U.S. Department of the Interior 

U.S. Geological Survey

Reservoir Quality and Diagenetic Evolution of 

Upper Mississippian Rocks in the Illinois Basin: 

Influence of a Regional Hydrothermal 

Fluid-Flow Event During Late Diagenesis

U.S. Geological Survey Professional Paper 1597
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Reservoir Quality and Diagenetic Evolution of 

Upper Mississippian Rocks in the Illinois Basin: 

Influence of a Regional Hydrothermal 

Fluid-Flow Event During Late Diagenesis

By Janet K. Pitman, Mitchell Henry, and Beverly Seyler

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1597

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1998
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Thomas J. Casadevall, Acting Director

For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

Pitman, Janet K.

Reservoir quality and diagenetic evolution of Upper Mississippian rocks in the 

Illinois Basin : Influence of a regional hydrothermal fluid-flow event during late 

diagenesis / by Janet K. Pitman. Mitchell Henry, and Beverly Seyler.

p. cm.—(U.S. Geological Survey professional paper : 1597) 

Includes bibliographical references. 

Supt.ofdocs.no. : I 19.16 : 1597

1. Sandstone—Illinois Basin. 2. Pertroleum—Geology—Illinois Basin. 

3. Geology, Stratigraphic—Mississippian. 4. Diagenesis—Illinois Basin. 

I. Henry, Mitchell E. II. Seyler. Beverly. III. Title. IV. Series 

QE471.15.S25P57 1999

553.2'82—dc21 98-6426

CIP
CONTENTS

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

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

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

Methods ............................................................................................................................... 3

Reservoir Characteristics ..................................................................................................... 6

Porosity ........................................................................................................................ 6

Permeability ................................................................................................................. 7

Porosity/Permeability Relationships and Distribution Patterns ................................... 8

Controls on Reservoir Quality ............................................................................................. 8

General Statement........................................................................................................ 8

Detrital Mineralogy ..................................................................................................... 8

Mineral Diagenesis ...................................................................................................... 10

Compaction Features............................................................................................ 10

Secondary Quartz ................................................................................................. 10

Carbonate Cements............................................................................................... 10

Authigenic Clay Minerals..................................................................................... 16

Other Phases......................................................................................................... 16

Secondary Porosity............................................................................................... 16

Evolution of Reservoir Quality ............................................................................................ 18

Quartz Cementation ..................................................................................................... 18

Carbonate Diagenesis .................................................................................................. 18

Clay Mineral Precipitation ........................................................................................... 20

Secondary Porosity Development ............................................................................... 20

Generation and Migration of Petroleum ...................................................................... 21

Thermal and Hydrologic Controls on Diagenesis ............................................................... 21

Conclusions ......................................................................................................................... 22

Acknowledgments ............................................................................................................... 23

References Cited .................................................................................................................. 23

FIGURES

1. Location map of the Illinois Basin............................................................................................................................. 2

2. Generalized lithologic section of Mississippian strata in the southern Illinois Basin ............................................... 3

3. Map showing location of wells sampled for petrographic and geochemical analysis............................................... 4

4-8. Graphs of:

4. Relationship between core porosity and permeability for sandstones within the Bethel-Cypress interval........ 6

5. Core porosity versus depth for reservoir sandstones .......................................................................................... 6

6. Relationship between compactional and cementational porosity loss in reservoir sandstones .......................... 7

7. Intergranular volume versus percent thin- section porosity in sandstones within the Cypress ........................... 7

8. Permeability versus depth for reservoir sandstones............................................................................................ 8

9. Isocontour maps depicting regional variations in core porosity and permeability in sandstones within

the Bethel-Cypress interval........................................................................................................................................ 9

10. Map showing late-diagenetic quartz and ankerite cementation relative to faults in southern Illinois....................... 10

11. Chart showing paragenetic sequence of diagenetic events in reservoir sandstones................................................... 11

12. Ternary QFL plots for sandstones from the Bethel and Cypress............................................................................... 11

in
IV CONTENTS

13. Graph showing stable isotope compositions of diagenetic carbonate cements in reservoir sandstones.................... 16

14. Burial and thermal history curves for the Bethel Sandstone-Cypress Sandstone interval showing

relative timing of major diagenetic events in southern and central Illinois............................................................... 17

15. Graphs showing equilibrium relationships between 818O of water, 818O of calcite and ankerite, and

crystallization temperature......................................................................................................................................... 19

TABLES

1. Locations of wells sampled for petrographic and geochemical analysis................................................................... 5

2. Mineralogic composition of sandstones within the Bethel-Cypress interval............................................................ 12

3. Stable isotope compositions of carbonate cements in sandstones within the Bethel-Cypress interval..................... 15
Reservoir Quality and Diagenetic Evolution of 

Upper Mississippian Rocks in the Illinois Basin:

Influence of a Regional Hydrothermal 

Fluid-Flow Event During Late Diagenesis

By Janet K. Pitman, 1 Mitchell Henry, 1 and Beverly Seyler2

ABSTRACT

Reduction and enhancement of porosity due to diagenetic alterations are common in Upper Mississippian Bethel 

and Cypress Sandstones in the Illinois Basin. Reduced porosity was caused by mechanical compaction and cementation 

by carbonate and quartz overgrowths. Enhanced porosity is 

mostly of secondary origin due to cement and framework 

grain dissolution, with only minor primary porosity preserved 

by early grain-rimming clay. Secondary porosity was caused 

by acidic meteoric waters and deep basin brines carrying 

organic and inorganic acids released during maturation of the 

Upper Devonian and Lower Mississippian New Albany Shale 

in the Early Permian. Burial-history reconstruction suggests 

that 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). 

Major diagenetic events observed in sandstones include (1) 

precompactional illite and early chlorite precipitation, (2) 

early quartz, and calcite and dolomite cementation, (3) calcite-cement and framework-grain dissolution, (4) illite and 

kaolinite formation, (5) late quartz and ankerite precipitation, 

(6) fracture cementation, and (7) hydrocarbon emplacement. 

On the basis of stable-isotope data and thermal modeling, 

early diagenetic nonferroan carbonate and quartz cement precipitated in a shallow meteoric environment governed by 

burial heat flow. In contrast, late-diagenetic ferroan carbonate, anhydrite, and quartz cementation occurred in a regime 

influenced by advective (hydrothermal) fluid flow. In southern Illinois, combined igneous and hydrothermal heat transport during the late Paleozoic increased the rate of 

hydrocarbon expulsion, which resulted in the migration of 

large amounts of petroleum into reservoir sandstones.

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

-Illinois State Geological Survey, 615 East Peabody Dr., Champaign, 

Illinois, 61820.

INTRODUCTION

Sixty percent of the oil produced in the Illinois Basin 

comes from Upper Mississippian rocks, and most of this production is from sandstones. Rocks of Mississippian age are 

limited to the southern half of Illinois, southwestern Indiana, 

and western Kentucky and reach their maximum thickness, 

about 430 m, in southern Illinois (fig. 1). This study examines the controls on reservoir quality and delineates general 

reservoir-quality trends, as governed by porosity and permeability in the Mississippian Bethel and Cypress Sandstones, 

two of the principal producing units in the Illinois Basin. 

Identifying the major influences on porosity and permeability distribution provides insight into some of the processes 

that must be accounted for in developing reliable porosity 

prediction models. Overall, sandstones within the BethelCypress interval have good porosity and permeability. In the 

main reservoir units, porosity varies from 10 to 25 percent, 

with permeabilities ranging over about four orders of magnitude (< 1-2,500 mD) at any given depth.

In the Illinois Basin, the reservoir quality in shallowly 

buried sandstones (<500 m) generally is high, but, in moderately buried sandstones (>500 m), reservoir quality can be 

quite variable because of extensive alteration. Reduction in 

porosity in sandstones is due primarily to mechanical compaction and the formation of quartz and carbonate cement; 

porosity enhancement is a result of carbonate- and framework-grain dissolution. Permeability variations at any given 

depth are a function of porosity, cement volume, and fracture 

development. In this study, petrographic and geochemical 

analysis is used to decipher the diagenetic controls on reservoir quality and the timing of diagenesis relative to migration 

and entrapment of hydrocarbons. An additional aspect of the 

study involves linking inorganic and organic diagenesis to 

late Paleozoic tectonic and hydrothermal fluid-flow events in 

the region. Some of these parameters are difficult to evaluate, 

but an understanding of their potential should enhance reservoir-quality prediction in undrilled areas of the basin.
RESERVOIR QUALITY AND DIAGENETIC EVOLUTION OF UPPER MISSISSIPPIAN ROCKS, ILLINOIS BASIN

GEOLOGIC AND 

DEPOSITIONAL HISTORY

In the Illinois Basin, the (Upper Mississippian) Bethel 

and Cypress Sandstone interval is part of the lower Chesterian 

Series in the upper part of the Kaskaskia sequence (Swann, 

1963). Stratigraphic relations for the Upper Mississippian are 

shown in figure 2. The Chesterian Series consists of alternating sandstone, shale, and limestone beds that record cyclic 

sedimentation in response to sea-level changes during the 

Mississippian (Pryor and Sable, 1974). Chesterian-age rocks 

together with sandstone and fossiliferous carbonate units in 

the upper part of the Valmeyeran Series are the principal reservoirs for most of the oil produced in the basin.

The geology and sedimentology of the Chesterian 

Series are given in Treworgy (1991) and Willman and others 

(1975). The Bethel and Cypress Sandstones represent facies 

of a large southeasterly prograding fluvial-deltaic system 

that deposited sediment shed from the eastern Canadian 

Shield. The Bethel Sandstone thickens downdip to more than 

30 m in southeastern Illinois and 75 m in western Kentucky 

(Sullivan, 1972). The facies is composed of tidal-dominated, 

deltaic to shallow-marine deposits that commonly display a 

northeast-south west trend, as do other Chesterian-age units 

in the basin (Treworgy, 1991). The Cypress Sandstone is 

more than 30 m thick, locally reaching as much as 60 m in 

the south-central part of the Illinois Basin (Willman and others, 1975). Depositional facies in the Cypress include distributary-bar, point-bar, and marine-sheet-sandstone deposits 

that are discontinuous and aligned parallel to major structural 

features in the basin (Pryor and others, 1991). Throughout 

the area of deposition, the Cypress Sandstone is comformably overlain by the Beech Creek Limestone, a widespread 

reservoir seal for Cypress Sandstone traps. The Cypress has 

been eroded along the southeast margin of the basin.

The Illinois Basin experienced periodic subsidence and 

multiple episodes of uplift and erosion before and after deposition of the Chesterian Series in Late Mississippian time. In 

southern Illinois, burial-history modeling indicates that maximum burial of Upper Mississippian strata was attained when 

the rocks reached depths of approximately 2 km during the 

Late Pennsylvanian to Early Permian. Somewhat lower maximum burial depths were reached near the Chesterian Series 

subcrop in central Illinois. During the Late Pennsylvanian to 

Early Permian, local igneous activity and compressional 

stresses associated with the Ouachita orogeny caused extensive folding and faulting, which resulted in the development 

of many of the structural traps in the basin. Erosion accompanied late Paleozoic uplift and was particularly widespread 

along the southeastern flank of the basin. Near-surface coal 

beds in Pennsylvanian strata across the basin display anomalously high thermal maturities (Damburger, 1971; Cluff and 

Brynes, 1991), which suggest that at one time they were 

more deeply buried. On the basis of thermal modeling and

0 40 80 KILOMETERS 

I____I

Figure 1. A, Location map showing the Illinois Basin. Black 

areas are ore districts. B, East-west cross section depicting the 

Upper Mississippian Chesterian Series and Upper Devonian and 

Lower Mississippian New Albany Shale; the subcrop of the 

Chesterian Series is shown as a dashed line on inset map (modified from Swann, 1967).

biomarker studies, the amount of section removed during 

erosion was approximately 1 km in southern Illinois, 

decreasing to several hundreds of meters in central Illinois 

(Rowan and others, in press). Although somewhat lower than 

the amount reported in others studies (1,400 m, Damburger, 

1971; 2,000 m, Cluff and Brynes, 1991), the estimated erosional loss is within the range of realistic values.
METHODS

FORMATION-RANK UNITS

Figure 2. Generalized lithologic section of Mississippian strata in the southern Illinois Basin (modified from Swann, 1963). Black dots indicate units analyzed in this study.

METHODS

Approximately 150 samples of core from 82 wells penetrating the Bethel-Cypress interval were collected from Illinois and Indiana for petrographic analysis (fig. 3; table 1). 

API location data, shown on table 1, were taken from the Illinois State Geological Survey's well inventory system. In 

Illinois, wells chosen for study in the Bethel Sandstone span 

a depth interval of 365 to 853 m; wells in the Cypress Sandstone that were sampled ranged from 305 to 884 m. Some 

sandstone samples in Indiana were at depths of only a few 

meters to several tens of meters below the surface.

To facilitate analysis, thin sections of individual 

sandstones were impregnated with blue epoxy for porosity identification and stained for mineral identification. 

Staining with Alizarin red-S and potassium ferricyanide 

permitted Fe-free and Fe-bearing carbonate minerals to be 

readily distinguished; sodium cobaltinitrate stain aided in 

the identification of K-feldspar. Routine point counts (300 

grains per sample) were performed on each sample to 

quantify rock composition and thin-section porosity. Mineralogical classification of the data is based on Folk 

(1974). In this study, the quartz category was modified to 

consist of only monocrystalline grains; poly crystalline

quartz grains were included as rock fragments. Grain size, 

based on the apparent long axis of framework grains, was 

determined for each sample.

In addition to petrographic analysis, representative 

sandstone samples were analyzed isotopically by Global 

Geochemistry (Canoga Park, Calif.) and Mountain Mass 

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). To prevent contamination from organic matter during acid digestion, 

kerogen was removed from the samples prior to analysis. 

Upon reaction with phosphoric acid, CO2 gas evolved in the 

first hour was attributed to calcite and CO2 gas evolved after 

several hours was assigned to dolomite. All isotope results 

are reported as the per mill (%o) difference relative to the 

PDB standard using the delta (8) notation. Data reproducibility is precise to ±0.2%o.

Conventional porosity and permeability data, provided 

by the Illinois and Indiana State Geological Surveys, were 

compiled in a reservoir-property database. Porosity analysis 

of individual core samples was performed by helium 

injection in a porosimeter using the Boyle's gas law method 

under ambient, or near-ambient, conditions. Permeability
RESERVOIR QUALITY AND DIAGENETIC EVOLUTION OF UPPER MISSISSIPPIAN ROCKS, ILLINOIS BASIN

38° —

37

0 20 40 KILOMETERS

I i I_____|

Figure 3. Map showing location of wells sampled for petrographic and geochemical analysis. Diamond symbols 

denote Cypress Sandstone samples, circles correspond to Bethel Sandstone samples.

measurements were made using nitrogen gas or air as the 

fluid. Wells incorporated in the database were selected to 

cover as wide a geographic area as possible. The porosity 

and permeability for each well were averaged as a whole 

rather than as individual facies because of complex lithogenetic relationships that are commonly difficult to distinguish on well logs and in drill cores. To minimize the 

potentially large effects of anomalous porosity and permeability on averages, median rather than mean porosity and 

permeability values were determined. The contribution of 

natural fractures to effective reservoir permeability was 

beyond the scope of the study.

Combined burial- and temperature-history curves 

(modified from Rowan and others, in press) were constructed 

for the Bethel-Cypress stratigraphic interval in the southern

Illinois Basin using the two-dimensional, finite-difference 

program, Basin2. The model takes into account differences 

in deposition and erosion, porosity and permeability variations, and the thermal effects of fluid flow during the basin's 

history. Thermal and petrographic data constrained the 

amount of uplift and erosion in the model to ~1 km. Coupled 

numerical fluid- and heat-flow calculations were used by 

Basin2 to produce a paleotemperature profile that reconciled 

observed biomarker and vitrinite reflectance maturities and 

measured fluid-inclusion homogenization temperatures. A 

temperature profile based on burial heat flow alone was 

constructed for comparison. The accuracy of the modeling 

results is subject to uncertainties involved in age determinations of stratigraphic intervals, amounts of erosion, and 

variations in heat flow.
METHODS

Table 1. Location of wells sampled for petrographic and geochemical analysis. 

[Latitude in decimal degrees north; longitude in decimal degrees west]

Core ID API no. Latitude Longitude

Bethel Sandstone

C0071

C0285

C0318

C0345

C0361

C0364

C0414

C0467

C0468

COS 18

C0525

C0573

C0595

C0597

C0607

C0651

C0735

C1018

C1113

C1125

C1169

C1270

C1511

C2069

C2085

C2114

C2230

C2341

IN303

IN643

1207700603

1214502037

1216501136

1208102322

1204700033

1202702511

1202900030

1219304604

1219306807

1202700823

1200500212

1202500986

1204900704

1207900017

1205500336

1218900150

1212101296

1212104149

1218900176

1212590017

1217300051

1208100499

1204900140

1212101475

1212104968

1202500549

1200500567

1208100199

No Data

No Data

37.928828

38.158661

37.878645

38.320668

38.481371

38.518618

39.595217

38.160159

37.929945

38.658891

38.978035

38.772606

38.958267

38.878071

37.867485

38.252045

38.705799

38.561675

38.470347

38.772307

39.326445

38.405546

39.065321

38.778051

38.779868

38.710497

38.827081

38.181586

38.185565

38.345879

89.283433

89.285138

88.578628

89.042866

88.014630

89.424022

88.388347

87.986364

88.285574

89.278940

89.309814

88.753333

88.624839

88.352995

88.975637

89.403828

89.055451

89.020984

89.210327

88.073440

88.906719

88.831430

88.765539

88.762495

88.755532

88.584495

89.474212

88.992598

86.508559

86.704695

Cypress Sandstone

C00071

C00285

C00374

C00385

C00467

C00491

COOS 20

C00592

C00597

C00642

C00671

C00693

C00735

C00962

C00996

CO 1049

C01096

1207700603

1214502037

1219901289

1215900044

1219304604

1205500563

1204700028

1219300424

1207900017

1206502267

1205500350

1205500244

1212101296

1216502428

1219105483

1213900006

1202700576

37.928828

38.158661

37.820457

38.725340

38.160159

38.029849

38.481310

38.153027

38.878071

38.006169

38.085232

37.922344

38.705799

37.661006

38.588268

39.581258

38.65244

89.283433

89.285138

88.825271

88.185230

87.986364

88.946832

87.999950

88.043539

88.352995

88.605524

88.760615

88.939580

89.055451

88.539391

88.322813

88.534095

89.198733

Core ID

C01103

C01113

C01135

C01160

C01183

CO 1309

C01353

C01439

CO 1445

C01511

C01546

C01558

CO 1698

CO 1704

C01717

C01735

CO 1740

C01760

CO 1794

CO 1806

C01813

CO 1834

C01931

CO 1968

C01975

C01981

C02114

C02149

C02174

C02230

C02357

C02621

C05279

C05834

C05874

C06058

C06079

C14018

C14019

C 14023

IN071

IN097

IN133

IN643

IN647

IN734B1

IN745

API no.

1218900387

1218900176

1207900014

1208100552

1219306873

1218500599

1204700182

1208100764

1219101791

1204900140

1202500525

1202901169

1202900325

1202900291

1202901131

1202901134

1202901301

1202900795

1202901319

1202900869

1203500074

1202901353

1202901019

1202901330

1202900788

1207900301

1202500549

1219101213

1204900813

1200500567

1212101986

1205901062

1218502475

1202501165

1218900972

1219302857

1203307971

1210127623

1210128363

1204724183

No Data

No Data

No Data

No Data

No Data

No Data

No Data

Latitude

38.277920

38.470347

39.164720

38.282282

37.926202

38.496459

38.398650

38.263679

38.589686

39.065321

38.789958

39.469611

39.460276

39.420342

39.463975

39.474788

39.433612

39.427991

39.437038

39.414467

39.337077

39.442305

39.502063

39.433810

39.424176

38.881033

38.710497

38.435874

38.946928

38.827081

38.765089

37.704520

38.260543

38.627951

38.233049

38.053129

38.867555

38.753134

38.759770

38.562076

38.086299

38.681153

38.238589

38.345879

38.330681

38.511973

39.062193

Longitude

89.247419

89.210327

88.233602

88.908279

88.278548

87.780426

87.983098

89.085271

88.279198

88.765539

88.419539

88.379941

88.396417

88.376784

88.403436

88.400756

88.398575

88.393260

88.386898

88.395643

88.433253

88.377306

88.398420

88.380040

88.386237

88.230995

88.584495

88.278806

88.480065

89.474212

88.902172

88.173993

87.944239

88.610797

89.690740

88.174550

87.788940

87.783338

87.786930

88.025117

87.381803

86.687115

87.932857

86.704695

87.65548

87.418606

86.85704
RESERVOIR QUALITY AND DIAGENETIC EVOLUTION OF UPPER MISSISSIPPIAN ROCKS, ILLINOIS BASIN

RESERVOIR CHARACTERISTICS

POROSITY

Whole-core porosity in sandstones within the BethelCypress interval varies by as much as 25 percent, with the 

majority of samples ranging from -13 to 22 percent (fig. 4). 

Porosity/depth trends of sandstones within the BethelCypress interval (fig. 5) depict a systematic decrease in 

median core porosity with increasing depth of burial. The 

relationship between porosity and burial depth generally is 

assumed to be exponential (e.g., Baldwin and Butler, 1985); 

however, given the limited depth range of the available data, 

a simple linear trend is a good approximation. Regression 

analysis of the sandstone data shows that core porosity 

decreases approximately 15 percent between 400 and 1,000 

m. Porosity values that fall to the right of the trend line (at 

depths >~750 m in fig. 5) reflect various amounts of erosion 

in different areas of the basin or the effects of diagenesis and 

mechanical compaction. These variations in core porosity 

also could reflect differences in framework grain size, shape, 

and sorting related to depositional-energy conditions and 

sediment source.

Measured whole-core porosities for sandstones within 

the Bethel-Cypress interval (-5-25 percent) compare very 

closely with the observed porosities in thin section (<l-25 

percent). Generally, thin-section porosities tend to be lower 

because of the difficulty in quantifying microporosity. Most 

visible porosity in thin section is macroporosity, thus the difference between core and thin-section porosity provides an 

estimate of the amount of microporosity in a sample. In Mississippian sandstones, average core porosity differs from 

thin-section porosity by <5 percent, suggesting that 

microporosity (mostly associated with authigenic and detrital 

clay) is quantitatively insignificant. It is important to note 

that thin-section porosity in sandstones within the BethelCypress interval does not vary with differences in grain size, 

nor does it show a depth-related trend as does core porosity. 

The latter is probably due to the limited size of the data set.

Intergranular volume (IGV), which is the sum of intergranular porosity plus mineral cements (Housekneckt, 1987), 

is a means of evaluating the relative importance of compaction versus cementation in initial porosity loss in sandstones. 

The IGV's in all sandstones within the Bethel average 21 

percent; sandstones within the Cypress average 26 percent 

IGV (fig. 6). Most sandstones within the Bethel-Cypress 

interval are very fine to medium grained (-70-380 jim) and 

moderately to well sorted with <5 percent detrital clay. Studies show that initial porosities of well-sorted sandstones are 

approximately 40 percent (Beard and Weyl, 1973; Atkins 

and McBride, 1992; Ehrenberg, 1995). Assuming an estimated 40 percent depositional porosity in sandstones within 

the Bethel-Cypress interval, an average of 47 percent of the 

original porosity in the Bethel was reduced by compaction,

10000 -3

1000

100 -

10 -

1 -

0.1

EXPLANATION

O Bethel Sandstone 

X Cypress Sandstone

X O

X

O

x

10 15 20 

POROSITY, IN PERCENT

25 30

Figure 4. Relationship between core porosity and permeability 

for the sandstones within the Bethel-Cypress interval.

EXPLANATION

O Bethel Sandstone 

X Cypress Sandstone

£ 400 -

fc 600

800 -

1000

10 15 20 

POROSITY, IN PERCENT

Figure 5. Plot of core porosity versus depth for reservoir sandstones. Note that porosity decreases with increasing depth. R2 corresponds to correlation coefficient.

and an additional 29 percent was lost by quartz and carbonate 

cementation (fig. 6). In the sandstones within the Cypress, 34 

percent of the porosity was lost by compaction and 34 percent by cementation. An average of about 9 percent secondary porosity in the Bethel Sandstone and 13 percent in the 

Cypress Sandstone was generated by carbonate-cement and 

minor framework-grain dissolution during later burial. The
RESERVOIR CHARACTERISTICS

40

30 -

20 -3

10 -

INTERGRANULAR CEMENT, IN PERCENT 

10 20 30

I________I________I

40

50 -

EXPLANATION 

Bethel Sandstone 

X Cypress Sandstone

Mean value, Bethel Sandstone 

Mean value, Cypress Sandstone

II

>_ CO

ORIGINAL POROSITY DESTROYED BY CEMENTATION, IN PERCENT

Figure 6. Relationship between compactional and cementational 

porosity loss in reservoir sandstones (after Houseknecht, 1987).

30

25

ro CD

* 15 

£

CD

8

o o 0

CO

0 00

10 20 

IGV, IN PERCENT

30 40

Figure 7. Plot showing percent intergranular volume (IGV) versus percent thin-section porosity in sandstones within the Cypress.

processes controlling porosity reduction are consistent with 

the mineralogic and textural differences observed in thin 

section. Petrographic analysis indicates that, during early 

burial, mechanical compaction was the main porosity-reducing process; however, as burial increased, mechanical compaction and mineral cementation were of approximately 

equal importance in reducing reservoir quality. A comparison of IGV versus thin-section porosity in sandstones within

the Cypress (fig. 7) shows no apparent trend with increasing 

porosity—this confirms petrographic observation that, 

despite IGV's varying from 13 to 38 percent (average 26 percent), low-porosity (cemented) sandstones are no more or 

less compacted than are the high-porosity sandstones. 

According to Stone and others (1993) and Stone and Siever 

(1994), IGV stabilizes at 24-26 percent between depths of 

1.5 and 2.5 km in quartz-rich sandstones. The average IGV 

in the Bethel Sandstone (21 percent) is slightly lower than 

the range of values (24-26 percent) that represents the configuration that typically can be reached by rearrangement of 

rigid grains at closest packing. Reduction of IGV below this 

value suggests that minor intergranular pressure solution, as 

observed in some sandstones, allowed framework grains to 

fit into a tighter packing arrangement. Variations in reservoir 

quality in sandstones within the Bethel-Cypress interval thus 

resulted from a combination of physical and diagenetic modifications at shallow to moderate burial depths.

PERMEABILITY

The positive correlation between core porosity and permeability in sandstones within the Bethel-Cypress interval 

(see fig. 4) suggests that the primary control on permeability 

is porosity, although other variables also can affect permeability. Some scatter in the permeability data may be artificial, 

caused by measurements made on samples containing coring - 

induced hairline fractures. Unfortunately, these samples were 

not available for examination; thus, the presence of unnatural 

fractures could not be fully evaluated. It is also important to 

note that total porosity does not distinguish a sample with 

mostly primary porosity from one that contains dominantly 

secondary porosity. Petrographic observations show that primary pores have better connectivity than secondary pores, 

resulting in a greater contribution to permeability.

Unlike porosity, permeability in sandstones within the 

Bethel-Cypress interval is unrelated to depth and varies 

markedly at any given depth (fig. 8). Substantial variations in 

permeability with depth are related to a number of parameters 

besides porosity, including variations in grain size and sorting, total cement volume, and primary and secondary porosity. Assuming the same degree of sorting, coarser grained 

sandstones would be expected to have higher permeability at 

the time of deposition than would finer grained sandstones 

(Beard and Weyl, 1973). The Bethel and Cypress Sandstones 

have undergone substantial diagenesis since they were 

deposited, but some of the original variations in permeability 

due to small grain size differences have been preserved.

Total cement volume is an important control on permeability in sandstones within the Bethel-Cypress interval and 

generally is a greater predictor of permeability than one or 

more authigenic mineral phases alone. Petrographic
RESERVOIR QUALITY AND DIAGENETIC EVOLUTION OF UPPER MISSISSIPPIAN ROCKS, ILLINOIS BASIN

observations suggest that increased amounts of carbonate 

and silica cement decreased permeability by occluding 

porosity and narrowing pore throats, particularly in primary 

intergranular pores. On the other hand, sandstones that have 

high amounts of quartz cement commonly have low permeability, but sandstones with low amounts of secondary quartz 

also can have low permeability because they contain abundant carbonate cement. The relationship between permeability and clay matrix in sandstones within the Bethel-Cypress 

interval is not well defined. Overall, permeability decreases 

with increasing detrital matrix content; however, low permeability commonly occurs in sandstones that have small 

amounts of matrix and large amounts of authigenic cement.

POROSITY/PERMEABILITY

RELATIONSHIPS AND 

DISTRIBUTION PATTERNS

The relationship between median core porosity and 

median permeability for sandstones within the BethelCypress interval (fig. 4) defines a linear decrease in porosity 

with the log of permeability. The significant overlap between 

these parameters suggests that similar depositional and 

diagenetic processes affected both units. Reservoir quality 

variations in the Bethel and Cypress are difficult to evaluate 

on a regional scale because of the lack of uniformly distributed porosity and permeability data across the study area; 

however, differences in porosity and permeability overall are 

of the same order of magnitude and basin-wide patterns in 

reservoir quality are generally comparable (fig. 9). The best 

porosity and permeability in both the Bethel and the Cypress 

occurs in the western portion of the area where values exceed 

20 percent and 2 mD, respectively. The Cypress Sandstone 

also shows high porosity and permeability locally in the eastern part of the basin. Reservoir quality in both units decreases 

in sandstones downdip to <15 percent and <1 mD in the 

southern part of the basin. In downdip parts of the study area 

close to faults (see fig. 9), petrographic analysis indicates that 

porosity in the Cypress Sandstone is as low as a few percent 

because of extensive mechanical compaction and late diagenetic cementation. Away from faults, porosity increases to 

approximately 15 percent due to the development of secondary porosity. In both units, higher porosity and permeability 

correspond to increasing secondary porosity.

Fracture porosity, used here to denote open pore space 

within the plane of a fracture, is present, particularly along 

parts of some major fault systems. Although several core 

samples indicate that some faults have porosity, the extent to 

which faults and fractures are open and permeable requires 

analysis of well tests and field-performance data that is 

beyond the scope of this paper.

200

400

£ 600

800 -

1000

EXPLANATION

O Bethel Sandstone 

X Cypress Sandstone

X

x

x

O

O X

X

X

X

o.

o o

V V f~^ Sjx^x0 < X*

X X° O XX

0.1 1 10 100

PERMEABILITY, IN MILLIDARCIES

1000 10000

Figure 8. Plot of permeability versus depth for reservoir sandstones. Note that permeability is highly variable at any given depth.

CONTROLS ON RESERVOIR QUALITY 

GENERAL STATEMENT

Regional controls on reservoir quality in sedimentary 

basins include depositional environment, initial framework 

grain composition, grain size and sorting, subsidence rate, 

pressure, temperature, pore-fluid composition, and diagenesis. In the Illinois Basin, reservoir quality in sandstones 

within the Bethel-Cypress interval is influenced primarily 

by (1) framework grain composition and (2) diagenesis. On 

a basin-wide scale, detrital and authigenic mineral suites 

show only minor differences in type and abundance and are 

depth independent. In addition, except for some late diagenetic phases that are locally restricted to the southern part 

of the basin (fig. 10), most detrital and authigenic minerals 

do not display systematic or regional variations. The 

paragenetic sequence of major diagenetic events in the 

Bethel-Cypress interval, as indicated by textural features, is 

shown in figure 11.

DETRITAL MINERALOGY

Sandstones within the Bethel-Cypress interval are subarkoses and sublitharenites with a combined average composition of 83 percent quartz, 8 percent feldspar, and 9 percent 

rock fragments or quartzarenites with >90 percent quartz 

(fig. 12; table 2). Quartz grains in sandstones typically are 

monocrystalline with optically continuous overgrowths and, 

in some samples, appear rounded, indicating they are 

reworked, second-cycle sediments. The feldspar content of
CONTROLS ON RESERVOIR QUALITY

87° 87°

39° —

38°

37°

A

Percent porosity 

Contour interval 4%

0 20 40 KILOMETERS

39°

38°

37°

C

ILLINOIS INDIANA

Valley /"'^

. fault / f * r 

;system| » 4,*% ~r

KENTUCKY

Percent porosity 

Contour interval 4%

0 20 40 KILOMETERS

39° —

Log permeability 

Contour interval 0.5

Log permeability 

Contour interval 0.5

Figure 9. Isocontour maps depicting regional variations in reservoir quality in sandstones within the Bethel-Cypress interval. A, percent 

porosity, Bethel Sandstone; B, log permeability. Bethel Sandstone; C, percent porosity, Cypress Sandstone; D, log permeability, Cypress 

Sandstone. Stippled pattern indicates Wabash Valley fault system.

sandstones comprises approximately equal amounts of 

potassium and sodium varieties. Most potassium feldspar 

grains exhibit the effects of alteration, such as replacement 

by authigenic clay or, less commonly, ferroan carbonate. 

Many sandstones display honeycombed grains or moldic and 

oversized pores, indicating partial to complete K-feldspar(?) 

dissolution. Na-feldspar, dominantly plagioclase, may show 

clay alteration, but leaching tends to be minor. Lithic grains 

include plutonic and metamorphic polycrystalline quartz

and, less commonly, gneiss, chert, argillite, and carbonate 

intraclasts. The relative proportion of polycrystalline quartz 

grains generally increases with increasing grain size. Fossil 

bioclasts (included in total lithics category of table 2) are 

present locally in some sandstones and include a variety of 

bryozoan and echinoderm fragments; similar fossil types 

have been reported in the upper Valmeyeran Series (Pryor 

and others, 1991). Illitic clay matrix is confined to thin, wavy 

laminations and finely crystalline hematite is present as a
10 RESERVOIR QUALITY AND DIAGENETIC EVOLUTION OF UPPER MISSISSIPPIAN ROCKS, ILLINOIS BASIN

87°

38° -

37° -

Figure 10. Map showing late-diagenetic quartz and ankerite cementation relative to faults in southern Illinois. Area 

of cementation defined by dashed line.

dark-red, patchy material in a few sandstones. This distribution of hematite strongly resembles a ferri-argillans mixture 

of clay minerals and iron oxides/hydroxides as described 

from soils (cf. Brewer, 1976). A variety of accessory minerals occur in trace amounts.

MINERAL DIAGENESIS 

COMPACTION FEATURES

Most sandstones within the Bethel-Cypress interval 

have undergone some degree of compaction, even in the 

most porous sandstones. Sandstones that have suffered 

porosity loss through mechanical compaction did so by 

means of a gradual increase in grain-packing density as a 

result of grain rotation and deformation of ductile components. In sandstones that experienced substantial mechanical 

compaction, there is little or no early cement to provide 

framework grain support, nor is there evidence of cementdissolution porosity. Chemical compaction, common in finer 

grained sandstones, affected quartz grains and early grain 

overgrowths in contact with early-infiltrated clay. Low 

amplitude (<5 mm) microstylolites, which account for minor 

porosity loss, also record the effects of chemical compaction.

SECONDARY QUARTZ

Early diagenetic quartz is the most abundant cement in 

sandstones (0-23 percent; 7 percent average; table 2) and is 

widespread throughout the basin. Typically, early authigenic 

quartz constitutes optically continuous, euhedral overgrowths that are embayed or partially developed. The largest 

overgrowths are in the coarser grained sandstones. Where 

early quartz is widespread, it produces well-cemented sandstones that have experienced substantial porosity reduction.

Late-diagenetic(?) quartz is observed in small amounts 

and is best developed in sandstones close to faults in the 

southern part of the basin (fig. 10). In thin section, late-stage 

quartz cement forms coarsely crystalline aggregates that 

engulf illite, chlorite, and kaolinite and locally coalesce into 

a clear cement.

CARBONATE CEMENTS

Diagenetic carbonate is common in some sandstones 

(0-42 percent; 4 percent average; table 2), but in most sandstones it is absent or poorly developed. On the basis of petrographic analysis, early and late stages of carbonate 

(including syntaxial nonferroan calcite, planar dolospar, 

poikilotopic ferroan calcite, planar ferroan dolospar
CONTROLS ON RESERVOIR QUALITY 11

Diagenetic stage Early Late

Mechanical compaction

Illite/lllite-smectite

Fe-chlorite

Kaolinite

Quartz overgrowths

Syntaxial calcite

Dolomicrospar

Planar dolospar

Carbonate cement dissolution

Poikilotopic calcite

Planar ankerite

Saddle ankerite

Anhydrite

Fracturing

Petroleum emplacement

Figure 11. Paragenetic sequence of diagenetic events in reservoir sandstones. Thick bars represent major periods of diagenesis; dashed 

lines indicate possible diagenesis.

(ankerite), and saddle ankerite) precipitated during the burial 

history of the Bethel and Cypress Sandstones (see fig. 11). 

Textural relations between individual phases are complex 

and often ambiguous, suggesting that there may be additional generations of carbonate that are not recognized. Stable isotope compositions of the major carbonate minerals are 

reported in table 3 and illustrated in figure 13.

Early- and late-diagenetic carbonates are dominantly 

pore-fill cements that generally postdate quartz overgrowths. 

Nonferroan calcite predates other carbonate cements and 

occurs sporadically in bioclastic sandstones as syntaxial

overgrowths that nucleated on fossil fragments. In some 

samples, this early calcite has been partially replaced by ferroan calcite cement. The 813C ratio of one sample of syntaxial calcite is -0.12%c, and the corresponding 818O ratio is 

-7.80%c (table 3; fig. 13). Ferroan calcite comprises optically 

continuous crystals that exhibit straight extinction and, in 

some sandstones, forms a poikilotopic cement that encloses 

framework grains. More commonly, ferroan calcite displays 

poorly defined crystals with irregular grain boundaries and is 

distributed as relic cement in secondary pores. In a few samples, poikilotopic ferroan calcite partially replaces earlier

Quartz

Bethel Sandstone

Mean QFL=

84:8:8

Cypress Sandstone

Mean QFL=

85:5:7

Sublitharenite 

75%

Feldspar 1:3 Lithic Fragments

Figure 12. Ternary QFL (quartz, feldspar, 

lithic fragments) plots for sandstones from 

the Bethel and Cypress based on the classification of Folk (1974). Note similarity in mineralogic compositions between units.
Table 2. Mineralogic composition of sandstones within the Bethel-Cypress interval.

<~> M- -J M-

[Latitude in decimal degrees north; longitude in decimal degrees west. Data reported in volume percent;

Core 

ID

API

no.

Latitude Longitude Grain 

size

Quartz Na- Kfeldspar feldsparTotal 

feldspar

grain size in micrometers; IGV, intergranular volume; Bi, bim

Total 

lithics

Quartz 

cement

Calcite 

cement

Dolomite 

cement

Total 

carbonate 

cement

To

cem

Bethel Sandstone

C0071

C0285

C0318

C0345

C0361

C0364

C0414

C0467

C0468

COS 18

C0525

C0573

C0595

C0597

C0607

C0651

C0735

C1018

C1113

C1125

C1169

C1270

C1511

C2069

C2085

C2114

C2230

C2341

IN303

IN643

1207700603

1214502037

1216501136

1208102322

1204700033

1202702511

1202900030

1219304604

1219306807

1202700823

1200500212

1202500986

1204900704

1207900017

1205500336

1218900150

1212101296

1212104149

1218900176

1212590017

1217300051

1208100499

1204900140

1212101475

1212104968

1202500549

1200500567

1208100199

no data

no data

37.928828

38.158661

37.878645

38.320668

38.481371

38.518618

39.595217

38.160159

37.929945

38.658891

38.978035

38.772606

38.958267

38.878071

37.867485

38.252045

38.705799

38.561675

38.470347

38.772307

39.326445

38.405546

39.065321

38.778051

38.779868

38.710497

38.827081

38.181586

38.185565

38.345879

89.283433 227

89.285138 152

88.578628 69

89.042866 152

88.014630 190

89.424022 265

88.388347 303

87.986364 190

88.285574 190

89.278940 127

89.309814 152

88.753333 190

88.624839 Bi

88.352995 152

88.975637 190

89.403828 114

89.055451 303

89.020984 152

89.210327 265

88.073440 152

88.906719 152

88.831430 265

88.765539 190

88.762495 152

88.755532 152

88.584495 114

89.474212 114

88.992598 190

86.508559 190

86.704695 152

79

64

52

65

58

67

50

49

60

69

40

65

69

68

61

60.5

66

59

69

68

54

58

61

58

49

66

60

59

53

39

1

1

5

1

4

4

0

5

5

5

1

2

3

2

7

0.5

1

3

2

1

2

5

2

1

2

1

6

3

3

0

0

4

2

3

0

1

0

2

6

2

2

3

1

4.5

0

7

1

5.5

1

3

3

4

3

4

2

5

3

4.5

2

2

1

5

7

4

4

5

0

7

11

7

3

5

4

6.5

7

7.5

2

8.5

3

4

5

9

5

5

4

6

9

7

5

2

4

7

9

5

4

2

22

7

9

5

1

4

3

5.5

6

6.5

3

7.5

3

11

6

5

5

6

4

6

4

7

6

11

3

4

15

6

6

3

1

1

9

0

0

6

7

5

4

4.5

9

3.5

5

6

9

15

9

14

0

8

4

5.5

11

0

0

0

0

18

2

2

22

0

0

0

0

0

0

1

2

0

4

3

1

0

1

4

5

3

36

1

0

1

0

0

0

0

12

1

0

4

3

0

0

0

1

0

0

0

4

0

0

0

0

0

1

0

0

0

3

0

0

1

12

1

0

0

12

19

2

6

25

0

1

0

1

0

0

1

6

0

4

3

1

0

2

4

5

3

39

1

0

2

12

1

2

2

1

2

1

1

1

1

1

2

1

2

3

1

1

2
CONTROLS ON RESERVOIR QUALITY 13

o

U

18

P _§ E

r- K

E y

,2 S

a (U 

U

P^

^§

S 9 

u

oinoo-nooinO'-H!^<Nr-'oinO

o^ooooofio —i r- <N o o in o

m_ 

<N

oo^om

(Nt^ooo-nooooooooooo

oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo

ro rn en en en en

O—lOOOOOOOO

8

^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ i—H i-—I ^H i—H i—H i-—I i-H i-H i-H ^H i-H i-—I i—H i-H i-H T—H i-H i——< 

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O

uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu
Table 2. Mineralogic composition of sandstones within the Bethel-Cypress interval — Continued.

Core API Latitude 

ID no.

C01760 1202900795 39.427991

C01794 1202901319 39.437038

CO 1806 1202900869 39.414467

C01813 1203500074 39.337077

C01834 1202901353 39.442305

C01931 1202901019 39.502063

C01968 1202901330 39.433810

C01975 1202900788 39.424176

C01981 1207900301 38.881033

C02114 1202500549 38.710497

C02149 1219101213 38.435874

C02174 1204900813 38.946928

C02230 1200500567 38.827081

C02357 1212101986 38.765089

C02621 1205901062 37.704520

C05279 1218502475 38.260543

C05834 1202501165 38.627951

C05874 1218900972 38.233049

C06058 1219302857 38.053129

C06079 1203307971 38.867555

C14018 1210127623 38.753134

C14019 1210128363 38.759770

C14023 1204724183 38.562076

IN071 no data 38.086299

IN097 no data 38.681 153

IN133 no data 38.238589

IN643 no data 38.345879

IN647 no data 38.330681

IN734B1 no data 38.511973

IN745 no data 39.062193

Longitude

88.393260

88.386898

88.395643

88.433253

88.377306

88.398420

88.380040

88.386237

88.230995

88.584495

88.278806

88.480065

89.474212

88.902172

88.173993

87.944239

88.610797

89.690740

88.174550

87.788940

87.783338

87.786930

88.025117

87.381803

86.687115

87.932857

86.704695

87.655480

87.418606

86.857040

Grain Quartz Na- K- Total 

size feldspar feldspar feldspar

114

152

190

114

152

152

152

190

152

227

227

190

190

190

190

265

190

152

227

152

114

114

114

190

190

190

152

152

227

152

65

60.5

57

53

60

57

50

65

59

52

67

67

61.5

58

55

68

70

52

60

58.5

44

62

67

61

63

61.5

48.5

70

70

69

0

2.5

0

5

0.5

1.5

0

0

4

4

3

4

4

2

7

2.5

0

5

5

5

1

1

2

1

0.5

3

0.5

2

1

0

0

0

2

2

0

0

1

0

0

1

0

0

1

0

2

0

2

1

0

2

0

0

0

2

2.5

0

3

0

3

4

0

2.5

2

7

0.5

1.5

1

0.5

4

5

3

4

5

2

9

1.5

2

6

5

7

1

1

2

3

3

3

3.5

2

4

4

Total 

lithics

10

3.5

7

6

6.5

4.5

10

6

7

6

2

5

2.5

4

4

2.5

8

4

6

6

6

4

2

5

3

6.5

5

7

6

5

Quartz 

cement

3

6

5

23

5

4.5

1

5

11

12

16

8

2

20

18

3

5

5

12

5

0

2

5

1

11

3.5

0.5

0

1

1.5

Calcite Dolomite Total 

cement cement carbonate 

cement

1

0.5

0

2

0.5

0

35

1

0

0

0

1

3

5

0

0

0

0

0

2

40

26

1

1

0

0

0

0

6

0

0

0

0

0

0

0

0

0

0

0

0

0

0

7

5

0

0

7.5

6

0.5

2

0

0

3

0

2.5

30.5

0

0

0

1

0.5

0

2

0.5

0

35

1

0

0

0

1

3

12

5

0

0

7.5

6

2.5

42

26

1

4

0

2.5

30.5

0

6

0

To

cem

2

1

3

1

1

2

1

3

2

1

2

4

2

1

3
CONTROLS ON RESERVOIR QUALITY 15

Table 3. Stable isotope compositions of carbonate cements in sandstones within the 

Bethel-Cypress interval.

[Values in parentheses are repeated analyses. 813C and 818O values in per mill, PDB]

Sample

ID

Depth

(m)

Unit 613C 6180

Syntaxial calcite

C00414

IN734

1,785

1,392

Bethel

Cypress

-0.12

-19.39 (-19.43)?

-7.80

-8.63 (-8.68)

Poikilotopic Fe-calcite

C00345

C00345

CO 1270

C00385

C00520

C00693

C01301

CO 1698

CO 1968

CO 1975

CO 1975

C08363

C14019

2,025

2,020

2,662

2,859

2,859

2,471

1,787

1,756

1,901

1,880

1,880

1,311

1,311

Bethel

Bethel

Bethel

Cypress

Cypress

Cypress

Cypress

Cypress

Cypress

Cypress

Cypress

Cypress

Cypress

-5.60

-3.09

-2.55

-2.59 (-2.59)

-2.71 (-2.71)

-15.83 (-15.75)

-11.27 (-11. 19)

-1.97 (-2.01)

-2.14 (-2.23)

-2. 17 (-2.35)

-2.27 (-2.27)

-4.22 (-4.22)

-4.26

-10.5

-9.90

-9.50

-10.35 (-10.35)

-8.76 (-8.76)

-8.70

-9.71 (-9.93)

-11.30 (-11.20)

-10.70 (-10.90)

-10.80 (-10.50)

-10.56 (-10.62)

-7.92 (-7.92)

-7.80

Planar ankerite

C00071

C00318

C00607

IN303

C00642

C00962

C05874

C05874

IN643

IN643

1,655

2,972

2,557

538.5

2,822

2,009

819

817

422

440

Bethel

Bethel

Bethel

Bethel

Cypress

Cypress

Cypress

Cypress

Cypress

Cypress

-1.97

-2.81

-1.52

-0.64

-0.56

0.02

-0.26

-0.34

1.65

0.32 (0.32)

-7.40

-6.60

-7.70

-6.60

-6.60

-9.00

-5.60

-5.80

-5.00

-5.56 (-5.56)

Saddle ankerite

C00285

C00374

C01183

C01353

C01981

IN71

1,157

2,584

2,584

2,717

2,752

1,672

Cypress

Cypress

Cypress

Cypress

Cypress

Cypress

-3.33

-8.85

-5.38

-2.63

-4.04

-2.68 (-2.61)

-11.80

-9.60

-7.40

-7.5

-8.20

-7.39(7.18)

formed syntaxial calcite and is in contact with early quartz 

overgrowths. It also occurs locally as a late-stage fracture-fill 

cement that crosscuts earlier carbonate and quartz overgrowth cements. The 813C values of poikilotopic ferroan calcite cluster in two groups; most samples display slightly 

negative 813C values ranging from -1.97 to -5.60%o, but two 

samples yielded highly negative 813C values of-11.27 and 

-15.83%o. 818O values of ferroan calcite samples vary from

-7.80 to -11.30%0 (table 3; fig. 13). A small component of 

dolomite in sandstones is classified as early, nonferroan, planar dolospar on the basis of its straight crystal boundaries and 

even extinction in polarized light. Most planar dolospar is 

compositionally homogeneous, but some dolospar grains 

exhibit well-developed Fe-rich overgrowths and are zoned. 

Except for its relationship with ankerite, the paragenesis of 

planar dolospar is ambiguous because it rarely occurs with
16 RESERVOIR QUALITY AND DIAGENETIC EVOLUTION OF UPPER MISSISSIPPIAN ROCKS, ILLINOIS BASIN

other carbonate cements. Unfortunately, planar dolospar is 

present in small amounts, or as complex intergrowths with 

ankerite, and thus could not be sampled for isotopic analysis. 

Late-diagenetic planar ankerite postdates early carbonate cements and predates hydrocarbon emplacement in reservoir sandstones. Planar ankerite typically occurs as 

solitary or clustered rhombs, and where abundant, forms 

irregular, coarsely crystalline or monocrystalline cement 

that infills secondary voids and replaces feldspar grains. 

Nowhere does it show evidence of dissolution. In some 

sandstones, a component of ankerite is anhedral to subhedral in habit with slightly to moderately curved crystal faces 

and broad sweeping extinction—features that classify it as 

saddle ankerite. This type of ankerite occurs as a pore-fill 

cement and fills fractures that crosscut earlier formed quartz 

and carbonate cements. Saddle ankerite (and planar ankerite) are most abundant in sandstones close to faults in the 

southern part of the basin (fig. 10). The isotope ratios of 

planar ankerite vary over a relatively narrow range, from 

1.65 to -2.8 1%0 (813C) and -5.00 to -7.70%0 (518O), and 

are consistently heavier than the ratios of saddle ankerite, 

which range from -2.63 to -8.85%0 (513C) and -7.39 to 

-11.80%0 (518O) (table 3; fig. 13).

AUTHIGENIC CLAY MINERALS

Minor authigenic clay (0-7 percent, average 2 percent; 

table 2) is observed in most sandstones within the BethelCypress interval. Illite and randomly ordered, interstratified 

illite/smectite are present as both precompaction and postcompaction clay. Precompaction (syndepositional) illite consists of thin tangential coatings on framework quartz grains 

devoid of authigenic overgrowths in finer grained sandstones. Where grain-rimming illite is widespread, quartz 

grains commonly display interdigitated, presolved contacts. 

Postcompaction illite is distributed as wispy, irregular flakes 

on detrital grains and, less commonly, as pore-bridging and 

pore-filling aggregates. Petrographic analysis suggests there 

are at least two stages of this later illite. The earlier phase is 

intergrown with authigenic quartz overgrowths, whereas a 

later phase is associated with residual pores and leached 

potassium-feldspar grains.

Other diagenetic clays in sandstones include Fe-rich 

chlorite and kaolinite. Authigenic chlorite occurs as euhedral 

platelets (1-5 JLim in diameter) that form tangential rims on 

framework grains, principally detrital quartz devoid of authigenic overgrowths. The chlorite rims commonly are discontinuous and bridge framework grains at grain contacts. 

Kaolinite forms small (2-5 JLim), randomly oriented, 

pseudohexagonal platelets and loosely to densely packed 

aggregates of platelets that line and infill secondary pores. In 

rare instances, it replaces partially dissolved feldspar grains. 

Kaolinite is most abundant near the basin margin where 

Chesterian-age sandstones were affected by subaerial erosion, but in southern Illinois, rare dickite, a high-temperature 

kaolin polymorph, occurs locally.

-10

-15

-20

vy Mississippian 

carbonate

X D

o

V 

Q

D i

EXPLANATION

l~1 Syntaxial calcite 

O Poikilotopic calcite 

X Planar ankerite 

V Saddle ankerite

-14 -12 -10

518 0,°/0

-6

PDB

-4 -2

Figure 13. 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). Note 

substantial overlap between the different carbonate types. PDB, 

Peedee belemnite standard.

OTHER PHASES

Anhydrite is present in volumetrically minor amounts in 

a few samples of Cypress Sandstone. Its mode of occurrence 

ranges from small scattered patches to large euhedral crystals 

that preferentially replace ferroan calcite and fill secondary (?) pores. There is no evidence to suggest that anhydrite 

was affected by dissolution processes.

Pyrobitumen is present locally as an opaque residue in 

some sandstones within the Bethel-Cypress interval. Within 

oil zones, pyrobitumen occupies moldic and intergranular 

(secondary) pores and, in a few samples, rims planar ankerite 

cement. Petroleum residue in sandstones also is observed 

coating minor quartz overgrowths and relic ferroan and nonferroan calcite cement.

Other authigenic phases in insignificant amounts 

include pyrite, siderite, and barite.

SECONDARY POROSITY

Secondary porosity occurs at all depths and was produced by the dissolution of carbonate cement, principally 

nonferroan and ferroan calcite, and framework grains. 

Removal of calcite ranges from minor corrosion to complete dissolution. In sandstones where calcite has been
CONTROLS ON RESERVOIR QUALITY 17

400 350 300 250

AGE, Ma 

200 150 100 50

5 10 —

20

I -y

Early illite and \ 

chlorite

30'

\

Syntaxial calcite x 

dolomicrospar

Quartz \ \ 

overgrowths \

Burial

Carbonate 

dissolution 

illite/chlorite /

Temperature (burial only)

Ankerite 

quartz

fracture cementation 

hydrocarbon emplacement

y

140°"" Hydrothermal 

flow event 

(-270 Ma)

Temperature (coupled igneous 

and hydrothermal heat)

200

160

120

80

40

Figure 14. Burial and thermal history curves for the Bethel Sandstone-Cypress Sandstone interval showing relative timing of major 

diagenetic events in southern and central Illinois. Age of fluorite and petrographic features constrain the timing of diagenesis; age of regional 

hydrothermal fluid-flow event (-270 Ma) is shown for comparison. Paleotemperature profiles (igneous plus hydrothermal heat flow and 

burial only heat flow) were determined using a coupled fluid-flow and burial heat-flow model. The temperatures bracketing diagenetic events 

were modified from Surdam and others (1989). Note that thermal curves are depicted relative to geologic age rather than burial depth.

dissolved, quartz grains have irregular embayments and 

overgrowths display partially developed or embayed 

boundaries. The size and shape of grain and overgrowth 

reentrants combined with relic calcite replacing framework 

grains attest to the dissolution of preexisting calcite 

cements. Additional lines of evidence in favor of calcitecement dissolution include well-developed intergranular 

calcite cement immediately adjacent to extensively leached 

areas and scattered dissolution voids in tightly cemented 

sandstones. Leaching of framework grains affected mainly 

detrital alkali feldspars and resulted in the formation of 

oversized and moldic pores. In some sandstones, there is no 

clear evidence that leaching was due to the dissolution of 

replacement carbonate. Overall, grain-dissolution porosity

typically is less than a few percent and did not substantially 

enhance the total sandstone porosity.

In moderately to highly porous sandstone, frameworkgrain-supporting cements clearly were removed, but all 

intergranular porosity may not have resulted from dissolution processes. If initial cementation was incomplete, some 

primary porosity should be preserved in reservoir sandstones. Distinguishing primary from secondary porosity 

proved to be difficult; thus, estimated amounts of primary 

porosity are not reported. Nevertheless, textural features 

together with heterogeneous grain-packing densities suggest that the intergranular pore system in sandstones is predominantly secondary with only minor primary porosity 

preserved locally.
18 RESERVOIR QUALITY AND DIAGENETIC EVOLUTION OF UPPER MISSISSIPPIAN ROCKS, ILLINOIS BASIN

EVOLUTION OF 

RESERVOIR QUALITY

The evolution of the diagenetic system in the Bethel and 

Cypress Sandstones in central and southern Illinois is shown 

on a reconstructed burial curve in figure 14. Major diagenetic 

events include (1) precompaction (syndepositional) illite and 

early chlorite precipitation; (2) quartz, calcite, and dolospar 

cementation; (3) calcite cement and framework grain dissolution; (4) illite and kaolinite formation; (5) late quartz and 

ankerite precipitation; (6) fracture cementation; and (7) 

hydrocarbon emplacement. The temperatures that correspond to individual diagenetic phases were modified from 

Surdam and others (1989) and correlated with the temperature profile that is based on the combined thermal effects of 

igneous heat and hydrothermal fluid flow (fig. 14). A temperature curve derived on the basis of burial heat flow alone is 

shown for comparison.

Late-diagenetic fluorite, containing oil-bearing fluid 

inclusions, in the Illinois/Kentucky Fluorspar district has 

been dated at -270 Ma (Richardson and others, 1988; Chesley and others, 1994). Although Upper Mississippian sandstones lack cements suitable for isotopic age dating and 

individual diagenetic events have no specific time connotation beyond "early" and "late," the timing of peak hydrocarbon migration in the basin (-270 Ma) places an upper limit 

on the age of much of the diagenesis in reservoir sandstones. 

The steep trend of the burial curve (fig. 14) indicates that 

diagenesis in sandstones occurred over a short geologic time 

interval (<~60 m.y.) and was largely complete by the time the 

rocks had reached their maximum burial in the late Paleozoic.

QUARTZ CEMENTATION

On the basis of burial-thermal modeling, most quartz 

cement that precipitated early in the burial history formed at 

relatively low temperatures (<~65°C), but minor late-diagenetic quartz in sandstones near faults in the southern part of 

the basin may have precipitated from fluids with temperatures as high as ~140°C (fig. 14). Feldspar leaching probably was not a major source of silica for early quartz 

overgrowth development because framework grain dissolution occurred after most quartz precipitation had ended. 

Pressure solution along stylolites and on grain contacts 

often is cited as an important source of silica for quartz 

cementation in sandstones (Bjorlykke and Egeberg, 1993; 

Houseknecht, 1984, 1988). Petrographic analysis shows 

evidence for incipient stylolitization and grain-to-grain contact dissolution in some sandstones within the Bethel, but 

the amount of silica released is difficult to determine 

directly because stylolite amplitudes and overlap quartz 

cannot be easily measured. In thin section, framework grain 

contacts and microstylolite seams commonly contain illitic 

clay. Studies by Heald (1955) and Weyl (1959)

demonstrated that, although pressure solution may occur 

without clay, clay coatings accelerate the grain-to-grain 

contact-dissolution process. Silica-rich pore fluids produced 

by pressure solution at stylolites or at grain contacts subsequently diffuse over relatively short distances to sites of 

quartz precipitation. Several studies have argued that the 

transformation of smectite to illite in interstratified clays is 

a potential silica source for quartz cementation (Siever, 

1962; Hower and others, 1976; Boles and Franks, 1979). 

Mixed-layer illite/smectite is minor in shallowly buried 

sandstones within the Bethel-Cypress interval, and shale 

interbeds containing mixed-layer clay generally are sparse; 

thus, the illite/smectite transformation process probably did 

not contribute a significant amount of silica for early quartz 

cement. Much of the silica for early quartz cementation 

might have originated from extrastratal sources, including 

saturated meteoric ground waters that circulated through the 

sandstones following deposition. In southern Illinois, a 

component of silica incorporated into late-stage quartz may 

have been derived from an igneous source.

CARBONATE DIAGENESIS

Carbonate cementation in sandstones began soon after 

early quartz precipitation and continued until the emplacement of hydrocarbons (fig. 14). Overall, carbonate cements 

become more coarsely crystalline, enriched in Fe, depleted in 

18O, and confined to preexisting intergranular porosity with 

increasing diagenesis. These phenomena have been noted in 

other sedimentary basins and have been attributed to precipitation from low-Eh pore waters under gradually increasing 

burial temperatures. In the Illinois Basin, increasing temperature with time records the increased effects of heat related 

to igneous activity and fluid flow in addition to burial. Most 

carbonate cements in sandstones within the Bethel-Cypress 

interval have slightly negative 813C values (-3%o average), 

indicating a strong influence from a marine carbon source 

(i.e., carbonate fossils and chemically unstable calcite 

cement). The restricted range in carbon-isotope compositions suggests that the carbonate system was controlled 

largely by the conditions that existed during initial carbonate 

precipitation, modified later by water-rock interaction. It is 

important to note that the marine carbon signature in carbonate cements has been preserved even when the 8 18O values 

reflect reequilibration, consistent with petrographic observations showing carbonate recrystallization. A few calcite samples in southern Illinois are depleted in J 3C with respect to 

inorganic carbon and have compositions of —11 to -16%o 

(see table 3 and fig. 13). These light values are unusual for 

the Bethel and Cypress and probably reflect a contribution of 

light carbon from decarboxylized organic matter in a system 

buffered by marine carbon. Between 10 and 50 percent of the 

organic carbon introduced into the diagenetic system must 

have had a 8J3C of -26%o in order to explain these light
EVOLUTION OF RESERVOIR QUALITY 19

0 5 

818 0 water (°/00, SMOW)

10

818 0 water (°/00 , SMOW)

Figure 15. Equilibrium relationship between 8 O of water, 8 O 

of calcite and ankerite, and crystallization temperature. Curves represent locus of temperature and possible 8 ^ 8O of water in equilibrium 

with a mineral having the indicated oxygen isotopic composition. 

Fractionation equations (calcite-water: 103 In a = 

2.78xl06xT-2-2.89 and ankerite-water: 103 In a = 

2.78x106xT~2+0.11) are from Friedman and O'Neil( 1977) and Fritz 

and Smith (1970), respectively. SMOW, Standard mean ocean water.

values. In the southern part of the basin, 5 13C values of saddle ankerite are slightly depleted relative to the 5 13C values 

of planar ankerite cement. The 8 13C values of saddle ankerite generally are within the range of marine values,

indicating that the carbon-isotope composition of the fluid 

from which the saddle ankerite precipitated was largely controlled by the carbon composition of the host sandstones.

The oxygen isotopic composition of carbonate cements 

in sandstones varies over a wide range (—5 to -12%c) and, 

as will be shown, was controlled by the isotopic composition 

of the precipitating fluids and the temperature of crystallization. By knowing the oxygen-isotope compositions of calcite 

and dolomite cement, the isotopic composition of carbonateprecipitating waters can be reconstructed by calculating 

equilibrium S^OH^O values, based on estimated temperatures of crystallization (fig. 15). Two assumptions are pertinent to this approach: the carbonate minerals formed in 

isotopic equilibrium with the pore waters and no temperature 

inversions occurred from the time of initial burial through 

ankerite precipitation. In making §18OH-,o calculations, we 

used the fractionation equation of Friedman and O'Neil 

(1977) for calcite and Fritz and Smith (1970) for dolomite. 

Precipitation temperatures of carbonate cements cannot be 

constrained directly because no fluid inclusions were found 

in individual carbonate minerals; therefore, estimated carbonate precipitation temperatures were taken from the 

burial-thermal profiles in figure 14. It is noteworthy that carbonate-precipitating temperatures at shallower depths of 

burial (<~ 1,000 m) were governed by burial (conductive) 

heat flow, whereas, at greater depths of burial (>~ 1,000 m), 

temperatures of crystallization were influenced by advective 

heat from hydrothermal fluids.

Using a near-surface temperature of ~30°C, early syntaxial calcite precipitated from waters with oxygen-isotope 

compositions of—6%c SMOW (standard mean ocean water) 

(fig. 15). Presumably, early dolomicrospar formed from similar waters at about the same temperatures. The negative oxygen isotope ratios of early calcite (and dolomite) relative to 

Mississippian marine carbonate (see fig. 13) indicate that the 

initial diagenetic environment was fresh-water influenced, if 

not fresh-water dominated, during precipitation of the earliest carbonate cements. Poikilotopic ferroan calcite postdates 

quartz overgrowths and early syntaxial calcite but predates 

hydrocarbon emplacement; thus it conceivably formed at a 

temperature as high as ~90°C (Surdam and others, 1989). 

Estimated pore-water compositions at these temperatures 

vary from ~0 to 4%o SMOW (fig. 15). Planar ankerite is a 

late-diagenetic carbonate phase that formed just prior to 

hydrocarbon emplacement when the host sandstones were 

close to their maximum depth (~2 km). The iron enrichment 

of planar ankerite (and ferroan calcite) is consistent with precipitation from Fe-enriched reducing waters, and the light 

518O isotope values indicate precipitation at elevated temperatures. Assuming an estimated formation temperature of 

~120°-140°C, planar ankerite precipitated from 18Oenriched waters with compositions ranging from ~3 to 9%c 

SMOW (fig. 15). Saddle ankerite commonly is associated 

with fractures in the southern portion of the basin (fig. 10) 

and postdates all other mineral cements in sandstones.
20 RESERVOIR QUALITY AND DIAGENETIC EVOLUTION OF UPPER MISSISSIPPIAN ROCKS, ILLINOIS BASIN

Assuming minimum temperatures on the order of 120°C, 

S18OH2o values of ~0 to 5%o SMOW are estimated, and 

slightly heavier values, ~6 %o SMOW, are plausible if fluids 

with temperatures as high as 140°C were involved in saddle 

ankerite cementation.

CLAY MINERAL PRECIPITATION

Clay mineral authigenesis began before significant compaction and continued until late in the burial history (fig. 14). 

Illite rims that predate mechanical compaction and underlie 

quartz overgrowths are regarded as a syndepositional or very 

early diagenetic precipitate. Postcompactional illite occludes 

some secondary pores, indicating it postdated carbonate dissolution but predated hydrocarbon emplacement. Postcompactional illite associated with leached detrital potassium 

feldspar grains indicates a genetic link between feldspar dissolution and illite that replaces feldspar. The illitization of 

smectite likely reflects increasing temperature, either as a 

consequence of burial or due to the introduction of an additional source of heat to the sediments late in the burial history.

The chlorite rims on framework grains are discontinuous and, in some sandstones, bridge framework grain contacts, indicating some mechanical compaction had occurred 

prior to chlorite precipitation. The relationship between Ferich chlorite and quartz overgrowths is ambiguous, but the 

lack of early quartz overgrowths on grains rimmed with 

authigenic chlorite suggests that the formation of chlorite 

inhibited quartz cementation. Iron-oxide grain coatings and 

dewatering of shales were potential sources of iron incorporated into chlorite.

Textural evidence is conflicting relative to the timing of 

kaolinite precipitation. Kaolinite is observed in dissolution(?) pores in shallowly buried sandstones, suggesting that 

it postdated calcite precipitation. Hydrocarbon emplacement 

followed carbonate dissolution and the formation of kaolinite, which indicates that kaolinite was not the most recent 

diagenetic event and thus precipitated at temperatures below 

the estimated maximum of 140°C. Kaolinite forms at low pH 

when there is an excess of silica and few K and Mg ions in 

solution, conditions that occurred during several periods of 

diagenesis. Assuming formation under these conditions, 

kaolinite could have precipitated during early diagenesis by 

ground waters entering sandstones at the post-Mississippian 

unconformity underlying Pennsylvanian strata, or it may 

have formed later as a consequence of meteoric diagenesis 

associated with late Paleozoic uplift and erosion. In southcentral Illinois, away from meteoric water invasion, minor 

kaolinite at moderate burial depths may reflect SiC>2 and 

Al2C>3 released into solution via pressure solution of quartz 

and decomposition of feldspar. On the southern tectonic 

flank of the basin, transformation of kaolinite to dickite probably occurred in response to the influx of high-temperature 

fluids into reservoir sandstones.

SECONDARY POROSITY DEVELOPMENT

Detailed petrographic evidence summarized above 

indicates that dissolution of carbonate cement (mostly calcite) and selective leaching of feldspar grains occurred late in 

the burial history and were responsible for most of the secondary porosity in reservoir sandstones. Three mechanisms—undersaturated meteoric water, organic acids and 

(or) CO2 derived from organic matter, and inorganic mineral 

reactions—might have caused the formation of secondary 

porosity. The first two are the most plausible mechanisms of 

dissolution. Leaching of carbonate by through-flowing meteoric waters probably was important at shallow depths and 

locally may have been of major importance; however, upon 

significant burial, meteoric flow most likely was not an 

important factor in secondary porosity development. In the 

deeper subsurface, the leaching capacity of low-acidity 

meteoric waters tends to be limited because the carbonate 

undersaturation is neutralized by reactions in the initial flow 

paths (Giles and Marshall, 1986; Giles; 1987). The exceptions are areas where well-developed fracture porosity may 

have helped to distribute undersaturated water over a larger 

distance than would have been possible if flow occurred 

through the pore network alone. The geochemical characteristics of petroleum in sandstones suggest that meteoric water 

was not a major factor in the mineral dissolution process during the time of hydrocarbon migration. Crude oils consistently have high API gravities (~30°^15 0 ) and water-soluble, 

saturated compounds are preserved (Hatch and others, 

1991), indicating that there was virtually no biodegradation 

or water washing of the hydrocarbons. This is the opposite of 

what would be expected if there had been large meteoric 

fluxes late in the basin's burial history.

Inasmuch as sandstones within the Bethel-Cypress 

interval are among the principal reservoirs for hydrocarbons 

in the basin, it follows that organic acids and CO2 produced 

during the early stages of hydrocarbon generation might have 

played a role in carbonate-cement and feldspar dissolution 

(Burley and others, 1985; Surdam and others, 1984). 

Carothers and Kharaka (1978) showed that short-chain 

organic acids can be present in formation waters throughout 

the 80°-200°C temperature range, assuming there has been 

no mixing with meteoric waters or dilution from deeper, hotter waters. The carboxylic acid anion dominance is particularly pronounced at temperatures of 80°-120°C and can 

buffer the alkalinity to relatively low pH values at low PCO,> 

which increases carbonate solubility (Surdam and Crossey, 

1987; Crossey and others, 1986). The reconstructed burialthermal history suggests that inorganic diagenesis and mineral dissolution in sandstones within the Bethel and Cypress 

spanned a temperature range of ~30°-140°C (fig. 14). 

Although no direct proof of the involvement of organic acids 

in the dissolution of carbonate cement is available, the temperature interval coincident with calcite dissolution 

(~90°-120°C) overlaps the thermal window in which the
THERMAL AND HYDROLOGIC CONTROLS ON DIAGENESIS 21

maximum concentrations of organic acids are produced during the early stages of hydrocarbon generation (80°-120°C) 

and thus provides support for the role of organic acids in secondary-porosity development.

Clay-carbonate reactions have been recognized as 

potential agents of mineral dissolution in many basins (Crossey and others, 1986; Lundegard and Land, 1986). In sandstones within the Bethel-Cypress interval, kaolinite is sparse, 

but, if it previously was common in the more thermally 

mature sandstones, its general absence may be due to clay/ 

carbonate reactions (i.e., kaolinite-calcite and (or) kaoliniteankerite) that largely eliminated the kaolinite. The volume of 

secondary porosity that can be generated from such mineral 

reactions was not calculated because the initial abundances 

of kaolinite are not known.

Selective K-feldspar leaching might be related to smectite/illite transformations in finer grained sandstones. Illitization of smectite takes place at temperatures of 70°-100°C 

(Hower and others, 1976; Pollastro, 1993). However, in this 

temperature range, the potassium uptake caused by smectiteto-illite conversion is much slower than the potassium 

released by K-feldspar dissolution, implying that the rate of 

K-feldspar dissolution should not be the rate-limiting factor 

in illitization of smectite (Altaner, 1986). The illite content 

in sandstones with high secondary porosity within the 

Bethel-Cypress interval does not exceed a few percent, and 

interstratified illite/smectite is insignificant; therefore, illitization of smectite may not have been associated with K-feldspar dissolution in sandstones. Upon dissolution of feldspar, 

aluminum is released, but due to its very low solubility (and 

mobility), aluminum must be consumed locally if dissolution is to proceed.

GENERATION AND MIGRATION 

OF PETROLEUM

Textural relations indicate that hydrocarbon entrapment 

is the latest diagenetic event that occurred in the Bethel and 

Cypress Sandstones (fig. 14). The origin of hydrocarbon residue in secondary pores has not been studied specifically, but 

it is best explained as the result of migrated oil sourced from 

the Upper Devonian to Lower Mississippian New Albany 

Shale, which underlies the Chesterian Series by -300 m 

(Swann, 1963). In the southern part of the Illinois Basin, 

hydrocarbon generation and migration in the New Albany 

started in the Middle to Late Pennsylvanian and reached 

their peak in the Early Permian when Upper Mississippian 

sandstones were close to their maximum burial depths and 

temperatures (Cluff and Byrnes, 1991). Generation and 

migration occurred later in the central part of the basin where 

the rocks were less deeply buried.

Most of the hydrocarbon production in sandstones 

within the Bethel-Cypress interval is concentrated along 

major structural features and faults, with many of the larger

reservoirs situated in the west-central part where sandstones 

have the best reservoir quality. The majority of the hydrocarbons reside in combination traps that overlie the generative 

portion of the basin (M. Lewan, 1997, unpub. data), which 

indicates that petroleum migrated vertically, primarily by 

buoyancy, along normal (extensional) faults and fractures 

with focused lateral flow locally updip where vertical flow 

paths intersected porous and permeable sandstone beds. 

Unconformity surfaces may have supplemented the primary 

migration pathways, especially in the center of the basin. 

Studies by Kolata and Nelson (1991) show that many of the 

major folds and faults that served as major conduits for 

hydrocarbon transport formed during or prior to hydrocarbon 

generation and expulsion in the Late Pennsylvanian and 

Early Permian. In southern Illinois and in the northern Reelfoot rift area, a period of igneous activity coupled with a brief 

but intense hydrothermal fluid-flow event accompanied late 

Paleozoic tectonism (Bethke, 1986). The exceptionally high 

rates of heat flow associated with the igneous activity and the 

advecting hydrothermal fluids resulted in temperatures 

exceeding those of normal burial in Upper Mississippian 

sandstones (see fig. 14). Presumably this high heat pulse also 

increased the rate of hydrocarbon expulsion in mature New 

Albany source rocks, which could account for the large volumes of petroleum that accumulated in late Paleozoic (and 

older) structures over a relatively short time period, approximately 60 m.y. (Abrams, 1995).

It is noteworthy that petroleum migration and Mississippi Valley-type mineralization were contemporaneous in 

southern Illinois. In the Illinois/Kentucky Fluorspar district, 

abundant oil in fluid inclusions within sphalerite and fluorite, 

and bitumen coating gangue minerals in fractures, are evidence that petroleum was present at the time of mineralization in the mid Permian (-270 Ma; Richardson and Pinkney, 

1984; Richardson and others, 1988; Chesley and others, 

1994). The temporal and spatial relationship between hydrocarbons and ore minerals indicate that the hydrocarbons 

migrated along the same fault and fracture systems that 

served as conduits for the mineralizing fluids. However, the 

discontinuous nature of individual reservoir units and the 

predominance of oil in traps overlying mature New Albany 

source rocks (Lewan, 1997, unpub. data) suggest that hydrothermal ore-forming fluids were not an important driving 

mechanism for oil migration in the basin as was previously 

thought (Bethke and others, 1991).

THERMAL AND HYDROLOGIC 

CONTROLS ON DIAGENESIS

Petrographic, stable-isotope and fluid-inclusion data 

provide good evidence for mixing of fluids of different temperatures from different sources during diagenesis in southern Illinois. The oxygen-isotopic evolution of pore water
22 RESERVOIR QUALITY AND DIAGENETIC EVOLUTION OF UPPER MISSISSIPPIAN ROCKS, ILLINOIS BASIN

suggests a rather constant composition during early non-ferroan carbonate (and quartz) precipitation, followed by a shift 

to relatively heavy values during late-stage ferroan carbonate 

and fracture cementation. Occurrences of early-diagenetic 

quartz overgrowths and syntaxial calcite and planar dolospar 

indicate precipitation at low to moderate temperatures in a 

meteoric-water-dominated, shallow-burial regime. In contrast, late-stage cements, including planar ankerite, saddle 

ankerite, and fracture-fill ferroan calcite, are compatible with 

precipitation from high-temperature, saline pore fluids in a 

system influenced by hydrothermal fluid flow. The strongest 

evidence for a hot, saline fluid source is based on a study of 

the Middle Ordovician St. Peter Sandstone in the Illinois 

Basin (Pitman and Spotl, 1996). Pitman and Spotl (1996) 

show that many of the petrographic and geochemical characteristics of burial-related carbonate cements in Ordovician 

sandstones resemble those of late-diagenetic carbonate 

cements in Mississippian sandstones, suggesting they may be 

genetically related. For example, saddle dolomite in the St. 

Peter Sandstone precipitated at minimum temperatures of 

~140°C; planar ankerite and saddle ankerite in sandstones 

within the Bethel-Cypress interval precipitated at about the 

same temperatures, ~120°-140°C. The stable-isotope 

geochemistry of these phases also is broadly comparable. In 

the St. Peter, 813C values vary from ~3 to -8%o and 818O values vary from —5 to -8%o; 813C and 518O values in the 

Bethel and the Cypress overlap this range, varying from ~0 

to -9%o and —5 to -12%o, respectively. Oxygen-isotope 

modeling coupled with fluid-inclusion analysis demonstrated that precipitating waters at minimum temperatures of 

140°C in the St. Peter were highly saline (>20 weight percent 

NaCl equivalent), with 818O values ranging from ~3 to 9%o 

(Pitman and Spotl, 1996). Pore-fluid compositions predicted 

at temperatures of 120°-140°C in the Bethel and the Cypress 

fall in the same range (818O= ~0 to 9%o SMOW), suggesting 

that the high-temperature, saline fluids that precipitated 

burial cements in Ordovician sandstones also may have 

formed ankerite in Mississippian sandstones.

The precipitation of late-diagenetic cements in Mississippian and Ordovician sandstones overlaps the timing of 

late Paleozoic tectonism and hydrothermal fluid migration in 

the Illinois Basin, which has been broadly constrained to 

have occurred during the Late Pennsylvanian and Early Permian. Thermal models show that the basin as a whole was 

affected by the influx of hot fluids and that transport of heat 

by these fluids significantly elevated the basin's thermal 

regime for a period of about 200,000 years (Rowan and 

Goldhaber, 1995). The change in fluid-flow dynamics in the 

basin to an advective, hydrothermal-flow system most likely 

was related to a change in the stress regime imposed by the 

Ouachita orogeny in the Late Pennsylvanian and Early Permian. During this time, gravity-driven recharge in the Appalachian-Ouachita highlands drove hot fluids from the deep 

Arkoma Basin northward through the Reelfoot rift and updip 

into the Illinois Basin proper (Bethke and others, 1988;

Bethke and Marshak, 1990; Garven and others, 1993). Data 

presented in this study and in Pitman and others (1996) provide evidence to suggest that these fluids (and hydrocarbons) 

moved through the basin by cross-formational flow along 

major bounding faults into porous and permeable reservoir 

units. Late-diagenetic mineral assemblages that formed from 

these hot fluids are found mainly in areas where folds and 

faults intersect these units.

CONCLUSIONS

Reservoir quality variations in sandstones within the 

Bethel-Cypress interval were evaluated on the basis of wholecore analyses and petrographic characteristics. Porosity in 

sandstones varies from 10 to 25 percent and decreases systematically with greater depth of burial due to the effects of 

mechanical compaction and quartz and carbonate cementation. Permeability is a function of porosity as well as a number 

of other factors and ranges over several orders of magnitude 

(< 1-2,500 mD) at any given depth. In the western part of the 

depositional area, porosity is highest and reflects preservation 

of initial pore space, enhanced by secondary porosity caused 

by leaching of framework grains and authigenic calcite 

cement. During shallow diagenesis, meteoric water was 

involved in the dissolution of calcite, but later, organic acids 

and CO2 produced during the early stages of hydrocarbon 

generation promoted more extensive carbonate dissolution. 

Reservoir quality in drilled and prospective undrilled areas 

should differ little because stratigraphically equivalent sandstones formed in similar depositional environments, had the 

same framework-grain compositions, and were subject to the 

same porosity reduction and enhancement processes.

Diagenetic alteration in sandstones within the BethelCypress interval commenced during the period of rapid sediment burial in the Late Mississippian and continued until 

migration of oil into sandstones in the Late Pennsylvanian 

and Early Permian. Major diagenetic events observed in 

sandstones include (1) precompactional illite and early chlorite precipitation, (2) early quartz, and calcite and dolomite 

cementation, (3) calcite-cement and framework-grain dissolution, (4) illite and kaolinite formation, (5) late quartz and 

ankerite precipitation, (6) fracture cementation, and (7) 

hydrocarbon emplacement. Syntaxial calcite, dolomicrospar, 

and quartz overgrowths formed during early burial at temperatures of ~30°-90°C in a regime dominated by freshwater 

inflow. By the Late Pennsylvanian to Early Permian, when 

the rocks were near maximum burial (~2 km), planar ankerite, saddle ankerite, and minor quartz precipitated at temperatures as high as 140°C in a diagenetic system influenced by 

igneous heat and advective heat from tectonically driven, 

hydrothermal fluids. In southern Illinois, hydrocarbons 

expelled as a consequence of this high heat pulse moved 

updip along faults and fractures, forming large accumulations of petroleum in sandstone reservoirs.
REFERENCES CITED 23

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. 

Special thanks are extended to Marty Goldhaber for generating the burial-thermal curves used in the study. The authors 

also are indebted to Jennie Ridgley and Neil Fishman for 

reviewing the manuscript and providing helpful suggestions.

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