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Robert S. Tye1, James J. Hickey1
(1) ARCO, Plano, TX

Abstract: Predicting Permeability in Distributary-Mouth Bar Reservoirs

Oil production rates and volumes from horizontal wells completed in distributary-mouth bar reservoirs can be erratic owing to the heterogeneous permeability patterns created by variable sedimentary processes and resultant lithofacies deposited at friction-dominated distributary-channel mouths. Horizontal cores from distributary-mouth bar deposits in Prudhoe Bay Field, Alaska, reveal seven fine-grained sandstone lithofacies. Amongst these lithofacies, average permeability ranges by greater than an order-of-magnitude (10 to 240 md). Permeability is depositionally controlled (sediment sorting) by the lateral segregation of supercritical versus subcritical flow over the bar.

Reservoir-quality trends can be predicted using this strong link between lithofacies and permeability, and documented sedimentation patterns on distributary-mouth bars. The best sorted and most permeable sediments are deposited on the distributary-mouth bar apex and along the margins of distributary channels. Sandstones deposited in central and distal distributary-mouth bar locations are less well-sorted and contain small amounts of argillaceous and lignitic material that serve as nucleation sites for carbonate cement precipitation and as catalysts for pressure solution of quartz grains.

These insights can reduce the risks associated with producing from distributary-mouth bar sandstones. Wells targeting distributary-mouth bar deposits can be optimized by: 1) using offset well data to map time-equivalent deltaic facies associations; 2) identifying transition zones between distributary channels and distributary-mouth bars; 3) extrapolating trends of high-permeability lithofacies within distributary-mouth bars; and 4) calculating a well’s trajectory and length, to optimally contact high-permeability rock.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana