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Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
Facies Mapping within the Late Permian Khuff-C High-Resolution Sequence Stratigraphic Framework in Hawiyah, Ghawar Field; Implications for Reservoir Predictions
(1) Saudi Aramco, Dhahran, Saudi Arabia.
Over 7,000 feet of core in 24 wells in the HWYH, S. UTMN and N. HRDH areas in Ghawar were utilized in a study to generate a high-resolution sequence stratigraphic and facies distribution framework for the Khuff-C reservoirs. Four high-frequency sequences make up the Late Permian Khuff-C in Ghawar, where each is bounded by well-defined sequence boundaries. Each sequence is made up of a Transgressive Systems Tract (TST) and a High Stand Systems Tract (HST), separated by a mappable Maximum Flooding Surface (MFS). Systems tracts are made up of mappable meter-scale shallowing-upward cycles, for a total of 32 cycles within the entire studied interval.
Two facies maps have been created for each of the 32 cycles across the entire studied area. Each cycle has a facies mosaic map at the surface of maximum retrogradation and another at the surface of maximum progradation. The accumulated result of all maps provides the spatial and temporal facies distribution for the four high-frequency sequences that make up the long-term Khuff C composite sequence.
These are not interval maps but rather they are time-facies maps that represent the facies evolution, position and facies migration (vertical and lateral) through time. The advantage of this mapping technique is that it places facies within the context of their times of deposition during a particular event of sea-level rise and fall, allowing better prediction of reservoir facies occurrences. One key control on reservoir quality is anhydrite cementation, which works to destroy pore networks. Petrographic studies suggest that such cementation is likely to have occurred at, or very shortly after, the time of deposition of reservoir (grainstone) facies. This is demonstrated through the preservation of original grains encased within tight anhydrite cement networks; all preserved porosity is otherwise moldic. The high-resolution surface mapping suggests, through superposition of facies maps, that anhydrite cementation is sourced through brine reflux processes from stacked, cycle-capping, tidal-flat facies lying directly above. In a down-dip direction, the same stacked tidal flats sourced lighter, magnesium-rich, brines that laterally dolomitized time-equivalent subtidal calci-silt facies.