Characterization of alkyl phenanthrene distributions in Permian Gondwana coals and coaly shales from the Barapukuria Basin, NW Bangladesh

Abstract

Coals and coaly shales from the Permian Gondwana sequence in the Barapukuria Basin (core GDH-40) of Bangladesh were investigated to determine the influence of rock type and organic source on methylphenanthrene (MP) maturity parameters such as methylphenanthrene ratio (MPR) and methylphenanthrene index 3 (MPI 3). The Tmax (oC) depth profile of the coals is almost constant, and the Tmax values (430 – 440 oC) correspond to about 0.6 – 0.7% vitrinite reflectance, which suggests the early stage of oil generation. In the constant maturation stage, the MPR and MPI 3 values of coals are clearly different from those of coaly shales. The 9-MP/1-MP and 9-MP/2-MP ratios of the coals are higher than those of the coaly shales because the activated methylation preferentially leads to 9-MP in coals during the early stage of oil generation. Also, these ratios increase in the coals rather than coaly shales attributed to selective biodegradation of the MP and conifer-rich sources. A negative correlation observed between 9-MP and 2-MP compositions indicates methylation effect and/or change in organic source input into the paleomire. The 1,7-dimethylphenanthrene (DMP) isomer, gymnosperm-derived pimanthrene, is more abundant in the coals than the coaly shales. The relative abundances of the 1,7-DMP in the coals tend to be higher in middle part of the drill core, and decrease with depths. Anthracene (A) and methylanthracene (MA) also tend to be more abundant in the coals. The abundance of 1,7-DMP, A and MA in the coals may be related to Gondwana coals characterized by the predominance of coniferous gymnosperms.