Organic Geochemistry 42 (2011) 727–738

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Organic Geochemistry

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The organic geochemistry of oil seeps from the Sierra de Perijá eastern foothills,Lake Maracaibo Basin, Venezuela

M. Escobar a,c,⇑, G. Márquez b, S. Inciarte c, J. Rojas c, I. Esteves d, G. Malandrino c

a CARBOZULIA, Av. 2 No. 55-185, Casa Mene Grande, Maracaibo 4002 A, Venezuelab Departamento de Ingeniería Minera, Mecánica y Energética, Escuela Técnica Superior de Ingeniería, Universidad de Huelva, 21819 Huelva, Spainc Postgrado de Geología Petrolera, Facultad de Ingeniería, Universidad del Zulia, Maracaibo 4002, Venezuelad INZIT, Km. 15 vía La Cañada, sector Palmarejo Viejo, Maracaibo 1114, Venezuela

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 February 2011Received in revised form 19 May 2011Accepted 7 June 2011Available online 13 June 2011

0146-6380/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.orggeochem.2011.06.005

⇑ Corresponding author at: CARBOZULIA, Av. 2 NoMaracaibo 4002 A, Venezuela. Tel./fax: +58 41436034

E-mail address: escomar24@gmail.com (M. Escoba

The organic geochemistry of samples from 11 oil seeps was studied. The samples were collected from theCachirí area, Carboniferous Region of Tulé (Lake Maracaibo Basin, Venezuela), associated with the TigreFault. Biomarkers (hopanes, steranes, n-alkanes, acyclic isoprenoids, and aromatic steroids) were ana-lyzed using gas chromatography-mass spectrometry (GC-MS). These hydrocarbon rich fluids have under-gone biodegradation (2–6 on the Peters and Moldowan scale), showing both the partial loss of n-alkanesand the microbial degradation of isoprenoids and steranes. These oil seeps were generated from a maturecalcareous source rock that was deposited in a marine paleoenvironment under reducing conditions.Moreover, these seeps are likely derived from the Cretaceous La Luna Formation that reached a levelof maturity near the peak of oil generation in the study area. The nature of the studied oil seeps, togetherwith the oil generation models reported for this rock unit in the study area, suggests that these oils are amixture of an initially heavy, altered oil and a second migrated light crude oil resulting from two gener-ation pulses from the La Luna Formation. Evidence for the presence of light oil trapped in the study areashould prompt re-exploration in the northwestern coast of Lake Maracaibo in shallow reservoirs, previ-ously discarded because they usually demonstrated a lack of light oils.

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1. Introduction

Although the relationship between petroleum migration andseep occurrence is complex (Abrams, 2005), oil seeps can be usedto provide evidence of a viable petroleum system and critical infor-mation about the nature of the source rock, its maturity and themigration of hydrocarbons in sedimentary basins (e.g., Rogerset al., 1999; Abrams et al., 2001). Sutton (1946) reported numerousoil seeps, gas emissions and asphalt deposits along the westernflank of Lake Maracaibo Basin (Zulia State). Some of these are veryclose to the Tigre Fault and occur along a line paralleling the faultin the foothills of the Sierra de Perijá, western Venezuela (Link,1952; Miller, 1962; Gallango et al., 2002; Fig. 1a). The author fo-cused on the oil seeps from Inciarte, an area of 0.12 km2 coveredby asphalt lagoons in the southern part of Mara municipality andon La Paz (current La Paz oilfield, some 0.06 km2 in area). In fact,these seeps were a guide to the discovery of the La Paz and La Con-cepción oilfields by the Royal Dutch Shell Group in 1925 (Sutton,1946).

ll rights reserved.

. 55-185, Casa Mene Grande,96.r).

The Lake Maracaibo Basin (containing 700 oil producing fieldsafter drilling about 16,500 wells) covers an area of approximately50,000 km2 (19,300 mi2). The main petroleum source rock is theCretaceous La Luna Formation, although other source rocks alsogenerated hydrocarbons (Talukdar et al., 1985, 1986; Tocco et al.,1997).

Seeps discovered between the 1900s and the 1930s led to thediscovery well that generally produced asphaltic (<10� API gravity)to heavy (10–20� API) crude oils (Table 1; Stalder, 1981). Excep-tions are paraffinic crude oils from the Amana Field (Amana-1and -7 wells, Fig. 1a). These light oils (around 40� API) display abimodal n-alkane distribution that are predominant over theC12–C20 and C25–C33 intervals, as well as pristane (Pr) to phytane(Ph) ratios of about 2.5 suggesting that they derived from an unde-termined continental siliciclastic source rocks distinct from thosefor the La Luna Formation in the northwestern coast of Lake Mar-acaibo (Escobar, 1987). This author applied McDowell’s model(1975) to estimate that 91% of the total oil accumulated in this areahas been derived from the Cretaceous La Luna source rocks.

Escobar (1987) also reported that Miocene reservoir rocks pro-duced high gravity (�38� API) fluids from the Peroc-1 and Perón-1wells (Fig. 1a), which were briefly reactivated in 1984. The phy-tane/n-C18 and pristane/n-C17 ratios of the oils are very low (0.3and 0.2, respectively), suggesting that these crude oils originated

Fig. 1. (a) Location of the wells and oilfields known in the northwestern part of the State of Zulia until 1946, as well as the seeps from the Sierra de Perijá foothills; (b) map ofthe Tulé Region; (c) Stratotype in the Cachirí area.

Table 1General characteristics of the oil wells drilled in the Sierra de Perijá foothills.

Well Year Depth (ft) API gravity (�) Reservoir formation

Amana-1 1926 4184 32 Lower Misoa (C-5)Marem-1 1930 6002 19 Misoa and MarcelinaAmboy-1 1928 5316 5–10 Lower MisoaVermor-1 1928 5364 10–20 Mito JuánCalentura-1 1929 6743 10 CogolloLeón-1 1929 1619 5–10 Granite BasementPeroc-1 1927 5600 10–20 CogolloPerón-1 1929 3233 17 La QuintaZancada-1 1918 2235 5–10 SocuyNeopam-1 1930 6412 15 GuasareNeopod-1 1929 7504 7–20 Icotea

728 M. Escobar et al. / Organic Geochemistry 42 (2011) 727–738

from a high maturity source rock. However, the low saturatedhydrocarbon proportions (29.4%) and relatively high concentra-tions in resins plus asphaltenes (45.5%) obtained for these crudesare not consistent with high maturation (Tissot and Welte, 1984).The origin of this light crude oil, in an area where the API gravityof the petroleum generally ranges from 18–25� (Hawkins and Chig-ne, 1982), has not been clearly established. A possible explanationfor these discrepancies may be that the heavy crude originally pro-

duced in these wells mixed with a lighter oil that migrated into thereservoir after the wells were shut in and reached sufficient pres-sure in the reservoir rock to once more produce by natural flow.Conduits for secondary migration can be set up when source rocksare juxtaposed against horizons with higher permeabilities (e.g.,sandstones) at faulting areas and other discontinuities (Mileset al., 1993). However, the faults served in many cases as verticalmigration paths to allow hydrocarbon migration from the Creta-ceous La Luna source rocks into the Eocene and Miocene sequences(Talukdar et al., 1986; Escalona and Mann, 2006).

These circumstances have engendered this work, which reviewsthe organic chemical signature of the 11 oil seeps in the Cachiríarea (Fig. 1b) in order to correlate these seepage oils with the LaLuna Formation rocks in the northwestern coast of Lake Maracaiboand to establish their relative degree of biodegradation and theirthermal maturity. Specifically, the study examines the hypothesisthat the study area produced heavy crude oils in the early 20thcentury and that these oils mixed with other lighter crude oilsyears later, indicating that crudes that had reached advancedstages of thermal maturity may have migrated to this part of theLake Maracaibo Basin in the past. This information is of great inter-

M. Escobar et al. / Organic Geochemistry 42 (2011) 727–738 729

est for oil companies, which can conduct exploratory surveys todiscover commercial oil accumulations at shallow stratigraphiclevels of the Tulé Carboniferous Region.

2. Geological setting

The Lake Maracaibo Basin is located on the southwestern edgeof the Caribbean Sea in western Venezuela near its border withColombia. Parnaud et al. (1995) divided the stratigraphic columnof this basin into five sequences based on tectonic events: (1) aJurassic rift sequence, (2) an Early–Late Cretaceous passive marginsuccession, (3) the transition to a compressive regime in the LateCretaceous–Early Paleocene deposits when collision and obductionof the Pacific volcanic arc overrode the South American plate andemplaced the Lara Nappes, (4) Late Paleocene–Middle Eocene fore-land basin in front of the volcanic arc and emplacement of the LaraNappes, and (5) a Late Eocene–Pleistocene sequence related to thecollision of the Panama arc with the South American plate.

The geology of the Sierra de Perijá has been previously studied(Sutton, 1946; González de Juana et al., 1980; Kellogg, 1981;Audemard, 1991; Lugo and Mann, 1995; Taboada et al., 2000;Duerto et al., 2006; Guzman and Fisher, 2006; Mann et al., 2006;James, 2009; Escalona and Mann, 2011; among others). Uplift ofthe Perijá Mountains occurred in the Oligocene; this orogenesisculminates in the Plio-Pleistocene (Kellogg, 1984). According toMiller (1962), the Sierra de Perijá forms a part of the complexbranching northern Andes chain located at the west margin of Ven-ezuela as a northward offshoot from the Eastern Cordillera ofColombia. At approximately 9�N latitude, there is a change in trendfrom N20�W, prevalent in the northern part of the Eastern Cordil-lera, to N25�E for the Sierra de Perijá. From this point, the Sierracontinues to a northern boundary at the edge of the Guajira Plainsat latitude 11�100N. Along its entire length, its crest forms theinternational boundary between Venezuela and Colombia.

The main fractures associated with the Sierra de Perijá fold beltat the Lake Maracaibo Basin in the study area include the Tigre,Perijá, and Cuiba sinistral transcurrent faults (González de Juanaet al., 1980). These faults are orientated approximately N60�Eand may have originated along normal fault trends in the riftedJurassic South America-African margin (Lugo and Mann, 1995).The Manuelote Syncline also is a well known fold in the area andits axial plane shares traits with the trend of the Tigre Fault (Miller,1962; Pindell et al., 1998).

This fault is a straight, high angle fault trending N35�E along thenorthwestern side of the Totumo-Inciarte anticline. It was classi-fied as a left lateral transcurrent fault (Rod, 1956; Alberding,1957). In the structural section of the Cachirí River and the Aljibemarsh creek, the Tigre Fault is evident at four points, dipping85�S. At some sites along the course of the Cachirí River, the TigreFault juxtaposes Tertiary rocks represented by the PaleoceneMarcelina Formation with Cretaceous rocks represented by CogolloGroup limestones (Fig. 1c). At these locations, there is a huge num-ber of hydrocarbon seeps along the plane of the fault, associatedwith a zone of fractures and variations in the dip of the strataplanes (Duerto et al., 2006). This evidence accounts for the seepsin the peneplain of the Cachirí River, associated with the TigreFault and its transverse fractures (Rojas, 2008). In addition, thestudy area contains transverse faults that juxtapose rocks fromthe Marcelina Formation with rocks from the Misoa Formation,both impregnated with hydrocarbons (Sutton, 1946).

3. Materials and methods

Eleven seep samples (OS-1 to OS-11) were collected in the Ca-chirí area (Fig. 2) and stored in glass jars prior to analysis. These

samples were collected from the bottom of the respective surfaceaccumulations, where fluids seep out of the rocks, and thereforewere taken from what appeared to be the most recently releasedmaterial. The oil seep samples were dehydrated with warm tolu-ene (ASTM D1796 standard method; American Society for Testingand Materials, 2004).

An aliquot of approximately 0.05 g of each oil seep was sepa-rated into its constituent fractions (SARA method) using conven-tional procedures. Briefly, asphaltenes were precipitated with n-heptane in a 1:40 v:v ratio in accordance with the ASTM D3279standard method (American Society for Testing and Materials,2007); each sample was passed through a batch reactor in constantagitation for 1 h at 60 �C, followed by a 12 h inactive period(Speight, 2007). Later, soluble remnants (maltenes) were separatedon activated alumina (Activity I, 80–200 mesh) into saturated, aro-matic and resin fractions by liquid adsorption chromatography (Dela Cruz et al., 1997).

The saturated and aromatic hydrocarbons were subsequentlyanalyzed using gas chromatography-mass spectrometry (AgilentTechnologies 6890 GC coupled to an Agilent 5973 Mass Detector).An HP-5MS capillary column (30 m � 0.25 mm, film thickness0.25 lm) was used. Oven temperature was programmed to runfrom the initial temperature of 80 �C (hold for 4 min) to 290 �C at4 �C/min, then to 290 �C for 20 min. Helium was used as carriergas with a flow rate of 2.4 ml/min. The gas chromatograph wasequipped with a splitless injector at 270 �C. Ions at m/z 99, 177,178, 184, 191, 192, 198, 217, 231 and 253 were scanned with adwell time of 0.1 s. The overall precision of the integrated peakareas from reconstructed ion chromatograms is 1–3%.

Inductively coupled plasma–atomic emission spectroscopy(ICP–AES) was used for the quantitative analyses of vanadiumand nickel concentrations (using the ASTM D-5708 standard;American Society for Testing and Materials, 2005) through a Per-kin-Elmer Optima 3000 spectrometer. Sulfur contents were mea-sured using a LECO S-144DR analyzer.

4. Results and discussion

4.1. Bulk geochemical data

The bulk, sulfur and trace metal (V, Ni) composition of the ele-ven oil seep samples is given in Table 2. Group type analyses (SARAfractions expressed as weight percentage) indicate that the oilseeps have a composition high in resins and asphaltenes (47–80%, average 63%), whereas the aromatic hydrocarbon fraction ran-ged from 6–27% (average 21%) and the saturates ranged between4% and 29% (average 16%). These values are generally characteristicof heavy crude oils subjected to microbial degradation (Tissot andWelte, 1984).

Table 2 also shows sulfur, vanadium and nickel concentrationsfor the samples analyzed. These values are typical of biodegradedoils (Hunt, 1996). Despite the fact that these concentrations canbe influenced by biodegradation or thermal maturity, the V/Ni ra-tios are similar due to the structural similarities among organome-tallic compounds that contain vanadium and nickel (Lewan, 1984).

Similar API gravities (11–15�), V (900–1100 ppm) and Ni (95–105 ppm) concentrations, V/Ni ratios (10.6–11.2) as well as sulfurcontents (5.5–6.5%) for the seep samples are observed in crude oilsfrom nearby reservoirs, such as Eocene Boscán (Escobar and Pas-quali, 1989), Eocene La Paz (Escobar et al., 1989b), Eocene Maraand Cretaceous Mara Oeste (López et al., 1995; López and Lo Móna-co, 2004). These similarities could indicate that the petroleumaccumulated in these reservoirs and the petroleum seeping outin the study area are very likely affected by similar alteration pro-cesses (Escobar, 1987). Moreover, high sulfur contents, V/Ni ratios

Fig. 2. Location of the study seeps in the Cachirí area.

Table 2Bulk geochemical, sulfur and trace metal data of the Cachirí oil seep samples.

Sample Latitude Length Saturates Aromatics Polars V (ppm) Ni (ppm) V/Ni S (wt%)

OS-01 N1201989 E802000 4 27 69 953 86 11.1 5.41OS-02 N1201989 E802000 22 6 72 931 85 10.9 5.47OS-03 N1201698 E801835 29 25 47 925 85 10.9 5.41OS-04 N1201177 E801518 27 22 51 945 89 10.6 5.52OS-05 N1201023 E801568 8 25 67 980 92 10.6 5.72OS-06 N1200202 E800805 21 26 53 954 89 10.7 5.33OS-07 N1200032 E800650 15 22 64 853 79 10.8 5.64OS-08 N1199866 E800490 17 25 59 990 90 11.0 5.61OS-09 N1199691 E800466 14 25 61 984 88 11.2 5.60OS-10 N1199140 E799594 9 22 69 1102 104 10.6 6.36OS-11 N1199328 E799930 7 13 80 1058 100 10.6 5.90Uncertainty ±3 ±3 ±3 ±1 ±1 ±0.1 ±0.01

Note: All crude oil fraction data are expressed in weight percentages.

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and Ni concentrations in all the samples may indicate that the cor-responding source rocks were deposited in a marine carbonateenvironment under euxinic or very reducing conditions (Galarragaet al., 2008). During deposition of carbonate facies, bacterial sulfideis not completely sequestered by iron and nickel ions will precipi-tate in metal sulfides rather than form organometallic compoundsin contrast to stable vanadyl ions leading to high V/Ni ratios (Le-wan, 1984; Lo Mónaco et al., 2007).

4.2. Organic matter source and depositional environment

Molecular parameters (pristane/phytane, dibenzothiophene/phenanthrene, homohopane index, among others) for saturatedand aromatic fractions separated from the oil seep samples arelisted in Table 3. GC–MS analyses (m/z 191) for terpane and sterane(m/z 217) biomarkers are shown in Fig. 3 for three representativesamples, two significantly biodegraded (OS-01 and OS-11) and athird slightly degraded sample (OS-08).

The nearly identical terpenoid distribution of the samples fromthe Tulé region (Fig. 3a, b and g) indicates a common origin. Thesimilarity among the m/z 191 fragmentograms is corroborated bythe homohopane index values (Table 3), with an average of 0.16and a relative standard deviation of 0.02, together with the pre-dominance of C35 regular hopane over the C34 homolog in all oilseeps (Fig. 3a, b and g). These results suggest marine input to theorganic matter deposited under highly reducing conditions (Petersand Moldowan, 1991). There is a high abundance of the C23 tricy-clic terpane with respect to the other tricyclic homologs (Fig. 3a, band g); consequently, the C24/C23 and C21/C23 tricyclic terpane ra-tios not exceeding 0.34 (Table 3) also are indicative of oil seepsderiving from organic matter deposited in a carbonate marineenvironment under reducing conditions (Waples and Machihara,1991). Finally, the near absence of tetracyclic terpanes and thesimilar values in the proportion of C30 hopane with respect to C23

tricyclic terpane (C30H/C23TT, Table 3) in the majority of the sam-ples (average 1.65 and standard deviation of 0.64) also indicate a

Table 3Geochemical indicators of source depositional environment for saturate and aromatic fractions in oil seep samples from the Tulé Region.

Sample C27/C29ST C35/C31–C35 C24/C23TT C21/C23TT Pr/Ph C30H/C23TT DBT/P

OS-01 0.31 0.15 0.30 0.21 0.47 2.33 1.32OS-02 – 0.14 0.29 0.18 – 1.50 1.36OS-03 1.01 0.17 0.25 0.20 – 2.17 1.35OS-04 – 0.15 0.27 0.23 0.81 1.17 1.37OS-05 – 0.15 0.26 0.22 0.72 1.22 1.31OS-06 – 0.18 0.22 0.22 0.60 1.38 1.40OS-07 0.71 0.12 0.24 0.22 – 2.84 1.35OS-08 1.30 0.16 0.24 0.19 0.72 1.56 1.44OS-09 1.19 0.17 0.26 0.21 0.47 1.27 1.42OS-10 1.18 0.19 0.26 0.20 – 1.38 1.38OS-11 – 0.13 0.24 0.22 0.94 1.32 1.30

Notes: C27/C29ST = C27-regular sterane/C29-regular sterane; C23/C24TT = C23-tricyclic terpane/C24-tricyclic terpane; C23/C21TT = C23-tricyclic terpane/C21-tricyclic terpane; DBT/P = dibenzothiophene/phenanthene; and homopane index or C35/C31–C35; C30H/C23TT = C30-hopane/C23-tricyclic terpane. Uncertainties of 0.02.

Fig. 3. (a), (c) and (h), respectively, m/z 191, m/z 217 and m/z 177 ion fragmentograms for the OS-08 sample; (g), (e) and (f), the same ion fragmentograms for the alteredOS-11 oil seep; (b) and (d), respectively, m/z 191 and m/z 217 ion fragmentograms showing hopanoid and sterane distributions for the OS-01oil seep.

M. Escobar et al. / Organic Geochemistry 42 (2011) 727–738 731

Fig. 4. (a), (b) and (c), respectively, plots of DBT/P vs. S (%), C24/C23TT against C22/C21TT and DBT/P vs. Pr/Ph for some of the studied seep oils.

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single geochemical type were of oils. It is noteworthy that three oilseeps (OS-01, 03 and 07) have C30H/C23TT values >2 (Table 3). Wecould not find any supporting evidence of different organic faciesin relation to the origin of these oil seeps (Hoffmann et al., 1984)or different maturity levels (Sajgó, 2000) among them. A slightdepletion of diterpanes with respect to triterpanes in experimen-tally water washed oils had been noted (Kuo, 1994). This couldbe an alternative explanation to account for these anomalous re-sults. Though the relative abundance of C27, C28 and C29 steranes(ST) and the TT/ST ratio are used to determine the type of organicmatter (Huang and Meinschein, 1979), these molecular parametersare dependent on the effects of biodegradation (Seifert and Moldo-wan, 1979). Only a few samples had been subjected to such severealteration that they contained no recognizable free steranes (OS-02, OS-04, OS-5, and OS-11; see Fig. 3e). The steranes also are de-pleted in the OS-01 sample (see Fig. 3d). By contrast, the remainingsamples (Table 3) could indicate a marine input to the organic mat-ter of the source rock (Volkman et al., 1998). Finally, the absence of18a(H)-oleanane has been commonly reported in the marinecrudes of the Lake Maracaibo Basin (Talukdar et al., 1986; Alberdiand López, 2000).

Both the cross plot of dibenzothiophene/phenanthrene (DBT/P)vs. the sulfur content and the relationship between C24/C23 andC22/C21 tricyclic terpane ratios are used as a paleodepositional-environmental indicator of sedimentary rocks (Hughes et al.,1995). The majority of the samples (OS-01, OS-04, OS-05, OS-06,OS-08, OS-09, and OS-11) display DBT/P ratios greater than one,sulfur contents of about six, Pr/Ph ratios <1 and C22/C21TT ratios>1 (Fig. 4a, b and c), indicating that the source unit for these seepsis lithologically a marine carbonate mixed with sulfur rich blackshales (Peters et al., 2005).

As shown in a representative m/z 184 + 198 fragmentogram(see Fig. 5b), the methyldibenzothiophene distribution in the aro-matic fraction shows a V pattern, with the first-eluting isomer(4-methyl) being largest and the last-eluting isomer (1-methyl)next largest. The middle peak of the V pattern (2 + 3 methyl) isthe lowest peak. These features are typical of crudes deriving froma carbonate marine source rock (Wang and Fingas, 1995). As statedabove, all data indicate that the oil seep samples are residualhydrocarbons from oils that were generated from marine carbon-ate rocks deposited under reducing conditions that subsequentlymigrated to the surface. All of the above corroborates the reportsof various authors (Hedberg, 1931; Sutton, 1946; González de Jua-na et al., 1980; Escobar, 1987; Pérez, 1991) that considered theCretaceous La Luna Formation from western Venezuela as the mostprobable source rock of these oil seeps.

4.3. Thermal maturity

The review of m/z 99 representative mass fragmentograms(samples OS-08 and OS-01) indicated that all the seep sampleshave low n-alkane contents (see Fig. 6a and b). These hydrocarbonrich fluids have a Pr/Ph ratio consistently <1 (Table 3) despite thefact that this value may have been affected (to a greater or lesserdegree) by biodegradation at several of the seeps (Peters et al.,2005). Alkanes are the first to be removed during early stages ofmicrobial degradation (Peters et al., 2005), and therefore the valuesof maturity parameters based on these hydrocarbons could be theresult of biodegradation and not of generation from mature sourcerocks.

Phenanthrene and methylphenanthrene homologs are presentin appreciable proportions in all the oil seep samples (m/z178 + 192; Fig. 5a); their relative intensities provide three ther-mally sensitive parameters, methylphenanthrene indices andmethylphenanthrene ratio (see Table 4). The methylphenanthreneindices, MPI-1 and MPI-2, are widely used as aromatic hydrocar-

bon based maturity parameters. MPI-1 and the methylphenanth-rene ratio are often used in estimating the equivalent vitrinitereflectance value �%Rc1 and %Rc2, respectively (Radke and Welte,1983; Radke et al., 1984). However, MPI-1 and MPI-2 ratios may

Fig. 5. (a), (b) and (d), m/z 178 + 192, m/z 184 + 198, and m/z 231 ion fragmentograms for a representative sample (OS-01); (c), (e) and (f), m/z 253 ion fragmentogram for thesamples OS-01, OS-11 and OS-08. Notes: Alpha and beta represent 5a(H) and 5b(H) series; dia, R and S refer to monoaromatic diasteroid hydrocarbons, 20R and 20S isomers,respectively.

Fig. 6. (a) and (b) m/z 99 ion fragmentograms showing n-alkane distribution for theOS-08 and OS-01 samples.

Table 4Molecular maturity parameters for the saturates and aromatics in natural hydrocar-bon seep samples from the Tulé Region.

Sample Ts/(Ts + Tm) % 22S MA ratio MPI-1 MPI-2 % R1 % R2

OS-01 0.71 61 0.62 0.87 0.94 0.92 0.98OS-02 0.78 57 0.64 0.94 1.00 0.96 1.00OS-03 0.69 61 0.51 0.89 0.95 0.93 0.96OS-04 0.75 64 0.53 0.95 0.98 0.96 1.00OS-05 0.78 60 0.48 0.96 1.01 0.96 0.99OS-06 0.68 60 0.42 0.91 0.96 0.94 0.96OS-07 0.68 58 0.53 0.90 0.95 0.94 0.96OS-08 0.65 63 0.50 0.89 0.99 0.93 1.03OS-09 0.70 57 0.58 0.92 0.94 0.96 0.96OS-10 0.74 60 0.56 0.93 0.98 0.96 0.97OS-11 0.75 60 0.44 0.86 0.97 0.92 1.02

Notes: % 22S or C31ab 22S/(22S + 22R) = 17a,21b(H)-29-homohopane ratio (%);MPR = 2-MP/1-MP; MPI-1 = 1.5�(2-MP + 3-MP)/(P + 1-MP + 9-MP); MPI-2 = 3�(2-MP)/(P + 1-MP + 9-MP);%Rc1 = 0.4 + 0.6�MPI-1;%Rc2 = 0.94 + 0.99�log MPR; Ts/Tm =18a(H)-22,29,30 trisnorneohopane/17a(H)-22,29,30 trisnorhopane; MA ratio orMA(I)/MA(I + II) = C21+22-monoaromatic steroids/C21+22+27+28+29-monoaromaticsteroids. Uncertainties of 0.02, except % 22S (2%), % R1 and % R2 (0.1%).

M. Escobar et al. / Organic Geochemistry 42 (2011) 727–738 733

be no longer valid above biodegradation degree 5 due to significantchanges in relative abundance of the methylphenanthrene isomersat advanced stages of biodegradation (Huang et al., 2004). Varia-tions in the type of organic matter or lithology in the source rockcan affect methylphenanthrene ratios. Indeed, Cassani et al.(1988b) noted that high MPI-1 and PP-1 corresponded with thehigh carbonate content in La Luna source rocks.

Despite this drawback, both ratios have been used as indicatorsfor the maturity, although they must be interpreted with caution.The MPI-1 and %Rc1 values calculated for the samples range from0.86–0.95 and 0.92–0.94, respectively. Table 4 also gives the %Rc2

(range 0.96–1.03) values. In accordance with results in the litera-

ture (e.g., Akinlua et al., 2007), when comparing MPI-1 and MPI-2 values, the latter are relatively higher. %Rc2 values also areslightly high compared to %Rc1 values. These results all show asimilar maturation level of organic matter for all the samples:around the peak oil generation of the oil window for type II kero-gens (Tissot and Welte, 1984).

Several molecular parameters were obtained from the hopanoiddistributions (m/z 191) and from the monoaromatic steroid (m/z253) characteristic signals (Figs. 3a, b, g, 5c, e and f). These datapermit the determination of the level of thermal maturation ofthe source rocks that generated these oil seeps, which are related

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to thermally mature light crudes. Additionally, the cracking ratioMA(I)/MA(I + II) can be used to determine the level of maturityreached by organic matter (Seifert and Moldowan, 1978; El-Gayar,2005); MA(I) and MA(II) are defined as the whole content of C21

plus C22 monoaromatic steroids and the sum of all C27–C29 com-pounds, respectively (Peters et al., 2005). The values on the orderof 50–60% (Table 4) are indicative of materials generated from ker-ogens covering a maturity range around the maximum peak oilgeneration of the oil window (Tissot and Welte, 1984). Unfortu-nately, the presence of numerous unresolved humps (Fig. 5d) madeit impossible to use C27–C29 triaromatic steroids (m/z 231) to cor-roborate the maturity level of the organic matter for the samples.This might be due to contamination with unknown products.

The predominance of 18a(H)-22,29,30 trisnorneohopane over17a(H)-22,29,30 trisnorhopane (Table 3) [Ts/(Ts + Tm)] exceeding0.65 and a low standard deviation of 0.2 confirms that the sourcerocks which generated the oil seeps were thermally mature (Seifertand Moldowan, 1981). The 22S hopane percentages (see Table 4)for the samples (averaging �60%, standard deviation of 2.7%) indi-cate maturity of the precursor organic matter (Seifert and Moldo-wan, 1978). The diagram for MA(I)/MA(I + II) vs. Ts/(Ts + Tm)shows that all oil seeps plot in the mature field (Fig. 7).

4.4. Biodegradation of oil seeps

Thermal maturity and source determination of organic matterin oil seeps can be problematic since n-alkanes, isoprenoids, ster-anes and the remaining biomarkers can be affected by geochemicalprocesses such as biodegradation (Bennett and Larter, 2008). In-deed, several common molecular parameters are not suitable forestablishing the thermal maturation of organic matter in the oilseeps studied since these ratios may be altered by microbial degra-dation. Table 5 provides information on its current state of preser-vation for each biomarker family and establishes its level ofalteration according to the biomarker biodegradation scale pro-posed by Peters et al. (2005). Representative ion chromatogramsare shown in Fig. 3 (peak identifications of terpanes and steranesare in the Appendix). The partial absence of n-alkanes in severalseep samples (OS-01, OS-05, OS-06, OS-08, OS-09, and OS-11) indi-cates a level 2 for biodegradation (Peters et al., 2005). The remain-ing samples are ranked at biodegradation degree 3, which meansthat they have a near total absence of n-alkanes (Peters et al.,2005). By contrast, no sample is ranked at biodegradation degree5, defined by the total absence of acyclic isoprenoids (Peterset al., 2005). In some of the samples (OS-01, OS-02, OS-04, OS-05,

Fig. 7. Correlation of thermal maturation parameters based on MA(I)/MA(I + II) andTs/(Ts + Tm) ratios.

OS-10, and OS-11), regular steranes are partially or totally depleted(biodegradation degree 6 or higher, see Fig. 3e), whereas the lowpresence of the 25-norhopanes (10-demethylhopanes) in thesesix samples is consistent with a severe process of microbial degra-dation (Bennett et al., 2006). The 25-norhopane biomarkers appearto result from the alteration of the C10 methyl group in the regularhopanes and are easily detected in a representative m/z 177 frag-mentogram (OS-11 sample; see Fig. 3f). Nevertheless, these bio-markers are absent in four samples (OS-03, OS-07, OS-08 and OS-09, see Fig. 3h). In addition, none of these oil seeps range betweenbiodegradation degrees 8 and 9, showing evidence of a very slightalteration of hopanes. Finally, monoaromatic steroids appear intactin all the samples, which suggests that none of them reaches de-gree 10.

It should be noted in Table 5 that eight of the 11 samples (allexcept for OS-06, OS-08, and OS-09) show various non-correlativeand very distinctive degrees of biodegradation. Various scales ofbiodegradation ranked according to a common alteration sequencefor each of a series of biomarker families with different resistancesto microbial degradation (e.g., Wenger et al., 2002; Peters et al.,2005). Nevertheless, exceptionally there are modifications in theusual order of the sequence (Larter et al., 2003). Additionally, thereare scarcely any cases in which bacteria have been able to degradeonly n-alkanes and steranes, as is observed in these oil seeps.While there have been a few studies dealing with oil seeps fromnearby areas and affected by very heterogeneous degrees of bio-degradation (Bojesen-Koefoed et al., 2005; Sánchez and Perman-yer, 2006), we are aware of few previous studies that havereported the occurrence of any oil seep affected simultaneouslyby very distinctive and uncorrelated degrees of biodegradation(e.g., Cassani et al., 1988a).

Although we cannot preclude other possibilities to account forthis anomalous pattern, the most probable explanation consistsof the dilution of an initial heavy crude that was severely degradednear the surface with a second, thermally mature light oil affectedby slight biodegradation and then the subsequent migration of thismixture to the surface. This would account for the heterogeneousdegrees of biodegradation observed in most of the samples. Theaforementioned light crude was not detected in the exploratorydrilling programs in the early part of the 20th century in this partof the Maracaibo Basin (Table 1 and Fig. 1), suggesting it mighthave migrated more recently. This migration may have occurredas a result of the movement of petroleum trapped in deeper strati-graphic levels through migration paths (e.g., faults) that were reac-tivated by very recent tectonics. Alternatively, this petroleum mayhave been generated and expelled extremely recently from La LunaFormation source rocks. The thermal evolution models of this unitthat have been reported (Blaser and White, 1984; Talukdar andMarcano, 1994), together with evidence of the primary petroleummigration established based on calcareous La Luna rock cores col-lected from some wells (Escobar et al., 1989a), suggest that the LaLuna Formation might currently be generating petroleum in thenorth-central region of the western shoreline of Maracaibo Lake(Escobar, 1987). A third permutation of this hypothesis also pre-sumes the existence of two oil pulses: a first oil charge would havebeen severely biodegraded when the reservoirs were shallow, incontrast with a second La Luna oil charge that would have under-gone slight biodegradation as the reservoirs subsided and becamehotter. Reservoirs that did not subside sufficiently to curtail micro-bial growth would contain only heavily degraded oil, such asencountered by the early exploration wells in the study area.

Lastly, the observations can be explained by a possible scenariothat requires a long period of continuous oil charging. Here, unal-tered La Luna oil was generated and migrated into stacked reser-voirs at varying depths in the past. Crude oils in the shallowerreservoirs were heavily biodegraded, while oils in the hotter

Table 5Brief description of the preservation levels for the analyzed biomarker families and biodegradation degrees, in accordance with the Peters et al. (2005).

Sample n-Alkanes Isoprenoids Steranes 25-Norhopanes Hopanes PM degrees

OS-01 Scarce Nearly intact Depleted Very scarce Intact 2/6OS-02 Very scarce Depleted Removed Scarce Nearly intact 3/4/7OS-03 Very scarce Depleted Intact Absent Intact 3/4OS-04 Very scarce Nearly intact Removed Very scarce Nearly intact 3/7OS-05 Scarce Nearly intact Removed Very scarce Nearly intact 2/7OS-06 Scarce Nearly intact Intact Scarce Intact 2OS-07 Very scarce Very depleted Intact Absent Intact 3/4OS-08 Scarce Nearly intact Intact Absent Intact 2OS-09 Scarce Nearly intact Intact Absent Intact 2OS-10 Very scarce Depleted Affected Very scarce Nearly intact 3/4/6OS-11 Very scarce Nearly intact Removed Scarce Nearly intact 2/7

Fig. 8. (a) and (b) m/z 217 and m/z 191 ion fragmentograms for the La Luna rock extracts of the Tulé region.

M. Escobar et al. / Organic Geochemistry 42 (2011) 727–738 735

736 M. Escobar et al. / Organic Geochemistry 42 (2011) 727–738

reservoirs would have remained non-biodegraded. Later in time, amigration conduit would have allowed the unaltered crude oil toflow into the shallower reservoir and mix with the oil that wouldhave previously undergone heavy biodegradation. However, thefact that the seep oils still retain a portion of the unaltered oilmight suggest that the seepage to the surface is fairly recent andthat their occurrence is likely due to recent tectonic activation ofthe local faulting systems.

Such a mixing of crude oils with distinct degrees of biodegrada-tion (characterized by the presence of 25-norhopanes in nearlynon-degraded light crudes) also has been reported from the BolivarCoast region of Lake Maracaibo, northwestern Venezuela (Gallangoet al., 1985; Alberdi et al., 2001). The high gravity oil accumulatesin shallow stratigraphic traps that are sealed by plugs of severelybiodegraded asphaltic crude.

4.5. Geochemical correlations

All the studied oil seepages are esentially of the same genetictype and were derived from the same marine source rocks (proba-bly La Luna Formation). The origin of these mature oil seeps fromthe marine organic source has been indicated by conventionalanalyses and molecular parameters from special analyses by GC–MS (Table 3 and Fig. 4).

An attempt has been made to identify the La Luna rock extractsas the source for the oil seeps in the Cachirí area based on the com-parison of geochemical data and chromatographic characteristics ofthe oil seep samples with those previously reported for some LaLuna rock extracts (P-114, 14RN-1X and 33F-1X; Fig. 1) from thenorthwestern coast of Lake Maracaibo by Escobar (1987). A positivecorrelation exists between the nickel, vanadium and sulfur contentsof the studied oil seeps and rocks of the La Luna Formation in thisregion. The La Luna rock extracts showed average concentrationsof sulfur, vanadium and nickel of about 8%, 1900 and 180 ppm,respectively, and V/Ni ratios of approximately 10.4 (Escobar,1987). This latter value is similar to those reported in Table 2. Withrespect to the distribution of steranes and terpanes, the oil seeps(Fig. 3a, b, c, d and g) show very similar patterns to those of theLa Luna rock extracts (Escobar, 1987) from the Tulé region(Fig. 8a and b). The tricyclic terpane distributions of the oil seepsamples can be used as evidence for their generation from the or-ganic rich facies of the aforementioned La Luna extracts. Both con-tain abundant C23 tricyclic terpane that predominates over otherhomologs. The La Luna source rocks also are characterized by theabsence of 18a(H)-oleanane, 22S hopane percentages >50% andthe predominance of the stereoisomers of C27 sterane over thoseof C29 sterane, suggesting a marine origin and a mature stage. Onthe basis of the relative abundance of C27, C28 and C29 steranes,the La Luna rocks appear to be well correlated to the majority ofthe oil seep samples. Variation in Ts/(Ts + Tm) between the La Lunarock extracts and the studied seeps may be related to factors such asmineral catalyzed rearrangement from Tm to Ts (Bakr and Wilkes,2002).

In summary, the geochemical source rock evaluation (Escobar,1987), identification of oil seepage type and source correlationshave indicated that the studied oil seep samples in the Cachirí areaare derived from the marine organic matter of the La LunaFormation.

5. Conclusions

After analyzing 11 oil seep samples along the Carboniferousregion of Tulé (Lake Maracaibo Basin), the values of several molec-ular geochemical parameters (known to be usually unaffected bybiodegradation) have led to the conclusion that these oil seeps

are residual hydrocarbons from a thermally mature crude oil, verylikely generated from the Cretaceous source rocks of the La LunaFormation. This crude oil may have been originally light and itshows evidence of being affected by microbial degradation, thusAPI gravity has certainly been reduced, which is confirmed bymolecular data and the average percentage of polar compounds(�60%).

The main outcome of this research is that most of the oil seepsamples analyzed show various degrees of biodegradation thatare not correlative and are very distinctive, revealing that the studysamples may be mixed fluids that displayed different levels of bio-degradation. In addition, there is a small presence of norhopanebiomarkers (degree 6) derived from the alteration of the C10

methyl group in the regular hopanes. It appears that a previouslyaltered heavy crude was diluted by a second migrated light crudeoil, with the resulting mixture reaching the surface through naturalhydrocarbon seeps. The two aforementioned oils may be the resultof two generation pulses from the La Luna source rock or from asingle oil charging event involving reservoirs are varying depththat later mixed.

The potential for lighter hydrocarbons being present in the re-gion suggests that the oil industry should take on new drilling pro-grams to explore for oilfields in the north-central region of LakeMaracaibo’s western shoreline. In the past, this zone’s Tertiaryshallow petroleum reservoirs have been systematically rejectedas the crudes found were generally heavy and of low commercialvalue.

Acknowledgments

The authors are grateful to the two reviewers (Dr. ManuelMartínez and a second anonymous referee) for their commentswhich helped to improve the original version of this work. We alsothank Dr. Clifford C. Walters (ExxonMobil Research & EngineeringCo., NJ) and Dr. Suhas C. Talukdar (Weatherford Laboratories,Texas) for critical reading of the manuscript.

Appendix A

Main terpanes, hopanes, steranes and diasteranes identified inthe mass chromatograms.

1

C1-Tricyclic terpane 2 C22-Tricyclic terpane 3 C23-Tricyclic terpane 4 C24-Tricyclic terpane 5 C25-Tricyclic terpane 6 C24-Tetracyclic terpane 7 C26-Tricyclic terpane 8 18a(H)-22,29,30 Trisnorneohopane 9 17a(H)-22,29,30 Trisnorhopane 10 17a,21b(H)-30-Norhopane 11 17a,21b(H)-Hopane 12 17a,21b(H)-29-Homohopane 22S 13 17a,21b(H)-29-Homohopane 22R 14 17a,21b(H)-29-Bishomohopane 22S 15 17a,21b(H)-29-Bishomohopane 22R 16 17a,21b(H)-29-Trishomohopane 22S 17 17a,21b(H)-29-Trishomohopane 22R 18 17a,21b(H)-29-Tetrahomohopane 22S 19 17a,21b(H)-29-Tetrahomohopane 22R 20 17a,21b(H)-29-Pentahomohopane 22S 21 17a,21b(H)-29-Pentahomohopane 22R 22 5a-Pregnane

M. Escobar et al. / Organic Geochemistry 42 (2011) 727–738 737

23

20-Methyl-5a-pregnane 24 13b(H),17a(H)-Diacholestane 20S 25 13b(H),17a(H)-Diacholestane 20R 26 5a(H),14a(H),17a(H)-Cholestane 20S 27 5a(H),14b(H),17 b(H)-Cholestane 20R 28 5a(H),14a(H),17b(H)-Cholestane 20S 29 5a(H),14a(H),17a(H)-Cholestane 20R 30 5a(H),14a(H),17a(H)-Ergostane 20S 31 5a(H),14b(H),17b(H)-Ergostane 20R 32 5a(H),14b(H),17b(H)-Ergostane 20S 33 5a(H),14a(H),17a(H)-Ergostane 20R 34 5a(H),14a(H),17a(H)-Stigmastane 20S 35 5a(H),14b(H),17b(H)-Stigmastane 20R 36 5a(H),14b(H),17b(H)-Stigmastane 20S 37 5a(H),14a(H),17a(H)-Stigmastane 20R 38 C20-Tricyclic terpane 39 C27-Pentacyclic terpane 40 13a(H),17b(H)-Diacholestane 20R 41 13b(H),17a(H)-Diaergostane 20S 42 13b(H),17a(H)-Diaergostane 20R 43 13a(H),17b(H)-Diaergostane 20R

Associate Editor—Cliff Walters

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