Combining data such as gravity, magnetics and satellite imagery with seismic, well and reservoir information can aid in exploration and development activities.

Hydrocarbon exploration is entering a new phase in its evolution. Although many companies have applied some type of integrated exploration in the past, truly integrated exploration has been accomplished in relatively few cases. Today, however, the availability and quality of data and the tools to integrate them have made such vast improvements that truly integrated exploration and interpretation should be routinely possible.

During the past two decades of the 20th century, seismic acquisition, processing and interpretation evolved in a manner almost unprecedented for a single industry. In this period, coverage and resolution improved by orders of magnitude. Largely unnoticed has been the development of "non-seismic" technologies, which have improved in a similar fashion. Today, these technologies can contribute to the integrated exploration process, not only in the early exploration or screening phase but in subsequent phases down to the identification of drilling locations on a worldwide basis. Of particular interest is the evolution of gravity data and its influence on this process. This particular type of data will be used as an example.

What are we talking about?

During the early years of exploration, gravity was used to locate basins for additional exploration or to delineate particular structures such as salt domes. Sometimes difficult structural areas such as overthrust zones were investigated. This was done mostly on land or in shallow water. With the advent of sea surface gravity meters, exploration was extended over the oceans of the world. The global coverage of sea surface data has been limited due to the cost of acquisition. Within the past 2 decades satellite radar altimeters have been used to map the variation in sea surface height, which can be converted into gravity. Thus global coverage of most of the oceans became a reality.

Today the accuracy and resolution for both sea surface and satellite gravity have improved by about two orders of magnitude. Sea surface gravity has continued to have resolutions and accuracy about 10 times better than satellite gravity, while the latter has much better coverage.

Why is gravity important?

Gravity is caused by variations in the density structure of the earth. Thus it responds to the results of all geological processes. Using the appropriate data coverage and enhancements of gravity, many of these processes can be recognized. Simple examples are sediment pathways, depositional centers, fan systems, salt basins and carbonate banks. On the other end of the scale, high-resolution gravity data inversion is used routinely by several companies in all stages of the prestack depth migration of seismic data to constrain velocity modeling. This is particularly true in the world's salt basins. An important factor in the utilization of gravity data has been the increases in coverage and resolution.

Improving coverage

About 15 years ago, a program was begun to compile gravity and magnetic data, continent by continent, over the entire world. Today this has been largely accomplished, although new data are added continuously. Figure 1 shows the current coverage, which includes satellite gravity data. This type of dataset allows exploration screening at various scales to be conducted routinely anywhere. To be useful in detailed exploration screening, both the resolution and accuracy of these data needs to be improved.

Improving resolution and accuracy

Because of its ocean-wide coverage, satellite-derived gravity is a logical candidate for improvement in resolution and accuracy. Through a process started in the mid-1990s, a research program has been developed to improve these data over the continental margins of the world. The current program, called the "Global Continental Margins Gravity Study," is sponsored by a number of leading oil companies. Data being delivered from this study has resolution and accuracy improvements about a factor of three better than currently available satellite products, at a fraction of the cost of sea surface gravity data.

Other data

Gravity is not the only type of data that are available on a worldwide basis.

Other data sources have various resolutions and accuracies but can be combined to form a powerful screening tool for basin and sub-basin analysis. For example, gravity and remote sensing data in the form of radar images can be used to screen large as well as small areas for the presence of hydrocarbons.
In the offshore, seeping oil leaking from reservoirs can reach the sea surface, usually in the form of oil-coated gas bubbles, and then form slicks identifiable from satellite. This is due to the dampening effect of the oil on the capillary wavelets, which produces an area of relative calm compared to the background waves. These rough-versus-calm areas can be easily detected on the satellite radar images as shown in Figure 2.

The platform

Current projects use ArcView 3.x and ArcGIS 8.x from ESRI for the integration and interpretation of these data. Other products such as MapInfo, Intergraph, GDMS, Manifold.net and the free software GRASS GIS also exist. These are all Geographical Information Systems (GIS). They provide methods for integrating spatial data and the tools for analysis and interpretation that speed the process. Using these tools can increase the speed of analysis and interpretation by a factor of three or more. This process also increases the accuracy of the final results. It has been found that almost all published and internal interpretation data contain inaccuracies because of the lack of integration with all data types. During the process outlined above, previous interpretations can be easily incorporated, corrected and extended.

Examples

GIS projects have been incorporated that provide the basis for the processes described above. These projects cover the South and North Atlantic, Southeast Asia, the Arabian Peninsula, north Africa, the west Appalachian Basin and various other areas worldwide. These projects are being used by more than 30 large and small companies as the basis for their initial and continuing exploration efforts.

Figure 3 shows an example of a display from one of these projects. This one is complex but illustrates the amount of information that is displayed effectively. A screening interpretation would indicate that an area to concentrate exploration efforts might be in the area of the intersection of the salt basin and turbidite fan where there may be trapping structures. This area also has a large number of hydrocarbon seeps as indicated by the darker square versus the lighter squares with fewer seeps. Examination of seismic data from the literature indicates that the interpreted salt may be either salt or mud diapirs. The nearby volcanics indicate higher heat flow, which would enhance hydrocarbon generation. The interpretations in this view are derived from enhancements of the gravity measured by satellite, integrated with published articles and based upon consultants' experience from around the world.

Conclusion

We have described a process to implement highly integrated interpretation projects. A few groups in several oil companies around the world are implementing this type of process. The learning curve to utilize GIS software in the exploration process is short and not steep. Integration of data into the GIS projects takes time, as it does in any project. Once the project is populated with data, however, the project itself is generally a non-volatile database. If a project is initiated within a more regional project, then the projects become screening tools for assessing other things such as farm-in opportunities in the area. These implementations save a company money and make its personnel more efficient.

Acknowledgements

The authors would like to thank Geophysical Exploration Technology (www.getech.com), for the use of the gravity data and ArcGIS interpretations shown in this article, Infoterra (www.infoterra-global.com) for the use of its hydrocarbon seep data and interpretations, and to our associated consultants, particularly Bill Dickson (DIGs) for their work and creative development of our joint integrated interpretation projects.

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