Geology Reference
In-Depth Information
At the same time, there is no contradiction between the actual data and
the functional correlation of formation temperature at subcrops vs. close-
ness to the surface of basement rocks heated by heat generation at depth. It
this case it would be legitimate to assume that heating of the section corre-
lates with the depth to basement. Wherever they are close to the surface the
conductive depth heat transfer into the upper intervals of the sediment fill
occurs with much lower dissipation compared to the regions of a thicker
stratisphere. This concept establishes the correlation between the section's
heating and the basement surface topography, the correlation defined by
variable heat exchange conditions in sedimentary sequences different in
thickness and relationship between heat-conductive and heat-insulating
intervals.
The minimum heating of deepest miogeosynclinal depressions and
troughs is caused to a significant extent also by lithofacies specifics of their
sedimentary section, by common development of thick heat-insulating
clay sequences in the depth range below 3-5 km. In the South Caspian
Depression this is the Paleogene-Miocene series, up to 6,000 m thick. In
the Zagros Trough this is schist-clayey Cambrian and Ordovician-Silurian
rocks (up to 2,300 m), Bajocian-Bathonian clay intervals of the Druma
Fm., 600-meter thick clay-limestone Kazhdumi Fm. (Albian), clay mem-
bers in the Tabur and Gurpi formations (Maastrichtian) and a 1,200 meter-
thick marly-limestone-clayey lowermost Oligocene formation. In the
Padan Depression there are Lower Permian and Lower Triassic deposits,
200-meters thick Lower Cretaceous clay member, mostly clayey Eocene-
Oligocene intervals, over 1,000 m in total thickness and 600-meters thick
marly-clayey Burdigalian series (Miocene). These clayey formations
in the stated regions serve as regional temperature barriers of low heat-
conductivity. They decrease intensity of conductive heat transfer up the
section and total heat amount entering the upper intervals.
Shielding of the depth heat flow by clayey sequences causes relative
overheating of the underlying section and a bounce in the geothermal
gradients. It may drastically increase within undercompacted or decom-
pacted clay as it is in direct proportion with their water-saturation
(which controls heat-conductivity) and thickness. In the Alpine folded
belt regions, such trend geologically goes together with the preservation
of primary or with the formation of secondary high porosity-decreased
density-elevated clay wetness at the shallow depth levels. This is in a
causal relationship in the former case with the lag of the pore water out-
flow from the deposit subsidence rate in the process of rapid permanent
immersion, and in the latter case with the volume increase of the clay
matrix to its dehydration.
