Kerogen origin, evolution and structure

Kerogen, commonly defined as the insoluble macromolecular organic matter (OM) dispersed in sedimentary rocks, is by far the most abundant form of OM on Earth. This fossil material is of prime importance as the source of oil and natural gas; moreover, kerogen can provide essential information on major topics such as past environments, climates and biota. This review reports the main advances in kerogen studies since the comprehensive synthesis edited by Durand [Durand, B. (Ed.), Kerogen, Insoluble Organic Matter from Sedimentary Rocks. Editions Technip, Paris, 1980.]. It is organized into eight sections. The first two are concerned with the successive definitions of kerogen and the definition used here, the different techniques used for kerogen isolation without loss or degradation and basic kerogen analysis. The third and fourth focus on sedimentary OM sources and preservation processes in relation to depositional environment, including sedimentation conditions favourable for kerogen accumulation, and extrapolation to past geological time. Great strides have been made in the latter topics over the last 25 years, owing to a combination of classical studies in organic geochemistry and studies in other domains such as biogeochemistry, oceanography, hydrology and soil science, along with the development of powerful analytical tools. The next two sections deal with the different kerogen classifications by type and kerogen evolution and maturation upon burial in sediments. Structural modelling of coal and kerogen, based on physical and/or chemical structural analysis, is described in the following section. Although, only statistical, the models thus derived provide a synthetic view of the main structural resemblances and differences among various samples in relation to source, maturity or physicochemical properties. Finally, the last section explores some of the advances in kerogen understanding expected for the near future. The review includes a list containing about 500 references. (C) 2007 Elsevier Ltd. All rights reserved.