oa Facies Mapping within the Late Permian Khuff-C High-Resolution Sequence Stratigraphic Framework in Hawiyah, Ghawar Field; Implications for Reservoir Predictions

Over 7,000 feet of core in 24 wells in the HWYH, S. UTMN and N. HRDH areas in Ghawar were utilized<br>in a study to generate a high-resolution sequence stratigraphic and facies distribution framework for<br>the Khuff-C reservoirs. Four high-frequency sequences make up the Late Permian Khuff-C in Ghawar,<br>where each is bounded by well-defined sequence boundaries. Each sequence is made up of a<br>Transgressive Systems Tract (TST) and a High Stand Systems Tract (HST), separated by a mappable<br>Maximum Flooding Surface (MFS). Systems tracts are made up of mappable meter-scale shallowingupward<br>cycles, for a total of 32 cycles within the entire studied interval.<br>Two facies maps have been created for each of the 32 cycles across the entire studied area. Each cycle<br>has a facies mosaic map at the surface of maximum retrogradation and another at the surface of<br>maximum progradation. The accumulated result of all maps provides the spatial and temporal facies<br>distribution for the four high-frequency sequences that make up the long-term Khuff C composite sequence.<br>These are not interval maps but rather they are time-facies maps that represent the facies evolution,<br>position and facies migration (vertical and lateral) through time. The advantage of this mapping<br>technique is that it places facies within the context of their times of deposition during a particular event<br>of sea-level rise and fall, allowing better prediction of reservoir facies occurrences. One key control on<br>reservoir quality is anhydrite cementation, which works to destroy pore networks. Petrographic studies<br>suggest that such cementation is likely to have occurred at, or very shortly after, the time of<br>deposition of reservoir (grainstone) facies. This is demonstrated through the preservation of original<br>grains encased within tight anhydrite cement networks; all preserved porosity is otherwise moldic. The<br>high-resolution surface mapping suggests, through superposition of facies maps, that anhydrite<br>cementation is sourced through brine reflux processes from stacked, cycle-capping, tidal-flat facies<br>lying directly above. In a down-dip direction, the same stacked tidal flats sourced lighter, magnesiumrich,<br>brines that laterally dolomitized time-equivalent subtidal calci-silt facies.