Kerogen Transformations in the Early Oil Window: Organic Petrology and Micro-Spectrometry of the Molecular Geochemistry of Tasmanites Microfossils

Abstract

The transformation of kerogen to hydrocarbons in the early stages of oil generation is critical for understanding the resource potential of liquid-rich shale plays. Organic petrology commonly is used to evaluate type, quality, and thermal maturity of organic matter, but the relationship of the visual changes to chemical transformations is not well characterized. To improve our understanding of these processes we have analyzed microfossils of the unicellular green alga Tasmanites in Upper Devonian Ohio Shale (Huron Member, Appalachian Basin) via micro-spectrometry (μ-FTIR, XPS, EMPA, fluorescence) in core and outcrop samples with solid bitumen Ro values from 0.45–0.80%. Hydrous pyrolysis of a low-maturity sample was used to simulate the natural maturation sequence. μ-FTIR spectrometry revealed a decrease in CH2/CH3 ratios with increasing maturity, indicating aliphatic chains become shorter and more branched. Oxygenated functional groups also decreased relative to aliphatic stretching bands. In samples pyrolyzed for 72 hrs at temperatures of 300–320°C (solid bitumen Ro range of 0.39–0.68%) Tasmanites showed similar trends, whereas at pyrolysis temperatures of 340°C and higher (bitumen Ro >0.71%), Tasmanites was pseudomorphed by accumulations of solid bitumen, carbonate and sulfide. Preliminary EMPA of Tasmanites in the natural sequence showed consistent decrease in S, Co, Mo, and U concentrations with increasing thermal maturity, possibly due to destruction of metallo-porphyrin complexes. XPS indicated the molar proportion of aliphatic C increases with increasing thermal maturity, accompanied by decreases in oxygenated functional groups and olefinic C. Fluorescence microscopy and spectrometry showed a red shift in spectral maximum and decreasing emission intensity with increasing maturity. The compositional data showed evidence of the loss of fluorophores such as conjugated polyenes (decrease in olefinic C observed by XPS) or metallo-porphyrins (decreasing trace metal concentrations) that is consistent with the changes observed via fluorescence. Replacement of Tasmanites by bitumen, carbonate, and sulfide in hydrous pyrolysis experiments at higher temperatures implies that a large fraction of this component of TOC is converted to hydrocarbons. Additional work is in progress to develop non-destructive methods to chemically differentiate kerogen, bitumen, and residual hydrocarbons to improve our understanding of both generation and expulsion.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015