Investigation of the Role of Rock Fabric on Gas Generation and Expulsion During Anhydrous Closed-System Pyrolysis Experiments

Abstract

The geochemistry of thermogenic hydrocarbon gases generated from petroleum source rocks depends on the level of maturity and the type of organic matter present. However, using gas compositional data from maturation experiments to predict natural gas geochemistry can be problematic. Laboratory studies of natural gas generation often involve higher temperatures, shorter reaction times, and lower confining pressures than those prevailing in natural systems. Reaction kinetics can be derived from experimental data and extrapolated to geologic conditions, but uncertainties remain as to whether the reaction mechanisms and physical processes in laboratory simulations are truly representative of petroleum generation in the subsurface.

Pyrolysis experiments were conducted on intact core plugs and powdered rock from a low-maturity shale sample to investigate the effects of rock fabric on gas geochemistry during experimental maturation. The experiments were conducted without additional water, at constant pressure and time, and held isothermally at five different temperatures corresponding to the following maturity levels: bitumen generation, early oil generation, peak oil generation, early oil cracking, and late oil cracking.

The results show that gas retention and compositional fractionation occur in the core plug experiments and vary with thermal maturity. During the bitumen generation stage, yields of C₁–C₅ from core plugs are significantly lower than those from rock powder, and gases from core plugs are enriched in C₁. However, the differences in C₁–C₅ gas yield and composition decrease throughout the oil generation stage, and by the oil cracking stage no obvious difference in C₁–C₅ gases exists. The decrease in the effect of rock fabric on gas yield and composition with increasing maturity is the result of an increase in gas expulsion efficiency. Pyrolysis of rock powder yields 4–16 times more CO₂ compared to core plugs, which is likely due to carbonate decomposition accelerated by reactions with organic acids. Furthermore, lower yields of gaseous alkenes and H₂ from core plug experiments suggest that the rock fabric plays a role in promoting hydrogenation reactions of alkenes.

Pyrolysis products from intact rock fabric samples are more representative of naturally occurring gases than those from rock powder, especially at lower thermal maturities. Experiments on powdered samples may produce products that are significantly different from those generated in nature.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018