Abstract

Seven crude oils and one biodegraded oil seep sample from four wells and one outcrop location was from Surma Basin, North East Bangladesh were analysed using the computerised gas chromatography and mass spectrometry. The detailed investigations of the saturated hydrocarbon fractions of these oils unveils the occurrences of a number of terrestrially derived biological marker compounds. Bicadinanes (C₃₀ pentacyclic triterpanes) have only been reported to date in crude oils and sediment extracts front South East Asia (Indonesia, Brunei, South China sea). The present investigation indicates that they are also widely present in Tertiary oils from the Surma Basin Bangladesh. In addition to bicadinanes the Surma Basin oils contain 24-norlupane (C₂₉H₅₀ pentacyclic triterpane), a probable lupane derivative characteristic of higher land plants. The occurrences of 24-norlupane has not yet been reported in crude oils. Oleanane is also abundant in these oils though its distribution does not correlate with those of the bicadinanes and 24-norlupane. This is consistent with separate specific land plant sources for these triterpanes, and with the formation of bicadinanes during maturation of polymerised angiosperm tree resin. The occurrences of highly specific biomarkers like bicadinanes, 24-norlupane, and oleanane in the Surma Basin oil indicates that the terrestrial organic matter has played a dominant role in the formation of these waxy oils.