Optical description of amorphous kerogen in both thin sections and isolated kerogen preparations of Precambrian to Eocene shale samples

Abstract

Abstract This paper presents a limited study of Precambrian kerogen and compares it to Phanerozoic kerogen. In both cases, most of the kerogen is amorphous. While the specific biological precursors of kerogen may be significantly different between the Precambrian and Phanerozoic, the mode of degradation and preservation in shales may be similar, and that is what is found in the description of the amorphous kerogen. It is suggested that similar methods of sample prepagation, description and classification can be applied to all sedimentary organic matter, regardless of age. Kerogen (disseminated organic matter of sedimentary rocks that is insoluble in nonoxidizing acids, bases and organic solvents, Hunt. 1979) occurs in many forms when observed with a microscope. Structured kerogens include spores pollen, unicellular algae, cuticle and cellular tissue of plants and coal fragments. The majority of kerogens have no structures relatable to precursor organisms and are called “amorphous”. This paper describes optical comparisons of various preparations of organic-rich sedimentary rocks and the information that can be derived from them. Optical descriptions of organic matter, in particular the amorphous fraction, in sedimentary rocks help us to understand the bulk chemical or petroleum-generating properties, as well as paleodepositional environments. There is no universally accepted method of preparing samples for optical study, nor is there a unified set of terms to describe the observations. Ideally, more information about the origin or preservation of organic matter can be obtained when it is studied in situ in the rock matrix in thin section, revealing relationships to depositional fabric and clues from mineralogy and microfossils. However, it is often necessary to physically concentrate the organic matter for study because it is too dispersed throughout the rock matrix to be seen in thin section. The concentrated, or isolated, organic matter is prepared either as polished mounts or strewn slides for optical study. More confident kerogen identifications can be made if the different preparations are examined with more than one microscopic lighting condition. Doubly polished thin sections of whole rock can be viewed with three common microscope lighting conditions (transmitted, reflected white and reflected blue light, or fluorescence). Visual kerogen strewn slides (which can also be viewed with the three lighting conditions) contain the isolated organic matter following acid removal of most of the mineral matrix and provide detailed morphological information about the kerogen itself. Polished mounts of isolated kerogen or whole rock (similar to polished coal plugs) can only be viewed with reflected white or blue light and are somewhat limited in the information they convey, despite their relative ease to prepare. Examples of amorphous kerogens from a variety of samples are shown including: thin section documentation of the occurrence of oil-prone amorphous kerogen in laminae of the Eocene Green River Formation: in stylolites of Cretaceous carbonates from Dubai; thin section illustration of solid bitumen rather than amorphous kerogen in paleoreservoir rocks from the Permian of Svalbard (Norway); detailed unicellar algal morphologies in strewn slides of Devonian samples from Russia; strewn slide details of gas-prone amorphous kerogen textures from Silurian Bolivian samples; and the occurrence of oil-prone amorphous kerogen in the Precambrian Nonesuch Formation. More Precambrian organic-rich rock need to be studied optically by the methods suggested here to verify the hypothesis that Precambrian kerogens are mostly amorphous. The best way to completely describe amorphous kerogens is to use both isolated kerogen strewn slides and whole rock polished thin sections.