High-Resolution Sequence Stratigraphy of the Triassic Khuff A and B Carbonates in the Subsurface of Haradh Area, Southern Ghawar Field, Saudi Arabia

Raed K. Al-Dukhayyil and Aus A. Al-Tawil
Reservoir Characterization, Saudi ARAMCO, Dhahran, Saudi Arabia

The Triassic Khuff Carbonates, up to 85 m thick, rest on the regionally mappable Permian-Triassic Boundary and are overlain by Sudair Formation fine siliciclastics. Two large scale sequences (the Khuff B and Khuff A) make up the bulk of the Triassic Khuff reservoirs. Three composite sequences make up the Khuff B sequence and two composite sequences make up the Khuff A sequence.

Composite sequences are upward thinning; and each composite sequence is made up of high frequency sequences (HFS) that also are upward thinning. These upward thinning patterns reflect an upward decrease in accommodation. The HFS is the highest resolution sequence in which its component retrogradational Transgressive Systems Tract (TST) to progradational Highstand Systems Tract (HST) can be defined and mapped across the study area. Cycle sets are the building-block of HFSs, where mappable (only in the Khuff A sequence). They lack retrograding TST-to-prograding HST facies tracts; instead they are bounded by exposure surfaces that are mappable across the study area.

The direction of progradation of the Triassic Khuff Sequences is to the south (down-structure), and retrogradation is to the north (up-structure). Syndepositional Ghawar basement block movements that affected the Triassic Khuff are relatively more significant starting at the MFS of the first of the two Khuff A Composite Sequences. Grainstone shoals occur down-dip of lagoonal carbonate mud and microbial deposits including thrombolitic/stromatolitic sheets and heads. These pass up-dip and are overlain by extensive supra-tidal anhydrite. Porosity-occluding anhydrite cementation is pervasive in the grainstone shoal deposits associated with supra-tidal anhydrite. Best reservoir development is in grainstone shoals behind and prograding over, retrogradational distal carbonate mud cycles.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas