Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Mahendra P. Singh ¹, Alok K. Singh ²

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi - 22 1 005, IN
2 R & D Division, The Tata Iron and Steel Company Limited, Jamshedpur - 831 007, IN

Abstract

Petrographic examination of Oligocene coals of Upper Assam has shown some morphological changes in the Maceral-Resinite, which is suggestive of their pyrolysis due to thermal maturation. It has been observed that the resinites of these coals resemble in optical properties, including the microstructures, to the artificially pyrolyscd resinite (up to 330°c/72hr) of Middle Jurassic coals of northeast Greenland. Since the Upper Assam crudes are high wax oil, this observation seems to be one of the potential evidences in support of the growing perception that coals of this basin (of the Barail Group) have acted as source for the oil in the same geological horizon.

Keywords

Pyrolysis, Resinite, Hydrocarbon Generation, Oligocene Coals, Upper Assam.

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Mahendra P. Singh ¹, Prakash K. Singh ¹, Alok K. Singh ¹

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi - 221 005, U P, IN

Abstract

In the present paper an attempt has been made to carry out the petrographic characterization of coals from the Deoghar basin and to reconstruct the palaeoenvironmental conditions of coal formation using macerals and microhthotypes as tools For this purpose a large number of pillar coal samples from the Karharban and Barakar seams were collected and were subjected to detailed petrographic examination The results indicate that the Karharban coals are rich in inerlimte while the Barakar coals are comparatively rich in vitnmte The rank of Deoghar coals (as per ASTM system), on the basis of the mean of random vitnnite reflectance, has been found to range between Sub-Bituminous 'B' and High Volatile 'C Bituminous The quantitative relationships of macerals, microhthotypes and mineral matter show that these coals have originated from the transitional plant communities in open to forest moor and under limno-telmatic to hmnic conditions Moreover, the Karharban coals have evolved under oxic moor conditions with sudden high flooding while the Barakar coals have evolved under comparatively wet moor conditions with intermittent moderate to high flooding Low values of TPI and GI indicate periodic dryness in the basin This contention is supported by the dominance of inertodetnnite particularly in the coals of Karharban Formation.

Keywords

Permian coals, Petrography, Depositional environment, Deoghar basin Bihar.

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Mahendra P. Singh ¹, Alok K. Singh ¹

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi - 221 005, IN

Abstract

Borjan coalfield is situated on the western flank of the Naga Hills in the Mon district of Nagaland. Pillar coal samples from Watking and Kongan seams were subjected to detailed petrographic and geochemical investigations for the assessment of their source rock characteristics. During petrographic examination, emigration of exsudatinite (waxy oil) from a microfracture in vitrinite (telocollinite/collotelenite) has been observed. This is a rarely reported visible evidence of primary migration of oil in coal through microfracturing.

Keywords

Oligocene Coals, Hydrocarbon Emigration, Microfracturing, Vitrinite, Borjan Coalfield, Nagaland.

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Mahendra P. Singh ¹, Alok K. Singh ¹

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi - 221 005, IN

Abstract

A large number of coal samples from the Palaeogene coal deposits of Northeast India were subjected to detailed chemical (proximate, ultimate and Rock-Eval pyrolysis) and petrographic (maceral, microlithotype and vitrinite reflectance) analyses to examine their source rock characteristics and to explore their participation in hydrocarbon generation. The results show that these coals are sub-bituminous 'C' to high volatile bituminous 'C' in rank and have requisite petrographic characteristics to act as a potential source for the generation of significant amount of normal hydrocarbons. The values of'vitrinitc reflectance, particularly of the coals of Meghalaya and Assam, suggest a maturation close to the 'top oil window' (birth line). This indicates the possibility of expulsion of oil and condensates in the basin. However, this contention needs to be supported by organic geochemical data, maturation modelling and time of hydrocarbon generation.

Keywords

Palaeogene Coals, Petrography, Source Rock Potential, Maturation, Oil Window, Northeast India.

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