POROSITY
CALCULATIONS
Sediment porosities can be
determined from analyses of recovered cores and from numerous borehole log
measurements (reviewed by Serra, 1984). At Sites 994, 995, and 997 we have
attempted to use data from the lithodensity (HLDT), neutron porosity (CNT-G),
and electrical resistivity (DITE) logs to calculate sediment porosities.
Core-derived physical property data, including porosities (Shipboard Scientific
Party, 1996a, 1996b, 1996c), have been used to both calibrate and evaluate the
log-derived sediment porosities.
Core
Porosities
On Leg 164, water content,
wet bulk density, dry bulk density, and grain density were routinely determined
from recovered sediment cores. Other related physical property data, including
sediment porosities, were calculated from these "index properties" (Paull,
Matsumoto, Wallace, et al., 1996). The core-derived porosities actually
represent the measured total water content of the sediments, which include
interlayer, bound, and free water. Most downhole logs also measure the total
water content of the sediments; thus the core- and log-derived sediment
porosities should be the same. Sediment core porosities determined from Sites
994, 995, and 997 are shown in Figure
6. In general, the core-derived sediment porosities decrease from about
80% near the top of each hole to about 50% at the bottom.
Density Log
Porosities
The HLDT log measurements
of bulk density in Holes 994D, 995B, and 997B (Fig.
4A-C) are highly variable and range from a maximum of about 1.9 g/cm3
to a minimum value of about 1.2 g/cm3. Other physical property data
from the Blake Ridge, including core-derived sediment wet bulk densities and
porosities (Fig. 6) (Shipboard
Scientific Party, 1996a, 1996b, 1996c), are not consistent with the density log
measurements. The core-derived bulk densities are relatively constant with depth
and are characterized by a relatively limited range of values. It is likely that
the density logs from all three sites have been severely degraded by both the
rugosity and the enlarged size of the boreholes. Before using the log-derived
bulk-density data to calculate sediment porosities, we have attempted to
systematically remove the erroneous data from the recorded density logs at Sites
994, 995, and 997. The detailed analysis of the recorded logs indicates that the
density log yields erroneous low values when the borehole exceeded a diameter of
about 36 cm. In addition, it appears that when the borehole size is reduced,
below a diameter of about 28 cm, the density log yields erroneous high values.
Therefore, we have systematically deleted all of the density log data from the
portion of Holes 994D, 995B, and 997B where the caliper log from the density
tool indicates that the hole diameter is more than 36 cm or less than 28 cm. The
edited density log curve still contained numerous unreasonably low density
"spikes" that usually consist of only one or two data points.
Therefore, any log measured bulk density values of less than 1.6 g/cm3
were also deleted from the recorded log traces. The edited bulk-density (
b)
log measurements were then used to calculate sediment porosities (
)
in Holes 994D, 995B, and 997B using the standard density-porosity relation:
= (m-b)/(m-f)
(Serra, 1984). Water densities (f)
were assumed to be constant and equal to 1.05 g/cm3 for each hole;
however, variable core-derived grain/matrix densities (m)
were assumed for each calculation. The core-derived grain densities (m)
in Holes 994C, 995A, and 997A range from an average value at the seafloor of
about 2.72 g/cm3 to about 2.69 g/cm3 at the bottom of each
hole (Shipboard Scientific Party, 1996a, 1996b, 1996c). The density log porosity
calculations from all three sites yielded values ranging from about 50% to near
70% (Fig. 6). The density
log-derived porosities are more variable and generally higher than the
core-derived porosities also shown in Figure
6. It appears that the density log porosities overestimate both the
range and absolute porosities for the sediments on the Blake Ridge. It is likely
the high clay content and unlithified nature of the sediments on the Blake Ridge
have contributed to the inability of the density tool to make good contact with
the borehole wall, which leads to erroneous density log measurements that cannot
be further corrected. Data from the density logs in Holes 994D, 995B, and 997B
can be used to assess general porosity trends but not for quantitative
calculations.
Neutron
Porosity Log
Because of poor hole
conditions, the CNT-G was run in only two holes (Holes 994C and 995B) on the
Blake Ridge. The CNT-G measures the amount of hydrogen within the pore space of
a sedimentary sequence, which is mostly controlled by the amount of water that
is present. The CNT-G has two pairs of detectors that indirectly measure both
epithermal (intermediate energy level) and thermal (low energy) neutrons, which
provide two porosity measurements. The recorded neutron porosity log data from
Holes 994C and 995B reveal an average thermal porosity of about 50%, and the
epithermal porosity averages about 100%. The thermal and epithermal porosity
logs are calibrated to read 50% and 100%, respectively, in water (no sediment).
Therefore, the neutron porosity log in Holes 994C and 995B only detected the
hydrogen in the borehole waters and the porosity data from the neutron log is of
no value. In "standard" industry applications the CNT-G is run with a
bowspring that keeps the tool near the wall of the hole, thus reducing the
effects of an enlarged borehole. Because of the size limitation of running the
logs through the drillpipe, it is impossible to use a bowspring on the CNT-G in
ODP holes.
Resistivity
Log Calculated Porosities
One approach to obtaining
sediment porosities from downhole logs is to use the electrical resistivity logs
(Fig. 4A-C) and Archie's
relationship between the resistivity of the formation (Rt)
and porosity (): Rt/Rw
= a -m,
where a and m are constants to be determined and Rw
is the resistivity of the pore-waters (Archie, 1942).
The resistivity of
pore-waters (Rw) is mainly a function of the temperature and
the dissolved salt content (salinity) of the pore waters. Pore-water salinity
data from Sites 994, 995, and 997 are available from the analyses of
interstitial water samples collected from recovered cores at each site
(Shipboard Scientific Party, 1996a, 1996b, 1996c). The interstitial water
salinity trends at all three core sites mimic the interstitial water chloride
trends discussed earlier in this report (Fig.
5). In general, the core-derived interstitial water salinities decrease
with depth from a maximum value of about 35 ppt near the sediment-water
interface to about 31 ppt within the upper part of logging Unit 2. Similar to
the chloride profiles in logging Unit 2, the interstitial water salinities are
also more variable within this inferred gas hydrate-bearing sedimentary section.
Formation and seabed temperatures have been directly measured at all three core
sites on the Blake Ridge, as described in Shipboard Scientific Party (1996a,
1996b, 1996c). Listed in Table 3
are the measured seabed temperatures and geothermal gradients for Sites 994,
995, and 997 (modified from Shipboard Scientific Party, 1996a, 1996b, 1996c).
Arps' formula (Serra, 1984) was used to calculate the pore-water resistivity (Rw)
at each site from the available core-derived interstitial water salinities and
measured formation temperatures (Table
3; Fig. 7). In general,
the calculated pore-water resistivities (Rw) reach a maximum
of about 0.34 
m
within 100 m of the seafloor and decrease with depth to a value below 0.20 m
at the bottom of each borehole. The small increase in water resistivities in
logging Unit 2, depicted in Figure 7,
are a result of the presence of freshwater in the analyzed cores, which was
expelled from gas hydrate that had disassociated in the cores. To avoid
introducing errors into the subsequent resistivity porosity calculations, the
gas hydrate-affected pore-water resistivities from logging Unit 2 have been
excluded and the pore-water resistivities from logging Units 1 and 3 have been
used to statistically project undisturbed pore-water resistivities (Rw)
for logging Unit 2 (Fig. 7).
To determine the Archie
constants a and m, we used the method described by Serra
(1984) and the log-measured resistivities and core-derived porosities (Paull,
Matsumoto, Wallace, et al., 1996) from each site drilled on the Blake Ridge. The
log-measured resistivity data from logging Unit 2 in each hole have been omitted
from this calculation of the Archie constants to avoid introducing an error
caused by using resistivity log-measurements that have been affected by the
occurrence of in situ gas hydrate. In addition, log-measured resistivities from
expected free-gas zones in Unit 3 of each hole have also been omitted from the
determination of the Archie constants. Linear trends in resistivity log and core
porosity data from logging Units 1 and 3 (exclusive of logging Unit 2) in each
hole have been used to calculate representative (100% water saturated) formation
resistivities (Ro) and porosities ().
From these representative values the slope, m, and the intercept, ln a,
of the function ln (Ro/Rw) = -m ln
+ ln a were
calculated for each of the logged boreholes. The calculated a and m
Archie constants for Holes 994D, 995B, and 997B have been listed in Table
3. The Archie constants (a and m) calculated for Holes
995B and 997B are similar and fall within the "normal" range of
expected values (Serra, 1984). However, the values of the a and m
constants for Hole 994D fall outside of the "normal" range of values.
The cause of these anomalous Archie constants in Hole 994D is likely because of
poor hole conditions in logging Unit 1, which has contributed to degraded
resistivity log measurements. Because all three boreholes penetrated similar
lithologic sections and because they are located in relatively close proximity
to each other, we decided to use an average value (calculated from Holes 995B
and 997B) for the a and m Archie constants throughout this
study of the Blake Ridge (Table 3:
a = 1.05, m = 2.56).
Given the Archie constants
(a and m) and pore-water resistivities (Rw),
we can now calculate sediment porosities ()
from the resistivity log using Archie's relation. The results of these
calculations are the porosity logs shown in Figure
8. The calculated resistivity porosities should be considered
"apparent" porosity values because the Archie relation assumes that
all of the void space within the sediments are filled with water (no gas
hydrate), which is not true. In all three holes, the resistivity-derived
porosities decrease with depth (Fig. 8)
though this is not the normal exponential consolidation trend that would be
expected if the pore space were decreasing with depth primarily from the weight
of increasing overburden (Lee and others, 1993); instead we see an almost linear
porosity decrease. Relative to Units 1 and 3, Unit 2 exhibits a baseline shift
to higher resistivities and lower calculated resistivity porosities. The
assumption that all of the pore space within the sediments of Unit 2 is filled
with only water is not valid. Some of the pore space in Unit 2 is occupied by
gas hydrate that exhibits higher electrical resistivities and would contribute
to an "apparent" reduction in resistivity-derived porosities.
Porosity
Calculations—Summary
The comparison of core-
and log-derived porosities in Figure 6
and Figure 8, reveals that the
resistivity log-derived porosities are generally similar to the core porosities.
The density log-derived porosities, however, are generally higher than the core
porosities. It is likely that the density log measurements have been degraded by
poor borehole conditions. The resistivity log-derived porosities in Figure
8 are the best downhole-derived porosity logs for all three holes on the
Blake Ridge. However, because of gas hydrate-induced resistivity effects in
logging Unit 2, the resistivity-derived porosity data from Leg 164 should be
used with caution and the core-derived sediment porosities are the best
available porosity data from the Blake Ridge.
