An integrated workflow for petrophysical characterization, microfacies analysis, and diagenetic attributes of the Lower Jurassic type section in northeastern Africa margin: Implications for subsurface gas prospection

Abstract

The Jurassic sedimentary sequences are an important source and reservoir rocks in the North Africa margin, with its type section, outcropped at the Gebel El-Maghara massif and some other sites along the southern Tethys. These exposed Jurassic outcrops, of the intercalated continental and marine sediments, allow us to conduct more detailed and integrated studies on the various depositional settings with diverse lithologies as analogous for the subsurface reservoirs. The present study concerns the implication of the mineralogical components and the diagenetic controls on the petrophysical properties and the reservoir characteristics of the Lower Jurassic in Gebel El-Maghara outcrop sequence (from base to top Mashaba, Rajabia, and Shusha Formations) in north Sinai, Egypt. Some field trips were conducted, and samples were collected, to characterize the petrographical and petrophysical characteristics of these deposits using comprehensive standard techniques including studying thin sections, SEM imaging, XRD analysis, and various petrophysical measurements. A workflow is proposed to apply a full set of petrophysical measurements on the collected samples including porosity (∅He), permeability (k), grain and bulk densities (ρg and ρb, respectively), and true formation resistivity factor. In this workflow, we have integrated the petrophysical data with detailed microfacies analysis to reveal the implication of diagenesis on the reservoir quality of the Lower Jurassic reservoirs. The Lower Jurassic sequence is subdivided into five hydraulic flow units (HFUs) that consist of three reservoir rock types (RRTs). These RRTs are composed of three microfacies associations (MFAs). The RRT1 plug samples consist of the mostly clastic MFA1; they are porous, permeable, and have good reservoir quality that is using the flow zone indicator (FZI), and the reservoir quality index (RQI). The lowest reservoir quality is assigned to the RRT samples which are mostly composed of tight carbonates. Based on the petrographical studies, SEM imaging, and the XRD analysis, dissolution and fracturing slightly enhanced the reservoir potentiality of the RRT1 samples, whereas physical compaction, tight cementation, and authigenic clay content (kaolinite, hematite, and goethite) are responsible for the deterioration of the reservoir properties of RRT3 samples and reduction of the RRT2 samples. The study outcrop has the potential to be an excellent analogue for the important subsurface Jurassic gas-bearing deposits on the African's northern margins. Furthermore, it may provide insights into the fluid flow units, mineralogical composition, and diagenetic controls on the unique Lower Jurassic deposits of Gebel El-Maghara for the assessment of subsurface gas-bearing sequences in the nearby Mediterranean and Gulf of Suez from the southern Tethys shorelines.