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About the Authors

Thomas X. Homza received a Bachelor of Science degree with honors from the University of Vermont in 1989. He received his Master of Science and Doctoral degrees in structural geology from the University of Alaska Fairbanks, in 1992 and 1995, respectively. Tom’s post-graduate work focused on the geometry and kinematics of detachment folding in the northeastern Brooks Range, where he spent three field seasons. He worked in various capacities for BP in Anchorage and London between 1995 and 2002. His roles included development geologist at Prudhoe Bay and Endicott Fields, structural geology specialist, and exploration geologist. His time in BP’s exploration department included work throughout the North Slope, NPR-A, the Brooks Range foothills area, and northeastern Siberia. Tom moved to EnCana Oil & Gas Company in Anchorage in 2002 where he served as lead exploration geologist. His efforts at EnCana were focused mainly in Alaska’s Beaufort Sea and Brooks Range foothills. Tom started at Shell in 2005 and his roles there have included exploration geoscience work throughout the North Slope, NPR-A, ANWR, Canadian Beaufort Sea, U.S. Beaufort Sea, Chukchi Sea, North Aleutian Basin, and Cook Inlet Basin. His current title is Principal Regional Geologist, Alaska. Tom has been a member of AAPG for more than 22 years and is active in mentoring students as part of the Imperial Barrel Award competition. He has served as an appointee to the Geologic Mapping Advisory Board for the State of Alaska for 13 years. He is past President and Vice President of the Alaska Geological Society. Tom has authored or coauthored 12 journal papers and 30 abstracts in addition to numerous internal company reports. He presently lives in Anchorage, Alaska.

Steven C. Bergman received a B.S. in Geology from the University of Dayton (1977) and M.A. and Ph.D. degrees in Geology from Princeton University (1979 and 1982, respectively). Until 2016, Steve was a Principal Regional Geologist in the Global Geology Upstream Exploration Research team at Shell International Exploration & Production Co. (Houston, Texas). Prior to joining Shell in 2005, he was Principal Research Geologist with ARCO R&D for 20 years in Dallas, and a Visiting Scholar at Bullard Laboratories, Cambridge University in 1996–1997. Steve is an exploration research geologist and geoscience educator with more than 35 years industry experience applying unconventional integrated field and laboratory approaches (completing more than a hundred worldwide minerals and petroleum exploration and production projects), five years University teaching at the University of Texas at Dallas and Southern Methodist University, serving on the advisory committees for more than 18 M.S. and Ph.D. students. Steve has had the pleasure of studying the tectonics, regional structure, field geology, basin analysis, hard rock petrology, volcanology, and geochronology with 30 months of field expeditions in 17 USA states and 22 countries, and the good fortune of spending more than two years on the ground in Alaska. He is coauthor of one textbook (Mitchell and Bergman, 1991, Petrology of Lamproites, Plenum Press, New York), more than 50 journal papers, 120 internal company reports, and 150 conference and seminar presentations. He has served on the National Academies Roundtable on Science and Technology for Sustainability since 2012, the NRC STS Landscape Analysis Committee in 2015, and as an advisor to the U.S. State Department on Arctic Geology Matters. Steve is an adjunct professor with Southern Methodist University (SMU, Dallas, Texas), a member of the AAPG, GSA, and AGU, a Fellow of the Geological Society of London, and serves on the Board of Directors of two not-for-profits: Zero Waste Vashon and Vashon Makerspace in Vashon Island, WA.

Acknowledgments

The laborious efforts of Peter Johnson, Kirk Sherwood, and Gregory Wilson, who carefully reviewed this work, are deeply appreciated. These professionals spent a significant amount of their valuable time providing thoughtful reviews of the manuscript that served to greatly improve it. We thank AAPG, especially Beverly Molyneux and her team, for shepherding this work through the many arduous processes between submission and publication. The authors alone, and not these reviewers and editors, are accountable for any mistakes, omissions, or other short comings of this work.

We are grateful to Shell Exploration and Production Company for enabling us to publish this work. Bob Scheidemann, a recently retired Shell geoscientist, maintained the company’s geological model for the U.S. Chukchi Sea for decades and we thank him for his organizational skills and ability to resurrect legacy data and interpretations, which form the interpretive basis for our study. We appreciate the unwavering support and sponsorship of Jonathan Holroyd during the production of this book. Although we hold no one but ourselves accountable for the interpretations in this book, a partial list of current and former Shell (and contract) geoscientists from both 20th- and 21st-century Chukchi Sea exploration campaigns who directly or indirectly influenced our interpretations includes Ziad Ahmadi, Zuwaina Al-Hari, Bert Bally, Steve Barker, Mark Barton, Fred Becker, Sherry Berana, Dale Bird, Andy Bishop, Alec Bray, Steven Bloemendaal, Mike Bourque, Pablo Buenafama, Scott Cameron, Glen Cayley, Doug Collins, Hugo Coumont, Steve Danielson, Tim Diggs, Mike DiMarco, Peter Doe, Eileen Donaghy, Myra Dria, Dominic Druke, Rob Foster, Larry Garmezy, Erik Goodwin, Joel Graves, Chris Griffith, Mario Gutierrez, Edith Hafkenscheid, Christine Hahn, Jan Harrell, Stewart Hayward, Mark Hempton, Dave Holland, Mark Hollanders, Jonathan Holroyd, Bill Hornbuckle, Esra Inan Villegas, Alan Jackson, Mohan Javalagi, Atilla Juhasz, Fred Keller, Stefan Kosanke, Paul Kralert, Stephane LaBonte, Jamie Lambrecht, Dave Lawrence, Klaus Leischner, Nina Lian, Calum Macdonald, Mike Mahaffie, Mark Martin, Legna Mendoza, Chuck Minero, Jake Mongrain, Zsolt Nagy, Steve Naruk, Mark Newell, Kyle Patterson, Marcus Patterson, Steve Phelps, Robert Pimentel, Brad Prather, Linda Poyser, Mike Roffall, Alejandra Rojas, Malcolm Ross, Bob Scheidemann, John Shepard, Mark Shuster, Bob Smith, Sig Snelson, Gary Scott Steffens, Rick Stocker, Tom Taylor, Elli Tchouparova, David Tilghman, Ron Tingook, Lana Van Der Hurst, Raphic Van Der Weiden, Hans Van Marle, Cees van Oosterhout, Esra Villegas, Mario Wannier, Chandler Wilhelm, Peter Winefield, Dan Worrall, and Matthew Zechmeister. Lastly, within Shell, we appreciate the efforts and diligence of Rutger de Kreij and Elke Delnooz.

The results of many decades of research in northern Alaska and the adjacent Arctic conducted by academia, the State of Alaska Division of Geological and Geophysical Surveys, the Minerals Management Service, and the U.S. and Canadian Geological Surveys provided the foundation for many of our interpretations. Although the individuals are too numerous to name, we thank the many able geoscientists at these organizations for their past and ongoing work.

We extend special appreciation to Jonathan Bujak, MCI, GeTech, and Ion Geophysical for granting us permission to publish data for which Shell purchased a license. Our gratitude is extended to ConocoPhillips for permitting us to show images from the jointly owned Burger, Klondike, and Steller 3-D seismic surveys.

Forward

The Chukchi Sea petroleum province beneath U.S. federal waters northwest of Alaska has the potential to be a prolific hydrocarbon producer due to its multiple, high-quality source rocks, reservoirs, traps, and seals. This potential is the result of a suitable yet complex tectono-stratigraphic evolution, the interpretation of which is the focus of this Memoir. Data from five deep exploratory wells and from a substantial 2-D and 3-D seismic database were integrated with potential fields data and interpretations of the well-documented geology beneath the adjacent Alaskan North Slope in order to establish our evolutionary models. We developed a terminology, defined in the glossary, to describe many previously unrecognized geologic features and events. For proprietary reasons, we do not present results of our Integrated Basin Model but we share key data and interpretations that could be used to support such a model. Nor are results of the 2015 Burger J well discussed. Beyond petroleum exploration, this report presents data and interpretations that bear on regional geologic/tectonic interpretations for North Alaska and the Arctic as a whole.

Our evolutionary model starts in the Devonian Period, when elements of the Pearya Terrane collided with the Laurentian craton to form the Ellesmerian Orogen. These collided blocks, the suture zone that separates them, as well as the internal and external structural domains of this east-vergent (present coordinates) orogen are preserved under a succession of Late Devonian (?) and younger rocks beneath the Chukchi Sea. Late Devonian-Mississippian orogenic collapse, manifested by asymmetric extension and failed rifting, resulted in the Hanna Trough that was filled with the syn-tectonic, largely non-marine rocks of the Endicott Group, which includes gas source rocks and clastic reservoirs. The Chukchi Evaporite Basin also formed during this collapse; it was separated from the Hanna Trough by a long, rotated fault block (the Greater Popcorn-Crackerjack Block) and began precipitating evaporitic material soon after the Ellesmerian orogenic collapse. As extension waned, the Hanna Trough evolved into a broad sag basin into which mixed clastic-carbonate sediments of the Lisburne Group were deposited. The Hanna Sag Basin experienced moderate deformation in Late Permian time that resulted in the pre-Echooka unconformity on the east side of the basin, which was transgressed by the siliciclastic reservoirs of the Echooka Formation. Non-reservoir distal equivalents of the North Slope’s Triassic Ivishak fluvio-deltaic reservoir were then deposited upon the Echooka Formation. Late Triassic source rocks of the Shublik Formation are present beneath the Chukchi Sea and include high TOC units that bled live oil into at least one well bore during drilling operations. The Sag River Formation, which we group with the Beaufortian Mega-Sequence, was deposited on the Shublik Formation and although it has not been penetrated in the basin, it is likely to be present above the eastern Hanna Sag Basin where it may be a viable reservoir.

The Jurassic section beneath the Chukchi Shelf was dominated by rift-related processes: pre-rift bulging and sub-aerial exposure that promoted karsting in some Ellesmerian rocks; the formation of rift basins with trends that differ from those of the underlying Hanna Trough; mafic volcanism; and syn- and post-rift deposition of high quality quartzose reservoirs. To the north, sea floor spreading in the adjacent Alaska sector of the Canada (Amerasian) Basin began in the latest Jurassic. In contrast to previous interpretations of the Lower Cretaceous unconformity (LCu) that associated it with continental break-up, we propose that the LCu resulted from flexural uplift related to Brookian orogenesis. Pre- and post-LCu Neocomian sandstone reservoirs were deposited in response to episodic forebulge uplift to the north and to the subsidence of the bulge, respectively.

Far-field effects associated with either Brookian orogenesis and/or the emplacement of a High Arctic Large Igneous Province (HALIP) impacted the northern part of the area in the late Neocomian when the North Chukchi High impinged upon the Arctic Platform. This tectonism inverted at least one Jurassic rift graben and formed the Blizzard Anticline. It was also responsible for deposition of the Kalubik Wedge, a northerly derived clastic wedge that, by continued siltation, depressed the otherwise high gamma ray signal of an interval that is an important source rock beneath the North Slope, the Highly Radioactive Zone (HRZ). Early elements of the Brookian Mega-Sequence inter-fingered with the Kalubik Wedge and later overwhelmed the Colville Basin’s offshore extension with northeast-prograding deposits of the Nanushuk and Torok Formation, both of which include reservoir facies. Further foreland flexure and/or regional uplift related to a HALIP plume-head caused erosion at the end of the Albian stage. This newly documented uplift was soon inundated by deposits of a recently appreciated Late Cretaceous section that overlies an Intra-Early Cenomanian unconformity (ECu) beneath the Chukchi Shelf. Maastrichtian-aged contraction and transpression involved deep-seated deformation in the north, where the entire Hanna Trough and all of its cover rocks were inverted, resulting in, among other elements, north-south closure at the nascent Burger Anticline. This also initiated major evaporite mobilization and diapir formation in the Chukchi Evaporite Basin.

During earliest Cenozoic time, the inverted basin rapidly collapsed, again reactivating remnants of deeply rooted Hanna Trough faults. This collapse formed the east-west closure at the Burger Anticline and formed sediment dispersal pathways to the North Chukchi Basin, into which thick sequences (>2-5 km) of Cenozoic siliciclastic sediments were deposited. At least two relatively poorly understood post-Paleocene uplift events further deformed strata beneath the Chukchi Sea: an Eocene event that may be associated with a similarly aged regional tilting event documented beneath the North Slope (perhaps related to the North Atlantic Plate reorganization) and a Pre-Pliocene event of unknown magnitude and extent.

It is in the context of this geologic history, with its generation (and potential degradation) of reservoirs, sources rocks, traps, and seals, that we qualitatively introduce the petroleum potential of the area. That discussion focusses on the presence of reservoirs, because we believe this to be the most critical variable for future hydrocarbon exploration in the area, with uplift and erosion events also important. Our presentation of the geologic evolutionary model, together with the discussion of the petroleum system elements, provides a framework to guide further exploratory and academic efforts in the region.

Note to the Reader

Due to the size and format of this Memoir, the reader will find terms in many figure captions that are not introduced in the text until later in the book. This is especially true in the early chapters. We recommend using the Glossary to help clarify those terms.

There are 10 Plates contained in various chapters within this Memoir. Full-sized digital versions of all of these Plates can be found online at www.aapg.org/publications/journals/bulletin/datashare.

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