AN ASSESSMENT OF THE GEOTHERMAL ... - Orkustofnun

Report 5 13 Akbar
region, the maximum
geothermal gradient was
found in the deep basin part
at Hatiya trough (Shahbajpur
1, 29.5°C/km), and the
minimum in the folded flank
of the Sylhet trough (Beani
Bazar 1 well, 19.8°C/km). A
single value of the
geothermal gradient for each
well was recorded and
expressed in units of °C/km
for the presentation. From
the plotted average curve,
temperatures at different
depths (at km interval) were
taken for drawing an
isotherm geothermal gradient.
Due to the different tectonostratigrahic
setup of
Bangladesh as well as the
distribution of deep
exploratory wells, the
geothermal gradients are
discussed under two broad
headings as follows.
4.1 Southeast basin part of
Bengal Basin
The Bengal Foredeep
comprising the southeastern
FIGURE 8: Subsurface temperature distribution in
a) Southeastern basin part of Bangladesh;
b) Northwestern stable part of Bangladesh
part of Bangladesh is a region of great subsidence of the earth´s crust, occupying the vast area between
the Hinge line and ArakanYoma folded system, and plays the most important role in the tectonic
history of the Bengal Basin. Based on its geotectonic behaviour, the Bengal Foredeep is again divided
into the western unfolded region or deep basin part and the eastern folded region known as the
Chittagong-Tripura folded belt, stretching parallel from north to south. The unfolded western part
consists of several highs and lows as observed from gravity and magnetic surveys. The Sylhet trough
to the northeast also known as the Sylhet basin, and the Faridpur trough to the southwest are separated
by a prominent high known as the Tangail-Tripura high and, in the southernmost depression of the
basin part, is the Hatiya trough; north of it is the Barisal-Chadpur gravity high. Sediment thickness
along the trough region is the highest and is assumed to be around 20 km.
Folded belt
The folded belt (or the folded eastern flank of the Bengal Foredeep) is the most prominent and the
youngest tectonic element of the western flank of the Indo-Burman Ranges. This zone of the Bengal
foredeep is well known for hydrocarbon prospects and most of the exploratory wells are drilled in this
area. The hydrocarbons discovered so far in the country are located in reservoir sands of the Neogene
Surma group (Imam and Shaw, 1987). The wells were drilled to depths in the range of about 2100-
4977 m below the surface. Temperature data from 30 exploratory wells was used to map the thermal
gradients of the region (Table 6). A high rate of sedimentation in the major part of the Tertiary
sedimentary sequence has a probable influence on the low geothermal gradient in the deep basin area.