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.