COAL FACIES, DEPOSITIONAL ENVIRONMENTS AND BASIN BASIN MODELING OF THAR COAL FIELD

Field: Earth and Planetary Sciences (Physical Sciences)

The coal deposits of Thar occur in the Sara Formation of Paleocene age. The coal bearing formation is considered to have been deposited in alluvial environments. Thar coal field is the largest coal field of Pakistan. The reserves of this coal field are estimated at 175,506 million tonnes. Thar coal field is divided into the four blocks and until 2002, 217 boreholes had been drilled. Core samples from five boreholes from block-l were selected for this study. \n \nDetailed organic petrographic work along with Rock-Eval pyrolysis and geochemical techniques were applied for this study. \n \nOn the basis of macroscopic examination the Thar Coal was mainly classified as â€˜matrix coal' facies. However, Xylite coal facies are observed at a few places. A number of pale and light layers are present in Thar coals which represent reed marsh environments. The matrix coals and pale and light layers suggest that peats were formed in treeless, low-lying swamps. The quantitative petrographic analysis shows that, on the average, the coals are huminite rich (91.0%, on mineral matter free basis) with low concentration of liptinite and inertinite (6.7 % and 2.7 %, mmf basis). Furthermore the Thar coals are rich in humodetrinite, its content varies from 43% to 56%. One-meter thick sapropelic coal facies which may grade into an oil shale facies was found in borehole SV -13. \n \nTissue Preservation Index and Gelification Index of Diesel, (1992), Ground Water Index and Vegetation index of Calder, (1991 ), Facies-critical Maceral Associations and Mineral Matter Contents, ABC ternary diagram of Kalkreuth(1991) and TFD ternary model of Marchioni and Kalkreuth (1991) were applied for the interpretation of the environments of deposition. All of these parameters suggest herbaceous vegetation, limnic and limno-telmatic environments for most of the coal seams, especially the main coal seam. While three to four samples from three small coal seams suggest wet forest environments. The sulphur content of the main seam ranges from 0.45 to 1.66% on as received basis (ar) and the ash content ranges from 3.24 to 8.46% (ar). There is positive correlation between the sulphur and pyrite contents. The low ash and low sulphur content or the main seam suggests that the mire was not frequently flooded and there was no marine influence on the hydrology of the peat. \n \nThe palynomorphs are represented by the prevalence of dicots, monocots and pteridophytes at the time of deposition of the investigated strata. Gymnosperms were rare, while the conifers were totally absent. Occurrence of high percentage of dicots, herbaceous monocots and complete absence of gymnospermic pollen in almost all samples (at all depths) indicate dominance of cool tropical to subtropical environment with low humidity. \n \nCoal petrographic, Rock Eval pyrolysis and gross calorific value based on moist and ash free data, classify these coals as Lignite B, Lignite A and Bituminous C. The geochemical, Rock- Eval pyrolysis and organic petrographic analyses on the coal and associated sediments indicate good to excellent potential for gas as well as for liquid hydrocarbons. The burial and thermal histories established using 1-0 basin modelling programme suggest that these coals were deeply buried (>500m) for a short period during late Oligocene and Early Miocene lime. The Tmax and vitrinite retlectance data show that these coals are not mature for hydrocarbon generation. However the presence of oil droplets in two boreholes point toward hydrocarbon generation. The oil droplets may be generated from resinite which can generate liquid hydrocarbons at low maturity.