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Some basic Log interpretation
Mastery Items:
What is the Archie Equation and why is it important?
Basic reservoir characteristics?
Basic source rock characteristics?
Lithology identification and determination
Archie Equation for Sw and Sxo
Water saturation (Sw) of reservoir’s uninvaded zone is
calculated by the Archie (1942) formula.
S
w

a  Rw
 m Rt

1
n
Where:
Sw = water saturation of the uninvaded zone (Archie method)
Rw = resistivity of formation water at formation temperature
Rt = true resistivity of formation (i.e. RIld or RLLd corrected for
invasion)
Ø = porosity
a = tortuosity factor
m = cementation exponent
n = saturation exponent which varies from 1.8 to 2.5 but is
normally equal to 2.0
Archie Equation for Sw and Sxo
S
xo


a  Rmf
 Rxo
m

1
n
Where:
Sw = water saturation of the uninvaded zone (Archie method)
Rmf = resistivity of the mud filtrate at formation temperature
Rxo = shallow resistivity from Laterolog-8*, Micropherically
Focused Log*, or Microlaterolog*
f
= porosity
a = tortuosity factor
m = cementation exponent
n
= saturation exponent which varies from 1.8 to 2.5 but is
normally equal to 2.0
Water saturation of the flushed zone (Sxo) can be used as an
indicator of hydrocarbon moveability
Ratio Method
The Ratio Method identifies hydrocarbons from the difference
between water saturations in the flushed zone (Sxo) and the
uninvaded zone (Sw). when water saturation of the uninvaded zone
(Sw) is divided by water saturation of the flushed zone (Sxo), the
following results:
 Rxo Rt 
 Sw 


  
 Rmf R 
 S xo 
w 

2
Where:
Sw = water saturation uninvaded zone
Sxo = water saturation flushed zone
Rxo = formation’s shallow resistivity from Laterolog-8*, Microspherically
Focused Log*, or Microlaterolog*
Rt = formation’s true resistivity (RIld or RLLd corrected for invasion)
Rmf = resistivity of the mud filtrate at formation temperature
Rw = resistivity of the formation water at formation temperature
Basic reservoir characteristics
 Porosity,φ
(Total, effective, primary,
secondary and intercrystalline)
 Permeability (K) (Absolute, relative)
 Saturation (S) (Sw, Sxo, Sh, So, Sg,
Shr, Shm) and their interrelationships.
POROUS MEDIUM
STANDARD
REALITY
Various
pore
geometries with
different effective
path length
Water-Wet Hydrocarbon-Bearing Formation
Basic source rock
characteristics

Determination of organic mattr content
 Determination of organic carbon content
Basic source rock
characteristics

Determination of organic mattr content
 Determination of organic carbon content
SOURCE ROCK EVALUATION
Organic matter content can he determined directly from laboratory
analyses of the source rock samples (shale, limestone or marl), but
indirect methods based on wireline data offer the advantages of
economic, ready availability of data and continuity of sampling of
vertically heterogeneous shale section .
A hydrocarbon source rock evaluation system is preliminary
revealed by showing the effect of the implicated organic material on
the responses of some specific types of logs as gamma-ray,
resistivity, density and sonic logs .
The total organic matter content of the shale can be calculated either
from the gamma-ray or the density logs.
Discrimination of Source Rocks from Non-Source Rocks
There is a simple classification rule for separating source rocks
from non-source rocks on the basis of quantitative wireline log
parameters. Two equations have been used to discriminate roughly
the source rocks from the non-source rocks, on the basis of log
combinations (Sonic-Resistivity) and (Density-Resistivity) .
Bulk Volume of Water
The product of a formation’s water saturation (Sw) and its porosity
(f) is the bulk volume of water (BVW).
BVW = Sw x Ø
Where:
BVW = bulk volume water
Sw
= water saturation of uninvaded zone (Archie equation)
Ø
= porosity
LITHOLOGY IDENTIFICATION AND DETERMINATION
In oil field logging applications, the prime importance is directed to
define the types and amounts of fluids encountered in the
formations. These determinations require the calculation of the
formation porosity, consequently the estimation of the
shale
volume. Thus, the shale content is an important quantitative
function of log analysis.
VOLUME OF SHALE
Shale volume is needed for correcting the porosity and water
saturation results for the biased effects of shale. It is considered as
an indicator for reservoir quality, in which the lower shale content
usually reveals a better reservoir quality. In addition to the shale
volume, it is important to determine the types of shale for choosing
the appropriate shale model, which can be utilized for selecting the
suitable water saturation model .
DETERMINATION OF MATRIX COMPONENTS
The simultaneous equations are considered an analytical way
used normally for accomplishing this target. Such equations
have important applications in the mathematical field. Their
uses are also extended to the geological fields, in which they
are utilized in the formation evaluation to determine the
lithologic contents and the accompanied porosities in fractions
. It can be more easy through the use of the computer system,
due to the complexity of their solution by manual treatment.
There are four equations reflecting the normal rock
constituents (porosity + shale + silica + carbonate) for every
type of the porosity tools (sonic, density and neutron).
DETERMINATION OF POROSITIES
Porosity is the volume of the non-solid portion of the rock, that is
filled with fluids divided by the total volume of the rock .
Primary porosity is the porosity developed by the original
sedimentation process, by which the rock was created. It is often
referred to in terms of percentage
Rock porosities are usually measured from well logs and
corrected using the porosity tools (Density, Neutron and Sonic).
These porosity tools are largely responsive to porosity and, also,
affected by other geologic factors, e.g. formation matrix
lithology, type of fluid present in the pores and the type of
porosity. This has allowed the use of these logs in combination
for the determination of the total porosity (T), as well as the
secondary porosity (sec) and effective porosity (E).