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Sandstone Petrography and Reservoir Quality of the Glacio-Fluvial Juwayl Member (Late Carboniferous-Permian) Western Saudi Arabia

Mohamed I. Mahgoub1 and Osman M. Abdullatif2
1ETS Engineering Consultants, Khobar, Saudi Arabia
2KFUPM, Dhahran, Saudi Arabia

Juwayl member has been correlated to the Unayzah Formation, one of the most important hydrocarbon reservoirs in Saudi Arabia.

The facies analysis of Juwayl Member revealed ten lithofacies and two main depositional environments namely glacio-fluvial and braided river environments. The glacio-fluvial environment is characterized by pebbly to conglomeratic sandstone facies suggesting medium to high energy within a proximal depositional setting. In contrast, the braided river facies are dominated by medium to coarse grained trough- and planner cross-bedded sandstone facies. These lithofacies suggest a distal depositional environment characterized by laterally switching channels within an extensive braid plain system.

This paper examines petrographical properties and reservoir quality of these lithofacies in the Juwayl Member and relationships among the facies and their petrophysical properties.Juwayl sandstone is classified as quartz arenite consisting mainly of 93 % quartz, feldspar (4 %) and micas (<1 %) in addition to calcite, clay minerals, rock fragments and iron oxides. Kaolinite is the dominant clay mineral with minor illite and chlorite. The petrophysical analysis revealed good- to very good porosity (17.7 to 32 %) and good- to excellent permeability (0.3 to 14.8 Darcy) in Juwayl Member. The Juwayl sandstone is affected by shallow burial conditions during its diagenesis history. The cementing materials, grain size variation, quartz overgrowth and matrix are the main factors controlling porosity and permeability in Juwayl Member.

Characterization of the hydraulic flow units in Juwayl Member revealed 15 hydraulic flow units. Relatively lower values of the Reservoir Quality Index and the Flow Zone Indicator are corresponding to the fluvial facies, whereas the higher values corresponding to the glacial facies.

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