Hydrocarbon Generation the generation of hydrocarbon oil and natural gas in the underground depends on three main factors, ie (i) amount of organic matter in the rock; a minimum of 0.5% of total organic carbon (TOC) is required for shale to generate hydrocarbon; (ii) type of organic matter; marine organic matter can generate both oil and gas whereas land derived woody type organic matter can only produce gas; (iii) thermal maturity of the organic matter, brought about by sufficient burial temperature in the subsurface.
Hydrocarbon generation takes place from organic rich rocks in the subsurface within a definite temperature range occurring within certain burial depth levels known as 'oil window' and 'gas window'. Oil and gas windows in Bangladesh are believed to occur at depth levels of about 5 to 8 km below the surface in the basinal area. Rocks rich in organic matter, when buried to this depth-generated hydrocarbons that subsequently migrated upward to accumulate in the present reservoirs at depth levels between 1 and 3 km.
Bangladesh has, under its surface, a very thick (about 20 km) sedimentary rock sequence of mainly Tertiary age (2 to 66 million years before present). These are divided into rock units known (from top to bottom) as Tipam formation, Bokabil formation, Bhuban formation, Barail formation and Pre-Barail Eocene sequence. The Tipam, Bokabil and most of Bhuban rock layers are poor to very poor in organic matter and are thus not considered to be the source of natural gas. The Barail rock unit has organic rich shales (TOC 0.6% to 2.4%) that are buried to depth levels in the range of 5 to 8 km. It is thus suggested that Barail shales occurring within the oil and gas windows have generated natural gas and oil found in Bangladesh. After generation, gas and oil have migrated upward through multi-kilometre rock layers above, to reach and accumulate in suitable sandstone reservoirs in the Bhuban and Bokabil rock units. The organic matter in the Barail shales are mainly of land derived woody types. Many geologists believe that because of predominance of woody type organic matter in the hydrocarbon source rocks, Bangladesh does not have as much oil as gas, since this type of organic matter can only produce gas. [Badrul Imam]