Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG 2002 Meeting, Houston, Texas, March 1013, 2002
Evaluation of Anhydrite Seals through Depositional, Structural, and Lithological Analysis; Example from the Jurassic Arab Formation, Al Rayyan Field, Qatar
Alton Brown, Consultant, and Robert G. Loucks
The Hith, Arab middle, and Arab lower anhydrites form seals for the Arab A, C, and D carbonate reservoirs respectively, whereas the upper anhydrite, intra "C" anhydrite, and intra "D" anhydrite in the Arab Formation do not form effective seals. Sedimentological, mineralogical, structural, and engineering data were collected to determine why different anhydrite units have differing sealing capability.
The sealing anhydrite units are thick, depositionally continuous bodies with low permeability and high capillary displacement pressure, necessary characteristics of good seals. Good anhydrite 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" anhydrite lack sufficient continuity to form an effective seal.
The major cause of seal failure in the thin, upper anhydrite 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 anhydrite 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.