Interpretation of the depth to Wilcox and apparent thickness maps along with published measured sections and cross sections indicates that portions of Texas, Louisiana, Mississippi and Alabama contain broad areas where a 20 to 40 foot net thickness of lignite and coal occur at shallow depths. The thicker coal zones are attributed to growth faulting or rift zones influencing peat deposition. The depth to Wilcox map shows several areas where dome-like uplifts and bench-like coal-bearing rock are buried to depths less than 5,000 ft.
Vitrinite reflectance data for the Wilcox Group is sparse. Searches of the AGI Georef, USGS Organic Geochemistry, Pennsylvania State University coal library, and State Geological Survey publications as well as other web data sources has turned up mostly information on shallow, mineable coals. Data from the deeper Wilcox group will mostly require new measurements of vitrinite reflectance from core and cutting samples.
What is known about rank is that shallow occurrences of the Wilcox "lignite" ranging in depth from outcrop to <4,000 ft, locally reach a maturity equal to bituminous coal. This rank level is an important benchmark for CBM prospects because it marks the onset of thermogenic gas generation.
In conclusion, the data indicate that CBM prospects for the Gulf Coast are better in South Texas (Webb, Dimmitt, LaSalle, and Frio counties) because the coal rank is anomalously high there and the coals occur at shallow depths over an extensive area. There are also local structures and apparent stratigraphic traps in this area that make attractive drilling targets. Finally, these higher rank southern Texas Wilcox coals are also plumbed to an apparently very thick section of Wilcox to the east that is mature with respect to gas generation and could provide significant thermogenic gas to these targets.
The AML program from the Illinois State Geological Survey, "convert-ism", assigns a value to the item DEPTH by taking one-half the contour interval and adding it to the minimum contour that defines a polygon. For example, if the contour interval is 100, and a polygon is bounded by the 100 and 200 contour lines, then the DEPTH value assigned to that polygon is 150.
The polygon coverage was projected to geographic, DD, NAD 83. The polygon attributes for depth to the top of the Wilcox Group range from 0 - 16,750 ft.
Lines from this file were used to create wlcxdptlg in order to label select contour lines.
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