Loss of Organic Carbon from Source Rocks During Thermal
Maturation
Alan R. Daly, Janell D. Edman
Preliminary source-rock evaluations of mature and overmature sediments are
often based solely on total organic carbon (TOC) content because pyrolysis
yields are substantially decreased during maturation
. Although such reductions
in pyrolysis yields during
maturation
are well established, possible decreases
in TOC as a result of
maturation
are commonly ignored. By using a series of
balanced equations, it is possible to show that appreciable reductions in TOC
may occur concurrently with hydrocarbon generation and expulsion. Our
calculations indicate that while TOC loss is relatively small (less than 10%)
for Type III organic matter, it may be as large as 80% for Type I organic
matter. Nonsource, Type IV organic matter is essentially unaffected.
Failure to consider organic carbon loss from mature sediments may result in
two types of error. First, source intervals are almost certainly overlooked
because the calculated decrease in carbon level is often sufficient to downgrade
source-rock ratings from mature source to mature nonsource. Second,
backcalculation of original pyrolysis yields from residual TOC levels will often
significantly underestimate the original source-rock potential. Such errors may
result in incorrectly condemning otherwise prospective source intervals.
Conversely, by making allowance for the reduction in TOC levels caused by
maturation
, the hydrocarbon-generating capacity of many otherwise poorly rated
intervals may be dramatically improved.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.