Quantitative identification of coal-type gas and oil-type gas in source-mixed gas at the northern margin of Qaidam Basin

The current study tested the gas component and carbon isotopic composition of gas samples from 6 oilgas fields at the northern margin of Qaidam Basin, and established a chart to quantitatively identify the mixing ratio of source-mixed gas. Besides, this research quantitatively investigated the natural gas generated by different types of organic matter. The results show that different ratios of source-mixed gas exist in the 6 oil-gas fields at the northern margin of Qaidam Basin. Among them, Mabei has the highest mixing ratio of coal-type gas, followed by Nanbaxian, Mahai, Lenghu-4, Lenghu-3 and Lenghu-5, with the ratios of coal-type gas 91%, 87%, 83%, 66%, 55% and 36%, respectively. Lenghu-3 and Lenghu-4 oil-gas fields were mainly filled by coal-type gas earlier. For Lenghu-3, the gas was mainly generated from low matured source rocks in lower Jurassic Series of Lengxi sub-sag. For Lenghu-4, the gas was mainly generated from humus-mature source rocks in lower Jurassic Series of the northern slope of Kunteyi sub-sag. Gas in Lenghu-5 was mainly later filled oil-type gas, which was generated from high matured sapropelics in lower Jurassic Series of Kunteyi sub-sag. Earlier filled coal-type gas was the main part of Mahai, Nanbaxian and Mabei oil-gas fields. Gas source of Mahai was mainly generated from high mature humics in lower Jurassic Series of Yibei sub-sag; for Nanbaxian, the gas was mainly generated from high matured humics in middle-lower Jurassic Series of Saishiteng sub-sag; for Mabei, the gas was mainly generated from humus-mature source rocks in middle Jurassic Series of Yuqia sub-sag.

In situ LA-ICP-MS zircon U-Pb age of ultrahigh-pressure eclogites in the Yukahe area,northern Qaidam Basin

Zircon grains were selected from two types of ultrahigh-pressure(UHP)eclogites,coarse-grained phengite eclogite and fine-grained massive eclogite,in the Yukahe area,the western part of the North Qaidam UHP metamorphic belt.Most zircon grains show typical metamorphic origin with residual cores in some irregular grains and sector,planar or misty internal textures on the cathodoluminescence(CL)images.The contents of REE and HREE of the core parts of grains range from 173 to 1680μg/g and 170 to 1634μg/g,respectively,in phengite eclogite,and from 37 to 2640(g/g and 25.7 to 1824μg/g,respectively,in massive eclogite.The core parts exhibit HREE-enriched patterns,representing the residual zircons of protolith of the Yukahe eclogite.The contents of REE and HREE of the rim parts and the grains free of residual cores are much lower than those for the core parts.They vary from 13.1 to 89.5(g/g and 12.5 to 85.7μg/g,respectively,in phengite eclogite,and from 9.92 to 45.8μg/g and 9.18 to 43.8(g/g,respectively,in massive eclogite.Negative Eu anomalies and Th/U ratios decrease from core to rim.Positive Eu anomalies are shown in some grains.These indicate that the presence of garnet and the absence of plagioclase in the peak metamorphic mineral assemblage,and the zircons formed under eclogite facies conditions.LA-ICP-MS zircon U-Pb age data indicate that phengite eclogite and massive eclogite have similar metamorphic age of 436±3Ma and 431±4Ma in the early Paleozoic and magmatic protolith age of 783―793 Ma and 748―759 Ma in the Neo-proterozoic.The weighted mean age of the metamorphic ages(434±2 Ma)may represent the UHP metamorphic age of the Yukahe eclogites.The metamorphic age is well consistent with their direct country rocks of gneisses(431(3 Ma and 432±19 Ma)and coesite-bearing pelitic schist in the Yematan UHP eclogite section(423―440 Ma).These age data together with field observation and lithology,allow us to conclude that the Yukahe eclogites were Neo-proterozoic igneous rocks and may have experienced subduction and UHP metamorphism with continental crust at deep mantle during the early Paleozoic,therefore the metamorphic age of 434±2 Ma of the Yukahe eclogites probably represents the continental deep subduction time in this area.