- Copyright © 2003 Society of Exploration Geophysicists
Wudayhi Field, in the eastern province of Saudi Arabia (Figure 1a), is an anticlinal structure in the Paleozoic-Cenozoic rocks; it is 30 km in length and 10 km in width, and trends ENE. The Wudayhi anticline is one in a group of forced folds related to draping of the sedimentary cover over basement block faults in the eastern margin of Arabia. The anticline is asymmetric, with southern vergence, and gentle dip angles that rarely exceed several degrees.
Surface seismic and borehole data show no clear evidence of major faults in the sedimentary rocks with resolvable offset. However, the dip-magnitude attribute of the seismic data shows an ENE-trending lineament on the steeper, southern limb of the anticline. This feature may reflect a fault zone. The trap is mainly structural with potential stratigraphic component.
The top of the Unayzah reservoir occurs at about 13 000 ft, ∼ 2.1 s TWT, and the total thickness of the formation is about 900 ft, ∼80–100ms (Figure 1a). Generalized stratigraphy and reservoir units are shown in Figure 1b. Unayzah Formation consists of the following units from top to bottom:
Unayzah A, which in turn is divided into two subunits—Unayzah A1 (fluvial/coastal plain grading to estuarine/shallow marine) and Unayzah A2 (alluvial fan/Elian).
Unayzah B (fluvial and lacustrine).
Unayzah C (glaciofluvial/eolian).
Figure 1c shows a typical section of the Unayzah reservoir and its porosity profile in Wudayhi Field. Unayzah B is tight with porosity of 3–9% and permeability not exceeding 10 mD. Therefore, fractures are essential to both porosity and permeability.
Well testing on the discovery well indicates an important potential …