Geochemical evidences linking the Lower Cretaceous source rocks to the biodegraded bitumen seeps and tar sands in the Gulf of Guinea Basin

IF 2.6 3区地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS

Organic Geochemistry Pub Date : 2024-10-04 DOI:10.1016/j.orggeochem.2024.104866

Abdulkareem Toyin , Ningning Zhong , Falilat Omotolani Idris , Olabisi Adekeye , Shengbao Shi

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Abstract

The source of biodegraded bitumen seeps and tar sands in the Nigerian axis of the Gulf of Guinea province has been a contentious issue for decades. The most prevalent opinion is that the biodegraded bitumen seeps are migration oils that were degraded by bacterial. In the present study, outcrop source rocks (Lower Cretaceous), core samples (Upper Cretaceous) from a shallow well-x, and bitumen seeps and tar sands from the eastern axis of the Dahomey Basin of Nigeria were analyzed. Rock-Eval™ pyrolysis show that the outcrop rocks are classified as having excellent-very good source quality; in contrast, those from well-x are classified as having only poor-fair quality. The cross plot of hydrogen index (HI) against T_max indicates that samples from the outcrops contain type II/III organic matter, whereas well samples contain type III organic matter and all source rocks are thermally immature, which is supported by vitrinite reflectance (R_o: 0.20–0.48 %), and biomarker maturity parameters (C₃₂ (S/S + R), MPI-I). In the bitumen seeps and tar sands, isoprenoids, n-alkanes, steranes and hopanes have been degraded, and tricyclic and tetracyclic terpanes are still mostly intact. In addition, C₃₀ hopane and homohopanes have been demethylated, and C₂₅ norhopanes are present in abundance, suggesting that the bitumen seeps and tar sands experienced heavy biodegradation in the range of PM-6 or greater. The distribution of 4,9-DMD, 4,8-DMD and 3,4-DMD diamondoids and the δ¹³C of saturate and aromatic fractions indicate that the bitumen seeps and tar sands were generated from Types II and III source rocks. The ratio of demethylated Ts/Tm indicates that the samples are low maturity and were generated from immature source rocks. Cross plots of tricyclic terpane ratios (C₂₅/C₂₆TT vs C₂₅TT/C₂₄TeT, C₂₃/C₂₁TT vs (C₁₉+₂₀)/C₂₃TT, and C₂₄TeT/C₂₄TeT + C₂₆TT vs (C₁₉+₂₀)/C₂₃TT) also reveal that the biodegraded bitumen seeps and tar sands are closely related to source rocks in outcrop groups 1 and 2. This conclusion is supported by the ternary plots of tricyclic terpanes, 4,9DMD, 4,8DMD and 3,4DMD diamondoids, and the cross plot of EAI against DMDI-1. The occurrence of exsudatinite in the outcrop source rocks in group 1, consistent with early hydrocarbon generation, and the low ratio of demethylated Ts/Tm (<1) in the bitumen samples confirms that the biodegraded bitumen seeps and tar sands in the Nigerian Dahomey Basin were generated by the Lower Cretaceous source rocks in group 1. The study advances on the previous propositions on the source of bitumen seeps and tar sands and contributes our knowledge in the Gulf of Guinea Basin.

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将下白垩统源岩与几内亚湾盆地生物降解沥青渗漏和焦油砂联系起来的地球化学证据

几十年来，几内亚湾省尼日利亚轴线地区生物降解沥青渗漏和焦油砂的来源一直是一个有争议的问题。最普遍的观点认为，生物降解沥青渗漏是被细菌降解的迁移油。本研究分析了尼日利亚达荷美盆地东轴线的露头源岩（下白垩统）、浅井岩芯样本（上白垩统）以及沥青渗漏和焦油砂。Rock-Eval™热解结果表明，露头岩的岩源质量被划分为极好-非常好；相比之下，来自 x 井的岩源质量被划分为较差-一般。氢指数（HI）与 Tmax 的交叉图表明，露头岩样品含有 II/III 类有机质，而井样品含有 III 类有机质，所有源岩的热成熟度都不高，这一点得到了玻璃光泽反射率（Ro：0.20-0.48 %）和生物标记成熟度参数（C32 (S/S + R)、MPI-I）的支持。在沥青渗漏和焦油砂中，异戊烯、正烷烃、甾烷和烷烃已经降解，而三环和四环萜类大部分仍完好无损。此外，C30 霍烷和均霍烷已经去甲基化，C25 诺霍烷也大量存在，这表明沥青渗漏和焦油砂经历了 PM-6 或更大范围的严重生物降解。4,9-DMD、4,8-DMD 和 3,4-DMD 金刚烷的分布以及饱和馏分和芳香馏分的 δ13C 表明，沥青渗漏和焦油砂产生于 II 类和 III 类源岩。脱甲基 Ts/Tm 的比率表明，样本的成熟度较低，是由不成熟的源岩生成的。三环萜烷比率（C25/C26TT 与 C25TT/C24TeT、C23/C21TT 与 (C19+20)/C23TT、C24TeT/C24TeT + C26TT 与 (C19+20)/C23TT）的交叉图也显示，生物降解沥青渗漏和焦油砂与第 1 和第 2 露头组的源岩密切相关。三环萜、4,9DMD、4,8DMD 和 3,4DMD 金刚烷的三元图以及 EAI 与 DMDI-1 的交叉图都支持这一结论。第 1 组露头源岩中出现的外渗岩与早期碳氢化合物的生成相一致，而沥青样品中去甲基化 Ts/Tm 的低比率（<1）证实了尼日利亚达荷美盆地的生物降解沥青渗漏和焦油砂是由第 1 组下白垩统源岩生成的。该研究推进了之前关于沥青渗漏和焦油砂来源的假设，并有助于我们对几内亚湾盆地的了解。

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来源期刊

Organic Geochemistry 地学-地球化学与地球物理

CiteScore

5.50

自引率

6.70%

发文量

100

审稿时长

61 days

期刊介绍： Organic Geochemistry serves as the only dedicated medium for the publication of peer-reviewed research on all phases of geochemistry in which organic compounds play a major role. The Editors welcome contributions covering a wide spectrum of subjects in the geosciences broadly based on organic chemistry (including molecular and isotopic geochemistry), and involving geology, biogeochemistry, environmental geochemistry, chemical oceanography and hydrology. The scope of the journal includes research involving petroleum (including natural gas), coal, organic matter in the aqueous environment and recent sediments, organic-rich rocks and soils and the role of organics in the geochemical cycling of the elements. Sedimentological, paleontological and organic petrographic studies will also be considered for publication, provided that they are geochemically oriented. Papers cover the full range of research activities in organic geochemistry, and include comprehensive review articles, technical communications, discussion/reply correspondence and short technical notes. Peer-reviews organised through three Chief Editors and a staff of Associate Editors, are conducted by well known, respected scientists from academia, government and industry. The journal also publishes reviews of books, announcements of important conferences and meetings and other matters of direct interest to the organic geochemical community.