Shuaiba Reservoir Geological Study, Yibal Field, Oman
Authors: Litsey Linus R. , MacBride William L. , Al-Hinai Khalifa M. , Dismukes N.B. 
Published: December 2007

Summary
The Yibal oil field in west central Oman is a large dome created by deep-seated salt movement. The maximum oil column is 370 ft [112.8 m] and the productive area is about 4.3 ×6.2 miles [7 × 10 km]. The structure is complicated by extensive tensional faulting. The main oil accumulation is in the Shuaiba chalk overlain disconformably by the Nahr Umr shale. The Shuaiba reservoir is in pressure communication with the underlying Kharaib formation. It is speculated that early accumulation of oil played a major role in preserving the high porosity and greater crestal reservoir thickness by inhibiting diagenetic processes. This field study provided the reservoir simulation engineer with a viable geological and petrophysical model of the Shuaiba reservoir to aid in understanding observed phenomena.

Introduction
This paper presents the results of a geological and petrophysical study of the Shuaiba chalk reservoir in the petrophysical study of the Shuaiba chalk reservoir in the Yibal field in Oman and provides a detailed reservoir description on a layer-by-layer basis for a reservoir-simulation study to determine the optimum development plan for the field. The geological analysis included plan for the field. The geological analysis included evaluation of the structure, determination of correlation markers within the reservoir, and consideration of the environment of deposition of the chalk. Analysis of open-hole and thermal decay time (TDTTM) logs established perosity and saturation profiles, values for irreducible perosity and saturation profiles, values for irreducible water saturation and residual oil saturation, plus information about movement of injected water and formation water in the reservoir.
The Yibal field is located in west central Oman and lies midway between the Persian Gulf coast of Sharjah and Abu Dhabi to the north and the Arabian Sea coast of Oman to the south. It is located on a plain between the Oman Mountains and the Rub Al Khali or Empty Quarter (Fig. 1). Yibal, southwest of the city of Muscat, can be reached by road through the Semail Pass in the Oman Mountains, a distance of slightly more than 186 miles [300 km].
The Yibal Shuaiba accumulation was discovered in Nov. 1962, when Well Y-2 penetrated the oil column near the crest of the structure and tested 40 deg. API [0.83-g/cm3] oil at 3,500 B/D [556 m3/d] oil; however, the field was not put on production until July 1969, because more productive reservoirs were discovered at the Fahud and productive reservoirs were discovered at the Fahud and Natih fields. Original oil in place in the Shuaiba is estimated to be about 2.85×10 STB [4.53 x 10–6 stock-tank m3]. Yibal also produces gas from the shallower but smaller Middle Cretaceous Natih "A" reservoirs (Fig. 2). It became apparent early in the productive history of the Shuaiba reservoir that some method of secondary recovery would be required. Initial evaluation confirmed by reservoir simulation studies indicated that waterinjection/pressure-maintenance would be the optimum production method. Consequently, a pattern waterflood production method. Consequently, a pattern waterflood was begun in 1972 with seven injection wells. The early injection pattern was modified to an inverted nine-spot pattern by infilling; expansion of that pattern is pattern by infilling; expansion of that pattern is continuing. Also, a 1979 study showed that water injected into the aquifer was more beneficial than water injected into the top of the reservoir in terms of total recovery. Ultimately, a ring of flank injectors is planned to complete the field development. Cumulative production as of Jan. 1, 1982, was 337×10-6 bbl [53.6×10-6 m3] oil from the Shuaiba. By mid-1981, 131 wells had been drilled in the Yibal field, although not all these wells reach the Shuaiba.

Structure
The Yibal oil field is a large domal feature developed primarily by deep-seated salt movement (Fig. 3). The primarily by deep-seated salt movement (Fig. 3). The dome is about 9.3 × 12.4 miles [15 ×20 km] in size with a northeast-southwest axial elongation that is probably a result of regional deformation. Vertical closure is about 1,000 ft [305 m]. Dips along the flanks are 4 to 5 deg. [0.07 to 0.09 rad]. The structure is complicated by extensive tensional faulting that affects the trapping mechanism in the oil reservoir.
Two Permian Khuff flexure zones appear to be related to pre-Jurassic structural movements that were followed by uplift and erosion before deposition of the overlying Jurassic and younger beds. There is no clear-cut evidence to indicate that the older (pre-Jurassic) faults extend upward across the pre-Jurassic erosional surface. Therefore, we assumed that (1) most of the faults in the Shuaiba formation and younger beds were developed as tensional release features as the dome developed and (2) the two prominent fault systems, the graben and the northern prominent fault systems, the graben and the northern crossfault, were developed and controlled by renewed but subtle movements of the two underlying pre-Jurassic flexure systems.
JPT
P. 651