Geology Reference
In-Depth Information
whereas in the Late-Pliocene-Quaternary time there was the generation of
gas-condensates.
The fluids in the underlying Mesozoic complex during this time
consisted mostly of gas. Substantial amounts of gas moved into the so
de-bituminized, immediately overlying sediments. The formation of gas-
condensate system with a high content of retrograde liquid component
dominated in the Paleogene-Miocene Series.
The Pliocene-Quaternary stratigraphic interval is currently generating
mostly the early catagenetic methane, which determines almost complete
domination of gas phase in the sedimentary section of South-Caspian
Basin (Kerimov and Rachinsky, 1989). This supports a previous conclusion
that the oil accumulations of PS/RBS are of secondary origin in relation
to the original condensate-rich, differentially entrapped vapor-gas hydro-
carbon mixture generated by the deep underlying complexes (Mekhtiyev
et al
., 1968).
The dominant generation within the sedimentary complex of South-
Caspian Depression of gas hydrocarbons, as well as the high-temperature
methane, carbon dioxide and nitrogen (with a substantial desorbing capac-
ity), provides for a high yield of dispersed organic matter from the rocks
through dissolution in the compressed gases. This explains the apparently
uniqueness in its magnitude commercial productivity of PS/RBS, which is
the only reservoir interval in the sedimentary section up to 25 km in thick-
ness, despite its relatively low original content of autochthonous dispersed
organic matter. We believe that the forming, high-temperature compressed
gases desorb the bulk of total dispersed organic matter in rocks and leave in
the “gassed-through” section small concentrations of the dispersed organic
matter, which is commonly mistaken for their original scarcity in that part
of sedimentary section.
As shown above, a significant role in the formation and accumulation
of PS/RBS hydrocarbon accumulations belongs to the vapor-gas phase
generated in the underlying stratigraphic complexes. This event enables
us to construct a thermodynamic model of these processes. Such a model
reflects with a reasonable reliability (1) the thermobaric environment in
the reservoirs, and (2) the quantitative distribution of hydrocarbons by
the phase type and their quantities. This also establishes the geochemical
composition of hydrocarbons in the various stratigraphic intervals (and
depths) of the Middle Pliocene Series.
The concept of formation of hydrocarbon accumulations in the PS/
RBS is illustrated by Figure 6.2. For an example assume the following: The
first bottle in a set of high-pressure bottles (all at the same temperature)
