Lithological and Petrophysical Controls on the Remaining Fluid Distribution in the Reservoirs of Gharif Formation Before the Halt in Oil Production
Authors: Al-Dhuhli Hajar , Taura Usman , Rudyk Svetlana 
Published: April 2022

Abstract
Gharif Formation is one of the most important hydrocarbon reservoirs in the Sultanate of Oman. In the oil field located in the central-west interior, the oil production occurred from 1980 to 2018 from three reservoirs of the Middle Gharif (Upper Haushi Sand (UHS), Dolomite Drain (DD), Sandstone Drain (SD)) and Lower Haushi Sand (LHS) of the Lower Gharif. This study presents a modelling workflow to evaluate the porosity, permeability, oil-water contact as well as changes in water saturation due to oil production for a field whose majority of the wells were drilled and logged after production had long started. The consequence of this is that fluid saturations may have greatly changed as the encroaching water from the aquifer and/or injected water from nearby earlier drilled wells may have shifted the contacts and the fluid distribution. Similarly, reservoir pressures might have been disrupted by the production and injection of fluid as well as by the distortion of localized vertical permeability barriers. Lithological and petrophysical factors controlling the remaining fluid distribution were studied. The portion of quartz (sand) in all reservoir units decreases from the eastern and southeastern parts toward the western and northwestern parts of the field while the portions of dolomite, calcite and shale volume increase in the opposite direction. These trends follow the transition from a more proximal setting (alluvial plain) in the southeast to a basinal setting (marginal marine) in the northwest. Petrophysical properties vary significantly in various directions. UHS shows an increasing trend of effective porosity when moving from the northwestern part of the field toward the southeastern part from poor to good porosity. Effective porosity of DD is lowest in the central part of the field. On the contrary, SD shows an increasing trend in effective porosity when moving from the southeastern part of the field toward the northwestern part from poor to good porosity. Effective porosity values of LHS increase in the northern part of the field and decrease to the southern part. Permeability gradually follows changes of porosity in Gharif units and increases from shallower depth (UHS) to deeper depth (LHS), from (poor and moderate permeability) to (good and very good permeability). Although the LHS is the most prolific of the reservoirs due to it having the best petrophysical properties (both porosity and permeability), it, however, experienced early water breakthrough with high watercut that is in part due to the high permeability streaks which resulted in water influx from the fringes of the aquifer. There were also integrity issues related to well casing leakage leading to dump flooding from the Khuff Formation overlying the Gharif Formation. Additionally, from the petrophysical correlation point of view, the fining upward sequence in the LHS may have resulted in early water breakthrough due to capillary effect that results in water coning. Wells that produced from the reservoir include 1, 3, 4, 5, 7 and 8. Petrophysical properties vary significantly in various directions. UHS has higher water saturation to the north and higher oil saturation to the south. DD is mostly oil-saturated, whereas SD is mostly water-saturated to the south-east because it is located predominantly below OWC. On the contrary, LHS demonstrates sharp change of fluid saturation from the north with oil to the south with water. By the halt in oil production, oil was fully extracted in UHS from well 14, 4 and 6 with the best porosity and permeability while overall oil recovery from the reservoir was very low because of the poor rock properties. Oil recovery was high from DD but the oil remained in two domes due to the absence of producing wells. Oil was concentrated in the elevated part of the ridge in SD, Sw reached more than 90% before abandonment. In LHS, oil remained in the ridge and in two domes. Integration of all petrophysical analysis data led to the building of a consistent 3D geological model that considers both structure and property of the reservoir, which can be used as input into a reservoir simulation model and provides a basis for a very effective reservoir management strategy.