2. TWO-MEASUREMENT POROSITY
CROSSPLOTS
• Two measurements determine two unknowns
– Formations with one lithology
• Lithology
• Porosity
– Formations of two known constituents
• Can determine a more accurate value of porosity
• Can determine the percentage of each mineral
– Complex lithologies
• Can determine a more accurate value of porosity
• Cannot determine percentage mineral makeup
3. COMMON POROSITY CROSSPLOTS
• Neutron-density
• Sonic-neutron
• Sonic-density
• All have complicating effects
– Shaliness
– Hydrocarbons (gas)
– Fractures
4. NEUTRON-DENSITY CROSSPLOTS
• Most frequently used
• Developed for clean, liquid-saturated
formations
• Boreholes filled with water or water-
based muds
• Several charts, depending on tools
– CP-1’s Schlumberger
– CNT-K-1’s and DSN-II-1’s Halliburton
5. • Axes
– Neutron limestone
– Density
• NE-SW Lith. Lines
– Sandstone
– Limestone
– Dolomite
NEUTRON
DENSITY
CROSSPLOT
• NW-SE Por. Lines
– Connect equal por pts
– Nearly parallel
φ = 20
Sulfur
Salt
1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
Trona
Polyhalite
Langbeinite
Bulk
density
(Mmg/m
3
or
g/cc)
40
30
20
10
0
Apparent neutron porosity (lspu)
φ = 10
φ = 30
22%
2.45
A
B
C
% Ls = BC / AC
% Dol = AB / AC
Matrix may be:
• Ls and dolomite
• Ss and dolomite
~2.83
8. • Parallel lith. lines
– Constant separation
– Distinct position
– Log presentation
• Limestone scaling
– Neutron 45 to -15 lspu
– Density 1.95 to 2.95 g/cc
– LS ρ and φ values overlie
in Ls
• Other scalings
possible
|
2.7
0
|
DENSITY-NEUTRON
PRESENTATION
φ=12 ss ρb =2.45; φn = 9
φ=30 ss ρb =2.15; φn = 27
φ=12 dol ρb =2.67; φn = 15
φ=30 dol ρb =2.33; φn = 33
φ=12 ls
ρb ~2.53
φ=30 ls ρb ~2.23
9. EXAMPLE LOG - WELL “X”
• 90-260ft Zone D
– OWC @ 150ft
– Pe = 1.8 - 2: SS
– Den-Neut
• LS compatible
• Ntrn to rt of Den
• SS lithology
• Large sep. above
150 ft
• Shales
– Pe = 2.5 - 3
– Neutron left of
Density
B
C
D
OWC
11. EXAMPLE LOG - DN PLOT, WELL “X”
Shale Effect
Borehole effects and calcite
stringers cause outliers
12. WELL “X”
DN PLOT
(EXCEL)
Density-Neutron Response
1.9
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
-5 0 5 10 15 20 25 30 35 40 45
Neutron Porosity, limestone
Bulk
density,
g/cc
Qtz Calcite Dolomite
GR < 30 & ILD < 1 GR > 80 GR < 30 & ILD > 1
Shale plots like dol; use GR
to differentiate
Oil
Water
Values do not all fall
on Ss line owing to:
• Borehole effects
• Carbonate cements
• Shaliness
• HC vs water
• Statistical fluctuations of tools
13. GAS EFFECT - 1
WELL “A”
• Zone (depth)
– A: 0 - 20
– B: 20 - 45
– C: 45 - 335
– D: 335 - 350
• A
– Low GR
– Den = 2.95 (high)
– Neut por. = -2
• B, D – high GR
• C
– D-N litho scaling
– GWC @ 290 ft.
– D-N near overlay
below
A
B
C
D
LS Calibration
Gas
Effect
0
15. DENSITY-NEUTRON - OTHER
COMMENTS
• Density log may be displayed as porosity
– Density-Neutron overlay for water- filled lithology
– Curve order as with g/cc scaling
• Shale/Gas effects
– Across litho lines: Lithology most affected
– Along porosity lines: Porosity least affected
• Response lines change with tool type
20. WELL “X,” SN
PLOT W/ GR
Sonic-Neutron (CNL), Fresh Mud
40
50
60
70
80
90
100
110
120
-5 0 5 10 15 20 25 30 35 40
Neutron, apparent LS pu
Sonic
slowness,
ms/ft
Quartz Calcite Dolomite GR < 30 GR > 80
• Shaliness
• Optimistic
porosity
• Lithology OK
• HC effect slight
21. COMMON POROSITY CROSSPLOTS
• Neutron-density
• Sonic-neutron
• Sonic-density
• Poor porosity resolution
• Useful for some evaporites
22. SONIC-DENSITY CROSSPLOTS
• Poor
lithology and
porosity
• Multiple lines
– WTA
– RHG
• Useful for
– Evaporites
– Vsh
Density-Sonic Response
Field Lines Only
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
40 60 80 100
Sonic DT
Density
Qtz Calcite Dolomite
23. WELL “X,”
DENSITY-SONIC
• Clean points cover
all lines
• Shale point distinct
Density-Sonic Response
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
40 60 80 100
Sonic DT
D
e
n
s
ity
Qtz Calcite Dolomite GR < 30 GR > 90
25. DENSITY-
SONIC
Vsh
• Locate 100%shale
and clean
formation
• Grid to give Vsh
Density-Sonic Response
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
40 60 80 100
Sonic DT
D
e
n
s
ity
Qtz Calcite Dolomite GR < 30 GR > 90
Vsh = 1
Vsh = 0
What is shale
content of
formation comprised
of 2 minerals?
26. MID PLOTS
PRINCIPLES
• Lith. lines
– 3 of many poss.
– Each line diff. lith.
1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
Bulk
density
(Mg/m
3
or
g/cc)
40
30
20
10
0
Apparent neutron porosity (lspu)
• Line definition
– Use endpoint loc’n
– Value of ρb for φ = 0
– Called ρmaa
ρmaa= 2.68
ρb= 2.30, φn = 21
• Example
Logs show ρb= 2.30, φn = 21
ρmaa = 2.68
1. DETERMINE APPARENT MATRIX DENSITY
27. MID PLOTS
PRINCIPLES
• Lithology Lines
– Similar to D-N
• Line Definition
– Use endpoint loc’n
– Value of Δt for φ = 0
– Called Δtmaa
• Example
Logs show Δt = 78, φn = 21
Δtmaa = 53
110
100
90
80
70
60
50
40
40
30
20
10
0
Syivite
Trona
t,
Sonic
transit
time
(μs/ft)
Field observation
Apparent neutron porosity (lspu)
Δt = 78, φn = 21
Δtmaa= 53
2. DETERMINE APPARENT MATRIX TRANSIT TIME
28. THREE - MEASUREMENT CROSSPLOTS
• For lithology determination only
• Used when 3 porosity logs available
• Three types
– Density-Neutron-Sonic (M-N plot)
– Density-Neutron-Sonic (MID plot)
– Density-Neutron-Pe (Umaa -- ρmaa plot)
30. Calculating Δtmaa and ρmaa
• At each depth: Δt, ρb, and φn
• 1. Obtain φDN and φSN from crossplots
• 2. ρmaa = (ρb − φDNρfl)/(1 - φDN)
• 3. Δtmaa = Δt − (φSNΔt)/0.7
– 0.7 factor may vary somewhat
31. SUMMARY
• 3 types of porosity logs
• Density, Neutron, Sonic
• These logs respond differently to matrix,
fluids and pore types
• Use for 2-log crossplots
• Crossplots allow determination of
porosity and proportions of 2 minerals,
if mineralogy is known
• Neutron – density is most commonly
used crossplot
