Carbon isotopic composition of plant-derived organic matter in tropical sedimentary sequences as a recorder of Late Cretaceous-Early Paleogene changes in the carbon cycle

The dynamics associated with the carbon cycle and the linkage between the oceans, the atmosphere, and land plants provide an opportunity to correlate marine and terrestrial sedimentary sequences using stable isotopes of carbon (delta13C), but few studies have tested this approach. To evaluate the possibility of using carbon isotope ratios of bulk sedimentary organic matter derived from land plants (delta13Cbulk ) as a chronostratigraphic tool, we are comparing the composite Paleocene-Eocene marine carbon-isotope (delta13Ccarbonate) record from Zachos et al., (2001) to that of a terrestrial sequence from Colombia. Sediments of the terrestrial rock units were deposited in a transitional setting dominated by mudstones, coals, and small lenses of sandstones (Catatumbo and Barco Formations) and in a mixture of deltaic and fluvial conditions (Cuervos Formation). The stratigraphic control was based on palynological zones for the region. The delta13Cbulk values for the studied terrestrial sequence show three carbon-isotope excursions, which correlate closely with those present in the marine record. The delta13C bulk values decreased from -24‰ to -26.5‰ in sediments accumulated during early to middle Paleocene. This shift is commonly associated with the slow recovery in marine primary production that occurred in the aftermath of the extinction event of the Cretaceous-Tertiary boundary. The positive shift in sediments accumulated during the late Paleocene shows delta 13Cbulk values increasing from -26.5‰ to -23.8‰. This event is commonly associated with the burial of large amounts of organic matter. The third excursion is found near the Paleocene-Eocene boundary, with values changing from -23.8‰ to -26.5‰. This shift is commonly interpreted to result from a long-term trend toward higher temperatures (52-50 million years ago, M.a.). The analysis of selected biomarker ratios (CPI, Pr/Ph, Paq, betabeta/betabeta+alphabeta hopanes) shows some diagenetic transformation. However, no correlation between diagenesis and delta 13Cbulk values was detected, thus suggesting that delta 13Cbulk could be correlated with delta13C carbonate values. The close correspondence that was found between delta 13Cbulk and delta13Ccarbonate values provides support to the hypothesis that a tight land plant-oceans linkage exists through geologic timescales via the atmosphere

Thermal history of the Northwestern Argentina, Central Andean Basin, based on first-ever reported graptolite reflectance data

The thermal maturity from the Early Paleozoic strata in Northwestern Argentina was studied using reflected light microscopy and Rock-Eval analyses. The graptolites were collected from the Acoite and Lipeón formations, at the Los Colorados section, Cordillera Oriental, and the Huaytiquina, and Muñayoc sections, Puna highland, corresponding to the “Coquena” Formation and the Cochinoca-Escaya Magmatic-Sedimentary Complex. Rock-Eval parameters were unreliable due to the low TOC, S1, and S2 values. The Cordillera Oriental region sediments have low maturity based on low reflectance (%GRo= 0.63%–1.11%) and anisotropy of graptolites. In contrast, the higher graptolite reflectance of samples from the Muñayoc and Huaytiquina sections (%GRo= 5.57%–6.62%), in the Puna region, indicates considerably higher maturity. This could result from the combination of hydrothermal fluids with a temperature range from 336 °C to 358 °C, associated volcanism, and deformation related to tectonics events, which produced a higher geothermal gradient in the Puna. The Los Colorados section's thermal maturity modeling shows a better fit considering erosion episodes at the Late Paleozoic and Early Cenozoic. However, more studies about geothermal parameters and stratigraphy are necessary to corroborate these preliminary models and propose new approaches for the Puna region.Fil: Herrera Sánchez, Nexxys Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Toro, Blanca Azucena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Ruiz Monroy, Ricardo. German Research Centre For Geosciences; AlemaniaFil: Gentzis, Thomas. Core Laboratories; Estados UnidosFil: Ocubalidet, Seare. Core Laboratories; Estados UnidosFil: Carvajal Ortiz, Humberto. Core Laboratories; Estados Unido

Carbon isotopic composition of plant-derived organic matter in tropical sedimentary sequences as a recorder of Late Cretaceous-Early Paleogene changes in the carbon cycle

The dynamics associated with the carbon cycle and the linkage between the oceans, the atmosphere, and land plants provide an opportunity to correlate marine and terrestrial sedimentary sequences using stable isotopes of carbon (delta13C), but few studies have tested this approach. To evaluate the possibility of using carbon isotope ratios of bulk sedimentary organic matter derived from land plants (delta13Cbulk ) as a chronostratigraphic tool, we are comparing the composite Paleocene-Eocene marine carbon-isotope (delta13Ccarbonate) record from Zachos et al., (2001) to that of a terrestrial sequence from Colombia. Sediments of the terrestrial rock units were deposited in a transitional setting dominated by mudstones, coals, and small lenses of sandstones (Catatumbo and Barco Formations) and in a mixture of deltaic and fluvial conditions (Cuervos Formation). The stratigraphic control was based on palynological zones for the region. The delta13Cbulk values for the studied terrestrial sequence show three carbon-isotope excursions, which correlate closely with those present in the marine record. The delta13C bulk values decreased from -24‰ to -26.5‰ in sediments accumulated during early to middle Paleocene. This shift is commonly associated with the slow recovery in marine primary production that occurred in the aftermath of the extinction event of the Cretaceous-Tertiary boundary. The positive shift in sediments accumulated during the late Paleocene shows delta 13Cbulk values increasing from -26.5‰ to -23.8‰. This event is commonly associated with the burial of large amounts of organic matter. The third excursion is found near the Paleocene-Eocene boundary, with values changing from -23.8‰ to -26.5‰. This shift is commonly interpreted to result from a long-term trend toward higher temperatures (52-50 million years ago, M.a.). The analysis of selected biomarker ratios (CPI, Pr/Ph, Paq, betabeta/betabeta+alphabeta hopanes) shows some diagenetic transformation. However, no correlation between diagenesis and delta 13Cbulk values was detected, thus suggesting that delta 13Cbulk could be correlated with delta13C carbonate values. The close correspondence that was found between delta 13Cbulk and delta13Ccarbonate values provides support to the hypothesis that a tight land plant-oceans linkage exists through geologic timescales via the atmosphere.</p

Evaluating Molecular Evolution of Kerogen by Raman Spectroscopy: Correlation with Optical Microscopy and Rock-Eval Pyrolysis

Vitrinite maturity and programmed pyrolysis are conventional methods to evaluate organic matter (OM) regarding its thermal maturity. Moreover, vitrinite reflectance analysis can be difficult if prepared samples have no primary vitrinite or dispersed widely. Raman spectroscopy is a nondestructive method that has been used in the last decade for maturity evaluation of organic matter by detecting structural transformations, however, it might suffer from fluorescence background in low mature samples. In this study, four samples of different maturities from both shale formations of Bakken (the upper and lower members) Formation were collected and analyzed with Rock-Eval (RE) and Raman spectroscopy. In the next step, portions of the same samples were then used for the isolation of kerogen and analyzed by Raman spectroscopy. Results showed that Raman spectroscopy, by detecting structural information of OM, could reflect thermal maturity parameters that were derived from programmed pyrolysis. Moreover, isolating kerogen will reduce the background noise (fluorescence) in the samples dramatically and yield a better spectrum. The study showed that thermal properties of OM could be precisely reflected in Raman signals

Organic geochemical, palynofacies, and petrographic analyses examining the hydrocarbon potential of the Cretaceous (Albian) Kharita Formation in the Matruh Basin, northwestern Egypt

A recent study of selected samples from the Cretaceous (Albian) Kharita Formation of Egypt revealed very good to excellent source rock (SR) potential for six intraformational, organic-rich intervals. This work investigates the SR potential of the entire Kharita Formation across the Matruh Basin, using samples from two wells: the Abu Tunis 1X well from the central part of the basin, and the Siqeifa 1X well drilled on the eastern margin of the basin. More strongly reducing conditions were developed in the centre of the basin, and resulted in the deposition of more organic-rich shales by comparison to the less reducing conditions that prevailed on the eastern basin margin, where the shales contain less organic matter. Deltaic intraformational shales and carbonates in the Kharita Formation of Abu Tunis 1X constitute a significant 120 m net of the potential SR. The lower Kharita Formation contains 34 m net shale SR of good to very good/excellent organic richness, yielding values of 1.14–11.59 wt % total organic carbon (TOC). The organic matter has low Hydrogen Index (HI) values (184–389 mg HC/g TOC) and amorphous organic matter (AOM) and relatively high non-opaque phytoclast frequencies indicating mainly gas/oil-prone organofacies (kerogen types II/III). The upper Kharita is more important, containing 86 m net shale/carbonate SR that has fair to good organic richness (0.8–1.8 wt % TOC), and lower HI (126–250 mg HC/g TOC), a dominance of non-opaque phytoclasts, and subordinate AOM frequencies, which together indicate gas/oil-prone organofacies (kerogen Types III/II). In the Siqeifa 1X well, Kharita deltaic intraformational shales and shaley dolostones comprise 80 m net SR, which has mainly fair to good to less very good organic richness (0.8–2.1 wt % TOC), whilst low HI (93–220 mg HC/g TOC), dominance of non-opaque phytoclasts and subordinate AOM indicate gas-prone organofacies (kerogen Type III). A relative upward increase in deposition of lignite and coaly carbonaceous material supports a gas-prone organofacies. Whilst thermal maturity indices only point to immature to early mature (pre- to early oil-window) SRs in both the Abu Tunis 1X and Siqeifa 1X wells, hydrocarbon exploration focussing on this potential source rock may be justified in areas to the southeast of the Matruh Basin, where modelling indicates this unit may have reached the late mature oil- to main gas-generation window

Experimental Study on the Impact of Thermal Maturity on Shale Microstructures Using Hydrous Pyrolysis

Hydrous pyrolysis was applied to four low-maturity aliquots from the Utica, Excello, Monterey, and Niobrara Shale Formations in North America to create artificial maturation sequences, which could be used to study the impact of maturation on geochemical and microstructural properties. Modified Rock-Eval pyrolysis, reflectance, organic petrology, and Fourier transform infrared spectroscopy (FTIR) were employed to analyze their geochemical properties, while gas adsorption (CO2 and N2) was used to characterize their pore structures (pores < 200 nm). Organic petrography using white and blue light (fluorescence) before and after hydrous pyrolysis showed that amorphous organic matter cracked into solid bitumen, oil, and gas during hydrous pyrolysis. A reduction of the CH2/CH3 ratio in hydrous pyrolysis residues was observed from FTIR analysis. Rock-Eval pyrolysis showed that kerogens in the four samples were dissimilar, and hydrous pyrolysis residues showed smaller hydrogen index and Sh2 values than starting materials. Results from CO2 and N2 gas adsorption analysis showed that pore structures (micropore volume, micropore surface area, meso-macropore volume, and meso-macropore surface area) changed significantly during hydrous pyrolysis. However, changes in pore structure were dissimilar among the four samples, which was attributed to different activation energies of organic matter. A thermodynamic fractal model showed a decrease in fractal dimensions of Utica, Monterey, and Excello after hydrous pyrolysis, indicating a decrease in surface roughness. The pore size heterogeneity in the Utica sample increased as hydrous pyrolysis temperature increased, whereas the pore size heterogeneity distributions in the Monterey and Excello decreased based on the N2 adsorption data