Etude de la diagenèse organique des séries paleozoiques du bassin de Sbaa (Algérie). Approche géochimique et pétrologique.

Abstract : The Sbaa basin is located in the West-Central Province of the Algerian Sahara. It essentially consists of Palaeozoic shales and sandstones ranging from Cambro-Ordovician to Namurian, overfolded by thin Mesozoic strata. An oil occurrence in the Sbaa sandstone Formation (Tournaisian age; 500 m deep, 22 millions tons of actual reserves) makes the basin distinctive in an area rich in gas fields. The geochemical and petrological studies of organic matter (OM) (160 samples from 9 wells) showed that although the sediments from Tournaisian, Strunian and Famennian have been deposited in a marine environment, important continental inputs occurred. However, the sediments from Givetian and Silurian were constituted of a marine OM dominated by planktonic material (Tasmanaceae, Acritarches, Chitinozoa).
OM maturity was estimated by Rock Eval pyrolysis and microspectrofluorimetry parameters. Results showed that (i) OM from Tournaisian was immature, i.e. before the oil-window maturity (ii) in the Southern part of the basin, maturity of OM from Strunian, Fammenian and Silurian corresponded to the beginning of oil window, (iii) in the northern part of the basin, OM maturity of Givetian and Silurian sediments corresponded to the middle of oil window.
The Silurian contained the highest contents of free hydrocarbons (HCs) whose composition showed a good correlation with Tournaisian reservoir oils: predominance of saturate aliphatic HCs and unimodal distribution of n-alkanes (n-C17). Therefore, the Silurian strata from the North of the basin was the actual HC source rock while the Givetian from the North of the basin as well as the Silurian in its whole were the potential HC source rocks. It has also been shown that Tasmanaceae contribution to oil generation was consistent in the Sbaa basin. Artificial maturation (confined system) of pure Tasmanaceae algae concentrate from Silurian generated HCs similar in composition to naturally generated oil from Tournaisian reservoir, confirming thus the identification of the HC source rock.
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Fatima Laggoun-Défarge. Etude de la diagenèse organique des séries paleozoiques du bassin de Sbaa (Algérie). Approche géochimique et pétrologique.. Géochimie. Université d'Orléans, 1987. Français. ⟨tel-00322456⟩

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