ORGANIC MATTER AND GEOCHEMISTRY OF KUNGURIAN COALS FROM NORTHERN PREURAL FOREDEEP

The article presents the results of coal-petrographical and geochemical studies of Permian Kungurian coals from Northern Preural foredeep. According to their coal-petrographical composition the coals are divided into twoand three-component with the predominance of vitrinite group. The coals formed within lowland bogs. The distribution of n-alkanes and isoprenoids in the hydrocarbon fraction of the bitumen indicates the presence of three organic matter sources: sapropel, humus and mixed.


Research Methodology
The results of coal petrography studies in conjunction with data of organic geochemistry make it possible to determine the type of OM, its composition and conditions for the formation of coals.Carried out coalpetrographical studies characterize the composition of OM, the conditions for its accumulation and the level of catagenetic transformation.In addition to coal petrography, the results of pyrolysis using the Rock-Eval method (VNIGNI, Moscow) were used to determine the type of OM, its generation potential (HI, mg HC/g C org ) and the degree of catagenetic conversion (T max , °C).Chemical-bituminological studies included determination of the content of organic carbon (C org , %) and chloroform bitumen (%).Another of the methods used is gas chromatography (GC), the results of which also characterize the dominant type of OM, conditions for its accumulation and transformation during catagenesis.In general, the conducted complex of methods gives more complete information on the OM composition and its properties.

Coal petrography and formation conditions
The coals in question correspond to different grades -from long-flame (Kozhym river) to the gas (in the mines of the Vorkuta district).Changes in the grade composition and accordingly metamorphism intensity occurs in northeastern direction, which was repeatedly pointed out by many researchers (Coalbearing formation..., 1990;Anischenko et al., 2004).The data of coal petrography studies, in particular the component composition, were plotted on the diagram of the OM microcomponents ratio (Fig. 2).The major part of coal concentrated in the upper corner and along the axis Vt-It of the diagram, characterized by inertinitevitrinite (It-Vt) composition.Part of the coals displaced to the center of the diagram consist of three groups of components: liptinite, inertinite and vitrinite.
The most diverse composition is determined in coals of the lekvorkutskian suite of the outcrop No.49 of the Vorkuta River and Vorkutinsky mine.The spread of the components Vt and It of these sections is 60-100%.According to the composition of microcomponents, the coals are divided into two groups: two-component and three-component.Coals consisting of three groups: vitrinite (Vt), inertinite (It), and liptinite (Lt), with a different fraction (5-40%) of the latter, were found in all the sections studied (Figure 3).
The vitrinite group is represented by telinite, collotelinite, and helinite.All components are found in coals in different ratios.The greatest content of telinite is determined in the coals of the Komsomolsky mine of the Vorkuta District (Figure 3a Inertinite components are represented by fusinite (Figure 4a, c), inertodetrinite (Figure 4b, f), semifusinite (Figure 3d), mikrinite, macrinite and funtinite (Fig. 4g).The group of liptinite is found in various co-forms of lenses and inclusions of rubberite (Figure 3c, d) in the collinite base mass, as well as various inclusionssporinite (Figure 4b), kutinite, and liptodetrinite (Figure 4 (a)), rarely alginite.The maximum contents of Lt are determined in coals of outcrop No. 49, where the greatest amount of cutinite, sporinite and, to a lesser extent, resinite were found.In the coals of the Vorkuta District, with small contents of liptinite group (up to 10%) and high ash content, the remains of algae (alginite) are found.The presence of mixed OM of the II-III type in these sections is confirmed by the data of pyrolytic studies, in particular the hydrogen index (HI).HI of mixed type coal varies from 150 to 400 mg HC/g of C org .According to the dominant composition of OM in coals, there are III, II-III, III-IV and VI types.The predominance of one or another type of OM is due to differences in the conditions of accumulation and sources of organic material.The presence of a large variety of OM types indicates the presence of various sources of material supply and, ultimately, the difference in the properties of coals.
Conditions for the formation of coal beds play a primary role in the coal composition.At the present time, various indices are used, on the basis of which diagrams are constructed, where regions are distinguished with differences in conditions of coal accumulation.In this paper, we used the Diessel indices (Diessel, 1992), taking into account the constancy of humid conditions and the rate of burial of the sediment.TPI and GI are indicators of structural integrity and gelification, respectively (Figure 5).The structural integrity index (TPI) is estimated using a fraction, in the numerator of which the sum of structural components, and in the denominator, structureless:
The second index (GI) -the index of gelification (homogenization) -serves to determine water cut.It is determined by the ratio of the gelified components to ungelified ones and shows the geochemical conditions of peat accumulation.An increase in the GI index is an indication of the water level.The transition is from anhydrous forest conditions to lake ones:

GI = (vitrinite + macrinite) / (semifusinite + fusinite + inertertetrinite).
The greatest differences in the conditions of coal accumulation are characteristic for the section of the lekvorkutskian suite on the outcrop No. 49 (River Vorkuta), in view of selection of coals in various parts of the coal cycle.According to the calculated indices, ears were formed under various conditions from small dry bogs, to lowland and reed bogs, i.e. coastal areas with the influence of marine conditions.The ears of the deposits of the Vorkuta region gravitate toward the formation of lowland bogs, as well as the coals of the Kungurian sediments on the river Kozhym and stream Bokovy.The vast majority of coals formed in coastal wetlands with the influence of marine conditions.Thus, the coals are characterized by the generality of formation conditions, but differ in the sources of the initial matter and in the composition of the OM.

Geochemistry of coals
The type of initial OM determines the differences in the geochemical indicators and products of its catagenetic transformation.Concentrations of organic carbon (C org , %) and chloroform bitumen (CFB, %) vary over a wide range: C org -25-90%, CFB -0,1 -1,4%.The maximum values are characteristic for less ash ears of the Vorkuta District.The bituminous factors (β хб , %) calculated from the ratio of CFB to C org do not exceed 3%.
According to the distribution of n-alkanes in bitumen (Figure 6), three groups can be distinguished among the examined coals.The first group of bitumen is characterized by a left-symmetric distribution with a predominance of low-molecular n-alkanes in the C 13 -C 18 area with a maximum distribution for n-C 16 , which is characteristic of the sapropel component of OM.The concentration of high molecular alkanes of the normal structure of composition C 25 -C 33 is extremely insignificant (Table ).It was possible to identify isoprenoids i-C 15 , i-C 16 , i-C 18 , i-C 19 (Pristan (Pr)) and i-C 20 (phytane (Ph)).Moreover, the concentration of i-C 19 is greater than i-C 20 .Such a distribution is characteristic for bitumen of coal of lekvorkutiskian suite from the outcrop No. 49 on the river Vorkuta and coal from the Vorgashorsky Mine.The composition of coals predominantly consists  Vorkuta River;2 -outcrop 5, Kozhym River;3 -outcrops 5, 9, Bokovoy Stream (numbering according to Pukhonto, 1998); 4 -Yunyaginsky Mine; 5 -Vorkutinsky Mine; 6 -Vorgashorsky Mine; 7 -Vorkutinsky field; 8 -Vorgashorsky field (7-8 finds of sapropelian-humus coals according to data (Korelsky, Margot, 1975)) of vitrinite, as well as inclusions of liptinite -resinite, kutinite and sporinite.
For the second group, characteristic for coals of the "thick" layer from the Vorkutinsky Mine, an increased content of medium-molecular n-alkanes of the composition Cl 9 -C 24 is observed, with a shift of the distribution maximum to the region of n-C 18 , n-C 20 .This group of bitumen is characterized by a very low content of alkanes of the isoprenoid structure in comparison with n-alkanes.
The first two types of distribution characterize the vast majority of ears, with predominance of low-and medium-molecular n-alkanes occurring in the northern sections.Each type of distribution is characterized by both 2-and 3-component composition of OM (Vt, It, Lt).Small differences in the ratio of n-alkanes are observed with a change in the composition of the microcomponents of the liptinite group, which is more pronounced in bitumen from the outcrops along the river Vorkuta and the stream Bokovoy (Figure 6).
The third group of bitumen is characterized by a high content of high-molecular n-alkanes of composition C 25 -C 33 C 33 (responsible for the contribution of humic OM), the relative concentration of which reaches 31.9%,Fig. 6.Distribution groups of saturated hydrocarbon coal bitumen (compiled with the addition of Anishchenko et al., 2004)