Clifford C. Walters | Geology Reports

USGS2026

Metalloporphyrins in the Eagle Ford Shale

Using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), Zheng et al. (2018, Energy & Fuels 32, 10382) reported abundant iron and vanadyl porphyrins and minor amounts of gallium and nickel porphyrins in asphaltenes extracted from a single lower Eagle Ford Shale sample. This finding is most unusual as iron and gallium porphyrins have been previously found only in coal. In this study, petroporphyrins in samples of the Eagle Ford Shale previously studied by French et al. (2020, Marine Petrol. Geol. 118, 104459), were examined using atmospheric pressure photoionization (APPI) FT-ICR-MS. Vanadyl porphyrins (N4VO) dominated the asphaltenes in thermally immature (VRo < 0.56%) samples decreasing in relative abundance with increasing maturity. Only minor amounts of nickel porphyrins were detected in the immature and early oil samples. The distribution of the vanadyl porphyrins is comparable to those reported for marine oils at varying levels of maturity. Immature samples contained porphyrins that were predominantly deoxophylloerythroetio- (DPEP: DBE = 18) and di- deoxophylloerythroetio (di-DPEP: DBE = 19) porphyrins, while ETIO- (DBE = 17), rhodo- (DBE = 20, 21, and 22) and higher condensed (DBE ≥ 23) porphyrins increased with increasing maturity. The vanadyl porphyrins included species with additional one to three oxygen atoms (N4VOx, x= 1 to 4) and one sulfur atom with one to two oxygen atoms (S1N4VOx, x=1 to 3). The degree of additional oxygen and sulfur atoms is consistent with O/C and Sorg/C of associated kerogen. No iron or gallium porphyrins were detected, showing that they are not a ubiquitous feature of the Eagle Ford. We hypothesize that the previously reported iron and gallium porphyrins (Zheng et al., 2018) were present because the specific sample that was analyzed in detail was from the early onset of the Cenomanian–Turonian oceanic anoxic event (OAE-2) in contrast to the samples investigated in this study that are primarily from the lower part of the Eagle Ford pre-dating OAE-2. Submarine volcanism, associated with eruption of large igneous provinces, occurred pre-OAE-2, injecting iron and other inorganic nutrients, giving rise to algal blooms and the acidification of the seawater. At the onset of OAE-2, boreal water masses flowed into the southern Western Interior Seaway, shifting the water column to more oxygenated conditions. Low pH-high Eh (oxic) conditions enhance the availability of iron and gallium such that these events abruptly changed the seawater chemistry, specifically enriching iron and gallium relative to vanadium and nickel. These pH-Eh conditions are similar to the depositional conditions associated with coals, which are known to contain iron and gallium porphyrins, suggesting similar conditions resulted in iron and gallium metalation of porphyrins in the marine setting of the Western Interior Seaway.

Texas↗

USGS2017

Organic petrology and micro-spectroscopy of Tasmanites microfossils: Applications to kerogen transformations in the early oil window

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 for visual evaluation of type, quality, and thermal maturity of organic matter, but the relationship of visual petrographic changes to chemical transformations is not well characterized. To improve understanding of these processes, organic-walled microfossils of the unicellular green alga Tasmanites (composed of algaenan) in Upper Devonian Ohio Shale (Huron Member, Appalachian Basin) were analyzed by micro-spectroscopy techniques including micro-Fourier transform infrared (micro-FTIR), X-ray photoelectron (XPS), electron probe microanalysis (EPMA), and fluorescence. Immature to mid-oil window maturation sequences of core and outcrop samples with solid bitumen reflectance (BR) and vitrinite reflectance (VR) values ranging from 0.45 to 0.80 %Ro were used. Hydrous pyrolysis was applied to low-maturity (BR: 0.25–0.39 %Ro) Huron and time-correlative New Albany shale samples to create similar artificial maturation sequences for comparison. Micro-FTIR spectroscopy revealed a decrease in the CH 2 /CH 3 ratio with increasing maturity, indicating Tasmanites aliphatic chains become shorter and more branched. Oxygenated functional groups decreased relative to aliphatic stretching bands and increased aromaticity was noted at the highest maturities. In samples that were pyrolyzed for 72 h at temperatures of 300–320 °C (BR: 0.56–0.68 %Ro), Tasmanites showed similar trends, whereas at pyrolysis temperatures of 340 °C and higher (BR > 1.0 %Ro), Tasmanites was pseudomorphed by accumulations of solid bitumen, carbonate and sulfide. Replacement of Tasmanites by these phases in hydrous pyrolysis experiments ≥340 °C and its absence at higher maturities (peak oil, VR and BR ≥ 0.9 %Ro) in naturally matured samples, as documented in a previous study, implies that a large fraction of the algaenan component of original organic carbon is converted to petroleum during thermal maturation. XPS analysis indicated the molar proportion of aliphatic carbon increases with increasing thermal maturity, accompanied by decreases in oxygenated functional groups and olefinic carbon. EPMA of Tasmanites showed highest concentrations of S, with concentrations of redox-sensitive trace elements U, Mo, Ni and V generally at or below detection limits. Decrease in organic S with increasing thermal maturity may be related to cleavage of Tasmanites at C S linkages; however, this relationship was inconsistent and presence of adjacent or entrained nanoscale silicate or sulfide phases may impact measured trace element concentrations. Fluorescence microscopy and spectroscopy showed a red shift in spectral maxima and decreased emission intensities with increasing maturity, interpreted as due to non-radiative energy loss possibly because of increased aromaticity. Collectively, these results provide new insights into the in situ chemical transformations that accompany petrographic changes as oil-prone kerogen converts to petroleum with thermal advance from immature conditions into the mid-oil window.

Organic Geochemistry↗