Petrology and thermal maturity of coals in the Jharia Basin: Implications for oil and gas origins

Abstract

Coals from the Karharbari Formation (Early Permian), the Barakar Formation (Early Permian) and the Raniganj Formation (Late Permian) of the Jharia Basin have distinctive petrographic characteristics. The Karharbari Formation coals are dominated by inertinite whereas the Barakar Formation and the Raniganj Formation coals are vitrinite-dominated. The vitrinite and exinite content of the Jharia Basin coals generally increases towards the upper parts of the sequence. Coal facies analyses based on petrographic composition that the Jharia coals were formed from woody peat facies in a fluvial environment. The Karharbari coals were formed in a predominantly forest swamp condition, whereas the lower Barakar coals were formed in mixed facies (wet/ dry) conditions. The middle and upper Barakar and the Raniganj coals were formed under wet forest swamp conditions. High rank Permian coals ( R v max 0.93–1.27%) occur near the surface in the Jharia Basin. Comparisons of the vitrinite reflectance versus depth profiles from the Jharia Basin samples with some known reflectance profiles indicate that 2.5–3.5 km of cover was lost in the Jharia Basin. Cover loss values extrapolated from a profile matching method show good correlation with those calculated from model temperatures. The thermal history of the Jharia Basin suggests that the Permian sections of the basin suffered a major Permian-Triassic thermal event. During this time a tensional tectonic regime probably prevailed in India, possibly centred in the Jharia Basin. This probably caused extensive subsidence and, consequently, a thicker sedimentary sequence was deposited in the Jharia Basin than in the other Gondwana basins of India. The Early Permian Barakar Formation is late mature to overmature. This section has a high potential for oil, condensate and gas generation, although only condensate and gas are likely to be preserved. The Late Permian Raniganj Formation is mature and the source potential for hydrocarbon generation of this section is assessed as very good; however, most structures within the Jharia Basin are probably fractured.