Late Jurassic Mrabtine formation, limited at the bottom by Tlalett formation and overlain by Early Cretaceous silicoclastics, is both reservoir and seal. It is a proven oil bearing complex sandstone reservoir, in southern Gulf of Gabes, onshore Tunisia. The seal consists in the upper evaporitic Member of Mrabtine formation. Detailed core study provided better knowledge of its heterogeneous lithology and petrophysical properties, in order to identify the different rock types and evaluate the reservoir potential.

In Jeffara Basin, the gross reservoir thickness can reach more than 300 m. Detailed sedimentological and petrophysical study of more than 40 meters of core, consisting in core logging, microfacies analysis, SEM analysis and routine core analysis (He-porosity, horizontal air-permeability and grain density) were undertaken to determine the different facies types and their relative petrophysical properties. In fact, the sampling was made considering the changes in the lithology. Both sedimentological and petrophysical analyses were used to define the different rock types characterizing Mrabtine reservoir.

The study revealed complex lithology and mineralogy of Mrabtine formation. The sandstone levels are, indeed, intercalated by several thin (millimetric to centimetric) and relatively thick carbonate layers (up to 60 cm). At the base of the reservoir, the carbonates thickness may extend to 7 m. Furthermore, there are frequent silty and shaly layers intercalated within the reservoir. Eight facies types may be distinguished: 1) sandstones with small scale cross-stratifications, 2) bioturbated sandstones, 3) silty sandstones, 4) bioturbated dolomites, 5) dolomitic limestones, 6) bioclastic limestones and/or dolomites, 7) siltstones and shales and 8) Evaporites. The sandstones are mostly fine quartzitic, with common occurrence of clay minerals (kaolinite, dickite and illite) and some iron sulfides and carbonates (pyrite, ankerite and siderite). The facies associations correspond to shallowing upward cycles, going from shaly, carbonate and silty deposits, to mostly fine sandy wave- and tidal-dominated shallow marine sediments, capped by evaporitic chicken-wire anhydrites. The routine core analysis confirmed the heterogeneity of the reservoir due to its complex lithology and mineralogy. The sandstones present the best reservoir potential (up to 25 % porosity and up to 425 mD permeability) and the best poroperms are recorded within cross-stratified sandstones.

The fine low poro-perm carbonate, silty and shaly layers constitute flow barriers which should be considered in geological reservoir modeling. The complex lithology of the Mrabtine formation should also be taken into account in case of scale prevention studies to avoid injecting fluids which are incompatible with the variable matrix nature of Mrabtine reservoir.

You can access this article if you purchase or spend a download.