Organic matter in limestones

Abstract

Analyses of about 1400 ancient rocks (Cambrian to late Tertiary) from many parts of the world show that the limestones contain much less organic matter than the shales. Mean (geometric) organic content of 346 limestones: 0.24 per cent Mean (geometric) organic content of 1066 shales: 1.14 per cent The organic matter in these limestones, however, is characterized by a higher proportion of hydrocarbons than the organic matter of shales. As a result, the average hydrocarbon content of these limestones and shales is very similar. Mean (geometric) hydrocarbon content of 346 limestones: 98 parts per million Moan (geometric) hydrocarbon content of 1066 shales: 96 parts per million Unlike the ancient rocks, Recent lime sediments (Gulf of Batabano, Florida Bay and Persian Gulf) and Recent clay sediments (Orinoco Delta, Cariaco Trench, Gulf of Mexico and Lake Maracaibo) contain similar quantities of organic matter, an amount approximately equal to that of the ancient shales. Moreover, the organic matter of the Recent lime sediments does not contain a higher proportion of hydrocarbons than the clay sediments. The data presents two questions: 1. (1) why do the ancient limestones contain so little organic matter, 2. (2) why does this organic matter contain a high proportion of hydrocarbons? One possible answer to these questions is that limestones lose much of their non-hydrocarbon organic matter during diagenesis, leaving a residue enriched in the more stable hydrocarbons. Greater adsorption of organic matter by clay minerals, as indicated by experiments with quinoline, may prevent loss of organic matter from shales.