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Geological Log Analysis

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Basement Rocks

Igneous and metamorphic basement rocks contain some significant oil and gas reservoirs in various parts of the world, particularly in Asian countries. So, for example, most of Vietnam's oil production comes from the fractured granite basement. However, other important basement reservoirs are found in North Africa, as well as one of the North Sea fields (Lancaster).

Consequently, logs are run routinely in Precambrian basement wells, but their interpretation is different from conventional sedimentary rocks, not only because the mineralogy is often more exotic but that interparticle porosity is minimal to non-existent.

However, some basic rules of log interpretation are outlined on the next page and match expectations of standard properties of igneous and metamorphic rocks.

Basic Log Interpretation Rules for Igneous and Metamorphic Rocks

The Gamma-Ray Log

Gamma-ray values

The photoelectric factor (PeF) log

In evaluating crystalline rocks, recall that the PeF value is a direct function of the aggregate atomic number of elements in the rock. Consequently:

The neutron porosity log

Open pores typically have very low volumes in igneous and metamorphic rocks. Consequently, the neutron response should be considered in terms of hydrogen content related to hydroxyls in mineral composition and bound water created by weathering processes. In summary:

The density log

The standard scaling of the density log in porosity units (either limestone or sandstone equivalent units) has only indirect meaning for igneous and metamorphic rocks, other than its use on an overlay convention with neutron porosity logs. However, the density log will also be recorded as bulk density in gm/cc on any logging run. The bulk density is a valuable diagnostic of igneous and metamorphic rocktype.

The great range in igneous and metamorphic rock composition means that individual formations should be considered on a case-by-case basis.

The rest of the lesson shows some examples of logs of basement rocks that occur in Kansas. First, a Precambrian granite.


The log below is from a well drilled in Russell County, where the Pennsylvanian LKC limestone-shale succession directly overlays a Granite Wash development from erosion of the Precambrian basement granite at the bottom of the well.

Log from a well drilled in Russell County, LKC limestone-shale succession directly overlays a Granite Wash.

The Precambrian granite has:

The overlying "Granite wash" is a general term for arkosic detrital material that immediately overlies Precambrian granitic rocks. Its age is assigned according to the overlying unit, so that when this is Upper Cambrian Lamotte Sandstone, then it is considered to be Precambrian. However, this well was drilled on an upwarped ridge, where the Granite wash is immediately succeeded by Pennsylvanian limestones and so is considered to be Pennsylvanian in age.

The contact between the Granite Wash and the granite can be seen in the marked increase in porosity to about 14%.

This well was completed with oil production from the Granite wash that was commingled with production from the Pennsylvanian limestones.

Some neighboring wells produce oil directly from the fractured Precambian granite, as documented by Walters (1953) including a well approximately a mile to the east that had an initial production of 429 barrels of oil when it was drilled in 1938.

Now let us look at the logs of metamorphic rocks.

Metamorphic Rocks in Kansas

Precambrian sediments and metasediments are found in a broad band across north-east Kansas. Rock types include schist, gneiss, quartzite, arkosic quartzite, arkose, and undifferentiated metamorphic clastics (Merriam, 1963).

Map of location of Precambrian sediments and metasediments in Kansas.

The first log example is of a chlorite schist.

Chlorite Schist

Terra Resources #1 Wangerin #1 is located in Smith County, Kansas and drilled through the Cambro-Ordovician Arbuckle Limestone into over a hundred feet of Precambrian metamorphics.

Log of Terra Resources #1 Wangerin #1 is located in Smith County, Kansas.

It is probable that the original plan had been to finish drilling when the hole encountered granitic basement. However this did not occur, so sidewall cores were taken and the Precambrian section was determined to be chlorite schist.

In looking at the logs, first notice the expanded scale with a range between -15 and 60 porosity units in order to accommodate the neutron porosity curve in the chlorite schist section

Now let us look at a well from a gneiss terrane.

Precambrian Gneiss Terrane

The basement in northern Kansas is generally considered to be a granitic terrane with patches of metasediments, while the Nebraskan basement appears to be composed of metamorphic gneiss and schist of island-arc origin penetrated by younger granite plutons.

A well was drilled into the Precambrian of Brown County close to the Nebraska border and penetrated metamorphic rocks. Details on this well were reported by Newell et al. (2007).

Map of Kansas and Nebraska showing basement terranes.

Part of the logged section is shown next.

Logs of Biotite Schist and Gneiss

The upper section of the Precambrian in this well was identified from cuttings as biotite schist, with a sharp contact and lower section of layered gneisses of differing composition ranging between aplitic gneiss, gneiss, and amphibolitic gneiss.

Logs of Biotite Schist and Gneiss.

The gneiss facies can be identified from the logs by:

Aplitic gneiss:

Amphibolitic gneiss:

Next, we review logs recorded from the deepest well in Kansas.

The Deepest Well in Kansas

The Midcontinent Rift System (MRS) is recognizable an elongate magnetic and gravity anomaly that extends from south-central Kansas into the Lake Superior region. The rocks that cause the geophysical anomalies are predominantly mafic volcanics interbedded with thousands of feet of clastic sediments deposited in response to subsidence of the rift.

Texaco Poersch #1 was drilled in 1984 to a depth of 11300 feet, penetrating the igneous rocks and sediments within the Kansas leg of the MRS. As the deepest hole in Kansas, it was over half a mile deeper than any other borehole.

Logs of Biotite Schist and Gneiss.

The rift rocks penetrated consisted of mafic volcanics and subsidiary mafic and acidic intrusives down to a depth of 7429 feet, which are probably about 900 million years old. Below this, the section is dominated by arkoses and subarkosic sediments, together with some minor mafic flows.

More details of this unique and historic hole are reported by Berendsen and others (1988).

Precambrian Rift Basaltic Lavas and Arkoses

The logs and cuttings are shown for an 800 feet section of the Precambrian. Note that in this instance, the neutron-density porosity scaling convention is calibrated with respect to an apparent sandstone scale.

Logs and cuttings are shown for an 800 feet section of the Precambrian.

The basaltic lava flows and gabbro of the upper sequence are characterized by

Some of the denser zones are matched with magnetite-rich basalts and sulfide-rich gabbro. The moderate neutron response is fairly typical of weathered basalts and appears to be caused by hydrothermal alteration mineral products.

The pegmatite zones are marked by much higher radioactivity,as a result of igneous fractionation, where potassium , thorium and uranium tend to be enriched in minerals of the later, more acid stages.

The sedimentary rocks of the lower sequence are mostly tight arkoses and subarkoses, with an average porosity of 1.8%.

The high gamma radiation of these sediments is caused by the potassium content of the feldspars, together with significant thorium and uranium retained from their granite source rocks.

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Kansas Geological Survey
Placed on web March 24, 2017.
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