* 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

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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

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study area shows wide variation. It ranges from 27˚C/km

Figure 2: Geothermal gradient of some representative wells

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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

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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.

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