Module 3Coal and Coal Bearing Environments

Module 3

Coal and Coal Bearing Environments

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Module 3Coal and Coal Bearing Environments

Coal and Coal Bearing Environments

Contents

Introduction Conditions of Peat Growth Peat Growth Rate Peat Forming Environments Compaction of Peat Coals in a Sequence Stratigraphic Framework Genesis of Laterally Extensive Coal Seams

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Module 3Coal and Coal Bearing Environments

Coal and Coal Bearing Environments

Introduction A key feature of many non-marine basins in tropical or cool-temperate settings since the Devonian are peat-forming environments that ultimately lead to the formation of coal measures. In Gondwana, these were dominant in the Permian and Late Triassic. The eastern Australian basins (Bowen-Sydney-Gunnedah, Galilee, Cooper Basins) contain one of the largest cool-temperate coal-bearing systems in the Palaeozoic world. It has long been recognised that much of the hydrocarbon found in onshore Eastern Australia is sourced from the extensive coal measures in these basins. In the Sydney and Bowen Basins coal seams are also exploited directly, both as world-class deposits of coking and steaming coal and for coal bed methane. Petroleum exploration in these basins widely utilises coal seams for correlation purposes due to their characteristic seismic response. These coals contrast with the dominantly tropical or subtropical coals characterised by the Carboniferous coals of North America and Europe, and the Tertiary coals of SE Asia (Land and Jones, 1987; Carbonele and Moyers, 1987). Before interpreting a coal-prone succession it is important to first establish the major depositional setting and to ensure that you are referring to the relevant literature (use the Gondwana literature for Permo-Triassic coals in Australia, Irian Jaya, India, South Africa, South America and Antarctica). Coal forms during burial from organic precursors (mainly peat, but rarely also algal mats). The process of coalification is complex and involves physical compaction, dewatering, and physico-chemical changes of the organic material, such as gelification and progressive alteration of cellulose material. Coal may form from organic material, which was buried in situ, at the site of plant growth (autochthonous coals), or result from transported plant material, such as rafted peat mats (allochthonous coals). Coals in eastern Australian basins are typically formed from peat precursors and are autochthonous. Microscopic examination of coal under reflected light shows constituent components (macerals), some of which can be identified as specific parts of plant precursors, such as lipid-rich cuticles.

Conditions of Peat Growth

Peat comprises predominantly humic organic material (>= 75%) with very little inorganic matter present. Peat-forming environments are characterised by both a high preservation potential of organic matter and a general absence or scarcity of clastic detrital input.

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Module 3Coal and Coal Bearing Environments

Where sediment invades the peat environment, as when a crevasse splay is deposited over the peat (Fig. 1.1), some splits occur resulting from the compaction of the peat. Peat forms in wetland environments:

marshes dominated by grasses and sedges, undergoing temporary inundation

swamps dominated by woody arboreal vegetation bogs dominated by mosses and ferns in areas of raised water

table. The term mire is often used generically to refer to waterlogged peat-forming environments.

Study of modern-day peat-forming environments provides important insights into the conditions that lead to prolific plant growth and a high preservation potential of organic matter. Decay of plant material is normally a rapid process and occurs either immediately before or after burial. It is caused by a combination of bacterial and fungal activity, as well as oxidation by contact with air or oxygenated porewaters. In peat-forming environments degradation of humic matter is prevented (or at least slowed down dramatically) by the presence of very acidic porewaters, which prove inhospitable to most fungi and microbes (Renton & Cecil, 1979; Renton et al., 1980; Teichmuller, 1982). The mostly stagnant water present within the pore space of peat is quickly depleted of oxygen, further preventing deterioration of organic tissues. The preserving qualities of peat mire porewaters is illustrated by bog corpsesexquisitely preserved human remains, dating from the bronze age, which have been prevented from decay by being totally submerged in bog waters.

The total water balance of a peat-forming environment has to be at least in equilibrium to maintain a raised water table in the peat at all times. Water flux into the peat through precipitation or capillary action has to be at least equal to the outflow. Consequently areas of high precipitation and/or low evaporation rates are favourable sites for peat accumulation. The most extensive areas of cool-temperate peat formation today occur in Siberia and Canada, in climatic settings characterised by low precipitation rates (~500mma-1 ), and low evaporation rates.

Variation in the local or regional water table has immediate effects on peat-forming environments. An example of this can be found in the modern day Rhine delta in the Netherlands. Here intensive agriculture and deliberate drainage of peat mire areas for land reclamation has resulted in a significant drop in the groundwater table and an associated subsidence of the land surface caused by the resulting oxidation of peat. The Netherlands Institute of Applied Geoscience (TNO) has investigated land subsidence using buried markers and sophisticated leveling techniques on a test farm operated for the last 25

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years (de Lange et al 2000). The soil profile at the site is representative of some 75% of the peat-covered areas of the Netherlands and comprises peat and soft clay resting on Pleistocene sand. With settlement indicators buried at several levels, the contributions of compaction and oxidation could be separated. Both processes exhibit a linear behaviour. A high phreatic level of 0.35 metres below the surface results in a subsidence of 6.7mm/y. A low phreatic level of 0.7 metres below the surface results in a subsidence of 15.8 mm/y. The increased subsidence is mainly due to the oxidation loss, which is more than doubled in the latter case (10.9mm/y). The subsidence of peat meadows is almost entirely attributed to oxidation. Since the beginning of agriculture in the 12th century about 2 metres thickness of peat has disappeared in this way.

Peat Growth Rate

The ability of peat mires to prevent either burial by clastic sediment or drowning by a rise in the local water table, depends on the growth rate of the plant communities that constitute the living outer layer of the peat. In this respect peat mires behave like coral reefs in carbonate depositional environments. Growth rates of modern peats have been measured by isotopic dating and correlation of marker beds within peats. Reported growth rates vary, but typical ranges are 0.10.7mma-1 in a raised bog in the Netherlands, (Kilian et al, 2000.) to 1.12.1mma-1 in a raised bog in Nova Scotia (Chague-Goff & Fyfe, 1996). Peat mires can adapt to environmental changes by variation in the plant communities, if the time frame of change is sufficiently long to allow this. Peat accumulates with a maximum thickness when the rate of peat production equals the accommodation rate (Fig. 1.2). Peat will be cleanest and thickest at some optimum away from the clastic input of the channel belt Fig. 1.3, 1.4). Often the coal prone interval will occur over the palaeohighs (Fig. 1.4, 1.5).

Peat Forming Environments

Low lying mires/swamps Areas of low-lying topography may develop peat mires that fill slight depressions and typically exhibit very high water tables with local development of small lakes. The flora of low-lying swamps has been shown to comprise a high species diversity (Teichmuller & Teichmuller, 1982). Slow but ongoing decomposition of organic material in the slightly acidic (pH 4.8-6.5) surface waters of these environments, results in an abundance of plant nutrients. Because low-lying mires form in topographically low-lying or low-relief areas, they are more prone than raised mires to the influx of clastic sediments such as splays and channels. A good example of this are

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Module 3Coal and Coal Bearing Environments

the delta plain coals that cap each deltaic progradational cycle. As with most coals, these will be distinctive on wireline logs because of the low density and slow sonic transit times (usually shown as a sharp kick to the left on the scale). However, depending on the amount of clastic input (ash as it is referred to in the coal literature) they can range from low gamma for clean coals to moderate or high gamma for dirty coals. Raised mires/swamps Raised mires have convex-up surfaces, which are elevated above the surrounding terrain and are independent of the underlying topography (Figs. 1.6, 1.7). Raised peats develop under favourable climatic conditions, which include low evaporation, abundant precipitation and low seasonality (Lottes & Ziegler, 1994). The peats become self-sustaining under these conditions, since they are topographically above the influence of nutrient-rich ground and surface waters. Raised peats are active ecosystems and can respond to climatic variations by shifts in the relative abundance of plant species making up the living, outer layer of the peat (Chague-Goff & Fyfe, 2000). In addition, the positive relief of the raised mire largely prevents the entry of clastic deposits such as fluvial channels or overbank facies, which would interfere with peat growth. Due to vertical variations of composition and changes in vegetation patterns between the margin and the edges of a raised mire, properties such as pH and brightness show characteristic profiles. The pH has been documented to decrease upward from 4.8 at the base to 3.6 at the surface within a 7.5 metre core retrieved from a raised bog in Nova Scotia (Chague-Goff & Fyfe, 2000). The zonation of plant communities and chemical environments within a raised bog results in correlatable profiles within contemporaneous bogs and their resulting coal seams. Vertical changes in trace element composition, isotopic variations, ash content and maceral abundances have been used successfully to correlate coal seams (Kilian et al, 2000; Hamilton & Tadros, 1994; McCabe, 1984; Glasspool, 2000). Similarly, the effects of fires on peat-forming wetlands during episodes of drought can be recorded in the stratigraphic record as bright bands showing an increase in the maceral inertinite. Because of the generally clean nature of raised mires, they will typically have low gamma, as well as low density and slow sonic transit times. Detrital peats/beach ridges In tropical regions (e.g. the Mahakam delta of Kalimantan) abundant detrital organic debris is transported down the rivers, and then, through gentle longshore drift processes, the organics are deposited on the low-energy beaches and can accumulate significant peat ridges (Gastaldo, Allen and Huc, 1993). On the present day Mahakam delta these can be seen accumulating adjacent and downdrift to the tidally influenced distributary channels, and form spongy ridges along and immediately behind the shoreline, settling during the high tides. On the

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Module 3Coal and Coal Bearing Environments

mouth bars, small deposits can accumulate typified by Nippa Nippa palm leaves broken down to various sizes, indeterminate leave litter, and commonly floating chunks of amber resin. Detrital coals can also form in lacustrine settings where they represent drifted detrital material that accumulates at one end of the lake where gentle breezes preferentially distribute the material. Finely-divided organic detritus can also accumulate at the base of lake successions, and it is useful to obtain palynological and coal maceral determinations to determine the origins of such coals.

Compaction of Peat

Peat undergoes a range of physico-chemical changes during burial, resulting ultimately in the formation of lignite, and later, black coal. The various macerals in a particular coal will form and change at different rates, a fact that is utilised in the measurement of the vitrinite reflectance as a hydrocarbon source rock maturity indicator. Peat undergoes a significant volume loss during compaction, which can lead to distorted geometries in adjacent sand or shale bodies. A wide range of ratios are proposed in the literature for the coalification of peat into black coal, ranging from 1.4:1 to 30:1,or 50% to 98% volume loss (Ryer & Langer, 1980). Most of these are estimates from observation of the compaction of modern day peats or from the reconstruction of depositional geometries of sandstone bodies encased within coals. Experimental coalification of peat samples was conducted by Shearer & Moore (1996), Orem et al. (1996), and Cohen & Bailey (1997). Shearer & Moore and Orem et al. were using samples from tropical raised mires in the Rajang delta, Sarawak, Malaysia. Cohen & Bailey analysed samples from the Everglades mangrove swamp (Florida) and raised bogs in the Okefenokee Swamp (Georgia, USA). The results of these studies are comparable and show compaction ratios of the peats studied between 5.7:1 and 8:1 (72% and 88%, respectively). Wood samples also used in the experiments showed much lower compaction ratios of 1.5:1 to 2.5:1 (33% to 60%). Shearer & Moore (1996) provided details of the nature of the compaction. They identified that most of the volume loss (45% of original porosity) was due to loss of inter-particle porosity within the peat, 17% due to compaction of peat particles, and the remaining 10% due to organic mass loss in outgassing and fluid expulsion. The observation that most of the volume loss during peat compaction is a result of physically pressing the humic particles closer together conforms with observations of dinosaur footprints in the roofs of underground coal mines in Utah. Individual footprints show a shallow depth of penetration and a preservation of foot morphology that is not possible unless the peat the animals walked upon was very firm.

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Module 3Coal and Coal Bearing Environments

Nadon (1998) is critical of the high compaction rates assigned to the peat-to-coal transformation. He points out that decompaction modeling using such values for coal seams in contact with penecontemporaneous channel sandstones leads to impossible depositional geometries for the sandstones, which have a final thickness that is 90% of their original thickness. Similarly, decompaction modeling of the fragments of organic material within channel lags, using the assumed large peat compaction value, results in the destruction of associated sedimentary structures such as trough cross-beds. The application of decompaction factors to coal in order to reconstruct depositional geometries is consequently fraught with a series of problems:

Compaction ratios for various components of the peat vary widely. A detailed knowledge of the components of peat (e.g. the proportion of woody material) is necessary to determine meaningful values.

Loss of matrix porosity (inter-particle porosity) from the peat

accounts for more than 60% of the total volume loss. This is likely to occur early during burial. A significant proportion of matrix porosity loss probably occurs almost spontaneously upon loading with clastic sediment, such as a splay (or dinosaurs).

Auto-compaction occurs in peat mires, so the lower parts of a

thick raised peat mire are more compacted than the near-surface layers. Application of a single decompaction factor to a coal seam will therefore greatly exaggerate the depositional thickness of the peat precursor.

The complications of peat compaction on sandstone geometry are illustrated in Figure 1.8. From our own observations in the West Siberian peat mires, the immediate compaction due to sediment loading is probably >95% in the sphagnum moss dominated peats, however woody peats are likely to compact considerably less (

Module 3Coal and Coal Bearing Environments

bypass (avulsion, stable channels, raised mires). When the increase in accommodation exceeds the rate of peat growth, the peats drown or are covered with clastic sediments. Where the peat growth rate exceeds the accommodation rate, peats dry out and are susceptible to oxidation, and fires (Bohacs & Suter, 1997). The growth rate of peat depends on a variety of local factors, such as the plant community of the peat, precipitation and evaporation rates, seasonality of the climate, nutrient supply, and others. However, for a given phytogeographic setting there will be a relatively narrow range of common peat growth rates and a corresponding but broader range of changes in accommodation rate, which facilitate growth and preservation of peat. The minimum rate of peat formation was found to be about one half of the maximum rate (Diessel & Boyd, 1994). Bohacs & Suter (1997) related the ratio of accommodation rate to peat production rate with the thickness and lateral extent of the peat mires, based on the theoretical considerations outlined above. If the accommodation rate is slightly lower than the peat accumulation rate, but above the minimum threshold, relatively thin, laterally extensive peat mires are predicted, resulting from the lateral spreading of peat environments. Where accommodation rate and peat growth are near equilibrium, thick, but isolated peats are predicted and the mire can accumulate peat to its full capacity and does not need to extend laterally. When accommodation rate outstrips the peat growth, mires become either flooded by lakes or invaded by clastic sediments. No peats will accumulate or be preserved where the accommodation rate is much lower than the peat growth rate. The association of coal occurrences and geometry within an accommodation cycle based on these generalisations is illustrated in Figures 1.9 - 1.12. This model of peat accumulation in a sequence stratigraphic framework summarises many significant features of peat growth and coal formation (Table 1.1). Potential problems arise because peat production rate, sediment accumulation rate, and accommodation are treated effectively as independent variables in the underlying assumptions. In reality, there are feedback mechanisms between these rates, particularly in fully continental settings. For example sediment supply, base level and peat production rate can be closely linked to climatic variations, and therefore are likely to show some dependence on each other.

Genesis of Laterally Extensive Coal Seams

Regionally extensive seams (100s to 1000s of km) of clean coal are common in coal-bearing basins and characterise many stratigraphic intervals in the Palaeozoic pan-Gondwanan basins of Australia, Antarctica, South Africa, South America and India. These seams

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Module 3Coal and Coal Bearing Environments

actually represent continuous areas of peat accumulation, as illustrated by regionally correlatable trace elements, brightness and palynomorph profiles from such seams (Glasspool, 2000). The origin of such widespread accumulation of peat to the apparent exclusion of clastic sedimentation is unclear. The change from clastic fluvial depositional environments to basin wide accumulation of coal with no apparent coeval clastic depositional systems is a change as dramatic as a change to evaporite or carbonate sedimentation (McCabe, 1984). It requires a change in climatic parameters favourable to the establishment and sustaining of peat mires. Additionally, the previously active supply and dispersal of clastic sediment in the basin has to be either diverted out of the area, or by some other means totally bypass the area of peat mires. McCabe (1984) favours widespread development of raised mires as responsible for the formation of regionally extensive clean coal seams. Their positive topographic relief forms an active barrier to the ingress of any clastic depositional systems, which may be active at the time. Hamilton & Tadros (1994) view the development of peat mires as representing accumulation of plant material under favourable conditions in what would otherwise be areas of non-deposition (interfluves) (Fig. 1.4).

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B

A

Fig. 1.1 A. Generalised section through ribbon split in the Top Hard Coal,Carboniferous, England. Coal has been decompacted to show approximate thicknessof original peat. Lines within coal represent bedding planes (from Elliott, 1965). B. Generalised sections through ribbon split (left) and washout (right) of theFolsomville/Dykersburg complex in Indiana and Illinois (from Eggert, 1982).

Fig. 1.2 A. Relation of the ratio of accommodation rate/peat production rate(normalized accommodation) with peat thickness. Peat accumulates withmaximum thickness when the two rates are approximately equal. B. Relation ofratio of accommodation rate/peat production to coal geometry. The mostwidespread coals accumulate at low to moderate values of this ratio.

Fig. 1.3 Relation of accommodation (subsidence) and coalthickness, Brunner seam, New Zealand. Coal peaks in thickness andquality at intermediate values (after Titheridge, 1993).

Fig. 1.4 Cross-section AA illustrates the distribution of the principal depositionalelements of the upper Black Jack stratigraphic section.

Fig. 1.5 Schematic illustration of examples, at several scales, of processes capable of shuttingoff sediment supply and providing opportunity for peat accumulation. A Disruption of sedimentsupply at a basin wide scale. Tectonic movement has tilted the thrust belt causing streamcapture and shedding of sediment to the northeast into an interthrust belt basin. B Sediment bypass at subregional scale where the axial channel complex occupied the eastern portion of the basin and peat accumulated uninterrupted in the west. C. Localized peat accumulation in a cut-off meander loop of a moderately sinuous mixed load fluvial system.

Fig. 1.6 Evolutionary sequence of swamp types showing the development of araised swamp with distinct peat zonations (based on Romanov, 1968).

Fig. 1.7 Theoretical model of fluvial architecture in an area of raised swamps. Theelevated swamp restricts overbank flooding and prevents avulsion, leading to thedevelopment of stacked channel sandstones.

Fig. 1.8 Possible compaction scenarios for two sandstone beds P and Q within a peat succession.

Ultimate compaction of peat and interseam sediments leads to coalification, and development of seam splits around compacted structures

Coal measure or other sedimentation proceeds steadily, increasing load and general compaction

Peat growth resumes and peat bed C is emplaced

Sedimentation at R, S &T proceed to the limit of compaction locally leveling peat bed A and bending peat bed B and the sediments deposited within the beds P & Q

Incipient sedimentation starts at R,S & T in zones compactive response. This has resulted in the channels

Peat growth is resumed and peat bed B emplaced

Peat bed A reaches its limit of compaction locally in response to sedimentation at P & Q

Peat bed A yields by compaction to successive sedimentation at P & Q

Athick peat bed A accumulates & is subjected to its first incipient sedimentation at P & Q

Fig. 1.9 A. Coal seams are conceptually equivalent to the hiatal surfaces of Frazier (1974).Localised or subregional coals can potentially cap the small-scale facies sequences anddepositional events, while regionally extensive coals can bound the depositional episodes. B. Depositional episode of Frazier (1974) in marginal basin setting. (Figures not to scale).

Table 1.1 Sequence Stratigraphic Distribution of Paralic Coaly Rocks

Fig. 1.10 Relation of rate of change of base level to coal thickness and geometry for a given peat production rate.

Fig. 1.12 For a given peat production rate, the occurrence of paralic coals mayvary significantly due to the local rate of change of accommodation. Loweraccommodation rates favor initiation of mires earlier in the lowstand systems tractand later termination in the highstand systems tract. Higher accommodation rateswould delay initiation of mires and, at very high rates, may prevent widespread peataccumulation, even in the transgressive systems tract.

Fig. 1.11 Lithofacies and sequence stratigraphic interpretation cross-section ofthe Teche, Saint Bernard, and LaFourche delta-plain complexes (after Kosters andSuter, 1993). Significant peat deposition is associated with aggradationalparasequence stacking in the late transgressive to early highstand systems tracts.

Introduction