Origin and classification of organic matter in clastic systems

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Abstract
A classification of organic particles based upon their degrada‐tional state and biological origin is outlined for use in palynologi‐cal analysis. The classification has been used in various forms since 1969 and has proved useful in analyzing organic matter from clastic rocks ranging from the Devonian to the Modern. The organic particles are referred to as macerals and are divided into phytoclasts (plant derived), zooclasts (animal derived), protistoclasts (protistan derived), and scleratoclasts (fungal derived). In addition, the term receptorclasts is used to include precipitated organic matter. Phytoclasts are subdivided into the following categories based upon their level of decay: well preserved, no evidence of biodegradation; poorly preserved, minor biodegradation; infested, highly disrupted cell walls; amorphous structured, remnant cell structure only; amorphous non‐structured, blocky mass. In addition to the amorphous non‐structured phytoclasts there are the amorphous non‐structured protistoclasts (fluffy masses), and amorphous infested indeterminate macerals (masses with inclusions). Other techniques of classifying macerals such as reflectance and UV‐fluorescence can be integrated into this classification. The residence time of an organic particle in the surficial environment affects its level of degradation. Prolonged exposure to an aerobic biocoenosis particularly enhances the decay rate of all organic matter. Among the plant fragments the proportion of the degraded matter is used to determine a “degradational index.”; The degradational index of ancient sediment is comparable to the carbon:nitrogen index of modern sediment as it measures the level of degradation which took place during deposition. Because organic matter from different environments shows different degradational indices as well as different biological origins for their entombed organic particles, it is possible to use the assemblage of organic particles as an indication of depositional environment. Maceral facies analysis clearly has a contribution to make in paleoenvironmental analyses. Using the dual approach of biologic origin and degradational level, maceral assemblages provide clues to the depositional environment as well as the petroleum generating potential (kinds and amounts of hydrocarbons) of a specific source rock. Changes in the attributes of macerals associated with thermal maturation are generally known for clastic environments. Important changes occur in vitrinite reflectance, fluorescence, and color and are used as a measure of thermal maturation level. Volumetric changes in macerals are associated with thermal maturation through catagenesis, and may cause micropressuring as hydrocarbons are generated.