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58

Chapter-4

INTERPRETATION OF PRODUCTIVE ZONES USING SPONTANEOUS POTENTIAL

(SP) LOG AND GAMMA RAY (GR) LOG

ABSTRACT

In this chapter, Spontaneous Potential (SP) and Gamma ray (GR) logs have been used for

identification of productive zones within the sedimentary sequences of Eocene (Chorgali and

Sakesar) and Paleocene (Patala) ages. The study encompasses samples from three wells D, E &

F. These formations mainly consist of limestone, sandstone and interbedded shale. The order of

permeability (reservoir property) from SP log was Chorgali > Sakesar > Patala. Shale contents

and organic matter i.e. source rock properties, increased with depth. Chorgali and Sakesar

showed permeable limestone with interbedded shale with some organic matter deposition. It was

shown to have reservoir properties. While Patala showed the presence of shale with interbedded

limestone and organic matter, it was shown to be source rock.

59

4.1. INTRODUCTION

The continuous recording of a geophysical parameter along a borehole produces a

geophysical well log. The value of the measurement is plotted continuously against depth in the

well. Various logs are used for different purposes e.g. SP log measures the difference in

electrical potential due to preferential diffusion and absorption of cations and anions in formation

fluid (Selley, 1985). Cations (small size) have high mobility than anions and creates charge

imbalance. The SP log is a measure of permeability. Limestones are low in permeability unless

they are porous or fractured. Sandstone usually shows a large deflection toward the negative pole

because of their permeability. If the sonde encounters a fluid that is a better conductor than the

drilling mud (such as salt water), the curve will deflect to the left and if the fluid is a poor

conductor (such as fresh water or oil), it will deflect to the right (Selley, 1985).

The gamma ray log is an extremely simple and useful technique that is used in all

petrophysical interpretations. The gamma ray log measures the total natural gamma radiation

emanating from a formation which originates from potassium-40 and the isotopes of the

Uranium, Radium and Thorium series. The gamma ray is measures by gamma ray tool i.e.

Geiger Muller counter. The total gamma ray log is expressed in API° scale goes from 0 to 200

API° but it is more common to see 0 to 100 API and 0 to 150 API° in log presentations. Organic

rich shales and volcanic ash show the highest gamma ray values, and halite, anhydrite, coal,

clean sandstones, dolomite and limestone have low gamma ray values (Selley, 1985). Care must

be taken not to generalize these rules too much.

4.2. INTERPRETATION OF PRODUCTIVE ZONES

In this chapter, SP and GR logs have been used for identification of productive zone

where productive zone is permeable and shale rich rocks within the study area.

4.2.1 Productive Zones Using Spontaneous Potential (SP) Log

Differences in salinity between the Formation water and the borehole fluid give rise to

spontaneous potential (SP). The SP log measures the spontaneous potential difference that exists

between the borehole fluid with conducting fluid electrode and a reference electrode at the

surface. SP response of shale and clay is same while opposite response is obtained for sandstone.

The difference of response is due to flow of ions i.e. sodium and chloride ions. Presence of these

natural occurring ions is related to permeability of the Formation (Selley, 1985). Deflection of

log from an arbitrarily determined shale base line indicates permeable and therefore porous

60

sandstones and carbonates. Deflection to left of the baseline is termed as normal or negative SP

and it is an indication of permeable sand and carbonates while deflection to right side of baseline

is reversed or positive SP (impermeable shale) while poor or absent SP deflection occurs in case

of uniformly impermeable Formation. In general, SP log is used to differentiate between

interbedded impermeable shale and permeable sandstone or carbonates (Selley, 1985).

Spontaneous potential (SP) log for Eocene (Chorgali and Sakesar) and Paleocene (Patala)

ages from wells D, E and F is shown in Figure-4.1 i. In Well-D, SP log response was normal in

the range of -68 to -54 mV on the left side of the baseline in linear manner showing occasional

fluctuations between -68 to -54 mV in Eocene Sakesar Formation and almost linear in Eocene

Chorgali and Paleocene Patala Formation as SP log was in the range of -61 to -57 mV and -65 to

-63 mV, respectively. This type of variations in SP response is associated with the permeability

of the Formation. More negative response means more permeability and less negative response

means less permeability (Selley, 1985). Permeability is the property of good reservoir rock.

More the permeability, good is the reservoir quality (Selley, 1985). So the SP response suggests

that the reservoir properties of Eocene Chorgali Formation are better than Eocene Sakesar

Formation which was in turn better than Paleocene Patala Formation.

The spontaneous potential (SP) log of three formation i.e. Chorgali and Sakesar (Eocene)

and Patala (Paleocene) from Well-E is shown in Figure-4.1 ii. SP log response at early depth of

Chorgali was between -60 to -55 mV showing good reservoir character but with increasing

depth, Chorgali gave a sudden increase in SP response and value shifted towards the base line

i.e. -40 mV. This sudden increase indicates the presence of some impermeable shale. The

presence of shale was also indicated by variation of SP log in Sakesar formation. The upper part

of Sakesar showed decreased SP response which increased with depth. Such a variation in SP

response indicated impermeable shale intervals although high overburden pressure contributes

compactness and decreases in permeability (Selley, 1985). The upper part of Paleocene Patala

formation showed impermeable formation (-5 mV) which continued till middle of formation

depth (-10 mV) while the lower part showed high negative response as compare to upper and

middle part i.e. -20 mV. This trend is different from SP log response of same formation in Well-

D which is due to the different burial depths. As Paleocene Patala Formation is considered as a

potential source rock in Potwar Basin so such behavior is not so much astonishing.

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Table-4.1: Description of geological information and total organic carbon in sediment

samples taken from different wells under study.

Sediment

I.D.*

Upper Depth

(m)

Geological

Age

Formation Ray

Log

SP

Log

Well-D

S-1 3756 Eocene Chorgali 50 -61





S-6 3821 Eocene Sakesar 18 -63




S-10 4062 Paleocene Patala 30 -63




Well-E















Well-F





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Sediment

I.D.*

Upper Depth

(m)

Geological

Age

Formation Ray

Log

SP

Log







*Sediments numbering is same used in chapter-6 & 7. Few sediments are missing as data is not

available

In Well-F, upper portion of Eocene Chorgali Formation showed almost a linear SP

response i.e. from -28 to -23 mV (Figure-4.1, iii). This behavior of Chorgali formation in Well-F

is different from same formation in wells D & E where this formation showed high negative

response. Perhaps this is due to the compactness of the Chorgali formation in Well-F. Sakesar

Formation has SP log value from -35 to -28 mV which is also a different trend than the SP log

values in wells D & E. Paleocene Patala Formation also showed SP log values within -32 to -23

mV which is similar to SP log response of same formation in Well-E but differs from Well-D.

49

20

20

Ch

org

ali

Sakesar

Pata

la

4650

4700

4750

4800

4850

-80

-60

-40

-20

0

ii

Ch

org

ali

Sakesar

Pata

la

4025

4045

4065

4085

4105

4125

4145

4165

-80

-60

-40

-20

020

iii

3750

3800

3850

3900

3950

4000

4050

-80

-60

-40

-20

0

i

Ch

org

ali

Sakesar

Pata

la

Well-D

Well-E

Well-F

Fig

ure

-4.1

: R

esp

on

se o

f S

P l

og

wit

h d

epth

fo

r C

ho

rga

li, S

ak

esa

r a

nd

Pa

tala

fo

rma

tio

ns

wit

hin

i)

Wel

l-D

, ii

) W

ell-

E

a

nd

iii

) W

ell-

F. R

efer

to

Fig

ure

-2.1

fo

r li

tho

log

y o

f fo

rmati

on

s

49

4.2.2. Identification of Lithology from Gamma Ray (GR) Log

Three types of logs that measure radioactivity are commonly used for formation

evaluation in oil or gas well drilling i.e. gamma ray log, neutron log and density log. The gamma

ray log uses a scintillatation counter to measure the natural radioactivity of Formation as the

sonde is drawn up the borehole. The main radioactive element in the rocks is potassium (K-40)

which is commonly found in illitic clay and to a lesser extent in feldspars, mica and glauconite.

Organic matter commonly scavenges uranium and thorium and thus oil source rock, oil shale,

sapropetlites and algal coals are radioactive. The gamma ray is measured in APIº units and

generally plotted on the scale of 0-100 or 0-120 APIº (Selley, 1985). Conventionally, the natural

gamma ray log reading is presented on the left hand column of the log in a manner similar to S.P.

log. A shale baseline is drawn and deflection from base line gives idea about rocks. Deflection to

left from baseline means clean lithology i.e. sandstone or carbonates. In general, limestone and

sandstones have a range of 0-75 APIº while organic shale and oil have a range of 50-120 APIº

and 110-200 APIº, respectively. So with the help of gamma ray log formation can be evaluated

(Selley, 1985).

In Well-D, Chorgali and Sakesar (Eocene) and Patala (Paleocene) were evaluated by

gamma ray log (Figure-4.2, i). The values of GR log for Chorgali Formation was in the range of

28-50 API° which is a typical value for Limestone and sandstone lithology (Selley, 1985). At the

start of Formation GR values was high i.e. 50 API° which was due to the presence of organic

shale (Selley, 1985). With increase in burial depth the GR values decreased which indicate

interbedded shale with limestone formation. Sakesar formation showed gamma ray response

from 18-50 API° (Table-4.1). the lower and upper part of formation have low API° values but

the middle part of formation showed high GR value i.e. 50 API° due to interbedded shale. Patala

formation being a potential source rock of Potwar Basin showed high value of gamma ray

response i.e. upto 75 API°. Such high value indicates shale having organic matter and oil

expulsion tendency is typical of potential source rock which Patala formation exhibits.

In Well-E, Eocene (Chorgali and Sakesar) and Paleocene (Patala) were analyzed by

gamma ray (Figure-4.2, ii). With increase in burial depth, a gradual decrease in gamma ray

response was observed in Chorgali formation (Table-4.1). The decrease continued till the lower

part of formation where GR log value increased upto 42 API° which indicates the presence of

interbedded shale content.

49

37

50

38

00

38

50

39

00

39

50

40

00

40

50

02

55

07

51

00

i

Ch

org

ali

Sa

ke

sa

r

Pa

tala

We

ll-D

46

50

47

00

47

50

48

00

48

50

02

55

07

51

00

Ch

org

ali

Sa

ke

sa

r

Pa

tala

ii

We

ll-E

iii

Ch

org

ali

Sa

ke

sa

r

Pa

tala

40

20

40

40

40

60

40

80

41

00

41

20

41

40

41

60

41

80

02

55

07

51

00

Fig

ure

-4.2

: R

esp

on

se o

f G

am

ma R

ay (

GR

) lo

g w

ith

dep

th f

or

Ch

org

ali

, S

ak

esar

an

d P

ata

la f

orm

ati

on

s w

ith

in i

)

W

ell-

D, ii

) W

ell-

E a

nd

iii

) W

ell-

F. R

efer

to

Fig

ure

-2.1

fo

r li

tho

lgy

of

form

ati

on

s.

49

Sakesar formation showed gamma ray response in a range of 15 to 40 API°. At the start of

formation depth, response decreased to its minimum value i.e. 15 API°, a clear indication of

presence of limestone and sandstone (Selley, 1985). At the middle of formation GR log reached

upto 40 API° which indicated the presence of organic shale (Selley, 1985). Patala formation

again showed behavior of a potential source rock. GR log response of Patala formation was in a

range of 20 to 67 API°. The GR log values continuously increased throughout formation depth

indicated the presence of organic shale with increasing burial depth in Patala formation. Such

response supported the concept of Patala formation as a potential source rock of the study area.

In Well-F, Chorgali and Sakesar (Eocene) and Patala (Paleocene) were analyzed by

gamma ray (Figure-4.2, iii). Chorgali formation showed almost a linear response having GR

values between 35-40 API°. These values indicated the presence of both shale and limestone but

shale was not a source rock rather act as reservoir rock. Sakesar formation also showed nearly

linear response except at the end of formation where GR log value reached upto 50 API°. This

behavior is similar to upper laying Chorgali formation indicated the presence of interbedded

shale with limestone. Patala formation showed GR log values (25-26 API°) which indicated the

presence of limestone.

4.3. CONCLUSIONS

Geophysical well logs showed the presence of limestone, sandstone and interbedded

shale in geological formations of study area. Permeability decreased with depth in order of

Chorgali > Sakesar > Patala while reverse order was observed for source rock property i.e. shale

contents and organic matter was maximum for Patala and minimum for Chorgali. Reservoir

characteristics of Chorgali formation were most and least for Patala formation.

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