Hackley, and two co-authors, reported in 20-5 that the reflectance of huminite in 19 cuttings samples was determined in support of ongoing investigations into the coal bed methane potential of subsurface Paleocene and Upper Cretaceous coals of South Texas. Coal cuttings were obtained from the Core Research Center of the Bureau of Economic Geology, The University of Texas at Austin.
Geophysical logs, mud-gas logs, driller's logs, completion cards, and scout tickets were used to select potentially coal-bearing sample suites and to
identify specific sample depths. The locations of the 16 boreholes from which the cuttings were collected are indicated in figure 1.
The authors concur with the interpretation of Barker and others (2003), in that increased depth of burial is most likely responsible for the apparent rank elevation in South Texas. When the reflectance data presented herein are plotted as a function of depth, a general trend toward increasing rank at depth is indicated, although there is considerable scatter in the data (fig. 7). More importantly, there is no apparent relationship between the X-Y spatial location of
samples and reflectance value, such as would be the case if the speculative isograds of SanFilipo (1999) were in fact as originally drawn. In fig. 8A, the
reflectance data are plotted as a function of depth and by their X-Y spatial location as shown in fig. 8B.
Arbitrary geographic zones numbered 1-5 from east to west are indicated by color in figs. 8A and 8B. The zones are drawn similar to the trend of the
speculative rank isograds presented by SanFilipo (1999). As is apparent from fig. 8A, sample depth appears to dominantly influence huminite reflectance,
rather than spatial location, supporting the interpretation that increased depth of burial is responsible for the apparent rank elevation in the South Texas area for most of the samples. However, the two samples collected from the farthest points west (ZAVALA 247 AND PW WILCOX 618) are slightly elevated in reflectance compared to others collected from the same depth farther east. These two data points and the higher reflectance values of 0.50-0.58 % reported for the Olmos coals farther to the west and southwest of the present study area (Verdugo and Ariciaga, 1991; Barker and others, 2000) may reflect thickening of the post-Olmos Cretaceous section towards the southwest into the Maverick Basin.
These areas would have been exhumed from deeper burial under the Sierra Madre Oriental piedmont than strata farther to the northeast (see Ewing, 2003, fig. 6, p. 20).
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