Geology Reference
In-Depth Information
The aforementioned enables making the following highly important
conclusion: the presence of significant pressure gradients between adja-
cent clay and reservoir sequences is a testimony not of hydrodynamic
communication between them but, on the contrary, of their mutual
hydraulic isolation. This conclusion introduces a new understanding in
the primary and secondary migration of hydrocarbons and formation of
oil and gas accumulations.
The AHFP spatial distribution within the penetrated stratigraphic sec-
tion in the South Caspian Basin (see Figure 3.1) displays generally a stressed
state of its regional geo-fluid-dynamic system on the local highs: for all
waterhead complexes the average value of formation pressure excess over
the hydrostatic is 33.5 MPa with the abnormality factorе of 1.89. ([These
values may actually be minimal. The reason is that the statistical selection
takes into account the rare data for the Oligocene-Miocene (Maikopian)
and Miocene (Chokrackian Horizon, Diatomaceous Formation) intervals
tested in wells located relatively closely to the surface exposures of the said
complexes or discharge zones.]
Based on the preceding we conclude that the main abnormal pressure
formation mechanism in the Pliocene reservoirs of the South Caspian
Basin is the injection of high-pressure fluids from the underlying sedimen-
tary complexes. This injection was mostly implemented within local highs
through a fault system and caused their predominantly epigenetic nature.
Additional factors are believed to have been elastic compression of natural
reservoirs due to neotectonic processes and smectite dehydration in clayey
parts of the section within the appropriate temperature range.
A conclusion was made that a single thick geo-fluid-dynamic system was
and is operational in the region. The system includes a stratigraphic range
from the Jurassic through the Pliocene. Within this interval, Middle Jurassic-
Valanginian Stage is the charge area and the Hauterivian-Pliocene is the dis-
charge area. The conclusion was made based on continuous association of
baric, geotemperature, hydro-geochemical and isotope anomalies with areas
of fault tectonics and mud volcanism and regular dynamics of their quantita-
tive parameters over the entire section of the South Caspian Basin.
The overall trend of continuous decline in average formation pressure
abnormality factor values up the section in combination with all afore-
mentioned localization within individual structures suggests that the main
mechanism in the formation of the geo-fluid-dynamic regime in the South
Caspian Basin was subvertical inter-formational fluid-mass transfer.
Several hydrodynamic (waterhead) complexes are identified within the
mentioned system based on a combination of lithofacies, reservoir proper-
ties, hydrogeological, geobaric and geothermal attributes. These complexes
