Techniques And Application Of Geochronology In Hydrocarbon Exploration
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TECHNIQUES AND APPLICATION OF GEOCHRONOLOGY IN HYDROCARBON EXPLORATION
Dr. S.S. RATHORE GEOCHRONOLOGY LABORATORY GEOLOGY DIVISION KDMIPE, ONGC DEHRADUN
GEOCHRONOLOGICAL METHODS USED Rb-Sr DATING METHOD K-Ar DATING METHOD 40 Ar-39 Ar DATING METHOD Sr ISOTOPE STRATIGRAPHY
Rb-Sr DATING METHOD Principle of Radiometric Dating Method Radioactive Decay
Rubidium has two naturally occurring isotopes i.e. 85 Rb and 87 Rb with isotopic abundances of 72.1654% and 27.8346%, respectively
Strontium has four naturally occurring isotopes (88 Sr, 87 Sr, 86 Sr, 84 Sr), all of which are stable. Their isotopic abundances are 82.53%, 7.04%, 9.87% and 0.56%, respectively
The average concentrations of Rubidium, Potassium, Strontium and Calcium in igneous and sedimentary rocks that figure in isotope age dating are given in Table 1.
The rubidium concentration of common igneous and sedimentary rocks range from less than a few ppm (ultrabasic rocks and carbonates) to more than 170 ppm in low calcium granitic rocks.
The concentration of strontium range from a few ppm (ultrabasic rock) to about 500 ppm in basaltic rocks and reach very high values in carbonates rocks (upto 2000 ppm or more).
Table: Average concentrations of Rubidium, Potassium, Strontium and Calcium in igneous and sedimentary rocks Rock Type 1. Ultrabasic 2. Basaltic 3. High Ca granitic 4. Low Ca granitic 5. Syenite 6. Shale 7. Sandstone 8. Carbonate 9. Deep sea carbonate 10. Deep sea clay
Rb ppm K ppm
Sr ppm
Ca ppm
0.2 30 110 170 110 140 60 3 10 110
1 465 440 100 200 300 20 610 2000 180
25000 76000 25300 5100 18000 22100 39100 302300 312400 29000
40 8300 25200 42000 48000 26600 10700 2700 2900 25000
Rb-Sr Clock β
87
Rb
87
Sr
Half life: 4.88 x 1010 years.
Basic Radioactive equation: D = P (eλ t – 1)
The growth of radiogenic 87 Sr* in a Rb rich mineral or rock can be described by an equation derivable from the law of radioactivity: 87 Sr* = 87 Rb (eλ t 1)
The total number of atoms of 87 Sr present in a mineral/rock whose age is t years is obtained from equation : 87 Sr = 87 Sri + 87 Sr* 87 Sr = 87 Sri +87 Rb(eλ t1) This equation can be modified by dividing each term by the number of 86 Sr atom which is constant because this isotope is stable and is not produced by decay of a naturally occurring isotope of another element. Thus, we obtain : 87
Sr/86 Sr = (87 Sr/86 Sr)i + (87 Rb/86 Sr) x (eλ t 1)
This equation is the basis for age determination by the RbSr method. The age of the rocks/minerals are determined by isochron method
87
Sr/86 Sr
Rb-Sr Isochron
t
=0
87
t=0
Rb/86 Sr
K-Ar METHOD Introduction
The KAr isotopic method is one of the most versatile and widely applied of the various geochronometers available for dating rocks.
In part this is because potassium is the eighth most abundant element in the Earth's continental crust, comprising about 1 wt %.
Minerals in which potassium is an essential element are fairly common in nature. Therefore, the KAr method, in principle, is applicable to many rocks and individual minerals (e.g., Feldspars, Micas etc.).
Another reason for its popularity as a dating method is that, with current techniques, there is a very high sensitivity for detection of radiogenic argon.
In favourable circumstances, the technique can be applied to igneous/sedimentary rocks as young as a few thousand years, with no older limit in terms of the physical measurements.
PRINCIPLE Potassium in nature consists of three isotopes, viz. 39 K, 40 K and 41 K with abundances of 93.2581%, 0.01167% and 6.7302%, respectively.
Of these 40 K is radioactive, decaying to 40 Ar (half life 1.25 X 109 years) by electron capture and to 40 Ca by β 1 decay. The branch yielding radioactive argon (40 Ar*) as daughter product provides the basis for the KAr dating technique through its accumulation over geological time.
Basic Assumptions As with all isotopic dating methods, there are number of assumptions that must be fulfilled for a KAr age to be related to any geological event. The most important assumptions are: (a) The radiogenic argon measured in a sample is produced by in-situ decay of 40 K in the interval since the rock crystallized or is recrystallized. (b) Corrections can be made for non-radiogenic 40 Ar present in the rock being dated. For terrestrial rocks the assumption generally is made that all such argon is atmospheric in composition with (40 Ar/36 Ar)atm = 295.5. (c) Potassium is homogenously distributed in the sample. (d) The sample must have remained in a closed system since the event being dated. Thus, there should have been no loss or gain of potassium or 40 Ar, other than by radiogenic decay of 40 K.
40
Ar‑39 Ar DATING PRINCIPLE
To overcome some of the shortcomings of the K‑Ar method viz. potassium inhomogeneity in the sample, the loss of argon due to thermal event or the presence of excess argon etc., which bring uncertainty in the age interpretation, a new method i.e. 40 Ar‑39 Ar method was developed, which is an analytical conversion of the conventional K‑Ar method.
In the 40 Ar‑39 Ar method, the sample to be dated is first irradiated in a nuclear reactor to transform a portion of the 39 K to 39 Ar by the fast neutron reaction i.e. 39 K(n,p)39 Ar. After irradiation, the sample is placed in an ultra‑high vacuum system and the argon extracted from it by fusion is purified and analyzed isotopically in a mass spectrometer.
AGE EQUATION If 40 Ar* is the radiognic daughter accumulated by spontaneous decay of 40 K in a rock of age 't', then 40 Ar* = (λ e/λ ) x 40 K (eλ t1) (1) The amount of 39 Ar produced, due to irradiation with fast neutrons, is given by: 39
ArK = 39 K ∆ t ∫ φ (ε ) σ (ε ) d(ε ) (2)
where ∆ t is the irradiation time, φ (ε ) is the neutron flux at energy ε and σ (ε ) is the neutron capture cross section at energy ε for 39 K(n,p)39 Ar reaction. The integration is for over all energies of the incident neutrons. Combining eqn. (1) and (2) will give: 40
Ar*
40
= 39
ArK
K (eλ t‑1)
(3) 39 K ∆ t ∫ φ (ε ) σ (ε ) d(ε )
It is convenient to define a dimensionless irradiation parameter "J" as follows: 39
K∆ t∫ φ (ε ) σ (ε ) d(ε )
J =
(4) 40
K
substituting eqn. (4) in (3) gives: Ar* 39 ArK 40
e λ t ‑1 J
=
which, upon rearrangement allows calculation of the age "t" of the sample as follows: t=
1 λ
ln (1 + J 40 Ar*/39 ArK)
where 40 Ar/39 Ar = the ratio of radiogenic neutron irradiation of the sample.
40
5
Ar to the potassium derived
39
Ar by
From eqn. (5) age of the sample can be calculated provided the irradiation parameter "J" is known, which is dependent upon the duration of the irradiation, the neutron flux and the reaction cross section.
Because of the difficulties encountered in accurately determining the relevant integrated fast neutron fluence that a sample has received, a monitor or standard sample, whose age is precisely known, is irradiated along with the unknown samples to monitor the fluence.
In the case of monitor sample, rearrangement of eqn. (5) will give: J =
(eλ tm ‑1) (40 Ar*/39 ArK)m
(6)
Since age of the monitor sample (tm) is known, so by simply measuring the (40 Ar*/39 ArK)m in the gas extracted from the monitor sample after irradiation, the parameter 'J' can be determined.
This value of J is then used in eqn. (5), together with the (40 Ar*/39 ArK) ratio measured on the unknown sample irradiated at the same time, to calculate its age "t".
Advantage of Ar-Ar Dating Method
Graphical depiction of analytical data.
The sample can be heated in incremental temperature steps starting from about 500 °C to fusion.
The ratio 40 Ar/39 Ar obtained at each step is plotted against the temperature thus a series of apparent ages can be determined on a single sample.
This approach known as the step heating technique provides a wealth of information.
The formation or crystallization ages of igneous rocks and depositional/provenance ages can be construed very precisely.
The most important application of step heating 40 Ar39 Ar method lies in elucidating the thermal history of the rocks visàvis that of the sedimentary basins quantitatively by evaluating the pattern of the age spectrum diagrams.
An example of excellent plateau consisting of 100% 39Ar released is shown in Fig. a (Rathore et al. 1996)
The plateau age of 64.1±0.6 Ma obtained from syenite of the Mundwara Igneous Complex, Rajasthan has been interpreted as the emplacement age of the syenite.
Fig. b illustrates simple thermal heating of hornblende from the Rameka Gabbro, New Zealand which was emplaced around 340 Ma ago (Shown by plateau of higher temperature steps) and was reheated about 114 Ma ago due to emplacement of a large scale intrusive body.
Applications
The Geochronology laboratory of the institute has KAr, RbSr, Sr isotope stratigraphy and ArAr dating facilities.
In case of igneous rocks absolute age of emplacement/formation and their petrogenesis.
In case of sedimentary rocks these methods can be used to know timing of diagenesis, sedimentation and carbonate precipitation.
Timing of metamorphism and illustration of thermal history of metamorphic rocks.
However, in petroliferous basins the KAr technique can be used to date the timing of petroleum migration and gas emplacement in the reservoirs.
Similarly, the ArAr isotope analysis of detrital Kfeldspar can provide thermochronological information in the temperature range of petroleum maturation and therefore the technique can provide a useful hydrocarbon exploration tool.
Timing of Petroleum Migration and Gas Emplacement in the Reservoirs
The KAr dating has been applied on diagenetic illites separated from Permian Rotliegende sandstone of Southern North Sea Region, Netherlands
Illite is a common diagenetic phase.
KAr ages of very fine grained illite should indicate the time, the illite forming process ceases.
Age of the finest fraction in the gas zone of the reservoir to indicate the time at which illite formation ceased as a result of gas emplacement and pore fluid displacement in the rock.
Illite growth and gas emplacement overlapped in time and that illite growth ceased when gas displaced most of the pore water.
Table: Mineralogy and KAr isotopic data of the clay fractions from the Rotliegende sandstone of Southern North Sea, Netherlands Sample
Fraction Size
Illite (%)
Chlorite (%)
K2O (%)
Age (Ma)
A 1380
< 0.2
65
35
4.1
172±5
0.20.5
60
40
4.2
209±5
0.52
50
50
3.4
236±7
<0.2
60
40
5.1
172±4
0.20.5
45
55
4.3
218±6
0.52
40
60
3.7
238±6
<0.2
65
35
6.3
167±7
0.20.5
55
45
5.2
205±5
0.52
55
45
3.5
245±6
A 1712
A 1434
Further, the SmNd isotope system, the facility that we are going to imbibe in near future, is a potential tool in petroleum exploration as it can be effectively used to date the provenence of sandstone/ shale reservoirs.
The system can also be used to correlate the reservoir lithologies within the petroleum producing basin and oil to source rock correlation even in adverse geological milieu where elevated temperatures and pressures during diagenesis have obliterated the organic biomarkers.
Application Dating of Basement Rocks of Western Offshore of India Geological Setting
The Mumbai Offshore Basin (MOB) covers an area of about 1,20, 000 sq. km and is limited to its north by the Saurashtra Arch and to south by the Vengurla Arch (Fig. 1).
The tectonic framework, stratigraphy, structural features and the depositional history of the MOB have been studied in detail by various authors (Rao and Talukdar 1980; Biswas 1987; Biswas and Deshpande 1983; Basu et al 1982).
However, little information is available about the nature and age of the basement, which plays an important role in the evolution of structures like horsts, grabens, rifts and regional faults and, therefore, controls the pattern of sedimentation and also at times the thickness of the formation within the basin.
The Deccan trap forms the floor of this basin with an exception of a few Precambrian (?) inliers.
The Precambrian rocks in different parts of the basin are of varied lithology, comprising biotite gneiss/chlorite gneiss, schist, syenite, granite/granodiorite etc.
N
North Tapti
SAURASHTRA COAST
Mid Tapti 0
20
40
South Tapti
Kms
DAMAN Saurashtra
Dahanu
Diu East
DAHANU
Tarapur
B-46-1
°
Bombay High
° D-12-1
TARAPUR MAHIM
°F
Bombay High East
° BH-36 ° SY-5
Panna Panna East Bassien
°B-192-5
°
Heera
HBM-1
MUMBAI
ALIBAG
RATNA
0 20 0 m 0 10 m 0 20 m
0 36 m
RATNAGIRI
Fig. Location map of studied wells of Mumbai Offshore
Well HBM-1 (Granulitic Basement)
Six samples from three successive cores viz. CC2 (1349.41356.65 m), CC3 (1356.5 1364.1 m) and CC4 (1364.11369.5 m) were analyzed for RbSr isotopic studies The studied samples have yielded RbSr isochron age of 502±25 Ma.
Two samples were dated by KAr method which had yielded a mean KAr age of 506±12 Ma.
The age of 500 Ma obtained by different dating methods has been interpreted as to represent the age of secondary thermal disturbance.
The isotopic ages in the range of 500550 Ma have been reported by various workers from the PanAfrican Zone (Fig.) extending from the Arabian Peninsula and Eastern Africa through Madagascar, southern India and Sri Lanka to East Antarctica.
Fig.
PanAfrican zone from the Arabian Peninsula and Eastern Africa covering Madagascar, southern India, Sri Lanka and East Antarctica [After Rogers et al. 1995]
The PanAfrican ages in southern India have mostly been reported form south of PalghatCauvery Shear Zone.
The secondary thermal disturbance around 500 Ma observed in the granulitic basement of the well HBM1 temporally coincides with the wide spread Neoproterozoic (PanAfrican) thermotectonic event.
The study further suggests that this part of the western offshore and southern granulite terrain probably shared a common tectonothermal history.
In the light of this study it was suggested that the PanAfrican Zone, which hitherto was thought to be confined to the western part (presently the southern part) of the Indian subcontinent, extended further eastward.
The paper published in Jour. Geol. Soc. India (2000), V. 56, 365372.
Well BH-36 (Granitic Basement)
Eight samples from two basement cores (CC5 and CC6) were analyzed for RbSr isotopic studies
The studied samples have yielded RbSr isochron age of 1446±67 Ma 0 .7 5 5 0 0 .7 5 0 0
535
WELL BH36 (Granitic Basement)
not included in regression
0 .7 4 5 0
536
0 .7 3 5 0
537
0 .7 3 0 0
511
510
538
87
Sr/86 Sr
0 .7 4 0 0
509
512
0 .7 2 5 0
Age = 1446 ± 67 Ma (2σ ) Sri = 0.7062 ± 0.0012 (2σ ) MSWD = 2.27
0 .7 2 0 0 0 .7 1 5 0 0 .7 1 0 0 0 .7 0 5 0 0
0 .2
0 .4
0 .6
0 .8
1
1 .2 87
Rb/86 Sr
1 .4
1 .6
1 .8
2
2 .2
2 .4
Two biotite fractions were also analyzed for RbSr isotopic studies which had yielded an isochron age of 1385±21 Ma.
1.700 1.600
1.400
Bio2
1.300 1.200 1.100 1.000
Age = 1385 ± 21 Ma (2σ ) Sri = 0.7061 ± 0.0012 (2σ ) MSWD = 0.16
0.900 0.800
WR
1 .7 0 0
0.700 5
10
15
20
25
87
30
35
40
45
1 .5 0 0
Age = 1394 ± 25 Ma (2σ ) Sr = 0.70712 ± 0.00051 (2σ ) i 1 .4 0 0 MSWD = 1.9
50
Rb/ Sr 86
Sr/86 Sr
0
Bio1
WELL BH36 (Whole Rocks + Biotite)
1 .6 0 0
1 .3 0 0
Bio2
1 .2 0 0 0 .7 5 5 0
87
Sr/86 Sr
1.500
87
Bio1
WELL BH36 (Whole Rock + Biotite)
1 .1 0 0
0 .7 4 5 0
1 .0 0 0
Whole Rock
0 .7 3 5 0
0 .7 2 5 0
0 .9 0 0
Age = 1446 ± 67 Ma (2σ ) Sri = 0.7062 ± 0.0012 (2σ ) MSWD = 2.27
0 .7 1 5 0
0 .8 0 0
WR
0 .7 0 5 0 0
0 .7 0 0 0
5
10
15
20
0 .4
25 87
Rb/86 Sr
30
0 .8
1 .2
35
1 .6
40
2
2 .4
45
50
Five biotites separated from different samples were also dated by KAr method to see the effect of thermal heating if any.
The analyzed samples have yielded concordant KAr ages with a mean age of 1438±19 Ma which is indistinguishable from the whole rock RbSr isochron age. Table
Analytical data and calculated KAr ages of biotite separates from basement rocks of well BH36
Sl. No. Sample Details
K (wt. %)
Total 40 Ar Rad 40 Ar (× 10-6 cc STP . g-1 )
Age (±2σ ) Ma
1
CC5B9T
6.80
603.858
589.671
1452±42
2
CC6B2T (4050 mesh)
7.94
735.794
692.654
1458±43
3
CC6B2T (4050 mesh)d
7.94
665.735
647.496
1392±40
4
CC6B2T (5070 meah)
7.68
682.939
674.381
1465±43
5
CC6B3B
7.90
699.833
666.597
1425±40
6
GLO#
6.46
31.419
24.679
95.71±291
# Gluconite standard with a reported age of 95.03±1.11 Ma (Odin et al. 1982) d Duplicate analysis
The similarity in the whole rock and biotite ages obtained by different isotopic methods suggests that no thermal disturbance has occurred in these rocks after their emplacement around 14001450 Ma ago.
The present study provides the first evidence for the existence of an important Middle Proterozoic Magmatic event around 14001450 Ma on the western offshore of India which, hitherto, was thought to be confined to the eastern Ghats, Satpura and Delhi fold belt of India.
This finding may have an important bearing on the reconstruction of Proterozoic crustal evolution of Western Indian Shield.
The paper was published in Proc. Indian Acad. Sci. (Earth Planet. Sci.) 2004, vol. 113 (1), 2736.
Well B-46-1 (Granitic Basement)
Five granitic samples from basement core CC3 were analyzed for RbSr isotopic studies.
The analyzed samples yielded RbSr isochron age of 1855±76 Ma suggesting granitic emplacement around 1850 Ma ago. 1.2 1.15
Well B-46-1 (Granitic Basement)
1.1 1.05
CC3B1M
1
87
Sr/86 Sr
0.95
CC3B2B CC3B2T
0.9 0.85 0.8
CC3B2M
0.75
CC3B4B
0.7 0
2
4
6
8
10
12
14
16
Age = 1855±76 Ma (2σ ) Sr initial = 0.70137 ± 0.0097 MSWD = 1.81
87
Rb/86 Sr
Fig . RbSr isochron of granitic basement of well B461
One sample was studied by ArAr method which had yielded a plateau age of 607± 6 Ma. 16.00 12.00 8.00 4.00 0.00
39
Ar/37 Ar
CC3B2B
Apparent Age (Ma)
1600.0 1200.0 800.0
607±6 Ma (2σ )
400.0 0.0
0
20
60 80 100 40 39 Cumulative yield of Ar (%)
Fig. 40 Ar-39 Ar age spectrum diagram of granitic basement (CC3B2B) of well B-46-1.
The basement samples studied by different methods suggests that the granitic rocks in this part of the basin were emplaced at around 1850 Ma ago and were thermally reheated around 600 Ma ago.
Well B-192-5 (Granitic Basement)
Four granitic samples from CC5 were analyzed for RbSr isotopic composition The studied samples have yielded RbSr isochron age of 1476±57 Ma. 1.2 1.15 1.1
Well B-192-5 Granitic Basement
1.05 1 0.95 0.9
Age = 1476± 57 Ma (2σ Sr 0.0098 (2σ ) ?) i )= 0.7306± MSWD = 1.48
0.85 0.8 0.75 0.7 0
2
4
6
8
10
12
14
16
18
Fig. 5. Rb/Sr Isochron of Granitic Basement from B-
20
This age has been interpreted as the time of secondary isotopic reequilibration coinciding with the PanAfrican thermotectonic event as reported from the well HBM1 of Heera field.
1600.0
39
0.80 0.60 0.40 0.20 0.00
Ar/37 Ar
Two samples were studied for ArAr dating Both the samples have yielded good concordant plateaus with a mean plateau age of 548±3 Ma.
Apparent Age (Ma)
Ar/37 Ar 39
Apparent Age (Ma)
1200.0 800.0
Age=540±2 Ma
400.0 0.0 0
100 20 60 80 40 cumulative yield of 39 Ar (%)
1.60 1.20 0.80 0.40 0.00 1600.0 1200.0 800.0
Age=556±4 Ma
400.0 0.0
0
20 40 60 80 cumulative yield of 39Ar (%) . 40 Ar39 Ar age spectra of samples from well B1925
100
Well SY-5 (Schistose Basement)
Five basement samples from four cores (CC5, 6, 9 and 11) were analyzed for RbSr studies.
The studied samples have yielded RbSr isochron age of 1465±40 Ma which has been interpreted as the time of metamorphism in the in the schistose basement of well SY5 around 14001450 Ma ago. 0.765
0.76
Not included in regression
Well SY-5 (Schistose Basement)
0.755
0.75
0.745
CC11B4T
0.74
87
Sr/86 Sr
0.735
0.73
CC5B3B CC9B7T
0.725
0.72
0.715 0
0.2
0 .4
0 .6
0.8
1
1.2
CC9B4T 1.4
1.6
CC6B3M
Age = 1465±40 Ma (2σ ) Sr initial = 0.71823 ± 0.00048 MSWD = 0.36 87
Rb/86 Sr
Fig 5. RbSr isochron of schistose basement of well SY5
The Best Isochron Diagram (BID) has advantage of displaying the analytical errors and deviations from the isochron simultaneously.
The BID, therefore, allows to simultaneously visualize the experimental data and their analytical precision and to judge the quality of the linear fit and to appraise the accuracy of the age and initial ratio. (87 Sr/86 Sr) Measured
0.724
0.726
0.73
0.74
0.75 0.76
0.8
1
+∞ 1900 1800
0.722
1700 1600
0.72 CC6B3M 0.718
1500
CC5B3B
CC11B4T CC9B7 T CC9B4T
1400 1300
0.716
1200 1100
0.714
1000 0.712
0.1
0.2
0.4
0.6 0.8 1
2
4 6
Rb/86 Sr Fig. 6. Best Isochron diagram of Schistose Basement of Well B 87
000 +∞
Well D-12-1 (Schistose Basement)
Four basement samples from core CC8 were analyzed.
The studied samples have not yielded good isochron but the best fit line suggests metamorphic event around 13001400 Ma ago.
The age of formation of the basement is still older.
Studies on a few more samples are under way to constrain precisely the metamorphic event.
Summary of the isotopic studies carried out on basement rocks of Westerrn offshore of India Sl No. Well NameField
Rock Type
Age Ma (Method)
1.
HBM1
Heera
Granulite
502 ± 25 (RbSr) 505 ± 16 (KAr) 507 ± 17 (KAr)
2.
BH36
Mumbai High
Granite
1446 ± 67 (RbSr) Whole rock 1385 ± 21(RbSr) Mineral Isochron 1458 ± 43 (KAr) Biotite 1465 ± 43 (KAr) Biotite 1452 ± 42 (KAr) Biotite 1425 ± 40 (KAr) Biotite
4.
B461
WNW of Mumbai High
Granite
1855 ± 76 (RbSr) 607 ± 6 (ArAr)
5.
B1925
SW of Mumbai High
Granite
14001450 (RbSr) 500 – 550 (ArAr)
6.
SY5
S of Mumbai High
Schist
14001450 (RbSr)
7.
D121
SW of Mumbai High
Schist
13501400 (RbSr) Partially Studied
Conclusions
The geochronological studies carried out on the Precambrian basement rocks of diverse composition from the Western Offshore of India suggest a complex emplacement and post crystallization tectonothermal history.
The studies suggest that the granitic/granulitic/schistose basement rocks of Western Offshore were emplaced at around 17002100 Ma and 14001450 Ma ago and were further subjected to secondary thermal activities between 14001450 Ma and 500600 Ma ago.
While the older thermal activity, observed in the wells SY5 and D121 might be related to the emplacement of Middle Proterozoic granites in the wells like B1925 and BH36, the younger thermal event as observed in the well B461, B1925 and HBM1are related to famous wide spread PanAfrican thermotectonic event.
The absence of PanAfrican thermal imprint on basement rocks from Mumbai high field like BH36 coupled with possible linkage of Mumbai high with Middle Proterozoic Mobile Belt of Delhi, Satpura and eastern Ghats suggests that these fields may in all probability be representing different entities with different geological histories.
However, this enunciation, based on isotopic studies alone on limited wells may be far fetched and needs to be substantiated by further isotopic studies from a few more wells of different fields of western offshore of India as well as by other geophysical studies.
Thank You
Introduction The evolution of any sedimentary basin is closely interlinked with global tectonics. The rifting and collision of the lithospheric plates in the geological past have carved the outlines of the tectonic framework and basinal architecture of most of the prolific basins. Basin formation and evolution are generally associated with mantle related geothermal phenomena, which also control the process of generation of hydrocarbons. Since geochemical and isotopic characteristics of the basement rocks provide the finger prints of various paleogeological processes associated with basin formation and evolution, it is necessary to carry out in detail the multiisotopic studies of the basement rocks. Today I will be talking on geochronological studies undertaken on different basement rock types with a view to understand the Precambrian basement evolution of the western offshore of India.
Geological Setting
The Mumbai Offshore Basin (MOB) covers an area of about 1,20, 000 sq. km and is limited to its north by the Saurashtra Arch and to south by the Vengurla Arch (Fig. 1).
The tectonic framework, stratigraphy, structural features and the depositional history of the MOB have been studied in detail by various authors (Rao and Talukdar 1980; Biswas 1987; Biswas and Deshpande 1983; Basu et al 1982).
However, little information is available about the nature and age of the basement, which plays an important role in the evolution of structures like horsts, grabens, rifts and regional faults and, therefore, controls the pattern of sedimentation and also at times the thickness of the formation within the basin.
The Deccan trap forms the floor of this basin with an exception of a few Precambrian (?) inliers.
The Precambrian rocks in different parts of the basin are of varied lithology, comprising biotite gneiss, chlorite gneiss, schist, syenite, granite and granodiorite etc.
Dr. S.S. RATHORE GEOCHRONOLOGY LABORATORY GEOLOGY DIVISION KDMIPE, ONGC DEHRADUN
GEOCHRONOLOGICAL METHODS USED Rb-Sr DATING METHOD K-Ar DATING METHOD 40 Ar-39 Ar DATING METHOD Sr ISOTOPE STRATIGRAPHY
Rb-Sr DATING METHOD Principle of Radiometric Dating Method Radioactive Decay
Rubidium has two naturally occurring isotopes i.e. 85 Rb and 87 Rb with isotopic abundances of 72.1654% and 27.8346%, respectively
Strontium has four naturally occurring isotopes (88 Sr, 87 Sr, 86 Sr, 84 Sr), all of which are stable. Their isotopic abundances are 82.53%, 7.04%, 9.87% and 0.56%, respectively
The average concentrations of Rubidium, Potassium, Strontium and Calcium in igneous and sedimentary rocks that figure in isotope age dating are given in Table 1.
The rubidium concentration of common igneous and sedimentary rocks range from less than a few ppm (ultrabasic rocks and carbonates) to more than 170 ppm in low calcium granitic rocks.
The concentration of strontium range from a few ppm (ultrabasic rock) to about 500 ppm in basaltic rocks and reach very high values in carbonates rocks (upto 2000 ppm or more).
Table: Average concentrations of Rubidium, Potassium, Strontium and Calcium in igneous and sedimentary rocks Rock Type 1. Ultrabasic 2. Basaltic 3. High Ca granitic 4. Low Ca granitic 5. Syenite 6. Shale 7. Sandstone 8. Carbonate 9. Deep sea carbonate 10. Deep sea clay
Rb ppm K ppm
Sr ppm
Ca ppm
0.2 30 110 170 110 140 60 3 10 110
1 465 440 100 200 300 20 610 2000 180
25000 76000 25300 5100 18000 22100 39100 302300 312400 29000
40 8300 25200 42000 48000 26600 10700 2700 2900 25000
Rb-Sr Clock β
87
Rb
87
Sr
Half life: 4.88 x 1010 years.
Basic Radioactive equation: D = P (eλ t – 1)
The growth of radiogenic 87 Sr* in a Rb rich mineral or rock can be described by an equation derivable from the law of radioactivity: 87 Sr* = 87 Rb (eλ t 1)
The total number of atoms of 87 Sr present in a mineral/rock whose age is t years is obtained from equation : 87 Sr = 87 Sri + 87 Sr* 87 Sr = 87 Sri +87 Rb(eλ t1) This equation can be modified by dividing each term by the number of 86 Sr atom which is constant because this isotope is stable and is not produced by decay of a naturally occurring isotope of another element. Thus, we obtain : 87
Sr/86 Sr = (87 Sr/86 Sr)i + (87 Rb/86 Sr) x (eλ t 1)
This equation is the basis for age determination by the RbSr method. The age of the rocks/minerals are determined by isochron method
87
Sr/86 Sr
Rb-Sr Isochron
t
=0
87
t=0
Rb/86 Sr
K-Ar METHOD Introduction
The KAr isotopic method is one of the most versatile and widely applied of the various geochronometers available for dating rocks.
In part this is because potassium is the eighth most abundant element in the Earth's continental crust, comprising about 1 wt %.
Minerals in which potassium is an essential element are fairly common in nature. Therefore, the KAr method, in principle, is applicable to many rocks and individual minerals (e.g., Feldspars, Micas etc.).
Another reason for its popularity as a dating method is that, with current techniques, there is a very high sensitivity for detection of radiogenic argon.
In favourable circumstances, the technique can be applied to igneous/sedimentary rocks as young as a few thousand years, with no older limit in terms of the physical measurements.
PRINCIPLE Potassium in nature consists of three isotopes, viz. 39 K, 40 K and 41 K with abundances of 93.2581%, 0.01167% and 6.7302%, respectively.
Of these 40 K is radioactive, decaying to 40 Ar (half life 1.25 X 109 years) by electron capture and to 40 Ca by β 1 decay. The branch yielding radioactive argon (40 Ar*) as daughter product provides the basis for the KAr dating technique through its accumulation over geological time.
Basic Assumptions As with all isotopic dating methods, there are number of assumptions that must be fulfilled for a KAr age to be related to any geological event. The most important assumptions are: (a) The radiogenic argon measured in a sample is produced by in-situ decay of 40 K in the interval since the rock crystallized or is recrystallized. (b) Corrections can be made for non-radiogenic 40 Ar present in the rock being dated. For terrestrial rocks the assumption generally is made that all such argon is atmospheric in composition with (40 Ar/36 Ar)atm = 295.5. (c) Potassium is homogenously distributed in the sample. (d) The sample must have remained in a closed system since the event being dated. Thus, there should have been no loss or gain of potassium or 40 Ar, other than by radiogenic decay of 40 K.
40
Ar‑39 Ar DATING PRINCIPLE
To overcome some of the shortcomings of the K‑Ar method viz. potassium inhomogeneity in the sample, the loss of argon due to thermal event or the presence of excess argon etc., which bring uncertainty in the age interpretation, a new method i.e. 40 Ar‑39 Ar method was developed, which is an analytical conversion of the conventional K‑Ar method.
In the 40 Ar‑39 Ar method, the sample to be dated is first irradiated in a nuclear reactor to transform a portion of the 39 K to 39 Ar by the fast neutron reaction i.e. 39 K(n,p)39 Ar. After irradiation, the sample is placed in an ultra‑high vacuum system and the argon extracted from it by fusion is purified and analyzed isotopically in a mass spectrometer.
AGE EQUATION If 40 Ar* is the radiognic daughter accumulated by spontaneous decay of 40 K in a rock of age 't', then 40 Ar* = (λ e/λ ) x 40 K (eλ t1) (1) The amount of 39 Ar produced, due to irradiation with fast neutrons, is given by: 39
ArK = 39 K ∆ t ∫ φ (ε ) σ (ε ) d(ε ) (2)
where ∆ t is the irradiation time, φ (ε ) is the neutron flux at energy ε and σ (ε ) is the neutron capture cross section at energy ε for 39 K(n,p)39 Ar reaction. The integration is for over all energies of the incident neutrons. Combining eqn. (1) and (2) will give: 40
Ar*
40
= 39
ArK
K (eλ t‑1)
(3) 39 K ∆ t ∫ φ (ε ) σ (ε ) d(ε )
It is convenient to define a dimensionless irradiation parameter "J" as follows: 39
K∆ t∫ φ (ε ) σ (ε ) d(ε )
J =
(4) 40
K
substituting eqn. (4) in (3) gives: Ar* 39 ArK 40
e λ t ‑1 J
=
which, upon rearrangement allows calculation of the age "t" of the sample as follows: t=
1 λ
ln (1 + J 40 Ar*/39 ArK)
where 40 Ar/39 Ar = the ratio of radiogenic neutron irradiation of the sample.
40
5
Ar to the potassium derived
39
Ar by
From eqn. (5) age of the sample can be calculated provided the irradiation parameter "J" is known, which is dependent upon the duration of the irradiation, the neutron flux and the reaction cross section.
Because of the difficulties encountered in accurately determining the relevant integrated fast neutron fluence that a sample has received, a monitor or standard sample, whose age is precisely known, is irradiated along with the unknown samples to monitor the fluence.
In the case of monitor sample, rearrangement of eqn. (5) will give: J =
(eλ tm ‑1) (40 Ar*/39 ArK)m
(6)
Since age of the monitor sample (tm) is known, so by simply measuring the (40 Ar*/39 ArK)m in the gas extracted from the monitor sample after irradiation, the parameter 'J' can be determined.
This value of J is then used in eqn. (5), together with the (40 Ar*/39 ArK) ratio measured on the unknown sample irradiated at the same time, to calculate its age "t".
Advantage of Ar-Ar Dating Method
Graphical depiction of analytical data.
The sample can be heated in incremental temperature steps starting from about 500 °C to fusion.
The ratio 40 Ar/39 Ar obtained at each step is plotted against the temperature thus a series of apparent ages can be determined on a single sample.
This approach known as the step heating technique provides a wealth of information.
The formation or crystallization ages of igneous rocks and depositional/provenance ages can be construed very precisely.
The most important application of step heating 40 Ar39 Ar method lies in elucidating the thermal history of the rocks visàvis that of the sedimentary basins quantitatively by evaluating the pattern of the age spectrum diagrams.
An example of excellent plateau consisting of 100% 39Ar released is shown in Fig. a (Rathore et al. 1996)
The plateau age of 64.1±0.6 Ma obtained from syenite of the Mundwara Igneous Complex, Rajasthan has been interpreted as the emplacement age of the syenite.
Fig. b illustrates simple thermal heating of hornblende from the Rameka Gabbro, New Zealand which was emplaced around 340 Ma ago (Shown by plateau of higher temperature steps) and was reheated about 114 Ma ago due to emplacement of a large scale intrusive body.
Applications
The Geochronology laboratory of the institute has KAr, RbSr, Sr isotope stratigraphy and ArAr dating facilities.
In case of igneous rocks absolute age of emplacement/formation and their petrogenesis.
In case of sedimentary rocks these methods can be used to know timing of diagenesis, sedimentation and carbonate precipitation.
Timing of metamorphism and illustration of thermal history of metamorphic rocks.
However, in petroliferous basins the KAr technique can be used to date the timing of petroleum migration and gas emplacement in the reservoirs.
Similarly, the ArAr isotope analysis of detrital Kfeldspar can provide thermochronological information in the temperature range of petroleum maturation and therefore the technique can provide a useful hydrocarbon exploration tool.
Timing of Petroleum Migration and Gas Emplacement in the Reservoirs
The KAr dating has been applied on diagenetic illites separated from Permian Rotliegende sandstone of Southern North Sea Region, Netherlands
Illite is a common diagenetic phase.
KAr ages of very fine grained illite should indicate the time, the illite forming process ceases.
Age of the finest fraction in the gas zone of the reservoir to indicate the time at which illite formation ceased as a result of gas emplacement and pore fluid displacement in the rock.
Illite growth and gas emplacement overlapped in time and that illite growth ceased when gas displaced most of the pore water.
Table: Mineralogy and KAr isotopic data of the clay fractions from the Rotliegende sandstone of Southern North Sea, Netherlands Sample
Fraction Size
Illite (%)
Chlorite (%)
K2O (%)
Age (Ma)
A 1380
< 0.2
65
35
4.1
172±5
0.20.5
60
40
4.2
209±5
0.52
50
50
3.4
236±7
<0.2
60
40
5.1
172±4
0.20.5
45
55
4.3
218±6
0.52
40
60
3.7
238±6
<0.2
65
35
6.3
167±7
0.20.5
55
45
5.2
205±5
0.52
55
45
3.5
245±6
A 1712
A 1434
Further, the SmNd isotope system, the facility that we are going to imbibe in near future, is a potential tool in petroleum exploration as it can be effectively used to date the provenence of sandstone/ shale reservoirs.
The system can also be used to correlate the reservoir lithologies within the petroleum producing basin and oil to source rock correlation even in adverse geological milieu where elevated temperatures and pressures during diagenesis have obliterated the organic biomarkers.
Application Dating of Basement Rocks of Western Offshore of India Geological Setting
The Mumbai Offshore Basin (MOB) covers an area of about 1,20, 000 sq. km and is limited to its north by the Saurashtra Arch and to south by the Vengurla Arch (Fig. 1).
The tectonic framework, stratigraphy, structural features and the depositional history of the MOB have been studied in detail by various authors (Rao and Talukdar 1980; Biswas 1987; Biswas and Deshpande 1983; Basu et al 1982).
However, little information is available about the nature and age of the basement, which plays an important role in the evolution of structures like horsts, grabens, rifts and regional faults and, therefore, controls the pattern of sedimentation and also at times the thickness of the formation within the basin.
The Deccan trap forms the floor of this basin with an exception of a few Precambrian (?) inliers.
The Precambrian rocks in different parts of the basin are of varied lithology, comprising biotite gneiss/chlorite gneiss, schist, syenite, granite/granodiorite etc.
N
North Tapti
SAURASHTRA COAST
Mid Tapti 0
20
40
South Tapti
Kms
DAMAN Saurashtra
Dahanu
Diu East
DAHANU
Tarapur
B-46-1
°
Bombay High
° D-12-1
TARAPUR MAHIM
°F
Bombay High East
° BH-36 ° SY-5
Panna Panna East Bassien
°B-192-5
°
Heera
HBM-1
MUMBAI
ALIBAG
RATNA
0 20 0 m 0 10 m 0 20 m
0 36 m
RATNAGIRI
Fig. Location map of studied wells of Mumbai Offshore
Well HBM-1 (Granulitic Basement)
Six samples from three successive cores viz. CC2 (1349.41356.65 m), CC3 (1356.5 1364.1 m) and CC4 (1364.11369.5 m) were analyzed for RbSr isotopic studies The studied samples have yielded RbSr isochron age of 502±25 Ma.
Two samples were dated by KAr method which had yielded a mean KAr age of 506±12 Ma.
The age of 500 Ma obtained by different dating methods has been interpreted as to represent the age of secondary thermal disturbance.
The isotopic ages in the range of 500550 Ma have been reported by various workers from the PanAfrican Zone (Fig.) extending from the Arabian Peninsula and Eastern Africa through Madagascar, southern India and Sri Lanka to East Antarctica.
Fig.
PanAfrican zone from the Arabian Peninsula and Eastern Africa covering Madagascar, southern India, Sri Lanka and East Antarctica [After Rogers et al. 1995]
The PanAfrican ages in southern India have mostly been reported form south of PalghatCauvery Shear Zone.
The secondary thermal disturbance around 500 Ma observed in the granulitic basement of the well HBM1 temporally coincides with the wide spread Neoproterozoic (PanAfrican) thermotectonic event.
The study further suggests that this part of the western offshore and southern granulite terrain probably shared a common tectonothermal history.
In the light of this study it was suggested that the PanAfrican Zone, which hitherto was thought to be confined to the western part (presently the southern part) of the Indian subcontinent, extended further eastward.
The paper published in Jour. Geol. Soc. India (2000), V. 56, 365372.
Well BH-36 (Granitic Basement)
Eight samples from two basement cores (CC5 and CC6) were analyzed for RbSr isotopic studies
The studied samples have yielded RbSr isochron age of 1446±67 Ma 0 .7 5 5 0 0 .7 5 0 0
535
WELL BH36 (Granitic Basement)
not included in regression
0 .7 4 5 0
536
0 .7 3 5 0
537
0 .7 3 0 0
511
510
538
87
Sr/86 Sr
0 .7 4 0 0
509
512
0 .7 2 5 0
Age = 1446 ± 67 Ma (2σ ) Sri = 0.7062 ± 0.0012 (2σ ) MSWD = 2.27
0 .7 2 0 0 0 .7 1 5 0 0 .7 1 0 0 0 .7 0 5 0 0
0 .2
0 .4
0 .6
0 .8
1
1 .2 87
Rb/86 Sr
1 .4
1 .6
1 .8
2
2 .2
2 .4
Two biotite fractions were also analyzed for RbSr isotopic studies which had yielded an isochron age of 1385±21 Ma.
1.700 1.600
1.400
Bio2
1.300 1.200 1.100 1.000
Age = 1385 ± 21 Ma (2σ ) Sri = 0.7061 ± 0.0012 (2σ ) MSWD = 0.16
0.900 0.800
WR
1 .7 0 0
0.700 5
10
15
20
25
87
30
35
40
45
1 .5 0 0
Age = 1394 ± 25 Ma (2σ ) Sr = 0.70712 ± 0.00051 (2σ ) i 1 .4 0 0 MSWD = 1.9
50
Rb/ Sr 86
Sr/86 Sr
0
Bio1
WELL BH36 (Whole Rocks + Biotite)
1 .6 0 0
1 .3 0 0
Bio2
1 .2 0 0 0 .7 5 5 0
87
Sr/86 Sr
1.500
87
Bio1
WELL BH36 (Whole Rock + Biotite)
1 .1 0 0
0 .7 4 5 0
1 .0 0 0
Whole Rock
0 .7 3 5 0
0 .7 2 5 0
0 .9 0 0
Age = 1446 ± 67 Ma (2σ ) Sri = 0.7062 ± 0.0012 (2σ ) MSWD = 2.27
0 .7 1 5 0
0 .8 0 0
WR
0 .7 0 5 0 0
0 .7 0 0 0
5
10
15
20
0 .4
25 87
Rb/86 Sr
30
0 .8
1 .2
35
1 .6
40
2
2 .4
45
50
Five biotites separated from different samples were also dated by KAr method to see the effect of thermal heating if any.
The analyzed samples have yielded concordant KAr ages with a mean age of 1438±19 Ma which is indistinguishable from the whole rock RbSr isochron age. Table
Analytical data and calculated KAr ages of biotite separates from basement rocks of well BH36
Sl. No. Sample Details
K (wt. %)
Total 40 Ar Rad 40 Ar (× 10-6 cc STP . g-1 )
Age (±2σ ) Ma
1
CC5B9T
6.80
603.858
589.671
1452±42
2
CC6B2T (4050 mesh)
7.94
735.794
692.654
1458±43
3
CC6B2T (4050 mesh)d
7.94
665.735
647.496
1392±40
4
CC6B2T (5070 meah)
7.68
682.939
674.381
1465±43
5
CC6B3B
7.90
699.833
666.597
1425±40
6
GLO#
6.46
31.419
24.679
95.71±291
# Gluconite standard with a reported age of 95.03±1.11 Ma (Odin et al. 1982) d Duplicate analysis
The similarity in the whole rock and biotite ages obtained by different isotopic methods suggests that no thermal disturbance has occurred in these rocks after their emplacement around 14001450 Ma ago.
The present study provides the first evidence for the existence of an important Middle Proterozoic Magmatic event around 14001450 Ma on the western offshore of India which, hitherto, was thought to be confined to the eastern Ghats, Satpura and Delhi fold belt of India.
This finding may have an important bearing on the reconstruction of Proterozoic crustal evolution of Western Indian Shield.
The paper was published in Proc. Indian Acad. Sci. (Earth Planet. Sci.) 2004, vol. 113 (1), 2736.
Well B-46-1 (Granitic Basement)
Five granitic samples from basement core CC3 were analyzed for RbSr isotopic studies.
The analyzed samples yielded RbSr isochron age of 1855±76 Ma suggesting granitic emplacement around 1850 Ma ago. 1.2 1.15
Well B-46-1 (Granitic Basement)
1.1 1.05
CC3B1M
1
87
Sr/86 Sr
0.95
CC3B2B CC3B2T
0.9 0.85 0.8
CC3B2M
0.75
CC3B4B
0.7 0
2
4
6
8
10
12
14
16
Age = 1855±76 Ma (2σ ) Sr initial = 0.70137 ± 0.0097 MSWD = 1.81
87
Rb/86 Sr
Fig . RbSr isochron of granitic basement of well B461
One sample was studied by ArAr method which had yielded a plateau age of 607± 6 Ma. 16.00 12.00 8.00 4.00 0.00
39
Ar/37 Ar
CC3B2B
Apparent Age (Ma)
1600.0 1200.0 800.0
607±6 Ma (2σ )
400.0 0.0
0
20
60 80 100 40 39 Cumulative yield of Ar (%)
Fig. 40 Ar-39 Ar age spectrum diagram of granitic basement (CC3B2B) of well B-46-1.
The basement samples studied by different methods suggests that the granitic rocks in this part of the basin were emplaced at around 1850 Ma ago and were thermally reheated around 600 Ma ago.
Well B-192-5 (Granitic Basement)
Four granitic samples from CC5 were analyzed for RbSr isotopic composition The studied samples have yielded RbSr isochron age of 1476±57 Ma. 1.2 1.15 1.1
Well B-192-5 Granitic Basement
1.05 1 0.95 0.9
Age = 1476± 57 Ma (2σ Sr 0.0098 (2σ ) ?) i )= 0.7306± MSWD = 1.48
0.85 0.8 0.75 0.7 0
2
4
6
8
10
12
14
16
18
Fig. 5. Rb/Sr Isochron of Granitic Basement from B-
20
This age has been interpreted as the time of secondary isotopic reequilibration coinciding with the PanAfrican thermotectonic event as reported from the well HBM1 of Heera field.
1600.0
39
0.80 0.60 0.40 0.20 0.00
Ar/37 Ar
Two samples were studied for ArAr dating Both the samples have yielded good concordant plateaus with a mean plateau age of 548±3 Ma.
Apparent Age (Ma)
Ar/37 Ar 39
Apparent Age (Ma)
1200.0 800.0
Age=540±2 Ma
400.0 0.0 0
100 20 60 80 40 cumulative yield of 39 Ar (%)
1.60 1.20 0.80 0.40 0.00 1600.0 1200.0 800.0
Age=556±4 Ma
400.0 0.0
0
20 40 60 80 cumulative yield of 39Ar (%) . 40 Ar39 Ar age spectra of samples from well B1925
100
Well SY-5 (Schistose Basement)
Five basement samples from four cores (CC5, 6, 9 and 11) were analyzed for RbSr studies.
The studied samples have yielded RbSr isochron age of 1465±40 Ma which has been interpreted as the time of metamorphism in the in the schistose basement of well SY5 around 14001450 Ma ago. 0.765
0.76
Not included in regression
Well SY-5 (Schistose Basement)
0.755
0.75
0.745
CC11B4T
0.74
87
Sr/86 Sr
0.735
0.73
CC5B3B CC9B7T
0.725
0.72
0.715 0
0.2
0 .4
0 .6
0.8
1
1.2
CC9B4T 1.4
1.6
CC6B3M
Age = 1465±40 Ma (2σ ) Sr initial = 0.71823 ± 0.00048 MSWD = 0.36 87
Rb/86 Sr
Fig 5. RbSr isochron of schistose basement of well SY5
The Best Isochron Diagram (BID) has advantage of displaying the analytical errors and deviations from the isochron simultaneously.
The BID, therefore, allows to simultaneously visualize the experimental data and their analytical precision and to judge the quality of the linear fit and to appraise the accuracy of the age and initial ratio. (87 Sr/86 Sr) Measured
0.724
0.726
0.73
0.74
0.75 0.76
0.8
1
+∞ 1900 1800
0.722
1700 1600
0.72 CC6B3M 0.718
1500
CC5B3B
CC11B4T CC9B7 T CC9B4T
1400 1300
0.716
1200 1100
0.714
1000 0.712
0.1
0.2
0.4
0.6 0.8 1
2
4 6
Rb/86 Sr Fig. 6. Best Isochron diagram of Schistose Basement of Well B 87
000 +∞
Well D-12-1 (Schistose Basement)
Four basement samples from core CC8 were analyzed.
The studied samples have not yielded good isochron but the best fit line suggests metamorphic event around 13001400 Ma ago.
The age of formation of the basement is still older.
Studies on a few more samples are under way to constrain precisely the metamorphic event.
Summary of the isotopic studies carried out on basement rocks of Westerrn offshore of India Sl No. Well NameField
Rock Type
Age Ma (Method)
1.
HBM1
Heera
Granulite
502 ± 25 (RbSr) 505 ± 16 (KAr) 507 ± 17 (KAr)
2.
BH36
Mumbai High
Granite
1446 ± 67 (RbSr) Whole rock 1385 ± 21(RbSr) Mineral Isochron 1458 ± 43 (KAr) Biotite 1465 ± 43 (KAr) Biotite 1452 ± 42 (KAr) Biotite 1425 ± 40 (KAr) Biotite
4.
B461
WNW of Mumbai High
Granite
1855 ± 76 (RbSr) 607 ± 6 (ArAr)
5.
B1925
SW of Mumbai High
Granite
14001450 (RbSr) 500 – 550 (ArAr)
6.
SY5
S of Mumbai High
Schist
14001450 (RbSr)
7.
D121
SW of Mumbai High
Schist
13501400 (RbSr) Partially Studied
Conclusions
The geochronological studies carried out on the Precambrian basement rocks of diverse composition from the Western Offshore of India suggest a complex emplacement and post crystallization tectonothermal history.
The studies suggest that the granitic/granulitic/schistose basement rocks of Western Offshore were emplaced at around 17002100 Ma and 14001450 Ma ago and were further subjected to secondary thermal activities between 14001450 Ma and 500600 Ma ago.
While the older thermal activity, observed in the wells SY5 and D121 might be related to the emplacement of Middle Proterozoic granites in the wells like B1925 and BH36, the younger thermal event as observed in the well B461, B1925 and HBM1are related to famous wide spread PanAfrican thermotectonic event.
The absence of PanAfrican thermal imprint on basement rocks from Mumbai high field like BH36 coupled with possible linkage of Mumbai high with Middle Proterozoic Mobile Belt of Delhi, Satpura and eastern Ghats suggests that these fields may in all probability be representing different entities with different geological histories.
However, this enunciation, based on isotopic studies alone on limited wells may be far fetched and needs to be substantiated by further isotopic studies from a few more wells of different fields of western offshore of India as well as by other geophysical studies.
Thank You
Introduction The evolution of any sedimentary basin is closely interlinked with global tectonics. The rifting and collision of the lithospheric plates in the geological past have carved the outlines of the tectonic framework and basinal architecture of most of the prolific basins. Basin formation and evolution are generally associated with mantle related geothermal phenomena, which also control the process of generation of hydrocarbons. Since geochemical and isotopic characteristics of the basement rocks provide the finger prints of various paleogeological processes associated with basin formation and evolution, it is necessary to carry out in detail the multiisotopic studies of the basement rocks. Today I will be talking on geochronological studies undertaken on different basement rock types with a view to understand the Precambrian basement evolution of the western offshore of India.
Geological Setting
The Mumbai Offshore Basin (MOB) covers an area of about 1,20, 000 sq. km and is limited to its north by the Saurashtra Arch and to south by the Vengurla Arch (Fig. 1).
The tectonic framework, stratigraphy, structural features and the depositional history of the MOB have been studied in detail by various authors (Rao and Talukdar 1980; Biswas 1987; Biswas and Deshpande 1983; Basu et al 1982).
However, little information is available about the nature and age of the basement, which plays an important role in the evolution of structures like horsts, grabens, rifts and regional faults and, therefore, controls the pattern of sedimentation and also at times the thickness of the formation within the basin.
The Deccan trap forms the floor of this basin with an exception of a few Precambrian (?) inliers.
The Precambrian rocks in different parts of the basin are of varied lithology, comprising biotite gneiss, chlorite gneiss, schist, syenite, granite and granodiorite etc.
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