Chapter three: QuantIfyIng Intra-Channel arChIteCture Of deep-water slOpe Channel strata usIng Channel MetrICs: a predICtIve MethOd

IntrOduCtIOn

Deep-water slope channel deposits have been the focus of extensive research over the last decade. These complex depositional systems have been thoroughly investigated using vivid three-dimensional images of subsurface turbidite complexes produced from extensive seismic reflection volumes (e.g. Kolla et al., 2001; Abreu et al., 2003; Deptuck et al., 2003; Posmentier and Kolla, 2003; Prather et al., 2003; Samuel et al., 2003; Saller et al., 2004; Schwenk et al., 2005; Gee et al., 2007; Hubbard et al., 2009; Labourdette and Bez, 2010; Sylvester et al., 2011). Despite increases in resolution produced as a result of technological advances in seismic data acquisition and processing, there remains a significant gap in resolution between what is imaged in these surveys and the stratigraphic detail needed to efficiently delineate and develop associated hydrocarbon reservoirs.

During the evaluation of deep-water petroleum reservoirs, seismic datasets are commonly combined with data collected from well penetrations in order to gain an understanding of a particular reservoir interval at multiple scales (e.g., Porter et al. 2006). Well data (ie., wireline or core) is spatially limited, which makes it difficult to predict lateral facies changes and variations in sub-seismic architectural geometries. Supplementing subsurface data with sedimentological insight drawn from outcrop analogues is therefore crucial for the prediction of bed-scale sediment distribution in slope-channel reservoirs (Sullivan et al., 2004; Pringle et al., 2010). Numerous workers have demonstrated the importance of subtle facies characteristics or architectural parameters that impart a strong influence on the connectivity of deep-water channel reservoir bodies (e.g., Barton et al., 2010; Alpak et al., 2011; Li and Caers, 2011). Deep-water architectural elements are commonly characterized with quantifiable metrics in outcrop (e.g., Chapin et al., 1994; Manzocchi et al., 2007; Romans et al., 2009; McHargue et al., 2011; Olariu et al., 2011; Pringle et al., 2010), an approach adopted in order to more objectively describe sedimentary units and generate robust data inputs for reservoir model construction.

The primary focus of this work is to compile quantitative data collected using outcrop observations of a slope channel complex-set 130 m thick from the Cretaceous Tres Pasos Formation of southern Chile (Fig. 3.1). This data characterizes sub-seismic-scale intricacies observed within 18 channel elements in a 2.5 km long outcrop belt. The

43

Lago Toro

10

km

51° 0

0’ S

51° 3

0’ S

72° 3

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L. S

arm

ient

o

Sou

thA

mer

ica

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ile

1000

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620

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ero

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ro S

ol

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uth

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ou

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on

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nd:

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nel s

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rbed

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ured

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ee b

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bels

in B

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12

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3 26

9

7

3

2

A

Fig.

3.1

. Stu

dy a

rea

over

view

; (A

) Hig

h-re

solu

tion

sate

llite

imag

e of

a p

ortio

n of

the

Ulti

ma

Espe

ranz

a D

istri

ct o

f Sou

ther

n C

hile

; ins

et m

ap o

f Sou

th A

mer

ica

prov

ides

con

text

. The

red

box

iden

tifies

the

loca

tion

of th

e ou

tcro

p be

lt st

udie

d ad

jace

nt to

Lag

una

Figu

eroa

. (B

) Pho

to-m

osai

c of

Tre

s Pas

os F

orm

atio

n ou

t-cr

op b

elt.

Blu

e lin

es id

entif

y th

e lo

catio

ns o

f eac

h of

the

stra

tigra

phic

sect

ions

mea

sure

d ac

ross

the

stud

y ar

ea. 1

607

m o

f det

aile

d (c

m-s

cale

) mea

sure

d se

ctio

n pr

ovid

es th

e fr

amew

ork

for t

his s

tudy

. (C

) Stra

tigra

phic

cro

ss-s

ectio

n sh

owin

g th

e ei

ghte

en sl

ope

chan

nel e

lem

ents

pre

serv

ed in

the

Tres

Pas

os F

orm

atio

n at

La

guna

Fig

uero

a. T

he 1

30 m

thic

k se

dim

enta

ry p

acka

ge re

pres

ents

a re

serv

oir-s

cale

, cha

nnel

com

plex

-set

. Thi

s com

plex

-set

is d

ivis

ible

into

thre

e ch

anne

l co

mpl

exes

(sho

wn

in d

ashe

d lin

es),

with

bas

al b

ound

arie

s dem

arca

ted

by w

ides

prea

d m

udst

one-

dom

inat

ed u

nits

acr

oss t

he e

ntire

out

crop

bel

t. Th

e no

rthea

st–

sout

hwes

t tre

ndin

g cr

oss-

sect

ion

is o

rient

ed h

ighl

y ob

lique

to p

aleo

flow,

whi

ch w

as ro

ughl

y so

uthw

ard

and

capt

ures

3D

out

crop

per

spec

tives

on

a fla

t pan

el.

44

data is derived from a series of measured sections, comparable to measurements that could be made from cores and wireline logs in subsurface datasets. These data are suited for interpretation of three-dimensional sediment distribution in subsurface reservoirs dominated by analogous slope channel strata (e.g., Mayall et al., 2006). In particular, the dataset compiled provides insight into the extrapolation and prediction of channel element architecture in the subsurface based on diagnostic quantitative criterion defined for intra-channel subenvironments, which include channel axis, off axis and margin.

study area and BaCkgrOund geOlOgy

This study focuses on the Late Cretaceous Tres Pasos Formation, part of a 4-5 km thick succession of deep-marine strata in the Magallanes foreland basin of southern Chile (Fig. 3.1; Romans et al., 2010, 2011). An exceptionally exposed outcrop of the formation located adjacent to Laguna Figueroa provides the foundation for this study (Figs. 3.1A and 3.1B). The Tres Pasos Formation consists primarily of mudstone- and siltstone-dominated strata associated with a graded clinoform system characterized by ~ 870 m of paleobathymetric relief (Hubbard et al., 2010). Architectural analysis was completed on a coarse clastic base of slope deposit that forms an outcrop belt 2.5 km long and ~130 m thick (Fig. 2.1C; Chapter 2). Southward paleoflow was determined from the measurement of hundreds of sole marks, which confirms the generally southward oriented paleoflow trend observed across the basin (Shultz et al., 2005; Shultz and Hubbard, 2005; Romans et al., 2009; Hubbard et al., 2010). The plane of the outcrop belt is oriented north-northeast (27 - 207°N), and is intersected at a highly oblique angle by slope channel bodies preserved in the strata (Fig. 3.1B and 3.1C). Numerous gullies crosscut the outcrop belt roughly perpendicular to paleoflow providing unique 2D and 3D exposures of channel geometries (Fig. 3.1B).

The architecture of channelized deep-water strata is commonly described hierarchically, which provides a means to organize observations and recognize persistent patterns at multiple scales (Mutti and Normark, 1991; Ghosh and Lowe, 1992; Pickering and Clarke 1996; Campion et al., 2000; Gardner and Borer 2000; Hubbard et al., 2008; McHargue et al, 2011). The hierarchical scheme used for the purpose of this study is a slightly modified version of Sprague et al. (2002, 2005). The entire 130 m of outcropping stratigraphy studied is defined as a single complex-set, which in turn, is comprised of three channel complexes 20-68 m thick, differentiated from one another by widespread erosive bases draped by siltstone-dominated deposits (Chapter 2; Fig 2.1C). Complexes are also composite features comprised of genetically related channel elements 6-18 m thick,which consist of sedimentation units measuring 0.1 - 4.5 m in thickness.

45

slOpe Channel MOdel

Slope channel elements in the study area consist of mappable low-sinuosity (1.01-1.05) bodies characterized by little change in internal architecture along the 2.5 km length of the outcrop belt. The cross-sectional fill of each channel is symmetric to slightly asymmetric (Fig 2.2; Chapter 2), and a typical Tres Pasos Formation channelform is ~15 m thick and estimated to be 300 m wide.

These channels are initiated by large, out-sized sediment gravity flows that scour the sea floor (cf., Elliot, 2000). Erosional relief is coupled with constructional internal levee build-up, which focuses turbidity currents and leads to overall aggradation of the channel system (cf., Deptuck et al., 2003; Kane and Hodgson, 2011). Channels largely back-fill with the deposits from collapsing high-concentration turbidity currents (Mutti and Normark, 1991; Clark and Pickering, 1996). An important component of channel fills are fine-grained drape deposits, which mantle the channel base and record deposition from the tails of largely bypassing turbidity currents (Fig. 3.2A; Mutti and Normark, 1987). If preserved, these 10 to 200 cm thick bypass deposits have the potential to impart considerable reservoir heterogeneity and in some instances could compartmentalize a sedimentary body (Figs. 3.1C, 3.2B and 3.2C; cf., Barton et al., 2010).

Channel element fills are composite features comprised of numerous stacked sedimentation units, which individually represent the deposit of a single sediment gravity flow (Fig. 3.2A). These sedimentation units are typically lenticular, thinning from the axis of a channel element towards the margins (Fig. 3.2A). Channel fills commonly transition from thick, amalgamated sandstone beds in the central portion of channel elements to thinly interbedded sandstone and mudstone-dominated deposits in the channelform flanks across < 30 m (Fig. 3.2A). Channel element fill is subdivided into three intra-channel architectural zones, which correspond directly to the sedimentation unit associations described in Chapter 2. For the purpose of this study, these distinct architectural associations are defined as axis, off-axis and margin. These zones are systematically distributed in each channel element (Fig. 3.2A) and are mapped across the outcrop belt (Fig. 3.3). Similar channel element architecture has been identified in numerous outcrop studies, including the Capistrano Formation, California (Campion et al., 2005), the Brushy Canyon Formation, West Texas (Beaubouef et al., 1998; Gardner et al., 2003), and the Ross Formation of Ireland (Sullivan et al., 2004). This internal channel fill architecture is quantitatively characterized in the Tres Pasos Formation by linking architectural observations to the detailed measured section data acquired across the study area (Fig. 1C).

46

05

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55

05

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5 m

25 m

vert

ical

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erat

ion

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SO

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ENT

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ND

ARY

erod

ed c

hann

elfo

rm

bypa

ss d

rape

dep

osit

BAC

sedi

men

tatio

n un

it

Fig.

3.2

. (A

) Cha

nnel

ele

men

t arc

hite

ctur

e is

illu

stra

ted

in a

dep

ositi

onal

strik

e or

ient

ed c

ross

-sec

tion.

Cha

nnel

form

geo

met

ry is

ero

ded

by la

rge

out-s

ized

sedi

-m

ent g

ravi

ty fl

ows t

hat p

redo

min

antly

byp

ass t

he c

hann

el e

nviro

nmen

t and

dep

osit

coar

se-g

rain

ed se

dim

ent o

n th

e de

ep se

afloo

r. C

hann

el e

lem

ent b

ases

are

co

mm

only

dra

ped

by si

lt- to

mud

ston

e-do

min

ated

dep

osits

resu

lting

form

the

low

-den

sity

trac

tion

depo

sitio

n an

d he

mip

elag

ic se

ttlin

g as

soci

ated

with

thes

e pr

edom

inan

tly b

ypas

sing

flow

s. C

hann

el e

lem

ent fi

lls a

re c

hara

cter

ized

by

an o

vera

ll co

arse

ning

upw

ard

sequ

ence

, whi

ch p

rese

rves

man

y ep

isod

es o

f ero

sion

, by

pass

and

dep

ositi

onal

bac

kfilli

ng. T

hree

sedi

men

tolo

gica

lly u

niqu

e po

rtion

s of a

cha

nnel

ele

men

t can

be

iden

tified

(axi

s, of

f-ax

is a

nd m

argi

n) a

nd c

hara

cter

-iz

ed q

uant

itativ

ely

by th

e m

etho

d de

scrib

ed in

this

stud

y. (B

) Cha

nnel

ele

men

ts st

ack

into

two

com

posi

te c

hann

el c

ompl

exes

(mea

sure

d se

ctio

n pr

ovid

es v

ertic

al

scal

e).

Das

hed

box

is e

xpan

ded

in p

art C

. (C

) Stra

tigra

phic

sect

ion

mea

sure

d th

roug

h th

e lo

wer

cha

nnel

com

plex

. A

rrow

s ide

ntify

ero

sion

al c

hann

el e

lem

ent

base

s, dr

aped

by

fine-

grai

ned

bypa

ss d

epos

its.

Whe

re p

rese

rved

, the

se fi

ne-g

rain

ed d

rape

s wou

ld p

rovi

de su

bsta

ntia

l ver

tical

bar

riers

to fl

ow.

47

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115

120

125

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100

105

110

115

Sub

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05101520253035404550556065707580859095

100

105

110

KJ1

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MM

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05101520253035404550556065707580859095100

105

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102

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m

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1716

14

13

12

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12

11

13

15

1818

6

5

15

3 26

debr

is �

owax

iso�

axi

sm

argi

nin

ner l

evee

Fig.

3.3

. Int

ra-e

lem

ent a

rchi

tect

ure

dist

ribut

ion

char

acte

rized

from

fiel

d ob

serv

atio

ns p

rese

nted

in th

e st

ratig

raph

ic c

ross

-sec

tion

in F

igur

e 3.

1C. R

ecog

nitio

n of

thes

e ar

chite

ctur

al d

ivis

ions

acr

oss t

he o

utcr

op b

elt r

elie

s hea

vily

on

wal

king

out

and

cor

rela

ting

beds

in th

e fie

ld. T

his s

chem

atic

pro

vide

s a si

mpl

ified

up

-sca

led

pers

pect

ive

of th

e ou

tcro

p be

lt, re

mov

ing

sedi

men

tolo

gica

l det

ail.

The

high

est q

ualit

y re

serv

oir r

ocks

are

ass

ocia

ted

with

cha

nnel

axi

s and

off

axis

de

posi

ts, a

nd a

ll ot

hers

wou

ld b

e of

subs

tant

ially

low

er q

ualit

y.

48

dataset and MethOds

Over 1600 m of detailed measured section were collected at the cm-scale, documenting sedimentation unit thickness, grain-size, primary sedimentary structures, and the nature of bed contacts across study area (Fig. 3.1C and 3.2C). Measured section through 185 channel elements were taken, and 3656 sedimentation units were documented. Lateral spacing between measured sections ranges from 100 to 250 m, although localized channel margins were characterized by 10-20 m section spacing (Fig. 3.1C). Measured sections were correlated through field and photomosaic mapping (Chapter 2; Fig. 2.1C).

Eighteen channel elements are present, defined through stratigraphic correlations, delineation of channel edges, facies analysis and paleocurrent measurements (Fig. 3.1C). Each channel fill is subdivided into axis, off axis and margin architectural zones along the length of the outcrop belt (Fig. 3.3). These architectural divisions are the foundation of the quantitative metrics tabulated. Linking the intra-element architecture to the bed-scale measurements collected allows these different zones to be differentiated based on a series of quantified 1-D characteristics. These characteristics, or metrics, include net-to-gross ratio, amalgamation ratio, and maximum amalgamated sandstone thickness.

Net-to-Gross Ratio Net-to-gross ratio (NTG) is a common quantitative parameter used to describe one-dimensional stratigraphic sections (from outcrop or subsurface) collected from deep-water strata. It is a measure of sandstone richness, recorded as a ratio of the total thickness of sandstone divided by the gross thickness of the interval of interest. In this study, channel element thickness is used to define the gross interval (Fig 3.1C and 3.2C), and net-to-gross values tabulated are expressed as a decimal between 0 and 1 (Fig. 3.4).

Amalgamation ratioAmalgamation ratio (AR) is a quantitative parameter defined by the number of

amalgamation surfaces in a gross interval (ie, sandstone on sandstone bedding contacts) divided by the number of sedimentation units in that same interval minus one (Fig. 3.4). Subtracting one from the denominator of the ratio normalizes the metric since there is always one fewer bed contact than sedimentation unit in an interval. The calculated value is also presented as decimal between 0 and 1 for ease of comparison with NTG. This metric is useful for the evaluation of vertical connectivity in deep-water sandstone accumulations (e.g., Chapin et al., 1994; Manzocchi et al., 2007; Romans et al., 2009).

49

Fig. 3.4. Quantitative metrics calculated in three stratigraphic sections measured through the axis to margin transition of a channel element. At left, graphic logs of sections indicate sedimentary unit thick-ness, grain-size, primary sedimentary structures, the nature of bed contacts and number of sedimenta-tion units in the interval of interest. To the right of the graphic logs, each sedimentation unit within the interval is classified Type 1 – 5. Net sandstone is tabulated and compared to the overall interval thick-ness to calculate net-to-gross ratio. Amalgamation surfaces (sandstone on sandstone contacts identified by arrows) are counted and compared to the number of beds in an interval to calculate an amalgamation ratio, and the maximum thickness of amalgamated sandstone is tabulated.

MM

2M

M 1

MM

102

0

5

10

0

5

10

0

5

cmfvfsltmud

cmfvfsltmud

cmfvfsltmud1

2

3

4

5

6

7

8

9

10

11

12 13 14 151617

1

2

3

4

5

6

7

89 10 11 12 131415

16

1718

2019

1 23

45 67 89 10 11 12 131415

1617

18

2019

2122

24

23

26 2725 28

measured section

measured section

measured section

sedimentationunit type

sedimentationunit type

sedimentationunit type

net sand

net sand

net sandcompiled metrics

compiled metrics

compiled metrics

Net to Gross Ratio:

Amalgamation Ratio:

0.97

0.63

Max. Amalgamated Sst. Thickness:

11.3 m

Total Unit Thickness:

11.6 m

Type 1:

73.6%Type 2:

24.1%Type 3:

1.1%Type 4:

1.2%

Net to Gross Ratio:

Amalgamation Ratio:

0.95

0.60

Max. Amalgamated Sst. Thickness:

7.5 m

Total Unit Thickness:

11.3 m

Type 1:

52.5%Type 2:

40.2%Type 3:

1.5%Type 4:

4.8%

Net to Gross Ratio:

Amalgamation Ratio:

0.53

0.11

Max. Amalgamated Sst. Thickness:

1.2 m

Total Unit Thickness:

6.3 m

Type 1:

4.8%Type 2:

27.1%Type 3:

15.0%Type 4:

53.1%

net sssed. unit type 3 sed. unit type 4sed. unit type 2sed. unit type 1 gross amalg. surface max. amalg. sst

Axis

O�-Axis

Margin

50

Maximum thickness of amalgamated sandstone The maximum thickness of amalgamated sandstone is defined as the greatest

thickness of continuous amalgamated sandstone present within a channel element succession. This metric is a useful measure of sandstone connectivity complimentary to both AR and NTG.

sedIMentatIOn unIts

Sedimentation units are the fundamental division used to capture stratigraphic details in the channelized deep-water deposits studied, and are defined using the well-established bed classification schemes of Lowe (1982) and Bouma (1964). Five sedimentation unit types characterize the channel elements of the Tres Pasos Formation (Table 3.1):

Type one: Type one sedimentation units consist of thick-bedded (25 – 450 cm) amalgamated sandstone with sharp, erosive bases. Beds contain a coarse- to very coarse-grained basal lag with occasional scattered granules, which normally grades into upper fine- to upper medium-grained structureless to planar laminated sandstone. Moderate to high concentrations of mudstone intraclasts (up to 25 cm in diameter) are commonly observed at the base of sandstone beds, rarely comprising layers > 20 cm thick. Intraclasts are sub-angular to sub-rounded and in some instances, where they are elongate, they are crudely aligned to imbricated. Bed tops are typically truncated by the erosive base of the overlying bed.

These highly erosive, amalgamated sedimentation units record deposition from erosive high-density turbidity currents (Lowe, 1982). Basal mudstone intraclast lags are locally transported by tractional processes on the channel floor. The structureless portions of sedimentation units record the rapid collapse of suspended material from the sediment-laden turbidity current (Ta division of Bouma, 1962; S3 division of Lowe, 1982) and planar laminations preserve waning stage traction sedimentation (Tb division of Bouma, 1962). Fine-grained deposits associated with the dilute low-density tails (ie, Td-e) of the large turbidity currents are usually absent due to erosion by subsequent high-density turbidity currents.

51

Tabl

e 3.

1. D

escr

iptio

n of

sedi

men

tatio

n un

it ty

pes a

nd in

terp

rete

d se

dim

enta

ry p

roce

sses

, Tre

s Pas

os F

orm

atio

n, L

agun

a Fi

guer

oa.

unit

disc

riptio

nTy

peD

omin

ant g

rain

-siz

eSe

dim

enta

ry s

truc

ture

sTu

rbid

ite

divi

sion

sBo

undi

ng

surf

aces

Thic

knes

sra

nges

Seco

ndar

y fe

atur

esD

epos

ition

al p

roce

ss

Thic

k-be

dded

sa

ndst

one

1U

pper

fine

to u

pper

m

ediu

m-g

rain

ed

sand

ston

e, c

oars

e to

ve

ry c

oars

e-gr

aine

d sa

ndst

one

and

gran

ules

at s

ed. u

nit

base

s

Nor

mal

ly g

rade

d;

stru

ctur

eles

s to

pla

ne

lam

inat

ed; c

rude

al

ignm

ent o

f cla

sts

and

rare

cla

st im

bric

atio

n in

sed.

uni

t bas

es; b

ed

amal

gam

atio

n co

mm

on

S1, T

a &

Tb

Eros

ive

base

; gr

adat

iona

l to

shar

p to

p

Mud

ston

e in

trac

last

s (u

p to

25

cm

in le

ngth

) com

mon

at s

ed. u

nit

base

s; ra

re d

ewat

erin

g st

ruct

ures

; m

inor

sof

t sed

imen

t def

orm

atio

n

Mix

ture

of t

ract

ion

and

rapi

d su

spen

sion

sed

imen

tatio

n fr

om h

igh-

dens

ity tu

rbid

ity c

urre

nts

2Lo

wer

fine

to u

pper

m

ediu

m-g

rain

ed

sand

ston

e

Nor

mal

ly g

rade

d;

stru

ctur

eles

s or

pla

ne-

lam

inat

ed to

ripp

le

lam

inat

ed; b

ed

amal

gam

atio

n co

mm

on

Ta, T

b &

TcSh

arp,

flat

ba

ses;

gr

adat

iona

l to

shar

p to

p

Min

or lo

w d

ensi

ty o

r iso

late

d m

udst

one

intr

acla

sts

(up

to 1

5 cm

in

leng

th) fl

oatin

g in

mat

rix o

r at s

ed.

unit

boun

darie

s; ra

re lo

adin

g an

d so

ft-s

edim

ent d

efor

mat

ion

Dep

osite

d ra

pidl

y fr

om s

uspe

nsio

n by

hig

h-de

nsity

turb

idity

cur

rent

s

Thic

k- to

thin

-bed

ded

non-

amal

gam

ated

sa

ndst

one

Low

er fi

ne to

upp

er

med

ium

-gra

ined

sa

ndst

one,

silt

ston

e to

m

udst

one

com

mon

ly

inte

rbed

ded

Nor

mal

ly g

rade

d;

stru

ctur

eles

s or

pla

ne-

lam

inat

ed to

ripp

le

lam

inat

ed; b

ed

amal

gam

atio

n ra

re

Part

ial t

o co

mpl

ete

Boum

a se

quen

ces

Shar

p, fl

at

base

s;

grad

atio

nal t

op

Abu

ndan

t pre

serv

atio

n of

silt

ston

e at

sed

. uni

t top

; mud

ston

e in

trac

last

s ar

e ra

re; r

are

load

ing

and

soft

-sed

imen

t def

orm

atio

n

Dep

osite

d ou

t of s

uspe

nsio

n by

a

mix

ture

of h

igh

and

low

den

sity

tu

rbid

ity c

urre

nts,

trac

tion

depo

sitio

n fr

om b

y-pa

ssin

g flo

ws

Shal

e w

ith ra

re

silts

tone

Pred

omin

antly

sha

le

with

rare

silt

ston

e an

d ve

ry fi

ne-g

rain

ed

sand

ston

e

Stru

ctur

eles

s to

fain

tly

lam

inat

ed; r

are

norm

al

grad

ing

Gra

datio

nal t

o sh

arp

base

s;

shar

p to

p

Rare

ver

y fin

e to

fine

-gra

ined

sa

ndst

one

beds

com

mon

ly <

10

cm th

ick

Dep

osite

d un

der n

orm

al m

arin

e co

nditi

ons

from

hem

ipel

agic

set

tling

or

by

colla

psin

g lo

w-d

ensi

ty

turb

idity

cur

rent

s, d

epos

ition

from

by-p

assi

ng �

ows

Chao

tic m

udst

one-

rich

depo

sits

Com

ingl

ed s

hale

, si

ltsto

ne a

nd

sand

ston

e

Non

e; c

haot

ic, f

olde

d an

d de

form

ed b

eddi

ngVa

riabl

e to

p an

d ba

seRa

re s

ands

tone

and

silt

ston

e ra

fted

bl

ocks

; poo

rly s

orte

d m

ixtu

res

of

sedi

men

t with

var

ying

pro

port

ions

of

san

d, s

ilt a

nd s

hale

; com

mon

so

ft s

edim

ent d

efor

mat

ion

Mas

s w

astin

g (b

ank

colla

pse,

sl

umpi

ng, s

lidin

g); d

epos

ition

by

mud

-ric

h de

bris

flow

s

3 4 5

Sedi

men

tatio

n

25 -

450

cm

10 -

250

cm

10 -

200

cm

0.5

- 10

cm

20 -

500

cm

52

Type two Type two sedimentation units consist of thick-bedded (10 to 250 cm) sandstone with flat bases, which commonly grade normally from upper medium- to upper fine-grained sandstone. These units are typically amalgamated. Sandstone is predominantly structureless with planar laminations common in the upper 5 to 30 cm of the unit, occasionally overlain by ripple lamination. Soft-sediment deformation characterizes sandstone bed tops in some instances. Thin siltstone beds rarely cap sedimentation units. Isolated mudstone intraclasts up to 15 cm in length are present at the tops of beds or in basal lags. Intraclasts are commonly angular and rarely imbricated.

Type two sedimentation units record rapid suspension sedimentation from high-density turbidity currents (dominated by S3/Ta divisions). Planar and ripple laminations (Tb and Tc divisions of Bouma, 1962) record traction deposition from waning turbulent flows.

Type three Type three sedimentation units are thin- to thick-bedded (5 to 200 cm), with sharp, flat bases. They are not typically associated with amalgamated, sand-on-sand bedding contacts. Sandstone beds normally grade from upper medium to upper fine-grained, are commonly structureless or faintly laminated, and grade rapidly to siltstone or mudstone deposits 2 to 10 cm thick. Planar and ripple lamination are sometimes preserved in the upper 10 to 15 cm of sedimentation units, and are often associated with soft sediment deformation. The siltstone- to mudstone-dominated intervals at the top of each sedimentation unit are occasionally characterized by diffuse planar laminations. Mudstone intraclasts are rare, randomly distributed in the upper half of sandstone beds.

Type three sedimentation units are the result of deposition from high to low-density turbidity currents. Thick structureless sandstone deposits record rapid sedimentation from the head of a large high-density turbidity current (Ta/S3). Waning of the current is recorded by the Tb, Tc and Td divisions of Bouma (1962).

Type four Type four sedimentation units consist of thinly interbedded (0.5-10 cm thick), non-amalgamated siltstone and very fine-grained sandstone. Units exhibit flat bases and where thicker than 5 cm, normal grading is typical. The upwards transition from sandstone to siltstone is gradational; sandstones are planar- to ripple-laminated and siltstones are diffusely laminated to massive.

Thin-bedded siltstone and sandstone units with gradational tops were deposited

53

by traction, associated with dilute, low-density turbulent gravity flows (Tb - Te divisions of Bouma, 1962). In some instances, these beds originated from the dilute tails of large, erosive turbidity currents, which predominantly bypassed the channel environment (cf., Mutti and Normark, 1987).

Type five Type five sedimentation units consist of poorly sorted mudstone-dominated deposits 20-500 m thick. They are chaotically bedded, characterized by folded and internally deformed fabric locally. Basal and upper contacts are sharply defined and often undulous. Small blocks (up to 40 cm in diameter) of rafted sandstone or siltstone are occasionally observed within a poorly sorted muddy matrix.

Chaotic deposits are attributed to mass wasting or slumping. They also could be the result of deposition by mud-rich debris flows (Lowe, 1982). Mass wasting deposits are an important constituent of many deep-water channel fills (e.g., Mayall et al., 2006; Hubbard et al., 2009), however they make up a minor proportion (1.2%) of the strata studied.

applICatIOn Of MetrICs tO well-expOsed Channel transeCts

In order to demonstrate the quantification of intrachannel architectural detail, the analysis of two channel axis to margin transects are described below (Fig. 3.5).

MM Margin Measured sections from the MM Margin characterize internal element architecture as it transitions from margin to axis (Fig. 3.5A). NTG increases systematically from 0.53 at the channel margin, to 0.95 in the off-axis position and then to 0.97 in the channel axis (Fig. 3.5A). AR also increases from 0.11 at the margin, to 0.60 in the off-axis section and 0.63 in the axial-most portion of the channel (Fig. 3.5A). Maximum amalgamated sandstone thicknesses increases from 1.2 m in the margin to 7.5 m in the off-axis position and 11.3 m in the axis. The characterization of the MM Margin produces idealized results, where all three quantitative metrics increase systematically from the channel margin to axis (Fig. 3.5A).

Gold Margin Four measured sections, A through D, are used to characterize Gold Margin architecture (Fig. 3.5B). Section A was measured through the margin and Section D

54

MM MARGIN

5 m

5 m

VE=1.5

5 m

5 m

VE=1.5

GOLD MARGIN

Max. Amalgamated Sst:

1.2 m

Net to Gross Ratio:

Amalgamation Ratio:

0.53

0.11Max. Amalgamated Sst:

7.5 m

Net to Gross Ratio:

Amalgamation Ratio:

0.95

0.60Max. Amalgamated Sst:

11.3 m

Net to Gross Ratio:

Amalgamation Ratio:

0.97

0.63

Max. Amalgamated Sst:

10.1 m

Net to Gross Ratio:

Amalgamation Ratio:

0.90

0.23Max. Amalgamated Sst:

8.8 m

Net to Gross Ratio:

Amalgamation Ratio:

0.88

0.37Max. Amalgamated Sst:

6.2 m

Net to Gross Ratio:

Amalgamation Ratio:

0.90

0.24Max. Amalgamated Sst:

0.88 m

Net to Gross Ratio:

Amalgamation Ratio:

0.56

0.05

0

2

4

6

8

10

12

0.0

0.2

0.4

0.6

0.8

1.0

01020304050607080

net t

o gr

oss

/ am

alg.

ratio

max

. am

alg.

sst

. (m

)

0

2

4

6

8

10

12

0.0

0.2

0.4

0.6

0.8

1.0

(m)

01020304050607080

net t

o gr

oss

/ am

alg.

ratio

max

. am

alg.

sst

. (m

)

amalgamation ratio max. amalgamation ss thicknessnet to gross

margin axis

margin axis

margin axis

east west

eastwest

B

0

5

0

5

10

MM 2

0

5

10

MM 1MM 102

0

5

10

D

0

5

10

15

C

0

5

10

A

0

5

10

B

A

Fig. 3.5. Stratigraphic cross-sections across two well-preserved channel margins in the study area. The graph beneath each channel element illustrates changes in channel metrics across the transition from margin to off-axis to axis.

55

towards the channel element axis, with sections B and C capturing characteristics of the intermediate off axis portion of the channel element (Fig. 3.5B). NTG values generally increase from margin to axis, from 0.56 to 0.90; intermediate values calculated in the off-axis position decrease from 0.90 in Section B to 0.88 in Section C (Fig. 3.5B). Minor variation in the systematic increase in NTG from margin to axis is locally recorded. AR increases as expected from Section A (0.05), through sections B (0.24) and C (0.37); however, the transition from Section C to Section D is characterized by a decrease in AR to 0.23 (Fig. 3.5B). With AR highly dependent on the number of sedimentation units present, the reduced amalgamation ratio in Section D results from a relatively high number of thin beds preserved within the drape deposits in the most deeply incised, axial-most portion of the channel element described. Maximum amalgamated sandstone values increase from 0.88 m in the margin (Section A), to 6.2 m and 8.8 m in the off-axis positions (sections B and C), and finally to 10.1 m as the channel axis is approached (Fig. 3.5B).

Comparison of the two margins The Gold and MM margin case examples demonstrate how quantitative metrics computed from measured sections help differentiate intra-channel architectural zones (Fig. 3.2A and 3.5). Perturbations in expected trends can commonly be attributed to increased presevation of drape deposits locally within the channel, however, an increase in NTG and AR is generally expected across the transition from margin to axis. With little variation in gross intra-element fill trends observed across the study area (Fig. 3.1C and 3.2A), this relationship forms the basis for computation of quantitative metrics for each intra-element architectural zone (axis, off-axis or margin). Examining the relative proportion of each sedimentation unit type across the channel fills also provides complimentary results that should add another quantifiable attribute to help differentiate each subdivision in poorly constrained reservoirs (Fig. 3.4).

results

Statistical analysis was completed on the tabulated channel metrics produced from the robust dataset collected at the Laguna Figueroa study area (Table 3.2). For the purpose of this discussion, NTG is considered high, moderate or low, when characterized by values of > 0.8, 0.5 to 0.8 and < 0.5, respectively. AR results are similarly subdivided into high (> 0.5), moderate (0.2 to 0.5) and low (< 0.2) degrees of amalgamation. Channel Axis Data

56

Eleme

nt

E186.7

016.

250.0

05.5

50.0

02.5

0--

----

--2.2

53.9

00.3

53.8

03.9

55.6

01.6

03.1

50.8

52.7

5--

----

----

----

----

----

----

--E17

----

3.00

6.55

1.80

11.30

1.65

5.00

2.30

6.15

1.10

5.60

1.50

5.25

4.60

4.60

3.63

6.95

5.85

7.30

5.60

6.95

3.10

5.20

----

2.70

7.20

0.00

4.77

----

----

E165.7

010.

003.5

86.8

01.9

56.8

03.9

010.

353.2

59.1

52.1

59.3

03.1

08.1

00.0

03.5

5--

----

----

----

----

----

----

----

----

--E15

0.10

3.10

5.15

5.15

0.80

5.90

0.00

3.65

0.00

3.60

0.00

3.50

0.00

1.65

0.00

3.70

5.65

8.10

4.10

6.25

6.75

9.95

8.20

11.00

----

4.10

10.00

0.19

9.19

----

----

E140.0

03.0

52.5

02.5

00.0

02.5

00.0

01.8

53.3

04.9

00.8

06.1

02.9

510.

705.9

011.

957.1

57.1

50.0

07.1

55.8

07.0

55.9

56.4

0--

--3.5

04.6

00.0

03.5

6--

----

--E13

0.30

3.45

6.85

6.85

5.45

7.55

7.20

7.20

3.75

3.75

2.10

2.10

0.00

1.30

2.50

3.25

1.90

4.20

0.68

2.75

1.90

3.90

3.20

3.20

----

3.85

4.10

0.81

6.48

----

----

E120.0

06.8

58.7

08.7

08.2

08.2

06.8

06.8

04.2

04.2

02.4

54.7

54.0

04.8

52.3

54.0

53.1

05.5

02.2

59.2

510.

6510.

6512.

5512.

55--

--12.

8012.

808.3

79.5

7--

----

--E11

1.70

11.65

4.30

4.30

7.90

10.95

3.70

4.70

5.85

6.45

0.95

3.10

0.00

2.00

0.00

2.65

0.00

4.45

4.05

4.05

2.75

2.90

4.05

4.05

----

5.40

5.40

4.30

6.91

----

----

E10--

----

----

--3.8

56.6

50.2

27.8

53.1

510.

152.5

38.4

02.8

05.4

01.5

51.5

5--

----

----

----

----

----

----

----

--E9

0.80

6.75

----

1.95

3.70

4.00

7.45

12.40

12.40

7.90

11.20

10.65

12.90

4.00

15.20

1.50

16.45

5.30

16.30

10.90

15.20

6.95

13.45

0.00

6.45

0.70

11.35

5.47

13.09

0.00

14.05

----

E89.4

016.

50--

--10.

2013.

101.3

05.4

00.0

01.4

00.0

05.2

5--

----

----

----

----

----

----

----

----

----

----

--E7

0.08

4.45

----

1.70

2.00

7.80

13.05

6.90

12.35

7.85

13.30

11.55

15.35

4.75

14.00

5.10

13.05

4.19

11.90

2.85

11.10

2.55

8.60

1.60

8.15

4.90

8.35

4.66

8.00

0.00

7.25

----

E61.6

08.5

0--

----

--3.0

05.3

52.0

05.2

56.0

07.7

52.0

58.6

50.0

06.8

00.0

02.9

5--

----

----

----

----

----

----

----

--E5

0.00

2.30

----

----

0.00

6.05

0.45

5.95

1.45

3.95

0.00

4.55

0.00

3.20

0.00

2.97

0.00

4.95

0.33

1.50

0.00

3.75

2.33

6.00

1.05

4.90

0.00

3.14

0.00

3.20

----

E4--

----

----

----

----

--0.7

50.7

51.9

04.5

07.9

09.0

56.0

012.

536.8

010.

654.8

713.

1211.

8011.

807.2

511.

20--

--2.7

111.

013.7

58.5

0--

--E3

2.90

7.40

----

----

0.85

8.15

0.69

7.55

9.65

13.10

6.45

6.45

7.95

8.35

3.75

8.50

3.75

5.55

6.18

6.93

8.50

8.50

3.05

8.20

----

8.08

8.26

9.35

9.95

2.85

11.50

E23.3

03.3

0--

----

--5.7

012.

855.1

05.7

59.2

09.2

04.9

013.

054.7

010.

258.0

511.

0513.

5017.

15--

--6.1

016.

907.6

07.6

0--

--8.1

512.

134.7

618.

83--

--E1

----

----

----

----

----

----

2.40

2.40

5.25

10.25

4.65

4.65

7.55

12.30

----

4.65

6.20

----

----

----

2.30

9.62

----

Darks

ideOP

1OP

2MM

102MM

101KJ1

Sub BB

3Sub

BB1

Pequen

a 2 Up

perCac

hana

Cachan

a 2Cac

hana 3

Sub BB

4Vac

a 1Vac

a 2Peq

uena

Pequen

a 2 Lo

wer

Eleme

nt

E18

0.31

0.72

0.00

0.16

0.00

0.45

----

----

0.64

0.69

0.06

0.32

0.15

0.89

0.05

0.80

0.50

0.31

----

----

----

----

----

----

----

E17

----

0.50

0.46

0.08

0.40

0.15

0.51

0.15

0.59

0.44

0.83

0.80

0.93

1.00

1.00

0.24

0.91

0.63

0.91

0.70

0.88

0.33

0.81

----

0.22

0.76

0.00

0.67

----

----

E16

0.70

0.69

0.35

0.82

0.19

0.79

0.21

0.78

0.37

0.58

0.64

0.72

0.39

0.78

1.00

1.00

----

----

----

----

----

----

----

----

----

E15

0.08

0.13

0.09

0.24

0.04

0.30

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.80

0.70

0.21

0.72

0.44

0.72

0.54

0.80

----

0.27

0.63

0.13

0.52

----

----

E14

0.00

0.36

0.13

0.74

0.00

0.85

0.00

1.00

0.53

0.80

0.20

0.47

0.30

0.84

0.42

1.00

1.00

1.00

0.00

0.59

0.55

0.98

0.63

0.94

----

0.30

0.89

0.00

0.29

----

----

E13

0.07

0.59

0.38

1.00

0.47

0.97

0.73

1.00

1.00

1.00

1.00

1.00

0.00

0.50

0.13

0.82

0.50

0.96

0.10

0.94

0.19

0.88

1.00

1.00

----

0.63

0.95

0.09

0.25

----

----

E12

0.00

0.31

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

0.25

0.91

0.61

0.85

0.50

0.58

0.13

0.95

0.11

0.98

1.00

1.00

1.00

1.00

----

1.00

1.00

0.80

1.00

----

----

E11

0.04

0.71

1.00

1.00

0.67

0.79

0.25

0.88

0.20

0.96

0.33

0.34

0.00

0.40

0.00

0.17

0.00

0.42

1.00

1.00

0.67

0.95

1.00

1.00

----

1.00

1.00

0.11

0.91

----

----

E10

----

----

----

0.93

0.89

0.04

0.24

0.05

0.69

0.10

0.78

0.19

0.85

1.00

1.00

----

----

----

----

----

----

----

----

E90.0

20.2

8--

--0.1

70.6

30.6

10.9

71.0

01.0

00.9

00.7

10.3

50.8

50.2

50.8

50.1

70.8

10.1

40.8

80.4

20.8

80.0

70.7

60.0

00.2

90.2

40.6

70.3

70.6

40.1

10.5

2--

--E8

0.61

0.87

----

0.75

0.82

0.82

0.79

0.00

0.14

0.00

0.00

----

----

----

----

----

----

----

----

----

----

----

E70.0

80.3

9--

--0.8

90.9

50.5

30.8

50.3

00.8

30.4

70.7

20.6

70.8

90.5

80.7

90.5

40.9

20.2

20.9

00.3

30.5

90.0

80.7

50.2

50.5

50.5

50.7

00.4

40.7

90.1

30.7

7--

--E6

0.14

0.72

----

----

0.50

0.56

0.40

0.44

0.17

0.81

0.15

0.75

0.00

0.00

0.00

0.65

----

----

----

----

----

----

----

----

E50.0

00.2

8--

----

--0.0

00.2

00.0

50.2

90.0

30.7

30.0

00.2

70.0

00.0

00.0

00.0

00.0

00.3

91.0

00.7

80.1

70.4

50.1

40.5

00.0

80.4

30.0

00.0

00.0

00.0

0--

--E4

----

----

----

----

----

1.00

1.00

0.25

1.00

0.90

0.97

0.17

0.91

0.46

0.99

0.53

0.99

1.00

1.00

0.43

0.98

----

0.55

0.94

0.74

0.72

----

E30.2

10.6

7--

----

--0.3

80.3

70.0

40.5

70.1

60.8

01.0

01.0

01.0

00.9

50.2

80.6

50.2

20.7

90.5

00.9

71.0

00.9

50.4

50.7

1--

--0.8

90.9

80.5

70.9

70.4

21.0

0E2

1.00

1.00

----

----

0.50

0.98

0.57

0.99

1.00

0.99

0.37

0.99

0.41

1.00

0.26

0.99

0.58

0.88

----

0.28

0.91

1.00

1.00

----

0.85

0.99

0.49

0.76

----

E1--

----

----

----

----

----

--1.0

00.7

90.5

00.9

71.0

01.0

00.3

90.9

7--

--0.8

20.9

0--

----

----

--0.8

00.2

4--

--

Darks

ideOP

1OP

2MM

102

MM10

1KJ

1Su

b BB3

Sub B

B1Pe

quen

a 2 U

pper

Cach

ana

Cach

ana 2

Cach

ana 3

Sub B

B4Va

ca 1

Vaca

2Pe

quen

aPe

quen

a 2 Lo

wer

BA

Axi

sO

�-ax

isM

argi

n

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

Mx Am

SS (m

)Thic

kness

(m)

AmRat

NTG

AmRat

NTG

AmRat

NTG

AmRat

NTG

AmRat

NTG

AmRat

NTG

AmRat

NTG

AmRat

NTG

AmRat

NTG

AmRat

NTG

AmRat

NTG

AmRat

NTG

AmRat

NTG

AmRat

NTG

AmRat

NTG

AmRat

NTG

AmRat

NTG

Tabl

e 3.

2. C

hann

el m

etric

s com

pute

d fo

r eac

h ch

anne

l ele

men

t fro

m F

ig 3

.1C

and

tabu

late

d in

col

umns

cor

resp

ondi

ng to

eac

h m

easu

red

sect

ion.

Eac

h el

emen

t is

cla

ssifi

ed b

y in

tra-e

lem

ent a

rchi

tect

ure

clas

sifie

d in

Fig

. 3.3

. NTG

= n

et-to

-gro

ss. A

mR

at =

Am

alga

mat

ion

ratio

. Max

Am

SS

= M

axim

um a

mal

gam

ated

sa

ndst

one.

Thi

ckne

ss =

Tot

al e

lem

ent t

hick

ness

. Blu

e bo

xes =

axi

s. R

ed b

oxes

= o

ff-ax

is. G

reen

box

es =

mar

gin.

57

Channel element axes are characterized by moderate to high NTG (0.71 to 1.00) with a mean value of 0.95 (Fig. 3.6 and 3.7A). The frequency distribution graph of NTG for channel element axes shows that >90% of the measurements are 0.8 or higher (Fig 3.7A). Amalgamation ratios within channel element axes are highly variable, with computed values ranging from low to high (0.1 to 1.00); however, channel element axes generally exhibit a high degree of amalgamation with a mean value of 0.68 (Fig. 3.6 and 3.7B). Maximum amalgamated sandstone measurements vary greatly within the axis of the channel elements studied, with values ranging from 0.1 m up to 12.8 m and an average value of 6.91 m (Fig 3.8A). The sedimentation unit proportions that characterize the axis of a channel element are: 61.4% type one, 23.2% type two, 8.1% type three, 7.3% type four and < 0.1% type five (Fig. 3.9). Based on the metrics tabulated, channel element axes are associated with the best reservoir properties within the studied strata (Fig. 3.6).Channel Off-axis Data Channel off-axis units are characterized by NTG ranging from low to high (0.29 to 1.00) with a mean of 0.81 (Fig. 3.7C). Distribution graphs indicate that > 50% of off-

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

Axis Off-Axis Margin

Intra-Element Architecture

Net

-to-g

ross

& A

mal

gam

atio

n R

atio

s(d

ec)

Max

imum

Am

alga

mat

ed S

S Th

ickn

ess

(m)

Amalgamation Ratio

Net-to-gross Ratio

Max Am. SS Thickness

MEAN METRIC VALUES

Fig. 3.6. Overview of mean quantitative metrics values tabulated from the Tres Pasos Formation.

58

0.10.20.30.40.50.60.70.80.91.0 0.00

5

15

25

35

45

20

10

30

40

0.10.20.30.40.50.60.70.80.91.0 0.00

5

15

25

35

45

20

10

30

40

0.10.20.30.40.50.60.70.80.91.0 0.00

5

15

25

35

45

20

10

30

40

N = 56mean = 0.68median = 0.65

N = 82mean = 0.40median = 0.32

N = 64mean = 0.17median = 0.04

Axis Amalgamation Ratio

Margin Amalgamation Ratio

Off-axis Amalgamation Ratio

Amalgamation Ratio

Amalgamation Ratio

Amalgamation Ratio

Freq

uenc

yFr

eque

ncy

Freq

uenc

y

0.10.20.30.40.50.60.70.80.91.0 0.00

5

15

25

35

45

20

10

30

40

0.10.20.30.40.50.60.70.80.91.0 0.00

5

15

25

35

45

20

10

30

40

0.10.20.30.40.50.60.70.80.91.0 0.00

5

15

25

35

45

20

10

30

40 N = 56mean = 0.95median = 0.98

N = 82mean = 0.81median = 0.81

N = 64mean = 0.39median = 0.38

Margin Net to Gross

Off-axis Net to Gross

Axis Net to Gross

Net-to-Gross Ratio

Net-to-Gross Ratio

Net-to-Gross Ratio

Freq

uenc

yFr

eque

ncy

Freq

uenc

y

BA

C

E

D

F

Fig. 3.7. Histograms of net-to-gross and amalgamation ratios computed from the Laguna Figueroa study area. N = the number of calculated ratios. SD = standard deviation. (A) Distribution of net-to-gross ratio tabulated from channel element portions characterized as axis. (B) Amalgamation ratio distribution tabu-lated from each channel element axis. (C) Distribution of net-to-gross ratio tabulated from channel ele-ment portions characterized as off-axis. (D) Amalgamation ratio distributions tabulated from each channel element portion label off-axis. (E) Distribution of net-to-gross ratios calculated from each channel element margin. (F) Amalgamation ratio distributions tabulated from channel element margins. Intra-element archi-tecture is classified in Figure 3.

59

axis NTG ratios are high (> 0.8), and 98% are above the moderate NTG cut off of 0.5. Off-axis portions of channel elements comprise highly variable AR ranging from 0.0 and 1.00. The mean is 0.40 indicating a moderate degree of amalgamation within this portion of the channel element (Fig. 3.6D). Maximum amalgamated sandstone measurements vary greatly from 0.0 m to 9.65 with a moderate mean value of 3.56 m (Fig. 3.8B). The sedimentation unit proportions that characterize the transitional off-axis portion of a channel element are: 36.1% type one, 30.1% type two, 11.6% type three, 21.4% type four and 0.8% type five (Fig. 3.9). Channel element off-axis intervals are commonly characterized by ideal reservoir metrics (Fig. 3.6); however, the preservation of fine-grained deposits interbedded between sandstone units leads to locally variable vertical sandstone bed connectivity.

Channel Margin Data Channel margin deposits are characterized by generally low NTG with a mean value of 0.39 (Fig. 3.6) and a range from low to high (0.29 to 0.99; Fig. 3.7E). AR is highly variable, ranging from 0.0 to 0.60, with a mean AR of 0.17 although it is typically < 0.1 (Fig. 3.7F). In this case the median (0.04) provides a much closer approximation to a typical AR. Maximum amalgamated sandstone measurements are generally low, characterized by a mean value of 0.93 m (Fig 3.8C). The sedimentation unit proportions that characterize the margins of channel elements are: 9.0% type one, 13.4% type two, 8.9% type three, 65.6% type four and 3.1% type five (Fig. 3.9). The margins of channel elements are of low reservoir quality, dominated by generally fine-grained deposits and low levels of sedimentation unit amalgamation (Fig. 3.6).

Amalgamation ratio vs net-to-grossAR versus NTG is plotted in order to emphasize the distinguishing characteristics

of intra-channel architectural zones (Fig. 3.10A). Overall, channel axis deposits possess both high net-to-gross and high amalgamation ratios, with the contrary characteristic of channel margin deposits where low values for both metrics are expected. Some discrepancies to expected end-member characteristics are notable (Fig 3.10A). For example, the moderate to high NTG in some channel margins is unexpected; however, these values usually reflect very thin or eroded channel margin packages (with low gross thickness) that preserve a high number of thin sandstone beds. As described, low amalgamation ratios in channel axial positions are controlled primarily by the presence of numerous non-amalgamated thin beds in drape deposits (Fig. 3.5). This perturbation in the data is also exacerbated where a significant portion of the channel element top

60

0

5

15

25

35

45

20

10

30

40

55

50

0

5

15

25

35

45

20

10

30

40

55

50

0

5

15

25

35

45

20

10

30

40

55

50

0

5

15

25

35

45

20

10

30

40

55

50

2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.00.00

5

15

25

35

45

20

10

30

40

55

50

0

5

15

25

35

45

20

10

30

40

55

50

2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.00.0

2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.00.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.00.0

2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.00.0

2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.00.0

N = 56mean = 6.91median = 6.75

N = 82mean = 3.82median = 3.67

N = 56mean = 9.86median = 10.10

N = 82mean = 8.19median = 7.68

N = 64mean = 4.56median = 4.00

N = 64mean = 0.93median = 0.32

Axis

Freq

uenc

yFr

eque

ncy

Freq

uenc

y

Freq

uenc

yFr

eque

ncy

Freq

uenc

y

Max amalgamated ss thickness

Max amalgamated ss thickness

Max amalgamated ss thickness Element thickness

Element thickness

Element thickness

Axis

Off-axisOff-axis

MarginMargin

B

A

C

E

D

F

Fig. 3.8. Histograms of maximum amalgamated sandstone thicknesses and element thicknesses from the Laguna Figueroa study area. N = the number of thickness measurements through each channel division. SD = standard deviation. (A) Distribution of maximum amalgamated sandstone thicknesses measured in chan-nel element axes. (B) Maximum amalgamated sandstone thickness distribution (off-axis). (C) Maximum amalgamated sandstone thicknesses measured in channel element margins. Element thickness distributions tabulated from each channel element division: axis (D) axis, off-axis (E) and margin (F).

61

has been removed by erosion at the location analyzed. Despite these discrepancies, it is possible to readily identify fields associated with intra-channel architectural end-members. Because off-axis deposits are intermediate between margin and axis architectural zones, they are more difficult to specifically delineate using these two metrics alone (Fig. 3.10A).

Maximum amalgamated sandstone vs. channel element thicknessMaximum amalgamated sandstone thickness versus overall element thickness

is presented in Figure 3.10B. If an element is preserved completely and not impacted by erosion from successive channel incision, it will be thickest in an axial position, tapering towards the margins (Fig. 3.2A). Likewise, a similar increase in amalgamated sandstone thickness is expected across this same transition. Two distinct data clusters are recognized in the graph associated with the two intra-channel architectural end members: axis and margin. The area between these two zones represents the intermediate off-axis architecture, although off-axis measurements are widely distributed (Fig 3.10B). Perturbations to the expected trends are exclusively due to differential erosion of the upper portions of channels, leaving a partially preserved record of the original channel fill.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Axis Off-axis Margin

Type 5

Type 4

Type 3

Type 2

Type 1

SEDIMENTATION UNIT PROPORTIONS

Fig. 3.9. Bar graph of the relative proportions of sedimentation unit types in each intra-element subdivision: axis, off-axis or margin.

62

Sedimentation unit proportions The proportion of each sedimentation unit type (1 through 5) within a measured interval can also be used to characterize intra-element architectural zones (Table 3.1; Fig. 3.9). Channel element margin deposits are associated with the most unique sedimentation unit signature, composed predominantly (> 65% by proportion) of thinly interbedded sandstone and mudstone deposits (type four sedimentation units) (Fig. 3.9). The thick-bedded sandstone deposits (type one and type two sedimentation units) dominate axis and off-axis architectures (> 65% by proportion; Fig 3.9). Channel element axes are

0

2

4

6

8

10

12

14

16

0 2 4 6 8 10 12 14 16 18 20

Element Thickness (m)

Max Amalgamated SS Thickness (meters)

Max

. Am

alga

mat

ed S

S Th

ickn

ess

(m)

axis

off a

xis

mar

gin

B

Am

alga

mat

ion

Rat

io

Net to Gross

axis

off axis

margin

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

Amalgamation Ratio (dec)

Axis

O� Axis

Margin

A

Fig. 3.10. (A) Scatter plot of amalgamation ratio vs. net-to-gross ratio; data points are classified to identify data fields for each zone: axis, off-axis or margin; see part A in Table 3.2. (B) Scatter plot of the maximum amalgamated sandstone vs. channel element thickness; data points are classified to identify data fields for each zone: axis, off-axis or margin; see part B in Table 3.2

63

impacted by the highest energy during deposition and are therefore dominated by type one sedimentation units (> 60% by proportion; Fig. 3.9), which have erosive bases and preserve the coarsest grained detritus. Off-axis architectural zones are characterized by a higher proportion of flat bedded, non-erosive, type two sedimentation units when compared to channel axis (30% in off-axis, 23% in axis; Fig. 3.9). This is attributed to the waning depositional energy away from the axis of the channelform (Chapter 2). In many instances it is straightforward to qualitatively distinguish off-axis architecture from axis architecture; however, in some cases it is particularly difficult. Sedimentation unit proportions are particularly useful for differentiating axis and off-axis architectural zones in the data collected.

pOtentIal applICatIOns Of results

Predicting/interpreting depositional model from limited well and seismic dataIntegrating this method with traditional subsurface exploration datasets (seismic

and well data) allows for the prediction of deep-water facies at the bed-scale. Seismic reflection surveys are useful for imaging system-scale depositional architecture, including the aerial extent of a deep-water slope channel complex or complex-set. Channel elements represent reservoir-scale sedimentary bodies (commonly < 15 m) thick with complex internal characteristics commonly not resolved in these surveys. Knowledge of critical bed-scale information, such as the distribution and character of shale drapes, is necessary for construction of reservoir models yet only available from core data in many instances. Predicting lateral facies changes beyond the well-bore from limited well data can be facilitated by using metrics data to determine what intra-element architecture zones are intersected by a core sample (axis, off-axis or margin). For example, based on the Tres Pasos Formation intra-element characteristics, if a channel margin is intersected by a wellbore, a substantial reservoir body is likely in close proximity because the transition to off-axis or axial sandstone commonly occurs across < 30 m (Fig. 3.5). It is important to note that thin sandstone beds in margin packages are connected with thicker, more aerially extensive channel axis deposits across the outcrop belt (Fig. 3.1C).

In many cases, discerning element boundaries is difficult in well data; however, their recognition is critical for the application of the metrics data tabulated from the Tres Pasos Formation. Element boundaries derived from outcrop mapping are highlighted in measured section data from the Tres Pasos Formation in Figure 3.11. A variety of intra-element architectural zones from successive channels are discerned in most cases through identification of the drape deposits at the base of each element (see OP2, Sub

64

BB3, and Vaca1 in Fig. 3.11). In areas where the axes of successive channel elements are vertically stacked, element boundaries are difficult to decipher due to erosion of drapes and amalgamation of sandstone units (see OP2, Vaca1 and Cach1 in Fig. 3.11). The preservation potential of basal drape deposits is greater in off-axis and margin deposits due to the lower energy imparted on the channel floor by turbidity currents (see OP2, MM 101, and Cach 1 in Fig. 3.11). This in practice should make identifying them in a subsurface dataset straightforward; however, these channel portions can potentially be very difficult to identify without lateral context (see MM101 and Vaca 1 in Fig. 3.11). All of the intra-element architectural zones can be recognized by an overall upward coarsening trend or upwards decrease in gamma-radiation through an individual channel element in cases where a basal drape is preserved (Fig. 3.12). In practice, evaluating and identifying architectural boundaries from limited subsurface datasets is difficult yet necessary in order to predict channel facies beyond the well-bore.

Incorporation of metrics data into reservoir modelsNumerous reservoirs on continental margins world round, target deep water

channel deposits (e.g., Samuel et al., 2003; Sullivan et al., 2004; Mayall et al., 2006; Porter et al., 2006). The architectures of these reservoirs are captured three-dimensionally in reservoir models by industry geoscientists and engineers for numerous reasons, which include development well planning and reservoir forecasting. Despite the importance of these models, they are commonly limited by a lack of available facies and architecture information. The high-resolution cross-section along the Laguna Figueroa outcrop belt records the architectural detail that should be expected in some deep-water channel reservoirs. The data is logically up-scaled across the transect into fundamental intra-channel divisions (ie., axis, off-axis or margin), providing a perspective of facies distribution (Fig. 3.3). Although it was not an objective of this study, the effectiveness of integrating traditional sedimentological data like that collected from the Laguna Figueroa outcrop belt into a high-resolution subsurface reservoir model has been demonstrated in numerous instances (e.g., Sullivan et al., 2004; Pringle et al., 2011)

Without direct information available for input from cores, reservoir models can be improved through incorporation of robust data collected from outcrop analogues. If the channel complex-set strata examined in this study of the Tres Pasos Formation is deemed an architecturally suitable analogue to a given reservoir, than the detailed metrics tabulated provide reasonable input values for the model constructed (Figs. 3.6-3.10).

65

MM

101

0510152025

Sub

BB3

404550556065

Vaca

1

657075808590

Cach

1

45

50

55

65

70

Elem

ent b

ound

ary

O�-axisAxis

Margin O�-axisMarginMargin

AxisAxis

AxisAxis O�-axis

Axis O�-axis

cm

fvf

slt

mu

d

cm

fvf

slt

mu

d

cm

fvf

slt

mu

dc

mf

vfsl

tm

ud

Axis

OP2

2025303540

cm

fvf

slt

mu

d

45

O�-axis

60

upward coarseningupward coarsening

upward coarseningupward coarseningupward coarsening

upward coarsening

Dra

peD

rape

Dra

pe

Dra

peD

rape

Dra

pe

Am

alga

mat

ed e

lem

ent

Dra

pe

Dra

pe

Fig.

3.1

1. S

ectio

ns m

easu

red

at th

e ce

ntim

eter

-sca

le fr

om th

e La

guna

Fig

uero

a st

udy

area

illu

stra

ting

chan

nel e

lem

ent

base

s, w

hich

are

com

mon

ly d

rape

d by

fine

-gra

ined

byp

ass d

epos

its.

With

out i

nsig

ht fr

om o

utcr

op th

ese

boun

darie

s are

dif-

ficul

t to

defin

e. O

rigin

al n

amin

g sc

hem

e an

d m

easu

rem

ents

are

show

n fo

r com

paris

on to

Fig

ure

3.1C

.

66

suMMary

• 3656 sedimentation units were documented in >1600 m of stratigraphic section, collected through 185 different portions of 18 slope channel elements exposed in an outcrop of the Tres Pasos Formation at Laguna Figueroa.• The quantitative metrics tabulated from this database, can be used to differentiate and characterize intra-channel architectural zones (ie, axis, off-axis, margin).• NTG is highest in channel axes (mean = 0.95), decreases towards the channel off-axes (mean = 0.81) and channel margins (mean = 0.39)• AR follows the same trend, of highest in channel axes (mean = 0.68), decreasing towards the channel off-axes (mean = 0.40) and channel margins (mean = 0.17) • Maximum amalgamated sandstone thicknesses also repeat this gradational trend, with mean measurements of 6.91 m in channel axes, 3.36 m in channel off-axes, and 0.93 in channel margins.• Proportions of sedimentation unit types can also be used distinguish intra-channel architecture, and are particularly useful in differentiating axis from off-axis deposits. • This robust database presents realistic input variables for reservoir models of slope channel deposits and illustrates the utility of this outcrop as a reservoir analog. • Finally, it is essential that the architectural boundaries separating elements be chosen precisely for this method to accurately predict intra-element architecture.

~10

m

~50 m

Gamma-Ray Curves

Fig. 3.12. Schematic cross-section of a representative channel element from the Laguna Figueroa study area. Theoretical gamma ray curves reflect grain-size trends observed in the outcrop belt across the transi-tion from channel axis to margin. Note the overall coarsening upward trend evident, especially in channel margins.

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