Petroleum Plays and Systems in the National Petroleum Reserve - Alaska

The North Slope of Alaska has re-emerged as one of the most active exploration provinces in the United States. Recent exploration successes, economic benefits of applying innovative exploration and production technologies, evolving industry demographics, rising oil and natural gas prices, and the anticipation that North Slope natural gas resources may become economically viable and marketable through a planned pipeline have stimulated a renewed intensity in leasing and exploration activity. The focus of NPRA exploration appears to include both structural and stratigraphic objectives that may include strata spanning much of the stratigraphic column. The purpose of the core workshop is to prove an opportunity to examine a large collection of core from all major stratigraphic units present in NPRA. The chapters in this volume provide a current perspective on the genesis and petroleum potential of each stratigraphic interval.
Depositional Sequences and Facies in the Torok Formation, National Petroleum Reserve – Alaska (NPRA) Available to Purchase
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Published:January 01, 2001
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CitationDavid W. Houseknecht, Christopher J. Schenk, 2001. "Depositional Sequences and Facies in the Torok Formation, National Petroleum Reserve – Alaska (NPRA)", Petroleum Plays and Systems in the National Petroleum Reserve - Alaska, David W. Houseknecht
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Abstract
Brookian turbidites (Cretaceous through Tertiary) have become oil exploration objectives on the North Slope of Alaska during the past decade, and it is likely this focus will extend into the National Petroleum Reserve - Alaska (NPRA). A regional grid of 2-D seismic data, sparse well control, and field work in the Brooks Range foothills provide constraints for an ongoing effort to establish a sequence stratigraphic framework for Brookian turbidites in the Torok Formation across NPRA.
The Torok Formation and overlying Nanushuk Formation (both mostly Albian) display the overall seismic geometry of bottomset-clinoform-topset strata indicating northeastward migration of a shelf margin. Within bottomset and clinoform strata of the Torok, depositional sequences have been identified that represent four distinct phases of shelf-margin sedimentation. (1) Regression, representing low relative sea level, is characterized by the development of an erosional surface on the shelf and upper slope, and the deposition of turbidite channel deposits on the middle to lower slope and submarine fan deposits at the base of slope. These deposits constitute a lowstand systems tract (LST). (2) Transgression, representing rising relative sea level, is characterized by the deposition of a mudstone drape on the basin floor, slope, and outer shelf. This drape comprises relatively condensed facies that constitute a transgressive systems tract (TST). (3) Aggradation, representing high relative sea level, is characterized by the deposition of relatively thick strata on the outer shelf and moderately thick mudstones on the slope. (4) Progradation, also representing high relative sea level, is characterized by the deposition of relatively thin strata on the outer shelf and very thick mudstones on the slope. Together, deposits of the aggradation and progradation phases constitute a highstand systems tract (HST).
Large scale geometries of Torok strata vary across the Colville basin.In southern NPRA, high rates of subsidence accommodated the deposition of a “fore-deep clinoform wedge” that contains a high proportion of sand-rich LST deposits. In northern NPRA, lower rates of subsidence favored the accumulation of mud-rich HST deposits.
The most favorable stratigraphic trapping geometries in the Torok Formation occur where amalgamated sandstones deposited in turbidite channels incised on the mid-to lower-slope and on the proximal parts of submarine fans during regression (LSTs) are capped by relatively condensed mudstone facies deposited during transgression (TSTs).
Common successions observed in Torok cores include a spectrum of slope and turbidite facies.Upper slope facies comprise laminated mudstones and siltstones that locally display evidence of slumping, sliding, and chaotic failure.Lower slope facies comprise heterolithic turbidites at some locations and interlaminated mudstones and thin, very fine-grained sandstones at others. Torok turbidites include amalgamated sandstones deposited in channel systems as well as thin-bedded, widespread sandstones deposited by unconfined flows on lobes or in channel overbank settings. These turbidite facies likely occur in both channel-lobe systems and slope-apron systems within the Torok.