分子・同位体組成が示す北東バングラデシュ，スルマ堆積盆地のオイル・コンデンセートの起源と貯留層内相分別作用

Abstract

The origin and source rocks of crude oils and condensates from seven oil and gas fields located in the Surma basin, Bangladesh were investigated based on saturated and aromatic hydrocarbon distributions. The relative compositions of pristane, phytane and adjacent n-alkanes suggest that the source rock was deposited in a non-marine setting. The abundance and similar distribution of biphenyls, cadalene and bicadinanes in most of the crude oils and condensates indicates a significant supply of higher-plant derived organic matter to the source rocks. Maturity levels of oils and condensates from the Surma basin were estimated based on the alkylnaphthalenes, alkylphenanthlenes, and diamondoids, correspond to calculated vitrinite reflectance (Rc) values of 1.0–1.3%. Rc (%) values indicating hydrocarbon expulsion from the source rock at a comparatively high maturity level. The Rc values of oils from the Titas field in the southern margin of the Surma basin are relatively low (0.8–1.0%). Some oils were severely biodegraded. The similar distribution of diamondoid hydrocarbons in both biodegraded and non-biodegraded oils indicated similar type of source rocks and similar maturity levels to those of oils from the Surma basin.Although the geochemical data set for the Oligocene and older sedimentary sequences is limited, the top of the Eocene Kopili Shale to lower Jenam Shale of early Oligocene sedimentary rocks rich in terrigenous organic matter are identified as possible source rocks. The relative abundance of C7 aromatic, cyclic, and straight chain hydrocarbons in oil and condensates and other molecular parameters inferred diversity of the petroleum from waxy crude oils to condensates in the study area. This diversity of the petroleum was likely due to evaporative fractionation, resulting in residual waxy oils and lighter condensates which subsequently underwent tertiary migration and re-accumulation. Evaporative fractionation is thought to be due to modification of the reservoir structure occurred during and after the Pliocene, when large-scale tectonic deformation occurred in and around the Bengal Basin