Coal petrology

Coal petrology (or coal petrography) is the study of the organic and inorganic constituents of coal and their transformation via metamorphism. Coal petrology is applied to the studies of the depositional environments of coals, correlation of coals for geological studies, and the investigation of coals for their industrial utilization. Traditionally, the latter has been dominated by the use of coal petrology in the optimization of coal blends for the production of metallurgical coke, but can also include the use of petrology in evaluating coals for beneficiation (coal preparation for downstream utilization) and combustion. Recently, significance of coal petrology has been demonstrated in coalbed methane exploration and in potential CO2 sequestration into the coal seams. Techniques developed in the study of coal are also used in the investigation of organic-rich rocks to evaluate source rocks in petroleum and natural gas exploration. The most recent fundamental reference book for coal and organic petrology is Taylor et al. (1998). The nomenclature has been developed and refined through the efforts of the International Committee for Coal and Organic Petrology , founded in 1951; the Society for Organic Petrology , founded in 1983; and other, smaller national and regional groups.
Coal is the product of the deposition of the peat, its degradation (for example, by insects or micro-organisms), selective preservation of the surface litter; the growth of roots though the peat; the subsurface action of aerobic and anaerobic micro-organisms; and the metamorphic changes of this organic mass through time. The organic constituents in coal and non-coal organic-rich rocks are termed macerals (in the broader sense, and particularly for dispersed organic material, the term kerogen is also used). By convention, maceral names always have an “-inite” suffix. Macerals are the (optical) microscopically identifiable constituents in coal, somewhat analogous to minerals in an inorganic rock (Stopes, 1935). Macerals are generally divided into the vitrinite (or huminite in lower rank coals), inertinite, and liptinite groups.
Huminite/vitrinite macerals are derived from humic substances, the alteration products of lignin and cellulose. Huminite refers to macerals in lignite and subbituminous rank (see below) coals (Sykorova et al, 2005), and vitrinite to maceral of bituminous and anthracitic ranks. The distinction with vitrinite (ICCP, 1998) is based on the textural and physical changes associated with gelification. Huminite is divided into three subgroups based on the texture/morphology of the maceral: telohuminite, with constituent macerals textinite and ulminite, the recognizably textured huminite macerals; detrohuminite, with macerals attrinite and densinite, the detrital huminite macerals; and gelohuminite, with macerals corpohuminite and gelinite, huminite macerals showing some degree of gelification. Vitrinite follows the same divisions: the telovitrinite subgroup, with constituent macerals telinite and collotelinite; the detrovitrinite subgroup, with macerals vitrodetrinite and collodetrinite; and the gelovitrinite subgroup, with macerals corpogelinite and gelinite.
Inertinite macerals are, to a certain degree, derived from the same starting materials as the huminite/vitrinite macerals (ICCP, 2001). In contrast to the latter maceral group, the inertinites have been oxidized, with fire thought to be the primary cause of their formation. The macerals fusinite and semifusinite are the products of such oxidation and, in most coals, are the most abundant inertinite macerals. Secretinite is a product of the oxidation of plant secretions. Macrinite is problematical, in part because it has been confused with what is now recognized as secretinite. Multiple pathways have been proposed for macrinite sensu stricto (ICCP, 2001). Funginite, with a fungus origin, is grouped with inertinites derived from plant cells by ICCP (2001). Fungi, however, are not plants, but encompass two distinct eukaryote kingdoms, Fungi and Protoctista-kingdom slime molds. Micrinite is thought to have originated as a secondary maceral from the breakdown of hydrogen-rich liptinites.
The liptinite group macerals originate from hydrogen-rich plant parts, including sporopollenin, cutin, suberin, resin, wax, etc.; from the bacterial degradation products of carbohydrates, cellulose, and proteins; and from algae. The maceral names of certain liptinite group macerals follow the origin of the maceral: sporinite, cutinite, suberinite, resinite, alginite. Secondary macerals can arise from the liptinite, for example, the fracture-filling exsudatinite, basically a variety of resinite, and fluorinite, a derivative of essential oils.
Coal petrographic nomenclature follows the scale of the observation. Macerals are components of the microscopic level. Microscopic assemblages (defined as being within a 50-micron diameter) of macerals are microlithotypes. The microlithotypes, all with the suffix “-ite,” are the monomaceral assemblages (vitrite, inertite, and liptite); the bimaceral assemblages (clarite [vitrinite + liptinite], durite [inertinite + liptinite], and vitrinertite [vitrinite + inertinite]; the trimaceral assemblages (duroclarite [vitrinite dominant], clarodurite [inertinite dominant], and vitrinertoliptite [liptinite dominant]); and the mineral-rich carbominerites. At the megascopic scale, lithotypes, all with an “-ain” suffix (e.g., vitrain, clarain, durain, fusain), are the assemblages of microlithotypes.
Inorganic elements can be included in coal in minerals or as elements incorporated in the organic structure of macerals. The most common example of the latter is the incorporation of sulfur into macerals as “organic sulfur.” Minerals can be incorporated into the peat during deposition, result from epigenetic processes, or be the consequence of metamorphic changes within the coal. Clay minerals, quartz, calcite, siderite, and pyrite/marcasite are the most common minerals in coals. All naturally occurring elements have been found in coal.
Coal petrography techniques, in addition to determining maceral/microlithotype composition, also include microscopic measurements of certain parameters that are indicators of the degree of coalification (metamorphism) termed coal rank. The coal rank series, from lowest to highest degree of metamorphism, is as follows: peat, lignite, subbituminous, high volatile bituminous, medium volatile bituminous, low volatile bituminous, semi-anthracite, anthracite, and meta-anthracite. Coal metamorphism is the consequence of burial, particularly in the dewatering from peat to lignite to subbituminous, and enhanced temperatures. Temperature increase can be the result of burial at varying geothermal gradients, influx of thermal waters or brines through the coal, or, in rare cases, contact metamorphism in the vicinity of igneous intrusions. Various chemical parameters are used to delineate coal rank. At lower ranks, the equilibrium moisture is used as a measure of coalification. Within the bituminous coal series, heating value, carbon, and volatile matter, both as defined by standards organizations, can be used. At higher ranks, the hydrogen content is used. Petrographers often use the reflectance of vitrinite as a coal rank parameter, determined using a reflected-light microscope, oil-immersion optics, and a 546-nm band pass filter, as a broad standard. Vitrinite reflectance is also used in the evaluation of organic-rich petroleum source rocks.