| "Scientific Drilling Project of Gas Hydrate in Qilian Mountain Permafrost" has successfully drilled gas hydrate samples in Muli area and has verified the presence of gas hydrate in Qilian Mountain Permafrost. China became the first country that discover natural gas hydrate in the mid-latitude alpine permafrost. Besides the background information, the paper also relies on the research result of 8 existing detection wells (DK1, DK2, DK3, DK4, DK5, DK6, DK7, DK8) in Qilian Mountain Area of a State-owned special project ("Qilian Mountain Natural Gas Hydrate Exploration" project). This paper mainly studies the rock core of DK9 detection well in Muli basin by petrological characteristics analysis and by geochemical analysis of mudstone trace element and rare earth elements. Combined with the analysis of logging and sedimentary facies, this paper discusses about the ancient sedimentary environments and sedimentary evolution of the formation of Qinghai Muli Basin during Jurassic period. The main conclusions are listed below:(1) By drilling and observing the rock core of DK9 well, there are two Jurassic formation:lower Muli group and upper Jiangcang group. Faulted contact appears between two formation and the formation repeats. Muli group formation mainly consists of fine sandstone mingled with siltstone and mudstone; Jiangcang group formation mainly consists of mudstone mingled with siltstone and rarely shale. By sampling analysis of the core from the significant interval, the main sandstone lithology is considered to be quartz sandstone, indicating a distant transport distance and pure particles. Its sedimentary environment is the delta plain-a branch river of the delta front and the underwater branch river section. Grain size analysis results show that the sandstone grain size of the region is mainly of fine-grained sandstone-siltstone. Physical test results show that it is a good reservoir with good porosity and permeability of the sediment in the area.(2) By using the method of trace element geochemistry to analyze the Muli formation and Jiangcang formation mudstone sedimentary environment, the sedimentary environment is considered to be low-salted and neutral with some alteration, oxidation of Muli formation and reduction of Jiangcang formation. The climate was humid climate-semi-humid climate. Through a comprehensive analysis of salinity, redox and the fluctuation of the trace elements and REE abundances, an apparent difference in Jiangcang group is found and proved to be accorded with the depositional-facies change from shallow lake to semi-deep lake. Analysis of the abundance and combination of REE and trace element showed its provenance should be from the upper crust felsic source area of active continental margin,and some basic rock material and ancient sediments was added to the sediment during he deposition process.(3) Sedimentary evolution of Jurassic group of the DK9 well is,the delta plain-delta front interactive sediment of lower Muli formation, with a gradually humid climate upwards and the trend of deepening water, increasing clay content, gradually thinning sandstone thickness, and an interactive deposition of mudstone and siltstone which replaced the sandstone,which manifest that the upper Muli group sedimentary environment has turned into delta front-lacustrine deposits. Jiangcang Group sediments is mainly of mudstone, mingled with a small amount of siltstone, a thin layer of shale is visible, and the shale deposits may indicate the location of the maximum flooding surface. Jiangcang group sedimentary environment is semi-deep lake to deep lake environment, with continuously fluctuating water, and the maximum flooding surface location is the area of shale. In conclusion, sedimentary evolution of Muli group and Jiangcang group during the Jurassic period is a water deepening process of delta plain-delta front-shallow lake-semi-deep. |
PDF Full Text Request