Introduction to incident-light microscopy of oil and gas source rocks

Abstract

Microscope investigation of oil and gas source rocks has experienced a rapid development during the last two decades. This is mainly due to the advent of improved fluorescence microscopes, the development of new sample preparation techniques and the availability of computers. The new observations have led to the recognition of three main source rock types and six sub-types. This classification is similar to that proposed by Van Krevelen and by Tissot and Welte. However, it is not based on time-consuming elemental analyses, but can be achieved by simple and efficient incident-light microscopy. Differences in optical behaviour of source rocks with increasing maturation can now be observed in much greater detail than in the past. With increasing levels of organic metamorphism the hydrogen-rich, insoluble organic matter (precursor of oil) in source rocks shows: 1) changes in fluorescence colour;2) a decrease in fluorescence intensity; 3) plastic behaviour;4) generation of mobile products and immobile residues. These changes occur in bacterial organic matter at lower levels of maturation than in well preserved algal source rocks.