density and mineralogy variations as a function of porosity in miocene monterey formation oil and...

AAPG Bulletin, v. 85, no. 1 (January 2001), pp. 149167 149

Density and mineralogyvariations as a function ofporosity in Miocene MontereyFormation oil and gasreservoirs in CaliforniaCaren Chaika and Loretta Ann Williams

ABSTRACT

The Miocene Monterey Formation, long known as the criticalsource rock in California, also includes significant fractured chertand porous diatomite reservoirs. What is not widely recognized isthat there are high matrix porosity reservoirs within the opal-CTand quartz-phase rocks. Using density, porosity, and mineralogydata, we have identified two distinct groups of Monterey Formationreservoirs. group 1 porosity changes gradually during silicadiagenesisporosities of 5570% exist not only in opal-Adominated samples but also in samples that have undergone thetransition from opal-A to opal-CT. In group 2, porosity decreasesabruptly at the transition, and rocks below the transition are tight.

The main mineralogic difference between groups 1 and 2 is ahigher clay content in group 1, resulting in different silica diagenesispathways for the two groups. This led us to expect all San Joaquinbasin samples to fall into group 1, and all coastal California samplesto fall into group 2, because of the different depositional historiesof the two areas. Although our prediction holds in most instances,we discovered that some San Joaquin basin samples exhibit group2 characteristics. Therefore, it is important to use petrophysical,seismic, and/or geological information to determine if a MontereyFormation reservoir is likely to be a fracture-dominated type (group2) or if there might be a matrix production component (group 1).

Once one has made this diagnosis, the characteristic density/porosity relationships of each group allow one to easily calculate anaccurate matrix porosity.

INTRODUCTION

The Monterey Formation has long been known as the critical hy-drocarbon source rock in California. Many petroleum geologists

Copyright 2001. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received October 28, 1999; revised manuscript received February 16, 2000; final acceptanceMarch 21, 2000.

AUTHORS

Caren Chaika Department of Geologicaland Environmental Sciences, StanfordUniversity, Stanford, California, 943052115;current address: Occidental of Elk Hills, Inc.,P. O. Box 1001, Tupman, California, 93276-1001; [email protected]

Caren Chaika received her B.A. degree ingeology from Rice University in 1993 and herPh.D. from Stanford University in 1998, whereshe combined geology and rock physics tobetter understand physical property changesduring silica diagenesis. She has been asummer geologist for the Naval ResearchLaboratory at Stennis Space Center,Mississippi, and for the U.S. Geological Surveyin Menlo Park, California. She is currently aproduction geologist with Occidental of ElkHills, located near Bakersfield, California.

Loretta Ann Williams IndependentConsultant, 2354 Moline Street, Aurora,Colorado, 80010; [email protected]

Loretta Ann Williams has specialized in fine-grained carbonate and silica source rocks thatserve as reservoirs. A past Sproule Awardrecipient, she holds a B.S. degree from SUNYStony Brook, an M.B.A. degree fromUniversity of Denver, and a Ph.D. fromPrinceton University. She served as a StanfordUniversity postdoctoral research affiliate from1981 to 1983 and as an exploration geologistfor Champlin Petroleum from 1984 to 1986.Her career has spanned exploration,production, and remediation consulting. She iswith PARSEC Group in Denver, Colorado.

150 Monterey Formation Oil and Gas Reservoirs

now also regard it as one of the last reservoir frontiers in the state.The focus of production to date has largely been on coastal Cali-fornia fractured cherts that have no matrix porosity and on the highmatrix porosity diatomites in which silica is in the form of billionsof opaline microfossils. We are beginning to realize, however, thatin some oil-producing areas of California, Monterey Formation res-ervoirs produce from high matrix porosity in rocks that have un-dergone diagenetic transformation from the microfossil opal-A tothe intermediate, denser phase opal-CT and even some that haveundergone final transition to the stable, dense quartz phase. Thisdiscovery has important implications for understanding the unitstrue reservoir potential.

Monterey Formation silica originated as biochemically precip-itated siliceous skeletal elements, such as sponge spicules, radiolar-ian tests, and diatom frustulesthat is, as amorphous silica colloids(Hurd, 1983). In the sediments, this opaline silica undergoes dia-genetic change over time to the stable silica polymorph, quartz,through the pathway opal-A r opal-CT r quartz (Murata and Lar-sen, 1975; Oehler, 1975; Isaacs, 1980; Williams et al., 1985; andmany others).

In deep-sea biogenic siliceous oozes, transitions have such amarked impact on rock properties that they sometimes createstrong acoustic reflectors (McManus et al., 1970; Wise and Weaver,1974; Hein et al., 1978; Bohrmann et al., 1992). Such reflectorssometimes occur in the Monterey Formation at the opal-A/opal-CT or the opal-CT/quartz boundary (Gayle Ehret, 1984, personalcommunication; Mayerson and Crouch, 1994; Mayerson et al.,1995; Rob Sterling, 1998, personal communication) and signal im-portant changes in petrophysical properties (Isaacs, 1981) that tiedirectly into reservoir behavior and sound reservoir productionpractices. A typical diatomite reservoir exhibits porosity of 5570%,permeability of less than 10 md, and poor induration leading topoor rock competence. After undergoing diagenesis to opal-CT, po-rosity typically drops from around 45 to below 25%, permeabilitydrops to negligible levels, and the rock becomes well-indurated andbrittle. At the transition to quartz, porosity decreases to 020%,and the rock becomes even more brittle. Thus, opal-A reservoirsare matrix porositydominated reservoirs. Opal-CT and quartz res-ervoirs, although sometimes maintaining some matrix porosity, arecommonly fracture dominated. As with almost every generalizationone can make about the enigmatic Monterey Formation, however,there are exceptions to this rule. The giant Elk Hills field, for ex-ample, has significant production from fine-grained Monterey For-mation biogenic silica reservoirs. As discussed by Reid and McIntyre(2001), extensive coring and analysis indicate that much of the pro-duction is from matrix porositydominated quartz reservoirs. Fur-ther, although a clearly defined opal-A/opal-CT boundary reflectoroccurs in some locales, there are many Monterey Formationcontaining parts of California where there is no reflector.

In this article, we examine matrix porosity preservation in Mon-terey Formation reservoirs, associated petrophysical properties, and

ACKNOWLEDGEMENTS

We appreciate the time and effort that JimRogers and Mark Longman have put into pre-paring this volume. We thank Mobil for corefrom South Belridge field and Chevron forcores from Cymric, Asphalto, and McKittrickfields and for funding for mineralogic analysis.The California State University at BakersfieldCore Repository provided a Shell core fromNorth Belridge field, and we are grateful tothis valuable facility, which would not haveexisted without the efforts of the late VicChurch. Loretta Ann Williams dedicates herwork in this article to his memory. CarenChaika expresses her gratitude to the StanfordRock Physics Project for use of laboratory fa-cilities, funding, and equipment training, andshe appreciates the guidance of Jack Dvorkin,J. G. Liou, and Stephan Graham in her thesisproject. We also thank Tony Reid and JanaMcIntyre of Occidental Petroleum for allowingus access to their data for Elk Hills field. Weare grateful for the work of John Comptonand Caroline Isaacs, which supplied ourcoastal California data. Finally, we are thank-ful for the work of the numerous investigatorswho have studied the Monterey Formation,California tectonics, and oceanic upwellingsystems before us. Without the body ofknowledge their work has provided, this arti-cle would not have been possible. This workwas supported by the Petroleum ResearchFund of the American Chemical Society GrantACS-PRF #32743-AC2.

Chaika and Williams 151

Pacific Ocean

Bakersfield

Los Angeles

Bakersfield

San JoaquinBasinSalinas Basin

San Andreasfault

Los Angeles

Point Pedernales

Santa Barbara

0 8 16scale in miles

North Belridge

South Belridge

Elk Hills

0 50 100 miles

Asphalto

McKittrick

CymricAreas of onshore Californiaunderlain by rocks broadly

Monterey

Sources of data for this study

Santa CruzBasin

BasinSanta Barbara

Santa Maria Basin

Ventura

Figure 1. Geographic extentof Monterey Formation andMonterey-type Miocene depos-its of onshore California, andspecific sample areas for thisstudy. Original analyses for thisarticle come from cores in As-phalto, North and South Bel-ridge, Cymric, and McKittrick oilfields and are supplemented bycore data from Elk Hills fieldcollected by Reid and McIntyre(2001) and outcrop data col-lected by Compton (1991) atPoint Pedernales and Isaacs(1980) along the Santa Barbaracoast.

physical and chemical reasons for porosity preservationbeyond the opal-A diagenetic stage.

GEOGRAPHIC AND DEPOSIT IONALSETT ING

The Monterey Formation is a Miocene biogenic sili-ceous deposit that underlies much of southern Cali-fornia (Figure 1), achieving thicknesses greater than2000 ft (610 m) in some places. It is an important pe-troleum source and reservoir rock in two general areas:(1) coastal California from Monterey south to near SanDiego, and (2) the San Joaquin Valley. Many of Cali-fornias largest oil fields produce in part from fine-grained Monterey Formation reservoirsfor example,the Ventura basin Oxnard field and the San Joaquinbasin Cymric field, which have so far produced 12 mil-lion and 19 million bbl of oil, respectively, from theMonterey Formation (CDOC, 1998). Cook (1994) es-timated a statewide total of 10 billion bbl of oil in placeremaining in Monterey Formation siliceous reservoirs.

In general, Monterey Formation style depositionoccurred owing to a unique Miocene combination ofclimatic, oceanographic, and tectonic conditions (In-

gle, 1981; Pisciotto and Garrison, 1981; Isaacs, 1983;Luyendyk and Hornafius, 1987; Schoell et al., 1994).Many investigators have studied the formations dep-ositional and diagenetic origin (e.g., Bramlette, 1946;Isaacs, 1980; Blake, 1981; Graham and Williams,1985; and many others). To summarize, California wastectonically active in the Miocene, primarily due to SanAndreas transform fault movement. Wrench tectonicscreated silled, restricted ocean basins in the areas thatnow contain Monterey Formation deposits. High ratesof coastal upwelling of nutrient-rich waters led to highbiologic productivity in these basins, including plank-tonic diatom blooms. Organic debris falling throughthe water column was oxidized, causing waters belowwave base to become oxygen depleted. Restricted wa-ter circulation due to the silled sea bottom topographycontributed to this development of low oxygen bottomwaters that are so important to preserving organic mat-ter in sediments. Although very fine-grained, like mostbiogenic siliceous deposits throughout time, there is agreat deal of subtle variation in Monterey Formationlithologiescommonly on a bed-to-bed scalethat di-rectly impacts source and reservoir quality. This is dueto a combination of original depositional componentsand fabrics and diagenetic history.


Coastal California Monterey Formation siliceousrocks are generally cleaner (contain fewer clastics) thanthe dirty San Joaquin Valley Monterey Formation bio-genic siliceous rocks. The San Joaquin Valley was aninboard basin in the Miocene, receiving clastic inputfrom the Sierra Nevada Mountains on the east and theemergent Salinian Block to the west. The coastal basinswere outboard basins, isolated from mainland clasticsby the Salinian Block and other sills. The resulting dif-ference in detrital influx resulted in two distinctly dif-ferent Monterey Formation reservoir styles. We havechosen our sample database to reflect both.

METHODS

Our study is based on dry bulk density (qb(dry)), graindensity (qs), and porosity () measurements, alongwith mineralogic analyses for cores from five fields inthe San Joaquin Valley (Asphalto, Cymric, McKittrick,North Belridge, and South Belridge) and outcrop sam-ples from Point Pedernales and the Santa Barbara area(Figure 1). San Joaquin data are a combination of origi-nal data and data collected by Core Laboratories (Ap-pendix). The coastal California data are from pub-lished information of Compton (1991) and Isaacs(1980). Our efforts to minimize unsystematic errorarising from measurements in different laboratoriesand/or using different methods are presented in theAppendix, along with a table of all unpublished dataused in this article. Most important, Isaacss (1980)methodology for measuring porosity was very differentfrom that used in collecting the other data. We there-fore only use her data in our mineralogic evaluations,where density and porosity are not considered.

We selected only samples having biogenic silica 40 wt. %. Based on the classification scheme ofWilliams (1982), this cutoff includes the followinglithologies: diatomite (80 wt. % opal-A), diato-maceous shale (4080 wt. % opal-A), chert (80wt. % opal-CT and/or quartz), porcelanite (5080wt. % opal-CT and/or quartz), and the higher endof the siliceous shale range (2050 wt. % opal-CTand/or quartz). It eliminates clastic-dominated andcarbonate-dominated lithologies.

Our analysis of density and porosity behavior ofMonterey Formation rocks relies on the basic relation-ship (e.g., Schlumberger, 1989)

q q (1 ) q (1)b(sat) s f

where qb(sat) saturated density of the rock, ex-pressed as total weight/total volume; qs total weightof rock solids/total volume of rock solids; and qf total weight of fluids in the rock/total volume of fluidsin the rock. Because analyses used in this study arefrom dried cores, qf 0, and the equation simplifiesto

q q (1 ) (2)b(dry) s

where qb(dry) total dry weight/total dry volume.For comparison, we used the standard quartz den-

sity of 2.654 g/cm3 (Schlumberger, 1989). Opal-CT,not a true mineral, varies widely in density. We usedthe density of a nonporous hydrothermal tridymitesample as our approximation of the ideal opal-CT(Appendix).

PATTERNS OF POROSITY REDUCTION

Silica phase transitions cause changes not only in po-rosity but in density as well. In the Monterey Forma-tion (Isaacs, 1981; Compton, 1991), qb(dry) rangesfrom 0.71.2 g/cm3 for opal-A rocks, to 1.12.0 g/cm3

for opal-CT rocks, to 1.92.6 g/cm3 for quartz rocks.This information, coupled with the associated porositychanges mentioned previously, suggests a distinctiveporosity/density pattern should emerge as a rock se-quence undergoes diagenesis. To examine this relation-ship, we plotted qb(dry) against (Figure 2). Because was directly measured, whereas qb(dry) was for themost part calculated (Appendix), qb(dry) was the req-uisite dependent variable. The result was two distinctlinear relationships:

2q 2.50 2.60; R 0.990; n 39 (3)b(dry)

and

2q 2.13 2.07; R 0.936; n 33 (4)b(dry)

where R 2 correlation coefficient, and n numberof samples upon which the regression is based.

Equation 3 characterizes the Belridge, Cymric, andAsphalto core samples, and equation 4 characterizesthe McKittrick core and Point Pedernales outcrop sam-ples. The lines intersect at 0.70, meaning thatthese high porosity rocks have the same dry bulk den-


0

0.5

1

1.5

2

2.5

0.0 0.10 0.20 0.30 0.40 0.50 0.60 0.70

Group 1Group 2

Porosity

Bulk Density = 2.50 2.60() R 2 = 0.993

Bulk Density = 2.13 2.07() R 2 = 0.942

Bul

k D

ensit

y (g/

cm3 )

Figure 2. Dry bulk density vs. porosity plot for core data fromAsphalto, North and South Belridge, Cymric, and McKittrick oilfields (Appendix) and Point Pedernales outcrop data of Compton(1991). All oilfield data except those for McKittrick field fall onthe group 1 line. McKittrick field core data and Point Pedernalesoutcrop data fall on the group 2 line, except for a Point Ped-ernales quartz chert (open square), which has a higher densitythan the other opal-A and opal-CT Point Pedernales samples.

sity regardless of which regression trend they follow.Indeed, the lines are practically indistinguishable for all greater than about 55% (predominantly diatoma-ceous rocks). Rocks lying on the equation 3 line thathave less than 55% have higher densities than rockslying on the equation 4 line. The higher negative slopeof equation 3 indicates a greater rate of density changewith porosity reduction than for equation 4 samples.If qb(dry) is a function of silica phase, this result suggeststhat equation 3 reflects either a more rapid rate ofphase transition or a longer porosity retention relativeto equation 4.

The high R2 value for both lines indicates that changes are almost wholly sufficient to explain qb(dry)changes. Because the trends are so strong, we hereafterdistinguish them as group 1 (rocks having /qb char-acteristics described by equation 3) and group 2 (rockshaving /qb characteristics described by equation 4).For each group, one can predict qb(dry) with a highdegree of certainty if one knows . It is more usefulto be able to predict from qb(dry). Therefore, as dis-cussed in a following section, we develop a method forporosity estimation from either dry bulk densities mea-sured directly from core or wire-line density log read-ings of saturated bulk density.

Note that one group 2 point plots directly on thegroup 1 line, at 0.037, qb(dry) 2.40. This sam-ple, the only quartz data point from the Point Peder-nales outcrop, illustrates an important point. Project-ing equation 4 out to qb(dry) 2.65 g/cm

3 yields a

porosity of 28%, which is impossible. In reality,equation 4 ends in a steep curve that must have aninflection point in the range of 010% porosity,as rocks of increasing density converge on 0% porosity.

For the relationship of qb(dry) to qs in equation 2to be linear, qs must be a constant that describes theregression line slope, an assumption critical to the stan-dard density method of estimating porosity from logs(Tom Zalan, 1997, personal communication). In thissituation, a density of zero must yield 100% porositythat is, where there are no grains to create density, therock is 100% pore space. Neither equation 3 nor equa-tion 4, however, passes through the 1, qb(dry)0 point (Figure 3), indicating that solid phase densitychanges with porosity reduction. We know from pre-vious authors work that silica phase density changesas porosity decreases. The fact that our data supportthis trend, having a strong R 2, indicates that silicaphase change is a dominating factor in porosity de-crease in these rocks.

Our next logical step is to plot qs vs. , resultingin two distinct nonlinear plots:

2Group 1: q 2.52 0.27 0.96 ;s2R 0.737 (5)

and

2Group 2: q 1.92 1.42 1.61 ;s2R 0.492 (6)

This confirms that grain density changes as poros-ity decreases, as one would expect if silica phasechanges are affecting the trends (Figure 3). We haveeliminated the aberrant quartz chert point in group 2that plots on the group 1 line, because it falls withinthe nonlinear part of group 2, which is outside of thescope of this study.

Porosity explains 74% of the grain density trendfor group 1 rocks but only 49% of the grain densitytrend for group 2 rocks. Thus, although there is a verystrong relationship between porosity and bulk density,porosity is not as useful in explaining grain density re-lationships. This is because different minerals have dif-ferent grain densities, but a rock made up of differentminerals can still have the same porosity. For example,for a pure quartz sandstone, qs is close to 2.65 g/cm

3,whereas for a pure dolomite, qs is close to 2.85 g/cm

3

(e.g., Schlumberger, 1989), but both rocks may have30% porosity. In the context of this data set, the fact


1.8

2

2.2

2.4

2.6

2.8

0.0 0.10 0.20 0.30 0.40 0.50 0.60 0.70

Group 1Group 2

Porosity

Grain Density = 2.52 + 0.27() 0.96()2

R 2 = 0.737

Grain Density = 1.92 + 1.42() 1.61 () 2

R 2 = 0.492

Quartz standard line

Tridymite standard lineGra

in D

ensit

y (g/

cm3 )

Figure 3. Grain density vs. porosity plot for the data of Figure2. Ideal quartz grain density is assumed to be 2.654 g/cm3

(Schlumberger, 1989), regardless of sample porosity. Ideal opal-CT density is assumed to lie somewhere around the grain den-sity of pure tridymite as determined in our study (Appendix),regardless of sample porosity.

that groups 1 and 2 separate so distinctly in solid den-sity indicates that they must be compositionally differ-ent. The low R 2 characteristic of each group, however,evidences a high degree of intragroup variabilityparticularly for group 2 rocks.

With decreasing , group 1 qs generally increasestoward the density of the mineral quartz, whereasgroup 2 qs remains low and even decreases toward theaverage density of the disordered mineraloid opal-CT.It appears that group 1 rocks must undergo diagenesisall the way to quartz before losing most of their po-rosity, whereas group 2 rocks lose most of their poros-ity in the opal-CT conversion. This is due to either amore rapid phase transition rate or slower rate of po-rosity decrease in group 1.

At high porosity, opal-A is the dominant biogenicsilica phase for both groups. Opal-A and opal-CT co-exist in group 1 rocks (Figure 4a) at midrange porosi-ties, with opal-CT becoming increasingly dominant. Atlower porosities (30% and less), quartz becomes thedominant polymorph. Group 2 rocks (Figure 4b) con-tain only opal-A at high porosities, which is abruptlyreplaced with opal-CT at around 45% porosity. Verylittle quartz (biogenic or detrital) is in any of thesegroup 2 rocks. At low porosities, the rocks are almosttotally made up of opal-CT. Thus, one of the distin-guishing characteristics of the two groups is that group1 rocks retain porosity longer during silica diagenesisthan do group 2 rocks. Using Isaacss (1980) data,which includes many group 2 quartz rocks, we confirmthe group 2 characteristics (Figure 4c).

If silica diagenetic phase fully explained group 1and group 2 distinctions, both Figure 3 curves wouldoverlap in the high porosity, low density opal-A andopal-CT range and would only diverge below 40% po-rosity, where group 1 rocks are entering the quartzphase and group 2 rocks remain in the opal-CT range.Instead, group 1 densities are consistently higher thangroup 2 densities, regardless of porosity and silica poly-morph. Group 1 rocks become increasingly dense asthey get tighter, whereas group 2 rocks become lessdense as porosity decreases below 40%. Thus, althoughsilica diagenesis impacts /qb trends, it does not ex-plain them. Figure 4 demonstrates that there is a sig-nificant nonsilica component in both group 1 andgroup 2 rocks, ranging from 5 to 55 wt. % of the rock.This nonsilica component is made up largely of clayminerals, zeolites, and feldspars, of both clastic anddiagenetic origin.

Although similar in the ratio of biogenic silica toother minerals in the opal-A stage, group 1 and group2 rocks differentiate with the transition to opal-CT.group 1 rocks, in which opal-A and opal-CT coexist,remain dirty and have a continuous porosity range (Fig-ure 5a). In group 2 rocks, opal-A and opal-CT appearto be mutually exclusive, and opal-CT rocks are sig-nificantly cleaner than their opal-A counterparts. Anobvious 8% porosity gap exists from about 45 to 53%at the opal-A to opal-CT transition (Figure 5b).

These observations suggest that group 1 rocks un-dergo a gradual change in rock properties, includingporosity, bulk density, grain density, and mineralogy,during diagenesis. No discontinuity in properties existsto produce an acoustic reflector. Group 2 rocks, how-ever, undergo an abrupt conversion of opal-A toopal-CT, and the boundary is characterized by a step-wise change in porosity, bulk density, grain density,and mineralogy, which is likely to produce an acousticreflector.

Note here that the acoustic reflector we are refer-ring to is a diagenetically induced reflector only. Silicadiagenetic reflectors tend to exhibit normal polarity,whereas gas-related reflectors, which can also occur inthe Monterey Formation, exhibit inverse polarity(Shipley et al., 1979; Lonsdale, 1990). This is what onewould expect, in that diagenetic reflectors result froman abrupt change from less dense to more dense ma-terial. A reflector caused by gas results from a lighterphase in porosity overlain by rocks that have a denserphase (water or ice) in the porosity. Lonsdale (1990)noted, however, that purely diagenetic horizons canexhibit inverse polarity if the transition zone is


0

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0.14

90.

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50.

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90.

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80.

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90.

266

0.29

60.

327

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443

0.44

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495

0.52

00.

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567

0.57

00.

572

0.57

70.

590

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614

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625

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80.

634

0.64

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646

0.65

40.

656

0.66

60.

683

0.68

6

opal-A

biogenic quartznonsilica clastics

Porosity

GROUP 1

(a)

opal-CT

Min

eral

Fra

ctio

n by

Wei

ght

0

0.2

0.4

0.6

0.8

1

0.02

50.

037

0.09

10.

100

0.24

00.

320

0.33

30.

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00.

341

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30.

348

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80.

366

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373

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30.

390

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397

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416

0.41

80.

433

0.45

40.

555

0.58

10.

583

0.58

50.

602

0.61

40.

670

0.67

0

opal-A

biogenic quartznonsilica clastics

Porosity

GROUP 2

(b)

opal-CT

Min

eral

Fra

ctio

n by

Wei

ght

0

0.2

0.4

0.6

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(c)

0 0.01

0.07

0.12

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0.29 0.3

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0.38

0.59

Frac

tion

byW

eigh

t

opal-Aopal-CTbiogenic quartz

ISAACS (1980)

Porosity

clastics nonsilicaMin

eral

Figure 4. Bar charts of sam-ple percentages by weight ofopal-A, opal-CT, biogenicquartz, and nonbiogenic miner-als plotted against porosity, ingroup 1 samples (a), group 2samples (b), and Isaacs (1980)data (c). Organic carbon andother minor components arenot included, measurementmethodologies are mixed, andtherefore component sums donot always equal 100%. Notethat opal-A and opal-CT do notcoexist in group 2 samples (band c). Instead, there is anabrupt transition from opal-A toopal-CT. The two phases do co-exist in group 1 samples.


0

0.2

0.4

0.6

0.8

1

0.1 0.2 0.3 0.4 0.5 0.6 0.7

biogenic silicanonsilica clastics

Porosity

GROUP 1

(a)

Min

eral

Fra

ctio

n by

Wei

ght

0

0.2

0.4

0.6

0.8

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

biogenic silicanonsilica clastics

Porosity

GROUP 2

(b)

Poro

sity

Gap

Min

eral

Fra

ctio

n by

Wei

ght

Figure 5. Biogenic silica content vs. other minerals (nonsilicaminerals and detrital quartz) as porosity decreases for group 1(a) and group 2 (b). Note the porosity gap in the group 2samples, between the opal-A phase at 53% porosity and theopal-CT phases at 45% porosity (b).

Table 1. Summary of Distinguishing Petrophysical andMineralogical Characteristics of Group 1 and Group 2Monterey Formation Reservoirs

Group 1 Group 2

Lies on group 1 regressionline

Lies on group 2 regressionline

Continuous porosity range Porosity gap at 4553%Opal-A and opal-CT

commonly coexistOpal-A and opal-CT rarely

coexistNo diagenetic reflector likely Can produce a diagenetic

reflector

complexthe sum of interferences from interbeddedopal-A/opal-CT or opal-CT/quartz zones can yield areverse polarity.

The foregoing analysis has defined two distinctivemineralogical and petrophysical groups, summarizedin Table 1. Differences between groups 1 and 2 couldbe due to stratigraphic, compactional, or geochemicaleffects, which we discuss in the next sections.

ORIGINS OF GROUP 1 AND GROUP 2TRENDS

Stratigraphy

A density difference due to relative clay content withporosity reduction may merely signal a stratigraphicchange with depth. Generalizing broadly, mostdiatomaceous rocks in the Monterey Formation and

equivalents are at the stratigraphic top of the forma-tion and were deposited as the basins were filling. Asa result, regardless of geographic location, this part ofthe section tends to be clastic rich. This probably ex-plains why group 1 and group 2 do not show strongdensity/porosity differentiation in the opal-A range.This is the only time-stratigraphic change likely to af-fect the data. Below the opal-A diagenetic boundary,stratigraphic members are largely defined based onclastic or carbonate content, and these stratigraphicdifferences were largely eliminated by our use of onlyrocks having at least 40 wt. % biogenic silica. Also,the opal-CT and quartz rocks from the five San Joa-quin basin cores were taken at different burial depthsbut in the same general stratigraphic interval, yet theMcKittrick field samples plot in group 2, whereas therest plot in group 1.

Compaction

Mechanical crushing and realignment of grains causessignificant volume reduction in any sedimentary rocksection during lithification. Visual evidence of me-chanical compaction exists in Monterey Formationthin sections, and pressure solution seams known asstylolites are also common (Williams, 1982, 1988).Most workers who have examined porosity reductionin biogenic siliceous rocks have invoked compactionas a primary or secondary mechanism (Iijima andTada, 1981; Isaacs, 1981; Tada and Iijima, 1983;Compton, 1991).

At first glance, the group 1 qb/ curve (Figure 3)is shaped very much like a typical compaction curve,and this makes sense. As porosity decreases, there aremore grains per unit volume of rock and so densityshould increase. Equation 2, however, by which solid


density is derived, corrects for this porosity variationeffect, and therefore the resemblance is an illusion.Also, rocks do not become less dense during compac-tion, as appears to be happening in group 2. There-fore, compaction, although an important part of theoverall picture, is not a likely factor in explaininggroup 1 and group 2 differences shown in Figure 3.

Although experimental velocity measurements ofcores (Chaika, 1998) provided evidence of some com-paction effects in opal-A/opal-CT transition rocks, acontinuous compaction curve model does not predictthe gap in porosity observed in group 2 rocks at theopal-A/opal-CT boundary (Figures 4b and 4c). Recallthat in group 2 rocks, opal-A and opal-CT do not co-exist (Figure 4b). Isaacs (1981) attributes the lack ofphase coexistence, plus the sharp porosity change atthe phase boundary, to a sudden compactional eventas silica dissolved and changed phase from opal-A toopal-CT. She suggests that these changes happenedquickly, causing the rock to lose its frameworkstrength during dissolution, allowing a compactionalcollapse. Thus, she sees compaction as an effect ofchemical diagenesis.

Although mechanical compaction is a factor inporosity reduction, we agree with Isaacs (1981) thatthe overriding control on porosity reduction withdepth in Monterey Formation rocks lies with geo-chemical effects, as we inferred from our qb/ graph-ical results.

SIL ICA DIAGENESIS AND POROSITYREDUCTION

Silica diagenesis follows the Ostwald step rule, whichpredicts that, given a constant chemical composition,the first solid to precipitate will be the phase of high-est entropy (opal-A), followed by intermediate steps(larger, less irregularly shaped opal-A and opal-CT) tothe lowest entropy phase (quartz) (Iler, 1973, 1979;Williams and Crerar, 1985; Chang and Yortsos,1994). Because silica diagenesis occurs through dis-solution-reprecipitation, not solid state transforma-tion, higher entropy phases dissolve and lower entropyphases precipitate. Therefore, the specific pathwaytaken (size/surface area and phase) depends upon sil-ica concentration in solution. Anything affecting pore-water silica concentration will affect the rates and na-ture of diagenetic reactions (Williams and Crerar,1985; Williams et al., 1985; Chang and Yortsos,1994).

Group 1 and Group 2 Diagenetic Pathways

Previous authors have empirically demonstrated thatan abundance of detrital clay minerals in the systemretards the opal-A/opal-CT transition and enhancesthe opal-CT/quartz reaction (Murata and Larsen,1975: Kastner et al., 1977; Isaacs, 1980, 1983). This ismost likely due to adsorption of dissolved silica ontoclay surfaces at naturally occurring pH values and thetendency of dissolved silica to complex with metal ionsin solution and precipitate as diagenetic clays (Hein etal., 1978; Badaut and Risacher, 1983; Williams et al.,1985).

Coexistence of opal-A and opal-CT in group 1rocks but uncommonly in group 2 rocks is a conse-quence of the different amounts of clays (both detritaland authigenic) between these two groups. As opal-Adissolves (having materials of highest surface area dis-solving first), silica in solution increases. In group 1rocks this silica is readily taken up by clays and clayauthigenesis, which keeps the degree of silica super-saturation (X) low. In the relatively detrital-free group2 rocks, X is larger. Reaction rates are generally pro-portional to X (Williams et al., 1985). Therefore, theopal-A transforms to opal-CT more slowly in group 1rocks than in group 2 rocks, increasing the likelihoodthat phases will coexist during the transformation.Also, because the crystals grow more slowly in group1, porosity is retained further into the diagenetic pro-cess. In contrast, in group 2 rocks, high X causes morerapid opal-A dissolution followed by opal-CTprecipitation.

The specifics of this model are best illustratedgraphically. Silica diagenesis of group 2 rocks, whichare relatively detrital-free, proceeds along the pathway1 r 2a r 4 of Figure 6a. Little competition for dis-solved silica from detrital phases exists, and so X, thedegree of supersaturation, becomes large enough to bein equilibrium with the nucleation of small, high sur-face area opal-CT crystals. Because reaction rates aregenerally proportional to X (Williams et al., 1985), ag-ing proceeds rapidlythat is, crystals grow rapidly towell-ordered, large, euhedral opal-CT crystals that in-terlock to occlude porosity.

For kinetic reasons, low pore-water silica concen-trations favor quartz crystal growth relative to higherconcentrations (Williams and Crerar, 1985; Williamset al., 1985)that is, as X decreases, quartz aging rateincreases. Although the group 2 opal-CT/quartz tran-sition proceeds along path 4a r 6 of Figure 6a, the rateof aging is impeded by elevated pore-water silica con-


fresh

dia

tom

sfre

sh d

iato

ms

60

30

b

(a)

(b)

S

A

S

A

GROUP 2

GROUP 1

Figure 6. Schematic graph of solubility (S) vs. surface area(A), showing the silica diagenetic pathway for group 2 rocks (a)and the more clay rich group 1 rocks (b). Note: solubility isplotted on a log scale. Group 2 represents the classic opal-A ropal-CT r quartz pathway. Opal-A diatoms lose surface areathrough dissolution of frustule ornamentation and fill in frustulepore spaces with coarse-grained, lower surface area precipitates,until surface area decreases to the point that tiny seed crystalsof high surface area opal-CT is preferred. This opal-CT ages tolower surface area, larger grained opal-CT, until the solution issaturated only with respect to quartz. At this point, fine seedcrystal quartz precipitates and ages to more coarsely crystallinequartz. The group 1 pathway represents the impact of clays onthis model. Modified from Williams et al. (1985).

centrations. Therefore, opal-CT persists in group 2rocks. By the time transition to quartz occurs, the com-bination of compaction and opal-CT aging has oc-cluded most matrix porosity.

Group 1 rocks follow a different pathway owingto their higher clay content. If we assume that the up-take of dissolved silica by clays depresses X to 30 mg/kg as per Williams et al. (1985, p. 308), group 1 sam-ples follow the pathway 1 r 1b r 4 during the opal-A/opal-CT transition (Figure 6b). In this situation,opal-A aging is interrupted at high surface areas, andopal-A diatoms that have diagenetic overgrowths andpore fillings do not occur. Instead, relatively pristineand/or partially dissolved diatoms intermix withopal-CT crystals of 60 m2/g surface arealarge rela-tive to the first opal-CT crystals precipitating in a group2 sample. At this lower X, Ostwald ripening rates (ag-ing to larger, more euhedral crystals) are also slower.Although the first opal-CT crystals to form are largerthan those in group 2, they age and grow more slowly.Diagenetic clays occlude some of the porosity, butthese clays have high surface areas (which implies ir-regular shapes), like the opal-CT crystals. Thus, poros-ity becomes more tortuous, and pore throats aresmaller, but porosity persists longer than in group 2rocks.

At point 4 of Figure 6b, opal-CT surface area/sizedifferences no longer affect solubility, but the solutionis still supersaturated with respect to quartz of surfacearea less than 150 m2/g, and quartz begins to precipi-tate, along pathway 4a r 6, just as for group 2. Highsurface area quartz crystals nucleate, decreasing pore-water silica saturation. This, plus precipitation of clayand zeolites, facilitates quartz precipitation, whichproceeds best at low silica concentrations. These idealsilica concentrations for quartz precipitation persist,because as X drops too low, clays that adsorbed silicaat higher pore-water silica concentrations are out ofequilibrium with the new pore-water concentrationsand begin to desorb silica. This process buffers porewaters at low silica concentrations where most of thesilica in solution is monomericthe ideal condition forgrowing quartz crystals and producing epitaxial over-growths on detrital quartz grains (Williams and Crerar,1985). Thus, chemical processes keep silica concentra-tion low, which allows quartz transformation at a shal-lower depth of burial. The group 1 trend toward in-complete porosity occlusion and increasingly tortuousporosity and smaller pore throats continues.

Note here that the effective opal-CT r quartz X/surface area pathway is the same (4a r 6 on Figures 6a


Figure 7. Dry bulk density vs. porosity data for two matrixporosity dominated C/D shale cores from the Elk Hills field plot-ted against the group 1 and group 2 regression lines of Figure2. The data are clearly classified as a group 1 reservoir, asexpected.

and 6b) for groups 1 and 2, but group 1 rocks movemore quickly through the opal-CT/quartz transitionthan group 2 rocks.

Thus, the differences between group 1 and group2 reservoirs depend largely on the relative amount ofdetritals and biogenic silica in the rocks. If there is alarge amount of clay minerals or potential to formclays, opal-A and opal-CT coexist, there is no porositygap, and the slow opal-A/opal-CT transition can pre-serve matrix porosity into the quartz phase. In contrast,if the rock is rich in biogenic silica and poor in detritals,opal-A dissolves rapidly. There is a porosity gap at theopal-A to opal-CT transition, opal-A and opal-CTrarely coexist, and opal-CT and quartz rocks are tight.

The ultimate result of the two pathways of dia-genesis are two distinctly different reservoirs. Group 2reservoirs proceed more rapidly from the high porosityopal-A phase to the opal-CT transition. Group 2opal-CT and quartz rocks, which have low porosityand low clay content, exhibit the near-homogeneoustextures of tightly interlocking crystals that are idealfor competent fracturing behavior under stress (McGilland Raney, 1970). Group 1 rocks retain high porosityopal-A longer and, thus, to a greater burial depth.group 1 opal-CT rocks proceed through the transitionto quartz more quickly, at a shallower burial depth andlower compaction level. Having higher clay content,these rocks behave more plastically under stress and soexhibit less large, long, through-going fractures (Griv-etti, 1982; Snyder et al., 1983; Williams, 1988; Gross,1995). Even where unfractured, however, group 1quartz rocks exhibit reservoir potential owing to oilstored in the 2040% matrix porosities.

APPLICATIONS

Statistically speaking, the model we present here is pre-liminary. It would require much more data to establishthe exact group 1 and group 2 regression line equa-tions. Even at this stage, however, the model has sev-eral practical applications.

Reservoir Classification

Opal-CT and quartz reservoirs that produce primarilyfrom matrix porosity are just beginning to be recog-nized in the Monterey Formation, and productionpractices are beginning to be adapted for the specialrequirements of such reservoirs. The C/D shale pro-ducing unit of Elk Hills field, as described by Reid and

McIntyre (2001), appears to be a clear example of agroup 1 reservoir. We plotted Elk Hills C/D shale dataon the qb(dry)/ graph of Figure 2 to determine if thequantitative model classifies this reservoir as we expectit should from qualitative characteristics. The modelclearly places the C/D shale reservoir in group 1 (Fig-ure 7).

Porosity Prediction from Density Logs

Although rigorous statistical practice required that weuse qb(dry) as the dependent variable in equations 3 and4, our 24 measured qb(dry) data points were consistentwith calculated data within a range of 0.010.08 g/cm3

for 22 out of the 24 samples, having the two outliersat a difference of 0.140.15 g/cm3. We argue that thecorrespondence is sufficient to allow us to flip the re-gressions and make the dependent variable. The re-sult is the following two regression lines:

Group 1: 0.956 0.382q ;b(dry)2R 0.993; n 39 (7)

and

Group 2: 0.987 0.454q ;b(dry)2R 0.942; n 33 (8)

To convert qb(dry) to well log bulk density readings,we must resaturate it. Recalling the definition of qb(dry)


given in equation 2, this resaturation can be expressedas

q q (9)b(sat) b(dry) f

Substituting equations 7 and 8 for qb(dry) in equa-tion 9 yields

Group 1: (0.956 0.382q )b(sat) (1 0.382q ) (10)f

and

Group 2: (0.987 0.454q )b(sat) (1 0.454q ) (11)f

Thus, can be directly calculated from qb(sat) usingthe group 1 and group 2 relationships. Given an aver-age qf of 0.95 g/cm

3 (Chaika, 1998, p. 74), we cansimplify these equations to

Group 1: 1.501 0.600q (12)b(sat)

and

Group 2: 1.736 0.798q (13)b(sat)

This gives us a direct relationship between and qb(sat)read from a density log. The qb(sat) readings are amongthe most reliable log information, whereas porosityread directly from the neutron porosity logs is com-monly much larger than directly measured helium po-rosity, because the tool detects water bound in clayminerals and other phases, as well as free fluids in thepores (Schlumberger, 1989). Equations 12 and 13 al-low us to circumvent the unreliable neutron porositylog, while replacing the standard density log methodof porosity estimation (Schlumberger, 1989) as well.This method assumes that grain density (our qs;

Schlumbergers qma) is constant, an assumption that isclearly erroneous for Monterey Formation rocks,whose grain densities range from less than 2.0 to about2.6 g/cm3 (Figure 3).

Our method requires only that one assume thatthe reservoir belong to either group 1 or group 2. Itthen becomes easy to calculate reservoir matrix poros-ity reliably from density log information, without hav-ing to make an assumption about grain densityevenas the well is being drilled! Figure 8 compares neutronporosity, standard density porosity estimation (as-sumed qs 2.5 g/cm

3, based on data for group 1 inFigure 3), directly measured porosity, and our calcu-lated porosity for three of the wells used in this studytwo from group 1 reservoirs in Cymric and Asphaltofields and one from a group 2 reservoir in McKittrickfield.

Neutron porosity tracks measured porosity accu-rately in the opal-A section of Cymric field but showspoor relationship to true porosity in the opal-CT andquartz reservoirs of Asphalto and McKittrick fields. Forthe standard density method, Figure 8 points out howcritical it is to correctly estimate grain density beforecalculating porosity. The 2.5 g/cm3 estimation that weassigned to all three logs works well for the group 1opal-CT and quartz reservoir of Asphalto field, butgreatly overestimates porosity in the group 1 opal-Areservoir of Cymric field and the group 2 opal-CT andquartz reservoir of McKittrick field. Our method esti-mates porosity well in all three cases.

CONCLUSION

This article indicates that Monterey Formation bio-genic siliceous reservoir rocks can be divided into twogroups based on petrophysical properties. Porosity,density, and mineralogy data indicate that porosity re-duction, although a function of compaction, is evenmore strongly affected by silica diagenetic conditions.In group 1 rocks, changes in porosity, density, and

Figure 8. Comparisons of neutron porosity logs, standard method density log porosity estimation (grain density assumption of 2.5g/cm3), our density method for porosity estimation, and direct porosity measurements on core samples. The neutron porosity logestimates porosity reasonably well in the Cymric opal-A reservoir (c) but overestimates porosity in quartz and opal-CT reservoirs ofAsphalto (a) and McKittrick (b) fields. The standard density method works well with our grain density assumption for the group 1opal-CT and quartz reservoir of Asphalto field (a) but not for the group 2 reservoir of McKittrick field (b) or the group 1 opal-Areservoir of Cymric field (c). In order for this method to work, the grain density assumption would have to be changed for eachreservoir. The method developed in this article is a fairly good estimator of porosity for all three reservoir types and requires onlythat one correctly choose the group to which the reservoir belongs.

neutron porosity

equations introduced in this paper

density-porosity

porosity as measured by the authorsCore Laboratories measurements

5850

5900

5950

6000

1800

1810

1820

0 0.1 0.2 0.3 0.4 0.5

Porosity

ASPHALTO FIELD

(a)

Dep

th (f

t)

Dep

th (m

)

3260

3280

3300

3320

3340

990

1000

1010

10200.2 0.3 0.4 0.5

Dep

th (f

t)

Dep

th (m

)

Porosity

MCKITTRICK FIELD

(b)

1230

1250

1260

1270

375

380

385

0.5 0.55 0.6 0.65 0.7 0.75

Dep

th (f

t)

Dep

th (m

)

Porosity

CYMRIC FIELD

(c)


mineralogy occur gradually, and there is no abruptdiagenetic transition to create an acoustic reflector.Because these rocks follow a diagenetic pathway dif-ferent from group 2 rocks, group 1 rocks retain ma-trix porosity to a greater burial depth and/or higherdegree of diagenetic grade than do group 2 rocks.group 2 rocks are characterized by sharp diageneticboundaries, accompanied by abrupt changes in po-rosity, density, and mineralogy. Matrix porosity isoccluded in these rocks by silica overgrowths addedduring the crystal aging process, creating brittle, ho-mogeneously textured rocks ideal for fracturing buthaving almost no matrix porosity. These rocks,which represent the stereotypical Monterey Forma-tion fractured reservoir, are more likely to be char-acterized by an acoustic reflector at silica diageneticboundaries.

We initially predicted that all San Joaquin basinrocks would plot as group 1 reservoirs and all coastalCalifornia rocks would plot as group 2. Because the SanJoaquin is an inboard basin, its rocks have a higher clas-tic content and are therefore likely to follow the dia-genetic pathway of impure rocks (Figure 6b). Thecoastal California outboard basins contain more purelysiliceous rocks and should follow the pathway of Figure6a to become fractured reservoirs.

The McKittrick field data from the San Joaquinbasin, however, proved this generalization wrong.Whereas most of the inboard basin samples plot asgroup 1 and most of the outboard basin samples plotas group 2, the McKittrick field samples act as an im-portant warning that there are exceptions to this rule.We recommend that investigators plot qb/ data fromthe logs of either their drilled prospect or a nearby an-alog well to determine which type of porosity systemtheir prospective reservoir contains. The differencesare important, as the drilling and completion fluids,hydraulic fracturing program, and other drilling, com-pletion, and production plans should be very differentfor a relatively pure, fracture porositydominatedgroup 2 reservoir than for a clay-rich group 1 reservoirthat has fewer fractures but much more oil and gasstorage in matrix porosity.

Preliminary results indicate our model can be usedto classify biogenic siliceous reservoirs of the MontereyFormation as fracture-dominated or matrix porositydominated. To test this, we used dry bulk density andporosity data from a Monterey Formation reservoirknown to be matrix porositydominated at Elk Hillsfield. The model correctly classified this reservoir asgroup 1.

Although the model was developed using dry bulkdensity, it can be resaturated and the regression linesflipped to allow porosity prediction from the saturatedbulk density given on well logs. Use of this model per-mits more accurate porosity prediction, based on thedensity log, than is presently possible using standardestimation methods.

The Monterey Formation, with its wide areal ex-tent, thick reservoirs, and rich oil reserves, representsone of the last frontiers of California oil and gas explo-ration. What we learn in successfully exploiting thisresource can be applied around the Pacific Rim and inany other area containing diagenetically immature bio-genic siliceous reservoir rocks.

APPENDIX : ANALYTICAL METHODS ANDDATA TABULATION

Analytical Methods

We performed laboratory measurements on samples from fivecores from San Joaquin Valley oil fields (Chaika, 1998): Shell (nowAera) well 565S1, from 1257 to 2493 ft (383760 m) in NorthBelridge field (Sec. 1, T28S, R20E); Mobil (now Aera) well8360A-2, from 1694 to 1976 ft (517603 m) in South Belridgefield (Sec. 2, T29S, R21E); Chevron well 1407R, from 1240 to1260 ft (378384 m) in Cymric field (Sec. 1, T30S, R21E); Chev-ron well 34217Z, from 3248 to 3684 ft (9911124 m) in Mc-Kittrick field (Sec. 17, T30S, R22E); and Chevron well 332X-25Z,from 5874 to 5958 ft (17921817 m) in Asphalto field (Sec. 25,T30S, R22E).

The samples had been dried by Core Laboratories Bakersfield(CLB) prior to our receiving them and were kept humidifiedthroughout analysis. We weighed the samples then measured di-ameter and length to calculate volume. Porosity of samples weremeasured using a helium porosimeter. We determined dry bulk den-sity by solving the equation

dry bulk density (q ) total weight/total volume (14)b(dry)

The CLB performed duplicate analyses on these samples andprovided mineralogy data. According to Allen Britton, manager atCLB, (1996, personal communication), pore fluid extraction wasdone by the Dean Stark method. This involves cleaning samples usingboiling toluene at approximately 115C and removing the residueusing methylene chloride, which boils at about 45C. All sampleswere dried at approximately 115C for between 4 and 48 hours.Rocks were weighed after all pore fluids were extracted, and thengrain volume was determined using Boyles law, using helium asthe gaseous medium. Grain density was calculated using theequation

grain density (q ) weight of solid/volume of solid (15)s


Dry bulk density was calculated from porosity and grain density usingthe relationship

q q (1 ) (16)b(dry) s

The CLB determined sample mineralogy using the semiquan-titative Fourier transform infrared spectroscopy (FTIR) method, asper Harville and Freeman (1988) and Rice et al. (1995). The FTIRmethod, which is becoming increasingly popular in a wide variety ofapplications, measures infrared spectra radiated by minerals, as mo-lecular bonds vibrate in response to application of infrared energy.Unlike x-ray diffraction (XRD), the FTIR method measures all fre-quencies at the same time, rather than moving sequentially througha range of wavelengths. The FTIR data collection is therefore easierand provides equally reliable qualitative to semiquantitative miner-alogical identifications. Note that CLB directly measured qs and and then calculated qb(dry), whereas we directly measured qb(dry) and and then calculated qs. Agreement between the two data sets isremarkably good.

For graphical analysis, we assumed measured data are more ac-curate than calculated data. Therefore, we preferentially used ourdirectly measured qb(dry) data and CLBs directly measured qs data.No clear reason exists to prefer either CLBs data or our own. Wechose to use the measurement that corresponds to the densitymeasurement used in each case.

Our sources for coastal California Monterey Formation data arepublished information of Compton (1991) for Point Pedernales andIsaacs (1980) for the Santa Barbara area (Chaika, 1998). Both au-thors used XRD as the primary tool for measuring mineralogy. LikeFTIR, this method is semiquantitative. It involves mounting a pow-dered sample on a slide, in such a way as to avoid preferred orien-tation of grains on the slide, and then subjecting it to alpha radiationfrom a copper source (CuK radiation). Minerals having strongcleavage planes, however, tend to orient along these planes and cre-ate artificially strong peak intensities. Also, high iron or organic mat-ter content in the sample interferes with CuK radiation, causingbackground fluorescence that interferes with peak intensity (Bloss,1971). Recognizing these limitations, both Isaacs (1980) and Comp-ton (1991) made efforts to maximize quantitative accuracy throughthe use of supplemental analyses, such as elemental analysis, organiccarbon analysis, x-ray fluorescence (XRF), thin-section petrography,and scanning electron microscopy (SEM). Indeed, Isaacs (1980)provides one of the most painstaking and rigorous attempts to reli-ably quantify mineralogy of fine-grained rocks in the geologicalliterature.

Comptons (1991) and Isaacss (1980) density and porosity datawere collected in slightly different ways than those we have discussedfor our data and that of CLB. For Compton (1991), cylindrical plugswere cut from rock samples, and bitumen was extracted using achloroform-acetone solution. Petroleum Testing Service, Inc. (PTS)measured and qs using Archimedes principle. Plugs were driedovernight at 105C and then weighed dry. They were then impreg-nated with toluene and immersed in a toluene bath and reweighed.Accuracy of porosity measurements was reported to be 0.5%(Compton, 1991).

Although there are slight differences in methodology betweenour work and that of CLB and Compton (1991), the most significantmethodological difference lies with Isaacs (1980). Porosity was cal-culated from direct qs and qb(dry) measurements, made after heatingsamples at 100105C for 24 hours to remove bound water in clay

and opaline grains. Samples were then kept in a dessicator to ensurethat no rehydration occurred before density measurements weretaken. All other studies for which density measurements are used inour present study were done on humidified samplesthat is, samplesthat were allowed to equilibrate with the atmosphere after drying.As Isaacs herself points out, Dry and humidified values of porositymay differ considerablyby 7 to 15 porosity % for opal-CT bearingrocks with 5 to 10% adsorbed water by weightand are not easilycompared (Isaacs, 1981, p. 260).

Given the potential for 715% discrepancies in density/porosityvalues between Isaacss (1980) dessicated samples and the humidi-fied sample data of all other studies, we omit Isaacs (1980) data fromour density/porosity discussion. An alternative would be to adjustdessicated porosities upward by some arbitrary value between 7 and15% to rehumidify them. This would, however, introduce a largesystematic error, and we chose not to do this. Therefore, we reserveIsaacs (1980) data for mineralogy-based discussions only.

Only rock samples having a silica content of at least 40 wt. %were used for this study. The 40 wt. % cutoff eliminated all highcarbonate Point Pedernales samples from the Compton (1991) dataset. We felt it was important to eliminate these data, because pres-ence of high concentrations of either calcite or dolomite significantlycomplicates the diagenetic story (Isaacs, 1980) and is a stratigraph-ically localized rather than a general phenomenon. Besides high car-bonate samples, this filtering process also eliminates clastic litholo-gies, such as sandstones, siltstones, and clay shales, that can beinterbedded with rocks of biogenic origin.

If the sample contained opal-A, we assumed all sample quartzwas detrital, because it is highly unlikely that there would be mea-surable diagenetic quartz in an opal-A dominated sample. If the sam-ple contained opal-CT, we assumed the sample quartz was biogenic.As a control sample for the disordered opal-CT phase, we used den-sity measurements performed on a zero-porosity, hydrothermal silicasample from Nevada, purchased from Wards catalog. The XRD anal-ysis indicated that this sample was pure tridymite, a high tempera-ture, moderate pressure polymorph of quartz (Deer et al., 1974).This is a reasonable standard to use, as the name opal-CT derivesfrom the fact that this mineraloid is made up of interlayered stacksof cristobalite and tridymite domains.

Data Tabulation

In Table 2, Other Rho(b), Other Rho(s), and Other Porosityare the qb(dry), qs, and data, respectively, that we received fromCLB. Our Rho(b), Our Rho(s), and Our Porosity are theqb(dry), qs, and data, respectively, that we collected. In qb(dry)/regressions, we preferentially used our measured qb(dry) rather thanthose calculated by CLB. In such cases, we used the associated from the Our Porosity column. Where we did not measure qb(dry),we used Other Rho(b) data and the associated Other Porosity.These selected data for graphical and statistical analysis of qb(dry)/regression trends are listed again in the Rho(b) for plotting andPor. to plot w/Rho(b) columns. In the qs analysis, we used CLBsmeasured qs (Other Rho(s)) exclusively and the associated poros-ities (Other Porosity). Missing data are represented by zeroes, asare nondetects. Total biogenic silica opal-A opal-CT bio-genic quartz. Total nonsilica clastics total clay detritalquartz other minerals. Depth is reported in feet, densities arereported in g/cm3, and all mineralogical reports are by weight. Po-rosity measurements are by volume.


Tabl

e2.

Prev

ious

lyUn

publ

ished

Data

Used

inRe

gres

sion

Anal

ysis*

Grou

pFi

eld

Dept

h(ft

)O

ther

Rho(

b)O

urRh

o(b)

Oth

erPo

rosit

yO

urPo

rosit

yO

ther

Rho(

s)O

urRh

o(s)

Por.

topl

otw

/Rho

(b)

Rho(

b)fo

rPl

ottin

gO

pal-A

(by

wt.)

Opa

l-CT

(by

wt.)

Biog

enic

Qua

rtz(b

yw

t.)

Tota

lBi

ogen

icSi

lica

Tota

lCl

ay(b

yw

t.)

Detri

tal

Qua

rtz(b

yw

t.)

Oth

erM

iner

als

(by

wt.)

Tota

lN

onsil

ica

Clas

tics

1Cy

mric

1240

.31.

09.0

0.5

20.0

002.

27.0

0.5

201.

090

.420

.00

.00

.420

.000

.00

.580

.580

1Cy

mric

1240

.7.7

5.0

0.6

66.0

002.

25.0

0.6

66.7

50.5

30.2

5.0

0.7

80.1

10.0

2.1

10.2

401

Cym

ric12

42.8

.89

.00

.625

.000

2.37

.00

.625

.890

.000

.74

.00

.740

.140

.00

.120

.260

1Cy

mric

1246

.7.8

4.0

0.6

28.0

002.

26.0

0.6

28.8

40.2

50.4

7.0

0.7

20.1

80.0

0.1

00.2

801

Cym

ric12

48.7

.80

.00

.656

.000

2.33

.00

.656

.800

.410

.36

.00

.770

.130

.00

.100

.230

1Cy

mric

1249

.0.8

0.0

0.6

46.0

002.

26.0

0.6

46.8

00.7

20.1

6.0

0.8

80.0

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0.0

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201

Cym

ric12

49.3

.70

.00

.683

.000

2.21

.00

.683

.700

.570

.28

.00

.850

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.00

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1Cy

mric

1255

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25.0

0.6

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301

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ric12

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1257

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010

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.780

.150

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alto

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phal

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910

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phal

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443

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101

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1975

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116

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2010

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048

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2107

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46.0

0.4

951.

242

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.47

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3248

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39.3

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702.

262.

21.3

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390

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17.4

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171.

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3268

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REFERENCES CITED

Badaut, D., and F. Risacher, 1983, Authigenic smectite on diatomfrustules in Bolivian saline lakes: Geochimica, CosmochimicaActa, v. 47, p. 363375.

Blake, G. H., 1981, Biostratigraphy and correlation: problems inmarginal basin development, San Joaquin Valley, California, inR. E. Garrison and R. G. Douglas, eds., The Monterey Forma-tion and related siliceous rocks of California: Pacific SectionSEPM Symposium Volume, book 15, p. 114.

Bloss, F. D., 1971, Crystallography and crystal chemistry: New York,Holt, Rhinehart and Winston, 544 p.

Bohrmann, G., V. Spiess, H. Hinze, and G. Kuhn, 1992, ReflectorPc, a prominent feature in the Maud Rise sediment sequence(eastern Weddell Sea): occurrence, regional distribution andimplications to silica diagenesis: Marine Geology, v. 106,p. 6987.

Bramlette, M. N., 1946, The Monterey Formation of California andthe origin of its siliceous rocks: U.S. Geological Survey Profes-sional Paper 212, 57 p.

California Department of Conservation (CDOC) Division of Oil,Gas and Geothermal Resources, 1998, 1997 Annual reportof the state oil and gas supervisor: Publication #PR06267 p.

Chaika, C., 1998, Physical properties and silica diagenesis:Ph.D. dissertation, Stanford University, Stanford, California,117 p.

Chang, J., and Y. C. Yortsos, 1994, Lamination during silicadiagenesiseffects of clay content and Ostwald ripening:American Journal of Science, v. 294, p. 137172.

Compton, J. S., 1991, Porosity reduction and burial history of sili-ceous rocks from the Monterey and Sisquoc formations, PointPedernales, California: Geological Society of America Bulletin,v. 103, p. 625636.

Cook, S. D., 1994, Advanced reservoir characterization in the An-telope Shale to establish the viability of CO2 enhanced oil re-covery in Californias Monterey Formation siliceous shale: Pro-ject proposal to the Department of Energy, DOE #DE-PS2294BC14973.

Deer, W. A., R. A. Howie, and J. Zussman, 1974, An introductionto the rock forming minerals, 7th ed.: London, Longman, 528 p.

Graham, S. A., and L. A. Williams, 1985, Tectonic, depositional anddiagenetic history of Monterey Formation (Miocene), centralSan Joaquin basin, California: AAPG Bulletin, v. 69, p. 385411.

Grivetti, M. C., 1982, Aspects of stratigraphy, diagenesis and defor-mation in the Monterey Formation near Santa Maria-Lompoc,California: Masters thesis, University of California at SantaBarbara, 155 p.

Gross, M. R., 1995, Fracture partitioning: failure mode as a functionof lithology in the Monterey Formation of coastal California:Geological Society of America Bulletin, v. 107, no. 7, p. 779792.

Harville, D. G., and D. L. Freeman, 1988, The benefits of applicationof rapid mineral analysis provided by Fourier transform infraredspectroscopy (abs.): Society of Petroleum Engineers 63rd An-nual Technical Conference and Exhibition Proceedings, ab-stract 18120, p. 141146.

Hein, J. R., D. W. Scholl, J. A. Barron, M. G. Jones, and J. Miller,1978, Diagenesis of late Cenozoic diatomaceous deposits andformation of the bottom simulating reflector in the South Be-ring Sea: Sedimentology, v. 25, p. 155181.

Hurd, D. C., 1983, Physical and chemical properties of siliceous skel-etons, in S. Aston, ed., Silicon geochemistry and biogeochem-istry: New York, Academic Press, p. 187243.

Iijima, A., and R. Tada, 1981, Silica diagenesis of Neogene diato-maceous and volcaniclastic sediments in northern Japan: Sedi-mentology, v. 28, p. 185200.

Iler, R. K., 1973, Colloidal silica, in E. Matijevic, ed., Surface andcolloid science, v. 6: New York, Wiley, p. 1100.

Iler, R. K., 1979, Chemistry of silica: New York, Wiley-Interscience,866 p.

Ingle, J. C., 1981, Origin of Neogene diatomites around the NorthPacific Rim, in R. E. Garrison and R. G. Douglas, eds., TheMonterey Formation and related siliceous rocks of California:Pacific Section SEPM Symposium Volume, book 15,p. 159180.

Isaacs, C. M., 1980, Diagenesis in the Monterey Formation examinedlaterally along the coast near Santa Barbara, California:Ph.D. dissertation, Stanford University, Stanford, California,329 p.

Isaacs, C. M., 1981, Porosity reduction during diagenesis of the Mon-terey Formation, Santa Barbara coastal area, California, in R. E.Garrison and R. G. Douglas, eds., The Monterey Formation andrelated siliceous rocks of California: Pacific Section SEPM Sym-posium Volume, book 15, p. 257272.

Isaacs, C. M., 1983, Compositional variation and sequence in theMiocene Monterey Formation, Santa Barbara coastal area, Cali-fornia, in D. K. LaRue and R. J. Steel, eds., Cenozoic marinesedimentation, Pacific margin, USA: Pacific Section SEPM Pub-lication, p. 117132.

Kastner, M., J. B. Keene, and J. M. Gieskes, 1977, Diagenesis ofsiliceous oozes, I: chemical controls on the rate of opal-A toopal-CT transformationan experimental study: Geochimica,Cosmochimica Acta, v. 41, p. 10411059.

Lonsdale, M. J., 1990, The relationship between silica diagenesis,methane, and seismic reflections on the South Orkney micro-continent, in P. F. Barker et al., eds., 1990 Proceedingsof the ocean drilling program, scientific results: v. 113,p. 2737.

Luyendyk, B. P., and J. S. Hornafius, 1987, Neogene crustal rotation,fault slip and basin development in Southern California, inR. V. Ingersoll and W. G. Ernst, eds., Rubey volume VI: Ce-nozoic basin development of coastal California: EnglewoodCliffs, New Jersey, Prentice-Hall, p. 259283.

Mayerson, D. A., and J. Crouch, 1994, The opal-CT/quartz diage-netic boundary within the Monterey Formation of the Califor-nia offshore Santa Maria Basin; an untapped exploration target(abs.): AAPG Bulletin, v. 78, no. 4, p. 669670.

Mayerson, D. A., C. A. Dunkel, K. A. Piper, and H. L. Cousminer,1995, Identification and correlation of the opal-CT/quartzphase transition in offshore central California (abs.): AAPGBulletin, v. 79, no. 4, p. 592.

McGill, G. E., and J. A. Raney, 1970, Experimental study of faultingin an anisotropic inhomogeneous dolomitic limestone: Geolog-ical Society of America Bulletin, v. 81, p. 29492958.

McManus, D. A., R. E. Burns, O. Weser, T. Vallier, C. V. von derBorch, R. K. Olsson, R. M. Goll, and E. D. Milow, 1970, Initialreports of the deep sea drilling project, volume V: Washington,D.C., U.S. Government Printing Office, 827 p.

Murata, K. J., and R. R. Larsen, 1975, Diagenesis of Monterey sili-ceous shales, Temblor Range, California: United States Geo-logical Survey Journal of Research, v. 3, p. 553566.

Oehler, J. H., 1975, Origin and distribution of silica lepispheres inporcelanite from the Monterey Formation of California: Journalof Sedimentary Petrology, v. 45, p. 252257.

Pisciotto, K. A., and R. E. Garrison, 1981, Lithofacies and deposi-tional environments of the Monterey Formation, California, inR. E. Garrison and R. G. Douglas, eds., The Monterey Forma-tion and related siliceous rocks of California: Pacific SectionSEPM Symposium Volume, book 15, p. 97122.


Reid, S. A., and J. L. McIntyre, 2001, Monterey Formation porce-lanite reservoirs of the Elk Hills field, Kern County, California:AAPG Bulletin, v. 85, no. 1, p. 169189.

Rice, S. B., H. Freund, W.-L. Huang, J. A. Clouse, and C. M. Isaacs,1995, Application of Fourier transform infrared spectroscopyto silica diagenesis: the opal-A to opal-CT transformation: Jour-nal of Sedimentary Research, v. A65, no. 4, p. 639647.

Schlumberger, 1989, Log interpretation principles/applications:Houston, Schlumberger Education Services, not paginated.

Schoell, M., S. Schouten, J. S. Sinninghe Damste, J. W. De Leeuw,and R. E. Summons, 1994, A molecular carbon isotope recordof Miocene climate changes: Science, v. 263, p. 11221125.

Shipley, T. H., M. H. Houston, R. T. Buffler, F. J. Shaub, K. J.McMillen, J. W. Ladd, and J. L. Worzel, 1979, Seismic evidencefor widespread possible gas hydrate horizons on continentalslopes and rises: AAPG Bulletin, v. 63, no. 12, p. 22042213.

Snyder, W. S., H. K. Brueckner, and R. A. Schweikert, 1983, De-formational styles in the Monterey Formation and other sili-ceous sedimentary rocks, in C. M. Isaacs and R. E. Garrison,eds., Petroleum generation and occurrence in the MioceneMonterey Formation, California: Pacific Section SEPM Vol-ume, p. 151170.

Tada, R., and A. Iijima, 1983, Petrology and diagenetic changes ofNeogene siliceous rocks in northern Japan: Journal of Sedimen-tary Petrology, v. 53, no. 3, p. 911930.

Williams, L. A., 1982, Lithology of the Monterey Formation (Mio-cene) in the San Joaquin Valley of California, in L. A. Williamsand S. A. Graham, eds., Monterey Formation and associatedcoarse clastic rocks, central San Joaquin basin, California: Pa-cific Section SEPM Publication 25, p. 1736.

Williams, L. A., 1988, Origins of reservoir complexity in the Mio-cene Monterey Formation of California, in S. A. Graham, ed.,Studies of the geology of the San Joaquin basin: Pacific SectionSEPM, v. 60, p. 261279.

Williams, L. A., and D. A. Crerar, 1985, Silica diagenesis II: generalmechanisms: Journal of Sedimentary Petrology, v. 55, p. 312321.

Williams, L. A., G. A. Parks, and D. A. Crerar, 1985, Silica diagenesisI: solubility controls: Journal of Sedimentary Petrology, v. 55,p. 301311.

Wise, S. W., and F. M. Weaver, 1974, Chertification of oceanic sed-iments, in K. J. Hsu and H. C. Jenkyns, eds., Pelagic sediments:on land and under the sea: International Association of Sedi-mentologists Special Publication 1, p. 301326.

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