ASTER Spectral Anomalies

In an area where hydrocarbons are seeping to the surface (even in very minute quantities), a reducing-acidic environment occurs.  Our Spectral Anomaly Analysis helps identify these areas based on the associated geochemical alteration of clays and iron.

MDA Federal uses information from the ASTER sensor to map mineral assemblages that are indicative of hydrocarbon seepage. ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) is an imaging instrument flying on Terra, a satellite that is a cooperative effort between NASA and the Japanese government and meant for mineral exploration. Previous studies performed in various hydrocarbon basins around the world have shown that a good correlation exists between spectral anomalies observed on ASTER data and existing production.

Figure 1 – Summary of MDA Federal’s use of the ASTER sensor to detect areas of mineral assemblages commonly associated with hydrocarbon microseepage.

A general seepage model has been developed through multiple case studies, and examination of the mineral assemblages that would be expected to occur at the surface under presently-seeping conditions. In an area where hydrocarbons are seeping to the surface (even in very minute quantities), a reducing-acidic environment occurs. In this reducing-acidic geochemical environment, mixed-layer clays and iron become altered. For example, clays are altered from montmorillonite to kaolinite (i.e., higher kaolinite and lower montmorillonite concentrations within the seep affected area). Ferric iron (Fe+++) will be affected in one of two ways:

• In most areas, ferric iron (Fe+++) is altered to become ferrous iron [Fe++] (i.e., low Fe+++ iron within an anomaly).
• In some instances, the reduced iron (Fe++) over the area of hydrocarbon microseepage is redeposited as a sulfide (pyrite / marcasite), which is then oxidized at the surface to limonite (i.e., yellow rather than red iron) but still a ferric (Fe+++) iron which looks like an area "enriched" in iron over the seep area.

ASTER, with several spectral bands in wavelengths sensitive to clay and iron absorption, is an excellent tool for identifying these differences in mineral assemblages. For the spectral analysis, the ASTER images are then processed using a proprietary Iron (as hematite), Kaolinite, Montmorillonite ratio image algorithm. We refer to this as MDA Federal’s IKM processing. Once the ASTER ratios are created, they are compared to areas of production. If the kaolinite response appears to be enhanced, the Fe+++ values around the production are noted (i.e., whether Fe+++ is relatively high or low). These criteria are then used to create a thematic map of areas that may be of potential interest. Spectral transects are run through anomalous areas to show how the mineralogy changes across them. The IKM spectral anomalies should not be used to “pinpoint” exact drilling locations, but rather as an indication that microseepage may have occurred in the area. They are most commonly used to focus future soil gas efforts, or in conjunction with other exploration data. Figures 2 through 4 show examples of how the anomalies may be expressed differently depending on the geologic setting. We customize the thematic maps accordingly to emphasize areas of potential interest.

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