Computerized Calculation of Lignite Resources in Texas

The Bureau of Economic Geology (BEG) is currently utilizing the National Coal Resources Data System (NCRDS) to estimate near-surface lignite resources in the state of Texas. Data provided to the BEG for this project primarily consist of geophysical logs from holes drilled for lignite exploration. These logs, obtained from industry sources on a proprietary basis, typically include density and resistivity traces, with occasional availability of natural gamma data. The boreholes average 150-200 ft in depth, with some reaching depths of up to 300 ft.

Due to the distribution of the data, accurate correlation of individual lignite seams is challenging. Therefore, resources are calculated by summing the thicknesses of seams encountered in each borehole (total lignite). There is no standardized NCRDS method for selecting data for the calculation of total coal resources; however, the BEG has developed a method through a series of trial methods.

The process begins with using PACER to search TXSTRAT for all information within a specified topographic quadrangle and a surrounding three-mile border. The additional border is included to ensure that resource calculation encompasses inferred resources from points located outside the quadrangle's latitudinal and longitudinal limits. All non-lignite (rock) lithologies in this initial file are then set to zero thickness, resulting in a file containing all data point locations with zero total thickness.

Next, the initial file is searched again for lignite lithologies in the three thickness categories specified by the USGS and two additional thickness categories used in Texas (2 to 3 ft, and greater than or equal to 3 ft). Five lignite files are generated based on these thickness categories and merged one at a time with the zero-thickness file. The thicknesses are then summed in each borehole for all five files. Data locations (boreholes) without coal or with seams thinner or thicker than the searched thickness category are assigned zero total thickness. This control mechanism improves interpolation of thicknesses between points.

While subsequent resource estimates may be conservative, this method prevents unrealistically large tonnages in the inferred resource category, addressing a major issue observed in earlier methods.