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Sedimentary characteristics and controls of a retreating, coarse‐grained fan‐delta system in the Lower Triassic, Mahu Depression, northwestern China
Author(s) -
Liu Beibei,
Tan Chengpeng,
Yu Xinghe,
Qu Jianhua,
Zhao Xiaoming,
Zhang Lei
Publication year - 2018
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3215
Subject(s) - geology , delta , facies , sedimentary rock , provenance , sedimentary depositional environment , subsidence , river delta , geomorphology , sediment , alluvial fan , structural basin , geochemistry , paleontology , aerospace engineering , engineering
In recent years, enriched hydrocarbon reservoirs have been largely founded in the Lower Triassic Baikouquan Formation, in the Mahu Depression of Junggar basin, northwestern China. Accordingly, the sedimentologic complexity of associated deposits has been extensively studied, though most of them focused on local oil‐producing fields separately. In this research, a detailed sedimentary study of the entire Mahu Depression has been conducted, which include several aspects of sedimentary characteristics, sediment provenance, evolution, and controls of the coarse‐grained fan‐delta system. According to facies features recognized from cores, thin sections, well logs, and seismic data, three sedimentary facies associations were identified: upper fan‐delta plain, lower fan‐delta plain, and fan‐delta front. Furthermore, four depositional systems (Xiazijie, Haungyangquan, Karamay, and Xiayan fan‐delta) around the Mahu Depression were recognized by the distribution of heavy mineral assemblages and change of ZTR index (percentage of Zircon, Tourmaline, and Rutile in the total transparent heavy mineral assemblage). Based on the analysis of facies and sediment provenance, the temporal and spatial distribution of the fan‐delta system were reconstructed associated with the variation of net/gross ratio. As characterized by decreasing‐upward of net/gross ratios and fining‐upward trend of grain size, the evolution of the fan‐delta succession showed an overall retreating pattern which was mainly controlled by lake level rising and rapid tectonic subsidence. Within the retreating pattern, hydrocarbons were trapped in the middle deposits with reservoir space produced by fan‐delta front and lower fan‐delta plain, which were encased between the top transgressive lacustrine mudstones and the bottom debris‐flow deposits produced by upper fan‐delta plain. These findings of the research throw light on the phenomena that the stratigraphy trap for hydrocarbon accumulation could be constructed by retreating, coarse‐grained fan‐delta systems.