Petrology and Reservoir Paragenesis 

in the Sussex "B" Sandstone of the 

Upper Cretaceous Cody Shale, 

House Creek and Porcupine Fields, 

Powder River Basin, Wyoming

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Chapter G

Petrology and Reservoir Paragenesis 

in the Sussex "B" Sandstone of the 

Upper Cretaceous Cody Shale, 

House Creek and Porcupine Fields, 

Powder River Basin, Wyoming

By DEBRA K. HIGLEY

A multidisciplinary approach to research studies of sedimentary 

rocks and their constituents and the evolution of sedimentary 

basins, both ancient and modern

U.S. GEOLOGICAL SURVEY BULLETIN 1917 

EVOLUTION OF SEDIMENTARY BASINS POWDER RIVER BASIN
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

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

purposes only and does not imply endorsement by the U.S. Government

UNITED STATES GOVERNMENT PRINTING OFFICE: 1992

For sale by the

Books and Open-File Reports Section

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Library of Congress Cataloging-in-Publication Data

Higley, Debra K.

Petrology and reservoir paragenesis in the Sussex "B" sandstone of the Upper 

Cretaceous Cody Shale, House Creek and Porcupine fields, Powder River Basin, 

Wyoming/by Debra K. Higley.

p. cm. (Evolution of sedimentary basins Powder River Basin ; ch. G) 

(U.S. Geological Survey bulletin ; 1917 G)

"A multidisciplinary approach to research studies of sedimentary rocks and 

their constituents and the evolution of sedimentary basins, both ancient and 

modern."

Includes bibliographical references.

1. Petroleum Geology Powder River Basin (Wyo. and Mont.). 2. Oil 

fields Powder River Basin (Wyo. and Mont). 3. Sandstone Powder River 

Basin (Wyo. and Mont.) I. Title. II. Series. III. Series: U.S. Geological 

Survey bulletin ; 1917-G. 

QE75.B9 no. 1917-G 

[TN870.5] 

557.3 s dc20

[553.2'8'097863] 91-24130

CIP
CONTENTS

Abstract Gl 

Introduction Gl 

Geologic setting G3 

Sandstone diagenesis G3

Reservoir paragenesis G4

Porosity relationships G5 

Petrologic characteristics of sandstone fades G7 

Inter-ridge facies G10

Inter-ridge sandstone and siltstone G10 

Wavy-bedded inter-ridge sandstone G12 

Ridge-margin and central-ridge sandstone G13 

Low-energy ridge-margin sandstone G13 

Central-ridge and high-energy ridge-margin sandstone G13 

Chert pebble sandstone G14 

Conclusions G15 

Acknowledgments G15 

References cited G15

FIGURES

1. Index and structure contour map of the Powder River Basin, Wyoming and

Montana G2 

2-3. Charts showing:

2. Stratigraphic nomenclature of Upper Cretaceous and Tertiary rocks, 

Powder River Basin G3

3. Generalized paragenetic sequence, Sussex "B" sandstone G5 

4 5. SEM photomicrographs of:

4. Sussex "B" sandstone porosity relationships G6

5. Authigenic clays, Sussex "B" sandstone G7

6. Crossplots of relationships between porosity, permeability, and cements, Sussex 

"B" sandstone G8

7. Thin section photomicrographs showing sandstone petrology and 

diagenesis G9

8. Sedimentologic, petrologic, and reservoir characteristics of Sussex "B"

sandstone facies Gil 

9 10. Thin section photomicrographs of:

9. Compaction and cementation relationships in inter-ridge facies G12

10. Porosity reduction in low-energy ridge-margin sandstones G13

TABLES

1. Names and locations of wells sampled for petrographic analysis G4

2. Selected thin-section point-count data G4

Contents III

EVOLUTION OF SEDIMENTARY BASINS-POWDER RIVER BASIN

Petrology and Reservoir Paragenesis 

in the Sussex "B" Sandstone of the 

Upper Cretaceous Cody Shale, 

House Creek and Porcupine Fields, 

Powder River Basin, Wyoming

ByDebra K. Higley

Abstract

Oil is produced in the House Creek and Porcupine fields 

in the Powder River Basin from central-ridge and ridge-margin 

facies of the Sussex "B" sandstone, Sussex Sandstone Member 

of the Upper Cretaceous Cody Shale. Mudstone forms overlying and updip reservoir seals in these fields. Hydrocarbons 

are also trapped by a thin, areally discontinuous chert pebble 

sandstone that may represent an unconformity surface.

Macroscopic reservoir heterogeneity results from interbedding of reservoir sandstones with inter-ridge and lowenergy ridge-margin sandstones. This interbedding forms sedimentologic permeability boundaries that isolate and 

compartmentalize individual sand bodies. Ductile deformation of glauconite, which is concentrated in reservoir sandstone trough cross-bedding ripple laminae, results in minor 

permeability boundaries to fluid flow.

Microscopic heterogeneity and porosity loss in the 

Sussex "B" sandstone results primarily from highly variable 

amounts and distributions of quartz and calcite cements. 

Minus-cement porosity that averages 30.2 percent for nonbioturbated sandstones suggests a compactional volume 

decrease of 5-10 percent porosity prior to lithification. 

Compactional processes in the Sussex "B" sandstone are 

important only in bioturbated sandstone these are found in 

mainly inter-ridge and low-energy ridge-margin depositional 

environments.

Oil-producing sandstones are commonly cemented by 

quartz, which averages about 8 volume percent of the rock. 

These sandstones contain only minor amounts of carbonate 

cement. However, the volumetrically most important cement

Manuscript approved for publication May 3, 1991.

in non-productive Sussex "B" sandstone is calcite. Authigenic 

kaolinite, illite-smectite, and chlorite influence reservoir 

properties by occluding pore spaces and pore throats. Most 

porosity in reservoir sandstones is primary; however, secondary porosity is created by dissolution of plagioclase and other 

unstable lithic grains and by dissolution of carbonate cements 

that filled pore spaces and replaced lithic grains and overgrowths. Microporosity is present within chert grains and 

clays.

INTRODUCTION

Using petrologic and sedimentologic studies, this 

paper characterizes the influence of sedimentologic and 

petrologic variations on reservoir heterogeneity in the Sussex "B" sandstone in the House Creek and Porcupine fields, 

Powder River Basin, Wyoming. Effects of authigenic minerals on reservoir properties are described in detail for 

selected inter-ridge and ridge facies sandstones.

The House Creek and Porcupine fields are about 1 mi 

(1.6 km) wide and have a combined length of more than 

36 mi (58 km). The fields form a major ridge system that is 

composed mainly of coalescing small- to medium-scale 

subaqueous dunes. The House Creek and Porcupine fields, 

and the ridge facies they outline, trend about N. 40° W., 

approximately parallel to basin structure contours (fig. 1).

The Sussex "B" sandstone is as much as 45 ft (14 m) 

thick, ranges in depth from about 7,000 to 9,000 ft 

(2,100-2,700 m), and is one of as many as twelve separate 

sandstone bodies that are located at different vertical and 

lateral positions within the Sussex Sandstone Member of the 

Upper Cretaceous Cody Shale (Anderman, 1976; Crews and

Petrology and Reservoir Paragenesis in Sussex "B" Sandstone G1
CARTER

45°

44°

43°

Sheridan

SHERIDAN

Outcrops of the Lance

Formation and 

Fox Hills Sandstone

Location of the 

Powder River Basin

107°

30 miles 

30 kilometers

105° 

Contour interval 500 feet

Figure 1. Powder River Basin structure map. Contours drawn on the top of the Sussex Sandstone Member of the Upper 

Cretaceous Cody Shale and its equivalents. Locations of the Salt Creek outcrop (large arrow) and House Creek and Porcupine 

fields are shown. Datum is mean sea level.

others, 1976). Oil is stratigraphically trapped in the Sussex 

"B" sandstone by overlying mudstone and ferroan calcitecemented sandstone and by a lateral change from permeable 

sandstone to relatively impermeable mudstone. Approximately 21 million barrels of oil (MMBO) and 19 million

cubic feet of gas (MMCFG) have been produced from the 

House Creek field, and more than 3 MMBO and 5 MMCFG 

have been produced from all reservoirs within the Porcupine 

field (as of December 1989). Porcupine field production 

is from the Muddy Sandstone Member of the Lower

G2 Evolution of Sedimentary Basins-Powder River Basin
Ma

1.6- 

5 -

24-

37.5.

55-

66-

78- 

96-

ERATHEM

CENOZOIC (PART)

| MESOZOIC (PART)

SYSTEM/

.QPRIFQ

TERTIARY

I CRETACEOUS (PART)

PLIOCENE

MIOCENE

OLIGOCENE

EOCENE

PALEOCENE

W

ID

o i

LJJ :

O s

<

LJJ 

DC 

O

DC 

UJ

O. 

O.

13

POWDER RIVER 

BASIN

WEST

White River 

Formation

 I

Wasatch Fm.

Wv^*v*v*'/V*V"vN'\* 

S

Fort Union Fm.

Lance Fm.

Fox Hills Ss.

Lewis Shale

Mesaverde 

Formation

Sussex Ss. Mem./

Shannon Ss. Mem./

Cody

Shale

A,

Frontier

Formation

Mowry 

Shale

EAST

White River 

Formation

 

Wasatch Fm.

/\<*V*v*v**W*vVv*v*v

Fort Union Fm.

Lance Fm.

Fox Hills Ss.

Pierre Shale

Niobrara Fm.

 VWWv^vV***vN/

Carlile Shale

Greenhorn Fm.

Belle Fourche Sh.

Mowry 

Shale

Figure 2. Generalized stratigraphic section showing nomenclature and correlations of Upper Cretaceous and Tertiary 

rocks in the Powder River Basin of Wyoming and Montana. 

Unconformities indicated by vertical ruled pattern and (or) 

jagged lines.

Cretaceous Thermopolis Shale, the Sussex "B" sandstone, 

and the Turner Sandy Member of the Upper Cretaceous 

Carlile Shale (fig. 2) (Lawyer and others, 1981). The mean 

reservoir porosity and permeability of pay sandstones in the 

House Creek field are 13 percent and 15 millidarcies (md), 

and the thickness of perforated sandstones ranges from 2 to 

32 ft (0.6-10 m) (Sabel, 1985). Reservoir porosity in the 

Porcupine field is 8-12 percent (Lawyer and others, 1981). 

Facies assignments were determined from outcrop 

and core studies and by correlation of stratigraphic data to 

geophysical well logs. Sussex "B" core from 21 wells was 

described (Higley, 1988); these cores are located at the U.S. 

Geological Survey core library in Denver, Colorado. Cores 

from 10 wells located in and near the House Creek and 

Porcupine oil fields were sampled for petrographic analysis 

(table 1). Thin sections of 54 samples, X-ray diffraction 

analyses, and scanning electron microscope studies were 

used to determine reservoir paragenesis, sandstone porosity, 

and mineralogic constituents of the Sussex "B" sandstone in 

the study area.

GEOLOGIC SETTING

The Powder River Basin is a Laramide-style structural basin with a steeply dipping western flank and a gently 

dipping eastern flank. The Cody Shale, including the Sussex 

Sandstone Member and its equivalents, underlies most of 

the basin and outcrops along the western flank, west of the 

Lance Formation and Fox Hills Sandstone outcrops shown 

on figure 1. Sandstone body and basin geometry indicate 

that the Sussex "B" sandstone was deposited as a narrow, 

low-relief, marine ridge complex in a shallow, offshore 

marine setting within the narrow Campanian seaway 

(Asquith, 1970; Berg, 1975; Hobson and others, 1982). This 

seaway extended from the Arctic to the Gulf of Mexico.

Sussex Sandstone Member deposition may represent 

transgressive marine reworking by oceanic, storm, and tidal 

currents of residual sand sheets that were deposited during 

a previous progradation (Hobson and others, 1982). Berg 

(1975), Brenner (1978), and Hobson and others (1982) 

believe that the Sussex Sandstone Member was deposited 

more than 100 mi (160 km) east of the Upper Cretaceous 

Eagle Sandstone shoreline as marine topsets in a progradation of marine shelf, slope, and basin sediments. 

Hobson and others (1982) further postulate that the sources 

of the sandstone for the Sussex Sandstone Member were the 

nearshore marine and deltaic sediments of the Eagle 

Sandstone. Brenner (1978) used core and well-log studies to 

hypothesize that Sussex Sandstone Member ridge facies 

were reworked and shaped by storm and tidal currents. 

Deposition in water as deep as 100-200 ft (30-60 m) is 

inferred based on Haplophragmoides and Reophax foraminiferal tests (Berg, 1975) and on the presence of trace 

fossils from oxygenated water (including Skolithos, Chondrites, Diphcraterion, and Arenicolites). Asquith (1970) 

inferred water depths as shallow as 50 ft (15 m) for 

sandstone deposition.

SANDSTONE DIAGENESIS

The Sussex "B" sandstone is a very fine to mediumgrained litharenite to feldspathic litharenite (Folk, 1970). 

Reservoir sandstones are composed of moderately sorted to 

fairly well sorted subangular to subrounded grains that 

commonly exhibit well-developed quartz overgrowths. 

Included in table 2 are average volume percentages of 

minerals and rock fragments determined from point-count 

data of 54 thin sections for all sedimentary facies of the 

Sussex "B" sandstone in the area of the House Creek and 

Porcupine fields. Porosity and mineralogic determinations 

were derived from more than 300 point-count measurements. Thin sections were stained for calcite, potassium 

feldspar, plagioclase, and iron. Mineralogical compositions 

were verified by X-ray diffraction and SEM analysis.

Petrology and Reservoir Paragenesis in Sussex "B" Sandstone G3
Table 1. Names and locations of wells sampled for petrographic analysis, Sussex "B" 

sandstone, Sussex Sandstone Member of the Upper Cretaceous Cody Shale

[HC, House Creek field; P, Porcupine field; *, estimated depth to top of Sussex "B" sandstone. Leaders (--) 

indicate that well is not within House Creek or Porcupine fields]

Well name

Location 

Field name (Section, township, range)

Depth to top of 

Sussex *B* fin feet)

14-9 Federal

1 Campbell

1 Government Miles "A"

1 Mandell Federal

1-23 House Creek Federal

1 Empire-Federal "C"

32-1 Marguiss "B"

2 House Creek Federal

1 Red Unit

1-1 lUcross State

P

P

HC

HC

HC

HC

HC

HC

-

-

9,T.42N.,R.71W.

21,T.42N,R.71W.

9, T. 44 N., R. 73 W.

22, T. 44 N., R. 73 W.

23, T. 44 N., R. 73 W.

29, T. 45 N., R. 73 W.

32, T. 45 N., R. 73 W.

13, T. 45 N., R. 74 W.

26, T, 48 N., R. 79 W.

11,T.52N.,R.81W.

Core

7,781

7,700

8,189

8,166

8,238

8,006

7,980

8,175

9,028*

8,233*

Well log

7,778

7,708

8,190

8,174

8,240

8,004

7,998

8,175

9,028

8,245

Table 2. Selected thin-section point-count data for depositional facies of the Sussex "B" sandstone, Sussex Sandstone Member 

of the Upper Cretaceous Cody Shale, Powder River Basin, Wyoming

[Data represent average volume percentages for number of thin sections listed. N, number of thin sections studied; qtz, quartz; poly-x qtz, polycrystalline 

quartz; ms, mudstone; sid, siderite; chl, chlorite; cmt, cement; cly clay; frag, fragments; -cmt por, minus-cement porosity]

Facies N Framework grains _______________ Mainly detrital matrix _____________ Cements, clays, porosity

Qtz Poly-x Feld- Rock Glauc- Matrix Drapes Pyr- Chl Qtz Chert Feld- Carb- Kaol- Poro- -Cmt 

______________qtz spar frag onite Ms Cly Ms Sid ite rims cmt cmt spar onate inite sity por 

All facies.........54 31.1 2.9 6.3 13.2 5.1 3.0 5.9 1.8 0.9 0.6 0.9 7.7 2.1 1.5 10.6 0.8 5.4 29.6

Chert pebble

sandstone .........3 26.8 0.8 3.5 36.3 3.0 0.0 2.1 0.0 0.0 0.2 0.0 1.9 0.0 0.2 20.9 0.0 1.6 24.8

Central-ridge

sandstone .......17 32.2 2.9 5.7 14.1 4.7 0.7 3.9 2.1 2.9 0.3 0.8 8.5 2.1 13 9.5 0.4 7.7 30.7

High-energy

ridge margin.....8 30.0 2.7 7.1 11.0 5.5 2.8 3.8 1.2 0.1 1.3 1.3 8.5 2.2 1.6 14.3 0.5 6.1 35.8

Low-energy

ridge margin.....3 26.8 3.1 8.5 11.8 6.7 1.4 7.6 1.2 0.8 1.0 1.1 11.3 1.9 3.8 9.7 0.9 2.4 32.1

Wavy-bedded

inter-ridge ........6 31.5 2.1 6.9 10.2 6.9 3.2 5.1 0.0 0.0 0.4 1.8 7.9 2.2 2.2 11.9 1.6 5.9 33.9

Inter-ridge

sandstone .......10 30.7 2.8 7.4 11.1 5.5 3.3 7.2 3.2 0.2 0.7 0.7 8.3 2.2 1.5 8.5 13 5.4 28.7

Inter-ridge

siltstone............7 33.3 2.3 5.3 9.9 3.4 103 13.3 3.3 0.2 0.6 0.6 4.5 2.6 0.7 6.8 0.6 2.1 18.6

Quartz grains in the Sussex "B" sandstone are almost 

entirely monocrystalline. Chert averages 5.8 percent by 

volume for the 54 samples and is included with mudstone 

clasts and minor amounts of sedimentary and volcanic rock 

fragments as part of the rock fragment category (table 2). 

Feldspar consists almost entirely of plagioclase (primarily 

albite); potassium feldspar is rare. The detrital matrix 

category contains mudstone and clay matrix categories 

(table 2). Clay matrix represents undifferentiated clays of 

mixed authigenic and detrital origin. Authigenic clay matrix 

may have formed by precipitation of clays into primary and 

secondary pore spaces and (or) by minor replacement of 

glauconite and lithic grains by illitic clay. X-ray diffraction 

analysis indicates that separates of clay from the Sussex "B"

sandstone are composed of illite, ordered and random-layer 

illite-smectite, iron-rich chlorite or chamosite, glauconite, 

and kaolinite. The minus-cement porosity category (table 2) 

combines intergranular primary porosity and secondary 

porosity (in addition to authigenic cements and clays) as an 

estimate of the amount of porosity at the time of lithification.

Reservoir Paragenesis

Reservoir paragenesis of the Sussex "B" sandstone in 

the House Creek and Porcupine fields is shown on figure 3. 

The inferred principal diagenetic events are:

G4 Evolution of Sedimentary Basins-Powder River Basin
BURIAL HISTORY EARLY MIDDLE LATE

Compaction 

Quartz Cement 

Carbonate Cement

Alteration of Lithics

Chlorite, Pyrite 

Glauconite,

Plagioclase Cement, 

Albitization

Oil Generation and 

Migration, Pyrite

Kaolinite, 

Illite-Smectite

( i

Figure 3. Generalized paragenetic sequence of the Upper Cretaceous Sussex "B" sandstone in the House Creek and 

Porcupine fields. Dashed line indicates dissolution of calcite and ferroan calcite cements.

1. Calcite precipitated during two major diagenetic events 

and is present primarily as poikilotopic crystals (fig. 4C). 

Siderite cementation occurred during the very early compactional history of the Sussex "B" sandstone. Dolomite 

and rare ankerite precipitated during the later stage of 

carbonate cementation and commonly occur as scattered 

rhombs. The average for all facies of 10.6 volume percent 

carbonate cement includes a mean 8.6 percent calcite and 

ferroan calcite and 2.0 percent dolomite and ferroan 

dolomite (determined from thin sections stained for calcite 

and iron).

2. An early stage of quartz cementation is characterized by 

minor syntaxial overgrowths, many of which are meniscal 

in appearance. Early quartz precipitation is interrupted by 

cementation by poikilotopic and radial-fibrous calcite.

3. Glauconite grains represent early diagenetic alteration 

of fecal pellets and biotite. The presence of glauconite, in 

addition to pyrite and chlorite, indicates reducing conditions 

in marine pore fluids.

4. Pyrite as framboids and minor euhedral crystals is 

present within organic-rich mudstone laminae and later 

hydrocarbons.

5. Rims of chlorite grow perpendicular to quartz grain 

surfaces. Chlorite crystals coat grains, are intergrown with, 

and may partially replace the quartz overgrowths of stage 6, 

below.

6. Syntaxial quartz overgrowths precipitate (figs. 4, 5A). 

This represents the major quartz cementation event.

7. Authigenic feldspar is present as plagioclase overgrowths on albite grains and as minor amounts of porefilling cement. Grains and overgrowths commonly exhibit 

albite twinning. Most albite appears detrital; however, 

chessboard twinning of some grains indicates albitization. 

Dissolution of plagioclase occurred during two stages: the 

first stage occurred prior to overgrowth formation, and the 

second stage may be related to changes in pore fluids that 

were associated with hydrocarbon generation and migration. Plagioclase may exhibit extensive replacement by 

calcite and dolomite cements; replacement by calcite 

occurred during both stages of carbonate cementation.

8. Oil from the House Creek and Porcupine fields 

migrated updip and up-fracture during early Eocene to 

Miocene time (Nuccio, 1990; Momper and Williams, 1984). 

Cores of organic-rich marine shale, adjacent to the Sussex 

"B" sandstone, were sampled from 6 locations across the 

fields. Samples range in depth from 7,776 to 9,803 ft 

(2,370-2,988 m). Vitrinite reflectance values range from 

0.51 to 0.58 percent, suggesting that hydrocarbon source 

rocks are only marginally mature for oil generation.

9. Following the onset of oil migration, kaolinite and 

minor amounts of associated illite-smectite precipitated in 

pore spaces (figs. 4, 5).

Porosity Relationships

Porosity and permeability increase generally upward 

from inter-ridge to central-ridge facies, reflecting depositional energy trends in the Sussex "B" sandstone. Increase 

in porosity is usually accompanied by an increase in

Petrology and Reservoir Paragenesis in Sussex "B" Sandstone G5
permeability, although scatter in the data is considerable 

(fig. 6A). Porosity relationships are complex petrology 

and diagenesis may vary greatly across small distances. The 

greatest amounts of porosity are present in central-ridge and 

high-energy ridge-margin facies. Lower depositional energy 

facies may contain moderate amounts of porosity; however, 

these thin, porous beds exhibit a spotty distribution and 

sedimentologic isolation that precludes their development 

as reservoir sandstones. Vertical stacking of thin beds of 

carbonate-cemented and non-cemented zones is common in 

the Sussex "B" sandstone; this may reflect isolation of 

cemented zones from migrating fluids that dissolved 

carbonate cements in other sandstone beds.

Porosity is mainly of the primary intergranular type; 

however, secondary porosity is present as corroded and 

dissolved lithic grains, and microporosity exists within chert 

and clays (fig. 4). Intergranular plus intragranular porosity 

in studied thin sections may total as much as 17.5 percent.

Figure 4 (above and facing column). SEM photomicrographs 

of: A, fine- to medium-grained trough cross-bedded sandstone 

from an oil-productive interval at 8,197.5 ft (2,498.6 m), No. 1 

Government Miles "A" well. Sandstone has well-developed 

quartz overgrowths and moderate intergranular porosity (17.5 

percent thin-section porosity). Porosity is mostly primary; 

however, secondary porosity results from dissolution of lithic 

grains (rim of chlorite and illitic clay (I)), and microporosity is 

present within clays. X is a fossil fragment. B, Close-up view of 

figure 4A shows a pore (P), adjacent quartz overgrowth and 

grain, a fossil fragment (X), and kaolinite. C, Poikilotopic 

calcite cement fills the pore space (P) in this inter-ridge 

sandstone from the No. 14-9 Federal well. Later chlorite 

rosettes and pyrite framboids (F) are present on calcite.

Average porosity for all studied thin sections is 5.2 percent; 

3.4 percent of this is primary intergranular, and as much as 

1.9 percent is secondary and intragranular porosity. Average 

porosity values for Sussex "B" facies are listed in table 2. 

Thin-section porosity values are lower than core and 

average reservoir porosity values for two main reasons: (1) 

Microporosity is not included in thin-section estimates but 

is measured by core analysis. The amount of microporosity 

may be considerable because clays comprise an average of 

12 percent by volume of the Sussex "B." The percentage of 

microporosity should decrease up-section because the 

amounts of suspended-load sediment and most clays 

decrease upward. (2) Three-dimensional effects of thin 

sections influence point-count results.

Sussex "B" sediment compaction was slowed by two 

processes: early calcite cementation and later formation of 

quartz overgrowths (fig. 3). Pry or (1973) analyzed porosity 

and permeability within unconsolidated beach sands of the 

Gulf Coast and found that depositional porosity of beach 

sands ranges from 39 to 56 percent, decreasing rapidly with 

burial. Initial porosity within central-ridge facies sandstone 

was probably 35-40 percent. Sussex "B" sandstone samples 

that contain the early calcite cement have minus-cement 

porosity (MCP) values of as much as 50.2 percent in this

G6 Evolution of Sedimentary Basins-Powder River Basin
case, calcite cement is 46.2 volume percent. This MCP 

value is misleading because early calcite within some 

intervals is moderately to highly displacive (fig. 7A), and 

calcite also partly to totally replaced some lithic grains. This 

results in substantial overestimation of porosity at the time 

of early calcite cementation. Diagenesis is commonly 

"frozen" within units that contain pervasive early calcite 

cement, suggesting that some permeability is required for 

calcite dissolution and for precipitation of later authigenic 

minerals.

Two periods of quartz cementation are shown on 

figure 3. Early quartz cementation is characterized by 

scattered, minor, poorly developed overgrowths; later overgrowths are well-developed and commonly have graincoating chlorite rims (figs. IB, IE). Some sandstone beds 

contain pervasive quartz cement (more than 12 volume 

percent) and display minimal evidence of prior cementation 

by calcite (fig. 6B). Therefore, the average 30 percent MCP 

in these intervals reflects porosity at the time of quartz

Figure 5 (above and facing column). SEM photomicrographs 

of authigenic clays in Sussex "B" sandstone. A, Chlorite rims a 

quartz grain in oil-productive central-ridge facies sandstone 

from 8,171.3 ft. (2,490.6m) depth, No. 1 Mandell Federal oil 

well. Quartz cement (O) overlies, and is intergrown with, 

chlorite. Quartz overgrowths have well-defined crystal faces 

that interlock and coalesce. B, Illite-smectite coats a pore; 

sample is from No. 1 Campbell well at a depth of 7,723.7 ft 

(2,354.2 m) in a planar-tabular cross-bedded ridge-margin 

sandstone. Close-up view is located within inset. C, Kaolinite 

covers a fractured quartz grain (Q) and fills porosity in a 

wavy-bedded inter-ridge sandstone at 8,245 ft. (2,513 m) 

depth in the No. 1-23 House Creek Federal well. Minor 

amounts of illitic clay are present as "wispy" coatings on grains 

and kaolinite.

cementation. Also, those intervals with the greatest porosity 

are generally quartz cemented and contain only minor 

amounts of calcite cement (fig. 6C). Dissolution of porefilling and replacive carbonate cements near maximum 

burial depths restored primary porosity and created locally 

extensive secondary porosity.

A second stage of carbonate cementation followed 

precipitation of well-developed quartz and feldspar overgrowths (fig. IE). Samples that exhibit this stage of 

carbonate cementation commonly contain intermediate 

amounts of the quartz and later calcite cements and are 

shown near the center of figure 6B. These later calcite and 

ferroan carbonate cements are commonly poikilotopic, with 

minor rhombs, and may be highly replacive of feldspar 

grains and overgrowths, glauconite, and rock fragments.

PETROLOGIC CHARACTERISTICS OF 

SANDSTONE FACIES

Facies assignments are based on core, well log, and 

outcrop studies. They are partly based on an evaluation of 

the Shannon Sandstone Member (fig. 2) in the Powder River 

Basin (Tillman and Martinsen, 1984) and on the Sussex 

Sandstone Member facies descriptions of Berg (1975) 

(fig. 8). The Sussex "B" sandstone is generally composed of

Petrology and Reservoir Paragenesis in Sussex "B" Sandstone G7
1000

CO 

UJ

o 10 °

DC

O

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UJ

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0.01

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5 10 15 20 

POROSITY, IN PERCENT

25

Figure 6 (facing column). Crossplots showing porosity, permeability, and cement relationships. A, Increase in core 

porosity is generally accompanied by increasing permeability, 

although considerable scatter is present. Graph represents core 

porosity and permeability for 792 samples of Sussex "B" 

sandstone from 30 drillholes across the study area. B and C, 

Volume percentages of intergranular plus intragranular 

porosity and quartz and calcite cements were determined for 

54 samples by thin-section petrology methods. Amounts of 

cements are highly variable, although certain trends are 

present. B, The central-ridge and high-energy ridge-margin 

reservoir facies generally contain 8 percent or greater quartz 

cement and highly variable amounts of calcite cement. 

Samples that contain large amounts of quartz cement (more 

than 1 2 percent) commonly have minimal calcite and ferroan 

calcite cement. C, Amounts of calcite cement in excess of 15 

percent are generally associated with low porosity. Quartzcemented sandstone with minor cementation by calcite 

exhibits the greatest porosity within these intervals, calcite 

cementation was minor, or porosity was restored by extensive 

late-stage dissolution of calcite.

25

UJ

20-

15-

10 -

5-

0-

B RESERVOIR FACIES

a NON-RESERVOIR FACIES

- °f

0 10 20 30 40 50 

CALCITE CEMENT, IN VOLUME PERCENT

50

(/)

I- 40-^

UJ

UJ 

Oy

UJDC 30-|

Q-J 20-

10-

0 -

. C D QUARTZ CEMENT 

 CALCITE CEMENT

 a

 D

 

0° . D°

 n a

o"a

0 10 20 

THIN-SECTION POROSITY, IN VOLUME PERCENT

Figure 7 (facing page). Photomicrographs showing paragenesis and porosity evolution in the Sussex "B" sandstone. 

Ferroan carbonates are stained blue. Blue epoxy fills pore 

spaces. Scale bars are 0.1mm. Transmitted light. A, 

Precipitation of "early" radial-fibrous calcite (C) results in a 

moderately replacive and highly displacive fabric with 

"floating" quartz grains (Q), minor deformation of glauconite 

(G) and mudstone clasts (M), and very low porosity (which is 

present as shrinkage cracks in mudstone and glauconite). The 

chevron pattern below the mudstone results from nucleation of 

calcite within mudstone layers and separation of laminae. This 

high-energy ridge-margin sandstone is at 7,723.7 ft (2,354.2 

m) depth in the No. 1 Campbell well. B, Oil-stained chlorite 

rims are beneath well-developed quartz overgrowths (arrow) 

in this sandstone from 8,012.7 ft (2,442.3 m) depth, No. 1 

Empire-Federal "C" well. Remnants of late dolomite (D) partly 

fill pores. Authigenic albite (F) is located within a secondary 

pore space. C, Ferroan calcite (dark blue) peripherally replaces 

grains and fills dissolution pores in plagioclase (F). This 

inter-ridge sandstone was sampled from 7,797.6 ft (2,376.7 m) 

depth, No. 14 9 Federal well. D, Soft-sediment deformation of 

glauconite and mudstone (M) that is concentrated in a ripple 

lamina creates a permeability boundary in this sandstone from 

8,014 ft (2,443 m) depth, No. 1 Empire-Federal "C" well. 

Kaolinite (K) fills a secondary pore, f, Late-stage calcite (C) 

covers quartz overgrowths in a sandstone from 8,167 ft (2,489 

m), No. 1 Federal well. Porosity includes primary (triangular 

pore) and dissolution (P) pores. F, Chert pebble sandstone from 

7,781.8 ft (2,371.8 m) in the No. 14-9 Federal well. Grain 

sorting is distinctly bimodal. Quartz grains are located in 

ferroan calcite cement between chert pebbles (CT). Quartz 

overgrowths are minor (arrow) and partially replaced by 

calcite. Porosity is present within a chert grain and as a 

dissolved foraminifera test (X). Scales are 0.1 and 1.0 mm.

G8 Evolution of Sedimentary Basins-Powder River Basin
Petrology and Reservoir Paragenesis in Sussex "B" Sandstone G9
one coarsening-upward marine ridge sequence, although 

there may be as many as three coarsening-upward progressions within it. The lower contact is transitional. The upper 

contact of the Sussex "B" sandstone grades rapidly upward 

into bioturbated siltstone and mudstone of inter-ridge or 

offshore depositional environments. Internal contacts of 

low-energy sandstones are sharp to gradational and nonerosional; higher energy ridge facies commonly have basal 

and internal scour contacts. The term bioturbated is used 

here to indicate sediments that contain more than 75 percent 

burrowing; almost all primary depositional features are 

destroyed by biologic activity and only relict, commonly 

ripple-laminated, bedding is preserved.

Inter-Ridge Facies

The Sussex "B" sandstone body includes interbedded 

and basal sheet sand of inter-ridge and offshore origin. This 

sandstone body is more than 7 mi (11 km) wide, and the 

thickest section is southwest of the House Creek producing 

area (Beaumont and others, 1980). Inter-ridge facies were 

probably deposited as sheet sands during periods of low- to 

moderate-energy oceanic and tidal currents; they may 

represent transgressive marine reworking of underlying 

nearshore and offshore marine sediments (Hobson and 

others, 1982). Inter-ridge strata are divided into three facies 

on the basis of: (1) degree of biologic activity, (2) type of 

sedimentary structures, and (3) differences in inferred 

depositional energy. Facies are generally less than 5 ft 

(1.5 m) thick; they are thinly bedded and are commonly 

interstratified. Wavy-bedded inter-ridge sandstone is distinguished from other inter-ridge sandstone by low biologic 

activity and predominantly wavy bedding. Biologic activity 

in other inter-ridge sandstone and siltstone facies may 

completely destroy bedforms.

Inter-ridge facies have generally low potential as 

reservoir rocks because of low porosity and numerous 

permeability boundaries; low porosity and permeability 

results from: (1) pervasive cementation by calcite and 

ferroan calcite, (2) cementation by quartz, or (3) by compaction and soft-sediment deformation. Within any interval, 

one of these three processes generally predominates.

Inter-Ridge Sandstone and Siltstone

Inter-ridge facies grade upward from bioturbated 

siltstone and shale to burrowed and bioturbated very fine 

grained ripple-laminated sandstone (fig. 8£). Sandstone 

may exhibit flaser bedding. Individual beds are several 

inches to 1 ft (4 30 cm) thick; they have tabular to lenticular 

geometry and can generally be traced more than 100 ft 

(30 m) across the length of the Salt Creek outcrop. 

Upwardly increasing depositional energy conditions are 

reflected by: (1) an upward increase in sandstone grain size,

(2) the upward transition from mudstone (which may 

exhibit "sand-starved" ripple lamination) to sandstone 

(which may be burrowed, with tabular bedding, and smallscale ripple laminae), to burrowed small- to medium-scale 

ripple-laminated bed sets, and (3) by the upward transition 

from a zone of mainly horizontal and back-fill burrows 

(Planolites, Teichichnus) to rocks that include some vertical 

burrows (Skolithos, Arenicolites).

Thin-section porosity of seven inter-ridge siltstone 

and ten inter-ridge sandstone samples averages 2.0 percent 

and 5.4 percent, respectively, although porosity in interridge sandstone is as great as 13.8 percent. More than half 

of the porosity is secondary intergranular and intragranular. 

Pore spaces are commonly pervasively cemented by quartz 

or calcite. Numerous mudstone drapes between these thinly 

bedded sandstones are effective boundaries to fluid flow: in 

the absence of fractures, these boundaries hydraulically 

isolate these facies from central-ridge and ridge-margin 

sandstone facies. This isolation would aid in secondary 

recovery of oil where reservoir facies overlie, and are not 

interbedded with, inter-ridge facies. Interbedding results in 

barriers to vertical flow of fluids in reservoir sandstones.

Bioturbated siltstone and mudstone are the only 

Sussex units in which compaction was a more important 

process than cementation for reducing porosity; glauconite 

and other soft-sediment grains are commonly extensively 

deformed (fig. 9C). Inter-ridge siltstone facies contain the 

least minus-cement porosity (18.6 percent) of the units 

studied (table 2). Porosity at the time of deposition was less 

for the siltstone unit than other facies mainly because of 

larger amounts of suspended-load sediment and poor sorting 

due to biologic and depositional processes.

Figure 8 (facing page). Sedimentologic, petrologic, and reservoir characteristics of Sussex "B" sandstone facies. Core width 

is about 3.5 in (9 cm). A, Core photograph of chert pebble 

sandstone which erosionally overlies medium-grained sandstone of the central-ridge facies (arrow at contact), 7,782 ft 

(2,372 m) depth, No. 14-9 Federal well. B, Core photograph of 

trough cross-bedded medium-grained central-ridge sandstone 

from 8,169 ft (2,490 m) depth in the No. 1 Mandell Federal 

well. Large siderite (S) clasts are located on trough crossbedding. Glauconite is concentrated in dark lineations. 

C, Outcrop of low-energy ridge-margin sandstone exhibits dips 

of 1 °-15° on trough and planar-tangential cross-bedding. Lowangle dips and lateral and vertical facies gradations into 

inter-ridge sandstone characterize this unit. The thin-bedded 

inter-ridge sandstone lenses above this unit are highly 

burrowed. A siderite drape marks the boundary between the 

ridge-margin sandstone and underlying slope-forming bioturbated sandstone and siltstone. Outcrop location is east of 

the Salt Creek anticline in SWV4 sec. 7, T. 40 N., R. 78 W. 

D, Core of wavy-bedded inter-ridge sandstone at 8,194 ft 

(2,498 m) depth in the No. 1 Mandell Federal well. A ripple 

reactivation surface is present to the right of a sideritecemented sandstone lamina (S). f, Extensively Teichichnus 

burrowed inter-ridge siltstone core exhibits minor, relict ripple 

laminae. Core width is about 2 in (5 cm). Sample location is 

8,038 ft (2,449 m) depth in the No. 1 Empire-Federal "C" well.

G10 Evolution of Sedimentary Basins-Powder River Basin
SUSSEX "B" FACIES SEDIMENTOLOGY

Chert pebble (CP) 

sandstone: a thin, 

lenticular, areally 

discontinuous, trough 

and planar-tangential 

bedded, very coarse 

grained sandstone. CP 

erosionally overlies 

ridge-facies sandstone. 

Sandstone may contain 

fossil debris.

Central-ridge (CF» and 

high-energy ridge-margin 

(HRM) sandstone: 

lenticular, fine to 

medium-grained, with 

moderate-angle trough 

crossbedding and minor 

planar-tabular and 

planar-tangential bedding. 

Rip-up clasts of mudstone 

and siderite may be 

present on crossbedding. Fades have 

erosional contacts and 

contain only minor 

burrowing.

'-energy ridge-margin 

(LRM) sandstone: very 

fine to fine-grained, ripplelaminated, and low-angle 

trough and planar bedded 

sandstone. Contacts are 

gradational to erosional. 

LRM is interbedded with 

IR and commonly grades 

upward into HRM 

sandstone. LRM may 

contain 0-25% burrows.

Inter-ridge (IR) 

sandstone: Facies 

include wavy-bedded 

subfacies. Very fine to 

fine-grained , 

thinly-bedded, mainly 

tabular bedded 

sandstone. Bedforms are 

ripple laminated with low 

to extensive burrowing. 

Individual sandstone 

beds are segregated by 

thin mudstone drapes. 

Internal and bounding 

contacts are gradational 

to sharp and nonerosional.

Inter-ridae (BIR) siltstone:

tabular siltstone to very 

fine grained sandstone. 

Sedimentary structures 

are commonly destroyed 

by biologic activity.

PETROLOGY

CP composition is different 

than other Sussex "B" 

fades. CP contains 32% 

chert granules (4% for 

other fades). Diagenesis 

is commonly frozen by 

early carbonate cement. 

Porosity is secondary, 

resulting mainly from 

dissolution of calcite, shell 

debris, and chert.

CR and HRM are quartzand carbonate-cemented, 

glauconitic, quartzose 

sandstones.

Oil-productive sandstone 

is quartz cemented (8% 

average) with negligible 

calcite; dissolution of 

lithics and of replacive 

carbonate cements 

restores primary porosity 

and generates locally 

extensive secondary 

porosity.

LRM sandstone shares 

similarities to IR and HRM 

sandstones; similar 

bedform types have 

similar petrology. Core 

evaluation indicates LRM 

facies are mainly calcite 

cemented.

IR sandstone contains 

variable but commonly 

pervasive amounts of 

quartz and calcite 

cement. Bioturbated 

sandstone exhibits 

extensive early porosity 

loss through compaction. 

Stacked sets of thin 

sandstones, which are 

variably cemented by 

carbonate and quartz, 

are common.

BIR is the only facies in 

which compactional 

processes predominate. 

BIR contains abundant 

ductile grains. Cements 

are minor.

RESERVOIR 

CHARACTERISTICS

The CP sandstone has low reservoir 

potential because it is thin, 

discontinuous, and completely 

cemented by calcite with negligible 

primary porosity and very minor 

secondary porosity. Because of the 

low porosity and permeability, it is a 

reservoir seal.

CR and HRM sandstone have good to 

excellent reservoir potential and would 

be a hydrologic flow unit. These facies 

have the greatest porosity and 

permeability. Maximum facies 

distribution corresponds to field 

outlines.

Ductile deformation of mudstone and 

glauconite, which are concentrated in 

ripple laminae, are minor barriers to fluid 

flow, and interbedding with low porosity 

and permeability inter-ridge sandstone 

may result in isolation and 

compartmentalization of sandstones.

LRM sandstone has low to moderate 

reservoir potential. Bioturbation and 

interbedding with lower depositional 

energy facies decreases reservoir 

potential. Because of the variation in 

bedforms, this sandstone is the most 

heterogeneous; trough cross-bedded 

units exhibit similar petrology to CR and 

HRM facies; ripple-laminated sandstone 

is similar to non-bioturbated inter-ridge 

facies; and bioturbated sandstone has 

low porosity due to compaction.

IR sandstone has low reservoir 

potential. Numerous thin mudstone 

drapes are effective permeability 

barriers to recovery of oil. Degree of 

cementation is highly variable in these 

units; however, core examination 

indicates that sandstones are commonly 

extensively cemented by carbonate 

and, to a lesser extent, quartz; this 

results in decrease of effective porosity 

and permeability.

BIR facies have very low reservoir 

potential due to extensive compaction 

and very low porosity and 

permeability. Interbedding with 

reservoir facies results in permeability 

boundaries to vertical flow.

Petrology and Reservoir Paragenesis in Sussex "B" Sandstone G11
Wavy-Bedded Inter-Ridge Sandstone

Wavy-bedded inter-ridge sandstone is composed of 

thinly bedded, cyclical mudstone-drape sandstone sets 

(fig. 8£>). Individual sets are generally less than 0.3 ft 

(0.1 m) thick and consist of fairly well sorted sandstone that 

exhibits small- to large-scale oscillation and current ripples;

Figure 9 (facing column). Compaction and cementation 

relationships in inter-ridge facies. Scale bars are 0.1 mm. A and 

B, Transmitted and plane-polarized light views of an oilstained wavy-bedded inter-ridge sandstone from 8,244 ft 

(2,513 m) depth in the No. 1-23 House Creek Federal well. 

This interval contains extensive (16.9 volume percent) quartz 

cement (O) that results in a close, interlocking framework with 

low porosity (5.4 percent thin-section) and low permeability. 

Cementation occurred prior to significant amounts of sediment 

compaction as indicated by undeformed glauconite (C) grains 

and minus-cement porosity of 38 percent. Peripheral dissolution of the albite grains (F) occurred prior to precipitation of 

overgrowths. Late-diagenetic dolomite and ferroan dolomite 

(D), comprising 2.3 volume percent, precipitated in available 

pores and automorphically replaced lithic grains. Grains, 

overgrowths, and dolomite rhombs are coated with later 

bitumen (arrows). Microporosity is present in authigenic kaolinite (K) and amounts to 2.7 percent, C, Transmitted light view 

of a bioturbated, very fine grained inter-ridge sandstone at 

7,809.1 ft (2,380.2 m) depth, No. 14-9 Federal well. The 

glauconite grain (G) is deformed around adjacent quartz 

grains. CH is chlorite grain. Carbonate is present as scattered 

dolomite rhombs (D). Dolomite (upper right) replaces a quartz 

grain.

they are separated from other sandstone sets by mudstone 

drapes 0.01-0.1 inch (0.25-2.5 mm) thick. Grain-size 

grading is uncommon in this sandstone. Internal contacts 

are sharp, wavy, and non-erosional; sandstone structure is 

preserved by mudstone drapes. This facies is interbedded 

with other inter-ridge facies and with ridge-margin facies. 

The wavy-bedded inter-ridge sandstone unit differs from 

other inter-ridge sandstones because it is essentially 

non-burrowed: sandstone samples contain no more than 

10 percent vertical and horizontal burrows. Mudstone 

drapes also display limited biological activity they contain 

minor Planolites burrows and rare back-filled burrows.

The presence of rhythmic sandstone and mudstone 

sets suggests that periods of low- to moderate-energy wave 

motion were separated by slack water periods during which 

deposition of suspended load was rapid enough to prevent 

significant amounts of reworking by biologic activity. 

Reactivation surfaces (fig. 8D) within some sandstone sets 

indicate flow reversals this suggests the influence of tides.

The wavy-bedded sandstone has limited potential as a 

petroleum reservoir. Primary and secondary oil recovery is 

adversely affected by numerous mudstone drapes that are 

effective boundaries to vertical and lateral fluid flow. 

Sandstone thin-section porosity averages 5.9 percent and 

may be as much as 13.9 percent. Sandstone is commonly 

pervasively cemented by quartz (fig. 9) or carbonate. 

Approximately half of the porosity is secondary and intragranular and results mainly from: (1) dissolution of plagioclase, or (2) replacement of feldspar, quartz, rock fragment, 

and glauconite grains by calcite, followed by carbonate 

dissolution.

G12 Evolution of Sedimentary Basins-Powder River Basin
0.1 mm , ~ Mf. 

 J-L _, -, - i'^%1

Figure 10. Transmitted I ight photomicrographs of low-energy 

ridge-margin sandstone showing two processes resulting in 

porosity reduction. A, Ripple-laminated sandstone in the No. 1 

Campbell well (7,704.3 ft depth) contains 17.4 percent quartz 

cement (O) that results in a volumetric decrease in porosity to 

4.1 percent. F is feldspar grain. Chlorite rims (arrows) comprise 

2 volume percent of this interval. Abundant clay matrix and 

minor dolomite rhombs (D) fill remaining pore spaces. Scale 

bar is 0.1 mm. B, This flaser-bedded unit is a ripple-laminated 

sandstone from 7,793.2 ft (2,375.4 m) in the No. 14-9 Federal 

well. Extensive mechanical deformation of glauconite (G) 

(10.3 percent by volume) reduced porosity to 4.1 percent 

intergranular and 20.7 percent minus-cement porosity. Mudstone laminae (M) are oil stained. This interval probably traps 

hydrocarbons in the underlying perforated interval in the well. 

Scale bars are 1.0 and 0.1 mm.

Ridge-Margin and Central-Ridge Sandstone

Ridge-margin and central-ridge sandstones are the 

reservoir facies in the House Creek and Porcupine fields. 

These facies generally grade upward from sets of lowenergy ridge-margin sandstone to high-energy ridge-margin 

and central-ridge sandstone. Low-energy ridge-margin sandstones include sedimentologic and petrologic characteristics

of inter-ridge and ridge facies and represent a transition 

zone between them. High-energy ridge-margin and centralridge sandstones are described together because they are 

almost identical petrologically.

Low-Energy Ridge-Margin Sandstone

Low-energy ridge-margin sandstones have a combined thickness of much as 8 ft (2.4 m) and consist of 

fine-grained trough or planar-tangential cross-beds that are 

interbedded with massive, wavy-bedded, and ripplelaminated sandstone (fig. 8C). Contacts between sets are 

generally transitional, and, partly because of erosion by 

higher-energy facies, the sets may be only a few inches (cm) 

thick. Low-energy ridge-margin sandstones have low to 

moderate reservoir potential. This sandstone is sedimentologically more heterogeneous than that of the central-ridge 

or high-energy ridge-margin facies: individual sandstone 

sets are thinner and are frequently bounded by mudstone 

drapes this facies is interbedded with inter-ridge sandstones and commonly contains greater amounts of carbonate 

cement and clay matrix.

Low-energy ridge-margin sandstones are moderately 

to highly glauconitic (3-13 percent) with minor, small clasts 

of mudstone rip-up and siderite. Drapes of mudstone are 

0.01-0.1 inch (0.25-2.54 mm) thick and may contain 

horizontal sand-filled burrows of Planolites. Sandstone 

generally contains less than 10 percent, vertical, U-shaped, 

and horizontal burrows of Arenicolites, Skolithos, Chondrites, Diplocraterion, and Terrebellina.

Evaluation of Sussex "B" core indicates that pore 

spaces in non-oil-productive sandstones are commonly 

filled with calcite cement. Extreme ranges in amounts and 

types of cement are characteristic of this facies and may 

reflect the interbedding of low and moderate depositional 

energy sandstone (fig. 10).

Central-Ridge and High-Energy Ridge-Margin 

Sandstone

Central-ridge and high-energy ridge-margin sandstones are the primary oil reservoir facies of the Sussex "B" 

sandstone. High-energy ridge-margin and central-ridge 

facies are grouped together because of very similar petrology and sedimentology: these facies form a hydrologic flow 

unit. High-energy ridge-margin sandstone is more heterogeneous than central-ridge sandstone because it contains 

more carbonate cement and exhibits greater interbedding 

with sandstones and siltstones of lower depositional energy.

The areal distribution of ridge facies sandstones in the 

House Creek field has been mapped and described by Berg 

(1975), Brenner (1978), and Hobson and others (1982); the 

distribution corresponds generally to the field outlines 

shown on figure 1. The central-ridge and high-energy 

ridge-margin facies in the House Creek and Porcupine fields

Petrology and Reservoir Paragenesis in Sussex "B" Sandstone G13
are as much as 20 ft (6 m) thick. Individual trough 

cross-bedded sets fine upward, are commonly less than 1 ft 

(0.3 m) thick, and are truncated at their upper surfaces. 

Sandstones exhibit small- to medium-scale trough crossbedding with minor planar-tabular and planar-tangential 

bedding; dip angles on trough cross-bedding are mainly 20° 

to 35° (fig. 8#). These facies were deposited primarily as 

small- to medium-sized subaqueous dunes. Biologic activity 

results in as much as 10 percent horizontal burrows in 

mudstone drapes. Uncommon vertical burrows of Skolithos, 

Arenicolites, and Diplocraterion may be present near the 

top of the trough sets or may penetrate into central-ridge 

sandstones where they are overlain by lower energy 

sandstone units. The vertical trace fossils were formed by 

suspension-feeding organisms, and burrows generally display evidence of rapid sediment deposition. Glauconite is 

concentrated within discrete trough cross-bedding ripplelaminae sets (dark lines on fig. 85). Extensive soft-sediment 

deformation within these laminae may result in minor 

permeability barriers to vertical fluid flow (fig. ID). Fluid 

should preferentially flow parallel to the long axes of trough 

cross-beds. Mudstone is present mainly as drapes on trough 

bedding planes; suspended-load sedimentation on dunes 

occurred primarily during slack water periods. Also present 

on the trough bedding planes are mudstone rip-up clasts and 

siderite clasts that may be as much as 3 inches (8 cm) long 

and 1 inch (3 cm) wide. Core and outcrop studies indicate 

that sources of the clasts are siderite drapes and sideritecemented sandstone; these are present in many of the facies 

located within and below the Sussex interval (fig. 8D). 

These siderite drapes may have been eroded by storm 

currents or other strong currents; the rounded clasts were 

subsequently deposited on subaqueous dunes. The volume 

percent for siderite drapes in central-ridge sandstone (table 

2) is anomalously large, primarily because of preferential 

sampling of some intervals containing the drapes.

Central-ridge and high-energy ridge-margin facies 

sandstones are fine to medium grained. Grains generally 

exhibit long and straight or point contacts with other grains. 

Cementation by quartz and calcite are important porosity 

reduction processes in central-ridge and high-energy ridgemargin facies this is indicated by Sussex "B" sandstone 

average minus-cement porosity values of 30.7 and 35.8 

percent, respectively (table 2). Quartz cement averages 8.5 

volume percent in both facies and ranges from 0.7 to 23.2 

volume percent. Thin-section porosity averages 7.7 and 6.1 

percent for central-ridge and high-energy ridge-margin 

sandstones; this is less than the average reservoir porosity of 

13 percent. Microporosity was not measured during thinsection point counting, resulting in low values relative to 

core porosity. Central-ridge and high-energy ridge-margin 

facies also average 9.5 and 14.3 volume percent carbonate 

cement, respectively, and oil is present mainly in those rock 

intervals in which calcite is absent or in which calcite 

dissolution has restored porosity and created secondary pore

spaces. The rare potassium feldspar grains generally are 

extensively corroded and are sometimes closely associated 

with pore-filling authigenic kaolinite. Plagioclase content 

ranges from 2.3 to 11.5 volume percent, and grains may 

exhibit extensive dissolution (fig. 7C) and minor to 

extensive replacement by calcite and dolomite. Secondary 

porosity results mainly from dissolution of: (1) plagioclase, 

(2) calcite that filled intragranular pore spaces and replaced 

feldspar and, (3), to a lesser extent, calcite that replaced rock 

fragments, quartz grains and overgrowths, and glauconite.

Chert Pebble Sandstone

Where present, chert pebble sandstone is the upper 

contact of the Sussex "B" sandstone (fig. 8A) and represents 

the highest depositional energy conditions of the facies 

studied (Hobson and others, 1982). This chert pebble 

sandstone facies is generally less than 1 ft (0.3 m) thick and 

is areally discontinuous across the House Creek and 

Porcupine fields. Cores from at least five of the 21 Sussex 

"B" wells in the area of the House Creek field and one well 

in the Porcupine field contain this chert unit (several of the 

cored sections did not include the upper contact). This 

sandstone is indistinguishable on well logs from underlying 

facies; it contains pervasive calcite cement and is not a 

reservoir for oil. However, where this unit is present, it acts 

as a seal that traps hydrocarbons in underlying reservoir 

sandstones.

The chert pebble sandstone exhibits distinct bimodal 

sorting and consists of medium-sized grains of subangular 

quartz and granule-sized grains of well-rounded chert that 

are 0.05-0.2 inch (1-5 mm) in diameter. This facies 

erosionally overlies central-ridge facies sandstones and is 

transitionally overlain by a unit that consists of offshore 

marine mudstone to very fine grained sandstone. The chert 

pebble sandstone facies exhibits small- to medium-scale 

trough cross-bedding and minor planar-tabular crossbedding. Individual trough cross-sets are several inches to 

1 ft (0.08-0.3 m) thick, fine upward, and have truncated 

upper surfaces. Glauconite, mudstone rip-up, and other soft 

sediment grains were winnowed from the sandstone and are 

concentrated within trough-laminae sets. The source of the 

chert is probably limestone that was replaced by chert and 

subsequently eroded this is indicated by inclusions of 

carbonate in chert, pseudomorphs after dolomite, and 

burrows and fossil fragments that are replaced by chert and 

chalcedony.

The chert pebble sandstone may have been deposited 

by high-energy storm currents. The sandstone may also 

have resulted from an event in which the pebbles were 

introduced by channelized flow (Berg, 1975) and reworked 

into the chert sandstone. However, the chert pebble 

sandstone probably represents a winnowed lag deposit on an 

unconformity surface: the unit is widely distributed across 

the House Creek and Porcupine fields, erosional surfaces

G14 Evolution of Sedimentary Basins-Powder River Basin
show low relief (less than 1 m), and deep channels are 

absent. Foraminiferal tests and bone fragments are occasionally preserved. Also, this unit is petrologically different 

from other facies of the Sussex: the chert pebble sandstone 

consists of 32.4 percent by volume chert pebbles (rock 

fragment category), whereas chert grains average 4.2 percent in the other facies. Chert grains in this unit exhibit 

coatings of oxidized iron (fig. 7F); this is unlike grains in 

the other units. Grain-to-grain contacts within this facies are 

point and long-straight contacts. Quartz grains incorporated 

from underlying central-ridge facies sandstones commonly 

"float" in a matrix of carbonate cement (fig. IF). Because of 

high depositional energy conditions, the sandstone does not 

contain burrows or tracks.

Porosity in the chert pebble sandstone is commonly 

secondary, resulting mainly from dissolution of lithic grains 

and fossil fragments (fig. IF}. Cementation is the major 

process of porosity reduction. Compactional loss of porosity 

results primarily from rearrangement and closer packing of 

lithic grains. The 24.8 percent average minus-cement 

porosity represents about a 5-10 percent loss of porosity 

from the initial depositional porosity. Initial porosity was 

probably less in this facies than in other facies because of 

bimodal sorting.

In core and thin sections, this chert pebble sandstone 

is completely cemented by calcite and ferroan calcite. 

Average calcite cement comprises 20.9 volume percent. 

This sandstone would have excellent porosity and permeability and would be a reservoir facies if the calcite cement 

were removed. Paragenesis is commonly "frozen" because 

of early calcite cementation. Kaolinite and illite-smectite 

(which precipitated in other sandstone facies after calcite 

dissolution) are rare in the chert pebble sandstone facies.

CONCLUSIONS

The Sussex "B" sandstone is composed generally of a 

coarsening-upward sequence of inter-ridge to central-ridge 

sandstones; the best developed reservoir sandstones are 

located at the top of each sequence. Sandstones are very fine 

to medium-grained litharenites to feldspathic litharenites. 

Porosity relationships are complex and consist of primary 

porosity, secondary porosity, and microporosity. Porosity is 

commonly greatest within the high depositional energy 

central-ridge and ridge-margin facies: these are the reservoir 

sandstones of the House Creek and Porcupine fields. 

Reservoir properties are controlled by: (1) types and 

amounts of cements, (2) enhancement of porosity through 

dissolution of carbonate cements, (3) sedimentologic factors 

that influence fluid flow and the degree of sediment 

compaction, and (4) amounts of pore-filling and graincoating clays. Fluid flow during oil production is slowed by 

the numerous permeability boundaries created by interbedding of inter-ridge with ridge facies sandstones and by 

numerous mudstone drapes.

ACKNOWLEDGMENTS

This research was supported by the Enhanced Oil 

Recovery project of the U.S. Geological Survey. I am 

especially grateful to Steve Fillingham, Dick Morrow, and 

Bill Walford of Woods Petroleum Corp., and William 

Brown and Oscar Peters of Meridian Oil Corp. for supplying reservoir and core analysis data. This paper benefited 

from critical reviews by Geoff Thyne of the University of 

Wyoming and by Bonnie Crysdale and Christopher Schenk. 

Mark Pawlewicz provided vitrinite reflectance analyses and 

Thomas Finn provided X-ray diffraction analyses.

REFERENCES CITED

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G16 Evolution of Sedimentary Basins-Powder River Basin

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