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Rare earth element geochemistry of petroleum source rocks from northwestern Niger Delta
Marine and Petroleum Geology 77 (2016) 409e417
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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Research paper
Rare earth element geochemistry of petroleum source rocks from northwestern Niger Delta A. Akinlua a, *, F.S. Olise b, A.O. Akomolafe a, R.I. McCrindle c a
Fossil Fuels and Environmental Geochemistry, Department of Chemistry, Obafemi Awolowo University, Ile-Ife, Nigeria Department of Physics, Obafemi Awolowo University, Ile-Ife, Nigeria c Department of Chemistry, Tshwane University of Technology, Arcadia Campus, 175 Nelson Mandela Drive, Private Bag X680, Pretoria, 0001, South Africa b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 April 2016 Received in revised form 16 June 2016 Accepted 23 June 2016 Available online 25 June 2016
Geochemical investigation of forty two rock samples from three fields in the northwestern Niger Delta was carried out in order to determine their rare earth elements (REEs) content and their geochemical significances. The rare earth elements and trace elements in the rock samples were determined using inductively coupled plasma e mass spectrometry (ICP-MS) and instrumental neutron activation analysis (INAA). The results indicate that nickel is most abundant trace elements in ML field while cobalt is most abundant in MF and MJ fields. Vanadium had the least concentration in the three fields. Nickel had enhanced concentrations over vanadium in the three fields. The concentrations and ratios of the trace elements indicated that the source rocks in the three fields had strong terrestrial organic matter input and were deposited under oxic conditions. Gadolinium is the most abundant rare earth element in three fields. Neodymium had the least concentration in ML and MJ fields while praseodymium had the least concentration in MF field. The REE distribution patterns of source rocks from the three fields are similar, indicating similar genetic origin. Pearson correlation matrix revealed that europium, dysprosium, lanthanum and samarium are potential redox and source indicators. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Rare earth element Source rock Niger Delta Origin Depositional
1. Introduction Source rocks are highly organic-rich type of sedimentary rock that are capable or may become capable or have been able to generate petroleum (Tissot and Welte, 1984). Source rock ultimately generates crude oil and contains measurable quantities of many trace metals and rare earth elements (REEs). Trace metals are incorporated into petroleum source rocks in the form of porphyrin complexes and may include direct incorporation from the biomass and formation during sedimentation. It may also involve diagenesis from organic molecules as well as metals derived from different biogenic and abiogenic sources (Barwise, 1990; Nwachukwu et al., 1995; Akinlua et al., 2007). The type and the thermal maturity of organic matter and their depositional environment have significant effects on the concentration of trace elements in source rocks (Lewan, 1984). Metals of proven association with organic matter may be used as reliable
* Corresponding author. E-mail addresses: [email protected], (A. Akinlua). http://dx.doi.org/10.1016/j.marpetgeo.2016.06.023 0264-8172/© 2016 Elsevier Ltd. All rights reserved.
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correlation tools. Nickel, vanadium, and cobalt (usually referred to as biophile elements) are such examples (Barwise, 1990; Udo et al., 1992; Akinlua et al., 2007). The geochemical characteristic and intrinsic distribution of rare earth elements in geological formation would make them useful geochemical tools to determine the origin, depositional environment, and thermal maturation of organic matter (Henderson, 1984; Takeda and Arikawa, 2005; Akinlua et al., 2008). A wide range of metals have been determined in crude oils, bituminous substance and organic sedimentary rock all over the world (Ajayi et al., 2009; Akinlua et al., 2007; Barwise, 1990; Nwadinigwe and Nworgu, 1999; Shtangeeva, 2006; Udo et al., 1992). Trace-metal accumulation is known to be controlled by redox conditions in marine sediments (Algeo and Rowe, 2012). Some trace metals have proven to be good paleoenvironmental redox indicators (Lewan and Maynard, 1982; Mongenot et al., 1996; Alberdi-Genolet and Tocco, 1999; Tribovillard et al., 2006). Several studies have been carried out on trace metals in oils from the Niger Delta area (Nwachukwu et al., 1995; Ajayi et al., 2009; Akinlua and Torto, 2006; Akinlua et al., 2007) and a very few study on rare earth elements in oils from the Niger Delta (Akinlua et al., 2008).
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However, there is paucity of data on the determination and geochemical assessment of rare earth elements content of petroleum source rocks from Niger Delta Basin. The present study is therefore an attempt to determine the rare earth elements content of source rock samples from Niger Delta in order to assess their suitability in the evaluation of origin and depositional environment of source rocks.
2. Geology of study area The Niger Delta is one of the world’s largest Tertiary deltaic systems and an extremely prolific hydrocarbon province. It is situated on the West African continental margin at the apex of the Gulf of Guinea (Doust, 1990). The area is located at the southern region (latitude 4 and 6 N and longitude 3 and 9 E) and occupies 2 an area of about 75,000 km with clastic sequence that reaches a maximum thickness of 9000e12,000 m of sediment and total sediment volume of 500,000 km2 and makes up 7.5% of Nigeria landmass (Frank and Cordry, 1967). The study area is bounded by older Cretaceous tectonic elements, such as the Abakaliki Anticlinorium and the Afikpo syncline (Ejedawe, 1981). The area is rich in oil and the host rocks are organic-rich sedimentary rocks. The Tertiary lithostratigraphic sequence of the Niger Delta consists of the Akata, Agbada, and Benin formations (Figs. 1 and 2) (Evamy et al., 1978). The lowest unit is the Akata formation with a uniform massive marine shale unit with age ranging from Paleocene to Recent. Akata Formation is overlain with Agbada Formation, which consists of intercalation of sandstone and shale with age ranging from Eocene to Recent (Short and Stauble, 1967). The Benin Formation that overlies the Agbada Formation consists of fluviatile sands and gravels with age ranging from Eocene to Recent (Avbovbo, 1978). The sandstones of the Agbada Formation are the reservoir rocks for liquid hydrocarbons that occur in this sedimentary basin (Evamy et al., 1978). Fig. 2. Stratigraphic columns showing the three Formations of the Niger Delta (Doust and Omatsola, 1990).
3. Experimental 3.2. Sample preparation for ICP-OES 3.1. Sample collection Source rock samples were collected at different depths, from three oil fields (ML, MF and MJ field) in the paralic sequence of Agbada Formation, northwestern Niger Delta (Fig. 3). Only rock samples with TOC above 0.5 wt% were selected for rare earth and trace elements analysis.
The source rock samples were prepared for inductively coupled plasma e optical emission spectrometric (ICP-OES) measurement by acid digestion into colourless aqueous solution. Dissolution of the source rock samples was achieved using microwave accelerated reaction system. 0.5 g of each sample was digested using 4 mL of
Fig. 1. Schematic cross section showing principal stratigraphic units of Tertiary Niger delta (Ekweozor and Okoye, 1980).
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Fig. 3. Map of Niger Delta showing location of the study area.
48% HF, 3 mL of 65% HNO3 and 1 mL of 30% H2O2, which resulted in complete dissolution of the rock sample. The experiment was performed in closed PTFE vessels at a digestion temperature of 200 C, pressure of 241 psi, at ramp time of 30 min and then held for 10 min. 4 mL of 7% boric acid was added to the digested sample and heated for 25 min of ramp time and 5 min hold time, and at temperature of 155 C. The experiments were performed at irradiation power of 800 W. After cooling the vessels, the resulting aqueous colourless solution was transferred into a 25 mL volumetric flask and made up to the mark with deionised water. Blank was prepared under the same conditions. Multi-element standard solution of the elements was prepared from the stock solutions of the elements. The stock solutions were diluted with deionised water to prepare each working solution for calibration. The calibration curves were obtained from the standard solutions of the elements. 3.3. Sample preparation for INAA Sample preparation for INAA is essentially limited to weighing a suitable amount of homogenous pulverized sample into small quart or plastic vials for reactor irradiation and subsequent gamma spectrometric analysis. The essential issues are to avoid contamination by careful selection of the vial material, cleaning the vials and verification of the blank levels. There must be tight specifications on the vial dimensions and wall thickness in order to produce counting efficiencies and gamma ray attenuation effects. At the laboratory, source rock samples were air-dried to approximately constant mass, and each sample was homogenised into a fine powder using agate pestle and mortal washed with nonmetallic liquid soap and thoroughly rinsed with distilled water and analytical grade ethanol. About 150 mg of each sample was weighed into ultrapure polyethylene containers using Sartorious Research Balance (model R 180D). Each sample was then put in the rabbit capsule and again heat-sealed. The polyethylene film and rabbit capsules were cleansed by soaking in 1:1 HNO3 and water for three days and rinsed with deionised water. 3.4. Inductively coupled plasma e optical emission spectrometric (ICP-OES) analysis Inductively coupled plasma e optical emission spectrometric
(ICP-OES) analysis of the digested samples were performed using Spectro Arcos, equipped with quartz spray chamber and concentric glass nebulizer as sample inlet system. The ICP-OES was operated under the following conditions; RF incident power of 1.29 kW, Plasma gas flow rate of 12 L min1, Auxiliary gas flow at 1.0 L min1, Nebulizer gas pressure of 26 psi, Counting precision of 0.1, Sample uptake time of 30 s and sample flow at 60 ml1. Indium was used as the internal standard.
3.5. Instrumental neutron activation analysis The samples and reference material together with the Au monitors (two Au monitors positioned at top and bottom of irradiation containers) were irradiated for 1 h on Cell 56 at C2TN/IST. The gold monitor was a disk (thickness: 125 mm; diameter: 5 mm) of an Ale 0.1% Au alloy used as a comparator. Irradiations were carried out with thermal neutron flux of about 1.15 1012 n cm2 s1. At the end of each irradiation, the capsule was returned from the reactor and allowed to decay until the activity level (count rate) was within the acceptable limit for handling. The gamma spectra were acquired with the liquid-N2-cooled, high purity calibrated HPGe detector with the full width at half maximum (FWHM) approximately 1.85 keV at 1.33 MeV (gamma ray line of 60Co). The detector’s relative efficiency was 30%, and it was connected to a 4096 multi-channel analyser (MCA), with associated electronic modules all made by ORTEC and a personal computer. The interpretation of samples to obtain the analytical results in terms of mass fractions of elements, accuracies and detection limits was performed by the K0-standardization method of NAA. This included the calibration of energy, peak-shape and full-energy peak detection efficiency as well as the correction of true-coincidence and sample geometry effects using k0-IAEA software. Elemental concentrations were determined with the K0-IAEA software (version 3.21). In order to perform quality control (QC) and to evaluate the analytical results, a certified reference material (CBW 07406 e Laterite soil) was employed in the same experimental conditions as the analytical samples. The analytical results of the reference material were compared with the certified values in order to know the level of agreement. The reference material was
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Table 1 Trace elements composition of northwestern Niger Delta source rocks. MF field (ppm)
MLField (ppm)
MJ field (ppm)
Sample
Depth (ft)
Co
Ni
V
Sample
Depth (ft)
Co
Ni
V
Sample
Depth (ft)
Co
Ni
V
ML1 ML2 ML3 ML4 ML5 ML6 ML7 ML8 ML9 ML10 ML11 ML12 ML13 ML14 Average
6020 6380 6680 7040 7280 7610 8030 4490 5060 5090 5210 5300 5630 5840
17.09 83.54 114 84.32 75.67 97.50 89.81 112.11 67.94 118.92 72.17 86.12 89.30 60.17 83.48
22.26 169.04 111.86 101.96 100.77 96.07 115.65 157.29 90.56 82 84.05 54.29 59.42 69.71 93.92
4.95 65.89 41.81 35.08 38.34 41.44 36.70 41.20 57.22 33.95 31.80 41.89 23.38 30.14 37.41
MF1 MF2 MF3 MF4 MF5 MF6 MF7 MF8 MF9 MF10 MF11 MF12 MF13 MF14
6790 6970 7360 7600 7930 8260 8560 6820 7030 7090 7150 7180 7210 7630
70.09 83.34 87.12 106.43 100.46 59.33 68.30 73.06 81.42 70.21 83.75 79.42 109.94 112.06 84.64
76.33 83.51 77.30 85.8 76.25 81.13 82.79 68.53 86.25 59.97 68.58 68.05 72.79 95.77 77.36
31.58 39.31 35.85 35.86 35.91 37.26 37.20 30.61 42.15 43.18 43.50 32.78 34.56 39.78 37.11
MJ1 MJ2 MJ3 MJ4 MJ5 MJ6 MJ7 MJ8 MJ9 MJ10 MJ11 MJ12 MJ13 MJ14
6040 6370 6700 7000 7300 7630 7960 8320 8440 8740 9070 9220 9550 9640
82.58 169.55 171.71 73.77 97.72 87.12 72.58 69.91 82.83 80.82 66.79 69 56.71 69.26 89.31
64.64 77.47 67.27 75.82 78.25 63.24 73.79 78.96 75.65 59.03 76.94 82.78 80.12 79.63 73.83
50.59 30.91 36.57 28.54 34.36 35.52 27.21 31.53 32.66 30.73 24.85 24.85 31.88 35.45 32.54
4. Results and discussion 4.1. Trace element geochemistry
Fig. 4. Distribution of trace elements in source rocks from three fields in the northwestern Niger Delta.
prepared in the same way as the samples and irradiated simultaneously with them. The irradiated samples were allowed to decay for two to three days and four weeks prior to performing the first (medium-lived radionuclides) and second (long-lived radionuclides) measurements, respectively, for each sample. The Au monitors were measured seven days after the irradiations on the same calibrated HPGe gamma-ray spectrometry detector.
Only the biophile elements (elements with proven association with organic matter) were determined in the source rock samples. The concentration of cobalt ranges from 17.09 to 118.92 ppm, 59.33e112.06 ppm, and 56.71e171.71 ppm for ML, MF and MJ oil fields, respectively, with an average of 83.48 ppm for ML field, 84.64 ppm for MF field and 89.31 ppm for MJ field (Table 1). Nickel concentration ranges from 22.26 to 169.04 ppm, 59.97e95.77 ppm and 59.03e82.78 ppm for ML, MF and MJ fields, respectively, with average values of 93.92 ppm, 77.36 ppm and 73.83 ppm for ML, MF and MJ fields, respectively. Vanadium concentration ranges from 4.95 to 65.89 ppm for ML field, 31.58e43.50 ppm for MF field and 24.85e50.59 ppm for MJ field with average values of 37.41 ppm, 37.11 ppm and 32.54 ppm for ML, MF and MJ fields, respectively. The distribution of the trace elements (Fig. 4) shows that nickel is most abundant trace elements in ML field while cobalt is most abundant in MF and MJ fields. Vanadium had the least concentration in the three fields. The ratios calculated from these elements have been used in assessing the origin, depositional environment and in some cases thermal maturity of organic matter (Udo et al., 1992; Galarraga et al., 2008; Akinlua et al., 2010). Enhanced concentration of vanadium over nickel is an indication of marine organic matter input while the reverse is indicative of terrestrial organic matter input. In other word, high V/Ni value indicates
Table 2 Geochemical ratios calculated from the concentrations of trace elements. ML field
MF field
MJ field
Samples
Depth (ft)
V/Ni
V/(VþNi)
Ni/Co
Samples
Depth (ft)
V/Ni
V/(VþNi)
Ni/Co
Samples
Depth (ft)
V/Ni
V/(VþNi)
Ni/Co
ML 1 ML2 ML3 ML4 ML5 ML6 ML7 ML8 ML9 ML10 ML11 ML12 ML13 ML14
6020 6380 6680 7040 7280 7610 8030 4490 5060 5090 5210 5300 5630 5840
0.22 0.39 0.37 0.34 0.38 0.43 0.32 0.26 0.63 0.41 0.38 0.77 0.39 0.43
0.18 0.28 0.27 0.26 0.28 0.3 0.24 0.21 0.39 0.29 0.27 0.44 0.28 0.3
1.30 2.02 0.98 1.21 1.33 0.99 1.29 1.40 1.33 0.69 1.16 0.63 0.67 1.16
MF1 MF2 MF3 MF4 MF5 MF6 MF7 MF8 MF9 MF10 MF11 MF12 MF13 MF14
6790 6970 7360 7600 7930 8260 8560 6820 7030 7090 7150 7180 7210 7630
0.41 0.47 0.46 0.42 0.47 0.46 0.45 0.45 0.49 0.72 0.63 0.48 0.47 0.42
0.29 0.32 0.32 0.29 0.32 0.31 0.31 0.31 0.33 0.42 0.39 0.33 0.32 0.29
1.09 1.00 0.89 0.81 0.76 1.37 1.21 0.94 1.06 0.85 0.82 0.86 0.66 0.85
MF1 MF2 MF3 MF4 MF5 MF6 MF7 MF8 MF9 MF10 MF11 MF12 MF13 MF14
6040 6370 6700 7000 7300 7630 7960 8320 8440 8740 9070 9220 9550 9640
0.78 0.4 0.54 0.38 0.44 0.56 0.37 0.40 0.43 0.52 0.32 0.30 0.40 0.45
0.44 0.29 0.35 0.27 0.31 0.36 0.27 0.29 0.3 0.34 0.24 0.23 0.28 0.31
0.78 0.46 0.39 1.03 0.80 0.73 1.02 1.13 0.91 0.73 1.15 1.19 1.41 1.15
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Fig. 5. Cross plot of trace element geochemical ratios of the source rocks showing classification according to their organic matter origin (After Galarraga et al., 2008). Fig. 8. Rare earth elements (RREs) distribution in source rocks from the northwestern Niger Delta.
Fig. 6. Cross plot of Nickel versus Vanadium showing depositional environment of the organic matter.
Fig. 9. a: Light rare earth elements (LREEs) fingerprinting of source rock samples from ML field in the northwestern Niger Delta. b: Heavy rare earth elements (HREEs) fingerprinting of source rock samples from ML field in the northwestern Niger Delta.
Fig. 7. Comparison of detection of the REEs by INAA and ICP-OES in the source rock samples from oil fields in the northwestern Niger Delta.
marine organic input while low value indicates terrestrial organic matter input. The study of Galarraga et al. (2008) showed V/Ni
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Fig. 10. a: Light rare earth elements (LREEs) fingerprinting of source rock samples from MF field in the northwestern Niger Delta. b: Heavy rare earth elements (HREEs) fingerprinting of source rock samples from MF field in the northwestern Niger Delta.
values less than 1.9 indicate terrestrial organic matter, V/Ni values from 1.9 to 3 indicate organic matter of mixed marine and terrestrial source input, while V/Ni ratio higher than 3 indicates marine organic matter. Source rock samples from ML field have V/Ni ratio ranging 0.22e0.77, MF source rock samples have values that range from 0.41 to 0.72, while source rocks from MJ field have V/Ni ratios ranging from 0.30 to 0.78 (Table 2). The values of V/Ni ratio of source rock samples from the three fields indicate that they have terrestrial organic matter input. Cross plot constructed from the values of Co/Ni and V/Ni (Fig. 5) also indicates that the source rocks from the three fields have terrestrial organic matter input. Depositional environment has a significant effect on the concentrations of vanadium and nickel in source rocks (Lewan, 1984). Enriched vanadium content compared to nickel in source rocks occurs in anoxic environments (Peters and Moldowan, 1993). V/Ni ratios for the source rocks from the three fields are generally low with the highest value of 0.78 (Table 2), which is an indication of deposition under oxic conditions (Lewan, 1984). Normalized ratio of vanadium and nickel (V/V þ Ni) can be used unambiguously as a redox condition indicator. V/V þ Ni values greater than 0.5 indicates anoxic environment while values less than 0.5 is an indication of oxic environment. The source rock samples from the three fields
Fig. 11. a: Light rare earth elements (LREEs) fingerprinting of source rock samples from MJ field in the northwestern Niger Delta. b: Heavy rare earth elements (HREEs) fingerprinting of source rock samples from MJ field in the northwestern Niger Delta.
have V/V þ Ni values less than 0.5 (Table 2), indicating that the organic matter of these source rocks were deposited under oxic conditions. Ni/Co ratio is also a good redox indicator (Dypvik, 1984; Dill, 1986; Jones and Manning, 1994). All the samples from the three fields have Ni/Co ratio less than 3, which indicates oxic conditions (Jones and Manning, 1994). Cross plot of nickel versus vanadium (Fig. 6) also revealed that source rocks from the three fields have organic matter content deposited in oxic environments.
4.2. Rare earth element geochemistry 4.2.1. Comparative detection of rare earth elements by INAA and ICP-OES Two analytical techniques, INAA and ICP-OES, were used to unravel the rare earth elements content of the source rock samples. The two techniques were to complement each other. Fig. 7 presented average concentrations of each rare earth element from the three fields. The results show that INAA gave better yields of La, Ce, Pr, Nd and Tb while ICP-OES gave better yields of Sm, Eu, Gd, Dy, Ho, Tm, Yb and Lu. Ce, Pr, Nd and Tb were only detected by INAA, while Eu, Gd, Dy, Tm and Lu were only detected by ICP-OES. The light rare earth elements (LREEs) were better detected with INAA. ICP-OES gave better detection of the heavy rare earth elements (HREEs).
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Fig. 13. Comparison of rare earth elements (REEs) distribution patterns of source rock samples from the three fields in the Niger Delta with REE distribution patterns of shale and oil samples from two Chinese Basins.
The order of occurrence of the REEs in the source rocks from ML field is Gd, Ce, Sm, La, Ho, Dy, Lu, Yb, Eu, Tm, Pr, Nd. In MF field, the most abundant REE in the source rock is Gd, follow by Ce, while Pr had the least concentration. The order of occurrence of REEs in this field is Gd, Ce, Sm, Ho, La, Lu, Dy, Yb, Eu, Tm, Nd, Pr. Gd also is the most abundant REE in the source rock from MJ field while Nd had the least concentration. The order of occurrence of the REEs in this field is as follows Gd, Ce, Sm, Ho, La, Lu, Dy, Yb, Tm Eu, Pr, Nd. The distribution of the REEs in the source rocks in the three fields is almost similar. Comparison of distribution of the REEs amongst the fields showed that ML field source rocks have the highest concentrations of the REEs except for Ce, Nd and Pr (Fig. 8). Source rocks from MF field are the most enriched in Ce and Nd, while MJ field source rocks have the highest abundance of Pr.
Fig. 12. a: Comparison of light rare earth elements (LREEs) fingerprintings of source rock samples from the three fields. b: Comparison of heavy rare earth elements (HREEs) fingerprintings of source rock samples from the three fields.
4.2.2. Distribution of REEs in Niger Delta source rocks Gd is most abundant rare earth element in the source rocks from ML field, follow by Ce while Nd had the least concentration (Fig. 8).
4.2.3. Geochemical significance of the rare earth elements The fingerprinting of the light rare earth elements (LREEs) of source rocks from ML field (Fig. 9a) constructed from their concentrations revealed similar distribution patterns for the samples except samples ML7 and ML8 that show positive anomaly of lanthanum (significant enrichment of lanthanum compared to other LREEs), and sample ML10 with positive anomaly of neodymium (enrichment of neodymium compared with other LREEs). The fingerprinting of the heavy rare earth elements (HREEs) of ML field source rocks also showed similar distribution patterns (Fig. 9b)
Table 3 Pearson correlation matrix of REEs data of source rock samples from the three fields in northwestern Niger Delta.
La Ce Nd Pr Sm Eu Gd Dy Ho Tm Yb Lu Co Ni V
La
Ce
Nd
Pr
Sm
Eu
Gd
Dy
Ho
Tm
Yb
Lu
Co
Ni
V
1 0.61 0.70 0.32 1.00 0.66 0.88 0.99 0.95 0.99 0.98 0.99 0.70 0.99 0.59
1 0.99 0.56 0.65 0.20 0.16 0.51 0.83 0.72 0.44 0.50 0.15 0.52 0.28
1 0.46 0.73 0.08 0.28 0.61 0.89 0.80 0.54 0.60 0.03 0.62 0.16
1 0.27 0.92 0.73 0.42 0.00 0.17 0.49 0.44 0.90 0.42 0.95
1 0.62 0.86 0.99 0.96 1.00 0.97 0.98 0.66 0.99 0.55
1 0.93 0.74 0.39 0.54 0.79 0.75 1.00 0.73 1.00
1 0.93 0.69 0.80 0.96 0.94 0.95 0.93 0.90
1 0.91 0.97 1.00 1.00 0.77 1.00 0.68
1 0.99 0.87 0.90 0.44 0.91 0.31
1 0.94 0.96 0.58 0.97 0.47
1 1.00 0.82 1.00 0.74
1 0.78 1.00 0.69
1 0.77 0.99
1 0.68
1
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except sample ML 8 that displays positive anomaly of ytterbium (slight enrichment of ytterbium compared to other HREEs). The similarity in the REE distribution patterns of the source rock samples is an indication of similar genetic origin. The LREE fingerprinting of the source rock samples from MF field show similar distribution patterns of the LREEs (Fig. 10a) except for MF2 sample that displays positive anomaly of neodymium. The distribution patterns of HREEs in source rock samples from MF field showed perfect similarity (Fig. 10b), which is indicative of similar genetic origin. The LREE fingerprinting of the source rocks from MJ field also showed similar distribution patterns (Fig. 11a) except for samples MJ1 and MJ3 that have positive anomaly of lanthanum and MJ9 sample that has positive anomaly of neodymium. The HREE distribution patterns of the source rocks in this field are also perfectly similar (Fig. 11b), which is also indicating similar genetic origin of their organic matter. The REE distribution patterns are not only similar amongst the samples from the same field but are also similar amongst source rock samples from the three fields (Fig. 12a and b). This is not surprising because the trace element geochemistry has revealed that the source rocks from the three fields have similar geochemical characteristics. The source rocks are of terrestrial organic matter origin deposited under oxic conditions. Therefore, the similarity in the REE distribution patterns could be a reflection of similar organic matter origin and depositional environment. ML field shows more enrichment of lanthanum than the others because it has more influence of terrestrial organic matter and oxic depositional environment than the other fields as indicated by the trace element data (Tables 1 and 2). Pearson correlation matrix performed on the REEs and trace elements data from the three fields shows (Table 3) very strong positive correlation between vanadium and europium (r ¼ 1.00), which suggests that as concentration of vanadium is increasing the concentration of europium is also increasing or vice versa. The geochemical implication of this is that there would be an enhanced concentration of europium in marine organic matter deposited under anoxic conditions as in the case of vanadium in anoxic conditions. Nickel is very strongly positive correlated with dysprosium, lanthanum and samarium, which suggests that as the concentration of nickel increases the concentrations of these rare earth elements also increase and vice versa. Nickel concentration is known to be enhanced in an oxic environment than an anoxic environment (Lewan, 1984). Thus, geochemically, dysprosium, lanthanum and samarium would have enhanced concentrations in organic matter deposited in oxic environments. The results of this study indicate europium, dysprosium, lanthanum and samarium are potential redox and source indicators. The results of rare earth elements in this study were compared with REE results from other basins (Fig. 13). The Results showed that the rock samples are more enriched in REE than the black shale samples from Early Cambrian Niutitang Foramtion, Guizhou Province, China (Pi et al., 2013) but less enriched compared with oil samples from southern margin of the Junggar Basin, China (Nakada et al., 2010). The REE distribution patterns of Niger Delta samples are different from those of the two basins in China. The REE distribution patterns of samples from the two Chinese basins are not similar. Thus, aside from genetic origin, REE distribution patterns are possibly influenced by geography. 5. Conclusions Concentrations of trace elements and their ratios indicated that the organic matter of the source rocks from ML, MF and MJ fields in the northwestern Niger Delta are of mainly terrestrial origin that
were deposited under oxic conditions. Rare earth elements distribution patterns of the source rocks within each field and across the three fields are similar, indicating similar genetic origin that could be a reflection of their similar organic matter origin and depositional environment. Pearson correlation matrix revealed that europium, dysprosium, lanthanum and samarium are potential redox and source indicators. Acknowledgment We thank the Department of Petroleum Resources (DPR) and Chevron Nigeria Limited for releasing the samples for this study. We also appreciate the financial support of the Africa Network of Analytical Chemists (SEANAC). The assistance of Susana M. Almeida ^ncias e Tecnologias Nucleares (C2TN), Instituto of Centro de Cie cnico (IST), Universidade de Lisboa Campus Tecnolo gico Superior Te e Nuclear, Estrada Nacional 10 (km 139,7), 2695-066 Bobadela LRS, Portugal is also appreciated for the instrumental neutron activation analysis. References Ajayi, T.R., Torto, N., Tchokossa, P., Akinlua, A., 2009. Natural radioactivity and trace metals in crude oils: implication for health. Environ. Geochem. Health 31, 61e69. Akinlua, A., Torto, N., 2006. Determination of selected metals in Niger Delta oils by graphite furnace atomic absorption spectrometry. Anal. Lett. 39 (9), 1993e2005. Akinlua, A., Torto, N., Ajayi, T.R., Oyekunle, J.A.O., 2007. Trace metals characterisation of Niger Delta kerogens. Fuel 86, 1358e1364. Akinlua, A., Adekola, S.A., Swakamisa, O., Fadipe, O.A., Akinyemi, S.A., 2010. Trace metals characterisation of Cretaceous Orange Basin hydrocarbon source rocks. App. Geochem. 25, 1587e1595. Akinlua, A., Torto, N., Ajayi, T.R., 2008. Determination of rare earth elements in Niger Delta crude oils by inductively coupled plasma e mass spectrometry. Fuel 87, 1469e1477. Alberdi-Genolet, M., Tocco, R., 1999. Trace metals and organic geochemistry of the machiques member (AptianeAlbian) and La luna formation (CenomanianeCampanian), Venezuela. Chem. Geol. 160, 19e38. Algeo, T.J., Rowe, H., 2012. Paleoceanographic applications of trace-metal concentration data. Chem. Geol. 324e325, 6e18. Avbovbo, A.A., 1978. Tertiary lithostratigraphy of Niger Delta. AAPG Bull. 62, 295e300. Barwise, A.J.G., 1990. Role of nickel and vanadium in petroleum classification. Energy Fuels 4, 647e652. Dill, H., 1986. Metallogenesis of early paleozoic graptolite shales from the graefenthal horst (northern Bavaria-Federal Republic Of Germany). Econ. Geol. 81, 889e903. Doust, H., 1990. Petroleum Geology of the Niger, vol. 50. Geological Society, London, Special Publications, Delta, p. 365. Doust, H., Omatsola, E., 1990. Niger Delta. Divergent/Passive margin basins. In: Edwards, J.D., Santogrossi, P.A. (Eds.), AAPG Mem. 48, 201e238. Dypvik, H., 1984. Geochemical Compositions and Depositional Conditions of Upper Jurassic and Lower Cretaceous Yorkshire Clays, England, vol. 121. Geological Magazine, pp. 489e504. Ejedawe, J.E., 1981. Patterns of incidence of oil reserves in Niger Delta Basin. AAPG Bull. 65, 1571e1585. Ekweozor, C.M., Okoye, N.V., 1980. Petroleum source-bed evaluation of tertiary Niger Delta. AAPG Bull. 64, 125e1259. Evamy, B.P., Haremboure, J., Kamerling, P., Knoop, W.A., Molly, F.A., Rowlands, P.H., 1978. Hydrocarbon habitat of tertiary Niger Delta. AAPG Bull. 62, 1e39. Frank, E.J., Cordry, E.A., 1967. The Niger Delta Oil Province. In: Recent World Petroleum Congress, Proc, vol. 2, pp. 195e209. Galarraga, F., Llamas, J.F., Martinez, A., Martinez, M., Marquez, G., Reategui, K., 2008. V/Ni ratio as a parameter in palaeoenvironmental characterization of nonmature medium-crude oils from several Latin American basins. J. Petrol. Sci. Eng. 61, 9e14. Henderson, P., 1984. General geochemical properties and abundances of the rare earth elements. In: Denderson, P. (Ed.), Rare Earth Element Geochemistry. Elsevier, New York, pp. 1e32. Jones, B., Manning, D.C., 1994. Comparison of geochemical indices used for the interpretation of paleo-redox conditions in Ancient mudstones. Chem. Geol. 111, 111e129. Lewan, M.D., 1984. Factors controlling the proportionality of vanadium to nickel in crude oils. Geochim. Cosmochim. Acta 48, 2231e2238. Lewan, M.D., Maynard, J.B., 1982. Factors controlling enrichment of vanadium and nickel in the bitumen of organic sedimentary rocks. Geochim. Cosmochim. Acta 46, 2547e2560. Mongenot, T., Tribovillard, N.P., Desprairies, A., Lallier-Verges, E., Laggoun-
A. Akinlua et al. / Marine and Petroleum Geology 77 (2016) 409e417 Defarge, F., 1996. Trace elements as palaeoenvironmental markers in strongly mature hydrocarbon source rocks: the Cretaceous La Luna Formation of Venezuela. Sediment. Geol. 103, 23e37. Nakada, R., Takahashi, Y., Zheng, G., Yamamoto, Y., Hiroshi Shimizu, H., 2010. Abundances of rare earth elements in crude oils and their partitions in water. Geochem. J. 44, 411e418. Nwadinigwe, C.A., Nworgu, O.N., 1999. Metal contaminants in some Nigerian wellhead crudes comparative analysis. J. Chem. Soc. Niger. 24, 118e121. Nwachukwu, J.I., Oluwole, A.F., Asubiojo, O.I., Filby, R.H., Grimm, C., Fitzgerald, S., 1995. A geochemical evaluation of Niger Delta crude oils. In: Oti, M.N., Postma, G. (Eds.), Geology of Deltas. A.A. Balkema, Rotterdam, Brookfield, pp. 287e300. Peters, K.E., Moldowan, J.M., 1993. The biomarker guide: interpreting molecular fossil in petroleum and ancient sediments. Prentice Hall, London, p. 363. Pi, D.-H., Liu, C.-Q., Shields-Zhou, G.A., Jiang, Y., 2013. Trace and rare earth element geochemistry of black shale and kerogen in the early Cambrian Niutitang
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Formation in Guizhou province, South China: constraints for redox environment and origin of metal enrichments. Precambrian Res. 225, 218e229. Short, K.C., Stauble, A.J., 1967. Outline of geology of Niger Delta. AAPG Bull. 51, 761e779. Shtangeeva, I., 2006. In: Prasad, M.N.V., Sajwan, K.S., Naidu, R. (Eds.), Trace Element in the Environment, Biogeochemistry, Biotechnology and Bioremediation. CRC Press, Boca Raton, pp. 549e581. Takeda, K., Arikawa, Y., 2005. Determination of rare earth elements in petroleum by ICP-MS. Bunseki Kagaku 54, 939e943 (in Japanese with English abstract). Tissot, B.P., Welte, D.H., 1984. Petroleum Formation and Occurrence, second ed. Springer-verlag, Berlin, Heidelberg, New York, Tokyo. 699pp. Tribovillard, N., Algeo, T.J., Lyons, T., Riboulleau, A., 2006. Trace metals as paleoredox and paleoproductivity proxies: an update. Chem. Geol. 232, 12e32. Udo, O.T., Ekwere, S., Abrakasa, S., 1992. Some trace metal in selected Niger Delta crude oils: application in oileoil correlation studies. J. Min. Geol. 28, 289e291.
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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Research paper
Rare earth element geochemistry of petroleum source rocks from northwestern Niger Delta A. Akinlua a, *, F.S. Olise b, A.O. Akomolafe a, R.I. McCrindle c a
Fossil Fuels and Environmental Geochemistry, Department of Chemistry, Obafemi Awolowo University, Ile-Ife, Nigeria Department of Physics, Obafemi Awolowo University, Ile-Ife, Nigeria c Department of Chemistry, Tshwane University of Technology, Arcadia Campus, 175 Nelson Mandela Drive, Private Bag X680, Pretoria, 0001, South Africa b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 April 2016 Received in revised form 16 June 2016 Accepted 23 June 2016 Available online 25 June 2016
Geochemical investigation of forty two rock samples from three fields in the northwestern Niger Delta was carried out in order to determine their rare earth elements (REEs) content and their geochemical significances. The rare earth elements and trace elements in the rock samples were determined using inductively coupled plasma e mass spectrometry (ICP-MS) and instrumental neutron activation analysis (INAA). The results indicate that nickel is most abundant trace elements in ML field while cobalt is most abundant in MF and MJ fields. Vanadium had the least concentration in the three fields. Nickel had enhanced concentrations over vanadium in the three fields. The concentrations and ratios of the trace elements indicated that the source rocks in the three fields had strong terrestrial organic matter input and were deposited under oxic conditions. Gadolinium is the most abundant rare earth element in three fields. Neodymium had the least concentration in ML and MJ fields while praseodymium had the least concentration in MF field. The REE distribution patterns of source rocks from the three fields are similar, indicating similar genetic origin. Pearson correlation matrix revealed that europium, dysprosium, lanthanum and samarium are potential redox and source indicators. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Rare earth element Source rock Niger Delta Origin Depositional
1. Introduction Source rocks are highly organic-rich type of sedimentary rock that are capable or may become capable or have been able to generate petroleum (Tissot and Welte, 1984). Source rock ultimately generates crude oil and contains measurable quantities of many trace metals and rare earth elements (REEs). Trace metals are incorporated into petroleum source rocks in the form of porphyrin complexes and may include direct incorporation from the biomass and formation during sedimentation. It may also involve diagenesis from organic molecules as well as metals derived from different biogenic and abiogenic sources (Barwise, 1990; Nwachukwu et al., 1995; Akinlua et al., 2007). The type and the thermal maturity of organic matter and their depositional environment have significant effects on the concentration of trace elements in source rocks (Lewan, 1984). Metals of proven association with organic matter may be used as reliable
* Corresponding author. E-mail addresses: [email protected], (A. Akinlua). http://dx.doi.org/10.1016/j.marpetgeo.2016.06.023 0264-8172/© 2016 Elsevier Ltd. All rights reserved.
[email protected]
correlation tools. Nickel, vanadium, and cobalt (usually referred to as biophile elements) are such examples (Barwise, 1990; Udo et al., 1992; Akinlua et al., 2007). The geochemical characteristic and intrinsic distribution of rare earth elements in geological formation would make them useful geochemical tools to determine the origin, depositional environment, and thermal maturation of organic matter (Henderson, 1984; Takeda and Arikawa, 2005; Akinlua et al., 2008). A wide range of metals have been determined in crude oils, bituminous substance and organic sedimentary rock all over the world (Ajayi et al., 2009; Akinlua et al., 2007; Barwise, 1990; Nwadinigwe and Nworgu, 1999; Shtangeeva, 2006; Udo et al., 1992). Trace-metal accumulation is known to be controlled by redox conditions in marine sediments (Algeo and Rowe, 2012). Some trace metals have proven to be good paleoenvironmental redox indicators (Lewan and Maynard, 1982; Mongenot et al., 1996; Alberdi-Genolet and Tocco, 1999; Tribovillard et al., 2006). Several studies have been carried out on trace metals in oils from the Niger Delta area (Nwachukwu et al., 1995; Ajayi et al., 2009; Akinlua and Torto, 2006; Akinlua et al., 2007) and a very few study on rare earth elements in oils from the Niger Delta (Akinlua et al., 2008).
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However, there is paucity of data on the determination and geochemical assessment of rare earth elements content of petroleum source rocks from Niger Delta Basin. The present study is therefore an attempt to determine the rare earth elements content of source rock samples from Niger Delta in order to assess their suitability in the evaluation of origin and depositional environment of source rocks.
2. Geology of study area The Niger Delta is one of the world’s largest Tertiary deltaic systems and an extremely prolific hydrocarbon province. It is situated on the West African continental margin at the apex of the Gulf of Guinea (Doust, 1990). The area is located at the southern region (latitude 4 and 6 N and longitude 3 and 9 E) and occupies 2 an area of about 75,000 km with clastic sequence that reaches a maximum thickness of 9000e12,000 m of sediment and total sediment volume of 500,000 km2 and makes up 7.5% of Nigeria landmass (Frank and Cordry, 1967). The study area is bounded by older Cretaceous tectonic elements, such as the Abakaliki Anticlinorium and the Afikpo syncline (Ejedawe, 1981). The area is rich in oil and the host rocks are organic-rich sedimentary rocks. The Tertiary lithostratigraphic sequence of the Niger Delta consists of the Akata, Agbada, and Benin formations (Figs. 1 and 2) (Evamy et al., 1978). The lowest unit is the Akata formation with a uniform massive marine shale unit with age ranging from Paleocene to Recent. Akata Formation is overlain with Agbada Formation, which consists of intercalation of sandstone and shale with age ranging from Eocene to Recent (Short and Stauble, 1967). The Benin Formation that overlies the Agbada Formation consists of fluviatile sands and gravels with age ranging from Eocene to Recent (Avbovbo, 1978). The sandstones of the Agbada Formation are the reservoir rocks for liquid hydrocarbons that occur in this sedimentary basin (Evamy et al., 1978). Fig. 2. Stratigraphic columns showing the three Formations of the Niger Delta (Doust and Omatsola, 1990).
3. Experimental 3.2. Sample preparation for ICP-OES 3.1. Sample collection Source rock samples were collected at different depths, from three oil fields (ML, MF and MJ field) in the paralic sequence of Agbada Formation, northwestern Niger Delta (Fig. 3). Only rock samples with TOC above 0.5 wt% were selected for rare earth and trace elements analysis.
The source rock samples were prepared for inductively coupled plasma e optical emission spectrometric (ICP-OES) measurement by acid digestion into colourless aqueous solution. Dissolution of the source rock samples was achieved using microwave accelerated reaction system. 0.5 g of each sample was digested using 4 mL of
Fig. 1. Schematic cross section showing principal stratigraphic units of Tertiary Niger delta (Ekweozor and Okoye, 1980).
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Fig. 3. Map of Niger Delta showing location of the study area.
48% HF, 3 mL of 65% HNO3 and 1 mL of 30% H2O2, which resulted in complete dissolution of the rock sample. The experiment was performed in closed PTFE vessels at a digestion temperature of 200 C, pressure of 241 psi, at ramp time of 30 min and then held for 10 min. 4 mL of 7% boric acid was added to the digested sample and heated for 25 min of ramp time and 5 min hold time, and at temperature of 155 C. The experiments were performed at irradiation power of 800 W. After cooling the vessels, the resulting aqueous colourless solution was transferred into a 25 mL volumetric flask and made up to the mark with deionised water. Blank was prepared under the same conditions. Multi-element standard solution of the elements was prepared from the stock solutions of the elements. The stock solutions were diluted with deionised water to prepare each working solution for calibration. The calibration curves were obtained from the standard solutions of the elements. 3.3. Sample preparation for INAA Sample preparation for INAA is essentially limited to weighing a suitable amount of homogenous pulverized sample into small quart or plastic vials for reactor irradiation and subsequent gamma spectrometric analysis. The essential issues are to avoid contamination by careful selection of the vial material, cleaning the vials and verification of the blank levels. There must be tight specifications on the vial dimensions and wall thickness in order to produce counting efficiencies and gamma ray attenuation effects. At the laboratory, source rock samples were air-dried to approximately constant mass, and each sample was homogenised into a fine powder using agate pestle and mortal washed with nonmetallic liquid soap and thoroughly rinsed with distilled water and analytical grade ethanol. About 150 mg of each sample was weighed into ultrapure polyethylene containers using Sartorious Research Balance (model R 180D). Each sample was then put in the rabbit capsule and again heat-sealed. The polyethylene film and rabbit capsules were cleansed by soaking in 1:1 HNO3 and water for three days and rinsed with deionised water. 3.4. Inductively coupled plasma e optical emission spectrometric (ICP-OES) analysis Inductively coupled plasma e optical emission spectrometric
(ICP-OES) analysis of the digested samples were performed using Spectro Arcos, equipped with quartz spray chamber and concentric glass nebulizer as sample inlet system. The ICP-OES was operated under the following conditions; RF incident power of 1.29 kW, Plasma gas flow rate of 12 L min1, Auxiliary gas flow at 1.0 L min1, Nebulizer gas pressure of 26 psi, Counting precision of 0.1, Sample uptake time of 30 s and sample flow at 60 ml1. Indium was used as the internal standard.
3.5. Instrumental neutron activation analysis The samples and reference material together with the Au monitors (two Au monitors positioned at top and bottom of irradiation containers) were irradiated for 1 h on Cell 56 at C2TN/IST. The gold monitor was a disk (thickness: 125 mm; diameter: 5 mm) of an Ale 0.1% Au alloy used as a comparator. Irradiations were carried out with thermal neutron flux of about 1.15 1012 n cm2 s1. At the end of each irradiation, the capsule was returned from the reactor and allowed to decay until the activity level (count rate) was within the acceptable limit for handling. The gamma spectra were acquired with the liquid-N2-cooled, high purity calibrated HPGe detector with the full width at half maximum (FWHM) approximately 1.85 keV at 1.33 MeV (gamma ray line of 60Co). The detector’s relative efficiency was 30%, and it was connected to a 4096 multi-channel analyser (MCA), with associated electronic modules all made by ORTEC and a personal computer. The interpretation of samples to obtain the analytical results in terms of mass fractions of elements, accuracies and detection limits was performed by the K0-standardization method of NAA. This included the calibration of energy, peak-shape and full-energy peak detection efficiency as well as the correction of true-coincidence and sample geometry effects using k0-IAEA software. Elemental concentrations were determined with the K0-IAEA software (version 3.21). In order to perform quality control (QC) and to evaluate the analytical results, a certified reference material (CBW 07406 e Laterite soil) was employed in the same experimental conditions as the analytical samples. The analytical results of the reference material were compared with the certified values in order to know the level of agreement. The reference material was
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Table 1 Trace elements composition of northwestern Niger Delta source rocks. MF field (ppm)
MLField (ppm)
MJ field (ppm)
Sample
Depth (ft)
Co
Ni
V
Sample
Depth (ft)
Co
Ni
V
Sample
Depth (ft)
Co
Ni
V
ML1 ML2 ML3 ML4 ML5 ML6 ML7 ML8 ML9 ML10 ML11 ML12 ML13 ML14 Average
6020 6380 6680 7040 7280 7610 8030 4490 5060 5090 5210 5300 5630 5840
17.09 83.54 114 84.32 75.67 97.50 89.81 112.11 67.94 118.92 72.17 86.12 89.30 60.17 83.48
22.26 169.04 111.86 101.96 100.77 96.07 115.65 157.29 90.56 82 84.05 54.29 59.42 69.71 93.92
4.95 65.89 41.81 35.08 38.34 41.44 36.70 41.20 57.22 33.95 31.80 41.89 23.38 30.14 37.41
MF1 MF2 MF3 MF4 MF5 MF6 MF7 MF8 MF9 MF10 MF11 MF12 MF13 MF14
6790 6970 7360 7600 7930 8260 8560 6820 7030 7090 7150 7180 7210 7630
70.09 83.34 87.12 106.43 100.46 59.33 68.30 73.06 81.42 70.21 83.75 79.42 109.94 112.06 84.64
76.33 83.51 77.30 85.8 76.25 81.13 82.79 68.53 86.25 59.97 68.58 68.05 72.79 95.77 77.36
31.58 39.31 35.85 35.86 35.91 37.26 37.20 30.61 42.15 43.18 43.50 32.78 34.56 39.78 37.11
MJ1 MJ2 MJ3 MJ4 MJ5 MJ6 MJ7 MJ8 MJ9 MJ10 MJ11 MJ12 MJ13 MJ14
6040 6370 6700 7000 7300 7630 7960 8320 8440 8740 9070 9220 9550 9640
82.58 169.55 171.71 73.77 97.72 87.12 72.58 69.91 82.83 80.82 66.79 69 56.71 69.26 89.31
64.64 77.47 67.27 75.82 78.25 63.24 73.79 78.96 75.65 59.03 76.94 82.78 80.12 79.63 73.83
50.59 30.91 36.57 28.54 34.36 35.52 27.21 31.53 32.66 30.73 24.85 24.85 31.88 35.45 32.54
4. Results and discussion 4.1. Trace element geochemistry
Fig. 4. Distribution of trace elements in source rocks from three fields in the northwestern Niger Delta.
prepared in the same way as the samples and irradiated simultaneously with them. The irradiated samples were allowed to decay for two to three days and four weeks prior to performing the first (medium-lived radionuclides) and second (long-lived radionuclides) measurements, respectively, for each sample. The Au monitors were measured seven days after the irradiations on the same calibrated HPGe gamma-ray spectrometry detector.
Only the biophile elements (elements with proven association with organic matter) were determined in the source rock samples. The concentration of cobalt ranges from 17.09 to 118.92 ppm, 59.33e112.06 ppm, and 56.71e171.71 ppm for ML, MF and MJ oil fields, respectively, with an average of 83.48 ppm for ML field, 84.64 ppm for MF field and 89.31 ppm for MJ field (Table 1). Nickel concentration ranges from 22.26 to 169.04 ppm, 59.97e95.77 ppm and 59.03e82.78 ppm for ML, MF and MJ fields, respectively, with average values of 93.92 ppm, 77.36 ppm and 73.83 ppm for ML, MF and MJ fields, respectively. Vanadium concentration ranges from 4.95 to 65.89 ppm for ML field, 31.58e43.50 ppm for MF field and 24.85e50.59 ppm for MJ field with average values of 37.41 ppm, 37.11 ppm and 32.54 ppm for ML, MF and MJ fields, respectively. The distribution of the trace elements (Fig. 4) shows that nickel is most abundant trace elements in ML field while cobalt is most abundant in MF and MJ fields. Vanadium had the least concentration in the three fields. The ratios calculated from these elements have been used in assessing the origin, depositional environment and in some cases thermal maturity of organic matter (Udo et al., 1992; Galarraga et al., 2008; Akinlua et al., 2010). Enhanced concentration of vanadium over nickel is an indication of marine organic matter input while the reverse is indicative of terrestrial organic matter input. In other word, high V/Ni value indicates
Table 2 Geochemical ratios calculated from the concentrations of trace elements. ML field
MF field
MJ field
Samples
Depth (ft)
V/Ni
V/(VþNi)
Ni/Co
Samples
Depth (ft)
V/Ni
V/(VþNi)
Ni/Co
Samples
Depth (ft)
V/Ni
V/(VþNi)
Ni/Co
ML 1 ML2 ML3 ML4 ML5 ML6 ML7 ML8 ML9 ML10 ML11 ML12 ML13 ML14
6020 6380 6680 7040 7280 7610 8030 4490 5060 5090 5210 5300 5630 5840
0.22 0.39 0.37 0.34 0.38 0.43 0.32 0.26 0.63 0.41 0.38 0.77 0.39 0.43
0.18 0.28 0.27 0.26 0.28 0.3 0.24 0.21 0.39 0.29 0.27 0.44 0.28 0.3
1.30 2.02 0.98 1.21 1.33 0.99 1.29 1.40 1.33 0.69 1.16 0.63 0.67 1.16
MF1 MF2 MF3 MF4 MF5 MF6 MF7 MF8 MF9 MF10 MF11 MF12 MF13 MF14
6790 6970 7360 7600 7930 8260 8560 6820 7030 7090 7150 7180 7210 7630
0.41 0.47 0.46 0.42 0.47 0.46 0.45 0.45 0.49 0.72 0.63 0.48 0.47 0.42
0.29 0.32 0.32 0.29 0.32 0.31 0.31 0.31 0.33 0.42 0.39 0.33 0.32 0.29
1.09 1.00 0.89 0.81 0.76 1.37 1.21 0.94 1.06 0.85 0.82 0.86 0.66 0.85
MF1 MF2 MF3 MF4 MF5 MF6 MF7 MF8 MF9 MF10 MF11 MF12 MF13 MF14
6040 6370 6700 7000 7300 7630 7960 8320 8440 8740 9070 9220 9550 9640
0.78 0.4 0.54 0.38 0.44 0.56 0.37 0.40 0.43 0.52 0.32 0.30 0.40 0.45
0.44 0.29 0.35 0.27 0.31 0.36 0.27 0.29 0.3 0.34 0.24 0.23 0.28 0.31
0.78 0.46 0.39 1.03 0.80 0.73 1.02 1.13 0.91 0.73 1.15 1.19 1.41 1.15
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Fig. 5. Cross plot of trace element geochemical ratios of the source rocks showing classification according to their organic matter origin (After Galarraga et al., 2008). Fig. 8. Rare earth elements (RREs) distribution in source rocks from the northwestern Niger Delta.
Fig. 6. Cross plot of Nickel versus Vanadium showing depositional environment of the organic matter.
Fig. 9. a: Light rare earth elements (LREEs) fingerprinting of source rock samples from ML field in the northwestern Niger Delta. b: Heavy rare earth elements (HREEs) fingerprinting of source rock samples from ML field in the northwestern Niger Delta.
Fig. 7. Comparison of detection of the REEs by INAA and ICP-OES in the source rock samples from oil fields in the northwestern Niger Delta.
marine organic input while low value indicates terrestrial organic matter input. The study of Galarraga et al. (2008) showed V/Ni
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Fig. 10. a: Light rare earth elements (LREEs) fingerprinting of source rock samples from MF field in the northwestern Niger Delta. b: Heavy rare earth elements (HREEs) fingerprinting of source rock samples from MF field in the northwestern Niger Delta.
values less than 1.9 indicate terrestrial organic matter, V/Ni values from 1.9 to 3 indicate organic matter of mixed marine and terrestrial source input, while V/Ni ratio higher than 3 indicates marine organic matter. Source rock samples from ML field have V/Ni ratio ranging 0.22e0.77, MF source rock samples have values that range from 0.41 to 0.72, while source rocks from MJ field have V/Ni ratios ranging from 0.30 to 0.78 (Table 2). The values of V/Ni ratio of source rock samples from the three fields indicate that they have terrestrial organic matter input. Cross plot constructed from the values of Co/Ni and V/Ni (Fig. 5) also indicates that the source rocks from the three fields have terrestrial organic matter input. Depositional environment has a significant effect on the concentrations of vanadium and nickel in source rocks (Lewan, 1984). Enriched vanadium content compared to nickel in source rocks occurs in anoxic environments (Peters and Moldowan, 1993). V/Ni ratios for the source rocks from the three fields are generally low with the highest value of 0.78 (Table 2), which is an indication of deposition under oxic conditions (Lewan, 1984). Normalized ratio of vanadium and nickel (V/V þ Ni) can be used unambiguously as a redox condition indicator. V/V þ Ni values greater than 0.5 indicates anoxic environment while values less than 0.5 is an indication of oxic environment. The source rock samples from the three fields
Fig. 11. a: Light rare earth elements (LREEs) fingerprinting of source rock samples from MJ field in the northwestern Niger Delta. b: Heavy rare earth elements (HREEs) fingerprinting of source rock samples from MJ field in the northwestern Niger Delta.
have V/V þ Ni values less than 0.5 (Table 2), indicating that the organic matter of these source rocks were deposited under oxic conditions. Ni/Co ratio is also a good redox indicator (Dypvik, 1984; Dill, 1986; Jones and Manning, 1994). All the samples from the three fields have Ni/Co ratio less than 3, which indicates oxic conditions (Jones and Manning, 1994). Cross plot of nickel versus vanadium (Fig. 6) also revealed that source rocks from the three fields have organic matter content deposited in oxic environments.
4.2. Rare earth element geochemistry 4.2.1. Comparative detection of rare earth elements by INAA and ICP-OES Two analytical techniques, INAA and ICP-OES, were used to unravel the rare earth elements content of the source rock samples. The two techniques were to complement each other. Fig. 7 presented average concentrations of each rare earth element from the three fields. The results show that INAA gave better yields of La, Ce, Pr, Nd and Tb while ICP-OES gave better yields of Sm, Eu, Gd, Dy, Ho, Tm, Yb and Lu. Ce, Pr, Nd and Tb were only detected by INAA, while Eu, Gd, Dy, Tm and Lu were only detected by ICP-OES. The light rare earth elements (LREEs) were better detected with INAA. ICP-OES gave better detection of the heavy rare earth elements (HREEs).
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415
Fig. 13. Comparison of rare earth elements (REEs) distribution patterns of source rock samples from the three fields in the Niger Delta with REE distribution patterns of shale and oil samples from two Chinese Basins.
The order of occurrence of the REEs in the source rocks from ML field is Gd, Ce, Sm, La, Ho, Dy, Lu, Yb, Eu, Tm, Pr, Nd. In MF field, the most abundant REE in the source rock is Gd, follow by Ce, while Pr had the least concentration. The order of occurrence of REEs in this field is Gd, Ce, Sm, Ho, La, Lu, Dy, Yb, Eu, Tm, Nd, Pr. Gd also is the most abundant REE in the source rock from MJ field while Nd had the least concentration. The order of occurrence of the REEs in this field is as follows Gd, Ce, Sm, Ho, La, Lu, Dy, Yb, Tm Eu, Pr, Nd. The distribution of the REEs in the source rocks in the three fields is almost similar. Comparison of distribution of the REEs amongst the fields showed that ML field source rocks have the highest concentrations of the REEs except for Ce, Nd and Pr (Fig. 8). Source rocks from MF field are the most enriched in Ce and Nd, while MJ field source rocks have the highest abundance of Pr.
Fig. 12. a: Comparison of light rare earth elements (LREEs) fingerprintings of source rock samples from the three fields. b: Comparison of heavy rare earth elements (HREEs) fingerprintings of source rock samples from the three fields.
4.2.2. Distribution of REEs in Niger Delta source rocks Gd is most abundant rare earth element in the source rocks from ML field, follow by Ce while Nd had the least concentration (Fig. 8).
4.2.3. Geochemical significance of the rare earth elements The fingerprinting of the light rare earth elements (LREEs) of source rocks from ML field (Fig. 9a) constructed from their concentrations revealed similar distribution patterns for the samples except samples ML7 and ML8 that show positive anomaly of lanthanum (significant enrichment of lanthanum compared to other LREEs), and sample ML10 with positive anomaly of neodymium (enrichment of neodymium compared with other LREEs). The fingerprinting of the heavy rare earth elements (HREEs) of ML field source rocks also showed similar distribution patterns (Fig. 9b)
Table 3 Pearson correlation matrix of REEs data of source rock samples from the three fields in northwestern Niger Delta.
La Ce Nd Pr Sm Eu Gd Dy Ho Tm Yb Lu Co Ni V
La
Ce
Nd
Pr
Sm
Eu
Gd
Dy
Ho
Tm
Yb
Lu
Co
Ni
V
1 0.61 0.70 0.32 1.00 0.66 0.88 0.99 0.95 0.99 0.98 0.99 0.70 0.99 0.59
1 0.99 0.56 0.65 0.20 0.16 0.51 0.83 0.72 0.44 0.50 0.15 0.52 0.28
1 0.46 0.73 0.08 0.28 0.61 0.89 0.80 0.54 0.60 0.03 0.62 0.16
1 0.27 0.92 0.73 0.42 0.00 0.17 0.49 0.44 0.90 0.42 0.95
1 0.62 0.86 0.99 0.96 1.00 0.97 0.98 0.66 0.99 0.55
1 0.93 0.74 0.39 0.54 0.79 0.75 1.00 0.73 1.00
1 0.93 0.69 0.80 0.96 0.94 0.95 0.93 0.90
1 0.91 0.97 1.00 1.00 0.77 1.00 0.68
1 0.99 0.87 0.90 0.44 0.91 0.31
1 0.94 0.96 0.58 0.97 0.47
1 1.00 0.82 1.00 0.74
1 0.78 1.00 0.69
1 0.77 0.99
1 0.68
1
416
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except sample ML 8 that displays positive anomaly of ytterbium (slight enrichment of ytterbium compared to other HREEs). The similarity in the REE distribution patterns of the source rock samples is an indication of similar genetic origin. The LREE fingerprinting of the source rock samples from MF field show similar distribution patterns of the LREEs (Fig. 10a) except for MF2 sample that displays positive anomaly of neodymium. The distribution patterns of HREEs in source rock samples from MF field showed perfect similarity (Fig. 10b), which is indicative of similar genetic origin. The LREE fingerprinting of the source rocks from MJ field also showed similar distribution patterns (Fig. 11a) except for samples MJ1 and MJ3 that have positive anomaly of lanthanum and MJ9 sample that has positive anomaly of neodymium. The HREE distribution patterns of the source rocks in this field are also perfectly similar (Fig. 11b), which is also indicating similar genetic origin of their organic matter. The REE distribution patterns are not only similar amongst the samples from the same field but are also similar amongst source rock samples from the three fields (Fig. 12a and b). This is not surprising because the trace element geochemistry has revealed that the source rocks from the three fields have similar geochemical characteristics. The source rocks are of terrestrial organic matter origin deposited under oxic conditions. Therefore, the similarity in the REE distribution patterns could be a reflection of similar organic matter origin and depositional environment. ML field shows more enrichment of lanthanum than the others because it has more influence of terrestrial organic matter and oxic depositional environment than the other fields as indicated by the trace element data (Tables 1 and 2). Pearson correlation matrix performed on the REEs and trace elements data from the three fields shows (Table 3) very strong positive correlation between vanadium and europium (r ¼ 1.00), which suggests that as concentration of vanadium is increasing the concentration of europium is also increasing or vice versa. The geochemical implication of this is that there would be an enhanced concentration of europium in marine organic matter deposited under anoxic conditions as in the case of vanadium in anoxic conditions. Nickel is very strongly positive correlated with dysprosium, lanthanum and samarium, which suggests that as the concentration of nickel increases the concentrations of these rare earth elements also increase and vice versa. Nickel concentration is known to be enhanced in an oxic environment than an anoxic environment (Lewan, 1984). Thus, geochemically, dysprosium, lanthanum and samarium would have enhanced concentrations in organic matter deposited in oxic environments. The results of this study indicate europium, dysprosium, lanthanum and samarium are potential redox and source indicators. The results of rare earth elements in this study were compared with REE results from other basins (Fig. 13). The Results showed that the rock samples are more enriched in REE than the black shale samples from Early Cambrian Niutitang Foramtion, Guizhou Province, China (Pi et al., 2013) but less enriched compared with oil samples from southern margin of the Junggar Basin, China (Nakada et al., 2010). The REE distribution patterns of Niger Delta samples are different from those of the two basins in China. The REE distribution patterns of samples from the two Chinese basins are not similar. Thus, aside from genetic origin, REE distribution patterns are possibly influenced by geography. 5. Conclusions Concentrations of trace elements and their ratios indicated that the organic matter of the source rocks from ML, MF and MJ fields in the northwestern Niger Delta are of mainly terrestrial origin that
were deposited under oxic conditions. Rare earth elements distribution patterns of the source rocks within each field and across the three fields are similar, indicating similar genetic origin that could be a reflection of their similar organic matter origin and depositional environment. Pearson correlation matrix revealed that europium, dysprosium, lanthanum and samarium are potential redox and source indicators. Acknowledgment We thank the Department of Petroleum Resources (DPR) and Chevron Nigeria Limited for releasing the samples for this study. We also appreciate the financial support of the Africa Network of Analytical Chemists (SEANAC). The assistance of Susana M. Almeida ^ncias e Tecnologias Nucleares (C2TN), Instituto of Centro de Cie cnico (IST), Universidade de Lisboa Campus Tecnolo gico Superior Te e Nuclear, Estrada Nacional 10 (km 139,7), 2695-066 Bobadela LRS, Portugal is also appreciated for the instrumental neutron activation analysis. References Ajayi, T.R., Torto, N., Tchokossa, P., Akinlua, A., 2009. Natural radioactivity and trace metals in crude oils: implication for health. Environ. Geochem. Health 31, 61e69. Akinlua, A., Torto, N., 2006. Determination of selected metals in Niger Delta oils by graphite furnace atomic absorption spectrometry. Anal. Lett. 39 (9), 1993e2005. Akinlua, A., Torto, N., Ajayi, T.R., Oyekunle, J.A.O., 2007. Trace metals characterisation of Niger Delta kerogens. Fuel 86, 1358e1364. Akinlua, A., Adekola, S.A., Swakamisa, O., Fadipe, O.A., Akinyemi, S.A., 2010. Trace metals characterisation of Cretaceous Orange Basin hydrocarbon source rocks. App. Geochem. 25, 1587e1595. Akinlua, A., Torto, N., Ajayi, T.R., 2008. Determination of rare earth elements in Niger Delta crude oils by inductively coupled plasma e mass spectrometry. Fuel 87, 1469e1477. Alberdi-Genolet, M., Tocco, R., 1999. Trace metals and organic geochemistry of the machiques member (AptianeAlbian) and La luna formation (CenomanianeCampanian), Venezuela. Chem. Geol. 160, 19e38. Algeo, T.J., Rowe, H., 2012. Paleoceanographic applications of trace-metal concentration data. Chem. Geol. 324e325, 6e18. Avbovbo, A.A., 1978. Tertiary lithostratigraphy of Niger Delta. AAPG Bull. 62, 295e300. Barwise, A.J.G., 1990. Role of nickel and vanadium in petroleum classification. Energy Fuels 4, 647e652. Dill, H., 1986. Metallogenesis of early paleozoic graptolite shales from the graefenthal horst (northern Bavaria-Federal Republic Of Germany). Econ. Geol. 81, 889e903. Doust, H., 1990. Petroleum Geology of the Niger, vol. 50. Geological Society, London, Special Publications, Delta, p. 365. Doust, H., Omatsola, E., 1990. Niger Delta. Divergent/Passive margin basins. In: Edwards, J.D., Santogrossi, P.A. (Eds.), AAPG Mem. 48, 201e238. Dypvik, H., 1984. Geochemical Compositions and Depositional Conditions of Upper Jurassic and Lower Cretaceous Yorkshire Clays, England, vol. 121. Geological Magazine, pp. 489e504. Ejedawe, J.E., 1981. Patterns of incidence of oil reserves in Niger Delta Basin. AAPG Bull. 65, 1571e1585. Ekweozor, C.M., Okoye, N.V., 1980. Petroleum source-bed evaluation of tertiary Niger Delta. AAPG Bull. 64, 125e1259. Evamy, B.P., Haremboure, J., Kamerling, P., Knoop, W.A., Molly, F.A., Rowlands, P.H., 1978. Hydrocarbon habitat of tertiary Niger Delta. AAPG Bull. 62, 1e39. Frank, E.J., Cordry, E.A., 1967. The Niger Delta Oil Province. In: Recent World Petroleum Congress, Proc, vol. 2, pp. 195e209. Galarraga, F., Llamas, J.F., Martinez, A., Martinez, M., Marquez, G., Reategui, K., 2008. V/Ni ratio as a parameter in palaeoenvironmental characterization of nonmature medium-crude oils from several Latin American basins. J. Petrol. Sci. Eng. 61, 9e14. Henderson, P., 1984. General geochemical properties and abundances of the rare earth elements. In: Denderson, P. (Ed.), Rare Earth Element Geochemistry. Elsevier, New York, pp. 1e32. Jones, B., Manning, D.C., 1994. Comparison of geochemical indices used for the interpretation of paleo-redox conditions in Ancient mudstones. Chem. Geol. 111, 111e129. Lewan, M.D., 1984. Factors controlling the proportionality of vanadium to nickel in crude oils. Geochim. Cosmochim. Acta 48, 2231e2238. Lewan, M.D., Maynard, J.B., 1982. Factors controlling enrichment of vanadium and nickel in the bitumen of organic sedimentary rocks. Geochim. Cosmochim. Acta 46, 2547e2560. Mongenot, T., Tribovillard, N.P., Desprairies, A., Lallier-Verges, E., Laggoun-
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