chapter 6 facies and depositional environments from...

Chapter 6

Facies and depositional environments fromsurface level composite logs of

SP and resistivity

6.1 Introduction

Recognition of depositional environment is made through the physical,

chemical, biological and geomorphologic imprints left in the deposits (Serra,

1985). These imprints define fades. And they characterize the environment and a

typical sequence of fades both space and time (Walker, 1976). This sequence

can be described by fades model. Lithofacies and microfacies analyses of

limestone and siliciclastics were utilized to decipher the depositional environment

(Singh and Bhat, 2002). In subsurface study involving well logs the equivalent of

facies model will be the concept of electrofacies model. An electrofacies is the set

of log responses which characterizes a bed and permits it to be distinguished

from others (Serra, 1979). Any environment can be characterized by an

arrangement of electrofacies which will be grouped into electrosequences. The

electrofacies analysis can be used to interpret the nature of the sedimentary

environments (Rao, 1993). In this work the fades interpretation, reconstruction of

depositional environment and depositional sequences and establishment of

lithology were made through the sequential sedimentological analysis of log

shape from the bottom to top of the basin. The essential steps of a proper

sequential analysis is given in the flow chart (Fig. 6.1)

174

Composite log of

QF^Cores and

SP and 3pp1rent s3.Ilea1s

resisi itv

LITHOLOGY

ELECTROFACIES I -

SEQUENTIAL ANALYSIS

Fades. depositional environments.depositional sequences

Figure 6.1 Flow chart for sequential analysis

Generally subsurface investigations are carried out through the

geophysical methods such as resistivity, self potential and seismic well log in oil

and ground water explorations. Logging is a highly sophisticated technique with

continuous recordings of geological parameter group of the formation met within

a well in terms of depth. Examinations of well logs indicate polarity of the grain

size and types of sedimentation and depositional environment (Serra and

Sulpice, 1975).

This chapter describes how surface level SP and resistivity logs can be

used for the identification of fades and depositional environment. By comparing

SP log with resistivity log the environment of deposition can be identified (Rao,

1993; Serra, 1985). The main geological characteristics of aeolian dune at

175

Manapad in Tuticorin district (Fig. 5.2) and fluvial environment at Neyveli in

Cudalore district (Fig. 3.11) on a single profile are described through geological

facies model concept.

6.1.1 Fades

The changes that take place owing to the differences in environmental

conditions in sedimentary formations are known as sedimentary facies.

Sedimentary geology is process oriented, focusing on how sediment was

deposited. Sedimentologists are geologists who attempt to interpret past

environments based on the observed characteristics, called facies, of

sedimentary rocks. Fades analysis uses physical, chemical, and biological

characteristics to reconstruct ancient environments. Facies analysis helps

sedimentologists to determine the features of the layers such as their geometry,

or layer shape; porosity, or how many pores the rocks in the layers have; and

permeability, or how permeable the layers are to fluids. This type of analysis is

important economically for understanding oil and gas reservoirs as well as

ground water supplies. The disappearance of permeability updip might be the

result of depositional environment producing different sedimentary facies

(Microsoft Encarta, 2002).

Originally a sedimentotogical study from logs involved examining the

shapes of various curves for indications of the types of sedimentation and

depositional environment. Classification of electrofacies by the shapes of

spontaneous potential response has been correlated with grain size distribution.

176

6.2 Electrofacies Analysis of the Litho Stratigraphy of NeyveliArea

The Neyveli deposit consists of a number of horizontal layers.

Knowledge of vertical variation is known from drilling data and the thicknesses of

each layer of sedimentary strata above the lignite seam are measured on the

walls of the opencast mine-I area (N) and Thoppulikuppam (Nm) in Neyveli mine

area. Vertical electrical sounding (yES) and self potential measurements were

carried out in the close proximity of the drilling sites of the study areas N and NTh

where the lithologs have been collected. The measured values of SP and

resistivity values are given in the Table 6.1. Then the graphical plots were made

using these values as in the previous chapters.

The manual identification of curve elements such as peak, and trough from

the curves of the apparent resistivity and the SP were used for electrofacies

analysis (Fig. 6.3a & 6.4a). The pattern recognition such as peak and peak (PP),

peak and trough (PT), trough and peak (TIP), trough and trough (TI) and trough

and flat (TF) of the composite log of SP and apparent resistivity helps to identify

the lithological sequences.

To interpret the lithology, a systematic approach is followed by knowing

the lithology from borehole data and by measuring the thickness of each

horizontal formation in the benches of the Neyveli mine area. These data are

utilized for depthwise correlation and for comparisons of gross lithology derived

from the composite logs of apparent resistivity and the SP values.

177

Table 6.1 Self potential and apparent resistivity values with respective electrodespacing 'a'

Neyveli N

a SP App. Res. a SP App.Res.

Jj (my) (ohm-m) (p) (mV) (ohm-m)

2 -20 19.47 118 128 61.51

6 -108 10.59 122 155 22.22

10 42 12.94 126 25 48.49

14 -31 16.18 132 114 48.38

16 153 17.18 138 66 43.60

20 237 21.35 1 144 211 46.42

26 -162 25.80 150 27 46.91

30 -83 24.12 156 62 45.82

34 31 29.47 162 113 1 44.20

38 171 27.44 168 70 43.41

42 67 29.54 174 188 43.67

46 1 -151 28.60 1 180 -121 38.43

50 128 30.77 186 -116 44.31

54 -141 20.01 192 -116 41.45

58 58 34.60 198 -155 39.00

62 -136 35.43 204 70 37.98

66 -161 37.30 206 24 53.61

70 -212 36.93 208 78 48.36

74 -182 38.57 212 23 41.23

78 138 40.17 216 24 40.92

82 110 33.99 220 -23 41.31

86 17 44.29 226 38 41.67

90 73 41.82 228 -44 47.29

94 107 42.50 232 44 37.15

98 -27 54.77 236 31 43.67

102 1 31 41.00 1 240 2 45.10

106 -17 42.60 244 1 7 48.82

110 -44 37.30

114 125 43.67

Neyveli Nm

a SP App.Res.

JJ (my) (ohm-m)

6 50 84.78

12 41 27.51

18 1

-14 47.82

24 -137 43.26

30 -74 40.88

36 -20 39.79

42 -9 26.38

48 -55 41.00

54 -14 38.32

60 56 39.19

66 39 40.62

72 -72 41.15

78 56 47.51

84 -32 49.06

90 83 49.74

92 1 111 50.27

94 74 50.77

96 -35 51.85

98 -78 53.54

100 1 -19 52.75

106 27 52.59

112 -27 56.27

118 193 55.58

120 250 54.26

122 164 58.23

124 1 165 59.96

126 93 62.51

138 95 58.93

150 14 64.06

178

The pattern recognition of the curve elements of SP and apparent resistivity

forms groups such as PP, PT, TP, TF and IT. These groupings of the curve

elements identify various types of lithology like sands, sandstones, clay, ball clay,

lignite, intercalated sand and clay and laterite. The comparison and corroboration

of gross lithology obtained from the composite logs with cores and outcrops were

carried out and it is found that all corroborate the same interpretation, and then

the lithology is marked. The interpreted depthwise lithology of Neyveli area from

the logs of apparent resistivity and SP is marked by the side of the plot (Fig. 6.3b

and 6.4b). The depth and bed thickness of the corresponding lithounits are

displayed in Table 6.2.

- - jFigure 6.2 Author with research scholars at the Neyveli mines

179

Apparent Resistivity (ohm-m) N

1015202530354045505560

0

10

20

30

40

50

60

70

80

90

100

110E

120CO

170

180

190

200

210

220

230

240

L itho logy

Sand-\Jternates.,M &ri

andsi one

CbSandClaySandCv

Sandstone

ClaySandstone

ClaySandstone

ClaySand ::::::::.:.........

ClaySandstone

ClaySandClay

Sandstone

ciSandClay

Lignite

Sand&ClavClay

Sandstone

50

App. Res

J75

SP

25

Lignite

100Sand & Clay

-225 -125 -25 75

175

Self-potential (mV)

Fig. 6.3bFig. 6.3a

Figure 6.3 Composite log of SP and Apparent resistivity and itsinterpreted lithology of N location of Neyveli area

180

Lithology j

Lateritesoil

Clay

MAOS.andslone

Sandidune

CbV

SandOL

CLaV

SandM one

Clay

Lignite

Clay

Lignite

Clay

Sandstone

App. Res.

Apparent Resstrvity(ohm-m) Nm

35 45 55 65 75 85

SP

-50 50 150Self-Potential(mV)

250-

10

20

30

40<

50

60

,_. 70

' 30

90

100

110

120

130

140

150-150

25

50

475250

Fig. 6.4bFig. 6.4a

Figure 6.4 Composite log of SP and Apparent resistivity and itsinterpreted lithology of Nm location of Neyveli area

181

Table 6.2: Depthwise litho units and their layer thickness of N & Nm locations of

Neyveli area

N locationS.No Litho unit Depth Bed

(m) Thickness

1 Top Sand - 4.1

2 Intercalated 4.1 7.7

3 Sandstone 11.8 4.1

4 Clay 15.9 5.3

5 Sand 21.2 3.5

6 Clay 24.7 2.3

7 Sand 27.0 1.8

8 Limestone 28.8 2.4


10 Clay 38.8 3.5


12 Clay 45.3 2.9


14 Clay 50.6 2.9

15 Sand 53.5 3.0


17 Clay 60.0 2.3



20 Sand 67.0 3.0



23 Clay 78.2 4.7

24 Sand 82.9 3.0

25 Clay 85.9 2.9

26 Lignite 88.8 11.8

27 Intercalated 100.6 4.1

28 Lignite 104.7 7.6


30 and 115.9 2.3

31 Sandstone 118.2 -

NTb location.

S.No Litho unit Depth Bed

(m) thickness

1 Top Soil - 4.0

2 Clay 4.0 4.5

3 Sandstone 8.5 7.5

4 Clay 16.0 6.5


6 Clay 28.5 6.0

7 Sand 34.0 5.5

8 Ball Clay 37.5 3.5


10 Clay 44.0 3.5 -

11 Lignite 47.5 11.0

12 Clay 58.5 2.2

13 Sand 60.7 1.8

14 Lignite 62.5 4.3

15 Clay 66.8 4.7

16 Sandstone 1 71.5 -

182

6.3 SP Log Shapes Analysis of Neyveli Area

Formation: A geometrical approach

The SP log shapes have been used to recognize the sedimentary fades

(Coleman and Prior, 1982). Based on geometrical consideration SP log shapes

are classified into bell, cylinder and funnel shapes (Fig. 6.5).

Smooth Serrated

•

MV

- rnjBell

Cylinder(Block)

Funnel

Figure 6.5 Log shape classification to analyze SP log shapes

The variation of log shape depends upon the grain size and amount of

clay content. Bell shaped SP log indicates increase of clay content or decrease of

grain size upward. Funnel shape SP log shows the reverse of the bell shape

indication. Third option of cylinder shape exhibits a sand body with constancy of

183

grain size or clay content in the upward direction (Rider, 1985). The examination

of log shapes indicates the types of sedimentation and depositional

environments. A basic scheme for the interpretation of depositional environment

and depositional sequence has been developed after establishing lithology and

electrofacies of the sedimentary rocks of the Neyveli formation. Sequential

analyses are carried out using the SP log shapes.

6.4 Sequences of Electofacies in Mine-1 Area

Although the horizontal routine is the basis for any lithological

interpretation the composite logs are examined vertically for trends. The

lithological sequences at mine-1 (N) and Thoppulikuppam (N Ih) of Neyveli basin

were divided into three cycles (Fig. 6.6 & 6.7).

The sequences of cycle I of the two locations of the basin are illustrating

two bell shapes and two cylinder shapes on the SP curve. The SP logs indicate

fining up sequences and lignite sequences at different depths. In the mine (N), it

starts from the depth of 140 m and ends at the depth of 90 m, whereas in the

Thoppulikuppam (NTh) the same cycle starts from 72 m and end at 47 m. In these

two locations, the top and the bottom of the two lignite seams are encompassed

by clay with carbonaceous matter. The general SP log shape of the two areas

exhibits serrated bell shape.

Cycle H of the mine-I area illustrates two funnel and a cylinder shape SP

logs indicating coarsening upward trend of grain size population. Core and

outcrop successions also clearly reiterate coarsening up trend. The overall log

184

trend also forms a serrated funnel shaped SP curve from 89 m to 37.5 m in

upward direction. Similar trends of the SP curve indicating coarsening up for

Thoppulikuppam area at the depth of 7 m to 45 m.

In the third cycle, two cylinders and one funnel shape SP curves indicate

intercalations of sand and clay and coarsening upward trends for the grain size

population in mine-I area. But in the Thoppulikuppam fining up trends for

sediment of 7 m thick is being recorded.

While comparing the thickness of each cycle in the two locations, the

thicknesses of cycle I of Neyveli mine area is greater than the cycle I of the

Thoppulikuppam area by 16 m, the thickness of cycle II of Neyveli mine area

greater by 17 m than the cycle II of the Thoppulikuppam area and the cycle III of

Neyveli mine area is greater thickness than the thickness of cycle III of

Thoppulikuppam area by 29 m. It is found that the cycle III, i.e. the top most

sequence, exhibits the highest difference in thickness between the two locations

of the Neyveli basin. In the Thoppulikuppam area the reduction of the bed

thickness in cycle III is due to the missing of the uppermost sequence of beds.

The reduction of the bed thickness in the other two cycles I and II of

Thoppulikuppam area are due to the diminution of beds because the deposition

in Thoppulikuppam area might have been at the periphery of the basin whereas

the Neveli mine-I area was at the centre of the basin.

185

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6.5 Depositional Environment

The thicknesses of each facies or sequence of facies and grain size

evolution are related to depositional environments (Selley, 1978; Miall, 1984).

The geometry of sedimentary bodies is an important parameter for facies and

environment recognition (Curry and Curry, 1972; Sabins, 1982). There is a close

relationship between SP log and grain size. This relationship leads to direct

correlation between facies and log shapes (Krueger, 1968; Galloway, 1968;

Fisher, 1969; Coleman and Prior, 1982; Galloway and Hobday, 1983). Shapes of

the SP log can also be interpreted as grain size trends. It is already noted that the

bell shaped and funnel shaped logs indicate channel point of fluvial and delta

border progradation respectively. The same log shapes also indicate the

transgressive marine shelf and prograding marine shelf. To differentiate between

the various environments which may give similar log shapes, a theoretical

scheme has been proposed (Selley, 1976). As per the scheme the presence of

glauconite along with bell shape or funnel shape log indicates the marine

environment and the presence of carbonaceous matter indicates nonmarine

environment.

In the present study the encounter of lignite and Marcasite in the lower

part of the basin indicates the depositional environment was nonmarine. In the

Neyveli formation the clay materials are dispersed throughout the sandy mass so

that fine grained sand had been entrapped a greater amount of clay than medium

grained sand. This explanation for the differences in relationship between clay

content and grain size is probably found in hydraulic regimes in Neyveli region.

This relationship has also been attested by the results of the textural studies of

188

the Neyveli sediment. The mean grain size of the sediment population varies

from 1.5(p to 3.4p. Of which the 64% of the sediment populations are of coarse

and medium grained and 29% are of medium and fine grained and remaining 7%

are of silt and clay. Even though the majority of sediment population belongs to

coarse and medium group the presence of little percentage of clay has reduced

the sorting efficiency. In the grain size analyses it is found that 58% of sediment

population are poorly sorted and 28% are moderately sorted and only 8% of the

sediment population are moderately well sorted (Sebastian Chandy et al, 1989).

This result exemplified that the correspondence between clay and grain size in

the fluviate environment shows that the winnowing action is low and so sorting is

poor.

The depositional environment of Neyveli sedimentary basin is reflected

primarily in the lithological sequences and distribution of lithofacies such as

sandstones, clays and biofacies like lignite found within the piles of sedimentary

strata.

The evidences obtained from the three depositional megacycles clearly

corresponded with the SP logs. They indicate the depositional environments of

the Neyveli basin. The SP log of lower most (cycle I) sequence is a serrated bell

indicating meander point bar into tidal channel connecting swampy lake to bay

and the SP log shape formed (block) cylinder shape towards the lignite seam.

This area was a transitional environment characterized by sediments that have

been transported to the end of a channel by a current of continental water of

ephemeral stream from the streams adjoining hilly terrain and deposited mostly

sub aqueous but partially sub aerially at the margin of standing swampy lake. The

189

bars are developed in front of swamps and fluvial channel complex and so these

deposits are quite distinctive with lignite deposits. The sediments found in this

cycle exhibit broad spectrum of grain size from clay to sand particles. These

sediments are terngenous sediments. Lignite formation in lower most cycle

indicates the depositional environment was nonmarine swamps in which large

scale accumulation of organic debris were deposited. Remnants of vegetable

matter like branches and leaves of trees found in the lignite seams in the lower

part of the Neyveli basin suggest that the swamp was saturated environment with

stagnant water and supporting a stand of trees. All saturated environments that

continued to accumulate peat, then bogs to lignite formation.

During the decay of the plant substance hydrogen sulfide generated was

precipitated as iron sulfide. Bacteria also induced the depositional process to

form a deposit of iron sulfide and reduced iron sulfate to sulfide by lignite was

possible as wall. Sulfide (Marcasite) is the most important form of sulfur in lignite

deposits in Neyveli basin.

In a rare exception the SP deflections towards left even when the

formation is not permeable are due to mineralization like Marcasite. The same

excessive left or negative side SP deflections for lignite bed which were formed

into extremely reduced condition are recorded in the cycle-1 of the Neyveli

formation. This excessive SP deflection may be due to electrical inequilibrium in

the subsurface condition (Hallenburg, 1978).

The overall log signature of sedimentary sequences of cycle II of Neyveli

basin shows the serrated funnel shape which includes funnel, block and bell

us 11

shapes. These sequences demonstrate the depositional environments of

distributaries mouth bar, distributing channel fill and meander point bar.

The distributary's mouth bars of the cycle II exhibits upward coarsening

sequences (89 m - 74 m) of 15 m thick consist of sandstone, clay and sand.

Distributary's channel fill comprises of clay, fine sand, sandstone and ball clay of

(74 m - 56 m) 18 m thick is present above the mouth bar deposits. The top most

deposits of cycle II, deposited in meander point bar consists of alternate layers of

sand stone and clay beds of (56 m - 40 m) 16 m and followed by 7 m thick

massive aquifer sandstone body. In this cycle the upward coarsening sequence

and cross bedding sedimentary structure evince the depositional environment as

distributaries mouth bar.

Sequences of cycle III consisting of serrated cylinder were deposited in

distributaries channel fill in which alternate layers of clay and sandstone capped

with latente at the top of the cycle are found. But laterite capping is missed in the

cycle Ill of Thoppulikkuppam area.

One of the main factors in environmental analysis through electrical study

is to find out the chemical and physical nature of medium from which the

sediments were deposited. In our study it has been seen that in all sequences,

alternate layers of clay, sand and sand stone beds are encountered. The sand

stone beds are interbedded with massive in character. The internal structure of

the sand bodies exhibits cross bedding, such as diagonal cross bedding which

indicates a shallow depositional environment and the transport of the

sedimentation was caused by swift current deposits of ephemeral streams.

191

Sequence of clays in the study area shows a cyclic stratification nature.

Thin and even lamination of the clay zones suggests that the deposition

environment was of quiet water. The alternate layers of sand bodies and clay

zones in each cycle reflect the seasonal and long term fluctuation in the weather

pattern dunng the deposition of Neyveli sedimentation. Both clay and sandstone

deposits were formed in an aerobic environment.

The chemical nature of the depositional setting of Neyveli basin is

retrieved from the clastic and nonclastic deposits. The clay sequences are spread

out throughout. The entire formation is mainly composed of kaolinite. Rangama

and Sahama (1952) categorically stated that the formation of kaolinite takes

place in river and bog water. The type of clay indicates an acid environment for

its formation (pH <7). The depositional environment of Neyveli basin is indeed a

complex one and is characterized by various sets of physical and chemical

parameters having good correspondence with log shapes. The physical nature of

depositional bodies is identified through the log signatures i.e. the SP log is

reliable indicator of day and sand body. In conclusion, the log shapes indicate

depositional environment.

It has been already stated that a strong deflection towards left side, when

the formation is not permeable, is definitely due to the chemical condition of the

formation. We have recorded strong negative deflections for lignite seam with

Marcasite mineral in an anaerobic environment.

192

In the environmental analysis, the physical nature of the depositional

media is identified through the combination of the logs. The SP log shape clearly

depicts the depositional environment and their depthwise grain size trends as

coarsening up or fining up. The thickness of each sedimentary strata and the

corresponding SP log shape clearly identifies the depositional environment as

meander point bar or distributary's mouth bar etc.

The heaping up of the organic matter has markedly reduced the

depositional environment of the swampy region of Neyveli as reducing facies.

The environment was reducing because carbon compound accumulates the

presence of the reducing agents such as plant debris and other by-products like

Marcasite established redox potential of the environment. The ability of a natural

environment to oxidize sulfur, or being about any other oxidation or deduction

process, is measured by a quantity called its redox potential (Eh). However, the

direct measurements of redox potentials of the order of -0.1 V to -0.5 V for the

modern depositional environment in which organic matter is accumulating have

been recorded (Krauskopf, 1989). Another possible measure of redox potential is

the nature of inorganic mineral such as Marcasite precipitated from circulating

water with lignite deposits. It can form only at Eh values below -0.3V in acidic

environment pH < 7 (Krumbein and Carrel, 1952). Electrofacies analyses also

indicate an abnormal left side indication with fiat apparent resistivity curve.

Though the depositional basin was very close to the Bay of Bengal, the presence

of lignite seam and Marcasite in the lower part of the basin clearly shows that the

depositional environment was high acidic and reducing and not contaminated

with sea water.

193

6.6 Main features of Aeolian environment in Manapad

A continental environment characterized by deposits resulting from

wind action is called Aeolian. Coastal dunes along the coasts of Manapad area

was developed from wind shadow deposits. These coastal dunes are

fundamentally like those of the inland desert. These dunes in coastal area are

located in tropical climatic condition and so they are dominated by carbonate

sediments largely by biogenic origin (Fig. 6.8).

-

Figure 6.8 Photograph showing the aerial view of Manapad

To model the Aeolian environment two sites with elevations of 50 m and

25 m above MSL and 500 m and 700 m away from the shoreline were selected

for electrical survey (Fig.6.9). The first and second locations Ml and M2 situated

200 m apart at Manapad village in the leeward side of the dune sediments exhibit

homogeneity. The cross stratification are noticed in the exposed part in the

vicinity of the study area. The composition of the dune is terrigenous with quartz

194

as dominating mineral with silt fragments. At the sites of Ml and M2 the lateral

spreading of 220 m, and 210 m for the SP and the resistivity measurements were

carried out in Wenner configuration (Table 6.3). As explained in the earlier

chapters the graphical plots were made (Fig. 6.1 Oa & Fig. 6.11 a).

Figure 6.9 Cross Section showing the development of the depositionalsequences in the Mana pad area

While analyzing electrostratigraphic sequences sharp boundary zones

were noted at the depths of 30 m, 22 m, 16 m and 8 m for Ml location

(Fig. 6.10a ) and for M2 sharp boundaries were marked at the depths of 28 m,

15 m respectively (Fig. 6.11a ). At these bounding surfaces, the resistivity values

are comparatively high along with right side deflections of the SP values. The

higher resistivity and right side SP deflections indicate that these zones were

cemented by systematic processes. In the dune environments sand, sandstone,

195

limestone and clay beds were identified through the analysis of the composite

logs of SP and resistivity (Fig. 6.10b & Fig. 6.11b). The encounted depthwise

lithological sequences and their layer thickness at Ml and M2 locations of

Manapad area are given in the Table 6.4.

Table 6.3 Self potential and apparent resistivity values with respect to electrodespacing of Mana pad area

Manapad Ml

a SP App. Res. a SP App. Res.

(m) (my) (ohm-m) (m) (mV) (ohm-m)

1 5940 289.02 34 6500 367.44

2 1130 2927.96 37 -115 40.89

4 6040 1163.64 40 1300 245.19

6 -84 2714.3 43 4830 245.32

8 -139 1291.82 46 196 303.18

10 5940 393.32 49 332 121.85

12 -57 741.91 52 6400 509.69

14 -117 550.65 55 4780 587.47

16 6100 1558.22 58 6400 1177.09

19 137 1117.4 61 6400 716.72

22 -86 1271.71 64 109 155.94

25 655 1514.24 67 6290 233.64

28 63 1108.35 70 6700 21.99

31 6300 1692.62 73 6300 254.56

Manapad M2

a SP App. Res.

(m) (mV) (ohm-m)

5 0 1058.71

10 -74 637.74

15 -68 1590

20 -47 727.59

25 -48 55449

30 101 640.88

35 0 307.87

40 -55 329.23

45 -83 187.45

50 -23 120.63

52 3 47.37

55 160 414.69

58 56 27.68

61 23 11.87

64 50 9.64

67 110 11.36

70 35 3.51

196

Ml ManapadApppare1 Resistivity(ohm-m)

1001

04Lithology

SandC1a

Sandstone

Clay:

Sandstone

Limestone z1t_

Sand

Limestone

Sand

Clay

Sand

:

Limestone :;____

Sand

Limestone

Sand

Limestone

Sand

Limestone

Sand

75 -p P

i

i j-

-200 1000 2200 3400 4600 5800 7000

Self Potential (mV) Fig.6.10a Fig.6.1Ob

Figure 6.10 Composite log of SP and Apparent resistivity and itsinterpreted lithology of Ml location of Manapad area

5

10

15

20

25

E

30

35

ce40

45

50

55

60

65

70

2

197

E

c)

cia]

-40 10 60 110 160

Self PentiaI(mV)Fig.6.1 Ia

1

0

5

10

15

20

25

30

cc

45

50

55

60

65

70

-90

Lithology

Sand

1

Sandstone

Clay

Sand :.....;.....::....:.

Limestone

Sand

Limestone

Sand

Limestone

Fig.6.l lb

M2 ManadApparent Ristivity(ohm-m)

100

1Eio

-* SP

/App.Res c

Figure 6.11 Composite log of SP and Apparent resistivity and it's interpretedlithology of M2 location of Mana pad area

1 ('O

Table 6.4 Depth wise lithology and its layer thickness of Ml & M2 locations inMana pad

Ml location

S.No Litho units Depth Bed

(m) Thickness

1 Top sand - 1.65

2 Clay 1.65 0.85


4 Clay 4.60 1.05



7 Sand 8.35 3.10


9 Sand 13.35 1.65

10 Limestone 15.00 1.65

11 Sand 17.50 2.50

12 Limestone 20.00 2.50

13 Sand 22.50 2.50

14 Limestone 25.00 2.50

15 Sand 27.10 2.10

16 Limestone 28.50 1.40

17 Sand 31.30 2.80

18 Limestone 32.90 1.60

19 Clay 34.20 1.30

20 Sandstone 36.30 -

M2 location

S.No Litho units Depth Bed

(m) Thickness

1 Top sand - 6.0

2 Sandstone 6.0 2.6

3 Clay 8.6 2.5

4 Sand 11.1 2.2


6 Sand 16.4 9.1


8 Sand 28.7 3.0

9 Limestone 1 31.7 3.3

6.7 The Sequences of Electrofacies Analysis in AeolianEnvironment at Manapad

The interpretation of the composite log at Ml location of Manapad area

shows that there are four aeolian cycles demarcated by consolidation of aeolian

sediments with almost constant interval of thickness. Each cycle of the aeolian

sediments is exhibited by the serrated cylinder in the SP log. That is the

199

constancy of the grain size variation in the depositional environment was

observed.

There are two aeolian cycles which were identified with cylindrical log

shape from the composite log of M2 location of Manapad area. These cylindrical

cycles of the SF log were less serrated with more thickness.

In both locations the composite log of the SP and the apparent resistivity

of the bottom cycle exhibit clay deposits. As the location is being in close

proximity to the seashore the presence of clay deposits indicates the bottom

formations that were originated from marine deposits.

The log shapes of SF of Ml and M2 locations indicate serrated cylindrical

shape. The frequency of serration is more in Ml than M2 (Fig. 6.12 & Fig. 6.13).

There is a close relationship between the SP log and grain size. The

sedimentological implication of this relationship leads to a direct correlation

between facies and log shape. In this case the log shapes are those of overall

sequences rather than individual bodies (Parker 1977). Since the dune sediments

in the Manapad region are medium sand in grain size and well-sorted and well-

rounded in nature (Marimuthu and Sivakumar, 2000) the depositional pattern in

dune environment maintains constancy in grain size distribution. As a result, the

shape of the log in the study area shows the cylindrical shape.

L itho logy Polarity I Interpretation General Trend

Serrated

Constant

Aeolian Cylinder

Grain size

Dunes

Fining up MarineFormationS

AeolianDunes

AeolianDunes

AeolianDunes

ConstantGrain size

ConstantGrain size

ConstantGrain size

Ml ManapadAppparent Resistivity(ohm-m)

0 500 1000 1500 2000 2500 3000f Litliology Sequence0

S,ntl

clav5 Aeolian

10 Cycle IV

15

20 Sinl

25App.Res

St AeolianCycle III

Snnd

Cd SP

50

55

60

65

70

75 --200 1000 2200 3400 4600 5800 700C

Self Potential (mV)Figure 6.12 Composite log of SP and apparent resistivity and their interpretation illustratingaeolian environment of Mana pad Ml area

201

' Aeolian

Sind

Cycle 11

1St

S.intl

Aeolian' S Cycle I

Sand

I_St

MarineClay Cycle

0

5

10

15

20

25

30

'- 35

40

45

50

55

60

65

7C

M2 ManapadAnrrrt Rpsistivitv(ohm-m

-90 -40 10 60 110 160

Self Potential(mV)Figure 6.13 Composite log of SP and apparent resistivity and theirinterpretation illustrating aeolian environment of Manapad M2 area

202

Sand dunes with cross bedding exhibit a series of bounding surface nearly

horizontal. Stokes (1968) showed how groundwater may control bounding

surfaces by anchoring dunes while their crests are blown off. The following

diagrams (Fig. 6.14) show the development of bounding surfaces in aeolian

dunes. In a similar manner anchoring of dune and the removal of crests were

repeated in the study area at Manapad. The phreatic water in these zones

dissolved the shells and consolidated the dune sediments to form series of

horizontal truncations. Analyses of the composite logs also attested the same

features in the study area.

Figure 6.14 Schematic diagrams showing development of bounding surfaces ataeolian environment in Manapad area (After Stokes, 1968)

Phase-1 Dune sand accumulates on previous level substratum

Water table

Phase-11 Sand accumulation continues, water table rises in sand

203

Water table

Phase-I!! Wind action removes sand upto water table

Water table

Phase-IV Second dune field accumulation on water table surface

Nino

......... ... .. ....Water table

Phase- V Water table rises to new position in dune field

Water table

Phase- VI Wind action removes sand upto second water table

Water table

Phase- VII Third dune field accumulates, etc

205

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