Ooid Diagenesis in the Khuff Formation as Indicators of Reservoir Development
Saudi Aramco, Dhahran, Saudi Arabia
In the subsurface of eastern Saudi Arabia, the Late Permian Khuff Formation is a carbonate-evaporite sequence that is divided into four major depositional cycles formally named Khuff-A, B, C, and D (from top down). The Khuff-B is characterized by a thick medial oolite. Diagenetic fabrics within the oolite have a direct impact on reservoir performance and provide valuable clues on the depositional history of the oolitic-bodies. The oolitic strata are mainly limestone over present-day structural highs but dolomite elsewhere. The limestone fabric is characterized by oomoldic porosity due to the selective leaching of ooid centers. The outer rims of the leached ooids are commonly retained and are cemented to adjacent grain rims, rendering permeability between the oomoldic pores poor. The oomoldic limestones produce a gas effect on the FDC/CNL wireline log track. Dolomite oolites, on the other hand, commonly have good intercrystalline porosity between the ooids, good intergranular porosity, good to very good permeability, and no gas effect.
The leached limestone oolitic grainstones are usually caused by fresh water phreatic diagenesis it is concluded that the lime oolites were deposited on offshore sand bars shallow enough to harbor fresh water lenses. In that case, the present day structural highs were probably paleotopographic highs in Late Permian times. Post-burial compaction of the leached oolites caused unleached grains to collapse into underlying oomoldic voids. If the voids were partially to totally filled and reinforced with precipitated minerals, the molds were spared collapse and distortion. The collapse of leached ooids caused pressure solution and the merging and interlocking of the squashed grains and rinds into phseudolaminae. Microcrystallization along the phseudolaminae transformed them into permeability barriers that consist of lens-shaped envelopes 1 to 5 millimeters thick within which the original fabric, with good oomoldic porosity, remains undisturbed. Mimetic dolomite ooids are common in the dolomitized oolites. They are taken to indicate marine or mixed marine diagenesis whereby early dolomitization occurs at concentric ooid surfaces and replaces the aragonite skin with fine sucrosic dolomite rhombs. Early leaching can remove the aragonite cores leaving behind the dolomite rhomb spheres. If the ooids are not leached they may be completely dolomitized by invasion through micropores created by algal boring.
2000 GeoArabia v 5,# 1, p 119-20.