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Subsurface Facies Analysis

Douglas J. Cant, Geological Survey of Canada, 3303 - 33rd St. N.W.

Calgary, Alberta T2L 2A7

INTRODUCTION

This chapter will attempt to bridge the

methodological and scale gap between

sedimentology based largely on out-

crops and modern environments, and

techniques and results of large-scale,

subsurface investigations. It is an intro-

duction to 1) geophysical logs and cor-

relation, 2) subsurface facies analysis,

3) seismic methods, and 4) larger-

scale analysis of sedimentary facies

successions and allostratigraphy.

Subsurface work lends itself to the

study of facies relationships on a scale

larger than can be accomplished on

most outcrops. Consequently, many

relatively new ideas concerning allo-

stratigraphy, sequence stratigraphy,

base-level changes, and global strati-

graphic correlations have emerged

!from subsurface geology, and are

based on both geological and geo-

physical data.

DIFFERENCES FROM

SURFACE WORK

Subsurface data provide a differently

biased sample of a rock unit than

outcrop data. Drill holes and cores are

concentrated in localities and zones of

economic interest whereas outcrops

preferentially expose rocks which are

harder and more resistant to weath-

ering. Because drill holes and geo-

physical logs normally sample a con-

tinuous, uninterrupted section (where-

as outcrops rarely do), subsurface cor-

relation is based on more complete

data. Seismic-reflection data may

provide a coherent three-dimensional

picture of a basin, along with informa-

tion on the relationships between the

sediments and structural features of a

basin.

Subsurface data cannot provide as

much local facies information. No mat-

ter how closely spaced wells may be,

data from geophysical logs, 3 to

20 cm-diameter cores, and cross sec-

tions constructed from them, cannot

match the level of local detail available

from an outcrop. Seismic data give a

view of a basin on a very much larger

scale than outcrop studies. The differ-

ences in the appropriate scales of

investigation for outcrop compared

with subsurface studies are extremely

significant, both scientifically and

economically.

Subsurface methods

The following sections will briefly

review the principles behind sub-

surface methods of investigation, in-

cluding both geological methods (well

logs and cores), and geophysical

methods (seismic-reflection data).

Other publications discuss these

techniques in more detail, notably

Krumbein and Sloss (1963), Allen

Table 1

Log

Log types, properties measured, and geological uses.

Property Measured

Natural electric potential (compared to

drilling mud)

Resistance to electric current flow

Natural radioactivity — related to

K, Th, U

Velocity of compressional sound wave

Size of hole

Concentrations of hydrogen (water and

hydrocarbons) in pores

Bulk density

(electron density) includes pore fluid

in measurement

Orientation of dipping surfaces by

resistivity changes

Units

Millivolts

Geological Uses

Lithology (in some cases), correlation,

curve shape analysis, identification

porous zones.

Identification of coals, bentonites, fluid

evaluation.

Lithology (shaliness), correlation, curve

shape analysis.

Identification of porous zones, coal,

tightly cemented zones.

Evaluate hole conditions and reliability

of other logs.

Identification of porous zones, cross

plots with sonic, density logs

for

empirical separation of lithologies.

Identification of some lithologies

such as anhydrite, halite, nonporous

carbonates.

Structural analysis, stratigraphic

analysis

Spontaneous

potential

Resistivity

Gamma-ray

Sonic

Caliper

Neutron

Ohm-metres

API units

Microseconds/metre

Centimetres

Per cent porosity

Density

Kilograms per cubic

metre (gm/cm

)

Degrees

(and direction)

Dipmeter

CANT

(1975), Payton (1977), Selley (1978),

Anstey (1982), and Miall (1984). Some

newly developed methods in subsur-

face sedimentology, such as the use

of resistivity microscanners to detect

sedimentary structures, and the anal-

ysis of high-resolution dipmeter data,

are as yet not widely applied, but are

discussed by Hurst

et al.

(1990). The

two major types of data, subsurface

geological and geophysical, will be

treated separately, but where avail-

able, both can be integrated.

GEOLOGICAL USES OF

WELL LOGS

Well logs are made by pulling an in-

strumented tool up the hole, and

recording the data as a function of

depth. They are used extensively in the

petroleum industry for evaluation of the

fluids in rocks, but this aspect will not

be covered here. The interested reader

is referred to the various logging

company manuals, or to Asquith

(1982). Geophysical logs are the fun-

damental source of data in many sub-

surface studies because virtually every

oil and gas well is logged from near the

top to the bottom. Almost all well

logging is done by pulling the mea-

surement tool up the hole on the end

of a wire. Different types of logs and

the properties they measure are

shown in Table 1 and are discussed

briefly below. On any well log, the ab-

solute elevation of any bed or bed

contact is obtained by subtracting its

depth in the well from the surveyed el-

evation of the Kelly Bushing (KB) on

the drilling platform; this elevation is

given on the top (header) of the well

log.

Spontaneous potential (SP) log

This log records the electric potential

between an electrode pulled up the

hole and a reference electrode at the

surface. This potential exists because

of electrochemical differences between

the waters within the formation and the

drilling mud, and because of ionic se-

lection effects in shales (the surfaces

of clay minerals selectively allow

passage of cations compared to

anions). The potential is measured in

millivolts on a relative scale only

(Fig. 1) because the absolute value

depends on the properties of the

drilling mud. In shaly sections, the

maximum SP response to the right

can be used to define a "shale line"

(Fig. 1). Deflections of the log from the

shale line indicate zones of permeable

rock containing interstitial fluid with

salinities different from the drilling

mud.

Experience in many basins has

shown that the SP log may be a good in-_

dicator of lithology in areas where sand-

stones are permeable and water

saturated However, where low-perme-

ability rocks occur, such as the tightly

cemented sandstones of the western

Alberta Basin or the bitumen-saturated

Athabasca Oil Sands, the SP log cannot

reliably distinguish lithologies If subsur-

face formations contain fresh rather than

saline water (such as some Upper

Cretaceous rocks of Alberta), SP deflec-

tion is suppressed or even reversed

Figure 1 Example of SP and resistivity logs from the Alberta Basin. A shale line is shown

on the SP — any deviation from this reflects porous rock. Two resistivity logs are shown —

one of medium depth (resistivity) and another (deep induction) which reads farther into the

rock beyond the influence of the drilling mud. The deep induction log shows lower resistivity

in porous zones, probably indicating salt-water saturation. The carbonates are Devonian

Winterburn dolomites overlain by Cretaceous Mannville Group sandstones and shales. The

curved arrows indicate individual iacies successions; these car, br. seen as progressive

upward deflections in both the SP and resistivity logs.

SUBSURFACE FACIES ANALYSIS

from normal, depending on the salinity

of the drilling mud. The best test of the

reliability of the SP log in determining

lithology is to calibrate the log against

cores and cuttings~[cTJUinys are" 1 to

3 mm fragments of rock brought to the

surface during normal drilling by the

circulating mud) and hence gain expe-

rience in a particular area.

Resistivity log

This log records the resistance of in-

terstitial fluids to the flow of an electric

current, either transmitted directly to

the rock through an electrode, or mag-

netically induced deeper into the for-

mation from the hole (induction logs;

Fig. 1). The term "deep" here refers to

horizontal distance from the well bore.

Resistivities at different depths into

the rock are measured by varying the

length of the tool and focusing the

induced current. Several resistivity and

induction curves are commonly shown

on the same track (Fig. 1).

Resistivity logs are used for evalua-

tion of fluids within formations. Thev

'can also be used for identification of

coals (high resistance), thin lime-

"sic gs in shales (high resistance), and

jjentonites (low resistance), as shown

in TaBte"-T. In older wells where few

types of logs were run, the resistivity

log may be useful for picking tops and

bottoms of formalions, and tor^coTre^

lating between wells. Freshwater-satu-

11-3-70-11W6

rated porous rocks have high resis-

tivities, so the log can be used in these

cases to^separate shales from porous

sandstonesjirjd rnrhnnntnt

Gamma-ray log

This log (Fig. 2) measures the natural

gamma-ray emission of the various

layers penetrated in the well, a prop-

erty related to their content of radio-

genic isotopes of potassium, uranium

and thorium. These elements (particu-

larly potassium) are common in clay_

minerals and some evaporites. In ter-

rigenous clastic successions the" log re-

flects the "cleanness" (lack of clays) or

"phgjiness" fhigh radioactivities on the

AP|_scale, Fig. 2) of the rock, averaged

over an interval of about 2 m. Because

of this property, gamma-ray log pat-

terns mimic v e r t i

r a l

sandsCQOlgpJ

carbonate-content trends of fades suc-

cessions. It must be emr

the gamma-ray reading is •'.•<•

tion of grain size or carbo; <: -

only of the proportion

faiMOcViviC

ements, which

may

be { - e

;

f e j IrA^C.

proportion of shale. For example, ciay-

free sandstones or conglomerates with

"any mix of sand and pebble clast sizes

generally give similar responses, and

lime mudstone gives the same re-

sponse as grainstone. Distinguishing

between clean (clay-free) lithologies

such as sandstones, conglomerates,

dolomites, and limestones is best done

by calibration of one or more lqqsJo__

jc^s^icujtings,

Once the main lithologies are known,

Figure 2

Gamma-ray, caliper, and sonic logs from the Alberta Basin. Because of space

limitations, coaly shales (higher gamma-ray readings) are labelled coals. The lower section

consists of the Ireton and Leduc Formations and the upper section shows regressive

shoreline successions of the Upper Mannville Spirit River Formation.

Figure 3

Relationship between gamma-

ray deflection and proportion of shale. A

cutoff half-way between maximum and

minimum values corresponds to about 28

per cent shale, and is commonly used as a

criterion for lithologic mapping.

CANT

the gamma-ray log can be calibrated to

lithology by establishing minimum and

m a x i m u m readings corresponding to

pure carbonate or sandstone versus

pure shale end members. The concen-

tration of radioactive elements in shale

increases with c o m p a c t i o n ^ so the

shale line s h o u l d be readjtFsled.if a

thick section is being studied. The tool

r e s p o n s e is n o n l i n e a r (Fig. 3), so a

cutoff halfway between the shale line

a n d s a n d s t o n e (or o t h e r c l e a n l i t h -

o l o g y ) l i n e h a s a v a l u e of r o u g h l y

30 per cent shale for purposes such

as lithologic mapping.

There are three main interpretation

problems with the gamma-ray log, 1)

the log response may be affected by

cliagenetic, radioactive clays in pores,

2) shales rich in illite (high K) are mi^re

radioactive man tnose ncn in montmo^

rMlonites or^chloriTeg, ana 3) ajj<os"ic~

sa^dstoTTes ( h i g h ^ - f j i j d s p a r s } a r e ,

more radioactive than those lacking

feldspar. Calibration of the log against

cores~or cuttings may be necessary to

distinguish lithologies in some cases.

Sonic log

This log (Fig. 2) measures the velocity

of sound waves in rock. This velocity

depends on 1) lithology, 2) amount of

interconnected porespace

_axid 3)

typ"e~T5T~fUiidJflJhe pores. The log is

useful for delineating beds of low-velo

city material such as coal (Fig. 2) o

poorly cemented sandstones, as well

as h i g h - v e l o c i t y m a t e r i a l s u c h a*.

iqhtly cemented sandstones and car-

onates or igneous basemenj. Sonic

logs are also important in understand-

ing and calibrating seismic lines, as

explained below.

Porosity

l o g s

Density and neutron logs can be d*> -

played as estim4tes_oX_p_orosities. The

density tool emits g a m m a radiai

which is scattered back to the deteaur

in amounts proportional to the electron

density of the rock. The electron density,

in most cases, is related to the density of

the solid material, and the amount and

density of pore fluids. Density porosity is

calculated by assuming a density of the

solid material (2650 kg/m

for sandstone

and shale, 2710 kg/m

for limestone) and

fluid (1146 kg/m

for salt water).

The neutron log measures the hy-

d r o g e n c o n c e n t r a t i o n (in w a t e r or~

petroleum) in the rock. The tool emits

neutrons of a known energy level, then

measures the .energy of neutrons re-

flected from the rock. Because energy

is transferred most easily to particles of

<^sjrnilar mass^the hydrogen concentration

can be estimated. Neutron porosities are

calculated by assuming that oil or water

fills the pore s p a c e s . G a s , or water

bound intoclay, minprak ijivp anoma-

lously low values.

Caliper log

This log records the diameter of the hole

(Fig. 2), and gives an indication of its

condition and hence the reliability of

other logs. A very large hole indicates

that dissolution, caving or falling in of the

rock wall has occurred, which can lead to

errors in log responses. This log is partic-

ularly useful in mixed evaporite succes-

sions where dissolution has preferentially

leached out soluble evaporites. A hole

smaller than the drill bit size may be

present because the fluid fraction of the

drilling mud invades permeable zones,

leaving the solid fraction (mud or filter

cake) on the inside of the hole. In one

gas f i e l d in the M a n n v i l l e G r o u p of

Alberta, very permeable, matrix-free

conglomerate can be recognized on the

caliper log where tne note bize is sfjiailei

than the bit size.

Figure 4 A gamma-ray cross section in the Upper Mannville Group of Alberta illustrating

correlation by pattern matching. The correlations have been made using the following cri-

teria, 1) facies successions do not show abrupt lateral changes in character, 2) facies suc-

cessions do not show abrupt changes in thickness, 3) facies successions do not show

seaward (right to left) coarsening, 4) correlated surfaces slo.-.e seaward (to the left;.

Coals (blank) were loentmea 0" sonir ions. ~-:etc logs are not spaced proportionally !c

'Mc

distances between wells.

SUBSURFACE FACIES ANALYSIS

Dipmeter log

This log is made by a resistivity tool

with three or four electrodesmounle_d_

on separate arms with a common

centre point. The orientation of the tool

is also continuously recorded. Wh£re

a dipping bed is enccajntered, the re-

3""fo the iitnofogic change takes

place atUilferent elevations on eacfi^

aTmrfrTie direction and magnitude of

Sip' can be calculated from this infor-

mation.^

T n e dip m eter

_ _

measures structural^

dig^ but can also detect various types

of sedjmervtary dips such as a

com-

paction drape over ,a reej. a sloping

^jTQucrarspeT' even some cross-stratF

ligation. TrTmany cases/it is difficult to

determine the nature of a dipping

surface unless a core has been cut.

CORRELATION OF LOGS

Correct correlation of stratigraphic

units is absolutely necessary to make

reliable cross sections and maps, and

to conduct regional facies analysis.

Complex numerical procedures for

matching and correlation of logs (such

as a method adapted from gene-typing

techniques; Griffiths and Bakke, 1990)

may be the primary tools in the future.

At present, most geologists match log

patterns by eye (or by tracing and

overlaying logs), allowing for variations

in lithologies, thicknesses, and com-

pleteness of section. Three major cor-

relation methods will be discussed, 1)

marker beds, 2) pattern matching, and

3) slice techniques.

| Marker

beds

The log response ("kick") of a distinc-

tive bed or series of beds can be used

as a marker (e.g., Fig. 19 in Chapter

12) even if the lithology or origin of the

bed is not known. Distinctive, laterally

extensive groups of beds commonly

result from transgressions or regres-

sions or erosional episodes which re-

distribute proximal sediment far across

the basin. Markers that can be

mapped regionally may therefore be

related to, or include, important al-

lostratigraphic surfaces. For example,

condensed sections (possibly ex-

pressing jna£imjjmJloj2dJiig_s^^

Chapter 1) are perhaps the most

extensive marker beds, and are ex-

cause_ihey_are es-

pes. In the Alberta

sh Scale Horizonji a

Basin

lis

shale rich in organic debris. It can be

identified over most of the basin by its

characteristic high-gamma ray, slightly

high-resistivity, ancMiigh-porosity

values. It is used in many studies 5t

the units above and below it (particu-

larly the Viking Formation), as a hori-

zontal datum for cross sections. The

top of the Middle Mannville Bluesky

Sandstone is also a prominent marker

in the Mannville Group. The Bluesky is

a shoreline deposit abruptly overlain

by marine shales; this contact is a

bounding discontinuity used to define

allostratigraphic units.

In other situations, markers are not

related to allostratigraphic units. For

example, volcanogenic bentonites are

easily recognized on logs (Table 1),

providing reliable markers as well as

time lines.

Pattern matching

This technique involves recognition

andjriatching of distinctive log patterns^

J3l_any origin.JTie correlated patterns

may represent vertical fgcies success-

i o n s (Fig. 4), jguperimpospH fanips,

Successions, or unconformity-bounded

units (Fig. 19 in Chapter 12). The sur-

faces of the units chosen may be

transgressive and separate individual

facies successions (Fig. 4). Alterna-

tively, surfaces of maximum transgres-

sjaa_separatet ^mposrFe-transqressiv1r>

fromrc^mpollle^gressive)units (to be

discussed in more detail below). The

bounding surfaces of the chosen units

may also be unconformities, such as

Cardium Formation surface E5 (Fig. 19

in Chapter 12).

By matching patterns, correlations

are made on the basis of log shapes

over intervals of metres or tens of

metres, rather than on individual peaks,

troughs, or markers within the succes-

sion. Pattern matching may allow_cjp£-

relation even

wb<*rf>

lateral variations

in lithologies, facies, and thicknesses

of units have occurred (Fig. 4). In diffi-

"cult ca'SesTTnatcningls facilitated by

tracing one log and overlaying it on an

adjacent log. The logs can be moved

up and down until the best overall fit is

obtained. Constantly changing posi-

tions of fit may indicate lateral facies

PL thickness changes, and may mdF"

cate synsedimentary tectonism. ..

Figure 5 Gamma-ray cross section correlated by pattern matching which shows facies

successions sloping seaward and downlapping against a lower surface. Correct pattern

matching a|lowj>ja^ntifjcati^^

such as this. D indicates downlap

and T indicates toplap. The logs are not spaced proportionally to distances between wells.

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