Petroleum geochemistry of the Tertiary sediments and oil samples from the Bengal Basin, Bangladesh
Tertiary sediments taken from ten exploration wells and seven outcrop sections in the Bengal Basin together with thirteen oil and condensates and one oil seep and three oil soaked sandstones have been analysed using a variety of organic geochemical techniques. Detailed investigations of the distribution of biological marker compounds indicates that these soils (Surma Basin) are paraffinic waxy oils with a varying amount of C21+ n-alkanes typical of generation from source rocks rich in land plant waxes. The common occurrences of 24-norlupane and oleanane limits the age of the source rocks to Cretaceous or younger and most likely Tertiary. The selective occurrence of bicadinanes in the Surma Basin samples grouped the oils and condensates into two families suggesting the existence of more than one source rock. All the maturity parameters concur that the condensate samples from the Bengal foredeep region are of lower maturity and had been generated at the early stage of oil generation whilst the Surma Basin oils had been generated at around peak maturity of the source rock. The Hararganj oil seep and Sitakund oil-sands are severely biodegraded. The abundances of bicadinanes in Surma Basin oils indicate that they are not restricted to a few South East Asian (Indonesia, Sabah, Brunei) basins only. The occurrence of 24-norlupane has not yet been reported in crude oils and their presence in the Surma Basin oil and condensate samples suggests that they may also occur in Tertiary oils rich in angiosperm markers from other basins. Source characterisation of the various sample suites indicates the existence of at least three organic facies. The Upper Jenam samples with moderate to high organic richness contain abundant plant derived amorphous organic matter sufficient to qualify as a oil regnerating source rock. Maturity measurements of the Upper Jenam formation generally concur that the exposed sequences of this formation are insufficiently mature to have generated and expelled significant quantities of petroleum although the same formation in well sections is around the threshold of oil generation. Despite their immaturity, the source specific triterpane distributions in the Upper Jenam sediments indicates that they correlate closely with the Surma Basin oils. In terms of triterpane distribution the Bhuban sediments show a correlation with condensate samples. Despite their lean organic content, the huge volume of these sediments suggests that the Bhuban formation might have generated minor amounts of early mature condensate.