Influence of organic matter type on the distribution of tri-aromatic hydrocarbons in Tertiary mudstones in the Sylhet Basin, Bangladesh

Abstract

The distribution and behavior of tri-aromatic hydrocarbons (especially alkylphenanthrenes) have been investigated in twenty-four mudstones from the Tertiary succession in the Sylhet Basin of Bangladesh. Phenanthrene (P) and methylphenanthrenes (MP) were abundant in most samples, whereas anthracene and methylanthracene (MA) were detected only in some samples. Abundances of the 1,7-dimethylphenanthrene (DMP) isomer are significant relative to other DMP isomers. Overall correlation between the methylphenanthrene index 3 (MPI 3) and T_max (426-449℃) is poor in this basin. Abundances of MA, pimanthrene (1,7-DMP) and 1-MP are relatively high in the lower part of the succession. Higher plant organic matter is abundant in the lower part of the succession (middle Eocene to early Miocene), whereas planktonic organic matter is relatively abundant in the middle and upper parts (middle Miocene to Pleistocene). These alkyl-isomers thus originated from terrigenous sources. High phenanthrene/alkylphenanthrene ratios in the mudstones can be related to oxic environmental conditions. Based on detailed aromatic distribution patterns and their isomer ratios, we conclude that the compositions of P, MP and DMP isomers in the Sylhet Basin were probably controlled by organic matter sources, rather than by thermal maturity in the early stage of the oil window.