Abstract:

The Juhugeng mining area in Qinghai Province is the primary discovery site of the gas hydrates in the mid-latitude of the global land, with the gas hydrates being preserved in the Middle Jurassic coal-bearing series of the Sanlutian mining field. This paper analyzed the depositional environments of the coal-bearing series in the Sanlutian mining field by using the borehole cores and logging curves. The palaeogeographic outlines of each lithological member of the Middle Jurassic have been reconstructed based on the single borehole facies analysis and the cross-section facies correlations. The lithology of the Lower Member of the Muli Formation is mainly fine conglomerates, pebbly sandstone and coarse-grained sandstone, reflecting a braided river-dominated environment. The Upper Member of the Muli Formation is characterized by the thick sandstone interbedded with thick coal seam, indicating a gradual deepening process and development of the deltaic environment. The Lower Member of the Jiangcang Formation is characterized by the fine -grained sandstones interbedded with mudstones and thin coal seams, signifying a further deepening process and development of the delta front and shore-shallow lake environments. The Upper Member of the Jiangcang Formation consists mainly of oil shale and dark mudstone containing bivalve fossils, reflecting a shallow to deep lake environment.

Key words: Middle Jurassic, depositional environment, gas hydrate, Juhugeng mining area, Sanlutian mining field