Petroleum geochemical aspects of the Mandawa Basin, coastal Tanzania: the origin of migrated oil occurring today as partly biodegraded bitumen

Abstract

The shallow-marine Upper Jurassic–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 analysed 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, where Rc is the calculated vitrinite reflectance), 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 an 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.

An Improved Computational Learning-Based Model for Estimating Total Organic Carbon in Unconventional Shale Reservoirs

“…From the Aptian to the Paleogene, a mid-to-outer shelf zone of coastal Mandawa Basin was declined at a constant speed which resulted to the formation of Kilwa group. [55][56][57][58][59][60][61][62][63] The source rock of the Mandawa basin consists of Nondwa shales of the lower Jurassic Pindiro Group and Mbuo Claystone of the upper Triassic Pindiro Group. 64…”

Lithofacies of early Jurassic successions derived from spectral gamma ray logging in the Mandawa Basin, SE Tanzania

“…Due to limited biostratigraphic information, the chronology of these formations is discussed controversially and deviating age assignments do exist for the base of Mbuo and Nondwa Formations as summarized in Fig. 2 (Kent et al 1971;Balduzz et al 1992;Wopfner 1994;Kagya 1996;Alconsult and TPDC 1997;Schlüter 1997;Quinton and Copestake 2007;Corelab and TPDC 2009;Hudson 2010;Robertson and CGG 2017;Davison and Steel 2018;Macgregor 2018;Godfray and Seetharamaiah 2019;Abay et al 2019).…”