ALBERT

Description: Beaufortian strata (Jurassic–Lower Cretaceous) in the National Petroleum Reserve in Alaska (NPRA) are a focus of exploration since the 1994 discovery of the nearby Alpine oil field (〉400 MMBO). These strata include the Kingak Shale, a succession of depositional sequences influenced by rift opening of the Arctic Ocean Basin. Interpretation of sequence stratigraphy and depositional facies from a regional two-dimensional seismic grid and well data allows the definition of four sequence sets that each displays unique stratal geometries and thickness trends across NPRA. A Lower to Middle Jurassic sequence set includes numerous transgressive-regressive sequences that collectively built a clastic shelf in north-central NPRA. Along the south-facing, lobate shelf margin, condensed shales in transgressive systems tracts downlap and coalesce into a basinal condensed section that is likely an important hydrocarbon source rock. An Oxfordian–Kimmeridgian sequence set, deposited during pulses of uplift on the Barrow arch, includes multiple transgressive-regressive sequences that locally contain well-winnowed, shoreface sandstones at the base of transgressive systems tracts. These shoreface sandstones and overlying shales, deposited during maximum flooding, form stratigraphic traps that are the main objective of exploration in the Alpine play in NPRA. A Valanginian sequence set includes at least two transgressive-regressive sequences that display relatively distal characteristics, suggesting high relative sea level. An important exception is the presence of a basal transgressive systems tract that locally contains shoreface sandstones of reservoir quality. A Hauterivian sequence set includes two transgressive-regressive sequences that constitute a shelf-margin wedge developed as the result of tectonic uplift along the Barrow arch during rift opening of the Arctic Ocean Basin. This sequence set displays stratal geometries suggesting incision and synsedimentary collapse of the shelf margin. Houseknecht joined the U.S. Geological Survey in 1992, serving as energy program manager until 1998. He has worked on Alaska North Slope basin analysis and petroleum resource assessment since 1995. Previously, Houseknecht was a professor of geology at the University of Missouri (1978–1992) and a consultant to the oil industry. He holds geology degrees from Penn State University (B.S. degree and Ph.D.) and Southern Illinois University (M.S. degree).Bird specializes in the petroleum geology of northern Alaska, where his experience spans more than 40 years. Currently, he is the leader of the U.S. Geological Survey Alaska Petroleum Studies Project. With interests primarily in stratigraphy and sedimentology, he has been extensively involved in petroleum resource assessments. He holds geology degrees from Oregon State University (B.S. degree) and the University of Wisconsin (M.S. degree and Ph.D.).

Description: Forty-one crude oil samples from the North Slope of Alaska have variable diamondoid and biomarker concentrations, indicating different extents of oil cracking. Some of the samples are mixtures of high- and low-maturity components containing high concentrations of both diamondoids and biomarkers. Compound-specific isotope analysis of diamondoids (CSIAD) shows that the Shublik Formation accounts for the higher maturity component in several mixed oil samples, whereas biomarkers, especially those providing information on the age of the source rock, show either a Cretaceous Hue-gamma ray zone (GRZ) or Triassic Shublik source for the lower maturity component. Oil samples in this study mainly correlate to six source rocks based on their biomarker characteristics and CSIAD. Chemometrics of selected source-related biomarker and isotope ratios helps to classify the oil samples into different genetic families. The source rocks include carbonate and shale organofacies of the Triassic Shublik Formation, Jurassic Kingak Shale, Lower Cretaceous Pebble shale, Lower Cretaceous Hue-GRZ, and Cenozoic Canning Formation. Oil presumed to originate from a seventh source rock interval, the Carboniferous–Permian Lisburne Group, was not clearly differentiated from well-established Shublik oil by any geochemical age-related parameter or CSIAD, which suggests that the Lisburne is not an effective source rock for any of the studied oil samples. Four oil samples collected from wells located north of the Barrow arch show unique biomarker characteristics, but age-related biomarker parameters indicate likely Triassic source rock organofacies that is not represented by any of the samples from south of the arch. The source rock for these four oil samples appears to be a clay-rich equivalent of the calcareous Shublik Formation that occurs to the north of the Barrow arch.

Description: Elemental sulfur (S) and high-pressure hydrogen sulfide (H2S), the first recorded occurrence of both from a well in northern Alaska, were encountered in the Husky 1 Inigok exploratory well in 1978. Located about 100 mi (160 km) southwest of Prudhoe Bay, the well was targeting an anticlinal structure on the northeast flank of a large late Paleozoic basin beneath the coastal plain and foothills region of the eastern National Petroleum Reserve in Alaska. The sulfur and gas first appeared at a depth of 17,570 ft (5355 m) while drilling in dark-gray to black organic carbon–depleted Carboniferous limestones (the Lisburne Group) at a temperature of 347°F, well above the melting point of S. Solidification of the molten sulfur by cooling during the drilling operation caused the drillstem to stick and delayed drilling operations for several months before being released. To determine the nature and source of the sulfur and H2S and to establish any relationship between them, we performed chemical analyses on S, drilling mud, and cuttings. Numerous holes and vesicles in some sulfur samples indicate the presence of gas, but other samples, without vesicles, contain sharp edges, fractures, and x-ray diffraction patterns characteristic of orthorhombic and not amorphous sulfur, thus arguing against a molten source in these samples. Isotopic values suggest that the sulfide in widely separated mud samples has a common source, and that the δ34S values for S are slightly more enriched in 32S in all deeper samples. The S from the drill pipe at 17,240 ft (5254 m) has a δ34S value similar to that of the sulfur in mud samples. The results appear to indicate that H2S is the source for the S. The source of the original H2S is an enigma. We consider several possibilities, among them, formation by biological sulfate reduction, thermal sulfate reduction during burial, a volcanic source, or generation in the deeply buried sediment from thermal disproportionation of pyrite. Here, we evaluate several possible origins based mostly on the properties of the enclosing limestone or shale host rocks, the downhole temperatures, and the stable isotope ratios measured and conclude that pyrite is the most probable source.

Description: The geology and reservoir-engineering data were integrated in the 2002 U.S. Geological Survey assessment of the National Petroleum Reserve in Alaska (NPRA). Whereas geology defined the analog pools and fields and provided the basic information on sizes and numbers of hypothesized petroleum accumulations, reservoir engineering helped develop necessary equations and correlations, which allowed the determination of reservoir parameters for better quantification of in-place petroleum volumes and recoverable reserves. Seismic- and sequence-stratigraphic study of the NPRA resulted in identification of 24 plays. Depth ranges in these 24 plays, however, were typically greater than depth ranges of analog plays for which there were available data, necessitating the need for establishing correlations. The basic parameters required were pressure, temperature, oil and gas formation volume factors, liquid/gas ratios for the associated and nonassociated gas, and recovery factors. Finally, the results of U.S. Geological Survey deposit simulation were used in carrying out an economic evaluation, which has been separately published. Mahendra Verma specializes in reservoir engineering and has more than 26 years of worldwide oil industry experience. Currently, he is a research petroleum engineer with the U.S. Geological Survey, providing engineering support to various geological assessments of fields and provinces in the United States, Canada, North Sea, Russia, and the Middle East. He holds petroleum engineering degrees from the Indian School of Mines, India (B.S. degree), the Imperial College of Science and Technology, London (Diploma of Imperial College), and Birmingham University, United Kingdom (Ph.D.).Kenneth Bird specializes in the petroleum geology of northern Alaska, where his experience spans more than 40 years. Currently, he is the coleader of the U.S. Geological Survey Alaska Petroleum Studies Project. With interests primarily in stratigraphy and sedimentology, he has been extensively involved in petroleum resource assessments. He holds geology degrees from Oregon State University (B.S. degree) and the University of Wisconsin (M.S. degree and Ph.D.).