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Origin and alteration of oils and oil seeps from the Sinú-San Jacinto Basin, Colombia
Organic Geochemistry Organic Geochemistry 37 (2006) 1831–1845 www.elsevier.com/locate/orggeochem
Origin and alteration of oils and oil seeps from the Sinu´-San Jacinto Basin, Colombia Christian Sa´nchez 1, Albert Permanyer
*
Dpt. Geoquı´mica, Petrologia i Prospeccio´ Geolo`gica, Facultat de Geologia, Universitat de Barcelona, Martı´ i Franque`s, s/n, 08028 Barcelona, Catalonia, Spain Available online 15 September 2006
Abstract Liquid hydrocarbons have been detected in the subsurface as well as in the surface in the Sinu´-San Jacinto Basin (northwestern Colombia). The origin of the oils has not been conclusively established especially in the southern part of the basin. The most likely source rocks in the basin are the Cie´naga de Oro Fm. of the Oligocene-Early Miocene and the Cansona Fm. of the Upper Cretaceous. In this study, oil samples, seeps and source rock extracts were analyzed by GC and GC/MS, and d13C was determined to identify the source facies. The sulphur content and gravity data were also considered. Two organic facies were identified: one constituted by terrestrial organic matter deposited in siliciclastic sediments in marginal marine to deltaic environments and the other made up of marine organic matter deposited in marine costal shelf to pelagic environments. The oils from the former organic facies have low sulphur contents, whereas the oils from the latter facies have high sulphur levels. Correlation of the oil seeps from the former facies with the Cie´naga de Oro Fm. has not been clearly established. The oil seeps from the latter facies correlate well with the extracts from the source rocks of the Cansona Fm., deposited along the fold belt of San Jacinto (east side). The oil seeps are affected by moderate to severe biodegradation, whereas the oil from the only oil producing well in the Sinu´ Basin (Floresanto-6 well) has not undergone biodegradation. 2006 Elsevier Ltd. All rights reserved.
1. Introduction The presence of hydrocarbons has been known for a long time in the Sinu´-San Jacinto fold belt. Numerous oil seeps have been found in the southern part of the Basin. In NW Colombia, exploitation *
Corresponding author. Tel.: +34 934021416; fax: +34 934021340. E-mail addresses: [email protected] (C. Sa´nchez), [email protected] (A. Permanyer). 1 Present address: Agencia Nacional de Hidrocarburos, Colombia.
started in 1908, when the Las Perdices well was drilled north of the San Jacinto fold belt. Between the 1940s and the 1960s various fields were discovered in the Plato and San Jorge sub basins as well as in the Sinu´ Basin (Floresanto field). At present, there is no longer any oil production in the Sinu´ Basin. In the San Jacinto fold belt some wells have been drilled but no oil and gas accumulations of economic importance have been discovered. Earlier studies identified a number of potential source rocks in the Sinu´-San Jacinto and the Lower Magdalena Valley basins, particularly in the Upper Cretaceous and in the Tertiary Oligocene to Miocene sediments
0146-6380/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2006.07.012
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C. Sa´nchez, A. Permanyer / Organic Geochemistry 37 (2006) 1831–1845
(Oppenheim, 1957; Haffer, 1963; Chevron, 1986; ESRI-ILEX, 1995; Ecopetrol-ICP, 1999; Caro and Spratt, 2003). The present study seeks to identify and re-evaluate the organic facies that account for the generation of oil in the Sinu´-San Jacinto fold belt, and aims to correlate the oil seeps with two of the candidate source rocks. To this end, the data of earlier studies were combined with those obtained in our study using techniques such as liquid and gas chromatography, biomarkers, and carbon isotopes. These techniques were applied to three classes of samples: source rock extracts, production oil, and oil seeps from the Sinu´-San Jacinto Basin. 2. Geological setting and earlier studies The Sinu´-San Jacinto Basin is located in the far northwest of Colombia along the boundary between the Caribbean and the South American plates (Fig. 1) where the subduction of the Caribbean plate under the South American plate determines the structural and stratigraphic patterns of the geological zones in western Colombia (Haffer, 1963; Duque-Caro, 1980; Chevron, 1986; Bowland,
1993; ESRI-ILEX, 1995; Laverde, 2000; Caro and Spratt, 2003). The Sinu´-San Jacinto Basin is located in the Colombian-Caribbean Basin, which is a marginal plate basin of the Cenozoic. It is made up of two belts with adjacent folds: (1) the San Jacinto fold belt of the Paleogene with an extension to the North and (2) the Sinu´ fold belt of the Neogene located along the western margin of the San Jacinto fold belt. Each belt has its own stratigraphic succession (Duque-Caro, 1980; Ecopetrol-ICP, 2000, 2003) (Fig. 2). Earlier studies by Chevron (1986), ESRI-ILEX (1995) and Ecopetrol-ICP (2003) showed that the best potential source rocks correspond to the intervals of the Maastrichtian and the Lower OligoceneEarly Miocene (Fig. 2). The former corresponds to the Cansona Fm. (Maastrichtian), which is only located in the San Jacinto fold belt. This unit has never been drilled by exploration wells. In outcrops it presents organic carbon (TOC) values that exceed 3% in the sections of Cantera San Sebastia´n, Cantera Purgatorio and Cerro Cansona (Fig. 1). The values of the hydrogen index are 360 mg HC/g TOC on average. Vitrinite reflectance indicates that
Fig. 1. General location map of the Sinu´-San Jacinto Basin and detailed emplacement of the studied samples: crude oil, oil seeps and outcrops (modified from Ecopetrol-ICP, 2003).
C. Sa´nchez, A. Permanyer / Organic Geochemistry 37 (2006) 1831–1845
SERIES SYSTEM Coastal SEQUENCE
onlap
PLIOCENE
QUATER- PLEISTO NARY CENE
H
U 3.8 L
LS8
U
G
M 15.5
LS7
NE
SW
SINU CORPA
NEOGENE
MIOCENE
PAJUIL
17.5
F
22.0
LS6
30.0
LS5
OLIGOCENE
TERTIARY
E
U
EOCENE
39.5
PA LEOCENE
RESERVOIR SEAL
SINCELEJO
TUBARA
CARMEN
CIENAGA DE ORO
LS4
SAN JACINTO MANANTIAL
TOLUVIEJO M
C CANDELARIA 49.5
UPPER
CRETACEOUS
PAVO PAVO
D
LS3
CHENGUE MACO
?
L
MESOZOIC SOZOIC
FLORESANTO
PETROLEUM SYSTEM SOURCE ROCK
MARALU
L
U
PAL EOGENE
CENOZOIC
L
SAN JACINTO
1833
U 61.0
L
94.0
SAN CAYETANO
LS2
A
Potential Source rock Reservoir rock
LS1
Potencial Reservoir rock Potential seal rock
CANSONA BASEMENT
Fig. 2. Stratigraphic units of Sinu´-San Jacinto Basin, and main hydrocarbon events (modified from Ecopetrol-ICP, 2003).
the Cansona Fm. reaches the oil window in the southern part of the basin. The Cie´naga de Oro Fm. forms part of Oligocene-Early Miocene sequence. It is located in the Sinu´-San Jacinto belts and in the basin of the Lower Magdalena Valley (LMV) situated to the west of the San Jacinto belt. This formation, which has been drilled in a number of wells, displays thicknesses between 60 and 800 m. Organic richness varies from poor to very good throughout the basin and the best potential was found in the San Jorge and the Plato basins (Fig. 1) in the upper part of the formation. TOC values range between 0.5% and 2.0% and the hydrogen index does not exceed 200. Vitrinite reflectance indicates that the Cie´naga de Oro Fm. reaches the oil window only in some areas of the Plato and San Jorge basins. Earlier studies of liquid hydrocarbons have mainly focused on samples of oil seeps, and three families of oils have been identified (Petrobras-
CENPES-CEGEQ, 1996; Ecopetrol-ICP, 1999, 2003). The first family constitutes oils derived from continental organic matter deposited in marine deltaic to nearshore environments of the Tertiary. The second family is also derived from continental organic matter deposited in nearshore siliciclastic environments of the Tertiary in a less proximal position than the first family. The third family is made up of oils derived from marine facies deposited in coastal shelf anoxic environments of the Cretaceous (Petrobras-CENPES-CEGEQ, 1996; EcopetrolICP, 1999, 2003). 3. Samples Five samples of rock extracts, one sample of crude oil and twelve samples of oil seeps from the Sinu´-San Jacinto Basin were analysed. The two rock extracts from the Cie´naga de Oro Fm. proceeded from the cores of the La Arena-1
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C. Sa´nchez, A. Permanyer / Organic Geochemistry 37 (2006) 1831–1845
Table 1 Pyrolysis Rock–Eval data of rock samples from the Cie´naga de Oro and Cansona formations ID Formation Locality Sample type
B-647 Cie´naga de Oro La Arena-1 Rock sample
B-648 Cie´naga de Oro La Arena-1 Rock sample
B-649 Cansona Cantera Golf Rock sample
B-651 Cansona Cantera Golf Rock sample
B-652 Cansona Cantera Golf Rock sample
TOC (%) S1 (mg HC/g of rock) S2 (mg HC/g of rock) S3 (mg CO2/g of rock) Hydrogen index Tmax (C)
25.0 0.8 34.7 8.2 138 411
16.1 0.6 15.7 6.3 97 402
1.3 4.17a 4.4 0.1 338 438
2.7 14.73a 8.5 0.3 314 330 nr
6.5 44.06a 20.9 0.3 321 315 nr
All data from ESRI-ILEX (1995); Ecopetrol-ICP (2003). a Denotes impregnated samples. nr = non representative.
well whereas those of the Cansona Fm came from the Cantera Golf outcrop (Fig. 1). Arena-1 is a shallow well drilled for stratigraphic purposes. Samples were taken at depths ranging between 450 and 780 m. The extracts and the Rock–Eval data from earlier studies were provided by the ICP (Table 1). The crude oil sample came from the Floresanto-6 well. Oil was produced in the Pajuil Fm. (base at 1370 ft 418 m). The well was shallow (1505 ft, 459 m) and had a low production (about 65 barrels per day in 1978). The oil seeps studied were from the following outcrops (from south to north, Fig. 1): Matamorito, Cienaguita, Pativilca, Buenavista, La Lucha, La Plancha, Cerro El Gas, Cruz de Guayabo, Finca Juan Banda, Mina San Sebastia´n, El Salvador and El Castillo. 4. Analytical methods The techniques used included liquid and gas chromatography, gas chromatography-mass spectrometry and carbon isotopes. Rock–Eval pyrolysis and total organic carbon (TOC) data of the samples from Cansona and Cie´naga de Oro formations were taken from earlier studies (Chevron, 1986; ESRIILEX, 1995; Ecopetrol-ICP, 2000, 2003) as were the values of API gravity and sulphur content for the oil seep samples. Samples were fractioned into saturated (SAT) and aromatic (ARO) hydrocarbons, and heavy compounds (NSO) using column liquid chromatography. Previously, oils had been topped for comparison with rock extracts. Topping was achieved at 50 C by using a 20–60 mb vacuum pressure for five hours. Gas chromatography was carried out on whole oil and on saturated fractions from crude oil and oil seeps, on a Delta Chrom Series 9980
model with a flame ionization detector. Separation was achieved on a fused silica capillary column J&W PONA (50 m · 0.2 mm · 0.5 lm). GC Chemstation Agilent Technologies 1990–2000 software was used. The GC operating conditions were as follows: temperature maintained at 35 C for 15 min, increased from 35 to 320 C at a rate of 2 C/min, and maintained at 320 C for 30 min. Saturated fractions were also analysed by gas chromatography–mass spectrometry (GC–MS) using a Thermo Electron MD 800 high resolution mass spectrometer coupled to a Thermo Electron 8060 gas chromatograph. The GC/MS operating conditions were as follows: a non polar J&W DB-5 column (60 m · 0.25 mm · 0.1 lm), temperature maintained at 40 C for 1 min, increased from 40 to 300 C at a rate of 2 C/min, and maintained at 300 C for 60 min. The mass spectrometer was operated in electron impact mode with an ionization voltage of 40 eV and a source temperature of 200 C and data were recorded by selected ion monitoring mode of eight ions. Terpane, sterane and diasterane ratios were calculated from the resulting m/z 177, m/z 191, m/z 217, m/z 218 and m/z 259 mass fragmentograms. Data were processed with a Masslab 1.4 de FinniganTM system. Carbon isotope measurements of saturate and aromatic hydrocarbons were performed on a Thermo Finnigan Series 1112 elemental analyser coupled to a Finnigan Mat Delta C mass spectrometer. The reference materials were Graphite (USGS 24), Saccharose (IAEA-CH6), Polyethylene (IAEACH7) and Oil (NBS-22). 5. Results The results of geochemical characterization of the two rock extracts, one crude oil and twelve oil
C. Sa´nchez, A. Permanyer / Organic Geochemistry 37 (2006) 1831–1845
seeps are presented in Tables 1–5 as well as in Figs. 3–5 and discussed below. 5.1. Geochemical characterization of rock extracts The five extracts from the Cie´naga de Oro and Cansona formations together with the Rock–Eval data were provided by the IPC (Table 1). The two rock extracts from the Cie´naga de Oro Fm. came from the cores of the La Arena-1 well, drilled for stratigraphic purposes (Fig. 1). Samples were taken at depths ranging between 450 and 780 m. The extracts from the Cie´naga de Oro Fm. show a composition high in resins and asphaltenes (more than 55%) whereas the fractions of saturated hydrocarbons ranged between 21% and 31% and the aromatics between 10% and 12% (Table 2). The chromatograms display a quasi absence of normal paraffins with the exception of C29, C31 and C33 nor-
1835
mal alkanes (Fig. 3A). The isoprenoids pristane and phytane are well developed with Pr/Ph ratios higher than four. The mass chromatograms show that the amount of terpanes between C27 and C35 is very low (Fig. 4A). The quasi absence of tricyclic terpanes is observed whereas C28 and C29 norhopanes appear well developed. Steranes are scarce and the absence of C27 is noted. The C29 steranes are dominant, as is the C29 diasterane with respect to C28 (Table 2). The Pr/Ph and (Pr/n-C17)/(Ph/n-C18) ratios and the dominance of the C29 steranes and diasteranes suggest a terrestrial origin of the organic matter deposited in an oxic environment (Tissot and Welte, 1984; Peters and Moldowan, 1993). Sterane isomerization ratios indicate immaturity of the organic matter (equivalent Ro 0.4%, Table 2), which is in agreement with the composition of the extracts that are high in resins and asphaltenes.
Table 2 Bulk composition, selected geochemical ratios and isotopic values of the rock extracts from the Cie´naga de Oro and Cansona formations ID Formation Locality Sample type
B-647 Cienaga de Oro La Arena-1 Rock extract
B-648 Cienaga de Oro La Arena-1 Rock extract
B-649 Cansona Cantera Golf Rock extract
B-651 Cansona Cantera Golf Rock extract
B-652 Cansona Cantera Golf Rock extract
Saturate fraction (%) Aromatic fraction (%) NSO (%) Pr/ph Pr/n-C17 Ph/n-C18 (Pr/n-C17)/(Ph/n-C18) n-C29/n-C17 CPI d13C SAT (&) d13C ARO (&) C27 Diasterane (%) C28 Diasterane (%) C29 Disterane (%) C27 Sterane (%) C28 Sterane (%) C29 Sterane (%) C27/29 Steranes Diasteranes/Reg. Steranes C29aa(20S)/C29aa(20R) Ts/Tm Tricyclic index C26/C25 Tricyclic terpanes C24/C23 Tricyclic terpanes C24 Tetracyclic/C26 Tricyclic C23 Tricyclic/C24 Tetracyclic C29 Norhopane/C30 Hopane Oleanane/C30 Hopane % Ro. Equivalent (from ster.Isom)
21.3 10.6 68.1 5.1 6.3 1.1 5.9 0.8 0.8 28.5 28.4 5.4 17.6 77.0 0.3 4.5 95.2 0.1 0.2 0.1 0.3 0.1 2.3 0.9 0.7 0.4 12.9 1.9 0.40
31.2 12.5 56.3 4.0 3.9 1.0 4.1 1.0 1.0 26.4 28.0 4.0 11.7 84.2 0.2 2.7 97.1 0.0 0.2 0.3 0.6 0.3 1.9 1.0 0.7 0.5 7.5 5.5 0.46
61.51a 20.4 18.1 Biodegr. Biodegr. Biodegr. Biodegr. Biodegr. Biodegr. 29.0 28.9 33.0 42.0 25.1 42.5 36.4 21.1 1.6 2.1 1.8 1.7 0.9 0.9 0.7 0.1 15.1 1.1 2.3 1.20
68.46a 17.9 13.6 Biodegr. Biodegr. Biodegr. Biodegr. Biodegr. Biodegr. 29.0 27.7 38.0 40.9 21.1 54.9 22.8 22.3 1.2 2.4 1.5 1.9 0.9 0.9 0.7 0.2 14.19.1 0.7 1.7 1.06
67.3a 21.5 11.2 Biodegr. Biodegr. Biodegr. Biodegr. Biodegr. Biodegr. 29.3 27.9 21.2 51.1 27.6 41.2 48.1 10.7 1.0 1.1 0.8 0.7 0.9 1.0 0.7 0.1 0.8 2.2 nr
a Denotes impregnated samples; nr = non representative; Tricyclic index = Ratio of tricyclic terpanes (C19-C30) over tricyclic terpanes plus hopanes (C29-C36).
1836
ID Locality
B-633 Floresanto-6
B-632 Matamorito
B-634 Pativilca
B-635 Cienaguita-3
B-636 Buenavista
B-638 La Plancha
Sample type Assigned facies type
Crude Oil
Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 1
API (+) Sulphur (%) (+) Saturate fraction (%) Aromatic fraction (%) NSO (%) Pr/Ph Pr/n-C17 Ph/n-C18 (Pr/n-C17)/(Ph/n-C18) n-C29/n-C17 CPI d 13C SAT (&) d 13C ARO (&)
44.98 0.10 76.19 19.49 4.32 5.15 1.07 0.18 6.02 0.34 1.02 27.40 26.24
20.72 0.08 51.94 42.30 5.76 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 24.67 24.54
16.05 0.14 53.40 32.04 14.56 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 22.85 22.77
12.17 0.18 40.31 31.83 27.86 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 24.13 23.57
18.16 0.16 43.66 41.35 14.99 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 23.18 23.48
(+) Data from ESRI-ILEX (1995); Ecopetrol-ICP (2003).
B-643 El Salvador
B-644 El Castillo
B-637 La Lucha
B-639 Cerro El Gas
B-640 Cruz de Cuayabo
B-642 Mina San Sebastian
Oil seep Facies 1
B-641 Finca Juan Banda Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 2
Oil seep Facies 2
Oil seep Facies 2
Oil seep Facies 2
n.a 0.25 49.96 33.03 17.01 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 26.11 25.80
15.18 0.17 41.28 49.02 9.70 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 25.00 24.60
18.28 0.18 43.36 43.85 12.80 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 24.70 24.30
18.49 0.11 46.49 33.60 19.91 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 25.50 25.61
15.59 1.96 40.37 38.92 20.71 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 28.51 27.60
9.18 2.13 27.92 30.45 41.63 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 28.35 27.80
14.39 1.68 40.71 39.29 20.00 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 28.52 27.52
15.79 1.79 38.55 34.86 26.59 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 28.60 28.20
C. Sa´nchez, A. Permanyer / Organic Geochemistry 37 (2006) 1831–1845
Table 3 Bulk composition, selected geochemical ratios and isotopic values of oil seeps and Floresanto-6 crude oil
Table 4 Main biomarker parameters and selected ratios of Floresanto-6 crude oil and oil seeps samples B-633 Floresanto-6
B-632 Matamorito
B-634 Pativilca
B-635 Cienaguita-3
B-636 Buenavista
B-638 La Plancha
B-643 El Salvador
B-644 El Castillo
B-637 La Lucha
B-639 Cerro El Gas
B-640 Cruz de Cuayabo
Oil seep Facies 1
B-641 Finca Juan Banda Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 2
Oil seep Facies 2
Oil seep Facies 2
B-642 Mina San Sebastian Oil seep Facies 2
Sample type Assigned facies type
Crude Oil
Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 1
C27 Diasterane (%) C28 Diasterane (%) C29 Diasterane (%) C27 Sterance (%) C28 Sterane (%) C29 Sterane (%) C27/C29 Steranes C29 aa(20S)/C29 aa(20R) Diasteranes/Reg. Steranes Steranes/Hopanes Ts/Tm Tricyclic index C26/C25 Tricyclic terpanes C24/C23 Tricyclic terpanes C24 Tetracyclic/ C26 Tricyclic C23 Tricyclic/C24 Tetracyclic C29 Nor-hopane/ C30 Hopane Oleanane/C30 Hopane C35/C34 Homohopanes Gammacerane/C30 Hopane Biodegradation degree (see Table 5) % Ro. Equivalent (from ster. lsom.)
22.22 32.54 45.23 24.42 33.37 42.21 0.49 0.87
15.72 59.65 24.62 24.49 32.60 42.91 0.54 0.87
27.02 40.73 32.25 Biodegr. Biodegr. Biodegr. Biodegr. 0.78
25.75 38.29 35.96 Biodegr. Biodegr. Biodegr. Biodegr. 1.33
26.84 43.35 29.81 28.38 38.20 33.42 0.93 0.42
26.49 37.13 36.38 Biodegr. Biodegr. Biodegr. Biodegr. 1.11
25.24 40.79 33.96 30.95 35.00 34.05 0.91 0.59
26.35 40.51 33.15 25.29 35.13 39.58 0.73 0.59
24.17 35.00 40.84 20.93 28.98 50.09 0.47 0.82
26.09 40.70 33.22 Biodegr. Biodegr. Biodegr. Biodegr. 0.65
20.32 36.81 42.88 Biodegr. Biodegr. Biodegr. Biodegr. 0.77
28.93 37.42 33.65 Biodegr. Biodegr. Biodegr. Biodegr. 0.86
26.61 38.22 35.17 35.62 34.49 29.89 1.11 0.65
0.70
0.96
Biodegr.
Biodegr.
0.39
Biodegr.
0.35
0.57
0.55
Biodegr.
Biodegr.
Biodegr.
0.19
0.36 1.66 0.22 1.14
Biodegr. 1.85 0.23 1.86
Biodegr. 1.37 0.34 1.87
Biodegr. 1.33 0.19 1.38
0.63 0.99 0.19 1.39
Biodegr. 0.80 0.21 1.03
0.27 1.20 0.11 1.40
0.41 1.13 0.16 1.16
0.19 1.16 0.09 1.41
Biodegr. 0.51 0.64 0.87
Biodegr. 0.54 0.17 0.88
Biodegr. 0.55 0.70 0.86
0.51 0.29 0.34 0.92
0.95
1.09
0.10
0.99
0.93
0.86
0.81
1.03
1.04
0.74
0.73
0.74
0.65
0.77
1.10
0.52
0.81
0.76
1.05
1.50
1.12
2.04
0.38
0.49
0.40
0.31
2.03
1.14
1.98
1.49
1.77
1.51
1.12
1.38
0.74
5.24
3.89
5.16
8.05
0.45
0.47
0.49
Biodegr.
0.56
Biodegr.
0.47
0.48
0.46
Biodegr.
0.50
Biodegr.
0.70
0.83
1.19
1.43
Biodegr.
1.16
Biodegr.
0.44
0.71
0.41
Biodegr.
0.50
Biodegr.
0.14
0.74
0.69
0.35
Biodegr.
0.41
Biodegr.
0.36
0.49
0.38
Biodegr.
0.81
Biodegr.
1.10
3.05
2.66
2.51
Biodegr.
2.65
Biodegr.
2.24
2.38
2.44
Biodegr.
3.26
Biodegr.
4.31
0
6
6
8
4–5
8
4–5
5–6
5–6
8
6
8–9
5–6
0.76
0.76
0.71
0.98
0.54
0.87
0.62
0.62
0.73
0.65
0.71
0.75
0.65
C. Sa´nchez, A. Permanyer / Organic Geochemistry 37 (2006) 1831–1845
ID Locality
Tricyclic index = Ratio of tricyclic terpanes (C19-C30) over tricyclic terpanes plus hopanes (C29-C36). 1837
1838
Table 5 Short description of the oil seeps current state of preservation and a proposal of level of biodegradation n-paraffin
Is oprenoids
Steranes
Hopanes
Diasternes
Biodegradation degree
API (+)
Sulphur (%) (+)
Facies 1 Matamorito
Depleted
Depleted
Intact Oln > C30H
Intact
6
20.72
0.08
Pativilca
Depleted
Depleted
Intact Oln > C30H
Intact
6
16.06
0.14
Cienagu¨ita-3
Depleted
Depleted
Partly degraded
Intact
8
12.17
0.18
Buenavista La Plancha
Depleted Depleted
Partly depleted Depleted
aaa20S partly degraded More to less degraded: C27 > C28 > C29 aaa20S partly degraded More to less degraded: C27 > C28 > C29 aaa 20(R+S) degraded abb 20(R+S) depleted Intact Nearly all degraded
Intact Oln > C30H Strongly degraded
4–5 8
18.16 n.a.
0.16 0.25
Finca Juan Banda El Salvador El Castillo
Depleted Depleted Depleted
Partly depleted Partly depleted Partly depleted
Intact aaa 20S partly depleted aaa 20S partly depleted aaa 20R moderately degraded
Intact Intact Oln > C30H Intact
Intact Slightly to moderately degraded Intact Intact Intact
4–5 5–6 5–6
15.48 18.28 18–49
0.17 0.18 0.11
Facies 2 La Lucha
Depleted
Depleted
Nearly all degraded
Strongly degraded
8
15.59
1.96
Cerro El Gas
Depleted
Depleted
Intact
6
9.18
2.13
Cruz de Guaybo
Depleted
Depleted
aaa 20S partly degraded More to less degraded: C27 > C28 > C29 Nearly all degraded
Slightly to moderately degraded Intact
Strongly degraded
8–9
14.39
1.68
Mina San Sebatia´n
Depleted
Partly depleted
aaa 20S partly depleted
Intact
5–6
15.79
1.79
Biodegradation degree is based on the scale of Peters and Moldowan (1993), ranging from 1 to 10 (low to high). (+) Data from ESRI-ILEX (1995); Ecopetrol-ICP (2003). Oln: Oleanane.
Slightly to moderately degraded Intact
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Oil seep
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Fig. 4. Mass chromatograms of terpanes and steranes (ions m/z 191 and m/z 217) from extracts from Cie´naga de Oro and Cansona Formations (TT: tricyclic terpanes; Ts: trisnorneohopane; Tm: trisnorhopane; C27, C28 and C29 refers to regular steranes).
Fig. 3. Gas chromatograms of rock samples (A and B), oil from Floresanto-6 well (C) and two oils seeps (D and E) (Pr: Pristane; Ph: Phytane; n-C: n-alkanes).
The d13C isotopic values range between 26.4& and 28.5& for saturates and 28& and 28.4& for aromatics (Table 2). The Rock–Eval pyrolysis data from the internal reports of the ICP (ESRI-ILEX, 1995; EcopetrolICP, 2003) are presented in Table 1. These data show high TOC, high S2 values and a low hydrogen index (HI < 150), denoting a type III kerogen. The Tmax between 402 and 411 C confirms the immaturity in the organic matter from the Cie´naga de Oro Fm. The three rock extracts from the Cansona Fm. came from the Cantera Golf outcrop (Fig. 1). These rock extracts have a composition that is high in sat-
Fig. 5. Terpane and sterane mass chromatograms from Floresanto-6 crude oil (TT: tricyclic terpanes; TeT: tetracyclic terpane; Ts: trisnorneohopane; Tm: trisnorhopane; H: homohopanes).
urated hydrocarbons (>62%) with small amounts of aromatic hydrocarbons (between 18% and 22%) and resins and asphaltenes (between 10% and 20%) (Table 2). The gas chromatograms display a high unresolved complex mixture (UCM) and a very
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small amount of n-alkanes and isoprenoids, suggesting severe biodegradation (Fig. 3B). The terpanes contained more tricyclic than pentacyclic biomarkers (Fig. 4B). Of these, oleanane and C30 hopane should be noted. The former for its notable presence and the latter for its degradation. An important depletion in the homohopane series was observed. Although the steranes were very scarce, there were more C27 diasteranes than C29 diasteranes despite severe biodegradation. The tricyclic terpanes are generated in a stage of greater thermal maturity and are more resistant to biodegradation than the homohopanes (Peters and Moldowan, 1993). Therefore, they constitute useful markers for environment determination and source-oil correlations. According to Peters and Moldowan (1993), the low C26/C25 and C24/C23 tricyclic terpane ratios (less than 1) are indicative of organic matter deposited in carbonate marine environments. The C28/C29 diasterane ratio exceeding 1.8 (Table 2) also allows us to confirm the marine input (Waples and Machihara, 1991; Peters and Moldowan, 1993). The isomerization ratios of regular steranes show values that suggest a high thermal maturity (equivalent Ro about 1%). These values should be considered with care because of the biodegradation of the steranes. However, the composition of the extracts that are high in saturated hydrocarbons confirms a high degree of maturity. The pyrolysis data presented in Table 1 show high S1 values (from 4 to 44 mg HC/g rock) as well as high S2 values ranging between 4 and 21 mg HC/ g rock. These results suggest that the rock samples from the Cansona Fm. are impregnated with oil and present a good source rock potential (Espitalie´ et al., 1985/1986). The Tmax values are not representative of maturity because the samples are impregnated with oil as indicated by the S1 values (S1 upto 14 mg HC/g rock) (Espitalie´ et al., 1985/1986). The d13C isotopic values are around 29& for saturates and range between 27.7& and 28.9& for aromatics (Table 2).
asphaltenes (<5%) (Table 3). The whole oil gas chromatogram (Fig. 3C) shows a classic distribution with a significant contribution of paraffinic compounds, and a minor depletion of C 13 compounds, which suggests that the oil was not affected by biodegradation. The Cþ 15 fraction shows no dominance of odd over even n-alkanes (carbon preference index, CPI = 1.02). The isoprenoids show a large predominance of pristane over phytane (Pr/ Ph > 5) (Table 3). The terpanes contain abundant tricyclic and pentacyclic biomarkers (Fig. 5, Table 4). Of these, the C24 tetracyclic, C29 norhopane, oleanane and the C30 hopane are well developed. Both steranes and diasteranes show a slight predominance of C29 over C27 (Fig. 5, Table 4). The isotopic values of the Floresanto-6 crude oil (d13C 27.4& for saturates and 26.24& for aromatics, Table 3) show an isotopic composition that is midway between the rock extracts from the Cie´naga del Oro Fm. and those of the Cansona Fm. The high Pr/Ph and (Pr/n-C17)/(Ph/n-C18) ratios, the raised oleanane content with respect to C30 hopane, the hopane/sterane ratio less than unity and the greater abundance of the C29 sterane with respect to C28 and C27 (Fig. 5) all suggest a terrestrial input to the organic matter of the source rock. The tricyclic C26/C25 ratio exceeding 1.1 also suggests the predominantly terrestrial origin of the organic matter of the source rock (Mello et al., 1987; Waples and Machihara, 1991; Peters and Moldowan, 1993). The carbon preference index near unity is characteristic of thermally mature crude oils generated in the maximum peak of the oil window (Marteau et al., 2002). This is corroborated by the isomerization of the regular steranes which reaches its end point and is evidenced by the high gravity value (45 API) and the composition of the oil (>75% saturated hydrocarbons).
5.2. Geochemical characterization of Floresanto-6 oil
The oil seeps studied in the Sinu´ and San Jacinto basins came from the following outcrops (from south to north, Fig. 1): Matamorito, Cienaguita, Pativilca, Buenavista, La Lucha, La Plancha, Cerro El Gas, Cruz de Guayabo, Finca Juan Banda, Mina San Sebastia´n, El Salvador and El Castillo. More than 80% of the oil seep samples studied have a composition of saturated hydrocarbons between 40% and 55%, aromatic hydrocarbons between
The crude oil sample came from the Floresanto-6 well located in the central part of the Sinu´ Basin. Oil was produced from carbonates in the Pajuil Fm. of Middle to Upper Miocene (Figs. 1 and 2). The Floresanto-6 oil has a composition that is high in saturated hydrocarbons (>75%) with small amounts of aromatic hydrocarbons (<20%) and resins and
5.3. Geochemical characterization of oil seeps
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32% and 50%, and resins plus asphaltenes between 6% and 28%. Less than 20% of the samples have a composition that is higher in heavy compounds (NSO between 27% and 42%) than in aromatic hydrocarbons (between 31% and 35%) and saturated hydrocarbons (between 28% and 39%) (Table 3). The gas chromatograms of all the oil seeps show a low amount of n-alkanes and isoprenoids and medium to high concentration of Unresolved Complex Mixture (UCM) (Figs. 3D and E), which is attributable to the effects of biodegradation. The terpanes contain tricyclic and pentacyclic biomarkers. In eight of the samples (Table 5), hopanes appear intact (Mina San Sebastia´n, El Castillo. . .) whereas in four samples hopanes are partially or strongly degraded (Cienaguita-3, Cruz de Guayabo. . .). The steranes are more or less affected by biodegradation. They appear intact only in two of the samples (Buenavista and Finca Juan Banda). By contrast, diasteranes are intact in nine of the samples and slightly or moderately degraded in the three others. The degree of biodegradation is determined in accordance with the scale proposed by Peters and Moldowan (1993). Table 5 provides for each biomarker family (n-alkanes, isoprenoids, steranes, hopanes and diasteranes) a short description of the current state of its preservation, and proposes a level of biodegradation. The absence of normal alkanes and isoprenoids in all the oil seeps indicates a level equal to or higher than 4 out of 10. Most of the oil seeps range between levels 4 and 6. The most biodegraded oil seep reaches level 8 out of 10. The presence of the 25-norhopane biomarker in these samples is consistent with the severe process of biodegradation (Fig. 6). The 25-norhopanes (10-demethylated) are a series of compounds that are often linked to processes of biodegradation (Philp, 1985). These compounds appear to result from the bacterial degradation of the C10 methyl group in the regular hopanes and are more easily detected in the mass chromatogram of triterpanes (m/z 177). The samples with intact hopanes have a higher content of oleanane than C30 hopane. There is a distribution in descending order of the homohopanes from C31 to C35 in almost all samples with the exception of the oil seep sample from Mina San Sebastia´n, which had a very low concentration of oleanane and a higher content of C35 than C34 homohopanes (Fig. 7A and B).
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Fig. 6. Mass chromatograms (ion m/z 177) from two oil seeps affected by severe biodegradation, showing the presence of 25norhopanes. Mass chromatogram (ion m/z 177) from Floresanto6 oil (non biodegraded) is presented for comparison.
Fig. 7. Terpane and sterane mass chromatograms corresponding to the two types of oil seeps (TT: tricyclic terpanes; TeT: tetracyclic terpane; Ts: trisnorneohopane; Tm: trisnorhopane; H: homohopanes; C27, C28 and C29 refers to regular steranes).
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6. Discussion 6.1. Oil seeps-oil correlation Oil–oil correlation requires parameters that are not altered by common geochemical processes such as biodegradation. In the oil seeps studied, biodegradation is severe with the result that n-alkanes, isoprenoids, steranes, and in some cases hopanes, are not suitable for establishing correlations. By contrast, the tricyclic and the tetracyclic terpanes are more resistant to biodegradation than hopanes and therefore constitute good tools for correlation purposes. Tricyclic terpanes are commonly found in oils and source rock extracts and probably originate in prokaryotic cell membranes (Ourisson et al., 1982). Tricyclic terpanes are formed in various depositional environments: the C23 member is frequently the dominant homologue in crude oils of marine origin while C25 is more abundant than C26 in terrestrial oils (Aquino Neto et al., 1983; Peters and Moldowan, 1993). The proportion of C24 tetracyclic terpanes with respect to tricyclic terpanes may also be facies dependent with a relatively high concentration in source rocks and oils with terrigenous input (Philp and Gilbert, 1986). Therefore, the difference in the distribution of these tricyclic and tetracyclic terpanes can be used for oil family classification. In eight of the oil seeps studied as well as in the Foresanto-6 oil, the C26/C25 ratios of the tricyclic terpanes (TT) exceed unity. Moreover, these oil seeps present high C24/C23 ratios of tricyclic terpanes and high C24 tetracyclic terpanes with respect to the C26 tricyclic terpane ratios (Table 4). The remaining oil seeps (Mina San Sebastian, Cerro El
Gas, Cruz de Guayabo and La Lucha) present values that are less than 1 in both tricyclic ratios as well as a low amount of C24 tetracyclic terpanes (C24Tet/ C23TT around 0.7) (Table 4). Based on these data (see Fig. 8), the first eight oils were generated from organic matter with terrestrial input deposited in marine deltaic environments. The other four oil seeps were generated from a more marine organic matter. Oleanane is postulated to have derived from angiosperms whose presence has been attributed to the Late Cretaceous and the Tertiary (Peters and Moldowan, 1993). Of the pentacyclic terpanes, oleanane appears to be one of the compounds that is most resistant to biodegradation. Oleanane was found in all the samples but it is especially abundant in the oils whose source is linked to terrestrial input. The carbon isotopic composition typically depends on the d 13C value of the kerogen in the source rocks from which it is derived. The d13C depends on both the type of organic matter and the depositional environmental conditions (Schoell, 1984; Peters et al., 1986). d13C values are consequently useful for determining oil–oil and oil-source rock correlations. A plot of the carbon isotope values of the saturated and aromatic hydrocarbons in oils proposed by Sofer (1984) is shown in Fig. 9. The plot divides the oil seeps into two groups which resemble those previously distinguished by the tricyclic and tetracyclic biomarkers. One group presents values ranging between 28& and 23& for saturates and between 26& and 23& for aromatics (Table 3). The other group offers lighter d13C isotopic values of about 29& for saturates and 28& for aromatics. Isotopic values from the Floresanto-6 oil fall between these two groups.
Fig. 8. Tricyclic terpanes C24/C23 vs. C26/C25 diagram from oil seeps, Floresanto-6 crude oil and rock extracts.
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Two groups of oils are identified in accordance with the biomarkers and the isotopic composition: – One group (facies 1) presents TT C24/C23 > 0.8, TT C26/C25 > 1, and d13C isotopic values ranging from 27& to 23& for saturates and between 26& and 23& for aromatics. It corresponds to eight oil seeps whose origin may be related to organic matter of mixed marine and terrestrial origin probably deposited in marine marginal to nearshore (deltaic) environments. C27/C29 sterane ratios around 0.9 in the less degraded samples are in agreement with this interpretation. According to Peters and Moldowan (1993), the high oleanane content may be related to the Tertiary. – The other group (facies 2) presents TT C24/ C23 < 0.8, TT C26/C25 < 1, and d13C isotopic values of about 29& for saturates and 28& for aromatics. It corresponds to four oil seeps whose origin is related to organic matter of marine origin. In the least degraded sample (Mina San Sebastian), the C27/C29 sterane ratio that is greater than unity corroborates the marine input of organic matter.
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and presented in Table 3, all the oil seeps show a low gravity (<21 API), suggesting an increase in density due to biodegradation (Tables 3 and 5). By contrast, the Floresanto-6 oil, which is genetically linked to some oil seeps, offers no evidence of biodegradation and has a gravity of 45 API. The sulphur content is low (<0.22%) in the Floresanto-6 oil and in the group of eight oil seeps related to the deltaic environment. The sulphur content is higher (>1.65%) for the oil seeps of marine origin. Surprisingly, there is no apparent relationship between the sulphur content and the degree of biodegradation (Tables 3 and 5). For example, the Cienaguita-3 oil seep presents a low sulphur content (0.18%), whereas the La Lucha oil seep has a high sulphur content (1.96%) despite the severe biodegradation undergone by both oils (level 8 out of 10). The plot of the sulphur content and the API gravity is shown in Fig. 10. This plot provides evidence of two groups of samples that correspond to the two groups previously distinguished by biomarkers and isotopic signatures. It has not been possible to account for the anomalous variations in sulphur content with respect to biodegradation. 6.2. Oil-source correlation
API gravity is a bulk parameter that characterizes whole oil. Biodegradation reduces oil API gravity and increases sulphur content and other heteroatoms by selectively removing saturate and aromatic hydrocarbons compared with NSO compounds (Peters and Moldowan, 1993). According to the data from the internal reports of the Ecopetrol-ICP (ESRI-ILEX, 1995; Ecopetrol-ICP, 2003)
Given the biodegradation of the seeps and the rock extracts, the most suitable geochemical parameters for oil-source correlation include tricyclic terpanes and carbon isotopes. The diagram of tricyclic terpane ratios (Fig. 8) shows that the oil seeps derived from organic matter of marine origin (facies 2) correlate well with the
Fig. 9. d13C isotopic values of saturated and aromatic fractions in the Sofer (1984) diagram: extracts from the Cie´naga de Oro and Cansona formations, crude oil from Floresanto-6 well and oil seeps.
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Fig. 10. Sulphur content vs. API gravity plot for Floresanto-6 crude oil and oil seeps (data from ESRI-ILEX, 1995; EcopetrolICP, 2003).
rock extracts from the Cansona Fm. A similar correlation is obtained from carbon isotopes showing relatively light values of d13C for the saturated and aromatic hydrocarbons (Fig. 9). The Cansona Fm. constitutes a good source rock of marine origin as deduced from the geochemical results. The source rock is mature where it outcrops in the central part of the Sinu´ Basin. It generated oil that impregnated the rock and underwent biodegradation. The extracts appear more representative of the generated oil than of the source rock itself. The results obtained from the Cie´naga de Oro extracts indicate that these rocks offer no source rock potential where the Arena-1 well was drilled in the San Jacinto fold belt (Fig. 1). The terrestrial origin of the organic matter is corroborated by the high isoprenoid ratios as well as by the dominance of C29 steranes. Despite the low tricyclic terpane content, the ratios of C24/C25 TT and C26/C25 TT were calculated. In the plot of tricyclic terpanes (Fig. 8), the Cie´naga de Oro Fm. falls in the same cluster as the Matamorito and Pativilca oil seeps in the non marine quarter. The isotopic values from the aromatic fraction in the Cie´naga de Oro extracts are lighter than those obtained for Floresanto crude oil and oil seeps from facies one. It has been postulated that heavier isotopic values could result from biodegradation (Sofer, 1984), which could explain the heavy values found in the oil seeps with respect to the Floresanto oil and to the Cie´naga de Oro extracts. The poor source rock potential of the Cie´naga de Oro observed in the Arena-1 well does not exclude this formation from being a good source rock in other areas. According to ESRI-ILEX (1995), Pet-
robras-CENPES-CEGEQ (1996) and EcopetrolICP (2003), the Cie´naga de Oro Fm. was deposited in the Sinu´ and San Jacinto belts, as well as in the Lower Magdalena Valley, where it offers a good potential. In particular, in the Plato Basin, crude oils were correlated with extracts from the Cie´naga de Oro Fm., confirming its oil generation capacity. As stated above, oil seeps have a large abundance of saturated hydrocarbons with respect to aromatic hydrocarbons, resins and asphaltenes. Because of biodegradation, the composition of oil seeps usually show greater relative contents of resins and asphaltenes than saturated and aromatic hydrocarbon. This relative abundance of saturated and aromatic hydrocarbons suggests a continued supply of oil in the oil seeps, which is consistent with the occurrence of active source rocks. In the Sinu´-San Jacinto Basin, the presence of oil seeps of two different origins indicates that at least two different source rocks are effective in the area. Finally, the presence of the two types of oil seeps in very restricted areas (such as Finca Juan Banda and Cruz de Guayabo, or La Lucha and La Plancha) provides evidence of the structural complexity of the migration paths in this basin. 7. Conclusions The results obtained from biomarkers and isotopes enabled us to identify two organic facies that generate hydrocarbons in the southern part of the Sinu´-San Jacinto Basin. The crude oil of the Floresanto-6 well and the oil seeps Matamorito, Pativilca, Cienaguita-3, Buenavista, La Plancha, Finca Juan Banda, El Salvador and El Castillo are generated in siliciclastic source rock sediments derived from organic matter with terrestrial input deposited in environments ranging from marine marginal to marine deltaic conditions during the Tertiary. The correlation with the Cie´naga de Oro Fm. has not been clearly established. The oil seeps from Mina San Sebastia´n, Cerro El Gas, La Lucha and Cruz de Guayabo are generated in a source rock derived from marine organic matter possibly deposited in coastal shelf to pelagic environments during the Upper Cretaceous. The rock extracts from the Cansona Fm. correlate well with this organic facies. The oil seeps analysed have undergone moderate to severe processes of biodegradation, levels 4 to 8 out of 10 on the scale of Peters and Moldowan.
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Most oil seeps that are heavily biodegraded present a composition that is high in saturated hydrocarbons, which suggests a continuous supply of oil that is rich in light hydrocarbons of low molecular weight. Acknowledgements C. Sa´nchez obtained a scholarship from the Universitat de Barcelona to carry out this work. This research was also supported by the Spanish Ministry of Education and Science (Grand BTE2003-06915) and DURSI from Catalonia Government (2001SGR00075, ‘‘Grup de Geologia Sedimenta`ria). We are indebted to the Instituto Colombiano del Petro´leo (ICP) for providing samples and some reports. The authors are grateful to Dr. Eugenio Vaz do Santos Neto (Petrobras) for his comments and suggestions, and to Drs. M. Kruge and D. Naafs for reviewing the manuscript. Guest Associate Editor—B. Artur Stankiewicz
References Aquino Neto, F.R., Restle, A., Connan, J., Albrecht, P., Ourisson, G., 1983. Novel tricyclic terpanes (C19, C20) in sediments and petroleums. Tetrahedron Letters 23, 2027– 2030. Bowland, C.L., 1993. Depositional history of the western Colombian Basin Caribbean Sea, revealed by seismic stratigraphy. Geological Society of America Bulletin 105, 1321– 1345. Caro, M., Spratt, D., 2003. Tectonic evolution of the San Jacinto fold belt, NW Colombia. CSEG Recorder, February, 2003, pp. 36–43. Chevron Overseas Petroleum Inc., 1986. Northwest Colombia hydrocarbon evaluation. Technical Report for Ecopetrol, Bogota´. Duque-Caro, C., 1980. Geotecto´nica y Evolucio´n de la Regio´n Noroccidental Colombiana. Boletı´n Geolo´gico Colombiano, vol. 23(3), pp. 4–37. Ecopetrol-Instituto Colombiano del Petro´leo, 1999. Evaluacio´n del Potencial Petrolı´fero del a´rea Sinu´-Uraba´. Informe Te´cnico, Piedecuesta. Ecopetrol-Instituto Colombiano del Petro´leo., 2003. Descripcio´n de Corazones y Ana´lisis Petrolo´gico-Petrofı´sico y Geoquı´mico. Cuenca del Sinu´. Technical Report, Piedecuesta, p. 204. Ecopetrol-Instituto Colombiano del Petro´leo y Ecopetrol-Divisio´n Adjunta de Exploracio´n, 2000. Evaluacio´n Regional Integrada Cuenca Valle Inferior del Magdalena. Informe Final. Piedecuesta, p. 367. Espitalie´, J., Deroo, G., Marquis, F., 1985/1986. Rock–Eval pyrolysis and its applications. Revue de l’Institut Franc¸ais du Pe´trole, part one 40 (5), 563–579 (part two 40(6) , 755–784; part three 41(1), 73–89).
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ESRI-ILEX, 1995. Evaluacio´n Geolo´gica Regional de la Cuenca del Sinu´-San Jacinto. Technical Report for Ecopetrol. Santafe´ de Bogota´, p. 250. Haffer, J., 1963. On the stratigraphy of the Upper Sinu-Upper San Jorge Region. Northwest Colombia. Colombian Petroleum Company, Geologic Report No. 302. Laverde, F., 2000. The Caribbean Basin of Colombia, a composite Cenozoic accretionary wedge with under-explored hydrocarbon potential. In: Memorias del VII Simposio Bolivariano de Cuencas Subandinas. Sociedad Venezolana de Geo´logos, pp. 394–410. Marteau, V., Groba, C., Romera, W., Labaye´n, I., Crotti, M., Bosco, S., 2002. Utilizacio´n de la geoquı´mica de reservorios para determinar la heterogeneidad de los petro´leos de produccio´n de la Formacio´n Rayoso, Cuenca Neuquina. In: Proceedings of V Congreso de Exploracio´n y Desarrollo de Hidrocarburos. Argentina. Mello, M., Telnaes, N., Gaglianone, P., Chicarelli, M., Brassell, C., Maxwell, R., 1987. Organic geochemical characterisation of depositional paleoenvironments of source rocks and oils in Brazilian marginal basins. In: Mattaveli, L., Novelli, L. (Eds.), Advances in Organic Geochemistry 1987. Part I. European Association of Organic Geochemists, Venice, pp. 31–45. Oppenheim, V., 1957. Sinu Basin in Colombia may produce new reserves. Wold Oil 44 (6), 151–162. Ourisson, G., Albrecht, P., Rohmer, M., 1982. Predictive microbial biogeochemistry – from molecular fossils to prokaryotic membranes. Trends in Biochemical Science 7, 236– 239. Peters, K., Moldowan, M., 1993. The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice-Hall, New Jersey, p. 212. Peters, K., Moldowan, J., Schoell, M., Hemppkins, W., 1986. Petroleum isotopic and biomarker composition related to source rock organic matter and depositional environment. Organic Geochemistry 10, 17–27. Petrobras-CENPES-EGEQ, 1996. The petroleum system of the Lower Magdalena basin, Colombia: a geochemical characterization of oil and potential sources rock. Technical Report for Ecopetrol. Philp, R.P., 1985. Fossil Fuel Biomarkers, Applications and Spectra. Elsevier, Amsterdam, p. 294. Philp, R.P., Gilbert, T.D., 1986. Biomarker distribution in Australian oils derived from terrigenous source material. Organic Geochemistry 10, 73–84. Schoell, M., 1984. Stable isotopes in petroleum research. In: Brooks, J., Welte, D.H. (Eds.), Advances in Petroleum Geochemistry, vol. 1, pp. 215–245. Sofer, Z., 1984. Stable carbon isotope compositions of crude oils: applications to source depositional environments and petroleum alteration. American Association Petroleum Geologists Bulletin 68, 31–49. Tissot, B., Welte, D., 1984. Petroleum Formation and Occurrence, 2nd ed. Springer-Verlag, New York, p. 699. Waples, D., Machihara, T., 1991. Biomarkers for geologists – a practical guide to the application of steranes and triterpanes in petroleum geology. American Association Petroleum Geologists, Methods in Exploration Series 9, 91.
Origin and alteration of oils and oil seeps from the Sinu´-San Jacinto Basin, Colombia Christian Sa´nchez 1, Albert Permanyer
*
Dpt. Geoquı´mica, Petrologia i Prospeccio´ Geolo`gica, Facultat de Geologia, Universitat de Barcelona, Martı´ i Franque`s, s/n, 08028 Barcelona, Catalonia, Spain Available online 15 September 2006
Abstract Liquid hydrocarbons have been detected in the subsurface as well as in the surface in the Sinu´-San Jacinto Basin (northwestern Colombia). The origin of the oils has not been conclusively established especially in the southern part of the basin. The most likely source rocks in the basin are the Cie´naga de Oro Fm. of the Oligocene-Early Miocene and the Cansona Fm. of the Upper Cretaceous. In this study, oil samples, seeps and source rock extracts were analyzed by GC and GC/MS, and d13C was determined to identify the source facies. The sulphur content and gravity data were also considered. Two organic facies were identified: one constituted by terrestrial organic matter deposited in siliciclastic sediments in marginal marine to deltaic environments and the other made up of marine organic matter deposited in marine costal shelf to pelagic environments. The oils from the former organic facies have low sulphur contents, whereas the oils from the latter facies have high sulphur levels. Correlation of the oil seeps from the former facies with the Cie´naga de Oro Fm. has not been clearly established. The oil seeps from the latter facies correlate well with the extracts from the source rocks of the Cansona Fm., deposited along the fold belt of San Jacinto (east side). The oil seeps are affected by moderate to severe biodegradation, whereas the oil from the only oil producing well in the Sinu´ Basin (Floresanto-6 well) has not undergone biodegradation. 2006 Elsevier Ltd. All rights reserved.
1. Introduction The presence of hydrocarbons has been known for a long time in the Sinu´-San Jacinto fold belt. Numerous oil seeps have been found in the southern part of the Basin. In NW Colombia, exploitation *
Corresponding author. Tel.: +34 934021416; fax: +34 934021340. E-mail addresses: [email protected] (C. Sa´nchez), [email protected] (A. Permanyer). 1 Present address: Agencia Nacional de Hidrocarburos, Colombia.
started in 1908, when the Las Perdices well was drilled north of the San Jacinto fold belt. Between the 1940s and the 1960s various fields were discovered in the Plato and San Jorge sub basins as well as in the Sinu´ Basin (Floresanto field). At present, there is no longer any oil production in the Sinu´ Basin. In the San Jacinto fold belt some wells have been drilled but no oil and gas accumulations of economic importance have been discovered. Earlier studies identified a number of potential source rocks in the Sinu´-San Jacinto and the Lower Magdalena Valley basins, particularly in the Upper Cretaceous and in the Tertiary Oligocene to Miocene sediments
0146-6380/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2006.07.012
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C. Sa´nchez, A. Permanyer / Organic Geochemistry 37 (2006) 1831–1845
(Oppenheim, 1957; Haffer, 1963; Chevron, 1986; ESRI-ILEX, 1995; Ecopetrol-ICP, 1999; Caro and Spratt, 2003). The present study seeks to identify and re-evaluate the organic facies that account for the generation of oil in the Sinu´-San Jacinto fold belt, and aims to correlate the oil seeps with two of the candidate source rocks. To this end, the data of earlier studies were combined with those obtained in our study using techniques such as liquid and gas chromatography, biomarkers, and carbon isotopes. These techniques were applied to three classes of samples: source rock extracts, production oil, and oil seeps from the Sinu´-San Jacinto Basin. 2. Geological setting and earlier studies The Sinu´-San Jacinto Basin is located in the far northwest of Colombia along the boundary between the Caribbean and the South American plates (Fig. 1) where the subduction of the Caribbean plate under the South American plate determines the structural and stratigraphic patterns of the geological zones in western Colombia (Haffer, 1963; Duque-Caro, 1980; Chevron, 1986; Bowland,
1993; ESRI-ILEX, 1995; Laverde, 2000; Caro and Spratt, 2003). The Sinu´-San Jacinto Basin is located in the Colombian-Caribbean Basin, which is a marginal plate basin of the Cenozoic. It is made up of two belts with adjacent folds: (1) the San Jacinto fold belt of the Paleogene with an extension to the North and (2) the Sinu´ fold belt of the Neogene located along the western margin of the San Jacinto fold belt. Each belt has its own stratigraphic succession (Duque-Caro, 1980; Ecopetrol-ICP, 2000, 2003) (Fig. 2). Earlier studies by Chevron (1986), ESRI-ILEX (1995) and Ecopetrol-ICP (2003) showed that the best potential source rocks correspond to the intervals of the Maastrichtian and the Lower OligoceneEarly Miocene (Fig. 2). The former corresponds to the Cansona Fm. (Maastrichtian), which is only located in the San Jacinto fold belt. This unit has never been drilled by exploration wells. In outcrops it presents organic carbon (TOC) values that exceed 3% in the sections of Cantera San Sebastia´n, Cantera Purgatorio and Cerro Cansona (Fig. 1). The values of the hydrogen index are 360 mg HC/g TOC on average. Vitrinite reflectance indicates that
Fig. 1. General location map of the Sinu´-San Jacinto Basin and detailed emplacement of the studied samples: crude oil, oil seeps and outcrops (modified from Ecopetrol-ICP, 2003).
C. Sa´nchez, A. Permanyer / Organic Geochemistry 37 (2006) 1831–1845
SERIES SYSTEM Coastal SEQUENCE
onlap
PLIOCENE
QUATER- PLEISTO NARY CENE
H
U 3.8 L
LS8
U
G
M 15.5
LS7
NE
SW
SINU CORPA
NEOGENE
MIOCENE
PAJUIL
17.5
F
22.0
LS6
30.0
LS5
OLIGOCENE
TERTIARY
E
U
EOCENE
39.5
PA LEOCENE
RESERVOIR SEAL
SINCELEJO
TUBARA
CARMEN
CIENAGA DE ORO
LS4
SAN JACINTO MANANTIAL
TOLUVIEJO M
C CANDELARIA 49.5
UPPER
CRETACEOUS
PAVO PAVO
D
LS3
CHENGUE MACO
?
L
MESOZOIC SOZOIC
FLORESANTO
PETROLEUM SYSTEM SOURCE ROCK
MARALU
L
U
PAL EOGENE
CENOZOIC
L
SAN JACINTO
1833
U 61.0
L
94.0
SAN CAYETANO
LS2
A
Potential Source rock Reservoir rock
LS1
Potencial Reservoir rock Potential seal rock
CANSONA BASEMENT
Fig. 2. Stratigraphic units of Sinu´-San Jacinto Basin, and main hydrocarbon events (modified from Ecopetrol-ICP, 2003).
the Cansona Fm. reaches the oil window in the southern part of the basin. The Cie´naga de Oro Fm. forms part of Oligocene-Early Miocene sequence. It is located in the Sinu´-San Jacinto belts and in the basin of the Lower Magdalena Valley (LMV) situated to the west of the San Jacinto belt. This formation, which has been drilled in a number of wells, displays thicknesses between 60 and 800 m. Organic richness varies from poor to very good throughout the basin and the best potential was found in the San Jorge and the Plato basins (Fig. 1) in the upper part of the formation. TOC values range between 0.5% and 2.0% and the hydrogen index does not exceed 200. Vitrinite reflectance indicates that the Cie´naga de Oro Fm. reaches the oil window only in some areas of the Plato and San Jorge basins. Earlier studies of liquid hydrocarbons have mainly focused on samples of oil seeps, and three families of oils have been identified (Petrobras-
CENPES-CEGEQ, 1996; Ecopetrol-ICP, 1999, 2003). The first family constitutes oils derived from continental organic matter deposited in marine deltaic to nearshore environments of the Tertiary. The second family is also derived from continental organic matter deposited in nearshore siliciclastic environments of the Tertiary in a less proximal position than the first family. The third family is made up of oils derived from marine facies deposited in coastal shelf anoxic environments of the Cretaceous (Petrobras-CENPES-CEGEQ, 1996; EcopetrolICP, 1999, 2003). 3. Samples Five samples of rock extracts, one sample of crude oil and twelve samples of oil seeps from the Sinu´-San Jacinto Basin were analysed. The two rock extracts from the Cie´naga de Oro Fm. proceeded from the cores of the La Arena-1
1834
C. Sa´nchez, A. Permanyer / Organic Geochemistry 37 (2006) 1831–1845
Table 1 Pyrolysis Rock–Eval data of rock samples from the Cie´naga de Oro and Cansona formations ID Formation Locality Sample type
B-647 Cie´naga de Oro La Arena-1 Rock sample
B-648 Cie´naga de Oro La Arena-1 Rock sample
B-649 Cansona Cantera Golf Rock sample
B-651 Cansona Cantera Golf Rock sample
B-652 Cansona Cantera Golf Rock sample
TOC (%) S1 (mg HC/g of rock) S2 (mg HC/g of rock) S3 (mg CO2/g of rock) Hydrogen index Tmax (C)
25.0 0.8 34.7 8.2 138 411
16.1 0.6 15.7 6.3 97 402
1.3 4.17a 4.4 0.1 338 438
2.7 14.73a 8.5 0.3 314 330 nr
6.5 44.06a 20.9 0.3 321 315 nr
All data from ESRI-ILEX (1995); Ecopetrol-ICP (2003). a Denotes impregnated samples. nr = non representative.
well whereas those of the Cansona Fm came from the Cantera Golf outcrop (Fig. 1). Arena-1 is a shallow well drilled for stratigraphic purposes. Samples were taken at depths ranging between 450 and 780 m. The extracts and the Rock–Eval data from earlier studies were provided by the ICP (Table 1). The crude oil sample came from the Floresanto-6 well. Oil was produced in the Pajuil Fm. (base at 1370 ft 418 m). The well was shallow (1505 ft, 459 m) and had a low production (about 65 barrels per day in 1978). The oil seeps studied were from the following outcrops (from south to north, Fig. 1): Matamorito, Cienaguita, Pativilca, Buenavista, La Lucha, La Plancha, Cerro El Gas, Cruz de Guayabo, Finca Juan Banda, Mina San Sebastia´n, El Salvador and El Castillo. 4. Analytical methods The techniques used included liquid and gas chromatography, gas chromatography-mass spectrometry and carbon isotopes. Rock–Eval pyrolysis and total organic carbon (TOC) data of the samples from Cansona and Cie´naga de Oro formations were taken from earlier studies (Chevron, 1986; ESRIILEX, 1995; Ecopetrol-ICP, 2000, 2003) as were the values of API gravity and sulphur content for the oil seep samples. Samples were fractioned into saturated (SAT) and aromatic (ARO) hydrocarbons, and heavy compounds (NSO) using column liquid chromatography. Previously, oils had been topped for comparison with rock extracts. Topping was achieved at 50 C by using a 20–60 mb vacuum pressure for five hours. Gas chromatography was carried out on whole oil and on saturated fractions from crude oil and oil seeps, on a Delta Chrom Series 9980
model with a flame ionization detector. Separation was achieved on a fused silica capillary column J&W PONA (50 m · 0.2 mm · 0.5 lm). GC Chemstation Agilent Technologies 1990–2000 software was used. The GC operating conditions were as follows: temperature maintained at 35 C for 15 min, increased from 35 to 320 C at a rate of 2 C/min, and maintained at 320 C for 30 min. Saturated fractions were also analysed by gas chromatography–mass spectrometry (GC–MS) using a Thermo Electron MD 800 high resolution mass spectrometer coupled to a Thermo Electron 8060 gas chromatograph. The GC/MS operating conditions were as follows: a non polar J&W DB-5 column (60 m · 0.25 mm · 0.1 lm), temperature maintained at 40 C for 1 min, increased from 40 to 300 C at a rate of 2 C/min, and maintained at 300 C for 60 min. The mass spectrometer was operated in electron impact mode with an ionization voltage of 40 eV and a source temperature of 200 C and data were recorded by selected ion monitoring mode of eight ions. Terpane, sterane and diasterane ratios were calculated from the resulting m/z 177, m/z 191, m/z 217, m/z 218 and m/z 259 mass fragmentograms. Data were processed with a Masslab 1.4 de FinniganTM system. Carbon isotope measurements of saturate and aromatic hydrocarbons were performed on a Thermo Finnigan Series 1112 elemental analyser coupled to a Finnigan Mat Delta C mass spectrometer. The reference materials were Graphite (USGS 24), Saccharose (IAEA-CH6), Polyethylene (IAEACH7) and Oil (NBS-22). 5. Results The results of geochemical characterization of the two rock extracts, one crude oil and twelve oil
C. Sa´nchez, A. Permanyer / Organic Geochemistry 37 (2006) 1831–1845
seeps are presented in Tables 1–5 as well as in Figs. 3–5 and discussed below. 5.1. Geochemical characterization of rock extracts The five extracts from the Cie´naga de Oro and Cansona formations together with the Rock–Eval data were provided by the IPC (Table 1). The two rock extracts from the Cie´naga de Oro Fm. came from the cores of the La Arena-1 well, drilled for stratigraphic purposes (Fig. 1). Samples were taken at depths ranging between 450 and 780 m. The extracts from the Cie´naga de Oro Fm. show a composition high in resins and asphaltenes (more than 55%) whereas the fractions of saturated hydrocarbons ranged between 21% and 31% and the aromatics between 10% and 12% (Table 2). The chromatograms display a quasi absence of normal paraffins with the exception of C29, C31 and C33 nor-
1835
mal alkanes (Fig. 3A). The isoprenoids pristane and phytane are well developed with Pr/Ph ratios higher than four. The mass chromatograms show that the amount of terpanes between C27 and C35 is very low (Fig. 4A). The quasi absence of tricyclic terpanes is observed whereas C28 and C29 norhopanes appear well developed. Steranes are scarce and the absence of C27 is noted. The C29 steranes are dominant, as is the C29 diasterane with respect to C28 (Table 2). The Pr/Ph and (Pr/n-C17)/(Ph/n-C18) ratios and the dominance of the C29 steranes and diasteranes suggest a terrestrial origin of the organic matter deposited in an oxic environment (Tissot and Welte, 1984; Peters and Moldowan, 1993). Sterane isomerization ratios indicate immaturity of the organic matter (equivalent Ro 0.4%, Table 2), which is in agreement with the composition of the extracts that are high in resins and asphaltenes.
Table 2 Bulk composition, selected geochemical ratios and isotopic values of the rock extracts from the Cie´naga de Oro and Cansona formations ID Formation Locality Sample type
B-647 Cienaga de Oro La Arena-1 Rock extract
B-648 Cienaga de Oro La Arena-1 Rock extract
B-649 Cansona Cantera Golf Rock extract
B-651 Cansona Cantera Golf Rock extract
B-652 Cansona Cantera Golf Rock extract
Saturate fraction (%) Aromatic fraction (%) NSO (%) Pr/ph Pr/n-C17 Ph/n-C18 (Pr/n-C17)/(Ph/n-C18) n-C29/n-C17 CPI d13C SAT (&) d13C ARO (&) C27 Diasterane (%) C28 Diasterane (%) C29 Disterane (%) C27 Sterane (%) C28 Sterane (%) C29 Sterane (%) C27/29 Steranes Diasteranes/Reg. Steranes C29aa(20S)/C29aa(20R) Ts/Tm Tricyclic index C26/C25 Tricyclic terpanes C24/C23 Tricyclic terpanes C24 Tetracyclic/C26 Tricyclic C23 Tricyclic/C24 Tetracyclic C29 Norhopane/C30 Hopane Oleanane/C30 Hopane % Ro. Equivalent (from ster.Isom)
21.3 10.6 68.1 5.1 6.3 1.1 5.9 0.8 0.8 28.5 28.4 5.4 17.6 77.0 0.3 4.5 95.2 0.1 0.2 0.1 0.3 0.1 2.3 0.9 0.7 0.4 12.9 1.9 0.40
31.2 12.5 56.3 4.0 3.9 1.0 4.1 1.0 1.0 26.4 28.0 4.0 11.7 84.2 0.2 2.7 97.1 0.0 0.2 0.3 0.6 0.3 1.9 1.0 0.7 0.5 7.5 5.5 0.46
61.51a 20.4 18.1 Biodegr. Biodegr. Biodegr. Biodegr. Biodegr. Biodegr. 29.0 28.9 33.0 42.0 25.1 42.5 36.4 21.1 1.6 2.1 1.8 1.7 0.9 0.9 0.7 0.1 15.1 1.1 2.3 1.20
68.46a 17.9 13.6 Biodegr. Biodegr. Biodegr. Biodegr. Biodegr. Biodegr. 29.0 27.7 38.0 40.9 21.1 54.9 22.8 22.3 1.2 2.4 1.5 1.9 0.9 0.9 0.7 0.2 14.19.1 0.7 1.7 1.06
67.3a 21.5 11.2 Biodegr. Biodegr. Biodegr. Biodegr. Biodegr. Biodegr. 29.3 27.9 21.2 51.1 27.6 41.2 48.1 10.7 1.0 1.1 0.8 0.7 0.9 1.0 0.7 0.1 0.8 2.2 nr
a Denotes impregnated samples; nr = non representative; Tricyclic index = Ratio of tricyclic terpanes (C19-C30) over tricyclic terpanes plus hopanes (C29-C36).
1836
ID Locality
B-633 Floresanto-6
B-632 Matamorito
B-634 Pativilca
B-635 Cienaguita-3
B-636 Buenavista
B-638 La Plancha
Sample type Assigned facies type
Crude Oil
Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 1
API (+) Sulphur (%) (+) Saturate fraction (%) Aromatic fraction (%) NSO (%) Pr/Ph Pr/n-C17 Ph/n-C18 (Pr/n-C17)/(Ph/n-C18) n-C29/n-C17 CPI d 13C SAT (&) d 13C ARO (&)
44.98 0.10 76.19 19.49 4.32 5.15 1.07 0.18 6.02 0.34 1.02 27.40 26.24
20.72 0.08 51.94 42.30 5.76 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 24.67 24.54
16.05 0.14 53.40 32.04 14.56 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 22.85 22.77
12.17 0.18 40.31 31.83 27.86 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 24.13 23.57
18.16 0.16 43.66 41.35 14.99 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 23.18 23.48
(+) Data from ESRI-ILEX (1995); Ecopetrol-ICP (2003).
B-643 El Salvador
B-644 El Castillo
B-637 La Lucha
B-639 Cerro El Gas
B-640 Cruz de Cuayabo
B-642 Mina San Sebastian
Oil seep Facies 1
B-641 Finca Juan Banda Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 2
Oil seep Facies 2
Oil seep Facies 2
Oil seep Facies 2
n.a 0.25 49.96 33.03 17.01 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 26.11 25.80
15.18 0.17 41.28 49.02 9.70 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 25.00 24.60
18.28 0.18 43.36 43.85 12.80 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 24.70 24.30
18.49 0.11 46.49 33.60 19.91 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 25.50 25.61
15.59 1.96 40.37 38.92 20.71 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 28.51 27.60
9.18 2.13 27.92 30.45 41.63 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 28.35 27.80
14.39 1.68 40.71 39.29 20.00 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 28.52 27.52
15.79 1.79 38.55 34.86 26.59 Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. Biodgr. 28.60 28.20
C. Sa´nchez, A. Permanyer / Organic Geochemistry 37 (2006) 1831–1845
Table 3 Bulk composition, selected geochemical ratios and isotopic values of oil seeps and Floresanto-6 crude oil
Table 4 Main biomarker parameters and selected ratios of Floresanto-6 crude oil and oil seeps samples B-633 Floresanto-6
B-632 Matamorito
B-634 Pativilca
B-635 Cienaguita-3
B-636 Buenavista
B-638 La Plancha
B-643 El Salvador
B-644 El Castillo
B-637 La Lucha
B-639 Cerro El Gas
B-640 Cruz de Cuayabo
Oil seep Facies 1
B-641 Finca Juan Banda Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 2
Oil seep Facies 2
Oil seep Facies 2
B-642 Mina San Sebastian Oil seep Facies 2
Sample type Assigned facies type
Crude Oil
Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 1
Oil seep Facies 1
C27 Diasterane (%) C28 Diasterane (%) C29 Diasterane (%) C27 Sterance (%) C28 Sterane (%) C29 Sterane (%) C27/C29 Steranes C29 aa(20S)/C29 aa(20R) Diasteranes/Reg. Steranes Steranes/Hopanes Ts/Tm Tricyclic index C26/C25 Tricyclic terpanes C24/C23 Tricyclic terpanes C24 Tetracyclic/ C26 Tricyclic C23 Tricyclic/C24 Tetracyclic C29 Nor-hopane/ C30 Hopane Oleanane/C30 Hopane C35/C34 Homohopanes Gammacerane/C30 Hopane Biodegradation degree (see Table 5) % Ro. Equivalent (from ster. lsom.)
22.22 32.54 45.23 24.42 33.37 42.21 0.49 0.87
15.72 59.65 24.62 24.49 32.60 42.91 0.54 0.87
27.02 40.73 32.25 Biodegr. Biodegr. Biodegr. Biodegr. 0.78
25.75 38.29 35.96 Biodegr. Biodegr. Biodegr. Biodegr. 1.33
26.84 43.35 29.81 28.38 38.20 33.42 0.93 0.42
26.49 37.13 36.38 Biodegr. Biodegr. Biodegr. Biodegr. 1.11
25.24 40.79 33.96 30.95 35.00 34.05 0.91 0.59
26.35 40.51 33.15 25.29 35.13 39.58 0.73 0.59
24.17 35.00 40.84 20.93 28.98 50.09 0.47 0.82
26.09 40.70 33.22 Biodegr. Biodegr. Biodegr. Biodegr. 0.65
20.32 36.81 42.88 Biodegr. Biodegr. Biodegr. Biodegr. 0.77
28.93 37.42 33.65 Biodegr. Biodegr. Biodegr. Biodegr. 0.86
26.61 38.22 35.17 35.62 34.49 29.89 1.11 0.65
0.70
0.96
Biodegr.
Biodegr.
0.39
Biodegr.
0.35
0.57
0.55
Biodegr.
Biodegr.
Biodegr.
0.19
0.36 1.66 0.22 1.14
Biodegr. 1.85 0.23 1.86
Biodegr. 1.37 0.34 1.87
Biodegr. 1.33 0.19 1.38
0.63 0.99 0.19 1.39
Biodegr. 0.80 0.21 1.03
0.27 1.20 0.11 1.40
0.41 1.13 0.16 1.16
0.19 1.16 0.09 1.41
Biodegr. 0.51 0.64 0.87
Biodegr. 0.54 0.17 0.88
Biodegr. 0.55 0.70 0.86
0.51 0.29 0.34 0.92
0.95
1.09
0.10
0.99
0.93
0.86
0.81
1.03
1.04
0.74
0.73
0.74
0.65
0.77
1.10
0.52
0.81
0.76
1.05
1.50
1.12
2.04
0.38
0.49
0.40
0.31
2.03
1.14
1.98
1.49
1.77
1.51
1.12
1.38
0.74
5.24
3.89
5.16
8.05
0.45
0.47
0.49
Biodegr.
0.56
Biodegr.
0.47
0.48
0.46
Biodegr.
0.50
Biodegr.
0.70
0.83
1.19
1.43
Biodegr.
1.16
Biodegr.
0.44
0.71
0.41
Biodegr.
0.50
Biodegr.
0.14
0.74
0.69
0.35
Biodegr.
0.41
Biodegr.
0.36
0.49
0.38
Biodegr.
0.81
Biodegr.
1.10
3.05
2.66
2.51
Biodegr.
2.65
Biodegr.
2.24
2.38
2.44
Biodegr.
3.26
Biodegr.
4.31
0
6
6
8
4–5
8
4–5
5–6
5–6
8
6
8–9
5–6
0.76
0.76
0.71
0.98
0.54
0.87
0.62
0.62
0.73
0.65
0.71
0.75
0.65
C. Sa´nchez, A. Permanyer / Organic Geochemistry 37 (2006) 1831–1845
ID Locality
Tricyclic index = Ratio of tricyclic terpanes (C19-C30) over tricyclic terpanes plus hopanes (C29-C36). 1837
1838
Table 5 Short description of the oil seeps current state of preservation and a proposal of level of biodegradation n-paraffin
Is oprenoids
Steranes
Hopanes
Diasternes
Biodegradation degree
API (+)
Sulphur (%) (+)
Facies 1 Matamorito
Depleted
Depleted
Intact Oln > C30H
Intact
6
20.72
0.08
Pativilca
Depleted
Depleted
Intact Oln > C30H
Intact
6
16.06
0.14
Cienagu¨ita-3
Depleted
Depleted
Partly degraded
Intact
8
12.17
0.18
Buenavista La Plancha
Depleted Depleted
Partly depleted Depleted
aaa20S partly degraded More to less degraded: C27 > C28 > C29 aaa20S partly degraded More to less degraded: C27 > C28 > C29 aaa 20(R+S) degraded abb 20(R+S) depleted Intact Nearly all degraded
Intact Oln > C30H Strongly degraded
4–5 8
18.16 n.a.
0.16 0.25
Finca Juan Banda El Salvador El Castillo
Depleted Depleted Depleted
Partly depleted Partly depleted Partly depleted
Intact aaa 20S partly depleted aaa 20S partly depleted aaa 20R moderately degraded
Intact Intact Oln > C30H Intact
Intact Slightly to moderately degraded Intact Intact Intact
4–5 5–6 5–6
15.48 18.28 18–49
0.17 0.18 0.11
Facies 2 La Lucha
Depleted
Depleted
Nearly all degraded
Strongly degraded
8
15.59
1.96
Cerro El Gas
Depleted
Depleted
Intact
6
9.18
2.13
Cruz de Guaybo
Depleted
Depleted
aaa 20S partly degraded More to less degraded: C27 > C28 > C29 Nearly all degraded
Slightly to moderately degraded Intact
Strongly degraded
8–9
14.39
1.68
Mina San Sebatia´n
Depleted
Partly depleted
aaa 20S partly depleted
Intact
5–6
15.79
1.79
Biodegradation degree is based on the scale of Peters and Moldowan (1993), ranging from 1 to 10 (low to high). (+) Data from ESRI-ILEX (1995); Ecopetrol-ICP (2003). Oln: Oleanane.
Slightly to moderately degraded Intact
C. Sa´nchez, A. Permanyer / Organic Geochemistry 37 (2006) 1831–1845
Oil seep
C. Sa´nchez, A. Permanyer / Organic Geochemistry 37 (2006) 1831–1845
1839
Fig. 4. Mass chromatograms of terpanes and steranes (ions m/z 191 and m/z 217) from extracts from Cie´naga de Oro and Cansona Formations (TT: tricyclic terpanes; Ts: trisnorneohopane; Tm: trisnorhopane; C27, C28 and C29 refers to regular steranes).
Fig. 3. Gas chromatograms of rock samples (A and B), oil from Floresanto-6 well (C) and two oils seeps (D and E) (Pr: Pristane; Ph: Phytane; n-C: n-alkanes).
The d13C isotopic values range between 26.4& and 28.5& for saturates and 28& and 28.4& for aromatics (Table 2). The Rock–Eval pyrolysis data from the internal reports of the ICP (ESRI-ILEX, 1995; EcopetrolICP, 2003) are presented in Table 1. These data show high TOC, high S2 values and a low hydrogen index (HI < 150), denoting a type III kerogen. The Tmax between 402 and 411 C confirms the immaturity in the organic matter from the Cie´naga de Oro Fm. The three rock extracts from the Cansona Fm. came from the Cantera Golf outcrop (Fig. 1). These rock extracts have a composition that is high in sat-
Fig. 5. Terpane and sterane mass chromatograms from Floresanto-6 crude oil (TT: tricyclic terpanes; TeT: tetracyclic terpane; Ts: trisnorneohopane; Tm: trisnorhopane; H: homohopanes).
urated hydrocarbons (>62%) with small amounts of aromatic hydrocarbons (between 18% and 22%) and resins and asphaltenes (between 10% and 20%) (Table 2). The gas chromatograms display a high unresolved complex mixture (UCM) and a very
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C. Sa´nchez, A. Permanyer / Organic Geochemistry 37 (2006) 1831–1845
small amount of n-alkanes and isoprenoids, suggesting severe biodegradation (Fig. 3B). The terpanes contained more tricyclic than pentacyclic biomarkers (Fig. 4B). Of these, oleanane and C30 hopane should be noted. The former for its notable presence and the latter for its degradation. An important depletion in the homohopane series was observed. Although the steranes were very scarce, there were more C27 diasteranes than C29 diasteranes despite severe biodegradation. The tricyclic terpanes are generated in a stage of greater thermal maturity and are more resistant to biodegradation than the homohopanes (Peters and Moldowan, 1993). Therefore, they constitute useful markers for environment determination and source-oil correlations. According to Peters and Moldowan (1993), the low C26/C25 and C24/C23 tricyclic terpane ratios (less than 1) are indicative of organic matter deposited in carbonate marine environments. The C28/C29 diasterane ratio exceeding 1.8 (Table 2) also allows us to confirm the marine input (Waples and Machihara, 1991; Peters and Moldowan, 1993). The isomerization ratios of regular steranes show values that suggest a high thermal maturity (equivalent Ro about 1%). These values should be considered with care because of the biodegradation of the steranes. However, the composition of the extracts that are high in saturated hydrocarbons confirms a high degree of maturity. The pyrolysis data presented in Table 1 show high S1 values (from 4 to 44 mg HC/g rock) as well as high S2 values ranging between 4 and 21 mg HC/ g rock. These results suggest that the rock samples from the Cansona Fm. are impregnated with oil and present a good source rock potential (Espitalie´ et al., 1985/1986). The Tmax values are not representative of maturity because the samples are impregnated with oil as indicated by the S1 values (S1 upto 14 mg HC/g rock) (Espitalie´ et al., 1985/1986). The d13C isotopic values are around 29& for saturates and range between 27.7& and 28.9& for aromatics (Table 2).
asphaltenes (<5%) (Table 3). The whole oil gas chromatogram (Fig. 3C) shows a classic distribution with a significant contribution of paraffinic compounds, and a minor depletion of C 13 compounds, which suggests that the oil was not affected by biodegradation. The Cþ 15 fraction shows no dominance of odd over even n-alkanes (carbon preference index, CPI = 1.02). The isoprenoids show a large predominance of pristane over phytane (Pr/ Ph > 5) (Table 3). The terpanes contain abundant tricyclic and pentacyclic biomarkers (Fig. 5, Table 4). Of these, the C24 tetracyclic, C29 norhopane, oleanane and the C30 hopane are well developed. Both steranes and diasteranes show a slight predominance of C29 over C27 (Fig. 5, Table 4). The isotopic values of the Floresanto-6 crude oil (d13C 27.4& for saturates and 26.24& for aromatics, Table 3) show an isotopic composition that is midway between the rock extracts from the Cie´naga del Oro Fm. and those of the Cansona Fm. The high Pr/Ph and (Pr/n-C17)/(Ph/n-C18) ratios, the raised oleanane content with respect to C30 hopane, the hopane/sterane ratio less than unity and the greater abundance of the C29 sterane with respect to C28 and C27 (Fig. 5) all suggest a terrestrial input to the organic matter of the source rock. The tricyclic C26/C25 ratio exceeding 1.1 also suggests the predominantly terrestrial origin of the organic matter of the source rock (Mello et al., 1987; Waples and Machihara, 1991; Peters and Moldowan, 1993). The carbon preference index near unity is characteristic of thermally mature crude oils generated in the maximum peak of the oil window (Marteau et al., 2002). This is corroborated by the isomerization of the regular steranes which reaches its end point and is evidenced by the high gravity value (45 API) and the composition of the oil (>75% saturated hydrocarbons).
5.2. Geochemical characterization of Floresanto-6 oil
The oil seeps studied in the Sinu´ and San Jacinto basins came from the following outcrops (from south to north, Fig. 1): Matamorito, Cienaguita, Pativilca, Buenavista, La Lucha, La Plancha, Cerro El Gas, Cruz de Guayabo, Finca Juan Banda, Mina San Sebastia´n, El Salvador and El Castillo. More than 80% of the oil seep samples studied have a composition of saturated hydrocarbons between 40% and 55%, aromatic hydrocarbons between
The crude oil sample came from the Floresanto-6 well located in the central part of the Sinu´ Basin. Oil was produced from carbonates in the Pajuil Fm. of Middle to Upper Miocene (Figs. 1 and 2). The Floresanto-6 oil has a composition that is high in saturated hydrocarbons (>75%) with small amounts of aromatic hydrocarbons (<20%) and resins and
5.3. Geochemical characterization of oil seeps
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32% and 50%, and resins plus asphaltenes between 6% and 28%. Less than 20% of the samples have a composition that is higher in heavy compounds (NSO between 27% and 42%) than in aromatic hydrocarbons (between 31% and 35%) and saturated hydrocarbons (between 28% and 39%) (Table 3). The gas chromatograms of all the oil seeps show a low amount of n-alkanes and isoprenoids and medium to high concentration of Unresolved Complex Mixture (UCM) (Figs. 3D and E), which is attributable to the effects of biodegradation. The terpanes contain tricyclic and pentacyclic biomarkers. In eight of the samples (Table 5), hopanes appear intact (Mina San Sebastia´n, El Castillo. . .) whereas in four samples hopanes are partially or strongly degraded (Cienaguita-3, Cruz de Guayabo. . .). The steranes are more or less affected by biodegradation. They appear intact only in two of the samples (Buenavista and Finca Juan Banda). By contrast, diasteranes are intact in nine of the samples and slightly or moderately degraded in the three others. The degree of biodegradation is determined in accordance with the scale proposed by Peters and Moldowan (1993). Table 5 provides for each biomarker family (n-alkanes, isoprenoids, steranes, hopanes and diasteranes) a short description of the current state of its preservation, and proposes a level of biodegradation. The absence of normal alkanes and isoprenoids in all the oil seeps indicates a level equal to or higher than 4 out of 10. Most of the oil seeps range between levels 4 and 6. The most biodegraded oil seep reaches level 8 out of 10. The presence of the 25-norhopane biomarker in these samples is consistent with the severe process of biodegradation (Fig. 6). The 25-norhopanes (10-demethylated) are a series of compounds that are often linked to processes of biodegradation (Philp, 1985). These compounds appear to result from the bacterial degradation of the C10 methyl group in the regular hopanes and are more easily detected in the mass chromatogram of triterpanes (m/z 177). The samples with intact hopanes have a higher content of oleanane than C30 hopane. There is a distribution in descending order of the homohopanes from C31 to C35 in almost all samples with the exception of the oil seep sample from Mina San Sebastia´n, which had a very low concentration of oleanane and a higher content of C35 than C34 homohopanes (Fig. 7A and B).
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Fig. 6. Mass chromatograms (ion m/z 177) from two oil seeps affected by severe biodegradation, showing the presence of 25norhopanes. Mass chromatogram (ion m/z 177) from Floresanto6 oil (non biodegraded) is presented for comparison.
Fig. 7. Terpane and sterane mass chromatograms corresponding to the two types of oil seeps (TT: tricyclic terpanes; TeT: tetracyclic terpane; Ts: trisnorneohopane; Tm: trisnorhopane; H: homohopanes; C27, C28 and C29 refers to regular steranes).
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6. Discussion 6.1. Oil seeps-oil correlation Oil–oil correlation requires parameters that are not altered by common geochemical processes such as biodegradation. In the oil seeps studied, biodegradation is severe with the result that n-alkanes, isoprenoids, steranes, and in some cases hopanes, are not suitable for establishing correlations. By contrast, the tricyclic and the tetracyclic terpanes are more resistant to biodegradation than hopanes and therefore constitute good tools for correlation purposes. Tricyclic terpanes are commonly found in oils and source rock extracts and probably originate in prokaryotic cell membranes (Ourisson et al., 1982). Tricyclic terpanes are formed in various depositional environments: the C23 member is frequently the dominant homologue in crude oils of marine origin while C25 is more abundant than C26 in terrestrial oils (Aquino Neto et al., 1983; Peters and Moldowan, 1993). The proportion of C24 tetracyclic terpanes with respect to tricyclic terpanes may also be facies dependent with a relatively high concentration in source rocks and oils with terrigenous input (Philp and Gilbert, 1986). Therefore, the difference in the distribution of these tricyclic and tetracyclic terpanes can be used for oil family classification. In eight of the oil seeps studied as well as in the Foresanto-6 oil, the C26/C25 ratios of the tricyclic terpanes (TT) exceed unity. Moreover, these oil seeps present high C24/C23 ratios of tricyclic terpanes and high C24 tetracyclic terpanes with respect to the C26 tricyclic terpane ratios (Table 4). The remaining oil seeps (Mina San Sebastian, Cerro El
Gas, Cruz de Guayabo and La Lucha) present values that are less than 1 in both tricyclic ratios as well as a low amount of C24 tetracyclic terpanes (C24Tet/ C23TT around 0.7) (Table 4). Based on these data (see Fig. 8), the first eight oils were generated from organic matter with terrestrial input deposited in marine deltaic environments. The other four oil seeps were generated from a more marine organic matter. Oleanane is postulated to have derived from angiosperms whose presence has been attributed to the Late Cretaceous and the Tertiary (Peters and Moldowan, 1993). Of the pentacyclic terpanes, oleanane appears to be one of the compounds that is most resistant to biodegradation. Oleanane was found in all the samples but it is especially abundant in the oils whose source is linked to terrestrial input. The carbon isotopic composition typically depends on the d 13C value of the kerogen in the source rocks from which it is derived. The d13C depends on both the type of organic matter and the depositional environmental conditions (Schoell, 1984; Peters et al., 1986). d13C values are consequently useful for determining oil–oil and oil-source rock correlations. A plot of the carbon isotope values of the saturated and aromatic hydrocarbons in oils proposed by Sofer (1984) is shown in Fig. 9. The plot divides the oil seeps into two groups which resemble those previously distinguished by the tricyclic and tetracyclic biomarkers. One group presents values ranging between 28& and 23& for saturates and between 26& and 23& for aromatics (Table 3). The other group offers lighter d13C isotopic values of about 29& for saturates and 28& for aromatics. Isotopic values from the Floresanto-6 oil fall between these two groups.
Fig. 8. Tricyclic terpanes C24/C23 vs. C26/C25 diagram from oil seeps, Floresanto-6 crude oil and rock extracts.
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Two groups of oils are identified in accordance with the biomarkers and the isotopic composition: – One group (facies 1) presents TT C24/C23 > 0.8, TT C26/C25 > 1, and d13C isotopic values ranging from 27& to 23& for saturates and between 26& and 23& for aromatics. It corresponds to eight oil seeps whose origin may be related to organic matter of mixed marine and terrestrial origin probably deposited in marine marginal to nearshore (deltaic) environments. C27/C29 sterane ratios around 0.9 in the less degraded samples are in agreement with this interpretation. According to Peters and Moldowan (1993), the high oleanane content may be related to the Tertiary. – The other group (facies 2) presents TT C24/ C23 < 0.8, TT C26/C25 < 1, and d13C isotopic values of about 29& for saturates and 28& for aromatics. It corresponds to four oil seeps whose origin is related to organic matter of marine origin. In the least degraded sample (Mina San Sebastian), the C27/C29 sterane ratio that is greater than unity corroborates the marine input of organic matter.
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and presented in Table 3, all the oil seeps show a low gravity (<21 API), suggesting an increase in density due to biodegradation (Tables 3 and 5). By contrast, the Floresanto-6 oil, which is genetically linked to some oil seeps, offers no evidence of biodegradation and has a gravity of 45 API. The sulphur content is low (<0.22%) in the Floresanto-6 oil and in the group of eight oil seeps related to the deltaic environment. The sulphur content is higher (>1.65%) for the oil seeps of marine origin. Surprisingly, there is no apparent relationship between the sulphur content and the degree of biodegradation (Tables 3 and 5). For example, the Cienaguita-3 oil seep presents a low sulphur content (0.18%), whereas the La Lucha oil seep has a high sulphur content (1.96%) despite the severe biodegradation undergone by both oils (level 8 out of 10). The plot of the sulphur content and the API gravity is shown in Fig. 10. This plot provides evidence of two groups of samples that correspond to the two groups previously distinguished by biomarkers and isotopic signatures. It has not been possible to account for the anomalous variations in sulphur content with respect to biodegradation. 6.2. Oil-source correlation
API gravity is a bulk parameter that characterizes whole oil. Biodegradation reduces oil API gravity and increases sulphur content and other heteroatoms by selectively removing saturate and aromatic hydrocarbons compared with NSO compounds (Peters and Moldowan, 1993). According to the data from the internal reports of the Ecopetrol-ICP (ESRI-ILEX, 1995; Ecopetrol-ICP, 2003)
Given the biodegradation of the seeps and the rock extracts, the most suitable geochemical parameters for oil-source correlation include tricyclic terpanes and carbon isotopes. The diagram of tricyclic terpane ratios (Fig. 8) shows that the oil seeps derived from organic matter of marine origin (facies 2) correlate well with the
Fig. 9. d13C isotopic values of saturated and aromatic fractions in the Sofer (1984) diagram: extracts from the Cie´naga de Oro and Cansona formations, crude oil from Floresanto-6 well and oil seeps.
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Fig. 10. Sulphur content vs. API gravity plot for Floresanto-6 crude oil and oil seeps (data from ESRI-ILEX, 1995; EcopetrolICP, 2003).
rock extracts from the Cansona Fm. A similar correlation is obtained from carbon isotopes showing relatively light values of d13C for the saturated and aromatic hydrocarbons (Fig. 9). The Cansona Fm. constitutes a good source rock of marine origin as deduced from the geochemical results. The source rock is mature where it outcrops in the central part of the Sinu´ Basin. It generated oil that impregnated the rock and underwent biodegradation. The extracts appear more representative of the generated oil than of the source rock itself. The results obtained from the Cie´naga de Oro extracts indicate that these rocks offer no source rock potential where the Arena-1 well was drilled in the San Jacinto fold belt (Fig. 1). The terrestrial origin of the organic matter is corroborated by the high isoprenoid ratios as well as by the dominance of C29 steranes. Despite the low tricyclic terpane content, the ratios of C24/C25 TT and C26/C25 TT were calculated. In the plot of tricyclic terpanes (Fig. 8), the Cie´naga de Oro Fm. falls in the same cluster as the Matamorito and Pativilca oil seeps in the non marine quarter. The isotopic values from the aromatic fraction in the Cie´naga de Oro extracts are lighter than those obtained for Floresanto crude oil and oil seeps from facies one. It has been postulated that heavier isotopic values could result from biodegradation (Sofer, 1984), which could explain the heavy values found in the oil seeps with respect to the Floresanto oil and to the Cie´naga de Oro extracts. The poor source rock potential of the Cie´naga de Oro observed in the Arena-1 well does not exclude this formation from being a good source rock in other areas. According to ESRI-ILEX (1995), Pet-
robras-CENPES-CEGEQ (1996) and EcopetrolICP (2003), the Cie´naga de Oro Fm. was deposited in the Sinu´ and San Jacinto belts, as well as in the Lower Magdalena Valley, where it offers a good potential. In particular, in the Plato Basin, crude oils were correlated with extracts from the Cie´naga de Oro Fm., confirming its oil generation capacity. As stated above, oil seeps have a large abundance of saturated hydrocarbons with respect to aromatic hydrocarbons, resins and asphaltenes. Because of biodegradation, the composition of oil seeps usually show greater relative contents of resins and asphaltenes than saturated and aromatic hydrocarbon. This relative abundance of saturated and aromatic hydrocarbons suggests a continued supply of oil in the oil seeps, which is consistent with the occurrence of active source rocks. In the Sinu´-San Jacinto Basin, the presence of oil seeps of two different origins indicates that at least two different source rocks are effective in the area. Finally, the presence of the two types of oil seeps in very restricted areas (such as Finca Juan Banda and Cruz de Guayabo, or La Lucha and La Plancha) provides evidence of the structural complexity of the migration paths in this basin. 7. Conclusions The results obtained from biomarkers and isotopes enabled us to identify two organic facies that generate hydrocarbons in the southern part of the Sinu´-San Jacinto Basin. The crude oil of the Floresanto-6 well and the oil seeps Matamorito, Pativilca, Cienaguita-3, Buenavista, La Plancha, Finca Juan Banda, El Salvador and El Castillo are generated in siliciclastic source rock sediments derived from organic matter with terrestrial input deposited in environments ranging from marine marginal to marine deltaic conditions during the Tertiary. The correlation with the Cie´naga de Oro Fm. has not been clearly established. The oil seeps from Mina San Sebastia´n, Cerro El Gas, La Lucha and Cruz de Guayabo are generated in a source rock derived from marine organic matter possibly deposited in coastal shelf to pelagic environments during the Upper Cretaceous. The rock extracts from the Cansona Fm. correlate well with this organic facies. The oil seeps analysed have undergone moderate to severe processes of biodegradation, levels 4 to 8 out of 10 on the scale of Peters and Moldowan.
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Most oil seeps that are heavily biodegraded present a composition that is high in saturated hydrocarbons, which suggests a continuous supply of oil that is rich in light hydrocarbons of low molecular weight. Acknowledgements C. Sa´nchez obtained a scholarship from the Universitat de Barcelona to carry out this work. This research was also supported by the Spanish Ministry of Education and Science (Grand BTE2003-06915) and DURSI from Catalonia Government (2001SGR00075, ‘‘Grup de Geologia Sedimenta`ria). We are indebted to the Instituto Colombiano del Petro´leo (ICP) for providing samples and some reports. The authors are grateful to Dr. Eugenio Vaz do Santos Neto (Petrobras) for his comments and suggestions, and to Drs. M. Kruge and D. Naafs for reviewing the manuscript. Guest Associate Editor—B. Artur Stankiewicz
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