Technical Note: Integration of Gravity, Magnetic, Well and Seismic Reflection Data in the National Petroleum Reserve, Alaska (NPRA)

By Gerry Connard, NGA, Inc., Corvallis, OR, USA and Ash Johnson, Geosoft Europe, Ltd.

Overview

This technical note provides a brief description of using gravity, magnetic, well, and seismic data for exploration in the National Petroleum Reserve in Alaska (NPRA). This note also demonstrates how modern PC-based software can be used for QA/QC and enhancement of gravity and magnetic data and how gravity and magnetic data can be integrated with exploration well data and seismic reflection data. These data are now freely available online in digital format from the US Geological Survey web site.

Introduction The NPRA was established in 1923 by President Harding and covers 96,000 km2 on Alaska’s North Slope to the west of the prolific Prudhoe Bay oil field (Figure 1). The NPRA has been explored and mapped by US government groups since 1901 generating 19,000-line km of seismic data and digital and analog data from 43 wells.

As part of a project to reassess oil and gas potential for northern Alaska, the U.S. Geological Survey (USGS) is applying gravity and magnetic analysis to the understanding of basement character in the NPRA (National Petroleum Reserve in Alaska) (Saltus, and others, 2001; Saltus, and others, 2002). An interpretation of the basement geology is important to developing and evaluating oil and gas plays for deeper parts of the basin as well as for understanding of regional heat flow and tectonic history. Integrating gravity and magnetic data with the existing wells and 2D seismic data answers many of the questions posed by the seismic interpreters and helps refine the interpretation of the prospective part of the basin. Phillips Petroleum Company recently announced a significant discovery in the Northwestern NPRA.

Most of the northern portion of the NPRA is covered by an aeromagnetic survey flown by the US Navy with 3.2 to 6.4 km (2-4 mile) line spacing 300 meters (1000 feet) above the ground.

This note focuses on the Northeastern part of the NPRA as shown in Figure 2. The upper-left map in Figure 2 displays the location of 2D regional seismic lines and selected wells. The upper-right map in Figure 2 is a shaded-relief version of the on-shore topography created from a 300-meter DEM. The lower-left map in Figure 2 is an isostatic residual gravity anomaly map created from the public-domain gravity data and the lower-right map displays the public-domain magnetic anomaly data.

Figure 1

Figure 2

Data QA/QC

As is typical of many public-domain data sets, the data quality of the gravity and magnetic data for NPRA is highly variable. Interactive PC-based software provides efficient tools for QA/QC of gravity and magnetic datasets. For example, Figure 3 shows a workspace containing a map of the gravity station locations plotted on top of a grid of the isostatic residual gravity with a grid cell size of 500 m. Using a “linked” database containing the gravity station data (shown in the bottom of Figure 3), it is very easy to locate and investigate suspicious

gravity stations in the gravity database spreadsheet. The user simply clicks on suspicious data points in the map window and the closest data point in the database is immediately highlighted.

In Figure 3, the gravity station highlighted in the map window is the same station highlighted in the spreadsheet window and the profile window. After deleting or adjusting the suspicious data points, users can quickly regrid the data to check the results of the editing process.

Figure 3

Figure 4

Figure 4, shows the same map area as Figure 3, with the gravity station causing the “bulls eye” removed.

Data Enhancements

A wide range of tools is available in modern PC-based software to enhance the gravity and magnetic data to focus on upper crustal structures. Figure 5

below shows a 3D view of the Complete Bouguer gravity anomaly grid draped on the topography of the NPRA. The view is looking south from the Beaufort Sea (declination = 210°, inclination = 20°).

Figure 5

The strong correlation between the higher elevations of the Brooks Range Mountains and the negative Bouguer gravity anomalies masks some of the gravity signal from the upper-crustal structures of interest in exploration.

Figure 6

Figure 6 shows the Isostatic Residual gravity anomaly draped on topography viewed from the same point as Figure 5. The isostatic residual process has removed most of the gravity signal caused by the deeper Moho under the Brooks Range, thereby enhancing the gravity signal from upper-crustal sources.

Data Integration

Recent exploration wells in the NPRA found mis-ties with initial interpretations of the seismic data by Tetra Tech (1982). The USGS has re-interpreted the seismic data using more modern techniques but many questions about the structures still remain. For example, Figure 7, from Saltus, and

others (2001), asks if the three features highlighted in NPRA Regional Seismic Line 8 are intrusives. By integrating the seismic data with the magnetic anomaly data in an integrated model (shown in Figure 8), one can see that two of the features are very likely intrusives and the one in

the middle is probably not. The top panel in Figure 8 shows the calculated magnetic response as a solid line and the observed magnetic data as a dotted line. The left and right intrusives have magnetic susceptibilities of 0.019 and 0.075 (SI) respectively.

Figure 7

Figure 8

Modeling provides a tool for integrating gravity and magnetic data with seismic data, exploration wells, and surface geology. Using modern PC-based modeling tools, interpreters can easily test a wide range of geologic models and examine the sensitivity of the

gravity and magnetic response to the variation in those geologic models. Figure 9 shows an example of a model integrating seismic, gravity, magnetic, and exploration well data along NPRA Regional Seismic Line 9.

All of the data used in this note are now freely available online in digital format from the US Geological Survey web site. The seismic and well data are available online at: http://energy.cr.usgs.gov/other/npra/npraindex.htm

Various public domain gravity surveys for the NPRA area have been compiled by the USGS and are available online at: http://wrgis.wr.usgs.gov/docs/gump/morin/alaska/gravity/gravity.html

A 300-meter digital elevation model (DEM) compiled by the USGS is

available online at: http://agdc.usgs.gov/

The aeromagnetic data are available on CD-ROM in Saltus, Connard, and Hill, (1999).

Acknowledgements

The authors wish to thank Dr. Richard Saltus of the USGS for his assistance in providing the data used in this paper. Oasis montaj software from Geosoft Inc. was used for the mapping, gridding and QA/QC examples shown. GM-SYS from NGA Inc. was used for the modeling examples.

Authors

Gerry Connard is a geophysicist with over 20 years experience in the exploration industry. He is president of Northwest Geophysical Associates, Inc.

Ash Johnson is a geophysicist with more than 10 years experience in the acquisition, processing and interpretation of potential filed data. He is the Managing Director, Geosoft Europe Ltd.

References

Saltus, R.W., Connard, G.G., and Hill, P.L., 1999, Alaska aeromagnetic compilation - digital grids and survey data: U.S. Geological Survey Open-File Report 99-0502, 1 CD-ROM.

Saltus, R.W., Hudson, T.L., and Phillips, J.D., 2001, Basement Geophysical Interpretation of the National Petroleum Reserve Alaska (NPRA), Northern Alaska; USGS Open-File report 01-0476, 3 poster panels with extended text (http://geology.cr.usgs.gov/pub/open-file-reports/ofr-01-0476).

Saltus, R.W., Hudson, T.L., Phillips, J.D., Kulander, C.S., and Potter, C.J., 2002, Basement Geology of the National Petroleum Reserve Alaska (NPRA), Northern Alaska: USGS Open-file report 02-0127. (http://geology.cr.usgs.gov/pub/open-file-reports/ofr-02-0127).

Tetra Tech, 1982, Petroleum exploration of NPRA 1974-1981 (Final Report): Tetra Tech Report No. 8200 (including maps at 1:500,000 scale).