Linking biomarkers with elemental geochemistry to reveal controls on organic richness in Devonian-Mississippian mudrocks of Oklahoma

Abstract

Paleoproductivity and depositional redox conditions play critical roles in the development of organic-rich mudrocks. Devonian and Mississippian mudrocks within the central region of Oklahoma, USA were assessed for their depositional redox conditions, paleoproductivity and the effects of these factors on organic matter enrichment using a combination of elemental and organic geochemical proxies.A number of lithofacies with a shallowing upward trend have been defined within Devonian and Mississippian formations. Kerogens in the examined samples are dominantly type II, with organic richness in Mississippian samples exhibiting cyclic fluctuations, whereas organic richness in Woodford samples exhibits a downward-increasing trend. Mississippian sections contained at least three beds of organic-rich mudrocks, with total organic carbon (TOC) ranging from 2 to 6% by weight, indicating good petroleum generation potential. Elemental redox proxies, along with biomarker proxies of photic-zone euxinia, show a strong anoxic signature within the middle Woodford member, while the Mississippian section exhibits signatures consistent with suboxic depositional conditions. Paleoproductivity in the Mississippian section suggests a cyclic trend, with a higher paleoproductivity signature than that preserved in the Woodford Shale. Examined geochemical parameters highlight the different processes that are critical for organic matter enrichment and preservation. Mississippian mudrock organics appear to be enriched because of variations in paleoproductivity, while Woodford Shale organics are likely preserved due to changes in environmental redox conditions. Our findings reflect the variability observed within unconventional tight reservoirs of liquid hydrocarbons and the utility of combining organic and inorganic geochemical approaches in identifying productive sweet spots.