Hydrocarbon Source Potential and Depositional Environment of the Surma Group Shales of Bengal Basin, Bangladesh

Abstract

Surma Group is the most important geological unit of Bengal basin, Bangladesh, because petroleum resources occur within this group. It is mainly composed of alternation of shale and sandstone and the shale fraction has long been considered as source rocks and the sandstone fraction as reservoir. These source and reservoir rocks have been studied by different authors by different approach but none of them adopted organic geochemistry and organic petrology as a means of study of source rock and their possible depositional environment. A total of thirty shale core samples have been collected from eight different gas fields to fulfill the short coming. The collected samples have been subjected to Source Rock Analysis (SRA) and/or Rock-Eval (RE) followed by pyrolysis gas chromatography (PyGC), gas chromatography mass spectrometry (GCMS), elemental analysis (EA) and organic petrological study such as vitrinite reflectance measurement and maceral analysis. The analyzed organic matter extracted from the shales of Surma Group consists mainly of Type III along with some Type II kerogen. The studied shales are mostly organically lean (TOC ±1%) and the extracted organic matter is fair to moderate. Based on these results, the analyzed shales have been ranked as poor (mostly) to fair quality source rock. The organic matter of the analyzed shale samples is thermally immature to early mature for hydrocarbon generation considering their Tmax and measured mean vitrinite reflectance values. The hopane 22S/(22S + 22R), moretane/hopane ratio and sterane parameters are also in good agreement with these thermal maturity assessments. The predominance of odd carbons over even carbons (most common) and/or even carbons over odd carbon numbered n-alkanes, moderate Pr/Ph ratio, low to high Tm/Ts ratio, comparative abundance of sterane C29 (i.e., C29 >C27>C28), Pr/nC17 - Ph/nC18 values, C/S ratio and dominance of vitrinite macerals group with the presence of liptinite macerals demonstrate that the organic matter has derived mainly from terrestrial inputs with an insignificant contribution from the marine sources. The condition of deposition alternates from oxic to anoxic.

Vertical distribution and release characteristics of nitrogen fractions in sediments in the estuaries of Dianchi Lake, China

“…Therefore, we divided the columnar samples of the sediments into superficial sediments (0-15 cm) and deeper sediments (15-40 cm) and analyzed the correlation between various N fractions in sediments at different depths and the correlation indicates that organic matter in the deeper sediments of the two study areas mainly originate from endogenous aquatic organisms rather than terrestrial plants. The C/N in sediments within the range of 0-40 cm in Areas B, D and E was higher than that in Areas A and C. This implies that terrestrial plants make a relatively large contribution to organic matter in sediments in Areas B, D and E [23,29]. Differences between the C/N ratios of sediments in different study areas indicate that different areas have different sources of organic matter.…”

“…In addition, TOC levels can also reflect the type and intensity of human activities [28]. The C/N ratio of sediments can be used to infer the source of organic matter and the productivity of a lake to reveal the influences of human activities on the evolution of lake eutrophication [29]. The C/N (by mass) in sediments of different depths in the five study areas indicated that C/N in the deeper sediments (20-40 cm) of Areas A and C were relatively stable and were characterized by a relatively small variation range (Figure 2).…”

Petroleum Source Rock Properties of the Neogene Bhuban Shales, Bengal Basin, Bangladesh

“…It is believed that the Bhuban Formation is acting both as source rock (shale part) and reservoir rock (sandstone part) for the existing petroleum system of the Bengal Basin (Imam 2005). So far, there is no detail work published on organic geochemical and petrographical aspects of the Bhuban shales except Farhaduzzaman et al (2014). Here, we present the data from both techniques: Organic geochemical and petrographical for Bhuban Formation shales.…”

Geochemical Characterization and Thermal Maturation of Cerrejón Formation: Implications for the Petroleum System in the Ranchería Sub-Basin, Colombia

“…Additionally, it may also have the capacity to generate oil-type hydrocarbons to a lesser quantity due to its organic matter type II/III kerogen. To assess the maturity and kerogen type, the obtained data were plotted on an HI versus Tmax diagram (e.g., [41,47,48]) (Figure 12). This concluded that the Cerrejón Formation in the central area of the Ranchería Sub-Basin has not reached the oil generation window, but it would have the capacity to generate mainly gas-type hydrocarbons due to its organic matter type III kerogen.…”