villamil, 1999), and the Mirador and Carbonera fms were deposited on-lapping and pinching out progressively eastward. A Mid-Miocene regional flooding (correlated with the Pebas lake to the south in Peru) covered the full area, the depositional context was fluvial-lacustrine and lagoonal with sporadic


marine incursions of the Leon Fm. (Bayona et al., 2006, 2008, 2013; Bande et al., 2011; Jaramillo et al., 2011). The Middle to Late Miocene abrupt increase of shortening in the Andes resulted in the eastward propagation of the compressive front that generated the fold and thrust structures east from the formal Guaycaramo thrust front of the Eastern Cordillera. In the foredeep, a fast subsidence took place with a molasse deposit (Guyabo Fm. deposit) which is continental. This phase of compression remains currently active. The result is a wedge of siliciclastic sediments pinching out eastward. A sequence of shale (source and top seal rock) and sandstone (reservoirs) interlayers is covered by a thick molassic deposit of the Late Miocene Guayabo Fm. (Fig. 2).

2.2. Main features of the petroleum system Hydrocarbon accumulations are found in several Cretaceous and Tertiary reservoirs. Hydrocarbons are trapped vertically by top seals (shale units along the siliciclastic sequence) and laterally by a series of antithetic normal faults and/or lateral facies variations. Westward, near the Cordillera, thrusting also formed structural traps. The larger fields (such as Cusiana and Cupiaga, Warren et al., 2003) are within the compressive wedge but stratigraphic traps eastward could also contain very large accumulations such as Rubiales (Gomez et al., 2009). Within the central part of the LBC, the short offset normal faults result in a series of medium size, widely distributed oil fields (almost 200 oil fields), localized mainly towards the central part of the basin. Shale units of the sedimentary sequence have valuable organic contents from the Cretaceous up to the Miocene (Moretti et al., 2009b). The Lower Cretaceous (Fomeque Fm.), a marine carbonate, is not present in the LBC but may be the source rock of some of the fluids. Indeed, some of the oils in the central western part contain markers suggesting a carbonate source rock, a feature missing in the current LBC (Mora, 2000). Uncertainties about the seal capacity towards the edge of the basin (mainly east and south) represent a major risk in exploration: (1) normal fault displacement decreases progressively eastward, (2) shaly units lose their top seal capacity becoming coarse grained and thinning. Source rocks are only mature in the west where the

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foreland is thick enough while in the east up-dip hydrocarbon migration took place over long distances (Moretti et al., 2009b). From west to east, oil fields present mainly 4 trap types: (1) in the foothills, thrusts, (2) in the central part of the foreland, small normal faults, (3) inverted structures and finally (4) pinchout and stratigraphic traps in the east. Respective examples of each types are (1) Cusiana and Cupiagua giant fields characterized by light oil, gas and condensates trapped in the fold and thrust foothills (eastern flank of the Eastern Cordillera), (2) Caracara (20 API, ~ o Limon, a deep giant light oil field located in the 73 C), (3) Can northwest of the basin (30 API at a depth of 2.2 km and 97 C, McCullough, 1990) and (4), Rubiales, a shallow heavy oil field toward the southeast (12 API at a depth < 1 km and a reservoir temperature of 40 C, Gomez et al., 2009). Crude oil analysis suggests that oil accumulations are derived from marine anoxic environments and continental sources, and often a large heterogeneous mixture of both sources at various n et al., 2001; degrees of maturation (Dzou et al., 1999; Ramo Aguilera et al., 2010; Emmerton et al., 2013). Heavy and ultraheavy oil accumulations have been found in shallow reservoirs (cooler than 80 C) experiencing biodegradation processes in variable degrees (presence of 25-norhopanes and 25-nortricyclic terpanes. However, heavy oils are also found at depths >3 km (Dzou et al., 1999) where temperatures preclude any bacterial activity.

2.3. Water regime A regime of fresh water circulation is present in the LBC. The average water/oil ratio in the basin is around 60% and can increase up to 95%. Previous studies proposed that an active topographydriven hydrodynamic system partly drives the up-dip hydrocarbon migration, generating hydrodynamic oil entrapment mechanisms (Villegas et al., 1994; Gomez et al., 2009; Person et al., 2012). However, an alternative model has been proposed (Gonzalez et al., 2011; Gonzalez-Penagos, 2013) taking into account the additional source of fresh water within the basin due to the shale diagenetic evolution. The distribution of bicarbonate-dominated waters and stable isotopes (d18O and dD) over the local meteoric water line in

Fig. 2. Generalized stratigraphic column of the siliciclastic sequence of sandstones and shales with the structural cross-section of the foreland monocline dipping westward. Thick deposit of the Guayabo Fm was reduced to highlight the formations were the data were collected.

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the LBC formation waters is an indicator of meteoric water influence (Gonzalez-Penagos et al., 2014). The data show a transitional decrease of meteoric water influence wit depth, where clay dewatering diagenetic processes takes place. 3. Sampling 3.1. Sampling strategy Natural gas production in the LBC oil fields varies widely from almost 0 to 0.1 MCFPD (data from 2012 Mines and Energy Minister of Colombia). The selection of natural gas samples is constrained by the distribution of the wells and by the production interval of a single formation (production from two or more intervals in the same well is common and precludes relevant analyses). Not only the samples were collected from the major hydrocarbon reservoirs, Une, Mirador and Carbonera fms, but also selected at different depths and with variable oil viscosity; these selections took place mainly in the central part of the basin. Because of the pinch-out of the Une and Mirador formations (below 2.4 km), shallow samples are all from the Carbonera Fm. (at a depth between 0.8 km and 3.7 km). All the analytical data for the Une, Mirador and Carbonera fms. are available in the supplementary data table. 3.2. Sampling procedure Samples were collected from the total fluid produced from 21 wells (a water-oil-gas mixture), recovered and stored in situ directly at the wellhead (before any phase separation process), avoiding as much as possible any phase partitioning and atmospheric contamination. Stainless steel cylinders (475 ml) were flushed several minutes, before the exit valve was closed allowing for the pressure to stabilize at well-head values. Samples were collected from Une (7 samples, pressure at the wellhead from 5 to 13 bars), Mirador (10 samples, from 5 to 14 bars) and Carbonera C7 (7 samples, 3-1 bars) fms. Temperature, water/oil ratio, API gravity and gas/oil ratio (GOR) were supplied by operating companies. The sample cylinders were shipped to the IFPEN laboratory and kept in vertical position at room temperature for several days in order to let a headspace accumulate at the top of the cylinders. The fluid pressure in the cylinders was measured by a manometer while transferring a fraction of the gas phase by static decompression to sub-sampling cylinders (stainless steel) adapted for our analytical instrument. The sampling cylinders were checked for both pressure (10 bar N2) and vacuum (102 mbar) leakage in the laboratory before sampling campaigns. However, 3 samples (M2, C-2, C-5) were affected by a relatively high contamination as indicated by important amounts of O2 (2.7e9.3%). The composition of such samples was corrected by subtracting the known air compounds, using the air elemental ratio for N2, He, Ne, Ar and Kr on the basis of the O2 content.

Fig. 3. Gas chromatography results for all samples sorted by formation. Logarithmic scale graphic showing compositional percentages for (a) Carbonera Fm. (green), (b) Mirador Fm. (red) and (c) Une Fm. (Yellow). Thick lines correspond to the deepest samples (>3 km) and dash lines to the shallowest ( Kr). The Une Fm. samples have mildly depleted ADNG (36Ar from 0.2 to 5.9 ppm) with very variable He contents (16e770 ppm). The samples from the Mirador Fm. show two distinct groups: one group

with ADNG compositions close to the air values (36Ar from 26 to 39 ppm) and with high 4He contents (from 130 to 1200 ppm) and the other group with depleted ADNG (36Ar from 0.1 to 0.4 ppm) and with lower 4He contents (14e265 ppm). As for samples from the Carbonera Fm they show various contents in ADNG (36Ar from 1.3 to 25 ppm) and He (37e24000 ppm). The 40Ar/36Ar ratio inversely correlates with the contents in ADNG from atmospheric values (295.5, Nier, 1950) up to 521. Most Une and Mirador samples have 3 He/4He ratios clustering around 2.2107, with the exception of samples M10 (2.0 108) and U-1 (6.0 108). The Carbonera Fm. (and Gadalupe Fm.) samples have variable 3He/4He ratios


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Fig. 8. 20Ne/4He vs R/Ra (3He/4He air-normalized) and three end member mixing lines Radiogenic, SCLM (Gautheron and Moreira, 2002): Sub continental lithospheric mantle and AEW: Air-Equilibrated Water.

ranging from 1.5 108 to near atmospheric values (1.4 106, Ozima and Podosek, 2002). 5. Discussion 5.1. Origins of hydrocarbon natural gas

Fig. 7. (a) Variation of the N2 abundance with depth. (b) 84Kr versus N2. The lines representing the air and the air equilibrated water compositions are represented for reference. (c) 1/36Ar versus N2/36Ar. The air value is shown. The solid line represents the compositional evolution for the composition of a gas phase resulting from the solubility equilibrium between a gas free from noble gases and N2, and an AEW (Air Equilibrated Water) reservoir. The compositional evolution is due to varying gas/AEW ratios, inducing progressive stripping and dilution of the N2 and noble gases from the AEW by the gas phase.

5.1.1. Thermogenic hydrocarbons and bacterial gas The d13C1 isotopic compositions of the deeper (2e5 km) Une and Mirador fms. show a trend of d13C1 values corresponding to a thermogenic origin (42‰ to 32‰). The two deeper samples of the Carbonera Fm. (C-6 and C-7 at depths of 2.4 km and 3.7 km) have lower d13C1 (51‰) compared to the Une and Mirador samples at similar depths (Fig. 6a). Shallower samples of the Carbonera Fm become progressively lighter (d13C1 ¼ 40‰ at 1.6 km to 68‰ at 0.8 km), indicating a bacterial origin (Whiticar, 1999). The molecular composition ratio C1/(C2 þ C3) of gases produced during a thermogenic maturity of organic matter present values below 20, while the gases produced by bacterial activity are characterized by values < 100 (Bernard, 1978, Fig. 6b). Most of the Une and Mirador samples fall in the thermogenic field, while the shallow Carbonera Fm. samples show a trend towards a bacterial origin, yet still in the mixing zone with thermogenic gas (Fig. 6b). In a diagram showing the covariation of the molecular and isotopic compositions of ethane and propane (Lorant et al., 1998), the LBC samples (Fig. 6c) follow a thermogenic maturity trend through open system primary cracking of kerogen for most of the Une, the Mirador and the deep Carbonera formations. The more mature Mirador Fm. samples extend to the secondary cracking domain of oil and gas (Fig. 6c). Other indications for thermogenic maturity and biodegradation relations are shown in a C2/C3 versus C2/i-C4 diagram (Fig. 6d, Bernard, 1978). Most Mirador and Une Fm samples lie on a thermogenic maturity trend through primary cracking, while the shallower Carbonera samples indicate a large contribution of biodegradation related gas as indicated by a large increase of the C2/ C3 ratio. The two deepest samples of the Carbonera Fm correspond to an early mature gas (C2/C3: 0.8e1), corroborating the low d13C1 values for these samples. Individual trends in d13C2 versus d13C3 for the Mirador and Une fms. may reflect different petroleum systems for both reservoir formations (Fig. 6d).

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Fig. 9. (a) d13CO2 vs %CO2 diagram. (b) d13CO2 vs. depth. Fields known for bacterial (B), inorganic (In), organo-mineral (O-M) and thermogenic (T) are shown.

5.2. Origin of non-hydrocarbon natural gas 5.2.1. Nitrogen The nitrogen accumulated in sedimentary basins can be supplied by various sources: the atmosphere (dissolved and transported by meteoric or paleometeoric water), diagenesis and metamorphism (ammonium substitution with potassium in clay minerals) or organic matter degradation (Boyd, 2001, and references therein). Large concentrations of nitrogen are present mainly in the Carbonera Fm. (up to 60%) and Mirador (62%) Fm. samples, while the Une Fm. samples have lower N2 percentages (up to 14%). Nitrogen concentrations in the natural gas of the LBC show a general negative trend with reservoir depths, suggesting a superficial source of N2 (Fig. 7a). A good linear correlation exists between N2 and the ADNG with a near zero origin, as presented for 84Kr (Fig. 7b), with ratios of N2/84Kr ranging between that of the air and that of air equilibrated water (AEW, at 21 C). Preserved atmospheric or meteoric water ratios are strong indicators that the bulk of the nitrogen derives from an atmospheric source, preserved as connate water or supplied by an active or paleo-hydrodynamic system. This is mainly due to the fact that ADNG and molecular nitrogen are inert in most geological environments, which prevents any fractionation of the molecular compositions for these elements. The addition of N2 by deeper sources could be inferred for samples with N2/36Ar above the AEW value, which is observed here for the most N2-depleted samples (Fig. 7c). We can firmly conclude that the nitrogen present in the gas phase of the produced fluids represents the contribution of formation water (meteoric, paleometeoric), with minor deeper sourced contributions in the deeper reservoirs. This result is in good agreement with the high water/oil ratio produced at wellheads in the LBC and the general low GOR (). The ADNG are therefore incorporated in hydrocarbon gas together with air-derived N2, by the stripping of the water-dissolved gas. This stripping follows Henry's law, where:

Ci ¼ Pi KiP; T with Ci the concentration of compound i in the liquid phase, Pi the partial pressure of compound i in the vapour phase, and Ki the coefficient of Henry of compound i, applicable for a defined pressure and temperature. In the case where a gas phase interacts with a water phase previously equilibrated with air, solubility effects will induce the transfer of atmospheric N2 and noble gases from the

water to the gas phase. This process is similar to the one described by Zhou et al. (2005) and Gilfillan et al. (2008) for the noble gases, which is also applicable here to N2. In the case studied here however, gas-water interactions are simple enough not to require the implication of multiple steps of stripping and re-dissolution. Indeed, the ratios of N2/36Ar and 1/36Ar vary along a gas-water equilibrium trend (Fig. 7c), with the exception of samples highly depleted in N2, which might contain a significant non-atmospheric proportion of N2. The variable concentrations and fractionation of the N2-ADNG reflect various degrees of interaction between hydrocarbons and formation water. Two types of end-member samples can be described: (1) the air-like compositions represent very low hydrocarbon/water ratios, with a strong dominance of N2 in the gas phase, with associated high abundances of ADNG. Such samples represent reservoirs with very low GOR; and (2) the N2-ADNG depleted samples reflect high hydrocarbon/water ratios resulting in minor N2 in the gas phase, with associated low abundances of ADNG. Such samples likely reflect reservoirs having a gas cap, or high levels of GOR. As a result, the ratio of N2/CH4, when the N2/ ADNG are comprised between AEW and air, may be used to track the degree of interaction of meteoric (or connate water) with the petroleum system. We can more generally conclude that GOR are very low in most of the Llanos Basin reservoirs, leading to a high N2 contribution at the wellhead. This depletion is correlated with the depth of the sampled reservoirs (Fig. 7a). This implies that during the life of the petroleum system, the hydrocarbon gases are separated from the oil in a process controlled by depth. It is difficult based on our results to conclude if such controlling factors are migration distance, PVT effects, hydrodynamism of the trapping reservoirs, or a combination of those. 5.2.2. Helium Helium is present in variable concentrations and with contrasting isotopic compositions within the LBC. In a 4He/20Ne versus 3 He/4He diagram, we can represent the contribution of the three major end-member reservoirs of noble gases, which are the atmosphere, the mantle and the crust (Fig. 8). Two distinct trends are observed, the first one made of the Carbonera Fm. samples, and the second one made of the bulk of the Mirador and Une fms. samples. The Carbonera Fm. trend corresponds to the mixing between radiogenic (crustal) and atmospheric noble gases, starting with highly radiogenic He (3He/4He ¼ 1.5 108, 4He/20Ne ¼ 5547) up to mixed He origins (3He/4He ¼ 2.4 107, 4He/20Ne ¼ 7.5). The


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mixing is more influenced by mantle fluids. Both groups of samples, which correspond to distinct lithostratigraphic members, have independent He sources (Fig. 8) indicating some stratigraphic compartmentalization of the LBC. The presence of regional stratigraphic seals between the Une-Mirador system and the shallower Carbonera system is therefore inferred. 5.2.3. Carbon dioxide origin The CO2 abundance in natural gas collected in the LBC samples is also highly variable (1.5e32%). The stable isotopes of CO2 carbon (d13CCO2) plotted versus the CO2 content (Fig. 9a) indicate an inorganic to organo-mineral origin of the CO2 for the richest samples, while at lower CO2 abundances, CO2 is likely derived from an organic source. The depth versus d13CCO2 plot (Fig. 9b) shows some distinct evolution for the two groups of (1) the Carbonera Fm. and (2) the Une and Mirador Fm. The Carbonera Fm. shows an isotopic trend with C becoming heavier at shallower depths (from 11‰ to 1.5‰). On the contrary, the Une and Mirador fms. present an isotopic trend with C getting heavier with depth (14 to 3,5‰). Both groups seem to have a common root of thermogenic-derived CO2 to the lighter end (30‰), which is mixed with a heavier CO2 source. A deep inorganic CO2 source could be inferred for the Une and Mirador fms. samples, whether as the decarbonation of sedimentary rocks in the deeper parts of the basin or in the basement as a result of metamorphic reactions; or of a mantle origin that could be correlated with the higher 3He/4He ratio in the Une and Mirador Fm (Fig. 8). The heavier CO2 source in the Carbonera Fm may be related either to the decarbonation of carbonates intervals of the Carbonera Fm itself, or to residual CO2 in shallower biodegraded reservoirs. 5.3. Regional trends and compartmentalization

Fig. 10. Geochemical data versus the distance of the sample locality to the Guaycaramo thrust front (a) C1/C2 compositions indicating gas wetness (b) Ratios of n-butane over isobutene as an indicator of biodegradation (c) R/Ra, representing the contribution of mantle, radiogenic or atmospheric sources.

Mirador and Une fms. trend is similar but slightly shifted towards the mantle end-member, with higher 3He/4He ratios well grouped around 2.2 107, and variable 4He/20Ne ratios from 7 to more than 10,000. A Mirador Fm outlier (M10) displays a purely radiogenic He origin, which is consistent with its remote location from the rest of the Mirador and Une Fm samples, in the northern part of the LBC. Helium in the LBC is therefore controlled in the easternmost and shallower levels (represented by the Carbonera Fm) by the mixing between an atmospheric end member (meteoric or connate water) and a radiogenic production (from the basement rocks and from sedimentary rocks of the LBC). In deeper levels to the west of the LBC, a distinct system of atmosphere/radiogenic He

The distribution of the geochemical characteristics of the fluids shows some symmetry with respect to the Guaycaramo Thrust Fault System (GTFS), which marks the western border of the Llanos Foreland Basin. The purely thermogenic accumulations are located in a “deep fluid” zone closest to the GTFS ( 2 km for the Une, Mirador and Carbonera fms. (Fig. 11). In this zone, the isotopic compositions of He and of hydrocarbon-C indicate some compartmentalization of at least the Une and Mirador fms. from the Carbonera Fm. The Carbonera reservoirs in this zone display a lower maturity, together with a more radiogenic He composition (Fig. 6c, d and 8). The Une and Mirador formations have among each other similar carbon isotopic compositions of methane (Fig. 6b), although the covariations of d13C2 and d13C3 may point to two different source rocks (Fig. 6e). The He isotopic compositions in Une and Mirador are similarly indicating some contribution from mantle fluids, which does not appear in the Carbonera samples (Fig. 8). He contents and the carbon and He isotopic compositions point to the presence of a regional seal between the Mirador and Carbonera formations (Fig. 8). The origin of the mantle contribution in the deeper parts of the deep fluid zone remains unclear. For the Une Fm., a trend indicating a larger proportion of mantle He moving towards the GTFS can be drawn (Fig. 10d). A first hypothesis could be the remains of mantle degassing recorded in the rift sediments predating the foreland basin development (Oxburgh et al., 1986). A second hypothesis could be the presence of mantle fluid fluxes in the GTFS percolating into the lowermost sedimentary units of the foreland basin as observed in the Karakoram fault system (e.g. Klemperer et al., 2013). It is difficult, however, to distinguish between these hypotheses. A more distant zone to the GTFS is characterized by shallow biodegraded oil reservoirs with a strong influence of meteoric

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Fig. 11. Cross section of the structural foreland monocline with three zones defined based on deep fluids, meteoric water infiltration and a transition zone.

fluids (Fig. 10a and b), as shown by the light methane carbon (Fig. 6a and b) and the high proportion of air derived noble gases and nitrogen (Figs. 6c and 7b). At the basin scale, the seal efficiency in the foreland decreases eastward as a result of the thinning of shale formations and fault throw decrease. Previous studies have proposed an active topography-driven recharge of meteoric water to the east flowing down-dip inside the basin (Villegas et al., 1994; Gomez et al., 2009; Person et al., 2012). This hydrodynamic setting would be an important driving mechanism for the decrease of the water/hydrocarbon ratio as evidenced by ADNG and N2 enrichment patterns (Figs. 5 and 7b), inducing either a phase separation of gaseous hydrocarbons due to pressure drop, or water washing during buoyancy driven oil migration against dip-down driven aquifer flow. 6. Conclusions New geochemical analysis of natural gases collected systematically from Une, Mirador and Carbonera formations at different depths allow to address the origins and mixing processes involved in the fluid flow history of the LBC. Three main different zones of influence are proposed based on compositional and isotopic data: a deep reservoir zone (70 km near to the GTFS), and a shallow reservoir zone (>180 km away from the GTFS) separated by a transition zone (70e180 km from the GTFS) characterized by a regional seal inducing little fluid migration. Deep Fluids accumulated in natural gases of the Une and Mirador formations show similar fluid origins and processes. The Une Fm. presents a non-atmospheric source of 3He with depth, related to a mantle source. The d13C values of CO2 for the Une and Mirador formations display inorganic values. Both sources can be associated with the heritage of the Cretaceous rifting phase or to an deep fluid leakage tied to the GTFS. Although heavy oils are present in deep reservoirs, hydrocarbon gas composition and isotopes show a lack of bacterial activity fingerprint at depth. The origin of natural gas accumulations for the Une and Mirador formations is thermogenic, likely representing

two different petroleum systems. The deep Carbonera Fm. samples are characterized by an early mature source rock origin. Shallower samples present a transitional influence towards a meteoric water influence, bacterial methane and biodegradation processes. The abundant nitrogen accumulated in the LBC natural gas was provided by an active meteoric or paleo-meteoric water recharge. This meteoric influence decreases progressively with depth to become marginal below a depth of 3 km. The Transition Zone is characterized by the absence of bacterial methane although the meteoric water influence is noticeable, and increasing towards the south. Acknowledgements Authors are grateful to ICP, Ecopetrol, Cepcolsa, Perenco, Canacol Energy and CandC Energy for allowing us to sample the water production and for providing additional material as complementary information. Anne Battani is thanked for fruitful discussions. le ne We would like to thank most particularly Sonia Noirez and He Vermesse at the geochemistry lab of IFPEN for the analytical work. This work was supported by a Ph.D. project of IFPEN. We are grateful to Barry Katz for the efficient editorial handling. Eugenio Vaz Dos Santos Neto and Mario Guzman are thanked for their constructive reviews. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.marpetgeo.2015.11.007. Appendix A table s1 Natural gas geochemical analyses from the Llanos basin of Colombia. Analytical procedures are described in the text provided in Appendix.


Analytical procedures Chemical composition of the gas: major compounds Analyses for determining the molecular composition of the gases are performed by gas chromatography (GC) on samples contained in vacutainers® and/or stainless steel tubes. The chromatograph is a VARIAN 3800 equipped with several columns in series and three detectors (2 TCD, 1 FID). This chromatograph allows direct injection of a given volume of gas under an injection pressure of 0.1e1 bar and analysis of the following compounds: H2, He, N2, O2, CH4, C2H6, C3H8 and C4H10. Their relative concentrations are calculated after calibration with an external standard that gives the chromatograph's response factors relatively to a given compound (methane in general). The analyses are given with a relative uncertainty (2s) of ±5% for nonhydrocarbon gases (TCD) and less than ±1% for hydrocarbon gases (FID). Each analysis is duplicated and bracketed with blanks. Carbon isotopic compositions The carbon isotopic compositions of the sampled gas were analysed on gas contained in vacutainer® and in stainless steel tubes. The measurement of the isotopic ratio 13C/12C for CO2 is performed on a triple collection mass spectrometer MAT253 (Thermo Fischer) coupled to a gas chromatograph. Hydrocarbon gases from methane to butane are oxidized to CO2 after chromatographic separation by a combustion interface. The instrument is calibrated by measuring an internal reference gas, calibrated itself with a standard of d13CPDB ¼ 40.6‰ (the d notation stands for [d13CPDB ¼ {1000.(13C/12C)ECH-(13C/12C)PDB}/(13C/12C)PDB], PDB being the Pee-Dee belemnite, an international reference standard). Repeatability and accuracy of the analysis of our internal reference allows us to obtain a relative uncertainty on the d13C value of ± 0.2‰ for gases with a CO2 molar fraction of over 1%. The procedure for a sample analysis is as follows: after 4 standard measurements, 3 injections are made from a first sample vacutainer®. After that, 2 standard measurements are performed and 3 injections are made from a second vacutainer® of the same sample. Thus repeatability and reproducibility of our measurements are determined for each sample. The procedure ends with 2 standard measurements. Noble gases composition and the 40Ar/36Ar ratio The noble gases elemental abundances and the 40Ar/36Ar isotopic ratio were determined by quadrupolar mass spectrometry after treatment of the gas sample through an ultra-high vacuum preparation line. Only samples in stainless steel tubes were analysed in order to guarantee a negligible air noble gas contamination after sampling. The ultra-high vacuum line is evacuated down to 109 mbar by the means of three turbomolecular pumps. The inlet part that connects to the sample tube is evacuated under primary vacuum (