Optical characteristics of amorphous kerogens and the hydrocarbon-generating potential of source rocks

Abstract

Four recurring types of amorphous kerogen are optically identified in petroleum source rock samples and are related to hydrocarbon-generating potential (defined here as geochemically interpreted oil-prone or gas-prone). At this time a genetic classification is not possible. The different amorphous kerogen types are distinguished on the basis of microscopic textural differences: Type A — chunky compact masses with mottled network or weak polygonal textures; Type B — very small, dense, elongate, oval, or rounded individual grains; Type C — clumpswith granular, fragmented or globular textures; Type D — thin, platy, or rectangular individual grains. Geochemistry defined oil-prone samples generally contain Types A and/or D separately or combined; geochemically defined gas-prone samples rarely contain Type A and vary in amounts of Types B, C, and/or D. The samples in this study contain dominantly one of the four types of amorphous kerogen, so the geochemical analyses of the whole rock or bulk concentrated kerogen samples are assumed to be analyses of the dominant amorphous type. The samples were chemically analyzed by several techniques to determine the oil- or gas-generating potential. The establishment of relations between specific optical and chemical properties of kerogen types can make visual kerogen analyses more meaningful in predicting the hydrocarbon generation potential of source rocks.