Organic constituents both in fine-grained rocks and reservoirs undergo chemical and physical changes in both the diagenetic and metamorphic realms. Four factors affect the final products--the original kind of organic material and its diagenetic state, heat due to geothermal gradient and metamorphism, time, and subsequent alteration in the reservoir.

In the diagenetic realm, algal debris is readily convertible to potentially hydrocarbon-rich, amorphous debris (floccules) through the action of organisms and suitable water chemistry. Phytoclasts, such as cuticle and spores, are more resistant, but also can be converted. High-carbon-structured fusinite is relatively inert to diagenesis or low-grade metamorphism.

Three facies of organic metamorphism with increasing temperature/time are recognized. The immature facies has abundant methane, trace quantities of C₂-C₁₄hydrocarbons and a C₁₅₊ fraction containing abundant NSO compounds. The mature facies exhibits a complete spectrum of hydrocarbons; its start marks the onset of oil generation. The metamorphic facies, characterized by abundant methane, only traces of heavier hydrocarbons, and practically no NSO material in the C₁₅₊ fraction, signifies the thermal destruction of preexisting oil pools. These facies, which can be mapped very early in the exploration of new-venture areas, can be recognized by combined cuttings-gas and organic-matter study. Chemical changes are paralleled by measurable physical hanges in the solid-organic components. A correlation of coal rank, vitrinite reflectance, and thermal alteration numbers, based on color of organic debris, is presented.

End_of_Article - Last_Page 806------------