Natural Shuaiba Influx Into Arab Reservoirs of the Bahrain Field
Authors: Khalaf A.S. 
Published: December 2007

Summary
Jurassic Arab D water salinity and composition vary considerably in the Bahrain field. These lateral variations result from the introduction of Cretaceous Shuaiba fresh water into the Arab D reservoir. The section that separates the Shuaiba and Arab D formations is about 2,500 ft thick. Shuaiba water influx into Arab D has caused many reservoir and production problems, such as a rapidly advancing freshwater front, asphaltene buildup on surface and downhole equipment, and the anomalous occurrence of water in structurally higher wells. These problems are observed in the southern part of the structure where Shuaiba water enters the Arab D. A collapse structure in the south is believed to serve as a conduit.

Introduction
The Arab formation occurs in the lower part of the Riyadh group of the Upper Jurassic Age and is penetrated 4,000 to 5,000 ft below mean sea level in the Bahrain field. Lithologically, the formation consists of four carbonate zones separated by anhydrite beds. These zones are designated as Arab A through D in descending order. In the off-structure wells, Arab D carbonate becomes more dolomitic and loses the good reservoir characteristics of the crestal wells. The Shuaiba aquifer is a porous and permeable limestone of the Upper Cretaceous Age. Its upper contact is unconformable with ahr Umr siltstones while the lower is conformable with the Hawar shale. Fig.1 depicts the Mesozoic stratigraphic column of the Bahrain field and its geographic location. Arab A and C are gas reservoirs; Arab D is a major oil reservoir with a large gas cap. Arab B is a minor reservoir with a thin oil column and a relatively large gas cap. Arab A and C were exploited during1938–76 and are considerably depleted. Oil production from Arab D started in1938. production from Arab D started in 1938. The water in the Arab formation in the Arabian Shelf oil fields and in exploratory areas around Bahrain Islandis supersaline (Fig. 1). The salinity of Arab D in the Bahrain field is very low, especially in the south where it averages 7,000 ppm (NaCl). The anomalous fresh nature of this water has puzzled geologists and engineers for years. The only source of abundant fresh water is the thick, porous Shuaiba aquifer, which lies about 2,500 ft above Arab D. Vertical communication between these formations is believed to freshen the Arab D water in the Bahrain field.

Classification and Composition of Arab D Waters
In this study, Arab D waters are classified into three types. Type C. This supersaline water occurs regionally and in the off-structure areas around Bahrain. It is characterized by low sulfate content (300 to 750 ppm) and very high NaCl content (greater than 150,000 ppm). Type B. This fresh water occurs only in the southern part of the field. This type is characterized by high sulfate content (greater than 1,000 ppm) and very low NaCl content (less than10,000 ppm). Type A. This water, with a salinity of 12,500 to 38,000 ppm NaCl, occurs in the northern part of the Bahrain field. The sulfate content varies from 750 to 1,000 ppm. The Shuaiba water is slightly fresher than Type B water (6,500 ppm NAC1), with a sulfate content of about 800 ppm. Tables 1 and 2 give the chemical composition of Types A and B, and Type C and Shuaiba waters, respectively. Type C water is believed to be the connate water of the Arab reservoirs. Each Arab reservoir is overlain by a thick anhydrite bed. Anhydriteis deposited in closed basins under extensive evaporation conditions. The evaporation during anhydrite deposition left heavier and more concentrated brines behind, possibly accounting for the high salinity of the Arab connatepossibly accounting for the high salinity of the Arab connate water. Most probably, the Arab D water in the field area also was originally supersaline as in the surrounding areas. However, the mixing of fresh water reduced its salinity. In the southern part of the field, which is close to the point of fresh water entry, flushing was more pronounced; the northern area had less mixing of fresh water. This gave rise to Type B and Type A waters, respectively. Fresh Arab D and Shuaiba waters are considered to be meteoric because of their very fresh nature and high sulfate content. On the other hand, Type C water is considered to be the connate water of Arab D, and type A is a mixture of the Arab D connate and Shuaiba meteoric waters.

Source of Fresh Arab D Water In the Bahrain Field
The Arab D water in the southern part of the field could have been freshened by lateral advancement of a freshwater front within the Arab D reservoir or by vertical communication between fresh and connate waters. Lateral advancement of fresh water from the Arab D aquifer to the field area is ruled out for the following reasons.
JPT
P. 363