Petroleum Systems Modeling of the Greater Alaska North Slope Based on a Revised Geometry and Paleo-Geometry Reconstruction

Oliver Schenk¹, Kenneth J. Bird², Kenneth E. Peters¹, Leslie B. Magoon², and Carolyn Lampe^1,3
¹IES, Integrated Exploration Systems - Schlumberger, Ritterstr. 23, 52072 Aachen, Germany
²U.S. Geological Survey, 345 Middlefield Road, MS 969, Menlo Park, CA 94025, USA
³ucon geoconsulting, Franz-Kreuter-Str. 4, 50823 Cologne, Germany

The Alaska North Slope, including the adjacent Beaufort and Chukchi continental shelves, is a prolific oil and gas province. It is estimated to contain the majority of undiscovered oil and gas resources throughout the Circum-Arctic (USGS, 2008). We present a calibrated 4D model (3D geometry evolving through geologic time) of this region that reconstructs and evaluates the development of the individual petroleum systems, burial history, and thermal evolution, as well as migration, accumulation, and preservation of hydrocarbons. This study is based on 3D compositional petroleum systems modeling.

Compared to previous studies (Lampe et al., 2003) the current model size was increased (832 × 520 km with a grid spacing of 1 km) and now includes the Chukchi platform and the foothills of the Brooks Range. In addition, the Beaufort continental shelf was incorporated to enhance understanding of the Hue-Thomson(!) petroleum system in the Prudhoe Bay area.

The present-day geometry is based on newly interpreted seismic data and a database that consists of more than 400 wells and related calibration and geochemical data (Fig. 1). The Beaufortian and Ellesmerian sequences were carefully mapped due to the importance of onlaps and truncations and their effect on hydrocarbon migration pathways and traps. The overlying Brookian sequence (121 Ma to present day) with a total thickness of up to 8,000 m was deposited in a prograding foreland basin predominantly from WSW to ENE (Bird, 2001). The reconstruction of this paleo-geometry—diachronous deposition, facies variation, and thickness distribution as well as variations in paleo-basin geometry—was one key element of this study. The designation of depocenters through time had a major impact on the timing of maturity, generation, and migration of hydrocarbons. These time-transgressive deposits were reconstructed by using timelines rather than formations. They were mapped from surface traces and shelf edges. During the Tertiary, most of the study area was uplifted and eroded. The total amount of uplift was estimated by using porosity-depth trends from sonic-porosity logs (Burns et al., 2005) and partitioned and assigned to the three dominant uplift episodes, i.e., at ~60, ~40, and ~24 Ma (O’Sullivan et al., 1993; 1997). The reconstruction of this complex interplay of diachronous deposition and erosion is shown in Figure 2. This approach allows assessment of variations in migration behavior and prediction of the present-day petroleum distribution.

The simulation results explain the different petroleum systems in this province. The model contains five source rock units, namely the Kekiktuk coals, Shublik source rocks, Kingak-K1 source rocks, Gamma Ray Zone, and Hue shales. The dominant reservoir units are the Kemik and Kuparuk sandstones and the Sag River and Ledge/Ivishak formations. Detailed mapping of the Ellesmerian and Beaufortian sequences allowed accurate identification of migration pathways and stratigraphic and structural traps.

Source rock characteristics, such as the distribution of original total organic carbon (TOC) and hydrogen index (HI), were adopted from Peters et al. (2006). We used kinetic parameters determined on four thermally immature Alaskan source rock samples (3 phases, 14 components) by the method of di Primio and Horsfield (2006) to predict PVT-controlled compositions and properties. Source rock tracking helped to calibrate the model according to existing mixed accumulations and thus to predict the geochemistry of undiscovered accumulations.

This dynamic 3D petroleum systems modeling study provides significantly improved understanding and quantifications of the generation, migration, accumulation, and loss of hydrocarbons in the region. It also provides a unique geological framework to help to reduce exploration risks and to assess the remaining potential hydrocarbon resources in this remote province.

References

Bird, K. J. (2001) Alaska: A twenty-first century petroleum province, in M. W. Downey, J. C. Threet, and W. A. Morgan, eds., Petroleum provinces of the twenty-first century, American Association of Petroleum Geologists Memoir 74, p. 137-165.

Burns, W. M., Hayba, D. O., Rowan, E. L., and Houseknecht, D. W. (2005) Estimating the amount of eroded section in partially exhumed basin from geophysical well logs: an example from the North Slope. U.S. Geological Survey Professional Paper 1732-D.

Di Primio, R., and Horsfield, B. (2006) From petroleum type organofacies to hydrocarbon phase prediction. American Association of Petroleum Geologists Bulletin 90, 1031-1058.

Lampe, C., Peters, K. E., Magoon, L. B., Bird, K. J., and Lillis, P. G. (2003) Petroleum systems of the Alaskan North Slope – a numerical journey from source to trap: U.S. Geological Survey Open-File Report 03-326, 3 sheets, http://geopubs.wr.usgs.gov/open-file/of03-326/

O’Sullivan, P. B., Green, P. F., Bergman, S. C., Decker, J., Duddy, I. R., Gleadow, A. J. W., and Turner, D. L. (1993) Multiple phases of Tertiary uplift and erosion in the Arctic National Wildlife Refuge, Alaska, revealed by apatite fission track analysis. American Association of Petroleum Geologists Bulletin 77, 359-385.

O’Sullivan P. B., Murphy, J. M., and Blythe, A. E. (1997) Late Mesozoic and Cenozoic thermotectonic evolution of the central Brooks Range and adjacent foreland basin, Alaska: including fission track results from the Trans-Alaska Crustal Transect (TACT). Journal of Geophysical Research. 102, B9, 20.821-20.845.

Peters, K. E., Magoon, L.B., Bird, K.J., Valin, Z.C., and Keller, M.A. (2006) North Slope, Alaska: source rock distribution, richness, thermal maturity, and petroleum charge. American Association of Petroleum Geologists Bulletin 90, 261-292.

USGS (2008) Circum-Arctic resource appraisal: estimates of undiscovered oil and gas north of the Arctic circle, USGS Fact Sheet 2008-3049, http://pubs.usgs.gov/fs/2008/3049

Figure 1. Location of the study area showing the seismic lines and wells used for structure and isopach maps.