Geochemical criteria of migration of natural gases in the Miocene sediments of the Bengal Foredeep, Bangladesh

Abstract

Thirteen trillion cubic feet of estimated recoverable gas along with 40 million barrels of condensates have been discovered in Miocene sandstones of the Bengal Foredeep, Bangladesh. The gases are mostly dry and sulphur free with a very low content of non-hydrocarbons. The terrestrial organic matter of Miocene shales at an early stage of maturation ( R 0 = 0.4–0.6%) is the source of natural gases in the region. Based on the relative ratio of C 2/C 3+ three gas accumulation zones could be distinguished in the region: northern ( C 2/ C 3+ = 1.98), central ( C 2/ C 3+ = 3.62), and southern ( C 2/ C 3+ = 4.90), which coincide with the three major tectonic elements of the region. Such types of distribution of C 2/C 3+ values in the three accumulation zones are related to the different adjacent hydrocarbon generative depression as well as the processes of migration. The physico-chemical parameters like C 2+, 100·C 2+/C I and 100·C 3+/C I decrease from the platform flank to folded flank, and from Sylhet and Hatia Troughs to Modhupur High, indicating the migration direction and pathways. A trend of change in the isotopic composition of methane ( δ 13 C I)is also observed in the same direction. The value δ 13 C 1 becomes heavier during the process of lateral migration and lighter during the tertiary migration within the multipay zone gas fields. The lateral migration plays the major role for the occurrence of hydrocarbon accumulation in the region. Maximum concentration of hydrocarbons may be expected to occur in the folded flank and the Tangail-Tripura High.