Abstract:

The Juhugeng mining area in Qinghai Province is not only a major site for abundant coal resources, but also the primary discovery site of gas hydrate in high plateau of the mid-latitude worldwide. The gas hydrate exploration horizon is mainly in the Middle Jurassic coal-bearing series. In this paper, we studied the sedimentary environments and palaeogeography of the Middle Jurassic coal-bearing series in the Juhugeng mining area by analysis of data from borehole cores and logging curves. The lithofacies paleogeography of each lithological member was reconstructed by using combination of the quantitative and qualitative analysis methods, and the single borehole and cross section depositional section analyses. The Lower Member of the Muli Formation is mainly composed of fine conglomerates, pebbly sandstones, and coarse-grained sandstones, reflecting a braided river depositional environment. The Upper member of the Muli Formation consists of sandstones, siltstones, and two thick coal seams, and this member was formed mainly in the deltaic sedimentary environment, with the shore-shallow lake environment being locally developed in the southwestern part of the mining area. The Lower Member of the Jiangcang Formation is dominated by the fine-grained sandstones and mudstones, intercalated with several coarse-grained sandstone beds and thin coal seams, and this member was mainly deposited in the lower delta plain and delta front environments, with the shore-shallow lake environment developed in the southwestern part of the area. The Upper Member of the Jiangcang Formation, where the gas hydrates were frequently discovered, is dominated by a succession of oil shale and dark mudstone containing bivalve fossils, reflecting a deep to semi-deep lacustrine environment. During the Middle Jurassic the Juhgeng area has experienced an overall deepening process from the braided fluvial, deltaic plain, delta front and shore-shallow lake, to shallow to deep lake environments.

Key words: Juhugeng mining area, Middle Jurassic, lithofacies paleogeography, gas hydrate