08_porosity in complex env

Schlumberger

(05/96)

Contents

H1.0 POROSITY IN COMPLEX LITHOLOGY......................................................................................1H1.1 INTRODUCTION....................................................................................................................1H1.2 DETERMINATION OF POROSITY AND LITHOLOGY ............................................................4

a) Crossplots............................................................................................................................4b) Apparent Matrix Density vs. Apparent Volumetric Cross Section Matrix Identification Plot.................................................................................................................4

H1.3 COMPLEX LITHOLOGY MIXTURES................................................................................... 12H2.0 WORK SESSION..................................................................................................................... 15

Introduction to Openhole Logging

(05/96)

Schlumberger

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H1.0 Porosity in Complex Lithology

H1.1 INTRODUCTIONAs previously mentioned, carbonate deposits

generally are complex in lithology. The min-eral composition of the nonclay fraction (i.e.,the matrix) usually varies within a given for-mation. The deposition may include

- shale (silt and clay)- limestone- dolomite- anhydrite/gypsum.

Accurate porosity determination becomesmore difficult when the matrix lithology is un-known or consists of two or more minerals ofunknown proportions. The content of the for-mation pore space, if other than water, can alsocomplicate analysis.

Sonic, density and neutron logs respond dif-ferently and independently to different matrixcombinations and to the presence of light hy-drocarbons. We use these characteristics to ouradvantage by combining (crossplotting) two ormore log responses to furnish more informa-tion about the formation and its contents thancan be obtained from a single measurement

(Figures H1 through H3). In evaluating com-plex lithologies it is essential that comparativeanalysis be made only within distinct geologicunits.

The minimum required logs are a deep re-sistivity, neutron porosity, bulk density, P

e,

sonic velocity and gamma ray. Only cleanzones should be evaluated (GR < 3045 API)because the addition of shale in carbonates hasan extremely variable affect on porosity andresistivity measurements. All measurementsshould also be evaluated as to their accuracywith respect to borehole conditions (e.g. toohigh a correction on the density measurementor invasion effect on the resistivity measure-ment). As an aid to evaluation, additionalmeasurements are available that simplify as-sumptions and aid in lithology identificationand saturation calculations. These include theAIT Array Induction Imager logs, EPT Elec-tromagnetic Propagation logs, Formation Mi-croScanner images, NGS logs, and R

xo logs

(MicroSFL and microlog) to name a few.


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Figure H1: Complex Lithology Evaluation

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Figure H2: Porosity Tool Response to Various Factors

0.5 0.4 0.3 0.2 0.1 0

Figure H3: Pe Response with Porosity Changes


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H1.2 DETERMINATION OFPOROSITY AND LITHOLOGY

a) CrossplotsCrossplotting two porosity logs is a conven-

ient, relatively simple method of assessingboth porosity and lithology information. Con-sider a clean (shale-free) water-filled forma-tion. Using neutron (CNT log) and density(Litho-Density log) porosities, charts CP-1(Figure H4) is scaled in limestone units. Thecharts are entered with porosity values com-puted assuming the matrix is a water-saturated limestone. Pure (water-filled)lithology lines are displayed for other matrices.

If the formation is water-filled limestone, thepoints will fall on the limestone line. A clean,water-saturated mixture of limestone anddolomite will fall between the limestone anddolomite line. Formation porosity may beevaluated and the matrix mixture estimated.

Beginning on the next page, charts for thefollowing crossplots are supplied:

a) Porosity and lithology determinationfrom Litho-Density log and CNLCompensated Neutron log (Chart CP-1)

b) Porosity and lithology determinationfrom sonic log and CNL Compen-sated Neutron log (Chart CP-2)

c) Lithology identification from formationdensity log and sonic log (Chart CP-7).

b) Apparent Matrix Density (maa

) versusapparent volumetric cross section(U

maa) Matrix Identification Plot

A more competent method of identifyinglithology uses data from the Litho-Density log.This common method requires two pieces ofinformation

maa and U

maa.

1. Solving for these parameters first re-quires apparent total porosity (

ta) us-

ing the appropriate neutron-densitycrossplot (CP-1e). Next, bulk densityand P

e values must be read from the

log over the section of interest.2. Next the apparent matrix grain density

is obtained. By equation:

b -

ta

f

maa

=1

ta

where:

bis bulk density from density log

f

is pore fluid density and

ta is apparent total porosity.

Chart CP-14 (Figure H7) can be usedto graphically obtain

maa. Using the

lower lefthand quadrant of the chart,values for

t a and

b are used to obtain

maa

from the x-axis.

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Porosity and Lithology Determination from Litho-Density*Log and CNL* Compensated Neutron Log

Liquid-Filled Holes f = 1.000 g/cc, Cf = 0 ppm

0 10 20 30 40

CNLcor, neutron porosity index (p.u.) (apparent limestone porosity)

1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3.0

b, b

ulk

dens

ity (

g/cm

3 )

D, d

ensi

ty p

oros

ity (

p.u.

) (

ma

= 2

.71;

f =

1.0

)

45

40

35

30

25

20

15

10

5

0

5

10

15Anhydrite

SulfurSalt

Approximategascorrection

Poro

sity

Calci

te (lim

eston

e)

0

5

10

15

20

25

30

35

40

45

Quart

z san

dston

e

0

5

10

15

20

25

30

35

40

Dolom

ite

0

5

10

15

20

25

30

35

Liquid-filled holes (f = 1.000 g/cm3; Cf = 0 ppm)

CP-1e

Figure H4


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Porosity and Lithology Determination from Sonic Logand CNL* Compensated Neutron Log

tf = 620 s/m, Cf = 0 ppm

0 10 20 30 40

360

340

320

300

280

260

240

220

200

180

160

140

CNLcor, neutron porosity index (p.u.) (apparent limestone porosity)

t,

soni

c tr

ansi

t tim

e (

sec/

m)

t f = 620 sec/m; Cf = 0 ppm

Salt

Anhy

drite

Dolo

mite

Cal

cite

(lim

esto

ne)

Qua

rtz s

ands

tone

Time averageField observation

Poro

sity

0

5

55

00

10

15

20

35

40

40

35

30

353530

20

15

10

25

20

15

10

10

15

20

3030

0

5

10

10

15

15

20

20

25

2530

30

0

5

0

5

2525

35

25

CP-2cm

Figure H5

Schlumberger

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Lithology Identification fromFormation Density Log and Sonic Log

tf = 620 s/m f = 1.0

150 200 250 300 350 400

1.8

1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3.0

t, sonic transit time (sec/m)

b, b

ulk

dens

ity (

g/cm

3 )

t f = 620 sec/m; f = 1.0Do

lom

ite

Calci

te (l

imes

tone

)

Time averageField observation

Anhydrite

Polyhalite

Gypsum

Trona

Salt

Sylvite

Sulfur

0

10

10

10 20

20

20

30

40

30

40 40

40

3030

20

20

0

0

10

0Q

uartz

san

dsto

ne

Poro

sity

40

30

20

0

0

10

10

CP-7m

Figure H6


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Determination of Apparent Matrix Parameters from BulkDensity or Interval Transit Time and Apparent Total Porosity

Fluid Density = 1.0

3 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2

350 325 300 275 250 225 200 175 150 125 100 3

2.9

2.8

2.7

2.6

2.5

2.4

2.3

2.2

2.1

2

350

325

300

275

250

225

200

175

150

125

100

Fluid density = 1.0

maa, apparent matrix density (g/cm3)

b, b

ulk

dens

ity (

g/c

m3 )

t, in

terv

al tr

ansi

t tim

e (

sec/

m)

t maa, apparent matrix transit time (sec/m)

40

30

20

10

10

20

30

40

Apparentcrossplotporosity

Dens

ity-n

eutro

n

Neut

ron-

sonic

CP-14m

Figure H7

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(05/96) H-9

3. Finally, the apparent matrix volumetriccross section is computed from thephotoelectric cross-section index, bulkdensity measurements and apparenttotal porosity by equation

Pe

e

taU

f

Umaa

=1

ta

whereP

e is photoelectric absorption cross-

section index,

b + 0.1883

e is electron density,

e =

1.0704and

ta is apparent total porosity.

Chart CP-20 (Figure H8) can be usedto graphically obtain U

maa.

Table H1 lists the photoelectric absorptioncross-section index, bulk density and thevolumetric cross section for common mineralsand fluids. For the minerals, the listed value isthe matrix value (

ma, U

ma); for the fluids, it is

the fluid value (f, U

f). Chart CP-21 (Figure

H9) shows the location of these minerals on a

maa versus U

maa crossplot. The triangle en-

compassing the three common matrix miner-als of quartz, calcite and dolomite is scaled inthe percentages of each mineral. For example,a point exhibiting an apparent matrix graindensity of 2.76 g/cm3 and volumetric crosssection of 10.2 barns/cm3 would be defined bythe crossplot as 40% calcite, 40% dolomite and20% quartz provided no other minerals existand the pores are liquid saturated.

On this crossplot, gas saturation displacespoints to the right. Clays and shales plotbelow the dolomite point.

Pe Specificgravity

bLOG U

QuartzCalciteDolomiteAnhydriteHaliteSideritePyriteBariteWater (fresh)Water (100K ppm NaCl)Water (200K ppm NaCl)Oil (n(CH2))Gas (CH4)

1.8105.0803.1405.0504.650

14.70017.000

267.0000.3580.7341.1200.1190.095

2.652.712.852.962.173.945.004.481.001.061.12

og

2.642.712.852.982.043.894.994.091.001.051.11

1.22 o 0.1181.33 g 0.188

4.78013.8009.000

14.9009.680

55.90082.100

1065.0000.3980.8501.360

0.136 o0.119g

Table H1


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Determination ofApparent Matrix Volumetric Photoelectric Factor

6 5 4 3 2 1 4 6 8 10 12 14

3.0

2.5

2.0

%

0

10

20

30

40

Pe, photoelectric factor

b, b

ulk

dens

ity (

g/c

m3 )

ta,

app

aren

t tot

al p

oros

ity (

%)

Umaa, apparent matrixvolumetric photoelectric factor

Fresh water (0 ppk), f = 1.0, Uf = 0.398Salt water (200 ppk), f = 1.11, Uf = 1.36

The Matrix Identification Plotmaa vs Umaa

MID Plot CP-21 identifies rock mineralogy through a comparison of apparent matrix grain density and apparent volumetricphotoelectric factor. To use, apparent matrix grain density, maa, and apparent volumetric photoelectric factor, Umaa, are entered in ordinate andabscissa, respectively, of the MID Plot. Rock mineralogy is identified by the proximity of the plotted data point to the la-beled points on the plot. To determine apparent matrix grain density, an apparent total porosity must first be determined (using, for example, a neu-tron-density crossplot). Then Chart CP-14 may be used with bulk density, b, to define the apparent matrix grain density,maa. To find the apparent matrix volumetric photoelectric factor, Umaa, enter the nomograph above with the photoelectric fac-tor, Pe; go vertically to the bulk density, b; then go horizontally across to the total porosity, t ; and finally, go verticallydownward to define the matrix volumetric photoelectric factor, U maa.

EXAMPLE: P e = 3.65 b = 2.52 g/cm

2 (f = 1.0 g/cm 2)ta = 16%Giving, maa = 2.81 g/cm

2 (from CP-14)and U maa = 10.9

Plotting these values on the MID Plot indicates the level to be a dolomite-limestone mixture approximately 60% dolomite -40% limestone.See Reference 27 for more information.

CP-20

Figure H8

Schlumberger

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Matrix Identification Plotmaa vs Umaa

Umaa, apparent matrix volumetric photoelectric factor

maa

, app

aren

t mat

rix g

rain

den

sity

(g

/cm

3 )

2 4 6 8 10 12 14 16

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3.0

3.1

Salt

K-Feldspar

Quartz

Dolomite

Kaolinite

Illite

Anhydrite

Heavy minerals

Barite

Calcite

Gas

dire

ctio

n

% Calcite

% Do

lomite

% Quartz

20

6080

40

60

40

20

80

60

40

20

80

maa versus Umaa

CP-21

Figure H9


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Additionally, the quartz point can be flippedabout the limestone-dolomite line to form alimestone-anhydrite-dolomite model. Thismodel is a useful variation of Chart CP-21(Figure H9) in carbonate sequences.

H1.3 COMPLEX LITHOLOGYMIXTURES

Mathematically, the transformation of thebasic measurement of a porosity or other ap-propriate log into porosity and/or lithologyand/or pore fluid identification is simply thesolution of one or more simultaneous equa-tions. When the rock matrix contains only asingle known mineral and the saturating fluidis also known, any one of the porosity logs canbe used for porosity identification. In otherwords, a single equation (single log measure-ment) is sufficient to solve for a single un-known (in this case, porosity).

If, however, in addition to porosity, the rockmatrix is an unknown mixture of two knownminerals, then two independent equations (twolog measurements) are needed to solve for thetwo unknowns (in this case, the porosity andthe mineral fractions). For example, in a lime-stone-dolomite mixture, the combination ofneutron and density logs could be used. Theirresponses to porosity and lithology are

b =

f + (1 )(L

maL + D

maD)

and

N = [HI]

f + (1 )(L[HI]

maL + D[HI]

maD),

where

b and

N are the measured bulk density and

apparent limestone porosity from the densityand neutron logs, respectively

HI is the hydrogen index

f and [HI]

f are the density and hydrogen index

of the fluid saturating the pores investigated bythe density and neutron logs

is the porosity;

maL

and maD

are the grain densities of lime-stone and dolomite, respectively;

[HI]maL

and [HI]maD

are the hydrogen indices oflimestone and dolomite

L and D are the fractions of limestone anddolomite in the rock matrix mixture.

Schlumberger

(05/96) H-13

Three unknowns exist in these two equa-tions: , L and D. However, because the min-eral fractions of the rock matrix must totalunity, the dolomite fraction could be expressedin terms of the limestone fraction as D = 1 L,thereby reducing the number of unknowns inthe above equation to two; or a third materialbalance equation of L + D = 1 could be in-cluded. In either event, solution for , L andD is possible because the number of equations(and independent log measurements) equalsthe number of unknowns.

The several crossplot charts that plot one logmeasurement against another are simply ap-proximate graphical solutions of the responsesof two log measurements for porosity andlithology determination. Charts CP-1, CP-2,and CP-7 (Figures H4, H5 and H6, respec-tively) are examples. These charts can also beused when the rock matrix is composed of asingle, but unknown, mineral. The problem isthe same; it is one of two equations and twounknowns. The unknowns, in this situation,are porosity and mineral identification (i.e., its

ma and

ma characteristics). It is presumed that

ma

and m a

are known for most minerals ex-pected in sedimentary rocks.

When more unknowns exist, such as in arock matrix made up of three minerals, anotherindependent equation (or log measurement) isrequired. Using sonic porosity as an example,the equations for a limestone-dolomite-quartzmixture become

b =

f + (1 )(L

maL + D

maD + S

maS)

N = [HI]

f + (1)(L[HI]

maL + D[HI]

maD +

S[HI]maS

)

t = tf + (1 - )(Lt

maL + Dt

maD + St

maS)

1 = L + D + S.

Simultaneous solution of these four equationsyields values for the four unknowns (L, D, Sand ). The

maa versus U

maa matrix identifica-

tion plot (Chart CP-21 in Figure H9) is agraphical solution to a four unknown fourequation system.

Even more complex mixtures can be unrav-elled by adding more equations (log measure-ments). Of course, the additional log meas-urements must respond to the same, but notnecessarily all, unknown petrophysical pa-rameters; they should not introduce additionalunknowns into the problem.


(05/96) H-14

Schlumberger

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H2.0 Work Session

1. Using the complex lithology example logs (Figures H10 H12) determine

a. Lithology and at 1377 m.

b. Lithology and from 1360-1370 m.

c. Lithology and at 1342-1349 m.

d. Is there any secondary in any of the zones?

2a. Find the crossplot porosities for points A and B (Figures H13 and H14).A = ________%B = ________%

b. What is the lithology in these zones?

3a. Cross plot Pe and DPHI for both points A and B (use chart CP-16, Figure H15).

A =________%B =________%

b. What is the lithology at points A and B?A _________B _________

c. What effect is occurring at point A?

d. Apply proper correction for point A to find correct crossplot porosity.A =________%


(05/96) H-16

1350

.45000 -.1500

DPHI(V/V )

.45000 -.1500

NPHI(V/V )

0.0 10.000

PEF

0.0 150.00

GR(GAPI)

125.00 375.00

CALI(MM )

125.00 375.00

BS1

CP 32.6 FILE 2 05-JUN-1992 11:26

FD = 1000 K/M3

MDEN = 2710 K/M3

LIMESTONE

PEFBS1

---PEF

NPHI---

DPHI---

---BS1

---CALI

1375

---GR

Figure H10: Complex Lithology

Schlumberger

(05/96) H-17

DT---

---BS1

---CALI

1350

---GR

CP 32.6 FILE 1 05-JUN-1992 11:17

500.00 300.00

DT(US/M)

0.0 150.00

GR(GAPI)

125.00 375.00

CALI(MM )

125.00 375.00

CALI(MM )

125.00 375.00

BS1

1375



(05/96) H-18

1350

---BS1

---DRHO

---CALI

---GR

1325

1350

RHOB---

CP 32.6 FILE 5 01-APR-1941 18:52

2000.0 3000.0

RHOB(K/M3)

250.00 -250.0

DRHO(K/M3)

0.0 150.00

GR(GAPI)

125.00 375.00

CALI(MM )

125.00 375.00

BS1

1375


Schlumberger

(05/96) H-19

---BS1

---GR

50

NPHI---

---CALI

DPHI---

---CALI

DPHI---

NPHI---

---GR

25

---BS1

1/240

1 09-JUN-1992 14:05 INPUT FILE(S) CREATION DATE

CP 32.6 FILE 7 09-JUN-1992 14:30

---CALI

.45000 -.1500

DPHI(V/V )

.45000 -.1500

NPHI(V/V )

125.00 375.00

CALI(MM )

0.0 150.00

GR(GAPI)

125.00 375.00

BS1

CP 32.6 FILE 7 09-JUN-1992 14:30

LIMESTONELIMESTONE

A

B



(05/96) H-20

1/240

1 09-JUN-1992 14:05 INPUT FILE(S) CREATION DATE

CP 32.6 FILE 5 09-JUN-1992 14:28

---PEF

---BS1

25

---GR

NPHI---

---CALI

DPHI---

.45000 -.1500

DPHI(V/V )

.45000 -.1500

NPHI(V/V )

0.0 10.000

PEF

125.00 375.00

CALI(MM )

0.0 150.00

GR(GAPI)

125.00 375.00

BS1

CP 32.6 FILE 5 09-JUN-1992 14:28

---PEF

---BS1

---GR

NPHI---

---CALI

50

DPHI---

A

B

LIMESTONE LIMESTONE


Schlumberger

(05/96) H-21

Porosity and Lithology Determinationfrom Litho-Density* Log

Fresh Water, Liquid-Filled Holes, f = 1.0

1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3.00 1 2 3 4 5 6

Pe, photoelectric factor

b, b

ulk

dens

ity (

g/c

m3 )

4030

2010

0

4030

2010

0

0

40

30

20

100

0

Qua

rtz

sand

ston

e

Dol

omite

Cal

cite

(lim

esto

ne)

Sal

t

Anh

ydrit

e

Fresh water, liquid-filled holes (f = 1.0)

See Reference 27 for more informationCP-16

Figure H15


(05/96) H-22

08_porosity in complex env

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