water saturation, fluid type (oil/gas/water)
From these, many corollary parameters can be derived by integration (and other means) to arrive at values for:
reserves (the recoverable fraction of hydrocarbons in-place)
mapping reservoir parameters
But not all users of wireline logs have quantitative analysis as their objective. Many of them are more concerned with the geological and geophysical aspects. These users are interested in interpretation for:
regional structural and sedimentary history
In quantitative log analysis, the objective is to define
the type of reservoir (lithology)
its storage capacity (porosity)
its hydrocarbon type and content (saturation)
its producibility (permeability)
Gamma Rays are high-energy electromagnetic waves which are emitted by atomic nuclei as a form of radiation
Gamma ray log is measurement of natural radioactivity in formation verses depth.
It measures the radiation emitting from naturally occurring U, Th, and K.
It is also known as shale log.
GR log reflects shale or clay content.
Clean formations have low radioactivity level.
Correlation between wells,
Determination of bed boundaries,
Evaluation of shale content within a formation,
Depth control for log tie-ins, side-wall coring, or perforating.
Particularly useful for defining shale beds when the sp is featureless
GR log can be run in both open and cased hole
The spontaneous potential (SP) curve records the naturally occurring electrical potential (voltage) produced by the interaction of formation connate water, conductive drilling fluid, and shale
The SP curve reflects a difference in the electrical potential between a movable electrode in the borehole and a fixed reference electrode at the surface
Though the SP is used primarily as a lithology indicator and as a correlation tool, it has other uses as well:
shale volume indicator
porosity indicator, and
measurement of Rw (hence formation water salinity).
The Neutron Log is primarily used to evaluate formation porosity, but the fact that it is really just a hydrogen detector should always be kept in mind
It is used to detect gas in certain situations, exploiting the lower hydrogen density, or hydrogen index
The Neutron Log can be summarized as the continuous measurement of the induced radiation produced by the bombardment of that formation with a neutron source contained in the logging tool which sources emit fast neutrons that are eventually slowed by collisions with hydrogen atoms until they are captured (think of a billiard ball metaphor where the similar size of the particles is a factor). The capture results in the emission of a secondary gamma ray; some tools, especially older ones, detect the capture gamma ray (neutron-gamma log). Other tools detect intermediate (epithermal) neutrons or slow (thermal) neutrons (both referred to as neutron-neutron logs). Modern neutron tools most commonly count thermal neutrons with an He-3 type detector.