ORGANIC MATTER CONTENT AND TYPE VARIATION IN THE SEQUENCE STRATIGRAPHIC CONTEXT OF THE UPPER DEVONIAN NEW ALBANY SHALE, ILLINOIS BASIN

Organic matter (OM) quantity and type are important parameters in conventional source rock evaluation and unconventional shale oil/gas reservoir characterization. Detailed examination of total organic carbon (TOC) content, organic petrographic composition, and high-resolution (8 cm spacing) geochemical proxies in the sequence stratigraphic framework were conducted on an early mature (0.55% R_o) New Albany Shale core to study the influence of relative sea level fluctuations on paleoproductivity, clastic supply, bottom water redox conditions, and their combined control on TOC distribution pattern and organic maceral variation stratigraphically in the New Albany Shale.

OM content and type in the New Albany Shale vary stratigraphically. The stratigraphic distribution of TOC and geochemical proxy profiles indicate that bottom water redox conditions play a fundamental control on OM accumulation. In contrast to bottom water redox conditions, paleoproductivity and clastic supply have dual controls (enrich or dilute) on OM accumulation. Within the sequence stratigraphic framework, TOC content increases in the transgressive systems tract (TST), reaches maximum before getting to maximum flooding surface (MFS), and maintains a relatively low value in the highstand systems tract (HST). The relatively low TOC content at MFS results from low burial efficiency, elevated bottom water oxygenation, and high biogenic silica dilution.

OM in the early mature New Albany Shale is dominated by either amorphous organic matter (AOM) or alginite (mainly derived from Tasmanites cysts). Terrestrial OM accounts for less than 10% of total OM. Within the sequence stratigraphic framework, AOM content increases in the TST, reaches maximum near MFS, and decreases in the HST. AOM content, however, decreases at MFS because of long-time exposure to oxygenated bottom water and inefficient burial by clastic materials. Enrichment of broken Tasmanites cysts and their detrital infills are an indicator of high-energy environments and could possibly indicate lowstand systems tract (LST). The stratigraphic distribution of OM content and type can result in cyclic variations of hydrocarbon generation potential, oil saturation, and influence the development of secondary organic pores when the New Albany Shale is within oil and gas windows.