Stratigraphy and sedimentology of a leveed-channel complex, Cretaceous Dad Sandstone Member, Lewis Shale, Wyoming, U.S.A.Roger M. Slatt1, Staffan K. Van Dyke2, David R. Pyles3, Alan J. Witten1, and Roger A. Young1

1School of Geology and Geophysics, University of Oklahoma, 2Vanco Energy Co., 3Bureau of Economic Geology, Jackson School of Geosciences, University of Texas as Austin

The Cretaceous Lewis Shale of southern Wyoming is an excellent outcrop example of a mud-dominated submarine fan system that was deposited basinward of a coeval, prograding shelf edge (Pyles and Slatt, this volume). Three outcrops of slope leveed-channel strata -- called Spine 1, Spine 2, and Rattlesnake Ridge—are exposed over a 2 mi (3.2km) area. Spine 1 is the main topic of this paper. This area has been the subject of considerable study for the past few years for two reasons: (1) it provides good outcrop exposures of the Lewis Shale in an area of active exploration for Lewis gas, and (2) Lewis strata here are considered to be an excellent, scaledanalog to delta-fed, deepwater (turbidite) sytems that produce oil and gas in areas such as the Gulf of Mexico and offshore west Africa (Pyles and Slatt, 2000).

Over a 200+ft. (67+m) interval, the vertical stratigraphy of Spine 1 consists of, from the base, two laterally continuous sheet sandstones separated by mudstone, overlain by 9 lenticular channel sandstones which form a 1750ft. (0.5km) wide ridge. Along with outcrop measured sections (121 of the channel sandstones), shallow boreholes, high-frequency seismic reflection, ground penetrating radar, and electro-magnetic induction surveys have been applied to parts of Spine 1 to document details of this leveed-channel complex. All outcrops, measured sections, and other locations have been spatially oriented by GPS.

A ‘master channel’ has been seismically imaged beneath the subcrop of the channel complex, and appears to have truncated parts of the sheet sandstones. Thin-bedded levee deposits occur in outcrop on both sides of the master channel; some of these beds show features typical of hyperpycnites. Within the master channel are the 9 smaller, lenticular channel sandstones. Internally, the channel sandstones are lithologically complex The following features indicate at least the lowermost channel (Channel #1) is sinuous: (a) the north side of the outcrop consists of interbedded sandy sediment gravity flows and shale clast conglomerates, while the south side (spaced <150m apart) consists of cross-bedded sandstones capped by massive sandstones with fluid escape pipes; (b) this cross-channel stratigraphy is considered to be analogous to the ‘cutbank’ and ‘point bar’ sides of a fluvial channel-fill; (3) a vertical well (CSM Strat Test #61) drilled behind the outcrop did not penetrate all 9 channel sandstones; (4) an electro-magnetic induction survey revealed a bend in the channel sandstone into the outcrop. Each of the 9 channel sandstones is separated by thin-bedded levee strata. Ground Penetrating Radar features---corroborated by outcrop observations--- include lateral accretion packages, slumps, and erosionalremnants within individual channel-fill. At the Rattlesnake Ridge outcrop, the margin between channel sandstone and levee beds is complex, and separated by a slumped, thin-bedded interval lining the channel.

All of the features documented on and within Spine 1, as well as Spine 2 and Rattlesnake Ridge, indicate extreme complexity might be expected in subsurface reservoir analogs.

Location Map

Country/Stateor Province

Spine 1

Spine 2

CSM well

Figure 1. Location of Spine 1 in Washakie Basin, southern Wyoming, U.S.A. Raised relief map shows Spine 1 and Spine 2. Dashed red lines are location of two sheet sandstones. Brown lenses on Spine 1 are channel-fill sandstones. Location of CSM Strat. Test #61 well is shown.

Location:

Specifics (town, Lat/long, etc.)Region/ province/ state

CountryFormation NameAge

Basin Setting:

Basin Size: shape and size (in km2 and mi2)

General Outcrop Description and Stacking Pattern:

Depositional Setting(s) Interpretation:

Deepwater strata are restricted to a circular depression that was >2,900 sq. mi. (>8000 sq. km) in area.

Outcrop Summary

Carbon County, south-central Wyoming, U.S.A. 20 mi (32 km) north of the town of Baggs and 65 mi (104 km) southwest of Rawlins, and 4 miles (6.4km) east of Highway 789 along a dirt road. Sections 24-25, T16NR92W.WyomingUSA

Spine 1: Dad Sandstone

Dad Sandstone Member of the Lewis ShaleUpper Cretaceous, Lower Maastrichtian.

The Lewis Shale is a mixed sand-mud system. The slope strata discussed here are dominantly mudstone with local sand-filled channel-form bodies. Spine 1 is comprised of 9 lenticular channel-fill sandstones, each <500ft. (170m) wide, separated by thin-bedded siltstones and v. fine sandstones. Total thickness of this sequence is about 200ft. (67m). It is these lenticular sandstones that form the ridge (app.1500 ft. or 0.5km wide) which is Spine 1. Valleys occur on both sides of Spine 1 due to modern differential erosion of thin-bedded strata which flanked the ridge. Underlying the channel sandstones are at least two sheet sandstones, each 5-12ft. (1.5-3.7m) thick, which can be traced laterally for at least 1 mi (1.6km).

Spine 1 is located on the flank of the Sierra Madre Uplift in the Washakie Basin of Wyoming. The basin was formed during the Laramide Orogeny. Prior to that, the depositonal site was part of a shallow epicontinental seaway which extended the length of the present-day U.S. Depositon of the Lewis Shale occurred during the final transgression and regression (Bearpaw cycle) of this seaway.

The 9 channel sandstones comprise a slope, leveed-channel complex that is about 0.5km wide. Shallow seismic and outcrop measurements indicate that thin-bedded levee strata flank both sides of the complex and that a master channel has focused the channel sandstones within this confined area. Thicknesses of the channel-sandstones vary from 8-34ft. (2.7-11m) and intervening thin-bedded mudstone intervals vary from 1 to 22 ft. thick. The following facies comprise the channel sandstones: massive sandstones with and without fluid escape pipes; rippled to parallel laminated sandstones; cross-bedded sandstones; shale-clast conglomerates, thin-bedded intervals of very fine sandstone-siltstone-mudstone; and slumped and contorted beds.

?

Lin

Line I-2 Line I-1

90

100

110

120

90

100

110

120

310

320

330

340

350

300

360

370

380

390

400

410

420

310

320

330

340

350

300

360

370

380

390

400

410

420

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

260

270

280

290

300

310

320

330

340

90

100

110

120

90

100

110

120

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

260

270

280

290

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

260

270

280

290

430

440

450

430

440

450

460

470

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

260

270

280

290

300

310

320

330

340

0

10

20

30

+ 700'

PE

PE

MARKER C

PE

VPE

PE

PE

VPE

3SistersMarker

25

PETrough shapedscour in sandstone.

40 N

Approximatelocation of

Kirn

5070

25

65

365

80

10

810

PE

70WW

?

?

10W

60W

6

9 10 11

12

15

1617

18 19 20 21

22 22b

232524

Shale 2

Shale 1 26-31

35 34

1

2

3

5

6

7

8

9

10

12

34

5

6

T1

T2

T

C1

S1

50ft.(17m)

Figure 2. Two measured stratigrapihc sections spaced 130ft. (40m) apart. T1, T2, S1, C1 are from Outcrop Summary table. Channel sandstones 1-10 are shown in boxes (4 and 5 are now combined to give 9 channels). After Witton (2000).

Depositional Setting(s) Interpretation (cont.):

Overall Outcrop Dimensions in PanelLength (meters and feet)

Thickness (meters and feet)Average Net/Gross

Grain size range of sand/conglomeratic elements

Other:

Sandstones are very fine to fine grained. Shale clasts in shale clast conglomerates are sometimes >1ft. (0.3m) in diameter.

?

The lowermost and thickest sandstone (Channel Sandstone #1) has been determined from shallow geophysics to be lenticular and to make a sinuous bend into the outcrop; shale-clast conglomerates are interbedded with massive sandstones on one side of the sandstone and cross-bedded sandstones occur on the other side, further indicating the channel is sinuous and asymmetric with a steep- and shallow-gradient side. Shale clasts in the conglomerate are thin-bedded, indicating they were shed off a steep sided levee wall; cross-bedded sandstones were deposited on the opposite side. This association is analogous to a point bar deposit with steep cutbank and shallow point bar sides. The two laterally continuous sandstones beneath the channel complex are interpreted as sheet sandstones.

Approximately 200ft. (67m)0.55

A total of 121 outcrop stratigraphic sections were measured along the 9 channel sandstones. Spatial distribution of the 9 sandstones and the measured sections were determined by GPS. Behind the outcrop, where it dips 12o into the subsurface, shallow boreholes were drilled and images were obtained with shallow hammer seismic, ground-penetrating radar, and electro-magnetic induction surveys.

Sheet-form Architectural ElementsLength (meters and feet)

Thickness (meters and feet)Net/Gross

Outcrop Orientation for this element (degrees)Average Paleocurrent (degrees)

Typical Facies succession:Texture: grain size range, average, and sorting

Channel-form Architectural ElementsComplete or Partial channel-form?

Width (meters and feet)Thickness (meters and feet)

Aspect Ratio: Width/thicknessNet/Gross

Outcrop Orientation for this element (degrees)Average Paleocurrent (degrees)

Typical Facies succession: Channel Infill Bedding Architecture:

Texture: grain size range, average, and sorting

Channel-base shale drape % Coverage (0-100%)

Thinbed Architectural Element

Length (meters and feet)Thickness: (meters and feet)

Net/Gross

Texture: grain size range, average, and sortingOutcrop Orientation for this element (degrees)

Average Paleocurrent (degrees)Typical Facies succession:

Analog Reservoirs or Fields Ram-Powell 'L' Sand

S1Minimum of 1mi (1.6km)

5-12ft. (1.5-3.7m)Individual sandstones = 0.99. Sandstone interval = 0.30

NW-SE; dip; 12o S.S

up to 500ft.(70m)_in outcrop

Ta to massiveFine-grained

C1Partial

8-34ft. (2.7-11m)??

0.55NW-SE; dip; 12o S.

VariableAs described above. Several lithofacies.

Complex interfingering of lithofacies.very fine-fine grained; mod.-poorly sorted. Shale clasts 'float' in

sandstones.Shale-clast conglomerates drape channel base. Thicknesses

variable.

T1

??

T3

??<0.5in(1.3cm)

mudstone,siltstone, shale

Same as sandstonessouthTd, Te

T2

??<4in (10cm)

siltstone with thin sandstones

Same as sandstones

Tb, Tc, Td, Te Td, Te

<4in (10cm)0.05

siltstone and clayey siltstone

Same as sandstones

#2 #3 #4 #5 #6#1

C

B

A

B

Figure 3. (A) Outcrop view along the presumed depositional axis of the channel complex. Channel sandstones 1 (arrow) through 6 are labelled. (B) Outcrop view perpendicular to depositional axis. S1 and S2 are sheet sandstones, CC is Spine 1 leveed channel complex, and F is Fox Hills Sandstone. (C) Ines A and B are shown on the raised relief map.

N

CSM wellChannel Max. SS

Thickness (ft)Max. Sh

Thickness (ft)10 8

19 8

68 12

37 10

196 8

94 16

33 8

222 10

221 34

Figure 5. (A) Channel Sandstone #1 showing (B) Alternating shale clastconglomerate and sandy sediment gravity flows (outlined in yellow)on the north, ‘cutbank’ side and

(C) cross-bedded sandstones on the south, ‘point bar’ side.

150m

Channel Sanstone #1A

C B

Figure 6. Sandy lateral accretion packages and erosional remnants of thin-bedded mudstones (Minken et al., 2003) seen on Ground Penetrating Radar (Young et al., 2003) and in outcrop.

20 nanoseconds

Channel Sandstone #1

Erosionalremnant

Onlapping sands Slump

Erosional remnant of levee beds

Onlap

Base of channel

Lateral Accretion surfaces

ch1

Channel Sandstone #1

Erosional remnants

Structureless sandstoneMassive sandstone with fluidEscape structuresShale clast conglomeratesCross bedded sandstone

Erosional Remnants

Figure 4. 3D model of Spine 1 and the 9 channel sandstones. Location of CSM well is shown. Red line is location of seismic line (Fig. 12). Maximum thickness of channel sandstones and intervening shales is given in table.

Figure 7. (A) Facies distribution across Channel Sandstone #1. (B) Interpreted edge of Channel Sandstone #1 beyond the outcrop (white lines). A

B

Distal levee beds Proximal levee beds 2 channel sandstones

A

B C

Gamma Ray ResistivityCurves

CoreGamma

Ray

A B

C

B

C

Figure 9. (A) Gamma ray and resistivity log of CSM StratTest #1 well. Gray intervals denote gamma-scan of cored intervals. (B) Base of channel fill at red line. (C) Thin-bedded facies. Location of cored intervals B and C are shown in A.

6

5

4

3

2

1

13FT

11

10

9

8

7

6

5

4

3

2

1

12FT

11

10

9

8

7

6

5

4

3

2

1

11FT

11

10

9

8

7

6

5

4

A

BC D

Figure 10. (A) Outcrop of thin-bedded strata interpreted as levee deposits (T3 in Outcrop Summary). These beds are outside the master channel. (B) Measured outcrop section. (C ) Close-up of thin-bedded mudstones. (D) Polished slab of thin-bedded mudstones. The mudstones within the red lines first become coarser grained upward, then fine upward, suggesting they might be hyperpycnites (Soyinkaand Slatt, 2003). (E) Part of measured Section; gray is mudstone and yellow is siltstone (Soyinka and Slatt, 2003).

Figure 11. Complex channel margin at Rattlesnake Ridge. (A) Channel sandstone in erosional contact with thin-bedded levee strata (covered by soil). (B) Ground Penetrating Radar line showing onlap of sandy channel fill onto slumped levee beds (transparent), indicating signficant time interval between levee deposition and channel back-filling. (C) Contact between channel sandstones and levee beds.

0

6.5

9.8

13

16B

A

Thin bedded Sandstones/Mudstones

CSlumpedbeds

Figure 12. High-resolution seismic line shot across CSM well (red vertical line; see Fig. 12 for location) and gamma ray (blue) and core gamma scan (red) logs to left. Blue = sheet sandstones,Red = base of internal channels, Tourquise = base of master channel, White = lateral accretion packages.

Dep

th in

m.

CSM Well location

150 mFigure 13. Spatial distribution of the 9 channel Fill sandstones showing facies and master channel (black dashed line). Inset in upper leftis a generalized model of Spine 1, modifiedfrom Mayall and O’Byrne (2002).

Figure 8. (A) Channel Sandstone #4, comprised of two amalgamatedchannels. (B) Distal levee beds exhibit low angle, uniform dips. (C) proximal levee beds exhibit variable dip and cut-and-fill. These criteria arediagnostic of other deepwater levee deposits (Browne and Slatt, 2002; Clemenceau et al., 2003).

E