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Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 241, part 2
Distribution of the Bandera Shale of the Marmaton Group, Middle Pennsylvanian of Southeastern Kansas--page 3 of 9

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Method of Investigation

Gamma-ray and Neutron Logs

The first step in determining the characteristics of sandstone units in the Bandera Shale in southeastern Kansas was to become familiar with the gamma-ray and neutron signatures of the Bandera Shale and the adjacent formations of the Marmaton Group. Because the thickness and general characteristics of the Altamont and Pawnee Limestones are variable, the thin, strongly radioactive black shales of the Lake Neosho and Anna Shale Members provided consistent stratigraphic markers that were found in all well logs, defining an easily recognized interval that was not truncated by post-depositional erosion.

The gamma-ray and neutron logs available for this study are from 469 wells, which are distributed across an area of about 1,280 square miles (3,315 km2) (fig. 3), averaging one log for every 2.73 sections. The distribution, however, determined in part by the distribution of potential petroleum accumulations, is not even. Another factor influencing data distribution is the fact that much of the petroleum-exploration drilling in the region predates the advent of geophysical well-logging.


Fig. 3. Distribution of Bourbon County outcrops, well logs, and cross sections used in this study.


Cross Sections

Cross sections (locations on fig. 3) were constructed using the Lake Neosho Shale Member as a datum. These cross sections (figs. 4, 5), combined with a grid of similar sections that were constructed, were used along with isopach and sandstone isolith maps (see figs. 6, 7) to verify the correlations and to study the character and distribution of the sandstones. Several cross sections were prepared to aid correlation and to help with paleoenvironmental interpretations. Two of these are shown for illustrative purposes: cross section A-A' (fig. 4) parallel to and west of the Bandera outcrop belt and section B-B' (fig. 5), along township 33 N., near and parallel to the Oklahoma state line.


Fig. 4. Gamma-ray logs along cross section A-A', showing a portion of the Marmaton Group between northeastern Allen County and south-central Montgomery County, Kansas, and illustrating the variation in stratigraphy and thickness parallel to the Bandera outcrop belt. See fig. 3 for location of cross section.


Fig. 5. Gamma-ray logs along cross section B-B', showing a portion of the Marmaton Group between south-central Cowley County and southwestern Labette County, Kansas, and illustrating the variation in stratigraphy and thickness perpendicular to the Bandera outcrop belt. See fig. 3 for location of cross section.


Sandstones were interpreted on each gamma-ray log by constructing a shale baseline and a sandstone baseline representing approximately 100% shale and approximately 100% sandstone, respectively. Deflections to the right of the sandstone baseline that were less than halfway to the shale baseline were considered to be either sandstone or limestone. Other than known limestone units such as the Amoret Limestone Member and Pawnee Limestone, intervals with low gamma counts were interpreted as sandstone.

The sandstones seem to appear at random in the section. No zones could be identified as likely to bear sandstones. Because of the low density of gamma-ray logs, few sandstone units could be carried to an adjacent log, indicating that most sand bodies were stratigraphically discontinuous. For the same reason the shapes of individual sand bodies could not be delineated.

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Kansas Geological Survey
Web version October 15, 1998
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email:lbrosius@kgs.ku.edu