Petrography, Palynology, and Organic Geochemistry of Neogene Paralic Coal, Mukah,Sarawak, Malaysia: Implications For Paleomires, and Peat-Forming Conditions

The eight coal seams of Neogene paralic coals from Mukah coaleld, Sarawak, Malaysia, were investigated using petrographical, palynological, and organic geochemical analyses to describe coal-forming vegetation, conditions during peat development and precursor mires, and their associations in a sequence-stratigraphic context. The petrographic data of the coals implies the existence of oxygen-decient and water-saturated conditions in the precursor mires. The condition of low mire oxidation was followed by biomass loss from the mires. The Mukah coals are suggested to be deposited in freshwater peat swamps, and the rich preservation of angiosperm pollens indicates that the organic matter in dense and lowland forest vegetation was mostly terrigenous. The overwhelming presence of Casuarina and Calamus types, suggesting the paleomires were closely linked to Kerapah/Kerangas peat forest and marginally bordered by rattan and supported by the biomarker data. Rheotrophic–ombrotrophic mires were temporarily formed because of water table uctuations, which strongly depend on ever-wet climate changes and syn-depositional tectonic during the Neogene, resulting in balanced to high peat accumulation and preservation. A maximum thickness of 35m of peat deposits that formed between 10,000 and 175,000 years ago is suggested. The coals are proposed to be inuenced by transgressive to initial highstand cycles within the paralic setting. Previous work on Mukah coals has emphasized the quality and re-estimation of coal reserves 1,5,6,7,8 . The main objective of this study is to provide signicant insights into how the composition and stratigraphy of coal at a single locality changes over time in response to changes in accommodation space, compaction, paleoclimate, and/or tectonic subsidence, allowing for the prediction of coal quality and its corresponding relation to the basis of sequence stratigraphy study of the Neogene paralic Mukah coal seams, using petrographical, geochemical, and palynological studies. by the huminite group (41.9– 91.9%) and mainly consist of humotelinite of textinite (1.5–26.2%) and ulminite (9.5–55.1%). The composition of liptinite does not signicantly vary with an average of 2.0–25.3% for all samples. Minor amounts of inertinite (1.4–5.5%) are found and characterized by fusinite, semifusinite, funginite, and inertodetrinite. Mineral matter mainly occurs as clay minerals and is typically dispersed. High amounts of argillaceous mineral matter (up to 50.6%) are reported in certain intra-seam. The amount of pyrite was almost non-existent, so it was not recorded in this study. For microlithotypes, the Mukah coals are characterized mainly by clarite, humite, and duroclarite with an average of 3.5–88.7%, 5.5–61.1%, and 0.0–8.5%, respectively. Liptite, inertite, durite, huminertite, and huminertoliptite were discovered infrequently (<1%). Carbominerite occurs in certain intra-seams and depends on the mineral matter occurrences. coals studied. n-C and n-C 31 the suggesting a contribution of higher CPI range between and 3.66 (mean 1.57). vs. Ph/n-C 18 and atomic ratios of H/C and O/C support the interpretation of organic matter source input, within the sub-oxic to oxic (mainly) condition 32 [100] and mainly from Type III kerogens and mixed Type II/III. The ratio of C/N is reported to be greater than 20 and 40 for the investigated samples, further representing the organic input from higher plants 32


Introduction
The petrographic composition shows values in range and average (avg.) as total (vol.%), and mineral-free basis (vol.%, m.m.f.)/ carbominerite free basis (vol.% Mukah coals. IV factor and correlation coe cients of variables are also shown.      The huminite/vitrinite of the coals being studied ranged from 0.39 to 0.49 R%. The accuracy of thermal maturity assessment is also supported by the percentage of non-hydrocarbon compound and biomarker distributions 22,23 .
Twenty-four of the 45 analyzed coal samples contained abundant and well-preserved palynomorphs, some of which are depicted in Fig. 3, and their vertical distribution is shown in Fig. 4 Bulk geochemical data.
The results of petrographic and organic geochemical studies of coals being studied are reported in Tables 2 and 3. High organic carbon values with TOC ranging from 37.62 to 75.8 wt% and high quantities of extractable organic matter (EOM) and hydrocarbon yields (HC), exceeding 29,000 and 18,000 ppm, respectively, were recorded. These results suggested that the analyzed samples are rich in organic matter and agree well with the plot of TOC content versus EOM ( Fig. 5a) and, hence, are capable of producing a substantial amount of gas with little to no oil product (Fig. 5b).  R dit -(19-norisopimarane + isopimarane + 16α-kaurane) / (ent-beyerane + 16β-phyllocladane + 16α-phyllocladane) Ts n-Alkanes (m/z 85, Fig. 6a) are mainly distributed between the n-C 16 to n-C 33 of the coals being studied. n-C 27 , n-C 29 , and n-C 31 compounds show the highest abundance, suggesting a contribution of higher molecular plant compounds, resulting in a relatively high CPI range between 0.86 and 3.66 (mean 1.57).
Pristane (Pr) being more abundant than phytane (Ph), with the ratio of Pr/Ph ranging from 0.09 to 3.67 (mean 1.82). Data show that the Pr/Ph ratio of most samples (80%) is higher than 3.0, whereas the remaining 20% is higher than 0.6, indicating a mainly oxic to a sub-oxic environment with dissolved oxygen in water between 1.0 and 0.1 ml/L. The ratio of Pr/n-C 17 varies from 0.31 to 6.67, with an average of 1.77, and Ph/n-C 18 ratio ranging from 0.14 to 1.00, with an average of 0.41. The cross plot of Pr/n-C 17 against Ph/n-C 18 24 (Fig. 7a) shows the organic matter source of the Mukah coals to be mainly composed of terrestrial input. Some plots indicate sources from mixed organic matter. This can be de ned from the distribution of the n-alkanes and acyclic isoprenoids, which show substantial occurrences of compounds of high molecular weight from n-C 23 upwards in most of the analyzed samples, implying that higher plant molecular compounds are the source inputs 25,26 . This interference is also in line with the high CPI values, which indicate the paleodeposition in the terrestrial environment 25 .
Diterpane (m/z 123) distributions were performed to the selected samples (Fig. 6b), indicating ranges from 1.13 to 1.59, with the majority falling into group 3.
Samples from the lower section of the Mukah coals (MC05-08, 53) fall into group 2 with R dit 1.59 (>1.5 R dit ). This nding implies the coals in the middle stratal section were deposited in mires in uenced by a high-water table, whereas uctuations in the water table may occur in the lower and upper sections of the Mukah coals, as represented by R dit values ranging from 1.13 to 1.59 (mean 1.32). Thus, the climate was probably wetter during the coal formation, as proposed by 27,28 .
Hopanes are found in signi cant amounts over tricyclic terpanes (m/z 191; Fig. 6c), predominantly over C 29 -norhorhopane, C 30 -hopane, and C 31 -C 32 homohopanes. The ratio C 29 /C 30 hopane values was found greater than 1 (1.17-4.50), indicating a typical source of terrestrial organic matter 24  The plot of S vs. TOC (Fig. 10) may further describe the development of peat in freshwater mires within topogenous to ombrogenous mire facies, as suggested by 31 . The vertical changes in hydrological regimes (Fig. 9) under which the precursor mires accumulated indicate that the peat-forming mires began with predominantly ombrotrophic mires and eventually ended in rheotrophic mires because of the moderate to rapid rise of the water table during peat accumulation. Temporary development of rheotrophic-ombrotrophic mires may have occurred because of the water table uctuations. Here, the mires were fed by rainfall and groundwater levels that formed during low to moderate water oods. In most cases, the intervals suggest that waterlogged and rheotrophic origins are related to the occurrence of increased mineral matter, which was probably in uenced by the clastic in ux from the uvial input into the mires with rising water table levels. Furthermore, the biomarker distribution and concentration of n-alkanes, high Pr/n-C 17 , low Ph/n-C 18 , high CPI, high C 29 /C 30 hopane, high Tm/Ts, high tetracyclic, and high C 29 sterane support the fact that the source of the organic matter in the Mukah coals is mainly terrestrial. In addition, the plots of Pr/n-C 17 vs. Ph/n-C 18 and atomic ratios of H/C and O/C support the interpretation of organic matter source input, within the sub-oxic to oxic (mainly) condition 32 [100] and mainly from Type III kerogens and mixed Type II/III. The ratio of C/N is reported to be greater than 20 and 40 for the investigated samples, further representing the organic input from higher plants 32 .
Peat-forming vegetation. References of peat-forming vegetation were made to the known ecological data of ancient peats in Malaysia and Brunei indicated by 33,34,35.36,37 .
The vertical pro le (Fig. 4) appears to be predominated by the freshwater peat swamp communities. Meanwhile, the riparian, strand forest, ferns, and mangrove swamp vegetation, which are present in low frequencies, support the ecology within the main lowland forest. The rich preservation of angiosperm pollens suggests that mostly terrigenous inputs are fed into the organic matter in dense and lowland forest vegetation. Overall, the plant communities appear to be represented mainly within freshwater-in uenced swamps. The presence of Calamus type, Barringtonia, Palaquium, Pandanus, and Stenochlaena palustris in a few coals being studied represent a former riparian fringe type of forest. The occurrence of mangrove pollens in sections MC12 and MC01 in small quantities suggests the mangrove pollens were within proximity to the peat swamp forest communities. In contrast, no indication of mangrove in uence is recorded in section MC05. Although pteridophytic spores of monolete smooth and Stenochlaena palustris are found in the entire coals being studied, they are not abundant, suggesting that the plant types were slightly in uenced by fern taxa. Moreover, the signi cant occurrence of Casuarina type in the middle part of section MC12 and the small amount of pollen in most of the samples suggests that the Mukah coals were deposited close to the sea under the in uence of the strand forest.
The assemblage list recovered in this study is similar to that provided by 2 . Compared with their work, this study yielded approximately similar assemblages of palynomorphs, but in slightly different proportions. The overwhelming presence of Casuarina type and Calamus type found in Mukah coals further suggests that the peat swamp vegetation was closely linked to Kerapah/Kerangas peat forest and marginally bordered by rattan 2 even though these pollens were found restricted to certain seam intervals of the coals being studied.
Controls on peat formation. Following the coal-forming models 38, 39,40,41,42,43,44,45 , the authors surmised that different characteristics, architecture, and thicknesses of coal seams are governed by the height of the mire water table, which can be correlated with the transgressive-regressive cycles and, hence, may re ect different system tracts and the signi cance of key surface of sequence-stratigraphic development in coal-bearing strata. Since the Mukah coals were deposited in a low-lying coastal setting, the groundwater and seawater are hydrologically connected. Thus, the rise in relative sea level can cause the groundwater table to rise in coastal mires, resulting in accommodation space for peat accumulation and upstream deposition of siliciclastics in the study area as the accumulation of peat requires a balance between the rate of accommodation space and peat accumulation (AR/PPR).
Overall, the Mukah coal formation initially occurred at a low water table that allowed the peat to form in peat-forming mires. The peat growth progressed within an overall rising water table (base level). During the peat formation, the creation of accommodation space was moderate to high in response to the rate of change in the base level. A gradual decrease in the rate is expected at the end of peat growth, which allowed the peat to accumulate and maintain a steady pace with the rise in the water table (Fig. 9). In some cases, in particular the upper section of the Mukah coals, the coal formation occurred in the mires during a high-water table, which resulted in high peat accumulation, followed by stable water table conditions, and balanced peat accumulation (Fig. 9). Since the Mukah coals were located in a coastal setting, peat accumulation and preservation were, therefore, associated with the sea-level rise in the paleo-mires, as indicated by the balanced to high AR/PPR. The balance in AR/PPR may cause partially expose paleo-peat bodies in the mires, whereas high AR/PPR may cause the drowning and inundations of paleo-peat bodies. Therefore, it is suggested that the proposed vertical variations in the lithotypes, microlithotypes, and macerals have been controlled by uctuations in the groundwater level in ancient freshwater mires, thereby resulting in different accommodation/peat preservation rates (Fig. 9). High huminite, low inertinite, and high humite and clarite contents in most of the coals being studied indicate permanently watersaturated peat with balanced to high accommodation creation 46 . The increased liptinite content in the middle section of the Mukah coals indicates a loss of biomass and poor preservation of woody tissues. This may re ect the high preservation of herbaceous vegetation (unstructured material) in the mires. The moderate detrital-mineral matter content in topogenous peats may imply the association of uvial input with the mires when peat accumulation was unable to keep up with the high rates of accommodation.
In this study, a large number of thin and clean coal seams topped by the tidal at Begrih Formation 7 indicates that frequent changes in the peat-forming mires occurred during the accumulation of peat, as evidenced by the temporary development of paleo-peat bodies from ombrotrophic to mesotrophic to rheotrophic and vice versa (Fig. 9). Based on the compositional variations of macerals, sub-macerals, and detrital-mineral matter, these changes were controlled by the moderate to high eustatic rise in sea level that led to a continuous rise in the base level of the region and subsequent rise in the water table of the coastal plain. The variations of coal lithotypes from being coarsely banded to dull in nature support intermittent moderate to high ooding of the uvial areas, resulting in moderate to high diversi cation of the macerals/submacerals and moderate to no amount of mineral matter (Fig. 9). Once the mires were formed, they were shielded from substantial clastic deposition, as shown by the moderate to low amounts of clastic material in the seams (Fig. 9). On the basis of these observations, it is suggested that the anticipated stability of the tectonic setting should be excluded during the development of peat. This is supported by the study conducted by 47 , which showed that shifts in the base-level from the Middle Miocene to the Pliocene were highly in uenced by eustasy, whereas during the Late Oligocene to Early Miocene, the effects of global sea-level changes were signi cantly obscured by major tectonic movements. Moreover, the syn-collision between Luconia Block-Dangerous Grounds and Borneo in a rapidly subsiding basin that in uenced the formation of Early Miocene Mukah coals 2 may be more important locally within the depositional basin in controlling the properties of coal seams (i.e., thickness, composition, continuity, and geometry) 48 and is comparable according to structural characteristics and overall subsidence that occurred on land within the Sarawak basin 49 .
The water table uctuations in the study, however, are strongly dependent on climate changes, as the in uence of relative sea level on the position of regional water tables diminishes further inland 38,50,51 . The compositional variations of coal lithotypes in Mukah coals have most likely been affected by climatic conditions as the changes may be attributed to the relative sea-level variables of the region. In this study, the obvious absence of major of re-generated inertinite coal seams, uncommon dull coal, and dulling-upward succession from the base to top coal seams could be related to the ever-wet climate during the Neogene, as the coals being studied may have experienced a widespread rise in rainfall 52,53,54,55 , which corresponded to the Asian monsoon from the earliest Miocene onward. As previously reported, the structured huminite maceral content (humotelinite) is predominant in the Mukah coals; thus, this observation supports permanently water-saturated mires with a minimum oxidation level during peat accumulation and was in uenced by the rise in the water table. Furthermore, the high content of humite-clarite in the entire coals being studied implies that the paleomires have experienced a prolonged rise in sea level, as the signatures are typical of "transgressive" coal 46 . From the petrographic evidence, the Mukah coal peat mires evolved on a wet, low-relief coastal plain in low-lying areas with continuously intermittent moderate to high ooding. The relationship between low inertinite (<10 vol. %), moderate to high R dit ratio (m/z 123), palynology signatures, and low IV factor suggests that a moderate to high-water table had in uenced the development of peat in the wet mires while the climate was probably more wet during these periods. The scarcity of gymnospermous pollen (Podocorpus) in the coals being studied may further indicate a low contribution from vegetation in the drier or upland areas. This is further supported by 2 , who reported the abundance of Casuarina-type pollen associated with common occurrences of Dacrydium in the Kerapah peat swamps, indicating an extremely wet climate. The presence of Casuarina type in high frequency is also the case of this study.
Peat accumulation rates vary considerably, particularly with respect to geographical latitudes 43 . It is proposed in this study that the average peat accumulation rate for low-lying Holocene peats at low latitudes (<10°) ranges from ~2 to ~5 mm/year. According to 56 , the compaction ratio to form low-rank coal seams from their original peats varies between 1.4:1 and 30:1. A ratio of 10:1 has been most commonly used 57,58,59,60 . Considering that the coals being studied are low in rank and accumulated in a tropical climate, a ratio of 10:1 and peat accumulation rates of 2-5mm/year have been used as conservative estimations, resulting in the original thickness of the peat deposits in the Mukah coals of the Balingian Formation to be between 5 and 35 m and that records peat-forming environments were established between 10,000 and 175,000 years ago. The calculation is strongly dependent on the assumed peat-coal ratio; thus, it shows that the deposition of peat for the Mukah coals occurred within a short time interval.
The Mukah coals in a sequence-stratigraphic context. Some of the key sequence-stratigraphic surfaces have been proposed in the paralic coal beds being studied of the Balingian Formation (Fig. 9). The coal that initially formed at a low water table, overlies the paludi cation surface (PaS); meanwhile, the gradual termination of the peat represents a give-up transgressive surface (GUTS) in response to an increasing accommodation rate. Furthermore, the coal bed overlies a terrestrialization surface (TeS) at its initiation of peat formation in response to a decreasing accommodation rate, resulting in a GUTS during a stable water table. An accommodation reversal surface (ARS) was identi ed during the transition of accommodation from decreasing to increasing (balanced to high AR/PPR) or increasing to decreasing (high to balanced AR/PPR).
In this study, the creation of peat accumulation is observed from the difference in the thickness of the coals, which could be linked to the period of the base level changes in the mires. A shorter period of rising base level is assumed to have occurred in the middle and top parts of the section. The lowest coal seam (05/01) within the lower stratal section (MC05) is thicker than the upper and middle coal seam, suggesting that the initial transgressive stage occurred during the formation of the peat 38 , as the increasing base level increases the rate of accommodation, resulting in the formation of a moderately thick coal bed (Fig.   11). Meanwhile, the development of the middle (05/02) and upper (05/03) coal seams in section MC05 is suggested to occur during the middle transgressive stage (Fig. 11), as their thicknesses are relatively thin and isolated owing to the high accommodation rate during peat accumulation in the mires 38 . The increase in the rate of change in the base level is supported by the transition to a coarsening-upward unit, which indicates the gradual decrease in the growth of the peat-forming environment into a more clay-and sand-rich deposition (Fig. 11). The greater thickness of the middle section coal at MC12 indicates an overall increase in base level with a moderate water table in the initial stage, and that a late transgressive stage (Fig. 11)  indicate that shifts in the base level have occurred over a relatively short period under stable peat-forming conditions (Fig. 11). The continuous formation of thin coal seams of the upper stratal section may suggest that the initial highstand stage (Fig. 11) controlled the low to moderate accommodation rate during the accumulation of peat in the mires 38 . The overall ning upward cycle indicates that hydraulic energy is low during deposition, thereby supporting the interpretation of gradual overgrowth peat in the mires.
In this study, the overall rising water table level during the growth of paleo-peat bodies suggests that the Mukah coals are deposited in a uvial system. In the sequence-stratigraphic framework, the architecture of the Mukah coals from the lower to upper section could represent the transition from transgressive (TST) to initial highstand (HST) cycles (Fig. 11). This interpretation is supported by the high content of humite/vitrite + clarite in the entire coals being studied (Fig.   11), which corresponds to a transgressive event in uencing the growth of peat in the mires 49 . The interpretation may also be linked to Sarawak's stratigraphic framework analysis 61 , the detailed offshore reservoir geological assessment in the Balingian province 62,63 , and the paleogeographic study of the Balingian Formation 7 , which suggested that shales and coals of offshore Cycles I-II were deposited in an overall transgressive environment 61,62,63 , while two deposition stages, comprising an early transgressive event, followed by a late regressive episode, in uenced the onshore Balingian Formation 7 .

Data And Methods
A total of 45 coal samples were collected from eight coal seams found from the three mining boreholes (MC01, MC05, and MC12) in the Mukah coal eld, Balingian Formation, with bed thicknesses ranging from 1 to 3.5 m (Fig. 2). Prior to sample selection, a detailed analysis of the lithologic and coal lithotypes from the base to the top of the seams was performed in high-resolution observation based on the brightness system indicated by 64 Organic geochemistry was performed on the samples' total organic carbon content (TOC, wt.%), using a Multi N/C 3100 analyzer, and element concentrations of carbon (C), hydrogen (H), nitrogen (N), sulfur (S), and oxygen (O) were studied using an Elementar Analysensysteme GmbH (vario MICRO cube) analyzer.
Selected samples were analyzed for bitumen extraction and biomarker analysis. Source rock characteristic assessments using TOC, bitumen, and hydrocarbon yields are based on the work by 73,74 . The aliphatic fractions of coal extracts were collected for gas chromatography and gas chromatographymass spectrometry. Biomarker traces were based on the published work by 27,28,75,76,77 .

Conclusion
The following conclusions can be drawn from the above: 1. Lignite to sub-bituminous B is de ned for the Mukah coals. A high huminite and clarite typi ed the coals. Little or no alginite and pyrite was observed. The vertical changes in coal-forming vegetation and temporary development of rheotrophic-ombrotrophic mires, as represented by the changing petrographic compositional variation, may have occurred because of the water table uctuations, which resulted in various ecological groups of plants.
2. The source of the Mukah coals is suggested to have been derived mainly from arborous vegetation, which appears to be predominated by the freshwater peat swamp communities. 3. Rapid subsidence and an ever-wet climate may be attributed to the relative sea-level variation in the region, which may have resulted in the water table uctuations, balanced to high peat accumulation and preservation ratios during the peat formation in mires, moderate diversi cation of the composition in the coals, and a large number of thin and clean coal seams coarsely banded to dull coal lithotypes with moderate mineral matter and low inertinite. 4. Two patterns of key sequence-stratigraphic surfaces are mainly proposed in this study: PaS-ARS-GUTS, in response to the increasing accommodation rate (balanced to high AR/PPR), and TeS-ARS-GUTS, in response to the decreasing accommodation rate (high to balanced AR/PPR) during periods of a stable water table. 5. Within the uvial setting, the formation of peat in the mires could be in uenced by TST to initial HST cycles. The Mukah coals may correspond to a minimum thickness of ~5 m of peat deposits, which are expected to have formed between 10,000 and 175,000 years ago. Figure 2 Complete vertical pro le of the eight coal seams from Mukah coal eld shows upward compositional changes in lithotypes, maceral groups, macerals, mineral matter, and microlithotypes. Note coal seam is labeled as 01/04.

Figure 4
Vertical distribution of different palynotaxas recovered in the Mukah coal samples being studied. The palynomorphs have been grouped into peat/freshwater swamp, mangrove, ferns, and others. The coal lithotypes are also shown.     Vertical compositional variations shown are vitrinite (mineral free), inertinite (mineral free), liptinite (mineral free), detrital-mineral matter, and microlithotypes, and interpreted accommodation curves for the Mukah coals, Balingian Formation. On the coal facies diagrams, the solid line represents an accommodation reversal surface (ARS).