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

Abstract

Organic matter (OM) in petroleum source rocks is a mixture of organic macerals that follow their own specific evolutionary pathways during thermal maturation. Understanding the transformation of each maceral into oil and gas 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, abundance, color, and fluorescence properties of primary organic macerals and solid bitumen (SB) in 14 Upper Devonian New Albany Shale samples (kerogen type II sequence) from early mature (vitrinite reflectance [VRo] of 0.55%) to postmature (VRo 1.42%). MicroFourier transform infrared (microFTIR) spectroscopy analyses were conducted on these samples to derive information on the evolution of the chemical structure of organic macerals and SB with increasing thermal maturity. Primary OM (amorphous organic matter, alginite, vitrinite, and inertinite) and secondary organic matter (SB) were identified in early mature samples. Amorphous organic matter (AOM) was the dominant organic component in early mature samples and was observed up to the maturity equivalent to VRo 0.79% but could not be identified at VRo 0.80%. An organic network composed of AOM and SB was observed from VRo 0.55 to 0.79%, which, together with the decrease in AOM content being accompanied by an increase in SB content, suggests that with the onset of petroleum generation, SB gradually replaced the original Liu, Bei, Juergen Schieber, and Maria Mastalerz, 2019, Petrographic and microFTIR study of organic matter in the Upper Devonian New Albany Shale during thermal maturation: Implications for kerogen transformation, in W. Camp, K. Milliken, K. Taylor, N. Fishman, P. Hackley, and J. Macquaker, eds., Mudstone diagenesis: Research perspectives for shale hydrocarbon reservoirs, seals, and source rocks: AAPG Memoir 120, p. 165–188. 14252_ch10_ptg01_165-188.indd 165 11/26/19 1:46 PM