The petroleum industry today is a product of American business and with its 8,900 producing companies it is one of the most competitive industries in. America. Because of this it has emerged from World War II as one of our most highly specialized businesses, and never before has science been drawn upon and applied more vigorously in shaping a maturing industry.

Because we can never physically see the oil or gas reservoir in any of its in-place conditions, we must indirectly rely upon measurements of certain physical parameters to guide us in operating the reservoirs prudently. The development and operation of an oil or gas accumulation has, therefore, become a reservoir engineering enterprise; and more arid more operations are logically being based on factual reservoir information in this manner


A formation is a body of rock with considerable thickness large enough to be map able and has a unique geophysical characteristics which make it different from adjacent rock unit.

In petroleum exploration and development, formation evaluation is used to determine the ability of a borehole to produce petroleum. Essentially, it is the process of “recognizing a commercial well when you drill one”.

Formation evaluation answers the following questions:

  1. Do any zones in the well contain producible hydrocarbons?
  2. How much? (volume)
  • How much, if any, water will be produced with them?

These questions are mainly answered using Archie’s equation

in a hydrocarbon environment


Where  :

But in a formation without hydrocarbon


a well with a geophysical log data sheet used to characterize it.BASIC FORMATION EVALUTION TOOLS.

Formation evaluation is divided into two:

  1. Wire line logging tricks.
  2. Mud logging tricks/ coring.







Logging, electro logging or well logging means continuous recording of a physical parameter of the formation with depth. The primary objectives of the wire line logging are:


  1. The identification of reservoir
  2. The estimation of hydrocarbon in place.
  • The estimation of recoverable hydrocarbon.


Well logs are results of several geophysical measurements recorded in a well bore. They consist of key information about formation drilled.




  1. To identify the productive zones of hydrocarbon.
  2. To define the petro physical parameters like porosity, permeability, hydrocarbon saturation and lithology of zones.
  3. To determine depth, thickness, formation temperature and pressure of a reservoir.
  4. To distinguish between oil, gas and water zones in a reservoir.

Wire line logs are further classified into: the following


  1. SP(spontaneous potential)
  2. Gamma ray log






. Sonic


Density logs.





SP arises due to salinity contrast between formation water and   mud filtrate against   permeable beds. No current is sent into the formation. The SP log is recorded by measuring the potential difference in mille-volts between an electrode in the borehole and a grounded electrode at the surface.  The change in voltage through the well bore is caused by a build up of charge on the well bore walls. Shale and clays will generate one charge and permeable formations such as sandstone will generate an opposite one. This builds up of charge in turn caused by differences in salt content and formation water.



To delineate porous and permeable reservoir rocks

To determine bed boundaries and bed thickness

To evaluate the formation water resistivity

To estimate the fraction of clay Correlation of  permeable beds




The production gamma ray tool comprises a sodium iodide scintillation crystal and photomultiplier to measure incident gamma radiation. The single conductor passing through the tool carries telemetry and power. The detector is unshielded and will thus accept radiation from any direction.


  1. Lithology identification
  2. Depth correction
  • Identification of radioactive scale
  1. Monitoring of radioactive flow tracer
  2. Evaluation of shale volume
  3. Delineation of nonradioactive mineral including coal beds




These logs are use to know the type of fluid present in the reservoir.


Formation  Resistivities higher than 1000 ohm-m are uncommon in permeable formations. In a formation containing oil or gas, both of which are electrical insulators resistivity is a function of formation factor, brine resistivity and water saturation which in term depends on true resistivity. Of the formation parameters resistivity is of particular importance because it is essential for saturation determination mainly of  the hydrocarbon.  Depending upon the environment under which resistivity logs are recorded. There are two types of resistivity Logs.  They are Latero logs and Induction logs. DUAL LATERAL LOG  The dual lateral log has been one focused electrode device d fo ons.  The DLL consists of an electronics section and a mandrel section. The mandrel supports the electrode  from centre electrode is forced to flow laterally into the formation by the focusing action of electrodes surrounding the centre electrode. It provides two measurements of the subsurface resistivity simultaneously. The two measurements have differing depth of investigation are called deep resistivity (Rd) and shallow resistivity (Rs).

DLL consist of a current emitting centre electrode positioned between guide electrodes. A known current is passed through the current electrode with a return electrode at the surface. Simultaneously a potential is applied to the focused electrode to keep zero potential difference between guard and centre electrode thereby the current is focused in to the formation. Thus the potential difference produced is equivalent to the formation resistivity. The lateral log current path is basically a series circuit consisting of the drilling fluid, Mud cake, flushed zone, invaded zone and the virgin zone, with the largest voltage drop occurring over the highest resistance zone.

The total amount of current emanating from an electrode must flow through any medium that encompasses the electrode.  The depth of investigation of a lateral log is defined M as the depth at which 50% of the total measured voltage is dropped.



MLL is pad device.MIL has small vertical resolution and depth of investigation.

Archie’s equation is use to calculate the saturation of the flushed zone.

Also  Used to determine

Rxo, Exact thickness of formation beds. Rxo can be used wit

Sxo = √ (a / øm) * (Rxo / Rmf)

Archie’s equation, rewritten for saturation of the flushed zone, to determine moveable oil.

Current from a measure electrode is forced into the flushed zone by guard electrodes to the return electrode. The current to the measure electrode is measured as is the voltage.

The MLL is a single tool contains an arm with the pad attached. The central electrode is the measure electrode. The eight other electrodes are guard electrodes.





Induction tools are based on principles of electromagnetic induction. A magnetic field generated by an AC electrical current flowing in a continuous loop/transmitter coil. The transmitter coil induces ground loop currents in the formation. This ground is magnetic field current loops will in turn have an associated alternating magnetic field which will induce a voltage in the receiver coil, the magnitude of which is proportional to the formation conductivity.


  1. It works in oil based mud and air filled holes where latero tool fails.
  2. Tool accuracy is excellent for formations having low to moderate resistivity (up
  3. The Dual Induction Latero (DIL) tool records   three resistivity different depths of investigation ( ILD,ILM & LL3)




Mud filtrate Invasion profile.

Is magnetic field f current loops will in turn have an associated alternating magnetic field  which will induce a voltage in the receiver coil, the magnitude of which is proportional to the formation conductivity.

It works in oil based mud’s and air filled holes where latero tool fails.

Tool accuracy is excellent for formations having low to moderate resistivity (The Dual Induction Latero (DIL) tool records   three resistivity different depths of investigation.

Applications of resistivity logs.

  1. True formation resistivity and flushed zone resistivity determination.
  2. A record of mud filtrate invasion profile.
  • Indication of producible hydrocarbon.
  1. Correlation of different formations.




Porositity is the ratio of pore space in rocks to the bulk volume.  Porosity values can be obtained from sonic logs, formation density or a neutron logs. In addition to porosity these logs are affected by parameters such as shalines lithology , nature of pore fluids



In neutron logs we use a chemical source {Americuim –beryluim neutron bulb} which provides as continuous source of energy of about 4.5MeV/14MeV. When neutron collides with the nucleus of atoms in the formation the neutron lose it energy and exits the nucleus of the atoms in the formation. When the exited nucleus returns back to its normal stage it emits Gamma rays characteristics to the atoms. The analysis of the gamma ray spectrum identifies the composition of the elements in the formation. When the energy of the formation reduces to the thermal level and collides with hydrogen atoms its energy reduces to 0.025ev also the neutrons are captured emitting gamma ray. These uncaptured neutron reaching the detector is a measure of the hydrogen index of the formation.




The sonic tool measures the interval transit time, Δt or the time in microsecond for an acoustic wave to travel through one foot formation, along a path parallel to the borehole,

Which is the reciprocal of the velocity of the compressional sound wave. Wyllie proposed the following empirical relation for determination of porosity from the sonic log: Ø = ( Δt  –  Δm)/  ( Δt,t  –  Δm)

Where Δt,t  and Δm are the transit times in the pore fluid and rock matrix, respectively. This time average relation is good for clean, compacted formations of intergranular porosity containing liquids.




The density measures formation bulk density and photo electric absorption index of the lithologic column penetrated. The δb density depends on fluid density and matrix density in porous formation, and Pe depends on atomic number used to determine the lithology of formation. To measure δb and Pe gamma rays are directed to the formation. The detectors measure the gamma ray flux resulting from scattering and absorption effect of the formation. The higher the formation density, the lower the gamma ray intensity at the detectors.



This refers to the picking up of samples from the well site. These samples may be in form of cuttings from the shale shaker or gases from the degasser. It may also be the placement of some logging tools inside the drill collar to record geophysical properties while drilling progresses [LWD]



  1. CORING: a core is a cylindrical sample of rock obtained by a centre of a mass by drilling. The process of obtaining and analysis of a core is known as coring.

    cores from diamond bits


  1. CONVENTIONAL CORES:these are cores recovered after drilling . Mostly used in hard formation.


  1. SIDE WALL CORE:in this case a hollow cylinder is driven into the formation with a charge of explosives, these cylinders penetrate only relatively shallow formations.


  1. RUBBER SLIP CORES: these cores are used for soft formation


  1. DIAMOND CORE:in this case a diamond studed bit is used. better recovery are and a better recovery is obtained as cores can be as long as 60fst.



  1. Porosity
  2. Resistivity
  • Rock compressibility
  1. Rock strength
  2. Fluid flow properties


Thus Logs are an explorationist’s eye.  Log measurements,  can give the majority of the parameters required by all. Specifically, logs can provide either a direct measurement or a good indication of porosity, both primary and secondary (fractures and vugs),  permeability, water saturation and hydrocarbon movability,  hydrocarbon type (oil, gas, or condensate), lithology, formation (bed) dip and strike, sedimentary environment, travel times of elastic waves in a formation

From this data, it is possible to obtain good estimates of the reservoir size and the petroleum     hydrocarbons in place.

Logging techniques in cased holes can provide much of the data needed to monitor primary production and also to gauge the applicability of water flooding and to monitor its progress when activated.

In producing wells, logging can provide measurements of flow rates, fluid type, pressure, residual oil saturation. From these measurements, dynamic well behaviour can be better understood, remedial work can be planned and secondary or tertiary recovery proposals can be evaluated and monitored.

In summary, when logging is properly applied, it can help to answer a great many questions from a wide spectrum of special interest groups on topics ranging from basic geology to economics. Of equal importance, however, is the fact that logging by itself cannot provide answers to all formation evaluation questions. Coring, core analysis, and formation testing are integral parts of any formation evaluation effort.This is an instrument which gives maximum information at a very minimal cost.  This also acts as a driller’s tool during complication- a third hand for a completion engineer.   Thus no hydrocarbon can be produced without the intervention of Logs.

UGOCHUKWU SOSTHENES -Petroleum Engineering and Geo-science department/ Petroleum Training Institute , Nigeria.


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