Origin and Accumulation Mechanism of Gas Condensate in Kailashtila Gas Field, Sylhet Basin, Bangladesh

The Kailashtila gas field (KGF) is situated in the northeastern part of Sylhet basin, Bangladesh. This paper presents chemical characteristics of extractable natural gas in drilled well KTL-2, in order to examine their potential source and maturity of organic matter, and hydrocarbon accumulation mechanism in the basin. The gas condensate in the KTL-2 composed primarily of methane (85.81 wt.%), ethane (6.68 wt.%), propane (2.13 wt.%), and traces of higher hydrocarbons (i-butane, 0.69 wt.%; n-butane, 0.73 wt.%; i-pentane, 0.50 wt.%; n-pentane, 0.44 wt.%; hexane, 1.27 wt.%; heptane, 0.99 wt.%; octane, 0.24 wt.%). Nitrogen and CO2 contents in the gas condensate are low (0.46 wt.% and 0.05 wt.%, respectively). Average dry coefficient (C1/C1–5) value in the gas condensate is 0.93 (0.91–0.95), which reflects relatively mature hydrocarbon migrating from nearby deeply buried source rocks. The δC1 (–39 to –40‰) and C1/C(2+3) (19.77) variation diagram show that gas condensate in the KGF is mainly controlled by type III kerogen, and the organic matter was thermally mature in nature. However, the relationships between stable isotope value of methane (δC1), ethane (δC2) and propane (δC3) indicate mainly thermogenic origin of the studied gas condensate, and minor input from mixed thermogenic and bacteriogenic processes.