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Deep‐water gravity depositional processes and evolution in arc systems have become topics of intense research focus in recent years. This study discusses the co‐evolution of volcanism and deep‐water gravity flow deposits at the southern margin of the Junggar Basin, based on petrology, geochronology and geochemical analyses. The results show that a massive collapse of unstable sediments from the slope was triggered by volcanism, resulting in the formation of slumping gravity flows. The occurrence of volcanic beds in the slump deposits confirm that synchronous volcanism likely affected sediment instability, triggering gravity flows. The Th/Yb, Ta/Yb and Th/Ta elemental ratios, U‐Pb ages of detrital zircons and paleocurrent directions indicate that the North Tianshan (NTS) island arc represents the provenance of the Qianxia Formation. Moreover, statistical data on the pyroclastic components in the gravity flow deposits reveal an intensity index of volcanism, indicating that volcanism is strongly related to gravity flow deposits, especially in terms of the type and distribution of the deposits. A model for volcanically‐triggered deep‐water gravity flow deposits is established, in order to provide a more in‐depth understanding of the co‐evolution of volcanism and gravity flow deposits within the depositional setting of the late Paleozoic NTS oceanic subduction margin in the Junggar Basin. [ABSTRACT FROM AUTHOR]

Liang, Z.; Ji, D.; Zhang, X.; Lingyun, W.; Zhang, T.; Li, S.; Yong, J.; Luo, J.; Liu, Y., and Zeng, J., 2023. Carboniferous oceanic basin sediment-gravity flows in the margin of the Junggar Basin, NW China: Characterisation and implications for hydrocarbon potential and evolution pacing. Journal of Coastal Research, 39(5), 890–906. Charlotte (North Carolina), ISSN 0749-0208. Several Carboniferous deep-marine-sandstone reservoirs have recently been discovered in the Junggar Basin, NW China. These reservoir sandstones were deposited by sediment-gravity flows that occurred on the continental slope of the Junggar Basin, and they have high hydrocarbon exploration potential. In this study, careful observations and petrological and geochemical analyses of samples from outcrop sections of Carboniferous rocks from the Junggar Basin were conducted to establish the deep-marine sedimentary characteristics, depositional environments, and mechanism of these gravity-flow deposits, as well as a sedimentary model for their occurrence. The deep-marine, gravity-flow deposit types include those of turbidity currents, debris flows, and slumps. The debris-flow deposits can be divided into pebbly debris-flow deposits and sandy debris-flow deposits. Results allow the relationship between the type and nature of gravity flows and ocean basin evolution to be established. During the early Carboniferous, the ocean basin had sufficient space to accommodate incoming sediment, and the sediment transport distance was long. The gravity-flow sediments of this period are dominated by sandy debris-flow deposits and turbidity-flow deposits characterised by thin individual layers but thick accumulations of multiple deposits. With continuous narrowing of the oceanic basin during the late Carboniferous, the basin had a smaller accommodation space. The main types of gravity-flow deposition during this period were pebbly and sandy debris flows, with rare thin turbidity-flow deposits. These deposits were coarser grained than their early Carboniferous equivalents, and their cumulative thicknesses were smaller. The results of the study provide insights into the prediction of deep-marine reservoir sandstones and the distribution of hydrocarbons in the Junggar Basin. [ABSTRACT FROM AUTHOR]

Igneous rocks in deep basins are important unconventional petroleum reservoirs. Storage space and pore–throat structure of igneous rocks are crucial to be considered as effective reservoirs. There developed large-scale petroleum reservoirs in the Carboniferous igneous rocks of Hongshan-Zhongguai Bulge in the Junggar Basin, western China. The storage space and pore–throat structures of various types of igneous rocks and their significance to petroleum accumulation were studied based on the analysis of core samples. The methods include macroscopic core observation, microscopic observation under microscopes, scanning electron microscopy, micro-resistivity imaging logging and mercury injection experiments. Results show that the storage space is made up of primary gas pores, secondary dissolution pores and various types of fractures. The fillings of fractures are mainly calcite, followed by asphalt, chlorite and gypsum. The igneous rocks have different pore-throat structures, which govern the reservoir properties. Four types of igneous rock reservoirs were recognized, and the most favorable type is volcanic breccia and part of andesite. Dissolution plays an important role in remodeling the pore-throat structure and storage space. Tuff is the most susceptible to dissolution and present greatest alteration in storage space among the studied igneous rocks. An igneous rock weathering crust with high porosity formed near the unconformity capping the Carboniferous rocks, and oil is most abundant within 150 m across the unconformity surface. Faults and the weathering crust jointly control the formation of igneous rock reservoirs. The fault-controlled reservoirs commonly form in structural high position, and stratigraphic reservoirs generally developed in slope area. • Storage space is mainly composed of secondary dissolution pores and various types of fractures. • Four types of igneous rock reservoir rocks were recognized, and volcanic breccia and andesite are most favorable. • Tuff is most susceptible to dissolution and greatly altered in storage space. • Dissolution and fractures play important roles in remodeling the pore-throat structure and storage space. • Oil is mainly stored within 150 m across the top Carboniferous weathering crust. [ABSTRACT FROM AUTHOR]

The Carboniferous igneous rocks in the western margin of Junggar Basin, western China developed large-scale oil reservoirs. Hongshan-Zhongguai Bulge, the location of Jinlong Oilfield where oil and gas exploration in igneous rocks has achieved efficient breakthroughs in recent years, was studied as an example, because a large number of igneous rocks and crude oil samples are available in this area. The Carboniferous igneous rock reservoirs are typical fault-controlled block reservoirs, and are distributed in a stepped-like manner. Based on analysis of biomarkers, inclusions and reservoir cores under microscope, the oil sources, filling history and major controls for the igneous rock reservoirs that associated with accumulation mechanisms were studied. The biomarker geochemistry reveals that the oil in the igneous rocks was derived from the Permian source rocks including formations of Jiamuhe, Fengcheng and Lower Wuerhe (P 2 w), and the P 2 w is the primary source rock. Three important periods of oil charging in the igneous rock reservoirs are suggested: Late Triassic to Early Jurassic, Early Cretaceous, and Late Cretaceous to Early Tertiary. Volcanic breccia of eruptive facies is most favorable for igneous rock reservoir, followed by andesite of effusive facies. Strike-slip faults, overthrust faults and secondary normal faults formed into a fault network, interwoven with the unconformities in the top of the Carboniferous system and within the Permian strata, forming a vertical and horizontal three-dimensional petroleum migration pathway. Hydrocarbons migrated to igneous rock reservoirs from the Permian source rocks along strike-slip faults. The faults communicate the hydrocarbon source rocks with the upper weathering crust reservoirs and control the petroleum distribution. Due to weathering and leaching below the unconformity at the top of the igneous rock reservoirs, oil and gas are most abundant in the upper 150 m. In the fault belt the rocks are cemented by minerals of carbonate, clay, zeolite, quartz, and serpentine, and these types of cementation along faults were recognized as important factors for laterally sealing oil and gas in igneous rock reservoirs. • Types and the characteristics of igneous rock reservoirs were revealed. • Oil sources based on oil-source rock correlation were identified. • Hydrocarbon accumulation period and process was demonstrated. • Fault and unconformities controlling on igneous rock reservoir was demonstrated. • Diagenetic cements were recognized in igneous rocks along fault belt. [ABSTRACT FROM AUTHOR]

The Chepaizi Bulge of the Junggar Basin contains large-scale fault-controlled petroleum reservoirs in the Carboniferous igneous rock basement. Breccia, tuffaceous breccia, andesite, basalt, and tuff are the major lithology of the igneous rocks, and the major hydrocarbon accumulation was found in the volcanic breccia, andesite, and basalt reservoirs. The types of igneous rocks were predicted by their different logging responses, and were differentiated in the plot of acoustic versus gamma ray loggings when the core is not available. Secondary dissolution pores and fractures (primary tectonic fractures) were the major reservoir spaces. Fractures mainly occurred within 250 m below the top surface of the Carboniferous system. The fractures formed in the early, middle and late stages with different causes were identified. The reservoirs were dominated by mesopores with excessively low permeability, and the volcanic breccia reservoirs exhibited higher porosity (2.0%–16.31%, averages at 7.95%) and permeability (0.01 md–3.13 md, average at 0.11 md) than the basalt–andesite reservoirs (averages at 5.8% and 0.07md) on the whole. Volcanic breccia samples show larger average throat radius, average capillary radius and maximum connected throat radius compared with the basalt samples. The major oil accumulation is mainly distributed along faults, because there is more reservoir space and superior oil migration channels as a result of the induced fractures. Diagenetic cements including minerals of carbonate, clay, zeolite, serpentine, and quartz, were commonly observed in the fault zone, and these types of diagenetic cementation are crucial to the fault sealing, and are of great significance in the formation of large igneous rock reservoirs. • The methods for the identification of various types of igneous rocks were raised. • The significance of fractures with various types in the igneous rocks was revealed. • The controlling of fault on volcanic rock reservoir was reflected. • Volcanic breccia is favorable for oil accumulation relatively. • Diagenetic cements in igneous rocks and their significance was demonstrated. [ABSTRACT FROM AUTHOR]