Ordovician–Upper Silurian–Triassic Petroleum System Assessment in the Chotts Area

Abstract

The Upper Silurian Fegaguira Formation is thought to be an active source rock in the Chotts Basin of southern Tunisia and is thought to have probably supplied Ordovician and Triassic clastic reservoirs in the area. However, various debates are still held regarding either the source rock distribution and thermal maturity but also the reservoirs viability and extension. Within this scope, this study focussed on the evaluation and the characterization of Ordovician and Triassic reservoirs through the integration of well logging data of twenty wells drilled in the southern Chotts Basin aiming to better delineate prolific levels. Furthermore, a 1D BasinMod modelling was achieved to reconstruct the Fegauira source rock burial and thermal histories and also to estimate its hydrocarbon generation and expulsion potential. The El Atchane and Hamra Ordovician reservoirs generally bear low to fair petrophysical characteristics and mostly fall within nearly tight reservoirs. Paleozoic orogenic phases, especially the Hercynian phase had a major impact on their lateral distribution. The Triassic TAGI (Trias argilo-grèseux inférieur) reservoir, with good porosity, changes to volcanic material to the West which is believed to have been settled through faulting during the Tethyan rifting. Strikingly, this volcanic material bears also good porosities which could have been enhanced through fracturing and diagenetic processes. The 1D Basin modelling shows that the Fegaguira source rock is mature in real wells and has begun hydrocarbon generation during Early Cretaceous. Hydrocarbon expulsion at a SATEX of 10% took place since Paleogene, the earliest. This initial evaluation of Ordovician and Triassic (TAGI) reservoirs combined to Fegaguira burial and thermal history modelling point to a functioning petroleum system in the Chotts area with more targets to be discovered further West. This study is an anticipation for possible more plays to be discovered in an area which needs to be further thoroughly explored for petroleum accumulations.