makowitz et al_2006

AUTHORS

A. Makowitz � Department of GeologicalSciences, University of Texas at Austin, Austin,Texas 78712; present address: BP America, 501Westlake Park Blvd., Houston, Texas 77079;[email protected]

Astrid Makowitz joined BP upon completion of herPh.D. at the University of Texas at Austin (2004).Both M.S. (1999) and B.S. (1997) geology degreeswere awarded from the Michigan State University.Astrid has enjoyed working as a reservoir qualityspecialist and is currently in the Onshore NorthAmerican Gas production setting. Her love for ge-ology remains with studying rocks on a pore tosubpore scale.

R. H. Lander � Geocosm LLC, 3311 San MateoDrive, Austin, Texas 78738

Robert Lander coinvented Geocosm’s Prism andTouchstone models and Geologica’s Exemplar1

model. Rob obtained a Ph.D. in geology from theUniversity of Illinois in 1991 and was a seniorresearch geologist at Exxon Production Researchfrom 1990 to 1993. He then worked for RogalandResearch and Geologica in Stavanger, Norway.Rob cofounded Geocosm in 2000 and is a researchfellow at the University of Texas at Austin.

K. L. Milliken � Department of GeologicalSciences, University of Texas at Austin, Austin,Texas 78712

Kitty Milliken has degrees in geology from Van-derbilt University (B.A.) and the University of Texasat Austin (M.A. degree, Ph.D.). At the University ofTexas at Austin, she currently serves as a researchscientist in the electron microbeam facility. Togetherwith students, she pursues research projects thatapply imaging and analysis to decipher the chem-ical histories of low-temperature systems. She isa coauthor of the recently released interactive teach-ing module Sandstone Petrology: A Tutorial Petro-graphic Image Atlas.

ACKNOWLEDGEMENTS

The authors are grateful to Zyihong He of Zetawarefor generously providing access to the GenesisSoftware. We thank Anadarko, BHPBillton, BP, Chev-ronTexaco, ConocoPhillips, ExxonMobil, Kerr-McGee,Petroleos de Venezuela SA, Petrobras, Saudi Aramco,Shell, Total, and Unocal for supporting Touchstoneresearch and development by virtue of their mem-bership in Geocosm’s Consortium for QuantitativePrediction of Sandstone Reservoir Quality. ReviewersOlav Walderhaug, Howard White, and Nick Wilsongave constructive suggestions for the improvementof our article.

Diagenetic modeling to assessthe relative timing of quartzcementation and brittle grainprocesses during compactionA. Makowitz, R. H. Lander, and K. L. Milliken

ABSTRACT

This study describes porosity reduction by brittle deformation and

the application of TouchstoneTM sandstone diagenesis modeling

software to assess the relative timing and interactions between

grain fracturing and cement formation during burial compaction.

Two examples from a previous study of compactional fracturing are

used: the Oligocene Frio Formation, Gulf of Mexico Basin, and the

Cambrian Mount Simon Formation, Illinois Basin, United States.

Grain fracturing during compaction creates intragranular fracture

surfaces that are favorable sites for quartz nucleation compared to

external grain surfaces that may bear coatings that inhibit the nu-

cleation and growth of quartz cement. Thus, the progress of brittle

fracture processes during diagenesis affects quartz cementation. In

turn, modeling of the quartz cementation process can serve to place

fracturing into its proper context in burial history.

In the Mount Simon Formation, the extent of brittle deforma-

tion of quartz grains correlates with reconstructed effective stress at

the onset of quartz cementation. For Frio Formation samples, how-

ever, the extent of brittle deformation does not correlate well with

reconstructed effective stress obtained using a one-dimensional basin

model that uses compaction disequilibrium as the dominant mecha-

nism for overpressure generation. Judging from the observed degree

of grain fracturing, significant fluid overpressures in the Frio may not

have developed at the shallow depths indicated by our basin models.

The degree of compactional fracturing in sandstones constitutes

observable evidence that can be used to decipher the complexities of

pressure history.

GEOLOGIC NOTE

AAPG Bulletin, v. 90, no. 6 (June 2006), pp. 873–885 873

Copyright #2006. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received March 5, 2005; provisional acceptance June 14, 2005; revised manuscript receivedNovember 15, 2005; final acceptance December 19, 2005.

DOI:10.1306/12190505044

INTRODUCTION

Here, we undertake to integrate observations of com-

pactional grain fracturing with quartz cementation

modeling. Because the brittle fracturing process in com-

paction creates significant new surfaces for quartz ce-

mentation, it is reasonable to seek linkages between

these two processes (Makowitz and Milliken, 2003).

Modeling adds a vital quantitative perspective to our

understanding of the timing and depth of quartz ce-

mentation (Lander and Walderhaug, 1999) and, fur-

ther, into the relative timing of cementation and grain

fracturing in the subsurface. Forecasting brittle grain

deformation influences on reservoir quality can pro-

vide important insights for hydrocarbon exploration,

especially in basins where deep sandstones are prolific.

PREVIOUS WORK

Compaction and cementation are the two mechanisms

whereby primary porosity is lost in sandstones (e.g.,

Lundegard, 1992; Ehrenberg, 1995), and an understand-

ing of the controls on these processes has significant

implications for predictions of reservoir quality. The

magnitude of mechanical compaction of sandstones

during burial, a process including grain slippage, ro-

tation, and deformation, is controlled by the composi-

tion, size, and shape of the constituent grains (Pittman

and Larese, 1991) and the burial history (Lander and

Walderhaug, 1999; Paxton et al., 2002). Brittle pro-

cesses in compaction are a particularly underestimated

process because intragranular fractures in quartz grains

are typically healed by quartz cement and are therefore

difficult to detect and measure and are commonly

missed using conventional transmitted light micros-

copy (e.g., Sippel, 1968; Milliken, 1994; Dickinson and

Milliken, 1995; Makowitz and Milliken, 2003).

Cementation hinders mechanical compaction; thus,

information on the timing and physical properties of

cement phases is necessary for predicting the extent

of mechanical compaction (Ehrenberg, 1989; Pittman

and Larese, 1991; Lundegard, 1992; Wilson and Stanton,

1994; Dutton, 1997; Stone and Siever, 1997; Lander

and Walderhaug, 1999; Paxton et al., 2002). Conversely,

the intergranular volume (‘‘IGV’’ is defined as the sum

of the intergranular porosity and cements and matrix

that fill intergranular pores) remaining at a particular

stage in the burial history places an upper limit on the

amount of space that is available for cement emplace-

ment at a given depth (e.g., Paxton et al., 2002).

Several recent investigations conclude that the sig-

nificance of brittle deformation in mechanical compac-

tion is greater than previously thought, especially for

rapidly and deeply buried sandstones (Milliken, 1994;

Chuhan et al., 2002; Makowitz and Milliken, 2003).

Cathodoluminescence (CL) imaging reveals the ubiq-

uity of microfractures initiating at quartz grain contacts,

where the deviatoric stress (condition in which stress

tensors are not the same in every direction) needed for

brittle failure can be achieved locally, at the grain scale,

under conditions that are below the critical conditions

for crack propagation through the sandstone as a whole

(e.g., Sippel, 1968; Walker and Burley, 1991; Milliken,

1994; Dickinson and Milliken, 1995). The fresh micro-

fracture creates a clean surface that is favorable for

quartz cement nucleation (Reed and Laubach, 1996).

Quantitative data on fracture aperture, morphology,

number of fractures, and volume of cement localized

within these fractures can be gathered readily using CL

imaging (Laubach and Milliken, 1996; Laubach, 1997;

Marrett and Laubach, 1997; Laubach et al., 2004). In-

herited fractures are discriminated on the basis of CL

textures and excluded from measurements of post-

compactional fractures using the criteria of Laubach

(1997).

Contrasts in the number of fractured grains per

sample versus maximum burial depth between the Frio

and Mount Simon formations and the differences in

fracture morphology were hypothesized in a previous

study to be dependent on the timing of quartz cemen-

tation, which, in turn, is governed by burial rate and

geothermal gradient differences between the Frio (Gulf

of Mexico Basin) and the Mount Simon (Illinois Basin),

together with compositional and textural differences

(e.g., Frio samples have lower quartz grain content and

larger grain size) (Makowitz and Milliken, 2002, 2003).

These earlier studies also discuss in detail the evidence

for the postburial timing of the intragranular fracturing

and its compactional association, correlations between

the degree of fracturing and grain size, and the para-

genetic sequence of cements in these sandstones.

GEOLOGIC CONTEXT AND PETROGRAPHYOF BRITTLE FEATURES

Frio Formation

The Oligocene Frio Formation sandstone has long served

as a natural laboratory for studying burial compaction

because more than 3500 m (11,400 ft) of sediment was

874 Geologic Note

rapidly deposited via subsidence and growth faulting

during the middle to late Oligocene and early Miocene

(e.g., Galloway et al., 1982) (Figure 1). Moreover, the

structural history does not involve significant uplift

or compression, the unit is at or near maximum buri-

al depth, and growth faults impose a wide range of

burial depths and temperatures on materials of rela-

tively uniform initial composition. The predominantly

lithic-rich sands of the Frio Formation of the lower

Gulf Coast were supplied by the ancient Rio Grande

draining the volcanic areas of west Texas and northern

Mexico (Loucks et al., 1984). Frio sandstones are mod-

erately sorted, fine to coarse grained, and range from

feldspathic litharenites to sublitharenites (Figure 2).

Although quartz cement is dominant in most samples,

for any given set of samples, there will be a few that

are dominantly calcite cemented. Zeolite cement is

abundant at shallow depths (maximum = 10%), asso-

ciated with volcanic-derived lithics, whereas quartz

cement generally increases systematically with depth

(Land, 1984; Land et al., 1987), as is widely observed

in many basins worldwide (e.g., Walderhaug, 1996;

Giles et al., 2000).

Quartz grains in the Frio Formation have a variety

of fracture morphologies, including wedge-shaped aper-

tures, intense comminution at grain contacts, and grains

with exploded fabrics (Makowitz and Milliken, 2002,

2003) (Figure 3A, B). Apparent fracture apertures in

the Frio grains are slightly wider (average 5 mm) than

in Mount Simon grains (average measurable aperture

width �4 mm). Fractures in both formations are gen-

erally confined to individual grains (intragranular frac-

tures) and do not transect two or more grains (trans-

granular fracturing).

Quartz cementation is expected to stabilize the

grain framework and thereby inhibit compactional grain

fracturing. Cathodoluminescence textures indicate that

most fractures precede significant cementation, given

that most do not crosscut overgrowths (Figure 3). The

minority of fractures that do crosscut overgrowths

(see Makowitz and Milliken, 2003, their figure 10E,

p. 1015) shows, however, that grain fracturing and

quartz cementation proceed synchronously, at least

to some degree. Shallowly buried quartz grains exhib-

iting intragranular grain fractures are generally filled

with quartz cement but lack cementation on external

grain surfaces (Figure 4), indicating faster surface area-

normalized growth rates on fracture surfaces com-

pared to outer grain surfaces. The fracture surface is

fresh and clean, allowing quartz cement to nucleate

and grow within the fracture, whereas the external

grain surface may contain irregularities and detrital

particles that slow the rate of quartz precipitation.

Mount Simon Formation

The Illinois Basin is an intracratonic basin in which up

to 6000 m (19,600 ft) of sediments accumulated dur-

ing the Paleozoic (Figure 1). The Mount Simon sand-

stones (Late Cambrian) are predominantly of quartz

arenite composition, medium to coarse grained, and

well rounded (Figure 2). Quartz is the most abundant

Illinois Basin

BBí

A

Aí

Gulf Coast

Illinois Basin

Sample Location Figure 1. Sample location map. TheFrio Formation was sampled from corefrom various depths in the south TexasGulf Coast. Samples from the MountSimon Formation were collected fromcore and outcrop localities in theIllinois Basin.

Makowitz et al. 875

cement, although calcite is locally abundant in shal-

low samples. During the Late Cambrian, the tectonic

setting of the proto-Illinois Basin was governed by

thermal subsidence, lasting until the early Mississippi-

an (Rowan et al., 2002). A second subsidence episode

(middle Mississippian through Early Permian), in re-

sponse to the Alleghanian–Hercynian orogeny (Klein

and Hsui, 1987), caused pronounced downwarping in

the more southerly parts of the basin, leading to thicker

sediment accumulation (Sargent, 1991).

Other tectonic events that effected Mount Simon

deposition included periodic uplift on bounding arches

(e.g., Wisconsin, Kankawee, and Pascola arches) that

separate the Michigan basin from the Illinois Basin.

Coal rank and two-dimensional burial-history models

calibrated to coal vitrinite reflectance and biomarkers

suggest that maximum burial was attained during the

Permian, approximately 1000–1500 m (3300–4900 ft)

deeper than present (Rowan et al., 1996; Damberger

et al., 1999). During the Quaternary, glacial outwash

was deposited over most of the Illinois Basin. Amounts

of uplift and erosion in the Illinois Basin vary, with up

to 2000 m (6600 ft) in the south and approximately

300 m (1000 ft) in the north (Hoholick, 1980). Other

estimates of burial depth provided by Wilson and Sib-

ley (1978) indicate nearly 900 m (2900 ft) of erosion

in the northerly area. Maximum burial depths of sam-

ples for this study are based on the model results of

Rowan et al. (2002). Their model considers the tem-

perature influence of burial (considered the most in-

fluential factor for temperature in past models) and

advective heat transport from a short period of mag-

matism and is consistent with both vitrinite reflectance

and fluid-inclusion data.

Fracture morphologies in the Mount Simon For-

mation are homogenous and occur as thin straight

traces transecting across the quartz grains. A few wedge-

shaped fractures are also present in some samples

(Figure 3).

MODELING APPROACH

Basin Modeling

Basin modeling was conducted using Genesis1 (devel-

oped by Zetaware) to reconstruct the thermal and ef-

fective stress histories of the analyzed samples. Data

for the one-dimensional (1-D) basin models were re-

trieved from well logs, including mud weights, bottom-

hole temperatures, circulation times, stratigraphy, and

Figure 2. Ternary plotof sandstone composi-tions according to Folk’s(1980) classificationscheme. Plot shows thevariation of sandstonecomposition between theMount Simon and Frioformations. Averagecompositions of the Frioand Mount Simon for-mations are feldspathicand quartz arenite,respectively.

876 Geologic Note

gross lithology for the Frio Formation. Although vi-

trinite reflectance data are scarce, when available, they

were used to constrain thermal histories. Where in-

put data were not available for some of the wells,

we estimated the values by interpolation with nearby

wells.

Although most of the modeled temperatures match

within ±5jC of measured temperatures, a substantial

number of measurements fall out of this range. In most

cases, measured temperatures are lower than modeled

temperatures. Most likely, the true temperatures are

higher than the measured values because of the effects

of drilling. Bottom-hole temperature data retrieved

from well logs match other such data from south Texas

(e.g., McKenna and Sharp, 1998).

Mount Simon Formation burial history data are

from the model of Rowan et al. (2002) for the burial

history of the intracratonic Illinois Basin (Figure 5).

Simulation of Quartz Cementation History

Sandstone diagenesis and reservoir quality models

such as ExemplarTM (Lander and Walderhaug, 1999) or

TouchstoneTM typically are used for reservoir quality

prediction (e.g., Bonnell et al., 1999; Lander and Wal-

derhaug, 1999; de Souza and McBride, 2000; Walder-

haug, 2000; Bloch et al., 2002; Bonnell and Lander,

2003; Taylor et al., 2004) or for constraining thermal

histories (Awwiller and Summa, 1997, 1998; Lander

et al., 1997a, b; Perez et al., 1999). Such models, how-

ever, also have the potential to provide improved tem-

poral constraints on the diagenetic evolution of sand-

stones (Bonnell et al., 1999; Helset et al., 2002). In

this study, we use Touchstone version 6.0 to constrain

the history of quartz cementation, so that we can bet-

ter delineate the precise timing and conditions of brit-

tle grain deformation relative to cement emplacement.

Model inputs include (1) textural and compositional

characteristics of each analyzed sample; (2) thermal

and effective stress histories derived from basin mod-

eling; and (3) and various model parameters discussed

below. We used the same model parameters for all

simulations with two important exceptions where pa-

rameters were optimized to match measurements: the

activation energy for quartz precipitation (E a) and the

stable packing arrangement (IGVf).

Following Walderhaug (1994, 1996), we assume that

the rate-limiting control on quartz cementation is the

rate of crystal growth and not the rate of silica supply. The

surface area-normalized rate of quartz precipitation, k,

Figure 3. Fracturestyles and morphologiescharacteristic of the Frio(A and B) and MountSimon quartz grains(C and D). Fractures inthe Frio Formation (A andB) are commonly wedgeshaped, exhibit spalling,and commonly havesmall-scale cataclasis as-sociated with grain-graincontacts. In the Mount Si-mon Formation, fracturesgenerally transect thequartz grains as straighttraces with fractureapertures more uniformand generally thinnerthan in the Frio.

Makowitz et al. 877

is modeled using an Arrhenius kinetic formulation

(Walderhaug, 1996):

k ¼ Aoe�EaRT

where E a is the activation energy for quartz precipi-

tation (kJ/mol); R is the universal gas law constant

(8.31 J/mol K); T is temperature (K); and Ao is the

pre-exponential constant (here taken to be 9 � 10�12

mol/cm2 s). The kinetic equation is integrated as a

function of time and temperature using thermal re-

constructions from basin models. We adjust the E a

value for each sample simulation to achieve a match

between the calculated and measured quartz cement

abundances for each individual sample (Table 1). The

adjusted E a values for a given stratigraphic unit gen-

erally fall within a narrow range.

An additional important control on quartz cemen-

tation is the nucleation surface area and how it changes

with diagenetic alteration. We follow an approach sim-

ilar to that of Lander and Walderhaug (1999), but as-

sume that cements concentrically line spherical pores

(Merino et al., 1983; Lichtner, 1988; Canals and Meunier,

1995). The timing of nonquartz cement precipitation

is defined by paragenetic rules and burial history re-

constructions as shown in Table 2.

Compaction reduces intergranular porosity and

therefore may reduce surface area for quartz cement

nucleation. The compaction state of the sample is de-

termined using the function of Lander and Walderhaug

(1999):

IGV ¼ IGVf þ ðIGVo � IGVfÞ�bse

where IGVf is a stable packing arrangement that rep-

resents the minimum likely intergranular volume (%);

IGVo is the intergranular volume upon deposition (%),

and b is the exponential rate of compaction (MPa�1)

with effective stress se (MPa). The compaction state

of the sample is determined through geologic time as

the effective stress (from basin modeling) changes, al-

though the compaction process is assumed to be ir-

reversible should effective stress decline (Lander and

Walderhaug, 1999). IGVo is determined using a pro-

prietary algorithm in Touchstone that is based on the

unpublished experimental work of R. E. Larese and L.

M. Bonnell, and a constant value of 0.6 MPa�1 is used

for b as suggested by Lander and Walderhaug (1999).

The IGVf value for each sample (Table 1) provides an

optimal match between the present-day calculated and

measured IGV values. These values vary considerably

among samples because of differences in the extent of

grain deformation and chemical compaction.

MODELING RESULTS

To evaluate the potential influence of quartz cemen-

tation on fracture characteristics, we used Touch-

stone simulations to reconstruct the burial conditions

at which small amounts of quartz cement (0.5, 1, and

Figure 4. Frio sample 3223 (A) scanning electron microscopy-cathodoluminescence image of grain exhibiting fractures filledwith quartz cement. (B) Secondary electron image (SEI) show-ing continuous smooth surface of grain, indicating that frac-tures are filled with quartz. Two possible reason for this pref-erential fracture annealing: (1) clays and byproducts fromdissolved grains (partially dissolved feldspar in upper left andcorner) adhered to the detrital grain surface and prohibitedquartz precipitation around the grain and (2) low temperaturesat this depth (�50jC) make it difficult for quartz cement toprecipitate.

878 Geologic Note

2%) formed in the analyzed samples (Table 1). Our

results show wide ranges in conditions. For example,

the reconstructed burial depth at which 2% quartz

cement formed ranges from approximately 1700 to

2600 m (5500 to 8500 ft) in Mount Simon samples

compared to about 2650–4400 m (8690–14,435 ft)

in Frio Formation samples (Figure 6A). These differ-

ences mainly reflect variations in the thermal histories

among the analyzed samples. Thermal history is im-

portant because modeled quartz precipitation rates

increase nearly exponentially with temperature, where-

as at a given temperature, the amount of quartz cement

increases nearly linearly with time. Sandstones with rap-

id burial rates, therefore, tend to be more deeply buried

by the time a small amount of quartz cement forms

because they have lower residence times at shallow

depths, where temperatures are cooler. Such samples

also tend to experience significant quartz cementa-

tion at earlier times given that they have earlier ex-

posure to higher temperatures that lead to faster rates

of quartz precipitation. Differences in the surface area

for quartz nucleation are an additional cause of varia-

tion in quartz cement abundances. Mount Simon For-

mation sandstones generally would be expected to have

somewhat more quartz cement than Frio Formation

samples of comparable grain size and thermal exposure

because of greater nucleation surface associated with

greater quartz grain abundance and lower grain coating

coverage.

The percentage of fractured quartz grains corre-

lates strongly with the reconstructed burial depth at

the time small amounts of quartz cementation formed

for samples from both data sets (Figure 6). This cor-

relation appears to be somewhat stronger for the depth

at which 2% quartz formed than it is for 1 or 0.5%

(Figure 6A, B). Burial depth is a driving force for com-

paction, however, only in as much as it relates to effec-

tive stress (and temperature when it involves chem-

ical processes). In the Frio Formation our 1-D basin

models indicate that those samples with the greatest

Figure 5. Thermal history for Frio and Mount Simon formations generated from 1-D Genesis basin models. Frio wells are depictedby name and are located in the following south Texas counties: (1) Jack Brown in Live Oak Co.; (2) Slick State in Starr Co.; (3) BaffinState in Kleberg Co.; (4) Hornsby in Brooks Co.; (5) Seeligson and McHaney in Jackson Co.; (6) Gerdts and McCullough in Willacy Co.;(7) Copano State in Aransas Co.; and (8) Pleasant Bayou in Brazoria Co.

Makowitz et al. 879

Table 1. Model Input and Output Parameters Including Modeling Results at 0.5, 1.0, and 2.0% Quartz Cement

2% Quartz 1% Quartz 0.5% Quartz

Sample Well Well Unit

E a(kJ/mol) IGVo IGVf

Time

(Ma)

Temperature

(jC)Depth

(m)

Effective

Stress

(MPa)

Effective

Stress

Hydrostatic

(MPa)

Time

(Ma)

Temperature

(jC)Depth

(m)

Effective

Stress

(MPa)

Effective

Stress

Hydrostatic

(MPa)

Time

(Ma)

Temperature

(jC)Depth

(m)

Effective

Stress

(MPa)

Effective

Stress

Hydrostatic

(MPa)

1164 Northern Illinois Mt. Simon 62.8 33.8 22.1 263 103.6 1963 24.5 24.5 338 57.9 1196.9 15.0 15.0 408 55.3 1075.7 13.5 13.5





3134.5 Northern Illinois Mt. Simon 63.8 34.2 15.9 378 79.6 1818 22.7 22.7 432 77.1 1662.1 20.8 20.8 460.33 76.7 1612.3 20.2 20.2


3793 Central Illinois Mt. Simon 61.4 33.1 15.4 337 70.3 1749 21.9 21.9 380 64.8 1583.2 19.8 19.8 414 56.7 1345.3 16.8 16.8

3581.5 Central Illinois Mt. Simon 61.2 32.4 21.3 333 69.8 1735 21.1 21.1 378 67.6 1665.0 20.8 20.8 412 59.7 1437.5 18.0 18.0





4477 Central Illinois Mt. Simon 61.8 34.3 10.8 354 78.8 1994 24.9 24.9 398 70.6 1755.5 22.0 22.0 430.5 61.9 1442.6 18.0 18.0




6154 Southern Illinois Mt. Simon 62.0 34.9 13.3 315 104.6 2113 26.4 26.4 340 90.2 1751.0 21.9 21.9 364 78.4 1432.4 17.9 17.9







3223 Jack Brown Frio * * * * * * * * * * * * * * * * * *

4908 Slick State Frio * * * * * * * * * * * * * * * * * *

6105 Seeligson Frio * * * * * * * * * * * * * * * * * *

8910 Baffin State Frio * 38.1 29.2 * * * * * * * * * * * * * * *

9001 Hornsby Frio 58.0 38.2 11.1 5 91.7 2650 30.2 33.1 13.88 84.7 2351.4 26.7 29.4 19.24 80.9 2191.3 24.7 27.4

9547 Gerdts Frio 57.2 38.4 25.5 6 108.5 2795 15.1 34.9 15.05 100.6 2493.2 12.6 31.2 19.85 98.3 2370.0 10.6 29.6

880

Geologic

Note

reconstructed burial depths at the time of significant

quartz cementation also have the lowest reconstructed

effective stresses because they experienced faster rates

of burial and, therefore, greater extents of fluid over-

pressure development because of compaction disequi-

librium (caused by the inability to expel pore fluids

in low-permeability shales and clay-rich sediments;

hence, most of the overlying sediment’s weight is

supported by the pore fluid instead of the grains)

(Figure 7). Thus, the Frio Formation samples with

the greatest degree of quartz grain fracturing also had

the lowest reconstructed effective stresses at the time

of significant quartz cementation. Such a result is in-

consistent with experimental and theoretical results,

which indicate that grain fracturing is promoted by

greater effective stresses (Chuhan et al., 2002; Chester

et al., 2004; Karner et al., 2005). The extent of grain

fracturing correlates much more strongly with effec-

tive stress if fluid pressures were near hydrostatic lev-

els at the time that small amounts of quartz cement

formed (hydrostatic case in Figure 7). These results

suggest that fluid overpressures in the Frio Formation

may have developed at significantly greater depths (and

later times) than would be expected in basin models

that rely mainly on compaction disequilibrium. Alter-

native mechanisms for fluid overpressure development

that could lead to a shift into overpressured conditions

late in the burial history include hydrocarbon reac-

tions (Luo and Vasseur, 1996; Osborne and Swarbrick,

1997; Hansom and Lee, 2005) and diagenetic reac-

tions (Waples and Kamata, 1993; Bjørkum and Nadeau,

1996, 1998; Lander 1998; Matthews et al., 2001; Helset

et al., 2002).

As discussed previously, fractures in the Mount

Simon Formation samples show thin-straight fracture

traces, whereas in Frio Formation samples, fractures9710

McHaney

Frio

61.0

38.0

25.3

**

**

**

**

**

0100.2

2962.7

34.6

37.0

9720

McHaney

Frio

62.5

38.8

14.1

**

**

*5.3

100.5

2917.6

33.6

36.5

15.37

96.7

2729.3

31.2

34.1

9744

McHaney

Frio

*36.7

26.6

**

**

**

**

**

**

**

*

10169

Gerdts

Frio

58.7

37.2

11.3

7113.7

2956

14.6

37.0

16.01

106.7

2668.4

11.9

33.4

20.82

105.2

2557.5

9.9

32.0

13833

McCullough

Frio

58.2

43.1

17.5

18108.3

2693

11.1

33.7

21.78

107.9

2615.7

9.4

32.7

23.7

107.1

2611.1

8.3

32.6

15620

Pleasant

Bayou

Frio

63.4

32.5

7.2

23177.2

4357

9.0

54.5

23.7

170.6

4366.5

8.5

54.6

24.6

156.5

4098.1

7.9

51.2

15640

Pleasant

Bayou

Frio

63.3

35.1

16.7

22177.6

4363

9.1

54.5

23.44

172.5

4370.7

8.6

54.6

24.45

159.5

4156.5

8.1

52.0

*Sam

ples

with

less

than

2%quartzcementthat

wewerenotableto

modelor

areinsignificant.

Table 2. Depth Constraints for the Paragenetic Sequence

Used in Modeling for Both the Frio and Mount Simon

Formations

Start (m) End (m)

Grain coating 0 100

Calcite 100 1000

Chlorite 200 1000

Kaolinite 1000 2000

Pyrite 0 100

K-feldspar 1000 3000

Dolomite 2000 4000

Iron oxides 100 1000

Makowitz et al. 881

have larger wedgelike forms. The difference in the

effective stress at the onset of significant quartz ce-

mentation may be one factor causing this change in

fracture geometry. It is also possible that fractures in

Mount Simon Formation samples had more restricted

dilation because of their greater abundance of rigid

quartz grains or their lower IGV values (average of

18.6 versus 24.8% for the Frio).

Figure 6. Depths at which quartz cement content reached 0.5% (A) and 2% (B) versus percentage of fractured quartz grains. Apositive correlation exists between the onset of quartz cementation and degree of grain fracturing for both the Mount Simon and Frioformations, and this correlation is best for the 2% level of quartz cement emplacement.

Figure 7. Effective stress at low amount of quartz cement, (A) at 0.5% quartz cement and (B) at 2.0% quartz cement, versuspercentage of fractured grains shows a positive correlation in both formations, considering a hydrostatic stress regime at this time inthe burial history. However, if deeper Frio sands are influenced by compaction disequlibrium, which causes overpressure, thus,reducing the effective stress, this trend would not hold true.

882 Geologic Note

CONCLUSIONS

� Data presented in this article demonstrate that the

effective stress at the time of quartz cement

initiation is an important constraint for predicting

the degree of grain fracturing in quartz-rich sands.� The deeper Frio data support the notion that ef-

fective stresses were much higher than would be

expected from 1-D disequilibrium compaction mod-

els at the time of quartz cement initiation, suggesting

that overpressure began at greater depths (later

times) in the burial history.� Differences in degree of fracturing and fracture mor-

phologies between the Frio and Mount Simon for-

mations can be attributed to (1) greater depth to

initiation of quartz cementation in the Frio than in

the Mount Simon, allowing for more and wider frac-

tures and apertures in the Frio; and (2) IGV, whereby

lower IGVs in the Mount Simon resulted in a re-

duced possibility of expansion of grains into the pore

space and, hence, thinner fracture apertures.

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