Vitrinite Reflectance and Palaeotemperature analysis of coals in Tikak Parbat Formation, Makum Coal basin, Assam, India

The present study is confined only to the coal seams and associated rocks of Tikak Parbat Formation of Barail Group(Oligocene) and based on the analysis of coal petrographic data and measured depth of sections in Tikak Hills and Tirap colliery areas of Makum Coal basin. It is observed from the palaeotemperature study that the palaeotemperature attained is 950C to 970C for 60ft.(18m) seam and 790C to 880C for 20ft.(6m) seam. Geothermal gradient (G) is found to be +_ 38.50C/Km and the Tikak Hills area was under higher geothermal regime than that of Tirap colliery area but both belong to a single palaeogeothermal regime. High heat flow is a reflection of subduction-collision processes related to Indian plate against Burmese plate and the loci is Naga-Patkai geotectonic belt of which Makum Coal basin is an integral part. INTRODUCTION: The question of heat flow or geothermal heat supply in a particular region for tectonic reasons is automatically brought in due to the fact that sometimes coal attains a very high rank at a lower depth of subsidence (e.g. Bransche massif in N.W. Germany) whereas at a reasonable higher depth, the maturity is much less (Ammosov, 1968). Due to varying palaeogeothermal histories from region to region the effect of duration of heating also changes. Therefore, it is difficult to relate a particular degree of coalification to a precise maximum temperature as the exact value of the two variables and the functional trend, has to be assumed from indirect evidences and or field/laboratory data. Published geothermal data on Upper Assam Basin and its adjoining areas provide some interesting information. The mean heat flow and geothermal gradient are 52.16 mW.m-2 and 22.86 ± 3.94°C/km respectively for Assam Basin in particular. The geothermal gradients within the Barail group are widely scattered and do not show any linear realtionship with depth. The estimated crustal temperature distribution within different depth ranges beneath the Naga-Patkai belt are 210°C at 10 km, 510°C at 25 km, and about 820°C at 45 km (Handique and Bharali, 1981; Panda, 1984; Cermak et al. 1990).Considering the above findings, the author has made an attempt to study the palaeotemperature distribution in Makum Coal basin based on measured exposed sections and vitrinite reflectance of coals in Tikak Parbat Formation of Barail Group(Oligocene). GEOLOGICAL SET-UP: Tectonically, the coalfield areas of Upper Assam, Arunachal Pradesh and Nagaland occur in the belt of Schuppen which is a narrow NE-SW trending linear overthrust zone on the NagaPatkai Range. This belt is delineated on the east by Halflong – Disang thrust and on the west by Naga thrust. Makum coal basin is an isolated tectonic block of this belt where the coal bearing Barail Group of rocks were sandwiched between two thrusts namely Margherita thrust on the north and HalflongDisang thrust on the south (Fig. 1). The litho stratigraphic succession as developed in the study area is shown in Table –1 (Sarmah, 1999).The coal bearing Barail Group is subdivided into Naogaon, Baragolai and Tikak Parbat Formation in ascending order. The upper Tikak Parbat Formation is named after the Tikak Hills (27° 15′ : 95° 45′) in Makum coalfield. This formation can be subdivided into lower Argillaceous unit and upper Arenaceous unit on the basis of gross lithofacies character and association (Sarmah, 1999). The lower unit houses thick, persistent and workable coalseams. The upper unit is arenaceous in character with more of massive to well bedded, medium to fine grained sandstone interbeded with sandy shale, grey shales, sandstone, sandy clay carbonaceous shale with several thin coal seams. AGE GROUP FORMATION LITHOLOGY Recent –


INTRODUCTION:
The question of heat flow or geothermal heat supply in a particular region for tectonic reasons is automatically brought in due to the fact that sometimes coal attains a very high rank at a lower depth of subsidence (e.g. Bransche massif in N.W. Germany) whereas at a reasonable higher depth, the maturity is much less (Ammosov, 1968). Due to varying palaeogeothermal histories from region to region the effect of duration of heating also changes. Therefore, it is difficult to relate a particular degree of coalification to a precise maximum temperature as the exact value of the two variables and the functional trend, has to be assumed from indirect evidences and or field/laboratory data. Published geothermal data on Upper Assam Basin and its adjoining areas provide some interesting information. The mean heat flow and geothermal gradient are 52.16 mW.m -2 and 22.86 ± 3.94°C/km respectively for Assam Basin in particular. The geothermal gradients within the Barail group are widely scattered and do not show any linear realtionship with depth. The estimated crustal temperature distribution within different depth ranges beneath the Naga-Patkai belt are 210°C at 10 km, 510°C at 25 km, and about 820°C at 45 km (Handique and Bharali, 1981;Panda, 1984;Cermak et al. 1990).Considering the above findings, the author has made an attempt to study the palaeotemperature distribution in Makum Coal basin based on measured exposed sections and vitrinite reflectance of coals in Tikak Parbat Formation of Barail Group(Oligocene).

GEOLOGICAL SET-UP:
Tectonically, the coalfield areas of Upper Assam, Arunachal Pradesh and Nagaland occur in the belt of Schuppen which is a narrow NE-SW trending linear overthrust zone on the Naga-Patkai Range. This belt is delineated on the east by Halflong -Disang thrust and on the west by Naga thrust. Makum coal basin is an isolated tectonic block of this belt where the coal bearing Barail Group of rocks were sandwiched between two thrusts namely Margherita thrust on the north and Halflong-Disang thrust on the south (Fig. 1). The litho stratigraphic succession as developed in the study area is shown in Table - 1 (Sarmah, 1999).The coal bearing Barail Group is subdivided into Naogaon, Baragolai and Tikak Parbat Formation in ascending order. The upper Tikak Parbat Formation is named after the Tikak Hills (27° 15′ : 95° 45′) in Makum coalfield. This formation can be subdivided into lower Argillaceous unit and upper Arenaceous unit on the basis of gross lithofacies character and association (Sarmah, 1999). The lower unit houses thick, persistent and workable coalseams. The upper unit is arenaceous in character with more of massive to well bedded, medium to fine grained sandstone interbeded with sandy shale, grey shales, sandstone, sandy clay carbona-ceous shale with several thin coal seams. Coarse, gritty and massive, bluish green to grey feldspathic and micaceous sandstone, variegated clay, sandy clay, shale, coal streaks and silicified woods. (+ 1000 m : thickness).

METHODOLOGY:
Coal samples from 60 ft, 7 ft , 20 ft and thin seams of Tikak Parbat Formation exposed in two sections-Tikak and Tirap were collected from Makum Coal basin. The polished coal pellets were examined under orthoplan microscope in oil media with 32x or 50x objectives and 10x or 8x ocular lens. Light source was stabilised sodium ray. For reflectance measurement a stabilised monochromatic sodium light with blue filter was imparted on the sample and the values were recorded from photovoltmeter calibrated against standard (natural glass with Ro 1.24% and Leucosapphire Ro 0.59%) following ICCP (1971) and IS 9127 (Part III) (IS 1992). The measured depth and vitrinite reflectance data are used to plot in binary diagrams and nomograms to calculate the palaeotemperature of the coal seams.

ReseaRch PaPeR
tion in the Upper Rhine Graben (Fig.2 ) on which the author has also tried his own exercises. It is evident from the graph that the Tikak hills section shows lower reflectance gradient 0.099% per 100 m than that of Tirap colliery section (i.e. 0.123% per 100 m). The curve of the Tikak hill section also shows slightly steeper slope in comparision to the Tirap colliery profile indicating gradual increase of geothermal gradient towards east of the studied basin. Fig.2. indicates that the paleogeothermal condition for the Tirap colliery area is the most warmer than the Tikak hills area to the west. The depth-reflectance trend in the Tirap colliery area is almost similar to that of the Upper Rhine Graben but has compartively warmer geothermal conditions. This variation may be due to the complex tectonic activity in the region. Bostick et al. (1979) established the relationship between vitrinite reflectance, rock temperature and burial history in number of boreholes in the region further north of California. Their results incorporated more than 5000 m of burial and supplemented the works of Hood et al. (1975). The modified nomogram presented by Bostick ( op. cit) is considered outstanding ( Fig.3 A & B ) as the present observations are found to be in agreement with that of field observations and other geological information as well. Since, the rank gradient varies in different coalification ranges, any comparison should be done within the same rank range. Teichmuller (1979) illustrated relationship between vitrinite reflectance (Ro) versus depth and vitrinite reflectance (Ro) versus rock temperature for the same well. The comparison clearly demonstrated the dominant role of temperature on coalification. This is also evident from the depth versus reflectance diagram (Fig.4) using the data obtained from the present study. Bostick (1973) gave an account on the relationship of time on coal rank and established that the rank of coal determined from vitrinite reflectance can go up to anthracite range even within a moderate temperature range of 100° -200° C provided, the time of coalification is substantial and in the tune of millions of years. He also agreed that the time factor is not tenable for coal attaining a temperature below 50° C, since the reaction cannot be triggered off. With determined reflectance value (Rm oil) in the laboratory and assumption of effective time, maximum temperature were read from the nomogram (Fig.3 A & B ) of Bostick et al. (1979), Shibaoka & Bennett (1976) respectively and incorporated systematically.  Shibaoka and Bennett (1977) and Nagornyy et al. (1977) have close resemblance and matched well with the geological history and the palaeodepth of the studied coal basin when the available data pertains to Mesozoic-Cenozoic basins of India is considered to be valid (Panda, 1984). The lower soaking time of 17 m. yrs. and 25 m. yrs. resulted in comparatively higher values.
The palaeotemperature values determined from the Karweil's and Ammosov et al. show much lower and higher range respectively compared to the methods already mentioned.
The plots in the diagram (Fig.5) (1975) near upper end. It clearly reveals a moderately higher geothermal gradient for the studied coal basin and a best fit line matches the soaking time as 35 m. yrs.
The plots in Fig.5 showing the relationship between palaeotemperature and Ro average (after Ammosov et al. 1975 andNagornyy &Nagornyy, 1977) reveal that the Tikak hills area was under relatively higher geothermal regime than that of the Tirap colliery area, but both belong to a single paleogeothermal regime. The depth -reflectance trend diagrams (Fig.5.12 ) on the other hand do not record this observation. This deviation may possibly relate to some differential subsidence of the basin floors in response to tectonic events in the studied coal basin or to the variable depth of burial of the basement in the area.
The present indirect approach may have some direct bearing on maturation of organic matter both for coalification process and hydrocarbon generation in the sedimentary basins of Upper Assam and adjoining areas (including Makum coal basin) on a regional scale during Late Miocene and Mio-Pliocene time due to the Himalayan orogenic activity and also due to the compression forces readjusting for subduction of Indian plate against Burmese plate in the region (Nandy, 1986).

GEOTHERMAL GRADIENT (G)
The estimated values of geothermal gradient are derived systematically from the maximum palaeotemperature calculated after the methods already described when divided by the maximum paleodepth (using the paleodepth data after Bostick, 1979) and presented in the table (Table-3). Besides these another method after Middleton (1982) was applied for the determination of palaeogeothermal gradient i.e.
Geothermal Gradient (m) = (Log Rmax/Rmin)/(Max depth -Min depth) x 194.8 (constant)  (1975) and Middleton (1982) seem to be quite high which indicate their limitation in applying the principles for the Tertiary coal basins of north eastern India.

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A critcal examination of the available geothermal gradient and heat flow data indicates that the Makum coal basin may not be normal to its setting with the adjoining oil -field areas of Upper Assam Basin to the further north west. The high heat flow and Moho temperature in the upper mantle beneath the mobile Naga -Patkai belt was attributed due to the subduction -collision related phenomenon in the Indian plate against the Burmese plate.

ACKNOWLEDGEMENT:
The author wishes to thank Mr. P. R. Sengupta and Dr. B. Bardhan, Geological Survey Of India for their help during his stay in Calcutta for reseach work .