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Geological Log Analysis

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Evaporite Log Responses

Marine evaporites occur extensively in the Permian of subsurface Kansas where they form thick deposits. As products of evaporation from sea-water, the minerals are deposited in a set order as was first demonstrated by Usiglio in 1884. He evaporated sea-water and identified the formation of minerals to define the Usiglio sequence. The evaporitic order is useful as an interpretation tool to guide stratigraphic analysis and the mapping of ancient evaporite formations.

Although about eighty different minerals have been recognized in subsequent evaporite research, the minerals that occur commonly and in amounts to form distinctive beds are relatively few.

In addition, their highly distinctive petrophysical properties generally make them easy to recognize on a standard log suite.

The log responses for halite, anhydrite, and gypsum are summarized graphically below:

Log responses for halite, anhydrite, and gypsum.

We will now apply this simple template to the pattern recognition of evaporite beds from standard nuclear log suites.


The Upper Permian Nippewalla Group consists of red bed-evaporite sequences located in the subsurface of western Kansas. Halite, anhydrite, and gypsum occur widely and a thick halite bed is developed in far west Kansas as shown on this cross-section.

Cross section of Upper Permian Nippewalla Group in Hamilton and Kearny counties.

Log was recorded of part of the Nippewalla Group Blaine Formation evaporites in a well located in Hamilton County.

This log was recorded of part of the Nippewalla Group Blaine Formation evaporites in a well located in Hamilton County, Kansas.

The section shows an anhydrite bed at the top (AN) that overlies halite.

In evaluating the logs, the first thing to notice is that the only appearance of the density porosity curve is in the anhydrite, where it has a negative porosity reading on the limestone-equivalent scale, caused by its high density of 2.98 gm/cc. Otherwise, the low density of the halite at 2.04 gm/cc results in a limestone-porosity equivalent of 39% which moves the density porosity curve off the scale to the left. However, the high density porosity relative to the neutron porosity is indicated on the log by the shading between the two curves that is routinely done as a potential "Gas effect indicator."

The most obvious indicator of halite is provided by the caliper log in Track 1. When the hole is on gauge, then the caliper's measurement of the borehole diameter will match the drill-bit size, which is commonly 8 inches. Dissolution of halite by a fresh mud results in significant hole enlargement as shown here, ranging up to 14 inches. Notice how the irregular caliper trace is mirrored to some extent by the gamma-ray curve, which picks up thin and less soluble shale beds.

The photoelectric factor (PeF) curve meanders towards the halite theoretical ideal of 4.7 barns/electron, but this would only be realized for pure salt. The composite variation of the gamma-ray, neutron porosity, and photoelectric curves are reflections of relative shale content.

There can be occasions of localized washouts that disrupt the lithology effects. However, in this section, the porosity tool pads appear to have maintained good contact with the formation. Neutron porosity curve spiking matches shale beds shown by the gamma-ray log which correspond to a decreased diameter of hole as measured by the caliper. Either way, a washed-out shale or a shale that was not washed out is still a shale!

Next, let's look at a thick anhydrite bed.


The Lower Permian Stone Corral Formation can be traced across most of the subsurface of western Kansas as a highly distinctive marker bed that is widely used in structural mapping. In outcrop, it is usually a dolomite, or shaly dolomite with some anhydrite. In the subsurface, it consists mostly of anhydrite.

The impedance contrast of this dense slab with the shales above and below it, cause the Stone Corral to be a major reflector of seismic energy that can be identified readily on geophysical records. The gamma ray-lithodensity-neutron logs below show a classic section of the Stone Corral Formation from Ellis County in west-central Kansas.

Log of Stone Corral Formation from Ellis County in west-central Kansas.

The high density of the anhydrite causes the Stone Corral to have an extreme negative equivalent limestone porosity. The lowest readings of -16% limestone porosity units matches an equivalent density of 2.98 gm/cc. The neutron porosity shows zero value and reflects the fact that anhydrite contains little or no hydrogen in the form of water of crystallization or pore water. The photoelectric factor value expected for pure anhydrite is 5.05 barns per electron. This value appears to be closely matched by zones in the lower part of this Stone Corral unit. However, there are systematic excursions in the upper part that approach 7 barns per electron and indicate an additional element with higher atomic number. Most commonly, these higher values indicate small amounts of some iron-bearing mineral.

The logging properties of the anhydrite bed contrast starkly with the shale sections above and below. The erratic excursions of the porosity logs immediately above and below the Stone Corral are caused by thin washout zones, as demonstrated by the caliper. Otherwise, these shales are fairly uniform for the most part, with minor fluctuations that probably reflect compositional changes in quartz-silt and evaporitic mineral contents. Some zones may be sandy, while others at the top of the section show "necking" in which the hole has contracted to a narrower diameter than the bit size and may indicate the action of swelling clays such as smectite.

Now, let's look at a log of gypsum beds.


In some locations, the Blaine Formation contains beds of gypsum interbedded with shales, together with subsidiary anhydrite.

Neutron and density porosity logs for the Nippewalla Group section.

The neutron and density porosity logs for the Nippewalla Group section are scaled on a conventional primary scale that ranges between an equivalent limestone porosity of -10 units to 30 units. However, notice that the crosshatching of the neutron porosity shows that the trace is referenced to the secondary scale of 30 to 70 units through most of the section. Porosity logs are commonly shown in a "wrap-around" display, so that traces that "disappear" to the right or left, reappear on the opposite side of the track scaled to the next range. Potential confusion is clarified by the crosshatching conventions or by the expedient of following the trace upwards (or downwards) from a "normal" section.

Gypsum is easily recognized by its low gamma -ray value, very high neutron porosity exceeding 60 (due to the hydrogen contained in its water of crystallization) and density of 2.35 gm/cc. The gypsum beds of the Blaine Formation are obvious on the example log section, and can be distinguished immediately from anhydrite, which has a neutron porosity of -2 p.u. and a heavy density of 2.98. ( An anhydrite bed is located at a depth of 1055 feet.)

The underlying Flower-pot Shale is recognized mostly by its elevated gamma-ray values and appears to contain significant amounts of gypsum. A major washout feature can be seen from the caliper at the base of the Flower-pot Shale and is the cause of the excessively high neutron and density porosities in this zone. In the Cedar Hills Sandstone, the density exceeds the neutron porosity in a characteristic silica crossover, and porosities of this "free-drilling" sandstone range around 30 percent.

Next, an analogue for Mars?

Kansas on Mars

A Martian analog in Kansas: Comparing Martian strata with Permian acid saline lake deposits

Kathleen C. Benison
Department of Geology,
Central Michigan University,
Mt. Pleasant, Michigan 48859, USA


An important result of the Mars Exploration Rover's (MER) mission has been the images of sedimentary structures and diagenetic features in the Burns Formation at Meridiani Planum. Bedding, cross-bedding, ripple marks, mud cracks, displacive evaporite crystal molds, and hematite concretions are contained in these Martian strata. Together, these features are evidence of past saline groundwater and ephemeral shallow surface waters on Mars. Geochemical analyses of these Martian outcrops have established the presence of sulfates, iron oxides, and jarosite, which strongly suggests that these waters were also acidic. The same assemblage of sedimentary structures and diagenetic features is found in the salt-bearing terrestrial red sandstones and shales of the middle Permian (ca. 270 Ma) Nippewalla Group of Kansas, which were deposited in and around acid saline ephemeral lakes. These striking sedimentological and mineralogical similarities make these Permian red beds and evaporites the best-known terrestrial analog for the Martian sedimentary rocks at Meridiani Planum.

Geology May 2006

Photographs of outcrops on Mars and in Kansas.

A. Burns Cliff within ENDURANCE Crater (courtesy NASA and JPL. Outcrop is approximately 7 m high. B. Representative outcrop of Nippewalla Group in Barber County, Kansas. Pale blue cap rock is composed of gypsum/anhydrite with some hematite. Underlying dark red rocks are siltstones and sandstones composed of quartz, hematite, and gypsum/anhydrite poorly cemented by easily dissolved halite. Note person in green coat for scale.

Photographs of outcrops on Mars and in Kansas.

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Kansas Geological Survey
Placed on web March 24, 2017.
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