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Forms, Mechanisms, and Rate of Hydrocarbon Migration in Rapidly Subsiding Basins

By

Ibrahim S. Guliev

Geology Institute, Azerbaijan Academy of Sciences

Among the structures of the earth’s crust, the rapidly subsiding basins (RSB) occupy a special place. Their peculiar features are as follows: domination of young terrigenous deposits in the section; contrast dynamic regime; high sedimentation rate; high reservoir and pore pressures; disconsolidated zones; and high specific density of hydrocarbons (HC).

Active migration of HC and existence of natural gas and oil seepages are typical RSB and they are fixed:

• In the atmosphere by anomalies of methane at different altitudes and contrast gradients of HC;

• In the surface of the land, traces of migration are fixed by geochemical anomalies of HC and different elements of the soil and plants;

• In the hydrosphere, migration can be studied by constant and dotted measurement;

• In the bottom sediments and in the sedimentary cover, migration of HC is fixed by acoustic anomalies and typical morphologic forms of biota communities.

Assessment of HC migration rate is up to the experiment requirements – from the maximum rate during eruptions of mud volcanoes 10^-2 m/s to 10^-10 m/s in microseepage.

The theoretical base for understanding the processes of migration is a provision about phase and mechanical instability in sedimentary series associated with the generation of HC.

• Accumulation of HC in terms of slow gas-water exchange results in saturation of pore extreme values when the phase transition of HC from the solution into a free state is possible.

• Initiation of phase transition may take place when reaching the pressure of hydrofracturing and also at account of the compression-extension wave when fissures and faults are formed. For a short period of time (fractions of a second) in the sedimentary cover, there occur impulses of a negative pressure. Possibility of their occurrence in the real solutions was proved by experiments (Veliev, 1999). Negative pressure stimulates the phase transition.

• The phase transition is characterized by a dramatic change of thermodynamical conditions, physical state of rocks and structure of layers and fluids and geochemical characteristics of the medium. There occurs “stimulation” of the system (Guliev, 1997, 1999).

• Migration of HC takes place along a system of canals and fissures that increases permeability of the medium in the subvertical direction.

• The process may be realized in the form of mud volcanism or in the form of tubular subvertical bodies or it may result in the formation of gas bodies of a different morphological form.

The most important consequences are:

• Generation and migration of HC are the most important dynamic factors which form structure and canals of migration of HC in the sedimentary cover;

• Process of migration of HC is periodical and it is associated with frequency of phase transitions in the zones of “stimulation;”

• The processes which take place in HC systems of RSB may be mapped and studied by geophysical and geochemical monitoring.

The above mentioned ideas supplement the classical theory of HC systems – formation of HC deposits.

• The sources of HC are not the rocks – sources (containing C_org>1 and subsided into the zone of oil and gas window), but any thickness of the sedimentary cover where conditions for the phase transitions (zones of stimulation) are reached.

• The most important condition of HC migration (phase transitions) is “stimulation of the system when there occurs a dramatic increase of pressure and decrease of permeability of the canals as a result of transition of matter into the pseudocondensate state.”

• Accumulation of HC occurs in any thickness of the sedimentary cover where conditions for stable gas bodies are observed.