Solid Hydrocarbons in Proterozoic Dolomites, Taoudeni Basin, Mauritania

Abstract

The Taoudeni Basin (Mauritania / Mali, West Africa) was formed as a result of pre‐ Pan‐African subsidence associated with rifting at the margins of the West African craton. Hydrocarbons in the Taoudeni Basin are derived from source rocks in the Meso‐Neoproterozoic Atar Group, which is composed of facies varying from stromatolite‐dominated carbonates to organic‐rich basinal shales. The stromatolitic carbonates are dolomitized and contain solid hydrocarbons (pyrobitumen). The pyrobitumen was formed in response to a Mesozoic hydrothermal event, with peak temperatures locally reaching 380°C, which resulted in hydraulic fracturing of the carbonates. Gas shows were recorded from these carbonates in the Abolag‐1 well and suggest that they may have potential as a reservoir rock. For this study, samples of Atar Group dolostones and black shales were collected from two localities in the Mauritanian part of the Taoudeni Basin and were analyzed by means of various geochemical and microscope‐based techniques including fluid inclusion analyses. The study suggests that Meso‐Neoproterozoic source rocks generated oil and gas during the Late Neoproterozoic – Early Palaeozoic. Later, in the Jurassic, a hydrothermal event caused in‐reservoir thermal cracking of the hydrocarbons to pyrobitumen and a second phase of gas generation and migration.

The Intraplate Stress Field of West Africa

“…Along the borders of the Reguibat Shield, the cluster of events is adjacent to fault zones developed in the Neoproterozoic Pan-African orogeny (750-550 Ma) (Black et al, 1994;Montero et al, 2016;, and the edges of a Neoproterozoic failed rift (underneath the Richat and Adar Basins, Figure 1b) (Affaton et al, 1991;Culver et al, 1991;Lécorché et al, 1991;Black et al, 1994;Grant, 1973). Several other rift segments and their brittle structures run through the platform beneath the Taoudeni Basin, the Trans-Saharan Mobile Belt, and the Western edge of the Sahara Metacraton (Figure 1b) (Albert-Villanueva et al, 2016;Begg et al, 2009;Caby et al, 1981). The Trans-Saharan Mobile Belt hosts earthquakes that are located along Precambrian shear zones, active during the Mesozoic strike-slip system in the region (Figure 1b) (Black et al, 1994;Caby & Bruguier, 2018;Fagbohun et al, 2020;J.…”

“…The seismically active domains are located within regions that have either (1) experienced rifting episodes in the Neoproterozoic and Mesozoic (Albert-Villanueva et al, 2016;Begg et al, 2009), or (2) are located along brittle shear zones developed in the Archean and Neoproterozoic (Koffi et al, 2020). In the southwestern edge of the craton (KMS, Figure 1b), the faults are sinistral passive transform structures that are related to shear zones developed in the Mesoarchean and Eburnean, and they are oriented N-S, NNE-SSW, and WNW-ESE (Thiéblemont et al, 2004;Egal et al, 2002).…”

The Intraplate Stress Field of West Africa

“…The seismically active domains are located within regions that have either (a) experienced rifting episodes in the Neoproterozoic and Mesozoic (Albert‐Villanueva et al., 2016; Begg et al., 2009), or (b) are located along brittle shear zones developed in the Archean and Neoproterozoic (Koffi et al., 2020). In the southwestern edge of the craton ( Kenema‐Man Shield (KMS) , Figure 1b), the faults are sinistral passive transform structures that are related to shear zones developed in the Mesoarchean and Eburnean, and they are oriented N‐S, NNE‐SSW, and WNW–ESE (Egal et al., 2002; Thiéblemont et al., 2004).…”

“…Along the borders of the Reguibat Shield (RS) , the cluster of events is adjacent to fault zones developed in the Neoproterozoic Pan‐African orogeny (750‐550 Ma) (Black et al., 1994; Montero et al., 2016; Rocci et al., 1991), and the edges of a Neoproterozoic failed rift (underneath the Richat and Adar Basins , Figure 1b) (Affaton et al., 1991; Black et al., 1994; Culver et al., 1991; Grant, 1973; Lécorché et al., 1991). Several other rift segments and their brittle structures run through the platform beneath the Taoudeni Basin (TB1) , the Trans‐Saharan Mobile Belt (TSMB) , and the Western edge of the Sahara metacraton (SMC) (Figure 1b) (Albert‐Villanueva et al., 2016; Begg et al., 2009; Caby et al., 1981). The TSMB hosts earthquakes that are located along Precambrian shear zones, active during the Mesozoic strike‐slip system in the region (Figure 1b) (Black et al., 1994; Caby & Bruguier, 2018; Fagbohun et al., 2020; J. Fairhead, 2023).…”

Seismic evidence of lithospheric delamination in North America that caused the termination of basin subsidence

“…The timing and duration of subsidence in cratonic basins inferred from sedimentation patterns preserve information about the tectonic and geodynamic modification of continental interiors. The termination of tectonic subsidence and even partial inversion of cratonic basins are important records of dynamic cratonic tectonics [1,[3][4][5][6][7][8][9]. However, these records are hard to observe in the surface geology and the mechanical process of subsidence termination in cratonic basins is understudied globally.…”