Geographic information systems as an advanced exploration tool

New technical developments and advances in the spatial presentation of geological data can be used to create regional databases of play fairways and petroleum systems for genetically-related sediment-ary basin units. A major goal for the explorationist is to produce a consistent, process-based database that can be used for comparative studies across the entire globe without any geographical, physi-cal, environmental, or political barriers.

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John Jacques
Robertson Research International Ltd.

New technical developments and advances in the spatial presentation of geological data can be used to create regional databases of play fairways and petroleum systems for genetically-related sediment-ary basin units. A major goal for the explorationist is to produce a consistent, process-based database that can be used for comparative studies across the entire globe without any geographical, physi-cal, environmental, or political barriers. Geogra-phic information systems (GIS) provide this tech-nology.

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Infrastructure and salt distribution map for the northern Gulf, from a regional digital structural and geology coverage. B) Inset map showing interpreted distribution of different crustal types.
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GIS can be viewed as the medium by which a diverse range of multiple geological datasets (structural, geophysical, geochemical, and sedimentological) can be presented in any combination. Spatial and temporal relationships can be observed across numerous data layers, and queries can be performed to evaluate data reliability and to perform multi-scenario analyses. This advanced query functionality provides a powerful means of assessing alternative geological models.

Activities

GIS not only provides the means of storing extremely large volumes of highly descriptive geospatial data from a wide range of data types, but also provides the following core activities:

  • Management
  • Integration
  • Analysis
  • Presentation.

GIS provides a data management system in which any set of data can be accessed and viewed in a shared environment. It can also be updated quickly and efficiently with new data.

When digitally captured, the geospatial data are organized into thematic layers in the database. The hierarchical data structure allows the data to be queried and accessed through a series of linked database tables, using a menu-driven user interface. Each layer can be viewed independently or integrated in any combination in a common geo-referenced grid. This creates dynamic geospatial displays that can be analyzed both visually (qualitatively) and statistically (quantitatively) to identify and evaluate important relationships.

Present day GIS functionality is due to three types of computer software:

•Computer-aided design systems (CAD) provide powerful vector data (points, lines, and polygons), visualization, and transformation (conversion) functions. On a structural geology map, points may include lithostratigraphic control and lines would represent structural elements (e.g., faults). Polygons would be used to define different rock types

•Image processing systems are largely concerned with processing raster (gridded) data, making them ideal for presenting satellite images and aerial photographs

•Database management systems are capable of organizing and searching extremely large geological datasets.

Spatial queries

Spatial queries can be performed in two ways, either by on-screen selection in which any information associated with the selected items is derived from the database, or by querying the database on an attribute or set of attributes allowing these items to be identified and displayed on-screen.

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A) Possible linear distribution of oil and gas fields along NNW transfer/transform faults in Texas. B) Possible spatial relationship of oil and gas fields with both NW and NNW continental/oceanic fracture systems in Louisiana.
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For example, on-screen selection could include source rock data control points. Hot-linked information could include total organic carbon (TOC) value, kerogen type, vitrinite reflectance, hydrocarbon index, and maturity value for a particular source rock horizon at some location.

The second method provides the explorationist with a very powerful way of spatially querying a heavily populated database on specified characteristics, for example, using a cut-off value of 1% TOC to define areas of reasonable source rock quality. By performing multiple queries on several parameters, locations with TOC values >2%, kerogen Type II, and hydrocarbon indexes between 400-600 could be identified, defining areas of high quality, oil-prone, source rock potential.

A similar exercise can identify areas of high reservoir quality, screening the database on lithology, facies, thickness, porosity, permeability, and fracturing. These, combined with the source rock maturation history and expulsion, trap timing or type, and seal integrity, provide the crucial elements for play fairway mapping.

GoM structural coverage

A first requirement for a GIS scheme for the Gulf of Mexico is the creation of a detailed descriptive database of the principal structural and geological elements. Using a combination of datasets, continental block outlines, first-order structural elements, and different crustal types need to be identified. This provides the tectonic platform from which future geospatial observations can be performed with confidence and provides the basic building blocks required for palaeogeographic analysis.

All structural and geological maps were compiled digitally as GIS coverages at basin scales of 1:500,000 through 1:2,000,000 to create a complete attributed structural coverage for the Gulf of Mexico basin. When combined with total sediment isopach coverage (contoured depth values to economic basement), basin boundaries were defined.

Where possible, a distinction was made bet-ween deep basement faults and crustal domain boundaries using potential fields data, and growth fault systems. Distinguishing growth faults propagating from deeper basement structure from those related to salt movement (hard/soft linkage) provided a further categorization.

Deep basement faults operative during the basin's opening cycle were categorised as either continental transfer or oceanic transform faults. Individual structures were attributed to reference source and values were assigned to each structure to illustrate confidence in their position and interpreted sense of movement. This provides a spatial representation of the quality of the data across the entire basin, enabling areas of poor coverage to be identified and updated efficiently as new data become available.

Structural lineaments and basement features from second-order derivative potential fields maps were interpreted by combining individual shaded relief images. Various illumination directions were analyzed, allowing features to be ranked based on confidence. These were merged into one interpretation. A distinction was made between basement features and structures confined to the sedimentary cover that sole out into a mobile substrate horizon (salt or shale).

Spatial analysis

GIS spatial analytical applications can perform advanced integrated rastor-vector analysis, map algebra, and grid statistics. They are particularly adept at integrating real-world physiographic variables, such as onshore topography and offshore bathymetry, with geospatial datasets to solve difficult problems. An example is the creation of detailed basin coverage using contoured isopach values of depth to economic basement, where land and/or seafloor relief are required to produce a total sediment thickness map.

Using this information, along with stratigraphic tops from well or seismic, a target horizon can be identified, contoured to produce a depth structure map, and distances determined for well penetration across any part of the basin. The basin's surface can be contoured to identify areas of economic viability using drilling depths.

Geostatistical analysis

These applications provide sophisticated, interpolation techniques, which can search for and identify data anomalies, then analyze the data for statistical trends. This analysis provides extreme flexibility. It allows the explorationist to cross-validate the data, quantitatively evaluating the accuracy of the geological model and predictions. In combination, geostatistical and geospatial GIS processing provide a very powerful exploration tool for identifying, evaluating and presenting geospatial relationships that could otherwise have gone unnoticed.

For example, in the northern offshore Gulf of Mexico, on visual and intuitive grounds, the spatial distribution of oil and gas fields with first-order NE-SW, NNW-SSE and NW-SE trending transfer/transform faults, and NE-SW growth fault systems, suggests a strong genetic relationship. All three transfer/transform fault sets have been shown recently to represent an integral part of the basin's opening cycle.

The basement rift geometry of fault-bounded grabens and half-grabens created during the basin's opening imposed an important control on the distribution and quality of Upper Jurassic source rocks. This extensional basement fabric had a profound effect on the original depositional thickness of Callovian salt. Laterally and vertically extensive, this mobile substrate:

  • Determined the sites of mini-basin formation by the creation of mainly Cenozoic growth fault systems related to salt mobilization and withdrawal
  • Retarded the thermal maturation of sub-salt petroleum source rocks
  • Formed major barriers to vertical petroleum migration
  • Created extensive structural traps and faults for petroleum migration.

From various lines of evidence, it is also becoming apparent that these first-order fault sets served as sediment fairways through the salt canopy throughout the Tertiary and provided primary vertical migration conduits for oil and gas. Thus, they exerted an important control on salt distribution, sediment influx patterns, and hydrocarbon migration. The advanced GIS techniques outlined above can be used to investigate this association.

Using both geospatial and geostatistical analyses, we can test whether the distribution of proven oil and gas fields show a close spatial relationship to one, two, or all three sets of basement faults than would be expected by chance. If this correlation is statistically strong, we can then use the model to make spatial predictions on the potential for gas in deep structural closures in the shallow shelf, as well as oil and gas distribution in frontier deepwater areas.

3D analysis

These GIS processes can create and query 3D data and, by draping vector and raster data over a surface, model real-world surface features. GIS provides the ability to generate dynamic interactive 3D views from multiple viewpoints and provides the option of creating interactive perspective views.

Through the user-interface it is possible to pan and zoom, rotate, tilt, and create fly-through simulations. The uses of this high quality presentation option are endless, from regional-scale draping of structural elements and basin boundaries over contoured elevation data to field-scale, 3D visualization of trap geometries.

Conclusions

GIS provides the explorationist with extremely powerful state-of-the-art software for geospatial data analyses. It can store and manage vast quantities of highly descriptive data, perform queries across multiple data types and levels of information for qualitative and quantitative interpretation, and provides extremely powerful presentation options. Although GIS possesses all these amazing functions, the usefulness of performing such analyses is totally dependent on the quality and quantity of the information in the database and on the explorationist's knowledge of the tectono-stratigraphic history of the region. With-out this in-depth understanding, useful descriptive data may not be captured during the compilation stage, valuable queries may not be performed, and results may be misunderstood.

References

Bonham-Carter, G. 2000, An overview of GIS in the geosciences, in T. C. Coburn and J. M. Yarus, eds., Geographic information systems in petroleum exploration and development: AAPG Computer Applications in Geology, No. 4, p. 17-26.

Jacques and Clegg 2002, Gulf of Mexico Late Jurassic source rock prediction: Integrating tectonics and geochemistry with GIS technology, Offshore Magazine, v.62, n. 10, pp 106-107, 164.


Future of GIS

Though we are far from exploring fully the capabilities of current GIS functions, we need to plan ahead for the exciting new challenges that will be provided by new GIS developments and its integration with the Internet. The Worldwide Web (WWW) already provides the ability to store, access, and update information via computer networks anywhere in the world. The next step will be a worldwide network of computers that can help interpreters understand the stored data.

According to Tim Berners-Lee, founder of WWW, and his colleagues, this may not be too far into the future. The focus will not be on the way computers work, but on the way the information is stored. They are currently developing a method by which computers will be able to extract information from one site to another as a means of answering complicated queries. Referred to as the Resource Descriptive Framework, it deals with "meaning" by breaking it down into a three-unit arrangement (RDF triples): the nature of the item described, its properties and the nature of those properties.

In the petroleum arena, the item "*named* Field" could have the following properties: location, fluid type, water depth, production unit, and trap type. Using this format, information from a multitude of scientific reports could be pieced together from anywhere in the world, saving days if not man-months of time.

Combining this Internet and web-based functionality with GIS will far exceed our current expectations for predicting geospatial relationships. The catch is, however, that this implementation is greatly dependent on the time it takes to begin writing RDF-compatible scientific documents.

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