Organic Matter In Sedimentary Rocks Research Articles

The long‐term legacy of subvolcanic intrusions on fluid migration in sedimentary basins: The Cerro Alquitrán case study, northern Neuquén Basin, Argentina

AbstractCooling subvolcanic igneous intrusions are known to have a substantial impact on fluid flow in the shallow Earth's crust, for example activation of geothermal systems, circulation of mineralized fluids in ore deposits, fast maturation of organic matter in sedimentary rocks and the potential release of large volumes of greenhouse gases, which have triggered mass extinctions during the Earth's history. However, the long‐term post‐cooling legacy of subvolcanic intrusions on fluid flow received much less attention. Here we describe a demonstrative geological example in the Andean foothills, Argentina, showing that igneous intrusions have long‐term effects on fluid flow after their emplacement and cooling. The case study is a ca. 11‐million‐year‐old, eroded subvolcanic conduit, at the rims of which large volumes of bitumen are naturally seeping out on the Earth's surface. This contribution highlights that intense syn‐emplacement fracturing of the magma has created high‐permeability pathways that affect the regional fluid circulations, even millions of years after cooling. Our observations reveal how extinct subvolcanic intrusions have long‐term consequences on subsurface fluid circulations, which need to be accounted for in the exploration of geothermal energy, drinkable groundwater, hydrocarbons and CO2 sequestration in volcanic basins and regions hosting ancient shallow magma intrusions.

Fundamental Study on Compressive Characteristics of Artificial Coal Seam and Chemical Characteristics of Pore Fluid while Accelerating Decomposition of Organic Matter Using Hydrogen Peroxide

We proposed a new gasification method to convert unused organic matter in sedimentary rocks to bio methane gas using microorganisms, known as Subsurface Cultivation and Gasification (SCG). Our approach uses hydrogen peroxide (H2O2) to decompose organic matter rapidly into usable substrates for methanogens. We previously reported that H2O2 would be useful for effective SCG at lignite, and conversion of organic matter from lignite into biogenic methane with the help of microorganisms is expected to be highly profitable. However, physical properties of the sedimentary rock are thought to change with the decomposition of sedimentary rock in the SCG method.

Quaternary uranium mineralization in the Qaidam Basin, northern Tibetan Plateau: Insights from petrographic and C-O isotopic evidences

There are several sandstone-type uraniferous basins around Tibetan Plateau. The Qaidam Basin is the largest sedimentary basin in the internal Tibetan Plateau, hosting important oil and coal resources, and has good potential for sandstone-hosted uranium resources. In this study, we examine the petrographic and mineralogical characteristics of a uranium mineralization interval at the Neogene-Quaternary transition (NQT) in the western Qaidam Basin using optical microscopy and electron probe microanalyzer (EPMA), including back scattered electron (BSE) imaging and energy-dispersive spectroscopy (EDS). The sedimentary lithofacies features and C-O stable isotopes of carbonate cement from two boreholes were analyzed, especially for the uraniferous interval. Whole-rock analyses of ores and barren rocks yielded highly variable U concentrations ranging from 1.5 to 2548 ppm. The main uranium minerals are coffinite and uraninite. The co-occurrence of uranium minerals with calcite indicates that uranium mineralization and calcite cementation were coeval. Coffinite mainly occurs at the interface between calcite cement and other minerals. The carbonates have δ13CV-PDB ranging from −29.67 to +3.93‰, suggesting involvement of organic carbon during uranium mineralization. This is also supported by the general trend of decreasing δ13CV-PDB and increasing U content from the conglomerate/coarse sandstone to the mudstone samples, and the overlap of δ13CV-PDB values between the calcite cement and organic matter in sedimentary rocks. The organic carbon may have been derived from hydrocarbons sourced from the deeper part of the basin and/or biogenic gases at shallow depths. The unconformity at the NQT may represent a favorable interval where oxidizing fluids carrying uranium met with reducing fluids carrying hydrocarbons and resulted in uranium mineralization. Thus, the NQT may serve as a guide for uranium exploration in this basin.

Features of the subsurface water composition formation in the Mesozoic hydrogeological basin of the Middle Ob region fields

The formation of the deep oil and gas bearing horizons hydrogeological conditions in the Middle Ob region and the West Siberian megabasin as a whole contains many questions and is a subject of discussion. This is due to numerous hydrogeodynamic and hydrogeochemical anomalies that do not have an unambiguous explanation. The hydrogeological conditions feature of the area under consideration is inversion hydrogeochemical zonation in the Lower Cretaceous and Upper Jurassic sediments, as well as the presence of low-mineralized formation waters of hydrocarbonate-sodium composition. The change in the genetic type of waters, the mineralization decrease, the calcium ion content decrease and an increase in the hydrocarbonate ion amount in the Mesozoic hydrogeological basin, is associated with the transformation of mineral and organic matter in sedimentary rocks during their immersion, occurring at the elision stage of the basin development.

Origin of hydrocarbons in the Bach Ho field (The Vietnamese shelf)

This article presents the results of a geochemical study of oil on the Vietnam shelf (Cuu Long basin), including those in the crystalline basement. The Bach Ho field oils in the basement have a hydrocarbon distribution that is no different than oil of numerous accumulations in Oligocene and Miocene sedimentary sequences. Similar to the organic origin oils of the world, oils from the Bach Ho field lack regular isoprenanes С12 and С17 and cheilanthanes (tri-cyclic terpanes) С22 and С27. A distinctive feature of these oils is a large amount of cheilanthanes С19-С29, and large neo-adiantane to adiantane and hopanes to steranes ratios. All these parameters indicate a large bacterial contribution in the generation of these oils. Studies have shown the similarity between oil biomarker parameters and the organic matter of sedimentary rocks, which supports the organic nature of the oils in the basement fields on the Vietnam shelf. It is shown that the hydrocarbon accumulations in the basement complexes of the Cuu Long basin are in a secondary occurrence, and their origin was the organic matter of the sedimentary source rocks.

Groundwater in the Mesozoic hydrogeological basin of the Middle Ob (a case study of the Ust-Balykskoye oil field)

Despite the fact that there is the large amount of accumulated factual material, formation of hydrogeochemical conditions of deep oil and gas horizons in the Middle Ob and the West Siberian megabasin still contains many questions. This is due to numerous hydrogeodynamic and hydrogeochemical anomalies that don't have an unambiguous explanation. The presence of inversion hydrogeochemical zoning in the Lower Cretaceous and Upper Jurassic deposits and the presence of low-mineralized reservoir water of a sodium-bicarbonate composition are the peculiarity of groundwater in the considered territory. A change in the genetic type of water, a decrease in mineralization, a decrease in the content of calcium ion, and an increase in the amount of bicarbonate ion in the Mesozoic hydrogeological basin are associated with the transformation of mineral and organic matter in sedimentary rocks, when they are immersed, at the water expelling stage. The determination of the origin of low-mineralized reservoir water of a sodium-bicarbonate lying at significant depths is of great practical importance, since the relationship between the inversion of groundwater and oil content has been revealed, so this fact can be considered a search criterion for petroleum potential.

Stratigraphy of the Paleocene continental sedimentary succession of the northern Pyrenean basin (Corbières, southern France) using δ 13 C org isotopes

Continental-dominated successions are often poorly constrained stratigraphically due to a lack of robust biostratigraphic markers. This study provides the first dataset of δ 13 C org together with magnetostratigraphic and biostratigraphic data from a thick continental-dominated succession at Lairière (northern Pyrenees, France). This section encompasses the latest Cretaceous up to the Paleocene–Eocene Thermal Maximum interval and is characterized by fluvial deposits, occasionally intercalated with continental carbonates, lacustrine deltaic deposits and shallow marine sediments. This work identifies δ 13 C org events and assigns them to global δ 13 C geochemical events defined in Pyrenean and Tethyan marine successions, in which the stage boundaries are well calibrated. As the isotopic measurements are performed on dispersed organic matter in sedimentary rocks with a low organic content, we propose that analyses of the signal should take into consideration the depositional environment because variations in the organic matter content might affect the signal. We establish a high-resolution chronostratigraphy in terrestrial Paleocene deposits in the NE Pyrenean zone, recognize a late Selandian interval and define the Paleocene–Eocene Thermal Maximum event. Supplementary material: Palaeomagnetic results, geochemistry details and palynological analysis are available at https://doi.org/10.6084/m9.figshare.c.4871214

Variations of organic matter transformation in response to hydrothermal fluids: Example from the Indiana part of the Illinois Basin

Thermal maturation of organic matter in sedimentary rocks is a complex process controlled by multiple parameters. In this study, we examined the thermal history of one location in the Indiana part of the Illinois Basin. A total of 21 samples spanning the time interval from the Ordovician to the Mississippian were selected for maturity assessment and chemical structure characterization of vitrinite/vitrinite-like particles, solid bitumen, and alginite. The reflectance of vitrinite/vitrinite-like particles (VRo) reveals erratic variations with depth from 0.61% to 1.18%. Solid bitumen reflectance (SBRo), ranging from 0.43 to 0.92%, is slightly lower than VRo across this maturity range in this borehole. In addition to documenting reflectance variations with depth, variations within single organic matter particles were analyzed in the Upper Devonian New Albany Shale and Upper Ordovician Maquoketa Shale samples; these samples were targeted because they contained more organic matter than the samples from other stratigraphic units. SBRo shows significant variations within a single solid bitumen particle with a difference up to 0.81%. Significant maturity-related chemical differences within individual particles of solid bitumen and alginite were also revealed using micro-FTIR technique. An increase in temperature with depth of burial is certainly the key factor in controlling the thermal transformation of organic matter in sedimentary rocks. However, it is apparent that this mechanism alone does not explain the stratigraphic variations of VRo throughout the core and reflectance variations within individual solid bitumen and alginite particles. We suggest that the flow of hydrothermal fluids through the New Albany and Maquoketa Shale caused the observed thermal maturity anomaly. High concentrations of chlorine, vanadium, chromium, and zinc also suggest that organic matter transformation was enhanced by the flow of hydrothermal fluids in the study area.

Magnetic mineral assemblage as a potential indicator of depositional environment in gas-bearing Silurian shales from Northern Poland

SUMMARYOrganic matter preservation and associated conditions during deposition, important in the context of fossil fuel exploration, are commonly determined by advanced geochemical analyses. However, the relation between organic matter preservation and magnetic mineral composition remains poorly constrained. The aim of the studies was to check the potential of magnetic mineral differentiation between facies containing various amounts of organic matter as a factor to better understand the processes which influence water chemistry at the bottom of sedimentary basins, and thus to better understand factors controlling the preservation of organic matter. To determine the composition and the properties of magnetic minerals, detailed low-temperature measurements of Saturation Isothermal Remanent Magnetization and hysteresis loops were performed on two types of rocks, Silurian shales from the Baltic Basin (northern Poland). The analysed shale facies are characterized by similar thermal evolution, but different amounts of organic matter: the Pelplin Formation, containing a modest content of organic matter, in which we also examined early diagenetic carbon concretions; and the Jantar Formation, which represents an organic-rich ‘sweet spot’ layer. In both facies, the results indicate the presence of multi- or pseudo-single domain magnetite, which is interpreted as detrital in origin. However, the main observation gained from this study is the relation between magnetic mineral assemblage in the studied shales and the amount of organic matter: in the rocks with modest amounts of organic matter we observed hematite, while in organic-rich layers hematite was absent. Hematite (mostly single-domain grains) preserved in the Pelplin Formation suggests that stable oxygen-rich conditions were present at the bottom of the sedimentary basin continuously during deposition, concretion cementation and compaction. In turn, its absence in the Jantar Formation suggests that during sedimentation and early diagenesis more anoxic conditions appeared. Generally, findings show that the presence of hematite is related to the significantly lower amount of organic matter in sedimentary rocks. Thus, presence of this mineral may be a useful indicator of organic matter preservation.

Acid demineralization with pyrite removal and critical point drying for kerogen microstructural analysis

Hydrocarbon storage and transport in unconventional shale resources occurs predominantly within pores hosted by kerogen (solid and insoluble organic matter in sedimentary rocks). Kerogen-hosted pores are small, so physiochemical interactions between pore fluids and pore surfaces (e.g., adsorption) are particularly important in shale. The understanding and prediction of hydrocarbon storage and transport in shale is dependent, therefore, upon the correct understanding of both chemical composition and pore geometry of kerogen. Several recent studies have attempted to construct molecular models of kerogen physical and/or chemical structure. Developing these models is challenging, in part because of the lack of laboratory samples of kerogen for experimental characterization that preserve both its chemical and microstructural properties representative of those in the subsurface.This study presents an integrated kerogen-isolation procedure that combines closed-system chemical demineralization with pyrite removal and critical point drying. The method produces a kerogen that is not only high in chemical purity but more representative of kerogen microstructure that occurs in the subsurface. Characterization of kerogen microstructure, which can be performed by direct measurement of the isolate obtained by this procedure, is essential to better understand and predict the storage, transport, and production hydrocarbons from unconventional resources.

Reflectance Spectroscopy of Organic Matter in Sedimentary Rocks at Mid-Infrared Wavelengths

AbstractReflectance spectroscopy is a rapid, non-destructive technique capable of characterizing mineral and organic components within geologic materials at spatial scales that range from μm to km. The degree to which reflectance spectra can be used to provide quantitative information about organic compounds remains poorly understood, particularly for rocks with low organic content that are common in the Earth’s ancient rock record and that may be present on other planetary bodies, such as Mars. In the present study, reflectance spectra (0.35–25 μm) were acquired for a suite of Proterozoic shales and the kerogen was isolated to assess how spectral properties of aliphatic and aromatic C-H absorption bands can be used to predict organic matter abundance (total organic content, TOC, and H/C ratio). A number of spectral parameters were evaluated for organic absorption bands observed in the 3–4 μm wavelength region for comparison with independently measured TOC and H/C values. Ratios of the strengths of aliphatic to aromatic absorption bands were directly correlated to H/C values, but the reflectance spectra for pure kerogens with H/C < 0.2 lacked clear evidence for C-H absorption bands in this spectral region. Organic absorption bands are routinely observed for bulk rock powders with >1 wt.% TOC, but the detection limits of reflectance spectra for TOC may be <1 wt.% or as high as 10 wt.%. Organic detection limits for reflectance spectra are, thus, controlled by both TOC and H/C values, but these parameters can be predicted for clay-rich, kerogen-dominated samples for a range of values that are relevant to drill cores, outcrops, meteorites, and planetary surfaces.

Assessing low-maturity organic matter in shales using Raman spectroscopy: Effects of sample preparation and operating procedure

Laser Raman spectroscopy is used to assess the thermal maturity of organic matter in sedimentary rocks, particularly organic-rich mudstones. However, discrepancies exist between quantified Raman spectral parameters and maturity values obtained by vitrinite reflectance. This has prevented the adoption of a standard protocol for the determination of thermal maturity of organic matter (OM) by Raman spectroscopy. We have examined the factors influencing the Raman spectra obtained from low-maturity OM in potential shale gas reservoir rocks. The inconsistencies in Raman results obtained are due to three main factors that are critically evaluated: (1) different operational procedures, including experiment setup and spectral processing methods; (2) different methods of sample preparation; (3) the analysis of diverse types of OM. These factors are scrutinized to determine the sources of inconsistency and potential bias in Raman results, and guidance is offered on the development of robust and reproducible analytical protocols. We present two new Raman parameters for un-deconvolved spectra named the DA1/GA ratio (area ratio of 1100–1400 cm−1/1550–1650 cm−1) and SSA (scaled spectrum area: sum of total area between 1100 and 1700 cm−1) that offer potential maturity proxies. An automated spreadsheet procedure is presented that processes raw Raman spectra and calculates several of the most commonly used Raman parameters, including the two new variables.

On the Physical Sense of 13C Values of Carbonate and Organic Matter of Sedimentary Rocks

On the Physical Sense of 13C Values of Carbonate and Organic Matter of Sedimentary Rocks

Sulfur Species in Source Rock Bitumen before and after Hydrous Pyrolysis Determined by X-ray Absorption Near-Edge Structure

The sulfur speciation of source rock bitumen (chloroform-extractable organic matter in sedimentary rocks) was examined using sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy for a suite of 11 source rocks from around the world. Sulfur speciation was determined for both the native bitumen in thermally immature rocks and the bitumen produced by thermal maturation of kerogen via hydrous pyrolysis (360 °C for 72 h) and retained within the rock matrix. In this study, the immature bitumens had higher sulfur concentrations than those extracted from samples after hydrous pyrolysis. In addition, dramatic and systematic evolution of the bitumen sulfur moiety distributions following artificial thermal maturation was observed consistently for all samples. Specifically, sulfoxide sulfur (sulfur double bonded to oxygen) is abundant in all immature bitumen samples but decreases substantially following hydrous pyrolysis. The loss in sulfoxide sulfur is associated with a relative increase in the fra...

A microscopic characterization of wettability in shale kerogen with varying maturity levels

Kerogen is defined as the insoluble macromolecular organic matter in sedimentary rocks and is a complex mixture of organic chemical compounds. Typically, a single chemical formula is inadequate to represent kerogen whose composition depends on a long history of geologic events and thermal maturity of the shale. The process of kerogen maturation is accompanied by the loss of functionalized molecules, leading to a decreased H/C and O/C ratios, as well as a reduction in molecular weight. The degree of maturation is well reflected in the van Krevelen diagram with H/C and O/C ratios as indicators. Even though the widely held view for kerogen pores is generally hydrocarbon-wetting, some recent experimental work indicates the existence of water content in kerogen. The key questions arising from this observation are whether kerogen may have mixed-wet characteristics and if so, is this likely to depend on kerogen maturity.Addressing these concerns is very essential because wettability is directly related to the dynamics of fluids, and is likely to be extremely relevant to developing models for reserves estimates and multiphase flow. Additionally it may provide some answers to the common observation of low recovery of hydraulic fracture water.In this paper, pore-scale molecular dynamics simulations are used to understand the relationships between kerogen maturity and its wettability. The traditional graphene model is chosen as a proxy for organic pores, and the varying degrees of maturity is approximated by grafting varying amounts of functionalized groups onto the pore surfaces. We study a mixture of water and alkanes in organic pores with effective width of 5 nm. We find that water molecules aggregate together to form clusters due to the polarity of water, and the shapes and the locations of the water clusters are strongly linked to the density of the functionalized groups. By determining the contact angles, we are able to quantify the relationship of kerogen wettability with the maturity level.The results demonstrate that kerogen maturity governs wettability of organic kerogen pores, due to the influence of surface heterogeneity and the distribution of polar molecules on kerogen. The wettability in kerogen is very likely to be heterogeneous. Depending on the maturity level, kerogen may be hydrocarbon wetting at high maturity, neutral wetting at intermediate maturities or even hydrophilic for organic surfaces of very low maturities. Additionally, our results indicate that in kerogen there may be a critical local density of surface functionalized sites that may create optimal conditions for hydrophilic kerogen surfaces.

地層内バイオメタン生成に関する宗谷夾炭層での有機物分解促進に関する検討

We propose a new gasification method that converts unused organic matter in sedimentary rocks to bio methane gas through the use of microorganisms, known as Subsurface Cultivation and Gasification (SCG). SCG was devised based on the product ion of biogenic methane gas in subsurface environments completely without a mining process of coal. With the goal of developing a method to produce biogenic methane gas in subsurface environments, this approach employed hydrogen peroxide to decompose organic matter rapidly. Conversion of organic matter from source rocks into bio-methane with the help of microorganisms is expected to be of high profitability. In northern Hokkaido, several coal seams called Soya coal-bearing formations form the Tenpoku coalfield. These seams consist of lignite, coaly shale, and tuff. This coalfield has not been mined since 1972. SCG will enable gas production from this source, thereby allowing the utilization of an unused resource in this area. In this study, immersion tests using a hydrogen peroxide solution were performed on lignite and coaly shale to estimate the quantity of low-molecular-weight organic acids and the producing potential for biogenic methane gas. Laboratory tests of both physical and mechanical properties were also performed on the lignite. Before the immersion tests, a pyrite in the coal was analysed by both the X-ray Fluorescence Analysis and the X-ray diffraction. Decomposition of organic matter using hydrogen peroxide depended on the amount of pyrite in the coal. The oxidative decomposition of lignite, which contains minimal pyrite, produced a high yield of lowmolecular-weight organic acids as substrates for methanogenic microorganisms, whereas there was no correlation between the yield of them and material characteristics of lignite. If SCG is used in the coal seams of Soya coalbearing formations, the methanogenic potential of minimal-pyrite-containing lignite should be greater than that of coaly shale, which contains much more pyrite.

Evolution of temperature, maturation level and realization of hydrocarbon potential in subtrappean sedimentary complex of the Bombay offshore – Results of 1-D basin modeling

The modified GALO program package for basin modeling is used to calculate the temperature and vitrinite reflectance distributions with depth at any time before, during and after the Decan Trap formation. An assessment of hydrocarbon generation by kerogen of type III is carried out for burial history of probable source rocks in the subtrappean sedimentary complex of the western passive margin of India near the Bombay offshore. Duration of trap formation is most critical and indefinite parameter in construction of maturity aureole in the subtrappean sedimentary complex. The analysis in the frame of the model of continuous formation of traps suggests that a maturation level of organic matter in sedimentary rocks under the trap depends strongly on duration of the trap formation. Even for traps thickness of 1.5 km or more, maturity level of organic matter in the rocks located on the 100–500 m below the base of the trap can be rather moderate if the trap has been formed over fairly a long time.

Fluid Inclusion and Carbon‐Oxygen Isotope Studies of the Hujiayu Cu Deposit, Zhongtiao Mountains, China: Implications for Syn‐metamorphic Copper Remobilization

The Hujiayu Cu deposit, representative of the “HuBi‐type” Cu deposits in the Zhongtiao Mountains district in the southern edge of the North China Craton, is primarily hosted in graphite‐bearing schists and carbonate rocks. The ore minerals comprise mainly chalcopyrite, with minor sphalerite, siegenite [(Co, Ni)3S4], and clausthalite [Pb(S,Se)]. The gangue minerals are mainly quartz and dolomite, with minor albite. Four fluid inclusion types were recognized in the chalcopyrite‐pyrite‐dolomite‐quartz veins, including CO2‐rich inclusions (type I), low‐salinity, liquid‐dominated, biphase aqueous inclusions (type II), solid‐bearing aqueous inclusions (type III), and solid‐bearing aqueous‐carbonic inclusions (type IV). Type I inclusion can be further divided into two sub‐types, i.e., monophase CO2 inclusions (type Ia) and biphase CO2‐rich inclusions (with a visible aqueous phase), and type III inclusion is divided into a subtype with a halite daughter mineral (type IIIa) and a subtype with multiple solids (type IIIb). Various fluid inclusion assemblages (FIAs) were identified through petrographic observations, and were classified into four groups. The group‐1 FIA, consisting of monophase CO2 inclusions (type Ia), homogenized into the liquid phase in a large range of temperatures from −1 to 28°C, suggesting post‐entrapment modification. The group‐2 FIA consists of type Ib, IIIb and IV inclusions, and is interpreted to reflect fluid immiscibility. The group‐3 FIA comprises type II and IIIa inclusions, and the group‐4 FIA consists of type II inclusions with consistent phase ratios. The group‐1 and group‐2 FIAs are interpreted to be entrapped during mineralization, whereas group‐3 and group‐4 FIAs probably represent the post‐mineralization fluids. The solid CO2 melting temperatures range from −60.6 to 56.6° C and from −66.0 to −63.4° C for type Ia and type IV inclusions, respectively. The homogenization temperatures for type II inclusions range from 132 to 170°C for group‐3 FIAs and 115 to 219°C for group‐4 FIAs. The halite melting temperatures range from 530 to 562°C for type IIIb and IV inclusions, whereas those for type IIIa inclusions range from 198 to 398°C. Laser Raman and SEM‐EDS results show that the gas species in fluid inclusions are mainly CO2 with minor CH4, and the solids are dominated by calcite and halite. The calcite in the hosting marble and dolomite in the hydrothermal veins have δ13CV‐PDB values of −1.2 to 1.2‰ and −1.2 to −6.3‰, and δ18OV‐SMOW values of 14.0 to 20.8 ‰ and 13.2 to 14.3 ‰, respectively. The fluid inclusion and carbon‐oxygen isotope data suggest that the ore‐forming fluids were probably derived from metamorphic fluids, which had reacted with organic matter in sedimentary rocks or graphite and undergone phase separation at 1.4–1.8 kbar and 230–240°C, after peak metamorphism. It is proposed that the Hujiayu Cu deposit consists of two mineralization stages. The early stage mineralization, characterized by disseminated and veinlet copper sulfides, probably took place in an environment similar to sediment‐hosted stratiform copper mineralization. Ore minerals formed in this precursor mineralization stage were remobilized and enriched in the late metamorphic hydrothermal stage, leading to the formation of thick quartz–dolomite–sulfides veins.

Thermal regime and amplitude of lithosphere extension in the Sirte basin, Libya: Numerical estimates in the plane basin modeling system

The GALO basin modeling system has been applied for the numerical reconstruction of the subsidence history, variations in temperature, and maturity of the organic matter of sedimentary rocks composing the main tectonic structures of Sirte Basin. The reconstruction was carried out for eight sedimentary cross sections along the profile stretching from the Cyrenaica Platform on the eastern coast of the basin to the Hun Graben on its western wall. The interval of depths for temperature calculations included the sedimentary layer, consolidated crust, and the mantle to below 100 km. This extensive depth coverage made it possible to use the analysis of the variations in the tectonic subsidence of the basin for estimating the amplitudes and duration of the events of thermal activation and extension of the lithosphere of the basin in the history of its evolution. The modeling suggests that thermal activations of the lithosphere in the Albian-Cenomanian and Oligocene-Pleistocene are common for all tectonic structures of the Sirte Basin and the Cyrenaica Platform and that a relatively high temperature regime is also characteristic of the present-day conditions in the Sirte Basin and Cyrenaica Platform. To a considerable extent, such a regime is caused by the thermal activation of the lithosphere of the basin during the last 10 Ma. The intensity of this activation is highest in the western part of the basin, where it is accompanied by the highest erosion amplitudes. The analysis of the variations in tectonic subsidence of the basement also suggests a series of intervals of lithospheric extension, which accounts for the stages of relatively rapid subsidence of the basin. Two intervals of significant extension of the lithosphere in the Upper Cretaceous and Paleocene are common for all areas within the basin. Here, the total amplitudes of the crustal extension attained 1.5 in the central part of the Sirte Basin (the Ajdabiya and Maradah troughs and Zelten and Dahra platforms), was equal to 1.3 in the Hameimat and Zallah troughs, and were minimal (1.11–1.17) in the periphery of the basin (within the Hun Graben and Cyrenaica Platform).

Influence of provenance and preservation on the carbon isotope variations of dispersed organic matter in ancient floodplain sediments

Carbon isotope ratios of bulk organic matter in sedimentary rocks (δ13CDOM) are a potential source of paleoenvironmental information in terrestrial stratigraphic sequences. However, insufficient understanding of the range of depositional and post‐depositional controls on δ13CDOM values makes interpretations of these data difficult. Here we evaluate the effects of organic matter (OM) provenance and preservation on δ13CDOM using records spanning the Paleocene Eocene Thermal Maximum (PETM) in the Bighorn Basin (Wyoming, USA) as a case study. We sampled sedimentary rocks spanning the PETM in two well‐studied locations—Polecat Bench (PB) and Highway 16 (HW16)—in the Bighorn Basin. Independent carbon isotope records from biomarkers and pedogenic carbonates at these sites suggest that local shifts in plant and soil δ13C values associated with the PETM CIE were broadly similar and were characterized by an abrupt ∼5‰ decrease followed by a plateau and a eventual return to pre‐PETM δ13C values. The δ13CDOM records from both sites differ significantly from these reference curves in both amplitude of change and in preserving high‐frequency isotopic fluctuations and large isotopic anomalies superimposed on the general pattern of isotopic change through the CIE. For each location, we separated organo‐mineral fractions (MOM), concentrated macerals from 20 stratigraphic levels and analyzed the carbon isotope ratio (δ13C) of each fraction. At both sites the δ13C of the fine and coarse MOM differ significantly from each other and from δ13CDOM. Concentration‐weighted mixing of these isotopically distinct OM fractions explains high resolution δ13CDOM fluctuations but does not explain the large isotopic anomalies observed at both sites. At HW16, we identified two thermally and isotopically distinct populations of macerals interpreted as being indigenous and recycled OM. At this site, one over total organic carbon (1/TOC) values correlate with δ13CDOM for pre‐PETM and PETM strata and both relationships converge toward the δ13C of recycled OM for low TOC. At PB, macerals display homogeneous thermal maturity, but the proportion of isotopically distinct vitrinite and liptinite varies between facies. Relationships between 1/TOC and δ13CDOM are also present within specific stratigraphic intervals at PB, but values do not converge on a single isotopic value across the sampled interval. These observations are consistent with variable mixing of OM fractions having different provenance—mixing of exotic recycled OM at HW16 and locally reworked OM at PB with indigenous “fresh” OM at both sites—and explain the large anomalies observed in the δ13CDOM records at both sites. Our findings raise questions about the assumption that OM in ancient sediments is indigenous and dominantly records δ13C variations of local plants.