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Using aromatic biological markers as a tool for assessing thermal maturity of source rocks in the Campano-Maastrichtian Mamu Formation, Southeastern Nigeria.

INTRODUCTION

Aromatic biomarkers and their structural isomers have been found useful for the maturity assessment of oils, source rocks and coals (Radke et al, 1982a; Radke and Welte, 1983; Alexander et al, 1985; Radke et al., 1986; Garrigues et al, 1988; Strachan et al, 1988; Akaegbobi et al, 2000; Akinlua et al, 2007; Sonibare et al, 2008). Complex mixtures of aromatic hydrocarbons have been identified in crude oils (Cumbers et al, 1986), organic extracts of coals (Radke et al., 1982a, b; Puttman and Villar, 1987) and ancient sediments (Alexander et al, 1985; Garrigues et al, 1988, 1990). These complex mixtures of polyaromatic hydrocarbons (PAHs) are usually not generated through biosynthetic processes by living organisms in the sediments. However, some living organisms may synthesize benzoyl-derivatives in large quantities (Akaegbobi et al., 2000).

Aromatic compounds found in crude oils and ancient sediments are believed to have been derived from modification through degradation of biologically produced compounds such as steroids and terpenoids (Hall et al., 1984; Strachan et al, 1988). They also observed that steroids give rise to substituted phenanthrenes, and terpenoids produce alkylnaphthalenes. Other potential sources of alkylnaphthalene include the thermal degradation of spores, coals, and sporopollenin (Allan and Larter, 1981), and of kerogens and cyclic sesquiterpenoids present in resinous components (Anderson et al, 1992). For the alkylated phenanthrenes, the terpenoids derived from higher plant constituents such as resins and waxes seem to be one major source. Radke et al. (1982a) suggested that direct methylation of phenanthrene during catagenesis could be a substantial source of alkyl phenanthrene. Furthermore, the conversion of higher-plant triterpenoids in sediments into aromatic hydrocarbons is through the release of oxygen functional groups and subsequent aromatization of the rings (Johns, 1986; Radke, 1987; Rullkotter et al, 1994; Akaegbobi et al, 2000). The ultimate products of such processes are therefore tetracyclic and pentacyclic aromatic hydrocarbons. Through cleavages of bonds within the rings coupled with photochemical and acid catalyzed processes during diagenesis/catagenesis, these tetracyclic and pentacyclic aromatic hydrocarbons could be broken into methyl-substituted naphthalenes and phenanthrenes, respectively. Therefore, relative variations in the concentrations of akylnaphthalenes and alkylphenanthrenes in the sediments could supply information on the maturity of organic matter, organic facies input and depositional environment (Seifert and Moldowan, 1978, 1986; Radke et al., 1982a, b; Radke, 1988).

The aim of this paper is to examine the ratios of aromatic compounds in coal and shale extracts from the Maastrichtian Mamu Formation in the Anambra Basin, southeastern Nigeria, and to assess the organic matter input and thermal maturity for the source rocks.

GEOLOGICAL SETTING

The Anambra Basin, constituting the southwestern portion of the Benue Trough (Fig. 1), is a roughly triangular-shaped sedimentary depression covering an area of approximately 40,000 km2 (Akaegbobi et al., 2000). The present study area lies within latitudes 6[degrees]20/ to 7[degrees]55/ N and longitudes 7[degrees]10/ to 8[degrees]00/ E (Fig. 2). The Anambra Basin was initiated in the Early Cretaceous with the formation of the Benue-Abakaliki Trough as a failed arm of a rift triple junction associated with the separation of the African and South American continents and the subsequent opening of the South Atlantic Ocean (Murat, 1972).The NE-SW trending Benue-Abakaliki Trough is believed to be the result of a pre-Albian rifting of the African Shield prior to the opening of the South Atlantic Ocean (Uzuakpunwa, 1974). The movements were reactivated by further plate activity in the Lower Tertiary soon after the intermittent Upper Cretaceous rifting (Petters, 1978). The Anambra Basin contains over 6 km thick Cretaceous/Tertiary sediments and provides a structural link between the Cretaceous Benue Trough and the Tertiary Niger Delta (Whiteman, 1982; Onuoha, 1986; Mohammed, 2005).

The stratigraphic succession ranges from Late Santonian to Eocene (Fig. 3), with sediments of fluvial, deltaic, pro-delta, and shelf facies origins. Accumulation of sediments in the Anambra Basin commenced with the Campano-Maastrichtian marine and paralic shales of the Enugu and Nkporo Formations, along with the deltaic Owelli Sandstone equivalent. These basal units are overlain successively by the coal measures of the Mamu Formation (Lower Coal Measures), the Ajali Sandstone (Middle Coal Measures) and the Nsukka Formations (Upper Coal Measures).The Nsukka Formation, which overlies the Ajali Sandstone, begins with coarse- to medium-grained sandstones and passes upward into well-bedded blue clays, fine-grained sandstones, and carbonaceous shales with thin bands of limestone. The marine shales of the Imo Formation were deposited in the Paleocene, and overlain by the Ameki Formation (lateral equivalents of the tidal Nanka Sandstones) of Eocene age which constitutes the Tertiary succession (Petters and Ekweozor, 1982)(Fig. 3).

MATERIALS AND METHODS

Seven core samples consisting of coal (4) and shale (3) having total organic carbon (TOC) values ranging from 3.35 to 55.07 wt.% were carefully selected at various depth intervals (75-221 m) from five boreholes penetrating the Mamu Formation in the Anambra Basin in three Coal fields at Enugu, Ezimo and Ogboyoga (Fig. 2). Pulverized samples were exhaustively extracted by Soxhlet extraction using dichloromethane as solvent. All samples were prepared according to standard organic geochemical sample preparation techniques (Radke et al., 1980, 1984). Aliquots of the extracts were separated into saturated and aromatic hydrocarbons and heterocyclic compounds by liquid chromatography using a dual column system and a back-flushing technique as a preparatory step for biomarker analysis. The asphaltene fraction of the sample was recovered by precipitation in pentane overnight at room temperature. The pentane soluble portion of the sample (i.e., the maltenes) was further separated by liquid chromatography into saturated, aromatic and resin fractions using a large column containing silica gel with fractions quantified by gravimetric analysis. Aromatic hydrocarbons were eluted using 40 mL of 90:10 (v/v) pentane/dichloromethane mixtures. The aromatic fractions were diluted with 10 [micro]l of carbon disulfide (C[S.sub.2]) per mg of fraction and analyzed by Gas Chromatography-Mass Spectrometry using a Hewlett Packard 6890 GC/5973 MSD system (Agilent Technologies, California, U.S.A.) equipped with a high-resolution column (60 m, DB-1 phase, 0.2 mm i.d., 0.2 |im film). A constant flow of hydrogen carrier gas was used during the entire gas chromatographic run.The GC was temperature programmed from 80 [degrees]C (2 minutes hold) to 3200C at 3.5 0C/minute with a final hold time of 20 minutes. There was a 7.5-minute solvent delay on acquisition, and selected ion monitoring (SIM) was the standard method for analysis.

RESULTS

Biomarker Chemistry

The mass chromatograms of naphthalenes, phenanthrenes and akyl derivatives of representative coal and shale extracts from the Maastrichtian Mamu Formation in the Anambra Basin are shown in Figures 4a-b and 5a-b, respectively. The identification of the peaks labeled in Figures 4a-b and 5a-b are reported in Tables 1 and 2. Two methylnaphthalenes (MN), two ethylnaphthalenes (EN), nine dimethylnaphthalenes (DMN), ten trimethylnaphthalenes (TMN), eleven tetramethylnaphthalenes (TeMN) and two ethyl-methyl-naphthalenes (EMN) isomers were identified in the aromatic fractions of the coal and shale extracts (Figs. 4a-b). Among the tricyclic aromatic hydrocarbons, four methylphenanthrenes (MP), and fourteen dimethylphenanthrene isomers were identified (Figs. 5a-b). Dimethylnaphthalene was the most abundant among the naphthalene family (Fig. 6). All samples have similar distributions of phenanthrenes (m/z 178).

Dimethylphenanthrene was the least abundant of the phenanthrene homologues (Fig. 7). The distribution of total naphthalenes and phenanthrenes in the coal and shale extracts from the Mamu Formation indicates that the total sum of the phenanthrenes and their isomers was greater than that of the naphthalenes in all the samples analyzed (Fig.8). The distribution of these aromatic hydrocarbons and their akyl derivatives was strongly controlled by a selective expulsion mechanism and thermal maturation of organic matter (Akaegbobi et al., 2000). In the present study, the relative abundance of the phenanthrenes and their akyl derivatives was greater than that of the naphthalenes. Radke et al. (1982a) suggested that a substantial source of alkyl phenanthrene could be the direct methylation of phenanthrene during catagenesis. The results obtained in this study corroborate with those reported by Akinlua et al. (2007).These isomers were applied in the calculation of temperature-sensitive maturity parameters such as methylphenanthrene indices (MPI-1 and MPI-2), methyldibenzothiophene ratio (MDR), dimethylnaphthalene ratios (DNR-1 and DNR-2) and trimethylnaphthalene ratio (TNR-1).

DISCUSSION

Thermal maturity

The concentration of aromatic compounds, such as naphthalenes, phenanthrenes, dibenzothiophenes and their structural isomers are at present attracting increasing attention as maturity indicators for coals and shale source rocks. These indicators rely either on an increase in maturity with the degree of alkylation of a given parent compound, or a shift in the isomer distribution of alkylaromatic homologues increasing towards isomers with greater thermal stability (Radke, 1988). The aromatic isomer ratios and indexes (DNR-1, DNR-2, TNR-1, TDE-1, TDE-2, MPI-1, MPI-2, and MDR) used in this study are reported in Table 5.

The total concentration of naphthalene and its isomers is greater than that of the phenanthrenes. The higher abundances of naphthalenes relative to phenanthrenes can be attributed to selective expulsion mechanisms and direct methylation during diagenesis/catagenesis (Akaegbobi et al, 2000; Radke et al, 1982a). The increase in the concentrations of naphthalenes along with an increase in the number of alkyl substitutes indicates that the oils are mature. This increase in the number of methyl substituents of naphthalene in mature oils occurs because of the thermal rearrangement of methylnaphthalene with increasing thermal stress (Akaegbobi et al., 2000). Furthermore, the coal and shale extracts contain enhanced concentrations of 1, 2, 5-trimethylnaphthalene relative to 1, 2, 7-trimethylnaphthalene (Table 4). This is typical of mature oils (Strachan et al., 1988; Akinlua et al., 2007). However, in the present study, the relative abundance of the total of phenanthrene and its isomers was greater than that of naphthalenes (Table 3, Fig.9). The abundance of phenanthrenes relative to naphthalenes suggests that the rock extracts are mature (Akinlua et al., 2007).

A number of aromatic maturity parameters based on the distribution of naphthalene, phenanthrene and alkyl isomers were calculated for the study samples after Radke et al. (1982a), Radke et al. (1986) and Alexander et al. (1985, 1986). Radke (1988) observed that aromatic hydrocarbons do change in a regular fashion with increasing maturity and thus developed maturity parameters based on the distributions of alkylnaphthalene and alkylphenanthrene.The maturity parameters computed from the aromatic biomarker distributions in the coal and shale extracts are listed in Table 5. The maturity parameters computed from naphthalene and its isomers, the dimethylnaphthalene ratios (DNR-1 and DNR-2) and trimethylnaphthalene ratio (TNR-1) ranged from 2.04 to 6.03, 1.21 to 3.37 and 0.73 to 1.22, respectively (Table 5).

The methylphenanthrene index (MPI-1) is one of the most widely used maturity parameters based on aromatic hydrocarbons (Radke et al., 1982a, 1982b; Radke, 1988). The parameter relies on a shift with maturity in the methylphenanthrene distribution towards a preponderance of P-type isomers. The methylphenanthrene indices (MPI-1 and MPI-2) were computed based on the relative concentrations of the 7-MP, 2-MP, 2-MP, 9-MP and phenanthrene (P). This was based on the assumption that the 2-MP and 2-MP are derived from the rearrangement of the 7-MP and 9-MP, and from phenanthrene (P) through methylation reactions. The relative abundances of the alkyl homologs of the aromatic hydrocarbons (phenanthrenes) were used to calculate the vitrinite reflectance (% [R.sub.c]) after the method of Radke et al, (1982a, b) and Radke (1988). The MPI-1 is often used for estimating the equivalent vitrinite reflectance values (% [R.sub.c]) for crude oils and source rocks. The MPI-1 and % [R.sub.c] values calculated for the coal and shale extracts range from 0.10 to 0.23 and 0.43 to 0.51, respectively (Table 5). MPI-1 calculated reflectance (%Rc) was carried out because of insufficient and good reflecting vitrinite in the samples.

Maturity parameters based on the isomers of the methyldibenzothiophene (MDBT) were calculated (Radke et al., 1986). This ratio (MDR) generally increases with increasing maturity and correlates well with vitrinite reflectance and the Rock-Eval [T.sub.max] of source rock (Radke, 1988). The ratio is also useful in calculating mean vitrinite reflectance (% [R.sub.m]). The methyldibenzothiophene ratio (MDR) and mean vitrinite reflectance (%[R.sub.m]) for the coal and shale extracts ranged from 0.40 to 6.56 and 0.51 to 1.43, respectively (Table 5). The high MDR exhibited by some of the samples may be a result of the effect of heating rates or thermal stress (Akaegbobi et al, 2000). The % [R.sub.m] values show that the coal and shale extracts range from immature to mature (Table 5).

Monoaromatic and triaromatic steroids were detected in all samples in the m/z 253 and m/z 231 fragmentograms (Figs. 10a-b). Both 20R and 20S homologues of [C.sub.26]-[C.sub.28], [C.sub.20] and [C.sub.21] triaromatic compounds are present (Figs. 10a-b).

Peters and Moldowan (1993) and El-Gayar (2005) suggested that the extent of cracking in the side chains of triaromatic steroids can be used to provide information about petroleum maturity. The triaromatic steroid maturity parameters [C.sub.20]/[C.sub.20]+[C.sub.28] (20R) and ([C.sub.20]+[C.sub.21])/ ([C.sub.20]+[C.sub.21]+[C.sub.26] to [C.sub.28] 20R and 20S) were used to evaluate the thermal maturity of the coal and shale extracts in this study (Peters and Moldowan, 1993; Peters et al, 2005). The maturity parameters computed from the peak areas of the m/z 231 ranged from 0.08 to 0.72 and 0.09 to 0.52, respectively (Table 5). These values indicate an immature to marginally mature status of the source rock.

The monoaromatic steroid ratios (MAS Dia/Reg [C.sub.27] and MAS [C.sub.21]+[C.sub.22]/all MAS) were measured from the m/z 253 fragmentograms (Table 5) and used to evaluate the source-rock depositional environment and thermal maturity (Peters and Moldowan, 1993). Both of these ratios increase with higher thermal maturity (Mackenzi et al, 1981; Moldowan and Fago, 1986). The MAS Dia/Reg [C.sub.27] sterane ratio ranges from 0.69 to 2.19 (Table 5). The coal and shale extracts in the Mamu Formation contain significant amounts of diasteranes, which appear to form through the interaction of steranes with clay mineral surfaces in source rocks (Rubinstein et al, 1975; Sieskind et al, 1979). Thus, the presence of significant amounts of diasteranes relative to regular steranes in oils has been used as evidence of petroleum generation from argillaceous source rocks, whereas low concentrations are considered an indication of a source rock lacking in clay minerals (Hughes, 1984; Mello et al, 1988a, 1988b; Hill et al, 2007). The calculated monoaromatic maturity ratio (MAS [C.sub.21]+[C.sub.22]/ all MAS) ranges from 0.04 to 0.05 (Table 5).

Thermal maturity was also evaluated on the basis of monoaromatic and triaromatic steroids because they are more resistant to the effects of biodegradation than alkane-type biological markers (Volkman et al., 1983). According to Tissot and Welte (1984), aromatization of C-ring steroid hydrocarbons occurs in mature oils. With an increase in thermal maturation, there is a conversion of C-ring monoaromatic steroid hydrocarbons to ABC-ring triaromatic steroid hydrocarbons (Akinlua and Ajayi, 2009). The TAS/ (TAS+MAS) and TAS 28/ (TAS 28+MAS 29) ratios ranged from 0.22 to 0.59 and 0.39 to 0.88, respectively (Table 5), indicating an immature to marginally mature status of the rock extracts.

Source of organic matter

The trimethylnaphthalene is dominated by 7, 2, 5- TMN in all the samples analyzed (Figs. 4a-b and Table 4). The high abundance of the naphthalenes, especially the dimethyl- and trimethyl-isomers, indicates organic matter derived mainly from higher plants (Strachan et al., 1988). High concentrations of 1, 2, 5- and 1, 2, 7- trimethylnaphthalene (TMN) in oils have been attributed to aromatization and cleavage of the ring-C of [beta]-amyrin in angiosperms (Strachan et al, 1988). 1, 2, 5- and 1, 2, 7- trimethylnaphthalenes can form as diagenetic products of oleanane-type triterpenoids (Chaffee and Johns, 1983; Chaffee et al, 1984; Sonibare et al, 2008). The presence of 1, 2, 5-TMN in the studied samples could be linked to aromatization of oleanane, as indicated by the absence of appreciable amounts of oleanane in the m/z 191 fragmentograms. This compound can also be formed from gymnosperm resins (van Aarssen et al, 1999) or from hopanoid precursors (Villar et al, 1988). Among the tetramethylnaphthalene family, 1, 2, 5, 6- tetramethylnaphthalene is the most abundant (Figs. 4a-b).

Strachan et al. (1986) used two related ratios (TDE-1 and TDE-2) of trimethylnaphthalenes to differentiate coal swamps from marine, lacustrine, and deltaic environments. 1, 2, 7- trimethylnaphthalene (TMN) has been used as a marker of angiosperm input. 1, 2, 5-trimethylnaphthalene and 1, 2, 7- trimethylnaphthalene can form as diagenetic products of oleanane-type triterpenoids (Chaffee and Johns, 1983; Chaffee et al, 1984). Strachan et al. (1988) compared the relative abundance of trimethylnaphthalene isomers in Southeast Asian samples ranging in age from Permian to Tertiary. They found that the relative concentrations of the 1, 2, 7-trimethylnaphthalenes were appreciably higher in most oils derived from post-Cretaceous source rocks dominated by the input of higher-plants, and in extracts from Cretaceous and younger samples, than in samples of older age or with little or no terrigenous plant input. Oils of this type yielded ratios of 1, 2, 7-/1, 3, 7-trimethylnaphthalenes from 0.46 to 1.36, while oils from older source rocks containing higher-plant material yielded ratios from 0.34 to 0.16, and oils from marine source rocks yielded ratios from 0.15 to 0.32. In this study, the rock extracts yielded ratios of 1,2, 7-/1,3, 7-trimethylnaphthalenes ranging from 0.40 to 1.07 (Table 5). The computed TDE-1 and TDE-2 ratios for the study samples range from 4.41-15.21 and 0.20-0.53, respectively (Table 5). The presence of 1, 2, 5- and 1, 2, 7-TMN in the rock extracts (Figs. 4a-b; Table 4) indicate both angiosperm and gymnosperm material contribution to the organic matter that formed the coals and shales (Killops and Killops, 2005; Adedosu et al, 2012). Furthermore, the occurrence of appreciable amounts of 1, 6- and 2, 6-DMNs in all the samples indicate terrestrial organic matter input (Achari et al, 1973). Additionally, the occurrence of 1, 7-DMP in the analyzed samples (Figs. 5a-b) is attributed to terrestrial organic matter (Simoneit et al, 1986), while plots of [C.sub.27] [C.sub.28] and [C.sub.29] monoaromatic steranes (Fig. 11) indicate that the kerogens were derived from mixed terrestrial and marine organic matter.

The distribution of the aromatic sulfur compounds including dibenzothiophenes (DBTs) and their methyl homologues in the coal and shale extracts are reported in Figures 12a-b. The dibenzothiophene/phenanthrene (DBT/PHEN) ratio together with the Pr/Ph ratio can be used to infer source rock depositional environments and lithologies (Hughes et al, 1995). High DBT/ PHEN ratios are found in oils from marine carbonates, and low ratios are found in oils from marine shales and most lacustrine rocks (Petersen et al, 2007). The coal and shale extracts (Table 5) have low DBT/PHEN ratios (0.01-0.06), indicating a strong input of terrigenous Type III organic matter. However, DBT/PHEN (Table 5) and Pr/Ph ratios only reflect the Eh and pH irrespective of depositional environment, and a cross-plot of the two ratios by itself cannot be used to distinguish lacustrine from marine oils (Petersen et al., 2007). The presence of organic sulfur compounds and sulfur-containing aromatics in rock extracts and crude oils indicates an anoxic depositional environment (Sinninghe Damste et al, 1988, 1989a, 1989b; Hughes et al, 1995). 1-methyldibenzothiophene (1-MDBT), 2+3-methyldibenzothiophene (2+3-MDBT) and 4-methyldibenzothiophene (4MDBT) were detected in all the samples analyzed (Figs. 12a-b; Table 6). The high relative abundance of dibenzothiophenes (DBTs) in all of the samples is indicative of probable oxic/anoxic conditions prevailing during deposition (Table 6). The distribution of DBTs has been proposed as an indicator of crude oils derived from carbonate versus siliciclastic sources (Hughes, 1984; Hegazi and El-Gayar, 2009). The presence of a definite V pattern (4-methyl > 2+3-methyl < 1-methyl) in the methyldibenzothiophenes is generally associated with oils from predominantly carbonate source rocks, while a stair-step pattern (4-methyl > 2+3-methyl > 1-methyl) is associated with predominantly siliciclastic source rocks, or oils of advanced maturity (late- to post-oil window) from carbonate sources (Hegazi and El-Gayar, 2009). Furthermore, oils from carbonate sources are characterized by an abundance of benzothiophenes (BTs) and a fairly equal distribution of substituted DBTs, while siliciclastic oils exhibit low concentrations of BTs and decreasing amounts of dimethyldibenzothiophenes and trimethyldibenzothiophenes relative to methyldibenzothiophenes (Hughes, 1984; Hegazi and El-Gayar, 2009). The results obtained in this study indicate that the 4-methyldibenzothiophenes (24.0%) are more abundant than the 2+3-methyldibenzothiophenes (20.4%) and 1-methyldibenzothiophenes (4.4%) (Table 6), indicating a siliciclastic source rock input for the organic matter (Hegazi and El-Gayar, 2009).

The mass chromatograms (m/z 184 and m/z 198) of the extracts representing the DBTs in this study show a high concentration of DBTs (51.2%) and decreasing amounts of dimethyldibenzothiophenes (24.0%) and trimethyldibenzothiophene (20.4%) relative to methyldibenzothiophenes (4.4%) (Figs. 12a-b and Table 6). These results are consistent with previous analyses that indicate that the coal and shale extracts in this study were generated by a siliciclastic source rock (Hegazi and El-Gayar, 2009).

Benzohopanes are formed by the cyclization of extended hopanoid side chains followed by aromatization (Hussler et al, 1984a, 1984b). Benzohopanes range in carbon number from [C.sub.32] to [C.sub.35]. He and Lu (1990) observed that oils and bitumens from evaporitic and carbonate source rocks have the highest concentrations of benzohopanes, which occur in trace amounts in most source rocks and crude oils. Benzohopanes were identified in all the rock extracts, but concentrations are low (Figs. 13a-b). All samples have similar distributions of benzohopanes (m/z 191). The distribution of benzohopanes (Figs. 13a-b) shows that [C.sub.32] benzohopane is the most abundant member of the bezohopane family. The relative concentrations of the various members of the aromatic hopane series seem to follow that of the regular hopanes in the saturated hydrocarbon fraction, and like the saturated hopanes, they most likely have a bacterial origin (Petersen et al., 2007).

CONCLUSION

The results of this study indicated that the aromatic hydrocarbons contained in the coal and shale samples from the Campano-Maastrichtian Mamu Formation of the Anambra Basin are marginally mature. The kerogens were formed from organic matter of mixed origin (terrestrial and marine). The distributions/concentrations of naphthalenes and phenanthrenes show that trimethylnaphthalene is the most abundant member of the naphthalene family and methylphenanthrene is the most abundant phenanthrene family member. All samples have similar distributions of phenanthrenes. The relative abundance of the total of phenanthrene and its isomers was greater than that of the naphthalenes. Thus, the distributions of aromatic hydrocarbons in the rock extracts are influenced by methylation reactions at elevated maturity. The slightly higher concentrations of [C.sub.29] monoaromatic steranes (49%), compared with [C.sub.28] (22%) and [C.sub.27] (29%) monoaromatic steranes, suggests a mixed input of terrestrial and marine organic matter.

ACKNOWLEDGEMENT

We are grateful to Daniel M. Jarvie and the staff of Humble Geochemical Laboratory, Texas, USA, for the assistance rendered during the geochemical analyses. We gratefully acknowledge the Nigerian Geological Survey Agency (NGSA) for providing the core samples used in this study, and the anonymous reviewers for their helpful comments.

Record

Manuscript received: 06/11/2012

Accepted for publication: 29/04/2014

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(1.) Jude E Ogala, Delta State University, Nigeria. Email: etunimogala@yahoo.com

(2.) Mike I Akaegbobi, University of Ibadan, Nigeria. Email: izumike20022002@yahoo.com

Table 1: Peak identification in the m/z 156,
170, 184, 198, 154, 168, 178 and 192 mass
chromatograms

Ion   Peak Label   Compound Name

128   NAPH         Naphthalene
142   2MN          2-Methylnaphthalene
142   1MN          1-Methylnaphthalene
156   2EN          2-Ethylnaphthalene
156   1EN          1-Ethylnaphthalene
156   26DMN        2,6-Dimethylnaphthalene
156   27DMN        2,7-Dimethylnaphthalene
156   13+17DMN     1,3 & 1,7-Dimethylnaphthalenes
156   16DMN        1,6-Dimethylnaphthalene
156   23DMN        2,3-Dimethylnaphthalene
156   14DMN        1,4-Dimethylnaphthalene
156   15DMN        1,5-Dimethylnaphthalene
156   12DMN        1,2-Dimethylnaphthalene
170   BB_EMN       Ethyl-methyl-Naphthalene
170   AB_EMN       Ethyl-methyl-Naphthalene
170   137TMN       1,3,7-Trimethylnaphthalene
170   136TMN       1,3,6-Trimethylnaphthalene
170   146+135T     (1,4,6+1,3,5)-Trimethylnaphthalenes
170   236TMN       2,3,6-Trimethylnaphthalene
170   127TMN       1,2,7-Trimethylnaphthalene
170   167+126T     (1,6,7+1,2,6)-Trimethylnaphthalenes
170   124TMN       1,2,4-Trimethylnaphthalene
170   125TMN       1,2,5-Trimethylnaphthalene
184   1357         1,3,5,7-Tetramethylnaphthalene
184   1367         1,3,6,7-Tetramethylnaphthalene
184   1247         (1,2,4,7+1,2,4,6+1,4,6,7)-TetraMNs
184   1257         1,2,5,7-Tetramethylnaphthalene
184   2367         2,3,6,7-Tetramethylnaphthalene
184   1267         1,2,6,7-Tetramethylnaphthalene
184   1237         1,2,3,7-Tetramethylnaphthalene
184   1236         1,2,3,6-Tetramethylnaphthalene
184   1256         1,2,5,6-Tetramethylnaphthalene
184   DBT          Dibenzothiophene
198   4MDBT        4 Methyl Dibenzothiophene
198   2+3MDBT      2 & 3 Methyl Dibenzothiophenes
198   1MDBT        1 Methyl Dibenzothiophene
154   BP           Biphenyl
168   2MBP         2-Methylbiphenyl
168   DPM          Diphenylmethane
168   3MBP         3-Methylbiphenyl
168   4MBP         4-Methylbiphenyl
168   DBF          Dibenzofuran
178   PHEN         Phenanthrene
192   3MP          3-Methylphenanthrene
192   2MP          2-Methylphenanthrene
192   9MP          9-Methylphenanthrene
192   1MP          1-Methylphenanthrene

Table 2: Peak identification in the m/z 206, m/z 191,
m/z 231 and m/z 253 mass chromatograms

Ion   Peak Label   Compound Name

206   36DMP        3,6-Dimethylphenanthrene
206   26DMP        2,6-Dimethylphenanthrene
206   27DMP        2,7-Dimethylphenanthrene
206   39DMP        (3,9+3,10+2,10+1,3)-Dimethylphenanthrenes
206   29DMP        (2,9+1,6)-Dimethylphenanthrenes
206   17DMP        1,7-Dimethylphenanthrene
206   23DMP        2,3-Dimethylphenanthrene
206   19DMP        1,9-Dimethylphenanthrene
206   18DMP        1,8-Dimethylphenanthrene
206   12DMP        1,2-Dimethylphenanthrene
191   BH32         [C.sub.32] Benzohopane
191   BH33         [C.sub.33] Benzohopane
191   BH34         [C.sub.34] Benzohopane
191   BH35         [C.sub.35] Benzohopane
231   1TA20        [C.sub.20] Triaromatic Steroid
231   2TA21        [C.sub.21] Triaromatic
231   3TA26        [C.sub.26] 20S Triaromatic 26
231   4TA26+27     [C.sub.27] 20S & C26 20R Triaromatic
231   5TA28        [C.sub.28] 20S Triaromatic
231   6TA27        [C.sub.27] 20R Triaromatic
231   7TA28        [C.sub.28] 20R Triaromatic
231   8TA29        [C.sub.29] 20R Triaromatic (24 n-propyl)
253   1MA21        [C.sub.21] Ring-C Monoaromatic Steroid
253   2MA22        [C.sub.22] Monoaromatic steroid
253   3MA27        [C.sub.27] Reg 5b(H),10b(CH3) 20S
253   4MA27        [C.sub.27] Dia 10b(H),5b(CH3) 20S
253   5MA27        [C.sub.27] Dia10bH,5bCH3 20R+Reg5bH,10bCH3 20R
253   6MA27        [C.sub.27] Reg 5a(H),10b(CH3) 20S
253   7MA28        [C.sub.28] Dia 10aH,5aCH3 20s+Reg5bH,10bCH3 20S
253   8MA27        [C.sub.27] Reg 5a(H),10b(CH3) 20R
253   9MA28        [C.sub.28] Reg 5a(H),10b(CH3) 20S
253   10MA28       [C.sub.28] Dia 10aH,5aCH3 20R+Reg5bH,10bCH3 20R
253   11MA29       [C.sub.27] Dia 10bH,5bCH3 20S+Reg5bH,10bCH3 20S
253   12MA29       [C.sub.29] Reg 5a(H),10b(CH3) 20S
253   13MA28       [C.sub.28] Reg 5a(H),10b(CH3) 20R
253   14MA29       [C.sub.29] Dia 10bH,5bCH3 20R+Reg5bH,10bCH3 20R
253   15MA29       [C.sub.29] Reg 5a(H),10b(CH3) 20R

Table 3: Abundance of aromatic hydrocarbons in rock
extracts (concentrations are in ppm)

Sample Number   Lithology   N        MN       EN

OG/1267/03      Shale       ND       16.70    9.40
ENG/1001/02     Shale       ND       759.50   153.10
ENG/1001/12     Coal        358.50   505.00   20.90
ENG/1001/16     Coal        ND       14.80    11.10
ENG/1002/06     Shale       ND       21.30    22.5.00
ENG/1008/13     Coal        39.50    402.20   25.00
EZ/1219/08      Coal        372.50   793.00   62.20

Sample Number   Lithology   DMN       EMN      TMN

OG/1267/03      Shale       532.90    65.60    1077.30
ENG/1001/02     Shale       8496.00   292.60   3016.60
ENG/1001/12     Coal        859.60    27.40    301.00
ENG/1001/16     Coal        858.20    86.80    1703.10
ENG/1002/06     Shale       1479.40   230.90   2852.50
ENG/1008/13     Coal        1175.80   49.30    581.90
EZ/1219/08      Coal        1991.50   76.30    1157.80

Sample Number   Lithology   TeMN      P           MP

OG/1267/03      Shale       499.80    293865.20   52542.10
ENG/1001/02     Shale       583.70    5554.50     2403.90
ENG/1001/12     Coal        357.30    1280.10     1157.40
ENG/1001/16     Coal        1053.80   14007.80    8875.70
ENG/1002/06     Shale       657.60    33841.70    3946.90
ENG/1008/13     Coal        287.20    2397.80     686.10
EZ/1219/08      Coal        526.30    63560.40    7983.40

Sample Number   Lithology   DMP        T-Naph     T-Phen

OG/1267/03      Shale       15581.00   2201.70    361988.30
ENG/1001/02     Shale       2799.50    13301.50   10757.90
ENG/1001/12     Coal        1275.00    2429.70    3712.50
ENG/1001/16     Coal        3444.00    3727.80    26327.50
ENG/1002/06     Shale       1240.40    5264.20    39029.00
ENG/1008/13     Coal        889.70     2560.90    3973.60
EZ/1219/08      Coal        2703.20    4979.60    74247.00

Abbreviations: N--naphthalene, MN--methylnaphthalene,
EN--ethylnaphthalene, DMN--dimethylnaphthalene,
EMN--ethylmethylnaphthalene,
TMN--trimethylnaphthalene, TeMN--tetramethylnaphthalene,
P--Phenanthrene, MP--methylphenenthrene,
DMP--dimethylphenenthrene, T-Naph--total naphthalene,
T-Phen--total phenanthrene, ND - not detected

Table 4: Abundance of isomers of trimethylnaphthalenes
and methylphenanthrenes in rock extracts (concentrations
are in ppm)

Sample Number   Depth (m)   Lithology   2-MP       3-MP

OG/1267/03      75-76       Shale       19203.20   13913.70
ENG/1001/02     157         Shale       519.90     369.80
ENG/1001/12     186-187     Coal        144.90     184.80
ENG/1001/16     205         Coal        1797.30    1065.80
ENG/1002/06     167-178     Shale       1446.70    1079.20
ENG/1008/13     220-221     Coal        144.50     120.40
EZ/1219/08      80-82       Coal        2567.10    1784.30

Sample Number   Depth (m)   1-MP       9-MP      125TMN

OG/1267/03      75-76       10818.90   8606.30   173.50
ENG/1001/02     157         595.40     918.80    389.90
ENG/1001/12     186-187     179.00     648.70    34.90
ENG/1001/16     205         2640.50    3372.10   418.20
ENG/1002/06     167-178     701.20     719.80    325.90
ENG/1008/13     220-221     108.90     312.30    62.50
EZ/1219/08      80-82       2113.60    1518.40   166.80

Sample Number   Depth (m)   127TMN

OG/1267/03      75-76       44.20
ENG/1001/02     157         159.40
ENG/1001/12     186-187     16.50
ENG/1001/16     205         144.80
ENG/1002/06     167-178     176.50
ENG/1008/13     220-221     30.60
EZ/1219/08      80-82       50.20

Abbreviations: 1-MP--1-methylphenanthrene,
2-MP--2-methylphenanthrene, 3-MP--3- methylphenanthrene,
9-MP--9- methylphenanthrene, 125TMN--1, 2,
5-trimethylnaphthalene, 127TMN--1, 2, 7-trimethylnaphthalene

Table 5: Thermal maturity parameters computed from the aromatic
biomarkers and aromatic sulphur compounds in the coal and shale
extracts

Interpretative Ratios         OG/1267/03   ENG/1001/02   ENG/1001/12

Naphthalenes DNR-1=           2.08         3.93          4.13
  (2,6+2,7)/
  (1,5)DMN
DNR-2=(2,6+2,7)/(1,4+2,3)     1.21         2.52          3.37
  DMN
TNR1=(2,3,6)/(1,4,6+1,3,5)    1.12         0.86          0.73
  TMN
TDE-1=1,2,5-TMN/1,2,4-TMN     10.43        10.81         4.41
TDE-2=1,2,7-TMN/1,2,6-TMN     0.20         0.30          0.33
1,2,7-TMN/1,3,7-TMN           0.45         0.46          0.44
Phenanthrenes MPI-2=3(2MP)/   0.18         0.22          0.21
  (PHEN+1MP+9MP)
MPI-1=1.5(2MP+3MP)/           0.16         0.19          0.23
  (PHEN+1MP+9MP)
Rc(a)=0.6(MPI-1)+0.37         0.47         0.48          0.51
  (for Ro<1.3)
Rc(b)=0.6(MPI-1)+2.3 (for     2.40         2.41          2.44
  Ro>1.3)
MDR=4MDBT/1MDBT               6.56         1.69          0.40
Rm=0.40+0.30(MDR)-0.094       1.43         0.69          0.51
  [(MDR).sup.2+]0.011
  [(MDR).sup.3]
MDR23=23MDBT/DBT              0.42         0.31          0.65
MDR1=1MDBT/DBT                0.07         0.23          0.66
DBT/Phenanthrene              0.06         0.02          0.04
Triaromatic Steroids TAS      0.52         0.13          0.06
  [C.sub.20]+[C.sub.21]/all
  TAS (20R and 20S)
TAS [C.sub.20]/[C.sub.20]+    0.70         0.22          0.12
  [C.sub.27]
TAS [C.sub.20]/[C.sub.20]+    0.72         0.26          0.08
  [C.sub.28] (20R)
TAS [C.sub.21]/[C.sub.21]+    0.15         0.08          0.04
  [C.sub.28]
TAS [C.sub.28]/[C.sub.26]     3.62         3.89          12.71
  (20S)
TAS [C.sub.28]/[C.sub.27]     2.12         2.16          3.96
  (20R)
% [C.sub.26] TAS              5.30         5.70          2.30
% [C.sub.27] TAS              36.70        35.90         23.70
% [C.sub.28] TAS              52.80        50.70         68.90
% [C.sub.29] TAS              5.20         7.60          5.10
Monoaromatic Steroids MAS     0.00         0.69          0.00
  Dia/Reg [C.sub.27]
MAS [C.sub.21]+[C.sub.22]/    0.05         0.04          0.04
  all MAS
% [C.sub.27] MA               50.10        43.00         11.00
% [C.sub.28] MA               29.30        21.70         14.50
% [C.sub.29] MA               20.60        35.20         74.50
TAS/(TAS+MAS)                 0.22         0.38          0.59
TAS [C.sub.28]/(TA            0.58         0.79          0.87
  [C.sub.28]+MA [C.sub.29])

Interpretative Ratios         ENG/1001/16   ENG/1002/06   ENG/1008/13

Naphthalenes DNR-1=           2.04          2.71          6.03
  (2,6+2,7)/
  (1,5)DMN
DNR-2=(2,6+2,7)/(1,4+2,3)     1.35          1.72          2.93
  DMN
TNR1=(2,3,6)/(1,4,6+1,3,5)    0.75          0.95          0.87
  TMN
TDE-1=1,2,5-TMN/1,2,4-TMN     15.21         7.22          7.22
TDE-2=1,2,7-TMN/1,2,6-TMN     0.53          0.33          0.31
1,2,7-TMN/1,3,7-TMN           1.07          0.54          0.39
Phenanthrenes MPI-2=3(2MP)/   0.27          0.12          0.15
  (PHEN+1MP+9MP)
MPI-1=1.5(2MP+3MP)/           0.21          0.11          0.14
  (PHEN+1MP+9MP)
Rc(a)=0.6(MPI-1)+0.37         0.50          0.43          0.45
  (for Ro<1.3)
Rc(b)=0.6(MPI-1)+2.3 (for     2.43          2.36          2.38
  Ro>1.3)
MDR=4MDBT/1MDBT               0.72          3.13          0.88
Rm=0.40+0.30(MDR)-0.094       0.57          0.76          0.60
  [(MDR).sup.2+]0.011
  [(MDR).sup.3]
MDR23=23MDBT/DBT              0.32          0.21          0.75
MDR1=1MDBT/DBT                0.75          0.13          0.67
DBT/Phenanthrene              0.01          0.01          0.02
Triaromatic Steroids TAS      0.11          0.21          0.09
  [C.sub.20]+[C.sub.21]/all
  TAS (20R and 20S)
TAS [C.sub.20]/[C.sub.20]+    0.19          0.31          0.20
  [C.sub.27]
TAS [C.sub.20]/[C.sub.20]+    0.20          0.42          0.17
  [C.sub.28] (20R)
TAS [C.sub.21]/[C.sub.21]+    0.07          0.13          0.05
  [C.sub.28]
TAS [C.sub.28]/[C.sub.26]     4.04          2.09          17.11
  (20S)
TAS [C.sub.28]/[C.sub.27]     2.92          2.27          2.87
  (20R)
% [C.sub.26] TAS              6.70          8.20          2.30
% [C.sub.27] TAS              32.20         40.5          23.9
% [C.sub.28] TAS              57.70         42.70         70.10
% [C.sub.29] TAS              3.40          8.60          3.60
Monoaromatic Steroids MAS     1.21          0.91          1.17
  Dia/Reg [C.sub.27]
MAS [C.sub.21]+[C.sub.22]/    0.04          0.05          0.04
  all MAS
% [C.sub.27] MA               26.50         46.30         14.20
% [C.sub.28] MA               21.80         21.00         20.90
% [C.sub.29] MA               51.70         32.70         65.00
TAS/(TAS+MAS)                 0.46          0.28          0.57
TAS [C.sub.28]/(TA            0.87          0.69          0.88
  [C.sub.28]+MA [C.sub.29])

Interpretative Ratios         EZ/1219/08

Naphthalenes DNR-1=           4.93
  (2,6+2,7)/
  (1,5)DMN
DNR-2=(2,6+2,7)/(1,4+2,3)     1.91
  DMN
TNR1=(2,3,6)/(1,4,6+1,3,5)    1.22
  TMN
TDE-1=1,2,5-TMN/1,2,4-TMN     11.51
TDE-2=1,2,7-TMN/1,2,6-TMN     0.20
1,2,7-TMN/1,3,7-TMN           0.41
Phenanthrenes MPI-2=3(2MP)/   0.11
  (PHEN+1MP+9MP)
MPI-1=1.5(2MP+3MP)/           0.10
  (PHEN+1MP+9MP)
Rc(a)=0.6(MPI-1)+0.37         0.43
  (for Ro<1.3)
Rc(b)=0.6(MPI-1)+2.3 (for     2.36
  Ro>1.3)
MDR=4MDBT/1MDBT               0.62
Rm=0.40+0.30(MDR)-0.094       0.55
  [(MDR).sup.2+]0.011
  [(MDR).sup.3]
MDR23=23MDBT/DBT              0.06
MDR1=1MDBT/DBT                0.14
DBT/Phenanthrene              0.01
Triaromatic Steroids TAS      0.13
  [C.sub.20]+[C.sub.21]/all
  TAS (20R and 20S)
TAS [C.sub.20]/[C.sub.20]+    0.20
  [C.sub.27]
TAS [C.sub.20]/[C.sub.20]+    0.35
  [C.sub.28] (20R)
TAS [C.sub.21]/[C.sub.21]+    0.10
  [C.sub.28]
TAS [C.sub.28]/[C.sub.26]     5.6
  (20S)
TAS [C.sub.28]/[C.sub.27]     1.33
  (20R)
% [C.sub.26] TAS              4.70
% [C.sub.27] TAS              35.80
% [C.sub.28] TAS              43.70
% [C.sub.29] TAS              15.70
Monoaromatic Steroids MAS     2.19
  Dia/Reg [C.sub.27]
MAS [C.sub.21]+[C.sub.22]/    0.06
  all MAS
% [C.sub.27] MA               11.60
% [C.sub.28] MA               22.60
% [C.sub.29] MA               65.70
TAS/(TAS+MAS)                 0.23
TAS [C.sub.28]/(TA            0.39
  [C.sub.28]+MA [C.sub.29])

Table 6: Abundance of dibenzothiophenes and their alkyl
homologues in rock extracts (concentrations are in ppm)

Sample Number   Lithology   DBT        4MDBT     2+3MDBT   1MDBT

OG/1267/03      Shale       17410.70   8454.20   7230.60   1287.90
ENG/1001/02     Shale       106.20     40.80     32.60     24.10
ENG/1001/12     Coal        48.80      13.10     32.00     32.40
ENG/1001/16     Coal        137.00     74.40     43.80     103.40
ENG/1002/06     Shale       281.80     116.50    59.00     37.20
ENG/1008/13     Coal        49.00      28.80     37.00     32.80
EZ/1219/08      Coal        726.40     63.50     41.60     102.90

Abbreviations: DBT--dibenzothiophene,
4MDBT--4-methyldibenzothiophene, 2+3MDBT--2,
3--methyldibenzothiophene, 1--methyldibenzothiophene
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Title Annotation:PETROLEUM GEOLOGY
Author:Ogala, Jude E.; Akaegbobi, Mike I.
Publication:Earth Sciences Research Journal
Article Type:Report
Geographic Code:6NIGR
Date:Jun 1, 2014
Words:8250
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