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Geological Log Analysis

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From the Dipmeter to Formation Image

Resistivity image logging extended the technology of the dipmeter by making microresistivity measurements from multiple buttons on each pad rather than a single button.

Although the records of microresistivity would be conventionally be presented as curves, the image presentation is made by displaying values on a gray-level scale and interpolating between buttons.

As a result, the most basic stratigraphy is immediately apparent from the image where:

The gray-scale is usually modified to incorporate a false color addition of yellow-orange-brown, so that image distinctions are more obvious to the human eye and tends to mimic the rock as it would appear visually.

However, it must always be remembered that the image is a resistivity map of the formation which is NOT the same as would be seen on a core photograph.

Composition plot was computed by the logging truck computer from gamma-ray, photoelectric factor, density, and neutron porosity logs.

Next, we will see how a section of Oread Limestone appears on an image log.

FMI Microresistivity Curves in the Oread Limestone

A formation microimaging (FMI) log was run in a well in the El Dorado field, Butler County, Kansas and the measurement curves for the Oread Limestone section are shown below.

  1. Track 1: Caliper and gamma-ray. The gamma-ray curve shows the shale-limestone stratigraphy of the Oread.
  2. Track 2: The borehole deviation from the vertical and the azimuth of Pad One
  3. Track 3: The microresistivity curves recorded by the buttons.

A formation microimaging (FMI) log was run in a well in the El Dorado field, Butler County.

Next, we will see the transformation of the multiple microresistivity curves into a gray-level image.

Resistivity Image of the Oread Limestone

Now that the microresistivity measurements are plotted as a gray-level image, the lithological implications are easy to relate to Oread outcrops in Lawrence, Kansas.

Gray-level image log from a well in the El Dorado field, Butler County.

In reviewing an image log, you should also refer to the other logs for a more complete geological interpretation. In this case, we have the gamma-ray and caliper curves in Track 1.

This example is a fairly old image log and there are ongoing improvements in borehole imaging technology as shown by a more recent image log of the Plattsmouth Limestone next.

Plattsmouth Limestone Resistivity Image Logs from Two Neighboring Wells

These images show part of the Plattsmouth Limestone from two wells in Sumner County, Kansas that are less than two miles apart.

Color image log from the Plattsmouth Limestone from two wells in Sumner County, Kansas.

Although paper copies of image logs are routinely distributed to the well contractor, image files are also supplied in graphic formats such as TIFF.

Graphic files can be processed by imaging software.

In this example, the bedding structure image has been subdivided by (free!) image software.

The result can be used to identify correlateable bed units or, because the processed bedding profile is a digital record, can be analysed for cyclicity structure by using Fourier analysis or other signal tools.

In the next example, we see how an image log of a Morrow Sandstone section compares with a core taken from the same interval.

A Morrow Sandstone Resistivity Image and Core Comparison

This example shows a comparison between a resistivity image log (left) and a core (right) of an Upper Morrow Sandstone channel from the Mustang East field in Morton County, Kansas, as described by DeVries(2006).

Comparison between a resistivity image log (left) and a core (right) of an Upper Morrow Sandstone channel.

The complete poster of image log analysis by DeVries is available as a downloadable pdf file from:

Adam DeVries made this study as part of his Master's thesis at Oklahoma State University, so image log studies are not confined to industry but are an academic option.

Resistivity image logs of clastic successions generally have an immediacy for visual interpretation by a geologist who has no background in traditional log analysis.

However two points should be borne in mind:

  1. This is an electrical image not an optical image, so that there will be strong similarities, but also some differences.
  2. The image is the rendition of a cylindrical borehole wall flattened into a plane, so that dipping planar features will sketch out sinusoids.

Next, we will make a more detailed examination of a Morrow Sandstone image log.

A Morrow Sandstone Resistivity Image Interpretation

The image log section corresponds to the interval marked as 'IMAGE' on the gamma-ray log.

The image log section corresponds to the interval marked as 'IMAGE' on the gamma-ray log.

The image orientation is marked at the top of the image log, with south in the center (180 degrees) and north to the right and left ( zero and 360 degrees).

The vertical white bands on the image correspond to the gaps between the pads and show a drift in orientation as the tool rotated when it was drawn upwards through the borehole.

The sinusoidal taces on the image are planes computed from dipmeter processing of the image microresistivity curves. Each sinusoid peak azimuth is the updip direction; the sinusoid trough azimuth is the downdip direction.

The thin limestone at the base (see gamma-ray log) is the white band at the bottom of the image log.

Many of the heterolithic features within the sandstones and shales are probably caused by bioturbation, while some distinctive burrows can be seen in the thinly interbedded sandstones and shales at the top of the image section.

Some black blobs in the middle sandstone may be pyrite. As a metallic mineral, pyrite is highly conductive and so will show strong image contrast, provided that the crystals are large enough to be registered by the microresistivity buttons.

Image logs of shales will often contain interesting geological features, as shown by the Hushpuckney Shale example next.

Image Log of a Black Shale: The Pennsylvanian Hushpuckney

On the left is a resistivity image log of a Hushpuckney Shale section in a well drilled in Barber County, Kansas.

On the right is an X-radiograph of a Hushpuckney Shale core from Leavenworth County, Kansas together with its interpretation by Algeo and Maynard (2004).

Image Log of a Black Shale: The Pennsylvanian Hushpuckney.

The Barber County Hushpuckney is from the extreme south of Kansas, where it is considerably thicker than the core from north-east Kansas.

However, it is easy to use the core interpretation as a basic guide to features seen on the image log.

In the lower, black shale phase, brighter yellow corresponds to higher resistivity, and picks out more carbonaceous-rich laminations that reflect varying anoxia in depositional environment.

In the upper, gray shale phase, the resistivity is lower, creating darker colors and burrowing features can be seen that disrupt the laminations.

Resistivity images of carbonates typically have less immediacy for a geologist trained in carbonate outcrop observation, but show distinctive geological features to a practised eye as shown by an Arbuckle Limestone section.

A Cambro-Ordovician Arbuckle Limestone Resistivity Image Example

This resistivity image is of an Arbuckle Limestone section from an injection well drilled in Finney County, Kansas.

Resistivity image is of an Arbuckle Limestone section from an injection well drilled in Finney County, Kansas.

The overlay of the density and neutron porosity logs in Track 1 show the lithology to be a dolomite.

The wellbore was drilled directionally at an angle of 30 degrees from the vertical towards the WNW.

The orientation of the borehole accounts for the strong sinusoidal pattern to the banding of lower resistivity, darker intervals and higher resistivity, lighter intervals, which have low dips as related to true horizontal.

The broad bands discriminating bedding and their true dip is shown by the processed tadpoles on the left of Track 3 as a few degrees to the NNW.

The banding highlights the cyclic character of peritidal platform deposits, with darker colors discriminating shalier and more conductive beds.

The blue pattern of tadpoles in the center pick up a distinctive feature which could, conceivably, be a stromatolite.

Notice the mottling of darker, isolated blobs which reflects "vugs", large pores that would be visible in core. The low resistivity of the vugs is caused by their conductive mud-filtrate content.

The sinusoidal traces that are discordant to the depositional banding are fractures discriminated by dipmeter processing.

The fracture tadpoles are shown on the right of Track 3 and are close to vertical in orientation. Notice that some fracture tadpoles point roughly south and others point north, which demonstrates that the difference of a degree or two from the vertical can change dip orientation by 180 degrees. More important is that the fracture strike is about WSW-ENE.

Image logging is more expensive than conventional logs, so let's review the reasons why resistivity imaging would be recommended as part of a logging program.

Reasons for Running a Resistivity Image Log

When image logs were introduced in the 1980's, the low price of oil discouraged the widespread use of this technology by industry. However, during this period image logs were run extensively on legs of the Ocean Drilling Program. Image logs were eagerly accepted by shipboard scientists, many of whom had been indifferent to conventional logs. Apart from the visual immediacy of the image logs, they had two major advantages:

Resistivity image logging has become progressively more common in industry applications, while tool technology continues to improve image quality and software image analysis becomes increasingly sophisticated.

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Kansas Geological Survey
Placed on web March 24, 2017.
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