C29 Steranes Research Articles

Unveiling the Mysteries of Fractured Basement Oil Residues in the Upper Benue Trough, Nigeria

This study offers additional evidence for the occurrence of oil residues within fractures of the basement rocks in the Upper Benue Trough, Nigeria. The data suggests that the observed oil residues were originally generated as light oils, which were later biodegraded into heavy oils. These Upper Benue Trough's oil stains are a valuable tool for hydrocarbon exploration in the region as they indicate the existence of a petroleum system in the basin. We find a strong similarity to the proportions of C27, C28, and C29 regular steranes of oils found in the sandstones and shales of the Bima Formation in the Upper Benue Trough when we plot the relative quantities of these steranes of the investigated basement rock oil types on a ternary graph. This shows a positive correlation between the oils from the Bima Formation and the fractures found in the Precambrian basement rocks. Additionally, the studied basement oils and the oils extracted from the Bima Formation had comparable sterane maturity criteria. Again, trisnorhopane thermal indicators are also identical in the studied oils. The C3222S/(22R+22S) homohopane isomerization ratio and Ts: Tm ratio of the examined basement rock oils are similar to those of the oils extracted from the Bima Formation's shales, suggesting that the Bima Formation's shale is the primary source rock for the examined basement rock oils. Oils have also been reported to develop similarly in the Precambrian basement rocks of the Bongar Basin in the Chad Republic, known as basement-buried hills. As members of the West African rift basins, the Nigerian Upper Benue Trough and the Bongar Basin share genetic ancestry. As a result, we should be looking for exploratory opportunities that mimic the Bongor Basin's basement buried hills, which are currently the location of commercial oil production.

An enhanced presence of tricyclic terpanes in the Triassic oils from the tight conglomerate reservoir in the Mahu Sag, Junggar Basin, NW China

A major exploration breakthrough has been made in the Mahu Sag, Junggar Basin. However, the origin of oils from Triassic Baikouquan Formation (T1 b) remains debated. In this study, 20 oils and 44 cores were selected to investigate their distributions of tricyclic terpanes (TTs). The results show that the relative abundance of TTs in oils is higher than those of hopanes due to a high maturity. Permian Wuerhe Formation (P2 w) cores are dominated by C20TT. Permian Fengcheng Formation (P1 f) samples, predominated by C23TT, are mature to high-mature, while Permian Jiamuhe Formation (P1 j) samples, having the same abundance of C21TT and C23TT, are in the over-mature stage. Maturity may be a likely possible reason for the different occurrences of C19–C23TTs between P1 f and P1 j as their organic matter source and depositional environment were similar. High gammacerane and C29 sterane occurred in T1 b oils, P1 f and P1 j cores, while those in P2 w are little, indicating P2 w had a minor contribution for T1 b oils. The δ13C values of oils and P1 f are similar and lower than those of P2 w and P1 j source rocks. Therefore, it is preliminarily speculated that the oils may be sourced from the deep buried P1 f in the over-mature stage.

Geochemical characteristics of lacustrine source rocks and oil-source correlation of the Yanchang Formation in the Zizhou area, Ordos Basin

The Zizhou area on the northeastern margin of the Ordos Basin has achieved major breakthroughs through continuous exploration over the years, demonstrating high potential for hydrocarbon exploration. However, due to the underdevelopment of source rocks, the origin of crude oil in the Chang 6 and Chang 4 + 5 Members remains unclear, which significantly hinders further hydrocarbon exploration. In this study, we use Rock-Eval pyrolysis, kerogen maceral analysis, quantitative gas chromatography (GC) of saturated hydrocarbons, and GC-mass spectrometry of saturated and aromatic hydrocarbons to determine the geochemical characteristics of source rocks and oils from different sedimentary environments. In addition, we select effective biomarker parameters to identify the source of oil in the Chang 6 and Chang 4 + 5 Members in the Zizhou area. The results indicate that the Chang 7 Member source rocks have organic matter abundances ranging from good to excellent, with types classified as type I-II1, and are in a mature stage. Source rocks from the semideep to deep lacustrine facies exhibit a clear predominance of low-carbon number hydrocarbons, a higher proportion of short-chain n-alkanes, relatively lower C24 tetracyclic terpane/C26 tricyclic terpane (C24TeT/C26TT) and C30 hopane/C30 diahopane (C30H/C30DiaH) ratios, and similar abundances of C27 and C29 steranes. In contrast, deltaic facies source rocks, which developed in suboxic to oxic environments, display opposite characteristics. The reservoir extracts from the Zizhou area indicate a high degree of similarity to the deltaic facies source rocks. Moreover, the subtle geochemical differences between the reservoir extracts and their potential source rocks are mainly attributed to geochromatographic effect during hydrocarbon migration and differences in preservation conditions. These findings provide new insights into the dynamics of hydrocarbon migration and accumulation in the northeastern margin of the Ordos Basin and further reinforce the reliability of using biomarker techniques to determine the relationship between crude oil and its source.

Petrographic and geochemical signatures of the Upper Permian Gondwana coals: Newcastle Coal Measures, Northern Sydney Basin, Australia

This research presents an integrated approach to organic petrographical and geochemical characteristics of the Upper Permian Newcastle Coal Measures outcropping in the northern Sydney Basin, southeast Australia. Twenty-seven coal, coaly mudstone and mudstone samples were collected and analyzed by using organic petrography (maceral composition and vitrinite reflectance) techniques, Rock–Eval pyrolysis, bitumen extraction and gas chromatography - mass spectrometry (GC–MS). The obtained results indicate that the samples exhibit promising gas-prone source rock potential and contain a predominantly terrestrial in origin organic matter, with limited contribution by marine organisms. The prevalence of terrestrial organic matter is inferred by the presence of collotelinite and sporinite macerals in most of the samples and the ternary plot of C27, C28 and C29 regular steranes. Further, the low C27/C29 ratio, the cross-plots of C27/(C27 + C29) regular steranes vs. Pr/Ph ratio, the C27/C29 vs. Pr/Ph and the C24 tetracyclic/C26 tricyclic terpane ratio point also to the terrestrial origin of the organic matter. The obtained values of vitrinite reflectance, Tmax, OEP27–31 and CPI25–33, along the cross plots of 20S/(20S + 20R) versus ββ/(ββ + αα) suggest that the samples are immature and were subjected to a low burial depth. The examined samples indicate accumulation in a delta-plain environment of deposition, under fluctuating oxic/anoxic conditions, in a humid climatic zone that is characterized by dry periods, interrupting the longer-termed humid climatic regime. This research adds knowledge on the source rock potential in the northern Sydney Basin and on the environmental and climatic setting of a time interval just prior to the major Permian - Triassic climatic crisis.

Intense Changes in the Main Source of Organic Carbon to the Gulf Coastal Plain Following the Cretaceous‐Paleogene Boundary

AbstractTo explore both environmental change and the response of non‐fossilizing phytoplankton across the Cretaceous‐Paleogene (K‐Pg) boundary mass extinction event, we determined changes in organic matter (OM) sources using a range of apolar (n‐alkanes, acyclic isoprenoids, steranes, and hopanes) and polar (BIT index) biomarkers. We analyzed two K‐Pg proximal sections, located in the Mississippi Embayment, Gulf Coastal Plain (USA), covering ∼300 kyrs prior to and ∼3 myrs after the K‐Pg event. The OM abundance and composition changed dramatically across the boundary. The post‐impact ejecta layer and burrowed unit are characterized by an increase in the mass accumulation rate (MAR) of plant and soil biomarkers, including high‐molecular‐weight n‐alkanes and C29 steranes as well as the BIT index, related to an erosive period which transported terrestrial OM to the ocean in the aftermath of the impact event. At the same time, MARs of putative aquatic biomarkers decrease (low‐molecular‐weight n‐alkanes, C27 steranes and pristane and phytane), which suggests a collapse of the marine phytoplankton community. The increase of terrestrial OM to the ocean, during the first 280 kyrs after the Chicxulub impact event, is a combination of reworked kerogen, soil and some plant material. Crucially, within the latter part of this erosion period, only ∼160 kyrs after the K‐Pg do biomarkers return to distributions similar to those in the upper Cretaceous, although not to pre‐impact MARs. Thus, our results suggest a long‐term interval for the full sedimentary and ecological recovery of the non‐fossilizing phytoplankton community after this event.

GEOCHEMISTRY OF OILS AND GASES FROM THE VERKHNECHONSKOYE FIELD, EAST SIBERIAN BASIN: APPLICATION OF ANALYTICAL RESULTS TO RESERVOIR CHARACTERISATION

A geochemical study was carried out on oil and gas samples from the Verkhnechonskoye field, located on the Nepa‐Botuoba Anteclise in the central‐southern part of the Siberian Platform. The goal of the study was to distinguish between fluids derived from the V10‐13 and B12 reservoir units in the Vendian (Neoproterozoic) Katanga and Nepa Formations and to identify the producing reservoir using geochemical data. The results of analyses of 12 oil and 13 associated gas samples from the two reservoirs showed that all the fluids have similar geochemical properties including: low Pr/Ph ratios (0.78‐1.00); a predominance of C29 over C27 and C28 steranes; a predominance of odd‐numbered C21‐C25 n‐alkylbenzenes over their even‐numbered homologues; the presence of 12‐ and 13‐methylalkanes; and a high relative abundance of tricyclic terpanes (cheilantanes). All these properties are consistent with those of the properties of petroleum from other fields on the Siberian Platform. The molecular and stable carbon isotope compositions of the oils and gases suggest that they were derived from marine organic matter with a high algal input deposited under reducing conditions. To date, specific source rocks which generated the oil and gas present at fields on the Nepa‐Botuaoba Anteclise have not conclusively been identified, but potential candidates include the Upper Riphean Iremeken and Ayan Formations and more probably the Vendian Zherbinskaya, Seralakh, Vanavara and Nepa Formations.The second part of the study demonstrates the application to reservoir geochemistry of C3‐ and C4‐ alkylbenzene compounds together with more conventional biomarkers. Key parameters were selected using statistical processing and displayed in graphic profiles. These profiles allowed the oil and gas samples to be classified according to the reservoir from which they were derived based on their geochemical properties. Parameters based on C3‐ and C4‐ alkylbenzene compounds were most effective in discriminating between oils from the two reservoirs. In addition, a new parameter is proposed based on the contents of 1‐methyl‐3‐isopropylbenzene, 1‐methyl‐2‐isopropylbenzene and 1‐methyl‐2‐propylbenzene; this parameter correlates closely with the pristane/phytane ratio and can be used as an additional indicator of the level of oxicity in the source rock depositional environment.

Reconstructing the palaeoecology of a middle Permian alkaline lake using molecular fossils, case study of the Lucaogou Formation in the Junggar Basin, NW China

The lacustrine Lucaogou Formation was deposited during the middle Permian in the Junggar Basin, northwestern China. The depositional environment was an alkaline, brackish to saline lake. The biological diversity of the Lucaogou Formation was reconstructed using biologically informative molecules such as n-alkanes, steranes, and hopanes, and the δ13C composition of organic matter. C30 4α-methyl-24-ethylsterane, C31 lanostane, and abundant C30 − C31 3β-methylhopanes were detected in the organic matter, suggesting the presence of type Ⅰ aerobic methanotrophs including the Methylococcaceae. C30 − C31 2α-methylhopanes and 7- and 8-methylheptadecanes are suggestive of the presence of cyanobacteria. Cyanobacteria and green algae are proposed to have made the dominant contribution to the organic matter. The presence of more 13C-enriched hopanoids in the lower unit compared to the upper unit of the Lucaogou Formation suggest a greater contribution of cyanobacteria to the organic matter in the lower unit. The high abundances of C28 and C29 steranes relative to C27 steranes are likely due to green algae from the Chlorophyta, including the class Chlorophyceae. Dinosteranes that are diagnostic for dinoflagellates were not detected. The low concentrations of C40 isorenieratane, renieratane, β-isorenieratane, renierapurpurane, and β-renierapurpurane that are indicative of photosynthetic green and purple sulfur bacteria indicate a minor contribution from Chlorobiaceae and Chromatiaceae to the organic matter. It is inferred that low to moderate amounts of ciliates grew at or below the relatively unstable chemocline in the lake. The presence of C19-norisopimarane, 8β(H)-labdane, and 4β(H)-19-norisopimarane suggests the presence of coniferous plants growing around the lake, probably including Pinaceae, Araucariaceae, Cupressaceae.

Petroleum potentiality of the Balambo Formation in Bekhme and Razan sections, Kurdistan region, northeastern Iraq

Balambo Formation (Early Cretaceous) had been studied in Razan and Bekhme sections, Erbil Governorate, Kurdistan Region-Iraq, to determine the hydrocarbon generation potentiality for the studied rocks. The samples were analyzed for TOC, Rock Eval pyrolysis and GC-MS technique. The Balambo Formation’s TOC wt. % content is relatively high, ranging between 0.95 to 7.59 wt.% (average 2.91 wt.%). The Formation has the potentiality to generate gas and oil owing to its suboxic-anoxic depositional environment, marine origin organisms, suitable maturity level, and the type of kerogen is predominantly types III and subordinate amount of mixed type II-III according to the Rock-Eval pyrolysis data and isoprenoid/n-alkane. The C27%, C28% and C29% regular steranes distribution suggested the studied formation is deposited in marine environment. The samples were proven to be mature based on the ratios of the Tmax, steranes 20S/ (20S+20R)C29 to ββS/(ββS+ααR)C29 steranes and C32 22S/(22S+22R) hopane. Although the Ts/(Ts+Tm) ratio data point to an early mature stage, vitrinite reflectance predictions ranged from 0.55 to 0.75%, once more pointing to a mature stage.

Organic geochemical characteristics of the Late Cretaceous coal and carbonaceous shale succession from the Taranaki Basin, New Zealand: Implications for sedimentary environmental setting and petroleum generation potential

This comprehensive study investigates the organic matter characteristics within the Late Cretaceous Rakopi and Taniwha formations, based on data from four exploration wells situated in New Zealand's Taranaki Basin. It employs a multifaceted approach, integrating bulk geochemical analyses, biomarker measurements and carbon isotopes to unveil the geological history of these formations. Analytical results include total organic carbon content, ranging from 7.27 to 75.78 wt%, and generation potentials spanning from 28.24 to 309.16 mg hydrocarbon/g rock. These observations underscore the source rock potential of these Late Cretaceous strata. These rocks show a mixed organic matter of hydrogen‐rich Type II and Type II/III kerogens, as evidenced by hydrogen index values (HI) between 237 to 428 mg hydrocarbon/g rock. These formations demonstrate promise potential for both oil and gas generation. Biomarker analysis uncovers distinct signatures, featuring a pristane/phytane (Pr/Ph) ratio ranging from 3.27 to 10.91, a Tm/Ts ratio surpassing 7 and elevated concentrations of C29 regular steranes relative to C27 and C28 regular steranes. These biomarker characteristics suggest a composite organic matter composition, influenced by terrigenous organic matter, likely deposited in oxygenated fluvial deltaic environments. Bulk carbon isotopic data corroborate these findings, highlighting the abundance of terrigenous organic matter. Collectively, these insights reveal that the coal and carbonaceous shale intervals in the examined wells are in the early stages of oil generation. Therefore, the Rakopi and Taniwha formations have not yet yielded commercially viable oil and/or gas quantities. In this case, these formations hold substantial promise for future exploration activities in relatively deep wells, with limited oil expulsion from coals.

Characteristics and origin of low-organic-matter carbonate source rocks in the Middle–Upper Ordovician, Tarim Basin, northwestern China

The continually-discovered large volumes of marine hydrocarbons indicate a huge exploration potential within the carbonate rocks of Ordovician in the Tarim Basin, northwestern China. The Middle–Upper Ordovician (O2+3) carbonate source rocks are at a highly mature stage. These rocks presently contain a low amount of total organic carbon (TOCpd ≤0.5%) (TOCpd means the present-day TOC). A mass balance approach is used to identify the source rocks that have expelled hydrocarbons. The characteristics and origin of hydrocarbon expulsion from low-TOCpd source rocks are studied, which are significant for oil exploration in the basin. The results showed that a low-TOCpd O2+3 source rock having expelled hydrocarbons was type I and had a very narrow oil window. With a high rock maturity and a high original TOC (TOCo) value exceeding 1%, nearly 80% of the generated hydrocarbons were expelled. The content of gammacerane and C28 steranes in the oil and source rock extracts were relatively lean, with a V-shaped distribution of regular steranes, suggesting a possible genetic relation between the low-TOCpd source rocks and the marine oil in the Tabei area. Hydrogen-rich planktonic algae or acritarchs were the main hydrocarbon parent materials, distributed in the subsiding platform-slope facies. Organic matter was preserved under reducing conditions, and source rocks were formed with a favorable kerogen type and a moderate hydrocarbon generation potential. This study illustrates the hydrocarbon expulsion of low TOCpd source rocks in northern Tarim Basin, which is greatly significant for analyzing the genesis of marine crude oil in Tarim Basin, and evaluating the distribution of marine source rocks. This research method is theoretically significant for oil and gas exploration in the same type of highly-mature carbonate paleobasins.

Organic Geochemical Evidence for the Formation of Condensate from Coaly Source Rocks in the Wumaying Buried Hill of the Huanghua Depression, Bohai Bay Basin.

Although oil and gas from coaly source rocks have been widely discovered worldwide, the role of oil generated from coal measures in marine-continental coaly deposits during the Carboniferous-Permian period in the Bohai Bay Basin has long been a subject of debate. The recent discovery of a condensate reservoir in the Wumaying buried hill within the Huanghua Depression of the Bohai Bay Basin offers new potential insights into this issue. In this study, we employed organic geochemical methods to explore the possibility of the Carboniferous-Permian coal deposit being a primary source of the condensate. The distribution of light hydrocarbons and the biomarker assemblage indicate that the condensate did not undergo significant secondary alterations such as thermal cracking, gas invasion fractionation, or biodegradation. The hydrocarbon generation potential of the Carboniferous-Permian coaly source rocks suggests that they could be an important contributor to the formation of condensate. High pristine/phytane ratios (1.0-7.5), an abundant presence of benzene series, and the dominance of C29 steranes (>50%) within the condensate could be indicative of coaly organic matter. These features are comparable to those found in coaly source rocks. Moreover, the stable carbon isotopic compositions of n-alkanes in the condensate, ranging from -26.0 to -30.0‰, correlate well with those from coaly mudstone (-25.4 to -30.0‰). This suggests that the condensate of the Wumaying buried hill may predominantly originate from the Carboniferous-Permian coaly mudstone. When integrated with the geological background, the results distinctly demonstrate that the Carboniferous-Permian coaly source rocks have significantly contributed to the formation of the condensate reservoir in the Wumaying buried hill. This provides an essential reference for future exploration of oil and gas resources derived from the carboniferous-Permian coaly source rocks in the Bohai Bay Basin.

Pristine biomarker compositions recovered from biodegraded oil samples using MSSV-Hy

Oil-source and oil-oil correlations are challenging when the primary chemical composition of petroleum was altered or destroyed by biodegradation. Biomarkers that are bound to macromolecular matrices rather than being in a free form are largely protected from alteration and may therefore offer a solution to the challenges. Using Microscale Sealed Vessel Catalytic Hydrogenation (MSSV-Hy), the bound biomarkers of two North Sea and two Barents Sea oil samples, with varying levels of biodegradation, were released from their respective asphaltenes. Despite biodegradation having changed the relative concentrations of steranes (C27–C30 steranes) and hopanes (C29–C30 hopanes) in the free biomarker fraction to varying degrees, the asphaltene-bound biomarkers were protected and show pristine ratios unaffected by biodegradation, e.g., the relative content of C27–C29 steranes, the C30/C29 sterane ratio, and the C29/C30 hopane ratio. Empirical calibration then allows these bound biomarkers to be converted into regular biomarker ratios. The MSSV-Hy technique has been shown to be an efficient and effective tool for releasing asphaltene-bound biomarkers from biodegraded oil and revealing genetic information when free biomarkers are altered by biodegradation.

Source rock characterization and biomarkers analysis of Adar and Galhak Formations, Rawat central sub-basin, White Nile basin, Sudan

Geochemical analysis was conducted on shaly-cutting samples from various formations in Rawat Central Sub-Basin using Rock-Eval pyrolysis, vitrinite reflectance, and GC-MS to determine the characteristics of different source rocks in the basin, including their hydrocarbon potential, maturity, and organic facies. It can be inferred that most samples exhibit characteristics of gas-prone source rocks. Only Adar and the shaly upper section of the Campanian Galhak formation are promising in having excellent source rocks with TOC reaching up to 12%, HI (563–919 mg/g TOC) Tmax (443–445 °C), and S2 (77 mgCO2/g rock) with a potential to generate oil. The maturity data of both Tmax and vitrinite reflectance revealed that these source rocks are within the early mature to early peak mature with dominant kerogen type of a mixed type II-III kerogen. Few samples reflect a pure type I and type II kerogen.The biomarkers data revealed that three possible organic faces can be identified. Slightly brackish anoxic to sub-anoxic deep lacustrine/marine shale to marginal marine shale (coastal plain estuary) characterized by (Pr/Ph < 1, High Hopanes, C23 TT, Pr/Ph < 1, C25TT/C26TT and C24TeT/C26TT < 1, C27 ∼ C29 sterane, high Gammacerene and β-carotane) (Organofacies I). Mixed facies of distal lacustrine to coastal plain proximal lacustrine (Organofacies II) characterized by (High waxiness, Pr/Ph > 2, C25TT/C26TT and C24TeT/C26TT > 1, high C28 sterane). Sub-oxic terrestrial to coastal plain estuary/bay facies characterized by (Pr/ph ∼1, C25TT/C26TT and C24TeT/C26TT > 1, low Gammacerane, High C29 sterane) (Organofacies III). Lower Galhak formation was deposited in slightly brackish, clay-rich, deep lacustrine to mixed marginal marine shale estuary representing Organofacies I with an average terrestrial input forming type II & III kerogen. For Galhak formation, the depositional environment ranged from proximal sub-oxic sallow lacustrine to distal sub-oxic lacustrine (Organofacies II) with a high terrestrial input in the Lates. Adar formation was deposited in a clay-rich, sub-oxic, terrestrial (Fluvial) to coastal plain (estuary/bay) (Organofacies III) with an average terrestrial input generating a mixed type II/III kerogen. The relatively low 20S/(20S + 20R) and ββ/(ββ + αα) C29 suggest an early maturation in Adar formation and early to peck mature source rocks in Galhak formation.

Common origin of sterol biosynthesis points to a feeding strategy shift in Neoproterozoic animals

Steranes preserved in sedimentary rocks serve as molecular fossils, which are thought to record the expansion of eukaryote life through the Neoproterozoic Era ( ~ 1000-541 Ma). Scientists hypothesize that ancient C27 steranes originated from cholesterol, the major sterol produced by living red algae and animals. Similarly, C28 and C29 steranes are thought to be derived from the sterols of prehistoric fungi, green algae, and other microbial eukaryotes. However, recent work on annelid worms–an advanced group of eumetazoan animals–shows that they are also capable of producing C28 and C29 sterols. In this paper, we explore the evolutionary history of the 24-C sterol methyltransferase (smt) gene in animals, which is required to make C28+ sterols. We find evidence that the smt gene was vertically inherited through animals, suggesting early eumetazoans were capable of C28+ sterol synthesis. Our molecular clock of the animal smt gene demonstrates that its diversification coincides with the rise of C28 and C29 steranes in the Neoproterozoic. This study supports the hypothesis that early eumetazoans were capable of making C28+ sterols and that many animal lineages independently abandoned its biosynthesis around the end-Neoproterozoic, coinciding with the rise of abundant eukaryotic prey.

Impact of aerobic reworking of biomass on steroid and hopanoid biomarker parameters recording ecological conditions and thermal maturity

Steranes and hopanes are the biomarkers of eukaryotic sterols and bacterial hopanols. Extracted from sedimentary rock, they are widely used to assess burial temperatures and palaeoecological conditions. The relative proportion of steranes and hopanes is commonly applied as a measure of the flux of eukaryotic versus bacterial biomass into sediments, and the relative abundances of C27, C28 and C29 steranes are proxies for shifts in eukaryote ecology. In Recent sediments, intact sterols provide additional information about particular eukaryotic origins. However, biological lipid distributions are not always recorded faithfully in sediments. Based on observations on modern algae and plants, and on 558 million year old fossil macroalgae from the Ediacaran of the White Sea, we suggest that these biomarker proxies can be severely altered by aerobic microbial reworking, to the extent that a complete loss of primary ecological information may occur. Network analysis on the biomarker data suggests that oxic degradation also affects isomer and homolog distributions of saturated and aromatic steroids, hopanes, cheilanthanes and n-alkanes, generating anomalies in apparent thermal maturity indicators and other proxies. In our dataset, between Ediacaran macroalgae that experienced the least and the most oxic degradation, the absolute concentration of biomarkers decreases 80-fold, and at the same time the proportion of steranes over hopanes decreases by a factor of 82, while the proportion of C29 steranes among total steranes decreases from 91% to 47%. Such redox dependent offsets may explain the recurrently erratic behaviour of numerous biomarker parameters. While these results impart constraints on the interpretation of biomarker distributions, they do provide a tool for evaluating the effects of oxygen exposure and microbial degradation on organic matter preservation in recent and ancient environments and may point towards a solution for the correction of such effects.

Stepwise deforestation during the Permian-Triassic boundary crisis linked to rising temperatures

Although the trajectory of the marine mass extinction at the ∼252-Ma Permian-Triassic (P-Tr) boundary has been well studied, details of the coeval collapse of terrestrial ecosystems remain murky. Here, we use hydrocarbon biomarker compositions and other geological records (i.e., organic carbon isotopes (δ13Corg), charcoal abundance, and Hg content) from a tropical peatland succession in southwestern China to reconstruct in detail the history of terrestrial ecosystem collapse during the P-Tr crisis. Our high-resolution hydrocarbon biomarker records reveal that this collapse proceeded in a stepwise manner with increasing intensity as the crisis unfolded. We recognize three discrete crisis stages: Stage I within the uppermost Xuanwei Formation and Stages II and III within the lowermost Kayitou Formation. Stage I, the early crisis stage, is marked by a significant decline in terrestrial biomass (continuing into the later stages), as recorded by reduced C29 steranes relative to total steranes and a concomitant reduction in the ratio of pristane to phytane (Pr/Ph). Stage II, the main crisis stage, records intensified soil erosion and sediment flux as revealed by rising dibenzofuran (DBF) content and high hopane/sterane ratios, the disappearance of coal seams, a sharp negative shift in δ13Corg, and peak concentrations of charcoal reflecting increased wildfire incidence. Stage III, the late crisis stage characterized by enhanced soil erosion, corresponds to peak values of Hg and Hg/TOC but no charcoal peak, suggesting intensified volcanism and a return to a humid climate. These stages closely follow temperature records, which show a stepwise rise during the crisis interval, implying that the deforestation process was strongly influenced by punctuated rises in temperature and/or its attendant effects (e.g., climate aridification). This P-Tr transition scenario suggests that global warming can trigger deforestation and reduce terrestrial carbon storage, thus serving as a positive climate feedback, with important implications for present-day climate change.

Implications of Organic Matter Input, Sedimentary Environmental Conditions, and Gas Generation Potential of the Organic-Rich Shale in the Onshore Jiza-Qamar Basin, Yemen.

The Jiza-Qamar Basin is one of the most important exploration sedimentary basins in Yemen. For over a decade, the exploration of hydrocarbons has been occurring in this basin. Late Cretaceous age rocks are the most occurring organic-rich sediments in this basin, including coals, coaly shales, and shales. The studied organic-rich shale beds are from the Late Cretaceous Mukalla Formation and associated with coal seams. These organic-rich shales can serve as source rocks for hydrocarbon generation potential. The current study investigates the geochemical characteristics, including assessing the organic matter (OM) input, sedimentary environmental conditions, and hydrocarbon generation potential of the organic-rich shale within the Mukalla Formation from three well locations in the onshore Jiza-Qamar Basin using organic geochemistry, biomarker, and carbon isotope measurements. The studied shale samples have high OM content with total organic carbon values between 0.74 and 19.48 wt %. Furthermore, they contain mainly hydrogen-poor Types III and IV kerogen, indicating the presence of the gas-prone source rock. The presence of these types of kerogen indicates the abundance of vitrinite and inertinite macerals, as established by microscopic investigation. However, the studied organic-rich shales had biomarker features, including high Ph/Ph ratio between 3.82 and 7.46, high Tm/Ts ratio of more than 7, and high C29 regular steranes compared to C27 and C28 regular steranes. Apart from the biomarker results, the studied Mukalla shales are characterized by the abundance of land-derived OM that deposited in fluvial to fluvial deltaic environments under highly oxic conditions. The finding of the considerable concentration of terrigenous OM is probably confirmed by the bulk carbon isotope and maceral composition data. The maturity indicators show that the examined organic-rich shale samples in the studied wells exhibit low VR values of up to 0.71%, and thereby, they have not been yet reached the high maturity for gas generation. This low maturity level in the studied wells is probably attributed to shallow burial depth, exhibiting depth of up to 2835 m. Therefore, the substantial gas exploration operations from the organic-rich shale source rock system of the Late Cretaceous Mukalla Formation can be recommended in the deeper stratigraphic succession in the offshore Jiza-Qamar Basin.

Depositional settings of the Eocene Suhostrel bituminous coal, SW Bulgaria, inferred from organic petrology and molecular proxies

The paper reports results from organic petrological and geochemical characterization of Suhostrel bituminous coal deposit, SW Bulgaria. Three high ash (21–47 wt%) and thin (up to 0.6 m) coal seams, representing splits from a locally compact coal bed (∼1.8 m thick), were formed around the southern and south-western rim of the Padesh Graben during its Eocene evolution. High total organic carbon contents (avg. ∼86 wt%, daf), vitrinite reflectance (avg. Ro = 1.05%) and Tmax (avg. 457 °C) argue for high vol. A/medium vol. bituminous coalification rank. Maceral composition is dominated by vitrinite (avg. ∼ 95 vol%), accompanied by small amounts of liptinite (avg. ∼ 5 vol%). Apart from few occurrences of funginite, inertinite is not present. The organic matter is mostly represented by detrovitrinite and collotelinite, the latter generally occurring in the form of gelified leaf-derived tissues. Liptinite is mainly represented by cutinite and microsporinite and display no evidence for organic matter contribution from algae. Low TPI and VI indices indicate peat formation from vegetation with poor preservation potential. Insignificant contents of gelovitrinite, coupled with high GI indices denote greater importance of the geochemical processes for the gelification of the organic matter. Striking predominance of mid- and long-chain n-alkanes argues for peat formation from a mixture of aquatic macrophytes and terrestrial vegetational communities. Presence of oleanane, albeit in low concentration, provides robust argument for contribution from angiosperm plants, whereas absence of resin-derived sesqui- and diterpenoid hydrocarbons denotes insignificant role of conifers during peat accumulation. Reconstructed depositional settings indicate organic matter accumulation under marginal aquatic, marsh/fen-type peatland with meso- to rheotrophic characteristics (GWI > 0.5). High mineral matter contents, coupled with the abundance of siliciclastic grains, imply hydrologically active environment. Plant remains deposited under oxidizing conditions (Pr/Ph = 1.5–2.5), but because of high water table were likely subjected to frequent (perhaps seasonal) changes in Eh settings. The resultant limited aerobic biodegradation of the plant remains is evidenced by low concentrations of hopanoid biomarkers and their diagenetic degradation products – drimane-type sesquiterpenoids. Post-depositional marine influence and downward infiltrating sulfate-rich waters is considered responsible for the presence of dibenzothiophene derivatives in Suhostrel coal.All molecular maturity-related ratios (e.g. CPI, Ts/(Ts + Tm), βα/(αβ + βα) C30 hopanes, ββ/(ββ + αα) C29 steranes, MPI-1, etc.) are fully compatible with the bulk geochemical parameters (e.g., TOC, Tmax, Ro) and confirm the bituminous coalification rank of the coal. Based on this data, temperatures of ∼130–140 °C were likely reached during burial of the sediments in a zone, characterized by high thermal influx (geothermal gradient ∼43–47 °C/km).

Study on the Applicability of Saturated Hydrocarbon Parameters in the Evaluation of Lacustrine Source Rocks and Oils Based on Thermal Simulation Experiments

Saturated hydrocarbons are widely used in the valuation of source rocks and oils, but the applicability of related parameters has received less attention. Based on the thermal simulation experiments on Mesozoic source rocks in the Yingen-Ejinaqi Basin, and the GC-MS analysis of the saturated hydrocarbon fractions of the expelled oils and extracts from solid residues, the applicability of the parameters related to lacustrine source rocks and oil were discussed. The results indicated that: Controlled by thermal degradation, both the relative abundance of the tricyclic terpenoids in terpenoids and the pregnane (including L pregnane) in steroids increased with the increase in maturity. Maturity had little effect on some environmental parameters of lacustrine source rocks and oils, such as Pr/Ph and gammacerane index, they were still effective even at the high-over maturity stage. But maturity had a significant influence on the method of using saturated hydrocarbons to identify the source of organic matter, because only at the thermal stage of Ro &lt; 1.45%, might it effectively identify the source of organic matter by using the dominant peak of n-alkanes method and the relative abundance of C27–C29 steranes method. Most saturated hydrocarbon maturity parameters had their valid scope of application, such as C29 20S/(20S + 20R) steranes, C29αββ/(ααα + αββ) steranes and C31αβ22S/(22S + 22R) homohopanes were only effective when the Ro value was below 2.06%, for the parameter’s value would be “inverted” in the stage of over-maturity (Ro &gt; 2.06%). However, the parameter Ts/(Ts + Tm) was effective in the whole thermal evolution process, reflecting good applicability. This study clarifies the validity of the application of commonly used saturated hydrocarbon parameters, and it can provide some reference for relevant studies.

Biomarkers applied to the assessment of organic matter sources, thermal maturation and depositional environment of the Devonian Pimenteiras Formation, Parnaíba Basin (Brazil)

The potential for hydrocarbon generation of Devonian shales from Pimenteiras Formation has been the object of study in many research works. In this study, the focus is to evaluate the organic matter sources, thermal maturation and depositional environments of the Pimenteiras Formation based on aliphatic and aromatics biomarkers. Thereby, a total of 30 samples from three wells were analyzed using gas chromatography and mass spectrometry (GC/MS). The n-alkanes, isoprenoids, and terpanes data revealed that shales from Pimenteiras Formation results of a mixture of marine, transitional and terrestrial organic matter. These findings are consistent with C27, C28 and C29 steranes distribution and abundance of methylphenanthrene and methyldibenzothiophene. The Pr/n-C17 and Ph/n-C18 ratios in the BP-77 well are greater than 1.0 (immature), while the most samples from BP-59 and BP-22 wells, the Pr/n-C17 and Ph/n-C18 ratios are less than 1.0 (mature). Findings are consistent with CPI, M30/H30, S/(S + R) (C29 ααα) and C29 ββ/(αα+ββ ratios. Additionally, the Ts/(Ts + Tm) ratios, MPI-1 versus Ro, 2-MP, 3-MP, MPI-1, MPR-1, dimethylnaphthalenes and dimethyldibenzothiophene indicate that the organic matter varies from immature to mature, and reaches the overmature stage in terms of thermal maturity. The Pr/Ph ratio, Pr/n-C17 vs Ph/n-C18 plot, high H35/H34, C29/C30, C27 Dia/C27 ααα (R + S) ratios and high concentration of dibenzothiophene suggests that the organic matter was deposited under oxic, suboxic and anoxic conditions. On the other hand, the low abundance of gammacerane and low Gam/H30 ratio suggests a low salinity and/or water column stratification. Additionally, the data showed that the organic matter was deposited in marine, marine carbonate platform and terrestrial environments.