Diagenetic Effects on Reservoir Properties of the Permian Khuff Formation in Eastern Saudi Arabia

Abstract

Abstract The Permian Khuff Formation represents the earliest major transgressive carbonates deposited on a shallow continental shelf in eastern Saudi Arabia. A detailed study was made of slabbed cores and thin sections from various wells recently drilled into this rock unit. The Khuff Formation was deposited in tidal flat environments including subtidal, intertidal and sabkha (supratidal). These depositional environments represent four major cycles. Each cycle starts with a transgressive grainstone facies which makes up the Khuff reservoirs and ends with regressive, muddy and anhydritic facies which make up the non-reservoir units. Development of reservoir quality appears to be complexly controlled by lateral continuity or discontinuity of depositional facies and, more effectively, by diagenesis. Because of these lateral changes, Khuff reservoir development does not exactly follow present structural position. Some wells located on present structural highs are in non-reservoir facies, while other structurally lower wells are very porous. Diagenesis has both increased and/or decreased reservoir quality. In places, dolomitization and leaching have enhanced the reservoir porosity and permeability. In other places, dolomitization and/ permeability. In other places, dolomitization and/ or cementation have seriously decreased the porosity and permeability. porosity and permeability. Good porosity and permeability are related to the preservation of primary interparticle pores in preservation of primary interparticle pores in totally dolomitized grainstones or leaching of remaining calcite in dolomitized mudstones. The later process produces secondary intercrystalline pores. Good porosity and poor permeability pores. Good porosity and poor permeability probably reflect an early exposure to fresh water probably reflect an early exposure to fresh water leaching in grainstones, producing unconnected or poorly connected moldic pores. Poor porosity and poorly connected moldic pores. Poor porosity and permeability are caused by one or more of the permeability are caused by one or more of the diagenetic effects of cementation, dolomitization, or absence of leaching in all facies types. Current studies show that Khuff reservoir changes can be described by a combination of depositional and diagenetic models. However, prediction of reservoir continuity is possible only for short distances from existing wells. Introduction The Khuff Formation in eastern Saudi Arabia has been the objective of exploratory and development drilling for the last few years. Recent drilling encountered non-reservoir rocks in the Khuff, s main reservoir zones. Core studies of these nonreservoir rocks revealed that the porosity is totally occluded by anhydrite cement. This development led the company to do more studies of cores and thin sections with techniques such as scanning electron microscopy. The scope of the study is to determine the effects of diagenesis on the depositional facies of the Khuff Formation. The study area includes most of Ghawar Field in eastern Saudi Arabia (Figure 1). GEOLOGIC SETTING The Khuff Formation (Powers et al., 1968) represents the earliest geologic record of major carbonate sedimentation in the region. The deposition took place during a wide-spread Late Permain marine transgression over the Arabian Permain marine transgression over the Arabian Foreland (Powers et al., 1968). P. 465