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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
(Begin page 459)
AAPG Bulletin, V.
Permeability
characterization of distributary mouth bar sandstones in
Prudhoe Bay field, Alaska: How horizontal cores reduce risk in developing deltaic
reservoirs


Robert S. Tye,1 James J. Hickey2
1Phillips Petroleum Company, 510B Plaza Office Building,
Bartlesville, Oklahoma, 74004; email: rstye@ppco.com
2Applied Reservoir Petrology, LLC, 7019 Southridge Drive, Dallas, Texas, 75214;
email: Clann_Hickey@msn.com
AUTHORS
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.
ACKNOWLEDGMENTS
We thank the Prudhoe Bay Working Interest Owners for their support and permission to publish this article. Many individuals aided our work by sharing their experience and knowledge. Darrel Bose and Tim Verseput of ARCO Prudhoe Bay Development strongly supported this project. Dick Owens planned the ARCO 18-34 PBU well and oversaw the coring operation. Ray Eastwood analyzed the wire-line data. Additional individuals to whom we are grateful are Eric West, Mark Stevenson, and J. A. Lorsong. Lillian Tiulana and Katherine Hale provided data and graphic support. D. Przyowjski and Mike McCracken cheerfully helped in the core lab. Early versions of this manuscript were reviewed by Beverly Burns and Rick Levinson. AAPG reviewers Michael Sweet, Kenneth Wolgemuth, and two anonymous reviewers are thanked for their comments and suggested improvements.
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 (Begin page 460) 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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