Sedimentary characteristics and controls of a retreating, coarse‐grained fan‐delta system in the Lower Triassic, Mahu Depression, northwestern China

Abstract

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.

The depositional evolution and controlling factors of the Lower Triassic Baikouquan Formation, Northern Mahu Slope, Junggar Basin, NW China

“…The basin coverage is about 13.4x10 4 km 2 with a triangular shape (He et al, 2004;He, Zhang, et al, 2018). The complex tectonic evolution from the Late Carboniferous to the Quaternary results in six first-order tectonic units, including: Wulungu Depression, Central Depression, Eastern Uplift, Western Uplift, Luliang Uplift, and Northern Tianshan Fold Thrust Belt (Figure 1a) (Liu et al, 2019;Yang et al, 2015;Zhang et al, 2006;Zhao, 1992;Zhao et al, 2003). In addition, it can be further divided into several secondorder tectonic units Zhao et al, 2003).…”

“…Mahu Sag, as one of the sags in the Central Depression, is situated in northwestern Junggar Basin (Figure 1a) (Zhang et al, 2015;Zhu et al, 2019). It is adjacent to Kebai and Wuxia Fault to the west, Dabasong and Xiayan Rise to the east, Zhongguai Rise to the south, and Shiyingtan Rise to the north (He et al, 2004;Liu et al, 2019). In general, Mahu Sag approximately covers 5,000 km 2 with length span of 50 km from the east to the west and 120 km from the north to the south (Chen et al, 2015;Zhu et al, 2019).…”

“…In the meantime, boundary faults (including syn-depositional faults) began to activate and led to various kinds of fan deposits on multiple scales which were characterized by thick coarse-grained successions in different structural belts (especially on the slope) (He et al, 2004;He, Zhang, et al, 2018). The Mahu Sag was bounded by Ke-Bai Fault Belt and Wu-Xia Fault Belt (Liu et al, 2019;Shan et al, 2017). It was filled by deposits from the Carboniferous to the Quaternary with stratal thickness of more than 10 km (Pan et al, 2019).…”

“…Previous studies have demonstrated that depositional evolution in fan delta systems is complex (Eberth et al, 2001;Li, Feng, Lu, & Hao, 2010). In addition, controlling factors which play an important role in controlling facies distributions are complicated and integrative (Huang et al, 2015;Liu et al, 2019). Therefore, it is essential to establish a suitable depositional model to reveal the depositional evolution and their responses to controlling factors which may provide the basis for hydrocarbon exploration.…”

“…A great deal of previous studies were carried out to address academic issues in Baikouquan Formation in the northern slope of Mahu Sag regarding the tectonics (Chen, Lu, Shu, Wang, & Zhang, 2002;Chen, Wang, & Wang, 2005;He et al, 2004;He, Zhang, Wu, Li, & Zhen, 2018;He, Zhou, & Wu, 2012;Jahn et al, 2004;Yang, Song, & He, 2015), sequence stratigraphy and sedimentology (Eberth et al, 2001;Huang et al, 2015;Li et al, 2010;Liu et al, 2019;Lu et al, 2019;Ma et al, 2015;Meng et al, 2015;Peng et al, 2017;Yang et al, 2013), source rocks (Tan et al, 2017;Zhang et al, 2016;Zhu et al, 2019), and reservoir distribution and prediction (Chen et al, 2015;Kuang et al, 2017;Kuang, Tang, Lei, Wu, & Qu, 2014;Shan et al, 2017;Xiao et al, 2019). The focus in the study area change from tectonics and sedimentation to source rocks and reservoirs, which indicate that the transition of hydrocarbon exploration changes from macro view to micro view.…”