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Contents H1.0 POROSITY IN COMPLEX LITHOLOGY ......................................................................................1 H1.1 INTRODUCTION ....................................................................................................................1 H1.2 DETERMINATION OF POROSITY AND LITHOLOGY ............................................................4 a) Crossplots............................................................................................................................4 b) Apparent Matrix Density vs. Apparent Volumetric Cross Section Matrix Identification Plot.................................................................................................................4 H1.3 COMPLEX LITHOLOGY MIXTURES................................................................................... 12 H2.0 WORK SESSION..................................................................................................................... 15
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Introduction to Openhole Logging
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Schlumberger
H1.0 Porosity in Complex Lithology H1.1 INTRODUCTION As previously mentioned, carbonate deposits generally are complex in lithology. The mineral composition of the nonclay fraction (i.e., the matrix) usually varies within a given formation. The deposition may include - shale (silt and clay) - limestone - dolomite - anhydrite/gypsum. Accurate porosity determination becomes more difficult when the matrix lithology is unknown or consists of two or more minerals of unknown proportions. The content of the formation pore space, if other than water, can also complicate analysis. Sonic, density and neutron logs respond differently and independently to different matrix combinations and to the presence of light hydrocarbons. We use these characteristics to our advantage by combining (crossplotting) two or more log responses to furnish more information about the formation and its contents than can be obtained from a single measurement
(Figures H1 through H3). In evaluating complex lithologies it is essential that comparative analysis be made only within distinct geologic units. The minimum required logs are a deep resistivity, neutron porosity, bulk density, Pe , sonic velocity and gamma ray. Only clean zones should be evaluated (GR < 30–45 API) because the addition of shale in carbonates has an extremely variable affect on porosity and resistivity measurements. All measurements should also be evaluated as to their accuracy with respect to borehole conditions (e.g. too high a correction on the density measurement or invasion effect on the resistivity measurement). As an aid to evaluation, additional measurements are available that simplify assumptions and aid in lithology identification and saturation calculations. These include the AIT Array Induction Imager logs, EPT Electromagnetic Propagation logs, Formation MicroScanner images, NGS logs, and Rxo logs (MicroSFL and microlog) to name a few.
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Introduction to Openhole Logging
Figure H1: Complex Lithology Evaluation
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Schlumberger
φ
φ
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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Introduction to Openhole Logging
H1.2 DETERMINATION OF POROSITY AND LITHOLOGY a) Crossplots Crossplotting two porosity logs is a convenient, relatively simple method of assessing both porosity and lithology information. Consider a clean (shale-free) water-filled formation. Using neutron (CNT log) and density (Litho-Density log) porosities, charts CP-1 (Figure H4) is scaled in limestone units. The charts are entered with porosity values computed assuming the matrix is a watersaturated limestone. Pure (water-filled) lithology lines are displayed for other matrices. If the formation is water-filled limestone, the points will fall on the limestone line. A clean, water-saturated mixture of limestone and dolomite will fall between the limestone and dolomite line. Formation porosity may be evaluated and the matrix mixture estimated. Beginning on the next page, charts for the following crossplots are supplied: a) Porosity and lithology determination from Litho-Density log and CNL Compensated Neutron log (Chart CP1) b) Porosity and lithology determination from sonic log and CNL Compensated Neutron log (Chart CP-2) c) Lithology identification from formation density log and sonic log (Chart CP-7).
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b) Apparent Matrix Density (ρmaa) versus apparent volumetric cross section (Umaa) Matrix Identification Plot A more competent method of identifying lithology uses data from the Litho-Density log. This common method requires two pieces of information ρmaa and Umaa . 1. Solving for these parameters first requires apparent total porosity (φta ) using the appropriate neutron-density crossplot (CP-1e). Next, bulk density and Pe values must be read from the log over the section of interest. 2. Next the apparent matrix grain density is obtained. By equation: ρb - φta ρf ρmaa = 1 – φta where: ρb is bulk density from density log ρf is pore fluid density and φ ta is apparent total porosity. Chart CP-14 (Figure H7) can be used to graphically obtain ρmaa . Using the lower lefthand quadrant of the chart, values for φt a and ρb are used to obtain ρmaa from the x-axis.
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Porosity and Lithology Determination from Litho-Density* Log and CNL* Compensated Neutron Log Liquid-Filled Holes ρf = 1.000 g/cc, Cf = 0 ppm Liquid-filled holes (ρf = 1.000 g/cm3; Cf = 0 ppm) 1.9
45
40
2.0
40
Sulfur Salt
35
ity os r Po
2.3
15
2.4
30
ne sto d n 25 sa tz ) r a ne to Qu s e (lim 20 te 25 lci Ca
20
15
20
2.5
10 2.6
35
30
25
10
5
35
30
30
25
20
15
ite m o l Do
10
15
5
5
0
10 0
0
5 –5
2.8
0 –10
2.9
3.0
–15
Anhydrite 0
10
20
30
40
φCNLcor, neutron porosity index (p.u.) (apparent limestone porosity)
CP-1e Figure H4
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φD, density porosity (p.u.) (ρma = 2.71; ρf = 1.0)
Ap pro xim co gas ate rre ctio n
2.2
ρb, bulk density (g/cm3)
40
35
2.1
2.7
45
Introduction to Openhole Logging
Porosity and Lithology Determination from Sonic Log and CNL* Compensated Neutron Log tf = 620 µs/m, Cf = 0 ppm
t f = 620 µsec/m; Cf = 0 ppm 40
360
40
Time average Field observation
20
10
15
10
200
25
15
10
5 5
20
15
10
15
20
30
20
15 240
Ca lci te Do 25 25 (lim lom es ton ite e) 25
20 20
260
Sa lt
t , sonic transit time (µsec/m)
280
220
35
30
30
Po ro sit y 25 25
300
35
320
3535
Qu 30 30 ar tz sa nd sto ne 30
35
340
10
15
0
0
5
180
10
0
5
0
160
5
An hy dr ite
5
0
0 140
0
10
20
30
φCNLcor , neutron porosity index (p.u.) (apparent limestone porosity)
CP-2cm Figure H5
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40
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Lithology Identification from Formation Density Log and Sonic Log tf = 620 µs/m ρf = 1.0
t f = 620 µsec/m; ρf = 1.0 1.8
Sylvite 1.9 Time average Field observation
40
40
2.0
40
Salt Sulfur 2.1
30
40
Trona
30
2.2
40
30
30
30
20 20 20
Gypsum
20
2.3
2.6
2.7
2.8
2.9
20
2.5
20 10 10
2.4
Qu 0 0 C ar 0 alc 0 D tz olo ite s m a (lim nd ite sto es to ne ne 10 ) 10 0 0 10 10
ρb, bulk density (g/cm3)
ty si ro o P
Polyhalite
Anhydrite 3.0 150
200
250
300
350
400
t , sonic transit time (µsec/m) CP-7m Figure H6
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Introduction to Openhole Logging
Determination of Apparent Matrix Parameters from Bulk Density or Interval Transit Time and Apparent Total Porosity Fluid Density = 1.0 Fluid density = 1.0
t maa, apparent matrix transit time (µsec/m) 350 3
325
300
275
250
225
200
175
150
125
100 350
2.9
325 40
30
N eu tro nso ni c
2.6
Apparent crossplot porosity
275
250
20
10
2.5
225
D en si ty -n eu tro n
ρb, bulk density (g/cm3)
2.7
300
2.4
200
10
2.3
175
20
2.2
150
30
2.1
125
40
2
100 3
2.9
2.8
2.7
2.6
2.5
2.4
2.3
ρmaa, apparent matrix density (g/cm3)
CP-14m Figure H7
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2.2
2.1
2
t , interval transit time (µsec/m)
2.8
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3. Finally, the apparent matrix volumetric cross section is computed from the photoelectric cross-section index, bulk density measurements and apparent total porosity by equation Pe ρe – φta Uf Umaa = 1 – φta where Pe is photoelectric absorption crosssection index,
ρb + 0.1883
ρe is electron density, ρe =
1.0704
and φ ta is apparent total porosity. Chart CP-20 (Figure H8) can be used to graphically obtain Umaa .
Pe Quartz Calcite Dolomite Anhydrite Halite Siderite Pyrite Barite Water (fresh) Water (100K ppm NaCl) Water (200K ppm NaCl) Oil (n(CH2)) Gas (CH4)
1.810 5.080 3.140 5.050 4.650 14.700 17.000 267.000 0.358 0.734 1.120 0.119 0.095
Table H1 lists the photoelectric absorption cross-section index, bulk density and the volumetric cross section for common minerals and fluids. For the minerals, the listed value is the matrix value (ρma , Uma ); for the fluids, it is the fluid value (ρf , Uf ). Chart CP-21 (Figure H9) shows the location of these minerals on a ρmaa versus Umaa crossplot. The triangle encompassing the three common matrix minerals of quartz, calcite and dolomite is scaled in the percentages of each mineral. For example, a point exhibiting an apparent matrix grain density of 2.76 g/cm3 and volumetric cross section of 10.2 barns/cm3 would be defined by the crossplot as 40% calcite, 40% dolomite and 20% quartz provided no other minerals exist and the pores are liquid saturated. On this crossplot, gas saturation displaces points to the right. Clays and shales plot below the dolomite point.
Specific gravity 2.65 2.71 2.85 2.96 2.17 3.94 5.00 4.48 1.00 1.06 1.12 ρo ρg
ρbLOG 2.64 2.71 2.85 2.98 2.04 3.89 4.99 4.09 1.00 1.05 1.11 1.22 ρo – 0.118 1.33 ρg – 0.188
U 4.780 13.800 9.000 14.900 9.680 55.900 82.100 1065.000 0.398 0.850 1.360 0.136 ρo 0.119ρg
Table H1
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Introduction to Openhole Logging
Determination of Apparent Matrix Volumetric Photoelectric Factor 3.0 Fresh water (0 ppk), ρf = 1.0, U f = 0.398 Salt water (200 ppk), ρf = 1.11, U f = 1.36
% 0
2.5
20
2.0
30 40
6
5
4
3
2
1
4
Pe, photoelectric factor
6
8
10
12
φ ta, apparent total porosity (%)
ρb, bulk density (g/cm3)
10
14
Umaa, apparent matrix volumetric photoelectric factor
The Matrix Identification Plot ρmaa vs Umaa
MID Plot CP-21 identifies rock mineralogy through a comparison of apparent matrix grain density and apparent volumetric ph otoelectric factor. To use, apparent matrix grain density, ρmaa, and apparent volumetric photoelectric factor, Umaa, are entered in ordinate and abscissa, respectively, of the MID Plot. Rock mineralogy is identified by the proximity of the plotted data point to the labeled points on the plot. To determine apparent matrix grain density, an apparent total porosity must first be determined (using, for example, a ne utron-density crossplot). Then Chart CP-14 may be used with bulk density, ρb , to define the apparent matrix grain density, ρmaa. To find the apparent matrix volumetric photoelectric factor, Umaa, enter the nomograph above with the photoelectric factor, Pe; go vertically to the bulk density, ρb; then go horizontally across to the total porosity, φt ; and finally, go vertically downward to define the matrix volumetric photoelectric factor, U maa. EXAMPLE:
P e = 3.65 ρ b = 2.52 g/cm 2 (ρf = 1.0 g/cm 2 ) φta = 16% Giving, ρ 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
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Matrix Identification Plot ρmaa vs Umaa
ρmaa versus Umaa 2.2
2.3 Salt
tion Gas direc
2.5
K-Feldspar
2.6
2.7
% Calcit e
20
Quartz
40
60
80
80
Calcite
60 %
2.8
rtz ua Q
ρmaa, apparent matrix grain density (g/cm3)
2.4
20 40
40 60
20
80
%
Dolomite
2.9
Barite
ite lom o D
Heavy minerals
Anhydrite
3.0 Kaolinite Illite 3.1 2
4
6
8
10
12
14
16
Umaa, apparent matrix volumetric photoelectric factor
CP-21 Figure H9
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Introduction to Openhole Logging
Additionally, the quartz point can be flipped about the limestone-dolomite line to form a limestone-anhydrite-dolomite model. This model is a useful variation of Chart CP-21 (Figure H9) in carbonate sequences. H1.3 COMPLEX LITHOLOGY MIXTURES Mathematically, the transformation of the basic measurement of a porosity or other appropriate log into porosity and/or lithology and/or pore fluid identification is simply the solution of one or more simultaneous equations. When the rock matrix contains only a single known mineral and the saturating fluid is also known, any one of the porosity logs can be used for porosity identification. In other words, a single equation (single log measurement) is sufficient to solve for a single unknown (in this case, porosity). If, however, in addition to porosity, the rock matrix is an unknown mixture of two known minerals, then two independent equations (two log measurements) are needed to solve for the two unknowns (in this case, the porosity and the mineral fractions). For example, in a limestone-dolomite mixture, the combination of neutron and density logs could be used. Their responses to porosity and lithology are ρ b = φρ f + (1 – φ)(LρmaL + DρmaD) and φ N = φ [HI]f + (1 – φ)(L[HI]maL + D[HI]maD),
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where ρb and φ N are the measured bulk density and apparent limestone porosity from the density and neutron logs, respectively HI is the hydrogen index
ρf and [HI]f are the density and hydrogen index of the fluid saturating the pores investigated by the density and neutron logs φ is the porosity; ρmaL and ρmaD are the grain densities of limestone and dolomite, respectively; [HI]maL and [HI]maD are the hydrogen indices of limestone and dolomite L and D are the fractions of limestone and dolomite in the rock matrix mixture.
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Three unknowns exist in these two equations: φ, L and D. However, because the mineral fractions of the rock matrix must total unity, the dolomite fraction could be expressed in terms of the limestone fraction as D = 1 – L, thereby reducing the number of unknowns in the above equation to two; or a third material balance equation of L + D = 1 could be included. In either event, solution for φ, L and D is possible because the number of equations (and independent log measurements) equals the number of unknowns. The several crossplot charts that plot one log measurement against another are simply approximate graphical solutions of the responses of two log measurements for porosity and lithology determination. Charts CP-1, CP-2, and CP-7 (Figures H4, H5 and H6, respectively) are examples. These charts can also be used when the rock matrix is composed of a single, but unknown, mineral. The problem is the same; it is one of two equations and two unknowns. The unknowns, in this situation, are porosity and mineral identification (i.e., its ρma and φma characteristics). It is presumed that ρma and φ m a are known for most minerals expected in sedimentary rocks.
When more unknowns exist, such as in a rock matrix made up of three minerals, another independent equation (or log measurement) is required. Using sonic porosity as an example, the equations for a limestone-dolomite-quartz mixture become ρ b = φρ f + (1 – φ)(LρmaL + DρmaD + SρmaS ) φ N = φ [HI]f + (1–φ)(L[HI]maL + D[HI]maD + S[HI]maS ) t = φ tf + (1 - φ)(LtmaL + DtmaD + StmaS ) 1 = L + D + S. Simultaneous solution of these four equations yields values for the four unknowns (L, D, S and φ). The ρmaa versus Umaa matrix identification plot (Chart CP-21 in Figure H9) is a graphical solution to a four unknown – four equation system. Even more complex mixtures can be unravelled by adding more equations (log measurements). Of course, the additional log measurements must respond to the same, but not necessarily all, unknown petrophysical parameters; they should not introduce additional unknowns into the problem.
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Introduction to Openhole Logging
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H2.0 Work Session 1. Using the complex lithology example logs (Figures H10 – H12) determine a. Lithology and φ at 1377 m. b. Lithology and φ from 1360-1370 m. c. Lithology and φ at 1342-1349 m. d. Is there any secondary φ in any of the zones? 2a. Find the crossplot porosities for points A and B (Figures H13 and H14). A φ = ________% B φ = ________% 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 φ =________% B φ =________% 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 φ =________%
(05/96) H-15
Introduction to Openhole Logging
BS1
PEF
125.00
375.00
0.0
10.000
CALI(MM )
NPHI(V/V )
125.00
375.00
.45000
-.1500
GR(GAPI)
DPHI(V/V )
0.0
150.00
.45000
-.1500
LIMESTONE CP 32.6
FILE
2
05-JUN-1992 11:26
MDEN = 2710 K/M3 FD = 1000 K/M3
1350
---PEF NPHI--DPHI-----BS1 ---CALI ---GR 1375
Figure H10: Complex Lithology (05/96) H-16
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BS1 125.00
375.00 CALI(MM )
125.00
375.00 GR(GAPI)
DT(US/M)
0.0
150.00
CP 32.6
FILE
1
500.00
300.00
05-JUN-1992 11:17
DT-----BS1 ---CALI ---GR
1350
1375
Figure H11: Complex Lithology (05/96) H-17
Introduction to Openhole Logging
BS1 125.00
375.00
1325
CALI(MM )
DRHO(K/M3)
125.00
375.00
250.00
150.00
2000.0
-250.0
GR(GAPI)
RHOB(K/M3)
0.0
CP 32.6
FILE
5
3000.0
01-APR-1941 18:52
RHOB--1350 ---BS1 ---DRHO ---CALI ---GR
1375
Figure H12: Complex Lithology (05/96) H-18
Schlumberger
BS1 125.00
375.00 GR(GAPI)
NPHI(V/V )
0.0
150.00
.45000
-.1500
CALI(MM )
DPHI(V/V )
125.00
375.00
.45000
LIMESTONE CP 32.6
FILE
7
-.1500
LIMESTONE 09-JUN-1992 14:30
INPUT FILE(S) CREATION DATE 1 09-JUN-1992 14:05 1/240
A ---BS1 25 ---GR NPHI-----CALI DPHI---
---BS1 ---GR NPHI--50
B
---CALI DPHI---
Figure H13: Complex Lithology (05/96) H-19
Introduction to Openhole Logging
BS1
PEF
125.00
375.00
0.0
150.00
.45000
10.000
GR(GAPI)
NPHI(V/V )
0.0
-.1500
CALI(MM )
DPHI(V/V )
125.00
375.00
.45000
-.1500
LIMESTONE
LIMESTONE CP 32.6
FILE
5
09-JUN-1992 14:28
INPUT FILE(S) CREATION DATE 1 09-JUN-1992 14:05 1/240
A ---PEF ---BS1 ---GR 25 NPHI-----CALI DPHI---
---PEF ---BS1 ---GR NPHI-----CALI DPHI--50
B
Figure H14: Complex Lithology (05/96) H-20
Schlumberger
Porosity and Lithology Determination from Litho-Density* Log Fresh Water, Liquid-Filled Holes, ρf = 1.0 Fresh water, liquid-filled holes (ρf = 1.0)
0
2.0
40 Salt
40
1.9
20
ne) (limesto Calcite
30
10
20
2.5
Dolomite
2.4
10
ρb, bulk density (g/cm3)
2.3
20
Quartz sandstone
2.2
30
30
40
2.1
10
0
2.6
0
2.7
0
2.8
0
Anhydrite
2.9
3.0 0
1
2
3
4
5
6
Pe, photoelectric factor
See Reference 27 for more information
CP-16 Figure H15 (05/96) H-21
Introduction to Openhole Logging
(05/96) H-22
