Origin of Shallow Jurassic Heavy Oils in the Northwestern Margin of the Junggar Basin, NW China: Constraints from Molecular, Isotopic and Elemental Geochemistry

Abstract

Oil group separation, gas chromatography–mass spectrometry analysis of saturated hydrocarbons, carbon isotope analysis of fractions and tests on trace elements were all carried out to determine the origin of shallow Jurassic heavy oils in the northwestern margin of the Junggar Basin, northwestern China. Results showed that all the crude oils had been subjected to different degrees of biodegradation, on an order ranging from PM 6 to 9, which yielded many unresolved complex mixtures (UCM) and formed a huge spike in the mass chromatogram (M/Z = 85). Two heavy oils from the Karamay area underwent slight biodegradation, characterized by the consistent ratios of biomarker parameters. C21/C23 and C23/H of the two samples were 0.81 and 0.85, while G/H, C27/C29 and C28/C29 were 0.38 and 0.40, 0.16 and 0.27, 0.87 and 0.86, respectively. The isomerization parameters of terpane and steranes were 0.50–0.53, and 0.48–0.49, respectively. The above geochemical indices indicated that the crude oils in the study area were in the marginally mature stage. The parent materials were a mixture, consisting of bacteria, algae and some higher plants, formed under reducing depositional conditions, which is in agreement with the source rocks of the Fengcheng Formation in the Mahu depression. The carbon isotopic compositions of saturated hydrocarbon, aromatic hydrocarbon, NSO and asphaltene were –31‰– to –30.3‰, –29.5‰ to –29.03‰, –29.4‰ to –28.78‰ and –28.62‰ to –28.61‰, respectively. These findings are in agreement with the light carbon isotope of kerogen from the lower Permian Fengcheng Formation. Furthermore, V/Ni and Cr/Mo of all the crude oils were 0.01 to 0.032, 0.837 to 10.649, which is in good agreement with the ratios of the corresponding elements of the extracts from the Fengcheng Fm. carbonate source rock. As a result, a two–stage formation model was established: (1) the oil generated from the carbonate source rocks of the Fengcheng Formation migrated to the Carboniferous, Permian and Triassic traps during the Late Triassic, forming the primary oil reservoirs; (2) during the Late Jurassic period, the intense tectonic activity of Yanshan Episode II resulted in the readjustment of early deep primary reservoirs, the escaped oils gradually migrating to the shallow Jurassic reservoir through cross‐cutting faults, unconformities and sand body layers. The oils then finally formed secondary heavy oil reservoirs, due to long–term biodegradation in the later stage. Therefore, joint methods of organic, isotopic and element geochemistry should be extensively applied in order to confirm the source of biodegradation oils.

Spatiotemporal relationship between hydrocarbon fluid and sandstone‐hosted uranium deposits in the Qianjiadian area, Songliao Basin

“…This is supported by C 1 /∑C and C 1 /C 2 + values ranging from 0.85 to 0.94 and C 1 /C 2 + values altering from 5.47 to 15.0. Furthermore, the light carbon isotope compositions (Table 3) of group components and saturated hydrocarbon indicate that organic carbon are mainly originated from lower organisms of sapropel type (I) or humic sapropel type (II‐1) (light 13 C value), rather than terrestrial high plant debris (heavy 13 C value) contained in strata itself (Huang et al, 2022; Zhang, Zhang, et al, 2022). These characteristics are highly consistent with the source rock characteristics of the deep‐buried Lower Cretaceous Jiufotang Formation in the Qianjiadian fault depression (Zhao, Liang, et al, 2018), also supported by a large number of organic geochemical indicators and organic carbon isotopes from previous studies (Cui et al, 2022; Zhao et al, 2022).…”

“…A small amount of black oil stains and asphaltenes were visible under the microscope (Figures 2i and 3e). Notably, a great deal of oxidized trivalent iron (Fe 3+ ), that developed during the sedimentary period underwent secondary reduction to divalent iron (Fe 2+ ) minerals (Huang et al, 2018; Vinokurov, 2021), sometimes forming a few incomplete‐reduced oxidation residual bodies in the reddish‐grey sand‐bodies (Figure 2h).…”

“…The formation of hydrogenic sandstone‐hosted uranium deposits in sedimentary basins is generally a result of coupled epigenetic processes involving various fluids during different stages, primarily controlled by multiple geological tectonic activities (Chen et al, 2022; Cheng et al, 2020). The exudation of deep fluids occurred during the strong structure activity era, with significant short‐term and multi‐stage characteristics (Huang et al, 2018; Zhao et al, 2022). Redox uranium mineralization will be progressively developed as the epigenetic oxidation groundwater penetrated into the grey secondary reducing sand‐bodies during the long‐term stable and calm stage after strong tectonic movement (Han et al, 2021; Jin et al, 2022).…”

“…Revived fault structures and associated secondary faults provided vertical channels for the migration of hydrocarbon fluids originating from deep Lower Cretaceous oil and gas reservoirs (Chen et al, 2022;Huang et al, 2018). Reduced hydrocarbons shall escape and migrate into the shallow Upper Cretaceous sand-bodies along the penetrating F 1 fault and its associated secondary faults in the Qianjiadian area (Zhao, Cai, et al, 2018).…”

Petroleum in the Jurassic Reservoirs within the Eastern Fukang Sub‐depression, Junggar Basin, NW China: Correlation and Source Rock

“…It is a superimposed basin developed on the Junggar terrane, which has a Precambrian crystal basement and a lower Paleozoic bilayer basement (Zhao, 1992;Zhang et al, 1999). The basin was subjected to the Hercynian, Indosinian, Yanshanian, and Himalayan orogenies and was proposed to have an evolutionary history comprising four stages, i.e., the late Carboniferous to early Permian foreland oceanic basin, middle to late Permian interior foreland basin, Triassic to Cretaceous intracontinental depression and the Paleogene to Quaternary rejuvenated foreland basin (Chen et al, 2003;Cao et al, 2005;Huang et al, 2022). The Junggar Basin contains six first-order structural units and the Fukang sub-depression is one of the second-order structural units of the Central depression (Fig.…”