Mutual relations between petrographical and petrophysical properties of Cretaceous rock samples for some wells in the North Western Desert, Egypt

The petrographical and petrophysical analyses are very important to understand the factors controlling the reservoir quality. So, these techniques have been applied on rock samples collected from the Cretaceous section for four wells drilled in the North Western Desert, Egypt. The wells are Gibb Afia-1, Betty-1, Salam-1X and Mersa Matruh-1. Lithostratigraphically, the studied rock samples are grouped into Lower Cretaceous sandstones, Upper Cretaceous sandstones and Upper Cretaceous carbonates. Petrographically, the Cretaceous rock samples can be differentiated into 14 microfacies, namely, chloritic ferrigenous quartz wacke, ferrigenous calcareous quartz wacke, laminated quartz wacke, calcareous glauconitic quartz arenite, laminated gypsiferous quartz arenite, fossiliferous glauconitic calcareous quartz arenite, clayey ferrigenous calcareous quartz arenite, ferrigenous calcareous glauconitic lithic arenite, feldspathic ferrigenous quartz arenite, fossiliferous biomicrite, glauconitic oolitic sandy biosparite, calcareous gypsiferous ferrigenous dolostone, calcareous algal siltstone and laminated sandstone and siltstone. Lower Cretaceous rocks have been deposited in a fluviatile in shallow marine environments, while the Upper Cretaceous rocks have been deposited in a fluviatile in marine environments. Diagenetic history has been controlled by diagenetic processes enhancing the porosity (dissolution of framework silicates, leaching and dolomitization) and diagenetic processes reducing porosity (mechanical infiltration of clays, formation of authigenic minerals, compaction and pressure solution, cementation and recrystallization). Porosity types are mainly interparticles, vugs and molds, in addition to fractures and channel porosities. The average porosity of the Lower Cretaceous sandstones, Upper Cretaceous sandstones and Upper Cretaceous carbonates are 16.6%, 16.7% and 22.0%, respectively, as depends mainly on the bulk density with average values of 2.22, 2.26 and 2.14 g/cm3, respectively. The permeability of both the Lower and Upper Cretaceous sandstones with average values of 114.14 and 50.40 mD, respectively, depends mainly on porosity with some other parameters, like pore throat sizes and it could be also related to the electrical tortuosity with average values of 2.27 and 2.40, respectively. The Upper Cretaceous carbonate rocks are characterized by high porosity (22.0%) and low permeability (3.91 mD), this could be attributed to the isolated pore space, and decreasing in pore throat sizes due the presence of significant amounts of fine particles (clay minerals having microporosity and reduce the pore throat radius). The reservoir quality index with average values of 0.43, 0.29 and 0.08 μm, depends mainly on permeability with average values of 114.14, 50.40 and 3.91 mD, of the Lower Cretaceous sandstones, Upper Cretaceous sandstones and the Upper Cretaceous carbonates, respectively. This indicates that, the sandstones of the Lower and Upper Cretaceous rocks are characterized by high to moderate reservoir quality, respectively, while the Upper Cretaceous carbonate rocks are characterized by low reservoir quality. This may be due to the combined effect of the porosity and permeability rather than the total porosity. The formation resistivity factor was measured at three consequent saline concentrations of 6, 30 and 60 kppm, it is controlled by the porosity and electrical tortuosity