Biomarker Evaluation of Hopanoid and Steroid Distributions ... · Biomarker Evaluation of Hopanoid and Steroid Distributions in some Nigerian Coals Uzoegbu and Onwualu 112 V-shape

Biomarker Evaluation of Hopanoid and Steroid Distributions in some Nigerian Coals


*1Uzoegbu M.U., 2Onwualu J.N.J 1,2Department of Geology, Faculty of Science, University of Port Harcourt, Port Harcourt, Nigeria.

A total of Twenty-one coal and carbonaceous shale samples were collected from four boreholes in Mamu and Awgu Formations of Lower and Middle Benue Trough, Nigeria. The homohopane index (C35/C31 - C35) and homohopane ratio (C35αβS/C34αβS) range from 0.02 to 0.12 and 0.15 to 0.92 indicates oxic condition during organic matter deposition from Awgu samples. The Moretane/Hopane, Hopane/Hopane + Moretane, Ts/Ts + Tm, 22S/22S + 22RC32 homohopane ratios range from 0.06 to 0.14; 0.88 to 0.94; 0.34 to 0.66; and 0.53 to 0.62 and 20S/20S+20R and αββ/αββ+ααα C29 ratios range from 0.43 to 0.58 and 0.42 to 0.55 indicate samples are within the late oil window/gas phase. Plots of 22S/22S+22R C32hopanes against C29αββ/αββ+ααα steranes show that Awgu samples are thermally mature. The C32-C35benzohopanes were detected in Onyeama and Okaba samples as a transformation product of C35 bacteriohopanepolyol. C35/C30hopane ratio range from 0.01 to 0.05 and 0.01 to 0.47 indicates fluvial/deltaic and lacustrine-fluvial/deltaic depositional environments for Onyeama and Okaba samples. The homohopane index and homohopane ratio for the samples range from 0.02-0.13 and 0.23-0.92 indicate oxic depositional environment for Onyeama samples and suboxic-oxic depositional environments for Okaba samples. Moretane/Hopane, Hopane/Hopane + Moretane, Ts/Ts + Tm, 22S/22S + 22R C32homohopane ratios in Onyeama samples range from 0.46 to 0.64; 0.61 to 0.69; 0.02 to 0.05; and 0.48 to 0.58 and Okaba ranging from 0.59-0.93; 0.54-0.64; 0.11-0.24; and 0.16-0.48 indicate that samples are thermally immature. The sterane and diasterane distributions for all the samples occur in the order of C29>C28>C27. The predominance of C29 sterane over C27 sterane reflects a greater input of terrestrial relative to marine organic matter.

Keywords: Coal, Depositional environment, Hopane, Organic matter, Sterane, Thermal maturity. INTRODUCTION Hopanoids are considered biomarkers for bacteria and cyanobacteria. Most hopanes molecular fossils originate from polar constituents of prokaryotic organisms (Nytoft et al., 2006). The most probable biological precursor of the hopane derivatives is bacteriohopanepolyol, which are present in the cell membranes of prokaryoticorganisms (bacteria and blue algae) where they played the rigidifying role by steroids in eukaryotic organisms (Ourisson et al., 1982; Durand, 2003; Bechtel et al., 2007a, b). The C30 Hopanoids have also been found in somecryptogams; moss, fern (Bechtel et al., 2007a, b). While hopanes with 30 or fewcarbon atoms are often interpreted as diagenetic products of C30 hopanoids (e.g. diploptene and diplopterol), the extended hopanes have been related to C35 precursors, such as bacteriohopane polyols,

aminopolyols and a number of composite hopanoids(Wang et al., 1996). High abundance of C35hopane usually found in marine carbonates and evaporiticsediments has been attributed to highly reducing depositional environments (Yangming et al., 2005).

*Corresponding Author: Uche Mmaduabuchi Uzoegbu, Department of Geology, Faculty of Science, University of Port Harcourt, Port Harcourt, Nigeria. Email: [email protected]

Vol. 6(1), pp. 111-122, August, 2019. © www.premierpublishers.org. ISSN: 9661-0255

Research Article

International Research Journal of Chemistry and Chemical Sciences


Uzoegbu and Onwualu 112

V-shape pattern of homohopane, i.e. C31>C32≥C33≤C34<C35 is indicative of salinelacustrine source rock deposited under anoxic, low Eh conditions (Peters and Moldowan, 1991; Yangming et al., 2005). Benzohopanes, which have not beenreported in living organisms, are thought to be secondary transformation products ofC35 bacteriohopanepolyol derivatives (Grice et al., 1998). Distribution and different ratios of hopanes are used to evaluate thermal maturity of oil and source rock. The isomerisation at C-22 can be used to assess the maturity of geological samples (22S/22S+22R). This ratio reflects the more thermally stable 22S isomer compared tothe biologically derived 22R stereochemistry (Farrimond et al., 1998; Peters et al.,2005; Killops and Killops, 2005). The parameter is usually measured using the C31homohopane, although the C32 homologues are commonly employed due to co-elution of gammacerane with C31homohopane (Farrimond et al., 1998). C27, 18a(H)-22,29,30-Trisnorneohopane (Ts) and C27, 17a(H)-22,29,30-Trisnorhopane (Tm) ratio are used as a maturity indicator. Ts is knownto exhibit a greater stability than the Tm, and with increase in maturation, Ts usuallyshows marked increase in apparent concentration relative to the Tm (Seifert and Moldowan, 1981). Ts/Ts+Tm ratios are not only related to maturity, but also toorganic facies and depositional environments. Oils derived from carbonates usually show low Ts/Ts+Tm compared to oil generated from shales. Bitumens of anoxic and acidic hypersaline source rocks generally show high Ts/Ts+Tm (Seifert and Moldowan, 1981).

Steranes are derived from sterols that are widely distributed in plants andmicroorganisms. The relative concentration of C27 and C29 steranes can indicatecharacteristics source inputs and sedimentary facies.The predominance of C27 steranes in non-marine strata indicates a deep lake faciesand source input of plankton (algae) while C29 sterane predominance shows a swampshallow water environment and a terrestrial higher plant input (Volkman and Maxwell, 1986; Peters and Moldowan, 1993; Volkman et al., 1999; Otto et al., 2005; Jauro et al., 2007).C28 sterane is a unique biomarker signature of organic matter deposited in saline lacustrine facies. Sterane/hopane ratio is often used as a measure of the relative inputsof eukaryotic versus prokaryotic debris (Peters and Moldowan, 1993). Low sterane/triterpane ratio (Norgate et al., 1999) has been postulated to favour terrestrial paralic facies rather than peat swamp facies as organic matter source. Sterane isomerisation at C-20 has been found useful in assessing the level of thermalmaturity of oil and sediments. The 20S/20S+20R ratio (usually measuredusing the C29ααα steranes) is one of the most widely applied molecular maturityparameters in petroleum geochemistry (Farrimond et al., 1998). It is based on therelative enrichment of the 20S isomer compared with the biologically inherited 20Rstereochemistry with increasing maturity.

Sterane nuclear isomerisation ratio, (αββ/ αββ+ααα) is widely applied owing to itsoperation beyond oil window

(Farrimond et al., 1998). In relatively immature samples, coelution of the ααα isomer with the αββ doublet is a common problem that isresponsible for the relatively high αββ values at shallow depth and apparent decreasesin the parameter with increasing depth (Farrimond et al., 1998). In highly maturesource rocks, an eventual decrease in this ratio occurs (Peters et al., 2005). The objective of this research is determining the depositional environment, organic matter source and distribution of hopanoids and steroids in some thermally immature Nigerian coals. STRATIGRAPHIC SETTING The infilling of the Anambra and Afikpo started during the Campanian to the Paleocene (Danian) under two major eustatic cycles; the more pronounced Nkporo transgression and the less active Nsukka transgression with the Anambra basin showing the most complete stratigraphic sections (Fig. 1). These cycles are also found in the Afikpo syncline SE of the Abakaliki anticlinorium and the Dahomey embayment, west of the Ilesha basement spur, although both are incomplete (Murat, 1972). The first cycle which took place during the Lower Campanian to the Maastrichian started with the deposition of the Nkporo whose lateral (age) equivalents are the Enugu and Owelli (Fig. 2). This is the basal unit of the Campano-Maastrichian transgression and comprises of dark mudstone, gray, fissile friable shales with thin beds of marl, sandy shale and limestone overlying an angular unconformity (Reyment, 1965). The regressive phase was marked with the development of a large offlap complex, starting with the paralic sequence of the Mamu (Lower coal measure) overlying the Nkporo (Reyment, 1965). It is thought to be lower Maastrichian in age with a basal part that contains thin marine intercalations, while the coal bearing part consist of fresh water and low salinity sandstones, shale, mudstone and sandy shales with coal seams occurring at several levels (Simpson, 1955). The Mamu formation is overlain by the continental sequence of the Ajali. This sandstone unit has received several names such as false bedded sandstone (Tattam, 1944), basal sandstone (Simpson, 1955) etc. Its present name was given by Reyment (1965) after establishing its type locality at the Ajali. Virtually all exposures of the formation are characterized by a lateritic profile at the top. It was deposited during the regressive phase of the Campano-Maastrichian transgression and the age is Maastricthian. The Ajali sandstone is overlain conformably by the Nsukka Formation (Upper coal measures), and it consists of alternating succession of gray sandy shales, sandstones, plant bearing beds and thin beds of coal (Reyment, 1965). Thin bands of marine limestone heralded the return of marine sedimentation at the top of the formation. These


Int. Res. J. Chem. Chem. Sci. 113

dark shales and the intensely bioturbated sandstones are well exposed at Ihube, along the Enugu – Port Harcourt expressway. The age range of the formation is late Maastrichian to Danian based on the fossil record. This

formation bears the K/T boundary which is described by Reyment (1965) as a period of transition in Nigeria. Mbuk et al. (1985) identified this boundary in the Nsukka Formation in OzuAbam area of Abia State.

Fig. 1: Generalized geological map of Nigeria (boxed areas of inset) showing the geological map of the Anambra Basin. Numbers indicate Cretaceous and Tertiary formations shown as follows: 1.Asu River Group; 2. Odikpani Formation; 3.Eze-Aku Shale; 4.Awgu Shale; 5. Enugu/Nkporo Shale; 6.Mamu Formation; 7.Ajali Sandstone; 8. Nsukka Formation; 9. Imo Shale; 10.Ameki Formation and 11.Ogwashi-Asaba Formation (after Akande et al., 2007). MATERIALS AND METHODS A total of nine samples comprising of six coals, two carbonaceous shales and one coaly shale were collected from 2 boreholes (BH94 and BH120) from Awgu Formation (BH). The coal seams and interbedded shale in BH94 and BH120 were sampled between 218-431 m and 131- 289 m depths respectively. In Mamu Formation, twelve samples consisting of nine coals and three carbonaceous shales were collected from Okaba (OBA) and Onyeama (AMA) with co-ordinates of 07o 28ˈN, 07o 43ˈE and 06o 28ˈ N, 07o 26ˈE). In the laboratory, the samples were reshaped using a rotating steel cutter to eliminate surface that could be affected by alteration. Chips were cut from the samples and dried in an oven at 105oC for 24 hours. The dried

sample was pulverized in a rotating disc mill to yield about 50 g of sample for analytical geochemistry. The samples were subjected to flame ionization detection (FID) for hydrocarbons thermal conductivity detection (TCD) for CO2. One milligram of bulk powder sample was added to 200 mg of KBr and the mixture homogenized using a pestle in an agate mortar. Pressing the mixture using a load of 10 t yielded a pellet for Fourier Transform Infrared (FT-IR) Spectroscopy using a Nicolet Bench 505P Spectrometer, with sample absorbance monitored using 256 scans with resolution of 4 cm-1 from a wave-number of 4000 – 400 cm-1. About 10 g of the sample was subjected to sohxlett extraction using a solvent mixture of acetone, chloroform and methanol (47: 30: 23 v/v) at 60oC for 24 hours to extract the soluble organic matter. The extract was concentrated by evaporation to dryness using a rotating vapour evaporator at 250 mb. The extract was



Fig. 2: The Stratigraphy of the Anambra Basin Southeastern Nigeria (After,Ladipo, 1988 and Akande et al., 1992; Modified in Uzoegbu et al., 2013b).

transferred to an 8 ml vial using the same solvent mixture and allowed to evaporate to dryness in a vented hood. The dried extract was fractionated by silica gel column chromatography with a column prepared using 2 g of baker silica gel calcined at 200oC for 24 hours to yield six fractions ranging from saturate to polar. The saturate fraction was subjected to urea adduction to separate isoprenoids from n-alkanes and subjected to gas chromatography-mass spectrometry (GC-MS) using a CE 5980 GC coupled to an HP Finnigan 8222 MS held at 80oC for three minutes and raised to 310oC at 3oC min-1 and held isothermally for 10 minutes in order to assess some molecular parameters used in source rock characterization.

RESULT AND DISCUSSION

The m/z 191 mass chromatograms showing the distribution of pentacyclic triterpanes in the samples are shown in Figs. 3, 4 and 5. C29 and C30αβ-hopane occur in appreciable amount in all the Awgu coaly shale samples (Fig. 3), indicating significant contribution of prokaryotic organisms(i.e. bacteria, cyanobacteria and blue algae) to the source organic matter (Adedosu, 2009).The sterane/hopane ratio is often used as a measure of relative

inputs of eukaryoticversus prokaryotic debris (Peters and Moldowan, 1993). The sterane/hopane ratio values range from 0.04-0.51(Table 1). The ratio values (<0.6) according to (Tissot and Welte, 1984; Peters and Moldowan, 1993; Sachsenhofer et al., 2000; Norgate et al., 1999) indicative of theincorporation of high level of bacterial inputs commonly associated with terrigenousorganic matter in coals (non-marine organic matter).The appreciable quantity of homohopanes (C31-C35) in all the samples, suggest that bacteriohopanetetrol and other polyfunctional C35 hopanoids; bacteriohopanepolyols, aminopolyols etc. (Wang et al., 1996; Adedosu, 2009), common in prokaryotic micro-organisms (Ourrisson et al., 1979; Rohmer, 1987) were significant contributors to the biomass.

The occurrence of 18α(H)-28-noroleanane, 18α(H)- and 18β(H)-oleanane in Awgu coaly shale samples is notable. Oleananes are regarded as reliable marker forangiosperm; being significant constituents of wood, roots and bark in Cretaceous oryounger effective source rocks in deltaic petroleum system (Moldowan et al., 1994;Nytoftet al., 2002; Peters et al., 2005; Ozcelik and Altunsoy, 2005; Otto et al., 2005;Bechtel et al., 2007b).

AGESEDIMENTARY

SEQUENCELITHOLOGY DESCRIPTION

DEPOSITIONALENVIRONMENT

REMARKS

ANKPASUB-

BASIN

ONITSHASUB-

BASIN

MIOCENE

OLIGOCENE

OGWASHI-ASABA FM.

Lignites, peats,

Intercalations of

Sandstones &

shales

Estuarine

(off shore bars;

Intertidal flats)

Liginites

EOCENE AMEKE NANKA

FM. SAND

Clays,shales,

Sandstones

& beds of grits

UnconformitySubtidal, intertidal

flats, shallow marine

PALEOCENEIMO SHALE Clays, shales

& siltstonesMarine

MA

ASTR

ICH

TIAN

Clays, shales, thin sandstones & coal

seams

Coarse sandstones,Lenticular shales,

beds of grits & Pebbls.

Clays, shales, carbonaceous

shale, sandy shale& coal seams

NSUKKA FM.

AJALI SST.

MAMU FM.

? Estuarine

Subtidal, shallow

marine

Estuarine/ off-shorebars/ tidal flats/

chernier ridges

CAMPANIANENUGU/

NKPORO SHALE Clays & shales Marine

CONIACIAN-

SANTONIANAWGU SHALE

TURONIAN EZEAKU SHALE

Clays &

shales Marine

ODUKPANI FM.CENOMANIAN

ALBIAN

L. PALEOZOIC

ASU RIVER GP.

B A S E M E N T C O M P L E X

Sub-

bituminous

Sub-

bituminous

3rd Marine

cycle

Unconformity

2nd Marine

cycle

Unconformity

1st Marine

cycle

Unconformity

NO

DE

PO

SIT

ION

(? MINOR

REGRESSION

REGRESSION

(Continued

Transgression

Due to geoidal

Sea level rise)

TRANSGRESSION(Geoidal sea level

Rise plus crustal

Movement)

Coal

Rank

~~ ~

~~~ ~

~ ~~ ~

~~~

~~

~~ ~

~~ ~

~~ ~

~ ~~ ~

~ ~~

~~ ~

~~~

~~

~~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

.. .. ..

... .... .



The C35/C30hopane ratio values range from 0.03 to 0.26, which reflects organicmatter deposited in lacustrine-fluvial/deltaic environments (Peters et al., 2005). The homohopane index (C35/C31 - C35) and homohopane ratio (C35αβS/C34αβS) range from 0.02 to 0.12 and 0.15 to 0.92 respectively (Table 1). The low homohopane index of the samples indicates oxic condition during organic matter deposition (Peters and Moldowan, 1991; Hanson et al., 2001; Killops and Killops, 2005; Peterset al., 2005; Yangming et al., 2005; Adedosu, 2009). Hopanes with αβ epimers are more prominent in all the samples (Table 2) while no ββ-epimeris detected.

Homohopanes ranging from C31-C35 showed notable predominance of the22S over the 22R epimer (Fig. 3). These observations reflect high maturity statusof the samples (Miranda et al., 1999; Peters et al., 2005; Tuo et al., 2007; Adedosu, 2009).The Moretane/Hopane, Hopane/Hopane + Moretane, Ts/Ts + Tm, 22S/22S + 22RC32homohopane ratios range from 0.06 to 0.14; 0.88 to 0.94; 0.34 to 0.66; and 0.53 to0.62 respectively. These values indicate samples are within the late oil window/gas phase (Seifert and Moldowan, 1986; Mackenzie, 1984; Seifert and Moldowan, 1986; Peters and Moldowan, 1993; Kagya, 1996; Tuo et al., 1999; Peters et al., 2005; Adedosu, 2009).

Fig. 3: m/z 191 Mass chromatogram showing the Fig. 4: m/z 191 Mass chromatogram showing the distribution of distribution of hopanes in Awgu samples. hopanes and benzohopanes in Mamu sample (Okaba).

Fig. 5: m/z 191 mass chromatogram showing the distribution of hopanes and benzohopanes in Mamu samples (Onyeama).


Uzoegbu and Onwualu 116 Table 1: Source and depositional environment parameters computed from the hopane and sterane distributions in the coals.

Sterane/Hopane=C27+C28+C29steranes/[(C29+C30)αβhopane + (C31+C32+C33)αβ(R+S) homohopane] C35/C30 = C35αβ(R+S) homohopane/ C30αβ hopane + C30βα moretane Homohopane ratio,C35/C34αβS = C35αβS/C34αβS homohopane Homohopane index = C35/ C31+C32+C33+C34+C35) αβ(R+S) homohopane Table 2: Maturity parameters computed from the hopane and sterane distributions in the coals.

Mor/Hop = Moretane/Hopane (C30) Hop/Hop + Mor = Hopane/Hopane + Moretane (C30) C32HH = C32homohopane



The C27 to C35hopanes are detected in all the Mamu samples but C28 was notdetected (Fig. 4 and 5). The most prominent hopane in Onyeama samples areC29αβ-norhopane and C30αβ-hopane while C29αβ norhopane is predominant in Okaba samples (Fig 4 and 5). In the Okaba samples, abundance of C29αβ-hopane in allthe samples reflects major contribution from terrestrial organic matter; however,contribution from prokaryotic organisms is not excluded while abundant C30αβ-hopane with notable presence of C29αβ in Onyeama samples reflects significant contribution from prokaryotic organisms as well as vitrinitic (terrestrial) organic matter (Adedosu, 2009). The unusual high abundance of 22R compared to 22S in the C31-17α (H),21β(H) homohopane is evident in all Okaba samples. This is likely due to co-elution of gammacerane (Peter and

Moldowan, 1993; Kagya, 1996; Farrimond et al., 1998; Peters et al., 2005). Benzohopanes with different distributions were found in Onyeama and Okaba coal samples (Fig. 4 and 5). There is no previous record of presence of benzohopanes in Nigerian coal and coaly organic matter. The C32-C35benzohopaneswere detected in Onyeama samples while C32-C33benzohopanes were detected in Okaba samples (Adedosu, 2009). Benzohopanes are thought to be secondary transformation products ofC35 bacteriohopanepolyol derivatives (Grice et al., 1998; Peters et al., 2005; Killops and Killops, 2005; Bechtel et al., 2007a).

Fig. 6: Mass frangmentogram and spectra of Olean-18-ene in Okaba coal samples.

Fig. 7: Mass frangmentogram and spectra of Olean-13(18)-ene in Okaba coal samples.



Fig. 8: Mass frangmentogram and spectra of Olean-12-ene in Okaba coal samples. Three isomers of oleanenes; olean-18-ene, olean-13 (18)-ene and olean-12-ene were identified in Okaba samples (Fig. 6, 7 and 8). Similar to the benzohopanes, this is the first time oleanene isomers are being identified in Nigerian coal and coaly organic matter (Adedosu, 2009). These three oleanene isomers are products of late diagenesis from taraxerol and β-amyrin, which are biomarkers for angiosperm (Ten Haven and Rullkötter, 1988; Ekweozor and Telnǽs, 1990; Rullkötter et al., 1994; Curiale, 1995). They have also been found useful as indicators of thermal immaturity (Eneogwe et al.,2002). In Onyeama sample, C35/C30hopane ratio range from 0.01 to 0.05 while Okaba samples have values ranging from 0.01 to 0.47. These values indicate fluvial/deltaicand lacustrine-fluvial/deltaic depositional environments for Onyeama and Okaba samples respectively. The homohopane index and homohopane ratio for the samples ranges from 0.02-0.13 and 0.23-0.92 respectively (Table 1). These values indicate oxic depositional environment for Onyeama samples and suboxic-oxic depositional environments for Okaba samples (Peters and Moldowan, 1991; Killops and Killops,2005; Peters et al., 2005; Yangming et al., 2005). There is presence of gammacerane in Okaba samples (Fig. 4 and 5), which indicate water column stratificationduring organic matter source deposition (Sinninghe-Damsté et al., 1995; Yangming et al., 2005; Adedosu, 2009). In Table 2 Moretane/Hopane, Hopane/Hopane + Moretane, Ts/Ts + Tm, 22S/22S + 22RC32homohopane ratios in Onyeama samples range from 0.46 to 0.64; 0.61 to 0.69;0.02 to 0.05; and 0.48 to 0.58 respectively. These values suggest low maturity status (Rullkötter et al., 1985; Kagya, 1996; Peters et al., 2005). The Okaba samples have Moretane/Hopane, Hopane/Hopane + Moretane,

Ts/Ts + Tm, 22S/22S + 22R C32homohopane ratios ranging from 0.59-0.93; 0.54-0.64; 0.11-0.24; and 0.16-0.48respectively. These values also indicate that Onyeama samples are thermally immature (Rullköteret al., 1985; Kagya, 1996; Peters et al., 2005). The m/z 217 mass chromatograms showing the distribution of steranes and diasteranes in all the samples are shown in Figs. 9, 10 and 11. The occurrence of C27 to C29 steranes and diasteranes were detected in Awgu coaly samples (Fig. 9). The sterane and diasterane distributions for all the samplesoccur in the order of C29>C28>C27 (Table 1). The predominance of C29 sterane overC27 sterane reflects a greater input of terrestrial relative to marine organic matter(Huang and Meinschein, 1979; Volkman 1988; Kagya, 1996; Sari and Bahtiyar, 1999;Otto et al., 2005; Peters et al., 2005). The ternary plots of sterane distribution in Awgu samples (Fig. 12) indicate organic matter derived from terrestrial materialsdeposited in lacustrine – fluvial/deltaic settings (Huang and Meinschein, 1979; Killops and Killops, 1993, 2005; Peters et al., 2005). The diasterane ternary plots (Fig. 13) also show that Awgu samples are from terrestrial organic matter. This observation is supported by C27/C29 ratios (Table 1), which range from 0.28 to 0.56(Peters et al., 2005). The dominance of dinosterol over C30 steranes in these samples reflects typical fresh water lacustrine source rocks (Köhler and Clausing, 2000; Peters et al., 2005). The 20S/20S+20R and αββ/αββ+ααα C29 ratios range from 0.43 to 0.58 and 0.42to 0.55 respectively. These values show that the samples are within the oil generativewindow (Peters et al., 2005; Adedosu, 2009). Plots of 22S/22S+22R C32hopanes against C29αββ/αββ+ααα steranes show that Awgu samples are thermally mature (Fig. 14).

Fig. 9: m/z 217 mass chromatograms showing the distribution of steranes and diasteranes in Awgu samples.



Fig. 10: m/z 217 mass chromatograms showing the distribution of steranes and diasteranes in Mamu samples (Okaba).

Fig. 11: m/z 217 mass chromatograms showing the distribution of steranes and diasteranes in Mamu samples (Onyeama).

Fig. 12: Ternary plots of C27, C28 and C29 steranes distributions in Nigeriancoal (After Huang and Meinschein, 1979).

Fig. 13: Ternary plots of C27, C28 and C29diasteranes distributions in Nigerian coal (After Huang and Meinschein, 1979).

Fig. 14: Plots of 22S/22S+22R C32hopanes against C29αββ/αββ+ααα steranes(After Inaba et al., 2001).

C29Diasteranes and steranes are the most abundant steranes in all the samplesexcept few samples from Okaba where C28 predominates. The sterane and diasterane distributions in Okaba samples are increasing in the order of C29>C28>C27.The predominance of C29 over C27 sterane reflects a greater input of terrestrial relativeto marine organic matter (Huang and Meinschein, 1979; Volkman, 1988; Kagya, 1996;Sari and Bahtiyar, 1999; Otto et al., 2005; Peters et al., 2005; Adedosu, 2009). The appreciablequantity of C27 and C28 in these samples also reflect contributions from phytoplankton; algae, diatoms, dinoflagellates (Volkman, 1986; Volkman et al., 1998; Sari and Bahtiyar, 1999; Peters et al., 2005). The ternary plot of C27, C28 and C29 sterane of Mamu samples (Fig. 12) reflects major terrestrial input in Onyeama samples while Okaba samples consist of both terrestrial and marine organic matter (Huang and Meinschein, 1979; Killops and Killops, 1993, 2005; Peters et al., 2005).The diasterane ternary plot (Fig. 13) shows that most of the Onyeama samples are derived from terrestrial organic matter with



few samples having mixed inputs (i.e. terrestrial and marine). Samples from Okaba are majorly derived from mixedorigin. There is little variation in sterane and diasterane distribution in Onyeama samples while significant variations are noticed in Okaba samples. This observationpossibly reflects same depositional environments for Onyeama samples(fluvial/deltaic) and lacustrine-fluvial/deltaic depositional environments for Okaba samples (Adedosu, 2009). This observation can be supported by the ratio of C27/C29 (Table 1) for thesamples. The values range from 0.2 to 0.7 and 0.18 to 0.43 in Okaba and Onyeama samples respectively. The dominance of dinosterol over C30 steranes in Okaba samples reflects typical fresh water lacustrine source rocks (Köhler and Clausing,2000; Peters et al., 2005; Adedosu, 2009). The 20S/20S+20R and αββ/αββ+ααα C29 ratios range from 0.04 to 0.19 and 0.16to 0.25 respectively in Onyeama samples while the values range from 0.1 to 0.39 and0.21 to 0.56 respectively in Okaba samples. The generally low values recorded according to Adedosu (2009) indicate that the samples are thermally immature (Table 2). The plot of 22S/22S+22R C32hopanes against C29αββ/αββ+ααα steranes also confirm the thermal immaturity status of Mamu samples (Fig. 14). CONCLUSION Coal and coaly organic matter samples were collected from coal bearing measures of Lower and Middle Benue Trough, Nigeria. A total of nine samples comprising of six coals, two carbonaceous shales and one coaly shale sample were collected from 2 boreholes (BH94 and BH120) from Awgu Formation (BH). In Mamu Formation, twelve samples consisting of nine coals and three carbonaceous shales were collected from Okaba and Onyeama coal seams. The C32-C35benzohopanes were identified in Onyeama and Okaba samples. The homohopane index (C35/C31 - C35) and homohopane ratio (C35αβS/C34αβS) indicates oxic - suboxic conditions during organic matter deposition from Awgu and Mamu samples. The Moretane/Hopane, Hopane/Hopane + Moretane, Ts/Ts + Tm, 22S/22S + 22RC32homohopane ratios and 20S/20S+20R and αββ/αββ+ααα C29 ratios indicate samples are within the early to late oil window/gas phase. Plots of 22S/22S+22R C32hopanes against C29αββ/αββ+ααα steranes show that Awgu and Mamu samples are thermally immature to mature stage. REFERENCES Adedosu TA (2009). Aspects of Organic Geochemistry of

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Accepted 26 July 2019 Citation: Uzoegbu M.U., Onwualu J.N.J (2019). Biomarker Evaluation of Hopanoid and Steroid Distributions in some Nigerian Coals. International Research Journal of Chemistry and Chemical Sciences 6(1): 111-122.

Copyright: © 2019. Uzoegbu and Onwualu. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

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