[First Hit]

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Alton A. Brown1, Robert G. Loucks2

(1) Consultant, Richardson, TX
(2) Bureau of Economic Geology, UT at Austin, Austin, TX

ABSTRACT: Evaluation of Previous HitAnhydriteNext Hit Seals through Depositional, Structural, and Lithological Analysis; Example from the Jurassic Arab Formation, Al Rayyan Field, Qatar

The Hith, Arab middle, and Arab lower anhydrites form seals for the Arab A, C, and D carbonate reservoirs respectively, whereas the upper Previous HitanhydriteNext Hit, intra "C" Previous HitanhydriteNext Hit, and intra "D" Previous HitanhydriteNext Hit in the Arab Formation do not form effective seals. Sedimentological, mineralogical, structural, and engineering data were collected to determine why different Previous HitanhydriteNext Hit units have differing sealing capability.

The sealing Previous HitanhydriteNext Hit units are thick, depositionally continuous bodies with low permeability and high capillary displacement pressure, necessary characteristics of good seals. Good Previous HitanhydriteNext Hit continuity results from deposition in widespread salinas and playas isolated from the ocean by permeable barriers that allow subsurface recharge.

The intra "C" and intra "D" beds are depositionally discontinuous and have higher dolomite content. This reflects deposition in sabkha, small subaqueous salina, and playa environments. Also, the intra "C" sabkha with its higher fraction of porous dolomite promotes seal failure. The discontinuous salina deposits in the intra "D" Previous HitanhydriteNext Hit lack sufficient continuity to form an effective seal.

The major cause of seal failure in the thin, upper Previous HitanhydriteNext Hit is fault offset, because fault throw exceeds its thickness. High fraction of brittle dolomite results in wider damage zones, allowing greater potential for along-fault leakage. Fault throw does not exceed the thickness of the Hith, middle, and lower anhydrites, so they are intact seals. Faults cause less damage due to their higher Previous HitanhydriteTop fraction, so along-fault leakage is minimal. Thick, continuous anhydrites deposited in widespread salinas and playas with low dolomite content form the most effective seals.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado