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Bulk sedimentary nitrogen isotope profiles are often used as proxies for depositional redox conditions, nitrogen cycling, and nutrient uptake in modern and ancient marine systems. The general preference in terms of analysis is that the sediments measured have undergone minimal thermal alteration, as post-depositional processes might have altered the initial δ15N signal, thus complicating the interpretation of these records. Although not a traditional proxy for petroleum evaluation purposes, recently the use of nitrogen isotopes in petroleum systems has been investigated as potential proxies to reconstruct paleoenvironmental conditions such as redox, and for organic matter evaluation. In this paper we review the use of nitrogen isotope data in petroleum systems, their interpretations, and factors that may complicate their use as proxies. We review the evidence for nitrogen isotopic fractionation during diagenesis, catagenesis, and fluid migration as determined by lab experiments, and how these might impact interpretation of δ15N data in petroleum systems. We also analyze the use and interpretation of δ15N data from petroleum-producing reservoir units, including unconventional reservoirs and lacustrine basins. Lastly, we discuss potential applications for nitrogen isotopes in petroleum systems with regards to their use as both geochemical proxies and as tools to evaluate petroleum reservoirs.

Оbject.The article presents the results of studies of secondary mineral formation in the lower Cretaceous sediments of the Bolshekhetskaya depression, which contain highly promising oil and gas deposits, with established industrial performance at Pyakyakhinskoye, Khalmerpayutinskoye and South-Messoyakhskoye fields. The objects under consideration are located in the most submerged part of the Nadym-Taz syneclise in the zone, where the Taz and Urengoy facies regions meet.Materials and methods.Main types of postsedimentary changes of reservoir rocks selected on the basis of study of core material Pokurskaya, Angelovskaya, Sortymskaya and Megionskaya fms. Petrography and Mineralogy of the rocks under consideration were studied on the basis of stage analysis using optical and electron microscopy, and X-ray diffraction study at the Center of core and reservoir fluids research of “LUKOIL-Engineering’s KogalymNIPIneft branch In Tyumen” (Kogalym). Determination of the mineral type of zeolite was performed by X-ray phase analysis at the Institute of Geology and Geochemistry UrO RAS (Ekaterinburg).Resuts.It was found that for rocks Pokurskaya, Angelovskaya, Sortymskaya and Megionskaya fms characteristic a diverse range of secondary minerals and structures, evidence of the background stage catagenesis and the superimposed processes. The most important among them is the regeneration of quartz, the transformation of feldspar, violation of the crystal structure of biotite, the formation of various clay minerals, crystallization of zeolites and calcite.Conclusions.Studies show that the upper part of the section of the lower Cretaceous deposits of Bolshekhetskaya depression corresponds to the area of the middle catagenesis (gradation MK2), the bottom – changed to the stage of deep catagenesis (gradation MK3). In this case, the epigenetic process proceeded unevenly, pulsation, due to changing tectonic conditions. Particular attention is paid to the origin of zeolites, which is still debatable. It is assumed that their formation is associated with the transformation of “camouflage pyroclastic” in particular, feldspar clastics.

Hydrothermal liquefaction (HTL) of sewage sludge is considered in the article as an analogue of the natural processes of oil formation (catagenesis). A comparison of the physicochemical composition of primary and secondary sludge with type II kerogen (natural precursor of oil) showed their similarity. Both types of sludge have a slightly higher level of oxygen and nitrogen. The study tested the hypothesis that the elements included in the inorganic part of the oil source rocks can have a catalytic effect on the oil formation processes. For the conducted studies of sludge HTL, the catalysts containing cations and substances found in oil source rocks were chosen: as homogeneous catalysts (KOH, NaOH, NH4Fe(SO4)2, CoCl6, NiSO4, CuSO4, ZnSO4, MoO3) and as heterogeneous catalysts (MgO, Zeolite, Al2O3). The effectiveness of catalysts containing metal ions, zeolite and aluminum oxide has been proven. The highest biocrude yield was achieved in a process with NiSO4 as the catalyst in a dose 2 g per 10 g of sludge: oil yield increased by 34.9% and 63.4% in the processing of primary and secondary sludge, respectively. The use of catalysts provided an increase in fuel HHV by 10.8–12.5%, which is associated with a decrease in oxygen content (by 10.8–43.2%) with a simultaneous increase in carbon (by 7.9–10.9%) and hydrogen (by 6.5–18.7%) content.