Organic rich Neogene strata are widespread throughout the onshore region of the West Baram delta. There have been numerous studies conducted in this region and adjacent Seria Formation in Brunei . Majority of the previous studies have established the hydrocarbon producing efficiency of the province [1-4], but have not adequately examined the source organic matter input and depositional conditions of the organic matter. Moreover, past interpretations were based primarily on pyrolysis methods, and petrography, without significant input from biomarkers and trace elements geochemistry [4-6].
This study focuses on the integration of biomarkers and trace elements distributions of the organic rich mudstones in the Lambir, Miri, and Tukau formations located in the onshore West Baram Delta Province. The purpose of this study is to provide information of the source input, depositional environment conditions and thermal maturation in the studied sediments. The study of trace element in potential source rocks are widely applied and successfully used because of their importance in exploration [7-10]. The concentration, distribution, and nature of metals in source rocks can give information on source input, depositional environment conditions and thermal maturity [9, 11]. Also, biomarker parameters have been widely used effectively in the characterization of the environmental conditions during the deposition of organic matter, the source input and the assessment of maturation level of potential source rocks [12, 13]. The outcome of this study provides better understanding to the type of source input, conditions of depositional environment and thermal maturity of the organic matter especially in the onshore formations in the West Baram Delta province.
2 Study Area and Geological Setting
The study area is located within the West Baram delta province, onshore Sarawak (Figure 1). A comprehensive geological history of the West Baram delta, both onshore and offshore has been studied by many workers [1, 14, 15]. The West Baram delta is characterized by the deposition of a northwestward prograding delta since Middle Miocene times. Periods of delta outbuilding were separated by rapid transgressions, represented by marine shale intervals that form the base of eight sedimentary cycles [1, 15]. The regressive sequences of each depositional cycle grade northwestwards from coastal-fluviomarine sands to neritic and marine shale. In the onshore, the West Baram Delta begins with a very dramatic change from Setap Shale (Lower Miocene) to very sandy Lambir and Miri formations (Middle to Upper Miocene) and the semi consolidated Tukau Formation of Late Miocene to Pliocene age .
The Lambir Formation (Middle - Upper Miocene) occupies approximately 220 square miles in the Lambir Hills, Bakong area, Teraja area and south-east of Marudi . The Formation consists predominantly of sandstones and alternating mudstones with minor limestone and marl in certain regions [1, 14, 16]. They are increasingly less consolidated towards the upper part of the formation, which consists of sandstone alternating with mudstones, with quartz pebbles . The Lambir Formation interfingers with the Miri and Tukau Formation in the study area where it sits comfortably on the older Setap Shale Formation [3, 15] as shown the Figure 2a and 2b.
The Miri Formation (Middle -Upper Miocene) is a siliciclastic sequence which consists of a succession of coarsening upwards clay-sand packages that is restricted to the coastal area between Miri in Sarawak and Jerudong in Brunei. The studied outcrops are located at the coastal region of Miri. Based on lithological differences and micropaleontology, the Miri Formation was subdivided into Lower Miri which consists of interbedded mudstones and sandstones overlying the Setap Shale Formation, and Upper Miri which is characterised by irregular sandstone and mudstones alternations [1, 15, 17].
The sedimentary rock of the Tukau Formation dates back between the Upper Miocene and Lower Pliocene and conformably overlies the Lambir Formation near Sungai Liku in the eastern Lambir Hill. They are preserved in a synclinal structure dissected by strike-slip faults systems and consists of both mudstone and sandstone beds . They are composed of medium to coarse grained sandstone with occasional micro-conglomerates at the channel base, representing deposition in a shoreface environment [18, 19]. The absence of brackish water forms of foraminifera, presence of lignite layers and amber balls in layered strata suggested that the Tukau Formation was possibly deposited in a coastal plain .
3 Samples and Methods
A total of fifteen samples were collected from outcrops along road cuts and construction sites in the South Miri district to represent the Lambir, Miri and Tukau formations (Table 1, Figure 2a). The sampling represents organic-rich shaly facies whereby attempts were made during sampling to avoid weathered materials. Collected samples were thoroughly cleaned prior to geochemical analyses.
Geochemical and various organic petrographic analyses including: determination of total organic matter (TOC) content, total sulphur (TS), bitumen extraction, column chromatography-mass spectrometry (GC-MS) and vitrinite reflectance measurements. Analyses were carried out in the Department of Geology, University of Malaya. Inductively coupled plasma mass spectrometry (ICP-MS) analysis for trace elements was carried out in the Department of Chemistry, University of Malaya.
Total Organic Carbon (TOC) and total sulphur measurement were performed on fine powdered samples using a Multi EA 2000 Analyser. Total nitrogen (TN) was measured using a Perkin Elmer 2400 Series II CHNS/O Analyser. Bitumen extraction was performed on approximately 25g of the powdered samples using Soxhlet apparatus for 72 hours with an azeotropic mixture of dichloromethane (DCM) and methanol (ratio 93:7). The extracts were separated into saturated hydrocarbon, aromatic hydrocarbon and NSO-compound by liquid column chromatography. The saturated hydrocarbon fraction was analysed by gas chromatography mass spectrometry on a V 5975B inert MSD mass spectrometer with a gas chromatograph attached directly to the ion source (70eV ionization voltage, 100 milliamps filament emission current, 230° C interface temperature). The identification of biomarkers was performed based on retention time and comparison with mass spectra of previously published data [e.g. [12, 20, 21]. The peaks identification are listed in Table 4. The abundance of the biomarker distributions was quantified based on the peak heights. Concentrations of trace elements were determined using an Agilent Technologies 7500 Series Inductively -coupled plasma mass spectrometer (ICP-MS). Approximately 250 mg of the ground samples were prepared for the analysis in duplicate by digestion with HNO3, HCL and HF in an Anton 3000 microwave oven. Boric acid was used for complexation after digestion and then diluted up to 100 times with ultimate pure water. Standard solutions of the elements with an analyte concentration of 10ppm were used for calibration which added to the reliability of the results.
Vitrinite reflectance measurement was conducted under normal reflected white light using a Leica CTR 6000-M microscope. The percentage of incident light reflected from the vitrinite particles in the samples were measured in comparison to a known standard of 0.589%. Mean vitrinite reflectance (% Ro) determinations were carried out on particles of vitrinite that are not associated with strong bitumen staining using a 50X oil immersion objective.
4 Results and Discussion
4.1 Organic-Carbon and Sulphur relationship
Organic Carbon and total sulphur contents in sediments may provide insight into the depositional environment and microbial sulphate reduction [10, 22-25]. According to Hedges and Keil , TOC/TS values of >5 indicates oxic conditions, while values between 1.5 and 5 indicates suboxic conditions. Anoxic conditions have TOC/TS values less than 1.5. The TOC/TS values in the analysed samples range from 3.06 to 5.85 (table 2), which suggests oxic to suboxic conditions. This is also supported by the plot of organic carbon and sulphur contents in Figure 3.
4.2 Organic -Carbon and Nitrogen relationship
The relationship between total organic carbon and nitrogen from bulk organic matter has been used in many studies to distinguish between phytoplankton and terrestrial plant sources [e.g. 26-29]. The former typically yields low values (4-10), and terrestrial plant sources tend to have ratios greater than 20 , while lacustrine and marine systems close to land generally show intermediate TOC/TN values [30, 31]. The TOC/TN ratios for all the analysed samples range from 22.08 to 27.60 (Table 2) which reflect major terrigenous organic input.
4.3 Trace elements distribution
Selected trace elements concentrations of Lambir, Miri and Tukau samples are listed in Table 2, along with the several widely used geochemical ratios. The trace elements data and ratios (Table 2) reveal the paleo-redox conditions during sedimentation of siliciclastic rocks [e.g. 10, 32-34].
Vanadium -nickel relationship: Vanadium (V) and Nickel (Ni) are important indicators for the redox conditions during deposition [9, 27]. The relative proportions of V and Ni are controlled by the depositional environment . A V/Ni ratio greater than 3 indicates deposition in a reducing environment; V/Ni ratios ranging from 1.9 to 3 or less than 1.9 indicate deposition under suboxic conditions and oxic conditions respectively . The V/Ni ratios for all the samples are in range of 1.20-2.67 (Table 2) thus show that the terrigenous organic matter was deposited under oxic to suboxic conditions. The cross plot of V and Ni Figure 4) shows the terrigenous organic matter in the studied samples is deposited under prevailing oxic to suboxic conditions.
Cobalt (Co) is also useful as an indicator for the depositional conditions . Co is usually enriched in comparison with Ni in oxic conditions . Co concentrations in all the analysed mudstone samples are relatively low and range from 3.46 to 15.69 ppm (Table 2). Ni/Co has been used as an indicator of oxygen levels [36, 37]. It has been highlighted that values of Ni/Co ratio below 5 indicate oxic environment, whereas values above 5 suggest suboxic/dysoxic environment . The organic matter in all the studied mudstones was considered to be deposited under oxic to suboxic conditions as indicated by the Ni/Co ranging from 2.04 to 5.21 ppm (Table 2).
Chromium (Cr) is thought to be associated only with the detrital faction and is not directly influenced by redox conditions, and thus high V/Cr values (>2) are thought to indicate anoxic conditions and low V/Cr values (<2) indicate oxic conditions . The V/Cr values in all the samples range from 1.14 to 2.61 ppm indicating oxic to suboxic conditions. There is a good agreement in the interpretations of redox conditions using thresholds established by  for Ni/Co and V/Cr (Figure 8) which suggest oxic to suboxic depositional conditions.
Molybdenum (Mo) can also be used as a proxy for depositional conditions. It has been observed that Mo is associated with humic acids in the organic matter [8, 38, 39]. Previous work suggested that the concentrations of Mo increase with increasing anoxic conditions [11, 40, 41]. It was highlighted that Mo concentrations between 5 to 40 ppm can be used as an indicator of anoxic conditions and that less than 5 ppm indicating oxic conditions [40, 41]. The concentration of Mo in most of the analyzed samples is less than 5ppm and this suggest prevalence of oxic conditions during the deposition of these sediments.
4.4 Biomarker distributions as indication for organic matter input and depositional environment conditions
4.4.1 n-Alkanes and isoprenoids
The use of n-alkanes to evaluate organic matter source input is based on short chain alkanes (n-C15 to n-C19) being predominantly derived from marine algae and long chain alkanes (n-C25 to n-C31) being derived from land plant waxes . The Terrigenous/Aquatic ratio (TAR) is defined as: (n-C27 + n-C29 + n-C31)/ (n-C15 + n-C17 + n-C19). Values >1 for this parameter indicate more land plant sources than marine algae sources and low values (<1) indicate more marine algae sources than land plant sources. TAR values for the analyzed samples range from 1.96 -2.86 for Lambir samples, 1.756-2.54 for Miri samples and 2.08-2.46 for Tukau samples (Table 2), which indicates that the biological source of organic matter in these formations is dominated by higher plants and supported by the dominance of long chains n-alkanes as recorded in m/z 85 fragmentogram (Figure 6).
A simplified parameter that reflects the relative contribution of marine algae versus land plants to preserved organic matter is the ratio of n-C17/n-C31. High values (>2) for this parameter indicate more marine algae sources than land plant sources, whilst low values (<2) indicate more land plant sources than marine algae sources . The n-C17/n-C31 values range from 0.15 to 0.35 in all the analysed samples (Table 2). These low values (<2) indicate that the source of organic matter within the analysed samples is dominated by higher plants, which is consistent with the TAR values.
The ratio of pristane to phytane is often used as an indicator of redox conditions in ancient sediments [13, 43]. Pristane and phytane may originate from the oxidation or reduction, respectively, of the phytol side chain of chlorophyll, which is controlled by oxic or anoxic conditions during sedimentation . Pr/Ph ratio values greater than 3.0 indicate oxic conditions, values below 1.0 indicate anoxic conditions, and values between 1.0 and 3.0 indicate alternating oxic and anoxic conditions [13, 43]. The Pr/Ph ratio values that range from 1.43-3.25 were obtained for the analysed samples (Table 2). Such values indicate these sediments have been deposited in oxic to suboxic conditions. Some researchers [13, 45] have raised objections to interpreting redox conditions from the Pr/Ph ratio, suggesting possible sources other than chlorophyll for pristane and phytane and possible influence of thermal maturity on the ratio. However, redox interpretations using the Pr/Ph ratio are consistent with interpretations using other biomarker redox indicators (e.g. Pr/n-C17 versus Ph/n-C18). The Pr/n- C17versus Ph/n-C18 plot (Figure 7) for the analysed samples indicates sediment deposition has occurred in peat accumulated swamp environment under prevalence oxic condition. This result supports the features of a marginal depositional environment .
4.4.2 Terpanes and Steranes
The distributions and relative abundances of pentacyclic terpanes obtained from m/z 191 ion fragmentograms are shown in Figure 6 and corresponding parameters in Table 2. The hopanoids are dominated by C29- norhopane, C30- hopane, and Oleanane.18α(H)-22,29,30-trisnorneohopane(Ts),17α(H)-22,29,30-trisnorhopane(Tm) and homohopanes (C31) are also present. Hopanoids are important biomarkers for indicating organic matter. The extended hopane or homohopanes are dominated by C31 with low concentration of C32 to C35 homohopanes. The distribution of the extended hopanes or homohopanes (C31 to C35) has been used to evaluate redox conditions [12, 13]. High concentration of C35 homohopanes is an indicator of highly reducing marine conditions, whereas low C35 homohopane concentrations are generally observed in oxidizing water conditions [12, 21]. This suggests that the studied mudstones from Lambir, Miri and Tukau formations were deposited under prevailing oxic conditions. The presence of oleananes in the samples indicates angiosperm input in rocks of late Cretaceous or younger age  or to a related plant type that possessed the availability to synthesis oleanane precursors [21, 48]. The measureable amounts of oleanane in all the studied mudstone samples is a strong indicator of terrestrial angiosperm plant input in the West Baram delta province.
The use of the distribution of C27 to C29 regular steranes in determining biological source of organic matter is based on observations that C27 steranes originate predominantly from marine algae; C28 steranes from yeast, fungi, bacterial plankton, and algae; and C29 steranes from land plants [13, 49]. The distributions based on integrated peak areas of steranes identified in each sample are listed in Table 2 and displayed in mass fragmentogram m/z 217 (Figure 6). All the formations show predominance of C29 over C28 and C27. The Lambir Formation samples show dominance of C27 over C28 while C28 is moderately higher than C27 in Miri and Tukau Formation. The αααR, αααS, ααβR, and ααβS isomers of C27 sterane, C28 sterane and C29 sterane were identified in all samples. The C29 sterane being most dominant comprising 42-58wt% of the total C27 to C28 steranes, followed by C28 comprising 19-35%, and C27 comprising 20-27% (Table 2).
The relationship between sterane composition and biological source of ancient sediments was developed by  and . A ternary plot of the C27 to C29 regular steranes indicates that land plants are the dominant biological sources of organic matter in the analysed samples (Figure 8). The ratio of C29 αααR to C27 αααR steranes (C29/C27) can be used to interpret the contribution of marine algae relative to the contribution of land plants . Low values (<1) of this ratio indicate more algal sources than land plant sources, and high values (>1) indicate greater land plant sources than algal sources . The C29/C27 values range from 1.62-2.9 (Table 2) for the analysed samples, indicating that the organic matter is dominated by land plant sources relative to marine algal source input.
4.5 Thermal Maturity
In this study, the thermal maturity of the Lambir, Miri and Tukau formation samples was evaluated based primarily on n-alkane data, biomarker maturity parameters and supported with the mean vitrinite reflectance (% Ro).
The proportion of odd versus even carbon numbered n-alkanes may be used to obtain a rough estimate of organic maturation level of sediments [12, 13, 53], These measurements include the carbon preference index (CPI) proposed by , which is an improved odd-even preference (OEP) by . CPI or OEP values significantly above 1.0 (odd preference) or below 1.0 (even preference) indicate thermal immaturity while values of 1.0 suggest that the organic matter is thermally mature . The CPI values for all the analysed samples have CPI more than 1.0 (Table 2) with prevalence of odd carbon number over even carbon number. This indicates that all the analysed samples are thermally immature in terms of hydrocarbon generation . However, the CPI values are greatly influenced by organic matter input and as a result, the maturity interpretation obtained from these values should be complemented with other maturity data .
The biomarker maturity parameters include the ratios of C31 homohopane 22S/(22S+22R), moretane/hopane and C29 (20S/(20S+20R) ratios. The (22S/(22S + 22R)) homohopane ratio is widely used as biomarker maturity indicator . The ratios of C31 22S/(22R + 22S) increase from 0 to about 0.6 at equilibrium ) during maturation. Values in the range 0.50-0.54 have barely entered oil generation, whereas ratios from 0.57 up to 0.62 indicate that the oil window has been reached. Homohopanes are dominated by C31 -hopane in all the analysed samples and R isomers are dominant over S isomers in all the analysed samples. All of the analysed samples have C31 22S/22S + 22R values less than 0.57, suggesting low level of thermal maturity for hydrocarbon generation. The 17β, 21α (H)-moretanes are thermally less stable than the 17α, 21β (H)-hopanes, so the concentrations of the C29-hopanes and C30-moretanes decrease relative to the corresponding hopanes with thermal maturity . The C30-moretane/C30-hopane ratio decreases with increasing maturity of organic matter from about 0.8 in immature bitumen to values of less than 0.15 in mature source rocks and in oils to a minimum of 0.05 [13, 56, 57]. The values obtained for the Lambir, Miri and Tukau formations samples range from 0.54 to 0.76 (Table 2) and suggest that they are immature. The ββ/ (ββ+AA) C29 sterane ratios are also increased with increasing maturity . These compounds (C29αββ-steranes) reach equilibrium more slowly than the C29 αα α- steranes, thereby allowing the establishment of the 20S/ (20S + 20R) ratio. The values less than 0.3 suggest immaturity. The analysed samples have ββ/ (ββ+αα) C29 sterane ratios in the range of 0.20 to 0.28 (Table 2). Crossplot of ββ/ (ββ+αα) C29sterane and C31 22S/22S +22R (Figure 9) shows that all the analysed samples are immature. This is consistent with the maturity parameters discussed above.
The immaturity of the Lambir, Miri and Tukau formations samples is further supported by the mean vitrinite reflectance (% Ro) from 0.39 to 0.48% (Table 2). Vitrinite reflectance values for main phase of oil generation ranges from (0.6-1.3)% and values greater than 2.0% indicate dry gas generation [58-60]. Thus, the thermal maturity determination based on biomarker parameters are in good agreement with the measured vitrinite reflectance data for all the samples analysed in this study.
The integration of biomarker and trace elements distribution on the shaly facies of Lambir, Miri and Tukau formation suggest the following:
There is no distinct variation in the geochemical characteristics between the Lambir, Miri and Tukau Formations in term of source input, depositional conditions and thermal maturity.
The distribution of n-alkanes, isoprenoids and biomarkers of saturated hydrocarbon are characterized by a dominance of high molecular weight compounds, high Pr/Ph ratios (>1.0), high abundance of C29 regular steranes and high C29/ C27 regular sterane ratios, indicating a strong terrigenous organic matter input that was preserved under relatively oxic to suboxic conditions as supported by TOC/TS, TOC/TN and trace elements concentrations.
Various biomarker distribution maturity data suggested that the sediments are yet to reach maturity stage of hydrocarbon generation which is also supported by mean vitrinite reflectance less than 0.6 Ro%.
Authors would like to thank the University of Malaya for providing IPPP Research Grant No. PG043-2013A.
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About the article
Published Online: 2017-08-10
Citation Information: Open Geosciences, Volume 9, Issue 1, Pages 302–313, ISSN (Online) 2391-5447, DOI: https://doi.org/10.1515/geo-2017-0025.
© 2017 O. S. Togunwa and W. H. Abdullah. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. BY-NC-ND 3.0