Definition of Well Log Curves

Written by kelvin o'donahue
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Definition of Well Log Curves
The use of well logs is critical to identification of hydrocarbon reservoirs. (oil well image by michael langley from

A well log is a collection of recorded measurements of the physical characteristics of the rocks lining the walls of an oil or water well, plotted against depth. A curve is any one such measurement.

Purpose of Logs

The different log curves have been developed as a means of remotely evaluating and describing the rocks at depth and the fluids that fill pore spaces in those rocks. Logs are mainly used to estimate quantitative properties such as porosity and permeability.

Types of Logs

There are thousands of well log curve types. Log types can be grossly separated into two types: direct measurements of rock properties and induced estimates of properties. Direct measurements include logs that calculate the speed of sound through a layer (sonic logs), measurements of natural radiation (gamma ray logs) and measurement of the difference in electrical charge between a layer and the Earth's surface (spontaneous potential). Induced measurements estimate rock and fluid properties based on transmission of electrical current or absorption of radioactive particles or other, more esoteric means.

Interpretation of Logs

Well logs provide a method for qualitative analysis of the rock and fluid properties at depth in an oil or water well, but there is no direct method of identifying the type and volume of resources present. Instead, log curves must be examined and combined with one another and with other information about the area. Professional well log analysts are highly trained and make use of sophisticated equipment and computer software to increase the probability that estimates made with well correctly describe the rocks at depth.

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