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Hunting for Old Oil – the Taoudenni Basin

Juergen Thurow¹, Bindra Thusu¹, Alexandra Nederbragt², and Jonathan Craig³
¹Department of Earth Sciences, University College London, UK
²School of Ocean and Earth Sciences, Cardiff, UK
³ENI Exploration & Production, Milan, Italy

The Taoudenni Basin is one of a series of Proterozoic/Palaeozoic basins in North Africa located on the West African craton covering an area of >500 000 km2. The sedimentary fill (>6km) ranges in age from at least Mesoproterozoic (>1100 Ma) to Palaeozoic. Intervals with potential source rocks and hydrocarbon shows have been identified in the Atar/Tifounke Group (ATG) (Meso-Neoproterozoic) and in the Silurian (“hot shales”).

Our understanding of the Taoudenni Basin’s Proterozoic history and its potential for hydrocarbon generation is limited by large areas covered in recent sediments and by the lack of a reliable chronostratigraphic framework. Re-Os geochronology from marginal outcrops of the ATG indicate that the oldest organic-rich sediments are Late Mesoproterozoic (±1100 Ma) in age. We used this reference age to develop a carbon isotope stratigraphy for the deeper part of the basin based on cuttings from Well Abolag-1 with the aim to better understand the deposition of organic-rich units in a sequence stratigraphy, chronostratigraphy and basin dynamics framework.

Samples were point-counted to reconstruct the lithostratigraphy of a 500 m thick sequence. The main lithologies are (stromatolitic) carbonates and shales. Dark shales are concentrated in the upper and the lower part of the carbonate unit. Results indicate that these shales are the main carrier of organic carbon.

Stable carbon isotope data were obtained from carbonates and from organic matter in dark shale. Both suites of data fluctuate in tandem and show stratigraphic fluctuations with an amplitude of ~4‰. The similarity of the pattern in δ13Corg and δ13Ccarb strongly suggest that the primary isotope signal is retained. Abrupt jumps in δ13Corg and δ13Ccarb occur at the base of a red carbonate interval and at the top of the ATG. These jumps suggest hiatuses of substantial duration. The top of the carbonate unit is known to represent an erosional surface, but relatively continuous sedimentation has been assumed within the ATG.

Correlation with a Neoptroterozoic δ13C reference curve confirms the early Neoproterozoic age of the studied interval as previously infered from palynological data. This suggests that the upper part of the ATG with the hiatus on top is the regional expression of the ~770 Ma old Bitter Springs Carbon Isotope Stage. Both the reference curve and Abolag-1 show minimum δ13C values in this interval. The older “jump” matches up with the next older minimum δ13C excursion around 875 Ma. The oldest part of the sequence below the negative excursion, only known from marginal outcrops, was probably deposited entirely in the late Mesoproterozoic. Based on these correlations one can assume that deposition in the studied sequence happened in two stages during times that global δ13C values were low, while the positive excursion in between is represented by a hiatus.

Results from Well Abolag-1 demonstrate that isotopic sequencing is potentially a powerful tool to improve predictions of source rock occurrence and accuracy of correlation on a basinwide scale and beyond.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013