Petrographic and Micro-FTIR Study of Organic Matter in the Devonian-Mississippian New Albany Shale During Thermal Maturation: Implications for Oil-Prone Kerogen Transformation

Abstract

Organic matter (OM) in petroleum source rocks is a mixture of organic macerals that follow their specific evolutionary pathways during thermal maturation. Understanding the transformation of each maceral into hydrocarbons with increasing thermal maturity is critical for both source rock evaluation and unconventional shale oil/gas reservoir characterization. In this study, organic petrology was used to document the reflectance, occurrence, color and fluorescence of organic macerals and solid bitumen in fourteen New Albany Shale samples (Devonian-Mississippian, kerogen type II sequence) from early mature (vitrinite reflectance R_o 0.55%) to post mature (R_o 1.42%). Micro-Fourier transform infrared spectroscopy analysis was conducted on samples of different maturities to derive information on the evolution of the chemical structure of organic macerals and solid bitumen. The results show that amorphous organic matter was observed up to the maturity equivalent to R_o 0.79%, but could not be identified at R_o 0.80%. An organic network composed of solid bitumen and amorphous organic matter was identified from R_o 0.55% to 0.79%, suggesting that solid bitumen network partially replaced the original amorphous organic matter network since the onset of hydrocarbon generation. Alginite represented by Tasmanites cysts starts to transform to pre-oil bitumen at the maturity of R_o 0.80%, and shows weak orange yellow fluorescence at this maturity, a change from strong greenish-yellow fluorescence of alginite in the early mature samples. Alginite could not be identified at the maturity of R_o 0.89%, and bitumen derived from it migrated over a limited distance. In the studied samples, oil-prone kerogens (amorphous organic matter and alginite) disappeared at the maturity of R_o 0.89% because of their thermal degradation, and solid bitumen became the dominant organic matter, forming the organic network beyond this maturity. The reflectance of vitrinite is higher than that of solid bitumen until R_o 0.99%, after which solid bitumen reflectance exceeds vitrinite reflectance. Migrated bitumen can fill void space including previously generated organic pores, and thus mask the presence of original and previously formed OM-hosted pores in the oil window. Secondary organic pores formed in the solid bitumen network, together with the oil wettability of solid bitumen, may facilitate the migration of oil and gas in unconventional shale reservoirs.

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