The thickness, lateral distribution, composition, and quality of a coal bed are determined to a great extent by the depositional environment. Moreover, Home, et al. (1978) found that the aforementioned characteristics were determined by the depositional environments that preceded, were coeval with, and that immediately followed deposition of the peat. The preceding environment shapes the topography on which the peat is deposited and therefore affects the thickness and lateral extent of the deposit. Contemporaneous environments affect seam continuity and composition whereas later environments may affect the peat by partial or complete removal of the deposit by erosion or, if brackish or marine waters are introduced, alteration of peat chemistry and therefore of coal quality.
Coal-forming environments can be divided into two broad categories: (1) paralic, which refers to coastal or near-coastal marine settings, and (2) limnic, which refers to coals formed inland, usually in intermontane regions and under freshwater conditions. Generally, limnic coals are characterized by thick beds of limited lateral extent. Although some of the coals in the western United States are limnic in origin, most North American coal deposits appear to have formed in paralic environments.
Paralic environments can occur in back barrier, deltaic, or coastal and interdeltaic settings (Bustin, 1983). Back barrier coals develop landward of barrier islands, frequently in abandoned lagoonal basins that are formed between the barrier islands and the mainland. Back barrier coals are typically rather thin, laterally discontinuous deposits that are elongate parallel with depositional strike and that are usually high in sulfur and ash.
Coastal plain coals develop on low, relatively flat, subsiding coasts that have a high water table and little influx of sediment. Some of the more persistent coals in the Appalachians of the eastern United States may have been deposited in coastal plain settings. Modern coastal plain swamps that are active sites of peat accumulation include the Everglades of Florida and the Okefenokee Swamp of Georgia (Bustin, 1983). Many ancient coals are interpreted to have formed in deltaic systems and thus depositional environments associated with deltas have been the subject of intensive investigation.
The following comments on coal-forming environments in deltaic systems are drawn from Home, et al. (1978). Depositional modeling can be used to predict large-scale trends in coal deposits on a regional scale and are therefore useful in the initial phases of coal exploration. Further, small-scale variations in coal thickness, quality, and lateral continuity frequently can be predicted, providing data that can be extremely valuable in mine planning and development.
The following illustration (Fig. 2.6.1) was derived from a detailed data base developed from the coal-bearing carboniferous-age rocks of eastern Kentucky and southwestern Virginia and from similar environments in contemporary coastal areas. Figure 2.6.1 illustrates the typical shape and lateral extent of coal deposits which form in the different environments within the deltaic setting.
Figure 2.6.1.
Coals that form in lower delta plain environments are typically elongate parallel with depositional dip because the only environments suitable for peat accumulation are adjacent to relatively narrow levees on either side of distributary channels. Interdistributary bays occur between the distributary channels and are sites of accumulation of fine-grained bay-fill detrital sediments. Sites of peat accumulation on the lower delta plain are generally restricted to the elongate, relatively narrow areas between the levees and the interdistributary bays. Lower delta plain coals are usually relatively thin and contain splits caused by crevasse splays that breach the poorly developed levees along the distributary channels.
Upper delta plain-fluvial coals also tend to be elongate in the direction of depositional dip although they are not as continuous in that direction as the lower delta plain coals. Deposits typically formed as pod-shaped bodies on flood plains adjacent to coexisting meandering channels and exhibit significant thickness variations over short distances. Also, as in the case with lower delta plain coals, numerous splits can occur near the levees bordering active channels because of splays. Post-deposition shifting of channels can also complicate the sedimentary sequence by eroding the coal deposit and creating “washouts.”
In some locales, a transitional zone exists between the lower and upper delta plain environments that exhibits characteristics of both lower and upper delta plain sequences. In the transition zone between the lower and upper delta plains, many of the large
interdistributary bays (flood basins) that occur between distributary channels have filled with sediment and provide broad basins in which large coal swamps can develop. These broad, relatively uninterrupted basins provide a favorable environment for the formation of coal deposits that are typically more laterally extensive than those of the lower and upper delta plain proper. Coals formed in this transitional zone share some characteristics with upper and lower delta plain coals such as splits that develop near levees and post-depositional washouts. Most of the more economically important coal beds in the Appalachian coal region are interpreted to have developed in this transitional zone between the lower and upper delta plains.
From the foregoing brief discussions, it is apparent that, in the initial phases of exploration, a knowledge of the depositional environments that control the shape and configuration of the coal body will enable explorationists to design a drilling program for maximum effectiveness and efficiency in defining the coal deposit. At the lease-tract or mine plan levels of exploration, more detailed drilling and evaluation may be desirable to predict areas of thick and/or high-quality coal.
Depositional environments also partially determine the sulfur content of coal deposits. Sulfur occurs in the form of iron sulfide (predominantly pyrite) in several ways in coal. A finely disseminated form sometimes referred to as framboidal pyrite is the most reactive form of pyrite and the most difficult to remove. It is so finely disseminated throughout the coal that it cannot be removed effectively in float-sink washability tests. Research suggests that framboidal pyrite originates from sulfur produced by microorganisms found in marine to brackish waters, but not in fresh water. It has been shown (Ferm, 1976; Caruccio, et al., 1977) that framboidal pyrite is most strongly associated with-coals overlain by roof rocks deposited in marine to brackish-water environments. Exceptions occur when a blanket of sediment (such as a crevasse splay) is introduced early enough to shield the peat deposit from later marine to brackish-water transgressions. It follows that coals which formed in back barrier to lower delta plain environments are more likely to be overlain by sediments deposited by marine to brackish water and hence will be more likely to contain higher amounts of framboidal pyrite.
Coals that formed in transitional lower delta plain environments are subject to a mix of fresh and brackish to marine water influences and hence are highly variable in their sulfur content. Generally, however, transitional lower delta plain coals are considered to be lower in framboidal pyrite than coals deposited in lower delta plain and back barrier settings. This trend is thought to continue for coals formed higher in the delta plain in fluvial-upper delta plain settings where marine influence is uncommon. These coals are generally considered to be lower in finely disseminated pyritic sulfur than coals formed in other delta plain depositional settings. An understanding of the depositional setting in which a coal bed formed can therefore be used to predict the amount and type of sulfur present and to guide the exploration for low-sulfur coals in areas where sulfur contents are usually high.
Investigations by Caruccio, et al. (1977) and Home, et al. (1976), serve as examples to illustrate the potential usefulness to mine developers of understanding the depositional history of a coal bed. Using a data base of 450 core holes in a 518-km2 (200-sq-mile area) located in the Appalachian coal region of the eastern United States, the investigators interpreted the target coal bed to have been deposited in a lower delta plain
setting. Typically, coals interpreted as lower delta plain coals, where overlain by brackish to marine rocks, have sulfur contents of greater than 2% with 75% or more of the sulfur occurring in the form of framboidal pyrite (Caruccio, et al., 1977). Where deposits interpreted as freshwater splays were emplaced over the peat surface prior to the deposition of the marine rocks, the peat apparently was shielded from the sulfur- reducing bacteria, causing the sulfur content in the peat to remain low (Home, et al., 1976).
Figures 2.6.2 and 2.6.3 summarize the investigative results of Horne (1978). Figure 2.6.2 is an interpretation of the depositional environments after deposition of a coal bed. The data suggest that the levees of a distributary channel in the southwestern part of the area were breached and splay deposits encroached to the north and east over the coal and into the marine-influenced interdistributary bay. Figure 2.6.3 shows the distribution of disseminated sulfur in a target bed. A comparison of Figs. 2.6.2 and 2.6.3 illustrates the expected association between areas where the coal is overlain by marine beds (the eastern part of Fig. 2.6.2) and higher sulfur concentrations. In the western and southern parts of the diagrams where the wedge of nonmarine splay deposits covered the coal, sulfur contents are correspondingly lower.
Figure 2.6.2.
Figure 2.6.3.
The relationships shown in these diagrams between disseminated sulfur content and specific depositional environments suggest that exploration drilling programs at the lease-tract level should be devised to gain an understanding of the depositional setting of a coal deposit and to define such depositional features as might cause significant variation in the physical or chemical characteristics of the coal.