Page 145 - Geology and Geochemistry of Oil and Gas

116 NATURAL GASES AND CONDENSATES

formation may be somewhat different [because the sources (oils or bitumens)
are different in composition]. The established distinctions may be utilized to reach
certain geochemical conclusions within the oil- and gas-producing sequences in the
same basin. Transposition of the established patterns outside of the oil and gas
deposits (and even more so outside of the productive basin) is fraught with possible
errors due to different compositions of compressed gases, oils, and bitumens.
It may be assumed that the ‘‘primary’’ gas-condensate mixtures are always un-
dersaturated compared to the ‘‘secondary’’ ones. However, all ‘‘secondary’’ gas-
condensates in the West Siberia were undersaturated regardless of their position
above the oil accumulations (or oil rims). This phenomenon is still awaiting an
explanation. The most likely reason is gravity separation. As mentioned earlier,
gravity separation was observed in very thick gas accumulations. If the density range
is wide, the gravity separation is likely. It may be expected that this situation will be
maintained until the accumulation approaches its critical stage, when the role of
gravity becomes insignificant. Until such time, samples obtained from different
depths within the transition zone will show different saturation of oil in gas. Thus,
the higher the sample location within the transition zone, the greater the difference.