Zeolite and clay-mineral induced resistivity in simulated reservoir sand

Journal of Petroleum Science and Engineering, 5 ( 1991 ) 163-172

163

Elsevier Science Publishers B.V., A m s t e r d a m

Zeolite and clay-mineral induced resistivity in simulated reservoir sand W.R. Reynoldsa and C.W. Williford b aDepartment of Geology and GeologicalEngineering, The University of Mississippi, University, MS 38677, USA bDepartment of Chemical Engineering, The University of Mississippi, University, MS 38677, USA (Revised and accepted January 17, 1990 )

ABSTRACT Reynolds, W.R. and Williford, C.W., 1991. Zeolite and clay-mineral induced resistivity in simulated reservoir sand. J. Pet. Sci. Eng., 5: 163-172. Clay-minerals dispersed in reservoir sands affect electric log response and register reduced resistivity values. Natural zeolites however, with large microporosities and water content could have a greater affect on resistivity measurements. Resistivity values were measured on a series of artificial cores prepared by mixing and compacting various percentages each of clinoptilolite, smectite and illite, with a medium-grained, moderately sorted quartz sand. Various concentrations of NaCl solution mixed with 39 API crude oil were circulated through each core. Impedance measurements were taken, resistance values segregated, and resistivities determined for each core. Water saturation values were calculated from the empirical resistivity values and porosities using a modified Simandoux equation. These values appeared to be much higher for those cores which contained the zeolite clinoptilolite. Clinoptilolite, when dispersed in a simulated reservoir sand and treated as a dispersed smectite or illite, produced inflated saturation values. This inflation effect is thought to be due to the more extensive microporosity and larger micropore water content of the zeolite. Therefore, from the empirical aspect, resistivity measurements of reservoir sands containing a dispersed zeolite, rather than a clay-mineral, would probably yield misleading water saturation values.

Introduction

Potential reservoir sands and sandstones often contain clay-minerals as part of the matrix or as cement. This has an affect on the resistivity of a reservoir. Smectites, for example, and occasionally illite under specific circumstances, will have an interlayer microporosity containing water which contributes to the overall values of porosity and water saturation. Authigenic clay-minerals, after deposition, have a tendency to react with the saline solutions commonly found in reservoir rocks. This reaction results in the formation of thin coatings of brine-saturated clay around individual sand grains. This, in turn, produces a condi-

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tion which affects electric log response by reducing the resistivity and thus indicating inflated water saturated values (Timmons, 1984; Clavier et al., 1977). Furthermore, detrital clay-minerals occupying intergranular pore space, contain interlayer and adsorbed waters that have a similar affect on resistivity values. The cation exchange capacity (CEC) of clayminerals, as well as the a m o u n t of absorbed micropore water has also been found to exert a strong influence on the conductivity of reservoir sands (Waxman and Smits, Mian and Hilchie, 1982 ). Natural zeolites are hydrated aluminosilicates. Basically, the structure of a natural zeolite consists of an extensive network of channels and cages filled with water and

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164

W.R. REYNOLDS AND C.W. WILLIFORD

exchangeable cations (Gottardi and Galli, 1985; Breck, 1974). There are 140 known zeolite species, 40 of which occur in igneous, metamorphic and sedimentary rocks. The most abundant occurrence however is as either diagenetic or authigenic minerals in sedimentary rocks. Sedimentary zeolites, like clay-minerals are microcrystalline but have higher cation exchange capacities plus a variety of catalytic properties which are related to their ability to adsorb numerous types of gas, vapor or fluid molecules. Common sedimentary zeolites are clinoptilolite, phillipsite, erionite, chabazite, mordenite and analcime. Sedimentary zeolites may have a similar, but magnified effect, as clay-minerals on the conductivities of sands and sandstones. Compared to smectites, zeolites can trap larger amounts of water, several times greater than their dry weight. An awareness of zeolitic minerals occurring as major constituents in sandstone cements or sand matrix is becoming more prominent (Curtis and Cornell, 1972; Surdam and Parker, 1972; Stewart, 1974; Walton, 1975; Franks and Hite, 1980). Zeolite minerals which occur Nomenclature Ccl

C~d C

F psig R~ Rcl

Ro R!

Rw Rz

s~ rb v~ v~

conductivity of dispersed clay conductivity of reservoir sand or sandstone formation conductivity formation factor pounds per square inch gauge clay resistivity resistivity of a clay slurry used to approximate an adjacent "shale" resistivity of sand 100% saturated with connate water formation resistivity resistivity of connate water portion of resistivity contributed by dispersed clay water saturation (%) volume of brine solution clay volume volume of clay slurry

y

[ Vc(1/Rw)(1/RJ]

¢

porosity (%)

as pore-linings in volcaniclastic sandstones were formed after the development of secondary porosity and diagenesis of volcanic ash (Mathison, 1984). Most authigenic zeolites found in volcaniclastic sandstones however were formed by the reaction of alkaline and saline pore-waters with buried precursor material usually an unstable detritus. Often, this unstable detritus includes detrital clay-minerals, feldspar, previously formed zeolite minerals, biogenic silica and volcanic ash (Deffeys, 1959; Hay, 1966; Surdam and Parker, 1972; Surdam and Boles, 1979; Moncure et al., 1981 ). Furthermore, the amount and composition of a stable zeolite species formed by the transformation of a precursor depends not only on the composition of the precursor, diagenetic environment and geologic time, but also on the chemistry, temperature and age of the pore-water reacting system (Hay, 1978 ). Zeolite cements have occasionally been observed in detrital units of Tertiary Age along the Gulf Coastal Plain of the southeastern United States (Wermund and Moiola, 1966 and Reynolds, 1970). Clinoptilolite and opal CT are the common constituents in cements in the lower and upper sandstone units of the Tallahatta Formation of Mississippi (Roquemore, 1984) (Fig. 1). Furthermore, Gulf Coast offshore petroleum exploration efforts have encountered sandstones containing clinoptilolite-rich cement (Fig. 2 ). Water saturation is directly related to intergranular porosity and permeability which are affected by the clay content of a sandstone cement or sand matrix. The concern about zeolite-rich cements however, centers around the affect the extra water content within the zeolite microstructure would have on the resistivity of a potential reservoir. Most likely, the saturated microporosity of a zeolite would cause an apparent lower resistivity measurement, hence an overall pessimistic water saturation value. This apparent inflationary proportion of water saturation could be accounted for and actually ignored assuming it would have little

ZEOLITE AND CLAY-MINERAL INDUCED RESISTIVITY IN SIMULATED RESERVOIR SAND

165

Fig. 1. Photomicrographs of clinoptilolite cemented sand, found in the lower part of the Tallahatta Formation, exposed south of Meridian, Mississippi. (A) Angular quartz grains in a clinoptilolite matrix. Diameter of scale dot in lower right is equivalent to 100 micrometers. (B) Clinoptilolite microcrystals as part of the matrix cement. Diameter of the scale dot in the lower left is equivalent to 25 micrometers. o r no affect o n h y d r o c a r b o n m o v e m e n t ( M a r tini a n d Vuagnat, 1965 ).

Experimental procedure C o n d u c t i v i t i e s were m e a s u r e d o n artificial cores m a d e u p o f m i x t u r e s o f s a n d a n d e i t h e r d i s p e r s e d zeolite or clay-mineral. N a t u r a l co-

res were n o t available a n d cost p r o h i b i t i v e to obtain. C o n s t r u c t i o n o f artificial cores d i d allow s o m e c o n t r o l o n the a m o u n t o f p o r o s i t y and brine percent. P r e p a r a t i o n o f f a b r i c a t e d cores c o n s i s t e d o f m i x i n g 30 v o l u m e p e r c e n t o f e i t h e r a claym i n e r a l or zeolite with m e d i u m - g r a i n e d , m o d erately s o r t e d q u a r t z sand. S o m e o f the cores

166

W.R. REYNOLDS AND C.W. W1LL1FORD

167

ZEOLITE AND CLAY-MINERAL I N D U C E D RESISTIVITY 1N S I M U L A T E D RESERVOIR SAND

-I:><3

l>-,:O-'--

I

I

0-I 11.

\\ I // \1/ ! \ / J \ FLUID

RESERVOIR

INSULATING HOOD

[-I

O 1

I

[I II I,

NITROGEN

NITROGEN

I ' HEATINGTAPE

m l

[ Q

t

Q

l

I

it

! I

[~ TEMPERATURE CONTROL

IMPEDANCE BRIDGE Fig. 3. Flow diagram of apparatus for the measurement ofconductivities of artificial sand/clay-mineral and sand/zeolite cores.

contained sand and dispersed zeolite while others, for the purpose of comparison were mixtures of sand and either dispersed smectite or illite. Both the clay-mineral and zeolite additives were ground to 300 mesh, randomly sedimented and compacted with the sand in a teflon sleeve. The zeolite species used was a clinoptilolite from Arizona. The smectite used was a calcium montmorillonite from Aberdeen, Mississippi and the illite used was from Fithian, Illinois. Each core within its teflon sleeve was fitted at both ends with a 2.5-cm diameter 300-mesh stainless steel screen, and the core-sleeve combination was then placed

within a stainless steel cylinder (Fig. 3 ). The cores were then secured within the cylinder between stationary and movable rams. The rams, electrically insulated from the cylinder body served as electrodes that were attached to the leads of an impedance bridge. The temperature of the cylinder containing the core within the core sleeve was maintained at 37°C in a controlled enclosure. This simulated an average formation temperature. Each experiment began by passing 300 ml of a 2% NaC1 solution, from a pressurized reservoir, through the core at a flow rate of 5-10 ml min -~. Fluid inlet pressure was 200-400 psig

Fig. 2. Scanning electron micrographs of clinoptilolite cemented sands encountered in offshore Louisiana exploration wells. (A) Subarkose with clinoptilolite (crystalline masses) occurring as grain coatings on quartz. The clinoptilolite was also observed to have grown into pores and pore throats. Scale bar is equal to 7 micrometers. (B) Clinoptilolite (c) filling a pore space in a quartzose sand. Scale bar is equal to 10 micrometers. (C) Clinoptilolite (c) coating quartz grains and filling a pore throat in a quartzose sand. Scale bar is equal to 12 micrometers.

W.R. REYNOLDS AND C.W. WILLIFORD

168

with an overpressure of 400-500 psig maintained around the outside of the sleeve. The solution and core material were then allowed to equilibrate for approximately one hour, after which electrical impedance was measured at 1 kHz. Subsequent measurements were made for 3, 6, 9 and 12% solutions, and 39 API crude oil. Resistance was calculated from the measured impedance and core dimensions. The crude oil was then displaced by 2% NaC1 solution. The core was carefully removed from the cylinder and sleeve and weighed. It was then slowly dried (8-15 hours at 100°F) to remove all bulk water and equilibrated with air moisture at 70°-75°F. Equilibration with air moisture replaced water that may have been desorbed from the zeolitic and clay-mineral material during drying. The weight difference between the wet and air equilibrated cores approximated the a m o u n t of void space water. An estimate of porosity was then determined as a ratio of measured void space volume to total core volume. Impedance measurements were also taken for zeolite and clay-mineral slurries in equilibrium with 3% NaC1. From these measurements resistivities were determined in order to simulate the resistivity of "adjacent shale". Thirty grams each of 300 mesh montmorillonite, illite and clinoptilolite were stirred with 500 ml of 3% NaCI solution. The resulting slurry was equilibrated at 70°F for two hours and allowed to settle overnight. The supernatant fluid was decanted and the remaining slurry centrifuged. The thickened slurry was then placed in the teflon sleeve and conductivity measurements were made in the same manner as for the sand, sand-clay and sandzeolite cores.

TABLE I

Data analysis

O& +

Experimentally derived resistivity values (Table 1 ) were obtained from a series of conductivity measurements for each brine concentration with crude oil and pure quartz sand,

Average values of resistivity for a series of experimental cores at 37°C, 30% clay volume and varying brine content Fluid

Rw

Ro values for sand, sand/clay and sand/zeolite Sand

Sand/ clinop,

Sand/ smectite

Sand/ illite

1.350 0.931 0.562 0.398 0.326

1.370 1.150 0.598 0.444 0.346

1.320 0.932 0.655 0.496 0.422

1.800 1.360 0.763 0.539 0.445

39 API crude 3.850 Measured 0.300 porosity Clay resistivity (Re)

1.430 0.275

2.470 0.245

3.670 0.293

0.480

0.320

0.620

% NaC! solution 2 3 6 9 12

0.275 0.205 0.114 0.082 0.069

plus separate mixtures of quartz sand mixed with clinoptilolite, calcium montmorillonite, and illite. Water saturation values were calculated from the empirically derived resistivity values and measured porosities. Water saturation calculations were based on a clay slurry model. The clay slurry model (DeWitt, 1950) assumes the clay portion to be dispersed within the pore spaces as a slurry with formation fluids, and considers the resistivity contribution of the dispersed clay (Rz) as:

Vs/R,~ = Vb/Rw + Vc/Re where Re is the resistivity of the dispersed clay and Rw is the resistivity of connate water. Therefore (q~Sw+ V~) /R~ =OSw/Rw + V~/Rc and: =

further;

1/Rt = (OSw +

gc)2/Rz

=q~Sw + V~(rbS~/Rw + V¢/Rc)

169

ZEOLITE AND CLAY-MINERAL INDUCED RESISTIVITY IN SIMULATED RESERVOIR SAND

Rewriting in the form, Ax+Bx+C=O, the above expression becomes:

1/FRw(S2w +OSw) ( 1/Rw + l /Rc) (Vc) + ( V~/Rc - 1/Rt) = 0 Solving for water saturation (Sw): Sw = 0.5Rw/¢[ - y + ~

- 4/nw ×

(V~/R 2 - 1/Rt) ] where y = Vc( 1/Rw+ 1/R~). Unfortunately, the above equation gives optimistic, and at times negative values for Sw. Furthermore, this condition was amplified by an increase in clay content. Another expression used for computing water saturation values in clay-bearing sands is a modification of the general Archie equation in the form, y = a + c x 2, (Hossin, 1982). This equation relates the resistivity of an argillaceous sand zone to the contributions of both the species of clay present and the fluid distribution in the pore spaces. The resulting expression becomes: C = ( r c X ~c| X Csd) -Ji-(¢m/a-t-Rw) XSnw Substituting Tixier's expression (0.81/02 ) and a saturation exponent of n = 2 the above expression becomes: Sw = 0 . 9 / 0 , / ( 1 / R , - Vc/Rc)Rw where C is the measured conductivity. Again, the above expression yielded estimates that were optimistic and often negative. Other modifications of the Archie equation were also found to be inappropriate when applied to laboratory derived data consisting of artificial sand/clay mixtures and slurries with fixed or artificial porosities. Waxman and Smits' cation exchange capacity (CEC) model (Waxman and Smits, 1968 ) also gave optimistic resuits when using standard CEC values for clinoptilolite. Recent studies have shown that CEC values of clinoptilolite can vary considerably from one locality to another (Lieu et al., 1988 ). The CEC model also seemed inappro-

priate because of being restricted by the perceivable difficulty of obtaining a downhole CEC measurement of a zeolite bearing sand. Consequently, it is not only difficult to standardize clinoptilolite CEC but there is no known way to measure this parameter downhole using standard logging methods. The Simandoux model (Simandoux, 1963) was found to be an appropriate model since it is designed specifically for laboratory derived data using an artificial media composed of sand and clay. With the incorporation of the Tixier expression and a saturation exponent of two ( n = 2 ) the conductivity of simulated Eocene Gulf Coastal Plain sands could be expressed as: Ct:

( Vc C c l ) X S w

+S~w(Om/aRw)

where Cd is an average conductivity of a dispersed clay and Vc is the clay volume. Use of the saturation exponent n = 2 further allows the assumption of the parabolic equation y = bx2+ cx. Water saturation values now can be determined as: Sw =0.4Rw/02{ - VURc + [ ( V j R c ) 2 + 5 ( 0 2 / R t R w ) ]},/2 where the resistivity of a dispersed clay is approximated by Rc = 0.3Rd. The factor Rcl is the resistivity in an adjacent shale zone and the value 0.3 is the fixed volume of clay material used in this study. Calculation of water saturation values using the Simandoux equation requires a value for R0 which is the resistivity of material 100% saturated with a fluid of resistivity Rw. Simulated formation resistivity, R,, is also required. Measured Ro is used as Rt. Other needed parameters are porosity (0), volume of dispersed clay (V~) and clay resistivity (Rc). The clay volume was fixed at 30% which left formation resistivity, porosity, clay resistivity and simulated formation fluid resistivity (Rw) as measured variables. Rw values were obtained from 2, 3, 6, 9 and 12% NaC1 solutions. Water saturation values were calculated

170

W.R. REYNOLDSAND C.W. WILLIFORD

TABLE2

TABLE3

Water saturation values (Sw) for clinoptilolite (A), smectite (B) and illite (C) in a medium-grained moderately-sorted quartz sand

Comparison of water saturation values (Sw) for smectite, illite and clinoptilolite mixtures in a dispersed system assuming a non-zeolitic "adjacent shale". Sw values were calculated for 30% by volume dispersion in randomly selected 3 and 9% brine solutions and 30% porosity. Parameters are porosity (0), clay resistivity (Re), adjusted clay resistivity (Rc), fluid resistivity (Rw) and measured resistivity of dispersed systems

A Sand/clinoptilolite % NaCI

Rw

2 0.275 3 0.205 6 0.114 9 0.082 12 0.069 Resistivity (Ro) Clay volume Porosity (q)) Clay resistivity (Rd)

Sw 0.324 0.321 0.306 0.280 0.280 = 1.43 =0.30 =0.275 = 0.48

Rw

2 0.275 3 0.205 6 0.114 9 0.082 12 0.069 Resistivity (Ro) Clay volume Porosity (0) Clay resistivity (Rd)

Sw 0.135 0.133 0.130 0.129 0.126 =2.47 =0.30 =0.245 =0.320

C Sand/illite % NaC1

Rw

2 0.275 3 0.205 6 0.114 9 0.082 12 0.069 Resistivity (R0) Clay volume Porosity (¢) Clay resistivity (R¢1)

Illite Illite Smectite Smectite

Matrix clay lllite Clinop. Smectite Clinop.

3% NaCl ¢

Rcl

Rc

Rw

0.30 0.30 0.30 0.30

0.62 0.62 0.32 0.32

0.186 0.186 0.096 0.096

0.205 3.67 0.160 0.205 1.43 0.383 0.205 2.47 0.126 0.205 1.43 0.216

Ro

Sw

9% NaC1

B Sand/smectite % NaCI

Adjacent clay

Sw 0.164 0.163 0.157 0.152 0.148 =3.67 =0.30 =0.293 =0.62

from data averages obtained for various salinities and clay/sand mixtures (Table 2 ). These values were higher when clinoptilolite was used as matrix material. This suggests that when all other constraints are equal the affect of zeoli-

Illite Illite Smectite Smectite

lllite Clinop. Smectite Clinop.

0

Re1

Rc

Rw

Ro

Sw

0.30 0,30 0.30 0.30

0.62 0.62 0.32 0.32

0.186 0.186 0.096 0.096

0.082 3.67 0.149 0.082 1.43 0.337 0.082 2.47 0.123 0.082 1.43 0.205

tic held waters seems to be significant. For example, the presence of clinoptilolite produced Sw values from 28% to 32.4% (Table 2A). On the other hand, montmorillonite and illite produced Sw values of only 12.6% to 13.5% (Table 2B) and 14.8% to 16.4% (Table 2C), respectively. A substantial portion of the Sw value for the clinoptilolite system is due to water held isolated in the micropore structure of the zeolite. This micropore water which inflates water saturation values is thought to have little impeding effect on hydrocarbon movement (Martini and Vuagnat, 1965 ). The above observations raise a question concerning well log interpretation. What would be the effects on saturation values when the dispersed clay in a reservoir sand or sandstone is a zeolite such as clinoptilolite but assumed to be either a smectite or illite? For example, Table 3 demonstrates that when an adjacent clay is composed mainly of illite yielding a clay resistivity of 0.62 in a 3% brine solution, and the dispersed clay in the sand being evaluated is illite, the calculated saturation value would

ZEOLITEANDDAY-MINERALINDUCEDRESISTIVITYIN SIMULATEDRESERVOIRSAND

be about 16%. Supposed however, the dispersed clay is mainly clinoptilolite. But, calculation of Sw was based on the resistivity for illite. In this case, the resulting calculated saturation value is 38%. This is an increase in Sw from 16 to 38% or about a 60% increase for a 3% brine solution. A similar change in Sw values is observed when the dispersed clay is assumed to be a smectite. In this case there is an increase in Sw from 13 to 22% or approximately 42%. Table 3 further shows that an increase in saturation values is not affected much by a change in the brine concentration. Conclusion Water saturation is directly related to intergranular porosity. Resistivity is inversely related to intergranular porosity and largely controlled by the amount and nature of the intergranular matrix or cement and formation fluids. Clay-minerals or zeolites occurring as grain coatings, intergranular matrix or as cement have microporosities which also contain fluids, principally water. Modeled sand-matrix systems containing clinoptilolite were found to have much lower resistivities than those experimental systems containing smectite or illite. The larger empirical and inflated values for water saturation in the sand-zeolite system are thought to be due to water held in the zeolite micropore structure. This study simulated a detrital matrix where a more realistic model would be that of an authigenic cement because clay-minerals and especially zeolites usually occur as constituents in an authigenic cement rather than as detrital matrix. The results of this study nonetheless do suggest that water saturation values are likely to be much more influenced by the presence of clinoptilolite in a sand rather than smectite or illite. Microporosities of zeolites and clay-minerals have an additive affect on the overall porosity by decreasing resistivity values and therefore yielding inflated water saturation values. However, the extensive mi-

171

croporosity of a zeolite structure such as clinoptilolite, due to the vast network of channels and cages has a much greater water uptake and storage capacity than that of a clay-mineral. Clay-minerals dispersed in a sand as detrital matrix or in a sandstone as cement are known to cause measured resistivity to indicate pessimistic water saturation values. These values are routinely taken into account and usually adjusted as part of the process of evaluating an exploration test well. What is not considered is the assumption that one or more species of clay-minerals are part of the matrix or cement composition rather than a zeolite such as clinoptilolite. Consequently, this assumption could produce very misleading water saturation values as shown in this study. These misleading values are due to the excess water held in the micropore structures of the zeolite which is not considered a hinderence to hydrocarbon movement. Therefore, because this apparent induced inflation of Sw, due to the unsuspected presence of zeolitic cement or matrix, some Gulf Coastal Plain and offshore exploration tests in the past may have been needlessly abandoned. References Breck, D.W., 1974. Zeolite Molecular Sieves; Structure, Chemistry and Use. Wiley, New York, N.Y., pp. 1132. Clavier, C., Coates, G. and Dumanoir, J., 1977. The theoretical and experimental basis for the "dual water" model for the interpretation of shaley sands. Soc. Pet. Eng. J., 6859 (Oct.): 3-18. Curtis, C.D. and Cornell, W.C., 1972. Unusual occurrence ofclinoptilolite, Fresno County, California. Geol. Soc. Am. Bull. 83(3): 833-838. Deffeys, K.S., 1959. Zeolites in sedimentary rocks. J. Sediment. Petrol., 29(4): 602-609. DeWitt, L., 1950. Relation between resistivities and fluid contents of porous rocks. Oil Gas J. (Aug.): 120-132. Franks, S.G. and Hite, D.M., 1980. Controls of zeolite cementation in upper Jurassic sandstones, lower Cook Inlet, Alaska. Am. Assoc. Pet. Geol., Bull., 64(5 ): 708709. Gottardi, G. and Galli, E., 1985. Natural zeolites. Springer, New York, N.Y., pp. 1-34. Hay, R.L., 1966. Zeolites and zeolitic reactions in sedi-

172 mentary rocks. Geol. Soc. Am., Spec. Pap., 85, 130 pp. Hay, R.L., 1978. Geologic occurrence ofzeolites. In: L.B. Sands and F.A. Mumpton (Editors), Natural Zeolites; Occurences, Properties and Use. Pergamon, New York, N.Y., pp. 135-143. Hossin, A., 1982. The generalized Archie Equation. In: Dresser Atlas, Well, Logging and Interpretation Techniques. Dresser Industries, Inc., Houston, TX. p. 180. Lieu, K., Williford, C.W. and Reynolds, W.R., 1988. Cation exchange characteristics of Gulf Coast clinoptilolire. In: D. Kallo and H.S. Sherry (Editors), Occurrence, Properties and Utilization of Natural Zeolites, H. Stillman, Boca Raton, Fla., pp. 449-461. Mian, M.A. and Hilchie, D.W., 1982. Comparison of resuits from three cation exchange capacity (CEC) analysis techniques. The Log Analyst, 23:10-16. Martini, J. and Vuagnat, M., 1965. Pr6sence du facibs a z6olites dans la formation des "gr6s" de Taveanne. Schweiz. Mineral. Petrogr. Mitt., 45 ( 1 ): 281-293. Mathison, M.E., 1984. Diagenesis of Plio-Pleistocene nonmarine sandstones, Cagayan Basin, Philippines: Early development of secondary porosity in volcanic sandstones. In: D.A. McDonald and R.C. Surdam (Edilors), Clastic Diagenesis. Am. Assoc. Pet. Geol. Mem., 37, pp. 177-193. Moncure, G.K., Surdam, R.C. and McKague, H.L., 1981. Zeolite diagenesis below Pahute Mesa, Nevada Test Site. Clays Clay Miner. 29 (5): 385-396. Murata, K.H. and Whiteley, K.R., 1973. Zeolites in the Miocene Briones Sandstone and related formations of the central Coast Ranges, California. U.S. Geol. Surv. J. Res., 1 (3): 255-265. Reynolds, W.R., 1970. Mineralogy and stratigraphy of

W.R. REYNOLDS AND C.W. W1LLIFORD

Lower Tertiary clays and claystones of Alabama. J. Sediment. Petrol., 40 (3): 829-838. Roquemore, S.K., 1984. Clinoptilolite occurrence in the Tallahatta Formation (Middle Eocene) of southeast Mississippi. Masters Thesis, The Univ. Mississippi, University, Miss., 106 pp. Simandoux, P., 1963. M6sures di61ectriques en milieu poreux, application a m6sure des saturations en eau 6tude du comportment des massifs argileux. Rev. Inst. Ft. Pet., Suppl. Issue, pp. 193-215. Stewart, R.J., 1974. Zeolite facies metamorphism of sandstone in the western Olympic Peninsula, Washington. Geol. Soc. Am. Bull., 85(7): 1139-1142. Surdam, R.C. and Parker, R.D., 1972. Authigenic aluminosilicate minerals in the tuffaceous rocks of the Green River Formation, Wyoming. Geol. Soc. Am. Bull., 83(3): 689-700. Surdam, R.C. and Boles, J.R., 1979. Diagenesis of volcanic sandstones. In: P.A. Scholle and P.R. Schluger (Editors), Aspects of Diagenesis. Soc. Econ. Paleontol. Mineral. Spec. Publ., 26: 227-242. Timmons, D.M., 1984. Tuscaloosa demands a close-up look. Gulf Coast Oil Reporter, 3(8): 20-23. Walton, A.W., 1975. Zeolitic diagenesis in Oligocene volcanic sediments, Trans-Pecos, Texas. Geol. Soc. Am. Bull., 86(5): 615-624. Waxman, M.H. and Smits, L.J.M., 1968. Electrical conductivities in oil-bearing shally sands. Soc. Pet. Eng. J. (June): 107-120. Wermund, M.H. and Moiola, R.J., 1966. Opal, zeolites and clays in an Eocene neritic bar sand. J. Sed. Petrol., 36(1 ): 248-253.