GEOLOGICAL LOG INTERPRETATION TUTORIAL Text and Figures by Geoff Bohling and John Doveton The following pages will familiarize you with the basics of the geological interpretation of common logs as they appear on hard-copy blue-line logs (or their electronic raster copies). The lithologies considered are those common in sedimentary successions, with particular emphasis on sandstones, limestones, and dolomites, which have potential for oil, gas, or water production. However, other log keys are given of evaporite sequences and coal-bearing clastic successions. The logs used in interpretation are all measurements of nuclear properties, which are sensitive to both fluids and gases within the pore space and the mineral composition of the rock framework. They are the gamma-ray, neutron porosity, bulk density, and photoelectric index logs. These logs are available for thousands of wells across Kansas, either as paper copy, raster or digital logs. Mastery of the principles described in this tutorial and practice with the Oz Machine will give you a start with the skills to "read the rocks" from these wireline logs. The Gamma Ray Log A majority of the elements are found in a variety of isotopic forms. Many of these isotopes are unstable and decay to a more stable form, while emitting radiation of several types. Gamma rays have significantly high penetrations and can be measured by simple devices such as Geiger counters or scintillation detectors on logging tools. Of the many radioactive isotopes which are known, only three types occur in any appreciable abundance in nature: • The uranium series • The thorium series, and • The potassium-40 isotope The measurement scale of the gamma-ray log is in API (American Petroleum Institute) units, accepted as the international reference standard that allows consistent comparisons to be made between a wide variety of gamma-ray counting devices. The API standard is set by the primary calibration test pit at the University of Houston where a radioactive cement calibrator is assigned a value
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GEOLOGICAL LOG INTERPRETATION TUTORIAL
Text and Figures by Geoff Bohling and John Doveton
The following pages will familiarize you with the basics of the geological
interpretation of common logs as they appear on hard-copy blue-line logs (or
their electronic raster copies). The lithologies considered are those common in
sedimentary successions, with particular emphasis on sandstones, limestones, and
dolomites, which have potential for oil, gas, or water production. However, other
log keys are given of evaporite sequences and coal-bearing clastic successions.
The logs used in interpretation are all measurements of nuclear properties, which
are sensitive to both fluids and gases within the pore space and the mineral
composition of the rock framework. They are the gamma-ray, neutron porosity,
bulk density, and photoelectric index logs. These logs are available for thousands
of wells across Kansas, either as paper copy, raster or digital logs. Mastery of the
principles described in this tutorial and practice with the Oz Machine will give
you a start with the skills to "read the rocks" from these wireline logs.
The Gamma Ray Log
A majority of the elements are found in a variety of isotopic forms. Many of
these isotopes are unstable and decay to a more stable form, while emitting
radiation of several types. Gamma rays have significantly high penetrations and
can be measured by simple devices such as Geiger counters or scintillation
detectors on logging tools. Of the many radioactive isotopes which are known,
only three types occur in any appreciable abundance in nature:
• The uranium series
• The thorium series, and
• The potassium-40 isotope
The measurement scale of the gamma-ray log is in API (American Petroleum
Institute) units, accepted as the international reference standard that allows
consistent comparisons to be made between a wide variety of gamma-ray
counting devices. The API standard is set by the primary calibration test pit at the
University of Houston where a radioactive cement calibrator is assigned a value
of 200 API units and conceived originally so that a typical Midcontinent shale
would register at about 100 API units.
Analyses of the North American Shale Composite (NASC) reference standard
reported values of Th 12.3 ppm, U 2.66 ppm., K 3.2%, which converts to an
equivalent gamma-ray log reading of 121.7 API units. Although higher than the
vague assertion that a typical Midcontinent shale should read about 100 API
units, the hypothetical log value of the NASC standard is a good match with the
gray shales of the Pennsylvanian succession shown in the figure at right. The
black shales, however, are prominent as thin anomalously radioactive zones.
Their markedly different character is produced by a high U content that
supplements radioactive sources in gray shales of 40K contained in illite and other
K-bearing minerals, and Th contained in monazite in the silt and clay fraction
and adsorbed at clay-mineral surfaces.
In the majority of stratigraphic and petroleum geological applications, the gamma
ray log is used as a "shale log", both to differentiate shales and "clean"
formations and to evaluate shale proportions in shaly formations. Typical
sandstones, limestones and dolomites have relatively low concentrations of
radioactive isotopes as contrasted with shales. Most carbonates show very low
levels of radioactivity unless they contain disseminated shale or have been
mineralized by uranium-bearing solutions. Simple orthoquartzites show similarly
low values, although relatively high readings may be introduced by significant
amounts of shale, felspar, mica or heavy minerals such as zircon.
Density and Neutron Log Overlay
The gamma-ray log generally allows a basic distinction of shales from non-shales
but is not usually diagnostic of the rock type in hydrocarbon reservoir or aquifer
formations. Neutron and density logs are used to evaluate porosity in these units
but are also affected by the neutron moderating characteristics and densities of
the formation minerals. By overlaying the two logs on a common reference scale,
a true volumetric porosity can be estimated and the formation lithology
interpreted. A scale of equivalent limestone percentage porosity is the most
commonly used reference because limestone is intermediate in its neutron-
density properties between sandstone and dolomite.
A hypothetical overlay is shown of neutron and density logs for some common
reservoir lithologies and a shale in the figure. Shales show a high gamma-ray
reading, a high neutron reading, and a moderate density reading. Limestones
generally have a low gamma-ray value, and a coincident density and neutron
response, because of common calibration to an assumed limestone porosity scale.
Dolomites have a low gamma-ray value, a relatively low density porosity
(because the grain density of dolomite is higher than calcite) and a relatively high
neutron reading (because the neutron moderating character of dolomite is higher
than calcite). Sandstones have a low gamma-ray value, a relatively high density
porosity (because the grain density of quartz is less than calcite), and a relatively
low neutron reading. The true, effective porosity of shale-free zones in the
reservoir lithologies is approximately midway between the two extremes of the
neutron and density porosities.
The Photoelectric Index
The photoelectric index (Pe) is a supplementary measurement by the latest
generation of density logging tools, and records the absorption of low-energy
gamma rays by the formation in units of barns per electron. The logged value is a
direct function of the aggregate atomic number (Z) of the elements in the
formation, and so is a sensitive indicator of mineralogy. The values are less
sensitive to pore volume changes than either the neutron or density logs, so that
the index is an excellent indicator of mineralogy. The common reservoir mineral