(1) Unocal Corporation, Sugar Land, TX
(2) Bangladesh Oil, Gas and Mineral Corporation, Dhaka, Bangladesh
Abstract: Origin of Oils and Gases of Bangladesh
Petroleum exploration in Bangladesh began early in this century. Significant commercial gas discoveries were made in Miocene sands in the eastern part of the country beginning in the 1950s. In an effort to understand the petroleum systems operative in Bangladesh, we have examined compositional and isotopic data for gases and condensates throughout the country, and interpret these data to define petroleum families and origins. With minor exceptions (e.g., Begumganj gases), the gases are compositionally and isotopically (CH4) uniform, and appear to be entirely thermogenic. Non-hydrocarbon gas concentrations (carbon dioxide and nitrogen) are below 5% in most cases. Most of the Bangladesh gases were expelled from source rocks that contained a mature kerogen (VR=0.5-1.0% and higher). This suggests the occurrence of a gas source that is deeper than most of the known oil and gas accumulations, and implies substantial vertical migration.
Most Bangladesh condensates possess molecular signatures indicating a major hydrogen-rich, angiospermous (flowering land plant) contribution to their source organic matter. Supporting data include pristane/phytane ratios, C29/C27 sterane ratios, sulfur, vanadium and nickel concentrations, V/(V+Ni) ratios, n-alkane carbon number distributions, and the occurrence of bicadinanes, oleananes, and possibly oleanenes. The occurrence of bicadinanes, derived from specific angiospermous trees called Dipterocarpacea, is particularly pronounced in a Surma basin oil subset. The molecular characteristics of the Bangladesh condensates have been modified by migration-fractionation, the process whereby deep-sourced gases dissolve and transport oils/condensates from deep in the section and exsolve these liquids at reduced pressures. This process, as well as accompanying migration-contamination, limits accurate condensate-source rock correlations.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana