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The siliceous rocks are well developed in Northwest Tarim Basin, which plays an important role in indicating its forming environment. In order to determine the origin of siliceous rock and its deposition settings, the paper studies the petrographic and geochemical characteristics of siliceous rock in well KP 1 and outcrops in Northwest Tarim Basin based on core observation, microscopic section identification and analysis of major and trace elements and rare earth elements. The Ordovician siliceous rocks of Northwest Tarim Basin are mainly developed in the middle and upper Penglaiba Formation and the bottom of the Yingshan Formation. Three types of cherts are identified, including cryptocrystalline silica, microcrystalline quartz and fibrous chalcedony depending on the mineral morphology. The terrigenous source is best evidenced by the high Ti (160∼310 ppm) and Al (1300∼3180 ppm), and positive δ30Si (0.9–3.6‰). Whereas, hydrothermal processes can be inferred from relative high content of V, Ni, Sr, Mo and Ba, and no obvious Ce and Eu anomaly. The chert of well KP 1 and Kuluxi profile is deposited in continental-margin settings, which is evidenced by its Al 2 O 3 /(Al 2 O 3 + Fe 2 O 3) (0.58–0.94, average of 0.81) and its La n /Ce n ratio (0.76–1.02, average of 0.93). The continental-margin settings are also evidenced by the close relation between silicification and dolomitization. Silicification is commonly well developed where dolomitization occurs, because the dolomitization mainly occurs in the regression background, where provides more terrigenous source silica to the sea water and results in more silicification. This study highlights the origin of Ordovician siliceous rocks and the outcome from this new contribution may provide useful guidelines for hydrocarbon exploration in the basin and other global analogues. • Based on the characteristics of main and trace elements and rare earth elements, it is concluded that the Ordovician siliceous fluid in well KP 1 is a fluid of mixed hydrothermal and terrigenous origin. • It is concluded that the precipitation form of siliceous fluid depends on the saturation degree and precipitation rate instead of the source of siliceous fluid. • The diagenetic coupling relationship between silicification and dolomitization is also studied. Since dolomitization occurs in the regression background, the decrease of sea level will result in the enhancement of terrigenous clastic weathering. Therefore, the formation with strong dolomitization is also characterized by strong silicification. [ABSTRACT FROM AUTHOR]

The Wensu uplift, to the northwest of the Tarim Basin, is located at the intersection of the marine Awati basin and the continental Kuche foreland basin. The origin and accumulation of hydrocarbons are complex processes in the Wensu uplift. The XWD 1 and 2 wells, drilled in the Wensu uplift in 2017, produced high-yield commercial oil flows from Neogene strata, thus realizing the first major breakthrough in the region's 50 years of oil and gas exploration. This discovery invited significant attention from the research community for shallow hydrocarbon accumulations along the margins of the Tarim Basin. This study focuses on the Neogene hydrocarbons of the Wensu uplift along with their geochemical characteristics and origin. We propose a hydrocarbon accumulation model, reconstructed by combining geology and geochemistry. The results show that crude oils from the Wensu uplift display typical lacustrine characteristics: 1) higher Pr/Ph values (1.68–2.04, average 2.33) that are indicative of an oxidizing sedimentary environment for the source rock; 2) higher content of rearranged steranes, C 29 Ts, and C 30 diasteranes and lower DBT/P value (0.09–0.11), indicating that the source rock is shale with a high content of clay minerals; and 3) lower sterane/hopane ratios (0.14–0.15) indicating high inputs of bacteria and microorganisms. The above characteristics are distinct from the adjacent marine crude oils which feature relatively lower ratios of Pr/Ph (1.27–1.37) and higher ratios of DBT/P (0.62–0.82) and steranes/hopanes (0.87–0.90). The oil-oil and oil-rock correlations conducted using hydrocarbon biomarkers and carbon isotope ratios show that the oils in Wensu uplift contain contributions from the lacustrine source rocks of the Jurassic Qiakemake (J 2 q) and Triassic Huangshanjie (T 3 h) formations of the eastern Wushi–Baicheng sag, suggesting that the Wensu uplift oils have a mixed origin. The age of stratigraphic traps in the Wensu uplift coincides with that of hydrocarbon expulsion peak and the hydrocarbons accumulated along the slope by long-distance migration. The results of this study aim to serve as an important guide for further studies on petroleum accumulation mechanisms and shallow hydrocarbon explorations in basin margins devoid of local hydrocarbon source rocks. • The Wensu uplift crude oils have typical non-marine characteristics. • The Wensu uplift crude oils were primarily derived from Jurassic and Triassic in the eastern Wushi–Baicheng sag. • The Wensu uplift crude oils has a mixed origin. • The 'allochthonous source' compound hydrocarbon conduit and accumulation model in Wensu uplift is reconstructed. [ABSTRACT FROM AUTHOR]

Abstract: Subtle traps have become an important petroleum play type for exploration in the Dongying Depression, Bohai Bay Basin, eastern China. The majority of the oil pools are in the middle interval of the Es 3 member of the Eocene–Oligocene Shahejie Formation, with the subtle traps being primarily in the lenticular, basin floor turbidite sands enclosed within basin floor mudstones. Detailed geochemical investigation of 68 oil and rock samples from the depression reveals that in contrast to previous models, the oils from the subtle traps correlate poorly with the surrounding source rocks. While the high pristane/phytane (Pr/Ph > 1) and low gammacerane/C30-hopane ratios in these source rocks represent a freshwater lacustrine setting, the oils feature low Pr/Ph ratios (<1) and relatively high gammacerane contents similar to the upper source rocks deposited in a brackish lacustrine setting. The compound-specific isotope ratios of n-alkanes in the oils have intermediate values between those observed in extracts from the lower Es 3 and upper Es 4 source rocks. Consequently, the oils in the subtle traps were most likely derived from mixed and sources, with the contribution from the source rocks being predominant. The oils generated from the source rocks must have migrated through a thick interval of the source rocks to reach the subtle traps in the , through which there are no apparent structural nor stratigraphic pathways. The traditional migration and accumulation model, i.e. indigenous crude oil with short migration distance to lenticular oil pools, should be revised to account for the charging of oils from source rocks. [ABSTRACT FROM AUTHOR]