The basic data for evaluation of the petroleum source=rock potential in the National Petroleum Reserve in Alaska are summarized on the accompanying 39 plates. These data consist of organic geochemical analyses and observed hydrocarbon occurrences in 63 Government-drilled wells located in and adjacent to the NPRA. To relate the geochemistry to the geology, the authors have also included geophysical well logs, lithology, rock units, paleontologic zones, and ages. Two additional wells, Iko Bay No. 1 and Teshekpuk No. 1, are included here even though they lack geochemical data because they were drilled during the latest exploration program and provide information on subsurface geology and hydrocarbon occurrences. These displays, in addition to being the culmination of a considerable team effort, also demonstrate the capabilities of the NPRA computer system. All of the data displayed are stored in various computerized files. All data were computer-plotted except for the columns showing age, paleontologic zones, and stratigraphic names. Magoon and Claypool are responsible for the geochemical data and plate format design, Bird for the geologic and paleontologic summary, Weitzman for the computer programming and file construction, and Thompson for the data on hydrocarbon occurrence.
The Alaska North Slope oil-rock correlation study was organized because several oil companies requested oil and rock samples for geochemical analyses that were recovered during the exploration drilling in the National Petroleum Reserve in Alaska (NPRA). Samples acquired with public funds could not be given to private organizations unless some guarantees could be provided that the information acquired from these samples could be made available to the public. For this reason, in August 1981, we sent out over 40 invitations to research laboratories in industry, government, and academia.Requirements to participate in this study included: (1) participation in an AAPG-sponsored research conference, (2) presentation of the data interpretations at the 1983 Annual AAPG Meeting in Dallas Texas, and (3) contribution of a manuscript, to include all acquired data and interpretations, that would be included in a symposium volume. If a research group wished to participate, they were to write a letter of intent that included their proposed analytical program and a statement indicating that the requirements would be adhered to by their group. Even with these stringent requirements, 30 research groups wished to participate. A balanced cross section of research groups are participating and are as follows: 15 from oil companies, 7 from commercial laboratories, 7 from government laboratories, and 1 university laboratory. These groups are listed in Table 1.In January 1982, each research group was sent 8 oils and 15 rocks recovered from NPRA drilling and 1 oil from the Prudhoe Bay field. Each group then proceeded to analyze these samples as they indicated in their letter of intent.
Northern Alaska is a prolific oil and gas province estimated to contain a significant proportion of the undiscovered oil and gas of the circum-Arctic. A three-dimensional petroleum system model was constructed with the aim of significantly improving the understanding of the generation, migration, accumulation, and loss of hydrocarbons in the region. This study provides a unique geologic perspective that will reduce exploration risk and assess the remaining potential hydrocarbon resources in this remote province. The present-day geometry is based on newly interpreted seismic data and a database of more than 400 wells. A key aspect of this model is an improved reconstruction of the progradation of the time-transgressive Cretaceous–Tertiary Brookian sequence and multiple erosion events in the Tertiary. The deposition of these overburden rocks controlled the timing of hydrocarbon generation in underlying source rocks and their principal migration from the Colville Basin northward to the Barrow Arch. The model provides a reconstruction of the complex and dynamic interplay of diachronous deposition and erosion and allows assessment of variations in migration behavior and prediction of the present-day petroleum distribution.
Three seismic reflectors are present throughout the lower Cook Inlet basin and can be correlated with onshore geologic features. The reflections come from unconformities at the base of the Tertiary sequence, at the base of Upper Cretaceous rocks, and near the base of Upper Jurassic strata. A contour map of the deepest horizon shows that Mesozoic rocks are formed into a northeast-trending syncline. Along the southeast flank of the basin, the northwest-dipping Mesozoic rocks are truncated at the base of Tertiary rocks. The Augustine-Seldovia arch trends across the basin axis between Augustine Island and Seldovia. Tertiary rocks thin onto the arch from the north and south. Numerous anticlines, smaller in structural relief and breadth than the Augustine-Seldovia arch, trend northeast parallel with the basin, and intersect the arch at oblique angles. The stratigraphic record shows four cycles of sedimentation and tectonism that are bounded by three regional unconformities in lower Cook Inlet and by four thrust faults and the modern Benioff zone in flysch rocks of the Kenai Peninsula and the Gulf of Alaska. The four cycles of sedimentation are, from oldest to youngest, the early Mesozoic, late Mesozoic, early Cenozoic, and late Cenozoic. Data on organic geochemistry of the rocks from one well suggest that Middle Jurassic strata may be a source of hydrocarbons. Seismic data show that structural traps are formed by northeast-trending anticlines and by structures formed at the intersections of these anticlines with the transbasin arch. Stratigraphic traps may be formed beneath the unconformity at the base of Tertiary strata and beneath unconformities within Mesozoic strata.
Petroleum source-rock richness, type, and thermal maturity for four rock units under the Alaskan North Slope are determined from four geochemical analyses (organic-carbon content, C15+ hydrocarbon content, elemental analyses, and vitrinite reflectance) of samples from 84 wells and 16 outcrops. Contour maps of organic-carbon content indicate that the average richness for the Shublik Formation, Kingak Shale, pebble shale unit, and Torok Formation is 1.7, 1.5, 2.4, and 1.2 wt%, respectively. The organic-carbon content of the Shublik Formation, Kingak Shale, and pebble shale unit increases from west to east and downdip in the Prudhoe Bay area. Elemental analyses of kerogen plotted on a van Krevelen diagram indicate: (1) the Shublik Formation is Type II/III in the w st but Type I in the Prudhoe Bay area; (2) the Kingak Shale is Type II/III across the Slope but Type II in the Prudhoe Bay area; (3) the pebble shale unit and Torok Formation both tend toward Type III even though the former is higher in organic-carbon content. Contour maps of vitrinite reflectance drawn on the pebble shale unit unconformity and at the top of the Torok indicate all four units are immature to marginally mature over the Barrow arch and mature to overmature in the Colville trough. Carbon isotope data for the saturated and aromatic fractions of the C15+ hydrocarbons from rock extracts suggest possible source-rock correlations with similar data for four North Slope oil types (Umiat, Simpson, pebble shale, and Kingak), but no obvious correlation of the Barrow-Prudhoe oil type with any of the four source rocks.
The Neogene Nonassociated Gas Assessment Unit (AU) of the Neogene Total Petroleum System consists of nonassociated gas accumulations in Pliocene marine and brackish-water sandstone located in the south and central San Joaquin Basin Province (Rudkin, 1968). Traps consist mainly of stratigraphic lenses in low-relief, elongate domes that trend northwest-southeast. Reservoir rocks typically occur as sands that pinch out at shallow depths (1,000 to 7,500 feet) within the Etchegoin and San Joaquin Formations. Map boundaries of the assessment unit are shown in figures 22.1 and 22.2; this assessment unit replaces the Pliocene Nonassociated Gas play 1001 (shown by purple line in fig. 22.1) considered by the U.S. Geological Survey (USGS) in its 1995 National Assessment (Beyer, 1996). The AU is drawn to include all existing fields containing nonassociated gas accumulations in the Pliocene to Pleistocene section, as was done in the 1995 assessment, but it was greatly expanded to include adjacent areas believed to contain similar source and reservoir rock relationships. Stratigraphically, the AU extends from the topographic surface to the base of the Etchegoin Formation (figs. 22.3 and 22.4). The boundaries of the AU explicitly exclude gas accumulations in Neogene rocks on the severely deformed west side of the basin and gas accumulations in underlying Miocene rocks; these resources, which primarily consist of a mixture of mostly thermogenic and some biogenic gas, are included in two other assessment units. Lillis and others (this volume, chapter 10) discuss the geochemical characteristics of biogenic gas in the San Joaquin Basin Province. Primary fields in the assessment unit are defined as those containing hydrocarbon resources greater than the USGS minimum threshold for assessment—3 billion cubic feet (BCF) of gas; secondary fields contain smaller volumes of gas but constitute a significant show of hydrocarbons. Although 12 fields meet the 3 BCF criterion for inclusion in the AU, only 5 fields were considered at the time of assessment.
Because gas hydrates from within a limited temperature range, subsurface equilibrium temperature data are necessary to calculate the depth and thickness of the gas-hydrate stability field. Acquiring these data is difficult because drilling activity often disrupts equilibrium temperatures in the subsurface, and a well mush lie undisturbed until thermal equilibrium is reestablished (Lachenbruch and Brewer, 1959). On the North Slope if Akaska, a series of 46 oil and gas exploratory wells, which were considered to be near thermal equilibrium (Lachenbruch and others, 1982; 1987), were surveyed with high-resolution temperature devices (see table 1). However, several thousand other exploratory and production wells have been drilled on the North Slope, and although they do not include temperature profiles, their geophysical logs often allow descrimination between ice-bearing and non-ice-bearing strata. At the outset of this study, the coincidence of the base of ice-bearing strata being near the same depth as the 0°C isotherm at Prudhoe Bay (Lachenbruch and others, 1982) appeared to offer an opportunity to quickly and inexpensively expand the size of our subsurface temperature data base merely by using well logs to identify the base of the ice-bearing strata.
