Schlumberger

(05/96)

Contents

E1.0 SHALY FORMATIONS.................................................................................................................1

E1.1 INTRODUCTION .....................................................................................................................1

E1.2 POROSITY IN SHALY FORMATIONS.....................................................................................3

E1.3 EVALUATION OF SHALE VOLUME (VSH

)................................................................................4

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Introduction to Openhole Logging

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E1.0 Shaly Formations

E1.1 INTRODUCTIONShales are one of the most important com-

mon constituents of rocks in log analysis.Aside from their effects on porosity and per-meability, this importance stems from theirelectrical properties, which have a great influ-ence on the determination of fluid saturations.

Archie's water saturation equation relatingformation resistivity to water saturation, as-sumes that formation water is the only electri-cally conductive material in the formation. Thepresence of another conductive material (e.g.,shale) requires changes to either Archie'sequation or the model relating resistivity towater saturation. As well, the presence of clayin the formation complicates the concept ofporosity. The water associated with the clayscan represent a significant amount of porosity.However, this porosity is not available as apotential reservoir for hydrocarbons. To thispoint, we have dealt with tool responses fromour porosity devices that yield total porosityφ

T. At this time we have to introduce a new

term, effective porosity, φe, which is that por-

tion of the formation porosity available tocontain and produce fluids.

The presence of shale in formations gener-ally affects the response of the logging devices.In our discussions we usually speak of shalysands; however, the presence of shale in car-bonates can often be treated in a similar man-ner.

As briefly mentioned before, we categorizethe distribution of shaly material in formationsin three possible ways (see Figure E1):

1) Laminar Shale: occurs when shaleexists in the form of laminae or thinlayers between thin layers of sand. Theshale streaks do not actually influencethe effective porosity of the sand lay-ers in the formation; however, as thebulk volume of shale increases, theoverall formation porosity decreases.The presence of the shale may haveconsiderable influence on the loggingtool responses.

2) Structural Shale: is defined as the typeof shale that exists as grains or nod-ules in the formation matrix. It is con-sidered to have properties similar tolaminar shale.

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3) Dispersed Shale: occurs where theshaly material is dispersed through thesand to occupy part of the intergranu-lar space. Dispersed shale reduces thepore space available for fluid accumu-lation and also reduces formationpermeability.

The evaluation of shaly sands requires thatwe assume some distribution model. With theadvent of computers we can analyze forma-tions on the basis of sedimentation principles.Here we determine the silt and wet clay contentof the shale; the former is a maximum near themain sand body (high-energy deposition) andthe wet clay becomes predominant as distance

from the main sand body increases (low-energy deposition).

When shales consist of wet clay and silt, thebulk volume fractions may be expressed as:

Vsh

= Vsilt

+ Vclay

Another commonly used expression isthe silt index (I

silt) where

Isilt

= Vsil t

/Vsh

also

Vclay

= Vsh

(I – Isilt

).

Figure E1: Forms of Shale Classified by Manner of Distribution in the FormationPictoral Representations Above, Volumetric Representations Below

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E1.2 POROSITY IN SHALYFORMATIONS

When a sand contains shale we cannot obtainan accurate value of effective porosity fromone porosity log. The responses of the densityand neutron logs to shale content in sands isconsidered to be the same as in nearby beddedshales, no matter what model of shale distri-bution is considered. On the other hand, soniclogs have quite a different response betweenlaminated-structural and dispersed shales.

a) Density Logs- When shale and sand matrix densities

are close to each other, the density logis least affected by shale and readsclose to the effective porosity.

- When the shale matrix density is lessthan 2650 kg/m3 the density log inshaly sands will record porositieshigher than the effective porosity.

- When shale matrix density is greaterthan 2650 kg/m3, the density log in theshaly sands will record porositieslower then the effective porosity.

- The relationship for liquid-filled shalysands can be written as

ρb = ρ

fφ

e + ρ

ma (1 - φ

e – V

sh) + ρ

sh V

sh

or

ρb = (1 – φ

e)ρ

ma + φ

eρ

f

+ Vsh

(ρsh

– ρma

)

b) Neutron (CNL/SNP) Logs- Neutron tools respond to the amount

of hydrogen in the formation. Becauseshales contain bound water, the poros-ity recorded by neutron devices inshaly sands is always higher than theeffective porosity.

- In liquid-filled shaly sands, the neutronrelationships may be written as

φN

= φe + V

sh(φ

Nsh )

c) Sonic Logs- Sonic traveltime in shales rises because

of the fluid content of the shales;hence, sonic porosities in shaly for-mations are always higher than the ef-fective porosity. To further enablesonic porosity determination, we mustalso know what shale model is pres-ent, and also whether a compactioncorrection is necessary.

- In compacted formations with shalespresent, a general sonic relationshipmay be written as

∆tlog

= (φe – V

sh)∆t

m a + (V

lam +

Vstr

)∆tsh

+ (φe – V

dis)∆t

f

- In uncompacted zones, sonic porositiesderived from this relationship mustalso be corrected downward for thelack of compaction.

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E1.3 EVALUATION OFSHALE VOLUME (V

sh)

Basic methods of shale (clay) volume calcu-lation use the following indicators:

- Gamma ray- NGS tool- Spontaneous potential- φ

N versus φ

D crossplot

φN versus φ

S crossplot

a) Gamma RayIf the radioactivity of the shale content is

constant and if no other mineral in the forma-tion is radioactive, the gamma ray reading maybe expressed as a function of clay content.

The formula can be written as

GRzone

– GRclean

†Vsh

= GR

shale – GR

clean

b) NGS Natural Gamma RaySpectrometry Tool

By using only thorium and potassium com-ponents of the gamma ray signal, the radioac-tive uranium element not associated withshales will be eliminated. The same method isthen applied to the NGS as that for a regulargamma ray.

CGRzone

- CGRclean

†Vsh

= CGR

shale - CGR

clean

These formulae will not hold true for zonesthat contain radioactive matrix materials or ra-dioactive waters (e.g., granite wash sands).Similarly, this method will not hold true wherenonradioactive shales occur.

Some typical values for formations are- Clean Sandstone: GR = 15–30 API- Clean Carbonates

- Dolomite: GR = 10–20 API- Limestone: GR = 8–15 A.P.I.

- Shallow Cretaceous Shale: GR = 100–140 API

†Strictly speaking, all GR values should becorrected for borehole effect and formationdensity. However, this approximation is usu-ally satisfactory.

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c) Chart CalculationThe linear equations in (a) and (b) of this

section are good first estimates of shale vol-ume. Chart V

sh-1 (Figure E2) allows us to

correct for the non-linear relationship betweenV

sh and the GR deflection denoted as “x”. Line

(1) is generally used, yielding good interpreta-tion results.

d) Spontaneous Potential (SP)In waterbearing sands of low to moderate re-

sistivity, the ratio of SSP (static SP) to PSP(pseudostatic SP) is indicative of clay content,where

α = PSP/SSP and Vsh

= 1 - α

If hydrocarbons are present, α will be de-creased because of the further reduction ofPSP by the hydrocarbons. Also, when usingthis method to calculate V

sh, suitable bed thick-

ness must be present to obtain PSP and SSP.

Figure E2: Chart Vsh - 1: Shale Model Correction

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