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Petroleum geochemical aspects of the Mandawa Basin, coastal Tanzania: the origin of migrated oil occurring today as partly biodegraded bitumen
Petroleum Geoscience ( IF 1.7 ) Pub Date : 2019-11-08 , DOI: 10.1144/petgeo2019-050
Tesfamariam Berhane Abay 1 , Katrine Fossum 1 , Dag Arild Karlsen 1 , Henning Dypvik 1 , Lars Jonas Jørgensen Narvhus 1 , Muna Hassan Mohamoud Haid 1 , Wellington Hudson 2
Affiliation  

The shallow marine Upper Jurassic to Lower Cretaceous sedimentary successions of the Mandawa Basin, coastal Tanzania, are located about 80 km away from the offshore gas discoveries of Block 2, Tanzania. In this paper we present petroleum geochemical data, including bitumen extracted from outcrop samples which are relevant to the understanding of the onshore “Petroleum System” and possibly also to the offshore basin. Despite some biodegradation and weathering, common to all outcrop samples, most bitumen samples analyzed contain mature, migrated oil. The maturity span of geomarkers (C13-C15 range) covers the entire oil and condensate/wet gas window (Rc = 0.7–2% Rc), with the biomarkers generally indicating the oil window (Rc = 0.7–1.3% Rc). This suggests that the bitumen extracts represent several phases of migrated oil and condensate, which shows that the samples are part of an active or recently active migration regime or “Petroleum System”. The source rock facies inferred for the bitumen is Type II/III kerogen of siliciclastic to carbonate facies. This is oil-prone kerogen, typical for a marine depositional system with influx of proximal derived terrigenous material blended in with in situ marine algal organic matter (OM). Application of age specific biomarkers such as the C28/C29 steranes, extended tricyclic terpane ratio (ETR), nordiacholestanes and the aromatic steroids suggest that more than one source rock have contributed to the bitumen. Possible ages are limited to the Mesozoic i.e. excluding the Late Paleozoic, with the most likely source rock belonging to the Early Jurassic. More geochemical and geological studies should be undertaken to further develop the general understanding of the petroleum system of the Mandawa Basin and its implications to the “Petroleum Systems” both offshore and onshore. This paper also presents a reinterpretation of published gas composition and isotope data on the Pande, Temane and Inhassoro gas fields (Mozambique) with implications for possible oil discoveries in the gas dominated region.

中文翻译:

坦桑尼亚沿海曼达瓦盆地的石油地球化学方面:今天作为部分生物降解沥青发生的迁移石油的起源

坦桑尼亚沿海曼达瓦盆地的浅海相上侏罗统至下白垩统沉积层序距坦桑尼亚 2 区块海上天然气发现约 80 公里。在本文中,我们提供了石油地球化学数据,包括从露头样品中提取的沥青,这些数据与了解陆上“石油系统”以及可能与近海盆地有关。尽管存在一些生物降解和风化,所有露头样品都很常见,但大多数分析的沥青样品都含有成熟的迁移油。地质标志物的成熟度跨度(C13-C15 范围)涵盖整个石油和凝析油/湿气窗口(Rc = 0.7-2% Rc),生物标志物通常指示油窗口(Rc = 0.7-1.3% Rc)。这表明沥青提取物代表了迁移油和凝析油的几个阶段,这表明样本是活跃或最近活跃的迁移制度或“石油系统”的一部分。为沥青推断的烃源岩相为硅质碎屑相至碳酸盐相的 II/III 型干酪根。这是油性干酪根,典型的海洋沉积系统,近端衍生的陆源物质与原位海藻有机物质 (OM) 混合流入。年龄特异性生物标志物的应用,如 C28/C29 甾烷、扩展的三环萜烷比率 (ETR)、降胆甾烷和芳香族类固醇表明不止一种烃源岩对沥青做出了贡献。可能的年龄仅限于中生代,即不包括晚古生代,最可能的烃源岩属于早侏罗世。应进行更多的地球化学和地质研究,以进一步加深对曼达瓦盆地石油系统及其对海上和陆上“石油系统”的影响的总体认识。本文还重新解释了 Pande、Temane 和 Inhassoro 气田(莫桑比克)已发表的天然气成分和同位素数据,对天然气主导地区可能的石油发现产生了影响。
更新日期:2019-11-08
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