Page 263 - Geochemical Remote Sensing of The Sub-Surface

236 tl. Yang, F.D. Van der Meet and,1. Zhang

The attraction of remote sensing is that it offers a rapid and cost-effective means of
conducting reconnaissance for hydrocarbon-induced alteration.

Bleached red beds

The presence at surface of bleached and discoloured red sandstones above petroleum
accumulation has been widely noted, but detailed studies are few. Bleaching occurs
whenever acidic, reducing fluids dissolve the ferric oxide (hematite) that gives the red
bed its characteristic colour. Reducing conditions also favour the formation of pyrite and
siderite from the iron that is released during the dissolution of hematite. Leakage from
petroleum accumulations of reducing agents such as hydrocarbons, H2S and CO 2 could
be responsible for bleaching overlying red beds (Schumacher, 1996).
The reflectance characteristics of various ferric and ferrous iron minerals, clay
minerals and calcite are shown in Fig. 7-2. Ferric iron (in hematite) exhibits its strongest
reflectance at wavelengths greater than 1.0 pm; at progressively shorter wavelengths
there is first a distinct absorption feature at 0.9 [am, then an increase in reflectance at 0.8
~tm and finally, at still shorter wavelengths, reflectance falls off sharply (Hunt et al.,
1973). On the other hand, the ferrous iron in non-transparent minerals such as pyrite and
magnetite shows a near-uniform low total reflectance, although transparent minerals
such as siderite have broad shallow reflectance at 1.0-1.1 ~tm (Hunt, 1970). These

'- ~ ---.. Kaolinite

_, ' ""~ ~ "", :- ..... Calcite
80
""
9 9 - .... i".. :"-''.
Montmorillonite
~ 80
.f./ .......... lURe

~ ,o
. . Hematite

20 ''' --- Pyrite

o "T"l"l "r "1" ]'" I'" I"l"l "] "l" "i" i'l I"1 I .... Magnetite
40( 1000 1500 2000 2500
Wavelength (nm)
Fig. 7-2. Spectra of minerals associated with hydrocarbon microseepage.