Petrology of the 

Eagle Sandstone, 

Bearpaw Mountains Area, 

North-Central Montana 

By DONALD L. GAUTIER 

INVESTIGATIONS OF SHALLOW GAS RESERVOIRS IN THE NORTHERN GREAT PLAINS 

GEOLOGICAL SURVEY BULLETIN 1521 

Prepared in cooperation with the 

U. S. Depart1Tl£nt of Energy 

Composition and burial history 

of an important conventional 

shallow methane reservoir in 

the northern Great Plains 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1981 
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary 

GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of Congress Cataloging in Publication Data . 

Gautier, Donald L. 

Petrology of the Eagle Sandstone, Bearpaw Mountains area, north-central Montana. 

(Investigations of shallow gas reservoirs in the northern Great Plains) 

(Geological Survey Bulletin 1521) 

Bibliography: p. 52 

1. Sandstone-Montana-Bearpaw Mountains region. 2. Gas, Natural-GeologyMontana-Bearpaw Mountains region. I. Title. II. Series. III. Series: 

Geological Survey Bulletin 1521. 

QE75.B9 no. 1521 [QE471.15.S25] 557.3s 81-607963 [552'.5] AACR2 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 
CONTENTS 

Page 

Absuact:---------------------------------------------------- 1 

In~uction------------------------------------------------- 2 

~~un~--------------------------------------------- 4 

Acknowledgments-------------------------------------- 5 

Geologic Setting----------------------------------- 6 

Hydrocarbons in the Eagle Sandstone------------------------ 8 

Lithology---------------------------------------- 9 

~udstones------------------------------------------------ 9 

Sandstonefrmnework---------------------------------------- 13 

QuwU------------------------------------------- 13 

Feldspars---------------------------------------- 17 

Lithic fragments:------------------------------ 23 

Dolomite grains:---------------------------------- 26 

Common varietal constituents:---------------------- 27 

Heavyminerrus------------------------------ 28 

Sources of the Eagle Sandstonte-------------------------- 28 

Diagenesis.---------------------------------------- 31 

Calcite------------------------------------------ 31 

Iron carbonatte----------------------------------------- 41 

Inuagranular porosity·-------------------------------- 46 

Authigenic phyllosilicates:--------------------------- 47 

Summary of burial history 48 

Conclusions 50 

References cited 52 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map of Bearpaw ~ountains study area, ~ontana, showing wellcore localities 3 

2. Diagrmn showing lithologies and electric log response for the 30-1 Anderson well 7 

3. Triangular diagrmn illusuating typical quwU, feldspar, and rockfragment proportions of the Eagle Sandston 12 

4-6. Diagrmns showing: 

4. Proportional variations in quwU, feldspar, and rock-fragment 

content with depth in the 30-1 Anderson co 14 

5. ~ean frmnework grain size distribution as a function of depth in 

the 30-1 Anderson cor 18 

III 
IV CONTENTS 

Page 

FIGURE 6. Framework, matrix, calcite cement, and pore space (FMCP) distribution as a function of depth in the 30-1 Anderson core-- 19 

7-8. Photomicrographs showing: 

7. Detrital constituents of the Eagle Sandston 22 

8. Dissolution and calcite replacement features of the Eagle Sandstone 37 

9. Diagram showing estimated framework, matrix, cement, and pore 

space distribution based upon minimum volume of calcite present 

prior to dissolution 39 

10. Photomicrographs showing noncalcitic authigenic minerals in the 

Eagle Sandston 44 

TABLES 

Page 

TABLE 1. Locations and descriptions of sampled cores from the Bearpaw Mountains area, Montana 4 

2. Clay mineral data for selected samples of the Eagle Sandston 11 

3. Petrographic data for selected thin sections of the Eagle Sandstone 

from the Bearpaw Mountains, Montana 15 

4. Point-count data used in calculating FMCP diagram for 30-1 Anderson co 20 

5. Quartz varieties determined by separate point counts of selected thin 

sections from the Bearpaw Mountains, Montana 21 

6. Potassium feldspar content in 10 selected thin sections of the 30-1 

Anderson co 24 

7. Rock-fragment varieties counted in 10 selected core samples of the 

Eagle Sandstone from the Bearpaw Mountains, Montana 25 

8. Summary of Eagle Sandstone burial history 32 

9. Carbon isotope data for selected samples of Eagle Sandstone carbonates 34 
INVESTIGATIONS OF SHALLOW GAS RESERVOIRS IN THE 

NORTHERN GREAT PLAINS 

PETROLOGY OF THE EAGLE SANDSTONE, 

BEARPAW MOUNTAINS AREA, NORTHCENTRAL MONTANA 

By DONALD L. GAUTIER 

ABSTRACT 

The Eagle Sandstone, in the Bearpaw Mountains area of Montana, is an important 

conventional reservoir for shallow gas. It represents shoreline sedimentation during a 

major regression of the Late Cretaceous sea. Hydrocarbons in the Eagle Sandstone 

mainly are isotopically light methane, which was probably generated in associated 

source rocks early in their burial history. 

The formation is composed of sandstone and lesser amounts of mudstone. The 

mudstones are of several varieties and contain large amounts of highly expansible 

mixed-layer clay and coarser grained minerals similar to those of the associated sandstones. Quartz of diverse origins makes up 30 to 50 percent of the sandstone; intermediate plagioclase (An30-An40), as much as 25 percent; K-feldspars, 5 to 10 percent; sedimentary, metasedimentary, and volcanic rock fragments, 10 to 30 percent; 

and transported dolomite grains, 5 to 10 percent. Glauconite is common, as are small 

amounts of biotite, muscovite, and detrital chlorite. Accessory heavy minerals are 

mainly apatite, zircon, garnet, tourmaline, and opaque grains. The source area for the 

Eagle Sandstone lay to the west and was a mixed terrane of Proterozoic Belt 

Supergroup metasediments, Paleozoic sedimentary rocks, and volcanic rocks of probable Cretaceous age. 

Clay mineralogy, carbon isotopes in gases, vitrinite reflectance, and stratigraphic 

reconstruction indicate the rocks have never been buried deeper than about 1,400 m 

and have not undergone thermal conditions sufficient for oil generation. Diagenesis 

was important in determining the reservoir properties of the Eagle Sandstone. 

Localized calcite precipitation began shortly after deposition. Compaction and minor 

quartz cementation occurred as burial depths increased. Then, extensive precipitation 

of sparry calcite tightly sealed intergranular pores and selectively replaced 

plagioclase. This calcite contains isotopically light (0'3{;= -7.5 permil) carbon. 

Siderite developed locally, usually because of increased iron activities associated with 

altering biotite. Dissolution of calcite cement and especially dissolution of calcite 

replacements of plagioclase significantly enhanced reservoir porosity. These processes provided space for precipitation of authigenic clays. The present distribution of 

calcite-cemented layers limits vertical permeability and compartmentalizes the reser1 
2 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

voirs. Early-formed carbonates may have acted as gas-trapping seals, particularly in 

lower permeability reservoirs to the east. Fluid-sensitive authigenic days and iron 

carbonates may cause formation damage if not taken into account during the development of gas recovery methods. The Eagle Sandstone displays many diagenetic 

features similar to those of rocks that have undergone thermal maturation sufficient 

for the generation of oil or gas. It is concluded that thermal maturation of organic 

matter is not required for the development of significant secondary porosity, nor for 

the occurrence of a complex diagenetic history. 

INTRODUCTION 

The Bearpaw Mountains, located in north-central Montana, constitute a complex physiographic feature, which includes a central northeast-trending composite anticlinal uplift (the Bearpaw Mountains 

arch) flanked by volcanic rocks of Eocene age (fig. 1). Numerous shallow intrusives occur in the area, and pre-volcanic sedimentary rocks 

are present in downfaulted blocks within and adjacent to the volcanic 

fields. The Eagle Sandstone in this vicinity is an important conventional reservoir for natural gas. Low-permeability facies of the Eagle 

Sandstone in Canada, which are included in the Milk River Formation, are prolific producers of natural gas. Similar rocks in eastern 

Montana are potentially important low-permeability reservoirs. 

Stratigraphic studies of the Eagle in the area (Rice, 1980), isotopic investigations of gases from the Eagle Sandstone at Tiger Ridge field 

in the Bearpaw Mountains (Rice, 1975), vitrinite reflectance of coal 

by N. H. Bostick (written commun., 1979), and clay mineral data reported herein all indicate that the Eagle Sandstone has never been 

subjected to burial depths great enough to produce the degree of thermal maturation of source rocks required for the generation of liquid 

hydrocarbons (oil). Burial depth has also been insufficient to cause 

major phyllosilicate transformations due to burial diagenesis. Nevertheless, postdepositional chemical and biological processes have 

played a significant role in developing both the reservoir properties of 

the Eagle Sandstone and the methane-rich natural gas entrapped 

within it. 

This .report summarizes the results of a petrologic examination of 

four cores from productive gas wells in the Eagle Sandstone of the 

Bearpaw Mountains area The purposes of this paper are to (1) describe this important conventional shallow methane reservoir, (2) develop a framework upon which to base related petrologic studies of 

low-permeability gas reservoirs to the east, and (3) discuss the extent 
INTRODUCTION 

30' 

MISSOURI 

Cleveland 

• 

3 

T. 32 N. 

T. 28 N. 

T. 24 N. 

~R~. ~E~ . ._.__._.~~--------~~--------~--_.--~T.22N. R. 20 E. 

30 KILOMETERS 

EXPLANATION 

Q Limit of volcanic outcrops 

~ Well-core localities 

FIGURE 1.-Index map of Bearpaw Mountains study area, Montana, showing wellcore localities. 
4 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

to which postdepositional processes have controlled the reservoir 

properties of a sandstone that has not been subjected to significant 

burial depths. 

PROCEDURE 

Samples were collected from five cores of producing wells in the 

Eagle Sandstone. Specimens from four of these cores are considered 

in detail in this paper; their names and locations are listed in table 1. 

The fifth core (High Crest Oils No. 12-1 Blackwood) is only discussed 

briefly, as it displays somewhat anomalous petrologic properties, 

some of which have been described previously (Gautier, 1979). 

Two thin sections were made from each of 170 samples. One of 

these was impregnated with a clear plastic resin and stained for 

plagioclase and potassium feldspar, and the duplicate was impregnated with a blue-dyed epoxy to facilitate observations of porosity 

distribution. One half of each color-impregnated slide was stained 

with Alizarin Red, for the determination of CaCOa. Petrographic 

point counts of mineral composition were made for most pairs 

(500 points on each thin section), and detailed petrographic examinations of these samples were used to determine textural and 

paragenetic relationships. Heavy mineral separates from 10 sandstones were examined petrographically. Approximately 40 samples 

of sandstone and mudstones were subjected to bulk X-ray diffraction 

TABLE I.-Locations and descriptions of sampled cores of Eagle Sandstone from the 

Bearpaw Mountains area, Montana 

Cored Producing 

Well name Location interval interval 

(meters) (meters) 

High Crest Oils NE 1/4 SW 1/4 sec. 30, 420-446 Most of 

No. 30-1 Anderson. T. 31 N., R. 16 E. core. 

High Crest Oils SE 1/4 NW 1/4 sec. 29, 511-557 490-546 

No. 29-3X Olson. T. 31 N.' R. 17 E. 

High Crest Oils NW 1/4 SE 1/4 sec. 6, 393-402 390-399 

No. 1 Morrison. T. 31 N.' R. 18 E. 

High Crest Oils NE 1/4 NW 1/4 sec. 12, 383-418 383-390 

No. 12-1 Blackwood. T. 30 N., R. 17 E. 

Fuel co No. 1 Federal SW 1/4 NW 1/4 sec. 26, 424-429 432-434 

26-24-18-B. T. 24 N., R. 18 E. 429-436 
INTRODUCTION 5 

analysis, and clay-size (< 4J.Lm) separates were made from each of 

these. Clay mineral identifications were made from diffractograms of 

oriented material mounted on porous unglazed porcelain tiles. Each 

oriented mount was scanned from 2° to 38° two-theta after each of 

four treatments: (1) air-dried but otherwise untreated, (2) solvated 

with ethylene glycol vapor, (3) heated to 400°C, and (4) heated to 

550°C. The approximate abundances of various clays were estimated 

according to the method of Schultz (1964, 1978) and reported in percent. However, these values are included for comparative purposes 

only. Other specific mineral identifications also employed X-ray diffraction. Detailed observations of pore textures and paragenesis for 

40 samples of sandstone were made with a scanning electron microscope (SEM). Stable carbon isotope content is reported for ten authigenic carbonates from sandstones: two of siderite cement, seven of 

calcite cement, and one from a calcite concretion. The determinations 

were made from C02 generated by the digestion of the carbonates 

with hydrochloric or phosphoric acid and analyzed with a NeirMcKinney double collecting mass spectrometer. Grain-size analyses 

of coarse silt and sand fractions were performed on an !MANCO electronic image analyzer1

, while the relative abundance of allogenic 

clays in mudstones was estimated by means of petrographic point 

counts. 

This investigation focused upon sandstone lithologies, as the dominant interest of the study was the gas-bearing reservoirs of the Bearpaw Mountains. However, mudstone lithologies are also briefly described, with particular emphasis on their clay-mineral content. Most 

of the diagrams and many of the photomicrographs included in this 

report refer to the 30-1 Anderson well, located in sec. 30, T. 31 N., R. 

16 E. (fig. 1). This emphasis is for illustrative purposes only, as most 

samples from all the cores display similar petrologic properties. 

Where significant differences exist, they are noted in the text and illustrated appropriately. 

ACKNOWLEDGMENTS 

Appreciation is expressed to personnel of the U.S. Geological 

Survey who provided technical assistance during the course of this 

investigation. Stratigraphic information, gas isotope data, and many 

helpful discussions were provided by Dudley D. Rice. Access to the 

mass spectrometer was obtained by George E. Claypool. Charles N. 

1 Use of brand names in this report is for descriptive purpose only and does not constitute endorsement by the 

U.S. Geological Survey. 
6 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

Threlkeld .prepared carbonate samples and analyzed them using the 

mass spectrometer. Pamela J. Ellerman made sample preparations 

and operated the X-ray diffractometer. Thin sections were prepared 

by Kenneth L. Gardner. Michael B. Sawyer provided access to the 

image analyzer and assisted in its operation. 

GEOLOGIC SETTING 

The Upper Cretaceous rocks of the northern Great Plains consist of 

eastward-thinning wedges of regressive sediments, which intertongue with westward-thinning transgressive deposits. The Eagle 

Sandstone in the vicinity of the Bearpaw Mountains represents 

shoreline sedimentation which culminated the Telegraph Creek-Eagle 

regression, the second of four major Late Cretaceous regressions recognized in the northern Great Plains. This retreat of the epicontinental sea began as early as 85 m.y. ago during the time of the ammonite 

Scaphites depressus (Gill and Cobban, 1973, p. 20). The regression 

was accompanied, in west-central Montana, by the formation of the 

lower portion of the Elkhorn Mountains Volcanics. In north-central 

Montana, this episode is represented by the upward tr~sition of the 

Niobrara Formation into the Telegraph Creek Formation, which, in 

turn, is conformably overlain by shoreline deposits of the Eagle Sandstone. 

In the Bearpaw Mountains area, the Eagle Sandstone was probably deposited under conditions similar to those described from subsurface and outcrop studies to the south by Rice (1980). It generally 

consists of three members (fig. 2): (1) The basal Virgelle Sandstone 

Member, poorly developed in boreholes of this study, which is dominated by sandstone deposited along a prograding coastal, interdeltaic 

shoreline; (2) a regionally variable, unnamed middle member, consisting of sandstones, siltstones, mudstones, shales, and minor impure lignite deposited in a coastal plain and deltaic setting; and (3) a 

disconformably overlying, unnamed upper member containing sandstone, siltstone, and shale primarily accumulated on a tidal flat or 

beachface. 

Deposition of the Eagle Sandstone was terminated abruptly by the 

rapid advance of the Claggett sea. This transgression began with 

subsidence and explosive volcanism, recorded by the Ardmore Bentonite Beds, and corresponds with the lowest part of the faunal zone 
GEOLOGIC SETTING 

High Crest Oils 

Anderson No. 30-1 

Sec. 30; T. 31 N., R. 16 E. 

~ r------------...... sp. res. 

7 

CLAGGETT SHALE METERS 

0 

UJ 

z 

0 

1-

UPPER MEMBER 

15 

b MIDDLE MEMBER .A-.~ .... -·~ z 

<( 

U") 

UJ 

_J 

(.9 

<( 

UJ 

VIRGELLE SANDSTONE 

MEMBER 

TELEGRAPH CREEK 

FORMATION 

EXPLANATION 

Q Sandstone 

~ 

~ Argillaceous sandstone 

I::>~<:J Shale 

- Calcite cemented 

interval 

FIGURE 2.-Lithologies and resistivity (res.) and spontaneous potential (s.p.) log 

response for the 30-1 Anderson well. 
8 EAGLE SANDSTONE,BEARPAWMOUNTAINSAREA,MONTANA 

of Baculites obtusus (Gill and Cobban, 1973, p. 20), approximately 

dated at 79 m.y. (Obradovich and Cobban, 1975, p. 36). In many 

places, the uppermost portion of the Eagle Sandstone and the lowermost part of the overlying Claggett Shale contain conglomeratic 

laminations of chert pebbles, possibly representing a lag deposit 

associated with this transgression. 

In the vicinity of the Bearpaw Mountains, the Eagle Sandstone 

was subsequently buried beneath marine and continental rocks of 

Late Cretaceous and early Tertiary age. These sediments, consisting 

of the· Claggett Shale, the Judith River Formation, the Bearpaw 

Shale, the Fox Hills Sandstone, the Hell Creek Formation, and the 

Fort Union Formation, accumulated to thicknesses that probably average 1,220 to 1,370 m (D. D. Rice, oral commun., 1979) and do not 

exceed a maximum of 1,650 min any location (Hearn, 1976). 

The onset of intrusion, uplift, and volcanism in late early Eocene or 

middle Eocene time terminated deposition in the Bearpaw Mountains 

area and initiated the major features of the modern Bearpaw Mountains anticlinal structure. Recent work by Hearn (1976), in general 

agreement with Maher (1969), has vindicated the interpretation made 

long ago by Reeves (1924, 1946) that this complex structure resulted 

from gravity sliding of sedimentary rocks, including the Eagle Sandstone, and overlying volcanics along gently dipping planes in the 

Colorado Shale. This sliding apparently occurred during or between 

episodes of volcanism, because older extrusives are involved in the 

structure to a much greater extent than younger ones, which may 

overlie both tilted sedimentary rocks and older volcanic rocks. 

HYDROCARBONS IN THE EAGLE SANDSTONE 

Large quantities of natural gas are present at shallow depths in the 

Bearpaw Mountains area. Schorning (1972, p. 153) estimated the 

proven reserves in the northern part of the field alone to be about 

20.8 billion cubic meters. This gas occurs as accumulations entrapped in the heads of gravity-slide blocks on the flanks of the Bearpaw 

Mountains anticlinal structure. Chemical and isotopic studieg by 

Rice (1975) indicate that the gas is composed of more than 99 percent 

methane enriched in the light isotope C12, with o13C values ranging between -66 and -70 permil, relative to the PDB Chicago standard. 

These values strongly suggest that the gas was formed through the 

reduction of C02 or HC03- by methane-generating bacteria (Rice, 

1975) under conditions analogous to those outlined by Claypool and 

Kaplan (197 4) for marine sediments examined during the Deep Sea 

Drilling Project. 
LITHOLOGY 9 

If the natural gas in the Bearpaw Mountains area was generated by 

a process similar to that observed in recent marine sediments, the 

methane must have formed early in the burial history of the rocks. 

Gas was probably produced prior to complete expulsion of interstitial 

water, by anaerobic bacteria. The bacteria acted upon the abundant 

organic matter contained in sediments associated with the Eagle 

Sandstone, such as the Niobrara Formation or the Claggett Shale. 

Progressive compaction injected both methane and interstitial bicarbonate-rich waters into the sandstone. Coals within the Eagle Sandstone, in this area, are volumetrically insignificant and could not 

have served as an important source of organic material for this 

process. 

LITHOLOGY 

The Eagle Sandstone may be divided into two broad lithologic 

categories. Most of the formation is a medium-gray (N5), well-sorted, 

angular, fine-grained sandstone. This lithology is generally highly 

porous (25 percent) and permeable (100-300 md), and it forms conventional gas reservoirs of the type now being explored for in the 

Bearpaw Mountains area. The second general lithologic category 

used in this report is termed mudstone. Characteristic lithologies and 

associated electric log responses for the 30-1 Anderson well are illustrated in figure 2. 

MUDSTONES 

Mudstones, as used in this report, comprise a diverse assemblage 

of lithologic types, including shales, siltstones, argillaceous sandstones, and rare thin coals. These sediments were probably deposited 

under a variety of conditions and occur in both nearshore marine and 

continental sequences. 

True shales, or silty shales, display strong phyllosilicate orientation parallel to bedding, which is commonly defined by horizontally 

arranged platelets and irregular masses of organic material. Strongly 

laminated rocks, consisting of intercalated finer and coarser lithologies, may be present and undisturbed. However, it is more common 

to observe disruptions in the bedding and mixing of lithologic types 

due to bioturbation. The effects of burrowing organisms are variable 

in intensity. These range from small sand-filled burrows within shaly 

sequences and clay-filled burrows in sandy units to completely 

mixed, unsorted rocks, which display no hint of their original orienta-
10 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

tion or bedding. Body fossils are essentially absent from the samples 

examined in this investigation, although both marine and nonmarine 

fossils have been reported from the Eagle Sandstone in the study 

area 

Silt- and sand-sized constituents of these mudstones are composed 

of materials similar to those that compose the reservoir sandstones 

with which they are associated. They differ only in their finer grain 

size and the resulting lower proportion of rock fragments, and in their 

association with abundant allogenic clay. Postdepositional, effects 

observable in thin section are limited to pyrite, often associated with 

organic material, dissolution features in silicate fragments, and small 

patches of iron-rich carbonate scattered about the rock. SEM studies 

reveal similar features, except that flamelike phyllosilicates are commonly observed upon allogenic clays. These are probably authigenic. 

Calcite concretions are occasionally seen. 

The clay-size fraction (< 4 p.m) from marine and nonmarine mudstones of the Eagle Sandstone displays a consistent mineralogical 

composition (table 2). It contains some quartz and feldspar but otherwise consists almost entirely of phyllosilicates. In mudstones, this 

assemblage is dominated by mixed-layer illite-smectite with lesser 

amounts of clay-size mica, kaolinite, and chlorite of undetermined 

composition. The mixed-layer mineral is randomly ordered and contains from 10 to 30 percent illite-like layers and 70 to 90 percent 

smectitic layers. This composition is similar to that reported by 

Schultz and others (1980) for clay minerals of continental and marine 

rocks of the Pierre Shale and its equivalents, which include, among 

others, the Eagle Sandstone and its westward continental equivalent, 

the Two Medicine Formation. They interpret most of the highly expansible mixed-layer illite-smectite as being volcanogenic. They further suggest that the similarity between marine and nonmarine 

clays, also observed in the present study, is probably indicative of a 

detrital origin for most of these minerals. I concur with these views. 

Convincing investigations of depth-related mineral transformations within mixed-layer illite-smectites in pelitic sediments of the 

Gulf Coast have been conducted by Powers (1959), Burst (1959), 

Weaver (1960), Perry and Hower (1970), Hower and others (1976), 

Aronson and Hower (1976), and many others. This work has 

demonstrated that, in the Gulf Coast area, highly expansible mixedlayer minerals undergo a temperature-controlled burial diagenesis 

which converts smectitic layers to illite-like layers at depths between 

1,525 m and approximately 4,575 m. Mixed-layer clays containing 

80 percent or more expanding layers were found to persist to depths 

of about 1,525 m. Beyond this, they become more illitic, until at 

depths of about 4,575 m no more than 20 percent expansible layers 
LITHOLOGY 11 

TABLE 2.-Clay mineral data for selected samples of Eagle Sandstone 

[Data in percent] 

Mixed-laler clal 

Depth Lithology Kao- Mica Chlo- Abun- Percent 1 (m) linite rite dance expansible 

30-1 Anderson Core 

420.3 Sandstone---- 68 20 2 10 N 

420.3 Sandy marine 

shale------ 28 19 4 49 70 

420.6 Sandstone---- 68 10 1 21 80 

421.2 ----do------- 66 10 1 23 N 

422.1 ----do------- 59 11 2 28 80 

425.2 Nonmarine 

mudstone--- 10 12 3 75 80 

426.4 ----do------- 20 20 3 57 90 

426.4 ----do------- 13 26 5 56 70 

426.4 ----do------- 32 21 2 45 70 

427.3 ----do------- 19 30 4 47 80 

427.9 ----do------- 13 15 3 69 90 

428.5 ----do------- 36 18 2 44 80 

429.2 ----do------- 13 22 3 62 70 

430.7 Sandstone---- 87 3 2 8 70 

431.3 ----do------- 88 6 0 6 N 

431.9 ----do------- 82 9 1 8 80 

438.0 ----do------- 86 5 2 7 N 

443.5 ----do------- 79 8 3 10 N 

445.3 ----do------- 85 6 2 7 N 

!-Morrison Core 

393.2 Sandstone---- 85 6 2 7 N 

399.6 ----do------- 67 13 1 19 N 

401.4 ----do------- 75 12 3 10 N 

29-3X Olson Core 

510.8 Marine 

mudstone--- 38 40 10 12 80 

515.1 Sandstone---- 70 24 1 5 N 

518.2 Argillaceous 

sandstone. 52 22 2 24 80 

524.6 Shale-------- 18 21 3 58 80 

527.6 Sandstone---- 75 7 1 17 N 

534.0 ----do------- 85 6 8 N 

541.3 ----do------- 82 12 5 N 

546.2 ----do------- 74 8 17 70 

550.2 Argillaceous 

sandstone. 61 11 27 85 

554.7 Marine(?) 

mudstone :!.4 24 3 49 85 

N=No determination. 
12 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

remain. A similar process has been proposed to explain the presence, 

within rocks of the Montana disturbed belt, of mixed-layer clays that 

display a regular alteration of illitic and smectitic layers (IS ordering) 

and contain less than 40 percent expanding layers (Hower and Hall, 

1970; Hoffman, 1976; Hoffman and others, 1976; Hoffman and 

Hower, 1979), because Cretaceous argillaceous sediments in this disturbed belt have been buried to great depths under thrust sheets. 

However, work by Schultz (1978) seems to indicate that burial diagenesis will not fully explain mineralogical variation within mixedlayer minerals of the Pierre Shale in other areas of the northern Great 

Plains. 

While a large percentage of illite-like layers within a mixed-layer 

clay may not demonstrate that burial diagenesis has occurred 

(Schultz, 1978), it is likely that a predominant content (> 80 percent) 

of expansible layers indicates that such alterations have not developed and that temperatures required for the thermocatylitic generation of oil from organic matter have never been attained (Burst, 

1959). The preponderance of expansible layers within mixed-layer 

QUARTZ 

FELDSPAR ROCK FRAGMENTS 

FIGURE 3.-Typical quartz, feldspar, and rock-fragment proportions of the Eagle 

Sandstone. 
LITHOLOGY 13 

clays of marine and nonmarine mudstones is considered strong evidence that no significant temperature-controlled clay-mineral transformations have occurred within the Eagle Sandstone in the vicinity 

of the Bearpaw Mountains. This interpretation is consistent with 

reconstructed burial depths and with vitrinite reflectance values in 

oil (R0) of 0.4 for coal from the 30-1 Anderson well (N. H. Bostick, 

written commun., 1979), which are well below those required for oil 

generation (Bostick, 1979, p. 28-29). All of these factors are consistent with the biogenic origin of the methane-rich gas at Tiger Ridge 

Field. 

SANDSTONE FRAMEWORK 

The majority of the Eagle Sandstone is composed of sandstone containing little or no allogenic clay. Although Rice (1980) has demonstrated that these rocks may have accumulated in a variety of environments, the petrologic consistency of their framework grain composition, grain size, and post-depositional history clearly warrants that 

they be considered as a group. This section describes detrital components of the sandstones, including quartz, feldspars, lithic 

fragments, varietal constituents, and accessory heavy minerals. 

Framework grain compositions, excluding dolomite, for typical sandstones are illustrated in figure 3; petrographic data are listed for 

many typical sandstones in table 3; variations of composition as a 

function of depth and lithology are shown in figure 4; typical grainsize distributions for silt and sand components of the 30-1 Anderson 

core are displayed in figure 5; and relative abundances of framework 

grains, allogenic clay, carbonate cement, and pore space are shown in 

figure 6 and listed in table 4. 

QUARTZ 

Quartz grains are generally the most abundant single component of 

the sandstone framework, typically constituting 30 to 50 percent of 

the rock. These grains are dominantly monocrystalline (table 5). Most 

of the monocrystalline grains display straight or slightly undulatory 

extinction, irregular grain boundaries, and inclusions, some of which 

are organized in "bubble trains." Some (10-20 percent) irregular 

quartz fragments show strongly undulatory extinction, numerous inclusions, and may have a "stretched" appearance. At least some of 

these fragments can be attributed to a metamorphic source. Virtually 

every sample contains a small quantity of quartz grains interpreted 

to be of volcanic origin. These grains commonly display the bipyramidal habit of beta quartz and always occur as euhedral, rounded 

euhedral, or fragmented euhedral crystals with absolutely straight 
14 EAGLESANDSTONE,BEARPAWMOUNTAINSAREA,MONTANA 

en 

cc 

w 

1-

w 

~ 

425 

~430 

(!) 

z 

J: 

en 

~ 

ID 

>- ..J 

..J 

w 

~ 

~435 

0 

..J 

w 

ID 

~ a.. 

w 

0 

440 

445 

0 

QUARTZ 

20 

CUMULATIVE PERCENT 

100 

FIGURE 4.-Proportional variations in quartz, feldspar, and rock-fragment content 

with depth in the 30-1 Anderson core. 

extinction and very few inclusions other than rare isotropic glass 

(fig. 7A). Many of these volcanogenic fragments have resorbed 

boundaries and may be attached to remnants of isotropic or slighty 

birefringent groundmass material. These grains were once phenocrysts in an extrusive or shallow intrusive igneous rock of siliceous 
LITHOLOGY 15 

TABLE 3.-Petrographic data for selected thin sections of the Eagle Sandstone from 

the Bearpaw Mountains, Montana 

[Abundances in percent, N, not determined] 

I Fe1dRpar Ul >. "0 Ul 0 ..... Ql I'd N 

::l ..... •.-I 0 ..... ..-1 ..... ,.-.~,...., 0 I'd Ql I'd u 1-1 

Depth u ..... Ql Ql 1-1 a 0 >< I'd,-... 

>< 1-1 Ul 1-1 a Ql bO Ql 0 u •.-I g.! (m) Ql Ql I'd I'd bO ..... Ql u .0 •.-I 1-1 '-",C ..-1 p.. I'd •.-I ..... Ql 1-1 0 ..... u u Ul 1-1 Ql 0 •.-I ..... Ql I'd Ql I'd Ql 

N 0 "0 Il-l ..... 0 :> •.-I ..... u bO a bOQl 

..... bO ..... ·.-I ..-1 ..-1 ·.-I u 0 a •.-I •.-I ..-1 I'd N 

1-1 0 1-1 bO Ql I'd .!<1 ..... ::l u 0 u 0 .c >. I'd 1-1 •.-1 

l'd•.-1 Ql I'd Il-l ..... u 0 I'd Ul ..-1 ..-1 0 ..... I'd ..... Ql Ul 

::l .c ..-1 I 0 ~ •.-I ..-1 ::l 0 I'd 1-1 ~ ..-1 0 0' u p... ::.:: E-< r:t:l c..? ::>:: 0 u H u E-< ~ 

30-1 Anderson core 

420.3 42 13 6 3 9 16 1 5 0 2 0 (l <1 11 99 N 

420.6 43 4 6 7 13 32 6 1 0 0 0 0 <1 <1 99 0.09 

421.2 26 3 7 9 16 26 2 1 <1 6 19 1 <1 0 100 .09 

422.1 29 1 6 5 11 16 1 <1 0 9 31 2 <1 0 100 . 1 

422.5 40 6' 9 8 17 15 2 (1 0 6 9 2 1 0 98 .12 

423.1 34 5 12 11 23 15 1 1 1 16 0 3 <1 <1 99 .09 

424.3 22 1 5 3 8 10 <l 1 0 3 0 1 <1 53 99 .09 

428.5 9 <1 N N 4 3 <1 0 <1 6 0 <1 <1 77 99 . 07 

430.1 42 6 7 5 12 5 0 0 0 3 0 1 <1 31 100 N 

430.7 42 8 15 6 21 14 0 0 0 <1 0 (1 <1 15 100 .15 

431.3 51 4 18 6 24 19 0 (l <1 1 0 (1 <l 0 99 .2 

431. 9 47 9 20 6 26 17 <1 <1 0 <1 (1 <1 1 0 100 . 27 

432.5 45 9 19 10 29 13 1 (1 (l <1 <1 (1 2 0 99 .22 

432.8 47 9 18 8 26 16 1 0 0 <1 (1 <1 1 0 100 .19 

433.1 48 9 20 7 27 14 1 <1 0 (l <1 <1 <1 0 99 .22 

434.3 43 7 22 10 32 13 1 1 0 0 <1 <1 2 0 99 .19 

435.3 31 8 11 9 20 8 2 0 0 0 30 0 <1 0 99 N 

435.8 39 13 15 15 30 16 1 1 0 0 <1 <1 <1 0 100 N 

436.2 49 11 12 8 20 17 2 0 0 0 0 (1 <1 0 99 N 

436.7 41 14 20 6 26 16 3 0 <1 0 0 <1 <1 0 100 N 

437.4 55 9 20 5 25 9 1 (1 0 0 <l 0 <1 0 98 N 

437.7 52 6 22 6 28 14 0 0 0 0 (1 0 <1 0 100 .16 

438.0 48 10 16 9 25 14 <1 <l 0 0 <1 <1 1 0 98 N 

438.5 40 8 20 13 33 17 2 0 0 <1 (1 0 <1 0 100 .19 

439.5 35 4 14 8 22 26 1 <l <1 2 9 1 <1 0 100 .16 

440.0 35 2 11 5 16 13 0 <1 0 2 30 <1 <1 <1 98 .22 

440.5 44 5 18 4 22 22 <l 1 0 3 (l <1 <1 0 97 .16 

441.0 45 2 18 7 25 22 <1 0 0 4 <1 2 <1 0 100 .16 

442.0 45 2 14 6 20 25 1 <l 0 4 2 1 <1 0 100 .16 

442.5 45 2 15 5 20 27 (1 <1 0 4 <1 (1 <1 0 98 .16 

442.9 41 3 12 5 17 22 1 1 0 5 10 (l <1 0 100 .16 

443.2 34 2 11 7 18 15 0 (l 0 4 30 <1 <1 <1 103 N 

443.5 47 3 18 7 25 18 (l <1 <1 4 <l 1 <1 0 98 .13 

444.1 41 4 16 9 25 18 <1 1 Tr (1 0 <1 <1 0 89 .13 

444.4 45 4 15 6 21 23 (l <l 0 5 0 1 <1 0 99 .15 

445.0 42 3 19 8 27 18 >1 <1 0 7 0 <1 <1 0 97 .16 

445.3 49 3 20 5 25 18 0 0 0 4 0 (l <1 0 99 .18 

445.6 42 4 21 8 29 16 2 <1 0 4 0 <1 1 0 98 .16 
16 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

TABLE a.-Petrographic data for selected thin sections of the Eagle Sandstone from 

the Bearpaw Mountains, Montana-Continued 

I Feldspar rn >.. "0 rn c:: ~ Q) co ~ 

:I ~ ·~ c:: ~ ..... ~ 

,....,...... c:: co Q) co () "' Depth ()~ Q) Q) "' e c:: >< co,-.. 

>< "' rn "' e Q) bO Q) 0 () ·~ 5-! (m) Q) Q) co co bO ~ Q) () .0 ·~ 

'-'.C ..... "' p.. co ·~ ~ Q) "' c:: ~ 

~ 

() () rn "' Q) c:: ·~ ~ Q) co Q) co Q) 

0 "0 "'" ~ 0 > ·~ ~ () bO e bOQ) 

~bO ~ ·~ ..... ..... ·~ () 0 e ·~ ·~ ..... co ~ 

"' c:: "' bO Q) co ~ ~ :I () 0 () c:: .c >.. co "' ·~ CO·~ Q) co "'" ~ () 0 co rn ..... ..... 0 ~ Ill ~ Q) rn 

:I .c ..... I 0 ~ ·~ ..... £ 0 co "' ~ ..... 0 

0' u p., ;:.:: E-< ~ (.!) ~ u 1-1 u E-< ~ 

Fuel co No. 1 Federal core 

425.0 28 8 10 6 16 5 2 3 (1 2 35 1 (1 0 100 N 

425.7 30 6 3 4 7 13 2 1 1 5 (1 1 (1 33 99 N 

425.8 33 12 2 4 6 14 (1 1 1 N N N (1 28 95 .07 

426.6 34 19 1 6 7 9 (1 3 0 0 24 2 (1 2 100 N 

426.9 48 18 4 5 9 18 (1 (1 (1 (1 (1 (1 (1 4 97 .16 

427.5 36 15 2 8 10 19 (1 4 0 5 (1 5 (1 6 100 N 

430.8 34 10 0 6 6 11 (1 7 (1 0 0 (1 (1 31 99 .11 

431.1 42 16 1 7 8 15 (1 3 (1 0 0 (1 (1 16 100 .12 

431.4 35 14 0 7 7 7 0 3 0 0 25 (1 (1 7 98 N 

433.6 54 14 5 7 12 15 <1 3 0 (1 0 (1 (1 0 98 .12 

433.9 46 13 5 10 15 20 (1 3 0 (1 0 (1 (1 0 97 N 

434.2 39 10 1 3 4 9 1 4 <1 (1 0 9 (1 20 96 N 

434.8 35 11 3 8 11 28 2 5 0 2 0 3 (1 2 99 N 

435.1 54 16 (1 4 4 16 (1 2 0 (1 0 (1 (1 6 98 N 

435.4 21 5 2 2 4 8 0 (1 0 <1 0 0 (1 60 98 N 

No. 1 Morrison core 

393.2 47 7 7 6 13 26 (1 6 (1 0 (1 0 (1 0 99 .18 

393.8 49 6 7 3 10 30 1 3 0 0 0 (1 (1 0 99 . 22 

394.4 51 5 11 6 17 25 (1 1 0 0 0 0 <1 0 99 N 

395.6 55 7 5 7 12 24 0 1 0 0 (1 0 <1 0 98 N 

396.2 48 9 2 5 7 26 (1 6 0 0 0 0 2 0 98 .28 

396.8 44 9 4 7 11 31 (1 2 0 0 0 0 (1 0 97 N 

397.4 51 9 7 6 13 21 (1 4 0 0 0 0 2 0 100 .25 

397.8 55 7 7 5 12 23 0 3 0 0 0 0 (1 0 100 N 

398.4 52 8 5 6 11 25 <1 3 0 0 0 0 1 0 100 .22 

399.0 55 8 7 2 9 23 0 4 0 0 0 0 1 0 100 N 

399.6 45 9 6 7 13 25 (1 7 0 0 0 0 1 0 100 .25 

400.2 56 5 4 4 8 27 0 3 0 0 0 0 (1 0 99 N 

400.8 52 6 6 7 13 22 0 6 0 0 (1 0 1 0 100 N 

401.4 50 6 11 6 17 22 <1 4 0 0 0 0 (1 0 99 N 

401.7 45 14 8 5 13 24 (1 1 (1 0 (1 0 2 0 98 .25 

composition. Polycrystalline fragments are also common and may 

form as much as 34 percent of all quartz in a particular sample. These 

grains are of diverse origin; some appear to be simply very coarse 

quartz chert, while others can be identified as fragments of quartzose 

siltstone, metasiltstone, or fine-grained quartzarenite. Many other 

polycrystalline quartz grains cannot be confidently attributed to any 

specific source. 
LITHOLOGY 17 

TABLE 3.-Petrographic data for selected thin sections of the Eagle Sandstone from 

the Bearpaw Mountains, Montana-Continued 

I Feldspar ell >. "0 ell c:: .... Q) Ill N 

:l .... •.-4 c:: .... .-4 .... ..-~,..... c:: Ill Q) Ill C) 1-1 

Depth C) .... Q) 

~ 1-1 e c:: >< Ill"' 

>< 1-1 ell 1-1 Q) !lO Q) 0 C) •.-4 5-! (m) Q) Q) Ill Ill !lO .... Q) 0 ,.0 •.-4 1-1 

'-'..C: .-4 c.. Ill •.-4 .... Q) 1-1 c:: .... 

N 0 0 ell 1-1 Q) c:: •.-4 .... Q) Ill Q) Ill Q) 

0 "0 1.1-l .... 0 > •.-4 .... C) C>O e !lOQ) 

.... !lO .... •.-4 .-4 .-4 •.-4 0 0 e •.-4 •.-4 .-4 Ill N 

1-1 c:: 1-1 C>O Q) Ill ~ .... :l 0 0 0 c:: ..c: >. Ill 1-1 •.-4 

lll·.-4 Q) Ill 1.1-l .... 0 0 Ill ell .-4 .-4 0 .... Ill .... Q) ell 

:l ..c: .-4 I 0 ~ •.-4 .-4 :l 0 Ill 1-1 ~ .-4 0 

0' u p.. :00:: E-t ~ t.!) ::E: 0 u 1-( u E-t ~ 

No. 29-3X Olson core 

516.0 46 10 4 7 11 14 0 3 0 2 4 7 <1 N 97 N 

518.2 46 11 7 4 11 12 1 3 0 <1 <1 12 (1 N 96 N 

519.4 43 9 4 2 6 24 <1 2 0 7 0 <1 3 N 94 N 

520.3 42 8 1 6 7 21 1 3 0 10 <1 1 2 N 95 N 

521.5 53 8 3 2 5 17 0 1 0 4 0 2 <1 N 90 N 

No. 29-3X Olson core--Continued 

522.4 44 3 6 2 8 22 <1 1 0 9 <1 8 <1 N 95 N 

523.6 48 5 7 2 9 18 <1 1 0 11 3 1 1 2 99 N 

527.6 32 4 7 4 11 13 <1 0 0 2 36 1 <1 0 99 N 

528.2 32 3 5 5 10 7 1 0 0 1 43 2 <1 0 99 N 

529.7 55 3 11 6 17 21 2 N N 0 2 0 <1 0 100 N 

531.0 61 2 14 3 17 19 0 <1 0 0 <1 N <1 0 99 N 

532.8 44 3 22 5 27 19 0 <1 0 0 6 0 <1 0 99 N 

534.0 40 17 15 9 24 15 <1 <1 0 0 2 0 <1 0 98 N 

535.2 53 10 11 3 14 15 0 0 0 0 7 <1 <1 0 99 N 

535.8 40 9 21 8 29 19 <1 <1 0 0 (1 0 <1 0 97 N 

536.4 56 5 18 6 24 15 <1 <1 0 0 <1 0 <1 0 100 N 

537.7 52 5 21 6 27 14 <1 0 0 <1 <1 <1 (1 0 98 N 

538.3 37 7 14 7 21 22 0 <1 <1 7 3 <1 1 0 97 N 

539.2 48 5 9 7 16 19 (1 <1 0 8 1 <1 (1 0 97 N 

540.1 43 3 9 5 14 24 <1 <1 0 N N N <1 0 84 N 

541.3 44 6 25 5 30 14 (1 (1 0 6 0 (1 <1 0 100 N 

542.2 50 4 12 6 18 20 <1 0 0 4 2 1 <1 0 99 N 

543.2 43 5 14 7 21 23 <1 0 0 2 5 <1 <1 0 99 N 

545.6 39 4 11 6 17 29 <1 1 0 N N N <1 1 91 N 

547.1 37 10 14 6 20 18 0 2 0 5 <1 5 <1 2 99 N 

549.2 34 5 9 9 18 33 <1 <1 0 2 2 2 1 2 99 N 

550.2 31 10 13 7 20 21 1 1 0 4 <1 6 <1 3 97 N 

555.0 22 1 13 5 18 12 1 0 0 8 0 3 <1 35 100 .06 

FELDSPARS 

Plagioclase forms as much as 25 percent of the entire rock. The 

grains may be irregular, angular fragments, or lath-like crystals. Albite twinning is observed frequently, but pericline and Carlsbad 

twins are uncommon. Optical determinations indicate that the composition of these grains is generally no more calcic than An.w (andesine) in unzoned crystals. However, strong normal or oscillatory zon-
18 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

420r-----------,------------.------------.-----------~ 

en 

a: 

w 

fw 

~ 

425 

z 430 

d 

z 

I 

en 

::::> 

Ill 

>- ....J 

....J 

UJ 

~ 

~ 435 

0 

....J 

w 

CD 

:r: 

IC.. 

UJ 

0 

440 

445 

SAND SILT 

2 3 4 5 6 

MEAN DIAMETER, IN PHI UNITS 

FIGURE 5.-Mean framework grain size distribution as a function of depth in the 

30-1 Anderson core. 
CJ) 

a: 

w 

~ 

w 

::?! 

LITHOLOGY 19 

EXPLANATION 

Framework 

Matrix 

20 40 60 100 

CUMULATIVE PERCENT 

FIGURE 6.-Fram.ework, matrix, calcite cement, and pore space (FMCP) distribution 

as a function of depth in the 30-1 Anderson core. 

ing precludes an accurate optical determination in many cases. Rare 

grains that are apparently immune to conventional staining techniques but display clear, closely spaced albite twin lamellae are interpreted as albite. Plagioclase fragments are generally not affected by 

any sort of clay alteration. However, kaolin booklets may form indis-
20 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

TABLE 4.-Point count data used in calculating FMCP diagram (fig. 6) for 30-1 Anderson core 

(Data in percent.] 

Depth (m) Framework Matrix Calcite cement Pore space 

420.3 62.1 11.0 0 26.9 

420.6 76.5 0 0 23.5 

421.2 71.0 0 5.0 24.0 

422.1 69.0 0 31.0 0 

422.5 65.4 0 11.0 23.6 

422.8 74.9 0 1.0 24.1 

423.1 75.3 0 0 24.7 

424.3 47.0 53.0 0 0 

425.2 14.0 86.0 0 0 

426.4 16.0 84.0 0 0 

427.3 34.0 66.0 0 0 

427.9 20.0 80.0 0 0 

428.5 23.0 77.0 0 0 

429.8 0 0 100.0 0 

430.1 69.0 31.0 0 0 

430.7 75.0 25.0 0 0 

431.3 72.6 0 0 27.4 

431.9 72.9 0 .5 26.6 

432.5 73.0 0 . 5 26.5 

433.1 72.3 0 .5 27.2 

434.3 70.4 0 .5 29.1 

435.3 70.0 0 16.0 14.0 

435.6 71.5 0 2.0 26.5 

435.9 72.5 0 .5 27.0 

436.2 73.0 0 0 27.0 

436.8 72.1 0 0 27.9 

437.4 72.5 0 .5 27.0 

438.0 70.9 0 .5 28.6 

438.6 72.0 0 .5 27.5 

438.9 70.1 3.0 0 26.9 

439.5 73.1 0 9.0 17.9 

439.8 70.0 0 30.0 0 

440.4 70.4 0 2.0 27.6 

440.7 72.4 0 .5 27.1 

441.0 72.3 0 0 27.7 

441.4 73.5 0 .5 26.0 

442.0 70.1 0 2.0 27.9 

442.6 71.8 0 . 5 27.7 

442.9 71.5 0 10.0 18.5 

443.2 70.0 0 30.0 0 

443.5 69.8 0 .5 29.7 

444.1 72.1 0 0 27.9 

444.4 71.6 0 0 28.4 

445.0 73.4 0 0 26.6 

445.6 73.0 0 0 27.0 
LITHOLOGY 21 

TABLE 5.-Quartz varieties determined by separate point counts of selected thin sections from the Bearpaw Mountains, Montana 

[Figures show number of grains] 

Depth Monocr:i:stalline Polycrys- Total 

(m) Straight Undulatory tal line grains 

extinction extinction counted 

30-1 Anderson core 

431.3 70 62 20 152 

435.3 79 17 9 105 

441.0 39 58 25 122 

445.3 52 69 31 152 

29-3X Olson core 

518.2 57 44 36 137 

521.5 48 81 28 157 

527.6 78 46 5 129 

531.9 88 82 5 175 

534.0 48 60 11 119 

535.8 47 55 16 118 

538.3 so 47 12 109 

541.3 45 69 13 127 

544.4 48 42 23 113 

546.2 43 59 9 111 

547.7 45 46 7 98 

550.2 34 46 12 92 

criminately upon them. Despite the absence of clay alteration, plagioclase grains are observed in various states of preservation, ranging 

from pristine to drastically diminished due to dissolution. Two processes are probably responsible for these effects: (1) dissolution of carbonate that had previously replaced plagioclase, and (2) dissolution 

of plagioclase directly by interstitial waters. These effects will be considered in a later section in more detail. 

Potassium feldspars are also a volumetrically important component of the framework with abundances generally ranging between 5 

and 10 percent, although they can constitute as little as 2 and as 

much as 15 percent of the entire rock. For this study, potassium feldspar was subdivided into three groups: complexly twinned grains 

(microcline), untwinned potassium feldspars with a small (< 40 o) optic 

angle (sanidine), and untwinned potassium feldspars with a large 

(> 40°) optic angle (orthoclase) (table 6). Microcline is an uncommon 
22 EAGLE SANDSTONE, BEARPAWMOUNTAINSAREA, MONTANA 

FIGURE 7.-Detrital constituents of the 

Eagle Sandstone. A, Euhedral grain of 

volcanogenic quartz (VQ) displaying hipyramidal beta habit and straight extinction. (Crossed polars.) B, sedimentary rock fragment ~omposed of finegrained, quartz-cemented quartzarenite. (Plane light.) C, Metasedimentary 

rock fragment consisting of quartz 

grains and interstitial white micachlorite matrix. (Crossed polars.) 

D, Volcanic rock fragment containing 

bipyramidal quartz phenocrysts in an 

isotropic groundmass. (Plane light.) 

E, Detrital moncrystalline dolomite (D) 

with a relict(?) quartz (Q) inclusion. 

(Crossed polars.) 
LITHOLOGY 23 

constituent with only a few grains usually present in any particular 

thin section. Sanidine is generally more abundant than orthoclase. 

Whereas many of the grains positively identified as orthoclase or microcline display irregular, angular grain boundaries, sanidine commonly possesses a subhedral to euhedral habit. In thin section, most 

potassium feldspars appear very fresh. However, the SEM reveals 

minor surficial dissolution features. Some potassium feldspars have 

served as a substrate for the growth of a potassium-bearing clay mineral, probably mixed-layer illite-sme<;tite. In no case, however, were 

these feldspars observed to have undergone dissolution extensive 

enough to significantly affect rock porosity. No pseudomorphous 

replacement of clay for potassium feldspar was seen. 

LITHIC FRAGMENTS 

Rock fragments, including chert in this report, compose 10 to 

30 percent of most samples (fig. 3) (table 7). These grains are of diverse origin and are found in various states of preservation. Fortu-
24 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

TABLE 6.-Potassium feldspar content in 10 selected thin sections of the 30-1 Ande~ 

son core 

[Figures show number of grains; 2Vis the angle between the two optic axes] 

Depth Total 2V>40" 2V<40" .Twinned Undeter-

(m) counts (orthoclase) (sa-;-idine) (microcline) mined 

420.6 34 9 12 1 12 

431.3 19 5 10 1 3 

435.3 28 5 13 0 10 

436.8 19 3 9 0 7 

439.5 33 7 15 1 10 

441.0 14 4 8 0 2 

442.0 19 5 7 0 7 

443.5 7 3 2 0 2 

445.0 23 6 9 1 7 

445.3 14 5 5 2 2 

nately, many lithologies represented by rock fragments are identifiable in thin section. 

Sedimentary rock fragments are dominated by chert, which occurs 

as sand grains throughout the formation and as chert pebbles in layers near the top of the unit and in the base of the overlying Claggett 

Shale (fig. SA). Several varieties of chert are common: one is clear, 

cryptocrystalline quartz free of any inclusions; another is deeply colored by opaque, carbonaceous material and contains numerous tiny 

euhedral crystals of pyrite. This latter variety may display spherical, 

elongate, or irregular internal structures, suggestive of microfossils. 

A third type of chert is brown or reddish-brown and contains 

numerous silt-sized grains of quartz or mica This type is gen~rally 

rich in isotropic material and niight best be described as siliceous 

siltstone. In larger fragments or pebbles, some chert appears to be a 

siliceous breccia Any of these chert fragments may be crosscut by 

chalcedony-filled fractures. Occasional tiny dolomite rhombohedra 

partially replace chert and may themselves be wholly or partly 

replaced by chalcedony. These features were probably produced long 

before erosion and transport. Many chert pebbles display a thin rind 

of black opaque material containing iron, magnesium, potassium, 

aluminum and silicon (D. D. Rice, personal commun., 1979). All the 

chert is composed of quartz of various crystal sizes, grading from 

truly cryptocrystalline quartz chert up to what would best be described as fine-grained polycrystalline quartz. This variation is often 

observed within a single fragment. 

Other occasionally identified sedimentary rock fragments include 

siltstone, carbonaceous shale or claystone, siliceous shale, and finegrained quartz sandstone (fig. 7 B). Sand-sized grains of dolomite are 
LITHOLOGY 25 

TABLE 7.-Rock-fragment varieties counted in 10 selected core samples of Eagle Sandstone from the Bearpaw Mountains, Montana 

[Figures show number of grains] 

Depth Sedimentary Total 

(m) (including Metamorphic Volcanic Unidentified grains 

chert) counted 

30-1 Anderson core 

431.3 11 43 3 17 74 

435.3 19 31 4 23 77 

441.0 31 37 8 44 120 

445.3 16 26 5 23 70 

29-3X Olson core 

521.5 30 18 0 25 73 

527.6 30 20 2 24 76 

531.9 14 16 1 16 47 

534.0 54 17 0 21 92 

535.8 27 25 1 30 83 

538.3 29 30 12 19 90 

also present and will be considered separately. Metamorphic rock 

fragments are the most abundant identifiable lithic component other 

than chert. These grains show considerable variation, which is apparent in all samples from all locations. Most are composed of quartz, 

white mica, and chlorite, with smaller amounts of biotite. The fragments may display well-developed schistosity. It is commonly possible to distinguish among metamorphic grains on the basis of the relative abundance of these components, as some are predominantly 

quartz while others contain mostly argillaceous material. Many 

quartz-rich fragments have textures that are probably relict sedimentary features, such as randomly oriented silt- or fine-sand-sized 

quartz crystals separated by a chloritic or micaceous matrix (fig. 7 C). 

Argillites usually show a strong preferred optical orientation. Most 

metamorphic fragments can be classified as quartz-muscovite-chlorite schist, quartzite, or argillite, which includes slate, phyllite, and 

mica schist. In most identifiable fragments, mineral constituents 

suggest that the metamorphic sources were of a rather low grade, 

such as greenschist or low amphibolite facies, and that they represent 

a dominantly metasedimentary assemblage. 

A few rock fragments in each sample are clearly of volcanic origin. 

The most easily recognized volcanic fragments consist of a dark, 

nearly isotropic groundmass, which may or may not be potassiumrich, containing one or more euhedral or fragmented euhedral phenocrysts of quartz (fig. 7D), K-feldspar, or plagioclase. 
26 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

Many lithic fragments are not identifiable in thin section. Identification is particularly difficult for many dark, inclusion-rich, finetextured grains, which are easily confused with sedimentary chert, 

and for highly altered lithic grains. Every sample also contains potassium-rich rock fragments that are nearly isotropic under crossed polars and that may contain crystals of potassium feldspar or plagioclase. These grains are commonly altered to potassium-rich clay and 

may contain quartz. In my opinion this sort of assemblage and alteration is probably volcaniclastic, like the anhedral, fine- grained fragments common in the Two Medicine Formation (Mudge, 1972, p. 7 4; 

Mudge and Sheppard, 1968). However, no definitive evidence was obtained, and the fragments are therefore labeled "unidentified" in this 

report. 

DOLOMITE GRAINS 

Numerous grains of dolomite constitute the only volumetrically 

significant nonsilicate component of the framework assemblage and 

may account for 5 or 10 percent of the rock. They are of approximately the same size as associated quartz grains and occur in pointto-point or line-to-line contact with adjacent framework silicates or as 

floating grains. Although some dolomite fragments may be polycrystalline, the majority are monocrystalline and display rounded rhombic morphology or subangular grain boundaries. Many of these exhibit twin lamellae and contain numerous black, nonmetallic, opaque 

inclusions. A small percentage of the dolomites contain readily identifiable polycrystalline or monocrystalline quartz (fig. 7 E) or quartz 

chert. These silicate inclusions may be present in either monocrystalline or polycrystalline grains, which are otherwise indistinguishable 

from other dolomite fragments. A few grains are composed of approximately equal proportions of dolomite and quartz or quartz chert, 

with the rhombic dolomite crystals crosscutting older silicate textures. Yet even these grains display a rounded and abraded appearance, and in no case do the dolomite rhombohedra crosscut grain 

boundaries. These grains may bear a genetic relationship to previously described chert fragments which contain small automorphic 

dolomite rhombs as a minor replacement. No other types of silicate 

are observed within the dolomite grains. Wherever dolomite is present in a sample, the grains occur throughout the rock in approximately the same proportion, regardless of its postdepositional 

history. For example, in samples where calcite cement has been par· 

tially removed through dissolution, dolomite grains are equally abundant in the calcite-cemented and porous portions of the rock. 
LITHOLOGY 27 

No evidence was observed to support an interpretation of intrastratal origin for the dolomite. Instead, the distribution, intergranular relationships, and morphology of these grains strongly suggest that 

they were deposited simultaneously with the remainder of the Eagle 

Sandstone framework assemblage. Internal textures and intragranular silicate relicts also suggest a detrital origin for the dolomite 

grains. This view is supported further by the presence of obviously 

detrital cobbles, pebbles, and sand grains of carbonate in the Two 

Medicine Formation (Mudge and Sheppard, 1968, p. 142; Mudge, 

1972; Schultz and others, 1980). The Two Medicine is a continental 

equivalent of the Eagle Sandstone and, therefore, was presumably 

derived from the same sources. 

This interpretation is at some variance with the views e~pressed by 

Sabins (1962) in his study of dolomite in Cretaceous sandstones of the 

Western Interior, including the Eagle Sandstone. He concluded that 

most monocrystalline dolomite grains that contain carbonaceous inclusions and display grain-to-grain contacts and abrasional features, 

are primary dolomite formed at the sediment-water interface not long 

prior to deposition of the remainder of the sediment. 

COMMON VARIETAL CONSTITUENTS 

Glauconite (used in the generic sense) is present in all but a few 

samples and may constitute a measurable percentage of the sand-size 

fraction. The glauconite occurs in the typical round or oblong microcrystalline grains of deep green color and relatively high birefringence. Internal structures are not usually seen, although occasional 

desiccation cracks are observed. Opaque inclusions are ubiquitous. 

No evidence of oxidation of iron in glauconite was noticed. Compaction has affected these grains to various degrees. Some examples display no evidence of glauconite deformation, while others are highly 

compacted. Original pelletoid morphology is generally best preserved 

in those samples where an early cement (calcite) has inhibited compaction. Glauconite is most abundant in sandstone containing measurable amounts of biotite. The pellets are entirely absent in nonmarine mudstones. 

Biotite may account for as much as several percent of an entire 

sample and may be concentrated with other heavy grains in placerlike laminae. The mineral is commonly a deep brown color, although 

reddish brown or green biotite is also sometimes seen. The grains are 

generally very fresh but may locally be extensively expanded owing 

to clay mineral alteration. Biotite may also display a distinctive iron 

carbonate alteration, which is described in a later section. Compaction-produced biotite deformation is common. 
28 EAGLE SANDSTONE; BEARPAW MOUNTAINS AREA, MONTANA 

Discrete muscovite fragments are much less common than biotite 

and rarely constitute as much as one percent of a sample. Muscovite 

is, however, a very important constituent of metamorphic lithic fragments and is far more abundant than biotite in these. 

Detrital chlorite, displaying strong pleochroism, micaceous habit, 

anomalous interference colors (berlin blue) and, less commonly, sagenitic texture is observed in most thin sections. This chlorite may represent pseudomorphs of pre-existing ferromagnesian minerals such 

as biotite or hornblende. 

HEAVY MINERALS 

Except for biotite and chlorite, which are ubiquitous and plentiful, 

detrital heavy minerals generally constitute less than 0.2 percent of 

the framework grain assemblage. The nonopaque suite is dominated 

by apatite, zircon, garnet, and tourmaline, whereas magnetite and ilmenite are the only common opaque detrital grains. Grains of staurolite, rutile, orthopyroxene, and clinozoisite are rare. 

Apatite usually occurs in rounded, prismatic, colorless grains and, 

less commonly, as angular fragments. Zircons are present in two populations: a perfectly euhedral, unabraded, clear form and a strongly 

rounded, pink or purple variety. The metamict and abraded condition 

of the latter variety suggests a probable Precambrian age. Garnets 

are generally rounded and colorless, with brown varieties less common. Angular fragments of garnet are also observed. Tourmalines are 

dominated by highly dichroic, well-rounded spherical or cylindrical 

grains of schorl. However, less pleochroic varieties are not unusual. 

SOURCES OF THE EAGLE SANDSTONE 

In the vicinity of the Bearpaw Mountains, the Eagle Sandstone 

was deposited along a north-northwest-trending shoreline, during one 

of the periodic uplifts that occurred in western Montana throughout 

the Cretaceous. Regional stratigraphic relations, sandstone distribution, and ammonite zone trends all suggest that the sediments had a 

westerly source. The materials were delivered to the shore through 

small deltas and transported for short distances by a southwardflowing longshore current (Rice, 1980). 

The metamorphic assemblage, which comprises the most abundant 

identifiable lithic component within the Eagle Sandstone, probably 

accounts for a large portion of the quartz grains as well. Observable 

fragments indicate the argillaceous and siliceous metasediments originally consisted of mudstones, shales, siltstones and fine-grained 
SOURCESOFTHESANDSTONE 29 

sandstones. Framework grains of quartz and feldspar in these sediments were largely unaffected by metamorphism, whereas clay components were altered to white mica and chlorite and, less commonly, 

to biotite. These properties suggest derivation from large exposures 

of the Belt Supergroup in orogenic highlands to the west. At present, 

the Belt Supergroup in northwestern Montana contains from 2,135 to 

15,250 m (Mudge, 1970, p. 378) of argillaceous, quartzitic, and carbonate rocks displaying low-grade regional metamorphism (Ross, 

1963). These lithologies agree well with those observed in the metasedimentary components of the Eagle Sandstone. Even today, very 

few exposures of pre-Belt crystalline rocks are present in western 

Montana. 

Fragments of chert, detrital dolomite, and very fine grained quartzarenite are abundant in the Eagle Sandstone, and siliceous shale, carbonaceous shale, and mudstone are common, demonstrating that sedimentary rocks were an important part of the source terrane. Although some of these materials were almost certainly derived from 

the Belt Supergroup, the abundant chert and dolomite fragments 

probably resulted from the rapid erosion of Paleozoic sediments. The 

whole Paleozoic section presently exposed in the Sun River Canyon 

area is approximately 1,220 m thick and is dominantly composed of 

carbonate rocks, many of which are dolomitic and cherty (Mudge, 

1972). In that area, chert- bearing dolomites in the Mississippian Castle Reef Dolomite are from 7 5 to more than 305 m thick (Mudge, 

1972). Other sedimentary materials in the Eagle could have been 

eroded from Mesozoic rocks or could represent intraformational erosion. However, no definitive evidence was documented for these 

sources. 

Volcaniclastic material is ubiquitous in the Eagle Sandstone. Distinctive bipyramidal quartz grains and some lithic fragments are 

clearly volcanogenic. Similarly, the abundant subhedral and fragmented grains of sanidine are indicative of a shallow intrusive or extrusive origin. Other components, such as andesine, biotite, orthoclase, and the common unidentified isotropic rock fragments, may 

have had similar origins. 

The exact location and nature of the contributing volcanic sources 

for the Eagle Sandstone are not known. Volcanism occurred sporadically during the later Mesozoic in western Montana and adjacent 

areas, yielding sediments to the Kootenai Formation (Aptian) and to 

the Vaughn Member of the Blackleaf Formation (upper Albian) prior 

to deposition of the Eagle Sandstone. Sources for these volcanogenic 

materials are believed to have lain to the west of the present mountain front in areas now buried by thrust plates in the vicinity of the 

Lewis and Clark Range (Mudge and Sheppard, 1968). Similar vol-
30 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

canic rocks and volcaniclastic sediments may have been exposed to 

erosion during accumulation of the Eagle Sandstone. Late Cretaceous volcanism began no later than Santonian time in western Montana, when the Slim Sam Formation, a sequence of volcano-sedimentary rocks, was deposited conformably upon the marine Colorado 

Shale (Klepper and others, 1957, p. 28; Smedes, 1966, p. 13). The transitional Slim Sam Formation grades upward into the Elkhorn Mountains Volcanics, an accumulation of calcalkaline extrusive and 

shallow intrusive rocks, which once covered as much as 25,900 km2 of 

western Montana (Smedes, 1966, p. 21). The lower part of the Elkhom Mountains Volcanics may be as old as the Telegraph Creek Formation (Klepper and others, 1957, p. 32) and is almost certainly contemporaneous, in part, with the Eagle Sandstone (Smedes, 1966, 

p. 27). The lower portion of the Elkhorn Mountains Volcani~s is dominantly andesitic or rhyodacitic in composition and contains phenocrysts of quartz, orthoclase, and intermediate plagioclase (Smedes, 

1966). These phenocrysts are petrographically similar to certain 

components of the Eagle Sandstone. The Elkhorn Mountains volcanic center definitely contributed clastics to the Eagle Sandstone in 

the Livingston, Mont., area (Roberts, 1972, p. 31). It is tempting to 

invoke the Elkhorn Mountains as a source for volcaniclastic material 

in the Eagle Sandstone of the Bearpaw Mountains area as well. However, this volcanic field may have lain too far southward to have actively contributed to the westerly sources of the Eagle Sandstone 

there. While derivation from the Elkhorn Mountains Volcanics cannot be positively discounted, sources in volcanic rocks to the west, 

now buried by thrust plates, and in older volcanic complexes and volcaniclastic sediments are also likely candidates. 

Plutonic rocks may have served as sources for the Eagle Sandstone, but their presence is not unambiguously demonstrated by the 

mineralogical evidence. Intermediate plagioclase and orthoclase, 

both common to the Eagle, are probably igneous in origin. These feldspars could have been derived from either a plutonic or a volcanic terrane. The striking scarcity of microcline might suggest that plutonic 

materials were unavailable. However, many of the great Cretaceous 

batholiths of the northwest had been in place for tens of millions of 

years by the beginning of Eagle Sandstone deposition and could have 

been exposed to erosion in Campanian time. The Idaho batholith, 

dated at between 92 and 135 m.y. by the lead-alpha method (Larsen 

and others, 1958), and the Nelson batholith of British Columbia, 

dated by the K-Ar method at 96 m.y. (Baadsgaard and others, 1961), 

are both located in likely source areas for Eagle Sandstone sediments. Descriptions by Cairnes (1934) and Little (1960) suggest that 

most coarse-grained potassium feldspars in the Nelson batholith are 
DIAGENESIS 31 

orthoclase rather than microcline. Larsen and others (1958) report 

large volumes of rock in the Idaho batholith with orthoclase phenocrysts, although some plutons apparently do contain coarsely crystalline microcline. 

DIAGENESIS 

Postdepositional processes have operated extensively within the 

Eagle Sandstone in the Bearpaw Mountains area. Petrologic investigations indicate that the high porosity and permeability of the rock, 

which make it a good conventional gas reservoir, were not entirely developed during deposition, but rather, that intrastratal effects operating throughout the burial history of the unit have played a dominant role. These effects have been reasonably simple and consistent 

throughout the study area. This consistency reflects the uniform biological processes, relatively shallow burial depths, and regionally 

similar orogenic activity in the Bearpaw Mountains area. This section describes observed postdepositional features within sandstone 

units of the Eagle Sandstone and discusses the processes that were 

responsible for them. Table 8 summarizes the inferred burial history 

of the Eagle Sandstone and the evidence upon which it is based. 

CALCITE 

Calcite cement tightly seals only a relatively small volume of the 

Eagle Sandstone, typically occupying layers a few tens of centimeters thick. These layers are readily identifiable in electric logs (fig. 2) 

and are apparent as resistant ledges in outcrop. Cemented layers are 

present in almost every section of the Eagle Sandstone, but all sections contain a larger volume of rock that is incompletely cemented. 

Where layers are completely cemented with calcite, this cement composes 30 percent or more of the rock volume. 

The results of stable-carbon-isotope analyses of seven selected samples of calcite cement and one sample of a calcareous concretion are 

presented in table 9. The samples were collected from three cores of 

the Eagle Sandstone: 30-1 Anderson, 29-30X Olson, and No.1 Federal (table 1). All eight samples yielded significantly and consistently 

negative o13C and o1110 values relative to the PDB Chicago standard, 

ranging from -6.52 to -7.56 permil for carbon and -13.83 to -15.65 

permil for oxygen. These values suggest equilibration with meteoric 

waters. However, the mechanism by which the isotopically light carbon and oxygen were incorporated in the calcite is not known. 
No. 

1 

2 

3A 

3B 

4 

5A 

Event 

Deposition--------------

Pyrite formation----------

Methane generation--------

Precipitation of early 

calcite cement. 

Compaction and minor quartz 

cementation. 

Extensive precipitation of 

and replacement by 

sparry calcite. 

TABLE B.-Summary of Eagle Sandstone burial history 

Evidence 

(1) Pyrite is included, along with organic matter, in early-formed calcite. (2) Inference based on chemical requirement of sulfate reduction for bacterially generated methane. 

Inference based on modern occurrences of biogenic methane generation (Claypool 

and Kaplan, 197 4) and on carbon isotope composition of gas. 

(1) Calcite contains abundant organic and pyritic inclusions (fig. 8A). (2) No 

preexisting authigenic phases are crosscut by this carbonate. (3) Included detrital grains display no evidence of compactional deformation. (4) Cement occupies 40 percent or more of rock volume. (5) Calcite displays fabric and optical 

qualities of concretions observed in the Eagle Sandstone. 

(1) Quartz overgrowths are often associated with somewhat compacted soft 

grains such as glauconite or biotite. (2) Sparry calcite fills fractures in framework grains and in inclusion-rich calcite cement (event 3B). (3) Quartz overgrowths are marginally embayed by sparry calcite cement (event 5A). 

(1) Sparry calcite embays quartz overgrowths. (2) Sparry calcite fills fractures in 

framework grains and in inclusion-rich calcite (event 3B). (3) Carbonate surrounds somewhat compacted grains of biotite and glauconite. (4) Extent of calcitization indicated by dissolution textures (event 6). 

~ 

~ 

tz:j 

e; 

~ 

~ a 

z 

_tr:l 

ll' 

tz:j 

> 

~ 

~ ~ 

0 

~ 

~ 1-4 

z 

00 

> 

~ 

tz:j 

..> 

~ 

0 

~ 
5B 

6 

7 

8A 

8B 

Siderite formation associated with biotite. 

Calcite dissolution----

Precipitation of abundant 

siderite near top of 

29-3X Olson core. 

Authigenic clay formationMajor migration of economically significant quantities of biogenic methane 

from associated sediments. 

(1) Siderite is associated with calcite or with evidence of preexisting calcite. 

(2) Siderite appears to crosscut calcite cement textures. (3) Siderite is generally 

associated with disrupted biotite (fig. lOA). 

(1) Ubiquitous calcite relicts in intragranular and intergranular pores (fig. 8F). 

(2) Dissolution fabric at margins of calcite and voids in optically continuous 

calcite (fig. 8C). (3) Application of textural criteria of Schmidt and others (1978) 

to porous portions of the Eagle Sandstone. 

(1) Rhombohedra of siderite are perfectly fresh and euhedral (fig. lOE). (2) Siderite 

occupies dissolution features in quartz overgrowths (fig. 10E). (3) Siderite 

occupies voids interpreted as dissolution features in calcite. (4) Siderite rhombs 

crosscut fabric of sparry calcite and quartz overgrowths. 

(1) Kaolinite occupies voids in optically continuous calcite cement.(2) Kaolinite 

overlies authigenic quartz and authigenic siderite (fig. lOE). 

Inferred on basis of presenc~ of authigenic tourmaline observed by Gautier (1979), 

which indicates migration subsequent to tourmaline precipitation during Bearpaw 

Mountains uplift and volcanism. 

t;j 

~ 

~ tlj 

z 

tlj 

00 

~ 

00 

~ 

~ 
34 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

TABLE 9.-Carbon and oxygen isotope data for selected samples 

of Eagle Sandstone carbonates 

(Expressed in parts per thousand 6 relative to the PDB Chicago standard. N indicates 

no determination) 

Core Depth (m) 

Determinations on calcite 

29-3X Olson 53-2.8 

Do------- 532.8 

30-1 Anderson 439.8 

Do------- 443.2 

Do. 

l _____ 429.8 

No. 1 Federal 424.7 

Do------- 425.0 

Do------- 426.6 

Determinations 

29-3X Olson--

Do-------

518.2 

520.0 

lcalcite concretion. 

-7.54 

-7.56 

-7.33 

-7.40 

-6.58 

-7.23 

-6.52 

-7.34 

on siderite 

-0.99 

-1.21 

N 

N 

N 

N 

-15.65 

-15.55 

-13.83 

-15.31 

N 

N 

Calcite has two basic modes of occurrence. The first is a finegrained, inclusion-rich variety. The second is a coarsely crystalline, 

clear, sparry variety (fig. SA). 

The first type of calcite appears as a finely crystalline mosaic, in 

patches, filling intergranular spaces. These patches are generally 

cloudy, owing to abundant inclusions of organic material. Blebs and 

grains of anhedral to euhedral pyrite may be sparsely disseminated 

within it. Framework grains within the rocks cemented by inclusionrich calcite generally display little evidence of compaction. Grain contacts are most often tangential, but many framework grains appear 

to "float" in calcite. The fine-grained carbonate may exist in areas 

larger than the combined diameters of several framework grains. In 

these more extensive areas, the fine-grained crystals are organized 

such that the entire patch displays radial extinction, similar to that 

observed in the calcite concretions that are occasionally seen in the 

Eagle Sandstone. Where carbonate development has been most extensive, clastic grains are generally excluded and glauconite, if present, may be seriously disrupted or replaced by the calcite. Excepting 

pyrite, no diagenetic effects were observed to precede this mineral. 

Most calcite occurs as a clear, coarse, sparry mosaic of anhedral 

crystals, typically 0.1 mm in diameter or larger, which may poikilo-
DIAGENESIS 35 

topically enclose silicate framework grains over large areas of the 

thin section. This carbonate is only rarely twinned and is generally 

free of inclusions. In addition to occupying intergranular spaces, it 

may fill fractures if they are present. A few of these rare calcite-filled 

fractures cut across areas of fine-grained, organic-rich calcite, thus 

demonstrating the paragenetic relationship between the two calcite 

varieties. Easily deformed grains, such as glauconite, may show evidence of compaction in rocks cemented with sparry calcite. Micaceous grains or argillaceous lithic fragments are commonly uncleformed. Labile constituents, such as feldspars and volcanic rock fragments, are perfectly preserved by the carbonate cement, displaying 

no clay alteration or dissolution effects. Calcite does, however, partially replace some clastic silicates, particularly plagioclase. It tends 

to follow cleavage planes and fractures within the grain. This replacement results in wedgelike, rhomboid, or irregular areas of calcite 

within the silicate fragment (fig. 8B). The carbonate completely fills 

intergranular pores. The calcite commonly embays the margins of 

quartz, feldspars, and other silicates~ producing "caries-type" boundaries. Where calcite cement is extensive, other cements or replacements are rare, although minor preexisting quartz overgrowths may 

be observed in areas of sparry cement. 

Near the upper or lower boundaries of the calcite-cemented layers, 

close to the more common highly porous portions of the sandstone, 

voids are apparent within the intergranular cement (fig. 8C). The 

voids farthest inside the cemented zone appear in the centers of intergranular spaces and exhibit the ragged boundaries characteristic 

of dissolution features within a formerly complete crystal mosaic 

rather than the clear, sharp boundaries that would result from incomplete cementation. In the more coarsely grained cement these pores 

occur within optically continuous crystals. It is concluded that these 

voids result from partial dissolution. Even closer to the porous portions of the sandstone calcite dissolution effects are more intensive. 

Fully open intergranular pores appear, bounded by slightly embayed 

framework materials. These pores contain small remnants of the 

once-continuous cement, now reduced to a few patches of completely 

isolated carbonate, which may still display optical continuity. Intragranular pores partly occupied by remnant calcite are also seen. 

These are most often present in plagioclase feldspars (fig. 8D). The intragranular pores are morphologically similar to the framework replacement features of calcite observed in completely cemented rocks, 

with voids often following cleavages or displaying wedgelike or rhombic outlines. These intragranular pores may be so extensive as to 

leave only skeletal remnants of the original framework silicate 

(fig. BE). These remnants are often surrounded by tiny relicts of the 
36 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 
DIAGENESIS 37 

FIGURE B.-Dissolution and calcite 

replacement features of the Eagle Sandstone. A, Chert-pebble (CH) conglomerate, from near top of Eagle Sandstone, cemented by two types of calcite: 

inclusion-rich, fine-grained early carbonate (EC) and later sparry calcite cement (SC). (Crossed polars.) B, Partial 

replacement of albite-twinned plagioclase by sparry calcite (SC). (Crossed 

polars.) C, Dissolution voids (V) in optically continous sparry calcite cement 

(SC) (Plane light.) D, Angular dissolution voids (V) in plagioclase, associated 

with relict calcite (C) replacement and 

cement. (Plane light.) E, Skeletal remnant of plagioclase grain. (SEM photomicrograph.) F, Oversize pore (V) containing relict calcite (RC). (Plane light.) 

G, Same view as BF, showing skeletal 

remnants of plagioclase (SP). (Crossed 

polars.) 
38 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

replacement calcite, which has been almost completely removed by 

dissolution (fig. SF, G). 

The highly porous, permeable rocks contain a small (< < 1 percent) 

but ubiquitous component of relict calcite, which occurs in either intergranular or intragranular spaces. The intergranular pores are of 

disparate sizes, including typical triangular-type intergranular voids 

connected by elongate passages. The grain boundaries along these 

pores may display the same development of caries texture seen in 

calcite-cemented rocks. Intragranular voids are extensive in the 

highly porous rocks. Nearly complete plagioclase grains grade into 

distinctly delicate skeletal remnants of plagioclase (fig. BE), which 

grade into large voids containing only a few bits of silicate material 

(fig. BF, G). Large pores, larger than the size of most sand grains, are 

also present. It is likely that these oversize pores were once partly occupied by a framework silicate, probably plagioclase, or by a large 

patch of fine-grained calcite. A continuous gradient of dissolution 

may thus be observed, ranging from completely cemented rocks into 

the highly porous and permeable sandstones. 

This petrographic evidence indicates that the fine-grained calcite 

formed prior to the sparry cement. Its finely crystalline texture and 

abundant organic inclusions indicate that it precipitated early in the 

burial history of the rock, probably immediately following deposition. Sparry calcite developed after some compaction had occurred 

and after the development of minor quartz overgrowths, which may 

have tacked together the quartzitic framework grains. 

The gradational transition from calcite-cemented to highly porous 

sandstone, the observable dissolution features in the calcite cement, 

and the presence of relict calcite within the highly porous rocks are 

unequivocal evidence that calcite once occupied a far larger volume of 

the Eagle Sandstone than at present. The minimum volume of sandstone once cemented by carbonate may be estimated from the present 

distribution of relict calcite (compare fig. 6 and fig. 9). The maximum 

volume formerly occupied is less easy to determine, however. In a 

highly influential paper, Schmidt and others (1977, p. 275) illustrated 

petrographic criteria applicable to the determination of the presence 

of secondary porosity. These include (1) partial dissolution of calcite 

cement, (2) molds of former framework grains, (3) inhomogeneous 

packing, (4) oversized pores and floating grains, (5) elongate intergranular pores, (6) corroded grain margins, (7) honeycombed 

grains, and (8) fractured grains (Schmidt and others, 1977). Every 

one of these criteria can be observed in most of the highly porous and 

permeable rocks of the Eagle Sandstone. The application of these 

criteria to the samples from the 30-1 Anderson core, for example, 

would indicate that a sequence of calcite cementation and replace-
C/) 

a: 

w 

1-

w 

~ 

EXPLANATION 

f"·j< :.:·/.:::_J Framework 

f~-===J Matrix 

~ Calcite cement 

c=J Porespace 

DIAGENESIS 39 

100 

FIGURE 9.-Estimated framework, matrix, cement, and pore space distribution based 

upon minimum volume of calcite present prior to dissolution. 

ment followed by calcite dissolution has produced most of the observable pores (fig. 9). 

Two interpretations seem to explain the present calcite distribution and accompanying textural qualities. The first is that calcite at 

one time simultaneously occupied most pore spaces in the Eagle 

Sandstone This theory suggests that the present-day conventional 

reservoirs were once completely tight nonreservoirs. If such were the 
40 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

case, the calcite must have been subsequently removed by dissolution processes which operated nearly everywhere, leaving only isolated remnants of the formerly extensive cement. This inference demands that the gas now occupying these reservoirs has migrated into 

the rocks since the dissolution of the carbonate, thus placing rather 

dramatic constraints upon reservoir quality and the timing of gas migration. This interpretation effectively explains the present textural 

properties of the formation as well as the observed distribution of 

calcite relicts and dissolution features throughout the rock. It requires, however, an explanation of where all the original calcite came 

from and where it has gone. It fails to account for the mechanism by 

which water moved through what would have been an impermeable 

rock and effectively dissolved most of the calcite cement while leaving isolated discrete ledges of carbonate. Furthermore, it does not account for the seemingly inexplicable absence of completely cemented 

Eagle Sandstone which, it would seem, should be preserved somewhere in the Bearpaw Mountains area Finally, it does not explain 

where the methane was during the time the reservoir was tightly 

sealed. Since the gas was certainly generated very early in the burial 

history, it must have occupied some reservoir during the time of calcitization. No such storage place exists, however, to my knowledge. 

This latter argument could be circumvented if the methane had remained in solution from the time of generation until after the dissolution of calcite. 

An alternative interpretation, also consistent with the petrographic evidence, is that calcite has at one time or another occupied 

virtually every pore in the reservoir but that at no time did it simultaneously occupy the entire rock. In this case, the distribution of calcite during much of the burial history of the Eagle Sandstone might 

have been very similar to that observed today. 

Layers, ledges, and patches of authigenic carbonate in sandstones 

are common in the geologic record. One explanation of their occurrence is based upon the nonlinear solubility of ionic compounds as a 

function of various geochemical parameters, such as the concentration of dissolved neutral electrolyte (Runnells, 1969), the partial 

pressure of carbon dioxide (Bogli, 1964), common-ion effects (Runnells, 1969), the concentration of dissolved organic compounds, and 

pH. Precipitation of calcite can occur from essentially undersaturated waters wherever they become locally supersaturated with 

respect to calcite. This local supersaturation may develop near chemical or physical interfaces or along chemical gradients whenever any 

of the critical parameters are affected. Because chemical interfaces or 

gradients are almost never static in nature, their movement may produce a sequence of precipitation and dissolution throughout their 
DIAGENESIS 41 

path. Effects of this movement may be preserved as petrographic evidence. Specific mechanisms of this sort that could feasibly have 

operated during the burial history of the Eagle Sandstone include 

movement of the gas-water contact and the mixing of natural waters, 

as described by Runnells (1969), along the shoreline early in the 

burial history or within the formation during later burial and uplift. 

I believe this interpretation successfully explains the ubiquitous 

presence of cemented layers throughout the Eagle Sandstone and accounts for the absence of any large volumes of tightly cemented rocks 

in the formation. This mechanism also explains the origin of calcite 

dissolution features and the apparent former presence of calcite 

throughout large portions of the reservoir. This mechanism requires 

only small amounts of carbonate to be present at any one time; thus 

it avoids the invocation of seemingly catastrophic events of cementation and replacement followed by equally catastrophic dissolution, 

and it does not. exclude the occupation of the reservoir by methane. A 

proposed mechanism of continual precipitation and dissolution would 

be expected to effectively homogenize divergent carbon isotope contents, thus producing isotopic compositions consistent with those 

observed within the calcites of the Eagle Sandstone. 

As in most cases where the former presence of extensive replacement or cementing minerals can be documented, petrographic relationships yield little information about the absolute timing of such 

events. Thus, the simultaneity of calcitization throughout the Eagle 

Sandstone is not definitively demonstrated. Rather, calcite can only 

be placed within a relative paragenetic framework, mostly postdating 

compaction and quartz cementation (except for the earliest calcite) 

and preceding calcite dissolution and clay mineral development (table 8). A final determination of which model best explains the observed petrographic properties of the Eagle Sandstone is not possible 

at present, although I favor the latter, nonsimultaneous interpretation. A possible means of testing the various hypotheses is suggested 

by the computer models of carbonate dissolution and precipitation 

during natural water mixing developed by L. N. Plummer (1975). 

IRON CARBONATE 

Iron-rich carbonate phases identified by X-ray diffraction analysis 

as siderite or, less commonly, as ankerite are present in most samples 

in amounts ranging from a trace to several percent of the rock volume. These ferrous minerals typically occur as accumulations of tiny 

subhedral, golden crystals in patches a few tenths of a millimeter in 

diameter. The carbonate is often associated with biotite and, less 

commonly, with detrital chlorite in various stages of preservation. 
42 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

This relationship covers a continuous spectrum ranging from fresh, 

reddish-brown biotite in which a few rhombohedra of iron carbonate 

have formed between cleavage plates to massive accumulations of 

crystals containing a few small fragments of widely dispersed biotite 

(fig. lOA). Some areas of iron carbonate contain no detectable relict 

biotite, and many biotite grains display no carbonate alteration. 

These effects may be observed in either calcite-cemented or highly 

porous rocks. The paragenetic placement of the formation of this iron 

carbonate is tenuous, but the crystals appear to cut across preexisting calcite cement textures, suggesting that the iron carbonate 

formed later than some of the calcite. 

The upper portion of the 29-3X Olson core contains morphologically distinct iron-rich carbonate in far greater abundance than is 

typical for the Eagle Sandstone as a whole. In addition. to forming 

small localized patches, fine-grained carbonate has developed extensively in some areas, where it may fill most intergranular pores, 

largely replace framework grains, and locally develop into widespread accumulations containing no silicate material at all. X-ray diffraction analysis of this fine-grained iron carbonate indicates it is 

composed entirely of siderite (d10i 4=2.81 A). Qualitative compositional information, obtained with an X-ray energy-dispersive spectrometer attached to an SEM, indicates that the variance in d-spacing, 

compared with d10i 4=2.79 A for pure FeC03, results from the isomorphous entry of CaC03 into the siderite. This type of substitution is 

known to produce a slight expansion of the crystal lattice (Zaritskiy, 

1964). Stable carbon isotope analyses from two samples of this mineral indicate it possesses a carbon isotope ratio of about zero, relative 

to the PDB standard (table 9). 

More coarsely grained siderite also occurs in the 29-3X Olson core: 

as clear brown rims formed in crystallographic continuity with dolomite fragments (fig. lOB) and as discrete, perfectly euhedral rhombohedra 0.004 to 0.01 mm in diameter. The rhombohedra are disseminated throughout some samples and tend to be concentrated along intergranular pores, some of which are interpreted as having resulted 

from calcite cement dissolution. The crystals are uncommon in areas 

of extensive calcite and do not occur within intragranular pores in 

framework silicates. However, the rhombs may automorphically cut 

across most mineral phases within the sandstone, including calcite. 

The scanning electron microscope reveals that the rhombic crystals 

of siderite display absolutely no evidence of dissolution etching and 

appear to have formed later than the embayments in the quartz, 

which they may partly occupy (fig. lOC). Kaolin crystals in well-developed booklets commonly have formed upon the euhedral siderite 

crystals (fig. lOD). Thus, textural relationships indicate that the 
DIAGENESIS 43 

coarsely crystalline siderite definitely developed prior to authigenic 

kaolin and may postdate calcite precipitation and dissolution: 

The chemical requirements for the precipitation of siderite from 

solution and the equilibrium conditions controlling the chemical relationships among iron minerals in the presence of dissolved carbon, 

sulfur, and iron species have been discussed by Garrels and Christ 

(1965, p. 201-225). According to their calculations, the formation of 

siderite under ideal conditions at moderate pH values requires a very 

low oxidation potential, which permits high ferrous iron activities, a 

high bicarbonate activity, and very low values for total dissolved 

sulfur. Under such conditions ferrous carbonates may be stable in the 

presence of ferrous sulfides such as pyrite. The relationship between 

calcite and siderite is simple under ideal conditions and is governed 

by the following relationship: 

or 

CaC03 + Fe+iq = FeC03 + Ca+iq' c~st c~st 

where Keq is the equilibrium constant. The value obtained for Keq 

depends upon whose thermodynamic data are used in evaluating the 

relationship and may vary between 150 (Krauskopf, 1967, p. 83) and 

794 (calculated from free energy of formation values given by Garrels 

and Christ, 1965). In any case, siderite may be formed from calcite 

whenever significant amounts of ferrous iron are present. For example, using the Keq value given by Krauskopf, if the activity of dissolved calcium is equal to 10-1, siderite should precipitate under 

ideal equilibrium conditions at dissolved ferrous iron concentrations 

of about 0. 7 moles per liter. If thermodynamic values cited by Garrels and Christ are used, this value drops to about 0.1 moles per liter. 

In most samples of the Eagle Sandstone, iron carbonates were 

found mainly in localized patches, associated with detrital biotite 

(fig. lOA). This pattern of occurrence is regarded as strong evidence 

that the biotite has yielded iron from its lattice to its surroundings, 

facilitating siderite formation. This process may be analogous to the 

movement of iron from ferromagnesian minerals under oxidizing conditions, as reported for Cenozoic sediments of the Sonoran Desert 

(Walker and others, 1967). While the diffusion gradient thus created 

will produce ferric oxides and hydroxides under aerated conditions 

like those observed by Walker and others, a similar chemical gradient 

under conditions of high bicarbonate activity and very low oxidation 

potential would produce an iron carbonate such as siderite. In the 

case of calcite-cemented Eagle Sandstone, it is likely that the development of an iron diffusion gradient away from a source of iron, such 
44 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

FIGURE 10.-Noncalcitic authigenic 

minerals in the Eagle Sandstone. A, 

Mass of siderite (S) associated with 

remnants of altering biotite (B). (Crossed 

polars.) B, Rim of authigenic siderite (S) 

in optical continuity with monocrystalline grain of detrital dolomite (D). 

(Plane light.) C, Euhedral rhombohedron of siderite and kaolin booklets in 

embayed quartz overgrowth. (SEM 

photomicrograph.) D, Authigenic kaolinite (K) precipitated on surface of 

authigenic siderite (S). (SEM photomicrograph.) E, Kaolinite booklets in an 

intergranular pore. (Plane light.) 
DIAGENESIS 45 

as biotite, would raise the ferrous iron activity in the vicinity of the 

biotite to levels sufficient to locally replace the calcite cement with 

siderite. It is reasonable, therefore, that the formation of local siderite patches within the carbonate- cemented Eagle Sandstone occurred 

once iron became locally mobilized by the breakdown of biotite. 

The extensive occurrence of siderite observed in the upper portion 

of the 29-3X Olson Core, however, cannot be explained in this 

fashion. The production of such large quantities of iron carbonate apparently required the introduction of iron from an external source, as 

insufficient amounts are present within the detrital constituents of 

the Olson core. Carbon isotope data (table 9) suggest an episode of 

formation separate from and unrelated to calcitization. Speculation 

about this iron source would probably be fruitless except to point out 

that stratigraphically equivalent rocks, such as the Gammon 

Member of the Pierre Shale farther east and the Virgelle Sandstone 

near the Montana disturbed belt, are also iron-rich. The Gammon 

contains abundant ferruginous concretions, and the Virgelle Sandstone displays prominent placer-like concentrations of magnetite. In 
46 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

addition, in the vicinity of the Bearpaw Mountains, the Eagle Sandstone may display ferruginous cements in outcrop. These ferruginous 

zones may have developed from the oxidation of siderite during 

weathering. Whether these occurrences are genetically related or 

only coincidental is not known. 

INTRAGRANULAR POROSITY 

Dissolution processes have produced significant amounts of intragranular porosity within framework grains of the reservoir sandstones. Point counts indicate that many samples contain intragranular pore space in volumes equal to or even in excess of intergranular 

pore volume. Most of this dissolution has affected plagioclase 

feldspars, although minor intragranular voids may be observed in 

quartz, potassium feldspar, or dolomite grains. Dissolution features 

range from wedgelike, rhombic, or irregular voids within otherwise 

pristine andesine fragments (fig. 8D) or small embayments in quartz 

grain margins, to voids the size of framework grains that contain 

only a few scattered remnants of the original silicate structure 

(figs. BE, G). Occasional observations of authigenic kaolinite enclosing grain-size voids (moldic porosity), of oversize pores, and of inhomogeneous packing of framework grains demonstrate that 

framework silicates have been completely removed by dissolution 

processes. 

The removal of framework grain material may be largely accomplished by the dissolution of calcite that had previously replaced 

silicate grains. The preferential replacement of plagioclase by calcite 

and the resultant proliferation of intragranular voids in these feldspars suggest an affinity of the carbonate for the calcium-bearing 

plagioclases. Schmidt and others (1977) expressed the view that most 

secondary intragranular pores are formed by dissolution of carbonates. However, the direct dissolution of detrital and authigenic silicates has been demonstrated by Walker, Waugh, and Crone (1978, 

p. 19-32) and by Heald and Larese (1973), among others. This process may produce significant amounts of pore space, particularly in 

sandstones containing a large proportion of labile constituents such 

as volcaniclastic material (Hayes, 1979; Hayes and others, 1976). 

Evidence for the development of porosity through the direct dissolution of silicate grains is also present in the Eagle Sandstone. For 

example, in argillaceous sandstones and mudstones, where a former 

episode of extensive calcitization cannot be demonstrated, skeletal 

remnants of plagioclase grains are occasionally observed. Such remnants reflect instances where intragranular pores developed through 

direct plagioclase dissolution. 
DIAGENESIS 47 

Experimental work by Lagache and others (1961a, b) and Wollast 

(1967) resulted in a theory of feldspar dissolution kinetics, which is 

summarized by Helgeson (1971), who developed a rate law for the dissolution. In general, at low temperatures and pressures, plagioclase 

dissolution is controlled by the outward diffusion of the large cations 

(N a, Ca). Where the hydrogen ion concentration is moderate, the 

dissolution rate depends upon the amount of grain surface area exposed and the rate of movement of migrating dilute solutions. Under 

these conditions, the amount of dissolution is time-dependent. 

No evidence of direct plagioclase dissolution was observed in 

calcite-cemented rocks, suggesting that the framework had been protected from migrating fluids. It is likely that dissolution features 

observed in plagioclase grains in porous rocks resulted from the extensive removal of replacement calcite and, to a lesser extent, from 

the direct dissolution of feldspar. The relative extent to which these 

processes have operated is not known, although the indirect removal 

of plagioclase through dissolution of replacement calcite is far easier 

to document. 

AUTHIGENIC PHYLLOSILICATES 

Authigenic clays are dominated by kaolinite, which may form as 

much as 1 or 2 percent of the total rock volume in porous sandstones. 

The kaolinite generally occurs as well-crystalized "booklets," which 

accumulated in the large intergranular pore spaces (fig. 10E) and, less 

commonly, in the intragranular voids of extensively dissolved 

plagioclase grains. The infrequent development of kaolinite in the intragranular spaces suggests that kaolinite formation is more favored 

in large, throughgoing pores than in the complex intragranular voids. 

This aluminous clay commonly occurs in dissolution voids within optically continuous sparry calcite cement. Locally, kaolin developed 

into very coarse crystals, which may actually replace sparry calcite 

cement in some rocks. Such crystals are particularly common in the 

strongly cemented samples of the 29-3X Olson core. The SEM indicates that well-crystalized · kaolinite booklets have formed upon 

quartz overgrowths and on euhedral siderite rhombohedra (fig. 10D). 

All this evidence supports the conclusion that kaolinite formation 

was among the latest events in the diagenetic history of the Eagle 

Sandstone. It certainly postdates most calcite formation and dissolution, quartz precipitation, and the formation of iron-rich carbonates. 

The wide variety of substrates that served for kaolinite growth and 

its euhedral habit indicate that the clay precipitated directly from 

dilute migrating pore waters and did not form as an alteration product of preexisting silicates. 
48 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

Authigenic phyllosilicates other than kaolinite are generally 

formed only in localized areas. Dominant among these is an iron-rich, 

mixed-layer illite-smectite clay, containing approximately 80 percent 

expansible layers. It is most common in samples from the No.1 Morrison core. This clay occurs as an alteration product of some lithic 

fragments and as a volumetrically unimportant surface alteration of 

potassium feldspar. The mixed-layer clay may also occur as sparse 

pore linings in porous portions of sandstones rich in allogenic clay, 

and as a flamelike authigenic development upon detrital clays in 

mudstones. Petrographic evidence and SEM observations similar to 

those previously described for kaolinite suggest that these clays 

formed late in the burial history of the Eagle Sandstone. The mixedlayer clay is occasionally associated with kaolinite in pore spaces, but 

ambiguous morphological relationships between the two clays 

precludes a relative paragenetic placement for them. The general but 

not invariable occurrence of the mixed-layer clay with feldspars and 

allogenic clays, rather than with random open pores in the silicate 

framework, suggests that locally increased activities of ions other 

than the alumina and silica required for kaolinite precipitation may 

have been the controlling factors in its development. No consistent 

relationship was observed between this mixed-layer clay and framework composition, grain size, sandstone depositional environment, or 

rock permeability. 

Other phyllosilicates are also observed locally in the Eagle Sandstone, although not in the rocks examined in this investigation. 

These include iron-rich chlorite and highly illitic mixed-layer illitesmectite, which are abundant in the No. 12-1 Blackwood core. To my 

knowledge, these clays are ~onfined to areas of specifically eontrolled 

water chemistry associated with Eocene volcanism, where they may 

occur with authigenic tourmaline (Gautier, 1979). 

SUMMARY OF BURIAL HISTORY 

The Eagle Sandstone was deposited along the western shoreline of 

the Late Cretaceous epicontinental seaway. The sediments were derived from a highland, probably in western Montana, consisting of a 

mixed terrane of low-grade metasedimentary, sedimentary, volcanic, 

and possibly plutonic rocks. Clastic materials were transported by 

fluvial systems through small deltas, and were delivered to the site of 

deposition by longshore currents. Trace fossils and other textural 

evidence of bioturbation indicate that the sedimentation interface 

was aerated during Eagle deposition but that oxygen depletion due 

to bacterial aerobic respiration must have produced anoxic condi-
SUMMARY OF BURIAL HISTORY 49 

tions at very shallow depths in the sediment column. It is likely that 

a low oxidation potential has persisted in the Eagle Sandstone from 

that time to the present day. 

The formation of concretions and the precipitation of calcite cement began very shortly after burial. This very early carbonate is fine 

grained and rich in inclusions of organic matter and authigenic pyrite. Inasmuch as bacterial processes in the Eagle Sandstone and associated sediments were probably similar to those observed in the 

modern marine environment, this pyrite probably developed as a byproduct of the bacterial reduction of sulfate in the anoxic interstitial 

pore waters. This sulfate reduction was a necessary prerequisite to 

biogenic methane generation (Claypool and Kaplan, 197 4; Presley 

and Kaplan, 1968). As sedimentation continued and burial depths increased, compaction and minor quartz cementation lithified the uncemented portions of the Eagle Sandstone. By this time quantities of 

isotopically light methane had been produced by bacterial activity in 

associated organic-rich sediments. Methane generation continued until compaction precluded bacterial metabolism. However, burial depths were never great enough to induce temperature-related alterations of detrital clays or to achieve detectable thermal maturation of 

included hydrocarbons. Burial depth probably never exceeded 

1,400 m. 

Clear, sparry calcite cement began precipitating in intergranular 

pore spaces, marginally replacing most framework silicates and 

preferentially replacing many of the abundant plagioclase grains on a 

large scale. The precise causes and timing of this calcitization are not 

known. It certainly postdates some compaction and minor quartz 

cementation. Consistent isotopic fractionation is evident in carbon 

from this calcite. The calcite has at one time or another occupied virtually every pore and void in the Eagle Sandstone. It is not known 

with certainty whether this occupation occurred simultaneously 

throughout the unit, eliminating effective porosity and permeability, 

or whether it occurred only in discrete layers as at present. I favor the 

latter view. 

Iron-rich carbonates, mainly siderite, formed locally as a result of 

increased ferrous iron activities, which are usually associated with 

altering detrital biotite or chlorite fragments. This iron carbonate 

may have developed as an alteration of preexisting calcite cement. It 

is likely that the locally extensive formation of isotopically neutral 

siderite was due to the introduction of iron-rich, bicarbonate-rich 

waters from outside the stratum. 

Intragranular pores presently account for 30 percent or more of the 

void space in the Eagle Sandstone. Petrographic evidence indicates 

that most of the pores result from the removal of calcite replacements 
50 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

of plagioclase. However, at least some direct plagioclase dissolution 

has occurred. 

Authigenic clay minerals, particularly kaolinite, precipitated from 

solution. Their formation seems to postdate all other diagenetic 

events and, in the case of kaolin, to be favored in the larger 

throughgoing pores. The kaolinite may have derived some of its constituent ions from dissolving silicate grains. Locally higher ion activities permitted the formation of compositionally more complex 

minerals, such as mixed-layer illite-smectite. 

Biogenic methane, which formed at very shallow depths early in 

the burial history of the rocks, must have been present during all 

phases of diagenesis in the Eagle Sandstone. However, depending 

upon the interpretation applied to the timing and extent of calcitization, the methane may not have entered its present reservoirs until 

sometime after the onset of calcite dissolution. Previous work 

(Gautier, 1979) suggests that at least some major methane migration 

occurred later than the Eocene tectonic activity and volcanism. 

CONCLUSIONS 

Results of this investigation suggest that postdepositional processes have affected reservoir properties, as well as hydrocarbon formation and entrapment in the Eagle Sandstone, to a perhaps unexpected extent. Virtually every sample of Eagle Sandstone examined 

during this study shows evidence that the present size, geometry, 

and distribution of pores results from diagenetic processes. In addition to the important porosity enhancement resulting from the development of intragranular voids, strong petrographic evidence indicates that most or all of these reservoirs have at one time been 

extensively occupied by calcite. The timing of this calcitization is not 

known with certainty, however. The simplest interpretation of this 

secondary porosity requires that most of the formation was once 

tightly sealed and thus unable to serve as a reservoir. Alternatively, 

this dissolution porosity may have been developed through time by 

mainly localized processes. It is suggested that, at least in the case of 

conventional methane reservoirs in the Bearpaw Mountains, convincing evidence of secondary porosity is subject to multiple interpretations. In the absence of decisive evidence of simultaneity for the 

events that produced these effects, their true role in the development 

of porosity, permeability, and other reservoir properties is an open 

question. 

The present distribution of tightly cemented layers within these 
CONCLUSIONS 51 

reservoirs limits vertical permeability, thus effectively compartmentalizing the reservoir. If these cemented portions of the reservoirs developed early in the burial history of the Eagle Sandstone, as seems 

likely at least for some of them, they may have been an important initial trapping mechanism for natural gas. This effect would be of particular importance in lower permeability facies to the east, where preliminary investigations indicate the prest~nce of similar horizontal 

layers of impermeable sandstone, generally at lithologic breaks. 

Authigenic phases potentially hazardous to well completion and 

treatment procedures have been identified. These include (I) ferrous 

carbonates, which are easily oxidized to insoluble ferric oxides and 

hydroxides by the introduction of fluids of relatively high oxidation 

potential; (2) smectitic niixed-layer clay and intergranular kaolinite, 

which may expand or migrate in the presence of dilute fluids, and 

(3) acid-soluble, iron-rich chlorite, the dissolution of which may also 

facilitate the precipitation of ferric oxides and hydroxides. Chlorite of 

this type was not observed in this study but has been identified in 

some Eagle Sandstone reservoirs (Gautier, 1979). Although these 

minerals may be present in sufficient quantities to affect hydrocarbon exploitation activities, their distribution is not generally predictable. However, intrastratal alterations associated with Eocene 

volcanism may provide an exception. The extensive formation of 

authigenic phases observed in the No. 12-1 Blackwood core probably 

resulted from the introduction of chemically enriched waters associated with volcanic activity (Gautier, 1979). These minerals, which include iron-rich chlorite, highly illitic mixed-layer illite-smectite clay, 

and quartz and tourmaline, are potentially detrimental to reservoir 

quality. According to William H. Bayliff (1975, p. 35), wells completed in the Eagle Sandstone near areas of igneous activity in the 

Bearpaw Mountains display comparatively low initial gas production 

rates and, subsequently, rapid production decline rates. It is likely 

that these effects are due to . alterations in reservoir sandstones 

similar to those present in the No. 12-1 Blackwood core. 

The extent to which postdepositional processes have operated 

within shallow gas reservoirs, now occupied by biogenic methane, has 

not been previously documented. This ·investigation indicates that 

the Eagle Sandstone displays many diagenetic features that are 

similar to those previously observed in rocks having a burial history 

suitable for the generation of oil or thermogenic gas. I conclude that 

the thermal maturation of organic matter and deep burial are not 

prerequisites for the occlusion of porosity by authigenic minerals, nor 

for the development of significant volumes of secondary intragranular and intergranular porosity. 
52 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA 

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