Influencing factors and significance of organic and inorganic nitrogen isotopic compositions in lacustrine sedimentary rocks

Comprehensive nitrogen biogeochemical cycle has been reconstructed for representative lacustrine organic-rich sedimentary rock in China,namely the Triassic Yanchang Formation(YF,199–230 Ma)in Ordos and the Cretaceous Qingshankou Formation(QF,86–92 Ma)in Songliao basins,by evaluating the organic and inorganic nitrogen isotopic compositions rather than only organic or bulk nitrogen isotopic compositions.The results indicate that the nitrogen isotope values of bulk rock(δ^(15)N_(bulk))in the non-metamorphic stage are significantly different from that of kerogen,which challenge the conceptual framework of sedimentary nitrogen isotope interpretation.Theδ^(15)N_(bulk)from the YF and QF were lower than their respective the nitrogen isotope values of kerogen(δ^(15)N_(ker)),with offsets up to5.1‰,which have the inverse relationship for the metamorphosed rock.Thermal evolution did not significantly modify the d15N of bulk rock and kerogen.The d15N of sediments from the YF(δ^(15)N_(bulk),1.6‰–5.6‰)were lower than that of rock from the QF(δ^(15)N_(bulk),10.2‰–15.3‰).The nitrogen isotope values of silicate incorporated nitrogen(δ^(15)N_(sil))were slightly lower than those of the d15Nker in the YF and obviously lower for the QF.The fact that different nitrogen cycles occur in the YF and QF due to the different depositional redox conditions leads to different isotopic results.The YF water environment dominated by oxic conditions is not conducive to the occurrence of denitrification and anammox,and no abundant N2 loss leads to the relatively lightδ^(15)N_(bulk).In the stratified water for the QF,redox transition zone promotes denitrification and anammox,resulting in the heavyδ^(15)N_(bulk)of rock and promotes the DNRA,resulting in heavyδ^(15)N_(ker)and lowδ^(15)N_(sil).

Geological implications of elements of the Pleistocene mudstone with different organism compositions and enrichment environments in the Qaidam Basin,China

Trace elements and rare earth elements(REEs)of two kinds of organic facies samples representing marginal and more basin-center deposits from Pleistocene lacustrine mudstones in the central Qaidam Basin were studied to understand the provenance,palaeotectonic setting,hydrothermal activity,palaeoredox conditions and sedimentary rate.The results show that the lacustrine mudstones were mainly derived from felsic sources with little contribution from ancient crustal sediments and no ultramafic(ophiolitic)source.The mudstones were deposited in a continental island arc tectonic setting,which is consistent with the tectonic evolution of the Cenozoic basin.Both two organic facies samples were hydrothermal in origin based on the ternary diagram of Ni–Zn–Co and normalized REE patterns.However,this does not mean that the water column in paleolake was affected by hydrothermal fluids in situ.This signal might indicate hydrothermal origins from hot springs related to active faults around the basin rather than the deep hydrothermal fluids entering the sediments via deep faculties based on the comprehensive analyses of normalized REE patterns,negative E_(uanom)(Eu anomaly),Y/Ho,Sm/Yb,and Eu/Sm.Redox proxies including U/Th,Ni/Co,and Mnanom values,are more sensitive for the studied samples indicating that most of the organic facies A samples were deposited under an oxygen-depleted condition,while the organic facies B samples were deposited under oxygen-rich conditions.Redox proxies of Ceanom values are unavailable for the organic facies B samples due to hypersaline environments,and V/C_(r)and V/(V+Ni)are invalid for the organic facies A samples,possibly because of their organism composition.The low Lan/Ybn values indicate high sedimentation rates,which is consistent with the average sedimentation rates of approximately 0.43 to 1.1 km/Ma.However,the La_(n)/Yb_(n) is more likely affected by the provenance of the studied samples,so it should be used with caution.

Influence of hydrogen fugacity on thermal transformation of sedimentary organic matter:Implications for hydrocarbon generation in the ultra-depth

Hydrocarbon generation from sedimentary organic matter(SOM)with an original net C oxidation state(OS,-1–-2)is a disproportionation reaction involving the transfer of internal(“organic”)hydrogen and producing oxidized C(up to OS=+4)and reduced C(down to OS=-4).The contribution of extrinsic(“inorganic”)hydrogen could lead to an increase of hydrocarbon yields,but mechanisms and potential sources are still unclear,although it is supported by some experimental evidence.In this study,we have analyzed quantitatively the effect of increasing hydrogen fugacity on the hydrocarbon generation behavior of the Cambro-Ordovician Alum shale at 350℃and 40 MPa.Hydrogen fugacity in the experimental system was controlled by the magnetite-hematite(MH)buffer.The experimental data confirm that a more reducing environment will increase overall hydrocarbon yields and depress CO_(2) yields.The elevated hydrogen fugacity was found to correlate with an increase in δ^(13)C isotopic composition of methane and a decrease of δ^(13)C values in CO_(2),ethane and propane.This demonstrates that elevated hydrogen fugacity constrained by water-rock reaction in sedimentary basins can enhance hydrocarbon generation by“hydrocracking”.This mechanism of hydrocarbon generation by organic-inorganic interactions might be ubiquitous in the deeper sections of sedimentary basins(approximately 4–10 km).Thus,it will extend the connotation of the current theory on petroleum generation.The level of hydrogen fugacity,constrained by water-rock reactions,increases with depth and may thus activate over-maturity sedimentary organic matter to produce more hydrocarbons.The end of hydrocarbon generation will thus move to a greater depth than predicted by the traditional theory.It will be correlated to the capacity of the buffer system,which should,in consequence,be taken into account in petroleum exploration,similar to the hydrogen index(HI)of SOM.This extension of the fundamental theory will help to enlarge the area of petroleum exploration to ultra-deep strata in sedimentary basins.