Carbonate/Evaporite Sequence Stratigraphy of the Subsurface Late Jurassic Arab D Formation from Three Fields, Saudi Arabia
The Jurassic Arab D Formation and its overlying anhydrite in Khurais, Harmaliyah and Abqaiq, Arabia, is an overall shallowing upward 3rd order composite sequence of three higher frequency sequences, each made up of several parasequence sets. Facies include from deep to shallow: lime mudstone, skeletal pellet wackestone, intraclastic-oncolitic storm beds, pellet packstone/wackestone, domal/encrusting stromatoporoid floatstone, Cladocoropsis peloid packstone/floatstone, peloid packstone/grainstone, Thaumatoporella-Clypeina fine packstone/wackestone, nearshore ooid/mollusk grainstone/packstone, peritidal laminites, and evaporites.
The lower Arab D Formation consists of upward deepening, meter-scale, cycles of intraclastic/oncoidal rudstone fining up to wackestone/mudstone, some with grainy burrow-fills beneath hardgrounds. Overlying are 3-5 m, upward shallowing cycles of domal/encrusting stromatoporoid carbonates, Cladocoropsis peloid packstone and lagoonal Thaumatoporella-Clypeina pellet wackestone/packstone caps. Above these are 2-5 m thick units dominated by Cladocoropsis peloid packstone/floatstone, with Thaumatoporella-Clypeina wackestone/packstone caps. Upper carbonates are thin ooid and mollusk grainstone, fining up into tidal flat microbial laminites.
The overlying C-D anhydrite is part of a high frequency sequence with 10 m of reservoir-carbonate-anhydrite with several cycles of ooid grainstone, capped by dololaminite, and/or non-displacive, sub-aqueous and displacive anhydrite. It is overlain by up to 40 meters of anhydrite, made up of cycles with very thin (0.3-1.5 m) flooding carbonate (restricted mudstone or dololaminite) and thick, subaqueous and displacive anhydrite.
The “brining-upward” Arab-D carbonates to anhydrite reflects basin-filling and relative sea level fall with superimposed higher frequency sea level fluctuations, following the Jubaila flooding, and long term accommodation of about 5 cm/k.y. During each relative sea level rise, the intrashelf basin became metahaline to hypersaline; sill depths allowed marine influx to maintain carbonate deposition. As relative sea level rise slowed and even fell, sill depths shallowed and the arid setting and evaporation pushed basin waters into the anhydrite field to the early phase of the next transgression. Accommodation allowed accumulation of anhydrites in subaqueous and supratidal settings. The upper carbonate-anhydrite cycles are transitional into the succeeding Arab C carbonate.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009