Page 266 - Geochemical Remote Sensing of The Sub-Surface

Aerospace detection of hydrocarbon-induced alteration 239

incorporates carbon that typically has a 13C content more negative than -20 per mil.
Depending on the proportion of carbon derived from hydrocarbon oxidation, the 13C
content of the resultant carbonate can range from-10 to-60 per mil (Schumacher, 1996).
Carbonates with isotopically anomalous carbon derived in part from hydrocarbons
have been reported from the Cement, Chikasha, Velma, and other southwestern
Oklahoma oil fields (Donovan, 1974; Lilburn and Alshaieb, 1984), the Ashland gas field
in the Arkoma basin of southeastern Oklahoma (Oehler and Stemberg, 1984), the
Recluse oil field in Wyoming (Dalziel and Donovan, 1980), the Gulf of Alaska (Barnes
et al., 1980), the Davenport oil field in Oklahoma (Donovan et al., 1974), the Ocho Juan
field in Texas, the Fox-Graham field in Oklahoma (Duchscherer, 1984), and from the
carbonate hardgrounds formed around modem gas seeps on the California continental
rise (Paull et al., 1995) and near Fredrikshavn, Denmark (Dando et al., 1994). These
isotopically anomalous carbonates are often present in both on-field and off-field wells,
but their ~3C content is more anomalous (more negative) on-field.
Remote sensing cannot detect the isotopic signature of carbonates, but it can detect
the increase in carbonate formation and carbonate cement that hydrocarbon oxidation
induces. Ground truth investigations can subsequently establish if this carbonate is
indeed isotopically anomalous. In relatively arid regions, such as west Texas, carbonates
may be visible on aerial photographs or on satellite imagery as light tonal anomalies
indicating an excessive development of caliche in surface soils (Thompson et al., 1994).
On the Landsat TM imagery of the Hugoton gas field in southwestern Kansas, Patton
and Manwaring (1984) found a tonal anomaly that corresponded to a slight increase in
calcium content. Carbonate enrichments can also be determined by selecting appropriate
wavelengths sensitive to variations in absorption and reflectance of calcite, which are
most pronounced in the 1.8 to 2.6 ~tm range (Fig. 7-2). The remote sensing bands used to
map carbonate (calcite) concentration are typically those at 1.8 ~tm, 2.0 ~tm, 2.16 ~tm,
2.35 ~tm and 2.55 ~tm (Hunt, 1971).
Laboratory spectra of brown sandstone from the Palm Valley gas field of the
Amadeus Basin, central Australia, show only about half the brightness of laterally-
equivalent red sandstone, with a reflectance maximum at 1.85 ~tm. Laboratory spectra of
calcrete, with a weak to moderate absorption feature at 2.32 ~tm caused by the carbonate
(calcite) content, are similar to those of the brown sandstone (but with increased
brightness towards the visible wavelengths), suggesting that the colour change of the
sandstone is due to the addition of carbonate. Based on these field spectral reflectance
data, a distinctive colour discrimination was obtained by digital image processing of
NASA NS-001 aircraft-borne thematic mapper simulator data. Using calibrated ratios of
bands 3:7 (in blue), 1:5 (green) and 7:6 (red), carbonate-enriched areas are picked out in
yellow. The carbonate can also be identified as a pale-dark magenta colour using
uncalibrated Landsat TM images with ratios of bands 4:1 (in blue), 7:1 (green) and 3:1
(red) (Simpson et al., 1991).
In the Junggar Basin, Xinjiang, China, airborne short-wave infrared split spectral
scanner data revealed an anomaly that proved to reflect a major increase in total