geologic summary and hydrocarbon plays, williston basin ... · geologic summary and hydrocarbon...

DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Geologic Summary and Hydrocarbon Plays,

Williston Basin, Montana, North and South Dakota,

and Sioux Arch, South Dakota and Nebraska, U.S.

by James A. Peterson

Open-File Report 87-450-N

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature

Missoula, Montana

1988

CONTENTS

PageIntroduction.......................................................... 1Principal plays....................................................... 17

I. Madison-Upper Devonian play................................... 17Geologic characterization................................... 17Exploration status.......................................... 24

II. Northeast Basin play......................................... 25Geologic Characterization................................... 25Exploration status.......................................... 26

III. Red River-Interlake-Middle Devonian play...................... 26Geologic characterization................................... 30Exploration status.......................................... 31

IV. Post-Madison-pre-Jurassic play................................ 31Geologic characterization................................... 31Exploration status.......................................... 33

V. Deep basin gas play........................................... 33Geological Characterization................................. 33Exploration status.......................................... 34

Sioux arch............................................................ 34Summary............................................................... 36Acknowledgments...................................................... 39Selected references................................................... 39

ILLUSTRATIONS

Figures 1. Map showing the major structural features inNorth and South Dakota, Montana, and Wyoming............. 2

2. Structure contour map top of Dakota Sandstone andequivalents (Cretaceous)................................. 3

3. Major Paleozoic structural trends, Williston basin andadjacent areas........................................... 4

4. Correlation chart - Williston basin...................... 55. Generalized west-east structural-stratigraphic cross

section A-A', Bighorn Mountains, Wyoming to northeastern North Dakota............................................. 6

6. Thickness of sedimentary cover in thousands of feet,U.S. portion of the Williston basin and adjacent areas... 7

7. Thickness of Paleozoic rocks in thousands of feet,U.S. portion of the Williston basin and adjacent areas... 8

8. Thermal gradient map (°F/100 ft)......................... 99. Thickness in hundreds of feet and generalized

carbonate facies distribution for the Madison Formation (Mississippian) and equivalent rocks, U.S. portion of the Williston basin and adjacent areas....................... 18

10. Structure contours in thousands of feet on top ofMadison Formation........................................ 19

11. North-south stratigraphic cross section B-B', Nessonanticline to south-central South Dakota.................. 20

12. West-east stratigraphic cross section C-C', north-centralMontana to Nesson anticline, North Dakota................ 21

CONTENTS Cont inued

13. Thickness of Devonian rocks in feet, showing approximate limit of Prairie salt (Prairie Formation, Middle Devonian), and source rocks in Bakken Formation (Devonian-Mississippian)................................... 22

14. Conceptual model of groundwater flow in the Paleozoicaquifers of the northern Great Plains...................... 23

15. Structure contours in thousands of feet on top of RedRiver Formation (Ordovician)............................... 27

16. Thickness in hundreds of feet and generalized carbonate facies distribution for the Red River Formation (Ordovician) and equivalent rocks, U.S. portion of the Williston basin and adjacent areas......................... 28

17. Thickness of Interlake Formation (Silurian anduppermost Ordovician) in hundreds of feet.................. 29

18. Thickness in feet and facies of Big Snowy Group(Mississippian)............................................ 32

19. Pool size distribution, Play I, Madison-Upper Devonian,showing size distribution of existing pools greater than 1 MMBO, 1951-1984............................................ 37

20. Pool size distribution, Play III, Red River-Interlake-Middle Devonian, showing size distribution of existing pools greater than 1 MMBO, 1951-1984............................. 38

TABLES

Tables. 1. Exploration and production summary for the U.S. portionof the Williston basin................................... 10

2. Data summary Williston basin, U.S. portion................. 12.3. Pool size distribution..................................... 154. Estimates of undiscovered petroleum resources.............. 16

11

GEOLOGIC SUMMARY AND HYDROCARBON PLAYS, WILLISTON BASIN, MONTANA, NORTH AND SOUTH DAKOTA,

AND SIOUX ARCH, SOUTH DAKOTA AND NEBRASKA, U.S.

By James A. Peterson

INTRODUCTION

The yilliston basin is a structural-sedimentary intracratonic basin located on the western shelf of the Paleozoic North American craton. The present-day basin occupies a large segment of the northern Great Plains and extends northward into Canada (figs. 1-3). The basin region is a generally flat-lying, moderately dissected plain with minimum topographic relief. Average elevation ranges from approximately 1,500 to 3,000 ft (460 to 915 m). The climate of the region is middle latitude semiarid with precipitation ranging between 15 and 20 in. (40 and 50 cm) per year. The temperature averages approximately 65-70 F during summer months and 10-25 F during the winter. The basin is bordered on the east and southeast by the Canadian Shield and the Sioux uplift. The western and southwestern borders are defined by the Black Hills uplift, Miles City arch, Porcupine dome and Bowdoin dome* The United States part of the basin covers approximately 143,000«mi with a total sedimentary rock volume of approximately 202,000 mi . Sedimentary rocks of Cambrian through Holocene age are present in the basin. Maximum thickness of Phanerozoic rocks is greater than 16,000 ft in northwestern North Dakota (figs. 4-8; tables 1, 2).

Paleozoic sedimentation, facies patterns, and paleogeography of the northern Great Plains are closely related to the tectonic history of the western border of the North American craton. The central part of the Paleozoic craton was made up of a stable core, the Canadian shelf of older Precambrian rocks and its southwestward extension, the Transcontinental arch (fig. 1). During the early to middle Paleozoic, the Transcontinental arch effectively separated the continent into eastern and western marine shelf and geosynclinal provinces as approximate mirror images of each other. To the west of the Canadian shelf and Transcontinental arch, the broad western flank of the Paleozoic craton made up the Cordilleran shelf, which was the site of shallow water marine cyclic sedimentation during most of Paleozoic and Mesozoic time. The shelf was bordered on the vest by the Cordilleran miogeosyncline (miogeocline of some authors), which was a slowly subsiding complex of marginal basins where primarily shallow-water marine Paleozoic carbonates, sandstones, and shales, as much as 50,000 ft or more thick, accumulated along a belt extending from southwestern United States to northwestern Canada. The Antler erogenic belt, which began active growth in Middle Devonian time, lay to the west of the miogeosyncline and formed a linear, relatively narrow system of thrusting, mountain building, and island growth that underwent several stages of development in late Paleozoic and early Mesozoic time. West of the Antler erogenic belt, thick eugeosynclinal deposits of deep-water shale, fine-grained limestone, coarse elastics, and submarine volcanic deposits accumulated during most of the Paleozoic time.

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Sand

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lliston

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n de

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and

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MONTANA PORTIONFORMATION

LITHOLOGIC SYMBOLS

^-T^-q Sholt ft Cloyiloni

Sdlilont

l=-*~l Block Organic Shaln

Glauconili

Limmtonn

Figure 4.0 Correlation chart, Williston ]Mi8in. Modified after Montana Geol. Soc., 19780

PEMBINA ESCARPMENT

North Dakota Minnesota

49° -

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of feet,

U.S.

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7.

Thic

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s of Pa

leozoic

rocks

in thousands

of fe

et,

U.S. po

rtion

of the

Villiston

basi

n and

adja

cent

ar

eas.

Ap

prox

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e limits'

of'

"Prairie salt"

.(Prai

rie fo

rmat

ion,

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salt (C

harles

. _For

mati

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Mississipp

iah),

and

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salt"

COoeche

Form

atio

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own.

Note location of

oil

accumulations concentrated

beneath one or

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als.

Modified after

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85.

too«

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re 8.

Th

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From Ma

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Table 1. Exploration and production summary, Williston Basin, U.S. Portion

North Dakota Montana South Dakota

9Area 142,537 mi*"; Sediment Volume,

Exploratory wells (1/84)

3,360 2,200

440Totals 6,000±

201,830 mi 3

Fields (1/84)

522141 20

683

Comparative production, reserves, undiscovered resources

Gas (Trillion cubic ft)Oil (Billion barrels) Associated

Cumulative production 1979January 1984

Measured reserves1979January 1984 estimate

Original reserves 1979January 1984 estimate

Undiscovered resources (1979) MedianMode

Recovery factor

MAIN PLAYS

I. Madison-Upper Devonian II. Northeast Basin

III. Red River-Interlake-MiddleIV. Post-Madison-pre- Jurassic V. Deep basin gas

0.895 1.5351.140

0.365 0.710.340

1.260 2.2451.700

1.18 2.130.82 1.480.24

Devonian

Non-associated

0.11

0.100

0.206

0.660.470.80

10

Table 1. Exploration and production summary, Williston Basin, U.S. Portion continued

Ultimate recovery for fields greater than 15 million barrels of oil

Estimated Ultimate recovery Oil Gas

Field and State

Fennel (Mont.) Beaver Lodge (N. Dak.)

Pine, Mont.

Little Knife, N. Dak.

Cabin Creek, Mont.

Tioga, N. Dak.

Poplar East, Mont. Charlson, N. Dak.

Antelope, N. Dak.

T. R., N. Dak. Newburg, N. Dak.

Lookout Butte, Mont. Dickinson, N. Dak.

Mondak, N. Dak.

Glenburn, N. Dak.

Flat Lake, Mont. Sherwood, N. Dak.

Rough Rider, No. Dak

Little Beaver, N. Dak. Hawkeye, No. Dak.

North Tioga, No. Dak.

Rival, N. Dak

(1) Trs - Spearfish Mh - HeathMe - Charles

Reservoir (1)

Si , Or , Mm Mm, D, Or

Or, Mra

Mm

Or, Si, Mm

Mm, Dd, Si,

(Mm, Mh, Dn Mm, Si, Mb,

Mm, Dtf

Mm, Dd, Or Trs, Me

Mm, Dd, Or Mh, Mm

Mm, Or, Dd

Mm

Or Mm

Mm, Mb

Or, MmMm

Mra

Mm

Mm - Madison Mb - BakkenD - Devonian

(millions of barrels)

138 125

115

110

105

Or 85

A8 Or 40

37

30 30

30 28

25

22

20 17

15

15 15

15

15

Dtf - Three Forks Dn - NiskuDd - Dupero

(billions of cubic feet)

55

40

136

40 62

66

1.5

3

10

1

5

7

20

21

25

Si - Interlake Or - Red River

11

Table 2. Data summary for

the

U.S. portion of the

Wil.

list

on basin

Prospective intervals

Reservoir

Average

gross

Reservoi r

depth ra

nge

Thickness

Age

Tertiary-Late

Cretaceous

Early Cretaceous

Jurassic

Triassic

Pennsylvanian-

Mississippian

Mississippian

Mississippian

Devonian

Devonian

Devonian

Mississippian-

Devonian

Silurian

Ordovician

Ordovician

Ordovician an

d

Rock unit

Several

intervals

Saud

e (economic usage)

Spearfish Formation

Tyler an

dHeath

formations

Big

Snowy

Group

Madison Group

Birdbear (Nis

ku)

Formation

Dupe row

Formation

Winnipegosis

Formation

Bakken Formation

Interlake Formation

Red

River

Formation

Winnipeg Formation

Deadwood Formation

Reservoir lithology

sandstone

do

do

do

do

do

limestone an

ddolomite

dolomite and

limestone

do

do

silt

s tone and

sandstone

dolomite

limestone an

ddolomite

sandstone

sandstone

(ft)

0-3,000

0-6,500

2,000-7,000

2,000-7,500

2,50

0-8,

500

3,000-8,500

500-9,000

500-10,000

500-

11,0

00500-12,000

7,00

0-10

,000

500-12,000

500-12,000

500-15,000

500-

16,0

00

(ft)

500-1,200

800

150

150

200

300

1,400 35

200

100 75

500

A 00 100

500

Oil

or

biogenic

biogenic

oil

oil

oil

oil

oil

oil

oil

oil

oil

oil

oil

and

oil

and

oil

and

Gas gas

gas

gas

gas

gas

"Cambrian

Table

2. Data su

mmar

y fo

r th

e U.S. portion

of the

Will

isto

n basin continued

Prob

able

Source Be

ds

Age

Rock Unit

Dept

h (ft)

Quality

Matu

rity

Oil

and

gas

Tert

iary

-Lat

e

Cretaceous

Late

Cr

etac

eous

Early

Cretac

eous

Pennsylvanian-

Mississippian

Miss

issi

ppia

n

Mississippian

Devo

nian

Devo

nian

Ordo

vici

an

Ordovician-

Cambrian

Fort

Un

ion

Form

atio

n 0-

3,00

0

and

Mont

ana

Group

Niobrara,

Carlile

and

1,000-5,000

Bell

e Fo

uche Fo

rmat

ions

Mowr

y Formation

Tyle

r an

d Heath

form

atio

ns

Lodgepole

Format

ion

Bakk

en Formation

Ston

y Mountain an

d

Red

Rive

r Formations

Winn

ipeg

and

Dead

wood

fo

rmat

ions

2,000-6,500

3,00

0-8,

500

7,00

0-9,

500

7,000-10,000

10,000-15,000

9,000-15,000

10,0

00-1

5,00

0

fair

fair

imma

ture

Immature

fair

to go

od

matu

re

good

mature

good

mature

matu

re

gas

gas

fair

immature

gas

fair

to go

od

mature

oil

oil

oil

oil

oil

and

gas

The northern Great Plains region lies within the eastern part of the Cordilleran shelf adjacent to the Transcontinental arch. Within this region, sedimentary processes were influenced to varying degrees by the growth of several paleostructural elements associated with the development and growth of the Cordilleran shelf (figs. 1,3). The major Paleozoic paleostructural element in the northern Great Plains region was the Williston basin, which has undergone mild tectonic subsidence since Late Cambrian or Orodovician time. Other important elements of regional size include the central Montana trough, Alberta shelf, and Wyoming shelf, all of which were present during most of Paleozoic time. Other features that exerted important local influences on Paleozoic sedimentary patterns in the northern Rocky Mountains region include the Sweetgrass arch, central Montana uplift, Cedar Creek anticline, Miles City arch, and the ancestral Black Hills uplift, Chadron arch, Laramie uplift, Bighorn uplift, and Powder River basin.

Prospective petroleum intervals in the U.S. part of the Williston basin are mainly Paleozoic in age and include carbonate reservoirs of, in order of importance, Mississippian, Ordovician, Devonian, and Silurian age (fig. 4, table 2) and sandstone reservoirs of, in order of importance, Ordovician, Mississippian, Pennsylvanian and Triassic age. The major reserves of petroleum occur in carbonate reservoirs mainly of Mississippian and Ordovician ages. Thermal gradient in the basin averages approximately 2 F/100 ft (fig. 8). Mature, organic-rich shale source rocksare present in the Mississippian (Bakken, lower Lodgepole, and Char Formations), Ordovician (upper Red River Formation), and Pennsylvanian-Mississippian (Tyler and Heath Formations). Mature source rocks of lesser importance are also present in the Ordovi cian-Cambr ian (Winnipeg and upper Deadwood Formations). Immature potential source beds of interest for biogenic gas are present in several parts of the Jurassic, Cretaceous, and lower Tertiary section.

Petroleum accumulations are almost exclusively oil and occur primarily in combination structural-stratigraphic traps related in part to carbonate buildups, discontinuous sandstone bodies, or intraformational porosity changes. As of January 1984, approximately 6,000 exploratory wells had been drilled in the basin, more than half of which are in North Dakota. As of that date, 683 oil fields had been discovered, 522 in North Dakota, 141 in Montana, and 20 in South Dakota, the most important of which are shown on figure 8. Cumulative production as of January 1984 was approximately 1.2 BBO (billion barrels oil) and 2.0 Tcf (trillion cubic feet) gas, mainly dissolved-associated gas (table 1). Historic pool sizes by plays are shown on table 3. Estimated undiscovered resources are summarized in table 4.

Five main plays are defined (figs. 4, 6):

I. Madison-Upper Devonian play, the main play of the basin. This play is based on the geographic and stratigraphic occurrence of several intervals of porous carbonate reservoirs with several interbedded anhydrite seals and a main seal (Charles salt and anhydrite) at the top.

14

Table

3. H

isto

ric

pool size di

stribution

Pool size

Play

I.

Madi

son-

Upper

Devo

nian

&

II.

North

East

Basin

III.

Red

Rive

r-

Winn

ipeg

, Siluri

an and

Middle

De

voni

an

IV.

Post

Ma

diso

n-

pre-

Jura

ssic

Greater

than

25-5

050 MMB

MMB

5 6

2 0

0 1

Approximate

mean

10-2

4 5-

9 1-5

Less than

Tota

l ac

cumu

lati

onMM

B MM

B MM

B 1 MM

B pools

size

24

56

113

220

424

3.1

MMB

(oil)

1.2

Bcf

(gas)

8 8

124

183

325

2.33 MM

B (oil)

2.33

Bc

f (gas)

161

6 15

1.74

MMB

(oil

) 0.174

Bcf

(gas)

Tota

ls33

70

238

409

764

2.15

MMB

(oil)

2.0

Bcf

(gas

)

Table A. Statistical estimates of undiscovered petroleum resources.

Fields greater than 1 MMBO or 6 BCF gas:

Play

I

II

Mean

166.9 MMBO66.8 BCF gas

9.8 MMBO2.9 BCF gas

F95


2.9 MMBO0.9 BCF gas

F50

15A.8 MMBO61.9 BCF gas

8.0 MMBO2. A BCF gas

F5



III & V

IV

223.7 MMBO 332.2 BCF gas


138.0 MMBO 20A.3 BCF gas


215.6 MMBO 336.9 MMBO320.1 BCF gas 501.5 BCF gas

9.0 MMBO 2A.8 MMBO0.9 BCF gas 2.5 BCF gas

Small fields (less than 1 MMBO or 6 BCF gas):

Oil 366.2 MMBO 290.0 MMBO 350.0 MMBO 480.0 MMBO

Non-assoc.gas

83.A BCF gas 68.0 BCF gas 80.0 BCF gas 105.0 BCF gas

Assoc.- 256.0 BCF gas 192.1 BCF gas 252.8 BCF gas 332.8 BCF gasdissolvedgas

Total for basin:

Oil 0

Assoc. & non- assoc. gas

780.0 MMBO A90.0 MMBO

7AO.O BCF gas A90.0 BCF gas

1,150.0 MMBO

1,070.0 BCF gas

16

II. Northeast basin play. This play is based on an area of the basin where updip stratigraphic changes and pinchouts in Paleozoic reservoirs occur beneath the Mesozoic unconformity, sealed by overlying Mesozoic shale.

III. Red River-Interlake-Middle Devonian play. This play, which is second in importance, is based on the geographic occurrence of major seals, the regional Prairie salt (Middle Devonian) and the upper Red River and Stony Mountain (Upper Ordovician) anhydrites, which are interbedded with carbonate reservoirs and source rocks.

IV. Post-Madison-pre-Jurassic play. This play is based on the geographic occurrence of Pennsylvanian-Mississippian discontinuous sandstone and minor carbonate reservoirs with associated shale source rocks, sealed by overlying Pennsylvanian shale beds.

V. Deep basin gas play. This play is based on the occurrence ofOrdovician carbonate and sandstone reservoirs buried below the oil window in the deeper part of the basin.

PRINCIPAL PLAYS

I. Madison-Upper Devonian Play

This is an oil play with moderate amounts of associated gas, primarily related to accumulations in Madison carbonate reservoirs (figs. 6, 9-14). Devonian dolomite reservoirs have been explored more heavily in the past ten years, but so far accumulations have been relatively small, and much of the Devonian production has been found in new pool discoveries involved with deeper drilling in areas of earlier production.

Geologic Characterization

Reservoirs - These are mainly dolomitized carbonate reservoirs in oolitic, crinoidal or bioclastic bank or mound buildups in several Madison carbonate-evaporite cycles, mainly in the Mission Canyon and Charles Formations. The largest reserves have been found in Mission Canyon reservoirs in several informally named zones, such as the Frobisher-Alida, Tilston, Midale, Rival, and Bottineau zones. Substantial reserves also have been found in Charles Formation zones, such as the Poplar, Ratcliffe, Charles A, B, and C and other zones. Upper Devonian reservoirs are porous dolomite beds in carbonate-evaporite cycles of the Duperow and Birdbear (Nisku) Formations. A few small oil accumulations have been found in siltstone, fine sandstone, or dolomite beds in the middle part of the Bakken Formation, an important source rock unit.

Source Rocks - Identified and probable source rocks are high-organic black shale and siltstone of the Mississippian-Devonian Bakken Formation (the primary source), black to dark gray marine shale and argillaceous limestone beds in the lower Lodgepole Formation, and black to dark gray shale and argillaceous carbonate of the Mission Canyon and Charles

17

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100°

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Figure 1

2.

West

-eas

t st

rati

grap

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cross

section

C-C'

, no

rth-

cent

ral

Mont

ana

to W

esso

n anticline, North

Dako

ta,

showing

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son marker s

ys-

v

PW.,M

,tern,, ori

noid

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nd o

olit

ic b

anks,

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d ma

in e

vapo

rite

beds. Location

«

sh

own on f

igur

e 9.

Approximate

boun

dary

of

cora

l zones

C-2

and

D '

o(Sando,

1978)

is s

hown

. Fr

ooP

ttw

son,

198

7.50

km.

- *T

----

-C"

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'-,110

°; ,-

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NTA

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onia

n or

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rfac

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tof

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evon

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it of

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rairie

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rma

l su

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us

age

. O

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onia

n re

serv

oir

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KA

Figu

re 13

, Thickness

'of De

voni

an ro.cks in fe

et,

shov

ing,

approximate limit

of "Prairie s

alt" (Prairie Fo

rmat

ion,

Mi

ddle

Devonian),, a

nd oi

l fi

elds

producing

from

Dev

onia

n re

serv

oirs

, Mo

difi

ed af

ter

Pete

rson

an

d Ma

cCar

y, 1987,

' '

' ' .

GENERAL AREA OF DISCHARGE AS* INTERFORMATIONAL FLOW TO ROCKS OF CAMBRIAN AND ORDOVICIAN AGE

MINNESOTA

GENERAL AREA OF DISCHARGE AS INTERFORMATIONAL FLOW TO ROCKS OF EARLY CRETACEOUS AGE\

limit of the Madison Limestone

GENERAL AREA OF RECHARGEORIGINATING AS PRECIPITATION

IN THE HIGHLANDS

Approximate limit of i Madison Limestone

EXPLANATION100

I I 100 200 KILOMETERS

HIGHLAND AREA Generally areas of i_ ground-water recharge. Numbers |~ represent rate of recharge in cubic feet 0 per second.

t"-"."r^|22j^ DISCHARGE RATE Approximate areas of principal »' ' ' '-' ' ground-waterdischarge.Numbers represent rate of

discharge in cubic feet per second.

|OOOOJ BRINE AREA Areas within which the concentration dissolved solids tAAyvAJ in ground water is greater than 100.000 milligrams per liter.

GENERAL DIRECTION OF GROUND-WATER FLOW

GEOLOGIC STRUCTURE Locations of paleostructures that may affect the flow of ground water.

Figure 14. Conceptual model of groundwater flow in the Paleozoic aouifera of the northern Great Plains. From Downey, 1984.

200 MILES _J

23

Formations, cyclically interbedded with carbonate and evaporite beds. Dark, marine shale and argillaceous carbonate beds are cyclically interbedded with porous Devonian dolomite beds, but they probably are of intermediate source rock quality.

Traps and Seals - Accumulations are found primarily in gentle folds and closures related to carbonate bank buildups overlain by cyclically interbedded anhydrite beds in the Mission Canyon and Charles Formations. Updip stratigraphic traps related to facies changes are common on the larger structures. Interbedded shale or argillaceous carbonates probably contribute to trapping in many cases, but evaporite beds make up almost all of the seals. The thick Charles salt at the top of the Madison section is a major regional seal in the basin interior. The largest traps and a major part of the reserves in this play are associated with major structures such as the Nesson anticline and Billings nose trends and the Poplar anticline (figs. 1, 3).

Generation, Timing and Migration - Oil generation probably began in the Madison Formation rocks by middle to Late Cretaceous time and probably has continued to the present. In most cases, migration was probably coincident with generation with some adjustment related to late structural growth.

Depth Range - 5,000 to 13,000 ft (1,650 to 4,300 m).

Exploration Status

This play is moderately to well explored, but use of refined seismic data probably will result in numerous additional new field or new pool discoveries, partly related to stratigraphic trap accumulations. Most of the new fields probably will be of moderate to small size, although rhere is potential for a few moderate to large sized accumulations. Federal lands in this play are moderately to well explored.

Cumulative Production - Approximately 685 MMBO (1/84); estimated 750 MMBO (1/85).

Estimated Ultimate Recovery from Existing Fields - Approximately 1 BBO, of which about one-third or more is in pools on the Nesson anticline.

Number of Pools - Approximately 325; 30 are greater than 10 MMB, about 12 are greater than 20 MMB, and about 5 greater than 50 MMB; largest is about 105 MMB, Mean pool size is 3.35 MMBO, 1.34 Bcf gas.

Mean Estimate of Undiscovered Petroleum Resources - 166.9 MMBO in fields greater than 1 MMB, 66.8 BCF gas in fields greater than 6 BCF.

Total area of play - 40,800 mi 2 (110,000 km2)

Area of Federal Lands - 20,400 mi 2 (53,000 k 2 )

24

II. Northeast Basin Play

This is an oil play, which involves updip stratigraphic facies changes and pinchouts in Paleozoic reservoirs of Cambrian through Permian age beneath the Mesozoic unconformity, as well as basal Triassic sandstones above the unconformity (figs. 6, 10, 11). Fractured Precambrian basement rocks also are involved to a minor degree. The main part of the play is in updip facies changes of dolomitized skeletal and oolitic Mississippian Madison carbonates, which grade eastward to anhydrite, anhydritic carbonates and dense carbonates. To some degree, similar stratigraphic changes occur in the Devonian carbonate section. However, these and older beds are probably in vertical communication with Madison carbonate reservoirs almost everywhere, diminishing the prospects for separate traps in pre-Madison reservoirs, except where they pinch out against the Mesozoic unconformity. Furthermore, source rocks are generally lacking or of lesser quality in the pre-Mississippian rocks in this region. Oil accumulations in the Triassic basal Spearfish and the Cambrian sandstone reservoirs and in fractured Precambrian are small and related to migration of oil from Madison carbonate reservoirs and source rocks.

Progressively older beds are truncated by Mesozoic clastic rocks from west to east toward the margin of the basin. Potentially, this play covers the entire eastern flank of the basin and was one of the earliest major exploration plays of the basin during the 1950's. However, after being tested by numerous exploratory wells, it is now apparent that, except for north-central North Dakota and Saskatchewan, where all the known oil accumulations of this play are located, most of the eastern flank of the basin has been hydrologically flushed by waters primarily from the Black Hills Uplift in South Dakota (figs. 1, 14).


Reservoirs - Important reservoirs are mainly dolomitized Madison oolitic, crinoidal, or bioclastic beds on the basin flanks. Updip truncated porous dolomite beds of reservoir quality also are present in the Ordovician Red River, Silurian Interlake, and the Devonian section. However, little or no accumulations are known in these beds, probably because of vertical communication with overlying Madison carbonate reservoirs and lack of good source rocks in the pre-Madison beds. Basal Triassic (Spearfish) sandstones are moderately porous, but accumulations depend on communication with underlying Madison reservoirs. Cambrian Deadwood and Precambrian accumulations are anomalous and small and probably are related to lateral fracture communication with Madison or other reservoirs.

Source Rocks - Organic-rich shale and siltstone of the Madison-Devonian Bakken Formation, mainly downdip, are the main source rocks. Dark gray or black moderately organic shale and argillaceous carbonate beds interbedded updip with Madison carbonate reservoirs are a secondary source.

25

Traps and Seals - Accumulations are present primarily in updip facies-related stratigraphic traps and gentle closure on carbonate buildups or mild tectonic folding. Seals are mainly upper Madison anhydrite beds cyclically deposited with the Madison carbonate beds. Triassic lower Spearfish red shale and perhaps Jurassic Piper red shale and evaporites provide effective seals in some cases.

Generation, Timing, and Migration - Oil probably migrated into these reservoirs from the west in slightly deeper parts of the basin where the Bakken and Madison source rocks are richer and thicker. Oil generation and migration probably began by middle to Late Cretaceous time and continued at least through middle Tertiary time. Stratigraphic traps were developed very early. Tectonically generated folding is very mild, and probably was fully developed by late in the Mesozoic or early Tertiary.

Depth Range - 3,000 to 6,000 ft (900 to 1,800 m)

Exploration Status

This play is moderately well explored, although numerous down-flank and other stratigraphic traps probably are undiscovered. Most of these will be of small to moderate size. Federal lands in this play are well explored, although very little Federal land is involved.

Cumulative Production - Approximately 95 MMBO (1/84); estimated 100 MMBO (1/85).

Estimated Ultimate Recovery from Existing Fields - 125 MMBO.

Number of Pools - About 75, of which about 25 are greater than 1 MMB and 5 are greater than 10 MMB; largest is about 20 MMB. Mean pool size, 1.34 MMBO, 0.52 Bcf gas.


Total Area of Play - 7,500 mi 2 (19,500 km2 )

Area of Federal Lands - 1,250 mi 2 (3,250 km2 )

III. Red River-Interlake-Middle Devonian Play

This play, the second most important play of the basin, is an oil and associated gas play primarily related to accumulations in carbonate reservoirs of the Upper Ordovician Red River Formation (figs. 6, 15-17). Some oil has been found in uppermost Ordovician Stony Mountain carbonate reservoirs, some in Silurian dolomite reservoirs of the Interlake Formation, and some in Middle Devonian carbonate reservoirs of the Winnipegosis and Souris River Formations. However, these latter reservoirs are not associated with good source rocks and probably most of the accumulations are related to vertical migration from underlying Red River carbonate reservoirs, or in isolated cases to vertical or lateral fracture communication with Upper Devonian or Madison reservoirs.

26

ALBERTA,

49°-.

I

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o B

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N

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1 1

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Figure -

15.

Stru

ctur

al contours in

thousands

.of

feet on to

p of

Red R

iver

Formation

(Ord

ovic

ian)

. Ar

ea of distribution o

f upper

Red

River

anhy

drit

es is

indicated

by b

arbe

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ne to sh

ow relationship a

mong o

il

pool

ac

cumu

lati

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Red

Rive

r carbonate-evaporite-bituminous

shal

e cy

cles

, and

regi

onal

evaporite

seals.

The

two

majo

r st

ruct

ural

trends in the

basin.(Cedar C

reek

and N

esso

n an

ticl

ines

) are

outl

ined

(s

ee fi

g. 1).

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fied

after

Pete

rsbh

and Ma

cCar

y, 1987.

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N

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ion

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ness

In

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et

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er

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mat

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des

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ne

and.

Dol

omite

fa

de

s

Oilf

ield

, Re

d R

iver

res

ervo

ir W

YOM

ING

.

Figure 16.

Thickness

in h

undreds

of feet.and g

ener

alli

ed ca

rbon

ate

facies

dist

ribu

tion

fo

r the

Red

Rive

r Formation

(Ordov.iclan)

and

equivalent

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s; U.

S. po

rtio

n ot

the

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basin

and

adja

cent

are

as..

Oil

fields pr

oduc

ing

from R

ed. River

carb

onat

e re

serv

oirs

are s

hovn.

Modified after

Peterspn'and H

acCa

ry,

1987.

.

MO

NTA

NA

\

}N

OR

TH D

AK

OTA

I

;48°

V

ro.W

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NA

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N

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ria

n

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r,

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I

IU

II

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////

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pose

d at

la

nd

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ace

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0**

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ckne

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ke

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mat

ion

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omite

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field

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terla

ke

rese

rvoi

r

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^0

K

ILO

ME

TE

RS

NEBR

ASKA

.

Figu

re.

17.

Thickness

o'f I

nter

lake

For

mati

on -

(Sil

uria

n an

d up

perm

ost

Ordovician)

in h

undreds

of fe

et.

Limy

car

bona

te fa

cies i

n

east-central p

art

of b

asin i

s sh

own.

- 'Formation is

dolomite in

re

main

der

of a

rea.

Oil

fields'producing f

rom

Inte

rlak

e re

serv

oirs

are

sh

own.

Modified a

f.ter

Pete

rson

and

Mac

Cary

, 1987.-


Reservoirs - Dolomite and dolomitic limestone reservoirs in cyclically deposited bioclastic carbonate and tidal flat dolomite beds in the upper part of the Upper Ordovician Red River Formation. Three main cycles are recognized, along with at least three additional carbonate-evaporite cycles in the overlying Upper Ordovician Stony Mountain Formation, from which minor production has been obtained in some fields. Silurian Interlake reservoirs are porous or fractured dolomite beds in the reefoid and tidal flat facies, which is present only in the subsurface of the interior basin. Middle Devonian Winnipegosis reservoirs are small dolomitized reef or mound buildups, capped by the Middle Devonian Prairie salt. Middle Devonian Souris River and Dawson Bay dolomitized cyclic beds are minor reservoirs in a few places where capped by the overlying upper Souris River shales or the basal Duperow shales.

Source Rocks - The main source rocks are organic-rich marine shales cyclically interbedded with carbonate and anhydrite beds in the upper Red River Formation. Shale beds in the overlying Stony Mountain Formation may be a secondary source. Some dark gray shale beds are present in the Middle Devonian Souris River Formation cyclic deposits but are not considered to be an important source.

Traps and Seals- -Major Red River traps are associated with the Cedar- Creek anticline trend and to a lesser extent with the Nesson anticline and Billings nose trends. Many small fields are on gently draped rootless folds associated with Red River carbonate mound or bank buildups identified by refined seismic methods. Success ratio in drilling these anomalies is very high, although many are very small accumulations; some are one- or two-well fields. Interlake traps are mainly related to porous dolomite reservoirs trapped by updip porosity changes or by the overlying Middle Devonian Ashern Formation shales. Middle Devonian Winnipegosis reservoir traps are related to dolomitized reefoid buildups overlain by the Prairie salt. Dawson Bay and Souris River carbonate accumulations are mainly in dolomite reservoirs trapped by updip porosity changes and overlying Middle Devonian shales.

A major share of the reserves in this play are on the Cedar Creek Anticline, where the upper Red River anhydrite and bituminous shale section is not present. This oil probably migrated from the basin interior into the regional Cedar Creek paleostructural trend.

Generation, Timing, and Migration - Source rocks probably reached the oil generation stage by late Paleozoic time. Migration into early-formed dolomitized carbonate stratigraphic traps was probably coincident with generation. The main tectonic features were in place by late Paleozoic or early Mesozoic time, and minor reactivation probably took place during Laramide tectonism, which only mildly affected the basin.

Depth Range - 7,000 to 15,000 ft (2,100 to 4,500 m).

30

Exploration Status

This play is moderately well explored. Almost all of the large accumulations probably have been found. There is good potential for many additional small accumulations and a few moderate-sized ones, although these will require good seismic work and relatively deep expensive exploratory drilling, with a resulting lesser payout.

Cumulative Production - 365 MMBO (1/84); estimated 390 MMBO (1/85)

Estimated Recovery from Existing Fields - Approximately 700 MMBO, of which about half is on the Cedar Creek anticline.

Number of Pools - About 325, of which about 140 are greater than 1 MMB, 8 or 10 are greater than 10 MMB, 3 or 4 are greater than 20 MMB. Almost all large pools are in Red River Formation; largest is 115 MMB, next largest is approximately 87 MMB. Mean pool size, 2.33 MMBO, 2.33 BCF gas.




IV. Post-Madison-Pre-Jurassic Play

This play is an oil and minor associated gas play involving primarily the marine and deltaic sandstone and shale with minor carbonate serrion of. the post-Madison, Upper Mississippian Big Snowy Group and the Pennsylvania!! Tyler Formation (figs. 6, 18). The overlying Upper Pennsylvania!!, Permian, and Triassic sequence of redbeds, sandstones, and some carbonates is part of the play, although thus far no production has been obtained from this section. The Big Snowy and Tyler, the main section of interest, is present only in the central part of the basin.


Reservoirs - Marine or deltaic discontinuous sandstone beds in the lower part of the Upper Mississippian Big Snowy Group and the basal part of the Pennsylvanian Tyler Formation are the main reservoirs. Carbonate beds in the upper part of the Big Snowy (Heath Formation) and in the Amsden Formation are minor producers.

Source Rocks - Dark gray and black organic-rich marine shales in the Tyler Formation are the main source rocks in North Dakota. Similar beds are present in the Tyler Formation in Montana. High-organic black shale and argillaceous dolomite are present in the Heath Formation in Montana.

Traps and Seals - Thus far, traps are mainly on anticlines with a strong stratigraphic trapping component. Isolated, perhaps small, accumulations may be expected in discontinuous Tyler sandstones but will be difficult to explore for.

31

T

1

\. ,

Ap

pro

xim

ate

lim

it o

f th

e S

no

wy

Gro

up

\14

8°

i

) I

' N

OR

TH D

AK

OTA

\

- -)

46°

i

SOU

TH D

AK

OTA

-200

Mis

siss

ipp

ian

or

ol

der

rock

s ex

pose

d at

la

nd

surf

ace

^

Thi

ckne

ss

in

feet

.o

f B

ig

Sno

wy

Gro

up

San

dy

shal

e an

d sa

ndst

one

faci

es

Gre

en,

gray

, an

d re

d sh

ale

with

m

inor

lim

esto

ne

too

(00

'ZO

O

KIL

OM

ETE

RS

NEB

RAS

KA

x/"

""*"^

200

MIL

ES 1

t I r \ /N

\ i

Oil. f

ield

, Big

Snowy

reservoir

Figu

re 18

. Th

ickn

ess

in feet and

facies of

Big S

nowy

Gro

up(Mississippian).

Modified..after

Pete

rson

and

MacCary, 1987.

Generation, Timing, and Migration - Tyler and Heath source rocks probably reached oil maturity by late Mesozoic or early Tertiary time in the deeper central part of the basin. Stratigraphic traps were formed very early, mainly by sandstone depositional processes. Structural traps were formed prior to early generation and migration, with only minor later modification. Generation and migration of oil from these source rocks probably is still underway.

Depth Range - 4,000 TO 8,000 FT (1,200 TO 2,400 M)

Exploration Status

This play is moderately well explored. Potential is fair to good for additional Stratigraphic accumulations, but the discovery rate probably will be low and mainly related to accumulations found in drilling for other better prospects in the pre-Upper Mississippian carbonate section. Federal lands are moderately well explored.

Cumulative Production - Approximately 61 MMBO (1/84); estimated approximately 65 MMBO (1/85).

Estimated Recovery from Existing Fields - 75 MMBO

Number of Pools - About 20, of which 7 are greater than 1 MMBO, 2 are greater than 10 MMBO; largest is about 25 MMBO. Mean pool size, 1.74 MMBO, 0.17 BCF gas.

Mean Estimate of Undiscovered Petroleum Resources - 10.0 MMBO in fields greater than 1 MMB, 1 ."0 BCF gas~in~fTeTds~greator than 6 BCF.

Total Area of Play - 38,000 mi 2 (100,000 km2 ).


V. Deep Basin Gas Play

During the past several years, gas has been found in Ordovician carbonate, sandstone, or quartzite rocks in several deep wells in the central basin region. The gas has a high H~S content, and so far drill stem test flows have been variable, although some are quite high (as high as 5-10 MMCF/D).


Reservoirs - Sandstone or quartzite beds, probably fractured, matrix porosity generally low, reservoir quality may be highly variable and related to fracturing of the Middle and Upper Ordovician Winnipeg Formation; dolomitized limestone beds of the Upper Ordovician Red River Formation, with variable porosity, some good porosity.

Source Rocks - Dark gray to black marine shale of the upper Deadwood Formation (Lower Ordovician part) and dark gray to black marine shales interbedded with Winnipeg sandstone beds. Shales are post-mature and in the gas window.

33

Traps and Seals - Mostly gentle folds of small closure. Carbonate traps are mainly stratigraphic, related to dolomitized mound or hank buildups of small vertical dimension, or to discontinuous sandstone bodies or variable fracture patterns in sandstones or quartzites. Ordovic.ian shales may form seals in some cases. The anhydrite beds are the probable source of the high H^S.

Generation, Timing, and Migration - Source rocks probably reached the oil generation stage by late Paleozoic time and the gas stage by Late Cretaceous or early Tertiary time. Stratigraphic traps and structure patterns probably were not changed substantially after the end of the Paleozoic, except for fracturing.

Depth Range - 13,000 to 16,000 ft (4,000 to 5,000 m) or slightly greater in places.

Comment - The deep gas play received considerable industry attention in the late 1970's, but is not being explored at this time because of high drilling costs related to depth of reservoirs, severe drilling and completion problems related to high KLS content of gas, and the present low market price of sulfur. The play may become important in the future.

Exploration Status

This play is in the early stage of exploration. Gas had not been produced, except on DST. Federal lands are lightly explored.

Because of difficulties at the present time in separating this play from III, resource estimates are combined with play III.



SIOUX ARCH

The Sioux arch province extends along the northeastern portion of the Transcontinental arch and separates the Williston basin to the north from the Denver basin and Salina to the south; the Powder River basin adjoins on the west. Three tectonic subdivisions are recognized along the Sioux arch, from east to west: the Sioux arch proper or Sioux ridge, the Kennedy basin, and the northern part of the Chadron arch. Each subdivision is characterized by unique structural styles and by different Precambrian basement rocks. The Sioux ridge is a broadly positive feature underlain by Sioux Quartzite (approximately 1.2 by); the Kennedy basin is a slight downwarp lying west of the main arch and probably underlain by granite and gneiss (approximately 1.8 by); the Chadron arch is a Laramide feature situated off the south flank of the province (greater than 2.5 by). The Chadron arch portion of the province is included in a separate Powder River basin assessment. Two plays are recognized in the remainder of the province, one with shallow gas potential in Cretaceous rocks present in both the Sioux ridge and the Kennedy basin tectonic subdivisions. The second is a minor oil play in Paleozoic rocks preserved in the Kennedy basin.

34

Cretaceous Gas - During the Paleozoic and Mesozoic, the Sioux arch bordered the Williston basin on the southeast. Coarse Cretaceous elastics were deposited in the vicinity of the arch; marine shales and chalks dominated deposition in the thick overlying section. The play is limited on the east by extensive subcrops around the high part of the uplift overlain by Pleistocene drift and on the west by outcrops and subcrops on the Chadron arch and Black Hills uplift.

Unconventional gas in chalk and shelf sandstone reservoirs constitute the main potential of this play. Enclosing marine shales provide seals for these reservoirs; organic-rich, low-porosity shaly carbonates constitute reservoirs and seals and probably also are source rocks for biogenic. gas generated shortly after deposition. Traps probably will be stratigraphic. and structural. Some small conventional gas accumulations may be locally present in thicker Cretaceous shelf sandstones. The unconventional, tight chalk and shelf sandstone reservoirs are fairly continuous throughout the area. At least two chalk units are extensive, but shelf sandstones may be more localized in the eastern and western parts of the area. Reservoirs are generally less than 2,000 ft (608 m) deep.

The play is relatively unexplored, fairly speculative, and is defined by analogy with production in shelf sandstones of the western Williston basin and in chalk (Niobrara and Greenhorn) of the northeastern Denver basin. There is no gas production from Cretaceous rocks on the Sioux arch. Federal lands include large Indian reservations, where subsurface control is particularly sparse, and Federal grasslands.

Paleozoic Rocks, Kennedy Basin - Erosional remnants of Paleozoic rocks are preserved in the Kennedy basin, including Ordovician and Mi.ssissippian carbonate rocks that produce oil to the north in the Williston basin. The petroleum potential is defined on the basis of known occurrences of. Paleozoic reservoirs; however, well control is very sparse. Consequently, distribution of Paleozoic rocks could be more widespread than currently documented, and correlation of specific lithostratigraphic units with productive units in the Williston basin is questionable.

The play is relatively unexplored and highly speculative. There is some possibility that Paleozoic oil could have migrated updip long distances out of the Williston basin. Potential reservoirs are generally at depths ranging from 2,000 ft (608 m) to 4,000 ft (1,216 m). There is no current production within the area and only a remote possibility exists for accumulations larger than 1 MMBO or 6 BCF gas. Two large Indian reservations are present.

The potential for significant oil or gas accumulations in the Sioux arch province, which includes the Sioux uplift, Kennedy basin, and northern Chadron arch areas, is considered remote, although some small localized accumulations of less than 1 MMBO or 6 BCF gas may be present. These are included with assessment figures for the Powder River, Williston, or Denver basin provinces.

35

SUMMARY

Petroleum exploration activity in the Williston basin falls into two main episodes, beginning in 1951 with the discovery of a Madison (Mississippian) oil pool at the Richey field in Montana and an Inter lake (Silurian) oil pool at the Beaver Lodge field in North Dakota. During this early phase, which lasted through the 1950's, most of the major Madison accumulations of the basin were discovered along the Nesson anticline trend in North Dakota and the major Red River and Interlake pools on the Cedar Creek anticline trend in Montana. The relatively quiescent period during the 1960's and early 1970's resulted mostly in the discovery of smaller pools, less than 2-5 MMB, except for a few larger discoveries such as Goose Lake, Flat Lake, Dwyer, Medora, and Tioga (Madison pools) and Cedar Creek (N. Dakota), Fairview, and Tioga (Red River pools) (figs. 19, 20). The second main exploration phase beginning in the mid-1970's coincided with the major increase in oil prices at that time and lasted until the rapid decline in prices in the mid-1980's. Several large Madison pools were discovered during this period of intense exploratory drilling and high discovery rates, the largest of which was Little Knife (approximately 75 MMB) in North Dakota. Increased emphasis on deeper drilling at this time also resulted in discovery of a large number of Red River and Interlake pools, most of which tend to be somewhat smaller than the larger Madison pools. This discovery phase reached a peak in 1981, when at least 80 new pools were found in the Madison, Duperow, Red River, and Interlake (figs. 19, 20).

Mean undiscovered resources in the basin foi all fields, including those less than 1 MMBO and less than 6 BCF gas, estimated as of 8/87, are 780 MMBO, 740 BCF natural gas, and 40 MMB natural gas liquids.

With somewhat over 6,000 exploratory wells having been drilled, much of the basin has been moderately well explored, particularly for the major structural prospects. Most of the remaining prospects probably contain a strong stratigraphic trapping component, and much of the potential involves deeper drilling. Prospects deserving particular attention in future exploration programs include the following considerations:

1. Carbonate buildups or porosity variations in the Ordovician Red River and Stony Mountain, Devonian Duperow, Winnipegosis, and Birdbear (Nisku), and Silurian Interlake units.

2. New pools, probably strongly stratigraphic-related, in existing fields of clusters of fields on structural trends.

3. Updip stratigraphic traps, primarily carbonate, related to porosity variations, evaporite sealing, or pinchouts on the eastern and southeastern basin flanks. Hydrology studies suggest that much of this region may be flushed by waters from the Black Hills (fig. 14). The region involves exploratory risk, but detailed hydrology studies and regional to semi-regional analysis of reservoir and other stratigraphic properties should enhance evaluation of these prospects.

4. Hydrodynamic traps in carbonate units of the basin interior,particularly those associated with porosity-permeability changes.

36

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6. Deep basin gas, particularly in the Red River, Winnipeg, and Deadwood, in part associated with fracture trends.

7. Shallow gas deposits, perhaps mostly unconventional, in Cretaceous and Jurassic sandstone, siltstone, or calcareous units, particularly the Judith River, Eagle, Niobrara, Greenhorn, Muddy (Newcastle), Inyan Kara (Dakota, Fall River, and Lakota), and lignite or coalbed methane in the Fort Union beds.

8. Increased oil recovery from the organic-rich Bakken Formation. Recent successes in horizontal drilling enhance the possibility of substantial future resources from this and other hydrocarbon-rich units, particularly in the central part of the basin.

Many of the future discoveries likely will be relatively small and the success ratio may tend to diminish. However, applications of refined geophysical, geological, and hydrologic studies should result in addition of significant reserves to fit the existing price structure.

ACKNOWLEDGMENTS

This report includes only a brief geological summary of the Williston basin. More detailed analyses are found in the references listed, many of which were freely used in developing the geologic background for this resource assessment. Critical review with many useful observations and recommendations were provided by G.L. Dolton, R.B Powers, and D.D. Rice, U.S. Geological Survey. G.W. Shurr provided information on the Sioux arch province. Team assessment of petroleum resources was made by D.L. Dolton, R.F. Mast, C.D. Masters, R.B. Powers, and J.A. Peterson.

SELECTED REFERENCES

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Anderson, S.B., Bluemle, J.P. and Gerhard, L.C., 1982, Oil exploration and development in the North Dakota Williston Basin: Saskatchewan Geological Society, 4th International Williston Basin Symposium, Publication 6, p. 3-10.

Ballard, W.W., 1969, Red River of northwest Montana and northwest North Dakota: Eastern Montana Symposium, Montana Geological Society 20th Annual Conference, p. 15-24.

Billings Geological Society, North Dakota Geological Society, andSaskatchewan Geological Society, 1964, Third International Williston Basin Symposium.

Brown, D.L., 1978, Wrench-style deformational patterns associated with a meridional stress axis recognized in Paleozoic rocks in parts of Montana, South Dakota, and Wyoming, in The economic geology of the Williston basin: Montana Geology Society, Williston Basin Symposium, 24th Annual Conference, p. 17-31.

39

Brown, D.L., Blankennagel, R.K., MacCary, L.M., and Peterson, J.A., 1982, Correlation of paleostructure and sediment deposition in the Madison Limestone and associated rocks in parts of Montana, North Dakota, South Dakota, Wyoming and Nebraska: U.S. Geological Survey Open-File Report 82-906, 71 p.

Carlson, C.G., 1960, Stratigraphy of the Winnipeg and Deadwood formations in North Dakota: North Dakota Geological Survey Bulletin 35, 149 p.

Carlson, C.G., and Anderson, S.B., 1965, Sedimentary and tectonic history of North Dakota part of Williston basin: American Association of Petroleum Geologists Bulletin, v. 49, p. 1833-1846.

Carroll, W.K., 1978, Depositional and paragenetic controls on porositydevelopment, upper Red River Formation, North Dakota, in The economic geology of the Williston basin: Montana Geological Society, Williston Basin Symposium, 24th Annual Conference, p. 79-94.

Christopher, J.E., 1987, Depositional patterns and oil field trends in the lower Mesozoic of the northern Williston basin, Canada, in Longman, M.W., ed., Williston basin Anatomy of a cratonic oil province: Denver, Colo., Rocky Mountain Association of Geologists, p. 223-244.

Clement, J.H., 1987, Cedar Creek A significant paleotectonic feature ofthe Williston basin, in Longman, M.W., ed., Williston basin Anatomy of a cratonic oil province: Denver, Colo., Rocky Mountain Association of Geologists, p. 323-336.

Dolton, G.L., Carlson, K.H., Charpentier, R.R., Coury, A.B., Crovelli, R.A., Frezon, S.E., Khan, A.S., Lister, J.H., McMullin, R.H., Pike, R.S., Powers, R.B., Scott, E.W. , and Varnes, K.L., 1981, Rstimar.es of undiscovered recoverable conventional resources of oil and gas in the United States: U.S. Geological Survey Circular 860, 87 p.

Dow, W.G., 1974, Application of oil-correlation and source-rock data toexploration in the Williston basin: American Association of Petroleum Geologists Bulletin, v. 58, p. 1253-1262.

Downey, J.S., 1984, Geohydrology of the Madison and associated aquifers in parts of Montana, North Dakota, South Dakota, and Wyoming: U.S. Geological Survey Professional Paper 1273-G, p. G1-G47.

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Gerhard, L.C., Anderson, S.B., and LeFever, J.A,, 1987, Structural history of the Nesson anticline, North Dakota, in Longman, M.W., ed., Williston basin Anatomy of a cratonic oil province: Denver, Colo., Rocky Mountain Association of Geologists, p. 337-353.

Gerhard, L.C., Anderson, S.B., LaFever, J.A., and Carlson, C.G., 1982, Geological development, origin, and energy mineral resources of Williston basin: American Association of Petroleum Geologists Bulletin, v. 66, p. 989-1020.

Hajnal, Z., and Fowler, C.M.R., 1982, The Earth's crust under the Williston basin in eastern Saskatchewan and western Manitoba: Fourth International Williston Basin Symposium, Saskatchewan Geological Society Special Publication 6, p. 13-18.

40

Hartling, A., Brewster, A., and Posehn, G., 1982, The geology andhydrocarbon trapping mechanisms of the Mississippian Oungre zone (Ratcliffe Beds) of the Williston basin: Fourth International Williston Basin Symposium, Saskatchewan Geological Society Special Publication 6, p. 217-223.

Hoganson, J.W., 1978, Microfacies analysis and depositional environments of the Duperow Formation (Frasnian) in the North Dakota part of the Williston basin, in The economic geology of the Williston basin: Montana Geological Society, Williston Basin Symposium, 24th Annual Conference, p. 131-144.

Kent, D.M., 1973, Paleozoic hydrocarbon reservoirs in Saskatchewan and their relationship to basement lineaments: Journal of Canadian Petroleum Technology, v. 12, p. 20-24.

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41

Maughan, E.K., 1984, Paleogeographic setting of Pennsylvanian TylerFormation and relation to underlying Mississippian rocks in Montana andNorth Dakota: American Association of Petroleum Geologists Bulletin,v. 68, p. 178-195.

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42

Sando, W.J., 1978, Coral zones and problems of Mississippian stratigraphy in the Williston basin: Montana Geological Society, 24th Annual Conference, Williston Basin Symposium, p. 231-238.

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