reservoir stratigraphic heterogeneity within the lower ... · shale interval (shell creek shale;...
TRANSCRIPT
Rocky Mountain Geology, v. 49, no. 2, p. 167–190, 13 figs., 3 tables, November 2014 167
Reservoir stratigraphic heterogeneity within the Lower Cretaceous Muddy Sandstone in the Powder River Basin, northeast Wyoming, U.S.A.: Implications for carbon dioxide sequestration
Majie Fan*
Department of Earth and Environmental Sciences, University of Texas at Arlington, P.O. Box 19049, Arlington, Texas
76019, U.S.A.
*Correspondence should be addressed to: [email protected]
ABSTRACT
The Muddy Sandstone in the Powder River Basin (PRB), northeast Wyoming, is a promising reservoir for CO2 sequestration because: (1) existing wells for hydrocarbon production can be used for CO2 injection when a field is depleted; and (2) data are available to assess the ability and capacity to trap CO2. Here, I provide new data and compile published results to: (1) characterize four oil and gas fields (the Amos Draw, Kitty, Hilight, and Sand Dunes fields) in the PRB with respect to lithofacies, sedimentary environment, sandstone composition, sand-body geometry, and porosity and permeability; and (2) assess the controls on reservoir heterogeneity and the CO2 sequestration potential.
Five lithofacies are recognized based on core description and log responses. They are interpreted as offshore, lower–middle wave-dominated shoreface, weathering zone, fluvial incised-valley, and tide-influenced estuarine depositional environments. The Muddy Sandstone contains predominantly quartz, with total quartz higher than 70 percent of the total framework grains. The percentage of feldspar is generally less than 5 percent, except for the Rozet Member in the Amos Draw Field, which is up to 22 percent. Sandstone petrographic examination also shows that the Muddy Sandstone can be divided into four groups based on the relative abundance of pore space, carbonate cement, and matrix. Sandstone with high porosity up to 23 percent is found in the shoreface lithofacies in the Amos Draw and Hilight fields and is also found in the estuarine lithofacies in the Kitty Field. The incised-valley lithofacies is of particularly low porosity due to high matrix content and carbonate cementation. The measured porosity in the sandstone varies between 1 percent and 23 percent, and the permeability is generally less than 10 millidarcys (mD). The variation of porosity is consistent with the observation in thin sections. XRD results show that the pore-filling clay minerals include kaolinite, chlorite, illite, and smectite. Core and well log correlation show that sandstone formed in lower–middle shoreface environments is laterally extensive and of uniform thickness, whereas sandstone of fluvial and estuarine origins is more variable in lateral extent and thickness. Based on examination of lithofacies, sandstone geometry, and thin section petrography, I suggest that the best reservoir interval for CO2 sequestration in the Amos Draw Field is the lower Rozet Member, in the Sand Dunes and Hilight fields is the Springen Ranch Member, and in the Kitty Field is the Ute Member. Variables examined in this study provide important inputs for calculating CO2 capacity potential and predicting chemical reactivity after CO2 injection.
Reservoir quality of the Muddy Sandstone is highly heterogeneous, and the complexity may be attributed to a combination of depositional environment, history of relative sea-level change during deposition, and type and extent of diagenetic alteration. The Muddy Sandstone, along with the overlying Mowry Shale, represents one third-order depositional sequence. Diagenetic processes include feldspar and lithic dissolution, secondary clay formation, quartz overgrowth, and four stages of carbonate cementation, which are early dolomite overgrowth, secondary calcite filling, dolomite replacement, and spotty siderite cementation. Carbonate cementation is interpreted as early diagenetic products formed during marine transgressions.
KEY WORDS: CO2 sequestration, Cretaceous, diagenesis, Muddy Sandstone, Powder River Basin (PRB), reservoir heterogeneity, sequence stratigraphy.
168 Rocky Mountain Geology, v. 49, no. 2, p. 167–190, 13 figs., 3 tables, November 2014
INTRODUCTION
Carbon dioxide (CO2) sequestration potentially plays a significant role in mitigating release of CO2 increase into the atmosphere, which may contribute to global warming. Some geologic formations may have the capacity to store a large amount of CO2. Depleted hydrocarbon fields are attractive CO2 sequestration reservoirs because: (1) they have been extensively investigated during hydrocarbon exploration and production; (2) they have data available for simulation modeling to assess the ability and capacity to trap CO2; and (3) they have existing wells for injection. In addition, CO2 injection can be used to enhance oil recovery if the reservoir is in depletion stages. In recent years, CO2 storage in depleted hydrocarbon fields has been extensively studied including simulation of CO2 migration, geochemical modeling, long-term storage integrity, and risk assessment (e.g., Bachu, 2000; Oldenburg et al., 2001; Li et al., 2006; Mito et al., 2008; Tarkowski and Wdowin, 2011). Generally, the feasibility of sequestration in depleted oil and gas fields depends on the storage capacity of the reservoirs.
Sequestration capacity may vary significantly across geological formations as a result of reservoir heterogeneity. Both structural and stratigraphic factors control reservoir heterogeneity. Structural factors include the abundance, character, and architecture of fractures, faults, and folds. Important stratigraphic factors of the target intervals include sand-body continuity and connectivity, and porosity and permeability. These factors are controlled by depositional environment, sequence stratigraphic architecture, and post-depositional diagenesis. These factors may vary across a range of scales, from: (1) basin-scale changes in sedimentary environment and tectonics; to (2) reservoir-scale changes in lithofacies, sand-body geometry, and stacking patterns; to (3) microscopic-scale changes in porosity/permeability and mineral composition. These reservoir properties constrain the volume available in a subsurface field for CO2 injection (capacity). Reservoir capacity for CO2 sequestration may be modeled by computer simulations to enable prediction of flow properties, chemical reactions, and potential environmental impacts after CO2 injection. For example, a heterogeneous rock
body may result in flow paths that disperse and increase storage capacity by increasing the rock–CO2 contact, whereas a homogeneous rock body may reduce storage capacity by bypassing CO2 into other formations (Hovorka et al., 2004).
In this paper, the Lower Cretaceous Muddy Sandstone in the Powder River Basin (PRB), northeast Wyoming, is used as an example to demonstrate stratigraphic reservoir heterogeneity and its influence on CO2 storage capacity. I examined four oil and gas reservoirs in the PRB to delineate the lithofacies, sedimentary environment, sand-body geometry, sandstone composition, and porosity and permeability of the potential sequestration intervals. These data were obtained from published literature as well as my analyses. The data include: (1) core lithofacies analysis and interpretation of depositional environment; (2) petrographic examination of representative sandstone thin sections; and (3) log correlation of wells. All measured porosity and permeability are compiled from publicly available data at the U.S. Geological Survey (USGS) Core Research Center and the Wyoming Oil and Gas Conservation Commission. The four studied fields are the Amos Draw, Kitty, Hilight, and Sand Dunes (Fig. 1). The fields are then compared to determine how the various controlling factors influence stratigraphic heterogeneity of the reservoirs and to explore how stratigraphic heterogeneity affects CO2 sequestration. The cores and wells I studied are represented by their API numbers.
GEOLOGIC SETTING
The PRB in northeast Wyoming is one of the Rocky Mountain intermontane basins that developed during the latest Cretaceous–early Eocene Laramide Orogeny (Dickinson et al., 1988; Fig. 1). The PRB is bounded on the west and east by the Precambrian basement-involved Bighorn Mountains and Black Hills. It is bounded on the southwest and south by the Casper Arch, Laramie Mountains, and Hartville Uplift. During the Late Cretaceous, the region was located in the Western Interior Seaway (e.g., DeCelles, 2004). The Cretaceous deposits in the PRB are at least 2 km thick (Robinson Roberts and Kirschbaum, 1995), which represents significant basin subsidence caused by flexural loading of the Sevier fold-and-thrust belt, and dynamic process
M. FAN
Rocky Mountain Geology, v. 49, no. 2, p. 167–190, 13 figs., 3 tables, November 2014 169
induced by the f lat-subduction of the Farallon Plate (DeCelles, 2004; Liu and Nummedal, 2004; Jones et al., 2011). The displacement of the Bighorn Mountains caused f lexure subsidence of the basin during the latest Cretaceous–early Cenozoic, and Cenozoic basin fill is up to 2 km thick in western parts of the PRB (Curry, 1971). Outcrops of the Muddy Sandstone are present along the flanks of the Black Hills and Bighorn Mountains (Fig. 1).
PREVIOUS STRATIGRAPHIC STUDIES
In the PRB, the Muddy Sandstone is a major oil- and gas-producing interval and has yielded more than 1.5 billion barrels of oil-equivalent hydrocarbons (Dolson et al., 1991). The Muddy Formation and equivalent strata are underlain by the Lower Cretaceous Skull Creek and Thermopolis Shales and are overlain by the Upper Cretaceous Mowry Shale (Fig. 2A). The Muddy Sandstone is considered as the lower sandy interval of the Muddy Formation, and it includes the Rozet, Recluse, Cyclone, Ute, and Springen Ranch Members (Fig. 2B; Martinsen, 1994). The Muddy Formation also contains an upper
shale interval (Shell Creek Shale; Fig. 2B). The Shell Creek Shale and overlying Mowry Shale are the sealing lithofacies of the Muddy reservoir rock in the four studied fields.
The stratigraphic framework and sedimentary environment of the Muddy Sandstone have been assessed from outcrop and core examinations as well as well-log characteristics in the PRB (Baker, 1962; Eicher, 1962; Prescott, 1970; Gustason et al., 1988; Odland et al., 1988; Weimer, 1988; Dolson et al., 1991; Martinsen, 1994). Throughout Cretaceous time, marine to marginal-marine sandstone and shale accumulated during transgression and regression of the Western Interior Seaway (Weimer, 1988). The Muddy Sandstone is one of several sandy intervals that formed in a marginal-marine environment. By studying outcrop, Eicher (1962) suggested that the Muddy Sandstone along the west margin of the PRB was deposited in a shallow-marine environment during a marine transgression. Baker (1962) studied the Newcastle Sandstone, a Muddy Sandstone equivalent, along the east margin of the PRB and interpreted it as fluvial incised-valley deposits. More recent studies, which focused on well-log correlation with l imited core descript ion, demonstrate
Figure 1. General map of Powder River Basin (PRB) and nearby area in northeastern Wyoming showing distribution of outcrop of Muddy Sandstone and four oil and gas fields characterized in this study. Black line represents Belle Fourche Arch.
Figure 2. A, Stratigraphic column and succession intervals of Lower Cretaceous strata in PRB; and B, Generalized stratigraphic cross section of Muddy Sandstone in studied fields (modified from Martinsen, 1994). Not scaled.
RESERVOIR HETEROGENEITY WITHIN THE MUDDY SANDSTONE
170 Rocky Mountain Geology, v. 49, no. 2, p. 167–190, 13 figs., 3 tables, November 2014
that the Muddy Sandstone is stratigraphically complex, containing a variety of marine and nonmarine lithofacies and many intraformational unconformities (Gustason et al., 1988; Odland et al., 1988; Dolson et al., 1991; Martinsen, 1994).
The Muddy Sandstone, as described below, has been divided into three genetic packages based on lithofacies and stratigraphic architecture (Dolson et al., 1991; Martinsen, 1994). They include: (1) an older marine interval, (2) an incised-valley interval, and (3) a younger marine interval (Dolson et al., 1991; Martinsen, 1994). Based on Martinsen (1994), the older marine interval (Rozet Member) consists of one or more coarsening-upward, shale-to-sandstone sequences with normal, open-marine trace fossils. The lower part of each sequence displays planar laminations and hummocky cross-stratification, and the upper parts show hummocky to high-angle cross-stratified or massive bedding. The Rozet Member is dominantly quartzarenites, sublitharenites, and litharenites. The incised-valley interval (Recluse Member) consists of variable proportions of shale, siltstone, and sandstone and commonly contains thin coal and coaly beds. The sandstone is massive and commonly shows evidence of soft-sediment deformation. The sandstone is quartzarenite and sublitharenite. The younger marine interval (Cyclone, Ute, and Springen Ranch Members) is quartz-rich and consists of coarsening-upward, shale-to-siltstone or shale-to-sandstone sequences showing a variety of well-preserved sedimentary structures.
STUDIED FIELDS
The Muddy Sandstone in the Amos Draw Field is divided into the Rozet (sandstone), Ute (shale), and Springen Ranch (shaly sandstone) Members (Fig. 2B; Von Drehle, 1985; Odland et al., 1988; Martinsen, 1994). The producing interval in this field is mainly the Rozet Member. There are few published studies from the Sand Dunes Field, where the producing interval is mainly the Springen Ranch Member. The Kitty and Hilight fields are located in two large incised-valley systems that developed to the north and south of the Belle Fourche Arch in the PRB (Fig. 1; Byrd Larberg, 1980; Wheeler et al., 1988; Dolson et al., 1991; Martinsen, 1994). The two valleys cut into the Skull Creek Shale locally, and thus the lower Muddy Sandstone is completely eroded in many
places in the two fields. The Muddy Sandstone presented in the Kitty and Hilight fields is divided into the Recluse, Cyclone, Ute, and Springen Ranch Members, and the Rozet Member exists only at edges of the valleys. The main producing intervals are the Ute Member in the Kitty Field and the Springen Ranch Member in the Hilight Field.
LITHOFACIES AND SEDIMENTARY ENVIRONMENTS
Core Description and Interpretation
I examined 15 cores from the Amos Draw, Kitty, Hilight, and Sand Dunes fields to characterize the lithofacies, constrain the depositional environments, and guide field-scale well-log correlations. The core intervals were subdivided into f ive major lithofacies, interpreted as offshore, lower–middle wave-dominated shoreface, weathering zone, fluvial incised-valley, and tidal-influenced estuarine depositional environments (Table 1 and Figs. 3–5).
Gamma-ray Log Responses and Lithofacies
Gamma-ray logs are presented to ensure consistent and accurate representation and interpretation of lithofacies observed in cores, though resistivity and spontaneous potential logs are studied in many wells (Fig. 6). In the Amos Draw and Sand Dunes fields, lithofacies F has high gamma-ray values and irregular log response, suggesting that the lithofacies is clay rich and thinly bedded. Lithofacies S1 and S2 show funnel trend representing upward coarsening. Lithofacies S2, however, has low resistivity compared to S1. This is because lithofacies S2 has high kaolinite, and kaolinite does not contain potassium to have high gamma-ray response (Odland et al., 1988). Lithofacies S4 displays irregular log response representing interbedded siltstone and mudstone. In the Kitty and Hilight fields, lithofacies S3 is dominated by blocky log response representing well-amalgamated sandstone. Locally the lithofacies is serrated, representing interbedded sandstone and siltstone. Lithofacies S4 displays bell- and serrated-shaped responses in the lower half, and blocky and serrated-shaped responses in the upper half. The bell and serrated lower half represents upward fining, and the blocky and serrated upper half represents
M. FAN
Rocky Mountain Geology, v. 49, no. 2, p. 167–190, 13 figs., 3 tables, November 2014 171
Lith
ofac
ies
code
Li
thof
acie
s na
me
Out
crop
and
cor
e de
scrip
tion
Inte
rpre
tatio
nD
epos
ition
al
envi
ronm
ent
FBi
otur
bate
d sil
tston
e an
d m
udsto
ne
Dar
k gr
ay, t
hinl
y be
dded
, or m
assiv
e sil
tston
e an
d m
udsto
ne.
Silts
tone
bed
s sho
w p
lana
r- a
nd ri
pple
-lam
inat
ion
and
good
so
rtin
g, a
nd th
ey c
onta
in fo
ram
inife
ra, b
ival
ve fr
agm
ents,
and
py
rites
. Thi
n, v
ery
fine-
grai
ned
sand
stone
with
shar
p ba
ses a
nd
tops
occ
asio
nally
occ
ur. B
iotu
rbat
ed b
y C
hond
rites,
Pal
eoph
ycus
, Pl
anol
ites,
and
Tere
belli
na.
Biot
urba
ted,
mas
sive
siltst
one
and
mud
stone
are
offs
hore
dep
osits
fo
rmed
bel
ow st
orm
-wav
e ba
se (E
lliot
t, 19
86).
Thi
nly
bedd
ed
siltst
one
and
fine-
grai
ned
sand
stone
wer
e de
posit
ed w
here
oc
casio
nal s
torm
s affe
cted
dep
ositi
on b
etw
een
fair-
wea
ther
and
sto
rm-w
ave
base
s (El
liott,
198
6; W
alke
r and
Plin
t, 19
92).
Offs
hore
S1C
oars
enin
g-up
war
d sa
ndsto
ne
Ligh
t bro
wn
and
gray
, pla
nar-
lam
inat
ed, i
nter
laye
red
very
fine
-gr
aine
d sa
ndsto
ne a
nd si
ltsto
ne c
oars
en u
pwar
d to
trou
gh a
nd
hum
moc
ky c
ross
-lam
inat
ed o
r pla
nar-
lam
inat
ed, m
ediu
m-
grai
ned
sand
stone
. Silt
stone
bed
s are
bio
turb
ated
by
Skol
ithos
, Te
ichich
nus,
Cho
ndrit
es, a
nd P
aleo
phyc
us. M
ediu
m-g
rain
ed
sand
stone
is p
oorly
to w
ell s
orte
d an
d is
mas
sive
in K
itty
and
Hili
ght f
ield
s.
Inte
rlaye
red
sand
stone
and
silts
tone
wer
e de
posit
ed a
bove
stor
m-
wav
e ba
se (W
alke
r and
Plin
t, 19
92; K
amol
a an
d Va
n W
agon
er,
1995
). H
umm
ocky
and
trou
gh c
ross
-str
atifi
ed sa
ndsto
ne w
ere
depo
sited
abo
ve fa
ir-w
eath
er w
ave
base
, and
dep
ositi
on w
as
dom
inat
ed b
y sh
oalin
g of
fair-
wea
ther
wav
es (E
lliot
t, 19
86; W
alke
r an
d Pl
int,
1992
).
Low
er–m
iddl
e w
ave-
dom
inat
ed
shor
efac
e
S2M
assiv
e sa
ndsto
ne
Cre
amy
whi
te m
ediu
m-g
rain
ed sa
ndsto
ne, ~
1.5
m th
ick,
occ
urs
only
in th
e up
per R
ozet
Mem
ber i
n th
e Am
os D
raw
Fie
ld.
Con
tain
s hig
h ka
olin
ite c
onte
nt a
nd a
bund
ant r
oot t
race
s (O
dlan
d et
al.,
198
8).
Hig
h ka
olin
ite c
onte
nt w
as d
ue to
wea
ther
ing
of fe
ldsp
ar g
rain
s (O
dlan
d et
al.,
1988
). T
he su
baer
ial e
rosio
n of
the
top
of th
e Ro
zet
Sand
stone
repr
esen
ts lo
ng-te
rm st
abili
zatio
n of
floo
dpla
in (M
iall,
19
88).
Wea
ther
ing
zone
S3Fi
ning
-up
war
d sa
ndsto
ne
Gra
y-ye
llow,
mas
sive,
coa
rse-
grai
ned
sand
stone
fine
s upw
ard
to
plan
ar-la
min
ated
, fin
e-gr
aine
d sa
ndsto
ne o
r to
mas
sive,
m
ediu
m-g
rain
ed sa
ndsto
ne. S
ands
tone
is p
oorly
to m
oder
atel
y so
rted
and
con
tain
s gra
nule
- to
pebb
le-s
ized
intr
acla
sts a
t the
ba
se. O
verli
es li
thof
acie
s S1
and
F w
ith e
rosio
nal b
ase
and
only
oc
curs
in K
itty
and
Hili
ght f
ield
s. C
onta
ins a
bund
ant w
ood
debr
is an
d th
in c
oal b
eds.
Soft-
sedi
men
t def
orm
atio
n is
com
mon
.
Fluv
ial i
ncise
d-va
lley
depo
sits (
Van
Wag
oner
et a
l., 1
990)
that
tr
unca
te o
lder
shor
efac
e de
posit
s and
loca
lly in
cise
offs
hore
de
posit
s und
erly
ing
the
shor
efac
e de
posit
s. T
he o
ccur
renc
es o
f the
lit
hofa
cies
in th
e K
itty
and
Hig
hlig
ht fi
elds
sugg
est t
hat t
here
may
be
at l
east
two
inci
sed-
valle
y sy
stem
s in
the
easte
rn p
ortio
n of
the
Pow
der R
iver
Bas
in. T
he m
axim
um th
ickn
ess o
f the
inci
sed-
valle
y de
posit
s ind
icat
es th
at th
e m
axim
um in
cisio
n re
lief i
s ~8
m.
Fluv
ial i
ncise
d-va
lley
S4
Inte
rbed
ded
sand
stone
, sil
tston
e,
mud
stone
, an
d co
al
Thi
nly
inte
rbed
ded
gray
-yel
low,
ver
y fin
e-to
med
ium
-gra
ined
, pl
anar
-and
ripp
le-la
min
ated
sand
stone
, silt
stone
, dar
k gr
ay
mud
stone
, and
coa
l ofte
n sta
ck a
s fin
ing-
upw
ard
sequ
ence
s w
ith e
rosio
nal b
ases
, and
to a
less
ext
ent a
s coa
rsen
ing-
upw
ard
sequ
ence
s. H
umm
ocky
cro
ss-la
min
atio
n on
ly sh
ows i
n th
e co
arse
ning
-upw
ard
sequ
ence
s. Sa
ndsto
ne is
poo
rly–w
ell s
orte
d an
d co
ntai
ns m
ud c
lasts
at t
he b
ase.
Cla
y dr
apes
and
sand
stone
-m
udsto
ne c
oupl
ets a
re c
omm
on. C
onta
ins a
bund
ant w
ood
debr
is an
d so
me
bent
onite
bed
s. G
lossi
fung
ites b
urro
ws a
re
com
mon
.
Fini
ng-u
pwar
d se
quen
ces w
ith e
rosio
nal b
ases
wer
e de
posit
ed a
s ba
yhea
d de
lta o
r dist
ribut
ary
chan
nels
deve
lope
d in
a w
ave-
dom
inat
ed e
stuar
y, an
d th
e m
udsto
ne a
nd c
oal w
ere
depo
sited
as
estu
arin
e ce
ntra
l-bas
in d
epos
its (D
alry
mpl
e et
al.,
199
2). C
lay
drap
es su
gges
t tha
t the
bay
head
del
ta w
as in
fluen
ced
by ti
des
(Fen
ies e
t al.,
199
9). C
oars
enin
g-up
war
d se
quen
ces a
re m
arin
e sh
oref
ace
depo
sits (
Wal
ker a
nd P
lint,
1992
), su
gges
ting
the
conn
ectio
n of
estu
ary
to o
pen
mar
ine
envi
ronm
ent i
n th
e es
tuar
y m
outh
, whe
re w
ave
ener
gy is
hig
h du
ring
storm
s (Ro
y et
al.,
198
0). Ti
de-in
fluen
ced
estu
arin
e
Tabl
e 1.
Lith
ofac
ies a
nd in
terp
reta
tions
use
d in
this
study
.Ta
ble
1. L
itho
faci
es a
nd in
terp
reta
tion
s use
d in
this
stud
y.RESERVOIR HETEROGENEITY WITHIN THE MUDDY SANDSTONE
172 Rocky Mountain Geology, v. 49, no. 2, p. 167–190, 13 figs., 3 tables, November 2014
variation of lithology from well-amalgamated sandstones to interbedded sandstone, siltstone, mudstone, and coal.
Correlation and Distribution of Lithofacies
In both the Amos Draw and Sand Dunes fields, the studied intervals are characterized by two
coarsening-upward sandstone bodies of lithofacies S1 sandwiching interbedded siltstone and mudstone of lithofacies S4 (Figs. 3–6). The lower coarsening-upward sandstone is the Rozet Member, deposited in shoreface environments, consistent with the interpretation by Odland et al. (1988), Dolson et al. (1991), and Martinsen (1994). The upper Rozet Member changes to lithofacies S2, which
Figure 3. Core descriptions and correlations in Amos Draw and Sand Dunes fields. Cores are denoted by well API numbers. Datum in Amos Draw Field is top of Rozet Member. Note: this study suggests best interval for CO2 sequestration is lower Rozet Member in Amos Draw Field and Springen Ranch Member in Sand Dunes Field.
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Rocky Mountain Geology, v. 49, no. 2, p. 167–190, 13 figs., 3 tables, November 2014 173
Figure 4. Core descriptions and correlations in Hilight Field. Datum is top of Cyclone Member. Note: this study suggests best interval for CO2 sequestration is Springen Ranch Member.
RESERVOIR HETEROGENEITY WITHIN THE MUDDY SANDSTONE
174 Rocky Mountain Geology, v. 49, no. 2, p. 167–190, 13 figs., 3 tables, November 2014
Figure 5. Core descriptions and correlations in the Kitty Field. Datum is top of Cyclone Member. Note: this study suggests best interval for CO2 sequestration is Ute Member.
Figure 6, facing page. Well log correlation along N–S cross sections in studied field. A, Amos Draw Field; B, Sand Dunes Field; C, Hilight Field; and D, Kitty Field. Black log lines are gamma-ray logs with sandstone in gray. Sections are marked as black lines in Figure 12. Sandstone beds of Rozet Member in Amos Draw and Sand Dunes fields and Springen Ranch Member in all fields are laterally consistent and display upward coarsening. Sandstone beds of Recluse, Cyclone, and Ute Members have complex stacking patterns and geometry.
M. FAN
Rocky Mountain Geology, v. 49, no. 2, p. 167–190, 13 figs., 3 tables, November 2014 175
4900528511 4900527938 4900527490 4900527251 4900527539 4900526958 4900526970 4900526955GRMD GR
1800MD GR
1800MD GR
1800MD GR
1800MD GR
1800MD
1800MD GR
1800MD GR
18009900
9960
9940
9920
9980
9860
9920
9900
9880
9940
9820
9880
9860
9840
9900
9920
9980
9960
9940
10000
9830
9890
9870
9850
9910
10000
10060
10040
10020
10080
10100
10160
10140
10120
10180
10120
10180
10160
10140
10200
4900922716 4900922633 4900922700 49009227554900922615 4900922764MD GR
1800MD GR
1800MD GR
1800MD GR
1800MD GR
1800MD
1800GR
12570
12630
12610
12590
12650
12620
12680
12660
12640
12700
12570
12630
12610
12590
12650
12890
12950
12930
12910
12970
12620
12680
12660
12640
12700
12690
12750
12730
12710
12770
4900522168MD GR
1800MD GR
1800MD GR
1800MD GR
1800MD GR
1800MD
1800MD GR
1800MD GR
1800
4900522028 49005218484900521473 4900521467 49005220824900521659 4900521741 490052271349005212105MD GR
1800MD GR
1800
8720
8780
8760
8740
8800
8850
8910
8890
8930
8820
9340
9400
9380
9360
9420
8870
9480
9540
9520
9500
9560
9080
9140
9120
9100
9160
9280 9580
9730
9790
9770
9750
9810
9730
9810
9790
9750
9830
9820
9880
9860
9840
9900
9650
9710
9690
9670
9730
9760
9820
9800
9780
9840
GR
4900521785MD GR
1800
49005205634900520664490052260749005210764900521860 4900520705 49005202084900520519 4900520719MD GR
1800MD GR
1800MD GR
1800MD GR
1800MD GR
1800MD
1800MD GR
1800MD GR
1800GR MD
1800GR
7860
7920
7900
7880
7940
8770
8830
8810
8790
8850
8510
8560
8540
8520
8580
8420
8480
8460
8440
8500
8190
8250
8230
8210
8270
8820
8820
8880
8860
8840
8900
8920
8950
9110
8990
8970
9130
8590
8650
8630
8610
8670
9170
9230
9210
9190
9250
8870
8930
8910
8890
8950
897092708690
Springen Ranch
Springen Ranch
Springen Ranch
Springen Ranch
Ute
Ute
Ute
Ute
Rozet
Cyclone
Cyclone
Recluse
Recluse
Rozet
A
B
C
D
RESERVOIR HETEROGENEITY WITHIN THE MUDDY SANDSTONE
176 Rocky Mountain Geology, v. 49, no. 2, p. 167–190, 13 figs., 3 tables, November 2014
is a weathering zone formed during a regional erosion event (Odland et al., 1988; Martinsen, 1994). The sandwiched interval of lithofacies S4 is the Ute Member, which represents brackish water or estuarine central-basin deposits. The upper coarsening-upward sandstone is the Springen Ranch Member, which is open-marine, shoreface deposits. In the Sand Dunes Field, the Ute Member contains more sandstone beds. The laterally most extensive and sandy lithofacies are the lower Rozet Member in the Amos Draw Field and the Springen Ranch Member in the Sand Dunes Field.
In the Kitty and Hilight fields, the studied intervals are in the order of lithofacies F changing to S3 with sharp contact, then to S4, and finally to S1 (Fig. 6). In some of the studied wells, a lower coarsening-upward sequence of lithofacies S1 overlies lithofacies F. Lithofacies S1 is the Rozet Member, which is completely eroded in the other wells. Lithofacies S3 is a blocky sandstone body, which is the Recluse Member of incised-valley deposits. Lithofacies S4 is mainly characterized by a fining-upward sequence overlain by a sandstone-dominated upper interval. In some wells, the upper interval consists of interbedded sandstone and shale. One layer of bentonite occurs in lithofacies S4 and is easily recognized as an interval of high gamma-ray and low resistivity. I correlate the bentonite with the top of the fining-upward sequence, defining the surface as the top of the Cyclone Member, and defining the upper lithofacies S4 as the Ute Member. The thickness of lithofacies S3 and S4 laterally varies. The coarsening-upward S1 at the top of the studied intervals is the Springen Ranch Member. Compared to the Kitty Field, the Recluse, Cyclone, and Ute Members are shale prone in the Hilight Field, and the Springen Ranch Member is sand prone and thick in the Hilight Field.
SANDSTONE PETROGRAPHY AND CLAY MINERALOGY
Thin sections from nine cores in the Amos Draw, Kitty, and Hilight fields were examined. These sandstone samples were selected from all the members in the Kitty Field to aid in recognizing relationships between sandstone composition and lithofacies. Samples from the Amos Draw and Hilight fields also were selected to characterize sandstone
composition. Forty-five fine- to medium-grained sandstone samples were point-counted following the modified Gazzi-Dickinson method (Ingersoll et al., 1984). Sandstone composition was determined by identification of 400 grains per slide. Matrix, cement, and pore space features also were counted. Sandstone petrography results are summarized in Table 2 and Figure 7. Matrix of four samples from the Hilight Field was examined for clay-mineral composition. Matrix (<63 mm) was separated by dry sieving and prepared for XRD analysis by hand grinding in a mortar and pestle. Oriented smears on glass slides were x-rayed with a Scintag XDS 2000 automated powder diffraction system equipped with a solid state X-ray detector at 2° per minute scanning speed. Relative abundance of clay minerals was determined based on peak height. These results (Fig. 8) combined with other clay-mineral compositional data made public by the USGS Core Research Center are presented in Table 3.
XRD results from this study show that clay matrix from the Muddy Sandstone in the Hilight Field is mainly kaolinite and chlorite (Fig. 8). Results compiled from the public data show that the clay matrix in the Kitty and Hilight fields is dominated by kaolinite and illite, with lesser amounts of smectite and chlorite (Table 3). Petrographic study show that total quartz grains, including monocrystalline and polycrystalline quartz, make up more than 70 percent of the total framework grains while feldspar and total lithic grains comprise at most 30 percent (Table 2, Fig. 7). Lithic fragments consist primarily of shale grains and to a lesser extent carbonate, biotite, glauconite, and wood fragments. Feldspar grains include potassium feldspar and plagioclase. Petrographic results show that the sandstone samples from the Ute, Recluse, Cyclone, and Springen Ranch Members in the Kitty and Hilight fields contain abundant monocrystalline quartz (>75 percent), but a lesser amount of polycrystalline quartz (Table 2). The abundance of feldspar and lithic fragments are generally low, with the sum less than 10 percent (Table 2). Samples from the Rozet Member in the Amos Draw Field, however, have feldspar abundance up to 22 percent. They also have slightly higher abundance of polycrystalline quartz and lithic fragments compared to samples from the Kitty and Hilight fields (Fig. 7).
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Rocky Mountain Geology, v. 49, no. 2, p. 167–190, 13 figs., 3 tables, November 2014 177
Sample† Member Qm(%)# Qp(%) C(%) K(%) P(%) Lsh(%) Oil(%) O(%)* Grain (%) Cement (%) Matrix (%) Pore (%) Group
Amos Draw Field
524520-9862 Rozet 54.0 12.5 8.4 0.2 1.1 17.3 0.0 6.4 56.1 0.8 41.9 1.2 4
524520-9868 Rozet 82.3 3.0 2.5 2.5 3.0 5.5 0.3 0.8 77.3 0.4 20.6 1.7 4
524520-9871 Rozet 74.2 12.0 0.0 1.3 8.3 2.5 0.0 1.8 81.3 1.2 15.9 1.6 4
524520-9873 Rozet 64.2 7.5 0.3 1.3 17.0 8.8 0.0 1.0 72.5 2.4 24.5 0.5 4
524520-9877 Rozet 74.6 3.5 0.0 4.5 12.4 4.2 0.0 0.7 86.6 3.4 6.7 3.2 4
524520-9880 Rozet 60.2 12.5 3.7 8.6 13.0 0.0 0.0 2.0 77.1 7.0 9.1 6.8 4
524520-9882 Rozet 65.8 5.0 0.0 2.3 18.0 8.5 0.0 0.5 73.1 3.8 22.3 0.7 4
Kitty Field
520750-8941 Ute 91.8 1.0 1.0 0.2 0.0 0.0 0.5 5.5 97.3 0.0 1.9 0.7 2
520750-8950.25 Recluse 75.4 10.1 14.1 0.4 0.0 0.0 0.0 0.0 70.3 20.3 8.8 0.6 3
520750-8953 Recluse 87.3 5.5 7.2 0.0 0.0 0.0 0.0 0.0 58.5 0.3 40.8 0.4 4
520750-8957 Recluse 72.0 5.0 16.1 0.2 0.0 6.0 0.0 0.7 71.2 21.6 7.2 0.0 3
520750-8958 Recluse 78.8 7.8 6.6 2.2 1.9 0.0 2.7 0.0 42.2 2.2 54.6 1.1 4
520750-8951.5 Recluse 89.4 6.9 1.4 0.0 0.0 0.0 0.0 2.3 86.3 0.0 13.7 0.0 4
521076-8438 Cyclone 90.8 7.2 0.0 0.0 0.0 1.0 1.0 0.0 62.0 38.0 0.0 0.0 3
521076-8439.5 Cyclone 83.2 5.9 2.4 0.0 0.0 8.6 0.0 0.0 100.0 0.0 0.0 0.0 2
521076-8448.4 Recluse 88.2 7.4 1.2 0.2 1.0 2.0 0.0 0.0 98.1 0.0 1.9 0.0 2
521076-8465.5 Recluse 83.8 1.9 5.9 0.0 3.0 5.4 0.0 0.0 82.3 0.0 17.7 0.0 4
521076-8467.6 Recluse 90.4 0.0 8.4 0.0 0.0 1.2 0.0 0.0 64.1 29.3 6.6 0.0 3
520429-9426 Ute 93.5 2.9 0.0 1.2 0.0 1.0 0.0 1.4 87.4 0.0 0.0 12.6 1
520429-9448 Recluse 87.1 2.0 10.6 0.0 0.0 0.0 0.3 0.0 66.4 0.0 31.9 1.7 4
520429-9451 Recluse 94.6 1.3 2.2 0.0 0.0 0.8 1.1 0.0 43.3 2.3 52.5 1.9 4
520429-9454 Recluse 98.8 0.0 0.0 0.0 0.3 0.0 0.9 0.0 56.3 0.0 43.7 0.0 4
520429-9456 Recluse 89.0 6.3 3.5 0.0 0.3 0.8 0.0 0.3 78.4 3.9 16.5 1.2 4
520429-9457 Recluse 87.3 4.5 6.2 0.0 0.3 0.3 0.0 1.4 71.7 0.7 27.0 0.5 4
520429-9465.1 Rozet 96.7 0.6 1.5 0.0 0.9 0.0 0.0 0.3 75.1 0.0 24.9 0.0 4
520664-8839 Ute 83.8 1.8 6.4 0.5 0.7 4.6 0.0 2.3 95.0 0.0 0.9 4.1 1
520664-8847 Ute 80.7 7.0 4.6 0.0 0.9 4.4 2.4 0.0 96.0 0.0 4.0 0.0 2
520664-8852 Ute 95.1 1.5 1.9 0.0 0.5 0.2 0.7 0.0 98.8 0.2 0.7 0.2 2
520664-8870 Recluse 93.9 2.3 0.3 0.0 1.2 0.3 0.0 2.0 99.7 0.0 0.3 0.0 2
520664-8883 Recluse 98.8 0.0 0.3 0.5 0.0 0.3 0.3 0.0 98.0 0.0 2.0 0.0 2
520664-8884 Recluse 95.3 2.0 0.8 0.0 0.0 0.3 0.0 1.8 93.7 0.0 0.2 6.1 1
521107-9194.5 Ute 94.6 0.2 0.0 0.5 0.2 0.7 2.7 1.0 89.7 0.0 0.0 10.3 1
521107-9196.5 Ute 96.2 0.5 0.0 0.0 0.0 0.0 0.0 3.3 85.1 0.0 1.5 13.4 1
521107-9208 Ute 95.2 4.8 0.0 0.0 0.0 0.0 0.0 0.0 86.7 0.0 0.7 12.6 1
521107-9212.2 Ute 98.0 0.0 0.2 0.0 0.0 0.0 0.7 1.0 97.1 0.0 0.0 2.9 2
521107-9233 Cyclone 85.1 1.3 2.6 0.5 0.8 5.7 3.4 0.8 68.9 5.3 13.9 11.9 3
Hilight Field
522168-8716.5 Springen Ranch 84.2 7.1 3.0 1.6 0.7 0.7 2.7 0.0 67.8 0.0 26.7 5.6 4
522168-8717 Springen Ranch 90.3 6.4 2.2 0.0 0.0 0.0 1.0 0.0 63.6 0.0 36.4 0.0 4
521742-9677.5 Cyclone 84.1 0.0 4.4 0.8 0.0 0.0 4.4 6.3 81.3 0.0 18.7 0.0 4
521474-9333 Springen Ranch 95.3 1.7 0.2 0.5 1.2 1.0 0.0 0.0 89.5 0.9 3.1 6.5 1
521474-9339 Springen Ranch 95.7 1.7 0.9 0.0 0.0 0.9 0.9 0.0 75.5 0.7 0.7 23.2 1
521474-9345 Springen Ranch 98.8 0.2 0.0 0.5 0.0 0.0 0.5 0.0 83.6 0.0 3.1 13.3 1
521474-9355 Springen Ranch 97.4 1.3 1.3 0.0 0.0 0.0 0.0 0.0 79.3 1.3 2.6 16.8 1
521474-9362 Ute 96.8 2.0 0.7 0.0 0.0 0.0 0.5 0.0 84.8 0.0 14.6 0.6 4
521474-9398 Rozet 95.7 1.7 1.7 0.0 0.2 0.5 0.0 0.2 71.2 0.0 19.6 9.2 4
Table 2. Modal petrographic data.
#Qm: monocrystalline quartz; Qp: polycrystalline quartz; C: chert; K: potassium feldspar; P: plagioclase; Lsh: shale lithic fragments; O: other.
*Other includes biotite, glauconite, zircon, wood fragment.
†Numbers beside last six digits of core APL represent stratigraphic depths of the samples below KB in feet.
Table 2. Modal petrographic data.
RESERVOIR HETEROGENEITY WITHIN THE MUDDY SANDSTONE
178 Rocky Mountain Geology, v. 49, no. 2, p. 167–190, 13 figs., 3 tables, November 2014
Sandstone petrography results from this study show that matrix content is generally less than 50 percent while the percentages of pore space and cement are low for most samples (Table 2). Sandstone in the Muddy Sandstone can be classified into four types based on percentage of pore space, carbonate cement, and matrix. Group 1 sandstone is tightly compacted and possesses pore space higher than 10 percent of the volume. This group contains more than 95 percent of monocrystalline quartz, but very small amounts of carbonate cement and clay matrix; sometimes, quartz overgrowth is observed. Microscopic inspection shows that some of the pore spaces were generated by dissolution of feldspars (Fig. 9A). Group 1 is found in the Ute Member in the Kitty Field and Springen Ranch Member in the Hilight Field (Table 2). Group 2 sandstone displays minimum pore space and common quartz overgrowths. Quartz overgrowths are recognized by the presence of a dusty rim that
separates overgrowth from host quartz grains or by a clay coating on the original quartz grain (Fig. 9B). Group 2 has more than 90 percent quartz, and some samples have up to 9 percent of shale and carbonate lithic fragments. This group is common in both the Ute and Recluse Members in the Kitty Field (Table 2). Group 3 sandstone displays minimal pore space, but contains 5–40 percent by volume of carbonate cement, including calcite, dolomite, and siderite. Calcite and dolomite cements are interpreted to have formed during three stages. An early stage of dolomite overgrowth on quartz grains is overprinted by a second stage of calcite filling the pore space between grains. A third stage of dolomite replaces calcite cement (Fig. 9C ). Siderite cement occurs within the calcite cement and displays radial fibrous texture with light brown color under polarized light (Fig. 9D). This group is found in both the Cyclone and Recluse Members in the Kitty Field (Table 2). Group 4 sandstone contains 6–55 percent
Table 3. Types of clays in representative core samples.
Core Percent of clay Dominant clay minerals Other Source
9200520750 (8920–8973
feet†, 13 analyses) 6-52 kaolinite illite, chlorite, smectite USGS9200521076 (8438–8489 feet, 5 analyses) 7-63 illite chlorite, smectite, kaolinite USGS9200520918 (8786–8836 feet, 9 analyses) 7-65 kaolinite illite, chlorite, smectite USGS9200520664 (8853–8885 feet, 6 analyses) 4-37 illite, kaolinite chlorite, smectite USGS9200520107 (9176–9235 feet, 10 analyses) 4-38 illite, kaolinite chlorite, smectite USGS
9200521742 (9652–9688 feet, 5 analyses) 23-59 illite chlorite, smectite, kaolinite USGS9200524953 (9374–9403 feet, 13 analyses) 5-60 illite, kaolinite chlorite, smectite USGS9200521742 (9651 feet, 1 analysis) Small amount chlorite, kaolinite smectite This study9200521742 (9658 feet, 1 analysis) Small amount chlorite, kaolinite smectite This study9200522310 (8722 feet, 1 analysis) Small amount chlorite, kaolinite smectite This study9200522310 (8698 feet, 1 analysis) Small amount chlorite, kaolinite smectite This study
Table 3. Types of clays in representative core samples.
Kitty Field
Hilight Field
†The stratigraphic depths of the samples are measured below KB.
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Rocky Mountain Geology, v. 49, no. 2, p. 167–190, 13 figs., 3 tables, November 2014 179
by volume of clay matrix and up to 10 percent pore space. This group is distributed in the Rozet Member in the Amos Draw Field, Recluse Member in the Kitty Field, and Springen Ranch Member in the Hilight Field (Table 2). Dissolution of feldspars and lithics are commonly observed in Group 4. In the Amos Draw Field, such sandstone also contains up to 27 percent of lithic fragments, consistent with observations by Odland et al. (1988) and Martinsen (1994).
Four types of post-depositional alteration are summarized from petrographic examination of the sandstone samples. They include: (1) mechanical compaction, which is common in all members; (2) dissolution of grains, particularly feldspars and lithic fragments (found only in Group 1 and 4 sandstone); (3) formation, dissolution, and recrystallization of pore-filling cements, including calcite, dolomite, and smectite, and development of quartz overgrowth (carbonate cements of different stages are common in Group 3 sandstone; quartz overgrowth is found in all sandstone, but is common in Group 1 sandstone); and (4) formation of grain-replacing and pore-filling kaolinite (this is
common only in Group 4 sandstone in the Rozet Member in the Amos Draw Field).
POROSITY AND PERMEABILITY
Petrographic study shows that Group 1 sandstone has high porosity (6–23 percent), Group 4 sandstone has porosity varying between 0 and 9 percent, and Group 2 and 3 have porosity generally less than 1 percent (Table 2). The measured porosity of the lower Rozet Member in the Amos Draw Field ranges from 5 to 13 percent, whereas the upper Rozet Member is <5 percent, which results from the high content of kaolinite formed during subaerial weathering of the strata (Martinsen, 1994). This agrees generally with my point-count data, which shows that sandstone of the upper Rozet Member in well 4900524520 has lower porosity than the lower Rozet Member (Table 2). The horizontal permeability of the lower Rozet Member ranges from 0.01 to 0.3 mD, and the upper Rozet is less than 0.01 mD. The porosity of five cores in the Sand Dunes Field varies between 1 percent and 23 percent, with the highest of 5–23 percent in the Springen Ranch Member (Fig. 10). The horizontal
Figure 7. Ternary QtFL diagram showing compositional variation of framework grains in Muddy Sandstone. See Table 2 for data. Qt includes monocrystalline and polycrystalline quartz. F includes potassium feldspar and plagioclase. L includes shale fragments, chert, and other grains.
Figure 8. X-ray diffractogram of four representative samples from Hilight Field. Gray is air-dried; black is glycolated. Minerals include: c-chlorite, ca-calcite, k-kaolinite, q-quartz, s-smectite.
RESERVOIR HETEROGENEITY WITHIN THE MUDDY SANDSTONE
180 Rocky Mountain Geology, v. 49, no. 2, p. 167–190, 13 figs., 3 tables, November 2014
permeability is up to 150 mD, and the highest values are in upper parts of the Springen Ranch Member. The porosity of five cores in the Hilight Field varies between 1 percent and 23 percent, with the highest in the Springen Ranch Member (Fig. 10). This is consistent with my observation that the porosity of studied sandstone samples of the Springen Ranch Member in the Hilight Field is up to 23 percent. The horizontal permeability of Springen Ranch Member is up to 15 mD (Fig. 10). There are no measured porosity and permeability data available for cores from the Kitty Field. My thin section study of the Kitty Field shows that many sandstone intervals in the Recluse and Cyclone Members are extensively cemented and
contain high amounts of clay matrix, whereas the Ute Member has high porosity up to ~13 percent (Table 2).
ISOPACH PATTERNS
Introduction
Isopach maps of the Muddy Sandstone and structural maps of the top of the Muddy Sandstone were constructed based on lithofacies and log responses. Twelve wells in the Sand Dunes Field, 173 wells in the Amos Draw Field, 12 wells in the Kitty Field, and 60 wells in the Hilight Field were correlated (Figs. 6, 11, and 12).
A
D C
B
100 µm
100 µm 100 µm
100 µm
F Q
Q
Q
Q
Q
R
R R
L
K
F
S
Figure 9. Photographs of representative thin sections of Muddy Sandstone in the Kitty Field. A, Secondary pore space (shown in blue) generated by dissolution of feldspars (F), well 4900520107. Thin section from 9,208 ft; B, No pore space due to quartz overgrowth, well 4900521076. Thin section from 8,439.5 ft; C, Dolomite overgrowth on quartz grains followed by calcite (stained to red) filling pore space, and subsequent dolomite veins invading calcite, well 4900520750. Thin section from 8,950 ft; and D, Secondary (post-depositional) light-brown siderite cement showing radial fibrous texture in sandstone of high clay matrix, well 4900520429. Thin section from 9,456 ft. All are in cross-polarized light. Q = monocrystalline quartz, F = feldspar, R = quartz overgrowth with a clay rim, K = kaolinite matrix, L = lithics, and S = siderite.
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Rocky Mountain Geology, v. 49, no. 2, p. 167–190, 13 figs., 3 tables, November 2014 181
Amos Draw Field
In the Amos Draw Field, the thickness of the Rozet Member is 15–35 ft (5–12 m) and increases to the west (Figs. 6A and 11A). Although the Springen Ranch Member is also regionally continuous, it is 5–10 ft (2–3 m) thick, significantly thinner than the Rozet Member (Fig. 6A). The measured depth of the top of the Muddy Sandstone varies between 9,500 ft (2,896 m) and 10,500 ft (3,200 m) below the kelly bushing (KB; Fig. 12A), and KB is at 4,200–4,700 ft (1,280–1,433 m) above sea level. The thickness of the Rozet Member is high in the east side of the field and low in the west (Fig. 11A).
Sand Dunes Field
In the Sand Dunes Field, the Rozet Member is shale prone in some wells, whereas the Springen Ranch Member is sandier and more laterally persistent (Figs. 6B and 11B). The Springen Ranch sandstone is 12–32 ft (4–10 m) thick across the field and thickens toward the south and southeast of the field (Fig. 11B). The measured depth of the top of the Muddy Sandstone varies between 12,400 ft (3,780 m) and 13,100 ft (3,993 m) below KB (Fig. 12B), and KB is at 5,600–6,000 ft (1,707–1,829 m) above sea level. The top of the Muddy Sandstone in the Sand Dunes Field is the lowest of the four studied fields; it is ~1500 ft (~457 m) lower compared to the top in the Amos Draw Field.
Hilight Field
The Recluse, Cyclone, and Ute Members are not laterally continuous in the Hilight Field, and they generally thin toward the south and north (Fig. 6C). This interpretation is consistent with that of Martinsen (1994), which documented that the three members progressively onlap the Skull Creek Shale from northwest to southeast within an incised-valley. The Ute Member is 4–22 ft (1–7 m) thick and is laterally continuous (Figs. 6C and 11C). The measured depth of the top of the Muddy Sandstone varies between 8,000 ft (2,438 m) and 10,000 ft (3,048 m) below KB (Fig. 12C), and KB is at 4,600–5,000 ft (1,402–1,524 m) above sea level. The top of the Muddy Sandstone, which deepens toward the southwest, is the highest in the four studied fields. It is ~2,300 ft (~701 m) higher compared to the top in the Amos Draw Field.
Kitty Field
The thickness of the Recluse, Cyclone, and Ute Members varies across the Kitty Field (Figs. 6D and 11D). In general, the Recluse Member contains quartz-dominated sandstone, and the thickness ranges from 5 to 30 ft (2 to 9 m). The overlying Cyclone Member is more silty and shaly and varies in thickness. The Ute Member is dominated by quartz-rich sandstone, of 18–28 ft (5–9 m) thick, and it thickens toward the south (Fig. 11D). The Springen Ranch Member is thin, but laterally extensive. The measured depth of the top of the Muddy Sandstone in the Kitty Field varies between 7,900 ft (2,408 m) and 9,200 ft (2,804 m) below KB (Fig. 12D), and KB is at 4,100–4,700 ft (1,250–1,433 m) above sea level. The top of the Muddy Sandstone deepens toward the southwest in the Kitty Field, and it is ~1,500 ft (~457 m) higher compared to the top in the Amos Draw Field.
The top of the Muddy Sandstone generally deepens toward the west in the four studied fields. The top is higher in the Hilight and Kitty fields compared to the top in the Amos Draw and Sand Dunes fields. This trend suggests that the basin limb was tilted down to the west after deposition, most likely caused by f lexural loading by growth of the Bighorn Mountains during the Laramide Orogeny (e.g., Flores and Ethridge, 1985).
Figure 10. Cross plot of permeability vs. porosity for core samples from Hilight, Amos Draw, and Sand Dunes fields in PRB. Springen Ranch Member has highest porosity and permeability.
RESERVOIR HETEROGENEITY WITHIN THE MUDDY SANDSTONE
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DISCUSSION
Reservoir Heterogeneity of Muddy Sandstone
Characteristics of high-quality subsurface reservoirs for CO2 sequestration include high porosity, high permeability, high abundance of reactive Ca-, Mg-, and Fe-bearing non-carbonate minerals, great thickness, and good continuity of sandstone intervals (e.g., Holloway, 2001; Kharaka et al., 2006). These characteristics vary for the Muddy Sandstone within each of the reservoir intervals, resulting in reservoir heterogeneity. The most favorable interval for sequestration within the Muddy Sandstone varies in the four studied fields. The primary reservoir in the Amos Draw Field is the lower Rozet Member because of its high thickness, high porosity and permeability, high feldspar abundance, and high sand/mud ratio compared to the Springen Ranch Member. The primary reservoir in the Kitty Field is the Ute
Member, resulting from its high porosity (up to ~30 percent). Both the valley-fill deposits of the Recluse Member and estuarine deposits of the Cyclone Member in this field are thick and sandy. They have low porosity and permeability, however, because of extensive cementation, high matrix content, and large vertical and lateral variations of lithofacies. These variations result in reduced CO2 sequestration capacity and ability. The primary reservoir in the Hilight Field is the Springen Ranch Member. The stratigraphic complexity in the Hilight Field is similar to the Kitty Field, and the valley-fill deposits in the Hilight are more shale prone in comparison to the Kitty. The Springen Ranch Member is laterally thick and consistent, with high porosity and high permeability, and thus it is the best interval for sequestration. The primary reservoir in the Sand Dunes Field is the Springen Ranch Member because of its greater thickness, high porosity and permeability, and high sand/mud ratio compared to the Rozet Member.
Figure 11. Isopach map of: A, Rozet Member in Amos Draw Field; B, Springen Ranch Member in Sand Dunes Field; C, Ute Member in Hilight Field; and D, Springen Ranch Member in Kitty Field. Squares represent well data used in study. Units are in feet.
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New Sequence Stratigraphic Framework
Sequence stratigraphy provides a useful way to interpret the large-scale architecture of reservoir development in the four fields studied in the PRB. Previous studies discussed the significance of major unconformities (Gustason, 1988; Gustason et al., 1988; Weimer, 1988; Ryer et al., 1990; Martinsen, 1994) in the Muddy Sandstone in the PRB. Sequence stratigraphic evolution of the Muddy Sandstone in the Hilight Field was also discussed previously (Donovan, 1995). In this paper, I use the sequence stratigraphic concepts of Van Wagoner et al. (1990) to explain the reservoir distribution within the Muddy Sandstone in each of the fields studied (Fig. 9). The sequence stratigraphic interpretation from this study is based on the work of authors listed in this section (‘New Sequence Stratigraphic Framework’) as well as my interpreted depositional environment (Table 1).
The underlying Skull Creek Shale and equivalent Thermopolis Shale were deposited during the first
incursion of the Cretaceous seaway into Wyoming during Albian time and are interpreted to represent an early highstand systems tract. Onset of Muddy deposition took place as a shoreface environment prograded from northwest to southeast across the PRB when sea level started to fall (Gustason, 1988; Gustason et al., 1988). This marine regression formed the lower-shoreface deposits of the Rozet Member in the Amos Draw and Sand Dunes fields and possibly in the Kitty and Hilight fields. The Rozet Member in the Sand Dunes Field to the south of the other fields is finer in grain size and is inferred to have been deposited farther offshore. The Rozet is interpreted as a late highstand systems tract. Subsequent sea level fall resulted in incision into the older shoreface deposits. The incision is evident as NW–SE trending paleovalleys in the Kitty and Hilight fields, whereas the upper Rozet Member in the Amos Draw Field and interfluve in the Kitty Field experienced erosion as the region remained as a paleohigh (Martinsen, 1994). This unconformity is described as lowstand
Figure 12. Structural contour map of top of Muddy Sandstone: A, Amos Draw Field; B, Sand Dunes Field; C, Hilight Field; and D, Kitty Field. Squares represent well data used in study. Black lines represent cross sections in Figure 6. Units are in feet.
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surface of erosion in previous studies (Weimer, 1988; Martinsen, 1994) and is interpreted herein as a sequence boundary (Fig. 13).
Subsequent infilling of the valleys consists of fluvial and estuarine Recluse and Cyclone Members in the Hilight and Kitty valleys and the Ute Member in all the fields. The depositional environment and thickness of the Ute Member vary laterally, depending on the degree to which the valleys were filled by the end of Cyclone deposition. The Ute Member in the Amos Draw and Sand Dunes fields was deposited in a restricted, brackish-water environment. As a result, the Ute Member in the two fields contains abundant shale. In contrast, the Ute Member in the Kitty and Hilight fields represents shallow-water environments including estuaries or barrier islands; as a result, it is locally very sandy (Martinsen, 1994; Fig. 13). This study interprets the Recluse, Cyclone, and Ute Members as a lowstand systems
tract and the depositional surface at the top of the Ute Member as a transgressive surface. Subsequent marine transgression toward the northwest resulted in a change from the estuarine environment to an open-marine environment. I interpret the lower-shoreface deposits of the Springen Ranch Member and open-marine deposits of the Skull Creek Member as a transgressive systems tract. The maximum flooding surface is situated within the overlying Mowry Shale (Bohacs et al., 2005). The Muddy Sandstone and the overlying Mowry Shale represent one depositional sequence. Because the depositional span of the Muddy Sandstone is at most five million years, my research suggests that the depositional sequence is a third-order depositional sequence (Van Wagoner et al., 1995).
Diagenesis
The distribution of diagenetic alterations observed in this study can be explained by the changes in relative sea level and rates of sediment supply during deposition. Late highstand systems tracts a re characterized by aggradation and progradation of shoreline in response to high sedimentation rate and decrease of relative sea level rise (Van Wagoner et al., 1990). Lowstand systems tracts are characterized by high sedimentation rate associated with decreased rate of relative sea level fall (Van Wagoner et al., 1990). Sediments formed in late highstand and early lowstand systems tracts, particularly in wet climatic conditions, may be subjected to pedogenic processes when the sea level drops, resulting in substantial silicate dissolution and kaolinization (Nedkvitne and Bjørlykke, 1992; Morad et al., 2000; Ketzer et al., 2003; Morad et al., 2010). The high kaolinite concentration in the upper Rozet Member in the Amos Draw Field is interpreted to have formed under these conditions (Odland et al., 1988). This also explains why sandstone in the fluvial Recluse Member in the Kitty and Hilight fields experienced substantial silicate grain dissolution. Silicate dissolution forms clay minerals and secondary silica, and it probably caused quartz overgrowth and high matrix content in some sandstone bodies in the two members.
Transgressive systems tracts are characterized by sediment starvation during rapid relative sea level rise (Van Wagoner et al., 1990). Carbonate cementation
Figure 13. A, Chronostratigraphic diagram of Muddy Sandstone. B, Sequence stratigraphic interpretation of A. Striped area in A = depositional lacuna. HST = highstand systems tract, in gray. TST = transgressive systems tract, in green. LST = lowstand systems tract in red. MFS = maximum flooding surface, in green. SB = sequence boundary, in red. TS = transgressive surface, in blue. Muddy Sandstone is shown in yellow. Sandstone bodies in HST and TST are dominantly shoreface deposits, and in LST are dominantly f luvial, and estuarine channels. Black triangle represents period of marine regression, and white triangle represents period of marine transgression.
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is a common feature of transgressive systems tracts. That is particularly the case on f looding surfaces, because of the abundance of carbonate bioclasts, organic matter, bioturbation, and prolonged settling time of the sediments (e.g., Al-Ramadan et al., 2005; Morad et al., 2000, 2010). Carbonate cementation is through diffusion of dissolved Ca2+, Mg2+, and HCO3
- from the overlying seawater. Diffusion is enhanced by the presence of abundant carbonate bioclasts, which act as nuclei for the precipitation of calcite (e.g., Taylor et al., 2000; Al-Ramadan et al., 2005). Therefore, the first stage of carbonate cement in the Recluse and Cyclone Members in the Kitty Field may be an early diagenetic product precipitated during the marine transgression of the Springen Ranch and Skull Creek Members. Subsequent sea level fall promotes migration of meteoric water and subsurface f luids, which facilitates dissolution of carbonate cement, thereby producing secondary pore space. The secondary pore space may be filled by carbonate cement during other transgressive systems tracts during the Late Cretaceous. Thus, I suggest that the second and third stages of carbonate cementation observed in the Recluse and Cyclone Members in the Kitty Field may have happened during subsequent transgression of the Western Interior Seaway.
Implications for CO2 Sequestration
Reservoir stratigraphic heterogeneity does not only impact CO2 sequestration capacity by controlling reservoir porosity, thickness, and lateral continuity. It may also impact ability for CO2 sequestration by influencing f luid-flow pathways, post-injection chemical reactions, and types and amounts of micro-porosity.
Reservoir heterogeneity affects the lateral and vertical continuity of permeable compartments and thus impacts patterns of f luid f low and potential CO2 migration (Meyer and Krause, 2006). Lower-shoreface sandstone of the Rozet and Springen Ranch Members are relatively homogeneous and laterally continuous. Permeability values tend to increase up-section in coarsening-upward sandstone sequences, as is typical for shoreface deposits. Simulation models demonstrate that CO2 injected into lower-shoreface sandstone tends to migrate upward to the top of the sandstone sequences and
then spread laterally as a thin plume (Hovorka et al., 2004). In addition, the subaerial erosion on the top of the Rozet Member in the Amos Draw Field represents long-term stabilization of f loodplain (Miall, 1988). The fine-grained sandstone of the Ute Member in the Amos Draw Field and the Shell Creek Shale in the Sand Dunes Field have good lateral continuity along with low porosity and permeability, and thus are an ideal seal for CO2 storage in this unit. Therefore, the Rozet Member in the Amos Draw Field and the Springen Ranch Member in the Sand Dunes Field have the greatest potential for CO2 sequestration. Fluvial and estuarine sandstone in the Kitty and Hilight fields are heterogeneous and discontinuous, which limit the volume for CO2 storage. These isolated mudstone, siltstone, sandstone, and coal beds are local in scale and would not be strong inhibitors to horizontal flow. They may act as a baffle to vertical flow, however, slowing the rise of buoyant fluids to the seal (Bryant et al., 2008).
Understanding the potential for dissolution of minerals in the reservoir is significant because the possibility for eventual precipitation of carbonate minerals capable of trapping CO2 over geologic time scales depends on the availability of cations that would be provided by dissolution of these mineral precursors (Xu et al., 2004; Peters, 2009). The Muddy Sandstone contains both detrital and authigenic minerals that could potentially be reactive when CO2 is injected. Mineral dissolution rates are several orders of magnitude higher at low pH (pH of ~3 or 4), which would be expected when CO2 is injected and which would cause the formation of brines to become undersaturated (Kharaka et al., 2006). Geochemical modeling also shows that the pH of formation brines can drop significantly (Parry et al., 2007). The pH drop related to CO2 injection could cause dissolution of Fe-bearing clay minerals, potentially mobilizing associated trace metals that are commonly adsorbed onto iron oxides or organic compounds (Kharaka et al., 2006). Carbonate minerals would be highly reactive in CO2-generated acid brine. Carbonate cements of four stages are found in the fluvial incised-valley lithofacies in the Kitty Field, and they are expected in the f luvial incised-valley lithofacies in the Hilight Field. The dissolution of carbonate cement potentially provides important ions for the formation of long-term carbon-sequestering mineral phases. Zones with
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abundant clay matrix are also found in the Recluse and Cyclone Members in the Kitty and Hilight fields. Clay matrix, particularly illite and chlorite, contains iron. These iron-rich silicate minerals may react with CO2 and increase the sequestration capacity. Remnants of dissolved feldspars are found in the Rozet Member in the Amos Draw Field, providing mineral surfaces for potential reaction with CO2 filling the pores.
Stratigraphic heterogeneity also affects the amount, types, and geometry of micropores within the sandstone. Clay authigenesis promotes an increase in specific surface area and the creation of microporosity (Eslinger and Pevear, 1988). Clay rims, especially illite, on quartz grains inhibit quartz overgrowth and maintain the sandstone porosity. The large surface area generated by authigenic clays can deteriorate the effectiveness of surfactant and polymer solutions in CO2 storage because of the tendency of these chemicals to adsorb on clay surfaces and be lost from circulation (Kalpakci, 1981). Diagenetic clay minerals can also control the retention rate of CO2. An example is that smectitic clays would favor a higher retention of CO2, especially in acidic environments (Venaruzzo et al., 2002). This aspect may particularly influence the CO2 capacity in the lower Rozet Member in the Amos Draw Field as well as the fluvial incised-valley lithofacies in the Kitty Field because of high matrix content.
Future Work
This study provides a background framework for identifying and evaluating reservoir heterogeneity on CO2 sequestration in the Muddy Sandstone of the PRB, northeastern Wyoming. Though this study identified four potential intervals for CO2 sequestration, realization of the maximum possible storage capacity requires more detailed understanding of the textural and mineralogical variability that will inf luence the fate of injected CO2. Additionally, simulation of CO2 injection in these fields is still lacking, which requires more data related to variables such as hydraulic pressure, temperature, structural setting, and brine chemistry. This study recommends that future work include: (1) more detailed studies of mineralogy focused on specific authigenic phases and pore characteristics; and (2) experimental evaluation of the effects of exposure to CO2-charged brines under conditions of elevated pressure and
temperature. Such studies should provide important parameters for reactive transport modeling, f luid-f low modeling, and f luid-pressure modeling, and they will improve understanding of CO2 injection into depleted oil and gas fields.
CONCLUSIONS
Sedimentologic, petrographic, and mineralogical analyses of the Muddy Sandstone in four oil and gas fields in the PRB, northeast Wyoming, demonstrate the reservoir heterogeneity that occurs throughout the formation. The heterogeneity is a result of spatially variable depositional environment, relative sea level change, and type and extent of diagenetic alteration.
The Muddy Sandstone contains f ive major lithofacies, representing offshore, lower–middle wave-dominated shoreface, weathering zone, fluvial incised-valley, and tidal-influenced estuarine depositional environments. The Muddy Sandstone in the Amos Draw and Sand Dunes fields includes the Rozet Member, deposited in wave-dominated shoreface, and Ute Member deposited in an estuarine central-basin environment, and the Springen Ranch Member of wave-dominated shoreface deposits. In the Kitty and Hilight fields, the incised-valley deposits of the Recluse Member eroded the Rozet Member locally, and the overlying Cyclone and Ute Members, deposited in estuarine environments, contain thick sandstone bodies. The Springen Ranch Member of lower-shoreface deposits occurs in both the Kitty and Hilight f ields. Sandstone bodies formed in wave-dominated shorefaces are laterally extensive and of equivalent thickness in each field. The ones formed in incised-valley and estuarine environments, in contrast, have variable thicknesses and lateral extents.
Sandstone petrographic analysis shows that quartz makes up more than 70 percent of the total framework grains, and the percentage of feldspar is generally less than 5 percent, except for the Rozet Member in the Amos Draw Field, which is up to 22 percent. Muddy Sandstone can be divided into four groups based on the relative abundance of pore space, carbonate cement, and matrix, and the sandstone types are closely related to depositional environment. Dissolution of feldspar and lithic fragments is commonly observed, enhancing the porosity. XRD
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results show that the pore-filling clay minerals include kaolinite, chlorite, illite, and smectite, which destroy porosity. Four stages of carbonate cementation are observed, including early dolomite overgrowth, secondary calcite filling, dolomite replacement, and spotty siderite cementation. Both the secondary porosity generated by feldspar dissolution and diagenetic carbonate cementation may enhance potential for CO2 sequestration by providing reactive mineral surfaces and calcium ions for the formation of long-term carbon-sequestering mineral phases.
A sequence stratigraphic model is proposed to explain reservoir development of the Muddy Sandstone in which it, along with the overlying Mowry Shale, represent one third-order depositional sequence. The Rozet Member represents an older, late highstand systems tract. The unconformity at the top of the Rozet Member is a sequence boundary. The Recluse, Cyclone, and Ute Members are components of a lowstand systems tract, and the depositional surface at the top of the Ute Member is a transgressive surface. The Springen Ranch Member and the overlying Skull Creek Member are components of a transgressive systems tract.
Re ser voi r cha rac ter i z at ion in the four fields suggest that the best reservoir interval for sequestration in the Amos Draw Field is the lower Rozet Member, in the Sand Dunes and Hilight fields is the Springen Ranch Member, and in the Kitty Field is the Ute Member. Reservoir heterogeneity is controlled by depositional environment, sea level changes, and post-depositional diagenesis. I suggest that realization of the maximum possible CO2 storage capacity in the Muddy Sandstone in the PRB requires detailed understanding of the textural and mineralogical variability that will influence the fate of injected CO2. This study recommends that computer simulations be conducted to improve understanding of injections in depleted oil and gas fields.
ACKNOWLEDGMENTS
The Wyoming State Legislature provided funding for this research. I am deeply indebted to the helpful comments and suggestions from an anonymous reviewer, Professor Mary Kraus, Dr. Gus Gustason, Dr. Brenda Bowen, and RMG editors Robert Waggener and Jay Lillegraven.
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Manuscript Received September 12, 2013
Revised Manuscript Submitted November 11, 2013
Manuscript Accepted March 31, 2014
M. FAN