Abstract: Depositional Facies and their Relationship to Reservoir Quality in the Sui Main Limestone, Eocene, Middle Indus Basin, Pakistan

SALLER, ARTHUR, HAYAT AHMAD, A.W. CHUGHTAI and SHAHID HAMEED

The Sui Main Limestone (SML), Eocene, is the main reservoir in several giant gas fields in central Pakistan. Pakistan's first gas discovery at Sui Field in 1952, remains the largest with ultimate recovery in excess of 8.6 TCF with 8.0 TCF in the SML. Uch (1955) and Qadirpur (1990) fields are other major fields in the SML. Ultimate recovery from the SML is estimated at 15.8 TCF (53% of gas reserves in Pakistan). Cores of the SML were studied to determine depositional facies and their reservoir characteristics. The vertical succession of facies in the middle to upper SML indicates a basinward progradation of shelfal environments followed by rapid deepening and drowning. The succession of facies and approximate thicknesses in the Uch and Sui fields are, from bottom to top: rotaline foram wackestone (greater than 40 m), bioclastic grainstone (40 m), miliolid-rich wackestone (90 m), and rotaline foram wackestone (5 m). The rotaline, 9 foram wackestones have variable porosity (2-25%) and permeability (less than 0.1-10 mD) and were deposited in outer shelf environments. Bioclastic grainstones accumulated in shelf-margin shoal complexes that were subaerially exposed locally. Bioclastic grainstones have generally lower porosity (5-20%), but open vugs and fractures cause some high permeabilities (0.1-226 mD). Miliolid wackestones were deposited in a lagoonal environment and are generally the most porous SML facies (15-25%; permeabilities of 0.5-5 mD). Intercrystalline microporosity is dominant in miliolid and rotaline foram wackestones resulting in moderate permeabilities.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria