Abstract

Oil production from upstructure drill sites at Prudhoe Bay field, Alaska, is almost exclusively from fine-grained deltaic sandstones. Distributary channel and distributary mouth bar facies associations in the Triassic Ivishak Formation comprise the pay zones, but wells are preferentially completed in the lower-permeability distributary mouth bar deposits in an attempt to avoid high gas/oil ratio wells. Thin light-oil columns combined with complex stratigraphy and an overlying, highly mobile gas cap make planning, drilling, and completing economic wells challenging.

To accurately assess lateral trends in permeability within distributary mouth bar sandstones, three conventional cores were cut (approximately 120 ft [36 m]) along the 1000 ft (304 m) horizontal reach of a recent development well. The cored interval consists of seven lithofacies, all composed of fine-grained sandstone. Comparison of permeability values to lithofacies demonstrates a striking and consistent trend. Six lithofacies possess average horizontal permeabilities ranging from 12 to 40 md. Average horizontal permeability for the seventh lithofacies (lithofacies 7) is 129 md. Porosity and vertical permeability follow similar patterns.

Distributary mouth bars in Prudhoe Bay field were deposited in fluvially dominated delta lobes in which sediment distribution at river mouths was controlled by friction between the sediment plume and basin bottom. During flood stage, the best sorted and most permeable sediments (i.e., lithofacies 7) were deposited on the apex or most proximal part of the distributary mouth bars between distributary channels. Sedimentologic and petrographic data corroborate a strong link between lithofacies and permeability. Sandstones deposited by unsteady flow conditions (e.g., turbulent scour, intermittent ripple migration) are likely to be less well sorted, and contain more clay and lignitic organic material (commonly as drapes and wisps), than lithofacies 7 sandstones deposited under more uniform high-energy transport conditions. Small amounts of argillaceous and lignitic laminae serve as nucleation sites for siderite cement precipitation and as catalysts for pressure solution of quartz grains, significantly degrading permeability in lithofacies 1 through 6.

Insights gained from analyzing these cores can reduce the risks associated with well completions in distributary mouth bar sandstones. Locations targeting distributary mouth bar deposits can be optimized by (1) using existing 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 the well trajectory and length, to optimally contact high-permeability rock (or moderate-permeability rock to inhibit gas coning).

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