The gas-reservoir-bearing Khuff Formation in Saudi Arabia is composed of carbonates, evaporites and minor fine siliciclastics. In Ghawar, the Permian Khuff C Carbonates contain four high-resolution sequences; Khuff C1-to-Khuff C4 (KC1-to-KC4), where only KC2, KC3, and KC4 are reservoir-bearing. Grainstone shoals and shoal-flank grain-dominated packstone of the late Transgressive Systems Tract (TST), as well as prograding shallow subtidal burrowed sand-flat facies of the High Stands Systems Tract (HST) are reservoir facies. This is true except for the same facies in KC1 everywhere in Ghawar and for KC3 in central Hawiyah area where anhydrite cementation plugged porosity and degraded reservoir quality.
The general types of anhydrites in the Khuff Formation are depositional sub-aqueous (Salina), sediment-displacing supra-tidal (Sabkha) and diagenetic cement in grainy facies. Mapping salina-type anhydrites in central Hawiyah forming behind, and interbedded with, grainstone shoals during the TST of KC3 has shown little to no direct spatial relationship to grainstone anhydrite cementation. Instead, multiple prograding tidal-flat facies in the HST of KC3 probably sourced anhydrite cementation to the directly underlying TST grainstone facies. When these facies are plugged with anhydrite cement, wire-line logs show a signal of anhydritic dolomite similar to that of tight peritidal facies. Cores and thin section data are essential to decipher the anhydrite cemented dolo-grainstone shoal facies from nodular anhydritic dolomite peritidal facies. Outside this limited area within the same stratigraphic interval, the grainstone facies tracts are major reservoirs in the Khuff C.
The nature of temporal and spatial relationships and controls on the source and mobilization of anhydrite are best understood using both cores and well logs for detailed mapping of reservoir and non reservoir lithofacies, and of different types of anhydrite and minor siliciclastic facies within a high resolution sequence stratigraphic framework. Reflux processes work to occlude porosity with anhydrite cement in local areas below prograding tidal flat facies while at the same time work to enhance porosity by dolomitizing sand flat facies ahead of these same prograding tidal flat facies. This detailed understanding offers a predictive model for reservoir modeling, simulation and development and perhaps as analogues for exploration outside these operating areas.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009