Well Log Suites


(source: Emery, 1996)
This log measures the bulk resistivity (the reciprocal of conductivity) of the formation. Resistivity is defined as the degree to which a substance resists the flow of electric current. Resistivity is a function of porosity and pore fluid in a rock. Porous rock containing conductive fluid (such as saline water) will have low resistivity. A non-porous rock or hydrocarbon-bearing formation has high resistivity. This log is very useful for determining the type of fluids in formations and is frequently used as an indicator of formation lithology.

Spontaneous potential (SP) logs
This log measures the electrical current that occurs naturally in boreholes as a result of salinity differences between the formation water and the borehole mud filtrate (formation and surface). These logs are used as indicators of permeable beds (including determining permeable sands and impermeable shales) or for locating bed boundaries. The SP log was one of the first tools to be used to distinguish shale from sand in clastic sequences (zero matches pure shale while high SP values match sand).

Gamma ray (GR) logs 
This log records the radioactivity of a formation. Shales (or clay-minerals) commonly have a relatively high gamma radioactive response, and consequently gamma ray logs are taken as good measures for grain size (and subsequently inferred depositional energy). Thus coarse-grain sand, which contains little mud, will have low gamma ray value, while a fine mud will have a high gamma ray value. The values range of gamma ray is measured in API (American Petroleum Institute) units and range from very few units (in anhydrite) to over 200 API units in shales.
Gamma ray logs are one of the most commonly used logs for sequence stratigraphic analysis. As explained above the variation in GR log character is interpreted to be a proxy for grain size and so enabled these logs to play a major role in the sequence stratigraphy analysis.The rationale behind this interpretation is that variations in the GR are related to the presence of organic matter that sequesters radioactive minerals responsible for the GR signal.In this case it is possible to:1) identify the portions of the section which had higher accumulations of organics and 2) those for which this has been reduced by winnowing and oxidizing effects! In turn, the presence or absence of organic matter is assumed to be a proxy for the winnowing and oxidizing effects of shallow marine waters. The result is that abrupt changes in the GR log response are interpreted to be related to sharp lithological breaks associated with unconformities and sequence boundaries. The principle shapes that GR log signal makes frequently used for interpreting the depositional setting of sedimentary cycles.

Neutron logs
This log measures the porosity of a formation, indicating in its response the quantity of hydrogen present in the formation. The log is calibrated to limestone. The linear limestone porosity units are calibrated using the API Neutron pit in 19% porosity, water-filled limestone is defined as 1000 API units. This log is useful in measuring lithology (usually in combination with density log).

Density logs
This log is a measure of the formation's bulk density and is mostly used as a porosity measure. Different lithologies can also be determined using density log based on returned density value. For example, pure quartz will have a bulk density (g/cm-3) up to 2.65, coal 1.2-1.8, halite 2.05, limestone up to 2.75, dolomite up to 2.87, anhydrite 2.98.
Density is mostly commonly used in conjunction with neutron logs to determine lithology of formation (density-neutron suites such as Schlumberger FDC-CNL suite).

Sonic (acoustic) Logs
This log measures of the speed of sound in the formation, and is related to both the porosity and lithology of the rock being measured. Thus, if the lithology of a formation is know, this log can be used to determine its porosity. shales have lower velocity (higher transit time) than sandstone of same porosity, making this log a good indicator of grain size.
sonic log values (in ms/ft) for some rock types are: sandstone 51-56, limestone 47.5, dolomite 43.5, anhydrite 50, halite 67.

 
Friday, March 27, 2015
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