View
9
Download
0
Category
Preview:
Citation preview
* Shed no. 26, Forward Base, ONGC, Ankleshwar.
sonamgupta18@gmail.com
10th Biennial International Conference & Exposition
P 386
Trend of Geothermal gradient from Bottom Hole Temperature studies in
South Cambay Basin (Narmada Broach Block)
Sonam*, B.S.Dhannawa & Varun Kumar
Summary
An endeavour has been made to ascertain the geothermal gradient and its trend in the South Cambay Basin. Knowledge of
geothermal gradient is required for many oilfield applications viz. evaluating well logs, designing cementing programs, basin
modeling for discerning source rock, to name a few. Bottom Hole Temperature (BHT) data from well logs of about 70 wells
were collected, corrected for equilibrium temperature and interpreted to understand the trend of geothermal gradient.
Regional geothermal gradient of the study area varies in the range from 27°C/km to 67°C/km and averages about 39°C/km.
The subsurface gradient map shows that the gradients are relatively lower towards the depocentre and increases towards its
eastern margin. An inverse relationship between the geothermal gradient and depth of basement (Deccan Trap) is observed
i.e. geothermal gradient is more in areas where basement is at shallow level & vice-versa.
Keywords: Bottom Hole Temperature, Geothermal Gradient, South Cambay Basin
Introduction
The recent thrust on energy has necessitated pooling of all
the available G&G tools to analyze and understand the data
available that can help in the future discovery of much
needed oil and gas. It is widely accepted & well known
that knowledge of subsurface temperature has a great
bearing & is an important tool, input & determinant in oil
exploration. It has a significant role in hydrocarbon
generation and migration of hydrocarbons and associated
pore fluids. Knowledge of geothermal gradient is required
for many oilfield applications. Some of these applications
include evaluating open and cased hole logs, designing
cementing programs, basin modeling for discerning source
rock, modeling steady and unsteady state of fluid and heat
flows in the wellbore for designing thermal recovery
projects, etc. Keeping this in view an endeavour has been
made in the present work to ascertain the geothermal
gradient of the South Cambay Basin. The geothermal
gradient is the rate of change of temperature with depth in
the earth. The temperature of the earth normally increases
with depth, and as a result when a well is drilled it shows
an increase in temperature with depth. The geothermal
gradient usually holds a linear relationship with depth, but
this holds good only in a completely homogeneous
medium. The geothermal gradient is greatly affected by
rock type and it’s thermal conductivity; i.e. a rock with
high thermal conductivity will show a low thermal
gradient. Therefore the real temperature gradient in a well
is not a straight line, but a series of gradient related to
thermal conductivities of various strata.
A good no. of sample points has been taken across the
basin to understand the trend of geothermal gradient
laterally and vertically. Interesting observations have been
made and the regional trends are brought out.
General Geological Setting of Study Area
Cambay Basin is a tertiary intracratonic rift basin, lying
between western and northwestern margin of Indian shield
covering an area of about 59000 sq. km which is 450 km
long and 55-100 km wide in dimension (Sarraf et al. 2000).
Upper Cretaceous-Lower Paleocene, tholeiitic continental
flood basalts represented by Deccan Trap forms the
technical basement over which more than 7-11 Km of
tertiary & quaternary sediments have been deposited.
These lava flows were the result of extensive volcanic
activity, associated with the northward drift of the Indian
plate near the end of the Mesozoic. They tend to thicken
towards the centre of the Cambay Basin, possibly
reflecting the onset of rifting (Yalcin et al., 1987). In the
2
basinal part, the continental sediments corresponding to
Olpad Formation are deposited over Trap. The area
initiated by the marine incursion from the south during the
early Eocene and remained as a narrow elongated Basin till
date i.e. Gulf of Khambhat. The alternating pulses of
regression and transgression of sea in the Basin have
deposited the Cambay, Kanwa and Tarkeshwar
argillaceous units separated by Hazad, Dadhar and
Babaguru arenaceous units. Cambay shale is the main
source rock in the area, Hazad sands are the reservoir and
Kanwa and Telwa and intervening shales within Hazad are
the cap rock in the area. The Cambay basin is divided into
four tectonic blocks based on recognizable basement fault
system (Raju et al. 1983) and the study area falls in the
Jambusar-Broach Block i.e in southern part of the NNW-
trending Cambay Basin which is bounded by the transverse
fault zone of Mahi in the north and Narmada river in the
south (Mathuria T. K. et al. 2011)
Figure 1: Location map of study area (Raju et al. 1993)
Methodology
Data Collection:
The efforts for determining geothermal gradients began
with compilation of information of exploratory wells
drilled in the study area. Bottom hole temperature used for
this analysis were taken from log headers collected during
well logging. More than 100 exploratory & development
wells in Narmada Broach block of South Cambay Basin
were taken, evaluated and analyzed for the study.
However, fewer than 70 exploratory wells were found to
have suitable technical conditions (depth range, time
elapsed between cessation of mud circulation and
temperature measurement, sufficient number of log run,
etc.) which could potentially help in eventual geothermal
measurements.
Data correction for equilibrium temperature:
Measured subsurface formation temperature from open
hole logs is always lower than the true or static formation
temperature. During the drilling of oil wells, a large
quantity of mud is circulated in the borehole to facilitate
the drilling, evacuate the cuttings and stabilize the hole.
The influence of this circulation and other drilling effects
like thermal properties of the drilling fluid, nature of heat
exchange between borehole fluid and the formation,
duration of drilling; provides a non-equilibrium
temperature at the time of temperature measurements.
Numerous methods have been adopted in the past to
correct the logged bottomhole temperature & estimate real
formation temperatures. These methods include those of
Hyodo et al (1994), Middleton M. F. (1982), Dowdle W.L.
and Cobb W. M. (1975), etc. The most common method
and the one used here is the Horner plot proposed by
Dowdle W.L. and Cobb W. M. (1975). The Horner
method, plots the measured temperature (at a given depth)
from each of several logging runs, against log(T/(t+T)).
The parameter t represents the length of time that the
borehole was subjected to the cooling effects of the fluid,
and T represents the time after circulation that the borehole
has had to partially reheat. The best fitted straight line
results from the plot is extrapolated to cut the temperature
axis where logT/(t+T) equals to zero, reflects the true
formation temperature at that particular depth. We have
assumed mean surface temperature to be 25˚C.
Geothermal gradient determination:
Assuming that there is a linear relationship between
temperature and depth, the equation of the straight line can
be expressed by a regression formula based on type of
available data as (Nwankwo et al. 2009):
The geothermal gradient (G) is determined from the given
formula:
Where; x= temperature, y = depths in metres
Results & Discussion
Computed geothermal gradient in different boreholes
along with well details (such as depth drilled, maximum
borehole temperature recorded, etc.) is presented in Table-
1. These wells are widely distributed in South Cambay
Basin and extend from shallow eastern margin to major
depocentre towards west. The geothermal gradient of the
3
study area shows wide variation. It ranges from 27˚C/km
Figure 2: Geothermal gradient of some representative wells
4
to 67˚C/km with an average of about 39˚C/km.
Trend of temperature variation with depth in respect of
selected wells is presented in figure 2. It is very obvious
from the trend that geothermal gradient is not uniform
through the entire borehole. Temperature data in trap
section indicates higher gradients and can be seen in Figure
2 (BH-26, BH-44, BH-47, BH-67, etc.). It is observed that
the rate of change of temperature with depth in
sedimentary section is lower as compared to the basement
igneous rocks.
Some wells are projected to E-W cross section line and
their corresponding geothermal gradient is plotted as
shown in Figure 3. This figure depicts that the gradient
profile follow the same trend as the depth to basement.
There is a general increasing trend of geothermal gradient
as we move from western to eastern margin of the basin.
The geothermal gradient is more in areas where basement
is at shallow level & vice-versa.
Figure 4 shows the location of the wells used in the study
and their corresponding geothermal gradient contour map.
The contour map shows that the gradients are relatively
lower at the depocentre of the basin compared to the
eastern marginal part where geothermal gradients are high.
Low gradient of 27˚C/Km is observed in Dahej &
North Harinagar fields which lie more or less in the
western and central part in Broach depression while higher
gradient of 67˚C/km in Karjan, Karvan and Padra fields
which lies in the eastern margin of the basin. The
gradients of Padra, Karjan and Karvan field in general are
on the higher side as Deccan trap has been encountered at
shallow depth. These differences in geothermal gradients
may reflect changes in thermal conductivity of rocks,
groundwater movement and endothermic reaction during
diagenesis. The higher gradient values observed in the
marginal part of the basin may have resulted in high heat
flow due to tectonic activities in the basin.
Table 1: Well Details and their calculated geothermal gradient
5
Figure 3: Cross section showing geothermal gradient from
western to eastern part of the Basin
Figure 4: Contour map showing geothermal gradient in the study
area
Conclusions
Regional geothermal gradient vary clearly from well to
well and with depths. It ranges from 27˚C/km to 67˚C/km.
It is observed that geothermal gradient depends on
thickness of sedimentary column and thus depth to
basement. A generalized correlation is established between
the geothermal gradient and depth of Deccan trap. An
inverse relationship between the geothermal gradient and
depth of basement (Deccan Trap) is observed i.e.
geothermal gradient is more in areas where basement is at
shallow level & vice-versa. The subsurface gradient map
shows that the gradients are relatively lower towards the
depocentre and increases towards its eastern margin.
Acknowledgement
The authors express their thanks to ONGC for providing
infrastructure and giving permission to publish the paper.
Authors are grateful to Shri S. K. Das, ED-Basin Manager,
WON Basin and Dr. M. C. Kandpal, GM, Block Manager-
I for facilitating publication of this paper and for their
encouragement and support. Authors acknowledge the
helps rendered by their seniors and colleagues for the
technical suggestions and helps during their work.
References
Biswas S. K., Rangaragu M. K., Thomas J., Bhattacharya,
S. K., (1994), Cambay-Hazad (!) petroleum system in the
South Cambay Basin, India, in Magoon, L.B., and Dow,
W.G., eds., The petroleum system – from source to trap:
AAPG Memoir, no. 60, p. 615-624.
Chen Z., Osadetz K. G., Issler D. R., Grasby S. E., (2008),
Hydrocarbon migration detected by regional temperature
field variation, Beaufort-Mackenzie Basin, Canada;
AAPG Bulletin Vol. 92 pp. 1639-1653.
Davis R. W., (2012), Deriving geothermal parameters
from bottom-hole temperatures in Wyoming; AAPG
Bulletin Vol. 96, pp. 1579-1592
Dowdle W. L., Cobb W. M., (1975), Static Formation
Temperature from well logs- an empirical method; J.
Petrol. Tech., 27, pp 1326-1330.
Gomes A. J. L., Hazma V. M., (2005), Geothermal
Gradient and Heat Flow in the state of Rio De Janerio;
RBGf 23 (4), pp 325-347.
Grisafi T. W., Rieke H. H., (1973), Approximation of
Geothermal Gradients in Northern West Virginia using
Bottom – Hole Temperatures from Electric Logs; AAPG
Bulletin.
Hyodo M., Takai K., Takasugi S., (1994), Evaluation of
curve fitting method for estimating the formation
temperature from logging data; preceedings of the 90th
SEGJ Conference, 285-289.
Jam L. P., Dickey P.A., Tryggvason E., (1969), Subsurface
Temperature in South Louisiana; AAPG Bulletin Vol. 50,
pp. 2141-2149.
6
Kundu J., Wani M. R., (1992), Structural Styles and
Tectono-Stratigraphic Framework of Cambay Rift Basin,
Western India; Indian Journal of Petroleum Geology, 1 (2),
pp. 181-202.
Kutasov I. M., Eppelbaum L. V., (2010), A new method
for determining the formation temperature from bottom
hole temperature logs; Journal of Petroleum and
GasEngineering Vol. 1(1), pp. 001-008
Mathuria T. K., Julka A. C., Dimri P. K., Pandey P. B.,
(2011), Search and Discovery Article # 10326,
Middleton M. F. (1982), Bottom-hole temperature
stabilization with continued circulation of drilling mud,
Geophysics, 47, pp. 1716-1723.
Mukharjee M. K., (1981), Evolution of Ankleshwar
Anticline, Cambay Basin, India; AAPG Bulletin, Geologic
Notes.
Nwankwo C.N., Ekine A. S., (2009), Geothermal gradients
in the Chad Basin, Nigeria, from bottom hole temperature
logs; International Lournal of Physical Sciences Vol.
4(12), pp 777-783.
Nwachukwu S. O., (1976), Approximate Geothermal
Gradients in Niger Delta Sedimentary Basin; AAPG
Bulletin Vol. 60, pp. 1073-1077.
Raju A.T.R., Srinivasan S., (1983), More Hydrocarbon
from Well Explored Cambay Basin, Petroleum Asia
Journal, pp. 25-35
Raju A.T.R., and Srinivasan S., (1993), Cambay Basin
petroleum habitat, in Biswas S.K., Alok D., Garg, P.
Pandey J., Maithani A., and Thomas, N.J., eds.,
Proceedings of the Second Seminar on Petroliferous
Basins of India, v. 2: Indian Petroleum Publishers, Dehra
Dun, pp. 33-78.
Sarraf S.C., Ray D. S., Kararia A. D., Lal N. K., (2000),
Geology, sedimentation and petroleum system of Cambay
Basin; Petroleum Geochemistry and Exploration in Afro
Asian Region, pp 215-226.
Yalcin M. N., Welte D. H., Misra K. N., Mandal S. K.,
Balan K. C. Mehrotra K. L., Lohar B. L., Kumar S. P.,
Misra G. S., (1987), 3-D computer aided basin modelling
of Cambay Basin, India - a case history of hydrocarbon
generation, in Kumar R., Dwivedi P., Banerjie P. V., Gupta
V., eds., Petroleum geochemistry and exploration in the
Afro-Asian region: Balkema, Rotterdam, p. 417-450.
Recommended