A New Layering Scheme for the Khuff-B Depositional Cycle in the Subsurface of Eastern Saudi Arabia
Saleh Al-Raimi and Rami Kamal
Saudi Aramco, Saudi Arabia
The Upper Permian Khuff Formation is a carbonate evaporite rock sequence that in the sub-surface of eastern Saudi Arabia has been formalized by Saudi Aramco geologists into four major depositional cycles. From top to bottom these cycles are the Khuff-A, -B, -C, and –D. Each cycle consists of a dominantly carbonate interval overlain by dominantly anhydrite. The carbonate intervals of the Khuff-A, -B, and _C, because of their reservoir potential, have been further subdivided into layers with the layer boundaries based mainly on regionally correlatable gamma-ray wireline markers.
A new layering scheme for the Khuff-B was developed, which is simpler than previous schemes (five layers rather than eight), sensitive to lithofacies distribution, and easily detected on compensated formation-density/compensated neutron wireline log curves and in rock core, in addition to gamma-ray signatures. Being sensitive to lithofacies, the new layers more meaningfully double as reservoir layers for reservoir modeling purposes.
The Khuff-B carbonates are characterized by a medial oolite that in the sub-surface of Eastern Saudi Arabia is from 68 to 81 feet thick. The other Khuff depositional cycles are devoid of prominent, thick oolites. The Khuff-B oolite volumetrically comprises the bulk of the Khuff-B reservoir. The oolite has several tight interbedded mudstones, the thickest of which averages 15 feet. It occurs in the lower one-third of the oolite and, unless fractured, is an effective seal between two oolitic lobes. The upper oolite lobe, the mudstone separator, and the smaller lower oolite, have been designated as layers 2, 3, and 4 of the Khuff-B reservoir. Over Ghawar field substructures, the Khuff-B oolite is a lime grainstone reservoir with good moldic porosity, but only poor to fair to good permeability. In saddle areas, the oolite is dolomitized, creating areas of very good to excellent combined moldic and intercrystalline porosity, and very good to excellent permeability. It has been shown that present-day structural highs closely correspond to Late Permian topographic highs that were spared early dolomitization.
Layer 1 overlies the oolite and averages 46 feet in thickness. The upper part of the layer is a peloidal dolograinstone rich in anhydrite and of poor reservoir quality. The lower most 8 to 12 feet of this layer is a dolomudstone with only a minor anhydrite content and is a good to excellent reservoir rock. Layer 5 underlies the oolite and consists of dense dolomudstone and skeletal dolograinstones with anhydrite cements. It averages 70 feet in thickness and is of poor to fair reservoir quality.
2000 Geo Arabia v5 #1, p.34