C-36. 12.2. 1,100 c-33. 15.7. 1,200. Ankleswar oil-. 21â35'. 72O55'. Ank-170. 23.3. 1,200 field. Ank-28. 13.0. 1,200. Ank-2. 12.5. 1,200 period (month) ficak/cm2.
Tectonophysics - Elsevier Publishing Company, Amsterdam Printed in The Netherlands
TERRESTRIAL HEAT FLOW AND TECTONICS OF THE CAMBAY BASIN, GUJARAT STATE (INDIA? M.L. GUPTA, R.U.M. RAO
R.K.
VERMA,
V.M.
HAMZA,
G.
VENKATESHWARRAO and
National Geophysical Research Institute, Hyderabad (India) (Received April 15, 1970)
SUMMARY
Heat flow values are presented for four sites in the Cenozoic Cambay Basin, Gujarat State, India, based on temperature measurements to a depth of 1,200m in seven water filled wells, drilled for oil exploration. Values obtained from oilfields of Kathana, (22O17’N ‘72O48’E) Nawagam (22O50’N 72O30’E) and Kalol (23O16’N 72O30’E), which are situated in different tectonic blocks along the northern part of the basin are varying and high (1.8 2.2 peal / cm2 esec). These heat flow measurements support the previously determined high heat flow (2.3) at Cambay gas field (22O23’N 72O35’E). Relatively low value of heat flux (1.62) has been obtained in the southern pa part of the basin at Anklesvar oilfield (21°35’N 27O55’E). The high heat flow values in the region extending from Cambay to Kalol can be explained as due to igneous intrusion in the crust underneath the basin during PlioceneMiocene times. Also discussed are other observations based on various geophysical studies that have been carried out in the basin. An attempt is also made in this paper to correlate the heat flow values to a few tectonic features of the basin.
INTRODUCTION
Terrestrial heat flow values so far published for India, range from 0.66 to 1.76 pal./cm2.sec (Gupta et al., 1967; Verma et al., 1967) in the Precambrian shield areas and from 1.06 to 2.49 in the Gondwana sedimentary basins (Verma et al., 1969, Rao et al., 1969). In the Cenozoic Cambay Basin, heat flow value at one place has already been reported (Verma et al., 1968). Heat flow values from four different new locations in the Cambay Basin, Kalol, Nawagam, Kathana and Anklesvar, are presented, involving observations in seven wells drilled for oil exploration by the Oil and Natural Gas Commission. Throughout the text, the gradients are given in OC/km, thermal conductivity in mcal./cm.sec°C and heat flow in pcal./cm2 ‘sec. N.G.R.I.
contribution
Tectonophysics,
no. 70-183
10 (1970)
147-163
147
GEOLOGY
AND TECTONICS
OF THE CAMBAY
BASIN
The Cambay Basin is located in the alluvial plains of Gujarat State of western India, extending approximately from latitude 21°N to 24*N and longitude 71a30’ to 73O45’E. It is a Cenozoic basin and runs as a narrow graben in an approximately north-northwest-south-southeast direction. South of latitude 21°45’ it takes a swing towards NNE-SSW and runs into the Gulf of Cambay. Extensive geophysical work in connection with oil exploration has been carried out in the basin since 1958. The geology and tectonics of the basin have been discussed in detail by many workers, including Marthur et al. (1968): Sen Gupta (1967); and Mathur and Evans (1964). Tectonically the basin is situated at thenorthwestern edge of the Peninsular Shield, and is surrounded by the Aravalli System of Precambrian age in the northeast, and by Deccan Traps on the east and west. The geological map [of the basin and the adjacent regions] is shown in Fig.1. The basin is of intracratonie type, formed by discontinuous normal faults. A complete sequence of sediments ranging in age from Recent to
m
CRETACEOUS EOCENE o.ccm Trap* UPPER, LOWER CRETACLOUS B#@hBads, Abamodnww SS
KHARACHODA Ov s
nAL”’ -
a
UPPER PRE CAMORIAW Grlhl Gyrtm e A~bGorbGutl.
UmIl
LOWER PRE CAMBRiAN Arovolli Gyst*m
SANANCI”
0
LOCATION OF ##IAT FLOW MEASURCYENTS y
C-A.. , ‘-. **
, qi’ \
JJQKm
MAP Of
INDIA AREA
SHOWING THE
Fig.1. Geology of Cambay Basin and adjacent region. (Geology from Geological Survey Map of India , 1963. ) 148
Eocene overlie an irregular surface of Deccan Traps, the basic extrusives of Cretaceous to Eocene age, which mark the close of the Mesozoic Era with an episode of volcanism. The sediments reach a thickness of over 5,000 m in the deepest part of the basin (Jambusar-Broach area). The sequence comprises greywackes, dark grey to black grey shales, coal cyclothems, silts, fine to medium grained sands and grey reddish-brown clays. The presence of sporadic outcrops of Jurassic and Cretaceous sediments STRUCY PAFAN 94
f-.3
OF
TRAP
URAL
MAP
SURFACE
Fig,% Map of the Trap surface in the Cambay Basin with four tectonic blocks (I-IV) after Avasthi et al. (1969). Tectonophysics, 10 (1970) 141-163
149
on the margins points to the likelihood that the basin may have existed even in the Mesozoic Era, It is believed that a deep-seated fault, formed probably during the end of the Gondwana Period, runs parallel to the western coast of India and into the Cambay Basin. This fault zone is cut into two parts by an important transverse basement fault running along the Narmada River and joining up with the southern coast of the Saurashtra Peninsula, The area to the south of the Narmada River forms the northeastern corner of the southern part of the fault zone. The other part of the fault zone comprises the rest of the Cambay Basin (Mathur et al., 1968). The whole basin is dissected into four structural blocks (Fig.2, I-IV) by faults within the Beccan Traps, which continue to some extent into the overlying sediments. These blocks are characterised by different fold and fault trends and basement depths (Fig.2). The pattern of sedimentation has been, to a certain extent, controlled by the block pattern. The southern part of the basin has structural trends parallel to the Narmada River, which flows in a west-southwest direction. In the northern part, the tectonic trend is markedly north-northeast. Another conspicuous feature of the basin is the reversals of block tilting throughout it’s Cenozoic history. During the Eocene, the general slope of the basin was towards the north, but it was reversed during the Oligocene, and the basin as a whole became tilted towards the south. The tilting of the basin towards the south continued during the Miocene Epoch. The Normada fault. which was dormant during the major part of the Miocene Epoch, was reactivated during the post-Miocene period, and the movement along this fault resulted in a vast syntectonic accumulation of sediments in Jambusar-Broach area and in the uplift and consequent erosion in the Narmada block to the south. The southerly tilt of the basin .eontinued during the post-Miocene. A slight but progressive westerly shift in the axis of sedimentation is noticed during Cenozoic history.
TEMPERATURE
MEASUREMENTS
Temperature measurements were made in seven unproductive, waterfilled wells. They were drilled by the Oil and Natural Gas Commission for oil exploration purposes. A thermistor probe, coupled to a three Conductor Vector Cable, was used for temperature observations. The resistance of the thermistor (Fenwal GB34 P21) was measured using a B.C. bridge with a circuit for compensation of leads resistance. The observations were made at intervals of 20 or 25 “9, to a m~lmum depth of 1,220 m. The location and a few other particulars of interest of the seven wells reported herein, are given in Table I. Least square gradients were calculated for those depth intervals within which the temperature vs. depth curves are linear. The gradients in all the seven wells and in the three wells reported earlier (Verma et al., 1968) are given in Table II. The temperature gradients are lowest at Anklesvar and highest at Cambay-Kathana region as compared to other parts of the basin.
150
Tectonophyeice,
10 (1970) 147-163
23“16’ 22”50’ 22“17’ 22O33’
21“35’
Nawagam oilfield
Kathana oilfield
Cambag gasfield
Ankleswar oilfield
Latitude (N)
72O55’
‘72O35’
7Z94%’
72O30’
72“30’
Longitude (E)
Coordinates
Ank-170 Ank-28 Ank-2
c-10 C-36 c-33
Kat-9
Naw-32
K-58 K-27
Well no.
23.3 13.0 12.5
11.9 12.2 15.7
26.2
-
73.8 65.2
1,200 1,200 1,200
1,225 1,100 1,200
1,200
900
1,200 1,220
Elevation Depth to which metres above logged (m) M.S.L.
of wells logged in Cambay Basin and the heat flow data.
KaIol oilfield
Localitv
Particulars
TABLE I
1.5 1.65 1.7
19 50 70
2.2
1.9
1.8 1.9
2.4 2.3 2.0
_-..
-
60 50 50
7
6 30
Undisturbed Heat flow,. ficak/cm2 . set period (month)
TABLE
II
Geothermal
gradients
Well no.
Depth interval
Kalol-58
Kalol-2
in various .----
7
in Cambay
Temperature gradient (“C/km)
560675 675875 875 9’75 975 - 1075 1075 - 1200
37.2 + 48.4 2 31.12 40.0 + 59.1 z
560740 740900 920 - 1020 1020 - 1140 1140 - 1220
39.1 48.7 30.1; 39.7 59.0
525 800 900
2.3 0.8 3.4 2.0 2.0
2 1.5 + 1.3 2.2 7 0.8 z 5.7
43.3 + 2.1 61.8 T 1.0 48.7 5 1.2
500 775 775900 900 - 1050 1075 - 1150
48.4 51.3 63.9 70.7
t + 7 z
0.7 2.7 2.0 2.0
Cambay-
450600 625775 875975 975 - 1125
53.3 48.8 74.9 65.7
+ + t z
0.7 1.0 8.0 1.7
Cambay-
450625 625750 775850 850 - 1100
50.0 44.5 46.9 63.9
2 2 + z
0.8 1.8 6.0 0.8
Cambayi33
525 700 700850 1025 - 1175
46.8 + 0.6 42.5 7 0.9 52.2 1; 1.3
Anklesvar-2
750 - 1200
47.3 f 0.3
350900 900 - 1075 1075 - 1175
41.9 + 0.9 53.17 1.9 38.9 z 3.0
525 675 6?5800 800 - 1125
32.3 + 1.1 33.5 2 2.2 41.6 2 1.9
Kathana-9
Anklaevar-2
Anklesvar-170
THERMAL
8
Basin
(m)
400550800-
Nawagam-32
wells
CONDUCTIVITY
MEASUREMENTS
It poses a major problem to make an accurate estimate of the mean thermal conductivity over depth intervals comprising a varied sequence of sediments. Core samples in wells drilled for oil prospecting are very scarce. No core samples could be obtained from the wells in which observations were made. However, samples of the major rock types met in the Cambay 152
Te~~~n~physi~s,
10 (1970) 147-163
Basin were obtained from the cores available from various other Wells in the basin at Kalol, Cambay and Ankfesvar. Thermal conductivity of 25 samples of various rock types from the Cambay Basin have been reported in a previous publication (Verma et al., 1968). Heat flow evaluated for Kalol wells based on the thermal conductivity data reported earlier, showed certain discrepancies, and so attempts Were made to obtain more core samples. Thermal conductivity of eight samples of silty shales and silty sandy clay stones from various depths from the Kalol oil field, three samples of limestone, four samples of siltstone and two more samples of sandstone have been measured. The conductivities were first determined in an oven-dry state, then after saturation with kerosene oil under vacuum. For the few samples which did not disintegrate in water, the conductivity values were also determined after saturation with water under vacuum. The values obtained for various types of sediments are given in Table III. The “water saturated” conductivity (K sw) of samples of clay stone, shales and silty sandy claystone or shales,coufd not be measured as these disintegrated easily in water. The ratio of “water saturatedi’~ond~~t~vity (ICsw) to “dry” conductivity (Ksa) could only be obtained for a few samples. This ratio varies from 1.32 to 1.59 for shales, clayey shales and sandstones, with an average of about 1.50. The ratio is much higher for Samples of fossiliferous limestone and the siltstones. However a value of 1.50 for K,,/K sa was adopted for estimating the TABLE II1 Thermal conductivity of core samples from Cambay Basin Sample no.
Depth from surface
Rock type
(m)
Thermal Conductivity mcal./cm . sec.V
Ratio
air saturated
oil saturated
water saturated
(Ksa)
($0)
%w)
K so K
Ksw -....-_ K sa
sa
K-13A
1,044
silty shale
2.99
-
-
-
-
K-13B K-11
1,044 1,200
silty shale silty shale
2.96 3.37
4.08 -
-
1.38 -
-
K-l K-14 K-14 K-8
1,055 1,040 1,040 695
silty shale silty shale silty shale silty clay
3.24 2.78 3.17 2.46
3.71 3.84 3.00
_
1.14 1.39 -
-
K-18 K-4 K-2
1,343 900 929
2.86 4.12 4.28
3.48 4.60 4.69
6.45 6.54
1.22 1.22 1.15 1.09
1,56 1.53
C-24 A-11 A-3
1,295 1,136 670
3.62 2.85
5.21 5.34 4.90
-
-
4.20 3.23
1.16 1.13
1.47 1.72
A-14 A-13 c-14 K-2
807 1,007 1,298 930
silty clay sandstone mottled sandy siltstone limestone limestone fossiliferous limestone siltstone siltstone siltstone siltstone
2.52 3.12 3.17 4.03
3.30 3.89 4.08 4.41
5.02 5.50 5.54 6.50
1.31 1.25 1.29 1.09
1.93 1.76 1.75 1.61
Tectonophysics,
10 (1970) 147-163
153
3.2320.54 3.21tO .62 4.15+0.x
3 4 4
Limestone Siltstone Sandstones
* Standard error of the mean.
Clayey or shaly ~andstonea
3.62+0.40
2.98+0.29 -
8
4.70~0.11
3.9220.46
3.7120.68
2.5720.1
2.02+0.1
10
Shales and clayey shales Silty or sandy shales or silty sandy claystones from Kalol oilfield
Clay stone8
2.6920.2*
. sece”C)
2.03~0.1*
(mcal./cm
8
Mean conductivity oil saturated
Number of samples air saturated
Rock type
Summary of the conductivity values measured and adopted
TABLE IV
5.520.3’
6.5+0.%4
5.620.62
5.220.23
4.5+0.45
3.0+0.2
3.0~0.2*
_-__
Water saturated conductivity values adopted
“water saturated” conductivity of the samples which distintegrated in water, from the )‘dry” conductivity values. The “water saturated” conductivity values finaIly adopted for various rock types are given in Table IV. For clayey and shaly sandstones, which are a few metres thick in the stratigraphic columns, a value of 5.520.3 mcal./cm.sec.°C, intermediate between sandstones and silty shales was adopted. No representative samples of this rock type could be obtained for conductivity studies.
EVALUATION OF HEAT FLOW
Heat flow values have been computed for all the seven wells fromplots of temperature against cumulative resistance (Hullard plot) over depth intervals for which the lithologic information was available. The llthologs were taken from the composite logs computed by the Oil and Natural Gas Commission, using S.P.,resistivity and cutting log data. The “water saturated” conductivity values were corrected for increase in temperature with depth as suggested by Joyner (1980). Using the corrected thermal conductivity values and the available lithologic information, the thermal resistance was calculated. The heat flow values for all the seven wells in different depth intervals are given in Table V. TABLE V
Heat flow data from different depth intervals for Kalol, Nawagam, Kathanaand Anklesvar wells Locality
Well no.
Kalol oilfield
K-58
975 550975 - 1,200
1.7 1.8
Kalol oilfield.
K-27
560 - 1,000 1,020 - 1,220
1.8 1.9
Depth interval (m)
Heat flow2 peal/cm . see
Nawagam oilfield
32
400-
900
1.9
Kathana oilfield
Kat- 9
725 525725 - 1,200
1.9 2.2
Ankievar oilfield
A-2 A-28
750 - 1,200 500 400500 - 1,175 700 - 1,200
1.7 1.5 1.65 1.5
A-170
KALOL OILFIELD Heat flow has been determined in two wells, Kalol 27 and 58. Well no. 27 is located in the northern part of the Kalol structure, which is an elongated asymmetrical anticline, the longer axis of which is trending NW-SE (Mathur and Evans, 1964). Kalol 58 is also located in the northern part of the structure and is about 11 km east-southeast of Kalol 27. TemperaTectonophysics, 10 (1970) 147-163
155
TEMPERATURE
'C
m
CLAY Or CLAYSTDNE
E.‘.1
SAND W SANOSTONE
m
SANDY CLAY
a
CLAYEY
m
SANDY SHALE
@
SHALY SAN0
I-_-I
SHALE
m
SILTY
lg
SHALY
SiLT
=
SILTY
SHALE
0
SWALY CLAY
SAND
SAND
Fig.3. Temperature profile, litholog and temperature vsI Z;Bi A’& curve, (Bullard plot) for Kalof wells where hZi is the depth interval and Ri is the thermal resistance, The top axis oF temperature is for well no. 27 md the bottom. temperature axis which is shifted to the right by Z°C is for Kalol well no. 58. ture profiles, the percentage of rock types and the BuUard plots for the two wells are shown in Fig.3, Dullard plots have been plotted from a depth of about 550 m. The plots for both the wells show a change in slope near about a depth of 9754,000 m. Therefore the heat flow has been calculated from the two different intervals separately for each well. The values are given in Table V.
The Nawagam anticline, which is located about 24 km south of &mad&ad, in shape, but has a general east-west trend. Well no. 32, in
is irregular 156
Tectonophy~lcs,10(1970)147-163
which the temperatures were measured, had a small flow of water from the perforated horizons below the depth of measurements. However, the temperature gradients do not seem to be much affected by the water movement. The Bullard plot for this well (Fig.4) is linear from 4X3-900 m and a heat flow value of 1.9 has been obtained. TEMPERATURE
“C
ROCK YYPE 1PERCENTAGE 1
Fig.4 Temperature well (no. 32),
profile,
m
CLAY Of CL&YSTOE
IX
SAND
m
SANDY
CLAY
a
SWALY
CLAY
CEil
SANDY
or
SAIIDST~NE
SHALE
litholog and Builard plot for the Nawagam
The field is on a NE-§W trending anticline, Beat flow by the Buflard technique has been determined in well no.9. The fiefd is situated east of the famous Cambay gasfield. The thermal conductivity value for silty sandy claystones or shales has been taken as 4.0 (Verma et al., 1968) since the field is very near to Cambay. An increase of heat flow has been noted from a depth of about 725m in this well. The heat flow values evaluated from the Bullard plot in the two intervals from 525-725 and 7254,200 m are 1.9 and 2.2 respectively, The Tectonophysics, 10 (1970) 147-163
157
TEMPERATURE 400
46
56 1
‘C
66 I
76 I
m
CLAY Or CLAYSTONE
=SAND
cn
8OOc
4CiOf-
E /
1g
+ I
moo-
BOO-
t
x
\\
86 i
OT
m
SANDY
m
CLAYEY
m
SHALY
SANDSTONE
CLAY SAND
SAN0
mSHALE
es P
“E I-G 4 1200-
1200-
tii
1400
1600 i
i
Fig.% Temperature no. 9 Kathana oilfield.
v6. depth curve, lithofog and Builard plot for well
temperature profile, the percentage plot are shown in Fig.% CAMBAY
of rock formations
and the Bullard
GASFIELD
The Cambay (or Lunej) field, 9 km northnorthwest of the town of Cambay and about 80 km west of Baroda, was the first commercially productive field to be found in the Cambay ~Basin. The field is on a north-south trending anticline with a fault on the east flank and minor faults on the west. (~a~ur and Evans, 1984). Heat flow values have been evaluated from temperature rne~ur~m~t~ af tsrate wells C-10 C-38 and C-33. The da.& has been reported earlier (Verma et a&.., 1988h The values from the three wells from the deeper intervals (below 850 m) are given in Table 1. The heat flow values are highest here and show a difference between C-10 and 38 on one side and C-33 on the other side. Temperatures are also lower for C-33 a8 in C-10 and C-36. compared to those at corraqon$tng Wells C-10 and C-38 we locatednear the top of the Lunej structure, while well no. 33 is located~ on the eastern flank of the StJWCttire. 158
Tectonophyafcs, 10 (1970) 147-163
ANKLESVAR
QILFIELD
The Anklesvar fieid, 80 km SSW of Baroda, lies on an elongated dome trending ~~-W~W~ and is associated with faulting in the trap on its southern flank. The anticline has a gentle northern limb-and a steeper southern limb, having a dip exceeding 20° and lacally reaching 40”-50°. Temperature observatrons were taken in three wells: A-28, A-170 and A-2 here, The temperature disturbance in weli no, A-2 is likely to be the result of artesian ground water moving up through the well and Leaving through horizons near a depth of 350 m. Temperature data of well no. A-110 also indicate a stow upward movement of water through the well,, Temperature vs. depth Curves for the three wells are shown in Fig.6, along wrth interpreted lithologs and temperature gradients each at 25m intervals. The heat flux has been computed by the Bullard technique for four different depth intervals in these wells. The heat flux varies from 1.5 to I,7 [Table VI_ The mean value for the Ankleswar we&s is l&2, The Buifard plots for the three wells are depicted in Fig,?,
l250/-
-”
1:.
Fig&. Temperature Anklesvar wells. Tectonophysics, 10 (1970)
vs, depth curves lithofogy and gradients in
147-J63,
Fig,‘l.
Bullard plots for Anklesvar wells.
DISCUSSION OF RESULTS
The heat flow computed for Kathana oil well no.0 shows an increase below a depth of about 725 m. A similar phenomenon was observed in all the three wells of the Cambay gasfieid, where the heat flow was particularfy lower above a depth of about 859 m than below this depth, The mean heat flow as calculated for all the three wells above a depth of 859 m was 1.76, and 2.3 in the depth interval 8594,200 m. The various reasons for the differences in heat fiow between the upper and the lower levels were discussed in our earlier paper of the Cambay Basin (Verma et al, 1968) and the same reasons probably hold good in the present case. The heat flow values from the lower intervals have been taken as the representative ones for the respective fields. The final values including the earlier reported values for Cambay gasfield are given in Table I. The error in the heat flow has been mainly due to the uncertainty in the conductivity, Keeping in view the various experimental uncer~inties, the error limits in the heat flow measurements may be taken as - 10%. On this basis, it appears that the heat flux in the Cambay Basin is higher than the world average of 1.05 (Rorai and Simmons, 1989) in the area north of the .Sahisagar River between Cambay and Kalof, and moderate at Anhlesvar in the southern part of the basin. All the values are considerably higher than the average heat flow for the shield areas. The region of high heat flow in tne basin now appears to extend from Cambay to Kathana and then further north to Nawagam and probabiy to the 160
Tec~~~~sics,
10 f1970)
147-163
Kalol area as well. A composite Bouguer anomaiy map of Gujarat area has been presented by Sen Gupta (1967). The most interesting feature of the gravity map is the prevalence of a gravity high along the axis of the basin, running almost through the central part of the Cambay Basin in the area north of the Mahisagar River. Several uplifts, which are situated near the central part of the basin and are associated with horsts in the trap, also exist in this part of the basin. A prominent gravity anomaly of the magnitude of 8 mgal, covering a wide area of about 500 sq. miles centred near Borsad about 25 km northeast of Cambay, has been attributed by Negi (1~52) to a probable change in density within the earth% crust. A strong magnetic high (500 gammas), slightly displaced to the southeast, has also been reported in this area. This magnetic high is probably of deep-seated origin (Ramachandra Rao, 1958). The high heat flow (2.3) near Cambay has been shown to be due toan igneous intrusive body, Based on gravity and heat flow data, it was deduced that the causative body with an area of about 50 x 50 km2 might have intruded at a depth of about 10 km under the Cambay Basin during Pliocene toMiocene times (Verma et al., 1968). These observations and the existence of high heat flux in the northern part of the basin indicate the probability of the extension of the above mentioned intrusive body within the crust underneath the Cambay Basin, from Cambay to Kalol, The gravity high is absent in the southern part of the basin and the only heat flow which could be obtained in this part of the basin at Ankleswar is also relatively lower. However, for a better understanding, more heat flow data is necessary in the southern part of the basin. As mentioned earlier, the reversals of block tilting have been a conspicuous feature of the basin throughout its Cenozoic history. The CambayKathana region has been a central relatively high zone between two areas of active subsidence to north and south, The highest heat flux is observed in this central zone.
CORRELATKON WITH BASEMENT TOPOGRAPHY
The decreasing trend in heat flow values from Cambay to Kalol appears to bear a correlation with basement topography, Avasthi et al., (1969) have prepared a map of the trap surface in the Cambay Basin (reTABLE VI Basement depths and average heat flux at different fields in the northern part of the Cambay Basin Locality
Cambay gasfield
Kathana oilfield Nawagam oilfield I&lo1 oilfield
Basement depth tm)
Average heat flow & cal./cm2 see
2,200 2,400 2,700 3,000-3,200
2.3 2.2 1.9 1.85
Tectonophysics, 10 (1970) 147-163
161
produced here in Fig.2). The depths to the trap surface for the different fields north of Mahisagar River have been given in Table VI. The table clearly indicates a decrease of heat flow with the increase in the depth of the basement complex. CONCLUSIONS A fe~gs~s~aI ~on~I~si~s rnzq be drawn: (1) The area north of the MahiSagar River of the Cambay &&sinfrom Cambay to Kalol seems to be a region of high heat flow. The association of high heat flow with gravity “high” in this part of the basin indicates the likelihood of a buried igneous ~ntrusi~ of Pliocene to JlEioceneage in the crust nnder~eath the basin, which probably &ends from Cambay towards Kalol. (2)The Cambay-Kathana region, which was a central relatiyely high zone separating two areas of active .subsiden$e during the Eocene Epoch, appears to be the area of h&he& heat flux in the Cambay Basin, (3) There seems to be a correlation between the heat flux ar!knd the structural relief of the basement, Geothermal gradients and heat flow are higher where the depth of the basement complex is less and vice versa. The above ~orre~ti~ also indicates the fragility that oil and gas stru~~re~ are also reflected in the thermal f&Id of the area,
We are very grateful to Shri I&S. Negi, Member (Rxploration), Oil and Natural Gas Commission for according permission to log these wells and to publish the information on the lithology, We are thankful to Shri C.K& Sastry, Chief Geologist, RIessrs S.V. ~ss~~ac~ar~ ~~*~~~~s~~~~ S, Aditya, N& Prabhu, Rr, IL Narayanan and to several. other officers of O.N.G.C., for extending all facilities and cooperation in carrying out these investigations. We wish to express our thanks to I)r, Hart Narain, Director, Native ~o~hysi~~ Research Institute, for his keen interest, guidance and support ta the ~rog~rnrn~ of heat Bow stftdies in various parts, of India and for permission to publish this paper.
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