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

Abstract

The Kailashtila gas field (KGF) is situated in the northeastern part of Sylhet basin, Bangladesh. This paperpresents chemical characteristics of extractable natural gas in drilled well KTL-2, in order to examine their potentialsource and maturity of organic matter, and hydrocarbon accumulation mechanism in the basin. The gas condensate inthe KTL-2 composed primarily of methane (85.81 wt.%), ethane (6.68 wt.%), propane (2.13 wt.%), and traces of higherhydrocarbons (i-butane, 0.69 wt.%; n-butane, 0.73 wt.%; i-pentane, 0.50 wt.%; n-pentane, 0.44 wt.%; hexane, 1.27wt.%; 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), whichreflects relatively mature hydrocarbon migrating from nearby deeply buried source rocks. The δ13C1 (–39 to –40‰) andC1/C(2+3) (19.77) variation diagram show that gas condensate in the KGF is mainly controlled by type III kerogen, andthe organic matter was thermally mature in nature. However, the relationships between stable isotope value of methane(δ13C1), ethane (δ13C2) and propane (δ13C3) indicate mainly thermogenic origin of the studied gas condensate, andminor input from mixed thermogenic and bacteriogenic processes.