Page 262 - Geochemical Remote Sensing of The Sub-Surface

Aerospace detection of hydrocarbon-induced alteration 235

Fig. 7-1. Surface alterations caused by migrating hydrocarbons (reproduced with permission from
Duchscherer, 1982).

directly or indirectly bring about significant changes in the pH and Eh of the surrounding
environment, thereby influencing mineral stability and chemical reactivity. Such
oxidation in the chimney above a leaking petroleum accumulation leads to dissolution or
precipitation of minerals and the mobilisation or immobilisation of certain elements in
the chimney, which thereby becomes mineralogically and chemically different from
laterally-equivalent rocks (Pirson, 1969; Oehler and Stemberg, 1984; Price, 1986). The
resulting alteration includes: the formation of calcite, pyrite, uraninite, elemental
sulphur, and certain magnetic iron oxides and iron sulphides; bleaching of red beds; clay
mineral alteration; electrochemical changes; radiation anomalies; and biogeochemical
and geobotanical anomalies (Fig. 7-1).
Where such changes can be measured and mapped at surface, they provide the basis
for a number of surface exploration methods for petroleum. Some of these changes are in
principle amenable to measurement and mapping by remote sensing techniques.

REMOTE DETECTION OF INDUCED SURFACE MANIFESTATIONS

Remote sensing has the potential to detect hydrocarbon-induced alteration in rocks,
soils and vegetation. Extensive studies have been performed on the reduction of ferric
iron (red-bed bleaching), the conversion of feldspars and mixed-layer clays to kaolinite,
the increase of carbonate content and the anomalous spectral reflectance of vegetation.