Abstract

The Upper Permian–Carboniferous Unayzah Formation in South Haradh, Saudi Arabia, includes two major mechanical and petrophysical layers that are separated by shale-rich zones. Open tectonic fracture clusters are rare and not essential for fluid flow in the Unayzah A zone, which has high porosity and permeability. However, such fracture clusters are essential to, and impact, the production performance in the Unayzah B/C tight-gas reservoir. The occurrence of the tectonic fractures in the Unayzah Formation is linked to the rock mechanical properties, which vary with porosity, shale volume, cement type, and texture. The B/C unit is more fractured than the A unit, but its layers vary in the degree of fracturing. The variation in fracture development within the B/C unit results in differences in fracture-enhanced permeability based on production profiles where flow is restricted to preferentially fractured mechanical layers that lack effective vertical fluid communication with other layers.

We identify two tectonic fracture systems: an older subordinate fully mineralized system and a younger primary mostly open system. Early extensional fractures including joints and faults developed parallel to the basement faults during the opening of the Neotethys. These are fully mineralized and have little or no function as fluid conduits. The younger system includes open-fracture clusters that are predominantly parallel or nearly parallel to the regional east-northeast–west-southwest maximum horizontal stress of the Zagros (that has been active since the Late Cretaceous) and is independent of local structures. Therefore, these fractures are controlled by remote stresses instead of the basement-rooted forced folds and faults. In this article, we demonstrate that in the Unayzah B/C, natural fractures are essential to permeability and, in some areas, to porosity, and thence, to reservoir performance. The results of this study are being implemented in well placement and completion design to optimize the intersection of open-fracture clusters with positive preliminary results.

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