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1 Phillips Petroleum Company, 510B Plaza Office Building, Bartlesville, Oklahoma, 74004; rstye{at}ppco.com
2 Applied Reservoir Petrology, LLC, 7019 Southridge Drive, Dallas, Texas, 75214; Clann_Hickey{at}msn.com
Bo Tye acquired his interest and passion for geology during his youth while exploring beaches and tidal rivers near Charleston, South Carolina. He holds degrees in geology, marine sciences, and environmental sciences from the College of Charleston, the University of South Carolina, Louisiana State University, and the University of Alaska-Anchorage. Presently, Bo is senior geological specialist in the Basin Evaluation Group at Phillips Petroleum. In addition to Alaska, he has worked projects in Algeria, Ecuador, Indonesia, Mozambique, Turkey, and Venezuela. Previous employers include Cities Service Company, the Texas Bureau of Economic Geology, and ARCO. Bo is keenly interested in the application of stratigraphy and depositional-systems analysis to reservoir characterization. He is a member of AAPG, SEPM, and the International Association of Sedimentologists.Jim Hickey is a geologist with Applied Reservoir Petrology in Dallas, Texas. He has more than 17 years experience with ARCO in a variety of geoscience technology and research positions and has considerable experience in several Alaska North Slope reservoirs. His primary responsibilities have been in the field of clastic petrology and its applications to exploration, reservoir development, and operational problems. He earned a B.S. degree in geology from CalTech and obtained a Ph.D. in geology from the University of California Santa Barbara. He began with ARCO in Houston in 1983 but spent most of his ARCO career at the research and technical center in Plano, Texas.
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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  C. R. Fielding, J. D. Trueman, and J. Alexander Sharp-Based, Flood-Dominated Mouth Bar Sands from the Burdekin River Delta of Northeastern Australia: Extending the Spectrum of Mouth-Bar Facies, Geometry, and Stacking Patterns Journal of Sedimentary Research, January 1, 2005; 75(1): 55 - 66. [Abstract] [Full Text] [PDF]
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