Page 74 - Geology and Geochemistry of Oil and Gas

50 TEMPERATURE AND PRESSURE IN THE SUBSURFACE

or even regularity in changes of some physical properties (such as, color, reflectance,
etc.) may be used as indicators of paleotemperatures.
Most common methods of determining the paleotemperature are the vitrinite
reflectance and, sometimes, the spore and pollen coloration. There are techniques
of determining the paleotemperatures from the crystallization temperature of
authigenic minerals (also gas and liquid inclusions), from the isotope composition of
oxygen and carbon in calcite, etc.
Currently used techniques are imperfect. Even the best developed and most
commonly used vitrinite reflection technique was widely criticized over the recent
decades. Price (1985) believes that the vitrinite crystalline structure is imperfect,
because the hydrogen content in vitrinite changes causing variations in R o . Dow
(1977) showed that R o changes not only with depth, but also with the age of
sediments (Fig. 3.9). The methodology developed by Ammosov et al. (1987) is
successfully applied and further updated. Saxby (1982) studied the chemistry of
organic matter alterations and the changes in the vitrinite reflectance R o .
Some investigators relate the R o values directly to the depth of burial. This makes
the problem much more difficult due to variability of geothermal gradient. For
instance, the R o vs. depth correlation (Fig. 3.10) becomes clear only together with the
temperature vs. depth correlation (Wescott and Hood, 1994).
To summarize, changes of the organic and mineral components in the course of
lithogenesis of petroleum sequences depend on the paleodepth, thickness and the
duration of exposure to high temperature. The main feature of such changes is in
their variability that result in the generation of different energy potentials in the
various portions of the sequence and in its total state of energy intensity.

Fig. 3.9. Vitrinite reflectance R o vs. depth of burial, temperature, and geological age (after Dow, 1977).
1 ¼ Chalk, 2 ¼ Eocene, 3 ¼ Oligocene, 4 ¼ Miocene, 5 ¼ Pleistocene.