High-resolution sequence stratigraphy of the Khuff A and B carbonates in the subsurface of Haradh Area, southern Ghawar Field, Saudi Arabia

Abstract

The oolitic ramp carbonates of the Khuff Formation are divided into five informal subsurface members: D, C and "Buraydah" of Permian age; and B and A of Triassic age. This study is based on detailed sedimentological logs of four cored wells, a total of 402.3 meter (1,320 feet), across the Triassic Khuff A and B interval in Haradh. The Triassic Khuff Carbonates are defined by two long term (3rd Order?) sequences: Khuff B and Khuff A Sequences. The Khuff B Sequence is defined at its base by the regionally mappable Permian-Triassic Boundary; and is made up of three upward-thinning composite sequences to reflect an overall upward decrease in accommodation. The first composite sequence is made up of three high-frequency sequences with transgressive open marine distal lime-mudstone or/and grainstone shoals and prograde with grainstone shoals with associated restricted facies. The second composite sequence is made up of three asymmetric, progressively upward-thinning high-frequency sequences. The transgressive systems tract (TST) of the first high-frequency sequence is defined by retrograding distal carbonate mud and the prograding shoal-to-back shoal packstone/grainstone define the highstands of this high frequency sequences. The upper two high frequency sequences have retrograding grainstone/packstone shoal/fore-shoal defining their TSTs and Prograding restricted lagoon and tidal flat facies defining the HSTs. The third composite sequence, itself is a high-frequency sequence of retrograding tidal-flats and their fringing grainstone shoals and prograding highstand supra-tidal capped meter scale peritidal cycles and their down-dip equivalent tidal-flat capped meter-scale peri-tidal cycles. The Khuff A Sequence is defined at the base by its sequence boundary, a mappable exposure surface throughout the study area at the top of the Triassic Khuff B Sequence. The Khuff A Sequence is made up of two, progressively upward-thinning, composite sequences to reflect an overall upward decrease in accommodation. The first composite sequence is made up of six high-frequency sequences. Transgressive facies are supra-tidal cycles followed by transgressive ooid shoals, tidal flats and supratidal facies; followed by regional transgressive ooid grainstone shoals. The culmination of the TST of the first Khuff A composite sequence is a regionally mappable 1-to-3 meters (4-to-9 feet) thick sub-aqueous anhydrite unit with the MFS at its top surface. Three high-frequency sequences define the HST of the first Khuff A Composite Sequence that are commonly tidal-flat capped on the highs. The sequence boundary of this composite sequence is defined by the top of the uppermost tidal-flat cap, where it is an exposure surface on the highs. The second composite sequence is made up of transgressive/regressive ooid grainstone shoals with associated with restricted facies prograding at the top. 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 of the Triassic Khuff are relatively more significant starting at the MFS of the first Khuff A Composite Sequence. Microbial deposits, Thrombolitic/stromatolitic sheets and heads, are abundant in distal and lagoonal carbonate mud. Anhydrite cementation is pervasive in grainstone shoal deposits that are in-front of and below extensive supra-tidal anhydrite deposits. Best reservoir development is in grainstones behind and prograding over, retrogradational distal carbonate mud cycles.