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Diagenesis and Reservoir Quality of the 

Upper Mississippian Aux Vases Sandstone, 

Illinois Basin

U.S. Geological Survey Professional Paper 1609

U.S. Department of the Interior 

U.S. Geological Survey
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Library of Congress Cataloging-in-Publication Data

Pitman, Janet K.

Diagenesis and reservoir quality of the Upper Mississippian Aux 

Vases Sandstone, Illinois Basin / Janet K. Pitman, Mitchell Henry, 

and Hannes Leetaru.

p. cm. — U.S. Geological Survey professional paper ; 1609) 

Includes bibliographical references.

1. Geology, Stratigraphic—Mississippian. 2. Diagenesis—Illinois 

Basin. 3. Petroleum—Geology—Illinois Basin. 4. Aux Vases 

Formation. I. Henry, Mitchell E. II. Leetaru, Hannes E. 

III. Title. IV Series. 

QE672.P58 2000

553.2'8'0977—dc21 99-39834

CIP
Diagenesis and Reservoir Quality of the 

Upper Mississippian Aux Vases Sandstone, 

Illinois Basin

By Janet K. Pitman, Mitchell Henry, and Hannes Leetaru

U.S. Geological Survey Professional Paper 1609

U.S. Department of the Interior 

U.S. Geological Survey
Contents

Abstract ................................................................................................................................................ 1

Introduction .......................................................................................................................................... 1

Methods................................................................................................................................................ 1

Geologic History and Depositional Framework............................................................................... 2

Mineralogy and Diagenesis ............................................................................................................... 4

Compaction and Pressure Solution ......................................................................................... 5

Mineral Cements ........................................................................................................................ 5

Clay Minerals .............................................................................................................................. 7

Other Phases............................................................................................................................... 7

Dissolution.................................................................................................................................... 10

Time-Temperature History................................................................................................................. 11

Diagenetic Evolution of Reservoir Sandstones .............................................................................. 11

Hydrocarbon Generation and Migration ......................................................................................... 14

Reservoir Quality ................................................................................................................................. 14

Conclusions........................................................................................................................................... 18

Acknowledgments .............................................................................................................................. 18

References Cited ................................................................................................................................. 18

Figures

1. Location map showing the Illinois Basin and east-west cross section 

depicting Upper Mississippian Chesterian Series and Upper Devonian 

and Lower Mississippian New Albany Shale................................................................. 2

2. Map showing location of core holes sampled for petrographic and

geochemical analysis......................................................................................................... 3

3. Stratigraphic column depicting the interval between Beech Creek Limestone

and St. Louis Limestone..................................................................................................... 5

4. Isolith map for the Aux Vases Sandstone....................................................................... 6

5. Ternary QFL plot for the Aux Vases Sandstone.............................................................. 7

6. Diagram of paragenetic sequence of diagenetic events in the

Aux Vases Sandstone......................................................................................................... 7

7. Plot of stable-isotope compositions of diagenetic carbonate cements in

reservoir sandstones.......................................................................................................... 10

8. Graph of relationship between thin-section porosity and total cement.................... 11

9. Burial- and thermal-history curves for the Aux Vases Sandstone interval 

showing relative timing of major diagenetic events in southern 

and central Illinois.............................................................................................................. 12

III
10-15. Plots showing:

10. Equilibrium relationship between 8180 of water, 8180 of 

early- and late-diagenetic calcite and dolomite, and 

crystallization temperature..................................................................................... 13

11. Relationship between percent core porosity and log permeability................. 14

12. Core porosity versus present depth for sandstones........................................... 14

13. Relationship between compactional and cementational

porosity loss in sandstones..................................................................................... 15

14. Percent intergranular volume versus percent thin-section

porosity in sandstones............................................................................................. 15

15. Core permeability versus present burial depth for sandstones........................ 15

16. Isocontour maps depicting regional variations in reservoir quality of

sandstones in the Aux Vases interval.............................................................................. 16

Tables

1. Location of well samples......................................................................................... 4

2. Mineralogic composition of Aux Vases sandstones........................................... 8

3. Stable-isotope compositions of carbonate cements in

Aux Vases sandstones............................................................................................. 10

IV
Diagenesis and Reservoir Quality of the 

Upper Mississippian Aux Vases Sandstone, 

Illinois Basin

By Janet K. Pitman, 1 Mitchell Henry, 1 and Hannes Leetaru2

Abstract

The Aux Vases Sandstone (Upper Mississippian) is extensively cored in the Illinois Basin and thus offers the opportunity 

to study diagenetic processes that occurred under different temperatures and burial conditions. Major diagenetic events 

observed in the Aux Vases Sandstone interval include (1) precipitation of precompactional illite and grain-rimming chlorite, 

(2) quartz overgrowth, syntaxial calcite and dolomicrospar 

cementation, (3) carbonate-cement and framework-grain dissolution, (4) illite and chlorite authigenesis, (5) ankerite precipitation 

and (6) hydrocarbon emplacement. On the basis of petrographic 

and stable-isotope analysis, early diagenetic quartz, syntaxial calcite, and dolomicrospar formed at relatively low temperatures 

(~20°-90°C) in a generally freshwater burial environment, 

whereas late-diagenetic ankerite precipitated at elevated temperatures (~120°C) in a diagenetic system dominated by basinal 

brines. Porosity reduction in sandstones was predominantly 

caused by mechanical compaction and cementation by carbonate 

and quartz overgrowths. Enhanced porosity, which is mostly of 

secondary origin, is due to carbonate cement and frameworkgrain dissolution; only minor primary porosity is preserved by 

early grain-rimming clay. Secondary porosity was mostly caused 

by acidic meteoric waters and (or) deep basin brines carrying 

organic and inorganic acids released during maturation of 

organic-rich shales. On the basis of burial-thermal reconstruction, sandstone diagenesis took place over a short time interval, 

culminating in the Late Pennsylvanian to Early Permian when the 

rocks were at or close to their maximum burial (~2 km). In southern Illinois, combined igneous and hydrothermal heat transport 

during the late Paleozoic influenced source-rock maturation and 

subsequent migration of large amounts of petroleum into reservoir sandstones.

Introduction

The Illinois Basin is a mature, hydrocarbon-producing province, and most of the oil that has been produced (approximately 

60 percent) comes from Upper Mississippian siliciclastic and carbonate rocks (fig. 1). The Aux Vases Sandstone, deposited during

1 U.S. Geological Survey, Mail Stop 939, Box 25046, Denver, Colorado, 80225.

Illinois State Geological Survey, 615 East Peabody Drive, Champaign, Illinois, 

61820.

Late Mississippian time, is an important reservoir unit that has 

yielded an estimated 1 billion barrels of oil (Seyler, 1984). To 

date, the diagenesis and reservoir quality of the Aux Vases generally have been studied on a field basis rather than a regional scale, 

and potential mechanisms and timing of diagenesis have received 

little attention. In this study we attempt to (1) document the depositional history and framework mineralogy of major clastic 

sequences within the Aux Vases Sandstone proper, (2) formulate 

a generalized diagenetic model and examine the extent to which 

diagenetic processes affected the reservoir quality of sandstones,

(3) construct regional reservoir-quality trends by mapping porosity and permeability variations across the area of deposition, and

(4) relate the diagenetic history of the unit to the migration and 

entrapment of hydrocarbons in the basin. Results of the study 

revealed that early- and late-diagenetic cements are good indicators of the types of fluids that migrated through the sandstone 

interval. The processes that controlled porosity enhancement and 

porosity reduction in potential reservoir units were constrained 

by identifying the origin and timing of these cements. A knowledge of the major controls on porosity and permeability distribution is necessary in order to develop reliable porosity-prediction 

models in underdrilled areas of the basin.

Methods

The regional stratigraphic framework of the Aux Vases 

Sandstone was defined by the correlation of wireline logs from 

more than 1,400 wells (Leetaru, 1997). Representative core samples of sandstones were taken from approximately 70 wells for 

petrographic study (fig. 2; table 1). The wells selected for analysis 

span a depth interval of -305-1,000 m and cover a wide geographic area. Thin sections of the sandstone samples were 

impregnated with blue epoxy for porosity recognition and stained 

with a combination of Alizarin red-S and potassium ferricyanide 

to distinguish Fe-free and Fe-bearing carbonate; thin sections 

were also stained with sodium cobaltinitrate to aid in the identification of K-feldspar. Three hundred counts were made on each 

sample to quantify rock composition and thin-section porosity. In 

this study, the classification scheme of Folk (1974) was modified 

so that the quartz category consisted of monocrystalline grains— 

polycrystalline quartz grains were included as rock fragments. 

Grain size, defined as the long axis of random framework grains, 

was computed for each sample.

Samples containing end-member carbonate were analyzed 

for their carbon and oxygen isotopic composition by Global 

Geochemistry (Canoga Park, Calif.) and Mountain Mass

1
Spectrometry (Evergreen, Colo.). Carbon- and oxygen-isotope 

ratios were obtained by a timed-dissolution procedure based on 

different reaction rates for chemically distinct carbonate phases 

(Walters and others, 1972). Upon reaction with phosphoric acid, 

CO2 gas that was evolved in the first hour was attributed to calcite, and CO2 gas evolved after several hours was assigned to 

dolomite. Isotope results are reported as the per mil (%o) difference relative to the PDB (Peedee belemnite) standard using the 

delta (6) notation. Data reproducibility is precise to ±0.2%o.

Conventional porosity and permeability analyses from 96 

wells that penetrated the Aux Vases Sandstone were compiled 

into a reservoir-quality database. The data were taken from the 

Illinois State Geological Survey's Well Inventory System. In 

order to minimize large variations in porosity and permeability 

within a well, median values were calculated for the cored interval. Determining a median value for a whole interval rather than 

a specific facies eliminated problems associated with complex 

lithogenetic relationships, which are difficult to distinguish on 

well logs and in drill cores.

./Mississippi c s 

Embayment 

'

Geologic History and Depositional Framework

The Aux Vases Sandstone was deposited during Late Mississippian time as part of a tidally influenced mixed carbonate-siliciclastic system. The unit is composed of alternating carbonate and 

siliciclastic rock and is bounded by major sequence boundaries 

(Leetaru, 1997) (fig. 3). The base of the Aux Vases, which corresponds to the lower sequence boundary, generally is at the top of 

the Upper Mississippian Ste. Genevieve Limestone, but, in some 

areas, the Ste. Genevieve has been eroded and the Aux Vases 

unconformably overlies the Upper Mississippian St. Louis Limestone. Where present, the Upper Mississippian Joppa Limestone 

forms the lower part of the Aux Vases Sandstone (Leetaru, 1997). 

The Joppa is composed of thinly interbedded limestone, shale, 

siltstone, and sandstone and corresponds to a marine transgressive event. No hydrocarbons have been recovered from the Joppa 

because of the lack of good reservoirs. The principal hydrocarbon 

reservoirs within the Aux Vases occur in sandstone sequences 

within the upper part of the unit. The upper part of the Aux Vases 

is predominantly a siliciclastic unit that was deposited during a 

marine highstand. A relative drop in sea level followed deposition 

and resulted in widespread erosion. The upper sequence boundary of the Aux Vases is represented by paleosols, mudcracks, and 

root traces, which are indicators of this erosional event.

On a regional scale, the Aux Vases Sandstone shows variations that correspond to different depositional environments 

within the siliciclastic-carbonate system (fig. 4). The Aux Vases 

in the western part of the study area is dominated by massive, 

crossbedded sandstone beds that are locally fossiliferous and 

conglomeratic at their bases. Stratigraphic and sedimentologic 

studies suggest that these fluvial and estuarine sandstones formed 

as valley-fill deposits during a period of sea-level change 

(Leetaru, 1997). At shallow depths, these sandstones are important aquifers; however, they have not been found to contain oil 

and gas. On the basis of sandstones geometries (fig. 4), the Aux 

Vases in the central region formed from northeast-southwesttrending offshore bars that were deposited parallel to the 

paleoshoreline. There is only minor oil production from the Aux

ILLINOIS INDIANA A'

SL

0 40 80 KILOMETERS

I I I

Figure 1. A, Location map showing the Illinois Basin. B, East-west 

cross section depicting Upper Mississippian Chesterian Series and 

Upper Devonian and Lower Mississippian New Albany Shale; subcrop of the Chesterian Series is shown as a dashed line on inset 

map (modified from Swann, 1967).

Vases in this region. Most oil in the Aux Vases is found in a crossbedded sandstone facies in the eastern part of the region. The reservoir facies contains isolated shale clasts and was deposited in a 

tidally influenced system (Seyler, 1998). In contrast, stratigraphically equivalent nonreservoir sandstones, including rhythmically 

bedded, and lenticular and flaser-bedded sequences, formed in a 

tidal-flat environment. The Aux Vases Sandstone generally 

becomes more calcareous to the east and southeast of the study 

area, and, at some localities, sandstones were deposited as channels that eroded into the underlying carbonate-rich facies of the

Diagenesis and Reservoir Quality of the Upper Mississippian Aux Vases Sandstone, Illinois Basin
38°-

37°-

0 10 20 30 40

KM

Figure 2. Map showing location of core holes sampled for petrographic and geochemical analysis.

Joppa (Leetaru, 1991). The crossbedded sandstone facies in the lateral and vertical seals that compartmentalize oil reservoirs 

eastern region may contain limestone with carbonate grains that (Oltz, 1994). Reservoir compartmentalization is recognized by 

were transported into a predominantly siliciclastic environment multiple oil-water contacts and higher-than-expected initial flowby storms. These impermeable limestones commonly create ing pressures within the Aux Vases interval (Leetaru, 1993).

Geologic History and Depositional Framework 3
Table 1. Location of well samples. Table 1. Location of well samples—Continued.

Core ID

C00071

C00285

C00345

C00381

C00433

C00460

C00467

C00494

C00592

C00597

C00598

C00606

C00627

C00674

C00735

C00752

C00771

C00946

C00992

C01049

C0 1064

C01 1 36

CO 1203

C01235

C01236

C0 1243

C01266

C0 1279

C01322

C01387

C01396

C01439

C0 1445

C01511

CO 1546

API

1 207700603

1214502037

1208102322

1217300063

1219100109

1219306796

1219304604

1219100012

1219300424

1207900017

1208100799

1207900128

1206500251

1 206500043

1212101296

1215900014

1207901699

1212103995

1218900369

1213900006

1212104128

1205100944

1219106348

1 204900446

1 205500679

1219101768

1205100167

1205900360

1202701515

1219101118

1208100709

1208100764

1219101791

1204900140

1 202500525

State

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Latitude

37.928828

38.158661

38.320668

39.326528

38.399507

37.933664

38.160159

38.556896

38.153027

38.878071

38.174662

38.980093

38.073224

38.254823

38.705799

38.586007

38.958428

38.589738

38.306769

39.581258

38.579448

38.945236

38.416416

38.967613

37.959383

38.561880

38.880339

37.739878

38.683297

38.531865

38.366666

38.263679

38.589686

39.065321

38.789958

Longitude

89.283433

89.285138

89.042866

88.916095

88.544023

88.294968

87.986364

88.618865

88.043539

88.352995

88.941924

88.284636

88.424937

88.520667

89.055451

88.081222

88.110717

88.997243

89.559301

88.534095

89.020696

88.840027

88.310185

88.541 509

88.723707

88.410092

88.69996

88.084803

89.562707

88.203934

89.11894

89.085271

88.279198

88.765539

88.419539

Core ID

CO 1626

C01635

C0 1697

C01742

C01813

C02055

C02059

C02085

C02096

C02114

C02157

C02170

C02235

C02253

C02323

C02341

C02521

C03339

C04683

C05279

C05817

C06040

C06069

C06071

C13071

C13892

C13974

C13975

C13976

C13977

C14007

C14011

No#

No#

INI 33

API

1219303068

1205101009

1202901120

1215900748

1203500074

1212900326

1212101476

1212104968

1212101475

1 202500549

1212104966

1212101410

1203500090

1219306226

1217300392

1208100199

1216501397

1205501226

1217300078

1218502475

1216503280

1216503273

1206503364

1206503362

1205523456

1219923491

1205523596

1219331131

1205523768

1205523769

1205523753

1205523750

1 205523744

1208124399

No data

State

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Indiana

Latitude

38.045577

38.961306

39.469346

38.691761

39.337077

39.453137

38.779823

38.779868

38.778051

38.710497

38.778053

38.541994

39.235724

37.992952

39.296958

38.181586

37.818689

38.052249

39.280430

38.260543

37.829886

37.829917

37.918572

37.911432

37.913560

37.783680

37.999733

38.014470

37.903527

37.905350

37.901550

37.903550

37.903550

38.351390

38.238589

Longitude

88.27385

88.860722

88.398547

88.21526

88.433253

88.400827

88.767094

88.755532

88.762495

88.584495

88.760168

88.788652

88.407222

88.1545

88.76281

88.992598

88.529178

88.732005

88.74881

87.944239

88.540589

88.545221

88.698900

88.703276

88.71468

88.98479

88.987413

88.16316

89.029376

89.0246

89.02454

89.0269

89.02454

88.73697

87.932857

Mineralogy and Diagenesis

The Aux Vases Sandstone is predominantly composed of 

well-sorted to poorly sorted, subangular to subrounded, very fine 

to medium-grained subarkoses and sublitharenites (fig. 5). 

Quartzarenites and lithic arkoses also are present within the 

sandstone interval but in subordinate amounts. Framework-grain 

compositions of individual sandstones average 81 percent 

quartz, 9 percent feldspar, and 10 percent rock fragments (table 

2). Detrital quartz, the major framework constituent, consists 

mostly of monocrystalline grains that commonly display small, 

rounded overgrowths, which suggests that they were derived 

from reworked, second-cycle sediments. Orthoclase is the dominant feldspar type in the sandstones, but minor amounts of plagioclase feldspar and microcline occur locally. Some feldspar 

grains have been altered to clay or exhibit a honeycombed (dissolution) texture, whereas other grains have been partially 

replaced by carbonate cement. Sedimentary rock fragments

consisting of chert and carbonate rip-up clasts are common lithic 

types in sandstones. Bioclastic grains, which also occur in the 

underlying upper Valmeyeran Series (Pryor and others, 1991), 

may comprise as much as 15 percent of the whole rock in sandstones adjacent to carbonate beds. Plutonic rock fragments and 

metamorphic polycrystalline quartz grains are present but in 

very minor amounts. Heavy minerals—including rutile, tourmaline, and apatite—generally are rare.

On the basis of petrographic analysis, the framework-grain 

mineralogy of the Aux Vases Sandstone interval remains relatively constant throughout the area of deposition with no distinct 

variations between individual facies. Outcrop studies and facies 

analysis suggest that the detrital assemblage was derived from a 

northwesterly source, possibly the Transcontinental arch or Wisconsin arch; the Ozark uplift also may have been a source for 

some of the siliciclastic sediment (Leetaru, 1997). In contrast, the 

Canadian Shield to the north and Appalachian Mountain belt east 

of the basin are reported to be the source of the sediment in 

younger Chesterian sandstones, which are also reservoirs for oil

4 Diagenesis and Reservoir Quality of the Upper Mississippian Aux Vases Sandstone, Illinois Basin
Series SP Resistivity

Beech Creek Ls

Ridenhower Sh

Bethel Ss

Downeys Bluff Ls

Yankeetown Ss

Renault Ls

Aux Vases Ss

i . i . i . i . i

i i i~r i

I.I.I. T~T

I 1 I T~ 1

i . i . i . i . i . i

Limestone

Sandstone

Figure 3. Stratigraphic column depicting the interval between the Beech Creek Limestone and the St. Louis Limestone (modified from Leetaru, 1998).

and gas (Swann, 1963). Our studies indicate that source-area differences are the most likely cause for the variations in framework-grain composition (and subsequent diagenetic alterations) 

observed in Chesterian-age reservoir sandstones.

The Aux Vases Sandstone was affected by a series of physical and diagenetic alterations during burial that included 

mechanical compaction, early quartz and carbonate cementation, 

framework-grain and cement dissolution, clay mineral precipitation, late-diagenetic carbonate development, and hydrocarbon 

emplacement (fig. 6). On a regional scale, there are no substantial variations in authigenic mineral type or abundance among

individual sandstone samples nor are there any obvious depthrelated trends. Minor variations that occur on a local scale most 

likely reflect the varied lithologies that comprise the unit. It is 

noteworthy that the paragenetic sequence in the Aux Vases generally is comparable to that found in other Chesterian sandstones 

despite variations in framework-grain mineralogy (Pitman and 

others, 1998).

Compaction and Pressure Solution

Most sandstones in the Aux Vases were affected by mechanical compaction, as indicated by the presence of long and concave-convex grain contacts. In the southern part of the basin, 

sandstones underwent substantial mechanical compaction, which 

resulted in only minor early-diagenetic cementation and subsequent dissolution porosity. Compaction in these sandstones 

occurred by means of increased grain-packing density due to 

grain rotation and deformation of ductile components. In some 

parts of the basin, quartz grains display interdigitated, presolved 

contacts, indicating that they experienced pressure solution. 

Grain-to-grain contact dissolution is most extensive in finegrained sandstones that contain precompactional, grain-rimming 

clay. Although pressure solution may occur without clay, clay 

coats have been shown to accelerate the dissolution process at 

framework-grain contacts (Heald, 1955; Weyl, 1959).

Mineral Cements

Total authigenic cement in the Aux Vases Sandstone ranges 

from approximately 2 percent in the most porous samples to about 

48 percent (mean 14 percent) in pervasively cemented beds (table 

2). Dominant authigenic mineral cements consist of calcite, dolomite, and quartz. Stable-isotope compositions of the major carbonate phases are reported in table 3 and shown in figure 7.

Calcite and dolomite are the dominant pore-fill cements in 

reservoir sandstones and are present in amounts ranging from 0- 

45 percent, mean 7 percent (calcite), and 0-47 percent, mean 6 

percent (dolomite) (table 2). On the basis of petrographic analysis, multiple generations of carbonate, including nonferroan and 

ferroan calcite, dolomicrospar, planar dolospar, and planar ankerite formed during early and late stages of diagenesis. The complex textural relations between individual carbonate phases 

suggest that other carbonate cements that are not recognized may 

also occur.

Nonferroan calcite predates other carbonate cements and is 

most abundant in sandstones containing bioclasts. The syntaxial 

nature of this calcite indicates that fossil fragments and detrital 

carbonate grains reworked from nearby carbonate environments 

served as nucleation sites for early carbonate growth. The carbonand oxygen-isotope compositions of syntaxial calcite vary over a 

relatively narrow range, from +1.02 to -3.40%c (813C) and -5.87 

to -8.63%c (818O) (table 3; fig. 7). In well-cemented sandstones, 

ferroan calcite comprises optically continuous crystals that form 

a poikilotopic, pore-fill cement with little to no intergranular 

porosity. The relatively high intergranular volumes characterizing 

these rocks (4-40 percent, average 25 percent) suggest that only 

minor mechanical compaction occurred prior to cementation.

Mineralogy and Diagenesis
90

60 ft-120 ft 

20 ft - 60 ft 

en Oft-20 ft

Absent or 

not mapped

Q 10 20 Kilometers 

0 20 Miles

Figure 4. Isolith map for the Aux Vases Sandstone. Contours depict net sand thickness.

More commonly, ferroan calcite has irregular grain boundaries 

and is distributed in irregular patches, or as remnant cement in 

sandstones that are porous and permeable. Such features suggest 

that ferroan calcite formerly may have been more widespread in 

reservoir sandstones. Locally, ferroan calcite replaces relict nonferroan calcite and is in contact with early quartz overgrowths; 

ferroan calcite commonly does not replace feldspar. Representative samples of poikilotopic calcite have 813C values ranging from 

+0.49 to -0.82%0 and 518O values varying from-8.00 to -8.88%o 

(table 3; fig. 7).

Dolomicrospar in reservoir sandstones comprises nonplanar 

interlocking crystals of nonferroan dolomite and, less commonly, 

tiny (-5-25 jam) planar crystals that occupy primary intergranular pores. The 813C values of dolomicrospar tend to be heavy, 

from +1.54 to +0.52%0. The 518O values range from -6.51 to 

-3.63%c (table 3; fig. 7). A large fraction of dolomite in the southern part of the basin is coarse-crystalline (-20-300 jam) planar 

dolospar that may or may not coexist with quartz and calcite. 

Most planar dolospar forms compositionally homogeneous euhedral crystals but some (detrital?) planar dolospar grains are

Diagenesis and Reservoir Quality of the Upper Mississippian Aux Vases Sandstone, Illinois Basin
Quartz

MeanQFL=81:9:10 

Quartzarenite 

Sublitharenite

Feldspar Lithic fragments

Figure 5. Ternary QFL (quartz, feldspar, lithic fragments) plot for the 

Aux Vases Sandstone based on the classification of Folk (1974).

partially rounded and exhibit Fe-rich overgrowths. The intergrown nature of planar ferroan and nonferroan dolospar and the 

small amounts of planar dolospar in most samples prevented stable-isotope analysis of these phases.

Planar ankerite postdates other carbonate cements in reservoir sandstones and is most abundant in southern Illinois in areas 

that are faulted. Most ankerite occurs as discrete or clustered 

rhombs with straight boundaries, but in well-cemented sandstones it forms a poikilotopic cement that infills secondary pores 

and replaces framework grains such as K-feldspar. One sample of 

planar ankerite analyzed isotopically has values of +0.04%0 

(513C) and -7.58%o (518O) (table 3; fig. 7). In a few samples, a 

small component of ankerite is anhedral to subhedral in habit and 

displays slightly curved crystal faces with broad sweeping extinction, which classify it as saddle ankerite. Unfortunately, it could 

not be sampled for isotopic analysis.

Authigenic quartz in reservoir sandstones constitutes discrete euhedral overgrowths that are partially developed or 

embayed. In a few samples, authigenic chlorite and illite are 

engulfed by quartz overgrowths. Generally, authigenic quartz 

averages a few percent, but in some coarser grained sandstones, 

commonly in the vicinity of major bounding faults in the southern portion of the basin, authigenic quartz cement is locally widespread (greater than 10 percent), which resulted in significant 

porosity reduction.

Clay Minerals

The clay-mineral assemblage in the Aux Vases Sandstone 

(0-18 percent, average 3 percent) is composed of illite, interstratified illite-smectite(?), and chlorite. In thin section, discrete clay 

phases are difficult to distinguish because they tend to be complexly intergrown. Illite and illite-smectite are predominantly 

precompactional clays that occur as thin tangential coatings on 

framework grains devoid of grain overgrowths. In sandstones 

where precompactional illite and illite-smectite are widespread, 

interdigitated, presolved grain contacts and incipient stylolites 

are common features. In a few samples, wispy flakes of illite (and 

chlorite) are engulfed in quartz overgrowths.

Authigenic chlorite occurs as platelets that are oriented perpendicular to detrital grain surfaces and as discontinuous rims 

that bridge framework grains in sandstones devoid of early secondary quartz overgrowths. In some sandstones with secondary 

porosity, dispersed chlorite crystals form loose aggregates in 

residual pores.

Other Phases

Authigenic phases present in volumetrically minor amounts 

in the Aux Vases Sandstone include anhydrite, pyrobitumen, 

hematite, and pyrite. Occurrences of anhydrite vary from scattered 

patches to individual euhedral crystals that replace earlier formed

DIAGENETIC STAGE

Mechanical compaction 

Illite, illite/smectite 

Fe-chlorite 

Quartz overgrowths 

Syntaxial calcite 

Dolomicrospar 

Planar dolospar 

Carbonate dissolution 

Poikilotopic calcite

Planar ankerite

Saddle ankerite

Anhydrite

Fracturing

Petroleum emplacement

EARLY LATE

Figure 6. Paragenetic sequence of diagenetic events in the Aux Vases Sandstone. Thick bar represents major period of diagenesis; dashed line indicates probable diagenesis.

Mineralogy and Diagenesis
Table 2. Mineralogic composition of Aux Vases sandstones.

[Data reported in volume percent, feld, feldspar; lith, lithics; Qtz, quartz; cmt, cement; dolo, dolomite; carb, carbonate; IGV, intergranular volume]

0)

ce

CD

3 

CD

££. 35'

0)

3

a.

30 

CD

CD

O

0

0)

5:

e

CD

C 

•o

•o

CD

i

55'

££. 55'

•5"

53'

3

X

0)

(A

CD

(A

00

0)

3 

Q.

i

3

0

3'

o_ 55'

03

0)

3"

Core ID

C00071

C00285

C00345

C00381

C00433

C00460

C00467

C00494

C00592

C00597

C00598

C00606

C00627

C00674

C00735

C00752

C00771

C00946

C00992

C01049

C01064

C01136

C01203

C01235

C01236

CO 1243

C01266

C01279

C01322

C01387

C01396

C01439

CO 1445

C01511

C01546

State

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Latitude

37.928828

38.158661

38.320668

39.326528

38.399507

37.933664

38.160159

38.556896

38.153027

38.878071

38.174662

38.980093

38.073224

38.254823

38.705799

38.586007

38.958428

38.589738

38.306769

39.581258

38.579448

38.945236

38.416416

38.967613

37.959383

38.561880

38.880339

37.739878

38.683297

38.531865

38.366666

38.263679

38.589686

39.065321

38.789958

Longitude

89.283433

89.285138

89.042866

88.916095

88.544023

88.294968

87.986364

88.618865

88.043539

88.352995

88.941924

88.284636

88.424937

88.520667

89.055451

88.081222

88.110717

88.997243

89.559301

88.534095

89.020696

88.840027

88.310185

88.541509

88.723707

88.410092

88.69996

88.084803

89.562707

88.203934

89.11894

89.085271

88.279198

88.765539

88.419539

Quartz

65

58

63.0

60

48

53

54

67

52

49.0

52

52.0

62

57.0

60

63

46

68

72

59

55.0

65

56

64

63.0

38.0

57

38.0

62

54

54

58

45.5

61

62

Nafeld

1

1

2.5

1

5

1.5

4

2

0

2.5

1

3.0

8

4.0

3

6

3

1

1

3

0.0

1

4

2

3.0

2.0

7

1.5

0.5

5

5

3

5.5

4

2

Kfeld

4

10

4.5

4

1

6

6

1

10

4.5

0

0.0

2

1.0

6

1

2

2

2

3

3.5

6

1

1

2.0

1.0

6

6.0

3

0

3

10

4.5

5

5

Total 

feld

5

11

7.0

5

6

7.5

10

3

10

7.0

1

3.0

10

5.0

9

7

5

3

3

6

3.5

7

5

3

4.0

3.0

13

7.5

3.5

5

8

13

10.0

9

7

Total 

lith

2

8

5.5

9

6

10

9

6

8

10.0

11

5.0

7

5.0

2

9

21

4

1

10

4.5

6

6

6

9.0

7.0

7

2.5

2

15

4

7

12.0

4

13

Qtz 

cmt

6

1

4.5

4

0

0

1

8

0

3.0

7

19.0

2

0.0

7

3

4

4

2

0

1.5

3

7

7

1.0

0.5

5

0.0

2

0

0

5

0.5

3

5

Total 

calcite

2

0

0.0

11

2

10

19

6

26

22.5

28

0.0

2

0.5

2

8

8

1

0

19

29.0

4

4

0

7.0

45.0

0

13.0

16.5

13

0

0

9.0

1

1

Total 

dolo

0

0

0

1

2

0

0

2

3

1.0

0

10.0

1

0.0

0

0

10

0

0

1

1.0

0

0

0

4.0

2.5

0

31.5

0.5

1

0

0

1.0

1

3

Total 

carb

2

0

0.0

12

4

10

19

8

29

23.5

28

10.0

3

0.5

2

8

18

1

0

20

30.0

4

4

0

11

47.5

0

44.5

17

14

0

0

10.0

2

4
Table 2. Mineralogic composition of Aux Vases sandstones—Continued.

3*

CD

5"

ca 

a)

3 

8.

Diagenes

M"

Core ID

C01626

CO 1635

C01697

CO 1742

C01813

C02055

C02059

C02085

C02096

C021 14

C02157

C02170

C02235

C02253

C02323

C02341

C02521

C03339

C04683

C05279

C05817

C06040

C06069

C06071

C13071

C13892

C13974

C13975 

C 13976 

C13977

C 14007 

C 14011

No#

No#

IN133

State

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois 

Illinois 

Illinois

Illinois 

Illinois 

Illinois

Illinois

Indiana

Latitude

38.045577

38.961306

39.469346

38.691761

39.337077

39.453137

38.779823

38.779868

38.778051

38.710497

38.778053

38.541994

39.235724

37.992952

39.296958

38.181586

37.818689

38.052249

39.280430

38.260543

37.829886

37.829917

37.918572

37.911432

37.913560

37.783680

37.999733

38.014470 

37.903527 

37.905350

37.901550 

37.903550 

37.903550

38.351390

38.238589

Longitude

88.27385

88.860722

88.398547

88.21526

88.433253

88.400827

88.767094

88.755532

88.762495

88.584495

88.760168

88.788652

88.407222

88.1545

88.76281

88.992598

88.529178

88.732005

88.74881

87.944239

88.540589

88.545221

88.698900

88.703276

88.71468

88.98479

88.987413

88.16316 

89.029376 

89.0246

89.02454 

89.0269 

89.02454

88.73697

87.932857

Quartz

54

40

69.5

46

58

58

62.5

44.0

58.0

59.5

58

57

64

45

59

72

53.0

59.5

61

52.0

60.5

58

60

54

65

64

64

54 

71 

70

61 

65.5 

64

52

54

Nafeld

4

0

0

3

6

4

2.5

1.0

3.0

3.5

4

6

3

5

0

0

3.0

5.5

0

2.5

3.5

1

1

0

1

0

2

1 

1 

0

1 

0.5 

1

0

4

Kfeld

5

4

2.5

2

0

0

3.5

5.0

5.5

6.5

5

5

8

6

3

1

8.0

3

4

4.0

5.5

6

3

3

2

2

1

3 

1 

1

2 

2.5

2

1.5

7

Total 

feld

9

4

2.5

5

6

4

6.0

6.0

8.5

10.0

9

11

11

11

3

1

11.0

9

4

6.5

9.0

7

4

3

3

2

3

4 

3

2

3.5 

3 

3

1.5

11

Total 

Nth

10

17

9.5

3

6

8

4.0

10.5

5.5

7.0

5

3

7

9

5

3

8.0

6

8

4.0

5.5

8

6

7

5

5

4

6

5 

5

8 

10

4

7

14

Qtz 

cmt

0

0

1

3

4

6

7.0

3.0

5.0

2.0

5

4

2

0

3

9

4

5

3

1.5

1.0

1

0

0

0

13

5

1 

12

3

0 

0 

0

0.5

1

Total 

calcite

6

37

4.5

0

0

6

0.0

23.0

0.0

8.5

0

0

0

5

0

2

6

8

6

1.0

6.0

15

1

3.5

0.5

6

6

6 

0 

0

2 

1 

1

0

1

Total 

dolo

1

1

0

37

0

0

0.0

0.5

0.0

0.0

0

4

0

2

1

0

9

0.5

0

16.0

1.0

0

1

0

1.5

2

0

1

3 

0

1 

0.5 

0

31

16

Total 

carb

7

38

4.5

37

0

6

0.0

23.5

0.0

8.5

0

4

0

7

1

2

15

8.5

6

17.0

7.0

15

2

3.5

2

8

6

7 

3 

1

3 

1.5 

1

31

17
carbonate cements. There is no evidence that anhydrite was 

affected by dissolution processes or that it was a pervasive cement.

Pyrobitumen is present locally as an opaque material in 

porous reservoir sandstones. In oil zones, bitumen occupies moldic and secondary intergranular pores and coats authigenic mineral cements. Authigenic minerals in oil-saturated sandstones 

generally are restricted to minor quartz overgrowths and relic carbonate cement. Fluorescent hydrocarbon inclusions were not 

observed in any diagenetic phase in the Aux Vases.

Dissolution

On the basis of visual estimates, approximately 95 percent 

of the porosity in the Aux Vases Sandstone is intergranular and 

less than 5 percent of the porosity is intragranular. Thus, in most 

porous sandstones, intergranular porosity dominates and intragranular porosity is minor. Although some workers have interpreted Chesterian sandstones as dominantly primary porosity 

reservoirs (Pryor and others, 1991), our studies indicate that most 

porosity in the Aux Vases sandstone interval is of secondary origin, resulting from the removal of carbonate cement and, to a 

lesser extent, framework grains, principally feldspar (fig. 8). In 

thin section, evidence of carbonate-cement dissolution includes 

embayed quartz overgrowths, well-developed carbonate cement 

adjacent to extensively leached areas, dissolution voids in tightly 

cemented sandstones, and relic carbonate cement in otherwise 

porous sandstones. Petrographic observations indicate that one or 

more generations of carbonate cement are present in various 

amounts in some reservoir sandstones, whereas other sandstones 

are still tightly cemented by carbonate.

Intragranular porosity resulting from partial dissolution of 

detrital grains is present throughout the Aux Vases Sandstone in

+6

+4

+2

-2

-4

Mississippian 

carbonate

O °x

o

n 

V

D

n

n

n

n

EXPLANATION

D Syntaxial calcite 

O Poikilotopic calcite 

V Dolomicrospar 

X Planar ankerite

10 -8 -6 -4

518O, PER MIL PDB

-2

Figure 7. Stable-isotope compositions of diagenetic carbonate 

cements in reservoir sandstones. Black square represents approximate composition of carbonate cement in equilibrium with Mississippian seawater(from Meyers and Lohmann, 1985). PDB, Peedee 

belemnite standard.

variable but small amounts. Overall, grain-dissolution porosity 

did not significantly enhance the total sandstone porosity. Leaching of framework grains affected mainly K-feldspar and resulted 

in the development of oversized and moldic pores. In sandstones 

where authigenic clay fills these voids, porosity is moderate and 

permeability is low.

Table 3. Stable-isotope compositions of carbonate cements in Aux Vases sandstones. 

[Values in parentheses are repeat analyses. PDB, Peedee belemnite standard]

Sample ID State Depth

(m)

513C

(per mil, PDB)

818 0

(per mil, PDB)

Syntaxial calcite

C0078

C1064

C1064

C1279

C13977

C13977

Illinois

Illinois

Illinois

Illinois

Illinois

Illinois

1,946

1,837

1,834

2,665

2,624.9

2,624

-1. 27 (-1.29)

-3.40

-2.74

-0.07

0.62

1.02(0.60)

-6. 14 (-6. 12)

-7.29

-8.63

-5.87

-6.46

-6.64 (-7. 17)

Poikilotopic Fe-calcite

C3339

C3362

C13977

Illinois

Illinois

Illinois

3,138.5

3,042

2,623

-0.82

0.49 (0.49)

0.10(0.10)

-8.91

-8.00 (-8.00)

-8.88 (-8.88)

Dolomicrospar

C1742

IN 133

Illinois

Indiana

2,921

2,825

1.54

0.52 (0.52)

-3.63

-6.51 (-6.59)

Planar ankerite

C4399 Illinois 2,891 0.04 (0.04) -7.58 (-7.58)

10 Diagenesis and Reservoir Quality of the Upper Mississippian Aux Vases Sandstone, Illinois Basin
1 I ""I

10 20 30 40 

TOTAL CEMENT, IN PERCENT

Figure 8. Relationship between thin-section porosity and total cement. The curve is a third-degree polynomial fit to the data.

Time-Temperature History

Generalized burial- and thermal-history curves of the Aux 

Vases Sandstone sequence (fig. 9) were generated using the 2- 

dimensional, finite-difference program, Basin2. The burial-thermal model takes into account differences in deposition and erosion, variations in porosity and permeability, and the thermal 

effects of fluid flow during the basin's history (Bethke and others, 

1993). Thermal data incorporated in the model constrained the 

amount of eroded section in southern Illinois to approximately 

1 km. Coupled numerical and heat-flow calculations were used to 

model paleotemperatures in the Aux Vases Sandstone because 

they have been shown to reconcile observed biomarker and vitrinite-reflectance maturities, and measured fluid-inclusion 

homogenization temperatures in the basin fill (Rowan and others, 

in press). The modeled temperatures based on these calculations 

(see temperature profile in fig. 9) reflect the strong influence that 

heat related to late Paleozoic hydrothermal and igneous activity 

had on the basin's thermal regime during late-stage diagenesis. 

Hypothetical temperatures calculated for the conductive heatflow model (see fig. 9) are significantly lower than the observed 

thermal values determined from the rock record (i.e., biomarker 

and vitrinite maturities, fluid-inclusion temperatures). This suggests that conductive heat flow from the basement played a relatively minor role in the overall thermal history of the basin.

Diagenetic Evolution of Reservoir Sandstones

In this section, we place the diagenetic processes that may 

have influenced the reservoir quality of the Aux Vases Sandstone 

in a time-temperature framework. We then use this framework to 

constrain our hypothesis that porosity evolution in individual

sandstones was controlled by the combined effects of mechanical 

compaction and mineral cementation.

The evolution of the Aux Vases diagenetic system is shown 

relative to its time-temperature history in figure 9. The temperatures that bracket individual diagenetic events were taken from 

Surdam and others (1989) and correlated with the temperature 

profile based on igneous and hydrothermal heat flow. The absolute age of individual diagenetic events is difficult to establish 

because Mississippian sandstones generally lack minerals suitable for isotopic dating; however, the upper limit can be constrained to Early Permian based on the age of peak hydrocarbon 

generation and migration in the basin (duff and Byrnes, 1991).

Sandstones in the Aux Vases were affected by multiple episodes of carbonate-mineral precipitation and dissolution, quartz 

cementation, and clay-mineral authigenesis. On the basis of petrographic analysis, precipitation of nonferroan and ferroan carbonate began soon after deposition and continued until 

hydrocarbons were emplaced in the Late Pennsylvanian and 

Early Permian. The temperature regime during carbonate precipitation varied widely (~30°-120°C) and was governed by the 

combined effects of burial heat flow, and coupled igneous and 

hydrothermal heat sources (Pitman and others, 1998). During 

early burial diagenesis, carbonate formed at low temperatures 

(~30°-80°C) in a meteoric environment, but later, during deeper 

burial, it precipitated at elevated temperatures (~80°-120°C) 

under conditions of high Pco2 when the pore-water alkalinity 

might have been buffered by organic acids. Burial-history reconstruction suggests that carbonate precipitation did not occur after 

Early Permian time (see fig. 9).

The carbonate for calcite and dolomite cements was derived 

mostly from the dissolution of detrital marine carbonate rock, as 

indicated by the relatively uniform carbon-isotope compositions 

of early- and late-diagenetic carbonate cements (average 0.3%c; 

table 3). The narrow range of carbon values (~+l to +3%o) further 

suggests recrystallization without CO2 exchange. The later carbonate cements (ferroan calcite and ankerite) formed in a rockbuffered system by remobilization of preexisting cements with 

little change in 813C.

Fractionation equations for calcite and dolomite (Friedman 

and O'Neil, 1977; Fritz and Smith, 1970) suggest that carbonate 

in sandstones precipitated in a burial regime that evolved from 

freshwater to saline conditions over a relatively short period of 

time. Paleosols, red beds, and root traces near the top of the Aux 

Vases (Leetaru, 1997) indicate a period of subaerial exposure during which meteoric water infiltrated the sandstone, precipitating 

the freshwater component of carbonate cement. Petrographic evidence of shallow burial at the time of syntaxial calcite cementation suggests that water of negative oxygen isotope composition 

was involved in the precipitation of early calcite cement. Assuming early calcite cementation was related to invasion of meteoric 

water at the end of Aux Vases deposition, burial temperatures 

probably would have been on the order of 30°C, which predicts 

pore-water oxygen-isotope compositions between -3 to -6%o 

SMOW (standard mean ocean water) (fig. 10).

Dolomicrospar, which formed in a shallow burial regime 

at about the same time as early calcite, is interpreted to have 

precipitated from waters with 818O values (-4 to -7%0 SMOW) 

comparable to those that precipitated calcite (fig. 10). The 

development of poikilotopic ferroan carbonate postdated the

Diagenetic Evolution of Reservoir Sandstones 11
400

o 10 —

— 120

Temperature 

(basement-controlled heat)

Temperature (coupled igneous 

and hydrothermal heat)

— 40

Figure 9. Burial- and thermal-history curves for the Aux Vases Sandstone interval showing relative timing of major diagenetic events in 

southern and central Illinois. Age of hydrocarbon generation and petrographic features constrain timing of diagenesis. Paleotemperature profiles (igneous plus hydrothermal heat flow and burial-only heat flow) were determined using the coupled fluid-flow and burial heat-flow model, 

Basin2. Note that thermal curves are depicted relative to geologic age, not burial depth. Temperatures bracketing diagenetic events modified 

from Surdam and others (1989).

formation of early syntaxial calcite but predated late-stage 

hydrocarbon emplacement, suggesting precipitation in a burial 

environment at temperatures as high as 90°C. Pore-water compositions at a temperature of 90°C should be slightly enriched 

in 818O, from +4 to +3%c SMOW (fig. 10). Late-diagenetic planar ankerite formed when the sandstone interval was close to 

its maximum burial depth (~2 km) and at about the same time 

that hydrocarbons entered the unit. Assuming crystallization 

temperatures approximating 120°C, ankerite formed from 

reducing, 818O-enriched waters with compositions on the order 

of +4%o SMOW (fig. 10). Saddle ankerite, also a late-diagenetic phase, may have precipitated from fluids with even 

heavier values if temperatures as high as 140°C were involved 

in cementation.

On the basis of petrographic observations, most quartz 

cement in the Aux Vases Sandstone precipitated at shallow burial 

depths early in the burial history before other mineral cements. In 

contrast to early quartz, which is the dominant cement in younger 

Mississippian sandstones, secondary quartz, is a minor diagenetic phase in the Aux Vases compared to early- and late-stage

carbonate cements. According to Surdam and others (1989), early 

quartz overgrowths generally form at fairly low temperatures, 

which, in the Aux Vases, might have ranged from about 40° to 

65°C (see fig. 9). Silica derived from external and internal 

sources likely was involved in early quartz cementation. As in the 

early carbonate cement, much of the silica might have originated 

from saturated meteoric ground waters circulating through the 

sandstone interval soon after deposition. Upon deeper burial, 

pressure solution along stylolites and framework-grain contacts 

might have added additional silica to pore fluids, which subsequently moved by diffusion to nearby sites of precipitation. Incipient stylolitization and grain-to-grain contact dissolution are 

observed in many sandstones in the Aux Vases and are features 

commonly associated with thin, illitic clay coats. According to 

Heald (1955) and Weyl (1959), even though pressure solution at 

grain contacts can occur without clay, clay coatings tend to accelerate the grain-contact dissolution process, which results in the 

release of silica into solution for uptake by quartz cement. The 

presence of interstratified illite and illite-smectite in small yet 

consistent amounts suggests that the illite-smectite

12 Diagenesis and Reservoir Quality of the Upper Mississippian Aux Vases Sandstone, Illinois Basin
160

120

-505 

618O, WATER, PER MIL, SMOW

10

180

20-10 -505 

618O, WATER, PER MIL, SMOW

Figure 10. Equilibrium relationship between 8180 of water, 8180 

of early- and late-diagenetic calcite and dolomite, and crystallization temperature. Curves represent locus of temperature and 

possible 818 0 of water in equilibrium with a mineral having the indicated oxygen isotopic composition. Fractionation equations 

(calcite-water: 103 Incc = 2.78x106xT2 - 2.89 and dolomite-water: 

103 1 na = 2.78x106xT"2 + 0.1 Dare from Friedman and O'Neil (1977) 

and Fritz and Smith (1970), respectively. SMOW, standard mean 

ocean water.

transformation process may have contributed minor silica to secondary quartz. Dissolution of framework grains such as feldspar 

was not a likely source of silica because quartz precipitation generally predated the main stage of framework-grain leaching.

Clay mineral authigenesis in the Aux Vases Sandstone commenced before significant mechanical compaction and continued 

until late in the burial history (see fig. 9). The earliest formed

clays are precompactional illite rims, regarded to be syndepositional or early diagenetic precipitates, and chlorite grain-rimming 

cement, which formed during shallow burial and before appreciable compaction. Iron oxide grain coatings on some detrital grains 

and the dewatering of shales may have provided the iron that was 

incorporated into chlorite. Minor void-filling and grain-replacement illite precipitated following moderate amounts of mechanical compaction. The potassium and silica released into solution 

upon dissolution of K-feldspar and other framework grains may 

have been a source of the ions necessary for late-stage illitization.

A combination of processes is required to explain the dissolution of carbonate cement and framework grains, which together 

account for most of the porosity enhancement in the sandstones 

studied. In the shallow subsurface, through-flowing meteoric 

waters generally are acidic and undersaturated with respect to 

carbonate; thus, they may have been important in the dissolution 

process. However, in the deeper subsurface, the leaching capacity 

of meteoric waters is limited because, in addition to being undersaturated with respect to carbonate, the waters tend to be neutralized by reactions along the flow paths.

The Aux Vases Sandstone interval is among the principal 

reservoirs for hydrocarbons in the Illinois Basin. Thus, it follows 

that the release of organic acids and the production of CO2 during 

the early stages of hydrocarbon generation might have been 

involved in the dissolution of carbonate cement and framework 

grains. Studies by Carothers and Kharaka (1978) have shown that 

organic acids are present in formation waters throughout the 80°- 

200°C temperature range and dominate the fluid alkalinity at 

temperatures of ~80°-120°C. Typically, production of organic 

acids, which reaches a maximum in the 80°-100°C temperature 

interval, gradually declines as a result of decarboxylation reactions (Surdam and others, 1989). From 80°-100°C, organic acids 

buffer the alkalinity at relatively low Pco,, which increases carbonate solubility, but, at temperatures of about 100°C, carboxylic 

acids dominate the fluid alkalinity at a high PCOr which are conditions that promote dolomite or ferroan carbonate stability (Surdam and Crossey, 1987; Crossey and others, 1986). The organicinorganic model for clastic diagenesis proposed by Surdam and 

others (1989) predicts that porosity-producing reactions occur 

throughout the same temperature range in which organic acids 

are present in formation waters (~80°-120°C ), suggesting that 

early-formed carbonate cements and framework grains in the Aux 

Vases Sandstone interval were subject to dissolution. At temperatures approaching 100°C, iron and aluminum commonly are 

mobile due to complexing with organic acids, which results in an 

environment that is generally reducing. Such conditions favor the 

formation of iron-bearing mineral phases such as ferroan carbonate and chlorite. Some chlorite that formed at temperatures of 

100°C may have nucleated on early chlorite that developed from 

clay/iron-oxide rims. In summary, the organic-inorganic model 

of sandstone diagenesis predicts that dissolution of calcite 

cement and feldspar followed by the formation of chlorite and 

dolospar cement took place when the sediments experienced temperatures of ~80°-120°C. Petrographic and stable-isotope data 

are consistent with this interpretation.

Diagenetic Evolution of Reservoir Sandstones 13
1000^,

100-

Z 10-

1 —

0.1-

R2=.39

15 30

POROSITY, PERCENT

Figure 11. Relationship between percent core porosity and log permeability. R2 is the variance of the power curve fit to the data.

Hydrocarbon Generation and Migration

The entrapment of petroleum generated from source rocks 

in the Upper Devonian-Lower Mississippian New Albany Shale 

was the last major diagenetic event that affected reservoir sandstones in the Aux Vases interval (see fig. 9). Petrographic observations suggest that the influx of hydrocarbons into reservoir 

sandstones inhibited mineral diagenesis in oil zones. In central 

and southern Illinois, the migration of oil into Aux Vases reservoirs began in the Middle to Late Pennsylvanian and reached its 

peak when the sedimentary section was close to its maximum 

burial in the Early Permian. The majority of oil in the Aux Vases 

Sandstone resides in combination (structural and stratigraphic) 

traps that overlie (by about 600 m) the generative portion of the 

New Albany Shale (M.D. Lewan, 1997, unpub. data), and the 

largest fields (i.e., Salem, Clay City, Dale Consolidated) are predominantly associated with faulted anticlines and related structural features that formed or were reactivated during the late 

Paleozoic (Howard, 1991). The spatial relationship between 

major oil accumulations in the Aux Vases and mature New 

Albany source rock suggests that the bulk of the oil moved vertically, by buoyancy, along faults and fractures into lenticular 

sandstone units and then migrated laterally updip until it reached 

a sealing unit.

Reservoir Quality

The Aux Vases Sandstone in the Illinois Basin has wholecore porosities as high as 24 percent, but most samples vary 

from 4-20 percent (fig. 11). Generally, measured core porosity 

increases with the log of permeability (fig. 11) but shows no 

systematic trend with present burial depth (fig. 12), as do other

Chesterian sandstone units in the basin (Pitman and others, 

1998). A combination of depositional and diagenetic factors 

(i.e., grain-size differences, postdepositional alterations) therefore must have influenced porosity preservation in reservoir 

sandstones.

Whole-core porosities (4-24 percent, median 15 percent) 

fall within the range of porosities determined in thin section (1- 

27 percent, mean 10 percent) but thin-section porosities generally 

are lower because microporosity (associated with authigenic and 

detrital clay) is difficult to quantify. Visible porosity in thin section usually classifies as macroporosity; thus, the amount of 

microporosity in a sandstone sample can be considered to be the 

difference between core porosity and thin-section porosity. The 

Aux Vases Sandstone average core porosity differs from thin-section porosity by ~5 percent, hence microporosity in these rocks 

may be quantitatively significant.

Intergranular volume (IGV) is a means of evaluating the 

importance of compaction versus cementation in initial porosity 

loss in sandstones (Houseknecht, 1987). The IGV in moderately 

to well-sorted sandstones in the Aux Vases averages 25 percent 

(fig. 13) and falls within the range of values (24-26 percent) that 

represents the configuration of grains at closest packing. Several 

studies have shown that the initial porosities of well-sorted sandstones approximate 40 percent (Beard and Weyl, 1973; Atkins 

and McBride, 1992; Ehrenberg, 1995). Assuming that depositional porosity in the Aux Vases was on the order of 40 percent, 

approximately 38 percent of the original porosity was destroyed 

by mechanical compaction and an additional 36 percent was lost 

due to mineral cementation (fig. 13). Mechanical compaction and 

mineral cementation thus were of nearly equal importance in 

reducing the reservoir quality of sandstones during burial. A comparison of IGV versus thin-section porosity (fig. 14) confirms the 

interpretation that low-porosity, well-cemented sandstones are no 

more or less compacted than are the high-porosity, noncemented 

sandstones. Figure 14 also shows that high-porosity sandstones 

contain less cement, whereas low-porosity sandstones contain 

more cement.

o.o-

^ 200.0-

LU

^ 400.0-

LL 

CC

5 600.0- 

O

LUm

H 800.0-

Q. 

LU 

Q

Im 1000.0-

LU 

CC

1200.0 I I I

10 15 20 

POROSITY, PERCENT

25 30

Figure 12. Plot of core porosity versus present depth for 

sandstones. Note absence of porosity-depth trend.

14 Diagenesis and Reservoir Quality of the Upper Mississippian Aux Vases Sandstone, Illinois Basin
INTERGRANULAR CEMENT, PERCENT 

0 10 20 30

0.0-

o<

< O LU 

- .50 ? O DC

1.00 "• ~ 

0 .50 1.00 

FRACTION OF ORIGINAL POROSITY 

DESTROYED BY CEMENTATION

Figure 13. Relationship between compactional and cementational porosity loss in sandstones (after Houseknecht, 1987). 

Solid circle is the mean intergranular volume (IGV) value. Shortdashed line represents linear fit to the data.

200.0-

£

CD CD I

S

CD CD I

800.0-

Lu 1000.0-

DC

1200.0

X

X

X X

X X

xX

x x

xxx*

X X

0.1

TT ^' '' r^ ' ' '' MM I

1 10 100 

PERMEABILITY, IN MILLIDARCIES

1000

Figure 15. Plot of core permeability versus present burial depth for 

sandstones.

Core permeability in the Aux Vases Sandstone ranges from 

0.1 to about 250 mD and varies markedly at any given depth (fig. 

15). Variations in permeability likely reflect differences in grain 

size, total cement volume, and amount of primary versus secondary porosity. As would be expected, the coarser grained sandstones initially had higher permeability than the finer grained 

sandstones (Beard and Weyl, 1973). According to petrographic 

observations, some of the original permeability variations due to 

grain size differences are preserved despite varying degrees of 

diagenesis subsequent to deposition.

30

25 -

20 -

15 -

10 -

Percent

30/

I T I ^I ^\ f* 

10 15 20 25 30 35 40 

IGV, PERCENT

Figure 14. Plot showing percent IGV (intergranular volume) 

versus percent thin-section porosity in sandstones.

In well-cemented sandstones, authigenic cements and clays 

decrease permeability by occluding porosity in intergranular 

pores. On the basis of textural relations, total cement volume 

played a greater role in decreasing permeability than did one or 

more mineral phases alone. For example, sandstones with large 

amounts of carbonate cement typically have low permeability, 

but sandstones with little or no carbonate cement also can have 

low permeability because they have large amounts of secondary 

quartz. The extent to which clay content affected sandstone permeability is less clear because most samples that have permeability values were not analyzed for their clay content. We speculate 

that reservoir sandstones with a significant component of detrital 

and authigenic clay overall have low permeability.

In moderately to highly porous sandstones, higher porosity 

and permeability correspond to increasing secondary porosity, 

but primary and secondary porosity are difficult to distinguish; 

thus, some porosity preserved locally may be of primary origin. 

In most sandstones, primary pores have better connectivity than 

secondary pores and should provide a greater contribution to permeability, if present.

Regional variations in porosity and permeability in the Aux 

Vases Sandstone interval (fig. 16) are generally comparable to the 

reservoir-quality trends that have been reported for younger Mississippian sandstones in the basin (Pitman and others, 1998). 

Porosity and permeability increase in western Illinois, reaching a 

high of 20 percent and 200 mD, and high porosity and permeability occur locally in southwestern Indiana (fig. 16). Reservoir 

quality in the Aux Vases declines downdip to the south to ~4 percent porosity and 50 mD in southern Illinois near the Cottage 

Grove fault system (fig. 16). On the basis of petrographic observations, sandstone porosity near faults decreases in response to 

extensive mechanical compaction and diagenetic cementation, 

whereas, away from faults, porosity significantly increases due to 

the development of secondary porosity.

Reservoir Quality 15
87°

I*

INDIANA

A 0 10 20 30 40 KILOMETERS

I I I I I

Contour interval = 4%

Figure 16 (above and facing page). Isocontour maps depicting regional variations in reservoir quality of sandstones in the Aux Vases interval. Horizontal lines indicate area of Wabash Valley fault system. A, percent porosity; B, log permeability.

16 Diagenesis and Reservoir Quality of the Upper Mississippian Aux Vases Sandstone, Illinois Basin
87°

INDIANA

B 0 10 20 30 40 KILOMETERS 

I I I I I

Contour interval = 0.5 log mD unit

Reservoir Quality 17
Conclusions

Diagenetic alteration in the Aux Vases Sandstone began 

soon after burial in the Late Mississippian and continued until oil 

entered the sandstones in the Late Pennsylvanian and Early Permian when the rocks were near maximum burial. Major diagenetic events observed in sandstones in the Aux Vases include 

(1) precipitation of precompactional illite and grain-rimming 

chlorite, (2) quartz overgrowth, syntaxial calcite, and dolomicrospar cementation, (3) carbonate cement and frame work-grain 

dissolution, (4) illite and chlorite authigenesis, (5) ankerite precipitation, and (6) hydrocarbon emplacement. On the basis of petrographic and stable-isotope analysis, early diagenetic quartz, 

syntaxial calcite, and dolomicrospar formed at relatively low 

temperatures (~20°-90°C ) in a dominantly freshwater environment, whereas late-diagenetic ankerite precipitated at elevated 

temperatures (~120°C ) in a diagenetic system dominated by 

basin brines.

The reservoir quality of the Aux Vases Sandstone varies 

spatially, ranging from 4 to 24 percent, porosity and 0.1 to 250 

mD permeability. Porosity reduction in sandstones is due to the 

effects of mechanical compaction and quartz and carbonate 

cementation, whereas porosity enhancement reflects dissolution 

of framework grains and carbonate cements. Initially, secondary-porosity development was caused by acidic meteoric waters, 

but, later, organic acids and CO2 produced during the early 

stages of hydrocarbon generation promoted more extensive dissolution. The burial-thermal model presented in this study demonstrates that heat associated with igneous activity and heat 

transported by hydrothermal fluids elevated the basin's heat 

regime above the level based on normal basement heat flow 

alone. This added source of heat significantly increased the thermal maturity of organic-matter-rich source rocks, which subsequently resulted in greater rates of hydrocarbon expulsion and 

oil accumulation in the basin.

The presence of freshwater carbonate and silica cements 

provides an alternative method of delineating sequence boundaries in marine cratonic basins. Frequently, good evidence for 

subaerial exposure such as paleosols and root traces are eroded 

after formation and are not present in core. Results of this study 

suggest that diagenetic alterations may, at times, be the only evidence of sequence boundaries in the local rock record.

Acknowledgments

This study was supported by the U.S. Geological Survey's 

Energy Resources Program in cooperation with the State Geological Surveys of Illinois, Indiana, and Kentucky. The authors are 

indebted to Marty Goldhaber for generating burial-thermal 

curves using Basin2. We also are grateful to Joe Hatch and Jim 

Schmoker for reviewing the manuscript and providing many 

helpful suggestions.

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18 Diagenesis and Reservoir Quality of the Upper Mississippian Aux Vases Sandstone, Illinois Basin
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2001-2025.

Published in the Central Region, Denver, Colorado 

Manuscript approved for publication January 14,1999 

Edited by Richard W. Scott, Jr. 

Graphics prepared by Norma J. Maes, David G. Walters, and

Carol A. Quesenberry. Use made of author-drafted material 

Photocomposition by Carol A. Quesenberry

References Cited 19

* U.S. GOVERNMENT PRINTING OFFICE: 1999 — 773-051 / 20161 Region No. 8

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indexes: primary metals, steel, copper, primary and secondary 

aluminum, and aluminum mill products.

Mineral Industry Surveys (MIS's) are free periodic statistical and economic reports designed to provide timely statistical 

data on production, distribution, stocks, and consumption of 

significant mineral commodities. The surveys are issued 

monthly, quarterly, annually, or at other regular intervals, 

depending on the need for current data. The MIS's are published by commodity as well as by State. A series of international MIS's is also available.

Published on an annual basis, Mineral Commodity Summaries is the earliest Government publication to furnish estimates covering nonfuel mineral industry data. Data sheets 

contain information on the domestic industry structure, Government programs, tariffs, and 5-year salient statistics for 

more than 90 individual minerals and materials.

The Minerals Yearbook discusses the performance of the 

worldwide minerals and materials industry during a calendar 

year, and it provides background information to assist in interpreting that performance. The Minerals Yearbook consists of 

three volumes. Volume I, Metals and Minerals, contains chapters about virtually all metallic and industrial mineral commodities important to the U.S. economy. Volume II, Area 

Reports: Domestic, contains a chapter on the minerals industry of each of the 50 States and Puerto Rico and the Administered Islands. Volume III, Area Reports: International, is 

published as four separate reports. These reports collectively 

contain the latest available mineral data on more than 190 foreign countries and discuss the importance of minerals to the 

economies of these nations and the United States.

Permanent Catalogs

"Publications of the U.S. Geological Survey, 1879-1961" 

and "Publications of the U.S. Geological Survey, 1962- 

1970" are available in paperback book form and as a set of 

microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" is

available in paperback book form (two volumes, publications 

listing and index) and as a set of microfiche.

Annual supplements for 1982, 1983, 1984, 1985, 1986, and 

subsequent years are available in paperback book form.