COAL RANKS IN THE HARTON BOREHOLE

Abstract

(Short communication) Ridd, Walker and Jones (1970) interpreted the top coals in the Harton borehole as of normally-expected rank, the middle coals as having been raised in rank by heat from three leaves of the Whin Sill, and the lowest coals to be of such a high rank that the steep rank gradient from top to bottom required a special explanation. It can be inferred that they expected no increase in rank with depth, for they stated “… instead of returning [at the bottom] to reflectivities of about 1.00 as occur at the top of the hole …”. Further, they considered that “… it seems likely that the high coal rank gradient was the result of a high heat flow from the buried granite basement”. The coals range through a depth of 1,610 metres. The carbon content of the top coal, judged by reflectivity measurements, was 82 per cent, and of the bottom coal 92 per cent. The need for a special explanation disappears if it can be shown that the increase of carbon content is within the range to be expected from the increase of depth. The carbon contents of the coals of the Pie Rough borehole in north Staffordshire (Millott et al., 1946) increase linearly from 81 to 89 per cent over a depth of 1,070 metres. Linear extrapolation of the carbon/depth gradient to 92 per cent suggests that an increase from 82 to 92 per cent carbon would be attained in 1,340 metres. Thus the depth of ...