54. ELECTRICAL RESISTIVITY MEASUREMENTS OF SEDIMENTS FROM THE BLACK SEAAS A GUIDE TO DIFFUSIVE PROPERTIES

F.T.Manheim,1 Department of Marine Science, University of South Florida, St. Petersburg, Florida

ABSTRACT

Formation factor (F) has been measured for Black Sea cores bymethods previously developed in the course of the Deep Sea DrillingProject (Manheim and Waterman, 1974). The inverse of F providesa measure of the tortuosity of the sediments with respect to theability of dissolved species to migrate through interconnected pores.The diffusive permeability of unlithified sediments down to morethan 600 meters depth decreases from about one-third to betweenone-tenth and one-twentieth of that of free solution. Lithified andcemented sediments such as limestones and dolomites in the loweststrata of Sites 380 and 381 display formation factors as high as 500,or as little as l/500th of the diffusive permeability of free solution.Though retaining finite permeability to salt and gas diffusion, suchsediments form substantial barriers to migration and permit partialretention of "paleosalinities" in pore fluids where the cementationtook place soon after sediment burial.

INTRODUCTION

The ability of sediments to permit ionic andmolecular diffusion through interconnected pore spacesis important for salt migration, diagenetic phenomena,and to predict movement of dissolved hydrocarbonssuch as methane, ethane and CO2. To measure theseproperties a resistivity probe similar to that employedpreviously on DSDP legs (Manheim and Waterman,1974; Manheim, 1974) was utilized.

The method is based on the analogy between theability of interconnected pores to conduct electricityand their ability to permit diffusion of dissolved speciesin response to concentration gradients. The chiefmeasurement is the "formation factor":

where Rs is rock or sediment resistivity and Rw is theresistivity of the enclosed pore water at a commontemperature.

d = dok

where d s is the diffusion coefficient for a given ion ormolecular species in sediments, Do is the coefficient infree solution, and k = \/F = the "tortuosity factor"measuring the increase in effective path length ofdiffusion. It should be noted that the diffusioncoefficient so measured does not take into accountadsorption or reaction effects, fluid movements, orconduction of electricity by the clay matrix of a rock.Normally, the conduction of electricity by the sediment

minerals has not been significant in strata penetrated byDSDP to date. It can be shown by comparing the rateof accumulation of sediments and effective diffusioncoefficient in common units that physical convection offluids owing to consolidation of sediment must beslower, at least in the vertical direction, than movementthrough molecular diffusion (see Manheim and Schug,this volume). Thus, measurement of diffusive perme-ability via electrical conductivity (resistivity) methodscan provide an effective guide to the behavior of inter-stitial components with depth.

METHODSThe details of operation of the measurement system

are given in Manheim and Waterman (1974). Briefly,cell constants for the 4-electrode probe are measured incore liner containers on fluid of known resistivity andsalinity. The core liner containers are chosen toresemble the configuration of sediment cores in theliners as shown in Figure 1. Electrodes are normallyinserted into the ends of the 10-cm sections used forpore fluid analysis before these are placed in theinterstitial fluid extraction device. In case of lithifiedsediments, guide holes are drilled with an electrical drilland carbide-tipped bit through a template matching theelectrode spacings. These holes permit insertion ofelectrodes without rupturing the core, even in very hardsediments. The holes may be moistened with electrolyteif necessary to provide better contact.

Partial immersion of electrodes will increaseelectrical resistance over full immersion, but this effectcan be calibrated as noted in Figure 1.

'Current address: U.S. Geological Survey, Woods Hole, Massa-chusetts. where C is the cell constant.

1125

F. T. MANHEIM

10.0

9.0

« 8.0

£ 7.0

6.0-

4 .0 -

3.01.0 2.0 2.5

Electrodeimmersion (cm) Full immersion

Figure 1. Correction diagram for resistivity measurementson cores (linear inner diameter = 2.56" = 6.51 cm).

These constants must be redetermined regularly. Rw,or resistivity of pore fluid may be obtained either bydirect analysis of pore water in a Schlumberger WellSurveying Co. or other minicell, or by interpolationfrom a curve established for various "salinities" (byrefractive index) and resistivities (Figure 2). Varyingionic relationships will cause deviations from linearrelationships, but these are usually reasonablyconsistent within a given site. Hence, a curve based on adiscrete site normally gives results well within the localvariability of resistivity, including variation based onorientation of cores. In the present studies, resistivitymeasurements have been principally made in thehorizontal direction, i.e., inserting probes into the endsof cores where disturbance was least.

RESULTSFormation factors (Rs/Rw) where Rs = sediment

resistivity and Rw = pore fluid resistivity tend toincrease with depth of sediment burial. This occurs as aresult of sediment consolidation, reduction of inter-connected pore space, cementation, as well as factors asyet poorly understood.

Data on the trends are given in Tables 1-3, and inFigures 3 and 4. The tables show that owing to themarked fluctuations in pore fluid salinity in the upperlevels of the core holes, F does not correlate well withgross sediment resistivity. With depth, however,diffusion has tended to smooth out changes in composi-tion with depth, and here better relationships arefound.

The figures show that the reduction of inter-connected pore space indicated by increase in F takesplace even though gross porosity does not change sig-nificantly. A part of the variation in F can be explainedby the presence and distribution of clays. Though claysgenerally display high porosity, they are generally lesspermeable by orders of magnitude than coarse clastic

10.00

5.00-

5 10 20

"SAL" (°/oo)

50 100

Figure 2. Plot of "salinity" (°/oO) against electrical resis-tivity for pore fluids, Sites 379, 380, 381.

Q

100

200

400

500

600

2 3 4 5 6 7 8 9 10

K~\

Po

20 30

s

»-— -

rosity (%)

40 50 60 7

J

Figure 3. Distribution of formation factor and porosity withdepth in Holes 379A and 379B. Note that both measure-ments are on the same scale. The heavy dashed line de-picts an approximate smoothed depth trend. Porositiesare calculated from water content using grain densityofZ67.

sediments such as sand, that have comparable porosity.This is because their fine grain tends to create highlytortuous structures and fine capillary channels.

1126

ELECTRICAL RESISTIVITY MEASUREMENTS OF SEDIMENTS

TABLE 1Electrical Resistivity-Diffusimetry Measurements, Holes 379-379Aa

SampleNo. Core Section

Depth(m)

Salinity07.*)

ProbeResistivity

Cell Resistivity CellConstant

Rock ResistivityCorr. (n-m) Remarks

1A2A3A4A5A6A7A8A9A

10A11A12A13A15A16A17A18A19A20A21A22A23A24A25 A26A27A28A29A

IB

2B

3B4B5B6B

147

12171921232527293134384043454749515456586064

6668

Punch

3

4446

Calibration

(Surface Water)(Surface Water)(Surface Water)(Surface Water)(Surface Water)

333215555452573223414

334

345

1

1424

3.932.460.9

110.3159.4166.9185.9204.9223.9250.0268.4285.9318.9358.4372.9380.9399.9429.9450.4463.4500.9519.9534.9553.4577583.2595.9621.4

5.9

17.5

50.054.581.097.6

20.25.37.56.056.004.425.34.14.04.15.50

12.07.106.57.39.7

11.314.317.619.824.228.630.331.236.638.540.538.511.2

5.55

5.806.207.86.8

17.9

17.917.917.917.917.9

0.360_

1.120?0.6201.0700.4800.6301.0400.5710.6220.5770.2420.5700.5710.5900.4070.4620.7480.4980.6840.330?0.5210.2430.4790.351

_0.3720.3470.235

591-691379589381572

37.5

37.558.7

67.544.257.5

0.3131.1300.8400.9101.0001.281.1701.4501.5201.4251.1000.5400.8350.9350.8480.6330.5890.4600.3900.3400.2910.2570.2410.2340.2080.1970.1900.1970.518

1.090

1.1101.0100.8320.894

0.353

0.3530.3530.3530.3530.353

9.419.419.419.419.419.419.419.419.419.419.419.419.419.419.419.419.419.419.419.419.419.419.419.419.419.419.419.417.06

9.41

9.419.419 419.41

9.41

7.066.016.237.996.14

3.39 10.8 (?)

10.55.83

10.74.525.939.795.375.885.422.285.365.375.883.824.357.044.676.453.10?4.902.284.513.30

3.503.251.66

5.6-6.53.575.543.585.38

12.56.41

10.13.535.076.753.534.134.934.226.425.736.546.037.38

15.311.9

9.4310.619.19.46

19.315.6

18.416.5

3.20

5.14-

3.215.484.306.01

Disturbed?

~18°C?Disturbed?

Full immers.

5-mm gap8-mm gap1.2-cm gaphalf-roundh-r, 6-mm gap

Note: Probe resistivity is given in arbitrary units; cell resistivity refers to interstitial water at lab temperature ("23.5°C) expressed in ohm-m.Cell constant refers to cell resistivity/cup resistivity when cup resembles geometry of sediment. Container is filled with water of knownresistivity. True resistivity for sediment = Probe Res. X Cell constant. F or formation factor is given by rock or sediment resistivity/interstitialwater resistivity.a F . T. Manheim and V. S. Sotelo, analysts.

Although the capillary forces restraining gross fluidmovements through sediments do not equally constrictor restrain movement of dissolved species through finechannels, at some point the surficial charged layers ofclay platelets are approached. One of the changesoccurring with time and loading is the greater approachtoward parallel orientation of clays. Bringing claysurfaces closer to each other will tend to obstructmovement of ions by the presence of surface-chargedlayers.

A second factor affecting diffusive permeability issorting. Poorly sorted sediments containing clays tendto block channels between larger grains by smallerparticles. Thus, we attribute the difference betweensamples at 204 meters and 223 meters at Site 379 (6.7

and 3.5 F values, respectively) to greater sorting andsand content at the latter horizon (Figure 3).

Cementation is an effective method of reducing netporosity of sediments. However, it does not appear tobe efficient in reducing the diffusional permeability ofsediments until porosity is reduced to low levels. Thismay be partly attributable to the fact that finercarbonate particles and pore channels appear to bepreferentially recrystallized, leaving the larger channelsas the last to be sealed. At Sites 380 and 381 lithificationand cementation are far advanced in limestones anddolomites at depths of 360 and 900 meters, approxi-mately. At these horizons formation factors of severalhundred are typical, whereas shales interbedded withthe carbonates have formation factors of only a few

1127

F. T. MANHEIM

TABLE 2Electrical Resistivity-Diffusimetry Measurements, Holes 380-380Aa

SampleNo.

373839404142434445464748

49

50515253545556575859606162636465666768

697071727374

7576777879

838589

Core Section

Pressure Barr.1469

10121415171921

23

2827293133353739

1579

11131517192123

252729313336394043464952

646979

53313352253

6

3311331524244145435

355424355552

543

Depth(m)

<517.93555.477.492

109.5133.5136.4,166.4b

179.9195

217

233245.5274293310318.5340.5359.4336376.9407415435450475492.5510527.4550

565.4587.4598.5614.5630.5652.5691.9728.5758.4787810.2_

975.510681068

Salinity (%)

20.416.523.131.940.236.94145.441.251.4—

48

56.7

40.757.458.362.062.262.263.865.264.365.068.269.369.366.467.869.370.470.472.1

72.673.772.671.571.775.0

(77.080.381.481.483.684.6

(95)95.390.0

90.0

ProbeResistivity

0.0750.1820.1880.122—

0.125—

0.118_

0.1520.1300.138

0.140

0.1520.1380.1310.141—

0.155_

0.111—

0.1130.2290.0940.1010.1160.1140.2720.1300.1280.148

0.1650.1200.1380.1070.1290.1150.150—

0.0870.1050.3100.0550.2000.1370.2490.182

5.47

Cell Resis-tivity (fi/m)

0.3190.3840.2970.2200.1795

(0.149)—

0.153—

0.140(0.135)0.151

0.130

0.1700.1280.148

(0.118)-

0.112_

0.115—

0.1080.102

(0.099)(0.101)(0.101)(0.101)(0.101)0.0970.0970.093

0.0980.096

(0.095)(0.095)0.097

(0.094)(0.086)(0.086)(0.085)(0.085)(0.083)0.08440.0844

(0.075)(0.073)(0.068)

(0.068)

CellConstant

9.419.419.419.41_

9.41—

9.41—

9.419.419.41

9.41

9.419.419.419.41_

9.41-

9.41—

9.419.415.59.419.419.419.419.419.419.41

5.55.59.419.419.419.419.419.415.54.19.419.419.419.419.416.10

6.10

RockResistivity

Corr. (fi-m)

.711.711.771.15—1.18-

1.11_1.321.221.30

1.33

1.431.301.341.33_1.46—

1.04-1.062.15

.52

.951.091.072.561.221.20

.88

1.551.131.301.011.211.081.41-

0.730.992.920.521.881.292.341.11

33.4

F

2.234.455.965.22—

7.9-

7.26—

8.79.18.6

10.2

8.417.21

10.611.2

-13.0

-9.5-

9.821.1

5.29.4

10.810.625.312.412.45.9

15.811.913.710.612.511.516.4-8.6

11.635

6.122.317.232.116.3

491

Remarks

Soft oozeSoft gray mud

Contam.-MushySmall

Contam.-disturbedDry, firm shaleContam.Gray-blackbanded clayDrilled holes infirm gray claySandy clay (cont.)Stiff clay

Disturbed

Black shale

Hard clayGray clayGray clay

Gray brittle shale

Soft carbonatevarved bands

Deformed?

Banded line Hardgreen shale

Hard green shaleBlack shale, withfracturingHard cemented car-bonate (watercontent 1.6)

Note: For explanation and notes see Table 1, Site 379, and accompanying text. Parentheses refer to pore fluid resistivities interpolated from curve

of resistivity vs. refractive index (Figure 1).a F. T. Manheim and V. S. Sotelo, analysts.

From this point and deeper, holes had to be pre-drilled into samples to prevent cracking.

tens. Providing that horizontal continuity of thelithified carbonate beds is maintained, it will not mattermuch that intervening shales are relatively permeable;the very low porosity carbonates will form effectiveseals to all but slow diffusion or movement alongfracture channels.

DISCUSSION

Key questions related to formation factor measure-ments are the extent to which diffusive permeabilitiesaffect ion and gas concentrations in the pore fluids ofthe rocks. Can paleosalinities be preserved long enough

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ELECTRICAL RESISTIVITY MEASUREMENTS OF SEDIMENTS

TABLE 3Electical Resistivity—Diffusimetry Measurements, Site 381 a

SampleNo. Core Section

Depth(m) Salinity (°/oo)

ProbeResistivity

Cell Resis-tivity

CellConstant

RockResistivity

Corr. (n-m) Remarks

9192939495969798

100103

105107108109110110

112113113

Note:

1-23579

12141625

273133353737

4851-51

See Table

2232554553

244522

544

1 for notes.a F . T. Manheim and V. S. Sotelo

/Mint C /-\

0.3017.427.043.065.084.0

111.0133.0171.0225.0

264.5301.0321.5336.0399.0399.0

472.0492.4492.4

, analysts.

f- ZΛ -t *• n *-* t i i-» »->

26.928.639.656.159.464.4(64)59.462.763.6

64.666.06465.564.8(65)

39.438.538.5

0.08050.1090.0650.0700.0510.0780.079

_0.0790.1370.1750.0710.0730.0760.1280.1047.06

0.1820.1980.225

0.2600.2570.1800.1260.1190.107

0.139-0.1080.1130.1100.1090.1080.1080.1080.1080.1080.1080.108

0.1820.1860.186

1 2 3

9.419.419.419.419.419.419.419.419.419.418.009.419.419.419.419.413.92

9.419.419.41

Poros

1

4 5 6 7 8 9 1 0

0.761.030.610.660.480.730.74—

0.740.1290.1400.670.690.721.200.98

27.68

1.901.862.12

ty (%)

20 40 6

2.923.993.405.224.036.866.88

-6.76

11.812.86.196.366.62

11.159.062.56

10.410.011.4

10

) 80100 200

HorizontalVertical

ShaleCementedlimestone

TopSide

100

400 700 1000

observed near the bottom of Site 381? Can perme-ability barriers allow significant gas buildups to occur?Can one set limits on the original salinities in strata?

Quantitative calculations for the latter will requirebetter dating control than is at hand at this writing.However, some limits can be set for the former twoquestions. Were the lithified carbonates fractured insuch a way as to permit long-distance transport ofBosporus waters into the Deep Basin, one mightanticipate a more seawater character in ioniccomposition. However, waters in the deepest horizonsof Site 381 show typical "deep basin" diagenesis, withSO4 absent (except for traces), K and Mg stronglydepleted, and calcium enriched (Manheim and Schug,this volume). In addition, the lithified dolomites andlimestones at Site 381 show a breccia-puddingstonetexture that suggests early lithification soon afterburial. Such lithification would help seal off waters ofmore normal marine composition (below) from thevarious salinity regimes that would occur later in BlackSea history. The formation factors ranging from 256 to500 or more apply to rocks having a water content ofl%-5%. One should point out that these determinationswere made on hard core fragments wedged in the corecatcher of otherwise empty barrels, in most cases. Wemay assume that the strata in question consist ofalternating hard and softer layers, based both ondrilling time logs, and on the typical pattern of verypoor recovery in such configurations of material.Continuously hard rocks such as limestones generallygive far better recovery.

Regarding gaseous diffusion1, one may note that atthe base of Site 380 a rather sudden increase of heavierhydrocarbons (ethane and higher alkanes) is obtained(Schmitt, this volume). Though the poor-recovery,lithified carbonate layer is considerably less lithified

Figure 4. Distribution of formation factor and porosity withdepth for Holes 380 and 380A. Porosities are derivedfrom water content assuming a grain density of 2.67.Note that a smaller logarithmic scale is used to accom-modate the larger variability than in Figure 3. The lastdata point (deepest) has been extended slightly deeperthan its true position near 1060 to obtain visual separa-tion of the shale and the dolomite.

than at Site 381, it nevertheless appears to form enoughof a seal to have retarded the movement of heavierhydrocarbons upward and concentrate them to aconsiderable degree below the poorly permeable forma-

1129

F. T. MANHEIM

tions. It is quite possible that greater concentrations of the Deep Sea Drilling Project, Volume 23: Washingtonhydrocarbons might be accumulated in updip trap (U.S. Government Printing Office), p. 975-998.closures, if the lithified carbonate sequences are Manheim, F.T. and Waterman, L.S., 1974. Diffusimetrycontinued. (diffusion constant estimation) on sediment cores by

resistivity probe. In von der Borch, C.C., Sclater, J.G., etREFERENCES al 5 initial Reports of the Deep Sea Drilling Project,

Manheim, F.T., 1974. Red Sea geochemistry. In Whitmarsh, Volume 22: Washington (U.S. Government PrintingR.B., Weser, O.E., Ross, D.A., et al., Initial Reports of Office), p. 663-670.

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