Petroleum Systems of the Bongor Basin and the Great Baobab Oilfield, Southern Chad

Abstract

The Bongor Basin in southern Chad is an inverted rift basin located on Precambrian crystalline basement which is linked regionally to the Mesozoic – Cenozoic Western and Central African Rift System. Pay zones present in nearby rift basins (e.g. Upper Cretaceous and Paleogene reservoirs overlying Lower Cretaceous source rocks) are absent from the Bongor Basin, having been removed during latest Cretaceous – Paleogene inversion‐related uplift and erosion. This study characterizes the petroleum system of the Bongor Basin through systematic analyses of source rocks, reservoirs and cap rocks.Geochemical analyses of core plug samples of dark mudstones indicate that source rock intervals are present in Lower Cretaceous lacustrine shales of the Mimosa and upper Prosopis Formations. In addition, these mudstones are confirmed as a regional seal. Reservoir units include both Lower Cretaceous sandstones and Precambrian basement rocks, and mature source rocks may also act as a potential reservoir for shale oil. Dominant structural styles are large‐scale inversion anticlines in the Lower Cretaceous succession whilst underlying “buried hill” ‐type basement plays may also be important. Accumulations of heavy to light oils and gas have been discovered in Lower Cretaceous sandstones and basement reservoirs.The Great Baobab field, the largest discovery in the Bongor Basin with about 1.5 billion barrels of oil in‐place, is located in the Northern Slope, a structural unit near the northern margin of the basin. Reservoirs are Lower Cretaceous syn‐rift sandstones and weathered and fractured zones in the crystalline basement. The field currently produces about 32,000 barrels of oil per day.

Neocomian – Barremian Lacustrine Shales in the Mayo Oulo‐lere Basin, North Cameroon: Depositional Environment and Petroleum Potential

“…The Fmlc and Fml facies samples from the Badesi and Tchontchi outcrops have average TOC contents of 2.7% and 1.4%, respectively, and contain Types I to III kerogen. Average values of the Rock-Eval S2 yield are 15.5 mg HC/g rock and 7 mg HC/g rock respectively, and these values are similar to those reported for source rocks in the Bongor Basin (Dou et al, 2020). Based on a plot of TOC versus Rock-Eval (S1+S2) (Fig.…”

“…Similar Early Cretaceous depositional environments have been interpreted in the Benue Trough (Babangida et al, 2015;Sarki Yandoka et al, 2017); in the Doba, Bongor, Termit and Muglad Basins (locations in Fig. 1; Dou et al, 2018aDou et al, , 2020Liu et al, 2015); in the Kribi/Campo, Douala and Rio-Del-Rey Basins of southern coastal Cameroon (Ntamak-Nida et al, 2008;Agyingi et al, 2019); in the North Gabon margin (N'Komi Basin: Mbina et al, 2009); and in the northern part of the Sergipe -Alagoas Basin along the conjugate margin of Brazil (Mello and Katz, 2000). The results therefore suggest that organic matter accumulated during the Neocomian -Barremian in the Mayo Oulo-Léré Basin in a lacustrine environment with variable depositional conditions.…”

“…Lower Cretaceous lacustrine shales in WCARS basins have also been studied by Liu et al (2015), Dou et al (2018a) and Sarki Yandoka et al (2015. Dou et al (2018aDou et al ( , 2020 studied Neocomian -Barremian source rocks in the Bongor Basin, Chad, and demonstrated the presence of immature to early mature Types I to II kerogen in the Cailcedra, Mimosa and Kubla I Formations with TOC contents ranging from 0.5 to 18.6 wt.%, highlighting the significant potential for petroleum generation. In the Upper Benue Trough, there is no evidence to show that wells have penetrated the Neocomian -Barremian succession.…”

“…1) which developed during the break-up of Gondwana (Fairhead, 1986;Benkhelil, 1988;Genik, 1992Genik, , 1993. Commercial hydrocarbon accumulations have been discovered in WCARS basins in Chad (Doba, Doseo and Bongor Basins), and Niger (Termit Basin) (Genik, 1993;Alalade and Tyson, 2010;Dou et al, 2018aDou et al, , 2020, where Barremian to Aptian lacustrine shales are proven source rocks (e.g. Mohammed et al, 1999;Obaje et al, 2004;Abubakar, 2014).…”

Geochemical characteristics and origins of biodegraded oils in the Bongor Basin (Chad) and their implications for petroleum exploration

“…Although the burial depth of the basement reservoir in Well BC-2 is 1267.0 m in Well BC-2, that of the reservoir in the early Oligocene is less than c.a 800 m (Figure 11). Previous studies have indicated that oil charging in the Great Baobab Oilfield occurred during the Campanian (Dou et al, 2020;Wen et al, 2018;Xiao et al, 2014), which means that the oil reservoir had already formed when uplift and erosion occurred. Strong fault activity in this region, and significant uplift and erosion of strata, caused heavy biodegradation of the oils in Well BC-2.…”