AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
Buried Evaporite Paleokarsts in the Arab Evaporites and the Hith Formation, Saudi Arabia
(1) Saudi Aramco, Dhahran, Saudi Arabia.
(2) Carbonate Research Consulting Inc., Conifer, CO.
Evaporites represent a major lithology in the hydrocarbon-rich Upper Jurassic section of Saudi Arabia. In many places these evaporites form competent seals and provide a framework for potential stratigraphic traps.
Despite this obvious importance, little is published about the internal architecture of Arab and Hith Formation evaporite sections. In-depth study of the anhydrite sections and their relationship to interbedded carbonate stringers and reservoirs help clarify interpretation issues concerning evaporite fabric patterns, internal organization and their sequence stratigraphic position in the Upper Jurassic.
Recently conducted high-resolution core studies identified several phases of evaporite karstification that punctuate Arab and Hith depositional successions. Available core material includes examples of evaporite karst solution pipes filled with transgressive and regressive or lowstand sediment.
The observed karst phenomena are small-scale features up to 12 feet deep. All solution pipes studied penetrate anhydrite sequences and document that a loss of accommodation space across the up-dip limits of the carbonate platform leads to the cessation of groundwater Arab and Hith anhydrite formation.
Transgressive carbonate deposits fill and define most of these evaporite karst phenomena. Typical fill sequences display a consistent facies evolution. Complex-depositional fill sequences begin with an intraclast packstone facies that passes gradationally upward through a peloid packstone into a capping thrombolite boundstone facies. Locally some of the facies types may be absent.
Closer to the continental limits of the platform, karst features display a combination of transgressive carbonate deposits and siliciclastics or only siliciclastics. Transgressive carbonates filling solution pipes are mainly a polymictic breccia. In addition, greenish shale may occupy centrally located space in solution pipes that only exhibit partially fills of transgressive carbonate accumulations. The green shale also forms beds overlying such solution pipe fill features and may occur as a solitary fill in some solution pipes.
These unconformities appear widespread and as such document unconformities at the top of the Arab B and C evaporite members as well as the middle of the Hith Formation.
The unconformities mark unequivocal sequence boundaries that terminate highstand deposition across the Arabian Platform during Arab and Hith deposition.