Reservoir Delineation by Wireline Techiniquess_Goetz 1977

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Paper

to

be

presented

at

the Sixth

Annual

Convention

of

the

Indonesian

Petroleum

Association

J akarta,

May

1977



3

RESERVOIR

DELINEATION

BY

WIRELINE

TECHNIQUES

J .F .

Goetz,

Schlumberger

Technical Services

Inc.,

Singapore

W.J .

Prins,

Schlumberger Overseas

S.A., J akarta,

Indonesia

J .F .

Logar,

Schlumberger Overseas

S.A.,

Balikpapan,

Indonesia

By

ABSTRACT

Wireline

logs provide complete

records

of

all

forma-

tions

in a well.

Log

responses

are

functions of

lithology,

porosity,

fluid

content

and

textural

variations

offorma-

tions. This

information,

coupled

with

characteristics

of

sedimentary

structures derived

from

high

resolution

dipmeter

surveys,

provide

clues to

the

sedimentary

en-

vironment

and

allow estimation

of

the

reservoir

geometry

and

orientation.

A

set

of

depositional models

is

estab-

lished

for

the

production

geologist.

A

new

technique

designed

to make rapid

and

accurate

multiple

formation

pressure

measurements

confirms

the hydraulic

separation

or

connection

of

reservoirs.

to predict the

location,

geometry

and orientation

of

reser

voir rocks. If the

origin of

a reservoir rock can be deter-

mined,

the

task

is

simplified,

because

reservoirs,

sand

bodies

for

example,

originating

under

similar

conditions

tend

to

have

similar

characteristics.

Therefore

definition

ol

the

sedimentary

environment

is

of

fundamental

impor-

tance,

fable

I

illustrates a

classification

ol depositional

environments. 'Ibis

is

not

the

only classification

which

could

have been

used,

but

it

is

convenient since

it

is

based on

oiigin and

geometry.

CLASSIFICATION

OF

DEPOSITIONAL

ENVIRONMENTS

INTRODUCTION

,

i

BRAIDED

_ «_. _-._.__._._

, L

MEANDERING

CONTINENTAL

I

LACUSTRINE

[

EOLIAN

Wireline

logs

are readily

available,

continuous re-

cords of the

phyiscal

characteristics

of all of the

forma-

tions

crossed

by

all

of

the

wells

in a

field.

As

such,

logs

are

ideal

tools,

not only

for

the

quantitative

evaluation

of

the

fluid content of each

potential

reservoir,

but

also

for the

estimation

of

the

si/c,

shape,

and orientation

of

reservoirs.

Methods

of determining the

geometry

and

trend

of

a

reservoir

are

of

great

assistance not only

to the

development

geologist

but more so to the

production

geologist and

the

reservoir engineer.

The

separation

and

identification

of

the

various

separate

units of

a

complex

sand

aggregation

is

important

knowledge

in

planning

an

optimum

completion

and

also

in workover

and

secondary recovery

operations.

The overall

objective

is

to

help

to

find

and recover

more

hydrocarbons,

more

economically.

DELTAIC i

TRANSITIONAL

I

[SHORELINE]

LINEAR

i

REEF

SHELF

TURBIDITE

PELABIC

MARINE

Table 1.

Classification

of

Depositional

F.nvhonments

ETHOD

Specifically,

the objective

is

to

increase

our ability

r

DISTRIBUTARIES

MARSHES

ESTUARIES

[

MOUTH

BARS

I BEACHES

j

LAOOONS

[

BARS & BARRIER

BARS



4

DEPOSITIONAL

Figure 1. Interrelationships

of

Depositional

Environments.



5

Figure

1

demonstrates

the

interrelationships

between

these

depositional

environments and

roughly

locates

tfiem on

the

continent and

adjacent

shelf.

Although

this

diagram

may

represent

depositional

environments

as we

see them today, it is

not

likely

representative

of the

distribution we

might

find in

the subsurface.

This

is

because

chances

of

preservation

decrease

with

distance to

the

left

of

the

shoreline.

Furthermore, our

interest

in

the

petroleum industry

is not

evenly

divided

over

all environ-

ments,

but tends

to concentrate

on

shoreline

and near

shoreline

deposits.

Figure 2

suggests

an integrated

approach

necessary

for

the

delineation of

reservoir

geometry;

observe,

interpret,

predict.

Through observations of items

in

the

left

hand

column,

we

attempt

to

interpret

the

sedimentary en-

vironment and

paleogeography,

and

then

predict

the

geometry

and trend

of

reservoir

rocks.

Figure

2.

Illustration

of

the

basic

approach

to

reservoir

geometry

delineation (modified

from

Selley).

Perhaps

the

most

important

item

to

observe is

the

type

of

fossils and

microfossils

present

since these

may

produce

the most

unequivocal

evidence

of

depositional

environment.

The

main assumption is

that the fossil lived

in

the

environment

where

it

was

buried. The identifica-

tion of fossils

can

not,

of

course,

be

performed

by

wire-

line

measurements

and

is

therefore

beyond

the

scope

of this

paper.

Lithology can

be

measured

efficiently by multiple

porosity

log

techniques.

Textural

sequences

affect the

readings

of

most logs

and

are

therefore

deduced by

changes

in

measured

parameter

versus

depth,

i.e.,

curve

shapes. Sedimentary

structures,

mainly in

the

form

of

current bedding,

are

measured

by

means

of

high

resolu-

tion

dipmeter

surveys.

The

directional

properties

of

this

current

bedding

are

measures

of

paleocurrent

directions.

All of

these observations should

be

combined

and

com-

pared to

recent

sediments or to a

set

of

models,

to in-

terpret

the

depositional

environment.

This

being

done,

the

directional

parameters

as

measured

by

the dipmeter

are

added

to

predict the

geometry

and

orientation

of

the

reservoir.

Finally,

multiple precision

pressure

measurements

are used

to

verify

the

hydraulic

connection

or

separation

of individual

reservoir

units,

both on a

single

well

and on

a

field basis.

RESERVOIR

GEOMETRY

BY

WIRELINE

METHODS

LITHOLOGY

The porosity

-lithology

logs,

Density, Neutron

and

Sonic,

are

sensitive

not

only to porosity

but

also to

fluid

content

and

rock matrix material. Combinations

of these

logs,

however,

along

with

a

value

of

the

flushed

zone

resistivity,

Rxo,

can be

used to

derive

true

porosity

and

matrix

material

density,

thus defining

the

rock

matrix.

In hydrocarbon

bearing

formations,

this

requires

a

complex

computation.

But in

clean

water

bearing

formations,

the

procedure

is

simplified

to

entering

bulk

density,

neutron

porosity,

and

transit time readings

on

cross

plots

of/*b

vs

0N

and A t

vs

0N

to

obtain values

of

apparent

matrix

density, (

t° ma)a,

and

apparent

matrix transit time

(

_.

tma)a. See Figures

3

and

4.

These

values

are

then entered on a Matrix Identification

(MID)

Plot to

determine

the rock

matrix.

See Figure 5

4

A

Density-Neutron

recording,

when compatibly

scaled

for

limestone,

offers

a

very

quick

and simple

way

to

check

lithology. Limestone

compatible

scaling

means

that

the Neutron Log

is

scaled

in

terms

of limestone

porosity units

at 30 p.u.

per

track

while the

Density

Log

is

scaled

in

grams per cubic

centimeter

at

0.5

gm/cc

per

track,

with the

2.70

gm/cc

point

positioned

to

coincide

with the

zero of

the Neutron

scaling.

With

this

scaling,

the

curves

should

be

almost

coincident

in a clean

water



6

MID

Chart

i

DETERMINAT ION Or

(

r'

ma

)

,,

FROM

FDC

*

NEUTRON

LOG

(

CNL,

fresh MurJ

)

NFUTI'ON

1.1.X

I

Api_r.nl

|„

n.

P . 0 .t

y

)

Figure

.

.

(.rossplot

for

determination

of

(Pniala

from

CNL

Neutron tnd

I-

DC

Density

oi<

M I

[..

(-,._.

i

H» MATRIX

II)[

N:lflf

AI

UIH

'MIO)

P j

ol'

_4__

70

0

50

A,

rn«)a,

fi»*c/li

80

Figure

J .

?*«"

*(flm_

Identification

(MID)

Plot. The

plot is

entered

with

(

rma)a

values

from

Figure

3,

and

{&tmaki

values

from

Figure

4,

for

mineral

identification.

MlO

Ch.i.l

2

Ot

RUMINATION

Of(/_

ma

)a FROM SONIC

LOG

4

NEUTRON

LOG

(CNL)

NFUTRON

INDEX

Upfront

Limwton.

P_romit»)

Figure 4.

Crossplot

for

determination

of

(A

tma)a

from

CNL

Neutron and

Sonic

Logs.

GA_MA

W BENSITY NEUTRON

RECORDING

Figure 6.

ldealhed

Density

Neutron

response

for

some

common

litbologies.



7

bearing limestone and

should

show

a

"positive"

separation

of

about

2 divisions in

a

clean water bearing

sandstone

and a

"negative" separation

of

about

5

divisions

in a clean

water

bearing

dolomite.

The response

for

these

and other

common

lithologies is

shown

in

Figure 6.

Care

must be

taken

to recognize hydrocarbon

bearing

intervals

since

the presence of

light

hydrocarbons

will

also

produce

a

"positive"

separation, in

which

case,

a

complete

flushed

zone

saturation

solution is a

lequisite.

Perhaps the easiest method

for

identifying lithology

is

by reading

the

"Average

Grain Density" curve

on

the

output

of

a

Computer Processed Interpretation,

"CPI".

This

curve,

recorded

in the

first

track,

is

an

end

product

of the CP I

program,

CORIBAND*.

The value reported

is

the

average

density of

the

reservoir

rock

matrix

material

and the

clay

content.

Therefore

in clean

formations this

curve

reports

the

grain

density

of

the reservoir

rock,

thus

identifying the lithology in

most

common cases.

In all

of

these

methods,

it is essential to

recognize

and

allow

for

the

effect

of

clay

content.

Clay

tends

to

produce

a

"negative"

separation

on the

Density-Neutron

log

as

illustrated

in

Figure

6.

Usually,

the Gamma Ray

curve can

be used as

a

first

approximation

of the

clay

content,

and

an

appropriate

correction

can

be

effected.

TEXTURAL

SEQUENCES

Textural

sequence

is

related

to

variations

versus

depth of

grain

size and

sorting.

Grain size of

a

clastic

rock

is

a

function of the

incoming

material

and the

deposi-

tional

energy.

Sorting

is

mainly

a

function

of

deposi-

tional energy. The

pattern

of

variations of

depositional

energy

versus depth

within

a

genetic unit is

characteristic

of

one,

or a

few,

specific

depositional

environments.

There

are,

of

course,

no

direct measurements

of

deposi-

tional

energy. Nor are

there

direct geophysical

measure-

ments

of

sorting

or

grain

size,

but

some

inferences can be

made.

We

can

relate porosity and permeability

as

func-

tions

of

sorting,

grain

size and

clay

content, Vcl.

Porosity

=

f

(Sorting,

1

/Vcl

) (I)

,-

,

,■ ,■

.

,

,

■

v h

' '

v

'

l'tgure

7. Recording

of

a

Computer

Processed

Inter-

pretation

(CPI).

Average

Grain

Density

curve

is

in the

Permeability

=

g

(Sorting,

Grain Size.

l/Vcl)

(2)

w

trackm Pormitv

and

clay

Volume

curves

are

left

hand

track.

Porosity and

Clay

Volume

curves

are

plotted

in

the

right

hand

track.

In

addition,

both

equations

should

include the distribu

tion

mode

of the

clay, as well as the

effects

ofdiagenesis

and

fracturing

but

these

are

ignored

in

this simplified

discussion.

♦Trademark of Schlumberger



8

With

modern

logging techniques

porosity

can

be

measured

very

accurately

and

clay content can be deter-

mined

with

reasonable

accuracy.

If there existed a

direct

geophysical

measurement

of

permeability,

these

equations

could

be

solved

for sorting

and grain size

after

establishing

empirical relationships.

Unfortunately

at this

time

per-

meability

cannot

be

measured

directly.

However,

by

making one

assumption,

qualitative

variations

of

sorting

and

gram size

versus

depth

can

be

recognized.

That

assumption is that

clay

content

increases

with decreasing

grain

size

of

rock

matrix

material.

Observation

tends to

bear

out this assumption.

Our

formulae

can

then

be

simplified

and rearranged.

Sorting

-

h

(Porosity,

l/Vcl)

(3)

.(4)

Grain

Size

-j

(l/Vcl)

Fquations 3

and

4 relate

porosity

and clay

content

to sorting

and

grain

size,

or,

by

inference,

to

depositional

energy.

Therefore

variations

of porosity

and clay content

versus depth

represent

variations

of

depositional

energy

and

such

variations

provide

clues

to

the identification

of

the

depositional

environment.

Porosity

is

measured

continuously and

quantitati-

vely by the

Density-Neutron-Sonic

techniques

for

any

lithology.

The

volume

of

clay

affects most

logging

tlevices to

some degree

and therefore

a

number

of

devices

can be

used

to

measure

it.

For

example, the Density-Neutron

crossplot is a reliable

clay

indicator when the

lithology

is

known

and

the

effects

of

light hydrocarbons

are

re-

cognized.

The

Spontaneous

Potential,

or SP curve is often

used

as a

clay

indicator, however,

its

sensitivity

depends

on

the

contrast

of fluid

salinities

in the

formation

and

the

borehole,

a problem in

many

offshore areas.

Further,

the

SP is

subject to distortion

from

hydrocarbon

effects

and numerous

sources

of

noise.

The

Gamma

Ray

curve

is

another

commonly

used

clay

indicator

but

its

quant-

itative

use

may be

adversely affected

by

the

presence of

glauconitc,

mica,

feldspars,

and

detrital

zircon.

The

Computer Processed Interpretation

system

uses a total

of seven clay

indicators,

all

of

which

are designed to

either

give

a

good

estimate

of Vcl or to

overestimate

it.

The program then

chooses

the

minimum

of

these

seven

values

as

being the value

most

representative

of

Vcl.

An

example

is

shown

in Figure

7,

in

which the Vcl

curve

appears

in

the

right

hand track of the

CPI.

Specific

depositional environments

exhibit

charac-

teristic

sequences

of

depositional

energy

with

time,

resulting in

characteristic

profiles

of

grain size

and

sorting

versus

depth. Since these

profiles

are

measured by

poro-

sity

and

Vcl,

recognizable

patterns

of porosity and

Vcl

curves

versus

depth

should

be

interpretable

in terms

of

depositional

environments.

Log curve shapes

of

potential

reservoirs

have

been

categorized

according

to their

appearance

as

follows:-

12

a)

cylindrical

(abrupt

lower

and

upper

contacts),

b)

bell shaped

(abrupt

lower

contact

and

gradational

upper

contact

)

g)

funnel shaped

(gradational

lowei

contact and

abrupt

upper

contact)

d)

combinations of the

above,

such

as

funnel-hell

shaped

(gradational

lower

and upper

contacts)

Further descriptive

terms

are added

such

as

smooth,

serrated,

concave, convex,

linear.

These

shapes

are

sum-

marized

in

figure

X. In

this

classification,

a bell

shaped

curve,

lot

example, would

suggest

a

transgressional

sequence

or

;i

fluvial

channel

fill,

while

a funnel

shaped

curve

would

suggest

a

regressive

sequence

or a

bar

type

deposit.

A cylindrical

curve shape

often

represents

a

delta

distributary channel fill.

Comparison

of

porosity

and Vcl

curve

shapes to those

Irom a

set

of

models

will

assist

in

the

definition of the

depositional

environment. When

combinations

of the

main

categories

are

used,

by

conven-

tion

we

start

from

the bottom

surface

of the

bed

and

move

upwards

emulating

the

direction

which

the

bed

was

deposited. For

example,

a

cylindrical

bell

de-

scription

means an

abrupt lower

contact,

cylindrical

lower

portion

and a

gradational

upper

contact.

SEDIMENTARY

STRUCTURES

Ihe

term

"sedimentary

structures"

covers a wide-

range

of

phenomena.

These

can

be subdivided

into pre-

depositional,

syn

-depositional

and

post

-depositional se-

dimentary

structures.

Some

sedimentary

structuies

can be

effectively

logged

by

means of the high

resolu-

tion

dipmeter

survey.

Pre-depositional

sedimentary

structures

are

those

observed

on

the

underside

of

a bed.

These

include

ero-

sional

features,

scour

marks,

flute

marks,

ripple

marks,

mud

cracks,

worm

burrowings,

grooves

and

channel

cutting. Of

these,

only

channel cutting may sometimes

be

recognized

by the dipmeter.

The

others

tend

to

pro-

duce

inconsistent,

and

confusing

dipmeter

results

(as

they also

confuse core

examinations).

Syn-depositional

sedimentary

structures

are

those

occurring

within

tire bed

and mainly

take

the form of

crossbedding

or

current

bedding.

Our

interest

here

is in

the

magnitude

of

current

bedding

angles, their

character-

istics

such

as

whether

the

current beds

are

planar

or

festoon

type,

and

their

variations

versus

depth.

These

factors

provide

clues to

the

depositional

energy.

It

is





10

probably

true

that

the

larger

the gram size, the

steeper

the

current bedding,

also,

the

larger

the

grain

size, the

greater

the depositional energy.

Therefore,

by

deduction,

steep

current

bedding

is

usually

interpreted

as indicative

of

high

energy deposition.

This

rule

usually holds when

deposition

occurs in a

place

away from

the

transporta-

tion

artery

such

as

in

a

delta

front

or when deposition

is

associated

with

ocean

wave

energy.

However,

this

rule

could

be

quite

wrong

when

deposition and tran-

sportation

occur

simultaneously,

as

in a

channel,

where

the

highest

energy

may

produce

the

flattest,

even revers-

ed,

current

bedding.

Figure

(

>

demonstrates

the

basic

differences

between

festoon

and tabular current

bedding.

its

counterpart

"sag"

can be measured

by

the

dipmeter

and can be

extremely

diagnostic.

As

with curve shapes, comparison of sedimentary

structure

patterns

to those

Irom

a set of models

will

facilitate

the

interprelation.

PALEOCURRENT PATTERNS

Tabular-

PSanar

Festoon

-

Trough

Figure 9. Comparison

of festoon

(trough)

and

tabular

(planar)

current

bedding

(After

Pettijohn, Potter,

and

Si

ever).

Specific

sedimentary

environments

give

rise

to

characteristic

patterns

of current

bedding

dips versus

depth.

Such

patterns

seen

on the

dipmeter

plot can

be

used to

help identify the

depositional environment.

For example,

most bar

type

deposits will exhibit a high

dip

spread

in

the

upper

part,

decreasing to a low

spread

near

the

base.

Post-deposttional

sedimentary structures are

those

observed

on

the

top

side of a bed.

These

include

load

casts,

quicksand

structures,

movement

by slump

or

creep

all

of

which

tend to

confuse

dipmeter

results.

However

"drape"

due

to differential compaction,

and

Paleocurrenls

are,

of

course,

not directly obser-

vable;

we

are

a lew

million

years

too

late

tor

thai. How-

ever,

the

orientation

of the current bedding

is

measured

using

the

dipmeter, and

that

direction

is

interpreted as

being

the

direction

of the

paleocurrent

(after

removal

ol

structuial dip).

This

represents

the

direction of the

depositional

current

and is a measurement

which

cannot

inherit

features from

outside

the actual

site

ofdeposition,

'I

he

variability

in

direction

reflects

the

vagaries

of the

depositing

currents.

CURRENT

BEDDING EVALUATION

A-

CHARACTERISTICS

Ssyn depositional sedimentary

structures]

TYPE

[festoon,

tabular)

*

ANGLE

PATTERN

SPREAD

B-

ORIENTATION

(pileocurrent

patterns]

DIRECTIONAL

MOOES

*

CATTER

DIRECTIONAL

RELATIONSHIPS

»)

to

paleosiope

bi

to

sand

unit

geometry

*

as

seen in

borehole,

not

large

scale

or

regional

Table 2.A

guide to

current

bedding

evaluation.



11

TABLE OF

CURRENT

BEDDING

CHARACTERISTICS

& ORIENTATION

CURRENT BEDDING

CHARACTERISTICS

CURRENT

BEDDING

ORIENTATION

DEPOSITIONAL

ENVIRONMENT

UNIMODAL LARGE SCATTER

(90°)

GENERALLY DOWN

PALEOSLOPE

DIRECTION

OF

SAND

ELONGATION

BRAIDED STREAM

ALLUVIUM

FESTOON

(TROUGH)

TYPE

LARGE

DIP

SPREAD

FESTOON

(TROUGH)

TYPE

LARGE

DIP

SPREAD

UNIMODAL SEVERE

SCATTER

(

1 80

)

MEANDERING

STREAM

POINT BARS

HIGHER ANGLE

AT

BASE

GENERALLY

DOWN

PALEOSLOPE

LOW

ANGLE

TABULAR AT TOP

DIRECTION

OF MEANDER BELT &

SAND BODY

ALIGNMENT

TABULAR HIGH

ANGLE

(30X

EXTREMELY

CONSISTENT

UNIMODAL

LITTLE SCATTER

NO

RELATION

TO PALEOSLOPE

NORMAL

TO

SAND

ELONGATION

EOLIAN DUNES

DECREASING

ANGLE

Al

BASE

UNIMODAL.

-

MODERATE

SCATTER

IN SEAWARD

DIRECTION

DELTA

DISTRIBUTARY

CHANNELS

FESTOON

TABULAR

HIGHER

ANGLE

AT

BASE

MODERATE

SPREAD

DIRECTION

OF SAND ELONGATION

TABULAR

MODERATE

ANGLE UNIMODAL

RADIATING

SEAWARD

DIRECTION

BUT

DISTRIBUTARY MOUTH

BARS

010°

)

INFLUENCED BY LONGSHORE

CURRENTS

HIGHER

ANGLE

AT

TOP

MODERATE

SPREAD

-

DIRECTION

OF

SAND ELONGATION

(LOBATE)

BIMODAL

(180°)

-

SCATTERED

NORMAL

TO COASTLINE

ESTUARINE

&

TIDAL

CHANNELS

TABULAR

LOW

ANGLE

(10°)

HIGHER

ANGLE

AT

BASE

FLATTER

AT

TOP

DIRECTION

OF

SAND

ELONGATION

UNIMODAL

POSSIBLY

BIMODAL

USUALLY DOWN

PALEOSLOPE

TABULAR

EACHES

& BARS

LOW

ANGLE

ON

SEAWARD SIDE

(

<OX

BUT

POSSIBLY

REVERSED

NORMAL

TO

SAND ELONGATION

IGH

ANGLE

ON LAGOONAL

SIDE

(

>

20

°

)

TABULAR

POLYMODAL RANDOM

MARINE

SHELF

SANDS

VERY

LOW

ANGLE THROUGHOUT

TABULAR

OR ABSENT

UNIMODAL

URBIDITES

DOWN

PALEOSLOPE

ERY LOW

ANGLE

THROUGHOUT

RARELY

OBSERVABLE

DIRECTION

OF

SAND

ELONGATION

terms

of

dip

spread

and

dip

se-

i

related to paleosiope

and sand

Table 3.

Listing

of

current

bedding

characteristics

in

i

quences versus

depth,

and current

bedding

orientation

body

geometry

for

several

depositional

environments.



12

SUMMARY

OF

EVALUATION

OF

SEDIMENTARY

STRUCTURES

AND

PALEOCURRENTS

The

geometry

of

a clastic

unit

is

related

to its

inter-

nal

structures

which

are

functions

ol

its

depositional

environment.

After data on

lithology.

textural sequences

(from

curve shapes), and current

bedding

patterns

are

combined

to

define the

depositional

environment,

draping

effects and paleocurrent

directions

are added

to

predict

the

geometry

and

orientation

of

the

reservoir.

A guide

to curient

bedding

evaluation

is

shown

m

Table 2. To evaluate current

bedding,

its

characteristics

(type,

angle,

pattern,

spread)

and its

orientation

(direc-

tional

mode,

scatter,

relationships)

are

considered

se

paratelv.

[allies

of

current

bedding

characteristics

and

01-

-ientation

are

presented

as an

aid

to

a systematic

analysis

in

1

able

s

At

this

point,

a

cautionary

note, should be sounded.

In

atteiiiptiii '

io evaluate

current

bedding

by

means ol

the

high

resolution

dipnielci

technique, it

is essential

to

appreciate

the

limitations

involved.

Although

the

111)1*

dipmetei tool

itself

is

capable of

reemding

a very high

degree

ot foimation

detail,

much

of

this

detail

may

be

lost

in

the

compulation

technique.

Indeed,

foi

struc-

tural

dip

applications

the

objective

is to

average

out

much

ot

the

anomalous

detail

in

order to obtain

results

which are

representative

of the

most

repetitive

resistivity

features

and

theieloie

most

indicative of

structural

dip.

Nol

so

in

stratigraphic applications;

here we

are

interested

m the

maximum

amount

of

detail,

however

inconsistent.

Most

automated

dipmeter

compulation

techniques

utilize a

system

which chooses

a discrete depth

interval

on

one

icsistivitv

curve

and

attempts

to

correlate

that

interval with

an

interval

of

equal

length on the oilier

curves

Of

necessity,

this

approach

lends to

aveiage

the

dip

information

within

the

correlation

interval.

To

com-

pute

the

dip

ot

anient

bedding,

it is necessary to

shrink

the correlation

interval

to lengths

sufficiently short

to

enable the

correlation

function

to

recognize

cross

beds.

This can

be

done,

but it

must be

realized

that

the

occurrence

of

spurious

results

or

noise

increases

as

the

length

of the correlation

interval

decreases. Such

noise

is

usually

due to

microrugosity of the

borehole wall.

The optimum

length

of correlation

interval

is a

function

of

the

current

bedding

thickness and

the

borehole

condi-

tion,

Tli

is

length

would

normally be

a

compromise

which

could

be

determined

only

by

trial

and

error

for a

given

depositional

environment.

A

new computation

technique

called

GEODIP*,

which

is

now

available,

is

designed specifically

for stra-

tigraphic dipmeter

analysis. This technique

analyzes

each

curve

and

attempts

to

recognize

certain

curve

charac-

teristics

or

patterns.

Correlations

are then made

on

these

characteristics which

will

usually be

representative

of

individual

geological

features

such

as

bedding

planes.

Such

a

system, therefore,

is geologically rathei

than

mathematically

oriented,

and much better suited

to the

measurement

of

current

bedding.

A second point

ol

caution arises from the

fact

that

the

dipmeter is a wall contact

type

tool and like

all

wall contact

tools,

it

is sensitive

to

borehole

rugosity

and microrugosity,

Microrugosity,

small

variations

in the

borehole wall

of the orilei

ol one

halt inch

in depth

and

wiih

a period

ot

six inches

to

two

feet,

is

nest

recognized

on

the

cahpei

curve

recoided

with

the

Formation

Density

log.

All

types

ol

hole wall

rugosity

are

deleterious

to

dipmeter

precision.

For

this

reason,

a

prudent

interpreter

correlates dip data

with

borehole

caliper

characteristics.

Fhirdlv.

stratigraphic

interpretation

attempts

to

recreate conditions

present

at

the

time

of

deposition. To

obtain

a

valid

picture

of dip

relationships at

the

time

of

deposition,

it is advisable

to

remove the

effects

of

post-

depositiunal

structural

[ill.

This

is achieved

easily

by

identifying

structural

dip

and

subjecting

all

dip

results

to

a rotation

equal

and

opposite

to the

structural

dip.

SE T OF

SEDIMENTARY

MODELS

I'he

science

ot

sedimentologv

is

extremely

complex.

indeed,

so

complex as to

discourage

efforts to

orga-

nize

it.

Yet,

detailed

studies

of

ancient

sedimentary

fades

reveal

that there

exist

recurring

patterns

of sedimentation

in

the

geologic

past.

These

patterns suggest

that instead

ol

a

vast

complexity

of

sedimentary

events that do

not

permit

generalization, there

are only

a

relatively

tew

ma|or types

of

sedimentation

patterns.

Some

of

these

patterns

can be represented

by

a

set of

models which

will

serve

as a basis

for

comparison.

The

following dis-

cussion develops

such a

set,

based on a

compilation of

actual

field

examples.

These

models

illustrate

both

porosity and

Vcl

curve

shapes as well

as the

curient

bedding

patterns.

In

case

of

a

conflict

between

evidence supplied

by

these two

approaches,

current bedding

patterns

should

be

allowed Io

overrule

curve

shapes.

This

extends

to

the

determination

of the

boundaries

of

genetic

units,

that

is,

units

coherent

in

terms

of

depositionalconditions.

Some-

*

Trademark of

Schlumberger



13

Figure

10 Origin

of

alluvial

sub-facies

deposited by

braided rivers. (After Selley)

times

the

incoming

material

may

change

while the same

depositional conditions persist, with

the result

that

lithological

unit

boundaries

may

not

match

those

ol

genetic

units.

One genetic

unil

may

be made

up ol more

than

one

lithological

unit

or vice versa. Interpretation

involving

sedimentary

structures

is

based

on

genetic

units.

BRAIDED

STREAM ALLUVIUM

Braided stream deposits are

the

products

of

an

interlaced

network

of

low

sinuosity

channels

exhibiting

flood

stage

scouring

and subsequent filling. Such streams

usually occur

on relatively

steep

gradients

where large

amounts

of sediments are available.

Fiosion

is

rapid,

discharge

is

sporadic and high. Because

of these

factors,

streams are generally

overloaded

with sediment.

A

channel

is no

sooner

cut than

it

chokes on its own detritus.

This is

dumped

in

the form of bars in the center

of the

channel

around

which

two

new

channels

are

diverted.

Repeated

bar

formation and channel

branching

generates

a

network

of

braided

channels

over

the entire deposi-

tional area. Individual bars

may

be

destroyed during

each

flood

stage.

Figure

10

shows

a

typical braided

stream

environment.

Braided

stream

alluvium

is

typically

composed

of

moderately

sorted

sand and

gravel

deposits

to the

ex-

clusion

of silts and

clays.

Because

of

repeated

channel

switching,

there aie

few

typical

"'fining

upward"

channel

sequences.

Braided

stream

deposits show

little

variation

either

vertically

or

laterally.

Both porosity

and permea-

bility are

high, forming

excellent

reservoirs.

The

amount

of silt

which

is

present

is generally

deposited in

aband-

oned channels.

This textural

sequence

gives

rise

to

serra-

ted

cylindrical

curve shapes.

Water

How during

deposition

is

highly turbulent

resulting in trough

or festoon

type

current bedding.

Dip-

meter

results can

be expected to

be

erratic

in

both

dip

angle and direction

because

the

relatively

small borehole

will

encounter

non-planar

bedding

surfaces

and

incom-

plete deposit

ional

sequences.

Dip

angle will

probably

vary sharply between

zero

and

35

while

direction

may

vary

up

to

lSO"but

will

probably

remain

within aX arc

which should reflect

the downstream

direction

and

the

direction

of

elongation

of

the sand

body.

Planarity

rating

of

dip

results

will

be

low.

Typical

curve

shapes

and

dip

patterns

for a

braided

stream

deposit

are shown

in

Figure 1 1.

MEANDERING

STREAM

POINT

BARS

High

sinuosity

meandering river

channels

typically

develop where

gradients

are low

and sediment

availability

is

relatively

low.

The

meandering action

results

from

erosion

on

the

outer bank

and

deposition

on

the

inner





15

Figure 12.

Meandering

Stream Fnvironment

(After

R.

J

. I.

e

ßlanc)

bank ol each curve. The stream bed thus

migiates

lateia-

lly. Most

deposition occuis

during

flood

stages

in

the

foi

in

of

point

bars

which

exhibit

a

chaiacteristic

sequence ol

grain

size

and

sedimentary

structures. At

the base

is

an

erosional

surface

overlain

by

pebbles

and

a

sequence of

sands

with a general

upward

decrease

in

gram

size.

Co-

arse lesioon crossbedded sands

grade

up into tabulat

crossbedded sands

of

diminishing

set

height. These in

turn

grade

into flat bedded line

sands

and then

into

silts. Cut

off

meander channels

form

ox-bow

lakes

which

fill

iir

with silts and

clays.

The

reservoirs

formed

do

not

take the

form

of

the meander channel

but rather that

of

curved,

tabular wedges of

sand

occupying

a

large

portion

of

the

meander

belt. These

may

be

separated

from each other

by

abandoned

channel

fades.

Ibis

meander belt

can

be

up

to 20 tunes the

width of

the

stream.

Figure

12

(after

R.

J .

LeBlanc)

demonstrates

a

meandering

stream

environment.

Repeated

reworkings

of

the deposits

within the

meander

belt

winnow

the line grained

material and

result

in

a

progressive

downstream

decrease

in

grain

size.

Such reworkings also

result in interrupted

sequences

and the stacking

of

severaJ of the basal coarse grained

parts.

'The

textural

sequence

is

basically

fining

upwards

with

a coarse grained

/one

of variable

thickness at

the

base.

This presence

gives

rise

to concave bell shaped

log

curves with a

cylindrical lobe at

the

base and serrated

in the upper

part-

Dip magnitude will

be

erratic

and high angled at

the base

in

the

festoon

bedded

sands,

progressively

becoming more

consistent

and

(latter

upwards in the

tabular

beds.

Because

of

the wide

swing

in the direction

of

the depositing

currents and the

type

of

current bed-

ding,

a variation

in

dip direction

of

180°

would

be

expected. But

statistically,

the

average dip direction

should

reilect

the overall downstream direction

of

the

meander belt and

the trend

of

the

separate

reservoirs.

Typical

curve shapes

and

dip patterns

for

a

mean-

dering

stream

point

bar

deposit

are shown in figure 1.

.

EOLIAN

DUNES

Wind blown dune deposits are

often

difficult

to

distinguish

from

those

laid down

by

water.

The

mechan-

ics

of

both

processes are

quite

similar. Although we

normally think of dunes as occurring in a

desert

environ-

ment,

dunes

often

form

on

beaches

and barrier

bars,

Tims

some

sediments

may

go

through

a

wind

phase

before

being finally depositedby

water.

Generally,

eolian

sands are better sorted

than

aqueous

ones,

leading

to uniformly

high

porosities

and

permeabilities. This

makes them excellent potential

reservoirs

if

they

come

in contact with source

rocks.

Curve

shapes are normally

cylindrical

with

possible

funnel

shaped

bases.

As dunes

migrate, sand

grains are carried

up the



16

ME ANDE RING STR EAM

POINT

BARS

Figure 13. Curve

shapes

and

dip

patterns

on

a

meandering

streampoint

bar

accumulation.

Although

no

perfectly

clean

zone

exists,

the

Density-Neutron plus Gamma Ray

suggest

sandstone. Porosity

and

Vcl

curves

are

of

the

cylindrical-serrated

hell

type

and

indicate good sorting in the bottom

with

sorting

and

gram

she

decreasing

upward.

Dip magnitude

and

scatter

suggest

decreasinggrain

she

upward

andprobably indicate

festoon

current

bedding

the

lowerzone



17

windward slope

and then

roll

down the

slip

face. This

results

in

crossbedding

of

enormous set heights.

As

seen by a

borehole,

this

crossbedding

is

tabular, high

angle and consistent in

magnitude

through

the

height

of

almost the

entire

dune.

Consistent

crossbedding

for

intervals

of

up

to 100 feet is

not

uncommon and it is

this

characteristic that

distinguishes

eolian

sands

from all

others.

The average

direction

of

the crossbedding

is in

the direction of

the

wind,

but

depending

on

the

type

of

dune,

there may be variations of

up

to 90° .

Figure 14

illustrates the

relationship

between crossbedding

direct-

ion and wind direction for

transverse,

barchan,

and

seif

dunes.

It is

to

be

noted

that

irrespective

of

the

type

of

dune,

the average crossbedding direction is roughly

normal

to

the direction

of

elongation

of

the

dune.

Figure

14.

Diagrams showing

relationship

between dune

morphology

and

crossbedding orientation.

Wind

blows

up

thepage, (After

Selley).

Figure

15

shows

the

curve shapes

and

dip

patterns

on

a

series

of

dune deposits.

The extreme consistency of the

dip angle and direction

over

long

intervals is

characteris-

tic

of

eolian dunes.

It is

also

characteristic

that

indi-

vidual dunes as seen in a

borehole,

bear

no

relationship

to

one

another

nor to

the

paleoslope.

DELTA

DISTRIBUTARY

CHANNELS

The

meandering

streams

of

the plains

areas

grade

into delta distributary

channels in

the

exposed

delta

areas.

These

channels

are

relatively

straight

and

are cut

into

young,

soft

sediments.

Natural

levees formed of

clays

and silts tend to

contain

the

channel in

a

fixed

position.

Such channels experience little

seasonal

variation in the

level

ofwater

they

carry.

By

various

mechanisms such as

blockage of

the mouth

or

bifurcation

upstream,

velocity

may drop and deposition

will

occur. The depositing

material is

relatively coarse grained

and moderately

well sorted. Normally more coarse

material

will

be

found

at

the

base

and

there

will

be a general fining upward.

However,

in many

cases,

the

entire channel

tends to

be-

come

clogged

with

fairly

uniform

sands. These give

rise

to

characteristic

cylindrical

curve shapes

possibly

grading

into

bell

shaped

at the

top.

The infilling

of

a

distributary channel may

be a

rapid

process

and

there

will be no

further

reworking

of

the infilling sediments.

Current

bedding

will

therefore

reflect

stream energy and direction at the time of de-

position. Current

bedding near the base

is usually of

the

festoon

type.

Dips

measured

are

likely

to

be fairly

erratic

near

the

base,

sometimes grading

upward

into more

consistent

dips near the

top.

The

direction ofthe channel

and thus the direction

of

sand

elongation, is

given

by

the

average of

the

current

bedding

directions.

In

the

U.

S,

Gulf

Coast,

a

second

mode

of

distri-

butary

channel

dips

has

been reported.

This mode pro-

duces

a

pattern

of

increasing

dipswith depth

within the

channel,

these

dips pointing

at

right angles

to

the strike.

These

relatively

low angle

dips, when

observable,

are

probably due to pre

-depositional

sedimentary

structures

in

the

form ofchannel

cutting, and

arise

from

the

channel

base changing in

a

series

of

progressively

shallower

con-

cave

surfaces

as

infilling proceeds.

Unlike

braided

or meandering stream

deposits,

which

tend

to

be quite

wide due to lateral migration

of

the

channels,

distributary

channel fills

will

produce

long,

narrow

reservoirs,

often

with

very

thick

sections.

Figure 16 shows curve shapes

and dip

patterns

for

a

delta

distributary

channel.



18

EOLIAN DUNES

Figure IS.

Curve

shapes

and

dip

patterns

on

a

series

of

eolian

dunes.

Lithology

is

shown to

be

sandstone by the Density-

Neutron.

Porosity and

Vcl

curve

shapes

are cylindrical

demonstrating excellent

sorting

and consistent

grain

she

throughout.

This

consistency

of

grain

size is

confirmed

by the dipmagnitude

which is

extremely

uniform.

Dip

angles

drop

to

structural

dipat the

base

of

each

dune

in

a

pattern

of

foreset

bedding.



19

DELTA DISTRIBUTARY

CHANNEL

FILL

Figure

16. Curve shapes and dip

patterns

on

a

delta

distributary

channel

fill.

Density-Neutron

response in the clean zone

defines

sandstone lithology. Porosity and

Vcl

curve shapes

are basically

cylindrical

with

a

short serrated hell shape

at

the

top.

The porosityrange

suggests

moderately well sorted

sand.

Dipmagnitude

and spread

indicate fining upward.



20

DISTRIBUTARY MOUTH BARS

Figure

17

illustrates

three common forms of deltas.

The form developed

by

a

delta depends on

the sediment

load

and the

leiative

strengths

of

fluvial

and

marine

processes. Where

river

currents

clearly

dominate,

a

highly

constructive

birdslool

delta,

such as

the modern

Miss-

issippi

will

form.

These are characterized by

elongate

bar fingers

containing

the

distributary

channels. On the

other

hand,

where marine

processes

such

as

longshore

currents are clearly more

powerful,

a

cuspate

type

land-

mass will develop.

An

example is

the

modern Baram

delta

of

Brunei-Sarawak. This

type

ol delta lias lew

distributaries

and grows

by

prograding

wave generated

beaches. A moie

balanced

situation results

in

a lobate

delta

lorm such as

the

modem

Niger.

A

cross

section

ol a

prograding

delta

front

in a

highly

constructive situation is

shown

in

Figure

18. This

shows

the

relative

positions

and

the

lithologies

of

delta

Ironl

deposits. A

highly

constructive

delta

is

most favour-

able

to

the

formation of distributary mouth

bars

or

delta

bar

lingers.

These are

sands

and silts dropped

in

front

of

the

mouths

of

distributary

channels which

suffei

little

or

no

reworking by

wave

motion.

The

river

currents are the

principal factor in

determining

sand

body

geometry.

Sand bodies usually take the

form ol

elongate or lobate

masses

extending

outward

from

the

river

mouth.

Current

bedding

is normally

tabular

in form

and

dips

in

the

seaward

direction,

normal

to the strand

line,

unless

deflected

by

longshore

currents. The direction of

current

bedding

dips

is

the direction of

sand elongation,

flic crossbedding

angle

is

steepest

at the

top

of

the

gross

sand

unit

and

decreases

downward.

Individual

sand

uniis

are normally

relatively

ihin. It

is not un-

common

to

have

a

dtstiihulaiy

channel

cutting

through

the

top of

a

distributaiy.

mouth bar.

Figure 17.An

illustration

of

the

effects

on delta

formation

of

the

relative

strengths

of

marine

processes

and

fluvial influences.

(AfterA.

J .

Scott)



21

Figure IH.

A

diagrammatic section

through

a

high-constructive

delta

front

showing seaward

movement

of

depositional

environments

and seaward

migration

of

a

distributary

mouth

bar.

Distributary

mouth bar sands are

relatively

tine

grained

and moderately sorted.

However,

curve

shapes

reflect a

general

coarsening upward

in a

highly serrated

funnel

type

configuration. The serrations probably

arise

from

irregular

downward

movements

due

to

growih

faulting,

common

in

a

highly

constructive

delta.

Curve shapes and dip

patterns

for a

typical distri-

butary

mouth bar

are

shown

in Figure

1

. .

ESTUARINE

AND TIDAL

CHANNEL

DEPOSITS

Certain delta areas are

strongly

tidal

dominated. In

this

case,

rather

than

the

distributaries building

outward,

the

effect of tidal currents

is

to

form

indentations

at

the

location of each

distributary

mouth.

The

modem

Mahakam is an example. The outer reaches of the dis-

tributary

channels will therefore

be

subject to tides

and

there

will be

significant

mixing

of

river water and

sea

water.

Because

of density

differences,

there

may

be

net

upstream

motion of sea

water,

at flood fide.

This

may

lead

to

bimodal crossbedding in the channel

deposits.

Narrow

estuaries

may develop

elongate

sand bodies

with

characteristics

similar

to

those

of

distributary

chan-

nel

tills

except

that

crossbedding may

be bimodal.

On

the

other

hand,

very

wide

estuaries

tend

to

develop

tidal

flats

which

contain

some sandbut are

often

predominant-

ly

mud.

Deposits

formed in

wide tidal

estuaries

will

tend

Io

he

groupings

of

roughly

parallel

elongate

sand

bars amid

silts

and

muds,

as

shown in

Figure

20.

In sec-

tion,

the profile would

show coarse sands at the

base,

grading

erratically

upwards

into

shales.

Tidal

estuary

deposits will

tend

to

generate

curve

shapes

of

the

serrated

bell

type.

Current bedding

will

be

polyniodal

or

possibly

bimodal. Figure

21

is an

ex

ample.



2

DISTRIBUTARY

MUUTH

BAR

Figure

19. Curve

shapes and dip

patterns

on a

distributary

mouth bar. Although

no

shale

free formation

exists

sandstone

lithology

is indicated. Curve shapes

are

of

the serrated

funnel

type

indicating

heller

sorting

upward.

Gram

size

increases

upward as

suggested by

increasing dip

magnitude.





24

ESTUARINE or

TIDAL CHANNEL

DEPOSITS

Figure

21.

Curve

shapes

and

dtp

patterns

on tidal

channel or tidal

flat

deposits.

TheDensity-Neutron

shows

sandstone litho-

logy.

Porosity

and

Vcl

curves

are

mainly

bell

shaped.

Clay

content

_

_____>__,

_.„_

/vX

■

...

«£.

decreasing

upward.

Dip

directions

may

be bimodal.

*

***

D

'

P

ma

V»

tude

«**«

"ruble

gram





26

Figure

23.

Curve shapes and

dip

patterns

on

a bar-type

sand.

Lithology is

sandstone. Porosity

and Vcl curves are

of

the

smooth

funnel

shaped

type

indicating better sorting

and coarsening

upward. Dip

magnitude

increases moderately

upward

suggesting

larger

grain

she at

the

top.

Beaches and most bars exhibit

low

angle

current

bedding

normal

to the elongation

of

the

sand

body.



27

MARINE

SHELF

BLANKET

SANDS

Figure 24. Curve

shapes and dip

patterns

on

marine

shelf

blanket

sands.

Density -Neutron

separation

indicates

sandstone.

possibly with lime

cement. The general

curve

shape is

funnel-bell.

The

porosity range

indicates

variable

sorting.

Dip

magni-

tude

suggests

fine

grain size.Dip direction may

be

polymodal.



28

A rule-of-thumb

has

been

established to calibrate

the

rate

of

build-up

of

the

reefmass.

Case

A:

Compaction

contemporaneous

with deposition

characterized

by low

resistivity

shales exhibiting

a

gradual

build-up of dip veisus

depth.:

teel

Iron

angle

-

maximum

diapingdip

plus

()'

Case

B: Compaction

mainly

after

deposition:

character-

ized

by

a

fairly constant

draping

dip

in the overlying

shale

which

contains

a

substantial

proportion

ol

calcilutite:

reef

front angle

-

twice

the

maximum

draping dip.

Reef

masses made

up

of

calearemte

oi

delntal

material

may show

crossbedding. paiiicularly on

the

red

Hanks. On the

other

hand,

no

meaningful dip

information

is

obtained

from biohthite

zones.

Back

reef

areas are

usually

filled in

by

calcilutite

which

has

neithei distinct

bedding

planes nor

diaping

ellects.

A

typical icef

outline is

shown

in Figure X

Figure

25.

A

cross-section

normal to

the shoreline

of

a

barrier

type

reef

build-up.

Reef

porosity

is

extremely

variable

and

follows

no

particular

pattern

veisus

depth.

The

biolithic

zones

will

normally be the

most

porous.

All three usual

rock

types,

biohthite,

calcarenite

and

calcilutite

may develop

porosity

in

the

form

of

vugs,

fractures

and

dolomitization.

Curve

shapes

are therefore

not

very

predictable other than

the

Vcl

or

Gamma Ray

which

will

be cylindrical.

The per-

centage

of

dolomite

can be determined

by

porosity

log

crossplots

and

calcarenite

can be distinguished

from cal-

cilutite

in

the

same

manner.

Figure 26

illustrates curve

shapes and dip

patterns

found on

the

flank ol

a

bioherin

reef.

'fhe

characlei

of

log

curve and

dip plots on shelf

caibonales is

very

dependent upon

the

degree

of

shaliness.

Massive

limestones,

ol couise.

give

rise to

cylindrical Vcl

curves. Dips

measured

in

massive

limestones

are

likely

to

present

an

incoherent

pattern,

being

mainly

the result

of

vugs

and

fractures.

To be able

to

measure

meaningful dips,

it is

necessary

to

have

lecogni/able

resistivity

bedding

planes which arc

mainly

due to variations in shaliness.

Hie degree

of

betiding is icvealed

by

the

character

ol the

dip

plot.

TURBIDITES

Density or

turbidity

currents,

currents caused

by

suspensions

ot mud

and sand that periodically

ti

a

vol

downslope

along the

bottom,

are

considered

by

most

sedi-

mentologists

to be

the principal mechanism for

tiansport

ol silt and

sand into

deep

water

basins. These

currents

result

in

thick

sequences of

marine

terrigenous

sedi-

ments

consisting,

mostly

of

rytlunically interhedded

shale

and

argillaceous,

poorly sorted sandstones.

Most of the

sandstones exhibit graded bedding

and evidence ofscour-

ing.

furbidiie

deposits

tend to

be

tabular,

elongate

or

fan

shaped.

Individual

sand beds

are

poorly

sorted,

but

the

upward

fining

of

grain

size should

produce

bell

shaped

log curves.

Rythmic

alternation

of

graded beds

with,

shales

produces

a

stacking

oi roughly

similar

curve shapes.

A

characteristic

of

turbidites,

in

spite

of

being

a

high energy

deposit,

is

the

absence

of

appreciable

crossbedding. Dipmeter

results will

therefore show

little

variation

from

structural

dip,

and

will

not be

very

help-

ful in

defining

sand

body

geometry.

Al

tins time,

no proven field example is

available

to

serve

as

a

model

of

curves

shapes and dip

patterns

for

turbidite

deposits.

Turbidites

deposited in deepmarine

basins

may be

interhedded with

muds

which

can be

hydrocarbon

source

rocks. However

turbidite sands

do

not

generally

make

good

reservoirs because

poor sorting and clay

matrix

inhibit

porosily

and permeability.

The thin

bedding of

turbidite

sands

and the intervening

shales

make

reser-

voirs

numerous,

hut thin and

disconnected.

CARBONATE

BUILD-UP

:

REEF

BACK

Rttf

fORE

Rfcfcf-

_ __ ~__ _̂_______ _______

ra

„___——

-,

*̂ ~*~ ^̂ ^̂

shale

calcilutite

g*x:-

r UCL

1

T

I

^

~

calcarenite

tongues

/

--— *»■■

*«■ «>«*;_

"__3iin«

/

£alcaren

tte

biohthite



29

CARBONATE BUILD-UP

FORE REEF

FACIES

Figure 26.

Curve shapes

and

dip

patterns

on

the

flank

of

a

reef.

The Vcl curve is

primarily cylindrical

while the porosity

curve

shows

no

characteristic

shape. Overlying shales

exhibit

draping

dip.

The

zone

immediately overlying the

reef

mass

is

made

up

largely

of

reef

detrital material

and

exhibits

a dip

scatter

due

to

crossbedding.

In

the hiolithic zone, no

coherent

dip

patterns

are

evident.



30

RESERVOIR DESCRIPTION

BY

PRESSURE MEASUREMENTS

Some methods

of

delineating

reservoirs by

wire

line

techniques

are described

above. To

a

great

extent,

these

methods

rely

on

the heterogeneous

characteristics

of

the

sedimentary rock

material,

and

variations of

porosity and clay

content,

which

produce

curve shapes

and dip

patterns

of

interpretive value. In general

these

methods

are usually

reliable,

such that formation

boun-

daries

can

be

extrapolated

beyond the

region

of

the

borehole

with

variable

degrees

of uncertainty

in

reservoir

mapping. It would be

of

interest

if

these

geometrical

predictions, and

the

implied

reservoir extent, could

be

verified

by

measurements

of

the

more

homogeneous

and

continuous

reservoir

properties

such

as

formation

pressure,

depth

to

fluid

contact

and hydrocarbon

density.

The various

units

of

a

complex

sand

aggregation

may

represent

one

or

several

independent reservoirs.

Similarly

a

single

correctable

unit may not

represent

a

single

reservoir,

if a

natural permeability

barrier exists.

Thus

it is of

importance

to have

information

relating to

lateral and vertical

hydraulic continuity

to

accurately

define reservoirs.

Until

recently,

efficient methods

of obtaining

multiple

pressure

measurements

in

open

hole were not

technologically

or

economically

feasible. However, the

development of the

wireline

Repeat

Formation Tester

(RFT*)

permits

rapid

testing

of

reservoirs,

of

whatever

economic

merit,

such that sufficient data are

available

to

assist

in the definition

of reservoir

extent

and cont-

inuity.

A

typical pressure test recording is shown in Figure

27.

The precision

of

the

downhole

measuring

system

is

reflected in

the resolution

shown

in

the data

presentation.

A

convenient

method

of

utilizing the

data is

by

means

of

pressure versus depth

curves.

Using multiple

pressure

measurements,

this

approach

is

simplified

since

less

reliance

needs

to

be

placed

upon

the

absolute

accu-

racy

of

each

measurement.

With the

RFT,

pressure

resolution

is

quoted

at 1

psi. Hence

slopes

of pressure

versus

depth

curves can be

expected to

be quite

precise.

Some

common reservoir

conditions are examined using

these curves.

"Trademark

of

Schlumberger

RESERVOIRS

UNDER A

DEPLETION

REGIME

Verification

of

lateral reservoir continuity is poss-

ible

where the reservoir is

being

produced

in

an

offset

well.

An

observable

pressure reduction is

definitive

of

hydraulic

connection,

while an indication

of

no

pressure

reduction implies separation

when drainage radii

intersect.

A few

measurements

in normally

pressured

water

bear-

ing

formations

will

establish

the

water gradient.

For

laterally connected

reservoirs,

fluid

gradients in the

/one

being depleted

will exhibit a

parallel

shift

in

the

direction

of reduced pressure, when compared with gradients

established

under

virgin

reservoir conditions.

Figure

28 is

a model lor

such

a

case. Pressure measurements

from

three

reservoirs in

the

same well

are plotted.

Reservoir 1

has

been

produced

from

an offset

well;

it can

be

said

to

be

hydrauhcally

connected

to

its

equivalent

in

the

offset

well.

Similarly,

reservoir

.2

is continuous

between

this

well

and

other

producing

wells.

Figure

27.

Fxample

of

RFT pressure

recording.

In

complex

sand aggregations, definition

of

verti-

cal hydraulic

continuity

can be

occasionally

less stra-

ightforward. However

from the

set

of

measurements,

a

valid

judgement

of reservoir

connection

can

often

be

deduced.

Referring

again

to Figure

28,

the pressure

measurements for

reservoir 3

establish

that

it is

not

ver-

tically

connected

to reservoirs 1

and

2.



31

DEPTHS

(aeti

ri)

RESERVOIRS

AT

VIRGIN

CONDITIONS

.1008

Sets of

pressure

measurements in virgin

formations

can

be helpful

in defining

reservoir

continuity.

II

cot-

relative units

in two

wells have

a

common

water

table,

this

fact

would

assisi in

substantiating

that

they are

laterally connected. Use

of pressure

measurements

to

determine distance to

fluid

contact in two

wells,

demands

extreme

precision,

and the presence

of

the same

fluid

in

bolh wells. The

fluid

densities

can

be compared by means

of

the

pressure

gradients. We would

suggest

the

following

method. Water gradients should

be

normalized

for abso-

lute

pressure

differences

by

referring these

gradients to

a

common

depth reference.

Then

comparison

of

the

inter-

cept

of the gradient in

hydrocarbon

with the

water

gradient will determine

the

water

fable.

Equivalent water

tables

substantiate lateral

continuity.

The

determination

of different

water tables

verifies lateral reservoir

separa-

tion. See figure 2

l

).

In this idealized model it would

be

of

interest to

verily

the

lateral

continuity

of

the

reservoir

in well B.

Reference

to pressure versus depth curves

suggests

that since

the

hydrocarbon gradient

determined

in

the

upper

reservoir

of

well

A

and

the reservoir

in

well

B

determine the

same

fluid contact,

verification

of

lateral

continuity

is

established. The

hydrocarbon gra-

RESERVOIR

.3

.1100

RESERVOIR

2

3200

RESERVOIR

1

3300

3400

r

.

1

4500 4hoo

4/00

4HOG 4900

_00

5100

PRESSURE

PS I

Figure

28.

F.xample

of

pressure versus

depth

plot

for

several reservoirs

under

a

depletion

regime.

dient in

the lowei reservoir

of well

A would

establish

Figure

29.

Model

of

pressure

versus

depth

curves in

virgin reservoirs

used

to

determine lateral

reservoir

continuity.



32

Well

A

Single

Reservoir

separated

by

shale

Figure 30. Model

of

pressure versus depth

curves

in virgin

reservoirs

used

determine

a

different fluid

contact

and verify that the

lower reser-

voir

in well

A

was not

laterally

connected to

the leser-

voir

in well

B.

Vertical

hydraulic continuity

of

a

complex sand

aggregation

can

often

be

deduced

by

pressure

versus

depth curves. II pressure

measurements

in several

mem-

bers

having

a common

fluid

type

define a

unique gradient,

hydraulic

continuity

is

implied. Where

separate

gradients

are observed hydraulic

discontinuity

is

indicated. Figure

30

is an

idealized

model

of these

cases. Logs

from both

wells would have nearly

the same

characteristics.

Pressure

measurements,

however,

would

be

definitive.

The single hydrocarbon

gradient

confirms

that

one

reservoir exists

m

well

A,

By

contrast,

the

separate

hydro-

carbon

gradients

demonstrate

that

two reservoirs

exist

in

well

B.

Well

B

,

Two

Reservoirs

separated by

shale

member

vertical

reservoir

continuity

EXAMPLE NO. 1

Figure 31.

Porosity

and Vcl curves

with

a

dipmeter

arrow

plot on

well

A. These data are

interpreted as

re-

presentative

of

a

delta distributary

channel

fill

trending

N6o°

F.



33

FIELD

EXAMPLES

EXAMPLE NO. 1

The

porosity and

Vcl curves,

as well as the

de-

tailed

dip

display from a section

of

hole

in

well

A,

are

shown in

Figure

31. The Density

-Neutron

log

(not

shown)

describes the

lithology

in

the cleanest

zones as sand-

stone and

identifies

a

coal bed. Curve shapes on

the main

sand

might be best

described as serrated

cylindrical.

Tins,

in

itself,

is

not

fully

definitive

but would

suggest

some

type

of channel

fill.

The dip

pattern

is

unimodal,

with

higher

dip

angles

near

the

base

and

reasonably

consistent

in

direction. Dip magnitude

and

spread

sug-

gest

relatively

coarse grain

size

in

a

generally

fining

upward

textural

sequence.

Overall,

these

characteristics,

with

the

possible

exception of the serrated curve

out-

line,

are

most diagnostic

of

a

delta

distributary

channel

fill.

Tlte

implied

trend is

southwest-northeast and the

channel

flow

direction

is

interpreted

as N6O°E.

EXAMPLE

NO

1

INTERPRETIVE ISOPACH

MAP

OF

GROSS

GENETIC

UNIT

THICKNESS

Figure 32.

Interpretive

isopach map

of

gross

genetic

units thickness

after

addingwells

H, C, D,

and

_".

Figure

32

illustrates an interpretive isopach map

of the

gross

genetic

unit,

made

using

thicknesses

measured in

the

subsequent

wells,

B,

C,

D,

and

E.

These thicknesses

support

the channel interpretation.

A

cross

section showing Induction log correlations

is

constructed in Figure

33. Note

that

the

hydrocarbon

bearing channel sand in well

A

does not extend to well

B as evidenced

by

the presence

of water

bearing

forma-

tions

the

equivalent

/one. Well B is somewhat lower

structurally,

but

not sufficiently

to explain the

lack

of

hydrocarbons.

To

the

south,

however,

structure

con-

trols

the

water-hydrocarbon

contacts.

EXAMPLE NO

1

LINi

Or

SECTION

O-C-A-B

Figure

33.

Cross

section through wells

/),

C, A,

and B

showing outline

of

distributary channel.

EXAMPLE

NO.

2

Curve

shapes

and

dip

patterns

from

a different

zone

in well

A

are

shown

in Figure

34. Allowing

for

shaliness,

the

lithology

in

the

cleanest

zone is sandstone

as

determined

by the

Density-Neutron

log.

Porosity

and

Vcl

curve

shapes

on

the

main

sand are

of

the

serrated

funnel

type

suggesting

coarsening

grain

size

and

better

sorting

upward.

Tire

upward

coarsening is confirmed

by

the

dip

spread.

Clearly, this is evidence ofa bar

type

deposit.



34

The

dip

magnitude is too

great

to

represent

either

a

beach or a

barrier

bar.

There

is

a

possibility

that it

could

be the landward

side of

an overwashed bar.

How-

ever,

the

serrated nature

of the

curves leads

to the

con-

clusion

that

it

is

a

distributary

mouth

bar.

Dip

directions

suggest

a lobe

prograding approximatelyN35 E.

EXAMPLE

NO

2

INTERPRETIVE

ISOPACH MAP

OF

CROSS

GENETIC

UNIT THICKNESS

Subsequent

wells

B,

C, D,

E, F,

G

and II

found

correlative thicknesses

as

shown in Figure 35.

The

inter-

pretive

isopach

map

fits

measured thicknesses and the

interpretation

on

well

A.

The cross

section

H,

Ig

A, B,

in Figure 36 demon-

strates the

shape of

the

gross

genetic

unit

relative

to

a

marker bed. Note that post-depositional

structural

de-

fornration

controls

water-hydrocarbon

contacts. Note

also that

the

SP

curve shape

on this

formation in well

A

is not

typical of

a bar

type

deposit.

This is

the

effect

of

distortion

due

to

hydrocarbon

and

illustrates the

danger of

using

a

single

clay

indicator.

tXAMPLE

NO.

2

Figure 35. Interpretive

isopach

map

of

gross genetic

unit

thickness

after

adding data

from

other wells.

Figure 34.

Porosity

and

Vcl curves with

dipmeter

arrow

plot on

another

zone

in

well

A.

A

distributary

mouth

bar

prograding northeast is interpreted.

H,

F, A,

and

B,

LINf. Ol

StCTION

H- (

-A

ft

H

f

fl

A ..■

_A

,_,

.a

i^

.

I

..

S

X'

-,

.x

l

X-'

*-

S

-'

I \ .X

■"'-

/

x

g

v

-,..

(

.-.

y,

I

V

<

J '

t

i

/

i-

■

.

'

->*

Figure

36.

Cross section

through

wells

H,

F,

showing the

outline

of

the

interpreted

geometry.



35

EXAMPLE NO.

3

Figure 37

shows

the

following curves

in well

A:

Gamma

Ray,

Caliper. Density-Neutron. In this well no

porosity

or Vcl

curves are available.

Lithology

indi-

cated

by the Density-Neutron

separation

in the

cleanest

non

gas

bearing zones

confirms

sandstone,

(e.g. the

zone

1584-1

590

m).

Several

coal

beds

stand

out

clearly.

Curve shapes

in the thickest sand P

(

1448-1

400

m)

are

serrated funnel-cylindrical-bell

shaped

combinations.

Porosity

is

varying

and

fairly

low

overall,

indicating

poor

sorting. From lithology

and

curve

shapes one may

deduce a

tidal flal

estuaiinc

condition.

The

dip

pattern,

displayed on the right

hand side

of Figure

37

shows,

after structural

dip subtraction

(1

1

v

at

275

v

azimuth),

a

polymodal,

gentle

current

bed-

ding

preferentially

to

the

west. The very

low

dip

spread

is associated with finer

grain

size. This

reservoir can be

considered

an

aggregation

of

low

energy

tidal

flat

deposits. Similarity

to the sand at

300 feet

in the es-

tuarine model

(Figure

21

)

is

to

be

noted.

The sand

0

at

I

385-1

367

mhas

a serrated funnel-

bell shape. Porosity

is

higher,

suggesting

betlei

sorting.

The

relatively

larger dip

spread indicates

coarser

gram

size but

without

preferential

lining

oi

coarsening

direc-

tions. Dip is

again

polymodal.

The

presence

of

coal

further

substantiates

the

tidal

flat-cstuarinc-deltaic en-

vironment.

Both

curve shapes and dipmeter

displays on

both

sands

P

and

0

indicate

a

complex

blanket

type

sand acc-

umulation. In

view

of

this

conclusion,

correlation

with

nearby

wells

should

be

possible.

Figure

38

shows

such

a

correlation

using

Resis-

tivity,

SP

and

Gamma Ray curves

in well

A

and well

B,

600

meters

awary

(S

28°

W). Also a dipmeter

result

from a

long interval

computation

is displayed on

the

right hand side

of

Figure

38. This

computation

shows

a

structural

dip

of

1 1

with an

azimuth of 275

°

,

read

preferentially

from

the

shalier

zones.

The correlation

is relatively straightforward if

one

can rely on

the con-

tinuity

of

the

coal streaks.

For

confirmation,

however,

it is a

good

practice

to utilize the

structural dip informa-

tion.

This

is

accomplished

by

calculating

the

apparent

dip along

the line

-of

section between

the

two

wells.

The

method

is

detailed

in

Figure

39.

The result

of

this

computation gives an

apparent

dip

of

4.3°

resulting

in

an

elevation

difference

of

approximately 45

meters.

EXAMPLE

NO 3

Figure 37. WellA.

Gamma Ray, Caliper,

Density,

Neutron

curves and detailed

dipmeter

results

after

subtraction

of

the

structured

dip

of

11° at

an

aximutb

of

275°. Note

the

effect

of

coal

streaks

and

gas on

the

Density-Neutron

log.



36

HUMPH

WO

3

Figure

38.

Correlation

of

logs between wells

A

and

B

using Gamma

Ray,

SP ,

and Resistivity logs.

Also sboivn is a

plot

of

long

interval

High

Resolution

Dipmeter

results and

a "Stick Plot"along

the line

of

section

between

the

two

wells.





38

EXAM. IE

NO

3

Figure 40. Pressure

versus

depth

curves

for

intervals

P, Q,

and

R

of

well

A.

Formations

Q

and

R

are producing in

well

B.





22.

I'oupon,

A.,

and

Gaymard,

R.,

1970,

"Tire Eva-

luation of

Clay

Content

from Logs":

SPWLA

Eleventh

Annual

Logging

Sympo-

sium,

May,

1970.

1970,

"Log

Analysis in

Formations

with

Complex

Lithologies":

Society

of

Pet-

roleum

Engineers

of

AIME,

45th

Annual

Fall

Meeting,

October,

1970.

and

Misk,

A., 1970,

"Log

Analysis

of

Sand-Shale Sequences

-

A

Systematic

App-

roach":

J ournal

of

Petroleum

Technology

J uly,

1970.

25.

Sabins,

F.F.

J r.,

1963,

"Anatomy of

Stratigraphic

Trap,

Bisti

Field,

New

Mexico":

Bulletin

of

the

American

Association

of

PetroleumGeologists,

vol.

47,

no.

2.

26.

Schlumberger

Ltd., 1972,

"Log

Interpretation

Principles".

27.

Schlumberger

Ltd.,

1974,

"Log

Interpretation-

Applications".

28.

Schlumberger

Ltd.,

1970.

"Fundamentals

of Dip-

meter

Interpretation

29.

Schultz,

A.L.,

Bell,

W.T.,

and

Urbanosky,

H.J .,

1974,

"Advancements

in

Uncased

Hole

Wireline

-

Formation

-

Tester

Techniques":

Society of

Petroleum

Engineers

of

AIME,

49th Annual

Fall Meeting,

October,

1974.

30.

Selley,

R.C., 1970,

"Ancient

Sedimentary

Environ-

ments :

Chapman

and

Hall, Ltd.,

London.

31.

Selley,

R.C.,

1976

"Environmental

Analysis

of

Subsurface

Sediments":

SPWLA,

London

Chapter,

Fourth

European

Symposium

Tran-

sactions,

October,

1976.

32.

Serra, 0.,

and

Sulpice,

L.,

1975,

"Sedimentological

Analysis of

Shale-Sand Series

From

well

Logs":

SPWLA

Sixteenth

Annual

Logging

Symposium,

J une,

1975.

33. Services

Techniques

Schlumberger,

1974,

"Well

Evaluation

Conference,

Nigeria".

34.

Services

Techniques

Schlumberger,

1974,

"Well

Evaluation

Conference,

North

Sea".

Evaluation

Conference,

Arabia

36.

Services

Techniques

Schlumberger,

1976.

"Well

Evaluation

Conference,

Iran".

37.

Shelton,

J .W., 1967,

"Stratigraphic

Models

and

General

Criteria

for

Recognition

of

Alluvial,

Barrier

Bar,

and

Turbidity-Current

Sand

Deposits".

Bulletin

of

the

American

Asso-

ciation of

Petroleum Geologists,

vol.

51,

no. 12.

38.

Truman,

R.B

,

Alger

R.P., Connell,

J .G.,

and

Smith,

R.L.,

1972,

"Progress

Report

on Interpreta-

tion

Of The Dual-Spacing

Neutron

Log

(CNL) In The

U.S.": SPWLA

Thirteenth

Annual

Logging

Symposium,

May.

1972.

39.

Visher,

G.S.,

1965,

"Use

of

Vertical

Profile in

Environmental

Reconstruction"

of

the

American Association

of

Bulletin

Petroleum

Geologists,

vol.

49,

no.

1.

23.

Poupon,

A.,

Hoyle,

W.R.,

and

Schmidt,

A.W.,

24.

Poupon,

A.,

Clavier,

C,

Dumanoir,

J .,

Gaymard,

R.,

Documents

Reservoir Delineation by Wireline Techiniquess_Goetz 1977