Depositional environments in an arid, closed basin and their implications for oil and gas exploration: The lower Permian Fengcheng Formation in the Junggar Basin, China

Yu, Kuanhong; Cao, Yingchang; Qiu, Longwei; Sun, Peipei

doi:10.1306/01301917414

Cited by 30 publications

(20 citation statements)

References 96 publications

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“…In contrast, the depocenter (i.e., the location of maximum thickness of fine-grained source rocks) of the Fengcheng Formation was located on the northwestern slope of the Mahu Sag and in the Fengcheng area of the Wu-Xia Fault Zone, where fine-grained sediments, such as argillaceous and dolomitic rocks, were mainly deposited. In the middle–late stages of the regression in the lower sedimentary cycle, extensive alkaline evaporites were deposited in the depocenter under hypersaline conditions (Figure b) . These rocks contain wegscheiderite and lesser amounts of trona, nahcolite, and northupite. , Volcanism occurred in parts of the study area during deposition of the Fengcheng Formation, and volcanic rocks are present in the Xiazijie area of the Wu-Xia Fault Zone and in parts of the southern slope in the Mahu Sag and Zhongguai Uplift.…”

Section: Geological Settingmentioning

confidence: 96%

“…The sediment provenance indicates the subsidence center (i.e., the location of maximum thickness of the entire Fengcheng Formation) was always on the northeastern side of the Ke-Bai Fault Zone. 27 The Fengcheng Formation has a maximum thickness of ∼1700 m. With increasing distance from the subsidence center, the formation thickness gradually decreases. At the subsidence center, ultrathick coarse-grained sediments were deposited in a fan-delta facies, which consist mainly of sandy conglomerates and pebbly sandstones (Figure 1b).…”

Section: Geological Settingmentioning

confidence: 99%

“…The lower cycle is represented by the first member (P 1 f 1 ), second member (P 1 f 2 ), and lower part of the third member (lower P 1 f 3 ), while the upper cycle is represented by the upper parts of the third member (upper P 1 f 3 ) (Figure b). The sediment provenance indicates the subsidence center (i.e., the location of maximum thickness of the entire Fengcheng Formation) was always on the northeastern side of the Ke-Bai Fault Zone . The Fengcheng Formation has a maximum thickness of ∼1700 m. With increasing distance from the subsidence center, the formation thickness gradually decreases.…”

Section: Geological Settingmentioning

confidence: 99%

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Chemometric Classification of Crude Oils in Complex Petroleum Systems Using t-Distributed Stochastic Neighbor Embedding Machine Learning Algorithm

et al. 2020

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The origin of crude oils is fundamental in the study of petroleum systems, but it encounters difficulties in complex systems because traditional geochemistry proxies are influenced by multiple factors (e.g., oil mixing, secondary alteration) and the interpretation of the data is challenging. To develop new potential approaches, a pilot study using the t-distributed stochastic neighbor embedding (t-SNE) machine learning algorithm was performed, based on a case study of the saline and alkaline lake petroleum systems in the lower Permian Mahu Sag, northwestern Junggar Basin, China. The algorithm revealed three main types of alkaline lacustrine related source rocks in the studied Fengcheng Formation: (i) argillaceous rocks deposited in brackish water and a weakly reducing environment; (ii) dolomitic mudstones deposited in saline water and a reducing environment; (iii) argillaceous dolomites deposited in hypersaline water and a strongly reducing environment. These organic facies are not time equivalent and vary temporally and spatially in the context of the alkaline lake evolution. Analysis of 43 crude oil samples showed that 5, 48, and 42% of the total number of samples were derived from argillaceous, dolomitic mudstone, and argillaceous dolomite source rocks, respectively, while the remaining 5% oil samples had a mixed origin from the former two end members. This suggests that hydrocarbon generation in the Fengcheng petroleum systems results mainly in large-scale oil generation from dolomitic source rocks. The biological precursors in the dolomitic rocks are dominated by haloduric algae, and the oil generation window is prolonged through organic–inorganic interactions during the hydrocarbon generation. This might be favorable for the preservation of an oil phase during deep burial and at high maturity. This represents a shale oil accumulation system in general as the source rocks and oils are the within the Fengcheng sequence. Our data suggest that the machine learning algorithm can find further application in this field with promising prospects.

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Section: Geological Settingmentioning

confidence: 96%

Section: Geological Settingmentioning

confidence: 99%

Section: Geological Settingmentioning

confidence: 99%

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Chemometric Classification of Crude Oils in Complex Petroleum Systems Using t-Distributed Stochastic Neighbor Embedding Machine Learning Algorithm

et al. 2020

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“…Quartz is relatively stable in acidic and neutral environments but is more easily dissolved in alkaline solutions, especially at higher pH values [69,70]. Experimental studies have shown that when the pH exceeds 6-7, the dissolution rate of quartz increases rapidly, thus forming curved quartz edges [71].…”

Section: Geofluidsmentioning

confidence: 99%

“…Due to the effects of organic matter evolution and fluid-rock reaction in each stage, the fluid pH will increase [73,74]. Therefore, alkaline fluids form because of 14 Geofluids burial diagenesis in clastic rocks [69][70][71][72]. Acid-labile minerals such as calcite and dolomite can be formed and preserved as cements in most clastic rock reservoirs, which also demonstrates that alkaline formation waters existed.…”

Section: Geofluidsmentioning

confidence: 99%

Chemically Active Elements of Reservoir Quartz Cement Trace Hydrocarbon Migration in the Mahu Sag, Junggar Basin, NW China

et al. 2021

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Element exchange and enrichment during fluid-rock interactions are common, providing potentially novel proxies to trace hydrocarbon migration in addition to the traditional organic geochemistry tracers. However, the processes, mechanisms, and geological and geochemical fingerprints of these interactions are complex, hampering the applications of hydrocarbon migration tracers. To investigate such interactions, we conducted a petrological, mineralogical, and in situ and bulk geochemical study of authigenic quartz and whole-rock samples from the Mahu Sag, northwestern Junggar Basin, northwest China. We found that dissolution, clay and chlorite formation, and overgrowth occurred on quartz grains in hydrocarbon fluid migration pathways, suggestive of strong fluid-rock interactions. In situ quantitative elemental analysis of quartz grains revealed elemental enrichment (e.g., Mn, Fe, Al, Sr, and W) in quartz overgrowth rims compared with their cores, indicating that migration of hydrocarbon-bearing fluids in reservoirs may promote elemental exchange between fluids and minerals. Whole-rock geochemical analysis showed that decreasing contents of some elements may reflect the direction of hydrocarbon-bearing fluid migration and can be monitored with three geochemical proxies, which are the MnO contents and MnO/Zr and Y/Ho ratios. Our data provide new constraints on fluid-rock interactions in petroleum reservoirs and have implications for using inorganic geochemical methods to trace hydrocarbon migration.

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Subaqueous felsic volcanic sequence and its contribution to the ancient alkaline lacustrine deposits in the Mahu Sag, Junggar Basin, NW China

Zhang

et al. 2023

Geological Journal

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Subaqueous eruptions alter the chemical conditions of the lake by intensifying the interaction between hot volcanic rocks and water, resulting in the exchange of alkalis. The lower part of the Fengcheng Formation in the Mahu Sag, NW China, has abundant rhyolitic welded tuff and related pyroclastic deposits, which were deposited under ancient alkaline lacustrine deposits of the Fengcheng Formation. Despite this, the effect of the eruption process on the formation of the alkali lake remains unclear. To gain a better understanding of the eruptive behaviour and emplacement processes, detailed drilling core descriptions, microscopic observations, and interpretations of textural characteristics of rhyolitic welded tuff units and adjacent volcano‐sedimentary associations have been conducted. The rhyolitic welded tuff can be categorized into seven lithofacies units based on the degree of welding, devitrification characteristics, pore development, and filling characteristics. The age of eruption was determined through zircon U–Pb dating of the rhyolitic welded tuff, revealing it to be approximately 300 Ma, thereby providing constraints on the deposition time of alkaline lacustrine deposits. Mudstones that encase the welded tuff unit and quenched structures such as perlitic fractures, quench cracks of spherulite and lithophysae, and chalcedony spherules indicate that the welded tuff unit was formed through subaqueous eruption. Alkali fractionation (sodium and potassium) occurs in the spherulite and lithophysae zone and at the top condensation boundary, demonstrating that the subaqueous eruption process significantly enhanced devitrification and hydration, causing sodium to escape from the welded tuff and released into the lake water as sodium ions. The rhyolitic welded tuff provides evidence of the evolution of a subaqueous felsic eruption where the water‐rock reaction played a fundamental role in alkalizing the lake by providing the lake with sodium ions in Mahu Sag.

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Depositional environments in an arid, closed basin and their implications for oil and gas exploration: The lower Permian Fengcheng Formation in the Junggar Basin, China

Cited by 30 publications

References 96 publications

Chemometric Classification of Crude Oils in Complex Petroleum Systems Using t-Distributed Stochastic Neighbor Embedding Machine Learning Algorithm

Chemometric Classification of Crude Oils in Complex Petroleum Systems Using t-Distributed Stochastic Neighbor Embedding Machine Learning Algorithm

Chemically Active Elements of Reservoir Quartz Cement Trace Hydrocarbon Migration in the Mahu Sag, Junggar Basin, NW China

Subaqueous felsic volcanic sequence and its contribution to the ancient alkaline lacustrine deposits in the Mahu Sag, Junggar Basin, NW China

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