## 2 Geological Setting  
Situated on the South China Sea's thinning palaeplatform, the Baiyun Sag is the principal sedimentary depocenter in the Pearl River Mouth Basin. The explored region is located in the northern part of the Baiyun Sag, near the Panyu Low Rise in the north and the Yunkai Low Bulge in the west (Figure 1a). The thickness of the sedimentary strata exhibits a steadily increasing trend from north to south. On the whole, the Zhuhai Formation displays more complex sedimentary structures and was dislocated by more developed faults [31; 32; 33; 34]. The Baiyun Sag is characterized by the bottom-up development of several sets of marine clastic strata, encompassing the Palaeocene--Shenhu Formation, the Palaeocene--Wenchang Formation, the Palaeocene--Enping Formation, the Palaeocene--Zhuhai Formation, the Neocene--Zhujiang Formation, the Neocene--Hanjiang Formation, and the Neocene--Yuehai Formation (Figure 1b) [19; 35; 36; 37; 38]. The target layer for this research is the Zhuhai Formation. During this period, the northern uplift area was the leading provenance supply area of the Baiyun Depression, while the Palaeo--Pearl River was the principal provenance transport channel. The thick shelf margin sedimentary strata of the Zhuhai Formation were formed by the change in relative sea level. The sedimentary period of the Zhuhai Formation was in the stage of comparatively sustained crustal uplift. The Baiyun Tectonic Movement gave rise to frequent faulting, and the Sag allowed for greater space. Simultaneously, the uplift of the Qinghai-Xizang Plateau and the eastern Himalayas elevated the sedimentary flux in the Pearl River Mouth Basin [39; 40; 41; 42; 43; 44; 45]. The Baiyun Sag has undergone the transformation of the Zhuqiong, Nanhai, and Dongsha Tectonic Movement. On that account, it has created four types of sedimentary combinations, namely, continental faulted basins, depression lake basins, shallow marine deltaic in transitional facies, and deep-water deposits in shelf facies. During the Eocene and earlier periods, the Baiyun Sag was predominantly deposited in a continental faulted basin, with braided-river delta deposits being the most ubiquitous kind. Following the Miocene, gravity flow deposits dominated the development of marine sedimentation in depressions. It is particularly noteworthy that the Oligocene Zhuhai Formation is a transitional facies deposit featured by shelf-margin deltas [46]. The shelf-margin delta is a massive delta wedge that forms along the continental shelf break, which is typical during periods of relative sea-level lowering. As the primary factor driving sand transport by rivers to the continental slope and basin, shelf-margin deltas can offer plentiful sediments for the deep water area in the basin's depression [47]. The Zhuhai Formation originated during the fault-depression transition phase. With sea-floor spreading, the fault depression evolved into a thermal subsidence depression, and the general tectonic context was extensional. The Zhuhai Formation is a transitional facies situated between the Nanhai and Baiyun Tectonic Movement. The top and bottom are separated by a regionally wide stratigraphic uncomortility. Furthermore, the base surface is a breakup uncomformity generated by uplift from the Nanhai movement against a background of tectonic denudation, revealing the basin's fault-depression transition. The  
Figure 1: The structural position of the study area and the comprehensive column chart [37] ((a) geographical location map of the study area, (b) composite bar chart).