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Highlights • The Lucaogou mudstones are still in the early oil window. • The biological sources are predominately algae and bacteria. • The Lucaogou mudstones were deposited in a hypersaline and anoxic environment. • The Lucaogou mudstones hold a significant exploration potential of tight oil. Abstract The mudstones of the Lucaogou Formation are the most important and organic-rich sediments and tight oil exploration target in the northwestern China. The organic and stable isotope geochemistry, and organic petrology were used to evaluate their sedimentary environments and hydrocarbon generative potential. The Lucaogou mudstones contain abundant organic matter, S 2 , hydrogen index (HI) and extractable organic matter (EOM), whereas S 1 values are low. The δ13C values of EOM are very light and display a negative correlation with HI. Rock pyrolysis data imply a low maturity (0.6–0.7 %R o equivalent) of the Lucaogou Formation, consistent with aliphatic and aromatic hydrocarbon ratios. The biological sources are predominately algae and bacteria, with less contribution of higher plants in the Lucaogou Formation as indicated by δ13C EOM , biomarkers and maceral composition. The Lucaogou mudstones contain low ratios of C 31 R/C 30 hopane, C 29 /C 30 hopane and C 22 /C 21 tricyclic terpane, and high C 24 /C 23 tricyclic terpane value, which are consistent with their depositional environments (i.e., lacustrine). The abundant β-carotane, gammacerane and hop-17(21)-enes indicate a hypersaline and anoxic environment. The studied samples are predominantly oil-prone with a dominant type I and II 1 kerogen. High contents of total organic carbon (TOC), S 2 , HI, EOM, saturated and aromatic hydrocarbons indicate that the Lucaogou Formation hold significant exploration potential of tight oil. [ABSTRACT FROM AUTHOR]

Abstract The Lower Permian Fengcheng Formation in the Mahu Sag of northwestern China is well-known for its alkaline lacustrine deposits. However, previous research is scarce due to this formation's notable burial depth (up to 5000 m) and limited oil and gas exploration. Using XRD, XRF, micro-XRF, and C-O isotope analyses, we determined: (1) the stratigraphic trends in mineral compositions, (2) the cyclic depositional processes, and (3) paleoclimate variations. The petrologic and geochemical data elucidate depositional and diagenetic processes in the mixed siliciclastic, volcaniclastic and chemical strata of the Fengcheng Formation. Sodium carbonate minerals in the studied core are dominated by wegscheiderite (Na 2 CO 3 ·3NaHCO 3) and trona (Na 2 CO 3 ·NaHCO 3 ·2H 2 O). Nahcolite (NaHCO 3) and a Mg-Na carbonate mineral, northupite (Na 2 CO 3 ·MgCO 3 ·NaCl), also occur, although their abundance is obviously lower than that of wegscheiderite and trona. These substances grew as bottom-growth crystals from lakes overlying tuffs. The volumetrically important Ca-Na carbonate mineral, shortite (Na 2 CO 3 ·2CaCO 3), and some northupite precipitated diagenetically from shallow groundwater brines in unlithified, dark-colored siliciclastic sediment and volcanic ash. Shortite mainly precipitated from the Ca-Na-CO 3 -rich shallow groundwater, whereas northupite precipitated from the Mg-Na-CO 3 -Cl-rich shallow groundwater. Sodium carbonates and tuffs, fine-grained tuffaceous rocks, and siliciclastic rocks formed repetitive centimeter- and meter-scale cycles that are shown as dark-colored lithological layers during lake expansion and contraction stages or during volcanic eruptions. The brine composition, which controlled the precipitation of sodium carbonates, was influenced strongly by volcanism and volcanism-related hydrothermal activity. Changes in climate and volcanic eruptions controlled the deposition of alternating beds of sodium carbonates and dark-colored lithologies. Changes in climate caused the expansion and contraction stages in the lakes. During relatively humid climate stages, diluted water entered the basin and resupplied the lake water with calcium and magnesium during flooding events. Sandy to silty beds were deposited by turbidity currents during this stage. During arid climate stages, bedded sodium carbonates grew in the lakes, and their co-occurrence suggests high atmospheric pCO 2 and high temperatures. The ashes that are provided by volcanic eruptions during arid climate stages can also stop the precipitation of sodium carbonates. Highlights • Sodium carbonates successions consist of alternating light-colored sodium carbonates layer and dark-colored tuff/tuffaceous layer. • The sodium carbonate minerals dominated by wegscheiderite and trona. Shortite and northupite occurred as transitional carbonate minerals. • Climates evolutions and periodic volcano eruptions joint to control the formation of depositional cycles in the sodium carbonates successions. [ABSTRACT FROM AUTHOR]

We report zircon U-Pb geochronologic and geochemical data for the post-collisional volcanic rocks from the Batamayineishan (BS) Formation in the Shuangjingzi area, northwestern China. The zircon U-Pb ages of seven volcanic samples from the BS Formation show that the magmatic activity in the study area occurred during 342-304Ma in the Carboniferous. The ages also indicate that the Palaeo-KaramailiOcean had already closed by 342Ma. Moreover, the volcanic rocks also contained 10 inherited zirconswith ages ranging from 565 to 2626 Ma, indicating that Precambrian continental crust or microcontinents with accretionary arcs are two possible interpretations for the basement underlying the East Junggar terrane. The sampled mafic-intermediate rocks belong to the medium-K to high-K calc-alkaline and shoshonitic series, and the formation of these rocks involved fractional crystallization with little crustal contamination. These Carboniferous mafic-intermediate rocks show depletions in Nb and Ta and enrichments in large ion lithophile elements (e.g. Rb, Ba, U, and Th) and light rare earth elements. The low initial 87Sr/86Sr values (0.7034-0.7042) and positive εNd(t) values (+2.63 to +6.46) of these rocks suggest that they formed from depleted mantle material. The mafic-intermediate rocks were most likely generated by 5-10% partial melting of a mantle source composed primarily of spinel lherzolite withminor garnet lherzolite that had been metasomatized by slab-derived fluids and minor slab melts. In contrast, the felsic rocks in the BS Formation are A-type rhyolites with positive εNd(t) values and young model ages. These rocks are interpreted to be derived from the partial melting of juvenile basaltic lower crustal material. Taken together, the mafic-intermediate rocks formed in a post-collisional extensional setting generated by slap breakoff in the early Carboniferous (342-330 Ma) and the A-type rhyolites formed in a post-collisional extensional setting triggered by the upwelling asthenosphere in the late Carboniferous (330-304 Ma). [ABSTRACT FROM AUTHOR]