Modeling: Integrating Structural Modeling, Fault Property Analysis, and Petroleum Systems Modeling – An Example from the Brooks Range Foothills of the Alaska North Slope

Carolyn Lampe¹, Kenneth J. Bird², Thomas E. Moore², Robert A. Ratliff³, and Brett Freeman⁴
¹UCON Geoconsulting, Franz-Kreuter-Str. 4, 50823 Cologne, Germany, [email protected]
²U.S. Geological Survey, 345 Middlefield Road, MS 969, Menlo Park, CA 94025, USA
³Geo-Logic Systems, 1435 Yarmouth Ave, Suite 106, Boulder, CO 80304, USA
⁴Badley Geoscience, North Beck House, North Beck Lane, Hundleby, Lincolnshire, PE23 5NB, UK

The North Slope of Alaska, including the adjacent continental shelves of the Beaufort and Chukchi Seas, is thought to hold significant amounts of undiscovered petroleum resources (Houseknecht and Bird, 2006). Most known petroleum accumulations involve structural or combination structural-stratigraphic traps related to closures along the Barrow arch, a regional basement high, yet this geologically complex region also includes prospective strata within passive-margin, rift, and foreland-basin sequences. Both extensional and compressional structures provide substantial exploration targets in the shelf and turbiditic sequences of Jurassic through Tertiary age (see Schenk et al. “Petroleum Systems Modeling of the Greater Alaska North Slope Based on a Revised Geometry and Paleo-Geometry Reconstruction”, this conference). While the area is maturely explored in the greater Prudhoe Bay area, little exploration has occurred elsewhere, with only a few wells and seismic surveys providing insight to the complex geology.

An approximately 50 km seismic section, trending N-S across the Brooks Range foothills in southern National Petroleum Reserve in Alaska (NPRA) and including the 3,414 m (11,200 ft) exploratory Awuna 1 well, is the basis for an integrated, comprehensive modeling study. The study area (gray rectangle in Fig. 1) lies in the Colville foreland basin between the Brooks Range in the South and the Barrow Arch in the North. The area is characterized by compressional tectonics, related to Tertiary deformation in the Cretaceous and Tertiary Brookian fold and thrust belt, the northernmost continuation of the back-arc fold and thrust belt of the Cordilleran orogen. Uplift of the Brooks Range, tectonic loading by the northward-moving deformation front, and extensive flysch and molasse deposits shed into the rapidly subsiding foreland basin in the Early Cretaceous resulted in strata more than 10 km thick in the southern part of the basin, thinning northwards towards the Barrow Arch. The stratigraphy can be subdivided into three major depositional sequences (Bird and Molenaar, 1987; Moore et al., 1994): (1) slightly metamorphosed and highly deformed Proterozoic to Devonian sedimentary and igneous rocks; (2) Mississippian to Lower Cretaceous northerly-derived passive margin clastic and carbonate rocks of the Ellesmerian sequence; and (3) Lower Cretaceous to recent clastic rocks of the Brookian Sequence, derived from the Brooks Range to the south.

In order to minimize technical uncertainty and drilling risk an improved understanding of the geological systems is required. For the first time, structural, fault, and petroleum system modeling have been combined and fully integrated to assess the processes and risks involved in the structural and geological development of the study area. The main advantage of this integrated approach is that feedback between the individual modeling processes can be ensured while the superior functionality of the respective tools/method is retained.

(1) Reprocessing and Interpretation of Seismic Data

The analyzed seismic section (line 89X-78) was acquired in 1978 as part of the 1974-1982 government exploration program of NPRA, during which more than 21,000 line-km of 2D seismic were collected. Stacked data provided by the seismic contractor for this line were migrated and then depth converted using posted stacking velocities. As with most foothills seismic lines, the time section is displayed with a sloping datum, which for this line ranged from sea level in the north to nearly 396 m in the south. The depth section is displayed with a horizontal datum of 396 m (1,300 ft) above sea level. Initially, four horizons were interpreted based on regional seismic reflectors and outcrop geology; later, two additional horizons within the Torok Formation were added based on seismic character and well logs, and help to define the local stratigraphy. Eroded units have been reconstructed based on the regional sonic log analysis of Burns et al. (2007), combined with vitrinite reflectance and apatite fission-track data.

(2) Palinspastic Reconstruction of Structural Architecture

Application of structural modeling to the present interpretation of the 2D section is designed to improve the structural interpretation and to palinspatically restore the geology to its undeformed configuration. The structural modeling requires a kinematically reasonable model of the deformation and results in iterative improvements of the structural interpretation through use of conservation (balancing) of area and line lengths between the undeformed and deformed geometries. Once balanced, the structural model can be portrayed as a series of quantitatively constrained paleomodels that can be integrated into petroleum systems analysis.
Palinspastic reconstruction of the 2D section, including the interpretation of several paleo-sections, has been performed using Geo-Logic Systems’ LithoTect software package. The reconstruction tracks movement through time and shows periods of deposition and subsequent erosion to account for possible impact of overburden and compaction during burial history of the basin. Understanding the evolution of the relative position of rocks involved in faulting allows us to establish the geometric and temporal controls on potential migration pathways.

(3) Estimation of Fault Rock Properties

About 75% of all hydrocarbon-bearing traps are fault-related, which makes fault interpretation a primary task when considering petroleum system modeling. Lithological information (derived from, for example, well data or outcrop), a thorough stratigraphic interpretation of the seismic section, and the evolution of displacement through time, provides the means to determine the sealing potential of faults using the shale gouge ratio method. Lateral and vertical fault correlation, fault relationships, fault-surface extent, and fault displacement and growth histories are addressed using Badley Geoscience’s TrapTester technology. Ultimately we condition the migration model with estimates of the capillary properties of the fault rocks through time.

(4) 2Dimensional Basin and Petroleum System Modeling with TecLink Technology

IES’ PetroMod TecLink technology (Hantschel et al., 2002) is used to integrate and combine the results of the structural- and fault property analyses in a classic petroleum system model. In this study, a two-dimensional, PVT-controlled, multi-component, 3-phase petroleum migration model demonstrates the integration of geometry changes, fault properties, thermal history, maturation, migration, and accumulation through time to understand the geologic evolution and the possible petroleum distribution in the study area.

Integrating palinspastic reconstruction work, fault seal analysis, and petroleum system modeling enables the simulation of dynamic processes and allows accurate assessment and prediction of the extent, timing, and potential of petroleum systems in tectonically complex areas such as the Colville foreland basin.

References
Bird, K.J., Molenaar, C.M., 1987. Stratigraphy of the northern part of the Arctic National Wildlife Refuge, northeastern Alaska. In: Bird, K.J., Magoon, L.B. (Eds.), Petroleum Geology of the Northern Part of the Arctic National Wildlife Refuge, Northeastern Alaska, U.S. Geological Survey Bulletin, 1778, pp. 37e59.

Burns, W.M., Hayba, D., O., Rowan, E. L., and Houseknecht, D. W., 2007. Estimating the amount of eroded section in a partially exhumed basin from geophysical well logs; an example from the North Slope. In: Haeussler, P. and Galloway, J. (Eds.), Studies by the U.S. Geological Survey in Alaska, 2005: U.S. Geological Survey Professional Paper 1732-D, p. 18; http://pubs.usgs.gov/pp/pp1732/pp1732d/

Hantschel, T. & Broichhausen, H., 2006. IES Integrated Exploration Systems, PetroMod 2D TecLink. – IES Technical Information Brochure, http://www.ies.de/files/public/brochures/English/PetroMod_2DTeclink.pdf

Houseknecht, D.W. & Bird, K.J, 2006. Oil and Gas Resources of the Arctic Alaska Petroleum Province. – U.S. Geological Survey Professional Paper 1732–A, http://pubs.usgs.gov/pp/pp1732/pp1732a/

Moore, T.E., Wallace, W.K., Bird, K.J., Karl, S.M., Mull, C.G., Dillon, J.T., 1994. The Geology of Alaska. In: The Geology of North America. Geological Society of America, Boulder, CO, G1, pp. 49e140.

Figure 1. Location of the study area (gray rectangle, Awuna-1 well) on the Alaska North Slope.