Creation of Porosity in Tight Shales during Organic Matter Maturation

It has been suggested that porosity and permeability are created in tight shales during the organic matter maturation process, i.e. kerogen conversion to oil and gas followed by oil cracking to gas. We have tested this hypothesis by examining changes in porosity and permeability using FIB-SEM in both artificially matured shales and by analysis of a variety of shales of varying maturities in the natural setting. Artificial maturation (laboratory pyrolysis) was carried out on low maturity samples of the Barnett Shale by heating them in pressure vessels under argon for a sequence of times and temperatures to induce oil generation and oil cracking. Porosity and permeability were recorded at various stages of maturation. These experiments were compared with results from a variety of naturally-matured shales of varying maturities. Kerogen maturity and hydrocarbon generation were determined by Rock-Eval pyrolysis. The extent of oil cracking to gas was measured by diamondoid analysis, i.e. quantitative measurements of nanometer-sized hydrogen-terminated diamonds dissolved in the generated liquids. Our results show that the process of organic matter maturation can produce porosity and permeability in tight shales. Furthermore, the amount of porosity and permeability creation differs in different shale types.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California