Contrasting tillage strategies not only affect the stability and formation of soil aggregates but also modify the concentration and thermostability of soil organic matter associated with soil ...aggregates. Understanding the thermostability and carbon retention ability of aggregates under different tillage systems is essential to ascertain potential terrestrial carbon storage. We characterized the concentration and thermostability of soil organic carbon (SOC) within various aggregate size classes under both zero and conventional tillage using novel Rock‐Eval pyrolysis. The nature of the pore systems was visualized and quantified by X‐ray computed tomography to link soil structure to organic carbon preservation and thermostability. Soil samples were collected from experimental fields in Botucatu, Brazil, which had been under zero‐tillage for 2, 15 and 31 years, and from adjacent fields under conventional tillage. Soils under zero‐tillage significantly increased pore connectivity whilst simultaneously decreasing interaggregate porosity, providing a potential physical mechanism for protection of SOC in the 0–20‐cm soil layer. Changes in the soil physical characteristics associated with the adoption of zero‐tillage resulted in improved aggregate formation compared to conventionally tilled soils, especially when implemented for at least 15 years. In addition, we identified a chemical change in composition of organic carbon to a more recalcitrant fraction following conversion to zero‐tillage, suggesting aggregates were accumulating rather than mineralizing SOC. These data reveal profound effects of different tillage systems upon soil structural modification, with important implications for the potential of zero‐tillage to increase carbon sequestration compared to conventional tillage.
Highlights
Different tillage systems may affect SOC thermostability and C retention potentials of soil aggregates.
SOC thermostability was characterised by Rock‐Eval pyrolysis and pore systems were quantified by X‐ray CT within aggregate size classes.
Profound effects of zero versus conventional tillage upon soil structural modification were observed
Important implications for zero‐tillage to increase C sequestration versus conventional tillage.
•Small angle neutron scattering (SANS) was employed in characterizing the pore structures of Bakken shale.•The pore surface area and pore size distribution from different methods were calculated and ...compared.•The multifractal analysis was used to quantify the pore heterogeneity from these three methods.•The impact of the rock compositions on the pore structure was analyzed.
Small angle neutron scattering (SANS) analysis was performed on six Bakken Shale samples with different maturities to reveal the complexities in the pore structure. Pore size distribution (PSD), porosity and specific surface area (SSA) were calculated from SANS data via the Polydisperse Spherical Pore (PDSP) model and compared with the data from N2 adsorption and mercury intrusion. The results showed that the Bakken samples have a very small porosity value (less than 1%) and a very larger specific surface area (larger than 180995 cm−1) in the measuring pore size range (pore diameter: 1–200 nm). SANS and N2 adsorption can detect pores in the similar size range (2–200 nm). The SSA measured by SANS and mercury intrusion was found larger than the one detected by N2 adsorption. Pore structure information that is obtained from SANS, N2 adsorption, and mercury intrusion methods exhibited a fractal and multifractal behavior. Moreover, the pore size distribution that is calculated from SANS data was the most heterogeneous. Finally, the effects of rock composition on pore structures demonstrated that organic matter hosts some isolated pores while clay minerals do not host a large quantity of pores that are either connected or isolated.
Raman spectroscopy on carbonaceous material has become the most used geothermometer in Earth science studies. However, its application in very different settings, associated with different heating ...rates, demands an updated review to understand the different paths of maturation that can occur in the geological record. A comparison between organic matter matured under slow (diagenesis and regional metamorphism) or fast (around shallow intrusion or under artificial pyrolysis) heating rates highlight a mismatch between Raman spectra at the same thermal maturity. Such differences are probably due to the interplay of kinetics and strain, and highlights that classical kinetics based on vitrinite reflectance is not appropriated when Raman parameters are used. In the controversial application to faults and shear zones a review of existing literature indicates that a distinction is needed between strain and frictional heating effects since they lead to different spectra evolution. The effect of strain enhances organic matter aromaticity, while data from frictional heating experiments show strong analogies with charcoal spectra even if the kinetics of the process still need to be understood. All this evidence emphasizes that Raman spectroscopy is a powerful tool to describe the aromatization experienced by carbon material in most of the natural conditions and is a good candidate for the development of a universal geothermometer based on a new kinetic model for Earth and planetary sciences.
Tropical peatlands hold large amounts of carbon but the influence of litter inputs and variation in peat properties with depth on carbon storage are poorly understood. Here we present a stratigraphy ...of peatland carbon stocks and accumulation through the peat profile in a tropical ombrotrophic wetland and assess shifts in vegetation inputs and organic matter degradation using n-alkane distributions and Rock-Eval 6 pyrolysis. Mixed forest (including canopy palms and tropical hardwood trees) contained the greatest total carbon stock in the soil (1884 Mg C ha−1), followed by Rhizophora mangle (mangrove, 1771 Mg C ha−1), Campnosperma panamensis (hardwood, 1694 Mg C ha−1) and Cyperus (sawgrass) bog plain (1488 Mg C ha−1). The long-term apparent rate of carbon accumulation, determined by 14C dating of the carbon stored in different layers in the peat profile, decreased from the edge to the interior of the peatland, with the highest accumulation rate in at the Rhizophora site (102.2 g C m−2 y−1) and the lowest in the deeper peat layers at the Cyperus site (45.6 g C m−2 y−1). High molecular weight n-alkanes dominated in surface peat in all four phasic communities, while deeper in the peat profile n-alkane profiles differed more among sites, suggesting contrasting litter inputs or decomposition environments. Deeper peat was depleted in carbohydrates and had a relatively larger thermostable C pool. Taken together our findings show (i) that different forest types hold varying C stocks and have different peat accumulation rates, even over relatively small distances, and (ii) progressive depletion of carbohydrates and thermolabile compounds with depth, despite strong variation in litter inputs throughout the peat profile.
•Mixed forest and Cyperus sites had highest and lowest C stocks, respectively.•LORCA's highest rate was at the Rhizophora and Cyperus site, respectively.•Long-chain n-alkanes dominated in surface peat•Deeper peat was depleted in carbohydrates and had a larger thermostable C pool.•There was depletion of carbohydrates and thermolabile compounds with depth.
This paper aims to develop a new approach to interpret soil organic matter (SOM) dynamics from Rock-Eval pyrolysis. Rock-Eval standard parameters (TpS2, HI, OI) are limited when applied to SOM, as ...they were defined for tracking the origin of sedimentary organic matter (i.e. terrestrial vs aquatic and lacustrine vs marine). This study proposes new Rock Eval based indices, projected on a new diagram plotting >1300 samples, including litter and soil horizons, pure compounds and organic material. These new parameters reflect the thermal stability of SOM rather than its bulk chemistry. Their calculations are based on the contribution of four different areas (A1 to A4) integrated below the S2 pyrogram (amounts of released hydrocarbon compounds during the pyrolysis step). Results demonstrate that the relative values of A1 to A4 parameters can be used to survey the evolution of SOM during pedogenesis. These Rock-Eval parameters revealed a consistent thermal differentiation of SOM with depth, from thermally labile biogenic SOM in soil organic horizons to thermally stable mineral-associated SOM compounds in organo-mineral and mineral soil horizons, indicating a pedogenetic stabilization of SOM. Finally, newly defined I- and R-indices integrate the respective parts of A1 to A4 parameters into SOM dynamics, the I-index emphasizing the degree of transformation of the immature organic fraction (related to SOM stabilization), the R-index highlighting the contribution of the most refractory fraction or persistent SOM (related to pedogenic and inherited contributions). Analyses of a wide range of soils and reference samples (cellulose, lignin, humic substances, lignite, charcoal, coal, etc.) allow end-members as well as particular trends (humic, spodic, inherited) to be drawn. Specific examples are provided in order to illustrate some applications and uses for this new Rock-Eval based I/R diagram, from the study of grain size fractions to the evolution of SOM in soil chronosequences.
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•A novel Rock Eval pyrolysis diagram to interpret soil organic matter dynamics•The diagram is based on more than 1400 samples of litter, soil horizons, and pure compounds.•The new I-index refers to the degree of decomposition of immature SOM fractions.•The new R-index highlights the contribution of SOM refractory/persistent fractions.•Particular stabilization trends (humic, spodic, inherited) are emphasized in the IR diagram.
The dramatic increase in exploration for unconventional hydrocarbon resources has inherently fuelled the need for new source-rock geochemical data. The need for new data comes along with an ...increasing number of new users, many of whom do not possess the background to interpret accurately and evaluate the quality of data sets generated by different geochemical screening techniques (e.g., Rock-Eval pyrolysis, vitrinite reflectance). Here, datasets from Rock-Eval pyrolysis, vitrinite reflectance, and LECO TOC analyses are scrutinized and compared to show how failing to recognize good vs. bad datasets can dramatically change interpretations during prospect or play appraisals. Detector saturation, contamination of the sample with drilling fluid, and suppression of both Tmax and vitrinite reflectance, are examples of complications that could compromise the validity of the results and the play or prospect reviews derived from them. In addition, misconceptions such as: “LECO TOC is better than Rock-Eval 6 TOC”, expressed by many users, are a consequence of the lack of understanding of how different screening techniques and instrumentation work. A solid understanding of these pitfalls and limitations can provide inexperienced geoscientists and engineers with the required support to improve risk maps for hydrocarbon-charge analysis and source-rock evaluations, when using classic guidelines for interpreting results.
•We discussed the major analytical pitfalls of source-rock evaluation techniques.•FID saturation during pyrolysis is a major and overlooked pitfall in the analysis of organic-rich rocks.•OBM contamination greatly affects pyrolysis and TOC yields.•Pyrograms are the only way to QC pyrolysis results.•We found that Rock-Eval 6 TOC values are comparable to LECO TOC values across lithologies and organic content.
Understanding organic matter properties in terms of maturity and production potential are crucial for the initial assessment of unconventional plays. This is important since the amount of hydrocarbon ...that can be generated is a function of organic matter type and content in the formation and its thermal maturity. The complexity of shale plays in terms of constituent components has demonstrated that new analytical methods should be acquired to better understand hydrocarbon generation processes. In this study, a few samples from the upper and lower members of the Bakken Formation in the USA were selected from different depths and maturity levels. The samples were analyzed by a high frequency (22 MHz) nuclear magnetic resonance (NMR relaxometry) equipment, followed by Rock-Eval pyrolysis (using the Basic/Bulk-Rock method for all samples and a multi-heating rate method, MHR, for the two least mature samples) and bitumen reflectance evaluations. Results showed NMR can detect different hydrogen populations within the samples and distinguish among phases, such as solid organic matter, hydrocarbons (mobile oil), and water by T1-T2 mapping. We were also able to relate different identified areas on NMR T1-T2 maps to geochemical parameters of the organic matter obtained from Rock-Eval pyrolysis (such as S1, S2, and HI) and with thermal maturity (vitrinite reflectance-equivalent).
Hydrocarbon generation in Taranaki Basin, New Zealand, has been proven in several source rocks within the syn- and post-rift basin-fill sedimentary successions. These source rocks are mainly hosted ...in the marginal and deep marine Upper Cretaceous shales, which generate marine and terrigenous oils. Some of the terrigenous oils could have been linked to the Paleogene coals; however, the capability of these coals to generate and expel liquid hydrocarbons is poorly-investigated. In this study, we present organic geochemistry, kerogen petrography, wireline logging, and basin modelling to evaluate the hydrocarbon generation potential of the Paleogene coals and coal-rich facies of the Kapuni Group sediments in Kapuni Field, Taranaki Basin. A compositional correlation between oil in the Kapuni Group reservoirs and coal extracts was performed to decipher the oil origin and define its parent source rock. Kapuni oils display high pristane/phytane (Pr/Ph) > 5, low dibenzothiophene (DBT)/phenanthrene <0.1 and high relative abundance of C29 regular steranes >60%, typifying their generation from a terrigenous source rock deposited in an oxic, fluvio-deltaic environment. Similar values were also observed in the muddy coal extracts of Farewell Formation, thereby reflecting a positive coal-oil correlation. The maturity-relevant biomarkers (Ts/Tm trisnorhopanes; C29 steranes isomers, and 22S-, 22R C32 homohopanes) are similar in both oils and Farewell coal extracts confirming oil expulsion from Farewell coals during the peak oil window maturity. These results demonstrate the generation and expulsion of coal-oil, thereby shedding light on the prospectively of the Paleogene plays in the Taranaki Basin.
•Studied Kapuni Paleogene coals have high levels of hydrocarbon saturation.•Vitrinite reflectance values confirm the oil-window maturity of Paleogene coals.•1D basin modelling demonstrates light oil expulsion from some coal seams.•Coal-oil correlation confirms a generation from deep Paleogene coals.
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•Multi-step Rock–Eval pyrolysis refines the shale oil potential evaluation.•The amount of adsorbed oil in saline lacustrine sequences was underestimated.•Lithologies and physical ...properties are joint controls on free oil, while adsorbed oil is mainly controlled by TOC.•Siltstones and carbonate rocks are important shale oil sweet spots.
Occurrence state of shale oil in saline lacustrine basins is complex, thus restricting the availability of high-efficiency shale oil exploration and exploitation. To address the complexity, we conducted a study of the middle Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin, China, in a lithofacies perspective, using comprehensively multi-step Rock–Eval pyrolysis (MREP) and mineralogical, petrological, and organic geochemical methods. Results show that the low-mature to mature Lucaogou Formation in this study consists mainly of four lithofacies types, i.e., carbonate rocks, dolomitic/calcareous mudstones, silty/tuffaceous mudstones, and siltstones. The amount of free hydrocarbon (S1–1 + S1–2) and revised free hydrocarbon (S1–1 + S1–2 + S2–1) values obtained by MREP exceeds the S1 value obtained by conventional REP, which refines the evaluation of the shale oil potential. Shale oil in siltstones and carbonate rocks occurs mostly in fractures and pores in the free state, whereas the shale oil in mudstones occurs mainly in the matrix and kerogen, and on mineral surfaces in an adsorbed state. The shale oil potential of the four main lithologies is overall good, with the siltstone having the best potential (i.e., oil saturation index OSI = S1 × 100/TOC OSIavg = 313.22 mg HC/g TOC) and highest free hydrocarbon content (S1–1 + S1–2avg = 13.53 mg HC/g rock). The tuffaceous/silty mudstone has the best hydrocarbon generation potential (TOCavg = 8.01 wt%; PG = S1 + S2avg = 46.11 mg HC/g rock) and the highest content of adsorbed hydrocarbon (S2–1 avg = 13.78 mg HC/g rock), but the lowest shale oil potential (OSIavg = 105.78 mg HC/g TOC). The controlling factors on the free hydrocarbon content (S1–1 + S1–2) are complex but are mainly controlled by petrophysical properties. The amount of adsorbed hydrocarbon (S2–1) is controlled by the organic matter content. When a rock in the Lucaogou Formation has OSI > 100 mg HC/g TOC, the ratio of free to revised free hydrocarbon is > 50%, and the free hydrocarbon content is > 10 mg HC/g rock, the shale oil is favorable for exploration. Exploration and development of shale oil resources in saline lacustrine basins needs to consider the shale oil potential and its occurrence state and focus on sweet spots with good petrophysical properties, such as siltstones and carbonate rocks.
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