Abstract

GDF SUEZ and Sonatrach will develop in partnership the main fields of the prolific Sbaa basin, SW Algeria. In this area, the main gas reservoir is composed by the glaciogenic Upper Ordovician, which exhibits high poroperm heterogeneities, with porosity and permeability ranging from 1 to 15% and 10−3 to 103 md respectively. The low permeability zones are generally encountered in the lower units of the reservoir and no obvious geological control has been interpreted so far. According to previous studies, quartz cementation and pressure dissolution are responsible for these low petrophysical properties. An extensive diagenetic study was then undertaken to understand the origin and the distribution of these diagenetic processes and their impact on the reservoir architecture (Tournier et al., submitted). It underlines the role of the glaciogenic processes, responsible of clay coating development in underlying clean sandstones. These clay coatings will determine afterwards diagnetic evolution of these sandstones by enhancing pressure dissolution. Produced silica is exported in low compacted upper layers and deposited as quartz cement (Tournier et al., submitted). In the present paper, two main petrofacies are distinguished by their dominant diagenetic features, either pressure dissolution or quartz cementation. This petrographical classification has strong petrophysical implication, and highlights the importance of these two diagenetic processes in the reservoir quality of these sandstones.

The two petrofacies described in this paper, QS1 and QS2, correspond to the distinction between conventional and tight reservoirs. The value of 5% porosity may be considered as the limit between the two types of petrofacies and provides a pertinent insight about porosity cut-off used in petrophysical properties evaluation of the glacial Ordovician sandstones of the Sbaa basin.

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