- GeoRef, Copyright 2006, American Geological Institute.
Organic carbon content of freshly cut core samples of Appalachian Devonian black shale has been measured using several methods. The results show several things of interest. The precision of the individual techniques is + or -3% or better. However, the comparison of results of organic carbon by-difference (total minus carbonate), acid leach-combustion, and wet oxidation techniques shows differences for most of the values (one sample excluded) within the range of +14% to -8% of the mean value of the three techniques. In general the organic carbon by-difference is 13% greater than the acid-leach and wet-oxidation techniques. The differences in organic carbon values are related to the fate of soluble. hydrolyzable, or volatile organic materials. Experimental measurements of volatile and leachable organics explain these results. Samples collected less than 30 cm up or down in the same core show variations in organic carbon content of from -59% to >+21% (as determined by wet oxidation technique) and demonstrate the inhomogeneity of the organic content of shales. These short range variations, due to depositional factors, must be taken into account when making resource assessments, such as natural gas, based on organic carbon content.