Page 265 - Geochemical Remote Sensing of The Sub-Surface

238 H. Yang, F.D. Van der Meer and J. Zhang

Kaolinisation

The acidic conditions resulting from the oxidation of hydrocarbons in near-surface
soils and sediments promotes the diagenetic weathering of feldspar to clay and the
conversion of smectite clay to kaolinite. The kaolinite thus formed remains chemically
stable unless the environment is changed (Schumacher, 1996).
Kaolinite exhibits a very strong absorption feature centred at 2.2 ~tm along with a
subordinate absorption feature at 2.16 ~tm (Fig. 7-2), forming a diagnostic doublet. This
can be picked out in remote-sensing imagery and used to indicate areas enriched in
kaolinite.
According to Segal et al. (1984, 1986) the bleached portions of the Wingate
Sandstone directly overlying the Lisbon Valley oil field in Utah contain three to five
times more kaolinite than the unbleached sandstone located away from the field, which
contains more plagioclase and muscovite. Using Landsat TM imagery, Segal and Merin
(1989) found that kaolinite-poor unbleached sandstone exposures have relatively low
ratios of bands 5:7, moderately kaolinite-rich sandstone (e.g., in an area known as Three
Step Hill) have higher ratios of bands 5:7 and kaolinite-rich bleached sandstone
exposures that overlie the Lisbon Valley field exhibit the highest ratios of bands 5:7.

Carbonate enrichment

The formation of diagenetic carbonates and carbonate cements, especially pore-
filling and replacement calcite, are amongst the most common alteration features
induced by hydrocarbon microseepage. These carbonates are a product of the oxidation
of hydrocarbons such a methane to carbon dioxide, which in groundwater hydrolyses to
bicarbonate anions. Dissolved calcium (and magnesium) in groundwater reacts with this
bicarbonate to precipitate as carbonate minerals or carbonate cement. One of two
reaction pathways applies, depending in the redox conditions, viz.,

aerobic CH4 + 202 + Ca 2+ = CaCOs + H20 + 2H +
anaerobic CH4 + SO42- + Ca 2+ = CaCO3 + HzS + H20

The resulting accumulation of carbonate at or near surface can be exploited as an
indicator of a hydrocarbon reservoir at depth (Patton and Manwaring, 1984;
Duchscherer, 1982; McDermott, 1940; MacElvain, 1963).
The carbon in this carbonate carries the isotopic signature of its parent
hydrocarbon(s). The carbon of most carbonate minerals is derived from the atmosphere,
freshwater or the marine environment and has a ~3C isotopic value of about -10 to +5 per
mil relative to the PDB standard (Fairbridge, 1972; Anderson and Arthur, 1983). The 13C
content of most crude oil ranges from about -20 to -32 per mil, and that of methane
ranges from -30 to -90 per mil. Thus carbonate formed from hydrocarbon oxidation