Journal of Petroleum Geology, Vol.1,
No.4, pp. 39-62, 1979
©Copyright 2000 Scientific Press,
Ltd.
ORGANIC MATTER AS INDICATOR OF
THE DEGREE OF METAMORPHISM OF THE CARBONIFEROUS ROCKS IN THE
SOUTH WALES COALFIELDS
W. D. Gill*, F. I. Khalaf** and
M. S. Massoud*
* University of London, lmperial
College of Science & Technology, Department of Geology,
Prince Consort Road, London, SW7 2AZ, England.
** Kuwait Institute for Scientific
Research, P.O. Box 24885, Kuwait.
Abstract
Isoreflectance maps of vitrinites
in coals and phytoclasts (coaly inclusions) in the Coal Measures
of S. Wales show a progressive increase in vitrinite reflectance
with increase in coal rank and grade of metamorphism towards the
anthracite area in the west, without significant variation in
reflectances of coals with present depth. Differences between
values of vitrinite reflectance in coals and phytoclasts in the
same profile are recorded and attributed to variations in
depositional environments, and to susceptibility to alteration of
the host rocks. Spores in the Coal Measures are progressively
carbonized (from golden brown to black) towards the west, and
this is not a function of present depth of burial. Reflectance of
vitrinites in coals, degree of spore carbonization and coal rank
are used in definition of three different diagenetic zones in the
coalfield. The Coal Measures could have acted as a source for
accumulations of gas in the Permo-Triassic sequence of the south
Irish Sea, if necessary geologic conditions are present. Burial
metamorphism (application of Hilt's law) could have caused the
coalification of S. Wales coals up to the high and medium
volatile bituminous stages (up to the late diagenetic stage). A
relationship is established between the lateral variation in coal
rank in the coalfield and the regional variation in coal rank in
the southern British Isles, where two regions of markedly
different geothermal history are recognised. The Diagenetic Zone
in the E and S parts of the coalfields marks the final stage in
the thermal history of the cold region (the W margin of London
Stable Massif), whilst the Anchimetamorphic Zone in the NW part
(the anthracite area) represents the final stage in the thermal
history of the hot region (the S margin of St. George's Land).
