Deep‐Water Sedimentation and Lake Level Changes of the Upper Triassic Huangshanjie Formation in the Tarim Basin

ABSTRACT The Upper Triassic in the Tarim Basin represents one of the most significant hydrocarbon source strata within the basin's Mesozoic‐Cenozoic petroleum system. However, oil and gas reservoirs within the Upper Triassic are still rarely discovered, and whether deep‐water sedimentation developed during this period remains controversial. In this study, a deep‐water identification framework integrating core structures, well‐log responses, and seismic facies was established based on extensive drilling, logging, and seismic data. Using wavelet transform analysis, a Triassic lake‐level fluctuation curve was reconstructed; in particular, the Huangshanjie Formation interval was highlighted and examined in greater detail, and the relationship between lake‐level variations and deep‐water sedimentary systems was evaluated. The results indicate that three large‐scale lake transgressions and two large‐scale lake regressions occurred during the Triassic. From the Ehuobulak Formation (T 1 e) through the Karamay Formation (T 2 k) to the Huangshanjie Formation (T 3 h), the basin underwent a transgressive‐regressive‐transgressive evolution, corresponding to a rise‐fall‐rise in lake level. During the Huangshanjie Formation, accommodation space reached its maximum, and the lake basin expanded to form a laterally extensive semi‐deep‐ to deep‐lacustrine environment, providing the necessary accommodation and slope‐break conditions for the development of deep‐water turbidite systems. In the northern part of the basin, multiple sets of deep‐water gravity‐flow deposits are developed within the Huangshanjie Formation, including turbidite sand bodies, turbidite channels, and sublacustrine fans. These sand bodies have a sand content of approximately 15%–20%, forming effective reservoir intervals. At the same time, thick dark mudstones (240–300 m) of semi‐deep‐ to deep‐lacustrine facies are developed, with total organic carbon (TOC) ranges from 0.31 to 5.0 wt%. These mudstones act not only as high‐quality source rocks but also as regional cap rocks. Comprehensive analysis suggests that the deep‐water sedimentary system of the Huangshanjie Formation was jointly controlled by lake‐level fluctuations and sediment supply. In the northern part of the basin, this system exhibits the geological conditions necessary to form a self‐generating and self‐storing lithologic reservoir, representing a favourable target for deep‐water exploration in the Upper Triassic.