Geology Reference
In-Depth Information
general background (associated with maximum scale of the zonal and local
commercial oil and gas occurrences) are contrasting.
Analysis of the geothermal component of the South Caspian Basin's
total geofluid-dynamic field enables the following conclusion. The pres-
ent-day geothermal regime of the region's sediment cover is controlled by
the section's lithofacies composition in individual regions, by the extent of
local structure faulting and thermal ground water mobility conditions. To
a substantial extent it is formed by the convective component of the total
heat flow (heat transfer by migrating fluids), which is in turn caused by the
cross-flow/injection processes that hydraulically join the lowermost and
the upper stages of the basin's lithosphere.
4.2.1
Forecasting Technique of Geotemperature at Depth
It is difficult to estimate objectively the present-day geothermal and ther-
mal regime at great depths intervals not penetrated by wells. The reasons
are obvious errors in assumptions regarding the values of porous medium
thermophysical parameters and in formalized solutions (based on these
assumptions) of complex heat conductivity equation systems. The appli-
cation of such temperature forecast technique results in that temperature
values estimated by different scientists for various deep stratigraphic sur-
faces and depth subcrops differ by tens and even hundreds of degrees. One
example is the South Caspian Basin. There, Geodekian (1968) estimated
temperature at the depth of 30 km at 525°С, Trotsyuk (1982) at the depth
of 20 km at 680°С; and Kutas (1978) at the base of the sediment cover at
740°С.
Similar discrepancies occur at extrapolating actual temperature values
in the deepest wells for great depths. Two conditions should be met for
such estimates. First, it is necessary to clearly determine and substantiate
the acceptable depth range of the forecast. Second, it is necessary to select
correctly the extrapolation function (linear, exponential, logarithmic, etc.).
Obviously, this technique is only acceptable for the temperature estimates
within relatively narrow depth interval and within a single stratigraphic
complex where thermophysical properties are reasonably consistent later-
ally and in the section.
For the South Caspian Basin, it means that within the PT-KT range such
extrapolation is justified only within this complex in compliance with the
general temperature trend between its top and base where actual obser-
vations are available. The same approach should be taken in forecasting
within the Miocene-Paleogene and Mesozoic sediments. Thick sedimen-
tary sequences comprise complex combinations of various lithofacies
