Tilted original oil/water contact in the Arab-D reservoir, Ghawar field, Saudi Arabia

Abstract

ABSTRACTA review of the electrical logs, fluid properties, and production history of 195 flank wells drilled in the Arab-D carbonate reservoir of the Ghawar field, Saudi Arabia, showed that the original oil/water contact was regionally tilted. The contact was about 200 ft higher in the southern Haradh sector than in the northern Shedgum and ‘Ain Dar sectors. In Haradh, the fluid contact was also locally tilted down from west to east by as much as 800 ft. In the reservoir, the oil and aquifer densities changed from lighter oil and denser water in the north to lighter water and denser oil in the south. Decreasing methane content caused the increase in oil density and a reduction in the water density was the result of a salinity decrease. The evolution of fluid densities was closely correlated to a decreasing regional-scale geothermal gradient, probably indicating that temperature controlled the distribution of fluid densities. Simple analytical calculations showed that the magnitude of the observed tilt of the original oil/water contact from north to south might be explained by changes in fluid densities. On the western flank of central Haradh, the Arab-D reservoir water was anomalously young and fresh and this created a large salinity gradient between the western and eastern aquifer legs. This anomaly was explained by pressure-dependent vertical leakage along the Wadi Sahba structural trough between the Arab-D reservoir and the shallower Biyadh aquifer. Consequently, the integrity of the Hith Formation seal above the Arab-D reservoir might be locally compromised under particular conditions. A full-field reservoir simulation model, specific geological features, and examples from the technical literature supported a static interpretation of the tilted original oil/water contact in the Arab-D reservoir of Ghawar through the combined effects of changes in oil and water densities.