vip

234

ABSTRACT

Petrographic analyses of dispersed organic matter (including macerals and palynomorphs), siliceous and calcareousmicrofossil assemblages and microtextures (e.g. stromatolitic) have been used to define and interpret five organic facies andregionally map their distribution for the following informal groupings of potential hydrocarbon source rocks in the WesternCanada Sedimentary Basin: Upper Devonian Woodbend group, Upper Devonian Winterburn group and Upper Devonian toLower Mississippian black shales of the Exshaw and Bakken formations. Five petrographic organic facies (A–E) are definedfor the potential source rocks based on assemblages of alginites, acritarchs, sporinites, siliceous microfossils and algal mat microtextures. Organic facies A, B (prasinophyte alginites and acritarchs) and C (coccoidal alginite), represent accumulation in relatively deep (basin), intermediate (shelf-platform), and shallow water depths (bank-reef margin tolagoonal). Organic facies D is defined by siliceous microfossils (e.g. Radiolaria) and accumulated in deep basinal to outershelf settings immediately east of an ancient Pacific Ocean, or south of an ancient Arctic Ocean. This facies may reflectregions of upwelling which extended into intracratonic and epicontinental settings. Organic facies E, characterized by stromatolitic microtextures with or without coccoidal alginite, only occur within Upper Devonian Winterburn Groupshallow water, restricted shelf to lagoonal dolostones associated with evaporites. As a whole, the regional distribution oforganic facies is related to paleogeography, paleobathymetry or paleostructure in the source rocks. Surprisingly, petrographic organic facies do not show strong positive correlation with kerogen type as defined by Hydrogen-Oxygenindices or TOC-S2 plots.

RÉSUMÉ

Des analyses pétrographiques de matières organiques dispersées (incluant des macéraux et des palynomorphes), desassemblages de micro-fossiles siliceux et calcaires et des micro-textures (ex. stromatolites), ont été utilisés afin de définir-interpréter 5 faciès organiques, et de cartographier leur distribution à l’échelle régionale en vue de groupementsinformels de sources potentielles de roches mères d’hydrocarbures dans le bassin sédimentaire de l’Ouest du Canada: leGroupe Woodbend du Devonien supérieur, le Groupe Winterburn du Devonien supérieur, et les argiles litées noires des formations Exshaw et Bakken du Devonien supérieur jusqu’au Mississippien inférieur. Cinq faciès organiques pétrographiques (A–E) sont définis en tant que sources potentielles de roches mères basées sur des assemblages d’alginites,d’acritarches, de sporinites, de micro-fossiles siliceux et de micro-textures de laminites algaires. Les faciès organiques A,B (alginites de prasinites et acritarches) et C (alginite de coccolithe), représentent une accumulation dans des niveaux d’eaurelativement profonds (bassins), intermédiaires (plateaux continentaux), et peu profonds (bordures des berges de récifs àlagunaires). Le faciès organique D est défini par des micro-fossiles siliceux (ex. Radiolaria) et se trouve accumulé dans desorganisations de bassins profonds à des plateaux continentaux externes situés immédiatement à l’est d’un Océan pacifiqueancien, ou au sud d’un Océan arctique ancien. Ce faciès peut représenter des régions ascendantes qui se sont prolongées en organisations intracratoniques et épicontinentales. Le faciès organique E est caractérisé par des micro-textures de stromatolites avec ou sans alginite de coccolithe, et se trouve présent uniquement à l’intérieur des eaux peu profondes du Groupe Winterburn du Devonien supérieur, dans les plateaux continentaux aux dolomites lagunaires protégés, associées aux évaporites. Dans l’ensemble, la distribution régionale de faciès organiques est reliée à la paléogéographie, à la paléobathymétrie ou à la paléostructure des roches mères. Curieusement, la pétrographie des faciès organiques ne révèle pas de forte corrélation positive avec un type de kérogène tel que défini par les indicesHydrogène –Oxygène ou par les tracés COT-S2.

BULLETIN OF CANADIAN PETROLEUM GEOLOGYVOL. 52, NO. 3 (SEPTEMBER, 2004), P. 234-255

Organic facies in Devonian and Mississippian strata of Western Canada Sedimentary Basin: relation to kerogen type, paleoenvironment, and paleogeography

L.D. STASIUK

Natural Resources CanadaGeological Survey of Canada

3303 – 33 St NWCalgary, AB T2L 2A7

M.G. FOWLER

Natural Resources CanadaGeological Survey of Canada

3303 – 33 St NWCalgary, AB T2L 2A7

INTRODUCTION

Organic facies were first defined by Rogers (1980) on thebasis of “organic matter type, its source, and paleodepositionalenvironment” (see Tyson, 1995). Peters et al. (1981) laterexpanded the definition stating that the “concept is based on thetype of organisms and biomass, the paleodepositional environ-ment, and the conditions prevailing during early diagenesis”.From a petroleum exploration and basin analysis perspective,organic facies are used primarily to predict the occurrence andquality of hydrocarbon source rocks as a function of paleo-environment (e.g. Tyson, 1995; Stasiuk, 1999). From a paleo-ecological and paleoclimatic perspective, organic facies andorganic microfacies can potentially identify short-term andlong-term cycles related to transgressions, regressions or bio-logical productivity (e.g. Lallier-Vergès et al., 1993; Chow etal., 1995; Stasiuk, 1999).

Lateral and vertical variations in organic facies within sedi-mentary rocks occur at a range of scales; e.g. basinal to sequenceto ‘lamina’ scale. A variety of geochemical and petrographicparameters have been used to characterize and define organicfacies variations within and between potential hydrocarbon

source rock units and how these relate to paleodepositional envi-ronments, sequence stratigraphy, sedimentology and cycles,source rock quality, hydrocarbon generation thresholds, and thechemistry of generated hydrocarbons (e.g. Jones, 1987; Tyson,1995). Most commonly, organic facies are defined by using thecovariance between bulk geochemical (C, H, O and S) andmicroscopic data (e.g. maceral or palynomorph composition; e.g.Jones, 1987; Tyson, 1995; Follows and Tyson, 1998). As part ofa major regional evaluation of Devonian petroleum systems inthe Western Canada Sedimentary Basin (WCSB) (Fowler et al.,2001), organic facies have been defined for whole rock samplesusing macerals, inorganic microfossils, and organic-inorganicmicrotextures such as those produced by cyanobacterial or algalmats (i.e. stromatolites) (see also Stasiuk et al., 1991; Chow etal., 1995; Fowler and Stasiuk, 1995, 1999; Stasiuk and Wendte,1995; Stasiuk, 1996; Obermajer et al., 1997; Stasiuk, 1999;Fowler et al., 2001; Fowler et al., 2004). Organic facies distri-bution at the basin-scale is mapped for Upper DevonianWoodbend and Winterburn groups and for latest Devonian toearliest Mississippian Bakken and Exshaw potential source rock units in Alberta, British Columbia, Northwest Territoriesand Saskatchewan (Figs. 1 and 2), and are compared with

ORGANIC FACIES IN DEVONIAN AND MISSISSIPPIAN OF WESTERN CANADA BASIN 235

Fig. 1. Location map showing study area in the Western Canada Sedimentary Basin.

236 L.D. STASIUK and M.G. FOWLER

Fig

. 2.

Str

atig

raph

y fo

r D

evon

ian

to L

ower

Mis

siss

ippi

an s

trat

a in

the

Wes

tern

Can

ada

Sed

imen

tary

Bas

in

(mod

ified

fro

m R

eins

on e

t al

., 19

93).

paleoenvironment, tectonic, and paleogeographic features,organic geochemical character, and hydrocarbon generationthresholds. Organic facies are also compared with total organiccarbon contents (TOC) and Hydrogen (HI) and Oxygen (OI)indices from Rock-Eval pyrolysis (see also Fowler et al., 2001)to assess potential correlations and variations in kerogen type(i.e. OI versus HI) and quality as a function of organic facies.

METHODS

Whole rock organic-rich core and cuttings samples wereprepared for incident light microscopy by mounting represen-tative, oriented (parallel and perpendicular to bedding) and ran-dom particulate rock fragments (approximately 2–4 mm ) in a3.2 cm diameter mold in epoxy resin to form pellets. The pel-lets were ground on 240 grit and then on 600 grit silicon car-bide papers and polished on pellon cloth in 0.3 µm aluminasuspension and then on a double silk cloth with 0.05 µm alu-mina suspension. Dispersed organic matter was analyzed andclassified using reflected white and fluorescent lightmicroscopy. Three reflected light microscope systems wereused for organic petrologic analysis: a Leitz MPV II system, aZeiss UMP system, and a Zeiss Axioplan II system with a totalmagnification ranging from 10 to 2500x. A Zeiss Invert 410confocal laser scanning microscope (up to x8500 magnifi-cation) equipped with 488 nm, 543 nm, and 633 nm lasers wasalso used for assisting in classifying vitrinite, structured andamorphous liptinite macerals.

During sample examination (approximately 300 to 350microscopic fields of view, 300µm across) detailed descriptivenotes were made on aspects of mineralogy, lithology (inc. vari-ations in mineralogy in micro-laminations), type and sizeranges of inorganic microfossils, prasinophytes, coccoidalalginites, acritarchs and sporinites, cell wall thicknesses ofprasinophytes and coccoidal alginite, micro-lamination oforganic and/or inorganic microtextures (e.g. lamination, algalmat pustules, pinnacles). A semi-quantitative visual assessmentof the amount of DOM, organic-walled microfossils, andsiliceous or calcareous microfossils in each sample was madeusing major, minor, very minor and trace to rare categories.Visual percentage assessment charts showing distribution ofparticle percentages were used as a guide for assessing compo-sition. At the end of each analysis, the sample was assigned anorganic facies in the same way a sedimentary petrologist orgeologist assigns a lithology or a lithofacies to a bed after thorough visual assessment. Constituents in the trace amount torare category had no bearing on organic facies definition of asample. For samples which contained at least 10% particleswith different organic components compared with the domi-nant particles, a primary and secondary organic facies wasdefined (see Tables 1–3).

Rock-Eval pyrolysis experiments were conducted usingDelsi Rock-Eval 6 unit equipped with a Total Organic Carbonanalysis module. Prior to pyrolysis, the samples were hand-pulverized in an agate mortar and weighed (about 100 mg) insteel crucibles. A typical Rock-Eval experiment is initiated

with heating of a pulverized rock sample at 300oC for 3 min inHe atmosphere, when naturally occurring hydrocarbons (freeand adsorbed) are volatilized. During the next stage, the oventemperature is steadily increased to 650oC at a rate of25oC/min. The final stage involves oxidation and combustionof the residual organic matter at 650oC. The amount of hydro-carbons volatilized at 300oC and evolved from kerogen at300oC to 650oC is quantitatively determined by a flame ioniza-tion detector, and recorded as the S1 and S2 peaks (mg hydro-carbons/gram Rock), respectively. The temperature measuredat the maximum of the S2 peak is referred to as Tmax.Percentage of carbon in CO2 formed during oxidation at 650oCand in the hydrocarbon peaks S1 and S2 is used to define thetotal organic carbon content (TOC), expressed as a weight per-centage. The amount of pyrolyzable carbon in the kerogen isexpressed as a percentage (PC %). The amount of hydrogen inthe kerogen is expressed as Hydrogen Index (HI = S2/TOCx100)and oxygen in the kerogen is expressed as Oxygen Index (OI =[mg CO2/gm sample/TOC] x100).

MACERALS, PALYNOMORPHS, MICROFOSSILS AND

MICROTEXTURES: ORGANIC FACIES MODEL

The palynomorph-, maceral- and microfossil-based organicfacies model used for Devonian and Mississippian source rockunits of the WCSB is shown in Figure 3. The validity of thismodel has been evaluated against lithostratigraphy, sedimentol-ogy, sequence stratigraphy, paleo-water depth estimates (e.g.Chow et al., 1995; Stasiuk et al., 1999; Wiebe et al., 2001) andorganic geochemistry (e.g. Obermajer et al., 1997; Fowler et al.,2001; Fowler et al., 2004). Organic facies A, B and C (Fig. 3;Chow et al., 1995; Stasiuk, 1999), defined by the amount andtype of unicellular prasinophyte alginite (e.g. Tasmanites andLeiosphaeridia planktonic green algae, “P” in Fig. 3), spinyacanthomorphic acritarchs (presumed planktonic algae; “H, M,V” in Fig. 3), coccoidal alginite (planktonic green algae and/orcyanobacteria; “Ca” in Fig. 3) and sporinite (land plant-derivedspores and pollen; “Sp” in Fig. 3), represent deposition in rela-tively deep, intermediate, and shallow water depths, respectively(Fig. 3). Organic facies D is found in potential source rock inter-vals enriched in siliceous microfossils (mainly Radiolaria; “Ra”in Fig. 3) deposited in deep through intermediate water depths inbasinal to platformal settings. Organic facies E is characterizedby algal mat micro-textures with or without coccoidal alginite,sporinite, funginite and chlorophyllinite (Fig. 3) and wasdeposited mainly in shallow water, evaporitic paleoenvironmentssuch as in restricted platforms or lagoons.

RESULTS AND DISCUSSION

Organic facies (A–E) were primarily defined for sampleshaving the best hydrocarbon source rock potential at any givenlocation (i.e. highest weight per cent organic carbon and thehighest Hydrogen Index values), although at many locationsless enriched potential source rocks were also analyzed.



Tab

le 1

.Lo

catio

ns,

orga

nic

faci

es a

nd R

ock-

Eva

l pyr

olys

is d

ata

for

sam

ples

exa

min

ed f

rom

the

Woo

dben

d gr

oup

and

appr

oxim

atel

y eq

uiva

len

t un

its —

see

tex

t fo

r st

ratig

raph

ic d

etai

ls.

TO

C =

wei

ght

%;

S1

and

S2

(mg

hydr

ocar

bons

/gra

m R

ock)

; T

max

(°C

); H

ydro

gen

Inde

x (H

I =

S2/

TO

Cx1

00)

and

Oxy

gen

Inde

x (O

I =

[m

g C

O2/

gm s

ampl

e/T

OC

] x1

00);

PI

= S

1/(S

1+S

2)


Tab

le 1

co

nti

nu

ed.

Organic facies and Rock-Eval data were collected for the fol-lowing informal groups of potential hydrocarbon source rocksin the WCSB (Fig. 2): (i) Upper Devonian Woodbend groupincluding shales in the Duvernay Formation of central Alberta,the Muskwa Formation shales of northern Alberta andNorthwest Territories, the Canol Formation of the NorthwestTerritories, and carbonates of the Duperow Formation ofSaskatchewan; (ii) Upper Devonian Winterburn group includingshaly and evaporitic carbonates of Nisku and Birdbear forma-tions of Alberta and Saskatchewan and the Camrose Member ofthe Ireton Formation in Alberta; and (iii) Upper Devonian toLower Mississippian black shales of the Exshaw and Bakken

formations of Alberta, Northwest Territories and Saskatchewan.Petrographic organic facies and Rock-Eval data for these group-ings of source rocks are provided in Tables 1 to 3.

ROCK-EVAL AND ORGANIC FACIES — GENERAL

Hydrogen Index versus Oxygen Index (pseudo-vanKrevelen diagrams) and S2 versus TOC by organic faciestype for all three source rock groups are shown in Figures 4and 5. The pseudo-van Krevelen diagrams (Figs. 4a–c) do notshow any clear relationship between organic facies and kero-gen type for any of the three intervals of potential hydro-carbon source rocks.


Table 2. Locations, organic facies and Rock-Eval pyrolysis data for samples examined from the Winterburn group and equivalent units in western Canada. TOC = weight %; S1 and S2 (mg hydrocarbons/gram Rock); Tmax (°C); Hydrogen Index (HI = S2/TOCx100)

and Oxygen Index (OI = [mg CO2/gm sample/TOC] x100); PI = S1/(S1+S2)


Tab

le 3

.Lo

catio

ns,

orga

nic

faci

es a

nd R

ock-

Eva

l pyr

olys

is d

ata

for

Exs

haw

and

Bak

ken

form

atio

ns in

wes

tern

Can

ada.

T

OC

= w

eigh

t %

; S

1 an

d S

2 (m

g hy

droc

arbo

ns/g

ram

Roc

k);

Tm

ax (

°C);

Hyd

roge

n In

dex

(HI

= S

2/T

OC

x100

) an

d O

xyge

n In

dex

(OI

= [

mg

CO

2/gm

sam

ple/

TO

C]

x100

); P

I =

S1/

(S1+

S2)


Tab

le 3

co

nti

nu

ed.

For low to mid maturity Upper Devonian Woodbend groupsamples (see also Stasiuk and Fowler, 2002), deep waterorganic facies A plots in the Type I to Type II kerogen field;intermediate water depth organic facies B data plots in Type Ito Type II, and the Type II to III kerogen fields (Fig. 4a).Organic facies C samples plot mainly in the Type I–II field(same as OF A) with a few of the samples in the Type II–III toType III fields. Organic facies D samples also plot in the Type Ito II field. S2 and TOC are highest for deepest water organicfacies A, intermediate for ‘shelf-platform’ organic facies B, andlowest for shallow water organic facies C (Fig. 5a). Petroleumpotential in the Woodbend group thus appears to be directlyrelated to organic facies and paleo-water depth.

Both organic facies B and C in the Winterburn samples plotacross the Type I–Type II–Type III kerogen fields (Fig. 4b).Organic facies A, E and D have too few data points to establisha trend although some of organic facies E samples fall in theType I to II field (Fig. 4b). S2 versus TOC for immature to mar-ginally mature Winterburn samples (Table 2; see also Stasiukand Fowler, 2002) shows that there is no definitive petroleumpotential difference between organic facies B and C, althoughorganic facies E samples have considerably less petroleumpotential (Fig. 5b).

Upper Devonian to Lower Mississippian Exshaw-Bakkenformation samples (Fig. 4c, Table 3) of organic facies A, B, andD plot mainly in the Type I to II field and between Types II andIII kerogen. The majority of organic facies C are in the Type IIto III kerogen field with a few samples plotting directly onType I kerogen trend. S2 and TOC (Fig. 5c) indicate that thedeep water organic facies A and intermediate water depth havethe best petroleum potential at low levels of thermal maturity.

REGIONAL DISTRIBUTION OF PETROGRAPHIC

ORGANIC FACIES

Woodbend Group

Table 1 lists the locations, depths, organic facies types,Rock-Eval and TOC data for Woodbend group samples.Photomicrographs of macerals and microfossils withinWoodbend group organic facies A to D are shown in Figure 6.Figures 7a, b illustrate the regional distribution of these organicfacies in the WCSB.

Intermediate water depth organic facies B potential sourcerocks (see model in Fig. 3) dominate in central to west centralAlberta whereas siliceous microfossil-enriched organic faciesD dominates in western and northern Alberta, and in southernNorthwest Territories (Figs. 7a, b). Shallow water bank marginto inner platformal coccoidal alginite-enriched organic facies Cpotential source rocks are less common although they areimportant in eastern Alberta and southern Saskatchewan(Figs. 7a, b).

The regional distribution of organic facies in Woodbendgroup source rocks appear to be primarily related to paleoba-thymetry, paleotectonic elements and paleogeographic features.In central Alberta, particularly within calcareous shales of theEast and West Shale basins in Alberta (Figs. 7a, b), intermediate


Tab

le 3

co

nti

nu

ed.


Fig

. 3.

Org

anic

fac

ies

(OF

) m

odel

for

Dev

onia

n an

d M

issi

ssip

pian

str

ata

in t

he W

este

rn C

anad

a S

edim

enta

ry B

asin

(se

e C

how

et

al.,

1995

). F

or d

etai

ls o

f m

etho

d us

ed f

or o

rgan

icfa

cies

def

initi

on s

ee t

ext

sect

ion:

DO

M (

Mac

eral

s, p

alyn

omor

phs)

, m

icro

foss

ils,

mic

rote

xtur

es:

orga

nic

faci

es m

odel

. T

he d

istr

ibut

ion

of a

lgin

ites

and

acrit

arch

s, s

ilice

ous

mar

ine

mic

ro-

foss

ils (

e.g.

Rad

iola

ria)

and

terr

estr

ial

spor

inite

s de

fines

org

anic

fac

ies

A–D

(se

e al

so D

orni

ng,

1987

; Ty

son,

198

7; s

ee S

tasi

uk,

1999

and

tex

t fo

r fu

rthe

r de

tails

). O

F A

, B

and

C a

rede

fined

by

unic

ellu

lar

pras

niop

hyte

alg

inite

s (e

.g.

plan

kton

ic g

reen

mic

roal

gae,

“P

”),

spin

y ac

anth

omor

phic

acr

itarc

hs (

plan

kto

nic,

gre

en m

icro

alga

e; “

H,

M,

V”;

see

als

o in

shor

e-of

fsho

reac

ritar

ch z

onat

ion

by t

ype

in M

olyn

eux

et a

l., 1

996)

, co

ccoi

dal a

lgin

ite (

plan

kton

ic g

reen

and

/or

blue

-gre

en m

icro

alga

e; “

Ca”

) an

d sp

orin

ite (

land

pla

nt-d

eriv

ed s

pore

s an

d po

llen;

“S

p”).

Org

anic

faci

es D

sam

ples

con

tain

min

or to

maj

or a

mou

nt o

f sili

ceou

s m

icro

foss

ils (

mai

nly

radi

olar

ian-

deriv

ed, “

R”)

. Org

anic

faci

es E

com

pris

es la

min

ated

am

orph

inite

and

/or

filam

ento

usal

gini

te w

ith m

icro

text

ures

con

side

red

indi

cativ

e of

alg

al m

ats.

Org

anic

fac

ies

Are

pres

ents

rel

ativ

ely

deep

wat

er,

orga

nic

faci

es B

int

erm

edia

te w

ater

dep

ths,

and

org

anic

fac

ies

C,

shal

low

wat

er d

epos

ition

. The

late

ral d

epos

ition

al li

mit

of o

rgan

ic fa

cies

D is

pro

babl

y w

ide,

ran

ging

from

bas

inal

to p

latfo

rmal

. Org

anic

faci

es E

is a

sha

llow

wat

er, s

ub-t

idal

set

ting.

Pal

eo-

wat

er d

epth

s fo

r A

–C a

re b

ased

on

corr

elat

ions

in C

how

et

al.

(199

5) f

or R

edw

ater

ree

fs w

here

bas

inal

to

bank

mar

gin

Duv

erna

y so

urce

roc

ks in

ter-

finge

r w

ith L

educ

ree

fs.


Fig. 4. Hydrogen Index versus Oxygen Index by organic facies for:(a) Woodbend group, (b) Winterburn group and; (c) Exshaw-Bakken formations, Western Canada Sedimentary Basin.

Fig. 5. Total organic carbon (wt %) versus S2 for: (a) Woodbendgroup, (b) Winterburn group and; (c) Exshaw-Bakken formations,Western Canada Sedimentary Basin.

are most characteristically associated with cold, deep watermasses and/or upwelling systems in open ocean settings (e.g.Jones, 1996; Barron, 1993), may imply connectivity to anancient Arctic Ocean to the north during Woodbend sourcerock deposition. Increased water circulation during depositionof the western and northern extent of Upper DevonianWoodbend group source rocks, relative to those deposited inthe West and East Shale basins of central Alberta (Figs. 7a, b),may account for the overall accumulation of lower qualitypotential source rocks, as documented by Jenden and Monnier(1997) and Fowler et al. (2001) in northern Alberta. This isparticularly evident adjacent to the Peace River landmasswhere oils contain evidence of terrestrial contribution near thelandmass (Fowler et al., 2001). The only significant break inthe extensive ‘radiolarian province’— organic facies D in theWoodbend group throughout northern Alberta and NorthwestTerritories — is the localized occurrence of coccoidalalginite-enriched shallow water organic facies C in northwestAlberta south of the Tathlina Arch (Figs. 7a, b; Table 1), probablyreflecting shallowing and deposition upon a narrow shelf associated with an active broad structural high (Moore, 1988).

Upper Devonian Winterburn Group

Regional Organic Facies

Table 2 lists the locations, depths, primary and secondaryorganic facies types, Rock-Eval and TOC data for UpperDevonian Winterburn group samples. Representative photomi-crographs of macerals and microfossils in Winterburn organicfacies can be found in Fowler et al. (2004). Potential hydrocar-bon source rocks in the Winterburn group are not as regionallyextensive as those within either Woodbend group or Exshawand Bakken formations and typically occur in shaly dolostonesassociated with evaporites deposited under restricted subtidalto supratidal conditions (see Creaney et al., 1994 and Fowler etal., 2001).

Figure 8c illustrates the distribution of organic facies A to Efor the Winterburn group in Alberta and Saskatchewan.Intermediate water depth organic facies B and shallow watercoccoidal alginite-enriched organic facies C (includingGloeocapsomorpha prisca; Fowler and Stasiuk, 1999; Fowler etal., 2004) and stromatolitic organic facies E are widespreadwithin Winterburn shelf paleoenvironments of central to south-ern Alberta and southwestern Saskatchewan (“Restricted Shelf”and “Nisku Shelf” in Figure 7c). Fowler and Stasiuk (1999) andFowler et al. (2004) have shown classic ‘Ordovician-like’ kuk-ersite characteristics for extracts from Type I kerogen in organicfacies C units of the Winterburn group (e.g. odd predominanceof C17 and C19 n-alkanes; very low amounts of higher molec-ular weight n-alkanes, acyclic isoprenoids), resulting from the


water depth organic facies B (Figs. 6a–e, lesser organic faciesA) were noted to be the most common within DuvernayFormation laminites (see Table 1; Figs. 7a, b). Paleo-waterdepths in these areas are estimated at >60 to >100 m (Chow etal., 1995; Stoakes, 1980). Along the eastern part of the WestShale basin and throughout the East Shale Basin of Alberta,shallow water coccoidal alginite-enriched organic facies C(Figs. 6f–h; Figs. 7a, b) contribute significantly to Woodbendgroup potential source rocks and are characterized by enrich-ment in chitinous microfossils (scolecodonts and chitinozons;Figs. 6k–m), terrestrial plant-derived macerals (e.g. sporinite;Figs. 6i, j), as well as by increased bioturbation. Organic faciesC is commonly associated with forestepping stages of carbonatereef complexes and bank margin environments where theDuvernay Formation interfingers with the Leduc Formation(Chow et al., 1995), and near Redwater, Alberta, paleowaterdepths for OF C are estimated at <40 m (Chow et al., 1995).

Southeast of the West and East Shale sub-basins in Alberta(Figs. 7a, b), intermediate water depth organic facies B andshallow water organic facies C are dominant within Woodbendgroup potential source rocks, extending into southwesternSaskatchewan where deposition occurred within platform tolagoonal carbonate environments on the Eastern Shelf(Figs. 7a, b). This region was also far removed from terrestrialhighlands (Kent, 1968; Switzer et al., 1994) which is consistentwith total absence of terrestrial macerals in OF C. There is alsoa lack of geochemical correlation between those samples in thisarea having lower HI values and those having higher abun-dances of C20+ n-alkanes with an odd carbon number prefer-ence, which might be expected if there was a significantcontribution from higher land plant derived organic matter(Fowler, 1999); i.e. there is no correlation of organic mattertype with the source of the organic matter that is present in theDuperow. Biomarkers from most Duperow Formation samplesin this area have pristane/phytane ratios of less than 1 which areconsistent with organic matter deposited under highly salineand/or reducing conditions (Fowler, 1999), such as within arestricted inner platform or lagoon setting.

Woodbend group potential source rocks in west-centralAlberta, northern Alberta and Northwest Territories are domi-nated by siliceous microfossil-enriched organic facies D(Figs. 6n–t, 7a, b; Table 1). Assemblages of Radiolaria, asso-ciated mainly with amorphinite and marine prasinophytealginites in this part of the WCSB, likely reflect a higherdegree of water circulation with connectivity to open oceancompared to Woodbend group source rocks deposited in theEast and West Shale basins (see also Fowler et al., 2001).Abundant siliceous plankton (e.g. OF D) such as diatoms, sil-icoflagellates and Radiolaria, that in modern ocean systems

Fig. 6. Macerals and microfossils in organic facies (OF) A, B, C and D of Upper Devonian Woodbend group; scale bar (50 µm) shown in “s” isfor photos without scale bar. Fluorescence and reflected white light (k, l, s, t), water or oil immersion. a. OF A: Large and small prasinophytealginites (a) , brown-fluorescing amorphinite matrix (am) and carbonate (c); b. Prasinophyte alginites (a). c–e. Acanthomorphic acritarchs (ac) inOF B. f–h. OF C: Coccoidal alginites (c); H shows degraded alginite. i, j. OF C: Sporinite (sp), bisaccate pollen grain in J. k–m. OF B–C: Chitinousmicrofossils (ch) are probably derived from scolecodonts and chitinozoans. n–r. OF D; siliceous microfossils (s) mainly derived from Chrysophytealginites like Radiolaria. s, t. OF D: Siliceous microfossils (c) infilled by granular bitumen (b).

significant contribution of G. prisca alginite. Evaporitic, shal-low water organic facies C and E in the Winterburn also com-monly contain red fluorescing amorphinite (see Fowler et al.,2004) suggesting preservation of the organic matter underhighly alkaline and highly reducing conditions (e.g. di Primoand Horsfield, 1996) which is consistent with deposition inshallow water lagoonal to tidal flat environments (see alsoSwitzer et al., 1994). Several stromatolitic organic facies Eunits also contain evidence for preferential sulphur-enrichmentof amorphinite where sulphide-rich lamina repeatedly alternatewith sulphide-depleted lamina. This association commonlyproduces abundant solid bitumens at relatively low levels ofthermal maturity. Deep water organic facies A and siliceousmicrofossil-enriched organic facies D are insignificant inWinterburn group source rocks compared with Woodbendgroup and Exshaw and Bakken formation source rocks(Fig. 7c). The occurrence of siliciclastic organic facies D isrestricted to the most westerly regions of the study area, per-haps representing a link to an open ocean westward of theCynthia Shale Basin in west central Alberta, and the outer shelfin southern Alberta (Fig. 7c).

Exshaw and Bakken Formations – Regional Organic Facies

Table 3 lists the locations, depths, organic facies types, Rock-Eval and TOC data for Exshaw-Bakken formation samples.Representative macerals within organic facies A through D areshown in Figure 8 and their regional distribution is shown inFigure 7d. Black shales of the Exshaw and Bakken formationsin the WCSB were deposited during Late Devonian to EarlyMississippian in an extensive epicontinental sea that extendedthroughout most of Alberta, northeast British Columbia, centralto southern Saskatchewan, and southern Manitoba.

In southern Alberta (between 49° and 52°N Latitude),Exshaw Formation black shales are dominated by siliceousmicrofossil-enriched organic facies D in the west, east of theRocky Mountain deformation front (Fig. 7d), with a region ofshallow paleo-water organic facies C shales in south-easternAlberta (between 110°W to 113°W longitude; Fig. 7d). Organicfacies C dominate up to the Alberta-Saskatchewan border andthen are replaced eastward by Bakken Formation shales con-sisting of deep to intermediate paleo-water depth organic faciesA and B. Shallow water deposition of OF C near theSaskatchewan-Alberta border is consistent with the ‘paleoba-thymetric high’ interpreted by Caplan and Bustin (2001) in thesame area. In central and northern Alberta, and extendingnorthward into southernmost Northwest Territories, organicfacies D and intermediate water depth organic facies B domi-nate within Exshaw black shales (Fig. 7d).

Across southern Saskatchewan, Bakken Formation blackshales are dominated almost exclusively by deep to intermediate

paleo-water depth organic facies A and B, with the only occur-rence of shallow water organic facies C restricted to the erosionalmargin of the Williston basin near the Manitoba-Saskatchewanborder, and at the Alberta-Saskatchewan border situateddirectly on the Sweet Grass Arch structural high (“NorthBattleford Arch”; Fig. 7d; Kent and Christopher, 1994). Theregional distribution of organic facies in the Bakken Formationin eastern and central Saskatchewan shows a progressivechange from shallow water organic facies C in the east, to a mixture of deep and intermediate water depth organic A, just north of the depocenter of the intracratonic Williston Basin (between Rge1W3 and Rge1W2; Fig. 7d) (Kent andChristopher, 1994; Caplan and Bustin, 1998, 2001).

Controls on Exshaw-Bakken Organic Facies in the WCSB

The regional distribution of Exshaw-Bakken organic faciesforms distinct belts in the WCSB, characterized by organicfacies that are persistent over large areas (Fig. 7d). This rela-tionship may suggest a first order basinal scale ‘tectonic’ con-trol on their formation within a vast epeiric sea east of thepresent day Rocky Mountain deformed belt. The distribution oforganic facies in the Exshaw and Bakken formations shows agood correlation between relative water depth predicted fromthe organic facies model (Fig. 3) and basinal scale physiogra-phy and paleobathymetry during late Famennian to Tournasiantime (Richards et al., 1994). Black shale organic faciesdeposited within Saskatchewan and Alberta during LateDevonian to Early Mississippian reflects variations in waterdepth related to structure, topography and tectonic influenceduring deposition.

The region of southwestern Saskatchewan (between Rge1W4and Rge1W3; Fig. 7d) was probably a structural low during deposition of the Bakken Formation black shales resulting indeep to intermediate water depth organic facies A and B; shallowwater organic facies C were deposited immediately to the weston the Saskatchewan-Alberta border in the vicinity of an activestructural high in a ‘bank’ margin type of setting (Sweet Grass-North Battleford Arch in Figure 7d; Kent and Christopher, 1994).In eastern Saskatchewan (between Rge1W3 and Rge1W2),organic facies in the Bakken reflect progressive deepening fromeast to west and are a reflection of the structural and paleobathy-metric influence of the intracratonic Williston Basin within thelarger WCSB (Kent and Christopher, 1994; Richards et al.,1994). East of the Sweet Grass-“North Battleford Arch”,siliceous microfossil assemblages and organic facies D blackshales do not occur in the Bakken Formation (Fig. 7d). This suggests that nutrient-rich paleo-Pacific Ocean waters movingout of the Prophet Trough may have been obstructed from fur-ther eastward movement by the North Battleford Arch. East of the North Battleford Arch and the stable cratonic platform of


Fig. 7. a, b. Distribution of organic facies within the Woodbend group equivalent strata of the WCSB. c. Distribution of organic facies withinWinterburn group. d. Distribution of organic facies within Exshaw and Bakken formations (Prophet Trough outline from Richards et al., 1994).Lower and Upper black shale members of the Bakken Formation have been plotted together to illustrate regional distribution of organic facies inthe Williston basin and southern Saskatchewan.


Fig. 8. Macerals and microfossils in organic facies (OF) A, B, C and D of Exshaw and Bakken formations. Fluorescence and reflectedwhite light (i–k, o, r, t), water or oil immersion; scale bar (50 µm) on photo f is for all photos. a. OF A: Large Tasmanites (T) andLeiosphaeridia (L) prasinophyte alginite within amorphinite matrix (am). b–e. OF B: Small prasinophytes alginites (p) and spiny acanthomorphic acritarchs (ac). f, g. OF A: Silicified prasinophyte alginite (p). h. OF C: Degraded coccoidal alginite (c). i– l. OF D: Siliceousmicrofossils (s), mainly derived from Radiolaria. m. OF A: Silicified prasinophyte (?). n–t. OF D: Siliceous microfossils (s), mainly derivedfrom Radiolaria.

south-eastern Alberta, a marked deepening occurs (also whereExshaw changes to Bakken Formation; Richards et al., 1994),with transition from deposition of shallow water organic faciesC to deposition of deep and intermediate paleowater depthorganic facies A and B east of the Alberta-Saskatchewan borderwithin a structurally low area in the western half ofSaskatchewan (Fig. 7d). This major change in organic faciescorresponds with an eastward thickening of black shales in theBakken Formation in this region with the depocenter above apaleo-low locally known as the Elbow sub-basin (Christopher,1961; Smith, 1996). The dominance of deep water organicfacies A in this area may imply that Middle Devonian salt dis-solution occurred sometime before Bakken Formation blackshale deposition thereby creating a deep water “sub-basinal set-ting” within a more regional platform environment.

Within Alberta, the distribution of organic facies in theExshaw Formation black shales show variations correspondingto pronounced paleogeographic features such as the ProphetTrough, the Peace River Embayment, the ‘Cratonic Platform’,and the “North Battleford” and Sweetgrass arches, all of whichwere important depositional-tectonic elements during latestDevonian to earliest Mississippian time (Richards et al., 1994;Kent and Christopher, 1994) (Fig. 8d). Throughout the westernhalf of Alberta, within, and east of the Prophet Trough includ-ing the Peace River Embayment, and up to the western marginof the Sweetgrass Arch in southeast Alberta, the association ofsiliceous microfossil enriched organic facies D and intermedi-ate water depth organic facies B are dominant (Fig. 7d;Table 3). Since radiolarian provinces commonly correspondwith zones of upwelling (e.g. Barron, 1993; Jones, 1996), per-haps nutrient-rich waters moved eastward out of the paleo-Pacific Ocean and Prophet Trough promoting siliceousplankton productivity, contributing to eutrophication and depo-sition of organic-rich, black shales in an expansive shelf orplatform setting (see also Caplan et al., 1996; Caplan andBustin, 1999). Furthermore, the assemblages of Radiolaria inorganic facies D of the Exshaw within western Alberta containmany forms with a morphology similar to the Spumellarians,which are typically characteristic of shelf waters (Casey, 1993).

Oil Character & Organic Facies

The regional distribution of organic facies in theExshaw-Bakken formation black shales at locations in WCSBmay in part control the threshold of hydrocarbon generationand potentially the organic geochemical character of expelledcrude oils. As for the Woodbend group discussed above, petro-graphic observations show that black shales with abundantsiliceous microfossils in organic facies D and some shallowwater organic facies C shales in the Exshaw Formation ofAlberta and NWT contain anomalous amounts of solid bitu-mens (e.g. infilling microfossils) which may have been gener-ated at low levels of thermal maturation (0.50 to 0.55 %Ro).Bazhenova and Arefiev (1990), Hunt et al. (1991) and Riedigerand Coniglio (1992) have also noted that biogenic siliceous orcalcareous, S-rich rocks generate heavy oils and bitumens atlow levels of thermal maturity. Further investigation is required

to fully understand and document the significance of theseobservations in the Exshaw Formation.

Crude oils sourced from the Exshaw Formation generallyshow very similar biomarker characteristics throughout theirarea of occurrence in Alberta, northeast British Columbia andwestern Saskatchewan (e.g. Allan and Creaney, 1991; Karavaset al., 1998; Riediger et al., 1999). While they share some char-acteristics in common with the Bakken-derived oils of south-east Saskatchewan, they also show some important differences.These include Exshaw oils having a lower pristane/phytaneratio, the presence of 28, 30-bisnorhopane, a C35 homohopaneprominence, a higher abundance of C24 tetracyclic terpane rel-ative to the C26 tricyclic terpanes and a higher relative abun-dance of C30 relative to C27–C29 4-desmethylsteranes (e.g.Osadetz et al., 1992; Creaney et al., 1994; Jiang et al., 2001;Manzano-Kareah, 2001). In most of the area where the ExshawFormation is mature in Alberta, it shows a predominance ofsiliceous microfossil enriched organic facies while the BakkenFormation in southeast Saskatchewan is dominated by deep tointermediate water depth organic facies A and B. Hence the dif-ferences in biomarker distributions between the Exshawsourced oils and Bakken sourced oils could be due, in part, totheir differing organic facies.

CONCLUSIONS

Five petrographic organic facies (A to E) are differentiatedusing maceral, micro-fossil and micro-texture criteria (i.e.alginites, acritarchs, sporinites, siliceous microfossils, algalmat microtextures) in Upper Devonian to Lower Mississippianpotential hydrocarbon source rocks in the Western CanadaSedimentary Basin. The distribution of organic facies corre-lates positively with paleogeography, paleobathymetry andpaleostructure. Organic facies A, B and C are defined by theamount and type of unicellular prasinophyte alginite, acantho-morphic acritarchs, coccoidal alginite, sporinite and inorganicmicro-fossils. Organic facies A, B and C represent depositionin relatively deep, intermediate, and shallow water depths,respectively. Prasinophyte-enriched organic facies A sourcerock units were deposited in the deepest basinal marine settingswhereas acanthomorphic acritarch-enriched organic facies Bwere deposited ‘landward’ in intermediate water depths in basi-nal to outer shelf marine settings. Potential source rocksdeposited in shallow water environments such as within arestricted inner shelf, around bank margins, near arches, or inlagoonal settings, typically host coccoidal alginite enrichedorganic facies C units. Organic facies E is characterized stro-matolitic microtextures with or without coccoidal alginite. Thisorganic facies only occurs within the Winterburn group andwas deposited in shallow water restricted shelf to lagoonal car-bonates, in association with evaporites. Organic facies D isdefined by the presence of siliceous microfossils and wasdeposited in deep basinal to outer shelf settings in proximity to,or in association with, organic facies A and B. The basinal dis-tribution of organic facies D in Devonian and Mississippian


shales may reflect regions of upwelling which extended intointracratonic and epicontinental settings, or, at least, areas withconnectivity to an open ocean. Kerogen type based onHydrogen-Oxygen indices are not strongly correlated with pet-rographic organic facies. This is probably a reflection of theabsolute dominance of marine kerogen and the general lack ofterrestrial organic matter indicating that Rock-Eval indices can-not always be used to define organic facies in isolation.

ACKNOWLEDGMENTS

This project was funded in part by the Devonian PetroleumSystems partners and Natural Resources Canada, GeologicalSurvey of Canada. Gary Addison mapped the organic facies.Maria Tomica of Ramtrec Research Projects and Dr. JudithPotter of JP Petrographics, Calgary, assisted in petrographic OFanalyses. Kim Dunn, GSC Calgary is thanked for providingexcellent technical assistance. Dave Sargeant drafted final mapfigures. Drs. Mark Caplan, Richard Tyson and CynthiaRiediger are thanked for their critical reviews. Dr. Riediger isthanked for her excellence in reviewing, editorial handling andcritique of this paper. This is Geological Survey of CanadaContribution # 2003132.

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Wiebe, N.S., Chow, N., Stasiuk, L.D. and Wendte, J. 2001. Sedimentology andorganic petrology of the Middle Devonian Keg River Formation, Rainbowsub-basin, northwestern Alberta: implications for source rock accumulationand reef growth. Canadian Society of Petroleum Geologists AnnualConvention, Calgary, AB., p. 121-121-8.

Manuscript Received: October 31, 2003

Date Accepted: May 10, 2004


vip

Documents

organic facies d

organic facies e

introduction organic

petrographic organic

macraux et

rsum des analyses

basis of organic matter

et peu profonds bordures