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Coupled use of carbon isotopes and noble gas isotopes in the Potiguar basin (Brazil): Fluids migration and mantle influence
Marine and Petroleum Geology 27 (2010) 1273e1284
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Coupled use of carbon isotopes and noble gas isotopes in the Potiguar basin (Brazil): Fluids migration and mantle influence Alain Prinzhofer a, *, Eugenio Vaz Dos Santos Neto b, Anne Battani a a b
Division de Géologie-Géochimie, IFP, 1 and 4, avenue de Bois-Préau, 92852 Rueil-Malmaison Cedex, France PETROBRAS/CENPES/PDEXP/GEOQ, Cidade Universitária-Quadra 7 e Ilha do Fundào 21949-900, Rio de Janeiro, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 March 2009 Received in revised form 20 January 2010 Accepted 5 March 2010 Available online 15 March 2010
We used the carbon isotope ratios of hydrocarbons and CO2, and the proportions of noble gas isotopes of associated gases from several geological provinces of the Potiguar Basin (Brazil) for gas/source rock correlation, and to determine maturity, post-genetic processes (migration, leakage, biodegradation), and to assess the possible interactions between hydrocarbons and surrounding waters. Barriers of permeability at the basin scale, the amount of water interacting with the accumulated hydrocarbons, proportion of meteoric water, and contamination of the fluids by the mantle were quantified for the distinct petroleum systems defined in this basin. Four hydrocarbon provinces have been defined in the Potiguar Basin. The first is located in the southern part of the basin, and the other three are in the northern part, with offshore pods of generation; migration occurred along NEeSW horst and grabben structures. All gases are mainly thermogenic (d13C of methane between 49 and 34, except for biodegraded gases with isotopically lighter methane), generated in the oil window, and only two fields from one of the trends seem to have been significantly biodegraded (d13C of propane up to 5.8 per mil). The source rocks were lacustrine for the gases from the southwest of the basin, and mixed marine/lacustrine for two trends. The third trend, as well as an offshore field, appears to have a lacustrine source rock, and the hydrocarbon generation and accumulation are older than for the other fields. Each of the three trends is generally isolated in terms of fluid migration, except in one location where noble gas isotopes indicate that the Guamaré Trend is invaded by fluids of the Macau Trend. An important contamination by mantle fluids has been confirmed for all the hydrocarbon fields close to the shore (3He/4He ratios up to 1.6 times the atmospheric ratio). The mantle contribution (as high as 20% of total helium) decreases inland. The mantle contribution provides some chronological constraints for the hydrocarbon accumulations, and may have had an influence in hydrocarbon generation, due to an igneous activity-associated influence in the hydrocarbon generation pod. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Petroleum Gas geochemistry Mantle fluids Gas migration Noble gas Isotopes
1. Introduction Geological reconstruction of petroleum systems is performed using different kinds of 2D and 3D basin modeling. For this purpose, more and more accurate geochemical information is required to constrain possible geological scenarios. Various sophisticated geochemical methodologies have emerged to answer some of the issues. Gas geochemistry in exploration has become a parameter as potent as the classical organic geochemistry interpretation, as several important analytical developments have allowed the characterization of major and minor gas
* Corresponding author. E-mail addresses: [email protected] (A. Prinzhofer), eugenioneto@ petrobras.com.br (E.V. Dos Santos Neto), [email protected] (A. Battani). 0264-8172/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2010.03.004
compounds, and correlated isotopic data. The use of stable isotopes (mainly carbon isotopes) has gained interest during the last decade due to the development of high resolution coupled Gas ChromatographyeCombustioneIsotopic Ratios Mass Spectrometry (GCeCeIRMS) instruments, which allows compound specific carbon isotopic analyses (methane, ethane, propane, iso-butane, normal-butane and carbon dioxide) in a single analysis. From the pioneering works of Stahl (1977), Sackett (1978), Schoell (1983) and Faber (1987), several new experimental and geological concepts emerged (Prinzhofer and Huc, 1995; Lorant et al., 1998; Prinzhofer et al., 2000a), giving access to several genetic and post-genetic quantitative interpretations (gas source, carbon isotope maturity, mixing, direction and distance of migration, biodegradation, etc.). In this study, we used trace and ultra-trace amounts of noble gas isotopes to study a basin-side petroleum system. The noble gas system provides valuable information about
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the sources of fluids, and about physical processes, which affect crustal fluids (physical interactions between fluids, diffusion, adsorption/desorption, etc.; Zartman et al., 1961; Bosch and Mazor, 1988; Ballentine and O’Nions, 1994; Battani et al., 2000; Prinzhofer and Battani, 2003). Moreover, we used the occurrence of radiogenic isotopes such as 4He and 40Ar to constrain chronological information about the hydrocarbon fluids in the subsurface. In order to illustrate the coupled use of carbon isotopes and noble gas geochemistry, we document a case study where natural hydrocarbon gas, oil and source rock data are available. The Potiguar Basin in Brazil is the focus of this study. It has been extensively studied geologically, and because there is already a good general understanding of the occurrence of different petroleum systems in the basin: the source rocks are well characterized, as well as their maturity, and the distance from the generating kitchens (petroleum generation area) to the oil accumulations. Some of the fields in the basin are known to be biodegraded, enabling the technique to be validated. The study will attempt to show that as the main reservoir for noble gas isotopes in the sedimentary rocks is the pore water, their concentration and possible fractionation in the hydrocarbons gives indications about the extent of interaction between oil and water.
The correlation between oil accumulation and source rock is complex. It appears that all the fields located in the southwestern part of the basin have hydrocarbons generated from the lacustrine sources of the Pendência Formation (Santos Neto et al., 1990; Trindade et al., 1992), whereas the oil accumulations in the two grabbens of Areia Branca and Guamaré have been generated mainly from marine source rocks of the Alagamar Formation, with a variable contribution of transitional deltaic and lagoonal environments, and with a contribution of the lacustrine Pendência source rocks in some offshore reservoirs (Santos Neto et al., 1990; Trindade et al., 1992; Morais, 2007). Biodegradation has severely affected the oils of the Guamaré Trend and the Belem field (not included in this study). The other oils are not altered. 3. Gas samples e analytical techniques and results The gas samples analyzed in this study come from different geological structures or petroleum systems of the basin. They are all associated with oil, and correspond to dissolved gas in the pressure/ temperature reservoir conditions, and collected as gas phases at the wellheads. Gases from an offshore field were collected in 1998, and were analyzed for chemistry and carbon isotopes, without any noble gas data. Gases from the southwestern part of the basin and from the Areia Branca Trend were collected in 1998 and 2001 for analysis of their chemistry, carbon isotopes, and noble gas concentrations of helium, neon and argon. Isotopic ratios of helium and argon were also measured. Gases from the two other trends of Macau and Guamaré were collected in 2003, and analyzed for the whole spectrum of noble gases (He, Ne, Ar, Kr, Xe), as well as whole chemistry and carbon isotopes. 10 cc VacutainerÒ glass tubes for gas sampling were conditioned with a secondary vacuum of the tubes. Injection of the gas during sampling, and withdrawal during analyses was performed with gas syringes through a septum. Samples in these glass tubes were used for molecular chemistry and carbon isotopes analyses. The gas was collected in stainless steel tubing for noble gas isotopic and chemical analyses, and isolated with helium proof valves. The collected volume was 20 cc with an absolute pressure of 3.5 bars. All the chemical analyses relevant to hydrocarbons and nonhydrocarbon proportions were performed with a gas chromatograph
2. Geological background The Potiguar Basin is the most northeastern basin of the Brazilian equatorial margin, and the second largest oil producing basin in Brazil after the Campos. It consists of onshore and offshore oil and gas fields, with a range of source rocks origins and compositions (Santos Neto et al., 1990; Trindade et al., 1992; Fig. 1). After a rift stage where lacustrine and deltaic Neocomian source rocks (Pendência Formation) were deposited (d13C of the source rocks around 29&), a transitional stage occurred, characterized as a drift stage, where organic matter is found in lagoonal deposits (Alagamar Formation) with different levels of marine influence (d13Csource around 24&). The hydrocarbon deposits occur inland along northwest-southeast trends, associated with offshore kitchens, and in the southern part of the basin within grabbens (Santos Neto et al., 1990).
SOUTH AMERICA
ca ran aB i e Ar D
E
F
G
A
BR A ZI L
W Offshore Potiguar
M
aré am u G
25 km
C B NATAL
South Potiguar Basement Studied well s
M
K
I L
H
Sediments
Studied wells
N
O
Q
J
S
Studied oil fields
10 km
Other oil f ie l ds
R
P
Ocean
T V
U
10 km
Dir ect ion of migr ati on
Areia Branca
Macau Trend + Guamaré Trend
Fig. 1. General map of the Potiguar Basin, with the detailed locations and names of the sampled wells in Areia Branca, Macau, Guamaré trends and in the offshore Potiguar field.
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Varian, using three columns and two detectors (FID and TCD). Carbon isotope analyses were obtained through a Fisons Optima mass spectrometer, coupled with a Fisons GC and a combustion interface (GCeCeIRMS), in order to measure the d13C values for all hydrocarbons from methane to butane, and CO2. Noble gas concentrations and isotopic ratios are analyzed with a GV5400 mass spectrometer. The apparatus is connected to a preparation line which separates and purifies the noble gas family from the other gas molecules (hydrocarbons, CO2, N2, H2, etc.). The analyses are performed in two sets, helium and neon were measured first, and then, after a new tuning of the mass spectrometer, argon, krypton and xenon are analyzed. 4. Results and discussion 4.1. Bulk chemistry and carbon isotopes Twenty wells from seven fields were sampled from the southwestern part of the basin. Nineteen wells from six fields from the Areia Branca Trend, four wells from different fields from the Macau Trend, five wells from different fields from the Guamaré Trend and five wells from the same field from offshore Potiguar. The results of the chemical analysis of the gases, as well as the carbon isotopic ratios from the different areas of the basin, are presented Table 1, and the approximate location of the fields at the basin scale is presented Fig. 1. Plotted in the diagram C2/C3 versus C2/iC4 (Prinzhofer et al., 2000b), the data set shows a well defined trend of maturity, except for two gases from the Guamaré Trend which suffered strong biodegradation (Fig. 2). The gases from the Macau Trend and from the Guamaré Trend show lower maturities than most of the gases from Areia Branca and from the south of the basin, whose range of maturity is very large. The most mature gases are found in the southwestern part of the basin. When plotted on a C2eC3 diagram (Fig. 3; Lorant et al., 1998), the two biodegraded gas samples from the Guamaré are out of the expected normal maturity trend, due to the partial alteration of propane. The other gases are mainly located in the oil window, with a trend corresponding to primary cracking in an open system (Fig. 3). Only some offshore gases appear to show a beginning of secondary cracking of the associated oil. It seems also that the petroleum system from the Macau Trend behaves more like a closed system, the other hydrocarbon provinces acting with a more open signature (Lorant et al., 1998). d13C profiles for gases from the different areas (Fig. 4) corroborate the idea that biodegradation occurred mainly in two gases (samples R and T) from the Guamaré Trend, as observed already in Fig. 2 from their molecular chemistry. This is confirmed by their carbon isotopic ratios, as propane and normal-butane become anomalously heavy, due to their preferential alteration during biodegradation (James and Burns, 1984). The residue is enriched in the heavier isotope 13C, as 12C is more readily mobilized. The d13C patterns of most other samples show very homogeneous signatures for offshore gases, and for the Macau and Areia Branca trends, a slightly larger spread for gases located in the southwestern part of the basin. If we try to consider the carbon isotopic signature of isobutane as the closest relative to the signature of the source rocks, because it is the heaviest measured hydrocarbon in the gas phase and because it is relatively insensitive to biodegradation, we observe different average signatures according to the observed areas, which may be related to different source signatures. Correlating the d13C of methane, ethane and propane (Fig. 5), it appears that the correlation between ethane and propane is linear (Fig. 5a), except for the two biodegraded gases of the Guamaré Trend (samples R and T), and shifted to heavier d13C values of propane. The main spread of data corresponds to either a possible source
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heterogeneity or a maturity trend. The difference of the slope between these two trends is very small, as source heterogeneity will not fractionate preferentially ethane versus propane, and the trend will have a slope of one. Fractionation due to maturity fractionates slightly more ethane relative to propane, as ethane is a smaller molecule. Even if the distinction between these two trends is not possible (Fig. 5), it is probable that both processes do occur, as we know from similar studies of oil (Santos Neto et al., 1990; Trindade et al., 1992) that two main source rocks of the basin have distinct carbon isotopic signatures (the lacustrine source rocks of Pendência Formation are isotopically lighter, averaging ca. 29&, relative to the marine/transitional marine source rocks of Alagamar Formation which are ca. 24&). Combining the relative maturity presented from the molecular diagram (Fig. 2), and the isotopic results (Fig. 5a), offshore gases are the most mature, and could be corrected to lighter d13C in order to compare with the other series in terms of source rock correlation. Gases from the Macau Trend indicate very low maturities, and should then be compared to others with slightly heavier d13C values. With this assumption, we obtain similar d13C signatures for the sources of the Macau Trend and of the offshore field, around 30&, and d13C for the sources of the other gas families (Areia Branca, Guamaré and Potiguar Southwest) around 29&. This would indicate that the offshore field and the Macau Trend come from a related transitional source rock. In contrast, the two families with the heaviest d13C for iso-butane are the trends of Areia Branca and Guamaré. These two families of hydrocarbons would preferentially be generated from marine source rocks (Alagamar Formation), as has been confirmed from biomarker studies of the oils (Santos Neto et al., 1990; Trindade et al., 1992). The fields located in the southwestern part of the basin have different organic matter depositional environments and a distinct lacustrine imprint, related to the Pendência Formation. The plot of the carbon isotopes of C1 and C2 (Fig. 5b) is scattered. Gases from Areia Branca and from the offshore W field exhibit a relatively regular trend of maturity, whereas the gases from the other areas are systematically shifted to isotopically lighter methane. One possibility is a post-genetic fractionation effect, partly due to migration, and also to a possible slight bacterial contribution of methane. We know that for the series of samples in Areia Branca, Macau and Guamaré, the associated hydrocarbon kitchens are located offshore (Trindade et al., 1992). The offshore field W is close to the same hydrocarbon generation pod, whereas the fields AeG, located southwest of the basin, have their hydrocarbons generated locally, close to the reservoirs (Trindade et al., 1992). Comparing the composition of the gases versus the offshore limit, one may check some key parameters and see if they indicate a fractionation linked to the migrating processes. Fig. 6 shows the C1/C2 ratios for the data set. It is observed that both families (W and A to G) which underwent small migration distances have relatively constant C1/ C2 values. In contrast, gases from three trends of Guamaré (ReV), Macau (NeQ) and Areia Branca (HeM) show evidence for large chemical fractionations. Guamaré and Macau trends have increasing C1/C2 ratios, increasing with the hydrocarbon’s distance of migration from the offshore kitchen, whereas gases associated with the Areia Branca Trend show an inverse pattern with the distance from the offshore hydrocarbon kitchen. Methane should be enriched during migration because it is lighter and smaller than ethane. This is what is observed for the Macau and Guamaré trends. For the Areia Branca Trend, the reverse trend may be caused by a partial leakage out of the reservoirs with leakage increasing with the distance from the kitchen to smaller depths of accumulation. The fact that no isotopic fractionation of methane seems to correlate with this process favors a process of tertiary migration (or dismigration) through phase changes, rather than a partial leakage
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Table 1 Carbon isotope composition of hydrocarbon gas molecules and CO2 expressed in per mil compared to the Pee Dee Belemnite standard (PDB), and chemical composition, in mol/mol percent, of the gases analyzed in this study. The uncertainties in the carbon isotope compositions are 0.3 per mil (2s), and 5% relative on the chemical proportions. (a) Gases analyzed from the southwestern part of the Potiguar Basin. (b) Gases analyzed from the Areia Branca Trend. (c) Gases analyzed from the Macau and Guamaré trends and from the offshore field. (a) Potiguar South-West Well 1a: 1998 sampling Well 1b: 2001 sampling
(b) Areia Branca Trend Well 3a: 1998 sampling Well 3b: 2001 sampling
Well 1a: 1998 sampling Well 1b: 2001 sampling
(c) Macau Trend
Guamaré Trend
Offshore Potiguar
d13X2
d13X3
d13iX4
d13nC4
31.0 31.2 29.7 30.9 30.0 30.6 30.1 30.2 29.6 29.3 30.2 33.4 36.0 33.2 26.3 30.5 30.4 30.3 33.4 30.8
28.9 28.7 27.4 27.9 27.4 28.4 27.5 27.8 27.1 27.7 27.8 30.8 32.1 29.9 25.2 26.4 27.4 27.0 28.3 28.1
29.4 29.7 28.2 31.4 29.5 28.2 27.7 29.0 29.7 29.0 27.1 31.2 32.8 32.1 26.4 28.4 28.5 28.5 29.3 25.4
28.2 28.8 27.3 28.2 28.4 27.0 26.9 27.5 27.4 26.9 28.3 29.5 32.1 30.1 26.8 26.0 26.9 25.8 28.2 28.0
0.25 0.71 0.26
40.0 39.0 39.8 39.1 38.5 39.8 40.3 40.2 38.4 39.9 39.4 39.6 40.4 40.2 39.1 43.8 39.5 39.7 39.0
30.0 29.9 29.9 29.5 29.5 30.2 29.8 30.1 30.7 29.4 29.5 29.4 30.1 30.3 29.6 29.3 30.1 29.4 29.6
27.3 27.6 27.5 27.3 28.3 27.4 27.1 27.9 28.7 26.8 27.0 27.0 27.7 28.3 27.2 26.8 26.3 26.8 26.9
27.9 28.2 28.3 28.8 28.8 27.7 28.5 27.6 29.0 28.5 28.1 28.7 28.5 30.6 25.3 28.1 28.1 28.2 27.2
26.1 26.3 26.8 26.6 27.4 25.7 26.2 26.0 27.2 26.0 25.5 26.2 26.1 28.0 26.9 25.6 26.0 25.6 26.2
0.29 1.83 2.46 4.07 13.74 12.41 10.22 56.41 20.75 2.59 0.33 2.81 2.41 2.77
50.5 45.2 49.1 60.1 49.3 55.8 47.6 47.8 45.7 40.4 44.1 40.4 40.2 40.6
30.6 29.9 34.2
27.5 27.5 27.1
33.1 33.1
30.9 30.9
26.6 27.9 26.9 30.2 34.2 31.6 35.8 31.5 31.4 31.8
5.8 27.0 15.6 24.9 30.3 29.3 32.4 29.1 29.1 29.1
27.8 28.4 29.7
21.0 26.3 22.5 23.4 24.8 27.3 32.3 28.0 27.7 28.0
C3
84.91 80.98 91.46 85.79 87.17 84.83 77.59 78.97 75.74 86.20 89.48 87.40 81.39 76.26 70.92 77.37 73.27 89.78 80.46 66.73
10.00 10.98 5.67 8.94 7.02 8.58 13.05 12.92 11.00 7.97 6.60 6.31 9.64 14.03 14.45 12.17 15.31 3.12 7.35 13.32
3.82 5.14 2.05 3.94 3.63 4.70 6.65 5.96 8.07 4.04 2.96 4.51 5.31 7.20 8.40 7.00 8.64 4.10 6.67 10.53
0.62 0.97 0.46 0.69 1.16 0.96 1.24 1.05 2.72 0.87 0.53 0.95 0.62 1.00 1.31 1.40 1.20 0.91 1.91 2.37
0.64 1.40 0.37 0.63 1.03 0.92 1.47 1.10 2.47 0.93 0.43 0.84 1.31 1.22 2.44 2.06 1.58 1.46 1.79 3.67
Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field
H, well 1 H, well 2 H, well 3a* H, well 3b* H, well 4 H, well 5 H, well 6 I J K, well 1 K, well 2 K, well 3 L, well 1a* L, well 1b* L, well 2 M, well 1 M, well 2 M, well 3 M, well 4
75.94 78.44 77.10 77.85 79.27 65.23 75.69 82.07 58.48 48.26 26.59 70.11 68.06 84.14 75.31 1.01 14.45 4.63 19.73
13.25 12.71 11.60 11.73 10.78 14.94 12.51 11.14 17.75 19.93 19.14 14.00 16.14 7.35 14.36 7.05 23.67 22.64 28.40
6.65 6.26 6.71 5.68 6.73 12.21 6.46 4.53 15.48 19.04 31.80 8.73 10.44 3.99 6.27 45.95 34.48 45.57 29.86
1.30 1.11 1.50 1.02 1.43 3.37 1.28 0.86 3.23 4.95 11.38 2.01 2.30 1.21 1.08 25.11 10.58 14.66 7.27
1.48 1.25 1.90 1.53 1.79 4.25 1.90 0.91 4.21 5.54 11.08 2.87 2.54 1.51 1.26 20.88 13.38 11.79 8.51
1.39 0.23 1.18 0.81
Field Field Field Field Field Field Field Field Field Field Field Field Field Field
N O P Q R S T U V W, W, W, W, W,
71.02 61.82 96.39 95.93 77.07 60.61 80.99 42.65 78.36 81.55 86.73 83.20 79.14 75.52
7.72 3.74 0.74 0.02 5.58 6.28 5.70 0.33 0.09 10.54 7.80 10.25 10.73 11.98
8.15 9.99 0.42
4.82 6.33
5.93 9.25
0.61 13.77 1.03 0.13 0.11 3.77 3.90 2.65 5.59 7.34
2.44 2.10 1.76
0.26 2.41 0.30 0.21 0.34 0.96 0.71 0.67 1.31 1.42
1 2 3 4 5
0.59 0.54 0.41 0.83 0.97
by diffusion through the caprocks. In the southwest of Potiguar (A to G fields) and in the offshore field (W), no specific trend is visible, and data cluster around C1/C2 values of 10. These values suggest that distances of secondary migration are significantly lower than those in the other studied trends, as the C1/C2 fractionation is relatively restricted. 4.2. Noble gases Noble gas isotopes may be classified in two main families: those that are derived from air, and those derived from formation water
nC4
d13X1
C2
A, well 1a* A, well 1b* A, well 2 A, well 3 A, well 4 A, well 5 A, well 6 A, well 7 A, well 8 A, well 9 A, well 10 B, well 1 B, well 2 C D, well 1 D, well 2inf D, well 2sup E F G
well well well well well
iC4
41.8 42.0 45.5 46.1 45.6 46.0 41.7 41.5 44.9 42.9 43.9 45.8 45.4 43.8 33.6 44.6 45.1 49.2 47.4 43.7
C1 Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field
CO2 0.14
0.19 0.29 0.01
0.07 0.03 0.00
0.97 0.49 0.84 0.13 0.10 0.52 0.08 0.15
30.3 32.7 29.8 30.0 30.1
d13XO2 5.5
4.6 15.4
0.5 25.4 10.0
6.1
7.2 5.7 17.5 17.6 20.6 21.3 18.7 18.2 9.3 16.0 9.6 3.6 6.6 5.1 18.1 6.0 17.0 10.3 10.9 13.0 11.7
(here 20Ne, 36Ar, 84Kr and 136Xe). These isotopes may enter a hydrocarbon phase, exchanging with aquifers equilibrated with the atmosphere. The radiogenic isotopes (4He and 40Ar*, where 40 Ar* ¼ the extra amount of 40Ar when removing the atmospheric contribution, with 40Ar/36Ar atmosphere ¼ 295.5), come from the natural radioactive decay of U/Th and K, respectively, in minerals. All these compounds are insensitive to chemical processes, subsequently tracing only different sources and physical processes occurring between phases or diffusive fractionation. All the available noble gas data from the fields in the Potiguar Basin are presented in Table 2.
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R
C2/C3
8
6
Potiguar Southwest
Biodegradation
10
Areia Branca Macau Trend Guamaré Trend Offshore Potiguar
T
4
Maturity
2
S
0 0
5
10
15
20
25
30
C2/iC4 Fig. 2. Diagram C2/C3 versus C2/iC4, showing the effects of increasing maturity and biodegradation. Only two samples from the Guamaré Trend show a clear degradation of gases.
The patterns of air normalized isotopes (20Ne, provide information regarding:
36
Ar,
84
Kr, 136Xe)
(1) The amount of water which interacted with hydrocarbons (a water/hydrocarbon ratio), as the concentrations of airderived isotopes may be directly linked to the amount of water equilibrated with the hydrocarbons. (2) The contribution of mantle noble gas, as these isotopes are present in small amounts in the mantle and have quite different patterns relative to air-equilibrated water. 20Ne concentrations are higher when compared to 36Ar, as are the 136 Xe versus 84Kr values. (3) The different physical processes which fractionate air-derived isotopes according to their mass (from neon to xenon), give different shapes to the patterns. For instance, it is a straightforward procedure to obtain a composition of a water at equilibrium with the atmosphere. The corresponding pattern will depend on the salinity of the water and the exchange temperature, but these variations are of second order (a factor 2 or 3 less) when the concentrations are represented in logarithmic scales (Fig. 7). 5
Primary cracking NSO cracking
0
S -10
gas cracking
n
-15
tio da ra
T
eg od Bi
δ 13C2- δ13C3
Oil cracking
-5
Potiguar Southwest
-20
Areia Branca Macau Trend
R
Guamaré Trend Offshore Potiguar
-25 0
5
10
15
C2/C3 Fig. 3. C2eC3 maturity diagram for all the studied gases. The two biodegraded gases from the Guamaré Trend are shifted from their normal location due to preferential alteration of propane.
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We calculate the equilibrium concentrations of fossil noble gas isotopes in freshwater in one pool represented by Lake Baikal water and in the other in the ocean, normalized to air concentrations (Fig. 8). A hydrocarbon phase equilibrated with such air-equilibration will produce the same pattern because of mass balance, as the major part of the noble gas compounds will be moving into the hydrocarbon phase. Gases from the Guamaré Trend have relatively similar patterns relative to a hydrocarbon phase equilibrated with such recharge aquifer waters (except well T), whereas the hydrocarbon fluids of the Macau Trend have distinct signatures, with a more concave shape (logarithmic scale; Fig. 7b). The abnormal pattern seen in well T from the Guamaré Trend (Fig. 7a) is very similar to the Macau Trend patterns (Fig. 7b). We interpret this similarity as resulting from contamination of fluids from the Macau Trend invading the Guamaré Trend at the level of the well T (Fig. 9). In fact, Lima Neto et al. (1990), describing the equipotentials of the basin, suggested a potential water flow from the Macau area to the Guamaré area, due to higher water potential in the Macau Trend. Associating this information with the possible calculation of water/ hydrocarbon ratios, linked to the absolute amounts of fossil noble gas isotopes in the hydrocarbon fluids, it is possible to model the relative degree of interaction of the hydrocarbons with water, and the water connectivity in the Macau/Guamaré area (Fig. 9). It appears that for the Macau Trend, the interaction with water increases inland, i.e. going further from the kitchen. For the Guamaré Trend, the interaction with water seems to be less well behaved, with a maximum of interaction around the field S. All the gases from the Macau Trend have higher 4He concentrations relative to the gases from the Guamaré Trend (Fig. 10). This is consistent with the curvature of the fossil isotope patterns, showing a relative enrichment in the light isotopes (36Ar and 20Ne). As it is impossible to mimic the patterns of fossil isotopes with a simple equilibration with atmosphere equilibrated aquifers, we interpret the patterns observed for the Macau Trend to be affected by a mantle contribution. In the Guamaré Trend, this possible mantle contribution is not visible for the light noble gas concentrations, but maybe seen in the ratios 84Kr/136Xe, which are smaller than expected from an air-equilibrated water source. Comparing the two radiogenic isotopes 4He and 40Ar*, we observe contrasting behaviors between the different analyzed families (Fig. 11). The hydrocarbons from the Guamaré Trend have very small concentrations of radiogenic isotopes, indicating a short residence time, without any contribution by old fluids (water and oil). The gases from the southwest of Potiguar also contain relatively small concentrations of 4He and 40Ar*. In contrast, the gases from the Areia Branca and Macau trends present good correlations between these two isotopes, but with different slopes. The average ratio of 4He/40Ar* in sediments is 5.9 (Steiger and Jager, 1977; Taylor and McLennan, 1985). The mobility of these radiogenic isotopes in the fluids tends to average their relative concentrations. The hydrocarbons from the Areia Branca Trend have a 4He/40Ar* ratio that is low compared to this average ratio, whereas the hydrocarbons from the Macau Trend have ratios slightly higher than the average production rate. Differential 40Ar expulsion from the crystal lattice of minerals has been used to explain the high apparent production ratios of 4He/40Ar* in sediments (Elliot et al., 1993; Prinzhofer, 2002). However, this may only be a minor factor in the Macau samples; the main abnormal 4He/40Ar ratio originates from their low value in the Areia Branca Trend. The hydrocarbon gas chemistry (Fig. 6) showed that the hydrocarbons of the Areia Branca Trend suffered drastic partial leakage due to tertiary migration. The deficit of 4He compared to 40Ar* supports this interpretation of a tertiary migration and partial leakage of hydrocarbons from the Areia Branca Trend. Helium would be enriched in the leaking light phase compared to argon, in the same
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A. Prinzhofer et al. / Marine and Petroleum Geology 27 (2010) 1273e1284
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
Fig. 4. d13C patterns for the gases from the different oil and gas provinces.
way that methane was leaking more efficiently relative to ethane. It is important to notice that for the other corresponding rift system of Macau, no similar trend of leakage is visible, indicating probably much better caprocks in the southeastern part of the basin. The 3He/4He ratios of the analyzed hydrocarbon fluids suggest an important mantle contribution. In fact, representing these isotope ratios normalized to the atmospheric value (in R/Ra
-20
ity gene tero e h rce Sou nd ity tre matur
T
-33
P
-25 -30
rit Matu -35 -37
y
erog ce h e t + sour
eneity
S
O
N
-31
tur ity
T physic al f & bio ractionatio degra n dation
Ma
S
-29
δ 13 C2
-15
-27
tion
-10
R
R Biodeg rada
-5
13 δ C3
b -25
0
Biodegradation
a
notation: 3He/4Heatm ¼ Ra ¼ 1.4 106), the oceanic upper mantle has a signature of approximately 8 Ra, the subcontinental mantle a signature of about 6 Ra, whereas the continental crust, richer in radiogenic 4He, has ratios of about 0.02 Ra. Values above 0.02 Ra for crustal fluids may be interpreted either as air-contaminated, or mantle-mixed. In our case, the fact that some helium isotopic ratios are above 1 Ra makes air contamination unlikely. We interpret that
-35
-37 -35
-33
-31
-29
-2 7
-25
-65
-60
-55
-50
-45
-40
-35
-30
δ 13C1
δ 13 C2 Potiguar Southwest Areia Branca Macau Trend Guamaré Trend Offshore Potiguar
Fig. 5. Graphs showing the correlation between the carbon isotopes of ethane and propane (5a) and of methane and ethane (5b).
A. Prinzhofer et al. / Marine and Petroleum Geology 27 (2010) 1273e1284
10000
Q
1000
V
C1/C2
P U
100
E
N W
10
n
Kitchen 2
Mig ratio
Kitchen 1
R
T
H
B
C
D G
e kag
K
1
A
Lea
J S L
O
F
M 0.1 -40
-20
0
20
40
60
80
Distance from the shore (km) Potiguar Southwest Areia Branca Macau Trend Guamaré Trend Offshore Potiguar
Fig. 6. C1/C2 molecular values for the gases of Potiguar, versus the distance from the modern shoreline.
these helium isotopic ratios are representative of the crustal fluids that have been affected by interactions with mantle fluids. The gases from the Macau Trend are the most contaminated, and reach ratios as high as 1.6 Ra, which means that more than 20% of the helium came from the mantle. This mantle contamination mainly affects the gases located on the three geologic trends of Areia Branca, Macau and Guamaré (Prinzhofer et al., 2004). The mantle contribution indicated by the 3He/4He ratio decreases inland, and all the fluids of the southwestern province of Potiguar are mantle contamination free (Fig. 12). Oxburgh et al. (1986) and O’Nions and Oxburgh (1988) found that mantle fluid contamination occurs in sedimentary basins during rifting. Volcanic activity in the vicinity of the Brazilian passive margin from 90 million years to the present
1279
(Mizusaki et al., 2002) does not seem to be the source for this mantle contribution, as the occurrence of mantle helium seems to occur along the entire margin. This implies that either a riftingrelated mantle fluid remains in the margin sediments since ocean opening, or that mantle helium memory is retained after the rifting for at least until the hydrocarbons have been generated and accumulated (several tens of millions years). If we try to correlate the concentrations of 3He and 4He (Fig. 13a), a mixture between two end-members should be represented as a straight line. It appears that any mixture between a mantle and a crustal end-member should present a straight line with a negative slope, as a mantle end-member would be enriched in 3He, and a crustal end-member enriched in 4He. All the analyzed gases from the three trends of Areia Branca, Macau and Guamaré, however, have a linear trend with a positive slope intermediate between mantle and crust values, without any evidence of a mixing trend between a mantle and a crustal end-member. The concentrations of helium (both 3He and 4He) may be affected by a preferential leakage of this compound versus the main components (oil and gas compounds), giving a correlated decrease of concentration (Fig. 13). However, as the variation of helium concentrations is high (approximately 500), from the higher to smaller helium concentrations, it is very unlikely that such a fractionating process would decrease the helium concentrations of more than two or three times lower than the maximum. In order to try to explain this positive correlation, we need to envision a two step process: first, mixing between mantle and crustal fluids, giving an intermediate 3 He/4He, and then a mixing of this homogenized source with an end-member close to the origin. This end-member is probably the atmosphere, or more precisely water equilibrated with the atmosphere, i.e. meteoric water. If this is the case, for each petroleum province (Areia Branca, Macau and Guamaré trends), the concentrations of 4He (or 3He) are a direct function of the proportion of meteoric water involved in the system. This makes possible a simple calculation of this proportion of meteoric water for each of these areas (Fig. 14). The Areia Branca Trend presents a smooth
Table 2 Noble gas chemical and isotopic compositions of the associated gases from the Potiguar Basin. The concentrations are expressed in mol/mol ppm, the helium isotopic ratio 3 He/4He is normalized to the atmospheric value (R/Ra with Ra ¼ 1.4 106), the 40Ar/36Ar ratio is expressed in mol/mol. All the concentrations of helium, neon and argon have an accuracy of 5% (2s). Krypton and Xenon have concentration accuracies of 10%. Helium and Argon isotopic ratios have an accuracy of 10% and 5%, respectively (2s). 4
He (ppm)
20
Ne (ppm)
36
Ar (ppm)
84
Kr (ppm)
136
Xe (ppm)
R/Ra
40
Ar/36Ar
Potiguar South-West
Field Field Field Field Field Field Field
A, well 1b* B, well 2 D, well 1 E F G G’
16.61 39.18 31.26 31.91 22.89 23.03 42.52
0.0079 0.0056 0.0082 0.0291 0.0100 0.0086 0.0041
11.175 0.063 0.120 0.133 0.453 0.201 0.020
0.14 0.21 0.01 0.03 0.02 0.03 0.02
295.5 692.5 696.5 378.6 438.0 446.9 863.6
Areia Branca Trend
Field Field Field Field Field Field Field Field
H, well 3b* H, well 6 K, well 1 K, well 3 L, well 1b* L, well 2 M, well 2 M, well 4
42.02 69.51 14.54 14.95 8.65 31.71 13.17 15.44
0.0179 0.1264 0.1097 0.0467 0.0567 0.0705 1.0958 0.0076
0.546 0.734 2.120 0.362 0.797 0.629 2.743 11.849
1.19 1.15 1.26 1.22 1.10 1.24 0.68 0.56
784.3 788.6 332.6 412.0 392.1 426.5 300.8 295.5
Macau Trend
Field Field Field Field
N O P Q
334.63 97.24 194.46 238.19
0.9977 0.2769 0.6865 1.2944
1.091 0.521 1.067 3.710
0.0329 0.0148 0.0239 0.1197
0.00244 0.00089 0.00070 0.00518
1.36 1.59 1.18 0.81
322.6 285.8 314.6 303.4
Guamaré Trend
Field Field Field Field Field
R S T U V
0.58 15.59 45.25 2.43 1.92
0.0153 0.2035 0.2205 0.0558 0.1652
0.345 1.840 0.474 0.478 1.209
0.0234 0.1034 0.0141 0.0202 0.0533
0.00183 0.00571 0.00062 0.00268 0.00367
0.90 1.06 1.05 0.46 0.18
305.2 307.4 310.3 287.6 295.3
S V
1
0,1
T 0,01
U 0,001
R
Concentrations normalized to air
A. Prinzhofer et al. / Marine and Petroleum Geology 27 (2010) 1273e1284
Concentrations normalized to air
1280
1
Q N
0,1
P O
0,01
0,001
Macau Trend
Guamaré Trend 0,0001
0,0001 20
Ne
36 Ar
84 Kr
36 Ar
20 Ne
136 Xe
84 Kr
136 Xe
Fig. 7. Noble gas fossil isotope concentrations patterns, normalized to the atmosphere for the analyzed gases from the Guamaré and Macau trends.
increase in the proportion of meteoric water inland, or to shallower reservoirs. This is consistent with water recharge from the southwestern part of the trend, where the reservoir rocks outcrop at the surface of the basin. The Guamaré Trend presents a similar pattern with higher contribution of meteoric water (reaching 80% of the whole water exchanging with the hydrocarbons based on the low helium concentrations in sample R for example), and it exhibits a larger scatter along the trend (Figs. 7 and 9). This may be due to local exchange between the Macau and Guamaré trends (Fig. 9), and a more widespread water exchange along the Guamaré Trend. The hydrocarbons from the Macau Trend suffered less interaction with meteoric water, with values decreasing slightly when moving inland along the trend (Fig. 9). Comparing the ratios of fossil and radiogenic noble gas isotopes gives some indication about the physical processes involved, and about the relative chronology of these processes. If a fractionating process occurs early in the history of the fluids, only the isotopes present will be affected, and the radiogenic daughter isotopes produced later will not be affected by the fractionation. A late fractionation will affect both families of compounds. Fig. 15 illustrates the ratios of the radiogenic isotopes 4He/40Ar* versus the ratios of the fossil isotopes 20Ne/36Ar. The two argon isotopes 36Ar and 40Ar behave the same, whereas 4He and 20Ne present similar if
not equal properties in terms of solubility and diffusion coefficients. He would be slightly more fractionated than 20Ne, as its mass and diameter are smaller. We observe contrasted behaviors between the different oil and gas provinces. The average value of non-fractionated crustal fluids is also represented, 20Ne/36Ar representing the average values of atmosphere equilibrated waters (from fresh water to sea water, equilibration between 4 and 20 C), the 4 He/40Ar* ratio being the average crustal production ratio. In this diagram, a trend with smaller isotope ratios would indicate a loss of the light isotope compared to the heavy one, interpreted as either a partial loss through diffusion, or a fractionation due to a water solubilization process. The reverse trend (enrichment of light isotopes) may correspond to either a fractionation due to a large distance of migration (relative enrichment of the smallest and lightest molecules) or to a water washing process (heavy noble gases are more soluble than light ones). The gases from the Macau Trend present a small fractionation of the radiogenic noble gas, and a fractionation of the fossil isotopes indicating an enrichment in lighter ones. This may be interpreted as a trend of fractionation due to migration, occurring before the main period of radiogenic isotope generation. However, the relative fractionation between the three available samples (the other one has a 40Ar* concentration that is too low to calculate a ratio 4He/40Ar*), 4
N
1
S V
Water flow communication
O ea rm Pe
Concentrations normalized to air
P
bi
y lit
i rr ba
er
R
S
0.1 Q T eabi Pe r m
l
arr ity b
ier
U
10 km
0.01
V Ocean 4°C
U
Ocean 20°C
Increasing interaction with water in the Macau trend
Baikal 4°C Baikal 20°C
0.001
R 20 Ne
36Ar
84Kr
136Xe
Fig. 8. Comparison of the fossil noble gas patterns from the Guamaré Trend to those of ocean and Lake Baikal waters (Aeschbach-Hertig et al., 1999) equilibrated with the atmosphere for different conditions of salinity and temperature equilibration.
Increasing interaction with water in the Guamaré trend Fig. 9. Schematic map suggesting the small interaction between aquifers and hydrocarbon fields in the area of the Macau and Guamaré trends showing the permeability barriers, zones of invading water from the north to the south, and the relative importance of water interaction for the studied fields (calculated from the 20Ne concentrations of gas samples).
A. Prinzhofer et al. / Marine and Petroleum Geology 27 (2010) 1273e1284
1281
100
Concentrtaions normalized to air
10
Field N to Q
Macau
Field R, S, U, V Field T
Guamaré
1
0.1
0.01
0.001 4He
20Ne
36Ar
84Kr
136Xe
Fig. 10. Noble gas isotope patterns for the Guamaré and Macau trends, including radiogenic 4He to the fossil isotopes, normalized to the atmospheric values (Ballentine et al., 2002).
the values of this ratio decrease from the shore to the inland, in opposition with the known increasing distance of migration (Fig.16). The geological history must then be adapted to this observation, assuming that after a fractionation due to migration, and increasing the ratios 20Ne/36Ar and a slight increase in the 4He/40Ar* ratio, a second fractionation occurred due to a small partial leakage, and affecting the shallower fluids preferentially inland. 400 350
N
4He
(ppm)
300
The case of the Guamaré fluids is somewhat different, as both fossil and radiogenic isotopes are affected by fractionation. This trend favors a loss of light isotopes (4He and 20Ne). However, it is shifted from the values of equilibrated aquifers, indicating a two step process: depletion of light isotopes first, followed by a reverse fractionation also affecting the 4He/40Ar* ratios, indicative of its recent occurrence. The 4He/40Ar* ratios versus the distance of migration (Fig. 16) presents a small increase of the two gases containing 40Ar* (R and S), and not contaminated by the Macau fluids (sample T), indicating that the main visible process is the fractionation linked to the migration, which starts with a very depleted initial ratio when compared to the average crustal production.
y = 9.704x
250
10
Q
Mantle contam inatio
200
n
P 1 y = 0.1865x
O
100
R/Ra
150
Cr us ta ls ig
0.1
50
K3
0 0
H6
L2 K1L1b 100
H3b 200 40Ar*
300
400
(ppm)
na t
ur e
0.01 -10
10
30
50
70
90
distance from the shore (km) Potiguar Southwest Areia Branca Macau Trend Guamaré Trend
Fig. 11. Correlation between the two radiogenic isotopes 4He and 40Ar* for the four oil provinces analyzed for noble gases.
Potiguar Southwest Areia Branca Macau Trend Guamaré Trend
Fig. 12. 3He/4He normalized to the atmospheric ratio versus the distance of the reservoirs from the shore for the Areia Branca, Macau and Guamaré trends.
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A. Prinzhofer et al. / Marine and Petroleum Geology 27 (2010) 1273e1284
tle an M
100
(ppt)
400
P
3He
(ppt)
500
N
nd
600
3He
b 1000
700
Mantle t re
a
300
K L1b
A
U
1
V
Air or meteoric water Crustal trend
0 100
200 4He
300
400
0.1 0.1
t us
D
1
10 4He
(ppm)
Cr
G’
F
R
0
S
E
200 100
H3b H 6 L2 T
Air or M meteoric water B
10
Q
O
100
1000
(ppm)
Potiguar Southwest Areia Branca Macau Trend Guamaré Trend Fig. 13. Correlation between the two helium isotopes 3He and 4He in linear (a) and logarithmic (b) scales. The lines representing the mantle and crustal ratios of 3He/4He are also represented (Ballentine et al., 2002).
The new results of this study are supported by noble gas geochemistry, and concern the discovery of a substantial mantle contamination in the hydrocarbons for all samples but not observed in the southwestern province. This widespread mantle contribution has several implications in terms of chronology of
a 1000 P Macau Trend
(ppm)
N
4He
In contrast, the gases from the southwestern part of Potiguar present a clear linear trend (Fig. 15) with a relative decrease of light isotopes, both fossil and radiogenic. This corresponds to a residual signature, that occurred after the radiogenic generation, and was probably due to a partial leakage out of the reservoirs. The fluids from the Areia Branca Trend also display a fractionation trend, severely shifted to smaller 4He/40Ar* values as does the one from the Guamaré Trend. The trend defined by the gases from Areia Branca (Fig. 15) corresponds closely to the mantle endmember based on estimates of the upper mantle (Moreira et al., 1998). The trend may then represent a residual signature of a source highly affected by mantle noble gases, then relatively enriched in heavier compounds. This fractionation is less important than for Guamaré, but a small increase of both ratios is nevertheless apparent with the distance of migration (Fig. 16).
O
100
T H
10
S
L
Areia Branc M a K
Guamaré Trend
5. Conclusions
U
1
V
R Contamination with the Macau water
b Proportion of meteoric water (%)
This study of gas geochemistry in the Potiguar Basin has expanded the understanding and quantitative modeling of this petroleum system. Some of the findings obtained from this study confirm previous studies of the geology and geochemistry in this basin. We also outline very new interpretations of several geological processes active in oil and gas generation and accumulation. Several separate petroleum systems in the basin have been characterized: the fields in the southwest of the Potiguar Basin have very different properties relative to the others. Their source rocks are different from the other fields (Pendência Formation), and have strictly crustal signatures without any alteration or contamination. Noble gas isotopes indicate a residual signature and trace of partial leakage out of the reservoirs. The fields of the three other trends (Areia Branca, Macau and Guamaré) are generated by source rocks in the Alagamar Formation. The Macau Trend gases have different facies, similar to the hydrocarbon source of the field located offshore. Biodegradation of gas is observed only in two of the Guamaré Trend samples. Strong fractionation occurs in all three areas, associated with long distance of hydrocarbon migration and leakage of gases in the Areia Branca reservoirs.
Q
0.1 100
Guamaré Trend
80 Areia Branca
60 40
Macau Trend
20 0 -10
-5
0
5 10 15 20 distance from the shore (km)
25
30
Fig. 14. Concentrations of 4He and proportions of meteoric water according to the model presented in the text versus the distance from the shore for the gases from the three trends of Areia Branca, Macau and Guamaré.
A. Prinzhofer et al. / Marine and Petroleum Geology 27 (2010) 1273e1284
100
Average continental crust P
Q
10 4He/40Ar*
G’
T
E
B 1
Mantle
G
S
M2
D F
R
L2
K1
K3
Acknowledgements
L1b
0.01 0.01
0.1
The analytical data were obtained in the Research Center of Petrobras (CENPES) for a part of the carbon isotopic data, and in IFP for the other carbon isotopic data and the noble gas concentrations and isotopic ratios. We thank Jorge Teixeira da Silva and Sonia Noirez for the analyses they performed, as well as the people working at PETROBRAS/UN-RNCE for support during the sampling campaign. We thank Petrobras and IFP for the support of this study, and for the authorization of publishing it. Two anonymous reviewers greatly improved the manuscript with a careful scientific reading and editing the initial text.
1
20Ne/36Ar
Potiguar Southwest Areia Branca Macau Trend Guamaré Trend Fig. 15. Correlation of the ratios of radiogenic isotopes 4He/40Ar* versus the ratios of fossil isotopes 20Ne/36Ar.
hydrocarbons generation, and general scheme of geodynamics of the Atlantic Ocean opening. We showed that it is possible to characterize the degree of connectivity between the provinces through the use of noble gas isotopes. The Macau Trend is isolated from the Guamaré Trend except between the two wells P and T, where water from the Macau area is interpreted to have invaded the Guamaré Trend. The associated water of the Guamaré hydrocarbon reservoirs looks like pristine meteoric water, whereas the water associated with the hydrocarbons of the Macau Trend corresponds to older water that has experienced complex fractionation processes. The combination of stable isotope analyses with noble gas geochemistry provides new insights into petroleum systems.
14 N
12
P
4He/40Ar*
10
Q 8 T
Average crustal production
6 4 2
H
S
R
L
M
K
0 -10
-5
0
Physical and chemical interactions between fluids present (water, oil and gas) in the rocks may be studied and quantified. These fluids may integrate an important contribution from deeper geological structures, and reflect the continental basement and the upper mantle. Noble gas isotopes allow the quantification of these different contributions. This changes our understanding of the thermal history of sedimentary basins (as these deeper fluids may induce thermal advection) and of the origins and associations of non-hydrocarbon gases (CO2, N2, He for example). It indicates the interest for extending the search for oil and gas exploration to areas considered in the past as sterile, such as the most offshore parts of passive margins.
H6
H3b
0.1
N
1283
5
10
15
20
25
30
distance from the shore (km) Areia Branca Macau Trend Guamaré Trend
Fig. 16. 4He/40Ar* versus the distance of migration from the shore for the three trends of Areia Branca, Macau and Guamaré.
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Prinzhofer, A., Battani, A., 2003. Gas isotope tracing: an important tool for hydrocarbon exploration. Revue de l’IFP, Special Publication for B. Tissot’s Jubilee, June 2003. Oil Gas Sci. Technol. Rev. IFP 58, 299e311. Prinzhofer, A., Mello, M.R., Takaki, T., 2000a. Geochemical characterization of natural gas: a physical multivariable approach and its applications in maturity and migration estimates. AAPG Bull. 84, 1152e1172. Prinzhofer, A., Mello, M.R., Da Silva Freitas, L.C., Takaki, T., 2000b. New geochemical characterization of natural gas and its use in oil and gas evaluation. In: Mello, M.R., Katz, B.J. (Eds.), Petroleum Systems of South Atlantic Margins. AAPG Memoir 73, 107e119. Prinzhofer, A., Battani, A., Santos Neto, E.V., 2004. Gas Geochemistry of the Potiguar Basin: Oil/Water Interaction and Mantle Contamination Alago meeting, Merida, Mexico, 5e9 December 2004. Sackett, W.M., 1978. Carbon and hydrogen isotope effects during the thermocatalytic production of hydrocarbons in laboratory simulation experiments. Geochim. Cosmochim. Acta 42, 571e580.
Santos Neto, E.V., Mello, M.R., Rodrigues, R., 1990. Caracterizacáo geoquímica dos óleos da Bacia Potiguar. Anais do Congresso Brasileiro de Geologia, Natal (RN) 1, 974e985. Schoell, M., 1983. Genetic characterization of natural gases. AAPG Bull. 67, 2225e2238. Stahl, W.J., 1977. Carbon and nitrogen isotopes in hydrocarbon research and exploration. Chem. Geol. 20, 121e149. Steiger, R.H., Jager, E., 1977. Subcommision on geochronology: convention on the use of decays constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359e362. Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell Scientific, Oxford, 312 pp. Trindade, L.A.F., Brassell, S.C., Santos Neto, E.V., 1992. Petroleum migration and mixing in the Potiguar Basin, Brazil. AAPG Bull. 76, 1903e1924. Zartman, R.E., Wasserburg, G.J., Reynolds, J.H., 1961. Helium, argon and carbon in some natural gases. J Geophys. Res. 66, 277e306.
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Coupled use of carbon isotopes and noble gas isotopes in the Potiguar basin (Brazil): Fluids migration and mantle influence Alain Prinzhofer a, *, Eugenio Vaz Dos Santos Neto b, Anne Battani a a b
Division de Géologie-Géochimie, IFP, 1 and 4, avenue de Bois-Préau, 92852 Rueil-Malmaison Cedex, France PETROBRAS/CENPES/PDEXP/GEOQ, Cidade Universitária-Quadra 7 e Ilha do Fundào 21949-900, Rio de Janeiro, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 March 2009 Received in revised form 20 January 2010 Accepted 5 March 2010 Available online 15 March 2010
We used the carbon isotope ratios of hydrocarbons and CO2, and the proportions of noble gas isotopes of associated gases from several geological provinces of the Potiguar Basin (Brazil) for gas/source rock correlation, and to determine maturity, post-genetic processes (migration, leakage, biodegradation), and to assess the possible interactions between hydrocarbons and surrounding waters. Barriers of permeability at the basin scale, the amount of water interacting with the accumulated hydrocarbons, proportion of meteoric water, and contamination of the fluids by the mantle were quantified for the distinct petroleum systems defined in this basin. Four hydrocarbon provinces have been defined in the Potiguar Basin. The first is located in the southern part of the basin, and the other three are in the northern part, with offshore pods of generation; migration occurred along NEeSW horst and grabben structures. All gases are mainly thermogenic (d13C of methane between 49 and 34, except for biodegraded gases with isotopically lighter methane), generated in the oil window, and only two fields from one of the trends seem to have been significantly biodegraded (d13C of propane up to 5.8 per mil). The source rocks were lacustrine for the gases from the southwest of the basin, and mixed marine/lacustrine for two trends. The third trend, as well as an offshore field, appears to have a lacustrine source rock, and the hydrocarbon generation and accumulation are older than for the other fields. Each of the three trends is generally isolated in terms of fluid migration, except in one location where noble gas isotopes indicate that the Guamaré Trend is invaded by fluids of the Macau Trend. An important contamination by mantle fluids has been confirmed for all the hydrocarbon fields close to the shore (3He/4He ratios up to 1.6 times the atmospheric ratio). The mantle contribution (as high as 20% of total helium) decreases inland. The mantle contribution provides some chronological constraints for the hydrocarbon accumulations, and may have had an influence in hydrocarbon generation, due to an igneous activity-associated influence in the hydrocarbon generation pod. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Petroleum Gas geochemistry Mantle fluids Gas migration Noble gas Isotopes
1. Introduction Geological reconstruction of petroleum systems is performed using different kinds of 2D and 3D basin modeling. For this purpose, more and more accurate geochemical information is required to constrain possible geological scenarios. Various sophisticated geochemical methodologies have emerged to answer some of the issues. Gas geochemistry in exploration has become a parameter as potent as the classical organic geochemistry interpretation, as several important analytical developments have allowed the characterization of major and minor gas
* Corresponding author. E-mail addresses: [email protected] (A. Prinzhofer), eugenioneto@ petrobras.com.br (E.V. Dos Santos Neto), [email protected] (A. Battani). 0264-8172/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2010.03.004
compounds, and correlated isotopic data. The use of stable isotopes (mainly carbon isotopes) has gained interest during the last decade due to the development of high resolution coupled Gas ChromatographyeCombustioneIsotopic Ratios Mass Spectrometry (GCeCeIRMS) instruments, which allows compound specific carbon isotopic analyses (methane, ethane, propane, iso-butane, normal-butane and carbon dioxide) in a single analysis. From the pioneering works of Stahl (1977), Sackett (1978), Schoell (1983) and Faber (1987), several new experimental and geological concepts emerged (Prinzhofer and Huc, 1995; Lorant et al., 1998; Prinzhofer et al., 2000a), giving access to several genetic and post-genetic quantitative interpretations (gas source, carbon isotope maturity, mixing, direction and distance of migration, biodegradation, etc.). In this study, we used trace and ultra-trace amounts of noble gas isotopes to study a basin-side petroleum system. The noble gas system provides valuable information about
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the sources of fluids, and about physical processes, which affect crustal fluids (physical interactions between fluids, diffusion, adsorption/desorption, etc.; Zartman et al., 1961; Bosch and Mazor, 1988; Ballentine and O’Nions, 1994; Battani et al., 2000; Prinzhofer and Battani, 2003). Moreover, we used the occurrence of radiogenic isotopes such as 4He and 40Ar to constrain chronological information about the hydrocarbon fluids in the subsurface. In order to illustrate the coupled use of carbon isotopes and noble gas geochemistry, we document a case study where natural hydrocarbon gas, oil and source rock data are available. The Potiguar Basin in Brazil is the focus of this study. It has been extensively studied geologically, and because there is already a good general understanding of the occurrence of different petroleum systems in the basin: the source rocks are well characterized, as well as their maturity, and the distance from the generating kitchens (petroleum generation area) to the oil accumulations. Some of the fields in the basin are known to be biodegraded, enabling the technique to be validated. The study will attempt to show that as the main reservoir for noble gas isotopes in the sedimentary rocks is the pore water, their concentration and possible fractionation in the hydrocarbons gives indications about the extent of interaction between oil and water.
The correlation between oil accumulation and source rock is complex. It appears that all the fields located in the southwestern part of the basin have hydrocarbons generated from the lacustrine sources of the Pendência Formation (Santos Neto et al., 1990; Trindade et al., 1992), whereas the oil accumulations in the two grabbens of Areia Branca and Guamaré have been generated mainly from marine source rocks of the Alagamar Formation, with a variable contribution of transitional deltaic and lagoonal environments, and with a contribution of the lacustrine Pendência source rocks in some offshore reservoirs (Santos Neto et al., 1990; Trindade et al., 1992; Morais, 2007). Biodegradation has severely affected the oils of the Guamaré Trend and the Belem field (not included in this study). The other oils are not altered. 3. Gas samples e analytical techniques and results The gas samples analyzed in this study come from different geological structures or petroleum systems of the basin. They are all associated with oil, and correspond to dissolved gas in the pressure/ temperature reservoir conditions, and collected as gas phases at the wellheads. Gases from an offshore field were collected in 1998, and were analyzed for chemistry and carbon isotopes, without any noble gas data. Gases from the southwestern part of the basin and from the Areia Branca Trend were collected in 1998 and 2001 for analysis of their chemistry, carbon isotopes, and noble gas concentrations of helium, neon and argon. Isotopic ratios of helium and argon were also measured. Gases from the two other trends of Macau and Guamaré were collected in 2003, and analyzed for the whole spectrum of noble gases (He, Ne, Ar, Kr, Xe), as well as whole chemistry and carbon isotopes. 10 cc VacutainerÒ glass tubes for gas sampling were conditioned with a secondary vacuum of the tubes. Injection of the gas during sampling, and withdrawal during analyses was performed with gas syringes through a septum. Samples in these glass tubes were used for molecular chemistry and carbon isotopes analyses. The gas was collected in stainless steel tubing for noble gas isotopic and chemical analyses, and isolated with helium proof valves. The collected volume was 20 cc with an absolute pressure of 3.5 bars. All the chemical analyses relevant to hydrocarbons and nonhydrocarbon proportions were performed with a gas chromatograph
2. Geological background The Potiguar Basin is the most northeastern basin of the Brazilian equatorial margin, and the second largest oil producing basin in Brazil after the Campos. It consists of onshore and offshore oil and gas fields, with a range of source rocks origins and compositions (Santos Neto et al., 1990; Trindade et al., 1992; Fig. 1). After a rift stage where lacustrine and deltaic Neocomian source rocks (Pendência Formation) were deposited (d13C of the source rocks around 29&), a transitional stage occurred, characterized as a drift stage, where organic matter is found in lagoonal deposits (Alagamar Formation) with different levels of marine influence (d13Csource around 24&). The hydrocarbon deposits occur inland along northwest-southeast trends, associated with offshore kitchens, and in the southern part of the basin within grabbens (Santos Neto et al., 1990).
SOUTH AMERICA
ca ran aB i e Ar D
E
F
G
A
BR A ZI L
W Offshore Potiguar
M
aré am u G
25 km
C B NATAL
South Potiguar Basement Studied well s
M
K
I L
H
Sediments
Studied wells
N
O
Q
J
S
Studied oil fields
10 km
Other oil f ie l ds
R
P
Ocean
T V
U
10 km
Dir ect ion of migr ati on
Areia Branca
Macau Trend + Guamaré Trend
Fig. 1. General map of the Potiguar Basin, with the detailed locations and names of the sampled wells in Areia Branca, Macau, Guamaré trends and in the offshore Potiguar field.
A. Prinzhofer et al. / Marine and Petroleum Geology 27 (2010) 1273e1284
Varian, using three columns and two detectors (FID and TCD). Carbon isotope analyses were obtained through a Fisons Optima mass spectrometer, coupled with a Fisons GC and a combustion interface (GCeCeIRMS), in order to measure the d13C values for all hydrocarbons from methane to butane, and CO2. Noble gas concentrations and isotopic ratios are analyzed with a GV5400 mass spectrometer. The apparatus is connected to a preparation line which separates and purifies the noble gas family from the other gas molecules (hydrocarbons, CO2, N2, H2, etc.). The analyses are performed in two sets, helium and neon were measured first, and then, after a new tuning of the mass spectrometer, argon, krypton and xenon are analyzed. 4. Results and discussion 4.1. Bulk chemistry and carbon isotopes Twenty wells from seven fields were sampled from the southwestern part of the basin. Nineteen wells from six fields from the Areia Branca Trend, four wells from different fields from the Macau Trend, five wells from different fields from the Guamaré Trend and five wells from the same field from offshore Potiguar. The results of the chemical analysis of the gases, as well as the carbon isotopic ratios from the different areas of the basin, are presented Table 1, and the approximate location of the fields at the basin scale is presented Fig. 1. Plotted in the diagram C2/C3 versus C2/iC4 (Prinzhofer et al., 2000b), the data set shows a well defined trend of maturity, except for two gases from the Guamaré Trend which suffered strong biodegradation (Fig. 2). The gases from the Macau Trend and from the Guamaré Trend show lower maturities than most of the gases from Areia Branca and from the south of the basin, whose range of maturity is very large. The most mature gases are found in the southwestern part of the basin. When plotted on a C2eC3 diagram (Fig. 3; Lorant et al., 1998), the two biodegraded gas samples from the Guamaré are out of the expected normal maturity trend, due to the partial alteration of propane. The other gases are mainly located in the oil window, with a trend corresponding to primary cracking in an open system (Fig. 3). Only some offshore gases appear to show a beginning of secondary cracking of the associated oil. It seems also that the petroleum system from the Macau Trend behaves more like a closed system, the other hydrocarbon provinces acting with a more open signature (Lorant et al., 1998). d13C profiles for gases from the different areas (Fig. 4) corroborate the idea that biodegradation occurred mainly in two gases (samples R and T) from the Guamaré Trend, as observed already in Fig. 2 from their molecular chemistry. This is confirmed by their carbon isotopic ratios, as propane and normal-butane become anomalously heavy, due to their preferential alteration during biodegradation (James and Burns, 1984). The residue is enriched in the heavier isotope 13C, as 12C is more readily mobilized. The d13C patterns of most other samples show very homogeneous signatures for offshore gases, and for the Macau and Areia Branca trends, a slightly larger spread for gases located in the southwestern part of the basin. If we try to consider the carbon isotopic signature of isobutane as the closest relative to the signature of the source rocks, because it is the heaviest measured hydrocarbon in the gas phase and because it is relatively insensitive to biodegradation, we observe different average signatures according to the observed areas, which may be related to different source signatures. Correlating the d13C of methane, ethane and propane (Fig. 5), it appears that the correlation between ethane and propane is linear (Fig. 5a), except for the two biodegraded gases of the Guamaré Trend (samples R and T), and shifted to heavier d13C values of propane. The main spread of data corresponds to either a possible source
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heterogeneity or a maturity trend. The difference of the slope between these two trends is very small, as source heterogeneity will not fractionate preferentially ethane versus propane, and the trend will have a slope of one. Fractionation due to maturity fractionates slightly more ethane relative to propane, as ethane is a smaller molecule. Even if the distinction between these two trends is not possible (Fig. 5), it is probable that both processes do occur, as we know from similar studies of oil (Santos Neto et al., 1990; Trindade et al., 1992) that two main source rocks of the basin have distinct carbon isotopic signatures (the lacustrine source rocks of Pendência Formation are isotopically lighter, averaging ca. 29&, relative to the marine/transitional marine source rocks of Alagamar Formation which are ca. 24&). Combining the relative maturity presented from the molecular diagram (Fig. 2), and the isotopic results (Fig. 5a), offshore gases are the most mature, and could be corrected to lighter d13C in order to compare with the other series in terms of source rock correlation. Gases from the Macau Trend indicate very low maturities, and should then be compared to others with slightly heavier d13C values. With this assumption, we obtain similar d13C signatures for the sources of the Macau Trend and of the offshore field, around 30&, and d13C for the sources of the other gas families (Areia Branca, Guamaré and Potiguar Southwest) around 29&. This would indicate that the offshore field and the Macau Trend come from a related transitional source rock. In contrast, the two families with the heaviest d13C for iso-butane are the trends of Areia Branca and Guamaré. These two families of hydrocarbons would preferentially be generated from marine source rocks (Alagamar Formation), as has been confirmed from biomarker studies of the oils (Santos Neto et al., 1990; Trindade et al., 1992). The fields located in the southwestern part of the basin have different organic matter depositional environments and a distinct lacustrine imprint, related to the Pendência Formation. The plot of the carbon isotopes of C1 and C2 (Fig. 5b) is scattered. Gases from Areia Branca and from the offshore W field exhibit a relatively regular trend of maturity, whereas the gases from the other areas are systematically shifted to isotopically lighter methane. One possibility is a post-genetic fractionation effect, partly due to migration, and also to a possible slight bacterial contribution of methane. We know that for the series of samples in Areia Branca, Macau and Guamaré, the associated hydrocarbon kitchens are located offshore (Trindade et al., 1992). The offshore field W is close to the same hydrocarbon generation pod, whereas the fields AeG, located southwest of the basin, have their hydrocarbons generated locally, close to the reservoirs (Trindade et al., 1992). Comparing the composition of the gases versus the offshore limit, one may check some key parameters and see if they indicate a fractionation linked to the migrating processes. Fig. 6 shows the C1/C2 ratios for the data set. It is observed that both families (W and A to G) which underwent small migration distances have relatively constant C1/ C2 values. In contrast, gases from three trends of Guamaré (ReV), Macau (NeQ) and Areia Branca (HeM) show evidence for large chemical fractionations. Guamaré and Macau trends have increasing C1/C2 ratios, increasing with the hydrocarbon’s distance of migration from the offshore kitchen, whereas gases associated with the Areia Branca Trend show an inverse pattern with the distance from the offshore hydrocarbon kitchen. Methane should be enriched during migration because it is lighter and smaller than ethane. This is what is observed for the Macau and Guamaré trends. For the Areia Branca Trend, the reverse trend may be caused by a partial leakage out of the reservoirs with leakage increasing with the distance from the kitchen to smaller depths of accumulation. The fact that no isotopic fractionation of methane seems to correlate with this process favors a process of tertiary migration (or dismigration) through phase changes, rather than a partial leakage
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Table 1 Carbon isotope composition of hydrocarbon gas molecules and CO2 expressed in per mil compared to the Pee Dee Belemnite standard (PDB), and chemical composition, in mol/mol percent, of the gases analyzed in this study. The uncertainties in the carbon isotope compositions are 0.3 per mil (2s), and 5% relative on the chemical proportions. (a) Gases analyzed from the southwestern part of the Potiguar Basin. (b) Gases analyzed from the Areia Branca Trend. (c) Gases analyzed from the Macau and Guamaré trends and from the offshore field. (a) Potiguar South-West Well 1a: 1998 sampling Well 1b: 2001 sampling
(b) Areia Branca Trend Well 3a: 1998 sampling Well 3b: 2001 sampling
Well 1a: 1998 sampling Well 1b: 2001 sampling
(c) Macau Trend
Guamaré Trend
Offshore Potiguar
d13X2
d13X3
d13iX4
d13nC4
31.0 31.2 29.7 30.9 30.0 30.6 30.1 30.2 29.6 29.3 30.2 33.4 36.0 33.2 26.3 30.5 30.4 30.3 33.4 30.8
28.9 28.7 27.4 27.9 27.4 28.4 27.5 27.8 27.1 27.7 27.8 30.8 32.1 29.9 25.2 26.4 27.4 27.0 28.3 28.1
29.4 29.7 28.2 31.4 29.5 28.2 27.7 29.0 29.7 29.0 27.1 31.2 32.8 32.1 26.4 28.4 28.5 28.5 29.3 25.4
28.2 28.8 27.3 28.2 28.4 27.0 26.9 27.5 27.4 26.9 28.3 29.5 32.1 30.1 26.8 26.0 26.9 25.8 28.2 28.0
0.25 0.71 0.26
40.0 39.0 39.8 39.1 38.5 39.8 40.3 40.2 38.4 39.9 39.4 39.6 40.4 40.2 39.1 43.8 39.5 39.7 39.0
30.0 29.9 29.9 29.5 29.5 30.2 29.8 30.1 30.7 29.4 29.5 29.4 30.1 30.3 29.6 29.3 30.1 29.4 29.6
27.3 27.6 27.5 27.3 28.3 27.4 27.1 27.9 28.7 26.8 27.0 27.0 27.7 28.3 27.2 26.8 26.3 26.8 26.9
27.9 28.2 28.3 28.8 28.8 27.7 28.5 27.6 29.0 28.5 28.1 28.7 28.5 30.6 25.3 28.1 28.1 28.2 27.2
26.1 26.3 26.8 26.6 27.4 25.7 26.2 26.0 27.2 26.0 25.5 26.2 26.1 28.0 26.9 25.6 26.0 25.6 26.2
0.29 1.83 2.46 4.07 13.74 12.41 10.22 56.41 20.75 2.59 0.33 2.81 2.41 2.77
50.5 45.2 49.1 60.1 49.3 55.8 47.6 47.8 45.7 40.4 44.1 40.4 40.2 40.6
30.6 29.9 34.2
27.5 27.5 27.1
33.1 33.1
30.9 30.9
26.6 27.9 26.9 30.2 34.2 31.6 35.8 31.5 31.4 31.8
5.8 27.0 15.6 24.9 30.3 29.3 32.4 29.1 29.1 29.1
27.8 28.4 29.7
21.0 26.3 22.5 23.4 24.8 27.3 32.3 28.0 27.7 28.0
C3
84.91 80.98 91.46 85.79 87.17 84.83 77.59 78.97 75.74 86.20 89.48 87.40 81.39 76.26 70.92 77.37 73.27 89.78 80.46 66.73
10.00 10.98 5.67 8.94 7.02 8.58 13.05 12.92 11.00 7.97 6.60 6.31 9.64 14.03 14.45 12.17 15.31 3.12 7.35 13.32
3.82 5.14 2.05 3.94 3.63 4.70 6.65 5.96 8.07 4.04 2.96 4.51 5.31 7.20 8.40 7.00 8.64 4.10 6.67 10.53
0.62 0.97 0.46 0.69 1.16 0.96 1.24 1.05 2.72 0.87 0.53 0.95 0.62 1.00 1.31 1.40 1.20 0.91 1.91 2.37
0.64 1.40 0.37 0.63 1.03 0.92 1.47 1.10 2.47 0.93 0.43 0.84 1.31 1.22 2.44 2.06 1.58 1.46 1.79 3.67
Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field
H, well 1 H, well 2 H, well 3a* H, well 3b* H, well 4 H, well 5 H, well 6 I J K, well 1 K, well 2 K, well 3 L, well 1a* L, well 1b* L, well 2 M, well 1 M, well 2 M, well 3 M, well 4
75.94 78.44 77.10 77.85 79.27 65.23 75.69 82.07 58.48 48.26 26.59 70.11 68.06 84.14 75.31 1.01 14.45 4.63 19.73
13.25 12.71 11.60 11.73 10.78 14.94 12.51 11.14 17.75 19.93 19.14 14.00 16.14 7.35 14.36 7.05 23.67 22.64 28.40
6.65 6.26 6.71 5.68 6.73 12.21 6.46 4.53 15.48 19.04 31.80 8.73 10.44 3.99 6.27 45.95 34.48 45.57 29.86
1.30 1.11 1.50 1.02 1.43 3.37 1.28 0.86 3.23 4.95 11.38 2.01 2.30 1.21 1.08 25.11 10.58 14.66 7.27
1.48 1.25 1.90 1.53 1.79 4.25 1.90 0.91 4.21 5.54 11.08 2.87 2.54 1.51 1.26 20.88 13.38 11.79 8.51
1.39 0.23 1.18 0.81
Field Field Field Field Field Field Field Field Field Field Field Field Field Field
N O P Q R S T U V W, W, W, W, W,
71.02 61.82 96.39 95.93 77.07 60.61 80.99 42.65 78.36 81.55 86.73 83.20 79.14 75.52
7.72 3.74 0.74 0.02 5.58 6.28 5.70 0.33 0.09 10.54 7.80 10.25 10.73 11.98
8.15 9.99 0.42
4.82 6.33
5.93 9.25
0.61 13.77 1.03 0.13 0.11 3.77 3.90 2.65 5.59 7.34
2.44 2.10 1.76
0.26 2.41 0.30 0.21 0.34 0.96 0.71 0.67 1.31 1.42
1 2 3 4 5
0.59 0.54 0.41 0.83 0.97
by diffusion through the caprocks. In the southwest of Potiguar (A to G fields) and in the offshore field (W), no specific trend is visible, and data cluster around C1/C2 values of 10. These values suggest that distances of secondary migration are significantly lower than those in the other studied trends, as the C1/C2 fractionation is relatively restricted. 4.2. Noble gases Noble gas isotopes may be classified in two main families: those that are derived from air, and those derived from formation water
nC4
d13X1
C2
A, well 1a* A, well 1b* A, well 2 A, well 3 A, well 4 A, well 5 A, well 6 A, well 7 A, well 8 A, well 9 A, well 10 B, well 1 B, well 2 C D, well 1 D, well 2inf D, well 2sup E F G
well well well well well
iC4
41.8 42.0 45.5 46.1 45.6 46.0 41.7 41.5 44.9 42.9 43.9 45.8 45.4 43.8 33.6 44.6 45.1 49.2 47.4 43.7
C1 Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field Field
CO2 0.14
0.19 0.29 0.01
0.07 0.03 0.00
0.97 0.49 0.84 0.13 0.10 0.52 0.08 0.15
30.3 32.7 29.8 30.0 30.1
d13XO2 5.5
4.6 15.4
0.5 25.4 10.0
6.1
7.2 5.7 17.5 17.6 20.6 21.3 18.7 18.2 9.3 16.0 9.6 3.6 6.6 5.1 18.1 6.0 17.0 10.3 10.9 13.0 11.7
(here 20Ne, 36Ar, 84Kr and 136Xe). These isotopes may enter a hydrocarbon phase, exchanging with aquifers equilibrated with the atmosphere. The radiogenic isotopes (4He and 40Ar*, where 40 Ar* ¼ the extra amount of 40Ar when removing the atmospheric contribution, with 40Ar/36Ar atmosphere ¼ 295.5), come from the natural radioactive decay of U/Th and K, respectively, in minerals. All these compounds are insensitive to chemical processes, subsequently tracing only different sources and physical processes occurring between phases or diffusive fractionation. All the available noble gas data from the fields in the Potiguar Basin are presented in Table 2.
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R
C2/C3
8
6
Potiguar Southwest
Biodegradation
10
Areia Branca Macau Trend Guamaré Trend Offshore Potiguar
T
4
Maturity
2
S
0 0
5
10
15
20
25
30
C2/iC4 Fig. 2. Diagram C2/C3 versus C2/iC4, showing the effects of increasing maturity and biodegradation. Only two samples from the Guamaré Trend show a clear degradation of gases.
The patterns of air normalized isotopes (20Ne, provide information regarding:
36
Ar,
84
Kr, 136Xe)
(1) The amount of water which interacted with hydrocarbons (a water/hydrocarbon ratio), as the concentrations of airderived isotopes may be directly linked to the amount of water equilibrated with the hydrocarbons. (2) The contribution of mantle noble gas, as these isotopes are present in small amounts in the mantle and have quite different patterns relative to air-equilibrated water. 20Ne concentrations are higher when compared to 36Ar, as are the 136 Xe versus 84Kr values. (3) The different physical processes which fractionate air-derived isotopes according to their mass (from neon to xenon), give different shapes to the patterns. For instance, it is a straightforward procedure to obtain a composition of a water at equilibrium with the atmosphere. The corresponding pattern will depend on the salinity of the water and the exchange temperature, but these variations are of second order (a factor 2 or 3 less) when the concentrations are represented in logarithmic scales (Fig. 7). 5
Primary cracking NSO cracking
0
S -10
gas cracking
n
-15
tio da ra
T
eg od Bi
δ 13C2- δ13C3
Oil cracking
-5
Potiguar Southwest
-20
Areia Branca Macau Trend
R
Guamaré Trend Offshore Potiguar
-25 0
5
10
15
C2/C3 Fig. 3. C2eC3 maturity diagram for all the studied gases. The two biodegraded gases from the Guamaré Trend are shifted from their normal location due to preferential alteration of propane.
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We calculate the equilibrium concentrations of fossil noble gas isotopes in freshwater in one pool represented by Lake Baikal water and in the other in the ocean, normalized to air concentrations (Fig. 8). A hydrocarbon phase equilibrated with such air-equilibration will produce the same pattern because of mass balance, as the major part of the noble gas compounds will be moving into the hydrocarbon phase. Gases from the Guamaré Trend have relatively similar patterns relative to a hydrocarbon phase equilibrated with such recharge aquifer waters (except well T), whereas the hydrocarbon fluids of the Macau Trend have distinct signatures, with a more concave shape (logarithmic scale; Fig. 7b). The abnormal pattern seen in well T from the Guamaré Trend (Fig. 7a) is very similar to the Macau Trend patterns (Fig. 7b). We interpret this similarity as resulting from contamination of fluids from the Macau Trend invading the Guamaré Trend at the level of the well T (Fig. 9). In fact, Lima Neto et al. (1990), describing the equipotentials of the basin, suggested a potential water flow from the Macau area to the Guamaré area, due to higher water potential in the Macau Trend. Associating this information with the possible calculation of water/ hydrocarbon ratios, linked to the absolute amounts of fossil noble gas isotopes in the hydrocarbon fluids, it is possible to model the relative degree of interaction of the hydrocarbons with water, and the water connectivity in the Macau/Guamaré area (Fig. 9). It appears that for the Macau Trend, the interaction with water increases inland, i.e. going further from the kitchen. For the Guamaré Trend, the interaction with water seems to be less well behaved, with a maximum of interaction around the field S. All the gases from the Macau Trend have higher 4He concentrations relative to the gases from the Guamaré Trend (Fig. 10). This is consistent with the curvature of the fossil isotope patterns, showing a relative enrichment in the light isotopes (36Ar and 20Ne). As it is impossible to mimic the patterns of fossil isotopes with a simple equilibration with atmosphere equilibrated aquifers, we interpret the patterns observed for the Macau Trend to be affected by a mantle contribution. In the Guamaré Trend, this possible mantle contribution is not visible for the light noble gas concentrations, but maybe seen in the ratios 84Kr/136Xe, which are smaller than expected from an air-equilibrated water source. Comparing the two radiogenic isotopes 4He and 40Ar*, we observe contrasting behaviors between the different analyzed families (Fig. 11). The hydrocarbons from the Guamaré Trend have very small concentrations of radiogenic isotopes, indicating a short residence time, without any contribution by old fluids (water and oil). The gases from the southwest of Potiguar also contain relatively small concentrations of 4He and 40Ar*. In contrast, the gases from the Areia Branca and Macau trends present good correlations between these two isotopes, but with different slopes. The average ratio of 4He/40Ar* in sediments is 5.9 (Steiger and Jager, 1977; Taylor and McLennan, 1985). The mobility of these radiogenic isotopes in the fluids tends to average their relative concentrations. The hydrocarbons from the Areia Branca Trend have a 4He/40Ar* ratio that is low compared to this average ratio, whereas the hydrocarbons from the Macau Trend have ratios slightly higher than the average production rate. Differential 40Ar expulsion from the crystal lattice of minerals has been used to explain the high apparent production ratios of 4He/40Ar* in sediments (Elliot et al., 1993; Prinzhofer, 2002). However, this may only be a minor factor in the Macau samples; the main abnormal 4He/40Ar ratio originates from their low value in the Areia Branca Trend. The hydrocarbon gas chemistry (Fig. 6) showed that the hydrocarbons of the Areia Branca Trend suffered drastic partial leakage due to tertiary migration. The deficit of 4He compared to 40Ar* supports this interpretation of a tertiary migration and partial leakage of hydrocarbons from the Areia Branca Trend. Helium would be enriched in the leaking light phase compared to argon, in the same
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A. Prinzhofer et al. / Marine and Petroleum Geology 27 (2010) 1273e1284
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
δ
Fig. 4. d13C patterns for the gases from the different oil and gas provinces.
way that methane was leaking more efficiently relative to ethane. It is important to notice that for the other corresponding rift system of Macau, no similar trend of leakage is visible, indicating probably much better caprocks in the southeastern part of the basin. The 3He/4He ratios of the analyzed hydrocarbon fluids suggest an important mantle contribution. In fact, representing these isotope ratios normalized to the atmospheric value (in R/Ra
-20
ity gene tero e h rce Sou nd ity tre matur
T
-33
P
-25 -30
rit Matu -35 -37
y
erog ce h e t + sour
eneity
S
O
N
-31
tur ity
T physic al f & bio ractionatio degra n dation
Ma
S
-29
δ 13 C2
-15
-27
tion
-10
R
R Biodeg rada
-5
13 δ C3
b -25
0
Biodegradation
a
notation: 3He/4Heatm ¼ Ra ¼ 1.4 106), the oceanic upper mantle has a signature of approximately 8 Ra, the subcontinental mantle a signature of about 6 Ra, whereas the continental crust, richer in radiogenic 4He, has ratios of about 0.02 Ra. Values above 0.02 Ra for crustal fluids may be interpreted either as air-contaminated, or mantle-mixed. In our case, the fact that some helium isotopic ratios are above 1 Ra makes air contamination unlikely. We interpret that
-35
-37 -35
-33
-31
-29
-2 7
-25
-65
-60
-55
-50
-45
-40
-35
-30
δ 13C1
δ 13 C2 Potiguar Southwest Areia Branca Macau Trend Guamaré Trend Offshore Potiguar
Fig. 5. Graphs showing the correlation between the carbon isotopes of ethane and propane (5a) and of methane and ethane (5b).
A. Prinzhofer et al. / Marine and Petroleum Geology 27 (2010) 1273e1284
10000
Q
1000
V
C1/C2
P U
100
E
N W
10
n
Kitchen 2
Mig ratio
Kitchen 1
R
T
H
B
C
D G
e kag
K
1
A
Lea
J S L
O
F
M 0.1 -40
-20
0
20
40
60
80
Distance from the shore (km) Potiguar Southwest Areia Branca Macau Trend Guamaré Trend Offshore Potiguar
Fig. 6. C1/C2 molecular values for the gases of Potiguar, versus the distance from the modern shoreline.
these helium isotopic ratios are representative of the crustal fluids that have been affected by interactions with mantle fluids. The gases from the Macau Trend are the most contaminated, and reach ratios as high as 1.6 Ra, which means that more than 20% of the helium came from the mantle. This mantle contamination mainly affects the gases located on the three geologic trends of Areia Branca, Macau and Guamaré (Prinzhofer et al., 2004). The mantle contribution indicated by the 3He/4He ratio decreases inland, and all the fluids of the southwestern province of Potiguar are mantle contamination free (Fig. 12). Oxburgh et al. (1986) and O’Nions and Oxburgh (1988) found that mantle fluid contamination occurs in sedimentary basins during rifting. Volcanic activity in the vicinity of the Brazilian passive margin from 90 million years to the present
1279
(Mizusaki et al., 2002) does not seem to be the source for this mantle contribution, as the occurrence of mantle helium seems to occur along the entire margin. This implies that either a riftingrelated mantle fluid remains in the margin sediments since ocean opening, or that mantle helium memory is retained after the rifting for at least until the hydrocarbons have been generated and accumulated (several tens of millions years). If we try to correlate the concentrations of 3He and 4He (Fig. 13a), a mixture between two end-members should be represented as a straight line. It appears that any mixture between a mantle and a crustal end-member should present a straight line with a negative slope, as a mantle end-member would be enriched in 3He, and a crustal end-member enriched in 4He. All the analyzed gases from the three trends of Areia Branca, Macau and Guamaré, however, have a linear trend with a positive slope intermediate between mantle and crust values, without any evidence of a mixing trend between a mantle and a crustal end-member. The concentrations of helium (both 3He and 4He) may be affected by a preferential leakage of this compound versus the main components (oil and gas compounds), giving a correlated decrease of concentration (Fig. 13). However, as the variation of helium concentrations is high (approximately 500), from the higher to smaller helium concentrations, it is very unlikely that such a fractionating process would decrease the helium concentrations of more than two or three times lower than the maximum. In order to try to explain this positive correlation, we need to envision a two step process: first, mixing between mantle and crustal fluids, giving an intermediate 3 He/4He, and then a mixing of this homogenized source with an end-member close to the origin. This end-member is probably the atmosphere, or more precisely water equilibrated with the atmosphere, i.e. meteoric water. If this is the case, for each petroleum province (Areia Branca, Macau and Guamaré trends), the concentrations of 4He (or 3He) are a direct function of the proportion of meteoric water involved in the system. This makes possible a simple calculation of this proportion of meteoric water for each of these areas (Fig. 14). The Areia Branca Trend presents a smooth
Table 2 Noble gas chemical and isotopic compositions of the associated gases from the Potiguar Basin. The concentrations are expressed in mol/mol ppm, the helium isotopic ratio 3 He/4He is normalized to the atmospheric value (R/Ra with Ra ¼ 1.4 106), the 40Ar/36Ar ratio is expressed in mol/mol. All the concentrations of helium, neon and argon have an accuracy of 5% (2s). Krypton and Xenon have concentration accuracies of 10%. Helium and Argon isotopic ratios have an accuracy of 10% and 5%, respectively (2s). 4
He (ppm)
20
Ne (ppm)
36
Ar (ppm)
84
Kr (ppm)
136
Xe (ppm)
R/Ra
40
Ar/36Ar
Potiguar South-West
Field Field Field Field Field Field Field
A, well 1b* B, well 2 D, well 1 E F G G’
16.61 39.18 31.26 31.91 22.89 23.03 42.52
0.0079 0.0056 0.0082 0.0291 0.0100 0.0086 0.0041
11.175 0.063 0.120 0.133 0.453 0.201 0.020
0.14 0.21 0.01 0.03 0.02 0.03 0.02
295.5 692.5 696.5 378.6 438.0 446.9 863.6
Areia Branca Trend
Field Field Field Field Field Field Field Field
H, well 3b* H, well 6 K, well 1 K, well 3 L, well 1b* L, well 2 M, well 2 M, well 4
42.02 69.51 14.54 14.95 8.65 31.71 13.17 15.44
0.0179 0.1264 0.1097 0.0467 0.0567 0.0705 1.0958 0.0076
0.546 0.734 2.120 0.362 0.797 0.629 2.743 11.849
1.19 1.15 1.26 1.22 1.10 1.24 0.68 0.56
784.3 788.6 332.6 412.0 392.1 426.5 300.8 295.5
Macau Trend
Field Field Field Field
N O P Q
334.63 97.24 194.46 238.19
0.9977 0.2769 0.6865 1.2944
1.091 0.521 1.067 3.710
0.0329 0.0148 0.0239 0.1197
0.00244 0.00089 0.00070 0.00518
1.36 1.59 1.18 0.81
322.6 285.8 314.6 303.4
Guamaré Trend
Field Field Field Field Field
R S T U V
0.58 15.59 45.25 2.43 1.92
0.0153 0.2035 0.2205 0.0558 0.1652
0.345 1.840 0.474 0.478 1.209
0.0234 0.1034 0.0141 0.0202 0.0533
0.00183 0.00571 0.00062 0.00268 0.00367
0.90 1.06 1.05 0.46 0.18
305.2 307.4 310.3 287.6 295.3
S V
1
0,1
T 0,01
U 0,001
R
Concentrations normalized to air
A. Prinzhofer et al. / Marine and Petroleum Geology 27 (2010) 1273e1284
Concentrations normalized to air
1280
1
Q N
0,1
P O
0,01
0,001
Macau Trend
Guamaré Trend 0,0001
0,0001 20
Ne
36 Ar
84 Kr
36 Ar
20 Ne
136 Xe
84 Kr
136 Xe
Fig. 7. Noble gas fossil isotope concentrations patterns, normalized to the atmosphere for the analyzed gases from the Guamaré and Macau trends.
increase in the proportion of meteoric water inland, or to shallower reservoirs. This is consistent with water recharge from the southwestern part of the trend, where the reservoir rocks outcrop at the surface of the basin. The Guamaré Trend presents a similar pattern with higher contribution of meteoric water (reaching 80% of the whole water exchanging with the hydrocarbons based on the low helium concentrations in sample R for example), and it exhibits a larger scatter along the trend (Figs. 7 and 9). This may be due to local exchange between the Macau and Guamaré trends (Fig. 9), and a more widespread water exchange along the Guamaré Trend. The hydrocarbons from the Macau Trend suffered less interaction with meteoric water, with values decreasing slightly when moving inland along the trend (Fig. 9). Comparing the ratios of fossil and radiogenic noble gas isotopes gives some indication about the physical processes involved, and about the relative chronology of these processes. If a fractionating process occurs early in the history of the fluids, only the isotopes present will be affected, and the radiogenic daughter isotopes produced later will not be affected by the fractionation. A late fractionation will affect both families of compounds. Fig. 15 illustrates the ratios of the radiogenic isotopes 4He/40Ar* versus the ratios of the fossil isotopes 20Ne/36Ar. The two argon isotopes 36Ar and 40Ar behave the same, whereas 4He and 20Ne present similar if
not equal properties in terms of solubility and diffusion coefficients. He would be slightly more fractionated than 20Ne, as its mass and diameter are smaller. We observe contrasted behaviors between the different oil and gas provinces. The average value of non-fractionated crustal fluids is also represented, 20Ne/36Ar representing the average values of atmosphere equilibrated waters (from fresh water to sea water, equilibration between 4 and 20 C), the 4 He/40Ar* ratio being the average crustal production ratio. In this diagram, a trend with smaller isotope ratios would indicate a loss of the light isotope compared to the heavy one, interpreted as either a partial loss through diffusion, or a fractionation due to a water solubilization process. The reverse trend (enrichment of light isotopes) may correspond to either a fractionation due to a large distance of migration (relative enrichment of the smallest and lightest molecules) or to a water washing process (heavy noble gases are more soluble than light ones). The gases from the Macau Trend present a small fractionation of the radiogenic noble gas, and a fractionation of the fossil isotopes indicating an enrichment in lighter ones. This may be interpreted as a trend of fractionation due to migration, occurring before the main period of radiogenic isotope generation. However, the relative fractionation between the three available samples (the other one has a 40Ar* concentration that is too low to calculate a ratio 4He/40Ar*), 4
N
1
S V
Water flow communication
O ea rm Pe
Concentrations normalized to air
P
bi
y lit
i rr ba
er
R
S
0.1 Q T eabi Pe r m
l
arr ity b
ier
U
10 km
0.01
V Ocean 4°C
U
Ocean 20°C
Increasing interaction with water in the Macau trend
Baikal 4°C Baikal 20°C
0.001
R 20 Ne
36Ar
84Kr
136Xe
Fig. 8. Comparison of the fossil noble gas patterns from the Guamaré Trend to those of ocean and Lake Baikal waters (Aeschbach-Hertig et al., 1999) equilibrated with the atmosphere for different conditions of salinity and temperature equilibration.
Increasing interaction with water in the Guamaré trend Fig. 9. Schematic map suggesting the small interaction between aquifers and hydrocarbon fields in the area of the Macau and Guamaré trends showing the permeability barriers, zones of invading water from the north to the south, and the relative importance of water interaction for the studied fields (calculated from the 20Ne concentrations of gas samples).
A. Prinzhofer et al. / Marine and Petroleum Geology 27 (2010) 1273e1284
1281
100
Concentrtaions normalized to air
10
Field N to Q
Macau
Field R, S, U, V Field T
Guamaré
1
0.1
0.01
0.001 4He
20Ne
36Ar
84Kr
136Xe
Fig. 10. Noble gas isotope patterns for the Guamaré and Macau trends, including radiogenic 4He to the fossil isotopes, normalized to the atmospheric values (Ballentine et al., 2002).
the values of this ratio decrease from the shore to the inland, in opposition with the known increasing distance of migration (Fig.16). The geological history must then be adapted to this observation, assuming that after a fractionation due to migration, and increasing the ratios 20Ne/36Ar and a slight increase in the 4He/40Ar* ratio, a second fractionation occurred due to a small partial leakage, and affecting the shallower fluids preferentially inland. 400 350
N
4He
(ppm)
300
The case of the Guamaré fluids is somewhat different, as both fossil and radiogenic isotopes are affected by fractionation. This trend favors a loss of light isotopes (4He and 20Ne). However, it is shifted from the values of equilibrated aquifers, indicating a two step process: depletion of light isotopes first, followed by a reverse fractionation also affecting the 4He/40Ar* ratios, indicative of its recent occurrence. The 4He/40Ar* ratios versus the distance of migration (Fig. 16) presents a small increase of the two gases containing 40Ar* (R and S), and not contaminated by the Macau fluids (sample T), indicating that the main visible process is the fractionation linked to the migration, which starts with a very depleted initial ratio when compared to the average crustal production.
y = 9.704x
250
10
Q
Mantle contam inatio
200
n
P 1 y = 0.1865x
O
100
R/Ra
150
Cr us ta ls ig
0.1
50
K3
0 0
H6
L2 K1L1b 100
H3b 200 40Ar*
300
400
(ppm)
na t
ur e
0.01 -10
10
30
50
70
90
distance from the shore (km) Potiguar Southwest Areia Branca Macau Trend Guamaré Trend
Fig. 11. Correlation between the two radiogenic isotopes 4He and 40Ar* for the four oil provinces analyzed for noble gases.
Potiguar Southwest Areia Branca Macau Trend Guamaré Trend
Fig. 12. 3He/4He normalized to the atmospheric ratio versus the distance of the reservoirs from the shore for the Areia Branca, Macau and Guamaré trends.
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A. Prinzhofer et al. / Marine and Petroleum Geology 27 (2010) 1273e1284
tle an M
100
(ppt)
400
P
3He
(ppt)
500
N
nd
600
3He
b 1000
700
Mantle t re
a
300
K L1b
A
U
1
V
Air or meteoric water Crustal trend
0 100
200 4He
300
400
0.1 0.1
t us
D
1
10 4He
(ppm)
Cr
G’
F
R
0
S
E
200 100
H3b H 6 L2 T
Air or M meteoric water B
10
Q
O
100
1000
(ppm)
Potiguar Southwest Areia Branca Macau Trend Guamaré Trend Fig. 13. Correlation between the two helium isotopes 3He and 4He in linear (a) and logarithmic (b) scales. The lines representing the mantle and crustal ratios of 3He/4He are also represented (Ballentine et al., 2002).
The new results of this study are supported by noble gas geochemistry, and concern the discovery of a substantial mantle contamination in the hydrocarbons for all samples but not observed in the southwestern province. This widespread mantle contribution has several implications in terms of chronology of
a 1000 P Macau Trend
(ppm)
N
4He
In contrast, the gases from the southwestern part of Potiguar present a clear linear trend (Fig. 15) with a relative decrease of light isotopes, both fossil and radiogenic. This corresponds to a residual signature, that occurred after the radiogenic generation, and was probably due to a partial leakage out of the reservoirs. The fluids from the Areia Branca Trend also display a fractionation trend, severely shifted to smaller 4He/40Ar* values as does the one from the Guamaré Trend. The trend defined by the gases from Areia Branca (Fig. 15) corresponds closely to the mantle endmember based on estimates of the upper mantle (Moreira et al., 1998). The trend may then represent a residual signature of a source highly affected by mantle noble gases, then relatively enriched in heavier compounds. This fractionation is less important than for Guamaré, but a small increase of both ratios is nevertheless apparent with the distance of migration (Fig. 16).
O
100
T H
10
S
L
Areia Branc M a K
Guamaré Trend
5. Conclusions
U
1
V
R Contamination with the Macau water
b Proportion of meteoric water (%)
This study of gas geochemistry in the Potiguar Basin has expanded the understanding and quantitative modeling of this petroleum system. Some of the findings obtained from this study confirm previous studies of the geology and geochemistry in this basin. We also outline very new interpretations of several geological processes active in oil and gas generation and accumulation. Several separate petroleum systems in the basin have been characterized: the fields in the southwest of the Potiguar Basin have very different properties relative to the others. Their source rocks are different from the other fields (Pendência Formation), and have strictly crustal signatures without any alteration or contamination. Noble gas isotopes indicate a residual signature and trace of partial leakage out of the reservoirs. The fields of the three other trends (Areia Branca, Macau and Guamaré) are generated by source rocks in the Alagamar Formation. The Macau Trend gases have different facies, similar to the hydrocarbon source of the field located offshore. Biodegradation of gas is observed only in two of the Guamaré Trend samples. Strong fractionation occurs in all three areas, associated with long distance of hydrocarbon migration and leakage of gases in the Areia Branca reservoirs.
Q
0.1 100
Guamaré Trend
80 Areia Branca
60 40
Macau Trend
20 0 -10
-5
0
5 10 15 20 distance from the shore (km)
25
30
Fig. 14. Concentrations of 4He and proportions of meteoric water according to the model presented in the text versus the distance from the shore for the gases from the three trends of Areia Branca, Macau and Guamaré.
A. Prinzhofer et al. / Marine and Petroleum Geology 27 (2010) 1273e1284
100
Average continental crust P
Q
10 4He/40Ar*
G’
T
E
B 1
Mantle
G
S
M2
D F
R
L2
K1
K3
Acknowledgements
L1b
0.01 0.01
0.1
The analytical data were obtained in the Research Center of Petrobras (CENPES) for a part of the carbon isotopic data, and in IFP for the other carbon isotopic data and the noble gas concentrations and isotopic ratios. We thank Jorge Teixeira da Silva and Sonia Noirez for the analyses they performed, as well as the people working at PETROBRAS/UN-RNCE for support during the sampling campaign. We thank Petrobras and IFP for the support of this study, and for the authorization of publishing it. Two anonymous reviewers greatly improved the manuscript with a careful scientific reading and editing the initial text.
1
20Ne/36Ar
Potiguar Southwest Areia Branca Macau Trend Guamaré Trend Fig. 15. Correlation of the ratios of radiogenic isotopes 4He/40Ar* versus the ratios of fossil isotopes 20Ne/36Ar.
hydrocarbons generation, and general scheme of geodynamics of the Atlantic Ocean opening. We showed that it is possible to characterize the degree of connectivity between the provinces through the use of noble gas isotopes. The Macau Trend is isolated from the Guamaré Trend except between the two wells P and T, where water from the Macau area is interpreted to have invaded the Guamaré Trend. The associated water of the Guamaré hydrocarbon reservoirs looks like pristine meteoric water, whereas the water associated with the hydrocarbons of the Macau Trend corresponds to older water that has experienced complex fractionation processes. The combination of stable isotope analyses with noble gas geochemistry provides new insights into petroleum systems.
14 N
12
P
4He/40Ar*
10
Q 8 T
Average crustal production
6 4 2
H
S
R
L
M
K
0 -10
-5
0
Physical and chemical interactions between fluids present (water, oil and gas) in the rocks may be studied and quantified. These fluids may integrate an important contribution from deeper geological structures, and reflect the continental basement and the upper mantle. Noble gas isotopes allow the quantification of these different contributions. This changes our understanding of the thermal history of sedimentary basins (as these deeper fluids may induce thermal advection) and of the origins and associations of non-hydrocarbon gases (CO2, N2, He for example). It indicates the interest for extending the search for oil and gas exploration to areas considered in the past as sterile, such as the most offshore parts of passive margins.
H6
H3b
0.1
N
1283
5
10
15
20
25
30
distance from the shore (km) Areia Branca Macau Trend Guamaré Trend
Fig. 16. 4He/40Ar* versus the distance of migration from the shore for the three trends of Areia Branca, Macau and Guamaré.
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