Origin and depositional environments of source rocks and crude oils from Niger Delta Basin: Carbon isotopic evidence

1.   Introduction

In oil-to-oil and oil-to-source rock correlations, isotope analyzes of individual compounds were implemented in oil and source rock extracts (Hayes et al., 1990; Rooney et al., 1998). Several writers have reported that biomarker isotope compositions can supply important genetic information (Hayes TM et al., 1990; Grice K et al., 2001; Lu H et al., 2003). Biomarkers’ isotopic structures can reveal parent organisms that indicate the carbon source consumed by the production. Isotopic fractionation takes place naturally during chemical, biochemical and physical processes and it relies on the strength of bonds (Hoefs J, 1987).

Geochemistry of carbon isotopes has been commonly applied in oil and gas production, maturation, migration and accumulation procedures in a sedimentary basin. It has many significant uses, one of which is its comprehensive use in the source and gaseous hydrocarbon correlation. This is due to the natural mobility and changes that may cover their origins and occurrence actual knowledge. Carbon isotopic compositions of light hydrocarbon gases, particularly methane, were used to identify various sources of prospective sources (Fuex AN, 1977; Hunt JM, 1996; Schouten S, 2000; Killops SD and Killops VJ, 2005). It has also been shown that n-alkane distribution in kerogen using standard geochemical instruments can sometimes lead to erroneous interpretations of the contributing precursors without accurate stable carbon isotope information (Audino M et al., 2002).

The measurement of a sample’s stable isotopic composition of total carbon is called bulk stable carbon analysis. The composition of all biomarkers in the entire blend is used to determine the isotope and only provides an average value for the entire complicated mixture (Sofer Z, 1984). Sofer Z (1984) successfully applied bulk carbon isotopic compositions of aromatics and saturated fractions of crude oils to oil-oil correlation study.

Chung HM et al. (1992) delineated post-Ordovician marine oils into four families based on their depositional settings and their source rock age using proportions of ¹³C and pristane/phytane ratios in combination with sulfur content. Andrusevich VE et al. (1998) recorded the bulk δ¹³C values of saturated and aromatic fractions of crude oils and stated that both fractions were enriched by ¹³C compositions at latest timescales. Unlike the isotope analysis of bulk stable carbon, carbon isotope particular assessment includes measuring ¹³C/¹²C of individual organic parts in complicated mixtures of crude oils and rock extracts (Matthews DE and Hayes JM, 1978). It has been reported that the distribution pattern of individual n-alkane carbon isotopes in various crude oil should reflect the differences in organic origin attributes (Murray AP et al., 1994; Zhu YM et al., 2005), with terrestrial organic matter often displaying depleted (light) values of n-alkane carbon isotopes (Murray AP et al., 1994; Zhu YM et al., 2005) while marine oils are characterized by enriched (heavy) values of carbon isotopes (Samuel OJ et al., 2009).

Maturity has been shown in the subsurface to affect stable crude oils isotope values (Clayton CJ, 1991). The thermal maturity of kerogen, rock source extracts and crude oils and their related fractions and individual compounds (e.g. n-alkanes) were noted to increase with δ¹³C values. The heat release of isotopically lighter products is believed to result in the enrichment of ¹³C in kerogen (Clayton CJ, 1991). It has been shown that secondary procedures such as biodegradation alter compositions of δ¹³C. Biodegradation can lead in an enrichment of ¹³C in remaining compounds, with the enrichment rate gradually decreasing as the molecular weight increases (Samuel OJ et al., 2009). Bulk and individual n-alkanes carbon isotopic compositions have been proved to be useful tools to indicate the origin and depositional environments of geological materials (Sofer Z, 1984; Murray AP et al., 1994; Samuel OJ et al., 2009).

Previous works on the determination of origin and depositional environments of the Niger Delta source rocks and crude oils have been limited to the use of biomarkers distributions and bulk isotopic compositions in the crude oils and rock extracts (Sonibare OO et al., 2008). In this work, the origin and depositional environments of crude oils and source rocks from the Niger Delta were determined from their bulk and individual n-alkanes carbon isotopic compositions.

2.   Geological and stratigraphic setting

The Niger Delta Basin is situated at the Gulf of Guinea, West Africa entrance (Fig. 1a; Short KC and Stauble AJ, 1967). The Niger Delta sub-aerial segment stretches over roughly 75000 km² and encompasses approximately 200 km from peak to entrance. The complete sedimentary prism encompasses 140000 km², with a highest stratigraphic thickness of approximately 12 km (Kulke H, 1995). The wide sedimentary order stratigraphy is divided into three lithostratigraphic units, namely the Akata, Agbada, and Benin Formations (Fig. 1b; Short KC and Stauble AJ, 1967). The uppermost unit, the Benin Formation ranging from Eocene to latest era, includes continental/fluviatile deposits of up to 2500 m dense sands, gravels and backswamps. The Agbada Formation of paralic, brackish to marine, coastal and fluvio-marine deposits underlines these. These are predominantly interbedded sandstones and shale organized into coarsening upward “offlap” cycles with minor lignite. Underlying this unit is the Akata Formation, which ranges in era from Paleocene to latest and up to 6500 m of marine clay with silty and sandy interbeds (Kulke H, 1995). Akata Formation shales are over-pressured. These over-pressured shales are regarded a mobile substratum comparable to natural evaporites that deforms in reaction to deltaic programming and sedimentary loading (Doust H and Omatsola E, 1990).

Figure  1.  Niger Delta map showing sample depobelts, formation and locations (after Stacher P, 1995).

DownLoad: Full-Size Img PowerPoint

The depobelts are divided into seven east-westbound blocks that correlate with the individual moments of the development record of deltas starting from the most mature in the northern delta to the immature offshore in the south (Doust H and Omatsola E, 1990). It is commonly recognized that the individual depobelt consists of a more or less independent section on sedimentation, structural deformation and the generation and accumulation of hydrocarbons (Evamy BD et al., 1978). In Agbada and Akata Formations (Evamy BD et al., 1978; Ekweozor CM and Daukoru CM, 1994), the source rocks in the basin are mostly present. The Niger Delta’s hydrocarbon habitat is mostly the Agbada Formation’s sandstone reservoir where oil and gas are generally trapped in growth-related rollover anticlines.

3.   Materials and methods

3.1   Sampling

A total of 39 crude oil samples were collected from four wells in four fields represented as ADL, OKN, MJI and MJO in Agbada Formation, at a depth ranging from 2602 m to 2101 m in the Northern and Offshore Niger Delta basin, Nigeria. Twenty-one rock samples from three wells in three of the fields were also collected for analysis. The map showing Niger Delta depobelts and the sample locations is shown in Fig. 1c.

3.2   Sample preparation and extraction

Before extraction, the samples were crushed with agate mortar and powdered to less than 100 mesh size. About 50 g of each powdered samples were subjected to Soxhlet extraction for 72 h using azeotropic mixture of dichloromethane: methanol (93∶7, v/v). To remove elemental sulfur from the extracts, activated copper powder has been added. Using a rotary evaporator, excess solvent was distilled to an aliquot quantity of about 3 mL. The aliquot was then transported with a micropipette to a weighed smooth vial and the remaining solvent was removed at temperatures below 50°C under the stream of nitrogen gas.

3.3   Sample fractionation and separation

The rock extracts and oils were fractionated by column chromatography using silica gel/alumina as stationary phase. The saturate fractions were eluted with 30 mL of n-hexane while the aromatic fractions were eluted with 30 mL of dichloromethane: n-hexane (2∶1, v∶v). The polar fractions were eluted with 30 mL of dichloromethane: methanol (1∶1, v∶v). Each fraction was concentrated on rotary evaporator before subjecting to elemental analysis-isotope ratio mass spectrometry (EA-IRMS). In preparation for the gas chromatography-isotope ratio mass spectrometry analysis, n-alkanes were isolated from saturated hydrocarbons by treating the saturated fractions with activated (250°C, 8 h) 5A molecular sieves (Dawson D et al., 2005; Grice K et al., 2008a) in cyclohexane. Half of a 2 mL vial was packed with activated 5A molecular sieves in a typical 5A molecular sieve separation. Then part of the saturated fraction in cyclohexane was added to the 2 mL vial. The vial was covered and placed overnight on a preheated block of aluminum (85°C). The resulting solution was then cooled and filtered with cotton wool (pre-rinsed with cyclohexane) through a tiny column of silica and the sieves were carefully rinsed with cyclohexane yielding n-alkane fraction.

3.4   Elemental analysis-isotope ratio mass spectrometry (EA-IRMS)

Bulk isotope analyzes of the whole oils and extracts, and their corresponding saturate, aromatic and polar fractions were conducted on an isotope ratio mass spectrometer (IsoPrime micromass) interfaced with an elemental analyzer EuroEA3000. 0.05–0.15 mg of each sample was weighed in a tiny tin capsule, then folded and accurately compressed to remove atmospheric gases. Using an autosampler, the tin capsule containing the sample was placed in a combustion reactor. In an atmosphere temporarily enriched with oxygen, the sample and capsule melted where the tin promoted flash combustion. The combustion materials passed through an oxidation catalyst (chromium oxide) in a steady stream of helium. The oxidation products then passed through a reduction reactor containing copper granules at 650°C where all nitrogen oxides (NO, N₂O and N₂O₂) are lowered to N₂ and SO₂ and separated at ambient temperature on a 3 m chromatographic column (PoropakQ). Oxidation products are then carried through a thermal conductivity detector (TCD) after removal of nitrogen oxides, followed by Elemental Analysis. The delta notation relative to Vienna Pee dee belemnite (VPDB) contains isotopic compositions.

3.5   Gas chromatography-isotope ratio mass spectrometry (GC-IRMS)

On a gas chromatography-combustion-isotope ratio mass spectrometer, the Carbon isotope analyzes of individual compounds was conducted. The gas chromatography was carried out using a Thermo Finnigan GC COMBUSTION III model fitted with a capillary column of DB-5 fused silica (30 m × 0.25 mm) and helium was used as a carrier gas with a flow rate of 1 ml/min. The temperature of the GC oven was isothermal at 70°C for 5 min, then programmed at 3°C/min from 70°C to 290°C and then isothermal at 290°C for 30 min. Isotopic values were computed by integrating the m/z 44, 45 and 46 ion currents of the peaks generated by combustion of chromatographically separated compounds (880°C) and those of normal CO₂ spikes admitted at periodic intervals. Reproductivity and precision were regularly assessed using laboratory norms of known δ¹³C (C₁₃-C₃₂ n-alkanes) values. After each six-sample assessment, the laboratory standard was injected. The isotope values for the PDB standard are provided.

4.   Results and discussion

4.1   Stable carbon isotopic composition of Niger Delta source rocks and crude oils

The origin and depositional environments of crude oils and source rocks from Niger Delta Basin were determined by bulk stable carbon isotopic compositions and compound specific isotopes of individual n-alkanes. Table 1 and Table 2 show the bulk stable carbon isotope information for rock extracts and crude oils, respectively, while Table 3 and Table 4 present the isotopic values for individual n-alkanes of rocks and crude oils, respectively.

Table  1.  Bulk stable carbon isotope data of the rock extracts from Niger Delta.

Field	Depth/m	δ13C/‰
		Extr	Sat	Aro	Polar
OKN	1537–1555	–28.2	–28.4	–26.7	–26.2
OKN	1729–1747	–28.0	–29.1	–28.1	–25.6
OKN	2625–2643	–28.5	–28.4	–27.8	–25.0
OKN	2780–2799	–28.2	–28.6	–27.3	–29.2
OKN	2863–2881	–28.2	–28.3	–26.8	–28.3
OKN	2909–2927	–28.5	–28.1	–27.6	–28.7
MJI	2079–2098	–28.7	–29.0	–28.5	–28.2
MJI	2299–2308	–28.4	–29.2	–28.4	–28.2
MJI	2637–2655	–28.0	–27.6	–27.6	–28.2
MJI	2857–2875	–27.9	–28.6	–28.0	–27.6
MJI	2994–3012	–28.2	–28.8	–27.8	–28.0
MJI	3085–3104	–28.2	–28.8	–27.8	–27.7
MJI	3232–3250	–27.6	–27.8	–27.2	–28.0
MJI	3323–3332	–27.9	–28.4	–27.8	–27.9
MJI	3405–3424	–27.5	–27.6	–27.2	–27.2
MJO	1616–1707	nd	–28.8	–27.7	–28.3
MJO	1771–1872	–27.7	–27.2	–26.9	–28.0
MJO	2091–2101	–28.1	–28.3	–27.7	–28.2
MJO	2293–2366	–27.8	–28.8	–27.1	–27.2
MJO	2570–2588	–27.6	–28.8	–27.9	–27.7
MJO	2808–2817	–26.8	–28.1	–26.8	–26.6
Notes: Extr–whole extract; Sat–saturate fraction; Aro–aromatic fraction; Polar–polar fraction; nd–not determined.

 | Show Table

DownLoad: CSV

Table  2.  Bulk stable carbon isotopic data of Niger Delta oils.

Sample	Depth/m	δ13C/‰
		Oil	Sat	Aro	Polar
ADL-1	2602–2607	–27.7	–28.1	–26.9	–27.2
ADL-2	2602–2607	–27.8	–27.8	–25.8	–27.1
ADL-3	2702–2704	–26.8	–27.7	–24.9	–27.4
ADL-4	2718–2720	–27.3	–28.2	–25.3	–27.2
ADL-5	2759–2763	–27.7	–27.7	–25.1	–27.1
ADL-6	2766–2770	–27.7	–27.9	–25.8	–27.6
ADL-7	2905–2908	–27.3	–27.7	–26.9	–27.4
ADL-8	2964–2967	–27.3	–27.8	–25.0	–27.2
ADL-9	3064–3052	–27.5	–28.3	–26.5	–27.1
OKN-1	1749–1750	–25.4	–26.1	–24.2	–25.5
OKN-2	1892–1895	–25.8	–26.4	–23.6	–25.0
OKN-3	1905–1907	–25.4	–26.5	–24.3	–25.6
OKN-4	1952–1955	–25.6	–26.0	–24.3	–25.6
OKN-5	2050–2059	–25.8	–26.3	–24.9	–24.9
OKN-6	2369–2555	–25.9	–26.6	–24.9	–24.9
OKN-7	2377–2672	–25.5	–26.4	–24.3	–25.2
OKN-8	2469–2782	–25.7	–25.9	–23.9	–24.9
OKN-9	2485–2793	–25.8	–26.5	–24.1	–25.2
OKN-10	2489–2491	–25.5	–26.9	–25.3	–25.8
OKN-11	2521–2523	–26.1	–26.9	–23.9	–26.1
OKN-12	2530–2537	–26.3	–27.0	–24.8	–25.7
OKN-13	2566–2568	–25.6	–26.2	–24.5	–24.6
OKN-14	2677–2683	–25.5	–26.3	–24.8	–25.8
OKN-15	3148–3154	–26.1	–26.3	–25.0	–25.5
OKN-16	3593–3605	–25.2	–26.2	–23.7	–25.0
MJI-1	1607–1611	–25.5	–27.1	–25.6	–26.8
MJI-2	1777–1779	–26.2	–27.0	–25.0	–26.8
MJI-3	1795–1797	–26.1	–28.0	–25.6	–27.6
MJI-4	1920–1921	–26.9	–27.9	–25.6	–26.7
MJI-5	1936–2342	–26.1	–26.2	–25.2	–26.5
MJI-6	1944–1947	–26.3	–27.1	–25.4	–27.0
MJI-7	1948–1950	–26.8	–28.0	–26.0	–26.9
MJI-8	1979–2398	–26.2	–26.9	–25.5	–28.5
MJI-9	2442–2444	–26.2	–27.7	–25.3	–27.6
MJI-10	3030–3036	–27.3	–28.4	–25.4	–26.8
MJO-1	2207–2216	–26.2	–26.6	–23.5	–24.9
MJO-2	2070–2081	–25.6	–27.0	–24.4	–25.4
MJO-3	2091–2104	–25.9	–26.6	–24.2	–24.5
MJO-4	2096–2101	–25.7	–26.5	–26.2	–26.6

 | Show Table

DownLoad: CSV

Table  3.  Carbon isotopic composition of n-alkanes in Niger Delta source rocks (‰).

Field	Depth/m	C12	C13	C14	C15	C17	C18	C19	C20	C21	C22	C23	C24
OKN	1729	–30.8	–30.7	–30.6	–30.0	nd	nd	nd	nd	–30.6	–30.0	–29.3	–28.7
OKN	2625	–29.4	–30.0	nd	nd	nd	nd	–30.0	–31.3	–28.9	–30.2	–31.4	–30.0
OKN	2780	–30.5	–29.9	–30.1	nd	nd	nd	–30.5	–30.4	–30.4	–29.7	–29.7	–30.5
OKN	2863	nd	nd	nd	nd	nd	–30.4	–29.5	–29.5	–30.4	–29.5	–29.7	–29.5
Ave.		–30.2	30.2	–30.3	–30.0		–30.4	–30.0	–30.4	–30.1	–29.9	–30.0	–29.7
MJI	2078						nd	–29.5	–30.3	–30.4	–30.6	–30.4	–30.7
MJI	2857						nd	–29.7	–29.5	–30.2	–30.2	–30.3	–30.3
MJI	3085						nd	–29.6	–29.9	–30.5	–30.7	–30.9	–30.7
MJI	3405						28.2	–28.1	–28.0	–27.9	–28.2	–28.2	–28.4
Ave.							28.2	–29.2	–29.4	–29.7	–29.9	–29.9	–30.0
MJO	1616					nd	–29.6	–28.9	–28.7	–30.9	–30.1	–31.1	–32.2
MJO	2293					nd	nd	nd	nd	nd	nd	–31.7	–32.6
MJO	2570					nd	–31.0	–31.5	–31.8	–33.1	33.2	–33.2	–32.4
MJO	2808					–29.2	–28.4	–28.0	–28.0	–28.3	–28.2	–28.3	–28.2
Ave.						–29.2	29.7	–29.5	–29.5	–30.8	–30.8	–31.1	–31.4
Field	Depth/m	C25	C26	C27	C28	C29	C30	C31	C32	C33	A	Pr	Ph
OKN	1729	–31.8	–29.7	–30.3	–29.8	–32.5	nd	nd	nd	nd	–30.4
OKN	2625	–30.3	–30.6	–32.3	–30.4	–30.4	nd	nd	nd	nd	–30.4	–28.0	–30.2
OKN	2780	–30.9	–32.0	–30.6	–32.8	–30.7	–32.0	nd	nd	nd	–30.7
OKN	2863	–32.6	–29.2	–30.0	–29.7	–31.3	–30.0	–32.0	–31.0	–32.3	–30.4	–29.2	–28.7
Ave.		–31.4	30.4	–30.8	–30.7	–31.2	–31.0	–32.0	–31.0	–32.3		–28.8	–29.5
MJI	2078	–29.9	–30.4	–31.0	–31.7	–32.1	–32.1	–33.5	nd	nd	–31.0
MJI	2857	–30.2	–30.8	–31.2	–31.8	–31.6	–32.2	–33.2	–33.2	32.6	–31.1	–29.0	–28.1
MJI	3085	–31.0	–31.3	–31.5	–32.0	–31.8	–32.4	–33.2	–33.1	–33.5	–31.5	–29.1	–29.0
MJI	3405	–28.4	–28.9	–29.2	–30.1	–30.8	–31.2	–31.4	–32.3	–32.7	–29.5	–28.3	–27.4
Ave.		–29.9	–30.3	–30.7	–31.4	–31.6	–32.0	–32.8	–32.9	–32.9		–28.6	–28.2
MJO	1616	–30.6	–30.9	–31.4	–31.6	–32.4	–32.8	–33.7	–33.8	nd	–31.3
MJO	2293	–31.0	–31.8	–31.7	–31.7	–32.1	–31.8	–31.3	–32.7	nd	–31.8	–29.0	–29.1
MJO	2570	–31.8	–32.0	–32.0	–31.7	–31.8	–32.2	–33.6	–34.5	nd	–32.4	–29.1	–28.9
MJO	2808	–28.4	–28.6	–28.8	–29.2	–30.2	–30.9	–32.4	–33.1	–34.9	–29.6	–28.0	–27.9
Ave.		–30.4	–30.8	–31.0	–31.1	–31.6	–31.9	–32.7	–33.5	–34.9		–29.0	–28.6
Note: Ave–average of individual n-alkanes; A–weighted average; Pr–pristane; Ph–phytane; nd–not determined.

 | Show Table

DownLoad: CSV

Table  4.  Stable carbon isotopic composition of n-alkanes in Niger Delta crude oils (δ¹³‰).

Sample	C13	C14	C15	C16	C17	C18	C19	C20	C21	C22	C23	C24	C25	C26	C27	C28	C29	C30	C31	C32	C33	Weighted Ave.	Pr	Ph
ADL-1	–27.7	–27.7	–27.9	–28.1	–28.4	–28.6	–28.8	–28.9	–29.0	–29.1	–29.1	–29.0	–29.0	–28.9	–29.2	–29.3	–28.8	–29.8	–30.2	–30.2	–29.4	–28.9	–32.0	–30.1
ADL-2	–27.5	–28.6	–28.0	–28.1	–28.4	–28.9	–29.3	–29.5	–29.7	–29.6	–29.9	–29.9	–30.0	–30.1	–30.3	–30.4	–30.4	–30.1	–30.2	–30.4	–22.6	–29.1	–30.3	–30.0
ADL-4	–28.0	–27.9	–28.1	–28.3	–28.5	–28.9	–29.2	–29.4	–29.5	–29.5	–29.8	–30.1	–29.4	–29.7	–29.0							–29.0	–26.2	–27.6
ADL-5	–28.7	–28.6	–28.7	–29.2	–29.6	–29.7	–30.0	–30.1	–30.3	–30.3	–30.7	–30.7	–30.9	–31.0	–31.2	–31.0	–30.3	–30.8	–30.9			–30.2	–27.2	–28.7
ADL-6	–28.0	–28.2	–28.1	–28.3	–28.4	–28.6	–28.8	–28.8	–29.0	–29.2	–29.5	–29.5	–29.5	–29.6	–30.0	–30.2	–30.5	–30.2	–31.3			–29.3	–31.4	–30.4
ADL-7	–27.8	–27.9	–27.6	–28.1	–28.8	–29.3	–29.7	–30.1	–30.5	–30.7	–31.0	–31.0	–31.1	–31.0	–31.2	–31.1	–30.8	–30.4	–30.2	–30.3	–30.9	–30.0	–27.9	–28.4
ADL-8		–28.7	–28.5	–28.7	–28.6	–28.8	–28.9	–29.1	–29.1	–29.1	–29.3	–29.2	–29.2	–29.2	–29.1	–29.3	–29.4	–29.3	–29.8	–30.5		–29.1	–31.9	–30.5
ADL-9	–28.6	–28.7	–29.0	–29.3	–29.5	–29.4	–29.5	–29.5	–29.6	–29.5	–29.8	–29.8	–29.9	–30.0	–29.9	–29.9	–29.8	–29.5	–30.2			–29.5	–28.0	–28.2
Ave.	–28.0	–28.3	–28.2	–28.5	–28.8	–29.0	–29.3	–29.4	–29.6	–29.6	–29.9	–29.9	–29.9	–29.9	–30.0	–30.2	–30.0	–30.0	–30.4	–30.4	–27.6		–29.3	–29.2
OKN-2		–27.0	–27.0	–27.0	–27.2	–27.6	–27.7	–27.9	–28.0	–28.0	–28.4	–28.3	–28.4	–28.3	–28.6	–28.3	–28.6	–28.5	–28.6	–27.4	–28.4	–28.0	–24.6	–27.8
OKN-3				–26.9	–26.9	–26.9	–27.0	–27.3	–27.4	–27.4	–27.7	–27.6	–27.8	–28.0	–28.0	–28.6	–28.8	–28.3	–29.7	–28.7	–28.3	–27.9	–29.7	–28.8
OKN-4		–26.5	–26.6	–26.3	–26.2	–26.2	–26.8	–26.8	–27.1	–27.2	–27.6	–27.3	–27.6	–27.5	–27.4	–28.3	–28.9	–29.0	–30.6	–30.4	–32.5	–27.8	–27.3	–28.3
OKN-5			–26.4	–26.5	–26.3	–26.5	–27.1	–27.9	–28.6	–29.1	–29.0	–28.9	–29.0	–29.0	–29.0	–29.3	–29.4	–29.1	–29.3	–28.8	–28.9	–28.3	–23.9	–27.0
OKN-7			–27.1	–26.6	–26.7	–26.5	–26.9	–27.0	–27.3	–27.5	–27.7	–27.7	–27.7	–27.7	–27.8	–28.3	–28.5	–28.3	–29.7	–28.5	–28.6	–27.7	–24.7	–27.0
OKN-8		–26.6	–27.1	–26.7	–27.0	–27.3	–27.4	–27.7	–27.9	–28.0	–28.2	–28.1	–28.4	–28.5	–29.0	–29.1	–29.8	–29.4	–29.4	–28.9	–29.0	–28.2	–29.8	–29.6
OKN-9	–25.9	–26.2	–26.0	–26.1	–26.3	–26.4	–26.5	–26.6	–27.2	–27.3	–27.9	–28.2	–28.9	–29.0	–29.3	–29.0	–29.3	–29.1	–29.7	–28.9	–28.6	–27.7	–24.6	–27.8
OKN-10	–25.8	–25.7	–25.7	–25.9	–26.1	–26.4	–26.6	–26.7	–26.9	–26.8	–26.9	–26.7	–26.8	–26.9	–27.1	–27.2	–27.5	–27.3	–27.9	–27.5	–28.1	–26.8	–24.9	–27.5
OKN-11		–27.0	–27.1	–27.0	–27.2	–27.4	–27.7	–27.7	–27.9	–27.8	–27.9	–27.6	–27.7	–27.5	–27.8	–27.7	–28.1	–27.8	–28.2	–27.9		–27.6	–28.2	–29.7
OKN-12	–26.6	–27.1	–27.5	–27.8	–27.8	–28.1	–28.2	–28.2	–28.5	–28.2	–28.2	–27.9	–27.6	–27.6	–27.3	–27.0	–27.0	–26.8	–26.3	–27.3	–24.8	–27.4	–25.4	–28.1
OKN-14	–26.4	–26.2	–25.9	–26.1	–26.3	–26.5	–26.7	–27.0	–27.3	–27.4	–27.6	–27.6	–27.6	–27.6	–27.7	–27.7	–27.8	–27.6	–27.9	–27.3	–27.3	–27.1	–29.9	–29.0
OKN-16	–25.2	–25.2	–25.8	–26.8	–27.2	–27.6	–27.9	–28.0	–27.8	–27.7	–27.3	–27.2	–26.9	–26.9	–26.7	–26.5	–26.6	–26.7	–27.0	–27.4		–26.9	–25.3	–27.2
Ave.	–26.0	–26.4	–26.6	–26.6	–26.8	–26.9	–27.2	–27.4	–27.7	–27.7	–27.9	–27.8	–27.9	–27.9	–28.0	–28.1	–28.4	–28.2	–28.7	–28.3	–28.4		–26.5	–28.1
MJI-1		–28.2	–28.4	–28.6	–28.9	–28.8	–29.1	–29.1	–29.4	–29.5	–29.7	–29.5	–30.0	–29.9	–29.9	–30.8	–30.8	–30.2	–30.8			–29.5	–26.0	–28.5
MJI-3		–28.2	–28.1	–28.2	–28.5	–28.5	–28.7	–28.7	–29.0	–28.9	–29.2	–29.2	–29.5	–29.4	–29.6	–30.7	–30.5	–30.5	–30.9	–30.0		–29.3	–26.4	–28.4
MJI-4	–28.0	–28.4	–28.8	–28.7	–28.9	–29.2	–29.5	–29.7	–29.9	–29.9	–30.1	–30.1	–30.1	–30.1	–30.4	–30.4	–30.6	–30.3	–30.8	–30.6	–30.3	–29.8	–32.1	–30.5
MJI-5		–28.0	–28.1	–28.2	–32.5	–28.7	–28.5	–28.7	–29.1	–29.2	–29.3	–29.5	–29.4	–29.5	–29.6	–30.3	–31.1	–30.2	–30.1	–27.1	–30.2	–29.4	–25.6	–27.6
MJI-7			–29.1	–28.9	–28.9	–29.0	–29.2	–29.4	–29.5	–29.4	–29.9	–29.6	–30.0	–30.1	–30.4	–30.7	–31.3	–31.2	–31.3	–32.1		–30.0	–29.1	–29.7
MJI-8				–28.8	–28.7	–28.6	–29.0	–29.3	–29.3	–29.4	–29.8	–29.7	–30.0	–30.5	–30.3	–30.8	–31.6	–32.6	–30.7			–29.9	–21.9	–29.0
MJI-9		–28.6	–28.7	–28.7	–29.0	–28.8	–28.7	–29.1	–29.5	–29.3	–29.7	–29.5	–30.2	–30.0	–30.5	–31.5	–31.2	–31.0	–31.5	–29.1		–29.7	–26.4	–28.9
MJI-10	–28.6	–28.6	–28.6	–28.9	–29.1	–29.4	–29.8	–30.1	–30.4	–30.5	–30.6	–30.7	–31.0	–31.1	–31.3	–31.5	–31.9	–31.6	–31.7	–31.3	–30.9	–30.4	–29.7	–30.4
Ave.	–28.0	–28.3	–28.6	–28.6	–29.3	–28.9	–29.1	–29.3	–29.5	–29.5	–29.8	–29.7	–30.0	–30.1	–30.2	–30.8	–31.1	–31.0	–31.0	–30.0	–30.5		–27.0	–29.1
MJO-1			–25.2	–24.8	–24.9	–24.9	–24.9	–24.9	–25.2	–25.1	–25.3	–24.7	–24.4	–24.3	–24.2	–24.2	–24.8	–24.3	–24.5	–23.8	–24.9	–24.7	–23.0	–26.9

 | Show Table

DownLoad: CSV

4.2   Bulk isotopic compositions

The bulk carbon isotopic values of the whole rock extracts, saturate and aromatic fractions range from –28.7‰ to –26.8‰, –29.2‰ to –27.2‰ and –28.5 to –26.7‰ (Table 1). The saturate fraction is more depleted in δ¹³C than the aromatic fraction with values not exceeding 1.1‰ in each of the samples, reflecting source rocks of mixed organic matter source rocks (Sofer Z, 1984; Peters KE and Moldowan JM, 1993). There are less than 2.5‰ isotopic variations between the whole rock extracts and the saturated and aromatic fractions of the rock extracts, suggesting that the rock samples were obtained from mixed origin (Terrestrial and Marine) (Sofer Z, 1984; Peters KE and Moldowan JM, 1993). The rock samples plotted between the terrigenous and marine areas on the plots of δ¹³C_aro versus δ¹³C_sat (Fig. 2). However, most rock samples fall within the terrigenous region, suggesting a greater input of higher plant organic matter into the organic matter that formed the source rocks. This interpretation is consistent with what has been inferred in recent studies on Niger Delta source rocks using aromatic biomarkers (Ogbesejana AB et al., 2018a, 2018b, 2019a, 2019b, 2019c).

Figure  2.  Plot of the δ¹³C values of aromatic fractions versus δ¹³C values of saturate fractions for rock samples from the Niger Delta (after Sofer Z, 1984).

DownLoad: Full-Size Img PowerPoint

The bulk carbon isotopic values of the whole oils, saturate and aromatic fractions range from –25.4‰ to –27.8‰, –25.9‰ to –28.4‰ and –23.5‰ to –26.9‰, respectively (Table 2). In each of the samples, the saturate fractions have greater negative values than the aromatic fractions with values not exceeding –3.1‰. Peters KE and Moldowan JM (1993) reported that a good correlation could be shown if the variations in isotopic composition between a group of crude oils of a similar range of maturity are not greater than 3‰. The isotopic differences between the whole oils and the saturated and aromatic fractions of the oils are less than 1.5‰, indicating that the crude oils were derived from mixed origin (Terrestrial and marine) (Table 1; Peters KE and Moldowan JM, 1993). In oil-oil correlation studies, the cross plots of δ¹³C_aro versus δ¹³C_sat were effectively implemented (Sofer Z, 1984). The oils plotted between terrigenous and marine area for the current samples (Fig. 3). Most oils, however, fall within the terrigenous region, indicating higher terrestrial source input for the oil samples (Audino M et al., 2002). This result is consistent with the previous reports on the Niger Delta crude oils using biomarkers (Sonibare OO et al., 2008; Ogbesejana AB, 2018; Ogbesejana AB et al., 2017, 2018b, 2019b, 2019c).

Figure  3.  Plot of the δ¹³C values of aromatic fractions versus δ¹³C values of saturate fractions for oil samples from the Niger Delta (after Sofer Z, 1984).

DownLoad: Full-Size Img PowerPoint

4.3   Carbon isotopic compositions of individual n-Alkanes

The isotopic values for the n-alkanes in the rock samples vary from –32.4‰ to –29.5‰ (Table 3). The isotope values are features of n-alkanes obtained from higher plants (Samuel OJ et al., 2009; Cai CF et al., 2015). The average isotopic carbon composition of individual alkanes (nC₁₂-nC₃₃) ranges from –34.9‰ to –28.2‰ in rock samples (Table 3). For the long chain alkanes, which are features of plant wax derived n-alkanes of C₃ plants, the most depleted values have been noted (Murray AP et al., 1994; Samuel OJ et al., 2009; Cai CF et al., 2015). Significant contribution of marine organic matter is reflected in light isotopic values found in all samples in the short chain (nC₁₂-nC₁₈) alkanes (Fig. 4). The pristane (Pr) and phytane (Ph) isotopic ratios range from –29.2‰ to –28.2‰ and –30.2‰ to –27.4‰, respectively in the rock samples (Table 3). There is no important distinction between Pr and Ph’s ¹³C-values, suggesting that the rocks were formed from similar organic source materials (Schwas VF and Spangenberg JE, 2007; Cai CF et al., 2015). However, in the n-alkane profile between nC₁₄-nC₂₄ and nC₂₇-nC₃₁ (Fig. 4), a particularly flat part was observed which is an indication of marine incursion (Murray AP et al., 1994). These characteristics indicate that the samples consist of both terrestrial and marine organic matter in lacustrine-fluvial/deltaic environments. Recent studies have shown that Niger Delta source rocks are formed from mixed input of terrestrial and marine organic matter and deposited under oxic to sub-oxic condition in lacustrine-fluvial/deltaic environments (Ogbesejana AB et al., 2018a, 2018b, 2019a, 2019b, 2019c).

Figure  4.  Carbon isotopic compositions of individual n-alkanes in Niger Delta rock samples (after Murray AP et al., 1994).

DownLoad: Full-Size Img PowerPoint

In the oil samples, the average carbon isotopic composition of individual alkanes (nC₁₃-nC₃₃) ranges from –31.1‰ to –23.8‰ (Table 4). For the long chain alkanes (nC₂₁-nC₃₂) the most depleted values were observed. These findings are features of greater organic matter derived from higher plants (Samuel OJ et al., 2009; Cai CF et al., 2015). Low contribution from marine organic matter (i.e. C₃ algae or cyanobacteria) is noted for ADL and MJI oil samples in enriched isotope values of short (nC₁₃-nC₁₈) n-alkanes. The ADL, MJI and OKN petroleum samples obtained a negatively sloping n-alkane profile showing a lighter (more negative) carbon isotope ratio with an increased n-alkane chain length (Fig. 5). This negative slope fingerprint was defined as characteristic of deltaic and terrestrial oils (Murray AP et al., 1994; Samuel OJ et al., 2009; Cai CF et al., 2015). The flat parts of the n-alkane profile between C₁₃ to C₂₂ and C₂₄ to C₂₈ (Fig. 5) in OKN samples showed a significant marine incursion. OKN samples were therefore derived from mixed-origin source rocks (terrestrial and marine) deposited in the lacustrine-fluvial/deltaic setting, while a positive to almost flat n-alkane isotope profile is acquired from MJO oil samples with increased chain length (Fig. 5). Previously, almost flat n-alkane δ¹³C profiles were diagnosed with marine and lacustrine kerogen oils (Murray AP et al., 1994; Samuel OJ et al., 2009; Cai CF et al., 2015). The n-alkanes carbon isotopic values for oils range from –30.4‰ to –24.7‰ (Table 4), supporting mixed origin (terrigenous and marine) (Murray AP et al., 1994; Cai CF et al., 2015) for the oils. The oil samples are therefore obtained from the mixed origin deposited in the lacustrine-fluvial/deltaic settings. Pr and Ph δ¹³C isotope ratios range from –32.1‰ to –21.9‰ and –30.5‰ to –26.9‰, respectively (Table 4). The oils’ δ¹³C-values of Pr and Ph demonstrate a significant variety suggesting that the oils were formed from mixed organic materials (Schwas VF and Spangenberg JE, 2007; Cai CF et al., 2015). These findings are consistent with earlier reports on the origin and depositional environment of Niger Delta crude oils based on biomarkers distributions (Eneogwe C and Ekundayo O, 2003; Sonibare OO et al., 2008; Samuel OJ et al., 2009; Ogbesejana AB, 2018; Ogbesejana AB et al., 2017, 2018b, 2019b, 2019c).

Figure  5.  Carbon isotopic distribution of individual n-alkanes in Niger Delta crude oils (after Murray AP et al., 1994).

DownLoad: Full-Size Img PowerPoint

4.4   Oil-to-oil correlation based on carbon isotopic compositions

The cross plots of δ¹³C_aro against δ¹³C_sat and n-alkane profile for the Niger Delta crude oils are shown in Fig. 3 and Fig. 5. It was observed that, oils from the same fields grouped together indicating similar origin (Fig. 3). However, on the n-alkanes profile (Fig. 5), the oils were delineated into three families. Oils from ADL and MJI oilfields (the most depleted and terrigenous oil samples) grouped together, suggesting that these oils have received similar organic materials. Oils from OKN and MJO oilfields were clearly separated from each other. It should be noted that oils from MJO field are more enriched in carbon isotopic composition than OKN oil samples, indicating that MJO oil samples were formed predominantly from marine organic matter while OKN oils were formed from the mixed origin (terrestrial and marine).

5.   Conclusion

This study investigated the origin and depositional environments of the crude oils and source rocks from Niger Delta basin, Nigeria by elemental analysis-isotope ratio mass spectrometry (EA-IRMS) and gas chromatography-isotope ratio mass spectrometry (GC-IRMS). The isotopic values recorded for the samples indicated that the crude oils and source rocks were formed from the mixture of organic matter (terrestrial and marine) deposited in lacustrine-fluvial/deltaic environments. This finding is consistent with earlier reports on the origin and depositional environment of Niger Delta crude oils and source rocks based on biomarkers distributions. This study showed that the stable carbon isotopic compositions can be used to assess the origin and depositional environments of crude oils and source rocks in the Niger Delta Basin.

CRediT authorship contribution statement

Abiodun B Ogbesejana, Oluwasesan M Bello, and Tijjani Ali conceived and planned the experiments. Abiodun B Ogbesejana contributed to sample preparation and carried out the experiments. Abiodun B Ogbesejana, Oluwasesan M Bello and Tijjani Ali interpreted the results. Abiodun B Ogbesejana took the lead in writing the manuscript. All authors provided critical feedback and helped shape the research, analysis and manuscript.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgment

The authors thank Nigeria’s Department of Petroleum Resources and Chevron Nigeria Limited for supplying samples of source rocks and crude oils. Abiodun B Ogbesejana appreciates the help of Cheng Quan and Zhao Jiang in the laboratory works. The authors gratefully acknowledge the State Key Laboratory of Petroleum Resources and Prospecting, College of Geosciences, China University of Petroleum, Beijing, China for granting Abiodun B Ogbesejana an international visiting research fellowship towards this research work.