Impact of Petrophysical and Mechanical Properties on Engineering Classification of the Facha Reservoir Rocks, Western Sirte Basin, Libya

Abstract

Rock identification and classification have contributed to the enhancement of drilling oil wells and production reservoir performance. Therefore, understanding the properties of reservoir rocks is a major concern in the petroleum industry. In this context, Young’s modulus and uniaxial compressive strength are major mechanical rock properties essential for defining the engineering classification and modulus ratio of rocks. However, these parameters are used in order to examine the impact of the petrophysical properties on mechanical strength in the Facha member (reservoir) of the Gir Formation in the western central of Sirte Basin, oilfields of central Libya. Dolomite, limestone, dolomitic limestone, and anhydrite are the main lithofacies rock components of this reservoir, as well as of a third member of the Gir Formation (Lower Eocene). Young’s modulus and uniaxial compressive strength were computed from sonic and bulk density well logs data from six wells in four different oil fields. This oil-rich reservoir has an average thickness of 104 m, an average total porosity of 16%, and argillaceous material content (clay) of 4%. According to the engineering classification chart of Deere and Miller, the Facha reservoir rocks have a very low class (E < 27.5 Mpa) to high strength class (B ≈ 110-220 Mpa) and have a medium-to-high modulus ratio (M ≈ 200–500 and H ≈ 500). Generally, the diagenesis process, for instance dolomitization, affected both the strength and the modulus ratio.