Introduction

The shale within the Timah Tasoh Formation has caught the attention of various studies of interest, particularly in sedimentary and sequence stratigraphy aspects (e.g. Foo 1983; Hassan and Lee 2005; Hassan et al. 2014; Baioumy et al. 2016). However, detailed research on the hydrocarbon source rock potential from this older sequence is still lacking. Numerous studies on this type of older and deeper sequence shales have shown a significant hydrocarbon potential and will most likely serve as unconventional and gaseous hydrocarbons (e.g. Hunt 1996; Zou et al. 2019). As examples, a lot of researchers around the world have recently been fascinated by the importance of unconventional hydrocarbons studies, especially in shale gas potential (e.g. Dardour et al. 2004; Soua 2014; Gasparrini et al. 2014; Tunstall 2015; Vanhazebroeck & Borrok 2016; Hakimi et al. 2020). The rapid development of unconventional hydrocarbons in the USA, especially shale gas from the sediment deposits of the Paleozoic Era, such as the Marcellus Formation, the Barnett Shale, the Woodford Shale and the Eagle Ford Formation, has shown that this resource can be commercialised (Scrotchman 2016). Therefore, the Paleozoic Era formation sequence quickly became the main hydrocarbon target areas (Soua 2014).

Nowadays, shale gas is becoming a vital hydrocarbon production goal, resulting in increased scientific research on mudrock or organic-rich shales, generating hydrocarbons (gas/oil) either through biogenic or thermogenic approaches (Tourtelot Harry 1979; Jarvie et al. 2007; Newport Leo et al. 2016). The shale gas could be produced from the sedimentary formations that show characteristics such as older stratigraphic layer, deep burial formations, and high thermal maturity area and had exposure to various tectonic activities (Dai et al. 2014; Zou et al. 2015). For instance, Zumberge et al. (2012) identified the geochemistry of the effective shale gas formations of North America, such as the Devonian Woodford Formation in the Arkoma Basin, with have > 2.0 wt% TOC, type I, II kerogen, and high thermal maturity from vitrinite reflectance values (1.1 to 3.0%Ro). Similarly, some countries, such as China, display a high TOC value (1.6 to 6.8 wt%) of shale source rock from the Silurian Longmaxi Formation of the Zhaotong block. That produced types I and II kerogens and high thermal maturity (2.1 to 3.0%Ro) produced from the burial of older deeper sediments (Zou et al. 2014).

The relationship between bitumen-to-thermogenic gas potential from the high thermal maturity source rock environments has now been widely studied around the world (e.g. Etiope et al. 2013; Pytlak et al. 2016; Xiong et al. 2016; Hakimi et al. 2020; Maeyama et al. 2020). Jarvie et al. (2007) and Zou et al. (2015) have classified the sources of thermogenic shale gas in the hydrocarbon basin into three categories which are kerogeneous (insoluble organic matter), bitumen (soluble organic matter) and pyrobitumen (cracking bitumen residue). Sources in which secondary bitumen cracking process of source rock can generate unconventional gases (Rooney et al. 1995). Therefore, the bitumen-to-gas production is highly dependent on the shale efficiency and efficacy for hydrocarbon expulsion, and the production of gas depends on its high levels of thermal maturity and geological environment (Rooney et al. 1995; Lewan and Henry 2001).

Palaeozoic age rocks are widely distributed in northern states of Malaysia, (e.g. Perlis, Fig. 1). This northern area is assumed to have a deposition of older shaly sediments that could act as an essential unconventional hydrocarbon resource (Baioumy et al. 2016). The geological characteristics of this area in terms of tectonic evolution, stratigraphy, and sedimentology have been studied by numerous researchers (Burton 1970; Jones 1981; Foo 1983; Lee 2001; Hassan & Lee 2005; Cocks et al. 2005; Azman 2009; Metcalfe 2013b; Hassan et al. 2014). However, shale gas studies especially thermogenic gas potential studies are still lacking in Malaysia and also Southeast Asia. Therefore, further analysis into the source and development process of these highly mature/overmature shale rocks is considered essential for shale gas exploration in Malaysia (Nugraheni et al. 2013), especially in terms of the potential of thermogenic gas resources.

Fig. 1
figure 1

The location of the study areas within central Perlis and simplified sedimentary logged section of the studied Paleozoic rock samples in the northern part of Peninsular Malaysia. The geological map and stratigraphy section shown is adapted after Hassan (2013).

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Therefore, in this study, an evaluation of the hydrocarbon generation potential of the Paleozoic shales from the central Perlis area in Malaysia was conducted. The main objectives of this study are (a) to understand the depositional condition setting of the source rock and (b) to determine the source rock potential for hydrocarbon generation. This study applies a combination of organic, inorganic geochemical methods and organic petrological analysis that incorporates biomarker fingerprints and elemental composition analysis to interpret the depositional environments and also to assess the remaining hydrocarbon source potential including the thermogenic gas potential in the study area. This would provide information on the shale gas potential generated by deep-oil source rocks that could be associated with many other potential thermogenic gas basins globally, thereby enabling future exploration of the new petroleum resource region.

Geological setting

During the Paleozoic Era, there were significant tectonic changes that occur on the Earth; thus, it is of great interest to geoscientists. The Paleozoic Era witnessed the breakup of supercontinent Rodinia and the formation of supercontinent Pangaea. The Paleozoic Era also reflects an enormous outbreak of life on Earth that started during the Cambrian Explosion and resulted in the largest mass extinction in the world, known as the Permian–Triassic event of extinction, extinguishing up to 95% of the planet’s life. In Peninsular Malaysia, the Paleozoic Era has seen various geological evolutions which resulted from the collision between the Sibumasu Terrane and the Sukhothai Arc along the Bentong-Raub suture zone that occurred in the early Triassic. The collision led to the development of three longitudinal north–south tectonostratigraphic belts which are Western, Central and Eastern belts with distinct variations in stratigraphy, composition, magmatism, volcanism, types of mineralisation, geophysical signatures and geological evolution (Scrivenor 1928; Khoo and Tan 1983; Metcalfe 2013a, 2017).

Around 25% of the Malay Peninsula is covered by Paleozoic rocks (Lee et al. 2004; Lee 2009), with Lower Palaeozoic rocks restricted to the Western Belt. However, the Upper Paleozoic rocks are distributed in all three tectonostratigraphic belts in Peninsular Malaysia. Metcalfe (1988) proposed that Peninsular Malaysia have consisted of two continental terranes, a Western Gondwana affinity continental Terrane (Sibumasu) and an Eastern Cathaysian affinity continental Terrane (Indochina/East Malaya). The Sibumasu Terrane is characterised by a Paleozoic passive margin sequence which includes a belt of Late Carboniferous to Early Permian glacial marine diamictite (Staufer & Mantajit 1981; Metcalfe 1988). These Early Permian faunas have a high-latitude, cold-water Gondwana affinity and thus correspond with the Western Belt of Peninsular Malaysia (Khoo & Tan 1983). In the north-western part of Peninsular Malaysia, where the study area is located, the complete Palaeozoic series on the peninsula, ranges in age from Upper Cambrian to Upper Permian, can be founded (Fig. 1) (Cocks et al. 2005; Hassan and Lee 2005; Lee et al. 2004; Lee 2009). Nevertheless, only shales containing formations, viz., the Early Devonian Timah Tasoh Formation, are being studied using organic geochemical methods to evaluate their hydrocarbon source potential and interpret their deposition conditions.

Tectonically, the sedimentary deposition of the area is situated at a far distance from a granitic intrusion and is therefore not significantly affected by metamorphic interaction. However, they were affected by regional metamorphism caused by the Western Belt of Peninsular Malaysia orogenic and subduction events (Ghani et al. 2013; Metcalfe 2013b). In addition, Madon (1993) and Tjia (1998) report that a significant deformation event has occurred during Late Cretaceous times as it was associated with a formation of a mantle spot located offshore of Peninsular Malaysia. The formation of mantle hot spot is due to the crustal extension in Malay Basin, which caused a circular regional uplift around 1000 km. Widespread from the hotspot location, which covered all parts of Peninsular Malaysia (Tjia 1998), gives a high heat flow to the surrounding area (Fig. 2).

Fig. 2
figure 2

The location of mantle plum or hotspot that created a Malay dome during Late Cretaceous

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Timah Tasoh Formation

The Timah Tasoh Formation, previously recognised as the “Upper Detrital Member” Setul Formation (Jones 1973, 1981), is an onshore sedimentary formation that formed during Paleozoic orogenic tectonic rifting events. This Devonian stratigraphic formation is characterised by graptolites and tentaculitids forming, with occasional cherts, black shales and mudstones. According to Hassan et al. (2013), the formation gives abundant tentaculitid dacryoconarides, including Styliolina sp., Metastyliolina? sp., M. Lardeuxi, Nowakia acuaria acuaria (Turkestanella), Nowakia (T.) acuaria posterior, and Nowakia (Alaina) matlockiensis. The graptolites Monograptus cf. uniformis and M. langgunensis and the brachiopod Plectodonta (P.) forteyi and the trilobite Plagiolaria are also found associated with the tentaculitids (Jones 1973; Hassan and Lee 2005; Hassan et al. 2014; Metcalfe 2017). The presence of tentaculitides and graptolites indicates that the formation during late Pragian (410 Ma) or earliest Emsian was deposited in an anoxic-dysoxic, marine outer shelf setting with a water depth of between 150 and 200 m (Brett and Baird 1993; Boucot et al. 1999; Hassan and Lee 2005; Hassan et al. 2014).

Samples and methods

Approximately 15 shale samples from the Timah Tasoh Formation outcrop were collected for this analysis. These samples were subsequently examined by applying organic geochemical methods to characterise the type and origin of the organic matter, paleodepositional environment, and thermal maturity to assess their hydrocarbon generation potential. All the samples were collected through the fieldwork by considering non-weathered exposures and oxidised surface (about 20 cm deep) to avoid possible weathering effect commonly associated with outcrop samples. The sampling locations are recorded and shown in Fig. 1 with the location coordinates presented in Table 1.

Table 1 Locations and lithology types of the analysed samples and results of geochemical and optical methods, including TOC content, Rock–Eval pyrolysis and vitrinite reflectance (%VRO) measurements
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Organic geochemical methods

The Rock–Eval/TOC analysis has been used as a screening method to identify and select samples for subsequent analysis such as vitrinite reflectance and biomarker analysis. The total organic carbon (TOC) values and Rock–Eval parameters are as provided in Table 1. During the preparation for Rock–Eval pyrolysis, the shales were pulverised into fine-grade powder and were examined using a Rock–Eval 6 machine fitted with a TOC module. Around 100 mg powder samples were heated and pyrolyzed from 100 to 850 °C to obtain the parameters needed in determining source rock potentials such as the TOC, S1, S2, S3, and the temperature of maximum pyrolysis yield (Tmax). The other parameters such as hydrogen index (HI), production index (PI) and production yield (PY) are as explained by previous researchers (e.g. Espitalie et al. 1977; Peters and Cassa 1994; McCarthy et al. 2011) were also presented (Table 2).

Table 2 Biomarker ratios of the analysed seven representative shale samples illustrating source organic matter, depositional environment conditions and thermal maturity
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About 10 to 15 g of finely pulverised shale samples with TOC more than 0.5 wt% were used for the bitumen extraction analysis for 72 h of extraction by using the Soxhlet apparatus. Bitumen extraction was performed to acquire the extractable organic matter (EOM) or bitumen and hydrocarbon content of the analysed samples. A combination solution of dichloromethane (CH2CI2) and methanol (CH3OH) with a ratio of 93:7 was used as the solvent to extract organic matter. The EOM was separated into specific components using a liquid column chromatography method. Increasing the polarity of the solvents such as petroleum ether, dichloromethane and methanol was applied to fractionate saturated hydrocarbon, aromatic hydrocarbon and NSO compounds for the biomarker analyses. An analysis using gas chromatography-mass spectrometry (GCMS) was executed using an Agilent 5975B inert MSD mass spectrometer instrument. The saturated and aromatic compounds were examined for biomarker fingerprint analysis. The GCMS analysis was conducted using an HP-5MS column heated with the temperature from 40 to 300 °C, rate of 4 °C/min and 30 min hold at temperature 300 °C. The mass spectrometer instrument was directly connected to the 70 eV ionisation voltage ion source, 100 mA filament emission current and 230 °C interface temperature.

Based on the GCMS, the n-alkanes and isoprenoids were analysed using a mass chromatogram of ion m/z 85, and terpanes and triterpanes were using m/z 191, while steranes and diasteranes were analysed using m/z 217 (Appendix Table 4). All of the components were examined and calculated by measuring the peak height and comparing the retention times with the relative abundances by referring to the procedure of calculation from the earliest researchers (e.g. Radke et al. 1986; Peters and Moldowan 1993; Makeen et al. 2015; Hakimi et al. 2016; Ayinla et al. 2017). The results of the analysed samples are presented in Table 2.

Inorganic geological methods

The amounts of major oxides and trace elements of the Timah Tasoh Formation were determined using a plasma mass spectrometer (ICP-MS) coupled with an Agilent inductive spectrometer following the GCMS analysis. To complete the digestion, a PerkinElmer Titan MPS microwave oven digested about 0.50 g of the samples using HNO3, HF, and HCl, and finally H3BO3 was applied to complete the digestion process. The solution was eventually diluted with ultimate pure water up to 100. The research used a standard solution of elements with an analytic concentration of 10 ppm, and a minimum detection limit of less than 1 ppb was used for the instrument calibration. The results of the major oxides and trace elements are shown in Table 3.

Table 3 Major (wt%) and trace element (ppm) compositions of the analysed Timah Tasoh Formation shale samples
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Organic petrological methods

In addition to geochemical methods, petrographic studies were also conducted to determine the type of kerogen and the thermal maturity of the samples analysed. Polished blocks of the samples were made by mounting fine grades of crushed rock samples (about 3 mm in size) inside a slow setting polyester of Serrifix resin mixed along with resin hardener and were left to harden. After 3 days, the hardened blocks were grounded using a grinder machine. Subsequently, they polished the surfaces using silicon carbide sandpaper from decreasing coarse texture by applying isopropyl alcohol as an emollient polished using finer alumina powder with decreasing grades to produce smooth reflecting surfaces. The petrographic analysis was carried out by observing under a Leica microscope (DM6000M) and Leica photometry system (CTR6000) which were fitted with fluorescence illuminators under a white plane-polarised reflected light. Vitrinite reflectance measurements were conducted using a × 50 objective under immersion oil with 1.518 refractive indexes at 23 °C. The mean random (15 to 30 measurements) vitrinite reflectance was calculated and performed using the Diskus Fossil software in the Windows programme. The calculated values for reflectance are as shown in Table 1.

Results and discussion

Petroleum source rock potential and organic matter richness

The richness of organic matter could be determined by calculating the amount of total organic carbon (TOC) conducted together with the analysis of bulk pyrolysis analysis (Behar et al. 2015). Total organic carbon higher than one (> 1 wt%) indicates a good source potential (Peters and Cassa 1994; Hunt 1996; Peters et al. 2005; McCarthy et al. 2011). The TOC content of the analysed shale samples varies from 0.10 to 5.28 wt%, suggesting poor to very good potential based on Peters and Cassa (1994) and McCarthy et al. (2011) classification. The hydrocarbon generative potential (S2) measured through the Rock–Eval pyrolysis together with other related parameters is presented in Table 1.

The shale samples show low S2 compared to S1, ranging from 0 to 0.17 mg HC/g rock-based on Table 1, indicating that only three samples are considered to have a poor hydrocarbon-generating potential. However, other samples show fair to excellent hydrocarbon-generating potential, regarding the minimum TOC required for further analysis is 0.5 wt% (e.g. Bordenave 1993; Peters and Cassa 1994; Hunt 1996). Therefore, as Peters and Cassa (1994) defined, only twelve analysed samples reach the standard 0.5 wt% TOC and subsequently selected for further biomarker analysis.

Kerogen type and hydrocarbon generation potential

Understanding kerogen type (quality of organic matter) and quantity of organic matter from several bulk pyrolysis data studied. Such as the hydrogen index (HI, mg HC/g TOC) against Tmax were used as performed by the earliest researchers (e.g. Peters and Cassa 1994; McCarthy et al. 2011; Hakimi et al. 2016; Ayinla et al. 2017). A cross-plot of HI vs Tmax is commonly applied, as shown in Fig. 3. Based on Fig. 3, the Timah Tasoh Formation shale samples are mostly plotted in the stage of the post-mature zone of the dry gas generation window (type IV kerogen). As the results show lower HI and S2 values (Table 1), the analysed Timah Tasoh Formation samples will not generate liquid hydrocarbons (Peters and Cassa 1994). As being dominated by type IV kerogen that composed of mostly inert constituents thus devoid of petroleum-generating potential, the capability to generate dry gaseous hydrocarbons could not be ruled out (Fuan 1991).

Fig. 3
figure 3

Hydrogen index (HI, mg/TOC) versus maximum temperature at S2 (Tmax, °C) plot showing the high thermal maturity stage of the examined samples and characterised as type IV kerogen

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Origin of organic matter and paleodepositional environment

n-Alkanes and isoprenoids

Biomarker fingerprint analysis of normal alkanes and isoprenoids is widely analysed, and selected measured ratios may interpret the origin of organic matter in both sediments and crude oil (Peters et al. 2005). Based on the bimodal fingerprints distribution in ion m/z 85 chromatogram (Fig. 4), a mixed source of organic matter input is the most probable interpretation for the analysed samples. Normal alkanes in these chromatograms are shown by the domination of medium to high molecular weight compounds of n-C15 to n-C33 and showing moderate height between odd and even n-alkanes. This n-alkane distribution supports the mixed source of organic matter input (Peters et al. 2005; Hakimi et al. 2020).

Fig. 4
figure 4

The m/z 85, 191 and 217 mass fragmentograms of selected shale samples representing the distribution of n-alkanes, isoprenoids, triterpanes, terpanes, steranes and diasteranes

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Acyclic isoprenoids especially pristane and phytane isoprenoids occur in a significant amount in the analysed samples. The pristane/phytane (Pr/Ph) ratios can provide an interpretation of the paleodepositional environment (Ten Haven et al. 1987; Peters and Moldowan 1993; Peters et al. 2005). Within the Timah Tasoh Formation shales, the value of Pr/Ph ranges from 0.71 to 1.67 (Table 2). These Pr/Ph values indicate that the analysed shales were deposited within suboxic conditions of possibility in a mixed terrestrial-marine environment during the deposition of the sediments. These fingerprints have produced moderate values of carbon preference index (CPI) as presented in Table 2 ranging from 0.94 to 1.16. This CPI index is calculated to provide some insights into the source input of the organic matter (Peters et al. 2005). On the other hand, the low molecular weight compounds especially those below n-C23 are depleted, and this could be due to the weathering effects on the outcrop. However, the collected samples appear to be fresh in hand specimen. This is partly consistent with the low S1 values determined from the Rock–Eval pyrolysis.

The results based on the Pr/Ph and CPI ratios can be corroborated by determining the correlation among the values of pristane/n-C17 and phytane/n-C18 ratios. Based on these ratios (Table 2), Pr/n-C17 and Ph/n-C18 ratios for the Timah Tasoh Formation range from 0.20 to 0.52 and 0.14 to 0.27 and have provided strong support for the origin of organic matter which is from a mixed organic matter source input. These sediments were most likely have been preserved under a suboxic condition within transitional environments deposited partially under reducing marine depositional environments (Fig. 5).

Fig. 5
figure 5

Cross-plot of Pr/n-C17 versus Ph/n-C18 showing the organic matter input in the examined samples of the Timah Tasoh Formation

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Hopanoid and steroid biomarker

The distribution of hopanoids and steroids among saturated hydrocarbon is commonly monitored on the fragmentograms of m/z 191 and 217 (Peters et al. 2005). According to Hunt (1996), hopanoids are important in petroleum hydrocarbon because they can maintain their habitual structure from the indigenous biological compounds. Ion m/z 191 of Timah Tasoh Formation is dominated by C27, Ts, Tm, C29, C30 hopane, and C30 to C34 homohopanes. The ion m/z 191 from the saturated hydrocarbon compounds shows many homohopanes than tricyclic terpanes, as shown in Fig. 4.

Peters et al. (2005) reported that the low abundance of tricyclic in these analysed samples suggests that these shale samples have considerable influence on terrestrial-derived organic matter, considering the high concentration of tricyclic terpanes is often associated with the marine environment. The homohopanes distributions from the analysed samples are controlled by the C31 homohopanes and decline to C34 homohopanes in a cascade pattern in the Timah Tasoh Formation (Fig. 4). According to Peters et al. (2005), Makeen et al. (2015) and Ayinla et al. (2017), the cascade pattern suggests that the organic matter was conserved in a suboxic condition. In the analysed ion m/z 191 fragmentograms, the C30 hopane in the Timah Tasoh Formation is more dominant than C29 hopane, which indicates the samples contain clay-rich sediments as reported from previous studies (e.g. Peters et al. 2005; Makeen et al. 2015).

In addition, the presence of gammacerane had initially been identified as an indicator of hypersalinity (Sinninghe Damsté et al. 1995; Makeen et al. 2015). Increasing salinity in both the marine and lacustrine environments has also been found to be associated with the presence of gammacerane (Waples and Machihara 1991; Peters and Moldowan 1993). In Table 2, the ratio of G/C30 from the analysed samples ranges from 0.04 to 0.11. The existence of gammacerane in the chromatograms of the Timah Tasoh Formation (Fig. 4) gives evidence of a slightly low salinity around the shale’s deposition environments (e.g. Peters et al. 2005; Makeen et al. 2015).

The distribution of steroids is characterised from the m/z 217 fragmentograms. Steroids are another group of essential biomarkers that are derived from sterols found in higher plant and algae, but rare or absent in prokaryotic organisms (Seifert and Moldowan 1979). As an indication of variations in source environments, the comparable abundance of steranes C27, C28, and C29 in ion m/z 217 can be used for the study (Hunt 1996). The standard sterane abundance of C27, C28, and C29 is determined and calculated in Table 2. A ternary diagram modified from Huang and Meinschein (1979) is used to represent the results by plotting the steranes according to the regular members of the steranes, as shown in Fig. 6A. As the C29 is in the ranges of 35.8 to 57.3% relative to C27 and C28 (Table 2), the high percentage of C29 steranes confirms mixed organic source material interpretation. In the Timah Tasoh Formation sediments, these components of mixed organic matter were characterised by compositions of the source of planktonic, bacterial, and land-derived organic matter input. The result is consistent with the reduced C27/C29 regular steranes deposition values in the suboxic condition (Fig. 6B), which was consistent with the decreasing hopane pattern in Fig. 4.

Fig. 6
figure 6

(A) Ternary diagram of C27, C28 and C29 regular steranes indicating a mixed organic matter input in the examined samples of the Timah Tasoh Formation. (B) Cross-plot of C31 22R/C30 hopane versus Pristane/Phytane ratios of the examined samples (adapted after Peters et al. 2005)

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Geochemistry of major and trace element

Paleoredox conditions during sedimentation

In addition to the GCMS biomarker identification, the geochemistry of the major oxides and trace elements measured from ICP-MS was also can be used to identify the paleodepositional condition and condition during sedimentation in the analysed sediments. From the concentration of trace elements, paleoredox conditions of sedimentary facies could be evaluated, primarily due to their strong association during the deposition in both sedimentary rocks and trace elements, as well as in the phases of kerogen transformation. Trace elements such as V, Ni, U, Th, Cr, Sr and Ba were used during sedimentation with specific standard ratios such as V/Ni and V/Cr applied to determine redox conditions (Peters et al. 2005; MacDonald et al. 2010; Fu et al. 2011).

Vanadium (V) and nickel (Ni) are the significant metals usually present during the early diagenesis of organic matter and are valuable indicators of redox conditions of sedimentation (Peters et al. 2005; Galarraga et al. 2008). The reduced depositional environment can be interpreted by V/Ni > 3 and the suboxic environment (V/Ni from 1.9 to 3) based on the V/Ni ratio as described by Galarraga et al. (2008). Preferential vanadium enrichment over its paleoenvironmental marker (nickel) results in V/Ni ratios of the analysed range from 2.54 to 2.81 (Table 3). As seen in the vanadium (V) versus nickel (Ni) cross-plot in Fig. 7A, this result indicates the terrestrial source of organic matter preserved under suboxic to oxic conditions. Based on these values, throughout the organic matter sedimentation, the cross-relationship between low total sulphur content (ranging from 0.21 to 0.34) and moderate values of V/(V + Ni) ranging from 0.71 to 0.74 generally confirms suboxic to oxic conditions of the analysed samples (Fig. 7B) (Barwise 1990).

Fig. 7
figure 7

Cross-plots of (A) vanadium (V) versus nickel (Ni) and (B) V/(V + Ni) versus total sulphur contents (TS, wt%) for paleodepositional environment conditions

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In addition, the identification of chromium (Cr) and vanadium (V) concentration in the analysed samples can be used for the interpretation of paleoredox condition during sedimentation processes (Jones and Manning 1994). In oxic environments, the concentration of Cr is higher than in V and vice versa for anoxic conditions; therefore, Jones and Manning (1994) identified the V/Cr ratio above 2 as an anoxic environment indicator below 2 suggests suboxic to oxic conditions. A V/Cr ratio of between 1.21 and 1.61 ppm was seen in the studied samples (Table 3). This V/Cr ratio of the analysed samples further indicates the predominance of suboxic to oxic conditions during the organic matter sedimentation. Since various physical and chemical processes control organic matter preservation, other factors such as sedimentation rate, clay mineral content, and oxygenation levels of the water column are also reported to be involved in the sample preservation control (Zonneveld et al. 2010).

The concentrations of strontium (Sr) and barium (Ba) are crucial indicators of paleosalinity with a higher Sr/Ba ratio, perceived as high salinity, and low Sr/Ba ratio as a low salinity (Makeen et al. 2015, 2019). The analysed samples have a reasonably low Sr/Ba ratio (0.28 to 0.34 ppm), meaning that saline water has a relatively low effect during deposition, suggesting that the samples will be deposited in a stratified water column under suboxic conditions. Perhaps the low abundance of gammacerane lends (Fig. 4) supports this low salinity condition as the occurrence of gammacerane is often indicative of high salinity (as discussed in “Hopanoid and steroid biomarker” section).

Paleoclimatic conditions

For assessing paleoclimatic conditions in the samples studied, the composition, concentration and abundance of significant major oxides such as SiO2, Al2O3, TiO2 and K2O are the important factors needed during the analysis (Suttner and Dutta 1986; Roy and Roser 2013; Makeen et al. 2019). Roy and Roser (2013) suggest that the amounts of SiO2 and Al2O3 (Table 3) prove that the examined shale samples are derived from clay groups such as illite and kaolinite. The existence of aluminium (Al) and gallium (Ga) is usually enriched with kaolinite. It is correlated with the warm and humid climate. At the same time, illite is also correlated with potassium (K) and rubidium (Rb), indicating poor chemical weathering and dry and cold weather (Beckmann et al. 2005; Ratcliffe et al. 2004).

Many illite-rich sediments would have lower Ga/Rb and higher K2O/Al2O3 ratios, while higher Ga/Rb and lower K2O/Al2O3 ratios will have high kaolinite-rich deposits (Ratcliffe et al. 2004). The shales of the Timah Tasoh Formation have relatively low Ga/Rb ratios and high K2O/Al2O3 ratios based on the results in Table 3. Indicating that illite clay mineral is higher than kaolinite clay mineral thus supports the interpretation of the predominance of illite mineral area (e.g. Baioumy et al. 2016). Therefore, Timah Tasoh Formation can be correlated to be formed during the arid climatic conditions. It is due to low water salinity during shale formation, as shown by low Sr/Br ratios (Table 3).

Organic matter optical characterisation and relation to thermal maturation

In this study, observed under the microscope, three types of macerals organic particles can be recognised under the microscope, which are recycled/reworked vitrinite, inertinite (Fig. 8A - D) and vitrinite-like bitumen or solid bitumen (Fig. 8E - H). Vitrinite-like bitumen or solid bitumen is a secondary organic matter substance mainly formed from converting the primary organic matter. Resulting from previous geological cycle exposure to relatively high temperatures (e.g. thermal alteration, burial tectonic pressure, and heating rate) that further caused the cracking of the primary organic matter (e.g. Jacob 1989; Jarvie et al. 2007; Mastalerz et al. 2018). In addition, the typical organic matter sequences of late to dry gas windows maturities are always significant to solid bitumen formation (Rippen et al. 2013; Hackley and Cardott 2016; Liu et al. 2019).

Fig. 8
figure 8

Selected photomicrographs (A to H) of the analysed samples from the Timah Tasoh Formation. A to D Small fragments of highly reflecting inertinite phytoclasts suggesting recycled organic matter and light grey vitrinite particles. E to H Grey to dark-coloured angular solid bitumen accumulation in pore spaces and natural fractures

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The optically observed bitumen was in the form of granular solid bitumen type in all of the analysed Timah Tasoh Formation samples which produced a dark grey in colour (Fig. 8E to H) with low reflections and displayed no fluorescence under blue light excitation. This solid bitumen was described either as fracture and void fillings or as dispersed particles inside the matrix of organic minerals (Landis and Castaño 1995). According to Cardott et al. (2015), as in the source rocks, the presence of solid bitumen suggests the existence of mature organic matter which can initially generate liquid hydrocarbons. The observation of the solid bitumen within samples indicates indigenous samples hosting post-oil solid bitumen, which originates and occurs in the exact location as the occurrence of trapped hydrocarbons suggesting that the examined solid bitumen is primarily a form of the residue of altered hydrocarbons. Post-oil solid bitumen could develop during thermal evolution as a primary solid bitumen or pre-existing hydrocarbons (Jarvie et al. 2007; Mastalerz et al. 2018). Therefore, it is likely an in situ solid primary oil migration remnant (Cardott et al. 2015).

The other organic components found in the analysed samples are reworked lighter grey vitrinite and macerals inertinite groups that give a slightly higher reflection than solid bitumen. Vitrinite reflectance measurement was carried out on the analysed samples of the Timah Tasoh Formation. The results are shown in Table 1. The formation contains few primary vitrinite particles, and this has caused difficulties in taking the reflectance measurements. The photomicrographs of Fig. 8 show the common occurrence of a reworked organic matter, plenty of amorphous organic matter (AOM) and inertinite macerals of type IV kerogen. The Timah Tasoh Formation has vitrinite reflectance values with a range of 1.22 to 1.93%Ro. These ranges of values suggest a high thermal maturity for oil generation of late oil to dry gas window. Therefore, the analysed rocks can be classified as mature to overmature source rocks (e.g. Tissot and Welte 1984; Peters et al. 2005).

The occurrence of solid bitumen is pertinent when vitrinite is missing or absent and always related to high thermal maturity indicator (Landis and Castaño 1995). The relation of solid bitumen reflectance values and vitrinite reflectance values is commonly used among previous researchers in conducting thermal maturity evaluations (e.g. Bertrand 1993; Ferreiro Mählmann and Bayon 2016; Liu et al. 2017). The reflectance values of solid bitumen vary from 1.05 to 1.39%SBRo (Table 1), which characterises the final stage of the oil window to the gas generation window. The results of vitrinite and solid bitumen reflectance reveal a clear linear association between these two organic matter components associated with an increase in the vitrinite reflectance maturity level and an increase in solid bitumen reflectance (e.g. Waliczek et al. 2019). Besides vitrinite reflectance, other various methods can be utilised for the interpretation of thermal maturity level in sedimentary rocks which includes pyrolysis (Espitalie et al. 1985), thermal alteration index (Staplin 1969) and conodont colour alteration index (Deaton et al. 1996). However, methods including vitrinite reflectance (%Ro), bulk pyrolysis (Tmax, HI, PI, S2) and interpretation of biomarker fingerprints remain the most common method used (Tissot and Welte 1984; Peters et al. 2005).

Based on the pyrolysis analysis, most of the analysed shale samples possess Tmax values above 550 ºC (Table 1); this characteristic suggests that the organic matter has subjected to a high level of maturity. The gas generation window has been reached (Fig. 3), which is generally supported by the vitrinite reflectance and solid bitumen reflectance values discussed above. Apart from Tmax, the production index (PI) ratio (S1/S1 + S2) can be implemented to examine thermal maturity (Merril 1991). All of the analysed samples possess PI values > 0.25 (Table 1), thus in support of the high thermal maturity level (e.g. Merril 1991). The cross-plot between PI and Tmax (Fig. 9A) shows that the analysed samples are in the gas generation window.

Fig. 9
figure 9

(A) Pyrolysis Tmax versus production index (PI), showing the maturation of the analysed shale samples. (B) The cross-plot of C29 sterane ββ/(ββ + αα) versus C29 sterane 22S/(22S + 22R) showing the thermal maturity level of the analysed Timah Tasoh Formation shales

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In addition, multiple biomarker parameters can also be used to assess the thermal maturity of the organic matter, such as the distribution of n-alkanes in the analysed samples, according to Waples and Machihara (1991) and Peters et al. (2005), as shown in m/z 85. The carbon preference index (CPI) ratio is commonly applied to determine maturity. CPI values below to one indicate as immature, while near to one indicates mature samples. Based on this study, the CPI ratios are in the range of 0.94 to 1.16 (Table 1) for the analysed samples, showing they are thermally matured. According to Peters et al. (2005) and Hakimi and Abdullah (2013), the C32 22S/(22R + 22S) homohopanes ratio from m/z 191 is also commonly applied to assess thermal maturity. For the Timah Tasoh Formation, the C32 22S/(22R + 22S) homohopanes range within 0.54 to 0.68 (Table 2) indicates the stage of oil generation already been outstretched or passed (Peters et al. 2005).

The moretane/hopane ratios concur with the organic matter being mature with values below 0.15 (e.g. Seifert and Moldowan 1979; Peters et al. 2005). According to Seifert and Moldowan (1986), the importance of moretane/hopane decreases with increasing thermal maturity. Moretane/hopane ratios of the Timah Tasoh samples range from 0.06 to 0.14 (Table 2). These calculated values reveal that the organic matter from the samples examined is at a high thermal maturity stage and thus supports the assessment based on the vitrinite reflectance measurements discussed earlier. In addition, according to Moldowan et al. (1986), although the ratio of Ts/Tm is known to be influenced by source facies, the Ts/Tm can be applied to measure and determine the thermal maturity level. As stated by Peters et al. (2005), during catagenesis, C27 18 α-trisnohorpane (Ts) is more stable than C27 17 α-trisnohorpane ratio (Tm). As can be observed in Fig. 4, Ts is more dominant compared to Tm, thus indicating that the analysed samples of the Timah Tasoh Formation are thermally matured. The cross-plot between C29 22S/(22S + 22R) and ββ/(ββ + αα) sterane from the m/z 217 fingerprints is particularly constructive in explaining the thermal maturity of source rocks (Seifert and Moldowan 1986; Peters et al. 2005). The analysed samples of the Timah Tasoh Formation are mostly already passed the thermal equilibrium, thus consistent with their generation of post-mature source rocks (Seifert and Moldowan 1986) (Fig. 9b).

As noticed with the combination of organic geochemical and petrological results presented, most of the organic matter in the analysed shale samples from the Timah Tasoh Formation is shown to attain a significantly high thermal maturation stage by the measured vitrinite reflectance (%Ro). It can be applied to determine the paleoheat flow (Botor et al. 2019), which was influenced by the burial temperature and geological exposure period (Sweeney and Burnham 1990; Hakimi et al. 2020). These results can be explained or attributed to factors such as regional tectonic setting caused by collisional orogenesis and subduction of the Western Belt of Peninsular Malaysia that produced various tectonic deformation (Ghani et al. 2013; Metcalfe 2013b). And also, a consequence of thick overburden rocks of Late Devonian to Permian in Paleozoic age times (Hassan and Lee 2005). Then, the major of high heat flow from a mantle hotspot during the late Cretaceous times (Tjia 1998) caused a sudden high thermal temperature that has cooked all the sediments in the surrounding (Fig. 3).

Generation of thermogenic gas and the impacts on the future hydrocarbon exploration

Having the occurrence of solid bitumen as observed under the microscope suggests that the shale samples initially have the capability to produce liquid hydrocarbons (Cardott et al. 2015; Waliczek et al. 2019) which supported the generation potential being fair to excellent based on the initial TOC content before the overburden rocks caused the cracking of primary kerogen which altered its characteristics to type IV, thus resulted in the low HI values. Moreover, the high Tmax indicates an increase in maturity (e.g. Peters et al. 2005; Dai et al. 2014). However, the high thermal temperature from the mantle hotspot caused the transformation of primary organic matter to secondary organic matter (secondary cracking of bitumen) which consequently transformed the liquid/wet gaseous hydrocarbons into thermogenic gas.

In the analysed Timah Tasoh shales, the initial types of organic matter and their preservation during deposition were primarily demonstrated from the biomarker fingerprints of the residual organic matter. The lipid biomarkers in the saturated hydrocarbons suggest that mixtures of organic matter are from phytoplanktonic algae, bacterial and land plant inputs which present in the analysed Timah Tasoh shales. These phytoplankton/bacterial lipids and terrigenous organic matter indicate a mixture of predominantly types II and III kerogens as the original organic matter during deposition of the Timah Tasoh shales. The distributions of the biomarker in the examined shale samples indicate that the analysed samples were accumulated under substantial environmental conditions of water column reduction and stratification. These environmental conditions have thus increased the preservation of organic matter and have consequently resulted in the organic matter being enriched during deposition.

However, these rich organic matter inputs derived from lipid biomarkers of the analysed Timah Tasoh shales are inconsistent with the geochemical pyrolysis results, in which the analysed samples consist mainly of type IV kerogen as indicated from their current low HI values. As shown by the high pyrolysis values (i.e. Tmax and PI) and optical VR values of more than 1%, this probably is due to its high thermal maturation of the gas generation window, which may have depleted possible liquid hydrocarbons in which the high maturity is known to reduce the HI and TOC sample (Dai et al. 2014; Hakimi et al. 2020). Therefore, the shale intervals in the Devonian Timah Tasoh Formation of northern Peninsular Malaysia were, as a result of this, postulated to possess potential as a shale gas source rock (Fig 10).

Fig. 10
figure 10

Relationship between TOC content and HI values of the analysed shale samples in the Timah Tasoh Formation in northern peninsular Malaysia, showing gas generation potential

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It is most evident within the analysed Timah Tasoh samples in the study area. The significantly high thermal maturation experienced on the organic matter can be attributed to the high heat flow (> 110 mW/m2) from a mantle hotspot during the end of Late Cretaceous times (Madon 1993; Tjia 1998), combined with the high overload rock thickness from late Triassic to late Cretaceous sequences. Consequently, in this analysis, the gas potential of the Timah Tasoh sediments is mainly thermogenic gas. It consists of two separate processes: the primary conversion of kerogen (types II and III) to oil and wet gas due to the high temperature of sedimentary rock thickness and the secondary cracking on the retention of generated oil and wet gas in the source rock system. This is regarded as the key to its inherent thermogenic gas potential.

Such a scenario of converting from the type II and III kerogen into type IV kerogen related to the high thermal maturation and reducing HI values from pyrolysis can also be found from other worldwide case studies. Hakimi et al. (2020) indicate that the Madbi Formation shales are a thermogenic gas potential in the Say'un-Masila Basin, Yemen. Most organic matter has TOC values of more than 1% and consists of large amounts of marine organic matter, influenced by the land plants environments. These Late Jurassic Madbi shales showing a good source rock, thus, indicate the Madbi Formation to have types II and III kerogen that initially could produce oil and gas.

The increase in burial temperature is due to overburden and the thickness of the sedimentary deposition that occurred from early Carboniferous to recent—leading to decreasing HI values of the organic matter—giving rise to high thermal maturity values (above 1.30%Ro) of this formation. Such high thermal maturity significantly changes the original organic matter (type II and III to type IV), which eventually caused the cracking of retained oil in the source rock. It conversely changes from oil into gas. It is the factor in the Say'un-Masila Basin generation of thermogenic gas.

Besides the high thermal maturity impact from the conditions of the tectonic event that generated thermogenic shale gas in the Timah Tasoh Formation, the conversions of this bitumen-to-gas generation can also be correlated with other potential petroleum sedimentary basin areas. Such as Katakolo Bay in Western Greece (e.g. Etiope et al. 2013) and Xiamaling Formation in the Xiahuayuan region (e.g. Xiong et al. 2016). Most of these thermogenic gas generation is mainly produced by secondary cracking of retained bitumen in highly mature and overmature shale. It is controlled by geological factors such as high thermal heat flow from the tectonic rifting events that caused various deformation events and controlled by the sediment burial history that influenced the increase of vitrinite reflectance values (Peters et al. 2005).

Therefore, the analysed Timah Tasoh Formation samples can generate thermogenic gas in the deeper parts of the Paleozoic succession. As the high maturity phase impacts the organic matter, which has already lost about 50% of its original organic carbon content (Hakimi et al. 2020), this could relate to the prospective hydrocarbon exploration in Malaysia and Southeast Asia.

Concluding remarks

At the end of the study, the integrated organic, inorganic geochemical and petrological study performed in this study on the Paleozoic shales of the Timah Tasoh Formation in northern Peninsular Malaysia has unravelled aspects of organic matter source input, paleodepositional environment, thermal maturity and the hydrocarbon generation potential. Therefore, the main research objectives which to understand the depositional condition setting of the source rock and also to determine the source rock potential for hydrocarbon generation can be fulfilled by the results as concluded as follows:

  1. 1.

    The analysed Timah Tasoh Formation shales possess TOC content ranging from 0.10 to 5.28 wt% and thus graded as poor to excellent source rocks.

  2. 2.

    The microscopical study showed that the shale samples are dominated by inertinite and recycled vitrinitic phytoclasts. The formation of solid bitumen supports the capability of the Timah Tasoh Formation shales to generate hydrocarbon.

  3. 3.

    Biomarker assemblages, petrographic, and pyrolysis data revealed that shales of the Timah Tasoh Formation contain primarily type IV kerogen of terrestrial-derived but showed the presence of marine influence. This mixed organic matter input suggests a transitional environment of deposition for the analysed sediments.

  4. 4.

    Based on the evaluation of the trace elements, the ratios of Sr, Ba, V, Ni and Cr indicated a stratified water column with low salinity and suboxic to oxic bottom water conditions.

  5. 5.

    Rock–Eval pyrolysis data, petrographic study and biomarker parameters of the analysed Timah Tasoh Formation shales show that the sediments are at a post-mature maturity level. However, they are presently most likely possessed gas potential for oil generation based on the high vitrinite reflectance and solid bitumen reflectance values.

  6. 6.

    This study, therefore, suggests that the shale sequences within the Paleozoic Timah Tasoh Formation have a variable potential of poor to excellent source rock with no remaining liquid hydrocarbon potential based on the very low HI values. And the organic matter is dominated by type IV kerogen, but they have thermogenic gas generation potential at deeper levels. The Timah Tasoh Formation itself can act as source rocks from the transformation of types II and III kerogens into type IV kerogen due to higher thermal maturation attributed to the high heat flow of a mantle hotspot during the later Late Cretaceous. Thus, the data produced can reference future petroleum exploration activities for conventional and unconventional hydrocarbon resources in this northern Peninsular Malaysia region.