H <
Cover. Leached ilmenite grain approximately 60 microns in diameter surrounded by 

quartz overgrowth in Lower Pennsylvanian sandstone in Indiana. Quartz overgrowths 

occlude much of the pore space in these rocks. Sample collected by Paula Hansley, U.S. 

Geological Survey.
Source Rock Potential of the 

Lower Part of the

Mississippian St. Louis Limestone in 

South-Central Indiana

By J.L. Ridgley and V.F. Nuccio

EVOLUTION OF SEDIMENTARY BASINS ILLINOIS BASIN 

J.L. Ridgley, Project Coordinator

U.S. GEOLOGICAL SURVEY BULLETIN 2094-D

A multidisciplinary approach to research studies of 

sedimentary rocks and their constituents and the 

evolution of sedimentary basins, both ancient and modern

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, 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

Ridgley, Jennie L.

Source rock potential of the lower part of the Mississippian St. Louis Limestone in southcentral Indiana / by J.L. Ridgley and V.F. Nuccio.

p. cm. (Evolution of sedimentary basins Illinois Basin ; D) (U.S. Geological 

Survey bulletin; 2094)

Includes bibliographical references.

Supt. of Docs, no.: 119.3:2094-D

1. Petroleum Geology Indiana. 2. Geology, Stratigraphic Mississippian. 

3. Limestone Indiana. I. Nuccio, Vito F. II. Title. HI. Series. IV. Series: U.S. 

Geological Survey bulletin ; 2094. 

TN872.I6R53 1996 

557.3 s dc20

[553.2'8'09772] 95-9486

CIP
CONTENTS

Abstract.......................................................................................................................... Dl

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

Methodology.................................................................................................................. 2

Stratigraphy of the St. Louis Limestone......................................................................... 3

General Geology of the St. Louis Limestone......................................................... 3

Depositional Facies in the Lower Part of the St. Louis Limestone........................ 4

Source Rock Potential.................................................................................................... 7

Organic Matter Quantity ........................................................................................ 7

Organic Matter Type.............................................................................................. 8

Thermal Maturity........................................................................................................... 9

Petroleum Potential........................................................................................................ 10

Summary........................................................................................................................ 11

References Cited............................................................................................................ 11

FIGURES

1. Map showing location of Illinois Basin, study area, and coreholes.......................................................................... D2

2, 3. Charts showing:

2. Nomenclature for Valmeyeran and Chesterian Mississippian rocks, Illinois Basin, Indiana............................. 3

3. Lithologic components of complete transgressive-regressive cycle................................................................... 4

4, 5. Generalized stratigraphic sections of the St. Louis Limestone in:

4. Survey Drill Hole 16........................................................................................................................................... 5

5. Survey Drill Hole 190......................................................................................................................................... 6

6. Photograph of limestone representing restricted lagoonal facies in lower part of the St. Louis Limestone in

Survey Drill Hole 190................................................................................................................................................ 8

7. Modified van Krevelen diagram of hydrogen and oxygen indexes determined by pyrolysis for samples in

Survey Drill Holes 16 and 190................................................................................................................................... 9

TABLES

1. Rock-Eval pyrolysis data for selected samples from Survey Drill Hole 16 and Survey Drill Hole 190 in the

lower part of the St. Louis Limestone, Illinois Basin, Indiana.................................................................................. D7

2. Lithology, depositional environment, and total organic content of samples from Survey Drill Hole 16 and

Survey Drill Hole 190 in the lower part of the St. Louis Limestone, Illinois Basin, Indiana.................................... 8

3. Pyrolysis parameters used to define source rock generative potential....................................................................... 10

in

Source Rock Potential of the Lower Part of the

Mississippian St. Louis Limestone in

South-Central Indiana

By J.L. Ridgley and V.F. Nuccio

ABSTRACT

The carbonate-evaporite sequence of the lower part of 

the Mississippian St. Louis Limestone in the Illinois Basin in 

south-central Indiana can be divided into as many as 19 

transgressive-regressive sedimentary cycles. Within each of 

these cycles, there are organic-rich strata that may be good to 

excellent hydrocarbon source rocks. The organic-rich strata 

are laminated or show evidence of compaction; they were 

deposited either during peak transgression (maximum flooding) or in restricted lagoonal environments when clastic 

input decreased or anoxic conditions were prevalent. In the 

two cores examined, the organic-rich strata contain from 

0.03 to 15.72 percent total organic carbon types II and III. 

The organic-rich intervals range from a few centimeters 

(inches) to as much as a meter (3.3 ft) in thickness; cumulative thickness may be as much as 15-20 percent of the lower 

part of the St. Louis, or as much as 73 m (250 ft).

The average Tmax and production index values both 

decrease slightly from south to north in the two coreholes 

examined. On the basis of vitrinite reflectance from the 

underlying Upper Devonian and Lower Mississippian New 

Albany Shale and coal-rank data from overlying Pennsylvanian coal measures, the trend in these values falls within the 

general pattern of thermal maturity in the basin. The organic 

geochemical and environmental data suggest that the lower 

part of the St. Louis Limestone warrants further study as a 

potential source of hydrocarbons in the Illinois Basin, especially in the more thermally mature parts of the basin.

INTRODUCTION

Globally, organic-rich strata are found within interbedded, laminated carbonate and evaporite rock sequences and 

are major sources of hydrocarbons in the Jurassic of the Arabian Gulf (Mums, 1980; Ayres and others, 1982) and Gulf 

Coast of the United States (Oehler, 1984), the Pennsylvanian 

of the Paradox Basin of the United States (Hite and others, 

1984), and the Lower Cretaceous of the South Florida Basin 

of the United States (Palacas and others, 1984). Some of the

organic-rich strata coincide with periods of peak transgression (maximum flooding) when euxinic conditions and 

reduced clastic input were prevalent. Other organic-rich 

strata were laid down in restricted marine environments in 

which the reducing environment associated with evaporite 

precipitation allowed preservation of the organic matter.

In the Illinois Basin (fig. 1), the source of most of the 

oil and gas has been attributed to the Upper Devonian and 

Lower Mississippian New Albany Shale (Hatch and others, 

1990), and carbonate rocks generally have been overlooked 

as potential sources of hydrocarbons. Consequently, their 

source rock potential, and thus their contribution to the oil 

resources in the basin, is relatively unknown. An exception 

is Ordovician carbonate rocks and calcareous shale, sometimes interbedded with anhydrite beds, which are known to 

have generated oil. If the presence of evaporite rocks in 

association with carbonate rocks is an important criterion 

for favorable nonclastic source rocks, then perhaps other 

formations in the basin might contain favorable source 

rocks. Most carbonate rock formations, however, contain 

few, if any, evaporite beds. The one exception is the lower 

part of the Mississippian (Valmeyeran) St. Louis Limestone, which contains interbedded carbonate-anhydrite/ 

gypsum sequences of variable thickness.

As a part of ongoing studies by the senior author to 

characterize the depositional environments, sedimentology, 

and diagenesis of interbedded evaporites and carbonate 

rocks in the lower part of the Mississippian St. Louis Limestone, we decided to test whether the St. Louis contains suitable source rock beds. In this report we discuss the 

distribution of organic carbon in two cores from the lower 

part of the St. Louis on the east side of the Illinois Basin and 

relate this distribution to depositional facies and thermal history. We show that the most organic rich strata in the St. 

Louis are associated with both peak transgression (maximum flooding) and restricted lagoons and that the potential 

for hydrocarbon generation is a function of the change in 

thermal history of the St. Louis from south to north in the 

basin. We do not address the source rock potential of other 

Paleozoic carbonate rock formations in the basin.

Dl
D2 EVOLUTION OF SEDIMENTARY BASINS ILLINOIS BASIN

METHODOLOGY

Two cores of the St. Louis Limestone in south-central 

Indiana were interpreted for depositional environments 

(Ridgley, 1992) and were evaluated for their hydrocarbon 

source rock potential. The cores are from Survey Drill Hole 

16 (SDH-16) (sec. 22, T. 3 N., R. 3 W., Martin County) and

Survey Drill Hole 190 (SDH-190) (sec. 25, T. 4 S., R. 1 E., 

Crawford County). Locations of the drillholes are shown in 

figure 1.

Representative chips of medium-gray to black limestone from the cores in the lower part of the St. Louis Limestone were evaluated by Rock-Eval pyrolysis (Espitalie and 

others, 1977). This technique measures the quantity, quality

94° 82°

46°

MINNESOTA

IOWA

42°

NEBRASKA s FOREST CITY 

~~ -*> BASIN

< 

A

"1

I

_KANSAs 

OKLAHOMA

36° I T'MISSOURI

ARKANSAS

100 200 KILOMETERS

_I

50 100 MILES

MICHIGAN

INDIANA j OHIO

Figure 1. Location of the Illinois Basin, study area (dark shaded), and coreholes Survey Drill Hole 16 (SDH-16) and Survey Drill 

Hole 190 (SDH-190), north-central United States.
SOURCE ROCK POTENTIAL OF MISSISSIPIAN ST. LOUIS LIMESTONE D3

(type), and level of thermal maturation of organic matter 

contained in a rock. Rock-Eval pyrolysis alone is by no 

means the only way to evaluate a source rock, but, if used 

with an understanding of the limitations, it can be very useful 

as a first screening tool and can yield a general characterization of the organic matter in a rock.

Both organic-rich-appearing (dark-gray to black) and 

non-organic-rich-appearing (light-gray) rocks were analyzed for total organic carbon (TOC) in drill core from 

SDH-16. Non-organic-rich-appearing rock was analyzed to 

provide a measure of background total organic carbon and to 

assess the effect of facies on the amount of preserved total 

organic carbon. Several organic-rich-appearing horizons in 

drill core from SDH-190 were analyzed for total organic 

carbon. Thin sections of samples from various intervals 

throughout the two coreholes and from samples analyzed by 

Rock-Eval pyrolysis were examined to determine depositional facies and rock fabric.

STRATIGRAPHY OF THE 

ST. LOUIS LIMESTONE

GENERAL GEOLOGY 

OF THE ST. LOUIS LIMESTONE

The St. Louis Limestone is a small part of a thick package of carbonate rocks that filled the Illinois Basin during the 

Mississippian (fig. 2). It was deposited over a broad area of 

the Mid-continent as shallow-shelf carbonates. In the Illinois 

Basin (fig. 1), the St. Louis is now preserved only in the 

southern two-thirds of the basin, due to removal below the 

sub-Pennsylvanian unconformity (Droste and Carpenter, 

1990). Outcrops of the St. Louis are confined to small areas 

on the east, west, and southeast sides of the basin. Maximum 

thickness of the St. Louis ranges from 136 m (450 ft) (Saxby 

and Lamar, 1957) to 166 m (550 ft) (Droste and Carpenter, 

1990) in the structurally deep part of the Illinois Basin in 

southeastern Illinois and southwestern Indiana.

In this report, the St. Louis is informally divided into a 

lower part and an upper part. The lower part is characterized 

by cyclic deposition of beds of grainstone, packstone, 

wackestone, mudstone, dolomite, and evaporite (anhydrite 

and gypsum), and organic-rich beds are interspersed 

throughout. Diagenetic chert "nodules" are sparse to absent 

in the lower part of the St. Louis, and dolomite and evaporite 

beds are locally common. Deposition of the lower part of the 

St. Louis took place in open to restricted shallow-water 

shelf, possibly lagoonal, to supratidal settings. Thickness of 

the lower part of the St. Louis ranges from zero at its erosional edge to 73 m (250 ft) (Saxby and Lamar, 1957; Droste 

and Carpenter, 1990) in the deeper part of the basin.

The upper part of the St. Louis contains diagenetic chert 

"nodules" scattered throughout, few dolomite beds, and no

MISSISSIPPIAN (part)

CHESTERIAN (part)

VALMEYERAN

RIVER BLUE GROUP

SANDERS GROUP

mestone

_i 

"5

s.

Ste. Genevieve Limestone

St. Louisimestone

Downeys Bluff Member

Yankeetown Member

Renault Member

Aux Vases Member

Joppa Member

Karnak Member

Fredonia Member

Upper part

Lower part

Salem Limestone

Harrodsburg Limestone 

(part)

Figure 2. Nomenclature for Valmeyeran and Chesterian Mississippian rocks in the Illinois Basin in Indiana. Modified from Droste 

and Carpenter (1990).

evaporite beds. Grainstone is more abundant than in the 

lower part. Depositional facies reflect open, shallow-water 

shelf environments that were more basinward than those of 

the lower part of the St. Louis.

The St. Louis is overlain by the Ste. Genevieve Limestone and underlain by the Salem Limestone (fig. 2); the contacts have been considered conformable and gradational. 

Placement of the Salem-St. Louis and St. Louis-Ste. Genevieve contacts (and thus the thickness of the St. Louis) varies 

with individual interpretation reflecting the complexity of 

facies relationships between the three formations. Only the
D4 EVOLUTION OF SEDIMENTARY BASINS ILLINOIS BASIN

lower part of the St. Louis was evaluated for its organic carbon content, and it is the focus of the remainder of the paper.

DEPOSITIONAL FACIES IN THE LOWER 

PART OF THE ST. LOUIS LIMESTONE

Detailed examination of core of the lower part of the St. 

Louis in coreholes SDH-16 and SDH-190 has resulted in 

identification of as many as 19 full or partial, short-term 

transgressive-regressive cycles within an overall regression 

(Ridgley, 1992). Cyclic sedimentation reflecting transgressive and regressive events in the lower part of the St. Louis 

has also been discussed by Jorgensen and Carr (1972). Figure 3 shows the lithologic, transgressive, and regressive 

components of an ideal cycle. The cyclic nature of the lower 

part of the St. Louis is demonstrated in a generalized representation of the major rock types and the relative positions of 

the transgressive and regressive parts of each cycle in each 

of the cores (figs. 4, 5). The transgressive and regressive

Anhydrite

Dolomite, with or without 

anhydrite stringers; no fossils

Dolomite, with or without 

ostracodes

Micrite, only ostracodes

Wackestone; more salinity 

tolerant marine fauna

Packstone with grainstone beds; 

more normal marine fauna

Micrite, thin, laminated

Packstone with grainstone beds; 

more normal marine fauna

Wackestone; more salinity 

tolerant marine fauna

Micrite, only ostracodes

Dolomite, with or without 

ostracodes

Dolomite, with or without 

anhydrite stringers; no fossils

Anhydrite

\/

Figure 3. Lithologic components of a complete transgressiveregressive cycle.

parts of each cycle vary in thickness depending, in part, on 

whether a complete cycle is present. In drill core from 

SDH-16, the regressive part of a cycle is generally thicker 

than the transgressive part (see fig. 4) in cycle 1 through 

cycle 8, which marks the end of the lower part of the St. 

Louis. In the lower part of the upper St. Louis, the reverse 

relationship is apparent (fig. 4). In drill core from SDH-190, 

the ratio of the thickness of the transgressive part of a cycle 

to the thickness of the regressive part of a cycle is more variable (fig. 5).

In the two cores examined, the maximum transgressive 

part of a cycle is characterized by thinly laminated micrite or 

fossiliferous limestone that indicates normal-marine salinity. 

The fossils, which are mostly stenohaline, include frondtype bryozoans, echinoderms, and solitary corals. Compared 

to limestone beds in the basal part of the transgressive cycle 

or in the regressive part of a cycle, fossils in the maximum 

transgressive limestone show a high faunal diversity; tests 

are robust. The maximum transgressive limestone is underlain and overlain by limestone, dolomite, or anhydrite (gypsum), depending on whether a complete or partial cycle is 

present (Ridgley, 1992).

Fossiliferous limestone at the base of a transgressive 

cycle or within the regressive part of a cycle was deposited 

nearer to the shore than was the maximum transgressive 

limestone; the contained fossils indicate either normal or 

restricted marine salinity. The regressive part of a cycle commonly is characterized by limestone that is overlain by silty 

dolomite and finally by anhydrite (gypsum) (Ridgley, 1992) 

but, in some cases, may be overlain by anhydrite and dolomite, depending on whether a complete or partial cycle is 

present. Limestone beds in the regressive part of a cycle 

show a decrease in diversity and size of fauna when compared to the maximum transgressive limestone beds. Dolomite beds that are associated with anhydrite beds contain 

either no fossils or only ostrocodes. The absence of fossils or 

the presence of only ostracodes in the dolomite beds indicates much higher salinity conditions during deposition.

The depositional facies interpreted in the cores and as 

expressed in the transgressive and regressive parts of cycles 

are indicative of shallow-water open-shelf (or open-lagoon), 

restricted lagoon, and supratidal environments. The oscillations in depositional environments reflect a change in water 

depth during short-term fourth-order sea-level rises and falls. 

Water depths ranged from zero in supratidal environments to 

just below wave base in the open lagoon or inner shelf during 

maximum flooding.

Figure 4 (facing page). Generalized stratigraphic section of the 

St. Louis Limestone in Survey Drill Hole 16, sec. 16, T. 3 N., R. 3 

W., Martin County, Indiana, showing principal lithologies, positions of transgressive-regressive cycles, and locations of samples 

containing 0.50 percent or greater total organic carbon.
SOURCE ROCK POTENTIAL OF MISSISSIPIAN ST. LOUIS LIMESTONE D5

Core depth 

(meters) (feet)

300 

320 

100 -

110 

120 

130 

140 

150 -H

340 

360 

380 

400 

420 

440 

460

480 

500 

Limestone

Dolomite

Anhydrite/gypsum

EXPLANATION

A Authigenic chert

|^ Dolomite/anhydrite, mixed

F Microfossils

Transgression 

R^ Regression 

~ Cycle boundary and number 

x Organic carbon sample
D6 EVOLUTION OF SEDIMENTARY BASINS ILLINOIS BASIN

Core depth 

(meters) (feet)

Limestone s 

3 

O

to

t

TO 

Q.

0! 

Q. 

Q.

13

part ower 

Salem

Ls.

40 

50 

60 

70 

90 

100 

110 

120 

120 

140 

160 

180 

200 

220 

240 

260 

280 

300 

320 

340 

360 

380 

400 

420 

EXPLANATION

Limestone 

Dolomite

Anhydrite/gypsum 

Limey shale 

Green shale

Dolomite/anhydrite, mixed

Authigenic chert

Microfossils

Breccia

Pisolites

Gypsum/anhydrite nodules

Transgression

Regression

Cycle boundary and numberdashed where approximate

Organic carbon sample

Figure 5. Generalized stratigraphic section of the St. Louis 

Limestone in Survey Drill Hole 

190, sec. 25, T. 4 S., R. 1 E., 

Crawford County, Indiana, showing principal lithologies, positions 

of transgressive-regressive cycles, 

and locations of samples containing 0.50 percent or greater total 

organic carbon
SOURCE ROCK POTENTIAL OF MISSISSIPIAN ST. LOUIS LIMESTONE D7

Table 1. Rock-Eval pyrolysis data for selected samples from Survey Drill Hole 16 (SDH-16) and Survey Drill Hole 190 (SDH-190) in 

the lower part of the Mississippian St. Louis Limestone, Illinois Basin, Indiana.

[Locations of drillholes are shown in figure 1. Tmax, temperature (°C) at which maximum yield of hydrocarbons occurs during pyrolysis of organic 

matter; TOC, total organic carbon (in weight percent); Si, integral of first peak (existing hydrocarbons volatized at 250°C for 5 minutes) (in milligrams 

per gram); 82, integral of second peak (hydrocarbons produced by pyrolysis of solid organic matter (kerogen) between 250°C and 550°C) (in milligrams 

per gram); 83, integral of third peak (CO2 produced by pyrolysis of kerogen between 250°C and 390°C) (in milligrams per gram); PI, production index 

(Si/Si+S2); HI, hydrogen index (S/TOC); OI, oxygen index (S3/TOC). Leaders ( ) indicate data not reliable]

Sample

No.

Depth

(feet) (meters) I max TOC S! S2 S3 PI HI OI

SDH-16

1

2

3

4

5

6

7

8

9

10

11

316.6

337.8

372.6

374.6

405.8

424.6

439.7

439.9

451.0

502.2

516.1

96.5

103.0

113.6

114.2

123.7

129.5

134.0

134.1

137.5

153.1

157.3

 

 

 

 

432

429

427

426

426

0.05

0.26

0.05

0.03

1.05

3.55

3.42

0.31

8.22

0.22

2.74

0.03

0.08

0.01

0.01

0.1

0.37

0.63

0.05

1.42

0.02

0.67

0.08

0.21

0.04

0.12

0.71

4.34

8.78

0.23

24.59

0.07

3.49

0.62

1.01

0.58

0.76

1.23

2.38

1.74

1.88

5.09

0.91

2.61

0.12

0.05

0.07

0.05

0.16

67

206

256

299

127

117

67

50

61

95

SDH-190

12

13

14

15

16

244.0

258.6

381.0

399.1

413.2

74.4

78.8

116.2

121.7

126.0

434

433

429

431

432

0.92

3.53

15.72

2.03

0.71

0.15

0.7

4.24

0.25

0.15

0.88

6.76

40.6

2.37

0.61

0.57

2.91

16.06

1.59

1.2

0.15

0.09

0.09

0.1

0.2

95

191

258

116

85

61

82

102

78

169

SOURCE ROCK POTENTIAL 

ORGANIC MATTER QUANTITY

The quantity of organic matter in samples from the 

lower part of the St. Louis Limestone is illustrated by the 

total organic carbon values (TOC) (table 1). Total organic 

carbon was determined using the Rock-Eval II instrument of 

Delsi, Inc., and is the sum of the carbon in the pyrolyzate 

plus the carbon from the residual oxidized organic matter. 

For this study we assume that a rock must contain a minimum of 0.50 weight percent total organic carbon to be considered a potential hydrocarbon source rock.

In drill core from SDH-16, total organic carbon values 

range from 0.03 to 8.22 percent (table 1). Samples 1, 2, 3,4, 

8, and 10 all have total organic carbon values of less than 

0.50 percent and are not considered potential source rocks. 

These samples are included in this report to provide information on the range of total organic carbon values in the various 

depositional environments in a vertical succession of the St. 

Louis. Too often, many reports discussing source rock 

potential of a formation highlight only the richest samples 

and fail to provide the reader with information on the vertical 

and lateral variability within the formation. Table 2 shows 

the lithology and depositional environment of each of these 

samples. The low total organic carbon values coincide with 

limestone of normal marine salinity that has an uncompacted 

fabric or with dolomite of restricted marine environments.

Samples 5, 6, 7, 9, and 11, however, have high total 

organic carbon values (1.05, 3.55, 3.42, 8.22, and 2.74 percent, respectively) and on the basis of total organic carbon 

are considered good to excellent source rocks. Stratigraphic 

locations of samples having total organic carbon values of 

0.50 percent or greater are shown in figure 4. Table 2 lists 

the lithology and depositional environment of each of these 

samples. The high total organic carbon values are associated 

with limestone of restricted lagoon environments that has a 

compacted rock fabric, with laminated, normal marine limestone indicative of maximum flooding, or with organic-rich 

dolomite of marine environments.

Samples 12, 13, 14, 15, and 16, from drillhole 

SDH-190, all have total organic carbon values of greater 

than 0.50 percent (0.92, 3.53, 15.72, 2.03, and 0.71 percent 

respectively), indicating they are good to excellent source 

rocks if only total organic carbon is considered. Their positions within the measured section are shown in figure 5. 

Table 2 lists the lithology and depositional environment of 

each sample. Again, the high total organic carbon values are 

associated with limestone of restricted lagoon environments 

that has a compacted rock fabric, with laminated, normal 

marine limestone indicative of maximum flooding, or with 

organic-rich dolomite of restricted marine environments.

Samples from drillholes SDH-16 and SDH-190 having 

total organic carbon values of 0.50 weight percent or greater 

(table 2) have either laminated or compacted rock fabrics. 

Laminated rock fabrics are primary and reflect deposition of
D8 EVOLUTION OF SEDIMENTARY BASINS ILLINOIS BASIN

Table 2. Lithology, depositional environment, and total organic carbon content of samples from Survey Drill Hole 16 (SDH-16) and 

and Survey Drill Hole 190 (SDH-190) in the lower part of the St. Louis Limestone, Illinois Basin, Indiana. 

[Locations of drill holes are shown in figure 1. Total organic carbon content from Rock-Eval pyrolysis]

Deoth

(feet) (meters)

Total

organic carbon

(wt. percent)

Lithology Depositional environment

SDH-16

316.6

337.8

372.6

374.6

405.8

424.5

439.7

439.9

451.0

502.2

516.1

96.5

102.9

113.6

114.2

123.7

129.5

134.0

134.1

137.5

153.1

157.3

0.05

0.26

0.05

0.03

1.05

3.55

3.42

0.31

8.22

0.22

2.74

Limestone, open framework

Dolomite, limey, clay-rich

Dolomite

Limestone, fossils, open framework

Limestone, shell debris, flattened, compacted

Limestone, laminated, dark, fissle

Limestone, laminated

Dolomite, limey, few laminae

Dolomite, black, laminated, fissile, minor anhydrite

Limestone, few laminae

Limestone, carbonaceous

Open lagoon.

Inner shelf, restricted marine.

Inner shelf, restricted lagoon to supratidal.

Inner shelf, restricted lagoon.

Inner shelf, restricted lagoon.

Open marine, maximum flooding.

Inner shelf, restricted lagoon.

Inner shelf, restricted lagoon to supratidal.

Inner shelf, restricted lagoon.

Open lagoon.

Open marine, near maximum flooding.

SDH-190

244.0

258.6

381.0

399.1

413.2

74.4

78.8

116.2

121.7

126.0

0.92

3.53

15.72

2.03

0.71

Limestone, laminated, fossils, microstylolites

Limestone, flattened shells, black laminae

Limestone, laminated, microstylolites

Limestone, laminated, burrowed, secondary dolomite

Limestone, wavy laminated, shells flattened and broken

Open marine, near-maximum flooding.

Inner shelf, restricted lagoon.

Inner shelf, restricted lagoon.

Inner shelf, restricted lagoon.

Inner shelf, restricted lagoon.

sediment in quiet water. Compacted rock fabrics are secondary and are due to vertical compression. Rocks showing evidence of compaction contain abundant microstylolites 

(compressed organic matter) and flattened and broken bioclasts (fig. 6). Samples having total organic carbon values of 

less than 0.50 weight percent (table 2) do not have a laminated fabric, although scattered laminae may be present. 

They have an open framework fabric and show no evidence 

of compaction. The correlation of a laminated or compacted 

rock fabric with high amounts of total organic carbon suggests that other strata, in the lower St. Louis, having these 

fabrics may potentially be the best source rocks.

ORGANIC MATTER TYPE

Another measure related to the quantity of pyrolyzable 

organic matter or hydrocarbons is hydrogen index (HI), 

defined as the 82 yield (remaining hydrocarbon-generating 

capacity of the organic matter) normalized by total organic 

carbon; in other words, it is the fraction of the total organic 

carbon that is generated as hydrocarbons during pyrolysis. 

Hydrogen index is also useful in describing the type of 

organic matter present in a rock and the type of hydrocarbons 

that will be ultimately generated. Hydrogen index values (in 

milligrams of hydrocarbons per gram of organic carbon) of 

0-150 yield mostly gas, of 150-300 yield gas and oil, and 

greater than 300 yield mostly oil (Peters, 1986).

The most widely used graphical tool for defining the 

type of organic matter contained in a source rock is the

Figure 6. Limestone representing restricted lagoonal facies in the 

lower part of the St. Louis Limestone at 115.45 m (381.0-381.25 ft) 

depth in Survey Drill Hole 190. Note the compacted nature of the 

rock; dark microstylolites are compacted organic matter. Anhydrite 

nodules (light areas) are early diagenetic and formed from pore fluids supersaturated with respect to calcium sulfate. Length of core is 

7.62 cm (3 in.). Total organic carbon value for this interval is 15.72 

percent.
SOURCE ROCK POTENTIAL OF MISSISSIPIAN ST. LOUIS LIMESTONE D9

modified van Krevelen diagram. This diagram is a plot of the 

hydrogen index and the oxygen index (OI) ($2 and 83 

normalized by total organic carbon respectively), and gives 

an indication of the type of organic matter (types I, II, III, and 

IV) and the degree of thermal evolution. Type I organic matter is hydrogen rich (sapropelic or lipid rich), is present primarily in marine and lacustrine rocks, and generates mainly 

oil during catagenesis. Type II organic matter is generally 

present in marine rocks and generates oil and gas during catagenesis. Type III kerogen is oxygen rich and hydrogen poor 

(huminite and vitrinite), is present mainly in terrestrial, marginal-lacustrine, or marginal-marine rocks, and generates 

mostly gas during catagenesis. Type IV organic matter is 

inert, contains very little hydrogen, and has no hydrocarbongenerating capability. Its path is not shown in figure 7 but 

would plot below that of type III.

Figure 7 illustrates the types of organic matter in the St. 

Louis Limestone samples using a modified van Krevelen 

diagram. Only samples that have total organic carbon values 

greater than 0.50 weight percent were plotted. Samples from 

drill cores SDH-16 and SDH-190 plot in the type II-III area, 

indicating that they are potentially source rocks for both oil 

and gas. A mixture of type II and III organic matter is 

expected for the depositional setting of the lower part of the 

St. Louis, in which semirestricted, nearshore environments 

acted as a sink for accumulation and preservation of both terrestrial and marine organic matter.

There is no apparent correlation between environment 

of deposition of St. Louis samples and the type of organic 

matter as represented by the position of the samples on the 

modified van Krevelen diagram (compare table 2 and figure 

7). One might expect that samples from the more restricted 

lagoonal facies would contain more terrestrial organic matter 

than samples from the maximum normal marine facies; however, this correlation is probably absent because the lateral 

shift in the depositional environments within the inner shelf 

in the study area is nominal.

THERMAL MATURITY

The level of thermal maturity of samples of the St. 

Louis Limestone that have total organic carbon values 

greater than 0.50 weight percent was estimated three ways. 

The first method used the relative position of the points plotted on the modified van Krevelen diagram. In general, plotted samples should migrate toward the origin of the diagram 

(HI=0, OI=0) with increasing maturity. The fact that the St. 

Louis Limestone samples fall well away from the origin suggests that they are relatively immature (fig. 7).

A second thermal maturity indicator used was Tmax, the 

temperature at which maximum yield of hydrocarbons 

occurs during pyrolysis or, in other words, the temperature at 

which the 82 peak occurs. The onset of oil generation (tran1000

900

800

= 700 I

O) 

O O

(ft 

0 O

*« 

0 0

300

200 

100

GAS PRONE

INCREASING f /INCREASING 

MATURITY JJ OIL PRONENESS

OIL AND GAS PRONE

POTENTIAL 

WEATHERING

OR

MINERAL MATRIX 

 . EFFECT

GAS PRONE 

-III

I I

0 20 40 60 80 100 120 140 160 180 200 

OXYGEN INDEX (OI)

Figure 7. Modified van Krevelen diagram of hydrogen and oxygen indexes determined by pyrolysis for samples in Survey Drill 

Hole 16 (solid circles) and Survey Drill Hole 190 (triangles) for 

samples containing 0.50 percent or greater total organic carbon.

sition from immature to mature) is considered to occur at a 

Tmax of 430°C (Humble Laboratory, 1993, based on written 

communication from J. Espitalie, 1982) or 435°C (Tissot and 

Welte, 1984), and the upper limit of oil generation occurs at 

about 460°C (Tissot and Welte, 1984). Peters (1986) suggested that the analytical error of a measured Tmax is commonly from 1° to 3° C. All of the St. Louis Limestone 

samples have Tmax values of less than 435°C, although a few 

have values only l°-2° C lower (table 1). Most samples 

from drill core SDH-190 have Tmax values between 430°C 

and 435°C, and these samples could be within the oil-generation window, depending the classification of thermal maturity used and on analytical error. Samples containing less 

than 0.50 percent total organic carbon have spurious Tmax 

values due to low Si and 82 values; thus, Tmax values for 

these samples are not shown in table 1.

Several variables (such as clay content, type of organic 

matter, and ratio of organic matter to mineral matrix) affect 

the temperature of onset of hydrocarbon generation, and 

therefore that temperature should vary from sample to sample (Peters, 1986). Thus, use of a rigid temperature value of 

435°C to depict the onset of hydrocarbon generation may 

have less validity than would a range of temperatures. 

Regardless of the classification used, Tmax values in the St.
D10 EVOLUTION OF SEDIMENTARY BASINS ILLINOIS BASIN

Table 3. Pyrolysis parameters used to define source rock generative potential. 

[Parameters are defined in table 1]

TOC

Quantity (wt. percent) 1

Poor 0-0.5

Fair 0.5-1

Good 1-2

Excellent 2+

s.

(mg/g) 1

0-0.5

0.5-1

1-2

2+

S2

(mg/g) 1

0-2.5

2.5-5

5-10

10+

S2

(mg/g)2

0-2.2

2.2-5.5

>5.5

>5.5

Si+S2

(mg/g)3

<2

2-6

>6

>6

'Peters (1986).

3Tissot and Welte (1984).

Louis Limestone suggest that the formation is immature to 

marginally mature where sampled.

The third thermal maturity indicator is the production 

index (PI) or transformation ratio. Production index is 

defined as the ratio of volatile hydrocarbon yield to total 

hydrocarbon yield S1 /(S1 + S2). The transition from immature to mature, or beginning of the oil window, is about 

PI=0.10, and the end of the oil window is about PI=0.50 

(Tissot and Welte, 1984). According to production index 

values, most of the samples are immature; however, samples 5, 11, 12, 15, and 16 have values slightly greater than 

0.10, indicating that they are marginally mature (table 1). 

Samples 13 and 14 have a PI=0.09, placing them just below 

the oil-generation window. Production index values for 

samples containing less than 0.50 percent total organic carbon and having low Si and 82 values are not reliable and 

thus are not shown in table 1.

The three maturity indicators used are internally consistent and indicate that the St. Louis Limestone samples are 

immature to marginally mature. Tmax data for the samples 

containing greater than 0.50 percent total organic carbon 

show a slight regional cooling from SDH-190 (average 

value 432°C) to SDH-16 (average value 428°C), consistent 

with the overall reduction of the paleothermal history from 

south to north in the basin. The production indices show a 

slight decrease from SDH-190 to SDH-16, which is also 

consistent with the overall reduction of the paleothermal gradient from south to north in the basin.

These trends are within the general pattern of thermal 

maturity in the basin determined on the basis of vitrinite 

reflectance data for the underlying Upper Devonian and 

Lower Mississippian New Albany Shale and coal-rank data 

for overlying Pennsylvanian coal measures. In the areas of 

the SDH-16 and SDH-190, vitrinite reflectance of the New 

Albany Shale is approximately 0.60 percent (Barrows and 

others, 1979; Barrows and Cluff, 1984). It should be noted 

that vitrinite reflectance values of the New Albany may be 

suppressed and that the corrected values could be as high as 

0.70-0.80 percent. If we extrapolate coal-rank trends of the

Herrin No. 6 coal bed in Illinois (Damberger, 1971) into 

western Indiana, vitrinite reflectance values in the area of the 

SDH-16 and SDH-190 would be in the 0.60-0.70 percent 

range. Thus, the St. Louis Limestone samples have an equivalent vitrinite reflectance (based on Tmax and production 

index values) of approximately 0.60-0.65 percent, values 

consistent with the vertical vitrinite reflectance profile or 

gradient in that part of the Illinois Basin.

PETROLEUM POTENTIAL

Several pyrolysis parameters are commonly used to 

define source rock favorability. These include Si, $2, and 

81+82, which is the genetic potential of Tissot and Welte 

(1984). The categories of source rock favorability for these 

parameters are shown in table 3. If only the Si peak is considered, then samples 7, 9, and 11 from drill core SDH-16 

and samples 13 and 14 from drill core SDH-190 are fair to 

excellent source rocks (tables 1, 3). If only the 82 peak is 

considered, samples 6, 7, 9, and 11 from drill core SDH-16 

and samples 13,14, and 15 from drill core SDH-190 are fair 

to excellent source rocks (tables 1, 3). If only genetic potential (81+82) is considered, samples 6, 7, 9, and 11 from 

SDH-16 and samples 13, 14, and 15 from SDH-190 are fair 

to excellent source rocks.

To date, no oils have been reported as sourced from 

Mississippian rocks in the Illinois Basin. Because biomarker 

information on organic matter in the St. Louis Limestone is 

not available, it is not known whether oil has actually been 

generated from the St. Louis and its signature masked by 

commingling with oils generated from the New Albany 

Shale. On the basis of the limited Rock-Eval pyrolysis data 

presented herein, the St. Louis Limestone could be considered a fair to excellent potential source rock for both oil and 

gas in the Illinois Basin. Our suite of data is fairly limited, so 

we cannot say for a certainty that the St. Louis Limestone has 

similar source rock potential in other parts of the basin; however, paleoreeonstructions in progress by the senior author
SOURCE ROCK POTENTIAL OF MISSISSIPIAN ST. LOUIS LIMESTONE Dll

indicate that favorable facies may be present in the deeper 

part of the basin. The distribution of organic-rich beds 

should coincide with the occurrence of evaporite beds or 

rocks of restricted marine origin because organic matter has 

the greatest potential for preservation in these depositional 

environments.

The samples analyzed are immature to marginally 

mature because the St. Louis on the eastern side of the Illinois Basin was never buried under significantly more overburden (303 m [1,000 ft] or less) (Cluff and Byrnes, 1990) 

than it is today. It is likely that, in the deeper parts of the 

basin, with additional depth of burial, or in areas with a 

higher paleothermal gradient, the St. Louis Limestone will 

be more thermally mature and hence further into the hydrocarbon-generation window.

Organic-rich strata are present in beds that range in 

thickness from a few centimeters to as much as a meter. 

These strata are thinner where they coincide with the maximum flooding surface and thickest in restricted lagoonal 

environments. Although individual beds are not unusually 

thick, the combined thickness of organic-rich strata in all the 

cycles represents a few percent to as much as 15-20 percent 

of the lower part of the St. Louis. Thus, the cumulative 

thickness of favorable source rock beds suggests that further 

study of the source rock potential of the St. Louis is 

warranted.

SUMMARY

The data presented herein suggest that the lower part of 

the Mississippian St. Louis Limestone could be a source of 

hydrocarbons in the Illinois Basin; however, additional studies of the source rock potential are needed. Such studies 

would include characterization of the organic matter in the 

St. Louis and the application of sequence stratigraphic concepts to paleoreconstruction of the lower part of the St. Louis 

to delineate favorable source rock facies. Source rock facies 

containing high amounts of preserved total organic carbon 

should have a laminated or compacted rock fabric; they 

should coincide either with maximum flooding during peak 

transgression or with restricted marine environments that are 

associated with evaporites. Potential for actual hydrocarbon 

generation from these rocks is greatest in the thermally 

mature parts of the Illinois Basin such as the deeper parts of 

the structural basin or the area around Hicks dome, which is 

an area of probable high heat flow in the southern part of the 

basin (fig. 1). Local oil-bearing reservoirs in the St. Louis 

and overlying Ste. Genevieve Limestones in proximity to 

organic-rich facies in the lower part of the St. Louis should 

be evaluated to see if the oils were sourced (at least in part) 

from the St. Louis. Evaporites in the St. Louis, although not 

very thick, could have served as local seals.

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D12 EVOLUTION OF SEDIMENTARY BASINS ILLINOIS BASIN

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Published in the Central Region, Denver, Colorado 

Manuscript approved for publication April 27,1995 

Edited by Judith Stoeser 

Graphics and photocomposition by Joan Nadeau

*U.S. GOVERNMENT PRINTING OFFICE: 1995-673-046/86125